Tool Wear in Titanium Machining

UPTEC K12 006 Examensarbete 30 hp Juni 2012

Tool wear in titanium machining

Stina Odelros

Abstract Tool wear in titanium machining

Stina Odelros

Teknisk- naturvetenskaplig fakultet UTH-enheten The present work was performed at AB Sandvik Coromant as a part in improving the knowledge and understanding about wear of uncoated WC/Co cutting tools during Besöksadress: turning of titanium alloy Ti-6Al-4V. Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 When machining titanium alloys, or any other material, wear of the cutting tools has a huge impact on the ability to shape the material as well as the manufacturing cost of Postadress: the finished product. Due to the low thermal conductivity of titanium, high cutting Box 536 751 21 Uppsala temperatures will occur in narrow regions near the cutting edge during machining. This will result in high reaction and diffusion rates, resulting in high cutting tool wear Telefon: rates. To be able to improve titanium machining, better knowledge and understanding 018 – 471 30 03 about wear during these tough conditions are needed.

Telefax: 018 – 471 30 00 Wear tests were performed during orthogonal turning of titanium alloy and the cutting tool inserts were analysed by SEM, EDS and optical imaging in Alicona Hemsida: InfiniteFocus. Simulations in AdvantEdge provided calculated values for cutting http://www.teknat.uu.se/student temperatures, cutting forces and contact stresses for the same conditions as used during wear tests.

It was found that turning titanium alloy with WC/Co cutting tools at cutting speeds 30-60 m/min causes chamfering of the cutting tool edge and adhesion of a build-up layer (BUL) of workpiece material on top of the rake face wear land. The wear rate for these low cutting speeds was found to be almost unchanging during cutting times up to 3 minutes. During cutting speeds of 90-115 m/min, crater wear was found to be the dominating wear mechanism and the wear rate was found to have a linear dependence of cutting speed. An Arrhenius-type temperature dependent wear mechanism was found for high cutting speeds, between 90 and 115 m/min.

Handledare: Jonas Östby Ämnesgranskare: Mats Boman Examinator: Rolf Berger ISSN: 1650-8297, UPTEC K12 006 Sponsor: AB Sandvik Coromant

Populärvetenskaplig sammanfattning Förslitning av skärverktyg vid svarvning av titan

För att kunna tillverka produkter utav metaller och andra hårda material, som ofta kan vara mycket svåra att forma, behöver man speciella skärverktyg. Sandvik Coromant är en tillverkare av sådana verktyg som till huvudsak består av volframkarbid och kobolt som pressas ihop och bildar ett s.k. hårdmetallskär efter uppvärmning. Då skäret används vid bearbetning är det önskvärt att det ska kunna användas så länge som möjligt innan det slits ut, samtidigt som man inte vill sänka skärhastigheten utan behålla så hög produktionstakt som möjligt. Användning av utslitna verktyg kan ge en dålig ytstruktur på produkten vilket i värsta fall leder till att bearbetade delar måste kasseras. För att kunna optimera tillverkningsprocessen vill man därför kunna förutspå livslängden hos skären och för att kunna göra detta behövs kunskap om vad som händer med material och verktyg under bearbetning.

Titan är ett material som har många bra egenskaper, t.ex. hög styrka jämfört med dess vikt i kombination med bra korrosionsmotstånd. Det tål att användas i konstruktioner som utsätts för temperaturer upp till några hundra grader vilket gör att man gärna vill använda materialet i produkter som ofta utsätts för påfrestningar, t.ex. i flygplanskonstruktioner och motorer. Titan är ett material med dålig värmeledningsförmåga och hög kemisk reaktivitet, vilket innebär att det gärna reagerar med andra material och ämnen som kan finnas i dess omgivning. Reaktionsbenägenheten ökar med ökad temperatur och eftersom titan leder värme dåligt så uppstår mycket höga temperaturer, upp till 1100˚C, i kontakten mellan arbetsmaterial och verktyg under bearbetning. Risken för att arbetsmaterialet reagerar med skärverktyget är därför mycket stor vid titanbearbetning och förmodligen en av de främsta orsakerna till varför livslängden för hårdmetallskär vid titanbearbetning är så låg. Med mer kunskap om vad som händer med skäret under bearbetning och vilka påfrestningar det utsätts för skulle man eventuellt kunna förbättra livslängden.

För att närmare undersöka vad som händer med hårdmetallskär vid titanbearbetning utfördes ett antal tester med olika skärdjup, skärhastighet och bearbetningstid. Obelagda hårdmetallskär användes för att svarva i titanlegeringen Ti-6Al-4V. För att se vad som hänt på ytan av skäreggen studerades och analyserades dessa i svepelektronmikroskop med röntgenanalys. Med hjälp av det optiska instrumentet Alicona InfiniteFocus, kunde skäreggarna avbildas och geometrin hos skären före och efter bearbetning kunde mätas. Detta gjorde det möjligt att beräkna bortnött volym och på så sätt uppskatta en förslitningshastighet. Eftersom temperaturer som uppstår vid kontaktytan mellan skär och spåna är mycket svåra att mäta så uppskattades dessa genom beräkningar i simuleringsprogrammet AdvantEdge. AdvantEdge är ett program som med hjälp av olika materialmodeller, givet olika skärdata, beräknar ett troligt jämviktsläge för olika parametrar under skärförloppet. Från dessa simuleringar kan man få fram t.ex. temperaturer, skärkrafter och kontaktspänningar.

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Resultaten visade på hur den ursprungliga skäreggen snabbt deformerades och avfasades varpå ett tunt skikt av titanlegeringen adderades till ytan av förslitningsärret. För låga skärhastigheter, mellan 30 och 60 m/min, och en bearbetningstid upp till tre minuter observerades en relativt låg förslitningshastighet och den dominerande förslitningsmekanismen verkar vara addition av arbetsmaterial i skärzonen. För hastigheter mellan 90 och 115 m/min och bearbetningstider upp till 3 minuter är gropförslitning den dominerande mekanismen och förslitningshastigheten är högre än för skärhastigheter 30-60 m/min. Gropförslitningen i sin tur verkar domineras av en kemisk förslitningsprocess, där adderat arbetsmaterial slits bort från hårdmetallskärets spånsida pga. dålig vidhäftning och samtidigt tar med sig underliggande material. För höga skärhastigheter, 90-115 m/min, visar resultaten även att det finns ett Arrhenius-liknande samband för förslitningens variation med kontakttemperatur.

Dictionary WC – Wolfram carbide

WC/Co – Cemented carbide

SEM – Scanning Electron Microscopy

EDS – Energy Dispersive X-ray Spectroscopy

BUL – Build up layer

BUE – Build up edge vc – Cutting speed fn – Feed rate ap – Depth of cut t – Cutting time

L – Cutting length

Fp – Back force

Ff – Feed force

Fc – Cutting force

θ – Cutting temperature

σt – Normal stress

Contents

Abstract…………………………………………………………………………………………i Populärvetenskaplig sammanfattning...... iii Dictionary ...... v 1. Introduction ...... 1 1.1 Background ...... 1 1.2 Assignment ...... 2 2. Theory...... 3 2.1 Machining ...... 3 2.1.1 Chip formation and cutting forces...... 4 2.1.2 Orthogonal cutting...... 5 2.2 Wear...... 6 2.2.1 Abrasive wear...... 6 2.2.2 Adhesive wear ...... 6 2.2.3 Diffusion/dissolution wear ...... 7 2.2.4 Chemical wear ...... 7 2.2.5 Wear due to plastic deformation ...... 7 2.3 Cutting tool wear ...... 7 2.3.1 Crater wear ...... 8 2.3.2 Flank wear ...... 8 2.3.3 Chipping ...... 9 2.3.4 Fracture...... 9 2.3.5 Notch wear ...... 9 2.3.6 Chatter ...... 9 2.4 Tool life prediction ...... 9 3. Literature study of Ti machining ...... 11 4. Experimental & Simulations ...... 13 4.1 Machining experiments ...... 13 4.2 Analysis methods ...... 15 4.2.1 Alicona ...... 15 4.2.2 Scanning Electron Microscopy ...... 15 4.2.3 Simulations...... 16

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5. Analysis and Results...... 17 5.1 Alicona...... 17 5.2 Scanning Electron Microscopy (SEM) ...... 21 5.2.1 Low cutting speeds...... 22 5.2.2 High cutting speeds ...... 25 5.3 Force measurements ...... 29 5.4 Simulations in AdvantEdge ...... 31 5.5 Wear modeling...... 34 6. Discussion...... 35 6.1 SEM and EDS ...... 35 6.1.1 Rake face wear land ...... 35 6.1.2 Flank face wear land ...... 37 6.1.3 Rake face ...... 38 6.2 Optical measurements...... 38 6.3 Cutting forces...... 39 6.4 Wear modeling...... 40 7. Conclusions ...... 41 8. Future outlooks ...... 43 9. Acknowledgements ...... 45 10. References ...... 47 APPENDIX A ...... 49 APPENDIX B...... 53 APPENDIX C...... 55 APPENDIX D ...... 59

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1. Introduction Sandvik Coromant is the world’s leading supplier of cutting tools and tooling systems, with 70 years of experience in the metalworking industry [1]. Today, Sandvik Coromant’s tools are used all over the world in manufacturing of all different kinds of products, from aeroplanes and smartphones to beverage cans. Being world leader demands knowledge of how different cutting tools are affected by different tool-workpiece combinations in order to recommend the cutting tool most suitable for different machining operations. The search for new knowledge and understanding of how different materials affect each other during different machining conditions can therefore never end.

1.1 Background Cemented carbide (WC/Co) cutting tool inserts are often used in the machining of hard materials such as different metals or alloys [2]. In most applications the use of WC/Co cutting tool inserts is well documented, but for some materials machining is difficult due to extensive tool wear and limited knowledge about the mechanisms involved. Titanium machining is one of these materials.

Titanium alloys are used in a wide range of light-weight applications in aerospace, energy and chemical industries because of good mechanical properties and excellent corrosion resistance in combination with the materials’ high strength-to-weight ratios [3, 4]. Titanium alloys have the potential to be used in a number of new manufacturing areas, but difficulties during machining of titanium parts and products due to their low thermal conductivity and high chemical reactivity often make the material change uneconomic.

During titanium machining, cutting temperatures up to 1100˚C [5] will be achieved at the tool-chip interface [3, 4]. Uncoated cemented carbide (WC/Co) cutting tools are frequently used during titanium machining, but the tool wear is often fast and extensive, making titanium difficult and expensive to machine. It is well known that higher contact temperatures give higher reaction rates and probably also higher tool wear rates during machining, but since temperatures at the cutting edge are very hard to measure they must often be simulated instead. If knowledge about different wear mechanisms and when they dominate is enhanced, improvements in the machining process and cutting tool life time could be achieved.

Pure titanium shows an allotropic behaviour with a reversible crystal structure transformation from alpha (α), hexagonal closed-packed, structure to beta (β), body-centred cubic, structure when the temperature is raised above 882°C [5]. By adding different alloying materials to titanium, the crystal structure transition temperature can be changed. Alpha-stabilizing materials, such as aluminium (Al) and oxygen (O), raise the transition temperature, while beta-stabilizing materials, such as vanadium (V) and tungsten (W), lower it [6]. The microstructures and properties of titanium alloys can be altered by adding various alloying elements or performing heat treatments on the material [3], but these factors have not been

1 considered in the present work. The most widely used titanium alloy, which has also been used in the present work, is the Ti-6Al-4V alloy, consisting of a base of Ti with 6 wt% Al and 4 wt% V [7]. Ti-6Al-4V is an α-β alloy, assessed as an ASTM grade 5 titanium alloy [8].

1.2 Assignment The assignment of this master thesis project is to enhance the knowledge and understanding of wear mechanisms during titanium machining. This will be performed by studying the worn surfaces of uncoated WC/Co cutting tools after orthogonal turning of titanium alloy Ti-6Al- 4V. The surfaces will be examined both by Scanning Electron Microscopy (SEM) with Energy-dispersive X-ray spectroscopy (EDS) and through optical 3D topography imaging of the attritional wear. Cutting temperature simulations will be performed in the FEM-based software AdvantEdge [9]. Comparison of cutting temperatures and wear mechanisms could hopefully reveal any obvious correlations between the two when machining titanium alloys. This thesis will hopefully contribute to a new understanding of the extensive tool wear and its mechanisms during titanium machining.

The goals of this work are to enhance the understanding of different wear mechanisms by:

 Determining if there is a relation between chemical reactions taking place at the worn surfaces and different wear mechanisms.  Connecting, if possible, different chemical reactions to different wear mechanisms, using temperature simulations and comparison to phase diagrams.

2. Theory

2.1 Machining Machining with cemented carbide (WC/Co) cutting tools is a well-documented versatile machining operation [2] often used for metals or other hard materials. The cutting tool usually consists of a steel shaft and a cemented carbide cutting tool insert which can be replaced when worn out. Cemented carbide is a metal matrix composite material consisting of a matrix of cobalt (Co) with tungsten carbide (WC) particles embedded inside. The cutting tool insert is created by pressing and sintering of WC and Co powder, together with other carbide additives (like TiC, CrC, ZrC or SiC) where applicable. The size of the WC grains and the amount of Co or other carbides added will change the properties of the insert. Normally 3-20 wt% Co is used depending on the desired properties of the insert. An increasing amount of Co added increases the toughness of the cutting tool insert while the hardness and strength at the same time decreases [2]. The cutting tools or their tips are often coated by another thin film material to enhance tool life and reduce friction. Cutting tools can vary in size, geometry and alloying materials, making a wide variety of different tool geometries and different material grades [2]. Different tool holders could also be used for different machining operations or different machines.

There are two basic standards for defining cutting tool geometry: the American National Standard B94.50-1975 [10] and the ISO standard 3002/1 [11]. These standards describe the cutting tool geometry in similar ways but with slightly different notations and reference planes. Cutting tool geometries include a number of different surfaces and angles defined in different planes [2]. In this thesis the most common faces and angles will be described according to ISO standards.

The cutting parameters presented in fig. 1 and 2, defined by ISO 3002/1 [11], have been found to be of great importance during machining and have impact on the cutting tool life time as well as the machined surface finish and structure [12]. The velocity with which the workpiece is moving relative to the cutting tool is referred to as cutting speed or vc. Feed rate, fn, is the feed motion that leads to continuous or repeated removal of workpiece material and creates the machined surface. Depth of cut, ap, is the width/depth of the cut (depending on cutting operation). Rake face is the cutting tool surface closest to the chip, on the front edge of the cutting tool, over which the chip formed slides. The edge that cuts the workpiece material is the cutting edge, which is theoretically the line of intersection between rake face and flank face. The flank face is the surface of the cutting tool that the newly created surface flows against [13].

Figure 1: Schematic 2D figure showing the geometry of cutting operation according to ISO 3002/1. Cutting tool insert acting on workpiece material with cutting speed vc at feed rate fn, equal to the uncut chip thickness t, causing a shear leading to chip formation and cut chip thickness t1. Cutting tool approach angles α and γ are the flank angle and rake angle, respectively.

There are two basic surfaces to consider during machining (defined in ISO 3002/1) [11], namely: work surface, which is the surface removed by machining operation, and machined surface, which is the surface produced after the cutting tool have passed [2].

Angles defined in fig. 1 are:

 Flank angle, α, which is the angle between the machined surface on the workpiece and the flank face of the cutting tool.  Rake angle, γ, which is the angle between rake face of the cutting tool and the plane perpendicular to the direction of the cutting motion.

2.1.1 Chip formation and cutting forces During all machining with cutting tools, severe plastic deformation of workpiece material occurs in front of the cutting edge, causing a shear that leads to chip formation. The workpiece will act on the cutting tool with a certain total force (F), which can be divided into different geometrical or physical components according to ISO 3002/4 [14]. Three of the geometrical force components described in ISO 3002/4 have been visualized in fig. 2. The cutting force (Fc) is the main component of the total force and is obtained by a perpendicular projection of the total force on the direction of primary motion. Fc is the force contribution giving rise to the cutting power. Feed force or in-feed (axial) force (Ff) is the force component obtained by perpendicular projection of the total force on the feed direction. Back force (Fp) is the force perpendicular to both the primary motion and the feed motion [14].

Figure 2: Geometrical cutting forces acting on the cutting tool during machining operation.

Cutting-force signals are highly sensitive carriers of information about the status of the machining process [15] and are therefore one of the most promising techniques for indirect detection of tool wear. Force measurements have been proven to correlate with cutting tool wear and can thereby be used as a tool wear indicator. Several tool wear prediction models have been suggested for trying to improve knowledge about when tools are worn out [16]. Force measurements can be performed during machining using a piezoelectric quartz crystal dynamometer. The cutting forces applying pressure on the piezoelectric crystal cause an electromagnetic force proportional to the force or pressure exerted on the cutting tool [17].

Analyses of cutting forces and/or moments have traditionally been important in metal machining operations, allowing estimations of for example tool deflection, wear, heat generation and power consumption.

2.1.2 Orthogonal cutting When the cutting edge is orientated as being perpendicular to the direction of cutting motion during a turning operation, the machining process is referred to as orthogonal or two- dimensional cutting [16]. Orthogonal cutting is regarded as the simplest machining process, ideally used for a simple cutting process model in theoretical and experimental work, but not very frequently used in industrial practice. In orthogonal cutting, effects of independent variables have been eliminated as much as possible so that influences of basic parameters can be studied more accurately [16].

When studying orthogonal cutting, a situation is often considered in which the cutting edge is wider than the depth of cut and does not pass through earlier machined surface edges [13]. The back force (Fp) is supposed to be non-contributing to the total cutting force (F) and the

5 geometrical force components presented earlier (in section 2.1.1) are narrowed down to two.

If the cutting nose radius is of significant size compared to depth of cut, Fp cannot be neglected and the operation is viewed as semi-orthogonal. During machining, when development of wear land on the cutting tool flank face occurs, additional cutting force contributions will be introduced [16], but those will be neglected in this thesis. For orthogonal cutting, another common idealization that has been made in the present work, is to treat a round turning workpiece with a large radius compared to feed depth as infinitely long and un- curved.

2.2 Wear Material wear processes are found at all places where materials are in mechanical contact with each other [17]. Wear is often present as combinations of several different physical wear mechanisms and the ones most likely to be present during mechanical contact are: abrasive, adhesive, diffusive, chemical, and wear due to plastic deformation [2, 18]. The dominating wear mechanism depends on the surfaces, the contact area between them, materials, topography, hardness, etc.

2.2.1 Abrasive wear Abrasion is a wear mechanism that arises from hard particles that abrade on a softer material [16]. The mechanism appears on almost all contact surfaces that have a relative velocity against each other and is dependent on the relative hardness of the abrading particles and abraded material. Most abrasion starts with two-body abrasion but switches to three-body abrasion at a certain point, when wear-off particles have been formed. Two-body abrasion often results in much higher wear rates than three-body abrasion does.

The abrasive wear often increases with increasing temperature, due to the decrease in material hardness at elevated temperatures [16, 19].

2.2.2 Adhesive wear Adhesion occurs when temperatures and pressure are high. Small particles are welded together when two metals are forced together [16, 18]. When the two metals also have a relative velocity against each other, as is the case in metal cutting, the small welds formed by adhesion will cause micropieces of the tool to break loose.

The adhesion of workpiece material on the cutting tool could form a built up edge (BUE) [19] and in that case the wear rate due to adhesive wear could be very high [17]. In the case when high speeds and temperatures cause adhesion junctions between workpiece material flowing past the flank face and the cutting tool material, carbide particles can be plucked from the WC/Co cutting tool into the chip. This wear mechanism has been referred to as attrition wear [19].

2.2.3 Diffusion/dissolution wear When two materials are in contact with each other, atoms from one material could diffuse into the other, causing diffusion or dissolution wear. Diffusion or dissolution wear mainly occurs at high temperatures and is strongly depending on the solubility of cutting tool material in workpiece material [18]. It is believed that diffusion between cutting tool and workpiece material is the dominating mechanism for crater wear (see section 2.3.1) at high cutting speeds [19]. It is also supposed that diffusion causes the tool to be depleted of some atoms, making the material softer and more sensitive to abrasive and adhesive wear.

2.2.4 Chemical wear Chemical wear arises from chemical reactions, such as oxidation or forming of compounds, which occur on clean surfaces at high temperatures [16]. Oxide layers could act as a protection against wear at the surface during mechanical contact, but they could also speed up the wear rate, depending on the formation rate of the oxide and the hardness and topography of the other surface.

In the case when the oxide formed on the surface is too brittle, chipping (see section 2.3.3) will occur and the oxide will fall off. Formation and breaking of junctions will lead to removal of tool material from the surface [19].

2.2.5 Wear due to plastic deformation The combination of high cutting forces and high temperatures could result in plastic deformation of the tool’s cutting edge and give the tool a new geometry so it loses its initial characteristics. Plastic deformation could occur both at the surfaces and at the cutting edge [16, 17].

2.3 Cutting tool wear The extent of cutting tool wear depends on the tool material and geometry, workpiece material, cutting parameters, cutting fluids and machine-tool characteristics. The wear land of the cutting tool insert is the area of the cutting tool, near the cutting edge, were the insert is worn during machining. The physical wear mechanisms presented in section 2.2 can be further divided and classified when discussing wear of cutting tools during machining. Two basic areas of tool wear are flank wear and crater wear, but several other mechanisms also occur [20]. Tool lifetime is often measured in terms of crater or flank wear according to ISO 3685:1993 [2]. Tool wear characteristics are often represented as a plot of material wear versus sliding distance, or time of cut, for a certain tool-workpiece combination.

Figure 3 shows a schematic graph of the width of flank wear land (VBB) vs. cutting length, including different wear regions describing the evolution of tool flank wear. After plotting wear versus cutting length, the curve often has three distinct regions [2]:

Figure 3: Schematic graph of tool wear evolution and its different regions.

 Region 1: primary or initial wear region with relatively high wear rate depending on accelerated wear due to damage of tool layer during manufacturing.  Region 2: steady-state region where normal operation for the cutting tool should occur.  Region 3: accelerated wear region, which stops with failure. This region is often accompanied by high cutting forces and temperatures in combination with severe tool vibrations.

2.3.1 Crater wear Crater wear is wear located at the rake face of the tool, in the form of a crater [16]. The rake face suffers from severe pressure and temperature loads, and crater wear is mainly caused by diffusive wear due to cutting tool material on the rake face dissolving into the chip material. Therefore, crater wear is very temperature sensitive and strongly depends on the solubility of tool material in the chip material. Crater wear is often measured by a profilometer as the maximum depth of the crater formed on the rake face of the tool [16].

2.3.2 Flank wear Flank wear is wear formed on the cutting tool’s relief surface as a flat-worn surface. Investigations of flank wear [16] suggest that flank wear depends mostly on abrasion from unwanted rubbing of clearance face against workpiece material. Flank wear is measured as the width of flank wear land, VBB, and is often measured microscopically [16].

2.3.3 Chipping Chipping is when a small material piece of the cutting tool edge breaks loose. This is an unpredictable wear mechanism that could occur when the cutting tool is subjected to sudden loads or thermal shocks due to low fracture toughness [16, 17].

2.3.4 Fracture Fracture wear is often observed on a heavily worn tool that runs under tough cutting conditions. The cutting tool edges break totally due to very high temperatures and cutting forces [19].

2.3.5 Notch wear Notching is mainly caused by a fracture process or a chemical reaction as discussed by Turkes et al. [21]. Notching happens when excessive localized damage occurs at the flank and rake face simultaneously, causing a single groove formation. Once formed, a notch will cause poor micro-finish on the machined part and many times proceeds into fracture wear.

2.3.6 Chatter Chatter, also known as machining vibrations, is a self-excited vibration problem, resulting in waves on the machined surface. Although chatter is not a proper wear mechanism on its own, it is often associated with accelerated tool wear and loud noise levels. Chatter in titanium machining occurs because of the low modulus of elasticity of titanium alloys, causing deflection of the material when subjected to cutting pressure [22]. Titanium alloys deflect nearly twice as much as carbon steel, giving a larger bouncing action when the cutting edge enters the cut [21].

2.4 Tool life prediction The prediction of tool life is an important feature for many cutting tool users in order to prevent unnecessary tool investments or scrapping of damaged products. One of the first equations for cutting tool wear estimation was proposed by F. W. Taylor in the early 1900’s [23], and it is now used commonly enough to be found in almost any text book on metal machining. The equation suggests that tool life is strongly dependent on cutting speed:

[Equation 1]

Here, v is the cutting speed and T is the tool life, and C and n are constants depending on the tool-workpiece combination. The value of n for cemented carbide is approximately 0.2 [16]. Prediction of tool lifetime from Taylor’s equation presented above could be a bit misleading. This is because tool life is not independent of parameters such as feed rate and depth of cut as

9 suggested by Taylor. Since Taylor presented the tool life equation (eq. 1), several researchers have been focusing on estimation and prediction of tool wear [23].

Usui et al. [23] proposed an equation for tool wear prediction suggesting temperature dependences:

( ) [Equation 2]

In eq. 2, dW is the change in volume, dL is the change in distance, σt is the normal stress on the contact area, θ is the temperature on the chip interface, C1 is a materials system constant that depends on the topologies of the surfaces and shapes of the worn-off particles, and , where ΔE is a theoretical activation energy of the wear mechanism, λ is Boltzmann’s constant and A is a constant which corrects for the temperature dependence of the hardness of the workpiece material.

According to equation 2, every wear mechanism can be considered to depend on the activation energy for a specific reaction. If there are several wear mechanisms, the total wear is predicted by summing up several terms of the type presented in eq. 2, all having different constants C1 and C2. The activation energies could be estimated from the values of C2, which can be calculated from plots of ln(dW/(σtdL)) versus (1/θ). In order to estimate the activation energies from C2, the system constant A must sometimes be known. The normal stress can be estimated in a number of ways, but in this thesis simulations in AdvantEdge will be used for this purpose.

3. Literature study of Ti machining

Hartung and Kramer [24] performed wear tests on a number of different cutting tool inserts machining titanium alloy Ti-6Al-4V. When machining with WC/Co cutting tools, they found that crater wear will be the dominating wear form and limit the tool life at cutting speeds (vc) between 61 and 122 m/min, while plastic deformation of the cutting edge will dominate between 122 and 610 m/min. During wear tests at vc being 61 m/min, after 30 s machining, the WC/Co cutting tools had an adherent layer of titanium over the entire crater and the crater wear rate was measured as 2.5 µm/min. The adherent layer was suggested to limit the wear rate by changing the diffusion rate of the cutting tool constituents through the layer, thus limiting the diffusion wear. To be able to study the actual worn surface, they removed the adhered titanium layer by etching in hydrofluoric acid for 20 minutes. The wear land area after etching showed a surface which appeared to be chemically polished. Further, the authors suggested a chemical reaction taking place between the titanium work piece and the cutting tool material forming an adherent layer of TiC at the rake face. The presence of oxygen was suggested to indicate the formation of oxycarbides on the surface. These suggestions were strengthened by Auger spectroscopy analysis [24]. Other results presented suggest that TiC grains will be removed from the cutting tool surface during machining, probably because of a decreasing toughness in the surface layers due to diffusion of cobalt matrix material. Replenishing of TiC grains on the tool surface will probably occur by obtaining C from WC grains below [5, 24].

Ezugwu and Wang [5] presented a review on the main problems associated with titanium machining, including tool wear and the mechanisms responsible for tool failure. They suggest that uncoated WC/Co cutting tools are better than most coated cutting tools for machining a titanium alloy. The high chemical reactivity of titanium causes welding of workpiece material on the cutting tool during machining, leading to chipping and premature tool failure. The prominent failure modes in titanium machining were: notching, flank wear, crater wear, chipping, and catastrophic failure. Different tool materials have different responses to different wear mechanisms. Crater wear is closely related to the chemical composition of the cutting tool. The conclusions presented by Ezugwu and Wang suggest that dissolution- diffusion wear dominates on the rake and flank face for uncoated cemented carbides used for the turning of titanium alloys. At very high cutting speeds and temperatures, the conclusion is that plastic deformation and development of cracks due to thermal shock will be the dominating wear mechanisms. Change of feed rate, depth of cut or cutting speed give changes in the wear rates. Ezugwu and Wang also suggest that cutting fluids have to be used during titanium machining to minimize high stresses and temperatures. The cutting fluid has to work both as coolant and lubricating agent to lower the cutting forces and avoid chip welding, which is a phenomenon often experienced during titanium machining [5].

Jianxin et al. [25] suggested that high cutting temperatures present during titanium machining with WC/Co cutting tools promote thermally related wear phenomena, such as element diffusion through tool-chip interface, which could give a change in composition of the

11 material and a very high tool wear rate. During dry cutting of Ti-6Al-4V with a WC/Co cutting tool, at a feed rate of 0.1 mm/rev, and a depth of cut 0.5 mm, the flank wear was found to increase when the cutting speed was increased in the range 20-120 m/min. Cutting forces measured during machining at 60 m/min were found to be essentially unchanged during machining a shorter distance than 600 meters. Through SEM analysis with an EDS detector, both diffusion and oxidation wear were observed by the authors during high cutting speeds. The main wear mechanisms occurring on the rake face during wear tests were found to be diffusion and adhesive wear. Through diffusion tests it was found that elements from the cutting tool material diffused into the workpiece material and that elements from the workpiece material diffused into cutting tool material at temperatures exceeding 400˚C [25].

Arrazola et al. [26] performed machining of titanium alloy Ti-6Al-4V and Ti555.3 with uncoated WC/Co cutting tools, holding fn = 0.1 mm/rev and ap = 2 mm while the cutting speed was varied between 40 and 90 m/min. The end of tool life criteria for flank wear defined in ISO 3685:1993 were used, setting the end of tool life as reached when average VBB = 0.3 mm for a 15 min cutting time. The maximum appropriate cutting speed for each alloy was decided from a characteristic wear plot where VBB was plotted vs. cutting speed for 15 min machining time. The maximum cutting speed was found to be 80 and 45 m/min for Ti-6Al-4V and Ti555.3, respectively. The results presented show how a protective build-up layer is formed during low cutting speeds, but at high cutting speeds the build-up layer is removed and high wear rates are observed [26].

In a work by Gerez et al. [27], an analysis of surface changes of WC/Co cutting tool inserts in dry turning of Ti-6Al-4V during 10 seconds was performed. Depth of cut used was 0.5 mm, feed rate 0.2-0.3 mm/rev and cutting speed 25-50 m/min. Results obtained and presented suggest the formation of BUL and BUE, where the length of the BUE can be compared to the depth of cut. Detection of abrasion traces was performed on the cutting tools rake face. Layered BUL material was observed, and the suggestion presented of formation of a thin titanium oxide layer on the cutting area on the tool. It was proposed by the authors that the BUE formed could be momentarily destroyed by the forces from the chip and be sheared onto the rake face wear land forming a BUL [27].

4. Experimental & Simulations

4.1 Machining experiments 90 cutting edges were marked on unworn cutting tool inserts, after which optical 3D topography measurements were performed with Alicona InfiniteFocus (see section 4.2.1) under 250X magnification.

Orthogonal turning of Ti-6Al-4V was performed in a George Fischer CNC (Computer Numerical Control) lathe, NDM-17/125, using a Sandvik Coromant tool holder type STFCR2525M 16 and cutting tool insert type TCMW 16T304 grade H13A. To get an orthogonal machining approach, it was decided to perform a radial turning operation, which meant that preparation of the workpiece was needed. Lamellas were created, 3 mm wide and 3 mm apart, with a 30 mm depth along the cylindrical workpiece, as can be seen in fig. 4. Before constructing the lamellar structure, the workpiece material top layer was removed to get a clean, smooth surface. The resulting workpiece diameter was 178 mm.

Coolant nozzle

Cutting tool

Workpiece material Cutting tool insert

Figure 4: Lamellar structure on workpiece with cutting tool above. 3 mm wide lamellas, 3 mm apart with a radial depth of 30 mm.

When the workpiece was prepared, the cutting tool insert was placed in the tool holder and cutting parameters were set in the CNC system. Experiments were performed holding the depth of cut (ap), in this case representing the lamella width, constant at 3 mm, varying cutting speed (vc) between 30 and 115 m/min, feed rate (fn) between 0.05 and 0.2 mm/rev and the time of cutting (t) between 0.5 and 3 min. A total of 80 experiments were performed, and they are detailed in appendix A. Turning experiments were performed during constant cutting operations with the cutting edge perpendicular to the direction of primary motion. The cutting fluid used in these experiments was a 6 % solution of a semisynthetic coolant (Hocut B50S) with pH of 9.5 and a maximum flow rate of 200 l/min.

Because of limitations in machining operations during force measurements, wear tests and measurements of cutting forces could not be performed in the same experiments, which is why 20 separate experiments were performed for this purpose. To make sure that no significant machining vibrations or interruptions during machining occurred, measurements of moment were performed during the wear tests.

Force measurements were performed in 20 wear tests with new cutting tool inserts. Samples were named A-T and the cutting parameters for each sample are presented in table 1.

Table 1: Cutting parameters used for force measurements. vc is the cutting speed, fn is the feed rate and t is the cutting time used.

vc (m/min) fn (mm/rev) t (min) Sample 30 0.05 0.3 A 30 0.1 0.3 B 30 0.15 0.3 C

30 0.2 0.3 D 45 0.05 0.3 E 45 0.1 0.3 F 45 0.15 0.3 G 45 0.2 0.3 H 60 0.05 0.3 I 60 0.1 0.3 J 60 0.15 0.3 K 60 0.2 0.3 L 90 0.05 0.3 M 90 0.1 0.3 N 90 0.15 0.3 O 90 0.2 0.3 P 115 0.05 0.3 Q 115 0.1 0.3 R 115 0.15 0.3 S 115 0.2 0.3 T

4.2 Analysis methods

4.2.1 Alicona Alicona InfiniteFocus is an instrument for optical 3D measurements of surfaces. It uses the focus variation principle to create an image of a surface. In Focus variation [28], the authors describes how coaxial white light is provided by a white light source, after which the light beam is guided through lens systems and beam splitter mirrors, before it reaches the infinity- corrected objectives in the nosepiece. When the modulated white light illuminates the specimen surface it reflects the light beam differently depending on the objects topography. Reflected light rays emerging from the surface, striking the objective, are gathered in a light sensitive sensor behind the beam splitter mirror. Topographic and colour information about the surface is generated through variation of focus in combination with a vertical scanning. Small regions of the object are sharply imaged due to the small depth of field of the optics. Full depth of field and complete detection of the surface is obtained when the precision optics is moved vertically along the optical axis, continually capturing data from the surface. The vertical resolution depends on the chosen objective and could be up to 10 nm [28].

4.2.2 Scanning Electron Microscopy A scanning electron microscope can be used for revealing information of a sample’s texture, chemical composition and crystalline structure. In most cases during SEM analyses, data are collected over a selected surface area (size variable between 1cm and 5µm) of the sample, creating a 2D image displaying spatial variations in properties. A conventional such microscope operates in vacuum of 10-5-10-6 torr, and provides images at 20-30 000X magnification with a spatial resolution of 50-100 nm.

When a high-energy source emits an electron beam, bombarding the sample, electrons interact with the atoms in the sample. The electrons striking the sample can either be transmitted, diffracted or scattered. Signals created from interactions between the electron beam and the sample could be backscattered electrons (BSE), secondary electrons (SE), Auger electrons or characteristic x-rays emerging from different depths in the sample.

Energy dispersive X-ray spectroscopy (EDS) can be used in combination with SEM to characterize the surface composition of the sample. An EDS detector collects and detects the amount of characteristic x-rays emitted from a sample, creating a spectrum. Spectra are often plotted as x-ray counts vs. energy, and the peaks correspond to different elements in the sample. Some elements have more than one peak, and some elements’ peaks overlap one another. At the same time, low contents of an element often result in small peaks “drowning” in background noise. The first elements of the periodic table cannot be detected by EDS [29].

Analysis with EDS is a method such that with proper calibration, quantitative analysis can be performed. The sample volume analysed in SEM and EDS depends on the analysed material’s composition (light or heavy elements), the acceleration voltage used, the type of signals detected and the detector used during analysis [30].

The microscope used for analysis in the present work was a Zeiss EvoMA25 with lanthanum hexaboride (LaB6) crystal as electron source.

4.2.3 Simulations Simulations of temperature, cutting force and normal pressure have been performed using Finite Element Method (FEM) calculations in the commercially available program AdvantEdge [9]. AdvantEdge is a CAE software (Computer-Aided Engineering) for the optimization of metal cutting. During simulations in AdvantEdge, material models describe the material’s behaviour during cutting operations, accounting for elasticity, plasticity and thermal dependence. AdvantEdge makes it possible to get detailed information about heat generation, heat flows, temperatures, tensions, cutting tool lifetimes and surface characteristics during machining processes [9].

Orthogonal cutting in titanium, using an uncoated WC/Co insert with the same geometry as in wear tests, was simulated in AdvantEdge, using FEM calculations and the default material and friction models included in the program (V5.9-011). The adhesion of workpiece material observed in the cutting zone after wear tests was consequently ignored during the simulations. Cutting speeds and feed rates corresponding to the experiments presented in table 1 were simulated. Simulations gave calculated values for cutting forces, normal stresses and cutting temperatures during the machining operations presented in section 4.1.

5. Analysis and Results After turning experiments in the lathe, wear lands on cutting tool edges were analysed to investigate the extent of cutting tool wear during machining. Optical measurements gave the opportunity to determine the volume of removed material from each edge during machining, while SEM analysis made it possible to take a closer look at the worn surfaces and to study the variation in elements present at the wear lands. Force and moment analyses have primarily been used for diagnostic purposes, ensuring that the cutting process was steady and vibration- free. Force measurements have also been used for comparison with model values from the AdvantEdge calculations.

5.1 Alicona Optical 3D topography measurements were performed for a chosen number of samples using Alicona InfiniteFocus, both before and after wear tests in the lathe. The before and after images were analysed and volume adhered or removed during machining was calculated. 3D topography measurements were performed under 250X magnification.

Optical images were studied to investigate the visual appearance of the wear lands of the cutting tool edges, and changes in geometry were measured. In figures 5-9, optical images for samples machined during 3 minutes with feed rate 0.15 mm/rev and cutting speeds between 30 and 115 m/min are presented. These images reveal visual information of the wear propagation for different cutting speeds. Typical images from volume measurements in Alicona InfiniteFocus are presented in figure 10.

Differences between worn and unworn cutting tool edges are measured as deviations in worn cutting tool inserts relative to unworn cutting tool inserts and are presented as:

 Volume below, which is the volume of negative deviations (presumed to be caused by wear).  Volume above, which is the volume of positive deviations (presumed to be caused by adhesion of material).  Minimum deviation, which is the maximum negative distance from the unworn cutting tool surface to the worn cutting tool surface.  Maximum deviation, which is the maximum positive distance from the unworn cutting tool surface to the worn cutting tool surface.  Mean deviation is the mean value of all deviations between the unworn and worn cutting tool.  Area below is the projected area of negative deviations on the original geometry, i.e. projection of the area of the removed material volume.  Area above is the projected area of positive deviations on the original geometry, i.e. projection of the area of the adhered material volume.

Tables with collected results from optical measurements are presented in appendix B.

Figure 5: Optical image of sample A4 3, vc 30 m/min Figure 6: Optical image of sample A10 1, vc 45 at 250X magnification. m/min at 250X magnification.

Figure 7: Optical image of sample B5 2, vc 60 m/min at 250X magnification.

Figure 8: Optical image of sample B10 3, vc 90 m/min Figure 9: Optical image of sample C6 3, vc 115 at 250X magnification. m/min at 250X magnification.

Flank face Flank face

Rake face Rake face

Figure 10: Optical images from analysis in Alicona InfiniteFocus for sample B10 2 (vc 90 m/min, fn 0.15 mm/rev, t 2 min). Left image is the optical image of the worn cutting tool edge. Right image is the result from comparison of before and after images. The green colour indicates zero deviation, bluer colour indicates negative deviations and more yellow or red colour indicates positive deviations.

In fig. 11, negative deviations are plotted versus cutting length. A linear dependence between volume removal and increasing cutting length is found for cutting speeds 90 and 115 m/min. For lower cutting speeds, between 30 and 60 m/min, volume removal is not showing an obvious dependence on cutting length or cutting time. For vc being 115 m/min and fn 0.2 mm/rev only two samples were analysed. This was because machining longer than 1 minute during these conditions made the cutting tool inserts break and the experiments had to be aborted. Linearity was assumed for vc = 115 m/min, fn = 0.2 mm/rev even though only two different cutting times could be analysed. This assumption is thought to be valid both because of the good linear fitting of a straight line starting in origin, passing through the two data points, and because clear observations of crater wear were made, just as for the other samples analysed for cutting speeds 90 and 115 m/min where linear dependences are clearly seen.

The total volume differences on the cutting tool inserts were calculated through subtraction of volume below from volume above, and the results are presented in fig. 12. As can be seen, the volume differences are positive for cutting speeds between 30 and 60 m/min, but for cutting speeds 90 and 115 m/min, the total volume change is negative for all cutting lengths, except for vc = 90 m/min, L = 45 m (t 0.5 min). Here a linear dependence can also be seen for vc = 90-115 m/min, but not for vc of 30-60 m/min.

Wear x105 x106 16 y = 0,359x 45

14 y = 0,1153x 40 Vc 30, fn 0.15 35 12 Vc 45, fn 0.15

30 ) 3 10 Vc 60, fn 0.15 25 8 20 Vc 90, fn 0.15

Wear Wear (µm 6 y = 0,0304x 15 Vc 115, fn 0.15 4 10 Vc 115, fn 0.1 2 5 y = 0,0276x Vc 115, fn 0.2 0 0 0 100 200 300 400 Cutting length (µm) x106

Figure 11: Variation of removed volume with cutting length. Left vertical axis represents vc 30-60 m/min, right vertical axis represents vc 90-115 m/min. Lines connecting data points for vc 30-60 m/min are there to clarify points hiding behind each other. Trendlines are adapted for vc 90-115 m/min with intercept set in origin.

Total volume difference x105 x106 45 5

40 0

) Vc 30, fn 0.15 3 35 -5 Vc 45, fn 0.15 30 -10 25 -15 Vc 60, fn 0.15 20 -20 Vc 90, fn 0.15

15 -25 Vc 115, fn 0.15 Volume Volume change (µm 10 -30 Vc 115, fn 0.1 5 -35 Vc 115, fn 0.2 0 -40 0 100 200 300 400 Cutting length (m)

Figure 12: Volume difference on cutting edges presented as volume above-volume below. Left vertical axis represents vc 30-60 m/min and right vertical axis represents cutting speeds 90-115 m/min.

For high cutting speeds, between 90 and 115 m/min, it seems as if the steady-state wear region discussed in section 2.3 (fig. 3) is reached, and a wear over time rate can be calculated by fitting a linear trend line passing through origin in a graph where volume change is plotted

20 versus cutting length, as in fig. 11. For low cutting speeds, between 30 and 60 m/min, it seems as if the wear still is in the initial stage after three minutes of machining.

5.2 Scanning Electron Microscopy (SEM) SEM micrographs of wear zones on the cutting edges were recorded for a chosen number of samples, and EDS analyses were performed to study the elements present at the surfaces of the worn cutting tool inserts. The cutting edges analysed by SEM and EDS are presented in table 2. During visualization in SEM, secondary electron (SE) and backscattered electron (BSE) micrographs were recorded and different areas on the wear lands were analysed with EDS. Acceleration voltages used for SEM analyses were changed between 10 and 15 kV for different samples, which gave slightly different analysis depths and could possibly have affected the results. Recorded EDS spectra were analysed for elements known or expected to be present at the surface of the attritional wear. In this section a compilation of results from SEM and EDS results are presented. The complete results from EDS spectra in terms of elemental composition in at% for the different samples are presented in appendix C. SEM micrographs with marked analysis areas are presented in appendix D.

Table 2: Samples analysed by SEM. vc is the cutting speed, fn is the feed rate, t is the cutting time and L is the cutting length.

vc (m/min) fn (mm/rev) t (min) L (m) Sample 30 0.15 0.5 15 A3 3 30 0.15 1 30 A4 1 30 0.15 2 60 A4 2 30 0.15 3 90 A4 3 45 0.15 0.5 22.5 A7 1 45 0.15 1 45 A9 3 45 0.15 2 90 A10 2 45 0.15 3 135 A10 1 60 0.15 0.5 30 B3 2 60 0.15 1 60 B4 3 60 0.15 2 120 B5 1 60 0.15 3 180 B5 2 90 0.15 0.5 45 B9 3 90 0.15 1 90 B10 1 90 0.15 2 180 B10 2 90 0.15 3 270 B10 3 115 0.15 0.5 57.5 C5 1 115 0.15 1 115 C5 2 115 0.15 2 230 C5 3 115 0.15 3 345 C6 3

After visual examination of SEM micrographs, wear patterns can be divided into two different groups, one for low cutting speeds between 30 and 60 m/min and one for high cutting speeds between 90 and 115 m/min.

5.2.1 Low cutting speeds Figure 13 shows a typical wear land for cutting speeds 30-60 m/min (here of sample A7 1) at 400X magnification. It looks as if the initially sharp cutting tool edge has been chamfered, leaving a sheared surface at the wear land.

Analyses of the elemental composition with EDS were performed on the material on the flank face wear land, rake face wear land and rake face. Figures 14, 15 and 16 below show how the elemental compositions vary with varying cutting speed and cutting time. For sample B5 1 (vc 60 m/min, t = 2 min), EDS analysis on the rake face is missing.

For recapitulation of the machining parameters, see section 2 and figures 1 and 2 on pages 4 and 5.

Rake face Rake face wear land

Adhered or plastically deformed material Flank face wear land

Figure 13: SEM micrographs of cutting edge wear land after 0.5 min machining at vc 45 m/min (sample A7 1). Left micrograph is a SE micrograph where typical areas for EDS analysis are shown. On the right, the same area is mapped with BSE.

Analyses of elemental compositions on rake face wear lands for cutting speeds 30-60 m/min, presented in fig. 14, show how compositions vary with cutting time and cutting speed. The detected amounts of titanium in at% seem to increase with increasing cutting speed, but no clear pattern has been found when studying the variation with increasing cutting time. Tungsten (W) was only detected in very small amounts for vc = 45 m/min after cutting time 0.5 and 1 minute and for vc = 60 m/min after cutting time 1 minute.

90 Rake face wear land Vc 30 t0.5

80 Vc 45, t0.5

70 Vc 60, t0.5 Vc 30, t1 60 Vc 45, t1

50 Vc 60, t1

At% 40 Vc 30, t2

30 Vc 45, t2 Vc 60, t2 20 Vc 30 t3 10 Vc 45, t3

0 Vc 60, t3 W C Co Ti Al V O

Figure 14: Variation in composition of analysis made at the rake face wear land for cutting speeds 30-60 m/min.

In fig. 15, results from EDS analyses on the flank face wear land for cutting speeds 30-60 m/min are presented. As can be seen, the flank face wear land seems to consist of mainly C, Ti, Al, V and O. Co was only detected for sample B3 2 (vc = 60 m/min, t = 0.5 min) and W was only detectable in very small amounts for a few samples. The most distinct difference from analyses on the rake face wear lands is the presence of oxygen. Oxygen was detected for all samples, except for samples A9 3 (vc = 45 m/min, t = 1 min) and B5 2 (vc = 60 m/min, t = 3 min).

The amounts of C detected here appear negatively correlated with the amounts of Ti detected at different cutting speeds. Where oxygen could not be detected, the amounts of C are lower than for the rest of the samples and the amounts of Ti are significantly higher, as indicated by the arrows in fig. 15.

Analyses performed at the rake face further away from the cutting edge show presence of cutting tool material as well as workpiece material and oxygen, as can be seen in fig. 16. The amounts of C, W and Co seem to follow the same variation pattern. Oxygen was detected for all samples except for samples A4 2 (vc = 30 m/min, t = 2 min) and A10 1 (vc = 45 m/min, t = 3 min), where the amounts of Ti detected once again were significantly higher than for other samples (just as for flank face wear land).

Flank face wear land 90 Vc 30 t 0.5 80 Vc 45 t 0.5

70 Vc 60 t 0.5 Vc 30 t 1 60 Vc 45 t 1 50 Vc 60 t 1

At% 40 Vc 30 t 2 Vc 45 t 2 30 Vc 60 t 2 20 Vc 30 t 3 10 Vc 45 t 3

0 Vc 60 t 3 W C Co Ti Al V O

Figure 15: Variation of elemental composition on flank face wear land adhered material. Arrows marking samples where no oxygen was detected but a significantly higher amount of Ti than samples where oxygen was detected.

Rake face 80 Vc 30, t 0.5 70 Vc 45, t 0.5

60 Vc 60, t 0.5 Vc 30, t 1 50

Vc 45, t 1

40 Vc 60, t 1 At%

30 Vc 30, t 2 Vc 45, t 2 20 Vc 30, t 3 10 Vc 45, t 3

0 Vc 60, t 3 W C Co Ti Al V O

Figure 16: Elemental composition variations from analysis on upper rake face. Sample B5 1 (vc 60 m/min, t 2 min) is missing. Arrows marking samples where no oxygen was detected but a significantly higher amount of Ti than samples where oxygen was detected.

For vc = 60 m/min, SEM micrographs reveal how the sheared layer formed on the wear land during machining is slightly different from what was observed after lower cutting speeds. What after lower cutting velocities looked as an evenly deformed, sheared surface (as in fig. 13) now looks as if it consists of two parts: one part where the sheared layer is still present and one part where it seems to have been worn off, forming a smooth subsurface beneath the old one. A micrograph revealing these two different kinds of surfaces can be found in fig. 18. Analyses of these two parts show how the rough part seems to be a surface similar to the one seen for cutting speeds 30-45 m/min, consisting of C, Ti, Al and V (presented in fig. 14). Analysis of the other, sub-surface, layer not only detects C, Ti, Al and V but also W and O.

5.2.2 High cutting speeds

For high cutting speeds, vc = 90-115 m/min, good observations of chamfered cutting edges cannot be made through SEM micrographs, possibly because the deformations have been worn away. For high cutting speeds at all cutting times used, formation of crater wear on the rake face seems to appear very fast. For vc = 115 m/min and cutting time 2 and 3 min, chipping was observed on the side of the crater.

Figure 17 shows micrographs of sample B9 3 at 400 X magnification. These micrographs show what appears to be the initial step of crater formation where sheared material on the rake face wear land starts to wear off in the middle region revealing the sub-surface beneath.

Rake face

Sub-surface Sheared material

Flank face

Figure 17: SEM micrographs of sample B9 3 (vc 90 m/min, fn 0.15 mm/min, t 0.5 min) at 400X magnification. To the left is a SE micrograph revealing areas of EDS analysis. On the right a BSE micrograph of the same area shows how the sheared layer starts to wear off in the middle.

Due to some procedural inconsistencies in areas analysed by EDS, some gaps will appear in presented data. Samples B9 3 (vc = 90 m/min, t = 0.5 min), B10 1 (vc = 90 m/min, t = 1 min) and B10 2 (vc = 90 m/min, t = 2 min) were not analysed on rake face. For sample C5 2 (vc = 115 m/min, t = 1 min), analysis of the composition at the sheared surface layer on the rake face wear land was not performed.

Figure 18 shows a typical wear land for high cutting speeds. A crater is clearly formed on the rake face wear land and chipping has occurred. In the right micrograph, two different kinds of surfaces are revealed at the rake face wear land, in this case inside the crater. Spectrum 1 is recorded on a smooth, sub-surface layer area and spectrum 2 is recorded at a rough, sheared surface layer area.

Rake face

Crater

Chipping

Figure 18: SEM micrographs of sample C6 3 (vc 115, t 3 min). Left micrograph is a SE micrograph of the cutting edge, revealing crater wear and chipping. The right micrograph reveals two different kinds of crater wear surface textures in spectrum 1 and spectrum 2.

Analysis with EDS on the sheared surface layer in the rake face wear land (spectrum 2, fig. 18) for high cutting speeds mainly detects C, Ti, Al and V. Oxygen is detected for vc = 90 m/min and longer than 30 sec of cutting time as presented in fig. 19.

Rake face wear land 90

70 Vc 90, t0.5

60 Vc 115, t0.5

50 Vc 90, t1

At% 40 Vc 90, t2

30 Vc 115, t2

20 Vc 90, t3

10 Vc 115, t3 0 W C Co Ti Al V O

Figure 19: Variation of composition on rake face wear land BUL for vc 90-115 m/min. Analysis is missing for sample C5 2 (vc 115, t 1 min).

Analysis with SEM and EDS shows differences between what appears to be sheared surface layer areas and smoother sub-surface layers on the rake face crater wear land. Comparisons between elemental compositions for smoother surfaces on different cutting edges are shown in fig. 20. By studying the compositions at places of smooth sub-surface layers, it is reasonably clear that the main constituents in this area are W, C, Ti, Al and V. Co was detected for three different samples, but in all three W and O were also detected and the amount Ti was lower than for other samples.

Analyses of compositions of smooth sub-layers (spectrum 1, fig. 18) were not performed for any sample following vc 30-45 m/min, due to the fact that no sublayer with such characteristics being found.

Analysis of elemental composition on the flank face wear land gives similar results as for low cutting speeds. The visual appearance of cutting tool wear on the flank face is not as clear as earlier, but still some deformed material can be seen on the flank side of the cutting edge. In fig. 21 the variations in elemental composition after cutting speeds 90-115 m/min are shown. As can be seen, W was only detected for some samples. C, Ti and O seem to be the main constituents, just as for lower cutting speeds. Oxygen was not detected for vc 90 m/min, t 3 min.

Rake face wear land (sub-surface) Vc 60, t 0.5 90 Vc 90, t 0.5

80 Vc 115, t 0.5 Vc 60, t 1 70 Vc 90, t 1 60 Vc 115, t 1

50 Vc 60, t 2

At% 40 Vc 90, t 2 30 Vc 115, t 2 20 Vc 60, t 3

10 Vc 90, t 3

0 Vc 115, t 3 W C Co Ti Al V O

Figure 20: Variations of elemental composition for cutting speeds 60-115 m/min where the sub-surface layers of rake face wear land for samples machined during cutting speeds between 60 and 115 m/min.

Flank face wear land

70 Vc 115 t 0.5 60 Vc 115 t 1 50

Vc 90 t 2 40 At% Vc 115 t 2 30 Vc 90 t 3 20 Vc 115 t 3 10

0 W C Co Ti Al V O

Figure 21: Variation in elemental composition on flank face wear land for vc 90-115 m/min. Analysis of sample B9 3 (vc 90 m/min, t 0.5 min) and sample B10 1 (vc 90 m/min, t 1 min) is missing.

Analysis of rake face composition was performed for cutting speeds 90 and 115 m/min and is presented in fig. 22. Main constituents on rake face seem to be W, C and O, with small amount of Ti and some traces from Al, V and possibly Co.

Rake face

80 Vc 90, t 0.5 70 Vc 115, t 0.5 60 Vc 90, t 1 50

Vc 115, t 1 40 At% Vc 115, t 2 30 Vc 90, t 3 20

10 Vc 115, t 3

0 W C Co Ti Al V O

Figure 22: Variations in elemental composition on rake face for vc 90-115 m/min. Sample B10 2 (vc 90 m/min, t 2 min) is missing.

5.3 Force measurements After recording cutting forces during 0.3 minutes of cutting time, the force was plotted to see variation over time, as in fig. 23. For all cutting forces, steady state was reached within the 400 first data points (corresponding to approximate 3 s) and the force was almost constant until the machining operation was aborted after 0.3 minutes. An arithmetic average was therefore calculated for cutting forces measured between 400 and 2000 data points, corresponding to a machining time between roughly 4 and 17 s. Cutting forces measured for sample L are presented in fig. 23.

Cutting forces for sample L 1200

1000

800

600 Ff Fc

Force Force (N) 400 Fp

200

0 0 500 1000 1500 2000 2500 -200 Measure points

Figure 23: Cutting force contributions measured during 0.3 minutes of cutting time. vc 60 m/min and fn 0.2 mm/rev. This was the overall trend for all force measurements performed. Arithmetic average was calculated for steady state region between 400 and 2000 data points.

Force measurements performed show that mean values for Fp range between -1 and -17 N for the different experiments, which corresponds to roughly one percent compared to Fc or two percent compared to Ff. Comparisons of moments measured during wear tests and force measurements performed show good correlations.

Arithmetic averages of measured cutting forces were plotted versus cutting speed and feed rate in fig. 24 and 25 respectively. The results show that the cutting force, Fc, is constant during different cutting speeds but grows linearly with increasing feed rate.

Cutting force vs. cutting speed

1200

1000 fn = 0.05 mm/rev

800 (N)

fn = 0.1 mm/rev c F 600 fn = 0.15 mm/rev 400

fn = 0.2 mm/rev 200 20 40 60 80 100 120

vc (m/min)

Figure 24: Experimental result of mean cutting force vs. cutting speed for different feed rates.

Cutting force vs. feed rate

1200

Vc = 30 m/min 1000

Vc = 45 m/min

800

(N)

F Vc = 60 m/min 600

400 Vc = 90 m/min

200 Vc = 115 m/min 0 0,05 0,1 0,15 0,2 0,25

fn (mm/rev)

Figure 25: Experimental results of mean cutting force vs. feed rate for different cutting speeds. Linear trendline adapted to vc = 115 m/min.

The feed force was plotted versus cutting speed in fig. 26, where for fn= 0.05 mm/rev the feed force is constant but at higher feed rates, Ff first decreases slightly at small vc and then increases at higher vc.

Feed force vs. cutting speed

600 550 500 fn = 0.05 mm/rev 450

fn = 0.1 mm/rev (N)

400

f F 350 fn = 0.15 mm/rev 300 fn = 0.2 mm/rev 250 200 20 40 60 80 100 120

vc (m/min)

Figure 26: Experimental measurements of Feed force plotted versus cutting speed for different feed rates.

5.4 Simulations in AdvantEdge Temperature simulations, as presented in fig. 28, indicated that machining titanium alloy according to the experimental matrix will result in temperatures up to 1100˚C at a feed rate of 0.2 mm/rev and cutting speed 115 m/min. The temperature along the cutting tool-workpiece contact surface will vary, giving the maximum contact temperature at the nose, as can be seen in fig. 27. Cutting temperatures presented in fig. 28 are the maximum temperature at the inserts surface in the rake face contact zone. Simulations also gave calculated values for the contributing forces Fc and Ff which are presented in fig. 29 and 30, respectively.

Figure 27: Temperature distribution in the tool-workpiece contact zone according to simulations in AdvantEdge for vc 115 m/min, fn 0.2 mm/rev.

Cutting temperature

1200

1000

800 fn=0.05 mm/rev

600 fn=0.1 mm/rev T T (˚C) fn=0.15 mm/rev 400 fn=0.2 mm/rev 200

0 0 20 40 60 80 100 120

vc (m/min)

Figure 28: Results from simulation of contact temperatures in AdvantEdge for different feed rates.

Cutting force values simulated in AdvantEdge and presented in fig. 29 show how the cutting force appears to be constant for feed rate 0.05 mm/rev. At higher feed rates the cutting force appears to decrease with increasing cutting speed up to 60 m/min after which the cutting force seems constant.

Simulated results presented in fig. 30 show how the feed force seems to decrease with increasing cutting speed and increase with increasing feed rate.

Simulated cutting force 1600

1400

1200

1000 fn=0.05 mm/rev

(N)

c F 800 fn=0.1 mm/rev fn=0.15 mm/rev 600 fn=0.2 mm/rev

400

200 0 20 40 60 80 100 120

vc (m/min)

Figure 29: Cutting forces calculated during simulation in AdvantEdge.

Simulated feed force 1100 1000 900 fn=0.05 mm/rev 800

700 fn=0.1 mm/rev

(N)

f fn=0.15 mm/rev

F 600 500 fn=0.2 mm/rev 400 300 200 0 20 40 60 80 100 120

vc (m/min)

Figure 30: Variations of estimated feed forces calculated during simulations in AdvantEdge.

5.5 Wear modeling Looking back at the negative volume deviations presented in fig. 11 (p. 20), wear rates can be calculated for cutting speeds 90-115 m/min by adapting linear trendlines to the data points. By using wear rates calculated and eq. 2 (p. 10), the temperature dependence can be examined for cutting speeds 90-115 m/min. This dependence has been plotted in fig. 31.

As can be seen, the data for vc 90 and 115 m/min is in good agreement with a wear mechanism that has a thermal dependence with an Arrhenius-type reaction rate constant.

Wear rate 0

-0,5

-1

) 2 -1,5 y = -25.4x + 17.6 -2

-2,5

-3 ln(dW/dL) ln(dW/dL) (µm

-3,5

-4

-4,5 0,72 0,74 0,76 0,78 0,8 0,82 0,84 0,86 1000/θ (1/K)

Figure 31: Wear rate for cutting speeds 90 and 115 m/min.

From eq. 2, with the equation for the trendline adapted to data points in fig. 31, the constant

C2 can be estimated. The normal stresses, σt, needed in equation 2 were estimated through simulations in AdvantEdge. However, the points of maximum contact temperature at the rake face are found in regions of very high stress gradients. Also, it should be expected that these points drift over the rake face during the machining process as crater wear becomes more pronounced. For the machining experiments for which wear was estimated during this work, calculations suggested that the normal stresses at maximum temperatures on the rake face varied between 1.5-2 GPa. For the sake of simplicity, it was decided to treat the pressure as constant, and the value that was used in the wear model was 1.8 GPa, which was the average of the normal stresses at maximum temperatures on the rake face. From this value, fig. 11 and eq. 2, it is found that the constant C1 and C2 in Usui's wear model can be determined as 0.024 and 25.4, respectively.

6. Discussion No extensive comparisons to phase diagrams were performed in this thesis due to the time limitations. After an introductory study on phase diagrams, the only result found to be relevant to the current results was the high-temperature phase transformation of Ti and its alloys.

6.1 SEM and EDS Results from analyses on the sheared layer on the cutting tools rake face wear land, where detection of C, Ti, Al and V were made, suggests the formation of a build-up layer from workpiece material in the contact zone. In fig. 13 (p. 22) one can see SEM micrographs of a typical wear land for low cutting speeds, where the cutting edge has been chamfered, possibly by abrasion and/or plastic deformation, and a sheared surface layer has been formed as a BUL on top. The horizontal line on the flank side of the cutting edge could be a trace from abrasion on the flank face during machining. On comparing results from EDS analysis from different samples, it should be noted that some variations in composition can occur due to the limited area analysed, so the local variations in composition at the surface will have an impact on the present results.

6.1.1 Rake face wear land Studies and comparisons of results from different cutting speeds suggest how the BUL is formed during low cutting speeds and starts to wear off at 60 m/min. At that cutting speed, differences in appearance of the sheared surface at the rake face wear land can be visualized in SEM micrographs (fig. 17) and optical images (figures 5-9). Comparison of detected amounts of various elements at the rake face wear land for cutting speeds 30-60 m/min (fig. 14) show how the amount of Ti present seem to increase with increasing cutting speed. This could indicate an increasing BUL formation, but since the amounts of Al and V present seems to decrease with increasing cutting speed, it could also be possible that only Ti adheres to the cutting tool material. When studying fig. 28, where simulated cutting temperatures are presented, it is clear that the cutting temperature increases with increasing cutting speed. It has therefore been considered reasonable to suggest that the BUL is formed by adhesion of Ti through some sort of bonding reaction, which has a reaction rate that increases with temperature. The suggestion of increasing adhesion with cutting speed and temperature should therefore be accurate if reaction rates increases with temperature.

Between cutting speeds 60 and 90 m/min the amount of Ti present seems to decrease and then increase again between 90 and 115 m/min as presented in fig. 19. For cutting speeds 90 and 115 m/min the amounts of Al and V present seem to slightly increase with cutting speed. The rate of formation of the BUL seems to be reduced for high cutting speeds compared to lower cutting speeds. Comparing with simulated contact temperatures from fig. 28, indications of dominating BUL formation between 600 and 800°C are found. At 900°C (vc 90 m/min) the

35 formation has been reduced, or the removal has increased, significantly. This could possibly be linked to the phase transformation occurring for pure titanium at approximately 880°C.

Several micrographs at higher velocities (90-115 m/min) suggest how crater wear could be formed by removal of cutting tool material and BUL in a horizontal zone parallel to the cutting edge, as can be seen in fig. 17. Removal of BUL in the middle of the wear land creates a ditch which will proceed to grow into a crater when continuing the machining. Analyses with EDS on the sheared areas detected C, Ti, Al and V, while most of the smooth subsurface areas also contained W. Because EDS analysis is limited to a certain analysis depth in the order of µm and W originally was present in the cutting tool material, it is reasonable to believe that the two different kinds of surfaces appearing in the rake face wear land are either both BULs with different thicknesses or one BUL without W and one smooth subsurface layer containing W.

The detection of carbon from the cutting tool material at the rake face wear land indicates C being transported into the adhered workpiece material layer. It would be difficult to explain if detection of C was made from the material beneath since no W or Co was detected. It is therefore concluded that the detected C is present in the BUL. The variation of the detected amount of carbon does not seem to have any clear correlation with cutting speed when comparing fig. 14 and 19. Examining the variation of C detected with cutting time for the different cutting speeds, on the other hand, gives indications of a periodical variation, in which the minimum and maximum amount are fairly the same for different cutting speeds between 30 and 60 m/min, only they appear after different cutting times. This indicates a continuous process of addition and removal of carbon in the BUL. It is therefore suggested that the detected carbon has been transported by diffusion from the tungsten carbide source material, into or through the adhered workpiece material layer.

The diffusion of carbon from the cutting tool material into the workpiece material agrees with the formation of TiC in the surface layer, as also suggested by earlier researchers [5, 24, 25]. The carbon diffusion and formation of TiC at the surface appears to be a continuous chemical wear process when comparing variations in detected amounts and visual appearance on the wear land. It is suggested that when carbon is diffusing away from the cutting tool material into the adhered workpiece material layer forming TiC, depletion of carbon will occur in the material beneath. Depletion of carbon in the cutting tool material could change the materials properties and cause weakened stress resistance close to the surface in the cutting tool material. Adhesive wear of the tool could then occur through removal of BUL and some of the underlaying material when the chip flows by. A new virgin surface will then be revealed, on which adhesion of new workpiece material can occur, creating additional carbon diffusion, creating further carbon depletion and so on. This chemical wear agrees with all observations, and could very likely be the wear mechanism observed, that forms the crater on the cutting tool’s rake face. Studying micrographs and images for high cutting speeds shows thinner and partially worn-away BUL, which would under the presently suggested model suggest that the carbon depletion of the underlying material occurs much faster, before the adhered layer has built up the same thickness as at lower cutting speeds. This extensive crater wear seems to appear at some point below cutting speed 90 m/min, where the cutting temperature is

36 approximately 900°C. At this temperature, the phase transformation temperature for pure titanium has been exceeded and it is possible that this is a reason for why the crater wear formation has increased, even though the phase transformation for Ti-6Al-4V is a bit higher.

Oxygen was only detected on the rake face wear land for vc 90 m/min, t 1-3 min (see fig. 19). The detection of oxygen only for one cutting speed could have several different explanations, such as local variations in the areas analysed or overlapping peaks in the spectra (for example O and V). It is therefore very likely that oxygen is present in more samples than analysis shows. During machining, the cutting edge will reach very high temperatures and the cooling time after the abortion of the machining operation could be long enough for the cutting tool to react chemically with the surrounding atmosphere at elevated temperatures, forming oxide at the surface. If this were the reaction occurring we would have expected to find oxygen present for samples machined at 115 m/min as well, since even higher temperatures is expected to be achieved for those. One other possible explanation could be diffusion of oxygen through the cutting tool or the adhered material layers, but then we would have expected to see an increase in oxygen detected with increased cutting time or cutting speed which could not be seen. Earlier researchers suggested the formation of titanium oxide [27] or oxycarbides [24] which could be possible, but this has to be further investigated before any conclusions can be drawn.

6.1.2 Flank face wear land At the flank face side of the cutting edge, visual examination and comparison of SEM micrographs and optical images from samples machined at low cutting speeds indicate the presence of an adhered material edge. Suspicions of adhered material can be partially confirmed from EDS analyses. The presence of Ti, Al and V suggests adhered workpiece material, just as for rake face wear land BUL, with carbon probably originating from cutting tool material diffusing through the adhered material. For high cutting speeds, comparison of micrographs and images cannot provide any clear sign of an adhered material edge. Analyses with EDS, on the other hand, detect the same elements as for lower cutting speeds. This indicates adhered workpiece material and the formation of oxide and carbide on the flank face wear land. The forces from the chip acting on the adhered material edge could most possibly lead to shearing of the adhered workpiece material onto the rake face wear land and hence be the mechanism for building the BUL. This mechanism for BUL formation was earlier suggested by Gerez et al. [27].

When oxygen was detected during analysis, the amount of C detected was higher and the detected amount of Ti was lower than for samples in which oxygen was not detected. This could possibly be explained by both oxygen and carbon rates being related to temperature on the flank face, but there is not presently enough EDS data to confirm or refute such a speculation.

6.1.3 Rake face Results from EDS analysis on the rake face outside the cutting zone presented in fig. 16 and 22 suggest the presence of cutting tool material and workpiece material together with oxygen. This could indicate adhesion or diffusion of workpiece material from the chip onto the rake face and reaction with oxygen from the surrounding atmosphere or coolant. As presented by Jianxin et al. [25], element diffusion between cutting tool and workpiece material has been observed at cutting temperatures over 400˚C. Comparing to temperatures simulated in this thesis (fig. 28), diffusion is then likely to have occurred.

Detection of cobalt (Co) is almost non-existent for high cutting speeds (fig. 19-23), and for lower cutting speeds, Co was almost only detected on the rake face (fig. 14-16). This could indicate Co diffusion away from the surface layer, and because Co is the matrix for the cutting tool it is reasonable to believe that depletion of Co in the surface layers will make the material more sensitive to wear. The combination of C and Co depletion from the cutting tool’s surface layer together with the contact stress applied from the chip could be the reason for crater wear observed. This theory is strengthened through results presented by Hartung et al. [24], in which indications were found of Co diffusion away from the cutting tools surfaces, leading to plucking of particles by the chip.

6.2 Optical measurements Volume changes calculated from measurements in Alicona InfiniteFocus and presented in fig. 11 and 12 represent not only worn off material, but also adhered workpiece material on the wear land. An increase in wear means a larger wear rate than material adhesion rate, and a decrease in wear suggests a higher material adhesion rate than material wear rate. During machining with a feed rate of 0.15 mm/rev and cutting speeds of 30-60 m/min, the almost unchanging results presented in fig. 11 suggest a material adhesion rate high enough to compensate for any material wear up to at least 3 minutes of machining in titanium alloy. For these low cutting speeds it seems as if the adhesion of workpiece material is often larger than the removal of material, which can be confirmed by fig. 12.

Observations of a high material adhesion rate at low cutting speeds and a high material removal rate at high cutting speeds for machining time up to 3 minutes have been supported by the SEM and EDS analyses performed. SEM micrographs and EDS analysis show the presence of a relatively thick adhered layer of workpiece material at the rake face wear land for low cutting speeds. At high cutting speeds, crater wear is clearly dominant and the adhesive mechanism seen for lower cutting speeds is almost gone. To strengthen the results in the present work further, it would be very beneficial to perform long-term wear experiments at lower cutting speeds in order to clarify if the observed material build-up is truly the initial stage of a slow material removal process through adhesive wear, as was found to be the case for higher cutting speeds.

Traditionally, wear is often not measured as a removed volume, but as the length, width or depth of the wear land. Because the present work calculates wear rate from total volume

38 changes and not pure removed material change it can be hard to compare wear rate results from this present work with earlier results presented by other authors. One way to calculate the true worn off volume is to remove adhered material, for example through etching in hydrofluoric acid as some earlier researchers did [24]. New measurements with Alicona InfiniteFocus could then be performed to measure the true cutting tool volume removed during machining from which the true wear rate could be calculated and compared with earlier results. One has to be careful though when etching, so that only adhered material is removed and so that no further wear is introduced. In the present work it was decided to perform no etching, in order to leave the worn inserts unharmed for possible future analyses.

6.3 Cutting forces Measurements of cutting forces and moments show that the cutting operations performed during the experiments are close to orthogonal, from which the assumptions of orthogonality made earlier should be reasonable.

When comparing fig. 24 and 29, forces simulated and forces measured at cutting speeds above 60 m/min are fairly the same, but for cutting speeds ≤ 60 m/min, simulated cutting forces are significantly higher than the experimental ones. From SEM and EDS analysis, a formation of BUL was detected, which could help reduce the friction and hence cutting forces for cutting speeds up to 60 m/min.

Comparison of experimental feed force in fig. 26 with simulated feed force in fig. 30 shows quite a big difference. The experimental feed force measured during experiments A-T (Table

1) shows an increase with increasing cutting speed and/or feed rate. At small feeds (fn=0.05 mm/rev) the feed force is almost constant while at higher feed rates, the feed force first decreases between cutting speeds 30 and 45 m/min and then increases between cutting speeds 60 and 115 m/min. Simulated feed forces decrease with increasing cutting speeds but increase with increasing feed rates.

One reason for these differences between measured and simulated forces could be flaws in the assumptions in the calculation model. As mentioned earlier, cutting tools are subjected to rapid and extensive tool wear during titanium machining and the cutting edge will change its geometrical form during machining. When the cutting edge changes its geometry, cutting forces will change due to changes in material flow and possibly also friction. Also, the cutting geometry will affect the shearing of the work piece material, and small changes can have effects on cutting forces.

Overall, the simulated cutting forces were in good agreement with the measured ones. Because of this, the estimated contact pressures and temperatures have also been considered trustworthy enough to be used in the assessment of the wear model. Estimation of the constant, C2, in eq. 2 was performed with the assumption that the contact pressure remains constant at the experimental conditions used. This assumption was made after simulating and calculating in AdvantEdge. Simulations showed that the contact stresses at the points for maximum contact temperature on the rake face varied between 1.5 and 2 GPa.

6.4 Wear modeling Assuming one dominating temperature dependent wear mechanism, the activation energy in a temperature region can be calculated from the equation obtained in fig. 31. If the same wear mechanism were dominating for all cutting speeds during titanium machining, it would be possible to calculate wear rates for lower cutting speeds as well. Assuming the same mechanism occurring for cutting speeds of 90-115 m/min as for a cutting speed of 60 m/min, one can calculate the expected wear rate. From simulated temperatures presented in fig. 28, the contact temperature during machining at 60 m/min and a feed rate of 0.15 mm/rev is estimated to be approximately 1050 K. Assuming 3 minutes cutting time gives a cutting length of 180 m. Using the same values for σt, C1 and C2 as presented in section 5.5, the removed volume from the machined cutting tool would be approximately 255 000 µm3. Studying results from volume measurements in fig. 11 shows a significantly higher wear, from which it is concluded that short-term machining of titanium alloy with WC/Co cutting tools at 60 m/min does not have the same dominating wear mechanism as machining at 90 or 115 m/min. At cutting speeds above 115 m/min we expect plastic deformation to be the dominating wear mechanism as earlier presented by Hartung and Kramer [24]. Machining titanium alloy at a cutting speed of 115 m/min, a feed rate 0.2 mm/rev and cutting times longer than 1 minute caused plastic deformation and catastrophic failure of the cutting edge.

7. Conclusions Results presented in this thesis suggest how the forces acting on the cutting tool edge cause chamfering, probably by abrasion and/or plastic deformation. Adhesion of workpiece material on the rake face wear land was observed for low cutting speeds. At high cutting speeds, the BUL starts to wear off and crater wear is becoming the dominating wear mechanism. On the flank face wear land, abrasion was observed. An adhered edge of workpiece material together with oxide formation was also found. On the rake face, further away from the cutting edge, both workpiece and cutting tool material were observed together with oxide formation.

For cutting speeds 30-60 m/min, the dominating wear mechanism was found to be adhesive and the total volume difference from before machining was found to be positive, as earlier presented in figure 12. Even though testing was only performed for up to 3 min of machining, it seems that the wear rate on the cutting tool at these low speeds is relatively low.

For cutting speeds 90-115 m/min, crater wear was found to be the dominating wear mechanism.

For all samples, it is clear that the most prominent wear is occurring on the rake face of the cutting tool. When the workpiece material adheres to the wear land of the cutting tool insert, carbon diffusion from WC grains in the cutting tool material into the adhered BUL is started. Diffusion of carbon from the insert causes carbon depletion at the interface between cutting tool material and BUL. This depletion will cause poor adhesion between cutting tool material and BUL and an increasing probability of material removal by the chip flowing by. This is the cause for the crater wear experienced in this present work. The wear mechanism which dominates at the rake face during titanium machining was hence found to be a chemical mechanism, controlled by the diffusion rate of carbon through the adhered material layer.

It was found that this diffusion controlled, chemical wear mechanism dominating the rake face wear (at least for cutting speeds 90-115 m/min) has a temperature dependence with an Arrhenius-type reaction rate. The equation found for this wear mechanism was:

( ) [ ( ) ]

This equation represents a complete expression for the wear model presented by Usui et al. [23] and can be used as presented in software such as AdvantEdge to calculate the wear of cemented carbide inserts during titanium machining.

The wear modelling equation presented is only proved to be valid for cutting speeds 90 and 115 m/min. Before using this equation for estimating tool wear for other cutting parameters, further investigations have to be performed.

8. Future outlooks It would be interesting to continue this investigation in tool wear during titanium machining. Some possible ways to continue the research could be:

 Evaluate and analyse samples not included in this thesis (because of time limitations) to see variations with feed rate.  Further investigations in the formation of TiC in the BUL, possibly by examinations with Auger spectroscopy and/or XRD.  Examine the depletion of atoms in the area between cutting tool material and BUL.  Further characterization of BUL and layer thickness together with examination of the tackiness between cutting tool material and BUL.  Further experiments for cutting speeds of 30-60 m/min to see what happens with the wear rate for cutting times longer than 3 minutes.  Measure width of wear land and see if it is possible to calculate a wear rate for lower cutting speeds as well as higher.  Diffusion experiments or literature studies to find the high-temperature diffusion rates of C through Ti, followed by attempts to correlate such results to the activation energy found for the wear mechanism in the present work. Some experiments of this type were presented by Jianxin et al. [25], but unfortunately they investigated only temperatures below the transition from hcp to bcc.

9. Acknowledgements First and foremost I would like to send my appreciations to my supervisor Jonas Östby: thank you for making this project possible and for all the help along the way. I would like to thank Kalle for helping with the experimental execution in the lathe, Patrik for help with optical analysis and Irene for helping during SEM and EDS analysis. Jonas and Vahid, thank you for helping me with the AdvantEdge simulations and thank you, Mats Boman at Uppsala University, for reviewing my report.

Special thanks to all my co-workers for all memorable coffee breaks and discussions, you really made me feel at home. Mikael, Anna, Linda, Daniel, Anders, Tomas, Martin x 3, Amil and the department CTA, I am going to miss you all!

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18. F. Luigino, M. Fabrizio, L. Settineri, D. Umbrello. Wear modelling in mild steel orthogonal cutting when using uncoated carbide tools. Wear. 2007, vol. 262, p. 545- 554. 19. P.K. Wright, A. Bagchi. Wear mechanisms that dominate Tool-life in Machining. Journal of Applied Metalworking. 1981, vol. 1, p. 15-23. 20. Y.C. Yen, J. Söhner, B. Lilly, T. Altan. Estimation of tool wear in orthogonal cutting using the finite element method. Journal of Materials Processing Technology. 2004, vol. 146, p. 82-91. 21. Machining Efficiencies. Tool wear photos. At www.machiningefficiencies.com/toolwearphotos.html (Retrieved: 2012-02-21) 22. E. Turkes, S. Orak, S. Neselie, S. Yaldiz. Linear analysis of chatter vibration and stability for orthogonal cutting in turning. International Journal of Refractory Metals and Hard Materials. 2011, vol. 29, p. 163-169. 23. E. Usui, T. Shirakashi, T. Kitagawa. Analytical prediction of cutting tool wear. Wear. 1984, vol. 100, p. 129-151. 24. P.D. Hartung, B.M. Kramer. Tool wear in titanium machining. CIRP Annals - Manufacturing Technology. 1982, vol. 31, p. 75-80. 25. D. Jianxin, L. Yousheng, S. Wenlong. Diffusion wear in dry cutting of Ti-6Al-4V with WC/Co carbide tools. Wear. 2008, vol. 265, p. 1776-1783. 26. P.J. Arrazola, A. Garay, L.M. Iriarte, M. Armendia, S. Marya, F. Le Maitre. Machinability of titanium alloys (Ti6Al4V and Ti555.3). Journal of Materials Processing Technology. 2009, vol. 209, p. 2223-2230. 27. J.M. Gerez, M. Sanchez-Carrilero, J. Salguero, M. Batista, M. Marcos. A SEM and EDS based Study of the Microstructural Modifications of Turning Inserts in the Dry Machining of Ti6Al4V Alloy. AIP Conference proceedings 1181. 2009, p. 567-574. 28. R. Danzl, F. Helmli, S. Scherer. Focus variation – A new technology for high resolution optical 3D surface metrology. 2009. 29. S. Swapp. Scanning Electron Microscopy. University of Wyoming. At http://Serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html (Retrieved: 2012-02-06) 30. B. Hafner. Scanning Electron Microscopy Primer. University of Minnesota, Characterization Facility. 2007.

APPENDIX A Experimental matrix for wear tests in lathe.

ap = 3 mm (constant)

vc (m/min) fn (mm/rev) t (min) length (m) Sample ap = dept of cut

30 0.05 0.5 15 A1 1 vc = cutting speed 30 0.05 1 30 A1 2 f = feed rate n 30 0.05 2 60 A1 3 t = cutting time 30 0.05 3 90 A2 1 30 0.1 0.5 15 A2 2 30 0.1 1 30 A2 3 30 0.1 2 60 A3 1

30 0.1 3 90 A3 2 30 0.15 0.5 15 A3 3 30 0.15 1 30 A4 1 30 0.15 2 60 A4 2 30 0.15 3 90 A4 3 30 0.2 0.5 15 A5 1 30 0.2 1 30 A5 2 30 0.2 3 90 A5 3 30 0.2 2 60 A6 1 45 0.05 0.5 22.5 A6 2 45 0.1 0.5 22.5 A6 3 45 0.15 0.5 22.5 A7 1 45 0.2 0.5 22.5 A7 2 45 0.05 1 45 C8 1 45 0.05 2 90 C8 2 45 0.05 3 135 C8 3 45 0.1 1 45 A8 3 45 0.1 2 90 A9 1 45 0.1 3 135 A9 2 45 0.15 1 45 A9 3 45 0.15 3 135 A10 1 45 0.15 2 90 A10 2 45 0.2 1 45 A10 3 45 0.2 2 90 B1 1 45 0.2 3 135 B1 2

49 vc (m/min) fn (mm/rev) t (min) length (m) Sample 60 0.05 0.5 30 B1 3 60 0.05 1 60 B2 1 60 0.05 2 120 B2 2 60 0.1 0.5 30 B2 3 60 0.05 3 180 B3 1 60 0.15 0.5 30 B3 2 60 0.1 1 60 B3 3 60 0.1 3 180 B4 1 60 0.1 2 120 B4 2 60 0.15 1 60 B4 3 60 0.15 2 120 B5 1 60 0.15 3 180 B5 2 60 0.2 0.5 30 B5 3 60 0.2 1 60 B6 1 60 0.2 2 120 B6 2 60 0.2 3 180 B6 3 90 0.05 0.5 45 B7 1 90 0.1 1 90 B7 2 90 0.05 1 90 B7 3 90 0.05 2 180 B8 1 90 0.05 3 270 B8 2 90 0.1 0.5 45 B8 3 90 0.1 2 180 B9 1 90 0.1 3 270 B9 2 90 0.15 0.5 45 B9 3 90 0.15 1 90 B10 1 90 0.15 2 180 B10 2 90 0.15 3 270 B10 3 90 0.2 0.5 45 C1 1 90 0.2 1 90 C1 2 90 0.2 2 180 C1 3 90 0.2 3 270 C2 1

50 vc (m/min) fn (mm/rev) t (min) length (m) Sample 115 0.05 0.5 57.5 C2 2 115 0.05 1 115 C2 3 115 0.05 3 345 C3 1 115 0.05 2 230 C3 2 115 0.1 0.5 57.5 C3 3 115 0.1 3 345 C4 1 115 0.1 1 115 C4 2 115 0.1 2 230 C4 3 115 0.15 0.5 57.5 C5 1 115 0.15 1 115 C5 2 115 0.15 2 230 C5 3 115 0.15 3 345 C6 1 115 0.2 0.5 57.5 C6 2 115 0.2 1 115 C6 3 115 0.2 2 230 C7 1 115 0.2 3 345 C7 2

APPENDIX B Results from optical measurements performed with Alicona InfiniteFocus and temperature simulations in AdvantEdge.

Volume Volume min max mean Area Area Contact Contact vc fn t length above below deviation deviation deviation above below temp temp (m/min) (mm/rev) (min) (m) Sample (µm3) (µm3) (µm) (µm) (µm) (µm2) (µm2) (˚C) (K) 30 0.15 0.5 15 A3 3 4890956 1221482 -12.772 60.16 0.7028 199392 263.84 620 893.15 30 0.15 1 30 A4 1 3580953 1480686 -30.634 42.963 0.3368 82136 7434.1 620 893.15 30 0.15 2 60 A4 2 3623847 1414106 -31.451 50.848 0.3836 86674 8888.7 620 893.15 30 0.15 3 90 A4 3 4982373 1015952 -16.993 57.101 0.9987 255663 2899.1 620 893.15 45 0.15 0.5 22.5 A7 1 2277177 944160 -67.692 57.469 0.3563 57848 5394.8 740 1013.15 45 0.15 1 45 A9 3 2621539 1009654 -21.909 33.743 0.3474 67517 2251.4 740 1013.15 45 0.15 2 90 A10 2 3053866 1019407 -23.365 44.126 0.4564 88141 2381.2 740 1013.15 45 0.15 3 135 A10 1 2776441 1210798 -34.958 29.792 0.3286 70119 7776.1 740 1013.15 60 0.15 0.5 30 B3 2 3515854 1422131 -16.889 25.18 0.3672 140151 5940.7 780 1053.15 60 0.15 1 60 B4 3 3095192 1468207 -40.295 35.918 0.2658 74010 9218.2 780 1053.15 60 0.15 2 120 B5 1 3760103 1311709 -21.584 38.688 0.5089 116508 3276.8 780 1053.15 60 0.15 3 180 B5 2 4274878 1433352 -8.0985 38.91 0.4621 121868 1002.6 780 1053.15 90 0.15 0.5 45 B9 3 2486371 2372647 -29.207 19.554 -0.1056 41245 99023 920 1193.15 90 0.15 1 90 B10 1 1813052 2896084 -15.471 13.465 -4.4054 4737.8 153300 920 1193.15 90 0.15 2 180 B10 2 2273092 5999470 -39.328 18.835 -1.113 33237 437016 920 1193.15 90 0.15 3 270 B10 3 1604257 6436633 -39.226 17.778 -1.3722 7669.2 477102 920 1193.15 115 0.15 0.5 57.5 C5 1 2428998 6637985 -18.941 26.55 -1.0698 34410 499989 1000 1273.15 115 0.15 1 115 C5 2 2199871 11266034 -35.678 32.928 -2.1982 29762 626441 1000 1273.15 115 0.15 2 230 C5 3 2171986 25792815 -94.435 21.501 -6.5415 74001 844353 1000 1273.15 115 0.15 3 345 C6 3 3982559 40951004 -152.2 41.337 -5.5097 78522 998298 1000 1273.15

Volume Volume min max mean Area Area Contact Contact vc fn t length above below deviation deviation deviation above below temp temp (m/min) (mm/rev) (min) (m) Sample (µm3) (µm3) (µm) (µm) (µm) (µm2) (µm2) (˚C) (K) 115 0.1 0.5 57.5 C3 3 1652230 2170386 -12.216 21.589 -0.2946 21586 128820 950 1223.15 115 0.1 1 115 C4 2 2138994 3986479 -28.523 18.928 -0.5322 14569 248538 950 1223.15 115 0.1 2 230 C4 3 2669828 6193393 -31.606 25.141 -0.855 26232 373319 950 1223.15 115 0.1 3 345 C4 1 2071178 10769093 -40.333 25.411 -2.0662 13756 511819 950 1223.15 115 0.2 0.5 57.5 C6 1 4683621 23839352 -60.696 18.961 -2.4334 82708 1.1598mm 1100 1373.15 115 0.2 1 115 C7 1 5251234 39679418 -75.697 60.489 -5.2421 186260 1.3392mm 1100 1373.15 30 0.05 3 90 A2 1 2246081 1263738 -17.717 34.242 0.1205 37442 1942.9 450 723.15 45 0.05 3 135 C8 3 2440184 1036875 -12.518 26.416 0.2691 52589 1208.4 570 843.15 60 0.05 3 180 B3 1 3032488 1283549 -32.04 37.562 0.2986 82066 4055.8 570 843.15 90 0.05 3 270 B8 2 2360005 1339396 -7.0222 28.374 0.1233 31944 500.41 690 963.15 115 0.05 3 345 C3 1 2017307 944153 -7.6522 19.806 0.2222 46169 557.85 760 1033.15

APPENDIX C Results from EDS analysis presented in at%.

Sample Spectrum W C Co Ti Al V O Cl K Na Mg S Ca Si P Zn Mo Fe Cu Mn Ni A3 3 9 7.4 26.6 50.6 1.5 12.9 0.9 10 70.2 5.6 0.5 0.5 19.4 1.4 0.5 1.3 0.4 0.1 0.1 11 8.9 71.2 10.2 6.9 2.8 12 15.8 47.8 1.6 18.2 1.9 1.0 13.7 A4 1 5 0.1 62.7 6.2 0.6 0.3 25.2 1.0 0.8 1.3 0.5 0.3 0.3 0.6 0.1 6 9.9 70.1 10.0 6.9 2.6 0.5 7 0.12 16.3 52.2 5.6 2.7 21.0 0.2 0.2 0.3 0.7 0.9 8 15.1 43.0 1.8 19.1 2.2 0.9 17.5 0.5 A4 2 01:27 23.2 55.8 6.1 2.7 11.3 0.9 01:28 7.2 74.9 10.5 7.4 01:29 8.1 17.0 3.2 62.2 6.8 2.7 A4 3 29 4.0 31.7 42.7 1.6 19.8 0.2 30 0.2 40.7 23.4 2.5 1.3 27.1 0.8 0.4 1.5 0.4 0.2 0.3 0.6 0.8 31 11.0 70.3 9.5 6.9 2.3 32 7.4 29.3 2.1 35.9 3.9 1.7 17.5 0.9 1.4 A7 1 44 9.0 33.1 29.7 0.5 26.2 0.5 1.0 45 0.6 15.0 72.9 7.1 3.9 0.4 46 0.1 57.7 14.4 1.0 0.6 21.8 1.3 0.4 1.9 0.2 0.2 0.2 0.3 47 6.6 52.7 1.1 16.1 1.6 0.8 18.3 1.1 0.2 1.5 A9 3 41 11.6 76.3 8.7 3.4 42 11.6 29.7 3.3 38.2 4.1 1.6 11.6 43 0.1 9.3 74.0 9.3 5.8 1.5

Sample Spectrum W C Co Ti Al V O Cl K Na Mg S Ca Si P Zn Mo Fe Cu Mn Ni A10 2 01:24 28.1 41.4 5.1 1.6 20.5 0.5 0.6 0.9 1.4 01:25 5.0 77.1 10.4 7.5 01:26 13.4 17.2 3.5 46.3 4.7 1.8 13.0 A10 1 37 15.4 73.2 7.7 3.3 0.4 38 15.0 57.4 4.9 2.8 17.1 0.5 0.3 0.7 0.5 0.6 0.3 39 45.8 20.4 2.2 1.1 26.4 0.8 0.4 1.3 0.3 0.2 0.3 0.6 0.2 40 15.1 71.9 7.8 3.7 0.3 0.6 0.3 0.4 B3 2 33 0.12 37.57 30.31 3.41 1.5 25.91 0.21 0.12 0.31 0.18 0.28 34 10.8 76.5 8.8 3.6 0.3 35 13.0 30.6 4.0 31.8 3.2 1.6 15.8 36 3.2 34.3 2.5 33.2 4.1 1.8 20.0 0.2 0.2 0.4 0.3 B4 3 20 2.5 25.3 33.0 0.7 38.6 21 61.0 7.5 0.7 0.4 26.5 0.9 0.5 1.0 0.2 0.3 0.5 0.4 0.1 22 0.5 9.2 74.2 8.8 5.9 1.4 23 0.4 14.3 75.0 6.4 3.9 24 19.0 54.9 1.5 4.9 17.0 1.5 1.3 B5 1 01:16 60.4 11.0 1.3 0.6 20.2 1.5 0.7 1.8 0.8 0.5 1.3 01:17 6.3 79.2 8.3 6.2 01:18 9.6 79.0 7.7 3.6 B5 2 17 1.3 14.4 71.6 8.3 4.0 0.5

18 9.5 78.7 6.9 4.6 0.3

19 13.9 37.0 2.2 17.8 2.0 1.0 16.1 3.8 1.0 0.6 4.0 0.7

Sample Spectrum W C Co Ti Al V O Cl K Na Mg S Ca Si P Zn Mo Fe Cu Mn Ni B9 3 13 0.1 15.2 72.55 7.01 4.41 0.26 0.46 14 24.7 21.0 1.0 39.6 4.3 2.1 7.4 15 8.8 79.2 8.2 3.8 16 13.7 58.0 7.7 0.8 0.4 18.5 0.4 0.5 B10 1 1 14.2 16.5 59.4 6.9 3.0 2 15.3 60.6 6.5 2.5 14.7 0.4 3 20.0 50.5 4.6 1.0 24.1 4 14.5 52.5 1.0 8.2 1.1 18.7 1.1 0.7 1.6 0.6 B10 2 01:12 0.2 16.9 61.1 6.3 2.8 10.9 0.6 0.3 0.8 01:13 0.7 24.7 56.2 6.5 2.8 9.0 01:14 4.6 8.6 74.4 8.4 4.0 01:15 0.2 17.1 58.7 6.9 2.6 12.7 0.4 0.8 0.8 B10 3 25 1.0 26.2 61.1 7.7 3.1 0.2 0.2 0.6 26 40.6 31.9 2.6 1.6 20.4 0.5 0.5 0.7 0.2 0.2 0.2 0.4 0.2 27 0.4 6.4 82.3 6.2 4.8 28 6.4 38.2 0.7 4.6 0.9 0.2 36.4 0.7 0.7 0.8 4.1 0.3 1.6 2.5 1.6 0.3 C5 1 01:06 0.9 72.2 3.4 0.6 19.0 1.2 0.9 1.1 0.3 0.3 0.3 01:07 61.53 11.69 1.5 0.51 21.94 0.77 0.62 0.77 0.19 0.24 0.26 01:08 10.9 76.9 8.9 3.3 01:09 2.8 12.4 72.7 8.8 3.4 01:11 0.3 80.8 0.2 14.7 1.3 0.9 1.2 0.4 0.2 C5 2 01:30 61.0 10.1 1.0 0.5 23.3 1.0 0.7 0.9 0.4 0.3 0.3 0.5 01:31 15.1 19.1 57.0 5.9 2.9 01:32 18.9 38.4 4.6 29.9 3.4 1.2 2.9 0.6

Sample Spectrum W C Co Ti Al V O Cl K Na Mg S Ca Si P Zn Mo Fe Cu Mn Ni C5 3 01:33 21.2 48.6 11.9 7.2 11.1 01:34 0.2 20.8 68.4 7.7 2.9 01:35 2.4 66.2 0.8 0.5 22.0 0.4 0.5 0.8 1.6 0.4 0.7 3.4 0.5 01:36 5.6 33.3 6.9 1.1 0.5 40.3 0.8 6.3 0.4 1.1 2.0 1.7 01:37 18.7 54.9 2.8 16.6 2.0 1.2 3.8 C6 3 01:01 24.1 20.4 1.3 34.5 3.8 1.4 14.6 01:02 9.1 80.2 7.1 3.7 01:03 0.5 11.8 70.1 9.5 3.8 4.4

APPENDIX D SEM micrographs with areas for EDS analysis marked out.

vc 30 m/min, fn 0.15 mm/rev, t 0.5 min. vc 30 m/min, fn 0.15 mm/rev, t 1 min.

vc 30 m/min, fn 0.15 mm/rev, t 2 min. vc 30 m/min, fn 0.15 mm/rev, t 3 min.

vc 45 m/min, fn 0.15 mm/rev, t 0.5 min. vc 45 m/min, fn 0.15 mm/rev, t 1 min.

vc 45 m/min, fn 0.15 mm/rev, t 2 min. vc 45 m/min, fn 0.15 mm/rev, t 3 min.

vc 60 m/min, fn 0.15 mm/rev, t 0.5 min. vc 60 m/min, fn 0.15 mm/rev, t 1 min.

vc 60 m/min, fn 0.15 mm/rev, t 2 min. vc 60 m/min, fn 0.15 mm/rev, t 3 min.

vc 90 m/min, fn 0.15 mm/rev, t 0.5 min. vc 90 m/min, fn 0.15 mm/rev, t 1 min.

vc 90 m/min, fn 0.15 mm/rev, t 2 min. vc 90 m/min, fn 0.15 mm/rev, t 3 min.

vc 115 m/min, fn 0.15 mm/rev, t 0.5 min. vc 115 m/min, fn 0.15 mm/rev, t 1 min.

vc 115 m/min, fn 0.15 mm/rev, t 2 min. vc 115 m/min, fn 0.15 mm/rev, t 3 min.