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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. iii 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. iv 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 v vi 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 ................................................................................................................
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