Microstructure and Mechanical Properties of Thixowelded AISI D2 Tool Steel

Article Microstructure and Mechanical Properties of Thixowelded AISI D2 Tool Steel

M. N. Mohammed 1,*, M. Z. Omar 2, Salah Al-Zubaidi 3, K. S. Alhawari 2 and M. A. Abdelgnei 2

1 Department of Engineering & Technology, Faculty of Information Sciences and Engineering, Management & Science University, Selangor 40100, Malaysia 2 Department of Mechanical and Materials Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Selangor 43600, Malaysia; [email protected] (M.Z.O.); [email protected] (K.S.A.); [email protected] (M.A.A.) 3 Department of Automated Manufacturing Engineering, Al-Khwarizmi college of Engineering, University of Baghdad, Baghdad 10071, Iraq; [email protected] * Correspondence: [email protected]; Tel.: +60-111-123-5993

Received: 9 March 2018; Accepted: 19 April 2018; Published: 4 May 2018

Abstract: Rigid perpetual joining of materials is one of the main demands in most of the manufacturing and assembling industries. AISI D2 cold work tool steels is commonly known as non-weldable metal that a high quality joint of this kind of material can be hardly achieved and almost impossible by conventional welding. In this study, a novel thixowelding technology was proposed for joining of AISI D2 tool steel. The effect of joining temperature, holding time and post-weld heat treatment on microstructural features and mechanical properties were also investigated. Acceptable joints without defect were achieved through the welding temperature of 1300 ◦C, while the welding at lower temperature resulted in a series of cracks across the entire joint that led to spontaneous fracture after joining. Tensile test results showed that maximum joint tensile strength of 271 MPa was achieved at 1300 ◦C and 10 min holding time, which was 35% of that of D2 base metal. Meanwhile, tensile strength of the joined parts after heat treatment showed a signiﬁcant improvement over the non-heat treated condition with 560 MPa, i.e., about 70% of that of the strength value of the D2 base metal. This improvement in the tensile strength attributed to the dissolution of some amounts of eutectic chromium carbides and changes in the microstructure of the matrix. The joints are fractured at the diffusion zone, and the fracture exhibits a typical brittle characteristic. The present study successfully conﬁrmed that by avoiding dendritic microstructure, as often resulted from the fusion welding, high joining quality components obtained in the semi-solid state. These results can be obtained without complex or additional apparatuses that are used in traditional joining process.

Keywords: thixowelding; thixojoining; semisolid joining; cold-work tool steel

1. Introduction Recently, AISI D2 tool steel get signiﬁcant attention in the wide applications of industrial sector owing to its attractive properties. Unfortunately, a high-quality joint of this type of material can be hardly achieved and almost impossible by conventional welding methods owing to its high carbon and alloying elements content with an enormous amount of carbides. The existing fusion welding technique is the most commonly used process and has been widely practiced in industries. In this process, the welding is used with preheating and post-weld heat treatment (PWHT) to avoid solidiﬁcation cracking and residual stresses that induced by the phase transformations throughout the welding. The most critical part in the process is the heat affected zone (HAZ), which can be very hard and brittle, along with susceptive to cracking if the tools are not heat-treated correctly.

Metals 2018, 8, 316; doi:10.3390/met8050316 www.mdpi.com/journal/metals Metals 2018, 8, 316 2 of 16

In addition, fusion welding technologies are characterized by high temperature gradients that results in high thermal stresses and a fast solidification, which lead to the occurrence of segregation phenomena. Furthermore, the interface morphology is characteristically dendritic and the natural progression of solidification commonly leads to interior structural defects, such as shrinkage porosities and non-homogenous microstructure [1]. Therefore, an innovative route that can avoid the above mentioned problems inherent in fusion welding is required. Thixojoining is a possible technique that can weld the AISI D2 cold-work tool steel free from the above mentioned problems, as the process is carried at the semisolid state where the mechanisms of solidification and heat flow are different from other welding processes [2–4]. During the last 30 years, several authors have shown that the thixojoining process has the potential to be used for a wide range of metal production processes. Notwithstanding some technical and technological differences between the available semi-solid joining processes, they can be classified into four categories: (1) joining metals by using semisolid slurries, (2) addition of functional features, (3) semisolid stir joining, and (4) semisolid diffusion joining [2]. The first method involves the utilization of a thixotropic metal as filler applied into the joint groove for the purpose of joining materials. This kind of joining has the distinctive advantage of allowing a controlled flow during deposition process. Moreover, the applied temperatures and thermal gradients are considered smaller if compared with fusion welding. Earlier research work in the 1990s has demonstrated the feasibility of the process when Mendez and Brown used Sn-5 wt. % Pb as a filler and applied it on the joint groove to join bars of Sn-15 wt. % Pb model alloy. The microstructure of the joint cross-sectional area showed excellent metallurgical joining with the interface connection and smooth and homogeneous welding between the bars and filler metal without defects or adverse effects in the heating zone [5]. Other researchers have more investigations in the field of semisolid joining revealed the possibility of producing prototype components by combining the forming operation with the simultaneous insertion of additional components [6]. One special approach that takes advantage of the material’s high flowability is the addition of functional features to a forged part. With this technology, it is possible to shorten conventional process chains and to create a new generation of components. Kiuchi et al. [6] studied the possibility of joining aluminum with aluminum and steel alloys. They found that thixojoining of simple insert shape with bulk material is promising approach. With decreasing of liquid fraction, there was increasing in joint strength. This clarifies the significant merits compared with conventional casting due to formation of intermetallic compounds and diffusion between base and insert materials. The big challenge of improvement this technique for other materials is accessible inserting component in mushy matrix with destruction of joint geometry by remelting or deformation. Further investigations in Semisolid stir joining (SSSJ) showed the applicability of this type of joining process to improve and overcome the difficulties accompanied with current joining techniques which were based on friction stir welding. Recently, there was trend toward carrying out a vacuum-free SSSJ process that enables using low joining temperature when applied to a semisolid base or filler metal. Hosseini et al. [7] have also carried out extensive experiments on semisolid stir joining (SSSJ) for AZ91 alloy by using mechanical stirring and a Mg-25 wt. % Zn interlayer. The results showed the significant effect of mechanical stirring rate on the joint-strength. This supposes that the formed oxide layer has been disrupted due to stirring effect and consequently improve metallurgical bonds. They claimed that this method can be used as an alternative joining for Mg-based alloys. More investigations to achieve joint with high-quality globular structure were undertaken by Mohammed et al. [8–10] in semisolid diffusion joining (SSDJ). Their research explored the possibilities of joining semisolid AISI D2 tool steel with functional elements of the same material as well as with other materials such as AISI 304 stainless steel by using a peculiar characteristic of the metals’ liquid components. In their proposed method, when the interfaces of two metals are pasted together, the liquid components present in both sides of these “semisolid metals” penetrate and diffuse through the interface and then solidify together as one body. The experimental results showed that, the use of Metals 2018, 8, 316 3 of 16 this technique can make homogeneous properties with high surface quality without any evidence of porosity or microcracking. The semisolid metal joining (SSMJ) process can be used for processing different net near net shape components with complicated shape and geometries where other joining processes cannot be utilized for many reasons such as: low melting temperature, poor mechanical properties, and insufficient plastic deformation. Several applications have been considered based on this new joining method like heating radiator components, car shock absorber, slider system components and roller way elements. In addition, this joining technology can be used for joining craft glass and stones to plates and bars with different shapes. The products can be utilized in decoration various wall plates and floor panels. Furthermore, metal/glass joining products have great capabilities to add other functional properties to materials to produce lubricated, corrosion-proof, antiwear or heatresistant items [2]. The thixojoining is new semi-solid-based process that should take their place among other joining processes. Nevertheless, it takes little considerations by researcher and investigators. This new specific area needs continuous studies and works to overcome the related problem and propose solutions and suggestions especially for high grade alloys and materials. In this work, the thixowelding process was applied to join AISI D2 cold work tool steels by using a direct partial remelting method (DPRM) method. The objectives of the present research are to study the microstructure evolution and mechanical properties of the joints under different welding conditions.

2. Experimental Section

2.1. Materials and Sample Preparation AISI D2 cold-work tool steel used in this study which was supplied after a soft annealing process, i.e., heating to 850 ◦C followed by cooling at 10 ◦C/h to 650 ◦C and finally air cooling. The chemical composition of the studied AISI D2 cold-work tool steel was determined by using X-ray fluorescence (XRF) (S8 TIGER ECO, Bruker, Karlsruhe, Germany) as shown in Table1. The base blank of AISI D2 was cut Ø 16× 100 mm while the length of the pin insert was 18 mm with diameter of 7 mm as shown in Figure1. Each specimen was disposed of rust and scale by submerging in pickling solution, washed with acetone, and finally rinsed using distilled water. The temperatures of liquidus and solidus, in addition to liquid fraction, were estimated by carrying out differential scanning calorimetry test (DSC). Small pieces samples (less than 20 mg) were cut from the as-received material for testing with The Netzsch-STA 449 F3 simultaneous thermogravimeter (TGA-DSC) (STA 449F3, NETZSCH, Bavaria, Germany). The samples were heated at a rate of 10 ◦C/min in nitrogen atmosphere to prevent oxidation. The common tangent procedure is adopted to estimate the liquid fraction. Finding out the fraction of liquid was done by integrating of partial areas under the heating curves as shown in Figure2.

Table 1. Chemical composition of AISI D2 cold-work tool steel (in Weight Percent).

C Si Mn P S Cr Ni Mo W Cu V Fe 1.50 0.258 0.24 0.02 0.01 11.2 0.21 0.78 0.2 0.08 0.781 Balance Metals 2018, 8, 316 4 of 16 Metals 2018, 8, x FOR PEER REVIEW 4 of 16

Metals 2018, 8, x FOR PEER REVIEW 4 of 16 16mm

16mm AISI D2

AISI D2 7mm 100mm

7mm 100mm AISI D2 AISI D2

20mm 20mm

FigureFigure 1. 1.Schematic Schematic drawing drawing of of the the AISI AISI D2 D2 parts parts combination. combination. Figure 1. Schematic drawing of the AISI D2 parts combination.

0.1870.187 100 100 90 90 0.1370.137 80 80 70 70 0.0870.087 60 60 50 50 0.0370.037 40 40

30 fraction Liquid

30 fraction Liquid Heat flow/(mW/mg) Heat -0.013 20 Heat flow/(mW/mg) Heat -0.013 DSC/(mW/mg) 20 DSC/(mW/mg)10 Liquid fraction 10 -0.063 Liquid fraction0 -0.063 1220 1240 1260 1280 1300 1320 1340 1360 1380 1400 1420 0 1220 1240 1260 1280 1300 1320 1340 1360 1380 1400 1420 Temperature (°C) Temperature (°C) FigureFigure 2. Liquid 2. Liquid fraction fraction vs. vs. temperature temperature andand heating flow ﬂow vs vs temperature temperature curves curves of the of AISI the AISID2 tool D2 tool steelFiguresteel calculated 2. calculated Liquid by fraction differentialby differential vs. temperature scanning scanning calorimetry calorimetry and heating (DSC). flow vs temperature curves of the AISI D2 tool steel calculated by differential scanning calorimetry (DSC). 2.2. Thixo2.2. Thixo Welding Welding Process Process High temperature carbolite furnace was utilised to carry out the thixo welding of AISI D2 tool 2.2. HighThixo temperatureWelding Process carbolite furnace was utilised to carry out the thixo welding of AISI D2 tool steel as shown in Figure 3. The furnace is vertical type and protected with argon gas. The moment steelthat asHigh shown furnace temperature in reached Figure 3the .carbolite The pre-selected furnace furnace istemperature vertical was utilised type (1250, and to1275, protectedcarry and out 1300 withthe °C), thixo argon the sample welding gas. The was of momentlowered AISI D2 that tool ◦ furnacesteelinto as reached theshown furnace thein Figureby pre-selected use chromehromel3. The temperaturefurnace wire is vertical for (1250, 0, 5 1275,andtype 10 and min 1300protected respectivelyC), thewith then sample argon it was was gas. allowed lowered The momentto into thethat furnacecool furnace slowly by reached useat room chromehromel the temperature pre-selected wireas shown temperature for 0,in 5Figure and (1250, 104. To min investigate1275, respectively and 1300the influence then°C), the it was ofsample post allowed welding was lowered to cool slowlyintoheat the at treatment roomfurnace temperature byon theuse microstructure chromehromel as shown and in wire Figuremechanical for4 .0, To 5properties investigateand 10 minof the the respectively welded inﬂuence plates, ofthen specimens post it was welding wereallowed heat to treatmentcoolheat slowly treated on theat byroom microstructure heating temperature the specimens and as mechanical shownto 850–900 in °CFigure properties followed 4. To by of investigate theslow welded rates of the furnace plates, influence specimenscooling of (22 post °C/h), were welding heat ◦ ◦ treatedheatwhen treatment by a heating steel oncools thethe to specimensmicrostructure 650 °C, air cooling to 850–900 and can mechanical beC used followed to bringproperties by the slow steel of rates theto room welded of furnace temperature. plates, cooling specimens (22 C/h), were ◦ whenheat treated a steel coolsby heating to 650 theC, specimens air cooling to can 850–900 be used °C followed to bring theby slow steel rates to room of furnace temperature. cooling (22 °C/h), 2.3. Metallography when a steel cools to 650 °C, air cooling can be used to bring the steel to room temperature. 2.3. MetallographyThe thixowelded joint samples were polished and then etched by using Villela reagent (1 g picric acid, 5 mL hydrochloric acid, and 95 mL ethyl alcohol) to reveal their microstructures. The 2.3. TheMetallography thixowelded joint samples were polished and then etched by using Villela reagent (1 g microstructures of the joints were characterized by Optics microscopy (BX51-P, Olympus, Tokyo, picricJapan),The acid, thixowelded 5 scanning mL hydrochloric electron joint samples microscopy acid, were and (SEM) 95 polished mL (LEO ethyl and 1455 alcohol) then VP etched SEM, to reveal Zeiss, by using their Oberkochen, Villela microstructures. reagent Germany) (1 g picric The microstructuresacid, 5 mL hydrochloric of the joints were acid, characterized and 95 mL by ethyl Optics alcohol) microscopy to reveal (BX51-P, their Olympus, microstructures. Tokyo, Japan), The microstructures of the joints were characterized by Optics microscopy (BX51-P, Olympus, Tokyo, Japan), scanning electron microscopy (SEM) (LEO 1455 VP SEM, Zeiss, Oberkochen, Germany)

Metals 2018, 8, 316 5 of 16 scanningMetalsMetals electron2018 2018, , 88,, xx FORFOR microscopy PEERPEER REVIEW REVIEW (SEM) (LEO 1455 VP SEM, Zeiss, Oberkochen, Germany) equipped 5 5 of of 16 16 with energy dispersive X-ray spectroscopy (EDX) (Oxford 7353, Oxford Instruments, Wiesbaden, Germany). equippedequipped with with energy energy dispersive dispersive X-ray X-ray spectroscopy spectroscopy (EDX) (EDX) (Oxford (Oxford 7353, 7353, Oxford Oxford Instruments, Instruments, X-rayWiesbaden,Wiesbaden, diffraction Germany).Germany). (XRD) (Bruker X-rayX-ray D8diffractiondiffraction Discover, (XRD)(XRD) Bruker, (Bruker(Bruker Karlsruhe, D8D8 Discover,Discover, Germany) Bruker,Bruker, analyses Karlsruhe,Karlsruhe, were Germany)Germany) carried out to analyzeanalysesanalyses the werewere phases carriedcarried present outout toto in analyzeanalyze the steel thethe specimen phasesphases presentpresent before inin and thethe aftersteelsteel specimen thespecimen thixojoining. beforebefore andand It alsoafterafter givesthethe an idea aboutthixojoining.thixojoining. the characteristics ItIt alsoalso givesgives anan of ideaidea joints aboutabout and thethe if characteristics therecharacteristics is any harmful ofof jointsjoints and phaseand ifif therethere or brittle isis anyany phase harmfulharmful have phasephase formed duringoror brittlethixojoining.brittle phasephase havehave formedformed duringduring thixojoining.thixojoining.

2.4. Test2.4.2.4. ofTestTest Mechanical ofof MechanicalMechanical Properties PropertiesProperties The mechanicalTheThe mechanicalmechanical characterization characterizationcharacterization of ofof the thethe joints jointsjoints waswas measuredmeasured usingusing using tensiletensile tensile strengthstrength strength andand andhardnesshardness hardness tests.tests.tests. The The tensileThe tensiletensile tests teststests were were were carried carriedcarried out outout on on on using using using ZwickZwick universaluniversal universal testing testing testing machinemachine machine (ProLine (ProLine (ProLine Z005/Z100, Z005/Z100, Z005/Z100, Zwick,Zwick,Zwick, Ulm, Ulm,Ulm, Germany) Germany)Germany) with withwith a cross-head aa cross-headcross-head speed speedspeed of 0.5 ofof mm/min0.50.5 mm/minmm/min at roomatat roomroom temperature temperaturetemperature according accordingaccording to toto ASTM ASTMASTM E8M E8M Standards. Standards. The The fracture fracture morphologies morphologies of of failed failed specimens specimens were were then then observed observed by by E8M Standards. The fracture morphologies of failed specimens were then observed by scanning electron scanningscanning electronelectron microscopymicroscopy (SEM).(SEM). VickersVickers micro-hardnessmicro-hardness testingtesting (FH5,(FH5, TiniusTinius Olsen,Olsen, Horsham,Horsham, microscopy (SEM). Vickers micro-hardness testing (FH5, Tinius Olsen, Horsham, PA, USA) was performed PA,PA, USA)USA) waswas performedperformed onon thethe polishedpolished specimensspecimens ofof thethe transversetransverse sectionsection ofof jointsjoints asas wellwell asas thethe on thebasebase polished materialmaterial specimens andand insertinsert toofto compare thecompare transverse thethe phasesphases section presentpresent of joints inin thethe asjointjoint well andand as thethe the joiningjoining base parametersmaterialparameters and onon insertthethe to comparebasebase themetal.metal. phases TheThe profilesprofiles present ofof in micro-hardnessmicro-hardness the joint and the werewere joining constructedconstructed parameters acrossacross the onthe thesample’ssample’s base metal.horizontalhorizontal The sectionsection profiles of micro-hardnessofof thethe D2D2 tooltool were steelsteel constructed basebase metal,metal, the acrossthe weldweld the zonezone sample’s andand thethe horizontal insertinsert byby usingusing section aa loadload of theofof 1010 D2 gg forfor tool 3030 steel ss alongalong base thethe metal, the weldlineslines zone perpendicularperpendicular and the insert toto thethe joint byjoint using interface.interface. a load HardnessHardness of 10 g measurementsmeasurements for 30 s along werewere the linescarriedcarried perpendicular outout byby usingusing VickersVickers to the joint interface.micro-hardnessmicro-hardness Hardness measurementstestertester FH5.FH5. were carried out by using Vickers micro-hardness tester FH5.

ArgonArgon InIn ThermocoupleThermocouple

FurnaceFurnace tubetube

SampleSample

HeatingHeating elementelement

FurnaceFurnace casingcasing 25mm25mm SteelSteel cupcup

SteelSteel plateplate ArgonArgon OutOut

FigureFigureFigure 3. 3.3.Schematic SchematicSchematic drawing drawingdrawing ofof thethe the furnacefurnace furnace set-up.set-up. set-up.

Temp.Temp. (°C)(°C)

ProcessProcess Temp.Temp. SemiSemi solidsolid zonezone HoldingHolding timetime

AirAir coolingcooling

TimeTime (minutes)(minutes)

Figure 4. Temperature proﬁle of direct partial remelting experiment. Metals 2018, 8, x FOR PEER REVIEW 6 of 16 Metals 2018, 8, 316 6 of 16 Figure 4. Temperature profile of direct partial remelting experiment.

3. Results Results and and Discussion

3.1. Microstructure and Joint Quality The microstructure of as-received AISI D2 consists of ferritic matrix parallel parallel to working working direction direction in which which different different surplus surplus of of large-coarse large-coarse and and fine ﬁne carbides carbides are aredistributed distributed as shown as shown in Figure in Figure 5. The5. revealedThe revealed microstructure microstructure refers refers to annealed to annealed tool steel tool steel[11] and [11] indicates and indicates that the that heat the heattreatment treatment was carriedwas carried out as out recommended. as recommended. The optical The optical microscopy microscopy examination examination performed performed on the welded on the joints welded at ◦ variousjoints at variousconditions conditions is shown is shown in Figure in Figure 6. From6. From the the complete complete weld weld section section at at 1250 1250 °CC heating temperature with with 0, 0, 5 5 and 10 min holding time, large large gap can been seen and big cracks at the joint ◦ interface can can be observed. With With the the increase increase of of welding welding temperature temperature to to 1275 1275 °CC with with 0 0 and 5 min holding time, time, a a series series of of cracks cracks resulted resulted and and the the weld weld seam seam could could not not be bewell well joined, joined, as asshown shown in Figurein Figure 6S4,S5.6S4,S5. In In addition, addition, big big fractures fractures across across the the entire entire joint joint were were shown with little and and local local interactions were were lactated lactated at at different different zones in the interface resulting in spontaneous fracture after joining.joining. However, However, when when the the holding time is adequate (10 (10 min) the the weldability weldability is is relatively good and there is no no macroscopic macroscopic voids voids and and fractures fractures across across the the entire entire joint joint which which is is occupied occupied by by eutectic eutectic liquid, liquid, ◦ as shown in Figure Figure 66S6.S6. Meanwhile,Meanwhile, byby furtherfurther increasingincreasing thethe weldingwelding temperaturetemperature toto 13001300 °C,C, strong strong thixojoint can be produced. Further, Further, fine ﬁne and and small small boundary boundary can can be be observed as transition zone between two alloys. No No presence presence of porosity and micro-cracking at this zone and the joint shape was smooth and not corrugated, as shown in Figure6 6S7–S9.S7–S9. TheThe thixoweldthixoweld jointjoint waswas characterisedcharacterised byby visible full penetration that indicate perfect thixojoning between base blank and the insert along the bounding border. Higher amount of of the interfacial reactions has been produced between two base metals [12]. [12]. The The structure structure is is distinguished distinguished by by two two features features that that can can be be determined determined by by grain grain direction, direction, as illustrated in arrows direction in Figure 77.. TheThe directiondirection ofof thethe grainsgrains areare inin bandsbands inin thethe formingforming direction and as a single pin of AISI D2 was inserted perpendicular into a base blank of AISI D2 to illustrateillustrate two two distinct distinct features. features. The The interfacial interfacial reactions reactions during during heating heating have have accelerated accelerated the the diffusion diffusion of elements through the interface and propagated the eutectic liquid across the joined surfaces that resulted in good thixojoint of the interface at both side of steels. Additionally, the properties of the thixojoining can be be improved improved by by subsequent subsequent heatheat- treatment. treatment. It is clearly visible that the changes in the microstructure patterns of the thixojoining occur after the heat treatment as shown in Figure 88.. ItIt isis obviouslyobviously toto seesee thatthat thethe primaryprimary distributiondistribution ofof graingrain boundary is is decreased decreased substantially substantially and and some some amounts amounts of of eutectic eutectic carbides carbides dissolve dissolve in inthe the matrix. matrix. It meansIt means that that the the primary primary grain grain boundaries boundaries have have distinctly distinctly pinning pinning effect effect on on growth growth of of grain grain size size in the heat treated condition [12]. [12]. The heat heat treated structures of the thixojoined parts only show improved contrast between solid and liquid phases, whereby the big solids appeared inin littlelittle liquidliquid matrix.matrix.

FigureFigure 5. OpticalOptical microstructure microstructure of as received AISI D2 tool steel.

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Figure 6. Microstructure of the thixojoined samples processed at different conditions: (S1) 1250 °C/0 Figure 6. Microstructure of the thixojoined samples processed at different conditions: Figuremin, (S2 6.) Microstructure1250 C/5 min, ( S3of )the 1250 thixojoined °C/10 min, samples (S4) 1275 processed °C/0 min, at (differentS5) 1275 conditions:°C/5 min, ( S6(S1) )1275 1250 °C/10 °C/0 (S1) 1250 ◦C/0 min, (S2) 1250 C/5 min, (S3) 1250 ◦C/10 min, (S4) 1275 ◦C/0 min, (S5) 1275 ◦C/5 min, min, (S7S2) 13001250 °C/0C/5 min, min, ( (S3S8)) 1250 1300 °C/10°C/5 min, min, ( (S9S4) )1300 1275 °C/10 °C/0 min,min. (S5) 1275 °C/5 min, (S6) 1275 °C/10 (S6) 1275 ◦C/10 min, (S7) 1300 ◦C/0 min, (S8) 1300 ◦C/5 min, (S9) 1300 ◦C/10 min. min, (S7) 1300 °C/0 min, (S8) 1300 °C/5 min, (S9) 1300 °C/10 min.

Figure 7. Microstructure of thixo-weld-joint of (a) base metal, (b) diffusion zone (c) insert, after Figure 7. Microstructure of thixo-weld-joint of (a) base metal, (b) diffusion zone (c) insert, after Figuresubjecting 7. Microstructureto 1300 °C heating of thixo-weld-jointtemperature for 10 of min. (a) base metal, (b) diffusion zone (c) insert, after ◦ subjecting to 1300 °CC heating temperature for 10 min.

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◦ Figure 8. Microstructure of thixojoint parts after application of 1300 °CC for 10 min and subjected to heat treatment. Figure 8. Microstructure of thixojoint parts after application of 1300 °C for 10 min and subjected to heat treatment. The samples were also examined using SEM in order order to get a a complete complete assessment assessment if if there there is is any any harmful or brittle phase have formed during thixojoining. Figure 9 shows the Scanning electron harmful orThe brittle samples phase were have also formed examined during using thixojoining. SEM in order Figureto get a9 complete shows the assessment Scanning if electronthere is any images images of the microstructures of the thixojoint parts and the EDS spectra corresponding to the of theharmful microstructures or brittle of phase the thixojoint have formed parts during and the thixojoining. EDS spectra Figure corresponding 9 shows the to Scanning the analyzed electron phases. analyzedWhile,images SEM-EDX phases. of the While, elemental microstructures SEM-EDX mapping of elemental the techniques thixojoint mapping was parts also techniques andperformed the EDS was spectra on also both performed corresponding sides at theon toboth periphery the sides atof the analyzed periphery joint in phases. order of the to While, determinejoint SEM-EDX in order the elemental compositionto determine mapping ofthe the techniquescomposition region near was of thealso the diffusionperformed region near zone on both the of thesidesdiffusion joined zonepart, atof as the shown peripheryjoined in Figurepart, of theas 10 shown joint. Regarding in inorder Figure to to determine Figures10. Regarding9 theand composition 10 to, Figures the EDX of9 resultsandthe region10, conﬁrmthe near EDX the that results diffusion the confirm contact thatarea the waszone contact identicalof the joinedarea in was part, microstructure, identical as shown in Figuremicrostructure, composition, 10. Regarding and composition, to properties Figures 9 and with 10,properties the the base EDX and resultswith insert. the confirm base On and the insert.basisthat of On element the the contact basis distribution, areaof element was identical it distribution, can be in deduced microstructure, it can that be the deducedcomposition, both sides that and at the the properties both periphery sides with at of thethe the baseperiphery joint and display of insert. On the basis of element distribution, it can be deduced that the both sides at the periphery of thea uniform joint display and homogeneous a uniform and distribution homogeneous in the distribution structure due in the thermal structure diffusion due thermal driving forcediffusion [12]. drivingthe force joint [12].display Along a uniform with thisand result,homogeneous it was observed distribution that in thethe intermetallicstructure due compoundsthermal diffusion were not Alongdriving with thisforce result, [12]. Along it was with observed this result, that it thewas intermetallicobserved that compoundsthe intermetallic were compounds not incorporated were not into incorporated into the joining region, providing further evidence of the reliability of this process. the joiningincorporated region, into providing the joining further region, evidence providing of further the reliability evidence ofof thisthe reliability process. of this process.

Figure 9. SEM micrographs of thixojoined part at 1300 °C with EDX of point (1) for base metal, (2) for Figure 9. SEM micrographs of thixojoined part at 1300 ◦C with EDX of point (1) for base metal, (2) for diffusion area and (3) for insert. Figurediffusion 9. SEM area andmicrographs (3) for insert. of thixojoined part at 1300 °C with EDX of point (1) for base metal, (2) for diffusion area and (3) for insert.

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FigureFigure 10. 10.SEM-EDX SEM-EDX elemental elemental mapping mapping of of thixojoined thixojoined part part at at 1300 1300◦ C°C of of results results near near joint: joint: ( a(a)) SEM SEM micrograph,micrograph, ( b(b)) Fe, Fe, ( c()c) Cr, Cr, (d ()d C,) C, (e ()e V,) V, (f )(f all) all four four elements. elements.

X-rayX-ray diffraction diffraction test test has has been been carried carried out out for for more more characterisation characterisation of of thixojoint thixojoint as as displayed displayed in in FigureFigure 11 11.. The The analysis analysis of of the the diffraction diffraction patterns patterns of of as-received as-received tool tool steel steel found found peaks peaks corresponding corresponding to the ferrite phase and iron-chromium carbide (M7C3). From the other side, the thixoweld at 1300◦ °C to the ferrite phase and iron-chromium carbide (M7C3). From the other side, the thixoweld at 1300 C is characterised by three peaks refer to austenite and ferrite phases in addition to M7C3 carbides. The is characterised by three peaks refer to austenite and ferrite phases in addition to M7C3 carbides. Thepresence presence of austenite of austenite is primarily is primarily connected connected with with the theincreasing increasing of carbon of carbon in the in thesolid solid solution solution that thatstabilizing stabilizing the the austenite austenite [13]. [13 Higher]. Higher contents contents of of these these elements elements have have a a significantsignificant effect effect on on the the martensitemartensite temperature temperature and and consequently consequently the transformation the transformation of austenite of austenite to martensite to martensite is very difficult. is very Indifficult. addition, In whenaddition, relatively when fastrelatively cooling fast from cooling the solidus–liquidus from the solidus–liquidus range is applied range thereis applied is relatively there is betterrelatively stability better for stability the austenitic for the phase austenitic at room phase temperature at room temperature [14]. [14]. Meanwhile,Meanwhile, analysisanalysis ofof the the XRD XRD patterns patterns of of the the joined joined parts parts after after post post welding welding heat heat treatment treatment conditioncondition revealed revealed a a change change of of the the structure structure character character over over the the non-heat non-heat treated treated condition condition as as shown shown inin Figure Figure 10 10.. AsAs cancan bebe observedobserved that,that, thethe microstructuresmicrostructures areare composedcomposed ofof different different amounts amounts of of retainedretained austenite, austenite, martensite martensite and and chromium chromium carbides. carbides. Retained Retained austenite austenite was was recognised recognised using using crystallographiccrystallographic data data for for austenite, austenite, while while martensite martensite was was recognised recognised with with data data for for ferrite ferrite because because of of similaritysimilarity inin the the crystal crystal structure structure and and slight slight differences differences in in lattice lattice constants. constants. The The decomposition decomposition of of metastablemetastable austeniteaustenite inin thethe structure structure was was probably probably caused caused by by the the changes changes of of carbon carbon content. content. TheThe effecteffect of of slow slow cooling cooling give give rise rise to to dissolution dissolution ofof a a large large number number of of carbide carbide particles particles causing causing inin anan augmentationaugmentation of of retained retained austenite austenite after after post post welding welding heat heat treatment treatment condition. condition. During During the the thermal thermal exposure,exposure, carbon carbon migrates migrates and and diffused diffused deeper deeper to the to grain the boundariesgrain boundaries while some while amounts some amounts of eutectic of chromiumeutectic chromium carbides dissolvecarbides in dissolve the matrix. in the It meansmatrix. that It means the primary that the grain primary boundaries grain boundaries have distinctly have pinningdistinctly effect pinning on growth effect ofon grain growth size of in grain the heat size treated in the conditionheat treated [15 condition]. Based on [15]. these Based findings, on these no harmfulfindings, or no brittle harmful phase or wasbrittle detected phase bywas XRD detected patterns by XRD at the patterns thixoweld at the zone thixoweld and these zone observations and these wereobservations in good agreementwere in good with agreement microstructure with microstructure examinations. examinations.

Metals 2018, 8, 316 10 of 16 Metals 2018, 8, x FOR PEER REVIEW 10 of 16

FigureFigure 11. 11. XRDXRD patterns patterns for for AISI AISI D2-AISI D2-AISI D2 D2 joining joining processed processed atat different different conditions. conditions.

3.2.3.2. Mechanical Mechanical Properties Properties

3.2.1.3.2.1. Tensile Tensile Test Test and and Fracture-surface Fracture-surface Investigation Investigation ForFor the the tensile tensile strength, strength, six six samples samples were were prepared prepared using using different different joining joining parameters parameters (the (the first first threethree specimens specimens with with 1250 1250 °C◦C heating temperaturetemperature waswas neglected). neglected). It It is is believed believed that that the the supplied supplied heat heatinput input not not providing providing sufficient sufficient diffusion diffusion within within the the joint joint interface interface even even when when the the holding holding time time was waslong. long. Typical Typical stress-strain stress-strain curves curves for for the the different different test test conditions conditions are are shown shown in Figure in Figure 12. With12. With high highthixowelding thixowelding temperature, temperature, the producedthe produced interface interface is free is from free voidsfrom andvoids pores and duepores to highdue diffusionto high diffusionrates of different rates of different elements elements across this across area. this Consequently, area. Consequently, when this when high temperature this high temperature accompany accompanywith long holdingwith long time, holding high tensiletime, high strength tensile could strength be achieved. could be It achieved. is well known It is well that known the strength that the of a strengthjoint primarily of a joint depends primarily on thedepends quality on of the its quality microstructure. of its microstructure. As can be observed, As can thebe tensileobserved, strength the tensilegraph strength shows a graph relatively shows identical a relatively shape of identical the applied shape heating of the temperature applied heating to 1300 temperature◦C at 0, 5 and to 130010 min °C at to 0, give 5 and 241 10 MPa, min 253 to MPagive 241 and MPa, 271 MPa 253 respectively.MPa and 271 A MPa minimum respectively. tensile strengthA minimum of 241 tensile MPa is strengthrecorded of for241 the MPa applied is recorded heating for temperature the applied of heating 1300 ◦C temperature at 0 min. At 5of min 1300 a slight°C at increment0 min. At 5 in min tensile a slightstrength increment was recoded. in tensile Meanwhile, strength was with recoded. 10 min. holdingMeanwhile, time with there 10 was min. remarkable holding time improvement there was in remarkabletensile strength improvement due to fact in tensile that diffusion strength rate due is to highly fact that affected diffusion by both rate of is temperaturehighly affected and by time both for ofnon-steady temperature state and diffusion. time for The non-steady tensile value state of the diffusion. D2 as-supplied The tensile material value (791 of MPa) the D2 is more as-supplied than two materialtimes if (791 compared MPa) is with more the than as-thixojoind two times valueif compared of 271 MPa. with Thisthe as-thixojoind is actually an value interesting of 271 resultMPa. whichThis iswas actually achieved an interesting by a process result which which is simpler was achieved than conventional by a process processes. which is simpler Meanwhile, than tensileconventional strength processes.of the joined Meanwhile, parts after tensile post weldingstrength heatof the treatment joined parts condition after post showed welding a significant heat treatment improvement condition over showedthe non-heat a significant treated improvement condition which over is the 560 non-heat MPa, i.e., treated about condition 70% the strength which is value 560 MPa, of D2 i.e., base about metal. 70%This the improvement strength value of strengthof D2 base is linkedmetal. toThis such improvement factors as the of size,strength distribution is linked and to such dissolvement factors as of thecarbides, size, distribution relative proportions and dissolvement of austenite of carbides, and martensite relative phases,proportions and grain of austenite size. This and explanation martensite is phases,also corroborated and grain size. by microscopic This explanation studies is which also corroborated resulted in improved by microscopic contrast studies between which solid andresulted liquid inphases, improved whereby contrast the between big solids solid appeared and liquid in littlephases, liquid whereby matrix the where big solids primary appeared distribution in little ofliquid grain matrixboundary where is decreasedprimary distribution substantially of andgrain some boundary amounts is decreased of eutectic substantially carbides dissolve and some in the amounts matrix. of eutecticThe carbides mechanical dissolve properties in the matrix. of similar substrates will have an important effect on the joints propertiesThe mechanical because the properties temperature of similar attained substrates by each substrate will have markedly an important depends effect on on the the properties joints properties because the temperature attained by each substrate markedly depends on the properties of the materials and on the selected joining parameters. According to the Arrhenius equation, an

Metals 2018, 8, 316 11 of 16 ofMetals the 2018 materials, 8, x FOR and PEER on REVIEW the selected joining parameters. According to the Arrhenius equation, 11 of an 16 increase in the temperature will occasion an exponential enhancement in the diffusivity of solute [16]. Consequently,increase in the temperature the heatingtemperature will occasion for an theexponential material enhancement will have a substantial in the diffusivity impact of on solute thejoint [16]. properties.Consequently, Weak the joint heating may betemperature produced iffor heating the material temperature will have is set a on substantial low level, impact even though on the if joint it is combinedproperties. with Weak high joint holding may be time. produced This resulted if heating in inadequatetemperature diffusion is set on alonglow level, the interface even though because if it theis combined elements with do not high have holding enough time. energy This toresulted migrate in and inadequate diffuse withindiffusion diffusion along the transition interface zone. because the elements do not have enough energy to migrate and diffuse within diffusion transition zone. D = D* exp (−Qd /RT) (1) D = D* exp (−Qd / RT) (1) 2 wherewhere DD isis the the diffusivity, diffusivity, D* D*is temperature-independentis temperature-independent preexponential preexponential (m /s), (mQd2 /s),is theQd activationis the activationenergy for diffusion energy for (J/mol diffusion or eV/atom), (J/mol R oris the eV/atom), gas constant Ris (8.31 the J/(mol·K) gas constant or 8.62×10−5 (8.31 J/(mol eV/(atom·K))·K) or 8.62and× T10 is− absolute5 eV/(atom temperature·K)) and T (K).is absolute temperature (K). ItIt isis worth mentioning mentioning that that the the joining joining strength strength increases increases with with the the increase increase of ofholding holding time time at the at theinitial initial stage stage of the of joining the joining process process due to due the better to the coalescence better coalescence of mating of surfaces mating surfacesto ensures to complete ensures completediffusion diffusion of atoms of and atoms attains and a attains maximum a maximum strength strength value [17].value In [17 general,]. In general, in non-steady in non-steady state statediffusion, diffusion, the concentration the concentration of diffusion of diffusion species species is function is function of both of of both time of and time position and position (Fick’s (Fick’ssecond secondlaw). It law). is thus It is expected thus expected with withthe aid the aidof more of more suitable suitable holding holding time, time, joints joints with with highly highly desirabledesirable microstructuremicrostructure can be obtained obtained and and in in turn, turn, highly highly reliable reliable bonded bonded joints joints with with good good performance performance can canbe anticipated. be anticipated. Moreover, Moreover, holding holding time time has an has effect an effect on the on quantity the quantity of atomic of atomic diffusion diffusion and the and creep the creepof the of protrusions the protrusions (i.e., (i.e.,elemental elemental diffusion diffusion of the of the joined joined alloy alloy can can be be improved improved by by increasing thethe holdingholding time)time) [[17].17]. 2 2 ∂C∂C/∂/t∂t= =D D( ∂(∂2CC//∂x∂x2)) (2) (2) wherewhere ∂∂CC//∂t∂t is the change in solute concentration with time at a given position in thethe basebase metal,metal, whichwhich couldcould provideprovide anan indicationindication ofof isothermalisothermal solidiﬁcationsolidification rate,rate, andand DD isis thethe diffusion coefﬁcient,coefficient, andand ∂∂22C/C/∂x∂x2 isis the the change change in in concentration gradient with distance.

Figure 12. Typical stress-strain curves for the different test conditions. Figure 12. Typical stress-strain curves for the different test conditions.

Metals 2018, 8, 316 12 of 16

Figure 13 displays the SEM morphology of the thixojoint that achieved with various joining conditions. Through observing the fracture surfaces created by the tensile test, it was found that the fracture appearance of the welded joint at 1300 ◦C was not completely along the interface between base Metals 2018, 8, x FOR PEER REVIEW 12 of 16 metal and insert. The fractures were characterised by brittleness, where intergranular fracture mode andis illustrative. grain boundary. The fracture The close-up path, which of the is fracture typically path dimpled, reveals isa located“cobbled” around surface liquid-solid appearance, zone which and couldgrain boundary.indicate that The during close-up solidification of the fracture the fracture path reveals occurred a “cobbled” through surface the liquid appearance, with the re-solidified which could grainsindicate then that becoming during solidiﬁcation exposed as the “cobbles”, fracture occurred a phenomenon through thealso liquid seen with in 7075 the re-solidiﬁedAl alloy [18]. grains The brittlenessthen becoming was exposeda result of as coarse “cobbles”, fracture a phenomenon with big solid also lump seen that in 7075 refer Al to alloy unbroken [18]. Thestructure. brittleness The wassolid aγ result (Fe) particles, of coarse mainly fracture with with globular big solid shape, lump surrounded that refer toby unbrokenthe carbide structure. networks The which solid is γa (Fe)typical particles, characteristic mainly with with respect globular to shape, semi-solid surrounded microstructure by the carbide of D2 tool networks steel. whichThe fracture is a typical then appearscharacteristic to have with quickly respect propagated to semi-solid along microstructure the grain of boundaries D2 tool steel. and The the fracture weak liquidus–solidus then appears to interfacehave quickly of the propagated lumps [19]. along From the the grain results boundaries it is believed and that the there weak is liquidus–solidus a good metallurgical interface joining of the at interface;lumps [19 ]. this From kind the of results compound it is believed fracture that mechanism there is a good means metallurgical that the rupture joining is at not interface; along this the interface.kind of compound fracture mechanism means that the rupture is not along the interface.

Figure 13. 13. FracturedFractured surface surface of welded of welded joints jointsof samples of samples processed processed at various at conditions: various conditions: (S7) 1300 (°C/0S7) 1300min, ◦(C/0S8) 1300 min, °C/5 (S8) min, 1300 (◦S9C/5) 1300 min, °C/10 (S9) 1300min. ◦C/10 min.

Figure 14 shows fractured surface of welded joints obtained after post welding heat treatment. Figure 14 shows fractured surface of welded joints obtained after post welding heat treatment. As can be understood from this figure, post welding heat treatment changes the fracture mode to As can be understood from this ﬁgure, post welding heat treatment changes the fracture mode to predominantly transgranular fracture, with few intergranular facets. On the contrary, in the tensile predominantly transgranular fracture, with few intergranular facets. On the contrary, in the tensile test of the non treated condition, disbanding of the solid particles has occurred as well as several test of the non treated condition, disbanding of the solid particles has occurred as well as several microcracks with nano wide which are found out in fracture surface. Stress applied on the sample microcracks with nano wide which are found out in fracture surface. Stress applied on the sample during the tensile tests led to the formation of big cracks out of small pores and in turn, fractures during the tensile tests led to the formation of big cracks out of small pores and in turn, fractures occurred when these cracks grew. It is worth mentioning that the main parameter responsible for the lowoccurred ductility when is thesethe grain cracks boundary. grew. ItThe is worthliquid mentioningphase appearance that the in maingrain parameterboundaries, responsible results in for the low ductility is the grain boundary. The liquid phase appearance in grain boundaries, formation of porosities after solidification, which reduces ductility. It is thus expected with the presenceresults in of formation grain boundary of porosities leads after to long solidiﬁcation, soft zones which which reduces distort discriminatory ductility. It is duringthus expected plastic deformation [18,20]. It is attractive to note that post-joining heat treatment is one of the key parameters for improving the mechanical properties of thixojoined parts (although this would have cost-implications and could reduce the attractiveness of the process).

Metals 2018, 8, 316 13 of 16 with the presence of grain boundary leads to long soft zones which distort discriminatory during plastic deformation [18,20]. It is attractive to note that post-joining heat treatment is one of the key parameters for improving the mechanical properties of thixojoined parts (although this would have Metalscost-implications 2018, 8, x FOR PEER and couldREVIEW reduce the attractiveness of the process). 13 of 16

Figure 14. Fractured surface of welded joints after post welding heat treatment. Figure 14. Fractured surface of welded joints after post welding heat treatment.

3.2.2. Hardness Measurements 3.2.2. Hardness Measurements Micro-hardness profiles were obtained for the AISI D2 tool steel base metal, the weld zone and Micro-hardness profiles were obtained for the AISI D2 tool steel base metal, the weld zone and the insert by using a load of 10 g for 30 s. Figure 15 presents the micro-hardness profile against the insert by using a load of 10 g for 30 s. Figure 15 presents the micro-hardness profile against distance from the interface and illustrates the variations of the HV value along the lines perpendicular distance from the interface and illustrates the variations of the HV value along the lines perpendicular to the joint interface. The hardness measurement of the base metal, the weld zone and the insert is to the joint interface. The hardness measurement of the base metal, the weld zone and the insert is fluctuated in the range 350 Hv to 370 Hv. This indicated that the hardness value of the welded joints fluctuated in the range 350 Hv to 370 Hv. This indicated that the hardness value of the welded joints was almost similar to those of base metal and insert. This indicated that the hardness profile gives a was almost similar to those of base metal and insert. This indicated that the hardness profile gives a good impression of joint microstructure and the degree of homogenization, this looks very good impression of joint microstructure and the degree of homogenization, this looks very promising. promising. However, the thixoweld-joint of AISI D2 exhibits different hardness values if compare to However, the thixoweld-joint of AISI D2 exhibits different hardness values if compare to as supplied as supplied material. This exposure to thixojoining temperature also results in the phase change to material. This exposure to thixojoining temperature also results in the phase change to metastable metastable austenite. While the remaining interspaces are filled by precipitated eutectic carbides on austenite. While the remaining interspaces are filled by precipitated eutectic carbides on the grain the grain boundaries and in the lamellar network. As can be seen from this figure, almost the similar boundaries and in the lamellar network. As can be seen from this figure, almost the similar trend is trend is observed in the microhardness profiles of all samples. Figure 15 clarifies small decrement in observed in the microhardness profiles of all samples. Figure 15 clarifies small decrement in hardness hardness values at same temperature with high holding time. The behaviour may be attributing to values at same temperature with high holding time. The behaviour may be attributing to grain growth grain growth with increasing holding time during evolution of microstructures. [21,22]. These results with increasing holding time during evolution of microstructures. [21,22]. These results evidenced that evidenced that diffusion has occurred and weld zone micro-hardness was free from harmful or brittle diffusion has occurred and weld zone micro-hardness was free from harmful or brittle phases. In the phases. In the case of post welding heat treatment condition (S10), hardness measurements were case of post welding heat treatment condition (S10), hardness measurements were somewhat higher somewhat higher than for the non-heat treated samples. This higher hardness can be attributed to the than for the non-heat treated samples. This higher hardness can be attributed to the some amounts of some amounts of eutectic chromium carbides dissolve in matrix during slow cooling. It means that eutectic chromium carbides dissolve in matrix during slow cooling. It means that the primary grain the primary grain boundaries have distinctly pinning effect on growth of grain size in the heat treated boundaries have distinctly pinning effect on growth of grain size in the heat treated condition [23]. In condition [23]. In addition, the phase transformation is probably the reason of hardness increase to addition, the phase transformation is probably the reason of hardness increase to the (490 Hv). This the (490 Hv). This value is higher in comparison with material hardness in the initial state after value is higher in comparison with material hardness in the initial state after thixojoining processing. thixojoining processing. During the isothermal heat treatment at higher temperatures, particular During the isothermal heat treatment at higher temperatures, particular decomposition of the austenite decomposition of the austenite and ferrite phases to the martensitic phases was achieved and the and ferrite phases to the martensitic phases was achieved and the carbides of chromium and retained carbides of chromium and retained austenite were included in the structure, as shown in XRD austenite were included in the structure, as shown in XRD analysis. In tool steels with high alloy analysis. In tool steels with high alloy content, high hardenability may cause martensite to form content, high hardenability may cause martensite to form during air cooling. Hardenability is the topic during air cooling. Hardenability is the topic which relates alloying to the phase transformations that which relates alloying to the phase transformations that occur on cooling [24]. occur on cooling [24].

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600

500

400

300

200 S7 S8 Micro-hardness HV0.2 100 S9 S10 0 -4 -3 -2 -1 0 1 2 3 4 Distance from joined interface (mm) FigureFigure 15. HardnessHardness distributions distributions ofof D2–D2 D2–D2 tool tool steel steel thixowelded thixowelded joints joints at various at various conditions: conditions: (S7) (1300S7) 1300 °C/0◦ C/0min, min,(S8) (1300S8) 1300 °C/5◦ C/5min, min,(S9) (1300S9) 1300 °C/10◦C/10 min, min,(S10) ( S101300) 1300°C/10◦C/10 min + min post + postwelding welding heat heattreatment. treatment.

4.4. ConclusionsConclusions TheThe novelnovel thixojoiningthixojoining processprocess ofof AISIAISI D2D2 similarsimilar tooltool steelsteel waswas carriedcarried outout byby usingusing thethe directdirect partialpartial re-meltingre-melting technique,technique, andand thisthis waswas investigatedinvestigated throughthrough thisthis research.research. TheThe effecteffect ofof joiningjoining temperature,temperature, holding holding time time and and post-weld post-weld heat heat treatment treatment on on microstructural microstructural characteristics characteristics and and mechanicalmechanical propertiesproperties of of AISI AISI D2 D2 tool tool steel steel joints joints have have been been studied. studied. The The summarised summarised conclusions conclusions can becan listed be listed as below: as below:

1.1. AdequateAdequate and defectless defectless thixoweld-joint thixoweld-joint were were successfully successfully produced produced with with thixowelding thixowelding temperaturetemperature of 1300 1300 °C,◦C, while while lower lower welding welding temperature temperature caused caused a series a series of cracks of cracks across across the theentire entire joint joint resulting resulting in in spontaneous spontaneous fracture fracture after after joining. joining. While While post post weld weld heat treatment, resultedresulted inin improvedimproved contrastcontrast betweenbetween solidsolid andand liquidliquid phases,phases, wherebywhereby thethe bigbig solidssolids appearedappeared inin littlelittle liquidliquid matrixmatrix wherewhere primaryprimary distributiondistribution ofof graingrain boundaryboundary isis decreaseddecreased substantiallysubstantially andand somesome amountsamounts ofof eutecticeutectic carbidescarbides dissolvedissolve inin thethe matrix.matrix. 2.2. AccordingAccording toto thethe X-rayX-ray line scan analyses across the joint interface conﬁrmsconfirms aa uniform elemental distributiondistribution inin thethe structure whilewhile thethe X-rayX-ray diffraction patternpattern ofof thethe thixoweldedthixowelded areaarea waswas freefree fromfrom anyany brittlebrittle oror harmfulharmful phases.phases. 3.3. MaximumMaximum tensiletensile streghthstreghth ofof 271271 MPaMPa waswas achievedachieved atat 13001300 ◦°CC andand 1010 minmin holdingholding time,time, whichwhich waswas 35% 35% that that of ofD2 D2 base base metal. metal. Meanwhile, Meanwhile, tensile tensile strength strength of the joined of the parts joined after parts heat after condition heat showedcondition a signiﬁcantshowed a significant improvement improvement over the non-heat over the treatednon-heat condition treated condition which was which 560 MPa, was i.e.,560 aboutMPa, i.e., 70% about that of 70% as receivedthat of as AISI received D2 tool AISI steel D2 basetool metal.steel base metal.

4.4. TheThe fracture ofof jointsjoints waswas locatedlocated alongalong thethe interfaceinterface betweenbetween basebase metalmetal andand insertinsert withwith typicaltypical brittlebrittle characteristicscharacteristics (intergranular).(intergranular). Meanwhile, thethe joints afterafter heatheat conditioncondition areare fracturedfractured atat thethe diffusiondiffusion zone,zone, andand thethe fracturefracture exhibitsexhibits typicaltypical brittlebrittle characteristiccharacteristic (tranasgranular).(tranasgranular). 5. The hardness measurement of the base metal, the weld zone and the insert is fluctuated in the 5. The hardness measurement of the base metal, the weld zone and the insert is ﬂuctuated in the range of 350 Hv to 370 Hv. This indicated that the hardness value of the welded joints was range of 350 Hv to 370 Hv. This indicated that the hardness value of the welded joints was almost almost similar to those of base metal and insert. While the hardness measurements of the similar to those of base metal and insert. While the hardness measurements of the (thixojoining (thixojoining + heat treated) condition (490 Hv) was somewhat higher than for the non-heat + heat treated) condition (490 Hv) was somewhat higher than for the non-heat treated samples. treated samples. This is more related to the phase transformation (austenite and ferrite phases This is more related to the phase transformation (austenite and ferrite phases to the martensitic to the martensitic phases). Moreover, some amounts of eutectic chromium carbides dissolve in phases). Moreover, some amounts of eutectic chromium carbides dissolve in the matrix. the matrix.

Acknowledgments: The authors would like to thank Universiti Kebangsaan Malaysia (UKM) and the ministry of Higher Education (MoHE), Malaysia, for the ﬁnancial support under Research Grants AP-2012-014.

Metals 2018, 8, 316 15 of 16

Author Contributions: M.N.A. and M.Z.O. conceived and designed the experiments; M.N.A. performed the experiments; M.N.A., M.Z.O., S.A.-Z., K.S.A., M.A.A. analyzed the data; M.N.A. wrote the paper. All authors discussed the result and contributed to the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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