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Friction welding of electron beam melted Ti-6Al-4V
Materials Science & Engineering A 761 (2019) 138045
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Friction welding of electron beam melted Ti-6Al-4V a,b
c
d
P.T. Qin , R. Damodaram , T. Maity , W.W. Zhang K.G. Prashanthe,f,**
a,b
a,b
, C. Yang
T a,b,*
, Z. Wang
,
a
Guangdong Key Laboratory for Processing and Forming of Advanced Metallic Materials, South China University of Technology, Guangzhou, 510640, China National Engineering Research Center of Near-net-shape Forming for Metallic Materials, South China University of Technology, Guangzhou, 510640, China SSN College of Engineering, Chennai, India d Materials Science and Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ, 85287-6106, United States e Department of Mechanical and Industrial Engineering, Tallinn University of Technology, Ehitajete tee 5, 19086, Tallinn, Estonia f Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700, Leoben, Austria b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Ti-6Al-4V alloy Electron beam melting Friction welding Tensile property Hardness
Ti-6Al-4V samples produced by electron beam melting (EBM) are welded using solid-state friction welding (FW) process. The microstructure of the weld sample shows the presence of fine equiaxed α grains with irregular β phase. Microstructural investigations reveal a pronounced change in the shape and size of the α phase in the weld metal as-compared to the base material along with the disappearance of columnar prior β grains. Such variations in the microstructure significantly change the mechanical properties of the FW material. The hardness in the weld zone increases and a decrease of hardness is observed along the heat affected zone (HAZ) with respect to the base metal as expected. Similarly, the room temperature tensile tests show an improvement of ductility in the welded EBM samples. However, the yield and the ultimate strength show a marginal drop in the welded samples compared to the as-prepared EBM specimens. The present work demonstrates that solid-state FW process not only permits successful joining of additively manufactured materials, but also helps in improving their ductility.
1. Introduction Electron beam melting (EBM) has attracted recent attention in the field of additive manufacturing (AM) and is being developed for wide variety of materials [1,2]. As the name suggests the EBM process uses an electron beam as heat source within a vacuum chamber, where electromagnetic coils providing extremely fast and accurate beam control that allows selective melting of the powder bed [3]. Since the EBM process heats the entire powder bed (400–800 °C) before melting, the alloy powder may be sintered partially. Since the powder bed is held hot throughout the process, the solidification of the samples takes place under near-equilibrium conditions. Hence, relatively low amount of internal stresses is present in the EBM sample compared to the samples produced by selective laser melting (SLM) [1]. The mechanical properties of the EBM processed samples are generally better than the cast and comparable to wrought material [4]. Since, the EBM process takes place under a vacuum atmosphere (unlike an inert atmosphere observed in the SLM process), oxidation of the parts is generally averted
[2,5–7]. The EBM process involves several process parameters, including: beam power, beam scanning speed, beam focus, beam diameter, beam line spacing, plate temperature, pre-heat temperature (including the repetitions, speed, and power of the beam), contour strategies, scan strategy and so on [8]. Therefore, the optimization of the process parameters is even more difficult than the selective laser melting (SLM) process and hence only limited materials are employed in EBM such as CP-Ti, Ti6Al4V, Inconel 718 and CoCrMo alloys [1] of which Ti6Al4V is extensively studied. Typically, planar growth is observed in Ti6Al4V fabricated by AM. In the EBM samples, columnar β grains formed firstly during cooling and subsequently were transformed to α + β with a lamellar or Widmanstatten microstructure, while α phase is formed along the prior columnar β-boundaries [3,9–11]. The yield strength of Ti6Al4V fabricated by EBM is found approximately 833–1116 MPa, with ductility ranging between 2.7 and 15% [12]. However, a major drawback for the additive manufactured parts is their size, in particular for materials produced by the EBM process,
*
Corresponding author. Guangdong Key Laboratory for Processing and Forming of Advanced Metallic Materials, South China University of Technology, Guangzhou 510640, China. ** Corresponding author. Department of Mechanical and Industrial Engineering, Tallinn University of Technology, Ehitajete tee 5, 19086, Tallinn, Estonia. E-mail addresses: [email protected] (Z. Wang), [email protected] (K.G. Prashanth). https://doi.org/10.1016/j.msea.2019.138045 Received 3 April 2019; Received in revised form 14 June 2019; Accepted 16 June 2019 Available online 18 June 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 761 (2019) 138045
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which depends greatly on the size of the chamber [1,12]. Increasing the size of the chamber will increase the requirements for auxiliary equipment such as cooling system, gas and powder feeder unit, etc. Moreover, large sized sample requires larger volume of powders, which makes it difficult in controlling the powder quality (including composition, shape and size). Additionally, the anisotropic microstructure in the AM samples will be more pronounced in increasing the size of these AM sample [12,13] and hence leads to other problems like anisotropic properties [14–16]. Hence, there is a need to look for alternative secondary processes like additional/conventional metal joining techniques that can join smaller EBM components at a later stage. Even though there are several metal joining processes available, only some of them may be suitable for joining EBM fabricated parts. This is because the EBM fabricated parts have a relatively unique microstructure, which is not possible to be produced using conventional manufacturing processes [17–19]. Hence, if any of the fusion welding processes are employed for joining, the weld zone will be completely re-melted and solidified again at slower cooling rates than the ones realized upon EBM processing. This leads to a relatively coarser microstructure and the advantages of the unique microstructures obtained using the EBM process will be lost and so the mechanical properties of the parts are expected to deteriorate. Moreover, EBM processed materials may contain high internal stresses (depending on the pre-heating temperature of the powder bed and the employed process parameters), especially at low bed pre-heating temperatures and high-energy input conditions. Hence it cannot accommodate additional stresses imparted during the conventional fusion welding processes [3,20,21]. Because of these reasons, the solid-state welding processes the friction welding (FW) or friction stir welding (FSW) processes may be preferred. To the best of our knowledge, only the welding of SLM processed parts were studied [22,23]. However, there is no work reporting the welding of the EBM processed parts. Since the microstructure and properties of the parts produced by SLM and EBM are completely different, because of the difference in the processing conditions, the welding behavior of these materials may also be completely different. Accordingly, the present work deals with the joining of Ti-6Al-4V parts fabricated by EBM using the FW process.
was measured according to the Oliver-Pharr method [24,25]. The contact stiffness S is then related to the reduced modulus, Er, by the following relation [24]:
Er =
π S 2β A
(1)
where, β is the constant related to the geometry of the Berkovich indenter, A is the projected contact area of indentation. The reduced elastic modulus can be related to the Young's modulus E as:
1 1 − ϑ2 1 − ϑ2 = + Er E Ei
(2)
where E and v are the Young's modulus and Poisson's ratio, respectively, and the subscript i refers to the indenters. In this study Ei was taken as 1141 GPa and vi as 0.07. The total strain rate recovery, ηr (ratio of the displacement after complete unloading hf to the maximum depth of penetration hmax) is an indicator of the relative portion of the plastic deformation in the total elasto - plastic deformation that occurs during indentation [25]. The hmax and the hf values were obtained from the nanoindentation P - h datum. The ηr can be estimated from the following equation:
ηr = (hmax − hmin)/ hmax
(3)
Tensile tests were performed using an Instron 8801 facility under quasistatic loading conditions at room temperature with a strain rate of (~1 × 10−3 s−1). Four parallel specimens were tested for an average. The tensile test samples (according to) were cylindrical tensile bars with 52 mm total length that were machined from the welded samples in such a way that the weldment lies along the center of the tensile bar. The dimensions along the gauge length of the tensile bar were: length ~17.5 mm and diameter ~3.5 mm. The fracture surfaces of the samples were investigated using SEM to analyze the causes for the failure. 3. Results and discussion 3.1. Structural analysis The XRD pattern (Fig. 1) shows the presence of hexagonally closed packed (hcp) α peaks in both FW and as-prepared Ti-6Al-4V samples, which is typical for EBM processed Ti6Al4V samples, similar to other published reports [26,27]. The XRD pattern of the EBM sample shows a 1 0) direction. However, the texcrystallographic texture along the (10‾ ture is partially reversed or the intensity of texture is significantly decreased for the FW-EBM sample and in the weld zone. The XRD pattern shows several differences, when compared with the SLM sample, where the texture is observed along the (0002) direction [23]. The peaks are
2. Experimental details Arcam A2X device was used to fabricate the sample using the EBM process. Conventional EBM process parameters including: vacuum 10−4–10−5 (mbar), accelerating voltage: 60 kV, layer thickness ~50 μm, scan speed ~0.50 m/sec and the process chamber was maintained at 903 K. A core shell scan strategy was used with the melting sequence varying between 0° and 90° between layers. FW was carried out using a continuous drive friction-welding machine with 200 kN capacity. The parameters used in the present study are: friction pressure 75 MPa, upset pressure 100 MPa, burn off length 3 mm and spindle speed 1000 rpm. Optical microscopy (OM, DMI5000), scanning electron microscope (SEM, NOVA NANSEM 430) and X-ray diffraction (XRD, Rigaku SmartLab SE) were used to examine the microstructure and structural characterization of the samples. Microhardness measurements were carried out with a load of 100 N applied for 10 s and a test spacing between adjacent indentations of 0.2 mm, using a computer-controlled SCTMC Vickers hardness tester (Shanghai Shangcai, Shanghai, China). They were tested following a matrix of 6 × 30 which cover the base metal, heat affected zone (HAZ) and weld zone. Nanoindentation investigations were performed by a TriboIndenter TI-950 (from the firm of Hysitron, United States) using fixed load control mode (with a maximum load Pmax of 2000 μN and depth hmax of 3 μm) at room temperature (RT). The contact area between the Berkovich indenter and the specimens was calibrated using a fusedquartz standard, and the Young's modulus E and nanohardness Hnano
Fig. 1. XRD patterns of the Ti-6Al-4V samples in the as-prepared EBM and friction welded condition. (The red vertical lines showing the standard peaks of the hexagonal α-Ti, Reference code of the PDF is 00-001-1198 [28]). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 2
Materials Science & Engineering A 761 (2019) 138045
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Fig. 2. (a-c) SEM micrographs of friction welded Ti-6Al-4V (d-h) OM micrographs of friction welded Ti-6Al-4V. The yellow solid circles indicate the intersections of the long columnar grain's boundary and the red line. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
along with the α phase or present at the boundaries. The microstructure of the heat affected zone has a thickness of 0.5–1 mm and shows that the columnar prior β grains (having a width of 140 ± 19 μm) are much coarser than both the base metal and welding zone due to the heat dissipation effect.
relatively less broad compared to the samples produced by SLM. This suggests that the Ti-6Al-4V samples produced by the EBM process have relatively less internal stresses than their SLM counterparts do. The relatively less internal stress in the EBM process may be due to the slow cooling rate observed because of the pre-heating of the powder bed in the EBM process. In addition, the relatively slow cooling rates observed in the EBM process as compared to the SLM process may leave to relative coarsening of the microstructure. In addition, β-phase is observed additionally to the α-hcp phase in the EBM processed Ti-6Al-4V sample. However, the intensity of β-phase peak decreases after welding (in the weld zone) which may be due to transition of the β-phase to α-phase as a result of excessive plastic deformation and dissipation of heat generated in the weld zone to attain thermal equilibrium during the FW process. The SEM and OM images of the FW Ti-6Al-4V samples are shown in Fig. 2. The OM images clearly indicate the presence of three different zones: base metal, welding zone and the heat-affected zone. The base metal shows the presence of long columnar prior β grains along with α + β phases. The bright colored lines indicate the single α phase, which is seen inside the columnar prior β grains. The dark colored areas correspond to the β phase. The columnar grains in the base metal is about 100 ± 15 μm. In welding zone, columnar prior β grains were broken into more equiaxed grains with irregular β phase distributed
3.2. Mechanical properties Fig. 3 shows the Vickers hardness, nanoindentation and modulusstrain rate recovery plot for the Ti-6Al-4V FW samples. The hardness profile along the weld interface measured using both micro- and nanoindentation can be observed in (Fig. 3(a)). Both these tests show similar hardness profile, where a nearly symmetrical behavior with respect to the weld interface is observed in the FW Ti-6Al-4V EBM samples. The micro hardness of welding zone is 383 ± 28 HV, which is significantly higher than the base metal tested using the same parameters (350 ± 14 HV), indicating that the hardening effect in the weld zone due to plastic deformation process. In addition, the increase of hardness along the weld zone may be attributed to the formation of a recrystallized α phase microstructure and its morphology, compared to the initial as-prepared EBM Ti-6Al-4V material. However, the hardness of the HAZ is even lower than the base metal, which is about 328 HV. The thickness of the HAZ from the hardness profile is about 0.5–1 mm, 3
Materials Science & Engineering A 761 (2019) 138045
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Fig. 3. (a) Vickers hardness and nanohardness profile measured across the weld interface of Ti-6Al-4V EBM samples (b) typical load-depth (P - h) curves of nanoindentation for weld zone and base alloy regions (c) modulus and strain rate recovery profiles for the Ti-6Al-4V weldment.
Firstly, the hardness of both the base metal and the weld zone of the FW SLM sample is much higher than the FW EBM sample. The higher hardness observed in the FW joints of the SLM materials compared to the EBM materials can be corroborated directly to their cooling rates. The high cooling rate observed in the SLM process leads to a very fine microstructure with the presence of martensitic α′ phase. On the other hand, the EBM samples with near-equilibrium solidification will lead to a typical α+β microstructure, which is relatively softer than α’ microstructure. During the EBM process, the powder bed is pre-heated to ~903 K, which significantly reduces the cooling rate, which is responsible for such differences in the microstructure and their mechanical properties. Characteristic tensile test curves of both as-prepared EBM and FW-EBM Ti-6Al-4V samples at room temperature are shown in Fig. 4. For comparison purposes, the SLM processed and FW-SLM Ti6Al-4V were included [23]. The as-prepared EBM samples show a yield strength of 970 ± 15 MPa and an ultimate strength of 1046 ± 13 MPa with 10 ± 1% ductility. The FW Ti-6Al-4V EBM samples exhibit a marginal deterioration in strength, with a yield strength marginally dropping (111 MPa) to 859 ± 10 MPa and similar ultimate strength of around 1034 ± 9 MPa. However, the ductility of the FW Ti-6Al-4V EBM samples reaches 13 ± 1%, which is ~3% higher than for the asprepared EBM sample. The strength effects observed in for the FW-EBM Ti-6Al-4V samples are similar to what was found for FW-SLM Ti-6Al-4V samples and FW-SLM Al-12Si samples. Both the yield and ultimate tensile strength of FW samples show a marginal decrease but also becomes more ductile, because of the microstructural transformation during the FW process. The hard martensitic phase observed after the SLM process transforms to α-phase and hence the materials become softer. In addition, the coarsening of the microstructure in HAZ plays a major role in improving the ductility and slight decrease of the strength of these samples. The fracture surfaces of the as-prepared EBM and FW-EBM samples
which is in accordance with the HAZ thickness measured from the OM images. The hardness measured from by nanoindentation also shows a similar trend with Vickers hardness, where the welded zone (5.65 ± 2.40 GPa) shows an improved hardness than the base metal (3.64 ± 1.47 GPa) and the HAZ shows a decreased hardness of lower than 2 GPa. The typical load vs. depth (P - h) curves extracted from the nano indentation test are shown in Fig. 3(b). The P – h curves for two different structures show different two different slopes both in the loading and unloading segments. Moreover, it can be observed from the P – h curves that the maximum indentation depth hmax of the base material is slightly greater than that of the weld zone, representing a small increase in the resistance to plastic deformation by weld zone (increase of hardness in the weld zone). Both the reduced modulus Er and strain rate recovery (ηr ) variations have similar profile between the base metal and weld zone (Fig. 3(c)). Generally higher ηr indicates better superplasticity/strain hardening, or lower the elasticity; therefore, ηr indeed exhibits improved strain hardening behavior [25]. In the present sample, the ηr increases gradually from base metal towards HAZ and then towards the center of the weld zone (ηr = 0.30). The maximum value of ηr is obtained at the center of the weld zone (positions 0 and/or -1). This suggests that the weld zone indicates better superplastic/strain hardening behavior than the heat affected zone and base metal. Observing such increase in the ηr value should increase the overall ductility in the material. It also suggests that the failure should happen in the base metal and not in the weld zone, which is preferred from the welding metallurgy point of view. The hardness profile of FW EBM processed Ti-6Al-4V is similar to the FW SLM processed Ti-6Al-4V in terms of increase of hardness in the weld zone compared to the base metal [23]. However, there are some distinct differences between the SLM and EBM processed FW samples.
4
Materials Science & Engineering A 761 (2019) 138045
P.T. Qin, et al.
Fig. 4. Room temperature tensile curves of the (a) EBM, (b) SLM specimens tested in the as-prepared and welded conditions [23].
Fig. 5. Fracture morphology of the Ti-6Al-4V samples in the (a-c) friction welded condition and (d-f) as-prepared EBM condition.
zones, but with different morphologies. What is the most interesting is that columnar β grain in base metal is broken into finer equiaxed α grains with irregular β phase. The HAZ has a coarser grain than the asprepared EBM sample due to the heat dissipation. Such microstructural variations influence the mechanical properties of the FW samples, where an increased hardness in the weld zone and decrease in the HAZ are observed. In addition, it can be observed from the nanoindentation data that the strain rate recovery is higher in the weld metal compared to base metal, suggesting that the weld metal is more ductile/superplastic/strain hardenable than the base metal, suggesting the tensile failure should happen in the base metal rather in the weld zone. Tensile tests revealed that the FW samples become softer compared to the as-prepared EBM specimens: the yield strength drops to 859 ± 10 MPa (970 ± 15 MPa for as-prepared EBM samples), the ultimate strength drops to 1034 ± 9 MPa (1046 ± 13 MPa) and the ductility increases to 13 ± 1% (from 10 ± 1%). Fracture surface analysis revealed that the fracture of the as-prepared EBM samples and the FW samples both exhibit dimple fracture. These findings demonstrate that solid-state processes like friction welding can be successfully used to join EBM fabricated materials and may improve the ductility of the sample with marginal drop in the yield strength. This offers a possible solution for overcoming the problem of limited dimensions achievable with additive manufacturing processes, especially for the powder bed fusion processes.
are shown in Fig. 5. The welding zone of the FW samples is located in the center of the gauge length of the tensile sample. Both the as-prepared EBM and FW samples fracture in the base alloy well away from the center of the gauge length (Fig. 5), which indicating that the welding zone and HAZ has better resistance to failure than the base metal. It is mostly because the welding zone possesses higher strength/ hardness, which is unlike the conventional FW samples where the welding zone is the weak place [27,29]. This may also be attributed using the strain rate recovery criteria, which suggests that the weld zone becomes relatively superplastic. The fracture didn't occur in the HAZ which may be owing to that the coarse microstructure may lead to higher ductility and better strain hardening effect. In addition, the less defects may exist in the HAZ which can help to suppress the crack propagation. For example, the heat in the HAZ during welding can lead to the relaxation of residual stresses. From the fracture surface analysis, it can be observed that both types of samples show dimples corresponding to ductile failure mode (Fig. 5) and the morphology of the dimples varies between the two samples depending on the degree of deformation before the failure.
4. Conclusions The effect of solid-state friction welding on the microstructure and mechanical properties of Ti-6Al-4V parts produced by EBM has been studied in detail. The microstructure of FW-EBM Ti-6Al-4V material shows the presence of three different regions: base metal, heat affected zone and weld zone, where α and β phases can be observed in all three 5
Materials Science & Engineering A 761 (2019) 138045
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Acknowledgement
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Friction welding of electron beam melted Ti-6Al-4V a,b
c
d
P.T. Qin , R. Damodaram , T. Maity , W.W. Zhang K.G. Prashanthe,f,**
a,b
a,b
, C. Yang
T a,b,*
, Z. Wang
,
a
Guangdong Key Laboratory for Processing and Forming of Advanced Metallic Materials, South China University of Technology, Guangzhou, 510640, China National Engineering Research Center of Near-net-shape Forming for Metallic Materials, South China University of Technology, Guangzhou, 510640, China SSN College of Engineering, Chennai, India d Materials Science and Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ, 85287-6106, United States e Department of Mechanical and Industrial Engineering, Tallinn University of Technology, Ehitajete tee 5, 19086, Tallinn, Estonia f Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700, Leoben, Austria b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Ti-6Al-4V alloy Electron beam melting Friction welding Tensile property Hardness
Ti-6Al-4V samples produced by electron beam melting (EBM) are welded using solid-state friction welding (FW) process. The microstructure of the weld sample shows the presence of fine equiaxed α grains with irregular β phase. Microstructural investigations reveal a pronounced change in the shape and size of the α phase in the weld metal as-compared to the base material along with the disappearance of columnar prior β grains. Such variations in the microstructure significantly change the mechanical properties of the FW material. The hardness in the weld zone increases and a decrease of hardness is observed along the heat affected zone (HAZ) with respect to the base metal as expected. Similarly, the room temperature tensile tests show an improvement of ductility in the welded EBM samples. However, the yield and the ultimate strength show a marginal drop in the welded samples compared to the as-prepared EBM specimens. The present work demonstrates that solid-state FW process not only permits successful joining of additively manufactured materials, but also helps in improving their ductility.
1. Introduction Electron beam melting (EBM) has attracted recent attention in the field of additive manufacturing (AM) and is being developed for wide variety of materials [1,2]. As the name suggests the EBM process uses an electron beam as heat source within a vacuum chamber, where electromagnetic coils providing extremely fast and accurate beam control that allows selective melting of the powder bed [3]. Since the EBM process heats the entire powder bed (400–800 °C) before melting, the alloy powder may be sintered partially. Since the powder bed is held hot throughout the process, the solidification of the samples takes place under near-equilibrium conditions. Hence, relatively low amount of internal stresses is present in the EBM sample compared to the samples produced by selective laser melting (SLM) [1]. The mechanical properties of the EBM processed samples are generally better than the cast and comparable to wrought material [4]. Since, the EBM process takes place under a vacuum atmosphere (unlike an inert atmosphere observed in the SLM process), oxidation of the parts is generally averted
[2,5–7]. The EBM process involves several process parameters, including: beam power, beam scanning speed, beam focus, beam diameter, beam line spacing, plate temperature, pre-heat temperature (including the repetitions, speed, and power of the beam), contour strategies, scan strategy and so on [8]. Therefore, the optimization of the process parameters is even more difficult than the selective laser melting (SLM) process and hence only limited materials are employed in EBM such as CP-Ti, Ti6Al4V, Inconel 718 and CoCrMo alloys [1] of which Ti6Al4V is extensively studied. Typically, planar growth is observed in Ti6Al4V fabricated by AM. In the EBM samples, columnar β grains formed firstly during cooling and subsequently were transformed to α + β with a lamellar or Widmanstatten microstructure, while α phase is formed along the prior columnar β-boundaries [3,9–11]. The yield strength of Ti6Al4V fabricated by EBM is found approximately 833–1116 MPa, with ductility ranging between 2.7 and 15% [12]. However, a major drawback for the additive manufactured parts is their size, in particular for materials produced by the EBM process,
*
Corresponding author. Guangdong Key Laboratory for Processing and Forming of Advanced Metallic Materials, South China University of Technology, Guangzhou 510640, China. ** Corresponding author. Department of Mechanical and Industrial Engineering, Tallinn University of Technology, Ehitajete tee 5, 19086, Tallinn, Estonia. E-mail addresses: [email protected] (Z. Wang), [email protected] (K.G. Prashanth). https://doi.org/10.1016/j.msea.2019.138045 Received 3 April 2019; Received in revised form 14 June 2019; Accepted 16 June 2019 Available online 18 June 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
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which depends greatly on the size of the chamber [1,12]. Increasing the size of the chamber will increase the requirements for auxiliary equipment such as cooling system, gas and powder feeder unit, etc. Moreover, large sized sample requires larger volume of powders, which makes it difficult in controlling the powder quality (including composition, shape and size). Additionally, the anisotropic microstructure in the AM samples will be more pronounced in increasing the size of these AM sample [12,13] and hence leads to other problems like anisotropic properties [14–16]. Hence, there is a need to look for alternative secondary processes like additional/conventional metal joining techniques that can join smaller EBM components at a later stage. Even though there are several metal joining processes available, only some of them may be suitable for joining EBM fabricated parts. This is because the EBM fabricated parts have a relatively unique microstructure, which is not possible to be produced using conventional manufacturing processes [17–19]. Hence, if any of the fusion welding processes are employed for joining, the weld zone will be completely re-melted and solidified again at slower cooling rates than the ones realized upon EBM processing. This leads to a relatively coarser microstructure and the advantages of the unique microstructures obtained using the EBM process will be lost and so the mechanical properties of the parts are expected to deteriorate. Moreover, EBM processed materials may contain high internal stresses (depending on the pre-heating temperature of the powder bed and the employed process parameters), especially at low bed pre-heating temperatures and high-energy input conditions. Hence it cannot accommodate additional stresses imparted during the conventional fusion welding processes [3,20,21]. Because of these reasons, the solid-state welding processes the friction welding (FW) or friction stir welding (FSW) processes may be preferred. To the best of our knowledge, only the welding of SLM processed parts were studied [22,23]. However, there is no work reporting the welding of the EBM processed parts. Since the microstructure and properties of the parts produced by SLM and EBM are completely different, because of the difference in the processing conditions, the welding behavior of these materials may also be completely different. Accordingly, the present work deals with the joining of Ti-6Al-4V parts fabricated by EBM using the FW process.
was measured according to the Oliver-Pharr method [24,25]. The contact stiffness S is then related to the reduced modulus, Er, by the following relation [24]:
Er =
π S 2β A
(1)
where, β is the constant related to the geometry of the Berkovich indenter, A is the projected contact area of indentation. The reduced elastic modulus can be related to the Young's modulus E as:
1 1 − ϑ2 1 − ϑ2 = + Er E Ei
(2)
where E and v are the Young's modulus and Poisson's ratio, respectively, and the subscript i refers to the indenters. In this study Ei was taken as 1141 GPa and vi as 0.07. The total strain rate recovery, ηr (ratio of the displacement after complete unloading hf to the maximum depth of penetration hmax) is an indicator of the relative portion of the plastic deformation in the total elasto - plastic deformation that occurs during indentation [25]. The hmax and the hf values were obtained from the nanoindentation P - h datum. The ηr can be estimated from the following equation:
ηr = (hmax − hmin)/ hmax
(3)
Tensile tests were performed using an Instron 8801 facility under quasistatic loading conditions at room temperature with a strain rate of (~1 × 10−3 s−1). Four parallel specimens were tested for an average. The tensile test samples (according to) were cylindrical tensile bars with 52 mm total length that were machined from the welded samples in such a way that the weldment lies along the center of the tensile bar. The dimensions along the gauge length of the tensile bar were: length ~17.5 mm and diameter ~3.5 mm. The fracture surfaces of the samples were investigated using SEM to analyze the causes for the failure. 3. Results and discussion 3.1. Structural analysis The XRD pattern (Fig. 1) shows the presence of hexagonally closed packed (hcp) α peaks in both FW and as-prepared Ti-6Al-4V samples, which is typical for EBM processed Ti6Al4V samples, similar to other published reports [26,27]. The XRD pattern of the EBM sample shows a 1 0) direction. However, the texcrystallographic texture along the (10‾ ture is partially reversed or the intensity of texture is significantly decreased for the FW-EBM sample and in the weld zone. The XRD pattern shows several differences, when compared with the SLM sample, where the texture is observed along the (0002) direction [23]. The peaks are
2. Experimental details Arcam A2X device was used to fabricate the sample using the EBM process. Conventional EBM process parameters including: vacuum 10−4–10−5 (mbar), accelerating voltage: 60 kV, layer thickness ~50 μm, scan speed ~0.50 m/sec and the process chamber was maintained at 903 K. A core shell scan strategy was used with the melting sequence varying between 0° and 90° between layers. FW was carried out using a continuous drive friction-welding machine with 200 kN capacity. The parameters used in the present study are: friction pressure 75 MPa, upset pressure 100 MPa, burn off length 3 mm and spindle speed 1000 rpm. Optical microscopy (OM, DMI5000), scanning electron microscope (SEM, NOVA NANSEM 430) and X-ray diffraction (XRD, Rigaku SmartLab SE) were used to examine the microstructure and structural characterization of the samples. Microhardness measurements were carried out with a load of 100 N applied for 10 s and a test spacing between adjacent indentations of 0.2 mm, using a computer-controlled SCTMC Vickers hardness tester (Shanghai Shangcai, Shanghai, China). They were tested following a matrix of 6 × 30 which cover the base metal, heat affected zone (HAZ) and weld zone. Nanoindentation investigations were performed by a TriboIndenter TI-950 (from the firm of Hysitron, United States) using fixed load control mode (with a maximum load Pmax of 2000 μN and depth hmax of 3 μm) at room temperature (RT). The contact area between the Berkovich indenter and the specimens was calibrated using a fusedquartz standard, and the Young's modulus E and nanohardness Hnano
Fig. 1. XRD patterns of the Ti-6Al-4V samples in the as-prepared EBM and friction welded condition. (The red vertical lines showing the standard peaks of the hexagonal α-Ti, Reference code of the PDF is 00-001-1198 [28]). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 2
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Fig. 2. (a-c) SEM micrographs of friction welded Ti-6Al-4V (d-h) OM micrographs of friction welded Ti-6Al-4V. The yellow solid circles indicate the intersections of the long columnar grain's boundary and the red line. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
along with the α phase or present at the boundaries. The microstructure of the heat affected zone has a thickness of 0.5–1 mm and shows that the columnar prior β grains (having a width of 140 ± 19 μm) are much coarser than both the base metal and welding zone due to the heat dissipation effect.
relatively less broad compared to the samples produced by SLM. This suggests that the Ti-6Al-4V samples produced by the EBM process have relatively less internal stresses than their SLM counterparts do. The relatively less internal stress in the EBM process may be due to the slow cooling rate observed because of the pre-heating of the powder bed in the EBM process. In addition, the relatively slow cooling rates observed in the EBM process as compared to the SLM process may leave to relative coarsening of the microstructure. In addition, β-phase is observed additionally to the α-hcp phase in the EBM processed Ti-6Al-4V sample. However, the intensity of β-phase peak decreases after welding (in the weld zone) which may be due to transition of the β-phase to α-phase as a result of excessive plastic deformation and dissipation of heat generated in the weld zone to attain thermal equilibrium during the FW process. The SEM and OM images of the FW Ti-6Al-4V samples are shown in Fig. 2. The OM images clearly indicate the presence of three different zones: base metal, welding zone and the heat-affected zone. The base metal shows the presence of long columnar prior β grains along with α + β phases. The bright colored lines indicate the single α phase, which is seen inside the columnar prior β grains. The dark colored areas correspond to the β phase. The columnar grains in the base metal is about 100 ± 15 μm. In welding zone, columnar prior β grains were broken into more equiaxed grains with irregular β phase distributed
3.2. Mechanical properties Fig. 3 shows the Vickers hardness, nanoindentation and modulusstrain rate recovery plot for the Ti-6Al-4V FW samples. The hardness profile along the weld interface measured using both micro- and nanoindentation can be observed in (Fig. 3(a)). Both these tests show similar hardness profile, where a nearly symmetrical behavior with respect to the weld interface is observed in the FW Ti-6Al-4V EBM samples. The micro hardness of welding zone is 383 ± 28 HV, which is significantly higher than the base metal tested using the same parameters (350 ± 14 HV), indicating that the hardening effect in the weld zone due to plastic deformation process. In addition, the increase of hardness along the weld zone may be attributed to the formation of a recrystallized α phase microstructure and its morphology, compared to the initial as-prepared EBM Ti-6Al-4V material. However, the hardness of the HAZ is even lower than the base metal, which is about 328 HV. The thickness of the HAZ from the hardness profile is about 0.5–1 mm, 3
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Fig. 3. (a) Vickers hardness and nanohardness profile measured across the weld interface of Ti-6Al-4V EBM samples (b) typical load-depth (P - h) curves of nanoindentation for weld zone and base alloy regions (c) modulus and strain rate recovery profiles for the Ti-6Al-4V weldment.
Firstly, the hardness of both the base metal and the weld zone of the FW SLM sample is much higher than the FW EBM sample. The higher hardness observed in the FW joints of the SLM materials compared to the EBM materials can be corroborated directly to their cooling rates. The high cooling rate observed in the SLM process leads to a very fine microstructure with the presence of martensitic α′ phase. On the other hand, the EBM samples with near-equilibrium solidification will lead to a typical α+β microstructure, which is relatively softer than α’ microstructure. During the EBM process, the powder bed is pre-heated to ~903 K, which significantly reduces the cooling rate, which is responsible for such differences in the microstructure and their mechanical properties. Characteristic tensile test curves of both as-prepared EBM and FW-EBM Ti-6Al-4V samples at room temperature are shown in Fig. 4. For comparison purposes, the SLM processed and FW-SLM Ti6Al-4V were included [23]. The as-prepared EBM samples show a yield strength of 970 ± 15 MPa and an ultimate strength of 1046 ± 13 MPa with 10 ± 1% ductility. The FW Ti-6Al-4V EBM samples exhibit a marginal deterioration in strength, with a yield strength marginally dropping (111 MPa) to 859 ± 10 MPa and similar ultimate strength of around 1034 ± 9 MPa. However, the ductility of the FW Ti-6Al-4V EBM samples reaches 13 ± 1%, which is ~3% higher than for the asprepared EBM sample. The strength effects observed in for the FW-EBM Ti-6Al-4V samples are similar to what was found for FW-SLM Ti-6Al-4V samples and FW-SLM Al-12Si samples. Both the yield and ultimate tensile strength of FW samples show a marginal decrease but also becomes more ductile, because of the microstructural transformation during the FW process. The hard martensitic phase observed after the SLM process transforms to α-phase and hence the materials become softer. In addition, the coarsening of the microstructure in HAZ plays a major role in improving the ductility and slight decrease of the strength of these samples. The fracture surfaces of the as-prepared EBM and FW-EBM samples
which is in accordance with the HAZ thickness measured from the OM images. The hardness measured from by nanoindentation also shows a similar trend with Vickers hardness, where the welded zone (5.65 ± 2.40 GPa) shows an improved hardness than the base metal (3.64 ± 1.47 GPa) and the HAZ shows a decreased hardness of lower than 2 GPa. The typical load vs. depth (P - h) curves extracted from the nano indentation test are shown in Fig. 3(b). The P – h curves for two different structures show different two different slopes both in the loading and unloading segments. Moreover, it can be observed from the P – h curves that the maximum indentation depth hmax of the base material is slightly greater than that of the weld zone, representing a small increase in the resistance to plastic deformation by weld zone (increase of hardness in the weld zone). Both the reduced modulus Er and strain rate recovery (ηr ) variations have similar profile between the base metal and weld zone (Fig. 3(c)). Generally higher ηr indicates better superplasticity/strain hardening, or lower the elasticity; therefore, ηr indeed exhibits improved strain hardening behavior [25]. In the present sample, the ηr increases gradually from base metal towards HAZ and then towards the center of the weld zone (ηr = 0.30). The maximum value of ηr is obtained at the center of the weld zone (positions 0 and/or -1). This suggests that the weld zone indicates better superplastic/strain hardening behavior than the heat affected zone and base metal. Observing such increase in the ηr value should increase the overall ductility in the material. It also suggests that the failure should happen in the base metal and not in the weld zone, which is preferred from the welding metallurgy point of view. The hardness profile of FW EBM processed Ti-6Al-4V is similar to the FW SLM processed Ti-6Al-4V in terms of increase of hardness in the weld zone compared to the base metal [23]. However, there are some distinct differences between the SLM and EBM processed FW samples.
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Fig. 4. Room temperature tensile curves of the (a) EBM, (b) SLM specimens tested in the as-prepared and welded conditions [23].
Fig. 5. Fracture morphology of the Ti-6Al-4V samples in the (a-c) friction welded condition and (d-f) as-prepared EBM condition.
zones, but with different morphologies. What is the most interesting is that columnar β grain in base metal is broken into finer equiaxed α grains with irregular β phase. The HAZ has a coarser grain than the asprepared EBM sample due to the heat dissipation. Such microstructural variations influence the mechanical properties of the FW samples, where an increased hardness in the weld zone and decrease in the HAZ are observed. In addition, it can be observed from the nanoindentation data that the strain rate recovery is higher in the weld metal compared to base metal, suggesting that the weld metal is more ductile/superplastic/strain hardenable than the base metal, suggesting the tensile failure should happen in the base metal rather in the weld zone. Tensile tests revealed that the FW samples become softer compared to the as-prepared EBM specimens: the yield strength drops to 859 ± 10 MPa (970 ± 15 MPa for as-prepared EBM samples), the ultimate strength drops to 1034 ± 9 MPa (1046 ± 13 MPa) and the ductility increases to 13 ± 1% (from 10 ± 1%). Fracture surface analysis revealed that the fracture of the as-prepared EBM samples and the FW samples both exhibit dimple fracture. These findings demonstrate that solid-state processes like friction welding can be successfully used to join EBM fabricated materials and may improve the ductility of the sample with marginal drop in the yield strength. This offers a possible solution for overcoming the problem of limited dimensions achievable with additive manufacturing processes, especially for the powder bed fusion processes.
are shown in Fig. 5. The welding zone of the FW samples is located in the center of the gauge length of the tensile sample. Both the as-prepared EBM and FW samples fracture in the base alloy well away from the center of the gauge length (Fig. 5), which indicating that the welding zone and HAZ has better resistance to failure than the base metal. It is mostly because the welding zone possesses higher strength/ hardness, which is unlike the conventional FW samples where the welding zone is the weak place [27,29]. This may also be attributed using the strain rate recovery criteria, which suggests that the weld zone becomes relatively superplastic. The fracture didn't occur in the HAZ which may be owing to that the coarse microstructure may lead to higher ductility and better strain hardening effect. In addition, the less defects may exist in the HAZ which can help to suppress the crack propagation. For example, the heat in the HAZ during welding can lead to the relaxation of residual stresses. From the fracture surface analysis, it can be observed that both types of samples show dimples corresponding to ductile failure mode (Fig. 5) and the morphology of the dimples varies between the two samples depending on the degree of deformation before the failure.
4. Conclusions The effect of solid-state friction welding on the microstructure and mechanical properties of Ti-6Al-4V parts produced by EBM has been studied in detail. The microstructure of FW-EBM Ti-6Al-4V material shows the presence of three different regions: base metal, heat affected zone and weld zone, where α and β phases can be observed in all three 5
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Acknowledgement
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