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Additive manufacturing of a functionally graded material from Inconel625 to Ti6Al4V by laser synchronous preheating
Journal of Materials Processing Tech. 275 (2020) 116368
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Additive manufacturing of a functionally graded material from Inconel625 to Ti6Al4V by laser synchronous preheating
T
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Wei Menga, Xiaohui Yina, , Wang Zhangb, Junfei Fanga, Lijie Guoa, Qunshuang Maa, Bing Cuia a b
School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan, 243032, Anhui, China School of Material Science and Engineering, Shanghai Dianji University, Shanghai 201306, China
A R T I C LE I N FO
A B S T R A C T
Associate Editor: M. Merklein
This work examines a functionally graded material, fabricated by directed energy deposition additive manufacturing and laser synchronous preheating that grades from Inconel625 to Ti6Al4V. The microstructure evolution, cracking behavior, phase characteristics and microhardness were determined as a function of position within the graded material. The cracks occurred in the transition zone between 80% Inconel625 + 20% Ti6Al4V and 70% Inconel625 + 30% Ti6Al4V for the non-preheated sample due to the formation of massive Cr- and Moenrich phases, while no cracks were found in preheated gradient samples. A series of phase evolutions with the increase of Ti6Al4V occurred: γ, γ + Ni3Ti, Ti2Ni + TiNi + β-Ti, β-Ti + Ti2Ni, α-Ti + β-Ti + Ti2Ni, α-Ti + β-Ti. The maximal hardness obtainable in the 60% Inconel625 and 40% Ti6Al4V deposition layer is determined largely regarding the presence of the various phases. Laser synchronous preheating was an effective measure on improving deposition and crack suppression in laser deposition for Inconel625/Ti6Al4V graded material.
Keywords: FGMs Laser metal deposition Synchronous preheating Cracking Precipitated phase Ti6Al4V Inconel625
1. Introduction Functionally graded materials (FGMs) are a class of material systems whose chemical composition or microstructure changes gradually with the variation of position, enabling performance to change in one or more dimensions. Currently, FGMs have found their place in some fields, especially extreme-environment applications, such as aerospace or nuclear power generation because different spatial locations of the components have different properties, has been reported by Zhou et al. (2004). However, functionally graded parts are generally not available using conventional metallurgical techniques, including vapor deposition (Xenidou et al., 2007), fusion welding (Veerababu et al., 2017), powder metallurgy (Chu et al., 1999), and centrifugation method (Watanabe et al., 2005). Laser directed energy deposition for metal additive manufacturing is a new type of solid-state free-forming process and a new technology for processing large-sized near-net-formed metal components has been applied in many industrial fields. It has been reported by Muller et al. (2013) and Hosson et al. (2000) that this technique also allows the design of material gradients at the microstructure level in laser rapid prototyping due to the small molten pool and the adjustable alloy components. Therefore, a variety of functionally graded materials can be obtained by laser melting deposition, and that, in particular, also contributes to reducing the thermal stress and the occurrence of cracking. ⁎
Recently, laser melting deposition (LMD) method has been used to prepare gradient structural materials for stainless steel, nickel, and titanium alloys (Shah et al., 2014; Wang et al., 2016). However, since LMD is a process based on melting and rapid solidification formation, the development of the undesirable intermetallic or precipitated phases must be solved. FGMs with various alloys constituents have been investigated, including Invar36/Ti6Al4V (Bobbio et al., 2017), TiC/Ti (Liu et al., 2003), Ti/Ti6A12ZrMoV (Liang et al., 2014), Inconel625/ 316L (Koikea et al., 2017), and CoCrMo/Ti6Al4V (Xue et al., 2008). LDM was used to prepare 304L stainless steel to Inconel625 functionally graded materials, and their composition, microstructure, microhardness, and precipitated phase were examined by Carroll et al., 2016. Reichardt et al., 2016 fabricated a Ti6Al4V to 304L functional grade component with a vanadium transition through a multi-hopper laser deposition system. It was found that the precipitation of brittle intermetallic compounds such as Fe-Ti and the formation of Fe-V-Cr sigma phase proved to be the cause of cracking of the gradient component. In addition, the researchers also investigated the gradient of Nibased alloys and Ti alloys. Lin and Yue (2005) conducted in-depth research on Ti-Ni-based FGMs, and the microstructure evolution of pure Ti/Rene88DT, Ti6Al4V/Rene88DT gradient materials were fully revealed. In addition, these have consisted of Ti6A14V graded to a precipitation-hardenable nickel-based alloy such as Inconel 718 and that focused on microstructure and phase evolution of gradient materials
Corresponding author. E-mail addresses: [email protected] (W. Meng), [email protected] (Y. Xiaohui).
https://doi.org/10.1016/j.jmatprotec.2019.116368 Received 3 January 2019; Received in revised form 15 April 2019; Accepted 13 August 2019 Available online 24 August 2019 0924-0136/ © 2019 Elsevier B.V. All rights reserved.
Journal of Materials Processing Tech. 275 (2020) 116368
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(Onuike et al., 2018). The nickel-based alloy Inconel625 and Ti6Al4V alloy were chosen for this research, due to their disparate thermal and mechanical properties. As a gradient alloy system, Inconel 625 alloy would be useful in applications that require both strength and corrosion resistance, and that is often used as a transition material for thermal expansion coefficient (TEC) at high temperatures. Ti6A14V is corrosion resistant and has a low density and a high strength to weight ratio compared to nickel-base alloy and correspondingly lower strength to weight ratio (Bobbio et al., 2017). These two alloys, when joined directly through dissimilar fusion welding, result in a joint in which intermetallics could be present and provide a point of weakness or failure (Mortensen and Suresh, 1995). In particular, the large difference in TEC between the two alloys could result in failure under significant temperature excursions as the joint would act as a location of stress concentration. However, because an FGM can provide a gradual change in elemental composition, the properties, including TEC, should change gradually across the gradient, resulting in a lower likelihood of component failure during temperature excursions. Because of this, the Ti6A14V to Inconel625 FGM has potential applications in extreme environment conditions, such as those found in aerospace or nuclear reactors. Preheating is usually considered to be an effective method for suppressing cracks in the welding process due to the release of internal thermal stress, as demonstrated by simulations and experiments by Snchez-Cabrera et al. (2007) and Zhu et al. (2018). In the present work, a FGM, fabricated using fiber laser metal deposition and synchronous preheating that grades from Inconel625 toTi6Al4V. Microstructure evolution, cracking behaviour and microhardness of the FGM were experimentally characterized as a function of position within the nonpreheating and preheating graded materials. The forming mechanism of local cracking for graded deposits under non-preheating were analyzed. The gradient forming feasibility from Inconel625 to Ti6Al4V by fiber laser metal deposition and a synchronous preheating process was finally determined.
Fig. 2. Schematic of the laser directed deposition process.
material, degreased with acetone and ethanol, and then deposited. The gradient components were fabricated in a closed atmosphere controlled deposition system as shown in Fig. 2. During the deposition process, high purity argon gas was introduced into the atmosphere box to eliminate air, and maintain the inert gas in the chamber. Experimental studies were performed using fiber laser deposition system. 6.0 kW IPG fiber laser was used with a focal length of 300 mm and 1064 nm wavelength for metal deposition, and the fiber laser was integrated with a KUKA robot. The powder feeding system was equipped with dual powder tubes. Powders were transported from the coaxial nozzle through pure argon to the molten pool. The Ti6Al4V and Inconel 625 powders were weighed separately and then mixed for 30 min using a mechanical stirrer according to designed weight percentage. Gradient layer deposition with different composition proportion was performed using the optimized process parameters. Pure argon gas was used as the powder feed gas and the shielding gas, and the flow rate was 5 L/min. During the laser deposition process, the laser scanning speed was 0.48 m/min, the laser power was 850 W, and the powder mass flow rate was maintained at 16 g/min. The deposition moved 15 mm in parallel along the scanning direction, and the z-direction for each layer deposition moved by 0.3 mm and the x-direction offset by 0.62 mm. Fig. 3 shows laser direct deposited Inconel625/Ti6Al4V functionally gradient samples under non-preheating and preheating. As shown in Fig. 3, the square block samples, 12.5 mm wide and 60 mm tall, was built on a substrate of 316L stainless steel. Initially, sixteen layers of 100% Inconel625 alloy were deposited on the substrate. In the graded region, the weight of Inconel625 powder was reduced by 10% and
2. Experimental procedure The deposited material used for this experiment consisted of Inconel625 powders and Ti6Al4V powders with the diameter ranging from 45 μm to 75 μm and 25 μm to 55 μm, respectively. The SEM morphology of Inconel 625 and Ti6Al4V alloy powders is shown in Fig.1. The chemical components of Inconel625 and Ti6Al4V powders were 0.01% C, 0.37% Mn, 0.32% Si, 21.53% Cr, 65.85% Ni, 8.47%Mo, 3.22%Nb, 0.13%Fe and 89.12% Ti, 6.63% Al and 4.11% V, respectively. A 10 mm thick 316L stainless steel plate was used as the base
Fig. 1. SEM image shows the characteristic morphology of (a) Inconel 625 alloy powders and (b) Ti6Al4V alloy powders. 2
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Fig. 3. Direct laser metal deposited Inconel625-Ti6Al4V functionally graded samples (a) Non-preheating, (b) Preheating.
Ti6Al4V powder increased by the same amount in each successive layer, and 16 layers for each gradient component. Nineteen layers of 100% Ti6Al4V were deposited on top of the gradient zone. Overall, the graded sample was divided into 11 different composition zones, which consisted of 100% Inconel625 the bottom of the sample, Inconel625/ Ti6Al4V gradient region at the middle and 100% Ti6Al4V at the top of the sample. The surface appearance of the gradient samples with nonpreheating and preheating were shown in Fig. 2. Fig. 4 shows a schematic diagram of the synchronous preheating process for Incone625/Ti6Al4V graded sample. As shown in Fig. 4(a), the substrate was firstly preheated by laser before deposition, and the preheating area was 100 mm × 100 mm, and the preheating time was 5 min. Owing to the benefits exhibited the thermal accumulation effect by laser preheating, the small temperature gradient could be guaranteed in the laser initial depositing as shown in Fig. 4(b). For the purpose of decreasing cooling speed, the laser was still heated to the gradient layer after depositing a powder of 11 layers, and that remained off for approximately 3 min each time that a new powder mixture was switched as shown in Fig. 4(c). After the Inconel625/Ti6Al4V gradient sample was prepared, the top would keep heating for 5 min by the laser for its temperature addition. The synchronous preheating parameters in laser deposition process were shown in Table 1. Although this preheated method added to the overall machine time, discontinuous processes have been shown to be beneficial for reducing residual stresses and distortion. Prepared square post specimens were sectioned in a transverse plane by wire cut electric discharge machine, defatted with acetone and polished to 2000 grit size. Then the samples were polished with abrasive paper and diamond slurry, and then the graded specimens were electrochemical etching (10% oxalic acid, 6V for 25 s) to reveal the microstructure. The graded microstructure was observed by an optical microscope (Leica S6 D) and SEM (FEI Quanta 200). X-ray diffraction
Table 1 The synchronous preheating process parameters. Preheating position
Laser power (W)
Scanning speed (m/s)
Offset Δx (mm)
Uplift ΔZ (mm)
Time (min)
Substrate Graded zone Top
650 650 850
0.008 0.008 0.008
0.62 0.62 0.62
0 0.3 0.3
5 3 5
(XRD) patterns were collected using a Bragg-Brentano-type diffractometer (Panalytical Empyrean) with a Cu K-α X-ray source operated at 45 kV and 40 mA (a = 1.54 Å). Microhardness was measured on the cross section of built gradient block using Leco V-100-C1 microhardness tester with a load of 100 g and a dwell time of 15 s. 3. Results and discussion 3.1. Cracking behaviour The appearance of the gradient sample under no-preheating conditions was observed by an optical microscope, and a plurality of cracks was found in the partial deposition layer close to the substrate. Fig. 5 shows the surface morphologies of cracks in gradient zone and EDS analysis of the secondary phase on the crack surface. As seen in Fig. 5(a), the cracks occurred in the transition zone between 80% Inconel625 + 20% Ti6Al4V and 70% Inconel625 + 30% Ti6Al4V, namely transverse crack parallel to the horizontal plane. Moreover, it was to be noted that the surface of macro cracks was a straight transgranular extension, and that presented the characteristics of cold cracking. Fig. 5(b) and (c) shows a high magnification morphology of crack surface and the EDS spectra of the precipitated phase. A large number of massive phases were observed on the crack surface and
Fig. 4. Schematic diagram of laser synchronous preheating in laser deposition process (a) Preheating for the substrate, (b) Laser deposition, (c) Preheating for gradient zone. 3
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Fig. 5. The crack for the non-preheated sample (a) surface appearance of transverse crack, (b) SEM image of the secondary phase on the crack surface, (c) EDS spectrum of the secondary phase, (d) Composition comparison for the precipitated phase and deposition layer.
obvious. Based on analysis, the fracture pattern of the Inconel625/ Ti6Al4V gradient specimen was confirmed as typical brittle cleavage. Xu et.al (2007) published a work on the trend of thermal expansion coefficient gradients of Ti-Ni alloy as shown in Fig. 7. The difference of TEC near both ends of the composition is greater than that of the similar proportion Ti and Ni zone under the same gradient conditions. Furthermore, the difference of TEC of the high nickel region is greater than that of high Ti region, and results in relatively large thermal stress near high Ni zone. Overall, the reasons for the cracking are chiefly attributable to low-melting eutectic formed in the grain along with the segregation elements such as Cr and Mo, and high internal stress in the gradient zone adjacent to the substrate. Therefore, it is of great importance to reduce the hard brittle phase and internal stress in the laser depositing for Inconel625/Ti6Al4V gradient materials. In order to reduce or avoid cracks, the preheating has also been proposed as a potentially effective measure to reduce the thermal stress and improve the morphology and distribution of the precipitated phase. According to the trend of the average thermal expansion coefficient of the uniform composition Ti-Ni alloy in Fig. 7, the Ni content gradually increases with the depositing when the composition gradient direction is from Ti to Ni, and the heat shrinkage also increases with the increases of the deposition height, resulting in an increasing tendency of cracks. On the contrary, when the composition gradient direction is Ni to Ti, the thermal expansion coefficient decreases with the depositing. Although the thermal stress also accumulates, the cumulative velocity is less compared to the former, resulting in less cracking tendency. Therefore, the deposition direction of the gradient layer chosen in this paper is from Inconel625 to Ti6Al4V. The cracking of Inconel625/Ti6Al4V gradient samples appeared in
nearby, which propagated along aggregation zone of the block phases. A block phase particle denoted by the arrow in Fig. 5(b) was analyzed by EDS with the spectrum. It can be seen that the Cr and Mo content of the precipitated phase was significantly higher than that of the deposition layers, thus, the block phases was identified as the Cr- and Moenriched phase as shown in Fig. 5(c) and (d). In addition, the massive phase was cut off by the crack under tensile stress together with a little displacement. The formation and aggregation of Cr- and Mo- enrich phases were considered as the essential internal condition for the cracking. EDS analysis were used to determine the deposited chemistry as a function of position in comparison with the anticipated composition. Fig. 5(d) shows the nominally anticipated and experimentally measured compositions, in weight percent, of the constituent elements present in the FGM for the non-preheated samples. Overall, the actual composition of the deposition layer did not deviate significantly from the anticipated composition. However, there was a significant amount of fluctuation in the measured elemental composition including Cr, Mo and Nb. These fluctuations were due to elemental segregation into separate phases, which were observed in the EDS maps and were discussed below. In the metallographic sample EDM cutting, the non-preheated gradient sample broke up, and fractured at the interface between 80% Inconel625 + 20% Ti6Al4V and 70% Inconel625 + 30% Ti6Al4V. Fig. 6 shows the macroscopic and microscopic morphology of the fracture. As shown in Fig. 6(a), the macroscopic fracture was smooth and bright, and appeared as brittle rupture characteristics. The microscopic topography of the fracture presented river patterns and step features as shown in Fig. 6(b)–(d), and the tearing edges were not 4
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Fig. 6. Fracture morphology characteristics of the non-preheated gradient sample (a) morphology, (b)–(d) microscopic topography.
σ=
E ΔαΔT 1−μ
Where σ is the internal stress, E is Young's modulus, Δα is the difference of the thermal expansion coefficients between the graded layers, and ΔT is the difference of the deposition temperature and room temperature, μ is the Poisson's ratio. According to the strength theory, brittle fracture occurs when the maximum stress is greater than the strength limit of the material, including the bond strength between the gradient interface and the strength of the enriched precipitate phase. The maximum stress is more than the bonding strength between the disposition layers, resulting in a transverse crack as shown in Fig. 5. From the calculation formula, the difference of the temperature gradient and the thermal expansion coefficient between the deposited layers is an important factor in the increase of the internal stress. From the above analysis, the difference of the temperature gradient and the TEC between the deposited layer and the substrate is a main factor affecting the internal stress. Previous researches (Chen et al., 2017; Zhang et al., 2014) has shown that the internal stress could be relieved by the preheating or ultrasonic vibration method. In this paper, a synchronous preheating process was used to reduce the temperature gradient and internal stress between deposition layers.
Fig. 7. The trend of thermal expansion coefficient gradients of Ti-Ni alloy.
the region near the substrate because of the high temperature gradient. In laser directed deposition for dissimilar materials, the composition difference of the gradient transition interface is small, resulting in change slightly of thermal expansion coefficient, elastic modulus, and Poisson's ratio. However, the difference of physical property parameters at the interface still leads to inconsistency in expansion and contraction and large stress between the layers, and the resulting internal stress is calculated as follows (Chen et al., 2017):
3.2. Microstructure morphology In order to investigate the effect of laser synchronous preheating on cracks and microstructure, the metallographic morphology of the 5
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Fig. 8. The optical images of microstructures at different locations along the direction from Inconel625 to Ti6Al4V for the preheated sample (a)–(k) Microstructure morphology, (l) the observation positions of the optical microscope.
between the dendrites. As 20% Ti6Al4V was reached, the microstructure of the deposition layer consisted of block-like γ phase (white), a dense granular phase (intermetallic, grey) and eutectic phase (black) in Fig. 8(c). It was to be noted here that the dendritic morphology had disappeared, while a small amount of intermetallic and eutectic phase formed. When Ti6Al4V increased to 30%, the microstructure mainly composed of a large number of star-shaped bulk segregated phases (white) and intermetallic compound matrix structure as shown in Fig. 8(d). A small quantity of star-shaped bulk phases distributed on the matrix when the content of Ti6Al4V was increased to 60%. A new irregular anomalous eutectic phase was found in Fig. 8(e). Microcracks appeared in the upper part of the deposition layer, and propagated along the star-shaped phases distributed on the surface. As increased to 50% Ti6Al4V, the petal-shaped phases (Dotted line) appeared, and interdendritic massive phases formed, while the twophase dendritic eutectic phase clearly grew, which was schematically illustrated in Fig. 8(f)). With the further increase of Ti6Al4V, although the irregular eutectic phase remained, the eutectic morphology changed
different deposited layers for the preheated Inconel625/Ti6Al4V graded samples was observed by optical microscopy. The observations of the optical microscope for the different deposited layers are shown in Fig. 8. The gradient samples were sectioned in a gradient direction by wire cut electric discharge machine, and the cutting surface was 3 mm away from the surface. The microstructure of the central portion of each deposited layer was observed as shown in Fig. 8(a)–(k), corresponding to the a–k regions in Fig. 8(l). Fig. 8 shows the optical image of microstructure characteristics of the preheated gradient sample. As shown in Fig. 8(a), γ columnar grain in 100% Inconel625 deposit was directed towards the energy source and transitioned into the gradient interface. With the addition of 10% Ti6Al4V, the γ columnar dendrite was transformed into equiaxed dendrite (white) in Fig. 8(b). The black zone around the dendrite was presumed to be weakly formed brittle intermetallic phases, and Cr, Mo and Nb could be segregated as secondary particles or other new phases, such as Laves formed. As experienced multiple thermal cycles, the equiaxed dendrite tended to coarsen and formed eutectic phases 6
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greatly. The eutectic dendrites appeared when Ti6Al4Vwas increased to 60% in Fig. 8(g). When Ti6Al4V was increased to 70%, the dendrites were refined as shown in Fig. 8(h). Under this condition, irregular eutectic abnormal solidification was observed from Fig. 8(h). With the further increase of Ti6Al4V content, the changes from cellular crystal to dendritic growth occurred, while the lath phases disappeared. Also, the precipitates ceased to exist within the prior grains. When the Ti6Al4V content reached 80%, the dendritic growth had been stabilized in Fig. 8(i). When the Ti6Al4V content was increased to 90%, the microstructure of the deposited layer consisted of a needle-like morphology phase and a block phase in an orthogonal orientation in Fig. 8(j). When the content of Ti6Al4V was 100%, the microstructure exhibited columnar cellular growth, and the different orientations of Widmanstatten α-Ti laths were found in the prior β-grains in Fig. 7(k). The uniformity of composition distribution had a great influence on the microstructure evolution, and the composition of different gradient layers for the preheated Inconel625/Ti6Al4V gradient samples was analyzed by EDS. To mark the locations for the EDS analysis, a series of indentations were formed by a microhardness tester along the compositional gradient, which were used to determine the deposited chemistry as a function of position. Moreover, five EDS points were tested for each deposition layer at intervals of 2 mm, and an average of composition for five points was obtained. Fig. 9 shows the average composition of the EDS analysis along the vertical direction of the graded deposit. As shown in Fig. 9, the element Ni, Ti, Al and V in graded samples presented a good linear relationship with the increase of Ti6Al4V, while for the other elements, such as Cr, Mo, Nb, the resulting alloy exhibited a certain degree of nonlinearity. According to the phase evolution analysis above, the reasons for the nonlinearity of some alloy elements could be the formation of many precipitated phases, and another possible explanation was the variation in element loss that occurred during laser depositing.
Fig. 10. Phase transformation based on the binary phase diagram of Ti-Ni.
stable intermetallic compounds. With the increase of Ni content, β-Ti phase decreases, whereas the β-Ti + Ti2Ni eutectic phase increase. In the equivalence of Ni-Ti proportion, TiNi phases are produced with accompanied by Ti2Ni. With further increase of Ni, TiNi3 phases are presented and then transform to γ phase. Based on analysis, the phase evolution of Inconel625/Ti6Al4V gradient materials can be forecast according to The Ti-Ni binary system. To determine the phase constitution of laser deposited layers, XRD analysis was performed, and the results were shown in Fig. 11. With the addition of Ti6Al4V, intermetallic compounds such as Ni3Ti, NiTi, and NiTi2 became the main phases in the Inconel625/Ti6Al4V graded material. It could be seen from the XRD pattern in Fig. 11 that a series of phase evolutions along the composition gradient were consistent with the Ti-Ni phase diagram. The observations and analysis of XRD, OM and SEM could be used to illustrate the phase transition in different components of the graded material. In order to investigate the effect of the laser synchronous preheating on the evolution of the phases that formed, the SEM and the EDS elemental composition maps must be examined in different deposition layers for the non-preheated and preheated samples. Fig. 12 and Fig. 13 show SEM images of microstructure morphology of the different gradient layer for non-preheated and preheated samples, respectively. As shown in Figs. 10(a) and 11 (a), at the composition of 90% Inconel625 + 10% Ti6Al4V, a noticeable change in microstructure
3.3. Phase evolution For the Ti-Ni-Cr and Ti-Mo-Cr alloys, as well as Inconel625/Ti6Al4V gradient materials, the main phases are a Ti-rich solid solution and (Ti, Ni) compound. Fig. 10 is a phase diagram of the Ti-Ni system, which was first proposed by Murray et al. (1987) and modified the extension of the possible phase fields by Nagarajan et al. (1994). The phase diagram illustrates that there exists a eutectic reaction between β-Ti and Ti2Ni, a peritectic reaction between Ti2Ni and TiNi, and a eutectoid decomposition of β-Ti. There are four phase regions in the Ti-Ni binary alloy, with six stable phases and two sub-stable phases. The possible phases in Ti-Ni gradient materials mainly include α-Ti, β-Ti, Ti2Ni, TiNi, TiNi3, Ti3Ni4, γ, and TiNi, Ti2Ni and TiNi3 are the three main
Fig. 9. The compositional gradient of the preheated Inconel625/Ti6Al4V graded sample.
Fig. 11. XRD analysis result of the typical laser deposited layer on the outmost surface of the preheated gradient sample. 7
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Fig. 12. SEM images of microstructure morphology of laser deposition layers in different gradient layer for the unpreheated samples.
occurred: γ columnar dendrites in 100% Inconel625 transform into a complex multiphase structure. The microstructure mainly consisted of γ equiaxed crystals and η (Ni3Ti) phase precipitated around γ phase, and both were about equal. γ + Ni3Ti lamellar eutectic phase (white) was generated together with γ phase. These phases in preheated sample grew bigger than that of the non-preheated sample. Moreover, a small platelet-like phase was also found in the interdendritic regions. Fig. 13(e) and (f), Fig. 15 shows EDS spectra of precipitated phase and deposition layers for the preheated and non-preheated samples. It was obvious that the Cr and Mo content of the massive phases were higher than the average composition of the base material by EDS analysis. As shown in Fig. 12(b) and (b), for 80% Inconel625 + 20% Ti6Al4V, γ equiaxed morphology degenerates, and a small amount of TiNi+η eutectic appeared around γ block-like crystal. Though phase morphology of the preheated sample was less different compared with the nonpreheated sample, more eutectic phases were produced for the latter. Cr- and Mo-rich phases formed on the TiNi (black) matrix as shown in Fig. 13(f) and Fig. 14(b). Fig. 13(c) and (d) shows the SEM images of the deposition layers of 70% Inconel625 + 30% Ti6Al4V and 60% Inconel625 + 40% Ti6Al4V for preheated samples. Microstructure of both mainly consisted of η
(Ni3Ti) and TiNi phase, and there was little difference in morphology. The microstructure was dominated by the primary TiNi equiaxed dendrites, and dense TiNi+η eutectic was produced between the dendrites. As seen in Fig. 13, polygonal Cr- and Mo-enrich phases were found in TiNi matrix phase, and that of 60% Inconel625 + 40% Ti6Al4V deposition layer were more. Moreover, a flower-like eutectic growth coupled with a TiNi matrix phase was also found at the interdendritic regions. It was interesting to note that the flower-like eutectic usually grew around the Cr and Mo enriched phase. As shown in Fig. 12, the microstructure of the unpreheated sample was similar to that of the preheated sample, while the former had a smaller grain size. It should be noted in Fig. 12(d) that the crack propagation path was along the Cr and Mo enriched phase and nearby, and that was tortuous. Therefore, Crack formation is closely related to the Cr- and Mo-enrich phases. The Cr and Mo enriched phase is a hard and brittle phase, and that have greatly effect on the mechanical properties. The size and morphology of Cr- and Mo-enrich phases are determined by high temperature residence time, growth rate and composition conditions. In the laser depositing for the Inconel625/Ti6Al4V gradient material, the Crand Mo-enrich phases grew up with the increase of laser deposition layer due to heat accumulation effect. As shown in Fig. 14, the Cr-and 8
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Fig. 13. SEM images of laser deposition layers for the preheated sample. (a)–(d) Microstructure morphology, (e) and (f) EDS spectra of secondary phase as shown in Fig.13(a) and (d).
Mo-enriched phases were distributed on the γ-Ni matrix, and that were obviously observed between the 90% Inconel625 + 10% Ti6Al4V and 60% Inconel625 + 40% Ti6Al4V deposition layers. As shown in Fig.15, EDS analysis showed that an average composition of Cr- and Mo-enrich phases with the deposition layers added 10% Ti6Al4V, 20% Ti6Al4V and 30% Ti6Al4V presented as Cr38.9Mo16.2Ni37.2Ti4.36Nb 2.5(at. %), Cr64.5Mo18.5Ni10.2Ti3.3Nb1.7 (at. %) and Cr66.3Mo18.3Ni4.5Ti5.1Nb 1.6(at. %), and the Cr and Mo atomic ratio of the precipitated phases was 2.4–3.6. Unfortunately, the diffraction peaks of the Cr- and Mo-enriched phases in the XRD pattern were not found, and that was
Mo-enrich phase presented a massive star shape, while a large number of eutectic phases appeared at the star-like tip. The segregation easily generated with the high content of Inconel625, resulting in the enrichment of local elements such as Cr and Mo. With the formation of the Cr- and Mo-enrich phase, the solidification tip frontier evolved into poor Cr and Mo region, resulting in the increase of other elements such as Ni and Ti. The formation conditions of eutectic precipitation were met, thus, many flower η+TiNi eutectic phases were produced at the tip of star-like Cr- and Mo-enrich phase in Fig. 14. Observed by optical and scanning electron microscopy, the Cr- and 9
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Fig. 14. The typical morphology of precipitated phase for the preheated sample and eutectic with (a) 70% Inconel625 + 30% Ti6Al4V and (b) 60% Inconel625 + 40% Ti6Al4V.
Fig. 15. (a) and (b) EDS analysis of the secondary phase for the unpreheated samples. (c) Composition comparison for the precipitated phase and the deposition layer. (d) Ni-Cr-Mo equilibrium phase diagram.
analysis. Fig. 16 shows the phase evolution of the composition ranging from 50% Ti6Al4V to 100%Ti6Al4V. For 50% Inconel625 + 50% Ti6Al4V deposit, the microstructure consisted of the dispersion TiNi + β-Ti dendrites on Ti2Ni matrix and TiNi + Ti2Ni lamellar eutectic structure. Some fishbone-like two-phase dendritic phases of β-Ti + Ti2Ni appeared in the interdendritic zones as shown in Fig. 16(a). The similar structures in some eutectic alloys were found by Villaret et al. (2016). For the composition 40% Inconel625 + 60% Ti6Al4V deposit, a class of anomalous eutectic structures, characterized by the appearance of a
qualitatively inferred to Cr3Mo based on the element contents. In addition, the multicomponent phase diagrams were used to determine the phases that precipitated during fabrication as a function of position in the gradient. It predicted by Perricone et al. (2003) that the formation of four secondary phases as shown in the Ni-Cr-Mo equilibrium phase diagram in Fig. 15(d). The Ni-Cr-Mo formula based on element contents could be used to estimate the precipitated phase of the different deposition layers. From the EDS analysis in Fig. 15, the composition range of Ni-Cr-Mo was found to be in the (Cr, Mo) phase region as shown in Fig. 15(d) (shadow zone), which coincided with the result of EDS 10
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Fig. 16. SEM images of microstructure morphology of laser deposition layers for the preheated samples.
granular β-Ti phase on a continuous Ti2Ni matrix. The microstructure mainly included two types of dendrites, a coarse and large volume fraction of TiNi dendrites, and the other small volume fraction of β-Ti dendrites in Fig. 16(b). For 30% Inconel625 + 70% Ti6Al4V, the microstructure consisted of a new two-phase TiNi + Ti2Ni dendritic growth. The β-Ti particles were remarkably refined and discretely distributed on the continuous network of Ti2Ni matrix, showing anomalous eutectic growth in Fig. 16(c). When 50% Ti6Al4V to 70% Ti6Al4V was added to the deposition layers, the microstructure of the unpreheated sample had a slight difference from that of the preheated sample, only the volume fraction of each phase was different. Fig. 16(d) and (e) shows the microstructure of a section close to 100% Ti6Al4V for the preheated sample. The microstructure of both was composed of β-Ti dendrites and intergranular β-Ti + Ti2Ni eutectic. For the composition 20% Inconel625 + 80% Ti6Al4V, β-Ti presented as an equiaxed dendritic morphology, and a rod-shaped βTi + Ti2Ni eutectic phase between the β-Ti dendrites was produced, which was similar to the microstructure of the unpreheated sample in Fig. 12(h). As shown in Fig. 16(e), when the composition reached 10% Inconel625 + 90% Ti6Al4V, a small quantity of β-Ti + Ti2Ni eutectoid phase had formed at the prior β grain boundaries, nevertheless, more βTi + Ti2Ni eutectoid phases were produced around the β grain for the non-preheated sample in Fig. 12(i). The depositional structure of 100% Ti6Al4V was composed of coarse primary β-Ti equiaxed crystals, and the disisotropic lath or acicular α-Ti distributed in the interior of the βTi grains, presenting typical Widmanstatten structure morphology in Fig. 16(f). For the purpose of further understanding the phase transition as a function of position within the graded material, Table 2 provides a summary of the phase constitution of the deposited layers in different compositions, and compare with the Ni-Ti binary phase diagrams. As shown in Table 2, the matrix phase of the deposited layer was substantially consistent with the phase obtained by the phase diagram. A series of phase evolutions of the preheated samples with the increase of Ti6Al4V occurred: γ, γ + Ni3Ti, Ti2Ni + TiNi + β-Ti, β-Ti + Ti2Ni, αTi +β-Ti + Ti2Ni, α-Ti + β-Ti. However, Cr and Mo enrich precipitated
Table 2 Phase evolution of phase diagram and experiments at different Ti-Ni ratios. Ni-Ti constituent
Phase diagram
Experimental results
Ti Ti-10%Ni Ti-20%Ni Ti-30%Ni Ti-40%Ni Ti-50%Ni Ti-60%Ni Ti-70%Ni Ti-80%Ni Ti-90%Ni Ni
β-Ti + α-Ti β-Ti + α-Ti + Ti2Ni α-Ti + β-Ti + (β-Ti + Ti2Ni) (Ti2Ni + β-Ti) + α-Ti Ti2Ni + TiNi + β-Ti TiNi + Ti2Ni + β-Ti TiNi + TiNi3 + Ti3Ni4 TiNi3 + TiNi TiNi3 + TiNi γ+(γ + TiNi3) γ
β-Ti + α-Ti β-Ti +(β-Ti + Ti2Ni) β-Ti+(β-Ti + Ti2Ni) (Ti2Ni + β-Ti) + TiNi Ti2Ni + TiNi + β-Ti TiNi + Ti2Ni + β-Ti TiNi + TiNi3 + (Cr, Mo) TiNi3 + TiNi + (Cr, Mo) TiNi3 + TiNi + γ + (Cr, Mo) γ + (γ + TiNi3) + (Cr, Mo) γ + NbC
phases were found in the Inconel625/Ti6Al4V gradient material due to the segregation of Cr and Mo elements, which had an important influence on the microhardness and crack of the Inconel625/ Ti6Al4V gradient material. 3.4. Microhardness Fig.17 shows the microhardness distribution of different deposited layers from the substrate to the top. The tests of Vickers microhardness were carried out five times for each deposited layer, and the measurement points of hardness were set at 2 mm intervals across the boundary. The average hardness of 5 points for each layer are shown in Fig. 17. As a function of the number of layers from the substrate, the hardness first increased with the increase of Ti6Al4V alloy. When the composition reached 70% Inconel625 + 30% Ti6Al4V deposition layer, the hardness reached the maximum value. An average value of about 821.8 HV in the 70% Inconel625 + 30% Ti6Al4V was detected, and the middle deposition layers had a much higher hardness compared with the bottom and the upper of the gradient sample. Moreover, it could be seen from the standard deviation in Fig. 1 that the distributions of hardness in each layer were uniform. In addition, the hardness 11
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Acknowledgements This work was supported by the University Scientific Research Project of Anhui Province Education Office No. KJ2018A0063, Anhui Provincial Natural Science Foundation NO. 1908085QE198 and the National Science Foundation of China under Grant No. 51505271 and No. 51375015. References Bobbio, L.D., Otis, R.A., Borgonia, J.P., Dillon, R.P., Shapiro, A.A., Liu, Z.K., Beese, A.M., 2017. Additive manufacturing of a functionally graded material from Ti-6Al-4V to Invar: experimental characterization and thermodynamic calculations. Acta Mater. 127, 133–142. Carroll, B.E., Oti, R.A., Borgonia, J.P., Suh, J., Dillon, R.P., Shapiro, A.A., Hofmann, D.C., Beese, A.M., 2016. Functionally graded material of 304L stainless steel and Inconel 625 fabricated by directed energy deposition: characterization and thermodynamic modeling. Acta Mater. 108, 46–54. Chen, Y.J., Yue, T.M., Guo, Z.N., 2017. Laser joining of metals to plastics with ultrasonic vibration. J. Mater. Process Tech. 249, 441–451. Chu, C.L., Zhu, J.C., Yin, Z.D., Lin, P.H., 1999. Hydroxyapatite-Ti functionally graded biomaterial fabricated by powder metallurgy. Mater. Sci. Eng. A 271, 95–100. Hosson, J.T.D., Pei, Y., 2000. Functionally graded materials produced by laser cladding. Acta Mater. 48, 2617–2624. Koikea, R., Unotoroa, I., Kakinuma, Y., Aoyama, T., Oda, Y., Kuriya, T., Fujishima, M., 2017. Evaluation for mechanical characteristics of Incone1625-SUS316L joint produced with direct energy deposition. Procedia Manuf. 14, 105–110. Liang, Y.J., Liu, D., Wang, H.M., 2014. Microstructure and mechanical behavior of commercial purity Ti/Ti6Al2Zr1Mo1V structurally graded material fabricated by laser additive manufacturing. Scr. Mater. 74, 80–83. Lin, X., Yue, T.M., 2005. Phase formation and microstructure evolution in laser rapid forming of graded SS316L/Rene88DT alloy. Mat. Sci. Eng. A. 402, 294–306. Liu, W., Dupont, J.N., 2003. Fabrication of functionally graded TiC/Ti composites by laser engineered net shaping. Scr. Mater. 48, 1337–1342. Mortensen, A., Suresh, S., 1995. Functionally graded metals and metal-ceramic composites: Part 1 Processing. Int. Mater. Rev. 40, 239–265. Muller, P., Mognol, P., Hascoet, J.Y., 2013. Modeling and control of a direct laser powder deposition process for functionally graded Materials (FGM) parts manufacturing. J. Mater. Process Tech. 213, 685–692. Murray, J.L., 1987. Phase Diagrams of Binary Titanium Alloys. ASM International pp: 354. Nagarajan, R., Aoki, K., 1994. A study of the glass-forming range in the ternary TiNiAl system by mechanical alloying. Mater. Sci. Eng. A 179–180, 168–172. Onuike, B., Bandyopadhyay, A., 2018. Additive manufacturing of Inconel 718-Ti6A14V bimetallic structures. Add Manuf. 22, 844–851. Perricone, M.J., DuPont, J.N., Cieslak, M.J., 2003. Solidification of Hastelloy alloys: an alternative interpretation. Metall. Mater. Trans. A 34, 1127–1132. Reichardt, A., Dillon, R.P., Borgonia, J.P., Shapiro, A.A., McEnerney, B.W., Momose, T., Hosemann, P., 2016. Development and characterization of Ti-6Al-4V to 304L stainless steel gradient components fabricated with laser deposition additive manufacturing. Mater. Des. 104, 404–413. Shah, K., Haq, I., Khan, A., Shah, S.A., Pinkerton, A.J., 2014. Parametric study of development of Inconel-steel functionally graded materials by laser direct metal deposition. Mater. Des. 54, 531–538. Veerababu, J., Goyal, S., Sandhya, R., Laha, K., 2017. Low cycle fatigue behaviour of Grade 92 steel weld joints. Int. J. Fatigue 105, 60–70. Villaret, V., Deschaux-Beaume, F., Bordreuil, C., 2016. A solidification model for the columnar to equiaxed transition in welding of a Cr-Mo ferritic stainless steel with Ti as inoculant. J. Mater. Process Tech. 233, 115–124. Wang, Z.Q., Palmer, T.A., Beese, A.M., 2016. Effect of processing parameters on microstructure and tensile properties of austenitic stainless steel 304L made by directed energy deposition additive manufacturing. Acta Mater. 110, 226–235. Watanabe, Y., Oike, S., 2005. Formation mechanism of graded composition in Al–AlCu functionally graded materials fabricated by a centrifugal in situ method. Acta Mater. 53, 1631–1641. Xenidou, T.C., Boudouvis, A.G., Markatos, N.C., Samélor, D., Senocq, F., 2007. An experimental and computational analysis of a MOCVD process for the growth of Al films using DMEAA. Surf. Coat. Technol. 201, 8868–8872. Xu, X.J., Lin, X., Chen, J., He, F., Huang, W.D., 2007. Laser rapid forming of Ti-Ni functionally graded alloy. Mater. Sci. Forum 561–565, 227–230. Xue, W., Bose, S., Bandyopadhyay, A., Krishna, B.V., 2008. Functionally graded CoCrMo coating on Ti6Al4V alloy structures. Acta Biomater. 4, 697–706. Sánchez-Cabrera, V.M., Rubio-González, C., Ruíz-Vela, J.I., Ramírez-Baltazar, C., 2007. Effect of preheating temperature and filler metal type on the microstructure, fracture toughness and fatigue crack growth of stainless-steel welded joints. Mater. Sci. Eng. A. 452–453, 235–243. Zhang, K., Wang, S.J., Liu, W.J., Long, R.S., 2014. Effects of substrate preheating on the thin-wall part built by laser metal deposition shaping. Appl. Surf. Sci. 317, 839–855. Zhou, M.Y., Xi, J.T., Yan, J.Q., 2004. Modeling and processing of functionally graded materials for rapid prototyping. J. Mater. Process Tech. 146, 396–402. Zhu, C.X., Tang, X.H., He, Y., Lu, F.G., Cui, H.C., 2018. Effect of preheating on the defects and microstructure in NG-GMA welding of 5083 Al-alloy. J. Mater. Process Tech. 251, 214–224.
Fig. 17. Microhardness distributions of each graded layer from the substrate to the top deposited layer for the preheated and unpreheated samples.
distribution of the unpreheated sample was consistent with that of the preheated sample, and the former was slightly higher than the latter, which may be due to the coarser phase structure for the preheated sample. Based on the results of the analysis of the microstructural changes, it could be concluded that two possible factors brought the initial rising of the microhardness with the increase of Ti6Al4V. One was the intermetallic compounds of Ni, Ti and other elements. Intermetallic compounds such as NiTi, Ni3Ti and Cr- and Mo-enrich phases having high hardness were found in the deposition layer, which was accompanied by the decrease of γ phase. The other was that the rapid solidification of the molten pool might capture more alloying elements in the solid solution phase of titanium and resulted in more effective solid solution strengthening. As can be seen from Fig. 12(c), the volume fraction of the intermetallic compound was higher, and the increase in microhardness was mainly related to the formation of intermetallic compounds. The Ti2Ni laths and Ti2Ni particles with a fine eutectoid lamellar structure within the TiNi dendrites were dispersed in the maximum microstructure region. The decrease in the hardness from the 70% Inconel625 + 30% Ti6Al4V deposition layer was due to the reduction of the Cr- and Mo-enrich phase and the disappearance of the eutectoid layered structure within the γ phase. Then, with the changeover from NiTi + Ti2Ni to β-Ti, the hardness decreased further. The drop in hardness in the10%Inconel625 + 90%Ti6Al4V zone that was formed towards the end of the deposition may be attributed to the formation of coarse β-Ti dendrites resulting from a decrease of thermal gradient due to the accumulation of heat as the deposit grew thicker.
4. Conclusion A graded component without sharp compositional boundary and crack was successfully fabricated from Inconel625 toTi6Al4V using directed energy deposition and laser synchronous preheating, with 10% composition interval. For the non-preheated samples, the cracks occurred in the transition zone between 80% Inconel625 + 20% Ti6Al4V and 70% Inconel625 + 30% Ti6Al4V due to the formation of massive Cr- and Mo-enrich phases. A series of phase evolutions with the increase of Ti6Al4V occurred: γ, γ + Ni3Ti, Ti2Ni + TiNi+β-Ti, β-Ti + Ti2Ni, αTi + β-Ti + Ti2Ni, α-Ti+β-Ti. The maximal hardness obtainable in the 60% Inconel625 and 40% Ti6Al4V deposition layer is determined largely regarding the presence of the various phases. This study demonstrates laser synchronous preheating is an effective measure on improving deposition and crack suppression in laser deposition for Inconel625/Ti6Al4V graded material. 12
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Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Additive manufacturing of a functionally graded material from Inconel625 to Ti6Al4V by laser synchronous preheating
T
⁎
Wei Menga, Xiaohui Yina, , Wang Zhangb, Junfei Fanga, Lijie Guoa, Qunshuang Maa, Bing Cuia a b
School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan, 243032, Anhui, China School of Material Science and Engineering, Shanghai Dianji University, Shanghai 201306, China
A R T I C LE I N FO
A B S T R A C T
Associate Editor: M. Merklein
This work examines a functionally graded material, fabricated by directed energy deposition additive manufacturing and laser synchronous preheating that grades from Inconel625 to Ti6Al4V. The microstructure evolution, cracking behavior, phase characteristics and microhardness were determined as a function of position within the graded material. The cracks occurred in the transition zone between 80% Inconel625 + 20% Ti6Al4V and 70% Inconel625 + 30% Ti6Al4V for the non-preheated sample due to the formation of massive Cr- and Moenrich phases, while no cracks were found in preheated gradient samples. A series of phase evolutions with the increase of Ti6Al4V occurred: γ, γ + Ni3Ti, Ti2Ni + TiNi + β-Ti, β-Ti + Ti2Ni, α-Ti + β-Ti + Ti2Ni, α-Ti + β-Ti. The maximal hardness obtainable in the 60% Inconel625 and 40% Ti6Al4V deposition layer is determined largely regarding the presence of the various phases. Laser synchronous preheating was an effective measure on improving deposition and crack suppression in laser deposition for Inconel625/Ti6Al4V graded material.
Keywords: FGMs Laser metal deposition Synchronous preheating Cracking Precipitated phase Ti6Al4V Inconel625
1. Introduction Functionally graded materials (FGMs) are a class of material systems whose chemical composition or microstructure changes gradually with the variation of position, enabling performance to change in one or more dimensions. Currently, FGMs have found their place in some fields, especially extreme-environment applications, such as aerospace or nuclear power generation because different spatial locations of the components have different properties, has been reported by Zhou et al. (2004). However, functionally graded parts are generally not available using conventional metallurgical techniques, including vapor deposition (Xenidou et al., 2007), fusion welding (Veerababu et al., 2017), powder metallurgy (Chu et al., 1999), and centrifugation method (Watanabe et al., 2005). Laser directed energy deposition for metal additive manufacturing is a new type of solid-state free-forming process and a new technology for processing large-sized near-net-formed metal components has been applied in many industrial fields. It has been reported by Muller et al. (2013) and Hosson et al. (2000) that this technique also allows the design of material gradients at the microstructure level in laser rapid prototyping due to the small molten pool and the adjustable alloy components. Therefore, a variety of functionally graded materials can be obtained by laser melting deposition, and that, in particular, also contributes to reducing the thermal stress and the occurrence of cracking. ⁎
Recently, laser melting deposition (LMD) method has been used to prepare gradient structural materials for stainless steel, nickel, and titanium alloys (Shah et al., 2014; Wang et al., 2016). However, since LMD is a process based on melting and rapid solidification formation, the development of the undesirable intermetallic or precipitated phases must be solved. FGMs with various alloys constituents have been investigated, including Invar36/Ti6Al4V (Bobbio et al., 2017), TiC/Ti (Liu et al., 2003), Ti/Ti6A12ZrMoV (Liang et al., 2014), Inconel625/ 316L (Koikea et al., 2017), and CoCrMo/Ti6Al4V (Xue et al., 2008). LDM was used to prepare 304L stainless steel to Inconel625 functionally graded materials, and their composition, microstructure, microhardness, and precipitated phase were examined by Carroll et al., 2016. Reichardt et al., 2016 fabricated a Ti6Al4V to 304L functional grade component with a vanadium transition through a multi-hopper laser deposition system. It was found that the precipitation of brittle intermetallic compounds such as Fe-Ti and the formation of Fe-V-Cr sigma phase proved to be the cause of cracking of the gradient component. In addition, the researchers also investigated the gradient of Nibased alloys and Ti alloys. Lin and Yue (2005) conducted in-depth research on Ti-Ni-based FGMs, and the microstructure evolution of pure Ti/Rene88DT, Ti6Al4V/Rene88DT gradient materials were fully revealed. In addition, these have consisted of Ti6A14V graded to a precipitation-hardenable nickel-based alloy such as Inconel 718 and that focused on microstructure and phase evolution of gradient materials
Corresponding author. E-mail addresses: [email protected] (W. Meng), [email protected] (Y. Xiaohui).
https://doi.org/10.1016/j.jmatprotec.2019.116368 Received 3 January 2019; Received in revised form 15 April 2019; Accepted 13 August 2019 Available online 24 August 2019 0924-0136/ © 2019 Elsevier B.V. All rights reserved.
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(Onuike et al., 2018). The nickel-based alloy Inconel625 and Ti6Al4V alloy were chosen for this research, due to their disparate thermal and mechanical properties. As a gradient alloy system, Inconel 625 alloy would be useful in applications that require both strength and corrosion resistance, and that is often used as a transition material for thermal expansion coefficient (TEC) at high temperatures. Ti6A14V is corrosion resistant and has a low density and a high strength to weight ratio compared to nickel-base alloy and correspondingly lower strength to weight ratio (Bobbio et al., 2017). These two alloys, when joined directly through dissimilar fusion welding, result in a joint in which intermetallics could be present and provide a point of weakness or failure (Mortensen and Suresh, 1995). In particular, the large difference in TEC between the two alloys could result in failure under significant temperature excursions as the joint would act as a location of stress concentration. However, because an FGM can provide a gradual change in elemental composition, the properties, including TEC, should change gradually across the gradient, resulting in a lower likelihood of component failure during temperature excursions. Because of this, the Ti6A14V to Inconel625 FGM has potential applications in extreme environment conditions, such as those found in aerospace or nuclear reactors. Preheating is usually considered to be an effective method for suppressing cracks in the welding process due to the release of internal thermal stress, as demonstrated by simulations and experiments by Snchez-Cabrera et al. (2007) and Zhu et al. (2018). In the present work, a FGM, fabricated using fiber laser metal deposition and synchronous preheating that grades from Inconel625 toTi6Al4V. Microstructure evolution, cracking behaviour and microhardness of the FGM were experimentally characterized as a function of position within the nonpreheating and preheating graded materials. The forming mechanism of local cracking for graded deposits under non-preheating were analyzed. The gradient forming feasibility from Inconel625 to Ti6Al4V by fiber laser metal deposition and a synchronous preheating process was finally determined.
Fig. 2. Schematic of the laser directed deposition process.
material, degreased with acetone and ethanol, and then deposited. The gradient components were fabricated in a closed atmosphere controlled deposition system as shown in Fig. 2. During the deposition process, high purity argon gas was introduced into the atmosphere box to eliminate air, and maintain the inert gas in the chamber. Experimental studies were performed using fiber laser deposition system. 6.0 kW IPG fiber laser was used with a focal length of 300 mm and 1064 nm wavelength for metal deposition, and the fiber laser was integrated with a KUKA robot. The powder feeding system was equipped with dual powder tubes. Powders were transported from the coaxial nozzle through pure argon to the molten pool. The Ti6Al4V and Inconel 625 powders were weighed separately and then mixed for 30 min using a mechanical stirrer according to designed weight percentage. Gradient layer deposition with different composition proportion was performed using the optimized process parameters. Pure argon gas was used as the powder feed gas and the shielding gas, and the flow rate was 5 L/min. During the laser deposition process, the laser scanning speed was 0.48 m/min, the laser power was 850 W, and the powder mass flow rate was maintained at 16 g/min. The deposition moved 15 mm in parallel along the scanning direction, and the z-direction for each layer deposition moved by 0.3 mm and the x-direction offset by 0.62 mm. Fig. 3 shows laser direct deposited Inconel625/Ti6Al4V functionally gradient samples under non-preheating and preheating. As shown in Fig. 3, the square block samples, 12.5 mm wide and 60 mm tall, was built on a substrate of 316L stainless steel. Initially, sixteen layers of 100% Inconel625 alloy were deposited on the substrate. In the graded region, the weight of Inconel625 powder was reduced by 10% and
2. Experimental procedure The deposited material used for this experiment consisted of Inconel625 powders and Ti6Al4V powders with the diameter ranging from 45 μm to 75 μm and 25 μm to 55 μm, respectively. The SEM morphology of Inconel 625 and Ti6Al4V alloy powders is shown in Fig.1. The chemical components of Inconel625 and Ti6Al4V powders were 0.01% C, 0.37% Mn, 0.32% Si, 21.53% Cr, 65.85% Ni, 8.47%Mo, 3.22%Nb, 0.13%Fe and 89.12% Ti, 6.63% Al and 4.11% V, respectively. A 10 mm thick 316L stainless steel plate was used as the base
Fig. 1. SEM image shows the characteristic morphology of (a) Inconel 625 alloy powders and (b) Ti6Al4V alloy powders. 2
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Fig. 3. Direct laser metal deposited Inconel625-Ti6Al4V functionally graded samples (a) Non-preheating, (b) Preheating.
Ti6Al4V powder increased by the same amount in each successive layer, and 16 layers for each gradient component. Nineteen layers of 100% Ti6Al4V were deposited on top of the gradient zone. Overall, the graded sample was divided into 11 different composition zones, which consisted of 100% Inconel625 the bottom of the sample, Inconel625/ Ti6Al4V gradient region at the middle and 100% Ti6Al4V at the top of the sample. The surface appearance of the gradient samples with nonpreheating and preheating were shown in Fig. 2. Fig. 4 shows a schematic diagram of the synchronous preheating process for Incone625/Ti6Al4V graded sample. As shown in Fig. 4(a), the substrate was firstly preheated by laser before deposition, and the preheating area was 100 mm × 100 mm, and the preheating time was 5 min. Owing to the benefits exhibited the thermal accumulation effect by laser preheating, the small temperature gradient could be guaranteed in the laser initial depositing as shown in Fig. 4(b). For the purpose of decreasing cooling speed, the laser was still heated to the gradient layer after depositing a powder of 11 layers, and that remained off for approximately 3 min each time that a new powder mixture was switched as shown in Fig. 4(c). After the Inconel625/Ti6Al4V gradient sample was prepared, the top would keep heating for 5 min by the laser for its temperature addition. The synchronous preheating parameters in laser deposition process were shown in Table 1. Although this preheated method added to the overall machine time, discontinuous processes have been shown to be beneficial for reducing residual stresses and distortion. Prepared square post specimens were sectioned in a transverse plane by wire cut electric discharge machine, defatted with acetone and polished to 2000 grit size. Then the samples were polished with abrasive paper and diamond slurry, and then the graded specimens were electrochemical etching (10% oxalic acid, 6V for 25 s) to reveal the microstructure. The graded microstructure was observed by an optical microscope (Leica S6 D) and SEM (FEI Quanta 200). X-ray diffraction
Table 1 The synchronous preheating process parameters. Preheating position
Laser power (W)
Scanning speed (m/s)
Offset Δx (mm)
Uplift ΔZ (mm)
Time (min)
Substrate Graded zone Top
650 650 850
0.008 0.008 0.008
0.62 0.62 0.62
0 0.3 0.3
5 3 5
(XRD) patterns were collected using a Bragg-Brentano-type diffractometer (Panalytical Empyrean) with a Cu K-α X-ray source operated at 45 kV and 40 mA (a = 1.54 Å). Microhardness was measured on the cross section of built gradient block using Leco V-100-C1 microhardness tester with a load of 100 g and a dwell time of 15 s. 3. Results and discussion 3.1. Cracking behaviour The appearance of the gradient sample under no-preheating conditions was observed by an optical microscope, and a plurality of cracks was found in the partial deposition layer close to the substrate. Fig. 5 shows the surface morphologies of cracks in gradient zone and EDS analysis of the secondary phase on the crack surface. As seen in Fig. 5(a), the cracks occurred in the transition zone between 80% Inconel625 + 20% Ti6Al4V and 70% Inconel625 + 30% Ti6Al4V, namely transverse crack parallel to the horizontal plane. Moreover, it was to be noted that the surface of macro cracks was a straight transgranular extension, and that presented the characteristics of cold cracking. Fig. 5(b) and (c) shows a high magnification morphology of crack surface and the EDS spectra of the precipitated phase. A large number of massive phases were observed on the crack surface and
Fig. 4. Schematic diagram of laser synchronous preheating in laser deposition process (a) Preheating for the substrate, (b) Laser deposition, (c) Preheating for gradient zone. 3
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Fig. 5. The crack for the non-preheated sample (a) surface appearance of transverse crack, (b) SEM image of the secondary phase on the crack surface, (c) EDS spectrum of the secondary phase, (d) Composition comparison for the precipitated phase and deposition layer.
obvious. Based on analysis, the fracture pattern of the Inconel625/ Ti6Al4V gradient specimen was confirmed as typical brittle cleavage. Xu et.al (2007) published a work on the trend of thermal expansion coefficient gradients of Ti-Ni alloy as shown in Fig. 7. The difference of TEC near both ends of the composition is greater than that of the similar proportion Ti and Ni zone under the same gradient conditions. Furthermore, the difference of TEC of the high nickel region is greater than that of high Ti region, and results in relatively large thermal stress near high Ni zone. Overall, the reasons for the cracking are chiefly attributable to low-melting eutectic formed in the grain along with the segregation elements such as Cr and Mo, and high internal stress in the gradient zone adjacent to the substrate. Therefore, it is of great importance to reduce the hard brittle phase and internal stress in the laser depositing for Inconel625/Ti6Al4V gradient materials. In order to reduce or avoid cracks, the preheating has also been proposed as a potentially effective measure to reduce the thermal stress and improve the morphology and distribution of the precipitated phase. According to the trend of the average thermal expansion coefficient of the uniform composition Ti-Ni alloy in Fig. 7, the Ni content gradually increases with the depositing when the composition gradient direction is from Ti to Ni, and the heat shrinkage also increases with the increases of the deposition height, resulting in an increasing tendency of cracks. On the contrary, when the composition gradient direction is Ni to Ti, the thermal expansion coefficient decreases with the depositing. Although the thermal stress also accumulates, the cumulative velocity is less compared to the former, resulting in less cracking tendency. Therefore, the deposition direction of the gradient layer chosen in this paper is from Inconel625 to Ti6Al4V. The cracking of Inconel625/Ti6Al4V gradient samples appeared in
nearby, which propagated along aggregation zone of the block phases. A block phase particle denoted by the arrow in Fig. 5(b) was analyzed by EDS with the spectrum. It can be seen that the Cr and Mo content of the precipitated phase was significantly higher than that of the deposition layers, thus, the block phases was identified as the Cr- and Moenriched phase as shown in Fig. 5(c) and (d). In addition, the massive phase was cut off by the crack under tensile stress together with a little displacement. The formation and aggregation of Cr- and Mo- enrich phases were considered as the essential internal condition for the cracking. EDS analysis were used to determine the deposited chemistry as a function of position in comparison with the anticipated composition. Fig. 5(d) shows the nominally anticipated and experimentally measured compositions, in weight percent, of the constituent elements present in the FGM for the non-preheated samples. Overall, the actual composition of the deposition layer did not deviate significantly from the anticipated composition. However, there was a significant amount of fluctuation in the measured elemental composition including Cr, Mo and Nb. These fluctuations were due to elemental segregation into separate phases, which were observed in the EDS maps and were discussed below. In the metallographic sample EDM cutting, the non-preheated gradient sample broke up, and fractured at the interface between 80% Inconel625 + 20% Ti6Al4V and 70% Inconel625 + 30% Ti6Al4V. Fig. 6 shows the macroscopic and microscopic morphology of the fracture. As shown in Fig. 6(a), the macroscopic fracture was smooth and bright, and appeared as brittle rupture characteristics. The microscopic topography of the fracture presented river patterns and step features as shown in Fig. 6(b)–(d), and the tearing edges were not 4
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Fig. 6. Fracture morphology characteristics of the non-preheated gradient sample (a) morphology, (b)–(d) microscopic topography.
σ=
E ΔαΔT 1−μ
Where σ is the internal stress, E is Young's modulus, Δα is the difference of the thermal expansion coefficients between the graded layers, and ΔT is the difference of the deposition temperature and room temperature, μ is the Poisson's ratio. According to the strength theory, brittle fracture occurs when the maximum stress is greater than the strength limit of the material, including the bond strength between the gradient interface and the strength of the enriched precipitate phase. The maximum stress is more than the bonding strength between the disposition layers, resulting in a transverse crack as shown in Fig. 5. From the calculation formula, the difference of the temperature gradient and the thermal expansion coefficient between the deposited layers is an important factor in the increase of the internal stress. From the above analysis, the difference of the temperature gradient and the TEC between the deposited layer and the substrate is a main factor affecting the internal stress. Previous researches (Chen et al., 2017; Zhang et al., 2014) has shown that the internal stress could be relieved by the preheating or ultrasonic vibration method. In this paper, a synchronous preheating process was used to reduce the temperature gradient and internal stress between deposition layers.
Fig. 7. The trend of thermal expansion coefficient gradients of Ti-Ni alloy.
the region near the substrate because of the high temperature gradient. In laser directed deposition for dissimilar materials, the composition difference of the gradient transition interface is small, resulting in change slightly of thermal expansion coefficient, elastic modulus, and Poisson's ratio. However, the difference of physical property parameters at the interface still leads to inconsistency in expansion and contraction and large stress between the layers, and the resulting internal stress is calculated as follows (Chen et al., 2017):
3.2. Microstructure morphology In order to investigate the effect of laser synchronous preheating on cracks and microstructure, the metallographic morphology of the 5
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Fig. 8. The optical images of microstructures at different locations along the direction from Inconel625 to Ti6Al4V for the preheated sample (a)–(k) Microstructure morphology, (l) the observation positions of the optical microscope.
between the dendrites. As 20% Ti6Al4V was reached, the microstructure of the deposition layer consisted of block-like γ phase (white), a dense granular phase (intermetallic, grey) and eutectic phase (black) in Fig. 8(c). It was to be noted here that the dendritic morphology had disappeared, while a small amount of intermetallic and eutectic phase formed. When Ti6Al4V increased to 30%, the microstructure mainly composed of a large number of star-shaped bulk segregated phases (white) and intermetallic compound matrix structure as shown in Fig. 8(d). A small quantity of star-shaped bulk phases distributed on the matrix when the content of Ti6Al4V was increased to 60%. A new irregular anomalous eutectic phase was found in Fig. 8(e). Microcracks appeared in the upper part of the deposition layer, and propagated along the star-shaped phases distributed on the surface. As increased to 50% Ti6Al4V, the petal-shaped phases (Dotted line) appeared, and interdendritic massive phases formed, while the twophase dendritic eutectic phase clearly grew, which was schematically illustrated in Fig. 8(f)). With the further increase of Ti6Al4V, although the irregular eutectic phase remained, the eutectic morphology changed
different deposited layers for the preheated Inconel625/Ti6Al4V graded samples was observed by optical microscopy. The observations of the optical microscope for the different deposited layers are shown in Fig. 8. The gradient samples were sectioned in a gradient direction by wire cut electric discharge machine, and the cutting surface was 3 mm away from the surface. The microstructure of the central portion of each deposited layer was observed as shown in Fig. 8(a)–(k), corresponding to the a–k regions in Fig. 8(l). Fig. 8 shows the optical image of microstructure characteristics of the preheated gradient sample. As shown in Fig. 8(a), γ columnar grain in 100% Inconel625 deposit was directed towards the energy source and transitioned into the gradient interface. With the addition of 10% Ti6Al4V, the γ columnar dendrite was transformed into equiaxed dendrite (white) in Fig. 8(b). The black zone around the dendrite was presumed to be weakly formed brittle intermetallic phases, and Cr, Mo and Nb could be segregated as secondary particles or other new phases, such as Laves formed. As experienced multiple thermal cycles, the equiaxed dendrite tended to coarsen and formed eutectic phases 6
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greatly. The eutectic dendrites appeared when Ti6Al4Vwas increased to 60% in Fig. 8(g). When Ti6Al4V was increased to 70%, the dendrites were refined as shown in Fig. 8(h). Under this condition, irregular eutectic abnormal solidification was observed from Fig. 8(h). With the further increase of Ti6Al4V content, the changes from cellular crystal to dendritic growth occurred, while the lath phases disappeared. Also, the precipitates ceased to exist within the prior grains. When the Ti6Al4V content reached 80%, the dendritic growth had been stabilized in Fig. 8(i). When the Ti6Al4V content was increased to 90%, the microstructure of the deposited layer consisted of a needle-like morphology phase and a block phase in an orthogonal orientation in Fig. 8(j). When the content of Ti6Al4V was 100%, the microstructure exhibited columnar cellular growth, and the different orientations of Widmanstatten α-Ti laths were found in the prior β-grains in Fig. 7(k). The uniformity of composition distribution had a great influence on the microstructure evolution, and the composition of different gradient layers for the preheated Inconel625/Ti6Al4V gradient samples was analyzed by EDS. To mark the locations for the EDS analysis, a series of indentations were formed by a microhardness tester along the compositional gradient, which were used to determine the deposited chemistry as a function of position. Moreover, five EDS points were tested for each deposition layer at intervals of 2 mm, and an average of composition for five points was obtained. Fig. 9 shows the average composition of the EDS analysis along the vertical direction of the graded deposit. As shown in Fig. 9, the element Ni, Ti, Al and V in graded samples presented a good linear relationship with the increase of Ti6Al4V, while for the other elements, such as Cr, Mo, Nb, the resulting alloy exhibited a certain degree of nonlinearity. According to the phase evolution analysis above, the reasons for the nonlinearity of some alloy elements could be the formation of many precipitated phases, and another possible explanation was the variation in element loss that occurred during laser depositing.
Fig. 10. Phase transformation based on the binary phase diagram of Ti-Ni.
stable intermetallic compounds. With the increase of Ni content, β-Ti phase decreases, whereas the β-Ti + Ti2Ni eutectic phase increase. In the equivalence of Ni-Ti proportion, TiNi phases are produced with accompanied by Ti2Ni. With further increase of Ni, TiNi3 phases are presented and then transform to γ phase. Based on analysis, the phase evolution of Inconel625/Ti6Al4V gradient materials can be forecast according to The Ti-Ni binary system. To determine the phase constitution of laser deposited layers, XRD analysis was performed, and the results were shown in Fig. 11. With the addition of Ti6Al4V, intermetallic compounds such as Ni3Ti, NiTi, and NiTi2 became the main phases in the Inconel625/Ti6Al4V graded material. It could be seen from the XRD pattern in Fig. 11 that a series of phase evolutions along the composition gradient were consistent with the Ti-Ni phase diagram. The observations and analysis of XRD, OM and SEM could be used to illustrate the phase transition in different components of the graded material. In order to investigate the effect of the laser synchronous preheating on the evolution of the phases that formed, the SEM and the EDS elemental composition maps must be examined in different deposition layers for the non-preheated and preheated samples. Fig. 12 and Fig. 13 show SEM images of microstructure morphology of the different gradient layer for non-preheated and preheated samples, respectively. As shown in Figs. 10(a) and 11 (a), at the composition of 90% Inconel625 + 10% Ti6Al4V, a noticeable change in microstructure
3.3. Phase evolution For the Ti-Ni-Cr and Ti-Mo-Cr alloys, as well as Inconel625/Ti6Al4V gradient materials, the main phases are a Ti-rich solid solution and (Ti, Ni) compound. Fig. 10 is a phase diagram of the Ti-Ni system, which was first proposed by Murray et al. (1987) and modified the extension of the possible phase fields by Nagarajan et al. (1994). The phase diagram illustrates that there exists a eutectic reaction between β-Ti and Ti2Ni, a peritectic reaction between Ti2Ni and TiNi, and a eutectoid decomposition of β-Ti. There are four phase regions in the Ti-Ni binary alloy, with six stable phases and two sub-stable phases. The possible phases in Ti-Ni gradient materials mainly include α-Ti, β-Ti, Ti2Ni, TiNi, TiNi3, Ti3Ni4, γ, and TiNi, Ti2Ni and TiNi3 are the three main
Fig. 9. The compositional gradient of the preheated Inconel625/Ti6Al4V graded sample.
Fig. 11. XRD analysis result of the typical laser deposited layer on the outmost surface of the preheated gradient sample. 7
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Fig. 12. SEM images of microstructure morphology of laser deposition layers in different gradient layer for the unpreheated samples.
occurred: γ columnar dendrites in 100% Inconel625 transform into a complex multiphase structure. The microstructure mainly consisted of γ equiaxed crystals and η (Ni3Ti) phase precipitated around γ phase, and both were about equal. γ + Ni3Ti lamellar eutectic phase (white) was generated together with γ phase. These phases in preheated sample grew bigger than that of the non-preheated sample. Moreover, a small platelet-like phase was also found in the interdendritic regions. Fig. 13(e) and (f), Fig. 15 shows EDS spectra of precipitated phase and deposition layers for the preheated and non-preheated samples. It was obvious that the Cr and Mo content of the massive phases were higher than the average composition of the base material by EDS analysis. As shown in Fig. 12(b) and (b), for 80% Inconel625 + 20% Ti6Al4V, γ equiaxed morphology degenerates, and a small amount of TiNi+η eutectic appeared around γ block-like crystal. Though phase morphology of the preheated sample was less different compared with the nonpreheated sample, more eutectic phases were produced for the latter. Cr- and Mo-rich phases formed on the TiNi (black) matrix as shown in Fig. 13(f) and Fig. 14(b). Fig. 13(c) and (d) shows the SEM images of the deposition layers of 70% Inconel625 + 30% Ti6Al4V and 60% Inconel625 + 40% Ti6Al4V for preheated samples. Microstructure of both mainly consisted of η
(Ni3Ti) and TiNi phase, and there was little difference in morphology. The microstructure was dominated by the primary TiNi equiaxed dendrites, and dense TiNi+η eutectic was produced between the dendrites. As seen in Fig. 13, polygonal Cr- and Mo-enrich phases were found in TiNi matrix phase, and that of 60% Inconel625 + 40% Ti6Al4V deposition layer were more. Moreover, a flower-like eutectic growth coupled with a TiNi matrix phase was also found at the interdendritic regions. It was interesting to note that the flower-like eutectic usually grew around the Cr and Mo enriched phase. As shown in Fig. 12, the microstructure of the unpreheated sample was similar to that of the preheated sample, while the former had a smaller grain size. It should be noted in Fig. 12(d) that the crack propagation path was along the Cr and Mo enriched phase and nearby, and that was tortuous. Therefore, Crack formation is closely related to the Cr- and Mo-enrich phases. The Cr and Mo enriched phase is a hard and brittle phase, and that have greatly effect on the mechanical properties. The size and morphology of Cr- and Mo-enrich phases are determined by high temperature residence time, growth rate and composition conditions. In the laser depositing for the Inconel625/Ti6Al4V gradient material, the Crand Mo-enrich phases grew up with the increase of laser deposition layer due to heat accumulation effect. As shown in Fig. 14, the Cr-and 8
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Fig. 13. SEM images of laser deposition layers for the preheated sample. (a)–(d) Microstructure morphology, (e) and (f) EDS spectra of secondary phase as shown in Fig.13(a) and (d).
Mo-enriched phases were distributed on the γ-Ni matrix, and that were obviously observed between the 90% Inconel625 + 10% Ti6Al4V and 60% Inconel625 + 40% Ti6Al4V deposition layers. As shown in Fig.15, EDS analysis showed that an average composition of Cr- and Mo-enrich phases with the deposition layers added 10% Ti6Al4V, 20% Ti6Al4V and 30% Ti6Al4V presented as Cr38.9Mo16.2Ni37.2Ti4.36Nb 2.5(at. %), Cr64.5Mo18.5Ni10.2Ti3.3Nb1.7 (at. %) and Cr66.3Mo18.3Ni4.5Ti5.1Nb 1.6(at. %), and the Cr and Mo atomic ratio of the precipitated phases was 2.4–3.6. Unfortunately, the diffraction peaks of the Cr- and Mo-enriched phases in the XRD pattern were not found, and that was
Mo-enrich phase presented a massive star shape, while a large number of eutectic phases appeared at the star-like tip. The segregation easily generated with the high content of Inconel625, resulting in the enrichment of local elements such as Cr and Mo. With the formation of the Cr- and Mo-enrich phase, the solidification tip frontier evolved into poor Cr and Mo region, resulting in the increase of other elements such as Ni and Ti. The formation conditions of eutectic precipitation were met, thus, many flower η+TiNi eutectic phases were produced at the tip of star-like Cr- and Mo-enrich phase in Fig. 14. Observed by optical and scanning electron microscopy, the Cr- and 9
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Fig. 14. The typical morphology of precipitated phase for the preheated sample and eutectic with (a) 70% Inconel625 + 30% Ti6Al4V and (b) 60% Inconel625 + 40% Ti6Al4V.
Fig. 15. (a) and (b) EDS analysis of the secondary phase for the unpreheated samples. (c) Composition comparison for the precipitated phase and the deposition layer. (d) Ni-Cr-Mo equilibrium phase diagram.
analysis. Fig. 16 shows the phase evolution of the composition ranging from 50% Ti6Al4V to 100%Ti6Al4V. For 50% Inconel625 + 50% Ti6Al4V deposit, the microstructure consisted of the dispersion TiNi + β-Ti dendrites on Ti2Ni matrix and TiNi + Ti2Ni lamellar eutectic structure. Some fishbone-like two-phase dendritic phases of β-Ti + Ti2Ni appeared in the interdendritic zones as shown in Fig. 16(a). The similar structures in some eutectic alloys were found by Villaret et al. (2016). For the composition 40% Inconel625 + 60% Ti6Al4V deposit, a class of anomalous eutectic structures, characterized by the appearance of a
qualitatively inferred to Cr3Mo based on the element contents. In addition, the multicomponent phase diagrams were used to determine the phases that precipitated during fabrication as a function of position in the gradient. It predicted by Perricone et al. (2003) that the formation of four secondary phases as shown in the Ni-Cr-Mo equilibrium phase diagram in Fig. 15(d). The Ni-Cr-Mo formula based on element contents could be used to estimate the precipitated phase of the different deposition layers. From the EDS analysis in Fig. 15, the composition range of Ni-Cr-Mo was found to be in the (Cr, Mo) phase region as shown in Fig. 15(d) (shadow zone), which coincided with the result of EDS 10
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Fig. 16. SEM images of microstructure morphology of laser deposition layers for the preheated samples.
granular β-Ti phase on a continuous Ti2Ni matrix. The microstructure mainly included two types of dendrites, a coarse and large volume fraction of TiNi dendrites, and the other small volume fraction of β-Ti dendrites in Fig. 16(b). For 30% Inconel625 + 70% Ti6Al4V, the microstructure consisted of a new two-phase TiNi + Ti2Ni dendritic growth. The β-Ti particles were remarkably refined and discretely distributed on the continuous network of Ti2Ni matrix, showing anomalous eutectic growth in Fig. 16(c). When 50% Ti6Al4V to 70% Ti6Al4V was added to the deposition layers, the microstructure of the unpreheated sample had a slight difference from that of the preheated sample, only the volume fraction of each phase was different. Fig. 16(d) and (e) shows the microstructure of a section close to 100% Ti6Al4V for the preheated sample. The microstructure of both was composed of β-Ti dendrites and intergranular β-Ti + Ti2Ni eutectic. For the composition 20% Inconel625 + 80% Ti6Al4V, β-Ti presented as an equiaxed dendritic morphology, and a rod-shaped βTi + Ti2Ni eutectic phase between the β-Ti dendrites was produced, which was similar to the microstructure of the unpreheated sample in Fig. 12(h). As shown in Fig. 16(e), when the composition reached 10% Inconel625 + 90% Ti6Al4V, a small quantity of β-Ti + Ti2Ni eutectoid phase had formed at the prior β grain boundaries, nevertheless, more βTi + Ti2Ni eutectoid phases were produced around the β grain for the non-preheated sample in Fig. 12(i). The depositional structure of 100% Ti6Al4V was composed of coarse primary β-Ti equiaxed crystals, and the disisotropic lath or acicular α-Ti distributed in the interior of the βTi grains, presenting typical Widmanstatten structure morphology in Fig. 16(f). For the purpose of further understanding the phase transition as a function of position within the graded material, Table 2 provides a summary of the phase constitution of the deposited layers in different compositions, and compare with the Ni-Ti binary phase diagrams. As shown in Table 2, the matrix phase of the deposited layer was substantially consistent with the phase obtained by the phase diagram. A series of phase evolutions of the preheated samples with the increase of Ti6Al4V occurred: γ, γ + Ni3Ti, Ti2Ni + TiNi + β-Ti, β-Ti + Ti2Ni, αTi +β-Ti + Ti2Ni, α-Ti + β-Ti. However, Cr and Mo enrich precipitated
Table 2 Phase evolution of phase diagram and experiments at different Ti-Ni ratios. Ni-Ti constituent
Phase diagram
Experimental results
Ti Ti-10%Ni Ti-20%Ni Ti-30%Ni Ti-40%Ni Ti-50%Ni Ti-60%Ni Ti-70%Ni Ti-80%Ni Ti-90%Ni Ni
β-Ti + α-Ti β-Ti + α-Ti + Ti2Ni α-Ti + β-Ti + (β-Ti + Ti2Ni) (Ti2Ni + β-Ti) + α-Ti Ti2Ni + TiNi + β-Ti TiNi + Ti2Ni + β-Ti TiNi + TiNi3 + Ti3Ni4 TiNi3 + TiNi TiNi3 + TiNi γ+(γ + TiNi3) γ
β-Ti + α-Ti β-Ti +(β-Ti + Ti2Ni) β-Ti+(β-Ti + Ti2Ni) (Ti2Ni + β-Ti) + TiNi Ti2Ni + TiNi + β-Ti TiNi + Ti2Ni + β-Ti TiNi + TiNi3 + (Cr, Mo) TiNi3 + TiNi + (Cr, Mo) TiNi3 + TiNi + γ + (Cr, Mo) γ + (γ + TiNi3) + (Cr, Mo) γ + NbC
phases were found in the Inconel625/Ti6Al4V gradient material due to the segregation of Cr and Mo elements, which had an important influence on the microhardness and crack of the Inconel625/ Ti6Al4V gradient material. 3.4. Microhardness Fig.17 shows the microhardness distribution of different deposited layers from the substrate to the top. The tests of Vickers microhardness were carried out five times for each deposited layer, and the measurement points of hardness were set at 2 mm intervals across the boundary. The average hardness of 5 points for each layer are shown in Fig. 17. As a function of the number of layers from the substrate, the hardness first increased with the increase of Ti6Al4V alloy. When the composition reached 70% Inconel625 + 30% Ti6Al4V deposition layer, the hardness reached the maximum value. An average value of about 821.8 HV in the 70% Inconel625 + 30% Ti6Al4V was detected, and the middle deposition layers had a much higher hardness compared with the bottom and the upper of the gradient sample. Moreover, it could be seen from the standard deviation in Fig. 1 that the distributions of hardness in each layer were uniform. In addition, the hardness 11
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Acknowledgements This work was supported by the University Scientific Research Project of Anhui Province Education Office No. KJ2018A0063, Anhui Provincial Natural Science Foundation NO. 1908085QE198 and the National Science Foundation of China under Grant No. 51505271 and No. 51375015. References Bobbio, L.D., Otis, R.A., Borgonia, J.P., Dillon, R.P., Shapiro, A.A., Liu, Z.K., Beese, A.M., 2017. Additive manufacturing of a functionally graded material from Ti-6Al-4V to Invar: experimental characterization and thermodynamic calculations. Acta Mater. 127, 133–142. Carroll, B.E., Oti, R.A., Borgonia, J.P., Suh, J., Dillon, R.P., Shapiro, A.A., Hofmann, D.C., Beese, A.M., 2016. Functionally graded material of 304L stainless steel and Inconel 625 fabricated by directed energy deposition: characterization and thermodynamic modeling. Acta Mater. 108, 46–54. Chen, Y.J., Yue, T.M., Guo, Z.N., 2017. Laser joining of metals to plastics with ultrasonic vibration. J. Mater. 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Fig. 17. Microhardness distributions of each graded layer from the substrate to the top deposited layer for the preheated and unpreheated samples.
distribution of the unpreheated sample was consistent with that of the preheated sample, and the former was slightly higher than the latter, which may be due to the coarser phase structure for the preheated sample. Based on the results of the analysis of the microstructural changes, it could be concluded that two possible factors brought the initial rising of the microhardness with the increase of Ti6Al4V. One was the intermetallic compounds of Ni, Ti and other elements. Intermetallic compounds such as NiTi, Ni3Ti and Cr- and Mo-enrich phases having high hardness were found in the deposition layer, which was accompanied by the decrease of γ phase. The other was that the rapid solidification of the molten pool might capture more alloying elements in the solid solution phase of titanium and resulted in more effective solid solution strengthening. As can be seen from Fig. 12(c), the volume fraction of the intermetallic compound was higher, and the increase in microhardness was mainly related to the formation of intermetallic compounds. The Ti2Ni laths and Ti2Ni particles with a fine eutectoid lamellar structure within the TiNi dendrites were dispersed in the maximum microstructure region. The decrease in the hardness from the 70% Inconel625 + 30% Ti6Al4V deposition layer was due to the reduction of the Cr- and Mo-enrich phase and the disappearance of the eutectoid layered structure within the γ phase. Then, with the changeover from NiTi + Ti2Ni to β-Ti, the hardness decreased further. The drop in hardness in the10%Inconel625 + 90%Ti6Al4V zone that was formed towards the end of the deposition may be attributed to the formation of coarse β-Ti dendrites resulting from a decrease of thermal gradient due to the accumulation of heat as the deposit grew thicker.
4. Conclusion A graded component without sharp compositional boundary and crack was successfully fabricated from Inconel625 toTi6Al4V using directed energy deposition and laser synchronous preheating, with 10% composition interval. For the non-preheated samples, the cracks occurred in the transition zone between 80% Inconel625 + 20% Ti6Al4V and 70% Inconel625 + 30% Ti6Al4V due to the formation of massive Cr- and Mo-enrich phases. A series of phase evolutions with the increase of Ti6Al4V occurred: γ, γ + Ni3Ti, Ti2Ni + TiNi+β-Ti, β-Ti + Ti2Ni, αTi + β-Ti + Ti2Ni, α-Ti+β-Ti. The maximal hardness obtainable in the 60% Inconel625 and 40% Ti6Al4V deposition layer is determined largely regarding the presence of the various phases. This study demonstrates laser synchronous preheating is an effective measure on improving deposition and crack suppression in laser deposition for Inconel625/Ti6Al4V graded material. 12