The most influential microstructural parameter that affects mechanical properties of LPBF fabricated lamellar structures is the α/α’ width35,64,76,96,117. Under extremely fast cooled condition, the effective slip length for dislocation is limited to the individual martensite width rather than the α colony size in thermomechanically processed Ti-6Al-4V1. The relationship between the strength and α/α’ width can be described by Hall-Petch relation as shown in Fig. 13a, in which a linear relationship can be established between the yield strength and the inverse square root of α/α’ width. Such a relationship is still valid after post-process annealing and HIP treatments in α+β phase field according to Fig. 13a. In general, the ductility exhibits an inverse relationship with the yield strength as shown in Fig. 13b. The as-fabricated LPBF Ti-6Al-4V has a fine α’ martensitic microstructure118, which results in a high strength but low ductility as shown in Fig. 13b and Table 1. Post-process heat treatments and HIP can promote the martensite decomposition to α+β lamellae, which reduce the strength and increase the ductility. A recent study found that heat treated LPBF Ti-6Al-4V with an equilibrium microstructure possesses a superior ductility (elongation at fracture (El.) > 18%) similar to that of wrought Ti-6Al-4V76. Details on the martensite decomposition in post-process treatments have been summarized in the previous section of annealing. With respect to the lamellar microstructure, the overall trend is a higher heat/HIP treatment temperature (below the β transus) and/or a longer heat/HIP treatment time lead to a lower strength but a higher ductility as shown in Table 1.
Fig. 13 The effect of microstructure on tensile properties.a A Hall-Petch relation between yield strength and the width of α’ or α laths in Ti-6Al-4V with a lamellar microstructure fabricated by LPBF31,54,63,64,94,112,119, LPBF and post-process heat treatment16,31,120,121,60,63,76,83,84,94,103,119, and LPBF and post-process HIP31,60,103,112,113,119,122. b Yield strength vs. total elongation to failure for Ti-6Al-4V fabricated by LPBF with α’ lamellar microstructures[16,31,54,60,63,76,84,119,120, α+β lamellar microstructures31,63,121,123,124,64,83,84,94,96,103,119,120, and bi-modal microstructures102,104,110.
Condition & Ref. Platform heating (°C) Post-LPBF treatment Microstructure YS (MPa) UTS (MPa) El. (%) As-LPBF
100 − α’ lamellar 1008 (V) 1080 (V) 1.6 (V) 116 100 − α’ lamellar − 1315 ± 16 (V) 4.0 ± 1.2 (V) 31 70 − α’ lamellar 1065 (H)
120 − − α’ lamellar 1055 (H) 1098 (H) 6.1 (H) 16 500 − α’ lamellar 1137 ± 20 (H)
926 ± 47 (V)
1206 ± 8 (H)
1116 ± 25 (V)
7.6 ± 2 (H)
1.7 ± 0.3 (V)
54 − − α’ lamellar 1140 ± 35 (H) 1130 ± 31 (H) 7.6 ± 1.1 (H) 76 100 − α’ lamellar 1040 ± 11 (H) 1201 ± 10 (H) 9.5 ± 0.2 (H) 63 − − α’ lamellar 978 ± 5 (H)
967 ± 10 (V)
1143 ± 6 (H)
1117 ± 3 (V)
11.8 ± 0.5 (H)
8.9 ±0.4 (V)
64 ,* 200 − α+β lamellar 1100 (V) - 11.4 (V) 96, * 200 − α+β lamellar 1022 ± 10 (V) 1109 ± 10 (V) 12.7 ± 2.1 (V) Heat treated
− 670 °C/5 h Partially decomposed α’ lamellar 1015 (V) 1090 (V) 10 (V) 103 − 700 °C/2 h/FC Partially decomposed α’ lamellar 1051 (V) 1115 (V) 11.3 (V) 63 − 730 °C/2 h/FC Partially decomposed α’ lamellar 958 ± 6 (H)
937 ± 9 (V)
1057 ± 8 (H)
1052 ± 11 (V)
12.4 ± 0.7 (H)
9.6 ± 0.9 (V)
76 100 700 °C/2 h/FC Partially decomposed α’ lamellar 1012 ± 9 (H) 1109 ± 10 (H) 9.5 ± 0.2 (H) 116 100 800 °C/2 h/FC Partially decomposed α’ lamellar - 1228 ± 32 (V) 8.0 ± 1.5 (V) 76 100 800 °C/6 h/FC α+β lamellar 937 ± 4 (H) 1041 ± 5 (H) 19 ± 1 (H) 124, ** − 920 °C/2 h/FC α+β lamellar 850 (V) 933 (V) 15 (V) 112 100 1050 °C/2 h/FC α+β lamellar 798 (V) 956 (V) 11.6 (V) 116 100 1050 °C/2 h/FC α+β lamellar − 986 ± 45 (V) 13.8 ± 0.8 (V) HIP treated
− 900 °C/100 MPa/2 h α+β lamellar 885 (V) 973 (V) 19 (V) 60 − 920 °C/100 MPa/2 h α+β lamellar 850 (V) 960 (V) 14 (V) 116 100 920 °C/100 MPa/2 h α+β lamellar − 1089 ± 26 (V) 13.8 ± 1.3 (V) 124, ** − 920 °C/120 MPa/2 h α+β lamellar 839 (V) 941 (V) 19 (V) 123 ,** − 930 °C/100 MPa/4 h α+β lamellar 866 ± 50 (H)
865 ± 3 (V)
938 ± 43 (H)
936 ± 4 (V)
14 ± 2 (H)
22 ± 2 (V)
116 100 1050 °C/100 MPa/2 h α+β lamellar − 1007 ± 15 (V) 13.5 ± 0.7 (V) Heat treated
− 910-930 °C/8 h/WQ + 750 °C/4 h/FC α+β bi-lamellar/bi-modal ~900 (V) ~950 (V) ~18 (V) 110 150 Thermal cycling between 975 °C and 875 °C/24 h/AC α+β bi-modal 865 ± 19 (H)
849 ± 12 (V)
1017 ± 16 (H)
1004 ± 23 (V)
18 ± 1 (H)
16 ± 1 (V)
102 − 900 °C/100 h/AC α+β bi-modal ~1080 (V) ~1120 (V) ~20 (V) * Specimens were fabricated on support structures. ** Specimen surface was polished.
Table 1. Microstructure and mechanical properties of LPBF fabricated Ti-6Al-4V in as-printed, heat treated, and HIP treated conditions. Air Cool (AC) and Furnace Cool (FC) are cooling methods after heat treatment. YS-Yield Strength, UTS- Ultimate Tensile Strength, and El.- Elongation at fracture are measured tensile properties.
In addition to the lamellar microstructure, another bi-lamellar/bi-modal microstructure can be achieved by post-process α+β STA and/or cyclic heat treatments which are described in the sections of solution treating and aging. As shown in Fig. 13b and Table 1, the bi-modal microstructure generally has an improved ductility without sacrificing strength when compared to the tensile properties of lamellar microstructure. The simultaneously enhanced strength and ductility are related to the fine αs region and the coarse equiaxed αp, respectively107.
A further information that can be extracted from Fig. 13 is the scattered tensile property in LPBF fabricated Ti-6Al-4V, which can be attributed to the various specimen orientations, compositions, porosity levels, and microstructures resulting from different powder characteristics (size and composition), LPBF systems, exposure parameters/strategies, and post-LPBF treatments.
Despite the efforts on transforming a non-equilibrium α’ martensitic microstructure into the an equilibrium α+β lamellar microstructure, a few studies have reported similar ductility in a fully martensitic microstructure when compared to that in an α+β microstructure80,126–128. In hot deformed Ti-6Al-4V, a rapid heating (> 50 °C/s) to the β phase field followed by water quenching lead to the formation of fine prior-β grains (~8 µm) containing only α’ martensite, which simultaneously improves strength and ductility126. The increase in strength is attributed to the refinement in both martensite width and prior-β grain size, and the enhanced ductility is related to the reduced strain localization due to the shortened martensite, which is limited by the prior β grain size126. Two similar observations have been reported in LPBF fabricated Ti-6Al-4V, in which a fully martensitic lamellar microstructure with a fine prior-β grain width between 50-100 µm results in high strength and ductility127,128. For such an approach to achieve good tensile properties, the key is to have a small prior-β grain size126. It is known that the prior-β grain width is related to the hatch distance in laser continuous scan strategy and the point distance in pulsed laser strategy. A small hatch distance can provide a narrow prior-β width, but it sacrifices the productivity. In addition, cautiously controlled laser processing parameters are required to avoid the β formation during the in-process thermal cycling, because the low volume of thin β film causes stress concentration, which decreases ductility127,128.
The mechanical properties of LPBF fabricated materials have been reported as anisotropic due to the columnar prior-β grains inclined close to the build direction113,129,130 and the manufacturing defects16,31. In the former case, horizontally built samples generally have a higher strength but a lower ductility than those of vertically built samples, because the width of columnar β grains is aligned in the tensile direction of horizontal samples113,129,130. The situation is changed when lack-of-fusion defects present between deposited layers and scan vectors. Vilaro et al. have reported higher elongation in horizontally built samples than that in vertically built samples in as-fabricated and post-process heat treated LPBF Ti-6Al-4V16. The reason is that tension in the vertical direction tends to open the lack-of-fusion defects and results in a low ductility. Recently, a plasticity model has been developed to predict the stress state dependent anisotropic plasticity behavior of LPBF fabricated Ti-6Al-4V131.
A recent study has revealed that β solution treatment can effectively mitigate the anisotropy in mechanical property due to the fact that β grains lost the columnar morphology after such a treatment132. However, the strength and ductility significantly decrease after β solution treatment, because excessive β grain growth occurs at temperature above the β transus84,132.
For LPBF fabricated Ti-6Al-4V, the fatigue performance is influenced by the surface finish, manufacturing defects, residual stress and microstructure133,134. The surface quality has been found to be the most crucial factor. Fig. 14b shows a rough as-built surface covered by partially melted powders. Without surface treatments, the fatigue performance of as-fabricated LPBF specimen is worse than that of the traditional cast products133. After surface treatment, the fatigue strength is significantly increased in LPBF Ti-6Al-4V with57,58,60,103 and without post-process HIP treatments58. Fig. 14a shows that various surface treatments can improve the fatigue performance, and a reduced surface roughness of Ra = 0.3 µm (Fig. 14c), achieved by milling, improves the tension-tension (R = 0.1) high cycle fatigue (HCF) strength to 775 MPa57, which is superior than that of wrought Ti-6Al-4V at 450–650 MPa135. Similarly, with respect to tension-compression fatigue (R = −1), Fig. 14d shows a similar observation that an improved surface quality increases the HCF strength, and a reduced surface roughness (Ra < 1 µm) provides a HCF strength comparable to that of wrought Ti-6Al-4V. This is consistent with the fractographs shown in Fig. 14e and f, which indicate that surface related defects are responsible for the inferior fatigue strength in the as-built sample, and fatigue crack initiation site is at the sample interior when the surface defects are removed by machining. The comparable or even higher HCF strength in improved surface conditions is because the LPBF fabricated Ti-6Al-4V has a much finer microstructure than wrought and cast Ti-6Al-4V, which means a short effective dislocation slip length and a high resistance to fatigue crack initiation.
Fig. 14 Effect of surface finish on HCF strength.a Fatigue performance of post-process HIP treated LPBF Ti-6Al-4V after different surface treatments with an axial fatigue stress of R =0.157. b SEM micrograph of as-built surface (Ra = 17.9 µm)57. c SEM image of milled surface (Ra = 0.3 µm)57. d Fatigue performance of post-process heat treated or HIP treated LPBF Ti-6Al-4V with an axial fatigue stress of R =-11,58. e Fatigue fractograph of a sample with as-built surface (Ra > 7 µm)58. f Fatigue fractograph of a sample with machined surface (Ra < 1 µm)58.
If the defects are on the surface and/or at the sub-surface which are connected to the surface, then HIP cannot close such the surface related defects and does not improve the fatigue strength of samples with an as-built surface quality as shown in Fig. 14d. In contrast, internal defects can be addressed by post-process HIP, which has been summarised in the section of hot isostatic pressing. Fig. 14d shows that HIP improves the fatigue performance of machined samples (Ra < 1 µm), which is through eliminating the internal defects. In addition, HIP can effectively narrow down the fatigue data scatter60. As described in the section of residual stress, tensile residual stress, which accelerates the fatigue crack propagation and is detrimental to the fatigue properties3, presents at the top and side surfaces of the part50. Therefore, surface peening treatments have been used to tackle the tensile residual stress, which result in a compressive stress below the surface and improves the fatigue strength60,136.
For the influence of microstructures, previous investigations are mainly focused on the lamellar microstructure. In general, a finer microstructure has a higher fatigue strength. A sub-transus HIP treatment at 920 °C shows a better fatigue performance than the super-transus HIP treatment at 1050 °C116. Similar observations has been reported in the heat treated conditions116. Whereas, the situation is different for fatigue crack growth (FCG). In the hypo-transitional region, FCG is generally influenced by the microstructure. A coarse microstructure has a higher fatigue crack threshold stress intensity factor (ΔKth) and a slower FCG rate137. At the threshold range, a coarser lamellar microstructure displays a stronger ability to deflect cracks along the lath boundaries137,138. Even at a higher ΔK, crack deflection can be controlled by the colony/packet size138. On the contrary, the cyclic plastic zone is sufficiently large in the hyper-transitional region, which exceeds the colony size, thus FCG is insensitive to the microstructure138.
As described in tensile and fatigue properties, a fine microstructure generally provides high yield and HCF strengths. In contrast, a fine microstructure leads to a low fracture toughness1. For LPBF Ti-6Al-4V, the fracture toughness has been reported at values below 30 MPa∙m−1/2 in the as-fabricated condition with α’ martensitic microstructure139. This is lower than that of the wrought Ti-6Al-4V with a lamellar microstructure at 45-75 MPa∙m−1/2 1,3. This is due to the extremely fine (sub-micro) and brittle α’ in the as-fabricated condition. It has been found that post-process stress-relieving and annealing can improve the fracture toughness to approximately 50 MPa∙m−1/2 139,140, which is comparable to that of wrought alloys. In addition, anisotropic fracture toughness has been observed in both as-fabricated and post-process stress-relieved/annealed conditions, which is related to porosity levels and columnar β grains aligned along the build direction. A recent study has showed that a two-stage heat treatment of α+β STA effectively improves the fracture toughness to approximately 100 MPa∙m−1/2 138. A short solution treatment at 920 °C followed by aging results in a coarse α+β Widmanstätten microstructure, and this microstructure can provide two benefits in terms of improving the fracture toughness. One is the α+β microstructure is more ductile than the α’ martensitic microstructure, and the other is a coarser lamellar microstructure means a rougher crack front profile and an increased crack tortuosity1,138.
In the temperature range between 450 °C and 650 °C, the as-fabricated Ti-6Al-4V has a comparable creep performance with that of the wrought Ti-6Al-4V141. After post-process heat treatments, a Widmanstätten microstructure showed a relatively lower creep strain and a lower steady-state creep rate than that in the as-fabricated condition. This indicates that the heat treated condition has a higher creep resistance142. This is due to the fact that the as-fabricated martensitic microstructure can be decomposed at creep testing temperatures, and such a microstructure evolution reduces the creep resistance142. The α/β interfaces in Widmanstätten microstructure act as obstacles to the dislocation glide during the creep deformation, which also improves the creep resistance143,144.
In titanium alloys, the corrosion resistance is attributed to the protective surface oxide film1. Therefore, the surface quality affects the corrosion performance of LPBF fabricated Ti-6Al-4V, and surface treatments can effectively improve the corrosion resistance by surface machining145 and electropolishing146. For the influence of microstructure, metastable α’ martensite is regarded as less corrosion resistant than the α phase, and the V-containing β phase is more corrosion resistant than α’ martensite147. Therefore, as-fabricated LPBF Ti-6Al-4V has an inferior corrosion resistance (in 3.5 wt.% NaCl aqueous solution at room temperature) when compared with conventionally processed Ti-6Al-4V with an α+β lamellar microstructure due to the non-equilibrium α’ martensitic microstructure147,148. The passive film thickness has been found to be thinner in laser fabricated Ti-6Al-4V than that in wrought counterpart, which also indicates a faster corrosion rate149. In terms of anisotropy, there is a negligible difference in corrosion properties between vertical and horizontal planes in 3.5 wt.% NaCl aqueous solution, while anisotropic corrosion behavior has been observed in a harsher environment of 1 M HCl aqueous solution, and the authors have related the anisotropy to different β volume fractions on different orientations150.
After post-process heat treatments, previous studies showed two distinctly different corrosion phenomena. A recent study has reported that both sub-transus and super-transus heat treatments deteriorated the corrosion resistance in 3.5 wt.% NaCl aqueous solution at room temperature due to the increased grain size151. While, other studies have suggested that post-process heat treatments improve the corrosion resistance in Ringer’s solution at 37 °C152 and in 3.5 wt.% NaCl aqueous solution at room temperature147. This is attributed to the limited element segregation in the μm-sized α+β lamellar microstructure after the heat treatment, which mitigates the galvanic corrosion between α and β phases147. Nevertheless, further investigations on the corrosion performance of SLM-produced Ti-6Al-4V are required to understand the influence of post-process heat treatments.
Review of laser powder bed fusion (LPBF) fabricated Ti-6Al-4V: process, post-process treatment, microstructure, and property
- Light: Advanced Manufacturing 2, Article number: 20 (2021)
- Received: 22 January 2021
- Revised: 15 July 2021
- Accepted: 16 July 2021 Published online: 13 August 2021
Abstract: Laser powder bed fusion (LPBF) is a timely important additive manufacturing technique that offers many opportunities for fabricating three-dimensional complex shaped components at a high resolution with short lead times. This technique has been extensively employed in manufacturing Ti-6Al-4V parts for aerospace and biomedical applications. However, many challenges, including poor surface quality, porosity, anisotropy in microstructure and property, and difficulty in tailoring microstructure, still exist. In this paper, we review the recent progress in post-process treatment and its influence on the microstructure evolution and material performance, including tensile, fatigue, fracture toughness, creep, and corrosion properties. The contradictions in simultaneously achieving high strength/ductility and strength/fracture toughness/creep resistance have been identified. Furthermore, research gaps in understanding the effects of the emerging bi-modal microstructure on fatigue properties and fracture toughness require further investigation.
Laser powder bed fusion (LPBF) is a timely important additive manufacturing technique that offers many opportunities for fabricating three-dimensional complex shaped components at a high resolution with short lead times. This technique has been extensively employed in manufacturing Ti-6Al-4V parts for aerospace and biomedical applications. However, many challenges, including poor surface quality, porosity, anisotropy in microstructure and property, and difficulty in tailoring microstructure, still exist. In this paper, we review the recent progress in post-process treatment and its influence on the microstructure evolution and material performance, including tensile, fatigue, fracture toughness, creep, and corrosion properties. The contradictions in simultaneously achieving high strength/ductility and strength/fracture toughness/creep resistance have been identified. Furthermore, research gaps in understanding the effects of the emerging bi-modal microstructure on fatigue properties and fracture toughness require further investigation.
Controllable microstructure and property in additive manufactured titanium alloy
Over the last two decades, laser powder bed fusion (LPBF) has become one of the most widely used additive manufacturing technique. Among different alloys, LPBF fabricated titanium alloys have attracted much attention due to their good mechanical properties, outstanding corrosion resistance, and superior biocompatibility. The microstructure and properties of LPBF fabricated titanium alloys are significantly affected by the 3D printing process and post-process treatments. Researchers at the University of Manchester and Monash University now give an overview of the LPBF fabricated Ti-6Al-4V in terms of the processing-microstructure-property relationship. Through applying appropriate process parameters and post-process treatments, LPBF fabricated Ti-6Al-4V has comparable or even superior tensile, fatigue, fracture toughness, and creep properties than those of cast and/or wrought counterparts. However, there are still contradictions in simultaneously achieving high strength/ductility and strength/fracture toughness/creep resistance.
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