Effects of build direction on tensile and fatigue performance of selective laser melting Ti6Al4V titanium alloy

International Journal of Fatigue 130 (2020) 105260

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Effects of build direction on tensile and fatigue performance of selective laser melting Ti6Al4V titanium alloy

T

⁎

Wenbo Suna, Yu'e Maa, , Wei Huanga, Weihong Zhangb, Xudong Qianc a

School of Aeronautics, Northwestern Polytechnical University, Xi’an 710072, China School of Mechanical Engineering, Northwestern Polytechnical University, Xi’an 710072, China c Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576, Singapore b

A R T I C LE I N FO

A B S T R A C T

Keywords: Selective laser melting Ti6Al4V Build direction Tensile strength S-N curves Fatigue crack growth rate

Additive manufacturing Ti6Al4V titanium alloy has a great potential to be applied in aviation structural components. During additive manufacturing process, the deposition layers present because of its intrinsic features, which may affect mechanical properties of additive manufactured parts and components. In order to study the effects of build direction on tensile and fatigue performance of Ti6Al4V manufactured by the selective laser melting technique, three different build direction Ti6Al4V samples (0° sample, 45° sample, 90° sample) were designed and manufactured in this work. Tensile and fatigue tests were performed. Stress versus strain curves, SN curves and crack growth rate curves were measured and studied. Fracture surfaces were investigated and compared with each other. It is shown that build direction can affect the fatigue performance, but has little effect on fatigue crack growth rate. The tensile and fatigue properties of the 45° sample are the best of the three build directions in this study.

1. Introduction Compared with the conventional manufacturing processes, additive manufacturing (AM) technology can be used to manufacture aircraft parts and components because it can increase material utilization, shorten the processing period, and reduce structural weight and produce complex structure. Selective laser melting (SLM) is one typical AM process. During SLM additive manufacturing process, the deposition layers present, which may affect mechanical properties of additive manufactured parts and components. Because of these deposition layers, build direction is an important factor that may lead to anisotropic tensile and fatigue performance of SLM Ti6Al4V. The loads applied in aircraft structures are complicated and multi-axial; when the angle between the load direction and the deposition layer is different, tensile and fatigue responses of AM material is affected. Hence, it is necessary to study the effects of build direction on mechanical properties of additive manufactured parts and components. According to papers published by some researchers, there are three main factors that can lead to the anisotropic mechanical performance of AM materials manufactured in different build directions: microstructure, porosity directionality and residual stress [1–4]. Some scholars have summarized the effects of microstructure on fatigue behaviours of AM materials [1,5,6]. The microstructure of SLM Ti6Al4V

⁎

alloy has a different morphology in different views because the columnar prior-β grains grow along the build direction altering the αphase grain boundary [7,8]. This can cause a lower ductility along the longitudinal direction with respect to the build direction for the reason that the grain boundary α-phase tends to fracture with tensile opening mode under a longitudinal tensile load [7]. Some researchers have revealed that porosity is another factor affecting the fatigue properties of SLM Ti6Al4V. Porosity is one of the most critical inevitable defects due to the complexity of the SLM process [7,9,10]. These pore defects are elongated in shape and oriented perpendicular to the build direction [11]. On the other hand, residual stress can lead to anisotropic mechanical behaviours [12–14]. Process variables can influence the residual stresses of SLM Ti6Al4V alloy, especially the scan vector length and preheating [15–18]. Increasing the scan vector length results in increased residual stress [15], while preheating of the base plate can help to reduce residual stress [19]. This characteristic may cause a state of complex residual stress in a compact tension (CT) specimen, when the build direction is not parallel or vertical to the crack path. Zhang [20] discussed the effect of residual stress on fatigue crack growth behaviour of AM Ti6Al4V alloy, and found that residual stresses can cause stress concentration near the crack tip and then affect the fatigue crack growth rate (FCGR). Although post-heat treatment can reduce residual stress and

Corresponding author. E-mail address: [email protected] (Y. Ma).

https://doi.org/10.1016/j.ijfatigue.2019.105260 Received 13 June 2019; Received in revised form 20 August 2019; Accepted 30 August 2019 Available online 31 August 2019 0142-1123/ © 2019 Elsevier Ltd. All rights reserved.

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porosity, it still does not close the pores at the sample surface. These pores will act as crack nucleation sites when samples are subjected to fatigue load [17,21]. Li [22] has summarized the effects of defects, microstructure, and processing parameters on fatigue performance of AM Ti6Al4V. Romano [23] developed a fatigue life prediction model considering the known defect population inside the material based on a fracture mechanics approach. The influence of defect size and population on fatigue performance was studied. Benedetti [24] discussed the effect of mean stress and defects on high cycle fatigue resistance of AM Ti6Al4V, and found that the high cycle fatigue performance is controlled by both the maximum stress and defects. The effects of pore shape, size, and location have been studied in Refs. [10,25–28]. Based on computed tomography, Shafaqat Siddique [26] evaluated the influence of porosity-incited stress concentration on the fatigue scatter, and found that pore size and location relative to the outer surface are two critical factors. Matthew [10] found that pores will not cause fatigue failure when the pore size in the microstructure is below a certain threshold value. Based on the high cycle fatigue test data, Eric [29] built a numerical model to analyze the fatigue properties of the additive manufactured Ti6Al4V. Edwards [30] compared the fatigue performance of SLM Ti6Al4V with that of wrought material. It was found that surface finish, porosity and residual stresses can influence the fatigue performance of SLM material. The fatigue strength of SLM materials at 106 cycles were over 75% lower than that of wrought material. In order to study build direction effects, most of researchers used two kind samples (vertical and horizontal build directions) and found that build direction may have little effect on Young’s modulus of AM material [31], but affects the pore distribution and fatigue life [1,4,30,32]. Junwen Zhao [4] found that the porosity fraction in the vertically built sample has a larger scatter. The build direction has a great effect on the average gas porosity fraction, size and distribution. Specimens built horizontally have a greater fatigue life than those built vertically for a similar pore size due to the weak bonded layer boundary. Based on the multi-scale damage mechanism, a meso-model considering the effects of build direction and porosity was established in Ref. [33]. The crack propagation behaviour of SLM Ti6Al4V manufactured in different build directions was studied by Walker [32]. He concluded that crack nucleating defects and inherent variability are two critical factors on FCGR characteristics of SLM materials. Yong Xie [2] found that the build direction can affect the stress intensity factor transition point. The stress intensity factor of vertical build direction sample at the transition point is typically 5% higher than that of the horizontal direction sample. Edwards [3] also investigated the effect of build direction on threshold and critical stress intensity factors, and found that the effect of build direction on crack growth rates is not obvious. But the crack growth rates of SLM materials are slower than that of wrought material in Region II of the Paris Relationship. Jiao [34] studied the influence of stress ratio on fatigue crack growth behaviour, and found that the FCG resistance at different stress ratios was very close in the high stress intensity factor region, and that there is no relationship between build direction and FCGR. The single edge notch tension (SENT) specimen was designed by Rans [35] to study the effect of build direction on crack growth properties. Build direction does not affect fatigue crack growth rate, but did affect the crack path. Although lots of researches have been done on the fatigue performance of AM materials, the information regarding the effect of build direction on fatigue performance of SLM Ti6Al4V material is still insufficient. Therefore, this work aims to investigate the effect of build direction on tensile and fatigue behaviour of SLM Ti6Al4V titanium alloy. Three kinds of specimens were designed and manufactured in different build directions (0°, 90° for fatigue specimens; 0°, 45°, 90° for tensile and fatigue crack growth rate test specimens). Tensile and fatigue tests were performed. Fracture morphologies were observed and analysed.

Table 1 Chemical compositions of Ti6Al4V powders (wt%). (%) Ti

Al

V

Fe

C

H

O

N

Rest

5.5–6.75

3.5–4.5

≤0.30

≤0.08

≤0.015

≤0.20

≤0.05

Table 2 SLM processing parameters. Laser power/ W

Laser beam diameter/mm

Scanning speed/ (mm/s)

Layer thickness/ mm

350

0.08

1000

0.06

2. Sample design and experimental set-up 2.1. Sample design Three types of specimens were designed in this study. Tensile and FCGR specimens with three different build directions (0°, 45°, 90°) were designed based on ISO 6892-1 and ASTM E647 standards. Fatigue specimens with two different build directions (0°, 90°) were designed according to ISO 1099 standard. All specimens were manufactured using a BLT-S300 machine (developed by Xi’an Bright Laser Technologies Co. Ltd) with the same laser parameters. The layer thickness was 0.06 mm. The chemical compositions of the Ti6Al4V powder and the SLM processing parameters are listed in Tables 1 and 2 respectively. To ensure that build direction was the only factor, all specimens were manufactured and post-processed in the same condition. A stress relief heat treatment process was performed once the specimens were manufactured. The specimens were annealed for 5 h at 750–850 °C, and then cooled in Argon gas to room temperature. The definition of build direction and diameters for all samples are shown in Figs. 1–3. The red arrows are the build direction, and the black arrows are the load direction. The sample with the deposition layer parallel to the load direction is defined as the 0° samples shown in Fig. 1(a) and 2(a), while the sample with the deposition layer perpendicular to the load direction are defined as the 90° samples shown in Figs. 1(c) and 2(b). The 45° sample has a 45° angle between the deposition layer and the load direction, as shown in Fig. 1(b). In Fig. 3(a), the CT specimen with the crack growth direction

Fig. 1. Geometry and build directions of tensile specimens: (a) 0° sample, (b) 45° sample, (c) 90° sample. 2

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Fig. 4. Nominal Stress-Nominal Strain curves. Fig. 2. Geometry and build directions of fatigue specimens: (a) 0° sample, (b) 90° sample.

Table 3 Tensile properties of SLM Ti6Al4V with different build directions.

parallel to deposition layers is defined as the 0° sample, that with the crack growth direction perpendicular to the deposition layers, as the 90° sample, shown in Fig. 3(a) and (c) respectively. As mentioned previously, the 45° sample mean that there is a 45° angle between the deposition layer and the crack growth direction, shown in Fig. 3(b). The width (W) and thickness (B) of all CT specimens was 50 mm and 10 mm respectively.

Build direction

Young’s Modulus/GPa

YS/MPa

UTS/MPa

Ultimate strain

0° 45° 90°

111.55 113.26 115.87

857.60 882.76 888.42

935.57 963.50 952.97

0.141 0.175 0.168

2.2. Experimental setup Tensile tests were performed at room temperature conditions using a DDL100 electronic universal testing machine at the tensile rate of 0.5 mm/min. Samples with different build directions were tested. Extensometer and strain gauges were used to measure the strain and calculate Young’s Modulus. Fatigue tests were performed at room temperature conditions using a MTS 810 fatigue testing machine. The test frequency was 20 Hz at a stress ratio R = −1. Twenty five specimens were built for each 0° and 90° sample respectively. The fatigue stress level was determined from the ultimate tensile strength. Fatigue crack growth rate tests were performed according to ASTM E647 using the same fatigue machine mentioned above. A sine wave cyclic load with fixed amplitude ΔP and frequency 20 Hz was used. The stress ratio R = 0.1 and the maximum load was 5.19 kN. The crack

Fig. 5. Comparison of yielding stress and elongation.

Fig. 3. Geometry and build directions of FCGR specimens: (a) 0° sample, (b) 45° sample, (c) 90° sample. 3

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Fig. 6. Macroscopic rough cup–cone features of tensile samples: (a) 0° sample, (b) 45° sample, (c) 90° sample.

Fig. 7. SEM images of fracture surfaces of the 0° tensile samples taken at: (a) 143×, (b) 650×, (c) 1500× magnifications.

propagation was monitored using a QM413T digital microscope.

higher than that of the 0° and 90° samples respectively.

3. Results and discussion

3.1.2. Fracture surfaces Fracture surfaces of SLM Ti6Al4V with different build directions are shown in Fig. 6. A cup-cone shaped failure surface and the shear lip, representative of a ductile fracture mode, can be observed in each sample. Many pores appear on the fracture surfaces, especially in the surface center. SEM was used to study fracture morphologies of the 0°, 45° and 90° samples, shown in Figs. 7–9. Fracture morphologies of the 0° sample at different magnifications are shown in Fig. 7. Many pores and long sharp cracks can be clearly observed in Fig. 7(a). These defects play a dominant role in fracture when subjected to uniaxial tensile load. At a large magnification, some incompletely melted particles and unmelted

3.1. Tensile properties 3.1.1. Stress-strain curves Nominal stress-strain curves of the SLM Ti6Al4V samples with three different build directions are shown in Fig. 4. Ultimate tensile stress (UTS), yielding stress (YS) and ultimate strain were obtained and given in Table 3 and Fig. 5. The 45° sample had a higher ultimate strength and elongation. The UTS of the 45° sample was 963.50 MPa, and it was 27.93 MPa and 10.53 MPa higher than that of the 0° and 90° samples respectively. Young’s Modulus of the 45° sample was 15.7 and 8.33 GPa 4

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Fig. 8. SEM images of fracture surfaces of the 45° tensile samples taken at: (a) 144×, (b) 500×, (c) 1500× magnifications.

process parameters, such as the laser energy and the hatch space et al. Every process parameter can affect microstructure and pore distribution of SLM material. Both of them can make the tensile properties relate to the build direction. In SLM, long columnar prior-β grains and α-phase grain boundaries can influence the ductility of SLM Ti6Al4V because the columnar prior-β grains grow along the build direction [8]. In addition, the pores also have a negative effect on ductility. The micro voids appearing at the interlayers may grow and coalesce. These can lead to the nucleation of microscopic cracks. If the pore defect in the interlayers is large, the tensile performance of the 90° sample will be worse. If the pore defect is small in the interlayers, the character of microstructure will dominate the effects. This can make the 90° sample have a better tensile performance. According to the geometry and the population of the defects in this test, it can be deduced that pores and some incompletely melted particles (shown in Fig. 7) are mainly responsibility for the lower ductility of the 0° sample manufactured by SLM. On the other hand, the dimples and the weak bond of layer interfaces due to incomplete melting also play an important role. All lead to the 0° sample having the lowest tensile property.

powders can be observed in the pores, shown in Fig. 7(b) and (c). It is important to note that all cracks were parallel to each other because the parallel deposition layers present in the fracture surface of the 0° sample. Small dimples, quasi-cleavage facets and some transgranular fracture can be observed on the fracture surface, and the fracture surface exhibits ductile and brittle mixed mode fracture features. In Fig. 8, the defects of the 45° sample were significantly less than those of the 0° sample shown in Fig. 7. Compared with the fracture morphologies of the 45° sample shown in Fig. 8, the large pores and quasi-cleavage facets account for the lower ductility of the 0° sample. Cone-shaped hills caused by large pores can also be observed. There is a certain angle between the crack plane and the fracture surface, shown in Fig. 8(c). A large number of fine and deeper dimples on the fracture surface of the 45° sample indicates the extent of plastic deformation and a good ductility. Fig. 9 shows the fracture morphologies of the 90° sample under the uniaxial tensile load. The fracture mode of the 90° sample was similar to that of the 45° sample except for the higher number of large pores. Staircase-like shapes observed at high magnification can be explained by small cracks occurring in the adjacent layer interfaces, and propagating along α grain boundaries. Small cracks are not visible on the fracture morphologies of the 90° sample, on account of the perpendicular relationship between the layer interface and the tensile loading direction. At present, there are two different conclusions about the effects of build direction on tensile performance of SLM Ti6Al4V according to the published papers [8,34,36–37]. Some researchers found that the 90° sample has better tensile performance than the 0° sample [8,36]. While some researchers found that the 0° sample has better tensile performance [34,37]. The effect of build direction on tensile performances is very complicated because that the AM material is highly sensitive to all

3.2. Fatigue performance of the 0° and 90° samples 3.2.1. S-N curves of the 0° and 90° samples S-N curves of the 0° and 90° samples are shown in Fig. 10. These values are compared with the fatigue performance of wrought titanium alloy illustrated by Kahlin [38]. It is apparent that SLM Ti6Al4V titanium alloys have a lower S-N curve than that of wrought materials regardless of the build direction. Fatigue cycles of the 0° and the 90° samples show a large scatter. The fatigue strength of the 0° and the 90° samples at 107 cycles is below 300 MPa and 350 MPa respectively. These strength values are 62.5% and 56.25% lower than the wrought 5

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Fig. 9. SEM images of fracture surfaces of the 90° tensile samples taken at: (a) 137×, (b) 500×, (c) 2500× magnifications.

3.2.2. Fracture surfaces of the 0° and 90° fatigue samples Figs. 11 and 12 show that fracture surfaces of the 0° and 90° samples at different stress levels respectively. Fatigue fracture surfaces under high stresses showed a shear fracture feature, and were accompanied by a heating ablation phenomenon regardless of the build directions, shown in Figs. 11(a) and 12(a)–(b). Fatigue stresses of 850 MPa and 750 MPa are close to the UTS of SLM Ti6Al4V, shown in Table 3. When 750 MPa and 850 MPa high fatigue stresses were applied to samples at a 20 Hz frequency, repeated strains accumulated quickly at the fracture surfaces. Several cracks nucleated and propagated rapidly leading to the sample fractured. High energy at a rapid frequency leads to heat ablation happening. The fracture surface is flat in the middle but rough elsewhere, shown in Figs. 11(a) and 12(a). In Fig. 11(b–f), fatigue crack nucleation sites (pointed by the red arrows) of the 0° samples can be clearly observed. Unlike that the crack starting at the center of the tensile fracture surface shown in Fig. 7, fatigue specimens cracks usually nucleated near the surface because of a pore defect. Fatigue cracks propagated longer as the stress was decreased from 650 MPa to 350 MPa. The 0° sample had a “cliff” shape before it fractured, shown in Fig. 11(c–f). Although the “cliff” shape was not as sharp as that in the 0° sample, the 90° samples still have similar features, shown in Fig. 12(c–f), Crack nucleation sites can be identified on fracture surfaces in Figs. 11 and 12. With the crack propagating, the effective bearing area deceases, leading to final fracture. The fracture surface area of Region II (the red dash line region) increases with the decrease of fatigue load, accompanying with better smoothness. For the 0° samples, a single crack nucleation site is hard to be discerned when the fatigue stress approaches 300 MPa, shown in Fig. 11(f). There may be multiple crack nucleation sites in this case, which can be seen in the SEM images shown in Fig. 13.

Fig. 10. S-N curves (R = −1, frequency = 20 Hz).

material for the 0° and the 90° samples respectively. There is a significant loss of fatigue strength at 107 cycles for SLM Ti6Al4V titanium alloy compared with the wrought titanium alloy in Ref. [38]. The reason for this decrease is that fatigue performance is highly sensitive to pores, and these defects are inevitable in the SLM titanium alloy. Build direction can affect the fatigue performance due to the weak bond between the layer interfaces. The 90° samples exhibited significantly better fatigue performance in this study.

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Fig. 11. Macroscopic fatigue fracture surface of the 0° samples at different load levels: (a) 750 MPa, (b) 650 MPa, (c) 550 MPa, (d) 500 MPa, (e) 400 MPa, (f) 350 MPa.

Fig. 12. Macroscopic fatigue fracture surface of the 90° samples at different load levels: (a) 850 MPa, (b) 750 MPa, (c) 650 MPa, (d) 550 MPa, (e) 500 MPa, (f) 400 MPa.

deposition layers present along the load direction in the 0° samples. The fracture surfaces of the 90° samples tested at 400 MPa are rather smooth, shown in Fig. 14. The crack nucleation site at a high magnification is shown in Fig. 14(c). It is a gas entrapment pore that leads to the crack nucleation site being near the specimen surface. Some incompletely melted particles and scars can be observed on the fracture surface, shown in Fig. 14(b).

3.2.3. SEM micrographs of the 0° and 90° samples Fig. 13(a) and (c) present crack nucleation zones of the 0° samples at 400 MPa. Several incompletely melted zones can be observed at a distance about 60–100 μm from the specimen surface, shown in Fig. 13(c). These defects cause crack nucleation. Fig. 13(b) is a high magnification of the incompletely melted particles shown in Fig. 13(a). Fig. 13(d) is a high magnification of the crack nucleation site indicated by the yellow arrow in Fig. 13(c). In addition, microcracks and facets can be seen on the fracture surfaces shown in Fig. 13(e) and (f) in line with the

7

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Fig. 13. SEM images of fatigue fracture surfaces of the 0° samples taken at different magnifications (maximum stress = 400 MPa).

the longest fatigue crack growth life and a good crack growth resistance. Fatigue crack growth life of the 45° sample is about 1.37 and 1.19 times longer than that of the 0° and 90° samples, respectively. The crack growth curves obtained by the incremental polynomial method are given in Fig. 17, and compared with the curves of SENT specimens tested by Rans [35] and the curves of wrought Ti6Al4V titanium tested by Oguma [39]. In both the SENT and CT specimens, the fatigue crack growth behaviour in different build directions has the same trend. When ΔK < 20 MPa m , the fatigue crack growth curve of the 0° sample is the highest of these three kinds of samples, while that of the 45° sample is the lowest. When ΔK > 20 MPa m , the differences between FCGR curves become smaller. The slopes of fatigue crack growth curves are higher than that of Rans’, but with a lower value. There is some fluctuation in the low ΔK regime (about 10–16 MPa m ). FCGRs of the 0° and 90° samples are slightly higher than that of the 45° sample. According to equation of Paris Relationship in the linear region (Region II),

3.2.4. Effects of test frequency on fatigue fracture In order to study the effects of test frequency on fatigue fracture, the 90° samples were tested at 750 MPa at three different frequencies (1 Hz, 10 Hz and 20 Hz). The shear fracture surface disappears after reducing the test frequency, shown in Fig. 15(a)–(c). The cycles to failure are given in Table 4. At 1 Hz and 10 Hz frequencies, the crack nucleation site and Region II (the red dash line region) can be observed on the fracture surface shown in Fig. 15(b) and (c). In addition, radial grooves can be observed near the crack nucleation site on the surface of the 1 Hz fatigue sample because of the difference in propagation resistance in front of the crack. 3.3. Fatigue crack growth rate 3.3.1. Effect of build directions The same size CT samples with three different directions (0°, 45° and 90°) were tested to measure their fatigue crack growth rates. Crack growth curves in the linear Paris fit (Region II) were calculated using the seven-point incremental polynomial method in ASTM E647. Crack length versus cycles is shown in Fig. 16. The 45° sample had 8

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Fig. 14. SEM images of fatigue fracture surfaces of the 90° samples taken at different magnifications (maximum stress = 400 MPa).

Fig. 15. Effect of test frequency: (a) 20 Hz sample, (b) 10 Hz sample, (c) 1 Hz sample. (Red arrows point to the crack nucleation site). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Constants, C and m, were fitted, and summarized in Table 5. Fracture surfaces of three specimens were examined and shown in Fig. 18. For the 0° sample, there are some intersecting small grooves on the fracture surface, shown in Fig. 18(a). The reason for this phenomenon is the fact that fracture surface is parallel to the deposition layer of SLM. The distance between hatch lines can cause less overlap and lead to lower fatigue performance. Under the fatigue load, the crack propagates along with the interaction layers, and then leads to the appearance of cross grooves. For the 45° sample, there is a 45° angle between the crack growth direction and the deposition layer. The fracture surface seems granular without other obvious features, shown in Fig. 18(b). For the 90° sample, the crack growth direction is perpendicular to the deposition layers. Lamellar structures can be observed clearly on the fracture surface shown in Fig. 18(c). Overall, the fatigue fracture surface of the 0° sample is relatively flat, while the 45° and 90° samples are granular. More inclusions and

Table 4 Cycles to failure at different frequencies. Frequency/Hz

Fatigue life/Cycles

Mean value/Cycles

20

4171 5192 3700

4354

10

7214 6763 6873

6950

1

7987 7584 7367

7646

da =C(ΔK) m dN

(1)

9

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Fig. 16. a-N curves (R = 0.1, frequency = 20 Hz) of SLM titanium alloy with different build directions.

Fig. 19. Effect of stress ratio on FCG (frequency = 20 Hz). Table 6 Paris constants at different stress ratios. Stress ratio

C (Standard error)

m (Standard error)

0.1 0.3 0.6

3.64789E−11 (2.65561e−12) 6.19689E−11 (6.18587e−12) 3.75858E−11 (4.06453e−12)

2.73492 (0.02075) 2.68615 (0.03078) 2.89 (0.03496)

pores can be observed on the fracture surface of the 90° sample. Different crack paths can also be observed on the fracture surfaces, and are similar to the observations in Ref. [35]. SLM Ti6Al4V titanium alloy with a 45° build direction has the best fatigue crack resistance in this study.

Fig. 17. Comparison frequency = 20 Hz).

of

fatigue

crack

growth

curves

3.3.2. Effect of stress ratio Stress ratio plays an important role in fatigue crack growth. Three stress ratios (R = 0.1, 0.3 and 0.6) were applied to investigate the effect of stress ratio on FCGR of SLM Ti6Al4V titanium alloy. Fatigue crack growth tests of the 90° samples were carried out under the same condition as mentioned previously. The fatigue crack growth curves of different stress ratio samples are shown in Fig. 19. These curves are compared with the fatigue crack growth rate of conventional Ti6Al4V reported by Boyce [40]. Corresponding Paris constants were fitted and shown in Table 6. All SLM samples have almost the same slope, and had lower da/dN values than conventional samples at the same stress ratio. With increasing stress ratio, the fatigue crack growth curves moved to the left. The curves of R = 0.3 and R = 0.6 are close to each other. The maximum value of ΔK decreased from 48.1 MPa m (R = 0.1) to 30.5 MPa m (R = 0.6).

(R = 0.1,

Table 5 Paris constants for different build directions. Build direction

C (Standard Error)

m (Standard Error)

0° 45° 90°

2.90621E−11 (3.48271e−12) 5.94416E−11 (7.5552e−12) 3.64789E−11 (2.65561e−12)

2.7901 (0.03442) 2.52554 (0.03628) 2.73492 (0.02075)

Fig. 18. Fracture surfaces of CT specimens: (a) 0° sample, (b) 45° sample, (c) 90° sample. 10

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4. Conclusions

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In this paper, the effects of build directions on tensile and fatigue performance of SLM Ti6Al4V are studied. Stress versus strain curves, SN curves and the fatigue crack growth curves are obtained. The fracture surfaces of different build direction specimens were analyzed. The main conclusions are as follows: (1). The tensile fracture of SLM Ti6Al4V titanium alloy is a mixed mode of ductile and brittle fracture. Build direction has a slight effect on tensile strength and ductility. The tensile strengths of the 45° sample was 963.50 MPa, and was 27.93 MPa and 10.53 MPa higher than that of the 0° and 90° samples respectively. The 0° sample had the poorest performance in this study. (2). Build direction has a significant influence on fatigue performance of SLM Ti6Al4V titanium alloy. Fatigue strength of the 0° and the 90° sample at 107 cycles are 300 MPa and 350 MPa respectively. Fatigue fracture under high loads shows a shear fracture, and is accompanied by a heating ablation phenomenon. The test frequency can affect the fracture mode under high loading. (3). Build direction can affect the fatigue crack growth life. The fatigue crack growth life of the 45° sample was about 1.37 and 1.19 times longer than that of the 0° and the 90° samples respectively. Curves of fatigue crack growth rate versus stress intensity factor range of three kind samples were similar. (4). Defects on fracture surfaces of the 0° samples are obvious in both tensile and fatigue tests. Voids, porosity and unmelted powders were the main reasons for low ductility and poor fatigue performance. The fracture surface of the 0° CT sample was relatively flat with some intersecting small grooves, while the 45° and the 90° sample’s fracture surfaces were granular. Lamellar structure can be observed on fracture surface of the 90° sample. Acknowledgement This study was financially supported by the National Natural Science Foundation of China under Grant No. 91860128, and supported by 2018 Innovation Ability Supporting Program of ShaanXi Province under Grant No. 2018KW-028. References [1] Yadollahi Aref, Shamsaei Nima. Additive manufacturing of fatigue resistant materials: challenges and opportunities. Int J Fatigue 2017;98:14–31. [2] Xie Yong, Gao Ming, Wang Fude, Zhang Chen, Hao Kangda, Wang Hongze, et al. Anisotropy of fatigue crack growth in wire arc additive manufactured Ti-6Al-4V. Mater Sci Eng, A 2018;709:265–9. [3] Edwards P, Ramulu M. Effect of build direction on the fracture toughness and fatigue crack growth in selective laser melted Ti-6Al-4 V. Fatigue Fract Eng Mater Struct 2015;38(10):1228–36. [4] Zhao Junwen, Easton Mark, Qian Ma, Leary Martin, Brandt Milan. Effect of build direction on porosity and fatigue life of selective laser melted AlSi12Mg alloy. Mater Sci Eng, A 2018;729:76–85. [5] Romero Carlos, Yang Fei, Bolzoni Leandro. Fatigue and fracture properties of Ti alloys from powder-based processes – a review. Int J Fatigue 2018;117:407–19. [6] Tan Jun Hao, Wong Wai Leong Eugene, Dalgarno Kenneth William. An overview of powder granulometry on feedstock and part performance in the selective laser melting process. Addit Manuf 2017;18:228–55. [7] Thijs Lore, Verhaeghe Frederik, Craeghs Tom, Van Humbeeck Jan, Kruth JeanPierre. A study of the microstructural evolution during selective laser melting of Ti–6Al–4V. Acta Mater 2010;58(9):3303–12. [8] Carroll Beth E, Palmer Todd A, Beese Allison M. Anisotropic tensile behavior of Ti–6Al–4V components fabricated with directed energy deposition additive manufacturing. Acta Mater 2015;87:309–20. [9] Stef Jonathan, Poulon-Quintin Angéline, Redjaimia Abdelkrim, Ghanbaja Jaafar, Ferry Olivier, De Sousa Matthias, et al. Mechanism of porosity formation and influence on mechanical properties in selective laser melting of Ti-6Al-4V parts. Mater Des 2018;156:480–93. [10] Dunstan Matthew K, Paramore James D, Zak Fang Z. The effects of microstructure and porosity on the competing fatigue failure mechanisms in powder metallurgy Ti6Al-4V. Int J Fatigue 2018;116:584–91.

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