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Evaluation of fatigue crack propagation behaviour in Ti-6Al-4V manufactured by selective laser melting
Accepted Manuscript Evaluation of fatigue crack propagation behaviour in Ti-6Al-4V manufactured by selective laser melting K.F. Walker, Q. Liu, M. Brandt PII: DOI: Reference:
S0142-1123(17)30304-3 http://dx.doi.org/10.1016/j.ijfatigue.2017.07.014 JIJF 4410
To appear in:
International Journal of Fatigue
Received Date: Revised Date: Accepted Date:
2 February 2017 4 July 2017 13 July 2017
Please cite this article as: Walker, K.F., Liu, Q., Brandt, M., Evaluation of fatigue crack propagation behaviour in Ti-6Al-4V manufactured by selective laser melting, International Journal of Fatigue (2017), doi: http://dx.doi.org/ 10.1016/j.ijfatigue.2017.07.014
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Evaluation of fatigue crack propagation behaviour in Ti6Al-4V manufactured by selective laser melting K. F. Walker *a ,Q. Liua, and M. Brandtb a Defence Science and Technology Group, Fisherman's Bend, Australia b Centre for Additive Manufacturing, RMIT University, GPO Box 2476, Melbourne VIC 3001, Australia Abstract: Total fatigue life performance of high strength titanium alloy Ti-6Al-4V manufactured by Additive Manufacturing (AM) based methods such as Selective Laser Melting (SLM) is of significant interest and has gained much attention recently. Researchers often compare the total fatigue life of materials manufactured from SLM compared with conventional manufacture, and often the SLM lives are reduced because cracks initiate from “defects” such as porosity or Lack of Fusion (LOF). But for repair and alternative manufacture of complex and critical aerospace structures for example, the crack growth/propagation phase of the fatigue process is also very important and it has not been studied as extensively. The aim of the work presented here was to evaluate and better understand crack propagation under constant amplitude loading in Ti-6Al-4V samples manufactured using SLM with a variety of layer thicknesses and build directions (vertical or horizontal). The “as-manufactured” condition was studied, with no post heat treatment. Cracks typically initiated at LOF features which had a negative impact on the total fatigue life. The focus here was on the crack growth phase. Modelling was performed with a conventional Linear Elastic Fracture Mechanics approach using literature data obtained from testing on Compact Tension (C(T)) specimens which were also in the asmanufactured condition with a variety of build directions. The modelling provided a very useful correlation of the data and provided a way of assessing the LOF features in terms of an equivalent initial crack. The crack growth properties of the SLM cases were also compared against literature data for conventionally manufactured material. The work will lead to a better understanding of fatigue crack growth characteristics for components manufactured by AM methods such as SLM. That understanding is an essential requirement for full certification and acceptance into service for critical applications such as aerospace structures. Keywords: Additive manufacturing, selective laser melting, Titanium alloy, fatigue, fatigue crack growth
*
Corresponding author. Tel.: +61 3 9626 7961; fax: +1 3 9626 7089 E-mail address: [email protected]
1
1. Introduction Additive Manufacturing (AM) or 3-D Printing is an emerging technology which offers significant advantages compared with conventional manufacturing processes such as casting, forging and machining [1]. The advantages offered by AM include rapid prototyping, design freedom (including the ability to manufacture very complex geometries not possible with conventional methods), minimal waste, and without the need for tools and dies. Selective Laser Melting (SLM) is a powder-bed-based AM technology [2, 3]. With SLM, a laser is used to selectively melt powder spread evenly over a thin layer on a build plate. The laser traverses the build area in a carefully controlled speed and scan pattern. After the first layer is melted, the build plate is lowered by a small, pre-set distance (the layer thickness) and a fresh layer of powder is introduced. The process then repeats and a three-dimensional part is therefore created (or printed). But before AM parts can be used in fatigue critical applications, the mechanical properties, including fatigue behaviour, need to be well understood and characterised. Titanium (Ti) alloys have been extensively used in fatigue critical applications in the aerospace and biomedical fields. Ti alloys offer high strength and stiffness to weight ratios, good high temperature performance and fatigue resistance, and they are compatible with biological and composite materials. One of the most useful and commonly used Ti alloys being produced by AM and SLM in particular is Ti-6Al-4V [4-7]. Fatigue behaviour of SLM manufactured Ti-6Al-4V has been investigated in many recent studies [8-13]. The results to date have revealed very significant variability in the fatigue life for a given applied constant amplitude stress [12]. This variability may be due to a range of factors, including initiating defects and microstructure. Total fatigue life is sometimes considered in two phases, i.e. an initiation or nucleation phase, followed by a phase of stable crack propagation to final failure. In some cases, particularly in the presence of defects of a significant size due to porosity or lack of fusion, the crack propagation phase may be the dominant factor. Porosity and lack of fusion defects are often present in SLM Ti-6Al-4V. The SLM process melts small, overlapping regions which can produce Lack of Fusion (LOF) defects. LOF is caused by insufficient energy density leading to irregular shaped cavities which may contain trapped, unmelted powder particles. Porosity is caused by gasses trapped within the powder, a result of the powder production process, which then expand during the melting thus creating a pore. Results from a recent test program [13] were used in the present work. Fatigue testing under constant amplitude loading at a stress ratio R of 0.1 (minimum stress/maximum stress) was performed on Ti-6Al-4V specimens manufactured by SLM in the vertical and horizontal build directions. Fatigue crack growth rate data were sourced from the open literature [8, 9]. The crack growth rate data were obtained from testing using Compact Tension (C(T)) specimens also manufactured by SLM in both the horizontal and vertical build directions. The rate data were compared with data from the literature for conventional Mil-Annealed material. The Mil-Annealed rate data were consistent with the 2
data for the SLM manufactured specimens. Conventional linear-elastic fracture mechanics (LEFM) methods were applied using the AFGROW fatigue crack growth analysis code to correlate the range of fatigue lives against the range of equivalent initial defects, i.e. the LOF and porosity. Finally, the typical LOF and porosity defects present in the SLM fatigue specimens were correlated against the analytical range of equivalent initial crack sizes. This showed reasonable agreement which suggests that the scatter in fatigue lives in such cases can largely be explained and understood through crack growth analysis and assessment. These methods are a useful tool for the evaluation of fatigue performance in additive manufactured Ti-6Al-4V. The fatigue life in these cases appears to be strongly a function of the LOF and porosity defects acting as crack starters from very early in the loading history.
2. Experimental Details 2.1 Fatigue test details The Ti-6Al-4V fatigue test specimens as detailed in [13] were fabricated by SLM processing in an SLM Solutions SLM250HL system, which incorporated a 250x250x250 mm3 chamber and a 400 W maximum laser power. The processing parameters used were: (1) laser power of 175 W; (2) hatch spacing of 0.125 mm (3) pre-layer thickness of 30 μm (4) scan speed of 710 mm/s. A commercial Ti-6Al-4V powder was used. This powder was a gas-atomized and pre-alloyed powder. The average spherical particle size was 43 μm. All samples were built in a single run and the location on the build plate was random. The specimen geometry was an hourglass type with a diameter of 5.5 mm at the gauge section (see Figure 1). The specimens were built in two orientations; vertically built and horizontally built. Specimens were manufactured in a cylindrical shape initially with a diameter of 12 mm and then machined to the final dimensions as shown in Figure 2. The fatigue fracture surfaces were examined using both optical and scanning electron microscopes (SEM). Optical microscopy was used to examine the manufacturing defects in cross-sectioned samples, while the SEM was used to examine the fatigue crack initiation sites in the fractured specimens. Fatigue testing was performed on a 100 kN, digitally controlled MTS testing machine, in a laboratory air environment under load control with a stress ratio R of 0.1 and a frequency of 10 Hz. Two applied stress levels, 600 and 700 MPa, were used and five samples were tested at each condition. Despite this being a small sample size, it was considered sufficient to identify trends and meet the objectives of the present work.
3
Figure 1 : Fatigue test specimen geometry
(a) Horizontal Build
(b) Vertical Build
(c) Final Machined
Figure 2 : Fatigue test specimen manufacture, build direction shown by arrow
2.2 Fatigue test results The fatigue life results compared with literature data from MMPDS [14] are shown in Figure 3. The scatter in life results is significant as has already been mentioned earlier.
4
Figure 3 : Fatigue test results [13] compared with MMPDS literature data [14] Table 1 : SLM fatigue specimen test lives Build Orientation
Vertically built
Horizontally built
σapp = 600 MPa
σapp = 700 MPa
Spec. No
Life (cycles)
Spec. No
Life (cycles)
SLM-B1
2480645
SLM-D1
221184
SLM-B2
102693
SLM-D2
36912
SLM-B3
312853
SLM-D3
10047
SLM-5a
15491
SLM-D4
27590
SLM-20a
53441
SLM-D5
22838
SLM-H7
181324
SLM-H12
40138
SLM-H8
155354
SLM-H13
1766199
SLM-H9
288879
SLM-H14
500855
SLM-H10
276335
SLM-H15
135639
SLM-H11
230695
SLM-H16
213009
5
Table 2 : Fatigue test lives and statistical analysis Stress Level (MPa) 600 700
Build Orientation
Avge Life (cycles)
Min Life (cycles)
Max Life (cycles)
Standard Deviation
SD/Avge Life
Vertical Horizontal Vertical Horizontal
593,024 226,517 63,714 531,168
15,491 155,354 10,047 40,138
2,480,645 288,879 221,184 1,766,199
949,392 51,952 79,210 636,396
1.60 0.23 1.24 1.20
The fatigue test data with some statistical analysis are shown in Table 1 and Table 2. Again, these results show very significant scatter/variability in the fatigue lives. The horizontal build at 600 MPa showed the least scatter, with the ratio of Standard Deviation/Average Life of 0.23. In the other three cases the same ratio ranged from 1.2 to 1.6 which suggests very significant variability. But significant scatter is also evident in the conventional Mil-Annealed data from the literature, refer to Figure 3. Power law fits to the MMPDS data show relatively poor fits with R2 values of 0.52 and 0.76 for the annealed sheet and annealed extrusion respectively. The AM material did show generally shorter fatigue life than the conventional Mil-Annealed material. The significant scatter for the AM material was thought to be a result of the variation in LOF and porosity defects and their shape, position and orientation relative to the loading axis.
2.3 Investigation into failure origins The origins of the fatigue failures in the SLM specimens were investigated in [13]. The failures typically initiated at porosity and LOF defects. The LOF defects often formed along the layer boundaries and were aligned parallel to them. This is thought to be due to insufficient energy to fuse the new layer with the previous layer. Cross-sections were taken and were polished and etched to reveal details of the LOF defects. Examples of the optical micrographs are shown in Figure 4. These defects are similar to others reported in the literature for SLM Ti-64 [10].
Figure 4 : Optical micrographs of the LOF defect in the cross-sections of the SLM Ti64 with the 30 μm layer thickness in the vertically built orientation at the high scan speed (both cross-sections 6
were etched to reveal the microstructures and defects): (a) Along the layer boundary and (b) LOF defect with unmelted powder particle (adapted from [13]) As reported in [13] the crack origin was investigated for the two shortest life specimens manufactured in the vertical orientation and subjected to the 700 MPa maximum stress. The shortest life result at 700 MPa vertical build was 10,047 cycles (Specimen SLM-D3) and the origin of the cracking was a LOF defect about 96 μm below the surface as shown in Figure 5 (a). The defect is irregular in shape, with a maximum dimension of about 400 μm. This is compared with the case with the next highest life (22,838 cycles for Specimen SLM-D5) which was also a LOF defect but this time about 222 μm below the surface, see Figure 5 (b). In that case the defect is also irregular in shape with a maximum dimension of about 250 μm . It was suggested in [13] that the longer life could be caused by the defect location, i.e. sub-surface compared with close to the surface. But the life will also be directly affected by the defect size, orientation, and sharpness (extent to which the defect is crack-like). The issue of equivalent crack size and a correlation against life is considered later.
Figure 5 : SEM micrographs of the fatigue crack origins at the different locations of the LOF defects in the SLM Ti64 specimens in the vertically built orientation: (a) ~ 96 μm below the specimen surface and (b) ~ 222 μm below the specimen surface. The crack origin for some of the horizontal build specimens was also investigated. One of the horizontal specimens tested at 600 MPa produced a life of 155,354 cycles and the origin of the fatigue cracking was an irregular shape LOF defect with a maximum dimension of about 300 μm and about 92 μm below the surface as shown in Figure 6(a). Another horizontal build specimen tested at 700 MPa produced a longer life of 213,009 cycles and the LOF defect at which the crack initiated was irregular in shape with a maximum dimension of about 300 μm and about 267 μm below the surface as shown in Figure 6(b). So it is evident that the shorter life results were associated with defects or features similar to those shown in Figure 5 and Figure 6. The defect/features are typically irregular in shape, with a maximum dimension around 250-400 μm.
7
Figure 6 : SEM images of the fatigue crack origins at the different locations of the LOF defects in the SLM Ti-64 specimens in the horizontally built orientation: (a) ~ 92 μm below the specimen surface and (b) ~267 μm below the specimen surface. Because of the irregular shape it is expected that the equivalent crack dimension would be smaller than 100 μm. This concept of an equivalent fatigue crack size has been considered in some detail elsewhere, see for example [15, 16]. In particular, fatigue cracking in a direct laser melting deposition case for high strength steel [17] showed that porosity with a maximum dimension of around 200 µm was equivalent to a crack length of 47 µm, giving a ratio of about 4:1 in terms of defect maximum dimension to equivalent crack dimension.
3. Analysis Details A fatigue crack growth analysis was performed using Linear Elastic Fracture Mechanics (LEFM) with the AFGROW code [18]. The cracking was modelled as a surface flaw in a solid circular cross-section bar as depicted in Figure 7. The standard stress intensity solution for this case provided in AFGROW was used, and the crack was assumed to maintain a consistent semi-circular shape.
8
Figure 7 : Schematic of assumed crack geometry A literature search was conducted to determine appropriate fatigue crack growth rate data to apply. Two suitable sources were found. Edwards and Ramalu [9] and Dhansay etal. [8]. Both sources conducted fatigue crack growth rate testing on SLM Ti-6Al-4V specimens in the as-manufactured condition with no heat treatment. The tests were conducted on Compact Tension specimens loaded under constant amplitude loading at R=0.1. They conducted tests on specimens manufactured in both the vertical and horizontal directions. The rate data are shown in Figure 8 along with data from MMPDS [14] for conventional Mil-annealed material.
9
Figure 8: Fatigue crack growth rate comparison, R=0.1 The extreme cases of the fastest and slowest rate data were identified and are as shown in Figure 9. It was interesting to note that in both cases the data came from specimens manufactured in the vertical orientation. When considered along with the variation in rates as seen in Figure 8, a clear link between build direction and rate is not apparent from these data. Any differences are masked by the variability overall in crack growth rate properties.
10
Figure 9 : Fastest and slowest crack growth rates, R=0.1 To determine if the range of fatigue lives observed could be correlated against the fatigue crack growth rate data, a parametric analysis was performed. The extreme cases of the slowest and fastest rates were considered and the initial crack size was varied in the range 2-200 μm. Based on the 4:1 ratio mentioned earlier and as determined in [17], this captured LOF/Porosity features ranging from about 8-800 μm which covers the expected LOF defect size range. The results for 600 MPa and 700 MPa are shown in Figure 10 and Figure 11 respectively. A bounded region which captures the range of initiating defect equivalent crack size and rate curve extremes is shown as the “life scatter region”.
11
Figure 10 : Fatigue life as a function of initiating crack size for the extreme rate properties, 600 MPa maximum stress, R=0.1
12
Figure 11 : Fatigue life as a function of initiating crack size for the extreme rate properties – 700 MPa maximum stress R=0.1 In both the 600 and 700 MPa maximum stress cases the full range of fatigue lives could be captured within the bounds of an initiating equivalent crack size of 4 - 40 μm which, based on the 4:1 ratio identified earlier, corresponds to LOF/porosity maximum dimension range of 16 – 160 μm and a rate curve between the slowest and fastest rates identified. This range compares reasonably well with the LOF/porosity defects and their shape and size as described earlier, although the upper end of the range at 160 µm is smaller than the maximum observed in these tests of about 400 µm for Specimen SLM-D3 tested at 700 MPa max stress. A detailed consideration of the range of initiating defects and the best correlation is summarised in Table 3. This comparison does not consider rate curves in between the slowest and fastest, so the initial crack sizes can be outside the 4 – 40 μm range. It should be noted that initial crack sizes would all be within the 4 – 40 μm range if rate curves in between the extreme fastest and slowest rates were considered. The life result is affected by a combination of the initial defect dimension and the growth rate properties.
13
Table 3 : Comparison of best life correlation achieved Maximum Stress (MPa)
Build Direction
Min. Life (cycles)
600
Vertical
15,491
Max. Life (cycles)
2,480,645 Horizontal
155,354 288,879
700
Vertical
10,047 221,184
Horizontal
40,139 1,766,199
Best Correlation Rate curve Initial Intitial (fast/slow) Equivalent Defect Crack (μm) Max Size (µm) Fast 35 140 Slow 5 20 Slow 85 340 Slow 45 180 Fast 35 140 Slow 35 140 Slow 160 640 Slow 4 16
4. Discussion Fatigue testing under constant amplitude R=0.1 loading at two high peak stress levels (600 and 700 MPa, about 70-80% of the uniaxial yield stress) revealed significant scatter/variability in the total fatigue life. This was evident even though the sample size here was small, i.e. five specimens at each of two stress levels and two build orientations (vertical and horizontal). In most cases the standard deviation was larger than the average specimen lives, and the range of specimen lives spanned across up to three orders of magnitude. The objective of the work reported here was to determine the extent to which a simple Linear Elastic Fracture mechanics based analysis combined with an understanding of the type and size of initiating defects could assist in explaining the variability. The assessment was performed by applying measured crack growth rate data from the literature for Ti6Al-4V material manufactured by SLM and tested in the as-manufactured condition. The resulting range of initial equivalent crack sizes ranging from 4-40 µm which is expected to relate to a maximum LOF/porosity defect range of about 16-160 µm. This compared reasonably well with the LOF/porosity defects/features identified in the lowest life fatigue results here. This suggests that the benefits to be gained by reducing/minimising the LOF/porosity could potentially be estimated using this approach.
5. Conclusion Ti-6Al-4V specimens manufactured by selective laser melting and tested under constant amplitude cyclic loading displayed very significant scatter/variability in total fatigue life. This behaviour is consistent with reported results in the literature. Significant scatter is also evident in conventional manufactured material. In the case of the SLM asmanufactured samples, the life is controlled by a combination of the initiating defects 14
(typically LOF and/or porosity) and inherent variability in the fatigue crack growth rate characteristics of the AM material. The method used here offers significant potential for the assessment of LOF/porosity features in terms of fatigue performance for Ti-6Al-4V manufactured by SLM.
6. References 1. Huang, Y., and Leu, M.C., Frontiers of Additive Manufacturing Research and Education, 2014, University of Florida: USA. 2. Abe, F., et al., The manufacturing of hard tools from metallic powders by selective laser melting. Journal of Materials Processing Tech., 2001. 111: pp. 210-213. 3. Shiomi, M., et al., Rapid manufacturing of metal components by laser forming. International Journal of Machine Tools & Manufacture, 2006. 46(12): pp. 1459-1468. 4. Murr, L.E., et al., Microstructure and mechanical behavior of Ti–6Al–4V produced by rapidlayer manufacturing, for biomedical applications. Journal of the Mechanical Behavior of Biomedical Materials, 2009. 2(1): pp. 20-32. 5. Thijs, L., et al., A study of the microstructural evolution during selective laser melting of Ti– 6Al–4V. Acta Materialia, 2010. 58(9): pp. 3303-3312. 6. Vandenbroucke, B. and J.P. Kruth, Selective laser melting of biocompatible metals for rapid manufacturing of medical parts. Rapid Prototyping Journal, 2007. 13(4): pp. 196-203. 7. Vilaro, T., C. Colin, and J.D. Bartout, As-Fabricated and Heat-Treated Microstructures of the Ti-6Al-4V Alloy Processed by Selective Laser Melting. Metallurgical and Materials Transactions A, 2011. 42(10): pp. 3190-3199. 8. Dhansay, N.M., R. Tait, and T. Becker, Fatigue and fracture toughness of Ti-6Al-4V titanium alloy manufactured by selective laser melting. Advanced Materials Research. Vol. 1019. 2014: Trans Tech Publications Ltd. 248-253. 9. Edwards, P. and M. Ramulu, Effect of build direction on the fracture toughness and fatigue crack growth in selective laser melted Ti-6Al-4V. Fatigue and Fracture of Engineering Materials and Structures, 2015. 38(10): pp. 1228-1236. 10. Günther, J., et al., Fatigue life of additively manufactured Ti–6Al–4V in the very high cycle fatigue regime. International Journal of Fatigue, 2017. 94(Part 2): pp. 236-245. 11. Leuders, S., et al., On the mechanical behaviour of titanium alloy TiAl6V4 manufactured by selective laser melting: Fatigue resistance and crack growth performance. International Journal of Fatigue, 2013. 48: pp. 300-307. 12. Li, P., et al., Critical assessment of the fatigue performance of additively manufactured Ti–6Al– 4V and perspective for future research. International Journal of Fatigue, 2016. 85: pp. 130-143. 13. Liu, Q., et al., The Effect of Manufacturing Defects on The Fatigue Behaviour of Ti-6Al- 4V Specimens Fabricated Using Selective Laser Melting. Advanced Materials Research, 2014(891892): pp. 1519. 14. Anon, Metallic Materials Properties Development and Standardization, 2016, Batelle Memorial Institute. 15. Barter, S.A., L. Molent, and R.J.H. Wanhill, Typical fatigue-initiating discontinuities in metallic aircraft structures. International Journal of Fatigue, 2012. 41: pp. 11-22. 16. Molent, L., et al., Using the lead crack concept and fractal geometry for fatigue lifing of metallic structural components. International Journal of Fatigue, 2016.
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17. Walker, K.F., Lourenco, J.M., Sun, S., Brandt, M., and Wang, C.H., Quantitative fractography and modelling of fatigue crack propagation in high strength AerMet100 steel repaired with a laser cladding process. International Journal of Fatigue, 2017. 94(http://dx.doi.org/10.1016/j.ijfatigue.2016.06.031): pp. 288-301. 18. Harter, J.A., AFGROW user's guide and technical manual, AFRL-VA-WP-TR-2004, 2004.
16
S0142-1123(17)30304-3 http://dx.doi.org/10.1016/j.ijfatigue.2017.07.014 JIJF 4410
To appear in:
International Journal of Fatigue
Received Date: Revised Date: Accepted Date:
2 February 2017 4 July 2017 13 July 2017
Please cite this article as: Walker, K.F., Liu, Q., Brandt, M., Evaluation of fatigue crack propagation behaviour in Ti-6Al-4V manufactured by selective laser melting, International Journal of Fatigue (2017), doi: http://dx.doi.org/ 10.1016/j.ijfatigue.2017.07.014
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Evaluation of fatigue crack propagation behaviour in Ti6Al-4V manufactured by selective laser melting K. F. Walker *a ,Q. Liua, and M. Brandtb a Defence Science and Technology Group, Fisherman's Bend, Australia b Centre for Additive Manufacturing, RMIT University, GPO Box 2476, Melbourne VIC 3001, Australia Abstract: Total fatigue life performance of high strength titanium alloy Ti-6Al-4V manufactured by Additive Manufacturing (AM) based methods such as Selective Laser Melting (SLM) is of significant interest and has gained much attention recently. Researchers often compare the total fatigue life of materials manufactured from SLM compared with conventional manufacture, and often the SLM lives are reduced because cracks initiate from “defects” such as porosity or Lack of Fusion (LOF). But for repair and alternative manufacture of complex and critical aerospace structures for example, the crack growth/propagation phase of the fatigue process is also very important and it has not been studied as extensively. The aim of the work presented here was to evaluate and better understand crack propagation under constant amplitude loading in Ti-6Al-4V samples manufactured using SLM with a variety of layer thicknesses and build directions (vertical or horizontal). The “as-manufactured” condition was studied, with no post heat treatment. Cracks typically initiated at LOF features which had a negative impact on the total fatigue life. The focus here was on the crack growth phase. Modelling was performed with a conventional Linear Elastic Fracture Mechanics approach using literature data obtained from testing on Compact Tension (C(T)) specimens which were also in the asmanufactured condition with a variety of build directions. The modelling provided a very useful correlation of the data and provided a way of assessing the LOF features in terms of an equivalent initial crack. The crack growth properties of the SLM cases were also compared against literature data for conventionally manufactured material. The work will lead to a better understanding of fatigue crack growth characteristics for components manufactured by AM methods such as SLM. That understanding is an essential requirement for full certification and acceptance into service for critical applications such as aerospace structures. Keywords: Additive manufacturing, selective laser melting, Titanium alloy, fatigue, fatigue crack growth
*
Corresponding author. Tel.: +61 3 9626 7961; fax: +1 3 9626 7089 E-mail address: [email protected]
1
1. Introduction Additive Manufacturing (AM) or 3-D Printing is an emerging technology which offers significant advantages compared with conventional manufacturing processes such as casting, forging and machining [1]. The advantages offered by AM include rapid prototyping, design freedom (including the ability to manufacture very complex geometries not possible with conventional methods), minimal waste, and without the need for tools and dies. Selective Laser Melting (SLM) is a powder-bed-based AM technology [2, 3]. With SLM, a laser is used to selectively melt powder spread evenly over a thin layer on a build plate. The laser traverses the build area in a carefully controlled speed and scan pattern. After the first layer is melted, the build plate is lowered by a small, pre-set distance (the layer thickness) and a fresh layer of powder is introduced. The process then repeats and a three-dimensional part is therefore created (or printed). But before AM parts can be used in fatigue critical applications, the mechanical properties, including fatigue behaviour, need to be well understood and characterised. Titanium (Ti) alloys have been extensively used in fatigue critical applications in the aerospace and biomedical fields. Ti alloys offer high strength and stiffness to weight ratios, good high temperature performance and fatigue resistance, and they are compatible with biological and composite materials. One of the most useful and commonly used Ti alloys being produced by AM and SLM in particular is Ti-6Al-4V [4-7]. Fatigue behaviour of SLM manufactured Ti-6Al-4V has been investigated in many recent studies [8-13]. The results to date have revealed very significant variability in the fatigue life for a given applied constant amplitude stress [12]. This variability may be due to a range of factors, including initiating defects and microstructure. Total fatigue life is sometimes considered in two phases, i.e. an initiation or nucleation phase, followed by a phase of stable crack propagation to final failure. In some cases, particularly in the presence of defects of a significant size due to porosity or lack of fusion, the crack propagation phase may be the dominant factor. Porosity and lack of fusion defects are often present in SLM Ti-6Al-4V. The SLM process melts small, overlapping regions which can produce Lack of Fusion (LOF) defects. LOF is caused by insufficient energy density leading to irregular shaped cavities which may contain trapped, unmelted powder particles. Porosity is caused by gasses trapped within the powder, a result of the powder production process, which then expand during the melting thus creating a pore. Results from a recent test program [13] were used in the present work. Fatigue testing under constant amplitude loading at a stress ratio R of 0.1 (minimum stress/maximum stress) was performed on Ti-6Al-4V specimens manufactured by SLM in the vertical and horizontal build directions. Fatigue crack growth rate data were sourced from the open literature [8, 9]. The crack growth rate data were obtained from testing using Compact Tension (C(T)) specimens also manufactured by SLM in both the horizontal and vertical build directions. The rate data were compared with data from the literature for conventional Mil-Annealed material. The Mil-Annealed rate data were consistent with the 2
data for the SLM manufactured specimens. Conventional linear-elastic fracture mechanics (LEFM) methods were applied using the AFGROW fatigue crack growth analysis code to correlate the range of fatigue lives against the range of equivalent initial defects, i.e. the LOF and porosity. Finally, the typical LOF and porosity defects present in the SLM fatigue specimens were correlated against the analytical range of equivalent initial crack sizes. This showed reasonable agreement which suggests that the scatter in fatigue lives in such cases can largely be explained and understood through crack growth analysis and assessment. These methods are a useful tool for the evaluation of fatigue performance in additive manufactured Ti-6Al-4V. The fatigue life in these cases appears to be strongly a function of the LOF and porosity defects acting as crack starters from very early in the loading history.
2. Experimental Details 2.1 Fatigue test details The Ti-6Al-4V fatigue test specimens as detailed in [13] were fabricated by SLM processing in an SLM Solutions SLM250HL system, which incorporated a 250x250x250 mm3 chamber and a 400 W maximum laser power. The processing parameters used were: (1) laser power of 175 W; (2) hatch spacing of 0.125 mm (3) pre-layer thickness of 30 μm (4) scan speed of 710 mm/s. A commercial Ti-6Al-4V powder was used. This powder was a gas-atomized and pre-alloyed powder. The average spherical particle size was 43 μm. All samples were built in a single run and the location on the build plate was random. The specimen geometry was an hourglass type with a diameter of 5.5 mm at the gauge section (see Figure 1). The specimens were built in two orientations; vertically built and horizontally built. Specimens were manufactured in a cylindrical shape initially with a diameter of 12 mm and then machined to the final dimensions as shown in Figure 2. The fatigue fracture surfaces were examined using both optical and scanning electron microscopes (SEM). Optical microscopy was used to examine the manufacturing defects in cross-sectioned samples, while the SEM was used to examine the fatigue crack initiation sites in the fractured specimens. Fatigue testing was performed on a 100 kN, digitally controlled MTS testing machine, in a laboratory air environment under load control with a stress ratio R of 0.1 and a frequency of 10 Hz. Two applied stress levels, 600 and 700 MPa, were used and five samples were tested at each condition. Despite this being a small sample size, it was considered sufficient to identify trends and meet the objectives of the present work.
3
Figure 1 : Fatigue test specimen geometry
(a) Horizontal Build
(b) Vertical Build
(c) Final Machined
Figure 2 : Fatigue test specimen manufacture, build direction shown by arrow
2.2 Fatigue test results The fatigue life results compared with literature data from MMPDS [14] are shown in Figure 3. The scatter in life results is significant as has already been mentioned earlier.
4
Figure 3 : Fatigue test results [13] compared with MMPDS literature data [14] Table 1 : SLM fatigue specimen test lives Build Orientation
Vertically built
Horizontally built
σapp = 600 MPa
σapp = 700 MPa
Spec. No
Life (cycles)
Spec. No
Life (cycles)
SLM-B1
2480645
SLM-D1
221184
SLM-B2
102693
SLM-D2
36912
SLM-B3
312853
SLM-D3
10047
SLM-5a
15491
SLM-D4
27590
SLM-20a
53441
SLM-D5
22838
SLM-H7
181324
SLM-H12
40138
SLM-H8
155354
SLM-H13
1766199
SLM-H9
288879
SLM-H14
500855
SLM-H10
276335
SLM-H15
135639
SLM-H11
230695
SLM-H16
213009
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Table 2 : Fatigue test lives and statistical analysis Stress Level (MPa) 600 700
Build Orientation
Avge Life (cycles)
Min Life (cycles)
Max Life (cycles)
Standard Deviation
SD/Avge Life
Vertical Horizontal Vertical Horizontal
593,024 226,517 63,714 531,168
15,491 155,354 10,047 40,138
2,480,645 288,879 221,184 1,766,199
949,392 51,952 79,210 636,396
1.60 0.23 1.24 1.20
The fatigue test data with some statistical analysis are shown in Table 1 and Table 2. Again, these results show very significant scatter/variability in the fatigue lives. The horizontal build at 600 MPa showed the least scatter, with the ratio of Standard Deviation/Average Life of 0.23. In the other three cases the same ratio ranged from 1.2 to 1.6 which suggests very significant variability. But significant scatter is also evident in the conventional Mil-Annealed data from the literature, refer to Figure 3. Power law fits to the MMPDS data show relatively poor fits with R2 values of 0.52 and 0.76 for the annealed sheet and annealed extrusion respectively. The AM material did show generally shorter fatigue life than the conventional Mil-Annealed material. The significant scatter for the AM material was thought to be a result of the variation in LOF and porosity defects and their shape, position and orientation relative to the loading axis.
2.3 Investigation into failure origins The origins of the fatigue failures in the SLM specimens were investigated in [13]. The failures typically initiated at porosity and LOF defects. The LOF defects often formed along the layer boundaries and were aligned parallel to them. This is thought to be due to insufficient energy to fuse the new layer with the previous layer. Cross-sections were taken and were polished and etched to reveal details of the LOF defects. Examples of the optical micrographs are shown in Figure 4. These defects are similar to others reported in the literature for SLM Ti-64 [10].
Figure 4 : Optical micrographs of the LOF defect in the cross-sections of the SLM Ti64 with the 30 μm layer thickness in the vertically built orientation at the high scan speed (both cross-sections 6
were etched to reveal the microstructures and defects): (a) Along the layer boundary and (b) LOF defect with unmelted powder particle (adapted from [13]) As reported in [13] the crack origin was investigated for the two shortest life specimens manufactured in the vertical orientation and subjected to the 700 MPa maximum stress. The shortest life result at 700 MPa vertical build was 10,047 cycles (Specimen SLM-D3) and the origin of the cracking was a LOF defect about 96 μm below the surface as shown in Figure 5 (a). The defect is irregular in shape, with a maximum dimension of about 400 μm. This is compared with the case with the next highest life (22,838 cycles for Specimen SLM-D5) which was also a LOF defect but this time about 222 μm below the surface, see Figure 5 (b). In that case the defect is also irregular in shape with a maximum dimension of about 250 μm . It was suggested in [13] that the longer life could be caused by the defect location, i.e. sub-surface compared with close to the surface. But the life will also be directly affected by the defect size, orientation, and sharpness (extent to which the defect is crack-like). The issue of equivalent crack size and a correlation against life is considered later.
Figure 5 : SEM micrographs of the fatigue crack origins at the different locations of the LOF defects in the SLM Ti64 specimens in the vertically built orientation: (a) ~ 96 μm below the specimen surface and (b) ~ 222 μm below the specimen surface. The crack origin for some of the horizontal build specimens was also investigated. One of the horizontal specimens tested at 600 MPa produced a life of 155,354 cycles and the origin of the fatigue cracking was an irregular shape LOF defect with a maximum dimension of about 300 μm and about 92 μm below the surface as shown in Figure 6(a). Another horizontal build specimen tested at 700 MPa produced a longer life of 213,009 cycles and the LOF defect at which the crack initiated was irregular in shape with a maximum dimension of about 300 μm and about 267 μm below the surface as shown in Figure 6(b). So it is evident that the shorter life results were associated with defects or features similar to those shown in Figure 5 and Figure 6. The defect/features are typically irregular in shape, with a maximum dimension around 250-400 μm.
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Figure 6 : SEM images of the fatigue crack origins at the different locations of the LOF defects in the SLM Ti-64 specimens in the horizontally built orientation: (a) ~ 92 μm below the specimen surface and (b) ~267 μm below the specimen surface. Because of the irregular shape it is expected that the equivalent crack dimension would be smaller than 100 μm. This concept of an equivalent fatigue crack size has been considered in some detail elsewhere, see for example [15, 16]. In particular, fatigue cracking in a direct laser melting deposition case for high strength steel [17] showed that porosity with a maximum dimension of around 200 µm was equivalent to a crack length of 47 µm, giving a ratio of about 4:1 in terms of defect maximum dimension to equivalent crack dimension.
3. Analysis Details A fatigue crack growth analysis was performed using Linear Elastic Fracture Mechanics (LEFM) with the AFGROW code [18]. The cracking was modelled as a surface flaw in a solid circular cross-section bar as depicted in Figure 7. The standard stress intensity solution for this case provided in AFGROW was used, and the crack was assumed to maintain a consistent semi-circular shape.
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Figure 7 : Schematic of assumed crack geometry A literature search was conducted to determine appropriate fatigue crack growth rate data to apply. Two suitable sources were found. Edwards and Ramalu [9] and Dhansay etal. [8]. Both sources conducted fatigue crack growth rate testing on SLM Ti-6Al-4V specimens in the as-manufactured condition with no heat treatment. The tests were conducted on Compact Tension specimens loaded under constant amplitude loading at R=0.1. They conducted tests on specimens manufactured in both the vertical and horizontal directions. The rate data are shown in Figure 8 along with data from MMPDS [14] for conventional Mil-annealed material.
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Figure 8: Fatigue crack growth rate comparison, R=0.1 The extreme cases of the fastest and slowest rate data were identified and are as shown in Figure 9. It was interesting to note that in both cases the data came from specimens manufactured in the vertical orientation. When considered along with the variation in rates as seen in Figure 8, a clear link between build direction and rate is not apparent from these data. Any differences are masked by the variability overall in crack growth rate properties.
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Figure 9 : Fastest and slowest crack growth rates, R=0.1 To determine if the range of fatigue lives observed could be correlated against the fatigue crack growth rate data, a parametric analysis was performed. The extreme cases of the slowest and fastest rates were considered and the initial crack size was varied in the range 2-200 μm. Based on the 4:1 ratio mentioned earlier and as determined in [17], this captured LOF/Porosity features ranging from about 8-800 μm which covers the expected LOF defect size range. The results for 600 MPa and 700 MPa are shown in Figure 10 and Figure 11 respectively. A bounded region which captures the range of initiating defect equivalent crack size and rate curve extremes is shown as the “life scatter region”.
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Figure 10 : Fatigue life as a function of initiating crack size for the extreme rate properties, 600 MPa maximum stress, R=0.1
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Figure 11 : Fatigue life as a function of initiating crack size for the extreme rate properties – 700 MPa maximum stress R=0.1 In both the 600 and 700 MPa maximum stress cases the full range of fatigue lives could be captured within the bounds of an initiating equivalent crack size of 4 - 40 μm which, based on the 4:1 ratio identified earlier, corresponds to LOF/porosity maximum dimension range of 16 – 160 μm and a rate curve between the slowest and fastest rates identified. This range compares reasonably well with the LOF/porosity defects and their shape and size as described earlier, although the upper end of the range at 160 µm is smaller than the maximum observed in these tests of about 400 µm for Specimen SLM-D3 tested at 700 MPa max stress. A detailed consideration of the range of initiating defects and the best correlation is summarised in Table 3. This comparison does not consider rate curves in between the slowest and fastest, so the initial crack sizes can be outside the 4 – 40 μm range. It should be noted that initial crack sizes would all be within the 4 – 40 μm range if rate curves in between the extreme fastest and slowest rates were considered. The life result is affected by a combination of the initial defect dimension and the growth rate properties.
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Table 3 : Comparison of best life correlation achieved Maximum Stress (MPa)
Build Direction
Min. Life (cycles)
600
Vertical
15,491
Max. Life (cycles)
2,480,645 Horizontal
155,354 288,879
700
Vertical
10,047 221,184
Horizontal
40,139 1,766,199
Best Correlation Rate curve Initial Intitial (fast/slow) Equivalent Defect Crack (μm) Max Size (µm) Fast 35 140 Slow 5 20 Slow 85 340 Slow 45 180 Fast 35 140 Slow 35 140 Slow 160 640 Slow 4 16
4. Discussion Fatigue testing under constant amplitude R=0.1 loading at two high peak stress levels (600 and 700 MPa, about 70-80% of the uniaxial yield stress) revealed significant scatter/variability in the total fatigue life. This was evident even though the sample size here was small, i.e. five specimens at each of two stress levels and two build orientations (vertical and horizontal). In most cases the standard deviation was larger than the average specimen lives, and the range of specimen lives spanned across up to three orders of magnitude. The objective of the work reported here was to determine the extent to which a simple Linear Elastic Fracture mechanics based analysis combined with an understanding of the type and size of initiating defects could assist in explaining the variability. The assessment was performed by applying measured crack growth rate data from the literature for Ti6Al-4V material manufactured by SLM and tested in the as-manufactured condition. The resulting range of initial equivalent crack sizes ranging from 4-40 µm which is expected to relate to a maximum LOF/porosity defect range of about 16-160 µm. This compared reasonably well with the LOF/porosity defects/features identified in the lowest life fatigue results here. This suggests that the benefits to be gained by reducing/minimising the LOF/porosity could potentially be estimated using this approach.
5. Conclusion Ti-6Al-4V specimens manufactured by selective laser melting and tested under constant amplitude cyclic loading displayed very significant scatter/variability in total fatigue life. This behaviour is consistent with reported results in the literature. Significant scatter is also evident in conventional manufactured material. In the case of the SLM asmanufactured samples, the life is controlled by a combination of the initiating defects 14
(typically LOF and/or porosity) and inherent variability in the fatigue crack growth rate characteristics of the AM material. The method used here offers significant potential for the assessment of LOF/porosity features in terms of fatigue performance for Ti-6Al-4V manufactured by SLM.
6. References 1. Huang, Y., and Leu, M.C., Frontiers of Additive Manufacturing Research and Education, 2014, University of Florida: USA. 2. Abe, F., et al., The manufacturing of hard tools from metallic powders by selective laser melting. Journal of Materials Processing Tech., 2001. 111: pp. 210-213. 3. Shiomi, M., et al., Rapid manufacturing of metal components by laser forming. International Journal of Machine Tools & Manufacture, 2006. 46(12): pp. 1459-1468. 4. Murr, L.E., et al., Microstructure and mechanical behavior of Ti–6Al–4V produced by rapidlayer manufacturing, for biomedical applications. Journal of the Mechanical Behavior of Biomedical Materials, 2009. 2(1): pp. 20-32. 5. Thijs, L., et al., A study of the microstructural evolution during selective laser melting of Ti– 6Al–4V. Acta Materialia, 2010. 58(9): pp. 3303-3312. 6. Vandenbroucke, B. and J.P. Kruth, Selective laser melting of biocompatible metals for rapid manufacturing of medical parts. Rapid Prototyping Journal, 2007. 13(4): pp. 196-203. 7. Vilaro, T., C. Colin, and J.D. Bartout, As-Fabricated and Heat-Treated Microstructures of the Ti-6Al-4V Alloy Processed by Selective Laser Melting. Metallurgical and Materials Transactions A, 2011. 42(10): pp. 3190-3199. 8. Dhansay, N.M., R. Tait, and T. Becker, Fatigue and fracture toughness of Ti-6Al-4V titanium alloy manufactured by selective laser melting. Advanced Materials Research. Vol. 1019. 2014: Trans Tech Publications Ltd. 248-253. 9. Edwards, P. and M. Ramulu, Effect of build direction on the fracture toughness and fatigue crack growth in selective laser melted Ti-6Al-4V. Fatigue and Fracture of Engineering Materials and Structures, 2015. 38(10): pp. 1228-1236. 10. Günther, J., et al., Fatigue life of additively manufactured Ti–6Al–4V in the very high cycle fatigue regime. International Journal of Fatigue, 2017. 94(Part 2): pp. 236-245. 11. Leuders, S., et al., On the mechanical behaviour of titanium alloy TiAl6V4 manufactured by selective laser melting: Fatigue resistance and crack growth performance. International Journal of Fatigue, 2013. 48: pp. 300-307. 12. Li, P., et al., Critical assessment of the fatigue performance of additively manufactured Ti–6Al– 4V and perspective for future research. International Journal of Fatigue, 2016. 85: pp. 130-143. 13. Liu, Q., et al., The Effect of Manufacturing Defects on The Fatigue Behaviour of Ti-6Al- 4V Specimens Fabricated Using Selective Laser Melting. Advanced Materials Research, 2014(891892): pp. 1519. 14. Anon, Metallic Materials Properties Development and Standardization, 2016, Batelle Memorial Institute. 15. Barter, S.A., L. Molent, and R.J.H. Wanhill, Typical fatigue-initiating discontinuities in metallic aircraft structures. International Journal of Fatigue, 2012. 41: pp. 11-22. 16. Molent, L., et al., Using the lead crack concept and fractal geometry for fatigue lifing of metallic structural components. International Journal of Fatigue, 2016.
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17. Walker, K.F., Lourenco, J.M., Sun, S., Brandt, M., and Wang, C.H., Quantitative fractography and modelling of fatigue crack propagation in high strength AerMet100 steel repaired with a laser cladding process. International Journal of Fatigue, 2017. 94(http://dx.doi.org/10.1016/j.ijfatigue.2016.06.031): pp. 288-301. 18. Harter, J.A., AFGROW user's guide and technical manual, AFRL-VA-WP-TR-2004, 2004.
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