How do SLM process defects impact Ti64 mechanical properties?

Obtaining high density components is a trade-off between build rate and powder irradiation time (ie scanning speed). Achieving high density is usually the 1st step in parameters development for SLM and EBM. But do we have to assume (close to) 100% density to obtain satisfactory mechanical properties? This blog post addresses the impact of defects obtained in SLM on mechanical properties of Ti64.

Fully dense ≠ flawless

Many studies assess the mechanical properties of SLM and EBM Ti–6Al–4 V parts [1–4]. The consensus is that porosity has a strong impact on the mechanical properties of SLM, most notably on the dynamic properties [5,6]. Pure titanium specimens fabricated by SLM with densities higher than 95% [7] showed comparable tensile strength to wrought material. However their low impact and torsional fatigue strengths was linked to porosity. Additional reports on inferior uniaxial fatigue performance [8-11] in Ti–6Al–4 V SLM parts are related to porosity. It is also suggested [12] that micron-sized pores mainly affect fatigue strength, while residual stresses have a strong bearing on the fatigue crack growth.

Process parameters critically influence defect generation in SLM and EBM [13–17]. This blog post discusses the impact of two broad types of SLM defects [18] on the mechancal properties of Ti64 components:

• defects due “under-melting” and generated by incomplete powder melting or improper fusion between successive tracks or layers. 
  
• defects due to “over-melting” and caused by entrapment of gases or improper closure of a keyhole.  
  

Influence of density on mechanical properties of SLMed Ti64

Density 1
Density 2
Density 3
Density 4

Rockwell Hardness Rc
36±2.5
34±4
33±2.5
27±2.5

UTS (MPa)
1257±74
1148±80
1112±13
978±78

YS (MPa)
1150±91
1066±91
932±16
813±23

E%
8±2
5.4±3.8
6.6±1.4
3.7±0.6

In SLM, microstructure evolution is governed by the maximum melt pool temperature Tmelt_max combined with the cooling rate achieved during processing [2,19,20]. Defects such as pores and voids have little effect on the intragranular microstructure: that is the phase constitution and morphology.

“Under-melting”

Defects caused by insufficient energy input – typically large irregularities due to lack of fusion and presence of unmelted particles - have a strong bearing on the mechanical properties even when present in small amounts such as 1vol.%.
When the defects quantity increases to 5vol.%, the mechanical properties fall outside acceptable standard range.

“Over-melting”

Comparatively, defects caused by excessive energy input – typically small size symmetrical pores and voids due to over-melting effects - are less detrimental to part mechanical properties when present in low amounts up to 1vol.%
Yet, when the amount of these defects rises up to 5vol%, it catastrophically affects tensile, fatigue, and hardness properties.

References
[1] L. Facchini, E. Magalini, P. Robotti, A. Molinari, S. Höges, K.Wissenbach, Ductility of a Ti–6Al–4 V alloy produced by selective laser melting of prealloyed powders, Rapid Prototyp. J. 16 (2010) 450–459.
[2] L. Facchini, E. Magalini, P. Robotti, A. Molinari, Microstructure and mechanical properties of Ti–6Al–4 V produced by electron beam melting of pre-alloyed powders, Rapid Prototyp. J. 15 (2009) 171–178.
[3] E. Wycisk, C. Emmelmann, S. Siddique, F. Walther, High cycle fatigue (HCF) performance of Ti–6Al–4 V alloy processed by selective laser melting, Adv. Mater. Res. 16–817 (2013) 134–139.
[4] G.V. Joshi, Y. Duan, J. Neidigh, M. Koike, G. Chahine, R. Kovacevic, et al., Fatigue testing of electron beam-melted Ti–6Al–4 V ELI alloy for dental implants, J. Biomed. Mater. Res. B Appl. Biomater. 101B (2013) 124–130
[5] T. Sercombe, N. Jones, R. Day, A. Kop, Heat treatment of Ti–6Al–7Nb components produced by selective laser melting, Rapid Prototyp. J. 14 (2008) 300–304.
[6] Q. Liu, J. Elambasseril, S. Sun, M. Leary, M. Brandt, P.K. Sharp, The effect of manufacturing defects on the fatigue behaviour of Ti–6Al–4 V specimens fabricated using selective laser melting, Adv. Mater. Res. 891–892 (2014) 1519–1524.
[7] E. Santos, F. Abe, Y. Kitamura, K. Osakada, M. Shiomi, Mechanical properties of pure titanium models processed by selective laser melting, 13rd Annual International Solid Freeform Fabrication Symposium, Austin TX 2002, pp. 180–186.
[8] H. Gong, K. Rafi, T. Starr, B. Stucker, Effect of defects on fatigue tests of as-built Ti–6Al–4 V parts fabricated by selective laser melting, 23rd Annual International Solid Freeform Fabrication Symposium, Austin TX 2012, pp. 499–506.
[9] P. Edwards, M. Ramulu, P. Edwards, M. Ramulu, Fatigue performance evaluation of selective laser melted Ti–6Al–4 V, Mater. Sci. Eng. A 598 (2014) 327–337.
[10] E. Wycisk, A. Solbach, S. Siddique, D. Herzog, F. Walther, C. Emmelmann, Effects of defects in laser additive manufactured Ti–6Al–4 V on fatigue properties, Phys. Procedia 56 (2014) 371–378.
[11] M. Simonelli, Y.Y. Tse, C. Tuck, Fracture mechanisms in high-cycle fatigue of selective laser melted Ti–6Al–4 V, Key Eng.Mater. 627 (2015) 125–128.
[12] S. Leuders, M. Thöne, A. Riemer, T. Niendorf, T. Tröster, H.A. Richard, et al., On the mechanical behaviour of titanium alloy TiAl6V4 manufactured by selective laser melting: fatigue resistance and crack growth performance, Int. J. Fatigue 48
(2013) 300–307.
[13] B. Song, S. Dong, B. Zhang, H. Liao, C. Coddet, Effects of processing parameters onmicrostructure and mechanical property of selective laser melted Ti6Al4V, Mater. Des. 35 (2012) 120–125.
[14] H. Gong, K. Rafi, H. Gu, T. Starr, B. Stucker, Analysis of defect generation in Ti–6Al–4 V parts made using powder bed fusion additive manufacturing processes, Addit. Manuf. 1 (2014) 87–98.
[15] H. Gong, K. Rafi, N.V. Karthik, T. Starr, B. Stucker, Defect morphology in Ti–6Al–4 V parts fabricated by selective laser melting and electron beam melting, 24th Annual International Solid Freeform Fabrication Symposium, Austin TX 2013, pp. 440–453.
[16] S. Zhang, Q.Wei, L. Cheng, S. Li, Y. Shi, Effects of scan line spacing on pore characteristics and mechanical properties of porous Ti6Al4V implants fabricated by selective laser melting, Mater. Des. 63 (2014) 185–193.
[17] N. Hrabe, T. Quinn, Effects of processing onmicrostructure andmechanical properties of a titaniumalloy (Ti–6Al–4 V) fabricated using electron beammelting (EBM), Part 2: energy input, orientation, and location, Mater. Sci. Eng. A 573 (2013) 271–277.
[18] H. Gong et al. / Materials and Design 86 (2015) 545–554
[19] M. Simonelli, Y.Y. Tse, C. Tuck, Microstructure of Ti–6Al–4 V produced by selective laser melting, J Phys Conf Ser, 371 2012, p. 012084.
[20] K. Rafi, N.V. Karthik, H. Gong, T. Starr, B. Stucker, Microstructures and mechanical properties of Ti–6Al–4 V parts made by selective laser melting and electron beam melting, J. Mater. Eng. Perform. 22 (2013) 3872–3883.