Microstructure and mechanical properties of a 3D printed titanium alloy
The selective laser melting (SLM) process is increasingly being adopted for the manufacturing of metal parts as it provides better design flexibility and has the capability to produce near net shape complex parts, which is otherwise impossible to build by conventional methods. Another advantage of the SLM process is the rapid cooling, which results in ultrafine microstructure and improved mechanical properties. However, the dynamic interaction between the laser power and powder bed is difficult to control and needs further analyses. To bridge this knowledge gap, this thesis is a systematic study of the effect of SLM parameters on the structure and properties of Ti-6Al-4V. In the present study, tensile properties, mode I fracture toughness (KIc), fatigue crack growth behaviour, and unnotched fatigue strength of additively manufactured Ti64 alloy using selective laser melting (SLM) technique were investigated. Four different layer thickness (t) - scan rotation between successive layers ()combinations, which resulted in mesostructures that range from through-thickness columnar prior grains with square cross-sections, whose side lengths equal to the scan spacing, to near-equiaxed mesostructures in both build and transverse directions, were explored. Anisotropy in mechanical properties, if any, was investigated by conducting tests on samples whose loading axis is either parallel or perpendicular to the build directions. In all cases, the microstructure consisted of fine ’ lath structure, where ' is the metastable martensitic Ti phase that is acicular in shape, within the prior grains. Experimental results show that the process parameter combinations of t = 60 m and = 67º results in an alloy that exhibits high yield strength (>1100 MPa) and ductility (>12%) simultaneously, KIc of 58 MPa√m , and unnotched fatigue strength that is similar to that of the same alloy but manufactured using conventional means. The anisotropy in properties, overall, was found to be not substantial, even in the case where the columnar growth of prior grains occurs in the build direction. The values of the Paris exponents for steady state fatigue crack growth (FCG) is much lower than those reported for conventionally manufactured Ti64, suggesting higher FCG resistance in SLM Ti64. Analysis of the effective microstructural length scale that controls the near-threshold FCG rate suggests that it is t that dominates this behaviour. Overall, the results of this study indicate directions for process parameter optimization that would lead to SLM Ti64 that does not only have high strength but also is damage tolerant Effect of the post-processing heat-treatment on the microstructure and mechanical properties of SLM Ti64 were also investigated. In the as-built condition, the combination of t and ϕ resulted in the α’ microstructure, and mesostructure ranging from columnar prior β grain to equiaxed grains. Heat-treatment below the β Transus temperature of these samples transformed the α’ microstructure into α + β while retaining the prior β mesostructure. However, heat-treatment above β Transus temperature completely transformed the prior β structure. The tensile properties, mode I fracture toughness (KQ), and fatigue crack growth behaviour of all the samples after heat-treatment were investigated. Uniaxial tensile testing results show, after the heat-treatment strength of the samples dropped by 20%. However, ductility improved up to 88% with maximum ductility of 16.6% being observed in the sample with the combination of ϕ = 67˚ and t = 60 μm. The fracture toughness of the samples improved up to 104%. However, the sample with columnar prior β grain resulted in 27% lower KQ when the loading direction was perpendicular to the columnar grain, i.e., perpendicular to the build direction. In the sample with equiaxed prior β structure KQ =101 and 95 MPa√𝑚 was observed in parallel and perpendicular to build direction respectively. Results indicate that prior β structure plays an important role in the fracture toughness, however, it has minimal effect on the steady state fatigue crack growth (FCG) rate. The value of Paris exponent for the steady state FCG (3.2-3.9) is comparable to the values of conventionally manufactured Ti64. Very high FCG threshold value (ΔK0 ~ 7.1-8.4 MPa√𝑚 ) was observed in all the sample because of the high crack path tortuosity caused by α + β basket weave microstructure. Analysis of the microstructural length scale and the crack tip plasticity shows higher crack tortuosity was caused by transition in the crack growth mechanism from trans-granular to intergranular near the threshold. Overall, the study shows that the post-processing heat-treatment can improve the damage tolerant properties of SLM Ti64 beyond attainable by the conventionally manufactured Ti64. The heat-treatment slightly improves the FS by delaying the crack initiation from the pores. The micro-CT analysis of the pores suggest that, higher energy density (J) results in higher porosity due to Marangoni convection effect, but ϕ = 67˚ significantly reduced the porosity.