Effect of Heat Treatment on the Microstructure and Hardness Property of Additively Manufactured Ti6Al-4V and Ti-6Al-4V-4.5wt.%316L Componentsfor Biomedical Applications.

AbstractThe continuous advancement of medical implants technology presents exciting possibilities.Laser Powder Bed Fusion (L-PBF) additive manufactured Ti-6Al-4V alloy implants showtremendous promise as they offer the potential for highly personalized implants, improved implantfunctionality and enhanced long-term outcomes. However, the microstructure and microhardnessproperty of components produced through L-PBF are inferior when compared to theircounterparts manufactured using traditional methods. Recent investigation presents aninnovative method that has potential to address these microstructure and microhardnesschallenges. This method creates a spatially modulated Ti alloy by adding 316L powder into Ti-6Al-4V powder before printing with L-PBF. The resulting alloy shows a reduced formation ofcoarse β columnar grains into more preferred equiaxed grains. However, the effect of heattreatment operation on this spatially modulated alloy has not yet been explored.Thus, this present study reports on the effect of heat treatment operation on the microstructureand hardness property of Laser Powder Bed Fusion (L-PBF)- fabricated Ti-6Al-4V and Ti-6Al-4V-4.5wt.%316L alloys respectively. The heat treatment processes include both super- and sub- betatransus temperature (Tβ). After the heat treatment processes, the samples were either water-quenched or air-cooled. X-ray Diffraction (XRD), Light Optical Microscopy (LOM), and ScanningElectron Microscopy (SEM) were the characterization techniques performed on these alloys andthese qualitative data gathered were correlated with their microhardness measurements. The as-fabricated Ti-6Al-4V alloy exhibited a martensite α՛ microstructure. On the other hand, the Ti-6Al-4V-4.5wt.%316L alloy possessed a β grain structure. Ti-6Al-4V alloy, subjecting it to the 1020˚Csuper-transus treatment followed by water quenching resulted in the formation of new α՛martensite microstructure and fine primary alpha. When the Ti-6Al-4V specimen was subjectedto 1020˚C and then air cooled, a bimodal microstructure comprising coarse primary alpha andα+β lamellar was formed. The 920˚C sub-transus heat treatment also produced α+β lamellarstructure. For the 1020˚C water-quenched Ti-6Al-4V sample, the microhardness value increasedby 7.3% and 11% in the transverse and cross-sections to the build direction axis, respectivelywhen compared to the as-built parts. Conversely, Ti-6Al-4V-4.5wt.%316L alloy exhibited evenlydistributed primary alpha plates in beta matrix for both the water-quenched and air-cooledsamples at 1020˚C and 920˚C. On the other hand, heat treatment at 800 ˚C retains the as-builtbeta grain morphology while alpha plates precipitate along the grain boundaries. Themicrohardness results suggest that performing only transus heat treatment on the Ti-6Al-4V-4.5wt.%316L alloy was insufficient to significantly alter its hardness properties.Key words: Additive manufacturing, Ti-6Al-4V, Laser powder bed fusion, microstructure control