Optimization and Microstructural Investigation of In-situ Fe-alloyed Ti6Al4V Fabricated by Laser Powder Bed Fusion method

Titanium alloys processed through laser powder bed fusion (LPBF) using in-situ alloying allow for a low-cost method to produce different microstructures and levels of solidification in titanium alloys. Pre-alloyed powder is not required to create these different alloys, as they are created using expensive powders. In this paper, iron additions of x = 1, 3, 5 weight percent were added to Ti6Al4V to explore how iron would affect both the processability of Ti6Al4V and the properties produced via LPBF. A process window was defined and an optimal processing condition was determined at P = 200 W, v = 800 mm/s, h = 0.105 mm, and t = 0.03 mm, which corresponds to VED ≈ 79.37 J/mm³. Powder blends containing iron additions of 1, 3, and 5 weight percent were mixed mechanically together before being printed into 67° rotated layers with bidirectional stripes on a TRUMPF TruPrint 1000 under high-purity argon (less than 100 ppm oxygen). Process consolidation was evaluated by three methods: optical image analysis (using ImageJ) of printed samples, Archimedes density of printed samples, and profilometry measurements of surface roughness (Ra, Rz). Microstructure/texture was evaluated using SEM/EBSD, and phase constitution using XRD. The 3 wt% Fe composition resulted in the highest level of consolidation among all compositions at the optimal VED condition; however, the improvements in relative density and reductions in apparent porosity of the 3 wt% Fe composition compared to the Ti6Al4V composition were small, but not dramatic. For example, at the optimal VED condition, porosity measured by Archimedes was approximately 1.5% (Ti6Al4V – 3 wt% Fe), down from approximately 2.2% (Ti6Al4V) and 2D optical image analysis indicated a decrease in area fraction porosity from approximately 0.10% (Ti6Al4V) to approximately 0.05% (Ti6Al4V – 3 wt% Fe); therefore, it appears there may be a shallow minimum in porosity at 3 wt% Fe. EBSD results indicate the presence of smaller beta grains and a weaker build direction texture in the Fe-containing samples when compared to the Ti6Al4V samples, consistent with improved melt pool wetting and increased track overlap. However, the magnitude of the microstructural changes observed in this study appears to follow the process optimization trends and not just the chemistry of the iron addition. Densification over the broader DOE (P = 160-200 W, v = 800-1200 mm/s, h = 0.09-0.115 mm, t = 0.03 mm; VED = 38.6-87.7 J/mm^3) followed the standard LPBF parabolic relationship between energy density and consolidation, with the maximum occurring at approximately 65-80 J/mm^3. Therefore, in-situ Fe alloying at 3 wt%, along with a process parameter set similar to 200 W / 800 mm/s / 0.105 mm / 0.03 mm (VED ≈ 79 J/mm^3), produces small increases in density and porosity control over the Ti6Al4V baseline. Additionally, this study presents a useful process-composition map and a multi-metric characterization methodology (Archimedes, ImageJ, XRD, SEM/EBSD) that can be applied to other Ti-based in-situ systems.