Fabrication of robust high-aspect-ratio Ti-6Al-4V ultra-thin-walled structures using powder bed fusion with in-situ focused infrared heating

Abstract

Thin-walled structures (thin walls) fabricated via laser-based powder bed fusion (PBF-LB/M) are prone to buckling at critical heights at which buckling of thin-walled structures begins (2.3 mm for ∼75 µm thin wall) due to excessive thermal residual stresses. The susceptibility of thin-wall buckling under thermal load is directly related to their high aspect ratio (height/thickness) and residual stress. When fabricating a part with a positive minimum feature size, minimizing the residual stress is crucial to achieving the designated part shape and height. Here, we demonstrate the printing of mechanically robust (high-strength) ultra-thin freeform Ti-6Al-4V thin walls at previously unprintable heights via a modified PBF-LB/M process. This was made possible by employing an in-situ focused infrared (IR) heating technique. Two focused IR heaters were employed to heat the powder bed and mitigate the thermal stresses. Residual stress measurements were conducted via X-ray diffraction (XRD), and the effect of IR heating temperature on residual stress was studied via thermomechanical simulations. Thin walls buckled at 2.3 mm build height without IR heating. When IR heating was employed with insufficient intensity (IR supply voltages <50 V), the maximum printable thin-wall height increased slightly; however, with optimized IR heating (50 V), the thin wall successfully reached its designated build height of 20 mm. This improvement was attributed to a maximum reduction of approximately 95 % in equivalent stress along the build height of the 20 mm tall thin wall. No major deviation in the wall thickness through the thin wall height was observed as a result of the IR heating. The ability to print tall sub-100 µm thin walls enabled us to analyze the tensile properties of these thin-walled samples for the first time. The ultimate tensile strength (UTS) values recorded for IR supply voltages of 40 V, 45 V, and 50 V were 1114 MPa, 1234 MPa, and 1184 MPa, respectively. Correspondingly, the maximum elongation values improved with increasing IR heating voltage, measured as 2.16 %, 2.28 %, and 2.45 % for 40 V, 45 V, and 50 V, respectively. The major finding from the testing indicated consistent material strength for thin walls below and above 100 µm thickness. However, the reduction in sample thickness led to a significant reduction in elongation.