Laser based additive manufacturing of tungsten: Multi-scale thermo-kinetic and thermo-mechanical computational model and experiments

Abstract

Coupling a multi-scale thermo-kinetic and thermo-mechanical model with critical experiments, the present study investigates the role of process parameters on the resulting densification, cracking behavior, and microstructural evolution in laser-based powder bed fusion (LPBF) of pure tungsten. Thermal management strategies comprising shorter scan length (1 mm) and/or substrate preheating (873 K) were explored to achieve higher densities and mitigate cracks. A finite element (FE) based thermo-kinetic and thermo-mechanical computational model was developed to simulate the process spanning from the melt pool scale to the component level. FE simulations indicated pronounced heat accumulation with reduced scanning lengths. Such increased heat accumulation resulted in melt pools with larger dimensions (width and depth) and more favorable rheological characteristics resulting in printed tungsten parts with higher densities. Using an input laser fluence (ILF) of 28.6 J/mm2 and a shorter scan length of 1 mm, a relative density of maximum 99.2% was achieved, the highest reported value using LPBF in case of pure tungsten. FE simulations indicated nearly 5-fold reduction in thermal gradients and 2-fold reduction in cooling rates associated with the processing conditions corresponding to low degree of cracking (ILFs:28.6 J/mm2 and ILF 15.9 J/mm2 augmented with substrate preheating at 873 K) compared to the processing conditions of high degree of cracking (ILF: 15.9 J/mm2). Further, high ILF samples were characterized by significant extent of in-grain misorientations and low angle boundaries indicating that process induced in-situ restoration mechanisms are operative, thereby potentially aiding in reduction of cracking.