Investigation of Thick Powder Layers in Laser Powder Bed Fusion of Ti6Al4V

Growing industrial adaption of additive manufacturing (AM) technologies has paved the way for designs, applications, and entire value streams which were previously impossible to realise. Within the group of metal AM, laser powder bed fusion (LPBF) offers outstanding technological maturity, which has resulted in widespread industrial implementation across the medical, aerospace, energy, and automotive sectors. A popular material in these demanding industries is Ti6Al4V alloy, combining high specific strength with excellent corrosion resistance and biocompatibility. The combination of LPBF and Ti6Al4V has received enormous attention in both research and industry, however several problems prevail in the additive fabrication of functional Ti6Al4V components. Due to rapid melting and solidification processes during LPBF, the as-fabricated microstructure is typically characterised by high strength and low ductility, caused by the formation of the metastable martensitic phase. For many applications, additional post-process heat treatment is required to tailor microstructure and mechanical performance, which adds time and cost to the manufacturing chain. Furthermore, high levels of residual stress arise within LPBF-fabricated material, leading to deformation and geometrical inaccuracy, deterioration of mechanical properties, and even part failure during fabrication. Lastly, the inherently low productivity of LPBF in comparison to conventional manufacturing, as well as other AM processes, drastically limits the number of technologically and economically viable business cases. It is well known that low productivity can be addressed by increasing the thickness of powder layers, which, melted successively on top of each other, from a three-dimensional part in LPBF. However, very few reports in literature have explored layer thicknesses above 100 µm, despite the potential for substantially reduced build times. This research aims to systematically investigate the limitations of thick-layer LPBF for Ti6Al4V alloy, while simultaneously evaluating the effects of increased layer thickness on microstructure, mechanical performance, and residual stress magnitude. This is achieved through process parameter studies for layers of up to 300 µm thickness, tensile testing, residual stress measurement, and extensive microstructural analysis. The results and findings of this research show that LPBF productivity can be improved from 2.55 mm3/s to 6.64 mm3/s without negatively impacting material quality (relative density > 99.9%) and tensile performance (~1 GPa yield strength, ~10% elongation) when the layer thickness is increased from 60 µm to 180 µm. However, limitations of the employed equipment, in particular a maximum laser power of 400 W, lead to porosity and poor ductility when the layer thickness is increased further. Thicker layers are also found to result in smaller magnitudes of residual stress, decreasing from 286 MPa at 60 µm to 236 MPa at 180 µm and 178 MPa at 300 µm layer thickness. The microstructure is observed to be similar to conventional LPBF Ti6Al4V in terms of phase constituents and morphology, however thicker layers were more susceptible to in-situ heat accumulation and formation of a decomposed, lamellar phase morphology. A novel process metric, combining laser parameters, layer thickness and layer-to-layer interval is proposed and its correlation with microstructural and mechanical properties demonstrated. The findings of this research could potentially be employed to reduce fabrication time in LPBF of Ti6Al4V, thus expanding the window of time- and cost-effective applications towards larger and bulkier components.