Argon-helium mixtures as Laser-Powder Bed Fusion atmospheres: Towards increased build rate of Ti-6Al-4V

Artikel i vetenskaplig tidskrift, 2020

The effect of the gas thermal properties and density on the laser powder bed fusion (L-PBF) process was investigated by using inert argon, characterized by high density, and helium, characterized by high thermal conductivity and heat capacity, and their gas mixtures. The results highlighted that for L-PBF of Ti-6Al-4V, the effect of residual impurities such as oxygen and nitrogen on the process stability and defect generation is prevailing the type of the process gas. However, by monitoring the residual oxygen level in the process atmosphere, the results showed that using the argon-helium mixtures allows to increase the produced material density upon higher build rates. High density, greater than 99.98 % is indeed achieved using a mixture of 50 % argon and 50 % helium, allowing for a build rate increase of 44 % in comparison to the standard build rate. The analysis of the produced material revealed the presence of thermal residual stresses attributed to an enhanced energy input when using the gas mixtures. The latter offer a positive balance of density and thermal properties, and in turn, probably reduce the accumulation of process by-products at the melt pool that interfere with and attenuate the laser radiation. The possible detrimental effect of the introduced residual stresses is efficiently eliminated by the conventional stress relieving treatment leading to the decomposition of α´martensite into a fine (α + β) microstructure. This study opens the perspective on the development of the gas recipes for improved process stability and increased productivity of L-PBF process.