Microcrack mitigation during laser scanning of tungsten via preheating and alloying strategies

Abstract

Abstract When tungsten is processed by laser powder bed fusion additive manufacturing, the combination of high residual stresses and tungsten’s inherent ductile-to-brittle transition leads to a network of microcracks. While preheating is widely accepted as the most efficient way to reduce residual stresses in additively manufactured parts, thus far it has proven ineffective in completely eliminating microcracking in tungsten. In addition to preheating, changing the alloy composition is an increasingly popular approach to circumvent cracking. This work utilizes in situ high-speed optical imaging combined with thermomechanical modeling to elucidate the effect of preheating and alloying with 2\xa0wt% rare earth oxides on the cracking behavior in powderless single track and hatch strategy laser scans. Optical imaging reveals two distinct cracking mechanisms in the fusion zone and the heat affected zone, both of which are eliminated by substrate preheating temperatures of 773–873\xa0K. Due to the lower melting point of the rare earth oxide inclusions, the melt behavior becomes more turbulent and processing parameters need to be adjusted accordingly, but the cracking characteristics remained the same. The response to preheating was similar for both pure tungsten and the alloys tested in this study; there was no beneficial effect of the alloy additions. This work shows that the fundamental metallurgical cause of microcracking in tungsten can be circumvented, in a step towards structural applications of additively manufactured tungsten.