Lack Of Fusion Pores Research Articles

Effect of heat treatments on pore morphology and microstructure of laser additive manufactured parts

The slit-shaped, lack-of-fusion pores (LFPs) persistent in the additively manufactured (AM) metals are detrimental to their fatigue resistance due to the associated stress concentrations at the edges. Postbuild treatments, such as hot isostatic pressing (HIP), are routinely used to eliminate pores resulting in, however, inconsistent improvements in fatigue performance. This work critically examines the effectiveness of HIP to eliminate such pores in AM Ti-6Al-4V. In addition, conventional heat treatment (HT) is proposed as a candidate remedial process to alleviate the harmful effects of the LFPs. Specifically, the spheroidization effect of HT on the sharp edges of the LFPs is investigated. Lastly, the effect of HT and HIP on the microstructure is carefully analyzed, safeguarding the beneficial refined microstructure of AM Ti-6Al-4V.

Effect of heat treatment on fatigue crack initiation of laser powder bed fusion stainless steel 316L

Laser powder bed fusion (L-PBF) is an emerging additive manufacturing technique for building structural components. L-PBF processing defects, such as lack of fusion pores, promote fatigue crack initiation and shorten the fatigue life. With well-controlled processing, critical pores can be avoided such that the microstructure-driven intergranular crack initiation mode becomes operative. In this work, the fatigue crack initiation behaviours of as-built and solution annealed L-PBF stainless steel 316L were studied. Crack initiation of the as-built samples is driven by de-bonding of the dendritic grain boundaries. High temperature annealing results in the formation of thermally-induced defects, possibly via the reheat cracking mechanism and the nucleation of pre-existing gas pores. As heat treating could have led to recrystallization and annihilation of the original grain boundary defects, the thermally-induced defects became the new sites for crack initiation. In addition, heat treatment incurred significant reduction in yield strength, such that the interaction of fatigue and ratcheting strain accumulation dominated the deformation behaviour of the material. The resulting fatigue strength in the finite life regime was reduced by about 13% but the fatigue endurance limit was not affected.