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Martensitic stainless steels are widely used in industries due to their high strength and good corrosion resistance performance. Precipitation-hardened (PH) martensitic stainless steels feature very high strength compared with other stainless steels, around 3-4 times the strength of austenitic stainless steels such as 304 and 316. However, the poor workability due to the high strength and hardness induced by precipitation hardening limits the extensive utilization of PH stainless steels as structural components of complex shapes. Laser powder bed fusion (L-PBF) is an attractive additive manufacturing technology, which not only exhibits the advantages of producing complex and precise parts with a short lead time, but also avoids or reduces the subsequent machining process. In this review, the microstructures of martensitic stainless steels in the as-built state, as well as the effects of process parameters, building atmosphere, and heat treatments on the microstructures, are reviewed. Then, the characteristics of defects in the as-built state and the causes are specifically analyzed. Afterward, the effect of process parameters and heat treatment conditions on mechanical properties are summarized and reviewed. Finally, the remaining issues and suggestions on future research on L-PBF of martensitic precipitation-hardened stainless steels are put forward.

In this study, an as-cast Mg–Gd binary alloy was bonded successfully via vacuum diffusion bonding at 500–560 °C in a closed graphite mould, and the microstructures and mechanical properties of the formed joints were analysed. It was found that Gd atoms diffuse to the bonding interface at temperatures lower than the eutectic temperature, with the resulting precipitated compounds improving the bonding strength. At temperatures higher than the eutectic temperature, the eutectic liquid produced at the interface gets squeezed into the residual voids, where it solidifies to form metallurgical bonds. Two types of grain boundary migration processes were observed at the interface in the joint formed at 550 °C. With an increase in the bonding temperature, the tensile strength of the joints first increased and then decreased, with the joint formed at 550 °C exhibiting a maximum efficiency of 88.3%. Casting defects in the base metals were eliminated during the bonding process with the use of a closed mould; this was also the reason that the joint produced at 550 °C showed a higher elongation than the as-cast alloy. However, the amount of the interfacial eutectic liquid increased sharply with the increase in the temperature. Thus, the joint formed at 560 °C showed a low elongation of 0.4% because a large number of microcracks formed in it owing to the solidification and shrinkage of the liquid phase.