Optimization of selective laser melting parameters and influence of post heat treatment on microstructure and mechanical properties of maraging steel

Additive manufacturing technologies such as selective laser melting enable the generation of 3D parts by monotonic and selective deposition of thin layers of material through computer aided operations. This technique enables the production of parts of intrinsic geometry that would be difficult to manufacture by conventional methods. The mechanical properties of additive manufactured parts depend on the materials, process parameters; and are expected to achieve properties superior or equal to those of conventionally manufactured parts. Maraging steels have in recent times found wide applicability owing to their excellent mechanical properties. During the selective laser melting process, the solidification process and the resulting microstructure assume a critical role that still demands a deeper understanding and control. To this end, there is need to carefully manipulate the selective laser melting parameters so as to yield optimum conditions for the desired mechanical properties.

A team of researchers led by Professor Zhong-Chun Chen from the Department of Mechanical and Aerospace Engineering, Graduate School of Engineering at Tottori University in Japan proposed a study whose main objective was to demystify the selective laser melting process in a bid to improve the end-use part properties. they purposed to optimize the process parameters and generate a process map, that would enable obtain high quality selective laser melted products and add on some new knowledge towards selective laser melting of maraging steels. Their work is currently published in the research journal, Materials and Design.

The research method employed commenced with the microstructural examination of the Nitrogen gas-atomized 18Ni (300-grade) maraging steel powder. Next, the design-of-experiment approach was employed in the optimization of the process parameters and post heat treatment process. The process parameters including scan speed, pitch, laser power, and spot diameter were varied and their effects on relative density, microstructure, surface quality, and hardness of selective laser melted 18Ni maraging steel were investigated.

The researchers observed that aging promoted reversion of martensite to austenite, but solution treatment and aging led to elimination of austenite. The authors of this work also noted that the specimens built parallel to loading direction exhibited a lower elongation when compared to those built perpendicular to loading direction. Additionally, it was seen that the as-built and aged specimens exhibited almost similar average grain size, while solution treatment/aging brought about grain growth of the martensite matrix and a noteworthy alteration in grain orientation.

The James Mutua and colleagues study has successfully presented an in-depth cross-examination of the influence of process parameters, such as laser power, spot diameter, scan speed and pitch, on densification behavior and surface morphology of selective laser melted maraging steel. Consequently, a process map has been constructed depicting the optimum SLM processing zone. The results obtained by Tottori University scientists indicate that the solution treatment caused almost complete disappearance of austenite phase in subsequently aged specimens. Altogether, the selective laser melted products exhibit higher strength and hardness than conventionally fabricated parts in their martensite state.