The Effects of Combined Heat-Assisted Additive Manufacturing and Laser Re-melting on Martensitic Stainless Steel

Abstract

Laser powder bed fusion (LPBF) has become an attractive manufacturing method due to its ability to fabricate metals with complex designs. LPBF can build accurate parts with geometries that are not possible in traditional manufacturing processes. Many applications of LPBF such as dental implants, replacement prosthetic joints, and gas turbine engines have demonstrated significant improvement in mechanical properties and microstructure. However, to qualify the LPBF process as an industry standard, more research and testing are necessary using different parameters to understand the influencing factors of the material properties. In LPBF, the produced parts require post-processing. In most cases, the fabricated part needs heat treatment to enhance the microstructure and mechanical properties and achieve better homogenous material. Some applications need post- processing to reduce surface roughness. These needs have motivated many research studies on how to improve material strength and obtain the desired surface roughness for LPBF materials in general. In addition, the quality and mechanical properties of the materials must be considered. The disadvantage of LPBF is the formation of internal stresses. When the laser fuses a layer, the rapid cooling results in residual stress as the laser moves across its path. Laser remelting with a heat-assisted additive manufacturing process can improve the surface quality and relieve residual stresses; however, if the laser power and laser speed are not suitable, it can reduce the strength and hardness of the material which was within the range of typical wrought properties. There is a lack of understanding of the effect of laser remelting and its parameters on the resultant material properties and any improvement that can be achieved for LPBF for metals such as steel. The integration of laser re-melting and heat-assisted additive manufacturing is also not fully understood in terms of its contribution to improving materials' microstructure and reduction in the high residual stresses usually formed during LPBF. The first objective of this dissertation is to investigate the effects of laser re-melting on stainless steel material fabricated by heat-assisted additive manufacturing in terms of mechanical properties. The second objective focuses on identifying the effects of laser re- melting on stainless steel fabricated by heat-assisted additive manufacturing to study the phase transformation and the formation of martensite in the stainless-steel material as the phase transformation is affected by the cooling rate. The retained austenite results in anisotropy of the mechanical properties. It was found the heat-treated samples have a reduction in the amount of retained austenite, homogeneous grain structure, and better mechanical properties. The transformation of the martensite-austenite phase was observed between 533 °C and 928 °C for austenite in heating, and between 175 °C and 74 °C for martensite in cooling. The martensite changes into reverted austenite if the heating is below the austenite phase starting temperature. The third objective is to predict the phase transformation in additively manufactured martensitic steel through a neural network with experimental validation. Retained austenite in martensitic steel is influenced by various temperature and time parameters. An increase in retained austenite tends to reduce the strength of the material. Neural networks are among the most dominant machine learning approaches used for their ability to fit into various classification or regression problems. In this context, the neural network is employed to predict the retained austenite based on post-processing heat treatment process parameters.