Martensitic to stainless steel gradient by laser-based directed energy deposition: Effect of thermal history on microstructure and hardness

This study presents the effects of laser-based directed energy deposition additive manufacturing processing conditions on local variations in microstructure and hardness across a sample graded from high-strength martensitic steel AF9628 to stainless steel 316L. Hardness was shown to decrease with increasing height in regions of constant composition. As the sample graded from AF9628 to stainless steel 316L, the phases changed gradually from martensite to austenite as expected, with austenite forming preferentially at solidification cell walls due to elemental segregation. Varying amounts of tempering were observed from varying heat accumulation along the height of the part and from processing pauses during fabrication. This study demonstrates how spatial variations in thermal history during functionally graded material (FGM) fabrication can affect intrinsic tempering of previous layers leading to significant variations in properties throughout a part and highlights the importance and complexity of path planning for FGM component fabrication.