Spatial and temporal variation of hardness of a printed steel part

Several key industries routinely make complex parts using metal printing, but its continued growth will require the ability to control the microstructure and properties of parts. Many process variables affect the spatially variable thermal cycles that affect the microstructure and properties of parts. Here we show that the evolution of hardness of a tool steel part at various locations can be calculated using computed thermal cycles and a Johnson-Mehl-Avrami kinetic relation. The calculated hardness values agreed well with the independent experimental data for various processing conditions. At a given location, the hardness continued to decrease with progressive thermal cycles. Lower layers of the part experienced continued thermal cycles during the deposition of upper layers and the hardness decreased with distance from the top of the deposit. High heat input due to high laser power and slow scanning speed resulted in low cooling rate, high temperature, more pronounced tempering of martensite, and low hardness. Since the model can predict the spatial variation of hardness as a function of process variables, the work can serve as a basis for tailoring the hardness of some additively manufactured parts.