Characterization of the microstructure and micro-segregation behavior on additively manufactured 17-4PH stainless steel using selective laser melting (SLM) was carried out. Additively manufactured alloys usually experience appreciable segregation of alloying elements. In 17-4PH steel, Cu segregates within the inter-dendritic regions of columnar ferrite grains during rapid cooling. This segregation of Cu results in an inhomogeneous matrix, where copper rich particles (CRPs) can precipitate in the segregated region during repetitive heating and cooling of the SLM. Thus, it is necessary to homogenize the as-built specimen with solution treatment to form and uniformly disperse the CRPs into small precipitates during aging.

In this study, segregation of alloying elements was investigated by using scanning electron microscopy (SEM) with secondary and back-scattered electron imaging. All solution treatment conditions were determined from thermodynamic predictions considering Cu segregation. Solution treatment was conducted on 1273, 1323, 1373, and 1473 K for 1, 2, 4, and 8 h. Subsequent aging conditions were selected according to the specifications of H900 (755 K for 1 h and cooled down with air cooling). Changes in the microstructure and CRPs after solution treatment was clarified with SEM, SEM-EBSD. Especially, changes in the content and size of CRPs were analyzed by image analysis of several SEM images, and the content of dissolved Cu was calculated to estimate the effect of solution treatment. In addition, the effect of homogenization was evaluated by measuring the difference in the Vickers hardness between the solution treated and aged samples.

Microstructure of solution-treated specimens were transformed into martensite. The CRPs didn’t disappear after all conditions of solution treatment. CRPs coarsened with time on 1273 K and the diameters of CRPs tended to decrease after solution treatment on 1323, 1373, and 1473 K with increasing duration. Homogenization effect measured by Vickers hardness increased with more intense solution treatments.