Microstructural characterization and mechanical properties of additively manufactured 17-4PH stainless steel

The aim of the present study is to carry out a comprehensive microstructural characterization on the Selective Laser Melting (SLM) printed 17-4PH stainless steel and to compare the results with commercial 17-4PH wrought alloy (in H1150 condition). It was found out that the middle region of as-printed SLM had less microporosities as compared to bottom and top regions which was attributed to the lower cooling rate in the middle-area and greater extent of fusion during manufacturing. The as-printed samples mainly consist of alpha-martensite and retained austenite (gamma). The electron back scatter diffraction (EBSD) results showed that the middle region had less retained austenite as compared to top and bottom ones. The fraction of high angle grain boundaries (HAGB) and twin boundaries (TB) in martensite in wrought alloy was found to be greater than the as-built samples. In contrast, a high fraction of HAGBs and low fraction of TBs were observed in austenite in as-printed samples. The SEM-TKD revealed that the fraction of phase boundaries close to Kurdjumov-Sachs (KS) orientation relationship (OR), i.e., x < 10 degrees, was 93.3% and 91.73% for the wrought alloy and SLM printed, respectively. The average Vickers hardness of as-printed sample was approximate to 25% less than the commercial wrought alloy (in H1150 condition) which can be attributed to more retained austenite, larger grain size, and more microporosities in the matrix. Monotonic uniaxial tensile test and in-situ tensile deformation with the aid of EBSD were used to examine the mechanical strength of wrought and SLM samples and microstructural evolution during plastic deformation. The ultimate tensile strength (UTS) of the as-printed sample (approximate to 1118 +/- 4 MPa) was quite comparable to commercial alloy (1017 +/- 2 MPa) but the uniform elongation was considerably higher in as-printed sample (21.8% +/- 0.3 in as-printed versus 9.00% +/- 0.56 in wrought alloy). EBSD analysis during the in-situ tensile test showed transformation induced by plasticity (TRIP effect) of gamma -> alpha. The area fraction of retained austenite in SLMed sample decreased from 26.2% in stage I (zero strain) to 0.49% in stage IV (failure). Lath and acicular shape retained austenite transformed faster to martensite than the blocky shape austenite.