Selective laser melting of IN738 superalloy with a low Mn+Si content: Effect of energy input on characteristics of molten pool, metallurgical defects, microstructures and mechanical properties

Abstract

IN738 superalloy with excellent high-temperature performance is of huge interest for hot-end component applications in aerospace, but its additive manufacturing via selective laser melting (SLM) is rather challenging due to the unsolved high crack susceptibility. Here, for the purpose of improving the formability of IN738 in SLM, Mn + Si content was then strictly controlled in raw powder. An orthogonal experiment was then carried out under different energy inputs to investigate the characteristics of the molten pool, metallurgical defects, microstructures, and mechanical properties of IN738 by SLM. The results showed that high laser power was more likely to shape large melting depth and depth-to-width ratio under the same linear energy density (LED), rather than scanning speed. Four morphologies of the molten pool were observed in the entire LED range. At lower LEDs, hot cracks were usually accompanied by irregular pores due to the stress concentration at the sharp corner of pores; at higher LEDs, cracks were located at the interface of grain boundaries with large misorientation angles. Energy dispersive spectrometry results revealed that the presence of Al-rich oxides and high Mn + Si content at the interface were the main reasons for cracking. The Scheil-Gulliver solidification modeling was used to examine the cracking susceptibility of Si and Mn elements. In addition, both dendrites and cellular dendrites were found in columnar grain structures. By reducing the Mn + Si content and optimizing process, crack-free and nearly fully dense samples of IN738 were obtained by SLM with excellent mechanical properties (ultimate tensile strengths = 1206 MPa, yield strength = 866 MPa and elongation = 16.5%), which were much higher than the mechanical properties of conventionally manufactured IN738.