Gradation of microstructure, phase aggregate, and mechanical properties with vertical depth of surface alloyed or melted zone synthesized by laser surface alloying or melting of Inconel 718

Abstract

A high-power (6 kW) diode laser has been utilized to develop compositionally and/or microstructurally graded alloyed zone (AZ) over Inconel 718 (IN718) substrate by laser surface alloying (LSA) with Al or Si and compared with the same with only laser surface melting (LSM). As the concentration of alloying elements decreases and thermal history varies with vertical depth, the microstructure, phase aggregate, and relevant mechanical properties significantly vary in the AZ or melted zone (MZ). Orientation imaging microscopy on the cross-sectional plane shows columnar grains growing from the solid-liquid interface along the anti-parallel direction of the heat flow in both AZ and MZ. The columnar grains following LSM seem to maintain < 001 > as the preferred growth direction. However, LSA with Al and Si develops a different fiber texture along < 112 > and < 012 > , respectively. Variation of microstructure, composition, phase aggregate, hardness, and selected tribological properties have been studied at four equidistant vertical depths of the AZ or MZ prepared by serial sectioning from the top surface till the AZ/MZ-substrate interface. LSA of IN718 with Al shows the maximum resistance to sliding and oxidative wear followed by that with Si. LSM is not as effective as LSA in improving the hardness and wear resistance. Heat generation during the wear test causes partial oxidation of the worn surface. The bare substrate or that after LSM shows the highest vulnerability to oxidative wear, while IN718 after LSA with Al or Si registers the maximum resistance against such damage.