X. Cao, M. Jahazi, NRC Institute for Aerospace, Montreal, QC, Canada; B. Rivaux, Ecole des Mines de Paris, Sophia-Antipolis, France; J. Cuddy, A. Birur, Standard Aero Ltd, Winnipeg, MB, Canada
Inconel 718 alloy is a precipitation-hardened nickel-iron base superalloy widely used in gas turbines, rocket motors, spacecraft, nuclear reactors, pumps and tooling due to its good corrosion resistance, high strength, and stable microstructure at elevated temperature. It has excellent weldability but suffers from liquation cracking in the heat-affected zone. However, very little has been reported on high power Nd:YAG laser welding of IN718 alloy. In this study, the weldability of IN718 alloy in two thicknesses (0.76 and 3.18 mm) was investigated using a 4 kW Nd:YAG laser system. The effects of laser power, welding speed, defocusing distance, heat treatment prior to welding and post-weld heat treatment conditions on butt joint quality are characterized from surface morphologies, joint shapes, welding defects, microstructure, hardness and tensile properties. It is found that (sound) welds without macrocracks and with minor (macro) porosity can be obtained, indicating that solid-state Nd:YAG laser welding is a suitable method for IN718 alloy. Optimum process window for welding of IN718 was constituted and sound weld joints with joint efficiencies of 90-100% were obtained. Tensile failure occurred predominantly in the fusion zone. There is a significant increase in hardness in the fusion zone probably due to the presence of the interdendritic Nb-rich Laves and the NbC precipitates inside the dendrites. The HAZ hardness was found to lie between the fusion zone hardness and that of the base material. Higher hardness values measured in the HAZ are probably due to the re-precipitation of the principal strengthening phases (γ’ and γ’’) during post weld cooling. The fusion zone is composed of fine and elongated dendrites. The heat affected zone has similar microstructure to that of the base metal but slight grain growth is observed.
Summary: Inconel 718 alloy with two thicknesses (0.76 mm and 3.18 mm) were welded using a 4 kW Nd:YAG laser system. The effects of processing parameters(laser power, welding speed and defocusing distance), pre- and post-weld heat treatment conditions have been investigated and characterized from surfaces morphologies, joint shapes, welding defects, microstructure, hardness and tensile properties.
