Experimental Study on Laser-Induced Surface Damage of a Single-Crystal Nickelbased Superalloy under CW Fiber Laser Scanning Process

Abstract

Single-crystal (SC) nickel-based superalloy castings offer high temperature mechanical properties that result in superior gas turbine engine performance and durability. These castings undergo various precision machining operations to remove a significant amount of material while manufacturing. Here, nickel-based superalloys are one of the most difficult materials to be cut. Therefore, novel concepts are being employed to improve their machinability including lowering their surface strength. This paper presents the introduction of laser-induced surface damage (LISD) on a second-generation SC nickel-based superalloy using a CW (continuous wave) fiber laser. Laser scanning experiments were performed on SC specimens in as-cast condition with a laser power of 1000 W, beam diameter of 1.2 mm and scanning speeds from 5.5 mm/s to 16.5 mm/s. The cross-sections of the laser irradiated surfaces were investigated by measuring the irradiated geometries (IRG), micro-structural changes, micro-segregations, solidification cracking and heat affected zone (HAZ). The IRG shows conduction mode of penetration with a high width-to-depth ratio under a bigger beam diameter and top-hat type beam profile. The IRG boundaries have irregular profiles due to the dissolution of inter-dendrite regions and eutectic phases. The IRG showed fine dendrites and solidification cracks with reduced micro-segregation levels. The solidification cracking is mainly attributed to thermal stresses and the micro-cracking in HAZ is attributed to the dissolution of low melting Mo and Ti eutectics. The evolved HAZ ranges from 15 to 20 % of the IRG depth. The LISD volume is evaluated as IRG plus HAZ for removal by machining process.