Crack,microstructure,and phase-evolution in pulsed-wave LMD of NiCoCrAlY coating

Abstract

Laser metal deposition or laser cladding, is increasingly utilized for creating coatings and bulk structures in engineering components. While the occurrence of tensile residual stress and crack formation in coatings has been extensively studied with continuous wave lasers, there is limited research on pulsed wave lasers despite their potential to mitigate these issues. This study explores the use of substrate preheating through an induction coil to examine the possibility of reducing cracks in a pulsed wave laser-deposited NiCoCrAlY coating, a material commonly applied in high-temperature environments like gas turbines and traditionally deposited through thermal spray methods. The microstructures were characterized using electron microscopy with an integrated element analyzer. Residual stress and phase formation were investigated using Xray diffraction. The dye penetration test was employed to detect surface cracks. The coating exhibited significant transverse cracking at room temperature, which was progressively eliminated by increasing the substrate temperature to 700 °C. This temperature increase led to a substantial reduction in residual stress. The as-deposited coating at room temperature consists of a β-dominated dual-phase microstructure, which is almost retained in the crack-free condition despite compromising on the grain growth and the γ-Ni content to some extent. The crack-free coating achieved a 2.7-fold increase in scratch hardness compared to the substrate. This experimental study on pulsed wave laser deposition provides an effective approach for eliminating cracks in NiCoCrAlY coatings, offering insights into how pulsed wave laser deposition can produce durable, crack-free coatings for brittle alloys in high-performance applications.