Laser Cladding of Ni-Based Superalloys

Abstract

Laser cladding of Ni-based superalloys has been studied extensively in the recent past owing to their combined properties of high strength and corrosion resistance at elevated temperatures, due to which they are finding wide applications in gas turbine engines, land-based power-generation gas turbines, nuclear and fossil fuel power plants, etc. The cladding process is basically a welding technique employing laser as a heat source for the deposition by melting feedstock in the form of powder or wire onto the substrate of a component. This is widely adopted for providing protective coatings as well as refurbishing functional components. With the technological advancement, the process has evolved into additive manufacturing technology wherein any complex shape component can be fabricated layer by layer using its 3D CAD model. Laser cladding process offers refined microstructure compared to any other conventional techniques like thermal spray coatings, wire arc depositions, etc., due to the rapid cooling rates experienced by self-quenching. Also, the process offers minimum thermal damage to the component with reduced dilution and distortion. Because of these advantages, laser cladding has been employed to deposit a number of different types of Ni-based superalloys which have been developed over the past several decades to meet the stringent requirements of elevated temperature operating conditions. During the laser cladding process, the resulting microstructure which determines the mechanical and corrosion properties is dictated mainly by the thermal history, i.e., the cooling rate, melt pool lifetime, etc. The thermal history in turn depends on various laser process parameters, such as laser wavelength, laser power, beam diameter, scan speed, and powder feed rate, among others. While laser cladding process has flexibility to precisely control the thermal history to get the desired microstructure, it is important to understand their correlations. Therefore, this chapter is focused on the laser cladding of nickel-based superalloys and its challenges which includes the development of the process-cooling rate-microstructure-property relationship. Methods to control and improve the microstructure and mechanical properties via controlling the cooling rates or by the use of external auxiliary methods have been briefly discussed. In addition, considering its potential application in developing wear-resistant coatings, the discussions have been also extended to the deposition of metal matrix composite coatings, challenges involved, and methods to overcome them.