Processing and Characterization of Laser-Cladded Coating Materials

Abstract

The feasibility of laser cladding as a surface modification process was experimentally investigated. Emphasis was placed on identification of the effects of independent critical process parameters such as laser power, process speed (interaction time), and feed rate of cladding powder mixture on the microstructure, compositional homogeneity, geometry (e.g., thickness and width), and mechanical properties of the developed coatings. Rapidly solidified coatings metallurgically bonded to AISI 1018 steel substrates were formed in situ by using a 10 kW continuous wave CO2 laser to melt a thin layer of the substrate as well as a powder mixture consisting of Fe, Cr, C, and W with a weight ratio of 10:5:1:1 delivered to the substrate by means of blown shielding gas and a pneumatic screw feed system. Various diagnostic methods, indentation hardness measurements, and scratch-resistance testing of the laser-cladded coating materials revealed a high degree of grain refinement, increased solid solubility and uniform distribution of alloying elements, high hardness, and appreciable resistance against plastic shear deformation when the important process parameters were optimized. Microstructure studies demonstrated that coatings with very fine or relatively coarse dendritic, feathery, and particulate type microstructures were obtained, depending on the processing conditions. This investigation verified that due to the inherent rapid solidification and high concentration of key elements in the surface, hard coating materials of novel microstructures and physical properties which can be tailored to the surface requirements of the application can be produced with minimum dilution and thermal distortion. Implications of the laser cladding process in tribological applications are also interpreted qualitatively in light of the obtained results.