The Importance of Laser Beam Power on the Microstructure and Wear Behavior of Al-WC Composite Layers Produced by Laser Surface Alloying

Laser alloying was used to form metal matrix composite layers strengthened by WC particles. The process parameters were selected in such a way that there was no complete melting of the WC particles. Four different laser beam powers (from 0.65 kW to 1.3 kW) were used, generating different temperature distributions during processing. The temperature across the laser track axis was determined according to the mathematical model proposed by Ashby and Esterling. All layers produced contained unmelted WC particles in an aluminum-based matrix. The depth of the WC-Al composite layers strongly depended on the applied laser beam power. The lowest thickness of 198 ± 36 µm was measured for the layer produced at a laser beam power of 0.65 kW. A twofold increase in power <i>P</i> was the reason for obtaining a thickness <i>th_AZ</i> = 387 ± 21 µm. The power of the laser beam also affected the percentage of the substrate material (7075 alloy) in the molten pool during the laser processing. As a result, the highest amount of substrate material was obtained for the WC-Al composite layer produced using the highest laser beam power <i>P</i> = 1.3 kW. Simultaneously, this layer was characterized by the lowest percentage of tungsten carbide particles in this layer. The temperature profile along the axis of the laser track and also the maximum temperature reached confirmed the difference in the bonding between the reinforcing WC particles and the metal matrix. For <i>P</i> = 0.65 kW, too low a temperature was reached for the tungsten carbide particles to overmelt, resulting in poor bonding to the metallic matrix in the layer. Moreover, the layer showed serious defects such as discontinuity, porosity, and cracks. As a result, the WC-Al composite layer produced at the lowest laser beam power was characterized by a wear resistance lower (<i>I_mw</i> = 6.094 mg/cm²/h) than the 7075 alloy without surface layer (<i>I_mw</i> = 5.288 mg/cm²). The highest wear resistance was characteristic of the 7075 alloy laser alloyed with a laser beam power equal to 1.17 kW (<i>I_mw</i> = 2.475 mg/cm²/h). This layer showed satisfactory quality and adhesion to the substrate material.