Laser developed Al-Mo surface alloys: Microstructure, mechanical and wear behaviour

Molybdenum forms a significant proportion of hard intermetallic compounds with Al, which greatly increase the strength of the material. Hence, Al-Mo alloys are potential candidates for coating materials to improve the surface hardness and wear resistance of Al-based light alloys. Surface alloys with composition ranging from 14.8 to 19.1 wt.% Mo were produced by laser surface alloying, injecting Mo powders into the melt pool created in Al substrates using a CO2 laser. Their microstructure consists of intermetallic compound particles dispersed in a matrix of alpha-Al solid solution. The intermetallic particles present acicular morphology at low scanning speeds and an equiaxial, flower-like shape at high scanning speeds. TEM characterisation shows that both types of particles correspond to the rhombohedral intermetallic compound Al5Mo(r), suggesting that the different morphologies result from changes in the local solidification conditions. The hardness of the Al-Mo alloys varies in the range 85-160 HV, while their Young's modulus, assessed by ultramicroindentation tests, varies from 84 to 92 GPa. Wear resistance was measured in dry sliding conditions against a hardened steel counterbody. The wear coefficient decreases with increasing load due to the protective effect of stable mechanically mixed layers that form on the surfaces of the samples and the counterbody during sliding. The wear mechanism in Al-Mo alloys is predominantly adhesion and material transfer and oxidation, with a minor contribution of abrasion. (c) 2005 Elsevier B.V. All rights reserved.