Product Code: ICAL05_P568

Laser Developed Al-Cr Surface Alloys: Microstructure, Mechanical and Wear Behavior
Authors:
Amelia Almeida, Instituto Superior Tecnico, Dep. Materials; Lisbon PORTUGAL
Patricia Carvalho, Instituto Superior Tecnico; Lisboa Portugal
Rui Vilar, Instituto Superior Tecnico; Lisboa Portugal
Presented at ICALEO 2005

Al alloys with enhanced surface hardness and wear resistance were produced by laser surface alloying Cr into Al. Surface alloys with composition ranging from 10 to 20 wt.%Cr were produced. Their microstructure consists of intermetallic compound particles dispersed in a matrix of a-Al. At low solidification rates the intermetallic compound particles are of Al4Cr phase with hexagonal crystal structure and a plate-like faceted morphology. At high solidification speed a transition to equiaxed solidification occurs at a critical solidification rate. The microstructure consists of equiaxed Al7Cr/a-Al eutectic cells formed by heterogeneous nucleation in the undercooled liquid ahead of the columnar solid-liquid interface.
The hardness of the surface layers varies in the range 130-260 HV. Their elastic and plastic behaviour was assessed by ultramicroindentation tests. Al-Cr alloys present Young?s modulus and hardness values that increase with increasing volume fraction of intermetallic compounds.
The wear behaviour of the alloys was investigated through microscale wear tests performed under dry sliding conditions. The influence of load on the prevailing wear mechanisms was analysed and related to the microstructure of the alloys. Wear resistance increases with increasing load due to the protective effect of stable mechanically mixed layers that form at the surface of the samples and the steel counterbody. Alloys with higher volume fraction of fine Al7Cr intermetallic compound particles organised in equiaxed cells provide better wear resistance than large plate-like particles of Al4Cr since a thinner, more stable and harder MML is formed, which offers best protection against wear.