Surface hardening of aluminium alloys through Cu electrolytic coating

Aluminium and its alloys are attractive for many applications in chemical, automotive and aerospace industries because of their excellent properties as height- to- weight ratio, high electrical and thermal conductivities and good formability. However their hardness and wear resistance are poor in comparison to steels. Continuous efforts are made in order to preserve the mechanical characteristics of the aluminium substrate while improving coating performance [1-12]. In this work, a surface hardening process based on Cu electrolytic coating and subsequent diffusion heat treatments was studied for 7075 and 2024 aluminium alloys (Tab. 1, 2). Heat treatments in furnace at temperatures ranging between 470 °C and 500 °C for different times were performed in order to obtain surface hardening of the aluminium alloy by diffusion of Cu into the substrate. The effect of temperature and time on the microstructure and the depth of hardening was studied by SEM, EDS, XRD and microhardness tests. The heat treatments at 470° C for 8 and 16 h carried out on the Cu coated - 7075 AA samples induced principally the formation of AlCu and Al2Cu intermetallic phases characterised by hardness values of 600-850 HV0,1 (Fig.1, 2, 4). The higher values (850 HV0,1 ) are due to the presence of Zn in solid solution or probably in form of intermetallich phase Al3Cu5Zn2 [20] (Tab. 3, 4). The heat treatments at 495° C for 24 h carried out on the Cu coated - 2024 AA samples induced principally the formation of a surface layer constituted of Al2Cu intemetallic phase, characterised by the hardness of about 500 HV0,1 (Fig. 5, 8).