Investigation of electron beam surface remelting of plasma nitrided spray-formed hypereutectic Al-Si alloy
Plasma nitriding of aluminium alloys is a suitable method for improving wear resistance because of the hard ceramic AlN layer formed. However, the surface’s load-bearing behaviour is greatly limited by the low hardness of the Al base material. Previous studies deal with pre-treatment (such as surface alloying) for improving hardness, but the effect is limited by the thermal instability of the layers formed at the nitriding temperature (approx. 475°C).
New investigations regarding improved load support of the thin AlN layer examine the treatment sequence of nitriding and subsequent EB remelting. This sequence is enabled solely by the higher melting temperature of AlN (approx. 2000°C) compared to the Al base material (approx. 515°C). Because of its broad range of beneficial alloying elements (Si, Fe, Cu, Mg), a hypereutectic Al-Si alloy (DISPAL S232), made by spray forming, was used as base material. The electron beam remelting process was carried out on samples with a nitride layer thickness of approx. 3 μm.
The focus of the present contribution is on the influence of both the beam deflection technique and the specific energy input on surface deformation, and therefore encompasses such factors as nitride layer damage, microstructure, phase formation and wear characteristics. As a result of the newly formed phases, grain refinement and oversaturation of the aluminium solid solution, the surface hardness beneath the nitride layer could be increased by up to three times compared to that of the initial base material. The estimated enhancement in load support was evaluated by unlubricated wear tests using a pin-on-disc configuration and scratch tests under constant loading conditions. Furthermore, the wear mechanisms were investigated by means of detailed SEM examination of the remelted surface layer.