An investigation on laser surface modification of an A356 aluminium alloy

Abstract

The laser surface modification of an A356 aluminium alloy was investigated as a means to improve the tribological behaviour of the surface of such alloy. Laser remelting and alloying were carried out using a CO2 laser, equipped with a co-axial powder delivery system. The parameters needed to create uniform and repeatable surface layers were identified and optimized. Although limited hardness improvement was achieved with laser remelting, considerable improvement was achieved with laser alloying with nickel. A hardness improvement of nearly 3 times that of the untreated material was reported for uniform crack-free surfaces, whilst a 7-fold improvement was reported for fairly uniform surfaces that experienced moderate cracking. Microstructural, chemical and phase analysis of such surfaces showed that dendrites of Al3Ni and Al3Ni2 intermetallics were formed in the microstructures; these being the cause of the high hardness achieved. The introduction of ceramics to this aluminium nickel structure proved to be successful. Alloying using a novel technique involving Ni-Ti-C resulted in a uniform distribution of fine TiC particles in an Al-Ni and Al-Si-Ti intermetallic structure. Using Ni-Ti-SiC on the other hand resulted in a similar structure but with additional SiC particles, less TiC particles and less uniform distribution of the particles. The hardness values obtained were related to the overall content of alloying elements in the alloyed surfaces, and were similar to those obtained when alloying with nickel. Due to the fact that surfaces alloyed with Ni-Ti-C resulted in better dispersion of the carbides compared to the surfaces alloyed with Ni-Ti-SiC, only the former were further investigated. The wear resistance of both surfaces alloyed with nickel and Ni-Ti-C was highly improved when using an overlap rate of 25% and powder flow rates of more than 0.04 g/s, reaching values of over 22 times less volume loss compared to the untreated material. The type of wear in fact changed from severe, for the untreated material, to mild for the surfaces treated with these parameters. Corrosion testing showed that the alloyed surfaces had reduced corrosion resistance compared to the untreated material, therefore when these surfaces would be exposed to a corrosive environment while resisting wear, processing parameters more suited to limit this degradation need to be chosen at the expense of wear resistance.