Aluminium alloys treated by laser melting and alloying are known to have improved corrosion resistance due to modification of the surface passive film and more uniform dispersion of second phase precipitates in the laser processed region. In large area applications, overlapping of individual tracks corresponding to the width of the laser beam is often applied. This involves the melting and reheating of a portion adjacent to the previous track. This has been found to cause some problems at the overlaps between tracks with coarsening and micro-segregation. When laser alloying is used to form amorphous surfaces, crystalline phases have been found to appear in the heat-affected zone at the border of the neighbouring laser irradiation tracks. Substantial detrimental effects on the corrosion behaviour of the overlapped areas are observed in these cases. It is therefore necessary to study the behaviour of this overlapped area in order to determine a strategy for large area laser surface treatment.
This paper reports the characteristics and corrosion behaviour of the overlapped areas in the laser surface melting of 2014 aluminium alloy and the laser surface alloying of the same material with Cr, W, Zr-Ni or Ti-Ni. Microstructure and chemical composition of the overlapped area are characterised using optical microscopy, SEM, TEM and EMPA. It has been shown that micro-segregation is formed within the planar front zone of laser melted samples and microstructural coarsening occurs in heat affected zones caused by the reheating effect. Electrochemical testing has indicated the effect on pitting corrosion behaviour of the overlapped zone. The micro-segregation within the overlapped areas has been found to act as an initiation site for pitting corrosion in most of the cases. The understanding of this problem within the overlapped area has guided research on the use of wide laser beams for large area applications. Initial work on the use of a 30 mm width CO2 laser beam at 18 kW power for surface melting of aluminium is presented.