Influence of Excimer Laser Surface Melting on Microstructure and Corrosion Behaviour of AA6061-T6 Alloy and SiCp/AA6061- T6 Composit

This thesis presents an investigation of excimer laser surface melting (LSM) on AA6061-T6 alloy and SiCp/AA6061-T6 composite in terms of microstructure and corrosion behaviour. Hardness and wear resistance of the melted layer for both materials were also evaluated and compared with the untreated specimen to understand if the improvement of the corrosion resistance could be achieved without sacrificing the wear resistance. The intermetallic particles within the aluminium matrix are believed to initiate of both galvanic and pitting corrosion for both materials. To effectively dissolve these intermetallic particles, laser fluence from 1 to 8.5 J/cm2 with a number of pulses from 10, 25 to 50 were selected to achieve an optimisation of the LSM condition. It was found that the increase of laser fluence increased the melt depth, but also promoted the formation of defects such as micro-cracks and pores. For AA6061-T6 alloy, under the best laser condition (3 J/cm2 with 50 pulses), the amount of large intermetallic particles (2-10 microm), such as AlFeSi, AlFeMnSi and Mg2Si, were significantly reduced resulting in the formation of a relatively homogeneous and defect-free melt layer with only some small randomly distributed of intermetallic precipitates. For the SiCp/AA6061-T6 composite, under the best laser condition (6 J/cm2 with 25 pulses), decomposition of SiC particles was achieved as well as the dissolution of large AlMgSiCr and AlFeSi intermetallic particles in the melt layer. The melt layer had a relatively complex microstructure consisting of three different regions. MgO was found at the bottom of the melted layer which was discontinuous along the interface between the melted region and bulk material or in some places, at the bottom of the melted layer. The corrosion behaviour of both materials before and after LSM was evaluated using electrochemical measurements and immersion test in deaerated 0.6 M NaCl solution. After LSM the AA6061-T6 with and without SiC showed a better corrosion resistance compared with untreated alloys. The pitting potential of the LSM for both materials was shifted to a more positive value with a significant reduction of the passivation current density. In addition, an intergranular corrosion test based on the standard ASTM G110-92 showed that the LSM had significantly improved the corrosion resistance of both materials due to dissolution of intermetallic particles as well as the removal of the SiC particles in the composite material within the melted regions. In addition, the wear resistance of as-received SiCp/AA6061-T6 composites was found to be much higher than that of the LSM specimen. This is attributed to the decomposition of the majority of the SiCp in the melted region since the contribution to the hardness from the SiC particle in the untreated specimen is much greater that the Si-rich region in the melt layer after LSM.