Laser welding of selected aerospace alloys
The literature relating to the laser welding of a number of alloys was studied, this revealed gaps in the current state of knowledge. It was found that no indepth parametric or inicrostructural investigations had been carried out into the laser welding of non-ferrous alloys. Although problems associated with the laser welding of aluminium alloys had been identified, there was little in the literature to suggest an indepth study into the effect of surface modifications in enhancing the laser weldability of this material had been carried out. The laser welding of aluminium based metal matrix composites has also yet to be studied in detail. A programme was therefore set up to carry out an indepth investigation into the laser welding of a number of alloys namely; Nimonics C263, and PE11, aluminium 6061, and aluminium 6061 based boron fibre reinforced, and silicon carbide particulate reinforced metal matrix composites. The study was aimed at developing an understanding of the microstructural effects of the laser welding process on the alloys, and assessing the structural integrity of the resultant welds. The effect of laser processing parameters such as laser power, laser beam traverse speed, lens focal length, and the manipulation of these parameters on the welding efficiency and weld area integrity was also investigated. Other tasks within the project included a study on the possibility of using an anodic film to enhance the laser weldability of Al 6061. Finally attempts were made to identify novel phases observed in the weld area of the composite materials. Nimonics C263, and PEll exhibited laser welds free of cracks and porosity. The difference in composition between the two alloys did not result in any significant dissimilarities in their response to the laser welding process. The welds in both alloys exhibited a fine columnar dendritic microstructure, and while carbides were observed in the interdendritic regions of the welds, electron optical analysis did not reveal any y precipitates in this region. It was concluded that for the welding of thin gauge materials above a threshold laser power the resultant welding efficiency shows a greater dependence on laser beam mode, and laser spot size, than on laser power, and beam traverse speed. Aluminium 6061 was not easily welded with a laser in its as received form, and the welds showed some degree of porosity. Anodising was found to improve the welding efficiency in this material. While the presence of an anodic film on the metal surface increased the welding efficiency of the alloy, no relationship was found between the thickness of the anodic film and welding efficiency in the range of film thicknesses investigated in this work. Weld regions were observed to be cellular dendritic in structure, with narrow heat affected zones. No precipitates or low melting point phases could be identified in the weld region. Melt zones were successfully produced in the composite materials, with the main problem encountered being that of porosity adjacent to the weld bead. It was shown that manipulation of the laser welding parameters resulted in a decrease in this porosity. In the weld beads, a number of novel phases were observed. These were identified with the aid of TEM, and SIMS analysis techniques.