High power Nd : YAG laser welding of SiC particle reinforced aluminium alloy 2124

Abstract

In this research, the effects of different YAG-laser output waveforms on the weldability of a SiC particle reinforced Al-alloy 2124 have been studied. In addition, a new laser joining technique has been developed with an aim to avoid the formation of harmful aluminium carbide phases in the fusion zone. This technique involved brush plating of nickel on the aluminium composite prior to laser welding. The results show that although the square waveform can produce the greatest depth-of-penetration amongst the three different waveforms studied, i.e. CW, sine-wave and square-wave, a high level of porosity was observed in the weld. This is believed due to the inherent fast cooling rate of the square-wave process. Whereas, porosity free welds with a reasonable depth-of-penetration could be obtained by using a sine-waveform operated at high peak powers. However, the results also clearly show that it would not be possible to stop the formation of aluminium carbides in the weld entirely simply by varying the laser output parameters and the waveform. The penetration depth of the fusion zone was successfully predicted by using a threshold laser intensity model. In order to stop the formation of aluminium carbides in the weld, nickel coating of a thickness between 0.15-0.25 mm was applied to the joining surfaces by means of a brush plating technique prior to laser welding. With an appropriate thickness of nickel coating, it was found possible to prevent the formation of aluminium carbides in the weld zone. Good metallurgical bond was obtained between the Ni coating and the composite matrix. Moreover, the results show that for the plated samples the power intensity required for achieving the same depth of weld penetration as in the case of the non-plated samples was at least 25% lower. Corrosion test results of the weld zone show that the overall corrosion current was reduced by a factor of 10 when nickel plating was employed. For the non-nickel plated joint the fusion zone would be completely destroyed after leaving it in laboratory air for two weeks. This was due to the hydrolysis of Al4C3 to Al(OH)3.