Study On Capacitor Discharge Welding Of Al₂O₃ Fibre Reinforced Al-Matrix Composites

The spot welding of the Al₂O₃ fibre reinforced Al-matrix composites (MMCs)(Volume fraction of fibreφf =5%,9%,19%,26%) is conducted using a micro-type capacitor discharge welding machine. The microstructural morphology of the joint and formation of nugget are investigated, the cooling rate of nugget is calculated. Moreover, the effects of welding parameters and volume fraction of Al₂O₃ fibre on weldability of MMCs are researched. It indicates that:The capacitor discharge welding can realize the joining of the MMCs. Under capacitor discharge welding conditions, A regular, flat nugget is formed in the joint and the microstructural transition from nugget to matrix is rather well. According to the theoretical calculation of temperature field of nugget, it is revealed that the solidification time of nugget is very short and the cooling rate is high up to 3.76×10⁶K/s, thus the forming process of nugget has typical rapid solidification characteristic. The microstructure of nugget is refined remarkably and the Al-rich phase and Si-rich phase are produced in the nugget, which results in the formation of microstructure different from that of MMCs. A few of Al₂O₃ fibres are broken down to some extent, while most of them remain their original morphology, distributing in the nugget matrix randomly.The content of Al₂O₃ fibre in the nugget are increased with the volume fraction of Al₂O₃ fibre in the MMCs. The electrode pressure and weld voltage have obviously effect on the formation of joint, but the capacitance has no obviously effect on it. During the solidification of Al-Si alloy, the eutectic Si always nucleates and grows along the surface of fibres, leading to the increasing of interface density of Si/matrix near the fibres.Moreover, the difference of linear expansion coefficient between Al₂O₃ fibre and matrix are the main reasons of creating the microcracks in the nugget. It is considered that an effective way to eliminate the porosity and improve the joint quality isincreasing electrode pressure properly, removing hydrogen from MMCs and holding the MMCs dry. The commended welding parameters are:5%φf MMCs:U=75V,C=3300μF,F=18N;9%φf MMCs:U=80V,C=3300μF,F=17N; 19%φf MMCs:U=85V,C=3300μF,F=17N;26%φf MMCs:U=85V,C=3300μF,F=16N。...