Investigation Of Nugget Quality Strengthening In Resistant Spot Welding Aluminum

As resistant spot welding of aluminum alloy is used more widely, there is a high demand on their performance and quality. The key issue to improve the quality is to improve the mechanical properties of the spot welding. But the traditional method by monitoring spot welding nugget size to improve the quality is difficult to be guaranteed. In view of the macro-properties are affected by the microstructure and compose, this paper introduces new ideas by changing the microstructure of nugget to achieve the objective of strengthening the quality.It was found that rare earth elements can significantly improve the aluminum alloys properties. And carbon nanotubes used in aluminum alloys attracted people's attention. In this paper, we used rare earth Er, Ce and carbon nanotubes as the enhancement elements in the aluminum spot welding. Their mechanical properties were tested and microstructure and composition was observed and analyzed. Through studying their effects on the experiments results, we investigated their strengthening mechanism in the nugget.First we studied the characteristics of aluminum spot welding and analyzed the nugget strengthened process. Based on the strengthening effects of the rare earth elements and nanotubes on aluminum, we established the strengthening theory model of them on nugget.By studying the strengthening role of the rare earth elements Er, Ce on the nugget, it was founded that after adding rare earth elements the hardness and strength was increased about 10%. The grain of nugget zone was refined significantly. In the boundary and interior of the grain, a number of rare elements and its compounds were dispersed. This showed that parts of rare earth elements existed as solid solution in Al Matrix, part as the rare earth compounds. This improved that the elements not only can promote nucleation and refinement, but also can inhibit plastic deformation by hindering the dislocation movement. Thus they could play a strengthened role. The strengthening mechanism of the rare earth elements Er, Ce in the alloy nugget is mainly the second phase strengthening, fine grain strengthening and solid solution strengthening.In the study of the nugget with adding carbon nanotubes, the hardness and strength increased by 30%, and the fine grain effect was remarkable. SEM results showed that it had a uniform distribution in the grain boundary, and substrate interface was good. So the nanotubes can play a very good strengthening effect. The presence of carbon nanotubes in the nugget decided that it is through the load transformation, grain refinement and the impact of dislocation behavior to achieve the strengthen role.Experimental results on different elements of comparison showed that the rare earth element Ce is slightly better than Er on the strengthening effect. This is due to the different dissolved rate in aluminum between them. The strengthening effect of carbon nanotubes was better than the rare earth elements, mainly because the two strengthening mechanisms are different. Through the comparative analysis of strengthening effect by different elements, the reinforced way in spot welding of aluminum alloy was discussed.