Investigation On Laser Welding Of Aluminum-steel Laser Lap Welding And Microstructure And Mechanical Properties

Aluminum-alloy laser tailor-welded blanks and Tailor-welded steel plate have been extensively applied in contemporary automotive industries. Despite the considerable economic benefits it gained filter wiring technology between aluminum and steel still has not been fully developed. This insufficient development is caused not only by general problems of aluminum-alloy laser welding, but also by high sensitivity of laser beam to the size of interface gap. Moreover, the distinctive physical and chemical performance differences between aluminum and steel lead to more problems on laser lap welding of dissimilar materials such as difficulty of weld shaping, uneven heating and stress concentration.The article raised a novel molten brazing method for Aluminum-steel laser lap welding, trying to fulfill the demands of automotive industry. This method successfully realized molten solder connection between 5A02 aluminum-alloy plate and ST04Z galvanized sheet steel. A laser lap welding process on equal-thickness 5A02 and ST04Z plates has been systematically investigated as initial step. And a set of experiments have been implemented with filter wiring and without filter wiring to determine the optimal range of Al-steel lap welding parameters. Furthermore, it provided the analysis on impacts and rules of various factors like welding speed, defocusing amount and galvanized layer thickness to laser absorption rate, weld shaping and welding macro quality. By using SEM to observe metallurgical structure of aluminum alloy laser connector, the types of intermetallics in welded joints are also determined based on EDS analysis. Besides, the article also gave performance tests on two kinds of laser welded taps and analysis to their fracture morphology.Experimental results indicated that during laser welding, the al-steel laser lap welding proces without filter wiring was instable and easy to cause welding defects like melt through and welding opaque. Yet the welding process with filter wiring was more stable with smooth and even welding lines. Under identical welding circumstances, the al-steel lap welding with filter wiring gave higher welding speed and welding width, but required smaller laser output power. After filter wiring, the tensile specimen ruptured in the heat affected zone of aluminum alloy and no necking phenomenon discovered on its fracture parts. Welding joints showed lower hardness but basically fulfilled the strength requirements. The tension break of filter-wired laser lap joints have been identified as tough fracture based on SEM.