Resistance Spot Welding Of Advanced High Strength Steels For Lightweight Vehicle Body Structure

With the progress of social civilization and sustainable development of the automotive industry, People’s demands on the environment as well as the performance of the car increasing, this prompted the automotive industry start to consider lightweight body structure. "Lightweight", "Green", "Fun to drive" has become an important direction of the automotive industry. The use of lightweight materials is an important way to achieve cars’ energy saving. And the birth of high-strength steel(HSS), advanced high strength steel(AHSS) means a low price, high performance lightweight materials has become an important solution to achieve lightweight car. But the widely use of high-strength steel especially AHSS was limited by the welding technology. Solve the welding problem of AHSS steels is of great practical significance and social value.This paper intended to solve nugget offset, deep indentation, serious splash, plate deformation and so on problems which always happened in the welding of Interstitial-Free Steel(IF) and Dual-Phase Steel(DP). The experiment plan was designed with control variable method and orthogonal test means. And optical analysis, tension and shear tests, positive pull test, micro-hardness analysis, spectroscopy and other means was used to invest the reason why the steels can not form a good weld nugget, and to reveal the mechanism of nucleation of welding. The experiment finally obtained the way to weld a good spot with good surface quality solder joints and its nugget size and position meet the requirements. A designed experiment was performed to test the stability of welding methods which this paper proposed. And the results show that this method can overcome the influence of the welding shunt and power fluctuations. Resistance spot welding can continue weld defect-free and qualified size nugget and neat surface welding spot with this welding method.The third generation of high-strength steels has overwhelming advantage compared to the second generation of high-strength steels. But welding research on the third generation of high-strength is in its infancy. This paper improved welding process by inserting cooling time between welding current pulse. The method successfully controlled the unsolvable welding spatter when welding 1200 MPa level AHSS. And the tensile shear strength of welding spot get a huge boost. Solder joint failure at the buttons interface changed from interface failure to button pull. The paper find the range of welding preheat current by experiment. It is unable to achieve effective weld of 1200 MPa level AHSS outside preheat current range, no matter how you adjust the welding current. In addition, the article studies the tempering current, and the best welding tempering time was obtained.