Study On Macro/Microstructures,Mechanical Properties And Welding Mechanisms Of Resistance Spot Welded Galvanized High Strength Low Alloy Steel

Galvanized high strength low alloy steel plate has been widely used as vehicle body sheet due to its high strength, excellent ductility and favorable welding performance, which can greatly meet the demands of automobiles for weight losing, security, energy saving, environmental protection and corrosion resistance, etc. Resistance spot welding is the most extensive approach for the connection of body-in-white, and there are usually approximately 4000 to 6000 welds in a single vehicle. Thus, the investigation of the resistance spot welding of Galvanized high strength low alloy steel plate(HSLA 350) is of great practical significance.In the present work, the galvanized steel HSLA350 was used as research object. Parameter optimization of welding process of two galvanized high strength low alloy steel plates with same thickness(1.5 mm/1.5 mm) was carried out by controlling variables method. The optimized welding parameters were obtained as 9.5 KA for welding current, 18 cycles for welding time, 2.6 KN for electrode force, by analyzing the tensile-shear strength, microstructure and hardness distribution of the weld formed at various parameters. Furthermore, the effects of welding current(I), welding time(t) and electrode force(F) on diameter of plasticity circle and nugget, indentation, welding spatter and tensile-shear strength of the weld were also discussed. Welding current and time shared similar effects on the weld, while electrode force exhibited the opposite effects.The weld could be divided into base metal(BM), heat affected zone(HAZ) and fusion zone(FZ) by the distinction of microstructure. The microstructure of BM consisted of ferrite matrix and martensite distributed along grain boundary, and the FZ was made up of lath martensite, while HAZ contained the combination of ferrite, martensite and pearlite. In HAZ, from BM-neared area to FZ-neared area, the percentage of martensite and pearlite was increasing which agreed with the variation of hardness value.The formation of the weld FZ of six groups of steel plates with different thickness was compared and analyzed when the welding current and electrode force were fixed at optimized value while the welding time was changed. And it was found that it trended to form welding defects in short welding time, such as porosities, cracks, etc. Additionally, the outline of FZ was likely to be butterfly-shaped when welding time was short and the plates were thick. Moreover, FZ would shift to thicker plate, and the deviation would decrease with increasing welding time.The fracture process and fracture surface morphology of the weld of six groups of steel plates with different thickness were investigated. In the tensile-shear test, the angle caused by deformation was increasing with the increase of displacement. It was found there were three different basic fracture modes including fracture along the interface, button-typed fracture and tearing-typed fracture, and the weld in these fracture modes cracked from FZ, BM and HAZ, respectively. The combination of button-typed fracture and tearing-typed fracture mode also cracked from HAZ.