| Heat-treatable 6000 series aluminum alloy is an ideal light-weight material for automotive industry.AA6016 aluminum alloy has received a great deal of attention because of good formability and corrosion resistance.The most widely used assembly method for automotive aluminium sheet is resistance spot welding.Due to the high electrical conductivity,thermal conductivity and thermal expansion coefficient of aluminum alloys,it is difficult to control the quality of resistance spot welding joint.This thesis investigated a series of welding process parameters for 1mm thickness AA6016 aluminum alloy sheet.The range of process parameters for high quality of welded joints was determined.Besides,the effect of different welding parameters on the quality of resistance spot welding joint was analyzed.According to the experimental results and finite element simulation,the failure modes of welded joints were discussed.The main conclusions can be summarized as follow:(1)The optimized parameters of resistance spot welding for AA6016 aluminum alloy were: welding current 24-30kA;welding time 60-100ms;electrode pressure 3.6-1.8 kN.It can be concluded that welding current was the major factor.The increase in the weld current and time increased the peeled nugget size.The electrode force has little influences on peeled nugget size.(2)Base metal(BM),heat-affected zone(HAZ)and nugget were classified by different microstructure.The nugget could be divided into equiaxed grain zone and columnar grain zone.Temperature field and constitutional supercooling affect the dendrite size and morphology of grains in nugget.The hardness of the BM was the highest,followed by HAZ,and the nugget was the lowest.The microstructure of aluminum in HAZ was recovered and recrystallized due to the thermal effect.The lowest hardness of nugget related to the melted and resolidified microstructure,resulting in the decrease of strengthening effect for work hardening.(3)The nugget diameter was the main factor affect the failure mode.With higher heat input,the failure mode of resistance spot welding transformed from interfacial fracture to button pull-out fracture.The relationship between nugget diameter and failure strength was obtained by linear fitting,predicting a critical nugget size of 5.38 mm.The finite element method was used to simulate the failure modes.Both interface fracture and button pull-out fracture were observed with different nugget diameters,indicating that nugget diameter is the main influence factor.(4)A great many of shear dimples were observed under different failure modes.The energy spectrum analysis combined with the fracture morphology revealed that the iron-bearing phases exist in the nugget,which is secondary influence factor of fracture mode. |
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