| With the increasing applications of aluminum alloys in lightweight automotive,the quality of aluminum alloy joining becomes critical in white body construction.Resistance spot welding(RSW)is the most widely used joining approach in automotive industry,but meets great challenges when applied to aluminum alloy due to its high electrical conductivity,thermal conductivity,thermal expansion coefficient and the oxidation film.Welding defects,including nugget size fluctuation,expulsion,pores and cracks,can occur easily without dedicated control,resulting in significant degradation of joint performance and great challenges to its extensive application.In current research,6000 series and 5000 series aluminum alloy RSW have been experimentally studied from microstructure to mechanical properties.A thermo-electro-mechanical coupling finite element model was established to simulate and analyze the welding process.The main conclusions can be summarized below:(1)Microstructure and property differences in different zones of the aluminum alloy RSW were investigated through optical observation and micro-hardness grid mapping.Combined with transmission electron microscopy analysis and numerical simulation results,a hardened thermo-mechanically affected zone(TMAZ)was identified between the weld nugget and base metal.The TMAZ is a few millimeters in width,and consists of grains aligned with local deformation.Strain hardening induced by the electrode force and deformation was determined to be the dominant hardening mechanism in the development of TMAZ.The soft nugget shows equiaxed dendritic structure in the center and columnar structure at the perimeter.As for base metal,it consists of equiaxed grains and elongated grains along rolling direction.(2)Quasi-static tensile tests of aluminum alloy RSW with different combinations of thicknesses and alloys were conducted under TS and CP loading conditions.It was found that the fracture modes for TS and CP specimens were interfacial fracture and button pull-out fracture respectively.The peak load and the nugget diameter were positively related.Given the characters of RSW geometry and fracture modes,a new failure criterion based on stress intensity and structural stress was established to predict the fracture mode and peak load for RSW with different sheet thicknesses and nugget sizes under both TS and CP loading conditions.(3)The effect of paint bake treatment to the aluminum alloy RSW was also studied in this research to match the real manufacture process,by comparing the nugget size,microstructure,micro-hardness and mechanical property before and after paint bake.It was found that the strength of the welds increased after paint bake due to the precipitation hardening.CP specimens showed better bake hardening response than TS ones.With the combined effect of heat and strain during the welding process,the hardening impact is non-uniform in different regions of TMAZ during paint bake.(4)A two-dimensional axisymmetric finite element model was established for aluminum alloy RSW,and numerical simulation of welding process was carried out to investigate the temperature,strain distribution and dynamic resistance evolution.It was found that the formation of the aluminum alloy RSW can be divided into four stages: random nucleation,extensive fusion,nugget growth and nugget solidification.The proposed finite element model can accurately predict the nugget size and indentation depth,and bring new way for the design and optimization of welding process. |
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