Enhancement of resistance element welding of AA6061 to DP600 steel by using external magnetic field

The application of aluminum (Al)/steel hybrid structure has a broad prospect in automobile weight reduction. In this work, AA6061/DP600 steel was joined using a resistance element welding (REW) method to achieve high strength and plasticity. To modify the deviation of weld nugget and increase the diameter of nugget, annular permanent magnet was designed and used. The effect of external magnetic field on microstructure evolution and mechanical properties was investigated. An electromagnetic stirring effect was generated inside weld nugget, resulting in a larger bonding area between Q235 rivet and DP600 steel. Convection in weld nugget contributed to restraint of dendrite growth and changes in heat distribution, leading to grain refinement at weld nugget. The microstructure of weld nugget and heat affected zone (HAZ) was bainite in magnetically assisted resistance element welding (MA-REW) specimens. The microhardness distribution presented a symmetrical distribution in both horizontal and vertical direction of cross section while the highest microhardness value occurred in weld nugget. Compared to REW specimens, MA-REW specimens achieved improvement of 10.2 % in peak load and 26.8 % in displacement during the tensile test. When the welding current reached 11 kA, the fracture mode of MA-REW specimen transformed from interfacial fracture to button fracture. Enlarged weld nugget size and refined grains were responsible for the improved performance.