Implementation of external magnetic field to improve strength of St37 steel resistance spot weld

Abstract

The magnetic assist technique involves the interaction between an external magnetic field and electrical current which produces Lorentz force that influences the flow pattern of molten metal and ultimately impacts the appearance and microstructure of the weld. Many parameters may influence on this process such as welding current, time, and force as well as the working magnet distance (MD). In this study, the effects of the distance between the permanent magnet’s MD on the heat-affected zone (HAZ) and weld nugget zone (NZ) were examined through mechanical and macro- and microstructural analyses. The results demonstrated that MD has a strong influence on the magnetic flux density which determines the joint appearance, quality, and microstructure of St37 steel. Results showed that with increasing MD, HAZ increased from 8 to 22 mm2 while NZ decreased from 26 to 19 mm2, and also, the grain size increased with increasing MD and reaching 48 µm at MD was set to 9 cm. Moreover, hardness decreased at both areas through increasing MD from (120–110) HV at HAZ and from (170–150) HV at NZ. Under the action of (electromagnetic filed) EMF, weld tensile shear strength and the cross tensile strength give the highest values equal to 172 MPa and 155 MPa, respectively, when MD is set to 4.5 cm. Besides, soundness joint was obtained at MD = 4.5 cm which confirms that this is the best distance between magnets.