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A novel surface modification method coupling surface melting and ultrasonic impact treatment (UIT) was developed to improve the wear resistance of S355 steel. The principle of ultrasonic impact electro-spark melting treatment (UIEMT) was presented, and the microstructure and surface plastic deformation layer were characterised by scanning electron microscopy, optical microscopy and X-ray diffraction (XRD). The obtained results revealed that UIEMT has more advantage than the UIT method. For UIEMT sample, fine microstructure with the average grain size of less than 2 μm was obtained, and the hardening layer was about 2.75 times deeper than the UIT one. The microhardness profile and residual compressive stress distribution showed a good correlation with the microstructural transition. In addition, no martensite transformation can be found by XRD. It was concluded that UIEMT can significantly improve the wear resistance without changing the chemical composition of the material.

This paper investigates the effect of workpiece magnetisation on the magnetic flux density and nugget shape during resistance spot welding (RSW) process. To this end, this paper analyses the magnetisation process of ferromagnetic workpieces. The magnetisation was calculated by the equivalent surface current. The effect of magnetisation on the nugget shape was studied for RSW of steel and a specially designed aluminium workpiece. The results show that the magnetisation of ferromagnetic workpieces will increase the dimensions of the weld nugget in the thickness direction. The magnetic flux density induced by workpiece magnetisation and that induced by welding current have the same order of magnitude (10 21 T). This indicates that the magnetisation of ferromagnetic workpiece is an important source of magnetic force acting on liquid nugget. Therefore, it should not be ignored in studying the physics of RSW process, especially for the fluid flow in RSW of ferromagnetic materials.