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The nugget growth and the residual stress evolution during a resistance spot welding process are calculated using the finite element simulation, based on the incrementally coupled electrical-thermal-mechanical modeling techniques. In the present simulations, the changes among liquid/solid phase and the evolution of microstructures are considered. In addition, differences among the stress-strain relations for the different microstructures are taken into account, because they seem to affect the residual stress field strongly. Case studies on welding of double sheet lap joints are demonstrated, and the simulated results showed good agreement with the experimental measurements.

Tensile testing technique for the small sample was newly developed. Small tensile specimens with gage length of 1mm were taken from spot welds of high tensile strength steel sheets, and stress-strain relationships and ductility of base metal, heat affected zone including corona bond and nugget were individually measured. Finite element analyses of spot-welded joints under the conditions of static and dynamic tensile-shear loading were carried out with these local mechanical properties to predict the fracture mode and strength of the joints. It was clarified that the effects of both nugget diameter and class of steels were evaluated with good accuracy.

The strength of steel sheets used in tailored blanks for automotive bodies has been rising. In order to obtain guidelines for the choice of appropriate welding processes and materials, the formability of welded steel sheets were investigated. The purposes of this work are 1) to determine an upper steel strength limit that can be welded and 2) to obtain the requirements of materials from a viewpoint of formability. Laser, mash seam, and plasma arc welding were employed up to 980MPa high strength steels. The results suggested that laser welding was the best welding process because of its small heat input. It could be applied to 980MPa steels. 590MPa was the maximum steel strength grade to which mash seam and plasma arc welding could be applied, because the mash seam and the plasma arc welding thermal cycles softened the HAZ of high strength steel. The requirement of materials for the formability of laser welded steels is high elongation of base metals.

This paper describes the lap welding of Zinc-coated steel sheet using a high power continuous wave YAG laser. The well-known problem of welding the Zinc-coated sheet is related to the low boiling point of zinc compared with the melting point of steel. During lap welding, zinc coating at the interface vaporize rapidly and causes defects1)2). In this study, therefore, lap welding was performed by YAG laser. The effects of type of coating layer, welding conditions, tensile strength and corrosion resistance after electro-deposition was examined. It was found that the weldability of coated steel is different by type of coating. Zn-Ni coated steel showed good weldability, but galvanealed steel inevitably pore pits with no gap set up. These defects not only lower the strength of joint, but also produce irregular bead where easily corroded after electro-deposition.

Vibration damping steel sheets (VDSS), which have sandwich structures with intermediate layers of resin, have been studied. The most important characteristics of VDSS for inner panels of automotive vehicles are the vibration damping properties, press formability and spot weldability. Vibration damping properties, which are quantified by loss factor,η, were influenced by both tanδ, which indicates damping capacity of resins, and elastic modulus of core resin. From a view point of vibration damping properties, resins with larger tanδ and relatively lower elastic modulus were favorable. Because these mechanical characteristics vary considerably with temperature, it is important to select the most suitable resin for the service temperature range. The relationship between noise reduction effect and loss factor of VDSS were also studied. It was experimentally confirmed that noise reduction effect of VDSS is proportional to the logarithm of their loss factor.