Effects of coating layer on weldability of electroconductive vibration damping sheet: Spot weldability of resin composite type vibration damping steel sheets (4th Report)

Abstract

Summary This paper describes an investigation of the effects of surface treatment of skin steel sheet on the resistance spot weldability of electroconductive vibration damping steel sheet containing metal particles in the resin layer. A Zn-Fe alloy plating layer is applied as the surface treatment. The effects of the volume fraction of metal particles and its optimisation are re-examined. In the weldability evaluation, the occurrence of spot defects is adopted as a quality evaluation index, and the changes in the weldability and resistance between the skin steel sheets depending on the surface treatment are investigated. Also considered are the changes in the weldability during electrode deformation. Pinholes occur more readily in vibration damping steel sheets provided with coated skin steel sheets than those provided with uncoated ones. The defect frequency decreases with a decreasing coating weight and an increasing volume fraction of metal particles. The pinholes are hollow-shaped and spread parallel to the sheet surface. They differ from the pinholes of globular type found in uncoated vibration damping steel sheets. When defects occur, the situation here is initially much the same as for normal welds, the voltage rises as high as around 20 V, the current decreases, and the initial high-voltage conditions are characterised by being long-lived for some 3–5 msec at a constant value. When pinholes occur, the steel sheets finally fuse with penetration of the skin steel sheets. However, the mechanisms differ: a Joule heating type in the uncoated steel sheets and an electric discharge heating type in the coated steel sheets. Arc discharge occurs by an electrical circuit being formed between the metal particles and skin steel sheets through the coated layer being vaporised. Pinholes occur more readily through the electrode face being deformed as the number of spot welds increases. In this case, local fusing of the skin steel sheets adjacent to the electrode is commonly found. This is due to the skin steel sheets being overheated through resin exclusion delay caused by electrode deformation. On the basis of the foregoing results, an attempt has been made to optimise the conditions of metal particle addition. This has led to the development of electroconductive vibration damping steel sheets exhibiting a favourable joint performance and being virtually free from defects even up to around 6000 spot welding cycles and even when the skin steel sheets are coated.