Electrode Life and Weldability Improvement in Resistance Spot Welding of DP600

Abstract

This thesis demonstrates effective methods to improve weldability and electrode life in resistance spot welding (RSW) of zinc coated advanced high strength steel. Six kinds of electrodes were investigated to determine the effect of electrode design/material on electrode life and welding performance. The variables were material (Class 2 and Class 3), electrode shape (Dome and Parabolic), and coating on the electrode surface (Ni/TiC coating). Weld qualities were evaluated with mechanical tests, such as tensile shear and peel test, and analyzed using electrical weld signals. Since electrode life and weldability is strongly related to electrode surface degradation, the electrode surface was printed using carbon paper to clearly record the progression of the electrode life cycle. After finishing the electrode life test, the metallurgy of the electrodes was analyzed using standard cross sectioning techniques, optical microscopy, hardness and scanning electron microscopy (SEM) techniques. A new type of electrode, with core inserts, is examined for improvements to weldability and increase electrode life when welding zinc coated advanced high strength steel. By using a refractory material insert, the electrode exhibited a local high resistance which promoted weld nugget growth. This type of electrode made a thicker and larger weld nugget, and it improved weldability and mechanical properties, especially with regards to cross tension (CT) strength. The effect and mechanism of inserted electrode were verified with commercial FEA software. The results of different weld current flow, nugget formation, and heat distribution were explored. Additionally, welding signal analysis was performed to verify the performance of the electrode. This type of electrode was also life tested with various diameters and insert materials to measure the effects of these changes.