Machine mechanical characteristics and their influences on resistance spot welding quality.

Abstract

Resistance spot welding (RSW) is a material joining process used predominantly in sheet metal assembly. RSW consists of three aspects:<italic> machine, process </italic>, and <italic>product</italic> (or <italic>weld</italic>). This research systematically studies all three aspects and targets their relationship. In this study, experimental investigation, analytical modeling, and FEA are utilized. hi addition, new approaches are developed. This research makes significant contributions in the following areas. (1) <italic>Characterization of welding force</italic>. Force change during welding is systematically characterized and analyzed under various conditions. The welding force increases for steel, which can be represented by an exponential model, but decreases for aluminum. The characteristics and mechanism of the force change are explored considering machine characteristics, such as stiffness, friction and moving mass; process parameters, such as preset force, current and time; and workpiece properties, especially yield strength. (2) <italic>Effect of forging force</italic>. Forging forces are investigated analytically and experimentally. The increase forging force can raise welding expulsion limits but reduce weld quality slightly; while the approach forging force can improve weld quality but reduces expulsion limits a little. Forging force should be applied optimally at about halfway through the weld stage for steel. In addition, the application of step current can enhance forging effect on weld quality. An approach based on the study of dynamic electrical resistance is developed. Accordingly, the relationship between forging force and weld quality is scientifically explained. The analytical results concur well with experimental observations. (3) <italic>Influences of machine mechanical characteristics on welding process and weld quality</italic>. The influences of machine stiffness, friction, and moving mass are investigated experimentally. Machine stiffness has a positive effect on weld performance; machine friction is unfavorable to weld quality except for aluminum weld indentation; and moving mass has no influence on weld quality. A methodology, process signature analysis, for investigating the relationship between RSW machine and weld quality is developed. Scientific understanding of the relationship is achieved using this approach. (4) <italic>Functionality and cost effectiveness of servo guns</italic>. The advantages of servo gun application are distinguished. It may improve welding productivity; upgrade process controllability, make forging force feasible, be cost effective, and be working-environment friendly.