Effects of Au plating on dynamic resistance during small-scale resistance spot welding of thin Ni sheets
- PDF / 1,949,120 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 59 Downloads / 222 Views
I. INTRODUCTION
IN resistance spot welding (RSW), the heat to form a weld is generated by the resistance to the flow of electric current through the sheets being joined. This resistance involves electrode and sheet bulk resistances as well as electrode-to-sheet and sheet-to-sheet contact resistances.[1] Study of the changes in resistance during welding (dynamic resistance) can help in understanding process mechanisms,[2] optimizing process parameters,[3] and developing quality control strategies.[4] Therefore, dynamic resistance behavior during RSW has attracted much research interest. Up to the present time, most detailed process studies of RSW have been concerned with “large scale” resistance spot welding (LSRSW) of relatively thick sheet steels (thicker than 0.6 to 0.8 mm) mainly for applications in the automotive and appliance industries.[5,6] For fabrication of electronic and medical devices, a small-scale version of RSW (SSRSW) is being increasingly used, requiring much more precise electrical and mechanical control, lower electrode force, and current/energy input. There is currently very little available information about fundamental process characteristics of SSRSW, including dynamic resistance behavior, and the present work is part of a series of investigations aimed at satisfying that demand. In a previous study,[7] weld nugget development has been characterized in SSRSW of 0.2-mm-thick Ni sheet. Both bare and gold-coated materials were welded, and large differences were found in the response of these materials related to differences in the physical/chemical nature of their surfaces. While the Au-coated sheet displayed solid-state bonding and brazing prior to the commencement of melting of Ni, the bare Ni sheet showed no bonding until base metal
melting began. The dynamic resistance behavior during welding of bare Ni sheets has been characterized,[8] and clear indications have been seen of oxide film breakdown and persistence of constriction resistance effects up to the beginning of melting. In the present work, the relationship between nugget development and dynamic resistance curves has been studied during SSRSW of Au-plated Ni, and the results are compared to those of welding on bare Ni. II. TECHNICAL BACKGROUND Contact resistance is a useful theoretical idealization of the real physical situation. For example, in experiments (Figure 1), the measured sheet-to-sheet resistances always include part of the sheets’ bulk resistance.[6] Therefore, the dynamic resistance, R, between two sheets will follow the equation: R RC RB
[1]
where RC and RB are the constriction resistance and bulk resistance, respectively. When sheet-to-sheet dynamic resistance is measured, as shown in Figure 1, the bulk resistance contribution RB involves an effective path length l of one sheet thickness and can be calculated using the following equation: l [2] RB r A where is the resistivity and A is the effective area of current path. For metals with clean surfaces, the contact resistance is equal to the constriction resi
Data Loading...
