| How can high-volume manufacturers resistance spot weld sheet steels with maximum quality, repeatability and efficiency? What are the key variables of this process? What are the requirements for feedback control? These are a few of the technical problems relevant in today's automotive industry. Effective answers to these questions stem from the knowledge of spot welding metallurgy, and the physical and cultural environment of the shop floor.; In the spot welding process, some of the classical variables are: material weldability, zinc coatings, metal fit, sheet thickness, electrode alignment, force, time, current density, cooling, and others. This work focused on ranking the known variables according to their relative influence on process control. Consequently, a number of variables were characterized in both laboratory and plant environments.; The results show that from a process control standpoint, peak efficiency and quality occur when operating close to the expulsion limit. Surprisingly, when welding about the expulsion limit, the key variables were found to be in the equipment maintenance area, rather than in the material properties and coatings area. Thus, through analysis, the long list of key variables was reduced to the following four important maintenance variables: (1) Electrical resistance of the total circuit. (2) Cooling water efficiency. (3) Mechanical condition of the weld guns. (4) Operating current.; A distant fifth variable was metal fit, but only when metal fit was extremely poor. Additionally, it was found that an electrode misalignment of less than 40% had little impact on quality, provided the process was operating at expulsion. And, the power factor was found to be an effective feedback signal for keeping the process centered at the expulsion limit. The results of the study, conducted in both laboratory and plant environments, are discussed at length in this dissertation. The recommendations were successfully implemented in a large automotive plant. |
