Multi-Objective Optimization of Complex Thermo-Fluid Phenomena in Welding

This work is to investigate optimization of Gas metal arc welding (GMAW) by employing Multi Objective Optimization method. GMAW is a process that joins pieces of metal by heating them with an electric arc. The heat of the arc melts the surface of the base metal and the tip of the electrode. The electrode molten metal is transferred through the arc to the molten base metal to form the weld pool. The quality of the weld pool is characterized by the penetration depth, the bead height and width. These characteristics are controlled by a number of welding parameters. The subject of this paper is to establish the welding parameters that yield a weld pool which has a predefined geometry. The approach to this goal is by casting the problem of optimization of GMAW in the framework of Multi–Objective Optimization. 1. Introduction Gas metal arc welding (GMAW) is a process used to join pieces of metal in the automotive manufacturing industry. In this process the pieces of metal are heated by an electric arc. The arc is between a continuously fed filler metal (consumable) electrode and the work piece. The heat of the arc melts the surface of the base metal and the tip of the electrode. The molten metal from the electrode is transferred through the arc to the molten base metal to form the weld pool. As the welding arc travels along the joint, the base metal is melted at the front edge of the pool, while it solidifies at the back edge. Externally supplied shielding gas protects the electrode and the weld pool from contamination. The GMAW process has become very popular in the last 40 years because of its speed and ease of use. GMAW is a very complex process which is a result of interplay of different physical phenomena. This includes heat conduction with change of phase (melting and solidification of the metal in the weld pool); melting of the electrode, droplet formation, its detachment, and impingement onto the work piece; liquid–metal convection with free surfaces; surface–tension-driven convection (Marangoni effect); electromagnetic forces due to the presence of electromagnetic induction; interaction of the free surface with the arc plasma; and fluid flow in the weld pool [14],[2].The mathematical analysis and computational modeling of GMAW process are very difficult, because of the multi-physics involved and the multiscale (in time and space) complexity of the problem. The presence of free–boundaries also makes the problem more complex. The quality of the weld joint is characterized by the penetration depth, the bead height and width. Weld penetration is the distance that the fusion line extends below the surface of the material being welded. These characteristics are controlled by a number of welding parameters. It is understood that the most important welding parameters in GMAW process are the welding current, arc length (the distance from the tip of the electrode to the work piece) and the arc travel speed (the rate that the arc moves along the work piece). These parameters will affect the weld pool characteristics to a great extent. The welding current is associated to the amount of heat applied to the process and the weld pool penetration is directly related to the welding current. An increase or decrease in the current will increase or decrease the weld penetration respectively. As the arc length increases, the bead height decreases and bead width increases. The arc travel speed is the linear rate that the arc moves along the work piece. Welding speed affects both the width and penetration of the weld pool. With the lower speeds, too much metal is deposited in the base metal resulting in an increase in the weld pool height. At the higher speeds, the heat generated by the arc does not have sufficient time to substantially melt the base material resulting in a decrease in the weld pool height and width.