Investigation of force-controlled friction stir welding for manufacturing and automation

Friction stir welding (FSW) is a solid-state joining process for materials with low melting points. The process uses a rotating tool that consists of a shoulder and a pin. The tool plastically deforms the material with its pin and then forges together the parent materials underneath the shoulder. Past research has established that the axial force on the tool that creates the forging pressure is a function of plunge depth, traverse speed, and rotation speed. Historically, force control of FSW has been accomplished by varying the plunge depth of the tool. The research presented in this paper examines the force control of FSW by varying each of the process parameters separately. A force controller was implemented on a retrofitted milling machine. The closed-loop proportional-integral-derivative (PID) control architecture was tuned using the Ziegler-Nichols method. Welding experiments were conducted by butt welding (1)/(4) inch (6.35mm) x 1(1)/(2) inch (38.1mm) x 8 inch (203.2mm) samples of aluminium 6061-T6511 with a (1)/(4) inch (6.35mm) FSW tool. The results indicate that force control via traverse speed is the most accurate and, as a by-product, heat distribution control along the weld seam occurs. Force control via plunge depth is the least accurate but it compensates for machine and robot deflection. Tensile test data show that greater strength can be obtained through force control via rotation speed. It is concluded that force is maintained by keeping the amount of tool surface area in contract with the workpiece constant throughout the welding process when plunge depth is used as the controlling variable. Force is maintained by varying the rate of heat generation when rotation speed is used as the controlling variable. Lastly, force is maintained by changing the amount of heat deposited per unit length along the weld seam when traverse speed is used as the controlling variable. Successful robotic FSW requires to be selected the appropriate controlling variable and the sensitivity of the interaction between the tool and the workpiece to be reduced.