William R. Longhurst

Bio: William R. Longhurst is an academic researcher from Austin Peay State University. The author has contributed to research in topics: Friction stir welding & Welding. The author has an hindex of 8, co-authored 13 publications receiving 541 citations. Previous affiliations of William R. Longhurst include Vanderbilt University.

Torque control of friction stir welding for manufacturing and automation

Abstract: Friction stir welding (FSW) is a solid-state welding process that utilizes a rotating tool to plastically deform and forge together the parent materials of a workpiece. The process involves plunging the rotating tool that consists of a shoulder and a pin into the workpiece and then traversing it along the intended weld seam. The welding process requires a large axial force to be maintained on the tool. Axial force control has been used in robotic FSW processes to compensate for the compliant nature of robots. Without force control, welding flaws would continuously emerge as the robot repositioned its linkages to traverse the tool along the intended weld seam. Insufficient plunge depth would result and cause the welding flaws as the robot’s linkages yielded from the resulting force in the welding environment. The research present in this paper investigates the use of torque instead of force to control the FSW process. To perform this research, a torque controller was implemented on a retrofitted Milwaukee Model K milling machine. The closed loop proportional, integral plus derivative control architecture was tuned using the Ziegler–Nichols method. Welding experiments were conducted by butt welding 0.25 in. (6.35 mm) × 1.5 in. (38.1 mm) × 8 in. (203.2 mm) samples of aluminum 6061 with a 0.25 in. (6.35 mm) threaded tool. The results indicate that controlling torque produces an acceptable weld process that adapts to the changing surface conditions of the workpiece. For this experiment, the torque was able to be controlled with standard deviation of 0.231 N-m. In addition, the torque controller was able to adjust the tool’s plunge depth in reaction to 1 mm step and ramp disturbances in the workpiece’s surface. It is shown that torque control is equivalent to weld power control and causes a uniform amount of energy per unit length to be deposited along the weld seam. It is concluded that the feedback signal of torque provides a better indicator of tool depth into the workpiece than axial force. Torque is more sensitive to tool depth than axial force. Thus, it is concluded that torque control is better suited for keeping a friction stir welding tool properly engaged with the workpiece for application to robotics, automation, and manufacturing.

Heated Friction Stir Welding: An Experimental and Theoretical Investigation into How Preheating Influences Process Forces

Abstract: As friction stir welding (FSW) has expanded to welding higher strength materials, large process forces and extreme tool wear have become issues. One possible solution is introducing an additional heating source in front of the FSW tool which softens the material and reduces the tool loads. We investigate the advantages of elevating temperature. Bead on plate welds were performed with a Trivex tool in aluminum alloy (AA 6061) heated to initial material temperatures up to 300°C. Macrograph cross-sections of the welds revealed a slight increase in material flow with increasing temperatures. More significant, the welding forces were analyzed to reveal up to a 43% reduction in the axial force with even moderate heating. An intriguing trend is observed that the process forces do not decrease steadily with increasing initial temperature, as might be expected, but exhibit a more complex polynomial shape, which actually increases for some heating intervals.

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

Abstract: 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 / 4inch (6.35mm) ·1 1 / 2inch (38.1mm) ·8inch (203.2mm) samples of aluminium 6061-T6511 with a 1 /4inch (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 con- trolling 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.

Development of friction stir welding technologies for in-space manufacturing

Abstract: Friction stir welding (FSW) has emerged as an attractive process for fabricating aerospace vehicles. Current FSW state-of-the-art uses large machines that are not portable. However, there is a growing need for fabrication and repair operations associated with in-space manufacturing. This need stems from a desire for prolonged missions and travel beyond low-earth orbit. To address this need, research and development is presented regarding two enabling technologies. The first is a self-adjusting and aligning (SAA) FSW tool that drastically reduces the axial force that has historically been quite large. The SAA-FSW tool is a bobbin style tool that floats freely, without any external actuators, along its vertical axis to adjust and align with the workpiece’s position and orientation. Successful butt welding of 1/8 in. (3.175 mm) thick aluminum 1100 was achieved in conjunction with a drastic reduction and near elimination of the axial process force. Along with the SAA-FSW, an innovative in-process monitor technique is presented in which a magnetoelastic force rate-of-change sensor is employed. The sensor consists of a magnetized FSW tool that is used to induce a voltage in a coil surrounding the tool when changes to the process forces occur. The sensor was able to detect 1/16 in. (1.5875 mm) diameter voids. It is concluded that these technologies could be applied toward the development of a portable FSW machine for use in space.

A Review on Dissimilar Friction Stir Welding of Copper to Aluminum: Process, Properties, and Variants

Abstract: Copper and aluminum materials are extensively used in different industries because of its great conductivities and corrosion resistant nature. It is important to join dissimilar materials such as c...