Robotic FSW causes stir in EV production efficiency

Machines solely dedicated to friction stir welding (FSW) exist, while others combine FSW with conventional subtractive CNC machining. Robotic FSW is a slightly different take on the technology; it incorporates a 6-axis robot, adaptive fixturing, and specific software. These systems are comparatively compact and, most importantly, provide more flexibility in terms of application part sizes and configurations than the other forms of the technology mentioned.

The tooling is straightforward with robotic FSW, and fixturing can be tailored to maximize versatility and accessibility to workpieces. Because the workpieces do not have to enter a machine, load/unload is simpler, and the robot can access the work from multiple sides and angles. Scaling of a robotic system with the acquisition of another machine is unnecessary; shops simply add a robot or additional fixturing to accommodate increased production levels.

Because of all its joining benefits, robotic FSW is playing a key role in the rapidly growing electric vehicle (EV) market. One significant application is joining castings, extrusions, and sheet aluminum to manufacture assemblies such as battery trays.

The trays hold the EV batteries and also serve as structural elements for the vehicle. In addition, a battery tray typically features cooling channels that must be leakproof. In sedan-style vehicles, the trays are large, typically about 2 metres long or more within the wheelbase, while extending the width of the vehicle. A typical tray can consist of a bottom casting with channels for coolant and containment for batteries with an aluminum plate welded to the top to seal it.

For this process, KUKA Robotics has engineered its KUKA cell4_FSW modules to provide up to 95 per cent more process efficiency and to maximize the available configuration options for EV manufacturers. The cell’s efficiency results from two workstations located at separate insertion areas.

Used for both 2D and 3D welding tasks, the cells are scalable and accommodate either one or two 6-axis robots. Shops can arrange several workpiece clamping tools in the cell’s working area so that robots can work simultaneously on larger components if needed.

Patented in 1991 by The Welding Institute, Cambridge, England, FSW offers a variety of advantages compared to traditional welding methods. A lower-temperature process (typically less than 500 degrees C in aluminum), FSW minimizes distortion and residual stresses in the workpieces while improving fatigue performance and causing little if any disturbance of material microhardness.

FSW facilitates welding of long, thin workpieces and is particularly suitable for joining non-ferrous metals with a low melting temperature, and also for mixed-material joints like aluminum with magnesium, copper, or steel. Because the work material never melts, the resulting welds do not suffer from solidification-related porosities and cracking. Unlike conventional welds that use filler material, such as rod or wire, the FSW joint has no unwanted phases resulting from a mix of filler and parent metal. From a sustainability point of view, lower temperatures consume less energy and the process generates no fumes or smoke, is quiet, and requires no gas or wire consumables.

When welding, the robot pushes the rotating metal FSW pin through workpieces while maintaining tight tolerances and high accuracy. As a result, an FSW robot has to be rigid and powerful enough to generate and control both strong vertical and traverse forces as well as torque.

For its cell, KUKA employs its KR 500 FORTEC robot, also applicable for heavy-duty machining operations such as milling and drilling. The robot, with a 500-kg-rated payload, weighs approximately 2,400 kg and has a 1,050-mm by 1,050-mm shop floor footprint. Additional gearing on the first three axes enables the robot to generate and handle up to 10,000 N (1,000 kg) of force, with pose repeatability of +/- 0.08 mm.

A robot’s ability to coordinate motion with multiple axes allows it to weld complex contours, reinforcing its utility and flexibility in FSW. Generally, joining two workpieces requires tooling to approach the work in a perpendicular direction. If the part is not flat, a robot’s ability to move in six degrees of freedom expedites fixturing and access to the part. When one part has a lip that extends over the other, or if there are variations in the joint resulting from an extrusion or casting process, a scanner like those used in conventional welding can track the seam, detect differences in height between the parts, and guide the robot’s path.

Conventional welding methods can be adapted to introduce more metal into a seam if the gap varies significantly. On the other hand, FSW is not as forgiving as conventional welding due to the high tolerance requirements for the parts. Nevertheless, KUKA can analyze the parts, develop the tooling, and set appropriate tolerance requirements for a specific project.

The rotating FSW tool consists of a shank with a protruding pin surrounded by an extended radial shoulder. In some tool designs, the pin and shoulder rotate together, but KUKA uses a stationary shoulder FSW (SSFSW) tool because the approach tends to put less heat into the process and therefore minimizes deformation of the welded result. The design helps support the seam and control welding forces, and it can improve the surface finish of the completed seam.

For precise weld guidance, KUKA FSW software provides positioning accuracy up to 0.5 mm with laser-supported path calibration to compensate for path deviations caused by varying stiffness in the work zone and process forces. As the robot works, KUKA Process Control and Documentation (PCD) software provides real-time system monitoring and documentation with numerical and graphic monitoring controls that record all welding parameters for continuous-path FSW. Results are consistently reliable and reproducible.

The speed with which EV development is currently taking place means that many technologies are being pushed into service to speed production and improve accuracy. FSW in configurations such as the one described here will no doubt play an important role.

KUKA Robotics, www.kuka.com