Tilting the tool spindle or the workpiece by 2 or 2.5° is advantageous during friction stir welding (FSW), to trap material near the leading edge of the tool, plasticise it by frictional heat underneath the shoulder and to move it around the pin to the trailing edge of the tool, where it is pressed downwards while it consolidates. If neither a tool tilt nor a scrolled shoulder profile are used, there is a significant risk of generating internal voids (i.e. tunnel defects), as the pressure underneath the tool are not sufficient to compress the plasticised material into the voids, especially if the set-up isn’t perfect.
The friction stir welding tool or the workpiece should be tilted by 2 or 2.5°, if a concave shoulder is used
If an old universal milling machine is used for friction stir welding, it is often possible to tilt the spindle axis manually and then lock it in position. Robots or bespoke friction stir welding machines can do this even automatically and change the direction of tilting angle during FSW, to make non linear welds. If only three axes (x-y-z) are available on CNC milling machines, the fixture and thus the workpiece might be tilted on the table, to make a straight weld downhill, but not to run around curves. Tilting the table by two axes is in possible in principle, but has not been put into practice.
Knowing actual tilt can be tricky, because machines can deflect under traverse load, changing the effective tilt. To translate a process condition from a machine of one stiffness to another, it can be helpful to measure machine deflection in the traverse direction and adjust the tilt target accordingly, because machine stiffness can affect the process.
Simple and thus easy to make FSW tools with a concave tool shoulder can be used, if the spindle or the workpiece are tilted during FSW. The downward pressure increases at the trailing edge of the FSW tool, if the tool heel plunge depth is set correctly and if it is visually controlled during FSW. Workpiece material is trapped underneath the shoulder and then trapped and compressed towards the trailing edge of the tool. The material flow is similar to that of a snow plow that moves material into the desired direction by tilting its shield.
On older, position controlled milling machines the tool spindle is commonly tilted by 2.5°, because less accuracy on setting the tool height, i.e. the tool heel plunge depth. On newer CNC controlled FSW machines, particularly if they are used in force control mode, a 2.0° angle is often preferred. The smaller angle has two advantages: First of all there is less concavity in the weld, i.e. thickness of the weld in the centre line is very similar to the thickness of the parent material, i.e. there is no weld undercut, apart from the volume of material that has been expelled as flash or burr. The 2.0° setting has also some benefits regarding the force control algorithms, i.e. the changes in tool height have an immediate effect on the pressure inside the plasticised material underneath the shoulder.
Larger tool tilt angles, up to 10° and more are used if tailor welded blanks are made, i.e. if sheets with dissimilar thickness are welded from the stepped side.
If non-linear welds are made with a CNC machine that has only three numerically controlled axes, it is recommended to keep the tool perpendicular to the surface of the workpiece. In most cases, scrolled shoulder profiles are required to drag plasticised material inwards, i.e. to scrape it from the edges of the weld inwards and towards the pin, and finally downwards using the threaded and/or helical profiles of the tool pin.
The additional cost and the limited rigidity of CNC machines with automatically tilting spindle axes makes them in many cases less attractive than using more rigid three-axis machines in combination with scrolled shoulder profiles. Due to the time required for calculating, setting and controlling the tilt angles, the speed in the sharp corners of a battery case is often limited to 1 m/min, when the tool spindle is automatically tilted around two axes, while higher speeds are possible with scrolled shoulder tools that are kept perpendicular to the workpieces. Even in the latter case, the speed in corners is limited, because the welding speed is larger at the outer edge of the weld than on the inner edge of the weld, even if the combination of welding direction and tool rotation is chosen in a way to minimise the effect. To calculate the effect, the actual welding speed depending on the curve’s diameters needs to be added to the tool rubbing velocity.
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