Friction stir welding (FSW) is ideal for joining copper to aluminium, because it is a solid state process that works below the melting point of the workpieces, while fusion welding processes cannot be used, because they produce brittle intermetallics, when the molten materials solidifies. Friction stir welding works below the melting point, because a high pressure is applied as in all pressure welding processes and the oxides are disrupted as in all friction welding processes. However, special know-how is required for welding dissimilar materials, because it is more complicated than welding aluminium to aluminium or welding copper to copper.
For dissimilar material welding, the axis of FSW tool is not traversed in the joint line but slightly off-set towards the softer material. The softer material is typically slightly thicker than the harder material, to generate a lubricating layer of the plasticised softer material underneath the shoulder of the FSW tool.
For conventional friction stir welding the axis of the rotating FSW tool crosses the joint line, i.e. the operator or seam tracker tries to align the tool as close as possible above the line, on which the two workpieces are butted together.
When welding dissimilar materials it is advantageous to offset the FSW tool so far towards the softer material that the profile of the tool pin just scratches the oxides off the harder material. For friction stir welding of copper to aluminium, the tool pin is, therefore, surrounded by aluminium, while it only touches the copper part. As a consequence, the generation of Cu-Al-alloys or Cu-Al-intermetallics is minimised. If the copper content gets too high in aluminium alloys, they become brittle, while a small amount of copper in aluminium increases their strength, which is useful for making 2000 series aerospace alloys.
As an exeption to the rule, it is in some cases it is perceived to be benefical to run the tool in the harder material. Song et al. performed friction stir butt welding of titanium and aluminum alloy by offsetting the probe edge into the titanium alloy (https://doi.org/10.1016/j.matdes.2013.12.040).
The aluminium part is often thicker than the copper part during friction stir welding for two reasons: First of all, this ensures that a lubricating layer of plasticised aluminium is forged underneath the FSW tool shoulder, to minimise the wear of the FSW tool and to achieve steady-state conditions. Furthermore, this is useful because aluminium has a lower electrical and thermal conductivity than copper when making bus bars or heat exchangers. In many cases the softer material has a lower strength than the harder material, when making tailor welded blanks from aluminium and steel. For these, the aluminium is most commonly 1.5 to 2 times thicker than the steel, depending on the strength of the alloys used.
For the lubricating effect, it is normally sufficient, if the aluminium is 0.25 mm or 0.5 mm thicker than the copper in most cases, although this depends on the thickness of the thinner workpiece. You will see that the ripples of the weld have often the same colour than the aluminium (or copper in the case of copper-to-steel). An actual example is 3.0 mm thick aluminium welded to 2.5 mm thick copper, as shown on Stirtec's web site.
For butt welds of sheets which have been cut with a guillotine or a laser in a manner that the edges are perpendicular to the surface, the threaded tool pin should be cylindrical. If one of the work piece materials is very hard, e.g. when welding thin aluminium sheets to steel a plain probe may be used, i.e. a cylindrical pin from a very hard material without any profile. For more complicated joint configurations, special pin shapes are available via AluStir.
Friction stir welding of dissimilar materials is possible with butt welds, overlap welds or fillet welds. At least for fillet welds a stationary shoulder needs to be used, while in the butt and overlap configuration.
The aluminium part can be set-up on the advancing or retreating side of the rotating tool, but the results will be very different. Therefore, we recommend to obtain consultancy from AluStir, to conduct a literature search or to do a lot of parameter development trials, if you want to achieve repetitively high-quality welds at high welding speeds.
Friction stir welding allows to join copper to aluminium either using a conventional milling machine or a bespoke FSW machine with a sophisticated control system. The FSW machine should have at least a force control system or even better also a temperature control system, which adjusts the welding speed and/or tool rotation speed according to the temperature measured in the pin and/or underneath the shoulder. These machines have often wire-less data transfer systems or in some cases mercury slip rings, because conventional slip rings are not reliable for transferring the data. It is beneficial to install a seam tracker, because the tool needs to be more accurately positioned relative to the joint line than in conventional FSW. Thermal imaging can be used during FSW, but it is complicated because heat of the hot FSW tool gets often reflected on the shiny surfaces of the metal work pieces.
Friction stir welding is ideal for welding aluminium to copper because it works without any filler material. In brazing the braze is often the weakest point of the joint, and has most commonly a higher electrical resistance and lower heat conductivity than the base metals. The friction stir welded joints have typically the average conductivity of the two base materials. The specific tool designs and parameters are often surrounded by a cloud of confidentiality, to give the successful user of FSW an advantage against his competitors.
The most common application is making bi-metal bus bars for connecting the copper minus pole to the aluminium plus pole to of batteries. If such Cu-Al bus bars are used, the manufacturer of electric batteries does only need to weld Al-to-Al and Cu-to-Cu in his factory, while the manufacture of the bus bars is often outsourced.
Friction stir welded Cu-to-Al joints are also often used for heat sinks, e.g. by welding a copper sheet onto an aluminium extrusion, so that electronic components can be glued or soldered onto the copper resulting in very effective cooling. In aluminium heat exchangers for air conditioning systems, thermal solar systems or coolers of large super markets, copper flanges are occasionally friction stir welded to aluminium radiators, so that the on-site plumbers can used well established copper-to-copper brazing when they connect to the cooling or heating circuit of a building.
One of the first industrial applications was in the production of aluminium cathode sheets for the zinc smelter industry, where the cathode sheets contacts the power supply via a friction stir welded copper plate. Some Chinese manufacturers of batteries for electric cars have several FSW machines in use to make Cu-Al-joints for bus bars in E-mobilitity batteries.
Industrial applications are still limited due to a lack of know-how, a lack of suitable FSW machines and the perceived difficulties on obtaining suitable FSW tools. AluStir can mitigate these limitations by providing consultancy, by conducting operator training, by conducting feasibility studies and parameter optimisation studies and by providing special FSW machines and or bespoke FSW tools. AluStir can supply off-the-shelf or customer-specific FSW tools help you to design and make your own FSW tools, e.g. for scientific studies.
Please contact firstname.lastname@example.org if you need help on FSW of copper to aluminium, FSW of copper to steel or FSW of aluminium to steel.