Train carriages benefit from multi-head friction stir welding

Friction welding, whereby two metals are joined by rubbing them together has been known for decades. Friction stir welding on the other hand, whereby one or more rod end tools rotate in a substrate softened but not melted by the heat, was only patented as a means of joining sheets of metal by TWI in 1991. Subsequently, the idea of using two contra-rotating tools was devised, one above and one below the sheet join, with one slightly displaced in the direction of travel relative to the other. This offers advantages that include reducing reactive torque, producing a more symmetrical weld and generating a full, through-thickness joint. The tools should not touch each other but should be sufficiently close that the softened material around the tool tips overlap. One of the most striking successes in the use of FSW is its suitability for successfully making very long, butt seam welds in aluminium railway carriages. Most aluminium alloys owe their strength to the formation of very small precipitate particles in the matrix. Traditional welding techniques that involve melting the material take these precipitates back into solution in the weld pool. This causes growth of precipitate particles in the heat-affected zone, which drastically reduces strength. Friction stir welding, because it requires only the brief softening of the alloy, is much less problematic. The end result of this is that the tensile strength of the weld can be within a few percentage points of the parent material, while the fatigue strength is very similar and the fracture resistance at least as good. Friction stir welding of train carriages was pioneered by Hitachi in Japan, although it has since been taken up by companies in many countries. More than 376 friction stir welded carriages have, for example, been ordered from Bombardier for the latest upgrade of London's Victoria underground line. The process is used to join stiff longitudinal extrusions, which constitute the carriage body's side walls. The welding has been sub-contracted to Swedish supplier Sapa Profiler, which holds a TWI friction stir welding licence. TWI says that FSW is likely to be used on future contracts for more than 2200 more new trains over the next decade. Aluminium train carriage bodies reduce weight and the amounts of energy needed to accelerate and decelerate them. They also cut rail wear. FSW is also appropriate for aerospace, commercial ship building and automotive applications. TWI says that there are currently more than 170 licence holders worldwide, with the greatest take up in Japan. Wayne Thomas, who with colleagues, published the first article about TWI's simultaneous multi-tool developments, claims that Hitachi is already using some of these techniques, as is Sapa. The original paper also describes a number of other variants, some of which are believed to be in commercial use. One such variant involves a machine with a series of contra rotating tools, side by side able to make a number of butt seam welds in parallel. Another idea that has been investigated at length by TWI is the use of one tool that pre-heats and another to weld. This is known to have been used elsewhere for lap welding steel plates. In this instance, the tools were being rotated in the same sense. TWI has investigated three variations involving tools rotating in opposite directions. One of these is to have two tools side by side, transverse to the welding direction. Another is to have two tools in line in the welding direction, while a third has the tools staggered to ensure that the edges of the weld regions partially overlap. The first variant, which TWI calls, 'Parallel twin-stir', enables defects associated with lap welding to be positioned between the two welds. The second variant, 'Tandem twin-stir', reduces reactive torque. It also improves weld integrity by having the following tool disrupt and fragment any residual oxide layer remaining within the first weld region. The second tool travels through already softened material and does not have to be as robust as the first. The third variant, named, 'Staggered twin-stir' produces an exceptionally wide common weld region. The tools are positioned with one in front and slightly to the side of the other so that the second probe partially overlaps the previous weld region. In lap welds, the wide weld region produced provides greater strength than a single pass weld. Residual oxides within the overlapping region of the two welds are further fragmented and dispersed. One particularly important advantage of the staggered variant is that the second tool can be set to overlap the previous weld region and eliminate any plate thinning that may have occurred in the first weld. Trials at TWI produced welds of good appearance using plates of Aluminium 6083-T6 alloy. The two exit holes weld showed that a similar footprint was achieved for both the lead and following tool. Metallographic observations revealed a marked refinement of grain size in the weld region and comminution of oxide remnants and particles. This is consistent with the microstructure features previously observed in conventional rotary stir welds in aluminium alloys. Friction surfacing, joining to ceramics and Bobbin stir welding Two more variants of friction stir welding that are becoming increasingly popular are friction surfacing, where the rotating tool is a consumable that is spread on the surface, and Bobbin stir welding, where the tool goes right through the parts to be joined, so there is no need for an anvil support plate. Friction surfacing is mainly used to apply a metal coating to a metal substrate. The coating material can either be used as is, as a coating that can be extruded with the substrate, or as a joining material. The surfacing material generally has a lower softening temperature than the substrate, but any increase in temperature differential enhances the deposition mechanism and allows comparatively harder materials to be deposited onto nominally softer materials. The substrate need not necessarily be metal and some years ago TWI developed an aluminium pin that had been friction welded onto an alumina ceramic wafer in order to ease fastening problems. This development subsequently led to the development of a process whereby aluminium conducting tracks can be laid down by friction onto an alumina substrate. In tests, the aluminium consumable was 3mm in diameter, the same diameter as the pin attachment. Aluminium becomes plastic at about 300°C, and TWI has had no trouble is laying down tracks less than 50µm thick. Examination of the cross sections of the joints between the aluminium and the ceramic show some mechanical keying. TWI says that from adhesion tests, bond strengths are excellent. Bobbin stir welding comes in two flavours: fixed gap, and 'adjustable' or 'adaptive', which pertains to the spacing between the shoulders. A variant of the fixed gap is the 'floating bobbin' tool, which is a fixed-gap tool that has is able to float in the direction perpendicular to the workpiece. The self-reacting principle of the bobbin technique means that the normal down force required by conventional FSW is reduced or eliminated. Most of the research developments at TWI to date have involved aluminium alloys, which reflect the biggest present day commercial applications. It should be borne in mind, however, that friction welding works with almost any metal alloy that softens at temperatures that are not especially high.