In reality an apparently smooth surface consists of many microscopic projections, called asperities. When one surface moves relative to another these asperities interact, see Figure 1, generating friction – the force resisting motion between two or more interacting surfaces. Interaction of these asperities through elastic and plastic yielding generates heat. Friction welding utilises this phenomenon for joining applications. The induced mechanical motion of friction welding generates heat, causing the materials to be joined to soften and become viscous. While in the softened state, the mechanical motion of the process mixes the materials to create a bond. The way by which the frictional heat and material mixing occurs is very dependent on the friction welding process utilised, of which there are four primary processes: Friction stir welding (FSW), friction stir spot welding (FSSW), linear friction welding (LFW) and rotary friction welding (RFW).
FSW and FSSW utilise a dedicated tool to generate the frictional heat and mechanical mixing. FSW works by using a non-consumable tool, which is rotated and plunged into the interface of two workpieces. The tool is then moved through the interface and the frictional heat causes the material to heat and soften. The rotating tool then mechanically mixes the softened material to produce a bond; see Figure 2(a). FSSW is a variant of FSW and works by rotating, plunging and retracting a non-consumable tool into two workpieces in a lap-joint configuration to make a “spot” weld. During FSSW there is no traversing of the tool through the workpieces; see Figure 2(b).
LFW and RFW do not require a non-consumable tool, i.e. the individual workpieces to be joined are used to generate the frictional heat and mechanical mixing. LFW works by linearly oscillating one workpiece relative to another while under a compressive force. The friction between the oscillating surfaces produces heat, causing the interface material to soften and mechanically mix; see Figure 2(c). RFW is similar to LFW except that the workpieces are often round and are rotated relative to each other; see Figure 2(d). During LFW and RFW the workpieces typically shorten (“burn-off”) in the direction of the compressive force, forming the flash. During the burn-off interface contaminants, such as oxides and foreign particles, are expelled into the flash. Once free from contaminants, pure metal to metal mixing occurs, resulting in an integral bond. Although the generated temperatures during friction welding are very hot, the material remains in a solid-state condition (i.e. no melting occurs).
Friction welding offers many advantages to the manufacturing sector, including:
- Remaining in the solid-state, therefore avoiding many of the defects associated with fusion welding, such as pores and solidification cracks.
- Producing comparably low temperatures when compared to fusion welding, which reduces intermetallic formation, allowing for a wide range of similar and dissimilar materials to be joined. The distortion of the welded component is also reduced.
- Being able to join many ‘non-weldable’ aluminium alloys, namely from the 2xxx and 7xxx series.
- Not requiring a filler metal, flux and shielding gas.
- Not requiring special edge preparation in most applications.
- Being easily automated, making the process highly repeatable and not dependant on human influence, resulting in very low defect rates.
- Being able to produce welds with mechanical properties that are comparable or superior to the parent material for a range of similar and dissimilar material combinations.
- Being able to reduce the materials required to make a component by joining smaller workpieces to produce a preform, which is subsequently machined to the desired dimensions.
To find out more about the individual friction welding processes, their microstructures, mechanical properties and industrial applications, please click on the following links:
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