Friction Stir Welding was invented and experimentally proven by Wayne Thomas and a tem of his colleagues at the TWI (The Welding Institute, Cambridge, UK) in December 1991. It is considered an exotic solid-state weld and is used for application where the original metal characteristics must remain unchanged as far as possible. This process is mainly used on Aluminium, and most often on large pieces, which cannot be easily treated post, weld to recover temper characteristics. Indeed, it has been convincingly demonstrated that the process results in strong and ductile joints, sometimes in systems, which have proved difficult using conventional welding techniques. The process is most suitable for components, which are flat and long (plates and sheets) but can be adapted for pipes, hollow sections and positional welding. The welds are created by the combined action of frictional heating and mechanical deformation due to a rotating tool. The maximum temperature reached is of the order of 0.8 of the melting temperature.
Friction stir welding (FSW) is one of the most remarkable and potentially useful solid state welding techniques to weld aluminium alloys. This process has made it possible to weld a number of aluminium alloys that were previously not recommended (2xxx and 7xxx) for welding, because of hot cracking and heavy distortion. Even though FSW can be used to join a number of similar and dissimilar materials, the primary research and industrial interest has been in optimizing the parameter to weld aluminium alloys. This article introduces the welding process and the welding of a 7xxx series alloy. Friction stir welding is a relatively new process of welding different materials. It is a solid state welding process where the materials to be welded are joined using a rotating tool, with a shoulder and pin, which moves along the weld line. The rotating tool stirs the material softened by the frictional heat generated, and consolidates the stirred material behind the tool. This process has several advantages when compared to conventional fusion welding process. The advantages include:
- There is no distortion of the material welded
- No filler material is required
- Consistent weld quality
- No protection gases are required
- It is easy to weld dissimilar materials.
The welding was carried out on a conventional milling machine that was adapted for this work. The tensile strength of the weld achieved during the process was as high as 90 per cent of the parent metal strength. Some defects were observed in some of the welds and happened due to non-filling of material in the advancing side of the weld. Over-aging of the 7xxx series alloys leads to strength deterioration away from the weld line and failure is seen to occur away from the weld. The Welding Institute (Cambridge, United Kingdom) invented the Friction stir welding (FSW) in 1991. FSW is one of the most remarkable and potentially useful solid state welding techniques to weld aluminium alloys. This process has made it possible to weld a number of aluminium alloys that were previously not recommended (2xxx and 7xxx) for welding, because of hot cracking and heavy distortion. Even though FSW can be used to join a number of similar and dissimilar materials, the primary research and industrial interest has been in optimising the parameter to weld aluminium alloys.
In FSW, frictional heat is generated by a rotating shouldered pin that is plunged at the interface of the two surfaces to be welded and moved laterally along the weld line. The material, which is softened by the thermo-mechanical process, moves from the leading edge to the trailing edge and gets consolidated by the application of an axial force. The process of FSW is schematically explained in Figure 1. In this process, parameters that control the quality of the weld are tool geometry, rotational speed, traverse speed, tool tilt angle, and axial force. At present, optimisation of these parameters is done experimentally and individually for each material and configuration. In general, the FSW zone is classified into three major regions, namely weld nugget (WN), thermo mechanically affected zone (TMAZ) and heat-affected zone (HAZ). The weld nugget is characterised by a well-defined plastically deformed region, which has very fine grains that is believed to be a dynamically re-crystallised region. The TMAZ is a region that undergoes some amount of plastic deformation but does not have a re-crystallised microstructure. The HAZ is a region in which the material is affected only by the heat generated during the welding process.
FSW has many advantages such as:
o The welding procedure is relatively simple, once the right combination of the process parameters are established
o It is an environmentally friendly process, without a consumable electrode, flux requirement or protection gases
o Recycling of welded parts is easier
o Ideally suited to automation
o Can be used for welding dissimilar metals
o Consistency in weld properties is better
o Single pass welds can be done for sheets with lesser than 1 mm thickness to greater than 25 mm thickness
o Weld distortion is low
o Weld strength and properties are better than other fusion welding processes have been achieved
o No major surface preparation is necessary before welding or after welding.
Using these advantages, a large weight reduction (10-30 per cent) of riveted aerospace structures can be achieved. The process has some disadvantages, especially the fact that clamping of the work piece rigidly is essential. This increases the initial investment requirement and poses many challenges for thin sheets. The process of FSW leaves a hole at the end of the weld that has to be plugged by other means.
A large number of studies, 340+ papers till date, as obtained by a search in Compendax (using Friction Stir Welding as title search) indicate the amount of work going on to understand the process of FSW. Many studies have been conducted on aluminium alloy series 2xxx, 6xxx, and 7xxx for micro structural characterisation and mechanical properties. Despite these large number of studies there is a lack of understanding in the various parameters governing FSW. More fundamental studies are required to understand the process parameters on the quality of weld and to determine the optimum parameters. Only a few of these papers are referred to in this article. Liu et al have conducted studies on 1050-H24, 2017-T351 and 6061-T6 to explain the effect of welding speed on tensile properties and fracture location during tensile testing. In FSW, tool geometry is one of the most important parameters that control the quality of the weld. There is not enough data available on the effect of tool geometry on defects in welding. This is primarily due to reluctance of various groups to give data on tensile properties and its influence on defects in the welding. In this study, the effect of shoulder diameter, pin diameter and pin profile on the mechanical properties, micro structural evolution and defect formation of an precipitation hardenable Al-Zn-Mg alloy are investigated using various shoulder diameter, pin diameter and pin profile. For this purpose, AFNOR 7020 alloy, an alloy used by aerospace industries, is used.
The base material used in this study was aluminium alloy AFNOR 7020-T851. The chemical composition of the material was analysed using a Specrovac and is given in Table 1. The material was in a solution, stress-relieved by stretching to the controlled straining and natural aged condition. Material was slightly over-aged, in order to decrease the stress corrosion, cracking problem. All FSW trials were carried out on vertical milling machine with the help of specially designed fixture plate. Square butt weld joint configuration was used in all trials and the square edges were prepared by milling. A pair of work pieces of dimension 100 mm x 150 mm x 4.4 mm were abutted along a longitudinal section and clamped rigidly on the 400 mm x 500 mm x 20 mm fixture on the backing plate. No special surface preparation or cleaning was done of the surfaces to be welded prior to welding. Two different tool profiles were used for this study. One was un-chamfered shoulder and cylindrical pin with a flat end, and the other was a chamfered shoulder and frustum shaped pin with rounded end. The tool profiles are shown in Figure 2. The tool shoulder diameter taken for the study was 10, 15, and 20 mm and the pin diameter taken was varied between 3 mm and 8 mm. The tool dimensions used for the welding trails are listed in the Table 2. For all the trials, rotational speed of 1400 rpm and traverse speed of 80 mm/min were kept constant. The tool backward tilt angle of 2o was used in some of the trials while there was no tool tilt for other trials.