Welding process and its parameters

The basic concept of FSW is simple. Non-consumable rotational tool with pin and shoulder shaped to provide required weld properties is inserted in between abutted edges of plates to be joined and traverse along the joint line. Joint is created by stirring the material of the two pieces by the tool pin [1]; plastic deformation caused by the pin and friction between the base material and tool shoulder creates heat that softens the basic material, thus easies the stirring process. There is no melting of the base material involved. [9, 22] The main source of heat in the weld is the friction between shoulder and base material [9]. The principle is illustrated on the schematic drawing (Figure 1) [23].

Figure 1: Schematic drawing of friction stir welding [23]

The main process parameters are (a) tool geometry, (b) welding speed and (c) tool revolutions. Their influence on resulting properties of the weld are discussed later.

Typical friction stir weld in its cross-section (Figure 2) consists of three main zones [9]: (a) Nugget, stirred zone, (b) thermo-mechanically affected zone (TMAZ) and (c)heat affected zone (HAZ). The three zones pose distinct mechanical properties [9, 24-27] and nugget and TMAZ has been often documented as being the weakest part of the joint [26, 28]. The weld is not symmetrical due to the rotational movement of the tool, thus moving material from advancing side to retreating side.

Figure 2: A typical macrograph showing various microstructural zones in FSP 7075Al-T651 (standard threaded pin, 400 rpm and 51 mm/min) [9]

The asymmetry of the weld can be used to improve mechanical properties of the weld of dissimilar alloys. A study showed that placement of the tool axis out of the joint axis by 0.5 to 1 mm improved mechanical properties of the weld [29].

According to [9, 20] temperatures in the weld do not reach melting point of alloys. Higher rotational speed of the tool lowers friction between the tool and base material which is reflected in lower heat production [30] and this prevents material from being melted within reasonable welding process parameters. Obviously welding affects structure of precipitates in nugget area as well as in both TMAZ and HAZ [31, 32].

2.1      Mechanical properties of welds

FSW can be used for welding of virtually any metal. In this thesis, we focused on 2XXX and 7XXX aluminium alloy series. Literature provides broad range of data for 2024 [2, 19, 22, 26, 29, 32-51], 2219 [10, 12, 52-55] and 7075 [49, 56, 57] as well as for dissimilar weld 2024-7075 [2, 29]. These aluminium alloys are considered as non-weldable by traditional fusion techniques. Unfortunately, not many resources provide detailed information about the conditions under which the mechanical properties were gained, reducing the usefulness of the data.

For the 2219-T6 Xu et al showed that the joint efficiency of the welds under optimized welding parameters varies from 71 to 80 % [58]. Another study done by Zadpoor et al showed very good results for 2024-T3 where the joint efficiency varied between 89 to 99 % [49], for 7075 was 90 % [49].

Generally, the optimized welding parameters can create a weld reaching joint efficiency of 70 % and higher. This is a good result, as these alloys are considered as non-weldable by traditional welding techniques [9].

2.2      Fatigue properties

Although FSW provides generally better fatigue properties than traditional fusion welding methods [7], fatigue properties are still significantly lower than of the base material. Several methods have been developed to prolong fatigue life of friction stir welds. A few of them have proven capable of improving fatigue properties of the weld. The technique of root flaw removal[51] and application of oxide array removal [50] appear to be effective. The fatigue characteristics of the alloy 2024-T3 weld exhibited improvement of up to 100 % [50, 51].

Studies on using of laser and shot peening on materials 2195 and 7075 [59, 60] did not show improvement of fatigue properties and crack growth speed as the residual stresses after FSW are already much lower than after welding by a traditional technology, which involves material melting.

2.2.1     Crack propagation within the weld

Propagation of cracks within the weld are most likely to start at so called “zigzag defect” of the weld. The defect is always initiated within the root of the weld and spreads through the nugget.

Zigzag defect is an inherent feature of the weld [35] and is considered as the main cause of the reduced fatigue life of FS welds in comparison to the base material. The typical zigzag defect is shown on Figure 3.

Figure 3: Propagation of fatigue crack within a FS weld in 2024-T4 alloy

2.3      Corrosion properties of welds

Several studies have been conducted on corrosion behaviour of friction stir welds [4, 40, 61]. Detailed study of corrosion behaviour of 2024-T351 alloy [40] showed that nugget and HAZ areas are both sensitised and susceptible to intergranular corrosion. Whereas study on 7108-T79 [4] has shown that this material is most sensitive in TMAZ and the character of the corrosion is also intergranular.

Dependence of corrosion behaviour on rotational speed of the welding tool was observed [40] and can be used as an explanation of the aforementioned difference in the observation of corrosion sensitivity of different part of the weld. For the lower rotational speeds of the tool, localised intergranular attack was observed in the nugget region; for the higher rotational speeds, attack occurred predominantly in HAZ.

Generally, sensitization of the microstructure that occurs during welding process is responsible for the corrosion susceptibility of HAZ region of the welds [61]. It was shown as well that the most of localized corrosion and environmental cracking in FS welds of aluminum alloys is of an intergranular-type [61].

The method how to improve corrosion resistance of the weld was proposed to be a short-term post-weld heat treatments with time–temperature exposures similar to that during the welding, as it would rehomogenize the sensitized microstructure of the welds [61].

2.4      Residual stresses

Due to complex thermo-mechanical process during FSW, residual stresses appear. It is believed that FSW produces lower residual stresses due to lower temperatures involved. Several studies were conducted on residual stresses in 2XXX and 7XXX alloys [36, 43, 62-64] and have shown that longitudinal stresses are generally higher than transverse ones not depending on pin diameter, tool revolutions or welding speed. Several methods have been developed to eliminate residual stresses by tensioning material during welding [43, 65]. It shows to be effective for longitudinal stresses whereas effect of transverse stresses on fatigue life can be eliminated by peening [33, 66].

A study on 7075-T651 alloy showed that the residual stresses associated with the weld nugget decreased, while those associated with HAZ increased with 
time [62].

For alloys 2024 and 7075 a study showed that unlike traditional welded joints, where the maximum residual stresses values occur close to the surface of a FSW joint, which is near the tool shoulder border of advancing side of the joint, the residual stresses are of negative values on the surface and become positive in the joint. Surface residual stresses vary from -15 to -40 MPa and increase with depth up to tensional 50 to 150 MPa [64].

2.5      Aging of friction stir welds

As the FSW process introduces heat into the aluminium alloy structure, changes in the microstructure occur [9]. Material has to return to the desired metastable state (with higher density of metastable precipitates) by natural or artificial aging. Natural aging is more commonly used as it does not require additional heating. Studies reported reaching metastable state (metastable precipitates) of the material after 7 [45] or up to 45 days [49] of natural aging for materials 2024 and 7075 depending on the thickness of the material and the welding parameters.

Study conducted by Linton [62] on 7075-T651 alloy has shown that residual stresses change during natural aging of welded material. Welding stresses associated with the nugget decrease, whereas the ones associated with HAZ increase. Mechanical properties in all areas of the weld increased with the time.