FSW application


3.1      Joint configuration

FSW allows the use of virtually any joint configuration as we know them from traditional fusion welding. The only limitation is the tool shoulder, which makes fillet joints more problematic than flat ones [9]. Typical FSW joints are shown (Figure 4).


Figure 4: Joint configurations for friction stir welding: (a) square butt, (b) edge butt, (c) T butt joint, (d) lap joint, (e) multiple lap joint, (f) T lap joint, and (g) fillet joint.

The most commonly used types of joint are the butt joints, in particular, square butt joint.

3.2      Tool geometry

Tool geometry has a great influence on resulting mechanical properties of the weld [10-14]. It provides in-situ heating, stirs base material, and thus creates weld. There has been variety of tool shapes used. FSW can be performed with tool of a simple geometry (Figure 5) yet having good mechanical properties [10]. Advanced tool design (Figure 6) provides intensified material flow in the stirred zone and better weld quality [23]

Figure 5 and Figure 6 provide basic overview on the basic and advanced types of FSW tools.


Figure 5: Basic FSW tool pin profiles

Figure 6: Advanced FSW tools developed at The Welding Institute, Cambridge

Experiments performed by Elangovan on tools with simple geometry [10] showed remarkable influence on mechanical properties of the weld as illustrated in the Figure 7 [10]. The best results were produced using tool with square cross-section pin that assured best stirring of the material in the weld area.

 

  Figure 7: Dependency of material properties on welding speed and tool shape [10]


The Figure 7 shows that the mechanical properties of the weld strongly depend on the volume of material stirred by the tool. The more material the tool stirs, comparing to its volume, the better the quality of the joint [23]. This is also the reason why tool producers try to create specially tapered and threaded tools to raise the ratio [23].



3.3      Welding speed and tool rotational speed

These two parameters of the welding process are the most important ones and determine overall mechanical properties of the welds.

A comprehensive study investigating dependency of mechanical properties of the weld on the welding and rotational speed was done by Elangovan et al [10, 12]. Figures below show the obtained results.

3.3.1     Dependency of mechanical properties on welding speed

Figure 8 and Figure 9 show the dependency of mechanical properties on welding speed as measured by Elangovan et al [10].


Figure 8 Dependency of elongation on welding speed and tool shape [10]

Figure 9: Dependency of elongation on welding speed and tool shape [10]

Even thought the number of different welding speed measurements is quite low, we can expect that there always is an optimal welding speed for the given material, rotational speed and tool shape. Optimal welding speed in the cases shown on the figures above does not strongly depend on the tool shape and optimal value of the welding speed for the given other parameters considering elongation and joint efficiency lies between 0.8 and 1 mm.s-1.

3.3.2     Dependency of mechanical properties on welding speed

Figure 10 and Figure 11 show the dependency as measured by Elangovan et al [12].


Figure 10: Dependency of elongation on rotational speed and tool shape [12]

Figure 11: Dependency of joint efficiency on rotational speed and tool shape [12]

In this case, the dependency of the value of elongation and joint efficiency on the tool shape is more significant than their dependency on rotational speed.

The joint efficiency graph in Figure 11 exhibits a peak around the rotational speed value of 1600 min-1, whereas elongation values grow with raising rotational speed throughout the measured range. The author does not specify aging time of the welds, so it is possible that higher values of elongation are caused by higher weld temperature at higher rotational speed of the tool. This could have lead to creation of unstable microstructure with higher elongation and would return to the metastable state with lower elongation and higher joint efficiency after aging.

3.3.3     Joint efficiency for optimised welding parameters

Attachment 2 summarises mechanical properties of butt welds done with optimised welding parameters from different source. The joint efficiency of yield strength is generally high, varies from 75 to 99 %. The table can be used as a base for setting welding parameters for FSW of 2024 and 7075 alloys. Unfortunately the detailed information about specific welding parameters resulting in specific mechanical properties are rare in open literature thus, the statistical base of the table is limited.

3.4      Welding forces

Welding forces are important for choosing appropriate machinery and tool design. Hattingh et al studied forces applied to variety of tools during friction stir welding [14]. At welding speed of 150 mm.min-1 and spindle speed 500 rpm the maximum radial forces never exceeded 6 kN. Maximal axial force varied from 4 to 14 kN for different tools.

The axial, also called vertical down force, is crucial for creation of sound weld, as it assures proper contact of the tool with welded work pieces and together with rotational speed induces release of frictional heat from the tool shoulder which is the main source of heat for the weld [9].

3.5      Machinery used

FSW is not demanding in regard of machinery requirements. There is no special welding machine necessary and utilisation of standard workshop equipment such as milling machines [67] is possible. Although specialised machines provide higher levels of productivity especially in the respect to the ease of gripping work pieces, welding speed (maximum speed around 2 m.min-1 for standard FSW machines [68]) and also number of welding heads allowing to work on multiple parts of the work piece at the same time.

Most of the currently used machinery is tailor made for the customers by several specialised producers that provide wide range of machine parameters suitable to customer needs.

3.6      Material preparation

Thanks to the principle of FSW there is no special preparation of welded pieces needed. The only requirement is degreasing [69] which prevents introduction of particles that would lower mechanical properties of the weld.

3.7      Heat treatment

3.7.1     Welding of heat treated material

Alloy 2024 with different heat treatment variants was by Aydin et al [70]. Results are summarised in Table 1 below.

 

Table 1: Comparison of heat treatment effects on FS weld mechanical properties

Sample

Base material

Joint

Joint efficiency

0.2% proof strength

Ultimate tensile strength

Enlongation

0.2% proof strength

Ultimate tensile strength

Enlongation

Rp0.2 [Mpa]

Rm [Mpa]

A [%]

Rp0.2 [Mpa]

Rm [Mpa]

A [%]

2024-W

171

336

28.4

165

318

5.0

0.95

2024-T4

351

492

21.9

279

389

9.0

0.79

2024-T6a(100  C – 10 h)

293

451

25.5

220

346

7.1

0.77

2024-T6b(190  C – 10 h)

403

496

8.6

341

416

5.1

0.84

2024-O

126

289

17.6

126

289

12.8

1.00

 

The best tensile properties were obtained for heat treatment T6a (aging at 190°C for 10 hours); for annealing the tensile strength of the weld reached the same level as the base material although the maximal elongation was lower. Strength efficiency of the welds made of heat-treated materials varied from 77 to 84 % and elongation efficiency from 28 to 60 %.

Generally, FSW of heat-treated materials does not encounter big problems as the temperatures during the welding process are relatively low.

3.7.2     Post-welding heat treatment

A study done by Paglia et al found that  25 minute at 180°C heat treatment after FS welding, shows a significant increase in the localized corrosion 
resistance [71].

On the other hand a study on precipitate microstructure of welded 2024-T351 with post weld heat treatment T6 (10 hours at 190°C) showed mixed results in different zones depending on their distance from the weld centre. Far from the weld line, the post-welding treatment increased the hardness of the 2024-T351 FS weld up to the one of the hardness of 2024-T6 alloy. However, at the TMAZ, at 4 mm from the weld centre, the hardness declines sharply after the post-welding heat 
treatment [37].

Generally, post-welding treatment cannot be recommended without detailed investigation of resulting mechanical properties of the weld after the heat treatment.