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Steel Friction Stir Welding

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Friction stir welding (FSW) of steels has now reached a level of technical maturity where weld lengths up to 30m can be attained in a wide range of engineering steels.

These welds demonstrate excellent mechanical properties and there are good indications that their corrosion and fatigue properties will exceed those of fusion welds. The process shows a degree of robustness, suiting it to industrial application, and can weld some steels, for example ODS steels, that are currently considered unweldable.

Hand-in-hand with this developing technical maturity, FSW is beginning to attain economic viability for a number of applications where the enhanced weld properties can be offset against its current higher cost when compared with arc or laser welding.

Table 1 Steels welded at TWI.

Carbon steels

Stainless Steels

ODS & Others

AISI 1018, 1060, 1085

304, 304 L, 316

20Cr5Al, PM2000, MA957

A36

420A, 420B, 420C

PH14, PH17

S275, S355, S690

RWL34

Armour steels

API X 65, X80, L80, X100

12C27

 

EH36, DH36, EH46

S32205 duplex

 

HSLA-65

   

HY-80, HY-100

   

D2 , O1

   

Friction stir welding dissimilar grades of steel

In addition to the single-material welds reported above, it has also been shown that it is possible to weld dissimilar grades of steel. For example, TWI has butt welded 2m-long plates of 6mm-thick S275 carbon steel to S32205 Duplex stainless steel.

Figure 1 shows a full penetration friction stir weld in 8mm-thick DH36 steel. Compared with a conventional fusion weld, the heat-affected zone is small and the microstructure in the weld zone is more akin to that of a forging than a casting. Unlike in fusion welding, there is no abrupt boundary between the parent and the weld metal, just a gradual change of structure across the HAZ until the TMAZ is reached, whereupon the flow of the stirred material gives rise to a slightly more pronounced boundary region where the material has undergone significant shear. The plastic flow of material in the weld zone is clearly visible.

Tailoring the weld microstructure

Friction stir welding of steels takes place at temperatures corresponding to those of the transformation between the BCC and the FCC crystal structure. Careful selection of the processing parameters used, for example the tool rotation rate (which primarily affects the heat input to the process) and the tool traverse rate (which primarily affects the cooling rate of the heated weld metal) therefore allow a degree of control over the phase transformations taking place.

This apparent ability to tailor the weld microstructure by careful selection of FSW process parameters and tool form holds out many possibilities and suggests how enhanced mechanical properties may be obtainable. Other potential property and performance benefits are also possible.

Adding to the attractiveness of friction stir welding in steel are the apparent enhancements of mechanical properties in addition to the accepted FSW benefits of high joint strength, minimal defects and low distortion. Initial indications also exist of good corrosion properties, supported by the fact that FSW welds in steel are autogenous as no filler metal is added to the weld. Friction stir welds may also be performed underwater, and in materials such as ODS steels that are generally regarded as not weldable by fusion techniques.

Table 2 Joint efficiencies of FSW butt welds in five steels.

Grade of Steel

Cross weld UTS, MPa

% of minimum parent material spec. value

RQT-701

831

105%

HSLA-65

626

107%

304L

635

131%

Duplex 2205

829

134%

S355J2 + N

609

110%


Joint industry project: FSW of high-performance steels

TWI is leading a joint industry project to investigate the techno-economic benefits of FSW in steels that are challenging to weld using existing techniques, and develop process parameters and techniques for the early commercialisation of FSW in these grades.

For more information on Friction Welding and Processing, please email:


contactus@twi.co.uk