Recent advances in friction-stir welding : Process, weldment structure and properties

The comprehensive body of knowledge that has built up with respect to the friction stir welding (FSW) of aluminium alloys since the technique was invented in 1991 is reviewed The basic principles of FSW are described, including thermal history and metal flow, before discussing how process parameters affect the weld microstructure and the likelihood of entraining defects After introducing the characteristic macroscopic features, the microstructural development and related distribution of hardness are reviewed in some detail for the two classes of wrought aluminium alloy (non-heat-treatable and heat-treatable) Finally, the range of mechanical properties that can be achieved is discussed, including consideration of residual stress, fracture, fatigue and corrosion It is demonstrated that FSW of aluminium is becoming an increasingly mature technology with numerous commercial applications In spite of this, much remains to be learned about the process and opportunities for further research a

Friction stir welding of aluminium alloys

The diversity and never-ending desire for a better life standard result in a continuous development of the existing manufacturing technologies. In line with these developments in the existing production technologies the demand for more complex products increases, which also stimulates new approaches in production routes of such products, e.g., novel welding procedures. For instance, the friction stir welding (FSW) technology, developed for joining difficult-to-weld Al-alloys, has been implemented by industry in manufacturing of several products. There are also numerous attempts to apply this method to other materials beyond Al-alloys. However, the process has not yet been implemented by industry for joining these materials with the exception of some limited applications. The microstructures and mechanical properties of friction stir welded Al-alloys existing in the open literature will be discussed in detail in this review. The correlations between weld parameters used during FSW and the microstructures evolved in the weld region and thus mechanical properties of the joints produced will be highlighted. However, the modeling studies, material flow, texture formation and developments in tool design are out of the scope of this work as well as the other variants of this technology, such as friction stir spot welding (FSSW).

Recent Developments in Friction Stir Welding of Al-alloys

Effect of welding speed on microstructures and mechanical properties of underwater friction stir welded 2219 aluminum alloy

Microstructure and mechanical properties of friction stir welded joints in 2219-T6 aluminum alloy

The microstructure, the microchemistry and the environmental assisted cracking susceptibility was investigated for friction stir welded 2219-T87 plates. It was found that the coarsening of the Al2Cu intragranular precipitates occurring within the thermo-mechanically and the heat affected zones after friction stir welding correlates with a decrease in the hardness values. The regions with the lowest hardness are relatively narrow in size and are limited to the boundary between the thermo-mechanically affected zone and the nugget. The weld microzones exhibited a different microstructure but a similar environmental assisted cracking susceptibility. The environmental cracking susceptibility of the weld was generally low. The constant extension rate tensile tests were carried out in air and in a 3.5 wt.% NaCl solution. For samples tested transverse to the welding direction failure occurred within the thermo-mechanically affected zone–nugget boundary. The thermo-mechanically affected zone–nugget boundary was also the softer region of the weld. The corrosion immersion tests indicated an increased resistance to corrosion for the friction stir welded region as compared to the weld unaffected parent metal.

Microstructure, microchemistry and environmental cracking susceptibility of friction stir welded 2219-T87

The temperature gradient occurring for a short time during friction stir welding greatly affects the localized corrosion properties of welded 7050-T7451 plates. An immersion experiment in a salty solution was carried out in order to verify the influence of short-term post-weld heat treatments at temperatures similar to those taking place during friction stir welding on the corrosion behaviour of friction stir welded 7050-T7451. The experiment consisted of inserting thermocouples at different weld regions for the registration of the temperature development with time, and partially immersing the welded plate in a salty solution at 480 °C. In this manner, the weld experienced different temperature expositions at different locations. It was found that a temperature exposition above 180 °C for 20 min significantly increases the general corrosion resistance of friction stir welded 7050-T7451. The re-exposition of 7050-T7451 friction stir weld to a time–temperature combination similar to that occurring during the welding process on the thermomechanically heat affected zones of the weld, significantly improves the corrosion properties and the environmental cracking resistance. After the short-term temperature exposition, the fracture location of samples tested in a 3.5 wt.% NaCl solution moved from the corrosion susceptible thermomechanically affected zone to the heat affected zone.

C.S. Paglia

Papers

Microstructure, microchemistry and environmental cracking susceptibility of friction stir welded 2219-T87

The time–temperature–corrosion susceptibility in a 7050-T7451 friction stir weld