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

The mass saving potential of light-weight materials, such as Al alloys, is beneficial for fuel economy and reducing CO2 emissions. However, the wide-spread use of these alloys has been long hindered due to the difficulty in fusion joining as well as their high cost. Welding of Al alloys, which are considered to be difficult to weld through conventional arc welding, is now possible by either of low heat input arc welding, high-power density fusion joining, such as laser beam welding and electron beam welding, or friction stir welding. Particularly, friction stir welding can be successfully applied to these materials owing to the fact that no melting takes place in the weld nugget. The aim of this overview is to summarize the developments in the joining of Al alloys over the recent years. This study is also intended to provide guidance for the industry and researchers dealing with joining of these alloys.

Recent developments in joining of aluminum alloys

Friction stir welding (FSW), a highly efficient solid-state joining technique, has been termed as “green” technology due to its energy efficiency and environment friendliness. It is an enabling technology for joining metallic materials, in particular lightweight high-strength aluminum and magnesium alloys which were classified as unweldable by traditional fusion welding. It is thus considered to be the most significant development in the area of material joining over the past two decades. Friction stir processing (FSP) was later developed based on the basic principles of FSW. FSP has been proven to be an effective and versatile metal-working technique for modifying and fabricating metallic materials. FSW/FSP of aluminum alloys has prompted considerable scientific and technological interest since it has a potential for revolutionizing the manufacturing process in the aerospace, defense, marine, automotive, and railway industries. To promote widespread applications of FSW/FSP technology and ensure t...

Recent Advances in Friction Stir Welding/Processing of Aluminum Alloys: Microstructural Evolution and Mechanical Properties

Prospects of laser beam welding and friction stir welding processes for aluminum airframe structural applications

Investigation on AISI 304 austenitic stainless steel to AISI 4140 low alloy steel dissimilar joints by gas tungsten arc, electron beam and friction welding

In this study, the effect of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of friction stir butt-joined AA6061 Al-alloy plates both in O and T6-temper conditions was investigated by detailed microstructural investigations and microhardness measurements, in combination with transverse tensile testing It was determined that the PWHT might result in abnormal grain growth (AGG) in the weld zone particularly in the joints produced in O-temper condition depending on the weld parameters used during friction stir welding The PWHT generally led to an improvement in the mechanical properties even if AGG took place Thus, the post-weld heat-treated joints exhibited mechanical properties much higher than those of respective as-welded plates and comparable to those of the respective base plates

Investigation into the Influence of Post-Weld Heat Treatment on the Friction Stir Welded AA6061 Al-Alloy Plates with Different Temper Conditions

Laser beam welding is considered to be a suitable joining process for high speed, low distortion, and high quality fabrication of aircraft structures manufactured from aluminum alloys, which are mainly preferred due to their favourable properties, such as high strength to weight ratio, ease of forming and high thermal and electrical conductivity. However, the laser beam welding of 6000 series aluminum alloys may exhibit a tendency to solidification cracking, and porosity may be a major problem unless appropriate welding parameters and filler metal are employed. In this study, the microstructural aspects and mechanical properties of laser beam welded new generation aluminum alloy, namely 6056, developed especially for aircraft structures, are investigated. A continuous wave CO 2 laser using AlSi12 filler wire was employed. A detailed microstructural examination of the weld region was carried out by Scanning Electron Microscopy (SEM). Standard tensile and microflat tensile specimens extracted from the welded plates were tested at room temperature for the determination of general and local mechanical properties of the welded joints. Extensive microhardness measurements were also conducted. Crack growth mechanisms of the joints produced were also determined by conducting fatigue tests under various stress ratios (i.e., 0.1 ≤  R  ≤ 0.7).

Microstructural and mechanical characterization of laser beam welded AA6056 Al-alloy

In this study, AA7075-O and AA7075-T6 Al alloy plates were friction stir butt welded using two sets of weld parameters in order to investigate the effects of temper condition prior to joining and post weld heat treatment on microstructure and mechanical properties of the joints. Another goal of the work is to determine the possibility of restoring the loss of strength in the joint area experienced in welding of this alloy in the age-hardened condition by subsequent heat treatment. The study revealed that the friction stir welding resulted in a strength undermatching when the alloy joined in T6 condition while a significant strength overmatching was obtained in the joints produced in O temper condition. The post weld heat treatment led to a significant recovery in the strength of the joints produced in T6 condition, thus a significant increase in the joint performance. On the other hand, it led to a decrease in the joint performance value of the joints produced in O temper condition although the strength was increased. Furthermore, the post weld heat treatment resulted in abnormal grain growth in the joint area the degree of which depends on weld parameters used and the prior temper condition.

Effects of temper condition and post weld heat treatment on the microstructure and mechanical properties of friction stir butt-welded AA7075 Al alloy plates

Laser beam (LB) welding is increasingly being used in welding of structural steels. The thermal cycles associated with laser beam welding are generally much faster than those involved in conventional arc welding processes, leading to a rather small weld zone that usually exhibits a high hardness for C-Mn structural steels due to the formation of martensite. It is rather difficult to determine the tensile properties of a laser weld joint area due to the small size of the fusion zone. Complete information on the tensile and fracture toughness properties of the fusion zone is essential for prequalification and a complete understanding of the joint performance in service, as well as for conducting the defect assessment procedure for such weld joints. Therefore, an experimental investigation on the mechanical properties of laser welded joints using flat microtensile specimens (0.5 mm thick, 2 mm wide was carried out to establish a testing procedure to determine the tensile properties of the weld metal and heat-affected zone (HAZ) of the laser beam welds. In the present work, two similar joints, namely, ferritic-ferritic and austenitic-austenitic and one dissimilar ferritic-austenitic joint were produced with a CO 2 laser using 6-mm-thick steel plates. In addition to the testing of flat microtensile specimens, the mechanical properties were examined by microhardness survey and conventional transverse and round tensile specimens. The results of the microtensile specimens were compared with standard round tensile specimens, and this clearly showed the suitability of the microtensile specimen technique for such joints. The crack tip opening displacement (CTOD) tests were also performed to determine the fracture toughness of the LB welds using three-point bend specimens. The effect of strength heterogeneity (mismatchingj across the weld joint and at the vicinity of the crack tip on the CTOD fracture toughness values was also discussed.

Determination of mechanical and fracture properties of laser beam welded steel joints

Al-alloys are widely used in the transportation industries due to their low density and excellent formability properties. These alloys, which are problematic in fusion-welding processes, can be successfully joined by friction-stir welding. Particularly high-strength AA7075 alloy is considered to be non-weldable by fusion welding. Friction-stir welding, being a solid-state joining technique, can also be successfully used in joining other difficult-to-weld alloys. In this study, 3.17-mm-thick, AA7075 Al-alloy plates both in the Oand T6-temper conditions were joined by friction-stir welding using four different weld parameters. Microstructural and mechanical characterizations of the joints produced were obtained with detailed optical microscopy investigations, extensive hardness measurements and tensile tests. The effect of temper condition on the joint performance was determined in addition to the effect of the weld parameters on the joint quality.

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Papers

Investigation into the Influence of Post-Weld Heat Treatment on the Friction Stir Welded AA6061 Al-Alloy Plates with Different Temper Conditions

Microstructural and mechanical characterization of laser beam welded AA6056 Al-alloy

Effects of temper condition and post weld heat treatment on the microstructure and mechanical properties of friction stir butt-welded AA7075 Al alloy plates

Determination of mechanical and fracture properties of laser beam welded steel joints

Investigation into the effect of temper condition on friction stir weldability of AA6061 Al-alloy plates