Friction stir welding (FSW), a mature solid-state joining method, has become a revolutionary welding technique over the past two decades because of its energy efficiency, environmental friendliness ...and high-quality joints. FSW is highly efficient in the joining of Al alloys, Mg alloys, Ti alloys, polymers and other dissimilar materials. Recently, FSW has gained considerable scientific and technological attention in several fields, including aerospace, railway, renewable energy and automobile. To broaden the adoption of FSW in manufacturing fields, three inherent issues—back support, weld thinning and keyhole defects—must be addressed to ensure the structural integrity, safety and service life of the manufactured products. This review covers the recent progress on the control strategies for these inherent issues, which are basically divided into self-supported FSW, non-weld-thinning FSW and friction stir-based remanufacturing. Herein, the aim is to focus on the corresponding technical development, process parameters, metallurgical features and mechanical properties. Additionally, the challenges and future outlooks are emphasized systematically.
Friction stir welding/processing (FSW/P) involving temperature, mechanics, metallurgy and interaction, is a complex solid state joining and processing technology. FSW has been widely applied to join ...aluminum alloy, titanium alloy and other materials which are difficult to weld by fusion welding. The last scientific study states that FSW has potential to join thermoplastic polymers and polymer matrix composites. In this review, current understanding and development about FSW of thermoplastic polymers and polymer matrix composites, multifunctional composites fabrication as well as dissimilar FSW of metal and polymer are reviewed. Future scientific research and engineering development related to FSW/P of thermoplastic polymers and polymer matrix composites are identified.
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Sound hybrid joint was achieved with a sufficient mixture of 6061-T6 aluminum (Al) alloy and polyether ether ketone (PEEK) via friction stir lap welding (FSLW) by a tapered thread pin ...with the triple facets. The larger length ratio between the Al anchor and the rotating pin reached approximately 0.8 at low welding speeds due to the dynamic flow induced by the triple facets. The mechanical interlocking and adhesive bonding attributed to the high-quality joining. Increasing welding speed resulted in the reduction of the adhesion area and the decrease in the size of the Al anchor, deteriorating the mechanical interlocking. A high-quality with the maximum shear bond strength of 20 MPa was achieved through the big Al anchor with a high hardness value. The tapered thread pin with the triple facets has feasible and potential to FSLW of metal and polymer in terms of enhancing mechanical interlocking.
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Tools with a high depth-to-width ratio of 0.6 were designed and tested. The thread structure of pin could promote material flow but increase fracture risk. The pins with milling ...facets were more beneficial to achieve sound joint. The tapered thread tool with triple facets was the optimal structure for high depth-to-width ratio friction stir welding at a rotational speed of 800rpm and a welding speed of 30mm/min. The measured temperature and joint formation agreed with the predicted data well. A narrower average heat affected zone with the width of 440μm was obtained, which was far lower than that underwater friction stir welding.
High strength and toughness carbon nanotubes (CNTs) reinforced Mg-6Zn composites were fabricated by stirring casting integrated with friction stir processing (FSP). The strengthening mechanisms of ...the CNTs/Mg-6Zn composites were expounded by the characterization of the microstructural evolution and the mechanical properties. The singly dispersed CNTs formed compact bonding with the matrix, which contributed to the grain refinement and the mechanical properties enhancement of the Mg-6Zn matrix. The strengthening contributions are based on the grain refinement, load transfer and Orowan looping mechanisms. The yield strength, ultimate tensile strength, elongation of the FSPed CNTs/Mg-6Zn composites reached 171 MPa, 330 MPa and 15%, which were 144%, 156% and 87% higher than those of the as-cast pristine Mg-6Zn alloy. The fabrication route is proved to be effective to develop innovative CNTs-reinforced metal matrix composites with exceptional mechanical properties.
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High-quality aluminum /steel joint was achieved via self-riveting friction stir lap welding (SRFSLW) characterized by the prefabricated holes in steel sheet. According ...to the origination of filling materials, the prefabricated holes were filled with the plasticized aluminum materials in this order: the deformed aluminum ahead of a pin firstly, the stirred aluminum by the pin body secondly and the driven aluminum by the pin tip finally. The strength of the SRFSLW joint reached maximum value of 317 N/mm at the hole diameter of 3 mm, which was 23% higher than that by conventional friction stir welding (FSW). The optimized joints failed at both the aluminum/steel interface and the formed aluminum rivets. The synergistic effect of mechanical bonding induced by the riveting and metallurgical bonding induced by the Al/Fe IMC layer contributed to high strength of the SRFSLW joint.
A novel technique named friction filling staking joining (FFSJ) was proposed to join 6082-T6 aluminum alloy and poly propylene (PP) polymer sheets. The FFSJ was realized with the filling of a ...pre-fabricated hole using an additional filling stud. The excellent FFSJ joint was achieved under the synthesis effects of frictional heat and thermo-mechanical behavior at the interface between the filling stud and polymer sheet. The intimate joining formed at the interface, while the pre-fabricated hole was completely filled with the stud. A macro mechanical interlocking formed between the metal and filling stud, attributing to the main joining mechanism. And the partial adhesive layer appeared between the re-solidified polymer and metal. The maximum tensile shear strength was comparable to that of state-of-the-art welding. The FFSJ combines the benefits of the improvement of assembly precision, simple process, short welding cycle and high strength compared with the state-of-the-art welding, indicating that the FFSJ technique has potential to join metal with polymer or polymer matrix composites.
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•Friction-based filling stacking joining (FFSJ) of metal and polymer is proposed.•Adhesive bonding and mechanical interlocking attribute to joining mechanism.•Shear tensile strength of FFSJ is comparable to the state-of-the-art welding.
To construct reliable heterojunctions between metals and polymer matrix composites, friction self-riveting welding (FSRW) was proposed, characterized by multi-scale mechanical interlocking and ...adhesive bonding, to improve mechanical properties. Holes and porous oxide structures were pre-fabricated on the metal sheet. High-quality heterogeneous joints were achieved. Under the frictional heat and forging force, the composites were softened and flowed into the pre-fabricated holes and porous structures via squeezing behaviors from a welding tool, improving macro/micro-mechanical interlocking. The oxide layers also enhanced adhesive bonding. The maximum tensile shear strength reached 27 MPa based on multi-scale mechanical interlocking and adhesive bonding. Joint fracture located at the interfaces between composites and metals, the composites rivets and the intermixing region of fibers. This strategy has the potential to construct reliable heterojunctions between polymers or polymer matrix composites and metals in the fields of aerospace and automotive.
Intermetallic compounds (IMCs) formed in the stir zone (SZ) of dissimilar friction stir welded aluminum/magnesium (Al/Mg) joint always deteriorate tensile properties. Joining of 6061-T6 Al and AZ31B ...Mg alloys via ultrasonic assisted friction stir welding (UaFSW) at low temperature was performed to reduce the disadvantages caused by IMCs. A sound Al/Mg joint with smooth surface was obtained based on the stationary shoulder at a high welding speed of 80 mm/min, rotational velocity of 1000 rpm, and ultrasonic power of 1400 W due to the improvement of material flow caused by ultrasonic, indicating the success of low-temperature welding. Meanwhile, joining interface, characterized by the IMCs, was attributed to inter diffusion caused by severe plastic deformation from ultrasonic and friction stirring. The tensile strength and elongation of the Al/Mg joint reached 134 MPa and 1.5%, respectively. The IMCs containing Al
2
Mg
3
formed in the SZ was the main reason of tensile fracture, and then resulted in cleavage fracture. The present study suggests that UaFSW has feasible and potential to join Al/Mg alloys at low temperature, which provides a significant reference for welding joint of dissimilar materials with the IMCs.
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The plunging depth of 0.05mm was optimum for joint formation of 0.5mm thick Al-6061 sheet by micro friction stir welding. Increasing rotational velocity from 1500rpm to 2000rpm was ...beneficial to sound surface formation, while the taper pin with three flats owned wider process window than the single taper pin. The minimum ratio of thickness reduction of 2% was attained, which enhanced the area of load bearing. The taper pin with three flats owing to the severe stirring actions resulted in the finer grain size, improving tensile property. The maximum tensile strength by the taper pin with three flats reached 217MPa, equivalent to 90% of base material.
