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.
The refill friction stir spot welding (refill FSSW) process is a solid-state joining process to produce welds without a keyhole in spot joint configuration. This study presents a thermo-mechanical ...model of refill FSSW, validated on experimental thermal cycles for thin aluminium sheets of AA7075-T6. The temperatures in the weld centre and outside the welding zone at selected points were recorded using K-type thermocouples for more accurate validation of the thermo-mechanical model. A thermo-mechanical three-dimensional refill FSSW model was built using DEFORM-3D. The temperature results from the refill FSSW numerical model are in good agreement with the experimental results. Three-dimensional material flow during plunging and refilling stages is analysed in detail and compared to experimental microstructure and hardness results. The simulation results obtained from the refill FSSW model correspond well with the experimental results. The developed 3D numerical model is able to predict the thermal cycles, material flow, strain, and strain rates which are key factors for the identification and characterization of zones as well for determining joint quality.
Friction spot joining is an alternative technique for joining metals with polymers and composites. This study investigated the fatigue performance of aluminum alloy 2024/carbon-fiber-reinforced ...poly(phenylene sulfide) joints that were produced with friction spot joining. The surface of the aluminum was pre-treated using various surface treatment methods. The joined specimens were tested under dynamic loading using a load ratio of R = 0.1 and a frequency of 5 Hz. The tests were performed at different percentages of the lap shear strength of the joint. Three models—exponential, power law, and wear-out—were used to statistically analyze the fatigue life of the joints and to draw the stress–life (S–N) curves. The joints showed an infinite life of 25–35% of their quasi-static strength at 106 cycles. The joints surpassing 106 cycles were subsequently tested under quasi-static loading, showing no considerable reduction compared to their initial lap shear strength.
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•A novel method of friction surfacing assisted hybrid friction stir welding was proposed to join dissimilar metals.•The tensile load of Ti/Al joints was improved to 12.2kN, reached ...85.3% of the Al base material.•Tool abrasion was totally avoided by introducing a friction surfaced Al-coating layer.•The bonding was combined contribution of nanoscale-IMCs and mechanical interlocking.
A new method of friction surfacing assisted hybrid friction stir welding (FS-HFSW) technique was developed to improve the joint efficiency and avoid the pin abrasion for joining of dissimilar Ti/Al joints. The FSW tool with enlarged head and concave end-face was designed to broaden the lap width and promote material flow. The maximum tensile load reached 12.2kN, representing 85.3% of the parent Al alloy, with a ductile fracture locating at the heat affected zone of base Al. The excellent bonding of Ti and Al was based on the combined effects of nanoscale TiAl3 IMCs layer and complex mechanical inter-locking.
To develop a suitable keyhole closure process in high-strength AlZnMg(Cu) alloys, refill friction stir spot welding was used to perform repair welds of through holes of 7.5mm diameter in AA 7075-T651 ...plates with 6mm thickness. The thermal cycle and the evolution of microstructural features were investigated in detail. The mechanical performance of the welds was studied based on the changes in microstructure and temperature exposure caused by the welding process.
Thermal cycle measurements revealed high heating rates and peak temperatures of up to 540°C in the weld center. Leftover grains from the base metal that did not recrystallize were determined in the stirred zone of the weld center. The welds showed a W-shaped hardness distribution with a lowest hardness of 70% of base metal values in the heat affected zone. Under quasi-static loading, two failure modes were determined, with mode 1 failure occurring in the heat affected zone and mode 2 failure occurring in the outer regions of the stirred zone with crack initiation in the lower portions of the weld. Post-weld natural aging was proven to be highly significant for the mechanical properties of the welds and is effective for up to 4weeks after welding.
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•Defect-free and highly efficient keyhole closure welds in AA 7075 - T651 are achieved using refill friction stir spot welding.•A comprehensive microstructural analysis revealed partially recrystallized grains in the stirred zone for the first time.•Microstructural features and thermal cycles explain the mechanical properties of the keyhole repair welds including an UTS of 74%.•The applied energy and peak temperatures in the HAZ show a linear dependency on the tool revolutions per mm plunge/retract.•The influence of the process parameter on the mechanical properties are analyzed in detail.
Friction surfacing (FS) is a solid-state layer deposition process for metallic materials at temperatures below their melting point. While the bonding of the deposited layers to the substrate is ...proven suitable for coating applications, so far the mechanical properties of additively manufactured stacks have not been systematically investigated. In particular, the effect of successive deposited FS layers, i.e., repetitive thermo-mechanical loading, on the interface properties as well as anisotropy and strength of the deposited stack is unknown. For this purpose, the mechanical properties of FS deposited multi-layer stacks from dissimilar aluminum alloys have been investigated, characterizing layer-to-layer as well as layer-to-substrate bonding interfaces via micro-flat tensile testing. Furthermore, directional dependencies in the stack and failure mechanisms are analyzed. The results show a homogeneous, fine-grained microstructure with average grain sizes between 4.2 and 4.6
μ
m within the deposited material. The resulting tensile properties with no significant directional dependency present an ultimate tensile strength between 320 and 326 MPa exceeding the strength of the AA5083 H112 consumable base material. No difference was obtained in terms of layer-to-layer or layer-to-substrate interface strength. Furthermore, homogeneous hardness was observed within the deposited structure, which is in the range of AA5083 base material’s hardness of 91 HV. The results indicate that the FS process in conjunction with the material used is suitable for additively generated structures and highlight the potential of this solid-state layer deposition technology.
An interplay between high degree of shear deformation and deformation-induced heating occurs during friction stir processing (FSP) of metals. In medium-to-low stacking fault energy Cu alloys, this ...can lead to a complex spatially heterogenous activation of dynamic recrystallization (DRX) and twinning mechanisms. Within the Cu-Nb system, the presence of Nb is further expected to influence the DRX mechanism of the Cu matrix. However, the microstructural changes induced by the co-deformation of Nb during FSP are still not well understood. Therefore, this study uses a combination of multimodal microstructural characterization, solution thermodynamics-based predictions, and computational crystal plasticity simulation to reveal the various microstructural evolution mechanisms that can occur during FSP of a Cu-4at%Nb binary model alloy. The formation of softer DRX zones, and harder shear localization regions are revealed using electron backscatter diffraction, transmission electron microscopy, atom probe tomography, and crystal plasticity modeling.
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•FSP of a binary Cu-Nb alloy activates complex spatially heterogenous dynamic recrystallization and twinning mechanisms.•Shear induced fragmentation and redistribution of Nb towards nano-sized particles leads to shear localization and strain hardening.•Super saturation of Nb was observed in ultra-fine Cu grains as a result of friction stir processing.
Ti–4Al-0.005B titanium alloy T-joints were produced with two different friction stir welding (FSW) sequences, and its effect on the low-cycle fatigue performance was investigated. Results show that ...hysteresis loop became a straight line with no significant fatigue damage occurring at low strain amplitudes (0.2% and 0.4%). As the strain amplitude increased to 0.6%, the area enclosed by the hysteresis loop increased for both T-joints and the base material (BM) due to fatigue damage accumulation. As the stress amplitude decreased gradually with increasing number of cycles, the fatigue life followed this decreasing trend. The fatigue life of single-weld T-joints is close to that of the double-weld T-joints, with a cyclic strain hardening index of the BM being in-between of the two T-joints. However, cyclic strength coefficient of BM is the lowest, and that of the double-weld T-joint is larger than that of the single-weld T-joint. The single-weld T-joint breaks at the heat affected zone (HAZ) on the advancing side, where the double-weld T-joint breaks at the HAZ of the second weld.
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•A comparative study was conducted between BT-FSW (bobbin tool) and semi-stationary BT-FSW.•Flawless welds were achieved in both cases with superior surface finish in the stationary ...variant.•The macrostructure of the stationary variant changed to an asymmetrical shape and material flow.•Higher mechanical performance was determined, the point of lowest mechanical performance was correlated with a most unfavorable precipitation state.
The aluminum lithium alloy AA2198-T851 has been bobbin tool friction stir welded using a tool concept with one stationary and one rotating shoulder. Defect free welds in 3mm thick sheet have been produced featuring a high quality surface finish on the stationary side. The macrostructure forms an asymmetrical shape with microstructural characteristics known from standard friction stir welding. Because of only one rotating side a material flow direction towards the stationary shoulder has been observed. A parameter survey shows that a weld pitch of one mm per rotation combined with high pressure between the shoulders lead to good results. Mechanical performance of 82% of base metal ultimate tensile strength and 77% of base metal hardness have been achieved. The fracture analysis indicates two competing fracture modes, one being in the heat affected zone and the other at the borderline of the stirred zone on the advancing side. The first mode forms due to thermal cycle influence, whereas the second location suffers from weak bonding as a result of the thermal cycle and experienced deformation.
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