There is a real demand for sustainable lightweight structures because of the growing environmental concerns. One important solution is developing structures through recycled scrap/waste thermoplastic ...materials. The current work studies the friction stir spot weldability of recycled thermoplastics, which will help to analyze the potential of friction stir-based welding techniques towards developing these sustainable structures. The combined behavior of recycling-welding procedures is investigated, as they may cause degradations; to ensure that the base thermoplastic polymer's chemical, thermal, and mechanical properties are retained. Scrapped milk bottles made from HDPE material are used as a case study. The highest lap-shear load of 1528 N was achieved at the optimum welding conditions of 1600 rpm rotational speed, 1 mm plunge depth, and 60 s dwell time. Fractographic studies (macroscopic and SEM-based) suggested four types of fracture morphologies depending on welding conditions used. The DSC results showed no significant differences in melting temperature and crystalline content of the polymeric material. The TGA tests showed no significant thermal degradations. The FTIR analysis of all the samples (bottle, recycled sheet, weld material) exhibited characteristic HDPE peaks. All these results suggest combined-welding recycling had a minimal impact on the polymeric structure. Thus, friction stir spot welding (FSSW) technique joins recycled thermoplastic scrap/waste materials with high lap-shear load and without any significant polymer degradations.
Defect-free dissimilar Al/zinc coated steel and Al/AlSi coated steel welds were successfully fabricated by refill friction stir spot welding. However, Al alloy and uncoated steel could not be welded ...under the same welding condition. Al-Zn eutectic layer formed at the Al/zinc coated steel interface showed non-uniformity in thickness and nanoscale intermetallic (IMC) produced was discontinuous. The bonding formation between the Al-Zn layer and the surrounding materials was attributed to a liquid/solid reaction mechanism. Bonding formation at Al alloy and AlSi coated steel interface was attributed to a solid/solid reaction mechanism, as the joining process did not involve with melting of base metals or AlSi coating materials. Kissing bond formed at the weld boundary acted as a crack initiation and propagation site, and the present study showed that weld strength of Al 5754/AlSi coated steel was greatly influenced by properties of original IMC layer.
Friction stir spot welding (FSSW) is one of the solid state welding methods and dissimilar material joining by FSSW has been investigated. "Scrubbing refill FSSW (Sc-RFSSW)" has developed as a new ...dissimilar material joining method, and it was shown that Sc-RFSSW provides higher joint strength than conventional FSSW methods in joining the aluminum alloy and non-coated steel. In this study, the microscale testing was performed to evaluate the dissimilar material interfacial strength of Sc-RFSSW joints. The relationship between the interfacial strength distribution and the macroscopic strength of the Sc-RFSSW joint was analyzed. In addition, the microstructure of the dissimilar material interface was observed, and the relationship between the microstructure and the interface strength was shown.
Unlocking the full potential of advanced high-strength bainitic steels in spot-welding is a pivotal endeavor for harnessing their extraordinary mechanical properties in the automotive industry. This ...study suggests a shift in paradigm to address the inferior fusion weldability of this type of steel because of the formation of coarse martensite within the welded zone of weldments obtained by liquid-state welding, through the strategy of refining the martensite within the weld zone using the solid-state friction stir spot welding (FSSW) process. This paper delves into an in-depth investigation of the microstructural evolution and load-bearing capacity of a Fe-0.25C-3Cr-1.63Mn-1.5Si (wt%) ultrafine bainitic steel, having a remarkable tensile strength of 1.7 GPa, during the intriguing process of FSSW at rotational speeds of 600 to 2000 rpm. The research uncovers that the stir zone (SZ) undergoes dynamic recrystallization, resulting in a significantly refined microstructure. Regardless of the rotational speed employed, the high hardenability of the steel and rapid cooling during the process inevitably results in martensite formation within this region. However, a comprehensive microstructural characterization conducted by the electron backscatter diffraction (EBSD) analysis proves that the different thermal cycles induced by different rotational speeds lead into various prior austenite grain sizes, martensitic/bainitic packets and blocks attributes, and density of high-angle grain boundaries (HAGBs). Through the evaluation of strengthening mechanisms in different weld zones, the study sheds light on the key factors influencing strength. Block boundaries emerge as the primary contributor to SZ hardening, surpassing the role of geometrically necessary dislocations (GNDs) and solid solution strengthening. The load-bearing capacity observed during the tensile-shear test is governed by a delicate interplay between bonding width and crystallographic features (block size and HAGBs density) of the martensite formed within the SZ. The highest peak load was achieved under optimal welding conditions (rotational speed of 1000 rpm), which enabled the production of a sufficiently large bonding width, providing ample bonding area for load-bearing, along with an abundance of blocks and HAGBs to effectively impeding crack propagation.
Display omitted
•Advanced ultrafine bainitic steel is welded successfully by FSSW.•Block boundaries surpass GNDs as primary contributor to strength in weld zones.•Load-bearing capacity is influenced by bonding width and features of martensite in SZ.•Block size and HAGBs density are crucial for SZ toughness and joint performance.•Optimal load-bearing achieved at 1000 rpm with ample bonding width, blocks, and HAGBs.
Display omitted
•Refill friction stir spot welding (refill FSSW) was successfully employed to weld AZ31 Mg alloy to galvanized DP600 steel for the first time.•Defect-free Mg/steel refill FSSW welds ...with excellent strength were obtained in a wide parameter window.•The “microstructure-strength-failure” observation revealed the joining mainly attributed to brazing and diffusion welding.•The growth of Fe-Al intermetallic compounds layer was identified, implying the important role of Al present in Mg alloy for obtaining high-performance welds.
The application of magnesium (Mg) inevitably involves dissimilar welding with steel. A novel solid state spot welding method, refill friction stir spot welding (refill FSSW), was utilized to weld AZ31 Mg alloy to galvanized DP600 steel. Although Mg/Fe is an immiscible alloy system, defect-free welds with high strength were successfully obtained in a wide parameter window. The results of microstructure, interfacial reactions, and mechanical properties are reported to reveal the underlying joining mechanism. Due to the melting of Zn coating and subsequent Mg-Zn reactions, Mg-Zn eutectic and intermetallic compounds were detected within welds. Heterogeneous interfacial reactions occur along Mg/steel interface, and the relationship between interfacial structure and fracture behavior was investigated. The joining mechanism is associated with Zn coating and Fe-Al layer: 1) the presence of Zn coating is beneficial for achieving high-quality welding between Mg and steel, it protects the interface from oxidation and contributes to brazing of the weld; 2) the Al present in Mg alloy reacts with Fe, resulting in the growth of Fe-Al layer, which contributes to the diffusion bonding in the interface. The overall results clearly show that refill FSSW is a competitive welding method for joining Mg and galvanized steel.
Alclad layer is a challenge to ensure the mechanical properties of refill friction stir spot welding (RFSSW) alclad 7050/2524 joint. In this study, RFSSW with suspension rotating process (SRP) was ...designed to improve the mechanical properties by strengthening the alclad layer. The thermal cycles, microstructures and mechanical properties of the joints welded without and with SRP were compared. The results indicated that SRP increased the high temperature dwell time of the stir zone bottom, which promoted more alloy being stirred into and mixing with the alclad, beneficial for strengthening the alclad. When with SRP, the tensile shear strength of joint reached 11 kN, improved by about 30% compared with joint without SRP.
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.
The hybrid structure composed of aluminum alloy and carbon fiber reinforced plastics could combine their advantages. In order to investigate the weldability of these two lightweight materials, the ...hybrid joints of 5052 aluminum alloy (AA5052) and carbon fiber reinforced polyether ether ketone composites (CF-PEEK) were fabricated by friction stir spot welding. The variance analysis revealed that the dwell time and plunge speed were the most significant factors. By optimizing the welding parameters, the ultimate tensile shear load reached 2690±64 N (the dwell time: 8 s, the plunge speed: 10 mm/min). The interface could be divided into pin-affected zone, shoulder-affected zone, resin adhesive zone and resin concentrated zone. Since resin concentrated zone could not provide interfacial bonding due to delamination, the shoulder-affected zone and pin-affected zone were decisive regions for mechanical properties. The bonding mechanism included three parts: adhesive bonding provided by re-solidified resin, macro-mechanical interlocking of aluminum alloy that entered CF-PEEK, and micro-mechanical interlocking of resin that was tightly trapped at surface slits as well as the carbon fibers beset into AA5052. This work clarifies the interfacial characteristics of AA5052/CF-PEEK hybrid joints and provides an approach to improve the mechanical properties.
Display omitted
•The ultimate tensile shear load of Al/PEEK hybrid FSSWed joints reached 2690 N.•Adhesive failure and cohesive failure coexisted in the hybrid joints.•Pin-affected zone, shoulder-affected zone are decisive areas for mechanical property.•Anchoring effect is the essence of micro-mechanical interlocking.
This paper studies the friction stir spot welding of AA2024-T3/AA7075-T6 Al alloys in the ambient and underwater environments by clarifying the nugget features, microstructure, fracture and ...mechanical properties of the joints. The results show that the water-cooling medium exhibits a significant heat absorption capacity in the AA2024-T3/AA7075-T6 welded joint. Nugget features such as stir zone width, circular imprints, average grain sizes, and angular inter-material hooking are reduced by the water-cooling effect in the joints. Narrower whitish (intercalated structures) bands are formed in the underwater joints while Mg2Si and Al2CuMg precipitates are formed in the ambient and the underwater welded joints respectively. An increase in tool rotational speed (600–1400 rpm) and plunge depth (0.1–0.5 mm) increases the tensile-shear force of the welded AA2024-T3/AA7075-T6 joints in both the ambient and underwater environments. The maximum tensile-shear forces of 5900 N and 6700 N were obtained in the ambient and the underwater welds respectively.
Display omitted
PDF
