The present study investigates a fatigue property and its microstructural and interface geometrical effect on the resistance element welded (REWed) aluminum (Al)/high-strength steel (Fe) lap joint. The assembled Al/Fe joints were subjected under the fatigue strength up to 80% of tensile-shear loads with 0.1 of the load ratio. As a result, an exceptional fatigue strength has been secured inducing the base material fracture because there are no noticeable interfacial defects at the 10.5 kA welding condition compared to assemblies joined with 3.5–4.5 kA of welding currents. Even, maximum tensile-shear loads exceed 9 kN at the 10.5 kA welding condition. Microstructural developments of base materials and welding interfaces were thoroughly analyzed by an optical microscopy (OM) and electron backscatter diffraction (EBSD), the development of prior-austenite-grain-boundaries (PAGBs) and α'-martensite phases were observed in the welding interfaces at all conditions. Heat affected zone (HAZ) developments of AA5052 were quantitatively analyzed in terms of recrystallization and grain growth, providing significant differences in the fatigue performances and striation, crack propagation developments. In addition, with the help of a finite element (FE) computational modeling, a mechanical stress distribution and strain behavior by geometries of the joining interface are discussed in detail. 

Microstructural and interface geometrical influence on the mechanical fatigue property of aluminum/high-strength steel lap joints using resistance element welding for lightweight vehicles: Experimental and computational investigation

Friction stir spot welding (FSSW) of dissimilar S45C steel and 6061-T6 aluminum alloy in a butt configuration is experimentally investigated. Butt spot welding is performed using a convex scrolled shoulder tool at different tool rotational speeds. FSSW butt joints are successfully fabricated by offsetting the tool to the steel side. The microstructures of the joints fabricated at three different tool rotational speeds are characterized using scanning electron microscopy and energy dispersive spectrometry. Microstructural analysis shows the presence of intermetallic compounds (IMCs) along the steel/aluminum interface. The thickness of the IMC layer and the tensile strength of the joint increase with increasing the tool rotational speed. The results of tensile tests and microstructural analysis show that the joint performance is closely related to the IMCs at the joint interface.

Friction Stir Spot Butt Welding of Dissimilar S45C Steel and 6061-T6 Aluminum Alloy

Enhanced mechanical properties of dissimilar Al and Mg alloys fabricated by pulse current assisted friction stir welding

Experimental studies are carried out on the friction stir butt-welded aluminum (Al)-clad-aluminum thin sheets. Aluminum alloy 4343 (AA4343, Al–Si series) and 3003 (AA3003, Al–Mn series) act as the clad layer and the core of the Al-clad-Al material system, respectively. Optical microscopy confirms that the joining is achieved successfully without macroscopic defects, while the clad layer is mixed into the core, mostly in the advancing side (AS) of the stir zone (SZ). Outside the SZ, no delamination between the clad layer and core is observed. Material flow analysis with electron probe microanalyzer exhibits the flow of silicon (Si) particles from the clad layer to the core resulting in several Si-induced precipitations of α-Al(Fe, Mn)Si, α-AlMnSi inside the SZ. The heterogeneous distribution of those precipitates is visualized by scanning electron microscopy and their compositions are analyzed using an energy dispersive X-ray spectrometer. Electron back-scattered diffraction maps reveal dynamically recrystallized ultra-fine grain structures (3.7–3.9 μm) inside the SZ. The mechanical properties of the joint are evaluated by cross-sectional 2D microhardness mapping and uniaxial tensile tests. Inside the SZ, the grain refinement with the formation of Si precipitation increases the tensile strength and microhardness of the joint. 

Friction stir butt-welding of roll cladded aluminum thin sheets: effect of microstructural and texture changes on mechanical properties

A bimetallic ring component is forged by a friction stir assisted forging (FS-forging). A cylindrical bimetallic blank, magnesium AZ31 cylinder (Mg core) tightly fitted inside an aluminum 6061-T6 tube (Al skin), is used. In the FS-forging, the frictional heat and stirring of rotating tool forge the blank into a desired shape, while simultaneously generate a solid-state joint between the Mg core and the Al skin, without additional external heating, as confirmed by the microstructural analysis and mechanical tests. The microstructural analysis also shows that the characteristics of the solid-state joint can be different depending on the process parameters.

Manufacturing of magnesium/aluminum bimetallic ring components by friction stir assisted simultaneous forging and solid-state joining

Lap joining of dissimilar steel S45C and aluminum 6061-T6 alloy sheets in the solid state is conducted by means of electrically assisted pressure joining (EAPJ). During joining, electric current is applied to a specimen assembly under continuously compressive plastic deformation. Additional holding time at the elevated temperature is also applied by periodically applying electric current after the completion of compressive deformation to enhance the diffusion between the steel and aluminum alloy sheets. Microstructural analysis confirms the solid-state joining and shows that the microstructure of the joint strongly depends on the process parameters. Quasi-static lap shear tensile testing shows that joint strength increases with increasing electric current density and with increasing holding time at the elevated temperature. Fracture surface morphology suggests that brittle fracture occurs at the interface between the steel and aluminum alloy, with some amount of intermetallic compound sticking to the steel side. The present study confirms that the EAPJ concept is applicable to the joining of steel and aluminum alloy in solid state.

Electrically assisted solid state lap joining of dissimilar steel S45C and aluminum 6061-T6 alloy

PF-LD is an advanced printing technique which introduces point-forging (PF) into the process of conventional laser-deposition (LD). Ti–22Al–25Nb intermetallic fabricated by PF-LD technique presented the excellent combination of room-temperature ultimate tensile strength (UTS: 1050.1–1169.3 MPa) and elongation after failure (EL: 20.8%–25.7%). An elaborately designed controlled trial by simultaneous use of LD and PF-LD techniques was employed to investigate the effect of PF on LD microstructural evolution. The results pointed out that the occurrence of static recrystallization (SRX) was the essential reason for the ultra-high mechanical properties. Both mechanisms of continuous-SRX (CSRX) and discontinuous-SRX (DSRX) played an important role in grain refinement. By in-situ forging and subsequent re-heating, the coarse columnar grains were first broken into fine equiaxed grains through progressive subgrain rotation. After two-pass PF-LD, the solidification texture <100>//DD disappeared while no obvious recrystallization texture was found. In subsequent PF-LD process, micron-scale CSRX grains were developed within original equiaxed grains through repeated polygonization and progressive subgrain rotation while DSRX grains appeared along grain boundaries driven by the differential dislocation density. We elucidated the excellent mechanical properties by detailed microstructural analysis. The results revealed that the finer equiaxed grains and the reasonable phase composition (O + BCC) greatly retarded the process of crack nucleation and propagation resulting in the typical micro-void accumulation ductile fracture while both high-density dislocations and finer recrystallized grains were beneficial to strength enhancement. 

Fabricating Ti–22Al–25Nb intermetallic with ductility higher than 25% by advanced printing technique: Point-forging and laser-deposition

Abstract In this paper, the surface texture parameters and distribution patterns are studied by numerical simulation and experiment. First, a three-dimensional micro-textured CFD fluid lubrication model with cavitation effect is established, and different texture arrays are designed to study the influence of different distribution modes on bearing capacity, friction coefficient and pressure distribution of the oil film. Then, the simulation results are further analyzed and verified by the visualized plane slider experimental platform, and the formation rules of cavitation bubbles in the micro-textured array, as well as the influences of the surface shape and different distribution modes of the micro-textured array on the cavitation bubbles are discussed. The results show that the existence of cavitation is one of the main reasons for the microtexture to increase the bearing capacity of the oil film, which cannot be ignored in the simulation study. The texture array with single symmetric orientation is the best to improve the oil film bearing capacity, and the bearing performance is the best when the texture inclination angle is 26.6°.The friction coefficient of the asymmetrically oriented textured array is 29.4% lower than that of the non-textured sample.The results in the experiment are consistent with the simulation. 

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Shengwei Zhang

Papers

Electrically assisted solid state lap joining of dissimilar steel S45C and aluminum 6061-T6 alloy

Friction Stir Spot Butt Welding of Dissimilar S45C Steel and 6061-T6 Aluminum Alloy

Fabricating Ti–22Al–25Nb intermetallic with ductility higher than 25% by advanced printing technique: Point-forging and laser-deposition

Research on cavitation effect of microtextured array

Hydrodynamic characteristics of the crane vessel-three-bucket jacket foundation coupling hoisting system during the process of lowering