Keywords:Friction stir welding FSW tool design Al-Li alloys Welding speed 2024 aluminum alloy Tensile properties DIC A B S T R A C T Different tool geometries were used to investigate the joining of aluminum alloys (AA2198 to AA2024) by friction stir welding (FSW). Three shoulder profiles (flat, raised spiral, and raised fan) and five different pin profiles (cone, half threaded cylindrical, straight cylindrical, tapered cylindrical and square) were selected. Preliminary investigations were conducted by moving the tool into a seamless sheet made of the AA2024-T3 in order to select the tools that produce defect-free joints. Preliminary investigations showed the raised fan shoulder profile helps the material flow from the edge of shoulder to the pin creating a smooth surface finish with no flash in comparison with flat and raised spiral shoulder profiles. Pins with a minimum diameter equal to half the plate thickness produced lack of penetration (LOP) defects, while increasing minimum pin diameter to the plate thickness eliminates the LOP defects. Half threaded cylindrical pin produced tunneling defect, whereas defect free joint made by straight cylindrical, tapered cylindrical and cubic pin profiles. So they were selected for joining AA2024 to AA2198. Fracture locations of different joint variants were observed the vicinity of the thermomechanical affected zone (TMAZ) of AA2198-T3 alloy, and in the nugget on the AA2198-T3 side which have the minimum hardness and highest strain localization as confirmed by hardness maps and digital image coronation (DIC). Higher measured temperature than dissolution temperature of AA2198 main strengthening precipitates could be the reason of low hardness and fracture in TMAZ and center of nugget. Furthermore a raised fan shoulder with a tapered cylindrical pin produced highest elongation and yield strength and it was selected as the best candidate for optimization of the welding parameters. It was found that higher rotational and traverse speeds enhance the formation of tunneling and kissing bond defects, suggesting that longer pins have to be used for higher traverse speeds. Welding speed 750 rpm with 450 mm min −1 could create joint with highest yield strength.
Edge ductility of dual phase steels is highly affected by the preparation of the edge and especially shear cutting has a detrimental effect on the formability. In this study, a novel in-plane bending test is used to characterize the edge ductility of dual-phase steels. Digital image correlation technique is employed to calculate the fracture strain along the edge in the desired material orientation. The proposed test setup isolates the material-dependent aspects, namely damage and hardening, from those resulting from testing procedure factors such as contact stresses and friction compared to conventional hole expansion capacity (HEC) test. The results show that the edge crack phenomenon depends on the material orientation.
A novel in-plane bending test was used to study edge ductility in DP800 as a common advanced high-strength steel in the car industry. The test utilized the digital image correlation technique to measure the local and average fracture strain values along the edge of the specimen. In contrast to the widely used hole expansion capacity test, the impact of punch friction, contact stress, and out-of-plane strain on edge ductility is eliminated by removing the punch. Also, the strain gradient inherent to the beam bending provides a controlled crack propagation path, making crack tracking easier than the sheared edge tensile test. The proposed bending test was utilized to investigate the influence of material orientation, cutting parameters, and global strain gradient on edge fracture strain. A correlation was observed between edge ductility, material orientation, and cutting tool sharpness, while the average fracture strain was independent of the strain gradient. The outcome shows that the in-plane bending test is reliable for determining edge ductility in any desired material orientation.
Dual-phase steels suffer from low edge ductility, which limits their formability. In this study, an in-plane bending test is used to investigate the initiation and evolution of edge cracks. The edges of samples were prepared by shear cutting and afterwards further deformed by the in-plane bending test. Void distribution and non-uniform plastic deformation were explored with the help of scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) analysis and microhardness measurement in different regions of the material edge. The extent of micro-cracks was revealed by microcomputed tomography (µCT) scan. The result shows that the blanking process creates inhomogeneous void distribution in the thickness direction. As the deformation increases during the subsequent in-plane bending test, the micro-cracks initiate at the burr region and grow towards the rollover region. Once they entirely pass the thickness of the material, they grow further, away from the edge. High roughness, plastic deformation, and void volume fraction were observed at the burr region, triggering crack initiation. The in-plane bending test successfully distinguished the dominant mechanism behind edge cracking.
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