Improving weld quality with optimized bobbin tools: an innovative approach to friction stir welding of aluminium

Friction stir welding (FSW) has gained significant attention as a viable method for joining aluminum alloys due to its ability to produce high-quality welds. In recent years, bobbin tools have emerged as an innovative tool geometry for FSW of aluminum. Because of their unique tool design and weld setup, there is no backing plate needed and weak points such as root defects cannot form. The creation of strong and high-quality joints in similar aluminum structures is a challenging task for welding processes. In this regard, the current study aims at investigating the effect of shape-optimized bobbin tools on the welding quality of the joints. For this purpose, a simulation of the critical run-in process was performed in an initial step. Thus, the contact conditions between the tool and the work-piece could be analyzed, and a qualitative impression was gained of the welding behavior of this welding set-up. Subsequently, the tool was shape-optimized by imposing ideal contact conditions. The optimized and non-optimized tools were then used to perform FSW on similar aluminum joints made of AA5754. The resulting joints were analyzed for their mechanical and microstructural properties, and it was found that the optimized tool led to a different microstructure and tensile strength than the non-optimized tool. Therefore, this study provides a new and effective approach to improve the weld quality of similar aluminum joints by optimizing the geometry of bobbin tools through simulation

Material characterization and finite element modelling of cyclic plasticity behavior for 304 stainless steel using a crystal plasticity model

Low cycle fatigue tests were carried out for a 304 stainless steel at room temperature. A series of experimental characterisations, including SEM, TEM, and XRD were conducted for the 304 stainless steel to facilitate the understanding of the mechanical responses and microstructural behaviour of the material under cyclic loading including nanostructure, crystal structure and the fractured surface. The crystal plasticity finite element method (CPFEM) is a powerful tool for studying the microstructure influence on the cyclic plasticity behaviour. This method was incorporated into the commercially available software ABAQUS by coding a UMAT user subroutine. Based on the results of fatigue tests and material characterisation, the full set of material constants for the crystal plasticity model was determined. The CPFEM framework used in this paper can be used to predict the crack initiation sites based on the local accumulated plastic deformation and local plastic dissipation energy criterion, but with limitation in predicting the crack initiation caused by precipitates

Experimental and modelling study of fatigue crack initiation in an aluminium beam with a hole under 4-point bending

Slip band formation and crack initiation during cyclic fatigue were investigated by in-situ experiments and non-local CPFEM simulations systematically. Experimental techniques including EBSD, digital image correlation (DIC) and SEM have been used to obtain consistent grain orientations, local strains, as well as the locations where slip bands and micro-cracks form on the sample surface. The realistic microstructure based on the EBSD map has been generated and used for finite element modelling. An advanced non-local crystal plasticity model, which considers the isotropic and kinematic hardening of the plastic strain gradient, has been adopted. The simulation results match well the corresponding experimental results. It was found that total strain and averaged slip on all slip systems, combined with accumulated slip on specific slip planes help predict the location and orientation of slip bands and micro-crack initiation correctly. Furthermore, a fatigue indicating parameter based on competition between maximum slip and the total slip has been proposed to reproduce the experimental observations

Role of bonding defects in a self-reinforced polypropylene (PURE) under fatigue loading

Self-reinforced plastics are manufactured by producing partly crystallized tapes, weaving fabrics, and hot-pressing of several layers of fabric. As there is no matrix accommodating the reinforcing elements the apes are locally welded to each other during hot-pressing. Upon loading pronounced strain localizations can be found using digital image correlation DIC. The localization occur on a mesoscopic scale near crossing points of the tapes and on microscopic scale between different fabric layers, if there are imperfect bondings of between the tapes. These imperfections are more likely to occur in the center of the specimen as can be visualized using computer tomography. The size and number of the bonding defects can be changed by modifying the parameters during hot pressing and by choosing different numbers of fabric layers. The paper deals with the fatigue behavior of this type of material and the role of the bonding imperfections on the damage accumulation process. For this purpose local deformation fields using DIC determined after quasi- static loading, step-wise increased loading and cyclic loading are compared with each other. These findings are related to the damage pattern observed after final fracture