NEED-The effect of dimensional variability of sheet thickness (tolerance) and tool misalignment is poorly understood for the clinching process. Finite element analysis (FEA) is valuable but requires ...a lot of and is difficult to verify in this situation due to the asymmetrical geometry and nonlinear plasticity. OBJECTIVE-The objective of this work was to determine the effect of thickness tolerance, tool misalignment and sheet placement (top vs. bottom) in the clinching process, by use of analogue modelling with plasticine. METHOD-Experiments used a scaled-up punch and die, with plasticine as the analogue. Thickness tolerances were represented by sheet thicknesses of 11 and 7 mm, 12 and 8 mm, 8 and 12 mm and 13 and 9 mm for upper and lower sheets, respectively. Two types of lubricant were tested between sheets: glycerine and silicone oil. Angular variability was also introduced. Measured parameters were interlock (also called undercut) and neck thickness. Analogue results for deformation were compared with microscopy of metal clinching. FINDINGS-The results reveal that the multiscale analogue model is an efficient tool for studying the effect of dimensional deviation on a clinch joint. Thickness tolerance showed a critical relationship with interlock, namely a reduction to about half that of the nominal, for both maximum and least material conditions. Increased angular misalignment also reduced the interlock. Compared with glycerine, silicone oil tests showed reduced interlock, possibly the result of a lower coefficient of friction. ORIGINALITY-This work demonstrates the usefulness of analogue modelling for exploring process variability in clinching. The results also show that significant effects for sheet placement are ductility, lubricant (friction), thickness of samples and tool misalignment.
One of the difficulties with bobbin friction stir welding (BFSW) has been the visualisation of microstructure, particularly grain boundaries, and this is especially problematic for materials with ...fine grain structure, such as AA6082-T6 aluminium as here. Welds of this material were examined using optical microscopy (OM) and electron backscatter diffraction (EBSD). Results show that the grain structures that form depend on a complex set of factors. The motion of the pin and shoulder features transports material around the weld, which induces shear. The shear deformation around the pin is non-uniform with a thermal and strain gradient across the weld, and hence the dynamic recrystallisation (DRX) processes are also variable, giving a range of observed polycrystalline and grain boundary structures. Partial DRX was observed at both hourglass boundaries, and full DRX at mid-stirring zone. The grain boundary mapping showed the formation of low-angle grain boundaries (LAGBs) at regions of high shear as a consequence of thermomechanical nature of the process.
Need—National legislative health and safety (H&S) frameworks impose requirements but grant self-management to organisations. Consequently variability arises in management systems, and some ...organisations struggle to achieve successful implementation. The risk assessment process is key to the H&S management system, and could benefit from greater consistency and better external alignment with the legislative framework of the jurisdiction. Approach—The harm categories in the New Zealand (NZ) Act were adapted into a consequence scale. A non-linear scale was developed for the consequence axis to represent the disproportional nature of catastrophic harm outcomes compared to minor injuries. A hazard assessment process was devised based on systems engineering methods. Organisational decision-criteria were derived from the communications requirement in the Act, and these thresholds linked to expected treatments. Originality—A method is providing for aligning risk assessments with a national legislative framework, and integrating the technical aspects of risk assessment with the management processes. The approach also more explicitly includes recovery actions in contrast to existing methods where prevention dominates. Regarding the management aspects, it shows how thresholds may be defined relative to the legislation, to give clear expectations regarding treatment and internal communication, thereby assisting executives (‘officers’ in terms of the NZ Act) meet their duties.
BACKGROUND-The solid-phase joining of A6082-T6 plates by bobbin friction stir welding (BFSW) is problematic. Better methods are needed to evaluate the microstructural evolution of the weld. However, ...conventional Al reagents (e.g., Keller’s and Kroll’s) do not elucidate the microstructure satisfactorily, specifically regarding grain size and morphology within the weld region. APPROACH-We developed innovative etchants for metallographic observations for optical microscopy. RESULTS-The macrostructure and microstructure of A6082-T6 BFSW welds were clearly demonstrated by optical microscopy analysis. The microetching results demonstrated different microstructures of the Stir Zone (S.Z) distinct from the Base Metal (B.M) and Heat Affected Zone (HAZ) & Thermo-mechanical Affected Zone (TMAZ). The micrographs showed a significant decrease in grain size from 100 μm in B.M to ultrafine 4-10 μm grains for the S.Z. Also, the grain morphology changed from directional columnar in the B.M to equiaxed in the S.Z. Furthermore, thermomechanical recrystallization was observed by the morphological flow of the grain distortion in HAZ and TMAZ. The etchants also clearly show the polycrystalline structure, microflow patterns, and the incoherent interface around inclusion defects. ORIGINALITY-Chemical compositions are identified for a suite of etchant reagents for metallographic examination of the friction-stir welded A6082-T6 alloy. The reagents have made it possible to reveal microstructures not previously evident with optical microscopy.
The profession depends on its practitioners developing management and leadership skills to achieve good client outcomes and robust, reliable products or services, delivered by profitable, ethically ...run engineering businesses. The difficulty is determining what those skills are, and where in the career they are needed. The New Zealand population of professional engineers was surveyed to rate the importance of a list of management and leadership topics. Results show the relative importance of various topics and how their importance is perceived differently with years of experience. The results also help differentiate the roles of teaching institutions and ongoing in-career professional development.
Bobbin friction stir welding (BFSW) is an innovative variant for the solid state welding process whereby a rotating symmetrical tool causes a fully penetrated bond. Despite the process development, ...there are still unknown variables in the characterization of the process parameters which can cause uncontrolled weld defects. The entry zone and the exit zone consist of two discontinuity-defects and removing them is one of the current challenges for improving the weld quality. In the present research, the characteristic features of the entry and exit defects in the weld structure and formation mechanism of them during the BFSW processing was investigated. Using stacked layers of multi-colour plasticine the material flow, analogous to metal flow, can be visualised. By using different colours as the path markers of the analogue model, the streamline flow can be easily delineated in the discontinuity defects compared with the metal welds. AA6082-T6 aluminium plates and multi-layered plasticine slabs were employed to replicate the entry-exit defects in the metal weld and analogue samples. The fixed-bobbin tool utilized for this research was optimized by adding a thread feature and tri-flat geometry to the pin and closed-end spiral scrolls on both shoulder surfaces. Samples were processed at different rotating and longitudinal speeds to show the degree of dependency on the welding parameters for the defects. The analogue models showed that the entry zone and the exit zone of the BFSW are affected by the inhomogeneity of the material flow regime which causes the ejection or disruption of the plastic flow in the gap between the bobbin shoulders. The trial aluminium welds showed that the elimination of entry-exit defects in the weld body is not completely possible but the size of the defects can be minimized by modification of the welding parameters. For the entry zone, the flow pattern evolution suggested formation mechanisms for a sprayed tail, island zone and discontinuity-channel. For the exit zone a keyhole-shaped discontinuity is discussed as a structural defect.
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