Abstract
The projection welding of nuts performed using the pneumatic (electrode) force system (PFS) was subjected to thorough numerical and experimental analysis, enabling the identification of the window of welding parameters taking into consideration boundary conditions including expulsion, torsional strength and the deformation of the nut thread. The welding process was subjected to optimisation involving the use of a new, i.e. electromechanical (electrode) force system (EFS). The optimisation-related approach involved the reduction of welding current and the extension of a welding current flow time in comparison with those obtained when using the pneumatic force system. It was assumed that the acceptance criterion would be the breaking torque not lower than that obtained under the most favourable welding conditions performed using the PFS. The research work involved comparative numerical calculations (performed using the SORPAS software) in relation to both, i.e. the PFS and EFS. The technological welding tests were performed using inverter welding machines (1 kHz) provided with various (electrode) force systems. The research work also included metallographic tests, hardness measurements and torsional strength tests. As a result of the application of EFS and a special innovative hybrid algorithm for controlling the electrode clamps, a wider and higher weld core was obtained. The depth of “penetration” into the sheet was greater. As a result, the welded joint has a higher resistance (by 30%). Technological welding tests for the new EFS system were carried out for a 25% lower welding current compared with PFS. Despite the lower welding current for EFS, the welding energy is slightly lower but the welding quality for EPS is significantly higher.
Highlights
Advanced car bodies contain approximately 300 welded and clamped fasteners, e.g. bolts, nuts and pins, to which key safety features including seat belts, steering columns or the earthing of electric circuits are fastened [1]
Analyses were performed in relation to 12 variants connected with the pneumatic force system (PFS) (P1 ÷ P12) and three variants related to the electromechanical force system (EFS) (E1 ÷ E3)
The welding process performed using the PFS was analysed with reference to the electrode
Summary
Advanced car bodies contain approximately 300 welded and clamped fasteners, e.g. bolts, nuts and pins, to which key safety features including seat belts, steering columns or the earthing of electric circuits are fastened [1]. The projection welding of nuts is characterised by numerous disadvantages and limitations, including (i) narrow window of welding parameters, (ii) lack of repeatability, (iii) sensitivity to the lack of tolerance in relation to the height of the projection and the hardness of the material subjected to. The above-named disadvantages could be primarily linked to the use of the pneumatic force system (PFS), characterised by significant mechanical inertia and the impossibility to control force, during the welding current flow [2]. A characteristic of projection welding, for PFS, is the fact that the joint is obtained in the solid state. The foregoing results from the fact that the molten material is pushed outside the welding area by excessive electrode force. This, in turn, leads to a significant increase in the area of contact between materials being welded (nut projection and sheet) and a rapid
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