Sheet Metal Forming

Hybrid metal–polymer joints were produced by mechanical clinching.Polystyrene sheets were pre-heated before joining according to different heating schemes.The mechanical behaviours of joints were investigated by single lap shear joints.The influence of process parameters on mechanical behaviours was assessed.The present investigation is carried out to assess the suitability of the clinching process for production of plastic–metal hybrid joints. To this end, a prototypal apparatus was developed to preheat the sheets before joining. A campaign of experimental tests was carried out on aluminium alloy AA5053 and polystyrene. Experimental tests were conducted by varying the main process parameters, i.e. pre-heating conditions (heating time and temperature of heating air) and forming pressure. The joints produced under different operating conditions were observed using optical and stereoscope microscope. In addition, the mechanical behaviours of joints were assessed by conducting single lap shear tests on single joints. Based on the achieved results, the main failure modes were identified and the effect of process parameters on mechanical behaviours of the joints were clarified.

The joinability of aluminium alloy sheets with reduced ductility produced by mechanical clinching is analysed. A modified tool set geometry was developed to reduce the localization and magnitude of plastic strain. Sheets pre-heating was adopted to increase the material formability. Optical microscopy and scanning electron microscopy were utilized to observe possible presence of cracks in clinched connections and to perform dimensional analysis. Mechanical characterization tests comprising micro-hardness test and single lap shear tests were conducted to evaluate the influence of the processing conditions on joints strength.

For a given sheet metal forming process, an accurate determination of the contact pressure distribution is an essential step towards the estimation of tool life. This investigation utilizes finite element (FE) analysis to model and explain the evolution and distribution of contact pressure over the die radius, throughout the duration of a channel forming process. It was found that a typical two-peak steady-state contact pressure response exists for the majority of the process. However, this was preceded by an initial transient response, characterized by extremely large and localized contact pressures, which were more than double the magnitude of the steady-state peak pressure. The validity of the predicted contact pressure behavior was assessed via detailed numerical analysis and by examining the wear response of an experimental stamping operation. The experimental results revealed that the high contact pressure zones of the transient response corresponded to a severe galling wear mechanism. Therefore, the transient response may be of primary significance to the tool wear response; thus questioning the applicability of traditional bending-under-tension wear tests for sheet metal stamping processes. Finally, a parametric study was conducted, examining the influence of the major process parameters on the steady-state and peak transient contact pressures, using the developed FE model. It was found that the bend ratio and the blank material ultimate tensile strength had the most influence on the peak contact pressures. The main process-related parameters, friction coefficient and blank holder force, were found to have only a minor influence.

An experimental investigation has been conducted on mechanically clinched joints, produced with fixed and extensible dies with different forming forces. Mechanical testing involving single lap shear tests, both with one and two joining points, and peeling tests were conducted under quasi-static conditions to assess the different mechanical behaviours of these joints. The effect of the processing conditions on the main mechanical response of the joints, namely the maximum strength, stiffness and absorbed energy were investigated.
The results showed that the joints produced with the extensible die exhibited a similar strength as compared to those produced with the fixed die in single lap shear tests, while they are characterized by a higher strength (up to 40%) when loaded during the peeling test because of a larger interlock. In addition, the employment of extensible dies allows a drastic reduction of the forming loads as compared to those required by adopting the fixed dies.

Rubber-pad forming process is widely used in aerospace components manufacturing. The problems associated in modelling this process is friction forces between sheet and forming tools play an important role because of their influence on the process performance and on the final product properties. This frictional behaviour is often taken into account by using a constant coefficient of friction in the finite element simulations of sheet metal forming processes. This is a poor description of the real frictional behaviour and therefore theoretical static and kinetic friction models are presented in this paper for an axisymmetric rubber-pad forming process. Both models are based on the local contact conditions. Furthermore, rubberpad forming experiments and simulations were performed. The measured friction curves using the new friction models were implemented in the finite element code ABAQUS. From the results of simulations it was found that the new friction models have better correlation with experimental results compare to traditional Coulomb friction model.

Gold is the most widely used electrode material for bioelectronic applications due to its high electrical conductivity, good chemical stability and proven biocompatibility. However, it adheres only weakly to widely used substrate materials such as glass and silicon oxide, typically requiring the use of a thin layer of chromium between the substrate and the metal to achieve adequate adhesion. Unfortunately, this approach can reduce biocompatibility relative to pure gold films due to the risk of the underlying layer of chromium becoming exposed. Here we report on an alternative adhesion layer for gold and other metals formed from a thin layer of the negative-tone photoresist SU-8, which we find to be significantly less cytotoxic than chromium, being broadly comparable to bare glass in terms of its biocompatibility. Various treatment protocols for SU-8 were investigated, with a view to attaining high transparency and good mechanical and biochemical stability. Thermal annealing to induce partial cross-linking of the SU-8 film prior to gold deposition, with further annealing after deposition to complete cross-linking, was found to yield the best electrode properties. The optimized glass/SU8-Au electrodes were highly transparent, resilient to delamination, stable in biological culture medium, and exhibited similar biocompatibility to glass. The development of bioelectrodes that are transparent, biocompatible and capable of extended, stable operation in a broad range of biological media is a critical challenge for the field of bioelectronics 1–3. Optoelectronic devices such as light-emitting diodes 4–8 , photodiodes 9–12 , and phototransistors 13 have been deployed in multiple media, ranging from relatively mild interstitial fluids 14–19 to corrosive solutions 20–22. The most important properties for bioelectrodes are high mechanical and chemical stability, high conductivity, high transparency, and excellent biocompatibility. For this reason, thin-film (≪100 nm) gold is usually considered the electrode of choice for bioe-lectronic applications 23–25. A further requirement of a bioelectrode, however, is strong adhesion to the surface on which it is deposited – often a transparent substrate material such as glass or quartz. Unfortunately, gold exhibits poor adhesion to both these materials, and typically requires the inclusion of a thin layer of an oxidative metal such as chromium to achieve adequate adhesion to the substrate 26. Chromium, however, has significant disadvantages from a bioelectronic perspective, including cumbersome patterning procedures and potential cytotoxicity, leading to a strong demand for alternative adhesion layers. (Note, while some other metals such as titanium have been successfully employed as adhesion layers for conventional electronic applications, the cytotoxicity of these metals and their oxides is also an open question, see e.g. ref. 27). The commercial negative-tone photoresist SU8 28 has previously been used as an alternative adhesion layer to chromium since it adheres well to multiple substrate materials and (once cross-linked) is chemically inert 29,30. SU8 has the further advantage of being photo-patternable, allowing patterned electrodes with feature sizes down to a few microns to be readily fabricated using a simple two-step expose/develop procedure 31. We have previously reported preliminary data 32,33 , showing SU8-supported gold electrodes to have favourable and moderately stable electro-optical characteristics in cell culturing medium, making them a potentially attractive choice for bioelectronics. Unfortunately, we found that, after prolonged periods of immersion in culture medium, gold deposited on SU8 had a tendency to develop wrinkles and cracks on the surface, reducing electrode durability and