Micro forming tribology for robust production

This Ph.D. thesis concerns tribology and robustness in micro-sheet metal forming. Sheet metal forming is a well-established field, but conventional theory becomes insufficient when transitioning to micro-sheet metal forming due to scale effects. Scale effects lead to changes in material behaviour and an increase in variation in process kinematics, making it difficult to apply established knowledge to design micro-sheet metal forming processes.
The robustness of a production system is a measure of how effective the system is at delivering consistent output despite variation in input. The robustness of micro-sheet metal forming systems is impacted by scale-effects, so a method for the robustification of these systems is therefore necessary to ensure consistent quality. Process monitoring can be used to warn operators of a system that is out of control due to e.g., wear. Conventional methods, such as intermittent inspection of formed parts, are not suitable in micro-sheet metal forming due to high production rates. The monitoring of thermoelectric current signals arising in the system is a viable candidate for use in industrial process monitoring. The ease by which a thermoelectric monitoring system is installed, the benefit of not needing direct access to the tool, and the low cost, make it a promising method. The robustness of micro-forming processes can be improved by reducing the influence of scale effects. The use of solid particles as lubricant additives is a promising approach to make friction in micro-sheet metal forming more consistent, as the particles help retain lubricant in open lubricant pockets on the surface of the workpiece. The scale effect described by the lubricant pocket model are thereby mitigated.
The investigation of industrial tribo-systems is typically performed as an off-line simulation of the industrial system in a laboratory to minimise expense and production stops. The fidelity of the simulation is highly dependent on the level of abstraction from the industrial counterpart that is necessary to allow laboratory-scale testing. It is beneficial to include as few abstractions as possible to ensure a high level of transferability of laboratory results to the industrial tribo- system. The effect of straining of workpiece material in progressive forming processes on specific steps in the process is often neglected. This was shown to affect the evaluation of coating performance, where testing of material that was not pre-strained gave conflicting results compared to testing with pre-strained material. Further, the influence of strain-induced surface-roughening should be included when testing friction in a progressive forming system as the surface development affects lubricated friction. The testing of systems in which recirculatory lubrication is used should take into account the influence of aging and use on the lubricant that is tested. Even though one fresh lubricant shows a better performance than another, it does not follow that the used lubricant does also. By accounting for these changes in tribological simulations, the results of the tests can more easily be transferred to decision making with regards to the real system.
A fundamental part of robustness in micro-sheet metal forming is the knowledge of what occurs to a material as it is deformed. Without this knowledge, process design is made more difficult as process limits cannot be gauged, and quality assurance is made almost impossible due to the high production rates and small size of parts. The reduced formability of thin foils inherently leads to a reduction in fracture strain, especially in tension. The characterisation of flow curves for thin foils at large strains by conventional methods such as tensile testing is therefore difficult. In extreme cases, e.g., when forming material that is not annealed, fracture strains of less than 0.05 are often found. To avoid excessive extrapolation from limited data, compression testing can be used instead. The use of plane-strain stack compression testing allows for high strains and requires very little specimen preparation. Strain-induced changes in materials include effects on the corrosion resistance, the martensite fraction, and electrical contact resistance, the quantification of which is necessary for designing of parts.