a b s t r a c tIn this study, T-Shape friction test was redesigned to make it more suitable for application to microforming processes. Workpiece with aspect ratio (length/diameter) of 5 was proposed in order to ease workpiece handling. The die geometry was also modified from the original test to improve friction sensitivity especially within the range of friction factors commonly observed in metal forming. Geometric deviation of the die was simulated using Deform-2D to establish the acceptable tolerance for the fabrication. The effect of variation in workpiece mechanical properties on the test behavior was also investigated through Deform-2D simulation. Based on simulations on a 1 mm diameter copper workpiece, a tolerance of 0.01 mm (1% of workpiece diameter) was found to be the most suitable for the die fabrication. In addition, it was shown that variations in workpiece mechanical properties of up to 10% do not significantly influence the friction test results. Ultimately, T-Shape test experiment was conducted using copper workpieces to examine how the test complied with the friction behavior observed in the experiment.
The frictional behaviors between metal forming tool and three different metallic materials were evaluated using the modified T-Shape test. A mathematical function is proposed to describe the calibration curves for different friction coefficients. Round bars of copper, aluminum and silver of diameter 1 mm and length 5 mm were used as the workpieces to study the material influence on friction factor, m, during unlubricated microforming process through comparison between simulation and experimental results. Furthermore, various lubricants were used with the aluminum and copper to examine their performance in microforming. The results have shown that the workpiece materials not only determine the friction factor, m, during unlubricated microforming, but also influence the performance of lubricants. Lubricant can be completely ineffective and may not produce discernible friction reduction in microforming, unlike in *Manuscript Click here to view linked References Highlights x Mathematical function for calibration curves in modified T-shape test is proposed. x The effect of workpiece materials on the microforming friction coefficients is studied. x The effect of lubricants on the microforming friction coefficients is extensively studied. x A phenomenological friction model is proposed for liquid lubrication microforming.
In this paper, the sensitivity of T-Shape test to friction conditions was evaluated by observing the extrusion height and load curves throughout the normalized stroke (relative to workpiece diameter). Using the finite element code Deform 2D and assuming plane strain approximation, the effects of changing die geometry to the T-Shape test results were investigated. The sensitivity of the T-Shape test was also improved by introducing the double-sloped T-Shape design. The double-sloped T-Shape test was able to separate the extrusion height curves for shear coefficient of friction 0.0 to 0.4 which was unable to distinguish using the original T-Shape setup.
Contact simulation involving asperities was developed by assuming that the deformation by asperities is equivalent to the deformation by an indenter in a hardness test. Consequently, depth dependent flow stress curves were derived from the indentation size effect model from Abu Al-Rub and were used to simulate the influence of the number of asperities involved during contact on the distribution of contact pressure and the value of effective friction coefficient. Results from simulations suggested that multiplying the number of asperities in contact, when the size of the asperities is comparable to the size of the apparent contact, is not followed by proportional multiplication of the reaction forces. The competing phenomena observed in the simulation are then proposed as an explanation to friction size effect occurring in microforming.
The effect of strain gradient on mechanical property of material is implemented through depth-dependent stress strain relation model in conventional finite element simulations for use in friction prediction. For the incorporation of strain gradient effect, contact simulation involving asperities was developed with the assumption that the deformation pattern created by asperities from tool surface in microforming is comparable to the deformation created by the indenter in a hardness test. Consequently, depth-dependent stress-strain relation was derived from the indentation size effect model and this stress-strain relation was used in a simulation to show the effect of strain gradient to friction behaviour in microforming at different surface roughness levels. Experiment was conducted alongside the simulation and the results showed that with asperity ploughing considered as major contributor to friction in microforming at room temperature, the simulation involving depth-dependent material properties is able to predict the better predict the friction behaviour as compared to its continuum simulation counterpart.
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