Entrapment and escape of liquid lubricant in metal forming
Introduction
In metal-forming operations using liquid lubricant, mixed lubrication is often occurring implying trapped lubricant in closed pockets of the workpiece surface and hydrodynamic lubrication on microscale. These mechanisms are of great influence on friction and lubrication as well as on the resulting surface topography. Increased knowledge about these lubrication mechanisms is required in order to improve modelling of friction and to design tailor-made surfaces like lasertex for specific forming operations.
The concept of micro Plasto Hydrodynamic Lubrication (PHL) was introduced by Mizuno and Okamoto [1]and later verified by Kudo et al. [2]and Azushima et al. 3, 4, 5; the latter by studying plane strip drawing through transparent, wedge-shaped dies introducing artificial lubricant pockets in the aluminium strip material by local indentations. Azushima et al. showed that lubricant escape from the pockets was influenced by the degree of reduction, drawing speed and lubricant viscosity.
In the present work, the mechanism of micro PHL is studied further applying the same experimental technique used by Azushima et al. Parameters investigated besides lubricant viscosity, drawing speed and degree of reduction are: die angle, back tension, strain-hardening of workpiece material and friction conditions along the lower die. Furthermore, theoretical models of two different mechanisms of lubricant escape from the pockets are developed and the calculated onset of these mechanisms is compared with experimental observations.
Section snippets
Experimental equipment
Fig. 1 shows the equipment developed for plane strip drawing where the tool/workpiece interface may be observed and recorded on videotape with a CCD-camera through a transparent die. As workpiece material, three different types of 140×20×2 mm3 aluminium strips are applied, all provided with lubricant pockets of pyramidal shape. The side angle of the pyramids is approximately 10°, the base is approximately 1×1 mm2. The pockets are placed in rows of four across the strip with a distance between
Experimental investigation
Fig. 2 shows an overview of the experimental investigation. Seven different parameters are analysed. For each parameter, a reference value, marked with REF, as well as two other values is investigated keeping the six other parameters as the reference value.
Theoretical analysis of MPHSL and MPHDL
The mechanisms of MPHSL and MPHDL lubrication are analysed by combined solid mechanics and fluid mechanics.
Calculation of force oscillations
In the experiments, it was observed that the strip drawing force oscillated with a frequency corresponding to the distance between the rows of lubricant pockets and a maximum variation (peak-to-peak) of up to 12% of the total drawing force. Recording an audible force signal synchronously on the soundtrack of the videotape, the connection between the size of the force and the lubricant escape was obvious.
The force was largest when no pockets were present in the die contact and it did not drop
Conclusions
Strip drawing experiments with different lubricants, workpiece materials and process parameters have shown the importance of these parameters as regards the escape of trapped lubricants from pockets during deformation. The phenomenon of backward lubricant escape by MPHDL in strip drawing has been verified. Distinguished from this phenomenon, forward lubricant escape has been characterised as MPHSL. Mathematical models have been established quantifying the onset of the two lubrication
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