Plastic deformation of workpiece during unloading in plate compression
Introduction
Plate forging is an integrated method of sheet metal forming processes and forging processes to form products with a machined precision [1]. In plate forging, the tool pressure is high and thus the tool distortion is considerable [2].
Elastic deformation of tools in metal forming has a great influence on the product precision and the tool shape is generally designed to anticipate the tool distortion. For example, a roll crown is commonly provided to make the rolled product into high accuracy shape [3]. The same idea is also applied to the tool design in cold forging to obtain forged parts with a good flatness [4].
A die crown corresponding to the elastic deflection of a flat die at the die stroke end is commonly considered to be necessary to improve the flatness of the products in plate compression since the flatness deviation due to the elastic recovery of thin plates during unloading is generally ignored [5].
In this paper, deformation behaviour of a plate in compression is investigated by experiment and FEM simulation. A surprising occurrence is reported that a noticeable plastic strain is induced in the compressed plate during unloading in plate compression.
Section snippets
Experiment
Fig. 1 shows the arrangement of the tools and sensors in plate compression. The die stroke is measured by two contact displacement sensors with a resolution of 0.1 µm installed on the die plate. The change of the plate diameter during loading and unloading is measured by four laser displacement sensors with a repeatability of 4.0 µm provided every 90° on the middle plane of the plate in the thickness direction.
Experiments are carried out on a universal testing machine of 1000 kN. The
Deformation behaviour in loading
In Fig. 3, the distribution of contacting pressure is shown. As the reduction in thickness at the plate edge et increases, the pressure peak moves towards the plate centre and the peak value rises.
Fig. 4 shows the velocity distributions in the plate and the die in loading. At the maximum point of contacting pressure, the flow is divided to outward and inward directions. The pressing velocity of the die outside of the flow dividing point is larger than that inside. This difference results from
Experimental results
Fig. 11 shows the variations of the plate diameter during loading and unloading in experiment. The increase of the measured plate diameter during loading is slower than the simulated result and that during unloading is a little larger than the simulated result, but the whole tendency in experiment is well represented by FEM simulation.
As shown in Fig. 12, the measured plate thickness coincides well with the simulated thickness.
Influence of friction and blank dimensions
As shown in Fig. 9, with increasing reduction in thickness, the compressed plate thickness increases by unloading and then turns to decrease. Let us express this turning point by the reduction in thickness at the plate edge, etcr.
Table 2 gives the influences on etcr of the friction coefficient μ and the ratio between the blank diameter and thickness d0/t0. etcr is investigated within the range of et ≤ 20%. A high friction and a large ratio d0/t0 cause a smaller etcr.
Fig. 13 shows the relation
Conclusion
To examine the optimum die crown in plate compression, deformation behaviour of an aluminium plate under a high-speed steel die coated with DLC is investigated by FEM simulation and experiment.
- (1)
As the reduction in thickness in loading increases, the change of the compressed plate thickness during unloading shifts to a decrease from an increase, and the additional plastic strain by unloading becomes noticeable.
- (2)
The plastic deformation during unloading is confirmed by measuring the increase in
Acknowledgements
The authors wish to thanks the Amada foundation for the financial support (AF-2020009-B2).
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