Experimental Research on Electro-Electroforming Metal Parts

1 Overview

Electro-hydraulic forming is a new high-energy forming process that utilizes the mechanical effect produced by the pulsed discharge of strong current in the liquid, ie, the electro-hydraulic effect. Compared with the conventional impact forming method, the electro-hydraulic forming has the following features.
1 High forming speed and supersonic shock waves generated by electro-hydraulic forming. Therefore, the forming speed of hydro-electroforming is very high, generally several hundred meters per second, while the forming speed of mechanical forming processing is approximately several to several tens of meters per second.
2 high forming accuracy, liquid electroforming can get high forming accuracy, generally believed that up to 0.02 ~ 0.05mm, foreign reports, the liquid electroforming in the formation of a long piece of 3660 ~ 4880mm, the precision can reach 0.13mm, No further processing after forming.
3 With fewer tooling, the electroforming method can be used for simultaneous deep drawing, punching, shearing, embossing, and flanging. Therefore, the processing of some complicated parts can simplify the process, reduce the tooling, and hydroforming. Only need to die, not only eliminates the male model, but also eliminates the yin and yang model matching problem, which greatly reduces the production cycle and reduces costs.
4 Especially suitable for processing tube bulging parts, the process of processing tube bulging parts by general mechanical methods is more complicated, and when the parts are asymmetrical or complex in shape, it is more difficult or even impossible to process. However, the use of hydroforming It can be solved relatively easily, and the energy utilization rate is high. In this way, bellows and asymmetrical parts can also be processed.
Due to the above-mentioned characteristics of electro-hydraulic forming, it is necessary to carry out experimental research on electro-hydraulic forming, which will lead the electro-hydraulic forming technology to practical production as soon as possible.

2 Electro-hydraulic forming experimental study

2.1 Experimental device

Figure 1 is a schematic diagram of the electro-hydraulic forming experimental device. The whole device can be divided into two parts: 1 charging circuit, which includes high-voltage transformer B, high-voltage rectifier G and charging resistor R, 2 discharge loop, discharge loop also called main loop It includes capacitor bank C, auxiliary, air, gap K and main gap F.

Fig.1 Schematic diagram of electro-hydraulic forming experimental device

2.2 Electroforming mechanism

When the power is turned on, the AC power is boosted by the transformer and rectified by the rectifier to charge the capacitor bank. At this time, the capacitor bank starts to store electric energy. When the voltage of the capacitor bank rises to the required value, the auxiliary gap is ignited and the capacitor bank is turned on. With auxiliary and main gap discharge, if the discharge loop parameters are properly matched, the entire discharge process can be controlled within several or several tens of microseconds.
Therefore, the energy accumulated by the capacitor is emitted in a very short time, and a huge instantaneous power is generated in the main circuit. As long as the resistance of the circuit is small enough, most of the energy will be concentrated in the liquid gap, so that the liquid gap is Breakdown, liquid gap breakdown, the large current in a very short period of time through the discharge gap, the liquid medium between the electrode water rapidly freed, while the temperature rose rapidly.
Due to the inertia of the liquid surrounding the discharge channel (the liquid is almost incompressible), the high temperature gas (ie, the plasma) cannot be expanded in proportion to the temperature, so the pressure rises to the peak in a very short time and the shock wave is generated in the liquid. Diffusion rapidly around the medium, the impact of the wave on the workpiece, to promote the formation of the workpiece, which is the mechanism of liquid electroforming.

Table 1 results of a deep drawing forming experiment

2.3 Forming experiment
2.3.1 Deep drawing Deep drawing, also called drawing, is a punching method that uses impact force to make a plane blank into an open hollow part. A deep drawing and multiple deep drawing of a sample made of L2Y210 is performed. For comparison experiments, the thickness of the test piece is 1mm, the maximum diameter of the forming is 150mm, the distance between the workpiece and the electrode is 40mm, the maximum diameter of the blank is 200mm, the depth of water is 500mm, and the experimental data of a deep drawing forming is shown in Table 1.
Note: thinning rate = [(t0-t)/t0] × 100%
Where t0—the thickness of the workpiece before deep drawing;
T—thickness after workpiece deep drawing.
For workpieces with relatively complex workpiece surfaces, where the radius of curvature is small in some locations and the mechanical strength of the material is large, the energy required for complete die-bonding at one discharge is larger, so multiple discharge draws are used, for which multiple pulls are performed. In deep experiments, the experimental conditions for multiple deep drawing are the same as one-time deep drawing. Only after each deep drawing, the specimen is re-clamped, the gap is adjusted, and the next deep drawing experiment is performed. The experimental results are shown in Table 2.

Table 2 Results of multiple drawing forming experiments

It can be seen from Table 1 that the maximum depth of deformation for a single deep drawing is 36.04 mm, and the maximum draw ratio is 1:0.24. At this time, the workpiece thinning rate is 18%.
From Table 2, it can be seen that the maximum deformation depth can reach 40.34mm by multiple deep drawing, the draw ratio can reach 1:0.269, and the thinning rate can be reduced to 14%.
From these data, it is possible to determine the drawing method used for a specific workpiece material based on the draw ratio and the thinning ratio of the actual workpiece to determine a reasonable process.
2.3.2 Punching Hole punching experiment results are shown in Table 3 for L2Y210 material with a thickness of 1mm. Through experiments, it can be shown that during punching, no gas or liquid can exist between the die and the workpiece. Otherwise, the workpiece will be reversed. Drums cannot be punched. To obtain high-quality punched parts, it is necessary to sharpen the edge of the punching of the mold. At the same time, the relative positions of the workpiece and the electrode must be arranged, and the seal must be strengthened to establish a vacuum device.

Table 3 Punching experiment results

No.

2.3.3 Composite Process For L2Y210 material with a thickness of 1mm, the forming and embossing are completed at a time. The results of the experiment are shown in Table 4.

Table 4 Composite process experiment results

The serial number 1 in Table 4 is performed without the seal of the mold and without the vent hole at the bottom of the mold. The number 2 is performed under the condition that there is a vent hole at the bottom of the mold. No. 3 is to strengthen the mold seal, but The vent hole uses a dummy seal, which is obtained when the seal can be washed off during molding.
From the comparison of three kinds of experiments, in the case that the mold has no vent hole and no seal, due to the water in the mold, the water can't be discharged rapidly with deformation when forming, so the workpiece can not be stuck at all, and when the workpiece has The exhaust hole, the water discharge speed is far less than the deformation speed of the workpiece, which can only be part of the mold, when the seal is strengthened to ensure that the mold only gas without water, and ensure that the gas can be discharged when the workpiece is formed, the workpiece is formed better Some, but also can not guarantee the accuracy of mold, therefore, to obtain a higher accuracy of the formation of vacuum devices must be established.
It can be seen from the experiment that the electroforming can be easily combined with forming, punching and embossing.
2.3.4 Tube bulging fluid Electroforming is particularly suitable for tube bulging, especially for shaped tubes, which is difficult to achieve with the general method, but no matter how complex the shape of the electro-hydraulic forming can be formed at one time, Figure 2 is the tube expansion Schematic processing.

Figure 2 Tube bulging processing

Pipe bulging experiments were carried out on a brass H68 material with a diameter of 65 mm. The length of the tube was 80 mm and the length of the deformation zone was 70 mm. The experimental results are shown in Table 5.

Table 5 Experimental results

Judging from the experimental results, the insulation measures should be taken when the tube is free bulging. Otherwise, the electrode will discharge the wall, which will result in poor discharge effect, irregular bulging of the tube, and even damage to the inner wall of the tube.

3 Conclusion

A preliminary experimental study was conducted on the electro-hydraulic forming process. The process apparatus needs to be further refined and the process parameters need to be further optimized. However, from the experimental results obtained, the following conclusions can be drawn:
1 Deep drawing can increase the ratio of forming diameter to forming depth and reduce the thinning rate.
2 sharp die can improve the quality of the stampings.
3 Insulation measures shall be taken for tube bulging.
4 The vacuum system is very important for the quality of electro-hydraulic forming.

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