Quality defects and improvement measures in wire EDM of die and mould

Because a wire electrical discharge machine (EDM) does not produce large cutting force and has high machining efficiency, wire EDM technology has been more and more widely used in the field of mould manufacturing. With the continuous improvement of the technical requirements of aerospace parts and precision moulds, the quality requirements of wire EDM are getting higher and higher. However, there are always some phenomena that affect the wire EDM machining efficiency and machining quality in the mould manufacturing process. Here, some improvement measures are put forward for several kinds of machining quality defects.

Quality defects and improvement measures

Collapse angle. Collapse refers to the phenomenon that when wire EDM turns, the mould corner is not the theoretical size, but is cut off by the electrode wire. The collapse angle does not meet the actual requirements, and different solutions should be taken according to the processing angle of the die.

For example, to prevent the outer corner collapse, the electrode wire program can continue to extend the 0.5mm at the corner, so as to avoid the electrode turning at the corner, thus eliminating the collapse. You can also move the 0.5mm forward at the corner, then cut back, and the program offsets the radius of the electrode wire, so that the collapse remains in the unwanted part. At the same time, increasing the tension of the electrode wire and replacing the vulnerable parts such as guide pulley and conductive block can reduce the jitter amplitude of the electrode wire and reduce the collapse of the die.

The surface is rough and scratched

The surface roughness of die wire EDM depends on many factors, such as machining conditions, discharge time, peak current, machining speed and so on. The faster the cutting speed, the rougher the cutting surface. Practice has proved that when wire EDM is carried out with a rectangular wave pulse power supply, regardless of the material, thickness and size of the workpiece, as long as the frequency conversion feed knob is adjusted to adjust the machining current (that is, the average current indicated on the ammeter) to about 70% 80% of the short-circuit current (that is, the current indicated on the meter when the pulse power supply is short-circuited), it is basically the best working condition.

To improve the machining accuracy of the die and ensure a certain processing speed, the method of multiple cutting can be adopted. In rough machining, we first use stronger processing conditions, obtain a larger offset, reserve some margin, cut quickly, and then use weaker processing conditions to reduce the offset and carry out finishing in the opposite direction. Especially when cutting the concave die, the machining margin is set aside in advance to carry out rough cutting at high speed, and then the pulse width of the power supply is reduced to carry out fine cutting. Compared with the multiple cutting method, when the same surface roughness is obtained, not only the average cutting speed is faster, but also the deformation of the workpiece is small and the dimensional accuracy is obviously improved due to the gradual release of the residual stress in the workpiece.

Deformation and cracking

In WEDM, due to the use of discharge heat for machining, the surface of the die melts because of the high temperature produced by discharge, and then the rapid cooling produces a metamorphic layer, and the material on the metamorphic layer is suddenly solidified due to rapid cooling. Shrinkage produces tensile thermal stress, resulting in many micro-cracks. Moreover, wire EDM is often arranged after die heat treatment to avoid various stresses in hot processing, cause deformation and cracking of mould cavity, and improve mould forming accuracy. However, in the process of material removal by WEDM, the symmetrically balanced stress of the workpiece on the cutting surface will change, causing new deformation, the stress balance of each part of the material will be destroyed during cutting, and the stress needs to be redistributed. this stress rebalancing process may also cause deformation and cracking of the workpiece. It is improved from the following aspects according to the causes of deformation and cracking.

The mould material should be selected according to the actual surface quality

The die materials are usually high carbon tool steel, such as T8A T10A and alloy tool steel Crl2,Crl2MV. Generally, high carbon tool steel can be used for dies with simple shape, small volume and low life requirements, while alloy tool steel is suitable for dies with complex shape, high life requirements and large volume. Because the composition of steel has a great influence on quenching deformation and residual internal stress, and because the hardenability of high carbon tool steel is poor, the thermal stress and microstructure stress of high carbon tool steel during quenching are larger than that of alloy tool steel, so quenching deformation cracking of high carbon tool steel often occurs. In addition, cracking often occurs when high carbon tool steel is machined by wire EDM due to the release of residual internal stress, so alloy tool steel should be selected as far as possible when using wire EDM die.

For those that need quenching, the materials with good hardenability should be selected, and the best materials should be determined by comparing the hardenability curve and comprehensive properties of the materials. If the precision is very high, cemented carbide materials must be selected. Stainless steel or unquenched high carbon steel should not be selected.

Correct formulation of heat treatment process

The pre-heat treatment is an effective measure to prevent the deformation and cracking of the heat treatment. The general quenching and tempering treatment is the pre-heat treatment, which is arranged after rough machining. Its technological process is: forging "spheroidising annealing" rough machining "quenching and tempering" semi-finishing "quenching and tempering" finishing. The practice shows that the addition of quenching and tempering treatment after rough machining is effective to control the deformation of heat treatment, which not only prepares the microstructure for the final heat treatment, but also eliminates the mechanical stress caused by rough machining.

When there is a large residual stress in the workpiece before quenching, it is bound to strengthen the non-uniformity of deformation during quenching, so the de-stress treatment should be supplemented before quenching.

To reduce its deformation, the correct selection of heating temperature is very important, too high quenching temperature is easy to produce irregular deformation, so under the premise of not affecting the performance, we should strive for lower heating temperature.

The spheroidising annealing process is strictly standardised, and the quenching adopts preheating and step quenching to reduce the quenching stress. The quenched steel should be tempered in time to eliminate the internal stress of quenching and reduce the brittleness; for the second type of tempered brittle die steel should be cooled quickly (water or oil cooling) after high temperature tempering, the second type of tempering brittleness can be eliminated.

Correct compilation of wire EDM process

For the precise CNC milling parts, the starting point, program direction and clamping position of the cutting should be arranged according to the shape characteristics of the parts, so as to minimise the deformation. Generally, the starting point of wire EDM should be set in the parts that are flat, finished or have little effect on the performance of the workpiece. At the same time, when compiling the part machining program, through the analysis of the drawing, when determining the cutting route, we should first cut from the side with precision requirements, so that the influence of machining deformation on the part accuracy can be reduced.

For punch parts, the wire hole at the starting point of the shape should be drilled in the convex blank before quenching, so that the internal stress balance of the workpiece will not be destroyed during cutting, so as to avoid cracking and deformation caused by cutting from the outside of the material.

Most of the parts machined by wire EDM are forged and quenched, and there is a large residual stress inside, which will be released after electrical machining to deform the die. To avoid this phenomenon, the traditional habit of once cutting is changed to rough and fine secondary cutting, so that the deformation after the first rough cutting can be corrected in time during precision cutting. Pre-processing can be carried out first, such as opening a stress narrow gap in the symmetrical centre of the online cutting part to release the internal stress, and then carry out finishing.

Conclusion

Above, some quality problems existing in wire EDM are analysed in detail, and the improvement measures are put forward. There are many factors that affect the machining accuracy and surface quality of WEDM, such as the internal stress of material, the stiffness of cutting part and fixed clamping part, the selection of processing technology and electrical parameters, and the factors of machine tool itself. it is necessary to carefully observe and analyse in the usual work, find out the existing reasons, constantly use new technological methods to practice, improve the production process and mode of production, and give full play to the advantages of wire EDM moulds.