Application of wire electric discharge machining technology in mold processing (2)

2. Measures to prevent deformation and cracking

After finding the cause of deformation and cracking, the corresponding measures can be taken to avoid the deformation and cracking. Specific measures can be started from the following aspects:

2.1 Select the material with less deformation and adopt the correct heat treatment process

In order to prevent and reduce deformation and cracking, the molds that require wire cutting processing should be fully concerned and valued in all aspects of material selection, heat processing and heat treatment until the finished product is finished;

1) Strictly check the chemical composition, metallographic structure and flaw detection of raw materials. It is not suitable for unqualified raw materials and coarse-grained steel materials and materials with excessive levels of harmful impurities;

2) Try to use vacuum smelting, refining outside the furnace or electroslag remelting steel;

3) Avoid using materials with poor hardenability and easy deformation;

4) The blank should be reasonably forged, obeying the forging rules such as upsetting, lengthening, and forging ratio. The ratio of the length to the diameter of the raw material, that is, the forging ratio is preferably between 2-3;

5) Improve the heat treatment process, using vacuum heating, protective atmosphere heating and full deoxidation salt bath furnace heating and step quenching, austempering;

6) Select the ideal cooling rate and cooling medium;

7) The quenched steel should be tempered in time to eliminate the internal stress of quenching and reduce the brittleness;

8) tempering for a longer period of time to increase the fracture toughness value of the mold;

9) Fully tempered to obtain stable tissue performance;

10) multiple tempering to fully transform the retained austenite and eliminate new stresses;

11) For the second type of tempered brittle mold steel after high temperature tempering should be cold (water or oil cooling), can eliminate the second type of temper brittleness;

12) Before the chemical treatment of the mold steel, diffusion annealing, spheroidizing annealing, and quenching and tempering are performed to fully refine the original structure.

2.2 Reasonable arrangement of mechanical processing technology

1) The size of the wire-cut workpiece blank should be determined according to the size of the part, and should not be too small. In general, the pattern should be located in the middle of the blank or at a position far from the edge of the blank and not easily deformed. Usually, the pattern should be taken to the blank margin of more than 10 mm;

2) Where a large cavity or a narrow and complex punch is used, the traditional solid sheet habit is changed when preparing the blank. Large frame cavity, narrow and long cavity and other easily deformable parts, the middle part should be hollowed out. In this way, the condition in the surface is improved when quenching, the temperature difference is small, the generated stress is small, and the volume to be cut at the time of cutting is small, and the stress is balanced and not damaged;

3) In the case where the mold is allowed to use, the corner of the large frame-shaped cavity part should be appropriately increased, or the blanking point should be drilled before the line cutting to alleviate the phenomenon of stress concentration;

4) For the punched part, before the quenching, the threading hole of the starting point of the shape should be drilled in the bump blank, so that the workpiece maintains the internal stress balance and is not damaged during cutting, so as to avoid cracking deformation from the outside of the material.

2.3 Optimize the process plan of wire cutting and select reasonable process parameters

2.3.1 The cutting method

1) The traditional habit of changing the cutting position in one place is the coarse and fine secondary cutting, so that the deformation amount after the first rough cutting is corrected in time in the fine cutting. Generally, the cutting amount during fine cutting should be determined according to the amount of deformation after the first cutting, and generally takes about 0.5 mm. This method is often applied to parts with complex shapes that are bound to be deformed or molds that require high precision and small clearance;

2) The habit of changing the two-point clamping is single-point clamping, so that the deformation during the cutting process can be freely stretched, and the interference of the two-point clamping on the deformation is prevented, but it should be noted that the reasonable part of the single-point clamping is usually at the end program. Where. The deformation produced in this way only affects the scrap portion and avoids the influence on the molded portion;

3) For the easily deformed cutting parts, the starting point of the cutting, the program direction and the clamping position should be arranged according to the shape characteristics of the parts to reduce the deformation amount. Generally, the starting point of the line cutting should be set in a flat, finished or incompletely affected part of the workpiece.

2.3.2 Selecting reasonable process parameters

1) Using high peak and narrow pulse electrical parameters, the workpiece material is thrown out in the gas phase, and the gasification temperature is much higher than the melting temperature to take away most of the heat to avoid deformation of the surface of the workpiece;

2) Effectively perform pulse-by-pulse detection, control the length of the concentrated discharge pulse train, and also solve the local overheating problem and eliminate the generation of cracks;

3) The influence of pulse energy on the crack is extremely obvious. The larger the energy, the wider and deeper the crack; the pulse energy is very small, for example, the finishing electrical standard and the surface roughness value.

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