In die processing, WEDM has been widely used. However, in the online cutting process, the die is prone to deformation and cracks, resulting in scrapping of parts and increasing costs. Therefore, the problem of deformation and cracking of molds in wire-cutting machining is also attracting more and more attention. Over the years, people have not known enough about the deformation and cracking of wire-cutting machining, which often results in the wire-cutting processing department and the processing of incoming materials. Reciprocal reciprocity and contradictions. In fact, the causes of deformation and cracking are many, such as material problems, heat treatment problems, structural design problems, process scheduling problems, and the problem of workpiece clamping and cutting line selection during line cutting. In this many factors, can we find the law of line cutting deformation and cracking it? Through years of in-depth research, the author proposes the following measures to prevent deformation and cracking. Vmc Machining Center,VMC650 vertical machining center, VMC850 vertical machining center, VMC1060 vertical machining center Dongguan Liyang Intelligent Technology Co., Ltd , https://www.leyocncmachine.com
1 The main factors of deformation and cracks
In the production practice, the author has analyzed a large number of cases and found that the deformation and cracks produced by the wire cutting process are related to the following factors.
1.1 related to the structure of the part
1) Where a narrow and long shape of the concave and convex dies is easily deformed, the amount of deformation is related to the shape complexity, aspect ratio, and the width ratio between the cavity and the frame. The more complex the shape, the greater the ratio of length to width and the width of the cavity and the frame, the greater the amount of mold deformation. The law of deformation is intrusion into the middle of the cavity, and the punch is usually warped;
2) In the case of quenched cavities with complex shapes and clear corners, cracks are easily generated at the sharp corners and even prone to bursting. The frequency of its occurrence is related to the composition of the material and the heat treatment process;
3) Cylindrical wall thickness compared to the book parts, if the inner wall for cutting, easy to produce deformation, generally from round to oval. If it cuts the notch, it will be easy to burst when it is about to be cut.
4) The deep notch cut into the outside of the part is prone to deformation. The law of deformation is the adduction of the mouth. The amount of deformation is related to the depth of the notch and the nature of the material.
1.2 related to thermal processing
1) Parts of the mold blank when the forging temperature is too high or too low, and the final forging temperature is low;
2) The final forging temperature is too high, the grain grows up, the cooling rate after the final forging is too slow, and there are mold blanks in which the network carbide precipitates;
3) The billet annealing is not carried out according to the spheroidizing annealing process, and the spheroidized pearlite exceeds 5 grades;
4) The quenching heating temperature is too high, the austenite grains are coarse, and the toughness of the material is reduced and the brittleness is increased;
5) Quenched parts that have not been tempered or tempered in time.
1.3 Related to machining process
1) The large area of ​​the die, the middle of a large area removed without hollowing out in advance, due to cut off the larger size of the frame, the frame size will produce a certain degree of deformation;
2) Where there is no starting point threading hole in the billet, it must be cut in from the outside of the billet. Irrespective of the tempering and shape of the punch, deformation is usually easy to occur, especially the quenched part is seriously deformed, and even cracking occurs in the cutting;
3) For grinding parts after heat treatment, there are no grinding wheel granularity, feed amount, cooling method and other technical requirements. After the grinding, there are parts such as burns and micro cracks.
1.4 related to materials
1) Severe carbide segregation exists in raw materials;
2) Materials with poor hardenability and easily deformed, such as T10A, T8A, etc.
1.5 related to wire cutting process
1) Improper selection of wire cutting path, easy to produce distortion;
2) The clamping method of the workpiece is not reliable, and the selection of the clamping pressure point is not proper, and they are easily deformed.
3) Improper selection of electricity standards, easy to crack.
2 Measures to prevent deformation and cracking
Found the cause of deformation and cracking, you can take appropriate measures to avoid the disease, prevent deformation, cracking. Specific measures can be taken from the following aspects:
2.1 Select the material with smaller deformation and adopt the correct heat treatment process
In order to prevent and reduce deformation and cracking, we must pay full attention to and pay attention to the selection of tools, heat processing, and heat treatment until the finished products are finished.
1) Strictly check the chemical composition, metallographic structure, and flaw detection of raw materials. For unqualified raw materials and coarse-grained steel and excessively hazardous impurities, the steel should not be selected;
2) Use vacuum smelting, furnace refining or electroslag remelting steel as far as possible;
3) Avoid using poor hardenability and easily deformable materials;
4) The billets should be forged reasonably, obey the forging rules such as upsetting, lengthening, and forging ratios, and the ratio of the length of the raw materials to the diameter, ie, the forging ratio, is preferably between 2-3;
5) Improve the heat treatment process, using vacuum heating, protection atmosphere heating and full deoxygenation salt bath heating and graded quenching, austempering;
6) Select the ideal cooling rate and cooling medium;
7) Quenched steel should be tempered in time to eliminate quenching stress and reduce brittleness;
8) Temper for a long time to increase the fracture toughness value of the mold;
9) Fully tempered to obtain stable tissue performance;
10) Tempering many times to fully transform the retained austenite and eliminate new stress;
11) For the second type of temper brittle die steel after quenching at high temperature (water-cooled or oil-cooled), the second-class temper brittleness can be eliminated;
12) Diffusion annealing, spheroidizing annealing, and quenching and tempering are performed prior to the chemical treatment of the die steel to fully refine the original structure.
2.2 Reasonable Arrangement of Mechanical Processing Technology
1) The size of the wire-cut workpiece blank must be determined according to the size of the part, and should not be too small. Under normal circumstances, graphics should be located in the middle of the blank or away from the edge of the blank and not easily deformed position, usually should take the figure to the blank margin is greater than 10mm;
2) Where larger cavities or narrow, long and complex punches are used, the traditional solid sheet habits have to be changed when preparing billets. Large frame cavity, narrow long cavity and other deformable parts, the middle part should be hollow. In this way, when the quenched and tempered condition is improved, the temperature difference is small and the stress generated is small. At the same time, the volume to be cut off when cutting is small, and the stress is not balanced even if it is balanced;
3) In the case where the use of the mold is permitted, the corner R of the large frame-shaped cavity part should be properly increased, or the empty corner should be drilled before the online cutting to relieve the stress concentration phenomenon;
4) For punch parts, the threading holes at the starting point of the profile should be drilled in the billet blank before quenching so that the internal stress of the workpiece is kept balanced and not damaged during cutting, so as to avoid cracking deformation caused by cutting into the material.
2.3 Optimize the process plan for wire cutting and select reasonable process parameters
2.3.1 The cutting method
1) The traditional habit of changing the position of cutting once is coarse and fine secondary cutting so that the amount of deformation after the first rough cutting is corrected in time when fine cutting is performed. The cutting amount in general fine cutting should be determined according to the size of the deformation after the first cutting, generally about 0.5mm. This method is often applied to parts with complex shapes that are bound to be deformed, or molds that require higher precision and smaller clearances;
2) The habit of changing the clamping pressure at two points is a single point clamping pressure, so that the deformation during the cutting process can be freely stretched to prevent the interference of the two point clamping pressure on the deformation, but it should be noted that the reasonable position of the single point clamping pressure is usually at the end of the program. Department. The resulting deformation only affects the waste part and avoids the impact on the molded part;
3) For easy-to-deform cutting parts, the cutting start point, program direction, and clamping position shall be arranged according to the part shape characteristics to reduce the deformation. In general, the starting point of the wire cutting should be set at a flat, finished or hardly affected part.
2.3.2 Choosing a Reasonable Process Parameter
1) Using high peak and narrow pulse electrical parameters, the workpiece material is thrown out in the gas phase, the gasification temperature is much higher than the melting temperature to take away most of the heat, avoiding the workpiece surface overheating and causing deformation;
2) Effectively perform pulse-by-pulse detection, control the length of the concentrated discharge pulse train, and also solve local overheating problems and eliminate cracks.
3) The influence of pulse energy on the crack is extremely obvious. The larger the energy, the wider and deeper the crack is. When the pulse energy is small, for example, using precision machining specifications, the surface roughness value is <Ra1.25μm, and cracks are unlikely to occur.
Analysis and Countermeasures of the Causes of Deformation and Cracking in Die Cutting