Mold structure and planning parameters
Kneading casting mold structure
The mold structure for kneading and casting of aluminum alloy wheels is shown in Figure 3. It mainly has the required cavity composed of convex mold, right concave mold, ejector block and left concave mold. The left die and the right die are separately fixed on the left die set template and the right die set template. The left die set template is fastened with screws to the human template, and the right die set template is opened on the guide post through the side cylinder. And closed.
1. Upper template 2. Male die fixing plate 3. Male die 4. Guide post 5. Right female die 6. Right female die
7. Backing plate 8. Lower formwork 9. Ejector block 10. Left die 11. Left die set template
2000 kN hydraulic machine is used for kneading and casting. The operation process is as follows: after pouring a certain amount of alloy melt into the mold groove, the convex die fixed on the movable cross beam is squeezed down into the cavity at a certain speed and the pressure reaches a certain value. After the aluminum alloy is condensed, the pressure is released, the punch moves upwards through the return cylinder of the working cylinder, and the ejector block exits from the casting through the lower ejection cylinder until it completely disengages from the casting. casting.
2.2 Primary parameters of mold planning
(1) Clearance The clearance between the male die and the left and right female die should be appropriate. If it is too small, it will collide or bite due to the installation error of the convex mold and the concave mold; if it is too large, the alloy melt will be sprayed out through the gap, which will cause a problem; perhaps longitudinal burrs will occur in the gap to reduce the pressure and prevent unloading. material. Reasonable gaps are related to the moment of pressure start, the speed of pressure, the size of the pressure, the size of the workpiece and the metal material. According to the practical production experience, the unilateral gap is 0.1 mm.
(2) Demolding slope The alloy melt is condensed into a casting under the pressure of the punch, and after cooling, it is tightly wrapped on the punch and the ejector block. In order to facilitate the ejection of the punch and the ejector block, a ejection slope of 3 ° is provided on the punch and the ejector block. Because the casting has a round shape and is divided into two left and right female molds, it only needs to move the right female mold to the right by a certain interval, and the casting can be easily taken out from the left female mold, so there is no need to set the demolding slope.
(3) Exhaust After the left and right two molds are completely closed, the alloy melt is slowly poured into the cavity, and the gas in the cavity can be exhausted completely. When kneading and casting, a small amount of gas remaining in the guide portion of the punch is exhausted through the gap between the punch and the punch.
(4) Mold material Kneading and casting is carried out under a certain pressure and a certain temperature, and there is no erosion by the metal liquid like the die. The working pressure is higher than that during die-casting, and only the mold must have a certain compressive strength at high temperature. In addition, in order to avoid thermal fatigue cracks on the surface of the mold that is in contact with the alloy melt, the left and right concave molds, convex molds and ejector inserts are made of 3Cr2W8V alloy mold steel. The hardness after heat treatment is HRC48 ～ 52, and the cavity surface is softened. Nitriding treatment.
3 Process parameters of kneading casting
Kneading and casting is a combination of casting and forging. The production process is: melting of alloys, preparation of molds (finishing, preheating, spraying lubricants), pouring of metals, pressurization of liquid metal, persistence of pressure, pressure Removal and removal of castings.
In order to ensure the quality of castings, the process parameters [1 ~ 2] must be reasonably selected.
(1) The specific pressure and pressure size have a direct impact on the physical and mechanical functions of castings, casting defects, arrangements, segregation, melting point and phase equilibrium. Therefore, it is important to determine the necessary unit pressure for forming. If the specific pressure is too small, neither the casting surface nor the internal quality can reach the technical indicators; if the specific pressure is too large, the progress of the function is not very significant, and the mold is simply damaged, and equipment with large clamping force is required. The kneading casting experiment was performed on a 2 000 kN hydraulic press. Experiments have shown that the specific pressure suitable for kneading and casting of this aluminum alloy wheel should be selected within the range of 50-60 MPa.
(2) Pressing start time From the results of wheel kneading and casting experiments, the interval between press start is too long, and the strength and elongation of the casting are reduced. The current starting pressure is 3 ~ 5 s, which is more suitable.
(3) Pressing speed Kneading casting requires a certain pressing speed. In some cases, it is better to press faster. Fast press speed, the punch can quickly apply pressure to the metal, which is convenient for forming, crystallization and plastic deformation. However, it should not be too fast. Otherwise, the surface of some alloy melts will splash and vortex, which will cause casting defects, and excessive alloy melt flowing out in the space between the convex and concave molds, which will cause longitudinal burrs that are difficult to remove. . Therefore, it is necessary to slowly press the punch into the liquid metal. The operating speed of the hydraulic press used is slow, so the speed of the working stroke is used to limit it.
(4) Holding time The pressure holding time depends primarily on the thickness of the casting. Under the conditions of forming and crystal solidification, the holding time should be short. However, if the pressure holding time is too short, shrinkage will easily occur in the casting. If the pressure holding time is too long, the production cycle will be extended, deformation resistance will be added, and the service life of the mold will be reduced.
Considering the wall thickness of the wheel, the holding time for kneading and casting should be about 12 s.
(5) Mold preheating temperature If the mold is not preheated, the alloy melt will condense quickly after being injected into the cavity, resulting in too little time to pressurize. Together they are also not conducive to spraying lubricants. The pre-heating temperature of the kneading casting mold of the wheel is 200-300 ° C, and the heating is usually performed by using a fire oil torch.
(6) Alloy pouring temperature The pouring temperature is too high or too low, which has a significant effect on alloy forming. If it is too low, the alloy is easy to condense and the required unit pressure is large; if it is too high, shrinkage is liable to occur. It is necessary to point out that the casting temperature of kneading casting alloy is higher than that of sand casting. It is generally desirable to control the pouring temperature to a relatively low value, because it is desirable to eliminate porosity, shrinkage and porosity during kneading. When the pouring temperature is low, the gas easily escapes from the inside of the alloy melt, rarely stays in the metal, and it is easy to eliminate pores. In addition, it is possible to reduce the timing of the shrinkage hole formation, because the pouring temperature is lower, the metal overflow is less, and the burr can be reduced. The casting temperature for kneading and casting of this wheel is preferably 720 to 740 ° C.
(7) Lubricant The function of the lubricant is to maintain the mold, improve the surface quality of the casting and facilitate the removal of the casting from the mold. Select engine oil graphite lubricant, that is, 5% of 200 ~ 300 intent graphite powder is added to 95% of engine oil, and it can be mixed evenly. Spray coating on the surface of the mold cavity with a thickness of 0.05 to 0.1 mm. Excessive thickness will affect the surface quality of the casting.
(8) Cooling After kneading, the mold should be released immediately after the pressure is released, so the casting temperature of the casting is high. In order to avoid the occurrence of cracks at the junction of thin and thick walls during high-temperature casting cooling, the casting after casting should be immediately placed in a sand pile, and then air-cooled after being cooled below 150 ° C.
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