Die casting is a method of casting die forging. Die casting die forging is a process completed on a special die casting die forging machine. Its basic technological process is to cast molten metal at low speed or high speed and fill it into the cavity of the mold. The die has a movable cavity surface. With the cooling process of molten metal, it is forged under pressure, which not only eliminates the shrinkage cavity and porosity defects of the blank, but also makes the internal structure of the blank reach the forging state. In addition, the comprehensive mechanical properties of the blank are significantly improved. The smoothness of the outer surface of the blank produced by this process reaches Grade 7 (Ra 1.6), which has the same metallic texture as the surface produced by cold extrusion or machining. Therefore, we call the die-casting die forging process "extreme forming process", which is a step further than "no cutting and less allowance forming process". Another advantage of die casting die forging process is that it can not only produce traditional casting materials, but also produce parts with complex structures. These alloy brands include: hard aluminum, superhard aluminum alloy, forged aluminum alloy, such as LY 1 1, LY 12, 606 1, 6063, LYC, LD, etc. The tensile strength of these materials is nearly twice that of ordinary cast alloys. For aluminum alloy automobile wheels,

I. Introduction to Die Casting Die casting, referred to as die casting, is a casting method in which molten alloy is poured into a pressure chamber at high speed to fill a steel mold cavity, and the alloy liquid solidifies under pressure to form a casting. The main characteristics of die casting are high pressure and high speed. ① Molten metal fills the cavity under pressure and crystallizes and solidifies under higher pressure. The common pressure is 15- 100 MPa. (2) High-speed mold filling of molten metal is generally 10-50 m/s, and some of them can exceed 80 m/s (the linear speed of introducing the mold cavity through the inner gate-the inner gate speed), so the mold filling time of molten metal is extremely short. The cavity can be filled in about 0.0 1-0.2 seconds (depending on the size of the casting). Die-casting machine, die-casting alloy and die-casting mold are the three elements of die-casting production, and they are indispensable. The so-called die casting process is to use these three elements organically to produce qualified castings with good appearance, good internal quality and dimensions meeting the requirements of drawings or agreements. Even the classification of high-quality castings. 1 and die casting machine (1) The heating conditions of the die casting machine can be divided into two categories: hot pressing chamber and cold pressing chamber. Cold chamber die casting machines can be divided into vertical, horizontal and full vertical die casting machines. Hot chamber die casting machine vertical cold chamber bedroom full vertical (2) Main parameters of die casting machine A- Forming force (clamping force) (kn)-KNB injection force (KN)-Maximum opening distance of KNC moving plate and fixed plate-Minimum opening distance between moving plate and sizing plate-stroke of ——mm e moving plate-spacing-MM F bar (horizontal× vertical) -mm g bar diameter-mm h jacking force-KNI jacking stroke-mm j injection position (center, eccentric)-mm k initial pouring metal amount (Zn, Al Cu)-kg l compression chamber inner diameter (Ф)-mm m empty cycle period-projection area of ——S N casting on parting surface under various specific pressure conditions Note: There should also be a movable plate. 2. Die-casting alloys The alloys used for die-casting are mainly non-ferrous alloys, as for ferrous metals (steel, iron, etc.). ), due to problems such as mold materials, it is rarely used at present, and non-ferrous alloy die castings mostly use aluminum alloy. Zinc alloy ranks second. Here is a brief introduction to die-casting nonferrous metals. (1), classification of die-casting nonferrous alloys: lead alloy with shrinkage inhibition, mixed shrinkage and free shrinkage -0.2-0.3%, 0.3-0.4%, 0.4-0.5%, low melting point alloy, tin alloy and zinc alloy -0.3-0.4%, 0.4-0.6%. Al-Mg series-0.5-0.7% 0.7-0.9% 0.9-1%Al-Zn series magnesium alloy-0.5-0.7% 0.7-0.9-1.65438+3mm Simple structure and complex structure.

Aluminum alloy aluminum-silicon series 610-650℃ 640-680℃ 600-620℃ 610-650℃

Al-Cu series 630-660℃ 660-700℃ 600-640℃ 630-660℃

Al-Mg series 640-680℃ 660-700℃ 640-670℃ 650-690℃

Al-Zn series 590-620℃ 620-660℃ 580-620℃ 600-650℃

Zinc alloy 420-440℃ 430-450℃ 400-420℃ 420-440℃

Magnesium alloy 640-680℃ 660-700℃ 640-670℃ 650-690℃

Copper alloy brass 910-930℃ 940-980℃ 900-930℃ 900-950℃

Silicon brass 900-920℃ 930-970℃ 910-940℃ 910-940℃

Note: ① The casting temperature is generally measured by the temperature of molten metal in the holding furnace. ② The casting temperature of zinc alloy should not exceed 450℃ to avoid coarse grains. Secondly, the die-casting die is one of the three major elements of die-casting production, and the correct and reasonable die structure is the premise of smooth die-casting production, which plays an important role in ensuring the quality of castings (off-machine qualification rate). Due to the characteristics of die casting process, the correct selection of process parameters is the decisive factor to obtain high-quality castings, and the mold is the premise to correctly select and adjust process parameters. Mold design is essentially a comprehensive reflection of various factors that may occur in die casting production. The mold design is reasonable, there are few problems in actual production and the qualified rate of castings is high. On the contrary, the mold design is unreasonable. For example, when designing castings, the wrapping force of the moving mold and the fixed mold is basically the same, while the gating system is mostly fixed mold. And put it on Guannan die casting machine, where the punch can't feed after injection, the castings can't be produced normally, and the castings always stick to the fixed mold. Although the smoothness of the cavity of the fixed die is very smooth, the phenomenon of sticking to the fixed die will still occur because of the deep cavity. Therefore, when designing the mold, it is necessary to comprehensively analyze the casting structure, be familiar with the operation process of the die casting machine, understand the possibility of adjusting the die casting machine and process parameters, and master the filling characteristics under different conditions. Only after considering the mold processing method, drilling and fixing form can we design a practical mold that meets the production requirements. As mentioned at the beginning, the filling time of molten metal is very short, and the specific pressure and flow rate of molten metal are very large, which is extremely unfavorable to the die-casting mold. Coupled with the influence of alternating cold and hot stress, it has a great influence on the service life of the die. The service life of molds usually refers to careful design and manufacture. Under normal use conditions, coupled with natural damage under good maintenance, the modulus of die casting (including the number of waste products in die casting production) is before it can be repaired and scrapped. In actual production, there are three main forms of mold failure: ① thermal fatigue cracking damage failure; ② Fracture failure; ③ Dissolution failed. There are many factors that lead to mold failure. There are external factors (such as casting temperature, whether the mold is preheated, the spraying amount of water-based paint, whether the tonnage of the die casting machine matches, the die casting pressure is too high, the speed of the inner gate is too fast, the cooling water is not started synchronously with the die casting production, the types and components of casting materials are Fe, the size and shape of the casting, the wall thickness of the casting, the types of coatings, etc.). ), as well as internal factors (such as metallurgical quality of mold materials, forging process of blank, rationality of mold structure design, rationality of gating system design, internal stress generated in the process of mold machining (electro-machining), and heat treatment process of mold, including various requirements for matching accuracy and flatness, etc. ). If the mold fails in the early stage, it is necessary to find out the internal or external reasons so as to improve it in the future. ① In the process of die casting production, the mold is repeatedly subjected to chilling and heating, and the mold surface and its interior are deformed, resulting in repeated thermal stress. It leads to the damage of secondary structure and the loss of toughness, which leads to the appearance and continuous expansion of microcracks. Once the crack propagates, molten metal is squeezed in, and repeated mechanical stress accelerates the crack propagation. Therefore, on the one hand, the die must be fully preheated at the beginning of die casting. In addition, in the process of die casting production, the mold must be kept within a certain working temperature range to avoid early cracking and failure. At the same time, it is necessary to ensure that there are no internal problems before the mold is put into production and during the manufacturing process. Because in actual production, most die failures are thermal fatigue cracking failures. ② Under the action of injection force, the mold will crack at the weakest place, especially when the scribing marks or electro-machining marks on the molding surface of the mold are not polished, or the corners of the mold are not cleaned up, tiny cracks will first appear. When there is brittle phase or coarse grain at the grain boundary, it is easy to fracture, and the crack expands rapidly when brittle fracture occurs, which is a very dangerous factor for the failure of die fracture. For this reason, on the one hand, all scratches and electro-machining marks on the mold surface must be polished, even if they are in the gating system. In addition, it is required that the mold materials used have high strength, good plasticity, good impact toughness and fracture toughness. ③ As mentioned above, the commonly used die casting alloys are zinc alloy, aluminum alloy, magnesium alloy and copper alloy. And pure aluminum die casting. Zinc, aluminum and magnesium are active metal elements and have good affinity with mold materials, especially aluminum is easy to bite the mold. When the die hardness is high, the corrosion resistance is good, but if there are soft spots on the molding surface, the corrosion resistance is unfavorable. However, in actual production, corrosion is only a part of the mold, and the parts directly washed by the inner gate (core and cavity) are easy to corrode. And soft hardness, aluminum alloys tend to stick to the mold. Problems often encountered in die casting production are: 1, examples of gating system and overflow system (1) Requirements for sprue of upper die of horizontal die casting machine with cold chamber: ① The inner diameter of the pressing chamber should be selected according to the required specific pressure and the fullness of the pressing chamber, and the deviation of the inner diameter of the sprue sleeve should be appropriately enlarged by several threads. Therefore, the problem that the punch is stuck or seriously worn due to the different inner diameters of the sprue sleeve and the pressure chamber can be avoided, and the wall thickness of the sprue sleeve cannot be too thin. The length of the sprue sleeve should usually be less than the conveying lead of the injection punch, so that the paint can escape from the pressure chamber. ② After heat treatment, the pressure chamber and the inner hole of the runner sleeve should be finely ground, and then ground along the axial direction, with the surface roughness less than or equal to Ra 0.2 μ m ③ The concave depth of the diverter plate and the paint forming cavity is equal to the runner depth, and its diameter matches the inner diameter of the runner sleeve, with a slope of 5 along the demoulding direction. When the gate with coating is used, the volume of the effective length of the pressurizing chamber can be shortened, and the fullness of the pressurizing chamber can be improved. (2) Requirements for the mold runner ① The runner inlet of the cold cross die should generally be located at more than 2/3 of the inner diameter of the upper part of the pressing chamber to avoid the molten metal in the pressing chamber from entering the runner prematurely under the action of gravity. Solidification begins early. ② The cross-sectional area of runner should be gradually reduced from the main runner to the inward gate. In order to enlarge the cross section, there will be negative pressure when the molten metal flows, which will easily absorb the gas on the parting surface and increase the vortex gas in the molten metal flow. Generally, the cross-sectional area at the outlet is smaller than that at the inlet 10-30%. ③ The runner should have a certain length and depth. The purpose of maintaining a certain length is to stabilize the flow and guide the flow. If the temperature of molten metal drops quickly, the depth is too deep and the condensation is too slow, it will not only affect the productivity, but also increase the consumption of burden. (4) The cross-sectional area of runner should be larger than that of inner gate to ensure the speed of molten metal entering the mold. The cross-sectional area of the main runner should be larger than that of each branch runner. ⑤ Both sides of the runner bottom should be rounded to avoid early cracks. The slopes on both sides can be about 5. Runner surface roughness is less than or equal to Ra 0.4 μm(3) Inner gate ① The parting surface should not be closed immediately after the molten metal enters the mold, and the overflow groove and exhaust groove should not directly impact the core. The flow direction of molten metal after entering the mold should be as long as possible along the casting ribs and fins, filling from thick wall to thin wall. (2) When selecting the position of the inner gate, the flow of molten metal should be as short as possible. When using multiple sprues, it is necessary to prevent several strands of molten metal from merging and colliding with each other after injection, resulting in defects such as vortex gas inclusions and oxidation inclusions. ③ The thick inner gate of thin-walled parts should be smaller to ensure the necessary filling speed, and the setting of the inner gate should be convenient for disassembly, so as not to make the casting body defective (eat meat). (4) Overflow trough ① The overflow trough should be easy to remove from the casting and try not to damage the casting body. (2) Pay attention to the overflow hole when an exhaust chute is arranged on the overflow chute. Avoid blocking the ventilation slot prematurely, so that the ventilation slot cannot work. ③ Don't open several overflow ports or a wide and thick overflow port in the same overflow tank, so as to avoid the coolant, slag, gas and coating in molten metal from the overflow tank flowing back to the cavity, resulting in casting defects. 2. The requirements such as unfilled fillet R2 are often indicated on the casting drawings of casting fillet (including corner), so don't ignore the function of these unfilled fillet when opening the mold. Never make rounded corners that are clear or too small. Casting fillet can make molten metal fill smoothly, make the gas in the cavity discharge orderly, reduce stress concentration and prolong the service life of the mold. Castings are not easy to crack there or produce various defects due to improper filling. For example, there are many corners on the standard oil pan mold, and the brother oil pan mold is the best at present. There are also many heavy oil trays. 3. The draft angle is forbidden to be artificially concave in the draft direction (it is often that the casting is stuck in the mold during the mold test, and when the treatment method is incorrect, it is partially concave by drilling and hard chiseling. ) .4. The surface roughness of molded parts and gating system shall be carefully polished as required. Grinding should be carried out in the direction of demoulding. Because the whole process of molten metal entering the gating system from the pressure chamber and filling the cavity takes only 0.0 1-0.2 seconds, in order to reduce the resistance of molten metal flow and minimize the pressure loss, it is necessary to have a high surface finish. At the same time, the heating and erosion conditions of the gating system are relatively poor. The worse the finish, the easier it is for the mold to be damaged. 5. Hardness of the mold forming part: aluminum alloy: about HRC46, copper: about HRC38. When processing, the mold should be trimmed as far as possible and the upper limit of size should be made. Avoid welding. Technical requirements for die casting die assembly: 1, parallelism between die parting surface and template plane. 2, the verticality of guide pillar and template. 3. The plane of the moving and fixed die inserts on the parting surface is 0. 1-0.05 mm higher than that of the moving and fixed die sets. 4. The push plate and the reset lever are flush with the parting surface. Generally, the push rod is recessed by 0. 1mm or according to user's requirements. 5. All the moving parts on the mold are reliable, and there is no hysteresis and pin movement. 6. The positioning of the slider is reliable, and the slider keeps a distance from the casting during core pulling. 7. After mold clamping, the matching position between the slider and the stopper exceeds 2/3. 8. The roughness of the runner is smooth and seamless. 8. When the mold is closed, part of the parting surface of the insert is clearance.