Pressure casting, as a special forming technology, has been widely used in many industries and fields, especially for the production of automobiles, motorcycles, internal combustion engines, electronics, instrumentation and aerospace industries, has become an integral part.
1. Aluminum alloy die casting technology overview
Aluminum is the most widely distributed crust, the largest reserves of metal elements, and aluminum is a reusable resources, is conducive to environmental protection. Pure aluminum is silver-white, its low melting point, conductive, good thermal conductivity, corrosion resistance. Aluminum alloy low density, small specific gravity, good strength, good thermal conductivity, good corrosion resistance, low price and easy to shape, suitable for processing all kinds of profiles, industrial use only after the steel, is the current amount of die-casting industry The largest class of nonmetallic structural materials. Aluminum alloy has a high melting point, the characteristics of light, high melting point means that it can be used as high temperature materials, is widely used in all walks of life, such as engines; quality advantages can be used in space equipment, China has been built Good on the moon car, the vast majority is built with high-strength aluminum alloy, there are many such examples, but also because of this, aluminum has become an automotive, aerospace and other industries irreplaceable metal materials.
2. Quality defects of aluminum alloy die-casting parts and improvement measures
porosity refers to the appearance of the die casting inside or the surface of the size of the hole, pin hole, a smooth surface, shape mostly round. porosity can lead to insufficient hardness of the die cast and affect the appearance of the surface.
2.1.1 porosity around the bolt hole for the die cast engine housing
Die-casting aluminum alloy box has a lot of bolt holes, oil holes and a variety of mounting holes, which directly affect the engine assembly quality and performance, in the die-casting process requires strict control of its quality.
Aluminum alloy engine housing die casting due to liquid metal filling cavity speed, mold cavity gas is not easy to discharge, easy to remain in the aluminum liquid, aluminum liquid cooling solidification after the residual gas in the casting to form a small bubble, Then
In the aluminum alloy die-casting production process, the temperature of aluminum pouring is generally around 660 ℃, but at this temperature aluminum liquid contains a lot of gas (the main hydrogen), hydrogen in the aluminum alloy solubility and temperature are closely related, at this temperature The gas content of about 0.69cm3 / 100g gas content is about 19-20 times the normal, so the aluminum alloy after solidification, these gases will lead to a large number of aluminum alloy castings have a lot of pores. In addition, due to the process caused by the volume of gas, release agent caused by the air hole can also account for a considerable proportion.
2.1.3 improvement measures
Mold exhaust channel design there is a certain structural problems or exhaust vent is not smooth, in the die-casting process will lead to mold cavity gas can not be completely ruled out. Casting system design also need to confirm whether the cross-sectional area is gradually reduced. Effective measures to ensure the quality of refining in the process of aluminum alloy smelting. Select the appropriate refining agent, the reaction of the bubble evenly generated, and then through the physical adsorption and aluminum impurities in the effective contact and bring to the surface. Adjust the process, the appropriate low-speed; to determine the release agent is too much spray. Vacuum die-casting may be considered for the above reasons.
In addition, the holes in the vicinity of the bolt holes having the processing steps are smaller than the length of the threads 1/3 and are not in the thread area, and the torque is not affected, and the use performance is not affected.
2.2.1 iron, manganese, chromium role
Aluminum alloy melting, often found by the heavy elements of solid compounds deposited in the hearth, this kind of sediment is generally called slag. Mainly by the aluminum, silicon and a large number of iron, manganese, chromium and other compounds at a certain temperature composition of the composition, into the casting to form inclusions. These grains have a high melting point, which is so large that they are deposited on the hearth. Slag deposition can produce harmful results, such as the formation of hard particles in the cast to increase the viscosity of the alloy, reducing the mobility of the alloy. In theory, when the iron content exceeds 0.8%, iron in the supersaturated aluminum and die steel in full contact with the case, the iron will not be dissolved. Therefore, die-casting aluminum alloy iron content is best in the 0.8-1.0% of the asked. Manganese and Chromium In die-cast aluminum alloys, manganese and chromium are often treated as impurities. In fact, after the combination of manganese and chromium or their combination, the structure of the phase containing more iron can be changed from acicular to cubic crystals. In this way, the toughness and strength of the die casting can be improved.
2.2.2 Oxidation of slag
Removal of heavy metal formation of slag, the other part of the main source of inclusions for the oxide, can be divided into primary oxide, secondary oxide. The primary oxide refers to the oxide which does not pass through the slag residue in the molten aluminum at the time of melting and enters the die casting directly. The secondary oxide refers to the oxide which enters the turbulence and is in contact with the air during the transportation and injection to Die Casting.
2.2.3 improvement measures
Strict control of the composition of aluminum ingots, especially heavy metal content can not be exceeded, in the incoming inspection must be strictly required. In addition, the melting furnace to be regularly cleaned up the bed, in the ladle transport of aluminum water, try to minimize the shock, one can avoid heavy metal oxide into the aluminum solution, the second is to avoid full contact with the air to form secondary oxide The Regular slag, generally every furnace to be transferred to the liquid aluminum slag treatment, continuous feeding smelting, the cycle can be adjusted according to the actual situation. In the ladle transport to be smooth, to avoid splashing. In the pouring to control the low speed, to avoid the formation of turbulence.
2.3 shrinkage hole
Shrinkage hole refers to the thickness of the die-casting section of irregular holes, the hole of the inner wall rough. Even can lead to the emergence of honeycomb within the die-casting organization, affecting the casting strength. Product processing hole exposed. The outer surface of the die-cast aluminum alloy product has a layer of very dense tissue, and the inside of the workpiece will have some small holes due to the loose phenomenon, and if the processing amount exceeds the thickness of the dense layer, the hole will be significantly increased.
In the die-casting process, the aluminum liquid is pressed into and filled with the cavity, and the molten aluminum starts to solidify. As the temperature of the mold surface is low and water is cooled, the molten aluminum starts to solidify from the surface in contact with the mold, Hard shell, and then gradually began to solidify inward. The aluminum liquid gradually shrinks as the temperature decreases, but the outer surface of the casting has formed a sealed hard shell, so that as the aluminum liquid gradually solidifies, the final solidified position will form some hollow space, that is, Shrinkage. Too large wall thickness caused by internal cooling solidification slow, liquid metal filled cavity, the shrinkage process can not be sufficient to add, prone to uneven thickness of the castings.
2.3.2 improvement measures
The method of eliminating the shrinkage can be achieved by reducing the wall thickness of the area where the shrinkage is located, enabling it to be quickly and uniformly coagulated, or by optimizing the casting and die structure. Sometimes due to the functional and structural reasons of a region, the wall thickness can not be thinned, you can consider increasing the casting processing prefabricated hole depth to change the processing area. From the process point of view, in the region of the mold to increase the cooling water, strengthen the cooling, speed up the solidification of aluminum liquid to reduce the volume of the shrinkage, and shrinkage control in the non-important areas; can also increase the pressure to enhance the organization Of the compactness. Shrinkage and stomata generally can not be 100% eliminated, can only be reduced or transferred, and sometimes even just change the processing of shrinkage problems.
The base of the aluminum alloy die casting is broken or broken, forming an elongated gap (length up to 50mm, linear or wavy lines and other irregular shape, in the external force under the extension of the trend, this defect is called crack.
Alloy composition abnormalities (such as magnesium content is too high), improve the sticky mold, in the top of the drawing die serious cracks; in the alloy composition under the premise of the same temperature, the higher the state will produce cracks, and the surrounding tissue There is a clear phenomenon of shrinkage. In the cooling and solidification, due to the different cooling order, the external area of ??the first contraction of the Department of the tensile stress generated in the loose parts of the crack caused.
2.4.2 Improvement measures
The correct control of alloy composition, in some cases can be added in the alloy aluminum ingots to reduce the amount of magnesium alloy in the alloy or aluminum alloy in the aluminum alloy to improve the silicon content; to alleviate the mold overheating, where the mold Increase the cooling waterway, reduce the mold temperature by the water temperature, keep the mold heat balance; change the aluminum alloy die casting structure, increase the fillet, change the mold inclination, reduce the mold difficulty, reduce the wall thickness difference; change or increase the top Out of the position, so that the top of the force evenly, to eliminate the local force is too large.
Along the open direction of the casting surface of the line-like traces of trauma, there is a certain depth, serious when the whole surface of the wound; metal and die surface adhesion, resulting in casting surface material shortage.
Mold cavity surface damage; the direction of the mold without slope or slope is too small; out of the imbalance; mold loose; pouring temperature is too high or too low, the mold temperature is too high lead to alloy liquid adhesion; release agent effect Bad; aluminum alloy composition of iron content of less than 0.8%; cooling time is too long or too short; die-casting machine parallelism is poor.
2.5.2 ways to improve
Repair the mold surface damage; modify the slope, improve the mold surface finish; adjust the mandrel, so that the top of the balance; fastening mold; control a reasonable pouring temperature and mold temperature 180-500℃; replacement mold release agent or anti- ; Adjust the iron content of aluminum; adjust the cooling time; modify the inner runner, change the direction of aluminum; adjust the parallel die-casting machine.
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