As a special forming technology, pressure casting has been widely used in many industries and fields, especially for large-scale production of automobiles, motorcycles, internal combustion engines, electronics, instrumentation and aerospace industries, which has become an integral part .
Overview of Aluminum Alloy Die Casting Technology
Aluminum is the most widely distributed and most abundant metal element in the earth’s crust, and aluminum is a recyclable resource, which is conducive to environmental protection. Pure aluminum is silver-white, with a low melting point, good electrical and thermal conductivity, and corrosion resistance. Aluminum alloy has low density, small specific gravity, high specific strength, good thermal conductivity, good corrosion resistance, low price and easy forming. It is suitable for machining various profiles. Its industrial usage is second only to steel.
It is currently used in the die casting industry. The largest category of non-ferrous metal structural materials. Aluminum alloy has the characteristics of high melting point and light weight. High melting point means that it can be used as a high temperature resistant material and is widely used in various industries, such as engines. The advantage of light weight can be used in aerospace equipment. Most of the completed moon landing vehicles are made of high-strength aluminum alloys. There are many examples of this. Because of this, aluminum machining have become irreplaceable metal materials in the automotive and aerospace industries.
The quality defects of aluminum alloy die-casting parts and improvement measures
Pores are holes and cavities of varying sizes that appear inside or on the surface of a die-casting, have smooth surfaces, and are mostly round in shape. The generation of pores will lead to insufficient hardness of the die casting and affect the appearance of the surface.
(1) Air holes around bolt holes in die-casting box
There are many bolt holes, oil holes, and various mounting holes on the die-cast aluminum alloy box, which directly affect the assembly quality and performance of the engine. The quality of the die-casting process needs to be strictly controlled.
When the aluminum alloy box is die-casting, because the liquid metal fills the cavity at a high speed, the gas in the mold cavity is not easy to be exhausted, and it is easy to remain in the aluminum liquid. The residual gas after the aluminum liquid is cooled and solidified forms small bubbles in the casting, that is, pore .
In the aluminum alloy die-casting production process, the temperature of the molten aluminum pouring is generally around 660 ° C, but at this temperature, the molten aluminum contains a large amount of gas (mainly hydrogen). The solubility of hydrogen in the aluminum alloy is closely related to the temperature. At this temperature, The lower gas content is about 0.69cm3 / 100g. The gas content is about 19-20 times that in the normal state. Therefore, after the aluminum alloy solidifies, these gases will be precipitated in large amounts, resulting in a large number of pores in the aluminum alloy casting. In addition, the blowholes caused by the process and the release of the release agent can also account for a considerable proportion.
(3) Improvement measures
There are certain structural problems in the design of the exhaust passage of the mold or the exhaust of the exhaust holes is not smooth. During the die casting process, the gas in the mold cavity cannot be completely eliminated. The casting system design also needs to confirm whether the cross-sectional area is gradually decreasing. Effective measures to ensure refining quality in the aluminum alloy melting process. Select an appropriate refining agent, bubbles are generated uniformly and continuously during the reaction, and then physical contact with impurities in the aluminum solution is effectively brought to the surface. Adjust the process to reduce the low speed appropriately; check if the release agent is sprayed too much. For these reasons, vacuum die casting can be considered.
In addition, the air holes around the bolt holes with machining steps are less than 1/3 of the thread length, and are not in the thread area. They have no effect on the torque and will not affect its use performance. The problem of air holes in this area can be eliminated.
(1) The role of iron, manganese and chromium
When aluminum alloys are smelted, it is often found that solid compounds composed of heavy elements are deposited on the hearth. This deposit is generally called slag. It is mainly composed of compound crystal grains containing aluminum, silicon and a large amount of iron, manganese, chromium, etc. at a certain temperature, and inclusions are formed when pressed into the casting. These crystal grains have a high melting point and a large specific gravity, so that they are deposited on the hearth. Slag deposits can have deleterious results, such as the formation of hard spots in the casting, increasing the alloy’s stickiness and reducing the alloy’s fluidity. Theoretically, when the iron content exceeds 0.8%, the iron will not dissolve in the case that the super-saturated molten aluminum and the mold steel are in full contact. Therefore, the iron content of the die-cast aluminum alloy is preferably between 0.81.0%. Manganese and chromium In die-cast aluminum alloys, manganese and chromium are often considered impurities. In fact, after the 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) Oxidation slag
After removing the slag formed by heavy metals, the main source of the other part of the inclusions is oxides, which can be divided into primary oxides and secondary oxides. Primary oxide refers to the oxide that remains in the aluminum liquid without slagging during smelting and directly enters the die casting; secondary oxide refers to the oxide that is generated by turbulence during transport and pouring into contact with the air. Die Casting.
(3) Improvement measures
Strictly control the composition of aluminum ingots, especially the content of heavy metals cannot exceed the standard, and must be strictly required during incoming inspection. In addition, the smelting furnace should be cleaned on a regular basis. When transferring aluminum water to the ladle, the vibration should be minimized. One is to prevent heavy metal oxides from entering the aluminum solution, and the other is to avoid contact with the air to form secondary oxides . Slagging is performed regularly. Generally, the slagging treatment is required for the aluminum liquid to be transferred in each furnace. During continuous feeding and smelting, the cycle can be adjusted according to the actual situation. When transferring the ladle, it should be stable to avoid splashing. Low speed should be controlled during pouring to avoid turbulence during propulsion.
Shrinkage refers to irregularly shaped holes at the thick section of the die casting, and the inner wall of the hole is rough. It can even cause a honeycomb structure to appear locally in the die casting, affecting the strength of the casting. Product machining face hole exposed. The outer surface of the die-cast aluminum alloy product has a very dense structure, and some small holes will appear inside the workpiece due to shrinkage. If the machining volume exceeds the thickness of the dense layer, the holes will increase significantly.
During the die casting process, the aluminum liquid begins to solidify after it is pressed into the cavity and filled with the cavity. Due to the low temperature of the mold surface and water cooling, the aluminum liquid starts to solidify from the surface in contact with the mold and forms a layer on the outermost surface The hard shell gradually solidifies inward. The molten aluminum gradually shrinks with decreasing temperature, but the outer surface of the casting has formed a sealed hard shell, so as the molten aluminum gradually solidifies, some hollow spaces will form in the final solidified position, that shrinkage cavity. Excessive wall thickness results in slow internal cooling and solidification. After the liquid metal fills the cavity, it cannot be replenished during the shrinkage process, which is likely to occur on uneven thickness castings.
(2) Improvement measures
The method of eliminating shrinkage can be achieved by reducing the wall thickness of the area where the shrinkage is located, so that it can solidify quickly and uniformly, or by optimizing the structure of the casting and mold. Sometimes due to the function and structure of a certain area, the wall thickness cannot be reduced. You can consider increasing the depth of the prefabricated hole in the casting to change the machining area. From a technological point of view, adding cooling water channels to the molds in this area to strengthen cooling and accelerate the solidification speed of the aluminum liquid to reduce the volume of shrinkage holes and control the shrinkage holes in non-critical areas; it can also increase the pressure to enhance the organization Of compactness. Shrinkage and pores cannot be eliminated 100% in general, but can only be reduced or transferred, and sometimes only the problem of shrinkage when machining is changed.
The matrix of the aluminum alloy die-casting is broken or broken to form an elongated gap (up to 50mm in length, with irregular shapes such as straight or wavy lines, which tends to extend under external forces. This defect is called crack.
Abnormal alloy composition (such as too high magnesium content), improved mold adhesion, severe cracking of the die during ejection; under the premise of unchanged alloy composition, cracks will also occur in a higher temperature state, and the surrounding structure There is obvious shrinkage. During cooling and solidification, due to the different cooling sequence, the outer area shrinks first to generate an outward tensile stress on the area, causing cracks in the loosened part.
(2) Improvement measures
Correctly control the alloy composition. In some cases, pure aluminum ingots can be added to the alloy to reduce the magnesium content in the alloy or aluminum-silicon alloys to increase the silicon content in the alloy. To alleviate mold overheating, Increase the cooling water path, reduce the mold temperature in the area through water cooling, and maintain the mold thermal balance; change the structure of aluminum alloy die castings, increase the fillet, change the mold tilt, reduce the difficulty of mold release, reduce the difference in wall thickness; change or increase The ejection position makes the ejection force uniform and eliminates excessive local stress.