As a special forming technology, die casting has been widely used in many industries and fields, especially for large-scale production of automobiles, motorcycles, internal combustion engines, electronics, instruments and aerospace industries, and has become an indispensable part of it .

 

Aluminum is the most widely distributed metal element with the largest reserves in earth's crust, and aluminum is a reusable resource that is conducive to environmental protection. Pure aluminum is silvery white, with 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 processing various profiles. Its industrial use is second only to steel. It is currently the largest type of non-ferrous metal structural material in die-casting industry. Aluminum alloy has 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. Taking advantage of advantages of light weight, it can be used in aerospace equipment. Most of built lunar landers are made of high-strength aluminum alloys. There are many such examples. Because of this, aluminum alloys have become irreplaceable metal materials in industries such as automobiles and aerospace.

Pores refer to holes and cavities of different sizes that appear inside or on the surface of die castings, have smooth surfaces, and are mostly round in shape. Generation of pores will lead to insufficient hardness of die castings and affect appearance of surface.

There are many bolt holes, oil holes and various installation holes on die-casting aluminum alloy box, which directly affect assembly quality and performance of engine, and its quality needs to be strictly controlled during die-casting process.

Due to high speed of filling cavity with liquid metal during die-casting of aluminum alloy box, gas in mold cavity is not easy to discharge, and it is easy to remain in aluminum liquid. After aluminum liquid cools and solidifies, residual gas forms small bubbles in casting, that is, pores .
In aluminum alloy die-casting production process, pouring temperature of molten aluminum is generally around 660℃, but at this temperature molten aluminum contains a large amount of gas (mainly hydrogen), and solubility of hydrogen in aluminum alloys is closely related to temperature. At this temperature, gas content is about 0.69cm3/100g, and gas content is about 19-20 times that of normal conditions. Therefore, after aluminum alloy is solidified, these gases will be precipitated in large quantities, resulting in a large number of pores in aluminum alloy casting. In addition, gas entrainment caused by process and pores caused by gas release agent can also account for a considerable proportion.

There are certain structural problems in design of exhaust channel of mold or exhaust of exhaust hole is not smooth, which will cause gas in mold cavity to be unable to be completely exhausted during die-casting process. Design of casting system also needs to confirm whether cross-sectional area is gradually reduced. Select appropriate refining agent, and bubbles will be generated uniformly and continuously during reaction, then effectively contact with impurities in aluminum liquid through physical adsorption and bring them to the surface. Adjust process, reduce low speed appropriately; confirm whether release agent is sprayed too much. Based on above reasons, vacuum die casting can be considered.
In addition, air holes around bolt holes with processing procedures are less than 1/3 of thread length and are not in thread area, so they have no effect on torque and performance, so problem of air holes in this place can not be solved.

 

When aluminum alloy is smelted, it is often found that solid compounds composed of heavy elements are deposited on hearth. This deposit is generally called slag. It is mainly composed of compound grains containing aluminum, silicon and a large amount of iron, manganese, chromium, etc. at a certain temperature, and inclusions are formed when pressed into casting. These grains have a high melting point and a high specific gravity, so that they are deposited on hearth. Slag deposits can have detrimental consequences, such as formation of hard spots in casting, increased alloy stickiness, and decreased alloy fluidity. Theoretically, when amount of iron exceeds 0.8%, iron will not dissolve when iron-containing supersaturated aluminum liquid is in full contact with mold steel. Therefore, iron content of die-cast aluminum alloy is preferably between 0.810%. Manganese and Chromium in die-casting aluminum alloys, manganese and chromium are often regarded as impurities. In fact, manganese and chromium or their combination can change structure of phase containing more iron from acicular to cubic crystal. In this way, toughness and strength of die castings can be improved.

Slag formed by heavy metals is removed, and main source of another part of inclusions is oxides, which can be divided into primary oxides and secondary oxides. Primary oxide refers to oxide that remains in molten aluminum without slag during smelting, and directly enters die-casting; secondary oxide refers to formation of turbulent flow and formation of oxides in contact with air during transfer and pouring into die casting.

Strictly control composition of aluminum ingots, especially content of heavy metals must not exceed standard, and must be strictly required when inspecting incoming materials. In addition, hearth of smelting furnace should be cleaned regularly, and vibration should be minimized when ladle is transferring aluminum water. First, it can prevent heavy metal oxides from entering aluminum solution, and second, it can avoid sufficient contact with air to form secondary oxides. . Periodic slag removal. Generally, each furnace of molten aluminum to be transferred must be slag removed. During continuous feeding and smelting, cycle can be adjusted according to actual situation. Be steady when transferring ladle to avoid splashing. When pouring, control low speed to avoid turbulent flow when advancing.

Shrinkage cavity refers to irregularly shaped holes in thick section of die casting, and inner wall of the hole is rough. It can even lead to local appearance of honeycomb structure in die casting, which affects strength of casting. Product processing surface holes are exposed. Outer surface of die-casting aluminum alloy product has a very dense structure, and some small holes will appear inside workpiece due to shrinkage. If processing amount exceeds thickness of dense layer, holes will increase significantly.

 

During die-casting process, molten aluminum starts to solidify after it is pressed into and fills cavity. Due to low temperature on the surface of mold and accompanying water cooling, molten aluminum starts to solidify from surface in contact with mold, forms a hard shell on outermost surface, and then gradually solidifies inward. Molten aluminum gradually shrinks and becomes smaller as temperature decreases, but a sealed hard shell has formed on outer surface of casting, so as molten aluminum gradually solidifies, some hollow spaces, namely shrinkage cavities, will be formed at final solidified position. Excessive wall thickness causes slow internal cooling and solidification. After liquid metal fills cavity, it cannot be replenished in sufficient amount during shrinkage process, which is easy to occur on castings with uneven thickness.

Method of eliminating shrinkage cavity can be achieved by reducing wall thickness of area where shrinkage cavity is located, so that it can be solidified quickly and uniformly, and can also be achieved by optimizing structure of casting and mold. Sometimes due to function and structure of a certain area, wall thickness cannot be reduced, and depth of prefabricated hole in casting can be considered to change processing area. From a technological point of view, add cooling water channels to mold in this area to enhance cooling and accelerate solidification speed of aluminum liquid to reduce volume of shrinkage cavities and control shrinkage cavities in non-important areas; it is also possible to increase pressure to improve structure of compactness. Shrinkage cavities and pores generally cannot be eliminated 100%, they can only be reduced or transferred, and sometimes even problem of shrinkage cavities occurs only after changing processing.

Matrix of aluminum alloy die-casting parts is damaged or disconnected, forming slender gaps (up to 50mm in length, irregular shapes such as straight or wavy lines, which tend to extend under action of external force, this defect is called crack.

Alloy composition is abnormal (such as magnesium content is too high), which improves stickiness of die, and severe cracks appear in drawing die during ejection; under premise that alloy composition remains unchanged, cracks will also occur in the state of higher temperature, surrounding tissue has obvious shrinkage and porosity. During cooling and solidification, due to different cooling sequences, outer area first shrinks to generate outward tensile stress on the area, causing cracks in shrinkage porosity area.

Correctly control alloy composition. In some cases, pure aluminum ingots can be added to alloy to reduce magnesium content in alloy or an aluminum-silicon alloy can be added to aluminum alloy to increase silicon content; In order to alleviate overheating of mold, a cooling water channel is added in mold at this place, and temperature of mold in this area is reduced by water cooling to maintain thermal balance of mold; change structure of aluminum alloy die-casting parts, increase fillet, change draft angle, reduce difficulty of mold release, and reduce difference in wall thickness; change or increase ejection position to make ejection force uniform and eliminate excessive local force.

 

Line-like strain marks on the surface of casting along direction of mold opening, with a certain depth, and in severe cases, the entire surface is strained; molten metal sticks to surface of mold, resulting in a lack of material on the surface of casting.

There is damage on the surface of mold cavity; there is no slope or too small slope in ejection direction; ejection is unbalanced; mold is loose; pouring temperature is too high or too low, and mold temperature is too high to cause alloy liquid to adhere; effect of release agent is not good; iron content of aluminum alloy composition is less than 0.8%; cooling time is too long or too short; parallelism of die-casting machine is poor.

Repair surface damage of mold; correct slope and improve surface finish of mold; adjust ejector pin to balance ejection force; tighten mold; control reasonable pouring temperature and mold temperature 180-500℃; replace release agent or use anti-strain coating ; Adjust iron content of aluminum alloy; adjust cooling time; modify inner runner, change direction of aluminum liquid; adjust parallelism of die-casting machine.