Aluminium alloy casting process performance is generally understood as a combination of those properties that perform more prominently during casting, crystallization, and cooling. Fluidity, shrinkage, airtightness, casting stress, air-intake. These characteristics of aluminum alloys depend on the composition of the alloy, but they are also related to casting factors, alloy heating temperature, mold complexity, pouring riser system, gate shape, and so on. (1) Liquidity Fluidity refers to the ability of the alloy liquid to fill the mold. The degree of fluidity determines whether the alloy can cast complex castings. The eutectic alloy has good fluidity in the aluminum alloy. There are many factors that affect fluidity, mainly components, temperature, and solid-phase particles of metal oxides, metal compounds, and other contaminants in the alloy liquid, but the external basic factors are the pouring temperature and pouring pressure (commonly known as pouring pressure head). High and low. In actual production, in addition to the strengthening of the smelting process (refining and slag removal) when the alloy has been established, it is necessary to improve the processability of the mold (sand mold permeability, metal mold exhaust, and temperature) without affecting The pouring temperature is increased under the premise of casting quality to ensure the fluidity of the alloy. (2) Shrinkage Shrinkage is one of the main features of cast aluminum alloys. In general, the alloy is poured from liquid to solidified until it cools to room temperature. It is divided into three stages: liquid shrinkage, solidification shrinkage and solid shrinkage. The shrinkage of the alloy has a decisive influence on the quality of the casting. It affects the size of the shrinkage cavity, the generation of stress, the formation of cracks and the change of dimensions. Usually casting shrinkage is divided into body shrinkage and line shrinkage, in the actual production of the general use of the line shrinkage to measure the alloy's shrinkage. Aluminum alloy shrinkage, usually expressed as a percentage, is called shrinkage. 1 body contraction The body contraction includes liquid contraction and solidification contraction. Casting alloy liquid from casting to solidification, macroscopic or microscopic shrinkage occurs in the later solidified place, this macroscopic shrinkage hole caused by shrinkage is visible to the naked eye, and is divided into a central shrinkage cavity and a dispersive shrinkage cavity. The central shrinkage pores have large and concentrated pore diameters and are distributed at the top of the casting or at sections with large cross sections. Dispersion shrinkage pores are scattered and small in size, most of which are distributed on the axis and hot section of the casting. Microscopic shrinkage pores are difficult to see with the naked eye, and the microscopic shrinkage pores are mostly distributed under the grain boundary or between the dendrite dendrites. Shrinkage and porosity are one of the major defects of castings, which is caused by the fact that liquid shrinkage is greater than solid shrinkage. Found in production, the smaller the solidification range of the cast aluminum alloy, the easier to form a centralized shrinkage hole, the wider the solidification range, the easier to form a dispersion shrinkage hole, therefore, in the design must be made to meet the principle of sequential solidification of the casting aluminum alloy, ie The body shrinkage during liquid to solidification should be supplemented by the alloy liquid, which is shrinkage and loosening concentrated in the riser outside the casting. For easily dispersive aluminum alloy castings, the number of risers is more than the number of concentrated shrinkage holes, and cold iron is easily set in loose places to increase the local cooling rate, allowing it to be simultaneously or rapidly solidified. 2-wire contraction The shrinkage of the wire will directly affect the quality of the casting. The greater the shrinkage of the wire, the greater the tendency of aluminum castings to crack and stress; the greater the change in size and shape of the casting after cooling. For different casting aluminum alloys, there are different casting shrinkage rates, even if the same alloy, different castings, the shrinkage is different, in the same casting, its length, width, high shrinkage is also different. It should be based on specific circumstances. (3) Hot cracking The hot cracking of aluminum castings is mainly due to the fact that the shrinkage stress of the castings exceeds the binding force between the metal crystal grains. Most of the cracks appear along the grain boundary and cracks observed from the cracks show that the metal is often oxidized and loses its metallic luster. The cracks extend along the grain boundary, the shape is sawtooth-shaped, the surface is wider, the interior is narrower, and some penetrate the entire face of the casting. The tendency of different aluminum alloy castings to crack is also different, because the greater the difference between the solidification temperature and the temperature at which the entire crystalline framework begins to form during the solidification of the cast aluminum alloy, the greater the alloy shrinkage and the greater the tendency for hot cracking to occur. Even the same alloys have different propensities for hot cracking due to mold resistance, casting structure, casting process, and other factors. Concentric casting molds are often used in production, or cast aluminum alloy casting systems are improved to prevent aluminum castings from cracking. Thermal cracking is usually used to detect hot cracks in aluminum castings. (4) Hermeticity The hermeticity of cast aluminum alloy refers to the degree of non-leakage of cavity aluminum castings under the action of high pressure gas or liquid, and the airtightness actually indicates the degree of compactness and purity of the internal structure of the casting. The hermeticity of the cast aluminum alloy is related to the nature of the alloy. The smaller the solidification range of the alloy, the smaller the tendency to loosen, and the smaller the occurrence of precipitated pores, the higher the hermeticity of the alloy. The hermeticity of the same cast aluminum alloy is also related to the casting process, such as reducing the casting temperature of the cast aluminum alloy, placing the cold iron to accelerate the cooling rate, and solidifying and crystallizing under pressure, which can make the aluminum casting hermetic. improve. Impregnation can also be used to plug leaking voids to improve the hermeticity of the casting. (5) Casting stress Casting stress includes thermal stress, phase change stress, and shrinkage stress. The causes of various stresses are not the same. 1 thermal stress Thermal stress is caused by inconsistent cooling due to uneven thickness at the intersection of different geometry of the casting. The formation of compressive stress at the thin wall results in residual stress in the casting. 2 phase change stress The phase change stress is due to the phase change during cooling of some cast aluminum alloys after solidification, which in turn brings about a change in the volume dimension. The main reason is that the thickness of the aluminum castings is uneven, and different parts of the aluminum castings have phase changes at different times. 3 contraction stress When the aluminum casting shrinks, it is caused by tensile stress due to the inhibition of the mold and the core. This stress is temporary and aluminum castings will automatically disappear when they are unpacked. But improperly opening the box, it will often cause hot cracks. In particular, cast aluminum alloys tend to produce hot cracks under such stress. The residual stress in cast aluminum alloy parts reduces the mechanical properties of the alloy and affects the machining accuracy of the casting. Residual stress in aluminum castings can be eliminated by annealing. Since the alloy has good thermal conductivity and no phase change in the cooling process, the residual stress of the aluminum casting is generally small as long as the structure design of the casting is reasonable. (6) Inhalation Aluminum alloy absorbs gas easily and is the main characteristic of cast aluminum alloy. The composition of liquid aluminum and aluminum alloys reacts with the moisture contained in the charge of the charge, the combustion products of the organic matter, the mold, etc., and the hydrogen gas generated is absorbed by the aluminum liquid. The higher the melt temperature of the aluminum alloy, the more hydrogen is absorbed; at 700° C., the solubility of hydrogen per 100 g of aluminum is 0.5-0.9, and when the temperature is increased to 850° C., the solubility of hydrogen increases by 2 to 3 times. When alkali metal impurities are contained, the solubility of hydrogen in the aluminum liquid increases significantly. In addition to suction during smelting, cast aluminum alloys also generate inspiration when poured into the mold. The liquid metal that enters the mold decreases with temperature, the solubility of the gas decreases, excess gas evolves, and part of the gas escapes. Stay in the casting to form pores, which is commonly known as the "pinhole." The gas sometimes combines with the shrinkage cavity, and the gas evolved in the aluminum liquid remains in the shrinkage cavity. If the pressure generated by bubble heating is large, the surface of the pores will be smooth and there will be a bright layer around the pores; if the pressure generated by the bubbles is small, the pores inside the pores will look like “flying feetâ€. Characteristics. The higher the hydrogen content in the cast aluminum alloy solution, the more pinholes are produced in the casting. Pin holes in aluminum castings not only reduce the hermeticity and corrosion resistance of the castings, but also reduce the mechanical properties of the alloy. To obtain a non-porous or less porous aluminum casting, the key is the smelting conditions. If the covering agent is added during smelting, the amount of inhalation of the alloy is greatly reduced. The refining of aluminum melt can effectively control the amount of hydrogen in the aluminum liquid.
