Aluminum alloy casting process performance
Aluminum alloy casting process performance is generally understood to be a combination of those properties that are most outstanding in casting, crystallization and cooling processes. Fluidity, shrinkage, air tightness, casting stress, and gettering. These properties of aluminum alloy depend on the composition of the alloy, but are also related to casting factors, alloy heating temperature, complexity of the mold, pouring riser system, gate shape, and the like.
Fluidity refers to the ability of an alloy liquid to fill a mold. The amount of fluidity determines whether the alloy can cast complex castings. The eutectic alloy has the best fluidity in aluminum alloys.
There are many factors affecting the fluidity, mainly the solid particles of the composition, temperature and metal oxides, metal compounds and other pollutants in the alloy liquid, but the external fundamental factors are the pouring temperature and the pouring pressure (commonly known as pouring indenter). High and low.
In actual production, in addition to the smelting process (refining and slag removal), the mold processability (sand mold permeability, metal mold venting and temperature) must be improved without affecting the alloy. Under the premise of casting quality, the pouring temperature is increased to ensure the fluidity of the alloy.
Shrinkage is one of the main features of cast aluminum alloys. Generally speaking, the alloy is poured from liquid to solidified until it is cooled to room temperature. It is divided into three stages, namely liquid shrinkage, solidification shrinkage and solid shrinkage. The shrinkage of the alloy has a decisive influence on the quality of the casting, which affects the size of the shrinkage of the casting, the generation of stress, the formation of cracks and the change in size. Usually the shrinkage of castings is divided into body shrinkage and line shrinkage. In actual production, line shrinkage is generally used to measure the shrinkage of the alloy.
The shrinkage of aluminum alloys, usually expressed as a percentage, is called shrinkage.
1 body contraction
Body shrinkage includes liquid shrinkage and solidification shrinkage.
The casting alloy liquid flows from casting to solidification, and macroscopic or microscopic shrinkage occurs at the final solidification. The macroscopic shrinkage holes caused by shrinkage are visible to the naked eye and are classified into concentrated shrinkage cavities and dispersive shrinkage cavities. The pores of the concentrated shrinkage holes are large and concentrated, and are distributed at the top of the casting or at a hot section with a large cross section. The dispersive shrinkage cavities are scattered and fine, and most of them are distributed in the axial center and hot section of the casting. Microscopic shrinkage holes are difficult to see with the naked eye, and most of the microscopic shrinkage holes are distributed under the grain boundaries or between the dendrites of the dendrites.
Shrinkage and porosity are one of the main drawbacks of castings, resulting in liquid shrinkage greater than solid shrinkage. It is found in production that the smaller the solidification range of the cast aluminum alloy, the easier it is to form concentrated shrinkage cavities, and the wider the solidification range, the easier it is to form dispersible shrinkage cavities. Therefore, in the design, the cast aluminum alloy must conform to the principle of sequential solidification, that is, the castings are in The body shrinkage during liquid to solidification should be supplemented by the alloy liquid, which is the shrinkage and looseness concentrated in the outer riser of the casting. For aluminum alloy castings which are prone to dispersion and looseness, the number of risers is larger than that of concentrated shrinkage cavities, and cold iron is provided at a loose place, and the local cooling rate is increased to make it solidify at the same time or rapidly.
The size of the wire shrinkage will directly affect the quality of the casting. The greater the line shrinkage, the greater the tendency of cracks and stresses in aluminum castings; the larger the size and shape of the casting after cooling.
Different casting alloys have different casting shrinkage rates. Even if the same alloy has different castings, the shrinkage rate is different. On the same casting, the shrinkage ratios of length, width and height are also different. It should be determined on a case-by-case basis.
3 .thermal cracking
The occurrence of hot cracks in aluminum castings is mainly due to the fact that the shrinkage stress of the casting exceeds the bonding force between the metal grains. Most of the metal along the grain boundary is observed. The metal is often oxidized and loses the metallic luster. The crack extends along the grain boundary and has a zigzag shape with a wide surface, a narrow interior, and some penetrate the entire end face of the casting.
Different aluminum alloy castings have different tendency to crack. This is because the greater the difference between the temperature and the solidification temperature at which the complete crystal frame begins to form during the solidification of the cast aluminum alloy, the greater the shrinkage of the alloy and the greater the tendency to generate hot cracks. Even if the same alloy has a tendency to generate hot cracks due to factors such as the resistance of the mold, the structure of the casting, and the casting process. In the production, the retracting mold is often used, or the casting system of the cast aluminum alloy is improved, so that the aluminum casting can avoid cracks. Hot cracking of aluminum castings is usually detected by a thermal cracking method.
4 .air tightness
The airtightness of the cast aluminum alloy refers to the degree of leakage of the cavity type aluminum casting under the action of high pressure gas or liquid, and the airtightness actually characterizes the degree of compactness and purity of the internal structure of the casting.
The airtightness of the cast aluminum alloy is related to the properties of the alloy. The smaller the solidification range of the alloy, the smaller the tendency to loosen, and the smaller the precipitated pores, the higher the airtightness of the alloy. The airtightness of the same cast aluminum alloy is also related to the casting process. For example, reducing the casting temperature of the cast aluminum alloy, placing the cold iron to accelerate the cooling rate, and solidifying the crystal under pressure can make the airtightness of the aluminum casting. improve. The impregnation method can also be used to block the leaking voids to improve the airtightness of the casting.
5. casting stress
The casting stress includes three kinds of thermal stress, phase transformation stress and shrinkage stress. The causes of various stresses vary.
1. thermal stress
The thermal stress is caused by the uneven thickness of the section where the different geometric shapes of the casting intersect, and the cooling is inconsistent. Compressive stress is formed at the thin wall, resulting in residual stress in the casting.
2 .phase transition stress
The phase transformation stress is due to the phase transition of some cast aluminum alloys during the cooling process after solidification, which leads to volumetric dimensional changes. Mainly due to the uneven wall thickness of aluminum castings, the phase changes of different parts in different time.
3 .shrinkage stress
When the aluminum casting shrinks, it is caused by the tensile stress caused by the mold and the core. This stress is temporary and the aluminum casting will automatically disappear when unpacked. However, if the unpacking time is improper, it will often cause hot cracks. In particular, metal-cast aluminum alloys tend to generate hot cracks under such stress.
The residual stress in the cast aluminum alloy parts reduces the mechanical properties of the alloy and affects the machining accuracy of the casting. The residual stress in the aluminum casting can be eliminated by annealing. The alloy has good thermal conductivity and no phase change during the cooling process. As long as the structural design of the casting is reasonable, the residual stress of the aluminum casting is generally small.
Aluminum alloy is easy to absorb gas and is the main characteristic of cast aluminum alloy. The hydrogen produced by the reaction of the components of the liquid aluminum and the aluminum alloy with the moisture contained in the charge, the organic combustion products and the mold is absorbed by the aluminum liquid.
The higher the temperature of the aluminum alloy melt, the more hydrogen is absorbed; at 700 ° C, the solubility of hydrogen per 100 g of aluminum is 0.5 to 0.9, and when the temperature is raised to 850 ° C, the solubility of hydrogen is increased by 2 to 3 times. When an alkali metal impurity is contained, the solubility of hydrogen in the aluminum liquid is remarkably increased.
In addition to inhalation during smelting, the cast aluminum alloy also generates suction when it is poured into the mold. The liquid metal entering the mold decreases with temperature, the solubility of the gas decreases, and excess gas is precipitated, and a part of the gas that escapes is released. The pores are left in the casting, which is commonly referred to 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 the bubble is very high, the surface of the pore is smooth, and there is a bright layer around the hole; if the pressure generated by the bubble is small, the inner surface of the hole is wrinkled, which looks like a "fly foot", and has a shrinkage hole after careful observation. Characteristics. The higher the hydrogen content in the cast aluminum alloy liquid, the more pinholes are produced in the casting. The pinhole in the aluminum casting not only reduces the airtightness and corrosion resistance of the casting, but also reduces the mechanical properties of the alloy. In order to obtain aluminum castings with no or less pores, the key is the melting conditions. If the cover agent is added during the smelting, the amount of suction of the alloy is greatly reduced. The refining treatment of the aluminum melt can effectively control the hydrogen content in the aluminum liquid.