1. Contractility
Shrinkage is one of the main characteristics of cast aluminum alloys. Generally speaking, the alloy is divided into three stages from liquid pouring to solidification and cooling to room temperature, namely liquid shrinkage, solidification shrinkage and solid state 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 the size of the casting. Usually casting shrinkage is divided into volume shrinkage and linear shrinkage. In actual production, linear shrinkage is generally used to measure the shrinkage of the alloy.
The shrinkage of aluminum alloy is usually expressed as a percentage, which is called the shrinkage rate.
(1) Body contraction Body contraction includes liquid contraction and solidification contraction.
From pouring to solidification, macroscopic or microscopic shrinkage occurs in the final solidification of the casting alloy liquid. This kind of macroscopic shrinkage caused by shrinkage is visible to the naked eye, and is divided into concentrated shrinkage and dispersive shrinkage. The pores of the concentrated shrinkage holes are large and concentrated, and are distributed on the top of the casting or at the hot section with a thick section. The dispersive shrinkage cavities are scattered and fine, most of which are distributed in the casting axis and hot joints. The microscopic shrinkage cavities are difficult to see with the naked eye, and most of the microscopic shrinkage cavities are distributed under the grain boundary or between the dendrites of the dendrites.
Shrinkage cavity and porosity are one of the main defects of castings. The reason is that the liquid shrinkage is greater than the solid shrinkage. It is found in production that the smaller the solidification range of cast aluminum alloys, the easier it is to form concentrated shrinkage cavities, and the wider the solidification range, the easier it is to form dispersive shrinkage cavities. Therefore, the casting aluminum alloy must conform to the principle of sequential solidification in the design, that is, the casting is in The body shrinkage from liquid to solidification should be supplemented by the alloy liquid, and shrinkage cavities and looseness are concentrated in the outer riser of the casting. For aluminum alloy castings that are prone to looseness, the number of risers is more than that of concentrated shrinkage holes, and chilled iron is set at the places that are prone to looseness, and the local cooling rate is increased to make it solidify at the same time or quickly.
(2) The shrinkage of the line shrinkage will directly affect the quality of the casting. The greater the linear shrinkage, the greater the tendency of aluminum castings to produce cracks and stress; the greater the size and shape of the castings will change after cooling.
Different casting aluminum alloys have different casting shrinkage rates. Even if the same alloy has different castings, the shrinkage rates are different. On the same casting, the shrinkage rates of length, width, and height are also different. It should be determined according to the specific situation.
2. Casting stress
Casting stress includes thermal stress, phase transformation stress and shrinkage stress. The causes of various stresses are not the same.
(1) Thermal stress Thermal stress is caused by uneven thickness and inconsistent cooling at the intersection of different geometric shapes of castings. Compressive stress is formed at the thin wall, resulting in residual stress in the casting.
(2) Phase transformation stress The phase transformation stress is due to the phase transformation of some cast aluminum alloys during the cooling process after solidification, which brings about the volume and size changes. It is mainly caused by uneven wall thickness of aluminum castings and phase changes in different parts at different times.
(3) Shrinkage stress Aluminum castings are hindered by the mold and core when they shrink, resulting in tensile stress. This kind of stress is temporary, and aluminum castings will disappear automatically when they are out of the box. However, improper unpacking time often causes hot cracks, especially for metal cast aluminum alloys that are prone to hot cracks under such stress.
The residual stress in the cast aluminum alloy 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. The alloy has good thermal conductivity and no phase change during the cooling process. As long as the casting structure is designed reasonably, the residual stress of the aluminum casting is generally small.
3. Inhalation
Aluminum alloy is easy to absorb gas, which is the main characteristic of cast aluminum alloy. The hydrogen produced by the reaction between the components of liquid aluminum and aluminum alloys and the moisture contained in the furnace charge, organic matter combustion products, and molds 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 in 100g of aluminum is 0.5 to 0.9, and when the temperature rises to 850°C, the solubility of hydrogen increases by 2 to 3 times. When alkali metal impurities are contained, the solubility of hydrogen in molten aluminum is significantly increased.
In addition to the inhalation of cast aluminum alloy during smelting, it will also produce inhalation when pouring into the mold. The liquid metal entering the mold decreases with the temperature, the solubility of the gas decreases, and excess gas is precipitated, and there is a part of the gas that cannot be escaped. It is left in the casting to form pores, which are usually called "pinholes". The gas sometimes combines with the shrinkage cavity, and the gas precipitated in the molten aluminum stays in the shrinkage cavity. If the pressure generated by the heating of the bubbles is large, the surface of the pores is smooth and there is a bright layer around the holes; if the pressure generated by the bubbles is small, the inner surface of the pores is wrinkled, which looks like "fly feet", and there are shrinkage holes on closer inspection. Characteristics.
The higher the hydrogen content in the cast aluminum alloy liquid, the more pinholes are produced in the casting. Pinholes in aluminum castings not only reduce the air tightness and corrosion resistance of the castings, but also reduce the mechanical properties of the alloy. To obtain aluminum castings with no or less pores, the key lies in the melting conditions. If a covering agent is added for protection during smelting, the amount of gas inhalation of the alloy is greatly reduced. Refining the molten aluminum can effectively control the hydrogen content in the molten aluminum.
4. Liquidity
Fluidity refers to the ability of alloy liquid to fill the mold. The fluidity determines whether the alloy can cast complex castings. Eutectic alloys have better fluidity among aluminum alloys.
There are many factors that affect fluidity, mainly the composition, temperature, and solid particles of metal oxides, metal compounds and other pollutants in the alloy liquid, but the fundamental external factors are the pouring temperature and pouring pressure (commonly known as pouring head) The heights.
In actual production, when the alloy has been determined, in addition to strengthening the smelting process (refining and slag removal), it is also necessary to improve the mold processability (sand mold air permeability, metal mold exhaust and temperature), and do not affect Under the premise of casting quality, increase the pouring temperature to ensure the fluidity of the alloy.
5. Hot cracking
The occurrence of hot cracks in aluminum castings is mainly due to the shrinkage stress of the castings exceeding the bonding force between the metal grains. Most of them occur along the grain boundaries. It can be seen from the crack fracture that the metal at the cracks is often oxidized and loses its metallic luster. The cracks extend along the grain boundary, with a zigzag shape, a wide surface and a narrow inside, and some penetrate the entire end surface of the casting.
Different aluminum alloy castings have different tendency to crack. This is because the greater the difference between the temperature at which a complete crystalline framework is formed during the solidification of the cast aluminum alloy and the solidification temperature, the greater the shrinkage rate of the alloy and the greater the tendency to produce hot cracks. , Even the same alloy has different hot cracking tendency due to the resistance of the mold, the structure of the casting, the pouring process and other factors. Measures such as regressive casting molds or improved casting aluminum alloy casting systems are often used in production to avoid cracks in aluminum castings. The hot crack ring method is usually used to detect hot cracks in aluminum castings.
6. Air tightness
The airtightness of cast aluminum alloy refers to the degree of non-leakage of cavity-type aluminum castings under the action of high-pressure gas or liquid. Airtightness actually characterizes the degree of compactness and purity of the internal structure of the casting.
The airtightness of cast aluminum alloy is related to the properties of the alloy. The smaller the solidification range of the alloy, the smaller the tendency for porosity. At the same time, the smaller the precipitation 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, lowering the casting temperature of the cast aluminum alloy, placing cold iron to accelerate the cooling rate, and solidifying and crystallization under pressure, etc., can all make the airtightness of aluminum castings. improve. The impregnation method can also be used to plug the leakage gap to improve the air tightness of the casting.The copyright belongs to the author www.jihoowheels.com .
