1. The contractility
Shrinkage is one of the main characteristics of cast aluminum alloy. Generally speaking, the alloy from liquid pouring to solidification, until cold to room temperature, is divided into three stages, respectively liquid shrinkage, solidification shrinkage and solid shrinkage. The shrinkage of the alloy has a decisive influence on the casting quality, which affects the size of shrinkage cavity, the generation of stress, the formation of crack and the change of size. Usually, casting shrinkage is divided into bulk shrinkage and linear shrinkage. In actual production, linear shrinkage is generally used to measure the shrinkage of alloy.
The shrinkage size, usually expressed as a percentage, is called shrinkage.
(1) body shrinkage body shrinkage includes liquid shrinkage and solidification shrinkage.
From pouring to solidification, there will be macroscopic or microscopic shrinkage in the final solidification place of casting alloy liquid. Such macroscopic shrinkage caused by shrinkage is visible to the naked eye, and can be divided into concentrated shrinkage and dispersed shrinkage. Concentrated shrinkage pores are large and concentrated, and are distributed on the top of the casting or at the hot joints with a thick section. The morphology of dispersed shrinkage pores is dispersed and fine, most of them are distributed in the center and hot spot of casting. Microshrinkage is hard to be seen by naked eye, and most of them are under grain boundary or between dendrites.
Shrinkage cavity and porosity are one of the main defects of castings. Found in production, casting aluminium alloy solidification range is smaller, the formation of concentrated shrinkage, solidification range is wide, the more easy dispersion shrinkage cavity formation, therefore, in the design must conform to the principle of directional solidification casting aluminum alloy, the casting liquid is to the body during the solidification shrinkage should be alloy liquid supplement, is loose focus on the outside of the castings and shrinkage cavity at the mouth. For aluminum alloy castings prone to dispersion and porosity, more risers are set than concentrated shrinkage holes, and chillers are set at the place prone to porosity to increase local cooling rate and make them solidify simultaneously or quickly.
(2) the shrinkage of the line will directly affect the quality of the casting. The larger the linear shrinkage, the greater the tendency of crack and stress. After cooling, the casting size and shape change more.
For different cast aluminum alloys, there are different shrinkage rates, even if the same alloy, casting different, shrinkage rates are different, in the same casting, the length, width, height of the shrinkage is also different. It depends on the circumstances.
2. Casting stress
Casting stress includes thermal stress, phase transition 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 change stress phase change stress is due to the phase change during the cooling process after solidification of some diecasted aluminum alloys, which brings about the change of volume size. This is mainly due to the uneven thickness of the aluminum casting wall and the phase transition in different parts at different times.
(3) shrinkage stress aluminum casting shrinkage by the mold, the core of the block caused by tensile stress. This stress is temporary, the aluminum casting will automatically disappear when the box is opened. However, if the time of unpacking is not appropriate, the hot crack is often caused, especially for the aluminum alloy in metal casting.
Residual stress in diecasted 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. Because the alloy has good thermal conductivity and no phase transition during the cooling process, the residual stress of aluminum castings is generally small as long as the casting structure design is reasonable.
3. Breathe in
Aluminum alloy absorbs gas easily, which is the main characteristic of cast aluminum alloy. The hydrogen produced by the reaction of liquid aluminum and aluminum alloy components with the moisture contained in charge charge, organic combustion products and casting mold is absorbed by the aluminum liquid.
The higher the melting temperature, the more hydrogen is absorbed. At 700℃, the solubility of hydrogen per 100g of aluminum is 0.5 ~ 0.9, and when the temperature rises to 850℃, the solubility of hydrogen increases by 2 ~ 3 times. When containing alkali metal impurities, the solubility of hydrogen in aluminum solution increases significantly.
In addition to the inhalation during smelting, the cast aluminum alloy will also produce inhalation when pouring into the casting mold. The liquid metal entering the casting mold will decrease with the temperature, and the solubility of the gas will decrease. The gas sometimes binds to the shrinkage cavity, and the gas precipitated from the liquid aluminum remains in the shrinkage cavity. If the pressure generated by heat is very great, the surface of the pores is smooth, and there is a ring of light layer around the pores. If the pressure produced by the bubble is small, the inner surface of the hole is wrinkled and looks like "fly feet".
The higher the hydrogen content, 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 alloys. The key to obtain aluminum castings without or without pores lies in the melting conditions. If the coating agent is added to protect the alloy during melting, the suction capacity of the alloy is greatly reduced. The content of hydrogen in aluminum melt can be controlled effectively by refining aluminum melt.