Aluminum alloy casting process performance is generally understood as a combination of those performances that are more prominent during the mold filling, crystallization, and cooling processes. Fluidity, shrinkage, air tightness, casting stress, getter. These characteristics of aluminum alloy depend on the composition of the alloy, but are also related to the casting factors, the heating temperature of the alloy, the complexity of the mold, the riser system, and the shape of the gate.
Flowability refers to the ability of the alloy liquid to fill the mold. The size of the fluidity determines whether the alloy can cast complex castings. Eutectic alloys have better fluidity in aluminum alloys.
There are many factors that affect the fluidity, mainly the composition, temperature and solid phase particles of metal oxides, metal compounds and other pollutants in the alloy liquid, but the external fundamental factors are pouring temperature and pouring pressure (commonly known as pouring head) The height.
In actual production, in addition to strengthening the smelting process (refining and slag removal), it is also necessary to improve the moldability (sand mold permeability, metal mold exhaust and temperature) without affecting the alloy. Under the premise of casting quality, increase the pouring temperature to ensure the fluidity of the alloy.
Shrinkage is one of the main characteristics of cast aluminum alloys. Generally speaking, the alloy is divided into three stages from liquid casting to solidification and cooling to room temperature, which are 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 cavity of the casting, the generation of stress, the formation of cracks and the change in size. Usually casting shrinkage is divided into body shrinkage and linear shrinkage. In actual production, linear shrinkage is generally used to measure the shrinkage of the alloy.
Aluminum alloy shrinkage, usually expressed as a percentage, is called shrinkage.
(1) Body contraction Body contraction includes liquid contraction and solidification contraction.
From casting to solidification, the casting alloy liquid will have macro or micro shrinkage in the later solidification area. This macro shrinkage hole caused by shrinkage is visible to the naked eye and is divided into concentrated shrinkage hole and dispersed shrinkage hole. The diameter of the concentrated shrinkage hole is large and concentrated, and is distributed on the top of the casting or the hot section with a thick section. The dispersive shrinkage pores are scattered and fine, most of them are distributed in the casting axis and hot joints. Micro shrinkage pores are difficult to see to the naked eye, and most of the micro shrinkage pores are distributed under the grain boundaries or between the dendrites of the dendrites.
Shrinkage and looseness are one of the main defects of castings. The reason for this is that liquid shrinkage is greater than solid shrinkage. It was 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 dispersive shrinkage cavities. Therefore, in the design, the cast aluminum alloy must conform to the principle of sequential solidification, that is, the casting is in The shrinkage of the body during the liquid state to solidification should be supplemented by the alloy liquid. Shrinkage and looseness are concentrated in the outer riser of the casting. For aluminum alloy castings that are prone to dispersion and loosening, the number of risers is more than that of concentrated shrinkage holes, and cold iron is set at the location where looseness is prone to increase the local cooling speed to make it solidify simultaneously or rapidly.
(2) Linear shrinkage Linear shrinkage will directly affect the quality of castings. The greater the linear shrinkage, the greater the tendency of aluminum castings to produce cracks and stress; the larger the size and shape changes of the casting 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 its length, width and height are also different. Should be based on specific circumstances.
3. Thermal cracking
The occurrence of thermal 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 are generated along the grain boundaries. From the crack fracture, it can be seen that the metal at the crack is often oxidized and loses its metallic luster. The cracks extend along the grain boundary, and the shape is zigzag, the surface is wider, the interior is narrower, and some penetrate the end face of the entire casting.
Different aluminum alloy castings have different tendency to crack. This is because the greater the difference between the temperature at which the complete crystalline frame begins to form during the solidification of the cast aluminum alloy and the solidification temperature, the greater the shrinkage of the alloy and the greater the tendency to generate thermal cracks. Even the same alloy has different tendency to generate hot cracks due to the resistance of the mold, the structure of the casting, the casting process and other factors. In production, measures such as concessional casting molds or improved casting systems for casting aluminum alloys are often used to avoid cracks in aluminum castings. The hot crack ring method is usually used to detect hot cracks in aluminum castings.
The airtightness of the cast aluminum alloy refers to the degree to which the cavity type aluminum casting does not leak under the action of high-pressure gas or liquid. 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 precipitating pores, the higher the airtightness of the alloy. The air tightness of the same type of cast aluminum alloy is also related to the casting process, such as reducing the casting aluminum alloy casting temperature, placing cold iron to speed up the cooling rate, and solidifying and crystallizing under pressure, etc., can make the airtightness of aluminum castings improve. Infiltration method can also be used to block the leakage gap to improve the airtightness of the casting.
5. Casting stress
The casting stress includes thermal stress, phase transformation stress and shrinkage stress. The causes of various stresses are different.
(1) Thermal stress Thermal stress is caused by uneven thickness at the intersection of different geometric shapes of the casting and inconsistent cooling. The 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 of some cast aluminum alloys during the cooling process after solidification, and the resulting change in volume and size. It is mainly caused by the uneven wall thickness of aluminum castings and the phase change of different parts at different times.
(3) Shrinkage stress Aluminum castings are caused by tensile stress due to the obstruction of the mold and core when shrinking. This stress is temporary, and the aluminum castings will disappear automatically when unpacked. However, improper unpacking time will often cause thermal cracking, especially aluminum alloys cast in metal molds are prone to thermal cracking 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 castings. The residual stress in aluminum castings can be eliminated by annealing. Due to the good thermal conductivity of the alloy, there is 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 alloys easily absorb gas and are the main characteristics of cast aluminum alloys. The hydrogen produced by the reaction of the components of the liquid aluminum and aluminum alloy with the moisture contained in the charge, the combustion products of organic matter, and the casting mold is absorbed by the aluminum liquid.
The higher the temperature of the aluminum alloy melt, the more hydrogen is absorbed; at 700 ℃, the solubility of hydrogen per 100g of aluminum is 0.5 to 0.9, when the temperature is increased to 850 ℃, the solubility of hydrogen increases by 2 to 3 times. When containing alkali metal impurities, the solubility of hydrogen in the aluminum liquid increases significantly.
In addition to gettering during melting, cast aluminum alloys also generate gettering when pouring into the mold. The liquid metal entering the mold decreases with temperature, the solubility of the gas decreases, excess gas is precipitated, and some of the gas cannot escape. Pore holes are formed in the casting, which is commonly referred to as “pinholes”. The gas is sometimes combined with the shrinkage cavity, and the gas precipitated 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 pore is wrinkled and looks like a “fly’s foot”. Characteristics.
The higher the hydrogen content in the aluminum alloy casting liquid, the more pinholes are produced in the casting. Pinholes in aluminum castings not only reduce the airtightness and corrosion resistance of castings, but also reduce the mechanical properties of the alloy. The key to obtaining aluminum castings with no or few pores is the melting conditions. If a covering agent is added for protection during smelting, the inspiratory volume of the alloy is greatly reduced. Refining the aluminum melt can effectively control the hydrogen content in the aluminum melt.