The types of aluminum alloy castings are as follows:
Due to the different composition of aluminum alloy, the physical and chemical properties of the alloy are different, and the crystallization process is also different. Therefore, it is necessary to choose a reasonable casting method according to the characteristics of aluminum alloy, prevent or reduce the occurrence of casting defects within the allowable range, and optimize casting.
1, aluminum alloy casting process performance
The technological properties of aluminum alloy castings are usually understood as the most outstanding combination of those properties during mold filling, crystallization and cooling. Fluidity, shrinkage, air tightness, casting stress, air suction. These characteristics of aluminum alloy depend on the composition of the alloy, but they are also related to casting factors, alloy heating temperature, mold complexity, gating and riser system, gate shape and so on.
(1) liquidity
Fluidity refers to the ability of alloy liquid to fill the mold. Fluidity determines whether the alloy can cast complex castings. Among aluminum alloys, * * * crystal alloy has the best fluidity.
There are many factors that affect the fluidity, mainly the composition, temperature and solid particles of pollutants such as metal oxides and metal compounds in the alloy liquid, but the external fundamental factors are pouring temperature and pouring pressure (commonly known as pouring head).
(2) contraction
Shrinkage is one of the main characteristics of cast aluminum alloy. Generally speaking, from liquid casting to solidification cooling to room temperature, the alloy can be divided into three stages, namely liquid shrinkage, solidification shrinkage and solid shrinkage. The shrinkage of alloy has a decisive influence on the quality of castings, which affects the shrinkage cavity size, stress generation, crack formation and size change of castings. Usually, the shrinkage of castings is divided into volume shrinkage and linear shrinkage, and the linear shrinkage is generally used to measure the shrinkage of alloys in actual production.
The shrinkage of aluminum alloy, usually expressed in percentage, is called shrinkage.
① Body contraction
Volume shrinkage includes liquid shrinkage and solidification shrinkage.
From pouring to solidification, macro or micro shrinkage will occur at the final solidification point of casting alloy liquid. Macroscopic shrinkage caused by shrinkage can be seen by naked eyes and can be divided into concentrated shrinkage and dispersed shrinkage. The concentrated shrinkage cavity is large in diameter and concentrated, and distributed at the top of the casting or at the hot spot with large cross section. The dispersed shrinkage cavity is finely dispersed, and most of them are distributed in the casting shaft and hot spots. Micro-pores are hard to see with naked eyes, and most of them are distributed under grain boundaries or between dendrites.
Shrinkage is one of the main defects of castings, and its reason is that liquid shrinkage is greater than solid shrinkage. It is found that the smaller the solidification range of cast aluminum alloy, the easier it is to form concentrated shrinkage cavities, and the wider the solidification range, the easier it is to form dispersed shrinkage cavities. Therefore, in the design, the cast aluminum alloy should conform to the principle of sequential solidification, that is, the volume shrinkage of the casting from liquid to solidification should be supplemented by the alloy liquid, so that the shrinkage cavity and porosity are concentrated in the external riser of the casting. For aluminum alloy castings that are easy to loosen and disperse, the number of risers is more than the number of concentrated shrinkage cavities, and cold iron is set at the parts that are easy to loosen, so as to increase the local cooling rate and make them solidify simultaneously or quickly.
② Linear contraction
Linear shrinkage will directly affect the quality of castings. The greater the linear shrinkage, the greater the trend of cracks and stresses in aluminum castings. After cooling, the size and shape of the casting changed greatly.
Different cast aluminum alloys have different casting shrinkage. Even if the shrinkage of the same alloy and different castings is different, the shrinkage of the same casting is different in length, width and height. It should be decided according to the specific situation.
(3) thermal cracking
Hot cracks in aluminum castings are mainly caused by the shrinkage stress of castings exceeding the bonding force between metal grains, and most of them occur along grain boundaries. It can be seen from the observation of crack fracture that the metal at the crack is often oxidized and loses its metallic luster. The cracks extend along the grain boundary in zigzag shape, with wide surface and narrow inside, and some of them run through the whole casting end face.
Different aluminum alloy castings have different tendencies to produce cracks, because the greater the difference between the temperature at which a complete crystal frame begins to form and the solidification temperature, the greater the shrinkage of the alloy and the greater the tendency to produce hot cracks. Even the same alloy has different tendencies to produce hot cracks due to factors such as mold resistance, casting structure and pouring process. In production, measures such as letting die or improving gating system of aluminum alloy castings are often adopted to avoid cracks in aluminum castings. Hot cracking ring method is usually used to detect hot cracks in aluminum castings.
(4) Air tightness
The air tightness of cast aluminum alloy refers to the degree of non-leakage of hollow aluminum castings under the action of high-pressure gas or liquid, and the air tightness actually represents the compactness and purity of the internal structure of the castings.
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 porosity tendency and the smaller the precipitation porosity, and the higher the air tightness of the alloy. Similarly, the airtightness of cast aluminum alloy is also related to the casting process, such as reducing the pouring temperature of cast aluminum alloy, placing chilled iron to speed up the cooling rate, and solidifying and crystallizing under pressure, which can improve the airtightness of aluminum castings. Infiltration can also be used to plug the leakage gap to improve the airtightness of castings.
(5) Casting stress
Casting stress includes thermal stress, phase transformation stress and shrinkage stress. The causes of various pressures are different.
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, which leads to residual stress in the casting.
② Phase transformation stress
Phase transformation stress is due to the phase transformation of some cast aluminum alloys during cooling after solidification, which brings about the change of volume size. It is mainly caused by uneven wall thickness of aluminum castings and phase transformation in different parts at different times.
③ Shrinkage stress
When aluminum castings shrink, they will be hindered by molds and cores, resulting in tensile stress. This stress is temporary, and the aluminum casting will disappear automatically after unpacking. However, improper unpacking time often leads to hot cracks, especially for aluminum alloys cast in permanent mold, which are prone to hot cracks under this stress.
Residual stress in cast aluminum alloy will reduce the mechanical properties of the alloy and affect the machining accuracy of the casting. Residual stress in aluminum castings can be eliminated by annealing. Because of its good thermal conductivity, the alloy has no phase change during cooling. As long as the structural design of castings is reasonable, the residual stress of aluminum castings is generally small.
(6) Inspiratory characteristics
Aluminum alloy absorbs gas easily, which is the main feature of cast aluminum alloy. Hydrogen produced by the reaction of liquid aluminum and aluminum alloy components with furnace charge, organic combustion products and moisture contained in the mold is absorbed by aluminum liquid.
The higher the melt temperature of aluminum alloy, the more hydrogen it absorbs. At 700℃, the solubility of hydrogen in 100g aluminum is 0.5 ~ 0.9. 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 liquid increases significantly.
Casting aluminum alloy not only sucks air when it is melted, but also sucks air when it is poured into the mold. As the temperature of liquid metal entering the mold decreases, the solubility of gas decreases, and excess gas is precipitated, and some inevitable gas remains in the casting to form pores, commonly known as "pinholes". The gas sometimes combines with the shrinkage cavity, and the gas precipitated from the aluminum liquid stays in the shrinkage cavity. If the pressure generated by bubble heating is large, the pore surface is smooth and there is a circle of bright layer around the hole; If the pressure generated by bubbles is small, the inner surface of the hole is wrinkled, which looks like a "fly's foot". Careful observation shows that it has the characteristics of contraction.
The higher the hydrogen content in molten aluminum alloy, the more pinholes will be 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 alloys. Melting conditions are the key to obtain aluminum castings without pores or with few pores. If covering agent is added to protect the alloy during smelting, the gas absorption of the alloy will be greatly reduced. Refining aluminum melt can effectively control the hydrogen content in aluminum melt.
Second, sand casting
The casting method of making molds with sand, clay and other auxiliary materials is called sand casting. The materials of sand mold are collectively called modeling materials. Sand molds for nonferrous metals are made of sand, clay or other binders and water.
The forming process of aluminum castings is the interaction between metal and mold. After the aluminum alloy liquid is injected into the mold, the heat is transferred to the mold, and the sand casting mold is subjected to the thermal, mechanical and chemical actions of the liquid metal. Therefore, in order to obtain high-quality castings, besides strictly mastering the melting process, we must also correctly design the proportion, modeling and pouring process of mold (core) sand.
Third, metal mold casting
1, introduction and process
Metal mold casting, also known as hard mold casting or permanent mold casting, is a method of pouring molten aluminum alloy into metal mold to obtain castings. Metal core, sand core or shell core are mostly used in aluminum alloy permanent mold casting. Compared with die casting, the service life of aluminum alloy metal mold is longer.
2, casting advantages
(1) Advantages
The cooling rate of metal mold is faster and the structure of casting is denser, which can be strengthened by heat treatment. The mechanical properties are improved by about 65438 05% compared with sand casting.
The quality of permanent mold casting is stable, the surface roughness is better than that of sand mold casting, and the rejection rate is low.
Good working conditions, high productivity, easy for workers to master.
(2) Disadvantages
Metal mold has high thermal conductivity and poor filling ability.
The metal mold itself has no air permeability. Corresponding measures must be taken to exhaust effectively.
The metal mold does not give in, and it is easy to crack and deform during solidification.
3. Common defects of metal mold castings and their prevention
(1) pinhole
Measures to prevent pinholes:
It is forbidden to use contaminated cast aluminum alloy materials, materials contaminated with organic compounds and materials severely oxidized and corroded.
Control the smelting process and strengthen degassing and refining.
Control the thickness of metal mold coating. If it is too thick, pinholes will easily appear.
Mold temperature should not be too high. The thick-walled parts of castings should be chilled by embedding copper blocks or watering.
When using sand mold, strictly control moisture and try to use dry core.
(2) Stomata
Measures to prevent blowholes:
Modify the unreasonable pouring and riser system to make the liquid flow stable and avoid gas involvement.
Mold and core should be preheated in advance, then coated, and must be thoroughly baked before use.
When designing dies and cores, adequate exhaust measures should be considered.
(3) oxidation slag inclusion
Measures to prevent oxidation slag inclusion;
Strictly control the smelting process, quickly smelt, reduce oxidation and thoroughly remove slag. Al-Mg alloy must be melted under the action of covering agent.
Melting furnaces and tools should be clean, oxide-free, preheated and dried after coating.
The designed gating system must have the ability of steady flow, buffering and skimming.
Inclined pouring system is adopted to make the liquid flow stable without secondary oxidation.
The selected coating has strong adhesion, does not fall off during pouring, and does not enter the casting to form slag inclusion.
(4) thermal cracking
Measures to prevent hot cracking:
In the actual pouring system, local overheating should be avoided to reduce internal stress.
The inclination of the mold and core must be greater than 2 degrees. Once the pouring riser is solidified, the core can be pulled out and the mold can be opened. If necessary, a sand core can be used instead of a metal core.
Control the coating thickness to make the cooling rate of all parts of the casting consistent.
Select the appropriate mold temperature according to the casting thickness.
Refine the alloy structure and improve the hot cracking ability.
Improve the casting structure, eliminate sharp corners and sudden changes in wall thickness, and reduce the tendency of hot cracking.
(5) Loose
Measures to prevent loosening:
Reasonable riser arrangement ensures its solidification and feeding ability.
Reduce the working temperature of metal mold appropriately.
Control the coating thickness and make it thinner at the thick wall.
Adjust the cooling speed of each part of the metal mold to make the thick wall of the casting have greater chilling ability.
Appropriately reduce the metal pouring temperature.