Deformation processing of aluminum and aluminum alloys should ensure stable, consistent dimensional accuracy, mechanical properties and good surface quality. Care must also be taken to prevent mechanical damage and corrosion, to control grain size and texture, and these quality requirements are primarily guaranteed by production processes and equipment. Aluminum and its alloys generally have good plasticity and are easy to plastically process. The phase composition of hard aluminum is complex, and there are brittle structures such as low-melting phase and intermetallic compound. Its plastic processing has some characteristics: such as homogenization to eliminate internal stress and intragranular segregation generated when billet cooling; billet The surface is milled to remove surface segregation from the low melting phase. Some aluminum alloys are also coated with aluminum for improved corrosion resistance and processability. Aluminum alloys are sensitive to overheating and the heating temperature must be strictly controlled.
Smelting is the provision of billets for plastic processing. The smelting furnace mostly uses a gas reverberatory furnace or a fuel reverberatory furnace, and the general capacity is 20 to 40 tons or more; and the resistance heating reverberatory furnace is also used, and the capacity is generally about 10 tons. In order to shorten the furnace charging time, increase the melting efficiency, reduce the absorption of gas and entrap the oxide film, the industry has adopted a tilting top-loading round furnace. It is best to use a rapid analytical instrument to analyze the alloy composition during smelting and adjust it in time. In order to ensure the purity of the melt, prevent the pollution of harmful gases and control the chemical composition, in addition to shortening the melting time as much as possible, it should be covered with powdered flux mainly composed of potassium chloride and sodium chloride. The general dosage is 0.4~ of the weight of the charge. 2%. The smelting temperature is usually controlled at 700 to 750 °C.
The molten metal also needs to be refined and filtered to remove harmful hydrogen and non-metallic inclusions from the metal to improve metal purity. Refining usually uses a solid refining agent or a gas refining agent. The solid refining agent is generally a chlorine salt-based refining agent which is also used to replace the chlorine salt with hexachloroethane. Early use of strong chlorine gas as a gas refining agent, although the purification effect is good, but the environmental pollution is serious, so the development of nitrogen-chlorine mixed gas, inert gas and three gas (N2, Cl2, CO) refining agent, the effect is better. In order to ensure the refining effect, the oxygen and moisture content in the refining gas should generally be less than 0.03% by volume and 0.3 g/m3, respectively. The dynamic vacuum degassing method also has better degassing and sodium removal effects.
Filtration is a filter in which the molten metal is made of a neutral or active material to remove suspended inclusions in the melt. Commonly used glass mesh, microporous ceramic tubes and plates, alumina particles for filtration, can also be refined with electric flux and flux layer.
In the next article, we will introduce more aluminum alloy processing and casting.