Views:0 Author:Site Editor Publish Time: 2021-09-23 Origin:Site
The aluminum processing industry covers smelting, casting (continuous casting, semi-continuous casting), hot rolling (hot continuous rolling), cast rolling (twin roll cast-rolling, continuous casting and rolling), cold rolling (cold continuous rolling), foil rolling, forging, extrusion (continuous extrusion), tube rolling, drawing, surface treatment, and aluminum door and window manufacturing.
Casting production includes casting of primary aluminum ingots, remelting and casting of the aluminum ingots for remelting and scrap aluminum. Aluminum processing plants usually melt primary aluminum ingots, secondary aluminum ingots, process wastes with master alloys or alloying element additives to produce aluminum alloy melt and then cast into slab ingots, round billets or casting-rolled coils as required for subsequent rolling or extrusion production.
At present, aluminum alloy casting methods can be divided into three types: iron mould casting, continuous casting and semi-continuous casting (DC), concatenation casting-rolling (CC). Iiron mould casting is suitable for small aluminum processing plants. DC casting is the mainstream casting method at present, with large output, high yield, many kinds of alloys, good product quality and improved working conditions.CC casting has less investment in primary equipment, short production process, high yield, low energy consumption, and product quality can meet the needs of many civil aluminum materials. CC casting includes the following three types: twin roll continuous casting-rolling, twin- belt continuous casting and rotary-wheel continuous casting.
On the basis of semi-continuous conventional mould casting, aluminum alloy casting technology has been continuously developed, such as hot top casting, same metal level casting, short mould casting, electromagnetic casting, and ultrasonic casting have been successively developed and applied one after another. In order to improve the surface quality of ingots, improve the uniformity of composition and microstructure of ingots, reduce the metallurgical waste and crack waste of ingots, and improve the mechanical properties of ingots, technology-leading enterprises have successively studied and applied airslip casting, airsol veil casting, low metal level casting, and composite ingot synchronous casting technologies.
In the traditional direct chilling casting (DC casting), during the solidification process, the temperature difference between the inner and outer parts of the ingot is large and the cooling intensity is different. Moreover, due to the large height of the crystallizer, the air gap formed by the shrinkage of the solidified shell in the crystallizer is easy to cause segregation tumor, casting crack, macrosegregation of alloy composition on the ingot surface and uneven microstructure in the cross section of the ingot. Unstable liquid level control is also prone to surface defects such as cold shuts and inclusions.
The main factors affecting the quality of the ingot include casting temperature, casting speed, cooling intensity, liquid level stability and mold height. In addition to optimizing the production operation and improving the management level, enterprises should constantly explore, research and use new casting technologies.
The quality of aluminum alloy ingots is directly related to the performance of subsequent aluminum alloy rolling or extrusion products. The quality requirements for aluminum alloy ingots mainly include: less hydrogen and inclusions, no cracks, shrinkage cavities, porosities and cold laps, less segregation of alloy elements, uniform metallurgical structure, fine grains and good plasticity, facilitating follow-up processing and deformation.
The new semi-solid casting technology is characterized by the temperature between liquidus and solidus, and the key technique is the preparation of semi-solid metal slurry, including two slurrying methods: rheoforming and thixoforming. Semi-solid die casting can produce high quality die castings, prolong the service life of the die, improve the yield and reduce the composite cost. The microstructure of semi-solid die castings is fine and uniform, with high comprehensive mechanical property and thermal conductivity, which can replace forgings, steel plates, cast iron parts, etc. and has advantages in the application of automobile lightweight and communication equipment components.
Modern aluminum processing enterprises produce high alloying, large-size duralium alloy ingots, generally equipped with self-guided hydraulic semi-continuous casting machine, using low head composite casting technology ( LHC ), air-slip or airsol veil casting technology, tilting holding furnace, pusher continuous homogenizing furnace, high automation slab sawing machine, Scalping machine, ingot flaw detector, etc.
The self-guided hydraulic casting machine adopts computer automatic control, its lifting speed is even and stable, the crystallizer is automatically lubricated, and the liquid level height is automatically controlled; The tilting angle of the tilting type holding furnace is automatically adjusted according to the liquid level setting height of the launder, the distribution plate and the crystallizer, so as to ensure a smooth flow rate and uniform temperature when the aluminum melt enters the crystallizer, and the controllability and stability of pour furnace and melt pouring of the whole aluminum melt casting process are realized; The soaking furnace homogenizes the ingot to eliminate Internal stress and microsegregation in the ingot and ensure uniform chemical composition of the ingot.
The high-quality ingots should have equiaxed fine grained structure, no component segregation, uniform distribution of the second phase, dense structure, isotropy, high strength and good plasticity. Dendritic segregation and macrosegregation are well controlled, and there are no feather crystal, planktonic grain and coarse intermetallic compound defects. Degassing and filtering are good without defects such as gas cavities, inclusions and looseness. The surface is smooth, the longitudinal direction is straight, and there are no defects such as cracks, cold-shuts, segregation tumors. Contemporary advanced semi-continuous casting machines have a casting speed control accuracy of 1%, ingot longitudinal straightness no more than 2mm/full length, grain fineness(G.F.) less than or equal to grade 1, porosity less than or equal to grade 1,cold-shut less than 3mm, and segregation tumor height less than 3mm.
In addition to the casting of aluminum ingots, there are three main casting methods for aluminum products: die casting, permanent mold casting and sand casting. Both die casting and permanent mold casting molds are made of metal, while sand casting mold is made of sand as the main molding material. Permanent mold casting is suitable for castings with simple shape and large wall thickness. Die casting is characterized by applying high pressure to the molten metal by using the mold cavity, which is especially suitable for high volume production of small and medium-sized castings with a smoother surface and higher dimensional consistency. Sand casting is often used for castings with complex design, small quantity and large size. Aluminum slab and round billet arecast in permanent mold.