What are the processes of aluminum alloy processing?
The density of pure aluminum is small, about 1/3 of iron, low melting point, aluminum is face-centered cubic structure, so it has high plasticity, easy to process, can be made into a variety of profiles, plates, good corrosion resistance; However, the strength of pure aluminum is very low, so it is not suitable for structural materials. Through long-term production practice and scientific experiments, people gradually add alloying elements and use heat treatment and other methods to strengthen aluminum, which gets a series of aluminum alloys. The alloy formed by adding certain elements can have high strength while maintaining the advantages of light weight of pure aluminum. This makes its "specific strength" better than a lot of alloy steel, become the ideal structural material, widely used in machinery manufacturing, transportation machinery, power machinery and aviation industry, aircraft fuselage, skin, compressor and other often made of aluminum alloy to reduce weight. The welding of aluminum alloy instead of steel plate can reduce the weight of the structure by more than 50%.
With the rapid development of science and technology and industrial economy in recent years, the demand for aluminum alloy structural parts is increasing day by day, which makes the processability research of aluminum alloy further. The wide application of aluminum alloy has promoted the development of aluminum alloy processing technology, and the development of processing technology has expanded the application field of aluminum alloy, so the processing technology of aluminum alloy is becoming one of the hot spots of research.
Smelting of aluminum alloy
Smelting provides billets for plastic processing. Melting furnace multi-purpose gas or oil reflector furnace, the general capacity of 20 to 40 tons or larger; Resistance heating reflector furnace is also used, the capacity is generally about 10 tons. In order to shorten the loading time, improve the melting efficiency, reduce the absorption of gas and the involvement of oxide film, the tilting top charging round furnace has been adopted in industry. It is best to use rapid analytical instruments to analyze the alloy composition 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 is appropriate to cover with powdery flux based on potassium chloride and sodium chloride, and the general dosage is 0.4 ~ 2% of the weight of the charge. The melting temperature is usually controlled at 700 ~ 750℃.
The melted metal also needs to be refined and filtered to remove the harmful gas hydrogen and non-metallic inclusions in the metal to improve the purity of the metal. Refining is usually done with a solid refining agent or a gas refining agent. The solid refining agent is mainly chlorine salt, and hexachloroethane is also used to replace the refining agent of chlorine salt. Early use of highly active chlorine gas as gas refining agent, although the purification effect is good, but serious pollution to the environment, so the development of nitrogen-chlorine mixed gas, inert gas and three-gas refining agent, the effect is better. In order to ensure the refining effect, the oxygen and water content of the refined gas should generally be less than 0.03% and 0.3 g/m 3, respectively. The dynamic vacuum degassing method also has better degassing and sodium removal effects.
Filtration is a filter made of melt metal through neutral or active materials to remove inclusions in suspension in the melt. Glass wire mesh, microporous ceramic tubes and plates, alumina particles are commonly used as filter beds for filtration, and electric flux refining and flux layer filtration are also available.
Second, aluminum alloy casting
Casting generally adopts vertical or horizontal water-cooled semi-continuous casting method. In order to improve the microstructure and surface quality of billets in vertical casting, electromagnetic crystallization tanks, dwarf crystallization tanks and hot top casting methods have also been developed (see solidification of metals). The water-cooled semi-continuous casting method is to introduce the liquid metal into the water-cooled mold through the flow trough, so that the liquid metal is cooled to form a solidified shell, which is pulled by the base of the casting machine or by its own weight evenly dropped out of the mold to form a billet. The technological parameters vary greatly due to the different alloy composition and billet size. Generally, the casting speed and cooling speed should be increased as much as possible, and the height of the crystallization tank should be reduced. The casting temperature is usually 50 to 110 ° C higher than the liquidus of the alloy. In addition, the continuous casting and rolling process of aluminum strip has been developed. Plate and strip production adopts flat roll rolling, the basic processes are hot rolling, cold rolling, heat treatment and finishing. The chemical composition of LY12, LC4 and other duralalloy should be homogenized before hot rolling. The treatment temperature is generally lower than the eutectic temperature of the low melting point phase of the alloy 10 ~ 15℃, and the heat preservation is 12 ~ 24 hours. The aluminum cladding of duralumin alloy is placed on both sides of the milled billet with the help of hot rolling welding. The thickness of the aluminum coating is generally 4% of the thickness of the plate. Hot rolling is generally carried out above the recrystallization temperature. Hot rolling can be carried out on a single stand reversing mill, or continuous rolling on multiple stands. In order to improve the yield and production efficiency to develop large ingot rolling, ingot weight of more than 10 ~ 15 tons. Factories with an annual output of less than 100,000 tons generally use four-roll reversible hot rolling and hot winding process, and the thickness of the hot rolled strip is about 6 to 8 mm. The output of more than 100,000 tons of factories, mostly in the four-roll reversible hot mill after the billet using a single stand or two stands, three stands, five stands continuous rolling, the implementation of hot finishing rolling, strip thickness up to 2.5 ~ 3.5 mm. Hot rolled strip is rolled as cold rolled blank. In order to ensure the best plasticity of the metal, hot rolling should be carried out in the single-phase microstructure state. The hot rolling opening temperature of LY11, LY12 and other alloys is 400 ~ 455℃. The first few times the deformation rate is generally less than 10%, and then gradually increase. The pass deformation rate of pure aluminum and soft aluminum alloy can reach 50%, and that of hard aluminum alloy is about 40%. The total deformation rate of hot rolling can reach more than 90%. Cold rolling is usually carried out at room temperature, and thinner sheets and strips with precise dimensions, smooth and flat surfaces can be obtained through cold rolling, and work-hardened sheets and strips with specific mechanical properties can be obtained. Cold rolling mainly adopts the production process of belt method, using four high reversible rolling mill or four high irreversible rolling mill for cold rolling, and currently developing irreversible rolling mill for cold rolling. The rolling mill is equipped with hydraulic pressure, hydraulic roll bending, automatic thickness control system or automatic thickness control system for measuring roll gap and plate shape control instrument, which is controlled by micro electronic computer, record and store various parameters to obtain accurate size and flat sheet and strip, such as 0.18mm strip tolerance up to ±5 microns. Small factories also have block production of sheet metal. After annealing, the cold deformation rate of aluminum can reach more than 90%. The polyphase duralumin alloy is obviously hardened by cold work and needs intermediate annealing. The cold deformation rate after intermediate annealing is 60 ~ 70%. Hot rolling with emulsion lubrication, cold rolling has developed from emulsion to full oil lubrication. Multi-stage cooling system with separate control nozzle to reduce friction between aluminum plate and roll, cool roll, control roll profile, wash aluminum powder and other impurities to obtain good surface quality and shape. After cold rolling and heat treatment, the coil is often finished on a roll straightener or on a stretch-bending continuous straightener line. The flat quenched plate should be carried out during the aging incubation period, generally completed within 30 to 40 minutes after quenching. The total deformation of flat calendering of the quenched plate should not exceed 2%. Continuous casting and rolling of aluminum strip successfully tested in 1955 can produce sheet metal and aluminum foil billets. China began to use this method to produce sheet metal in the early 1970s.
3. Related processes of melting and casting
In order to ensure the final performance of the product, the heat treatment is called the finished product heat treatment, including the finished product annealing, solution treatment, quenching, natural aging and artificial aging treatment; In addition, fractional aging and deformation heat treatment processes have been developed. Aging treatment not only improves the mechanical properties of the product, but also improves the stress corrosion resistance and fracture toughness of the product. The transfer time from solution heating to quenching should generally be controlled within 30 seconds. The cooling rate of quenching should not only ensure the supersaturated solid solution, but also cause the product to produce excessive quenching stress and bending deformation. The coil and the sheet are annealed in a box furnace, well furnace, vertical plate continuous annealing furnace or air cushion continuous annealing furnace with strong circulation ventilation, and the new furnace uses a protective atmosphere. Quenching is generally heated in a salt bath, and the new process uses an air cushion type continuous quenching furnace.
(1) According to the stress and deformation mode of the workpiece in the deformation process
According to this method, aluminum and aluminum alloy processing can be divided into rolling, extrusion, drawing, forging, spinning, forming and deep processing.
A. Rolling
Rolling is a plastic deformation process in which the ingot is pulled into the rotating roll by the force of friction, and its cross section is reduced, its shape is changed, its thickness is thinned and its length is increased by the pressure exerted by the roll. According to the different rotation direction of the roll, rolling can be divided into longitudinal rolling, horizontal rolling and diagonal rolling. In longitudinal rolling, the rotation direction of the working roll is opposite, and the longitudinal axis of the rolled piece is perpendicular to the axis of the roll, which is the most common method in the flat roll rolling of aluminum alloy plate, strip and foil. In cross rolling, the working roll rotates in the same direction, and the longitudinal axis of the rolled piece is parallel to the rolling roller line, which is rarely used in the rolling of aluminum alloy plate and strip. In diagonal rolling, the rotation direction of the working roll is the same, and the longitudinal axis of the rolling piece is inclined to a certain Angle with the roll axis. In the production of aluminum alloy pipes and some special-shaped products, double or multi-roll diagonal rolling is often used. According to the different roll system, aluminum alloy rolling can be divided into two-roll rolling, multi-roll rolling and special roll system (such as planetary rolling, V-shaped rolling, etc.) rolling: according to the different roll shape, aluminum alloy rolling can be divided into flat roll rolling and pass roll rolling. According to different product varieties, aluminum alloy rolling can be divided into plate, strip, foil rolling, bar, flat strip and shaped profile rolling, pipe and hollow profile rolling.
B. Extrusion
Extrusion is a processing method in which the billet is loaded into the extrusion cylinder, and the metal is pressed through the extrusion shaft to extrude it from the die hole of the given shape and size, resulting in plastic deformation and obtaining the required extruded product. The direction of metal flow is different when pressed. Extrusion can be divided into forward extrusion, reverse extrusion and combined extrusion. In forward extrusion, the movement direction of the extrusion shaft is consistent with the flow direction of the extruded metal, while in reverse extrusion, the movement direction of the extrusion shaft is opposite to the flow direction of the extruded metal. According to the heating temperature of the billet, extrusion can be divided into hot extrusion and cold extrusion. Hot extrusion is to heat the billet above the recrystallization temperature for extrusion, cold extrusion is to extrude at room temperature.
C. Pull out
Drawing is a processing method in which the drawing machine (or drawing machine) pulls aluminum and aluminum alloy billets (wire billets or tube billets) from the mold of a given shape and size through a clamp to produce plastic deformation and obtain the required tubes, rods, shapes and wires. According to the variety and shape of the products produced, the drawing can be divided into wire drawing, tube drawing, bar drawing and profile drawing. Pipe drawing can be divided into empty drawing, drawing with core head and drawing with floating core head. The main elements of drawing process are drawing machine, drawing die and drawing drum. According to the drawing mold, drawing can be divided into single drawing and multi-drawing.
D. Forging
Forging is a forging hammer or press (mechanical or hydraulic) through the hammer or indenter to exert pressure on aluminum and aluminum alloy ingots or forging billets, so that the metal produces plastic deformation. Aluminum alloy forging has two basic methods: free forging and die forging. Free forging is the forging of the workpiece between the flat anvil (or shape anvil); Die forging is to place the workpiece in the mold of a given size and shape, and then apply pressure to the workpiece to forge deformation, and obtain the required die forging. The repair process of forging is an important part of the die forging process of aluminum alloy. Because aluminum alloy is soft at high temperature, large viscosity, poor liquidity, easy to stick and produce a variety of surface cracks (folding, burr, cracks, etc.), before the next process, it must be polished, repaired, and the surface defects removed, otherwise the defects will be further expanded in the subsequent process, and even cause the scrap of the forging.
The tools used for repair include pneumatic grinder, pneumatic milling cutter, electric milling cutter and flat shovel. Before repair, the defect is identified by corrosion, and the repair should be smooth and transition, and its width should be 5 to 10 times the depth.
E. Other plastic forming methods for aluminum
At present, people have also researched and developed a variety of new aluminum processing methods, which are mainly:
(1) Pressure casting forming method, such as low, medium and high pressure forming, extrusion forming, etc.
(2) semi-solid forming method, such as semi-solid rolling, semi-solid extrusion, semi-solid drawing, liquid die forging, etc.
(3) Continuous forming method, such as continuous casting and extrusion, high-speed continuous casting and rolling, Conform continuous extrusion method, etc.
(4) composite forming method, such as laminated rolling method, multi-billet extrusion method, etc.
(5) deformation heat treatment method. Aluminum and aluminum alloy processing materials to roll (plate, strip, strip, foil) and extrusion (pipe, rod, type, wire) the most widely used, the largest output, according to recent statistics, the annual output of these two types of materials accounted for the world's total annual output of aluminum (average) 58% and 39%, the rest of the aluminum processing materials, such as forging products, only accounted for a few percent of the total output of aluminum.
(2) According to the temperature characteristics of the workpiece in the process of processing
Aluminum and aluminum alloy plastic forming methods are many, usually according to the temperature characteristics and work of the workpiece during processing
The stress and deformation mode (stress-strain state) of the parts in the deformation process are classified. According to the temperature characteristics of the workpiece in the processing process, aluminum and aluminum alloy processing can be divided into hot processing, cold processing and warm processing.
A. Hot processing
Hot working refers to the plastic forming process of aluminum and aluminum alloy ingot above the recrystallization temperature. When hot working, the plasticity of the ingot is high and the deformation resistance is low, and the equipment with small capacity can be used to produce the product with large deformation amount. In order to ensure the organizational properties of the product, the heating temperature, deformation temperature and deformation speed of the workpiece should be strictly controlled, as well as the deformation end temperature and the cooling speed after deformation. The common hot processing methods of aluminum alloy include hot extrusion, hot rolling, hot forging, hot top forging, liquid die forging cold, solid state forming with half work, continuous casting and rolling, continuous casting and extrusion.
B. Cold processing
Cold working refers to the plastic forming process that is completed at temperatures below which recovery and recrystallization do not occur. The essence of cold working is the combination process of cold working and intermediate annealing. Cold processing can obtain smooth surface, accurate size, good organizational properties and can meet different performance requirements of the final product: the most common cold processing methods are cold extrusion, cold top forging, cold rolling, cold drawing, plate and foil cold rolling, cold stamping, cold bending, spinning and so on.
C. Warm processing
Warm processing is a plastic forming process between cold and hot processing. The main purpose of warm processing is to reduce the deformation resistance of the metal and improve the plastic properties of the metal (workability) : the most common warm processing methods are warm extrusion, warm rolling, warm top forging and so on.
Fourth, the surface treatment of aluminum alloy
Aluminum alloy sheet can be divided into two categories according to the surface treatment of non-painted products and painted products.
(1) Non-painted products
It can be divided into hammer aluminum plate, embossed plate and pre-passivated alumina surface treatment plate. Such products are not painted on the surface of the plate, the appearance of the surface is not high, and the price is low.
(2) Paint products
According to the coating process can be divided into: spray plate products and pre-roll coating plate; According to the type of paint can be divided into: polyester, polyurethane, polyamide, modified silicon, epoxy resin, fluorocarbon and so on.
Among various coatings, the main performance difference is the resistance to ultraviolet rays of sunlight, among which the most commonly used coating on the front is fluorocarbon paint (PVDF), which has a strong ability to resist ultraviolet rays; The back can be coated with polyester or epoxy resin as a protective paint. In addition, the front can also be affixed with a layer of removable protective film.
Fifth, aluminum alloy defect repair
In the process of production, aluminum alloy is prone to shrinkage hole, sand hole, porosity and slag inclusion. How to repair defects such as pores in aluminum alloy castings? If using electric welding, argon welding and other equipment to repair, due to large heat release, easy to produce thermal deformation and other side effects, can not meet the requirements of repair welding.
Cold welding repair machine is the use of high-frequency electrical spark instant discharge, no heat surfacing principle to repair casting defects. Due to the small thermal influence area of cold welding, it will not cause annealing deformation of the substrate, no cracks, no hard points, hardening phenomenon. Moreover, the welding strength is high, the re-solidification of the filler and the matrix after melting at the same time, the combination is firm, and can be grinding, milling, filing and other processing
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