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Several New Technologies Of Aluminum Machining
Confronted with structure transformation and product improvement，the aluminum machining industry has set the research and application of innovated technology， process，material and equipment as its primary task in progress．Key points in innovation in‘ clude reasonable utilization and allocation of resources，energy saving and emission reduc— tion，simplification of procedures，new product development，stability of product quality，im‘ provement of yield and productivity，and cost reduction．Introduction has been made Off the research and application of continuous extrusion，effective friction backward extrusion，multi— billet extrusion，vacuum／atmosphere protection continuous casting，semi—solid processing technology．The development and promotion of new technologies, new processes, new materials and new equipment for aluminum processing are of great significance to the structural transformation of the aluminum processing industry, energy conservation and emission reduction, and improvement of product quality and production efficiency. For this reason, this paper briefly introduces several new extrusion technologies, vacuum/atmosphere protection continuous casting, and semi-solid processing technologies for reference by peers.
Continuous aluminum extrusion technology
Continuous extrusion is another major technological breakthrough in the field of metal material processing industry (especially aluminum processing industry) following the practical application of continuous rolling, continuous casting and continuous casting and rolling technologies. In principle, it effectively utilizes sufficient extrusion force and temperature generated by the strong friction between the billet and the rotating extrusion wheel to directly extrude rods, pellets or molten metal into products in a truly continuous and large shear deformation mode. ,
Features of Continuous Extrusion Technology
1) Compared with conventional extrusion, Conform continuous extrusion has the following characteristics
- ①Low energy consumption. Due to the combined effect of friction and deformation heat, the cold material is directly fed, which greatly reduces power consumption. It is estimated that about 3/4 of heat and electricity costs can be saved compared to conventional extrusion.
- ②The material utilization rate is high. Since there is no excess extrusion pressure, the amount of laser cutting ends and tails is very small, so the material utilization rate is very high. The material utilization rate is as high as 96% or more.
- ③The length of the product is large. It can continuously extrude several lengths of solidification boots. Figure 3 Schematic diagram of Castex continuous casting 3 Diagram of Castex continuous cast extrusion Thousands of meters, even 10,000 meters of thin-walled soft aluminum alloy coils, electromagnetic flat wires, aluminum wires and aluminum-clad steel wires, etc.
- ④Material organization and performance are uniform. It is very similar to an isothermal or temperature gradient extrusion process, so that the extruded product has a uniform structure and performance.
- ⑤ The billet has strong adaptability. The continuous casting, rolling or continuous casting disc rods can be used as billets, or metal particles or powders can be used as billets to directly extrude into materials; various continuous casting technologies can also be organically combined with Conform continuous extrusion to form Castex. Continuous casting and extrusion, that is, directly using the metal melt as a billet to extrude into a product.
- ⑥ Flexible production and high efficiency.
- ⑦ The equipment is light and light, occupying a small area, less investment, low cost of infrastructure construction, good production environment, and easy to realize automatic control of the whole process.
2) Disadvantages of Conform continuous extrusion compared with conventional extrusion
- ① The requirements for the surface quality of the blank are higher. Contaminants such as oxide film, oil stains and water vapor on the surface of the billet are easily extruded directly into the product, which seriously affects the product quality.
- ②Conform continuous extrusion process is difficult to master.
At present, there are various forms of continuous extrusion and continuous coating extrusion machines, such as single-wheel single-tank, single-wheel double-tank, double-wheel single-tank, etc. The serialized continuous extrusion equipment includes C250. C1000 Continuous Extruder. my country has started to produce LJ300 aluminum continuous extrusion equipment in 1990, which is used to produce aluminum and aluminum alloy coils, small and medium complex profiles, wires and cables, metal-coated wires, etc.
Process flow and temperature and speed control of Conform aluminum continuous extrusion
The general production process of Conform aluminum continuous extrusion method is: billet surface pretreatment _ unloading _ straightening _ online cleaning _ continuous extrusion _ ÷ product cooling – + tension straightening – ÷ coiling _ ÷ inspection leaf packaging warehouse .
In the continuous extrusion process of Conform, the preheating and extrusion temperature of the die are provided by the combined action of the friction between the feeding metal and the extrusion wheel and the heat of plastic deformation of the metal. Reasonable control of the extrusion wheel speed in the continuous aluminum extrusion process is an important process parameter for maintaining a constant extrusion temperature, ensuring product structure, performance and surface quality, and improving tool life.
- 1) In the heating and extrusion stage, short material must be fed into the groove of the extrusion wheel intermittently every time the extrusion is started, so that the extrusion wheel is operated at a low speed (7 r/min-8 r/min) to heat up to ensure The tool and die gradually and uniformly reach the desired extrusion temperature, especially the temperature of the extrusion die cavity must reach the extrusion temperature.
- 2) Stable extrusion stage The main factors affecting the extrusion temperature-speed relationship in the stable extrusion stage are: alloy properties, product specifications, running clearance, groove sealing block wrapping angle, extrusion ratio and cooling strength of cooling system, etc. Usually the main process parameters to stabilize the extrusion process are: the temperature of the wheel groove is 400℃～4500C; the die temperature is 3500C～4000C; the temperature of the shoe body should be less than 4000C; the rotation speed of the extrusion wheel should be less than 24 r/min; the outflow speed of the product should be Less than 70 m/min; operating current should be less than 300 A; operating voltage should be less than 400 V.
1) Conform aluminum continuous extrusion production line 1) Basic composition of Conform continuous aluminum extrusion production line The layout of Conform aluminum continuous extrusion production line is shown in Figure 4, which usually consists of the following parts:
- ①Blank pretreatment unit: including unwinding, straightening and online cleaning;
- ②Conform continuous extrusion machine host;
- ③ Product post-processing unit: including product cooling, tension straightening and coiling machine.
2) Main products and uses of Conform aluminum continuous extrusion At present, Conform aluminum continuous extrusion is widely used in the production of aluminum and aluminum alloy coils, small and medium complex profiles, wires and cables, metal-coated wires, etc.
Effective friction reverse extrusion technology
Effective friction reverse extrusion, also known as high-efficiency reverse extrusion, essentially changes the frictional force that originally acted as a hindrance to an effective frictional force that acts as a driving force. When K. . > Under the condition of K (suck. The speed of the extrusion cylinder relative to the extrusion die, the speed of the extrusion shaft), in the direction of metal flow, due to the effective friction between the extrusion cylinder and the extruded blank A high-speed extrusion process produced by the complementary deformation of.
Advantages of effective friction back extrusion
In the process of efficient and effective frictional reverse extrusion, due to the formation of an advanced circumferential flow in the extrusion cylinder and the supplementary compression of the metal flow near the die, the hydrostatic pressure is increased and the front of the metal flow is leveled. edge, eliminating the non-uniformity of deformation. This dynamic mechanism of deformation ensures: whether compared with the forward extrusion (in terms of energy consumption, volume uniformity of the structure, dimensional accuracy and yield in the length direction of the profile), or with the reverse extrusion. Ratio (uniformity of mechanical properties, dimensional accuracy of profile cross-section and surface quality) are superior. At the same time, there are advantages in terms of flow rate, production rate and the possibility of staged control by deformation and metal flow, as well as in the production of extruded products with special properties.
- ①It can increase the outflow speed of metal. When the non-lubricated high-efficiency friction reverse extrusion, reverse extrusion, and forward extrusion of 2024 aluminum alloy 060 mE bars are carried out under similar conditions, the corresponding ratio of the maximum flow velocity of the metal is (2.9±0.2): (1.7-4-1): 1.0, thus improving productivity.
- ②Because the acceleration of the peripheral flow and the deceleration of the central flow also exist in the extruded billet, the extruded product has good surface quality, uniform organization and stable mechanical properties.
- ③ The thickness of the extruded residual material is reduced by 50%, and the depth of the coarse grain ring at the end of the extruded product is reduced to 1 mm. Generally, there will be no bubbles, scars and scratches on the surface of the product, and there will be no overburning or cracking. The yield can reach 89%. .5%, up to 92%.
- ④ It can greatly increase the plastic deformation ability of metals, such as powder alloys and granular alloys to obtain higher extrudability, thereby obtaining various complex extrusion products.
- ⑤ It can realize semi-continuous and pseudo-isotropic flow, greatly improve and stabilize the dimensional accuracy of products, and eliminate the anisotropy of extruded products. The 35% to 45% of the extrusion is reduced to 1% to 3%; it can make the K of the 2024 aluminum alloy large material. . An increase of 20% increases exfoliation corrosion resistance by 35% to 40%.
- ⑥It can realize the assembly line of hard aluminum alloy profiles, and the product quality has good stability.
Application of High Efficiency Friction Reverse Extrusion
Applying the principle of high-efficiency friction reverse extrusion, various types of new extrusion machines have been developed
According to the varieties, specifications and uses of aluminum alloy products, a complete set of high-efficiency friction reverse extrusion tools has been developed, including: a complete extrusion pad with a round hole; 2 to 3 extrusion pads with forming grooves ; Can be squeezed by the combination of multi-slot sections. Table 4 shows the outer diameter of the limiting hollow spacer through the cross section. Squeeze blocks consist of several work blocks and a steel sleeve that is annularly connected with structural steel pipes to assemble the work blocks. Steel sleeves accurately hold and install these work blocks and can remove the press blocks from the outside of the extruder. Extrusion dies are available in both removable and fixed (with elastic parts). The change from integral to combined extrusion pads can increase the outer diameter of single-hole and multi-hole extrusions by 25% to 30%. The complete set of extrusion pads can ensure the production of profiles with many specifications and varieties. For example, a 5 MN extrusion machine can produce profiles with a circumscribed circle diameter of 8 mm to 80 mm and a wall thickness of 0.7 ITUTI-8 mm; 25 MN The extruder can produce profiles with a circumscribed circle diameter of 30 mm to 180 mm and a wall thickness of 1.5 mm to 8 mm. An extrusion line based on a high-efficiency friction reverse extruder, including an extruder with a full set of tools; a device for grading and heating of billets; a drawing system; an in-line profile quenching system; and a complete set of typical automation equipment.
Multiple billet extrusion
The multi-blank extrusion method requires opening a plurality of extrusion cylinder holes on one cylinder, and inserting blanks of the same or different sizes and materials into each cylinder hole, and then extruding them at the same time to make them flow into the hollow cavity. The extrusion die is welded into one body and then extruded through the die hole to obtain products of the desired shape and size. The multi-blank extrusion method is proposed in view of the high deformation resistance or high extrusion temperature of high-strength aluminum alloys, and the conventional split-die extrusion method cannot be used to form special-shaped hollow profiles. The basic idea is that if each shunt cavity L of the shunt die is regarded as an extrusion cylinder cavity L, the billet flows into the welding cavity through the shunt hole after the shunt bridge is divided into several strands, and then re-welds and extrudes from the die hole. out process. In fact, it is equivalent to the process of extruding the billet from several extrusion cylinders simultaneously into an extrusion die with a concave cavity (equivalent to a welding cavity). Strictly speaking, this idea is similar in principle to the previously existing method of lateral extrusion of coated cables. However, through the use of extrusion dies with special structures to control the flow of metals to form various layered composite materials (or cladding materials), the possible application fields of the multi-blank extrusion method have been greatly expanded.
The main features of the multi-blank extrusion method
- ① Hollow profiles can be directly formed from solid round blanks, without perforation or use of split dies. The mold structure is simple and the strength conditions are greatly improved, which is suitable for forming special high-strength alloy profiles.
- ②The solid round billet can be directly used to form various cladding materials and bimetallic tubes, omitting the process of preparing the composite billet and simplifying the forming process.
- ③ When extruding layered composite materials such as cladding materials and bimetallic pipes, the size uniformity of the products along the length and the circumferential direction is good, and it is not easy to produce defects such as bamboo knots, faults, and peeling. This is difficult or even impossible to achieve by conventional composite billet extrusion methods.
- ④ When extruding various layered composite materials, the billet combination has a large degree of freedom, and even the combination of two materials with a large difference in deformation resistance can be formed normally.
- ⑤ The powder can be used as raw material, plasticized and extruded, and then sintered and solidified to prepare high-strength and low-plastic layered composite materials that cannot be extruded from block blanks, or existing powder metallurgy methods (including conventional powder plasticizing extrusion) can be used. Gradient composites that are difficult to prepare by pressing, injection molding, etc.
- ⑥Because more than 2 blanks need to be used in one extrusion, the structure of tools such as extrusion cylinder and extrusion shaft, and the extrusion operation are more complicated than conventional extrusion.
- ⑦ When extruding with block blanks, the extrusion cylinder and extrusion gasket cannot be lubricated, otherwise the welding between the blanks will be affected; during hot extrusion, the oxidation of the blank surface during the heating process will prevent the welding. Quality has a significant impact and requires special measures.
It can be seen from the above characteristics that the multi-blank extrusion method is mainly suitable for the forming of special profiles, layered composite materials and gradient materials with high added value, difficult to form by other methods, or the existing product quality cannot meet the requirements of use. The raw material can be block. The body blank can also be a powder material.
3.3 Application of multi-blank extrusion method
- 1) Multi-blank extrusion method to form bimetallic pipe The forming process of the bimetallic pipe is as follows: the billets for the outer layer pipe are loaded into the two extrusion cylinders located on the OA section, and the billet for the inner layer pipe is loaded into the two extrusion cylinders located on the OB section. During extrusion, the two billets on the OB section are welded into the welding cavity of the inner extrusion die, see Figure 8, and then flow into the welding cavity of the outer extrusion die through the die hole of the inner extrusion die. Combined cavity. In the outer layer die welding cavity, the inner layer tube is covered by the outer layer tube material extruded from the two extrusion cylinders on the OA section under the condition that the new surface is kept free of oxidation, subjected to high temperature and a certain pressure, and then It flows out from the die hole of the outer extrusion die to become a double-layer tube. As described above, since the inner layer tube is combined with the outer layer tube in a state where the surface is not oxidized, subjected to high temperature and a certain pressure, a well-bonded inner and outer layer interface can be obtained. With the same principle as the forming of aluminum/aluminum alloy double-layer composite pipe, the multi-blank extrusion method can be used for forming the double-layer composite pipe of aluminum/copper, copper/titanium and other metals.
- 2) Preparation of layered composite materials and gradient materials by powder plasticizing extrusion Due to the problems of high temperature, high pressure, oxidation, etc., the above-mentioned block billet extrusion method is difficult to apply to the forming of layered composite materials such as high melting point materials and high temperature alloys; the same , Due to plasticity and other reasons, this method is also difficult to form layered composite materials of ceramic/ceramic, ceramic/metal and other combinations. However, if the powder is used as the raw material and an appropriate binder (plasticizer) is added to the powder, various metal/metal, metal/ceramic, and ceramic/ceramic layered composites can be formed using the principle of the multi-blank extrusion method. The powder blank is then sintered and solidified to prepare special composite materials, such as w—Cu gradient materials, stainless steel/ceramic gradient composite tubes, etc., which is one of the important application directions of the multi-blank extrusion method.
Air/Atmosphere Protection Continuous Casting Technology
Traditional smelting and processing methods have been unable to meet the requirements of high-tech industries such as electronics industry, aviation, and aerospace for some high-tech materials: such as special aluminum alloys (Al-Li, weldable Al-” with welding wire), metal matrix composite materials ( SiC/A1, A1203/A1), intermetallic compounds (Ti3A1), etc. The above materials have some special physical and chemical properties, such as oxidation in liquid state, serious inhalation, volatile and easy to burn components, brittleness and difficult to process , easy to segregate, ultra-pure, homogenized, low gas content, and generally small in batches and expensive, traditional continuous casting and block casting methods cannot or are difficult to produce such materials. At present, foreign non-ferrous metal continuous casting equipment manufacturers are all Actively developing small and precise continuous casting technology and equipment, such as the horizontal continuous casting machine designed and manufactured by the British RAUTOMEAD company using graphite resistance heating holding furnace. ASABA Co., Ltd. has developed a series of mini continuous casting machines, which can cast plates and rods. Using a high-frequency furnace as a melting and holding furnace, it has developed vacuum horizontal continuous casting (15 k ~ 30 kg), Vacuum vertical continuous casting (3 kg ~ 10 kg) and other series of continuous casting equipment are used to produce electronic materials and special alloys, which meet the manufacturing requirements of high purity and high performance.
4.1 Characteristics of vacuum/atmosphere protection continuous casting
Vacuum/atmosphere protection continuous casting technology has the characteristics of continuous casting technology: high-quality billets can be obtained; low energy consumption, short production cycle, low production cost; low investment in equipment and infrastructure; easy to connect with subsequent processes to form a continuous production line. The vacuum/atmosphere protection continuous casting technology is a method to realize the continuous preparation of materials in a special metallurgical environment. Since some materials are smelted under atmospheric conditions, they often react with most gases in the air, or absorb a large amount of gases, and the material properties cannot meet the requirements. And vacuum/atmosphere protection continuous casting technology also has all the advantages of vacuum metallurgy:
- ①Special metallurgical environment can achieve the goal of high-purity material preparation and reduce the gas and impurity content in the melt;
- ② For materials containing volatile components, pre-vacuum and then pass through the smelting and casting under the protection of inert gas, which can ensure that the alloy composition and the content of trace elements are very accurate;
- ③For alloys with severe air intake, under vacuum/protective gas melting conditions, continuous casting with complete isolation of air can avoid air intake;
- ④ Various metallurgical atmospheres can be created to meet the needs of preparation of different materials. Alloys containing easily burnt-out components can be smelted and cast in a reducing atmosphere, a neutral atmosphere, and an anti-component volatile medium after pre-vacuuming the gas CO:, Ar, H:, CO, etc.
4.2 Process layout and technical points
1) Vacuum/atmosphere protection continuous casting unit and its configuration
The vacuum/atmosphere protection continuous casting unit consists of melting and holding furnace, vacuum and atmosphere protection facilities, mold, secondary cooling system, traction system and coiler, hydraulic system and electrical control system.
2) Process flow
- ① Install the furnace and vacuumize. Put the pre-prepared alloy or pure metal charge into the crucible, install the dummy bar, seal each port, and start the vacuum pump to evacuate.
- ② melt the alloy. When the vacuum in the tank reaches a certain value, the power is turned on to rapidly melt the alloy.
- ③ Thermal insulation of molten metal or protection by atmosphere: keep the molten metal in the furnace warm at a certain overheating temperature. For materials that require atmosphere protection, turn off the vacuum pump to pass through the atmosphere protection, and maintain a certain pressure in the tank.
- ④ Start pulling. Start the tractor, pass cooling water to the mold and the secondary device, and start continuous casting.
- ⑤ Normal continuous casting. Adjust parameters such as cooling intensity and drawing process to the values of normal continuous casting.
3) Key points of production technology
- ① Billet ingot technology. The pulling technology of the continuous casting billet is extremely important, which directly affects the success of continuous casting. The technology includes the control of furnace mouth temperature and melt temperature, and the selection of pulling, stopping and withdrawing parameters.
- ②Dynamic sealing technology of vacuum continuous casting. Dynamic sealing is a key that directly affects material vacuum/protective atmosphere continuous casting. Without real dynamic sealing, the continuous preparation of materials cannot be achieved, and the high efficiency advantages of continuous production can be fully exploited.
- ③Continuous casting process expert system. Different materials have different physical properties, so the continuous casting process (including the melt temperature during continuous casting, the drawing system, the cooling system, etc.) is very different. The physical parameters affecting the process are multivariate, thermophysical properties (density, thermal conductivity, thermal conductivity, thermal expansion coefficient), solidification characteristics (melting point, solidification temperature zone, phase transition law during solidification), rheology (viscosity, elasticity and Plasticity, high temperature strength) all play a role in the continuous casting process.
- ④Coagulation tissue control technology. The solidification structure is affected by the cooling system, subcooling degree, addition of grain refiners, liquid convection conditions, etc., and different structures such as columnar crystals and equiaxed crystals can be obtained. Therefore, it can be achieved by controlling the continuous casting system and stirring conditions. The purpose of controlling the coagulated tissue.
3 Application examples
1) Preparation of low hydrogen aluminum alloy wire material
Aluminum alloys are prone to porosity during the welding process. In order to ensure the quality of the weld, the welding material must have low hydrogen content and impurity content; A1. Sc alloy welding filler also has very special requirements, such as extremely low hydrogen content and impurity content, uniform and accurate composition, anti-oxidation, etc. Vacuum/protective atmosphere continuous casting can effectively solve the problem of preparation of such materials. Table 5 lists the performance test results of 1420 AI-Li alloy welding wire prepared by Beijing General Research Institute of Nonferrous Metals using vacuum/protective atmosphere continuous casting technology. Vacuum smelting can remove hydrogen in the aluminum melt, and continuous casting under argon protection can avoid the gas involved in casting and the gas inhaled when contacting the mold. Therefore, the hydrogen content of the welding wire is controlled in a very low range (0. 24 × 10″). The weld has no defects as a whole, harmful impurities are controlled within a low range, and argon shielded continuous casting avoids element burning. Stirring and rapid solidification during the melting process avoid Al, zr sedimentation, so the composition Uniform, the difference of all main components in each batch is less than 8%; the grains are fine, the structure is dense, the gas inclusions are reduced, the mechanical properties of the weld are improved, the strength coefficient is above 0.80, and the butt welded weld reaches the first level.
2) Ultrafine bonding wire for industry
The diameter of ultra-fine silicon-aluminum wire used for diode lead bonding wire generally reaches 20 Ixm to 30 Ixm. This ultra-fine wire has high requirements for raw materials and processing technology. Foreign countries require a single roll length of more than 300 m, in order to facilitate Automated manufacturing. At present, the domestic silicon-alumina ultra-fine wire is widely used in casting. Extrusion technology route. The material cannot meet the requirements in terms of strength and elongation, and it often breaks when it is processed to ultra-fine conditions. The maximum length is 100 m to 200 m, which is difficult to meet the requirements of the length of a single coil; the method of vacuum continuous casting and drawing is produced. This ultra-fine filament improves product quality and simplifies processes. The metallurgical quality of the material is improved, the strength and elongation of the welding wire are improved, and the welding performance of the bonding wire is greatly improved. The length of a single coil of ultra-fine silicon aluminum wire produced by vacuum continuous casting generally exceeds 300 m, and all performance indicators meet the requirements of use.
Semi-solid aluminum alloy processing technology
Semi-solid metal processing is that during the solidification process of the metal, strong stirring or by controlling the solidification conditions, the formation of dendrites is suppressed or the dendrites are broken, forming equiaxed, uniform and fine primary phases evenly distributed in the liquid phase. The suspended semi-solid slurry, this slurry still has good fluidity under the action of external force, that is, the solid phase ratio reaches 60%. It can be processed and formed by conventional processes such as die casting, extrusion, and die forging, and can also be formed into parts by other special processing methods. This method of forming a metal slurry that is neither completely liquid nor completely solid is called Semi-Solid Metal Forming or Semi-Solid Metal Process (SSM for short).
5.1 Characteristics of semi-solid machining
1) The main advantages of semi-solid forming
- A wide range of applications, all alloys with solid-liquid two-phase region can achieve semi-solid forming. Can be used in a variety of forming processes such as casting, extrusion, forging and welding.
- SSM has stable filling, no turbulence and splash, low processing temperature, and small solidification shrinkage, so the casting has high dimensional accuracy. SSM formed parts are nearly the same size as finished parts, greatly reducing the amount of machining, and can be machined with little or no chipping, saving resources. At the same time, SSM has a short solidification time, which is beneficial to improve productivity.
- The semi-solid alloy has released part of the latent heat of crystallization, thus reducing the thermal shock to the forming device, especially the mold, and greatly improving its life.
- The surface of the @SSM forming part is smooth and smooth, the internal structure of the casting is dense, the internal pores and segregation and other defects are small, the grain size is small, and the mechanical properties are high, which can approach or reach the performance of deformed materials.
- The application of semi-solid forming process can improve the technical difficulties of non-metallic reinforcement phase floating, segregation and non-wetting with metal matrix during the preparation of composite materials, which provides favorable conditions for the preparation and forming of composite materials.
- Compared with solid metal die forging, the flow stress of SSM is significantly reduced, so the forming speed of SSM die forging is higher, and very complex parts can be formed.
2) Microstructure characteristics of semi-solid processing materials
With conventional casting methods, the nucleation and growth of grains are free to move during the early solidification phase of the alloy melt. However, if the grains grow further into large dendrites, they will form a network of intertwined dendrites, and the mutual movement of the grains becomes difficult, resulting in a typical dendritic structure; in semi-solid processing During the solidification of the metal, the dendrites generated are broken due to strong stirring, or the formation of dendrites is suppressed by controlling the solidification conditions. Therefore, the primary phase in the solidified structure has an equiaxed, uniform, fine and nearly spherical shape.
3) Mechanical properties after semi-solid machining
Due to the structure of semi-solid processing and forming parts, that is, the primary phase is uniform, fine, and nearly spherical. Therefore, the mechanical properties of the formed parts are also high. Table 7 shows the comparison of the mechanical properties of several aluminum alloys after semi-solid forming and other methods. The advantages of semi-solid processing forming technology can be seen from the table. For example, the thixoformed A356 aluminum alloy has better mechanical properties than the aluminum alloy obtained by ordinary sand casting in the T6 heat treatment state, and its mechanical properties are close to those of forgings.
Process flow of semi-solid processing technology
1) The main process flow of semi-solid processing technology
Semi-solid metal processing technology is suitable for alloy systems with a wide liquid-solid coexistence region: aluminum alloys, magnesium alloys, zinc alloys, nickel alloys, copper alloys and steel alloys. Among them, aluminum alloys and magnesium alloys have special application significance in the transportation and aerospace industries because of their low density. At the same time, they have low melting points and are easy to achieve in industrial production. The main forming methods of semi-solid processing are die casting and forging. In addition, some people experiment with methods such as extrusion and rolling. There are two main process routes: one is to directly shape the semi-solid slurry obtained by stirring under the condition of maintaining its semi-solid temperature, which is usually called Rheocasting; the other is to prepare the semi-solid slurry. It is formed into a billet, which is cut according to the size of the product, and then reheated to a semi-solid temperature for forming, which is usually called thixogenesis.
For thixoforming, because the semi-solid billet is easy to transport and easy to realize automation, it has been widely used in the industry earlier. For rheological casting, because the stirred semi-solid slurry is directly formed, it has the characteristics of high efficiency, energy saving and short process, and it has developed rapidly in recent years.
2) Preparation techniques of several semi-solid metal slurries
Although semi-solid processing technology has many advantages, it also has some disadvantages, such as a long process. Therefore, the development trend of semi-solid processing technology is: to further simplify the processing process; to further reduce the cost of JJi-r_; to expand the application range of semi-solid processing technology. The first and very critical step of semi-solid processing technology is how to prepare semi-solid metal slurry efficiently and at low cost. In recent years, there have been many new methods, new processes and new technologies for the preparation of semi-solid slurries. To a certain extent, these new processes and technologies have the advantages of low production cost, low energy consumption, high production efficiency, simple equipment, convenient operation, and can produce semi-solid billets of larger size. The main methods are introduced later.
- ①New MIT process (New MIT Process). Figure 10 shows the new MIT process: the alloy in the vessel is kept within a few degrees above the liquidus temperature; a copper-coated rod stirrer with cooling The temperature of the alloy is lowered to the liquidus temperature, and the volume percentage of the solid phase in the molten alloy is very small; take out the stirrer, put the alloy in the semi-solid interval, cool slowly for a short time or keep it in adiabatic state, and then cool the alloy to the specified forming temperature. Stirring-cooling near the liquidus temperature will generate a large number of nuclei in the alloy melt.
- ②Cooling Slope method. The principle of the cooling chute method is: pour the molten metal slightly higher than the liquidus temperature on the cooling chute, and the metal melt flows down the inner surface of the chute. The grain nucleates and grows, and the impact of the metal fluid and the self-weight of the material make the grain fall off and turn over from the wall of the chute to achieve the stirring effect. The metal slurry in the cooling chute is dropped into the container, and the temperature of the container is controlled, that is, slow cooling, cooling to a certain semi-solid temperature, and then keeping warm to reach the required solid volume fraction, and then rheological deformation and thixotropic deformation can be carried out. . rz is the technological process of the cooling chute method, Fig. 11a and b are the direct casting and rolling of the prepared semi-solid slurry into a slab; Fig. 11c shows that after the prepared semi-solid slurry is cast into an ingot, the second The process of thixoforming after heating and remelting. The volume percentage of solid phase of semi-solid slurry prepared by cooling chute method is between 3% and 10%, and its fluidity is the same as that of molten metal. In rheological casting, the lower the solids volume percentage, the easier it is to cast. Therefore, the cooling chute method can be applied to rheological casting to form very thin castings.
- ③ Double helix semi-solid metal rheological injection molding method. The double-helix rheological injection machine is composed of a crucible, a double-helix shearing device and a central controller. The double-helix shearing device is composed of a barrel and a pair of co-rotating helixes that are closely meshed with each other. The tooth profile of the helix shaft is specially designed. , which can make the metal melt obtain higher shear rate and higher turbulence intensity. The heating unit and the cooling unit are distributed along the direction of the axis of the outer layer of the cylinder, forming a set of heating-cooling belts to control the temperature of the device. The temperature control accuracy can reach 4-1 °C, and the volume percentage of solid phase in the semi-solid metal slurry can be accurately controlled.
- ④Cut. Cooling humiliation L method (Shear.cooling Roll, referred to as SCR method). As shown in Figure 13, the main structure of the SCR method includes shearing/cooling rolls, shoe-shaped seats made of refractory materials and strippers. Water is cooled in the rolls, and the rotation of the rolls guides the molten metal to flow along the shoe-shaped seats. The shoe needs to be preheated to a certain temperature to prevent the metal from solidifying on the shoe. A stripper is installed at the outlet of the metal slurry to scrape off the metal that may be adhering to the roll. The working principle is: the molten metal heated to a certain temperature is injected into the guide groove above the roll gap through the nozzle, leaving a certain gap between the roll and the shoe-shaped seat, and the surface of the roll has a certain roughness, and water is passed through the roll. cool down. Due to the cooling effect of the roll and the shoe-shaped seat, the alloy melt is solidified, and the rotating roll produces a shearing and stirring effect on the partially solidified alloy, so that the alloy liquid is converted into a semi-solid slurry. And the friction force exerted by the roller is pulled out from the gap between the roller and the shoe-shaped seat, and the semi-solid slurry flow is guided by the stripper installed at the outlet. The outflowing semi-solid slurry can be directly rheologically formed, or it can be made into a billet with a non-dendritic structure of the required size, and then thixoformed
- ⑤ Different melt mixing methods. Different melt mixing methods are methods of mixing molten metals of two or three hypoeutectic compositions, or by mixing molten metals of hypoeutectic and hypereutectic compositions to obtain semi-solid slurries. The method firstly prepares the molten metals to be mixed separately, and keeps all the molten metals to be mixed above the liquidus line, and there is no crystal nucleus or few crystal nuclei in the molten metal. Mixing is carried out in an insulated vessel or in a stationary mixing tank by coating the surface of the insulated vessel with graphite; the mixing tank needs to be preheated to ensure that the temperature of the melt is maintained at the liquidus as it flows through the mixing tank above temperature. The melt flows in a turbulent manner over the mixing tank, resulting in good mixing. The mixing of the two melts leads to spontaneous heat conduction (heat release), and the hypoeutectic absorbs the heat of the hypereutectic. The temperature of the new alloy obtained after mixing is around the liquidus temperature and contains a large number of crystal nuclei, which can be formed into fine spheres by further processing. Semi-solid slurry of tissue. By controlling the composition and quality of different melts, the desired semi-solid slurry can be obtained. If the two melts to be mixed are of different compositions, control the heat and mass transfer (or diffusion) to obtain particles containing a large amount of the initial phase. The eutectic alloy temperature, convection strength and particle size of the initial phase have a great influence on the final microstructure of the semi-solid slurry.
- ⑥ Near-liquidus casting method (Near-liquidus cast). The near-liquidus casting method mainly uses factors such as controlling the casting temperature, standing time, casting speed and cooling intensity, and the temperature of the molten metal is above the liquidus temperature or close to the liquidus temperature. Pouring at a certain cooling rate to obtain a fine, nearly spherical, non-dendritic semi-solid structure. At present, it has been successfully used in the preparation of semi-solid billets of aluminum alloys such as 7075, 2168 and A356 and magnesium alloys such as AZ91D and ZK60.
5.3 Industrial application of semi-solid processing technology
Semi-solid processing technology is widely used in various countries. Many countries have already started the research and development application of this technology. At present, the United States, Italy, Switzerland, France, the United Kingdom, Germany, Elben and other countries are in the leading position. Currently, the most successful and widespread application of semi-solid processing technology is in the automotive industry. Table 8 lists the weight savings of replacing cast iron parts with semi-solid machined aluminum alloy automotive parts
New technologies such as rational utilization and allocation of resources, simplification of technological processes, energy saving and casting reduction, and semi-solid processing are of great significance for accelerating industrial transformation and product upgrading, developing new products, improving product quality and production efficiency, and reducing production levels. Production cost is the focus of technological innovation, and it is also the basic condition for promoting the transformation of the aluminum processing industry and the upgrading of products.