Introduction Semi-solid forming technology has become one of the most promising metal forming processes in the 21st century due to its advantages of energy-saving, high-efficiency, near-end forming, and high performance of formed parts. It has outstanding advantages especially in non-ferrous metal forming. Semi-solid forming can be divided into semi-solid rheology and semi-solid thixotropic depending on the process flow. At present, semi-solid thixoforming is widely used, especially thixotropic die-casting technology to "emerging" semi-solid forming technology and The combination of mature die-casting technology has achieved excellent results and has been widely used in the transportation and consumer goods industries. Compared with semi-solid rheology, thixoforming has the advantages of easy clamping, easy transfer, and easy industrialization. Therefore, it has received extensive attention. However, thixotropy also has its drawbacks. The process complexity is often referred to as the "three-step method." Energy consumption requires secondary heating. Of particular importance is the superior performance of rheological parts. Therefore, I believe that the prospect of semi-solid forming is rheo-diecasting.
I. Rheological Technology Introduction Semi-solid forming technology originated from rheology. However, due to the difficulties in the storage and transportation of semi-solid billets in the middle of the flow, the research and application of semi-solid forming technology are relatively stagnant. Therefore, thixotropic development is relatively rapid, but after many years of research and practice, it has been found that thixoforming has the following process problems: (1) The cost of the semi-solid aluminum alloy billet is high, and is about 40% higher when the billet is prepared. (2) traditional electromagnetic stirring power, low efficiency, high energy consumption; (3) traditional electromagnetic induction remelting heating high energy consumption, the surface of the billet oxidation is serious, and the billet will always lose part of the metal heating, accounting for the billet 5% to 12% of the mass; (4) The liquid fraction of the billet must not be too high, otherwise the formation of very complex parts is difficult; (5) Sawdust, billet reflow, the flow of the soup, pouring system (accounting for the quality of the billet 10% to 20%) and waste products (all return materials account for about 40% to 50% of the billet quality) cannot be recycled immediately, and must be returned to the billet preparation plant or billet supply production plant, increasing the cost of forming production; (6) Billets The composition, grain shape, and non-uniform grain distribution directly affect slurry quality. As a result, the flow became re-emphasized. The rheology is that the prepared semi-solid slurry is directly sent to a forming apparatus for forming without undergoing secondary heating remelting. Rheological die-casting is a kind of net final forming method that transfers the prepared semi-solid metal slurry directly to the injection chamber for die casting. Not only has the semi-solid forming casting less solidification shrinkage, and the formation is not easy to gas, so the casting is dense, can be general characteristics of heat treatment and strengthening, and compared with other semi-solid forming, the grain is finer, there is no macro segregation, and the performance is more excellent. Therefore, the rheology technique is widely expected.
Second, the advantages of rheological die-casting From the semi-solid slurry structure, semi-solid alloy forming is based on semi-solid non-dendritic rheology and thixotropy, but this feature is determined by its microstructure. It is considered that for a given semi-solid alloy slurry, the degree of aggregation of solid particles determines the apparent viscosity of the system, and the shear rate and shear time indirectly affect the viscosity by changing the degree of aggregation. The microscopic basis for thixotropy is that the aggregation rate and the rate of depolymerization of solid particles are not equal at any time. If the deagglomeration process is much faster than the agglomeration process, it means that the semi-solid slurry produced by the rheological route is smaller than the semi-solid slurry produced by thixotropy heating, and the solid particles are more spherical. And the degree of aggregation is lower. Therefore, it has better rheological characteristics, good filling ability, and is more suitable for die casting. Compared with traditional die-casting parts, the semi-solid parts have obvious heat-treating properties. Because in the traditional liquid die-casting process, the alloy liquid is entrapped in a high-speed turbulent filling cavity, and the gas generated in the cavity and the coating gas cannot be effectively eliminated during forming, and is dissolved in the alloy under high pressure. Or form a diffuse distribution of high pressure micro pores. The gas and micro-pores are precipitated and expanded at high temperatures, causing deformation of the die casting and bubbling of the surface, which will result in the inability of the traditional liquid die-cast parts to be strengthened by the heat treatment. In the semi-solid forming technique, semi-solid forming blanks generally have a solid fraction of 40% to 60%, and the forming temperature is about 100° C. lower than that of liquid forming. In addition, the semi-solid forming blanks fill the cavity in a laminar manner when semi-solid filling is performed. This greatly reduces the defects such as subcutaneous pores and shrinkage and increases the density. The literature shows that the porosity of the microstructure of AZ91D semi-solid molded parts is significantly reduced, and the porosity is reduced by at least 50%. Compared with traditional die-casting magnesium alloy parts, the semi-solid molded parts have a porosity of 0.4%-0.8%, while the former has reached 2.5%-3.0%. Therefore, heat treatment has obvious feasibility for improving the mechanical properties of semi-solid molded parts. Especially rheological die-casting, its microstructure is more dense, and after heat treatment, its strengthening phase is more dispersed and uniform, making its heat-treatability more outstanding.
In terms of energy consumption, rheo-diecasting does not require thixo-reheating for thixotropic die-casting, which will not cause surface oxidation and flow-through during secondary heating, but also reduce additional processing costs; rheological die-casting pastes can be Thixotropic die-casting slurries have a higher liquid-phase ratio, so that they have better filling ability and can form more complex parts; die-casting surplus materials, casting system materials, waste materials, and stream soup can be immediately reused. It is not necessary to return to the billet preparation plant or the billet supply production plant like a thixotropic one. This will not only make full use of waste materials, save costs, but also reduce energy consumption. In short, rheo-diecasting has the advantages of more energy-saving than thixotropic die-casting, better performance of formed parts and shorter process flow. Therefore, rheo-diecasting will be an important development direction in semi-solid metal forming technology.
3. New Progress in Rheological Die Casting 3.1 Low Superheat Pulp Rheological Die Casting Recently, a number of rheoforming techniques based on the mechanism of "spheroidal direct growth" semi-solid tissue growth have emerged. The theory is that pouring at low superheat conditions can nucleate a large amount in the alloy melt; at a certain cooling rate, as long as the alloy forms enough nucleus at the initial stage of solidification, it can be directly obtained from the melt of the alloy. Nearly spherical or spherical tissue without the need for dendritic spheroidization. The new rheology technologies based on this theory are: low superheat tilted casting rheocasting, low superheat casting, weak mechanical stirring rheocasting, low superheat pouring and weak electromagnetic stirring rheology. Die casting and so on.
3.1.1 Low Superheat Tilt Plate Casting Flow Formed Low-Superheat Tilt Plate Casting rheoforming technology was developed by Japan UBE, called New Rheocasting, or NRC for short, and its process is shown in Figure 1. . First, the superheat of the cast alloy is reduced, the alloy liquid is poured on a tilting plate, the alloy melt flows to collect the crucible, and then solidified by proper cooling; at this time, a spherical primary solid phase is generated in the molten metal, and uniformly distributed in the low melting point. In the residual liquid phase, the temperature is finally adjusted to obtain a uniform temperature field, and the semi-solid alloy slurry collected in the crucible can be sent to a die casting machine. The quality of the slurry of this technology is difficult to guarantee. UBE has improved its cooling method and post-processing and developed the UNRC process, as shown in Figure 2. In this process, the alloy melt is injected into the crucible first, and the cooling rate is controlled by blowing air around the crucible, so that the primary near-spherical particles are evenly distributed, and the semi-solid slurry is initially obtained; then, the temperature is adjusted and the mechanical turnover is as uniform as possible. Organize; Finally, the semi-solid slurry is sent into the die casting machine die casting.
Nanchang University developed a shear-casting, semi-solid, rheo-diecasting process on the basis of a pouring-type rheology at low superheat. This process is to melt the alloy melt with a certain degree of superheat into the conveying pipe under the action of gravity and the rotation of the conveying pipe. At the end of the conveying pipe, the temperature of the melt is controlled at 2 to 5°C below the liquidus, and the flow Human slurry accumulator, in this case the alloy melt has a large number of small primary crystals. The melt is slowly cooled to the desired temperature and then poured into the injection chamber of the die casting machine for die casting.
3.1.2 Low Superheat Pouring and Weak Mechanically Stirred Rheological Diecasting Low superheat casting and weak mechanical agitation rheocasting technology is SSR (Semi Solid Rheocasting) technology, as shown in Fig. 3. The technical principle is that the low superheat alloy slurry is poured into the preparation crucible, and the alloy liquid is subjected to short-term mechanical agitation using the coated copper rod, and the temperature of the alloy slurry is reduced below the liquidus line to make the alloy. The semi-solid slurry is at a suitable temperature, and the alloy slurry is finally cast into the injection chamber of a die casting machine.
3.1.3 Low Superheat Pouring and Weak Electromagnetic Stirred Rheological Die Casting The process flow of low superheat casting and weak electromagnetic stirrer rheological die casting is to cast the low superheated alloy melt into the preparation crucible, using electromagnetic stirring technology. The alloy melt exerts a weak shearing action while cooling the melt in the crucible to a suitable temperature. Finally, the semi-solid slurry in the crucible is poured into the injection chamber of the die casting machine for die casting.
3.2. Injection chamber rheology die casting In the face of semi-solid slurry storage and transportation problems, Hitachi Metals Co., Ltd. of Japan proposed a semi-solid slurry prepared in the injection chamber, and then directly die-casting method, as shown in Fig. 4 Show. The alloy melt is injected into the injection chamber, and under the action of the injection-proof outdoor electromagnetic stirrer, shearing is effected on the alloy melt, and at the same time, the alloy is cooled to a suitable temperature to obtain a suitable semi-solid slurry, which is then directly die casted. However, in the process of forming, the protection of the alloy melt is a problem to be solved. Shibata et al. use an electromagnetic pump and a hot suction pipe to directly send the alloy liquid in the melting furnace to the die casting machine to avoid contact with air, and then further protect it by argon gas. Reduce the oxidation inclusions in the slurry, as shown in Figure 5.
3.3 Twin-helix rheological die-casting technology Twin-helix rheological die-casting is developed on the basis of rheological casting. Solid-state rheological casting is a new technology that combines the plastic forming process with semi-solid technology. The earliest research on rheological injection molding was Wang KK of Concell University in the United States. The structure of the rheological injection molding they studied is shown in Figure 6. After the liquid alloy enters the cylinder, it is subjected to continuous mechanical agitation of the spiral during the downward process, and simultaneously cooled to obtain a semi-solid slurry. When the slurry has accumulated to a certain amount, it is injection molded by the injection device. On this basis, Fan and Bevis of the Brunel University in the United Kingdom proposed a double screw mechanical stirring rheological casting process, as shown in Figure 7. This double-screw stirring mechanism makes the movement of the metal liquid very unique: the metal flows outside the spiral in an "8" shape, and from one slope to the other slope, it spirals in an "8" shape to push the metal along the spiral axis Flow, from one helix to another, metal undergoes a cycle of stretching, folding, and adjustment. In addition, due to the periodic variation of the spiral and cylinder gaps, the shear rate of the metal is periodically changed (the minimum shear rate occurs at the root of the thread, and the maximum shear rate occurs at the meshing interval of the double helix). This gives the alloy melt a higher periodic shear deformation and a higher turbulence intensity. Under forced convection conditions, the metal is sufficiently cold. Due to the violent stirring effect, the high melting point metal melt is dispersed, the potential nucleation sites are increased, the nucleation rate is increased, and the initial crystal grains are refined. As the shear rate and turbulence intensity increase, the crystallites form spherical crystals from rosettes, thereby obtaining a semi-solid microstructure of fine uniform spherical crystals.
In 2001, Huazhong University of Science and Technology Luo Jirong et al. "improved the twin-screw structure by changing the threads of the two screws from the original single lead, single staggered angle to multiple lead, multiple staggered column angles, thus making the slurry Different temperature stages are subjected to different shearing actions to make the resulting semi-solid structure more ideal.Fan et al. developed a double-screw mechanically agitated rheo-diecasting process on this basis, but it is still in the experimental research stage.
3.4 Other New Types of Rheological Die Casting Technology Beijing University of Science and Technology has developed a new type of rheology-shaped device, a cone-barrel rheology, which combines semi-solid slurry preparation with die-casting, as shown in the figure. 8 shows. The agitator is composed of an inner barrel and a outer barrel. The two barrels are cone-shaped. The two are flip-mounted and coaxial and rotate under the upper motor. The prepared alloy melt is poured into a stirring device with a double barrel structure. When the alloy melt passes through the gap between the two barrels under the rotation of the outer wall of the inner barrel and the inner wall of the outer barrel, the alloy is severely sheared. Dissolved in a stirrer while cooling, in order to produce a fine grain, uniform organization of semi-solid tissue. A certain amount of semi-solid slurry is injected into the injection chamber.
Korean scholar Hong Junhao proposed a new electromagnetic stirring rheocasting system, as shown in Figure 9. The system starts electromagnetic stirring before the alloy liquid is fed into the pressure chamber, which can not only promote nucleation, shorten pulping time, improve die casting efficiency, but also can effectively stir the alloy liquid between the center and the inner wall of the pressure chamber to make heat flow. The transmission is sufficient and the temperature is even, so that the slurry composition of each part is uniform, and the occurrence of dendrite due to chilling at the inner wall of the pressure chamber is suppressed.
Mao Weimin proposed two-stage electromagnetic stirring slurry rheological casting system. The process is as follows: the molten alloy of low superheat degree (usually overheated 5-30°C) is poured into the container and weak electromagnetic stirring is applied for a short time. A nearly spherical primary crystal is preliminarily obtained in the alloy. After further cooling or heat preservation, the spherical primary crystal is further rounded to obtain an excellent semi-solid slurry. Finally, the rounded slurry is sent to the pressure chamber for die casting. Yidela's SSR technology developed by the Yidela Group of Companies integrates the SSR workstation into the die-casting unit, making it easy to use conventional alloys, existing die-casting equipment and processes. The PLC in the SSR workstation calculates the time required for agitation of the molten metal through the various process parameter variables detected in order to produce a stable slurry. This method is characterized by easy and accurate control, low solid phase rate (less than 20%), and can reduce the subsequent casting processing (such as infiltration), which greatly reduces the cost.
IV. Problems faced by rheological die-casting applications The main problems that currently limit the development of rheo-diecasting are still the preparation, storage, and transportation of semi-solid slurry. In the preparation of semi-solid slurry, there are currently many methods, such as electromagnetic stirring method, deformation induced mutagenic activation method, ultrasonic vibration method, single-roller rotation method, grain refining method, spray deposition method, powder metallurgy method, low superheat Casting method and turbulence effect method, but the commonly used electromagnetic induction method and deformation induced mutagenic activation method, not only in the pulping there are deficiencies, and after the pulping will be facing the slurry storage and transportation problems.
The advantages of electromagnetic stirring pulping are that the electromagnetic parameters are easy to control, and the slurry does not directly contact the stirring device to cause pollution, but also can extend the service life of the pulping device and facilitate the automatic pulping. However, the electromagnetic stirring method also has its disadvantages. Due to the uneven distribution of the magnetic field, the shear force generated in the overall slurry is uneven, especially in the center region, which causes the primary particles of the semi-solid slurry to have different sizes. And some of the pellets are also poor in roundness, and can also generate gas, which affects the quality of the semi-solid slurry. The common practice at present is to change the distribution of the windings and the solution of the coil to equalize the magnetic field. In the center region of the slurry, the core rod is added to reduce the depth of the liquid hole or eliminate the liquid hole, but this has strict requirements on the selection of the core rod. . Deformation mutagenesis activation method has shown its great advantages in the semi-solid thixoforming, such as the production of clean billet, high productivity, but the raw materials need a large extrusion deformation, and the prepared semi-solid slurry is mostly small diameter .
Both the pulping method and the die-casting method must solve the storage and transportation of the slurry. There are generally two ideas for these two problems. First, pulping and die-casting are performed separately. After the semi-solid slurry is prepared, the semi-solid slurry is filled with a crucible or other container having a temperature control function through an intermediate transportation process, and then injected into the injection chamber of the die casting machine. However, in this process, the semi-solid slurry is loaded and oxidized, the temperature setting and time during transportation must be strictly controlled, and the intermediate process also complicates the overall process. The complicated structure of the dispensing container also increases the cost. When injecting slurry into the injection chamber by the dispensing container, additional measures such as shielding gas are needed to prevent oxidation and gas entrainment of the slurry, complicating the structure of the corresponding injection chamber. This series of problems will affect Rheological die casting automation. Another idea is to synchronize pulping and die casting. One approach is to pass the semi-solid slurry into the injection chamber under its own gravity or other external force (as described above), but this must be solved by the infusion tube. Structure, material selection, temperature control of the slurry in the infusion process, setting of the interface mechanism of the infusion tube and the injection chamber; another approach is to prepare a semi-solid slurry in the injection chamber, and then die-cast directly (as described above), but this The special injection chamber required for this practice not only needs sufficient capacity, but also must quantify the semi-solid slurry required for each die casting. This makes the structure of the injection chamber complex, the cost increase, and every specific time Need to open the shot chamber charge, which is not conducive to automation.
In addition, the materials used in rheological die-casting are also relatively limited, and the current suitable rheocast die-cast alloys are mainly aluminum alloys and magnesium alloys in non-ferrous metals. Due to the lower melting point of these alloys and the relatively low requirements for pulping equipment, the study is relatively extensive compared to other alloys; the die casting equipment used is also mostly improved on traditional die casting machines, but it is difficult to meet the rheological die casting. The use of scale, such as injection pressure and enhanced pressure can not meet the requirements, the injection chamber is not complete. Therefore, in order to broaden the field of rheological die casting, it is necessary to develop a series of alloys suitable for semi-solid forming, and to increase the research and investment in semi-solid slurry manufacturing equipment and die casting equipment for steel and other materials.
In summary, due to the outstanding problems in rheological die-casting, it is very limited to apply rheo-diecasting to the actual field, and most of them still remain in the laboratory research stage.
V. Looking to the rheological die-casting industry, there are many advantages. This has become the consensus of the people and will certainly be the direction of future development of semi-solid forming, but it must also face the difficulties that exist. Therefore, we must update our thinking on pulping methods, slurry transportation, die casting machines, etc., and we must develop new alloy systems that are suitable for semi-solid forming, such as the development of new pulping methods to synchronize with existing die-casting equipment. We have developed die casting machines for rheological die casting to promote the development and application of rheo-diecasting. Recent studies have found that composite pulping produces finer, rounder, more uniform distribution and better slurry quality than primary pellets produced by a single method. Wang Ping found that the slurry prepared by the near-liquidus electromagnetic stirring method has finer and more uniform equiaxed grains than the slurry produced by the near-liquidus method and the electromagnetic stirring method. It can be seen that the slurry prepared by this method is half. Solid tissue performance is better. Nanchang University, based on the low superheat inclined plate pouring rheocasting technology, developed a shearing low-temperature casting semi-solid rheo-diecasting process. This process is: Under a certain degree of superheat, the alloy melt enters the slurry accumulator through the conveying pipe, the melt is slowly cooled in the slurry accumulator, and a suitable semi-solid slurry is obtained, and then the man-made die casting machine is Injection chamber for die casting. Feng Pengfa et al. studied the bidirectional multi-speed electromagnetic stirring device and developed a rheology technique for the separation of aluminum alloy from a slurry preparation system and a workpiece forming system. This method separates the preparation and the forming of the slurry, thereby making the structure simple and easy to implement. These practices have provided new ideas for semi-solid pulping. The die casting machine dedicated to rheology differs from traditional liquid die casting machines in that the apparent viscosity of the semi-solid metal is much higher than that of the liquid metal, and the apparent viscosity continues to change as the filling process progresses. Great resistance. Therefore, the rheological die casting machine should have higher injection pressure and enhanced pressure than the traditional liquid die casting machine. At the same time, it should have the ability to digitally control the injection pressure and the injection speed. The injection curve can be arbitrarily changed to meet the stability stratigraphic flow. Fill the cavity and reduce turbulent gassing. In addition, a special die-casting chamber for placing semi-solid metal blanks should also be provided. At present, the H-630SC thixotropic die-casting machine manufactured by Buhler of Switzerland adopts a large shot-spray cylinder, which can generate high injection pressure and enhanced pressure, and control the injection process through the central control valve and control loop; the US EPCO Thixotropic die casting machines manufactured by the company are mainly used for automotive die castings; In addition, Italpresse of America, Prince Machine Corp, etc. also manufacture die casting machines for semi-solid thixotropic die casting.
At present, energy shortages and environmental pollution are increasingly serious. For this reason, it is possible to develop and apply a large number of new energy-saving and environmental-friendly materials, and it is expected that more energy-saving forming processes will be used. This brings good opportunities for the development and application of non-ferrous metal materials, especially light-weight magnesium alloys and aluminum alloys, and also brings opportunities for the research and development of rheo-diecasting. China is in a critical stage of accelerated development, with large energy demand and environmental issues. However, there is still a large gap between the development and application of energy-saving and environmental protection materials and developed countries and regions. With the development of our country's economy and society, it is imperative to increase the investment in the research and development of new materials and new processes. Therefore, new materials and new processes suitable for semi-solid forming will emerge in an endless stream and the development prospects are promising.
I. Rheological Technology Introduction Semi-solid forming technology originated from rheology. However, due to the difficulties in the storage and transportation of semi-solid billets in the middle of the flow, the research and application of semi-solid forming technology are relatively stagnant. Therefore, thixotropic development is relatively rapid, but after many years of research and practice, it has been found that thixoforming has the following process problems: (1) The cost of the semi-solid aluminum alloy billet is high, and is about 40% higher when the billet is prepared. (2) traditional electromagnetic stirring power, low efficiency, high energy consumption; (3) traditional electromagnetic induction remelting heating high energy consumption, the surface of the billet oxidation is serious, and the billet will always lose part of the metal heating, accounting for the billet 5% to 12% of the mass; (4) The liquid fraction of the billet must not be too high, otherwise the formation of very complex parts is difficult; (5) Sawdust, billet reflow, the flow of the soup, pouring system (accounting for the quality of the billet 10% to 20%) and waste products (all return materials account for about 40% to 50% of the billet quality) cannot be recycled immediately, and must be returned to the billet preparation plant or billet supply production plant, increasing the cost of forming production; (6) Billets The composition, grain shape, and non-uniform grain distribution directly affect slurry quality. As a result, the flow became re-emphasized. The rheology is that the prepared semi-solid slurry is directly sent to a forming apparatus for forming without undergoing secondary heating remelting. Rheological die-casting is a kind of net final forming method that transfers the prepared semi-solid metal slurry directly to the injection chamber for die casting. Not only has the semi-solid forming casting less solidification shrinkage, and the formation is not easy to gas, so the casting is dense, can be general characteristics of heat treatment and strengthening, and compared with other semi-solid forming, the grain is finer, there is no macro segregation, and the performance is more excellent. Therefore, the rheology technique is widely expected.
Second, the advantages of rheological die-casting From the semi-solid slurry structure, semi-solid alloy forming is based on semi-solid non-dendritic rheology and thixotropy, but this feature is determined by its microstructure. It is considered that for a given semi-solid alloy slurry, the degree of aggregation of solid particles determines the apparent viscosity of the system, and the shear rate and shear time indirectly affect the viscosity by changing the degree of aggregation. The microscopic basis for thixotropy is that the aggregation rate and the rate of depolymerization of solid particles are not equal at any time. If the deagglomeration process is much faster than the agglomeration process, it means that the semi-solid slurry produced by the rheological route is smaller than the semi-solid slurry produced by thixotropy heating, and the solid particles are more spherical. And the degree of aggregation is lower. Therefore, it has better rheological characteristics, good filling ability, and is more suitable for die casting. Compared with traditional die-casting parts, the semi-solid parts have obvious heat-treating properties. Because in the traditional liquid die-casting process, the alloy liquid is entrapped in a high-speed turbulent filling cavity, and the gas generated in the cavity and the coating gas cannot be effectively eliminated during forming, and is dissolved in the alloy under high pressure. Or form a diffuse distribution of high pressure micro pores. The gas and micro-pores are precipitated and expanded at high temperatures, causing deformation of the die casting and bubbling of the surface, which will result in the inability of the traditional liquid die-cast parts to be strengthened by the heat treatment. In the semi-solid forming technique, semi-solid forming blanks generally have a solid fraction of 40% to 60%, and the forming temperature is about 100° C. lower than that of liquid forming. In addition, the semi-solid forming blanks fill the cavity in a laminar manner when semi-solid filling is performed. This greatly reduces the defects such as subcutaneous pores and shrinkage and increases the density. The literature shows that the porosity of the microstructure of AZ91D semi-solid molded parts is significantly reduced, and the porosity is reduced by at least 50%. Compared with traditional die-casting magnesium alloy parts, the semi-solid molded parts have a porosity of 0.4%-0.8%, while the former has reached 2.5%-3.0%. Therefore, heat treatment has obvious feasibility for improving the mechanical properties of semi-solid molded parts. Especially rheological die-casting, its microstructure is more dense, and after heat treatment, its strengthening phase is more dispersed and uniform, making its heat-treatability more outstanding.
In terms of energy consumption, rheo-diecasting does not require thixo-reheating for thixotropic die-casting, which will not cause surface oxidation and flow-through during secondary heating, but also reduce additional processing costs; rheological die-casting pastes can be Thixotropic die-casting slurries have a higher liquid-phase ratio, so that they have better filling ability and can form more complex parts; die-casting surplus materials, casting system materials, waste materials, and stream soup can be immediately reused. It is not necessary to return to the billet preparation plant or the billet supply production plant like a thixotropic one. This will not only make full use of waste materials, save costs, but also reduce energy consumption. In short, rheo-diecasting has the advantages of more energy-saving than thixotropic die-casting, better performance of formed parts and shorter process flow. Therefore, rheo-diecasting will be an important development direction in semi-solid metal forming technology.
3. New Progress in Rheological Die Casting 3.1 Low Superheat Pulp Rheological Die Casting Recently, a number of rheoforming techniques based on the mechanism of "spheroidal direct growth" semi-solid tissue growth have emerged. The theory is that pouring at low superheat conditions can nucleate a large amount in the alloy melt; at a certain cooling rate, as long as the alloy forms enough nucleus at the initial stage of solidification, it can be directly obtained from the melt of the alloy. Nearly spherical or spherical tissue without the need for dendritic spheroidization. The new rheology technologies based on this theory are: low superheat tilted casting rheocasting, low superheat casting, weak mechanical stirring rheocasting, low superheat pouring and weak electromagnetic stirring rheology. Die casting and so on.
3.1.1 Low Superheat Tilt Plate Casting Flow Formed Low-Superheat Tilt Plate Casting rheoforming technology was developed by Japan UBE, called New Rheocasting, or NRC for short, and its process is shown in Figure 1. . First, the superheat of the cast alloy is reduced, the alloy liquid is poured on a tilting plate, the alloy melt flows to collect the crucible, and then solidified by proper cooling; at this time, a spherical primary solid phase is generated in the molten metal, and uniformly distributed in the low melting point. In the residual liquid phase, the temperature is finally adjusted to obtain a uniform temperature field, and the semi-solid alloy slurry collected in the crucible can be sent to a die casting machine. The quality of the slurry of this technology is difficult to guarantee. UBE has improved its cooling method and post-processing and developed the UNRC process, as shown in Figure 2. In this process, the alloy melt is injected into the crucible first, and the cooling rate is controlled by blowing air around the crucible, so that the primary near-spherical particles are evenly distributed, and the semi-solid slurry is initially obtained; then, the temperature is adjusted and the mechanical turnover is as uniform as possible. Organize; Finally, the semi-solid slurry is sent into the die casting machine die casting.
Nanchang University developed a shear-casting, semi-solid, rheo-diecasting process on the basis of a pouring-type rheology at low superheat. This process is to melt the alloy melt with a certain degree of superheat into the conveying pipe under the action of gravity and the rotation of the conveying pipe. At the end of the conveying pipe, the temperature of the melt is controlled at 2 to 5°C below the liquidus, and the flow Human slurry accumulator, in this case the alloy melt has a large number of small primary crystals. The melt is slowly cooled to the desired temperature and then poured into the injection chamber of the die casting machine for die casting.
3.1.2 Low Superheat Pouring and Weak Mechanically Stirred Rheological Diecasting Low superheat casting and weak mechanical agitation rheocasting technology is SSR (Semi Solid Rheocasting) technology, as shown in Fig. 3. The technical principle is that the low superheat alloy slurry is poured into the preparation crucible, and the alloy liquid is subjected to short-term mechanical agitation using the coated copper rod, and the temperature of the alloy slurry is reduced below the liquidus line to make the alloy. The semi-solid slurry is at a suitable temperature, and the alloy slurry is finally cast into the injection chamber of a die casting machine.
3.1.3 Low Superheat Pouring and Weak Electromagnetic Stirred Rheological Die Casting The process flow of low superheat casting and weak electromagnetic stirrer rheological die casting is to cast the low superheated alloy melt into the preparation crucible, using electromagnetic stirring technology. The alloy melt exerts a weak shearing action while cooling the melt in the crucible to a suitable temperature. Finally, the semi-solid slurry in the crucible is poured into the injection chamber of the die casting machine for die casting.
3.2. Injection chamber rheology die casting In the face of semi-solid slurry storage and transportation problems, Hitachi Metals Co., Ltd. of Japan proposed a semi-solid slurry prepared in the injection chamber, and then directly die-casting method, as shown in Fig. 4 Show. The alloy melt is injected into the injection chamber, and under the action of the injection-proof outdoor electromagnetic stirrer, shearing is effected on the alloy melt, and at the same time, the alloy is cooled to a suitable temperature to obtain a suitable semi-solid slurry, which is then directly die casted. However, in the process of forming, the protection of the alloy melt is a problem to be solved. Shibata et al. use an electromagnetic pump and a hot suction pipe to directly send the alloy liquid in the melting furnace to the die casting machine to avoid contact with air, and then further protect it by argon gas. Reduce the oxidation inclusions in the slurry, as shown in Figure 5.
3.3 Twin-helix rheological die-casting technology Twin-helix rheological die-casting is developed on the basis of rheological casting. Solid-state rheological casting is a new technology that combines the plastic forming process with semi-solid technology. The earliest research on rheological injection molding was Wang KK of Concell University in the United States. The structure of the rheological injection molding they studied is shown in Figure 6. After the liquid alloy enters the cylinder, it is subjected to continuous mechanical agitation of the spiral during the downward process, and simultaneously cooled to obtain a semi-solid slurry. When the slurry has accumulated to a certain amount, it is injection molded by the injection device. On this basis, Fan and Bevis of the Brunel University in the United Kingdom proposed a double screw mechanical stirring rheological casting process, as shown in Figure 7. This double-screw stirring mechanism makes the movement of the metal liquid very unique: the metal flows outside the spiral in an "8" shape, and from one slope to the other slope, it spirals in an "8" shape to push the metal along the spiral axis Flow, from one helix to another, metal undergoes a cycle of stretching, folding, and adjustment. In addition, due to the periodic variation of the spiral and cylinder gaps, the shear rate of the metal is periodically changed (the minimum shear rate occurs at the root of the thread, and the maximum shear rate occurs at the meshing interval of the double helix). This gives the alloy melt a higher periodic shear deformation and a higher turbulence intensity. Under forced convection conditions, the metal is sufficiently cold. Due to the violent stirring effect, the high melting point metal melt is dispersed, the potential nucleation sites are increased, the nucleation rate is increased, and the initial crystal grains are refined. As the shear rate and turbulence intensity increase, the crystallites form spherical crystals from rosettes, thereby obtaining a semi-solid microstructure of fine uniform spherical crystals.
In 2001, Huazhong University of Science and Technology Luo Jirong et al. "improved the twin-screw structure by changing the threads of the two screws from the original single lead, single staggered angle to multiple lead, multiple staggered column angles, thus making the slurry Different temperature stages are subjected to different shearing actions to make the resulting semi-solid structure more ideal.Fan et al. developed a double-screw mechanically agitated rheo-diecasting process on this basis, but it is still in the experimental research stage.
3.4 Other New Types of Rheological Die Casting Technology Beijing University of Science and Technology has developed a new type of rheology-shaped device, a cone-barrel rheology, which combines semi-solid slurry preparation with die-casting, as shown in the figure. 8 shows. The agitator is composed of an inner barrel and a outer barrel. The two barrels are cone-shaped. The two are flip-mounted and coaxial and rotate under the upper motor. The prepared alloy melt is poured into a stirring device with a double barrel structure. When the alloy melt passes through the gap between the two barrels under the rotation of the outer wall of the inner barrel and the inner wall of the outer barrel, the alloy is severely sheared. Dissolved in a stirrer while cooling, in order to produce a fine grain, uniform organization of semi-solid tissue. A certain amount of semi-solid slurry is injected into the injection chamber.
Korean scholar Hong Junhao proposed a new electromagnetic stirring rheocasting system, as shown in Figure 9. The system starts electromagnetic stirring before the alloy liquid is fed into the pressure chamber, which can not only promote nucleation, shorten pulping time, improve die casting efficiency, but also can effectively stir the alloy liquid between the center and the inner wall of the pressure chamber to make heat flow. The transmission is sufficient and the temperature is even, so that the slurry composition of each part is uniform, and the occurrence of dendrite due to chilling at the inner wall of the pressure chamber is suppressed.
Mao Weimin proposed two-stage electromagnetic stirring slurry rheological casting system. The process is as follows: the molten alloy of low superheat degree (usually overheated 5-30°C) is poured into the container and weak electromagnetic stirring is applied for a short time. A nearly spherical primary crystal is preliminarily obtained in the alloy. After further cooling or heat preservation, the spherical primary crystal is further rounded to obtain an excellent semi-solid slurry. Finally, the rounded slurry is sent to the pressure chamber for die casting. Yidela's SSR technology developed by the Yidela Group of Companies integrates the SSR workstation into the die-casting unit, making it easy to use conventional alloys, existing die-casting equipment and processes. The PLC in the SSR workstation calculates the time required for agitation of the molten metal through the various process parameter variables detected in order to produce a stable slurry. This method is characterized by easy and accurate control, low solid phase rate (less than 20%), and can reduce the subsequent casting processing (such as infiltration), which greatly reduces the cost.
IV. Problems faced by rheological die-casting applications The main problems that currently limit the development of rheo-diecasting are still the preparation, storage, and transportation of semi-solid slurry. In the preparation of semi-solid slurry, there are currently many methods, such as electromagnetic stirring method, deformation induced mutagenic activation method, ultrasonic vibration method, single-roller rotation method, grain refining method, spray deposition method, powder metallurgy method, low superheat Casting method and turbulence effect method, but the commonly used electromagnetic induction method and deformation induced mutagenic activation method, not only in the pulping there are deficiencies, and after the pulping will be facing the slurry storage and transportation problems.
The advantages of electromagnetic stirring pulping are that the electromagnetic parameters are easy to control, and the slurry does not directly contact the stirring device to cause pollution, but also can extend the service life of the pulping device and facilitate the automatic pulping. However, the electromagnetic stirring method also has its disadvantages. Due to the uneven distribution of the magnetic field, the shear force generated in the overall slurry is uneven, especially in the center region, which causes the primary particles of the semi-solid slurry to have different sizes. And some of the pellets are also poor in roundness, and can also generate gas, which affects the quality of the semi-solid slurry. The common practice at present is to change the distribution of the windings and the solution of the coil to equalize the magnetic field. In the center region of the slurry, the core rod is added to reduce the depth of the liquid hole or eliminate the liquid hole, but this has strict requirements on the selection of the core rod. . Deformation mutagenesis activation method has shown its great advantages in the semi-solid thixoforming, such as the production of clean billet, high productivity, but the raw materials need a large extrusion deformation, and the prepared semi-solid slurry is mostly small diameter .
Both the pulping method and the die-casting method must solve the storage and transportation of the slurry. There are generally two ideas for these two problems. First, pulping and die-casting are performed separately. After the semi-solid slurry is prepared, the semi-solid slurry is filled with a crucible or other container having a temperature control function through an intermediate transportation process, and then injected into the injection chamber of the die casting machine. However, in this process, the semi-solid slurry is loaded and oxidized, the temperature setting and time during transportation must be strictly controlled, and the intermediate process also complicates the overall process. The complicated structure of the dispensing container also increases the cost. When injecting slurry into the injection chamber by the dispensing container, additional measures such as shielding gas are needed to prevent oxidation and gas entrainment of the slurry, complicating the structure of the corresponding injection chamber. This series of problems will affect Rheological die casting automation. Another idea is to synchronize pulping and die casting. One approach is to pass the semi-solid slurry into the injection chamber under its own gravity or other external force (as described above), but this must be solved by the infusion tube. Structure, material selection, temperature control of the slurry in the infusion process, setting of the interface mechanism of the infusion tube and the injection chamber; another approach is to prepare a semi-solid slurry in the injection chamber, and then die-cast directly (as described above), but this The special injection chamber required for this practice not only needs sufficient capacity, but also must quantify the semi-solid slurry required for each die casting. This makes the structure of the injection chamber complex, the cost increase, and every specific time Need to open the shot chamber charge, which is not conducive to automation.
In addition, the materials used in rheological die-casting are also relatively limited, and the current suitable rheocast die-cast alloys are mainly aluminum alloys and magnesium alloys in non-ferrous metals. Due to the lower melting point of these alloys and the relatively low requirements for pulping equipment, the study is relatively extensive compared to other alloys; the die casting equipment used is also mostly improved on traditional die casting machines, but it is difficult to meet the rheological die casting. The use of scale, such as injection pressure and enhanced pressure can not meet the requirements, the injection chamber is not complete. Therefore, in order to broaden the field of rheological die casting, it is necessary to develop a series of alloys suitable for semi-solid forming, and to increase the research and investment in semi-solid slurry manufacturing equipment and die casting equipment for steel and other materials.
In summary, due to the outstanding problems in rheological die-casting, it is very limited to apply rheo-diecasting to the actual field, and most of them still remain in the laboratory research stage.
V. Looking to the rheological die-casting industry, there are many advantages. This has become the consensus of the people and will certainly be the direction of future development of semi-solid forming, but it must also face the difficulties that exist. Therefore, we must update our thinking on pulping methods, slurry transportation, die casting machines, etc., and we must develop new alloy systems that are suitable for semi-solid forming, such as the development of new pulping methods to synchronize with existing die-casting equipment. We have developed die casting machines for rheological die casting to promote the development and application of rheo-diecasting. Recent studies have found that composite pulping produces finer, rounder, more uniform distribution and better slurry quality than primary pellets produced by a single method. Wang Ping found that the slurry prepared by the near-liquidus electromagnetic stirring method has finer and more uniform equiaxed grains than the slurry produced by the near-liquidus method and the electromagnetic stirring method. It can be seen that the slurry prepared by this method is half. Solid tissue performance is better. Nanchang University, based on the low superheat inclined plate pouring rheocasting technology, developed a shearing low-temperature casting semi-solid rheo-diecasting process. This process is: Under a certain degree of superheat, the alloy melt enters the slurry accumulator through the conveying pipe, the melt is slowly cooled in the slurry accumulator, and a suitable semi-solid slurry is obtained, and then the man-made die casting machine is Injection chamber for die casting. Feng Pengfa et al. studied the bidirectional multi-speed electromagnetic stirring device and developed a rheology technique for the separation of aluminum alloy from a slurry preparation system and a workpiece forming system. This method separates the preparation and the forming of the slurry, thereby making the structure simple and easy to implement. These practices have provided new ideas for semi-solid pulping. The die casting machine dedicated to rheology differs from traditional liquid die casting machines in that the apparent viscosity of the semi-solid metal is much higher than that of the liquid metal, and the apparent viscosity continues to change as the filling process progresses. Great resistance. Therefore, the rheological die casting machine should have higher injection pressure and enhanced pressure than the traditional liquid die casting machine. At the same time, it should have the ability to digitally control the injection pressure and the injection speed. The injection curve can be arbitrarily changed to meet the stability stratigraphic flow. Fill the cavity and reduce turbulent gassing. In addition, a special die-casting chamber for placing semi-solid metal blanks should also be provided. At present, the H-630SC thixotropic die-casting machine manufactured by Buhler of Switzerland adopts a large shot-spray cylinder, which can generate high injection pressure and enhanced pressure, and control the injection process through the central control valve and control loop; the US EPCO Thixotropic die casting machines manufactured by the company are mainly used for automotive die castings; In addition, Italpresse of America, Prince Machine Corp, etc. also manufacture die casting machines for semi-solid thixotropic die casting.
At present, energy shortages and environmental pollution are increasingly serious. For this reason, it is possible to develop and apply a large number of new energy-saving and environmental-friendly materials, and it is expected that more energy-saving forming processes will be used. This brings good opportunities for the development and application of non-ferrous metal materials, especially light-weight magnesium alloys and aluminum alloys, and also brings opportunities for the research and development of rheo-diecasting. China is in a critical stage of accelerated development, with large energy demand and environmental issues. However, there is still a large gap between the development and application of energy-saving and environmental protection materials and developed countries and regions. With the development of our country's economy and society, it is imperative to increase the investment in the research and development of new materials and new processes. Therefore, new materials and new processes suitable for semi-solid forming will emerge in an endless stream and the development prospects are promising.
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