Development and Application of Semi-solid Metal Forming Technology

In the early 1970s, M. Flemigs and D. Spencer of the Massachusetts Institute of Technology (MIT) discovered that alloys in the solid-liquid phase show a low apparent viscosity after continuous stirring, and the dendrites formed during the crystallization process are granular. Crystal instead. This slurry is easily deformed, and it can be filled with a very small force to fill a complex cavity, thereby developing a new metal forming method - semi-solid metal forming. Semi-solid metal forming can be divided into rheology and thixoforming. The former uses the rheological properties of semi-solid metals to pressurize the strongly stirred metal slurry. The latter utilizes the thixotropy of metals to heat the solidified stirred metal slurry to a semi-solid state and press-form it. Semi-solid metal forming has the advantages of eliminating air holes and shrinkage holes, improving the mechanical properties of the parts and the life of the mold, reducing the solidification shrinkage, and improving the part dimensional accuracy. Semi-solid metals are easy to handle and transport, creating the conditions for continuous and efficient automation. It is also superior to traditional casting methods in energy saving and environmental protection. At present, semi-solid processing forming technology has been applied to production in the United States and Western Europe. The U.S. military has listed the manufacture of composite tank parts using rheocasting as one of the five-year process development plans. Under the support of 18 large-scale companies such as Kawasaki Steel, Japan established a specialized research institute from 1988 to 1994 to conduct a comprehensive study on semi-solid metal properties, manufacturing and processing technologies, and has now begun industrial production. The research on semi-solid metal forming technology in China is still in the laboratory stage and there is still a certain distance from industrial production.

1 Rheological properties of semi-solid metals <br> The rheological properties of semi-solid metals refer to the flow and deformation properties of semi-solid metals under external forces. The study of rheological properties of semi-solid metals has important guiding significance for the preparation and forming of semi-solid metals. When the composition of the solid metal particles in the molten metal is greater than 0.05 to 0.1, the rheological behavior is non-Newtonian. At higher solids fractions (0.5-0.6), the slurry exhibits a non-linear viscoplastic behavior with Binghan flow characteristics. Although the alloy composition, the manufacturing conditions of the semi-solid metal, and the shape and size of the solid phase all affect the rheological properties of the semi-solid metal, the number of solid components has the greatest influence on the rheological properties. The apparent viscosity of a semi-solid metal is generally used as an indicator of its rheological properties. Through the stirring test under certain shear deformation speed and cooling conditions, the apparent viscosity of semi-solid metal of aluminum, copper, and iron under different solid components was determined. See Fig. 1 and the viscosity formula of the suspension was used. The relationship between the apparent viscosity and the solid fraction was analyzed by regression analysis to obtain the expression of the apparent viscosity of the semi-solid metal as shown in formula (1) [1]:

Fig. 1 Relationship between solid fraction and apparent viscosity (curve is regression result)

(1)

Where: ηa—semi-solid metal apparent viscosity, Pa.s, ηLa—aluminum metal apparent viscosity (Pa.s), ρm—alloy density (kg.m-3), C—solidification rate, s-1, - shear deformation rate, s-1, fs-solid fraction.
Since the solid fraction in the semi-solid metal paste is mainly determined by the temperature of the semi-solid metal, the temperature control in practical applications is very important. The shear strain rate when the semi-solid metal is deformed also has a great influence on the apparent viscosity. The apparent viscosity of the semi-solid A356 aluminum alloy in a stable state was measured with a high-temperature rotational viscometer. The results are shown in FIG. 2 . The apparent steady-state viscosity can be expressed in formula (2) [2]:

Fig.2 The relationship between apparent viscosity and shear rate in A356 aluminum alloy slurry

(2)

Where: η - apparent viscosity, -Shear rate, C-consistency, m- is an exponent and its value is -1.2 to -1.3.
The above conditions are the results obtained when the stirring test is performed for several tens of minutes and the viscosity no longer changes and reaches a steady state. For continuous cooling, the apparent viscosity is slightly higher than the steady state. In the actual molding process, the semi-solid metal filling cavity only lasts for a few seconds. At this moment, due to the viscosity of the liquid phase, the number, size, and morphology of the solid particles are all changing, and the situation becomes very complicated. In [3], after testing tin-15% lead, it was pointed out that the instantaneous structural characteristics of the semi-solid slurry under a given structure is an increase in the apparent viscosity as the shear rate increases.
The stirred semi-solid metal slurry is solidified and then reheated to a semi-solid state. Due to the thixotropy of the semi-solid metal, when the shear rate is very small or equal to zero, the viscosity of the semi-solid metal is high and can be retained like a solid. When handling, and when it is subjected to higher shear stress, resulting in a larger shear rate, the viscosity drops rapidly and becomes as easy to form as a fluid. Like other thixotropic materials, semi-solid metal slurries also exhibit hysteresis loops, as shown in Figure 3. For semi-solid alloys with a dendritic primary crystal, when the solid fraction reaches about 0.3, it cannot flow, and the primary crystal shape is a nearly round semi-solid alloy, and even if the solid fraction exceeds 0.5, there is still mobility. Grain morphology during solidification has a major influence on rheology. In the production of semi-solid metal slurries, the stirring speed, the cooling rate, and the formation of non-dendritic structures with solid components have the effect shown in Figure 4 [4].

Fig. 3 Thixotropic phenomenon of shear stress and viscosity of semi-solid metal

Figure 4: Mechanism of formation of non-dendritic structures 2 Preparation of semi-solid metals Mechanistic or electromagnetic stirring methods are commonly used in the production of metal slurries. Using these two methods, the particle size of solid components can be in the range of 50 to 100 μm. Slurry. Figure 5 shows the continuous production of metal slurries using mechanical stirring [5]. For aluminum, copper alloys and cast iron, this method can achieve continuous production of a slurry with a solid fraction of 0.5. Mechanical agitation can also use shear cooling roller mode [6]. Compared with the mechanical stirring, the electromagnetic stirring method reduces the contamination of the slurry by the agitator. However, in the preparation of a slurry with a high solid fraction, the agitation speed sharply decreases, and the apparent viscosity rapidly increases, making it difficult to discharge the slurry. . Figure 6 shows an electromagnetic stirring device [7] using semi-solid metal to make aluminum matrix composites. The four pairs of magnetic poles in this device rotate at a speed of 0 to 3000 r/min. In order to generate a three-dimensional movement of the slurry, a deflection angle of 10[deg.] is provided between the magnet and the central axis of rotation, and is placed in a spiral shape. With this device, an A356 aluminum alloy as a matrix has been produced, and a composite ingot having 20 vol% SiC particles having an average particle size of 29 μm has been added. Figure 5 Mechanically agitated semi-solid metal manufacturing device

Figure 6 Electromagnetic stirring device for manufacturing aluminum matrix composites

Japan invented a method for preparing thixoforming blanks by adding 0.001% to 0.01% of B and 0.005% to 0.30% of Ti in an aluminum alloy containing 4% to 6% of Si. The heat does not exceed 30°C above the liquidus, and it is cooled in the solidification section at a cooling rate of 1.0°C/s or more to obtain a fine equiaxed crystal of 200 μm or less [8]. It is also possible to prepare semi-solid metal forming raw materials by means of strain-induced melt activation and the like.

3 Semi-solid metal forming and application <br> All kinds of alloys can be semi-solid metal forming processing as long as they have solidification and liquid-phase solidification intervals. Many experimental studies have been conducted on aluminum, magnesium, zinc, copper alloys and steels, cast irons, nickel-base superalloys, and composites. Currently applied alloys are directly taken from existing cast or forged alloy series. For example, aluminum alloys are 3XXX series aluminum silicon casting alloys and 2XXX, 7XXX series wrought alloys. The most widely used alloy is A356, which has a solidification range of about 60°C. Magnesium alloy is mainly AZ91D. Until now, little research has been done on the semi-solid forming alloys. In the United States and Western Europe, semi-solid forming of aluminum and magnesium alloys is mainly used for the production of automotive parts. Japan has done more research on semi-solid forming of ferrous metals.
3.1 Die Casting The current production mainly uses thixotropic die casting aluminum alloy castings, as shown in Figure 7. The most representative companies in Western Europe are Alusisse/Alusingen in Switzerland and Germany, Stampal in Italy, and Pechiney in France. The Alingen plant in Singer, Germany, was equipped with a 9800 kN die casting machine and a heating section capable of heating 12 billets at the same time. The production line was put into operation in 1996 and mainly produced automobile parts [9]. The mechanical properties of specimens cut from semi-solid metal die castings are shown in Table 1 [10]. In addition to the mass production of automotive parts using this method, Stampal also produces components for aerospace and aerospace applications. Its typical product is a fuel dispenser for the Ford Zeta engine. Concurrent Technologies Corp. (CTC) of Johnstown, Pennsylvania, United States, plays the role of the Department of Defense's National Center for Metal Processing and Manufacturing Technology (NCEMT). The mechanical properties of the company's A356 aluminum alloy castings produced using a thixotropic die casting method are σb = 315 MPa, σs = 266 MPa, and δ = 12% [9]. A challenging new research project is underway to produce titanium hydraulically actuated valves for the LPD-17 Amphibious Assault Ship. At present, the largest component produced by the semi-solid metal die-casting method has a mass of 6.7 kg and is a rear suspension component of the “European” car, which was put into operation in 1995. The automobile components that are die-casted by this method include a master brake cylinder, a rack-and-pinion-operated steering housing, steering tie rod heads, injection rails, and brackets. Thixotropic die-casting can also be performed on a vertical die-casting machine, and Japan has obtained a patent in this regard [11].

Figure 7 Flow chart of semi-solid metal die-casting

Table 1 Mechanical properties of cut specimens on semi-solid metal die castings

Japan Koji Co., Ltd. made a semi-solid AC4C alloy (Al-7%Si-0.3%Mg) by die casting using the rheology method. At this time, the mechanically agitated metal paste was placed in the refractory material. The vertical die casting machine is directly fed into the container. The dimensional accuracy and density of the resulting connecting rod are better than that of metal hydraulic casting [5]. Because semi-solid metal die casting can significantly reduce the temperature of the mold, it creates favorable conditions for the die casting of black solid metal. Tests have shown that cast iron can be die cast using rheological or thixotropic casting methods. The cast iron is melted at 1637K and then stirred at 500r/min to 1408K (65K below liquidus temperature) to make a semi-solid slurry, which can be successfully die cast to 100 x 150 x 6 (mm) on a 2450kN vertical die casting machine. ) The plate. Thixotropic die-casting forming, the cast iron billet is placed on the pressure chamber of a horizontal die casting machine, heated to a semi-solid state with a solid phase rate of 0.2, and cast into a casting with a wall thickness of 3 mm. The mechanical properties are shown in Table 2. [12]. Japan also invented related patents [13]. In addition to the above methods, different metal powders can also be compacted and heated to semi-solid die-cast parts [14].

Table 2 Mechanical properties of cast iron die castings

3.2 Forging semi-solid metal forging and semi-solid metal thixotropic die casting are essentially no significant difference, the main difference lies in semi-solid metal processing in the forging equipment. Forging semi-solid metals can be performed at a lower pressure, as shown in Fig. 8 [6]. This allows some of the traditional forge shape complex components to be produced using semi-solid metal forging methods. Leading the field in semi-solid metal forging is Alumax Engineered Metal Processes (AEMP), a subsidiary of Alumax, USA [9]. The US$75 million plant at Jackson, Tennessee, uses the company's proprietary semi-solid metal forging technology to produce 22,500 tons of high-quality automotive parts annually. The company recently built a factory to manufacture automotive parts in Bentonville, Arkansas, which is equipped with two complete semi-solid metal forging production lines. The production process is to cool the aluminum alloy liquid to a semi-solid state and stir it with a magnetic stirrer to cast a blank on a horizontal continuous casting machine with a grain diameter of about 30 μm. The cut blanks were inductively heated to a semi-solid state (solid phase rate of approximately 0.5) and forged on a vertical press. Forging speed of several hundred mm to more than one thousand mm per second, molding from a few MPa to more than ten MPa, or even higher. Heating, transporting, clamping and forging of materials are automated. The first semi-solid forgings produced by Alumax are Ford automotive air conditioning compressor front and rear housings. Alumax's semi-solid forged aluminum alloy rocker shaft supports were also used for the first time on the 214 HP 3.5L24 V-6 engine. Due to the reduction of machining, the unit price of the 357 aluminum alloy semi-solid forging support is 13 cents lower than that of the ductile iron. Another part replacing the ductile iron is a pulley pivot bracket, the weight of which is reduced from 0.31 kg of ductile iron to 0.16 kg. The use of semi-solid forged bushings and pulley mounting studs can be integrally formed in the pivot brackets. Each semi-solid forged aluminum part can save a cost of $2.15 [15] compared to the ball-iron parts.

Figure 8 Effect of solid fraction on resistance to compression deformation

3.3 Injection Molding In 1988, DOW Chemical Co of the United States invented a new semi-solid metal forming method that combines common die casting and injection molding. It eliminates the usual melting equipment and is a one-step forming process. The semi-solid magnesium alloy processing method has been patented. After 1990, Thixomat, Inc., an independent company founded in Ann Arbor, Michigan, engaged in the commercial operation of the technology. The second generation of equipment was put into use in October 1991. The 3920kN semi-solid thixotropic injection equipment manufactured by HPM is mainly composed of two parts. The mold locking mechanism of the mold is the same as that of an ordinary die casting machine, while the injection mechanism uses a screw type injection mechanism with an electric heating device. The principle is shown in Figure 9 [16]. The granular AZ91D magnesium alloy is fed into a multi-stage temperature-controlled cylinder through a feeder. To prevent oxidation from passing through argon from the feeder, the cylinder is equipped with a spiral stirrer that can move forwards and backwards. The cylinder is heated in both induction and resistance modes. The rotating screw conveys the raw material heated to the semi-solid forward, and the material is subjected to the shear force while mixing. After a certain amount of the semi-solid magnesium alloy enters the storage chamber in front of the spiral, the screw is advanced at a predetermined speed. During the movement, the metal slurry is injected into the mold cavity. After the injection is completed, the screw is returned to the original position. The production rate of this equipment is 123kg/h, and the maximum mass that can be produced is 1.5kg. For the AZ91D magnesium alloy shot temperature is 580 °C, 70 ~ 80 °C lower than the ordinary die-casting, the solid phase rate of the metal slurry is 0.3. It takes approximately 90 minutes for the device to reach the operating temperature from room temperature. The speed of the spiral injection is 250-380 cm/s, and the pressure of the semi-solid metal is 31-55 MPa. The equipment is controlled by a computer and the average energy consumption for a 1h run is about 29 kW. The dimensions of the parts produced by this method are precise and the performance is also superior to that of die-casting, see Table 3. The price of magnesium alloy parts is 10% lower than that of hot chamber die casting machines [17]. With this technology, more than 500,000 gearbox housings for adjusters have been produced, and parts such as industrial electronic display frames, hinged parts, and electronic instrument housings have been produced. With this technology, more than 40 parts have passed prototype tests in various fields of automotive, electronics and consumer products.

Figure 9 Magnesium alloy shotcrete thixotropic equipment

Table 3 Comparison of performance of magnesium alloy semi-solid shot and die casting

Method Alloys σs/MPa σb/MPa δ/% Method Parts Porosity/% Method Parts Porosity/% Die Cast AZ91 158 209 3.3 Die Cast Test Rod 3.2 Die Cast Gearbox Shell 3.4 Semi-Solid Shot AZ91 161 210 3.9 Semi-Solid Shot Test Rod 1.7 Semi-Solid Injection Transmission Case 1.4

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