Research and application of semi-solid metal forming technology

Professor Flemings of the Massachusetts Institute of Technology in the 1970s developed a new metal forming method called semi-solid processing technology [1]. In a paper by Flemings, it is reported that the metal material is strongly stirred during the solidification process, which can break up the dendritic network structure formed by solidification of the metal to form a nearly spherical structure, and obtain a liquid metal mother liquor uniformly suspended in a certain particle. The solid-liquid (solid phase component is generally 50%) mixed slurry of the solid phase component, the semi-solid metal at this time has excellent rheology and thixotropy [2,3]. Thus, it is easy to achieve the forming by conventional processing techniques such as die casting, extrusion, die forging, and the like. A method of forming a metal paste that is neither liquid nor completely solid is referred to as a semi-solid forming technique of metal. It can be seen that semi-solid processing is a method of forming a metal using a metal to solid-state transition or a solid-to-liquid transition (ie, liquid-solid coexistence) process. This new forming method combines the advantages of solidification processing and plastic processing, that is, the processing temperature is lower than that of the liquid, and the deformation resistance is smaller than the solid state, and the parts with complex shape and high precision and performance quality can be processed with one large deformation amount at a time, so Semi-solid processing technology is known as the most promising material forming method for the 21st century.

1. Formation mechanism of semi-solid metal

When the molten metal starts to crystallize, the crystal nucleus rapidly forms and grows with strong stirring. As the temperature decreases, although the grains are still grown by dendrite growth, due to the agitation, the grains are worn and sheared, and the liquid is strongly washed against the crystal grains, so that the dendrite arms are interrupted. , more fine grains are formed, and its own structure is gradually evolving. As the temperature continues to decline, this rose-shaped structure eventually evolves into a simpler spherical structure, and the evolution process is shown in Figure 1 [4].

Regarding the mechanism of semi-solid metal non-dendritic spheroidization, there is no conclusion yet. Flemings et al. believe that the formation of initial spherical particles is related to the following dendritic fracture mechanism [2, 5]:

(1) The mechanism of the root fracture of the dendritic arm. The dendrite arm breaks at the root due to the shear force. The initially formed dendrites are ideal crystals without dislocations and cuts, and the shear forces generated by the application of strong agitation cause the dendrite arms to break at the roots.
(2) The melting mechanism of the dendritic roots. The crystal grows during the normal growth of the surface area, and the liquid flow accelerates the diffusion in the liquid, causing thermal vibration and generating stress at the root to contribute to melting, which is favorable for fusing. At the same time, a higher mass fraction of solutes in the root solids will also lower the melting point, facilitating the action of this mechanism.
(3) Dendritic bending mechanism. This mechanism suggests that the dendrite arms are bent under flow stress, causing dislocations resulting in plastic deformation. At temperatures above the solidus line, the dislocations migrate and combine to form a grain boundary. When the orientation difference of adjacent grains exceeds 20°, and the grain boundary can exceed twice the solid-liquid interface energy, the liquid will Wetting the grain boundary and rapidly infiltrating along the grain boundary, separating the dendrite arm from the stem.

It should be noted that at the beginning of solidification, the liquid is vigorously stirred, and the detached dendrites are not spherical dendritic arms, and each dendrite arm continues to grow. However, as the shearing continues during the solidification process and the time increases, the dendrites become rose-like due to growth and shearing and abrasion with other grains, and the grains are crystallized during further cooling. Continue to deepen until the particles are sufficiently cold and at a high shear rate, the particles become spherical. As the shear rate, the amount of solidification increase, and the cooling rate decrease, the tendency of grains to change from dendritic shape to spherical shape increases.

2. Method for preparing semi-solid metal

The improvement and development of the preparation method is mainly the improvement and development of the manner in which the molten metal provides the stirring force during the solidification process. Since the 1970s, researchers at home and abroad have conducted relatively thorough research on semi-solid processing methods for aluminum alloys, magnesium alloys, and copper alloys, and successively developed a variety of semi-solid preparation methods with different characteristics [2,6 ~11].

2.1 Mechanical stirring method

This method was the earliest method used by the Massachusetts Institute of Technology Flemings et al. in the early 1970s. The working principle is as follows: during the solidification process of the molten metal in the stirring chamber, the stirring rod is used for vigorous stirring, and the dendritic crystal which has been solidified is fully broken, thereby forming a non-dendritic half in which the granular solid phase is uniformly distributed in the liquid phase. Solid slurry. The method has complicated structure, high cost and low yield, so this method is only suitable for laboratory research.

2.2 electromagnetic stirring method

The electromagnetic stirring method uses the alternating electromagnetic induction force to cause a violent flow of the metal being solidified in the stirring chamber, and the dendrites which are solidified and precipitated are continuously broken and spheroidized, thereby forming a non-dendritic half in which the granular solid phase is uniformly distributed in the liquid phase. Solid slurry. The method is not easy to be entangled with gas, does not pollute the metal liquid, the metal slurry is pure, and the semi-solid slurry can be continuously produced, and the output can be large, which is one of the main methods for producing aluminum alloy slurry at present.

2.3 strain activation method

The variable activation method is to continuously cast the ingots with fine grains in advance, and then to deform the metal ingots, and the deformation amount is large enough. Finally, the deformed metal ingots are cut into a certain size as needed, and then heated to the solid-liquid region. Recovery and recrystallization occur during heating to refine the grains, and then some of the grain boundaries are melted to obtain a semi-solid metal slurry in which solid-liquid two phases coexist. The metal ingot produced by the method is pure and has a large output, but the production cost is increased due to the increase of the pre-deformation process. At the same time, the method can only be used to produce small parts with limited application range.

2.4 Electromagnetic mechanical compound mixing method

The author's research group has recently developed an electromagnetic mechanical compound mixing technology, and its stirring device is shown in Figure 3. The special agitator in the figure is a device for changing the flow of semi-solid slurry in the stirring chamber during electromagnetic stirring. It can move up and down in the vertical direction in the stirring chamber, which can make the semi-solid slurry of the whole mixing chamber large. Range flow, the up and down movement of the special mechanical agitator is controlled by the motor. Electromagnetic stirring is achieved by three pairs of relatively placed N- and S-pole semi-solid slurry forces generated by the circumferential electromagnetic force. The cooling holes and heating holes on the graphite crucible are used to place the cooling tube and the heating tube. These devices function to precisely control the temperature of the semi-solid slurry to obtain a semi-solid slurry with a stable solid phase ratio. Argon gas protection is applied throughout the agitation process to prevent oxidation of the semi-solid slurry and affect the quality of the semi-solid slurry.

The modified electromagnetic stirring technology is obviously superior to the traditional electromagnetic stirring technology, and on the one hand, it can effectively avoid the segregation of the alloy composition during stirring, and on the other hand, can ensure the uniform distribution of the primary solid phase particles. The method has obvious advantages and application prospects in preparing semi-solid composite slurry.

3.5 Other methods

In addition to the above methods, there are many preparation methods in the research or development stage. Such as single roll rotation method, powder metallurgy method, chemical grain refining method, liquid phase solidification method, slope cooling method, shear-cooling-rolling method, pulse method, ultrasonic treatment method, and the like. These methods have not yet been put into industrial production [12~13].

3. Semi-solid metal forming process

The basic forming process of semi-solid metal is divided into two types: rheoforming and Thixoforming. The process is shown in Figure 3 [13]. The heated raw material of the alloy raw material is formed into a semi-solid slurry during mechanical solidification by mechanical stirring, electromagnetic stirring or other compound stirring. The subsequent process is divided into two types: one is to directly press the semi-solid slurry into the mold cavity. Further, the die-casting or the semi-solid slurry is directly formed by rolling, extrusion, etc., that is, the flow is deformed; the other is to make the semi-solid slurry into an ingot and reheating to a semi-solid temperature to form The semi-solid slurry is then subjected to a forming process, which is thixoforming.
The semi-solid metal forming process produces a better quality than conventional processing methods because the thixotropic material is more viscous than the liquid metal and has a lower forming temperature. In the case of die casting, semi-solid metal is filled with a "sticky solid front end" [14], while the molten metal is "splash-like" filling, which makes it easier to get into the gas and The inclusions cause defects; since the solidification shrinkage of the semi-solid metal is significantly reduced compared to the total liquid metal, the integrity of the part is improved and the size is nearly net shape. Table 1 shows the performance comparison of aluminum alloy semi-solid molded parts and ordinary formed parts [15]. It can be seen that semi-solid formed parts have very obvious performance advantages.

Table 1 Comparison of mechanical properties of A356 and A357 molded parts

alloy	Process	Heat treatment method	Yield strength / Mpa	Tensile strength / Mpa	Elongation /%	Hardness / HB
A356 ( Al7Si0.3Mg )	SSM	Cast state	110	220	14	60
	SSM	T6	240	320	12	105
	PM	T6	186	262	5	80
A357 (Al7Si0.6Mg)	SSM	Cast state	115	220	7	75
	SSm	T6	260	330	9	115
	PM	T6	296	359	5	100

4. Research and application status of semi-solid metal and its development trend in China

4.1 Status of foreign research and application

Since the 1980s, semi-solid processing technology has been widely recognized by scientists and technicians in various countries. At present, many process experiments and theoretical research have been carried out for this processing technology, and many achievements have been made.

The semi-solid state originated in the United States, so this technology in the United States is basically mature and is a global leader. Alumax pioneered the conversion of this technology into productivity. In 1978, it used MHD technology to produce round ingots for thixoforming. In 1994 and 1996, Alumax built two semi-solid aluminum alloy forming auto parts production plants; the semi-solid die-wrought aluminum alloy brake master cylinders produced by the company, because the blank size is close to the part size, the machining volume is only It accounts for 13% of the casting quality, and the same metal castings account for 40% of the casting quality. In addition, the quality of the semi-solid formed automobile brake pump is 13% lighter than that of the metal casting; by 1997, the production capacity of the semi-solid aluminum alloys of the above two plants reached 50 million respectively. The company has more than 60 related patents; Thixoma uses semi-solid injection molding patents to produce magnesium alloy parts. ITT uses semi-solid processing technology for the production of brass electrical connectors. At present, Alumax Aluminium's Mt Holly and Intalco aluminum plants have produced MHD ingots with diameters of 76.2mm and 152.4mm. HMM produces 50,000 parts for general purpose automotive parts (A356-T6) and plans to increase production by four times. In addition, EPCO Division, HPM Corporation, Italpresse of America, Prince Machine Corporation and other companies have been able to produce semi-solid aluminum alloy thixoforming special equipment, and through the dynamic monitoring of the die casting process, improved die castings, reducing scrap rate , saving energy [16-20].

In Europe, since the 1980s, Italy, the United Kingdom, Sweden, Germany and other countries have done a lot of research and application work in semi-solid applications. Italy is one of the earliest countries in the application of semi-solid processing technology, and Stampal-Saa uses this technology to produce domestic parts for gearbox covers and rocker arms for Ford Motor Company. MM (Magneti Marelli) produces Fuelinjection Rail parts for semi-solid aluminum alloys for the automotive industry. In 2000, Nissan reached 7,500 pieces. Weber began producing oil injections for Nuova Lanacia Delta in 1993 using semi-solid technology. The Swiss company Bubler has produced a special secondary heating device for the special SC type die casting machine for aluminum alloy thixoforming and semi-solid aluminum alloy blank. At the same time, internationally renowned companies such as Germany's EFU, France's Pechiney SA, Switzerland's Alusuisse-Lonza, and Italy's Fiat have also adopted semi-solid processing technology [21~24].

In the late 1980s, Japan established the first Rheotech company by 17 companies including the Center for Basic Technology Research and Promotion, and the steel, nonferrous and heavy industries [25]. Subsequently, the company conducted a systematic study of semi-solid processing technology and strengthened communication with famous universities and companies in Europe and America. A total of 3 billion yen was invested in research from March 1988 to June 1994, and the next step will be to industrial applications to develop semi-solid metal forming parts, and automotive parts will be one of their preferred targets [26] . At present, Japan's Speed ​​Star Wheel has used semi-solid metal forming technology to produce aluminum alloy wheels (weight about 5kg) [27].

4.2 Status of domestic research and application

Semi-solid metal processing technology started late in China and began in the late 1970s. Many research institutes have carried out research in this area, but they mainly use the mechanical stirring method for rheological casting and thixotropic casting research. The Institute of Metals of the Chinese Academy of Sciences is one of the earliest units to conduct semi-solid processing technology research [28]. Earlier research on "aluminum alloy semi-solid casting" and other "self-developed semi-solid slurry preparation equipment". In the late 1980s, most scholars began to work on semi-solid composite materials and small parts used in the automotive industry. Northeastern University used liquid-phase casting method to modify aluminum alloys 2618 and 7075 as examples. The semi-solid formability, heat treatment system and mechanical properties of finished products were systematically studied and industrial applications were being promoted [29]. In recent years, with the support of national major fund projects, researchers have made breakthrough research on the key technologies of semi-solid processing. With the support of the national “863”, Beijing Nonferrous Metal Research Institute has studied the semi-solid processing technology of aluminum alloy, and designed a semi-solid material preparation test line with a design capacity of 100t/a, which was realized in October 1998. Semi-solid continuous rheological casting of aluminum alloy [30]. Under the support of the National Science and Technology Foundation of the University of Science and Technology of Beijing and the Chinese Academy of Sciences, the basic research on the semi-solid direct forming of steel materials, in the semi-solid processing technology of high melting point alloy materials such as cast iron, spring steel, stainless steel and high carbon steel. Great progress and phased results have been achieved [31–33].

At present, the application of semi-solid processing technology to prepare metal matrix composites has become a new research hotspot. Utilizing the principle of large viscosity and adjustable semi-solid metal, it can overcome the difficulty of hardening of most of the reinforcing materials and the molten metal and difficult to recombine. The semi-solid metal is used to prevent the reinforcing material from floating up due to the difference in specific gravity. A metal matrix composite in which the material is uniformly distributed in the mother liquor. Wuhan University of Technology and Harbin Institute of Technology respectively carried out research on aluminum alloy Al2O3 particle reinforcement and short carbon fiber reinforced semi-solid processing. Reinforced aluminum-based composites with superior performance over liquid-cast composites have been obtained [34–35].

The author's research group has long been engaged in semi-solid composite research of metal matrix composites. The self-designed semi-solid electromagnetic mechanical compound mixing equipment has achieved fruitful research results in the fields of semi-solid steel-aluminum composite and steel-Al semi-solid immersion plating. Under the support of the National Natural Science Foundation of China and the Ministry of Education Key Fund, the research team is conducting a semi-solid composite study of copper graphite self-lubricating composite materials under the leadership of Professor Zhang Peng. The method breaks through the limitations of the traditional powder metallurgy method for preparing copper-graphite composite materials, and can well solve the floating phenomenon of graphite particles in copper liquid, and obtain copper graphite self-lubricating composite material with uniform distribution of graphite particles, as shown in the figure. 4 is shown. Compared with the traditional powder metallurgy method, not only the preparation process is simple, the cost is saved, and the performance is also improved.

4.3 Trends in semi-solid metal processing technology in China

Compared with foreign countries, China's research in the field of semi-solid metal forming technology is still very backward. For the development of the national economy, especially the development of China's automobile industry, to improve the level of China's automobile industry and the competitiveness in the international market, it is necessary to use various new technologies and materials to equip China's automobile industry and promote semi-solid metals. The application of forming technology in the automotive industry is currently the key [3]. As far as China's current research status is concerned, the development trend of semi-solid metal forming technology is as follows:

(1) Semi-solid metal thixoforming technology has been basically mature, and the development of flow-forming technology is slow, and there is not much breakthrough technological progress. As a result, more researchers will turn to semi-solid flow of metals to shape theory and application research to reduce the cost of semi-solid products and save energy. At the same time, it will also pay attention to the application of mature thixoforming technology in industry and promote the development of China's automobile industry.

(2) At present, the semi-solid metal forming technology is mainly applied to the formation of low-melting metals such as aluminum, magnesium and lead, and the application to high-melting ferrous metals is less, and the theory is not mature. Because ferrous metals are widely used in industry and agriculture, they have an important role that other materials cannot replace. Therefore, the theoretical research and industrial application of ferrous metal semi-solid forming in the future development will be a key research area.

(3) At present, domestic and foreign scholars have developed numerical simulation software for semi-solid forming process, but there are still many shortcomings, such as not considering the thixotropic properties of the alloy, and the application range is greatly limited. Therefore, increasing the application of computer technology in semi-solid metal forming process, making full use of computer technology, computer simulation of convection forming and thixoforming process, and promoting theoretical research of semi-solid metal forming will be another hot spot.

 

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