Preparation technology of particle-reinforced metal matrix composite by casting method

Metal matrix composites are multiphase materials with a special second phase dispersed in a metal or alloy matrix. The second phase with special physical and mechanical properties greatly enhances the strength, hardness, wear resistance, heat resistance and other properties of the material. Therefore, this second phase is also called the reinforcing phase. Reinforcement phase is usually divided into particle reinforcement phase and fiber (whisker) reinforcement phase. Due to reasons such as the price of the reinforcement and the composite technology, most of the composite materials made by the casting method are particle-reinforced.

In recent years, the preparation method of particle-reinforced composite materials has continuously improved in the physical method of adding the reinforcement, and the in-situ reaction synthesis of the reinforcement method has been developed.

No matter what kind of casting composite method is used, to successfully prepare composite products with excellent performance and stable quality, the following problems must be solved technically:

• ①Select the appropriate matrix and reinforcement based on the different requirements of the material,
• ②Improve The wettability of the matrix melt to the reinforcement,
• ③control the reasonable distribution of the reinforcement in the matrix,
• ④solve the problem of the casting forming process caused by the effect of the reinforcement particles on the viscosity of the matrix melt.

This article summarizes the above-mentioned problems on the preparation technology of particle-reinforced metal matrix composite material casting, in order to promote the production of particle-reinforced composite material.

• Reinforcement selection: The performance of the matrix, the reinforcement and the good combination of the reinforcement and the matrix determine the performance of the composite material, so the reinforcement should be selected reasonably according to the type of substrate and the performance requirements of the composite material. When selecting a reinforcement, the elastic modulus, tensile strength, hardness, thermal stability, density, melting point, price and other factors of the reinforcement should be considered. At the same time, the linear expansion coefficient and chemical reactivity between the reinforcement and the matrix should be matched. Claim. The matrix with aluminum as the main component, the commonly used reinforcements are graphite, Al2O3, SiC, TiC, Al3Ti, TiB, Al3Zr, etc. The research on the composite materials with steel as the matrix is ​​not as mature as the aluminum matrix composites, and the commonly used reinforcements are more mature. Less, mainly WC, VC, TiC, TiN, Al2O3, SiC, etc. After the type of reinforcement is selected, the size of the reinforcement and its volume fraction in the matrix should also be determined through experiments.
• Methods to improve the wettability of the reinforcement/matrix: improve the wettability of the reinforcement by the matrix melt, help reduce the agglomeration of the reinforcement, help improve the bonding strength of the matrix/reinforcement interface, and improve the overall composite material In other words, it also contributes to the uniform distribution of the reinforcement in the matrix. The following measures are usually used to improve the wettability of the matrix melt to the reinforcement.
• Reinforcement surface coating treatment: Coating a certain metal or compound on the reinforcement surface by methods such as electroless plating or vapor deposition can effectively improve the wettability of the matrix melt to the reinforcement body. Studies have shown that copper coating on the surface of graphite and vapor deposition of TiN on the surface of Al2O3 particles have effectively improved the wettability of the matrix to the reinforcement particles.
• Additional surfactants: According to reports, adding magnesium blocks while adding graphite or silica powder to the aluminum melt can improve the wettability of the aluminum liquid to the reinforcement. Research in recent years has also shown that in addition to Mg, Ca, RE, alkali metal elements, and elements of group VI and group VIa all have the effect of improving the wettability of molten aluminum on reinforcements such as Al2O3 and SiC.
• Heat treatment of reinforcement: Heat treatment to remove oil and water on the surface of the reinforcement particles can increase the surface energy of the reinforcement and increase the wettability of the melt to the reinforcement. Someone has prepared graphite/aluminum composite materials using heat-treated graphite powder without copper coating. The heating treatment method is: heating the graphite powder to about 600°C for 8 hours to activate the surface, then cooling, and reheating to 200°C before adding the aluminum liquid to remove moisture. Some people also used the method of heating the reinforcement to improve the wettability of the reinforcement and the matrix to produce composite materials.
• High-energy ultrasonic treatment of the melt: Someone prepared SiC/ZA27 composite material with high-energy ultrasonic treatment of the melt. The study believes that the cavitation effect produced by high-energy ultrasound when propagating in the melt cleans the surface of the reinforced particles, increases the surface tension of the particles, and at the same time reduces the surface tension of the melt, and significantly improves the relationship between the SiC particles and the ZA27 melt. Wettability.
• In-situ reaction synthesis of reinforcement: the application of in-situ reaction synthesis technology to prepare a master alloy containing reinforcing particles, and then adding this master alloy to the matrix melt to prepare composite materials technology has become a hot spot in composite materials research in recent years. The in-situ reaction synthesis method usually adds some pure metals, alloys, compounds or salt substances to the master alloy, and the reinforcement is obtained through chemical reactions between the additives or between the additives and the components of the master alloy. Because the reinforcement is generated by in-situ reaction, the surface is clean and pollution-free, and the thermal stability is good, which well solves the problem of the wettability of the reinforcement by the matrix melt, and the reinforcement and the matrix are very firmly combined with a better reinforcement effect. Some scientists mixed Al, Ti, and C powders in a certain proportion and then pressed them into small pieces. The small pieces were vacuum sintered in a high-temperature diffusion furnace to obtain a TiC/Al master alloy, and the TiC/2618 composite was prepared from the master alloy. material. Others have also successfully prepared composite materials with uniform distribution of reinforced particles and excellent properties by in-situ reaction synthesis.
• Reinforcement distribution control technology: Controlling the distribution of reinforcements in the matrix and giving full play to the effective strengthening of the matrix is ​​the fundamental guarantee for preparing composite products that meet certain performance requirements.
• The control method of the non-uniform distribution of reinforcements: For wear-resistant/wear-reducing materials, the working surface is required to have high anti-wear performance, while the remaining parts have better comprehensive mechanical properties to ensure that the working surface is effectively supported.

Therefore, such products that need to be locally strengthened require the reinforcement to be distributed in a certain range near the working surface of the product. There are several commonly used methods:

• Pre-reinforcement method: The pre-reinforcement method is one of the main methods for preparing surface-strengthened composite materials by the additive method. It is an application of cast-infiltration method surface modification technology in the preparation of composite materials. Mainly suitable for the preparation of wear-resistant composite products. The specific method is: pre-place the reinforcement in the form of paint or paste block on the part where the product needs to be strengthened, and then pour it into the matrix alloy liquid. The matrix alloy liquid penetrates into the gap of the reinforcement body by the capillary siphon effect and the pressure of the alloy liquid, and solidifies Then, a surface-reinforced composite product in which the reinforcement body and the matrix are tightly combined is formed. Scientists have done in-depth research on this technology and believe that the key to the pre-reinforced surface composite technology lies in: ①appropriate reinforcement particle size and good wettability of the reinforcement by the matrix melt; ②binder selection and coating, Paste preparation and brushing process, ③ pouring temperature control and pouring process; the preset reinforcement method has the characteristics of simple process, low cost, and excellent effect. It is a surface composite technology that has been applied successfully at present and has a very broad Application prospects.
• Centrifugal control method: Based on the difference in the specific gravity of the reinforcement and the matrix melt, the method of making the reinforcement in a gradient segregation distribution along the radial direction with the help of centrifugal force is called the centrifugal control method. An important development direction of gradient composite materials. Someone used a centrifugal control method to prepare a graphite/aluminum composite material with a gradient distribution of graphite. Scientists centrifugal casting Al-Fe alloy, obtained the self-generating gradient composite cast pipe in which the primary Fe phase is distributed along the radial gradient. The sand-coated metallic centrifugal casting Al-Fe alloy self-generated gradient composite material pointed out: ①As the number of reinforcements increases, the distribution gradient of the reinforcements in the radial direction gradually decreases, and the distribution range in the radial direction gradually expands; In the range of 0～2000r/min, with the increase of the rotation speed, the distribution gradient of the reinforcement along the radial direction gradually increases, while the distribution range in the radial direction gradually decreases.
• Electromagnetic stirring control method: Scientists applied strong AC electromagnetic stirring to the (Mg2Si)20Al80 alloy melt during the solidification process to prepare a gradient composite material enriched in Mg2Si on the outer surface. According to analysis, when electromagnetic stirring is carried out, the molten metal will be subjected to electromagnetic force directed to the axis in the alternating magnetic field. Due to the low conductivity of the reinforcement (nascent Mg2Si), it is basically not affected by the electromagnetic force directed to the axis, while the electromagnetic force exerted on the melt is relatively large, resulting in an unbalanced force field around the reinforcement, and the reinforcement is subject to The squeezing force exerted by the metal melt away from the shaft center moves to the outside in the radial direction, thereby obtaining a gradient composite material with an outer surface enriched and reinforced. The study pointed out that the greater the voltage of the three-phase alternating current applied by the electromagnetic stirrer, the greater the shear force between the liquid/solid interface and the melt, and the easier it is for the primary Mg2Si particles to be pushed to the outer surface of the specimen, and the segregation layer The thicker.
• Control method for uniform distribution of reinforcement: For the overall reinforced composite material, the uniform distribution of the reinforcement in the matrix is ​​very important. Strengthening the stirring of the melt is the fundamental means to achieve the goal of homogenization of the reinforcement. Here are several effective mixing methods:
• Mechanical stirring: Mechanical stirring is the most traditional method of stirring the melt. Due to the limitation of the material of the stirrer blades, there are few examples of the mechanical stirring method used in steel alloys. Pay attention to the mechanical stirring of the melt: ① Reasonably choose the material and form of the stirrer blade: the stirrer blade is directly in contact with the melt, which can easily cause pollution to the alloy. For non-ferrous alloys, non-ferrous blades should be used or steel blades should be coated with an external coating (such as a layer of white clay). And the blade rotation direction should be selected according to the particle density of the reinforcement. ②Good mixing: The immersion depth of the agitator blades should be controlled appropriately to produce a stable vortex. The shaking of the stirring rod or improper stirrer blades will increase the probability of the reinforcement being repelled by the melt, thereby deteriorating the distribution of the reinforcement in the matrix. ③Mixing time: After adding the reinforcement, the stirring time should be as long as possible, and the waiting time for pouring after the stirring should be as short as possible. There is a U.S. patent for "a device for mixing solid particles into a liquid," which states that the composite material produced by this device "overcomes the common defects of particle-reinforced composite materials, and has short mixing time, high productivity, and low cost."
• Gas stirring: Stirring the melt with a large amount of gas generated by the external gas flow or melt reaction can also achieve the purpose of evenly distributing the reinforcement in the melt. Scientists used a large amount of gas generated by the in-situ reaction to agitate the melt and prepared Al3Zr(p).Al2O3(p)/A356 and (TiB2+TiAl3)/AlSi6Cu4 composites with evenly distributed reinforcements.
• High-energy ultrasonic treatment: Scientists add SiC particles to the surface of ZA27 alloy melt at 600°C, and treat the melt with high-energy ultrasonic for 60-90s to obtain a melt-particle suspension, and the overall particle distribution (as-cast) is obtained. Uniform SiCp/ZA27 composite material. The study believes that the finite amplitude attenuation of high-energy ultrasound in the melt causes a certain sound pressure gradient in the melt to form a fluid jet, which directly leaves the end face of the ultrasonic horn and forms in the entire melt. Circulation (ie, acoustic current effect), the velocity of the acoustic current can reach 10 to 103 times the convection velocity of the melt. While the sound flow removes the impurities from the surface of the reinforcing particles, it also sends the particles into the deep part of the melt and makes them evenly dispersed.
• Composite melt performance characteristics and forming process key points: The biggest difference between composite melt and ordinary melt is the introduction of solid particles of reinforcement. Due to the introduction of solid particles, the viscosity of the composite melt will suddenly and significantly increase with traces of TiC and TiB2, causing a sudden change in the viscosity of the aluminum melt. Scientists pointed out the theory of casting formation. The viscosity of liquid metal has a significant effect on the flow characteristics of the metal in the mold, the filling of the mold, the floating of the gas in the liquid metal, and the feeding of the metal.

In order to obtain a sound product, the forming process of a composite melt with a sudden increase in viscosity must deal with the following two problems:

• ①Improve the fluidity of the melt and increase its filling ability;
• ②Prevent the melt from inhaling gas and strengthen the removal of gas in the melt.

Manchang Gui and others developed a vacuum differential pressure pouring process consisting of a filter screen and a sprue. The composite melt is directly filled into the mold after passing through the filter. After the molten metal is filled, the sprue always has a pressure effect, and it is finally solidified, so it can feed the casting. The characteristics of this casting process are specifically manifested in:

• ①Essentially eliminate the source of gas, and basically eliminate the pore defects generated in the pouring process;
• ②Simplify the pouring system, the weight of the pouring system and the weight of the casting are reduced from non-vacuum free pouring (5-10):1 (0.5～1.5): 1;
• ③Overcoming the shortcomings of poor fluidity of composite melt, it can cast complex thin-wall composite castings.

Outlook

The casting method is one of the most promising methods for preparing composite materials. Future research should focus on the following aspects:

• ① For the ferrous metal matrix, select reinforcements based on the performance requirements of composite materials;
• ② Develop preparation methods that are easier to realize and apply to industrial production;
• ③ Significantly reduce the material and manufacturing costs of composite products;
• ④ Research the recovery of composite materials Reuse technology.

Therefore, it is believed that the casting method will do a lot in the production of wear-resistant and heat-resistant composite products.　　 

Metal matrix composite material is a multiphase material with a special second phase dispersed in a metal or alloy matrix. The second phase with special physical and mechanical properties greatly enhances the strength, hardness, wear resistance, heat resistance and other properties of the material. Therefore, this second phase is also called the reinforcing phase. Reinforcement phase is usually divided into particle reinforcement phase and fiber (whisker) reinforcement phase. Due to reasons such as the price of the reinforcement and the composite technology, most of the composite materials made by the casting method are particle-reinforced.

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