Copper industry knowledge: Introduction to high-performance copper-based composite materials
Copper and copper alloys have good mechanical properties and excellent process performance. They are easy to cast and plastic process. More importantly, copper and copper alloys have good corrosion resistance, thermal conductivity, and electrical conductivity, so they can be widely used in electronic and electrical, mechanical manufacturing and other industrial fields. However, copper's room temperature strength, high temperature performance, and wear performance are insufficient, which limits its wider application. With the rapid development of modern aerospace and electronic technology, more and higher requirements are put forward for the use of copper, that is, on the basis of ensuring copper's good electrical conductivity, thermal conductivity and other physical properties, copper is required to have high strength, especially good high temperature mechanical properties, and the material is required to have low thermal expansion coefficient and good friction and wear performance. The total investment of my country's first high-speed railway Beijing-Shanghai line is about 20 billion US dollars. Construction started in 2008. The annual demand for contact wire is nearly 10,000 tons. Obviously, the research and development of contact wire, that is, the research and development of high-strength, high-conductivity and high-wear-resistant copper alloy functional materials, has a large domestic and foreign market. Resistance welding electrodes, seam welding rollers, and integrated circuit lead frames also require high-strength and high-conductivity copper alloys. It is difficult to take into account the high strength and high conductivity of existing copper and copper alloys. Therefore, by introducing appropriate reinforcing phase composite strengthening methods, giving full play to the synergistic effect of matrix and functional strengthening phase, the research and development of high-performance copper (alloy) based functional composite materials has become a hot topic in the world today.
The so-called high-strength and high-conductivity copper alloy generally refers to a copper alloy with a tensile strength (Gb) of 2-10 times that of pure copper (350-2000MPa) and a conductivity of 50%~95% of copper, that is, 50-95% IACS copper alloy. The internationally recognized ideal index is δb=600-800MPa, and the conductivity is ≥80% IACSE. The main application areas of high-strength and high-conductivity copper alloys are ultra-large-scale integrated circuit lead frames in the electronic information industry, electronic countermeasures for national defense and military industry, radars, high-power military microwave tubes, high-pulse magnetic field conductors, nuclear equipment and launch vehicles, overhead wires for high-speed rail transit, 300-1250Kw high-power frequency-modulated speed-regulating asynchronous traction motor bars and end rings, resistance welding electrode heads for the automotive industry, continuous casting machine crystallizers for the metallurgical industry, electric vacuum devices and switch contact bridges for electrical engineering, etc. Therefore, this type of material has broad application prospects in many high-tech fields.
Introduction to high-performance copper-based composite materials-classification:
1. Particle-reinforced copper-based composite materials
The reinforcement is mainly silicon carbide and aluminum oxide, and there are also a small amount of titanium oxide and titanium boride particles (the particle size is generally about 10μm). Whiskers not only have superior mechanical properties themselves, but also have a certain aspect ratio, so they have a more significant reinforcement effect on the metal matrix than particles. Whiskers are commonly used silicon carbide and aluminum borate whiskers. Alloying process can prepare oxide dispersion strengthened and carbide dispersion strengthened copper-based composite materials.
2. Fiber-reinforced copper-based composite materials
Composites made of copper or copper alloys and non-metallic or metal fibers not only maintain the high electrical conductivity and thermal conductivity of copper, but also have high strength and high temperature resistance. When manufacturing such copper-based composite materials, both long fibers and short fibers are used. Carbon fiber-copper composite materials have the characteristics of good thermal conductivity and electrical conductivity of copper, as well as self-lubrication, wear resistance, and low thermal expansion coefficient of carbon fiber, so they are used in sliding electrical contact materials, brushes, power semiconductor support electrodes, integrated circuit heat sinks, etc. Another application example of copper-carbon fiber composite materials in industrial production is the slider on the electric pantograph of trams, and the vulnerable parts of slider trams and electric locomotives. Metal sliders were used at first, and carbon sliders are used at present, but both have shortcomings. After using carbon fiber-copper composite materials, the contact resistance is reduced, overheating is avoided, and the strength and overload current are improved at the same time, and it has excellent lubrication and wear resistance.
3. High-performance micro-composite copper alloy
High-performance micro-composite copper alloy materials were discovered in the 1970s when studying superconducting materials. In 1978, Bark et al. from Harvard University in the United States first proposed the concept of high-performance Cu-X alloy, Cu-X binary alloy, X includes refractory metals W, Mo, Nb, Ta and Cr, Fe, V and other elements. After forging, drawing or rolling, the X metal is distributed in the direction of deformation in the form of wire or ribbon to form a micro-composite material. This micro-composite copper alloy material is characterized by ultra-high strength (the highest tensile strength can reach more than 2000MPa), electrical conductivity can reach 82% IACS, good heat resistance, micro-composite structure and grain orientation. In addition to being used as spot welding electrodes, this material can also be used as a propeller and heat exchanger. Compared with traditional copper alloy materials, it contains more total alloy elements, but fewer types of alloy elements. Cu-X alloy has attracted people's attention with its ultra-high strength, high electrical conductivity and good heat resistance. At present, the University of Iowa, Harvard University Department of Materials, AMES Laboratory, Michigan Institute of Technology, and Zhejiang University in China have done a lot of research in this regard, but there are still many theoretical and practical application problems to be solved.
Introduction to high-strength and high-conductivity copper-based composite materials-preparation methods:
1. Powder metallurgy method
Powder metallurgy was first developed for the preparation of particle-reinforced metal-based composite materials, generally including powder mixing, compaction, degassing, sintering and other processes. Powder metallurgy is a near-net forming process with high material utilization, which can eliminate organizational and component segregation, and the particle size and volume fraction of the particle reinforcement phase can be adjusted within a large range. This method is the main means of producing structural parts, friction materials, and high-conductivity materials in copper-based composites. Due to the poor wettability of copper and most ceramic reinforcement particles and the large difference in density, it is easy to produce reinforcement aggregation when preparing composite materials by liquid method, resulting in uneven distribution of the second phase. Powder metallurgy can mix metal powder and reinforcement evenly in the required proportion, solving the problem of reinforcement distribution. In order to enhance the interface bonding strength between copper and reinforcing particles, chemical deposition and other methods are usually used to coat the surface of reinforcing particles with metal coatings such as Cu and Ni, and then the particles are evenly mixed with copper powder to obtain composite materials using powder metallurgy [11]. Since the reinforcing particles are more evenly distributed in the matrix metal after being coated with metal coatings, the direct contact between the reinforcing materials is reduced, and the reinforcing effect is more effectively exerted. At the same time, by coating with different metals, the interface structure can be improved, the interface bonding strength can be enhanced, and the comprehensive performance of the composite material can be improved.
2. Composite casting method
Casting is the preferred method for industrial mass production. However, after casting, there is generally an auxiliary deformation process for this composite material. The deformation strengthening effect will be invalidated due to the recrystallization of the cold-deformed metal. Since the recrystallization temperature of most metals is only about 40% of their melting point, the high temperature resistance of the material obtained by casting is relatively poor. The composite casting process was proposed by M. C. Flemings et al. of the Massachusetts Institute of Technology. This method has a good solution to the segregation of the reinforcing phase, a simple production process, and adapts to the trend of large-scale industrial production of composite materials, with great development advantages. However, due to the high viscosity of the melt, composite casting is not conducive to the discharge of gas and inclusions, so there are often pores and inclusions in the prepared material; in addition, this method is also difficult to control the temperature.
3. Internal oxidation method
The internal oxidation method is one of the most commonly used methods for preparing copper-based composite materials. It can obtain uniformly distributed fine dispersed particles and can accurately control the number of strengthening phases. The typical application of this process is to prepare Cu-A1203 dispersion-strengthened copper-based composite materials. In this process, a small amount of aluminum, an alloying element that is solid-dissolved in copper but has a greater tendency to form oxides than copper, is added to copper to make copper-aluminum alloy powder. Oxygen is diffused from the surface of the powder to the inside, so that the alloy atomized powder undergoes internal oxidation at high temperature and oxygen atmosphere, and aluminum is converted into aluminum oxide. Then, the oxidized copper is reduced in a hydrogen atmosphere, but aluminum oxide cannot be reduced, and a copper and aluminum oxide mixed powder is made, and finally sintered under a certain pressure. There are some problems in the forming and curing technology of Cu-A1203 made by the internal oxidation method. It is extremely difficult to sinter powder, and the process is complicated and the cost is high. The disadvantages of the internal oxidation method are that the process is complicated, there are many factors that affect the preparation process, the material quality is difficult to control and the production cost is high, which greatly limits the application of this process. .
4. Liquid metal in-situ method
The liquid metal in-situ reaction method is one of the new preparation technologies for copper-based composite materials that has been developed in recent years. Lee et al. first successfully prepared TiB2/Cu composite materials. This method fully stirs and mixes two or more alloy liquids and produces uniformly dispersed nano-scale reinforcements through chemical reactions. The conductivity of the Cu-based composite material containing 5vol1% TiB2 was 76% IACS. Chrysanthou et al. added carbon black, B203 or W carbon black to the Cu-Ti solution respectively, and reacted to generate fine and uniformly distributed TiC, TiB2, and WC particles in-situ to reinforce the copper-based composite material. Since the reinforcement in the composite prepared by this process has no interface contamination and has good interface compatibility with the matrix, it has higher conductivity and mechanical strength than traditional composite materials.
5. Rapid solidification method
Due to the fast cooling rate, large initial nucleation supercooling and high growth rate during the solidification process, the rapid solidification method causes the solid-liquid interface to deviate from equilibrium, thus presenting a series of organizational and structural characteristics different from conventional alloys. The rapid solidification method has the following characteristics for preparing copper-based composite materials:
(1) The solid solubility of the alloying element copper is significantly increased;
(2) The grains are greatly refined;
(3) The microsegregation of chemical components is significantly reduced;
(4) The density of crystal defects is greatly increased;
(5) A new metastable phase structure is formed;
(6) After aging treatment, the content of the second phase in the copper matrix is increased and the degree of dispersion is increased.
With a slight decrease in conductivity, the strength of the alloy is significantly improved, and the wear and corrosion resistance of the alloy are improved. Rapid solidification technology has opened up a new field for the preparation of high-strength and high-conductivity copper-based composite materials. In the future, the research focus of rapid solidification preparation of high-strength and high-conductivity copper-based composite materials will be to optimize the material composition, solidification kinetic parameters and aging process through analysis of the solidification process and aging process, and improve the microstructure and performance.
6. Mechanical alloying method
Mechanical alloying uses a high-energy ball mill to mix metal powder or ceramic particles in a certain proportion, and grinds them repeatedly. The composite powder undergoes repeated deformation, cold welding, crushing, re-welding, and re-crushing processes, which can refine the grains to the nanometer level and have great surface activity [17]. Due to the introduction of a large number of distortion defects, the mutual diffusion ability is enhanced and the activation energy is reduced, making the alloying process different from the ordinary solid-state process. Therefore, it is possible to prepare many new materials that are difficult to synthesize under conventional conditions. The disadvantage of mechanical alloying to prepare copper-based composite materials is that impurity elements are easily introduced during the ball milling process, which reduces the material properties, especially the conductivity. At the same time, the production efficiency is low due to the long ball milling time.
