According to the different physical states of the component materials in the preparation process of multilayer composites, the preparation methods of multilayer composites can be roughly divided into three categories: solid-solid composite, liquid-solid composite and liquid-liquid composite. The main methods of solid-solid composite are explosive welding, rolling, diffusion and different combinations of methods. The vacuum hot pressing sintering equipment and SPS sintering equipment developed by our company can also be used in this field. Liquid solid composite mainly includes casting rolling method, reverse solidification method, surfacing method, spray deposition method and so on. Liquid liquid composite mainly includes electromagnetic continuous casting. This issue focuses on the solid-solid composite method.
Explosive welding method: explosive welding method uses explosive as energy, and uses the high-speed impact produced by explosion to act on different connecting components, resulting in violent collision, deformation and even melting in a very short time, so as to realize the solid connection of interface. Because the interaction time between the interfaces is very short, no obvious diffusion layer will appear, and no brittle intermetallic compound will be formed. This process has low requirements for equipment, relatively simple process and low cost, and is especially suitable for the preparation of alloys and metal composites with large differences in strength and melting point. However, due to the characteristics of high energy and fast explosion, the interface is wavy, low bonding strength, poor controllability, and it is not suitable for the preparation of thin plate and strip. In addition, the noise pollution caused by explosion and environmental protection are often the important factors limiting its application.
Rolling method: rolling method is divided into hot rolling compound and cold rolling compound. The basic principle is that under the action of the extreme pressure of the rolling mill (or coupled with the thermal effect of high temperature), the oxide film on the surface of different material components is broken and the fresh metal surface is exposed. Under the action of continuous and large plastic deformation, mechanical occlusion and bonding between atoms are formed on the exposed fresh contact surface, thus forming a firm bonding interface. In the process of hot rolling compound, the problem of interface oxidation is one of the most important problems. In order to prevent the oxidation of interface in hot rolling process, the method of pre welding is often used, that is, the components are assembled together in a certain order, and then the surrounding is sealed by welding to minimize the oxidation phenomenon. For cold rolling compound, there is always a critical value of reduction, when the deformation exceeds the critical value, good interfacial bonding can be obtained. It is traditionally believed that a good interface can be obtained when the first pass reduction is more than 50%. However, for some materials, the single pass reduction is not a sufficient and necessary condition, and there are corresponding requirements for the total reduction. At present, three-step process, namely "surface treatment + emulsification + annealing", is often used to obtain composite slabs with good interface bonding and high dimensional accuracy. On the whole, the rolling method has strict requirements on component material composition, surface quality and process parameters, and the process is relatively complex, and the material size is limited.
Diffusion method: the diffusion method is to stack the metal plates after surface cleaning in a certain order, keep them at a high temperature for a certain period of time, and combine different materials by mutual diffusion between atoms to form a firm combination. In the process of diffusion, a certain pressure is often added to maintain a compression state between different components, but the pressure does not cause obvious macroscopic plastic deformation of different components, and there is less residual stress near the interface. In the process of solid-state diffusion of heterogeneous metals, the interface bonding has experienced three stages: point contact, surface contact and volume contact. In the early stage of diffusion, it is mainly the stage of multi-point physical contact. The atoms near the interface depend on the deformation, which makes the distance between the heterogeneous atoms gradually smaller, reaching the condition of forming weak chemical bonds. The second stage is chemical interaction. With the diffusion, the contact position directly changes from point contact to surface contact, and forms the activation center, which produces physical and chemical interaction on both sides of the interface, and finally forms a stable chemical bond. After the formation of the surface contact, the metallurgical bonding is formed by the diffusion of the bonding face to the three-dimensional space around the interface, which is called the "bulk" diffusion stage.
Accumulative roll over (ARB): ARB is a method to prepare ultra-fine grained structural materials based on severe plastic deformation. The main process is: base metal pretreatment + stacking + rolling + half cutting + stacking + rolling Each process cycle is called a pass. The thickness of the sample obtained by this method is the same as that of the initial sample, but the number of layers increases gradually, and each layer has a huge deformation. If the initial number of layers is 2, after n passes, the final number of layers is 2n. Through ARB method, on the one hand, the combination of heterogeneous materials can be realized, on the other hand, the severe plastic deformation of multilayer composites can be realized, and the ultra-fine grain structure of multilayer composites can be obtained.
Hot pressing sintering method:
The specific process is to apply high temperature and external pressure to the loose powder or powder compacts at the same time. The loose powder in the graphite mold is heated and sintered under high pressure to form a fixed shape product. Heating methods include resistance type and induction type, and pressurization mainly uses mechanical pressurization or hydraulic pressurization. Characteristics of hot press sintering: hot press sintering is carried out at the same time due to heating and pressure, and the powder is in a thermoplastic state, which is conducive to the contact diffusion of particles, flow and mass transfer process, so the molding pressure is only 1 / 10 of that of cold press, and it can also reduce the sintering temperature
Fig. 1 appearance (left) and inner structure (right) of hot pressing sintering furnace produced by Shanghai Haoyue
Spark plasma sintering (SPS): spark plasma sintering (SPS) is a low-temperature, short-term rapid sintering method, which can be used to prepare metals, ceramics, nano materials, amorphous materials, composite materials, gradient materials, etc. It is a direct on-off DC pulse current sintering method. The main function of on-off DC pulse current is to produce discharge plasma, discharge shock pressure, Joule heat and electric field diffusion. During the sintering process, the discharge plasma generated instantaneously when the electrode is connected with DC pulse current makes the particles in the sintering body generate Joule heat uniformly and activate the surface of the particles. SPS sintering process can be regarded as the result of particle discharge, conductive heating and pressurization. In addition to heating and pressure, the effective discharge between particles can produce local high temperature, which can make the surface melt and peel off. The high temperature plasma sputtering and discharge impact can remove the surface impurities (such as surface oxide) and adsorbed gas.
Figure 2 appearance (left) and structure (right) of spark plasma sintering furnace produced by Shanghai Haoyue
According to the market demand, Haoyue technology continues to research and develop, improve technology, launch heat treatment equipment with excellent performance, and provide corresponding technical support to assist the domestic composite market and provide corresponding technical consultation for the heat treatment process of composite industry.
Haoyue technology is a high-tech enterprise integrating R & D, production and sales of electric furnace. The company has been focusing on advanced ceramics and composite materials, semiconductor materials, carbon materials and lithium and new energy materials and equipment in four major fields, with rich industry experience and professional technology, dedicated to serving customers, providing perfect integrated industrial solutions.