Preparation Process of Multilayer Metal Composites

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According to the different physical states of the member 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, liquid-liquid composite composite. The solid phase-solid phase composite methods mainly include explosive welding method, rolling method, diffusion method, and a combination of different methods. The vacuum hot pressing sintering equipment and sps sintering equipment developed by our company can also be applied in this field. Liquid-solid composite mainly includes casting and rolling method, reverse solidification method, surfacing method, spray deposition method, etc. Liquid-liquid phase composite mainly includes electromagnetic continuous casting and so on. This issue focuses on the solid-phase composite method.

Explosive welding method: The explosive welding method uses explosives as an energy source, and uses the high-speed impact generated by the explosion to act between different connecting components, resulting in violent collision, deformation, and even melting in a very short period of time, so as to achieve a firm interface. connect. Since the interaction time between the interfaces is extremely short, there will be no obvious diffusion layer, and no brittle intermetallic compounds will be formed. This process does not require high equipment, the process is relatively simple, and the cost is low. It is especially suitable for strength and melting point. Preparation of different alloys and metal composites. However, due to the high-energy and rapid explosion characteristics, the interface is often wavy, the bonding strength is low, and the controllability is poor, and it is not suitable for the preparation of thin plates and strips. In addition, the noise pollution and environmental protection problems caused by explosions are often important factors that limit their application.

Rolling method: The rolling method is divided into hot-rolled composite and cold-rolled composite. 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, exposing the fresh metal surface. Under the action of continuous and large plastic deformation, mechanical occlusion and interatomic bonding are formed on the fresh contact surface of the exposed part, thereby forming a firm bonding interface. In the process of hot rolling and cladding, the problem of interfacial oxidation is a major problem affecting the interfacial bonding. In order to prevent the oxidation of the interface during the hot rolling process, the pre-welding method is currently 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-rolled cladding, there is often a critical down-pressure value, and good interfacial bonding can be obtained only when the deformation exceeds the critical value. Traditionally, it is believed that a good interface can be obtained when the amount of depression in the first pass is greater than 50%, but for some materials, the amount of depression in a single pass is not a sufficient and necessary condition, and there are corresponding requirements for the total amount of depression. . At present, a three-step process, that is, “surface treatment + rolling + annealing”, is mostly used to obtain composite slabs with good interface bonding and high dimensional accuracy. On the whole, the rolling method has strict requirements on the component material composition, surface quality and process parameters, the process is relatively complex, and the material size is limited.

Diffusion method: The diffusion method is to stack the surface-cleaned metal sheets together in a certain order, keep them at a high temperature for a certain period of time, and combine different materials through inter-atomic diffusion to form a firm bond. In the process of diffusion, a certain pressure is often added to keep different components in a compressed state, but the pressure does not cause obvious macroscopic plastic deformation of different components, and the residual stress near the interface is relatively high. few. In the process of solid-state diffusion of heterogeneous metals, the bonding of the interface goes through three stages: point contact, surface contact and bulk contact. In the initial stage of diffusion, it is mainly a multi-point physical contact stage. The atoms in the vicinity of the interface rely on deformation, so that the distance between the heteroatoms gradually decreases, reaching the conditions for the formation of weak chemical bonds. The second stage is the chemical interaction stage. As the diffusion progresses, the contact position is directly changed from point contact to surface contact, and an activation center is formed, resulting in physical and chemical interactions on both sides of the interface, and finally a stable chemical bond is formed. After the surface contact is formed, metallurgical bonding is finally formed through the three-dimensional space diffusion around the bonding face interface, which is the “bulk” diffusion stage.

Accumulation Rolling Rolling (ARB): The cumulative rolling rolling method is a method based on severe plastic deformation to prepare materials with ultrafine grain structure. The main process flow is: base metal pretreatment + stacking + rolling + half cutting + stacking + rolling…, each completed process cycle is called a pass. The thickness of the sample obtained by this method is the same as the thickness of the original sample, but the number of layers is gradually increased, and each layer is greatly deformed. If the initial number of layers is 2, after n passes, the final number of layers is 2n. Through the ARB method, on the one hand, the combination of heterogeneous materials can be realized, and on the other hand, the severe plastic deformation of the multi-layer composite material can be realized, and the ultra-fine grain structure of the multi-layer composite material can be obtained.

※Hot pressing sintering method:

The specific process is to apply high temperature and external pressure to the loose powder or powder compact at the same time, and the loose powder in the graphite mold is heated and sintered under high pressure to form a product with a fixed shape. Heating methods include resistance type, induction type and other methods; pressurization mainly uses mechanical pressurization or hydraulic pressurization. The characteristics of hot-pressing sintering: due to the simultaneous heating and pressing, the powder is in a thermoplastic state, which is conducive to the contact and diffusion of particles and the process of flow and mass transfer, so the molding pressure is only 1/10 of that of cold pressing; It can reduce the sintering temperature and shorten the sintering time, thereby resisting the growth of grains, and obtaining products with fine grains, high density and good mechanical and electrical properties. Ultra-high purity ceramic products can be produced without the addition of sintering aids or forming aids. The disadvantage of hot pressing sintering is that the process and equipment are complicated, the production control requirements are strict, the mold material requirements are high, the energy consumption is large, the production efficiency is low, and the production cost is high.

Spark plasma (SPS) sintering method: Spark plasma sintering (SPS) is a low-temperature, short-time rapid sintering method that can be used to prepare metals, ceramics, nanomaterials, amorphous materials, composite materials, gradient materials, etc. It is a pressure sintering method that utilizes on-off DC pulse current to directly electrify sintering. The main function of the on-off DC pulse current is to generate discharge plasma, discharge impulse pressure, Joule heat and electric field diffusion. During the sintering process, the discharge plasma generated instantaneously when the electrode is supplied with a DC pulse current makes each particle inside the sintered body uniformly generate Joule heat and activate the particle surface. The SPS sintering process can be regarded as the result of the combined action of particle discharge, conductive heating and pressure. In addition to the two factors that promote sintering, such as heating and pressure, in SPS technology, the effective discharge between particles can generate local high temperature, which can locally melt the surface and peel off the surface material; the sputtering and discharge impact of high temperature plasma removes the powder Particle surface impurities (such as removal of surface oxides, etc.) and adsorbed gases.

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