Research Progress Of New Functional Material Aluminum Foam

1. Introduction

Foamed aluminum is a new type of functional material, which is a foam-like ultra-light metal material with numerous bubbles dispersed in the metal aluminum matrix, and the general porosity is 40%~98%. In 1948, Sonik proposed a method for preparing foamed aluminum by vaporizing and foaming mercury in aluminum, and then Elliot successfully prepared foamed aluminum in 1951. In the 1960s, American Ethyl Company became a scientific research center base for the development of aluminum foam. In 1991, Japan's Kyushu Institute of Industrial Metals developed a process route for the industrialization of foamed aluminum, and now large and small parts can be produced by metal foaming and percolation methods. Until today, the United States, Japan, the United Kingdom, France, Canada and other countries have successively developed a number of technical patents. In my country, some exploratory work has also been carried out on the basic research on the preparation process of foamed aluminum, and some progress has been made.

Aluminum foam has many excellent properties due to its unique structure. It not only has the lightweight properties of porous materials, but also has excellent mechanical properties and thermal, electrical and other physical properties that metals have, such as penetration, damping, energy Absorption, high specific surface area, electromagnetic shielding and other properties. Now, the main applications of foamed aluminum are: fireproof and sound-absorbing panels, impact energy absorbing materials, building panels, semiconductor gas diffusion plates, heat exchangers, electromagnetic shielding and so on. It can be used in the fields of metallurgy, chemical industry, aerospace, shipbuilding, electronics, automobile manufacturing and construction, and the scope of application is constantly expanding.

2. Preparation method of aluminum foam

2.1 Casting method

The casting method is widely used in the manufacture of foamed aluminum, and the preparation method is relatively mature. Several related methods are introduced below.

2.1.1 Seepage casting method

The percolation casting method is to infiltrate high-temperature liquid metal aluminum into the voids of the filler particles under a certain pressure, and remove the soluble particles enclosed in the aluminum alloy after solidification, that is, to obtain foamed aluminum with a connected cell structure. This method can be further divided into upper pressure infiltration casting method and negative pressure infiltration casting method. The upper pressure sulphurizing casting method is an electrode material with strength, high conductivity, excellent heat transfer and creep resistance compared with the negative pressure infiltration casting method.

(3) The average service life of Al2O3 dispersion-strengthened copper electrodes for welding 1.5mm thick aluminized steel plates is 7200 points, which can well meet the needs of CPT explosion-proof belt welding.

It has the advantages of simple process operation, fast preheating speed, high productivity and yield, large safety factor, small equipment investment, and easy mechanized production. Among them, the selection and processing of filler particles is very critical, which has the following characteristics: sufficient heat resistance, not melting at the pouring temperature; sufficient strength and stiffness, not broken and deformed under the action of seepage pressure; good removal The filler particles in the three-dimensional pores can be completely removed with a solvent or aqueous solution after the aluminum liquid solidifies; chemical stability, does not react with the aluminum liquid. This method is simple to operate, and the average porosity can reach 70%, which is convenient for large-scale production. At present, the percolation method is not only used in the manufacture of aluminum foam, but also in the production of foam materials such as zinc, magnesium, lead, tin, and iron, which can be used to manufacture parts with complex shapes.

2.1.2 Add ball method

The ball addition method is to add particles or hollow spheres to the liquid aluminum alloy, strengthen the stirring, and cast the aluminum liquid that is still in relative flow to obtain an aluminum alloy and particle composite, and then dissolve and remove the aluminum alloy matrix. Soluble particles, and a connected-cell aluminum foam was obtained.

2.1.3 Investment casting method

First, a three-dimensional continuous foam sponge material with a certain porosity is used as the parent material, and then an easy-to-remove refractory material is flushed into the sponge-like foam, dried and hardened to form a prefabricated shape, and then fired to harden the refractory material and make it The foam sponge is vaporized and decomposed, and then the preform is placed in a metal mold, poured into molten metal, and a certain pressure is applied to it or vacuum casting is performed to fill the pores of the mold with molten metal, and it is removed after cooling. The block of refractory material can be obtained three-dimensional network through-cell aluminum foam. The aluminum foam produced by this investment casting preparation process has good three-dimensional penetration, and the process has a wide range of applications and is non-corrosive, and has a good prospect in the manufacture of fluid-permeable products.

2.1.4 Foaming method

There are two kinds of gas foaming method and metal liquid foaming method. The gas foaming method is to blow gas into the molten metal melt to foam the metal melt. The gas used for foaming can be oxygen, argon, air, water vapor, carbon dioxide, etc. The technical key is that the melt should have a suitable viscosity, the composition of the metal should ensure a wide enough foaming temperature range, and the formed foam should have enough stability to ensure that the foam does not break during the subsequent collection and molding process. The gas foaming method is currently the cheapest method for producing metal foam. The size of the foam is very large, and the porosity of foamed aluminum products can reach 97%. The liquid metal foaming method was widely used in the early days, mainly adding TiH2, ZrH2, CaH2 and other foaming agents to the liquid aluminum alloy, and then heating to decompose the foaming agent to release the gas, and the expansion of the gas made the aluminum alloy into a foam, and then cooled That is, the foamed aluminum product is obtained. The aluminum foam prepared by this method belongs to closed-cell aluminum foam. The structure and porosity of the pores are related to the viscosity of the molten metal and the foaming time. Controlling the viscosity of the molten metal and the foaming time during the melting process can control the porosity. However, the pores are independent of each other, their size and distribution are uneven, and the control is complicated.

2.1.5 Eutectic solidification method

The gas has a certain solubility in the molten metal, and it increases with the increase of pressure and temperature. When the solubility of the gas in the metal reaches a predetermined value, the metal and the gas phase undergo eutectic solidification to obtain the desired aluminum foam. , by precisely controlling the cooling conditions (pressure, cooling rate, heat dissipation direction), isotropic and anisotropic high-porosity aluminum foams with various pore shapes can be obtained [18].

2.2 Deposition method

2.2.1 Sputter deposition method

The sputter deposition method is to use the sputtering technology to uniformly spray the powder with inert gas on the aluminum alloy metal, and heat it to the melting point of the metal, so that the gas added in the metal matrix expands into holes, and after cooling, it is A foamed aluminum with a foamed structure is obtained. The pore volume fraction of the product obtained by this method can be controlled by controlling the partial pressure of the inert gas in the deposition, and the mass fraction of the inclusion gas can be varied in the range of 0.015% to 0.23%.

2.2.2 Vapor Evaporation Deposition

In a relatively high pressure inert atmosphere (102~103Pa), metal aluminum is slowly evaporated, and the evaporated metal atoms undergo a series of collisions and scattering with the inert gas in the process of advancing, and rapidly lose kinetic energy, thus partially condensing to form metal smoke, The metal fume is deposited under the action of its own gravity and carried by the inert gas flow, and continues to cool down during the downward process, and finally reaches the base. Because of its low temperature, it is difficult for atoms to migrate or diffuse, so the metal fume particles are only loosely piled up to form Cellular foam structure. The aluminum foam produced by this technique is different from the foamed aluminum with a macroscopic structure. It is composed of a large number of sub-micron-scale metal particles and micropores, and its density is about 1% of the density of the parent metal aluminum, and the minimum is 0.15%.

2.2.3 Electrodeposition

This method is to cover the foamed plastic with multiple steps such as roughening treatment, sensitization and activation treatment, chemical pre-plating and chemical plating, etc., and then heat the foamed plastic to decompose to obtain foamed aluminum. Among them, the two processes of sensitization and activation treatment play a very important role. Sensitization treatment is to make the surface of the roughened part adsorb the reducing ions, so that during the ion activation treatment, the surface of the part forms a layer with catalytic properties. The role of the precious metal layer, so that the electroless plating can be carried out spontaneously, so that the foam becomes a semiconductor, which in turn promotes the success of electroplating aluminum. The aluminum foam prepared by this method has good pore connectivity, uniform distribution, and large porosity, but the thickness of the product is limited and the cost is high.

2.2.4 Molten Salt Electroplating Aluminum

Molten salt electroplating aluminum is a kind of porous aluminum foam made by electrodeposition process in molten salt containing aluminum salt, with foam plastic as electrode cathode and aluminum plate as anode. The aluminum foam made by this method has high porosity and uniform pores.

2.3 Powder metallurgy

2.3.1 Powder foaming method

The powder foaming method is to mix aluminum powder with a foaming agent (TiH2), and axially compress it at a certain temperature to obtain a preform with an airtight structure. A method of expanding the product to obtain foamed aluminum. Mixing, pressing and foaming are the three important links in the powder foaming method. At the same time, by adjusting the parameters in the foaming process (the amount of foaming agent, foaming temperature, and foaming time), foamed aluminum with different cell structures can be obtained.

2.3.2 Slurry forming method

The slurry molding method is to form a suspension of metal aluminum powder, foaming agent (hydrofluoric acid, aluminum hydroxide or orthophosphoric acid) and organic carrier, stir it into a state containing foam, and then put it into a mold and heat it After calcination, solid metal aluminum with porous structure can be obtained. This method was initially used to make foamed Be, Fe, Cu, and stainless steel materials, and later to create foamed aluminum. But the product strength is very low.

2.3.3 Powder molding method

The powder molding method is a method of mixing metal aluminum powder with a foaming agent (TiH2), forming it by cold or hot pressure, and then sintering it into foamed aluminum. It has two important advantages: one is that it can be used for a wider range of alloy compositions than other methods, thereby controlling the mechanical properties of aluminum foam; Sandwich-shaped aluminum foam, and aluminum foam-filled turbine structures

2.3.4 Loose powder sintering method

The principle of the loose powder sintering method is that when the fine particles are in contact with each other, they can be bonded to each other through the action of surface tension. These fine metal aluminum powders are put into the mold for sintering, and the particles are bonded to each other to form a porous sintered body. This method can also be used to produce filters with porosity typically 40% to 60%.

2.3.5 Impregnation sponge sintering method

In powder metallurgy, the spongy material can also be used as a temporary support structure to generate high-porosity, uniform aluminum foams. The spongy organic matter is cut into the desired shape and then infiltrated with a slurry containing the metal aluminum powder to be processed (the carrier of the suspension is water and organic liquid). The immersed sponge-like organic matter is dried to remove the flux, and after cooling, the foamed metal aluminum with a high porosity three-dimensional structure can be obtained. The method is also used to produce foamed silver sheets with porosity of 70% to 90%.

2.3.6 Fiber sintering method

The metal aluminum wire used in this method is a metal wire obtained by mechanical drawing or other methods. The metal aluminum wire is made into a felt ring by slip casting or mechanical felt ring method, and then sintered to achieve the required strength. and porosity. The porous aluminum foam prepared by this method has quite outstanding advantages: higher porosity than powder sintering can be obtained, up to 95%; all pores are interconnected pores; the maximum porosity is still maintained. Structural properties; at the same porosity, the strength and toughness are several times higher than those of powder metallurgy. It can also be used to manufacture various metal filters, such as stainless steel, copper, nickel, nichrome, etc.

2.3.7 Sintering dissolution method

The sintering dissolution method is a new method for manufacturing foamed aluminum developed in recent years. It is a method of preparing through-cell aluminum foam by mixing, pressurizing, sintering and dissolving aluminum powder and filler particles. It has the characteristics that the shape, size, porosity and distribution of pores can be precisely controlled, and it has a good comprehensive index of quality and price. It is an effective method to produce medium-density aluminum foam with uniform or gradient fine openings.

3. Properties and applications of aluminum foam

In terms of structure, aluminum foam can be divided into two types: closed-cell aluminum foam and open-cell aluminum foam. The organizational and morphological characteristics of aluminum foam, including the structure of pores (open or closed), relative density, pore size, pore shape, pore wall thickness, anisotropy, etc. Electron microscopy and X-ray tomography are used for analysis and research, and research in this area is developing rapidly. Due to different structures, its properties are very different, so it has different uses. Compared with traditional metal aluminum, foamed aluminum has the following characteristics and is widely used in metallurgy, chemical industry, aerospace, shipbuilding, electronics, automobile manufacturing and construction industries. and other fields have been widely used.

3.1 Low density

Because aluminum foam has large and small pores in the aluminum matrix, it has a smaller density. The density of aluminum foam can vary in a wide range, and the maximum porosity that can be obtained at present can reach 97%, and its pore size ranges from several micrometers to tens of millimeters. The lower the density. Can be used in packing boxes, especially air freight containers.

3.2 Strong heat resistance

Foamed aluminum has high heat resistance, and it will not melt even if it reaches the melting point of the alloy. Generally, the melting temperature of aluminum alloys ranges from 560 to 700 e, but the foamed aluminum does not melt even when heated to 1400 °C. Moreover, it does not release harmful gases at high temperature, and can replace foamed resin and asbestos products as heat-insulating and heat-resistant materials and core parts of various heat exchangers in many occasions.

3.3 Good permeability

Aluminum foam can be used as a filter material to filter out solid particles from liquid or gas. Generally, the permeability increases with the increase of pore size, but it is also affected by surface roughness. permeability. It can be used to make filters for various liquids and gases.

3.4 Large specific surface area

Using the large specific surface area of aluminum foam to achieve high heat exchange, it is a good material for making heaters and heat exchangers. In addition, it can also be used as a carrier for chemical reactions that require a huge surface, such as a catalyst carrier. , porous electrodes, plate materials for rechargeable batteries, heat exchangers, energy absorbers and catalyst carriers, etc.

3.5 Strong sound insulation

Aluminum foam can absorb the energy of sound through the vibration of the hole wall, and can be used to muffle and remove noise. Generally speaking, the sound absorption performance of through-cell aluminum foam is better. The size of the hole affects its absorption performance for the entire sound wave frequency range. The smaller the hole, the greater the sound absorption capacity.

High sound absorption performance can be obtained by changing the size and shape of the aluminum foam pores. It can be used for interior and exterior decorative parts, curtain walls, partition movable door panels in the construction industry, manufacturing high-performance sound-absorbing panels, sound-insulating walls, various mufflers, etc.

3.6 Has a high performance of absorbing impact energy

Foamed aluminum is not directional like honeycomb materials, nor does it have a rebound effect like polymer foam materials. It has good shock absorption properties and is a good material for making impact-resistant parts. It can be used in automotive brakes, tensioners, and protective sleeves and bumpers in aerospace equipment. The size of its damping is related to the size of the pore size of the pores. It can be used for safety pads for elevators and conveyors, shock absorption and energy absorption linings for high-speed grinder guards, bases for high-precision machine tools, etc.

3.7 Mechanical properties

The mechanical properties of aluminum foam are mainly determined by its density, but the size, structure and distribution of pores are also important parameters that determine the mechanical properties. Under the action of compressive stress, after the material undergoes initial elastic deformation, the aluminum foam enters the stress curve platform, that is, the aluminum foam begins to rupture, and the stress remains basically unchanged during the foam crushing stage. After a large amount of plastic deformation, the foam has been completely broken, and the material enters the dense During the chemical phase, the stress increases rapidly. Both the Young's modulus and the shear modulus of aluminum foam increase with the increase of density.

3.8 Electromagnetic shielding performance

Aluminum foam has a good shielding effect on high-frequency electromagnetic waves, which can reduce electromagnetic interference by more than 80%. Closed-cell aluminum foam with a thickness of 5mm and a porosity of 90%, the electromagnetic shielding performance at 60 ~ 1000MHz is 35 ~ 75dB, which can be used in electromagnetic shielding rooms (covers), electronic instrument enclosures, radio recording rooms, electromagnetic shielding and other occasions.

3.9 Other properties

Aluminum foam also has gas sensitivity, fire resistance, catalysis, etc. The electrical conductivity of foamed aluminum is greatly affected by the relative density, while the size of the pore size has little effect on the electrical conductivity. It is expected to have good application prospects in new fields such as aerospace, telecommunications and environmental protection.