Preparation Method And Application Of Porous Metal Foam

Porous metal foam is a functional material developed in recent decades. Its concept and classification are not uniform in the academic world, but basically there are the following definitions: Porous metal foam is a metal matrix containing a certain amount and a certain size. A metal material with a pore size and a certain porosity. Porous metal foam was first produced by SoSnik in the United States in 1948 by vaporizing mercury in molten aluminum, which made people's understanding of metals have undergone a major change. It expands, thus breaking the traditional concept that metals have only dense structures. Porous foam metal material is actually a composite material of metal and gas. It is precisely because of this special structure that it has both metal characteristics and bubble characteristics, such as small density, large surface area, good energy absorption, and low thermal conductivity. (closed-hole body), high heat exchange and heat dissipation capacity (through-hole body), good sound absorption (through-hole body), excellent permeability (through-hole body), good electromagnetic wave absorption (through-hole body), flame resistance, resistance Thermal refractory, thermal shock resistance, gas sensitivity (some porous metals are very sensitive to certain gases), regenerable, good workability, etc. Therefore, as a new type of functional material, it has a wide range of uses in electronics, communications, chemical industry, metallurgy, machinery, construction, transportation, and even aerospace technology.

1. Preparation method of porous metal foam

1.1 Processes based on metal melts

1.1.1 Air blowing foaming process

SiC is first added to the molten metal. Al2O3, etc. to increase the viscosity of the molten metal" and then use a special rotating nozzle to blow gas (such as air. Argon. Nitrogen) into the melt [4!5]) Currently, Hydro Aluminium in Norway and Cymat Aluminium in Canada This method is being used to produce foamed aluminum "such as cast aluminum alloy AlSi10Mg (A359) or deformed aluminum alloy 1060" 3003 "6016" 6061, etc.) The foamed aluminum produced can be arbitrarily long in principle" and the width is the same as that of the aluminum liquid container) The porosity of aluminum foam prepared by this method is 80%~98%", the density is 0.069~0.54 g/cm3", the average pore size is 3~25 mm", and the wall thickness is 50~85!m) The advantage of the direct foaming process is that it can Continuous production of large blocks. Low density metal foam) Compared with other methods "This method has the lowest cost) Cymat can produce aluminum 1 000 kg/h" Length 1.5 m" Thick 2.5~15 cm) Foam aluminum produced by Hydro Sheet width 70 cm" Thick 8~12 cm" Length 2 m" Productivity is 500~600 kg/h) The disadvantage of this process is that it needs to be cut at end use "resulting in exposed pores" and processing due to the use of reinforcing particles " difficulty).

1.1.2 Add foaming agent method

Another way to directly foam the melt is to add a blowing agent to the melt) The blowing agent decomposes under the action of heat and releases gas" to foam the metal melt [6!7]) The method in 1986 Developed by Japan's Shinco Wire company "daily output up to 1 000 kg of aluminum foam) In this method "first add Ca" and then stir to increase viscosity" This is because CaO is formed in the melt. CaAl2O4 or Al4Ca) and then Add TiH2" it can release hydrogen in the hot melt) The melt soon begins to slowly expand "after cooling, it forms solid aluminum foam) The aluminum foam produced by this method" is one of the most widely available aluminum foams. The most uniform porosity) In some literature "ZrH2 is also used to produce aluminum foam" The foaming temperature is controlled at 670~7056" and the addition amount is 0.5%~0.6%) The size of the aluminum foam block produced by Shinco Wire Company [8] 2050mm!! 650 mm!! 450 mm" Weight is about 160 kg" Including the overall density of the shell is 0.27 g/cm3) After cutting off the edge " Density is generally 0.18~0.24 g/cm3" Average pore size is 2~10 mm) there is a density gradient in the horizontal and vertical directions "and the density is lowest in the middle of the top) It is reported that this aluminum foam is more expensive) Therefore, some other methods have also been proposed" to achieve continuous production and production of complex shapes. Foamed metal parts) Using a similar process "tungsten powder and a blowing agent can be added to the molten iron" to produce foamed iron) In addition to using Ca to adjust the melt properties, "Oxygen can also be blown into the melt. Air or other gases to increase the viscosity" can also add powdered Al2O3. MnO2 and SiC, etc.) In order to overcome the problems caused by the addition of metal vapors to the melt, the decomposition rate is too fast (" It is possible to prepare foam containing undecomposed first The low melting point eutectic compound of the agent "such as Al-Mg preform" and then the preform is added to the high melting point alloy for the foaming process) In addition, "the foaming agent can also be slightly higher than the solidus temperature. Below the decomposition temperature The metal melt is added when it is "stirred and solidified) and the composite is then heated above the decomposition temperature of the blowing agent) so the actual foaming process is carried out in the second stage)

1.2.3 Solid-gas eutectic solidification method

Ukrainian metallurgist Shapovalov et al. developed a new method for the preparation of porous metals by solid-gas eutectic transformation [9]) Certain liquid metals can form eutectic systems with hydrogen) Melting metals in a high-pressure hydrogen environment" can be obtained Metal melts containing hydrogen. When the temperature is lowered "the melt will eventually undergo a eutectic reaction" forming a solid-gas two-phase system. If the composition of the system is close enough to the eutectic composition, solid-gas separation will occur at the same temperature reaction. When the solidification speed is between 0.05 and 5 mm/s, "the hydrogen content of the solidification front increases" to form bubbles. The process parameters "must be tightly controlled" to prevent bubbles from escaping from the liquid phase. The resulting pore shape depends primarily on the hydrogen content, the pressure to which the melt is subjected, the direction and rate of heat dissipation, and the chemical composition of the melt. Usually, large pores elongated along the solidification direction are formed, "pore size 10!m~10 mm" pore length 100 mm~300 mm" aspect ratio 1~300" porosity 5%~75%. This method is called GASAR" which is the Russian acronym for Gas Augmentation. The method has been used to produce porous nickel, copper, aluminium, etc. In addition to this" the process can also be used to produce porous steel, cobalt, Chromium, molybdenum and even ceramics. However, the uniformity of the porous structure prepared by this method is sometimes unsatisfactory and needs to be further improved.

1.1.4 Seepage casting method

Porous metals can also be obtained by injecting liquid metal into the voids formed by inorganic or organic particles or hollow spheres. After casting, "the particles can remain in the metal" forming so-called composite structures, also in suitable solvents, acids, or Removal of particles by heat treatment Vermiculite, refractory clay spheres, soluble salts, loose expanded clay, sand particles, foamed glass spheres, and alumina hollow spheres can all be used as inorganic fillers that can form voids. If the solidification rate of the melt is fast enough, plastic spheres can also act as a support material for void formation. Porous metals with an open cell structure can be produced using this method. The advantage of the percolation casting method is that the pore size distribution can be precisely controlled by adjusting the size of the filler particles. "But the porosity is less than 80 percent. The pore size and its distribution obtained in the foaming technique are not controllable" and

The porosity can be as high as 98%. Parts made of porous material with this open-cell structure can be mounted on the air outlet of a pneumatic device to reduce vibration.

1.1.5 Investment casting method

The principle of the method is to infiltrate the fluid refractory material into the foam sponge, then air-dry, harden, and bake to decompose the foam sponge to form a prefabricated shape with a three-dimensional network skeleton, pour liquid metal into the prefabricated shape, and remove the refractory material after solidification. A metal foam with a three-dimensional network structure can be obtained. At present, both Japan and our institute have successfully prepared foam aluminum samples by this method. The sample prepared by this method has inheritance to the parent material, the pores are three-dimensionally connected, the structure is uniform, and is not limited by material, shape and size, and can provide through-hole metal foam for various purposes. The disadvantage is that the metal skeleton The strength is low and the process is more complicated. In addition to the above-mentioned preparation processes, there are several other methods, such as: adding hollow ball method, loose powder sintering method, fiber metallurgy method and so on. With the continuous in-depth research on porous metal materials, many countries have proposed various preparation methods. It is reported in the US patent that the US ERG company has developed a preparation process called "Duocel". A method of directly preparing foamed aluminum from superheated aluminum melt in a vacuum environment. The foamed aluminum produced by this method has low density but high strength. The Canadian aluminum company has developed a unique preparation process: air is passed into the solidifying molten metal, and the gas is condensed into a foam after the gas is discharged. This method can produce large metal foam materials, and the density of the resulting material is small. Sanders Jr. designed a production process of aluminum foam called through-shaft nozzle hollow spherical aluminum bubble, which is especially suitable for the preparation of eutectic Al-Si alloy foam.

1.2 Powder-based preparation process

1.2.1 Powder metallurgy

Powder metallurgy is also a common method for manufacturing foamed metal, which has a wide range of applications. Many metals (such as aluminum, tin, iron, gold, zinc, lead, etc.) and their alloys can be foamed by this method. The process firstly mixes the metal powder with an appropriate amount of foaming agent uniformly, and then processes the mixed powder into a dense pre-product by extrusion, hot pressing or rolling, and then heats the pre-product to the vicinity of the melting point of the mixed powder to make the foaming agent. Decomposition produces gas, and closed-cell metal foam can be obtained after cooling.

Compared with the melt foaming method, the powder metallurgy method is easier to operate and control; by selecting the foaming time and foaming temperature reasonably, foam metal with different density values can be obtained. However, the production cost of powder metallurgy is higher than that of melt foaming, and it is difficult to prepare large-volume components.

1.2.2 Gas injection foaming method

The gas injection foaming method, which is similar to the melt blowing agent foaming method, is currently the cheapest method for producing porous metal foams. The method is to blow gas directly into the molten metal melt to foam the metal melt, and the gas used for foaming can be oxygen, argon, air, water vapor, carbon dioxide and the like. Like the melt foaming agent foaming method, there are problems such as difficulty in controlling the size of the pores and their distribution in the metal matrix. The key technology is to make the molten metal have a suitable viscosity. Generally, measures such as adding calcium and silicon carbide powder tackifier are used to increase the viscosity of the metal melt. The composition of the metal should ensure a wide enough foaming temperature range, so that the formed The foam cells have sufficient uniformity and stability to ensure that the foam does not break during the subsequent collection and molding process. The biggest advantage of this method is its low cost and easy industrialized mass production

1.2.3 Sintering method

That is, at a higher temperature, the material produces an initial liquid phase. Under the action of surface tension and capillary phenomenon, the material particles contact and interact with each other. After cooling, the material consolidates and becomes a foam metal. Binder, but the binder must be removed during sintering. In order to improve the porosity of metal foam, fillers can be used. The fillers also need to sublime, dissolve or decompose. Ammonium chloride and methyl cellulose can be used as fillers agent. When preparing high-porosity metal foam, the method of sintering with organic supports can be used. First, the natural sponge or artificial sponge is cut into the required shape, so that it can fully absorb the slurry containing metal powder, and then heated to decompose the sponge after drying. , Continue to heat to decompose the organometallic compound and sinter the material. After cooling, a foamed metal with high porosity can be obtained. This method also uses metal fibers instead of powder particles to manufacture porous metals. The permeability of porous metals prepared by this method is dozens of times higher than that obtained by powder methods. In addition, it also has high mechanical strength, corrosion resistance and thermal stability.

1.3 Preparation process based on deposition technology

1.3.1 Electrodeposition method

A method of using foamed organic matter of the required specification and shape as the matrix, volatilizing liquid metal into metal vapor and depositing it on the foamed organic matter under vacuum, removing the organic matter matrix after cooling, and sintering to obtain a foamed metal material. The advantage of this method is that the preparation is fine, the porosity is high, and the pore size is regular; the disadvantage is that the investment is large, the production cost is high, and the operating conditions are strict. This method is mainly applicable to the preparation of electrode materials.

1.3.2 Vapor deposition method

The non-conductive foam organic matter is used as the matrix, and it is first roughened, that is, the organic matter is corroded with a strong oxidant under acidic conditions, so that the surface becomes easily wetted by water and produces micro-marks. After roughening, sensitization is carried out, that is, a layer of metal ions with reducing properties is adsorbed on the surface of the organic foam. Activation is carried out after sensitization, that is, another layer of metal ions with catalytic properties is adsorbed on the surface of the organic foam, and then placed in a plating solution for electroless plating to obtain a uniform metal layer that is conductively attached to the surface of the organic material. The electroless plated organics are finally electroplated to obtain the desired type of metal and thickness. The high-temperature treatment decomposes the organic matter to obtain a foamed metal material. The advantages of this method are high porosity and regular pore size; the disadvantages are troublesome operation, large investment and high production cost. This method is mainly suitable for the preparation of foamed nickel, aluminum, copper, silver, etc.

2. Performance characteristics and applications of porous metal foam

Since its inception, the porous metal foam material has the characteristics of light weight and high specific strength as a structural material; as a functional material, it has the characteristics of porous, vibration reduction, damping, sound absorption, sound insulation, heat dissipation, impact energy absorption, electromagnetic shielding, etc. Therefore, it has been more and more widely used in general industrial fields and high-tech fields at home and abroad. The specific applications are as follows: Use its vibration reduction and damping properties to make buffers and vibration absorbers, such as the landing gear of spaceships, elevator transmission safety pads, various packaging boxes, especially air transport packaging boxes, machine bed, base, shock absorber, etc. Damping ring for pinion vibration and noise, energy-absorbing lining of high-speed grinder, this application can also be regarded as the application of sound-absorbing and sound-insulating properties of porous foam metal; It has been used to make structures such as soundproofing panels, housings for electronic instruments and electrical shielding rooms in the construction industry; its porosity has been used in chemical filters, gasifiers for water purification, and oil-impregnated bearings for automatic refueling , scented decorations, etc.; using its light weight and high specific strength characteristics, it is used to make water floats, sports equipment (such as sleds, etc.), and the corresponding parts of aerospace vehicles. According to relevant information, the use of porous metal foam materials to manufacture aircraft not only has the advantages of reducing weight and saving energy, but also has the advantage that when the space station ends its mission, it can re-enter the atmosphere and burn quickly and completely in the atmosphere. It can be converted into gas to reduce space waste; using its heat dissipation performance, it has been used to make radiators; using its shock absorption, vibration reduction and damping performance,

It has been used to make impact parts for the sides and fronts of automobiles, trains, and impact protection materials for military armored vehicles.

2.1 Electrode material

With the rapid development of high-end electrical appliances (portable computers, cordless phones, etc.), the consumption of reusable rechargeable batteries with high volume ratio and high quality specific capacity is also increasing. Porous metal foams with high porosity (>95%) offer the opportunity to improve these battery properties. For example, when nickel foam is used as the electrode material for the electrode of Ni-Cd battery, the gas-liquid separation of the electrode is good, the overvoltage is low, the energy efficiency can be increased by 90%, the capacity can be increased by 40%, and it can be charged quickly. Cadmium batteries, nickel-metal hydride batteries, and rechargeable alkaline batteries tend to use nickel foam as positive and negative plates to increase capacity, which is a breakthrough in the battery industry.

2.2 Catalyst

In chemical reactions, especially in organic chemical reactions, catalysts often play a very important role. The larger the surface area of the catalyst, the better, and the high porosity makes the porous metal foam have a large specific surface area. In the chemical industry, nickel foam can be directly used as a nickel catalyst, or nickel foam can be made into a catalyst carrier. Porous metal foam with high porosity as a support may make the catalyst highly dispersed and play a greater role, and its performance is far superior to that of ceramic catalyst supports.

2.3 Fluid pressure buffer material

Porous metal foam can be installed in a gas or liquid pipeline. When the fluid pressure or flow rate on one side fluctuates strongly, the porous metal foam material can absorb part of the kinetic energy of the fluid and retard the penetration of the fluid, so that the porous metal foam can be absorbed. The fluctuations on the other side of the metal body are greatly reduced, and this effect can be used to protect precision instruments.

2.4 Mechanical vibration buffer material

When the porous metal foam is placed at the joint of the vibration part, a part of the mechanical impact energy can be absorbed by the elastic deformation of the porous foam material. According to reports, the energy absorption of aluminum foam with a density ratio of 0.05 to 0.15 g/cm3 is 20 to 180 MJ/m3. The strong energy absorption capacity makes it possible to use it in the bumper of the car and even the landing gear of the spacecraft. It can also be used as a buffer in the manufacture of lift transportation systems, energy-absorbing linings in grinding machinery, deformable materials in front and rear of car passenger seats to improve safety, and excellent vibration damping properties also make foam technology possible for rockets and jets. Engine support material.

2.5 Sound absorbing material

Sound wave is also a kind of vibration, so when the sound passes through the porous metal foam, it can be scattered and interfered in the material, and the sound energy is absorbed by the material, so the porous metal foam can also be used as a sound absorbing material, that is, a sound-absorbing material, which is a sound-absorbing material. Applications are available in both gas pipelines and steam pipelines.

2.6 Flame retardant and explosion-proof materials

Porous metal foam has good fluid penetration and can effectively prevent the spread of flame and has a certain fire resistance, so it can be placed in the pipeline for transporting flammable liquid or gas to prevent the spread of flame, because the fluid Ignition is possible when the transport speed is increased (the speed of sound produces a pressure of about 15 MPa near the explosion limit). Experiments show that [13], 6 mm thick porous metal foam can stop the flame of hydrocarbon burning speed of 210 m/s. The mechanism can be explained that when the high temperature gas or particles in the flame pass through the porous metal foam material, Due to rapid heat exchange, heat is absorbed and dissipated, causing the temperature of the gas or particles to drop below the ignition point and the spread of the flame is prevented.

2.7 Spontaneous perspiration cooling material

The solid coolant is melted and infiltrated into the porous skeleton made of heat-resistant metal. When subjected to high temperature, the coolant inside the material will melt and vaporize and absorb a large amount of heat energy, so that the material can maintain the coolant gas for a certain period of time. At the level of temperature, the escaping liquid and gas will form a liquid film or gas film on the surface of the material, which can isolate the material from the external high temperature environment. This process can be carried out until the coolant is exhausted. Since the cooling mechanism is equivalent to The material itself "sweates", so it is called a self-perspiring cooling material.

2.8 Divergent cooling material

Divergent cooling is an advanced cooling technology that forces a gaseous or liquid cooling medium to pass through a porous material, so that a continuous and stable gas boundary layer with good thermal insulation performance is established on the surface of the material to isolate the material from heat flow. open to get a very ideal cooling effect. Taking the liquid hydrogen-liquid oxygen engine thrust chamber injector panel as an example, after using divergent cooling, one side of it is hydrogen at -150 °C, and the other side is gas at 3500 °C, and the hot surface temperature of the material is only 80-200 °C. °C between [14]. The porous material used for divergent cooling must be able to accurately control the amount of infiltration within a reasonable range, with uniform ventilation, small tortuous pores, and smooth flow of the medium, and must meet the basic requirements as a heat-proof structural material, with certain strength and stiffness. and toughness, select materials with good anti-oxidation properties to prevent accidental oxidation blocking pores, sintered wire mesh porous foam material is the best choice.

2.9 Filter material

The porous metal foam is prepared into the appropriate shape, and it can be used as a filter material to filter out solids or suspensions from fluids (such as water, solutions, gasoline, lubricating oils, refrigerants, polymer melts). Commonly used porous metal foam materials are bronze or stainless steel. In very corrosive fluids, precious metals such as Au are used.

3. Preparation of aluminum alloy solder by powder metallurgy

3.1 Experimental materials and methods

The A1-Si solder powder with a particle size of 45-105 ^m and the KAlF4 flux powder with a particle size of 25-45 were uniformly mixed in a mass ratio of 9:1, and pressed into a quasi-40mm cylindrical powder on a cold isostatic press. The unit pressing pressure is 100-300MPa. Then in a vacuum sintering furnace with a vacuum degree of 10-3Pa, sintered at 300-550 °C for 2 hours, and cooled to room temperature with the furnace. Then, the sintered blank was extruded with a loot hot extruder, with an extrusion ratio of 64:1, an extrusion speed of 2.2m/min, and an extrusion temperature of 400°C to extrude a quasi-5mm brazing filler metal. Density was measured using the drainage method. The metallographic samples were mechanically polished and etched with standard Keller reagent (0.5% HF + 1.5% HCl + 2.5% HNO3 + 95.5% H2O), and the microstructure of the material before and after hot extrusion was observed with a QUANTA200 scanning electron microscope.

3.2 Experimental conclusion

(1) The size of the pressing force determines the density of the self-fluxed aluminum solder powder. The higher the pressing force, the higher the density of the powder. When the pressing pressure is low, the density of the powder increases rapidly with the increase of the pressing force; when the pressing force is high, with the increase of the pressure, the density of the powder increases slowly. When the pressing force is about 150MPa, the relative density of the powder can reach 80%, and the powder has the conditions for subsequent sintering and hot extrusion.

(2) The conventional sintering process (including vacuum sintering) cannot increase the density of the self-fluxed aluminum solder powder. When sintered at a temperature lower than the solidus, the density of the sample does not increase, but decreases; higher than the solidus Temperature sintering, the sample will melt. And the sintering temperature increases, the powder sintered density will not increase accordingly.

(3) During the hot extrusion process, the sintered billet undergoes plastic deformation, the voids and boundaries between the internal particles disappear, the voids are reduced, and the relative density of the sample reaches 96.7%. From the phase composition point of view, white particles KAlF4, small black dots and primary crystal Si are relatively uniformly dispersed on the A1-Si matrix.

Porous metal foam has various physical properties such as porosity, vibration reduction, damping, sound absorption, sound insulation, heat dissipation, impact energy absorption, electromagnetic shielding, etc. Therefore, it has been more and more widely used in general industrial fields and high-tech fields at home and abroad. . The current research on porous metal foams is mostly carried out by metallurgical or metal material workers using single-disciplinary methods, and the research on porous metal foams should start from the integration of multiple disciplines and knowledge. It is difficult to achieve breakthroughs in single-disciplinary research, and It is advisable to decouple research from application. Future research should adopt multi-disciplinary cross-penetration, overcome the phenomenon of material preparation and application being disconnected, and conduct targeted research with demand as the object, so as to speed up the process of transforming science and technology into real productivity.