Foam aluminum has a series of excellent properties, such as high porosity, large specific surface area, high specific strength, high specific stiffness, good energy absorption, damping and shock absorption performance, corrosion resistance, high temperature resistance, electromagnetic shielding, non-toxic, easy processing, coating surface treatment, etc. Because of its excellent physical and mechanical properties, foam metal can be used as both structural materials and functional materials.
Features of foam aluminum
Foam aluminum is a kind of porous material with numerous bubbles distributed in the metal aluminum matrix. Its special structure determines that it has special properties that many compact metals do not have.
Structural characteristics of foam aluminum
The metal skeleton composition and pore structure are controllable, which can meet different needs.
Large aperture: 0.3-7mm;
Diversified pore structure: closed cell, through-hole and micro through-hole foam aluminum;
High and controllable porosity: 63%-90% ;
Large specific surface area: 10-45cm/cm2.
Performance features of foam aluminum
Lightweight
The density is only 10% - 40% of the metal aluminum;
High specific stiffness
The bending stiffness is 1.5 times of steel;
High damping and shock energy absorption
The damping performance is 5-10 times of that of metal aluminum;
Good sound insulation (closed hole) and sound absorption (through hole)
When the acoustic frequency is between 800~4000HZ, the sound insulation coefficient of closed cell foam aluminum is more than 0.9;
When the sound frequency is between 125~4000HZ, the sound absorption coefficient of through-hole foam aluminum can reach 0.8;
Excellent electromagnetic shielding performance
When the electromagnetic wave frequency is between 2.6~18 GHz. Electromagnetic shielding capacity of foam aluminum can reach 60~90dB ;
Low thermal conductivity
The thermal conductivity of closed cell foam aluminum is equivalent to that of marble; The through-hole foam aluminum has good heat dissipation.
The performance of foam mainly depends on its porosity, pore diameter, through porosity, pore type, specific surface area and other pore structure parameters. The pore structure parameters mainly depend on the preparation process.
Preparation technology of foam aluminum
The preparation technology of foam has become a research focus in the field of new materials. The following is a detailed introduction to the preparation process of foam aluminum:
1. Solid metal sintering method
Most of the foam aluminum produced by this method has a through-hole structure. This is due to the fact that most of the aluminum particles are connected with each other through sintering, and the aluminum is always kept solid.
1.1 Powder metallurgy foaming method
The process principle is to mix aluminum powder and foaming agent powder and compress them to obtain a preform with a gas tight structure. Heating the preform will cause the foaming agent to decompose and release gas, forcing the preform to expand to obtain foam aluminum.
Process flow of powder metallurgy foaming method:
Features: First, compared with other methods, the available alloy composition is more extensive, which is conducive to improving the mechanical properties of foam aluminum; Second, it can directly manufacture components with complex shapes.
The disadvantage is that the process parameter range of this method is narrow, the cost is high, and the size of foam aluminum produced is limited.
1.2 Loose powder sintering
This method is mostly used to prepare foam copper. Because the dense oxide film on the surface of aluminum powder will prevent the particles from sintering together, it is relatively difficult to prepare foam aluminum by loose powder sintering method. At this time, the oxide film can be destroyed by means of deformation to make the particles stick together more easily; Or add magnesium, copper and other elements to form eutectic alloy when sintering at 595~625 ℃.
This production method includes three processes:
Features: The advantages are simple process and low cost. The disadvantages are low porosity and low material strength. If fiber is used instead of powder, porous materials can also be obtained.
1.3 Slurry molding method
The slurry forming method is to form a suspension of metal aluminum powder, foaming agent (hydrofluoric acid, aluminum hydroxide or orthophosphoric acid), reaction additive and organic carrier. Stir it into a state containing foam, and then put it into the mold for heating and roasting. Then the slurry starts to become sticky, and with the gas generated, it starts to expand, and finally gets foam aluminum with certain strength.
If the slurry is directly poured into the polymer foam, the polymer material can be pyrolyzed by heating up, and the open cell foam material can also be made after sintering.
This production method includes:
Features: The foam aluminum produced has low strength and cracks.
1.4 Sintering dissolution method
The aluminum powder and salt powder are evenly mixed and pressed into billets. During the pressing process, the salt powder basically keeps its original appearance. The aluminum powder undergoes plastic deformation and fills the gap between the salt particles to form a continuous network matrix. Then, the billets are sintered to combine the reticulated aluminum matrix into a whole. Finally, the sintered billet sample is placed in hot water, and the salt particles in the billet are filtered out to obtain uniform open cell foam aluminum parts.
The process includes:
Features: The advantage is that by selecting the shape and particle size of the salt powder, the shape and size of the holes can be controlled within a certain range; The porosity can be precisely controlled by the volume ratio of mixed powder; Can produce gradient foam materials; It can manufacture net products; The equipment is simple and easy to realize mass production.
The limitation is that only medium density foam aluminum with a pore range of 50%~80% can be obtained; Sodium chloride is easy to remain in the finished product, causing local corrosion of aluminum base; The process cycle is long.
1.5 Hollow three-dimensional skeleton method
The liquid metal is diecasted into a ceramic with a hollow skeleton 3D mesh structure, cooled, and then the skeleton is removed.
The process includes:
Features: The porosity of foam is adjustable, the operation is cumbersome, the cost is slightly high, and the product range is limited, so its promotion and application are limited.
1.6 Fiber sintering
The process of this method is to first obtain aluminum wire by mechanical drawing or other methods, then make aluminum wire into felt ring by slurry casting or mechanical felt ring, and then sinter it to achieve the required strength and porosity.
The process is as follows:
The advantage of fiber sintering method is that it can obtain higher porosity than powder sintering. The structural properties of the material are maintained at the maximum porosity. At the same porosity, the strength and toughness of foam aluminum produced by this method are higher than those by powder metallurgy method. However, the cost of this method is high.
1.7 Slurry soaked sponge sintering method
The method is to make spongy organic substances into organic precursors of the desired shape, and then use the slurry containing the metal aluminum powder to be processed to penetrate (the carrier of the suspension is water and organic liquid). The soaked organic precursor is dried to remove solvent, sintered and cooled to obtain foam aluminum with high porosity and three-dimensional structure.
The process flow is roughly as follows:
Features: It is mainly affected by the selection and pretreatment of organic precursors, the composition of slurry, the selection of additives, sintering temperature and other factors.
2. Solidification of liquid metal
This method is to produce foam structure through liquid aluminum. It can be directly foamed through aluminum liquid; Porous materials can also be obtained by casting foam materials or closely packed pore forming agents.
2.1 Direct blowing foaming method
First, add the SiC, Al2O3, etc. into the molten metal, and uniformly dispersed to improve the melt viscosity. Then blow gas (such as nitrogen, inert gas, etc.) into the bottom of the melt. A large number of pores are formed in the liquid metal and then cooled and solidified.
Features: continuous preparation of products can be realized; Simple requirements for equipment; The porosity of the product is controllable; Low cost.
2.2 Foaming agent foaming method
Add foaming agent into the aluminum melt and stir it evenly. Heat it to make the foaming agent decompose to produce gas. The gas expands and foams. After cooling, foam metal is obtained. The foaming agent used is usually metal hydride such as TiH2 or ZrH2.
Features: simple equipment requirements, low cost, and continuous product preparation. Short foaming time interval, difficult control of foaming temperature, uneven bubble distribution, poor product reproducibility, etc.
2.3 Seepage casting
Percolation casting is to stack removable particles (such as NaCl) in the mold, press them into billets, pour metal after preheating, and then remove the particles to prepare a through-hole foam structure with interconnected holes.
Process principle:
Schematic Diagram of Vacuum Seepage Method Device
Features: The preparation process has controllable pore diameter parameters, high through-hole ratio, large specific surface area, low cost, and is suitable for large-scale industrial production. The disadvantage is that due to the high surface tension of liquid metal, the particles can not be completely wetted, so the gap between particles can not be completely filled.
2.4 Investment casting
The investment casting method is to immerse the molded polymer foam material into the liquid refractory to fill the gap of the refractory. After the refractory is hardened, heating will vaporize and decompose the foam material to form a three-dimensional framework with the shape of the original foam material. Pour the molten aluminum into the casting mold, remove the refractory after solidification, and then obtain foam aluminum with three-dimensional mesh through holes.
Schematic diagram of process principle:
Schematic Diagram of Porous Materials Prepared by Investment Casting
Advantages: various foam metals can be prepared; Opening structure, good production repeatability and relatively stable density.
Disadvantages: low output; high price.
2.5 Solid-gas eutectic solidification
Many metal liquids can form eutectic systems with gases (such as hydrogen). If these metals are melted in a high-pressure hydrogen atmosphere, a uniform melt containing supersaturated hydrogen can be obtained. In the subsequent cooling and solidification process, the melt will undergo eutectic transformation and decompose into solid and gas phases. During directional solidification, because the solubility of hydrogen in the solid phase and liquid phase differs greatly, the supersaturated hydrogen will separate from the solid phase to form bubbles, thus obtaining the required foam aluminum.
Schematic Diagram of Process Principle:
Schematic Diagram of Solid Gas Eutectic Solidification Process for Preparing Porous Materials
Features: High porosity foam aluminum with isotropic and anisotropic pore shapes can be obtained by precisely controlling cooling conditions (pressure, cooling rate, heat dissipation direction).
2.6 Ball feeding method
The method of adding ball material is to add particles or hollow balls into the aluminum alloy melt and strengthen the stirring. And casting while the melt is still in relative flow to obtain the composite of aluminum alloy and particles. Then the soluble particles in the aluminum alloy collective are dissolved and removed, and finally a connected porous foam aluminum is obtained.
The process flow is roughly as follows:
Features: The surface tension of liquid metal is high, and the particles or hollow balls cannot be completely wetted, so the gaps between particles cannot be completely filled. The structural continuity of the obtained foam aluminum is poor.
3. Metal deposition
3.1 Electrodeposition
The principle is that foam aluminum is electroplated in alkyl aluminum solution with pretreated foam plastic as cathode and industrial pure aluminum plate as anode.
Process flow of electrodeposition method:
Features: easy to control pore structure, small pore size, uniform pore size, high porosity, and its heat insulation and damping characteristics are better than that of foam aluminum produced by casting method. However, this method has long process, complicated operation, slightly high cost and limited product thickness, so its popularization and application are limited.
3.2 Vapor phase evaporation deposition method
This method is to slowly evaporate the metal aluminum in a high inert atmosphere (102~104Pa). The evaporated metal atoms collide and scatter with the inert gas molecules, rapidly losing kinetic energy. This process is shown in the macroscopic view as the metal vapor temperature decreases. Then the evaporated metal atoms combine with each other to form atomic clusters before reaching the substrate, so "metal smoke" can be seen in the evaporation process. These clusters continue to cool down and deposit on the substrate with inert gas. Because atoms at low temperature are difficult to migrate or diffuse, "metal smoke" particles are loosely stacked to form a hollow foam structure.
Schematic Diagram of Process Principle:
Schematic Diagram of Vapor Phase Evaporative Deposition Process
Features: The formation of metal foam is affected by many factors, such as metal materials, heating power, inert gas pressure, type of vaporization source heater and its distance from the substrate, and substrate materials. The heating power, inert gas pressure and inert gas flow rate are the most important control parameters.
3.3 Splash deposition
Splashing deposition is to evenly spray the powder with inert gas onto the aluminum alloy metal by using the spraying technology. Then it is heated to the melting point of the metal, so that the gas added to the metal expands and forms uniformly distributed and dense holes. After cooling, it is made of foam aluminum products with dense network.
Schematic diagram of process principle:
Schematic Diagram of Splash Deposition
Features: By controlling the partial pressure of inert gas in deposition, the volume fraction of pores of the obtained product can be controlled.
3.4 Molten salt electroplating
Foam aluminum is prepared by electrodeposition in molten salt with foamed plastic as cathode and aluminum plate as anode.
Schematic diagram of process principle:
Schematic Diagram of Electroplating Process Principle
Features: foam aluminum has high porosity and even pores.
4. Others
The following methods are mainly used for scientific research or small batch trial production, and are not widely used in industrial production.
4.1 Secondary foaming method
The secondary foaming method is a preparation method of foam aluminum that combines the advantages of powder metallurgy foaming method and melt foaming method. The technical process is to add viscosity increasing agent (Ca, Al2O3, etc.) into the aluminum melt and stir it evenly. Add foaming agent (pretreated TiH2) under appropriate temperature and viscosity conditions. It is uniformly dispersed, and the melt is cast into the mold for rapid cooling and solidification before TiH2 is decomposed to obtain foaming precursor. When the foaming precursor is heated to a certain temperature, TiH2 in the precursor begins to decompose and foam, and finally foam aluminum is prepared.
4.2 Metal hollow ball method
The method is to form a porous structure by bonding metal hollow spheres together through sintering. Metal hollow spheres can be obtained by chemical synthesis and electrodeposition of a layer of metal on the surface of polymer spheres, and then the polymer spheres are removed.
There are many preparation processes of foam aluminum, and each method has its own advantages and disadvantages. In practical production, melt foaming method, percolation casting method, powder metallurgy foaming method and electrochemical method are widely used. Other processes are mainly used for scientific research or small batch trial production.
Application of foam aluminum
Application of foam aluminum in cars
The application of foam aluminum in the automotive industry mainly includes lightweight structure, energy absorption structure and damping heat transfer structure, as shown in the figure below. The three circles represent different application fields, and the characters on the outside of the circle illustrate the advantages and characteristics of foam aluminum corresponding to the three application fields. The overlapping part of two circles represents the dual function integration of foam aluminum. The ideal application is the multi-function integration represented by three circles coincidence.
This foam aluminum is produced by "gas blowing into liquid metal" method. In this process, the matrix material (forged aluminum or cast aluminum alloy) is usually melted with traditional casting equipment, and then 10%~30% (vol) of viscosity increasing agent (SiC or Al2O3 particles) is added, and the viscosity increasing agent is uniformly distributed by stirring in an agitator.
Pour the mixed melt into a container with a funnel, and inject the gas into the agitator through a small nozzle to form dispersed small bubbles. The bubble size can be controlled by adjusting the gas flow rate, agitator design (number and size of nozzles) and mixing speed. Bubbles rise to the surface and gather.
The ceramic particles surrounded by bubbles can stabilize the pore wall and delay the coalescence of bubbles with proper interfaces; At the same time, it can also increase the melt viscosity and slow down the rising rate of bubbles. The liquid metal foam is transported through the conveyor belt and cooled and solidified at the same time to obtain closed cell foam.
The relative density of foam is mainly affected by process parameters, such as stirring speed, gas flow rate, number of particles in the melt and solidification conditions. As the filler of automobile products, foam aluminum has been widely used in German GAOFISHER automobiles.
Chalco foam aluminum has excellent performance. The engine hood made of Chalco foam aluminum can resist the impact of 11m/s headshape, which can effectively protect passengers. Chalco foam aluminum filled impact energy absorption box can withstand the impact of 5m/s.
The bending strength of A and B columns filled with Chalco foam aluminum is three times that of hollow columns. Chalco foam aluminum can cast complex ribbed structure, thus simplifying mold design and processing methods.
Main performance parameters of Chalco foam aluminum:
relative density ρ/ρS=0.02GPa～0.2GPa,
Young's modulus E=0.02GPa～210GPa,
shear modulus G=0.01GPa～1.0GPa,
bending modulus Ef=0.03GPa～3.3GPa,
Poisson's ratio υ= 0.31～0.34,
Compressive strength σ C=0.04MPa～7.0MPa,
tensile strength σ T=0105MPa～8.5MPa,
thermal conductivity λ= 0.3W/(m・K)～10W/(m・K).
The application of auto parts made of foam aluminum sandwich structure in cars is shown in the figure.
In addition to the applications identified in the figure, foam aluminum can also be used in the impact energy absorption structure between the front longitudinal beam, rear longitudinal beam, bumper and chassis, internal appliances and decorative parts, fender, top cover plate, top cover longitudinal beam, rear diaphragm, connecting rod, piston, lower control arm, transmission gear, cylinder block, brake cylinder piston, muffler, etc.
Application of foam aluminum in high-speed railway and transportation
Anti crash structure of high-speed rail train
There are serious security hidden danger of collision of high-speed trains, due to the trains without effective anti-collision structure. Using aluminum tube energy absorbing column filled aluminum foam made locomotive head anti-collision structure shown below, anti collision structure between carriages shown below, when the train speed is below 70Km, the anti collision structure can absorb all the impact energy, so that the train will buffer and stop, so as to ensure the life safety of the passengers.
The sound barrier made of aluminum foam on high-speed rail line
The sound barrier of high-speed rail line at present is a counterfeit product, its sound absorb box is of 140mm thickness, suitable for the frequency of 100 ~ 200Hz of low frequency noise, does not have the sound absorption and noise reduction function, but High-speed Rail line noise is high frequency (frequency of 1000 ~ 2000Hz). The aluminum foam sound box is made of 10~15mm thickness foam aluminum sound absorption board and the folded type galvanized steel partition board at the middle, its cavity is 0 ~ 30mm. The backplane is made of 1mm thick galvanized plate, as shown below. This kind of sound barrier to adapt the frequency of High-speed Rail noise of 500Hz ~ 2000Hz, can be more than 20 dB noise reduced.
Safety school bus made by aluminum foam
Before and after the safety school bus anti-collision beam is made by using aluminum tube absorption column filled with aluminum foam, its surface made of 1～2mm thick steel plate. The anti-collision beam can absorb most of the impact energy when the school bus collision. The body upright column and cross beam are made of square steel tube filled with aluminum foam, its ability of anti rollover will be increased doubled.
The aluminum foam sound barrier on highway
Aluminum foam sound barrier has been build in many cities, aluminium foam absorption board is
800mmx 2000mm, its cavity 70mm, the noise can be decreased 20dB.
Application of foam aluminum in military industry
Light missile manhole cover
The missile manhole cover made of reinforced concrete materials, its weight reach to more than 600 tons, very heavy, opening and closing difficulty. The new missile manhole cover made of armor plate and aluminum foam, can make anti explosion strength more than 10000Mpa, the anti armor piercing capability than the original cover, and its weight be reduced to 1/6.
Wartime quick mounting large span bridge made of aluminum foam
In wartime, the armored forces need quick mounting support bridge to across the river and ditch. The quick mounting support bridge at home and abroad is made of steel, its maximum length is 53 meters. The 70 meters long bridge can be manufacture by aluminum foam materials, as shown below.
Light explosion door
The existing explosion-proof door adopts reinforced concrete and steel plate structure, its total thickness is 300 ~ 400mm. Maximum antiknock ability of the explosion door is about 3Mpa, but its weight is 20 ~ 30 tons, so opening and closing difficulty. The light explosion-proof door structure made of aluminum foam, its panel are made of carbon steel plate of 5mm thickness and intermediate aluminum foam is about 100mm thickness. The antiknock ability of this kind of 870 explosion door reach more than 1000Mpa, but its weight reduce to several tons.
Aluminum foam sandwich flight deck of aircraft carrier
When the plane landed on the aircraft carrier flight deck, due to the rebound effect , can not easily be arrested by the cable attachment, resulting in some aircraft cannot stop on the deck. Using aluminum foam sandwich flight deck, the plane bounce height is reduced by more than 50%, ensuring plane smooth glide, increasing the chance to be the arrested by cable attachment. In addition, the blast resistance and anti missile combat ability will significantly improved.
Heavy equipment airborne buffer table made of aluminum foam
Russian airborne 20 tons of equipment adopt 4 meters high big balloon, do not stable when landing. Using the filling foam aluminum aluminum tube energy absorbing column, the heavy equipment airborne buffer pad can be made as shown below its height is about 500mm, 20 tons of equipment can be dropped, to ensure the smooth landing and safety equipment.
