How is Aluminum Foam Made? — Understanding the Different Production Processes of Aluminum Foam

Aluminum foamis a new type of lightweight metal material produced by Foaming aluminum alloy. It has received widespread attention worldwide due to its excellent properties, including energy absorption and cushioning, electromagnetic shielding, and sound absorption and noise reduction.

So, how is Aluminum Foam transformed from solid aluminum into a porous structure? Or, in other words, what methods are used to manufacture it?

Generally speaking, the production methods that have been used for large-scale production both domestically and internationally include infiltration, melt foaming, and air blowing. Other methods, such as precision casting, powder metallurgy, and electrodeposition, are more expensive and difficult to mass-produce. They are generally used in demanding or specialized fields with small usage volumes, such as aerospace and high-end equipment.

Aluminum foam is divided into two types: open-celled and closed-celled. In open-celled foam, the cells are interconnected, even exhibiting a three-dimensional spatial structure like a loofah sponge; in closed-celled foam, the cells are independent, with occasional slight gaps that do not affect the independence of the cells.

1. Mainstream Production Process: Percolation Method (Percolation Casting)

The percolation method is a simple and well-developed production process. The foamed aluminum material produced is through-cell, with interconnected cells. The appearance of foamed aluminum manufactured by the percolation method is shown in the image:

[Image of percolation method foamed aluminum sample]

1.1 Brief Process Principle

The process flow of the percolation method is relatively simple:

• Place salt particles into a container and heat to a suitable temperature;
• Pour molten aluminum into the container and pressurize to ensure full penetration;
• After cooling, cut the blank into thin sheets of a certain thickness;
• Use water or other solvents to wash away the salt particles trapped within the aluminum.

1.2 Process Characteristics and Advantages/Disadvantages

Advantages:

• Aesthetically pleasing appearance: High uniformity, and the appearance of freshly washed percolation method foamed aluminum is excellent.

• Higher acoustic properties: The through-cell structure is more conducive to sound absorption; in practical acoustic applications, closed-cell foamed aluminum generally requires a thicker layer.

• Low technical barriers: Production can be carried out under simple laboratory conditions, and the capital and technical barriers to small-scale mass production are not high.

Disadvantages:

• Corrosion and short service life: Salts are difficult to remove completely from porous structures. Foamed aluminum becomes damp in the air, and the high concentration of salt solution causes an electrochemical reaction, leading to aluminum corrosion. In actual use, most percolation-processed foamed aluminum exhibits significant discoloration, corrosion, and even severe salt leakage.

• Poor mechanical properties: Salt particles inevitably form sharp corners, which are detrimental to mechanical properties, making the material generally brittle. Electrochemical corrosion further exacerbates this drawback. Therefore, in practical engineering applications, the percolation method is not suitable for scenarios requiring high mechanical performance, such as energy absorption and buffering.

• Limited porosity: The natural accumulation of salt particles has a clear upper limit to their volume ratio in space. For example, the maximum space ratio of a spherical structure is 67%, meaning that the maximum porosity can only be 67% in this case. Closed-cell foamed aluminum, on the other hand, can easily achieve porosities of 90%, 95%, or even 97%. High Cost: Due to poor mechanical properties and high density, large amounts of aluminum are used, resulting in high costs for foamed aluminum produced by the infiltration method.

• Size and Thickness Limitations: Generally, the maximum size is 600mm x 600mm, with a thickness of less than 20mm.

• Wastewater Discharge: It's easy to see that washing salt particles requires a large amount of water, especially during mass production, resulting in significant wastewater discharge.

1.3 Industry Status

Poor mechanical properties, high cost, and environmental unfriendliness are all shortcomings that make the infiltration method uncompetitive in the market, while also subjecting it to significant pressure from environmental policies.

2. Mainstream Production Process: Melt Foaming Method (Melt Casting Method)
The melt foaming method was first invented in Japan and is currently the predominant method for manufacturing foam aluminum in China. Sample appearance is as follows:

Melt-foamed aluminum foam

2.1 Brief Process Principle

The process principle of melt foaming is similar to steaming bread:

• First, thicken the molten aluminum and stir thoroughly;

• Add a pre-treated foaming agent (usually titanium hydride or zirconium hydride) and stir evenly;

• Put it into a heat-insulating furnace for foaming;

• Cool and solidify, then machine.

Sounds simple, right?

Consider the following questions:

• Is the pore inside your steamed bread uniform?

• If you were to steam a two-meter-long loaf of bread, would the pores still be uniform?

• Air bubbles don't move around much in dough, but they do in liquids, a phenomenon known as "bubbling." Now, if we replace the dough with molten aluminum at 700 degrees Celsius, will the pores in this "aluminum bread" still be uniform?

• After doing all of the above, how do we ensure cost control?

See? The melt foaming method isn't difficult at all. If you can handle these dozen or so contradictory, mutually influential, and mutually restrictive process parameters, you're halfway there.

Why half and not all? You'll have to figure out what secrets I'm hiding.

As for how big the difference is, take a look:

Side cross-sections of aluminum foam at different process control levels, note the uniformity of pores and the absence of a bubble layer at the bottom.

Above are side cross-sections of melt foaming aluminum materials at different process control levels. If not handled properly, the products will be drastically different. Note that even with the highly refined melt foaming process on the left, significant differences remain in pore size, density, wall thickness, and strength between the materials at the bottom and top. This is an inherent characteristic of melt foaming and can only be mitigated, not eliminated.

2.2 Process Characteristics and Advantages/Disadvantages

Advantages:

• Good mechanical properties; capable of manufacturing aluminum foam materials with pressures ranging from 1MPa to 35MPa.
• Large porosity range; theoretical porosity 0-95%, with 60-92% commonly used in actual production.
• Furthermore, under certain processes, an effect similar to molded pearls can be achieved, as shown in the sample above.

Disadvantages:

• High process requirements: As mentioned above, precise process control is extremely difficult.
• Product consistency: Due to gravity and aluminum melt drainage, there are significant differences in density and strength between different batches. Even within the same blank, differences in density and strength exist horizontally and vertically, posing a significant challenge to performance control.
• High cost: For example, the cost of foaming agent raw materials is over ten thousand yuan per ton, and even higher after processing.
• Large production line footprint and limited capacity.

2.3 Industry Status Kyushu Steel Works in Japan, which invented the melt foaming method, has long since ceased production due to cost reasons. Some companies in South Korea and the Middle East also use the melt foaming method.

3. Mainstream Production Process: Air Blowing Method (Melted Air Blowing, Melt Injection)

The air blowing method is the most challenging and difficult mainstream aluminum foam production process.

Taking university research as an example, dozens of universities and research institutes in China have conducted research on the melt foaming method and produced prototype samples in the laboratory; however, only a handful of universities and research institutes have conducted research on the air blowing method and successfully produced samples, and all of them have now ceased research.

Even for the "relatively simple" foaming method, among the dozens of research institutes mentioned above, only two or three have achieved large-scale industrial manufacturing. If economic indicators are also considered, only one or two universities can achieve commercial operation.

From technological prototype to large-scale production, to mature products, and then to commercial distribution, every step is a challenging test.

A sample of foamed aluminum material manufactured by the air-blowing method is shown in the figure:

3.1 Brief Process Principle

Why all the preamble? Because the process principle of the air-blowing method seems simpler: Molten aluminum is melted and thickened; air is blown from below; the bubbles are collected and cooled from above.

If the bubbles are allowed to stabilize and cool uniformly after blowing, it's the static air-blowing method; if air is continuously blown while the collected bubbles are continuously removed and cooled, it's the dynamic continuous air-blowing method.

That's right, producing foamed aluminum by the air-blowing method is that simple.

3.2 Process Characteristics and Advantages/Disadvantages

The advantages of the air-blowing method are as follows:

• The only process to date capable of continuous production;
• Good mechanical and functional properties;
• High consistency and strong controllability;
• Low manufacturing cost: high material utilization and low energy consumption;
• Small production line footprint and large capacity (compared to melt foaming method).

The disadvantages or difficulties lie in: high barriers to entry in equipment design and manufacturing, complex equipment, and difficult control; continuous high-temperature production places extremely high demands on the machining accuracy and reliability of core components; and difficulties in process control and high requirements for matching process parameters.

3.3 Industry Status Cymat of Canada has been the longest-standing player in the air-blowing method field, developing a series of products with a high market positioning.