Aluminum foam material composition design and optimization

Aluminum foam is a lightweight porous metallic material with excellent performance. This performance depends largely on the composition design and optimization of the material. The following summarizes the composition design and optimization of aluminum foam materials, from main raw material selection to alloying element addition and microstructure control in the foaming process.
1. Selection of main raw materials
Aluminum foam core material generally chooses the following two materials as the main raw material:

• Pure aluminum (Al): It has a low melting point and good processing performance.
• Al-Si alloy: Because its melt properties (melting point, viscosity, surface tension, etc.) are superior to pure aluminum, it is most widely studied in the preparation of aluminum foam.
  In the Al-Si alloy system, the alloy with Si mass fraction of 5%~12% is usually used, and the advantages of Al-Si alloy include: the melt has moderate viscosity and fluidity, which makes it easy to regulate the foaming performance. The properties can be further optimized by adding other alloying elements.

2. Addition of alloying elements and their effects
To improve the structural parameters and overall performance of aluminum foam, appropriate amounts of alloying elements such as Cu, Mg, Zn, etc. are usually added to Al-Si alloys. The functions of different elements are as follows:

• Addition of Cu
  Principle: Adding about 3% Cu to the Al-Mg-Si composite system can reduce the size of Mg₂Si particles from 30 μm to 10 μm, which can significantly increase the total surface area of the particles and the viscosity of the melt. When 5% Cu is added, the solidification mode changes from layer-by-layer solidification to simultaneous solidification, which improves the organization of the material.
  Effect: Prolong the stabilization time of the pores in the foaming process. Reduce the occurrence of pore wall defects and cracks. Improve the hardness and corrosion resistance of the material.
• Addition of Mg
  Principle of action: Adding an appropriate amount of Mg (about 2%) can effectively improve the structure of the bubble pore and increase the stability of the foaming process. The addition of Mg can help to reduce the entanglement of oxide particle clusters, thus making the structure of the bubble pore more regular and the pore wall more uniform.
  Effect: Improve the homogeneity and stability of the pores. Reduce the collapse phenomenon.
• Optimization of Si
  In Al-Si alloys, reducing the particle size of Si particles has an obvious effect on improving the vesicle structure. Relevant experiments have proved that adding 1% Mg to the powder system of Al-Si alloy can significantly improve the uniformity and smoothness of the pore wall, and make the foaming process more stable.

3. Melt flow and viscosity control
Aerospace and automotive industry requirements: A good flow of aluminum melt is essential for the manufacture of thin and lightweight aluminum alloy castings.
Aluminum foam preparation needs: In order to achieve a better foaming effect, a relatively low melt fluidity (high viscosity) needs to be maintained.
Methods of viscosity adjustment: Adding alloying elements (e.g. Cu, Mg) increases the melt viscosity by changing the particle size and distribution. Adjust the ratio of alloy composition to optimize the solidification zone and solidification mode.
4. Microstructure optimization
Through microstructure analysis (e.g. optical microscopy and SEM observation), the following optimization strategies can be derived:

• Reduce the entanglement problem of oxide particles: Simple oxide particles do not improve the foaming performance significantly but will lead to irregular bubble holes and easy to collapse. The addition of Mg can significantly improve the distribution of oxide particles and make the bubble structure more regular.
• Optimization of eutectic composition: Adding Mg to Al-Si alloy to form a eutectic system (e.g. Al-Mg-Si) can significantly improve the foaming performance. The eutectic alloy powder system shows higher stability in the foaming process.

5. Related Research
Research: To study the effects of Mg and Si on the pore wall structure, pore size, and distribution uniformity in the foaming process. It was found that the addition of single Si or Mg to pure Al powder did not have a prominent effect, whereas the addition of about 1% Mg to Al-Si alloys could significantly improve the foaming stability, resulting in more uniform pore size and smoother pore walls.
Experimental comparison: Experiments under optical microscope show that single oxide particles cannot improve the foaming effect, but lead to irregular pore structure. With the addition of an appropriate amount of Mg, the distribution of foam pores is more uniform and the stability is significantly improved.
The core of the compositional design of aluminum foam materials is to optimize the melt properties and foaming properties of the materials through alloying means and microstructure modulation. The following are the main points of the optimization strategy:
1. Prefer Al-Si alloy (Si content 5%~12%) as the main raw material.
2. Add an appropriate amount of Cu (about 3%~5%) to improve the melt viscosity, improve the structure of the bubble holes, and prolong the stabilization time of the bubble holes.
3. Add an appropriate amount of Mg (about 1%~2%) to improve the stability of the foaming process, the smoothness of the pore wall, and the uniformity of the pores.
4. Reduce the particle size of Si to optimize the eutectic composition of the alloy melt.
Through the above optimization, the performance of aluminum foam materials in the aerospace and automotive industries will be significantly improved.