Preparation process of porous powder metallurgy materials of aluminum magnesium alloy

Aluminum magnesium alloy porous powder metallurgy materials have the characteristics of lightweight, high specific surface area, and functional integration by regulating the pore structure (pore size, porosity, connectivity). The core of their preparation process lies in the control of powder properties and the design of pore formation mechanisms. The following analysis will focus on traditional processes, emerging technologies, and process optimization directions.

1、 Traditional preparation process: pore forming agent and foaming method

Adding pore forming agent method

Process principle: Mix aluminum magnesium alloy powder (such as AZ91, particle size 20-100 μ m) with a removable pore forming agent, and after compression molding and sintering, remove the pore forming agent through dissolution or volatilization to form pores.

Type of pore forming agent:

Soluble type: Sodium chloride (NaCl, particle size 50-200 μ m), ammonium bicarbonate (NH ₄ HCO ∝, particle size 20-100 μ m), suitable for preparing connected pore structures, with a porosity of up to 30% -60%, and a positive correlation between pore size and pore forming agent size (such as 80-120 μ m pore size for 100 μ m NaCl particles).

Ablative type: Polystyrene (PS) microspheres, starch granules, decompose and evaporate during high-temperature sintering, suitable for preparing closed or semi connected pores with a porosity of up to 70%, but residual carbides may affect corrosion resistance.

Key control: The dispersion uniformity of the pore forming agent determines the pore distribution, which needs to be achieved through ball milling mixing (rotation speed of 200-300 rpm, time of 2-4 hours) to achieve micrometer level uniform dispersion; The sintering temperature should be lower than the melting point of the pore forming agent (such as the melting point of NaCl at 801 ℃, and the sintering temperature of aluminum magnesium alloy should be controlled at 500-550 ℃) to avoid premature melting.

foaming

Chemical foaming: Adding decomposition foaming agents (such as TiH ₂, ZrH ₂) to aluminum magnesium alloy powder, releasing gas (H ₂) during the sintering process, forming a closed cell structure. The foaming agent content (0.5% -2%) and decomposition temperature (such as TiH ₂ starting to decompose at 400 ℃) need to be matched with the sintering heating rate, with a typical porosity of 20% -40% and pore size of 50-300 μ m.

Physical foaming: introduce bubbles into the molten aluminum magnesium alloy liquid through mechanical stirring or ultrasonic vibration, and then rapidly cool and solidify. It is suitable for preparing foam materials with large pore size (>500 μ m), but it is less used in powder metallurgy solid process, and more used for direct foaming of liquid metal.

2、 Emerging Preparation Technologies: Additive Manufacturing and Template Method

Additive Manufacturing (3D Printing)

Selective Laser Melting (SLM): Using laser to melt aluminum magnesium alloy powder (particle size 15-53 μ m) layer by layer, with unmelted powder as a support to form pores. By adjusting the scanning strategy (such as scanning spacing of 0.1-0.2 mm, laser power of 100-200 W), complex connected pore structures with pore sizes of 50-500 μ m and porosity of 10% -50% can be prepared. The advantage lies in the ability to form biomimetic porous structures (such as honeycomb and truss structures), but it needs to address the volatilization loss of magnesium during laser melting (volatilization rate of about 5% -10%) and oxide scale defects.

Electron Beam Melting (EBM): Using electron beam heating in a vacuum environment to reduce magnesium oxidation, it is suitable for preparing high-purity porous components with better porosity controllability than SLM, but the equipment cost is high.

templating

Biological template method: Using natural porous materials such as wood and sponge as templates, the template is removed by impregnating aluminum magnesium alloy slurry (powder to liquid ratio 1:1-2:1), drying and sintering (temperature 600-700 ℃) to replicate the natural pore network. The obtained material has graded pores (micrometer level main pores+nanometer level micropores) with a pore size of 100-1000 μ m, suitable for biomimetic catalysis or energy storage carriers.

Mesoporous template method: Using mesoporous silica templates such as SBA-15 and MCM-41, aluminum magnesium alloy materials with nanoscale pores (pore size 2-50 nm) are prepared for high-capacity hydrogen storage or catalyst support, but the process is complex (requiring hydrofluoric acid to remove the template) and the cost is high.

3、 Process optimization and composite technology

Multi level pore construction

Preparation of multi-level pore structure through the synergistic effect of "pore forming agent+foaming agent": First, micron sized NaCl particles are added to form large pores (100-200 μ m), and then nano-sized foaming agents (such as nano TiH ₂) are introduced to form micropores (<10 μ m) at grain boundaries, increasing the specific surface area to over 10 m ²/g, suitable for battery electrodes or adsorbent materials.

Surface modification and densification

Surface metallization of pores: By chemically nickel plating or vapor deposition (CVD), Ni or Al ₂ O3 layers are coated on the pore walls to improve corrosion resistance (such as increasing salt spray life from 300 hours to 800 hours), while enhancing the mechanical strength of the pore structure (compressive strength increased by 20% -30%).

Local densification treatment: Hot isostatic pressing (HIP, temperature 550 ℃, pressure 100 MPa) is applied to the pressure bearing parts of porous materials (such as bearing seats) to reduce the local porosity from 40% to below 5%, achieving a gradient design of "porous functional area dense load-bearing area".

Exploration of Green Technology

Developing water-based formaldehyde free molding processes to replace traditional organic binders (such as PVB) and reduce volatile organic compound (VOCs) emissions; Using microwave sintering technology (heating rate 10-20 ℃/min), it saves more than 40% energy compared to traditional resistance furnaces, while suppressing grain coarsening (average grain size reduced from 20 μ m to 8 μ m).