1. Introduction to SPS Technology

Spark plasma sintering is an advanced powder rapid solidification technology. It achieves internal heating and sintering by directly passing pulsed DC electricity through graphite molds and powder particles. Compared with traditional hot pressing (HP) or hot isostatic pressing (HIP), SPS has the following unique features:

• Rapid heating and cooling: The heating rate can reach several hundred degrees Celsius per minute, which can greatly shorten the process cycle.

• Discharge effect: The instantaneous pulse discharge generated between powder particles can activate the particle surface, remove oxide pollution, and promote material migration and diffusion.

• Pressure assistance: Applying axial pressure throughout the sintering process helps to densify the powder.

These characteristics make SPS very suitable for preparing fine-grained, high-performance metal materials, including magnesium alloys that are sensitive to oxidation and difficult to sinter.

2. Process flow for preparing AZ91D alloy using SPS

Step 1: Raw material preparation

1. Powder selection: AZ91D pre alloyed powder prepared by atomization method. Aerosolized powders are usually spherical in shape, with good flowability, relatively low oxygen content, and uniform composition.

• Composition: Mg-9Al-1Zn (main alloying element), as well as trace amounts of Mn, Si, Cu, Fe, Ni, etc.

2. Powder pretreatment:

• Screening: Select powders with a suitable particle size range (e.g. 20-100 µ m) to ensure uniform filling and sintering.

• Drying: Low temperature drying in vacuum or inert atmosphere (such as argon) to remove adsorbed moisture is crucial for preventing magnesium powder oxidation and hydrogenation.

Step 2: Loading and Setting

1. Mold system: High strength graphite molds and punches are usually used. Place graphite paper between the powder and the inner wall of the mold to prevent adhesion and facilitate demolding.

2. Powder filling: Load the weighed AZ91D powder into a graphite mold and gently vibrate to ensure even powder filling.

3. System assembly: Place the assembled mold into the SPS furnace chamber.

Step 3: Sintering process

This is the core of the entire process, usually carried out under vacuum or inert atmosphere protection.

1. Vacuuming and preloading: First, evacuate the furnace chamber to a medium high vacuum (such as below 10 Pa), and then apply a small initial pressure (such as 5-10 MPa) to preliminarily fix the powder.

2. Sintering cycle:

• Heating stage: Raise the sintering temperature at a high heating rate (such as 100 ° C/min or higher).

• Sintering temperature: For AZ91D, the sintering temperature is usually below its solidus temperature (about 468 ° C), typically between 400 ° C and 450 ° C. Excessive temperature can cause the melting of low melting point eutectic phase (β - Mg ₁₇ Al ₁₂), resulting in "overburning" and damaging performance.

• Sintering pressure: The pressure range is usually between 30-50 MPa. Pressure helps with plastic deformation, breaks the oxide film on the surface of particles, and promotes densification.

• Insulation time: Due to the rapid sintering characteristics of SPS, the insulation time is usually very short, ranging from a few minutes to over ten minutes. Long term insulation can lead to grain coarsening.

3. Cooling and pressure relief: After the insulation is completed, stop heating and cool down with the furnace to room temperature under pressure or after pressure relief.

Step 4: Post processing

1. Demoulding: Remove the mold from the SPS equipment and use a demoulding machine to eject the sintered sample.

2. Machining: Use a lathe or wire cutting to remove graphite paper and possible contamination layers attached to the surface of the sample.

3. Heat treatment (optional): According to performance requirements, AZ91D prepared by SPS can be subjected to T4 (solution treatment) or T6 (solution+aging treatment) to regulate the morphology and distribution of β - Mg ₁₇ Al ₁ ₂ phase, thereby optimizing mechanical properties.

3. Advantages of SPS preparation of AZ91D alloy

1. Grain refinement: The rapid sintering process effectively suppresses grain growth, resulting in a fine and uniform microstructure, which is the key to achieving high strength and high plasticity.

2. High density: The synergistic effect of discharge, pressure, and rapid sintering allows the sample to reach a density close to theoretical (>99%), reducing defects such as pores.

3. Inhibition of oxidation: The rapid sintering cycle and vacuum/inert atmosphere environment minimize the oxidation of magnesium alloy powder to the greatest extent possible.

4. Retaining non-equilibrium phases: Rapid cooling may retain supersaturated solid solutions or other non-equilibrium phases at high temperatures, providing a unique initial state for subsequent heat treatment.

4. Challenges and Key Control Factors

1. Oxidation problem: Magnesium powder is extremely reactive, and any exposure in any process may introduce oxides, becoming a source of cracks and deteriorating performance. The atmosphere must be strictly controlled.

2. Narrow sintering window: The sintering temperature must be below the solidus line of AZ91D, but high enough to achieve densification. High precision is required for temperature control.

3. Graphite contamination: Graphite molds may undergo slight reactions with magnesium at high temperatures or leave carbon contamination on the surface of the sample, which requires subsequent machining to remove.

4. Parameter optimization: There is a complex interaction between sintering temperature, pressure, and holding time, which needs to be optimized through systematic experiments to balance density, grain size, and phase composition.

5. Microstructure and Properties

• Microstructure: AZ91D prepared by SPS is usually composed of small α - Mg grains and dispersed β - Mg ₁₇ Al ₁₂ phases at grain boundaries. Due to rapid solidification, the size and distribution of β phase are smaller and more uniform than traditional casting materials.

• Mechanical properties:

• Strength: Due to fine grain strengthening and dispersion strengthening, the yield strength and tensile strength of AZ91D are usually significantly higher than those of traditional casting (such as die casting).

• Plasticity: Fine grains and uniform structure also contribute to improving the ductility of alloys.

• Corrosion resistance: A dense structure and reduced pores and inclusions are beneficial for improving corrosion resistance. However, the continuous distribution of β phase at grain boundaries may form galvanic corrosion, and the distribution of phases needs to be optimized through heat treatment.

summary

Spark plasma sintering (SPS) provides a promising technological path for the preparation of high-performance AZ91D magnesium alloys. It can overcome the defects of coarse grains, segregation, and porosity caused by traditional casting methods, and obtain near net formed products with fine grains, density, and high performance. Despite challenges in process control and cost, SPS technology has broad application prospects in the context of increasing demand for lightweight, high-strength magnesium alloys in fields such as aerospace, biomedical, and 3C products. Future research will focus on further optimizing process parameters, developing new composite/gradient materials, and gaining a deeper understanding of the physical metallurgical behavior during sintering processes.