How to Choose the Right Stainless Steel Powder for MIM Processes? A Comparison of Three Major Atomization Methods

With the widespread application of Metal Injection Molding (MIM) technology in medical devices, consumer electronics, automotive precision parts, and other fields, the performance requirements for raw materials have become increasingly stringent. As the core material of the MIM process, the choice of stainless steel powder directly affects the product’s density, mechanical properties, and surface quality.

This article will provide an in-depth comparison of gas-atomized stainless steel powder, water-atomized stainless steel powder, and water-gas combined atomized stainless steel powder used in MIM processes.

Overview of Powder Production Methods

Water Atomization:

Principle: Molten metal is impacted by high-speed water jets to form powder.
Powder Morphology: Irregular polygonal shapes.

Gas Atomization:

Principle: Molten metal is atomized into fine droplets by high-pressure inert gas jets, which then cool and solidify into powder.
Powder Morphology: Spherical or near-spherical particles.

Water-Gas Combined Atomization:

Principle: Initial atomization by gas, followed by further fragmentation using water.
Powder Morphology: Morphology lies between the two; partially spherical but with a significant amount of irregular particles.

Comparative Analysis

Differences in Powder Morphology and Particle Size Distribution

Water atomized powders cool rapidly and generally exhibit irregular shapes with rough surfaces and a wide particle size distribution. They often contain satellite and fragmented powders. This morphology leads to poor flowability during feeding and injection molding, potentially causing defects like dead zones and porosity in mold filling.

Gas atomized powders form spherical particles under the high-temperature molten metal flow sheared by inert gases (e.g., nitrogen or argon). These powders have a concentrated particle size distribution, smooth surfaces, and excellent flowability, making them better suited for precise MIM injection and high-density sintering requirements.

Water-gas combined atomized powders combine the high cooling rate of water atomization and the high kinetic energy of gas atomization, usually resulting in near-spherical or spherical particles but with some irregular shapes such as satellite spheres or flaky fragments. When the gas atomization proportion is high, particle sphericity improves (surface tension dominates); when water atomization proportion increases, irregular particles increase due to rapid solidification suppressing surface tension effects.

Chemical Composition Stability

Water atomization uses water as the cooling medium, which in a highly oxidative environment tends to increase the oxygen content on the powder surface, often reaching 0.3–0.8 wt.%. Especially in ferritic stainless steels, oxide inclusions easily form, adversely affecting the mechanical properties and corrosion resistance of sintered parts.

Gas atomization is typically performed under protective atmospheres of nitrogen or argon, effectively preventing oxidation. The powder oxygen content is stably controlled at very low levels (<0.05 wt.%), enabling superior sintering densification and corrosion resistance.

Water-gas combined atomized powders have moderate oxygen content (about 0.15–0.3 wt.%), with greater difficulty in impurity control.

Sintering Densification and Mechanical Properties

The final performance of MIM products largely depends on the sintered density:

Gas atomized powders with smaller particle sizes and high sphericity → uniform sintering shrinkage → high density and mechanical properties approaching those of forged parts.

Water atomized powders with irregular morphology → more sintering porosity and uneven shrinkage → relatively lower density.

Water-gas combined atomized powders show moderate densification with more fluctuations in sintering behavior.

Surface Quality and Post-Processing Effects

Gas atomized powders produce dense particles with fewer defects, resulting in smooth MIM part surfaces free of peeling, which are easier to polish.

Water atomized powders have sharp particle edges, causing rough surfaces on formed parts and making polishing more difficult.

Water-gas combined atomized powders offer limited surface quality improvement, requiring enhanced grinding during post-processing.

Process Stability and Batch Consistency

High-end MIM manufacturing demands very high batch-to-batch consistency. Gas atomized powders, produced under automated controlled conditions, achieve high uniformity in key parameters such as particle size, composition, and tap density, minimizing process variability and improving product yield. This is crucial for large-scale, stable MIM production.

Cost Advantages

Although gas atomized stainless steel powders generally have a higher unit price compared to water atomized and combined atomized powders, their comprehensive cost advantages become apparent in MIM projects with annual outputs exceeding 500,000 parts, mainly reflected in:

Higher Binder Efficiency: Thanks to the spherical structure and excellent flowability of gas atomized powders, the binder ratio in feedstock can be significantly reduced. For example, in 100 tons of feedstock, binder consumption can be reduced by about 12–15 tons, saving approximately 80,000 RMB.

Significantly Improved Yield: Better powder density and sintering consistency reduce defect rates sharply. A 1% reduction in defect rate can cut about 5,000 defective parts annually in a 500,000-part project. Assuming a cost of 100 RMB per part, this saves about 500,000 RMB per year.

Lower Post-Processing Costs: Better sintered surface quality reduces or eliminates traditional polishing and grinding. Assuming a 0.5 RMB saving per part, an annual output of 500,000 parts can save about 250,000 RMB in post-processing.

Conclusion

In the manufacturing of precision MIM products that demand high density, high strength, and high stability, gas atomized stainless steel powder is undoubtedly the more ideal choice. However, for cost-sensitive applications or those with lower performance requirements, water atomized and combined atomized powders remain viable alternative options.