Metal Powder Part 1

Metal Powder Manufacturing

The production of metallic particles suitable for use as feedstock in additive manufacturing and other powder-processing methods.

Major Metal Powder Manufacturing Routes in the Source

Plasma processes, atomization, ground or machined processes, hydride–dehydride processing, and direct reduction.

Plasma Powder Manufacturing

A family of processes that uses plasma to melt solid metal or alloy feedstock and form generally spherical powder particles.

Composition of Plasma-Produced Powder

Barring pickup of undesired gases, the powder retains the composition of the original feedstock.

Three Plasma Powder Processes in the Source

Plasma Rotating Electrode Process (PREP), plasma atomization, and spheroidization.

Plasma Rotating Electrode Process (PREP)

A powder-production process in which a cylindrical alloy bar is rapidly rotated while a plasma torch melts its end, causing molten metal to fly outward and solidify into spherical particles.

PREP Feedstock

A cylindrical bar of the desired metal or alloy.

PREP Particle-Formation Mechanism

A plasma torch melts the rotating bar, and rapid rotation causes molten metal to leave the bar and form spherical particles.

Factors Affecting PREP Particle Size

Alloy composition or surface tension, electrode diameter, and rotation speed influence the resulting particle size.

Typical PREP Powder Characteristics

Relatively coarse particle-size distribution, nearly perfect spheres, almost no satellites, and almost no porosity.

Satellite

A small powder particle adhered to a larger powder particle.

Plasma Atomization (PA)

A plasma powder-production process in which wire feedstock is melted using plasma torches to produce powder.

Plasma Atomization Feedstock

Wire.

Major Powder-Quality Benefit of Plasma Atomization

It generally produces lower satellite and porosity content than gas atomization.

PREP vs. Plasma Atomization

PREP melts a rapidly rotating cylindrical bar, whereas plasma atomization uses wire feedstock melted by plasma torches.

Spheroidization

A plasma process used to convert existing irregularly shaped or porous particles into more spherical powder particles.

Spheroidization Process

Existing particles pass through a plasma, melt, and then resolidify into spheres while falling.

Major Factor Controlling Powder Size During Spheroidization

The initial particle size, together with alloy surface tension, influences the resulting particle size.

Spheroidized Powder vs. PREP Powder

Spheroidized powder can be finer than PREP powder but generally contains more satellites and pores.

Spheroidized Powder vs. Typical Atomized Powder

Spheroidized powder is generally coarser than atomized powder but can have fewer satellites and pores than typical atomized powder.

Atomization

A powder-production method that generally mixes a liquid metal stream with a flowing fluid, disperses the liquid into droplets, and allows the droplets to solidify into powder.

General Atomization Sequence

Liquid metal stream → interaction with flowing fluid → dispersion into droplets → solidification into powder particles.

Key Variables Affecting Atomized Powder Size Distribution

Molten-stream size, metal-to-gas ratio, fluid velocities, additional gas flows, and orifice size.

Important Characteristics of Atomized Powder

Particle-size distribution, sphericity, satellite content, and porosity.

Inert Gas Atomization (IGA)

An atomization process in which molten alloy is dispersed using an inert gas, usually argon, to reduce reactions with the metal and oxidation-sensitive alloying elements.

Why is inert gas used in IGA?

It protects reactive base metals and oxidation-sensitive alloying elements from reaction during atomization.

Examples of Alloys Commonly Associated with IGA or EIGA in the Source

Titanium, aluminum, magnesium, nickel-base alloys, and higher-value tool and stainless steels.

Electrode Inert Gas Atomization (EIGA)

An inert-gas atomization process in which a bar is melted with an electrode and the resulting molten metal is impacted by an inert-gas stream.

IGA vs. EIGA

Standard IGA melts alloy in a crucible and pours it into a gas stream, whereas EIGA melts a bar with an electrode before the molten metal is impacted by the gas stream.

Does inert-gas atomization guarantee good powder flow and packing?

No. Protection from oxidation does not guarantee good flow, packing characteristics, or homogeneity.

Gas Atomization (GA)

An atomization process primarily used in the source for non-reactive, non-ferrous alloys and lower-grade tool, stainless, and carbon steels.

Typical Gas Used in Gas Atomization

The source identifies nitrogen as the typical gas, although air is sometimes used.

Why is ordinary gas atomization not used for reactive metals?

Reactive metals can react catastrophically with the non-inert atomizing environment.

Water Atomization

An atomization process that uses water rather than gas as the fluid for breaking molten metal into particles.

Effect of Water Atomization on Cooling

Water provides more rapid cooling of the particles than the gas-based process described in the source.

Typical Characteristics of Water-Atomized Powder

Relatively coarse particle-size distribution and irregular particle shapes.

Cost Characteristic of Water Atomization

The source identifies water atomization as the lowest-cost atomization process discussed.

Why is water atomization unsuitable for reactive or higher-grade powders?

The reactive environment and resulting powder characteristics make it unsuitable for the reactive and higher-grade materials described in the source.

Ground/Machined Powder

Powder produced from existing solid stock such as bar, plate, or granules using mechanical grinding or machining.

Major Advantage of Solid-State Powder Production

Solid-state production avoids melt-related defects such as gas pores.

Major Contamination Concern in Ground/Machined Powder

Care must be taken to prevent contamination from cutting or grinding media.

Milling/Turning Powder Production

The use of machining chips, turning scrap, or deliberately machined material followed by additional grinding or cutting to produce powder.

Benefit of Milling/Turning Powder Production

Almost any wrought alloy can potentially be converted into powder without the complexities associated with melting and oxidation.

Hydride–Dehydride (HdH)

A titanium-specific powder-production process that uses hydrogen absorption to embrittle titanium, allowing it to be ground before the hydrogen is subsequently removed.

Why is HdH particularly applicable to titanium?

Titanium and its alloys have the combination of hydrogen absorption and hydrogen-induced embrittlement needed for the process.

Hydriding Step of HdH

Titanium or titanium-alloy particles absorb hydrogen at elevated temperature, causing them to become embrittled.

Grinding Step of HdH

After hydrogen embrittlement and cooling, the brittle titanium material is ground to the desired particle size.

Dehydrogenation Step of HdH

The ground material is heated in a vacuum so that the absorbed hydrogen leaves the titanium or titanium alloy.

Typical Shape of HdH Powder

HdH produces irregularly shaped particles unless the powder receives additional spheroidization.

HdH Followed by Spheroidization

Irregular HdH titanium powder can subsequently undergo spheroidization to produce more spherical particles.

Contamination Concern for HdH Feedstock

Machining-chip feedstock must be thoroughly cleaned to remove contamination such as cutting-tool material and cutting fluids.

Direct Reduced Powder

Powder produced by starting with an oxide or another metal-containing compound and using reacting gases, liquids, thermal decomposition, or related methods to produce metallic powder.

Typical Product of Direct Reduction

Direct-reduction processes are generally used to make pure-metal powders, although some variants can produce alloys.

Typical Shape of Direct-Reduced Powder

Most direct-reduced powders are irregular, and some can have a spongy structure.

Tap Density

The density of a compacted container of powder expressed as a percentage of the density of the fully dense alloy.

Why may some direct-reduced powders require secondary processing?

Their irregular or spongy morphology and low tap density can produce poor flow characteristics, so processes such as spheroidization may be used to improve them.

Powder-Manufacturing Route–Property Relationship

The method used to manufacture a powder influences characteristics such as particle shape, size distribution, satellites, porosity, contamination, and flow-related behavior.