Metal Powder Production

Overview

Metal powders are fine metal particles used as feedstock for manufacturing techniques like additive manufacturing, metal injection molding, and powder metallurgy pressing and sintering. Producing advanced specialty metal powders with precise control of chemistry, particle size distribution, morphology, and microstructure is critical to properties of finished components.

There are various methods used for large scale production of metal powder production from different alloy systems including:

• Gas atomization
• Water atomization
• Plasma atomization
• Electrode induction melting gas atomization
• Rotating electrode process
• Carbonyl process
• Electrolytic process
• Metal reduction processes

Each process results in powders with different characteristics suited to specific applications.

Metal Powder Production Methods

Method	Metals Used	Key Characteristics	Main Applications
Gas Atomization	Titanium, aluminum, stainless steel, tool steel, superalloys	Spherical powders, moderate production rate	Metal injection molding, Hot isostatic pressing
Water Atomization	Low-alloy steel, iron, copper	Irregular powder shapes, higher oxygen content	Press and sinter process
Plasma Atomization	Titanium alloys, superalloys	Very fine spherical powders	Additive manufacturing
Rotating Electrode	Tungsten, molybdenum, tantalum	Controlled grain structure	Filaments, cutting tools
Carbonyl Process	Iron, nickel, cobalt	Ultrafine high purity powders	Electronic components, magnets
Electrolytic	Copper, nickel	Dendritic flake morphology	Surface coatings

Metal Powder Production Methods

There are a variety of commercial methods used for producing metallic powders from different alloy systems. The choice of production method depends on factors like:

• Type of alloy material
• Purity requirements
• Desired powder characteristics like particle size, shape, grain structure
• Scale of production in tons per year
• Powder end use application

Here are some of the most common industrial processes for metal powder production:

Gas Atomization Process

In gas atomization process, a stream of molten metal alloy is disintegrated by high pressure jets of gas, usually nitrogen or argon. The metal stream breaks up into fine droplets, which solidify into powder particles.

Gas atomized powders have a spherical shape and smooth surface morphology. Particle size distribution can be controlled by adjusting process parameters. This is a widely used technique for reactive materials like titanium, aluminum, magnesium alloys as well as stainless steels, tool steels and nickel superalloys.

Parameter	Description
Metals used	Titanium alloys, aluminum, magnesium, stainless steel, tool steel, superalloys
Particle shape	Spherical morphology
Particle size	50 – 150 μm typical
Purity	High, inert gas prevents contamination
Oxygen pickup	Minimal compared to liquid metal atomization
Production scale	Up to 10,000 metric tons per year

Water Atomization

In water atomization, the molten metal stream is hit by high velocity water jets. The sudden cooling causes an explosion that breaks the metal into fine particles. The powders have irregular shapes and contain higher oxygen content from water contact.

Water atomization is lower cost process used for producing large volumes of stainless steel, alloy steel, iron and copper powders for pressing and sintering type applications.

Parameter	Description
Metals used	Carbon steels, low alloy steels, stainless steels, copper, iron powders
Particle shape	Irregular morphology from explosive water breakup
Particle size	10 – 300 μm typical
Purity	Lower, water contact increases oxygen levels by 200-500 ppm
Production scale	Very high, over 50,000 tons per year

Plasma Atomization Process

In plasma atomization process, a plasma torch is used to melt the metal alloy before disintegration into fine droplets through gas jets. The ultra-high temperatures enable highly reactive elements like titanium aluminides to be successfully atomized.

The powders have a very spherical shape and narrow size distribution suitable for additive manufacturing methods like laser melting and electron beam melting.

Parameter	Description
Metals used	Titanium alloys, nickel superalloys, titanium aluminides
Particle shape	Highly spherical
Particle size	15 – 45 μm typical
Purity	Very high purity due to melting under inert atmosphere
Production scale	Lower, about 100 – 1000 tons per year

Rotating Electrode Process (REP)

In the rotating electrode process, a cylindrical metallic electrode is spun at high speeds in an evacuated chamber. It is melted using an electric arc and the molten metal droplets flung off through centrifugal forces cool to form powders.

REP powders have a grain structure and morphology ideal for hot extrusion into fine wires and rods for aerospace alloys like tungsten, molybdenum, tantalum.

Parameter	Description
Metals used	Tungsten, molybdenum, tantalum
Particle shape	Irregular, controlled microstructure
Particle size	45 – 150 μm typical
Purity	Very high from processing under vacuum
Production scale	Small volumes of high value powders

Electrode Induction Gas Atomization (EIGA)

The EIGA process uses induction heating to melt consumable electrode tips in an inert gas atmosphere. The droplets undergo secondary gas atomization by argon jets into fine spherical powders.

EIGA enables very high purity of reactive nickel superalloys for critical aerospace components through controlled melting and minimizing contamination.

Parameter	Description
Metals used	Nickel superalloys, titanium aluminides
Particle shape	Spherical
Particle size	15 – 53 μm typical
Purity	Extremely high, customized for critical alloys
Production scale	R&D/prototyping to mid-volume

Carbonyl Process

In the carbonyl process, metal is converted into a volatile carbonyl, which decomposes under controlled conditions to produce uniform, ultrafine metallic particles. This approach is suitable for producing highly pure iron, nickel and cobalt powders.

Parameter	Description
Metals used	Iron, nickel, cobalt
Particle shape	Spherical to polyhedral
Particle size	1 – 10 μm typical
Purity	Extremely high 99.9%+ purity
Production scale	Up to 30,000 tons per year

Other Powder Production Methods

Some other techniques used for specialty metal powder production include:

• Electrolytic Process: Used for producing irregular shaped copper and nickel powders with dendritic morphology by electro-deposition process
• Metal Reduction Processes: Reduction of metal oxides using hydrogen or carbon to produce titanium, zirconium, tungsten, molybdenum powders
• Mechanical Alloying: High energy ball milling to synthesize composite and nanostructured alloys

Metal Powder Specifications

Critical quality attributes and specifications tested for metal powders depend on production method and end-use application but typically include:

Powder Chemistry

• Alloy composition using optical emission or X-ray fluorescence spectroscopy
• Minor alloying elements
• Impurity elements like oxygen, nitrogen, hydrogen
• Loss on ignition testing at high temperature

Particle Size Distribution

• Volume mean particle size
• Distribution widths like D10, D50, D90

Particle Shape Characterization

• Scanning electron microscopy for morphology
• Shape factors like aspect ratio and form factor

Microstructure

• Phases present using X-ray diffraction
• Grain characteristics from imaging

Powder Properties

• Apparent/tap density
• Flow rates through Hall flowmeter funnel tests
• Compressibility levels

Specification requirements for powders vary widely depending on end use in different applications:

Parameter	Metal Injection Molding (MIM)	Additive Manufacturing	Press & Sinter
Particle size range	3 – 25 μm	15 – 45 μm	150 – 300 μm
Aspect ratio	1 – 1.25 preferred	<1.5 spherical	Not critical
Oxygen levels	<1000 ppm	<500 ppm	2000 – 4000 ppm
Apparent density	>2.5 g/cm3	>2.8 g/cm3	2 – 3 g/cm3
Hall flow rate	15 – 35 s/50g	25 – 35 s/50g	>12 s/50g

Characterization Methods

There are several analytical methods used to characterize the properties of metal powders essential to product performance:

Particle Size Analysis

Laser diffraction methods are most widely used to characterize the particle size distribution. This technique passes a laser beam through a dispersed powder sample which scatters light at an angle dependent on particle sizes. Computer analysis of the diffraction pattern provides detailed statistically relevant size distribution data within seconds.

Morphology and Surface Imaging

Scanning electron microscopy (SEM) provides high resolution images of powder particle shape, surface topographies and features at much higher magnification and depth of focus compared to optical microscopy.

SEM imaging is used to study particle rounding, satellite formation, surface smoothness and defects like porosity.

Density and Flow Property Measurement

Standard test methods have been established to quantify bulk behavior using:

• Hall flowmetry funnel to measure powder flow rates through an orifice
• Carney funnel to assess flowability by angle of repose
• Scott volumeter to determine tap density and compressibility

These methods help predict ease of handling, blending, die filling and spreading during component manufacturing.

X-ray Methods for Composition and Crystal Structure

• X-ray fluorescence spectroscopy accurately identifies and quantifies elemental composition of metals
• X-ray diffraction analyzes the atomic arrangements and phases present by diffraction peak patterns

Applications of Metal Powders

Some major end uses of engineering metal powders include:

Additive Manufacturing

Also known as 3D printing techniques like selective laser melting (SLM), direct metal laser sintering (DMLS) and electron beam melting (EBM) to build complex geometries from titanium, aluminum, stainless steel, superalloy, cobalt chrome powders.

Metal Injection Molding (MIM)

Powders like stainless steels, titanium alloys and tool steels are combined with a binder, injection molded then sintered to manufacture small, complex parts at high volumes for lower costs.

Powder Metallurgy Press and Sinter

Compacting and sintering iron, copper and alloy steel powders into high volume components like gears, bushings and magnets.

Application	Metals Used	Key Property Needs
Additive manufacturing	Titanium alloys, nickel superalloys, aluminum, tool steel, stainless steel, cobalt chrome	Spherical morphology Good flowability High purity
Metal injection molding	Stainless steel, titanium, tool steel, tungsten heavy alloys	Fine <25 μm powder Good packed density
Press and sinter	Iron, steel, stainless steel, copper	Cost effective powder Lubricant coatings

There are also niche applications in areas like welding, diamond tools, electronics and surface coatings that use specialty metal powders.

Suppliers and Pricing

Some leading global suppliers of various metal powders are:

Company	Production Methods	Materials
Sandvik Osprey	Gas atomization	Titanium, aluminum, nickel alloys
AP&C	Plasma atomization	Titanium aluminides, superalloys
Carpenter Technology	Gas, water atomization	Tool steels, stainless steels, alloys
Höganäs	Water atomization	Iron, stainless steels
JFE Steel	Water atomization	Stainless steel powders
Rio Tinto	Aluminum powder	Carbonyl nickel and iron

Pricing for metal powders varies widely by:

• Alloy material and composition
• Production method used
• Processing to achieve particle characteristics
• Purity levels and degree of contamination
• Purchase volumes – very high volume contracts bring lower pricing

Typical base prices per kilogram are:

Material	Pricing Estimate
Stainless steel 316L	$12 – $30 per kg
Aluminum AlSi10Mg	$15 – $45 per kg
Titanium Ti-6Al-4V	$80 – $220 per kg
Nickel superalloy Inconel 718	$90 – $250 per kg
Specialty alloys for AM	$250 – $1000 per kg

Prices go up significantly for highly customized particle size distributions, controlled oxygen and nitrogen levels below 100 ppm, and small lot purchases.

Advantages and Limitations of Powder Metallurgy

Benefits of Powder Metallurgy

• Ability to produce complex geometries not possible through casting or machining
• Near-net-shape manufacturing reduces material waste
• Higher performance metals and alloys can be used
• Consistent porosity structures not possible in ingot metallurgy
• Components can be mass customized

Limitations of Powder Production and Processing

• Capital investment for production and handling equipment is very high
• Increased surface area makes handling pyrophoric reactive powders risky
• Achieving high compaction densities can require high pressures
• Additional process steps compared to casting
• Portability of AM machines due to powder being LO/NO

Here is a quick comparison of powder metallurgy against the conventional casting process:

Parameter	Powder Metallurgy	Casting
Complex shapes	✅ Excellent for layered AM builds	Limited for typical castings
Mechanical properties	Can approach cast properties after Hot Isostatic Pressing	✅ Predictable properties
Cycle time	Slower process for AM methods	✅ Faster for volume production
Dimensional accuracy	Varies, depends on post-processing	Very good for precision investment castings
Equipment costs	Very high for industrial AM machines	✅ Lower capital costs
Types of metals	Continually expanding options	✅ Broadest selection

FAQs

Q: What is the typical particle size range used in metal 3D printing powders?

A: In powder bed technologies like selective laser melting (SLM) and electron beam melting (EBM), the optimal particle size range is 15-45 microns. Finer powders improve resolution but can be challenging to handle and process.

Q: What determines morphology of metal powders from different methods?

A: Production factors like intensity of melt stream breakdown forces from gas jets or water impacts and subsequent cooling rates determine particle shapes. Faster cooling produces irregular, dendritic particles while slower solidification (spherical atomization) enables smooth rounded structures.

Q: Why is high purity important for metal powders in additive manufacturing?

A: Impurities can cause defects, porosity issues, alter alloy microstructures, reduce density, affect performance under loads and temperatures – negatively impacting mechanical properties. Target oxygen levels below 500 ppm and nitrogen levels below 100 ppm have become typical.

Q: How are metal powders handled safely during transportation and storage?

A: Reactive metal powders are passivated to create oxidized surfaces minimizing flammability risk. Powders are sealed in drums under inert gases like argon instead of air during shipment to prevent ignition. Storage containers must be properly grounded. Personnel wear specialized PPE while handling.

Q: What are common powder characterization methods?

A: Hall flowmetry, tap density tests, pycnometry, LOI testing, spectrographic analysis, metallography and particle size distribution using laser or sieve techniques are vital to quantifying behavior, building quality process control for metal powder production and assessing batch suitability for given applications.

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