Almost 70 years ago, the industries of Metal Powders manufacturing and Powder Metallurgy parts manufacturing came into modern day prominence. Since then a number of different types of both metal powder and powdered parts manufacturing processes have evolved and are still practiced. The key processes used are described below.
Ceramic parts and ceramic powders manufacturing are similar industries which also use the same powder quality control technologies.
Metal Powders Processes
Direct reduction (sponge iron), liquid atomization, gas atomization, and centrifugal atomization are all processes that are presently today.
Purified iron oxide ore, blended with a carbon source like coke, is heated to high temperatures in a rotary kiln. The product is sponge iron which is removed from surplus solid carbon, ground, annealed (in order to remove extra oxygen and carbon) and finally reground for final use for manufactured parts.
A molten metal, which can be alloyed or pure metals, passes via an orifice under high pressure into a gas filled chamber where it cools and then solidifies as it falls via the chamber. The powder is collected and then annealed for subsequent parts manufacture.
Similar to gas atomization, but the metal stream is hit by a high-pressure liquid spray which cools and then solidifies the droplets in a more rapid manner, resulting in less porous, smaller, cleaner particles with a wider size distribution compared to gas atomized powders. The product is finally annealed.
A rod of the metal to be powdered is sent inside a chamber into a rotating spindle. An electric arc across the gap melts the end of the rod from which melted droplets are thrown into the neighbouring chamber and solidified. This method can develop a much narrower size distribution than either atomization method.
Figure 1. Direct Reduced Powder: Blocky, rough - results in high green strength after compaction.
Figure 2. Liquid Atomized Powder: Spherical, smooth (required for Selective Laser Melting).
Commonly available metal powders include bronze, aluminum, metal carbides, chromium, cobalt, hafnium, iron, molybdenum, nickel, copper, niobium, platinum, rhenium, silicon, silver, tungsten, tantalum, vanadium and many different alloys of these.
Powder Metallurgy Parts Manufacturing
Powdered metal components are produced from powdered metal using a wide range of manufacturing techniques. These techniques comprise of pressing and sintering, hot isostatic pressing, powder forging, electric current assisted sintering, selective laser melting, and metal injection molding.
Pressing and Sintering
The part is first compressed by die compaction at room temperature. In a few cases, this is enough to develop a finished part. In most cases, die compaction (pressing) is followed by sintering, at high enough temperatures for the particles to coalesce or diffuse together, not melt completely. The final part has some porosity to it, unlike a molten cast part. If the final strength and hardness is higher, the porosity of the final part will be lower.
A pressed and sintered part is heated to high temperatures and hot-forged. The final part has properties near those of wrought parts.
Hot Isostatic Pressing (HIP)
Powder fills a mold which is evacuated and then heated to high temperatures while it is subjected to external gas pressure up to 15,000 psi. The final part has near wrought density and strength.
Electric Current Assisted Sintering (EACS)
Similar to HIP except the heat is electrical localized and massive resistive heat, sometimes complemented by electric currents which can trigger other mechanisms such as surface oxide removal. The massive heat concentrates at particle surfaces, and the localized heat improves plastic deformation during sintering.
Metal Injection Molding (MIM)
MIM can develop more complex parts, since a mixture of the powder with a binder gives it fluid properties which can pass into small passages and spaces. The mixture gets compacted into a "green" part, after which the binder is removed, either chemically or thermally, to produce a "brown" part, which is sintered and shrinks to provide a complex part with 97 - 99% density.
Selective Laser Melting (SLM)
SLM is the latest, and considered by most to be the most enhanced, PM process technology. (See diagram below) It employs a rotating mirror, which, by following a CAD pattern, directs a laser beam onto the top powder layer, melting the powder layer on top of the earlier layer of the part. All particles not melted onto the part are scraped off while the next layer is loaded. Attempts have been performed to successfully re-use the un-melted particles for as many cycles as possible before it gives out too much wear in order to meet shape and size criteria. It can take 10 pounds of metal powder to produce a 1-pound part if the left-behind powder cannot be recycled.
Below right is an example of the complexity of a metal part that can be produced by laser melting. Parts can also be extremely durable as they are made in one structural piece needing no subassembly. They are also much more customizable on demand. One machine supplied with a number of varied available CAD programs can be employed for making individual custom parts on demand saving the massive expense of tooling to produce a single part type.
Figure 3. Diagram of Selective Laser Melting (SLM) Process
Figure 4. Complex part made by SLM
The size specifications for an atomized metal powder are frequently tighter than for most other parts manufacturing processes. The mean size might be smaller, and the distribution narrower for a complex part with extremely thin surfaces. Or a bimodal distribution might be called for in order to maximize the loose-packed density on the bed of the laser melter, which would maximize the strength and density and minimize voids of the finished part.
The individual particle shapes are now also extremely important to control. The particles must be smooth-surfaced and highly spherical for 1) good flowability and packing as the bed of the laser melter is recreated after every layer is deposited, and 2) the most consistent structural integrity as the part is fused. Moreover, as contaminants are detrimental in any metal powder, they are especially a problem in feed to laser melting, since even a single contaminant could cause a point defect in an extremely thin section of a part. Contaminants can be identified by image analysis if they are rough-surfaced, non-spherical, or translucent. They can also be quantified as a proportion of the sample by number or volume.
Recycling the metal powder means the powder will wear and pick up a few contaminants on each recycle. Hence, the recycle stream must be re-measured for both shape and size before re-use. When it goes out of spec it must be atomized and melted into quality powder again.
Quality Control for Metal Powders
Metal powders will have to meet quality specifications of both the powder metallurgy parts manufacturers (incoming inspection) and the powder manufacturers (outgoing inspection). Basic powder morphology (shapes and sizes) is a specification in itself and influences all other specifications, depicted in the chart below.
Figure 5. Importance of Powder Morphology: Powder morphology affects properties of both the powders (left) and the manufactured parts (right). Powder size is, and has been, for decades now, measured by Laser Diffraction (LD) technology. A large number of powder shapes (and sizes) can now be measured by the more recent Dynamic Image Analysis (DIA) technology.
Laser Diffraction (LD)
Laser light hitting a stream of flowing powder is scattered at lower intensities and higher angles, the smaller the particle. Detectors at many angles around the sample stream measure the distribution of the scattered light, and an iterative algorithm calculates the size distribution which scattered it. Laser Diffraction has become the de facto standard method for QC size measurement in both the powder metallurgy industries and the metal powders.
Figure 6. Diagram of Laser Diffraction (LD) Technology: Two blue and one red laser diodes at different angles provide scattered light to the array detectors at angles from 0 to 165 degrees. The blue lasers, at lower wavelengths, detect the smallest particles more accurately.
Dynamic Image Analysis (DIA)
Particles flow via a sample cell between a digital camera and high-speed strobe light. A video file of the particle images is sent to a computer. All the analysis takes place on the recorded images. The size of the pixels is calibrated so all shape and size data are effortlessly calculated and then reported. The video image file is saved and can be re-measured under varied Standard Operating Conditions (SOP).
Figure 7. Diagram of Dynamic Image Analysis (DIA) Technology: Rapid strobe on left illuminates the sample cell. Particles flowing though cell are photographed by digital camera on the right. Video image output is recorded in image file in computer.
Figure 8. Combination LD/DIA Analyzer: Each measuring unit measures and reports al results simultaneously on the same sample.
The instrument pictured in Figure 8 measures one sample simultaneously employing both the Laser Diffraction and Dynamic Image Analysis technologies. They report all parameters as they were earlier discussed. This is considered to be the only combination LD/DIA system commercially available today.
- Sizes and shapes (morphology) of metal powders need to be measured:
- To meet suppliers and users' qc requirements
- To identify/quantify off-spec and contaminant quantities for all processes
- To monitor recycle streams in laser melting additive manufacturing
- Laser diffraction (LD) is the size technology predominantly employed in the metal powders/powder metallurgy industries for qc data
- Dynamic image analysis (DIA) is the technology used for morphological data
- A grouping LD/DIA system can presently be used to make both measurements simultaneously on the same sample in a matter of minutes
This information has been sourced, reviewed and adapted from materials provided by Microtrac, Inc.
For more information on this source, please visit Microtrac, Inc.