Particle Size and Morphology for Quality Control of Powder Metallurgy Processes and Metal Powders

Manufacturing processes centered on metal powder and powder metallurgy (PM) parts production became extremely important approximately 70 years ago. The field has since seen major strides taken in the development of newer processes of various types, some of which are described in this article. Many of these processes have to do with quality control of the powder, and this is of equal importance in the ceramic powder and ceramic parts manufacturing arena as well.

Metal Powders Processes

The major metal powder processes today include Direct Reduction, Gas Atomization, Liquid Atomization, and Centrifugal Atomization.

Direct Reduction

This process involves the heating of pure iron oxide ore along with coke, or other similar carbon source, to a high temperature, using a rotary kiln. This yields sponge iron which is separated from the remaining solid carbon, ground fine and then annealed, to get rid of any excess carbon and oxygen, before being finally ground again to be used in parts manufacturing.

Gas Atomization

Here, the molten pure or alloyed metal is extruded through an opening at high pressure into a chamber filled with gas, which causes it to cool and become solid as it passes through the gas. The powder is then collected for annealing before being fed into the manufacturing process.

Liquid Atomization

The process resembles gas atomization except that the extruded metal stream is atomized by a high-pressure spray of liquid, causing very rapid cooling and solidification of the metal droplets. This yields smaller, denser and cleaner metal powder particles which have, however, an increased size distribution than with gas-atomized powder. The powder is annealed before use.

Centrifugal Atomization

For the method, the metal is in the form of a rod. It is passed into a rotating spindle within a chamber, and the end of the rod is melted by an electric arc passing across the gap. This causes the droplets of molten metal to scatter violently into the chamber and become solid. The advantage is the smaller particle size distribution width than atomization methods.

The powders most frequently available include bronze, aluminum, metal carbides, cobalt, copper, chromium, iron, niobium, molybdenum, nickel, platinum, silicon, silver, hafnium, rhenium, tantalum, tungsten, vanadium, or their alloys.

Powder Metallurgy Parts Manufacturing

The production of PM parts begins with powdered metal being fed into any of the current production methods, such as pressing and sintering, hot isostatic pressing, metal injection molding, powder forging, electric current assisted sintering, and selective laser melting.

Pressing & Sintering

In this technique, the part undergoes die compaction or pressing at room temperature. While this is sufficient in some cases, in the majority of situations the part is then sintered at a temperature adequate to allow the metal particles to fuse by diffusion or coalescence, but below the melting point. The final part is not as dense as a typical molten cast object, and this higher porosity corresponds to less strength and hardness of the end product.

Powder Forging

In this process the part is not only pressed and sintered but exposed to intense heat before being hot-forged. This gives it mechanical properties which resemble those of wrought parts.

Hot Isostatic Pressing (HIP)

This technique employs a mold filled with powder, which is then subjected to vacuum and high heat in an externally pressurized environment of gas pressures up to 15,000 psi. This, again, produces parts equivalent to wrought parts.

Electric Current Assisted Sintering (EACS)

EACS is quite close to HIP, but the source of heat is electric current and this is used to produce local intense resistive heat. If required, complementary electric currents are used for other purposes as well, such as removing surface oxide. The application of great heat at the surface of the particles causes more plastic deformation during the process of sintering.

Metal Injection Molding (MIM)

The use of this technique permits complex parts to be formed by employing powder mixed with a binder, thus generating a more fluid mixture which flows into and fills crevices and cracks. The compacted mixture forms a “green” part which is subjected to binder removal by heat or chemical means. The result is a “brown” part which is then heated to produce sintering. It then shrinks to produce a part with 97-99% density.

Selective Laser Melting (SLM)

SLM is cutting-edge technology in this field of PM part manufacture. As Figure 3 shows, a rotating mirror focuses a laser beam onto a metal powder layer in accordance to a CAD pattern, melting it in the outline of the pattern just above the previous layer. The remaining metal powder is scraped off for recycling and another layer is loaded on top of the newly created top surface of the part. The powder is thus re-used as many times as possible until it is too worn, oxidized or deformed to meet the quality specifications. Without this recycling, up to 90% of the metal powder material can be wasted during production.

Figure 4 depicts a complex metal part created by SLM. In addition, these parts are durable since they are made in one piece, avoiding the need to assemble smaller parts. It is simpler to customize this process for individually modified parts, using different CAD programs with a single SLM machine. This saves a lot of money on tooling for individual parts.

Atomized metal powders typically need to be produced in narrow size distribution widths compared to metal powders used for other types of manufacturing processes. Complex parts with extremely thin surfaces may require a smaller mean size with narrower distribution, while powder loading on the laser melter bed may require a bimodal distribution to ensure that loose packing occurs with the highest possible density, resulting in a dense and strong end product with minimal voids.

Particle shape is another important parameter, which must be tightly controlled to produce spherical smooth particles so that they flow smoothly and pack closely in the laser melter bed, which is recreated with each new layer of added powder. This shape is also necessary to ensure structural strength during the build.

Contaminants are a particular problem in this type of process because even one can result in point defects in that layer of the SLM build. These can, however, be detected by image analysis, identifying their differences in their shape, surface and transparency, and the amount can be expressed as volume or number percentage.

Metal powder recycling inevitably involves the inclusion of particles which have undergone some wear and stress, or which have been contaminated during handling. To ensure that these particles meet the quality specifications, the stream of recycled particles must be measured for particle shape and size before it is fed into the process. Once the results show that it has gone beyond the limits the decision can be made to melt and re-atomize the powder again.

Quality Control for Metal Powders

Metal powders used in part production must meet stringent quality standards from both the manufacturers of the powder and those who manufacture the PM parts. The basic size and shape of the powder particles are the fundamental specification which affects all the other specifications at both these ends. This is shown in Figure 5.

Laser Diffraction (LD)

When a laser beam is incident on a flowing metal powder stream, the particles of metal scatter the light to varying intensities and angles. Smaller particles produce greater deviation and lower intensity of the final light. Detectors placed in array at angles to the original sample stream pick up the scattered light and these provide data for the calculation of the size distribution of the particles responsible for the scattering, by an iterative algorithm. LD is now the standard method for measuring powder quality in the manufacture of metal powders and PM parts.

Dynamic Image Analysis (DIA)

The DIA technology relies on high-resolution imaging of a stream of particles passing through a sample cell, backlit by a high-speed strobe light, using a digital camera. The images acquired are compiled into a video files which is then analyzed on a computer after pixel calibration. This is so that size and shape data can be easily obtained and reported in a standard format. The video file is then saved so that it can be measured repeatedly if another set of SOP is required.

Figure 8 shows the Microtrac analyzer which allows LD and DIA to be measured at the same time on the same sample, with all the parameters discussed above being reported. This is the only commercial system currently available that uses these two techniques together.

Summary

Metal powders need to have their size and shape measured for various purposes, such as meeting quality standards of both suppliers and users, to identify and quantify particles which are off-specification, and to detect contaminants. This morphological assessment is also important in keeping watch over recycle streams in the SLM process.

LD is the standard technology used to measure the particle size in metal powders or PM part production. DIA is used to evaluate particle morphology. The combination of these two in a single machine from Microtrac is powerful, yielding simultaneous measurements of size and shape on the same sample within a few 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.

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