Basics of Powder Metallurgy

We just covered casting and now we’ll talk about a different manufacturing process called powder metallurgy.  Powder metallurgy is a very neat way to make a component, for lack of a better word.  It’s also known as powder processing or sintering.  With this process, a die/mold is still necessary, so it shares that similarity with casting (although the dies and molds are different).  Instead of using molten metal, very fine particles of material (we’re talking less than 200 microns, about the width of human hair) are compressed under high pressure.  This is a useful process when the material has a high melting temperature, making it difficult to cast.  On the face of it, it’s an easy process: create tiny particles of the material; compress the material; sinter.  Sintering is an interesting step.  When you compress the articles, they are still discrete but packed very tightly together.  Sintering is heating the particle packed component up to some temperature under it’s melting point, so that it doesn’t melt, but so that the energy of the particles is increased, allowing for diffusion and bonding to take place between all of the atoms in the discrete particles – what you’re left with is a solid piece.  See?  It’s a neat process.  Some important stuff is powder processed, like turbine blades on aircraft (I can’t make the claim that all turbine blades are powder processed, but some certainly are).  Other examples are gears in cars, bearings, and engine parts.  All important parts, that are typically made of extremely hard material (especially bearings).  Commonly used materials are ones that are hard and with high melting temperatures, like iron and steel, and titanium and tungsten alloys, and cemented carbides.  Aluminum too.  Anything with significant tungsten is almost always made with powder processing because of it’s ridiculously high melting temperature of 3,422 degrees celsius (if you’re on the surface of the sun you would be able to easily cast tungsten, since the temperature is around 5,500 degrees celsius.  Actually tungsten is pretty interesting – it has the highest tensile strength of any pure metal, and the lowest coefficient of thermal expansion.  One of it’s applications is in creating what are called kinetic energy penetrator, which are designed to bust through armour). Iron and steel are the most common.  Cemented carbides, which are extremely hard and used for cutting tools, are also commonly manufactured using this process.

General Advantages of Powder Metallurgy

One of the advantages of powder processes is that you get nice uniform material properties throughout, and it’s easy to distribute particles such as carbides throughout.  It’s not easy to do this with casting.  Of course, the metal powder has to be made before we can powder process.  This can be done mechanically (grinding up the material), or atomization or chemical means.  Once the material is thoroughly ground and mixed (in the case of multiple materials), lubricant is often added to reduce friction before it is compacted.  Generally, better mechanical properties are produced at greater pressures – the particulars are packed closer together, and the voids between particles diminish.  The pressure is usually so great that the plastic deformation begins to occur – of course, after elastic deformation.  This step is called the preform – you have the component in the correct shape, but it has not be sintered yet. The more compacted the preform, the better dimensional tolerances you’ll get when it’s been sintered.  Keep in mind that how dense your preformed part ends up is a function of how hard you compress it.  Usually the pressures applied are somewhere in the region of 100 to 100 psi, although much larger pressures – even 1,000,000 psi – are possible.

Hot or Cold?

There’s a few options when it comes to the temperature and the method used for compacting the powder.  It can be done hot or cold.  If it’s cold, generally the powder is compressed in one a few ways: a common method is a pressed, in which two punches come together to compress the material.  This can happen in several directions, depending on the shape of the part.  This is known as rigid-die compaction.  Another method is flexible mold compaction, where a reasonably flexible mold applies pressure to the particles; the pressure is applied to the flexible mold via an incompressible liquids and a compressor.  Additionally, powders can be compacted by being rolled into sheets.  Hot methods are usually reserved for materials where the process will have a marked impact on the mechanical properties, and it’s not as common because it’s more difficult and expensive.  It’s sometimes used for complex parts with fancy materials – like superalloys.  

Sintering

Sintering is the last stage, which removes some of the porosity remaining in the compacted component.  The thermal bonding creates a continuous solid part in the same way that casting would.  Note that sintering can mess up the dimensional tolerances as the pores are eliminated and the density increases.  As the amount of material remains the same, the amount of volume it occupies has to decrease.  This is another reason why it is desirable to compact the material as much as possible in the preform stage.  What’s interesting about the sintering process is that it actually decreases the internal surface energy through elimination of pores and increasing grain size as diffusion occurs (if you recall from the materials chapter, grain boundaries are essentially internal surfaces).  An apt example of sintering is when you compact loose snow into a snowball.  Originally, the snow falls as individual flakes, and by the end the process you’ve create a solid mass of snow.

Design for Powder Metallurgy

Components created by powder processing don’t require much additional processes – i.e., they don’t require extensive machining or anything like that to produce the desired component.  In this way they are considered ‘net shape’, which is a term you might hear about castings as well.  This is desirable, it’s a big advantage that you don’t waste much material with this process, because material cost money.  You have to keep in mind that with high production volumes, manufacturers will fight to remove cents from each component.  If you’re producing thousands or hundred of thousands or millions of components, it adds up.  So no wasted material is an excellent thing.  There are some limitations and things to remember with powder processing.  Remember that the powder must flow and must be able to be compacted, so right off the bat there are a couple of limitations – there can’t be extremely thin details (powder flow) and the component can’t be too large, or else compacting the powder becomes too difficult.  You shouldn’t have long thin sections were the length exceeds the width by more than roughly a factor of 3 – sometimes even a factor of 6 or 7 is recommended.  Keep in mind that you have to removed the preformed part from the press afterwards  All in all, the part should be relatively simple in order for powder processing to be a good manufacturing method, but as well, complex geometries can definitely be made using this method.  As for production rate?  Well it all depends on the equipment and the manufacturer, but double digits should be possible, per minute.