Friday, December 14, 2018

Understanding the Sintering Process

Powder metallurgy is the process of creating metal objects from metal powder rather than formed billets.
It is highly useful for smaller items in which casting or forging would create a great deal of loss, for
metals such as tungsten with extremely high melting points, and for creating alloys of mutually insoluble
metals. It is also used when a highly porous item is desired.
Sintering is the process of heating and compacting the metal powder to create the desired object. It is
done at temperatures higher than room temperature but lower than the melting point of the metal.
Solid State Sintering
If solid state sintering is desired, two things must happen after the powder is heated and compacted.
First, the pressing lubricant must be removed through evaporation and vapor burning. Then, the surface
oxides of the powder particles must be removed. At this point, the contact points of the object will grow
“sintering necks” that join them together, which can later be cold welded for strength. Most commonly,
the entire process occurs in a single, continuous furnace kept at an appropriate temperature.
Sinter Hardening
To enhance the strength imparted by sintering, it is possible to accelerate the cooling rate of the
sintering furnace. Special material grades are available that create tougher microstructures at specific
cooling rates.
Liquid Phase Sintering
There are two types of liquid phase sintering: transient and permanent.
Transient liquid phase sintering: When working solely with iron powder, solid state sintering would
lead to shrinkage as the sintering necks develop. Adding fine copper powder, though, guards against
shrinkage and adds extra strength to the finished product. In transient liquid phase sintering, the copper
melts at sintering temperature, diffusing throughout the iron powder.
Permanent liquid phase sintering: In some cases, such as cemented carbides, a permanent liquid
phase is desired throughout the sintering process. This involves adding a melting binder to the
powdered metal. There are three distinct stages to this process:

Rearrangement: During melting, the liquid will be pulled into pores and the grains will rearrange
through capillary action.
Solution-precipitation: Where capillary action is high, atoms will move into solution and then precipitate into areas of lower chemical potential. The result is grain boundary diffusion and
Final densification: At this point, grains are efficiently packed, allowing liquid to easily flow into pores, gradually densifying the full solid skeletal network.
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Atlas Bronze is a leading U.S. distributor of bronze, copper, brass, iron, and more. Contact us today at
1-800-478-0887 to place an order or learn about our custom products.

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