Wednesday, January 23, 2019

Sintered Bronze: What You Should Know

Sintered bronze is a popular choice for bearings, bushings, and similar applications, due to the
material’s high strength, rigidity, and temperature resistance. Sintering is a form of powder metallurgy
in which the metal powder is heated to a temperature below its melting point and then compressed
into the desired shape. Here is what you should know.
Creating the Metal Powder
Metal can be powdered in many different ways, but atomization is the most common commercial
process. The raw components of bronze (copper and tin) are individually melted in a high-heat furnace
and then transferred to a reservoir known as a tundish. This provides a controlled and consistent flow
of metal into the atomizing chamber, where it meets a high velocity stream of water, air, or an inert gas.
This disintegrates the molten metal into fine droplets that solidify into particles as they fall through the
atomizing chamber and are collected from the bottom of the tank.
If needed, mechanical methods such as milling, lathe turning, or chipping can be used to further reduce
the particles to the desired shape and size. Forces such as compression, shear, and impact all play a
role in the final size and shape of the particles.
In this phase, the base metals, along with a powdered lubricant, are fed into a blender and mixed to a
homogenous blend. The lubricant is necessary to reduce friction and even out variations in density, as
well as to reduce ejection forces and thus minimize the risk of cracking. Commonly used lubricants
include stearic acid, metallic stearates, and stearin. If needed, the mix may be tumbled in a large mixing
vessel for further blending.
Next, the blended bronze powder must be compressed, or compacted, to create cold welds of sufficient
“green strength” for handling. Precisely designed dies and punches are used to ensure uniform density.
Compaction is carried out at ambient temperature.
Finally, the compacted metal is put into a furnace and heated to a temperature below its melting point.
This bonds the particles, increases strength, and controls porosity. Diffusion and recrystallization occur,
creating a strong, rigid, but somewhat porous finished product.
Finishing Processes
Finishing processes depend on the specific application for the sintered bronze product. Sintering tends
to create small dimensional changes, so parts with very tight tolerances may be sized in a separate
die. If additional lubrication is required, a vacuum process may be used to impregnate the part with oil.
If additional strength or hardness is desired, heat treating and oil quenching may be performed.
Finally, if a unique shape is needed, the finished product may be machined.
Ready to Start?

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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