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.

Wednesday, January 16, 2019

Understanding the Cold Drawing Process

Cold drawn metal, particularly steel, is common in many different types of consumer products. Drawn
metal is passed through a series of dies until it achieves the desired shape. Each die applies a specific
amount of pressure to the metal, assisted by a machine press. Drawn metal typically passes through
the dies multiple times. Cold drawn metal is manufactured at room temperature. Here is what you
should know about the process.
Before cold drawing takes place, the raw material must undergo several preparatory steps:
Hot Rolling: Bars or coils of the raw metal are created through hot rolling at temperatures of
approximately 1700 to 2200 F. This produces a scaled, rough surface, and may also create size
Cleaning: The bar or coil is carefully cleaned to remove abrasive scaling and smooth the surface.
Coating: The cleaned surface of the bar or coil is liberally coated with a lubricant to reduce friction
and improve the drawing process.
Pointing: The lead end has several inches removed from its diameter to allow the piece to pass
smoothly through the drawing dies. This is necessary because the die openings must be smaller than
the original bar or coil to apply the required pressure during drawing.
Cold Drawing
After the metal bar or coil has cooled to room temperature, the pointed end of the bar or coil passes
through the die and enters a gripping device on the drawing machine. The machine then pulls (draws)
the remainder of the bar or coil through the die. This reduces its diameter, increases its length, and
shapes its profile.
The finished product, referred to as either cold drawn or cold finish, appears bright and polished. Cold
drawing allows for more precise measurements and tighter dimensional tolerances than other forms of
metalworking, as well as sharper corners. Cold drawn metal also has a higher yield strength and
tensile strength than hot rolled metal.
Additional Processing
Depending on the product that is being created, cold drawn metal may undergo additional processing:
Multi-Pass Drawing: For complex shapes or profiles, the bar or coil may need to be drawn multiple
times. Multi-pass drawing pulls the metal through a series of gradually smaller die openings.
Annealing: Annealing is a thermal treatment that softens the material, modifies the microstructure,
and changes the mechanical properties. It can also remove internal stresses. Annealing may be
performed at any point in the drawing process. It is almost always performed between passes in
multi-pass drawing to improve ductility.
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.

Closed Die Forging vs. Open Die Forging

Die forging is the process of forming heated metal into a specified shape through the use of dies.
These specialized pieces of hardened steel or ceramic use pressure and impact forces to form the
metal into a specific shape. Open die forging does not enclose the heated metal, allowing it to flow
freely except where the dies are placed. In closed die forging, the dies come together to fully enclose
the metal that is being forged. Here is a comparison of the two forging techniques.
For closed die forging, specific molding dies are created in advance. The metal is then heated and
poured into the bottom die. As the top die closes, it creates pressure and impact that forges the metal
into the desired shape.
For open die forging, dies that resemble tools are under the control of the metalworker. He or she
continuously orients and positions the metal as a forging hammer strikes it.
Forging modifies the internal grain structure of the worked metal, but open die forging and closed die
forging create different results and, thus, are suited to different applications.
Open die forging is best for very large pieces. In fact, some open die forges can create products that
are 80 feet long and weigh up to 136 metric tons. The process improves resistance to fatigue and
wear, boosts strength, reduces porosity, and minimizes the likelihood of voids in the finished piece.
However, the resulting process tends to be rough and somewhat imprecise. Open die forging is ideal
for simple shapes and large parts used in the railroad and aircraft industries.
Closed die forging is best for small pieces and those that require precise dimensions with very tight
tolerances. The finished pieces are tighter and have a smoother surface. However, it can be cost
prohibitive for small runs, as custom dies must be created. Fatigue resistance is also lower in pieces
that are closed die forged. Pieces created through closed die forging are often used in the mining, oil,
and automotive industries.
Advantages and Disadvantages
Benefits of Open Die Forging:
·         Improved microstructure and better fatigue resistance
·         Finer grain size and smoother grain flow
·         Increased strength
·         Longer life
·         Less waste
·         Fewer voids
·         Less expensive
Drawbacks of Open Die Forging:
·         Incapable of producing precision parts with tight tolerances
·         Further refining may be needed to create desired features
Benefits of Closed Die Forging:
·         Tighter grain formation
·         No limitation on materials
·         Smoother surface finish
·         Better precision and tolerances
·         Little to no refinement required for complex pieces
·         Economical for large runs
Drawbacks of Closed Die Forging:
·         High setup costs
·         Powerful vibrations require special building provisions
·         Potentially hazardous work environment
·         Costly for small runs
Each forging method is best for specific applications. It is always best to consult with a professional to
determine which production method is right for your unique needs.
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.