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.

Blending/Mixing

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.

Compacting

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.

Sintering

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.

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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.

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.

Preparation

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
variations.

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.

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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.

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.

Processes

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.

Applications

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.

Understanding the Hot Rolling Process

In metalworking, rolling is a fabrication process that consists of passing the metal through a pair of
rollers. Flat rolling creates a sheet, while profile rolling creates a bar or a rod. In addition, rolling
processes are temperature-dependent. Hot rolling is performed when the material is above its
recrystallization temperature, while cold rolling is performed below the recrystallization temperature.

Advantages of Hot Rolling

As you might suspect, hot rolling and cold rolling create products with very different characteristics,
meaning that each process has its place. Hot rolling requires less processing than cold rolling,
meaning that hot rolled products are significantly less expensive. In addition, hot rolled metals cool at
room temperature, avoiding the internal stresses of quenching or work-hardening. Hot rolling allows
metals to be easily manipulated into a variety of shapes, and it creates a denser finished product.

It is important to note that dimensional tolerances are not as precise with hot rolling as with cold rolling,
but the material strength is higher. Therefore, it is best suited to applications in which strength matters
more than minute distortions, such as construction projects and railroad tracks.

Hot rolled metals do not have the same smooth surface as cold rolled metals, as scaling is a byproduct
of the cooling process. However, this scaling can be removed through acid-bath pickling, sandblasting,
or grinding, and a new finish can be applied. In addition, descaled steel provides an optimal surface for
coatings such as paint.

How Is Hot Rolling Done?

The first step is to heat a large, rectangular billet of metal to an extremely high temperature based on
the temperature at which the molten metal recrystallizes (for example, more than 1700 degrees F for
steel), and then pre-process it by flattening it into a single large roll. While maintaining the temperature,
the metal is then pushed through a series of rollers at high speeds to create its finished shape and size.
To create sheet metal, the rolled metal is spun into coils before cooling. For bars, plates, or other
profiles, the material is sectioned, cooled, and then packaged. Note that hot rolled metal is allowed to
cool naturally, which will result in a bit of shrinkage and perhaps minute warping.

Each metal and each fabrication process has its own unique pros and cons, and each is best for
specific applications. Because there is no one size fits all solution, it is vital to work with an expert to
ensure that you choose just the right metal and fabrication process for your specific project.

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.

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
densification.
Final densification: At this point, grains are efficiently packed, allowing liquid to easily flow into pores, gradually densifying the full solid skeletal network.

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.

A Guide to Open Die Forging

Also known as smith forging, open die forging takes its name from the fact that the dies—the surfaces
that make contact with the metal being forged—do not enclose the piece. Instead, the metal flows
freely except in the limited areas where the dies are placed. Open die forging is an important technique
for many manufacturing applications. Here is what you should know.

Capabilities of Open Die Forging

Open die forging allows both rough and finishing shaping work to be done on metal of sizes ranging
from very small to incredibly large. In fact, some open die forges can handle pieces that are 80 feet
long and weigh a whopping 136 metric tons.

Open die forging is difficult, though not impossible, to use for pieces with a great deal of fine detail.
Therefore, except for art pieces and other specialty operations, open die forging is typically used to
create discs, blocks, hugs, shafts, cylinders, flats, plates, sleeves, and other relatively simple shapes.
It is often used to rough-shape ingots in preparation for further forging of a different type.

Common Metals

Steel and similar alloys are the most common metals used in open die forging. However, the process
also works well with a variety of other metals, including alloys of copper, nickel, and titanium.

Open Die Forging Process Operations

While closed die forging can be automated, open die forging is entirely under the control of the
metalworker. The forge operator must continuously orient and position the metal being worked as a
hammer strikes and deforms it to create the desired shape. The dies resemble tools rather than molds
and since the metal flows freely except where it is in contact with the dies, the metalworker must
carefully manage the forging process.

There are three common open die forging processes that are typically performed first, regardless of the
type of metal or what the finished product will be. Taken together, they prepare the metal for further
forging:

Cogging: Cogging is used to reduce a raw bar of metal to the desired thickness. It uses an open die
drop forge to successively deform the bar along its length.

Edging: Generally performed after cogging, edging uses a concave open die to form the metal into the
desired width.

Fullering: Fullering uses a convex open die to thin out sections of the metal as needed.

Benefits of Open Die Forging

Open die forging has a number of benefits over other types of forging. It refines the metal grain,
improving strength and reducing porosity. It improves wear resistance and fatigue resistance. The
improved microstructure reduces the chance of voids.

No single forging process is right for all applications. It is always important to speak with an expert who
can help you decide which process has the most benefits and the fewest drawbacks for your particular
project.

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.