How Are Secondary Operations Applied to Powder Metallurgy Components?

Secondary OperationsA sintered powder metallurgy (PM) component can be finished or treated just like any other metal component to achieve desired characteristics—corrosion resistance, improved strength and hardness, surface wear resistance, edge-sharpness relief, porosity sealing, and control of size and surface finish.

Plating, coating, deburring, welding, furnace brazing, heat treating, and steam treating are among the secondary operations that are used successfully in the process of fabricating finished PM parts.

Unlike components made using other metal forming processes, powder metallurgy components can be repressed or coined, as well as sized, to densify or modify the surface shape and provide stricter dimensional control. Through oil impregnation, used on PM self-lubricating bearing components since the late 1920s, components can absorb 12%–30% oil by volume. Resin impregnation can also be performed on PM components to improve machinability or to prepare the surface for plating. Infiltration is a secondary process step used to either improve strength or seal parts and make them gas- or liquid-tight. Optionally, like resin impregnation, it can also be used to enhance machinability, improve ductility and prepare parts for plating.

Read a whitepaper on machining powder metallurgy parts for even more detail.

The following tables detail how various secondary operations are applied to PM components.

Machining

Secondary Operation Typical Parts Objectives
Drill and Tap All To install set screws or assembly fasteners
Turning All Machine to exact tolerances. Form undercuts or features not possible with compaction tooling.
Milling All Form undercuts or slots
Grinding All Remove stock. Make faces flat and parallel. Improve surface finish and dimensional tolerances.
Bathtub Drain Tool
Bathtub Drain Tool

This bathtub drain tool is an assembly of five PM components. Machining was used as a secondary process during the manufacturing of this component.

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Trigger
Trigger

The trigger goes into an adjustable trigger system on a pump-action shotgun. Precise tooling was needed to address sufficient machine stock for effective secondary operations…
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Repressing

Secondary Operation Typical Parts Objectives
Coining All Densification, modify surface shape (coining), dimensional control
Sizing All Dimensional control

Impregnation

Secondary Operation Typical Parts Objectives
Oil Bearing Make bearings self-lubricating
Resin Structural Improve machinability. Prepare surface for plating with other metals. Seal parts gas- or liquid-tight. Provide lubrication.

Infiltration

Typical Parts Objectives
Ferrous Improve strength and hardness. Seal parts gas- or liquid-tight
Structural Prepare surface for plating with other metals. Improve ductility and machinability.

Heat Treating

Secondary Operation Typical Parts Objectives
Quench and Temper Ferrous Structural Improve strength and hardness.
Steam Treat Ferrous Make surface hard and wear resistant. Improve corrosion resistance and seal porosity.

Finishing

Secondary Operation Typical Parts Objectives
Deburring All Remove sharp edges related to punch and die tooling
Burnishing Ferrous Structural Ball size, roller burnish for size control and surface finish
Coating Oil Dip Ferrous Corrosion resistance
Copper, Nickel, Cadmium, Zinc, Chromium Plating Ferrous Corrosion resistance. Appearance.
Welding Ferrous, 6.8 g/cm3 min. density Make assemblies from two or more parts
Black Oxide Ferrous, resin impregnated Corrosion resistance. Paint base.
Mechanical Surface Treatments:
Glass Beading
Wire Brush
Sanding, Tumbling/
Vibratory Finishing
All Clean/improved surfaces
Shot Peening Ferrous Structural, Forgings Improve surface fatigue life