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 OperationTypical PartsObjectives
Drill and TapAllTo install set screws or assembly fasteners
TurningAllMachine to exact tolerances. Form undercuts or features not possible with compaction tooling.
MillingAllForm undercuts or slots
GrindingAllRemove 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 OperationTypical PartsObjectives
CoiningAllDensification, modify surface shape (coining), dimensional control
SizingAllDimensional control

Impregnation

Secondary OperationTypical PartsObjectives
OilBearingMake bearings self-lubricating
ResinStructuralImprove machinability. Prepare surface for plating with other metals. Seal parts gas- or liquid-tight. Provide lubrication.

Infiltration

Typical PartsObjectives
FerrousImprove strength and hardness. Seal parts gas- or liquid-tight
StructuralPrepare surface for plating with other metals. Improve ductility and machinability.

Heat Treating

Secondary OperationTypical PartsObjectives
Quench and TemperFerrous StructuralImprove strength and hardness.
Steam TreatFerrousMake surface hard and wear resistant. Improve corrosion resistance and seal porosity.

Finishing

Secondary OperationTypical PartsObjectives
DeburringAllRemove sharp edges related to punch and die tooling
BurnishingFerrous StructuralBall size, roller burnish for size control and surface finish
Coating Oil DipFerrousCorrosion resistance
Copper, Nickel, Cadmium, Zinc, Chromium PlatingFerrousCorrosion resistance. Appearance.
WeldingFerrous, 6.8 g/cm3 min. densityMake assemblies from two or more parts
Black OxideFerrous, resin impregnatedCorrosion resistance. Paint base.
Mechanical Surface Treatments:
Glass Beading
Wire Brush
Sanding, Tumbling/
Vibratory Finishing
AllClean/improved surfaces
Shot PeeningFerrous Structural, ForgingsImprove surface fatigue life