SECONDARY TREATMENTS OF POWDER METALLURGY COMPONENTS

Presented by

Bhukya Srinu

Reg.No:14ETMM08 M.Tech.(Materials Engineering)

School of Engineering Sciences and Technology

University of Hyderabad

OUTLINE

Introduction

Necessity

Secondary Treatmentsa) Sizing and coining

b) Machining

c) Impregnation

d) Infiltration

e) Surface engineering

f) Heat treatment

g) Joining

Conclusion

Powder Metallurgy

• Powder Metallurgy is a fabrication technique involves blending and compaction of powdered material into a desired shape, followed by a heat treatment (sintering) to produce a component.

• Powder metallurgy is especially suitable for metals having high melting temperatures.

Powder Metallurgy Process

Powder production

Blending or mixing

Powder compaction

Sintering

Finishing Operations

4/18/2015 Powder Metallurgy

Secondary Operations

Most powder metallurgy products are ready to useafter the sintering process.

Some products may use secondary operation toprovide enhanced precision, improved properties, orspecial characteristics.

Distortion may occur during non uniform cool-downso the product may be repressed, coined, or sized toimprove dimensional precision.

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Secondary treatments:

Sizing and coining

Machining

Impregnation

Infiltration

Surface engineering

Heat treatment

Joining

Sizing:

Sizing is done to refine dimensional accuracy.

Pressures used are no more than the initial compacting pressure.

Sized parts will be straighter, dimensional tolerances will be closer and surface finish will be improved.

Little or no increase in density is achieved.

Coining:

Pressures used are more than the initial compacting pressure.

Increased part density part density in addition to improving dimensional accuracy.

Increased mechanical properties.

Machining:

Machining is any of various processes in which a piece of raw material is cut into a desired final shape and size by a controlled material removal process.

Machining creates geometric features that cannot be achieved by pressing, such as threads, side holes, and other details

Impregnation:

Controlled porosity permits PM parts to be impregnated with oils or resins.

This can be done simply by submerging the PM components in an oil bath for several hours.

Vacuum impregnation process gives best results.

In resin impregnation interconnected pores are filled with resins, usually polyesters of the thermosetting type.

This process greatly improves the machinability of PM parts by filling the pores and increasing the density.

Entrapment of other fluids is prevented.

Infiltration:

Operation in which the pores of the PM part are filled with a molten metal.

The melting point of the filler metal must be below that of the PM part.

TM (filler) < TM (Part)

Involves heating the filler metal in contact with the sintered component so capillary action draws the filler into the pores.

Resulting structure is relatively nonporous, and the infiltrated part has a more uniform density, as well as improved toughness and strength.

Surface engineering:

Surface engineering refers to a wide range of technologies designed to modify the surface properties of metallic and non-metallic components for functional and/or decorative purposes.

Steam treatment

Coating Vapour deposition processes

PVD CVD

Thermal spraying

Short peening

Steam treatment:

• In this process, all exposed surfaces are coated with hard black iron oxide.

• Used to improve the wear properties of PM components.

• Improved corrosion resistance.

• Dimensional changes are minimal.

Physical Vapour Deposition:

• Deposition of a material in the vapor phaseonto a solid in a vacuum.

• The coating method involves purely physical processes such as high-temperature vacuum evaporation with subsequent condensation, or plasma sputter bombardment

• Evaporated atoms travel through the evacuatedspace between the source and the sample and stick to the sample.

• Usually no chemical reactions take place• Carried out in a vacuum atmosphere• Used for thin and uniform coating or films

Chemical Vapour Deposition:

• Chemical vapor deposition (CVD) is a chemical process used to produce high-purity, high-performance solid materials or coatings

• In a typical CVD process, the substrate is exposed to one or more volatile precursors which react and decompose on the substrate surface to produce the desired deposit

• Precursors include Halides (eg TiCl4), Hydrides (eg SiH4) and other compounds etc.

• During this process, volatile by-products are also produced, which are removed by gas flow through the reaction chamber.

Thermal Spraying:

• Thermal spraying is an industrial coating process that consists of a heat source and a coating material in a powder or wire form which is melted into tiny droplets and sprayed onto surfaces at high velocity.

Shot peening:

• Shot peening is a cold work process used to finish metal parts to prevent fatigue and stress corrosion failures and prolong product life for the part.

• In shot peening, small spherical shot bombards the surface of the part to be finished.

• The shot acts like a peen hammer, dimpling the surface and causing compression stresses under the dimple.

• As the media continues to strike the part, it forms multiple overlapping dimples throughout the metal surface being treated.

• Significant improvement in fatigue properties can result from the formation of harder and stronger surfaces.

Heat treatment:

Joining:

• Powder metallurgy itself makes it possible to obtain complex shapes, and hence the joining operation is not often required.

• Conventional welding methods (TIG, MIG, electron beam, resistance and friction welding) are used to join PM parts.

• Best welding results for PM steels are achieved when the component density is 6.8 g/cm3 (88% of theoretical density) or higher.

• Sinter welding can also be done by inserting suitable powder of the same metal between the parts to be joined, and then hot pressing the assembly in a suitable atmosphere.

Conclusion:

References:

• Powder metallurgy science, technology and materials- Anish Upadhyaya, GS Upadhyaya.

Thank you

Powder Metallurgy Process

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Finishing• The porosity of a fully sintered part is still significant (4-

15%).

• Density is often kept intentionally low to preserveinterconnected porosity for bearings, filters, acousticbarriers, and battery electrodes.

• However, to improve properties, finishing processes areneeded:

• Cold restriking, resintering, and heat treatment.

• Impregnation of heated oil.

• Infiltration with metal (e.g., Cu for ferrous parts).

• Machining to tighter tolerance.

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Secondary Operations

Most powder metallurgy products are ready to useafter the sintering process.

Some products may use secondary operation toprovide enhanced precision, improved properties, orspecial characteristics.

Distortion may occur during non uniform cool-downso the product may be repressed, coined, or sized toimprove dimensional precision.

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Secondary Operations• If massive metal deformation takes place in the second

pressing, the operation is known as P/M forging

• Increases density and adds precision

• Infiltration and impregnation- oil or other liquid is forcedinto the porous network to offer lubrication over anextended product lifetime

• Metal infiltration fills in pores with other alloying elementsthat can improve properties

• P/M products can also be subjected to the conventionalfinishing operations: heat treatment, machining, andsurface treatments

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Densification and SizingSecondary operations are performed to increase density,improve accuracy, or accomplish additional shaping ofthe sintered part.

• Repressing - pressing sintered part in a closed die to increasedensity and improve properties

• Sizing - pressing a sintered part to improve dimensionalaccuracy

• Coining - pressworking operation on a sintered part to pressdetails into its surface

• Machining - creates geometric features that cannot be achievedby pressing, such as threads, side holes, and other details

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Impregnation and Infiltration

Porosity is a unique and inherent characteristic ofPM technology

It can be exploited to create special products byfilling the available pore space with oils, polymers,or metals

Two categories:

1. Impregnation

2. Infiltration

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InfiltrationOperation in which the pores of the PM part are filled

with a molten metal.

The melting point of the filler metal must be below that ofthe PM part.

TM (filler) < TM (Part)

Involves heating the filler metal in contact with thesintered component so capillary action draws the fillerinto the pores.

• Resulting structure is relatively nonporous, and theinfiltrated part has a more uniform density, as well asimproved toughness and strength.