Manufacturing Technology II (ME-202)

Powder Metallurgy

Dr. Chaitanya Sharma

PhD. IIT Roorkee

Powder Metallurgy

Lesson Objectives In this chapter we shall discuss the following: 1. What is powder metallurgy (PM) 2. Need of PM 3. Advantages, Limitations & Application of

PM 4. Basic steps in PM 5. Design considerations in PM 6. Secondary & finishing operations

Learning Activities 1. Look up

Keywords 2. View Slides; 3. Read Notes, 4. Listen to

lecture

Keywords: Powder, Blending, Sintering, Particle size and shape, Infiltration etc.

What Is Powder Metallurgy ?

OR

It may also be defined as “material processing technique used to consolidate particulate matter i.e. powders both metal and/or non-metals.”

Powder metallurgy may defined as, “the art and science of producing metal powders and utilizing them to make serviceable objects.”

Why PM?

Because:

• PM parts can be mass produced to net shape or near

net shape.

• PM products have doctored properties.

• No need for subsequent machining

• PM process wastes very little material ~ 3%.

• PM parts can be made with a specified level of

porosity, to produce porous metal parts

− Examples: filters, oil-impregnated bearings and gears

Some More Reasons For PM …

• Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy

− Example: Tungsten filaments for incandescent lamp bulbs

• Certain alloy combinations and cermets made by PM cannot be produced in other ways

• PM compares favorably to most casting processes in dimensional control

• PM production methods can be automated for

economical production.

Parts Made by PM

Fig (a) Examples of typical parts made by PM processes.

(b)

(c)

Fig (c) Main-bearing metal-powder caps for 3.8 and 3.1 liter General Motors automotive engines.

Fig(b) Upper trip lever for a commercial sprinkler made by PM.

This part replaces a die-cast part of unleaded brass alloy; with a 60% savings.

(b

(a)

Applications of PM

• Gears

• Cams

• Cranks

• Bearings

• Roller bearing cages

• Housings

• Light bulb tungsten filaments

• Sprinkler mechanisms

• Cemented carbide cutting tools

• Electrical contacts, brushes

• Metallic coating

• Metal to glass seal

Advantages of PM

Cost Advantages:

1. Zero or minimal scrap.

2. High production rates

3. Avoids high machining cost

needed for holes, gear teeth,

key-ways etc.

4. Extremely good surface

finish

5. Very close tolerance

without a machining

operation;

6. Assembly of two or more

parts (by I/M) can be made

in one piece;

Properties Advantages of sintered

components: 1. Complex shapes can be produced

2. Wide composition / property variations are

possible

3. Physical properties are comparable with cast materials and wrought materials.

4. Ability to retain lubricants reduces wear and lengthens life of bearings;

5. Improved surface finish with close control of

mass, volume and density;

6. Components are malleable and can be bent

without cracking.

7. Hard tools like diamond impregnated are

made for cutting porcelain, glass & WC.

8. Reactive and non-reactive metals can be

processed.

Limitations of PM Process

Major limitations are as follows:

1. Principal limitations of the process are those imposed by the size and shape of the part, the compacting pressure required and material used.

2. High initial investment in machinery and dies.

3. Economically viable for production ranges in excess of 10,000.

4. High material cost.

5. Inferior strength properties.

6. Limitations on part geometry due to limited flowability of powders.

7. Varying density of part may be a problem, for complex geometries.

8. Can not make undercuts and re-entrant angles.

9. Problems in storing and handling metal powders e.g. degradation over time, fire hazards with certain metals.

10. Limited cross-sectional area and length of the component .

11. Copper-based materials which are hot-worked have not so far been made

by PM successfully.

Basic Steps In PM

Powder metallurgy is the process of blending fine powdered

materials, compacting the same into a desired shape or

form inside a mould followed by heating of the compacted

powder in a controlled atmosphere (sintering) to facilitate

the formation of bonding of the powder particles to form

the final part.

The four basic steps of PM include:

(1) powder manufacture,

(2) blending of powders,

(3) compacting of powders in a mould or die, and

(4) sintering.

Steps In Making PM

Fig 2 Outline of processes and operations involved in making powder-metallurgy parts.

Powder Blending

• A single powder may not have all the requisite properties and hence, powders of different materials are blended to form a final part with desired properties.

• Blending is carried out for several purposes as follows:

1. To imparts uniformity in the shapes of the powder particles.

2. To facilitates mixing of different powder particles.

3. To impart wide ranging physical and mechanical properties.

4. To improve the flow characteristics of the powder particles reducing friction between particles and dies.

5. To enhance green strength of parts by adding binders.

Is Blending & mixing same?

• Blending: process of mixing powder of the same chemical composition but different sizes.

• Mixing: process of combining powders of different chemistries.

Devices For Blending & Mixing

Blending and mixing are accomplished by mechanical means. Some bowl geometries are shown below:

Rotating drum Rotating double cone

Screw Mixture Blade Mixture

Since metal powders are abrasive,

mixers rely on the rotation or tumbling

of enclosed geometries as opposed to

using aggressive agitators.

A mixer

Compaction

• Compaction: Blended powers are pressed in dies under high pressure to pressurize & bond the particles to form a cohesion among powder particles to impart. required shape.

• The work part after compaction is called a green compact or simply a green, (green means not yet fully processed.)

The compaction exercise imparts the following effects. 1. Reduces voids and enhance density of consolidated

powder.

2.Improves green strength of powder particles.

3.Facilitates plastic deformation of the powder particles to conform to the final desired shape of the part.

4.Enhances the contact area among the powder particles and facilitates the subsequent sintering process.

Guidelines For Compaction

General guidelines for metal powder compaction are:

1. Powder must fill die orifice completely.

2. A constant volume or constant weight may be used.

3. Use vibration filling to create denser packing to avoid bridging and high porosity defects.

4. Apply pressure along more than one axis to minimize defects.

5. Filling, Pressing and Ejection may be done automatically.

6. To facilitate compaction add additives to powder i.e.

– Lubricants: to reduce the particles-die friction – Binders: to achieve enough strength before sintering – Deflocculants: to improve the flow characteristics

during feeding

Compaction: Process & Variables

Compaction process is shown below:

Main variables are:

(a) Method of compaction

(b) Compaction pressure, time and temperature

(c) Rate of compaction

(d) Compacting atmosphere

(e) Lubricants and other additives of mix, and

(f) Die design

(g) Die materials

(h) Punch

(i) Carbide inserts

(j) Tolerances, clearances and finishes

Further during compaction tooling materials, clearances and tolerances require expertise.

Mechanism of Compaction

• Consolidation generally occurs in three stages

(a) rearrangement of particles.

(b) particles contacting by plastic deformation.

(c) mechanical locking and cold welding of particles due to surface shear strains.

• It is, therefore, easier to cold compact irregular particles than spherical powder particles.

• During compaction green density increases rapidly with compaction pressure.

• Compaction pressure determines mechanical properties of parts

Methods of Compaction

1. With application of pressure

a) Unidirectional pressing (single action or double action pressing)

b) Isostatic pressing c) Rocking die compaction d) Powder rolling e) Powder extrusion f) Powder swaging g) Powder forging h) Powder Injection Molding

2. Without applying pressure

a) Slip mixing/ slip casting

b) Vibrational compaction

Single action Double action

Tool For Compaction (Presses)

• The basic types of compacting presses are: 1. Mechanical (single punch or rotary type) presses.

2. Hydraulic presses.

3. Hybrid-type presses (mechanical presses may make use of auxiliary pneumatic or hydraulic devices).

• Minimum requirements for any powder metal press: 1. Adequate total pressure capability

2. Part ejection capability.

3. Controlled length and speed of

compression and ejection strokes.

4. Adjustable die fill arrangements.

5. Synchronized timing of press strokes.

6. Material feed and part removal systems.

A 7.3-MN

(825-ton

Compacting Presses: Parts & Attachments The presses systems used are;

(a) Single action press system consisting of:

• a die to form the outer contour of the part;

• an upper punch to form the top surface of the part;

• a lower punch to form the bottom surface of the part;

• if required, core rods to form any through holes (for class I parts). (b) Double action opposed ram system consists of • a die, upper punch, lower punch and core rods (for class I and class

II parts). (c) Double action floating die system consists of • moving upper punch, stationary lower punch, moving die table and

core rods (for class I – IV parts).

Density as a Function of Pressure and Effects of Density on Other Properties

Figure (b) Effect of density on tensile strength, elongation, and electrical conductivity of copper powder.

Fig: (a) Density of copper- and iron-powder compacts as a function of compacting pressure.

Density greatly influences mechanical & physical properties of PM parts.

Density Variation in Compacting Metal Powders

Fig: Density variation in compacting metal powders in various dies:

(a) and (c) single-action press; (b) and (d) double-action press.

Note in (d) the greater uniformity of density from pressing with two punches with separate movements when compared with (c).

(e) Pressure contours in compacted copper powder in a single-action press

Compacting Pressures for Various Powders

Sintering

• Sintering bonds individual metallic particles, thereby increases strength and hardness of final part.

• Compressed metal powder is heated in a controlled-atmosphere furnace to a temperature (70% and 90% of Tm) below its melting point, but high enough to cause diffusion thereby bonding of neighboring particles.

• Powder performs are heated in a controlled, inert or reducing atmosphere or in vacuum prevent oxidation.

• The primary driving force for sintering is not the fusion of material, but formation and growth of bonds between particles due to reduced of surface energy.

• Part shrinkage occurs during sintering due to pore size reduction.

• Density increases due to filling up incipient holes and increasing area of contact among powder particles in compact perform.

Movements of Atoms During Sintering

Fig: A three particle sketch of sintering, showing several possible paths of atomic motion involved with particle bonding (neck growth) and pore shrinkage (densification).

Mechanisms For Sintering Metal Powders

Fig: Schematic illustration of two mechanisms for sintering metal powders: (a) solid-state material transport; and (b) vapor-phase material transport.

Where R = particle radius, r = neck radius, and p = neck-profile radius.

Bonding among the powder particles takes places in three ways: (1) melting of minor constituents in the powder particles, (2) diffusion between the powder particles, and (3) mechanical bonding.

Solid State Sintering

• Solid state sintering involves heating the powder below the melting point to allow solid-state diffusion and bonding the particles together.

• Particle bonding is initiated at contact point, which then grow into necks, reducing pores between particles.

• Prolonged heating develops grain boundaries between particle in place of necked regions.

Liquid Phase Sintering

Liquid phase sintering usually involves mixing an iron powder With a liquid forming powder ( Boride, carbide, phosphide, copper ,tin And heating to a temperature where the liquid forms, spread and contributes to particle bonding and densifications.

Fig: Liquid phase sintering

Factors In Sintering

• The nature and strength of the bond between the particles depends on: 1. The mechanism of diffusion,

2. Plastic flow of the powder particles, and

3. Evaporation of volatile material from the compacted preform.

• The three critical factors that control the sintering process are:

1) time, 2) temperature and 3) the furnace atmosphere

Sintering Time and Temperature for Metals

Examples of Sintering Production Lines

Mechanical Properties of P/M Materials

Comparison of Properties of Wrought and Equivalent P/M Metals

Finishing Operations

• A number of secondary and finishing operations can be applied after sintering, some of them are:

1. Sizing: cold pressing to improve dimensional accuracy

2. Coining: cold pressing to press details into surface

3. Impregnation: oil fills the pores of the part

4. Infiltration: pores are filled with a molten metal

5. Heat treating, plating, painting

Impregnation and Infiltration

• Porosity is a unique and inherent characteristic of PM technology.

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

• Two categories:

1. Impregnation

2. Infiltration

Impregnation

• The term used when oil or other fluid is permeated into the pores of a sintered PM part

• Common products are oil-impregnated bearings, gears, and similar components.

• An alternative application is when parts are impregnated with polymer resins that seep into the pore spaces in liquid form and then solidify to create a pressure tight part.

Infiltration

• An 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.

• Involves heating the filler metal in contact with the

sintered component so capillary action draws the

filler into the pores

• The resulting structure is relatively nonporous, and

the infiltrated part has a more uniform density, as

well as improved toughness and strength.

General Classification of Powder Metallurgy Parts 1) Class I parts with a diameter (or

thickness) up to 65 mm and single level parts of any contour that can be pressed with a force from one direction.

2) Class II parts are single level components of any thickness and any contour that must be pressed from two directions.

3) Class III parts are two level components of any thickness and contour that must be pressed from two directions.

4) Class IV parts are multilevel components of any thickness and contour that must be pressed from two direction.

(a) Class I,(b) Class II

(c) Class III,(d) Class IV

Design Considerations for P/M

1. Shape of compact must be kept as simple and uniform as possible.

2. Provision must be made for ejection of the green compact without damaging the compact.

3. P/M parts should be made with the widest acceptable tolerances to maximize tool life.

4. Part walls should not be less than 1.5 mm thick;

5. Walls with length to thickness ratios above 8:1 are difficult to press.

6. Steps in parts can be produced if they are simple and their size doesn’t exceed 15% of the overall part length.

7. Letters can be pressed if oriented perpendicular to pressing direction.

8. Raised letters are more susceptible to damage in the green stage and prevent stacking.

9. Flanges or overhangs can be produced by a step in the die.

10. A true radius cannot be pressed; instead use a chamfer.

11. Dimensional tolerances are on the order of ±0.05 to 0.1 mm.

12. Tolerances improve significantly with additional operations such as sizing, machining and grinding.

Poor & Good Designs of P/M Parts

Fig: Examples of P/M parts showing poor and good designs.

Note that sharp radii and reentry corners should be avoided and that threads and transverse holes have to be produced separately by additional machining operations.

Design Features for Use with Unsupported Flanges or Grooves

Fig: (a) Design features for use with unsupported flanges.

(b) Design features for use with grooves.

Die Design for Powder-Metal Compaction

Fig: Die geometry & design features for P/M compaction.

Further reading

• Fundamentals of powder metallurgy W. D. Jones

• Powder Metallurgy: Principles & Applications F. V. Lenel

• Fundamentals of P/M I. H. Khan