Powder Metallurgy

Powder Metallurgy, Powder Production methods,

Powder characteristics, Micromachining

ME 010 803 PRODUCTION ENGINEERINGModule III

Overview The Characterization of Engineering Powders

Production of Metallic Powders

Using them to make finished/semi-finished products.

Conventional Pressing and Sintering

Alternative Pressing and Sintering Techniques

Materials and Products for PM

Design Considerations in Powder Metallurgy

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Modern powder metallurgy dates only back to the

early 1800

Powder Metallurgy (PM) Metal processing technology in which parts

are produced from metallic powders PM parts can be mass produced to net shape

or near net shape, eliminating or reducing the need for subsequent machining

PM process wastes very little material ~ 97% of starting powders are converted to product

PM parts can be made with a specified level of porosity, to produce porous metal parts

Examples: filters, oil‑impregnated bearings and gears

Certain metals that are difficult to fabricate by other methods can be shaped by PM

Tungsten filaments for lamp bulbs are made by PM

Introduction• Earliest use of iron powder dates back to 3000 BC.

Egyptians used it for making tools

• Modern era of P/M began when W lamp filaments were developed by Edison

• Components can be made from pure metals, alloys, or mixture of metallic and non-metallic powders

• Commonly used materials are iron, copper, aluminium, nickel, titanium, brass, bronze, steels and refractory metals

• Used widely for manufacturing gears, cams, bushings, cutting tools, piston rings, connecting rods, impellers etc.

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Powder Metallurgy

. . . is a forming technique

Essentially, Powder Metallurgy (PM) is an art & science of producing metal or metallic powders, and using them to make finished or semi-finished products. Particulate technology is probably the oldest forming technique known to man.

There are archeological evidences to prove that the ancient man knew something about it.

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History of Powder Metallurgy

IRON Metallurgy >

How did Man make iron in 3000 BC?

Did he have furnaces to melt iron air blasts, and

The reduced material, which would then be spongy, [DRI ], used to be hammered to a solid or to a near solid mass.

Example: The IRON PILLER at Delhi

Quite unlikely, then how ???

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History of P/M Going further back in Time . . .

The art of pottery, (terracotta), was known to the pre-historic man (Upper Paleolithic period, around 30,000 years ago)!

Dough for making bread is also a powder material, bound together by water and the inherent starch in it. Baked bread, in all its variety, is perhaps one of the first few types of processed food man ate.

(Roti is a form of bread.)

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Renaissance of P/M The modern renaissance of powder metallurgy began

in the early part of last century, when technologists tried to replace the carbon filament in the Edison lamp.

The commercially successful method was the one developed by William Coolidge. He described it in 1910, and got a patent for it in 1913.

This method is still being used for manufacturing filaments.

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The Wars and the post-war era brought about huge leaps in science, technology and engineering.

New methods of melting and casting were perfected, thereby slowly changing the metallurgy of refractory materials.

P/M techniques have thereafter been used only when their special properties were needed.

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Renaissance of P/M

Powder Metallurgy An important point that comes out :

The entire material need not be melted to fuse it.

The working temperature is well below the melting point of the major constituent, making it a very suitable method to work with refractory materials, such as: W, Mo, Ta, Nb, oxides, carbides, etc.

It began with Platinum technology about 4 centuries ago… in those days, Platinum, [mp = 1774°C], was "refractory", and could not be melted.

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Metal processing technology in which parts are produced from metallic powders.

Usually during PM production, the powder is compressed (pressed) into the desired shape and then heated (sintered) to bond the particles into a hard, rigid mass.

− Pressing is accomplished in a press-type machine using punch-and-die tooling designed specifically for the part to be manufactured

− Sintering is performed at a temperature below the melting point of the metal

Powder Metallurgy

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PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machining.

PM process wastes very little material - about 97% of the starting powders are converted to product.

PM parts can be made with a specified level of porosity, to produce porous metal parts.

Examples: filters, oil-impregnated bearings, gears….

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Why Powder Metallurgy is Important?

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

Example: Tungsten filaments for incandescent lamp bulbs are made by PM

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.

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Why Powder Metallurgy is Important?

When to use PM• Competitive with processes such as

casting, forging, and machining. • Used when melting point is too high (W,

Mo).• reaction occurs at melting (Zr).• too hard to machine.• very large quantity. • Near 70% of the P/M part production is

for automotive applications.• Good dimensional accuracy.• Controllable porosity.• Size range from tiny balls for ball-point

pens to parts weighing 100 lb. Most are around 5 lb.

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P/M Applications Electrical Contact materials

Heavy-duty Friction materials

Self-Lubricating Porous bearings

P/M filters

Carbide, Alumina, Diamond cutting tools

Structural parts

P/M magnets

Cermets

and more, such as high tech applications15

Hi-Tech Applications of P/MAnti-friction products

Friction products

Filters

Electrical Contacts

Sliding Electrical Contacts

Very Hard Magnets

Very Soft Magnets

Refractory Material Products

Hard and Wear Resistant Tools

Ferrous & Non-ferrous Structural parts etc . . .

THESE COMPONENTS ARE USED IN AIR & SPACE

CRAFTS, HEAVY MACHINERY, COMPUTERS,

AUTOMOBILES, etc…

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Powder Metallurgy Merits The main constituent need not be melted

The product is porous – [note: the porosity can be controlled]

Controlled porosity for self lubrication or filtration uses

Constituents that do not mix can be used to make composites, each constituent retaining its individual property

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Powder Metallurgy Merits Near Nett Shape is possible, thereby reducing the post-

production costs, therefore:

Precision parts can be produced

The production can be fully automated, therefore,

Mass production is possible

Production rate is high

Over-head costs are low

Break even point is not too large

Material loss is small

Control can be exercised at every stage18

Advantages of P/M Virtually unlimited choice of alloys, composites, and

associated properties – Refractory materials are popular by this process

Can be very economical at large run sizes (100,000 parts)

Long term reliability through close control of dimensions and physical properties

Wide latitude of shape and design

Very good material utilization

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Limitations and Disadvantages High tooling and equipment costs.

Metallic powders are expensive.

Problems in storing and handling metal powders.

– Degradation over time, fire hazards with certain metals

Limitations on part geometry because metal powders do not readily flow laterally in the die during pressing.

Variations in density throughout part may be a problem, especially for complex geometries.

Powder Metallurgy Disadvantages

Porous !! Not always desired.

Large components cannot be produced on a large scale.

Some shapes are difficult to be produced by the conventional p/m route.

Whatever, the merits are so many that P/M, as a forming technique, is gaining popularity

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Powder Metallurgy Products1. Porous or permeable products such as bearings, filters, and

pressure or flow regulators

2. Products of complex shapes that would require considerable machining when made by other processes

3. Products made from materials that are difficult to machine or materials with high melting points

4. Products where the combined properties of two or more metals are desired

5. Products where the P/M process produces clearly superior properties

6. Products where the P/M process offers economic advantage

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Materials and Products for PM• Raw materials for PM are more expensive than for

other metalworking because of the additional energy required to reduce the metal to powder form.

• Accordingly, PM is competitive only in a certain range of applications.

• What are the materials and products that seem most suited to powder metallurgy?

Gears, bearings, sprockets, fasteners, electrical contacts, cutting tools, and various machinery parts

When produced in large quantities, gears and bearings are ideal for PM because:

− The geometry is defined in two dimensions

− There is a need for porosity in the part to serve as a reservoir for lubricant

PM Products

25A collection of powder metallurgy parts.

PM Parts

Connecting Rods: Forged on left; P/M on right

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Powdered Metal Transmission Gear Warm compaction method with 1650-ton press Teeth are molded net shape: No machining UTS = 155,000 psi 30% cost savings over the original forged part

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P M as a Forming Technique

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PM Work Materials Largest tonnage of metals are alloys of iron,

steel, and aluminum

Other PM metals include copper, nickel, and refractory metals such as molybdenum and tungsten

Metallic carbides such as tungsten carbide are often included within the scope of powder metallurgy

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Engineering Powders

A powder can be defined as a finely divided particulate solid

Engineering powders include metals and ceramics

Geometric features of engineering powders:– Particle size and distribution

– Particle shape and internal structure

– Surface area

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Chemistry and Surface Films• Metallic powders are classified as either

– Elemental - consisting of a pure metal

– Pre-alloyed - each particle is an alloy

• Possible surface films include oxides, silica, adsorbed organic materials, and moisture

– As a general rule, these films must be removed prior to shape processing

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PM Materials – Elemental Powders

A pure metal in particulate form Applications where high purity is important Common elemental powders:

– Iron– Aluminum– Copper

Elemental powders can be mixed with other metal powders to produce alloys that are difficult to formulate by conventional methods.

– Example: tool steels

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PM Materials – Pre Alloyed Powders

Each particle is an alloy comprised of the desired chemical composition

Used for alloys that cannot be formulated by mixing elemental powders

Common pre-alloyed powders: – Stainless steels– Certain copper alloys– High speed steel

Powder Characterization

Different Shapes and Sizes

Sizes and shapes are important in blending and

compaction.

Often a mixed size is beneficial.

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Figure: Several of the possible (ideal) particle shapes in powder metallurgy.

Particle Shapes in PM

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Inter-particle Friction and Powder Flow

Friction between particles affects ability of a powder to flow readily and pack tightly

A common test of inter-particle friction is the angle of repose, which is the angle formed by a pile of powders as they are poured from a narrow funnel.

Figure: Inter-particle friction as indicated by the angle of repose of a pile of powders poured from a narrow funnel.

Larger angles indicate greater inter-particle friction.

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Let’s check!

Smaller particle sizes show steeper angles or larger particle sizes?!

Smaller particle sizes generally show greater friction and steeper angles!

Finer particles

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Let’s check!

Which shape has the lowest inter-particle friction?

Spherical shapes have the lowest inter-particle friction!

Little friction between spherical particles!

As shape deviates from spherical, friction between particles tends to increase

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Observations Easier flow of particles correlates with lower inter-

particle friction.

Lubricants are often added to powders to reduce inter-particle friction and facilitate flow during pressing.

Smaller particle sizes generally show greater friction and steeper angles.

Spherical shapes have the lowest inter-particle friction.

As shape deviates from spherical, friction between particles tends to increase.

Powder Size Parameters

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Measuring Particle SizeMost common method uses screens of different mesh sizes

Mesh count It refers to the number of openings per linear inch of screen

– A mesh count of 200 means there are 200 openings per linear inch

– Since the mesh is square, the count is the same in both directions, and the total number of openings per square inch is 2002 = 40,000

– Higher mesh count = smaller particle size

Figure: Screen mesh for sorting particle sizes.

Powder Sieving

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MESH SIZES

-100/+200 mesh: negative means particle goes through, positive means particle does not go through. Thus, this mesh means particle sizes between 75 and 150 microns.

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Sedimentation Technique

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Sedimentation Technique

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Sedimentation Technique

Laser Technique

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Particle Density MeasuresTrue density

density of the true volume of the material

– The density of the material if the powders were melted into a solid mass

Bulk densitydensity of the powders in the loose state after

pouring

Which one is smaller?!

Because of pores between particles, bulk density is less than true density.

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Packing Factor

Typical values for loose powders range between 0.5 and 0.7

Bulk densitytrue densityPacking factor =

• If powders of various sizes are present, smaller powders will fit into spaces between larger ones, thus higher packing factor

• Packing can be increased by vibrating the powders, causing them to settle more tightly

• Pressure applied during compaction greatly increases packing of powders through rearrangement and deformation of particles

How can we increase the bulk density?

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Porosity Ratio of volume of the pores (empty spaces) in the

powder to the bulk volume

• In principlePorosity + Packing factor = 1.0

• The issue is complicated by possible existence of closed pores in some of the particles

• If internal pore volumes are included in above porosity, then equation is exact

Powder Metallurgy Process

Powder production

Blending or mixing

Powder compaction

Sintering

Finishing Operations

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Usual PM production sequenceBlending and mixing (Rotating drums, blade and screw mixers)

Pressing - powders are compressed into desired shape to produce green compactAccomplished in press using punch-and-die tooling designed for the part

Sintering – green compacts are heated to bond the particles into a hard, rigid mass.

Performed at temperatures below the melting point of the metal

Powder Metallurgy Processing

POWDER

PROCESSING

PROPERTIES

Powder fabrication

Size and shape characterization

Microstructure (eg. dendrite size)

Chemical homogeneity, and ppt. size

Compaction

Sintering

Forging/Hot pressing

Density, Porosity

Ductility, Strength

Conductivity

Other functional properties 52

Powder metallurgy

(P/M) consists of several steps.

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Powder Metallurgy Process

Powder Metallurgy Process

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Production of Metallic Powders In general, producers of metallic powders are not the same

companies as those that make PM parts

Any metal can be made into powder form

Three principal methods by which metallic powders are commercially produced

1. Atomization (by gas, water, also centrifugal one)

2. Chemical

3. Electrolytic

In addition, mechanical methods are occasionally used to reduce powder sizes

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High velocity gas stream flows through expansion nozzle, siphoning molten metal from below and spraying it into container.

Figure: (a) gas atomization method

Gas Atomization Method

Produces a liquid-metal stream by injecting molten metal through a small orifice.

Stream is broken by jets of inert gas, air, or water.

The size of the particle formed depends on the temperature of the metal, metal flow rate through the orifice, nozzle size and jet characteristics.

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Gas Atomization Method

HORIZONTAL GAS ATOMIZATION

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VERTICAL GAS ATOMIZER

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DETAILS OF DROPLET FORMATION

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Centrifugal Atomization

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Electrode CentrifugationA consumable electrode is rotated rapidly in a helium-filled chamber.

The rotating electrode is melted by an arc using a tungsten cathode or plasma torch

The centrifugal force breaks up the molten tip of the electrode into metal particles.

The powder is formed from melt electrode and is solidified in a vacuum or inert gas environment.

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OXIDE REDUCTION PROCESS FOR METAL POWDER FABRICATION

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• Reduce metal oxides with H2/CO• Powders are spongy and porous and they have

uniformly sized spherical or angular shapes

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OXIDE REDUCTION

CARBONYL PROCESS

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CARBONYL PROCESS• React high purity Fe or Ni with CO to form gaseous

carbonyls• Carbonyl decomposes to Fe and Ni• Small, dense, uniformly spherical powders of high

purity.

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Electrolytic Process for Powder Production

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ELECTROLYSIS PROCESS

• Metal powder deposits at the cathode from aqueous solution.

• Powders are among the purest available.

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MECHANICAL MILLING

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Comminution Crushing

Milling in a ball mill

Powder produced

– Brittle: Angular– Ductile: flaky and not particularly suitable for P/M

operations

Mechanical Alloying Powders of two or more metals are mixed in a ball mill

Under the impact of hard balls, powders fracture and join together by diffusion

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Mechanical Comminution to Obtain Fine Particles

Figure: Methods of mechanical comminution to obtain fine particles:

(a) roll crushing, (b) ball mill, and (c) hammer milling.

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Mechanical Alloying

Figure Mechanical alloying of nickel particles with dispersed smaller particles. As nickel particles are flattened between the two balls, the

second smaller phase impresses into the nickel surface and eventually is dispersed throughout the particle due to successive flattening, fracture,

and welding events.72

Particle Shapes in Metal Powders

Figure: Particle shapes in metal powders, and the processes by which they are produced. Iron powders are produced by many of these processes.

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Conventional Press and SinterAfter metallic powders have been produced, the conventional PM sequence consists of:

1.Blending and mixing of powders

2.Compaction - pressing into desired shape.

3.Sintering - heating to temperature below melting point to cause solid‑state bonding of particles and strengthening of part.

In addition, secondary operations are sometimes performed to improve dimensional accuracy, increase density, and for other reasons.

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Figure: Conventional powder metallurgy

production sequence:

(1)blending,

(2) compacting,

shows the operation and/or work part during the

sequence.

(3) Sintering;

shows the condition of the particles while sintering

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Blending and Mixing of Powders

• Blending - powders of same chemistry but possibly different particle sizes are intermingled

– Different particle sizes are often blended to reduce porosity

• Mixing - powders of different chemistries are combined .

PM technology allows mixing various metals into alloys that would be difficult or impossible to produce by other means.

For successful results in compaction and sintering, the starting powders must be homogenized (powders should be blended and mixed).

Blending or Mixing• Blending a coarser fraction with a finer fraction ensures that

the interstices between large particles will be filled out.

• Powders of different metals and other materials may be mixed in order to impart special physical and mechanical properties through metallic alloying.

• Lubricants may be mixed to improve the powder’s flow characteristics.

• Binders such as wax or thermoplastic polymers are added to improve green strength.

• Sintering aids are added to accelerate densification on heating.

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BlendingTo make a homogeneous mass with uniform distribution of particle size and composition.

– Powders made by different processes have different sizes and shapes

– Mixing powders of different metals/materials

– Add lubricants (<5%), such as graphite and stearic acid, to improve the flow characteristics and compressibility of mixtures.

Combining is generally carried out in– Air or inert gases to avoid oxidation

– Liquids for better mixing, elimination of dusts and reduced explosion hazards

Hazards– Metal powders, because of high surface area to volume ratio are

explosive, particularly Al, Mg, Ti, Zr, Th78

Bowl Geometries for Blending Powders

Figure: (e) A mixer suitable for blending metal powders.

Since metal powders are abrasive, mixers rely on the rotation or tumbling of enclosed geometries as opposed to using aggressive agitators.

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Some common equipment geometries used for blending powders

(a) Cylindrical, (b) rotating cube, (c) double cone, (d) twin shell

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CompactionApplication of high pressure to the powders to form them into the required shape.

Conventional compaction method is pressing, in which opposing punches squeeze the powders contained in a die.

– The work part after pressing is called a green compact, the word green meaning not yet fully processed.

– The green strength of the part when pressed is adequate for handling but far less than after sintering.

Compaction• Press powder into the desired shape and size in dies using

a hydraulic or mechanical press• Pressed powder is known as “green compact”• Stages of metal powder compaction:

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Compaction

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Powders do not flow like liquid, they simply compress until an equal and opposing force is created.

– This opposing force is created from a combination of

(1)resistance by the bottom punch and

(2)friction between the particles and die surface

Compacting consolidates and densifies the component for transportation to the sintering furnace.

Compacting consists of automatically feeding a controlled amount of mixed powder into a precision die, after which it is compacted.

Compacting is usually performed at room temperature. Pressures range from 10 tons per square inch (tons/in2) (138 MPa) to 60 tons/in2 (827 MPa), or more.

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Compacting

Compacting• Loose powder is compacted and densified into a shape,

known as green compact.• Most compacting is done with mechanical presses and rigid

tools.– Hydraulic and pneumatic presses are also used.

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Figure: (Left) Typical press for the compacting of metal powders. A removable die set (right) allows the machine to be producing parts with one die set while

another is being fitted to produce a second product. 85

Compaction Sequence

Figure: Typical compaction sequence for a single-level part, showing the functions of the feed shoe, die core rod, and upper and lower punches.

Loose powder is shaded; compacted powder is solid black. 86

Additional Considerations During Compacting

When the pressure is applied by only one punch, the maximum density occurs right below the punch surface and decreases away from the punch.

For complex shapes, multiple punches should be used.

Compaction with a single moving punch, showing the resultant non uniform density (shaded), highest

where particle movement is the greatest.

Density distribution obtained with a double-acting press and two moving punches. Note the

increased uniformity than in a single punch. Thicker parts can be effectively compacted.

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Friction problem in cold compaction

The effectiveness of pressing with a single-acting punch is limited. Wall friction opposes compaction. The pressure tapers off rapidly and density diminishes away from the punch. Floating container and two counteracting punches help alleviate the problem.

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• Smaller particles provide greater strength mainly due to reduction in porosity

• Size distribution of particles is very important. For same size particles minimum porosity of 24% will always be there– Box filled with tennis balls will always have open space

between balls

– Introduction of finer particles will fill voids and result in ↑ density.

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Because of friction between (i) the metal particles and (ii) between the punches and the die, the density within the compact may vary considerably. Density variation can be minimized by proper punch and die design

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.

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Increased compaction pressure– Provides better packing of particles and leads to ↓

porosity

– ↑ localized deformation allowing new contacts to be formed between particles

Effects of Compaction

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• At higher pressures, the green density approaches density of the bulk metal

• Pressed density greater than 90% of the bulk density is difficult to obtain

• Compaction pressure used depends on desired density.

Effects of Compaction

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Complex Compacting• If an extremely complex shape is desired, the powder

may be encapsulated in a flexible mold, which is then immersed in a pressurized gas or liquid

– Process is known as isostatic compaction

• In warm compaction, the powder is heated prior to pressing

• The amount of lubricant can be increased in the powder to reduce friction

• Because particles tend to be abrasive, tool wear is a concern in powder forming

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(a)Compaction of metal powder to form bushing(b)Typical tool and die set for compacting spur gear 94

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SinteringHeat treatment to bond the metallic particles, thereby

increasing strength and hardness.

Usually carried out at between 70% and 90% of the

metal's melting point (absolute scale)

– Generally agreed among researchers that the primary

driving force for sintering is reduction of surface energy

– Part shrinkage occurs during sintering due to pore size

reduction

Sintering

Parts are heated to ~80% of melting temperature. Transforms compacted mechanical bonds to much

stronger metal bonds. Many parts are done at this stage. Some will require

additional processing.96

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Figure: Sintering on a microscopic scale: (1) particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) the pores

between particles are reduced in size; and (4) grain boundaries develop between particles in place of the necked regions.

Sintering Sequence Parts are heated to 0.7~0.9 Tm. Transforms compacted mechanical bonds to much

stronger metallic bonds.

SinteringIn the sintering operation, the pressed-powder compacts are

heated in a controlled atmosphere to right below the melting point.

Three stages of sintering

Burn-off (purge)- combusts any air and removes lubricants or binders that would interfere with good bonding

High-temperature- desired solid-state diffusion and bonding occurs

Cooling period- lowers the temperature of the products in a controlled atmosphere.

All three stages must be conducted in oxygen-free conditions of a vacuum or protective atmosphere.

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• Green compact obtained after compaction is brittle and low in strength

• Green compacts are heated in a controlled-atmosphere furnace to allow packed metal powders to bond together

Carried out in three stages:

• First stage: Temperature is slowly increased so that all volatile materials in the green compact that would interfere with good bonding is removed

– Rapid heating in this stage may entrap gases and produce high internal pressure which may fracture the compact.

Sintering – Three Stages

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Promotes solid-state bonding by diffusion. Diffusion is time-temperature sensitive. Needs sufficient time.

Promotes vapor-phase transport. As material is heated very close to MP, metal atoms will be released in the vapor phase from the particles. Vapor phase resolidifies at the interface.

Sintering: High temperature stage

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Third stage:

Sintered product is cooled in a controlled atmosphere.– Prevents oxidation and thermal shock

Gases commonly used for sintering:

H2, N2, inert gases or vacuum

Sintering: 3rd stage

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Figure: (a) Typical heat treatment cycle in sintering; and (b) schematic cross section of a continuous sintering furnace.

Sintering Cycle and Furnace

Sintering Time, Temperature, and Indicated Properties

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Sintering Time and Temperature for Metals

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sintered

sintered green

greenV

VshrinkageVolume

3/1

sintered

greenshrinkageLinear

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Sintering

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

15%).

• Density is often kept intentionally low to preserve interconnected porosity for bearings, filters, acoustic barriers, and battery electrodes.

• However, to improve properties, finishing processes are needed:– 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 use

after the sintering process.

Some products may use secondary operation to provide enhanced precision, improved properties, or special characteristics.

Distortion may occur during non uniform cool-down so the product may be repressed, coined, or sized to improve 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 forced into the porous network to offer lubrication over an extended product lifetime

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

• P/M products can also be subjected to the conventional finishing operations: heat treatment, machining, and surface treatments

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

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

•Sizing - pressing a sintered part to improve dimensional accuracy

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

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

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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

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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

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

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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 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.

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• Conventional press and sinter sequence is the most widely used shaping technology in powder metallurgy.

• Additional methods for processing PM parts include:

– Isostatic pressing

– Powder injection molding

– Powder rolling, extrusion and forging

– Combined pressing and sintering

– Liquid phase sintering

Alternative Pressing and Sintering Techniques

Isostatic Pressing

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Cold isostatic pressing is performed at room temperature with liquid as the pressure medium. Hot isostatic pressing is performed at elevated temperature with gas as the pressure medium.

Special Process: Hot compaction• Advantages can be gained by combining consolidation and

sintering,

• High pressure is applied at the sintering temperature to bring the particles together and thus accelerate sintering.

• Methods include

– Hot pressing

– Spark sintering

– Hot isostatic pressing (HIP)

– Hot rolling and extrusion

– Hot forging of powder preform

– Spray deposition115

Hot - Iso static Pressing• Hot-isostatic pressing (HIP) combines powder compaction

and sintering into a single operation

– Gas-pressure squeezing at high temperatures

• Heated powders may need to be protected from harmful environments.

• Products emerge at full density with uniform, isotropic properties.

• Near-net shapes are possible.

• The process is attractive for reactive or brittle materials, such as beryllium (Be), uranium (U), zirconium (Zr), and titanium (Ti).

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Hot-Isostatic PressingHIP is used to

Densify existing parts

Heal internal porosity in casting

Seal internal cracks in a variety of products

Improve strength, toughness, fatigue resistance, and creep life.

HIP is relative long, expensive and unattractive for high-volume production

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Hot Iso static Pressing (HIP)

Steps in HIP

Schematic illustration of hot isostatic pressing. The pressure and temperature variation versus time are shown in the diagram.

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Combined StagesSimultaneous compaction + sintering

Produces compacts with almost 100% density.

Good metallurgical bonding between particles and good mechanical strength.

Container: High MP sheet metal

Container subjected to elevated temperature and a very high vacuum to remove air and moisture from the powder.

Pressurizing medium: Inert gas

Operating conditions -100 MPa at 1100 C

Uses– Superalloy components for aerospace industries– Final densification step for WC cutting tools and PM tool steels.

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• Metal powder placed in a flexible rubber mold

• Assembly pressurized hydrostatically by water (400 – 1000 MPa)

• Typical: Automotive cylinder liners →

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Cold Isostatic Pressing

Cold Isostatic Pressing

Figure: Schematic diagram of cold isostatic pressing, as applied to forming a tube. The powder is enclosed in a flexible container around a solid-core rod. Pressure is applied isostatically to the assembly inside a

high-pressure chamber121

Liquid Phase SinteringDuring sintering, a liquid phase from the lower MP component, may exist.

Alloying may take place at the particle-particle interface.

Molten component may surround the particle that has not melted.

High compact density can be quickly attained.

Important variables:

– Nature of alloy, molten component/particle wetting, capillary action of the liquid.

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Powder Injection Molding

Metal injection molding Pellets made of powders and binder Heated to molding temperature and

injected into a mold Can create complex designs

Metal Injection Molding (MIM)

Figure: Flow chart of MIM process used to produce small, intricate shaped

parts from metal powder.

Figure: Metal injection molding is ideal for producing

small, complex parts.

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Metal Injection Molding (MIM)/ Powder Injection Molding (PIM)

• Ultra-fine spherical-shaped metal, ceramic, or carbide powders are combined with a thermoplastic or wax.

– Becomes the feedstock for the injection process

• The material is heated to a paste like consistency and injected into a heated mold cavity.

• After cooling and ejection, the binder material is removed.

–Most expensive step in MIM and PIM

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MIMTable: Comparison of conventional powder metallurgy and metal

injection molding

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Feature P/M MIM

Particle size 20-250 mm <20 mm

Particle response Deforms plastically Un deformed

Porosity (% nonmetal) 10 – 20% 30 - 40%

Amount of binder/Lubricant 0.5 – 2% 30 – 40%

Homogeneity of green part Non homogeneous Homogeneous

Final sintered density <92% > 96%

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Powder Rolling

Figure 17.17 Schematic illustration of powder rolling.128

Powder Rolling

Figure: One method of producing continuous sheet products from powdered feed stock.

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Slip: Suspension of colloidal (small particles that do not settle) in an immiscible liquid (generally water).

Slip is poured in a porous mold made of plaster of paris. Air entrapment can be a major problem.

After mold has absorbed some water, it is inverted and the remaining suspension poured out.

The top of the part is then trimmed, the mold opened, and the part removed.

Application: Large and complex parts such as plumbing ware, art objects and dinnerware.

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Slip-Casting

(i) Slip is first poured into an absorbent mould(ii)a layer of clay forms as the mould surface absorbs water

(iii)when the shell is of suitable thickness excess slip is poured away(iv)the resultant casting

Slip-Casting

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Other Techniques to Produce High-Density P/M Products

High-temperature metal deformation processes can be used to produce high density P/M parts

Ceracon process- a heated preform is surrounded by hot granular material, transmitting uniform pressure.

Spray forming- inert gases propel molten droplets onto a mold.

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Spray Deposition

Figure: Spray deposition (Osprey Process) in which molten metal is sprayed over a rotating mandrel to produce seamless tubing and pipe

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Properties of P/M ProductsThe properties of P/M products depend on multiple variables

– Type and size of powder

– Amount and type of lubricant

– Pressing pressure

– Sintering temperature and time

– Finishing treatments

Mechanical properties are dependent on density

Products should be designed (and materials selected) so that the final properties will be achieved with the anticipated final porosity.

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P/M Materials

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• The Metal Powder Industries Federation (MPIF) defines four classes of powder metallurgy part designs, by level of difficulty in conventional pressing.

• Useful because it indicates some of the limitations on shape that can be achieved with conventional PM processing.

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PM Parts Classification System

Classes of P M Equipment•The complexity of the part dictates the complexity of equipment

•Equipment has been grouped into four classes.

Figure: Sample geometries of the four basic classes of press-and-sinter powder metallurgy parts. Note the increased pressing complexity that would be

required as class increases. 137

Classes of Powder Metallurgy Equipment

Table: Features that define the various classes of press-and –sinter P/M parts

Class Levels Press Actions 1 1 Single 2 1 Double 3 2 Double 4 more than 2 Double or multiple

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• Economics usually require large quantities to justify cost of equipment and special tooling

− Minimum quantities of 10,000 units are suggested

• PM is unique in its capability to fabricate parts with a controlled level of porosity

− Porosities up to 50% are possible

• PM can be used to make parts out of unusual metals and alloys - materials that would be difficult if not impossible to produce by other means

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Design Guidelines for PM Parts - I

The part geometry must permit ejection from die after pressing

− This generally means that part must have vertical or near-vertical sides, although steps are allowed

− Design features such as undercuts and holes on the part sides must be avoided

− Vertical undercuts and holes are permissible because they do not interfere with ejection

− Vertical holes can be of cross-sectional shapes other than round without significant difficulty

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Design Guidelines for PM Parts - II

• Screw threads cannot be fabricated by PM; if required, they must be machined into the part.

• Chamfers and corner radii are possible by PM pressing, but problems arise in punch rigidity when angles are too acute.

• Wall thickness should be a minimum of 1.5 mm (0.060in) between holes or a hole and outside wall.

• Minimum recommended hole diameter is 1.5 mm (0.060in).

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Design Guidelines for PM Parts - III

Design Aspects

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(a) Length to thickness ratio limited to 2-4;(b) Steps limited to avoid density variation;

(c) Radii provided to extend die life, sleeves greater than 1 mm, through hole greater than 5 mm;

(d) Feather-edged punches with flat face;

Design Aspects

(e) Internal cavity requires a draft; (f) Sharp corner should be avoided;

(g) Large wall thickness difference should be avoided;(h) Wall thickness should be larger than 1 mm.

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Design Considerations for P/M• The shape of the compact must be kept as simple and

uniform as possible.

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

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

• Part walls should not be less than 1.5 mm thick; thinner walls can be achieved on small parts; walls with length-to-thickness ratios above 8:1 are difficult to press.

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

• Letters can be pressed if oriented perpendicular to the pressing direction. Raised letters are more susceptible to damage in the green stage and prevent stacking.

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

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

• Dimensional tolerances are on the order of ±0.05 to 0.1 mm. Tolerances improve significantly with additional operations such as sizing, machining and grinding.

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Design Considerations for P/M

Die Design for Powder-Metal Compaction

Figure: Die geometry and design features for powder-metal compaction.146

Poor and Good Designs of P/M Parts

Figure: 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.147

Design Features for Use with Unsupported Flanges or Grooves

Figure: (a) Design features for use with unsupported flanges. (b) Design features for use with grooves.

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Die Design for P/M• Thin walls and projections create fragile tooling.

• Holes in pressing direction can be round, square, D-shaped, keyed, splined or any straight-through shape.

• Draft is generally not required.

• Generous radii and fillets are desirable to extend tool life.

• Chamfers, rather the radii, are necessary on part edges to prevent burring.

• Flats are necessary on chamfers to eliminate feather-edges on tools, which break easily.

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Design of Powder Metallurgy Parts

Basic rules for the design of P/M parts

– Shape of the part must permit ejection from die

– Powder should not be required to flow into small cavities

– The shape of the part should permit the construction of strong tooling

– The thickness of the part should be within the range for which P/M parts can be adequately compacted

– The part should be designed with as few changes in section thickness as possible

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Basic Rules for P/M Parts• Parts can be designed to take advantage of the fact that

certain forms and properties can be produced by P/M that are impossible, impractical, or uneconomical by any other method

• The design should be consistent with available equipment

• Consideration should be made for product tolerances

• Design should consider and compensate for dimensional changes that will occur after pressing

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Figure: Examples of poor and good design features for powder metallurgy products. Recommendations are based on ease of pressing, design of tooling,

uniformity of properties, and ultimate performance. 152

Financial Considerations

Typical size part for automation is 1” cube– Larger parts may require special machines (larger surface

area, same pressure equals larger forces involved)153

Die design must withstand 700 MPa, requiring specialty designs.

Can be very automated 1500 parts per hour not uncommon

for average size part

60,000 parts per hour achievable for small, low complexity parts in a rolling press.

P/M Summarizing:Powder Metallurgy is sought when -

a) It is impossible to form the metal or material by any other technique

b) When p/m gives unique properties which can be put to good use

c) When the p/m route is economical.

There may be over-lapping of these three points.

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