Powder Metallurgy

Presented To :Prof.Dr.Nahed El Mahalawy

Presented By : Reham Mohamed

Outline Introduction .Metal Powder Production .Metal Powder characteristic .Metal Powder Heat-treatment compaction .Sintering .Secondary Operation (treatment) .

Powder metallurgy is the process of blending fine

powdered materials, pressing them into a desired shape (compacted), and then heating the compressed material in a controlled atmosphere to bond the material (sintering).

Why Powder Metallurgy is Important

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 like filters, oil-impregnated bearings and gears

Why Powder Metallurgy is ImportantCertain metals that are difficult to

fabricate by other methods can be shaped by powder metallurgy like 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

The powder metallurgy process generally consists of four basic steps shows :(1) Powder Production(2) Powder mixing and blending(3) Compacting(4) Sintering

Metal Powder ProductionThe significant manufacturing methods of metal powder production may be classed as follows:1. Chemical methods (Chemical

reduction).2. Physical methods (Electrolytic

Method & Atomization Method).3. Mechanical methods

Chemical methods (Chemical reduction)

from the solid state : as in the reduction of iron oxide with carbon or of tungsten oxide with hydrogen

Physical methods (Electrolytic Method)

Physical methods (Atomization Method)(a) Gas atomization

(b) Water atomization

Mechanical methods A common method is the

use of a ball mill consisting of a rotating drum with hard wear resistant balls.

The critical factor is the speed of the drum’s rotation.

A very high speed will cause the material and the ball to be pressed against the walls of the drum,

because of the centrifugal forces and prevent relative motion between the material and the balls

The performance of metal powders during processing and the properties of powder metallurgy are dependent upon the characteristics of the metal powders that are used.

characteristics of metal powders:(a) Particle shape (b) Particle size(c) Particle size distribution (d) Flow rate(e) Compressibility (f) Apparentdensity(g) Purity

CHARACTERISTICS OF METAL POWDER

(a)Particle Shape: The particle shape depends largely on the method of

powder manufacture.The particle shape influences the flow characteristics

of powders.The desired product depends on the particle shape .Spherical particles can be used when high porosity is

desired. If high strength is required, we can select coarse

particles.The shape usually described in terms of aspect ratio.Aspect ratio : is a ratio of largest dimension to

smallest dimension, this ratio ranges from 1 (for spherical particle) to about 10 (for flakelike or needllike particle).

(b)Particle Size:• The particle size influences the control of porosity,

compressibility and amount of shrinkage. • It is determined by passing the powder through

standard sieves with various mesh size or by microscopic measurement.

(c)Particle Size Distribution:• means quantity of each standard particle size in

mixture.(which pass through standard sieves)• Particle size distribution influences the packing of

powder and its behaviour during moulding and sintering.

(d)Flow Rate:• a measure of the ease by which powder can be fed

and distributed into the die.• This determines the fineness of the particles.

(e)Compressibility: It is defined as volume of initial powder (powder

loosely filled in cavity) to the volume of compact part.

It depends on particle size, distribution and shape.Affects the green strength of the compact.The mechanical strength which a compacted

powder must have ,in order to withstand mechanical operations to which it is subjected after pressing and before sintering, without damaging its fine details and sharp edges.

(f) Apparent Density:ability to fill available space without the application

of external pressure. It dependence on powder size and size

distribution.

(g)Purity:High purity is required for a better

product. For maintaining this, the particles

must be isolated from the atmospheric oxidation or any contamination.

Metal Powder Heat treatment

Before the consolidation process there is treatment process to the powder, this called pre-compaction process.

(I) Pre-compaction (metal powder treatment): Annealing: It is customary that the

powder producer delivers the powder to the fabricator ready for mixing.

The aims of annealing are:1) To soften the powder.2) To reduce the residual amount of

oxygen, carbon and/or nitrogen from the powder.

Mixing and blendingMixing and blending are the two most

common pre-compaction steps in powder metallurgy. Blending means the combination of different sized powders of the same chemistry.

It is carried out to obtain a desired powder size distribution.

Various variables in the powder mixing process have been highlighted by Hausner. They are:

1. Type of mixer 2. Volume of the mixer 3. Geometry of the mixer

Types of Mixers Among various types of mixers

available, the following are most common for metal powders:

(i) Double Cone Mixer(ii) V-Mixer

Particle Size ReductionSuitable size reduction processes

normally produce an increase in the surface area (as a result of decreasing the average particle size) with narrow particle size distribution.

These results in increased homogeneity of non-uniform mixtures, increased chemical reaction rates, the actual requirements of a suitable size reduction process are extremely varied and depend on several parameters.

COMPACTIONPress blended powder into the desired shape and size in dies

using a hydraulic or mechanical pressThe purposes of compaction are to obtain the req. shape,

density and particle to particle contact to make the part sufficiently strong for further process

Pressed powder is known as “green compact”

Increased compaction pressure◦ Provides better packing of particles and leads to ↓ porosity◦ ↑ localized deformation allowing new contacts to be formed

between particles

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

Smaller particles provide greater strength mainly due to reduction in porosity

Because of friction between the metal particles and 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

(a) and (c) Single action press; (b) and (d) Double action press

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

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

Compacting cycle for a single level component

(a) Compaction of metal powder to form bushing

(b) Typical tool and die set for compacting spur gear

Cold isostatic pressing (CIP)The metal powder is placed in a

flexible rubber mold, the assemble is then pressurized hydrostatically in a chamber (usually by water)

The pressure range is from 400-1000 MPa

SINTERINGGreen compact obtained after compaction is brittle and low in

strengthGreen compacts are heated in a controlled-atmosphere furnace to

allow packed metal powders to bond togetherSintering temperature and time is usually 0.6 to 0.8 times the

melting point of the powder.

Sintering process 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

Second stage: High temperature stagePromotes solid-state bonding by diffusion. In some cases, the sintering temperature is above the melting point of one of the materials but below the melting point of the other. This is called liquid phase sintering.

(a) Loose powder (start of bond growth). (b) Initial stage (the pore volume shrinks). (c) Intermediate stage (grain boundaries

form at the contacts).  (d) Final stage (pores become smoother).

Optical microscope images taken from a stainless steel formed by PIM. This image sequence from various temperatures during the sintering cycle. The pores are black and diminish in size and content during heating to progressively higher temperatures: a) 1000C, b) 1100C, c) 1200C, d) 1260C, e) 1300C, and f) 1365C (the final sintering temperature).

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 to avoid oxidation

Secondary Operations•Sizing: To tighten dimensional tolerances, usually in the radial direction, relative to the direction of compacting pressure•Coining: To change axial dimensions and tolerances•Machining: To obtain shapes that cannot be compacted, such as by tapping holes or cutting undercut grooves•Forming: To change the shape of the part; can be done hot or cold•Re-pressing: To reduce porosity and increase strength and ductility; may be accompanied by resintering•Infiltration: To increase strength and decrease porosity•Heat treating: To increase hardness or strength•Joining: By sinter bonding, staking, brazing, infiltrating, or welding•Finishing: Includes deburring, polishing, impregnating, and plating

Poor and Good Designs of P/M Parts

Data about material composites

Advantages & disadvantages of P/M

Advantages Disadvantages

The P/M part can be produced to the neat net-shape requiring very little finishing operations

P/M process does not cause any waste products during the processing

Reasonably complex shape can be produced by P/M

It is possible to produce parts with a combination of materials(metal & ceramic)

Automation of the P/M process cab be easily accomplished.

The products of the tungsten & tungsten carbide can also be produced by P/M

The tooling cost is generally high so can only be justified for mass production.

The raw material cost is high. Because of the presence of

residual porosity mechanical properties are inferior.

With complex part geometries, the flow of metal powder into deep cavities & corner is a problem.