Design Project Presentationon

Fabrication of Microwave Sintering Setup for Powder Metallurgical Components

Presented By:Anjali Thomas (121720)Dhruv Raj (121736)Drishti Kurre (121737)Gaurav Singh (121738)Shashank Shekhar Singh (121779)

Guided By :Dr. Amit Sharma

Contents

Powder Metallurgy- An Introduction

• Science of producing metal powders and making finished /semi finished objects from mixed or alloyed powders with or without the addition of nonmetallic constituents.

• The P/M process is a rapid, economical and high volume production method for making precious components from powders.

• New types of powder allow the production of larger and higher strength materials.

• P/M is a choice when requirement for strength, wear resistance or high operating temperatures exceeds the capability of die casting alloys.

•P/M offers greater precision eliminating most or all of the finish machining operations required for casting.

•It avoids casting defects such as blow holes, shrinkage and inclusion.

•The biggest consumers of P/M parts at presents are automotive industries.

•Hardware, tools, cameras, farm and garden equipment's industries, business machine, sporting goods and military products are few more areas.

Basic Steps of Powder Metallurgy

Metal Powder Production

• Manufacturing methods for metal powder production may be classed as follows:-

1. Mechanical Methods

2. Physical Methods

3. Chemical Methods

Metal powder are the main constituent of a P/M product , final properties of the finished P/M part depends on size, shape, and surface area of powder particles.

Mechanical Methods:-• It is the cheapest powder production methods.

• These methods involve using mechanical forces such as compressive forces, attrition, shear or impact to facilitate particle size reduction of bulk materials. Eg- Milling, Grinding etc.

• These processes are not used as primary methods for the production of metal powders.

• Such methods have been used as the primary process for the following cases:-

1. Material which are relatively easy to fracture.

2. Reactive materials.

3. Common metals which are required in the form of flake powder.

Milling:-•During milling, impact, attrition, shear and compression forces are acted upon particles. •During impact, striking of one powder particle against another occurs. Attrition refers to the production of wear debris due to the rubbing action between two particles.

Ball Milling:- •It consist of rotating drums with hard wear resistant balls. The critical factors is the speed of drums rotation.•This contains cylindrical vessel rotating horizontally along the axis. Length of the cylinder is more or less equal to diameter.

• This contains cylindrical vessel rotating horizontally along the axis. Length of the cylinder is more or less equal to diameter.

• The vessel is charged with the grinding media. The grinding media may be made of hardened steel, or tungsten carbide, ceramics like agate, porcelain, alumina, zirconia.

• This process grinds the powder materials by impact/collision & attrition.

Physical Method:- Under this, two methods are prevalent.

1. Electrolytic Method

2. Atomization Method

Electrolytic Method:- •In this method, the processing conditions are so chosen that metals of high purity are precipitated from aqueous solution on the cathode of an electrolytic cell. • This method is mainly used for producing copper, iron powders. This method is also used for producing zinc, tin, nickel, cadmium, antimony, silver, lead, beryllium powders.

Copper powder :- Solution containing copper sulphate and sulphuric acid; crude copper as anode

Reaction:

At anode: Cu Cu+ + e-

At cathode: Cu+ + e- Cu

•The copper powder is washed and filtered and finally given an annealing and reducing treatment at temperature between 800 K – 1100 K in an atmosphere of partially combusted hydrocarbon gas.

Advantages:•Powders of high purity with excellent sinterability

•Wide range of powder quality can be produced by altering bath composition

Disadvantages:•Time consuming process; Pollution of work place because of toxic chemicals;

•Waste disposal is another issue, cost involved in oxidation of powders and hence they should be washed thoroughly.

Atomization• This uses high pressure fluid jets to break up a

molten metal stream into very fine droplets, which then solidify into fine particles.

• Types:-

1. Gas Atomization

2. Water Atomization

3. Centrifugal Atomization

4. Vacuum Atomization

Gas Atomization-

High velocity argon, nitrogen and helium gas jets are used. The molten metal is disintegrated and collected as atomized powder in a water bath. Fluidized bed cooling is used when certain powder characteristics are required.

Water Atomization-•High pressure water jets are used to bring about the disintegration of molten metal stream.•Water jets are used mainly because of their higher viscosity and quenching ability. •This is an inexpensive process and can be used for small or large scale production. But water should not chemically react with metals or alloys used.

Centrifugal Atomization-•One end of the metal bar is heated and melted by bringing it into contact with a non-consumable tungsten electrode, while rotating it longitudinally at very high speeds.•The centrifugal force created causes the metal drops to be thrown off outwards. This will then be solidified as spherical shaped particles inside an evacuated chamber.

Vacuum Atomization-In this method, when a molten metal supersaturated with a gas under pressure is suddenly exposed into vacuum, the gas coming from metal solution expands, causing atomization of the metal stream. This process gives very high purity powder. Usually hydrogen is used as gas. Hydrogen and argon mixture can also be used.

Chemical methods-This method consist of two process1.Chemical reduction2.Chemical decomposition of compounds.

Chemical Reduction-Chemical reduction involves chemical compounds most frequently an oxide, but sometimes a halide or other salt of the metal. This may be carried out-1.From the solid state- Reduction of iron oxide with carbon or of tungsten oxide with hydrogen. 2.From the gaseous state- reduction of titanium tetra chloride vapour with molten magnesium – the well known Kroll process.3.From the aqueous solution- Precipitation of cement copper from copper sulphate solution with iron or In the reduction of an ammonia nickel salt solution with hydrogen under pressure(hydrometallurgical method).

Chemical Decomposition of Compound-Under this category of powder production two methods are very common. These are:

1.Decomposition of metal hydrides

2.Decomposition of metal carbonyls

Metal Powder TreatmentAnnealing- It is a heat process whereby a metal is heated to a specific temperature / colour and then allowed to cool slowly.

The aims of annealing are:-

1.To soften the powder

2.To reduce the residual amount of oxygen, carbon and nitrogen from the powder.The annealing operation may be done in an atmosphere furnace or a vacuum furnace.Annealing temperature are kept as low as possible to minimize sintering.

Powder Mixing

It is mixing of solid lubricant with a metal powder or powders of several other metals. Sometimes the additives act as lubricant as well as alloying addition. Eg– Graphite in Iron Powder.

Types of Mixtures-Double Cone Mixtures:- This consist of vertical cylinder with conical ends which rotates about a horizontal axis.

V -Mixtures:-This is constructed by joining two cylinder of equal length into a ‘V’ as the ‘V’ rotates about its horizontal axis. The powder charge splits and refold.

Particle size reduction:- By reducing the particle size, it increases the surface area increases the homogeneity of non uniform mixture increases chemical reaction rate

Various method of size reduction can be classified as :-

1.Crushing:- The material Is drawn down into the tapered crushing chamber which reduces the size by compression and frictional forces.

2.Ball Milling:- These are of two types : Centrifugal and Planetary mills

In Centrifugal ball mills, a single bowl fastener is merely horizontally and eccentrically driven while not rotating itself.

In Planetary ball mills, two or four bowls fastener, each of which accommodate one grinding bowl are attached to supporting disc.

3.Disc Grinder:- The disc grinder are suited for processing hard brittle

material.

4. Attritor Milling:- This milling is achieved by high energy ball milling under condition such that powder are not only fragmented but also rewelded together.

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

conventional compaction method is pressing, in which opposite punches squeeze the powder 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.

Metal powder compaction

Compacting is usually performed at room temperature . Pressure ranges from 10 tons per square inch (138 MPa) to 60 tons per square inch827 (827 MPa).

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, highest where particle movement is greatest.

Density distribution obtained with double acting press and two moving punches. More uniformity and thicker parts can be compacted easily

Pressureless

Slip Casting Gravity Compaction Continuous Pressureless

Compaction

Pressure

Cold Isostatic Pressing Hot Isostatic Pressing High Energy Rte

techniques Vibratory Compaction Continuous Compaction Forging or Extrusion

Compaction Techniques

COLD ISOSTATIC PRESSINGPlacement of powder in flexible rubber mold.Hydrostatic pressurization in chamber by water.Most common pressure 400 mpa.

Typical application: automotive cylinder liner

Pressure

Single action uniaxial press forming and cold isostatic press forming

Hot isostatic pressing

Powder container is usually made up of high melting point sheet metal.Pressurizing media: inert gas

Common condition: 1100°, °, 100 mpa

Advantages: compacts of 100 density , good mechanical properties, good metallurgical bonding.

Limitation: wider dimensional tolerances, greater cost and time.

Compaction Pressure

Hot Isostatic pressing

High energy rate techniquesExplosive or spark discharge methods are applied in closed die.Short time and high pressuresHigh punch and die wear, limited tolerance, high cost.

Vibratory compaction: Simultaneous application of pressure and vibrationUse of much lower pressureComplicated equipment design.

Compaction- Pressure

CONTINUOUS COMPACTION

Applied to simple shapes (rod, sheet, tube, plate, etc)Flowing loose powder between a set of vertically oriented olls at much lower speed.

FORGING OR EXTRUSION Canning of powder Heating or evacuation of sealed container

followed by forging or extrusion Mechanical or chemical removal of container

material.

Slip Casting Preparation of slip (powder suspended in liquid and

additives) Keeping slip in mold made up of fluid absorbing material Formation of Slip casting Removal of slip followed by drying operation.

Continuous Pressureless Technique Application of powder in the form of slurry Consolidation and drying Used to produce porous sheets for electrodes of nickel-

cadmium batteries

Compaction Pressureless

SinteringSintering is defined as the thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles.

Important Parameters in Sintering

We can divide parameters into four broad categoriesPowder preparation:

-- Particle size

-- Shape

-- Size distribution

Important Parameters in Sintering

Distribution of: Dopants, Second phases

Powder Consolidation: Density, Pore size distribution

Firing: Heating rate, Temperature, Applied pressure, Atmosphere

Types of Sintering

1. Solid state sinteringOnly solid phases are present at the sinter temperature

2. Liquid phase sinteringSmall amounts of liquid phase are present during

sintering

3. Reactive sinteringParticles react with each other to new product phases

Important Parameters in Sintering

Some parameters, such as the sintering temperature, applied pressure, average particle size and atmosphere can be controlled with sufficient accuracy

Others, such as the powder characteristics and particle packing are more difficult to control but have a significant effect on sintering

What Happens During Sintering?

Atomic diffusion takes place and the welded areas formed during compaction grow until eventually they may be lost completely

Recrystallisation and grain growth may follow, and the pores tend to become rounded and the total porosity, as a percentage of the whole volume tends to decrease

In the pressing operation the powder particles are brought together and deformed at the points of contact

What Happens During Sintering?

At elevated temperature - the sintering temperature - the atoms can move more easily and quickly migrate along the particle surfaces (the technical term is Diffusion)

At the sintering temperature new crystallites form at the points of contact so that the original inter-particle boundaries disappear, or become recognizable merely as grain boundaries (This process is called Recrystallisation)

What Happens During Sintering?

The total internal surface area of the pressed body is reduced by sintering

Neck-like junctions are formed between adjacent particles as can be seen on the adjoining scanning electron micrograph

Driving Force for Sintering

As with all processes, sintering is accompanied by an increase in the free energy of the system. The sources that give rise to the amount of free energy are commonly referred to as the driving forces for sintering. The main possible driving forces are

The curvature of the particle surfacesAn externally applied pressureA chemical reaction

Driving Force for Sintering

Schematically it can be shown as

Stages of Sintering

Three stages are distinguished in sintering

First Stage

After burn out of any organic additives, two things happen to the powder particles when the mobility of the surface atoms has become high enough; initially rough surface of the particles is smoothed and neck formation occurs

Stages of Sintering

Second Stage

Densification and pore shrinkage. If grain boundaries are formed after the first stage, these are new source of atoms for filling up the concave areas which diminishes the outer surface of the particle

Third Stage

Grain growth takes place, the pores break up and form closed spherical bubbles

Stages of Sintering

The three stages in the dry sintering can be shown as

Advantages of Sintering Particular advantages of this powder technology include:

1. the possibility of very high purity for the starting materials and their great uniformity

2. preservation of purity due to the restricted nature of subsequent fabrication steps

3. stabilization of the details of repetitive operations by control of grain size in the input stages

Microwave sintering Microwave heating and sintering is fundamentally different from

the conventional sintering, which involves radiant/resistance heating followed by transfer of thermal energy via conduction to the inside of the body being processed.

Microwave heating is a volumetric heating involving conversion of electromagnetic energy into thermal energy, which is instantaneous, rapid and highly efficient. powdered metals are very good absorbers of microwaves and heat up effectively, with heating rates as high as 100°C min-1.

Most other materials are either transparent or absorb microwaves to varying degrees at ambient temperature. The degree of microwave absorption, and consequently of heating, changes dramatically with temperature.

Microwave vs. Conventional Heating

The use of microwave energy for materials processing has major potential, and real advantages over conventional heating. These include:

Time and energy savings Rapid heating rates Considerably reduced processing time and

temperature Fine microstructures and hence improved mechanical

properties and better product performance Lower environmental impact

Why Microwave Sintering ?

There are two main reasons why the microwave process yields better mechanical properties, especially in the case of powder metals –

It produces a finer grain size, and the shape of the porosity, if any, is quite different than in a conventional part.In microwave-processed powder metal components, we have observed round-edged porosities producing higher ductility and toughness.

OBJECTIVES

To study previous researches on microwave heating mechanism

To modify a domestic microwave oven for sintering

To Study the microwave sintered properties of test component

References

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