Manufacturing Technology II (ME-202)

Production &

Characteristics of Metal Powder

Dr. Chaitanya Sharma

PhD. IIT Roorkee

Powder Metallurgy

Lesson Objectives In this chapter we shall discuss the following: 1. Powder properties 2. What is powder metallurgy (PM) 3. Need of PM 4. Advantages, Limitations & Application of

PM 5. Basic steps in PM 6. Powder Production and its characteristics 7. Design considerations in PM

Learning Activities 1. Look up

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

lecture

Keywords: Powder metallurgy, powder, Blending, Compacting, Sintering, Particle size and shape, Atomization, Communication etc.

Steps In Making P/M

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

Metal Powders

• A powder can be defined as a finely divided particulate solid.

• Engg powders include metals, refractory & ceramics such as – Metals: Alloys of iron, steel, copper, nickel and aluminum

– Refractory : such as molybdenum and tungsten

– Metallic carbides: such as tungsten carbide

• Metallic powders are classified as either − Elemental - consisting of a pure metal − Pre-alloyed - each particle is an alloy

• Geometric features of engineering powders: • Particle size and distribution

• Particle shape and internal structure

• Surface area

• In general powder surface are covered with oxides, silica, adsorbed organic materials, and moisture; these films must be removed prior to shape processing

Some Applications of Metal Powders

and many more…

Particle Shapes in Metal Powders

Powder Particles

Fig (a) SEM photograph of iron-powder particles made by atomization.

(b) Nickel-based superalloy powder particles made by the rotating electrode process

(a) (b)

Interparticle Friction & Flow Characteristics

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

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

• Larger angle indicate greater interparticle friction

Knowledge Check

• Which powder (fine or coarse) does experience greater friction?.

– Smaller particle sizes generally show greater friction and steeper angles

• Which particle shape does offer lowest interpartical friction?

– Spherical shapes have the lowest interpartical friction

How to Measure Particle Size?

• Most common method uses screens of different mesh sizes.

• Mesh count - 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 means smaller particle size

Particle size that would

pass through mesh Particle size that would

not pass through mesh

Particle Density Measures

• True density - density of the true volume of the material

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

• Bulk density - density of the powders in the loose state after pouring

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

Porosity

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

powder to the bulk volume

• In principle, Porosity + Packing factor = 1.0

• The issue is complicated by the possible existence of

closed pores in some of the particles

• If internal pore volumes are included in above porosity,

then equation is exact.

• Mixing particles of different sizes allows decreased

porosity and a higher packing ratio.

Packing Factor

• Packing Factor = Bulk Density/ True Density

• Typical values for loose powders range between 0.5 & 0.7

• Powders of various sizes give 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

Production of Metal Powders

• The selection of materials in powder metallurgy is determined by two factors.

i) The alloy required in the finished part.

ii) Physical characteristics needed in the powder.

• Both of these factors are influenced by the process used for making powder.

How to Select Method for Powder Production?

• The choice of a specific technique for powder production depends on following factors:

1. Particle size,

2. Shape,

3. Microstructure and chemistry of powder and

4. Cost of the process.

Methods for Producing Metal Powders

The different methods used for producing metal powder are: 1. Mechanical methods of powder production: i) Chopping or Cutting ii) Abrasion methods iii) Machining methods iv) Milling v) Cold-stream Process. 2. Atomization methods of powder production:

i) Gas atomization ii) Water atomization iii) Atomization with a rotating consumable electrode; and iv) Centrifugal atomization

3. Chemical methods of powder production: i) Reduction of oxides ii) Precipitation from solutions iii) Thermal decomposition of compounds iv) Hydride decomposition v) Thermite reaction vi) Electro- chemical methods 4. Electrolysis Process:

Mechanical Methods for Powder Production

1. Chopping or Cutting:

In this process, strands of hard steel wire, in diameter as small as 0.0313 inches are cut up into small pieces by means of a milling cutter.

This technique is actually employed in the manufacturing of cut wire shots which are used for peening or shot cleaning.

Limitations:

• Difficult and costly to make powders

• No control on purity of powders

Mechanical Methods Continued…

2. Rubbing or Abrasion Methods:

a) Rubbing of Two Surfaces: When we rub two surfaces against each other, hard surface removes the material from the surface of soft material.

* Contamination

b) Filing: Filing as a production method has been frequently employed, especially to alloy powders, when supplies from conventional sources have been unobtainable.

Such methods are also used for manufacture of coarse powders of dental alloys.

Filing can also be used to produce finer powder if its teeth are smaller.

* commercially not feasible.

Rubbing or Abrasion Methods continued...

c) Scratching: If a hard pin is rubbed on some soft metal the powder flakes are produced. Scratching is a technique actually used on a large scale for the preparation of coarse magnesium powders. Either by hardened steel pin/ scratching belt against mg block/revolving drum.

Advantages: •For consuming scrap from another process, machining is a useful process. •Presently the machined powder is used with high carbon steel and some dental amalgam powders. Disadvantages: • Lack of control on powder characteristics, including chemical contamination such as oxidation, oil and other metal impurities. • The shape of the powder is irregular and coarse.

Mechanical Comminution to Obtain Fine Particles

Figure 17.6 Methods of mechanical comminution to obtain fine

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

Milling Process

Milling Process is a high production rate of mechanical method of powder production.

Principle:

The basic principal of milling process is the application of impact and shear forces between two materials, a hard and a soft, causing soft material to be ground into fine particles.

Milling techniques are suitable for brittle materials.

Two types of milling are;

i) Ball Milling

ii) Attrition Milling.

Ball Milling

Ball milling is an old and relatively simple method for grinding large lumps of materials into smaller pieces and powder form.

Principle of the process:

The principle is simple and is based on the impact and shear forces.

Hard balls are used for mechanical comminution of brittle materials and producing powders.

Milling Unit:

The basic apparatus consists of the following;

• A ball mill or jar mill which mainly consists of

a rotating drum lined from inside with a hard material.

• Hard balls, as a grinding medium, which continue to impact the material inside the drum as it rotates/rolls.

Important Parameters of Milling

1. The most important parameter to consider is the speed of rotation of the drum. An

optimum/critical speed is adjusted for maximum impact velocity. * Critical speed is the

speed above which the ball will centrifuge.

• Very slow speed of rotation will not carry the balls to the top, these will roll back down the

drum sides.

• Very fast speed (higher than critical speed) will not let the balls drop down as they will be

carried around due to centrifugal forces. Thus, an optimum speed is required. This speed of

rotation varies with the inverse square root of the drum diameter.

2. The material of grinding media and its size and density.

• The size and density of the milling medium is selected according to the deformation and fracture resistance for metals.

• For hard and brittle materials large and dense media is used. Whereas, small balls are used for finer grinding.

• As a general rule, the balls should be small and their surface should be a little rough. The material of the balls and lining of the drum should be same as that of the material being ground.

3. The rate of milling of a powder is a function of quantity in the total space between the balls.

4. Lubricants and surface active agents are used to nullify the welding forces which causes

agglomeration.

ATTRITION MILLING

Attrition is the term which means to wear or rub away. It is a process of grinding down by friction.

Milling Unit:

•In attrition milling a very high efficiency ball mill is agitated by a vertical rotating shaft with horizontal arms.

•In these mills the rotational speeds are nearly 6 – 80 rpm while the size of medium (balls) used is 3 – 6 mm.

•Power is used to rotate the agitator and not the vessel as in case of ball mills. The central rotating shaft of attrition mill is equipped with several horizontal arms. When rotated, it exerts the stirring action to tumble the grinding medium randomly throughout the entire chamber.

COLD STREAM PROCESS

• This process is based on impact phenomenon caused by impingement of high velocity particles against a cemented carbide plate.

• The unit consists of:

A feed container;

A compressor capable of producing a high velocity stream of air (56 m3/min.) operating at 7 MPa (1000 psi);

A target plate, made of cemented tungsten carbide, for producing impact;

A classifying chamber lined with WC while the supersonic nozzle and target generally are made of cemented tungsten carbide.

Mechanism of the Process:

The material to be powdered is fed in the chamber and from there falls in front of high velocity stream of air.

This air causes the impingement of material against target plate, where material due to impaction is shattered into powder form. This powder is sucked and is classified in the classifying chamber. Oversize is recycled and fine powder is removed from discharge area.

* Rapidly expanding gases leaving the nozzle create a strong cooling effect through adiabatic expansion. This effect is greater than the heat produced by pulverization.

Atomization

• In this process the molten metal is forced through an

orifice into a stream of high velocity air, steam or inert

gas .This causes rapid cooling and disintegration into very

fine powder particles and the use of this process is limited

to metals with relatively low melting point.

Major atomization techniques are:

i) Gas atomization

ii) Water atomization

iii) Centrifugal atomization

iii) Atomization with a rotating

consumable electrode; and

Gas/ Liquid Atomization

• Atomization breaks molten metal into small droplets by rapidly

freezing them before the droplets come into contact with each other or

with solid surface.

• The atomization is achieved by bringing the thin molten metal stream in

contact with the impact of high energy jets of gas or liquid.

• Air, nitrogen and argon are commonly used gases and water is the most

widely used liquid.

If gases are used for

atomization then it

is known as gas

atomization while

atomization with

water is known as

water atomization

Factors Affecting Particle Shape & size in Gas/ Liquid Atomization

Following factor influences shape and size of powder:

• Rate of solidification determines the particle shape.

– Spherical if low heat capacity gas is employed.

– Highly irregular if water is employed

• Particle size distribution can be controlled by

o Design and configuration of jets

o Pressure and volume of atomizing fluid

o Thickness of metal stream

Centrifugal Atomization

• In this technique, molten metal falls on to a disc rotating at high speed. The liquid metal that is impinging on the disc will be thrown out rapidly into small droplets by the disc owing to centrifugal forces. These particles are solidified and collected in the collection chamber.

Advantages:

i) low operating costs, ii) very good yields in narrow size ranges.

iii)Capability to produce only one size of droplet

iv)Modest energy consumption

Application:

Magnesium powders in the ~0.5mm range for

military pyrotechnic applications. Superalloys

etc.

Disadvantage:

Requires large diameter atomizing chamber owing to

spray with an included angle of 1800

Atomization with Rotating Consumable Electrode • In this method, arc between the rotating consumable

electrode and the stationary electrode is utilized for melting the metal. Since the electrode is rotating, the molten metal is atomized by the centrifugal force and collected in the chamber filled with inert gas.

Electrochemical Process

These methods are based on the electrolysis of molten solutions

of metals or fused salts.

The metals are electrically deposited on the cathode of an

electrolytic cell as a sponge or powder or at least in a physical

state form which it can be easily disintegrated into a powder.

Components

The equipment used is an

electrolytic bath made of steel,

and lined from inside with rubber.

Two electrodes are inserted in the

bath.

Cathode is made of lead while

anode is made of the same metal

whose powder is being produced. Figure: Electrolytic Cell Operation for deposition of Powder

Electrochemical Process

Principle & Operation:

•The basic principle is the electrolysis process in which decomposition of a

molten salt/aqueous solution into its ions is obtained by the passage of

electric current.

•The metallic ions are deposited at the cathode which can be removed with a

brush and collected at the bottom.

The electrolytic tanks have conical

bottoms with a valve. Suction pipes

are connected to these bottoms and

powder is removed from the tank.

The efficiency of the tank/process

depends on the deposition rate.

Advantages of the process:

The technique has a number of advantages, e.g.

• The product is usually of a high commercial purity.

• A considerable range of powder qualities can be obtained by varying

bath compositions.

• Frequently the product has excellent pressing and sintering properties.

• The cost of the operation may in some cases be low.

Limitations:

• Alloy powders cannot be produced.

• The product of process is frequently in active condition (presence of

chemicals on powder particles) which may cause difficulties in

washing and drying it (contamination/oxidation with atmospheric

oxygen may occur).

• The cost of operation may be high in some cases.

Powder production at cathode is favored by:

high current density;

weak metal concentration;

addition of acids;

low temperature;

avoidance of agitation, and;

suppression of convection.

* Very fine powder can be obtained when the current flowing is so strong in relation to the strength of the solution that hydrogen is strongly evolved from the cathode.

• Hydrogen evolution is encouraged by:

(i) increasing cell voltage;

(ii) diminishing the size of the cathode;

(iii) bringing the anode and cathode closer together;

(iv) increasing the temperature;

(v) weakening the strength of the metallic solution

(vi) adding acid

• * When metal is deposited without evolution of

hydrogen, the deposit may be ductile and

compact if the current is just not great enough

to cause hydrogen formation, or very hard with

large crystals using strong solutions and large

quantities of electricity, or sandy and brittle

with little cohesion using very small current.

Chemical Methods

• Almost all metallic elements can be produced in the form of

powders by suitable chemical reactions or decomposition. For

example all chemical compounds can be decomposed into

their elements if heated to sufficient high temperature. If the

non-metallic redical could be removed, for example by

continuous evacuation or by entrainment in an inert gas, then

practical methods of making metal powders might be feasible.

Theory of the process

Mostly chemical methods are based on the decomposition of a

compound into the elemental with heating or with the help of

some catalyst.

In most cases such processes involve at least two reactants.

(i) a compound of the metal

(ii) a reducing agent

Either of the two may be in the state of a solid, liquid (melt), solution or gas and it would seem therefore that from this point of view at least sixteen types of such reactions could be possible.

Solid

Liquid

Solid

Solution

Gas

REDUCTION OF METAL OXIDES

Manufacturing of metal powder by reduction of oxides is extensively employed, particularly for Fe, Cu, W and Mo. As a manufacturing technique, oxide reduction may exhibit certain advantages and disadvantages. These are listed below;

Advantages:

A variety of reducing agents can be used and process can be economical when carbon is used.

Close control over particle size --- because oxides are generally friable, easily pulverized and easily graded by sieving.

Porous powders can be produced which have good compressive properties.

Adoptability either to very small or large manufacturing units and either batch or continuous processes.

Limitations:

Process may be costly if reducing agents are gases.

Large volumes of reducing gas may be required, and circumstances where this

is economically available may be limited; in some cases, however, costs may be

reduced by recirculation of the gas.

The purity of the finished product usually depends entirely upon the purity of

the raw material, and economic or technical considerations may set a limitation

to that which can be attained.

Mechanism of Reaction:

Most metal powders manufactured by reduction of oxides are produced using solid

carbon or hydrogen, cracked ammonia, carbon monoxide, or mixture of such

gases. As a reducing agent for metal oxides, carbon holds an important and

peculiar position – because of its general cheapness and availability, and peculiar

for the following reasons.

According to circumstances and temperature, three carbon/oxygen reactions can

occur: (i) C + O2 = CO2

In this reaction, the number of gaseous molecules remain constant and the entropy

change is very small. The free energy change of the reaction is almost constant

from room temperature to 2000 oC.

(ii) 2CO + O2 = 2CO2

The reaction is accompanied by a decrease in the number of gas molecules and in entropy with a considerable free energy change.

(iii) 2C + O2 = 2CO

This reaction involves an increase in the number of gaseous molecules and a considerable increase in entropy and a considerable free energy change. This implies that within temperatures normally used metallurgically, carbon monoxide becomes increasingly stable the higher the temperature.

Consequently, the free energy change temperature curves for these reactions intersect ------- at about 700 oC

Further reading

• Fundamentals of powder metallurgy W. D. Jones

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

• Fundamentals of P/M I. H. Khan