Chapter#08

Powder metallurgy and welding.

Introduction.

• Powder metallurgy is an art and science of producing fine metal powders and then making objects/components from individual mixed or alloyed with or without the inclusion of non metallic constituent.

• Components are produce in their final shape by pressing metal powders in to the desired shape usually in a metal mold, in dies and heating/sintering the compacted powder either concurrently or subsequently for a period of time at a temperature below the melting point of the major constituent.

• The principle steps in a powder metallurgy process include.

• Obtaining/producing metal powders in a suitable degree of fineness and purity.

• Weighing and mixing of the necessary powders (lubricants) to arrive at a composition which possesses satisfactorily and which produce desire properties in the fabrication part.

• Pressing the powder (mixture) in a suitable mold or die of required size and shape to cause cohesion to occur between the powder particles.

• Pre sintering the powder compact by heating and holding it at a moderate temperature. Pre sintering develops additional green strength and drive off mixing lubricants and or moisture.

• Sintering the compacted mass at a temperature high enough to cause diffusion and intergranular crystal growth to occur.

Powder metallurgy process.

Methods for producing metal powders.

• For forming metals by the technique of powder metallurgy these must first of all be powdered.

• For this purpose different techniques are employed.

• The most important techniques of making metals in powdered form are as under.

• Atomization. This technique also called as operation of metal spraying include the forcing of molten metal through small orifice and is broken up by a power full jet of compressed air.

• The powders produce by this method are usually spherical shape.

• It is an excellent means of making powders from many of the low temperature metals like aluminum, cadmium, lead, zinc, tin, copper, as well as certain alloys like brasses, bronze etc.

• Chemical method. These methods of powder making include reduction, condensation.

• Chemical methods are generally employed for metals like iron, cobalt, nickel and tungsten having high melting point by reducing them from oxides.

• Reduction. In this process for example iron oxide or ore of iron is used to make pure iron from hematite.

• The pure iron then first of all goes through heavy crushing machine then through crushing rolls and finally through a ball mill to produce successively finer grades of powder.

• Condensation. This technique is used in case of metals such as Zn, Cadmium, an Mg which can be boiled and the vapors are condensed in to the powder form.

• For example. A rod of metal say zinc is fed into a high temperature flame.

• The vaporized droplets of metal are allowed to condense on a cool surface of a material to which they can not adhere.

• Electrolysis. It is principally used for the production of extremely pure powders of copper and aluminum.

• For making copper powder copper plates are placed as anode in a tank of electrolyte where as aluminum plates are placed in to the tank as cathode.

• After a definite time period the cathode plates are taken out from the tank are rinsed to remove electrolyte and are dried.

• The cu deposited on cathode plates is then scraped off and pulverized to produce powder.

• Mechanical pulverized. It is a process by which the metal powder is made by applying the mechanical force on the metal piece.

• Many varieties of mechanical pulverization are used.• In this some of these have counter rotating plates of rapidly

hammers by which the powder may produce in maximum fineness.• Usually this method is followed by ball milling are also included in to

the mechanical pulverization method.• It is principally used for Mg, aluminum, copper and so on. Generally

this method is mostly used for brittle metals like antimony. • N.B. the above processes are used to bring the particle sizes of the

metal very fine as much as possible.

Characteristics of metal powders.

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

• Most important characteristics of metal powders are.• Chemical composition. It implies the type and percentage of

alloying elements and impurities which are present. The chemical composition is determined by chemical analysis methods.

• Particle size. It is the most important property of metal powders.• It express by the diameter for spherical shape particles and by the

average diameter of non spherical particles as determine by the sieve analysis method.

• The metal powders are used for powder metallurgy should more spherical.

• Fine particle size influence die strength, density, compaction, flow and mixing and dimensional stability etc.

• Particle size distribution. It is specified in term of sieve analysis.• In the sieving method the particle size is measured by a square

mesh screen of standard size which will just retain the particles.• The particle size is commonly reported by screening out the coarse,

medium and then the fine. For example.• 66% at 200 mesh, 17% at 300 mesh, 17% at 350 mesh.• This mean that the 66% are coarse, 17% medium and 17% fine.• Particle size distribution influences packing of the powders and its

behavior during molding and sintering.• Flow ability. It is defined as the rate at which metal flow fastly and

fill the mold cavity. • Flow rate is an important property of metal because the die must

be filled rapidly with powder to achieve high rate of production and economy.

• Flow rate depends upon particle size, shape etc.• Spherical shaped metal powders posses maximum flow rates where

as non spherical ones least.

Fabrication methods.• Before the powders are pressed in to the shapes they are usually

blended/mixing for the following reasons.• To add lubricants (oils, glycerin) to powders to reduce friction during

pressing operations.• Powders are get coated with lubricants this reduces die wear and

lowers the pressure required for pressing.• To mix the powders of different materials like alloying action in order

to obtain properties of heat resistance friction heavy weight and hardness.

• To obtain the uniform distribution of particle sizes.• Different powders in correct proportions are thoroughly mixed either

in wet or dry in ball mill.• Proper blending and mixing of powders are essential for uniformity

of finished products.• How ever over mixing should be avoided since it may decrease

particle size and work harden the particles.

Compacting.

• After blending and mixing the next step is that of compacting or pressing the powders in to their semi finished form preparatory to sintering.

• The purpose of compacting is to consolidate the powders in to the desire shape and as closely as possible to final dimensions, taking into account any dimensional changes that result from sintering.

• Compacting also imparts. • The desired level and type of porosity and adequate strength for

handling.• Powders are compressed by using high pressure. The degree of

pressure required depends upon.• The required density of the final product and the ease with which the

powder particles will weld/joint together.• Compacting pressures may be applied in following ways.

• Die pressing. Die pressing is done in special presses that include a feed hopper for the powder flow to the die cavity.

• The shaping die to form the product an upper punch and lower punch to apply correct pressure on to the powder being compacted.

• Weighed quantity of powder is placed in the die through the hopper and is compressed under pressure ranging from 8 to 200 Kg per square mm.

• Roll pressing. Roll pressing is used for production of continuous strip section.

• There are two rolls of appropriate size in to which a regulated stream of powder is guided so that the rolls are able to apply the necessary compacting pressure in continuous sequence.

Application of powder metallurgy.

• Porous products like bearings and filters.• Automotive components such as crank shafts, piston

rings, door mechanism etc.• Products made from materials are very difficult to

machine like wire drawing dies, deep drawing punches, rock drilling bits etc.

• Parts made by powder metallurgy have also been used in clocks and timing devices, typewriters and calculators etc.

• Grinding wheels that incorporate steel and diamond powder may manufactured by powder metallurgy.

• Use as parts in military and defense system like military arms.

Advantages and disadvantages.

• The dimensional accuracy and surface finish obtained.• High production rate.• No material is wasted as scrape.• Quite complex shapes may be produce.• Highly and qualified or skill labor is not required.• Disadvantages.• Parts can not be made which are made by casting.• Parts made by this do not have good physical properties.• It may be difficult some times to obtained particular alloy

powders.• Powdered metals are considered more expensive.• Relatively high tool and die cost is associated with the

process.

Welding, soldering and brazing.

• Welding. It is the process of joining two or more two parts of similar metals by heat.

• Welding can be done with or without the application of pressure.

• While welding the edges of metal pieces are either melted or brought to plastic condition.

• Welding can be done with the addition of filler material or without it.

• Welding is used for permanent joint.• It is used in manufacturing of

automobile bodies, air craft frames, railway wagons, machine frames, structural works, furniture, boiler plates and general repair work and ship building.

• Almost in all metal working industries welding is used.

Types of welding.• There are two main types of welding process. They are• Fusion welding.• Pressure welding.• Fusion welding. In this type of process the metal pieces are heated

up to plastic and add the electrode and filler rod.• Then it is allowed to solidify, no pressure is applied, so it is called

pressure less welding.• Electric arc welding and gas welding are the examples of fusion

welding process.• Electric arc welding. This is simply called arc welding.• Arc welding is the process of joining two or more than two metal

parts by melting/ fuse their edges by an arc.• The electric arc is produces between two conductors.• One is electrode and other one is work piece.• The electrode and work piece are brought nearer with a small gape

approximately 3mm.

• When current is passed an electric arc is produces between the electrode and the work piece.

• The work piece is melted by arc, the electrode also melted, both molten metals becomes one.

• Temperature of arc is about 4000oc. The transformer or generator is used for supplying the current.

Gas welding.• Oxy acetylene gas welding. Gas

welding process consists of joining the metals with the help of high temperature flame and filler rod.

• High temperature flame is generated by combusting a mixture of two gases oxygen plus acetylene and the flame is used to heat the work piece and filler rod fills the gap.

• The filler rod melts along with the parent metal and fills the parent metal cavity.

Advantages.The gas flame generally more easily

controlled.It may be used for brazing.It is portable.Disadvantages.Gas welding process is much slower than

arc welding.Harmful thermal effect may be produced on

the metal properties.There safety problem in handling.

Pressure welding.

• In this type of welding process the work piece is heated at plastic state.

• They are pressed together to make a joint.• This type of welding is also known as pressure

welding.• Electric resistance is a best example of

pressure welding.• Electric resistance welding. In this type of

welding both heat and pressure are used at a time to induce the coalescence the metal joined over a limited area.

• In this process two electrodes are used and the metals to be welded are place in between two electrodes.

• The current is passed through the resistance to the electrode for some periods of time.

• When maximum heat is produce at that time the pressure be applied.

• After some time the current should shut off and allow to cool the joint.

Advantages and

disadvantages.

• Merits.• It is possible to weld dissimilar metals.• Skilled operation or welders are not required.• Welds are quickly made.• Demerits.• High initial cost of the resistance welding

equipments.• Certain resistance welding processes are

limited.

Soldering and brazing.

• Soldering and brazing processes are differ from the welding processes in the sense that there is no direct melting of base metal or metals being joined.

• Rather the brazing or soldering filler alloys flows between the two closely adjacent surfaces of work piece by capillary action.

• Both soldering and brazing processes are particularly use full for joining of two dissimilar metals.

• More over the brazing is again differ from soldering in sense of in brazing the filler metal is fill with capillary and apply some pressure for joint.

• Both the brazing or soldering alloys have lower melting point than the metals to be joined.

• Brazing and soldering processes differ in the following way.• Filler metal used in soldering has melting point below 427oc where as that in

brazing has melting point above 427oc.• Brazing produces joints stronger than those made by soldering and they can

be used in service at high temperatures. • THE END