1

Chapter-1: Literature Review

1.1 Introduction

Powder metallurgy is a science of producing metal powders and making finished / semi-finished

objects from mixed or alloyed powders with or without the addition of nonmetallic constituents.

It is the process of blending fine powdered materials, compacting the same into a desired shape

or form inside a mould followed by heating of the compacted powder in a controlled atmosphere,

referred to as sintering to facilitate the formation of bonding of the powder particles to form the

final part. Compacting is generally performed at room temperature and at high pressure.

Sintering is usually done at elevated temperature and at atmospheric pressure. Powder

Metallurgy route is very suitable for parts that are required to be manufactured from a single or

multiple materials (in powder form) with very high strength and melting temperature that pose

challenge for the application of casting or deformation processes.

In many cases individual engineering components are produced directly by the process such as

components being referred to indiscriminately as sintered components, sintered parts, or PM

parts. However , wrought products also can be produced from powder and recently a number of

scientifically exciting developments of great industrial potential have taken place.

Powder metallurgy is useful in making parts that have irregular curves, or recesses that are hard

to machine. It is suitable for high volume production with very little wastage of material.

Secondary machining is virtually eliminated. Typical parts that can be made with this process

include cams, ratchets, sprockets, pawls, sintered bronze and iron bearings (impregnated with

oil) and carbide tool tips.

The growth of the P/M industry during the past few decades is largely attributable to the cost

savings associated with net (or near-net) shape processing compared to other metalworking

methods, such as casting or forging. In some cases, the conversion of a cast or wrought

component to powder metal provides a cost savings of 40% or higher.

PM typically uses more than 97% of the starting raw material in the finished part and is

especially suited to high volume components production requirements. There are two principal

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reasons for using a powder metallurgy product:

(1) cost savings compared with alternative processes, and

(2) unique properties attainable only by the PM route

PM process enables products to be made that are capable of absorbing up to 35% of selected

fluids.

India is today an important producer and consumer of PM parts. The story of Powder Metallurgy

in India dates back to the latter half of 1950’s, when two entrepreneurs, Mr. Upadhyaye of M/s

Siemetals Ltd., Mumbai, and Mr. Bhat of M/s Flexicons Ltd., Udhana, established the

production of porous components. The significant growth of PM in India began when Mahindra

Group established Mahindra Sintered Products in Pune. This was followed by Goa Sintered

Products in Margoa, Sundram Fasteners in Hosur, and Widia in Bangalore. The pie chart below

shows the market for powder metallurgy in India due to its cost effectiveness and various

properties

PIE CHART Showing The Market For Powder Metallurgy In India

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1.2 Literature survey

Kurgan Naci, Sun Yavuz, Cicek Bunyamin & Ahlatci Hayrettin [1] investigated the mechanical

and wear properties of AISI 316L stainless steel implant materials, produced by powder

metallurgy. AISI 316L stainless steel powder was cold-pressed with 800 MPa of pressure and

then sintered at 1200, 1250 and 1300°C for 30 min as three sample groups. The microstructure,

and mechanical and wear properties of the resulting steels were investigated. Light optical and

scanning electron microscopies were used to characterize the microstructure of the steels. Room

temperature mechanical properties of the steels were determined by hardness measurements and

impact tests. Wear was determined using the pin-on-disc wear test, and the results were

evaluated according to weight loss. The results indicate that the sintering temperature, time and

atmosphere are important parameters that affect the porous ratio of materials produced by P/M.

Sintering at high temperature can eliminate small pores and make the residual pores spherical.

The wear tests showed that the wear of the AISI 316L stainless steel implants changed

depending on the sintering temperature and load. Spherical pores in the samples increase the

wear resistance. Moreover, decreasing the porosity ratio of these materials improves all of their

mechanical properties.

Suttha Amaranan and Anchalee Manonukul [2] investigated compaction, sintering and physical

properties of silver powder. The silver powder was uniaxially compacted into a cylindrical

specimen using compaction pressures of 13.79, 27.58, 41.37 and 55.16 MPa. Compacted parts

were sintered at 700, 800 and 900°C in argon atmospheres. In addition, compacted parts were

also sintered in vacuum at 900°C. For the sintering temperatures of 700 and 800°C, it was found

that the sintered density increased as the compaction pressure increased below 40 MPa, while the

sintered density decreased at a compaction pressure above 40 MPa. At the sintering temperature

of 900°C, the sintered density decreased with increasing compaction pressure. The highest

sintered density of 10.22 g⋅cm3 (67.41 % relative density) was obtained at a temperature of

900°C under argon atmosphere for compaction pressure of 13.79 MPa. At this sintering

temperature, vacuum sintering gave a slightly higher sintered density than argon atmosphere.

Moreover, the difference in shrinkage of thickness and diameter of the sintered parts was

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observed. The diameter has higher shrinkage than the thickness. The weight of the specimen did

not affect the sintered density, whereas the compaction pressure, sintering temperature and

atmosphere influenced the sintered density.

Ali Osman Kurt, Tom J. Davies [3] investigated the compaction of metal powders. Two types of

irregular atomized iron powders, a spherical copper powder and a bronze powder were uniaxially

compacted in a floating-action die at pressures up to 950 MPa. Displacements during compaction

were recorded continuously and density-compaction pressure relationships were obtained.

Sequential stages of compaction were identified and interrelated in terms of particle

rearrangement and deformation. The role of work hardening during compaction is described for

the different powders.

O. Smuk, P. Nenonen, H. Hänninen, and J. Liimatainen [4] investigated the microstructures of a

powder metallurgy/hot-isostatically pressed super duplex stainless steel, designed and

manufactured for massive components of paper machines, after heat treatments simulating the

industrial production. It was shown that copper precipitates in the ferrite phase as phase.

Morphologically, the copper precipitates are of two types—nearly spherical particles of typical

size from 30 to 50 nm, and rod like particles 30- to 35-nm wide, and up to 700-nm long. The

main observations on copper precipitation in modern super duplex stainless steels are similar to

those in low-alloy steels. Copper particles were shown to be the nucleation sites for the

formation of secondary austenite and to pin the boundaries of sigma phase.

V. D. Khramtsov [5] investigated the computational equations for determining the packing

density of particles that are two and three fractions of metal powders (whose ratio of the average

particle diameter of fractions is >10, which correctly predicts the packing density of the mixture

particles and optimal ratios of their amounts to provide the maximum density as a function of

individual packing densities and the amounts of powders). The relative density was used after

bumping-down as the quantity that characterizes the packing density of the particles. The

regularities are experimentally verified for the mixtures of three pairs of different powders, and

the correctness of obtained equations is confirmed.

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W. M. Khairaldien, A.A. Khalil and M. R. Bayoumi [6] investigated the extensive utilization of

aluminum reinforced with silicon carbide composites in different structural applications and the

need to find a cost effective technological production method for these composites.

Homogeneity, machinability, and interfacial reaction of the constituents represent the large

problems pertaining to these composites. Production of a homogenous, high strength and net

shape structural components made from aluminum-silicon carbide composites can be achieved

using powder metallurgy (PM) technology. In the present work the problem of low strength of

the aluminum silicon carbide produced by powder metallurgy (PM) is solved by rising the

sintering temperature of the composite above the melting temperature of the aluminum this

method produce a local fusing and welding of the aluminum particles while using aluminum

powder with thick oxide layer surrounding the particles prevents the all melting of the

composite. Compression tests are carried out for compositions containing 0%, 5%, 10%, 15%,

20%, 25% and 30% silicon carbide simultaneously at sintering temperature of 650, 700, 750,

800, 850 and 900 C. The microstructure for each composition examined to study both the

homogeneity and the interaction between the constituents. Generally the results show the

tendency for both the strength and ductility to increase upon increasing the sintering temperature

and almost there are very small changes above specific sintering temperature which depends

upon the silicon carbide content. These specific sintering temperature levels are found to be

6500C for the aluminum with no silicon carbide content, 700C for composite containing both

5% and 10% SiC, 750C for composite containing 15% SiC, 800C for composite containing

20% SiC, 850C for containing 25% SiC and 900C for composite containing 30% SiC.

S.Szczepanik, [7] investigated Fe-Al PM preforms from powder mixes with 30 and 40at% Al

when heated and closed-die forged at 500 or 550ºC to yield materials with porosity lower than

10%. Brinell hardness, bend and compression strengths were determined, also after 4 hour heat

treatment at 540ºC. The content of the Fe-Al inter-metallics depends on forging temperature and

heat treatment. Optimum bend strength reached, 153 MPa, was for 30%Al alloy, forged at 550ºC

and heat-treated. Larstran Shape program was used to simulate shape manufacturing by closed -

die forging.

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L. N. Smith and P. S. Midha, [8] investigated the elements of a prototype knowledge based

system (KBS), which have been developed to provide advice relating to the manufacture of

components through the use of powder metallurgy (PM) technology. The system comprises

modules which address the following three important areas of product development: component

design; materials selection; and powder packing and compaction. The design module employs a

combination of feature based modelling and rule based design analysis, to assist with PM design

optimization. Materials selection advice is generated through utilization of empirical rules

relating, for example, the tensile strength of PM steels to their density and percentage carbon

content. The third KBS module is concerned with quantification of the effects of variation in

various critical powder packing and compaction parameters. A deficiency was identified in the

existing knowledge relating to the effects on packing density, of combinations of more than one

particle size, and particle irregularity. Consequently new modelling techniques have been

developed which employ Monte Carlo type simulations for prediction of powder packing

behaviour. It is expected that future developments based upon the methodologies described here,

will enable production of comprehensive KBS modules, able to advise on optimum and

concurrent design and manufacture of powder metallurgy components.

R.L.Orban, [9] investigated on the need of present research and development directions in

Powder Metallurgy (PM). There are analysed investigations focused on the mechanical

properties of sintered structural parts improving, for overcoming the PM limitations concerning

the complex shape and large parts realization as well as for the advanced materials obtaining,

like nanocrystalline materials, intermetallics and composites, the most recent methods for this

purpose being considered.

Sorush Parvizia, Vahid Hasannaeimib, Ehsan Saebnoorib, Taghi Shahrabib, and Sayed

Khatiboleslam Sadrnezhaada, [10] investigated on the production of porous NiTi shape memory

alloy by mechanical alloying of the elemental Ni and Ti powders. The compacting process was

done at two temperatures (warm and cold press) and then sintering at 980 and 1050°C was

performed on the specimens. Microstructure and mechanical properties of the samples were

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investigated by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction.

Moreover, shear punch test (SPT) employed to investigate the effect of compaction pressure and

sintering temperature on the mechanical properties of the fabricated samples. It was revealed that

warm compaction/sintering resulted in 15% yield stress improvement and 20% ultimate tensile

strength (UTS) enhancement with respect to conventionally produced specimens. The proposed

approach in this paper seems a step forward towards fulfillment of the demand for heavy load-

bearing artificial bone.

L. Ma. Flores-Ve´lez, J. Cha´vez, L. Herna´ndez, and O. Domı´ngue, [11] investigated on the

preliminary results of aluminum metal matrix composites (MMCs) reinforced with granulated

slag (GS) and electric arc furnace dust (EAFD). The present work concerns the synthesis and

properties of Al/GS and Al/EAFD composites based on powder metallurgy techniques. The

hardness and compressive strength of the sintering compacts were determined to compare the

mechanical properties of the composite material as a function of the GS and EAFD content. The

best results were obtained with the Al/GS composites, which reached compressive strengths up

to 372 MPa.

S. G. Agbalyan, A. S. Petrosyan, É. S. Amalyan, and G. A. Vasilyan, [12] investigated The

extrusion dynamics of high-strength powder metallurgy composite materials was studied. The

extrusion parameters for porous compacts of copper fibers and Cu − Mo composites were

optimized. It was shown that orientation of fibers is possible only when they are sufficiently

widely dispersed in the powder matrix and also when the fiber length is much greater than its

diameter l >> d. The mechanical properties of the composites were investigated. A practically

pore-free structure was obtained at a degree of reduction λ = 4-6. With this the strengthening

effect of fibers in the matrix was fully realized.

S. K. Sadrnezhaad and A. R. Selahi, [13] performed an experiment where pure nickel-titanium

powders were mechanically milled in a vertical attritor mill under protective atmosphere for

various times from 10 to 24 hr. Products were then compacted and sintered at different

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temperatures for different times. Amorphization and interatomic phase formation were

determined by X-ray diffractometry, scanning electron microscopy, and differential scanning

calorimetry. Porosity, virtual density, transition temperatures, and the amount of Ni 3Ti first

increased and then decreased with the milling time. Presence of oxygen in the milling

atmosphere showed partial crystallization of NiTi intermetallic compound accompanied by

titanium oxide formation.

Punya Talagala, Ratna Naik, and Lowell Wenger, [14] investigated a soft magnetic material (iron

with 1 wt.% aluminum) which has been developed using a powder metallurgy processing route.

The addition of aluminum in iron has resulted in an alloy with significant grain growth during

sintering and correspondingly superior magnetic properties characterized by a low coercivity

(Hc 0.9 Oe), a relatively high permeability ( max. 5000) and a high saturated magnetic

induction (Bs 13 kG at 8 Oe) after sintering at 1482°C for 12 hours. The effects of sintering

time and temperature on the magnetic properties of this alloy have also been examined.

G. A. Baglyuk and L. A. Poznyak, [15] considered the basic laws of vacuum sintering of high-

speed steel powder with activating additions of 10-30% B and 70-90% Ni. It is shown that for

each of the compositions an optimal temperature and time of isothermal sintering, and also initial

compact density exists, above which the density of the sintered ingot does not noticeably

increase.

D. A. Levina, L. I. Chernyshev, and N. V. Mikhaylovskaya, [16] The state of the production of

metallic and ceramic powders (including nano-dimensional powders) and of articles fabricated

from these powders in North America is discussed. New technologies that produce an increase in

the density and improvement in the mechanical properties of powder-based articles are

considered. Technological developments and articles for which prizes were awarded by the

North American Metal Powders Industry Federation in 2003 are adduced.

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I. I. Ivanova and A. N. Demidik,[17] distinguished the alloys of the Ni − Mo system by high

strength and corrosion resistance in numerous acidic solutions. However their deformability in

hot working is low, which is connected with structural inhomogeneity. A method for obtaining

such alloys from mixtures of nickel and molybdenum powders is proposed. Methods for

protecting compacts during hot working and the basic conditions for heat treatment and hot

extrusion are given. The mechanical properties of prepared alloys were determined, and their

microstructures investigated. The techniques developed can be used to produce a nickel-

molybdenum alloy in the form of a practically homogeneous solid solution with high strength

and ductility.

R. Q. Guo, P. K. Rohatgi, D. Nath [18] investigated the aluminium-fly ash mixtures containing

different weight percentages of fly ash were prepared and compacted at pressures from 138 - 414

MPa. The compacts prepared at 414 MPa were sintered in nitrogen atmosphere at 600, 625 and

645 C, respectively. The time of sintering ranged from 0.5 - 6 h. The densification parameter

and the green densities of the compacts were determined as a function of compacting pressure

and fly ash weight per cent. Density, hardness and strength of the sintered compacts were

determined as a function of weight per cent of fly ash particles. Volume changes during sintering

of green compacts were also evaluated as a function of increasing fly ash weight per cent.

Microscopic studies of green and sintered compacts were done to study the effectiveness of

sintering. Green and sintered densities of the compacts were found to decrease with increasing

weight per cents of fly ash. Sintering results in slight decrease in density and increase in volume

of green compacts within the range investigated. Strength of the sintered compacts decreased

with increasing weight per cent of fly ash under the present experimental conditions; however,

the hardness was found to increase slightly up to 10 wt% fly ash, beyond which it decreased.

K. V. Sudhakar, [19] This paper presents a failure analysis of tool steel and brass powder

metallurgy (P/M) parts that failed during service. A detailed failure investigation of fractured

tool steel and brass parts was carried out to assess the causes for their premature failures. The

fractured surfaces of the broken pieces and the component surfaces were subjected to detailed

examination. Investigations were carried out by visual methods, micro-hardness measurements,

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and using optical and scanning electron microscopes. In the case of the brass sample, visual

examination of the surface indicated flat surface features. Detailed optical and electron

microscopic studies corroborated by micro-hardness indentations have conclusively established

that the failure was mainly due to the presence of very small impurities in the brass component

material. In the case of the tool steel sample, the fractured surfaces of the component were

subjected to destructive and nondestructive tests. Representative fractured pieces were examined

visually and tested for their yield strength using simple tensile tests. Optical and scanning

electron microscopies at appropriate magnifications were also performed to characterize its

microstructure and fracture morphology. Detailed investigations of the tool steel part established

that the failure was mainly due to inferior yield strength of the component resulting from

improper heat treatment.

Sermin Ozan & Seda Bilhan, [20] investigated, the effect of fabrication parameters on the pore

concentration of aluminum metal foam manufactured by powder metallurgy process is studied.

Aluminum metal foam specimens were fabricated from the mixture of aluminum powders (mean

particle size 60 μm) and NaCl at 10,20,30,40(wt) % content under 200, 250, 300, MPa Pressures.

All specimens were then sintered at 630°C for 2.5 hours in an argon atmosphere. For pore

formation (foaming), sintered specimens were immersed into 70°C hot running water. Finally

the pore concentration of specimens was recorded to analyze the effect of fabrication parameters

(namely NaCl ratio, NaCl particle size and compacting pressure) on the foaming behavior of

compacted specimens. As a result of the study, it has been recorded that the above mentioned

fabrication parameters are effective on pore concentration profile while pore diameters remained

unchanged.

Based on the above literature review it can be said that powder metallurgy is an effective process

for making complicated shapes with desired properties. In this project we plan to fabricate setup

for making components by powder metallurgy.

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1.3 Summary of literature review

Material Property

investigated

Compaction

process

Compaction

pressure(MPa)

Sintering

atmos.

Sintering

temp(C)

1

Stainless steel

Mechanical and

wear properties

Cold isostatic

compaction

800

vaccum

1200

1250

1300

2

Silver

Physical prop. at

diff. temp. pressure

Uni axial die

compaction

13.75

27.58

55.16

argon

700

800

900

3 Iron copper

and bronze

powder

Compaction

properties and

strength

Uni axial die

compaction

950

--------

--------

4 Fe-Al Porosity at different

composition

Closed die

forged

200 atmosphe

re

500

5 Fe-Al Porosity<10% Closed die

forged

153 vaccum 550

6

NiTi porosity Cold comp.

Warm comp.

330

330

vaccum 980

1050

7 Al fly ash Density hardness

and strength

Uni axial die

compaction

414 nitrogen 625

8 Al metal foam

(Al +Nacl)

manufacturing Uni axial die

compaction

300 argon 630

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Chapter-2: Identification of Project

2.1 Reasons for Selection of Project

The project selected by us is of great use in the field of mechanical engineering this project will

not only complete the requirement of our core mechanical project but also of great use for future

researches. Parts produced by powder metallurgy have lower cost as compared to the cost of

product made by general casting method; this is because of low scrap and fewer processing steps.

Parts produced by this technique are close to the required final dimension, there is very little

waste of material about 95% of the starting powders is converted into product. Powder

metallurgy technique not only consists of manufacturing any product by the combination of

powders but also allow us research out various combination of powder which will give out more

durable and strong alloy. The reason for selection of this project is though a lot but few are listed

below:

A core mechanical project

Can be used for further research work in our department.

It will increase the lab. Varsity of School of Mechanical Engineering.

Various combination of powder can be researched out which will give out more strong

and durable alloys.

The final manufacturing product received here can be used in Strength of Material lab for

testing its strength, toughness, hardness and other mechanical properties.

The project also tilts toward changing of dies as per requirement thus allowing various

kinds of end product as per use in the future thus making it versatile.

PM consumes only around 43% of the energy compared with forging and machining, and

the numbers of process steps are less.

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Conventional metal forming or shaping processes, against which PM competes, generally

involve significant machining operations from bar stock or from forged or cast blanks. These

machining operations can be costly and are wasteful of material and energy.

2.2 Expected Outcome of Project

Fabrication of powder mixing blender

Fabrication of hydraulic press of a capacity of 300MN/m2.

A test rig of powder metallurgy to manufacture various products based on this technique.

Analysis of different mechanical properties of the component by varying either

compaction pressure or sintering temperature at various lab of our department.

2.3 Cost Analysis

Blender : Rs. 5,000

Hydraulic press : Rs. 25,000

Powder : Rs. 5,000

Miscellaneous : Rs. 5,000

Total : Rs. 40,000

The cost indicated above is as per search on the data and the price available on the internet. It

may vary from place to place. The cost analysis can see a change during real time visit to the

market. However Cost analysis done above is keeping in view the changes in market and basic

requirements of our project.

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2.4 Feasibility Analysis

The project is feasible because of its core mechanical nature. The various parts of this test rig ie,

blender, hydraulic press, sintering can be fabricated without much problem. The blender can be

fabricated by the same principle as of sand blender in our foundry shop of workshop but of

smaller size. It will have a capacity of mixing 2 Kg of powder at a time. Since size of blender is

small the capacity of motor required will also be small therefore its cost of manufacturing will be

low.

According to cost, the project is feasible because of differential cost of various part of test rig.

The major investment of the project will be on the fabrication of hydraulic press. The die used

for compaction of powder is made of very hard metal like chromium which we would be

purchasing. The other parts can be manufactured in a small investment range. The major part of

our project will be the fabrication of the hydraulic press.

The powder of various metals can be easily procured from the market. Additives and lubricant

can also be purchased from the market. The mechanical analysis of the product can be done in

the SOM LAB and the MMC LAB of our department. Overall the requirements and the process

for completion of this project is feasible and can be done without much problem.

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Chapter-3: Powder Metallurgy Process

The powder metallurgy process generally consists of four basic steps:

(1) Powder manufacture,

(2) Powder blending,

(3) Compacting,

(4) Sintering.

3.1 Powder Manufacture

The manufacturing of the material powder is the first step in powder metallurgy processing route

that It involves making, characterizing, and treating the powder which have a strong influence on

the quality of the end product.[21] Different techniques of powder making are:

a) Atomizing Process

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.

Methods of metal-powder production by atomization; (a) melt atomization; (b) atomization with a

rotating consumable electrode.

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b) Gaseous Reduction

This process consists of grinding the metallic oxides to a fine state and subsequently, reducing it

by hydrogen or carbon monoxide. This method is employed for metals such as iron, tungsten,

copper, etc.

c) Electrolysis Process

In this process the conditions of electrode position are controlled in such a way that a soft spongy

deposit is formed, which is subsequently pulverised to form the metallic powder. The particle

size can be varied over a wide range by varying the electrolyte compositions and the electrical

parameters.

d) Mechanical Alloying

In this method, powders of two or more pure metals are mixed in a ball mill. Under the impact of

the hard balls, the powders are repeatedly fractured and welded together by forming alloy under

diffusion.

Different grinding processes

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e) Stamp and Ball mills

These are mechanical methods which produce a relatively coarse powder. Ball mill is employed

for brittle materials whereas stamps are used for ductile material.

f) Granulation Process

This process consists in the formation of an oxide film in individual particles when a bath of

metal is stirred in contact with air.

g) Carbonyl Process

This process is based upon the fact that a number of metals can react with carbon monoxide to

form carbonyls such as iron carbonyl can be made by passing carbon monoxide over heated iron

at 50 – 200 bar pressure. The resulting carbonyl is then decomposed by heating it to a

temperature of 200 – 3000C yielding powder of high purity.

h) Other methods

The other less commonly used methods to form metallic powder are by (i) precipitation from a

chemical solution, (ii) production of fine metals by machining, and (iii) vapour condensation.

3.2 Powder treatment & Handling

In powder conditioning, the powders prepared by various methods are subjected to a variety of

treatments to improve or modify their physical, chemical characteristics Powder treatments.

Powders manufactured for P/M applications can be classified into – elemental powders, and pre-

alloyed powders

Elemental powders => powders of single metallic element; eg.: iron for magnetic applications

Pre-alloyed powders => more than one element; made by alloying elemental powders during

manufacturing process itself; In this case, all the particles have same nominal composition and

each particle is equivalent to small ingot

Majority of powders undergo treatments prior to compaction like:

i. Drying to remove moisture,

ii. grinding/crushing to obtain fine sizes,

iii. particle size classification to obtain the desired particle size distribution,

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iv. annealing,

v. mixing and blending of powders,

vi. lubricant addition for powder compaction,

vii. powder coating

a) Cleaning of powders: Refers to the removal of contaminants, solid or gaseous, from the

powder particles

Solid contaminants come from several sources like nozzles or crucible linings. They interfere

during compaction and sintering preventing proper mechanical bonding

Most of these contaminants are non-reactive, but they act as sites for crack nucleation and

reduce the dynamic properties of the sintered part; Non-metallic solid impurities can be

removed from superalloy powders by particle separators, electrostatic separation

techniques.

Gaseous impurities like hydrogen and oxygen get into powders during processing, storage or

handling if proper care is not taken. Finer the powders, contamination will be more because of

large powder surface area.

These gaseous impurities can form undesirable oxides during processing at relatively

high temperature or gets trapped inside the material as pores, reducing the in situ

performance of the P/M part; Degassing techniques like cold, hot static or dynamic

degassing methods are used to remove adsorbed gases from the powders

Lubricants added to the powders for better compaction has to be removed for desirable

final P/M part

b) Grinding: similar to the mechanical methods seen earlier; Milling is widely used for reducing

the aggregates of powder; Milling time, speed, type can be selected for getting required degree of

grinding.

c) Powder classification & screening: Powder size and shape, size distribution varied within

specified range is required for better behavior of P/M parts; In this method, the desired particle

size distributions with particle sizes within specific limits can be obtained. These variations

depend on lot also.

d) Blending & mixing:

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Blending is the process in which powders of the same nominal composition but having different

particle sizes are intermingled. This is done to:

(i) obtain a uniform distribution of particle sizes, i.e. powders consisting of different particle

sizes are often blended to reduce porosity,

(ii) for intermingling of lubricant with powders to modify metal to powder interaction during

compaction

Mixing is the process of combining powders of different chemistries such as elemental powder

mixes (Cu-Sn) or metal-nonmetal powders. This may be done in dry or wet condition. Liquid

medium like alcohol, acetone, benzene or distilled water are used as milling medium in wet

milling. Ball mills or rod mills are employed for mixing hard metals such as carbides.

3.2.1 Heat treatment of powders

Heat treatment is generally carried out before mixing or blending the metal powders. Some of the

important objectives are,

i) Improving the purity of powder: Reduction of surface oxides from powders by annealing in

hydrogen or other reducing atmosphere. Dissolved gases like hydrogen and oxygen, other

impurities are removed by annealing of powders. Lowering impurities like carbon results in

lower hardness of the powder and hence lower compaction pressures & lower die wear during

compaction. For eg., atomized powders having a combined carbon and oxygen content as high as

1% can be reduced after annealing to about 0.01% carbon and 0.2% oxygen. Heat treatment is

done at protective atmosphere like hydrogen, vacuum.

ii) Improving the powder softness: Aim is to reduce the work hardening effect of powders that

has be crushed to obtain fine powders; while many powders are made by milling, crushing or

grinding of bulk materials. Powder particles are annealed under reducing atmosphere like

hydrogen. The annealing temperature is kept low to avoid fusion of the particles.

iii) Modification of powder characteristics: The apparent density of the powders can be

modified to a higher or lower value by changing the temperature of treatment.

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3.3 Blending

It is the process in which powders of the same nominal composition but having different particle

sizes are intermingled. This is done to

(i) Obtain a uniform distribution of particle sizes, i.e. powders consisting of different particle

sizes are often blended to reduce porosity,

(ii) For intermingling of lubricant with powders to modify metal to powder interaction during

compaction

Mixing is the process of combining powders of different chemistries such as elemental powder

mixes or metal-nonmetal powders. This may be done in dry or wet condition. Liquid medium

like alcohol, acetone, benzene or distilled water are used as milling medium in wet milling.[22]

Ball mills or rod mills are employed for mixing hard metals such as carbides.

The objective of mixing is to provide a homogeneous mixture and to incorporate the lubricant.

Popular lubricants are stearic acid, stearin, metallic stearates, especially zinc stearate, and

increasingly, other organic compounds of a waxy nature. The main function of the lubricant

is to reduce the friction between the powder mass and the surfaces of the tools – die walls, core

rods, etc. - along which the power must slide during compaction, thus assisting the achievement

of the desired uniformity of density from top bottom of the compact. Of equal importance is the

fact that the reduction of friction also makes it easier to eject the compact and so minimises the

tendency to from cracks.

It has been suggested that an additional function of the lubricant is to help the particles to slide

over each other, but it seems doubtful whether this factor is of much significance: -good

compacts can be obtained without any admixed lubricant, e.g. using die wall lubrication or

isostatic pressing.

Care in the selection of lubricant is necessary, since it may adversely affect both green and

sintered strengths especially if any residue is left after the organic part has decomposed. Over-

mixing should be avoided, since this increases the apparent density of the mix. Additionally,

over-mixing usually further reduces the green strength of the subsequent compacts probably by

21

componentry coating the whole surface of the particles, thereby reducing the area of metal

contact on which the green strength depends. The flow properties also are impaired good flow is

essential for the next step i.e. loading the powder into the die. In the special case of cemented

carbides, the mixing process is carried out in ball mill, one of the objects being to coat the

individual particles with powders involved do not flow, the mixture is subsequently granulated to

form agglomerates.

BLENDER

3.4 Powder Compaction

Powder compaction is the process of compacting metal powder in a die through the application

of high pressures. Typically the tools are held in the vertical orientation with the punch tool

forming the bottom of the cavity. The powder is then compacted into a shape and then ejected

from the die cavity.

The forming of a sintered component begins with the densification of the metal powder in a rigid

die having a cavity of more or less complicated contour. In this operation, high pressures are

exerted upon the powder in the die cavity, simultaneously from top and bottom, via two or more

vertically moving compacting punches. Under the influence of such high compacting pressures,

the powder particles are being squeezed together so closely that their surface irregularities

interlock and a certain amount of cold welding takes place between their surfaces. After ejection

from the die, if the compacting operation was successful, the compact owns sufficient strength

(so called green strength) to withstand further handling without damage. In order to facilitate the

22

compacting operation and reduce tool wear to a minimum, a lubricant is admixed to the powder

before compacting.

The density of the compacted powder is directly proportional to the amount of pressure applied.

Typical pressures range from 80 psi to 1000 psi, pressures from 1000 psi to 1,000,000 psi have

been obtained. Pressure of 10 tons/in² to 50 tons/in² are commonly used for metal powder

compaction. To attain the same compression ratio across a component with more than one level

or height, it is necessary to work with multiple lower punches. A cylindrical work piece is made

by single-level tooling. A more complex shape can be made by the common multiple-level

tooling.

The compaction exercise imparts the following effects.

1. Reduces voids between the power particles and enhance the density of the consolidated

powder,

2. Produces adhesion and bonding of the powder particles to improve green strength in the

consolidated powder particles,

3. Facilitates plastic deformation of the powder particles to conform to the final desired shape of

the part,

4. Enhances the contact area among the powder particles and facilitates the subsequent sintering

process.

Die compaction lubricants

It is known that presence of frictional forces limits the degree of densification.

Usage of lubricants either mixed or applied to contact surfaces can be done to minimize

Friction

Lubricants are organic compounds such as waxes or metallic stearates or salts and they

generally have low boiling points; Amount of lubricant added can be 0.5 to 2 % by weight of

charge

23

Mixed lubrication: Reduce the inter-particle friction and aid better packing. But they may

affect the densification property depending on their volume and density. The mixed lubricants

should be removed before sintering to avoid distortion of compact. Even 1 wt% of lubricant can

occupy large volume of app. 5% and maximum attainable density will be 95% (assuming zero

porosity) only.

Die wall lubrication: Graphite & MoS2 can be applied physically on the die, punch surfaces.

They can be easily removed, but takes longer production times.

Commonly used lubricants in P/M: Paraffin wax, Aluminium stearate, Lithium stearate, Zinc

stearate, Magnesium stearate, stearic acid, Oleic acid, Talc, Graphite, boron nitride, MoS2

Properties of different materials:

Typical work

piece materials

Density

(grams/cc)

Yield strength

(psi)

Tensile strength

(psi)

Hardness (HB)

Iron 5.2 to 7.0 5.1*103to 2.3*104 7.3*103 to 2.9*104 40 to 70

Low alloy steel 6.3 to 7.4 1.5*104 to 2.9*104 2.00*104 to 4.4*104 60 to 100

Alloyed steel 6.8 to 7.4 2.6*104 to 8.4*104 2.9*104 to 9.4*104 60 and up

Stainless steel 6.3 to 7.6 3.6*104 to 7.3*104 4.4*104 to 8.7*104 60 and up

Bronze 5.5 to 7.5 1.1*104 to 2.9*104 1.5*104 to 4.4*104 50 to 70

a) Uniaxial (Die) Pressing

Die pressing is the powder compaction method involving uniaxial pressure applied to the powder

placed in a die between two rigid punches. Uniaxial (die) pressing is effectively used for mass

production of simple parts (alternative method is isostatical pressing).

The pressing process consists of the following stages:

Die filling: At this stage a controlled amount of the powder is fed into the die cavity.

24

Compaction: Upper punch moves down and presses the powder with a predetermined

pressure. The pressure varies between 10,000 psi to 120,000 psi (69 MPa to 830 MPa).

“Green” compact part ejection and removal (“green” compact – unsintered powder

compact). The pressing cycle repeats 400 to 5000 times/hour, depending on the press

type, powder filling properties and the part size and geometry [23]. Hydraulic and

mechanical presses with load up to 750 tons are used for the powder die pressing.

The scheme of the die pressing method is presented in the picture:

Die pressing, which is conducted at the room temperature is called cold pressing.

If the pressing process is conducted at increased temperature it is called hot pressing.

25

b) Isostatic Powder Compacting

Isostatic powder compacting is a mass-conserving shaping process. It is the powder compaction

method involving applying pressure from multiple directions through a liquid or gaseous

medium surrounding the compacted part. Fine metal particles are placed into a flexible mould

and then high gas or fluid pressure is applied to the mould. The resulting article is then sintered

in a furnace. This increases the strength of the part by bonding the metal particles. Compacting

pressures range from 100,000 kPa to 280,000 kPa for most metals and approximately 14,000

kPa to 69,000 kPa for non-metals. The density of isostatic compacted parts is 5% to 10%

higher than with other powder metallurgy processes. There are two types of processes:

I. Cold Isostaic Process

Cold isostatic pressing (CIP) uses fluid as a means of applying pressure to the mold at room

temperature. For Cold (or room temperature) Isostatic Pressing (CIP), the container is typically a

rubber or elastomeric material; the pressurizing medium is a liquid such as water or oil. Free of

die frictional forces, the powder compact reaches a higher and more uniform density than would

be obtained using conventional cold die compaction at the same pressure. In CIP processing, the

part must be sintered (solid-state diffused) after removal from the mold.

26

II. Hot Isostatic Pressing

For Hot Isostatic Pressing (HIP), the hermetic container for the powder is made of metal or glass

and the pressurizing medium is a gas (inert argon or helium). At the elevated temperatures the

process employs, the hermetic container deforms plastically to compact the powder within it.

The combination of heat and pressure during the process eliminates the need for a supplemental

sintering step. Removal of the HIP "can" (container) after processing is an additional

requirement not found in other PM processes.

3.5 Sintering

Sintering is a method involving consolidation of powder grains by heating the “green” compact

part to a high temperature below the melting point, when the material of the separate particles

diffuse to the neighboring powder particles. During the diffusion process the pores, taking place

in the “green compact”, diminish or even close up, resulting in densification of the part,

improvement of its mechanical properties

27

The ISO definition of the term reads: - “ 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”.

The operation is almost invariably carried out under a protective atmosphere, because of the

large surface areas involved, and at temperatures between 60 and 90% of the melting point of the

particular metal or alloys. For powder mixtures, however, the sintering temperature may be

above the melting-point of the lower-melting constituent, e.g. copper/tin alloys, iron/copper

structural parts, tungsten carbide/cobalt cemented carbides, so that sintering in all these cases

takes place, hence the term liquid phase sintering. It is, of course, essential to restrict the amount

of liquid phase in order to avoid impairing the shape of the part. Control over heating rate, time,

temperature and atmosphere is required for reproducible results.

Sintering can be considered to proceed in three stages:

1. During the first, neck growth proceeds rapidly but powder particles remain discrete.

2. During the second, most densification occurs, the structure recrystallizes and particles

diffuse into each other.

3. During the third, isolated pores tend to become spheroidal and densification continues at

a much lower rate.

The words Solid State in Solid State Sintering simply refer to the state the material is in when it

bonds, solid meaning the material was not turned molten to bond together as alloys are formed.

[24] To allow efficient stacking of product in the furnace during sintering and prevent parts

sticking together, wares are separated by using Ceramic Powder Separator Sheets. These sheets

are available in various materials such as alumina, zirconia, tin and magnesia. They are also

available in fine medium and coarse particle sizes.

28

Figure below shows the diffusion process of sintering.

METAL

SINTERING TEMP.(C)

SINTERING TIME

(MIN)

BRASS

850-900

10-45 MIN

BRONZE

750-880

16-20 MIN

IRON

1000-1150

10-45 MIN

COPPER

850-900

10-45 MIN

Table: Sintering temp. and time of some metals

29

3.6 Tools

The basic parts of a tool set are the die in which the powder is contained, and punches which are

used to apply the compacting pressure. If, as is frequently the case, the part has holes running

through it, these are formed by core rods located in the powder is introduced. Multiple punches

acting independently are used if the component being pressed different levels.

The die and core rod(s) from the contour of the compact parallel to the direction of pressing, and

must, of course, be free from projections and re-entrants at right angles to the pressing direction;

otherwise it would be impossible to eject the compact from the die.

Materials used are hardened tool steels or hard metals (cemented carbides). The use of the more

expensive carbide is increasing because of the life it affords, and the increasing cost of tool

changes both in lost production and tool setters wages. PM high-speed steels are finding,

increasing application in this field.

For short runs, ordinary steel dies may, of course, be more economical. The importance of

precise dimensions and high quality of the surface finish scarcely needs emphasis bearing in

mind that one of the major features justifying the use of sintered parts is the ability to produce

30

such parts accurately as regards size and with a surface finish that obviates the necessity for

subsequent machining operations.

Die life is another important aspect, and here it is impossible to give more than an indication.

The life depends not only on what material is being pressed, and to what density, what

lubrication is provided and the degree of die wear that can be tolerated, but also on the skill of

the toolsetter, and the complexity of the tools. With steel dies up to about 200,000 compacts can

be achieved, with carbide dies 1,000,000 parts or more are possible.

3.6.1 Die materials

Soft powders like aluminium, copper, lead use abrasion resistant steel such as air hardened steels

as adie, die steels are used for making die.

Relatively hard powders use dies made of tool steel.

More hard & abrasive powders like steel use dies made of tungsten carbide. But carbide dies are

costly & have high hardness and therefore difficult to machine.

Coated dies with hard & wear resistant coating material like titanium nitride or titaniumcarbide

can be used.

Defects occurring in die pressing of powders:

1) lamination cracking – this is caused by trapped air in compact sample. This cracking occurs

perpendicular to load direction. This trapped air prevents the interlocking of particles.

2) Blowout – occurs when all the entrapped air tries to escape at the interface between the die

and punch

31

3.7 Design Considerations

Several design rules must be considered to make parts efficiently and economically by the

powder metallurgy process:

1. The design must be such that the part can be ejected from the mould or die. Parts with straight

wall are preferred. No draft should be required for the ejection of a part from a lubricated die

2. In designing the part, consideration should be given to the need for the powder particles to

flow properly into all parts of the mould or die. Therefore, thin walls, narrow splines, or sharp

corner should be avoided (should be thicker than 0.762 mm).

3. The shape of the part should permit the construction of strong tooling. Dies and punches

should have no sharp edges. Reasonable clearance must be provided between the top and the

bottom dies during pressing.

4. Since pressure is not transmitted uniformly through a deep bed of powder, the length of the

part should not exceed about two and half times of the diameter.

5. Very close tolerance in the direction of compression should be avoided.

6. Shape of the parts should be kept as simple as possible and should contain with few levels and

axial variation. Holes should not be designed in the direction of pressing.

7. Provide sufficiently wide dimensional tolerance whenever possible. Wide tolerance means that

the part can be made more economically with a longer tool life.

32

Chapter 4: Action Plan for Execution of the

Project

ACTIONS TIME STATUS

Phase -1

Taking up/ Deciding of Project August 2012 Completed

Project Analysis September 2012 Completed

Literature Survey October 2012 Completed

Phase -2

Pre –Final Project PPT October 2012 Completed

PPT for MINOR PROJECT November2012 ------

Submission of project REPORT November 2012 -------

Phase-3

Market Survey December 2012 --------

Procurement of Parts January 2013 --------

Fabrication February- March 2013 -------

Final Report April 2013 -------

Submission of report and Final PPT May 2013 -------

33

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