RT60 Reverberation Time

 RT60 reverberation time is the main room acoustic parameter. Following ISO 3382, it is the duration required for the sound energy in a room to decrease by 60 dB after the source emission has stopped. The values of RT60 may range from fractions of a second to a few seconds and depend upon the size of the room and the nature of the materials used in its construction.

What is RT60 used for?

Reverberation time is used to determine the required acoustics for a room. The reverberation time RT60 in a room is determined by the absorptive properties of the reflecting surfaces and the distances between them. The purpose of this measurement is to obtain an objective, quantitative indication of the acoustic quality of a room in a building. In an empty room, sound waves reflect off of the walls, ceiling, and floor, and these reflections build up over time. This build-up of sound is known as reverberation, and it can be a major problem in large rooms with hard surfaces.

When designing a room for optimal acoustics, it is important to ensure that the reverberation time is appropriate for the intended use of the room. If the reverberation time is too long, the speech will be unintelligible and music will sound muddy.

On the other hand, if the reverberation time is too short, the room will sound sterile and uninviting. By carefully considering the absorption characteristics of the materials used in a room, it is possible to achieve ideal reverberation times for any given application.

Depending on the use of the room, more direct and less indirect (reflected) sounds are required. For example with a long reverberation time, a speech becomes less understandable and background noise levels increases,  and with a shorter reverberation time background noise reduces but voice muffles.

RT60 Reverberation Time Examples:

Type of room	Reverberation time
Church	2 – 10 s
Concert Hall	1 – 2 s
Office	0.5 – 1.1 s
Classroom	0.4 – 0.7 s

De stratification Fans

civil basic toilet drawings

nuclear power plant.

Why Transformer Rated In kVA, Not in KW?

Transformer Always Rated In kVA instead of KW

As the name suggest, transformer only transfer the power from one circuit to another without changing the value of power and frequency. In other words, It can only step up or step down the value of current and voltage while the power and frequency would remain same. A general date on transformer nameplate are printed for further details, such as rating in VA, single phase / three phase (power or distribution transformer), step up / step down, connection etc.

There are two type of losses in a transformer;

1. Copper Losses

2. Iron Losses or Core Losses or Insulation Losses

Let’s explain in more details to get the idea that why a transformer rated in VA instead of kW?

When manufactures design a transformer, they have no idea which kind of load will be connected to the transformer. The load may be resistive (R), inductive (L), capacitve (C) or mixed load (R, L and C). Its mean, there would be different power factor (p.f) at the secondary (load) side on different kind of connected loads depends on R, L and C. This way, they go for VA instead of W in case of Transformer.

Lets clear the rating of transformer in VA instead of W  with solved example.

Losses of transformer will remain same as  long as the magnitude of current / voltage is same. No matter what power factor of the load current / voltage is

Example 

Suppose for a single phase step up transformer

Transformer rating in kVA = 11kVA

Primary Voltages  = 110V

Primary Current = 100 A

Secondary Voltages = 220V

Secondary Current = 50 A.

Equivalent resistant on Secondary = 5Ω

Iron losses = 30W

In first scenario, If we connect a resistive load to the secondary of the transformer at unity power factor θ = 1,

Then total losses of transformer would be copper losses + iron losses, i.e.

I²R + Iron losses

Putting the values,

(502 x 5 ) + 30W = 12.53kW

i.e. losses on primary and secondary of transfer is still same. (See below example for secondary losses as well)

The transformer output will be:

P = V x I x Cos θ

Again putting the value from secondary (Same value if we put the values from primary)

P = 220 x 50 x 1 = 11kW.

Now rating of transformer

kVA = VA / 1000

kVA = 220 x 50 / 1000 = 11kVA.

Now, In second scenario, connect a capacitive or inductive load to the secondary of the transformer at power factor θ = 0.6.

Again, total losses of transformer would be copper losses + iron losses, i.e.

I²R + Iron losses

Putting the values,

(502 x 5 ) + 30W = 12.53kW

Hence proved that losses in both of primary and secondary is same.

But The transformer output will be:

 

P = V x I x Cos θ

Again putting the value from secondary (Same value if we put the values from primary)

P = 220 x 50 x 0.6 = 6.6kW.

Now rating of transformer

kVA = VA / 1000

kVA = 220 x 50 / 1000 = 11kVA.

Now, In second scenario, connect a capacitive or inductive load to the secondary of the transformer at power factor θ = 0.6.

Again, total losses of transformer would be copper losses + iron losses, i.e.

I²R + Iron losses

Putting the values,

(502 x 5 ) + 30W = 12.53kW

Hence proved that losses in both of primary and secondary is same.

But The transformer output will be:

P = V x I x Cos θ

Again putting the value from secondary (Same value if we put the values from primary)

P = 220 x 50 x 0.6 = 6.6kW.

Now rating of transformer

kVA = VA / 1000

kVA = 220 x 50 / 1000 = 11kVA.

Related Post: What are the Colored Aerial Marker Balls on Power Lines For?

Its mean, 11kVA transformer rating means it can handle of 11kVA. It is our turn to transform and utilize the 11kVA as 11kW (we can do it by improving the power factor to 1 in case of pure resistive load) which is not predictable and even very hard to get in case of inductive and capacitive loads where power factor would have different values.

From the above example, it is clear that the rating of transformer is same (11kVA) but different output in power (11kW and 6.6kW) due to different power factor values after connecting different kind of load which is not predictable for transformer manufactures where the losses are same in both cases.

So these are the exact reason for transformer rating in kVA instead of kWA.

Distributed pumping solutions represent a new paradigm in chilled water air conditioning

Chilled water systems with modulating valves are common air conditioning systems in today’s commercial and residential buildings.

However, these systems face challenges with balancing and poor dynamic flow regulation, which leads to severe energy loss, inadequate climate control, and an often uncomfortable environment.

Focus on the heart of the HVAC system

All chilled water distribution systems require pumps for moving the chilled water, and all buildings have several terminal units such as AHUs, FAHUs, FCUs with different needs.

There are several reasons why chilled water loops can get out of balance, such as improper commissioning, components deteriorating over time, aging of the building, and changes to other parts of the system installation.

An imbalanced water loop can lead to a low Delta T, causing the chillers to work outside the best efficiency point and over pumping the loop.

This leads to excessive energy consumption and can result in an uncomfortable environment.

As a solution to these challenges, distributed pumping solutions are growing in popularity.

Replacing valves with pumps on each floor of the building, instead of centralizing them in the basement, provides continuous automatic balancing, reducing pump energy consumption and providing a more consistent and comfortable indoor climate.

Holistic solution to imbalanced water loops 

Distributed pumping solutions are a paradigm shift away from centralised pumps in distribution networks towards decentralised pumps throughout the building.

By replacing balancing and motorised valves with pumps, the system is equipped only with components that generate pressure only when and where it is needed.

This reduces the time spent on balancing the system, as once the correctly sized pumps are selected, there are no valves needed to 

balance the system.

Additionally, the main pumps can be downsized as distributed pumps generate the needed pressure individually, saving pump energy that way as well.

Distributed pumping solutions can be applied to existing chilled water systems that need refurbishment, or to new commercial buildings planned with chilled water air conditioning.

How distributed pumping works

Distributed pumping systems consist of five key components: primary pumps, distributed pumps, primary pump controller, check valves, and sensors located throughout the building.

The primary pump controller uses a control algorithm to manage the primary pumps, which are variable speed pumps that are regulated by sensor measurements from the decoupled line to avoid over or under pumping the system. Dedicated distributed pumps are installed with a non-return valve at each air handling unit (AHU) or a branch containing multiple FCUs.

The distributed pumps measure the air temperature using the AHU air duct sensor and will automatically regulate the speed to achieve the desired temperature.

Interfaces with the building management system (BMS), if installed, and other control options can be discussed during the design process, ensuring seamless integration based on the sequence of operations.

For multiples FCUs in the branch, the pumps’ in-built differential pressure control enables perfect proportional pressure control, so even the further FCU is adequately fed with flow and pressure to create perfect indoor climate

A well-balanced loop system creates a well-balanced indoor climate

Distributed pumping solutions represent a new paradigm in chilled water air conditioning.

By providing consistent, accurate load balancing, distributed pumping solutions save energy and provide optimal comfort for people in the building.

They are also fast and easy to commission, reducing the initial investment and the time spent on system balancing.

For all these reasons and more, distributed pumping is becoming widely spread in commercial building projects around the world.

Hermetic Compressor & Semi-hermetic compressors

A hermetic or sealed compressor is one in which both compressor and motor are confined in a single outer welded steel shell. The motor and compressor are directly coupled on the same shaft, with the motor inside the refrigeration circuit. Thus the need for a shaft seal with the consequent refrigerant leakage problem was eliminated. All the refrigerant pipeline connections to the outer steel shell are by welding or brazing. The electrical conductors to the motor are taken out of the steel shell by sealed terminals made of fused glass. The figure below shows the cut-away view of a hermetic compressor. One can see the cooper windings inside the outer shell and also the refrigerant conections (copper pipes). Hermetic compressors are ideal for small refrigeration systems, where continuous maintenance (replenishing refrigerant and oil charge etc) cannot be ensured. Hence they are widely used in domestic refrigerators, room air conditioners etc. Since, the motor is in the refrigerant circuit, the efficiency of hermetic compressor based systems is lower as the heat dissipated by the motor and compressor becomes a part of the system load. Also material compatibility between the electrical windings, refrigerant and oil must be ensured. Since the complete system is kept in a welded steel shell, the hermetic compressors are not meant for servicing. A variation of hermetic compressor is a semi-hermetic compressor, in which the bolted construction offers limited serviceability.

Semi-hermetic compressors are identical sealed type, but the motor and compressor built in manufactured housing with screw sections or access panels for ease of maintenance. These compressors are manufactured in small and medium capacities and their engine power may be up to 300 kW. For this reason, they are cheap, and another advantage is that they are compact. In addition, they have no problems with leaking. On Fig. 3.5 shows a new type of semi-hermetic reciprocating compressors for medium-and low-temperature commercial refrigeration equipment. They are issued to alternative refrigerants (e.g.. R-134a, R-404A and R-507). Fig. 3.5a shows a cutaway view of a single-stage octagon series semi-hermetic reciprocating compressors with a nominal engines with a capacity of 60 and 70 HP With integrated ripple mufflers and performance management (100-75-50%), smooth, efficient and compact piston semihermetics now available for this category of potential. They can work with refrigerants R-134a, R-407C, R-404A, R-507A, R-22. Fig. 3.5b shows used a two-stage semi-hermetic reciprocating compressors for extremely low temperatures and its main feature is the two-stage compression in a single package. A two-stage compression, the compression ratio of the share, thus avoiding extreme temperatures and achieve very reliable operation. In particular, for commercial refrigeration systems with high load variations, energy-efficient operation at full and partial load (capacity up to four stages) to all common refrigerants can be at a reasonable price. In addition, it is recognized features of the octagon, compressors, which even pay with a double in tandem configuration.

Advanced Casting - 1

Introduction

The Melter Should not only understand the operation of the equipment he required to use .
He should know Nature & Metallurgy of Various Cast metals , their behaviour During solidification & cooling , their physical & mechanical properties
Therefore the need arise to understand in depth about the advanced casting Process.

Melting Equipments

We are Discussing some of the Melting Equipments in Detailed here

i)Crucible Furnace

a) Coke Fire Furnace
b) Oil & Gas Fired furnace
ii)Open hearth Furnace
iii)Air Furnace
iv)Rotary Furnace
v)Cupola Furnace
vi)Electric Furnace

a) Direct Arc Furnace.
b)Indirect Arc Furnace.
c)Electric Induction Furnace
Crucible Furnaces
Simplest of all the Foundries
Used for Melting many Ferrous & non Ferrous Metals, Copper based Alloys .
Crucible is made of Chamotte or clay or graphite For melting said materials.
while for melting Al & zinc base alloys these are made of Steel & CI .

one of the Crucible Furnance is a Coke Fired Furnance
Used for Melting non ferrous metals such as brass , Bronze & aluminium .
Generally installed in Pit
Has a Steel cylindrical Steel Shell, lined on inner side with refractory bricks , closed at the bottom with a grate & covered at top with a removable lid.
Metal to be Melted is contained in a crucible which is embedded in burning coke .

Reform Movement in kerala

Samathwa samajam:1836 - nadar
Ayya Vaikundar (Vaikunda swamikal) founded Samathwa samajam for reform of nadar community.
He organized SAMA PANTHI BHOJANA in each and every place of worship in the name of ANNA DHANAM.

Sri Narayana Dharma Paripalana Yogam :1903 –Ezhava
• 1903 May 15 :The S.N.D.P. Yogam came into existence under the guidance of Sri Narayana Guru
• 1904:Its first annual session held at Aruvippuram ,Trivandrum
• The basic aim of was to popularize Guru’s messages and bring about the social regeneration of the Ezhavas and other backward communities.
• Dr. Palpu and Kumaran Asan were active leaders.
• Some Newspapers also helped to spread Gurus’s message of social reform.
Eg: Sujananandini NewsPaper :1891 (Published by Paravoor Kesavanasan) Kerala Kaumudi 1911-(Started by KV Kunhiraman)Yogadaanam is a well known publication from SNDP

Islam Dharma Paripalana Sangham:1906

Vakkom Abdul Khader Moulavi established Islam Dharma Paripalana Sangham for the reform of Muslims.

Sadhujana Paripalana Sangham: 1907 -For Dalits
Ayyankali’s Sathujana Paripalana Sangham was established for education for Dalits with the support of government of Travancore.
Thomas Vaidyar was given the responsibility of organization correspondence.

SJPS published a monthly magazine, Sadhujana Paripalini, the first ever magazine to be brought out by the Dalit community. Kali Chodikkuruppan was the founder editor.
Later this sangham became Pulaya Mahasabha.

Yoga kshema Movement: 1908 -Namboothiri
Slogan: “Make Namboothiri a human being”.

Aim: the marriage of all the junior Namboothiri males within the community itself, to popularise the study of English and to abolish the purdah system from Namboothiri females
Leaders: E.M.S. Namboothiripad and V.T.Bhattatiripad.
"Unni Namboothiri" was a famous publication from Yoga kshema Sabha.

Prathyaksha Raksha Daiva Sabha:1909
Prathyaksha Raksha Daiva Sabha ("God's Church of Visible Salvation") was a Dalit religious protest movement founded at Eraviperoor, Pathanamthitta by Poikayil Yohannan.

The PRDS rejected both Christianity and Hinduism, and preached that God would send an incarnation to liberate the Dalits.
They spread message to leave superstitious beliefs, and to stop practicing black magic and sacrificing the animals
Vaala Samudaya Parishkarani Sabha : 1912


Fishermen community reform society.
It was organized under Pandit K P Karuppan, the "Lincoln" of Kerala"
Leaders: N Krishnan, VV Velukkuttty Arayan and rao Bahadur VV Govindan
Initially it was a small group called kalyana dayini sabha
Aim: abolish outdated customs, spread discipline, hygiene , education and freedom of movement.
Nair Service Socety: 1914
Founded by: Mannathu padmanabhan on October 31, 1914
Inspiration: “Servants of Indian Society” by GK Gokhale
Areas: Reform Nair society, abolition of Talikettukalyanam, Tirandukuli, untouchability, joint family system

Sahodara Sangham :1917
Founded by the noted Ezhava leader, K.Ayyappan(also known as Sahodaran Ayyappan) at Cherai, Kochi in 1917

Aim: eradication of the evils of caste and popularizing the idea of misra-bhojanam among the Ezhavas and other castes considered inferior to them .

Yukthivadi Sangham :1935
Yukthivadi Sangham was registered at Cochin M. C. Joseph as secretary and Panampilly Govinda Menon as treasurer. M C Joseph was the sole editor-publisher of "Yukthivaadi" Magazine by Sahodara Sangham.
The existing Kerala Yukthivadi Sangham (KYS) was formed at Kozikode in 1969 May Adv. M. Prabha as president and P.S. Raman Kutty as Secretary

Secondary operations on P/M part

Final Step in powder Metallurgy is the Secondary Opeartion inorder to Finish the product manufactured by the powder Metallurgy
Some techniques Used are

: Machining – In general, machining is not necessary for porous parts. But it can be done to produce specific shape and size in which case a very sharp tooling with a slight rake is employed. The machined surface is then treated to remove the cutting fluids. EDM and laser cutting are also performed to obtain specific shape and size.
Joining – Joining of porous parts can be done with one another or with solid part mainly in the case of stainless steel porous components. TIG, LASER or electron beam welding are recommended for satisfactory joining of porous parts. Soldering and brazing are not used. Epoxy resins are also used for bonding of porous parts.
Insert moulding, sinter bonding, press fitting are other secondary operations.

Sintering -P/M

After Cold or hot Compaction Sintering Take place
Sintering refers to the heating of the compacted powder perform to a specific temperature (below the melting temperature of the principle powder particles while well above the temperature that would allow diffusion between the neighbouring particles).

Sintering facilitates the bonding action between the individual powder particles and increase in the strength of the final part. The heating process must be carried out in a controlled, inert or reducing atmosphere or in vacuum for very critical parts to prevent oxidation.

Prior to the sintering process, the compacted powder perform is brittle and confirm to very low green strength. The nature and strength of the bond between the particles depends on the mechanism of diffusion and plastic flow of the powder particles, and evaporation of volatile material from the in the compacted preform.
Bonding among the powder particles takes places in three ways: (1) melting of minor constituents in the powder particles, (2) diffusion between the powder particles, and (3) mechanical bonding. The time, temperature and the furnace atmosphere are the three critical factors that control the sintering process. Sintering process enhances the density of the final part by filling up the incipient holes and increasing the area of contact among the powder particles in the compact perform.
• It is the process of consolidating either loose aggregate of powder or a green compact of the desired composition under controlled conditions of temperature and time.
• Types of sintering: a) solid state sintering – This is the commonly occurring consolidation of metal and alloy powders. In this, densification occurs mainly because of atomic diffusion in solid state.
b) Liquid phase sintering – The densification is improved by employing a small amount of liquid phase (1-10% vol). The liquid phase existing within the powders at the sintering temperature has some solubility for the solid. Sufficient amount of liquid is formed between the solid particles of the compact sample. During sintering, the liquid phase crystallizes at the grain boundaries binding the grains. During this stage, there is a rapid rearrangement of solid particles leading to density increase. In later stage, solid phase sintering occurs resulting in grain coarsening and densification rate slows down. Used for sintering of systems like tungsten-copper and copper-tin. Also covalent compounds like silicon nitride, silicon carbide can be made, that are difficult to sinter.
c) Activated sintering – IN this, an alloying element called ‘doping’ is added in small amount improves the densification by as much as 100 times than undoped compact samples. Example is the doping of nickel in tungsten compacts d) Reaction sintering – IN this process, high temperature materials resulting from chemical reaction between the individual constituents, giving very good bonding. Reaction sintering occurs when two or more components reacts chemically during sintering to create final part. A typical example is the reaction between alumina and titania to form aluminium titanate at 1553 K which then sinters to form a densified product.
Other than mentioned above, rate controlled sintering, microwave sintering, gas plasma sintering, spark plasma sintering are also developed and practiced.
Sintering theory
- Sintering may involve, 1) single component system – here self-diffusion is the major material transport mechanism and the driving force resulting from a chemical potential gradient due to surface tension and capillary forces between particles, 2) multi-component system (involve more than one phase) – inter-diffusion occurs with the concentration gradient being the major driving force for sintering in addition to self-diffusion caused by surface tension and capillary forces. IN this sintering, liquid phase formation and solid solution formation also occurs with densification.
- First theory was proposed by Sauerwald in 1922. This theory says that two stages are involved in sintering namely adhesion and recrystallisation. Adhesion occurs during heating due to atomic attraction and recrystallisation occurs at recrystallisation temperature (above 0.5 Tm). In recrystallisation, microstructure changes, phase changes, grain growth, shrinkage occurs.

Property changes during sintering
• Densification is proportional to the shrinkage or the amount of pores removed in the case of single component system
• IN multicomponent system, expansion rather than shrinkage will result in densification and hence densification can not be treated as equal to the amount of porosity removed.
• densification results in mechanical property change like hardness, strength, toughness, physical properties like electrical, thermal conductivity, magnetic properties etc. Also change in composition is expected due to the formation of solid solution.
Solid state sintering process
Condition for sintering: 1) densification occurs during sintering and solid state sintering is carried out at temperatures where material transport due to diffusion is appreciable. Surface diffusion is not sufficient, atomic diffusion is required.
2) This occurs by replacing high energy solid-vapour interfaces (with free energy SV) with the low energy solid-solid interface (particle-particle) of free energy SS. This reduction in surface energy causes densification.
3) Initially free energy of solid-solid interface must be lower than free energy of solid vapour interface. The process of sintering will stop if the overall change in free energy of the system (dE) becomes zero, i.e., dE = ϒSS dASS + ϒSV dASV< 0 Where dASS &dASV are the interfacial area of solid-solid and solid-vapour interfaces.

4) Initially, the surface area of compact represent the free surface area, since no grain boundaries have developed and hence ASV = ASV0 & ASS = 0. As sintering proceeds, ASV decreases and ASS increases. The sintering process will stop when dE = 0, i.e.,ϒSS dASS + ϒSV dASV = 0 => ϒSS / ϒSV = - dASV / dASS
5) Densification stops when - dASV / dASS is close to zero. To achieve densification without grain growth, the solid-solid interface must be maximized. Such conditions can be achieved by doping or by using suitable sintering conditions for surface free energy maximization.

Stages in solid state sintering

• In general, solid state sintering can be divided into three stages – 1st stage: Necks are formed at the contact points between the particles, which continue to grow. During this rapid neck growth takes place. Also the pores are interconnected and the pore shapes are irregular.
• 2nd stage: In this stage, with sufficient neck growth, the pore channels become more cylindrical in nature. The curvature gradient is high for small neck size leading to faster sintering. With sufficient time at the sintering temperature, the pore eventually becomes rounded. As the neck grows, the curvature gradient decreases and sintering also decreases. This means there is no change in pore volume but with change in pore shape => pores may become spherical and isolated. With continued sintering, a network of pores and a skeleton of solid particle is formed. The pores continue to form a connected phase throughout the compact.
• 3rd or final stage: In this stage, pore channel closure occurs and the pores become isolated and no longer interconnected. Porosity does not change and small pores remain even after long sintering times.

Driving force for sintering
.
• The main driving force is excess surface free energy in solid state sintering. The surface energy can be reduced by transporting material from different areas by various material transport mechanisms so as to eliminate pores.
• material transport during solid state sintering occurs mainly by surface transport, grain boundary transportation. This surface transport can be through adhesion, surface diffusion. Many models available to describe sintering process – like viscous flow, plastic flow, grain boundary and volume diffusion models. These models will be briefly described here.

Mechanism in solid state sintering
As discussed earlier, material or atom transport forms the basic mechanism for sintering process. A number of mechanisms have been proposed for sintering operation.
These are,
1. Evaporation condensation, 2. diffusion (can be volume diffusion, grain boundary diffusion, surface diffusion), 3. plastic flow
1. Evaporation and condensation mechanism
The basic principle of the mechanism is that the equilibrium vapor pressure over a concave surface (like neck) is lower compared to a convex surface (like particle surface). This creates the vapor pressure gradient between the neck region and particle surface. Hence mass transport occurs because of vapor pressure gradient from neck (concave surface) to particle surface (convex surface).

2. Diffusion mechanism
- Diffusion occurs because of vacancy concentration gradient. In the case of two spheres in contact with each other, a vacancy gradient is generated between the two surfaces. This condition can be given by, μ-μ0 = RT ln(C/C0) = (-ϒ)(Ω)/r Where C and C0 are the vacancy concentration gradient around the curved and flat surface.
- Neck growth due to surface diffusion, lattice diffusion, vapour transport, grain boundary transport from GB source, lattice diffusion from sources on GB, lattice diffusion from dislocation sources
3. Plastic flow mechanism
Bulk flow of material by movement of dislocations has been proposed as possible mechanism for densification during sintering. Importance was given to identify dislocation sources during the sintering process. Even if frank read sources are present in the neck region, the stress available for dislocation generation is very small, indicating that the generation of dislocation must come from free surfaces.
Only if the surface is very small of the order of 40 nm, the stress required for dislocation generation will be sufficient. But experimental results have shown the absence of applied stress and plastic flow is expected to occur during early stages of sintering. Plastic flow mechanism is predominant during hot pressing.

LIQUID PHASE SINTERING
Mechanism during liquid phase sintering
In the sintering of multi-component systems, the material transport mechanisms involve self diffusion and interdiffusion of components to one another through vacancy movement. Sintering of such systems may also involve liquid phase formation, if the powder aggregate consists of a low melting component whose melting point is below the sintering temperature
Liquid phase sintering: In this, the liquid phase formed during sintering aids in densification of the compacts. Liquid phase sintering employs a small amount of a second constituent having relatively low melting point.
This liquid phase helps to bind the solid particles together and also aids in densification of the compact. This process is widely used for ceramics – porcelain, refractories.
Three main considerations are necessary for this process to occurs, 1. presence of appreciable amount of liquid phase, 2. appreciable solubility of solid in liquid, 3. complete wetting of the solid by liquid.
Three main stages are observed in liquid phase sintering:
1. Initial particle rearrangement occurs once the liquid phase is formed. The solid particles flow under the influence of surface tension forces
2. Solution & reprecipitation process:
in this stage, smaller particles dissolve from areas where they are in contact. This causes the particle centers to come closer causing densification. The dissolved material is carried away from the contact area and reprecipitate on larger particles 3. solid state sintering
This form of liquid phase sintering has been used for W-Ni-Fe, W-Mo-Ni-Fe, W-Cu systems. The three stage densification is schematically shown in figure.

IN solid phase sintering, the solid particles are coated by the liquid in the initial stage. In liquid phase sintering, the grains are separated by a liquid film. For the figure shown here, the surface energy for the solid-liquid-vapour system : θ = ϒs-s/2ϒl-s where s-s& l-s are the interfacial energies between two solid particles and liquid-solid interfaces respectively. For complete wetting θ should be zero. This means that two liquid-solid interface can be maintained at low energy than a single solid-solid interface. This pressure gradient will make the particles to come closer.

Powder Rolling

This process involves feeding of powders between rolls to produce a coherent and brittle green strip. This green strip is then sintered & re-rolled to obtain a dense, finished product.
Steps: 1) preparation of green strip, 2) sintering, 3) densification of sintered strip, 4) final cold rolling and annealing Parameters affecting powder rolling are roll gap, roll diameter, roll speed, powder
characteristics; Roll gap => large roll gap leads to decrease in green density; very small roll gap leads to edge cracking; roll diameter => increase in density and strength with increase in roll dia. for a given strip thickness; roll speed => Kept low, 0.3-0.5 m/s;
Powder => irregular powder with rough surfaces provide better strip density
In densification stage, either repeated cold rolling followed by annealing or hot rolling of strip can be followed Applications: nickel strips for coinage, nickel-iron strips for controlled expansion properties, Cu-Ni-Sn alloys for electronic applications, porous nickel strip for alkaline batteries and fuel cell applications.

HOT ISOSTATIC PRESSING

Ideal method for consolidation of powders of nickel and cobalt base super alloys, tool steels, maraging steels, titanium alloys, refractory metal powders, cermets. It has got variety of applications including bonding of dissimilar materials, consolidation of plasma coatings, processing hard and soft magnetic materials etc. - HIP is the application of pressure at elevated temperatures to obtain net or near net shape parts from metal, ceramic, cermet powders. - HIP unit consists of a pressure vessel, high temperature furnace, pressurizing system,controls and auxiliary systems (material handling, vacuum pumps, metering pumps). - The pressure vessel is made of low alloy steel. Its function is to heat the powders while applying uniform gas pressure on all the sides. Furnaces are of radiation or convection type heating furnaces with graphite or molybdenum heating elements. Nichrome is also used. The furnace heats the powder part, while pressurizing medium (a gas) is used to apply a high pressure during the process. Generally, argon, nitrogen, helium or even air is used as pressurizing medium. - The pressurizing gas, usually argon, is let into the vessel and then a compressor is used to increase the pressure to the desired level. The furnace is then started and both temperature and pressure are increased to a required value.

HIP presses are available in diameters up to 2m with pressures ranges from 40 to 300 MPa with temperature range from 500 to 2200 °C. The processing time can last up to 4 hours depending on the material and size of the part. - during HIP, the pores are closed by flow of matter by diffusion and creep, but also bonded across the interface to form a continuous material.
- Commonly used heating elements: Kanthal heating element – up to 1200 °C; Molybdenum heating element – 1200 to 1700 °C; Graphite heating element – 2000 to 2600 °C

Cold Isostatic Compaction (CIP)

• CIP is a compaction process in which isostatic fluid pressure is applied to a powder mass at room temperature to compact it into desired shape. The powder parts can be compacted up to 80-95 % of their theoretical densities. Water or oil can be used as pressuring medium. • Process details: High density near-net shape green parts, long thin walled cylinders, parts with undercuts can be readily fabricated. In this process, pressure is applied simultaneously and equally in all directions using a fluid to an elastomeric fluid with powder at room temperature. Sintered CIP component can reach up to 97 % of theoretical density. Steps in this process is shown in flowchart.

Good mould filling is required in CIP because the initial powder distribution and density affect the preform shape. Powder size, shape, density and mechanical properties affect the flowability of powder into the mould and the packing density. Optimum pressing is obtained by using a free-flowing powder along with controlled vibration or mould tapping. Materials used for flexible moulds are natural, synthetic rubber like neoprene, urethane, nitrile, silicones. During pressing, high density is achieved at a low pressure, while the green strength of the compact rises linearly with pressure. The pressure applied can range from 100- 400 MPa. Initially the applied stress (exactly shear stress) serves to improve the density of the compact by particle sliding and rotation. In the next stage, deformation of powder particles occur and particle characteristics like shape play vital role in deciding this stage. • Irregular particles which interlock with one another and also deform during both the stages, tend to densify much easily than spherical powders. In the case of spherical powders, in spite of their higher initial packing densities, particles do not mechanically interlock with one another and hence do not easily deform. Hence high pressures are required for their compaction.

Types of cold isostatic pressing:
Wet bag process: IN this, the mould is directly in contact with the fluid. This reduces the productivity, since the bag has to be removed every time before refilling. Tooling costs are reduced in this.
Fixed mould process: the mould is fixed in the pressure vessel and powders are filled in situ. The tooling has internal channel into which fluid is pumped. This is an automated process in which the powder filling, compaction, depressurization and removal of green parts are done continuously. This involves higher tooling cost, but has higher production rate.

Advantages of CIP:

Uniform, controlled, reproducible densification of powder; long, slender parts can be pressed; neat net shape forming; short production times; economy of operation for complex and large parts.

Applications:

Metallic filters made from bronze, brass, stainless steel, Inconel, Monel, Titanium, high speed tools, carbide tools. Also ceramic parts such as sparks plugs and insulators are made by this method.

Powder Compaction - Post 1

Next Step is Called powder Compaction
The principle goal of the compaction process is to apply pressurize and bond the particles to form a cohesion among the powder particles. This is usually termed as the green strength. 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.
Compaction is carried out by pouring a measured amount of metallic powder into the die cavity and applying pressure by means of one or more plungers. To improve uniformity of pressure and reduce porosity in the compacted part, compressive forces from both the top and the bottom sides are necessary. The requisite compacting pressure depends on the specific characteristics and initial shape of the particles, the method of blending and the application of the lubricants. Extremely hard powders are slower and more difficult to press. Some organic binder is usually required to hold the hard particles together after pressing until the sintering process is performed

Compaction is an important step in powder processing as it enables the forming of loose metal powders into required shapes with sufficient strength to withstand till sintering is completed.
• In general, compaction is done without the application of heat. Loose powders are converted into required shape with sufficient strength to withstand ejection from the tools and subsequent sintering process. IN cases like cemented carbide, hot compaction is done followed by sintering. One cannot call this as compaction strictly, as sintering is also involved in this.
Powder compaction methods
Powder compaction techniques can be classified as,
1. Methods without application of pressure – i) loose powder sintering in mould, ii)vibratory compaction, iii) slip casting, iv) slurry casting, v) injection moulding
2. Methods with applied pressure – i) cold die compaction (single action pressing, double action pressing, floating die pressing), ii) isostatic pressing, iii) powder rolling, iv) powder extrusion, v) explosive compaction.

Pressureless compaction techniques
-Used for the production of simple and low density parts such as filters, other parts that are porous in nature; these techniques involve no external force and depend upon gravity for powder packing
I) Loose powder sintering: - Also known as loose powder shaping, gravity sintering, pressureless sintering. In this method, the metal powder is vibrated mechanically into the mould, which is the negative impression of the product and heated to sintering temperature. This is the simplest method and involve low cost equipment. The main reasons for not using this method for part production are, difficulty of part removal from the mould after sintering, & considerable shrinkage during sintering.
- Applications: Amount of porosity ranges from 40 vol% to 90 vol%; Highly porous filter materials made of bronze, stainless steel, and monel, porous nickel membrane for use as electrodes in alkaline storage batteries and fuel cells are typical examples.
II) Slip casting: - Used for compacting metal and ceramic powders to make large & complex shapes for limited production runs - A slip is a suspension of metal or ceramic powder (finer than 5 m) in water or other soluble liquid which is poured into a mould, dried and further sintered.

•Steps in slip casting: i) Preparing assembled plaster mould, ii) filling the mould, iii)absorption of water from the slip into the porous mould, iv) removal of part from the mould,v) trimming of finished parts from the mould • Sometimes mould release agents like oil, graphite can be used.
• Hollow and multiple parts can be produced
• Advantages of slip casting: Products that can not be produced by pressing operation can be made, no expensive equipment is required, works best with finest powder particles
• Disadvantage: slow process, limited commercial applications
• Applications: tubes, boats, crucibles, cones, turbine blades, rocket guidance fins; Also products with excellent surface finish like basins, water closets.
III) Slurry casting: This process is similar to slip casting except that a slurry of metal powders with suitable liquids, various additives, and binders is poured into a mould and dried. The solvent is removed either by absorption into the POP or by evaporation. Very high porous sheet for use as electrodes in fuel cells and nickel cadmium rechargeable batteries are produced by this method.
IV) Tape casting (doctor blade casting): - This is a variation of slurry casting process and is used to produce thin flat sheets.
-This process involves preparing a dispersion of metal or ceramic powder in a suitable solvent with the addition of dispersion agent (to improve the dispersion of the particles). Then a binder is added and fed to a reservoir. Whole mixture is fed on to a moving carrier film from the bottom of the reservoir.
-This slurry layer is deposited on the film by the shearing action of a blade. The slurry should be free of air bubbles, otherwise result in porosity. During sintering, the binder is burnt off first and densification of material occurs.
- In present days, endless stainless steel belt is used instead of carrier tape. This process can be used for making very thin tapes between 50 to 1000 m thickness. This method is used for making electronic substrates, dielectrics for capacitors and piezoelectric actuators.

V) Vibratory compaction: - Vibratory compaction uses vibration energy to compact the powder mass. During this process, smaller voids can be filled with particles of still smaller size and this sequence is carried out till a high packing density of powder is achieved even before consolidation. Mechanical vibration facilitates the formation of nearly closed packed powder by settling particles in the voids present in the powder agglomerate. During vibration, small pneumatic pressure is usually superimposed on the powder mass.
- Brittle powders can be compacted by this method as they develop crack if done by pressure compaction - This method is generally used when, 1) powders have irregular shape, 2) use of plasticizers for forming is not desirable, 3) sintered density is required to be very close theoretical density
- Important variables in vibratory compaction:
1. Inertia of system: larger the system, more the energy required for packing
2. Friction force between particles: more friction results in need of more KE for compaction
3. Particle size distribution: more frequency required if more large particles are present. Vibration cycle is important and not period of vibration.

Pressure compaction techniques
• These techniques involve application of external pressure to compact the loose powder particles; Pressure applied can be unidirectional, bidirectional or hydrostatic in nature.<
> • Die compaction: In this process, loose powder is shaped in a die using a mechanical or hydraulic press giving rise to densification. The mechanisms of densification depend on the material and structural characteristics of powder particles.
• Unidirectional and bidirectional compaction involves same number of stages and are described in this figure. They are, i) charging the powder mix, ii) applying load using a punch (uni-) or double punch (bi-) to compact powders, iii) removal of load by retracting the punch, iv) ejection of green compact. The table gives compaction pressure ranges for metals and ceramics.
Effect of powder characteristics
For a good compaction, 1) irregular shaped particles are preferred as they give better interlocking and hence high green strength,
2) apparent density of powders decides the die fill during compaction. Hence powder size, shape & density affect the apparent density,
3)flow rate affects the die fill time, and once again powder size, shape & density affect theflow rate.
Powder behavior during compaction
- Compaction involves
, 1) flow of powder particles past one another interacting with each other and with die-punch,
2) deformation of particles. In the case of homogeneous compaction, two stages are observed. First stage => rapid densification occurs when pressure is applied due to particle movement and rearrangement resulting in improved packing; Second stage => increase in applied pressure leads to elastic and plastic deformation resulting in locking and cold welding of particles. In the second stage, large increments in pressures are seen to effect a small increase in density.

• The green compact produced can be considered as a two-phase aggregate consisting of powder particles and porosity each having own shape and size.
• Compaction can be done at low and high temperatures. Room temperature compaction employs pressures in the range of 100-700 MPa and produce density in the range of 60- 90% of the theoretical density. At higher temperatures, pressures are kept low within the limits for preventing die damage.
• In single die compaction, powders close to the punch and die walls experience much better force than in center. This results in green density variation across the sample length. Longer the sample more the density difference. This non-uniformity can result in non-uniformity in properties of sintered part.
• This density variation and hence final property variation can be greatly reduced by having double ended die compaction. In this case, powder experiences more uniform pressure from both top and bottom, resulting in minimization of density variation. But this variation will still be considerable if the components have high aspect ratio (length to diameter ratio). This means that long rods and tubes cannot be produced by die compaction. In this case, isostatic pressing can be used.

Powder Blending

Powder Blending
The Second Stepin Powder Metallurgy process is Called Powder Blending A single powder may not fulfil all the requisite properties and hence, powders of different materials with wide range of mechanical properties are blended to form a final part. Blending is carried out for several purposes as follows. 1. Blending imparts uniformity in the shapes of the powder particles,
2. Blending facilitates mixing of different powder particles to impart wide ranging physical and mechanical properties, 3. Lubricants can be added during the blending process to improve the flow characteristics of the powder particles reducing friction between particles and dies,
4. Binders can be added to the mixture of the powder particles to enhance the green strength during the powder compaction process.

Powder Manufacture in P/M - Atomization & other Techniques

Powder Manufacture
The manufacturing of the material powder is the first step in powder metallurgy processing route that It involves making, characterising, and treating the powder which have a strong influence on the quality of the end product. Different techniques of powder making are: Atomising 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.

Atomization This uses high pressure fluid jets to break up a molten metal stream into very fine droplets, which then solidify into fine particles .High quality powders of Al, brass, iron, stainless steel, tool steel, superalloys are produced commercially Types: water atomization, gas atomization, soluble gas or vacuum atomization, centrifugal atomization, rotating disk atomization, ultrarapid solidification process, ultrasonic atomization

Mechanism of atomization:

In conventional (gas or water) atomization, a liquid metal is produced by pouring molten metal through a tundish with a nozzle at its base. The stream of liquid is then broken into droplets by the impingement of high pressure gas or water. This disintegration of liquid stream is shown in fig. This has five stages
i) Formation of wavy surface of the liquid due to small disturbances
ii) Wave fragmentation and ligament formation
iii) Disintegration of ligament into fine droplets
iv) Further breakdown of fragments into fine particles
v) Collision and coalescence of particles.

The interaction between jets and liquid metal stream begins with the creation of small disturbances at liquid surfaces, which grow into shearing forces that fragment the liquid into ligaments. The broken ligaments are further made to fine particles because of high energy in impacting jet.
• Lower surface tension of molten metal, high cooling rate => formation of irregular surface => like in water atomization
• High surface tension, low cooling rates => spherical shape formation => like in inert gas atomization
• The liquid metal stream velocity, v = A [2g (Pi – Pg)]0.5 where Pi – injection pressure of the liquid, Pg – pressure of atomizing medium, – density of the liquid
Types of atomization
Atomization of molten metal can be done in different ways depending upon the factors like economy and required powder characteristics. At present, water or gas atomizing medium can be used to disintegrate a molten metal stream. The various types of atomization techniques used are,
1. 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.
2. Gas atomization: Here instead of water, 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.
3. 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.
4. Centrifugal atomization: In this method, 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. Titanium powder can be made using this technique
5. Rotating disk atomization: Impinging of a stream of molten metal on to the surface of rapidly spinning disk. This causes mechanical atomization of metal stream and causes the droplets to be thrown off the edges of the disk. The particles are spherical in shape and their size decreases with increasing disk speed.
6. Ultra rapid solidification processes: A solidification rate of 1000C/s is achieved in this process. This results in enhanced chemical homogeneity, formation of metastable crystalline phases, amorphous materials.

Atomization Unit

Melting and superheating facility: Standard melting furnaces can be used for producing the liquid metal. This is usually done by air melting, inert gas or vacuum induction melting. Complex alloys that are susceptible to contamination are melted in vacuum induction furnaces. The metal is transferred to a tundish, which serves as reservoir for molten metal.
Atomization chamber: An atomization nozzle system is necessary. The nozzle that controls the size and shape of the metal stream if fixed at the bottom of the atomizing chamber. In order to avoid oxidation of powders, the tank is purged with inert gas like nitrogen.

Powder collection tank:

The powders are collected in tank. It could be dry collection or wet collection. In dry collection, the powder particles solidify before reaching the bottom of the tank. In wet collection, powder particles collected in the bottom of the water tank. The tank has to be cooled extremely if used for large scale production.
During operation, the atomization unit is kept evacuated to 10-3 mm of Hg, tested for leak and filled with argon gas.

Atomizing nozzles

• Function is to control the flow and the pattern of atomizing medium to provide for efficient disintegration of powders
• For a given nozzle design, the average particle size is controlled by the pressure of the atomizing medium and also by the apex angle between the axes of the gas jets
• Higher apex angle lead to smaller particle size
• Apex angle for water atomization is smaller than for gas atomization
• Nozzle design: i) annular type, ii) discrete jet type;
i) free falling, ii) confined design
• In free falling, molten metal comes in contact with atomizing medium after some distance. Here free falling of metal is seen. This is mainly used in water atomization.
• In confined design used with annular nozzle, atomization occurs at the exit of the nozzle. Gas atomization is used generally for this. This has higher efficiency than free falling type. One has to be cautious that “freeze up” of metal in the nozzle has to be avoided.

Atomizing mediums
• The selection of the atomizing jet medium is based mainly on the reactivity of the metal and the cost of the medium
• Air and water are inexpensive, but are reactive in nature
• Inert gases like Ni, Ar, He can be used but are expensive and hence have to be recycled
• Pumping of cold gas along with the atomizing jet => this will increase the solidification rate
• recently, synthetic oils are used instead of gas or water => this yields high cooling rate & lower oxygen content compared to water atomized powders
• Oil atomization is suitable for high carbon steel, high speed steels, bearing steals, steel containing high quantities of carbide forming elements like Cr, Molybdenum
• This method is not good for powders of low carbon steels.

Important atomization processes
Inert gas atomization
- Production of high grade metal powders with spherical shape, high bulk density, flowability along with low oxygen content and high purity
- Eg. Ni based super alloys
- Controlling parameters: (1) viscosity, surface tension, temperature, flow rate of molten metal; (2) flow rate, velocity, viscosity of atomizing medium; (3) jet angle, jet distance of the atomizing system; (4) nature of quenching media
Water atomization
- Water jet is used instead of inert gas
- Fit for high volume and low cost production
- Powders of average size from 150 micron to 400 micron; cooling rates from 103 to 105 K/s. Rapid extraction of heat results in irregular particle shape => less time to spheroidize in comparison to gas atomization
- Water pressure of 70 MPa for fine powders in 10 micron range
- important parameters:
1. Water pressure: Increase water pressure => size decrease => increased impact
2. water jet thickness: increase thickness => finer particles => volume of atomizing medium increases
3. Angle of water impingement with molten metal & distance of jet travel

Atomization process parameters
1. Effect of pressure of metal head: r = a + b√h; r – rate of atomization
2. Effect of atomizing medium pressure: r = a√p + b; Increase in air pressure increases the fineness of powder up to a limit, after which no increase is seen
3. Molten metal temperature: As temperature increases, both surface tension and viscosity decrease; so available energy can efficiently disintegrate the metal stream producing fine powders than at lower temperature; Temperature effect on particle shape is dependent on particle temperature at the instant of formation and time interval between formation of the particle and its Solidification; Temperature increase will reduce surface tension and hence formation of spherical particle is minimal; however spherical particles can still be formed if the disintegrated particles remain as liquid for longer times.
4. Orifice area: negligible effect
5. Molten metal properties:
Iron and Cu powder => fine spherical size; Pb, Sn => irregular shape powder;
Al powders => irregular shape even at high surface tension (oxidation effect)
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. 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. 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 – 3000 Stamp and Ball mills C yielding powder of high purity, however, at higher cost. These are mechanical methods which produce a relatively coarse powder. Ball mill is employed for brittle materials whereas stamps are used for ductile material. 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. 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.

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
Some other Techniques for Powder Manufactures are 1. 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. Shear refers to cutting of particles resulting in fracture. The particles are broken into fine particles by squeezing action in compression force type. Main objective of milling: Particle size reduction (main purpose), shape change, agglomeration (joining of particles together), solid state alloying, mechanical or solid state mixing, modification of material properties. Mechanism of milling: Changes in the morphology of powder particles during milling results in the following events. 1. Microforging, 2. Fracture, 3. Agglomeration, 4. Deagglomeration Micro forging => Individual particles or group of particles are impacted repeatedly so that they flatten with very less change in mass Fracture => Individual particles deform and cracks initiate and propagate resulting in fracture Agglomeration => Mechanical interlocking due to atomic bonding or vander Waals forces Deagglomeration => Breaking of agglomerates The different powder characteristics influenced by milling are shape, size, texture, particle size distribution, crystalline size, chemical composition, hardness, density, flow ability, compressibility, sinterability, sintered density Milling equipment: The equipments are generally classified as crushers & mills Crushing => for making ceramic materials, oxides of metals; Grinding => for reactive metals such as titanium, zirconium, niobium, tantalum

Vibratory ball mill • Finer powder particles need longer periods for grinding • In this case, vibratory ball mill is better => here high amount of energy is imparted to the particles and milling is accelerated by vibrating the container •During operation, 80% of the container is filled with grinding bodies and the starting material. Here vibratory motion is obtained by an eccentric shaft that is mounted on a frame inside the mill. The rotation of eccentric shaft causes the drum of the vibrating mill to oscillate. • In general, vibration frequency is equal to 1500 to 3000 oscillations/min. The amplitude of oscillations is 2 to 3 mm. The grinding bodies are made of steel or carbide balls, that are 10-20 mm in diameter. The mass of the balls is 8-10 times the charged particles. Final particle size is of the order of 5-100 microns Attrition mill: IN this case, the charge is ground to fine size by the action of a vertical shaft with side arms attached to it. The ball to charge ratio may be 5:1, 10:1, 15:1. This method is more efficient in achieving fine particle size. Rod mills: Horizontal rods are used instead of balls to grind. Granularity of the discharge material is 40-100 mm. The mill speed varies from 12 to 30 rpm. Planetary mill: High energy mill widely used for producing metal, alloy, and composite powders. Fluid energy grinding or Jet milling: The basic principle of fluid energy mill is to induce particles to collide against each other at high velocity, causing them to fracture into fine particles • Multiple collisions enhance the reduction process and therefore, multiple jet arrangements are normally incorporated in the mill design. The fluid used is either air about 0.7 MPa or stream at 2 MPa. In the case of volatile materials, protective atmosphere of nitrogen and carbon-di-oxide is used. • The pressurized fluid is introduced into the grinding zone through specially designed nozzles which convert the applied pressure to kinetic energy. Also materials to be powdered are introduced simultaneously into the turbulent zone. • The velocity of fluid coming out from the nozzles is directly proportional to the square root of the absolute temperature of the fluid entering the nozzle. Hence it is preferable to raise the temperature of fluid to the maximum possible level without affecting the feed material. • If further powdering is required, large size particles are separated from the rest centrifugal forces and re-circulated into the turbulent zone for size reduction. Fine particles are taken to the exit by viscous drag of the exhaust gases to be carried away for collection. • This Jet milling process can create powders of average particle size less than 5 m Machining: Mg, Be, Ag, solder, dental alloy are specifically made by machining; Turning and chips thus formed during machining are subsequently crushed or ground into powders Shotting: Fine stream of molten metal is poured through a vibratory screen into air or protective gas medium. When the molten metals falls through screen, it disintegrates and solidifies as spherical particles. These particles get oxidized. The particles thus obtained depends on pore size of screen, temperature, gas used, frequency of vibration. Metal produced by the method are Cu, Brass, Al, Zn, Sn, Pb, Ni. (this method is like making Boondhi) Graining: Same as shotting except that the falling material through sieve is collected in water; Powders of cadmium, Bismuth, antimony are produced. 2. Physical methods Electrolytic deposition • 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 • Iron powder => anode is low carbon steel; cathode is stainless steel. The iron powder deposits are subsequently pulverized by milling in hammer mill. The milled powders are annealed in hydrogen atmosphere to make them soft • Mg powder => electrodeposition from a purified magnesium sulphate electrolyte using insoluble lead anodes and stainless steel cathodes • Powders of thorium, tantalum, vanadium => fused salt electrolysis is carried out at a temperature below melting point of the metal. Here deposition will occur in the form of small crystals with dendritic shape. In this method, final deposition occurs in three ways, 1. A hard brittle layer of pure metal which is subsequently milled to obtain powder (eg. iron powder) 2. A soft, spongy substance which is loosely adherent and easily removed by scrubbing 3. A direct powder deposit from the electrolyte that collects at the bottom of the cell Factors promoting powder deposits are, high current density, low metal concentration, pH of the bath, low temperature, high viscosity, circulation of electrolyte to avoid of convection 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.

Powder Metallurgy - Post 2

Characteristics of Metal Powder
1.Particle Shape : shapes may be special nodular , irregular , angular & dendritic. It influence the flow characteristics of powder
. 2.Particle Size : Influence the control of porosity , compressibility & amount of shrinkage . It is determined by passing the powder through the sieves or microscopic measurement .

3.Particle Size Distribution : Specified in term of sieve analysis , the amount of powder passing through 100, 200 etc mess sieves.
4. Flow rate : ability of powder to flow readily & conform to the mould cavity .
5. Compressibility : Defined as volume of initial powder to the volume of compact part.
6.Apparent Density : Depends on the particle size & is defined as the ratio of volume to weight of loosely filled mixture.
7. Purity : Impurities reduce the life of dies & affect the sintering process. Oxides & gaseous impurities can be removed from part during sintering .
BASIC Powder Metallurgy Steps

PRODUCTION OF POWDERS
• Metal powders => 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
• Single powder production method is not sufficient for all applications
Powder production methods
: 1. Mechanical methods, 2. Physical methods, 3. Chemical Methods 1. Mechanical methods
=> cheapest of the powder production methods; These methods involve using mechanical forces such as compressive forces, shear or impact to facilitate particle size reduction of bulk materials; Eg.:Milling 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. Shear refers to cutting of particles resulting in fracture. The particles are broken into fine particles by squeezing action in compression force type. Main objective of milling
: Particle size reduction (main purpose), shape change, agglomeration (joining of particles together), solid state alloying, mechanical or solid state mixing, modification of material properties.