Friday, 7 October 2016

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.