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    7.7 PLASMA ARC WELDING

    Plasma arc welding is an arc welding process where in coalescence is produced by the heat

    obtained from a plasma arc. Practically all welding arcs arc plasma, but the one used in plasma

    arc welding is a constrictive arc, which considerably reduces the profile o f t h e plasma column

    and thus, increases the forces acting along the axis of the arc. Plasma arc is two types i.e.,

    transferred arc and nori-transferred arc. Transferred arc is produced between tungsten electrode

    and the work piece, where as non-transferred arc is produced between the electrode and the

    constrictive nozzle. Arrangement of plasma ar c welding process is shown in Fig. 7.11.

    Plasma arc is the temporary state of a gas. The gas gets ionized after passage of electric current

    through it and becomes conductor of electricity. In ionized stale the gas atoms beak in to

    electrons and ions and system contains a mixture of ions, electrons and highly excited atoms.

    The energy of the plasma jet and thus the temperature is dependent on the electrical power em

    ployed to create plasma arc. The range of temperature in plasma arc welding is 20,000C-

    25JBU0PC.

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    In this process, pressure is not employed and f i l l e r metal may or may not be used. The

    process employs two inert gases, one forms the plasma arc and second one shields the plasma

    arc. This process requires d.c. power source, high frequency generator and plasma torch.

    Voltage used in this process is 27V-31V, current range is 50A-350A and the flow rate of the

    gas is 2-40 Ib/lir. For welding stainless steel Thoariated or Zirconialed tungsten electrode and

    for aluminum copper electrode is used.

    Many of the base metals such as stainless steels, carbon steels, low alloy steels, copper alloys,

    aluminum alloys, titanium alloys, nickel and cobalt based alloys can be welded by this process.

    Applications

    1. Circumferential pipe welding

    2. Used in welding cryogenic, aerospace and corrosion resistant alloys3. Used in welding rocket motor cases, nuclear submarine pipe systems.

    Advantages

    1. Plasma arc is stable and provides uniform penetration in to the work piece.\

    2. Good weld quality is obtained with simple welding fixtures.

    3. High quality welds can be produced at high speed.

    Limitations

    1. High noise is produced and special protections are required from infra-red and

    ultraviolet radiations.

    2. Skilled worker is required to operate plasma arc welding equipment.

    3. Inert gas consumption is more.

    4. Electrical hazards are more in th is process.

    Explosive welding (EXW)

    Explosion welding (EXW) is a metal-working process that is sometimes also

    called explosivebonding or explosive cladding. Considered a solid state process, EXW allowstwo different types of metals to be forced together with a clean vacuum-tight weld. This is done

    without heating either type of metal to its melting point or jeopardizing the original properties of

    either. This is achieved when the velocity from controlled detonations is used to produce an

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    atomic bond that is often impossible if other welding processes were used. Historically,

    thisprocess has not been among the most widely used, but is often considered a specialty.

    EXW is a welding process that was introduced in the latter half of the 20th century. The idea ofexplosion welding is said to be inspired by events in World War I. The process is believed to

    have arisen from observations about the manner in which shrapnel managed to weld itself to

    soldiers armor.Explosion welding is considered a solid state process because two different metals can be joined

    without either reaching its melting point. One of the major benefits of EXW is that it can be used

    to weld almost any pair of metals and most alloys. This possibility is significant because many

    pairs of metals or alloys are considered incompatible if welding is attempted using otherprocesses.

    The weld from the EXW process is achieved by using the energy generated from controlledexplosions. The force from those explosions causes the outer layers of each metal surface to take

    a plasma-like form that allows for coalescence. Although neither item reaches its melting point,the surfaces may appear molten, and some amount of heat is commonly generated. The heat is

    produced partly due to the impact from the collision of the two surfaces.

    There are several notable benefits to explosion welding. First, the metals do not lose their

    original individual properties. Second, the joint that results from this explosive process tends tobe exceptionally clean and vacuum tight. Third, the process is carried out very quickly and can

    be used on large surfaces.

    In the decades following its introduction, explosion welding has generally been considered more

    of a specialized process. The use of this method is considered to be minimal when compared to

    its potential. This may be attributed in part to the fact that the process requires an extensiveknowledge of explosives, which many metal workers do not have.

    Advantages and disadvantages

    Explosion welding can produce a bond between two metals that cannot necessarily be welded by

    conventional means. The process does not melt either metal, instead it plasticizes the surfaces of

    both metals, causing them to come into intimate contact sufficient to create a weld. This is a

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    similar principle to other non-fusion welding techniques, such as friction welding. Large areas

    can be bonded extremely quickly and the weld itself is very clean, due to the fact that the surface

    material of both metals is violently expelled during the reaction.

    A major disadvantage of this method is that an expansive knowledge of explosives is needed

    before the procedure may be attempted. Explosion welding is therefore far less commonly used

    than fusion welding alternatives.

    Ultrasonic welding process: When bonding material through ultrasonic welding, the energy

    required comes in the form of mechanical vibrations. The welding tool (sonotrode) couples to thepart to be welded and moves it in longitudinal direction. The part to be welded on remains static.

    Now the parts to be bonded are simultaneously pressed together. The simultaneous action of

    static and dynamic forces causes a fusion of the parts without having to use additional material.

    This procedure is used on an industrial scale for linking both plastics and metals (figure 1).

    Ultrasonic welding of plastics When welding thermoplastics, the thermal rise in the bonding

    area is produced by the absorption of mechanical vibrations, the reflection of the vibrations in theconnecting area, and the friction of the surfaces of the parts. The ultrasonic vibrations are

    introduced vertically and are usually subjected at a frequency of 20, 30 or 40kHz.In the

    contraction area, frictional heat is produced so that material plasticizes locally, forging aninsoluble connection between both parts within a very short period of time. The prerequisite is

    that both working pieces have a near equivalent melting point. The joint quality is very uniform

    because the energy transfer and the released internal heat remains constant and is limited to the

    joining area. In order to obtain an optimum result, the joining areas are prepared to make themsuitable for ultrasonic bonding. Besides plastics welding, ultrasonics can also be used to rivet

    working parts or embed metal parts into plastic.

    Equipment

    An ultrasonic welding machine consists of four main components: a power supply, a converter,

    an amplitude modifying device (commonly called a Booster) and an acoustic tool known as the

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    horn (or sonotrode). The power supply changes mains electricity at a frequency of 50-60Hz, into

    a high frequency electrical supply operating at 20, 30 or 40kHz. This electrical energy is

    supplied to the converter. Within the converter, discs of piezoelectric material are sandwichedbetween two metal sections. The converter changes the electrical energy into mechanical

    vibratory energy at ultrasonic frequencies. The vibratory energy is then transmitted through the

    booster, which increases the amplitude of the sound wave. The sound waves are then transmittedto the horn. The horn is an acoustic tool that transfers the vibratory energy directly to the partsbeing assembled, and it also applies a welding pressure. The vibrations are transmitted through

    the workpiece to the joint area. Here the vibratory energy is converted to heat through friction -

    this then softens or melts the thermoplastic, and joins the parts together. Benefits of the process

    include: energy efficiency, high productivity with low costs and ease of automated assembly line

    production. The main limitation of the process is that the maximum component length that can

    be welded by a single horn is approximately 250 mm. This is due to limitations in the power

    output capability of a single transducer, the inability of the horns to transmit very high power,and amplitude control difficulties due to the fact that joints of this length are comparable to the

    wavelength of the ultrasound.

    Typical applications: Ultrasonic assembly is the method of choice for many applications in the

    automotive, appliance, medical, textile, packaging, toy and electronics markets, among others.

    The basic advantages of ultrasonic assembly - fast, strong, clean and reliable welds - arecommon to all markets. However, each market has specialised needs that they rely on ultrasonic

    assembly to meet.

    Stud welding

    Stud welding is a form ofspot welding where a bolt or specially formed nut is welded ontoanother metal part. The bolts may be automatically fed into the spot welder. Weld nuts generally

    have a flange with small nubs that melt to form the weld. Studs have a necked down, un-threadedarea for the same purpose. Weld studs are used in stud welding systems.

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    Capacitor Discharge Stud Welding: The Process

    Capacitor Discharge (CD) stud welding is an extremely efficient method of welding fasteners toa wide variety of metals like:

    mild steel

    stainless steel

    aluminum

    brass

    copper

    titanium

    The process utilizes a powerful bank of capacitors to store energy at a specific voltage

    determined by stud size and material. When a weld is initiated, this energy is "discharged"through a special "ignition tip" at the base of the stud, creating an instantaneous arc which melts

    both the base of the stud and the adjoining surface on the work piece. At the same time, the

    welding gun forces the stud into the work piece, resulting in a permanent bond as the moltenmaterial solidifies........all in 0.004 seconds!

    Capacitor Discharge stud welding eliminates drilling, tapping, punching, riveting, gluing, andscrewing; and is especially beneficial when working with thin gauge materials due to the absence

    of reverse-side marring or discoloration. This process is suitable for studs ranging in size from

    #4-40 (M3) thru 3/8-16 (M10).

    Drawn Arc Stud Welding: The Process

    Drawn-arc stud welding is an extremely efficient method of attaching fasteners primarily to mildsteel and stainless steel by utilizing a constant-current DC power supply, typically a 3-phase

    transformer-rectifier, equipped with integral controls to operate a special drawn-arc stud welding

    gun. When a weld is initiated, current begins to flow through the stud while the weld gunsimultaneously lifts the stud to "draw an arc", which melts the base of the stud and adjoining

    surface on the work piece. Upon completion of the weld time, the gun plunges the stud back to

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    the work piece, resulting in a permanent bond as the molten material solidifies...all in less than

    one second.

    In general, the drawn-arc process is capable of welding a broad range of stud diameters (up thru

    1-" or 32 mm) and to almost any material thickness above 0.040" (1 mm). To accommodate

    such a broad range, the process is actually split into two categories: Standard Drawn-Arc andShort-Cycle.

    Standard drawn-arc is used with studs " (6 mm) diameter and larger, and base materialthicknesses at least 1/3 the stud diameter. This process requires the use of flux-loaded studs and

    ceramic ferrules, included with the studs, to contain the molten material during the weld and

    form a fillet around the stud base.

    Short-cycle is an abbreviated version of drawn-arc for studs 3/8" (10 mm) and smaller, especially

    when welding to base materials as thin as 1/5 the stud diameter. With this method, the weldcurrent is usually much higher and weld time much shorter (short-cycle), resulting in lighter

    penetration and less molten material, thereby eliminating the need for ceramic ferrules and flux-

    loaded studs, though shielding gas is often recommended.

    Friction Welding

    Friction welding (FW) is a class of solid-state joining processesGenerates heat through mechanical friction between a moving work piece and a stationary

    component, with the addition of a lateral force called "thrust load" to plastically displace and

    fuse the materials.When sufficient energy input has occurred (length loss), the rotation is stopped and thrust load

    increased, to forge the parts together and form a solid state bond.

    Friction welding is used with metals and thermoplastics in a wide variety of aviation and

    automotive applications

    Spin Welding Linear Welding Friction Surfacing Friction Stir Welding

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    Spin Welding

    Spin Welding uses rotational motionand high axial pressures (thrust load) to

    convert rotational energy into frictional

    heat at a circular interface. The heat produced by this rubbing

    action raises the inter-surfacetemperature of the two parts to the

    plastic state where the high thrust load

    extrudes metal from the weld region to

    form an upset.

    When sufficient energy input hasoccurred (length loss), the rotation is

    stopped and thrust load increased, to

    forge the parts together and form a

    solid state bond. Rotary friction welding is very energy

    efficient compared to most competitivewelding processes

    Linear Welding

    Linear friction welding(LFW) is similar to spin welding except that the moving chuckoscillates laterally instead of spinning.

    The speeds are much lower in general, which requires the pieces to be kept underpressure at all times.

    This also requires the parts to have a high shear strength. Linear friction welding requires more complex machinery than spin welding, but has the

    advantage that parts of any shape can be joined, as opposed to parts with a circularmeeting point.

    Friction Stir Welding

    A constantly rotated cylindrical-shouldered tool with a profiled nib is transversely fed ata constant rate into a butt joint between two clamped pieces of material.

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    The nib is slightly shorter than the weld depth required, with the tool shoulder riding atopthe work surface.

    Frictional heat is generated between the wear-resistant welding components and the workpieces. This heat, along with that generated by the mechanical mixing process and

    the adiabatic heat within the material, cause the stirred materials to soften

    without melting. As the pin is moved forward, a special profile on its leading face forces plasticised

    material to the rear where clamping force assists in a forged consolidation of the weld.

    Advantages

    The combination of fast joining times, and direct heat input at the weld interface, yieldsrelatively small heat-affected zones.

    Friction welding techniques are generally melt-free, which avoids grain growth inengineered materials.

    Clean weld surface. Allows dissimilar materials to be joined. Friction welding can be used with thermoplastics. Friction welds often cost less

    UNDER WATER WELDING

    Underwater welding provides a means to assemble or repair underwater. This is a highly useful

    technology available that allows repairs of ships damaged during hurricanes or wars. There are a

    couple of alternatives available, which include clamped and grouted repairs and bolted flanges.

    However, these alternatives do not always provide satisfactory results and also introduces high

    loading at offshore structures. This is a highly specialized trade and most people are employed inthe oil or shipping industry and the military.

    Underwater welding process can be classified into the following two categories:

    Wet Welding

    Dry Welding

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    In the case of Wet Welding, the welding is performed underwater, directly exposed to the wet

    environment. A special electrode is used and welding is carried out manually as in open air

    welding. As freedom of movement is permissible, wet welding is most effective, efficient and

    economical. The welding power and supply is located on the surface and is connected to the

    welder using cables or hoses.

    The advantages of wet welding are:

    The cost is very low especially when compared to dry welding.

    The speed with which the task can be carried out is very high.

    As the equipment required are very less, the welding can be carried out immediately with

    minimal planning.

    The disadvantages of wet welding are:

    The weld is quenched rapidly in the water. Although, this is good in a way as quenching

    increases the tensile strength of the weld, it decreases the ductility and impact strength and also

    makes the weld very porous.

    The visibility of the welder is poor.

    The amount of voltage that can be used is very limited and a lot of precaution has to be taken to

    ensure that the welder does not receive electric shocks.

    Dry Welding

    In the case of Dry Welding, also known as Hyperbaric welding, it is carried out in a chamber

    sealed around the structure to be welded. The chamber is filled with a gas, normally helium

    containing 0.5 bar oxygen. The chamber is fitted on to the pipeline and filled with the breathable

    mixture of air described above and the pressure is maintained slightly above the pressure at

    which welding is to take place. The gas tungsten arc welding process is used.

    The advantages of dry welding are:

    Welding can be performed immune to ocean currents and marine animals.

    The welds are of a better quality when compared to the welds of wet welding as water does not

    quench the welds.

    Joint preparation and pipe alignment can be monitored visually from the surface.

    The disadvantages of dry welding are:Large support equipment is required at the surface to support the chamber which by itself is very

    complex.

    The cost involved in dry welding is high and the cost increases with increase in depth where the

    welding is to be carried out.

    The reusability of the chamber is very limited.

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    Risks and Precautions

    Both the welder and the structures being welded are at risk when underwater welding is used.

    The welder has to be very careful so as to avoid receiving an electric shock. For this, adequate

    precaution must be taken to insulate the welder and limiting the voltage of welding sets.

    Secondly, pockets of hydrogen and oxygen are built up by the arc and are potentially explosive.

    Lastly, the welder has to keep in mind that nitrogen is built up in the bloodstream when exposed

    to air at high pressure under the water and hence must take adequate precaution.

    For the structures that are being welded underwater, although inspection is very difficult when

    compared to surface welding, it is a must. The weld must be inspected very carefully to confirm

    that no defects remain.

    Diffusion Bonding

    Diffusion welding (DFW) is a solid state weldingprocess by which two metals (which may be

    dissimilar) can be bonded together. Diffusion involves the migration ofatoms across the joint,

    due to concentration gradients. The two materials are pressed together at an elevated temperature

    usually between 50 and 70% of the melting point. The pressure is used to relieve the void that

    may occur due to the different surface topographies.

    Applications

    DFW is usually used on sheet metal structures. Typical materials that are welded

    include titanium, beryllium, and zirconium. It is usually used on low volume workpieces mainly

    foraerospace,nuclear, and electronics industries.

    In many military aircraft diffusion bonding will help to allow for the conservation of expensive

    strategic materials and the reduction of manufacturing costs. Some aircraft have over 100

    diffusion-bonded parts, including; fuselages, outboard and inboard actuator fittings, landing

    geartrunnions, and nacelle frames.

    Cold Welding (CW)

    Cold Welding is a Solid State Weldingprocess, in which two work pieces are joined together at

    room temperature and under a pressure, causing a substantial deformation of the welded parts

    and providing an intimate contact between the welded surfaces.

    As a result of the deformation, the oxide film covering the welded parts breaks up, and clean

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    metal surfaces reveal. Intimate contact between these pure surfaces provide a strong and

    defectless bonding.

    It uses mechanical force or pressure to bring two metallic surfaces in intimate contact.

    Then interatomic forces are developed to complete the weld, while considerable plasticdeformation is taking place.

    The need for metal distortion and flow to perform Cold welding, excludes that brittle materials

    could be joined by this process.

    However many combinations of ductile dissimilar metals can be joined by Cold-welding, eventhose that could never be joined by fusion welding.

    There is no fusion of either metal, so that welding performed cold is included in the class ofsolid

    state welding processes.

    To be successful cold welding must provide for the disruption of surface oxides if they arepresent.

    Only by reducing them to separate tiny particles, they do not interfere with the process.

    Shattering of the oxides during material flow is due to theirbrittle nature.

    Microscopic observations confirm that the interface becomes a phase boundaryand that oxides

    are not interfering with the soundness of the bond.

    Many variants of the basic Cold-welding process are known to have been implemented.

    Deformations can be made in lap or butt configuration and can involve press forming, drawing

    and extrusion.

    Roll Welding is a process variant, dealt with separately. (Click on the link to see the page).

    Lap Cold welding could be explained as a variant of resistance spot welding, where two

    overlapping sheets are joined at separate spots.

    Except that no current and no heating are involved, and there is no fusion.

    Instead considerable deformation is generated in the transversal direction to that of the appliedpressure.

    Aluminum alloys, Copper alloys, low carbon steels, Nickel alloys, and otherductile metals may

    be welded by Cold Welding.

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    Cold Welding is widely used for manufacturing bi-metal steel - aluminum alloy strips, for

    cladding of aluminum alloy strips by other aluminum alloys or pure aluminum (Corrosion

    protection coatings). Bi-metal strips are produced by Rolling technology. Presses are also used

    for ColdWelding.

    Cold Welding may be easily automated.

    3.6.2 Processes Based on Organic Binders

    Various types of organic binders have long been in use and have lately become very popular for

    the production of high quality castings of small to medium size. These processes were initially

    developed for making cores, but in spite of the high cost involved they have found use in mould

    making as well. This is because they produce castings of superior quality and can considerably

    shorten the production cycles. The processes using organic binders include:

    (1) shell moulding:

    (2) hot box moulding;

    (3) cold box moulding;

    (4) alkyd and phenolic no-bake processes;

    (5) furan process; and

    (6) gas-setting resin process.

    (2) Hot Box Moulding This process is a refinement of shell moulding. A resin similar to that

    used in shell moulding is applied for coating the sand grains. Then the resin-sand mix is blown

    over the heated pattern or core box. which in this case is not turned over to allow shell formation,

    but instead the bulk of sand in the mould is heated and allowed to form a solid mass. The cores

    or moulds obtained are relatively free from distortion or shrinkage and the accuracy of

    dimensions is greater than in the case of shell moulding.

    For small sizes, for which the method has been largely adopted, very high rates of

    production are achieved- The process has been applied particularly in core making. Today, there

    are special hot box machines where the sand mix is blown over the heated pattern, the blown

    sand cured, and then the mould or core stripped from the pattern or core box. Proprietary resins

    are available for hot box work for cast iron, steel, and non-ferrous metals. Those for cast iron

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    are usually the phenolic thermosetting type, modified with urea. For steel, S.G. iron, etc., zero

    nitrogen furan resin is excellent and for non-ferrous work, a straight furan type resin is found

    suitable. Hot box resins are covered by IS: 12424-1988.

    (3) Cold Box MouldingThis method is the latest development in moulding using organic resin

    binders and is now tending to replace even the hot box process. Its overwhelming advantage is

    that no heating of patterns is required, the curing being obtained simply by passing a gaseous

    catalyst through the blown mass of sand in the core box. The process is excellent for mass

    production as its production rate is much faster than that of the shell or the hot box method.

    Besides saving heat energy and its high productivity, this process gives lower gas evolution,

    higher

    dimensional accuracy and better surface finish to the casting, good collapsibility, better

    resistance to erosion and lower scrap rate. The resins for the cold box process are proprietary

    and are available under different brand names.

    The difficulty with the process is that special equipment, which is expensive, is required for

    blowing the catalyst gas through the core box. Further, the gas is toxic and poisonous and is

    dangerous if inhaled and, therefore, it must not be allowed to leak into the atmosphere. Still, the

    process is extremely suitable for small-sized castings required in big quantities. The total time to

    prepare a core could be well within 20-30 seconds.

    The cold box process for core making involves mixing fine dry sand with a polyisocyanate

    binder and an alkyd phenolic resin, blowing the vapour (TEA liquid atomised in air) through the

    core box. After the excess gas passes through the core box, it is introduced into a scrubbersolution where all of it is dissolved to prevent it from escaping into the air (Fig. 3.44). After

    gassing, air or C02 is passed through the system to remove unused gas and pump it out through

    the exhaust manifold. The cured core is then ready for ejection from the core box.

    The cores made by the cold box process do not require dressing, provided the base sand is of

    good quality. However, if dressing is absolutely necessary, a water-based type is desirable. It is

    applied after curing, and is torched thoroughly.

    The shell, hot box and cold box processes are essentially for mechanised production of small

    castings required in large quantities. In fact these processes are economical only when, they are

    used with an adequate degree ofmechanisation. There remain a large range of castings which,

    though not ordered in bulk, are required to

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    achieve a high degree of accuracy and finish at short notice with low fettling and machining

    costs. For fulfilling the need of such castings, no-bake, cold curing processes using liquid

    catalysts have been developed.

    (4) Alkyd No-Bake Sands With normal drying oils resins, which are rigid and brittle materials,

    are added to strengthen and harden the drying oil film. Resins are derivatives of petrochemicals.

    The Alkyd Resins, used in core oils, are formed by the reaction of phthalic anhydride with

    glycerol). The hydroxyl groups in the glycerol react with the acid groups in the anhydride to

    produce, long chains. These chains act to bond the aggregate further and will produce three

    dimensional structures. These resins can be modified by substituting fatty acid for some of the

    phthalic anhydride to get more plasticity. Solvents like turpentine, kerosene or mineral spirits are

    added to resin oil mixtures to improve flowability. Some alkyd resins are accelerated by

    isocyanate to harden into a solid sand mass more quickly. Cold Box process uses this type.

    As in the case of Air-hardening process, Alkyd no-bake system consists of Binder, Hardener

    and Catalyst

    The binder is drying oil containing some oil modified alkyd resins, which is of

    proprietary nature. Manufacturers can formulate different grades suiting to the customer's

    need.

    The hardener is cobalt- or lead-napthanate which is the reacted product between

    cobalt/lead salts and napthanic acid.

    The catalyst is isocyanate.

    In this process, curing of the binder takes place in two stages. Initially the isocyanate reacts

    with the reduced moisture in the sand and with hydroxyl groups of the oil modified alkyd resin to

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    give a bond, which is relatively weak, but strong enough to allow the stripping of the core from

    the pattern.

    The second stage of the curing involves reaction of the unsaturated drying oils with oxygen

    in the air. resulting in polymerization as in the core oil system. Metallic napthanates will also

    catalyse the oxygen cross-linking in combination with isocyanate.

    A 3-part Alkyd system for a particular recipe can be described as "2.5:5:20". (i) Part-A: The

    first figure is the proportion of Binder (i.e. Part-A) in percentage. It is based on the weight of

    sand.

    (ii) Part-B: The middle figure is the proportion of Accelerator or Hardner

    (i.e. Part-B) in percentage. It is based on the weight ofBinder (i.e. Part-A).

    (iii) Part-C: The last figure is the proportion of Catalyst or Cross Linking Agent (i.e. Part-C) inpercentage. It is also based on the weight ofBinder (i.e. Part-A). Such a recipe will work better

    and give best results when used in that proportion only. Any deviation from this will give poor

    results. In this regard the major role is played by part-C. Thus an alkyd designed for20%part-C

    (No-Bake), when used as Semi-Bake (8%- part-C) will give slower setting. If proportion of part-

    C is increased it will result in drastically reduced bench life and final strength will also be lower.

    Order of Addition of Ingredients While mixing Sand. Resin Binder Part-A and Accelerator

    Part-B should be premixed first and added to the sand, and then the addition of cross-linking

    Agent Part-C should be made. In this case, the initial strength development will be relativelyslow and steady. The final strength development will be good.

    But. if Binder Part-A and Cross-linking Agent Part-C are pre-mixed and added to the sand,

    followed by Accelerator Part-B. initial strength development will be faster, sacrificing the final

    strength. The reason could be due to:

    1. Alkyd is the binder for sand grains and cross-linking agent is the active hardener. When

    cross-linking agent is added first to the sand, the initial coating of the sand grains is done

    with the cross-linking agent. Bond strength of cross linking agent and sand is lower and

    so is final strength.2. Initial faster rate of reaction can be explained by the fact that, in this system mulling time

    after addition of cross-linking agent is more than with conventional mixing, resulting in

    initial high strength.

    Hence it is advisable to pre-mix Part-A and Part-B and add to the sand, followed by Part-C.

    After adding Part-C, i.e. cross-linking agent excess mulling should be avoided as it will lead to

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    friability and low final strength development. Optimum mixing/mulling time should be

    determined and followed.

    (5) Phenolic No-Bake SandsPhenolic no-bake binders were introduced around 1960. This 2-partsystem works well for both smaller and larger castings. Continuous mixers are used to get desired

    work/strip time and ihc production is faster. There is no need to bake the moulds or cores in the

    oven. The binder gives better hot strength.

    Phenolic no-bake are alkaline condensates ofPhenolandformaldehyde. The phenolics are

    also completely nitrogen free and are well suited for the production of steel and iron castings,

    where pin-hole avoidance is necessary. With the introduction of strong acid catalysts based on

    sulfonic acids, some of the problems associated with curing and deep set properties were

    overcome.

    (7) Graphite Mould Casting Graphite has been successfully used as a mould material for

    producing steel castings, particularly wheels for railway wagons and coaches. Graphite does not

    fuse with molten steel at high temperatures, has high resistance to burn-in allowing clean

    withdrawal of the casting from the mould, high resistance to thermal shock, low coefficient of

    expansion and ability to resist distortion. Thus, the mould once prepared from graphite blocks by

    machining can be used repetitively, though special measures, such as pressure pouring have to be

    adopted to prevent erosion of mould walls and to regulate the rate of entry of metal into the

    mould.

    (5) Vacuum Moulding Vacuum molding was developed a few years ago in Japan. Its main

    advantage is that it does away with the use of binders and moisture as sand ingredients. The mould

    is prepared with dry sand and the required compaction and the shape of mould cavity are obtained

    by using vacuum. The procedure used to prepare the mould is shown schematically in Fig. 3.48. As

    neither binder nor water is required in the mould, a clean environment is possible, fettlingproblems are eliminated, and no sand conditioning is necessary. Mould production is quite fast

    and moulding machines can produce 90 to 100 moulds per hour.

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    (A) Pattern set on hollow carrier plate.

    (B) Thin plastic sheet softened by heater.(C) Softened-sheet dropped over pattern and vacuum created in carrier plate.

    (D) Double-walled flask set on pattern, flask filled with dry sand and vibrated, sprue formed,

    mould levelled. Sprue opening and mould top covered with plastic sheet. Vacuum applied

    to flask. Sand gets compacted.

    (E) Vacuum in carrier plate released and mould stripped.

    (F) Cope and drag moulds assembled, having plastic-lined cavity. Vacuum in flasks main tained

    during pouring and later, till casting solidifies. On releasing vacuum, sand drops leaving

    clean casting.

    The process however requires special pattern plates and double walled flasks for effecting

    vacuum, efficient venting system in the inner face of flasks to prevent the sand particles from

    being sucked by the vacuum pump, a device of stretching and heating plastic sheet and a

    powerful vacuum pump. Sand grains must be carefully selected for successful operation. For

    maximum compaction, a two-screen sand (70% of 70 mesh size and 30% of 270 mesh size) is

    employed. A vibratory frequency of 3000 cycles per minutes is used for a few seconds to cause

    compaction.

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    Modified V process makes use of an EPS pattern, duly applied with a refractory coating. Loose

    dry sand grains are poured around the pattern and compacted by vibration followed by application

    of vacuum to achieve full consolidation of sand. Thus, vacuum in the process needs to be applied

    only during pouring and for a short time thereafter. While plain V process is suitable for simple-

    shaped castings needing minimum coring, modified process can be applied for complex-shapedcastings, as well.

    (3) Dicalcium Silicate or Fluid Sand Process Dicalcium silicate has been found to be a very

    effective hardening agent when used with sodium silicate as a binder. Unlike the Fe-Si process where

    hardening is due to exothermic reaction, the chemical reactions taking place in this process do not

    cause any evolution of heat. Synthetically produced dicalcium silicate can be used for the purpose.

    Slags from certain melting or reduction processes, such as basic-lined hot blast cupola, open-hearth

    furnace, and ferro-chrome production, contain an appreciable quantity of dicalcium silicate and are

    quite suitable. The rate of hardening depends on the grain fineness of the silicate and the temperature

    of the sand. The fineness of the silicate grains should not be less than 200 mesh. The higher the

    temperature, the quicker the reaction and the shorter the bench life.

    To prepare the mix, about 2-3% of dicalcium silicate and 5% sodium silicate arc mixed with sand,

    along with suitable foaming chemicals. The mix can flow easily in the mould, thus eliminating any

    need for ramming. Mixing time is from 3-5 minutes, and the bench life in temperate climate, as in

    this country, is not more than 25-30 minutes. The sodium silicate used in this process should have

    a high mass ratio (1:2.3 to 1:2.8) and specific gravity 1.48 to 1.50.

    The fluid sand process finds its main application in medium and heavy castings, both in grey iron

    and steel, such as ingot moulds, heavy machine tool castings, steel mill rolls, castings for cement

    and the mining industry, and pump castings. The

    main advantage of the process is the great saving in labour input and moulding equipment. No drying

    or baking facilities arc needed and high-quality defect-free castings are produced. The process has

    gained wide popularity in many European countries, the USSR, and Japan, and has been introduced

    in Indian foundries also.' In order to guide foundries to adopt the fluid sand process, test methods

    have been laid down in IS: 9674-1980. Tests are prescribed for flowability, bench life, bulk density,

    compressive strength, permeability, sagging tendency, collapsibility, hardener fineness, water-

    absorption capacity, hardener activity, foamability, and residual water content.

    High Pressure Moulding High pressure moulding uses a compacting process for the production

    of green sand moulds. The force of compaction required is much higher (about 5-10 times) than thatfor conventional moulding machine work. The squeeze force is usually applied hydraulically as this

    enables high production rates and noiseless operation. Since the process has been found to offer

    technical and economic advantages for both the producer and the consumer, special machines have

    been developed to achieve the necessary high compaction pressures. It is worthy of note that once

    the machines and related equipment arc installed, production can be carried on uninterruptedly with

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    the same moulding materials as are used in ordinary moulding work, abiding by green sand practice.

    Figure 3.38 shows the construction of a high pressure moulding machine.

    The process is highly flexible and is suitable for all types of cast metals, both ferrous and non-

    ferrous. Closer dimensional tolerances can be maintained and less machining allowances are required,

    causing casting to be lighter by 3-7%. But since the initial cost of the equipment is high and the sandcontrol requirements precise, high pressure moulding is considered worthwhile only for mass

    production work using close cycle operation. Flaskless moulds are generally used with high pressure

    moulding (Fig. 3.39). The moulding boxes, in which the moulds arc made, are not fixed but of the

    hinged type so that moulds can be removed from the box after they are compacted. To protect these

    moulds from damage or distortion, a sheet metal jacket is placed around them. The boxes used are

    snap flask moulding boxes and their design and specifications are covered by IS: 10518-1983. The

    use of flaskless moulds helps in considerably reducing the inventory of regular moulding boxes,

    reduced weights to be transported over conveyers, easier shake-out, saving of foundry space and

    reduced transportation.

    The range of squeeze pressures used in conventional jolt squeeze moulding machines is from 1.5

    kg/cm2

    to 5 kg/cm2. In high pressure moulding, however, pressures vary from 7 kg/cm

    2to 25 kg/cm

    2

    and may be as high as 40 kg/cm2. Contrary to the common belief that mould hardness increases in

    proportion with the increase in squeeze pressure, in practice it is seen that hardness increases at a

    steep rate with pressure only up to about 4.6 kg/cm2. Any further exertion of squeeze pressure causes

    negligible rise in mould hardness. Properly controlled sand composition, which is a prerequisite,

    enables just adequate permeability for the gases to escape, coupled with sufficient strength, hardness,

    and other characteristics. The moisture content used in sand mix is usually not more than 2.5%.

    Comparatively higher clay content is used with suitable additives such-as dextrin so as to control the

    spring-back tendency of sand and prevent mould distortion.

    For uniform composition of sand all over the pattern, especially when the latter is of varying height,

    a contoured squeeze head is used. A self-contoured squeeze head, which can automatically adjust

    itself, is found ideal for overconvng the difficulty of preparing a squeeze head of a fixed contour for

    each pattern. The head consists of a large number of squeeze feet, each with its own piston and

    hydraulic cylinder. Each cylinder is hydraulically connected through one or more manifolds. During

    the squeeze operation, the pistons are fully extended and apply pressure on the sand. Each piston

    continues the squeeze action through its squeeze feet till a pre-set value of pressure is reached. At this

    instant, the cylinder is no longer capable of resisting any further pressure and a relief valve operates

    causing the pistons to yield and a contoured squeeze head is formed to suit the shape of the pattern.

    It is essential for patterns used in high pressure moulding to have fine finish and polish, high

    strength and rigidity, so as to enable easy stripping, good wear resistance and hardness, and

    accuracy. The patterns are made of cast iron, steel, or epoxy resin. Sometimes, aluminium

    patterns are also used; wooden patterns are not favoured. Large patterns are provided with

    suitable ribs and may also have supporting plates. Wear strips may also be inserted at places

    where greater wear is expected. In the case of cast iron patterns, chrome-plating ensures good

    finish.

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    Various processes used to achieve compac-tion in high pressure moulding are:

    1.Blow Squeeze The most common practice followed in high pressure moulding is blow squeeze

    where the moulding sand is blown pneumatically over the pattern and then the squeezing of

    sand is carried out. The process is satisfactory, producing good quality rigid moulds and is

    quieter in operation than the jolt squeeze machine but the prccompaction obtained by blowingis limited and not uniform in large moulds. Improved methods have been lately incorporated in

    the new generation moulding machines which are more efficient and productive.

    2. Impact Moulding This process uses cither air impulse or gas injection system in order to use

    impact force to achieve a high degree of compaction in a very short time, the latter being

    controlled by the fast build-up of pressure in a closed moulding chamber. Gas injection utilises

    combustion of a gas/air mixture {natural gas, methane or propane), as in case of I.C. engine. In

    an air-impulse system, the pressure build-up is caused by releasing compressed air at high

    pressure on the back of loosely filled sand.

    Both systems are effective and allow compaction in one operating cycle, have high operationalreliability, less down time, reduced maintenance costs, reduced noise and dust emissions and

    controlled shock-free movements. However, the compaction at the edges in the fast may be low,

    due to which the pattern plate cannot be fully utilised.

    3.Shoot Squeeze The sand in this system is ejected from a shorter head and is made to impinge

    on the pattern under heavy force. The shooting operation, thus causes pre-compaction of sand,

    which is then followed by a high pressure squeeze for full compaction. The process is efficient and

    productive, less noisy than blow squeeze machines and incurs lower operating costs. The machine can

    produce moulds weighing 500 kg in a cycle time of 45 to 60 seconds.

    Magnetic Moulding

    In Magnetic Moulding method the pattern is embedded in iron sand containing fine iron shots,

    iron oxide powder, or grinding swarf. The flask carrying the iron sand and pattern is surrounded

    by an electromagnet and a magnetic field of about 2000 gauss created (for a flask size of 600 mm

    x 600 mm x 200 mm). The magnetic field causes easy flow of moulding material into and around

    the pattern recesses and enables it to get fully consolidated. The mould is highly permeable as it

    is completely dry. The molten metal is then poured as in full mould casting. When the casting

    has solidified sufficiently, the magnetic field is broken by switching off the current to the

    electromagnet. The mould instantaneously disintegrates and the casting becomes available

    without requiring any cleaning or fettling. The same iron sand can be used over and over againafter cooling. This process, called magnetic moulding, has been adopted in some foundries for

    the production of automobile components.

    Metal Injection Moulding

    Many of the manufacturing processes applied to plastics can be traced to metal processingtechniques. However, metal injection moulding (MIM) is an example where the reverse holds.

    Figure 6.14 illustrates a typical MIM process flowchart. The essential difference from more

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    traditional P/M processing is that the metal powder is intimately mixed with organic

    thermoplastic binders to form a plastic jell that is injected into moulds, as opposed to the gravity

    filling of powder into dies. Early applications of MIM utilized moulding equipment developedfor plastic injection moulding. After removing the moulded 'green' part from the moulding

    machine, the binder is removed by evaporation or a solvent extraction technique, possibly

    followed by curing to burn off any remaining binder. Depending on the part geometry, binderremoval may take a few hours to a few days to complete. Finally, the parts are sintered and, ifnecessary, secondary forming operations applied.

    For the powder to be readily injectable requires fine powder, typically from 0.5-20 micro m indiameter. Such fine powders, sometimes referred to as metallic dust, are relatively easily

    dispersed into the air, potentially causing a health hazard if inhaled or, as virgin metal dusts are

    pyrophoric, causing an explosion hazard. The explosive nature of metallic powders and dusts is

    associated with a high surface energy per unit mass and exothermic oxidation reactions. In some

    cases, metallic powders and dusts are slowly exposed to an oxidizing atmosphere to passivate theparticle surfaces in a controlled manner. Metal powders and dusts are most suitably stored and

    shipped in containers pressurized with an inert gas.

    The primary advantage of the MIM process is that complex shapes with excellent dimensional

    accuracy and greater than 99% density can be produced. The process tends to be more expensive

    than traditional P/M processes. However, for the complex shapes produced, other manufacturing

    options such as machining may be equally or more expensive than MIM. Since the late 1980s

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    MIM has found increasing use for the manufacture of jewellery, computer hardware, and other

    relatively small but intricately shaped parts.

    Compocasting. One unusual technique for producing paniculate-reinforced castings is

    based on the thixotropic behavior of partly liquid-partly solid melts. A liquid alloy is

    allowed to cool until about 40% solids have formed; during solidification the solid-liquid

    mixture is vigorously stirred to break up the dendritic structure (Figure 16-9). The

    resulting solid-liquid slurry has thixotropicbehaviorthe slurry behaves as a solid when

    no stress is applied, but flows like a liquid when pressure is exerted.

    If a particulate material is introduced to the molten metal during cooling and stirring, a

    uniform dispersion is produced. The thixotropic slurry containing the paniculate is

    injected into a die under pressure; this process is termed compocast ing. A variety of

    materials, including Al2O3, SiC, TiC, and glass beads, have been incorporated into

    aluminum and magnesium alloys by this technique.

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    thixocasting

    In thixocasting process semi solid metal is subjected to compressive force inside a die and the

    metal shaped as required.

    Working Principle of Thixocasting Process

    In Thixocasting, a known quantity of metal is heated in a furnace to its softening point then

    subjected to external force when kept inside a die cavity. Since the metal is subjected to

    squeezing action, when it is plastic state or semi solid state, it is referred as Thixocasting.

    In thixocasting ingots produced by conventional routes are cut to size as required. These cut

    pieces are kept in a furnace maintained at a known temperature. When the metal reaches a pasty

    zone as indicated by an indicator semisolid metal is transferred into the cavity and forced

    between two dies. The dies used for t he purpose are capable of withstanding wear and tear and

    high temperature. The semisolid metal cools very fast and gets solidified. The component is

    withdrawn, the casting is ejected out.

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    The thixocasting setup details will exactly look similar to a pressure die casting or squeeze

    casting.

    RheoCasting

    In rhcocasting, a slurry (a solid suspended in a liquid) is received from a melt furnace and is

    cooled, then magnetically stirred prior to injecting it into a mold or die. Rhcocasting has been

    successfully applied with aluminum and magnesium and has been used to produce engine blocks,

    crankcases (as for motorcycles or lawn-mowers), and various marine applications.