spot & tig welding

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    Study and observation of a Spot weldingmachine and Tungsten inert gas weldingmachineSpot welding machine and TIG welding machine

    Welding is the process of permanently joining two or more metal parts,by melting both materials. The molten materials quickly cool, and the two

    metals are permanently bonded. There are various types of weldingprocess among them Spot welding and Tungsten inert gas welding areused widely to produce a better type of welding

    2009

    Sobuj

    SUST

    12/2/2009SpotandTIGweldingoperation

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    Name of the experiment:Study and observation of a Spot

    welding and Tungsten inert gas welding machine.

    Objectives:1. To learn about the spot welding machine.2. To learn about the welding principal of spot welding.3. To learn about the Tungsten inert gas welding machine.4. To learn about the welding principal of Tungsten inert gas welding.

    Introduction:Welding is the process of permanently joining two or more metal parts, bymelting both materials. The molten materials quickly cool, and the two metals are

    permanentlybonded.There are various types of welding process among them Spot weldingand Tungsten inert gas welding are used widely to produce a better type of welding.

    Spot welding is one of the oldest welding processes. It is used in a wide range of

    industries but notably for the assembly of sheet steel vehicle bodies. It is one form of

    resistance welding, which is a method of welding two or more metal sheets together without

    using any filler material by applying pressure and heat to the area to be welded.

    On the other hand Gas tungsten arc welding (GTAW), also known as Tungsten inert

    gas(TIG) welding, is an arc welding process that uses a nonconsumable tungsten electrode to

    produce the weld. The weld zone is well protected from the atmospheric impurities because

    of using the inert gas as a shielding agent thus a better welding can be produced by Tungsten

    inert gas welding.

    Figure:Spot welding operation

    Figure:Tungsten inert gas welding operation

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    Spot welding machine: A typical resistance spot welding machine is shown bellow. It

    essentially consists of two electrodes, out of which one is fixed. The other electrode is fixed

    to a rocker arm (to provide mechanical advantage) for transmitting the mechanical force from

    a pneumatic cylinder

    Push button control box: Push button control box helps to control various welding

    parameters.

    Hydraulic cy linder: Hydraulic cylinder helps to create pressure to the welding zone.

    Rocker arm: Rocker arm is connected to the electrode and supplies the pressure to welding

    zone.

    Electrode: Electrode supplies the current for welding.

    Welding control pedals: Welding control pedals helps to control the welding operation time.

    Push button

    control boxElectrode

    Rocker arm

    Hydraulic

    cylinder

    Welding control pedals

    Figure:Spot welding machine

    Part A: Spot welding

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    Principles of spot welding:Spot welding is accomplished when current is caused to flow through electrode tips and the

    separate pieces of metal to be joined. The resistance of the base metal to electrical current

    flow causes localized heating in the joint, and the weld is made.

    Spot welding involves three stages:

    1. The first of which involves the electrodes being brought to the surface of the metaland applying a slight amount of pressure.

    2. . The material between the electrodes yields and is squeezed together. It then melts,destroying the interface between the parts. The current is switched off and the

    "nugget" of molten materials solidifies forming the joint.

    3. After the current is removed from the workpiece, it is cooled via the coolant holes inthe center of the electrodes. Both water and a brine solution may be used as coolants

    in spot welding mechanisms

    Figure: Resistance spot weld section

    Figure:Process of spot welding

    Nugget

    Figure:Formation of a nugget in spot welding

    Welded section

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    1. Welding time Cycle:Weld times range from 0.01 sec to 0.63 sec depending on the thickness of the metal. A

    resistance welding schedule is the sequence of events that normally take place in each of thewelds,

    The events are:

    Squeeze time:The squeeze time is the time required for the electrodes to align and clamp the

    two workpieces.

    Weld time: The weld time of the current flow through the workpieces till they are heated to

    the melting temperature.

    Hold time: the hold time is the time is the time when the pressure is to be maintained on the

    molten metal without the electric current. During the time, the pieces are expected to be

    forging welded.

    Off time: The off time is time during which, the pressure on the electrode is taken off so that

    the plates can be positioned for the next spot. The off time is not normally, specified for

    simple spot welding, but only when a series of spots are to be made in predetermined pitch.

    2. Depth of penetration:The depth of penetration of the weld world normally varies from 0.3 to 0.8 times the

    thickness of the joining members. A depth of penetration less than this would not give rise to

    enough strength whereas a very high penetration nearing the thickness would bring the

    nugget outside and disfigure the appearance of the sheet. This also reduces the life of the

    electrode tip. Also, the penetration should be equal from both the sides.

    Figure: Spot Welding Time Cycle

    Welding parameters:

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    3. Width & thickness of the workpiece:The two materials being welded together are known as the workpieces and must conduct

    electricity. The width of the workpieces is limited by the throat length of the welding

    apparatus and ranges typically from 5 to 50 inches. Workpiece thickness can range from

    0.008 inch to 1.25 inch.

    4. The Tip diameter:The formula generally used for low carbon steel is as follows:

    Electrode tip diameter = 0.100 + 2twhere t is the thickness in inches of one thickness of

    the metal to be welded. This formula is applicable to the welding of metals of dissimilar

    thicknesses. The formula is applied to each thickness individually, and the proper electrode

    tip diameter selected for each size of the joint. If the two pieces were unequal in thickness,

    such as one piece 0.062 and the other 0.094, two calculations would have to be made. Eachthickness would be treated as the basis for one electrode tip diameter determination.

    5. Required current for welding:The open circuit voltage from the transformer is typically in the 5-10 V range, but there is a

    large voltage drop in the electrodes and secondary side of the transformer when the circuit is

    closed. And the weld currents can range from 4000 to 24,000 amps for different types of mild

    steel.

    6. Pressure Or Welding Force:The pressure exerted by the tongs and the electrode tips on the workpiece have a great effect

    on the amount of weld current that flows through the joint. The greater the pressure, the

    higher the welding current value will be, within the capacity of the resistance spot welding

    machine.

    7. Time Calculation:Spot welding setup can be calculated using the following equation:

    Total weld time = (L + W + U) x N

    Where L is the load time in seconds, N is the number of welds, U is the unload time inseconds, and W is the weld time in seconds.

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    8. Heat balance: One of the very important characteristics of the resistance weldingprocess is the transfer of heat to the two parts being joined differently so that proper fusion

    obtained even when the plates are dissimilar from the stand point of material of thickness.

    Advantages of resistance spot welding process:There are many advantages of resistance spot welding among them some of important

    advantages are given bellow.

    1. Very little skill is required to operate the resistance welding machine.2. These are very well suited for mass production, as they give a high production rate.3. Heating of the workpiece is confined to a very small part which results in less

    distortion.

    4. It is possible to weld dissimilar metals as well as metal plates of different thicknesses.5. There are no consumables used in this process except for the electrical power and a

    relatively smaller electrode wear. As a result it is a very economical process.

    Disadvantages of resistance spot welding process:In spite of these advantages, there are certain limitations and disadvantages of the

    resistance welding process. They are,

    1. The resistance welding machine is highly complex with various elements such as aheavy transformer, electrodes and heavy conductors for carrying the high currents, the

    electrode force applying mechanism such as a pneumatic cylinder and its supply, the

    heavy machine structure to support the large forces and an expensive timing

    arrangement. All these make the resistance welding machine highly expensive, unless

    it is used for large scale production.

    2. Certain resistance welding processes are limited only to lap joints. This limits its useto sheet metal whose thickness is less than 3 mm. the lap joints have an inherent

    crevice between the sheets which is responsible for stress concentration and a loss o f

    fatigue life. Also for materials that are prone to corrosion, the lap joint may be asource of trouble.

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    Applications of spot welding:1. Spot welding is typically used when welding particular types ofsheet metal.

    Aluminium alloys can also be spot welded. However, their much higher thermal

    conductivity and electrical conductivity mean that up to three times higherwelding currents are needed. This requires larger, more powerful, and moreexpensive welding transformers.

    2. Perhaps the most common application of spot welding is in the automobilemanufacturing industry, where it is used almost universally to weld the sheet

    metal to form a car.3. Spot welding is also used is in the orthodontist's clinic, where small scale spot

    welding equipment is used when resizing metal "molar bands" used in

    orthodontics. 4. Another application is spot welding straps to nickel-cadmium or nickel-metal-

    hydride cells in order to make batteries. The cells are joined by spot weldingthin nickel straps to the battery terminals. Spot welding can keep the batteryfrom getting too hot, as might happen if conventional soldering were done.

    Manufacturing Considerations:1. Thickness of the parts to be welded should be equal or the ratio of thicknesses should

    be less than 3:1.

    2. Spacing of welds:o Minimum weld to weld spacing = 10 x Stock thickness.

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    o Center of weld to edge distance = 2 x weld diameter, minimum.o Weld to form distance = Bend Radius + 1 weld diameter, minimum.3. Adequate access for spot welding should be considered. Small flanges in U channels

    for example may restrict the electrode from entering the part.

    4. Flat surfaces are easier to spot weld due to easy access. Multiple bends impose accessrestrictions, and special fixtures may have to be designed to handle the parts, if accessis not a problem.

    5. Prior to finishing, the spot welds have to be sanded or ground to blend the welds withthe rest of the surface.

    6. It is best to choose the same spot weld size, to minimize setups and increasethroughput.

    7. Plating of spot welded assemblies can cause problems when the sheet metal isoverlapped. This can cause plating salts to be trapped-requiring special cleaning, or

    potential long-term corrosion problems. By carefully designing the assembly to alloweasy draining of plating solutions this can be avoided

    8. The mating parts can be self-jigged for easy location prior to welding. This can bedone by lancing one part and locating in a corresponding slot in the other part; or by

    boss type extrusion, weld buttons, in part locating to a slot in the other. This type ofdesign can often eliminate the need for external fixtures.

    Discussion: The process description made so far is called the resistance spot welding orsimply spot welding. This is the most common resistance welding process. Spot weldingoffers a number of advantages over other techniques, including high speed, ease of

    automation and energy efficiency. Although it has so much application in the modernmanufacturing system but the sad news is, we dont perform the spot welding operationbecause of some technical problem of that machine although weve the spot welding machine

    in our workshop. I think itll be more effective and meaningful if we perform the spotwelding operation. I think our respective teacher has already taken the necessary steps to fix

    the problem.

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    Part B: Tungsten inert gas weldingTungsten inert gas welding machine: A typical tungsten inert gas welding setup is shown

    bellow. It consists of a welding torch at the centre of which is the tungsten electrode.

    The inert gas is supplied to the welding zone through the annular path surrounding the

    tungsten electrode to effectively displace the atmosphere around the weld puddle.

    The inert gas is supplied to the welding zone through the annular path surrounding the

    tungsten electrode to effectively displace the atmosphere around the weld puddle.

    Figure: A typical Tungsten inert gas (TIG) welding setup

    Inert gas supplyShilding gasWorkpiece

    Weldingmachine

    Electricalconductor

    Insulatingsheath

    Electrode holder

    Gas passages

    Tungsten electrode

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    Equipments used in TIG welding:1. Power supply:Gas tungsten arc welding uses a constant current power source,

    meaning that the current (and thus the heat) remains relatively constant, even if the

    arc distance and voltage change. The preferred polarity of the GTAW system dependslargely on the type of metal being welded. Direct current with a negatively charged

    electrode (DCEN) is often employed when welding steels, nickel, titanium, and other

    metals.

    2. Electrode: The electrode used in GTAW is made oftungsten or a tungsten alloy,because tungsten has the highest melting temperature among pure metals, at 3,422 C

    (6,192 F). As a result, the electrode is not consumed during welding, though some

    erosion (called burn-off) can occur.

    The diameter of the electrode can vary between 0.5 and 6.4 millimeters (0.02 and 0.25 in),

    and their length can range from 75 to 610 millimeters (3.0 to 24.0 in).

    Figure: Power supplier of TIG welding

    Figure:GTAW torch with various electrodes,

    cups, collets and gas diffusers

    Electrode

    Gas diffusers

    GTAW torch

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    3. Welding torch: The welding torch holds the electrode and supply the necessarycurrent to the weld zone. GTAW welding torches are designed for either automatic or

    manual operation and are equipped with cooling systems using air or water. But in our

    workshop the TIG welding machine doesnt contain the cooling system.

    The internal metal parts of a torch are made of hard alloys ofcopper or brass in order to

    transmit current and heat effectively. The tungsten electrode must be held firmly in the center

    of the torch with an appropriately sized collect, and ports around the electrode provide a

    constant flow of shielding gas. Collets are sized according to the diameter of the tungsten

    electrode they hold. The body of the torch is made of heat-resistant, insulating plastics

    covering the metal components, providing insulation from heat and electricity to protect the

    welder.

    4. Shielding gas supply:As with other welding processes such as gas metal arc welding,shielding gases are necessary in GTAW to protect the welding area from atmosphericgases such as nitrogen and oxygen, which can cause fusion defects, porosity, and weldmetal embrittlement if they come in contact with the electrode, the arc, or the welding

    metal. The gas also transfers heat from the tungsten electrode to the metal, and ithelps start and maintain a stable arc.

    Figure:Welding torch

    Figure:Inert Gas cylinder

    Ar onArgonHelium

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    Welding principals: The welding technique used for TIG is essentially similar tothat of the gas welding. The welding principal is given bellow.

    1. GTAW normally requires two hands, since most applications require that the weldermanually feed a filler metal into the weld area with one hand while manipulating thewelding torch in the other.

    2. To strike the welding arc, a high frequency generator provides a spark; this spark is aconductive path for the welding current through the shielding gas and a llows the arc

    to be initiated while the electrode and the workpiece are separated, typically about

    1.53 mm (0.060.12 in) apart.

    3. Once the arc is struck, the welder moves the torch in a small circle to create awelding pool, the size of which depends on the size of the electrode and the amount

    of current. While maintaining a constant separation between the electrode and theworkpiece, the operator then moves the torch back slightly and tilts it backward

    about 1015 degrees from vertical. Filler metal is added manually to the front end of

    the weld pool as it is needed.

    4. The filler rod is withdrawn from the weld pool each time the electrode advances, butit is never removed from the gas shield to prevent oxidation of its surface and

    contamination of the weld. Filler rods composed of metals with low melting

    temperature, such as aluminium, require that the operator maintain some distance

    from the arc while staying inside the gas shield. If held too close to the arc, the filler

    rod can melt before it makes contact with the weld puddle. As the weld nears

    completion, the arc current is often gradually reduced to allow the weld crater to

    solidify and prevent the formation of crater cracks at the end of the weld.

    Figure: Welding operation

    http://en.wikipedia.org/wiki/File:GTAW.svg
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    Welding parameters:1. Selection of polarity: The preferred polarity of the GTAW system depends largely on

    the type of metal being welded. Direct current with a negatively charged electrode

    (DCEN) is often employed when welding steels, nickel, titanium, and other metals. Itcan also be used in automatic GTA welding of aluminium or magnesium when helium is

    used as a shielding gas. Direct current with a positively charged electrode (DCEP) is less

    common, and is used primarily for shallow welds since less heat is generated in the base

    material.

    A table is given bellow to show the selection of proper polarity

    Table: The metals that can be commonly welded by TIG welding and their choice of setups

    Material Electrodes Power supply used Preferred shielding

    gas.Stainless steel Thoriated tungsten DCEN Argon

    Aluminium All types AC Argon

    Magnesium Tungsten AC Argon

    Deoxidized copperMonel

    Thoriated tungsten DCEN Argon

    High carbon steel

    cast iron

    Thoriated tungsten AC or DCEN Argon

    2.

    Selection of power supply (AC or DC):The preferred polarity of the GTAW system

    depends largely on the type of metal being welded. Direct current (DC) with a negatively

    charged electrode (DCEN) is often employed when welding steels, nickel, titanium, and

    other metals on the other hand Alternating current (AC), commonly used when welding

    aluminium and magnesium manually or semi-automatically, combines the two direct

    currents by making the electrode and base material alternate between positive and

    negative charge.

    3. Selection of shielding gas: The selection of a shielding gas depends on several factors,including the type of material being welded, joint design, and desired final weld

    appearance. Argon is the most commonly used shielding gas for GTAW, since it helpsprevent defects due to a varying arc length. When used with alternating current, the useof argon results in high weld quality and good appearance. Another common shielding

    gas, helium, is most often used to increase the weld penetration in a joint, to increase thewelding speed, and to weld metals with high heat conductivity, such as copper and

    aluminium. Argon-helium mixtures are also frequently utilized in GTAW, since they canincrease control of the heat input while maintaining the benefits of using argon.Normally, the mixtures are made with primarily helium (often about 75% or higher) and

    a balance of argon. These mixtures increase the speed and quality of the AC welding ofaluminium, and also make it easier to strike an arc.

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    4. Selection of electrode tip shapes: The electrode tip should be prepared for proper weldpenetration. The typical shapes that can be used are shown bellow though it is possible to use

    these electrodes without any tip preparation, it would be better to prepare the top since itenhances the weld quality.

    For AC welding with high frequency unbalanced machines, the tip should be pencilpointedas shown in fig. So that the HF current gets concentrated and the arc is easily initiated, also

    once the arc is initiated the tip gets heated and a ball of tungsten is formed which reduces theeffect of current rectification and thus, stabilizes the AC arc. With DCEN, the electrodewould be made conical as shown above. Pure tungsten electrodes are never made into conical

    point since the end is likely to melt and contaminate the weld metal. Instead, it is better tomake full round ball at the tip, as shown above.

    Applications:There are many applications of TIG welding some of them are given bellow.

    1. This welding is useful for joining aluminium, magnesium and stainless steel.

    2. Better penetration can be achieved by TIG welding so it uses for high strength joining.

    3. Many industries use GTAW for welding thin workpieces, especially nonferrous metals.

    Figure:GTAW fillet weld

    Welded area

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    4. It is used extensively in the manufacture of space vehicles, and is also frequentlyemployed to weld small-diameter, thin-wall tubing such as those used in the bicycle industry.

    5. As more heat is generated thus it is used for welding thick section.

    6. In addition, GTAW is often used to make root or first pass welds for piping of varioussizes. In maintenance and repair work, the process is commonly used to repair tools and dies,especially components made of aluminium and magnesium.

    Discussion: In modern days the application of TIG welding is getting increase. It hasmuch application in manufacturing products. Its fast application and reliability of applicationattracts many jobs. So as were going to be anIndustrial and production engineer so we

    should aware of TIG welding processbut in our workshop we dont perform the TIG weldingoperation although weve the TIG welding machine because of some technical problem. We

    just observe the TIG welding machine. I think itll be very essential for us to perform the

    operation with our hand. We hope our respective teacher will take the necessary step to fixthis problem.

    Conclusion: Spot welding is one form of resistance welding, which is a method ofwelding two or more metal sheets together without using any filler material by applyingpressure and heat to the area to be welded. Resistance spot welding is welding technique that

    is used for almost all known metals. Welding procedures for each type of material must bedeveloped for the most satisfactory results. This process is used for joining sheet materials

    and uses shaped copper alloy electrodes to apply pressure and convey the electrical currentthrough the workpieces. Spot welding offers a number of advantages over other techniques,including high speed, ease of automation and energy efficiency.

    Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an

    arc welding process that uses a nonconsumable tungsten electrode to produce the weld. Theweld area is protected from atmospheric contamination by a shielding gas (usually an inert

    gas such as argon), and a filler metal is normally used. This type of welding ensures thequality of welded zone. The TIG welding process can be used for the joining of a number ofmaterials though the most common ones are aluminium, magnesium and stainless steel.

    The whole operation gives us a comprehensive concept about spot welding and Tungsten

    inert gas welding process.

    Figure: A pipe is joined by TIG welding

    Welded area

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    Assignment:Question: Write down the difference between arc welding and Tungsten inert gas welding.

    Answer: The basic difference between conventional arc welding and Tungsten inert gaswelding are given bellow.

    Tungsten inert gas welding Conventional arc welding1. Tungsten inert gas welding use inert gas as

    a shielding agent.

    1. Conventional arc welding use flux as a

    shielding agent.

    2. Tungsten inert gas welding use nonconsumable tungsten electrode.

    2. Conventional arc welding use consumableelectrodes.

    3. Because of the use of shielding gases, no

    fluxes are required to be used in inert gasshielded arc welding.

    3. Conventional arc welding required flux, it

    is now found in belt in state with coatedelectrodes.

    4. More heat is generated in TIG welding

    process thus better penetration can beachieved.

    4. Conventional arc welding doesnt generate

    so much heat compare to TIG welding thusthe penetration is much poor than TIGwelding

    5. The inert gas protect the molten weld

    puddle from the atmospheric contaminationthus very high quality welding can be

    achieve by TIG welding process.

    5. Although in conventional welding, we use

    flux as a shielding agent, but it is notsufficient to protect the entire weld puddle

    and thus various contamination is occurredby atmospheric impurities.

    6. As its quality of welding, it is usedextensively in the manufacture of space

    vehicles, and is also frequently employed toweld small-diameter, thin-wall tubing such as

    those used in the bicycle industry.

    6. The conventional arc welding is onlyemployed where the quality is not so

    important.

    Edited by SobujSUSTDept of Industrial & ProductionEngineeringRegistration No. [email protected]