Welding

Welding:

          Welding is the process of joining two metals either those metals are similar or dissimilar, with or without the application of pressure and with or without the use of filler metal.

Weldability:

          In is the property of metal which indicates the ease with which two similar or dissimilar metals are joint together.

Advantages of welding:

1. Strength is more.

2. Takes less time.

3. Weight of welding joints is less.

4. Smooth appearance.

5. Less wastage of material.

6. Efficiency is Maximum which not possible in other types of joints.

7. Complicated shape can be easily welded.

8. It can be done at any point of structure.

Disadvantages of welding:

1. Skilled workers are required.

2. It is permanent joint.

3. Jigs and fixtures are required.

4. Preparation of edges.

5. Personal protective equipments.

Tungsten Inert Gas Welding (TIG)

Tungsten Inert Gas Welding (TIG): 

          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 area is protected from atmospheric contamination by a shielding gas (usually an inert gas such as argon), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.

          GTAW is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.

OPERATION :
          Manual gas tungsten arc welding is often considered the most difficult of all the welding processes commonly used in industry. Because the welder must maintain a short arc length, great care and skill are required to prevent contact between the electrode and the workpiece. Similar to torch welding, GTAW normally requires two hands, since most applications require that the welder manually feed a filler metal into the weld area with one hand while manipulating the welding torch in the other. However, some welds combining thin materials (known as autogenous or fusion welds) can be accomplished without filler metal; most notably edge, corner, and butt joints.
          To strike the welding arc, a high frequency generator (similar to a Tesla coil) provides an electric spark; this spark is a conductive path for the welding current through the shielding gas and allows the arc to be initiated while the electrode and the workpiece are separated, typically about 1.5–3 mm (0.06–0.12 in) apart. This high voltage, high frequency burst can be damaging to some vehicle electrical systems and electronics, because induced voltages on vehicle wiring can also cause small conductive sparks in the vehicle wiring or within semiconductor packaging. Vehicle 12V power may conduct across these ionized paths, driven by the high-current 12V vehicle battery. These currents can be sufficiently destructive as to disable the vehicle; thus the warning to disconnect the vehicle battery power from both +12 and ground before using welding equipment on vehicles.
          An alternate way to initiate the arc is the “scratch start”. Scratching the electrode against the work with the power on also serve to strike an arc, in the same way as SMAW (“stick”) arc welding. However, scratch starting can cause contamination of the weld and electrode. Some GTAW equipment is capable of a mode called “touch start” or “lift arc”; here the equipment reduces the voltage on the electrode to only a few volts, with a current limit of one or two amps (well below the limit that causes metal to transfer and contamination of the weld or electrode). When the GTAW equipment detects that the electrode has left the surface and a spark is present, it immediately (within microseconds) increases power, converting the spark to a full arc.
Once the arc is struck, the welder moves the torch in a small circle to create a welding 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 the workpiece, the operator then moves the torch back slightly and tilts it backward about 10–15 degrees from vertical. Filler metal is added manually to the front end of the weld pool as it is needed.
          Welders often develop a technique of rapidly alternating between moving the torch forward (to advance the weld pool) and adding filler metal. The filler rod is withdrawn from the weld pool each time the electrode advances, but it 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 aluminum, 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.

Operation modes :

          GTAW can use a positive direct current, negative direct current or an alternating current, depending on the power supply set up. A negative direct current from the electrode causes a stream of electrons to collide with the surface, generating large amounts of heat at the weld region. This creates a deep, narrow weld. In the opposite process where the electrode is connected to the positive power supply terminal, electrons flow from the part being welded to the tip of the electrode instead, so the heating action of the electrons is mostly on the electrode. This mode also helps to remove oxide layers from the surface of the region to be welded, which is good for metals such as aluminum or magnesium. A shallow, wide weld is produced from this mode, with minimum heat input. Alternating current gives a combination of negative and positive modes, giving a cleaning effect and imparts a lot of heat as well.
ADVANTAGES:
1.No flux is used, hence there is no danger of flux entrapment when welding refrigerator and air conditioner components.
2.Because of clear visibility of the arc and the job, the operator can exercise a better control on the welding process.
3.This process can weld in all positions and produces smooth and sound welds with less spatter.
4.TIG welding is very much suitable for high quality welding of thin materials (as thin as 0.125 mm).
5.It is a very good process for welding nonferrous metals (aluminium etc.) and stainless steel.

DISADVANTAGES:
1. Under similar applications, MIG welding is a much faster process as compared to TIG welding, since. TIG welding requires a separate filler rod.
2. Tungsten if it transfers to molten weld pool can contaminate the same. Tungsten inclusion is hard and brittle.
3. Filler rod end if it by chance comes out of the inert gas shield can cause weld metal contamination.
4. Equipment costs are higher than that for flux shielded metal arc welding.

APPLICATIONS:
1. Welding aluminium, magnesium, copper, nickel and their alloys, carbon, alloy or stainless steels, inconel, high temperature and hard surfacing alloys like zirconium, titanium etc.
2. Welding sheet metal and thinner sections.
3. Welding of expansion bellows, transistor cases, instrument diaphragms, and can sealing joints.
4. Precision welding in atomic energy, aircraft, chemical and instrument industries.
5. Rocket motor chamber fabrications in launch vehicles.

Metal Inert Gas Welding ( MIG )

Metal Inert Gas Welding ( MIG ):
           Gas metal arc welding (GMAW), sometimes referred to by its subtypesmetal inert gas (MIG) welding or metal active gas (MAG) welding, is a semi-automatic or automatic arc welding process in which a continuous and consumable wire electrode and a shielding gas are fed through a welding gun. A constant voltage, direct current power source is most commonly used with GMAW, but constant current systems, as well as alternating current, can be used. There are four primary methods of metal transfer in GMAW, called globular, short-circuiting, spray, and pulsed-spray, each of which has distinct properties and corresponding advantages and limitations.
         Originally developed for welding aluminum and other non-ferrous materials in the 1940s, GMAW was soon applied to steels because it allowed for lower welding time compared to other welding processes. The cost of inert gas limited its use in steels until several years later, when the use of semi-inert gases such as carbon dioxide became common. Further developments during the 1950s and 1960s gave the process more versatility and as a result, it became a highly used industrial process. Today, GMAW is the most common industrial welding process, preferred for its versatility, speed and the relative ease of adapting the process to robotic automation. The automobile industry in particular uses GMAW welding almost exclusively. Unlike welding processes that do not employ a shielding gas, such as shielded metal arc welding, it is rarely used outdoors or in other areas of air volatility. A related process, flux cored arc welding, often does not utilize a shielding gas, instead employing a hollow electrode wire that is filled with flux on the inside.
           For most of its applications gas metal arc welding is a fairly simple welding process to learn requiring no more than a week or two to master basic welding technique. Even when welding is performed by well-trained operators weld quality can fluctuate since it depends on a number of external factors. All GMAW is dangerous, though perhaps less so than some other welding methods, such as shielded metal arc welding.

Technique :

          The basic technique for GMAW is quite simple, since the electrode is fed automatically through the torch. By contrast, in gas tungsten arc welding, the welder must handle a welding torch in one hand and a separate filler wire in the other, and in shielded metal arc welding, the operator must frequently chip off slag and change welding electrodes. GMAW requires only that the operator guide the welding gun with proper position and orientation along the area being welded. Keeping a consistent contact tip-to-work distance (the stick out distance) is important, because a long stick-out distance can cause the electrode to overheat and will also waste shielding gas. Stick-out distance varies for different GMAW weld processes and applications.
            For short-circuit transfer, the stick-out is generally 1/4 inch to 1/2 inch, for spray transfer the stickout is generally 1/2 inch. The position of the end of the contact tip to the gas nozzle are related to the stickout distance and also varies with transfer type and application. The orientation of the gun is also important—it should be held so as to bisect the angle between the workpieces; that is, at 45 degrees for a fillet weld and 90 degrees for welding a flat surface. The travel angle, or lead angle, is the angle of the torch with respect to the direction of travel, and it should generally remain approximately vertical. However, the desirable angle changes somewhat depending on the type of shielding gas used—with pure inert gases, the bottom of the torch is often slightly in front of the upper section, while the opposite is true when the welding atmosphere is carbon dioxide.

Quality :

          Two of the most prevalent quality problems in GMAW are dross and porosity. If not controlled, they can lead to weaker, less ductile welds. Dross is an especially common problem in aluminum GMAW welds, normally coming from particles of aluminum oxide or aluminum nitride present in the electrode or base materials. Electrodes and workpieces must be brushed with a wire brush or chemically treated to remove oxides on the surface. Any oxygen in contact with the weld pool, whether from the atmosphere or the shielding gas, causes dross as well. As a result, sufficient flow of inert shielding gases is necessary, and welding in volatile air should be avoided.
          In GMAW the primary cause of porosity is gas entrapment in the weld pool, which occurs when the metal solidifies before the gas escapes. The gas can come from impurities in the shielding gas or on the workpiece, as well as from an excessively long or violent arc. Generally, the amount of gas entrapped is directly related to the cooling rate of the weld pool. Because of its higher thermal conductivity, aluminum welds are especially susceptible to greater cooling rates and thus additional porosity. To reduce it, the workpiece and electrode should be clean, the welding speed diminished and the current set high enough to provide sufficient heat input and stable metal transfer but low enough that the arc remains steady. Preheating can also help reduce the cooling rate in some cases by reducing the temperature gradient between the weld area and the base material.

ADVANTAGES:
1) Higher welding speeds.
2) Greater deposition rates.
3) Less post welding cleaning (e.g. no slag to chip off weld).
4) Better weld pool visibility.
5) No stub end losses or wasted man hours caused by changing electrodes.
6) Low skill factor required to operate M.I.G / M.A.G.S welding torch.
7) Positional welding offers no problems when compared to other processes. (Use dip or pulsed mode of transfer).
8) The process is easily automated.
9) No fluxes required in most cases.
10) Ultra low hydrogen process.

DISADVANTAGES:
1) Higher initial setup cost
2) Atmosphere surrounding the welding process has to be stable (hence the shielding gasses), therefore this process is limited to draught free conditions
3) Higher maintenance costs due to extra electronic components
4) The setting of plant variables requires a high skill level
5) Less efficient where high duty cycle requirements are necessary
6) Radiation effects are more severe

Thermit Welding

THERMIT WELDING:
          Thermit welding is an effective, highly mobile, method of joining heavy section steel structures such as rails. Essentially a casting process, the high heat input and metallurgical properties of the Thermit steel make the process ideal for welding high strength, high hardness steels such as those used for modern rails.
          Thermit Welding is a skilled welding process and must not be undertaken by anyone who has not been trained and certificated to use it.
Detailed operating instructions are provided for each of our processes, but the welding methods all comprise of 6 main elements:
  1. A carefully prepared gap must be produced between the two rails, which must then be accurately aligned by means of straightedges to ensure the finished joint is perfectly straight and flat.
  2. Pre-formed refractory moulds which are manufactured to accurately fit around the specific rail profile are clamped around the rail gap, and then sealed in position. Equipment for locating the preheating burner and the Thermitt container is then assembled.
  3. The weld cavity formed inside the mould is preheated using an oxy fuel gas burner with accurately set gas pressures for a prescribed time. The quality of the finished weld will depend upon the precision of this preheating process.
  4. The Thermit® Portion is manufactured to produce steel with metallurgy compatible with the specific type of rail to be welded. On completion of the preheating, the container is fitted to the top of the moulds, the portion is ignited and the subsequent exothermic reaction produces the molten Thermitt Steel. The container incorporates an automatic tapping system enabling the liquid steel – which is at a temperature in excess of 2,500°C – to discharge directly into the weld cavity.
  5. The welded joint is allowed to cool for a predetermined time before the excess steel and the mould material is removed from around the top of the rail with the aid of a hydraulic trimming device.
  6. When cold the joint is cleaned of all debris, and the rail running surfaces are precision ground the profile. The finished weld must then be inspected before it is passed as ready for service.

ADVANTAGES:
1.The heat necessary for welding is obtained  from a chemical reaction and thus no costly power supply is required. Therefore broken parts (rails etc.) can be welded on the site itself.
2.For welding large fractured crankshafts.
3.For welding broken frames of machines.
4.For building up worn wobblers.
5.For welding sections of castings where size prevents there being cast in one piece.
6.For replacing broken teeth on large gears.
7.Forgings and flame cut sections may be welded  together to make huge parts.
8.For welding new necks to rolling mill rolls and pinions.
9.For welding cables for electrical conductors.
10.For end welding of reinforcing bars to be used in concrete (building) construction.
LIMITATIONS:
1.Thermit welding is applicable only to ferrous metal parts of heavy sections, i.e., mill housings and heavy rail sections.
2.The process is uneconomical if used to weld cheap metals or light parts.