Welding distortion

One of the major problems found with weldments is called distortion. Distortion is caused mainly because of the shrinkage that takes place in weldments. The shrinkage taking place is a weldment depends upon the geometry and type of the weld. 

There are three types of distortions possible in welding :

• Transverse shrinkage occurring perpendicular to the weld line.
• Longitudinal shrinkage occurring parallel to the weld line, which is very small of the order of about 0.1 % of the weld length and hence can be neglected.
• Angular change as a rotation about the weld line.

Now we can check the methods for reducing all three types of shrinkage :

The methods for reducing the transverse shrinkage are :

• Decrease the total weight of the weld metal. 
• Increase the metal deposited in the first pass.

Some other factors which influence the transverse shrinkage are :

• Root opening.
• Joint design.
• Electrode diameter.
• Degree of constraint.

The methods for reducing the angular shrinkage are :

• Fillet welds in a structure are affected by the way the structure is designed and the type of restraint provided.

Control of distortion :

As we saw above, distortions are inevitable in a welding operation. It is, therefore, necessary to find ways by which these can be minimized to prepare to satisfactory weldment.

• One of the important ways to control the distortions is a good design of the product with a minimum number of joints.
• Proper design of the joint helps in reducing the magnitude of the problem.

In spite of this, there still will be distortion and therefore during the assembly process care should be taken to see that the distortions are controlled. 

There are two possibilities one is to present the members to compensate for this distortion another one is to assemble parts correctly and then apply a proper restraint to minimize the distortion during the welding process. 

The most generally preferred method in the industry is using restraint. There are many ways used for restraining such as clamps, fixtures and even tack welds. Through this method reduces the distortion, it causes high residual stresses, which may lead to cracking. Hence it is necessary to carefully apply restraint without causing too high a magnitude of harmful residual stresses. 

Another method available for reducing the distortions is the preheating of the members of the weldment such that the heat of welding would be properly balanced. 

Friction welding

It is a solid-state welding process that generates heat through mechanical friction between workpieces in relative motion to one another with the use of lateral force and fuses the materials. 
Friction welding is not a fusion welding process because no melting occurs. 

Working principle :

In this process of welding, the heat required for welding is obtained by the friction between the ends of the two parts to be joined. One of the parts to be joined is rotated at a high speed around 3000 RPM and the other part is axially aligned with the second one and pressed tightly against it.

Also, the friction between the two parts raises the temperature of both the ends. After that, the rotation of the part is stopped abruptly and the pressure on the fixed part is increased so that the joining takes place. 

Machine set up :

The machine for friction welding is similar to a centre lathe. Through a centre lathe could be used for smaller sized jobs, the bigger ones required a special welding machine because in a lathe machine power available would not be sufficient. The power requirements of friction welding may be between 25 kVA to 175 kVA, which is far beyond that of the many general-purpose centre lathes. 

Major parameter :

The major parameters in friction welding are the rotational speed and the axial pressure applied. The axial pressure applied depends on the strength and hardness of the metals being joined. 

The pressure may range from 40 Mpa for low carbon steels to as high as 450 Mpa for alloy steels.

The rotational speed may also change the requirement of the pressure. It may be of the order of 1500 to 3000 RPM.  

The other variable that needs to be closely controlled is the time of contact between the two parts. The total welding time that is taken in the friction welding is between 2 to 30 seconds. 

Advantages of friction welding :

• The major advantage of friction welding is the ease with which the joining can take place. 
• Edge cleaning is not a problem since the oxides and contaminants present would easily be removed during the initial rubbing. 
• The heat generated is small and well below the melting temperature, there will be no distortion and warping. 
• The quality of the weld achieved is very high and it is economical in operation. 
• No skilled operator required since it completely automatic in operation. 

Because of the above advantages, the quality of weld obtained is very high so that the friction welding has been widely accepted in the aerospace industry as well as the automobile industry for the welding of critical parts. 

Disadvantages of friction welding :

• This welding process mostly used only for round bars of some cross-section.
• Non-forgeable materiel can not be weld.
• Preparation of workpiece is more critical.
• High machine setup cost. 
• Joint design is limited. 
• It can only be used for smaller parts of machines, big parts are not compatible with it. 

Atomic hydrogen welding

Introduction :

Atomic hydrogen welding is an arc welding process. In this process the arc used in between two tungsten electrodes in a shielding atmosphere of hydrogen. This process was invented by Irving Langmuir in the course of his studies of atomic hydrogen. When Hydrogen is in its atomic state, is a strong reducing gas which prevents oxidation of weld metal and rapid burning of electrodes. Any oxygen present in the surrounding combines with hydrogen forming water which is converted into steam.

Equipment set up :

Set up of this operation consist of hydrogen cylinder, an AC welding machine and the welding torch to accommodate to tungsten electrons, with provision for changing the distance between them. The normal voltage range of the power supply is between 50 to 75 volt with the current varying from 15 to 150 A. This measure is good enough for an electrode size of 1 to 5 mm.

The path of electron travel between the two electrodes is not a straight path as in other arc welding process. Instead, they travel in the form of a fan. This is because the hydrogen atoms formed by the arc causes a downward force because of with electron flow slightly deviates. This fan shape can be changed by altering the distance between the electrodes and the current level. The DC machines could also be used in atomic hydrogen welding, but because of the electron flow is only in one direction, the wear of electron is particularly high and as a result, only AC power supply is used.

When hydrogen atoms recombine near the workpiece surface, they generate a temperature of the order of the 300 0C. Because of this heat, the molten metal becomes highly fluid and therefore, atomic hydrogen welding is used for the flat position only. Filler metal when needed is melted intermittently in the arc fan for fusing with the base metal.

Working principle :

Atomic hydrogen welding the atomic hydrogen welding is an inert gas welding arc welding process done with non-consumable electrodes. The main difference between tungsten inert gas welding and this process is that in atomic hydrogen welding, the arc is obtained between the two tungsten electrodes rather than between the tungsten electrode and the workpiece. This shielding gas used here is hydrogen, which is reactive in nature compared to argon. The hydrogen molecule (H₂), when passing through the electric arc, get this dissociated into two hydrogen atoms (H+). The hydrogen atoms are highly reactive. They form hydrogen molecule and combine with oxygen if present to form water vapour enters release intense heat for the necessary melting of the joint. Because of its type reactivity, the atomic hydrogen is able to break the oxide on the base metal and thus allow the formation of a clean weld. 

Application :

When properly performed, atomic hydrogen welding gives an extremely clean weld with excellent quality. It is generally used for welding of tool Steels containing tungsten, Nickel and Molybdenum as also for hard surfacing and repairing of moulds, dies and tools. Though it can be used for any job, its high cost prohibits its general usage.

Oxy hydrogen welding

Working principle :

In oxy-hydrogen welding, hydrogen combines with oxygen to generate steam and attains a flame temperature of around 2800⁰C. But the weld pool is not protected from the atmosphere when the oxygen for combustion is completely provided by the torch itself. So, Oxygen is an amount slightly less than that required for complete combustion is provided by the torch, whereas atmospheric oxygen accounts for the burning of the remaining hydrogen. This gives rise to a protective preheating flame that surrounds the main flame. But this reduces the flame temperature to some extent. Because of the lower flame temperature, oxy-hydrogen welding is a generally slow process. It is normally used to weld thin sheets of steels and alloys with low melting temperatures.

Key features :

• Operation is convenient & safe

Oxy hydrogen generator produces oxygen and hydrogen gas that you required and also no gas cylinder is required. There is no risk of explosion.

• Environmental friendly 

In this process, fuel comes from water and there is water vapour after finishing this process so this process is environmentally friendly.

• Welding features 

Welding work is fast, precision, smooth and beautiful welding spot. The oxyhydrogen flame is concentrated up to 2800 ⁰C so it can heat the welding spot to melting point very quickly.

• Energy-saving and low cost 

This process is done with very low electricity and pure water. The cost of electricity and water is reduced by more than 40% compared with LPG and other welding processes.

Tungsten inert gas welding

Tungsten inert gas welding is also known as gas tungsten arc welding.

In this process, an inert gas shielded arc welding process using a non-consumable electrode. The electrode may also contain 1 to 2% thoria mix along with the core tungsten or tungsten with 0.15 to 0.40% zirconia. The pure tungsten electrodes are less expensive but also it will carry less current. The thoriated tungsten electrodes carry high current and more desirable because they can strike and maintain a stable at with relative ease. The zirconia added tungsten electrode better than pure tungsten but inferior to thoriated tungsten electrodes.

The process set up :

Tungsten inert gas welding setup is shown in figure it consists of a welding torch at centre electronic inert gas is supplied to the welding zone through the Anil apart surrounding the tungsten electrode to effectively displays the atmosphere Around The World Patel the smaller world tours may not be provided with Anni cooling device for the electrodes but larger ones are provided with circulating cooling water 30 welding process can be used for joining a number of materials through the more aluminium magnesium and stainless steel.

Components used :

• Power Supply (A.C or D.C)
• Filler Rod
• Non-consumable Tungsten electrode
• Welding Head
• Inert Gas Supply

Working principle :

TIG welding process is relatively difficult to perform out of another welding process because it normally requires two hands for the process to be performed while other processes require that the welder manually feed a filler metal into the weld area with one hand.

In this process, first of all, strike the welding arc that can be produced by torch. A high-frequency generator provides an electric spark. This spark is a conductive path for the welding current through the shielding gas and it allows the arc to be initiated while the electrode and the workpiece are separated. The inert gas forms a gas shielding around the weld. It protects the weld from the external atmosphere. 

Once the arc is struck, the welder moves the torch in a small circle to create a welding pool, the size of that depends on the size of the electrode and the amount of current then torch moves back and filler metal is added manually to the front end of the weld pool as it needed.
The arc current is often gradually reduced to allow the weld crater to solidify and prevent the formation of cracks at the end of the weld. Thus, this process can be done.

Advantages of TIG welding :

• It produces high quality and clean welds.
• No slag is produced so welds are not weakened.
• The weld is automatically protected by the inert gas during the welding process so welds are corrosion resistance, more ductile, and stronger.
• This process can apply for any position of welding like wise horizontal, vertical or flat.
• It can be performed by both automatic and manual technique.
• It easily applied to thin material and used for a wide range of metal thickness.
• There is less distortion of work piece because the small heat affected zone.
• Only the necessary amount of filler metal is added to the welding puddle so there is no spatter or sparks are produced.
• Use one shielding gas mainly Argon for all applications.
• In this process finishing process required little or less. Sometimes grinding or preparation before it can be painted.
• It is the preferred choice in most of the intricate works, where shape of each and every weld joint counts.

Main most common limitations of TIG welding is low deposition rate of the filler and metal per pass so that time increased to completed the welds that's why it is mostly used for thinner metal. More than that it has certain disadvantages we can check it below :

Disadvantages of TIG welding :

• As we seen it is slow process.
• More complicated process so that highly skilled labour is required.
• Less economical than consumable electrode for sections thicker than 3/8 inch.
• Tungsten inclusion.
• Welder is exposed to the huge intensities of light.
• This process is also more expensive.
• Sensitive to drafts.

Tungsten Inert Gas Welding offers several advantages that account for its popularity and its use in agriculture and many other industries. 

Applications of Tungsten Inert Gas Welding :

• Stainless steel
• Alloy steel
• Aluminium
• Titanium
• Copper
• Magnesium
• Nickel alloys

Submerged arc welding

Introduction :

Submerged are welding is a common arc welding process in which the formation of an arc between a continuously fed electrode and the workpiece. It is mainly used for doing faster welding joints. The arc is produced while the consumable electrode wire is continuously fed into the weld zone as in gas metal arc welding. The welding zone is completely covered by means of a large amount of flux. The arc occurring between the electrode and the workpiece is completely submerged under the flux and is not visible from outside. A part of the flux melts and forms the slag, which covers the weld metal. The unused flux is collected and reused.

How it works?

This welding process may be automatic or semi-automatic. The main principle of this process is the flux starts depositing on the joint to be welded. The power source used with submerged arc welding can either be AC or DC. Both constant-voltage and constant current type machines can e effectively used. The arc may be stuck either by touching the electrode with the workpiece or by placing steel wool between electrode and job before switching on the welding current. The arc is maintained between the end of the bare wire electrode and the weld. The electrode is constantly fed into the arc as it is melted. In all cases, the arc is stuck under the cover of flux. Flux otherwise is an insulator but once it melts due to the heat of arc, it becomes highly conductive and hence the current flow is maintained between the electrode and workpiece through the molten flux. The upper portion of flux is in contact with the atmosphere which is visible and remains unused. The lower portion of flux becomes slag, which is waste material and it is removed after the welding process is done.

The electrode at a constant predetermined speed continuously fed to the joint to be welded. The arc length is also kept constant by using the principle of self-adjusting arc because if the arc length decreases the voltage will increases. Therefore burn off rate will increases. 

In this process also some backing plate of the material like steel or copper may be used for control penetration and to support a large amount of molten metal associated with this process. 

SAW process mainly depending upon the following factor :

• Arc voltage or Arc length 
• Electrode travel speed 
• Electrode stick out or Contact tip to work 
• Current type AC or DC 
• Wire feed speed 

Advantages of submerged arc welding :

• High deposition rate.
• High operating factors in some applications.
• Deep weld penetration so welded joints are strong.
• It prevents hot materials from splattering and splashing onto workers because arc is always covered under the blanket of flux.
• High speed welding for thin plates.
• SAW is suitable for indoor and also outdoor works.
• Single pass welds can be made in thick plates. 
• Most of the flux are reused in this process. 
• Very neat appearance and smooth weld shapes can be got.
• Good ductility and corrosion resistance and good impact strength are formed in joints.
• In this process we did not required to add pressure because it already generated by electrode.

Limitations of submerged arc welding :

• This process is limited to steel, stainless steels and nickel.
• Limited to the 1F, 1G and 2F positions. 
• Limited to high thickness metal plates.
• Flux is subjected to contamination that may caused porosity in welded joints.
• The flux needs replacing on the joint which is not always possible.
• Requires backing strips for proper penetration.
• Flux and slag residue can present a health and safety concern.

Electroslag welding

Introduction :

The electro slag welding process is developed mainly to weld very large metal plates without any edge preparation. This process is most of the cases single-pass process using a consumable electrode for filling the gap between the two heavy plates. The heat required for melting the plates and the electrode is obtained initially by means of an arc so that flux will form the molten slag. Once the molten slag is formed, the arc is extinguished and the heat of welding is obtained by the resistance heating of the slag itself. 

How it works?

First of all, in this process, the gap between the two workpieces is filled with a welding flux. It required to maintain a satisfactory amount of slag is fairly small in the order of 0.2 to 0.3 kg per meter of weld length, irrespective of plate thickness. The welding is initiated by an arc between the electrode and workpiece thus, heat utilized for melting the slag is much less. Most of the heat supplied in electroslag welding it melts the fluxing powder and forms the molten slag. The slag, having low electrical conductivity and is maintained in a liquid state due to the heat produced by the electric current.

By this process, a plate of 200 mm thickness can easily be welded in a single pass. The slag reaches a temperature of about the 1930 ⁰C. This temperature is sufficient for melting the consumable electrode and workpiece edges. Metal droplets fall to the weld pool and join the workpiece.

For effective welding, it is necessary to maintain a continuous slag pool and therefore the best way to maintain it. The slag pool is contained in the groove with the help of two water-cooled copper dam plates which move along with the weld.

Electroslag welding is mainly used for welding the steels.

The quality of weld in this process mainly depends upon :

• Slag depth.
• The ratio of a width of the weld pool and its maximum depth is known as the form factor.
• Weld current and voltage.
• A number of electrodes used.

Advantages of electro slag welding :

• Most of the cases welding are done easily by a single pass.
• If any gas is present easily bubbles out through the slag and therefore, a better weld can be done.
• The heating and cooling of the edge are more gradual.
• Whatever be the thickness of the plate,  no edge preparation is required.
• It is also useful for very thick plates. 
• High deposition rate - up to 45 lbs/h (20 kg/h).
• Low slag consumption and low distortion.

Disadvantages of electro slag welding :

• The coarse grain structure of the weld.
• An only vertical position of the weld is possible.
• Low toughness of the weld. 

Application of electro slag welding :

• Fabrication of high-pressure vessels.
• Frames of heavy mechanical and hydraulic presses. 
• In Rolling mill frames. 
• Ship hulls and locomotive frames.

Resistance welding

Introduction :

All welding process are fusion welding process. The resistance welding process is also a fusion welding process where both heat and pressure are applied on the joint but no filler metal or flux is added. The heat necessary for the melting of the joint is obtained by the heating effect of the electrical resistance of the joint and hence, the name is resistance welding.

How it works?

In this process, a low voltage and very high current are passed through the joint for a very short time. This high heats the joint, due to the contact resistance at the joint and melts it. The pressure on the joint continuously maintained and the metal fuses together under this pressure. The heat generated in resistance welding can be expressed as 

H = k I² R t

Where 

H = The total heat generated in the work ( J )

I = electric current ( A )

t = time for which the electric current is passing through the joint ( s )

R = Resistance of the joint ( ohms ) 

k = A constant to account for the heat losses from the welded joint 

In this process, the amount of heat released is directly proportional to the resistance. It is likely to be released at all of the mentioned points below :

• The resistance of the electrodes
• Contact resistance between the electrode and workpiece
• Contact resistance between the two workpiece plates
• The resistance of the workpiece plates

All of above a large amount of heat is to be generated to have an effective fusion is at the interface between the two workpiece plates. 

Because of the squaring in the above equation, the current I needs to be precisely controlled for any proper joint.

At first, apply force and current through the electrodes contacted with metal parts to be welded and resistance heat is generated at the interface of metal parts and hence the metal parts melt and form the joint. Through a large current flows, there is no danger of an electric shock because the only low voltage is impressed.

Electrodes for resistance welding :

The electrodes in resistance welding carry the very high current required for fusion, as also transmit the mechanical force to keep the plates under pressure and in alignment during fusion. They also help to remove the heat from the weld zone thus preventing overheating and surface fusion of the work. For both of the purpose in this process, the electrode should have higher electrical conductivity as well as high hardness. 
Copper is alloyed form is generally used for making electrodes. Though pure copper has high electrical and thermal conductivity, it is poor in mechanical properties.

Copper-cadmium alloys have the highest electrical conductivity with moderate strengths and therefore are used for welding non-ferrous materials such as aluminium and magnesium alloys.

Copper-chromium alloys have slightly lower electrical conductivities than above but better mechanical strength. These are used for resistance welding of low-strength steels such as mild steels and low alloy steels. 
When cobalt and beryllium are added to copper, its conductivity is decreased to a great extent but the strength is increased. Hence, these are used for welding higher heat-resisting alloys such as stainless steels and steels with tungsten and other such alloying elements.  

Advantages of resistance welding :

• High-speed welding.
• Economical welding process.
• Easily automated.
• Easy operation so that suitable for high rate of production.
• No flux require such as solder is necessary.
• Electric facility is required in some cases due to use of large current.
• Possible to weld dissimilar metals as well as metal plates of different thickness.
• Very little skill is required to operate resistance welding machine.

Disadvantages of resistance welding :

• Visual inspection is difficult because welded portion can't be checked from the outside.
• Initial equipment cost is high.
• Lower tensile and fatigue strength of welded joint that formed by using this process.
• This welding process are limited only to lap joints.

The various resistance welding processes of interest are mentioned below :

• Resistance spot welding 
• Resistance seam welding 
• Projection welding 
• Upset welding 
• Flash welding

Thermit welding

Introduction :

Thermit welding is a process, which was traditionally used for the welding of very thick plates. Though this was used for welding large sections such as locomotive rails, ship hulls and broken large castings, its use has decreased nowadays because of the availability of other simpler methods such as submerged arc welding. 

A thermit mixture that uses for welding steels is aluminium and iron oxide. One question arises in your mind that what is thermit mixture?
The heat source utilized for fusion in this welding process is the exothermic reaction of the thermit mixtures. The thermit reaction starts when the mixed thermit powder is brought to its ignition temperature of 1200 ⁰C. 

How it works?

The thermit welding in which the molten metal obtained by the thermit reaction is poured into the refractory cavity made around the joint. It is a similar process as casting. The two pieces to be joined are properly cleaned and the edge is prepared. Then wax is poured into the joint so that a pattern is formed where the weld is to be obtained. Around the joint moulding, a flask is kept and sand is rammed carefully around the wax patterns as shown in below figure providing necessary pouring basin, risers and sprue. For the run off the molten wax, the bottom opening is also provided. The wax is melted through this opening which is also used to preheat the joint and make it ready for welding.

The thermit mixture which is mixed with fluxes is filled into a ladle through the bottom opening. The opening is initially closed. The igniting mixture which is normally barium peroxide or magnesium is placed at the top of the thermit mixture. The igniting mixture is lighted by means of a heated metal rod, whereby the complete reaction takes place and molten metal is produced. The bottom plug of the ladle is opened and the metal is allowed to flow into the mould prepared. The weld joint is allowed to cool slowly. Thus weld is formed. If we making a fast weld, thermit welding also provides a reasonably strong weld. The strength if thermit welded joint reaches that of a forged metal without any defects.

Applications :

The main application of this welding technique is in the repair works of rails in railways.

Advantages of thermit welding :

• Very large and heavy parts are also joined.
• No external power source is required it just use the heat of the chemical reaction.
• It is also used for building up large wobblers.

Disadvantages of thermit welding :

• Weld may contain gas mainly hydrogen so that the slag inclusion problem occurs.
• The metal which has low melting point can't weld by this process.
• Low deposition rate with operating factor.
• Only ferrous parts may be weld by this process.
• It is the high-temperature process so that cause distortion and changes in Grain structure in the weld region.

Electron beam welding

Introduction :

Electron beam welding is a powerful beam welding process. The heat source in electron beam welding for melting joints is a focused beam of high-velocity electrons. The electron beam upon impinging on the workpiece releases the necessary heat by converting its kinetic energy. 
Electron beam welding is a fusion welding process in which a beam of high-velocity electrons is applied to two materials to be joined. It is often performed under vacuum conditions to prevent dissipation of the electron beam.

How it works?

The cathode within the electron gun is the source of a stream of electrons. The electrons are accelerated towards the anode because of the large potential difference that exists between them. The potential difference between that are used are of the order of 30 kV to 175 kV. The higher the potential difference, the higher would be the acceleration. The current levels are low, ranging between 50 mA to 1000 mA. Depending on the accelerating voltage, the electrons would travel at the speed of 50000 to 200000 km/s. The depth of penetration of the weld depends on this electron speed which in turn is dependent upon the accelerating voltage.

The electron beam is focused by means of an electron magnetic lens so that the energy is released in a small area. When the high-velocity electron beam strikes the workpiece all the kinetic energy is converted to heat. As these electrons penetrate the metal, the material that is directly in the path is melted and a keyhole is formed melting the metal around the beam. As the beam traverses, the keyhole would also travel along, with the molten metal being pushed back which when solidified and thus forms the joint.

Advantages of electron beam welding :

• The penetration of the beam is high. 
• The depth-to-width ratios between 10:1 to 30:1 can be easily realized with electron beam welding.
• It is also possible to closely control this penetration by controlling the accelerating voltage, beam current and beam focus. 
• The process can be used at higher welding speeds, typically between 125 to 200 mm/s.
• Filler metal or flux are not needed to be used in this process of welding.
• The heat liberated is low and also is in a narrow zone. Thus, the heat-affected zone is minimal as well as weld distortion is eliminated.
• It is also possible to carry out electron beam welding with workpieces in an open atmosphere.
• The other advantage of using a vacuum is that the weld metal is not contaminated.

Limitations of electron beam welding :

• EBW is the most costly welding process due to vacuum enclosure.
• This process requires a vacuum chamber containing a hard vacuum.
• Only small to medium size items can be welded.
• Though the welding itself can be done very fast, overall EBW is time-consuming.
• The equipment is complex and there are quite a few process variables involved.

You can also check it out :

• Applications of electron beam welding
• Adcvanatages and disadvantages of electron beam welding

Laser beam welding

Introduction :

Laser beam welding is a technique of welding which used to join multiple pieces of metal through the use of laser. It is frequently used in high volume applications using automation, such as in the automotive industries.
Now we all know that laser is a concentrated beam of coherent monochromatic radiation. Normal white light consists of a number of colours and a number of waves. As a result, it is not possible to use a monochromatic source as laser, provided all waves are of a single phase. This is achieved by means of stimulation. Because of coherency, it is possible to concentrate the laser beam by means of an optical lens to a spot of any desired size without appreciably losing any of its intensity. Thus, the laser beam is a high-energy source of heat to melt a joint for fusion welding in laser beam welding.

Laser beam welding operates in two fundamentally different modes :

• Conduction limited welding 
• Keyhole welding 

The mode in which the laser beam will interact with the material it is welding will depend on the power density of the focus laser spot on the workpiece.

How it works?

There are generally two types of laser which are used in welding operation such as solid-state lasers and gas lasers.

In solid-state lasers, light is emitted from a glass on a single crystal that is doped with transition elements such as chromium for ruby. When a beam of normal white light impinges on the crystal, the outer shell electrons of the dope elements go to a higher energy metastable state. They return to the normal state after emitting the extra energy in the form of a photon. All the photons that are stimulated to emit at a given instant will form coherent radiation, which can be concentrated by optical lenses. Thus, the output would be normally in pulses. The power ratings of such units may be up to 2 kW.

In gas lasers, the gas such as carbon dioxide molecules is excited to the higher vibrational energy level by means of an electric discharge. The transition from this high-energy level to the normal level generates the radiation which is coherent and gets focused by means of the usual optical lenses. Continuous-wave gas lasers using carbon dioxide gas with powers up to 20 kW are used for laser beam welding. 

With low power lasers typically less than 1 kW, the penetration would not be much and the weld is obtained by means of complete welding of the joint near the surface. But as the power increased because of that the large heat density obtained. That large density causes the metal at the centre of the jet to be vaporized with a keyhole being formed similar to that of electron beam welding. The temperature within this keyhole can reach as high as 25000 ⁰C. This technique permits large welding speeds depending on laser size.  

There are three types of lasers that are commonly used in welding operation such as CO₂, Nd: YAG and fibre laser. The Nd: YAG laser light is absorbed quite well by conductive materials, with a typical reflectance of about 20 to 30 % for most metals. Using standard optics, it is possible to achieve focused spot sizes diameter as small as 0.025 mm. On the other side, the CO₂ laser has an initial reflectance of about 80 to 90 % for most metals and requires special optics to focus the beam to a minimum spot size of about 0.075 to 0.100 mm diameter. However, whereas the Nd: YAG lasers might produce power outputs up to 500 watts, the CO₂ system can easily supply 10 kW of power and more.


Advantages of laser beam welding :

• Heat input is close to the in the minimum required to fuse the weld metal, thus heat-affected zones are reduced and workpiece distortions are minimized.
• Time for welding thick sections is reduced and the need for filler wires and elaborate joint preparations is eliminated by employing the single pass laser welding procedures.
• No electrodes are required.  
• LBM being a non-contact process so distortions are minimized and tool wears are eliminated.
• Welding in areas that are not easily accessible with other means of welding can be done by LBM.
• The joining of small spaced components with tiny welds very easily because of laser beam can focused on a small area.
• Wide variety of materials including various combinations can be welded very easily.
• Thin welds on small diameter wires are less susceptible to burn back than is the case with arc welding. 
• Metals with dissimilar physical properties, such as electric resistance can also be welded by LBW.
• No vacuum or X-Ray shielding is required. 
• Welds magnetic materials also. 
• Aspect ratios means depth-to-width ratio of the order of 10:1 are attainable in LBM.
• Faster welding rate.
• No flux or filler metal required.
• Single-pass two-side welding. 
• Shorter cycle and higher up times. 

Limitations of laser beam welding :

• Joints must be accurately positioned laterally under the beam.
• Final position of the joint is accurately aligned with the beam impingement point.
• The maximum joint thickness that can be welded by laser beam is somewhat limited. 
• The materials have high thermal conductivity and reflectivity like Al and Cu alloys can affect the weldability with lasers. 
• An appropriate plasma control device must be employed to ensure the weld reproducibility while performing moderate to high power laser welding. 
• Lasers tend to have low energy conversion efficiency less than 10 percent. 
• Some weld-porosity and brittleness can be expected, as a consequence of the rapid solidification characteristics of the LBM. 

Laser beam welded parts are often required additional steps before and after the actual weld. we can see this additional steps below :

Process before welding :

• CAD/CAM product design and weld design.
• Tooling design and fabrication.
• Parts cleaning.
• Strategic Sourcing and Subcontractor Contract Management

Process after welding :

• Leak test.
• Metallurgic evaluations.
• Post weld thermal treatment.
• Not destructive testing.

You also check it out the :

• laser beam welding application

Plasma arc welding

Introduction :

In plasma arc welding plasma is the state of the matter when part of the gas is ionized making it a conductor of electric current. It is the state of matter present in between the electrode in an arc. The plasma arc welding closely resembles that of the TIG welding in that it also uses a non-consumable tungsten electrode and a shielding gas such as argon.
The plasma welding process was introduced to the welding industry as a method of bringing better control to the arc welding process in lower current ranges. 

How does it work?

In plasma arc welding, the plasma arc is tightly constrained and a small amount of pure argon gas flow is allowed through the inner orifice surrounding the tungsten electrode to form the plasma gas.

To initiate the arc in PAW, a low current pilot arc is obtained between the electrode and the constricting nozzle which is heated to an extremely high temperature and ionized so that it becomes electrically conductive. The arc in this type of welding is concentrated and straight. and similar to tungsten inert gas welding plasma to transfer an electric arc to a workpiece. The metal to be welded is melted by the intense heat of the arc and metal fuses together thus welding is done.

In plasma welding torch a tungsten electrode is located within a copper nozzle having a small opening at the tip. A pilot arc is produced between the torch electrode and nozzle tip. This arc is then transferred to the metal to be welded.

The plasma gas itself is not sufficient to protect the weld metal therefore, a large volume of inert-shielding gas is allowed to flow through an outer gas nozzle surrounding the inner nozzle. Plasma gases are normally argon. The torch also uses a secondary gas surcharge, argon/hydrogen or helium which assists in shielding the molten weld puddle thus minimizing the oxidation of the weld.

The power source used is DC with electrode negative for better electrode life. 
The actual welding by PAW is done by means of a technique called a keyhole.

Equipment Required List :

• Power Supply
• Plasma Console (sometimes external, sometimes built-in)
• Water re-circulator (sometimes external, sometimes built-in)
• Plasma Welding Torch
• Torch Accessory Kit (Tips, ceramics, collets, electrodes set-up gages)

Advantages of plasma arc welding :

• The main advantages of this are lies in the control and quality produced in the part being welded.
• The torch design allows for better control of the arc, as well as a higher tolerance for in torch standoff distance.
• Welds are typically cleaner and smoother in PAW process.
• Smaller heat affected zone results welds are very strong.
• Metal deposit rates are high.

Limitations of plasma arc welding :

• Welding equipment is expensive.
• Nozzle surrounding the electrode needs a frequent replacement.
• Relatively high startup costs.
• High skilled workers required.