Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the work pieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the work pieces to form a bond between them, without melting the work pieces.
Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding may be performed in many different environments, including open air, water and in outer space. Welding is a potentially hazardous undertaking and precautions are required to avoid burns, electric shock, vision damage, inhalation of poisonous gases and fumes, and exposure to intense ultraviolet radiation.
Classification of welding process:
Welding process can be classified into different categories depending upon the following criteria :
(a) It can be classified as fusion welding or pressure welding depending upon on the application of heat. If application of heat is not required, it is called pressure welding.
(b) In case of fusion welding it can classified low temperature welding and high temperature welding. When heat is generated to develop low temperature it is called low temperature welding like soldering and brazing. Other fusion welding methods are high temperature welding methods.
(c) Fusion welding can also be classified on the basis of method of heat generation like gas welding, electric arc welding, resistance welding, thermit welding, etc.
(d) On the basis of the type of joint produced it can be categorized as butt welding, seam welding, spot welding, lap joint welding, etc.
Each of the above type of welding can be further classified depending on other micro level characteristics.
It is a fusion welding in which strong gas flame is used to generate heat and raise temperature of metal pieces localized at the place where joint is to be made. In this welding metal pieces to be joined are heated. The metal thus melted starts flowing along the edges where joint is to be made. A filler metal may also be added to the flowing molten metal to fill up the cavity at the edges. The cavity filed with molten metal is allowed to solidify to get the strong joint. Different combinations of gases can be used to obtain a heating flame. The popular gas combinations are oxy-hydrogen mixture, oxygen-acetylene, etc. different mixing proportion of two gases in a mixture can generate different types of flames with different characteristics.
Oxy-acetylene welding can used for welding of wide range of metals and alloys. Acetylene mixed with oxygen when burnt under a controlled environment produces large amount of heat giving higher temperature rise. This burning also produces carbon dioxide which helps in preventing oxidation of metals being welded. Highest temperature that can be produced by this welding is 3200⁰C. The chemical reaction involved in burning of acetylene is
2C2 H2 + 5O2 = 4CO2 + 2H2 O + Heat
on the basis of supply pressure of gases oxy-acetylene welding is categorized as high pressure welding in this system both gases oxygen and acetylene supplied to welding zone are high pressure from their respective high pressure cylinders. The other one is low pressure welding in which oxygen is supplied from high pressure cylinder but acetylene is generated by the action of water on calcium carbide and supplied at low pressure. In this case high pressure supply of oxygen pulls acetylene at the welding zone.
A comparison can be drawn between low pressure and high pressure welding. High pressure welding equipment is handy, supplies pure acetylene at constant pressure, with better control and low expenses as compared to low pressure welding.
Electric resistance welding (ERW) refers to a group of welding processes such as spot and seam welding that produce coalescence of faying surfaces where heat to form the weld is generated by the electrical resistance of material v/s. the time and the force used to hold the materials together during welding. Some factors influencing heat or welding temperatures are the proportions of the work pieces, the metal coating or the lack of coating, the electrode materials, electrode geometry, electrode pressing force, electrical current and length of welding time. Small pools of molten metal are formed at the point of most electrical resistance (the connecting or “faying” surfaces) as an electrical current (100–100,000 A) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials and the equipment cost can be high.
Submerged arc welding (SAW)
Submerged arc welding (SAW) is a common arc welding process. It requires a non-continuously fed consumable solid or tubular (flux cored) electrode. The molten weld and the arc zone are protected from atmospheric contamination by being “submerged” under a blanket of granular fusible flux consisting of lime,silica, manganese oxide, calcium fluoride, and other compounds. When molten, the flux becomes conductive, and provides a current path between the electrode and the work. This thick layer of flux completely covers the molten metal thus preventing spatter and sparks as well as suppressing the intense ultraviolet radiation and fumes that are a part of the shielded metal arc welding (SMAW) process.
SAW is normally operated in the automatic or mechanized mode, however, semi-automatic (hand-held) SAW guns with pressurized or gravity flux feed delivery are available. The process is normally limited to the flat or horizontal-fillet welding positions (although horizontal groove position welds have been done with a special arrangement to support the flux). Deposition rates approaching 100 lb/h (45 kg/h) have
been reported — this compares to ~10 lb/h (5 kg/h) (max) for shielded metal arc welding. Although Currents ranging from 300 to 2000 A are commonly utilized, currents of up to 5000 A have also been used (multiple arcs).
Single or multiple (2 to 5) electrode wire variations of the process exist. SAW strip-cladding utilizes a flat strip electrode (e.g. 60 mm wide x 0.5 mm thick). DC or AC power can be used, and combinations of DC and AC are common on multiple electrode systems. Constant voltage welding power supplies are most commonly used; however, constant current systems in combination with a voltage sensing wire-feeder are available.
· High deposition rates (over 100 lb/h (45 kg/h) have been reported).
· High operating factors in mechanized applications.
· Deep weld penetration.
· Sound welds are readily made (with good process design and control).
· High speed welding of thin sheet steels up to 5 m/min (16 ft/min) is possible.
· Minimal welding fume or arc light is emitted.
· Practically no edge preparation is necessary.
· The process is suitable for both indoor and outdoor works.
· Low distortion
· Welds produced are sound, uniform, ductile, corrosion resistant and have good impact value.
· Single pass welds can be made in thick plates with normal equipment.
· The arc is always covered under a blanket of flux, thus there is no chance of spatter of weld.
· 50% to 90% of the flux is recoverable.
· Limited to ferrous (steel or stainless steels) and some nickel-based alloys.
· Normally limited to the 1F, 1G, and 2F positions.
· Normally limited to long straight seams or rotated pipes or vessels.
· Requires relatively troublesome flux handling systems.
· Flux and slag residue can present a health and safety concern.
· Requires inter-pass and post weld slag removal.
Gas tungsten arc welding (GTAW),
Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by an inert shielding gas (argon or helium), and a filler metal is normally used, though some welds, known as autogenously 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.
Fig: GTAW weld area
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 serves 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.
Gas metal arc welding (GMAW):
Gas metal arc welding (GMAW), sometimes referred to by its subtypes metal inert gas (MIG) welding or metal active gas (MAG)welding, is a welding process in which an electric arc forms between a consumable wire electrode and the workpiece metal(s), which heats the workpiece metal(s), causing them to melt, and join. Along with the wire electrode, a shielding gas feeds through the welding gun, which shields the process from contaminants in the air. The process can be semi-automatic or automatic. A constant voltage, direct 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.
Fig:GMAW weld area. (1) Direction of travel, (2) Contact tube, (3) Electrode, (4) Shielding gas, (5) Molten weld metal,(6) Solidified weld metal, (7) Workpiece.
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.
The basic technique for GMAW is quite simple, since the electrode is fed automatically through the torch (head of tip). 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 stickout distance can cause the electrode to overheat and also wastes shielding gas. Stickout distance varies for different GMAW weld processes and applications. 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.
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.
Gas metal arc welding can be dangerous if proper precautions are not taken. Since GMAW employs an electric arc, welders wear protective clothing, including heavy leather gloves and protective long sleeve jackets, to avoid exposure to extreme heat and flames. In addition, the brightness of the electric arc is a source of the condition known as arc eye, an inflammation of the cornea caused by ultraviolet light and, in prolonged exposure, possible burning of the retina in the eye. Conventional welding helmets contain dark face plates to prevent this exposure. Newer helmet designs feature a liquid crystal-type face plate that self-darken upon exposure to high amounts of UV light. Transparent welding curtains, made of a polyvinyl chloride plastic film, are often used to shield nearby workers and bystanders from exposure to the UV light from the electric arc.
Welders are also often exposed to dangerous gases and particulate matter. GMAW produces smoke containing particles of various types of oxides, and the size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. Additionally, carbon dioxide and ozone gases can prove dangerous if ventilation is inadequate. Furthermore, because the use of compressed gases in GMAW pose an explosion and fire risk, some common precautions include limiting the amount of oxygen in the air and keeping combustible materials away from the workplace.
Plasma arc welding (PAW):
Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an electrode (which is usually but not always made of sintered tungsten) and the workpiece. The key difference from GTAW is that in PAW, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. The plasma is then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities (approaching the speed of sound) and a temperature approaching 28,000 °C (50,000 °F) or higher. Arc plasma is the temporary state of a gas. The gas gets ionized after passage of electric current through it and it becomes a conductor of electricity. In ionized state atoms break into electrons(-) and ions(+) and the system contains a mixture of ions, electrons and highly exited atoms. The degree of ionization may be between 1% and greater than 100% i.e; double and triple degrees of ionization. Such states exist as more number of electrons are pulled from their orbits.
The energy of the plasma jet and thus the temperature is dependent upon the electrical power employed to create arc plasma. A typical value of temperature obtained in a plasma jet torch may be of the order of 500000F against about 100000F in ordinary electric welding arc. Actually all welding arcs are (partially ionized) plasmas but, the one in plasma arc welding is a constricted arc plasma.
Principle of operation:
Plasma arc welding is a constricted arc process. The arc is constricted with the help of a water-cooled small diameter nozzle which squeezes the arc, increases its pressure, temperature and heat intensely and thus improves arc stability, arc shape and heat transfer characteristics. Plasma arc welding process can be divided into two basic types:
(A)Non-transferred arc process: The arc is formed between the electrode(-) and the water cooled constricting nozzle(+). Arc plasma comes out of the nozzle as a flame. The arc is independent of the work piece and the work piece does not form a part of the electrical circuit. Just as an arc flame (as in atomic hydrogen welding), it can be moved from one place to another and can be better controlled. The non
transferred arc plasma possesses comparatively less energy density as compared to a transferred arc plasma and it is employed for welding and in applications involving ceramics or metal plating (spraying). High density metal coatings can be produced by this process. A non-transferred arc is initiated by using a high frequency unit in the circuit.
(B)Transferred arc process: The arc is formed between the electrode(-) and the work piece(+). In other words, arc is transferred from the electrode to the work piece. A transferred arc possesses high energy density and plasma jet velocity. For this reason it is employed to cut and melt metals. Besides carbon steels this process can cut stainless steel and nonferrous metals also where oxyacetylene torch does not succeed. Transferred arc can also be used for welding at high arc travel speeds. For initiating a transferred arc, a current limiting resistor is put in the circuit, which permits a flow of about 50 amps, between the nozzle and electrode and a pilot arc is established between the electrode and the nozzle. As the pilot arc touches the job main current starts flowing between electrode and job, thus igniting the transferred arc. The pilot arc initiating unit gets disconnected and pilot arc extinguishes as soon as the arc between the electrode and the job is started. The temperature of a constricted plasma arc may be of the order of 8000 – 250000C.
Thermite welding (TW),
Exothermic welding, also known as exothermic bonding, thermite welding (TW), and thermit welding, is a welding process for joining materials that employs molten metal to permanently join the conductors. The process employs an exothermic reaction of a thermite composition to heat the metal, and requires no external source of heat or current. The chemical reaction that produces the heat is an aluminothermic reaction between aluminium powder and a metal oxide.
An exothermic weld has higher mechanical strength than other forms of weld, and excellent corrosion resistance. It is also highly stable when subject to repeated short-circuit pulses, and does not suffer from increased electrical resistance over the lifetime of the installation. However, the process is costly relative to other welding processes, requires a supply of replaceable moulds, suffers from a lack of repeatability, and can be impeded by wet conditions or bad weather (when performed outdoors).
Exothermic welding is usually used for welding copper conductors but is suitable for welding a wide range of metals, including stainless steel, cast iron, common steel, brass, bronze, and Monel. It is especially useful for joining dissimilar metals. The process is marketed under a variety of names such as Quikweld, Tectoweld, Ultraweld,Cadweld, Techweld,Thermoweld and Kumwell.
Because of the good electrical conductivity and high stability in the face of short-circuit pulses, exothermic welds are one of the options specified by §250.7 of the United States National for grounding conductors and bonding jumpers. It is the preferred method of bonding, and indeed it is the only acceptable means of bonding copper to galvanized cable. The NEC does not require such exothermically welded connections to be listed or labeled, but some engineering specifications require that completed exothermic welds be examined using X-ray equipment.
Electron beam welding (EBW):
Electron beam welding (EBW) is a fusion welding process in which a beam of high-velocity electrons is applied to two materials to be joined. The work pieces melt and flow together as the kinetic energy of the electrons is transformed into heat upon impact. EBW is often performed under vacuum conditions to prevent dissipation of the electron beam.
Electron beam welding equipment:
Fig:Electron beam welder
To cover the most various requirements, countless welder types have been designed, differing in construction, working space volume, workpiece manipulators and beam power. Electron beam generators (electron guns) designed for welding applications can supply beams with power ranging from a few watts up to about one hundred kilowatts. “Micro-welds” of tiny components can be realized, as well as deep welds up to 300 mm (or even more if needed). Vacuum working chambers of various design may have a volume of only a few liters, but vacuum chambers with the volume of several hundred cubic meters have also been built.
Specifically, the equipment comprises:
1 Electron gun, generating the electron beam,
2 Working chamber, mostly evacuated to “low” or “high” vacuum,
3 Workpiece manipulator (positioning mechanism),
4 Supply and control/monitoring electronics.
In the electron gun, the free electrons are gained by thermo-emission from a hot metal strap (or wire). They are then accelerated and formed into a narrow convergent beam by an electric field produced by three electrodes: the electron emitting strap, the cathode connected to the negative pole of the high (accelerating) voltage power supply (30 – 200 kV) and the positive high voltage electrode, the anode. There is a third electrode charged negatively with respect to the cathode, called the Wehnelt or control electrode. Its negative potential controls the portion of emitted electrons entering into the accelerating field, i.e., the electron beam current.
After passing the anode opening, the electrons move with constant speed in a slightly divergent cone. For technological applications the divergent beam has to be focused, which is realized by the magnetic field of a coil, the magnetic focusing lens.
For proper functioning of the electron gun, it is necessary that the beam be perfectly adjusted with respect to the optical axes of the accelerating electrical lens and the magnetic focusing lens. This can be done by applying a magnetic field of some specific radial direction and strength perpendicular to the optical axis before the focusing lens. This is usually realized by a simple correction system consisting of two pairs of coils. By adjusting the currents in these coils any required correcting field can be produced.
After passing the focusing lens, the beam can be applied for welding, either directly or after being deflected by a deflection system. This consists of two pairs of coils, one for each X and Y direction. These can be used for “static” or “dynamic” deflection. Static deflection is useful for exact positioning of the beam by welding. Dynamic deflection is realized by supplying the deflection coils with currents which can be controlled by the computer. This opens new possibilities for electron beam applications, like surface hardening or annealing, exact beam positioning, etc.
The fast deflection system can also be applied (if provided with appropriate electronics) for imaging and engraving. In this case the equipment is operated like a scanning electron microscope, with a resolution of about 0,1 mm (limited by the beam diameter). In a similar mode the fine computer controlled beam can “write” or “draw” a picture on the metal surface by melting a thin surface layer.
Since the appearance of the first electron beam welding machines at the end of the 1950s, the application of electron beam welding spread rapidly into industry and research in all highly developed countries. Up to now, uncountable numbers of various types of electron beam equipment have been designed and realized. In most of them the welding takes place in a working vacuum chamber in a high or low vacuum environment.
The vacuum working chamber may have any desired volume, from a few liters up to hundreds of cubic meters. They can be provided with electron guns supplying an electron beam with any required power up to 100 kW, or even more if needed. In micro-electron beam devices, components with dimensions in tenths of a millimeter can be precisely welded. In welders with electron beams of high enough power, welds up to 300 mm deep can be realized.
There are also welding machines in which the electron beam is brought out of vacuum into the atmosphere. With such equipment very large objects can be welded without huge working chambers.
Electron beam welding can never be “hand-manipulated”, even if not realized in vacuum, as there is always strong X-radiation. The relative motion of the beam and the workpiece is most often achieved by rotation or linear travel of the workpiece. In some cases the welding is realized by moving the beam with the help of a computer controlled deflection system. Workpiece manipulators are mostly designed individually to meet the specific requirements of the welding equipment.
Power supply and control/monitoring electronics
Electron beam equipment must be provided with an appropriate power supply for the beam generator. The accelerating voltage may be chosen between 30 and 200 kV. Usually it is about 60 or 150 kV, depending on various conditions. With rising voltage the technical problems and the price of the equipment rapidly increase, hence, whenever it is possible a lower voltage of about 60 kV is to be chosen. The maximum power of the high voltage supply depends on the maximum depth of weld required.
The high voltage equipment must also supply the low voltage, above 5 V, for the cathode heating, and negative voltage up to about 1000 V for the control electrode.
The electron gun also needs low voltage supplies for the correction system, the focusing lens, and the deflection system. The last mentioned may be very complex if it is to provide computer controlled imaging, engraving and similar beam applications.
Complex electronics may also be needed to control the work-piece manipulator.
Laser beam welding (LBW):
Laser beam welding (LBW) is a welding technique used to join multiple pieces of metal through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume applications, such as in the automotive industry.
Like electron beam welding (EBW), laser beam welding has high power density (on the order of 1 MW/cm2) resulting in small heat-affected zones and high heating and cooling rates. The spot size of the laser can vary between 0.2 mm and 13 mm, though only smaller sizes are used for welding. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point: penetration is maximized when the focal point is slightly below the surface of the workpiece
A continuous or pulsed laser beam may be used depending upon the application. Millisecond-long pulses are used to weld thin materials such as razor blades while continuous laser systems are employed for deep welds.
LBW is a versatile process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum, and titanium. Due to high cooling rates, cracking is a concern when welding high-carbon steels. The weld quality is high, similar to that of electron beam welding. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the workpieces. The high power capability of gas lasers make them especially suitable for high volume applications. LBW is particularly dominant in the automotive industry.
Some of the advantages of LBW in comparison to EBW are as follows:
– the laser beam can be transmitted through air rather than requiring a vacuum,
– the process is easily automated with robotic machinery,
– x-rays are not generated, and
– LBW results in higher quality welds.
A derivative of LBW, laser-hybrid welding, combines the laser of LBW with an arc welding method such as gas metal arc welding. This combination allows for greater positioning flexibility, since GMAW supplies molten metal to fill the joint, and due to the use of a laser, increases the welding speed over what is normally possible with GMAW. Weld quality tends to be higher as well, since the potential for undercutting is reduced.
· The first type uses one of several solid media, including synthetic ruby and chromium in aluminum oxide, neodymium in glass (Nd:glass), and the most common type, crystal composed of yttrium aluminum garnet doped with neodymium (Nd:YAG).
· Regardless of type, however, when the medium is excited, it emits photons and forms the laser beam.
Solid state laser
Solid-state lasers operate at wavelengths on the order of 1 micrometer, much shorter than gas lasers, and as a result require that operators wear special eyewear or use special screens to avoid damage. Nd:YAG lasers can operate in both pulsed and continuous mode, but the other types are limited to pulsed mode. The original and still popular solid-state design is a single crystal shaped as a rod approximately 20 mm in diameter and 200 mm long, and the ends are ground flat. This rod is surrounded by a flash tube containing xenon or krypton. When flashed, a pulse of light lasting about two milliseconds is emitted by the laser. Disk shaped crystals are growing in popularity in the industry, and flash lamps are giving way to diodes due to their high efficiency. Typical power output for ruby lasers is 10–20 W, while the Nd:YAG laser outputs between 0.04–6,000 W. To deliver the laser beam to the weld area, fiber optics are usually employed.
Gas lasers use high-voltage, low-current power sources to supply the energy needed to excite the gas mixture used as a lasing medium. These lasers can operate in both continuous and pulsed mode, and the wavelength of the laser beam is 10.6 μm. Fiber optic cable absorbs and is destroyed by this wavelength, so a rigid lens and mirror delivery system is used. Power outputs for gas lasers can be much higher than solid-state lasers, reaching 25 kW.
In fiber lasers, the gain medium is the optical fiber itself. They are capable of power up to 50 kW and are increasingly being used for robotic industrial welding.
Laser beam delivery
Modern laser beam welding machines can be grouped into two types. In the traditional type, the laser output is moved to follow the seam. This is usually achieved with a robot. In many modern applications, remote laser beam welding is used. In this method, the laser beam is moved along the seam with the help of a laser scanner, so that the robotic arm does not need to follow the seam any more. The advantages of remote laser welding are the higher speed and the higher precision of the welding process.
Common weld defects include:
i. Lack of fusion
ii. Lack of penetration or excess penetration
vii. Lamellar tearing
Any of these defects are potentially disastrous as they can all give rise to high stress intensities which may result in sudden unexpected failure below the design load or in the case of cyclic loading, failure after fewer load cycles than predicted.
2. Types of Defects
i To achieve a good quality join it is essential that the fusion zone extends the full thickness of the sheets being joined. Thin sheet material can be joined with a single pass and a clean square edge will be a satisfactory basis for a join. However thicker material will normally need edges cut at a V angle and may need several passes to fill the V with weld metal. Where both sides are accessible one or more passes may be made along the reverse side to ensure the joint extends the full thickness of the metal.
Lack of fusion results from too little heat input and / or too rapid traverse of the welding torch (gas or electric).
Excess penetration arises from to high a heat input and / or too slow transverse of the welding torch (gas or electric). Excess penetration – burning through – is more of a problem with thin sheet as a higher level of skill is needed to balance heat input and torch traverse when welding thin metal.
ii. Porosity – This occurs when gases are trapped in the solidifying weld metal. These may arise from damp consumables or metal or, from dirt, particularly oil or grease, on the metal in the vicinity of the weld. This can be avoided by ensuring all consumables are stored in dry conditions and work is carefully cleaned and degreased prior to welding.
iv. Inclusions – These can occur when several runs are made along a V join when joining thick plate using flux cored or flux coated rods and the slag covering a run is not totally removed after every run before the following run.
v. Cracking – This can occur due just to thermal shrinkage or due to a combination of strain accompanying phase change and thermal shrinkage.
In the case of welded stiff frames, a combination of poor design and inappropriate procedure may result in high residual stresses and cracking.
Where alloy steels or steels with a carbon content greater than about 0.2% are being welded, self cooling may be rapid enough to cause some (brittle) martensite to form. This will easily develop cracks.
To prevent these problems a process of pre-heating in stages may be needed and after welding a slow controlled post cooling in stages will be required. This can greatly increase the cost of welded joins, but for high strength steels, such as those used in petrochemical plant and piping, there may well be no alternative.
This is also called centreline or hot cracking. They are called hot cracks because they occur immediately after welds are completed and sometimes while the welds are being made. These defects, which are often caused by sulphur and phosphorus, are more likely to occur in higher carbon steels.
Solidification cracks are normally distinguishable from other types of cracks by the following features:
they occur only in the weld metal – although the parent metal is almost always the source of the low melting point contaminants associated with the cracking
they normally appear in straight lines along the centreline of the weld bead, but may occasionally appear as transverse cracking
solidification cracks in the final crater may have a branching appearance
as the cracks are ‘open’ they are visible to the naked eye
A schematic diagram of a centreline crack is shown below:
On breaking open the weld the crack surface may have a blue appearance, showing the cracks formed while the metal was still hot. The cracks form at the solidification boundaries and are characteristically inter dendritic. There may be evidence of segregation associated with the solidification boundary.
The main cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors which increase the risk include:
insufficient weld bead size or inappropriate shape
welding under excessive restraint
material properties – such as a high impurity content or a relatively large shrinkage on solidification
Joint design can have an influence on the level of residual stresses. Large gaps between components will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Hence weld beads with a small depth to width ratio, such as is formed when bridging a large wide gap with a thin bead, will be more susceptible to solidification cracking.
In steels, cracking is associated with impurities, particularly sulphur and phosphorus and is promoted by carbon, whereas manganese and sulphur can help to reduce the risk. To minimize the risk of cracking, fillers with low carbon and impurity levels and a relatively high manganese content are preferred. As a general rule, for carbon manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%. However when welding a highly restrained joint using high strength steels, a combined level below 0.03% might be needed.
Weld metal composition is dominated by the filler and as this is usually cleaner than the metal being welded, cracking is less likely with low dilution processes such as MMA and MIG. Parent metal composition becomes more important with autogenous welding techniques, such as TIG with no filler.
Avoiding Solidification Cracking
Apart from choice of material and filler, the main techniques for avoiding solidification cracking are:
control the joint fit up to reduce the gaps
clean off all contaminants before welding
ensure that the welding sequence will not lead to a buildup of thermally induced stresses
choose welding parameters to produce a weld bead with adequate depth to width ratio or with sufficient throat thickness (fillet weld) to ensure the bead has sufficient resistance to solidification stresses. Recommended minimum depth to width ratio is 0.5:1
avoid producing too large a depth to width ratio which will encourage segregation and excessive transverse strains. As a rule, weld beads with a depth to width ratio exceeds 2:1 will be prone to solidification cracking
avoid high welding speeds (at high current levels) which increase segregation and stress levels across the weld bead
at the run stop, ensure adequate filling of the crater to avoid an unfavorable concave shape
Hydrogen induced cracking (HIC) – also referred to as hydrogen cracking or hydrogen assisted cracking, can occur in steels during manufacture, during fabrication or during service. When HIC occurs as a result of welding, the cracks are in the heat affected zone (HAZ) or in the weld metal itself.
Four requirements for HIC to occur are:
a) Hydrogen be present, this may come from moisture in any flux or from other sources. It is absorbed by the weld pool and diffuses into the HAZ.
b) A HAZ microstructure susceptible to hydrogen cracking.
c) Tensile stresses act on the weld
d) The assembly has cooled to close to ambient – less than 150oC
HIC in the HAZ is often at the weld toe, but can be under the weld bead or at the weld root. In fillet welds cracks are normally parallel to the weld run but in butt welds cracks can be transverse to the welding direction.
vi Undercutting – In this case the thickness of one (or both) of the sheets is reduced at the toe of the weld. This is due to incorrect settings / procedure. There is already a stress concentration at the toe of the weld and any undercut will reduce the strength of the join.
vii Lamellar tearing – This is mainly a problem with low quality steels. It occurs in plate that has a low ductility in the through thickness direction, which is caused by non metallic inclusions, such as sulphides and oxides that have been elongated during the rolling process. These inclusions mean that the plate can not tolerate the contraction stresses in the short transverse direction.
Lamellar tearing can occur in both fillet and butt welds, but the most vulnerable joints are ‘T’ and corner joints, where the fusion boundary is parallel to the rolling plane.
These problem can be overcome by using better quality steel, ‘buttering’ the weld area with a ductile material and possibly by redesigning the joint.
Prior to any welding, the materials should be visually inspected to see that they are clean, aligned correctly, machine settings, filler selection checked, etc.
As a first stage of inspection of all completed welds, visual inspected under good lighting should be carried out. A magnifying glass and straight edge may be used as a part of this process.
Undercutting can be detected with the naked eye and (provided there is access to the reverse side) excess penetration can often be visually detected.
Liquid Penetrant Inspection
Serious cases of surface cracking can be detected by the naked eye but for most cases some type of aid is needed and the use of dye penetrant methods are quite efficient when used by a trained operator.
This procedure is as follows:
Clean the surface of the weld and the weld vicinity
Spray the surface with a liquid dye that has good penetrating properties
Carefully wipe all the die off the surface
Spray the surface with a white powder
Any cracks will have trapped some die which will weep out and discolour the white coating and be clearly visible
X – Ray Inspection
Sub-surface cracks and inclusions can be detected ‘X’ ray examination. This is expensive, but for safety critical joints – eg in submarines and nuclear power plants – 100% ‘X’ ray examination of welded joints will normally be carried out.
Surface and sub-surface defects can also be detected by ultrasonic inspection. This involves directing a high frequency sound beam through the base metal and weld on a predictable path. When the beam strikes a discontinuity some of it is reflected beck. This reflected beam is received and amplified and processed and from the time delay, the location of a flaw estimated.
Porosity, however, in the form of numerous gas bubbles causes a lot of low amplitude reflections which are difficult to separate from the background noise.
Results from any ultrasonic inspection require skilled interpretation.
Magnetic Particle Inspection
This process can be used to detect surface and slightly sub-surface cracks in ferro-magnetic materials (it can not therefore be used with austenitic stainless steels).
The process involves placing a probe on each side of the area to be inspected and passing a high current between them. This produces a magnetic flux at right angles to the flow of the current. When these lines of force meet a discontinuity, such as a longitudinal crack, they are diverted and leak through the surface, creating magnetic poles or points of attraction. A magnetic powder dusted onto the surface will cling to the leakage area more than elsewhere, indicating the location of any discontinuities.
This process may be carried out wet or dry, the wet process is more sensitive as finer particles may be used which can detect very small defects. Fluorescent powders can also be used to enhance sensitivity when used in conjunction with ultra violet illumination.
Any detected cracks must be ground out and the area re-welded to give the required profile and then the joint must be inspected again.
Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Soldering differs from welding in that soldering does not involve melting the work pieces. In brazing, the filler metal melts at a higher temperature, but the work piece metal does not melt. In the past, nearly all solders contained lead, but environmental concerns have increasingly dictated use of lead-free alloys for electronics and plumbing purposes.
Soldering is used in plumbing, electronics, and metalwork from flashing to jewelry.
Soldering provides reasonably permanent but reversible connections between copper pipes in plumbing systems as well as joints in sheet metal objects such as food cans, roof flashing, rain gutters and automobile radiators.
Jewelry components, machine tools and some refrigeration and plumbing components are often assembled and repaired by the higher temperature silver soldering process. Small mechanical parts are often soldered or brazed as well. Soldering is also used to join lead came and copper foil in stained glass work. It can also be used as a semi-permanent patch for a leak in a container or cooking vessel.
Soldering and brazing
The distinction between soldering and brazing is based on the melting temperature of the filler alloy. A temperature of 450 °C is usually used as a practical delineating point between soldering and brazing . Soft soldering can be done with a heated iron whereas the other methods require a higher temperature torch or furnace to melt the filler metal.
Different equipment is usually required since a soldering iron cannot achieve high enough temperatures for hard soldering or brazing. Brazing filler metal is stronger than silver solder, which is stronger than lead-based soft solder. Brazing solders are formulated primarily for strength, silver solder is used by jewelers to protect the precious metal and by machinists and refrigeration technicians for its tensile strength but lower melting temperature than brazing, and the primary benefit of soft solder is the low temperature used (to prevent heat damage to electronic components and insulation).
Since the joint is produced using a metal with a lower melting temperature than the workpiece, the joint will weaken as the ambient temperature approaches the melting point of the filler metal. For that reason, the higher temperature processes produce joints which are effective at higher temperatures. Brazed connections can be as strong or nearly as strong as the parts they connect, even at elevated temperatures.