7.a. Gas tungsten arc weldingFrom Wikipedia, the free encyclopedia

TIG welding of a bronze sculpture

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 aninert shielding gas (argon or helium), 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 electrical 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.[1]

Applications[edit]

While the aerospace industry is one of the primary users of gas tungsten arc welding, the process is used in a number of other areas. Many industries use GTAW for welding thin workpieces, especially nonferrous metals. It is used extensively in the manufacture of space vehicles, and is also frequently employed to weld small-diameter, thin-wall tubing such as those used in the bicycle industry. In addition, GTAW is often used to make root or first-pass welds for piping of various sizes. In maintenance and repair work, the process is commonly used to repair tools and dies, especially components made of aluminum and magnesium.[18] Because the weld metal is not transferred directly across the electric arc like most open arc welding processes, a vast assortment of welding filler metal is available to the welding engineer. In fact, no other welding process permits the welding of so many alloys in so many product configurations. Filler metal alloys, such as elemental aluminum and chromium, can be lost through the electric arc from volatilization. This loss does not occur with the GTAW process. Because the resulting welds have the same chemical integrity as the original

base metal or match the base metals more closely, GTAW welds are highly resistant to corrosion and cracking over long time periods, making GTAW the welding procedure of choice for critical operations like sealing spent nuclear fuelcanisters before burial.

7.b. Here are 7 advantages of Friction (inertial) welding:

1. Easily joins dissimilar metals. This means the ability to use more expensive corrosion resisting materials where needed, and less resistant but sufficiently strong materials where there is no need- ON THE SAME PART.

2. The full surface of the cross section is made up of bothmetals, airtight and absent of voids.3. Friction welds are higher strength than other means of joining.4. Friction welds often cost less as there are no consumables like filler metals fluxes etc. (This

would be the bottom line for most businessmen, but I chose another, see # 7 below.)5. Friction welds minimize the Heat Affected Zone (HAZ).6. Friction welding minimizes the need to clean  furnace residues from the entire part, post

welding.7. The ability of a designer to optimize material choices by using friction welding cannot be

overstated.

8.a. Arc welding is a welding process, in which heat is generated by an electric arc struck between an electrode and the work piece.Electric arc is luminous electrical discharge between two electrodes through ionized gas.Arc welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or non-consumableelectrodes. The welding region is usually protected by some type of shielding gas, vapor, or slag. Arc welding processes may be manual, semi-automatic, or fully automated. First developed in the late part of the 19th century, arc welding became commercially important in shipbuilding during the Second World War. Today it remains an important process for the fabrication of steel structures and vehicles.

8.b. ELECTRICAL: Mounting components on panels and in cabinets. Earthing components. Bonding doors and panels. Fixing wiring looms.

ELECTRONICS:Fixing fascia panels. Mounting switches, push buttons and instruments. Mounted printed circuit boards.

MECHANICAL: 

Fixing cover plates and maintenance inspection hatches. Attaching machinery guards. Fixing fluid and air lines. Mounting handles and other components. HEATING AND VENTILATION:Attaching flanges, covers and hatches. Mounting fluid chambers. Fixing burners and heating elements. Securing pipes and insulating material.

DECORATIVE AND COMSUMER:Fixing of signs, plates, panels and badges. Attaching pins, findings and clasps to jewellery. Attaching feet to kettles and handles to pans.

MISCELLANEOUS INDUSTRIAL:Hygienic fixing of legs, brackets and stiffener strips to counter and table tops. Securing acoustic insulation. Fixing fireproofing material.

Advantages of using percussion welding types include a shallow heat affected zone, and the time cycle involved is very short. Typical times can be found to be less than 16 milliseconds.

9.a. Submerged arc welding (SAW) is a common arc welding process. The first patent on the submerged-arc welding (SAW) process was taken out in 1935 and covered an electric arc beneath a bed of granulated flux. Originally developed and patented by Jones, Kennedy and Rothermund, the process requires a continuously fed consumable solid or tubular (metal cored) electrode.[1] 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 45 kg/h (100 lb/h) have been reported — this compares to ~5 kg/h (10 lb/h) (max) for shielded metal arc welding. Although currents ranging from 300 to 2000 A are commonly utilized,[2] 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 voltagewelding power supplies are most commonly used; however, constant current systems in combination with a voltage sensing wire-feeder are available.

Advantages[edit]

High deposition rates (over 45 kg/h (100 lb/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 depending on joint configuration and required penetration.

The process is suitable for both indoor and outdoor works.

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, recycled and reused.[4]

Limitations[edit]

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.

9.b. Exothermic welding, also known as exothermic bonding, thermite welding (TW),[1] and thermit welding,[1] is a welding 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 reactionbetween aluminium powder and a metal oxide.

10.a. Friction welding (FRW) is a solid-state welding process that generates heat through mechanical friction between workpieces in relative motion to one another, with the addition of a lateral force called "upset" to plastically displace and fuse the materials. Technically, because no melt occurs, friction welding is not actually a welding process in the traditional sense, but a forging technique. However, due to the similarities between these techniques and traditional welding, the term has become common. Friction welding is used with metals and thermoplastics in a wide variety of aviation and automotive applications.

Here are 7 advantages of Friction (inertial) welding:

1. Easily joins dissimilar metals. This means the ability to use more expensive corrosion resisting materials where needed, and less resistant but sufficiently strong materials where there is no need- ON THE SAME PART.

2. The full surface of the cross section is made up of bothmetals, airtight and absent of voids.3. Friction welds are higher strength than other means of joining.4. Friction welds often cost less as there are no consumables like filler metals fluxes etc. (This

would be the bottom line for most businessmen, but I chose another, see # 7 below.)5. Friction welds minimize the Heat Affected Zone (HAZ).6. Friction welding minimizes the need to clean  furnace residues from the entire part, post

welding.7. The ability of a designer to optimize material choices by using friction welding cannot be

overstated.

10.b. 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 workpieces 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. Weldability[edit]

For welding thin-walled parts, appropriate welding aids are generally needed. Their construction must provide perfect contact of the parts and prevent their movement during welding. Usually they have to be designed individually for a given workpiece.

Not all materials can be welded by an electron beam in a vacuum. This technology cannot be applied to materials with high vapour pressure at the melting temperature, like zinc, cadmium, magnesium and practically all non-metals.

Another limitation to weldability may be the change of material properties induced by the welding process, such as a high speed of cooling. As detailed discussion of this matter exceeds the scope of this article, the reader is recommended to seek more information in the appropriate literature. [1]

Titanium-to-aluminium joints

Joining dissimilar materials[edit]

It is often not possible to join two metal components by welding, i.e. to melt part of both in the vicinity of the joint, if the two materials have very different properties from their alloy, due to the creation of brittle, inter-metallic compounds. This situation cannot be changed, even by electron beam heating in vacuum, but this nevertheless makes it possible to realize joints meeting high demands for mechanical compactness and that are perfectly vacuum-tight. The principal approach is not to melt both parts, but only the one with the lower melting point, while the other remains solid. The advantage of electron beam welding is its ability to localize heating to a precise point and to control exactly the energy needed for the process. A high-vacuum atmosphere substantially contributes to a positive result. A general rule for construction of joints to be made this way is that the part with the lower melting point should be directly accessible for the beam.

Possible problems and limitations[edit]

Cracks in weld

The material melted by the beam shrinks during cooling after solidification, which may have unwanted consequences like cracking, deformation and changes of shape, depending on conditions.

The butt weld of two plates results in bending of the weldment because more material has been melted at the head than at the root of the weld. This effect is of course not as substantial as in arc welding.

Another potential danger is the emergence of cracks in the weld. If both parts are rigid, the shrinkage of the weld produces high stress in the weld which may lead to cracks if the material is brittle (even if only after remelting by welding). The consequences of weld contraction should always be considered when constructing the parts to be welded.

11.b. Gas metal arc weldingFrom Wikipedia, the free encyclopedia

Gas metal arc welding

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 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 provided faster 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. 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 use a shielding gas, but instead employs an electrode wire that is hollow and filled with flux.

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 aninert shielding gas (argon or helium), 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 electrical 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.[1]

12.b. 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. It is based on keyhole or Penetration mode welding.

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 workpieces 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.

Bi-metal saw blades are produced in large numbers by electron beam welding. The high speed steel required for the cutting teeth is electron beam welded in a narrow strip to the main part of the blade, which is in low carbon steel. Material costs are reduced, and the bi-metal blade is considerably more flexible and thus less prone to breakage. This results in an improved product, both in terms of cost and performance.

Transmission assemblies are frequently fabricated by electron beam welding. Complex parts can be fabricated with great savings in machining and materials. The parts can be finish-machined and hardened prior to welding, which is the final operation. This example illustrates the ability of electron beam welding to restrict distortion to a minimal level, thus allowing easy fabrication of otherwise difficult or even impossible components.

Aerospace components in titanium alloys are fabricated by electron beam welding. Again low distortion means intricate components can be accurately joined. Due to the clean vacuum welding environment, there is no risk of oxygen pick-up and consequent weld embrittlement when electron beam welding these titanium alloys.

Laser beam welding is a fusion joining process that uses the energy from a laser beam to melt and subsequently crystallize a metal, resulting in a bond between parts. Laser beam welding can be successfully used to join many metals to themselves as well as to dissimilar metals. Main applications are related to welding steels, titanium, and nickel alloys.

 

Laser welding of steels Low-carbon steels are readily laser weldable provided that sulfur and phosphorus levels are kept below 0.04%. A higher content can promote solidification cracking. In low-carbon steel the welding zone is martensiticand exhibits increased hardness, the level of which depends upon carbon and alloying elements contents. The structure of the heat-affected zone depends upon initial material state. In the case of annealed (normalized) condition there is fine martensite in the place of former pearlite grains and conserved ferrite colonies, appearing as if austenite transformation occurs separately in these two locations. This is due to the typically short thermal cycle of laser welding and lack of time for carbon diffusion to occur throughout the whole material. However, time is enough for carbon redistribution to proceed in pearlite that results in martensite formation with eutectoid carbon content upon cooling.

The heat-affected zone in steel after standard heat treatment (quenching and tempering) consists of two areas: a re-quenching zone and a tempering zone. In the first area tempered martensite undergoes reverse martensitetransformation when heating up and martensite transformation upon cooling that results in increased hardness. In the tempering zone, temperature is not high enough for austenization, and additional tempering proceeds.

Knowing these metallurgical features is essential for choosing correct welding parameters and materials. At Alabama Laser (Munford, AL) lever-shaft assemblies made of A36 and C1018 have been successfully welded (see Figure 1). A circumferencial butt weld 3 mm deep was made with 2 kW power providing 30 inches/min welding speed. At the weld end the welding head was gradually retracted so as to move the focal point above the surface to prevent sudden collapse of a keyhole and associated weld defects. Another example is laser welding cylindrical corrosion probe assemblies made of C1010.

Figure 1. Laser welded lever shaft assembly composed of two steels.

 

As carbon content in steel increases, martensite created in the weld zone, as well as in heat-affected zone, becomes more brittle. Reduced toughness combined with a high level of residual stress may lead to cracking in the weld or heat-affected zone, and also have an adverse effect on weld embrittlement and other alloying elements.

When the carbon equivalent is less than 0.4, steel is readily weldable, but when it exceeds this value, special precautions should be taken, for example, pre-heating and/or use of filler material to change the metallurgy of the weld. Often pre-heating is a preferred method, because it affects not only the weld zone but also the heat-affected zone. Pre-heating can be performed with the use of an induction heater, hot air blast, or auxiliary laser beam. Pre-heating reduces the cooling rate, resulting in a formation of different phases other than martensite (bainite, pearlite) that eliminates cracking. There are no ready-to-use recipes on pre-heating temperature and dwell time. They are dependent on the material to be welded, part geometry, and mass. Proper choice of pre-heating and welding regimes requires experimentation.

 

Stainless steel Austenitic stainless steels are good candidates for laser welding if their sulfur and phosphor contents are kept low. Specific laser beam welding features, such as high welding speed, small heat-affected zone, and low material exposure at elevated temperatures are beneficial for welding this type of steel, because

prolonged periods at high temperatures may lead to chromium precipitation on grain boundaries and reduction of corrosion resistance. Alabama Laser has a great deal of experience in laser welding different grades of austenitic stainless steel: 304, 316L, 309, which are used for corrosion probes.

Martensitic stainless steels produce relatively brittle welds due to their high carbon content. To avoid cracking, the same measures used for high carbon steels may be performed; pre-heating and use of filler material (in particular, austenitic wire or powder).

Alabama Laser has designed a precision micro-wire feeder that assures full control over the wire feeding process. When utilizing wire feeding, the wire diameter should be smaller than the size of the weld pool (1 mm) and directed precisely into the weld. The correct wire feed rate is determined by the gap, wire cross section, and required welding speed.

Normal practice is to deliver the wire at a 45° angle, intersecting the surface slightly ahead of the laser beam thus preventing excessive spatter.

Powder feeding has an advantage because more alloys are available than with wire. However, obtaining a constant mass flow rate of powder over time is a problem. Selection of the method of material delivery to the weld joint depends upon the specific task to be performed and available resources.

 

Nickel alloys Nickel and its alloys are widely used in industry due to their corrosion, heat, and creep resistance. There are two major types of nickel alloys, solid solution and precipitation hardenable.

The solid solution type is readily weldable with conventional methods as well as with laser welding. Alabama Laser has welded a number of corrosion probes made of solid-solution-type alloys (Monel 400, Inconel 600, 825) as well as heat exchanger fin assemblies of Monel 400.

The precipitation-hardenable type is generally difficult to weld due to cracking. Strain-age cracking can occur under some combinations of temperature and stresses in the heat-affected zone during or after welding. Alloys of the Nb-Al-Ti system are generally less susceptible to this type of cracking.

Some tips on laser welding of precipitation hardenable alloys include proper fit-up and attention to start and closeout of the weld. If post-welding heat treatment is necessary, weldments should be annealed first. Also helpful are a good inert gas shield and a convex bead shape.

 

Titanium alloys Titanium alloys are widely used in welded structures due to their high specific strength and corrosion resistance. The main difficulties in welding titanium alloys are high reactivity with gases at elevated temperatures, especially in liquid state, that produce weld embrittlement, and rapid grain growth at elevated temperatures. The second drawback is easily overcome with laser beam welding, because high welding speed and temperature gradients lead to a short material exposure to elevated temperatures in a narrow heat-affected zone and suppressed grain growth.

Special precautions should be taken to protect the titanium weld area from the ambient atmosphere, including the use of welding grade argon or argon-helium mixture as a shield gas and the use of trailing and back-up shields. The function of trailing and back-up shield coverage is to shroud the weld and adjacent hot material with inert gas until the surface temperature cools to 300°-400°C. Dendrite structure of the weld zone in a cross section is a good indication of improper shielding.

The weld structure of titanium alloys consists generally of a’ martensite, which differs from martensite in steels by its higher toughness and lower susceptibility to cracking. Short thermal cycles in laser welding accounts for smaller a’ martensite needles as compared to arc welding and, consequently, enhanced mechanical properties. A distinctive feature of laser beam welding of titanium alloys is a very small heat-affected zone. This is due to a low diffusion rate of a’ stabilizing elements as compared to a carbon diffusion rate in steels.

Another factor, which has a major impact on mechanical properties of the weld, is its chemical composition. A number of publications indicate the superiority of laser welding over the electron beam and arc welding with regard to fatigue strength. Laser welding is considered superior because of its typical short welding cycle, which causes little or no evaporation of the alloying elements.

 

Dissimilar metals When welding dissimilar metals certain considerations with regard to their properties must be taken into account; difference in melting point, heat conductivity, reflectivity, possible formation of brittle phases, and wettability. To achieve better welding performance the laser beam can be shifted towards the material with higher melting point, heat conductivity, and reflectivity.

A number of projects related to laser welding of dissimilar metal have been accomplished at Alabama Laser. The most significant is the laser welding of wear-resistant nose tips to steel guide bars of chain saw blades forSandvik Windsor Corporation (see Figure 2). Previously an oxyacetylene process was used to deposit a layer of Stellite 6 on the surface of the steel. This process was labor intensive, hazardous, led to excessive waste of expensive Stellite 6, and required extensive post-welding machining operations. Laser welding of Stellite 6 tips to steel bars produced sound welds. Laser welding has many benefits; the manufacturing environment is safer because of significant reduction of subsequent grinding, productivity is increased by eliminating the need to pre-heat and post-weld cool, the fusion bonding achieved with laser welding between the Stellite and the steel bar has excellent resistance to mechanical and thermal shock.