Fabrication Processes - Welding, Brazing, and Soldering

Welding

  • Welding joins similar or dissimilar metals by fusion, with or without pressure or filler metal.
  • A weldment is an assemblage of parts joined by welding.

Welding Applications

  • Construction Industry
  • Oil and Gas Industry
  • Energy Sector
  • Automotive Industry
  • Shipbuilding Industries

Technological Advantages of Welding

  • Provides a permanent joint.
  • Most economical for material usage and fabrication costs.
  • Can be done in the field.
  • Welded joint can be stronger than parent materials if appropriate filler metal and techniques are used.

Limitations of Welding

  • Often performed manually, increasing labor costs.
  • Requires skilled labor, which may be scarce.
  • Involves high energy, making it dangerous.
  • Permanent bond does not allow for easy disassembly.

Types of Joints

  • Butt Joint: Parts lie in the same plane and are joined at their edges.
  • Corner Joint: Parts form a right angle and are joined at the corner.
  • Lap Joint: Two overlapping parts.
  • Tee Joint: One part is perpendicular to the other, forming a T shape.
  • Edge Joint: Parts are parallel with a common edge.

Types of Welds

  • Fillet Weld
    • Used to fill edges of corner, lap, and tee joints.
    • Filler metal forms a right triangle shape.
  • Groove Welds
    • Edges shaped into a groove for weld penetration.
    • Shapes: square, bevel, V, U, and J, single or double sided.
    • Used on all joint types except lap.
  • Plug and Slot Welds
    • Attaching flat plates using holes or slots in the top part.
    • Filler metal fuses the parts together.
  • Spot and Seam Welds
    • Used for lap joints.

Welding Positions

  • Flat
  • Horizontal
  • Vertical
  • Overhead

Classification of Welding

  1. Solid-State or Pressure Welding
    • Coalescence results from pressure or a combination of heat and pressure.
    • Temperature below the melting point of the metals.
    • No filler metal is used.
  2. Fusion or Non-Pressure Welding
    • Heat melts the base metals.
    • Filler metal may be added.
    • Autogenous weld: no filler metal is added.

Classification of Welding Operations

  • Filler/Electrode Material
    • Autogenous
    • Homogeneous
    • Heterogeneous
  • Fusion / Non Pressure
    • Arc Welding
    • Gas Welding
    • Thermit Welding
    • ElectroSlag Welding
    • Radiant Energy (Electron Beam Welding, Laser Beam Welding)
  • Non-Fusion / Solid state / Pressure
    • Forge, Resistance, Ultrasonic, Friction, Explosive, Induction welding, cold pressure, diffusion Bonding, etc
  • Allied Joining Processes
    • Soldering, Brazing, Braze Welding, Adhesive Bonding

Arc Welding

  • Carbon Arc
  • Shielded Metal Arc (SMAW)
  • Submerged Arc (SAW)
  • Gas Tungsten Arc (GTAW / TIG)
  • Gas Metal Arc (GMAW/MIG)
  • Plasma Arc
  • Atomic Hydrogen Arc
  • Stud Welding
  • Electroslag
  • Electro gas

Resistance Welding

  • Spot Welding
  • Seam Welding
  • Projection Welding
  • Friction Welding
  • Percussion Welding
  • Flash Welding
  • High Frequency Resistance Welding
  • High Frequency Induction Welding

Gas Welding

  • Air-Acetylene
  • Oxy - Acetylene

Oxyfuel Gas Welding

  • Uses various fuels mixed with oxygen for welding.
  • Also used in cutting torches.
  • Oxyacetylene welding (OAW) is the most important.

Oxyacetylene Welding

  • Fusion welding using a high-temperature flame from combustion of acetylene and oxygen.
  • Flame is directed by a welding torch.
  • Filler metal is sometimes added.
  • Pressure is occasionally applied.
  • Filler metal composition should be similar to base metals.
  • Filler is often coated with flux to clean surfaces and prevent oxidation.
  • Acetylene (C2H2) is the most popular fuel, reaching temperatures up to 3480^\circ C (6300^\circ F).
  • The flame is produced in two stages:
    1. C2H2 + O2 = 2CO + H2 + \text{heat}
    2. 2CO + H2 + 1.5O2 = 2CO2 + H2O + \text{heat}
  • When the mixture of acetylene and oxygen is in the ratio 1:1, the resulting neutral flame is shown in Figure.

Neutral Flame

  • Achieved with equal proportions of acetylene and oxygen.
  • Complete combustion, consuming all carbon and releasing maximum heat.
  • White rounded cone is clearly visible.
  • Temperature at the inner cone tip is approximately 3232^\circ C, dropping to 1260^\circ C at the outer envelope.
  • Preferred for welding cast iron, mild steel, and stainless steel.

Carburizing/Reducing Flame

  • Excess acetylene, showing two cones and an outer envelope.
  • Lower temperature due to unconsumed carbon.
  • Not suitable for welding steel as it may introduce carbon into the weld, creating a hard & brittle deposit.
  • Best used for high carbon steels, hard surfacing, and non-ferrous alloys like Monel.
  • Temperature is approximately 3149^\circ C at the inner cone tips.

Oxidizing Flame

  • Excess oxygen.
  • Inner cone is shorter and sharper, with a deeper purple color.
  • Molten metal is less fluid, with excessive sparking.
  • Used to braze steel and cast iron.
  • A stronger oxidizing flame is used for fusion welding brass and bronze.
  • Temperature is approximately 3482^\circ C at the inner cone tip.
  • Used for welding metals such as brass, copper, bronze, and zinc.

Arc Welding

  • Fusion-welding process where coalescence is achieved by the heat of an electric arc between an electrode and the work.
  • Electric arc is a discharge of electric current across a gap in a circuit.
  • Sustained by a thermally ionized column of gas (plasma) through which current flows.
  • Electrode is brought into contact with the work and then quickly separated by a short distance to initiate the arc.
  • Temperatures of 5500^\circ C (10,000^\circ F) or higher are reached, melting any metal.

Components of Arc Welding

  • Electrodes: consumable or non-consumable.
  • Arc Shielding: Protects against oxygen, nitrogen, and hydrogen in the air.

Power Source in Arc Welding

  • Both direct current (DC) and alternating current (AC) are used.
  • AC machines are less expensive but restricted to ferrous metals.
  • DC equipment can be used on all metals and provides better arc control.

Different Types of Arc Welding Processes—CONSUMABLE ELECTRODES

  1. Shielded Metal Arc Welding (SMAW)
    • Uses a consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielding.
    • Coating consists of powdered cellulose mixed with oxides, carbonates, and other ingredients, held together by a silicate binder.
    • Metal powders may be included in the coating to increase the amount of filler metal and to add alloying elements.
  2. Gas Metal Arc Welding (GMAW)
    • Electrode is a consumable bare metal wire, and shielding is accomplished by flooding the arc with a gas.
    • Bare wire is fed continuously and automatically from a spool through the welding gun.
    • Gases: inert gases (argon and helium) and active gases (carbon dioxide).
    • Inert gases are used for welding aluminum alloys and stainless steels, while CO_2 is commonly used for welding low and medium carbon steels.
    • No slag covering, eliminating manual grinding and cleaning.
    • Ideal for making multiple welding passes.
  3. Flux-Cored Arc Welding (FCAW)
    • Electrode is a continuous consumable tubing that contains flux and other ingredients in its core.
    • Tubular flux cored “wire” is flexible and supplied in coils.
    • Two versions: self-shielded and gas shielded.
    • Used primarily for welding steels and stainless steels.
    • Produces high-quality weld joints.
  4. Electro-gas Welding (EGW)
    • Uses a continuous consumable electrode (flux-cored wire or bare wire with externally supplied shielding gases) and molding shoes to contain the molten metal.
    • Primarily applied to vertical butt welding.
    • The shoes are water cooled to prevent their being added to the weld pool.
    • Process is performed automatically.
  5. Submerged Arc Welding (SAW)
    • Uses a continuous, consumable bare wire electrode, and arc shielding is provided by a cover of granular flux.
    • Electrode wire is fed automatically from a coil into the arc.
    • The blanket of granular flux completely submerges the welding operation, preventing sparks, spatter, and radiation.
    • Steel plates of 25-mm (1.0-in) thickness and heavier are routinely welded by this process.
    • Low-carbon, low-alloy, and stainless steels can be readily welded by SAW; but not high-carbon steels, tool steels, and most nonferrous metals.
    • Parts must always be in a horizontal orientation.

AW PROCESSES—NON CONSUMABLE ELECTRODES

  1. Gas Tungsten Arc Welding (GTAW)
    • Uses a non-consumable tungsten electrode and an inert gas for arc shielding.
    • Also known as TIG welding.
    • Can be implemented with or without a filler metal.
    • Tungsten has a high melting point of 3410^\circ C (6170^\circ F).
    • Shielding gases include argon, helium, or a mixture of these gas elements.
    • Applicable to nearly all metals in a wide range of stock thicknesses.
    • Used for joining various combinations of dissimilar metals.
  2. Plasma Arc Welding (PAW)
    • Special form of gas tungsten arc welding in which a constricted plasma arc is directed at the weld area.
    • Tungsten electrode is contained in a specially designed nozzle that focuses a high-velocity stream of inert gas into the region of the arc to form a high-velocity, intensely hot plasma arc stream.
    • Temperatures reach 17,000^\circ C (30,000^\circ F) or greater.
    • The power is highly concentrated to produce a plasma jet of small diameter and very high power density.

Other Arc-Welding and Related Processes

  1. Carbon Arc Welding (CAW)
    • Uses a non consumable carbon (graphite) electrode.
    • It has historical importance because it was the first arc-welding process to be developed.
    • The carbon arc process is used as a heat source for brazing and for repairing iron castings.
  2. Stud Welding (SW)
    • Specialized AW process for joining studs or similar components to base parts.
    • Shielding is obtained by the use of a ceramic ferrule.
    • Automatic timing and power parameter control.
    • Applications include threaded fasteners for attaching handles to cookware, heat radiation fins on machinery, and similar assembly situations.

Resistance Welding

  • Uses a combination of heat and pressure to accomplish coalescence.
  • Heat is generated by electrical resistance to current flow at the junction to be welded.
  • Uses no shielding gases, flux, or filler metal.
  • Electrodes are non-consumable.

Resistance-Welding Processes

  1. Resistance Spot Welding
    • Components: work parts, opposing electrodes, pressure application, and AC power supply.
    • Fused zone is called a weld nugget.
  2. Resistance Seam Welding
    • Stick-shaped electrodes are replaced by rotating wheels.
    • A series of overlapping spot welds are made along the lap joint.
    • Capable of producing air-tight joints.
  3. Resistance Projection Welding
    • Coalescence occurs at one or more relatively small contact points on the parts.
    • Contact points are determined by projections, embossments, or localized intersections of the parts.

Other Resistance-Welding Operations

  • Flash Welding (FW)
    • Two surfaces are brought into contact or near contact, and electric current is applied.
    • Surfaces are forced together to form the weld.
  • Upset Welding (UW)
    • Faying surfaces are pressed together during heating and upsetting.
    • Heating is accomplished entirely by electrical resistance.
  • Percussion Welding (PEW)
    • Duration of the weld cycle is extremely short (1 to 10 ms).
    • Fast heating is accomplished by rapid discharge of electrical energy.
  • High-Frequency Resistance Welding (HFRW)
    • Uses a high-frequency alternating current for heating.
    • Followed by the rapid application of an upsetting force to cause coalescence.
    • High-Frequency Induction Welding (HFIW)
    • Heating current is induced in the parts by a high-frequency induction coil.

Brazing

  • Joining process in which a filler metal is melted and distributed by capillary action between the faying surfaces.
  • No melting of the base metals occurs.
  • Filler metal has a melting temperature above 450^\circ C (840^\circ F) but below the melting point of the base metal(s).
  • The brazed joint will be stronger than the filler metal.

Advantages of Brazing

  • Any metals can be joined, including dissimilar metals.
  • Certain brazing methods can be performed quickly and consistently.
  • Some methods allow multiple joints to be brazed simultaneously.
  • Can be applied to join thin-walled parts that cannot be welded.
  • Less heat and power are required than in fusion welding.
  • Problems with the heat-affected zone are reduced.
  • Inaccessible joints can be brazed.

Disadvantages and Limitations of Brazing

  • Joint strength is generally less than that of a welded joint.
  • High service temperatures may weaken a brazed joint.
  • Color of the metal in the brazed joint may not match the color of the base metal parts.

Brazed Joints

  • Conventional butt joint, and adaptations of the butt joint for brazing: scarf joint, stepped butt joint, increased cross section of the part at the joint.
  • Conventional lap joint and adaptations of the lap joint for brazing: cylindrical parts, sandwiched parts, and use of sleeve to convert butt joint into lap joint.

Filler Metals and Fluxes

  • Characteristics of a good brazing/filler metal:
    1. Melting temperature must be compatible with the base metal.
    2. Surface tension in the liquid phase must be low for good wettability.
    3. Fluidity of the molten metal must be high for penetration into the interface.
    4. The metal must be capable of being brazed into a joint of adequate strength for the application.
    5. Chemical and physical interactions with base metal must be avoided.
  • Filler metals are applied in various ways, including wire, rod, sheet sand strips, powders, pastes, preformed parts, and cladding.
  • Braze metal pastes consist of filler metal powders mixed with fluid fluxes and binders.
  • Brazing fluxes dissolve, combine with, and otherwise inhibit the formation of oxides.
  • Characteristics of a good flux:
    1. Low melting temperature.
    2. Low viscosity.
    3. Facilitates wetting.
    4. Protects the joint until solidification of the filler metal.
    5. Easy to remove after brazing.
  • Alternatives to using a flux are to perform the operation in a vacuum or a reducing atmosphere.

Brazing Methods

  • Differentiated by their heating sources.
  1. Torch Brazing
    • Uses a torch to direct a flame against the work in the vicinity of the joint.
    • Reducing flame is used to inhibit oxidation.
    • Filler wire is added to the joint.
  2. Furnace Brazing
    • Uses a furnace to supply heat for brazing.
    • Suited to medium and high production.
    • Temperature and atmosphere control are important.
    • Vacuum furnaces are sometimes used.
  3. Induction Brazing
    • Utilizes heat from electrical resistance to a high frequency current induced in the work.
    • The parts do not directly contact the induction coil.
  4. Resistance Brazing
    • Heat is obtained by resistance to flow of electrical current through the parts.
    • Parts are directly connected to the electrical circuit.
  5. Dip Brazing
    • Either a molten salt bath or a molten metal bath accomplishes heating.
    • Assembled parts are immersed in the baths contained in a heating pot.
  6. Infrared Brazing
    • Uses heat from a high-intensity infrared lamp.
    • Limited to thin sections.
  7. Braze Welding
    • Used for filling a more conventional weld joint, such as the V-joint.
    • A greater quantity of filler metal is deposited than in brazing, and no capillary action occurs.
    • The joint consists entirely of filler metal.
    • The principal application of braze welding is repair work.

Soldering

  • Joining process in which a filler metal with a melting point not exceeding 450^\circ C (840^\circ F) is melted and distributed by capillary action between the faying surfaces.
  • No melting of the base metals occurs.
  • Soldering details are similar to those of brazing, and many heating methods are the same.
  • Surfaces must be precleaned and an appropriate flux must be applied.
  • Filler metal, called solder, is added to the joint.
  • In some applications, the solder is precoated onto one or both of the surfaces - a process called tinning.

Advantages of Soldering

  1. Low energy input relative to brazing and fusion welding.
  2. Variety of heating methods available
  3. Good electrical and thermal conductivity in the joint
  4. Capability to make air-tight and liquid-tight seams for containers
  5. Easy to repair and rework.

Disadvantages of Soldering

  1. Low joint strength
  2. Possible weakening or melting of the joint in elevated temperature service.

Solders and Fluxes

  • Most solders are alloys of tin and lead.
  • Lead is poisonous and its percentage is minimized in most solder compositions.
  • Soldering fluxes should:
    1. Be molten at soldering temperatures
    2. Remove oxide films and tarnish from the base part surfaces
    3. Prevent oxidation during heating
    4. Promote wetting of the faying surfaces
    5. Be readily displaced by the molten solder during the process
    6. Leave a residue that is noncorrosive and nonconductive.
  • Soldering fluxes can be classified as organic or inorganic.

Soldering Methods

  • Many of the methods used in soldering are the same as those used in brazing, except that less heat and lower temperatures are required for soldering.
  1. Hand Soldering
    • Performed manually using a hot soldering iron.
    • A bit, made of copper, is the working end of a soldering iron.
    • Functions: deliver heat, melt the solder, convey molten solder to the joint, and withdraw excess solder.
  2. Wave Soldering
    • Mechanized technique that allows multiple lead wires to be soldered to a printed circuit board (PCB) as it passes over a wave of molten solder.
  3. Reflow Soldering
    • Widely used in electronics to assemble surface mount components to printed circuit boards.
    • A solder paste consisting of solder powders in a flux binder is applied to spots on the board.