Welding Processes Notes

Welding (Part 2)

Fusion Welding Processes

  • Based on Kalpakjian & Schmid Chapter 30 and Groover Chapters 29, 30.

Casting Metals

  • Reactivity with air and mould materials is a key consideration.
  • Gas solubility, particularly hydrogen gas in Aluminum, needs to be managed.
  • Safety concerns include metal fires (e.g., Magnesium).

Protection of Welds

  • Hot metals are reactive to their environments (e.g., air).
  • Reactions with O2O_2 are a concern.
  • N<em>2N<em>2 and H</em>2H</em>2 are very soluble in molten metals.
  • Hydrogen leads to hydrogen cracking.
  • Nitrides are relatively benign.
  • Most become supersaturated solids at high temperature.
  • Welding processes must include means of protection.
  • Fluxes
    • Consumable, e.g., SMAW (Shielded Metal-Arc-Welding) - carbon dioxide
    • Separate flux feed, e.g., submerged arc welding (SAW)
  • Gas shielding
    • GTAW (gas tungsten arc welding) - Argon gas
    • GMAW (gas metal arc welding) - Argon or carbon dioxide

Classification of Fusion Welding Processes by Energy Source

  • Electrical Resistance
    • Fusion Welding
  • Chemical
    • SMAW
    • SAW
    • GMAW
    • FCAW
    • EGW
    • ESW
    • RSW
    • OAW
    • OHW
    • AAW
  • Arc
    • Radiation
    • EBW
    • LBW
    • Consumable Electrode
    • Non-Consumable Electrode
      • GTAW
  • Gas

Consumable Electrode AW Processes

  • Shielded Metal Arc Welding (SMAW)
  • Gas Metal Arc Welding (GMAW)
  • Flux-Cored Arc Welding (FCAW)
  • Submerged Arc Welding (SAW)
  • Other Arc Welding Processes
  • Electrogas Welding (EGW)
  • Electroslag Welding (ESW)
  • Arc Stud Welding (SW)

Shielded Metal Arc Welding (SMAW) AKA Metal Manual Arc (MMA) Welding

  • About 50% of all large-scale industrial-welding operations use this process.
  • A manual welding process uses a flux-coated consumable electrode.
    • A welding helmet is a type of personal protective equipment used in performing certain types of welding to protect the eyes, face, and neck from flash burn, sparks, infrared and ultraviolet light and intense heat.
  • The electric arc forms between the electrode and the workpiece, melting both to create the weld.
  • Also known as “stick welding.”
  • Uses consumable electrodes (sticks) consisting of a filler metal long rod that are held manually.
  • The electrode coating melts to produce shielding gas and slag, which:
    • Protect the molten weld from atmospheric contamination (oxygen, nitrogen).
    • Help shape and cool the weld.
  • The welder must manually feed the electrode and control the arc.
  • Weld pool: A mixture of materials from base metal and electrode forms the weld when it solidifies. It is protected by shielding gas and solidified slag.
Advantages of SMAW
  • Can be used in remote locations or places where electricity is not readily available.
  • Used to weld a wide range of metals (steel, stainless steel, cast iron).
  • Power supply, connecting cables, and electrode holder available for a few thousand pounds.
  • High-quality welds.
Disadvantages of SMAW
  • Consumable electrode. Sticks periodically changed increasing cost, defects, and time reducing productivity.
  • Skill Requirement. Required welder level of skill is high.
  • High current levels may melt coating prematurely.
  • Difficult to weld thin materials as it can lead to excessive heat input, causing the material to warp or distort.

Gas Metal Arc Welding (GMAW) Process AKA Metal Inert/Active Gas (MIG/MAG)

  • Uses a continuous and consumable bare metal wire electrode and a shielding gas to join two metal pieces.
  • Shielding gases include inert (argon, helium) and active (CO2CO_2 or a blend).
  • Electrode wire is fed continuously and automatically from a spool through the welding gun.
  • Used in the automotive, construction, and manufacturing industries.
Advantages of GMAW
  • GMAW can weld any metal and most commercial alloys.
  • GMAW can be easily operated or automated.
  • GMAW is a high-speed welding process that can produce high-quality welds quickly and efficiently. This makes it an ideal choice for applications that require a large amount of welding in a short amount of time.
  • Better production efficiency versus SMAW since the electrode or filler wire does not need to be continuously replaced.
  • Produces very little splatter and slag inclusions.
Disadvantages of GMAW
  • Equipment more expensive and less portable than SMAW.
  • Torch is heavy and bulky so joint access might be a problem.
  • Various metal transfer modes add complexity and limitations.
  • Surface preparation is required.

Flux Cored Arc Welding (FCAW)

  • Uses a continuously fed electrode coil that has a flux core in its core.
  • Similar to MIG/MAG welding, but instead of using a solid wire, it uses a tubular wire that is filled with flux.
  • Self-shielded FCAW - core includes compounds that produce shielding gases making it more portable.
  • Gas-shielded FCAW - uses externally applied shielding gases.
Advantages of FCAW
  • Popular choice as a replacement for GMAW and/or SMAW for welding steels and stainless steels over a wide stock thickness range.
  • Higher deposition rates than other welding processes, which means that more weld metal can be deposited per unit of time, resulting in faster weld completion.
  • Self‐shielded version is tolerant to drafty conditions.
  • More tolerant to weld metal contamination than GMAW.
Disadvantages of FCAW
  • Filler metal is more expensive than GMAW filler metal.
  • Requirement to remove slag after welding (self-shielded).
  • Fume production can be extremely high (especially self‐shielded version).
  • Limited to steels and nickel‐based alloys only.
  • Gas‐shielded version is not very tolerant to drafty conditions.
  • Spatter expelled from the welding arc can sometimes be significant.
  • More complex and expensive equipment compared to SMAW.

Submerged Arc Welding (SAW)

  • Uses a continuous, consumable bare wire electrode, with arc shielding by a cover of granular flux.
  • The arc is submerged under a layer of flux, which prevents spatter, sparks, and fumes from escaping the weld area.
  • Commonly used in the fabrication of large structures such as ships, pressure vessels, and offshore platforms.
Advantages of SAW
  • Extremely high weld metal deposition rates, especially when multiple wires are used.
  • No arc radiation. Minimal smoke and fumes
  • Significant opportunity to customise weld metal properties through the selection of the flux.
  • Mechanised process (usually) does not depend on welder skill.
Disadvantages of SAW
  • Restricted to flat position for groove welds, and flat and horizontal positions for fillet welds.
  • Flux handling equipment adds complexity.
  • Not suitable for thin sections.

Non-Consumable Electrode Processes

  • Gas Tungsten Arc Welding (GTAW)
  • Plasma Arc Welding
  • Carbon Arc Welding
  • Stud Welding

Gas Tungsten Arc Welding (GTAW) AKA Tungsten Inert Gas (TIG)

  • Uses a non-consumable tungsten or tungsten alloy electrode held in a torch to produce the weld.
  • Filler wire could be fed into the weld pool to add material to the joint.
  • Inert shielding gas is used (e.g., argon, helium, or their mixture)
  • No flux is needed.
  • Commonly used in aerospace, automotive, and other industries where high-quality welds are required.
Advantages of GTAW
  • High quality welds and finish.
  • Minimal post-weld cleaning is required and no possibility for slag defects.
  • Works well for complex geometries and thin sheets
Disadvantages of GTAW
  • Expensive due to the use of inert gases.
  • Possibility of tungsten inclusions in the weld.
  • Slower and more costly than consumable electrode AW processes.
  • Requires a high level of welder skill and experience

Resistance Welding (RW)

  • A group of fusion welding processes that use a combination of heat and pressure to accomplish coalescence
  • Heat generated by electrical resistance to current flow at the junction to be welded
  • Principal RW process is resistance spot welding (RSW)

Resistance Spot Welding (RSW)

  • Resistance welding process in which fusion of faying surfaces of a lap joint is achieved at one location by opposing electrodes
  • Used to join sheet-metal parts
  • Widely used in the mass production of automobiles, metal furniture, appliances, and other products
  • Example: Spot welding car bodies by robots on an automotive assembly line. A typical car body has ~10,000 spot welds. Annual production of automobiles in the world is measured in tens of millions of units
Advantages of Resistance Welding
  • No filler metal
  • High production rates possible
  • Lends itself to mechanisation and automation
  • Lower operator skill level than for arc welding
  • Good repeatability and reliability
Disadvantages of Resistance Welding
  • High initial equipment cost
  • Limited to lap joints for most RW processes

Summary

  • In these fusion welding processes, heat is used to bring the joint being welded to a liquid state
  • Protection of liquid metal in the pool is necessary using shielding gases or fluxes
  • The metallurgy of the welded joint is important, which will determine the strength and toughness of the joint due to microstructure changes by HAZ
  • Residual stresses are normally induced in the weld, and it is important to consider this by pre-heating/post heating or post-treatment to relieve the stress, which in turn can lead to distortions of the part.

Concept Questions

  • Name the principal groups of processes included in fusion welding.
  • What is an electrical arc?
  • Name and define the categories in which electrodes in arc welding are divided into.
  • What is the heat-affected zone (HAZ) in a fusion weld?