Welding Flashcards
Joining
These processes form a permanent joint between parts. Sometimes also referred to as assembly processes.
Mechanical Assembly
Some of these methods allow for easy disassembly, while others do not.
Welding
Joining process in which two (or more) parts are coalesced at their faying (contacting) surfaces by application of heat and/or pressure.
Advantages
Permanent joint with welded components becoming a single entity.
Usually the most economical way to join parts in terms of material usage and fabrication costs.
Can be accomplished outside a factory environment.
Limitations
Most welding operations are performed manually and are expensive in terms of labour cost.
Most welding processes utilise high energy and are inherently dangerous.
Welded joints do not allow for convenient disassembly.
Welded joints can have quality defects that are difficult to detect.
Fusion Welding
Joining processes that melt the base metals.
In many fusion welding operations, a filler metal is added to the molten pool to facilitate the process and provide bulk and added strength to the welded joint.
Autogenous weld
A fusion welding operation in which no filler metal is added.
Solid State Welding
Joining processes in which coalescence results from application of pressure alone or a combination of heat and pressure.
If heat is used, temperature is below melting point of metals being welded.
No filler metal is added in solid state welding.
High-density heat energy
To accomplish fusion, a source of high density heat energy must be supplied to the faying surfaces, so the resulting temperatures cause localised melting of base metals. For metallurgical reasons, it is desirable to melt the metal with minimum energy but high heat densities.
Power density is measured in W/mm^2.
If power density is too low
Heat is conducted into work, so melting never occurs.
If power density too high
Localised temperatures vapourise metal in affected region.
Fusion Zone
Mixture of filler metal and base metal that have completely melted. Characterised by a high degree of homogeneity.
Weld Interface
Narrow boundary consisting of a thin band of base material that was melted or partially melted but immediately solidified before any mixing with the metal from the fusion zone. Chemical composition identical to that of the base material.
Heat-Affected Zone (HAZ)
Metal has experienced temperatures below melting point, but high enough to cause micro-structural changes in the solid metal. Chemical composition same as base metal, but this region has been heat treated so that its properties and structure have been altered. Effect on mechanical properties in HAZ is usually negative, and it is here that welding failures often occur.
Arc welding
Coalescence of metals is achieved by heat from an electric arc between the electrode and the work. A pool of molten metal is formed near electrode tip, and as electrode is moved along joint, molten weld pool solidifies in its wake. Sustained by an ionized column of gas (plasma) through which the current flows. To initiate the arc in AW, electrode is brought into contact with work and then quickly separated from it by a short distance.
Consumable Electrodes
Welding rods (sticks) must be changed frequently or weld wire can be continuously fed from spools with long lengths of wire, avoiding frequent interruptions. Source of filler metal in arc welding.
Non-consumable Electrodes
Made of tungsten which resists melting. Gradually depleted during welding (vapourisation is principal mechanism). Filler metal must be added separately if it is used.
Shielding
At high temperatures in AW, metals are chemically reactive to oxygen, nitrogen, and hydrogen in air. Mechanical properties of joint can be degraded by these reactions. To protect operation, arc must be shielded from surrounding air in AW processes. Arc shielding is accomplished by: Shielding gases (e.g. argon, helium, CO2) or Flux.
Shielded Metal Arc Welding
SMAW.
Consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielding. Used for steels, stainless steels, cast irons, and certain nonferrous alloys.
Gas Metal Arc Welding
GMAW or MIG.
Consumable bare metal wire as electrode with shielding by flooding arc with a gas. Wire is fed continuously and automatically from a spool through the welding gun.
Flux-Cored Arc Welding
FCAW.
Adaptation of shielded metal arc welding, to overcome limitations of stick electrodes. Electrode is a continuous consumable tubing (in coils) containing flux and other ingredients (e.g. alloying elements) in its core.
Submerged Arc Welding
SAW.
Uses a continuous, consumable bare wire electrode, with arc shielding by a cover of granular flux. Electrode wire is fed automatically from a coil. Flux introduced into joint slightly ahead of arc by gravity from a hopper.
Gas Tungsten Arc Welding
GTAW or TIG.
Uses a non-consumable tungsten electrode and an inert gas for arc shielding. Used with or without a filler metal (added to weld pool from separate rod or wire). Aluminium and stainless steel mostly.
Plasma Arc Welding
PAW.
Tungsten electrode is contained in a nozzle that focuses a high velocity stream of inert gas (argon) into arc region to form a high velocity, intensely hot plasma arc stream. Temperatures in PAW reach 28,000C, due to constriction of arc, producing a plasma jet of small diameter and very high energy density.
Oxyfuel gas welding
OFW.
Is a fusion welding process. Burning of various fuels mixed with oxygen to perform welding. Oxyacetylene welding (OAW) uses a high temperature flame from combustion of acetylene and oxygen (is the most important one).
Resistance welding
RW.
Is a fusion welding process. Coalescence is accomplished through a combination of heat and pressure melting the contacting surfaces. Heat is generated by electrical resistance to current flow between faying surfaces to be welded. Used for spot welds in cars.
Solid State Welding
Joining processes in which coalescence results from application of pressure alone or a combination of heat and pressure. If heat is used, temperature is below melting point of metals being welded. No filler metal is added in solid state welding.
Cold Welding
Is a solid state welding process. Application of high pressure between clean contacting surfaces at room temperature.
Roll Welding
Is a solid state welding process. Application of sufficient pressure to cause coalescence by means of rolls, either with or without external heat
Friction Welding
Is a solid state welding process. Coalescence is achieved by frictional heat combined with pressure without melting of the faying surfaces. Is often done by having one part rotating and one stationary.
Butt joint
The parts lie in the same plane and are joined at their edges.
Corner joint
The parts form a right angle and are joined at the corner of the angle.
Lap joint
The parts are overlapping.
Tee joint
One part is perpendicular to the other in the shape of the letter “T”.
Edge joint
Parts are parallel with at least one of their edges in common, and the joint is made at the common edge(s).
Fillet Weld
Used to fill in the edges of plates created by corner, lap, and tee joints. Filler metal used to provide cross section in approximate shape of a right triangle. Requires minimum edge preparation.
Groove Weld
Most closely associated with butt joints. Usually requires part edges to be shaped into a groove to facilitate weld penetration. Grooved shapes include square, bevel, V, U, and J, in single or double sides.
