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.
