Welding:
- A materials joining process in which two or more parts (usually metals) are coalesced at their contact surfaces by the application of heat and/or pressure.
– Many welding processes are accomplished by heat alone, with no pressure applied.
– Others by a combination of heat and pressure.
– Still others by pressure alone with no external heat.
- Faying surfaces – the part surfaces in contact or close proximity that are to be joined.
- In some welding processes a filler material is added to facilitate coalescence.
Why is welding important?
Provide a permanent joint.
Usually the most economical way to join parts in terms of material usage and fabrication costs
– Mechanical fastening usually requires additional hardware (e.g., screws) and geometrical alternations of the assembled parts (e.g., holes).
Not restricted to a factory environment. Welding can be accomplished “in the field”.
Principal applications of welding:
- Construction – buildings and bridges
- Piping, pressure vessels, boilers and storage tanks
- Shipbuilding
- Aircraft and aerospace
- Automotive
- Railroad
Limitations and drawbacks of welding:
- 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.
Types of welding processes:
Fusion welding
Solid state welding
Fusion welding:
Joining processes that melt the base metals by heat alone or a combination of heat and pressure.
– 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.
– Examples: arc welding (AW), resistance welding (RW)
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 the melting point of metals being welded.
– No filler is added in solid state welding.
– Examples: diffusion welding (DFW), friction welding (FRW)
Some fusion welding processes:
- Arc welding (AW) – melting of the metals is accomplished by an electric arc.
- Resistance welding (RW) – melting is accomplished by heat from resistance to an electrical current between faying surfaces held together under pressure.
- Oxyfuel gas welding (OFW) – melting is accomplished by an oxyfuel gas such as acetylene.
Arc welding (AW):
- A fusion welding process in which coalescence of the metals is achieved by the heat from an electric arc between an electrode and the work.
– An electric arc is a discharge of electric current across a gap in a circuit.
- Pool of molten metal is formed near the electrode tip, and as the electrode is moved along the joint, the molten weld pool solidifies in its wake.
Resistance welding (RW):
- A fusion welding process that uses a
combination of heat and pressure to
accomplish coalescence, the heat
being generated by electrical
resistance at the junction. - Components include
– Parts to be welded (usually sheet metal parts)
– Two opposing electrodes
– A means of applying pressure to squeeze the parts between the electrodes
– Power supply from which a controlled current can be applied for a specific time duration.
- Weld nugget – the fused zone between the two parts.
Friction welding (FRW):
- A solid state welding process in which
coalescence is achieved by frictional
heat combined with pressure.
– When properly carried out, no melting occurs at faying surfaces.
Features of fusion welded joint:
Typical fusion welded joint in which a filler metal has been added consists of:
– Fusion zone: mixture of filler metal and base metal
– Weld interface: quickly solidified base metal
– Heat affected zone (HAZ): temperature < Tm
– Unaffected base metal zone
Heat affected zone (HAZ):
- Metal has experienced temperatures below melting point, but high enough to cause microstructural changes in the
solid metal.
– Chemical composition is the same as the 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.
– It is here that welding failures often occur.
