Joining Processes Flashcards
What is fusion welding and what are the four main types?
Components to be joined are melted along their common interface by an external heat source
Filler metal may be used to fix gap
Four main processes:
- Resistance welding
- Gas welding
- Arc welding
- Power beam welding (laser)
What is resistance welding?
Uses electric current to heat the metal locally by power dissipation in resistance at contact region
Often used for sheet material
Commongly usefd to join steel sheet in robotic car body/ substructure and domestic white goods fabrication
What is gas welding?
Simple and cheap methid
Uses a manually operated gas-torch building acetlyne in oxygen
Filter rod supplied manually to the melt pool, and molten metal is prevented from oxidizing by combustion gases
What is arc welding?
An electrical arc disharge is generated between electrode and work piece using high voltage/ current
A meltable inorganic material or inert gas (argon) is provided to prevent oxidation of the melt pool
processes differ in source of filler material
Very common in structural uses of steel and Al
What us power beam welding?
Laser and electron beam welding uses a beam of high power density as heat source
Beam vapoursies a small surface spot, giving high energy absorption from beam - a deep narrow cavity forms called a ‘Key hole”
Thic plates can be welded without filler in a single pass, giving narrow melt and HAZ
Explain the weld metal
The ‘weld metal’ is formed by solidification of the molten pool in fusion welding.
It is like a miniature casting, with new metal being melted in from the filler and the leading edge of the pool, and solidification occurring around the trailing edge of the pool.
The shape of the pool depends on the heat input and the welding speed - which control the shape of the temperature distribution around the weld (including the melt isotherm).
What is epitaxial solidification
Occurs as grains grow into the melt pool form the unmelted regions
What happens with segregation?
Secregation can occur, with solute and impurities concentrated on the centre line
Segregation of sulpher for example gives brittle grain boundaries
Explain the importance of the Heat Affected Zone
Haz surrounds the weld metal but does not actuallly melt
Several processes occur in HAZ during heating and cooling cycles:
- Recovery, recrstallization and grain growth
- Precipitate coarsening/ dissolution
- Phases transformations
Haz microstructure is depedent on
- Alloy composition
- Initial microstructure
- welding processes (which determines heat input, size of melted zone, and the heating/cooling cycles)
Explain the HAZ in steels
The most important property in fusion welding of steels is the final toughness. Since there is an inverse correlation between hardness and toughness in steels, it is usual to specify a maximum allowable final hardness in the HAZ.
In steel welding, the cooling rate at a given point in the HAZ is commonly described by the time taken to cool between 800 oC and 500 oC: ‘Δt8-5’.
- small Δt8-5: fine structure, more likely to form martensite.
- large Δt8-5: coarse structure, no martensite, not brittle.
Hardness in the HAZ and weld metal depends on:
- the amount of austenite formed on heating
- the products of the austenite decomposition (i.e. proportions of martensite, ferrite/pearlite, bainite)
- changes to the original grains which do not change to austenite
What should be done for strong tough welds in steels?
use low CE steels (< 0.4)
use a fine-grained ‘microalloyed’ steel (to prevent grain growth)
preheat to ~ 200 oC to reduce subsequent cooling rates, and reduce the likelihood of martensite formation.
post-weld heat treat at 650 oC - this tempers any martensite (and also relieves residual stresses)
Explain the HAZ in aluminium alloys
Many heat-treatable and non-heat-treatable Al alloys are weldable, but suffer some strength loss in the HAZ. The thermal cycle may be regarded as a brief annealing treatment - the extent of microstructural change depends on the softening mechanisms in the two classes of alloy.
A weld in a non-heat-treatable wrought alloy, which will produce a cast microstructure, will be inevitably weaker than the surrounding material, as is the HAZ (which has been annealed by the heating cycle).
For an age-hardening (heat-treatable) aluminium alloy it is possible to recover much of the strength after welding.
Explain stregnth recovery in age hardended aluminium alloys
two possible softening routes - coarsening the precipitates and overaging the alloy (path A), or dissolving the precipitates, to return to a supersaturated solid solution (path B).
In most weldable aluminium alloys, we get re-solution (i.e. path B), a process known as reversion. This is very important, since it offers the potential to restore strength to the HAZ by re-ageing
Welds cannot be aged at elevated temperatures to give the full T6 strength as that would cause the surrounding material to over-age and soften. They can however be naturally aged at room temperature
What is surface engineering?
Modification or coating of a surface to achieve a combination of properties in the surface and in the substrate which cannot otherwise be achieved.
Reasons
Wear resistance
fatigue life
corrosion resistance
Aesthetic appearance
Explain Transformaton hardening.
The surface is subjected to rapid heating and quenching.
Austenite is formed to a limited depth on heating, and is then cooled sufficiently fast to form martensite (by conduction into the underlying colder material, or by a water quench.
Hardening can be local or global, depending on the heat source:
- ‘Flame hardening” - gas flame
- ‘induction hardening’ - large area
- ‘Laser hardening’- intense localised treatment
Exaplain Carburising of steels
Widely used.
The carbon concentration is locally increased near the surface, by diffusion of C at high temperature (e.g. 900 ºC) from a carbon-rich environment (e.g. hydrocarbon gas).
Carbon diffuses rapidly in the austenite phase.
Usually followed by quenching, to give a high carbon martensitic surface
Explain Nitiriding of steels
Differs from carburising in two main ways:
- conducted at lower temperature, where the steel is still ferritic (alpha, b.c.c.) so there is less distortion
- only suitable for alloy steels containing strong nitride formers (Al, Cr, Mo, Ti, V)
Diffusion of nitrogen in ferrtic phase
Forms nitride precipitates which give hardening effect
