Laser Welding Flashcards
What are the 2 main methods of welding?
Conduction limited welding and keyhole (plasma ) welding
What is welding?
- Welding is a materials joining process
which produces coalescence of materials
by heating them to suitable temperatures. - Sometimes a filler material is added to the
molten pool to strengthen the weld.
What are the main steps of welding?
- Heating
- Melting / Vaporisation
- Melt pool dynamic
- Rapid solidification
What are the advantages of laser?
Advantage of Laser Welding
* Close tolerance (±0.1 mm).
* Excellent repeatability.
* Minimal distortion and heat affected zones.
* Little or no burring can make secondary finishing unnecessary
What is the difference between the 2 types of laser welding?
Conduction limited welding - melting
keyhol welding - melting and vaporisation
What is conduction limited welding?
Laser absorbed at the surface(skin depth) and heat is transferred to the
lower part of the body entirely by conduction.
* Conduction limited welding has substrate melting only. The meltpool is
stirred gently by surface tension forces caused by surface temperature
gradients.
* The weld depth is limited by the heat conduction therefore welding is
slow.
What are the benefits of conduction welding?
- Very stable
- Extremely high quality welds (no keyhole or arc)
- No loss of material – no joint prep required
- Large spot sizes
- Low cost diode sources
- Large standoff distances
- Very tolerant of joint mismatch
What are the issues of conduction welding?
- Surface reflectivity
- Relatively slow
What radiation is used for conduction limited welding?
Near IR radiation.
The interface is coated in an infrared absorbing film.
Heat energy absorbed is suffience to form a weld
What is the transmission welding system?
- The laser (Nd:Yag, CO2, Fibre) is focussed to a large spot (0.3 - 0.5 mm in
diameter) using a 100-200 mm focal length lens - A high flow of low pressure shield gas is used e.g. argon, nitrogen, helium to
prevent oxidation. This uses a large aperture nozzle (> 3 mm diameter) - The gas also acts to keep the interaction temperature low (5,000 K !) which helps to keep the weld stable
What are different types of weld geometries?
Butt, overlap , lip seal weld, lap edge joint etc
What is the difference between conduction and keyhole welding?
Key hole uses a higher power density therefore achieving a great weld depth and penetration.
What are the specific characteristics?
- With high laser power density,
fast vaporisation of materials
occurs and the pressure of
vapour causes the depression of
the molten material and
formation of a keyhole. - The “keyhole” is the vaporised
channel filled with ionised
substrate material immediately
below the interaction region. - Energy absorption during keyhole
welding is very different from
conduction limited processing.
What are the two absorption mechanisms in key hole welding?
- Energy is absorbed through inverse bremsstrahlung in the plasma and Fresnel
absorption at the keyhole walls. - The inverse bremsstrahlung leads to a point like heat source.
- The Fresnel absorption leads to a line-like heat source.
- Deep meltpool as energy is absorbed at the surface and the keyhole walls.
What are the benefits to keyhole welding?
Benefits
* High aspect ratio deep penetration welds.
* High welding speeds.
* Low heat input leading to reduced distortion.
* Heat can be directed very accurately – useful for fillet welding.
What are the issues with keyhole welding?
Issues
* Joint fit up.
* Incorporation of filler material
What are the 4 main process variables for keyhole and conduction welding?
Laser beam properties
* Power density; Pulse, Spot size;
Polarisation; Wavelength.
Transport properties
* Welding speed; Beam focal
position; Joint geometries.
Shroud gas properties
* Composition; Shroud design;
Mass flow / Pressure .
Material properties
* Composition; Surface condition.
What are the main 4 defects in keyhole welding?
Defects
* Fusion
* Porosity
* Cracking
* Oxidation
Of CO2, Nd:YAG, Diode and Fibre whihc is best suited use case for the 2 welding modes?
CO2 - Conducting welding
Nd:YAG- conduction and keyhole spot weld
Diode-conduction and transmission plastic welding
Fibre - keyhole welding
How does irradiance effect welding?
A lower irradiance is for conduction limited welding as it will cause a shallower melt pool whereas for keyhole it will be achieve from a higher irradiance achieving a greater penetration depth.
How does beam shaping effect welding?
The gaussian beam can achieve the deeper penetration for keyhole welding whereas a DOE/modified beam will have a more controlled Heat affected Zone (HAZ) with a evenly distributed energy distribution of hot zone that are shallower in conduction limited.
How does transport properties effect welding?
Melt Flow Dynamics
* The steep temperature changes in the
meltpool cause high speed melt
velocities.
* The high melt velocities give a large
convective heat flow effect. This is
called Marangoni flow.
* Molten pools are typically double the
expected length leading to fault
formation Welds can suffer from
“drop- out” or “humping”.
How do processing speeds effect the weld?
-Too high a speed causes humping and undercut, low fusion zone and holes.
-low speed causes high HAZ, porosity and dropout
How does the focus position effect the weld?
0 to 1 mm below the surface.
* Influence the penetration depth and type of welding;
Keyhole or conduction-limited welding.
IF the focus is below the material surface TOO DEEP then there is shallow, inconsistent weld pool and increase HAZ.
IF the focus is JUST ABOVE the material surface then there is appropriate conditions for welding and not causing insufficiency melt pools.
