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Laser Materials Processing > Laser Welding > Flashcards

Laser Welding Flashcards

1
Q

What are the 2 main methods of welding?

A

Conduction limited welding and keyhole (plasma ) welding

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2
Q

What is welding?

A
  • 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.
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3
Q

What are the main steps of welding?

A
  1. Heating
  2. Melting / Vaporisation
  3. Melt pool dynamic
  4. Rapid solidification
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4
Q

What are the advantages of laser?

A

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

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5
Q

What is the difference between the 2 types of laser welding?

A

Conduction limited welding - melting
keyhol welding - melting and vaporisation

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6
Q

What is conduction limited welding?

A

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.

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7
Q

What are the benefits of conduction welding?

A
  • 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
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8
Q

What are the issues of conduction welding?

A
  • Surface reflectivity
  • Relatively slow
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9
Q

What radiation is used for conduction limited welding?

A

Near IR radiation.
The interface is coated in an infrared absorbing film.
Heat energy absorbed is suffience to form a weld

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10
Q

What is the transmission welding system?

A
  • 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
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11
Q

What are different types of weld geometries?

A

Butt, overlap , lip seal weld, lap edge joint etc

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12
Q

What is the difference between conduction and keyhole welding?

A

Key hole uses a higher power density therefore achieving a great weld depth and penetration.

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13
Q

What are the specific characteristics?

A
  • 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.
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14
Q

What are the two absorption mechanisms in key hole welding?

A
  • 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.
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15
Q

What are the benefits to keyhole welding?

A

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.

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16
Q

What are the issues with keyhole welding?

A

Issues
* Joint fit up.
* Incorporation of filler material

17
Q

What are the 4 main process variables for keyhole and conduction welding?

A

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.

18
Q

What are the main 4 defects in keyhole welding?

A

Defects
* Fusion
* Porosity
* Cracking
* Oxidation

19
Q

Of CO2, Nd:YAG, Diode and Fibre whihc is best suited use case for the 2 welding modes?

A

CO2 - Conducting welding
Nd:YAG- conduction and keyhole spot weld
Diode-conduction and transmission plastic welding
Fibre - keyhole welding

20
Q

How does irradiance effect welding?

A

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.

21
Q

How does beam shaping effect welding?

A

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.

22
Q

How does transport properties effect welding?

A

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”.

23
Q

How do processing speeds effect the weld?

A

-Too high a speed causes humping and undercut, low fusion zone and holes.
-low speed causes high HAZ, porosity and dropout

24
Q

How does the focus position effect the weld?

A

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.

25
Q

How do gas properties effect the weld?

A

a) To cool and blow away the plasma.
b) To protect melt pool and hot workpiece from atmospheric
contamination and oxidation.
c) To protect the laser optics from metal vapour.

26
Q

What are the two ways gas can be delivered in welding?

A

It will either be through the nozzle at the top or it will be a seperate tube to the nozzle.

27
Q

What is the 4 main gas options and which would be ?

A

Carbon Dioxide gas: increases absorption by generating FeO at high
temperature. It has low ionisation threshold, thus producing more plasma which is bad.
* Nitrogen gas: low cost, but has low ionisation threshold, thus easy to be
ionised - not so good.
* Argon gas: heavy (good for shroud, Reduce Oxidisation) but low ionisation threshold.
* Helium gas: light (bad for shroud), expensive, highest ionisation threshold (Good for Penetration) of all.

28
Q

Which gas has the highest ionisation potential and what is ionization potential?

A

Nitrogen LOWEST
Argon 2ND LOWEST
Neon SECOND HIGHEST
Helium HIGHEST
ionization is what causes the plasma arc for welding whereas higher the more it is used to form plasma in welding.

29
Q

How do material properties affect the weld?

A

Should be looking for ejection, seam width, irregular surface, surface quality post weld, pore formation (bubbles), bulging etc

30
Q

What are 6 main process defects for welds? and what to check for if these happen?

A

Lack of fusion -beam joint alignment
Lack of penetration - check focus position, laser power and speed, gas shielding arrangement
Porosity - check gas shielding, levels of moisture in atmosphere, other surface contamination.
Cracking - check material spec, welding speed or weld shape
Misalignment of sheets - check fit up
Oxidation - check gas shielding

31
Q

What is the melt pool geometry in welding?

A

The long narrow weldpool (1)
encourages capillary flow (2) through Marangoni flow.
This removes molten material from the
body of the meltpool which then
freezes behind the capillary leaving a
“hump” (3) which must be removed
by finish machining. This problem is
found during high speed processing

32
Q

What is the porosity defect in welding?

A

Large, wide weldpools
cannot support themselves
under gravity. This lead to
“drop-out”. Boiling and shroud gas entrapment can cause this.

33
Q

What is the case theory of single stationary spot?

A
  • In spot welding, you have either
    – an instantaneous source
    – a continuous source
    – a pulse
  • The source shape may be
    – a point
    – distributed on the surface e.g. a circle
    – a line
  • The substrate may be
    – infinite
    – semi-infinite
    – thin sheet
    There is an absorbed power density at the surface to the depth of melt at time. There can be losses to conduction through the eq.
34
Q

What is Rosenthal’s theory?

A

There is a single line moving heat source assessing the depth from the surface to the material thickness and absorbed power of velocity calculating.
The temperature field calculated is that due to conduction only
If fluid flow is taken into account the meltpool is typically 1.5 - 2
times longer due to convection

35
Q

What is the swift hook and gick weld analysis?

A

Swifthook and Gick analysed Rosenthal’ solution, with an analytical solution for the Bessel functions at high speed. This identified a relationship between two dimensionless terms. Real experimental data for thin sheet welding ( 1 - 5 mm) shows
reduced efficiency due to loss of energy transmitted through the
workpiece

36
Q

Which welding process is most energy
efficient: conduction limited or keyhole?

A

Keyhole is more because the energy is more concentrated on the material causing a deeper penetration and smaller heat affected zone.

37
Q

how does processing rate and specific energy consumption interact with each other?

A

As the specific energy consumption decreases, the processing rate increases.

38
Q

What to consider for a laser welding criteria?

A
  • Laser, wavelength, power, Pulsed/CW, mode (beam shape)
  • Optics, beam expansion, guidance method, focusing optics, focal length, position
    with respect to focus
  • Shroud gas, type, flow rate, delivery method, delivery geometry
  • Traverse, optics or table? Automated tracking?
  • Process control, operating parameter monitoring, in-process quality monitoring,
    post process monitoring

Laser Materials Processing (13 decks)

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