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Laser Materials Processing > Laser Cladding > Flashcards

Laser Cladding Flashcards

1
Q

What is cladding?

A

To overlay one material with another material to form a sound interfacial bond without diluting the cladding metal with the substrate

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2
Q

What is contamination to cladding?

A

Dilution

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3
Q

What is the clad track characteristics for the deposited material for cladding?

A
  • The material deposited is well adhered to the substrate because of the molten surface.
  • Excessive melting causes “dilution” - the uptake of substrate elements by
    the cladding alloy.
  • This can cause detrimental effects; e.g., reduced corrosion resistance,
    loss of high temperature properties, embrittlement.
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4
Q

What is the difference between cladding and alloying?

A

Alloying is within the material wheras cladding is to form a sound interfacial bond without diluting the cladding metal placed on top.

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5
Q

What are the 3 main steps of laser cladding?

A
  1. Melting
  2. Material feeding
  3. Rapid
    solidification
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6
Q

What are the 7 steps of the laser cladding process?

A
  1. Irradation of substrate
    2.molten pool established
    3.powder ejected into pool
    4.Powder melts and mixes with molten substrate
    5.laser irradation stops
    6.laser and nozzle movement continues
    7.path of track retraced
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7
Q

Why do laser cladding?

A
  • Fabrication of functional prototype.
  • Repair of components.
  • Applying functionally graded coatings.
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8
Q

What are the 3 main components of laser cladding?

A
  1. laser source
    2.material delivery
    3.cladding nozzle
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9
Q

Compare laser cladding between CO2 laser, Nd:YAG laser and Fibre laser

A

CO2 laser - high power available, poor absorption
Nd:YAG laser - medium powers, good absorption and can be fibre coupled
Fibre laser - high powers, good absorption and fibre delivered

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10
Q

What are the 3 methods of supplying clad material?

A

Wire feeding
blow powder feeding
pre placed powder

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11
Q

what is wire feeding in laser cladding?

A

This process is similar to arc welding
methods.
* The wire takes a lot of heat energy from the melt pool, so a larger melt pool is required.
* High deposition rate.
* Dense clad tracks.
* Straight line cladding only.

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12
Q

what is blown powder feeding in laser cladding?

A

This technique offers the most versatility as it has a well defined heated region, a fusion bond with low dilution and can be automated.
* The tracks are generally pore free and
with good surface strength.
* Cladding can proceed in any direction
and on any surface geometry.
* The powder is blown by an inert gas
into the meltpool.

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13
Q

What are the 3 main settings of powder feeding?

A

Off axis powder feed
Continuouse and discontinuous Co-axial powder feed

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14
Q

Compare Off axis powder feed &
Continuous Co-axial powder feed
Discontinuous Co-axial powder feed

A

Off axis powder feed (separate to the nozzle) - Higher powder catchment efficiency
Needs head repositioning
Less laser/powder coincidence
PROCESS ROBUST

Co-axial powder feed(inside the nozzle on both sides)- Less powder efficient
Omnidirectional
Increased laser/powder coincidence
FINE DETAILED WORK

Continuous - steady powder flow, precision in material from smooth flow and improved process stability

Discontinuous - intermittent powder flow with a controlled deposition saving powder consumption and more adaptable to different materials.
ALL POSITIONAL CLADDING

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15
Q

What is pre-place powder in laser cladding?

A

A powder layer is laid on the substrate surface prior to laser irradiation.

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16
Q

What is laser cladding advantages and disadvantages for pre placed powder?

A

Advantages
* Low volume manufacturing.
* Capability to clad small profiles with high precision.
* Manufacturing of highly complex geometries.
* Internal features

Disadvantages
* One of the main issue is that the process is very slow
* Can only build in one direction
* Issues with material shrinkage and density
* Sintering vs Melting

17
Q

What are the controllable input variables for laser cladding?

A
  • Power
  • Beam size
  • Wavelength
  • Laser traverse speed
  • Optical system
  • Material Characteristics
  • Geometry
  • Flow/feed rate
  • Composition
  • Powder size
  • Common Materials
18
Q

What are the main output variables of laser cladding?

A
  • Microstructure
  • Geometric tolerance and surface
    Roughness
19
Q

What are the main defects from laser cladding?

A
  • Crack
  • Dilution
  • Oxidisation
  • Spallation
  • Porosity
20
Q

What are cracks in laser cladding defects?

A

Cracks can be easily generated during
laser cladding, due to:
* small melt pool,
* large temperature gradient,
* narrow bonding zone
* thermal stress caused by differential
cooling and expansion rates of base
and clad materials

21
Q

What is dilution in laser cladding defects?

A
  • Dilution is the mixing percentage between the substrate and clad.
  • Some dilution occurs to fuse the material together
  • Over dilution can cause unfavourable effects
    aim for optimal not over or under diluted.
22
Q

What is oxidation in laser cladding defects?

A

As with many laser process, unwanted oxidation in laser cladding tends
to occur.
* Increasing the flow of the inert shroud gas to compensate may cause
spattering of the molten track.
* Industrial systems use a skirt to provide a large shroud gas rich region.

23
Q

What is spallation in laser cladding defects? and what is it caused by?

A

The clad track may cluster into
separate balls instead of forming
one molten region.
This can be caused by:
* lack of wetting of the substrate
surface.
* improper melting of the
additional material.

24
Q

What is porosity in laser cladding defects? - 2 main causes

A
  • Insufficient heating leading to
    incomplete melting of powder. The
    pore is the resultant gap between
    particles.
  • Excessive heating causing boiling of
    a volatile alloy constituent; e.g., lead,
    zinc, aluminium, which can cause
    the evolution of gases.
25
Q

How can crack generation be controlled/reduced?

A
  • Substrate pre-heating to reduce
    thermal gradients
  • Intermediate layers to increase
    material compatibility
26
Q

What are the advantages of laser cladding over traditional forming?

A

Reduced number of pre-production stages
* Greater material efficiency
* Cost not as dependent on geometric complexity
* No tool wear during manufacture
* Complex internal geometry possible
* Compositionally graded structures

27
Q

What are the advantages of laser cladding over other laser cladding techniques?

A
  • A wide range of metallic materials can be used
  • Standard powders can be used
  • A fully dense part is produced in a single stage (no post processing)
  • Self quenching – fine microstructure. Good final material properties.
  • Small heat affected zone and little part distortion (when used for repair /
    refurbishment)
28
Q

What are the disadvantages of laser cladding?

A
  • Accuracy
  • Secondary machining can be used for finishing
  • Surface finish
  • Adherent particles
  • The ‘staircase effect’ of layers
  • Part size limitations
  • Initial capital cost of equipment
  • Low deposition rate
  • Part re-orientation required for overhang structure.
29
Q

How does the gaussian beam temperature gradient cause undesired effects?

A

Excess heat at central region
* Degradation of alloy constituents
* Excess heat input to process
Uneven cooling
* Large grain growth
* Radial growth directions
* Formation of residual stresses
* Cracking
Uneven deposition surface leading to
overlapping of tracks
* Microstructure changes as previous
tracks are reheated
* Unstable foundations for the build-up
of multiple layers

30
Q

Why are diffractive optical elements ideal for welding?

A

DOE provides bespoke beam irradiance distribution

31
Q

What are advantages of using DOE?

A
  • Allows the beam distribution to be
    optimised for the process
  • Rapid beam distribution change –
    switch optics
  • No maintenance
  • No moving parts
  • Increased depth of field
  • ± 30 mm with DOE (Top Hat)
  • ± 12 mm with equivalent
    Gaussian lens
32
Q

Why is DOE good for the user?

A

Provides:
* Heating profile at the material surface
* The temperatures reached
* For how long
* Heating / Cooling rates.

33
Q

What is the difference in thermal gradient for gaussian beam vs modified beam

A

Gaussian beam has a high thermal gradient and the modified beam has a controlled thermal gradient

34
Q

What happens to the grain size in gaussian vs modified beam? why?

A

There is larger amounts of grain growth in gaussian making it weaker than in the modified where it is over a smaller area.
Gaussian has excessive heating and high intensity of the ORP dposition casuing the grain growth and the gaussian deposition has more columnar grains - intergranular cracking and corrosion=weak

35
Q

Compare alloy element segregation in gaussian vs modified beam

A

using an energy dispersive x ray there is great segregation in the gaussian beam throughout the whole material vs in the modified beam. This is an indicator of internal material transport mechanisms so there is a lower dilution through DOE.

36
Q

What are the main problem with scanning method?

A

Moving object whether the table or nozzle = low scanning speed
Beam reflection on a galvo scanner = fast but may require beam reconstruction

Laser Materials Processing (13 decks)

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