Laser Cladding and Deposition

Question 1

How does the blown powder method work?

Answer 1

• powder is mobilised in a stream of inert gas that is carried to the melt pool
• A computer controlled powder hopper/feeder is needed to feed the powder via a suitable nozzle into the laser-generated melt pool

Question 2

Explain laser cladding fusion bonding and what level dilution should be ? (‘dilution” is dissolution in cladding terminology)

Answer 2

• Aim is to deposit a relatively thick layer of metal onto the substrate, which is fusion bonded to the surface, but with a low level of dissolution of the added materail (<10% as a rule)
• Dilution >10% is generally considered to be a contamination of the cladding which might degrade its intended mechanical or corrosion-resistance properties

Question 3

What are the key control parameters for laser cladding?

Answer 3

Principal factors are:

1. Laser intensity
2. Powder flow rate
3. Substrate translation velocity

Question 4

Explain why resolidification in the tranquil zone is a key advantage of laser cladding?

Answer 4

Re-solidification in the ‘tranquil zone’ under the front edge of the clad profile leads to intrinsinctly low dilution, even though a strong fustion bond is formed.

Question 5

Explain what happens if resolidification occurs above the tranquil zone

Answer 5

There is a high velocity backward of molten material that accounts for the ability to develop thick, porosity-free layers.

Question 6

Why is the angle of inclination for the melt front to the laser important?

Answer 6

• Important in attaining a metallurgical bond through careful melting of a thin layer of the substrate at the leading edge.

Question 7

What is aspect ratio and why is it important?

Answer 7

• Defined as width/height
• For a given transverse speed and powder flow rate, there is an optimum laser power and spot diameter for maximising the cladding rate
• A discontinuous clad or ‘balling’ can occur when there is a lack of fusion bonding

Question 8

What are the three limits for low dilution, fusion bonded, porosity free cladding?

Answer 8

1. onset of porosity (aspect ratio< (or equal to) 5)
2. onset of significant dilution (upper energy balance)
3. Loss of clad continuity (lower energy balance)

Question 9

What is multi-track laser cladding?

Answer 9

Multiple overlapping tracks for large area coverage

Question 10

When does inter run porosity occur?

Answer 10

• Occurs when aspect ratio falls below 5, resulting in thicker clad layers and higher mass flow rates.
• Aspect ratio>5 is required fopr successful overlapping

Question 11

Whatis laser deposition and what are the main fratures of the laser deposition process?

Answer 11

• Layer-by-layer deposition of powder based materials
• Fabricates 3D 100% dense parts of complex shape, or to add functionality to existing surfaces.
• integrates blown powder laser cladding technology with 3D part/laser beam manipulation, CAD, fluid dynamics, control and metallurgy

Question 12

Why is type 2 the best type of clad profile?

Answer 12

Has the minimum dilution consistent with fusion bonding and it corresponds to the highest ‘catchment efficiency’

Question 13

Why is powder jet geometries important in laser deposition?

Answer 13

cross sectional size of powder jet in relation to molten layer is important for determining catchment efficiency.

25-30 % most common for this process.

Question 14

Why are Coaxial and balanced multi-jet nozzles better than single side jets?

Answer 14

They avoid single jet problems like:

1. directional effects on the clad bead shape
2. Critical alignment of the powder stream with the melt pool

They can ‘write in metal’ any direction they want.