Ch.10 Directed Energy Deposition

Question 1

What are three characteristics of DED similar to extrusion-based processes?

Answer 1

material extruded through nozzle, deposited layer by layer in different orientations

requires support material depending on complexity of structure

post-processing required to removal supports

Question 2

what are 3 characteristics not similar to extrusion-based processes?

Answer 2

material melted as its being deposited, multi-axis deposition

focused heat source

powder feeding enable overlapping scan without swelling/overfeeding

Question 3

Why is solidification rate considered the key characteristic to control in DED processing?

Answer 3

because of small molten pool (0.25-1 mm
diameter and 0.1-0.5 mm in depth)
and quick traverse speed –> produces high cooling rates, large thermal gradients –> unique grain structures

Question 4

what are some microstructural advantages due to the high cooling rate?

Answer 4

suppression of diffusion controlled solid-state phase transformation

formation of supersaturated solutions/nonequilibrium phases

extremely fine microstructures

very fine secondary phase particles

Question 5

Why are DED processes particularly suitable for repair?

Answer 5

multi-axis deposition head allows deposits onto existing parts, not limited to flat plate

Question 6

List and discuss the important process parameters in DED processes.

Which parameters
are inter-related and how do they interact?

Answer 6

track scan spacing, powder feed rate, beam traverse speed, beam power, spot size

inter-related parameters: powder feed rate, beam power, traverse speed

interactions:
increase feed rate –>lower power
increase speed –> lowers energy

Question 7

Benefits and limitations of powder feed

Answer 7

benefits:
- most versatile
- Ability to overlap scan lines without swelling/overfeeding
- Dynamically changing melt pool size
- Easy to change orientation in between layers
- Eliminates preferential grain growth and anisotropic properties leading to minimal residual stresses

Limitations: <100% powder capture

Question 8

Benefits and limitations of wire feed

Answer 8

Benefits:
- 100% feedstock capture efficiency
- high deposition rate
- suitable for “blocky” geometries

limitations:
- difficult dynamic leveling of thickness transitions
- Careful control of geometry process parameters needed
- Design porosity into large parts with geometric complexity to maintain accuracy

Question 9

Discuss the process parameters that determine how much material is added to the surface
(i.e. layer thickness) in DED processes?

Answer 9

Beam power
powder feed rate
beam traverse speed

Beam power/powder feed rate increase and lowering beam speed → deposit thickness goes up

Question 10

Why is the layer thickness dynamic?

Answer 10

first layer dependent on process parameters and focal plane location

excess powder material flow enables the melt
pool size to dynamically change

melt pool size change affects first few layers and then converges to right thickness