Lecture 9 - Metal AM

Question 1

What is a metal ?

• closely …….. …….. structure, lose their ……. shell electrons
• …….. (exception Hg)
• ……….. density than most non metals
• Typically ……
• Generally malleable,…….. & fusible
• Shiny, good conductors of ………/…….

Answer 1

packed, atomic, outer

electrons

solid

higher

hard

ductile

electricity , heat

Question 2

_Conventional Metal Processing -1 _

What are the 4 conventional metal processes?

Answer 2

Casting

Subtractive/cutting(material removal)

Forming(no material removal)

Joining

Question 3

Conventional Metal Processing - 2

Give examples of the different conventional processes

Casting (4 examples)

Forming (6 examples)

Subtractive (3 examples)

Joining ( 4 examples)

In which category is AM in?

Answer 3

Casting - Investment casting,Die casting,Spin casting,Sand casting etc.

Forming - Extrusion,Drawing,Bending,Forging,Powder Metallurgy,Rolling etc.

Subtractive - Milling,Turning,Grinding etc.

AM is in the joining category

Question 4

Metal Additive Manufacturing

What’s good about it? (6 examples)

What is the Metal AM economic climate like?

Answer 4

• Geometric freedom
• Customisation
• Quicker to Market
• Cost Savings
• Improved Design/Performance
• Growing economy - Revenue from metals grew 38.3% to an estimated $24.9 million in 2012, up from $18 million the year before.
  Terry Wohlers Report 2013

Question 5

Metal AM Techniques

What are the 5 categories of Metal AM techniques?

Answer 5

• Powder bed fusion
• Powder/Wire feed
• Binder Jetting
• Sheet lamination
• other

Question 6

_Powder Bed Fusion -1 _

Most widely and extensively used AM process for the production of metal parts.

How does it work ?

what are 3 advantages of this technique?

Answer 6

Uses thermal energy, typically laser or electron beam

3 advantages are:

• Can produce high density fully functional parts in one step
• Excellent mechanical properties
• Good material variety and part properties

Question 7

Powder Bed Fusion -2

sketch a diagram of PBF and lable the parts

Answer 7

Question 8

Powder Bed Fusion - 3

…… layers of metallic powder deposited onto ………./…… plate and ……. with thermal energy from laser or ………… ……..

What happends to the unused powder?

Sketch a diagram of the point of interaction between the energy source and the powder

Answer 8

thin

substrate,base

melted

electron beam

Unused powder can be recycled

Question 9

PBF – Post-processing

Name and describe the 4 PBF post-processing techniques?

Answer 9

• Removal of excess powder – tapping, compressed air,ultrasonic
• Thermal processing – relieve stress of or improve mechanical properties. Furnace cycles or HIPPING to reduce pores and heal micro-cracks
• Support removal – Wire EDM or band saw to cut parts off platform. Often hand finishing (with pliers) to pull off remainingsupports
• Surface finishing operations - machining, shot-peening, tumbling and hand benching, electro-polishing, abrasive flow machining (for internal cavities). Micro-machining, chemical reaction at surface of material driven by fluid flow

Question 10

PBF - Process Control

Control of ………., ………… parameters and ………. conditions have major influence on the final ………… of the part, e.g. microstructure, …….., surface roughness etc.

sketch a diagram of the PBF process controls

Answer 10

material, process , environment

Properties

density

Question 11

Powder Bed Fusion - Part Properties

…….. Density
Complete melting in single step, therefore parts are produced to full density

Can match /exceed properties of …… and
approaches that of wrought

Less than full density compromises fracture …………. and ……….. properties. Could lead to …………. failure, act as crack initiation sites when subjected to cyclic stress

Compliance to specifications for toughness and
fatigue properties critical in the ……….. industry and
…………. and dental implants

Answer 11

High

cast

toughness, fatigue

premature

aerospace

orthopaedic

Question 12

PBF – Part Properties

Fine Microstructure

• Rapid melting and cooling of thin layers of material produces a …………. microstructure
• Some ……….. …………… may occur,but on a smaller scale compared to …………. processes.
• Chemical composition is more ………. than casting resulting in better ……….. properties

Answer 12

uniform

material segregation

casting

uniform

mechanical

Question 13

PBF – Part Properties

Metal AM allows you to ………… the micro structure and therefore tailor your ………… ………… to some extent.

EBM AM titainium has been found to have better mechical properties than cast or wrought.

This process produces ………….. material properties

Answer 13

control

mechanical properties

anisotropic

Question 14

Powder Bed Fusion - Limitations

what are the two main limitations of PBF ?

Answer 14

Residual stresses (rapid heating of material and the thermal gradients developed)

Surface finish

Question 15

PBF - Limitations - 2

Some parts require support/anchors because of thermal warpage. This is due to :

• Rapid heating/cooling
• Large thermal variations
• Stresses/warpage

What geometries are affected? sketch a diagram

How does this limit Metal AM ?

How can you reduces the number of supports required?

Answer 15

overhanging geometries are affected

This limits Metal AM by:

• Limiting geometric freedom due to anchor structures.
• Incurs post processing and cost for anchor removal
• Cannot easily stack parts on top of each other(as with polymer SLS)

changing the design orientaion may reduce the amount of support structure required.

Question 16

PBF - Limitations - 3

Why cannot you stack parts with PBF unlike polymer SLS?

Answer 16

because it must be connected to the substrate plate, therefore you cannot maximise the build volume.

Question 17

PBF - Limitations - 4

Stress can cause problems even after
support have been removed.

How can this be relieved ?

How do you remove the support structure?

Answer 17

This can be relieved with thermal processing (e.g. furnace cycles)

Wire EDM or band saw to cut parts off platform. Often hand finishing (with pliers) to pull off remaining supports

Question 18

PBF - Limitations - 5

What causes the surface roughness ?

how can you reduce surface roughness during the build? (3 methods)

Answer 18

the stair stepping effect causes surface roughness.

3 ways to reduce this by:

• Using smaller powder particles
• Laser re-melt strategies(pass over with laser , faster speed lower energy)
• Orientating parts differently

Question 19

PBF - Limitations - 6

Surface roughness can be reduced after a build by:

How can you reduce surface roughness in internal geometries?

Answer 19

Machining,

Shot-peening (high pressure ball bearings)

tumbling and hand benching,

electo-polishing,

(for internal cavities)abrasive flow machining is used (highpressure abrasive paste) .Chemical reaction on surface of material driven by fluid flow