Transcript Materials Processing and Design

Materials Processing and
Design
Process Attributes
Material Class
Characterized by melting point and hardness
Size
Minimum and Maximum overall size, measured by
volume and weight
Shape
Aspect ratio; web thickness-to-depth ratio; surfaceto-volume ratio
Complexity
Information content, symmetry, etc.
Tolerance
Dimensional accuracy or precision
Roughness
Surface finish measured by RMS surface roughness
Surface Detail
Smallest radius of curvature at corner
Min. Batch Size
Minimum number of components to be made
Production Rate Time to produce one component; cycle time
Cost
Cost per component
Process Selection
Design
Process
Process Attributes
Materials
Size
Shape
Tolerance
Precision
Process cost
Material
Capital
Lalbour
Process Choice
Classes of Processes
Raw Material
Casting Methods
Gravity, Pressure
Die Casting
Pressure Moulding
Polymer Moulding
Glass Moulding
Deformation Processing
Roll, Forge
Draw, Press
Machining
Cut, Turn, Plane
Drill, Grind
Heat Treat
Quench, Temper Steels
Age-hardened Al-alloys
Joining
Bolt, Rivet, Weld
Braze, Adhesive
Finish
Polish, Plate
Anodise, Paint
Powder Methods
Sinter, Slip-cast
Hot Isostatic Press
Special Methods
Lay-up, CVD
Electroform
Process Selection Charts
Size-Shape chart
Information Content-Size chart
Size-Melting Point chart
Hardness-Melting Point chart
Tolerance and Surface Finish
Process Cost
Size-Shape Chart
Volume contours V = At
Aspect ratio  = t/l  t/A1/2
There are inaccessible zones on the
chart – it is not possible to create shape
with smaller surface-to-volume ratio
than that of a sphere
Information Content-Size chart
Complexity of shape can be measured in
terms of:



Number of independent dimensions
Precision with which these dimensions are
specified
Symmetry, or lack of it.
The first two aspects are captured
approximately by the quantity
 l
C  n log 2 
 l



Size-Melting Point Chart
Low melting metals can be cast by any one of
the casting techniques; as Tm rises, the range
of primary-shaping techniques becomes more
limited
The ‘surface-tension limit’ is a lower size limit
for gravity-fed castings
The addition of a pressure, e.g. in pressure
die casting or centrifugal casting, overcomes
this limit
Hardness-Melting Point Chart
Yield strength limits the ability to deform and
machine
Forging and rolling pressure, tool loading and
the heat generated during machining
depends on the flow strength or UTS
Real materials occupy only the region
between the two heavy lines because
hardness (H) and Tm are inter-dependent.
H
0.03 
 20
kTm
 Is the atomic or molecular volume
Tolerance and Surface Finish Chart
Tolerance is the permitted slack in the dimension of
a part, e.g. 100±0.1 mm
Surface finish is measured by the RMS amplitude of
the irregularities on the surface, e.g R = 10 m.
Obviously, T > 2R. Real processes gives T which
range from 10R to 1000R.
Processing cost increase almost exponentially as the
requirement for T and R.
Polymer can easily attain high surface smoothness
but T < 0.2 mm is seldom achievable.
Tolerance and Surface Finish Chart
Finish (R), m
Process
Typical Application
0.01
Lapping
Mirrors
0.1
Precision grind or lap
High-quality bearings
Precision grinding
Cylinders, pistons, cams,
bearings
0.5-2
Precision machining
Gears, ordinary machine
parts
2-10
Machining
Light-loaded bearings,
Non-critical components
3-50
Unfinished castings
Non-bearings surfaces
0.2-0.5
Process Cost
Commonsense rules for minimizing cost
Keep things standard and simple
 Do not specify more performance than is
necessary
Breakdown of Cost C  Cm  Cc  CL
n
n
 C : material cost

m




Cc: capital investment
CL: labour cost (per unit time)
n: batch size
n : batch rate
Case Studies – Forming a Fan
To make a fan of radius 60 mm with 20
blades of average thickness 3 mm
Must be cheap, quiet and efficient
Materials selection procedure identified
aluminium alloys and nylon
Form in a single operation to minimize
process costs, i.e. net-shape forming –
leaving the hub to be machined
Case Studies – Forming a Fan
Constraint
Value
Material
Nylons Tm = 550 –573 K
H = 150 – 270 MPa
Al-alloys
Tm = 860 – 933 K
H = 150 – 1500 MPa
Complexity
160 – 330
Minimum section
1.5 – 6 mm
Surface area
0.01 – 0.04 m2
Volume
1.5  10-5 to 2.4  10-4 m3
Weight
0.03 – 0.5 kg
Mean precision l l
10-2
Roughness
< 1 m
Case Studies – Forming a Fan
Surface smoothness is the discriminating requirement
Process
Comment
Machine from solid
Expensive. Not a net-shape
process
Cold deformation
Investment casting
Cold forging meets design
constraints
Accurate but slow
Die casting
Meets all design constraints
Injection moulding
Meets design constraints
Resin transfer moulding
Meets all design constraints
Case Studies – Fabricating a
Pressure Vessel
Tough steel was chosen as the material
Inside radius is 0.5 m and height is 2m,
with removable end-caps; operating
pressure is 100 MPa.
Outside radius is calculated as 0.7m,
surface area  15 m2 and volume  1.5
m3; weight  12 tonnes
Precision and surface roughness are
both not important
Case Studies – Fabricating a
Pressure Vessel
Size is the discriminating requirement
Process
Comment
Machining
Machine from solid (rolled or forged) billet. Much
material discarded, but reliable
Hot working
Steel forged to thick-walled tube, and finished by
machining end faces, ports, etc. Preferred route for
economy of material use.
Casting
Cast cylinder tube, finished by machining end-faces
and ports. Casting-defects a problem
Fabrication
Weld previously-shaped plates. Not suitable for the
HIP; use for very large vessels (e.g. nuclear
pressure vessels.)
Case Studies – Fabricating a
Pressure Vessel
Other consideration includes:



Casting is prone to including defects;
elaborate ultrasonic testing needed
Welding is also defect-prone and requires
elaborate inspection
Forging or machining from a forged billet
are best because the large compressive
deformation during forging heals defects
and aligns oxides and inclusions in a less
harmful way
Case Studies – Forming a
Silicon Nitride Microbeam
The ultimate in precision mechanical
metrology is the atomic force microscope
Design requirements:



Minimum thermal distortion
High resonant frequency
Low damping
Silicon carbide and silicon nitride are
suitable materials
Case Studies – Forming a
Silicon Nitride Microbeam
Constraint
Value
Material
Silicon carbide Tm = 2973-3200 K
H = 30 - 33 GPa
Al-alloys
Tm = 2170 - 2300 K
H = 30 - 34 GPa
Complexity
40 - 60
Minimum section
2 – 8 m
Surface area
5  10-7 to 2  10-6 m2
Volume
2  10-12 to 10-11 m3
Weight
6  10-9 - 3  10-8 kg
Mean precision l l
10-2 to 10-3
Roughness
 0.04 m
Case Studies – Forming a
Silicon Nitride Microbeam
Casting or deformation methods are
impossible for the materials
Powder methods cannot achieve the
size or precision required
CVD and evaporation methods of
microfabrication are the best bet here