materials processing and design. process attributes material classcharacterized by melting point and...
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Materials Processing and Design
Process AttributesMaterial 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; surface-to-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
Process AttributesMaterials
SizeShape
TolerancePrecision
Design Process
Process costMaterialCapitalLalbour
Process Choice
Classes of ProcessesR aw M ate ria l
C as tin g M eth od sG ravity, P ressu re
D ie C as tin g
P ressu re M ou ld in gP o lym er M ou ld in g
G lass M ou ld in g
D efo rm ation P rocess in gR o ll, F org e
D raw , P ress
P ow d er M eth od sS in te r, S lip -cas t
H o t Isos ta tic P ress
S p ec ia l M e th od sL ay-u p , C V DE lec tro fo rm
F in ishP o lish , P la te
A n od ise , P a in t
Jo in in gB o lt, R ive t, W e ldB raze , A d h es ive
H eat Trea tQ u en ch , Tem p er S tee lsA g e-h ard en ed A l-a lloys
M ach in in gC u t, Tu rn , P lan e
D rill, G rin d
Process Selection Charts
Size-Shape chartInformation Content-Size chartSize-Melting Point chartHardness-Melting Point chartTolerance and Surface FinishProcess Cost
Size-Shape Chart
Volume contours V = AtAspect 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
lnC 2log
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 limitedThe ‘surface-tension limit’ is a lower size limit for gravity-fed castingsThe 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 machineForging and rolling pressure, tool loading and the heat generated during machining depends on the flow strength or UTSReal materials occupy only the region between the two heavy lines because hardness (H) and Tm are inter-dependent.
2003.0
mkT
H 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 mmSurface 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 ChartFinish (R), m
Process Typical Application
0.01 Lapping Mirrors
0.1 Precision grind or lap
High-quality bearings
0.2-0.5 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
Process Cost
Commonsense rules for minimizing cost Keep things standard and simple Do not specify more performance than is
necessary
Breakdown of Cost Cm: material cost Cc: capital investment CL: labour cost (per unit time) n: batch size : batch rate
n
C
n
CCC Lcm
n
Case Studies – Forming a Fan
To make a fan of radius 60 mm with 20 blades of average thickness 3 mmMust be cheap, quiet and efficientMaterials selection procedure identified aluminium alloys and nylonForm in a single operation to minimize process costs, i.e. net-shape forming – leaving the hub to be machined
Case Studies – Forming a FanConstraint 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 10-2
Roughness < 1 m
ll
Case Studies – Forming a Fan
Process Comment
Machine from solid Expensive. Not a net-shape process
Cold deformation Cold forging meets design constraints
Investment casting Accurate but slow
Die casting Meets all design constraints
Injection moulding Meets design constraints
Resin transfer moulding
Meets all design constraints
Surface smoothness is the discriminating requirement
Case Studies – Fabricating a Pressure Vessel
Tough steel was chosen as the materialInside 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 tonnesPrecision and surface roughness are both not important
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.)
Size is the discriminating requirement
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 – Fabricating a Pressure Vessel
Case Studies – Forming a Silicon Nitride Microbeam
The ultimate in precision mechanical metrology is the atomic force microscopeDesign requirements: Minimum thermal distortion High resonant frequency Low damping
Silicon carbide and silicon nitride are suitable materials
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 10-2 to 10-3
Roughness 0.04 m
ll
Case Studies – Forming a Silicon Nitride Microbeam
Casting or deformation methods are impossible for the materialsPowder methods cannot achieve the size or precision requiredCVD and evaporation methods of microfabrication are the best bet here
Case Studies – Forming a Silicon Nitride Microbeam