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