Materials Processing and Design

Process Attributes Material ClassCharacterized by melting point and hardness SizeMinimum and Maximum overall size, measured by volume and weight ShapeAspect ratio; web thickness-to-depth ratio; surface- to-volume ratio ComplexityInformation content, symmetry, etc. ToleranceDimensional accuracy or precision RoughnessSurface finish measured by RMS surface roughness Surface DetailSmallest radius of curvature at corner Min. Batch SizeMinimum number of components to be made Production RateTime to produce one component; cycle time CostCost per component

Process Selection

Classes of Processes

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/A 1/2 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

Size-Melting Point Chart Low melting metals can be cast by any one of the casting techniques; as T m 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 T m are inter-dependent. 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 ProcessTypical Application 0.01LappingMirrors 0.1Precision grind or lapHigh-quality bearings Precision grindingCylinders, pistons, cams, bearings 0.5-2Precision machiningGears, ordinary machine parts 2-10MachiningLight-loaded bearings, Non-critical components 3-50Unfinished castingsNon-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 C m : material cost C c : capital investment C L : labour cost (per unit time) n: batch size : 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 ConstraintValue MaterialNylonsT m = 550 –573 K H = 150 – 270 MPa Al-alloysT m = 860 – 933 K H = 150 – 1500 MPa Complexity160 – 330 Minimum section1.5 – 6 mm Surface area0.01 – 0.04 m 2 Volume to m 3 Weight0.03 – 0.5 kg Mean precision10 -2 Roughness < 1 m

Case Studies – Forming a Fan ProcessComment Machine from solidExpensive. Not a net-shape process Cold deformationCold forging meets design constraints Investment castingAccurate but slow Die castingMeets all design constraints Injection mouldingMeets design constraints Resin transfer mouldingMeets all design constraints Surface smoothness is the discriminating requirement

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 m 2 and volume 1.5 m 3 ; weight 12 tonnes Precision and surface roughness are both not important

ProcessComment MachiningMachine from solid (rolled or forged) billet. Much material discarded, but reliable Hot workingSteel forged to thick-walled tube, and finished by machining end faces, ports, etc. Preferred route for economy of material use. CastingCast cylinder tube, finished by machining end-faces and ports. Casting-defects a problem FabricationWeld 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 microscope Design requirements: Minimum thermal distortion High resonant frequency Low damping Silicon carbide and silicon nitride are suitable materials

ConstraintValue MaterialSilicon carbideT m = K H = GPa Al-alloysT m = K H = GPa Complexity Minimum section 2 – 8 m Surface area to m 2 Volume to m 3 Weight kg Mean precision10 -2 to 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 Case Studies – Forming a Silicon Nitride Microbeam