Selection Criteria Properties Availability Cost Manufacturability Most designers select materials considering Properties Availability Cost Manufacturability

Properties Mechanical Yield strength (ductile) / Ultimate strength (brittle) Fatigue (endurance limit) Stiffness or rigidity (modulus of elasticity) Hardness (Rockwell, Brinell) Impact strength (fracture toughness) Toughness (sharpy test) Ductility (% reduction is area) Hardenability Creep (elevated temperature properties) High/Low temperature properties Wear characteristics

Properties Physical Chemical Corrosion resistance Oxidation resistance Density Electrical properties Thermal properties (conductivity) Magnetic Melting point Thermal expansion Flamability Optical Chemical Corrosion resistance Oxidation resistance Toxicity Resistance to radiation Resistance to elements Dimensional Dimensional stability Surface finish

Availability Important questions: Is the supply plenty, stable, and predictable Is the material available in the desired form? Gage 10 sheets of 304L stainless steel? Do we have to order 100,000 lbs? Is the material available from company-certified vendors and multiple suppliers?

COST The primary factor in material selection in industry Comparative cost of engineering metals per pound Hook design: Tungsten In school: Properties important / cost no issue In industry: Cost also very important

COST Cost per volume may be a better choice. The following table is $/in3

Other Cost Factors What is the cost of material in relation to the production cost or selling price? Is it an orbiting satellite, B1 bomber, or a hammer? What is the production volume? A one of a kind design, or Garden Weasel? What are the manufacturing processes?

Manufacturability Primary processes Forming Machining Casting Welding Rolling (hot or cold) + forging + upsetting + bending + drawing Machining Turning + milling + drilling + broaching + sawing Casting Sand casting, investment casting, die casting Welding Laser cutting + water jet cutting PM technology

Manufacturability Secondary processes Forming Machining Heat treatment Finish forming Machining Turning + milling + reaming + grinding Heat treatment Deburring Plating + coating

Manufacturing Machinability Weldability Formability Castability Hardenability

Designer’s Choices Know about the properties of a few carbon steels Know about the properties of a few cast irons Class 20, Class 60, Ductile 60-45-18 Know about the properties of a few alloy steel 4140, 4340, 9310 Know about the properties of a few tool steels 01, A2, D2, S1, H13, M2 Know about the properties of a few stainless steels 304, 304, 316,420,440C, 17-4 PH

Designer’s Repertoire Know about the properties of a few Aluminum alloys: 3003, 5052, 6061, 7075 Know about the properties of a few nickel alloys: Monel, Inconel Know about the properties of a few copper alloys: C36000 yellow brass, C52100 phosphor bronze Know about the properties of a few titanium alloys Know about the properties of a few Magnesium alloys

Designer’s Repertoire Know about the properties of a few ceramics Aluminum oxides Silicon nitride / Silicon carbide Cemented carbides Carbon products Ceramic coatings (Chromium oxide, Tungsten carbide) Know about the properties of a few plastics: Polyethylene / Polyimides / Polyamides Polycarbonate PVC /ABS Nylons Elastomers

Example Suppose a designer has created a drawing for a shaft of a hypothetical device. The list of operational conditions have been determined as follows: 1.25” diameters are to fit ball bearings The shaft is subjected to maximum shear stress of 10 ksi There is possibility of moderate shock load Small end must resist damage from frequent removal of a keyed gear. There are no inertial requirements. Surface roughness to be 32 micro inches max. Diameters must be concentric to 0.001 inch.

Example Based on the operational requirements, time constraint, and cost, the following selection factors have been established: Hardness of at least 30 HRC Fatigue strength of 30 ksi Impact strength must be high Stiffness must be high The part must not rust in 50% RH room air. Must be dimensionally stable. Parts are needed in one week. Three units are required. Expected service life is 5 years