Selection Criteria Properties Availability Cost Most designers select materials with equal consideration of: Properties Availability Cost

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 material available in the desired form? Gage 10 sheets of 304L stainless steel? If not available, can it be obtained? Do we have to order 100,000 lbs? Is the material available from 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 orbiting satellite, B1 bomber, or a hammer? What is the production volume? A one of a kind design, or graden weisel? What is the anticipated service life? A toy is expected to last for a year or two, low quality gears and other components do fine. Tooling materials for a one-year production can wear more quickly

Other Cost Factors Manufacturability (Fabricability) If considerable machining is required, it may well be more economical to choose a more expensive material with higher machinability rating than a lower cost material with lower machinability. Some materials can not be machined at all (ceramics). A “non-common” metal may require more lead time, different tooling and set up, or special techniques for fabrication. Only softer materials with good formabilty can be easily formed.

Manufacturability The size and weight of a part can limit its formability. Part size and geometry limit parts fabricated with PM technology Materials to be heat treated for strength must have sufficient hardenability. Materials that require heat treatment may have to go through further processes. Hot rolled materials may require further processing. Cold-rolled materials may distort when machined Casting processes have weight, wall thickness, size, accuracy, surface finish, and material limitations.

Manufacturing Parts fabricated by welding require good material weldability. Most metals can be welded but with a wide range of methods and very different costs. Low alloy steels cost the least, high strength steels cost more, and tool steels are most expensive to weld. Some materials require pre and post-weld heat treatment. Welding induces distortion in parts, further processing may be required. It is difficult to join aluminum and steels together.

Designer’s Repertoire Know about the properties of a few carbon steels 1010, 1030, 1050, 1080, B1112 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

Example