Selection Criteria

• 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– 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

Material Price $ Material Price Cast irons .15 Cobalt 12

Carbon steels .25 Titanium 17 Low alloy steels .30 Tungsten 30

Aluminum .60 Silver 75 Copper alloys .80 Beryllium 200 Stainless steels 1.25 Gold 6100

Magnesuim 1.5 Tool steels 1.75

Nickle alloys 4.00

COST

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

Material Price $ Material Price Cast irons .04 Titanium 2

Carbon steels .07 Cobalt 4 Low alloy steels .09 Beryllium 15

Magnesuim .10 Silver 31 Aluminum .10 Gold 4257

Copper alloys .22 Stainless steels .35

Tool steels .50 Nickle alloys 1.25

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

• 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

• 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– 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