Material Selection

• What do you think you should know about material science to make you an good designer?– The structure of crystals?

– The molecular composition of polymers?

– Methods of refinement of iron ore?

• Where does the material selection takes place in the design cycle? Imagine you are designing a “table saw” for home use, or a “train set” for children.– At the time conceptual design forms?

– At the time the drawing are made?

– At the time the analysis is performed?

Material Selection

• Good designers make a list of the part’s required properties and start a thorough literature search to find materials that meet or exceed the requirements.– Arguments in favor

– Arguments is opposition

• What other criteria is important in material selection? How important are they?

Selection Criteria

• Most designers select materials with equal consideration of:

– Properties

– Availability– Cost

Material Properties

• Team exercise: Team up with a fellow student and write down as many criteria as you can remember. Try to organize the criteria into groups. [5 minutes]– Mechanical

– Physical

– Chemical

– Dimensional

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 material available in the desired form?

• Gage 10 sheets of 304L stainless steel?

– If not available, can it be obtained in a month?

– Do we have to order 100,000 lbs?

– Is the material available from multiple suppliers?

– Does a particular material impose time delays in part manufacturing.

COST

• The primary factor in material selection in industry

• Comparative cost of engineering metals per pound

Material Price $ Material PriceCast irons .15 Cobalt 12

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

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

Magnesuim 1.5Tool steels 1.75

Nickle alloys 4.00

COST

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

Material Price $ Material PriceCast irons .04 Titanium 2

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

Magnesuim .10 Silver 31Aluminum .10 Gold 4257

Copper alloys .22Stainless steels .35

Tool steels .50Nickle alloys 1.25

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 mateials for a one-year production can wear more quickly

– A hydoelectric turbine requires highest quality materials

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 weigth, 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