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

2. 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

3. 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

4. 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?

5. 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

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

7. 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

8. 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.

9. 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.

10. 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.

11. 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
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

12. 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

13. 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

14. 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 inch.

15. 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

16. Example