Materials Selection Guidelines

Or how to avoid breakage, berylliosisand/or bankruptcy

Overview

Important material propertiesDesign-centered EnvironmentalFabrication

Material classes and common examplesMetalsPolymersCeramicsComposites

When in doubt, ask your machinist!

Material Properties - Overview

Design-Centered PropertiesSatisfy functional requirements

Environmental PropertiesConstraints based on operating environment of apparatus

Fabrication PropertiesConstraints based on cost, available processes, and manufacturing quantity

Material Properties - Overview

ResourcesMatWeb (www.matweb.com)Machinery’s HandbookChemical Resistance Database (http://www.coleparmer.com/TechInfo/ChemComp.asp)McMaster-Carr (www.mcmaster.com)

Material Properties - Design

Mechanical PropertiesYoung’s modulus, density, fracture toughness

Electromagnetic PropertiesDielectric strength, transparency, permeability

Thermal PropertiesThermal conductivity, heat capacity

Material Properties – DesignMechanical Properties

Young’s ModulusDensityYield Strength (???)

Material Properties – DesignMechanical Properties

Failure is not as simple as yield stress!Yield stress

Processing sensitive, maybe anisotropicCreep

Stay below 1/3 of melting point (in K)Fatigue

Some materials fatigue even at low stressFracture

Very common failure mode, but very complex

Material Properties – DesignMechanical Properties

Watch for tempers, fillers, and/or temperatures used for mechanical property measurementsBrittleness indicators

Low elongation (< ~10%)UTS close to yield stressVery different notched/unnotched impact energy (Izod or Charpy tests)

Material Properties – DesignElectromagnetic Properties

Dielectric StrengthWatch out for short circuits!

Transparency / ColorThere are very few clear materials

Electrical ConductivityMagnetic Permeability / Hysteresis

Special alloys with tailored properties are available

Material Properties – DesignThermal Properties

Thermal ConductivityGood insulators are not strong materials

Thermal Diffusivity

Material Properties – Environment

Operating temperature rangeRemember to watch the low end, too!

Chemical resistanceIf in doubt, buy a material sample and test itCleaners matter too!

Radiation resistanceIncludes solar UV radiation!

AppearanceDoes it need to be shiny and impressive?

Material Properties - Fabrication

You are making one machine, not 106

Casting, molding, forging, and stamping are not economicalBenefits of rapid prototyping may offset poor material properties

Machinability directly impacts costHazardous materials more expensive to work

DON’T TRY THESE YOURSELF!Beryllium, magnesium, glass-reinforced plastics, etc.

A Reminder:

When in doubt, ask your machinist!

Materials - Metals

Aluminum PropertiesHigh strength/weight ratioNo fatigue limit (vibration = trouble!)Widely variable toughnessMost alloys resistant to air, humidity, solvents

Acids and bases very bad!Inexpensive and easy to work

Materials - Metals

Aluminum AlloysMachining

6061, 2024, 7075Forming

5052, 3003, 6063Welding

6061, 5052

Materials - Metals

Carbon and Alloy Steel PropertiesHeat treatment allows even a single alloy to have widely variable propertiesIn general, toughness and yield strength inversely proportionalWell-defined fatigue limitPoor corrosion resistanceInexpensive, but can be hard to machineWeldable

Materials - Metals

Steel Alloys1018: General purpose, low strength4140: Higher strength, heat treatableTool Steels

Many proprietary varieties availableDifficult to machine or formS7 good for high-impact structural applications

Maraging SteelsUltra-high strength and toughness, but very expensive

Materials - Metals

Stainless Steel PropertiesWide range of corrosion resistance

Some little better than alloy steel!Difficult to machineModerate costMost corrosion-resistant alloys not strong

Good at low temperaturesCan be easy to weld

Materials - Metals

Stainless Steel Alloys304: Most common, resistant to common conditions316: Extreme corrosion resistance, very difficult to machine, low strength440C: High strength, relatively low corrosion resistance

Materials - Metals

Copper Alloy PropertiesHigh thermal and electrical conductivityWidely variable strength and machinabilityModerate costCorrosion resistant, but can tarnishEasy to join by brazing/soldering

Materials - Metals

Copper AlloysBrass

Alloy 360 is strong and extremely machinableBronze

Alloy 630 is very hard and wear-resistantBeryllium Copper

Superb properties, but VERY TOXICFew shops will work it, and only for big $$$

Materials - Metals

MagnesiumLightest structural metalFlammable during machiningTarnishes and corrodes easily

TitaniumVery high strength/weight ratioMore corrosion resistant than stainless steelVery difficult to machine or form

Materials - Plastics

AcrylicTransparent and UV resistantStiff and strongVery brittleEasily solvent-weldedEasily laser cutDifficult to drill or machine

Materials - Plastics

PolycarbonateTransparent, but not UV resistantImpact resistant, but notch-sensitiveEasy to machine

Materials - Plastics

DelrinVery easy to machineSomewhat brittleLow friction

TeflonVery easy to machineVery solvent resistantExpensive

Materials - Ceramics

Not machinable without special equipment/experience

Even “machinable” ceramics aren’tHigh strength with high dielectric strengthHigh temperature capabilityVery brittleVery expensive

Materials - Composites

FR-4 Fiberglass-Epoxy CompositeCircuit board material; inexpensiveMachining dust is hazardousStrong, but somewhat brittle

Carbon FiberVery expensiveSheets, tubes available w/o custom toolingVery strong, but a bit brittle