©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
PHYSICAL PROPERTIESOF MATERIALS
•Volumetric and Melting Properties•Thermal Properties•Mass Diffusion•Electrical Properties•Electrochemical Processes
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Physical Properties Defined
Properties that define the behavior of materials inresponse to physical forces other than mechanical
•Components in a product must do more than simplywithstand mechanical stresses
•They must conduct electricity (or prevent conduction),allow heat to transfer (or allow its escape), transmitlight (or block transmission), and satisfy many otherfunctions
• Includes: volumetric, thermal, electrical, andelectrochemical properties
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Physical Properties in Manufacturing
• Important in manufacturing because they ofteninfluence process performance
•Examples:In machining, thermal properties of the work
material determine the cutting temperature, whichaffects how long tool can be used before failure
In microelectronics, electrical properties of siliconand how these properties can be altered bychemical and physical processes is the basis ofsemiconductor manufacturing
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Volumetric and Melting Properties
Properties related to the volume of solids and how theproperties are affected by temperature
• Includes:DensityThermal expansionMelting point
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Density Defined
Weight per unit volume•Typical units are g/cm3 (lb/in3)•Determined by atomic number and other factors such
as atomic radius, and atomic packingSpecific gravity = density of a material relative to density
of water and is a ratio with no units
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Why Density is Important
Important consideration in material selection for a givenapplication, but it is generally not the only property ofinterest
•Strength is also important, and the two properties areoften related in a strength-to-weight ratio, which istensile strength divided by its densityUseful ratio in comparing materials for structural
applications in aircraft, automobiles, and otherproducts where weight and energy are of concern
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Thermal Expansion
•Density of a material is a function of temperatureIn general, density decreases with increasing
temperatureVolume per unit weight increases with increasing
temperatureThermal expansion is the name for this effect of
temperature on densityMeasured by coefficient of thermal expansion
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Coefficient of Thermal Expansion
Change in length per degree of temperature, such asmm/mm/C (in/in/F)
•Length ratio rather than volume ratio because this iseasier to measure and apply
Change in length for a given temperature change is:L2 - L1 = L1 (T2 - T1)
where = coefficient of thermal expansion; L1 andL2 are lengths corresponding respectively totemperatures T1 and T2
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Thermal Expansion in Manufacturing
•Thermal expansion is used in shrink fit andexpansion fit assembliesPart is heated to increase size or cooled to
decrease size to permit insertion into another partWhen part returns to ambient temperature, a
tightly-fitted assembly is obtained•Thermal expansion can be a problem in heat
treatment and welding due to thermal stresses thatdevelop in material during these processes
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Melting Characteristics for Elements
Melting point Tm of a pure element = temperature atwhich it transforms from solid to liquid state
•The reverse transformation occurs at the sametemperature and is called the freezing point
Heat of fusion = heat energy required at Tm toaccomplish transformation from solid to liquid
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Melting of Metal Alloys
•Unlike pure metals, most alloys do not have a singlemelting point
• Instead, melting begins at a temperature called thesolidus and continues as temperature increases untilconverting completely to liquid at a temperaturecalled the liquidusBetween the two temperatures, the alloy is a
mixture of solid and molten metalsException: eutectic alloys melt (and freeze) at a
single temperature
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Melting of Noncrystalline Materials
• In noncrystalline materials (glasses), a gradualtransition from solid to liquid states occurs
•The solid material gradually softens as temperatureincreases, finally becoming liquid at the melting point
•During softening, the material has a consistency ofincreasing plasticity (increasingly like a fluid) as itgets closer to the melting point
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Figure 4.1 - Changes in volume per unit weight (1/density) as afunction of temperature for a hypothetical pure metal, alloy, andglass; all exhibiting similar thermal expansion and meltingcharacteristics
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Importance of Meltingin Manufacturing
•Metal casting - the metal is melted and then pouredinto a mold cavityMetals with lower melting points are generally
easier to cast•Plastic molding - melting characteristics of polymers
are important in nearly all polymer shaping processes•Sintering of powdered metals - sintering does not
melt the material, but temperatures must approachthe melting point in order to achieve the requiredbonding of powders
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Thermal Properties
•Thermal expansion, melting, and heat of fusion arethermal properties because temperature determinesthe thermal energy level of the atoms, leading to thechanges in materials
•Additional thermal properties:Specific heatThermal conductivityThese properties relate to the storage and flow of
heat within a substance
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Specific Heat
The quantity of heat energy required to increase thetemperature of a unit mass of material by one degree
To determine the energy to heat a certain weight ofmetal to a given elevated temperature:
H = C W (T2 - T1)where H = amount of heat energy; C = specific heatof the material; W = its weight; and (T2 - T1) = changein temperature
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Volumetric Specific Heat
The quantity of heat energy required to raise thetemperature of a unit volume of material by onedegree
•Density multiplied by specific heat C•Volumetric specific heat = C
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Thermal Conductivity
Thermal conduction - transfer of thermal energy within amaterial from molecule to molecule by purely thermalmotions; no transfer of mass
Thermal conductivity of a material = capability totransfer heat through itself by this physicalmechanism
•Measured by coefficient of thermal conductivity k.Units: J/s mm C (Btu/in hr F)
•Coefficient of thermal conductivity is generally high inmetals, low in ceramics and plastics
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Thermal Diffusivity
The ratio of thermal conductivity to volumetric specificheat is frequently encountered in heat transferanalysis
Ck
K
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Thermal Properties in Manufacturing
• Important in manufacturing because heat generationis common in so many processesIn some cases, heat is the energy that
accomplishes the processExamples: heat treating, sintering of powder
metals and ceramics
In other cases, heat is generated as a result of theprocessExamples: cold forming and machining of
metals
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Mass Diffusion
Movement of atoms or molecules within a material oracross a boundary between two materials in contact
•Because of thermal agitation of the atoms in amaterial (solid, liquid, or gas), atoms are continuouslymoving aboutIn liquids and gases, where the level of thermal
agitation is high, it is a free-roaming movementIn metals, atomic motion is facilitated by vacancies
and other imperfections in the crystal structure
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Figure 4.2 - Mass diffusion: (a) model of atoms in two solid blocksin contact: (1) when two pieces are first brought together, eachhas its own compositions; (2) after time, an exchange of atomsoccurs; and (3) eventually, a uniform concentration occurs. Theconcentration gradient dc/dx for metal A is plotted in (b).
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Mass Diffusion in Manufacturing
•Surface hardening treatments based on diffusioninclude carburizing and nitriding
•Diffusion welding - used to join two components bypressing them together and allowing diffusion tooccur across boundary to create a permanent bond
•Diffusion is also used in electronics manufacturing toalter the surface chemistry of a semiconductor chip invery localized regions to create circuit details
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Electrical Properties
•Engineering materials exhibit a great variation in theircapability to conduct electricity
•Flow of electrical current involves movement ofcharge carriers - infinitesimally small particlespossessing an electrical chargeIn solids, these charge carriers are electronsIn a liquid solution, charge carriers are positive
and negative ions
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Electrical Properties
•Movement of charge carriers is driven by thepresence of electric voltage
•And resisted by the inherent characteristics of thematerial, such as atomic structure and bondingbetween atoms and molecules
RE
I Ohm's law:
where I = current, A, E = voltage, V, and R = electricalresistance,
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Electrical Resistance
Resistance in a uniform section of material (e.g., a wire)depends on its length L, cross-sectional area A, andresistivity of the material r
or
where resistivity r has units of -m2/m or -m (-in.)
AL
rR LA
Rr
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Resistivity
Property that defines a material's capability to resistcurrent flow
•Resistivity is not a constant; it varies, as do so manyother properties, with temperature
•For metals, resistivity increases with temperature
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Conductivity
Often more convenient to consider a material asconducting electrical current rather than resisting itsflow
Conductivity of a material is simply the reciprocal ofresistivity:
Electrical conductivity =
where conductivity has units of (-m)-1 ((-in)-1)
r1
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Engineering Materials andElectrical Properties
•Metals are the best conductors of electricity, becauseof their metallic bonding
•Most ceramics and polymers, whose electrons aretightly bound by covalent and/or ionic bonding, arepoor conductors
•Many of these materials are used as insulatorsbecause they possess high resistivities
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Semiconductors
A material whose resistivity lies between insulators andconductors
•Most common semiconductor material is silicon,largely because of its abundance in nature, relativelow cost, and ease of processing
•What makes semiconductors unique is the capacityto significantly alter conductivities in their surfacechemistries in very localized areas to fabricateintegrated circuits
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Electrical Properties in Manufacturing
•Electric discharge machining - uses electrical energyin the form of sparks to remove material from metals
•The important welding processes, such as arcwelding and resistance spot welding, use electricalenergy to melt the joint metal
•Capacity to alter electrical properties ofsemiconductor materials is the basis formicroelectronics manufacturing
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Electrochemistry
Field of science concerned with the relationshipbetween electricity and chemical changes, and withthe conversion of electrical and chemical energy
• In a water solution, molecules of an acid, base, orsalt are dissociated into positively and negativelycharged ions
• Ions are the charge carriers in the solution - theyallow electric current to be conducted, playing thesame role that electrons play in metallic conduction
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Terms in Electrochemical Processes
•Electrolyte - the ionized solution•Electrodes –where current enters and leaves the
solution in electrolytic conductionAnode - positive electrodeCathode - negative electrode
•The whole arrangement is called an electrolytic cell
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Electrolysis
The name given to these chemical changes occurring inthe solution
•At each electrode, chemical reaction occurs, such as:Deposition or dissolution of materialDecomposition of gas from the solution
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Figure 4.3 –Example of electrolysis: decomposition of waterDilute sulfuric acid (H2SO4) = electrolytePlatinum and carbon (both chemically inert) = electrodes
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Chemical Reactions inDecomposition of Water
•The electrolyte dissociates into the ions H+ and SO4=
•H+ ions are attracted to negatively charged cathode;upon reaching it they acquire an electron andcombine into molecules of hydrogen gas
2H+ + 2e H2 (gas)
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Chemical Reactions inDecomposition of Water
•The SO4= ions are attracted to the anode, transferring
electrons to it to form additional sulfuric acid andliberate oxygen
2SO4= - 4e + 2 H2O 2H2SO4 + O2
•The product H2SO4 is dissociated into ions of andSO4
= again and so the process continues
©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Electrolysis inManufacturing Processes
•Electroplating - an operation that adds a thin coatingof one metal (e.g., chromium) to the surface of asecond metal (e.g., steel) for decorative or otherpurposes
•Electrochemical machining - a process in whichmaterial is removed from the surface of a metal part
•Production of hydrogen and oxygen gases