UEET 601 Modern Manufacturing Introduction to structure and properties of materials
Introduction • What is manufacturing? • Conversion of a material from a primary form into a more valuable form - adding VALUE to a material • List examples of ANYTHING you know and how you think they were produced • Involves product • Design, • selection of Materials and • selection of Process UEET 601 Modern Manufacturing
Manufacturing demands/trends: • product design requirements, specs. and standards • environmentally conscious and economic methods of manufacture • Quality issues • flexibility in manufacturing methods (Why?) • New developments in materials, methods, CIM • System dynamics, productivity UEET 601 Modern Manufacturing
Product design considerations: • product requirements and performance • design considered together with manufacturing • product design cycle and life cycle characteristics • CONCURRENT ENGINEERING - integrated product development and design: • CAD, CAM, CAE • Rapid prototyping • Design for manufacture and assembly UEET 601 Modern Manufacturing
What materials? • There are a wide variety of materials available today with diverse characteristics that suit various applications. They are: • Metals and alloys • Ferrous or non-ferrous (Examples?) • Plastics • Thermoplastics, Thermosets • Ceramics, glass and diamond • Composites • Engineered, Natural (examples?) • Nano-materials, shape memory alloys, armorphous alloys, superconductors UEET 601 Modern Manufacturing
Other considerations in the selection of materials: • Properties of Materials • Mechanical - how a material will respond to its service condition loading - strength, stiffness, hardness, e.t.c. • Physical properties - density, thermal, electrical and magnetic properties, • Chemical properties - oxidation, corrosion, toxicity, flammability • Manufacturing properties - machinability, weldability, formability, castability, heat treatment • Cost and availability • Appearance, service life and recyclability UEET 601 Modern Manufacturing
What Process? • A wide variety; usually a product goes through a combination of processes • Choice depends on properties of material and product requirements, costs • casting - molten material allowed to solidify into shape in a mold cavity • forming and shaping - rolling, forging, extrusion, drawing, sheet forming, P/M, molding • machining - shape formed by removal of material • joining - welding, soldering, adhesive joining, brazing • Finishing operations - polishing, coating, e.t.c. UEET 601 Modern Manufacturing
The most important concept in materials science Structure – Property Relationships Composition Structure Properties Processing Useful applications
Compositionally Identical • Diamond • hardest known material • transparent to light • electrically insulating • highest thermal conduction of any material known • Graphite • one of the softest materials known • opaque • electrically conductive (in the basal plane) • thermally conductive (in basal plane) Why? Processing, that’s why.
Structure of Materials States of Matter Gas – molecules are free to move, no definite shape, no definite volume container determines volume Liquid - molecules are free to move but not as free as in a gas, definite volume, no definite shape container determines the shape Solids – molecules cannot move freely, definite volume, definite shape Plasmas – high temperature, similar to a gas, but many electrons are free leaving many charged ions While most industrial products are solids, liquids, or gasses, plasmas are important for industrial processing. *we’re going to forget about the Bose-Einstein condensate for this class.
Structure of Materials Bonding Ionic – electron transfer from one atom to another, bonding is electrostatic, common in salts Covalent – electrons are shared by nearby atoms, common in ceramics, semiconductors, and polymers Metallic – electrons in the valence shells become delocalized and are shared by the now positively charged metal atoms, common in metals Hydrogen bond – this is an electrostatic bond between an electronegative atom and a hydrogen atom bonded to nitrogen, oxygen, or fluorine, important for water and for nucleic acid and protein structures Van der Waals bond – a relatively weak bond caused by electric dipoles, which in turn are caused by random motion of electrons, occurs in all materials, important for noble gases, colloids (paint, polishing and cutting formulations, etc.,)
Structure of Materials - Metals The vast majority of metals are crystalline (atoms have a regular repeating spacing and orientation with respect to one another). There are a number of different possible symmetries for atomic arrangement, some common ones: bcc fcc
Structure of Materials - Metals The 14 Bravais lattices These represent the only possible ways to stack hard, uniform, spheres in 3-D space. This is true for all materials, not just metals. Many more possibilities arise when multiple atom types are present. * James F. Shackelford, Introduction to Materials Science for Engineers, Macmillan Publishing, 1988.
Structure of Materials - Metals Consequences of crystal structure: FCC crystals have a close packed plane along the diagonal of the cube, it is relatively easy to shear parallel to this plane. In general fcc metals are more ductile, and have lower melting points than bcc metals. fcc – planes can slip easily bcc – large corrugations, slippage is more difficult Crystal structure also plays a very significant role in electronic properties, very important for semiconductors.
Structure of Materials - Metals Formation of crystals: During cooling from a molten state crystal growth starts (nucleates) in many different places, these nuclei grow until they run into one another. Since the crystals nucleate in random orientation, when they meet there will be a boundary. These crystals are called grains. Most metals are polycrystalline, production of single crystals is possible in many cases but requires specialized processing. * http://chemical-quantum-images.blogspot.com/2007/03/shaping-copper.html
Structure of Materials - Metals Defects in Crystals: Point - Impurity (present in all materials) - Thermally Generated vacancies – a missing atom interstitial – an atom in a position that isn’t supposed to have one Line - dislocations Planar - twins - grain boundaries
Structure of Materials - Ceramics Most are crystalline (except for glasses) and often polycrystalline, with many grains like metals. The difference is in bonding, covalent (or ionic) instead of metallic. Much more difficult for dislocations to move, low ductility/brittle. Consider Al and Al2O3: Al Melting point 660 °C Mohs hardness 2.75 Electrical resistivity 2.65 x 10-6Ωcm Al2O3 Melting point 2054 °C Mohs hardness 9 (about 100X harder) Electrical resistivity 2.0 x 1013Ωcm Semiconductors are generally similar in bonding, but with greater ease of freeing an electron.
Structure of Materials - Semiconductors Silicon is FCC with two atoms per lattice point, this is the same as diamond and germanium. Diamond is not considered a semiconductor because it requires too much energy to free an electron. In most applications semiconductors are used in single crystal form (no grain boundaries). * wikipedia.org
Structure of Materials - Semiconductors Conductivity of Semiconductors is modified by controlling defect populations. Adding small quantities of an element with one too many electrons makes that extra electron very easy to free. Adding small quantities of an element with too few electrons makes a missing bond in the structure, this is also easy to move. * wikipedia.org
Structure of Materials - Glass Sometimes classified as a ceramic. A covalently bonded network that does not have a well defined repeating structure, it is amorphous. • Generally formed by cooling a melt of mostly silica (SiO2) containing other glass formers, intermediates, and modifiers (B2O3, P2O5, Na2O, CaO, Al2O3, PbO, etc.) fast enough that it cannot order itself into crystals. Unlike in metals this is not difficult to achieve. • While there is no long range order there is typically short range order, Si atoms are mostly bonded to four O atoms. • Melting point is not as well defined as in other materials, glass transition temperature.
Structure of Materials - Polymers Covalently bonded chains, made from repeating monomer units – polymerization • Covalently bonded within the chain, but with the ability to twist. • Between chains bonding can range from Van der Waals to covalent cross-linking H H H H H H Catalyst, heat, light C C C C C C H H H H H H n ethylene polyethylene
Structure of Materials - Polymers Huge variety of polymer types Addition – polyethylene, PVC, pAA, pAMPS, polystyrene, etc. Condensation – polyurethane, nylon, polycarbonate, silicones, etc. Can also be co-polymers (mixed monomer types, block or random, cross-linked or not, etc.
Mechanical Properties • Manufacturing often involves application of external forces. • The response of a material to external forces is important for its use in different applications
Types of Forces • Tension • Compression • Torsion • Bending • Shear Tensile testing is a common way to evaluate the strength of a material, though other types of testing are also done.
Tension Test • A material loaded in tension will stretch. F A L0 stress units are force per area [Mpa, psi] strain Dimensionless, expressed as in/in or % What is the relationship between stress and strain? It depends on the material.
Stress – Strain Curves *http://irc.nrc-cnrc.gc.ca/images/cbd/157f02e.gif
Stress – Strain Curves Proportional, Hook’s law, Young’s modulus, E=s/e *http://www.mech.uwa.edu.au/unit/MECH2402/lectures/hot_cold_working/degarmo_17-7.gif
Ductility The extent to which plastic deformation takes place before fracture: Elongation Percent reduction in cross sectional area
Hardness Ability to resist permanent indentation from a scratch. The result depends both on the material and on the shape of the indenter, it is not a fundamental material property. Wear resistance is related and sometimes tested also with a sliding stylus or indenter.
Hardness Tests Brinell Hardness (BHN) – uses a hard ball indenter Multiple different sizes and materials can be used for the ball Vickers Hardness – uses a diamond pyramid indenter Knoop (KHN) – also uses a diamond pyramid A microhardness test, for thin sheets Rockwell – multiple types of tests
Fatigue • Components may undergo cyclic or otherwise fluctuating loads that may cause a part to fail at lower stresses than if under a static load. • Its cause is the movement of dislocations that eventually form small cracks which weaken the material. • Fatigue failure is responsible for the majority of failure of mechanical components.
Creep Permanent elongation over time under a static load. • caused by disslocation slipping, grain boundary sliding, and diffusional flow • often worse at elevated temperature but that is material dependent (W > 1000 °C, ice even at sub-zero temps), typically 30% of melting temp for metals and 40-50% for ceramics (glass does NOT creep near room temperature) • very important for high temperature applications – nuclear plants, turbine blades, steam power plants, etc • also important for more mundane applications – paper clips, light bulbs
Impact Resistance The ability to withstand impact loads. It is a function of both ultimate tensile strength and ductility (the area under stress-strain curve)
Physical Properties Other physical properties are also improtant in material selection and manufacturing decisions. (examples) • Density • Melting point • Heat capacity • Thermal expansion • Thermal conductivity • Electrical conductivity • Magnetic properties (permittivity, magnetoresistance, magnetorestriction • Other dielectric properties (dielectric constant, breakdown strength) • Chemical compatibility/corrosion resistance • Optical properties
Density Mass per unit volume [g/cm3, lb/ft3] Important for transportation. Strength (of the type required) per weight is another way to look at this one. Melting Point Important for casting, refractories, others…
Heat Capacity Energy required to change temperature [cal/g°C, J/g°C, cal/lb°F] Important for machining, forming, and thermal management, why? Thermal Expansion Dimensional change per unit temperature [1/°C] Important for stress management, expansion joins, glass metal seals, shrink fits, thermal fatigue, etc.
Thermal Conductivity Rate at which a material can transport heat. [W/mK] Important for machining, thermal management (extrusion, microelectronics, etc.) Electrical Conductivity Ability of a material to carry electrical current, inverse of resistivity. [1/ Ωcm] Important for electrical applications, examples?
Chemical Compatibility This is a major issue that needs to be considered along with all of the other physical properties. Examples: Corrosion in transportation (air, sea, land), refractories, bridges and buildings, … Dielectric strength Amount of applied electric field before failure. [V/cm] Important in integrated circuits (driving away from SiO2 gates), electrical insulation.
Magnetic properties Important in hard disk industry, transformers, RF processing, others? Other properties Piezolectric, ferroelectric, thermoelectric, magnetorestriction, magnetoresistance. What might these be useful for?
Changing Properties of Metals, Heat Treatment and Strengthening Processes
Structure of Alloys Alloy = composed of two or more types of atoms, at least one of which must be a metal. Both solid solutions and intermetallic compounds are alloys. Steel – the most famous class of alloys
Solid Solutions What it sounds like, analogous to a solution of liquids. The solvent must maintain its original crystal structure. Either because the solute can occupy the same sites (with about 15% of the same size), or because the solute can occupy interstices.
Intermetallic Compounds Compounds that form between metals. Rather than a solution in the same structure a new structure is formed. Many are hard and brittle. Fe3C is the most famous of these.
Phase Diagrams • In pure metals solidification takes place at constant temperature • Mixtures solidify over a range of temperature. • Phase diagrams show the EQUILLIBRIUM situation, kinetics are not considered * http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/sciviz/html/clicktuta.html
The Iron-Carbon System Polymorphic transformation BCC to FCC (austenite) Partial transformation to ferrite (ductile and soft) Transformation to ferrite and pearlite (alternating layers of cementite and ferrite) * Materials Science and Metallurgy, 4th ed., Pollack, Prentice-Hall, 1988
General classes of steels • Low carbon (mild steels) <0.3% C - high ductility, low strength, for general use, sheets, plate. • Medium carbon steel 0.3-0.6% C – higher strength, higher hardness, less ductility, gears, axles, railroad, etc. • High carbon steels >0.6% C – hard, strong, brittle, tool steel, springs, cutting tools
Heat Treatments Both microstructure and composition affect a material’s properties. Heat treatment is one way to manipulate microstructure. These changes to microstructure are caused by phase transformations and changes in grain size. These effects are both thermodynamically and kinetically driven.