CH 6604 – Materials Science and Technology Dr. D. Balaji
Associate Professor Department of Chemical Engineering
[email protected]
OBJECTIVE: To provide students with a strong foundation in materials science with emphasis on the fundamental scientific and engineering principles which underlie the knowledge and implementation of material structure, processing, properties, and performance of all classes of materials used in
engineering systems. OUTCOME: At the end of this course, the students would understand various material, their properties and manufacturing methods.
CH6604
MATERIALS SCIENCE AND TECHNOLOGY
LTPC 3 00 3
UNIT I INTRODUCTION
10
Structure – Property relationship - Selection criteria and processes: General criteria of selection of materials in process industries. Properties: Mechanical, Thermal, Physical, Chemical, Electrical, Magnetic and Technological properties. Processing of Metals and Alloys- Casting, Hot and cold rolling, Forging, Extrusion, Deep drawing. UNIT II MECHANICAL BEHAVIOUR
8
Elastic, An elastic and Viscoelastic Behaviour – Introduction to Slip, Slip planes, Plastic Deformation by Slip: Critical resolved shear stress, Mechanism of Creep, Creep Resistant .Materials – Fracture: Ductile and Brittle, Fatigue fracture, Griffith’s theory, S-N curves, Fracture toughness. UNIT III PHASE DIAGRAMS AND PHASE TRANSFORMATIONS
Gibb’s Phase rule : Uniary and Binary phase diagrams , Al
8 2 CO 3
- Cr 2 O 3 , Pb-Sn, Ag-Pt and Iron-
Iron Carbide Phase Diagram – Lever rule – Invariant reactions- TTT diagrams – Micro structural changes – Nucleation and growth – Martensitic transformations – Solidification and Crystallization – Glass transition – Recrystallization and Grain growth UNIT IV FERROUS, NON-FERROUS METALS AND COMPOSITES
10
Pig iron, Cast iron, Mild Steel-Manufacturing process, properties &, Applications Stainless steels, Special Alloy steels-properties and uses; Heat treatment of plain-carbon steels Manufacturing methods of Lead, Tin and Magnesium. Properties and applications in process industries. FRP-Fiber Reinforced Plastics (FRP), manufacturing methods; Asphalt and Asphalt mixtures; Wood. UNIT V NANOMATERIALS
9
Introduction to Nanotechnology- Zero Dimensional Nano Structures – Nano particles – One .Dimensional Nano Structures- Nano wires and Nano rods – Two Dimensional Nano Structures, Films – Special Nano Materials – Nano Structures fabricated by Physical Techniques – Characterisation and Properties of Nano Materials – Applications of Nano Structures. TOTAL : 45 PERIODS
Course Outcomes On successful completion of this course, the student will be able to • CO 1 : Knowledge on material properties and material selection. • CO 2: Understanding mechanical behavior of materials. • CO 3: Understanding phase diagram and phase transformation of materials • CO 4: Understanding the Properties, manufacturing methods and applications of ferrous and non-ferrous metals • CO 5: Knowledge on nanomaterial preparation, properties and characterization.
TEXT BOOKS • Khanna O P, “Material Science and metallurgy” Dhanpat Rai Publications (1995) • Raghavan V, “Materials and Engineering” Prentice Hall of India, Newdelhi (2006) • Materials Science & Engineering, Callister’s
UNIT I • Selection criteria and processes: General criteria of selection of materials in process industries. • Properties: Mechanical, Thermal, Chemical, Electrical, Magnetic and Technological properties. • Processing of metals and alloys-Casting-hot and cold rolling, forging-extrusion-deep drawing.
Rocket
Ammonia Reactor
Glass Reactor
Science of Materials or Material Science Material is something that consists of metals and non-metals
matter – wide range of
Attempts to relate the properties of materials with their at the electronic, atomic and micro levels
structure
Material Science
Materials science is based on the
physics and chemistry of
the internal structure of the materials
Investigates relationships existing between the structure of materials and their properties.
Concern
with
inter-disciplinary
study
of
materials
for
engineering and other practical purposes.
Deals with all material – metal, ceramic, glasses, organic plastics and polymers
Engineering Materials Material classification a. Metals i.
Ferrous
ii.
Non-ferrous
b. Ceramics c. Organics d. Composites e. Semiconductors
Metals
Metals are composed of elements which readily give up electrons to provide a metallic bond and electrical conductivity . Distinguishing Characteristics • Good conductors of heat and electricity, lustre, hard, shiny, reflect light, malleable, ductile, typically have one to three valence electrons
Examples
Metallic Materials Distinguishing Characteristics Pure metal elements (Not commonly found or used)
Metal element compounds (alloy) (Commonlyofused due to the engineered properties the compound)
Thermal and electrical conductors Mechanical properties include strength and plasticity
Metalloids Distinguishing Characteristics • Possess both metallic and nonmetallic properties • B oron , silicon , germanium , arsenic , antimony and tellurium .
Periodic Table of Elements
19
Ceramic Materials (Cont’d) Applications Clay – Shaped, dried, and fired inorganic material Examples: Brick, tile, sewer pipe, chimney flue, china, porcelain, etc. Refractory – Designed to provide acceptable mechanical or chemical properties while at high temperatures Example: Space shuttle all-silica insulating tiles
Ceramic Materials Applications Electrical Resistors – Create desired voltage drops and limit current Thermistors – Application of heat regulates current flow Rectifiers – Allow current to flow in one direction Heating elements for furnaces
Organic Materials Polymeric materials composed of carbon compounds. Countless organic materials, natural, synthetic or manufactured and based chemically on carbon.
Distinguishing Characteristics • Light weight • Soft • Combustible • Ductile • Poor conductors of heat and electricity
Polymeric Materials Plastics Thermoplastic Formed into a desired shape by applying heat and pressure and being cooled May be heated and remolded
Thermosetting Formed into a desired shape by applying heat and pressure and being cooled May not be heated and remolded
Polymeric Materials Elastomers Natural or synthetic material Can be stretched 200 percent of their length at room temperature and can return quickly to srcinal length after force is released
Vulcanization Chemical process used to form strong bonds between adjacent polymers to produce a tough, strong, hard rubber (automobile tires)
Composite Materials Distinguishing Characteristics Composed of more then one material Designed to obtain desirable properties from each individual material. Example: Fiberglass
Composite Materials Layer Composites – Alternate layers of materials bonded together
Particulate Composites – Discrete particles of one material surrounded by a matrix of another material
Fiber-Reinforced Composites – Composed of continuous or discontinuous fibers embedded in a matrix of another material
Selection Criteria Criteria of selection of optimum material: • What are the properties required ? • What are the processing requirements & their implications for the choice of materials ? • What is the availability of material ? • What is the cost ?
Best performance for least mass
Factors for Selection • Properties- to suit the function without failure • Reliability- degree of probability that the material will remain stable enough to function in service for the intended life of product without failure • Safety- No catastropic failure • Size, mass, shape etc • Environmental conditions • Availability • Disposability & Recyclability • Cost
Best performance for least mass
Selection of Materials Main Engineering Requirements
Fabrication requirements
Service requirements
Economic requirements
Fabrication requirements
Material should be able to get shaped (Eg. Cast, forged, formed, machined, sintered etc) and
joined
(eg. Welded, brazed etc) easily.
Fabrication requirements relate themselves with materials machinability, ductility, castability, heattreatability, weldability, etc
Service requirements
Imply that the material selected for the
purpose must
stand up to service demands , eg. Proper strength, wear resistance, corrosion resistance etc.
Economic requirements
Demand that the engineering part should be made with minimum overall cost .
Minimum overall cost may be achieved by proper selection of both technical and marketing variables .
Engineering properties of Materials
Property A property can be defined as the behavior of the material when an external stimuli or a constraints are applied. Those stimuli or the constraint are in the form of Forces Temperature A property of material is independent of its size and it decides whether a material is suitable for an application or not
1. Elasticity Ability of mechanically stressed material to restore to its srcinal dimension on the removal of the applied stress . The recovery from the distorting loads may be Instantaneous Partial Delayed Gradual
The instantaneous and complete restoration from the deformation is referred to as perfect elasticity. In the given curve the region O-A represent the elastic behavior.
2. Plasticity Ability of material to be deformed permanently on an applied stress which is enough to exceed the elastic limit . This property indicates the ability of the material to be shaped under the applied force. The minimum stress required to induce a plastic deformation in a material is called as its BOND STRENGTH . Different plastic deformation mechanisms followed in crystalline and amorphous materials. Crystalline material – slip of the crystallographic plane Amorphous material – slide of the individual or grouped molecules
3. Tensile strength • Tensile strength is the ratio of the maximum load to the srcinal cross sectional area. • The ultimate tensile strength refer to the force which is required to break the material. • It measures the strength and ductility of the material. • The tensile strength value will be using as a standard for fixing the working strength of the materials. • Universal testing machine and dumbbell shaped specimens are being used for estimation of these property.
4. Yield strength • Up on continuous application of stress, a material deforms as the stress increases and at a particular point the deformation suddenly increases without the proportional increment in the stress . This point is known as Yield point. • Ability of the materials to resist the plastic deformation is known as Yield Strength . • Up to this point the material exhibits a constant Young’s modulus and will be elastic in nature.
5. Toughness • The ability of the material to absorb energy during plastic deformation up to rupture especially on bending. • It is taken as an index for the material to asses its ability to withstand the mechanical stresses with undergoing the failure or a rupture. • The area under the Stress-Strain curve up to the fracture point indicates the magnitude of the Toughness.
7. Impact strength • These are the indices for the strength and toughness of the material. • The capacity of a material to resist and absorb shock before break or fail is known as Impact Strength of a material. • This property having close relation with structure of the material. • Izod or Charpy test will estimate the above property of the materials.
8. Ductility
The ability of the material to undergo deformation under tension without rupture. It is an important property when deep drawing processes of the materials as concern. It is normally given in terms of Percentage elongation. Percentage area reduction.
10. Brittleness • The property of the material that breaks even without a little deformation is known as Brittleness • If a material breaks on loading without undergoing a deformation greater than 5% in 50 mm gauge wire is considered to be brittle • This property is closely related to the internal micro structure of the material considered.
14. Wear resistance • Wear is the unintentional removal of the material from the material surface. • Wear resistance is the property of the material to resist the above. • The types of material wearing • Adhesive wear • Abrasive wear
Factors affecting the mechanical properties 1. The constituents present in the material 2. The grain size 3. Crystal imperfections 4. Excessive cold working (strain hardening) 5. Manufacturing defects
1. Hygroscopic nature • Hygroscopic nature is the ability of a substance to attract water molecules from the surrounding environment through either absorption or adsorption . • Materials such as calcium chloride and silica gel absorb water readily so that they are used as drying agents.
2. Surface energy • Surface energy quantifies the disruption of intermolecular bonds that occurs when a surface is created. • The surface energy may therefore be defined as the excess energy at the surface of a material compared to the bulk.
3. Reactivity
• Reactivity then refers to the rate at which a chemical substance tends to undergo a chemical reaction in time. • In pure compounds, reactivity is regulated by the physical properties of the sample. • For example higher specific surface area increases its reactivity that increases its reactivity. • In impure compounds, the reactivity is also affected by the inclusion of contaminants. • In crystalline compounds, the crystalline form can also affect reactivity . • Reactivity is primarily due to the sub-atomic properties of the compound.
Thermal properties The response of a material on the application of heat is known as a thermal property. They important thermal properties are, Heat capacity Specific heat Thermal expansion Melting point Thermal conductivity Thermal shock resistance Thermal stability Thermal diffusivity Glass transition temperature
5. Thermal shock resistance A thermal shock is the condition of the material when it is subjected to severe and sudden changes in temperature caused either by change in the external environment or by internal heat generation. The ability of the material to withstand a thermal shock condition is known as thermal shock resistance. A ductile material is more resistive to the thermal shock as compared to a brittle material of comparable strength. Thermal fatigue also will be occurring in the materials as there is cyclic changes in the temperature.
Failures due to low thermal stability • Thermal cracking • Quench cracking • Spalling
8. Thermal diffusivity • The thermal diffusivity is a measure of the transient heat flow through a material. • In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. • It measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy
9. Glass transition temperature • The glass transition temperature, or T g is an important property of polymers. The glass transition temperature is a temperature range which marks a change in mechanical behavior. • Above the glass transition temperature a polymer will behave like a ductile solid or highly viscous liquid. • Below T g the material will behave as a brittle solid. Depending on the desired properties materials may be used both above and below their glass transition temperature.
Electrical resistance
• The electrical resistance: Ability of the material to impede or resist the flow of electric current through the same. • The electrical resistance is proportional to the ratio of area to the length of the material • The unit for measuring the above property is ohm (Ω) . R α L/A R is the resistance of the material. L is the length of the material. A is the cross sectional area of the material. The proportionality constant is known as Electrical Resistivity of the material.
Temperature coefficient of resistance. • This is employed to indicate the variation of resistivity with respect to the temperature • It has the unit of inverse of temperature. • It can be given by, α T = (R 0 - R)/ (T-T o)R o Where R o is the resistivity at the reference temperature, R is the resistivity at the measured temperature,
To. T.
Electrical conductivity • The conductivity (or specific conductance ) of a material is a measure of its ability to conduct electricity in terms of the flow of electrons. • It is the reciprocal of Specific Resistivity. • When an electrical potential difference is placed across conductor, its movable charges flow giving rise to an a electric current. • The conductivity σ is defined as the ratio of the current density J to the electric field strength E
Dielectric strength The dielectric strength of a material is the maximum electric field strength that it can withstand intrinsically without breaking down, i.e. , without experiencing failure of its insulating properties. • The material with high dielectric strength is considered to be an insulator.
Factors affecting Dielectric strength It decreases slightly with increased sample thickness. It decreases with increased operating temperature. It decreases with increased frequency. For gases (e.g. hexafluoride) it normally decreases with nitrogen, increasedsulfur humidity. For air, dielectric strength increases slightly as humidity increases It depends up on the type of the material and the type of the electrodes is being used for the application of the electric field.
4. Thermoelectricity • This forms the basics of the working of a thermocouple . • A junction formed by two dissimilar material is heated, a small electrical potential in the milli voltage range is produced. This is known as thermoelectric effect.
Magnetic properties • The study of the magnetic properties are important because the science of magnetism explains many aspects of structure and behavior of matter. Ex. Transformer • The important magnetic properties are; – Permeability – Coercive force – Hysteresis – Super conductivity – Curie point
Magnetic Permeability(µ)
• It is the ratio of magnetic induction (B) to the intensity of magnetizing field (H) µ= B/H • Paramagnetic materials have a permeability more than one. Example: aluminium, sulphate, crown glassplatinum, chromium, manganese, copper • Diamagnetic materials have a permeability less than one. Example: bismuth, antimony, copper, gold, quartz, mercury, water, alcohol, air, hydrogen. • The commonly used unit for the magnetic permeability is henrys/m .
2. Hysteresis • External magnetic field is applied to a ferromagnetic material such as iron, the atomic dipoles align themselves with it. • Even when the field is removed, part of the alignment will be retained. That means the material has become magnetized. • Once magnetized, the magnet will stay magnetized indefinitely. To demagnetize it requires heat or a magnetic field in the opposite direction. This property is known as the magnetic hysteresis.
• The relationship between field strength H and magnetization M is not linear in such materials which are having hysteresis. • If the relationship between H and B is plotted for increasing levels of field strength, B follows the initial magnetization curve . This curve increases rapidly at first and then approaches an asymptote called magnetic saturation.
If the magnetic field is now reduced monotonically, a different curve will be followed. At zero field strength, the magnetization is offset from the srcin by an amount called the remanence. If the H-M relationship is plotted for all strengths of applied magnetic field the result is a hysteresis loop called the main loop . The width of the middle section is twice the coercivity of the material.
4. Superconductivity • The state at which the abrupt drop in electrical resistance of certain materials become zero at certain critical temperatures . • At this temperature the material becomeatathe perfect diamagnetic material except surface (about 50nm depth) is called Meissner effect. • Eg: Al, Pb, Sn, W, Ti, Nb-Ti Alloy etc They are getting into the superconductive behavior when their temperature is lowered.
5. Curie point • The temperature at which certain magnetic materials undergo a sharp change in their magnetic properties .
Physical properties • Physical properties are employed to describe a material under conditions in which external forces are not concerned. • Physical properties include i) Dimensions – size & shape ii) Appearance iii) Density iv) Colour v) Melting point vi) Porosity vii) Structure etc.,
Castability • Castability is that complex property of a metal or alloy which allows it, when molten, to fill a mould so as to give a flawless casting . It depends on many factors and cannot be determined simply by one test. • Viscosity plays an important role. • Castability increases linearly with temperature above the melting point. • The shrinkage of the metal in the mould on solidification and cooling is also of importance : it depends on the coefficient of expansion of the metal, the casting temperature and the nature of the mould.
Imperfection
• The accuracy of machining depends not only on the quality of the tool machine, but also on the form of the built-up cutting edge, which is largely influenced by the nature of the material to be machined. • Machinability index is determined by comparing the Machinability of a given material under specified conditions with that of a much-used material arbitrarily taken as standard . If this standard material is given an index of 100, the index of the material in question can be expressed as a percentage of this.
Weldability • Weldability is the capacity of a material to be welded under the fabrication conditions imposed into a specific suitably designed structure and to perform satisfactorily in the intended service. • This implies that a metal with good Weldability can be welded readily so as to perform satisfactorily in the fabricated structure. • Weldability depends upon the welding process employed, the material and the form in which it must be welded, as well as on the nature of the added weld material
• Poor Weldability manifests itself by brittle fractures as a result of the stresses produced in the material by heating and cooling this effect is found in particular in the electrical are welding of insufficiently tough materials. • Poor Weldability can also give microcracks in the weld ; of importance in this connection are the qualities of the electrode material and of the welding process. Cracks next to (and alongside) the weld are also a sign of unsatisfactory welding. The formation of such crack is strongly influenced by the thickness of the plate being welded, and also depends on the welding process. • Good Weldability means that the weld is free from pores, slag inclusions, cracks and hard patches or zones and has properties that differ very little from those of the material being welded.
Solderability • The solderability of a material is determined by the purity of its surface and by the choice of solder and flux . • Solders are classified as
soft solders and hard solders
• Soft solders have a relatively low melting point and consist mainly of lead – tin alloys . • Hard solders have a considerably higher melting point, so that it is generally impossible to use them with an electric soldering iron. • Hard solders include the silver solders, which are alloys of silver, copper and zinc. • When brass is used as the filler the process is termed as brazing instead of hard soldering.
Taxonomy of Metals Metal Alloys
Ferrous Steels Steels <1.4wt%C <1.4wt%C
Cast Irons Cast Irons 3-4.5wt%C
3-4.5wt%C
Cu
Al
d
L
1400 1200 1000
a 800 ferrite
600 400
0 (Fe)
g austenite
Eutectoid:
0.76
1
L+Fe 3C
1148°C 4.30
727°C
2
Eutectic:
g+Fe 3C
4
Ti
Adapted from Fig. 9.24, Callister 7e . (Fig. 9.24 adapted from Binary Alloy Phase Diagrams , 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)
Fe 3 C
cementite
a +Fe 3C 3
Mg
microstructure: ferrite, graphite cementite
1600T (°C)
g+L
Adapted from Fig. 11.1, Callister 7e .
Nonferrous
5
C o , wt% C
6
6.7
Steels High Alloy
Low Alloy low carbon <0.25wt%C Name
plain
HSLA
Med carbon 0.25-0.6wt%C plain
Cr,V
heat treatable Cr, Ni
Additions none Ni, Mo none Example 1010 4310 1040 Hardenability 0 + + TS 0 + EL + + 0 Uses
auto struc. sheet
bridges towers press. vessels
crank shafts bolts hammers blades
high carbon 0.6-1.4wt%C plain
Cr, V,
Mo none 4340 1095 ++ ++ +++ pistons gears wear applic.
tool
austenitic stainless
Mo, W 4190 +++ ++ --
Cr, Ni, Mo 304 0 0 ++
drills saws dies
high T applic. turbines furnaces V. corros. resistant
wear applic.
increasing strength, cost, decreasing ductility
Ferrous alloys • Plain carbon steels • Alloy steels • Stainless steels • Cast irons
Low-carbon steels • Contain less than 0.25%C • Not very responsive to heat treatments • soft, weak, tough and ductile • Machinable, weldable, not expensive • YS~275 MPa, TS~415-550MPa, 25% el.
High strength low alloy steels (HSLA steels) • Contain alloying elements such as Cu, V, Ni, Mo in combined concentrations of >10 wt% • Stronger than plain low-C steels • Ductile, formable and machinable
Medium-carbon steels • Contain 0.25-0.60 wt.% carbon • Can be heat-treated but only in thin sections • Stronger than low-C steels but less ductile and less tough • Good wear resistance • Railway wheels & tracks, gears
High carbon steels • • • • •
0.60 -1.4 wt.% C Hardest, strongest, least ductile of all steels Almost always used in tempered condition Especially wear resistant Form hard and wear resistant carbides with alloying elements • Used in cutting tools, dies, knives, razors, springs and high strength wires
Stainless steels • Highly resistant to corrosion in many environments • Predominant alloying element is at least 11% Chromium • Corrosion resistance may be enhanced by Ni and Mo • 4additions classes: ferritic, austenitic, martensitic and precipitation-hardening • Used at high temperatures (upto ~ 1000 environments • Gas turbines, steam boilers, aircraft, missiles
C) and severe
Cast irons • Theoretically contains > 2.14 wt.% carbon • Usually contains between 3.0-4.5 wt.% C, hence very brittle • Also 1-3 wt.% silicon • Since they become liquid easily between 1150 C and 1300 C, they can be easily cast • Inexpensive • Machinable, wear resistant • 4 types: gray cast iron, nodular cast iron, white cast iron, malleable cast iron
Nonferrous alloys • Aluminum alloys • Copper alloys • Magesnium alloys • • • •
Nickel alloys Titanium alloys Refractory metals Superalloys
Aluminum alloys • Low density - 2.7 gm/cc • High electrical and thermal conductivities • High ductility • Low melting point and strengths • Cast or wrought • Temper designation
Copper alloys • • • •
Soft, ductile, difficult to machine Highly resistant to corrosion Excellent electrical & thermal conductivity Can be alloyed to improve hardness
• Cold worked to get the maximum hardness • Cu-Zn = brass; Cu-X = bronzes
Magnesium alloys • • • • • •
Lowest density of all structural metals= 1.7 gm/cc Relatively soft and low elastic modulus (45 GPa) Have to be heated to be deformation processed Burns easily in the molten and powder states Susceptible to corrosion in marine environments Competing with plastics
Nickel alloys • Quite ductile and formable • Highly corrosion resistant, especially at high temperature •• Essential part ofvalves austenitic stainlessand steels Used in pumps, in seawater petroleum environments
Titanium alloys • Low density, high melting point • High specific strength and elastic modulus • superior corrosion resistance in many environments • Absorb interstitials at high temperatures • Highly reactive with other materials and hence non-conventional processing techniques have been developed • Highly used in aerospace applications
Refractory metals • • • •
These are extremely high melting metals Nb, Ta, Mo, W Very high strengths and hardness Very high elastic modulus
• W alloys used in x-ray tubes, filaments • Ta & Mo used with stainless steels for corrosion resistance • Ta is virtually immune to all environments below 150 C
Casting Manufacturing Processes
Casting Pouring molten metal into a mold shaped after the part to be produced, allowing it to harden, and removing it from the mold
Introduction -
Can be used to create complex internal and external part geometries
-
Some casting processes can produce parts to net shape (no further manufacturing operations are required)
-
Can very large parts (cast parts weighing over 100 tons haveproduce been made)
-
Can be used with any metal that can be heated to its liquid phase
-
Some types of casting are suited to mass production
Examples of Cast Parts
Crank handle formed by casting; some areas were machined and assembled after casting
Examples of Cast Parts
C-clamps formed by casting (left) and machining (right)
Examples of Cast Parts
Complex part formed by casting Courtesy of Toth Industries
Forms of Casting and Terminology
Introduction Requirements: - Mold cavity with desired shape and size - Melting process to provide molten metal - Pouring process to introduce the metal into the mold - Solidification process controlled to prevent defects -
Ability to remove the casting from the mold Cleaning, finishing and inspection operations
Casting Terminology Flask The box containing the mold Cope The top half of any part of a 2-part mold Drag The bottom half of any part of a 2-part mold Core A shape inserted into the mold to form internal cavities Core Print A region used to support the core
Casting Terminology Mold Cavity The hollow mold area in which metal solidifies into the part Riser An extra cavity to store additional metal to prevent shrinkage Gating System Channels used to deliver metal into the mold cavity Pouring Cupof the gating system that receives poured metal The part Sprue Vertical channel Runners Horizontal channels
Casting Terminology Parting Line / Parting Surface Interface that separates the cope and drag of a 2-part mold Draft Taper on a pattern or casting that allows removal from the mold Core Box Mold or die used to produce cores Casting The process and product of solidifying metal in a mold
Cooling Rate Rapid cooling produces equiaxed (roughly round) grains Slow cooling towards the interior forms long columnar grains that grow towards the center
Metal Solidification Dendrites Tree-like structures that form during the solidification of alloys
Slow cooling rates produce dendrites with larger branch spacing; faster cooling rates produce finer spacing; very fast cooling rates produce no dendrites or grains
Fluidity of Molten Metal Mold design The design and size of the sprue, runners, and risers affect fluidity Mold material and surface Thermal conductivity and roughness decrease fluidity Superheating The temperature increment above the melting point increases fluidity Pouring Lower pouring rates decrease fluidity because of faster cooling Heat transfer Affects the viscosity of the metal
Heat Transfer The metal that solidifies first is at the wall of the mold; this solid layer thickens as time passes Shrinkage during cooling can change the part dimensions and sometimes cause cracking; it is caused by the metal’s thermal expansion properties and the phase change between liquid and solid.
Heat Transfer
Casting Defects A.Metallic Projections B.Cavities C.Discontinuities D.Defective surface E.Incomplete Casting F. Incorrect dimensions or shape G.Inclusions
Casting Defects Porosity may be caused by shrinkage and/or gases Thin sections solidify faster than thick sections; therefore the molten metal cannot be supplied to thick regions that are solidifying Gases become less soluble in a metal as it cools and solidifies, causing it to be expelled and sometimes form or expand porosity
Summary Casting involves melting metal and allowing it to solidify in the desired shape Casting allows the creation of parts that would be difficult or uneconomical to make by machining
1) COLD WORKING Plastic deformation which is carried out in a temperature region and over a time interval such that the strain hardening is not relieved is called cold working.
Some Cold Working Processes: Cold rolling Cold forging Cold extrusion Bending Drawing Shearing
Reason for Cold Working: Provides better surface finish and dimensional precision.
The advantages of cold working are •
A better surface finish may be achieved
•
Dimensional accuracy can be excellent because the work is not hot so it doesn't shrink on cooling; also the low temperatures mean the tools such as dies and rollers can last a long time without wearing out.
•
Usually there is no problem with oxidative effects such as scale formation. In fact, cold rolling (for example) can make such scale come off the surface of a previously hot-worked object.
•
Controlled amounts of cold work may be introduced.
Effect of cold working on tensile strength, hardness, ductility and grain size. (The curve below ductility represents the change in grain size)
•
As with hot working, the grain structure of the material is made to follow the deformation direction, which can be good for the strength of the final product.
•
Strength and hardness are increased, although at the expense of ductility.
•
OH & S problems related to working near hot metal are eliminated.
•
There is a limit to how much cold work can be done on a given piece of metal. See the discussion above about accumulation of damage in the form of piled up dislocations.
•
Higher forces are required to produce a given deformation, which means we need heavily built, strong forming machines .
HOT WORKING •
Hot working refers to the process where metals are defromed above their recrystallizatıon tempereture and strain hardening does not occur.Hot working performed at elevated tempreature.However is hot working at room temperature because of its low melting temperature.
Hot ingots
Some Hot Working Processes: • • • • • •
Rolling Forging Extrusion Hot drawing Pipe welding Piercing
Reason for Hot Working: At elevated temperatures, metals weaken and become more ductile.
The most important continuous hot working processes
The advantages of hot working are •
Lower working forces to produce a given shape, which means the machines involved don't have to be as strong, which means they can be built more cheaply;
•
The possibility of producing a very dramatic shape change in a single working step, without causing large amounts of internal stress, cracks or cold working;
•
Sometimes hot working can be combined with a casting process so that metal is cast and then immediately hot worked. This saves money because we don't have to pay for the energy to reheat the metal.
•
Hot working tends to break up large crystals in the metal and can produce a favourable alignment of elongated crystals
•
Hot working can remove some kinds of defects that occur in cast metals. It can close gas pockets (bubbles) or voids in a cast billet; and it may also break up non-metallic slag which can sometimes get caught in the melt (inclusions).
Forging •
Forging is manufacturing process where metal is pressed, pounded or squeezed under great pressure into high strength parts known as forgings. The process is normally (but not always) performed hot by preheating the metal to a desired temperature before it is worked. It is important to note that the forging process is entirely different from the casting (or foundry) process, as metal used to make forged parts is never melted and poured (as in the casting process).
165
Rolling Aluminium is first passed through a hot rolling mill and then transferred to a cold rolling mill.
•
Hot rolling mils:
Prior to rolling the aluminium is in the form of an ingot which can be up toheated 600mmtothick. This ingotand is then around 500°C passed several times through the hot rolling mill. This gradually reduces the thickness of the metal to around 6mm . •
This thinner aluminium is then coiled and transported to the cold rolling mill for further processing. 168
Rolling is a fabricating process in which the metal, plastic, paper, glass, etc. is passed through a pair (or pairs) of rolls. There are two types of rolling process, flat and profile rolling . In flat rolling the final shape of the product is either classed as sheet (typically thickness less than 3 mm, also called " strip ") or plate (typically thickness more than 3 mm). In profile rolling, the final product may be a round rod
or other
shaped bar such as a structural section (beam, channel, joist etc). Rolling is also classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature then the process is termed as hot rolling, If the temperature of metal is below its recrystallization temperature the process is termed as cold rolling.
Extrusion and Drawing of Metals
WHAT is DRAWING? Drawing is an operation in which the cross-section of solid rod, wire or tubing is reduced or changed in shape by pulling it through a die .
The principle of this procedure consist of reducing the thickness of a pointed ,tapered wire by drawing it through a conical opening in a tool made of a hard material.The wire will take shape of the hole.
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Drawing improves strength and hardness when these properties are to be developed by cold work and not by subsequent heat treatment
• Where is it used? This process is widely used for the production of thicker walled seamless tubes and cylinders therefore; shafts, spindles, and small pistons and as the raw material for fasteners such as rivets, bolts, screws.
DRAWING TOOLS
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The most important tool in the drawing process is without doubt the drawplate.This consist of a plate of high grade steel into which similar shaped holes have been placed whose size in evenly reduced from one hole to another.
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The most common drawplate have round holes and are used to reduce the size of round wire.
Drawing wirewiththe draw tongs
drawbench
How such a drawplate hole is made
Deep Drawing Deep drawing and pressing involve a combination of bending and stretching .The simplest example of this process involves the fabrication of a cup from a circular sheet blank . For deep drawing operations the quality of strip required should be non-directional and of the correct combination of hardness and grain size for the tooling .
Extrusion Introduction Extrusion –A material is pushed or drawn through a die of the desired cross-section. Any solid or hollow cross-section may be produced by extrusion, which can create essentially semi-finished parts. The metal can forcing through a die in the same direction or opposite direction. • Parts have constant cross-section • Typical Products of Extrusion – Sliding Doors, tubing having various cross-sections, structural and architectural shapes and door and window frames.
Extrusions
Fig : Extrusions and examples of products made by sectioning off extrusions.
Extruded products •
Typical products made by extrusion are railings for sliding doors, tubing having carious cross-sections, structural and architectural shapes, and door and windows frames. Extruded products
The Extrusion Process Types of Extrusion : Direct Extrusion (or) Forward Extrusion
– Billet is placed in a chamber and forced through a die opening by a hydraulically-driven ram or pressing stem.
Indirect Extrusion
– Die moves towards the billet.
Hydrostatic Extrusion
– The billet is smaller in diameter that the chamber, which is filled with a fluid, and the pressure is transmitted to the billet by a ram.
Extrusion Ratio = A o/A f A o – cross-sectional area of the billet A f - cross-sectional area of extruded product
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Direct extrusion: A metal billet is located into a container, and a ram compresses the material, forcing it to flow through one or more openings in a die at the opposite end of the container.
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Indirect extrusion: The die is mounted to the ram rather than at the opposite end of the container. One advantage of the indirect extrusion process is that there is no friction, during the process, between the billet and the container liner.
Direct Extrusion
Fig : Schematic illustration of direct extrusion process
Types of Extrusion
Fig : Types of Extrusion (a) indirect (b) hydrostatic (c) lateral
Hot Extrusion • Extrusion is carried out at elevated temperatures – for metals and alloys that do not have sufficient ductility at room temperature, or in order to reduce the forces required.
Cold Extrusion •
Cold extrusion is performed at temperatures significantly below the melting temperature of the alloy being deformed, and generally at room temperature.
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The process can be used for most materials, sufficiently robust machinery can be designed.
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Products of cold extrusion include aluminium cans, collapsible tubes and gear blanks.
provided that
Cold Extrusion Combination of operations, such as direct and indirect extrusion and forging. Advantages : – Improved mechanical properties – Good control of dimensional tolerances – Improved surface finish – Elimination of the need for billet heating;
Fig : Two examples of cold extrusion. Thin arrows indicate the direction of metal flow during extrusion.
Impact Extrusion • Similar to indirect extrusion • Punch descends rapidly on the blank, which is extruded backward
Fig : Schematic illustration of the impact-extrusion process. The extruded parts are stripped by the use of a stripper plate, because they tend to stick to the punch.
Examples of Impact Extrusion
Fig : (a) Two examples of products made by impact extrusion. These parts may also be made by casting, by forging, or by machining; the choice of process depends on the dimensions and the materials involved and on the properties desires. Economic considerations are also important in final process selection. (b) and (c) Impact extrusion of a collapsible tube by the Hooker process.
Hydrostatic Extrusion • • • • •
The pressure required for extrusion is supplied through and incompressible fluid medium surrounding the billet Usually carried at room temperature, typically using vegetable oils as the fluid Brittle materials are extruded generally by this method It increases ductility of the material It has complex nature of the tooling
Fig : General view of a 9-MN (1000-ton) hydraulic-extrusion press.
