Metallurgy Part 1 Fall 2024-2025 PDF

Field of Dental medicine Dental biomaterials I (PDD131) Lecture : Structure of perfect solids Metallurgy I Assoc.Prof.Dr : Enas Mahmoud Elddamony Date : 24/ 11 /2024  References Philips science of dental materials Introduction to dental material...

Field of Dental medicine Dental biomaterials I (PDD131) Lecture : Structure of perfect solids Metallurgy I Assoc.Prof.Dr : Enas Mahmoud Elddamony Date : 24/ 11 /2024  References Philips science of dental materials Introduction to dental materials 12th edition (Richard van Noort) Chapter 5: Structure and properties 4th edition of cast dental alloys Chapter 1: 4. structure of metals Pp: 69 -90 and alloys Chapter 17: Wrought wires Pp: 17- 22 Pp: 396-406  Objectives 1. Enumerate different properties of metals and alloys 2. Know different methods of metal shaping 3. Understand the solidification mechanism of molten metal 4. Get an idea about the effect of cold working and annealing on the structure and properties of metals 5. Know how to control both the grain size and shape of solidifying metal  Introduction Metal is the element ionizes positively in the solutions 95-118 elements in the periodic table Almost, all metals properties are related to the free movement of valence electrons  Properties of metals 1. Ionize positively in solutions 2. Crystalline solids, except mercury and gallium are liquid metals and hydrogen is gaseous metal 3. Have metallic luster (free electrons re-emit electromagnetic waves) 4. Good electrical and thermal conductivity (due to free movement of valence electrons) 5. High hardness, melting and boiling point (due to strength of the primary interatomic bonding) 6. High density (high atomic weight ) 7. High ductility and malleability (Crystal structure and imperfections that facilitate plastic deformation) 8. Metallic ring when struck 9. Most metals are white in color except gold is yellow, copper is red Metals in dentistry  Gold foil Platinum foil Mercury in amalgam restorations Titanium in implants and crown and bridge work  Cold Casting working Sintering Electroforming Shaping of metals  1. Casting Shaping of molten metal or alloy in a mould of the required shape This involves melting the metal or alloy and forcing it into the investment mold. For easy casting, which is better???? Low or high density alloy?  2. Cold working (plastic forming) Block of metal can be shaped into wires, sheets, tubes by mechanical working at room temperature Stresses applied should be above yield strength of the metal  3. Sintering (powder metallurgy) Bonding of metal particles by pressure or heat in absence of any liquid Involves elimination of porosity that gives denser product Associated with amount of shrinkage 3. Sintering (powder metallurgy)  4. Electroforming The reverse of corrosion process By which a metal can be plated on to a conductive surface As silver and copper electroplated dies 4. Electroforming The theoretical strength of the material is higher than the actual strength  Types of crystalline imperfections a) Point defects 1.Vacancy 2. Impurities -interstitial -substitutional b) Line defects (Dislocation ( Displacement of a raw of atoms from their normal position Plastic deformation of metals c) Plane defects Grain boundaries in metals. Solidification of metals Cooling curve of pure metal (temperature-time relationship) A Liquid C B B’ Solid  Mechanism of solidification 1. Nuclei of crystallization 2. Growth of crystals  If metal is melted and allowed to cool: 1. Temperature decreases from A to B’, then increases to B 2. Temperature is constant until C 3. After C, temperature decreases gradually to room temperature Tf is the freezing or fusion temperature BB̀ is the latent heat of fusion(super cooling ) Above Tf , metal is completely liquid Below Tf , metal is completely solid A Liquid At Tf, metal is solid and liquid B C B’ Solid 1 2 3 4  Mechanism of solidification 1. When molten metal reaches melting point, solidification starts at different centers called nuclei of crystallization 2. Growth of crystals in the 3 dimensions 3. Growth continues till contact between adjacent growing crystals occur 4. Each crystal is called grain 5. Transition from grain to another is called grain boundary 6. Atoms at grain boundary are highly stressed and distorted and have higher energy, so it affects the mechanical properties of the metal and corrosion resistance Grain boundaries Grain proper  Control of grain size Size is dependent on the time and temperature The smaller the grain size, the better are the mechanical properties 1. Amount and rate of cooling: Rapid rate of cooling more nuclei of crystallization smaller grains Rapid cooling Slow cooling Small grains Large grains 2. Rate of crystallization and nucleation: During solidification, if the rate of nucleation is higher than the rate of crystallization , the grains will be smaller 3. Nucleating agents (grain refiners): Addition of pre-fab nuclei of crystallization will result in smaller grains May be impurities or added intentionally Example: Irridium in gold alloys To act as grain refiner Pre-fab nuclei of crystallization Increase number of grains Decrease size of grains Increase mechanical properties Relation between microstructure and mechanical properties  1. Elastic deformation When metal is stressed under its elastic limit, it deforms temporarily due to stretching of interatomic bonds  2. Plastic deformation When metal is stressed above its elastic limit, it deforms permanently due to dislocation and slip planes  Wrought metals These are metals that are changed by mechanical cold working from microscopically grain ( crystal) structure to fibrous structure Grain (crystal) structure Fibrous structure Example: Hammering, rolling or drawing into wires  Cold working and strain hardening Wrought or fibrous structure is plastically formed by stresses above its yield strength Cold worked metal has: 1. High strength 2. High hardness 3. High proportional limit 4. Low ductility 5. Low corrosion resistance  Heat treatment annealing The effect of cold working can be reversed by heating Heating of cold worked structure passes in three stages:- 1. Stress relief anneal or recovery 2. Recrystallization 3. Grain growth  1. Recovery On heating, No change in fibrous structure Slight decrease in tensile strength No change in ductility *Recovery involves mainly relief of internal stresses induced during cold working *These internal stresses if released may lead to warpage or unpredicted fracture during service  2. Recrystallization Occurs after recovery If temperature is held for longer times, 1. The fibrous structure converted into crystal (grain) structure with: 2. decrease in hardness 3. decrease in strength 4. increase in ductility  3. Grain growth If further increase in temperature and/ or time, The grains begin to grow Forming of coarse grains with: 1. Increase in ductility 2. Detrimental decrease in strength and proportional limit  Factors controlling size and shape of grains 1. Rate of cooling (rapid….small grains) 2. Nucleating agents (small grains) 3. Cold working (fibrous structure) 4. Recrystallization (cast or grain structure) 5. Grain growth (large grains)  Casting Cold working Pouring molten metal or alloy Shaping metal into sheets or into mold wires at room temperature Grain or crystal Fibrous or wrought structure High ductility Low ductility Low strength High strength Low hardness High hardness High corrosion resistance Low corrosion resistance  Self assessment questions Choose the correct answer: 1. Increase in rate of cooling of the metal results in: A. Large number of small sized grains B. Small number of large sized grains C. Large number of large sized grains D. Small number of small sized grains 2. The figure represents which method of shaping metals? A.Cold working B.Sintering C.Casting D.Electroforming 3. Cold working of the metals results in: A. Increase in ductility B. Increase in strength C. Decrease in strength D. (A&B) 4. Electroplating process done by: A. Melting metal and shaping it into mold B. Plating metal on a conducting surface C. Pressing powder metal under high pressure D. None of the above 5. Lattice imperfection which is described by grain boundaries, is called: A. Impurity B. Plane defect C. Vacancy D. Line defect