METALS What is METAL? Different Types of Metals – Ferrous, Non-Ferrous, Alloys Main focus of Ferrous Metals Properties of Metals Characteristics of Ferrous Metals with examples Advantages/Disadvantages of Ferrous Metals
METALS Metals are… Solid at room temperature, except mercury, which is liquid ! Metals have… very high melting point. Metals are… shiny when they cut. Metals are… good conductors of heat and electricity. Metals are… usually strong & malleable so they can be hammered into shape.
Metal source and availability The basic elements of all metals are found naturally occurring within the earth. After they are extracted in the from of ore they are refined and processed in a variety of ways to produce usable materials. Metals are commonly available for manufacturing use in a wide range of forms. www.afcd.gov.hk/.../cou_vis_cou_mos.html Mining for Iron Ore in Ma On Shan until 1981 http://thomonthenet.com/ContemporaryGlass/JoshSimpson_Megaworld13_large.jpg www.ferretrollforming.com/aboutus.htm
Metals are sometimes described as a lattice of positive ions surrounded by a cloud of delocalized electrons.
METALS FerrousNon-FerrousAlloys Containing iron & Do not contain iron. A mixture of almost all are e.g. aluminium, metals, or a magnetic. copper, silver, gold, metal & small e.g. mild-steel, lid, tin etc. amount of cast-iron, tool- other substance Steel etc. Ferrous AlloysNon-Ferrous Alloys e.g. e.g. brass (copper + zinc) stainless steel bronze (copper + tin ) steel + chromium
METALS METALS & ALLOYS Metals are available in pure or alloy form. Pure Metals such as pure aluminium or pure copper, contain only one type of metal. They are not mixed with any other metal. Alloys are mixture of two or more pure metals. Alloys tend to have better strength properties than pure metals. Alloys and pure metals often have special physical properties.
FERROUS-METALS Mild Steel Composition: Iron alloy with 0.3% carbonProperties: Malleable and ductile, and therefore bends fairly easilyUses: nuts, bolts, screws, tubes etc. Methods of Identification Appearance: Bright drawn mild steel has a smooth, bright surface; black mild steel is covered with a blue-grey oxideDropping: Gives out a ringing noteGrinding: Gives off a shower of long white sparksEffect of Heating: Slightly tougher but little change
FERROUS-METALS 2. Tool Steel / cast steel / carbon steel Composition: Iron alloy with 0.5%-1.5% carbonProperties: Tough rather than hard, and fairly ductileUses: Springs and most tools such as hammer heads, drills, chisels, shears etc Methods of Identification Appearance: Has a smooth skin of black oxideDropping: Gives out a high ringing noteGrinding: Moderate number of red sparksEffect of Heating: Becomes hard and brittle
FERROUS-METALS Cast iron Composition: Iron alloy with 2%-4% carbonProperties: Brittle, snaps before it will bend. Strong in compressionUses: Vices, cylinder blocks for car engines, frames for most machines Methods of Identification Appearance: Grey with a granular surfaceDropping: gives out a dull noteGrinding: Gives off a few dull sparksEffect of Heating: No change
ADVANTAGES OF FERROUS METALS High strength to weight ratio it minimise the substructures cost, which beneficial in poor ground condition. E.g. The Newark Dyke Rail Bridge – comprises 77 meter long, 11.25 meter wide bowstring with 820 tonnes of S355 steel. This bridge use IMD (Interactive Model Technique) – reduced the time required to assess the dynamic response of the structure. This bridge was the first UK steel bridge to be designed for the next generation of 225 km/hr trains.
ADVANTAGES OF FERROUS METALS 2. High quality material readily available worldwide in various certificate grades. Speed of construction Versatility steel suits range of construction methods & sequences. Modification & repair Recycling Durability 8. Aesthetics steel has a broad architectural possibilities.
DISADVANTAGES OF FERROUS METALS Costly waste High cost of final finishing & polishing Environmental issue Heavy for transpsort applications
Solidification of a substance • Liquid metal solidifies when cooled (except mercury) • Minute nuclei (crystals) of a solid form when a pure, molten metal is cooled to just below its freezing temperature. Impurities in the molten material provide the centre for growth for the nuclei. • All metals (except mercury) are solid at room temperature. • A process of nucleation & growth achieves solidification. • ‘Dendrites’ grow out from the nuclei forming a tree-like structure in the direction of the fastest heat loss.
Metallic Structures • This crystallinestructure gives metals their properties (strength, stiffness, ductility, conductivity & toughness). • Each dendrite grows in a geometricpattern consistent with the lattice structure until each one touches its neighbour. At this point the dendrites begin to thicken to form a totally solid grain of metal. • The grain boundaries between are visible under a microscope, each grain having the same structure but a different orientation. This boundary is a narrow zone (perhaps three atoms) in which the atoms are not properly spaced according to the lattice structure.
Alloys Definition : Alloys are materials made up of more than one chemical element, at least one of which must be a metal.
For example a small amount of carbon is added to iron to form steel. • A certain amount of chromium is added to iron and carbon to form stainless steel, an alloy that is highly corrosion resistant. • Although pure gold is highly corrosion resistance, copper is added to it to enhance its strength .
Solidification of metals • A pure metal solidifies at one fixed temperature, a fact which can be checked by plotting a cooling curve. • A cooling curve may be obtained by melting a small amount of a metal and recording the temperature drop at suitable time intervals as this metal solidifies (the metal must be allowed to cool very slowly i.e. under equilibrium conditions) .
Graph of temperature against time - gives the cooling curve for that particular metal.
Nucleus formation • As the metal is cooled, clusters of atoms come together from the liquid to form solid crystal nuclei. • Nuclei become stable and grow into crystallites or grains. • Nucleation can occur by two processes – • Homogenous nucleation • Heterogeneous nucleation
Mechanism of crystallization • Crystallization is controlled by atomic diffusion from melt to the nuclei. • Characteristically a pure metal may crystallize in a tree branch pattern from a nucleus. Such formations are called dendrites.
Microstructure of copper-tin alloy showing branch like dendritic formations
In crystallization growth starts from the centre of the nuclei and crystals grow towards each other. • When two or more crystals collide their growth is stopped. • Finally the entire space is solidified.
A metal part is typically made up of millions of tiny crystals. Such a metal is said to be polycrystalline. • Each crystal in the structure is known as a grain.
Grain size Factors affecting grain size :– • Number and location of the nuclei at time of solidification • Shape of the mould in which the metal solidifies • Rate of crystallisation • Rate of cooling • Cold working • Nucleating agents
In polycrystalline metal shape of the grains is influenced by the shape of the mold.
Control of grain size • Typically – the smaller the grain size in the metal, the better its physical properties. • Can be controlled by rate of cooling - super cooling leads to finer grain sizes.
Latent heat given up by initial solidification raises the temperature in the vicinity of the solidification front and this condition promotes primary axes of the dendrite to grow in the direction of this front resulting in columnar grains. • In a cylindrical mould, grains would in this case grow perpendicular to the wall resulting in radial grains.
Decreasing the grain size can have a number of beneficial effects on the cast alloy structure of a crown or removable partial denture. • The finer grain size can raise the yield stress, increase the ductility and raise the ultimate strength. • The change in the grain size is related to the process of plastic deformation and fracture.
Grain boundaries • The grain boundary is a region of transition between differently the crystal lattices of two neighboring grains (typically oriented differently). • Grain boundary structure is more like a non-crystalline structure, particularly towards its central region. • Impurities in the metal may be found in greater concentration at the grain boundaries. • This region is typically readily attacked by chemicals.
The position of the neighboring atoms surrounding every atom of a crystal lattice is identical in a pure crystalline metal. • When the property of identical periodic points in space was explored mathematically it was discovered there are 14 ways to arrange points in space.
Body centered cubic Simple cubic Face centered orthorhombic Face centered cubic Body centered orthorhombic Simple triclinic Simple monoclinic Base centered monoclinic
Modification of grain size • Cooling rate and amount of impurities in the molten metal will affect grain size: • Gradual cooling - a fewnuclei are formed - largegrain size • Rapid cooling - manynuclei formed - small grain size. • Reheating a solid metal / alloy allows the grain structure to re-align itself. • Directionalcooling in a structure by selectivelycoolingone area. • A large amount of small impurities (or additives) in a molten metal can induce a largenumberof fine grains that will give a stronger and harder metal. This addition must be carefully controlled as too many impurities may cause an accumulation at the grain boundaries, which will weaken the material.
Chemical properties • Metals are usually inclined to form cations through electron loss. • They react with oxygen in the air to form oxides . • Iron rusts over years, while potassium burns in seconds.
The transition metals (such as iron, copper, zinc, and nickel) take much longer to oxidize. Others, like palladium, platinum and gold, do not react with the atmosphere at all. Some metals form a barrier layer of oxide on their surface which cannot be penetrated by further oxygen molecules and thus retain their shiny appearance and good conductivity for many decades (like aluminium, some steels, and titanium). The oxides of metals are generally basic (as opposed to those of non-metals, which are acidic.
Prosthodontic Considerations • Cobalt- chromium & titanium alloys are used in prosthetic dentistry for fabrication of implants • Nickel chromium alloys are used for porcelain fused to metal restorations • Nickel chromium alloys were introduced for crowns bridges and partial denture frame works
Nonferrous metals and alloys • Common - aluminum, copper, and magnesium • High-strength high-temperature alloys include: tungsten, tantalum, and molybdenum • Higher cost than ferrous metals but have good properties such as: • Corrosion resistance • High thermal and electrical conductivity • Low density and ease of fabrication
Aluminum and Aluminum Alloys • Most abundant metallic element (8% crust) • High strength to weight ratio • Resistant to corrosion • High thermal and electrical conductivity • Nonmagnetic • Easy formability and machinability
UNS – Unified Numbering System • UNS-Unified Numbering System - Common system used to describe condition of metal or alloy. • Generally has 4 numbers and a temper designation - Temper designation tells the condition of the material. • Example: 2024 wrought aluminum is A92024
UNS - Wrought Aluminum • Form: 1XXX • 1st # - major alloying element • 2nd # - modifications of alloy • 3rd & 4th # - minimum amount of aluminum alloy • EX: 1050 is aluminum with minimum 99.50% Al • Ex: 1090 shows a minimum of 99.90% Al
UNS - Cast Aluminum • Form: 1XX.X • 2nd & 3rd # - minimum amount of aluminum • 4th # - product form
Temper Designation • F: as fabricated (by cold or hot working or by casting) • O: Annealed (from the cold worked or cast state) • H: strain hardened by cold working (for wrought products only) • T: heat treated • W: solution treated only (unstable temper)
Magnesium and Magnesium Alloys • Lightest of all metals • Not sufficiently strong in pure form - alloyed to increase strength. • Uses • Aircraft and missile components, bikes, luggage, portable power tools, hard drive covers… • Designations for magnesium • A. 1 or 2 prefix letters • B. 2 or 3 numbers
Copper and Copper Alloys • First produced in 4000 BC • Properties: • Best conductors of electricity and heat, good corrosion resistance, and easily processed. • Uses: • Electronics, springs, cartridges, plumbing, heat exchangers, and marine equipment. • Common alloys: • Brass, Bronze, Beryllium copper