Metals and Metallic Alloys

What is a Metal?

• Crystalline structures comprised of individual grains.
• Pure metals consist of a single element.
  • For example: copper, gold, silver, lead, tungsten, tin, etc.
• Metals are typically hard and shiny with good electrical and thermal conductivity.
• Generally, atoms (the smallest part of a chemical element) of a metal contain one or more loosely bound electrons.
• These are considered free electrons, and they make metals good conductors.
• When one end of a metal wire is heated, these free electrons start moving to another end of a metal, thus conducting heat.
• The same is true for electricity.

• When most solids form, the atoms arrange themselves in a regular three-dimensional pattern.
• This regular three-dimensional arrangement is known as a crystal structure.
• All metal alloys solidify as collections of crystals, also known as grains.
• These grains are not usually visible, but can be seen on galvanized lamp posts for example.

Grain Size

• The grain size of metals can vary and be determined by heat treatment, particularly how quickly a metal is cooled.
  • Quick cooling results in small grains.
  • Slow cooling results in large grains.
• Grain size in metals can affect the density, tensile strength, and flexibility.

Mining Metals

Ores

• An ore is a rock that is usually metallic, that can be mined and processed at a profit.
• Ore is a deposit in the Earth's crust of one or more valuable minerals.
• The most valuable ore deposits contain metals crucial to industry and trade, like copper, gold and iron.

Mines

• A mine is an opening or excavation of the earth from which ores are extracted.

Mining Definition

• Mining is the process of extracting ore from the ground.

Extracting Metal

• When miners find rock containing mineral ore, they first extract the rock from the earth.
• This can be a huge process, sometimes displacing millions of tons of dirt.
• The rock is then crushed by powerful machinery.

• Metal is extracted from the crushed ore by one of two major methods: smelting or electrolysis

Smelting

• Smelting uses heat to separate the valuable metal from the rest of the ore.
• Smelting usually requires a reduction agent, or another chemical, to separate metal from its ore.
• In the earliest smelters, the reduction agent was carbon in the form of charcoal.
  • Charcoal burned with hematite ore, for instance, smelts iron.
  • Charcoal burned with cuprite ore smelts copper.

Electrolysis

• Electrolysis separates metal from ore by using acid and electricity.
  • Aluminum, which burns at a very high temperature, is extracted from bauxite ore by electrolysis.
  • Bauxite is placed in a pool of acid, and an electrical current is run through the pool.
  • The electrons in the current attach to the oxygen and hydrogen, which are the other elements in bauxite, leaving the aluminum.

Metallic Alloys

Pure Metals

• Metals are typically hard and shiny with good electrical and thermal conductivity.
• Therefore metals are a very useful resource for the manufacturing industry.
• Pure metals are either too soft, too brittle, or too chemically reactive for practical use and so understanding how to manipulate these materials is vital to the success of any application.
• Alloys are one of the primary methods of manipulating metals.

Alloys

• Alloys are metals that have been mixed with other metals or elements to take on new properties.
• This can be a mixture of two or more metals; such as bronze = Cu (copper) + Sn (tin).
• This can also be a mixture of metals and non-metals; such as carbon steels = Fe (iron) + C (carbon).

• Example:
  • The pure metal iron consists only of iron atoms.
  • Steel, an alloy of iron and carbon, contains mostly iron atoms with isolated atoms of carbon that lend it strength.
  • Adding the metals chromium or molybdenum to the steel produces yet another alloy: stainless steel.

• Some examples of metal alloys include:
  • Steel
  • Bronze
  • Brass
  • Nichrome
  • Berrylim-copper

Metallic Alloys Compared to Metals

• One reason that manufacturers combine pure metals to form alloys is to change the physical properties of metals.
• Metallic alloys typically offer greater strength, durability and flexibility over the base metal.

Use of Different Alloys

Jewellery

• An alloy of gold and silver, copper or zinc is often used instead of pure gold to improve the durability.

• Pure metals may be too soft to hold up to regular use, but alloying them makes them tougher.
• As a pure metal, gold bends and stretches so easily that it would quickly pull out of shape if it were formed into a ring and worn on the finger.
• Jewelry manufacturers alloy pure gold with silver, copper or zinc to improve the metal’s durability and rigidity.
• The gold contributes its color and resistance to corrosion; the other metals contribute their strength.

Cooking

• Stainless steel is used instead of pure iron to improve the reactivity to its surroundings.

• In their natural elemental state, some pure metals react strongly with their surroundings, oxidizing and corroding until they become unusable.
• Blending these metals with less reactive metals such as Chromium and other metals changes their reactivity.
• Stainless steel does not readily rust or pit the way pure iron would.

Aerospace

• Aluminium or titanium is used instead of pure iron to decrease the mass.

• Light metals such as aluminum and titanium reduce the mass of pure metals with which they alloy.
• These lighter alloys allow manufacturers to design and build lighter aircraft.
• A lighter vehicle can carry more load, such as fuel.

Ferrous Alloys

• Ferrous alloys contain iron which makes them magnetic.

Mild Steel

• Steel with a carbon content of 0.1% to 0.3% and iron content of 99.7% to 99.9%.
• As iron is magnetic, the high iron makes mild steel magnetic too.
• Used for engineering purposes and in general non-specialized metal products.

Stainless Steel

• Made up of iron, nickel and chromium.
• Resists staining and corrosion and is therefore used for the likes of cutlery and surgical instrumentation.

Cast Iron

• Carbon 2 to 6% and iron at 94 to 98%.
• Very strong but brittle.
• Used to manufacture items such as engine blocks and manhole covers.

Non-ferrous Alloys and Metals

• Non-ferrous alloys contain no iron, and are therefore not magnetic.

Aluminium Alloy

• An alloy of aluminium, copper and manganese.
• Very lightweight and easily worked.
• Used in aircraft manufacture, window frames and some kitchenware.
• Pure aluminium can be used in drink cans.

Copper

• Copper is a natural occurring substance and is often used as a pure metal.
• The fact that it conducts heat and electricity means that it is used for wiring, tubing and pipe work.

Brass

• A combination of copper and zinc, usually in the proportions of 65% to 35% respectively.
• Is used for ornamental purposes and within electrical fittings.

Silver

• Mainly a used as a pure substance, but mixing with copper creates sterling silver.
• Used for decorative impact in jewelry and ornaments, and also to solder different metals together.

Lead

• Lead is a naturally occurring substance.
• It is heavy and very soft and is often used in roofing, in batteries and to make pipes.

Metal Treatments

Work Hardening

• Also called strain hardening or cold working.

• The process of toughening a metal through plastic deformation.
• When a metal is stressed beyond its elastic limit it enters the plastic region.

• When the load is increased further it makes the metal harder and stronger through the resulting plastic deformation.
• It’s more difficult to deform the metal as the strain increases and hence it’s called “strain hardening”.

Properties of Work Hardening

Advantages:

• Tensile strength
  • The ability of a material to withstand pulling forces.
• Hardness
  • The resistance a material offers to penetration or scratching

Disadvantages:

• Ductility
  • The ability of a material to be drawn or extruded into a wire or other extended shape.

Usage

• It is mostly used when a surface with a better finish is required.
• Common applications include flat steel products for automotive parts and construction works.

Annealing

• A heat-treating process designed to increase the toughness of an iron-based metal by heating it slowly and allowing it to cool in air (slowly).

• This gives the finished product looser tolerances than the initial material used, unlike work hardened steel products.
• This also makes the finished product free from internal stresses that may arise during work-hardening processes.

Usage

• Tempered metals are more malleable and can be cast into a variety of different shapes, making it a good choice for producing structural components, such as l-beams and rail tracks.

Quenching

• A metal is heated above its critical temperature and shaped and cooled rapidly using water, oils, or air.

• This increases the hardness and strength of the metal part but makes it more brittle.
• Uses include cutting tools, blades and gears.

Tempering

• Once quenched, metal is reheated to a lower temperature and allowed to cool slowly.

• This reduces the brittleness incurred through quenching improving toughness while keeping some of the hardness.
• Different temperature may be used when tempering so as to achieve the right properties for the function of the product.
• Typical uses are springs, structural parts and tools.

Properties of Tempering

Advantages:

• Toughness
  • The ability of a material to resist the propagation of cracks.
• Ductility
  • The ability of a material to be drawn or extruded into a wire or other extended shape.

Disadvantages

• Brittleness
  • Breaks into numerous sharp shards.
• Hardness
  • The resistance a material offers to penetration or scratching.

Super Alloys

• A super alloy that exhibits excellent mechanical strength, resistance to thermal creep deformation, good surface stability and resistance to corrosion.
• Super alloys are primarily made from nickel, cobalt and iron.

• The term was first used after WWII to describe a group of alloys developed for use in aircraft turbine engines that required high performance at elevated temperatures.

• Super alloys can be used at high temperatures, very close to their melting point.
• Unlike super alloys, the strength of most metals decreases as the temperature increases.

• Super alloys are used in aerospace (turbine blades and jet/rocket engines) marine industry (submarines), chemical processing industry, nuclear reactors, heat exchange tubing and industrial gas turbines and many more.
• Two design criteria for super alloys are creep resistance and oxidation resistance.

Resistances of Super Alloys

Creep Resistance

• Creep is the slow permanent deformation of a solid material under the influence of a continuous mechanical stress.
• The mechanical stress in creep is generally a lot lower than the yield strength of the material, and wears it out over time.

• Thermal creep occurs similarly, but due to high temperatures instead; leading to deformation even if the heat is much lower than the critical temperature.

• Super alloys need to be creep resistant as they generally operate under extreme conditions with strong mechanical stresses and high temperatures.

Oxidation Resistance

• A property of a metal that means that it does not readily react with oxygen and degrade.

• Oxidation is the interaction between oxygen and different substances when they make contact, such as rust Fe2O3.
• Oxidation resistance is the ability of a material to resist the direct and indirect attack of oxygen (oxidation).

Recycling and Reusing Metals

Design for Disassembly

• Design for disassembly, or DfD, is the process of designing a product so that when it becomes obsolete it can easily and economically be taken apart, the components reused or repaired, and the materials recycled.
• Design for disassembly is an important aspect of sustainable design.

• Valuable metals, such as gold and copper, are recovered from millions of mobile phones that have gone out of use following the end of product life.
• Some laptops and mobile phones can be disassembled very quickly without tools to allow materials to be recovered easily.

• DfD involves the designers of future products in choosing material combinations with recycling in mind.
• They can reverse the current trend of greater material mixing.
  • Current designs are less recyclable than those from a few decades ago.
  • Applications such as nano materials and microelectronics introduce major recycling challenges.

Recovery and Disposal

• When the lifetime of a product ends there are, generally, two options:
  • Put it somewhere as waste (disposal), resulting in environmental pollution
  • re-use the materials (recovery)

• Unlike other materials, such as paper and plastic, metals can be repeatedly recycled without degradation of their properties.

• Recycling of metals results in significant savings in energy consumption.
• For each metal and metallic alloy the possibilities for recycling are different.

Aluminium

• Recycling aluminium requires 20 times less energy than making new aluminium.
  • The big advantage of aluminium is that you can reuse it time and time again without loss of quality/capability.
  • Due to the low required energy for recycling and it’s “easy” process, Aluminium is wanted for recycling.

Alloys

• Recycling alloys is more difficult but possible and essential.
• All metals are used to a lesser or greater extent in a wide variety of alloys.
• When the metal equipment reaches end-of-life, it is very likely to end up with scrap of different alloys compositions.
• Not only are these alloys diverse, often they are unknown.

Finished Products

• It is difficult to recycle metals from finished products, like cars and fridges or metal that is used in many applications such as zippers for clothing, electronics equipment etc.

• The activity with the greatest potential to improve metal recycling is collection: if materials such as cans, pots or an old taps are discarded in the common trash, it can take up to 500 years to decompose and the way of recycling is stopped.