Untitled Flashcard Set

Engineering Materials Classification of Materials − Elements are building blocks of nature, can’t be broken down − Solutions occur where one substance dissolves in another − Compounds are a combination of two or more elements combined chemically − Mixtures are the result of two or more pure substances (elements or compounds) which are mechanically mixed together Polymer Structures − Thermoplastics − Thermosets − Thermoplastics • Are also known as thermosoftening polymers cause they can be resoftened and reformed • With covalent bonds and weak secondary bonds between the chains − Thermosets • Cannot be resoftened by heating. Once they are formed, heat cannot reshape them Properties of Materials Mechanical Properties − Strength: withstand applied loads without failure − Hardness: the ability of material to resist scratching, abrasion or indentation − Elasticity: to return to its original shape and dimension − Stiffness: to resist elastic deformation under load − Plasticity: to undergo some degree of permanent deformation without rupture − Malleability: to be hammered and rolled into thin sheets − Ductility: to be drawn out into thin wire Electrical Properties − Electrical conductivity is the ability to conduct electricity. Metals and carbon are good conductors; pure water & timber are poor conductors. And air, glass, most polymers and ceramics are good insulators. − Semi-conductors are materials that are manufactured to be poor conductors Structure of Materials − Each atom is made up of three components: protons, neutrons and electrons. − Protons and neutrons are located at the centre of the atom called the nucleus, while the electrons orbit the nucleus. o Neutrons- − Have no charge − Are located in the nucleus o Protons − Have a single positive charge − Located in the nucleus o Electrons − Have an equal value negative charge − Electrons In the outer shell are called valence electrons as they are used in bonding Bonding − Noble (inert (not reactive)) gases do not react at normal temperatures and pressures − They have a full outer shell making them stable and not needing to bond − They are useful for welding applications (MIG) where it is desirable to exclude oxygen and other reactive gases Crystal Structure − When a material is in its liquid form there is little or no order to the structure. As the material solidifies, however, the atoms arrange themselves into regular crystal structure. − Three most important structures are: ● Body centred cubic (BCC) ● Face centred cubic (FCC) ● Hexagonal close packed (HCP) Atomic Structure: Primary Bonds- Atoms form three types of primary bonds: ionic, covalent & metallic. Primary bonds are the strong bonds between the tightly clustered atoms that give any pure substance its characteristic properties. Secondary Bonds- these are van der Waals and hydrogen bonds and are relatively weak Predominant bonds: Metals- metallic Polymers- covalent Ceramics- ionic and/or covalent bonding Ionic Bond − Large differences in valance electrons usually metals and non-metals (such as NaCI) and involves the transfer of one or more electrons − Donor atom loses its valance electrons − Recipient fills its outer shell − The imbalance (electron: proton: ratio) creates an ionic charge of attraction Covalent Bond − Generally occur between non-metal elements − The strong attraction results in a sharing of valance electrons − Covalent bonding is important in polymers Secondary Bonds − Molecular or van der Waals bonds − The weak bond is produced by the concentration of –ve electrons on one side of an atom at one particular time which leaves the rest of the atom with a +ve charge − The change can vary with time and can be easily be broken down but heat, candle wax, graphite and polymers Polymorphism − Polymorphism or allotropy is the ability of a single substance to exist in multiple forms or crystal structures Crystal Structure − Liquids have little or no ordered structure − A crystal is a homogenous solid of definite chemical composition, with internal order, bounded by plane faces ● BCC ● FCC ● HCP Non-crystalline materials (amorphous) ● Amorphous means without form ● Eg are all liquids and gases, glass, which is technically a liquid Crystalline ● Regular ordered patterns ● All metals, majority of ceramics, some polymers and most minerals are crystalline Metals Ferrous metals ● Iron is the primary constituent in ferrous metals ● Mild steels contain low carbon and magnesium ● MS can easily be formed (i.e ductility) machined and welded. ● Issues are corrosion ● Stainless steel ● The chromium reacts with oxygen to forma chromium oxide layer that prevents further corrosion Non-ferrous metals ● Iron is not the primary constituent metal ● Copper and Aluminium are the most commercial non-ferrous metals Copper ● The main electrical conductor used due to high electrical conductivity ● High ductility, malleable and good corrosion resistance ● Used in electrical wiring, electrical contacts, motor windings Brass ● Alloy of Copper & Zinc ● Can contain up to 40% zinc, but beyond that is too brittle ● All brasses are corrosion-resistant and harder then pure copper ● Good wear, conduction and corrosion resistance makes them useful for switchgear and contacts ● Outdoor taps are cast using 60/40 brass. Bronze ● An alloy of copper and tin ● By pressing and sintering bronze powder, a porous sleeve may be produced ● The porous article is then impregnated with oil, graphite or polytertrafluroethylene (PTFE or Teflon) Aluminium ● Is a highly used metal, which has low density and excellent corrosion resistance; low strength and as such is usually used in alloyed form. ● Aluminium foil is almost pure aluminium ● Usually aluminium is alloyed with materials like copper, zinc, magnesium, lithium and other metals to gain excellent strength ● Is lightweight, offers strength to weight ratios better than most ferrous alloys Basic forming processes suitable for materials Casting ● Casting Is a forming process that involves heating up a material, such as a metal alloy and then placing it in a mould ● Moulds may be permanent moulds made of metal or they may be disposable moulds made of sand ● Die casting uses permanent moulds and is extensively used for nonferrous alloy casting. Sand casting is used for a lot of ferrous alloy casting. Rolling ● Many metals can be cast in the form of ingots or bars ● For example circular or square bar ● Rolling may be done at a high temperature (Hot rolling)- easier ● Slightly elevated temperature (cold rolling) ● Cold rolling is harder to do and the final metals structure will be stressed and deformed, but it will have a better surface finish and be more dimensionally accurate. Extruding ● Extruding may be likened to squeezing toothpaste from a tube ● Aluminium alloy window frames are generally made up of extruded sections. Cutting ● Removal of unwanted material ● Most familiar with cutting with a hacksaw ● Turning, grinding, sawing, drilling, etc Joining o Various methods dependant on the material and the use of the metal o Metallurgical: • Electric arc welding ● Method: metal is melted by an electrode, which doubles as the filter metal covered by flux to prevent oxidation of molten metal. ● Applications: joining thick steel sections and small runs • Oxy-acetylene welding ● Method: metal is melted by flame and filler metal added ● Applications: joining steel fan cages • Bronze welding ● Method: a flame heats the parent metal and bronze filler metal which is added to the joint. There is little or no metal of the parent metal. ● Applications: low strength uses • MIG welding ● Method: metal inert gas uses a continuous feed wire (electrode) and inert gas (eg argon) preventing rapid oxidisation ● Applications: suited for automation and can be set up for aluminium using a suitable feed wire and special gas mixture • TIG welding ● Method: Tungsten Inert Gas uses a tungsten electrode (that doesn’t melt) and a manually fed filler rod or wire ● Applications: joining aluminium and stainless steel, especially thick sections o Mechanical Joining ● Bolts, nuts and screws used to fasten materials together ● Hole drilling and corrosion pose issues for this form Fabricating ● Is the process of assembling an item from various components ● Eg mild steel welded together Polymers − Polymers are generally solid materials made up of long molecular chains that are created by adding or connecting smaller molecules together. − Polymers are often termed organic, due to carbon being the primary constituent. − Although most polymers are synthetic, there are some natural polymers, such as natural rubber and cellulose fibres. − The term polymer is a very broad term; the field has a wide group of materials, each having distinct structures and properties. − They are used for a wide range of applications − Synthetic polymers make up the remainder, known as “plastics” e.g PET, HDPE, PVC, LDPE, PP, PS − They tend to exhibit good strength-to-weight ratios − They are generally formed into shape by moulding, rolling, extruding or other heat forming processes − Derived from crude oil Ceramics − Engineers use ceramics for high temperature applications and situations where abrasion resistance or thermal stability is required − Ceramics are hard and brittle, high compressive strength, low tensile strength, low electrical & thermal conductivities. − Ceramics have been used for centuries and are now finding new uses due to their hardness and good thermal properties. − Any ceramic material that requires some form of purification, mixing or firing is a synthetic ceramic, e.g. clay-body ceramics, glass, refractories and cement. − Clay body ceramics • Made from a combination of different clay materials • Classified as earthenware, china, stoneware and porcelain ● Porcelain and china are two most common seen in homes Porcelain has low porosity (1%)- this is important for plates and cups that hold beverages and food. Good electrical and thermal insulator. Glazed to improve aesthetics and reduce surface porosity to zero. However brittle & heavier than polymers. Glass: ● Transparent, brittle, most used glass is soda lime ● No atomic order- represent liquids in their atomic structure ● Maximum theoretical strength of glass is 30,000 MPs ● Ceramic glass cooktops also use special glass tops which prevent the saucepan from directly contacting the heating element. Gorilla Glass: ● Modern smartphones use a capacitive touchscreen that must be strong and resistant to cracking yet be exposed so a finger can contact it. ● By an ion exchange a compression stress is a set-up in the surface, which makes it more difficult to introduce tensile failure in the glass. ● Higher density Composite materials − Are made of different materials combined together to capitalise on the desirable properties in each. − Fibreglass as an example uses fine glass fibre with high tensile strength in a thermosetting resin matrix. The glass fibre provides good tensile strength while the resin provides toughness usually absent in glass. − Concrete & timber are examples of composites. − Properties include specific strength (strength- to weight-ratio) − Concrete is sand, cement, aggregate (acts as a binder/glue) Timber is a natural composite that is composed of cellulose fibres, the tracheids, are held together by the lignin. Advantages- ● High specific strength (strength-mass ratio) ● Wood is a regenerative and if managed professionally can be a permanent resource ● Easily handled, worked and joined Disadvantages: ● Wood is combustible ● Strength of wood varies with the species and direction of the applied force