Metals and Cold & Hot working Flashcards
What is a metal?
Are opaque, lustrous elements that are good conductors of heat and electricity.
- Most metals malleable and ductile and are usually denser than other elemental substances
What is a bond?
A bond is a link between atoms in molecules or compounds and between ions and molecules in crystals.
What is a metallic bond?
Metallic bond, force that holds atoms together in a metallic substance. Such a solid consists of closely packed atoms.
Name 3 types of bonds/structures
- Hexagonal Closed Packed (HCP)
- Face Centred Cubic (FCC)
- Body Centred Cubic (BCC)
What is Hextagonal Closed Packed (HCP)?
Provide Examples.
Refers to layers of spheres packed so that spheres in alternating layers overlie one another.
- pattern is hexagonal
- brittle
- has three layers of atoms
Examples:
- Zinc
- Cobalt
- Titanium
What is Face Centered Cubic (FCC)?
Provide Examples.
Has atoms at each corner of the cube and six atoms at each face of the cube.
- in the shape of a cube
- More ductile than other structures
- has no whole actom
Examples:
- lead
- nickel
- gold
- Silver
What is Body Centered Cubic?
Provide examples.
The unit cell has a quater of an atoms at each corner, and a WHOLE atom in the centre.
- in the shape of a cube
- does not allow the atoms to pack together as closely as the fcc or hcp arrangements
- stronger but less ductile
Examples:
- iron
- chronnium
- tunsten
What are some defects in metalic crystals? And explain how these would effect the metals?
Vacancies: the empty spaces where an atom is missing
Dislocations: Which are lines of defective boundary
- Metalic crystals are not perfect
These imperfections determine many of the mechanical properties of metals
When STRESS is applied to a metal, dislocations are generated and move, allowing the metal to deform.
What is a metal alloy?
An alloy is a mixture of metals, or a metal combined with one or more other elements.
- If the added element is a non-metal, alloys ay still have metallic properties
- alloys are mixtures and their % composition can vary and manipulated the properties of the alloy
WHat are some exmaples of alloys?
- Brass: copper + zinc
- Bronze: copper + tin
- Solder: tin + lead
- Cast iron: iron + 2-4% carbon
- Steel: iron carbon
- Stainless steel: iron + carbon
What is iron/steel and explain their proporties?
Iron is simply a metal element that occurs naturally on Earth. In comparison, steel is a man-made alloy that’s made by mixing iron and carbon together
What is Elastic Deformation?
Is a small change in shape at low stress which is recoverable after the stress is removed.
This type of deformation involves the stechting of the metals bonds, but the atoms do not slide past each other
What is Plastic Deformation?
Occurs when stress is suffucent to permately deform the metal.
This type of deformation required the breaking of bonds
- ‘slip’ mechanism of plastic deformation
Name the elements of a stress strain graph
- Proportional limit
- Elastic limit / yeild point
- UTS
- Necking
- Fracture
What is Quenching?
Quenching involves the rapid cooling of a metal to adjust the mechanical properties of its original state.
- most steels are hardened by heating & quenching
- the temperature the metal is heated to, is dependent on the metal and the desired level of hardness
Quenching medians: oils, salt solutions or inert gases are generally used for cooling.
What is metal hardening?
Hardening is a metalworking technique which is used to increase the hardness of a metal.
What is tempering?
Tempering is the heating metal and letting it cool slowly. This allows us to create metal which is still hard but less brittle.
What is cold working? What is hot working?
Cold working is the shaping of a metal while it’s still cold.
Hot working is the process of plastically deforming metals above their crystallisation temperature.
What is casting?
- casting involves the pouring of molten metal into a prepared mould cavity
What is an ingot?
An ingot is a block of metal, which is typically oblong in shape.
Ingots are changed through rolling or extrusion.
Provide examples of casting?
Sand casting
- green sand casting
- dry sand casting
- CO2 sand moulds
- synthetic binders
Shell moulding
Permanent Mould Casting
Die casting
Centrifugal Casting
What is sand casting (General)?
Sand is > SiO2 mixed with small amounts of clay
This is used to form the moulds and each sand moulds producers only one casting ( is not reusable).
Are always gravity feed
What is green sand casting?
No attempt to dry sand out. This means that small amounts of moisture still remain.
What is dry sand casting?
• The sand is completely dried by heating it to 250*C.
• This increases the strength of the sand, while effectively reducing the amount of steam produced when the hot metal enters the mould
What is a CO2 sand mould?
• uses a mixture of sand and 4% sodium silicate.
•after ramming the sand is ‘gassed’ with CO2 and becomes quite hard but not impermeable.
• used for core making and more detailed/ accurate moulds.
What is a Synthetic binder moulds?
No water is required. Binders at set to room temperature Nd break down after casting
What is shell moulding?
the mold is a thin-walled shell Created from applying a sand-resin mixture around a pattern. The pattern, a metal piece in the shape of the desired part, is reused to form multiple shell molds
very high dimensional accuracy can be obtained with tolerances of 0.05 mm
• only finishing required is the grinding of all bearing surfaces
• suitable for mass production although the equipment is expensive this is offset by the high production rate
• recent development
What is permanent mould casting? 
Permanent mold casting is a metal casting process that employs reusable molds usually made from metal.
• permanent mould or dies last for several thousand cast
• reduces cost for long production runs
• usually made from iron cast
What is Die casting?
• Similar to permanent mould casting, instead of being gravity feed the metal is forced into the mould up to 140 Mpa
• shapes are limited due to the problems associated with the removal
• this process is fully automated • typically requires little to no finishing
• has a good surface finish
What is centrifugal casting?
Centrifugal casting
is a casting technique that is typically used to cast thin-walled cylinders.
Centrifugal allows the creation of hollow objects such as pipes and tubes without the use of a core
The metal can be made to flow more rapidly into mould cavities resulting in a denser material
Name 3 cast structure defects?
- crystalline segregation
- gas porosity
- inclusions 
What is crystalline segregation?
are slips in the bonds that haven’t yet fully formed. these compounds are in-soluble in the solid alloy
What is gas porosity?
Occurs when gas is trapped in the mould try to escape (bubbles?)
What are inclusions?
Inclusions are usually slags of one kind or another. They are lighter than the molten metal and can be held back during the pouring process or alternately skimmed off the surface. 
What are grain sizes?
Grains are the individual crystalline areas / structures within a material.
A materials grain size of the solidified metal is determined by the rate of nucleus formations. Which is in turn controlled by three very important techniques.
Grain size affects the material properties of the metal.
Name the 3 techniques which can influence grain sizes
1) The cooling rate
2) Agitation of the metal
3) the induction of impurities
How does the cooling rate affect the grain sizes of a metal? 
• slow cooling result in a few nuclei allowing for larger dendrites to grow
• fast calling produces mini nuclei and consequently many smaller dendrites
• if the metal is super cool (truth is freezing is overshot) the rate of nuclearisation is increased enormously
How does the agitation of melt effect the grain size?
Also increases the rate of nucleasation if agitation is continued doing the ‘mushy’ state the larger dendrites will be broken up
What does the introduction of impurities do to the gain structure of a metal?
These small insoluble particles also act as nuclei.
Melts (metals) may be ‘seeded’ with such impurities to produce finer a grain size
What is piping?
Piping in ingots are formed due to the contractions of the cooling and solidifying metals, and can be serious defects.
what is a slip?
Slip is by far the most common mechanism of plastic deformation and occurs when one part of the crystal has become displaced with respect to another part along a particular crystallographic plane of the crystal
What is hot working?
Hot working is controlled plastic deformation carried out at temperatures above the re-crystallisation temperature of the metal.
Exmaples of hot working?
Rolling
Forging: hammer (smith), drop forging, press, upsetting, swaging
Extrusion
Drawing
Piercing
Spinning
What is rolling?
Hot rolling is usually carried out at temperatures 40’C to 95’C above the re-crystallisation (or critical) temperature of the metal.
Metal is feed between two rollers once.
What is forging?
Forging is the shaping of metal by the application of localised compressive forces:
Hammer: village blacksmith
Drop forging: Essential for mass production. Single or double action machines up to 10MN capacity stamp articles such as spanners.
Press: Squeezing between special dies.
Upsetting: section is increased while length is reduced (opposite to drawn out)
Dimensional accuracy is not high with drop forged components and machining is always necessary.
how does forging affect gains?
Forging usually produces a particular type of grain structure known as “grain flow” or “fibre” which results in the development of directional strength properties within the finished forging
What is extrusion ?
Extrusion can be likened to the squeezing of toothpaste from a tube. Extrusion is the shaping of a billet of metal to a given cross-section by pushing it through a die of suitable shape.
What is drawing?
Is a hot woking technique whereby a heated metal is pulled through two dies using tensile force.
- simple cross-sections
- wires and tubes are made from this process
Advanatges and Disafvanatges of cold-working?
Advantages of Cold Working
No necessity for heating
Improved dimensional control
Improved surface finish
Improved strength properties
Development of directional properties
Less metal loss and tool wear from scaling
Disadvantages of Cold Working
Heavier, more powerful and more rigid equipment required
Higher deformation forces required
Strain hardening occurs
Metals are usually less ductile at ordinary temperatures
Undesirable directional properties may develop
Metal surfaces must be free from oxides and scale prior to cold working
What is a ferrous metal?
A material which contains iron, is magentic, oxides, hard.
What is the difference between a machines, forged and casted item?
differnet gain structures
How does forging affect grain structures?
- Forging usually produces a particular type of grain structure known as “grain flow” which results in the the development of directional strength properties within the finished forging
- re-crystallisation in preferred directions
- that the elastic limit, ultimate strength, ductility and toughness are all greater when measured across the direction of grain flow that when measured along it.
How does machining affect grain structure?
Sections through typical forged and machined yokes showing fibre patterns. Arrows indicate planes of weakness in the yoke machined from bar stock.
Benefits of machining and forging?
Machining:
Advantages - cheaper, quicker, better & safer working conditions
Disadvantages - have weak points, less strong
Forging:
Advantages - stronger along the grain
Disadvantages : poor work conidtions, intense labour, tiemconsumer ing
What is annealing?
Annealing is a softening process in which metals are heated and then allowed to cool slowly.
- It makes the metal as soft as possible.
- More ductile, easier to machine material.
- Refinement of grain structures
What is quenching?
Most steels may be hardened by heating and quenching (cooling rapidly).
The desired temperature it is heated to depends on the metal and the desired hardness.
Water, oil, salt solutions, or inert gas are generally used for the cooling process.
Quenching results in a metal that is very hard but also brittle.
What is tempering?
Gently heating a hardened metal and allowing it to cool slowly will produce a metal that is still hard but also less brittle. This process is known as tempering.
Advantages and disadvantages of Hot Woking?
Advantages
- easier to shape (larger deformation is possible)
- forging creates ‘grain flow’
- removes imperfections
Disadvantage
- heating is required
- tools used have a shorter life span
- creates a lot of waste
- poor surface finish
- difficult to attain high dimensional accuracy
How does grain size affect material properties?
Metals with larger grain structures are usually more ductile.
Metals with smaller, refined grain structures are stronger, as there is a decrease in build up on their boundaries. More grain boundaries means more strength. This is due to the fact that these boundaries help stop dislocations.
