Sheet Metal Forming

Sheet steel is very widely used. It has very good formability. It can go into very complex processes such as drawing. In sheet steel the formability will decrease with an increase in carbon content. A higher carbon content results in higher carbides and a finer structure. Adding alloying elements more or less they have some form of strengthening will make formability inferior. To control the grain orientation. The BCC structure has a lot of planes of atoms, within the steel sheet the planes would have a certain direction that can create properties in the steel which are anisotropic. We can adjust the microstructure that when the sheet is being pulled the strain ratio would be greater than 1. In order to create a preferred grain orientation. Aluminium is very important since it increases the drain ratio,r. A thicker sheet can be drawn into a higher depth. Using a thicker sheet you need more forces during forming and you are using more material which increases the cost. The idea is to use as little alloying elements as possible.

Bulk Formability

forming process that change the surface ratio of the material. Similar to forging and extrusion. Unlike sheet metal forming it is less dependent on the chemistry(carbon content and alloy content). It is more dependent on the working temperature. However, Sulphur affects bulk formability by decreasing it. We increase the temperature accordingly and we get the right formability.

Weldability of steels

Steels have the advantage which can be joined easily via welding. The weldability of steels changes a lot with the chemistry. Low carbon steels less than 0.3 % can be welded without any problems. Especially if we are welding thick sections one might consider to preheat in order to minimise stresses. A higher carbon content medium carbon steels we need to be very careful during welding since it would involve the heat effective zone to cool and form martensite of relatively high carbon which makes the weld susceptible to cracking. high carbon steels are very difficult to weld. Welding procedures which are low in hydrogen and which would then need to be tempered. The electrode material used needs to match the type of metal which is going to be welded.

Cast Irons

Also Ferrous materials. Contain a carbon content which is higher than the maximum

solubility if carbon in austenite. (>2.1%) Surplus carbon can either manifest itself as carbides or graphite. With cast irons it is not fair to say that they are just iron carbon materials. They include also Silicon. Normal cast irons the silicon can range 1-4%. Silicon is an element which promotes the formation of graphite rather than carbides. As the cast iron cools the carbon will come out as graphite. There are cast irons which do not contain graphite these are called white cast irons. In grey cast irons graphite forms as flakes. In spheroidal cast iron graphite forms as nodules.

Advantages of cast iron

The big majority of steels are produced into shape via the wrought rule. With cast irons their chemistry makes them very suitable material for casting. One big advantage is that it is a material which is very easy to cast. It does not oxidise - no oxide on the surface. On solidification, the precipitation of graphite will cause an expansion in the liquid and this will give a lot of fluidity*. Cast irons have a lower melting temperature when compared to steels. Grey cast irons are cheaper than steels. Cast irons are quite abrasion resistant. The presence of graphite will also help to lower the coefficient of friction. They have very good machinability. Excellent vibration damping ability. Cast irons have a relatively high surface temperature.

One of the major drawbacks of grey cast iron is the ductility. Strength is also limited.

The properties of cast iron are influenced a lot by the shape of the graphite.

White Cast Iron - No graphite.The chemical composition and the cooling rate in production are adjusted to have the carbon present in the form of carbides. Also, it is used a lot as a starting material to produce the malleable cast iron.

Graphite is basically composed of 100%Carbon. The carbon of a pencil is made out of graphite. The structure of graphite consists of layers of carbon atoms in a hexagonal configuration. These are very well bonded together via covalent bonds. These sheets of atoms are bonded together via secondary bonds. Graphite has a ver low density due to its open structure. It is rather soft and shears easily.

Alloying elements and impurities in Cast iron

They do contain some impurities: Sulphur and Phosphorus.

Silicon is always present in cast iron. In most cast irons it would be present between 1-4%. This Silicon promotes the formation of graphite. Also, Silicon will promote second stage graphitisation. It softens the cast iron. In grey cast iron we have to play with the amount of Silicon depending on the cooling rate. The cooling rate would vary from one casting to another. If we would like to form grey cast iron then we use a slow cooling rate in order to graphite to form. If we consider a grey cast iron with a composition of 3.5%C

Two factors that affect graphite formation: chemistry and cooling rate. Elements that stabilise the carbides and high cooling rates will favour the formation of carbides in the microstructure. Excess silicon will make the cast iron more brittle. Silicon promotes first stage graphitisation as we keep cooling down more graphite will form. As we go through the eutectoid silicon will transform the carbon in austenite into graphite instead of pearlite. The deposition of graphite on primary graphite. Bulls eyes microstructure. The eutectic point will shift towards the 3.5%C due to the Silicon present. Sulphur will hinder the formation of graphite and promotes the formation of carbides. When we want a spheroidal shape sulphur is the least desirable. Manganese has high affinity to Sulphur forming Manganese Sulphide. Initially Manganese has the effect to soften the cast iron. However, once we exceed the amount of Manganese required then it will promote the formation of carbides (Fe3C) - it will hinder graphite formation. (In malleable cast irons we add manganese to form a matrix of pearlite.) Therefore when adjusting the chemical composition of cast irons we need to be careful of those elements which favour the formation of carbides and those that favour the formation of graphite. Phosphorus makes the cast iron more fluid. It is added in values up to around 1% to improve even more the cast ability. Phosphorus will make the cast iron much more prone to fracture. It is used a lot in cast irons which are used for aesthetic purposes.

The carbon equivalent formula sums up the elements of the cast iron. A higher carbon content means higher tendency to form graphite upon cooling. Increasing the carbon equivalent has the effect of reducing the strength contrary to steels.

In grey cast irons carbon content is normally between 3.3-3.5%C. Bigger castings would usually require less silicon. A high phosphorus content enables the formation of an iron phosphide eutectic which its main role is to promote the fluidity. Being close to the eutectic would increase the fluidity. The composition of cast iron has to take into the consideration the functionality and the size needed.

One problem with cast iron is that when cooling the casting can have different parts of different cross sections. These will cool down at different rates. We can start getting temp gradients, micro structural differences between surface and core and the formation

of residual stresses-which will cause problems during machining.

The chill effect - when the surface is cooled so quickly that it forms white cast iron within the grey cast iron. This can cause problems during machining.

Cast irons are sometimes heat treated, either to relief stresses or remove unnecessary carbides that form or in some cases to change the ferritic microstructure into pearlite. 3 common types of heat treatment:

Annealing: 2 effects- to get rid of the unwanted cementite and to relieve the internal stresses.

Quench Hardening: only carried out on cast iron with a ferritic microstructure. It is done in order to convert the ferrite into pearlite to make such cast iron stronger.

Stress Relieving: This will relieve any stresses which are formed during the quenching cycle.

One of the big limitations of grey cast iron is that that results from the morphology of the graphite which forms as flakes. One way to get some malleability: deformed without failure is by changing the morphology of this graphitic phase. One way of doing as such is to produce malleable cast irons. These are produced by heat treating a product which is initially in the form of a white cast iron. Following that we have to subject the component to a heat treatment: The Black Hearth Process. Involves a very prolong heat treatment. We heat to around 900 deg C. We hold it at that temp for tens of hours even up to a week. During this heat treatment the cast iron is protected from oxygen-placed in an air tight box in order not to lose the carbon. The more stable the carbides the longer the heat treatment. The white cast structure will transform itself in to the formation of graphite rossets (spherical in shape but not exactly spherical) in a matrix of ferrite. When we cool through the eutectoid the carbon in solution will deposit itself on the rossets. We have a limit of the component which we are going to produce.

Mottle refers to graphite formation.

The white Hearth process- we start with white cast iron. During the heat treatment we surround our product with an iron oxide ore. Then we heat our component to about 1000 deg C. The heat treatment may take up to a number of days. We might end up losing most of the carbon from the component to the atmosphere. The microstructure is very

similar to that of steel however white cast iron has a much better castability than steel.

The production of components to form malleable cast iron is not as popular as it used be. Another production method to produce cast irons where the shape of graphite is nodular in shape. This is called Nodular cast iron. We can produce them straight away from the melt. We need to be careful not have sulphur in the melts. On cooling graphite will start forming but the presence of magnesium will tend to poison the preferential growth direction of the graphite. So graphite will form uniformly in all directions as nodules. With this type of cast iron we can adjust the composition and the cooling rate to get different matrices. We can have a range of properties by getting different matrices with one common thing that is graphite in the shape of nodules.

White cast iron has no ductility and high hardness. Its main use is to act as the base material of malleable cast irons. Carbon, Silicon contents are usually low in order not to form graphite.

Grey Cast iron has low ductility due the shape of the graphite i.e flakes. They have excellent cast ability. The paralytic cast irons can also be surface hardened by the use of chills or flame hardening. Is much more effective to dampen vibration.

Malleable Cast irons - have some ductility. Different types: Black Hearth, White Hearth, Perlitic Malleable.

Spheroidal Cast irons: an increase in strength and a reduction in ductility (from Ferritic to Martensitic) These cast irons are in direct competition with steels regarding properties. They are chosen more than steels since they are cheaper to produce.

Alloy Cast Irons

Welding of cast irons is very difficult. However sometimes welding is done for repair purposes. It has to be done under a very strict protocol. The choice of fillers is very important in order to prevent cracking.

Stainless Steels

They are so resistant to corrosion due to the addition of chromium. It is an essential element in these materials -12wt% Chromium. When the chrome will corrode it will form chrome oxide and this layer of chrome oxide will form a continuous thin layer (5nm thick) it covers the whole surface and this layer is not soluble in the surrounding environment. It will act as a barrier to corrosion. This is what prevents stainless steels from corrosion. This thin layer as soon as it is damaged it will reform immediately, self-healing. Therefore, stainless steels are not resistant in all environments but they are corrosion resistant in many environments. An increase in popularity during the last few years.

We always have iron and enough chromium to form the passive layer. In a lot of stainless steels we do not want carbon. Carbon is kept to a minimal amount. If we have a high carbon content during high temp treatments carbon will react with chromium and therefore there will be a depletion in chromium leading to corrosion. Sensitisation effect: the depletion of chrome along the grain boundaries. There are some stainless steels were we do add carbon are called the martensitic stainless steels. The idea to add carbon is to produce martensite which is hard and gives us wear resistant. Chemistry in stainless steels effects what structure is going to form inside the material. By adding Nickel we would be able to produce steels with an austenitic microstructure even at room temperatures. Other elements similar to Nickel are called austenite stabilisers. Some other elements help to promote the ferritic structure at room temperature: ferrite stabilisers. There are stainless steels which are nickel free. The austenitic structure is obtained by adding other austenite stabilisers.

We etch the material by formulating a chemical ( a mixture of nitric acid and hydrochloric acid and water). As soon as the reaction starts we just rinse. The acid is in contact with the passive layer. The passive layer starts to dissolve. Once it is dissolved, ideally uniformly, the acid comes into contact with the surface of the material. The passive layer didn’t form again. In order to have a functional passive layer we would need to have a layer that is insoluble, self healing and covers the whole area. These attributes can be realised by adding enough chromium.

Stainless Steels Classification

Martensitic

Ferritic

Austenitic

Duplex(Austenitic and Ferritic)

Precipitation Hardened-Martensitic, Semi austenitic, Austenitic

The Iron Chromium Phase Diagram

We have the alpha phase which is ferrite(BCC structure) iron and chrome are in solution together. Delta ferrite is the ferrite that forms at high temperatures. The gamma phase is austenite where the structure is FCC. The sigma phase is an inter metallic which forms between iron and chromium. A very complex unit cell and is brittle. It is not desired because it will embrittle stainless steel. The dotted lines show the transformation which is very slow and sluggish. We limit chrome to less than 25% so that the sigma phase is limited to form. At a temperature less than 500 under the dotted line ferrite forms. Some of this ferrite is rich in chromium and some is rich in iron. The gamma loop is the area where austenite can form. The alpha plus gamma loop. Increasing the chrome will stabilise the ferrite. Other elements expand the gamma loop and the gamma plus alpha loop. These are called austenite stabilisers.

A steel with 12%chromium and a tiny amount of carbon, once you cool down to room temp we get martensitic stainless steel. The austenitic stainless steels are obtained by adding the austenitic stabilisers these expand the loop or shift the martensite start and martensite finish temperatures to lower temperatures.

Fe-Cr alloys containing 0.1%C :

the gamma loop is extended

the gamma plus alpha loop extends more than the gamma loop

Gamma will transform to martensite if the martensite start and martensite finish are above the room temperature, if they are below the room temperature the Gamma will not transform but remains as gamma. Alpha in both cases remains as alpha.

*In iron chromium slide from Fe-Cr alloys… line we are talking on the diagram on the next slide!*

The Iron Nickel Phase Diagram

Pure Iron has a ferritic microstructure at room temperature. As we start adding Nickel we start entering a phase field where the entire phase is austenite. It is promoting the formation of austenite. It is an austenite stabiliser. It will expand the gamma loop and the gamma+alpha loop. It will act to suppress the ms and mf temperatures.

Fe-Cr-Ni stainless steels - region of delta ferrite(horizontal line) and then region of martensite and ferrite(sloped line) *see slide

17%Chromium 0.1%C 0% Ni gives us 60% martensite

17%Chromium 0.1%C 2%Ni ferrite decreases and the hardness increases, more martensite is forming because the ms and mf are still above the room temp. At high temperature we have 100% austenite. the Ms is around 100 degC. On cooling some of the austenite transforms into martensite and some of it is retained. Therefore, we keep on increasing the nickel content to lower the Ms temp below the room temperature until the austenite that forms at high temp is retained at room temperature.

The Shaeffler helps to predict the microstructure at room temperature of stainless steels. Nickel equivalent vs. Chromium Equivalent. Used a lot for welding.

Stainless are designated using the euronorm, also by using the AISI and UNS. AISI: 3 digits those between 200 and 300 are largely used for the austenitic grades and the 400 series are used to designate both the ferritic and martensitic stainless steels. There are some designations which include letters as suffix. In the UNS designation the last two digits show modification of the basic grades.

ex: AISI 316L (L implies that it has low carbon content less than 0.3%) Sensitisation the formation of chrome carbides at grain boundaries which form localised corrosion.

AISI 316 LVM (VM implies vacuum melted)

AISI 316 LN (Low carbon contains Nitrogen)

For the UNS: AISI 316 = S31600

The 304 is the most common austenitic stainless steel. It has around 18-20%Chrome

and 8 to 10%Ni. It has good corrosion resistance to atmospheric corrosion. It is not very resistant to stress corrosion. Has limited corrosion resistance in environments that include chlorine. If we had to use 304 in place very close to the sea it is very likely that it would start to pit. A low pitting resistance. Pit is a localised type of corrosion. By adding about 2-3% Molybdenum it would form the 316 type of stainless steels. This has a higher resistance to pitting. It is also called the marine grade stainless steel. Adding more Molybdenum we increase the pitting resistance thus forming the 317 type. Molybdenum is a ferrite stabiliser so we add more nickel to keep its austenitic structure. When adding more Ni,Mo and N we form the superaustenitic stainless steels. The austenitic stainless steels are tough materials by having an FCC microstructure they do not display a ductile to brittle transition. One of the applications of austenitic stainless steel are to contain liquid nitrogen. They have significant amount of work hardening and are quite difficult to marine. From 304 to 316 the formability does go down and also the machinability tends to suffer.

By lowering the carbon content of the 316(which already has a very low carbon content) we make it more resistant to sensitisation. We go to a steel of less than 0.03%C . Used for welding applications. (304L, 316L, 317L) There is another way how we can reduce the sensitisation problem is by adding Titanium(321) or Niobium (347 includes both titanium and niobium) which have a higher affinity to carbon than chromium. They will react with carbon forming carbides and leave chromium in solution. It prevents chromium from reacting with carbon.

No Nickel results in ferritic stainless steels. With the 430 we can add more chromium and molybdenum to increase the corrosion resistance of the steel. Adding high chrome contents will embrittle the steel.

Adding Chromium and Nickel we increase the strength and oxidisation resistance. By adding high amounts of Ni we increase the corrosion resistance furthermore.

Sometimes Sulphur is added to make the steel more machinable. Se is added to form more globular manganese sulphide inclusions.

Another very popular type of stainless steels is the duplex stainless steel we get around 50% ferrite and 50% austenite. We need to have a composition which will result in the microstructure to fall in the alpha and gamma loop at high temp. The austenite has to be retained at room temp. They are high alloyed stainless steels. We need to increase the

Cr and lower the Ni to form a duplex stainless steel. Apart from their very good corrosion resistance they will attain twice as high the yield strength as their austenite or ferrite counterparts.

From 304 we can add Cu,Ti,Al but lowering the Ni content we can get precipitation hardened stainless steels. We can have a martensitic, austenitic and semiaustenitic structures. These steels we heat them and get all the elements in solution and then we cool them. These are machined and then age hardened. We get the formation of precipitates that will strengthen the lattice. The strongest are the martensitic precipitation hardened stainless steels, but these cannot be cold hardened. The austenitic precipitation hardened stainless steels are non magnetic. (check for more properties)

Adding manganese and nitrogen and lowering Ni form the 201 and 202. These are cheap and high in strength. They respond very well to work hardening.

The martensitic stainless steels are characterised by no nickel and low chromium. We can have martensitic stainless steels with different carbon content(403,410 and 420). Increasing carbon increases the hardness and strength but reduces the toughness. Patients who are allergic to nickel we use nickel free stainless steels. We usually use stainless steel that include manganese and nitrogen.

Ferritic Stainless steels are entirely composed of a BCC structure, entirely ferritic. They are also magnetic. Cannot be hardened by heat treatment. If they are exposed to high temp we start getting grain growth and this grain growth cannot be rectified by heat treatment. The only way to refine them is to cold work them and recrystallise them but this changes the shape. It has low strength compared to other stainless steels. They have low work hardening rate and usually they display a poor formability. They have low toughness specially at low temperature applications. Around 11-30%Cr we try to limit Cr to 25-27% because this will lead to problems. They do have a very good stress corrosion resistance.

The 409 contains around 11%Cr. We get also some other ferrite stabilisers normally a little Ti. Applications include: Automative exhaust systems

The 430 type stainless steel have a higher chrome content 13%.

The 446 contains over 25%Cr. Have a very high level of corrosion and oxidisation resistance.

Austenitic Stainless Steels their microstructure is austenitic, an FCC microstructure. As we cool down these steel will remain austenitic. They cannot be hardened by heat treatment. They usually contain low carbon content. They contain a minimum of 1618%Cr . They are non magnetic. They have a very good toughness. Their yield strength is slightly less than that of the ferritic stainless steels. Have higher cold workability and formability and higher degree of workability than the ferritic stainless steels. In general they have a good corrosion resistance but they have a problem with stress corrosion cracking. In order to make them less susceptible we increase the amount of Ni. To avoid pitting corrosion we usually add molybdenum. Superaustenitics are extremely corrosion resistance. Can be strengthen by work hardening. *see notes for different types of austenitic stainless steels

Martensitic Stainless Steels contains the least amount of alloying elements. We have to limit the amount of alloying elements in order to make sure that the Ms and Mf are above room temp. This makes them less expensive. These steels have moderate corrosion resistance nowhere as good as the austenitic cousins. High hardness depending on the carbon level and high strength. Good tensile and good fatigue properties. These are magnetic. The hardenability is very high so we do not need a severe quenching medium, air cooling would be enough. They are supplied in the annealed state. *check heat treatment of such steels Sometimes with these steels if they are used at high temp other elements are added to make the steel more resistant against tempering. Sometimes some molybdenum is added to improve corrosion resistance however this is limited. * see notes for applications

Duplex Stainless steels.The austenitizing temp would have an effect on the percentage of alpha and gamma. Will ideally contain around 50%alpha phase and 50%gamma phase in their microstructure. This makes them much stronger than austenitic or ferritic stainless steel. They typically contain around 20%Cr and 5%Ni. They are widely used in the chemical industry due to the high corrosion resistance. They are magnetic. Compared to the ferritic they have higher ductility and they display high formability mainly due to the alpha phase in their microstructure. Are not prone to stress corrosion cracking compared to the austenitic stainless steel. Very good corrosion resistance. They are used in a variety of applications esp. where we require good strength and good corrosion resistance.

Precipitation Hardenable Stainless steel(PH). We get certain elements sometimes Cu or sometimes a combination of Al,Ti and Nb with Nickel this makes it age hardenable, they

form very fine coherent precipitate result in very high strengthening of the alloy with minimal effect in the corrosion resistance.

Low carbon content, if we had to increase the carbon content would spoil their ductility and toughness. They are very strong and good corrosion resistance much better than that of the martensitic stainless steel.

Austenitic(highly alloyed, if we cool them the austenite does not form into martensite since the austenite is very stable).

Semi Austenitic(a microstructure of partly martensite and partly ferrite in the annealed condition, during aging we get the formation of precipitates and during cooling it will convert to total martensite).

Martensitic (are the stronger within this family and they have the least corrosion resistance and they are the least alloyed)

These PH stainless steels are divided into these 3 groups by their microstructure in the annealed state. These material can attain very high strength and can be machined in the solution annealed condition. During the aging treatment the amount of distortion is minimal. In terms of corrosion resistance they are moderate to good. They have lower Ni content therefore their stress corrosion resistance is high.

The Controlled Transformation stainless steels have a chemical composition that put the Ms and Mf is just below room temperature. Therefore the alloying element content is very important so that not to shift the Ms and Mf temp. In cases where we get tiny variations in alloy content during production we can adjust the heat treatment to shift the Ms and Mf line. This can be done by austenitizing or tempering. As we austenitize at high temp we put back all the alloys back into solution and this suppresses the Ms and Mf. We can encourage some alloys out of the solution and this is done by tempering. During the tempering cycle there would be elements which are not soluble. These form carbides and therefore result in shifting the Ms and Mf upwards.

Some forms of corrosion of stainless steels.

General corrosion(is the least detrimental)

Localised Corrosion

Pitting corrosion( often seen in stainless steel, Mo and N prevents it from happening, Manganese Sulphide tends to increase pitting corrosion by a lot, once it starts it continues getting worse)

Crevice corrosion(it occurs within crevices, can be avoided during design, initially we

start getting uniform corrosion and then oxidation reactions start to cur and then reduction reactions, within the crevice we get a self catalytic corrosion)

Inter-granular corrosion(occurs along the grain boundaries, a cause of such corrosion is sensitisation,)

Galvanic corrosion(when we have a metal in contact with a different metal in a solution, can be classified as uniform)

Stress Corrosion Cracking(is caused by an aggressive environment in combination with stress, it could be an internal stress in the material which resulted in the production, we can eliminate this stress by stress relieving processes, materials which are susceptible to such corrosion contain about 8%Ni)

Aluminium and its alloys

Are a very versatile class of materials. Al has a good electrical conductivity and good thermal conductivity. Pure Al has rather good corrosion resistance in air or in environments which are neutral or slightly acidic. Al and sea water are not compatible due to water being an alkali. Pure Al is rather weak in terms of strength, a yield strength which is less than 10MPa. In its pure form it is not good for structural applications. We add a whole range of different elements to achieve much higher strengths. Aluminium alloys are materials of high strength to weight ratio that makes them ideal in applications of weight saving. Its surface most of the time has to be protected this is done either by a powder coating(ex: polymer coating by manufacturer) or else its protected by anodising aluminium, we encourage aluminium to corrode thus forming an oxide layer.

Characteristics

It has an FCC structure. It is non toxic despite it cannot be ingested at high temp. It is non magnetic and non sparking(metals in contact can spark for example in a firework factory tools must be non sparking to avoid explosions). Low melting point(660deg C) The yield strength is very low. Aluminium alloys are less stiff compared to steels. Aluminium alloys are not good for high temperature applications.

The effect of composition, mechanical working and heat treatment on mechanical and physical properties.

There are a number of elements which are used in Al alloys to change their properties. The most common elements are those alloys which contain one or a combination of : Cu,Mg,Mn,Si and Zn. These elements have some solid solubility this increases with an increase in temperature. There are few elements which have solubility greater than 10% such as Magnesium, others have a solubility greater than 1% such as Silicon. Copper, Manganese and Silicon have a low solubility.

The Al-Cu eutectic phase diagram

Cu has a relatively high solubility but this solubility decreases with decreasing temperature. Cu is not soluble in Al below 200 degC. In order to obtain Cu and Al in solution it is heated to a temp where they are in solution and then quench. This results in a super saturated solid solution. Then we do precipitation hardening by heating to the right temp thus forming the intermediate precipitates. Not all alloys can be precipitation heat-treated. These are clusters of copper embedded in the microstructure that is coherent with the microstructure. This will cause significant strengthening. By heating the material below the GP solves line will encourage the copper atoms to start their process to form the equilibrium phase, but copper will not go straight to the equilibrium phase but it will go through an intermediate phase.

Strengthening Mechanisms

Solid Solution Strengthening- when we have other elements in solution with Al that are not of the same size of the Al atoms, this causes a stress on the Al lattice. This stress field will hinder the movement of dislocations within the material. By hindering such movement we will be increasing the yield strength of such a material.

Second Phase or Dispersion Strengthening- the CuAl2 will form a distinct second phase which is harder and hinders also the dislocation movement.

At elevated temp. the main strengthening mechanism will be solid solution strengthening and second phase.

At lower temperatures close to room temperature, those alloys which are not heat treatable turn to those alloys which can be strengthened by precipitation hardening while those alloys which are heat treatable turn to those which cannot be strengthened by precipitation hardening.

Strain Hardening only applies for wrought alloys.

Grain Size Reduction, reducing the grain size will produce more grain boundaries therefore more hindering to dislocations which would mean higher strength.

The Hall-Petch Relation - a smaller grain size a bigger yield stress. For aluminium the k

value is very small(0.07). When it comes to steels it is much larger(0.7). So the grain size effect on steel products is much bigger than the effect on aluminium and its alloys. Al alloys are less responsive to grain size reduction.

Grain size refinement by heat treatment can only be done for allotropic materials.

In non heat treatable alloys we can get an increase in strength by solid solution, second phase and grain size refinement.

Heat treatable alloys can be strengthened by precipitation hardening.

Strain Hardening is used especially for heat treatable alloys. Not all alloys will have the same increase in strength for a given amount of cold work. The Al Mg type alloys are very responsive to strain hardening. They can double their yield strength in the strain hardened condition.

Precipitation hardened alloys are the Al-Cu alloys, the 6 series alloys Al-Mg-Si alloys, the 7 series alloys which are composed of Al-Zi-Mg. These are the strongest alloys.

Grain Size hardening is hardening through grain size refinement.

Solid Solution Strengthening

Most effective solid solution strengthening are Mn and Cu. Mg has a reasonably high solubility in Al. Zi is the most soluble in Al almost 30%. The strengthening effect of Zi is rather weak compared to the other alloying elements.

Precipitation Hardening heat treatment would involve 3 steps:

1. Heat treatment to get the solute in solution by heating above the solves line but below the liquidus.

2. A rapid cooling to room temperature, it will ensure that that solute is retain in solution to get a super saturated solid solution.

3. To age the alloy, heating below the GP line and we get the formation of precipitates, metastable precipitates which are coherent with the matrix.

The lattice for an Al-Cu alloy.

The alloy is heated above the solves line all the alloys go into solution and then after cooling we form a super saturated solid solution. A super saturated structure obtained via this rapid cooling (Picture 1). The main strengthening mechanism is solid solution strengthening. We are going to heat the super saturated solid solution below the GP line, the solute will start diffusing within the lattice. In the case of Cu, the Cu atoms will start segregating together on specific lattice planes. We start getting a layer composed of single atoms which are clustered together. This clustering of the atoms together will result in a very high lattice stress. The precipitate is coherent with the matrix; all the planes are there, no dislocations. This produces a lot of stress because none of that strain is relieved for example through a dislocation. (Picture 2) If the alloy is subjected to longer time we get multiple planes.(Picture 3) If we hold the alloy for a longer time the precipitates will get bigger in size but some of the stress is relieved due to the formation of a dislocation. From 3 to 4 we start to observe a decrease in strength. (Picture 4) By 5 we would have lost all the strength due it not being coherent with the matrix and due to precipitation. The strength in picture 5 would be by second phase which is not strong.

Why is it important to heat below the GP solves line?

If we heat above the GP zone it will just immediately form the equilibrium precipitates. We heat below to go through the intermediate phases.

A 6 series alloy composed of Al, Mg and Si. (Al alloy 6061) The yield strength if this material at different duration of the precipitation heat treatment at different temperature. The higher the temperature (always below the GP zone), the quicker we get the maximum strength but the maximum strength that we get is lower than if the ageing heat treatment was carried at a lower temp.

Al alloys can be divided in to two:

Casting Alloys

Wrought Alloys

These two can be further divided into:

heat treatable - age hardened alloys

non heat treatable alloys

Wrought Al alloys

IADS divided these alloys into families. The first number indicates the major alloying element. If it is 1-Al, 2-Cu, 3-Mn, 4-Si, 5-Mg, 6- , 7-Al-Zi. The second number indicates the modification impurity. The last two numbers will give us the name of the number. For the case of the 1 series only, which is pure Al, the last two numbers indicate the Al impurity. If the last two numbers are 45 then the it is 99.45% pure Al.

The 1 series are mainly Al with some impurities. They have very low strength and can be increased a bit by strain hardening. These alloys have very high thermal and electrical conductivity. They have very good corrosion resistance in neutral or near neutral environments. These materials are used a lot in chemical equipment, reflectors, heat exchangers, packaging foil, and decorative trim. It is a single-phase material an FCC structure.

The 3 series Aluminium Manganese alloys. They are non-heat treatable. Mn has a very high solid solution effect. Can produce around 20% increase in strength when compared to the 1 series alloys. These materials are stronger than the 1 series, very good formability and ductility and excellent corrosion resistance. Used in applications such as highway signs. Architectural applications, and used for beverage cans-the cylindrical face. It can resist the internal pressure and can be formed into a thin sheet. The top part is made of a stronger material.

Aluminium Magnesium Alloys. (5xxx series)

Magnesium forms a solid solution with aluminium. Magnesium is very good to increase the strength by solid solution strengthening. Increasing the Mg to the alloys makes the alloys more responsive to work hardening. Their strength increases further by cold working. When subjected to work hardening the yield strength doubles. It also has very good corrosion resistance in marine environments. They are much stronger than the Al-Mn. It has good formability and weldability. The ductility is unaffected when adding Mn in solid solution. When we workharden the alloy we sacrifice the ductility. When these alloys are workhardened they tend to lose their strength over time. The dislocations undergo recovery process and some of the strength is lost. Are used in welding applications. They can also be used as plates for dump truck bodies. The problem is for those alloys having a weight of 5%Mg since at 65deg C they tend to have issues with corrosion since they start forming precipitation of Mg5Al8 along the grain boundaries.

Al-Si alloys (4xxx series) are very important for casting. In the case of wrought alloys they are less used. The problem with this alloy is that increasing the Si content above 12% will form proeutectoid Si crystals, which will embrittle the material. They are often used in the wrought form as brazing materials. Al-Si alloys are not heat treatable. Their strength is obtained from the solid solution of Si in the Al phase. Al-Si alloys

Miscellaneous Alloys (8xxx series)

Very important alloys are those alloys alloyed with tin. These are used in bearing materials.

Heat Treatable Wrought Al alloys

These are alloys that can be strengthened via precipitation strengthening. These alloys consist of 3 families: the 2, 6, and 7 series. Those that display medium strength but are readily weldable: Al-Mg-Si and Al-Zn-Mg. Those that display high strength: Al-Cu, Al-Cu-Mg, and Al-Zn-Mg-Cu, most of the time at the detriment of weldability and corrosion resistance.

The Al-Cu Series (2xxx series) can produce the highest strength alloys. Are much stronger in the precipitation-hardened condition. These alloys can be strengthened by precipitation hardening. These can be used in the solution treated condition; they can also be subjected to an artificial aging treatment. Can produce materials with very high strength to weight ratio. They are used mostly in the aerospace industry. Fatigue is the creation or the formation of small cracks resulting from the fluctuation of stresses. These alloys display good fatigue resistance. In the age condition have limited ductility and they don’t display very good corrosion resistance. Their corrosion resistance is inferior to pure Al or Al-Mg. These alloys have very limited weldability. The very important alloy is the 2024 sine they can produce very high strength aluminium parts. The 2219 aluminium alloy contains more Cu around 6%, it is usually supplied in a tempered condition- first cold worked and then artificially aged. This is an exception from the other alloys since it can be welded.

The Al-Mg-Si alloys (6 series alloys) are also strengthened via precipitation hardening. Compared to the Al-Cu alloys they don’t achieve the same level of strength (300MPa and even less). They can give us very good corrosion resistance, weldability and resistant to some other forms of corrosion attacks. They are used as body panels in certain car models on the outer side of the car. They are used a lot for the production of bicycle frames. These are alloys, which are strong, corrosion resistant and are responsive to anodizing treatments.

The Al-Zn-Mg alloys (7 series alloys) are heavily utilized in the aerospace industry. The 7075 alloys contain around 6%Zn and a considerable amount of Cu as well has a very good fatigue resistance. They have very limited weldability and inferior corrosion resistance when compared to other alloys. These alloys can naturally age at room temperature. They won’t require a heat treatment. Also these alloys don’t require a quench as severe as the other Al alloys in order to get a solid solution condition. Sometimes these alloys are refrigerated after annealing in order to prevent natural ageing.

Cast Aluminium Alloys

They have very good fluidity; the melt has a very good ability to fill intricate details in the mold. Hydrogen gives us problems with the solubility; this can be controlled with certain processes.

The designation of cast Al-alloys (1xx.x, 2xx.x, 3xx.x etc.)

Cast Alloy Systems

They cannot be strengthened by strain hardening. They are divided into alloys, which are heat treatable, and those, which are non-heat treatable. The alloys with the best castability, the addition of Si is preferred.

The 2 series are based on Al and Cu with some tiny amounts of Si. They have generally poor castability and are not very popular. We can subdivide them in groups based on their Cu contents. These alloys are very difficult to cast especially when the casting geometry is difficult in shape. They have very low fluidity. They are not very responsive to heat treatment via precipitation hardening.

The 3 series is more widely used then the 2 series. It’s a system where it contains significant amounts of Si that improves the castability. They are also heat-treated; one can obtain castings that are more complicated. The Si phase gives us better fluidity.

The 4 series is the Al-Si system. The principle-alloying element is Si. Are used extensively where we need very good castability and corrosion resistance. They have very limited ductility and a way to improve this ductility is by modifying the eutectic phase. This is done by adding Sodium in the hypoeutectic phase and Phosphorus in the hyper eutectic phase.

The 5 series is the Al-Mg casting alloys. These alloys are used as cast alloys when really needed in the cast condition. In applications where we require high corrosion resistance, Al-Mg cast alloys give us a leading etch. It is also suitable for welding applications.

Al-Zn-Mg Alloys (7 series) compared to the AL-Si system have limited castability. The main advantage of these alloys is that these alloys can achieve very high strength and these alloys can age naturally.

Copper and its Alloys

Copper out of the commercial metal has the highest thermal conductivity. It also has a very low electrical resistivity. Pure copper is very ductile: can be joined by a number of techniques, can be formed and it is can be fabricated rather easily. It is a metal, which can be easily soldered. It is much more expansive than steel. Copper has a very good corrosion resistance in different environments. It is used in marine and chemical environments. It is also used for decorative purposes due to their pleasant appearance. Copper alloys are also used for the production of statues. It forms a protective oxide that changes colour over years. Copper is very strong in fact it can also be used unalloyed. Pure Copper can have its yield strength increased considerably by cold working. Cold working has a big effect on the tensile strength, it can double the tensile strength of copper from 200MPa to 400MPa. There are copper alloys, which can undergo other strengthening mechanisms like strengthening by solid solution. In hazardous places such as facilities dealing with explosive gases, the tools have to be made of material, which do not spark. It is heavier than iron, its density is greater than that of steel. Copper is not often used as structural material. When we alloy copper usually we will sacrifice electrical conductivity. Various impurities can be found in copper as well. Copper and its alloys are used in a variety of applications.

They have a very good corrosion resistance. There are some copper alloys, which do suffer from certain degradation mechanisms such as Hydrogen embrittlement: Copper contains a significant number of oxygen thus we get copper oxide. This hydrogen, which forms at the surface, can react with the copper oxide in the material to reduce that oxide and the byproduct would be water (hydrogen+oxygen). This water will exert pressure in the material resulting in embrittlement in the material. Copper undergoing welding procedures must be free of dissolved oxygen. Another way is to add elements to react with that oxygen for example we add phosphorous. Phosphorous will react with the oxygen and won’t let the hydrogen embritlement to take place. Tough pitch coppers contain a minor amount of Oxygen and thus make them susceptible to this degradation phenomenon. Stress corrosion cracking is a type of corrosion, which originates from stress: residual stress or tensile stress in the material or due to chemical corrosion environments such as ammonia. This causes cracks in the material, which will eventually lead to the failure of the material. Also called as seasonal cracking. Annealing can solve this problem given that no tensile loading is subjected to the component. Another mechanism is dealloying, which is mainly associated with brasses. The copper and the zinc have very different nobility; zinc is very reactive while copper is very noble. In dealloying, the zinc will be preferentially corroded from the material.

Electrical and Thermal conductivity are two characteristics of copper. We have to be careful with the impurities and the alloying elements since they affect the thermal and electrical conductivity. In oxygen free coppers the impurities have a more detrimental effect on the thermal and electrical conductivities. When conductivity is the prime quality that we want then pure copper is the best selection.

If we had to look at pure copper as soon as it is polished it has a pinkish colour, then it starts to turn into a brownish colour due to the oxide that forms. The brownish colour continues to darken due to the conversion of copper oxide to copper sulphide. Eventually, the copper sulphide will change into copper sulphate resulting in a greenish colour: patina.

Copper and its alloys if you look at them as a family are very easy to fabricate. There are certain alloys which can be cold worked very well and other which can be hot worked very well, and some alloys, which have a good castability. In cold work, we can adjust the annealing cycle to control the mechanical properties for example the grain size of the material. There are certain copper alloys which can be strengthened by precipitation hardening. Also there are certain alloys, which can be heat treated similarly to steels: heating, quenching and tempering.

Wrought alloys include:

Coppers, which have a copper content around 99%. High copper alloys, which have less than 2%, copper alloys. In brasses, copper and zinc are the two main elements. Sometimes we add tin to

reduce dezinfication. Phosphor bronzes and leaded phosphour bronzes. Nickel Silvers have a silvery appearance mainly composed of Copper, Nickel and

Zinc.

In the cast alloys we get a high concentration for bronzes.

Copper family can be divided into:

Coppers

The first family is the coppers. These contain a very high amount of copper. They have less than 0.7% of impurities. They are mainly renowned for the good conductivity and high corrosion resistance. They can be easily joined and they have good soldering characteristics. These coppers are again subdivided into 3 categories: the oxygen free copper, the tough pitch copper and the phosphorous deoxidized copper. All of them have a copper content greater than 99%.

The Oxygen Free Copper is free from dissolved oxygen and free from impurities. We do this be forming a copper anode which is already partially defined by a melting in environments which limit certain impurities. These anodes are placed in a bath usually in a solution

containing a copper sulphate solution, which is acidic. We connect the anode to a positive terminal of a power supply. The negative terminal is the cathode. Copper metal will start dissolving and the impurities will form at the bottom. The copper cathode formed is turned into a liquid to form ingots in a controlled environment. We form an ingot, which is free of oxygen. Copper, which is produced this way, is used in electric conductors and heat exchangers. It is a copper, which is not susceptible to hydrogen embrittlement. It has the best electrical conductivity. It is important to limit any other impurities since this has a very negative effect on conductivity.

The tough pitch coppers can be obtained by fire refining. These coppers are not suitable for welding and brazing because they can get susceptible to hydrogen embrittlement. However, they can still be used for electrical conductivity purposes.

The phosphorous deoxidized copper, where the phosphorous reacts with the oxygen present. The removal of dissolve oxygen prevents gassing and porosity.

High Copper Content Alloys generally content less than 4wt% alloying elements. We add tiny amounts of elements that have little effect on the conductivity but they strengthen the material. Silver copper contains a small amount of silver. This silver does not affect the conductivity. We add silver primarily because it raises the annealing temp by around 100 deg C and by doing so it enables this copper to be used at high temp without losing its properties. Another alloy is cadmium copper, which contains around 0.5-1 % cadmium content. The higher the cadmium content the lower the conductivity and the higher the strength. Mainly used for electrical conductivity purposes. This increases the strength considerably which goes above the 500MPa. It is stronger than oxygen free copper. It is also very resistant to fatigue failure. It is used a lot in aircraft wiring and electrified railway systems. It also raises the annealing temperature without losing its properties. The negative side of cadmium copper is production; melting results in toxic fumes. Another high copper content alloy is chromium copper. We add tiny amounts of chromium. We reduce a little bit the conductivity by at most 15%. It is added for conductivity purposes. This alloy can be precipitation strengthened. It can be heat treated where we get clusters of chromium copper. It can be used in welding electrodes for spot welding and seam welding. Chromuim copper also improves the corrosion resistance of the material. Tellurium copper is another high copper content alloy. Tellurium and copper form compounds that facilitate the machining of the material. The conductivity is affected negatively but by a very small amount. It still makes the copper corrosion resistance. Beryllium copper is another high copper content alloy that usually contains somewhere around 2% Beryllium. Such alloys are not meant for electrical conductivity purposes. However, these alloys are very responsive to age hardening. During age hardening we get clusters of Beryllium that continue to get bigger till the gamma phase and through the intermediate phase we get a lot of coherency strain and therefore a very high strength. It is the strongest among the high copper content alloys. It is used in applications where we require high strengths, hand tools. There are certain beryllium copper alloys that are used in applications were electrical conductivity is needed; these would have a beryllium content of around 1.7%.

Brasses

Used to produce coins, jewellery, ammunition, and hot shaped products. The properties of the brass are very dependent on the zinc content. The alpha brasses contain one phase and a very soft and ductile. There are brasses that are less ductile and stronger these are called the alpha and beta brasses. Gilding metal is an alpha brass that contains small amounts of zinc and is very soft especially in the annealed state. It is usually used as caps for ammunition. There are some brasses that are sometimes referred to bronzes. These include the commercial bronze, jewellery bronze or rich low class brass. Red Brass and Low Brass are also alpha brasses. A very popular brass is the cartridge brass it is the most ductile out of the all the brasses. Because of this characteristic it is mainly used as sheets and it also used in cartridge bullets. Increasing the zinc content increases the ductility and strength in brasses up to 30% zinc. It is susceptible to stress corrosion cracking. By adding small amounts of tin will make the alloy resistant to zincification. The higher the zinc content the yellower the brass. Yellow brass is suitable for most engineering processes. It has a ductility that is grater than 30% therefore the ductility starts decreasing. The Basis Brass has zinc content of 37%, it is at the borderline between the alpha brasses and the alpha beta brasses. Alpha brasses are cold worked. The basis brass can be cold worked and hot worked as well. An alpha beta brass is the muntz metal it has a much lower ductility and cannot be cold worked. However, it has excellent hot work characteristics. Is susceptible to zincification and therefore we add about 1% tin. This is usually referred to as naval brass.

Mechanical properties of brasses

The low brasses are extremely ductile since ductility increases with the zinc content up to 30% zinc. The high brasses are stronger they contain a higher amount of zinc content. The alpha beta brasses are high brasses.

The copper zinc phase diagram

The copper and zinc exist together as one solid solution up to about 36% zinc. The brasses, which are used commercially usually, contain up to about 45% zinc or less.

The beta prime phase in alpha beta brasses reduces the cold ductility. The beta phase at high temperature will make the material suitable for die casting without problems of cracking upon cooling. These brasses are stronger than the alpha brasses because they have the beta phase and they have a low corrosion resistance when it comes to zincification.

Lead is added to brass to increase the machinability. Sometimes we also add some tin and tiny amounts of arsenic to make the alloy more corrosion resistant. Alloys with high zinc content are more suitable to marine applications. Aluminium is also added sometimes to increase the corrosion resistance. Brasses can also be cast into products.

Tin bronze is an alloy of copper and tin together with a deoxidizer. Phosphor Bronzes are alloys of copper, tin and phosphorous. The phosphorous has an effect of acting as a deoxidizer, it stiffens the material and it further reduces the friction of the alloy and hence

improves the wear resistance. Bronzes, which are higher in tin content, are usually deoxidized with tin and they are more suitable for casting products. Bronzes unlike brasses can produce very good castings with no porosity

Aluminium Bronzes are used in applications where good corrosion resistance is required. These bronzes up to around 9% al give us single-phase bronzes, which are ductile. They can be heated similarly to steel. Heated into the beat phase, quenched and then tempered.

Silicon Bronzes

Copper Nickels, Nickel Silvers are very good corrosion resistant.