Materials Assignment Comparing Material Properties Engineering Essay

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Steel is by far the most commonly manufactured metal on earth. There are a huge number of different types of steel, with extensively varying material properties and manufacturing costs. These include spring steel, which has a huge tensile strength allowing it to be used in all kinds of springs; stainless steel, which is alloyed with Zinc to almost eliminate oxidisation, and high temperature steel which has a melting point of over 1500°C.

One of the most common types of steel is carbon steel, this is where carbon is added to the steel to make an alloy. Low carbon steel, (approximately 0.16-0.29% carbon) also known as mild steel, is the most common and cheapest form of carbon steel. It has a relative low tensile strength but is cheap and malleable, making it an ideal material for most applications. Medium carbon steel (approximately 0.3-0.59% carbon) is stronger and has better wear resistance than mild steel, but is less ductile. This is commonly used in forging and casting and to make automotive parts such as chassis. High carbon steel (approximately 0.6-1.0% carbon) is very strong and commonly used to make springs and high-tensile wires. Ultra-high carbon steel (>1.0% carbon) is extremely hard wearing, is very strong and has excellent impact resistance, however, it can be slightly brittle. It is generally used in special purposes such as engine cam shafts, punches and cutting surfaces like drill and lathe bits.

Most variations of steel are available in high or low grades depending on the required performance and cost limitations. (Taylor & Francis, 1996)

Grey Cast Iron

Grey cast iron is the most common type of cast iron, the name grey cast iron comes from its colour along a fracture surface. It is also known as flake graphite cast iron because of the flaky texture of a fracture surface and the fact it is categorized by its graphite microstructure.

Grey cast iron consists of 2.5-4% carbon, and 1-3% silicon with the remainder being iron. In general it has less tensile strength and shock resistance than steel, however, similar compressive strength and thermal performance whilst being considerably cheaper than steel.

Below are two microscope images showing the microstructure of grey cast iron.

Figure 1. Grey cast iron, Fe-3.2C-2.5Si wt%, containing graphite flakes in a matrix which is pearlitic. The speckled white regions represent a phosphide eutectic. Etchant: Nital 2%

Figure 2. Grey cast iron, Fe-3.2C-2.5Si wt%, containing graphite flakes in a matrix which is pearlitic. The lamellar structure of the pearlite can be resolved, appearing to consist of alternating layers of cementite and ferrite. The speckled white regions represent a phosphide eutectic. Etchant: Nital 2%

One of the main advantages of cast iron is its exceptional surface finish after casting and other moulding techniques. This means iron can often be used straight out of the casting mould where as alternative materials such as steel will have to under-go a second finishing procedure to improve its surface quality before the item can be used. This secondary finishing process means that although the initial material cost of steel can be similar to that of iron the final production cost could well be higher.

(http://www.msm.cam.ac.uk/phasetrans/2001/adi/cast.iron.html (11th Jan 2010))

Aluminium

Aluminium is very widely used in the automotive and aeronautics industries, its the most widely used non-ferrous metal in the world. Manufacturing aluminium is a two phase operation, starting with the Bayer process of refining bauxite ore to obtain aluminium oxide. This aluminium oxide is then used in a smelting process to release pure aluminium. Most aluminium is then tempered in order to toughen the material improving its strength and other properties.

Aluminium has grown hugely in its application and uses over the past 30 years due to its many useful attributes. The main strong point of aluminium is its very high strength to weight ratio, which makes it an ideal structural material for vehicles; where reducing weight is always important to improve fuel efficiency. For this reason it has been used extensively in aviation to build aircraft fuselages and structural compartments. More recently it has started to be adopted into car design; being used to build car bodies and chassis for the more expensive high performance cars. In both cases this is owing to the reduced weight of aluminium reducing the overall weight of the vehicle, in turn reducing the vehicles inertia and consequently improves the vehicles fuel efficiency. In these cases the financial saving of the increased fuel efficiency outweighs the increase in initial manufacturing costs, as aluminium is around 25-50% more expensive than steel. (http://www.madehow.com/Volume-5/Aluminum.html (Jan 9th 2010))

Titanium

Titanium in generally considered to be 'best' metal available at the moment. This is because its extremely strong, having a yield and tensile strength of approximately 25% more than steel, whilst also having around half the density of steel, making it much lighter. Like aluminium this means titanium has a very high strength to weight ratio which makes it ideal for use as a structural material in vehicles. The only reason titanium is not widely used in industry is its cost, titanium is around 17.4 times more expensive than carbon steel. So if a car chassis cost £2000 to manufacture from carbon steel, is would cost approximately £34,800 to manufacture from titanium (not including the additional moulding costs). For this reason titanium is only used for certain critical components where only small quantities of the material are required. Such as hinges and locks on aeroplane fuselages as well as cams and cam-followers in high performance car engines. Formula 1 engines are built entirely from titanium but cost around £500,000 each. Below are two microscope images showing the microstructure of titanium.

Figure 3. The typical microstructure of Ti-6V-4Al wt% alloy, cooled from the α phase field to produce Widmanstätten β.

Figure 4. Diagram showing the face centred cubic atomic structure of titanium.

(http://www.msm.cam.ac.uk/phasetrans/2004/titanium/titanium.html (Jan 8th 2010))

Properties Comparison

The table below directly compares the material properties of Cast Iron, Aluminium, Cast Steel and Titanium.

 

 

Cast Iron

Aluminium

Cast Steel

Titanium

Property

Unit

GGG-40

G-AlSi7Mg (Thixo)

HA

Ti-6V-4Al

Young's Modulus

MPa

167000-170000

72000-73000

200000

116000

Tensile Strength

MPa

370-390

230-250

740

900

Yield Strength

MPa

250

190-200

660

864

Thermal Expansion

e-6/K

10-11.5

21-22

13.5

8.6

Thermal Conductivity

W/m.K

25-42

160-170

26.2

21.9

Melting Temperature

°C

1150-1250

557-582

1510

1725

Service Temperature

°C

-100 - 400

-50 - 450

 

 

Density

kg/m^3

7100-7300

2650

7700

4506

Estimated Cost

 

Low

Moderatly High

Moderate

Extremaly High

Figure 5. Table of comparison showing the material properties of Cast Iron, Aluminium, Cast Steel and Titanium.

As the Table shows, titanium is extremely strong while also having half the density of cast iron or steel, however, its estimated cost is extremely high although titanium also has exceptional thermal performance. Cast iron and cast steel are quite similar, apart from steel being stronger and more expensive, however, the table does not show details of the casting nature of the materials, where steel has a much poorer surface finish after casting than iron. As shown, aluminium is by far the lightest having half the density of titanium and a third the density of steel or iron. It is however also the weakest but its thermal conductivity is excellent making it a good material in a high temperature environment. (http://www.matbase.com/index.php (Jan 8th 2010))

Why Cast Iron and Aluminium are used in Cylinder Heads, rather than Steel or Titanium

Cast Iron (ASTM A48) has been used to make engine cylinder blocks ever since the car was invented. It was used originally as it was the only viable material that was available at the time. Its extensive use in other industry such as blacksmithing meant that is was cheap and widely available. It was also easy to cast and had excellent material properties for engine manufacturing. These properties include:

High tensile and yield strength.

Excellent wearing resistance.

Very low thermal expansion.

Very high melting point.

High quality surface finish from casting.

The main downside of using cast iron in engine cylinder heads is generally considered to be its extremely high density (7100-7300 kg/m^3) resulting in the engine being very heavy. This excess weight can drastically reduce a cars overall performance. For this reason the majority of modern engines are being build from cast aluminium. Aluminium has a much lower density than cast iron (2650kg/m^3) while still having a reasonable high tensile and yield strength, meaning an aluminium engine can be built much lighter than a cast iron one.

Modern HGV engines and engines used in heavy machinery, cranes, etc are still typically made from cast iron. This is because they are very heavy duty diesel engines running at very high compression ratios. Which means the engines cylinder head is under a much higher load than a typical petrol engine. Therefore, in this case the extra yield strength of cast iron is favourable over aluminium, and the lower weight of aluminium is less important. Performance diesel engines used in modern sport saloon cars are often built from aluminium. As in these performance vehicles the difference in weight will have a large effect on the performance of the car, and the additional cost of the material and engineering design are more justifiable.

An experiment was carried out by CarCraft.com on a Chevrolet small block 6.3ltr V8 engine to compare the difference in performance between a cast iron cylinder head and an identical aluminium one. The experiment involved changing no parts of the original engine apart from the cylinder heads and head gaskets. The engine was then dyno tested and the ignition timing was optimised with each cylinder head. To test the thermal performance of the two cylinder head materials the engines coolant water was regulated at 43°C and 85°C. The two cylinder heads ran using the same 91-octane and then 118-octain fuel and were tested under identical test conditions and under the same loading sequence.

The conclusion was that the torque, power and temperature dependency of the both cylinder heads was identical throughout the whole of the engines rev range. At 43°C (coolant temperature) the engine with either cylinder head produced 502hp and 481lb-ft of torque, and at 85°C the engine output 483hp and 461lb-ft of torque.

This shows that the material used to manufacture the cylinder heads has no direct effect on the engines performance, however, it also shows that a very hot engine does not perform as well as a cooler engine. Suggesting that in real life situations a cylinder head that is able to disperse more heat will result in a less powerful cooling system being required and accordingly improve the overall performance of the engine. Aluminium has a much higher thermal conductivity than cast iron (160-170W/m.K compared to 25-42W/m.K), meaning an aluminium cylinder head will disperse more heat. In addition, the much lower density of aluminium means that an aluminium cylinder head will be much lighter than a cast iron one, and as a result further improving the cars overall performance.

The downsides of using aluminium to build cylinder heads is that its more expensive and has a larger thermal expansion than cast iron, whilst also having a lower quality surface finish after casting. However, with modern engineering techniques and in the modern economic climate all of these problems can be addressed.

The larger thermal expansion of aluminium is usually compensated for by engineering the clearances in a cylinder head to be slightly smaller than if it was to be made from cast iron. This means that as the engines temperature increased after its started running the cylinder block expands and the clearances reduce to there optimum size.

The lower surface quality after casting a cylinder head from aluminium can be corrected by carrying out a finishing process; this will reduce the excessive wear brought on by a poor surface finish at the same time as increasing the engines efficiency, this will however increase the final cost of the engine. Nonetheless in the modern climate of eco-friendliness, green cars and a substantial increase in fuel prices a more expensive but cheaper to run car is often very desirable. So the reduction in weight and thus increase in fuel economy brought of by an aluminium engine often outweighs the additional manufacturing costs.

Steel is rarely used to manufacture engine cylinder heads simple because it offers no significant advantage over cast iron other than its strength (cast steel being around 50% stronger than cast iron). Steel also has very similar thermal properties to cast iron but it heavier and much more expensive. Furthermore steel doesn't cast as well as iron meaning it has to undergo a finishing process to improve its surface quality, but steel is harder to machine than iron due to the burring. This means that the overall manufacturing costs of a steel engine are very similar to that of an aluminium engine, and the aluminium engine will be much lighter and have significantly better thermal performance.

Titanium is generally considered to be the ideal material for building a cylinder head. Its tensile and yield strength is in the region of three times that of cast iron or aluminium. Its thermal expansion is lower than cast iron and less than half that of aluminium. Its melting temperature is much higher than either alternative and yet its density is only slightly higher than aluminium. Meaning titanium is much stronger, less effected by heat, and lighter than either aluminium or cast iron. The downside of titanium is that it is very expensive to produce and difficult to mould, this limits its use to high budget race teams such as Formula 1 where the performance gain is vitally important and the additional cost is justifiable.

The future of engine manufacturing could very well be in ceramics. Zirconia ceramics can operate at extremely high temperatures (up to 800°C) while also containing the heat within the engine. This increased operating temperature allows the fuel to burn more fully therefore outputting more energy per unit of fuel, while the very low thermal conductivity reduced the energy loss due to heat transfer. Zirconia ceramics are still in the early stages of development meaning they are unlikely to be used in engines in the near future, nevertheless, as there development continues they may offer a viable manufacturing alternative in high performance engines.

(http://www.carcraft.com/techarticles/ccrp_0602_iron_versus_aluminum_cylinder_heads_test/dyno_results.html (Jan 9th 2010). http://www.ultrahardmaterials.co.uk/engine.html (Jan 7th 2010). http://news.bbc.co.uk/sport1/hi/motorsport/formula_one/7718682.stm (Jan 8th 2010)

Taylor & Francis, 1996)

Cylinder Head Manufacturing Processes

Mass Production Sand Casting

When mass production of engines is required, automated machines carryout most of the manual work. First glue and hardener are mixed with Zircon sand and the mixture is blown into a master mould which is made of iron, a gas is then injected into the mould to activate the hardener, this solidifies the mixture. Once all seventeen sand moulds have been formed they travel down an assembly line where they are assembled into a full cylinder head mould. After assembly they are coated with talcum powder, this prevents sand particles from sticking to the aluminium and getting into the engines oil. Meanwhile, aluminium ingots are melted in a gas furnace. The sand moulds are filled with molten aluminium from the bottom up to avoid contaminating the metal with aluminium oxide. This is caused by the metal coming into contact with the air, filling from the top would allow the metal to come into contact with oxygen and subsequently the pouring motion itself would mix in the oxide, causing impurities.

After moulding the casts spend six hours in a sand reclaim oven which breaks down the glue so the sand simply falls away, the heat also strengthens the metal. Robots then turn the engines over to spill out any loose sand. Large milling machines then saw off the risers; these contain extra metal that is fed into the mould cavity to compensate for the 7% shrinkage that occurs when the liquid aluminium solidifies. A machine then mills the surface of the cylinder head ensuring a smooth finish for the head gasket. Every cylinder head then passes through an automated vision system for quality control. (How It's Made 2008)

High End, Small Quantity Sand Casting

Engines used in some high end sports cars such as Ferraris and Lamborghinis are almost entirely hand made. One new set of casting moulds are made for every individual engine out of compacted sand and resin. This mould is then removed from its pressing machine and its surface finished by hand, taking around five hours to sand down the moulds and scrape off any deformities. These sand moulds are then fitted together on a jig and sprayed with a this layer of carbon which acts as a lubricant allowing the metal to flow more smoothly. Moulton aluminium alloy (of a highly confidential chemical make up) is then poured into the moulds. After seven minutes the aluminium has set hard enough for the moulds to be removed. They are removed by a simple shaking process which crumbles the sand revealing the cylinder head. Each cylinder head is then x-rayed to check for any microscopic cracks or flaws in the metal. It is then policed by hand and visually checked again before being used in the cars. (Megafactories 2006)

CNC Machining

One off manufacturing of cylinder heads for racing or testing purposes is carried out using CNC machining, or computer controlled machining. The cylinder head is first designed in a CAD system and then loaded into the CNC software (e.g. Transcut). This then generates the CNC code in XYZ coordinates, each port in the head requires approximately 80,000 lines of code. The code is then loaded into the CNC machine and the incremental stopover is set to around 0.5mm to ensure a near perfect finish. Once this is set the CNC machine can start machining the head, this can take several hours depending on the complexity of the design. Once complete the head requires no further processing as the surface finish is almost perfect. This manufacturing method can provide much higher quality cylinder heads than any form of casting, however, the process is extremely expensive and very time consuming. For this reason only Formula 1 teams and other high end motorsport teams use this method for engine manufacturing. (http://www.cncheads.co.uk/index.html (Jan 11th 2010))

Composite Wing for Formula 1 Car

Manufacturing

What is Carbon Fibre?

Carbon fibre is a cloth material comprising of many featherweight strands, containing mainly carbon elements. This cloth can be made rigid when imbedded in an epoxy resin. There are varying grades of carbon fibre for numerous different purposes. It is resistant to corrosion, fire and has high stress tolerance levels in addition to being chemically inert. The structure of carbon fibre, chemically, is made up of extremely thin fibres around 0.005mm in diameter of mostly carbon atoms, all arranged in a parallel pattern, a close up x-ray of a carbon fibre can be seen below in figure 6.

Figure 6. X-Ray to show the Chemical Alignment of Carbon Fibre in a Cross Section.

(Figure, http://www.chem.wisc.edu (Jan 12th 2010))

Several thousand of these thin threads are spun to make a yarn and from that a sheet of carbon fibre is woven. There are many different weave patterns such as single and double cross along with linear weave, there are also different resins including epoxy and polyester resin, plastic may also be added to form varying compounds of carbon fibre. It also has a very low weight and high rigidity making it ideal for the intended purpose, where as years ago steel, fibreglass or wood would have been the optimal material. The table below compares the rigidity and density of carbon fibre and steel, it can be seen that carbon fibre is almost three times more rigid than steel; yet 4.5 times less dense.

Material

Tensile Strength, GPa

Density, g/cc

Specific Strength MN*M/kg

Carbon Fibre

3.50

1.75

2

Steel

1.30

7.90

0.17

Figure 7 Table Comparing Material Properties of Carbon Fibre and Steel

In Formula 1, cars can reach speeds of 200mph and are subjected to centrifugal forces that can give momentary weights of up to 2.5 tones. It is due to carbon fibre that the car does not suffer structural failure under this relentless and repeated treatment; this demonstrates how strong a compound carbon fibre is. In fact, when properly treated and manufactured carbon fibre can be three times stiffer and seven times stronger than the equivalent mass off aluminium.

Epoxy resin is just as important in the manufacture of carbon fibre as the material its self. Epoxy resin is extremely flexible, which allows the product to absorb a high level of impact force without breaking, it does not shrink and is 100% UV resistant, in addition it will not "spider crack" - once the epoxy reaches its maximum bending potential (MBP) it will form a single crack at the maximum stress point,

(www.Carkeys.co.uk (Jan 12th 2010), www.autospeed.com/CMS/A_108673/article.html (Jan 12th 2010), T. Gutowski, John Wiley & Sons, 1997. Smith (2009))

General Manufacturing with Carbon Fibre

Carbon fibre objects are made up of layers of the sheet material, aluminium honeycomb sheets and resin. Firstly, a computer based finite element modelling system is used to decide on an exact design and a mould is created on the basis of those calculations. However, the actual calculation of how many sheets of the laminated 'sandwich' aluminium honeycomb and carbons composite skins are used is done by hand. The combination is then heated in an autoclave for several hours and then trimmed and supplied for final assembly. Lola Racing which is one of the largest manufactures of carbon fibre components, list their manufacturing process online as: "Unlike steel, carbon fibre has a non uniform structure so by orientating the pieces of carbon fibre in particular directions, it is possible to direct externally applied forces across a wing, thereby dissipating them. Within the product, 2 layers of carbon fibre actually sandwich a layer of aluminium honeycomb core material, which produces a very strong, rigid and light weight structure." - www.lolaracing.co.uk (Jan 12th 2010).

When the carbon fibre is layered into the mould it is vital that the carbon fibre follows the exact contours of the mould and that the fibre is orientated in the pre-determined directions, in order to introduce the exact structural requirements of the specific panel. In addition, it must be ensured that no pockets of air or other imperfections are introduced. Research suggests that the industry standard method is to use a total of five layers to form the outer skin of a part in order to achieve an approximate cured thickness of 1mm per layer. The process is as follows: once the initial layer of fibre is applied and cured (as described above), the part is allowed to cool before a layer of pre-cut aluminium honeycomb core material is placed on the outer skin. A sheet of resin is applied between the outer skin and the aluminium to ensure a strong bond that will endure the extreme conditions the cars are subjected to. The part is then returned to the autoclave for further curing. This process is then repeated to build up the required amount of layers usually four or five.

(www.chm.birs.ac.uk (Jan 12th 2010). www.autospeed.com (Jan 12th 2010). T. Gutowski, John Wiley & Sons, 1997. Smith (2009))

Manufacturing Costs

The cost of carbon fibre is on average £4 per lbs, with a wing requiring around £1500 worth of raw material. From direct discussions with Lola Racing labour and tooling is charged at around £60 an hour, with the average wing (not a Formula 1 wing) requiring around 100 hours of work to produce a completed part. This would include the production of a mould, moulding of the carbon fibre material, and production of a finished part. This would result in a material cost of £1500 and a labour cost £6000. However, a Formula 1 wing will be more complex in design and have to be built to much higher standards. Lola estimate that the manufacturing cost of a single Formula 1 rear wing to be in the region of £30,000 to £40,000. This does not include the additional cost of any motorised or otherwise movable elements in the wing.

(Telephone conversation with Lola Racing (11th Jan 2010). Smith (2009))

Formula 1 Wing Construction

Once the complete formula 1 wing has been designed using CAD and CFD analysis the manufacturing process can begin. This starts with the mould being built from CNC machined aluminium, though expensive this gives the most consistent reusable moulds. The moulds are the exact shape of the reverse of the wing, so when the 2 moulds are placed together the void in the centre is the shape of the wing. The wing moulds are then coated with a lacquer and then a very thick coat of wax, this allows the carbon fibre to be separated from the mould after curing.

The first layer of carbon fibre to be applied will be the outer layer when the wing is completed. A thin layer of epoxy resin is applied to the surface of the mould, a layer of plain weave carbon fibre is then applied and more epoxy resin is poured on top. The material is then pressed into place and an aluminium roller is used to force out any air pockets which will cause a structural defect if not removed. The next layer applied is either satin weave or unidirectional carbon fibre, this gives the wing the strongest possible bending rigidity. A total of 4 layers are applied to the upper surface of the wing and between 7 and 9 layers are applied to the lower wing surface.

The core of the wing is made of aluminium honeycomb, this is extremely light and acts to hold the upper and lower surface of the wing apart when the aerodynamic forces are applied, this maintains the wings structural rigidity. The aluminium is cut to shape and then the surface is hammered in order to fold over the honeycomb creating a large surface area for the adhesive to bond to.

Once all layers of carbon fibre have been applied to the mould of the wings lower surface the aluminium honeycomb is pressed into place. Meanwhile the wings upper surface will be completed and allowed to cure until the adhesive is tacky. A fresh layer of epoxy resin is applied to the aluminium surface and the two moulds are places together. They are then clamped in place and put into vacuum bags before being placed into a high temperature oven where the resin cures. After the resin has cured then moulds are prized off.

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