HNC Material Science Steve Goddard

Assignment 3 – Select Materials and processing for a specified product

For the first section of my assignment I am going to concentrate on car road wheels.I will look at the function and material property characteristics of the product and also the design from which I will suggest the most appropriate processing method and most suitable material.I will then identify any possible limitations on the product imposed by the processing.

Function:

The wheel is a device that enables efficient movement of an object across a surface where there is a force pressing the object to the surface. Common examples are a cart drawn by a horse, and the rollers on an aircraft flap mechanism.

Alloy wheels are automobile (car, motorcycle and truck) wheels which are made from an alloy of aluminum or magnesium metals (or sometimes a mixture of both). Alloy wheels differ from normal steel wheels because of their lighter weight, which improves the steering and the speed of the car, however some alloy wheels are heavier than the equivalent size steel wheel. Alloy wheels are also better heat conductors than steel wheels, improving heat dissipation from the brakes, which reduces the chance of brake failure in more demanding driving conditions. Alloy wheels are also considered more visually attractive than hubcaps.

Materials

Steel

Most genuine wheels are made of steel. A stamped inner part is junctioned to a rolled outer rim and welded together. Many manufacturers use the same production method but take stronger and/or thicker materials to increase load capacity and they also make them in sizes better suited to aftermarket tires. The biggest disadvantage of steel is it's weight or better it's lack of performance. An alloy wheel will accelerate faster and stop quicker as well as reducing load on shocks and steering linkages. A steel wheel will also bend much earlier than an alloy wheel. But you can hammer a steel wheel back in shape while an alloy wheel will usually break. Also it's not uncommon for steel to rust to a point where structural integrity is affected.

Aluminium Alloys

Alloys can't be knocked back into shape as easily as steel wheels, and if they are knocked back into shape the structure could be seriously weakened. 

In theory Alloy wheels as opposed to weight steel ones improve the handling and ride of your car by reducing the unsprung weight. This gives the springs an easier time in controlling the bouncing wheel and reduces its gyroscopic effect making it easier to turn. As Alloys are generally accompanied by wider bigger tyres (heavier) of lower profile (stiffer side walls) you probably would not notice these benefits.  Carbon fibre

These wheels are very rarely seen, they are extremely light, less than half that of a steel rim of the same capacity. However they are prone to breakage if not constructed for off road use- and I know of none produced in greater quantities. They are also extremely expensive due to initial cost of raw material and the lot of manual work involved. Cost about 10 times the price of a forged alloy wheel.

HNC Material Science Steve Goddard

Material Property Characteristics

Specific material property characteristics for a car road wheel would be:

o Colour – Car wheels make up a major part of the cars overall appearance and aesthetics. Although it is worth mentioning that metals can be coloured by chemical means.

o Density – A wheel is preferably light and a low density is essential to keeping this weight down, keeping the density down will decrease the cars unsprung weight resulting in better steering feel and increased brake response.

o Strength and toughness – A wheel would require this to make sure that it doesn't deform or buckle with the weight and forces of the car acting upon it.

o Hardness – This will stop the material from denting, such as when it is fit with debris from the road.

o Specific Strength (strength to weight ratio)

o Cost – For standard wheels this could be more important, although for customized alloy wheels normally price isn't considered as much and more quality.

o Feasibility of mass production

o Thermal Heat Dissipation – To dissipate heat from the brake pads

Material selection

There are two main materials in use for the production of wheels, other more exotic materials could include magnesium alloys and composite materials but these would either be extremely expensive or rare to come across. So In general, alloy wheels are lighter, more attractive, and better at dissipating brake heat that their steel counterparts. They tend to be available in standardised sizes which mean competition amongst tyre vendors giving low prices and good availability!

Alloy metals provide superior strength and dramatic weight reductions over ferrous metals such as steel, and as such they represent the ideal material from which to create a high performance wheel. In fact, today it is hard to imagine a world class racing car or high performance road vehicle that doesn't utilize the benefits of alloy wheels.The alloy used in the finest road wheels today is a blend of aluminum and other elements. The term "mag wheel" is sometimes incorrectly used to describe alloy wheels. Magnesium is generally considered to be an unsuitable alloy for road usage due to its brittle nature and susceptibility to corrosion. (Flammability doesn't help either!)

Most genuine wheels are made of steel. A stamped inner part is junctioned to a rolled outer rim and nowadays welded together. The setup is strong, easy to repair but most important, cheap to fabricate. They can be painted over and over again if years of off roading take their toll. Many manufacturers use the same production method but take stronger and/or thicker materials to increase load capacity and they also make them in sizes better suited to aftermarket tires. The biggest disadvantage of steel is its weight or better its lack of performance. An alloy wheel will accelerate faster and stop quicker as well as reducing load on shocks and steering linkages.A steel wheel will also bend much earlier than an alloy wheel. But you can hammer a steel wheel back in shape while an alloy wheel will usually break. So if you intend to do many miles far away from civilization keep the steel.

HNC Material Science Steve Goddard

Alloy Wheels

· Enhance the look of your vehicle

· Are manufactured to precise standards to meet exact fitment and performance needs

· Weigh less than steel wheels and have superior strength

· May be the preferred option for your vehicle based on fitment requirements

· Will allow for better brake clearance (depending on wheel style and brake components installed)

Steel Wheels

· Meet the basic needs of drivers who want the convenience tire package without the additional cost of an alloy wheel

· Typically available in black or silver finish depending on the application

· Basic styling can often be updated with wheel covers

· Cost less than alloy wheels due to ease of manufacturing and lower material costs

Overall it really depends upon the major factor of: Cost or Performance.Buying an alloy wheel will give you better handling, more responsive braking and a nicer look. Btu will cost you more then the steel wheel, a steel wheel offers you the basics for an largely reduced price. I think that an alloy wheel will last longer, and with the extra added performance benefit to the car it is used on, in the long run the cost will even itself out.

Suggesting the most appropriate processing method

Forging

Forging is a non-machined manufacturing technique carried out while the material is heated but still solid. The so-called blanks are aluminium cylinders about 30 cm in diameter and 60 cm in height. Great pressure and high temperatures are applied to the blank, which is pressed into the shape of the mould. This takes place over three production steps using moulds with various degrees of contouring. The moulds are clamped into the forging press and pressed together under 5000-8000t of locking pressure. Compressing the material during forging boosts the density of the material, but also disproportionately increases the maximum load capacity. A silicon aluminium alloy is used in the forging process, enabling the surface to be polished to an extremely high shine. This shine is maintained even when the wheel surface is sealed using an acrylic finish and is protected from the elements and scratching. The acrylic coating makes it impossible, and unnecessary, to repolish the wheel.The materials are heated to approximately 400°C before every step.Finally, the wheel is polished on the front and on the interior, and the surface is sealed using a liquid acrylic finish. Forging allows to give the wheel high-performance properties, and achieve sportier handling and improved braking ability, while keeping wheel mass to a minimum. The only bad point to this method is forged wheels entail much higher manufacturing costs than cast wheels.

Material Property: Priority:Aluminum

Alloy Steel Carbon Fiber MagnesiumTensile Strength 5 5 2 3 4

Weight 6 4 2 5 4Cost 4 3 5 1 1

Wear Resistance 3 3 4 1 4Ease of Manufacture 2 4 5 2 2Thermal Dissipation 1 4 3 3 4

82 67 59 68

HNC Material Science Steve Goddard

Casting

Aluminium Ingot, one of the raw materials of alloy wheels will be subjected to "Spectrometer Analysing" on a sampling method. This is to ensure the composition of the raw material conforms to specifications.

Once confirmed to specifications, the aluminium ingot is then taken to the foundry melting area for the melting process.  The liquid aluminium would then undergo the flux treatment to take out the unwanted particles or dross and undergo the degazing process.  A sample of aluminium would be taken out again for "Spectrometer Analyzing" to analyze the Molten Composition and then go through a "Vacuum Test" to ensure the degazing process has been properly done.

After all the testing in ensuring the quality of the aluminium, the molten alloy is ready to be formed through Six Axis Robotic Gravity Casting, Tuting Casting or Low Pressure Casting. Once the wheel has been casted, each wheel will be inspected through x-ray machine which delivers precise and reliable results of quality standards and safety at all times.

The next process is the Riser Cutting where the point edge & the center core of the wheels would be cut off by way of riser & sprue cutting.  Then, all wheels will undergo the Solid Solution Process or in short the T4 heat treatment.  Wheels are placed in a heat treatment furnace with very high temperature for four hours, after which they are soaked in water. 

This process is to reinforce the microstructure of the wheel and with the sudden change in temperature while soaking into the water, it hardens the metal. Continuing with that is the T6 Aging Process where the wheels would go through six hours of heat treatment at a lower temperature.  Besides the hardness, the heat treatment also ensures the fundamental elasticity and strent of the wheels.

Upon completion of the heat treatment process, the new wheels are the passed onto the next stage which is the CNC stage.  It is in this stage that the Center holes, Bolt/Nut holes and valve holes are bore turned and drilled in order to remove the roughness from the wheel as well as to drill the holes to conform with the necessary specifications according to the vehicles they are meant to be fitted on.  

After the machining process, the wheels will be checked to ensure that the PCD holes and other dimensions comply with specifications.   After which the wheels would be tested for balance and checked for leaks. 

HNC Material Science Steve Goddard

In addition to the normal wheel finishes of full silver and silver machine polish, there is also a method recently introduced into production called the "Mirror" finished alloy wheels through the introduction of the latest Vacuum Sputtering Coating (VSC) technology.  This technology enables wheels to have a chrome like finish while eliminating the issues of pollution and high costs associated with conventional chrome electro-plating methods thus making VSC technology environmentally friendly.

In order to impart a VSC finish to the wheels, the wheels would be loaded into a VSC painting chamber.  The chamber would then be sealed and the air inside would be pumped out thus generating a vacuum in the chamber.  Electrical voltage would then be passed onto the target (usually aluminium) and the charged paint particles would then sputter out and stick to the surface of the wheels which will have an opposing charge. Upon completion of this process, the wheels would then be sprayed with a top coat to protect the finish and baked.

From the CNC, wheels are moved to the painting area. Before the wheels is on the conveyer for onwards cleaning as well as applying the process of electrostatic, deburring of the wheels take place.(The deburring is to ensure smooth finishing).A base coat is applied using electrostatic powder coating and then the wheels undergo the polishing process to ensure the good absorption of paint.  The colour coat paint is then applied and finally, a clear coat of lacquer to the front face.

In producing cast aluminum alloy products, such as vehicle wheels, it is generally necessary, after the initial casting operation, to subject the casting to a series of metal treatment steps in order to produce a casting having the desired tensile strength, yield strength, elongation, and fatigue strength properties. These steps include: (1) a "solution heat treatment" (SHT) process and (2) an "aging" (i.e. , precipitation hardening) process. In the SHT process, an aluminum alloy casting is first heated to a "solution" temperature of about 1000° F. for a predetermined time such that certain soluble constituents contained in the alloy (such as age hardening constituent magnesium silicide Mg2 Si) are dissolved into "solid solution". The casting is then immediately and rapidly cooled (such as by quenching in a water bath) to retain the constituents in solid solution. This prevents rapid precipitation of the associated constituents which would otherwise occur if the casting were allowed to slowly cool through a certain temperature range. Next, during the "aging" process, the hardening constituents are precipitated out of the solution in a controlled manner to produce a casting having the desired mechanical properties. The aging is effected either "naturally" at room temperature over a period of at least 10-12 hours, or it can be "accelerated" by heating the casting to an elevated temperature for a shorter period of time (e.g. 450° F. for 30 minutes).

The conventional method of producing gravity-cast aluminum wheels includes initially pouring a suitable molten aluminum alloy, such as A356 aluminum, into a mold through its gate channel until the molten alloy flows upwardly through one or more mold risers. After the molten alloy has completely solidified, the casting is removed from the mold, at which time it can be degated (i.e., the portion of the casting which solidified in the gate channel is cut off) and quenched in water to cool the casting to room temperature. The casting is then derisered (i.e., the riser portions of the casting are removed) and subjected to fluoroscope inspection to locate any obvious casting defects.

Next, a group of wheels (typically between about 70 and 350), are loaded onto racks and subjected to a "batch" solution heat treatment process. The batch solution heat treatment process is effected by placing the racks in a large gas-fired or electrical-resistance forced air convection oven. In the convection oven, the castings are heated to a desired "solution" temperature (approximately 1000° F.) and are maintained at this temperature for approximately 2 to 8 hours. Following heating, the batches of wheels are immediately quenched in water to rapidly cool the wheels. Following cooling, the wheels are machined and painted and/or clear coated, during which time they are naturally aged at room temperature.

HNC Material Science Steve Goddard

One of the problems associated with the above method for producing cast aluminum wheels relates to the amount of "work-in-process" which occurs as a result of the long process times. It is known that if a casting is heated to the correct "solution" temperature, proper solution heat treatment will occur within about 5 minutes. However, since a large number of wheels are heated during the batch solution heat treatment process, it is difficult to maintain even and uniform temperatures in all the wheels. Thus, to ensure that all the wheels are properly heat treated, the time to solution heat treat the wheels are usually at least two hours.

Limitations on the product imposed by processing

Melting and Metal Treatment

Aluminium and aluminium alloys can be melted in a variety of ways. Coreless and channel induction furnaces, crucible and open-hearth reverberatory furnaces fired by natural gas or fuel oil, and electric resistance and electric radiation furnaces are all in routine use. The nature of the furnace charge is as varied and important as the choice of furnace type for metal casting operations. The furnace charge may range from prealloyed ingot of high quality to charges made up exclusively from low-grade scrap.

Even under optimum melting and melt-holding conditions, molten aluminium is susceptible to three types of degradation:

With time at temperature, adsorption of hydrogen results in increased dissolved hydrogen content up to an equilibrium value for the specific composition and temperature

With time at temperature, oxidation of the melt occurs; in alloys containing magnesium, oxidation losses and the formation of complex oxides may not be self-limiting

Transient elements characterized by low vapour pressure and high reactivity are reduced as a function of time at temperature; magnesium, sodium, calcium, and strontium, upon which mechanical properties directly or indirectly rely, are examples of elements that display transient characteristics.

Hydrogen Influence on Aluminium

During the cooling and solidification of molten aluminium, dissolved hydrogen in excess of the extremely low solid solubility may precipitate in molecular form, resulting in the formation of primary and/or secondary voids.

Oxidization

Aluminium and its alloys oxidize readily in both the solid and molten states to provide a continuous self-limiting film. The rate of oxidation increases with temperature and is substantially greater in molten than in solid aluminium. The reactive elements contained in alloys such as magnesium, strontium, sodium, calcium, beryllium, and titanium are also factors in oxide formation. In both the molten and solid states, oxide formed at the surface offers benefits in self-limitation and as a barrier to hydrogen diffusion and solution. Induced turbulence, however, results in the entrainment of oxide particles, which resist gravity separation because their density is similar to that of molten aluminium.

Oxides are formed by direct oxidation in air, by reaction with water vapour, or by aluminothermic reaction with oxides of other metals, such as iron or silicon, contained in tools and refractories. Aluminium oxide is polymorphic, but at molten metal temperature the common forms of oxide encountered are crystalline and of a variety of types depending on exposure, temperature, and time. Some crystallographic oxide forms affect the appearance and coloration of castings, without other significant effects.

HNC Material Science Steve Goddard

For the second section of my assignment I have been asked to choose a critical/ important component which my company is involved in. For this component I have chosen the Intermediate gearbox housing of the A129 Helicopter.

Function

The function of the IGB (Intermediate gear box) housing is to hold and protect the gears which transfer power from the main drive shafts to the tail drive shafts which is subsequently transfer to the TRGB (Tail rotor gearbox). Weight on this part are particularly important so this is one of the driving factors on the methods and materials used to produce this component.

Material Property Characteristics Required

Density – The IGB has a specific weight allowance that it must comply to.

Strength – The SHP (Shaft horse power) going through these gearboxes require the housing to be able to withstand a lot of stress.

Hardness – The material used to make the IGB must be hard enough so that for example if It was hit by a bullet the bullet would not pierce the interior workings of the IGB. If it did this would cause the IGB to function incorrectly and potentially disable the tail rotor causing the helicopter to spin out of control.

Toughness – The IGB will be under constant loading from the rest of the helicopter, it is important that the material used can withstand this.

Machineability – This is important from a cost and manufacturing point of view, with a good machineable material, manufacture will be easier and cheaper.

Thermal Conductivity – This is a useful property to disperse the heat generated inside the housing.

Selecting the most appropriate processing method

Sand Casting

Sand casting is used to make large parts (typically Iron, but also Bronze, Brass, Aluminium). Molten metal is poured into a mold cavity formed out of sand (natural or synthetic).

The cavity in the sand is formed by using a pattern (an approximate duplicate of the real part), which are typically made out of wood, sometimes metal. The cavity is contained in an aggregate housed in a box called the flask. Core is a sand shape inserted into the mold to produce the internal features of the part such as holes or internal passages. Cores are

HNC Material Science Steve Goddard

placed in the cavity to form holes of the desired shapes. Core print is the region added to the pattern, core, or mold that is used to locate and support the core within the mold. A riser is an extra void created in the mold to contain excessive molten material. The purpose of this is feed the molten metal to the mold cavity as the molten metal solidifies and shrinks, and thereby prevents voids in the main casting.

Typical Components of a Two-part Sand Casting Mold.

In a two-part mold, which is typical of sand castings, the upper half, including the top half of the pattern, flask, and core is called cope and the lower half is called drag. The parting line or the parting surface is line or surface that separates the cope and drag. The drag is first filled partially with sand, and the core print, the cores, and the gating system are placed near the parting line. The cope is then assembled to the drag, and the sand is poured on the cope half, covering the pattern, core and the gating system. The sand is compacted by vibration and mechanical means. Next, the cope is removed from the drag, and the pattern is carefully removed. The object is to remove the pattern without breaking the mold cavity. This is facilitated by designing a draft, a slight angular offset from the vertical to the vertical surfaces of the pattern. This is usually a minimum of 1° or 1.5 mm (0.060 in), whichever is greater. The rougher the surface of the pattern, the more the draft to be provided.

Investment casting

This is also known as the lost wax process. Intricate shapes can be made with high accuracy. In addition, metals that are hard to machine or fabricate are good candidates for this process. It can be used to make parts that cannot be produced by normal manufacturing techniques, such as turbine blades that have complex shapes, or airplane parts that have to withstand high temperatures.

The mold is made by making a pattern using wax or some other material that can be melted away. This wax pattern is dipped in refractory slurry, which coats the wax pattern and forms a skin. This is dried and the process of dipping in the slurry and drying is repeated until a robust thickness is achieved. After this, the entire pattern is placed in an oven and the wax is melted away. This leads to a mold that can be filled with the molten metal. Because the mold is formed around a one-piece pattern, (which does not have to be pulled out from the mold as in a traditional sand casting process), very intricate parts and undercuts can be made. The wax pattern itself is made by duplicating using a stereo lithography or similar model-which has been fabricated using a computer solid model master.

The materials used for the slurry are a mixture of plaster of Paris, a binder and powdered silica, a refractory, for low temperature melts. For higher temperature melts, sillimanite an alumina-silicate is used as a refractory, and silica is used as a binder. Depending on the fineness of the finish desired additional coatings of sillimanite and ethyl silicate may be applied. The mold thus produced can be used directly for light castings, or be reinforced by placing it in a larger container and reinforcing it more slurry.

Just before the pour, the mold is pre-heated to about 1000 ºC (1832 ºF) to remove any residues of wax, harden the binder. The pour in the pre-heated mold also ensures that the

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mold will fill completely. Pouring can be done using gravity, pressure or vacuum conditions. Attention must be paid to mold permeability when using pressure, to allow the air to escape as the pour is done.

Tolerances of 0.5 % of length are routinely possible, and as low as 0.15 % is possible for small dimensions. Castings can weigh from a few grams to 35 kg (0.1 oz to 80 lb), although the normal size ranges from 200 g to about 8 kg (7 oz to 15 lb). Normal minimum wall thicknesses are about 1 mm to about 0.5 mm (0.040-0.020 in) for alloys that can be cast easily.

The types of materials that can be cast are Aluminium alloys, Bronzes, tool steels, stainless steels, Stellite, Hastelloys, and precious metals. Parts made with investment castings often do not require any further machining, because of the close tolerances that can be achieved.

For this particular gearbox housing, cost was a deciding factor on the decision, the housing is manufactured an Agusta owned plant in Italy and they specialise in sand casting, this means that the housing will have thicker walls resulting in it weighing more but cost will be reduced because the process is done in house and not by an external manufacturer.

Materials

I have researched into possible material that could be used for the manufacture of the gearbox housing

Material Description

Aluminium Alloy Aluminum alloys, alloys of aluminum, often with copper, zinc, manganese, silicon, or magnesium. They are much lighter and more corrosion resistant than plain carbon steel, but not quite as corrosion resistant as pure aluminum.

Steel Steel is an alloy consisting mostly of iron, with a carbon content between 0.2 and 1.7 depending on grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten. Carbon and other elements act as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount of alloying elements and form of their presence in the steel controls qualities such as the hardness, ductility and tensile strength of the resulting steel.

Titanium Alloy Titanium alloys are metallic materials which contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness (even at extreme temperatures), light weight, extraordinary corrosion resistance, and ability to withstand extreme temperatures. However, the high cost of both raw materials and processing limit their use to military applications, aircraft, spacecraft, medical devices, and some premium sports equipment and consumer electronics.

Magnesium Alloy

Magnesium alloy developments have traditionally been driven by aerospace industry requirements for lightweight materials to operate under increasingly demanding conditions. Magnesium alloys have always been attractive to designers due to their low density, only two thirds that of aluminium. This has been a major factor in the widespread use of magnesium alloy castings and wrought products.

HNC Material Science Steve Goddard

Improvements in mechanical properties and corrosion resistance have led to greater interest in magnesium alloys for aerospace applications.

The material used to produce the gearbox housing is a magnesium alloy this is due to the fact that it is significantly lighter than any other material that could be used for this. The cost is quite different but from the designer's point of view "We'd spend double on the component if we could make the weight less!"The composition, mechanical and thermal properties are shown below.

Element  Weight % 

Zn  4.20 

Zr  0.70 

Re  1.2 

Properties Conditions 

T (°C) Treatment

Density (×1000 kg/m3) 1.82 25  

Poisson's Ratio 0.35 25  

Elastic Modulus (GPa) 44.8 25  

Tensile Strength (Mpa) 205.0 

25  T5 (sand casting, permanent mold casting) Yield Strength (Mpa) 140.0 

Elongation (%) 3.5 

Reduction in Area (%)  

Hardness (HB500) 62  25  T5 (sand casting, permanent mold casting) 

Shear Strength (MPa) 160  25  T5 (sand casting, permanent mold casting) 

Properties Conditions 

T (°C) Treatment

Thermal Conductivity (W/m-K) 112.97  0  T5 

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Possible Limitations on the product, imposed by processing

DISADVANTAGES OF SAND CASTING

1) Poor surface finish requires extensive further treatment to produce acceptable finish.

2) Some shapes of keys could very difficult or impossible to cast. Necessitating the splitting of keys down into separate pieces to make casting possible then assemble them and solder them together.

3) Due to long narrow shape of most keys, very high risk of inclusions and cold shuts.

4) Nickel silver material used for most keys is not very fluid when molten and this process only uses gravity to fill the mould making thin sections difficult.

5) Final quality of castings depends on the skill of the caster. (Nothing to do with wheels)

6) Sand casting is cheaper but you pay the price by having to increase wall thickness compared to an investment casting.

HNC Material Science Steve Goddard

Bibliography

Pete Watson's class notes

Evaluation of fatigue life of aluminium alloy wheels under radial loads – P. Ramamurty Raju

www.Wikipedia.org

www.sciencedirect.com

Aluminum and Aluminum Alloys Casting Problems – Key-to-Metas.com

www.alcar.co.uk

www.bsa.com.ny

www.difflock.com

www.ukcar.com

www.channel4.com

http://www.carbibles.com/tyre_bible.html

www.efunda.com