Material selection for brake disc: The effectiveness of the component depends on the material of the component therefore it is crucial that the material chosen provides performance, as well as cost effectiveness and ease of manufacturability. For our project different types of disc rotor types were considered such as: Ventilated type, solid type. And from which solid type brake disc was chosen because the design will be lighter and cost effective, because there will be less material involved. Materials considered for the brake disc: 1. 2. 3. 4. 5.
Carbon ceramics Stainless steel 321 Grey cast iron GG-20 Titanium alloys Aluminium matrix composite 2
1. Carbon ceramics A special feature of carbon-ceramic brake disks is the ceramic composite material they are made from. Both the carbon-ceramic brake disk body and the friction layers applied to each side consist of carbon fibre-reinforced silicon carbide. Advantages: The use of the high-tech material had revolutionized the brake technology: In comparison to the conventional grey cast iron brake disk the carbon-ceramic brake disk weighed round 50 per cent less reducing the unsprung mass by almost 20 kilograms. Further significant advantages are: improved brake response and fading data, high thermal stableness, no hot judder, excellent pedal feel, improved steering behaviour, high abrasion resistance and thus longer life time and the advantage of avoiding almost completely brake dust. Due to the factors mentioned above premium brands use the advantages of this innovative brake technology for more security and comfort. These are for example sports cars and luxury class limousines from Audi, Bentley, Bugatti and Lamborghini.
The mechanical properties of the material are given in the following figure: (Carbon-Ceramic Brake Disks)
Figure 1
Well despite having cutting edge advantages and outstanding mechanical performance the material was not chosen for the project because of its high cost which is about 20 times higher than other brake disc designs in the market. Therefore this particular material is not suited for the project, where the formula team has limited budget.
2. Stainless steel 321 Stainless steels are widely used for brake discs and automotive components in general because of their cost effectiveness, mechanical properties and their ease of manufacturability. The reasons for considering this particular steel are as follows: -
Type 304L is not good for high temperature strength Type 3CR12 is good enough for only mild high temperatures (up to 600 0C) Type 304H is good enough for only mild high temperatures (up to 800 0C) Type 310: the operating temperature of the material is only up to (1100 0C)
Advantages: -
Corrosion resistant Heat resistant Heat treatable Weldability
The mechanical properties of the material are given in the following Figure: (AISI Type 321 Stainless Steel, annealed sheet) (Stainless Steel - Grade 321, 2011) Hardness, Rockwell B
80
Tensile Strength, Ultimate
620 MPa
Tensile Strength, Yield
240 MPa
Elongation at Break
45 %
Modulus of Elasticity
193 - 200 GPa
Charpy Impact
165 J
Izod Impact
135 J Figure 2
According to a case study I have conducted where a company named as United performance Metal Joins participated in SAE Race, which is a similar kind of competition as our Student formula team is participating in, they used Type 321 stainless steel for their brake disc. The following figure shows their vehicle design:
Figure 3
The following figure shows the brake disc design, made of Type 321 steel: (United Performance Metals Joins the Formula SAE Race, 2012)
Figure 4
3. Grey cast iron
Gray cast iron is an attractive material used in industrial applications due to its some advantageous properties such as good castability, corrosion resistance, machinability, low melting point, low cost, and high damping capacity. It is used widely in the manufacturing of some machine components, disc brake rotors and hydraulic valves. Advantages: Cast iron is also widely used for automotive components such as engine block, and especially brake discs or drums. The reason of their wide use is its low cost and ease of manufacturability.
The mechanical properties of GG-20 are given in the following figure: (TYPES OF IRON POURED BY WASHBURN IRON WORKS, 2010) Tensile Strength
Compressive strength
Yield Strength
Elongation rate
σb/MPa
/MPa
σ0.2/MPa
(%)
500
210
320
7
Brinell Hardness 170~230
Figure 5
4. Titanium alloys Like carbon ceramics this material is also widely used as a high performance automotive material. And like carbon ceramics this material is also used in exotic car brands due to its high performance. But reasons for not selecting this material are similar as carbon ceramic.
5. Aluminium composite matrix 2 I found out about this particular material in a study that I have conducted where it has it has been justified that this material can perform better that any other conventional materials used for brake discs, such as Grey cast iron, Titanium alloys, etc. Advantages: Generally aluminium alloys offer some key advantages such as: - Durability - Light weight - Heat dissipation But this specific material, 20% SiC reinforced Al-Cu alloy (AMC 2), is having a lower density and higher thermal conductivity as compared to the conventionally used gray cast irons are expected to result in weight reduction of up to 50 – 60 % in brake systems. Moreover, these advanced materials have the potential to perform better under severe service conditions like higher speed, higher load etc. which are increasingly being encountered in modern automobiles. (M.A. Maleque, 2010) Because this is an experimental material, the mechanical properties of the material could not be found in much detail, therefore only relevant figure could be found in the study conducted. The following figure shows the properties of AMC 2: (M.A. Maleque, 2010) Material
Compressive strength (MPa)
Friction coefficient
Wear rate Specific heat, (x10-6mm3/N/m) Cp (KJ/Kg. K)
Specific gravity (Mg/m3)
AMC 2
761
0.44
2.91
2.8
Figure 6
0.92
Comparison and Justification
Comparison between Cast iron and stainless steels:
There have been two principal materials used for brake discs, Cast Iron and Stainless Steel. Cast iron is very cheap to produce and produces very good friction coefficients but it is also fragile, it is not compatible with many modern pad materials, particularly sintered pads, it is heavy and of course it rusts. Grey cast iron discs can shatter and ductile cast iron is fragile, very fickle with pads and in our experience can warp very easily. Some companies still believe it is the right material to use but there are just too many disadvantages and not enough advantages. Stainless steel on the other hand is a little more expensive has a lot more advantages. It doesn’t rust, or at least not to any great extent. It is very robust and is tolerant to almost all brake pads and particularly to sintered brake pads. It is highly resistant to wear, it doesn’t shatter and it resists heat very well. Therefore among these two types of material it is concluded that the Grey cast will definitely will not be used for our design.
Comparison between Grey cast iron, Ti-6Al-4V, Ti-Composites (TMC), and Aluminium Matrix Composite 2 (AMC 2):
The following figure taken from the study conducted shows the mechanical and thermal properties of the materials mentioned above. (M.A. Maleque, 2010)
Figure 7
The following figure shows the comparison in performance index of the materials mentioned above: (M.A. Maleque, 2010)
Figure 8
The following figure shows the relative cost, performance index, figure of merit, and Rank of the materials mentioned above: (M.A. Maleque, 2010)
Figure 9
The following figure shows a plot of performance index against the material mentioned above:
Figure 10
Among the materials mentioned above it is concluded that AMC 2 will be used for the brake disc design bevause it has well balaced mechanical and thermal properties as shown figure 7, has a reasonable cost as shown in figure 9, and with that it ranks highest in performance index as shown in figure 8 and 9. (M.A. Maleque, 2010)
Manufacturing and weight reduction: Manufacturing: The manufacturing cost of Aluminum alloy and grey cast iron is almost similar but compare with carbon fibre, it is really low. The reason was it took 3 times as much time to develop increasing the labour and manufacturing cost. The increase in time was because carbon has properties that resisted high heat therefore it took great time to break the element and cast it into another shape. Also the casting is different we cannot just melt it and give it another form but it was shaped and even woven like we make cloths. This is the reason carbon is so expensive. For the Aluminum alloy it was simple the manufacturing costs were low which helped us to keep in budget and improve the efficiency of the Student formula car. Some of the preferable processes of manufacturing include:
Mill balancing: in this process the disc is equally flattened from both side it is also called ‘surfacing’. The reason for this is to make the disc smooth and balanced so that it does not wobble while in motion.
Ground finishing: this is done to smoothen the edges and shape up the disc so that it does not produce any unnecessary friction.
Chamfered holes: this is one of the important processes as it smoothes the edges of hole and reduces the wear of braking pad.
Cast holes: the holes could be casted instead of drilling and later on the burrs could be removed from the casted holes, casting will also maintain the strength of the disc and make it more durable, such that it will an extra set of brakes will not be required for the race.
But as for our disc design, following manufacturing methods will be followed: 320mm circular shaft of AMC 2 will be cut into 7mm of sheets, and then they will be ground down to precisely 5mm thickness using facing. Then using water jet cutting, the patter based on design idea 2 will be cut, the advantage of using a water jet cutter is that it will not cause tempering and hence will not deform the design.
Weight Reduction: The design pattern chosen for the brake disc has plenty of room for alterations, in order to make it even more lightweight. For example more number of holes could be added or the diameter of existing holes could be enlarged, with that the thickness of the ventilation slots could also be increased. Other than the pattern, the thickness of the brake disc itself could be reduced. In conventional use the weight of a 320mm brake disc made of either Cast iron, or stainless steel is about 2000 grams but the material chosen for our design is about 50-60% lighter than cast iron and stainless steel, and therefore the weight of the brake disc is expected to be about 900-1000 grams.
References: M.A. Maleque, S. a. (2010). Material Selection Method in Design of Automotive Brake Disc. London: WCE. United Performance Metals Joins the Formula SAE Race. (2012, 02 14). Retrieved from United Performance Metals: http://upmet.blogspot.co.uk/2012/02/united-performance-metalsjoins-formula.html Brake Calculations. (2013). Retrieved from Engineering Inspiration: http://www.engineeringinspiration.co.uk/brakecalcs.html JIANG Lan, J. Y.-l.-d. (2012). Thermal analysis for brake disks of SiC/6061 Al alloy co-continuous composite for CRH3 during emergency braking considering airflow cooling. Shenyang: Science Press. AA 2011. (2013). Retrieved from eFunda: http://www.efunda.com/Materials/alloys/aluminum/show_aluminum.cfm?ID=AA_2011&sh ow_prop=all&Page_Title=AA%202011 Aalco. (2013). Aluminium - Specifications, Properties, Classifications and Classes, Supplier Data by Aalco. Retrieved from azom.com: http://www.azom.com/article.aspx?ArticleID=2863#_Mechanical_Properties AISI Type 321 Stainless Steel, annealed sheet. (n.d.). Retrieved from ASM Aerospace Specifications Metals Inc.: http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MQ321A Brake problems. (2011, 11 29). Retrieved from TheAA.com: http://www.theaa.com/motoring_advice/general-advice/brakes-discs-drums-pads.html Carbon-Ceramic Brake Disks. (n.d.). Retrieved from SGL group the carbon company: http://www.sglgroup.com/cms/international/products/product-groups/bd/carbon-ceramicbrake-disks/index.html?__locale=en Cast Iron or Stainless Steel Discs? (n.d.). Retrieved from Bike torque Racing: http://www.biketorqueracing.co.uk/btr-tech-station/btr-tech-station-brake-systems/brakediscs/cast-iron-or-stainless-steel-discs Hartman, D. (2013). The Advantages of Aluminum Brake Discs. Retrieved from eHow: http://www.ehow.co.uk/list_6072875_advantages-aluminum-brake-discs.html Lexus. (2007). wvpress7. wvpress7. Stainless Steel - Grade 321. (2011, 05 04). Retrieved from Azom.com: http://www.azom.com/article.aspx?ArticleID=967 TYPES OF IRON POURED BY WASHBURN IRON WORKS. (2010, 06). Retrieved from Washburn Iron Works: http://washburnironworks.com/110040
