Introduction
The aim of the project is to have a detailed study of fracture mechanics, wear and friction behaviour, temperature dependence of material properties, and material development from monolithic (polymers/metals/ceramics) to composites, and other physical properties which are an essential driving force for material development in a specific engineering application. Cast iron is the traditionally used as the braking disc material. In recent years lot of composites are been developed to replace the Cast iron such as metal matrix composite (MMC) and carbon-carbon-composite (CCC). The objective is to research on the functions of the brake disc and corresponding fundamental material property requirement, review the characteristic microstructure and properties of a specific material and its manufacturing process, and lastly is to outline and justify the change of material for the brake disc.
Brake Disc
The brake disc is used for slowing or stopping the rotation of a wheel. When the brakes are applied disc brakes squeezes the rotor and the force is transmitted hydraulically. There friction created between pads and disc which slows down the disc. When the brakes are applied the kinetic energy of the wheel is converted to the heat by the friction between the pads and the disc. When the brakes are applied there is high amount of heat produced therefore the material should have good thermal conductivity. As the brakes have to convert the kinetic energy in to heat, the brake needs to be strong enough to hold the disc. As there is a friction so there would be wear and tear, so there should be high wear resistance. Therefore, fundamental material property requirements are: high strength, good thermal conductivity and fracture toughness.
High strength
"Strength is the ability of a material to resist deformation". Maximum load which a material can withstand before is fails, is generally regarded as strength (engineeringedge, 2010). "Yield strength refers to the point on the engineering stress-strain curve (as opposed to true stress-strain curve) beyond which the material begins deformation that cannot be reversed upon removal of the loading". "Ultimate strength refers to the point on the engineering stress-strain curve corresponding to the maximum stress" (Wikipedia, 2010).
A stress can be defined by the following equation.
,where, = stress, L = Load, A = Area
The applied stress may be tensile, compressive, or shear. Tensile means that the object tends to elongate. Compressive stress means that the object is reducing the length of the material. Shear stress means a pair of opposite forces acting along parallel lines through material.
Strain means a measure of deformation of the material, it can be represented as
where, ϵ is Strain, ∆L is change in length, L is original length.
"Young's modulus (E) is a measure of stiffness of an isotropic elastic material". It is defined as the ratio of Stress over strain.
where, E = Young's modulus, σ = Stress, ϵ = strain, F = Force, L = specimen length, A = area of cross section, δ = elastic extension. (Material selection lecture notes, 2009). Thus the material having the good value of young's modulus and ultimate strength is one of the best materials for the brake disc.
Thermal conductivity
The capacity of a material to conduct heat is known as Thermal conductivity. Simply speaking it is a transfer of energy within a material without motion of its whole. Its equation is,
Where, λ = Thermal conductivity, Q = heat, L = distance, A = area, ∆T = temperature gradient
As the brake disc has to dissipate high amount of heat energy very quickly, therefore, material with the high amount of thermal conductivity would be appropriate (ndt-ed, 2009).
Fracture toughness
Fracture toughness is that stress intensity ahead of crack tip which leads to unstable crack propagation and then fracture of the part. It's resistance to the brittle fracture when a crack is already present.
Stress intensity is function of loading, crack size and structural geometry which can be represented by
Where, K1 = fracture toughness, σ = applied stress, a = crack length, β = crack length and component (ndt-ed, 2009).
Therefore, the fundamental properties which needs to be considered during design of brake disc are strength, thermal conductivity, fracture toughness. But there are also other properties which need to be considered secondarily such as density, endurance limit, modulus of rupture, specific heat capacity, melting temperature, glass transition, thermal expansion coefficient, thermal shock resistance.
Metal Matrix composite
Metal matrix composites are combination of two material in which 1 is primarily a metal. And another can be any other metal or material. The metal matrix composites can be used at high temperatures; they have high yield strength, high young's modulus which accounts for high transverse strength and metal matrix composite modulus. Metal matrix composites also have high wear resistance. These are lightweight and have superior mechanical and thermal properties which make them very attractive in many structural applications. There are different types of Metal Matrix composites such as fibers, short fibers, particle reinforced. In this particle reinforced are the most common due to good properties, ease of fabrication and low cost. There are lots of metal matrix composites available in the market such as Aluminum, Zinc, Lead, Copper, magnesium, Titanium etc. but the particular metal matrix composite described in this assignment will be Aluminum metal matrix composite (Mallick, 1997).
Properties
The Aluminum composite consists of different composite of aluminum, Al2O3, Alumina, Copper, Magnesium, Manganese and Silicon. The percentage of each of the materials varies depending on the requirement of the properties.
CES Edu Pack (2009) software provides a dictionary of materials, which gives information regarding all the materials, there properties and prices. On reviewing the list from the software, there were lots of composites found. The aim was to select best composite for the give assignment. Therefore, all the composites where tested on the basis of the strength wise prices. By analyzing the graph, it was found that Aluminum composite AL-20Al203 was best suited in terms of price and strength. This can be seen by below graph. C:\Users\jish\Desktop\material selection graph.png
(Figure 1: Comparison of different AL MMC)
After having decide the metal, different properties needs to be considered. In all of the Aluminum composites the best combination of the strength, thermal conductivity, fracture toughness and price was Duralcoan AL-20Al203 composite. Below is the overview of the properties of the selected composite. Highlight boxes are the ones which were give particular attention during selection of the material.
Duralcan Al-20Al2O3 (p) wrought (W2A20A-T6)
General properties
MMC: Duralcan W2A20A-T6
Density
3.04E+03
3.08E+03
Price
4.37
5.83
Composition overview
Composition (summary)
Al-4Cu-Si-Mg/20%Al2O3(p)
Base
Al (Aluminum)
Composition detail
Al (aluminum)
75
Al2O3 (alumina)
20
Alumina (particulate)
20
Cu (copper)
3.6
Mg (magnesium)
0.4
Mn (manganese)
0.64
Si (silicon)
0.64
Mechanical properties
Young's modulus
96
97
Shear modulus
36.3
36.7
Bulk modulus
100
102
Poisson's ratio
0.29
0.31
Shape factor
18
Yield strength (elastic limit)
460
480
Tensile strength
485
500
Compressive strength
460
480
Flexural strength (modulus of rupture)
460
480
Elongation
0.8
1
Hardness - Vickers
152
158
Fatigue strength at 10^7 cycles
173
180
Fatigue strength model (stress range)
126
134
Parameters: Stress Ratio = 0, Number of Cycles = 1e7
Fracture toughness
12
15
Mechanical loss coefficient (tan delta)
0.002
0.004
Thermal properties
Melting point
620
650
Maximum service temperature
280
290
Minimum service temperature
-273
Thermal conductivity
120
130
Specific heat capacity
825
875
Thermal expansion coefficient
15.6
16.5
Latent heat of fusion
315
320
Electrical properties
Electrical resistivity
4.79
5.25
Primary material production: energy, CO2 and water
Embodied energy, primary production
266
294
CO2 footprint, primary production
16.7
18.5
Material processing: energy
Casting energy
2.38
2.63
Metal powder forming energy
7.93
8.76
Vaporization energy
16.6
18.3
Non-conventional machining energy (per unit wt removed)
31.1
34.4
Advanced composite molding energy
10
22
Material processing: CO2 footprint
Casting CO2
0.143
0.158
Metal powder forming CO2
0.634
0.701
Vaporization CO2
1.33
1.46
Non-conventional machining CO2 (per unit wt removed)
2.49
2.75
Advanced composite molding CO2
1.5
1.65
Material recycling: energy, CO2 and recycle fraction
Recycle
Embodied energy, recycling
23.9
26.4
CO2 footprint, recycling
1.5
1.66
Recycle fraction in current supply
0.1
C:\Users\jish\Documents\final year\materials\W2A20A-T6.png
Manufacturing process
There are many manufacturing processes for the manufacture of the composites like,
Powder blending and consolidation: where powdered metal and discontinuous reinforcement are mixed and then bonded with the help of the process such as compaction, degassing and thermo-mechanical treatment.
Stir casting: reinforcement of a discontinuous type into molten metal which is then allowed to solidify.
Spray deposition: in this process molten metal is sprayed on the fibres.
Reactive processing: in this process one material is a reactant and forms matrix and other material forms the reactant. A chemical reaction occurs between both of the metal and the composite is formed.
Squeeze casting: a liquid mould metal is injected into a furnace containing fibres.
Physical vapour deposition; in this process the metal are liquidised in vapour form and then fibres are passed through this vapour which are coated with the metal.
(Wikipedia, 2010)
From all the process above, the most suitable process for the Aluminium composite will be Squeeze casting.
Squeeze casting
Squeeze casting process is a compromise between the casting and forging processes. In this process a melted metal has been created and kept on the lower mould die. Then an upper mould die lowers down and fills the mould cavity. The melt forms the shape of the mould cavity. Until the mould fully solidifies, squeezing pressure is applied to it. These techniques are really useful for the fibre-reinforced casting from a pre form cake (azom, 2002).C:\Users\jish\Documents\final year\materials\squeeze casting2.gif
(Figure2: Squeeze casting prcess, (azom, 2009))
Advantages
High production, Low cost
Enhanced mechanical properties
Both cast and wrought Aluminium and Manganese alloys can done
The aluminium composites produced in this manner doubles the fatigue strength at 300 C.
Good surface quality and fine grain structure.
Microstructure
The essential aspect of the microstructure is the distribution of the reinforcing particles. This reinforcement of particle depends on the processing and fabrication.
Reinforcement distribution
The reinforcement distribution depends on the blending and consolidation procedure as well as on the relative size of the matrix and reinforcing particles. The reinforcing particles will agglomerate in the interstices of the coarse particles and it will be distributed very inhomogeneously in the final product if the matrix powder particle is large relative to that of the coarse particles. In molten metal mixing methods, the reinforcement distribution is influenced by many factors such as
distribution in the liquid due to mixing,
distribution in liquid after mixing and
Redistribution due to solidification.
The distribution is dependent on the mixing process and it's important to produce a uniform mix because the gas bubbles will be left lined with reinforcing particles. After mixing is completed, segregation of the particles will occur due to gravity. Due to high volume of fraction of particles and a range of particle sizes, the settling of the particles is given by
Where, Vc = particle velocity, Vo = stokes velocity, C = particle concentration, and
p = 4.65 + 19.5 (d/D) for Re < 0.2 or,
= (4.35 + 17.5 d/D) Re ^0.03 for 0.2 Re <1
Here, d = particle diameter, D = container diameter, Re = Reynolds number.
The settling rate is dependent on the particle density, size and shape of the particles.
The third factor which influences the reinforcement is solidification itself. The reinforcing particles don't solidify in the primary phase. These particles are rejected and are segregated to the interdendritic region which solidifies last. If the solidification is done in the molten state then solidification rate influences the particle distribution because cell size is governed by solidification rate (Mallick 1997).
Effects from manufacturing process
The main processing variables governing the microstructures in squeeze casting process are: fibre and melt preheat temperature, infiltration speed and pressure and inter fibre spacing.
If the preheat temperature is very low then the casting produced is poor or porous. High temperatures lead to degradation of the casting properties due to excessive fibre-metal reaction. A threshold pressure and speed is required to maintain which keeps the flow moving between the preform cakes in the lower mould die. A wetting is required to reduce the friction between the melt and fibres. This wetting is created by the whiskers. Whiskers are dispersed in the cast during the squeeze casting technique. This solves the problem of the inter fibre spacing (Rohatgi, 1993)
Conclusion
Below is the comparison of the cast iron which is in use at present as the brake disc material with the Aluminium Metal matrix composite. The graphs show the major property comparison of this selection of the material.
From the graph it can be clearly seen that the young's modulus, yield strength, tensile strength, fracture toughness, melting point and thermal conductivity of the Aluminium Metal matrix composite is higher or nearly equal to the cast iron. The main difference is between the prices both of the materials. Price of Al MMC varies from 4.37 to 5.83 GBP/kg and cast iron varies from 0.4 to 0.44 GBP/kg. Due to the changing rules and regulations, the requirement of the passenger safety is utmost important. Moreover, there are no major manufacturing concerns for both of the materials. Therefore, Aluminium Metal Matrix composite will be the ideal choice for the brake disc.
(Figure 3: Comparison between Cast iron and AL MMC)
