Aluminum matrix

* Continuous fibers: boron, silicon carbide, alumina, graphite * Discontinuous fibers: alumina, alumina-silica * Whiskers: silicon carbide * Particulates: silicon carbide, boron carbide

STIR CASTING METHOD OF FABRICATION OF MMCs * Liquid state fabrication of Metal Matrix Composites involves incorporation of dispersed phase * into a molten matrix metal, followed by its Solidification. * In order to provide high level of mechanical properties of the composite, good interfacial * bonding (wetting) between the dispersed phase and the liquid matrix should be obtained. * Wetting improvement may be achieved by coating the dispersed phase particles (fibers). Proper * coating not only reduces interfacial energy, but also prevents chemical interaction between the * dispersed phase and the matrix. * The simplest and the most cost effective method of liquid state fabrication is Stir Casting. * 26 * Stir Casting * Stir Casting is a liquid state method of composite materials fabrication, in which a dispersed * phase (ceramic particles, short fibers) is mixed with a molten matrix metal by means of * mechanical stirring. * The liquid composite material is then cast by conventional casting methods and may also be * processed by conventional Metal forming technologies. * Stir Casting is characterized by the following features: * Content of dispersed phase is limited (usually not more than 30 vol. %). * Distribution of dispersed phase throughout the matrix is not perfectly homogeneous: * 1. There are local clouds (clusters) of the dispersed particles (fibers); * 2. There may be gravity segregation of the dispersed phase due to a difference in * the densities of the dispersed and matrix phase. * The technology is relatively simple and low cost. * Distribution of dispersed phase may be improved if the matrix is in semi-solid condition. * The method using stirring metal composite materials in semi-solid state is called Rheocasting. * High viscosity of the semi-solid matrix material enables better mixing of the dispersed phase.

FIGS 2.5 STIR CASTING

* 1-Motor with stirring system,2-Heating Furnace,3-Crucible,4-Stirring blade,5-Plug.

STRENGTHENING MECHANISM OF PARTICULATE COMPOSITE
In the particulate reinforced composite the size of the particulate is more than 1 μm, so it strengthens the composite in two ways. First one is the particulate carry the load along with the matrix materials and another way is by formation of incoherent interface between the particles and the matrix. So a larger number of dislocations are generated at the interface, thus material gets strengthened. The degree of strengthening depends on the amount of particulate (volume fraction), distribution, size and shape of the particulate etc.

3.1 EXPERIMENTAL PROCEDURE
First of all, 400 gm of commercially pure aluminium was melted in a resistance heated muffle furnace and casted in a clay graphite crucible. For this the melt temperature was raised to 993K and it was degassed by purging hexachloro ethane tablets. Then the aluminium-fly ash (10%) composite was prepared by stir casting route. For this we took 400 gm of commercially pure aluminium and 40 gm of fly ash. The fly ash particles were preheated to 373K for two hours to remove the moisture. Commercially pure aluminium was melted by raising its temperature to
993K and it was degassed by purging hexachloro ethane tablets. Then the melt was stirred using a mild steel stirrer. Fly-ash particles were added to the melt at the time of formation of vortex in the melt due to stirring. The melt temperature was maintained at 953K-993K during the addition of the particles. Then the melt was casted in a clay graphite crucible. The particle size analysis and chemical composition analysis was done for fly ash. The hardness testing and density measurement was carried out for both commercially pure Al and Al-10% fly ash composite. The hardness of the samples was determined by Brinell hardness testing machine with 500 kg load and 10 mm diameter steel ball indenter. The detention time for the hardness measurement was 30 seconds.

WORKS DONE:
1. Commercially pure Al was melted and casted.
2. Al-10% fly ash composite was fabricated by stir casting method.
3. Chemical composition analysis was done for fly ash used.
4. Particle size analysis was done for fly ash used.
5. Density and hardness measurement was carried out for both commercially pure Al sample and Al-10% fly ash composite sample.
6. The wear characteristics of both commercially pure Al and Al-10% fly ash composite was evaluated and compared.
7. SEM analysis was done for both the samples.
8. EDS microanalysis was done for both the samples.

SEM ANALYSIS
SEM photographs were taken to analyze the surfaces of as cast Al and Al-10% fly ash composite. EDS (ENERGY DISPERSIVE SPECTROSCOPY) MICROANALYSIS
FIG 4.6(a) EDS microanalysis for as cast Al

End Milling
Flat surface as well as various profiles can be produced by end milling. The cutter in end milling has either straight or tapered shanks for smaller and larger cutter sizes respectively. The cutter usually rotates on an axis perpendicular to the workpiece, although it can be tilted to machine-tapered surface (Kalpakjian and Schmid 2003).

TYPE OF MILLING MACHINE
2.5.1 Vertical Milling Machine
In the vertical mill the spindle axis is vertically oriented. Milling cutters are held in the spindle and rotate on its axis. The spindle can generally be extended (or the table can be raised/lowered, giving the same effect), allowing plunge cuts and drilling. There are two subcategories of vertical mills: the bedmill and the turret mill. Turret mills, are generally smaller than bedmills, and are considered by some to be more versatile. In a turret mill the spindle remains stationary during cutting operations and the table is moved both perpendicular to and parallel to the spindle axis to accomplish cutting. In the bedmill, however, the table moves only perpendicular to the spindle's axis, while the spindle itself moves parallel to its own axis. Also of note is a lighter machine, called a mill-drill. It is quite popular with hobbyists, due to its small size and lower price. These are frequently of lower quality than other types of machines (Kalpakjian and Schmid
2003).

Vertical Milling Machine
Source: Krar, et al 1994

Computerized Numerical Control Machine (CNC machine)
CNC machine is a Computerized Numerical Control machine that the tool is controlled by a computer and is programmed with a machine code system that enables it to be operated with minimal supervision and with a great deal of repeatability. CNC mills can perform the functions of drilling and often turning. CNC mills are classified according to the number of axes that they posses. Axes are labelled as x and y for horizontal movement, and z for vertical movement. The same principles used in operating a manual machine are used in programming a CNC machine The main difference is instead of cranking handles to position a slide to a certain point, the dimension is stored in the memory of the machine control once. The control will then move the machine to these positions each time the program is run. CNC machine also economic to use for big size capacity for production and for special case CNC can be use (Armarego et al, 1999).

CNC milling machine

TOOL
In the context of machining, a cutting tool (or cutter) is any tool that is used to remove material from the workpiece by means of shear deformation. Cutting tools must be made of a material harder than the material which is to be cut, and the tool must be able to withstand the heat generated in the metal-cutting process. Also, the tool must have a specific geometry, with clearance angles designed so that the cutting edge can contact the workpiece without the rest of the tool dragging on the workpiece surface.
The cutter are generally made from high speed steel (HSS) and coated carbide which means they will cut through metals such as mild steel and aluminium. There are many variables, opinions and lore to consider before selects a milling cutter (S. Krar, et al
1994).

Tool Material
Various cutting-tool materials with a wide range of mechanical, physical, and chemical properties have been developed over the years. The desirable tool-material characteristics are chosen based on the criteria below:
(i) Hardness and strength are important with regard to the hardness and strength of the workpiece material to be machined.
(ii) Impact strength is important in making interrupted cuts in machining, such as milling. (iii) Melting temperature of the tool material is important versus the temperatures developed in the cutting zone.
(iv) The physical properties of thermal conductivity and coefficient of thermal expansion are important in determining the resistance of the tool materials to thermal fatigue and shock.
Tool materials generally are divided into the following categories, including:
(i) High-speed steels
(ii) Cast-cobalt alloys
(iii)Carbides
(iv) Coated tools
(v) Alumina-based ceramics
(vi) Cubic boron nitride

Metal Matrix Composites (MMC’s): Metal matrix composites are increasingly found in the aerospace and automotive industry. These materials use a metal such as aluminum as the matrix, and reinforce it with fibres, particulates or whiskers such as silicon carbide.

The metal-matrix composites offer a spectrum of advantages that are important for their selection and use as structural materials. A few such advantages include the combination of high strength, high elastic modulus, high toughness and impact resistance, low sensitivity -to changes in temperature or thermal shock, high surface durability, low sensitivity to surface flaws, high electrical and thermal conductivity, minimum exposure to the potential problem of moisture

absorption resulting in environmental degradation, and improved fabricability with conventional metal working equipment [5].
Metal-matrix composite reinforcements can be generally divided into five major categories:
a) Continuous fibers
b) Discontinuous fibers
c) Whiskers
d) Wires
e) Particulates

With the exception of wires, which are metals, reinforcements are generally ceramics. Typically these ceramics are oxides, carbides and nitrides, which are used because of their excellent combinations of specific strength and stiffness at both ambient temperature and at elevated temperatures. The typical reinforcements used in metal-matrix composites are listed in Table 1.1. Silicon carbide, boron carbide and aluminum oxide are the key particulate reinforcements and can be obtained in varying levels of purity and size distribution.
Silicon carbide particulates are also produced as a by-product of the processes used to make whiskers of these materials [5].

Table 1.1 The typical reinforcements used in metal-matrix composites [5].
Reinforcement Matrices
Boron, fiber (including coated) Aluminum, titanium
Graphite fiber Aluminum, magnesium, copper
Alumina fiber Aluminum, magnesium
Silicon carbide fiber Aluminum, titanium
Alumina-silica fiber Aluminum
Silicon carbide whisker Aluminum, magnesium
Silicon carbide particulate Aluminum, magnesium
Boron carbide particulate Aluminum, magnesium
The particulate-reinforced metal-matrix composites have emerged as attractive candidates for use in a spectrum of applications to include industrial, military and space-related. The renewed interest in metal-matrix composites has been aided by the development of reinforcement material, which provides either improved properties or reduced cost when compared with existing monolithic materials [5].
Particulate reinforced metal-matrix composites have attracted considerable attention on account of:
a) Availability of a spectrum of reinforcements at competitive costs,
b) Successful development of manufacturing processes to produce metalmatrix composites with reproducible microstructures and properties
c) Availability of standard and near standard metal working methods, which can be utilized to form these materials.
Furthermore, use of discontinuous reinforcements minimizes problems associated with fabrication of continuously reinforced metal-matrix composites such as fiber damage, micro-structural heterogeneity, and fiber mismatch and inter-facial reactions. For applications subjected to severe loads or extreme thermal fluctuations such as in automotive components, discontinuously-reinforced metal matrix composites have been shown to offer near isotropic properties with substantial improvements in strength and stiffness, relative to those available with monolithic materials. [6]
Several metallic systems have been considered for use as a matrix material for metal matrix composites Table 1.2.
Table 1.2 Typical matrix alloys [5].
Aluminum
Titanium
Magnesium
Copper
Bronze
Nickel
Lead
Silver
Superalloys (nickel- and iron-based)
Niobium (columbium)
Intermetallics
Nickel aluminides
Titanium aluminides
The most important have been the non-ferrous lightweight materials for structural use such as aluminum, titanium and magnesium because specific properties of these materials can be enhanced to replace heavier monolithic materials. Aluminum is the most attractive non-ferrous matrix material used particularly in the aerospace and transportation industries where weight of structural components is critical [5].
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The most common particulate composite system is aluminum reinforced with silicon carbide. So far most of the alloys that have been employed as matrices in aluminum have been focused on the A356, 2xxx and 6xxx series alloys.
Although very few studies have been reported on the 7xxx series alloys reinforced with silicon carbide particles, much less attention has been paid to the 7xxx Al alloy matrix composites, which show the highest strength of all commercial Al alloys and widely used for structural applications [7].
Stronger matrix alloys tend to produce stronger composites, but within these composite systems there are many variables such as ageing conditions, weight/volume fraction of particulate, particulate size, which can affect mechanical properties [8].
Therefore the objective of this study is to investigate the fracture behavior of silicon carbide reinforced aluminum matrix composite.
2.3.2. Effect of Silicon Carbide Particle Size
T.J.A. Doel and P. Bowen made a study about the effect of particulate size on the mechanical properties of silicon carbide reinforced metal matrix composites. As matrix alloy, aluminum 7075 (nominally 5,6wt% Zn – 2,5wt% Mg – 1,6wt%Cu) is chosen. Three grades of silicon carbide are used; F1000 (nominal average particle size d= 5μm), F600 (d=13 μm) and F230 (d=60 μm). The nominal volume fraction of all of the materials used is 15%. Three aging conditions are selected; under-aged, peak-aged and over-aged.
It is generally found that, 0.2% proof stress, tensile strength and ductility tend to improve with decreasing particle size for a given volume fraction of reinforcement. The effect of particle size can be seen in Table 2.1. There are only small differences in the 0,2% proof stress and tensile strength of the 5 μm and 13 μm silicon carbide reinforced composites but the 60 μm particulate reinforced has a much lower yield stress and a much lower fracture strength. It is important to note that the materials reinforced with 5 μm and 13 μm silicon carbide have greater 0,2% proof stress and tensile strength than the unreinforced material in the same ageing condition.
However, in the case of material containing 60 μm silicon carbide the proof stress and the tensile strength are lower than the equivalent unreinforced material [22].

Figure 2.4. Effect of aluminum matrix alloy on stress-strain behavior of composites with 20 vol% SiCw reinforcement (tested in direction parallel to final rolling direction) [23].

Table 2.6. Temper Designations and their explanations [27].
Temper
Designation Explanation
T1 Cooled from an Elevated-Temperature Shaping Process and
Naturally Aged to a Substantially Stable Condition
T2 Cooled from an Elevated-Temperature Shaping Process, Cold-
Worked, and Naturally Aged to a Substantially Stable Condition
T3 Solution Heat-Treated, Cold-Worked, and Naturally Aged to
Substantially Stable Condition
T4 Solution Heat-Treated and Naturally Aged to a Substantially
Stable Condition
T5 Cooled from an Elevated-Temperature -Shaping Process and
Artifically Aged
T6 Solution Heat-Treated and Artificially Aged
T7 Solution Heat-Treated and Over-aged or Stabilized
T8 Solution Heat-Treated, Cold-worked and Artificially Aged
T9 Solution Heat-Treated, Artificially Aged and Cold-worked
T10 Cooled from an Elevated-Temperature Shaping Process, Cold-
Worked and Artificially Aged

CHAPTER 3
EXPERIMENTAL
3.1. Matrix Material
Al 7075 was used as matrix material. The main alloying element is zinc. The second is magnesium, which is predominantly added to increase the wetting between matrix and reinforcement. Composition of aluminum 7075 was tabulated in Table 3.1.
Table 3.1. Composition (wt%) of Aluminum 7075.
Cu Mg Zn Cr Si Ti Al
1,2 - 2,0 2,1 - 2,9 5,1 - 6,1 0,18 - 0,28 0 -0,40 0 - 0,2 Bal.
Table 3.1 gives ranges of the alloying elements. The actual composition in our casting process is given in Table 3.2. Al-Ti-B (Al-5wt% Ti-1wt%B) was used to refine and decrease grain size of the matrix.
Table 3.2. The actual composition (wt%) of the matrix material.
Alloying Element Cu Mg Zn Cr Al-Ti-B Si Al
Weight (gr) 15,0 28,0 60,0 2,5 4,0 0,0 900,0
Percentage (%) 1,49 2,77 5,94 0,25 0,40 0,00 89,15
3.2. Reinforcement Material
Silicon carbide particulates are used as reinforcement material. The powder was obtained from EGESAN. The type of the silicon carbide is F320. Density of silicon carbide is between 1,29-1,35 g/cm3 and the mesh size is 29,2 ± 1,5 μm. Surface chemical values are given in Table 3.3.
Table 3.3. Surface chemical values of F 320 silicon carbide.
Product %SiC %Free C %Si %SiO2 %Fe2O3
F 240 - F 800 99,50 0,10 0,10 0,10 0,05
Silicon carbide powders was supplied from KLA Exalon, Norway. The structure of the silicon carbide is hexagonal 6H with some rhombohedral 15R and sometimes some hexagonal 4H.
3.3. Casting of Silicon Carbide Reinforced Aluminum 7075 Matrix Composite:
The existing vertical filling squeeze casting process was developed at METU. Die assembly made of hot work tool steel, specially machined and heat treated, was used to perform metal matrix composite processing. The three point bending and tensile test specimens were directly and simultaneously. Figure 3.1 shows the squeeze casting machine and Figure 3.2. shows the die. Inducto-therm induction furnace is used to melt the aluminum 7075 alloy.

3.4. Mechanical Testing and Testing Apparatus
Tensile testing and three point bending tests were done. Seven specimens of three point bending of all compositions, as-cast and heat treated are tested. Also three specimens of tensile testing of all compositions, as cast and heat treated are tested.
Hardness tests data was obtained from Emco Test Automatic hardness test machine.
Vickers 10 kg hardness values were acquired. Average values of hardness values were taken. Both sides of the specimens were tested.
3.5. Calculations
Load (P) versus deflection (δ) data were recorded during tensile testing. Also the ultimate tensile strengths was evaluated. Recorded maximum loads are in kilograms
41
and they were converted to maximum stress values (MPa). Cross-sectional areas of tensile testing samples were measured and lengths are compared before and after fracture. All of the burrs were grinded in order to prevent notch effect.
In three-point bending tests, the maximum fracture loads were evaluated. These kilogram values were converted into flexural stress (MPa) values.
The flexural stress formula is given as; σ = My/I where σ flexural stress, M the bending moment, y the distance from the natural axis and I the moment of inertia.
The maximum flexural surface stress occurs in the mid-point of the specimen.
Therefore:
M = P*L/4; y = t/2; I = b*t3/12 σmax = (3*P*L) / (2*b*t2)
P: Load applied by the testing machine, t: Thickness of the specimen b: Width of the specimen, and
L: Span length respectively.
3.7. Metallographic Examination and Image Analyzer Studies:
Microstructures of as-cast and heat treated aluminum composite samples were examined by metallographically. The photographs of samples were taken. Samples were firstly cut and mounted. Then they were grinded, polished and etched with
Keller solution which contains 1,5% HCl, 2,5% HNO3, 1% HF, 95% H2O. At the end, representative photographs were taken by a digital camera.
In order to have an information about the volume fraction of SiC reinforced aluminum 7075 alloy composites, image analyzer study was performed. With the help of Clemex software, area percentages of SiC and aluminum matrix was calculated and this gives an approximate value about the volume percentages of reinforcement and the matrix.
3.8. X-Ray Study
X-ray studies were made to find out the crystal structure of silicon carbide particulates. The second phases that may form during casting and heat treatment were revealed by x-ray structure analysis. X-ray analysis was made by 100 kV
Philips twin tube X-ray diffractometer.
3.9. SEM Study
In order to get detailed and close views of interior structures of aluminum samples
SEM studies were done. Especially the precipitates that should form after heat treatment were examined. The percentages of alloying elements were analyzed and their graphs were obtained. SEM studies were done with JSM-6400 Electron
Microscope (JEOL), equipped with NORAN System.

CHAPTER 4
RESULTS AND DISCUSSION
Effects of silicon carbide addition on the fracture behavior of aluminum matrix alloy composites was examined in this study. Both as-cast and heat treated matrixes were examined. Hardness tests were also evaluated in order to find out the optimum heat treatment procedure. Five different additions of silicon carbide was carried out and samples were investigated. They are listed in Table 4.1.
Table 4.1. List of silicon carbide (wt%) reinforced aluminum matrix composites.
% SiC Addition Explanation
Al – 0% SiCp 0 wt% particulate reinforced, squeeze cast aluminum matrix
Al - 10% SiCp 10 wt% particulate reinforced, squeeze cast aluminum matrix
Al - 15% SiCp 15 wt% particulate reinforced, squeeze cast aluminum matrix
Al - 20% SiCp 20 wt% particulate reinforced, squeeze cast aluminum matrix
Al - 30% SiCp 30 wt% particulate reinforced, squeeze cast aluminum matrix
4.1. Hardness Test Results
Hardness tests were carried out to observe the effects of heat treatment and effects of wt% addition of silicon carbide on aluminum alloy matrix since hardness is an indicator of a materials resistance to plastic deformation. Figure 4.1. shows the variation of hardness values with wt% silicon carbide. Hardness test results are listed in Table 4.2.

Table 4.2. Hardness test results of the as-cast specimens measured by Vickers test
(10 kg).
Measurement No. 1 2 3 4 5 6 7 Average
Al - % 0 SiC 131 131 131 136 136 132 135 133
Al - %10 SiC 139 138 138 138 136 138 137 138
Al - %15 SiC 134 138 153 154 153 154 146 147
Al - %20 SiC 155 163 166 155 160 157 160 159
Al - %30 SiC 172 205 175 185 182 192 206 188
Al-SiC As-Cast Hardness Values
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