Question

You are required to design and fabricate a composite material to be used for machining hard steels. Use the information presented in your lecture notes and:

i) Identify the type of a composite that you would like to design, then select the appropriate reinforcement and matrix and fully justify your selection.

ii) Select a fabrication route for the designed composite and justify your fabrication process selection with respect to temperature and time

Answer 1

Al2O3–TiC composites have been widely used as cutting tools recently, because of their high strength, hardness, chemical stability and excellent wear resistance. However, chemical reactions between Al2O3 and TiC (e.g.
Al2O3+TiC=Al2O#+TiO+CO#) can occur at high temperatures, introducing pores into the composites which deteriorate the mechanical properties. Thus, due to these gas-generating reactions, fully dense Al2O3–TiC
composites are difficult to obtain by conventional sintering. In order to prevent these reactions and to obtain a high density material, hot pressing (HP) , hot isostatic pressing (HIP) and pressureless sintering (PS)
at 1950 C with a fast heating rate (400 C/min) have been used. Comparing with HP and HIP, PS gives somewhat worse mechanical property materials, however, its production costs are also lower. Therefore, preparation Al2O3–TiC composites by PS has still attracted attention. Chae et al. studied the effect of Y2O3 addition on the densification of Al2O3–TiC. Their results show that Y2O3 can effectively inhibit the gas-generating reactions. In this work, PS and Al as an additive were selected to prepare Al2O3–TiC composites. The purpose of the Al additive was to lower the sintering temperature and, possibly, to suppress the gas-generating
reaction. The mechanical properties (hardness, toughness and wear resistance) as functions of the volume fraction of TiC in the composites were studied, which has not been previously done in a systematic way.

2. Experimental procedures
Al2O3 (AKP-50, 0.3 mm), TiC (average grain size 1.5mm) and Al (5 mm) were used as starting powders. The Al2O3 powder, various amounts of TiC powders and 1wt.% Al powder were homogenized by planetary ball milling for 5 h in hexane. The milled powders were then dried under vacuum and uniaxially pressed into pellets (20 mm in diameter and 5 mm thick). The pellets were
embedded in the powders of the same composition and sintered at 1750–1800 C for 30 min under argon in an induction furnace. The heating rate was about 30 C/min. After grinding and polishing, the density of the samples was measured using the methods.
The theoretical density of the samples was determined by the rule of mixtures, using 3.986 and 4.92 g/cm3 for the theoretical densities of Al2O3 and TiC, respectively. After painting silver paste electrical contacts on both
surfaces, the d.c. electrical conductivity was measured using a Keithley 617 programmable electrometer and an LR400 self-balancing bridge, respectively for high and low resistance samples. Vickers hardness was evaluated
at a load of 10 kg. Fracture toughness was determined from radial cracks produced by Vickers indentation at a load of 1 kg [9]. Each data point was obtained from the average of 3–5 indentations. The wear resistance was
tested using a commercial dimple grinder [10,11]. The samples were ground under a load of 20 g for 1 min. The wear volume was calculated from the diameter of the crater, which was measured using optical microscopy. Each data point was obtained from the average of 3–5 dimples. The microstructure of polished, fractured and worn surfaces of the samples was observed by scanning electron microscopy (SEM).
3. Results and discussion
The relative densities of 16, 20, 23, 27 and 30 vol.% TiC samples are 98.0, 97.5, 98.2, 97.7 and 98.4%, respectively, which indicates that using the embedding powder and the Al additive can efficiently reduce porosity creating chemical reactions between Al2O3 and TiC at high temperatures.

Al2O3–TiC composites with various amounts of TiC, from 16 to 30 vol.%, were pressureless-sintered using 1 wt.% Al as an additive. The electrical conductivity, Vickers hardness, fracture toughness and wear resistance of the composites were studied. The hardness and toughness increased gradually with the increase of TiC volume fraction. The electrical conductivity increased with the increase of TiC volume fraction and increased about 10 orders of magnitude as the TiC volume fraction changed from 0.20 to 0.23. A sample whose TiC volume fraction is 0.23, which is close to its percolation threshold, had the best wear resistance.

3.1. Mechanical properties
3.1.1. Vickers hardness and fracture toughness
The Vickers hardness and fracture toughness of the samples are plotted in Fig. 3 vs. the TiC volume fraction. Both properties increase with increasing TiC volume fraction up to 30 vol.% and can, almost certainly, be extrapolated to still higher values. Wahi and Ilshner [2] also found that the fracture toughness of HPed Al2O3–TiC composites increased with TiC content up to 35.1 vol.%. Both the hardness and toughness are somewhat lower for the present materials than for HPed or HIPed composites with 98.7–99.5% porosity and fully dense , probably on account of porosity. The hardness of the composites increases gradually as the TiC content increases, because TiC is relatively harder than Al2O3. The fracture toughness of the samples also increases with the TiC content. This is almost certainly due to effects of crack deflection and grain bridging by TiC grains as shown in Fig. 4. Crack deflection is due to the intrinsic stresses produced by the thermal mismatch of the two phases during cooling. As the TiC content increases, the TiC grains or clusters become coarser as can be seen in Fig. 1. The coarser the TiC grains, the more the cracks deflect and bridge. Therefore, as the TiC content increases, the fracture toughness increases.
3.1.2. Wear resistance
The wear volume of the samples as a function of the volume fraction of TiC is plotted . It is seen. rom Fig. 5 that the wear volume of the samples does not have a linear relation with the volume fraction of
TiC. The lowest value of the wear volume corresponds to a TiC volume fraction of 0.23, i.e. the higher limit of the estimated ’p. Fig. 6a and b are typical micrographs taken in a dimple of the 16 vol.% sample after abrasion test, for a low wear loss area and a high wear loss area, respectively. Comparing Fig. 6a with b, it can be seen that there are fewer pits in Fig. 6a than in Fig. 6b and that the content of the white phase (TiC) is similar in the two
different wear loss positions. This implies that the TiC is more difficult to wear off than Al2O3. In other words, adding TiC particle can increase the wear resistance of the composites. However, as the TiC volume fraction increases, the TiC forms ever larger clusters, isolated below ’p and continuous above ’p . Note that we only considered the effect of grain size of the TiC because we do not see any major difference in the grain size of the Al2O3. It is also known that the wear resistance decreases with increasing of the grain size . Therefore, the two competing processes lead to the smallest value of wear volume when the volume fraction of TiC is around ’p.
The above results emphasize the fact that the distribution of the grains and the grain size of the TiC phase are very important for both the electrical conductivity and wear resistance of the composites. The value of ’p depends strongly on the relative grain size and shape of the Al2O3 and TiC components, therefore using different starting powders and processing will lead to different values of ’p. This means that the composition of the composite with the optimum wear resistance will change with the starting materials and processing, as will the composition with the best overall mechanical properties. Recall, from 3.2.1 that the hardness and toughness increase monotonically with TiC content. It is reported that composite in which the TiC content is about 30 wt.% (25.8 vol.%) has the best mechanical properties. However, our work shows that the best wear resistance is about 23 vol.%, while both
the hardness and toughness increase with TiC content in the range of 16 to 30 vol.%.
. Conclusions
Using embedding powder and 1 wt Al% as an additive, a high density Al2O3–TiC composite can be prepared using pressureless sintering. The electrical conductivity, hardness, toughness and wear resistance of the composites are studied. Both the hardness and toughness increased with TiC content up to the max- imum of 30 vol.% studied in this paper. Crack deflection and bridging mechanisms are revealed as the mechanisms responsible for the toughening behavior. When the TiC content is close to the percolation threshold, the electrical conductivity changes rapidly and the wear resistance reaches its highest value.

With a wide range of applications, aluminum matrix composites (AMCs) play an important role in many of the engineering applications and industries, i.e., automobile and aerospace. AMCs are the composites having aluminum as matrix and ceramics or some other metals as reinforcement. The aim of the present study is to develop the Al2O3–TiC-reinforced AMCs and study the effect of reinforcements on the mechanical properties. In the present case, the Al2O3–TiC content is varying in composition of (0, 5, 10, 15, and 20 wt%) in aluminum matrix, fabricated by stir casting technique. It is found that reinforced AMCs show the better mechanical properties as compared to unreinforced AMCs. Various properties which were improved include tensile strength, hardness, and wear resistance. It is expected that the present composite will be useful for developing lightweight aerospace components.

Stir casting process
The stir casting process is a cost-effective simple process and used for the manufacturing of particle reinforced MMCs. The simplest and cheapest way to make AlMMCs is to blend the solid reinforcement into liquid metal and then to solidify the mix into a suitable mold, while the reinforcing material is slowly being added in melted material . Before the reinforcing material is added, the matrix material is heated above its melting temperature. Stir casting is an extensively chosen method for the fabrication of MMCs because in comparison to the traditional metal processing route, it is a very simple method . During the Fabrication of MMCs through the stir casting process generally, a variety of factors that require significant consideration, which includes to achieve a homogeneous dispersion of reinforcement material, porosity, and a good bonding between reinforcement material
and matrix material . In order to achieve the desired dispersed phase distribution in the matrix during casting, the solidification of melted material contacting reinforcement material must be under specified conditions .
The basic concept of this process is that the reinforcement materials are directly added to the molten metal and the particles are uniformly distributed by means of stirring, reinforcement material must be evenly distributed in the matrix material to achieve the best possible characteristics. In this process, reinforcement material particles are introduced into the liquid matrix material . The size of the reinforcement particles is generally small and the wettability with liquid metal is poor, which makes a problem in uniform distribution of reinforcement particles in the molten metal. Because of the weak wetting in between reinforcement and matrix materials, it is necessary to apply a mechanical force to combine all materials, usually by stirring . In this process, reinforcements are usually in the form of powder, which is homogeneously mixed in the molten metal with the help of mechanical
stirring, since generally the ceramic reinforcement materials having a higher density in comparison to matrix materials, so reinforcement material moves toward the bottom of the crucible if not mixed in a proper manner .
The mechanical stirring used for mixing the molten metal and the reinforcement in the furnace in this process. The resulting mixture is then used to make the desired shape, either by molding or casting. Degassing of the melt is required by an appropriate medium, as molten metal reacts with atmospheric elements which became the cause of degradation of the characteristics of the matrix material . However, this process experiences some
specific drawbacks which cause issues in manufacturing

Working method
As indicated in the diagram, the casting setup consists of a feeder for feeding the reinforcement material into the furnace, a furnace for heating and melting the matrix material and a mechanical stirrer used to make a vortex for proper mixing of the reinforcement material into the melted material. The stirrer is connected to the motor which can operate at various speeds and this speed of rotation controlled through a regulator attached to a motor
. When the reinforcement material has been mixed into the melted material, immediate poring is required to ensure that reinforced particles should not settle at the bottom of the furnace, by heating in a crucible furnace, the matrix material melted in the crucible. The stirring process started through a motor which positioned on the top of the stirrer. The reinforcement material is added to the matrix after stirring. In addition, preheating is done to prevent the composite material’s thermal distortion . The double or two-step casting method is an advanced form of the typical stir casting method. In the initial stage, the matrix material is usually heated to above its melting temperature and after that cooled down up to a temperature where it maintains in a semi-solid state and reinforcement material which is preheated now added and mixed with the help of a mechanical stirrer, once again the slurry is heated to molten temperature and mixed completely. Currently this process is widely used in the fabrication of AlMMCs to obtain more uniform microstructure in comparison with the typical stir casting process. Stir casting machine shown in Figure.
5.2. Process parameters
The factors which play a vital role in the dispersion of reinforcement in molten metal and good interface bonding discussed below. The factors are shown in Fig.

5.2.1. Stirrer speed
Stirrer speed is an incredibly necessary parameter in the stir casting method. Stirring speed influences and determine the creation of vortex which is liable for the distribution of reinforcement
particulates in molten metal. So it is clear that wettability is promoted by stirring. Molten metal’s flow pattern directly depends on the stirring speed [38].Stirring considered an important factor
that facilitates the homogeneous distribution of the reinforcement particles in the molten metal, the effective mechanical stirring can be accomplished by high-speed rotary mechanical stirrer or ultrasonic stirrer to improve the wetting between the metal melt and the reinforcing particles .
5.2.2. Stirring period
In the stir casting method stirring period is a critical process parameter. Less stirring time becomes a cause of the non- uniform dispersion of reinforcement particles. Because of the higher stirring period there is big possibility of deformation of stirrer blade so adequate stirring time is necessary. The stirring period is considered important, more stirring contributes to consistent particle distribution and less stirring creates particle clustering at certain places
5.2.3. Pouring temperature
Pouring rate, pouring temperature, the size of reinforcement, distance in between mold and crucible are important factors for the quality of casting, to avoid entrapping of gases, the Pouring rate
should be uniform and pouring temperature should be sufficiently high .
5.2.4. Wetting elements
The addition of alloying elements such as calcium, magnesium into the aluminum melt can enhance the wettability with reinforcement particles. Magnesium can be added in liquid aluminumto enhance the wettability because it reduces the surface tension. The presence of magnesium also enhances the strength of the alloy, it also reacts with oxygen and forms magnesium oxide, this becomes the cause of the reduction of the blow holes in the casting

5.2.5. Reinforcement preheating
Reinforcement material preheated to get rid of moisture and any type of other gases that existing within this material. The simpler method is to heat the reinforcement particles at high
temperature to remove substances. This process enhanced the wettability of reinforcement with the matrix .
5.2.6. Preheating of mold
Preheating of the mold is important as it helps remove the entrapped gas from slurry otherwise it becomes the cause of porosity so it is a good solution for preventing porosity .

STIR CASTING MACHINE

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