Mechanical and Thermal Behavior of Chilled Aluminum Alloy (Lm-25) Reinforced With Borosilicate Glass Particulate Metal Matrix Composites (MMCs)

Hemanth J

Published on: 2019-12-31

Abstract

Present investigation aims at developing chilled aluminum alloy (LM-25) reinforced with borosilicate glass using stir casting method. Matrix alloy was melted in a composite making furnace to a temperature of about 700oC to which preheated reinforcement particles was added (3 wt.% to 15 wt.% in steps of 3 wt.%), stirred well and finally poured in to a AFS standard mold containing different metallic and nonmetallic end chills placed judiciously for directional solidification. The resulting chilled composites were subjected to microstructural, EDAX, mechanical (strength and hardness) and thermal (carburization, coefficient of thermal expansion and thermal conductivity) tests. Results of the microstructural and EDAX analysis indicate that the chilled castings were sound with good distribution of particles and bonding between ceramic particulate (reinforcement) material and Al alloy matrix (LM-25) possessing excellent isotropic properties without any shrinkage or micro porosity. Mechanical characterization indicate that both strength (170 Mpa) and hardness (159 HV) was maximum in the case of copper chilled MMC containing 9 wt. % reinforcement. It is observed that because of the ceramic reinforcement in aluminum alloy, the coefficient of thermal expansion (CTE) decreased linearly with reinforcement addition whereas the thermal conductivity was decreased. Carburization tests (which is time dependent) reveal that near the interfacial regions, a flakey type of carbon layer was present that has refined the microstructure and slightly increased the hardness.

Keywords

Composite Chill casting Thermal Reinforcement Mechanical

Introduction

Composite materials are the class of materials in which two phases are combined usually with strong interfaces between them. Metal Matrix Composites are those in which the matrix phase is composed of a ductile metal and/or alloy and the reinforcement phase is composed of a ceramic materials, carbon materials, intermetallic etc. Metal matrix composites are gaining widespread application and acceptance due to the relation of structure to properties such as specific stiffness or specific strength. The main aim involved in designing the metal matrix composites is to combine the desirable properties of metals and ceramics. The addition of refractory particles which possess high strength, hardness and high modulus to a ductile metal matrix results in a composite material whose mechanical properties are intermediate between the matrix alloy and ceramic reinforcements. Aluminum qualifies as one of the best matrix material due to its availability. Of all the structural materials, aluminum is abundantly found in the earth’s crust, and is the third most abundant element after oxygen and silicon. Apart from ease of availability, aluminum also possesses desirable characteristics such as high strength to weight ratio, corrosion resistant, ease in machining, good durability with sufficient ductility. Aluminum is a very light metal with a specific weight of 2.7 gm/cc. These characteristics have enabled the use of aluminum as the primary material of choice for automobile and aerospace applications. Aluminum alloys probably form the widely used matrix materials for Metal Matrix Composites (MMCs) since it is light in weight. Metal Matrix Composite will replace conventional materials in many commercial and industrial applications. Al-based particulate reinforced MMCs which possess an excellent strength to weight ratio are attractive for practical applications like automobile, aerospace and defense industries. Toughness and formability of aluminum can be combined with strength and hardness of borosilicate glass on weight adjusted basis to outperform the conventional material. Driven by the ever increasing demand for high strength, low weight materials and advancements in manufacturing technologies, the growth in the field of composites has been many fold. It goes without the saying that the technological advancements in the design and manufacturing of composite materials has fueled the growth of auto sector and has enabled various nations around the world to achieve the impossible, which is low cost spatial exploration. It also goes without saying that the ease with which the composites adapt themselves to the specific requirements has also contributed immensely to the popularity of composite materials. Composite materials which can be tailor-made to meet specific requirement, have successfully overtaken the monolithic materials and find their presence in almost all fields. It is well known that Al alloys that freeze over a wide range of temperature are difficult to feed during solidification. The dispersed porosity caused by the pasty mode of solidification can be effectively reduced by the use of chills. Chills extract heat at a faster rate and promote directional solidification. Therefore chills are widely used by foundry engineers for the production of sound and quality castings. There have been several investigations [1-5] on the influence of chills on the solidification and soundness of alloys. With the increase in the demand for quality composites, it has become essential to produce Al composites free from unsoundness. Search of open literature indicates that so far no investigation has been done on microstructure, mechanical and thermal properties of Al-alloy (LM-25) and borosilicate glass chilled particulate MMCs and nothing is available on the thermal properties of the same manufactured by chill casting technique. Hence the present investigation is undertaken to fill the void since this chilled MMC has great importance in the aerospace as well as in the automobile sector. Thus, the current research work focus on chill casting of borosilicate glass reinforced aluminum alloy matrix composites with regard to fabrication, microstructure, mechanical properties and thermal analysis.

Literature Review

During the late 1980s and early 1990s the research focused mainly on aluminum and titanium matrix composites. This period saw the emergence of aluminum metal matrix composites cast with discontinuous reinforcements. This was due to massive funding by the U.S. Air Force in a project called Title III. This has firmly laid the foundation for the development of numerous applications employing discontinuously reinforced aluminum. The researchers around the world focused on developing lighter and stronger materials. Massive research on light metal matrix composites set forth, in which the leading matrix metal was aluminum followed by titanium to some extent. These matrices were reinforced by both continuous as well as discontinuous reinforcements. Powder reinforcements were added in high volume percentage to the metal matrices [6-10] and the composites thus developed were employed in electronic packaging which required an excellent thermal management [11]. There was a considerable shift from continuous reinforcement composites to discontinuous reinforcement composites [12-14]. Concentrated effort was directed towards the study of microstructural properties of MMCs. The success of discontinuously reinforced metal matrix composites through improved affordability and ease of process ability expanded the application areas of the MMCs. Fukuda and Chou published “An Advanced Shear-Lag Model Applicable to Discontinuous Fiber Composites” in 1981. Researchers, during this period (1980s-1990s) concentrated their efforts in tackling the problem of residual stresses that developed during the production of metal matrix composites. Researchers from the University of Texas, Austin published “Residual Stresses Measurement in Metal Matrix Composites”. The period also witnessed the investigation of interface behavior such as interface reaction, interface coating and its effect on thermal cycling and fracture toughness, between the matrix and the reinforcement. The MMCs were investigated for their suitability in high temperature applications. Thermal properties like thermal degradation, thermal expansion and creep were investigated during this period of time [15-20]. Formability and fiber alignment aspects in aluminum composites reinforced with silicon carbide were extensively investigated [21-26]. Composite production processes such as powder metallurgy and sintering methods were developed further to fabricate metal matrix composites. Various research papers on the fabrication of metal matrix composites through chill casting route was extensively investigated during 2000 [27-33]. Thus it can be concluded that 1990s belonged to aluminum metal matrix composites reinforced with particulate reinforcements in general and SiC in particular. Response of the MMC at elevated temperatures on parameters such as ageing, ultimate tensile strength and fatigue behavior also came under the study [34-38]. Titanium matrix composites began to evolve only during the late 1990s as researchers came up with specific applications for these materials in the field of aerospace. Formability and fiber alignment aspects in aluminum composites reinforced with silicon carbide were also extensively investigated [39-44]. Composite production processes such as powder metallurgy and sintering methods were developed further to fabricate metal matrix composites. Various research on the fabrication of metal matrix composites through squeeze casting route was extensively investigated in the 1990s [45-47]. Thus it can be concluded that 1990s belonged to aluminum metal matrix composites reinforced with particulate reinforcements in general, and SiC in particular. Response of the MMC at elevated temperatures on parameters such as ageing, ultimate tensile strength and fatigue behavior also came under the study [48-55]. Titanium matrix composites began to evolve only during the late 1990s as researchers came up with specific applications for these materials in the field of aerospace. A notable development during the year 2000 has happened in the field of Nano composites. Nano particle reinforced metal matrix composites came to be heavily researched around the world. Carbon Nano tubes were extensively used for incorporating in almost all types of metal matrices. The fabrication technique along with heat treatment methods of nano composites were discussed [56-58]. During this period, the investigators focused on the fabrication of nano metal matrix composites. This was due to the fact that MMC had firmly made their mark in various applications which now called for mass production of these materials. Later the researchers tried to evolve ways to weld aluminum and titanium matrix composites via friction stir welding, in particular that attracted numerous research interest. Many innovative methods such as spark plasma sintering of titanium matrix composites, drilling of SiC particle reinforced aluminum matrix composites, milling of magnesium matrix composites came into be existence [59-60]. Squeeze casting technique that had evolved as a proven fabrication technique in the past decade was successfully adopted by the engineers to produce aluminum matrix composites reinforced by discontinuous phase [61]. Investigators from India, Japan and China worked mainly with aluminum, titanium-nickel, and nickel-Molybdenum and stainless steel matrices [62]. This period also witnessed the research in copper and magnesium as matrix materials. Alloys, in specific, garnered a lot of attention and experiments were undertaken to alloy magnesium with aluminum and titanium metal matrices [63-65]. Hybrid composites, have recently became the most researched materials. Another point of interest proved to be in the field of fiber metal laminates. The significant contributors to this area were the scientists and engineers from Netherland. Incorporation of sandwich panels and metallic foams in various matrices was also studied during this period. Aluminum matrices came to be reinforced with ceramic foams (mainly silicon carbide foam), while aluminum foam was used to reinforce a sandwich panel. The latter half of this decade belonged to aluminum and magnesium matrix composites [66-68]. A healthy research in this material is due to the fact that magnesium matrix composites have very low density and an excellent mechanical properties. Due to this fact, the aluminum matrix composites find their extensive use in automotive and aerospace applications as well as in engine parts and in parts of compressors.

Experimental Method And Testing Procedure

Matrix material and the reinforcement

Commercial grade aluminum alloy 356 also known as LM-25 (popularly used in automotive industries) was selected as the matrix material which is of great importance in automotive sector. LM-25 is known for its excellent mechanical and thermal properties due to the presence of a good percentage of silicon along with the right amount of magnesium and manganese that has made LM-25 is one of the most suitable material for the matrix. (Table 1) shows the composition of the matrix al- alloy (LM-25). Borosilicate glass was selected as the reinforcement because of its high hardness, toughness, and high melting point. Borosilicate glass powder of range 80-100 μm is procured from Nano and Amorphous Materials Ltd, INC, USA.

Following are composition of the reinforcement

Si-dioxide- 80.6, Boron trioxide-13.0, Sodium oxide-4.0, Al-oxide- 0.3, other traces – 0.1.

Table 1: Chemical composition (wt.%) of Al-alloy (LM-25).

Element                                  

Si

Mg        

Cu    

Fe    

Mn  

Zn   

Al

Comp.        

7

0.37

0.2

0.2

0.1

0.1

 Bal

Molding and composite making

Molds were prepared according to American Foundrymen Society (AFS) standards (5% bentonite as binder and 5% moisture with special additives) along with standard runners and risers. End chills were inserted in the mold at the desired location and finally molds were dried in the air furnace. In the present investigation two metallic chills (copper and mild steel) and two non-metallic chills (graphite and silicon carbide) were used. The reason for using various types of chills is to vary the volumetric heat capacity (VHC) of the chill which in turn varies the heat extraction capacity and hence the properties. Matrix material (LM-25) was melted and super-heated to 710oC to which preheated reinforcement (up to 500oC) was introduced evenly into the molten alloy by means of special attachment and stirred well by a stirrer at a speed of 360 rpm to create vortex for uniform and thorough mixing of the reinforcement in the matrix and at the end of stirring the degasing tablet was added in to the melt. Finally the reinforcement treated melt was poured into the mold cavity containing the end chill and allowed to solidify. For testing purpose specimens were selected from the chill end. Note that all the test samples before mechanical testing were subject to aging heat treatment process (at a temperature of 400oC for 5 hours) to remove unwanted residual stresses. (Figure 1) shows the dimension of the mold cavity (150 * 20 mm) and the end chill (150 * 25 mm) in position. Note that the thickness of the mold cavity and the chill is 25 mm.

Figure 1:  AFS standard mold cavity along with the end chill in position.

Testing

The microstructural examination was performed on the neatly polished specimens using Nikon Microscope LV150 with CEMEX image analyzer according to ASTME 3 standards. EDAX analysis was also carried out for copper chilled composites to confirm the presence of reinforcement in the matrix alloy. Tensile strength test was performed on a tensometer specimen using Instron testing machine according to ASTM-E 8M standard. Hardness tests of all the heat treated specimens were performed on the polished specimens used for microstructural studies using a digital Vickers micro-hardness tester (model-MMT X7A) under a load of 100 gm for 15 seconds. Coefficient of thermal expansion (CTE) was done using Thermo-Mechanical Analyzer (TMA Q 400) which operates over a temperature range of –150C to 1000C using heating rates up to 200C/min. In the present research CTE test was conducted by heating the specimens from ambient temperature to 250ºC with the heating rate of 5 ºC/min along with an expansion probe with an applied force of 0.05 N. The specimens used for this test are rectangular in shape of dimension 10 x 5 x 5 mm. Thermal conductivity tests were done using the laser flash technique (ETZCH LFA 447 Nano Flash tester) according to ASTM E-1461 standards. This apparatus is equipped with a furnace that operates between room temperature and 300ºC. Computerized data acquisition was accomplished by integrating the apparatus with a computer using PROTEUS software. Carburization tests were conducted on the specimens used for hardness tests by the pack carburization method using a muffle furnace. The test was conducted at a temperature 450oC for a time duration of 4 hours. The amount of weight gain after carburization due to the deposition of carbon was recorded and weight gain will be measured using an electronic balance. Note that all the above tests were conducted on the chilled composites developed and each test result is obtained from an average of three samples drawn out form the same location near the chill end.

 

Results And Discussion

Microstructure of the Chilled Composite

Figure 2: (a) Copper chilled MMC with 3 wt. % reinforcement; (b) Copper chilled MMC with 6 wt. % reinforcement.

Figure 2: (c) Copper chilled MMC with 9 wt. % reinforcement; (d) Copper chilled MMC with 12 wt. % reinforcement.

(Figure 2) (A to d) illustrates the photo-micrographs taken at 500 X magnification for etched specimens with varied reinforcement content (3, 6, 9 and 12 wt. %) cast using the copper end chill and (Figure 2) (e) shows the graphite chilled MMC containing 12 wt.% reinforcement. But at higher percent of addition (beyond 12 wt, %), it is observed that there is segregation/clustering of the reinforcement (microstructure not shown). It can be seen that the particles are evenly distributed at the inter dendritic regions of 3, 6 and 9 wt% reinforcement content chilled composite whereas for 12 wt.% reinforcement composite, micrographs show fewer particles at the inter-dendritic region.

Figure 2: (e) Graphite chilled MMC with 12 wt. % reinforcement.

This may be due to the increase in density difference ensued at higher concentration of reinforcement within the matrix resulting in settling down and/or clustering of the reinforcement. Therefore adding reinforcement content beyond 12 wt. % do not serve the purpose. Microstructure of the composite developed in the present research primarily depends on the rate of chilling as well as on the reinforcement content that influences solidification. The photo micrographs as well as the EDAX analysis reveal the fact that there is an almost linear relationship between the uniform distribution of the reinforcement within the matrix alloy and the weight percent of reinforcement present in the matrix. Microstructure of the chilled MMCs is also discussed in terms of distribution of reinforcement and matrix reinforcement interfacial bonding. It is observed from the microstructure that for the chilled MMCs containing lower amount of reinforcement (3 to 9 wt. %) revealed that there is no formation of clusters but with good matrix and reinforcement integrity. This may be due to gravity of the reinforcement particles associated with judicious selection of the stirring mechanism (vortex route) and good wettability of the pre-heated reinforcement by the matrix melt. It is observed from the microstructure that the MMC fabricated using copper chill (having highest VHC) has a very fine grained structure because of the steep temperature gradient setup during solidification as compared against the graphite chilled MMC. Thus copper chilled MMC has made proper flow of liquid metal in to the inter-dendritic spaces making the bond strong without any porosity. Examination of the photo micrographs indicate that the microstructure consisting of fine eutectic silicon (dark gray) dispersed within the inter-dendritic region (light gray matrix) and fine precipitates of alloying elements in the matrix of aluminum solid solution. It is also observed that the fine grain structure due to grain refinement was extremely good in the case of copper chilled (metallic chill) MMCs whereas coarse grain structure was observed in the case of graphite chilled (nonmetallic, (Figure 2) (e) MMCs. However the grain refinement and reinforcement distribution is primarily attributed to the capacity of the reinforcement particulate to nucleate from matrix aluminum alloy during the directional solidification under the influence of chills that restricted the growth of aluminum grains because of the presence of finer reinforcement and effect of chilling. Note that he rate of solidification has a direct influence on the size and structure of the dendrite arm spacing. Steeper and higher cooling rates (effect of chilling) result in smaller dendritic arm spacing than the reinforcement size. This drastically reduces the movement of the reinforcement within the matrix resulting in a fewer segregation during solidification. Hence, it is always desirable to have a dendritic arm spacing of either less than or equal to the particle size to ensure a uniform distribution of the reinforcement within the matrix. The rate of solidification in the present work is directly related to the VHC of the chill used. Therefore a smaller dendritic arm spacing is obtained in composites cast with the help of metallic end chill, copper which is having a greater VHC as compared to other end chills (nonmetallic) used in the present research. This has resulted in a more refined grain structure of the chilled composite developed. It is observed that the reinforcing particles during solidification are generally trapped between the dendrites. This trapping is usually more pronounced at the dendritic tip and for the first few secondary branches. The reinforcement that are placed within the secondary branches tend to remain trapped between the branches as the dendritic growth propagates. (Figure 3) shows the EDAX analysis test report of the composite developed (copper chilled MMC with 9 wt.% reinforcement)indicating the presence of constituents of the matrix alloy as well as the reinforcement indicating the reinforcement is thoroughly mixed and present in the composite.

Figure 3: EDAX photograph for copper chilled MMC with 9 wt. % reinforcement content.

Mechanical Properties

Hardness of Chilled Composite

In comparison to ferrous materials, aluminum alloy is soft, ductile and possesses lower hardness. Hence, it becomes extremely difficult to use aluminum and its alloys in abrasive/wear environments. To overcome this difficulty, aluminum alloys are reinforced with hard ceramic reinforcements so that the composite is rendered wear resistant. In the current research, hard borosilicate glass particulates are embedded in softer LM-25 aluminum alloy to render the composite harder than the matrix material. Hardness tests were conducted only after subjecting the test samples to aging heat treatment process. From the past research of the present author, if all other factors are kept constant, the aging rate of a composite is generally faster than that of the matrix alloy [33]. After solution treatment, optimum aging conditions can be determined by observing the hardness of the MMCs cast using chills for different aging durations. It is known that the optimum aging conditions are strongly dependent upon the amount of reinforcement present [27]. It can be seen that for each MMC, as the aging time increases, the hardness of the MMCs increases to a peak value and then drops again. As reinforcement content is increased, there is a tendency for the peak aging time to be reduced because reinforcement provide more nucleation sites for precipitation. As expected, for any fixed aging temperature and duration, increasing the reinforcement content causes the hardness of the MMC to increase since reinforcement particulates are so much harder than the aluminum alloy matrix [32].

Figure   4:  Plot of Hardness Vs. wt. % reinforcement for MMCs cast using different chil.

(Figure 4) shows hardness of chilled MMCs cast using various types of chills. The results of micro hardness test (HV) conducted on chilled MMCs samples revealed an increasing trend in matrix hardness with an increase in reinforcement content (up to 9 wt. %). Results of hardness measurements also revealed that the type of chill has an effect on hardness of the composite. This significant increase in the hardness can be attributed primarily to presence of harder ceramic particulates in the ductile matrix that has higher constraint to the localized deformation during indentation because of reduced grain size due to chilling. In ceramic-reinforced composites, there is generally a big difference between the mechanical properties of the reinforcement and those of the matrix. These results in incoherence and a high density of dislocations near the interface between the reinforcement and the matrix [30]. Precipitation reactions are accelerated because of incoherence and the high density of dislocations act as heterogeneous nucleation sites for precipitation [31]. It is observed from the results of the hardness test that hardness of the composite increases linearly (up to 9 wt. % addition) as the reinforcement increases and maximum harness was observed in the case of composite cast using the copper chill. This increase in hardness in turn increases the wear resistance of the composite developed that suits automotive application. It is observed form the Figure 4 that beyond 9 wt. % addition, the hardness decreases because of cluster formation and segregation of the reinforcement as evident from the microstructural studies. VHC of the chill has an effect on the hardness of the composites developed since chilling refines the grains in reducing the grain size that again offers resistance to indentation. It is observed that of all the metallic and nonmetallic chills, copper chilled composites have attained the maximum hardness because of its high heat extraction capacity. It is also observed that maximum hardness was observed for all the composites developed was near the chill end and it decreases towards the riser end. This again shows that chilling has an effect on the hardness.

Ultimate Tensile Strength (UTS) of the Chilled Composite

Over a few decades, metal matrix composites are being widely used as structural material owing to their increased strength coupled with an extremely good strength-to-weight ratio. This is primarily due to the fact that the composites transfer the applied load onto the harder reinforcements via a soft and ductile matrix metal. In order to realize this, an imperative chilling was introduced in the present research to ensure a strong interfacial bonding between the different phases making up the composite.

Figure 5:  Plot of UTS Vs. wt. % reinforcement for MMCs cast using different chills.

(Figure 5) shows the effect of reinforcement content on Ultimate Tensile strength UTS for various MMCs cast using different types of chills. It is observed that UTS is again maximum for the chilled composite cast using copper chill and least for the graphite chill (low VHC). Further, it may be observed from Fig.5 that, the effect of increasing the VHC of the chill increases UTS. It is also observed from the microstructure as well from the strength test results that, increase in the reinforcement content increases UTS up to 9 wt.% and beyond which it decreases due to uneven distribution of the reinforcement and cluster formation. These clusters act as stress risers concentrated at that location fails to transfer the load and hence the strength falls. In most cases, ceramic reinforced MMCs have superior mechanical properties compared to the un-reinforced matrix alloy because these MMCs have high dislocation densities due to dislocation generation as a result of differences in coefficient of thermal expansion [32]. As in the study however, with the incorporation of glass particulate reinforcement has a major effect in improving mechanical properties. Standard results reveal that the UTS of the matrix aluminum alloy (LM-25) used in the present research is around 100 N/mm2. Whereas in the present research, the UTS obtained for aluminum alloy (LM-25) reinforced with borosilicate glass particulate cast using different chills of different VHCs ranges from 118 to 170.0 N/mm2. The increase in UTS suggests that borosilicate glass particulate as reinforcement and chilling has an effect on the UTS. Past research indicates that the strength of the composite depends on the reinforcement, interfacial bond between the matrix and the reinforcement, distribution of the particles as well as on the size of the particles. But in the present investigation in addition to all the above parameters, sound casting with fine grain structure was obtained due to chilling, vortex method of stirring and preheating of the reinforcement has played a vital role in further increasing the UTS. From the results of the mechanical characterization of chilled Al-alloy (LM-25) and borosilicate glass MMCs, one can conclude that strength depends on reinforcement content and chilling effect. In order to confirm the above discussions, fractographic analysis of the specimens failed in strength test was under taken using SEM. (Figure 6) (a to d) shows the photomicrographs (at 100 X and 100 µm) of the fractured specimens containing 3, 6, 9 and 12 wt. % reinforcement MMCs cast using the copper chill. It is observed that for chilled MMCs containing lower percent of the reinforcement (less than 6 wt, %), fracture was in ductile mode containing large amount of dimples and failure is due to propagation of the cracks. Here again the size of the dimples present in the fractured surface depends on the reinforcement content as well as on the heat extraction capacity of the chill i.e., VHC. Further increase in the reinforcement content up to 9 wt. %, the composite become brittle and the SEM of the fractured sample showed cleavage mode of fracture with voids. At higher percent of addition of reinforcement (beyond 9wt,%), cluster formation of the reinforcement was observed in the microstructure with poor interfacial bonding that has caused the trans- granular type of brittle fracture. Therefore the size of the dimples in the fracture surface exhibits a direct proportional relationship with strength and ductility of the MMC i.e., finer the dimple size increases the strength and ductility of the MMCs developed.

Figure 6:  (a to d) Fractographs of composite failed in tension test.

 

Thermal Properties

Coefficient of Thermal Expansion (CTE) of the Chilled Composite

Coefficient of Thermal Expansion (CTE) is a term used to describe the relationship between the dimensions of an object with temperature. Thermal characterization of the ,composites along with mechanical characterization aids in exploring and exploiting the properties of the composites exclusively for automotive, aerospace and electronics applications due to their customizable property i.e., coefficient of thermal expansion suiting to the product requirement. This property especially aids in the employment of such materials in severe operating conditions calling for excellent structural rigidity with sustained mechanical and thermal properties at higher temperatures. An effort has been made in the present research to evaluate the CTE of the chilled MMCs with the help of a Thermo-Mechanical Analyzer (TMA). The thermo-grams were recorded for the samples and the plot of dimension change in micron meter (µm) versus temperature in oC for the copper chilled MMCs with 9 wt. % reinforcement is as shown in (Figure 7). It is observed from the plot that thermal expansion of the chilled composite increases as the temperature increases and the change in dimension for all the specimens ranges from 0 to 55 μm when the specimens are heated from ambient temperature of 30ºC to 250ºC. The specimens exhibit linear relationship when they are heated from ambient temperature to 0ºC. However, as the heating is continued beyond 200ºC, the change in dimension for all the specimen increases at a rapid pace and at 250ºC, it was observed that with a dimension change was to 40 - 55 μm. This is mainly because that at higher temperatures (>100ºC) aluminum alloy tends to lose its strength and the alloy tends to become malleable. However, due the presence of the reinforcement which is extremely resistant to deformation at elevated temperatures, restricts the plastic flow that sets up in the chilled composite up to a temperature of 200ºC. This shows that reinforcement embedded in the matrix strengthens the dimensional stability at elevated temperatures. From the open search of literature, it is well known that the CTE of aluminum alloy is around 37 μm/mºC at 100ºC. Whereas the CTE of the MMCs containing different wt. % reinforcement in the present research was found to be in the range of 13 to 23 μm/mºC which is far less compared to the matrix alloy (Figure 8). Such a reduction in CTE compared with the matrix alloy is due to presence of the ceramic reinforcement (CTE of the reinforcement used: 3.3 μm/mº C) which is a thermal insulator embedded in the matrix alloy. The analysis of CTE values at different temperatures obtained for MMCs cast using different end chills reinstate the fact that copper end chill (high VHC) with 12 wt.% reinforcement possesses lower values of CTE (13 μm/mºC) compared with other MMCs containing 3, 6, 9 wt.% reinforcement at different temperatures (Figure 8 for copper chilled MMC). This is again attributed to the fact that copper chilled MMCs have fine grain structure compared with other chilled MMCs. Therefore it is concluded that temperature, reinforcement content and VHC of the chill has an effect on CTE of the composite developed.

Figure 7:   Dimension change v/s Temperature for MMC (9wt. % reinforcement) cast.

Effect of Carburization on Hardness of the Chilled Composite

Past research indicate that aluminum and its alloys are less hampered by carburizing environments as they tend to form protective oxide layers over them. However, the recent research on Al based composites have showed that in severe carbonaceous environments, carbon monoxide effects are sufficiently high on the surface of aluminum alloys that carbon can seep into the material through the matrix-reinforcement interface layers. Under such circumstances, it becomes all the more important to study the effect of carburization on aluminum composites that are employed in automotive applications which operate under severe carbonaceous environments. In the present research an attempt has been made to study the effect of carburization on the hardness and weight gain of aluminum alloy (LM-25) reinforced with borosilicate glass particulates.

Figure 8:  Plot of CTE Vs. wt. % reinforcement for different temperatures.

 

Figure 9:  Effect of carburization on the weight gain for different MMCs.

Figure 10: Microstructure after carburization.

Figure 11:  SEM photograph after Carburization.

(Figure 9) illustrates graphically the weight gain due to carbon deposition on copper chilled composites specimens after 4 hours of carburization. Form the percent research it was found that increase in weight gain of all the specimens has a relationship with duration of carburization. As the duration of carburization increases from one hour to four hours it is observed that the weight gain also increases for all the chilled MMCs. It is observed from the microstructure (Figure 10) and SEM photographs (Figure 11) that that after carburization there is formation of flaky carbide layer due to prolonged heating (9wt.% . Cu chilled MMC). Also carburizing beyond nine hours causes’ excess carburization that leads to embrittlement of the chilled composite which reduces the hardness (Figure 12).

Figure 12:  Effect of carburization on hardness for Cu chilled MMCs.

Figure 13:  Effect of temperature on thermal conductivity for different Cu chilled MMCs.

 

It is observed that the hardness of the chilled MMC increases as the duration to carburization increases (up to 9 hours as in (Figure 12) and microstructural studies reveal that this increase is attributed to the formation of brittle aluminum carbide layer on the surface. Hence, it is imperative to evaluate the hardness of the material before and after carburization to assess the effect of carbon deposition on the chilled MMCs property. (Figure 12) shows the variation of hardness of the copper chilled MMC before and after carburization (four hours of testing). Form (Figure 12) it is observed that there is a relation between the hardness values before and after carburization for the MMCs cast using copper chill containing different wt. % reinforcements. It is also evident from the results that the hardness of the carburized specimens increases due to the deposition of carbon in the interfaces of the matrix and the reinforcement and also due to the formation of extremely hard carbide layers over the surfaces.

Thermal Conductivity (TC) of the Chilled Composite

It is well known that the thermal conductivity of a particle reinforced composite depend mainly on the type of particle used, the particle size, their distribution within the matrix and the quality of the interfacial bonding between the various phases of the composites. In the present research an effort has been made to evaluate the effect reinforcement and chilling on the thermal conductivity of the composites developed. Thermal conductivity of different MMCs cast using copper chill is shown in (Figure 13) as a function of temperature. Thermal conductivity test results indicate that thermal conductivity of the chilled MMC decreases with temperature and as well as with the reinforcement content in the matrix. This decrease is because of very low thermal conductivity of the reinforcement particles (1.2 W/m K at 90°C) that offers an excellent resistance to heat flow. (Table 2) shows the details of the thermal characterization viz. thermal conductivity, thermal diffusivity, specific heat capacity of the MMC containing 3 wt. % reinforcement cast using copper end chill at different temperatures. (Table 2) Thermal conductivity, thermal diffusivity, specific heat capacity of the MMC (3 wt. % reinforcement) cast using copper end chill at different temperatures. It is observed from the test results that the composites cast using the copper chill, highest thermal conductivity of 159.12 w/mk was obtained at a temperature of 50 °C for MMC with 3 wt. % reinforcement content. Thermal conductivity of MMCs cast using other chills viz. mild steel, silicon carbide and graphite decreases for the same temperature (50 °C) and for reinforcement content i.e., 3 wt.%. Results of the TC test reveal that copper chilled MMC containing 3 wt. % reinforcement has highest TC because of high VHC of the copper chill and fine grain structure with strong interfacial bonding. In MMCs containing higher amount of reinforcements, TC decreases because of uniform distribution of thermally resistant reinforcement in the matrix. Therefore VHC of the chill has a significant effect on TC since the grain structure depends on the type of chill used viz. metallic or nonmetallic. However, thermal conductivity of different chilled MMCs are less than the thermal conductivity of the matrix alloy (170 w/m K).

Table 2: Thermal conductivity, thermal diffusivity, specific heat capacity of the MMC (3 wt. % Reinforcement) cast using copper end chill at different temperatures.

Temperature (°C)

Thermal diffusivity (m2/s)

Sp. Heat capacity Cp, (J/kgk

Thermal conductivity, k, (w/mk)

 

50

5.2

940

128.99

100

4.9

940

121.55

150

4.6

940

114.11

200

4.3

940

106.17

250

3.9

940

96.74

300

3.7

940

91.78

 

Conclusion

The objective of the present research is to fabricate a superior quality and sound Al-alloy (LM-25) reinforced with borosilicate glass particle chilled metal matrix composites. Microstructural, mechanical and thermal characterization of the chilled composite developed reveal the following. EDAX analysis of the composite reveal that the reinforcement is thoroughly mixed and present in the composite. Microstructural studies reveal that the reinforcement is uniformly distributed within the matrix with perfect bonding and fine grain structure (effect of chilling). Hardness of the chilled MMCs developed depends on the aging heat treatment process, VHC of the chill as well as mainly on the reinforcement content. UTS of the chilled MMCs developed mainly depends on the fine grain structure, embedding the ceramic reinforcement and the strong bond between the matrix and the reinforcement which is the effect of chilling, It is observed that both strength and hardness increases up to 9 wt. % reinforcement indicating that further addition has no effect on mechanical properties of the chilled MMCs. CTE rest results reveal that fine grain structure, reinforcement content has an effect since the reinforcement strengthens the dimensional stability of the composite which restricts the expansion at higher temperatures. It is observed that CTE bares a linear relationship with the temperature. Carburization test results indicate that there is weight gain and increase in hardness of the chilled MMCs developed because of formation of carbide layer on the surface. Thermal conductivity test results indicate that TC decreases linearly with the temperature because of thermal resistant reinforcement also it was found that fine grain structure and perfect bonding (effect of chilling) has an effect on the TC. Therefore it is concluded that since the chilled MMC developed in the present research possesses superior mechanical and thermal properties, the composite developed can be recommended for automotive applications.

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