CHAPTER -2 LITERATURE REVIEW. During the literature review, an intensified research work is assessed

Transcription

1 8 CHAPTER -2 LITERATURE REVIEW 2.1 INTRODUCTION During the literature review, an intensified research work is assessed on aluminum and its alloy based metal matrix composites because of low density, good corrosion resistance and excellent mechanical properties for various engineering applications. Early MMCs find their tradition confined to military and aerospace applications. Their extensive usage is hindered due to high production costs, limited production methods, and restricted product forms. The factors influencing the type and form of reinforcement are the desired material properties, ease of processing, and part fabrication. In the early stages of development, only a limited range of reinforcements have been used. The stability between the components and the differences in their thermal properties such as coefficient of thermal expansion and coefficient of thermal conductivity are the limiting factors in the compatibility of the two materials used to make the composite. The particulate reinforced metal matrix composites possessing isotropic properties are found to be thermally stable and wear resistant as compared to monolithic materials. Based on the literature review, an attempt has been made to exploit the possibility of usage of fly-ash as a reinforcement material for Al-Pb metal matrix composite.

2 9 2.2 ROLE OF REINFORCEMENTS IN ALUMINUM MMCs The literature review reveals that most of the works have concentrated on Al-SiC metal matrix composites produced using different techniques. Most of the researchers have used silicone carbide (SiC) because of its availability in the wide range of grades. Mazen and Emara [7] and Tan et.al [8] have observed that the presence of SiC in aluminum can increase its yield strength, young s modulus and wears resistance. It has been reported that, alumina (Al2O3) can be the alternate reinforcement material for SiC because of its stable, inert, high temperature behavior, and high corrosion resistance [9-11]. Dobrzanski et.al [12] have stated that the presence of Al2O3 particles can increase the hardness and impede the deformation of the composite. Titanium Carbide (TiC), borate whiskers, silicon dioxide (SiO2), diamond, graphite, granulated slag, fly-ash, alumino silicate, quartz, zirconium dioxide (ZrO2), mica and titanium dioxide (TiO2) are also being employed as reinforcements in the aluminum based metal matrix composites. Despite their potential properties, limited manufacturing processes have hindered their wide commercial usage. The material used in the present work is fly-ash (waste by-product from thermal power plants) reinforced Al-Pb matrix alloy. In the metal matrix composite, the reinforcing particles with different physical characteristics may result in mismatch at the interface between matrix and reinforcement. This situation is a favorable condition to increase the strength since it can increase the dislocation density and effective in nucleating new grains. The

3 10 reinforcement particles may also stabilize grain size by pinning of the grain boundaries. It has been presented that, the pinning effect can increase the strain rate sensitivity and result in super plasticity at high strain rate [13-15]. The formation of microscopic cavities, which primarily may occur in the grain boundaries during the high temperature deformation, is referred to as cavitation. The cavitation may limit the elongation in the metal matrix composites. Ganguly and Warren [16] have concluded that, the extent of cavitation has been increased by the grain boundary sliding due to the presence of reinforcing particles. They have also suggested that, the use of very fine reinforcing particles or application of hydrostatic pressure may minimize the cavitation problem. They have also remarked that the particle clusters may be the prone areas of crack initiation and the cracks can be minimized by the uniform distribution of reinforcing particles. It has been noticed that, the volume fraction of reinforcement as a critical factor may control the elastic modulus of the composite. Tan et. al [8] have observed that the elastic module of composites is higher when compared to non reinforced matrix at elevated temperatures. They have also found that both interfacial bond strength and volume fraction of reinforcement are crucial for effective transfer of load from matrix to the reinforcement and consequently in strengthening the composite. By using variety of reinforcements with different volume fractions it may be possible to optimize the wear and tear properties of the composites. Mazen and Ahmed [9] and Arpon

4 11 et.al [17] while experimenting separately have examined that the ceramic reinforcements may contribute low coefficient of thermal expansion which in turn may increase hardness, stiffness and specific strength of the composites. In some cases, these characteristics might have increased the density of the composite slightly depending upon the volume and density of the reinforcement material. 2.3 MANUFACTURING PROCESSES OF ALUMINUM MMCs The general processing techniques used to manufacture the aluminum metal matrix composites are either solid state processing techniques or liquid state processing techniques. The choice of a particular process may depend on the matrix, reinforcement, and service requirement of the composite. In a composite material, the distribution of reinforcement is the major factor as it can dictate the morphology, microstructure and finally the mechanical properties. Proper mixing method must be employed to minimize agglomeration of reinforcements. Quick pouring and chill casting technique have been employed to reduce settling of particles [18, 19]. Sometimes, the secondary processes like extrusion, forging, and rolling operations may promote better distribution of reinforcements in the composites. In another study, it has been observed that the reactivity between reinforcement and the matrix can significantly affect chemistry of the matrix and the microstructure [20]. In addition, the interfacial strength may play a vital role during the deformation and fracture of composite materials. These problems can be minimized by the powder

5 12 metallurgy technique because of its low processing temperatures. Hence in the present experimental work, the powder metallurgy technique has been chosen as the processing method for preparing Al- Pb/fly-ash composites Powder Metallurgy Technique of Processing Al MMC In the literature review, it has been noticed that the aluminum metal matrix composites consisting of dispersions, particulate whiskers, fibers have been produced by a variety of powder metallurgical techniques such as: (1) Conventional powder metallurgical process involving pressing and sintering of elemental powders to produce near net shapes. (2) Hot extrusion, vacuum hot pressing, hot isostatic pressing, vacuum sintering to produce billets. (3) Powder forging and powder rolling to produce the components directly. (4) Pressure-less sintering and spray forming processes. The above methods can offer different combinations of cost, shape, capability and potential properties. 2.4 ALUMINUM POWDER METALLURGY The powder metallurgy techniques have been observed possessing the advantage of controlled porosity, attainment of close tolerances, refined micro structure, near-net shape formability and elimination of machining scrap losses. It has been noticed that the powder

6 13 metallurgy techniques have shown the capability of developing new or extended alloy families which is not possible by the casting techniques. The powder metallurgy process can be extended to the aluminum materials to increase their utility. The aluminum powder metallurgy components are having potential applications in the automotive market because of the need to reduce weight, to lower emission and to boost fuel economy. In the literature review, some important powder metallurgy components are employed for engine cam caps, shock absorber parts, air conditioning compressor parts, connecting rods, and mirror brackets Processing Aluminum by Powder Metallurgy Technique During literature review the following points are noticed to be the advantages of processing Al by the powder metallurgy techniques: (1) Better green strength is obtainable for aluminum alloy powders as compared to ferrous powders even at a lower compaction pressures. (2) Low energy is sufficient to process aluminum alloys in contrast to other materials due to their low sintering temperatures in the range of 500 to C and sintering times in the range of 10 to 60 minutes. (3) Post sintering treatments like coining, cold forming, sizing, heat treating, hot forging, anodizing, etc. are applicable with greater ease on the aluminum alloys. This is owing to higher ductility and unique characteristics of aluminum. (4) It is possible to have improved fracture toughness, resistance to stress corrosion cracking, strength due to fine grain size, improved

7 14 microstructural control, compositional and homogeneity of aluminum components. (5) New aluminum alloys may be possible to produce by incorporating insoluble elements like lead and cobalt in the aluminum. (6) Aluminum matrix composites with particle, fiber and whisker reinforcements with wide range of reinforcement levels and improved uniformity of reinforcement distribution can be easily produced. Besides these advantages, certain disadvantages are also noticed during the processing of Al by the powder metallurgy. These may occur during the compacting or sintering processes Compacting Problems Compacting of aluminum in the normal steel dies may generate serious problems because of its tremendous seizing and galling characteristics [21, 22]. It has been examined that the presence of non-reducible oxide film may not permit the development of sufficient strength in the green briquettes. High compacting pressures are required to produce briquettes with sufficient strength and density because of low apparent densities of aluminum powder (0.8 to 1.1gram/cm 3 ) and inferior flow characteristics [21]. The high compacting pressure in turn may cause more seizure, scoring and galling of the die wall. The success of compacting and sintering process may depend to a great extent on the selection of aluminum powder particle size, shape and composition [21]. Too fine or flake form of powder may lead to

8 15 poor flow resulting a tendency for cold welding and seizing in the die. Coarse or spherical powder may exhibit better flow but develop inferior strength in both green and sintered conditions. In the powder metallurgy, it has been observed the necessity of admixed or die wall lubricants to reduce friction between metal powders and die walls and to minimize die wear. The admixed lubricants can simplify the compaction and can minimize the interaction between the tooling and compact during the ejection [23]. However, the admixed lubricants may have some deleterious effects on some properties of the briquettes. They usually decrease the green and sintered strengths of the aluminum powder metallurgical briquettes. Low green strength results in the formation of cracks, edge blunting, part laminations and breakage of briquettes prior to sintering. The admixed lubricants may also cause some difficulties during sintering. It has been noticed that the faded, weak, and dimensionally non-uniform compacts have been produced during sintering the green briquettes admixed with lubricants. Therefore, prior sintering (at temperatures lower than C) must be carried out for aluminum powder metallurgical components with admixed lubricant. This can avoid the reaction of decomposition products with aluminum during sintering. During sintering, if the internal lubricant leaves the residual products in the composite, it impedes the formation of strong metallurgical bond and the desired mechanical properties. The lubricant must burn out in a non-oxidizing atmosphere to prevent the oxidation of aluminum in the

9 16 presence of oxygen. Embrittlement, distortion and discoloring have also been observed during sintering of the pre-alloyed powders. Kehl et. al [24] have presented a comparative study on the effect of lubricant admixtures and die wall lubricants on the dimensional stability, green and sintered strength of Al Cu briquettes. They have used metal-free stearamide wax upto 3wt% as lubricant. When it is used as die wall lubricant, it is applied with an atomizer to the walls of pressing tool in the form of 2wt% suspension in ethyl alcohol. The admixed lubricant in comparison with die wall lubricant lowers the green strength, decreases true density, and reduces the strength during sintering. This may be on account of poor wetting and expansion of the compact during de-waxing. The residues of the wax reduce the wetting of aluminum particles with the eutectic melt and thereby reduce the subsequent shrinkage. The result is a net dimensional change. Some investigators [5, 25, 26, 27] have used silicone spray with or without fine graphite powder as die wall lubricant. This produces the briquettes without scoring on the surface of the compact. Therefore, the silicone spray is used as the die wall lubricant in the present experimental investigation Sintering Problems In the aluminum powder metallurgy, the presence of aluminum oxide may cause a major problem during sintering because it is not reduced by the common furnace atmospheres at the temperatures of sintering

10 17 [21]. The aluminum oxide, which is dense and stable, may acts as a skin on the aluminum powder particle. This may hinder the diffusion of particles during sintering. Because of stable oxide layer which can not be removed by the reducing atmospheres during sintering, weak sinter necks may develop especially at the places where the oxide layer is damaged during the compaction. Because of this, the properties of briquettes sintered from pure aluminum may remain unsatisfactory even when compacted at high compacting pressures [28]. Some of the problems of aluminum during sintering can be overcome using the liquid phase sintering [22]. In liquid phase sintering, the liquid metal penetrates and diffuses into the oxide layer through the cracks caused by the compaction [29]. The stable oxide film present on the aluminum particles gets disrupted and in due course the oxide layer is lifted out. This may result in the inter particle bridges to fully establish and give good particle bonding. The liquid phase may also assist the material transport and the remainders of the oxide layers may settle as fine particles at the grain boundaries. In any powder metallurgical process, the sintering atmosphere is governed by the material characteristics [30]. The sintering atmosphere, temperature and humidity conditions may affect the decrease in density and growth of the sintered briquettes. Nitrogen, dissociated ammonia, vacuum and argon are observed to be the common sintering atmospheres used for aluminum products. Dudas

11 18 and Thompson [31] have reported that the surface nitriding has occurred in the nitrogen atmosphere when the sintering time extends beyond 40 minutes. When dissociated ammonia is used, the mechanical properties of the sintered parts are slightly lower than those sintered in nitrogen. The lower properties of sintered parts in the dissociated ammonia may be due to the presence of hydrogen and/or un-dissociated ammonia [31]. Low hardness values have been reported for nitrogen sintered briquettes [32]. The better properties are attributed to argon sintered briquettes compared to vacuum sintered ones due to the absence of any volatization loss of alloying additions. The maximum ultimate tensile strength is imparted to the aluminium briquettes with argon sintering. The maximum linear expansion is observed for aluminum alloy graphite composites sintered in the nitrogen atmosphere instead of argon and vacuum atmosphere [33]. Therefore, in the present experimental work, the aluminum composites are sintered in an argon gas atmosphere. 2.5 DEVELOPMENT OF ALUMINUM BASED BEARING MATERIALS During literature review it has been found that the generally used bearing materials are white metals. Though these materials have good seizure resistance, embedability and conformability; the lack of strength constrain them to be used as bearing materials for the applications requiring heavy loads, high speeds and high operating temperatures. This may be the reason for the development of aluminum alloys and its composites as the bearing materials. The

12 19 various alloying elements are added to improve bearing characteristics of aluminum based bearing materials [34]. The anti-welding and antiscoring characteristics of aluminum alloys can be improved by the addition of alloying elements which may form discrete soft phase constituents. The important factor, which governs the seizure resistance of the bearing metal against the journal, is the mutual miscibility of bearing metal and journal and the nature of bond between the atoms of bearing metal and the journal material. For steel journals, the bearing metal must have an atomic diameter greater by at least 15% than that of iron and it must have covalent bond [35]. The commercially available materials, which can meet the above criteria, are Cd, silver (Ag), In, Sn, Pb, gold (Au) and Bi; out of these both Pb and Sn can offer most attractive combinations of engineering properties, cost and availability. Pb is found to be soft and cheaper and has low modulus of elasticity when compared to Sn. Therefore, Pb is chosen as an alloying element in the aluminum in the present work Powder Metallurgy Technique of Processing Al-Pb Alloy The leaded aluminum alloys in the form of pre-alloyed powder are produced by the powder rolling process [36]. The rolling process causes the mechanical rupturing of the oxide film. This causes clean, fresh and active aluminum surfaces which can contact with each other producing metal to metal bond between the neighboring particles. The rolled green strips have sufficient strength and

13 20 ductility. The coils produced from this material are sintered. It is found that during the sintering process the diffusion between the particles improves strength and ductility. Gopinath [37] has developed sintered Al-17.5% Pb alloy using the conventional powder metallurgy technique. In this technique, the powder mix is blended in the double cone blender for 30 minutes and compacted at 392MPa and 568MPa in a double acting die. The green briquettes are sintered in the nitrogen atmosphere at 600 o C for 1, 3 and 6 hours. For the prepared briquettes, the effect of compaction pressure and sintering time has been studied. Sastry et. al [38] have studied the densification behavior of leaded aluminum alloys processed through attrition milling routes and conventional ball milling. It has been reported that the attrition milling can be an effective method for densification of experimental alloys. Nath et. al [5, 39] have used the conventional powder metallurgy to produce Al-Pb alloys containing 10, 15, 20 and 25mass% Pb. In this work, the powder mixtures are compacted in the pressure range of 400 to 600MPa. It has been reported that Al-15mass% Pb alloy have minimum spring back and maximum green strength and green hardness. Al-4.5%Cu-Pb admixed alloys are produced by compacting the powders in the pressure range of 98 to 490MPa using the conventional powder metallurgy technique. It has also been reported that the compaction pressure can increase the green and sintering properties [40, 41].

14 FLY-ASH In the literature, the fly-ash is described to be a particulate waste byproduct formed as a result of coal combustion in thermal power plants. It has been reported that the disposal of fly-ash is a major challenge for the power plants with minimum pollution to the environment. The composition of fly-ash may depend upon the coal being burned in the thermal power plants. In general, the constituents of fly-ash are silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3) in major quantity and oxides of magnesium (Mg), calcium (Ca), sodium (Na), potassium (K) etc in minor amount. The trace amounts of vanadium oxide and manganese oxide are also observed in the fly-ash [42]. The morphology of fly-ash is revealed to comprise of smooth and tiny spherical particles, either solid or hollow. The solid sphere fly-ash is termed as precipitator fly-ash and the hollow sphere fly-ash is termed as cenosphere fly-ash. The density of the precipitator fly-ash is in the range of 2 to 2.6g/cm 3 and its particle size range from 1 to 150µm, whereas the density of cenosphere fly-ash is in the range of 0.4 to 0.6 grams/cm 3 and its particle size range from 10 to 250 µm Disposal and Utilization of Fly-ash It has been reported that the thermal power plants throughout the world produce hundreds of million tons of fly-ash. Out of this, only a small portion of the fly-ash is being reused for productive purposes. The remaining amount of fly-ash is either disposed off in controlled

15 22 landfills or stockpiled for future use. As a result, significant amount of cost is associated with disposing these vast quantities of fly-ash, and there is a need to develop new and innovative, yet environmentally safe applications for the utilization of coal fly-ash. During the last few decades, extensive research has been carried out to utilize fly-ash as an engineering material which turns waste into useful product [43]. 2.7 ALUMINUM FLY-ASH COMPOSITES The Al-fly-ash composites are produced by the casting and powder metallurgy techniques Casting Technique The fly-ash is successfully used as a filler material in the light metals and alloys by various researchers. Dean Golden [44] has reported that it is possible to produce the ash alloy cast products by the standard foundry techniques. Most of the ash alloys find promising applications in the automotive industry. Rohatgi [45] has demonstrated the production of ash alloy castings of different shapes and dimensions. It has been reported that the matrix hardness increases from 65 to 82 HB with the addition of 8volume% of fly-ash. The addition of fly-ash significantly increases the abrasive wear resistance of aluminum and consequently leads to wide spread applications in automotive, small engine and electro mechanical machinery sectors. It has been observed that the fluidity of the ash alloys is adequate to make variety of castings and also the cenosphere fly-ash decreases casting densities and improves its economics further.

16 23 Rohatgi et. al [46] have produced Al-Si alloy (A356) containing 3 10 volume% fly-ash using the stir casting technique. They have observed uniform distribution of fly-ash particles in the small castings. These composites appear to be attractive products for the engineering applications. The composite comprising of A356 Al and fly-ash exhibits high damping capacity as compared to un-reinforced alloy [47]. For Al-7Si-0.35Mg/fly-ash composites, the interfacial reactions in liquid metal stir cast components are more when compared to components produced by the compo-casting technique [48]. The dimensional stability of A535 alloy is improved by the addition of fly-ash [49]. Rohatgi et. al [50, 51] have studied the infiltration of nickel coated and uncoated cenosphere fly-ash particles with pure aluminum and A356 aluminum alloy manufactured by the pressure infiltration technique. By the use of pressure infiltration technique, segregation of fly-ash particles in the casting has been reduced. The infiltrated length is longer at high pressure or high temperature. It is also reported that the nickel coating can reduce the presence of un-infiltrated agglomerates of fly-ash in the composites and can reduce the infiltration of aluminum into the cavities within the cenospheres. The abrasive wear of stir cast A356/5volume% flyash composite is similar to the aluminum alloy containing alumina fibers but superior to the base alloy [52, 53]. The stability of Al/40volume% fly-ash composite system is studied by Guo et. al [54] using differential thermal analysis (DTA). The aging characteristics of aluminum alloy containing hollow spherical particles are studied by

17 24 Rohatgi et. al [55]. Even though the hardness of the as-cast composite is higher than that of the base alloy, no significant changes in the aging kinetics are observed. It has been noticed that AK12/fly-ash composite has high pitting corrosion in comparison to AK12 aluminum alloy [56]. The presence of fly-ash particles in the aluminum may decrease its coefficient of thermal expansion [2]. It has been reported that up-to 20% fly-ash can be successfully added to the pure aluminum by the stir casting technique [57]. The hardness, wear resistance and ultimate tensile strength increase with increase in fly-ash content but the ductility decreases. In another research, Al 4.5%Cu/fly-ash metal matrix composite is cast by the stir casting technique [3]. It has been reported that Al 4.5%Cu/fly-ash metal matrix composite can be used as a bearing material. Addition of cenosphere fly-ash particles to Al-Si alloy can increase its hardness and ultimate tensile strength whereas it can decrease the density and wear loss [58]. The wear resistance of Al-12wt% Si/fly-ash composites increase with increase in wt% of flyash but decrease with the increase in normal load and track velocity [59]. Al-4Si Mg reinforced with fly-ash particles are fabricated by the stir casting process [60]. It has been observed that increasing of flyash content can increase the porosity in the composite. The 15wt% fly-ash composite shows highest porosity and lowest hardness. The tensile, compressive, and impact strengths and hardness are improved with the increase in fly-ash content in Al-4.5%Cu alloy [4].

18 25 The resistance to dry wear and slurry erosive wear also increase with increasing fly-ash content Powder Metallurgy Technique Green briquettes of pure aluminum powder upto 20wt% fly-ash are produced by Guo et. al [61] using the conventional powder metallurgy technique. Green strength and green density increase with increasing compaction pressure and decrease with increasing fly-ash content. The hardness does not change significantly for the briquettes containing up to 10wt% fly-ash, but it decreases for above 10wt% flyash levels. The sintering has been carried out at 600, 625 and C for 0.5 to 6 hours in the nitrogen atmosphere for the briquettes prepared at 414MPa. Upon sintering, density of the green briquettes decreases. With the increase in fly-ash content, the sintered strength decreases. Ramana et. al [26] have prepared mixtures of aluminum powder containing 0, 10 & 20% fly-ash and compacted at 96, 128 and 160MPa. They have also prepared the briquettes for Al 10wt% fly-ash with various metallic and non-metallic additions [62]. It has been observed that spring back, ejection pressure, green density, green strength and hardness increase with increase in compacting pressure while the true porosity decreases. Angeliki et. al [63] have prepared Al/fly-ash composites by the powder metallurgy technique and have reported a decrease in density and an increase in hardness with the increase in wt% fly-ash. Fly-ash aluminum alloy composites have been produced by compacting the powder particles in the pressure

19 26 range of 63 to 316MPa [64]. It has been observed that as the compaction pressure increases the green density increases and the density decreases with the increase in wt% of fly-ash. The fly-ash reinforced AA6061 metal matrix composites have been produced using the cold pressing followed by the hot extrusion [65]. It has been reported that the hardness and tensile strength of 2wt% fly-ash composite are better when compared to monolithic alloy after age hardening. The composites also exhibit better wear resistance compared to the matrix alloy [66]. From the above, it can be concluded that the aluminum fly-ash composite components may be produced by the casting and powder metallurgy techniques. When compared to casting, the powder metallurgy technique is capable of producing uniform distribution of particles in the composite with near-net shaped products. 2.8 FLY-ASH AS AN ADDITIVE IN Al-Pb ALLOY It has been reported that the fly-ash as a waste by-product from thermal power plants in India may reach million tons by the end of 2012 [67]. Despite the extensive research, the utilization of flyash is found to be low. The shape of fly-ash particle is spherical. The metal matrix ash composites encompass lower density and lower stress concentration than the composites with alumina, silicon carbide particles. This is mainly because of the spherical shape and low density of fly-ash when compared to the angular shape and high density of alumina and silicon carbide particle. It has been observed

20 27 that, the discontinuous reinforced composites incorporating inexpensive particles in view of their low cost find widest application in the automotive industry. Among the metal matrix composites, the greatest attention is focused on aluminum matrix composites. The addition of fly-ash particles may serve as filler and reinforcement material and reduces the cost of aluminum composites. It may also improve selected properties while maintaining others at adequate levels. Thus, an attempt is made to use the fly-ash particles as an additive to the metals (Al-Pb) to promote the use of this low cost waste by product. From the literature review, it has been clearly observed that almost all the research has concentrated on the development of aluminum fly-ash composites using the casting technique. An important requirement is that the fly-ash must be uniformly distributed in the aluminum matrix, but this distribution is influenced by the tendency of the particles to float due to density differences and interaction with the solidifying metal. Guo et. al [61] have stated that these castings have exhibited segregation and non-uniform distribution of particles because of difference in density between fly-ash particles and the melts. The poor wettability of fly-ash particles with the molten metal has also been reported with the casting techniques. These problems can be minimized if the components are produced by the powder metallurgy technique.

21 28 Upto now no significant information has been noticed on the compacting and sintering behavior of Aluminum-Lead-Fly-ash particles. The information on compacting and sintering of composite powder mixture as well as spring back behavior is vital in making high performance near-net shaped parts by the powder metallurgy technique. Hence, in the present work Al-Pb/fly-ash composites are prepared by the powder metallurgy technique and their compacting and sintering characteristics are evaluated.