Development and characterization of Al15Si2,5Cu0,5Mg matriksli ve CeO2, Y2O3, La2O3 reinforced composites synthesized by mechanical alloying

Technological deveploment in this era has never been as fast as before throughout history. This technological motion brings new material requirements for keeping pace with this fast development. At this point, the interest have been focused on the composite materials which can shelter different material properties within. Composite materials can be formed from different combinations of countless materials. This advantage simplifies the production of materials which need to answer the necessary and spesific features differing with application fields. Metal matrix composite materials are the subgroup of composite materials. Metallic materials are used to form the matrix and mostly ceramics are used to form the reinforcements in the metal matrix compsite materials. So that, metal matrix composites combine the ductility and the strength of metals with the hardness and the high temperature resistance of ceramics. Aluminum and its alloys are used at the fields like aviation and aerospace technologies in which low density is a significant requirement. These materials also perform high corrosion resistance, high ductility and strength, high wear resistance (esspecially in Al- Si alloys). Additionally, aluminum and their alloys are recyclable materials and they are also cheaper from titanium, magnesium and their alloys. These reasons expand the application fields of aluminum and its alloys from transportation to food industry. High temperature resistance is one the most vital feature in some applications like aerospace field. So it is necessary to develop materials which must withstand this conditions. In connection with this example, the research and development studies have been focused on Al- based metal matrix composites rather than Al-alloys since the developing technology needed to be taken into account in materials science and engineering. Oxides, borides, carbides, nitrides and even silicides are added as reinforcements into the metal matrix to develop the mechanical properties of the metal matrix composites. These reinforcements significantly provides esspecially high hardness and wear resistance to the metal matrix composites compared with Al and its alloys. These advanced properties are also protected easily at higher temperatures. Radius/Length ratio detects the type of the reinforcements used in metal matrix composites. Particulate, short fibres and continious fibres are diffrent form of reinforcements added into the metal matrix composites. Short or continious fibers are hard to product and they are more expensive than particulate reinforcements. So particulate metal matrix composites find more area in industrial aplicantions rahter than fiber reinforced metal matrix composites. SiC, Al2O3, TiB2, TiN are some of the most used reinforcements in metal matrix composites. Particulate size and shape, hardness and the other charactheristic features of the particulate reinforcements affect directly to the mechanical and pyhsical properties of the metal matrix composite materials. And also it is very important to disperse the reinforcement homogeniously in the metal matrix with no agglomeration. Otherwise, a matrix in which segregations exist will not perform same properties in all its points. Al-based metal matrix composites are mostly produced with casting and powder metallurgy techniques. It is hard to obtain homogenious metal matrix composites produced with traditional casting. Esspecially sub-micron sized particulates tend to aglomerate in the molten metal at the production of metal matrix composites with traditional casting technologies. So the final products usually have an unhomogenious microstrucre when they are produced with liquid state processing methods. On the contrary to liquid state processing methods, powder metallurgy is a solid state processing method. Mixing and/or milling of the powders is the first process step of powder metalurgy. Compaction of the mixed and/or milled powders is the following step of the process. Generally, sintering of the compacted samples is the final step of this method. Sometimes, secondary shaping methods like cold rolling, extrusion are applicated on the sintered samples to form the final shape of the products. The particulate reinforcements are found in a solid state matrix during this process rather than traditional casting technologies. Accordingly, unhomogeous microstructres are not seen at the metal matrix composites synthesized with powder metallurgy method. In addition to the homogenious microstructre, it is easer to mix the powders at the same chemical composition in mixing step which is also a increasing factor for the reproducibility of the final products. Mechanical alloying is a high energy ball milling technique under powder metallurgy. In difference with casting methods, alloying occurs in solid state like in all powder metallurgy techniques during mechanical alloying. The required energy for the solutionizing of the elementel powders within each other are provided from the high mechanical energy effect. Particle dimensions dramatically decrease with high energy ball milling process of the powders. Also the particle size distrubition becomes more narrow with increasing mechanical alloying time. Overmore, powders are exposed to high deformation which causes deformation strengthening. Besides, cyrstalline sizes of the powders significantly reduce like particulate size with increasing mechanical alloying time. The reduction at the crystalline sizes decreases the driving force for the sintering process. Subsequently, relative densities of the sintered samples show higher values when they are compared with the samples producted with traditional powder metallurgy methods. In this study, Al-based particulate reinforced metal matrix composites are synthesized with mechanical alloying technique. Al-15Si-2,5Cu-0,5Mg matrix alloy is formed from Al, Si, Cu and Mg elemental powders. Silicon is added to the alloy system increase the hardness and the wear resistance of the soft aluminum metal. Because of Al- Si- Cu alloys are age hardenable materials, copper is added to the composition to advance the mechanical properties of the products with age hardening. Magnesium is an element, of which the oxygen affinity is higher than aluminum. Accordingly, a small amount of magnesium is added to the powder mixture to minimize the oxidation of aluminum for a possibility of an oxygen presence during mechanical alloying. Al- Si- Cu- Mg alloys are commonly used in automobile industry at the parts like brake discs which hardness and wear resistance are needed. In the first step, Al-15Si-2,5Cu-0,5Mg powder mixture is mechanically alloyed for 0, 1, 2, 3, 4 and 8 hours in a high energy ball milling enviroment. Powders are poured into the millling vials in the Glove Box under Argon atmosphere and also the vials are opened in here after milling process. Following the milling process, particle size analysis, phase anaylses and microstructral characterizations of the unmilled and milled Al-15Si-2,5Cu-0,5Mg powders are conducted. Phase analyses showed that there was no chemical reaction between the particles during mechanical alloying. Additionally, in comprasion with the micron sized unmilled powders, average particle sizes of the milled powders were under micrometer. Particle sizes of the milled powders were supported and particle shapes were observed with scanning electron microscope analysis. Al-15Si-2,5Cu-0,5Mg milled for 4 hours showed better results including particle size, particle shape and microstrucre rather than other powders milled for different durations. Rewelding between particles after 4 hours mechanical alloying was detected during scanning electron microscope analyses. Accordingly, 8 hours milled powders exhibited higher average particle size and unhomogenious particle size distrubitons. Therefore, powders mechanically alloyed for 4 hours are selected for fixed milling time of the reinforced composite powders. Mechanically alloyed powders are opened under Argon atmosphere in Glove Box and poured into heptan. %2 wt. molten paraffin wax embed into the heptan- powder solution later. Afterwars, heptan was evaporated at normal atmosphere conditions. Surface area of the milled powders dramatically increases with reduced particle size after high energy ball milling. This phenemona increases the oxygen affinity of aluminum based powders. Parrafin wax covered the particle surfaces and minimized the contact of the powders with oxygen. Different amounts of CeO2, Y2O3 ve La2O3 powders (%wt. 0,5, 1, 2 ve 5) were reinforced into Al-15Si-2,5Cu-0,5Mg powders mixture. In this way, the effect of different reinforcement materials and diffrent amounts of reinforcements to the mechanical and pyscial properties of Al-15Si-2,5Cu-0,5Mg alloy were examined. Particle size analyses, phase analyses and microstructral characterizations were perfomed on 4 hours mechanically alloyed composite powders next. There was not seen any important affect of reinforcements and thier amounts, to he particle sizes and shaped. Additionally, phase analyses showed that not any new phases formed between reinforcements ant alloying element.during mechanical alloying. The various analyses and characterization of the milled and unmilled powders revealed that mechanical alloying has a highly positive effect on powders. Furthermore, the increasing resudial stress in the powders with increasing mechanical alloying time indicates a deformation strengthening mechanism. Thermal analyses were conducted on the unmilled and milled powder mixtures. DTA analyses showed a endothermic peak belonging to unmilled powders nearly at 655 ◦C which is the melting temperature of aluminum. In a comprasion with the unmilled powders, endothermic peak shifted to nearly 577 ◦C at the temperature where Al- Si point is. This endothermic peak belonging to the eutectic point of Al- Si, was seen even at the powders milled for 1 hour. This means, mechanical aloying is actualized even if in a 1 hour milling time. Additionally, a smaller endothermic peak was alson seen at nearly 521 ◦C on DTA curves. It was considered that this endothermic peak was belonging to Al2Cu melting phase. DTA curves showed no remarkable difference between .reinforced and unreinforced powders. Sintering temperature was detected at 570 ◦C and it had been seen that the sintering mechanism would be liquid phase sintering because of the endothermic peak at 521 ◦C. After thermal analyses, powders were compacted at 450 MPa with a single action cold press. After measuring the appearent densities of the compacted samples, stearic acid was removaled out from the sample under Argın atmosphere at 420 ◦C with a 2 ◦C/ minute heating rate and 2 hours hold time. After removal of the stearic acid, samples were sintered under Argon atmosphere at 570 ◦C, with a 5 ◦C/minute heating/cooling rate and 2 hours hold time. After sintering process there was no remarkabe dimensional change at the sintered samples. Relative densities of the sintered powders were measured at the next step. Measured densities showed that, unreinforced Al-15Si-2,5Cu-0,5Mg powders reached up to %95-95 of the theorical density. Additionally, reinforced Al-15Si-2,5Cu-0,5Mg powders reached up to %90-99 of the theorical densities of the composites. Phase anaylses and microstctral characterizations revealed that, Al2Cu was formed during sintering process. There were significant differences between the microstructres of unmilled and milled samples. Both the optical microscope and scanning electron microscope images showed that there were many deep and wide cracks and porosities on the unmilled sample surface. Milled sample microstructes exhibited a homogenious and the surface was lack of caracks and prorosities. No remarkable effect of the reinforcement materials or amounts was noticed compared with unreinforced samples. The final step of the experiments was mechanical tests including microhardness and wear tests. Microhardness and wear tests showed that, increasing mechanical alloying time and reinforment amount makes positive effects on the mechanical properties of the sintered samples. Microharndness values of the metal matrix composites reached up to 2,5 times higher unmilled sintered samples. And also, wieght loss during the wear test of the composite samples reached down to %65 smaller from the unmilled sintered samples. As a result, Al-15Si-2,5Cu-0,5Mg based CeO2, Y2O3 ve La2O3 reinforced metal matrix composites are synthesized with a homogenious microstructre. Additionally, Al15Si2,5Cu0,5Mg-5CeO2, Al15Si2,5Cu0,5Mg-5Y2O3 and Al15Si2,5Cu0,5Mg- 5La2O3 exhibited the best mechanical properties.