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The aim of this paper is to investigate an effect of aging time on mechanical properties and wear characteristics of Al12Si3Cu1Mg1.78Ni alloy. This alloy is used for aluminium piston parts. The solution treatment was carried out at 500oC for 5 hours followed by water quenching. Subsequent aging was carried out at 1800C with various solution treatments times for 1, 2, 4, 6, 8, 9, 10, 12, 16, 20 and 24 hours. As-cast microstructure of this alloy contains eutectic Si, Mg2Si, Al2Cu, Al3Ni and various other intermetallic phases. After solution treatment, the flake like and acicular eutectic silicon appears to be fragmented. The maximum hardness and ultimate tensile strength was found for an aging time of 8 to 9 Hours. Also, it was observed that the wear rate is minimum for an aging time of 8 to 9 hours for this alloy. Therefore, the solution treatment time significantly affects the hardness and microstructure evolution of the Al12Si3Cu1Mg1.78Ni alloy.
The automotive industry is increasingly using aluminium-silicon alloys cast at near eutectic and aging-hardened due to their good wear resistance, low thermal expansion, relatively high specific strength at low specific weight and low cost, resulting in affordable improvements in fuel efficiency. By optimized heat treatment, the eutectic structure of near eutectic aluminium silicon alloys can be refined and its properties improved. Factors like the temperature and holding time affect not only the as-cast microstructure and alloying addition but also the mechanical properties and wear characteristics of heat treated Al-Si alloys. Copper is added for arresting natural aging but increases the kinetics of artificial aging, magnesium to increase the strength. An improvement in wear resistance of 390 Al alloys after heat treatment was observed. Yasmin et al found that full heat treatment has a great influence on the wear and mechanical properties of the Al-Si piston alloy after studying the mechanical and wear properties of Al-Si eutectic alloy. Haque and Sharif also noted that heat treatment show a great influence on the mechanical and wear properties of the Al-Si piston alloy as it reduces the wear rate and increases the ultimate tensile strength. Modern casting processes like squeeze casting are used to cast Al-Si alloys to overcome the disadvantages of the conventional die-casting process. In the present study influence of aging time on mechanical and wear properties of cast Al-Si-Cu-Mg-Ni piston alloy has been investigated. The study also aims to understand the effect of heat treatment on the mechanical and wear properties of both as cast and squeeze cast Al-Si-Cu-Mg-Ni piston alloy.
Preparation of castings.
The experimental alloys are prepared using as received LM6 alloy ingots, Al30wt. %Cu master alloy, Al20%Mg master alloy and Al75%Ni master alloy. The LM6 alloys were melted in graphite crucibles in an electrical resistance pit furnace at 800 C. After the alloy melted completely, the alloying elements (Cu, Mg, and Ni) are added to obtain the expected compositions. A graphite lance is used to bubble pure, dry nitrogen gas for about 60 minutes into the melt to degas it and remove the hydrogen and inclusions. After alloy additions and degassing the slag on the top of the melt is removed. It is then poured into the preheated molds for gravity die-cast alloy. For squeeze casting, a measured quantity of the liquid metal (2 Kg) is poured into the die cavity and 110MPa pressure is applied to the casting for 2 min. All the alloys under investigation were solutionized at 500C for 5 h followed by water quenching and artificial aging at 180C for 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h and 9 h.
Preparation of samples:
An optical emission spectrometer was used to determine the chemical compositions of the alloys investigated in the study. A Leica DMRX 82 optical microscope was employed for observing the microstructures of the as-cast and heat-treated specimens.The tensile properties of the alloy samples were evaluated using a universal testing machine (Instron Model 1195 -5500R) at room temperature. The dimensions of the specimens used for the tensile tests are shown in Figure 2. A Brinell hardness machine (Indentec) was employed to determine the hardness values of the samples (diameter of indenter ball 2.5 mm and load 62.5 kg). For conducting wear tests, DUCOM TR-20LE pin-on-disc wear testing apparatus under dry sliding conditions in the ambient air at room temperature was used. The sliding distance of the pin and the velocity of the disc are fixed as 1800m and 2m/s. Each test was for fifteen minutes duration with a load of 20N. Wear surface morphology and fracture surface of the tensile samples were studied under scanning electron microscope.
Role of aging time on wear properties and mechanical Properties:
The wear behaviour, hardness and ultimate tensile strength of the alloy as a function of aging time are shown in the table. It is observed the wear rate decreases with an increase in aging time from 1H to 9H. A significant reduction in wear rate is observed at the aging time of 9H. At earlier stages of aging, the BHN increase with aging time until it reaches the peak. After reaching the peak, the BHN decreases as a result of over aging. The hardness values exhibit another peak at 20 H, resulting from the presence of several hardening phases, including Al2Cu, Mg2Si and Al7CuNi which contribute to the precipitation hardening of the alloys. Similar observations were recorded and explained for Al-Si –Cu-Mg 380 alloy by Morin. The ultimate tensile strength shows the maximum after aging at 180C for 9H. The ultimate tensile strength increase with an increase in aging time but decreases after aging for 9 H. After 9H the specimen is over aging with an increase in aging time.
The optical micrographs of gravity die-cast as-cast, represented by Fig 1, gravity die-cast and heat treated represented by Fig 2, and squeeze cast and heat treated microstructure represented by Fig 3 of the alloy are presented here. The predominant phase (light grey) in the as-cast microstructure of these alloys is α (Al) face-centred-cubic solid solution. A dendritic network is formed by α-phase which is usually cores, participating in several multi-phase eutectic reactions. Aluminium and silicone binary eutectic is an anomalous eutectic phase as it is it constitutes a metal (aluminium) and non-metal (Silicon). Primary silicon particles as well as the eutectic silicon particles which are present as coarse plates for this alloy. Other intermetallic phase particles are also present in the aluminium dendrite arms. After heat treatment, it is observed a very remarkable fragmentation and spherification of eutectic Si particles in comparison to plate-shaped as cast specimen. The particle distribution including eutectic silicon and intermetallic phases is more homogeneous after heat treatment.
Microstructure parameters such as amount of massive primary silicon particles, eutectic silicon morphology, silicon crystal and bonding of these particles with soft and tough aluminium matrix are the important factors which should be considered in the analysis of wear behaviour of these alloys. Spherification of eutectic silicon grains may be attributed to a reduction in wear rate and increase of mechanical properties with an increase in aging time. Spherical silicon morphology would discourage the crack origination and propagation. Since particles of near-spherical shape would cause low-stress concentration at particle-matrix interfaces. The morphology of eutectic silicon is changed to smooth edges near- spherical-shaped, after the heat treatment.
It is clear from the microstructures (Fig 3) that the increase in the applied pressure causes a decrease in the grain size of the primary α phase. The application of pressure during solidification would affect the freezing temperature of the melt and its effect may be deduced from the Clausius-Clapeyron equation.
The mechanical properties of Al-Si alloy not only depend on the chemical composition but also on the microstructural features such as morphologies of dendritic α(Al), spherification of eutectic Silicon and other intermetallic phases within the microstructure. The variation of the morphology and size of eutectic silicon particles results in the improvement in mechanical properties. During solution heat treatment, some particles dissolve back into Al matrix, yielding a solid solution. The degree of solid solution strengthening depends on the number of solute atoms in the Al matrix. Table 5 indicates that the hardness value of the heat treated alloy is higher than the hardness value of as-cast alloy. During ageing, the precipitation of strengthening phases increases the hardness of the alloy. The variation in the tensile strength is usually in good agreement with the variation in hardness, for example, an alloy with higher hardness typically has a higher tensile strength.
Fractography of heat treated samples is presented in fig 6. Linkage of cracks between eutectic particles, shrinkage pores and microcracks inside the silicon particles were observed on the fracture surface. Shrinkage pores affect the alloys tensile strength and ductility. SEM fractograph of the alloy shows the following fracture sequence; (1) initiation of microcrack inside Si particle, formation of slipping band in the Al dendrite, a links between the cracks both micro and macro, and the growth of crack. During tensile strain, heterogeneous deformation in the microstructure induces external stresses in the eutectic silicon and Fe-bearing intermetallic particles.
SEM micrographs of the wear surface of gravity die-cast as-cast, gravity die-cast and heat treated, and squeeze cast and heat treated alloys is shown in the figure. It can be observed that heat treated alloys are subjected to shallow and narrow micro grooves than cast alloys. Common surface features like micro groove, craters due to micro-cutting and scoring marks on the wear surface due to abrasions of all the alloys are revealed. The wear behaviour of materials is largely dependent on surface material, physical structures and surface conditions as well as the sliding conditions of the normal load and abrasive medium. Materials under abrasive wear conditions are primarily removed by ploughing and micro-cutting. These mechanisms need penetration of materials by hard abrasive particles which in turn controlled by the hardness of material. Therefore, hardness of alloys plays a significant role in controlling the wear rate during the sliding against the abrasive medium. Micro-cutting is a major factor in abrasive wear. The volume of the wear groove produced is completely related to the material loss due to micro-cutting.
For the alloy, solution treatment at 495C for 5H and then quenching in cold water and aging at 180C for 9 h is optimum.
Heat treatment of alloy showed an improvement in the abrasive wear rate and mechanical properties. Heat treatment of alloy showed spherification of the eutectic silicon and destroys cast dendritic structure. An increase in aging time refines and distributes the eutectic silicon particles in the aluminium matrix and breakdown the dendritic structure.
The overall investigation shows that the heat treated squeeze cast alloy has higher UTS, hardness and wear resistance properties. However in order to obtain the best combination of the structure and properties of this material, further investigation is needed.
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