Introduction
Near eutectic Al-Si alloys are used in applications that require high resistance to wear and corrosion, good mechanical properties, low thermal expansion and reduce density [1,2]. Their properties are of great interest to the automobile industry for the production of fuel efficient vehicles using light weight components produced from these alloys such as connecting rod, pistons, cylinder liners and engine blocks [3-7]. The good mechanical properties and high resistance to wear are essentially attributed to the presence of hard Si particles distributed in the metal matrix [8]. In order to improve the mechanical properties of near eutectic Al-Si alloys it is necessary to add alloying elements such as Cu,Mg and Ni. These form several inter metallic phases with very complex morphologies. Generally the intermetallic phases include Mg2Si,Al2Cu,Al5Cu2Mg8Si6,Al3Ni,Al3CuNi and Al7Cu4Ni phases and so on[1,2]. Obviously, the mechanical properties of Al-Si multicomponent piston alloys depend on the chemical composition, morphology features and evolutions of intermetallic phases.[8,9]. To further improve the mechanical and wear properties of cast components these alloys can be heat treated. Sjolander[1] have recently summarised the sequence of microstructural changes that occurs during the solution treatments applied by different studies and their influence on mechanical properties. Structural changes that occurs during the solution treatments applied by different studies and their influence on mechanical properties. Mg in Al-Si cast alloys is intentionally added to increase the tensile strength on heat treatment and it predominantly affects the micro structure[3]. According to Joenoes and Gruzlyski[4] Mg neither clearly refines nor clearly coarsens the eutectic , but definitely reduces the degree of homogeneity of the micro structure. Micro structure has significant effect on the fracture mechanism of Al-Si-Mg-Cu-Ni alloys. Studies of Lasa and Rodriguezlbabe[12] focused on wear resistance of Al-Si-Cu-Mg-Ni alloys formed by various processing routes using a pin on disc method with different speed. The wear test results showed improved wear resistance for alloys with high Mg content []. Generally the mechanical and wear properties of Al-Si cast alloys are strongly related to the casting process besides the alloy composition and the past casting operations such as heat treatment. A distinctly different approach to component making is possible with squeeze casting an emerging metal forming process. Desirable features of both casting and forging are combined in this hybrid method. The squeeze casting process uses a technique by which metred and poured molten metal solidifies under pressure within closed die halves. The applied pressure and instantaneous contact of molten metal with die surface produce a rapid heat transfer condition that yields a pore free and grained casting with improved mechanical and wear properties since the as-fabricated components can be readily used in service or after a minor post fabrication treatment. Squeeze casting is regarded as a net or near net shape fabrication route [3]. Lynch and Olley[4] have carried out investigation with squeeze casting of aluminium and presented various aspects of the process. The objective of the present study is to investigate the wear behaviour and mechanical properties of Al-12Si-xMg-2Cu-3Ni[x varies from 0.3 to 1.5 wt% Mg) in the as cast and in the heat treated condition. Also in this work, an attempt is made to compare the mechanical and wear properties of squeeze cast and gravity die cast Al-12Si-1Mg-2Cu-3Ni. Fractographic studies and wear analysis were performed on the fractured surface and wear surfaces of squeeze cast and gravity die cast heat treated Al-12Si-1Mg-2Cu-3Ni using a scanning electron microscope.
Result and Discussion
Microstructure
The microstructures of the Al-Si alloy made in permanent mould (32x200x250 mm plate casting) with varying Magnesium content, in as-cast form is shown in figure 6.1. The microstructure consists of α-aluminium dendritic halos with eutectic Si and complex intermetallic compounds segregated into the inter-dendritic regions and primary Si. The polygonal primary Si particles and irregular eutectic Si are dark grey, while the intermetallic phases are light grey in colour. The high magnification micrograph in figure6.1. reveals that the eutectic Si flakes are lamellar and rod type shapes and that the intermetallic phases precipitate in fibrous or blocky morphologies in the interdendritic regions. The alloy has some other coexisting elements such as copper, nickel, magnesium, and iron which forms the intermetallic Al2Cu, Al7Cu4Ni, Al4Cu2Mg8Si7 and Al-Si-Fe-Ni-Cu. The solubility of these elements in aluminium usually increases with increasing temperature. This decrease from high concentrations at elevated temperatures to relatively low concentrations during solidification as well as during heat treatment results in the formation of secondary intermetallic phases [34].
The features of the microstructures of the alloy in T6 condition undergoes changes upon heat treatment. Figure 6.2 shows the comparison between the microstructures of the alloy in as-cast and T6 heat treatment condition. The differences in the size and shape of different features are evident. Microstructure observation reveals that the secondary arm spacing is reduced for the heat treated alloys. Most of the intermetallic phases dissolved and tended to spherodize, i.e., sharp corners have become rounded. The morphology change of the eutectic Si is obvious after heat treatment. The plate-like eutectic Si in as-cast case is broken into small particles. Spheroidization and coarsening of the discontinuous phase occur at elevated temperatures, because the interfacial energy of a system decreases with the reduction in interfacial surface area per unit volume of the discontinuous phase. Solution heat treatment results in the microstructural changes due to the instability of the interface between two phases. Plate-like eutectics are more resistant to interfacial instabilities and subsequent spheroidization than the fibrous kind. Spheroidization and coarsening of the discontinuous phase occur at elevated temperatures, because the interfacial energy of a system decreases with the reduction in interfacial surface area per unit volume of the discontinuous phase [35]. Thus combination of alloying elements and heat treatments is a satisfactory option for obtaining improved control of the microstructure and hence of improving the properties of the alloy. Actually, due to heat treatment both the primary silicon crystals and eutectic silicon needles show some spheroidizing. Increasing in Mg content results in the precipitation of intermetallic particles of Mg2Si. Heat treatment promotes rounding of the eutectic Si particles particularly for high Mg content alloys. The micro structure of the sample under gravity die cast and squeeze cast are shown in figure. Application of pressure and its consequent increase in the cooling rate causes the rounding of the eutectic Si particles and this improves the mechanical properties and wear characteristics of the squeeze cast alloy.
Mechanical Properties and Wear characteristics
The hardness and tensile properties of as cast and heat treated samples obtained are tabulated in table1. It is observed that the hardness and ultimate tensile strength (UTS) values are found to increase with increase in Mg content. Alloys with Mg level increases, exhibit a micro structure in which the Si particles are inherently refined and well distributed. When Mg level increases large Mg2Si particles tend to form and their number and size increases with increased Mg. The Mg2Si phase is desirable in Al alloys because of its high melting point (1085C) low density (1.9 gm/cc), high hardness (4.5 GPa) low thermal coefficient of thermal expansion (7.5X10-6/K) and reasonably high elastic modulus [14]. Its presence in the form of large blocky particles significantly detracts from the alloy’s mechanical properties. The UTS and hardness values are higher than that of as cast alloys. Heat treatment of Al-Si-Cu-Mg-Ni shows cumulative effect of precipitation hardening(Mg2Si), breaking of cast dendritic structure , reducing this segregation of alloying elements, spheroidization of silicon crystals and improved bonding between the second phase particles and matrix aluminium. To get the benefits of precipitation hardening it is necessary that alloy elements are dissolved in aluminium matrix during the solutionizing. Solution temperature determines diffusion and solubility limits of alloying elements in the aluminium. An increase in temperature [(500C) increases both the above parameters which intern increases the effect of age hardening and their effect on mechanical and wear resistance. Enhanced distribution of refined and spheroidizaised silicon crystals would retard the crack nucleation and propensities and can be attributed to improvement of wear resistance with heat treatment. Wear rate decreases with increase in Mg content for both as cast and heat treated alloy. After heat treatment the wear resistance of all samples in greatly improved. Harun et al [20] and Ott etal[21] have also found that the wear resistance of Al-Si-Cu-Ni-Mg alloys is affected by heat treatment in a favourable way. Improvement of yield strength obtained after heat treatment was also known to delay or inhibit wear[22].
Effect of Pressure
The application of pressure during solidification has resulted in increase of hardness, increase of UTS and decrease in wear rate. The rate of increase of hardness and UTS which is the result of both increase of heat transfer rates and decrease of inter atomic distances. Squeeze cast components are characterised by a refines micro structure having fine grains, close dendrite arm spacing and small constituent particles. The fine grains size result from a high level of nucleation and subsequent rate of growth. Nucleation occurs initially in the under cooled region at the die wall and is enhanced as meatal movement promotes back melting, dendrite sharing mixed with rapid cooling. The mechanical shock encounter at the instant of the die closer is felt to contribute further to nucleation.
SEM analysis of fracture behaviour and wear surface
Figure2 reveals the SEM micro graphs of the typical fracture surfaces of as cast and heat treated squeeze casted samples. A mixed mode of brittle cleavage and ductile fracture with dimples was observed at both heat treated and as cast samples. Application of squeeze pressure improve the fracture surfaces. It indicates more ductile fracture mode. The fracture behaviour of the alloys is affected by the size of α particles and Si morphology.Figure 3 shows SEM images of worn surfaces of and heat treated gravity die cast and squeeze cast Al-Si-Cu-Mg-Ni .Nucleation of cracks in Al-Si-Cu-Ni-Mg alloy mostly occurs at particle matrix interfaces. The mechanism of material removable in the alloy was found to be micro cutting. The material accumulated around the groove, deformed plastically and subsequently detached from the wear surface by nucleation and propagation of the cracks. The silicon particles for the high Mg alloys are surrounded by Mg2Si particles resulting in a better bonding of Si particles to the matrix []. The Inferior wear properties of low Mg alloy may be attributed due to the debonding of large primary Si at their interface with the matrix[].
Conclusions
From the above study, the following conclusions are made:
1. The UTS and hardness values are found to increase with increase in Mg content and attain the maximum at 1% Mg content alloy.
2. Heat treatment increase the strength of the investigated alloys. The eutectic silicon particles start to fragmentize and spherodize almost immediately with heat treatment. This leads to pronounce improvement in mechanical properties and reduces the wear rate.
3. Increase of squeeze pressure promotes rapid solidification and refined cell structure, decreases the α- Al grain size and modified the eutectic Si, which increases the mechanical properties and decreases the wear rate.
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