Good Research Paper On Results And Discussion.

Abstract:

In this research, the effects of pressure on the wear resistance characteristics, mechanical properties and the microstructures of Al-Si piston alloys that have variable Magnesium (Mg) content is studied. The paper begins with an explanation of the desirable properties of eutectic Al-Si alloys and why their chemical and mechanical properties are desirable in the fabrication of light weight machine components. The methods of strengthening the alloys further using alloying elements such as Ni, Cu and Mg, and applying heat treatment are also discussed. The paper also places major emphasis on the addition of Magnesium, and compares the traditional gravity die casting with a novel hybrid technology known as squeeze casting. In the results and discussion section, the microstructure properties of the Al-Si both as-cast and after heat treatment conditions are discussed. The mechanical and wear properties as well as the implications of pressure on the alloys are also discussed in detail. SEM analysis of wear surface and fracture behavior on the Al-Si alloys as-cast and after heat treatment reveals that squeeze pressure increases fracture ductility and increase resistance to wear, and more so upon heat treatment. It is also determined that the hardness and UTS values increase with the increase in Magnesium content to reach the maximum values when Mg content is at 1% of the alloy’s composition.

Introduction:

The near eutectic Al-Si alloys are applied in areas where qualities such good mechanical properties, high resistance to wear and tear, reduced density and low thermal expansion are desired [1, 2]. The properties of these alloys are of paramount interest to players in the automobile industry since they are used to fabricate light weight engine components such as pistons, connecting rods, engine blocks and cylinder lines that are used in the manufacture of fuel efficient vehicles [3-7]. The high resistance to wear and tear, and the good mechanical properties are generally attributed to the existence of hard Si particles that are well distributed in the metal alloy matrix [8].
Improvement of the mechanical properties of the near eutectic Al-Si alloys requires the addition of alloying elements that include Ni, Mg, and Cu. The alloying elements added lead to formation of various inter-metallic phases that have very complex chemical and mechanical structures. Some of the inter-metallic phases created include Al3CuNi, Al2Cu, Mg2Si, Al7Cu4Ni, Al3Ni, and Al5Cu2Mg8Si6 among others [1, 2]. Essentially, the mechanical properties of multi-component pistons made from Al-Si alloys relies heavily on the chemical structure and composition, the evolution of inter-metallic phases, and the morphology features [8, 9]. In order to further improve the wear resistance and mechanical properties of components cast from these alloys, heat treatment can be applied. Sjolander [1] recently performed a summary of the microstructural changes in sequence that occur when treatment solutions proposed by various studies are applied, and their implications on the overall mechanical properties. In Al-Si cast alloys, Mg is added to increase the alloys’ tensile strength upon heat treatment, since it has been found to have a predominant effect on the material microstructure [3]. Joenoes and Gruzlyski [4] argue that while Mg does not clearly refine or coarsen the eutectic, it does reduce the microstructure’s level of homogeneity. In this regard, the effects of Mg on the microstructure are well desired since they have a significant implication on the fracture mechanisms of Al-Si-Cu-Mg-Ni alloys. Lasa and Rodriguez-Ibabe [12] conducted studies focusing on the wear resistance of Al-Si-Mg-Cu-Ni alloys formed through several processing routes using pin on disc techniques with variable speed. The results of wear testing showed that alloys with a high Mg content had improved wear resistance. In general, apart from the composition of the alloy and previous casting procedures such as heat treatment, the wear resistance and mechanical properties of alloys cast from Al-Si is also highly dependent on the casting process.
A new unique approach to component fabrication is made possible using an emerging metal forming process known as squeeze casting. Squeeze casting is a hybrid metal forming process that combines the desirable features of metal forging and casting processes. The process uses a technique where metal is melted and the molten metal is poured into closed die halves and solidified under high pressure. The instantaneous contact of molten metal with the die surfaces combined with the high pressure applied causes a rapid heat transfer condition that results to the yielding of a grained casting free of pores and with improved wear resistance and mechanical properties. The resulting fabricated components can be used immediately for service, or after some minor post-fabrication treatment operations. Squeeze casting is usually considered as a near net or net shape route of fabrication [3].
Lynch and Olley [4] performed some investigations on aluminum squeeze casting and presented some aspects of the process. The objective of the current study is to carry out an investigation on the mechanical properties and wear resistance behavior of Al-12Si-XMg-2Cu-3Ni (X varies between 0.3 and 1.5% wt. Mg) in the as-cast, and after heat treatment conditions respectively. In this work, attempts are also made to compare the wear resistance and mechanical properties when gravity die casting and squeeze casting techniques are used on Al-12Si-1Mg-2Cu-3Ni. A scanning electron microscope was used to conduct wear analysis and fractography studies on the wear surfaces and fractured surfaces of the heat treated Al-12Si-1Mg-2Cu-3Ni gravity and squeeze die casts.

Microstructure
The microstructure of the permanently molded (using 32x200x250 plate casting) Al-Si alloy with a variable Magnesium content in its as-cast form is as shown in the Figure 6.1 below.
The microstructure comprises of α-aluminum dendritic halos that have eutectic Silicon (Si) and complex intermetallic compounds that are segregated into the primary Si and inter-dendritic regions. The primary irregular eutectic Si particles and the primary polygonal Si are dark grey, while the intermetallic phases have a light grey color. The highly magnified micrograph in the Figure 6.1 above shows that the eutectic Si flakes have lamellar and rod style shapes while the intermetallic phases are occur in blocky or fibrous morphologies in the inter-dendritic regions. The alloy also contains some coexisting elements such as nickel, copper, iron and magnesium which form the intermetallic Al7Cu4Ni, Al2Cu, Al-Si-Fe-Ni-Cu and Al4Cu2Mg8Si7. An increase in temperature results to a corresponding increase in the solubility of these elements in aluminum. Consequently the decrease from high concentration at high temperatures to relatively lower concentration during heat treatment and solidification leads to the formation of intermetallic phases [34].
The Figure 6.2 below shows compares the changes undergone by the Al-Si alloy from its as-cast condition to the condition after T6 heat treatment. There is a significant difference exhibited in the sizes and shapes of different features. For example, the heat treated alloys indicate a significant reduction in the spacing of the secondary arm. The intermetallic phases are also seen to dissolve and tend to spherodize i.e. the one sharp corners 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

The UTS and hardness values are found to increase with increase in Mg content and attain the maximum at 1% Mg content alloy.
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.
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.