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A Study on the effects of Squeeze Pressure on Microstructure, Mechanical Properties, and Wear Characteristics of Near-Eutectic Al-Si Piston Alloys with variable Cu contents.
This paper discusses the effects of squeeze pressure on the microstructure, wear characteristics and the mechanical properties of near eutectic Al-Si piston alloys with variable Copper (Cu) content. The paper starts with a description of Al-Si alloys and the features that make them desirable for use as piston alloys. The impact of alloying elements on the mechanical properties, microstructure and wear characteristics are also discussed where it is determined that adding alloying elements such as Cu, Mg and Ni results in improved strength of Al-Si alloys and good casting properties. The paper also discusses the microstructural improvements and material strength that is achieved using the hybrid squeeze casting process and various heat treatment solutions. In order to evaluate the validity of these methods, the strength of Cu enriched Al-Si alloy as-cast and after heat treatment is analysed, and the results compared. The results are for both gravity die casts and squeeze casting. Various techniques are used to analyse the chemical, mechanical and wear properties of the Al-Si alloys. Some of the analyses include chemical analysis using optical spectrometry, heat treatment testing, microstructural observation, tensile testing and wear testing using Scanning Electron Microscopy.
The increase in Cu content in Al-Si alloys from 2.5 to 4% wt. shows an increase in UTS (ultimate tensile strength) from 210Mpa to 250Mpa while material hardness raises from 100 to 120 BHN. Heat treatment and pressure application to squeeze cast sample also result in improve mechanical properties, a fine and modified microstructure, improve wear resistance and lack of surface porosity compared to gravity die cast samples. These characteristics are suitable for reducing mechanical losses of Al-Si pistons, as well as reduced fuel consumption.
The expansion group of aluminium silicon eutectic or near eutectic alloys are referred to as piston alloys, which provide the best overall balance of properties . The mechanical properties of cast Al-Si alloy parts largely depend on grain size and is morphology, the size and spacing of dendrite arms, and the distribution of secondary phases . Significant research has been carried out to understand the effects of alloying elements on the microstructure and mechanical behaviour of Al-Si casting alloys. The alloying elements often used in the Al-Si alloy include magnesium, copper, nickel. Manganese, zinc, lead and phosphorous. [13, 14]. It is well known that adding small amounts of Cu, Mg or Ni strengthens Al-Si alloys and also the presence of Si provides good casting properties [3, 15-17]. The addition of alloying elements strengthens a metal and cause both precipitation and solution hardening, which results in a high strength to weight ratio. The as cast Al-Si alloys having low thermal expansion coefficient and high wear resistance when alloyed with the other elements such as Cu, Mg and nickel can be extensively used in piston .The amount of useful alloy elements that can be incorporated is controlled by their solid solubility rates and the cooling rate achieved during casting [3,16,19]. The addition of alloying elements such as Cu, Ni and Mg forms several intermetallic phases with very complex morphologies. Generally the intermetallic phases mainly include Mg2Si, Al2Cu, Al5Cu2Mg8Si6, Al3Ni, Al3CuNi and Al7Cu4Ni phases and so on[1,2].the complex microstructural characters lead to excellent mechanical and tribological properties[3,4]. The addition of Cu and Ni is the most effective and practical method to improve the mechanical and wear properties of piston alloy .Li et al.  and Zeren  have investigated the effect of Cu on the mechanical properties and precipitation behaviour of Al-Si alloys. It has also been reported by early researchers [3, 20, & 21] that copper substantially improves the strength and hardness in the as-cast and heat treated conditions. Shabestari and Moemeni  studied the effect of adding copper to the microstructure and the mechanical properties of Al-Si-Mg alloys and concluded that the best mechanical properties were obtained with 1.5% wt. copper solidified in the graphite moulds. They also reported that, dendrites have partially refined with increasing Cu. This will improve the mechanical properties of piston alloy. The alloy addition may also improve the wear characteristics of Al-Si [17, 23]. Several authors have investigated the influence of various alloying elements on wear resistance of Al-Si alloys [17, 23-25]. The mechanical properties of Al-Si casting alloys containing Cu and Mg are known to be improved by heat treatment. It is reported that the microstructure and mechanical properties of Al-Si alloys are sensitive to heat treatment and subsequent deformation condition [5,6]. In order to obtain improved mechanical properties aluminium alloys often subjected to different heat treatments [7-10]. The mechanical properties of Al-Si multi-component piston alloys do not only depend on the chemical composition morphology features and evolution of intermetallic phases, but also on the casting techniques. In the recent years, squeeze casting has been developed. It is casting process in which liquid metal solidifies under the direct action of pressure.The major advantages of squeeze casting are that the produced parts are free of gas and shrinkage porosity, feeders or risers are not required, Squeeze casting can have enhanced mechanical properties.In this study, the effect Cu content on the mechanical properties and the tribological properties of Al-12Si-xCu-1Mg-3Ni alloys under gravity die casting was studied in the as cast and heat treated condition. Also in this study an experimental investigation was carried out on microstructure, mechanical and wear properties of squeeze cast Al-12Si-3Cu-1Mg-3Ni. To study the tensile fracture surface and wear behaviour of the heat treated gravity die cast and squeeze cast alloy, SEM is also used
Melting and Casting of Test Specimens:
Ingots of Al-17Si alloy were used to prepare the experimental alloy. Al-30Cu, Al-20Mg, and Al-75Ni master alloys were added to the molten metal to achieve the required composition. The melting process was done using an 18kW electrical resistance pit furnace. The melting temperature was maintained at 760 ±5°C. Temperature of the melt was monitored using a Chrome-Alumel thermocouple. The melt was degassed for 60 minutes by bubbling pure, dry nitrogen gas into the melt using a graphite lance to remove the dissolved hydrogen in the melt. After alloy additions and degassing, the slag on the top of the melt was removed, and the molten metal poured into the preheated moulds. For squeeze casting, the melt was poured at 680C into a die cavity of dimensions 100100100mm. Immediately after the melt had been poured into the cavity, and pressure in the excess of 100MPa applied to the melt in the cavity using a ram for 2 minutes. For squeeze casting, the moulds were preheated to 200C before the pouring of the melt.
An optical emission spectrometer was used in the measurement of chemical compositions of the alloys studied. The measurements were done using arc spark excitation.
Heat Treatment of Test Specimens:
The tensile, hardness (20x20x20mm) and microstructure (20x20x20mm) samples machined out from the unmodified and modified Si gravity die casts and squeeze cast were subjected to T6 heat treatment. The samples were heated at a temperature of 500C for 5 hours and quenched with cold water (20C). After samples quenching, the samples were air dried, heated again to 180C for 9 hours in an electric oven for ageing, and then cooled in air.
For microstructural observations, metallographic specimens were polished using normal polishing techniques. A Leica DMRX 82 optical microscope was employed for observing the microstructures in the as cast and T6 condition.
Tensile properties were evaluated using the universal testing machine (Instron Model 1195 -5500R). The tensile properties of the samples in the as-cast and heat-treated conditions were evaluated at room temperature. The dimensions of the tensile tests specimens are shown in figure 2.
Samples of dimensions of 1520 mm were machined out and prepared for hardness test. A Brinell hardness machine (Indentec) was employed to determine the hardness values. The Brinell hardness numbers of the samples were measured using an indenter ball of diameter 2.5 mm at a load of 62.5 kg.
Experiments are conducted using the DUCOM TR-20LE pin-on-disc wear testing apparatus under dry sliding conditions in the ambient air at room temperature. A pin on disc tribometer comprises of a stationary "pin" found under an applied load in contact with a rotating disc. The pin is of 30mm long and 6mm diameter which is in specific contact while rotating the disc (55HRC), but the circular flat tips are often used to simplify the contact geometry. The sliding distance of the pin and the velocity of the disc are fixed as 1800m and 2m/s. time duration for each test was 15 minutes, and the test is repeated with four different loads. The pin was ﬁrst weighed and then placed in the sample holder of the lever. After testing had been done, the worn surface of the pin was examined using an optical microscope. The pin was then cleaned thoroughly under running water, dried with acetone and again examined. It was then weighed, and the weight lost.
Scanning Electron Microscopy (SEM):
Results and discussion.
Figure 1 represents the optical micrograph of the as-cast microstructure of the alloys. The α-Al face centred cubic solid solution is the predominant phase (light grey) in the as-cast microstructure of these alloys. The α-phase forms a dendritic network and also participates in several multi-phase eutectic reactions. The silicon phase which is soluble in aluminium and other alloying elements forms a binary eutectic with the α-Al. In the as-cast alloys, there is a lack of uniformity in both the morphology and orientation of dendritic α(Al). There are also some primary silicon particles in block-like shapes, whereas the eutectic silicon is present as coarse plates. The microstructure of as-cast alloy consist of large grains, including the dendrites of the aluminium matrix, interdendritic networks of eutectic silicon plate, block like primary silicon particles, and particles of the large intermetallic compounds in the aluminium dendrite arms are also shown in the figure. Copper is only soluble in α-Al up to a small concentration of 5.65% in the binary alloy and is a major component in the intermetallic phase CuAl2
Figure 2 represents the optical micrographs of heat treated alloy. Many intermetallic phases tend to dissolve while the eutectic Si particles spheroidize after heat treatment. Microstructural investigations show that the particle shapes of the eutectic Si change to granular shapes in all alloys after heat treatment. A very remarkable spheroidization of eutectic Si was also observed compared to the plate-shaped as-cast sample. The particle distribution also becomes more homogenous for both the eutectic silicon and intermetallic compounds upon heat treatment.
Testing of the tensile properties of the as-cast alloys was done, and the ultimate tensile strength and elongation are listed in Table 1. The result shows that the UTS increases with Copper content up to 1.5% wt. UTS increases because of precipitation of copper bearing phase in interdendritic spaces caused by increasing copper. The as cast tensile properties of near eutectic Al-Si alloys are controlled by the microstructures which depends on the characteristics of primary α-Al grains and eutectic Si particle, the intermetallic phases precipitated and the casting defect such as porosities and inclusions. With more copper addition, the presence of Al2Cu + Al5Cu2Mg8Si6 phases significantly promotes the UTS.
It was found that hardness increases with increase in Copper content and reaches a maximum for 1.5% Cu. The adequate solution heat treatment process in this study leads to significant improvement in the eutectic silicon particles morphology. During solution treatment, the eutectic Si particles undergo shape perturbations. The particles are fragmented into small segments then begin to spheroidize. A careful control on processing conditions is required to produce defect free castings with fine microstructure, which in turn, may exhibit a more uniform particle distribution. The particle distribution including eutectic silicon and intermetallic compounds is more homogenous after heat treatment, and thus heat treatment increases the mechanic properties of the alloy.
In Al-Si-Cu-Ni-Mg alloys the temperature is usually limited to about 500οC because high temperature leads to incipient melting of Cu-rich phase and lowers the mechanical properties of casting .The Increase in Cu content leads to increased hardness and the relationship between the hardness of matrices of similar alloys, and the Cu content is in agreement with that Ref [8,11]. Rapid cooling after a heat treatment makes GP zones to form disc shapes. Zones with uniform distribution in the α(Al)matrix form preferentially with Cu atoms in the aluminium lattice. A general characteristic of these zones is to have a coherent interface between the matrix that results in local strain and improved hardness. As copper content increases, the formation of GP zone is promoted due to rapidly cooling with homogeneous nucleation .
An increase in Cu content decreases the wear rate the wear rate is low when the copper content is 3%. This is due to the increase in the strength and hardness of Metrix. Heat treated alloy was also found to have a lesser rate of wear than as-cast alloy due to the spheroidization of eutectic Si particles. Increasing the matrix strength of alloys with the addition of Cu results in decreased wear rate. When 2% Cu was added, the severity of surface damage was less due to change in the microstructure, resulting in improvement in strength and hardness of the alloy owing to lesser wear rate.
Effect of Pressure
The figure shows the microstructure of samples solidified under 0 and 100MPa external applied pressure. The main two constituents in the microstructure of each sample include a block like primary silicon phase a eutectic silicon phase. Closer loops at the eutectic region of the samples are shown in the figure, where the changes in eutectic Si morphology are observed after applying a pressure of 100MPa and the sudden increase in the cooling rate caused by the improved contact between and die surface. The melting of most metals and alloys increase under pressure according to Clausius-Clapeyron equation [*].
The microstructure of the eutectic region of the sample under gravity die casting, and squeeze casting are shown in the figure. In the gravity die-cast microstructure relatively long needle-like eutectic particles can be observed. These are expected to reduce the mechanical properties of the casting. Pressure application and the ensuing increase in the cooling rate results in the modification of the eutectic silicon particles. This increases the mechanical properties and the wear resistance characteristics of the squeeze cast alloy. The shape of Si particles is important because large and elongated Si particles fracture more frequently than spherical ones. Caceres and Griffiths  reported that the number of cracked particles increases with the applied strain and that larger and longer particles are more prone to cracking. In coarse structures, particle cracking occurs at low strain, whereas, in finer structures, the progression of damage is more gradual.
SEM Analysis of Fracture Behaviour
Fracture surface of the heat treated gravity die cast and squeeze cast alloy are shown in the figure. For the gravity die cast and squeeze cast alloy are shown in the figure. For the gravity die cast alloy, it shows dimples and quasi-cleavage fracture. From the macroscopic view of the squeeze cast alloy, the fracture shows cup and cone in shape and obvious plastic deformation can be observed on the exterior surface of the tensile failed specimen. This means that it undergoes a large amount of plastic deformation prior to fracture.
The figure shows the morphology for the squeeze casted alloy. In this figure, fine equiaxed dimples that are uniform can be observed which means that the ductility of the material is superior to that of the die-cast alloy. The ductile fracture is determined by the size of dimples, and its size is governed by the number and the distribution of microvoids that are nucleated. Most of the small round eutectic Si particles are not cracked, and longish needle-like eutectic Si particles are prone to crack easily [ *]. Since silicon crystals possess low strength and high hardness, the do not deform but easily fracture on the application of a tensile load. The hard and brittle eutectic Silicon particles present in the soft Al matrix increase the crack nucleation tendency. In the gravity die-cast, alloy damage is initiated by the cracking of eutectic silicon. For the squeeze cast alloy, eutectic silicon particles of smaller size generate smaller stresses and are thus less probable to crack. The near-spherical shape eutectic Si particles crack nucleation results in a resistance to plastic deformation thus improving strength and ductility [16, 19 & 23].
SEM Analysis of wear surfaces
At the beginning of the pin and sample surface contact, the counter face is in direct contact with an oxide layer that covers aluminium and its related alloys, which has a high friction coefficient. The oxide layer is highly brittle thus weight loss and the rate of wear are high due to its fracture caused by applying force on the counter face at the beginning of the test and wear debris are large in size. With sliding distance increment, the counter face touches the alloy instead of oxide layer that has lower friction coefficient that is the reason of gradual reduction in wear rate and smaller wear debris.
Squeeze samples are in better conditions. This means they have more desirable wear behaviour which is related to their higher mechanical properties. The higher surface quality observed in the squeeze cast samples leads to a reduction in the friction coefficient. The small and highly dispersed surface porosities on the gravity cast samples seem highly serrated, and so gravity cast sample surfaces seemed to have an exaggeratedly rough and unpleasant surface. The existence of small beneath surface porosities acts as pits thus making pin motion difficult.
The figure shows surface of the samples indicating that both squeeze and gravity samples have tracks parallel to wear line. This indicates that one of the dominant mechanism in the both samples is the abrasive wear mechanism. In wear test, the pin is accompanied by weight increase and thus the other dominant mechanism for both samples is the adhesive mechanism. It should be mentioned that abrasive wear occurs when there is a high coefficient of friction between the surfaces in contact and, therefore, this mechanism is not unexpected here due to the high hardness of pin versus samples. During pin sliding, the stress applied to the contact point is severely high and, as a result, plastic deformation of the fracture occurs. The high relative softness of the alloy with respect to the pin causes some material transfer from the sample’s surface to the counter face during splitting debris.
With the increase of copper content from 2.5 to 4wt%, the UTS increase from 210 Mpa to 250MPA and hardness increases from 100 to 120BHN.
Mechanical properties of AL-Si-Cu-Ni-Mg alloys largely depend on the heat treatment. These characteristics of heat treatment play a vital role for a good combination of microstructure and mechanical properties. By increasing in copper content, UTS and hardness increase from 210MPa to 250 Mpa and 100 BHN to 120MPa respectively for the heat treated alloys due to precipitation hardening.
Pressure application on molten metal during solidification of squeeze sample causes better surface quality. Lack of surface porosity and on the inside of the squeeze samples was caused application of pressure on the molten metal during solidification, while the application of heat treatment on squeeze cast samples leads to better mechanical properties, fine and modified microstructure in comparison with gravity samples.
The squeeze samples have better mechanical properties which makes them more reliable and possess better tribological properties. This results in fewer mechanical losses, and fuel consumption decreases.
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