Introduction
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 [1]. The mechanical properties of cast Al-Si alloy parts largely depend on grain size and is morphology, dendrite arm spicing, size and distribution of secondary phases [12]. Significant research has been carried out to understand the effects of alloying elements on the micro structure and mechanical behavior 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 of small amounts of Cu, Mg or Ni strengthens Al-Si alloys and also the presence of Si provides good casting properties [3,15-17]. 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 co-efficient and high wear resistance when alloyed with the other elements such as Cu, Mg and nickel can be extensively used in piston [18]The amount of useful alloy elements ,which can be in-corporate, is controlled by their solid solubility and the cooling rate achieved during casting [3,16,19]. The addition of alloying elements such as Cu, Ni and Mg form several inter metallic phases with very complex morphologies. Generally the intermetallic phases mainly includeMg2 Si ,Al2Cu,Al5Cu2Mg8Si6,Al3Ni,Al3CuNi and Al7Cu4Ni phases and so on[1,2]the complex micro structural 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 [10].Li et al [11] and Zeren[12] have investigated the effect of Cu on the mechanical properties and precipitation behaviour of Al-Si alloys . Also it has been reported by early researches [3,20,21] that ,copper substantially improves strength and hardness in the as cast and heat treated conditions. Shabestari and Moemeni[12] studied the effect of copper addition on the micro structure and 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 molds. 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 micro structure 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 not only depend on the chemical composition morphology features and evolution of inter metallic phases but also depend 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 produced parts are free of gas porosity or 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 tribilogical 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 micro structure, 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
Experimental
Melting and Casting of Test Specimens
Ingots of Al-17Si alloy was used to preparing the experimental alloy. Al-30Cu, Al-20Mg, and Al-75Ni master alloys were added to the melt to achieve the required composition. The melting process was carried out in an 18kW electrical resistance pit furnace. The melting temperature was maintained at 760 ±5°C. Temperature of the melt was monitored using a Chromel–Alumel thermocouple. The melt was degassed for 60 minutes by bubbling pure, dry nitrogen gas into the melt by means of 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 melt was poured into the preheated molds. For squeeze casting the melt was poured at 680C into a die cavity with dimensions of 100100 100mm. Immediately after the melt had been poured in to the cavity, a pressure of more than 100MPa was exerted on the melt in the cavity using ram for 2 min. For squeeze casting the molds were preheated to200C before the pouring of the melt.
Chemical Analysis
An optical emission spectrometer was used to measure the chemical compositions of the alloys investigated in the study. The measurements were performed using arc spark excitation.
Heat Treatment of Test Specimens
The tensile, hardness (20 X 20 X 20mm) and micro structure (20 X 20 X 20mm) samples machined out from unmodified and Sr modified die cast and squeeze cast were subjected to T6 heat treatment. For solutionizing the samples were heated at a temperature of 500C for 5 h and quenched in cold water (20C). After quenching, the samples were air dried and then heated to 180C for 9 h in an electric oven for ageing and cooled in air.
Microstructural Observations
For microstructural observations metallographic specimens were polished using normal polishing techniques. A Leica DMRX 82 optical microscope was employed for observing the micro structures in the as cast and T6 condition.
Tensile Tests
Tensile properties were evaluated using 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.
Hardness Tests
Samples of dimensions of 15 20 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 ..
Wear Tests
Experiments are conducted using 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 consists of a stationary “pin” 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, the worn surface of the pin was examined by optical microscopy. The pin was then cleaned thoroughly in running water, dried with acetone and again examined. It was then weighed and the loss in weight was calculated.
Scanning Electron Microscopy (SEM)
To describe the type of wear exhibited by the Sr modified squeeze casted T6 conditioned samples, the worn-out surfaces of samples of the castings subjected to 40N loads were viewed using SEM (JEOL JSM 5600LV). Similarly, to study the fracture behavior of the castings, tensile-fractured samples of the castings were observed using SEM. The secondary electron mode was used for all the observations; the excitation potential was 10 or 25kV.
Result and discussion
Micro structure
Figure 1 represented the optical micro graph of as cast micro structure of alloys. α Al face centred cubic solid solution is the predominant phase(light grey) in the as cast micro structure of these alloys. α –phase forms dendritic network and also participates in several multi phase eutectic reactions. The silicon phase which is soluble into aluminium, and the other alloying elements form a binary eutectic with α(Al). In the as cast alloys, the morphology and orientation of dendritic α(Al) are not uniform. There are some primary silicon particles in block like shape, whereas the eutectic silicon is present as coarse plates. The micro structure of as cast alloy consist of large grains, including the dendrites of the aluminium matrix, inter dendritic networks of eutectic silicon plate, block like primary silicon particles and other large inter metallic compound particles present between the aluminium dendrite arms as shown in the figure. Copper is soluble to a low concentration in α(Al)(5.65%) in the binary alloy[13] and is a major constituent in the intermetallic phase CuAl2
Figure 2 represents the optical micro graphs of heat treated alloy. Many inter metallic phases dissolve and the eutectic silicon particles tend to spherodize after heat treatment. The micro structural investigations show that the shape of the eutectic silicon particles changes to granular type in all alloys after heat treatment. It is observed a very remarkable spheroidization of eutectic silicon particles in comparison to plate shaped as cast specimen. The particle distribution including eutectic silicon and inter metallic compounds is more homogeneous after heat treatment.
Mechanical Properties
The tensile properties of the as cast alloys were tested and the ultimate tensile strength and elongation were listed in table 1. The result shows that the UTS increases with Copper content up to 1.5wt%. UTS increases because of precipitation of copper bearing phase in inter dendritic spaces caused by increasing copper. The as cast tensile properties of near eutectic Al-Si alloys are controlled by the micro structures which depends on the characteristics of primary α-Al grains and eutectic Si particle, the inter metallic 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 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 spherodize. A careful control on processing conditions is required to produce defect free castings with fine micro structure, which in turn, may exhibit a more uniform particle distribution. The particle distribution including eutectic silicon and inter metallic 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 500C, because high temperature leads to incipient melting of Cu rich phase and lowers the mechanical properties of casting [5].Increase in Cu content leads to increase in hardness and this relationship between the hardness of the matrix of similar alloy and Cu content is in agreement with that Ref[8,11].
Rapid cooling after a solution treatment makes GP zones form disc shape. These zones with uniform distribution in the α(Al)matrix form preferentially with Cu atoms in the aluminium lattice. A general characteristics of the zones is to have a coherent interface with in the matrix, which results in local strain and provides higher hardness. As copper content increases the formation of GP zone is promoted due to rapidly cooling with homogeneous nucleation [14].
Wear Studies
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. Also heat treated alloy shows lesser wear rate 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. With the addition of 2% Cu the severity of surface damage was very less this is due to change in the micro structure, resulting in improvement in strength and hardness of the alloy owing to lesser wear rate.
Effect of Pressure
Figure shows the micro structure of samples solidified under 0 and 100MPa external applied pressure. The main two constituents in the micro structure of each sample include a block like primary silicon phase an eutectic silicon phase. Closer loops at the eutectic region of the samples are shown in figure, where the changes in the morphology of eutectic silicon particles by applying a pressure of 100MPa. 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 [ *].
Micro structure of the eutectic region of the sample under gravity die casting and squeeze casting are shown in figure. In gravity die cast micro structure relatively long needle like eutectic particle are present. These expected to reduce the mechanical properties of the casting. Application of pressure and its consequent increase in the cooling rate cause modification of the eutectic silicon particles. This increases the mechanical properties and wear 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[15] reported that the number of cracked particles increases with the applied strain and that larger and longer particles are more prone to cracking. Also in coarse structure particle cracking occurs at low strain, where as in finer structure 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 figure. For the gravity die cast and squeeze cast alloy are shown in 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 in the exterior surface of the tensile failed specimen. This means that it under goes a large amount of plastic deformation prior to fracture.
Figure shows the morphology for the squeeze casted alloy. Fine and uniformly distributed equiaxed dimples are clearly observed, which means that ductile 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 micro voids 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 no deform but fracture easily when the tensile load is applied. The presence of hard and brittle eutectic Si particles in a soft Al matrix increases the crack nucleation tendency. For the gravity die cast alloy damage is initiated by the cracking of eutectic silicon. For the squeeze cast alloy eutectic silicon particles with small size would generate smaller stresses and in turn have a smaller probability of cracking near spherical shaped eutectic Si particles crack nucleation and resist plastic deformation which improves the strength and ductility[16,19,23].
SEM Analysis of wear surfaces
At the beginning of the pin and sample surface contact, counter face is in direct contact with oxide layer that covers aluminium and its related alloys, which has high friction coefficient. Since oxide layer is brittle weight loss and wear rate is high due to its fracture resulting from applied force by counter face at the beginning of 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. Higher surface quality in squeeze samples reduces friction coefficient. On the other hand small and, dispersed surface porosities in gravity samples act like hillock and dip, so gravity samples surface is seemed to be like and exaggeratedly rough and unpleasant surface. The existence of small beneath surface porosities act like pits and as a result, pin motion encountered difficulties.
¬ 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 abrasive wear mechanism. In wear test, pin is accompanied with weight increase, therefore the other dominant mechanism for both samples is adhesive mechanism. It should me mentioned that abrasive wear occurs when the difference between contacting surfaces hardness is very high. Therefore, this mechanism is not unexpected here due to high hardness of pin versus samples. During pin sliding, applied stress in contact point is severely high. As a result plastic deformation and fracture will occur. High relative softness of alloy with respect to pin will cause some materials transfer from surface of samples to counter face during splitting debris.
Conclusions
1. 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.
2. Mechanical properties of AL-Si-Cu-Ni-Mg alloys largely depend on the heat treatment. This 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 increases 210MPa to 250 MPa and 100 BHN to 120MPa respectively for the heat treated alloys due to precipitation hardening.
3. Pressure application on molten metal during solidification of squeeze sample causes better surface quality. Lack of porosity on the surface and inside of the squeeze samples resulted by pressure application on molten metal during solidification and application on heat treatment on squeeze samples leads to better mechanical properties, fine and modified microstructure in comparison with gravity samples.
4. The squeeze samples have better mechanical properties and therefore are more reliable and have better tribological properties and therefore mechanical losses and fuel consumption decreases.
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