Mechanical Properties as a Function of Casting Process of Aluminum–Silicon Alloy Matrix Composites

Applying aluminum composite in the defense, aerospace and automotive industries depends on how they behave during the elasto-plastic form change. In addition to the factors responsible for changing the form of the alloy, many other factors have an impact on the behavior of the composite form change. In this study, the effect of casting type on the mechanical properties of Al-Si nano composites has been investigated. Due to the proper distribution of reinforcing particles, tensile strength in compo casting sample in semi-solid state is higher than sand casting and squeeze casting. In all samples, the tensile strength of the heat-treated samples has increased by about 30%. Tensile strength in compo casting sample in semi-solid state was obtained with higher nano particle reinforcing particles, which can be explained by the fact that the percentage of elongation in micro samples was lower than that of nano composite samples.


5.
Physical properties, such as the coefficient of thermal expansion of each component.
Assuming complete connection and uniform distribution of particles, and ineffectiveness of changing the shape of the matrix and reinforcing phases on each other, the simplest way to predict the behavior of the composition of the composite is the classic law of the mixtures [8,9]. But in most cases, the strength and formulation obtained from empirical calculations is significantly lower than the values obtained from the mixing rule. There are several reasons for this, including the following:  Existence of thermal incompatibility stress due to differences in thermal expansion coefficients between field phase and reinforcing phase;  Inefficient transmission of loads between phases due to the relatively weak connection;  The presence of imperfections on the reinforcing phase;  Clustering particles The density of dislocations in the field increases due to the thermal incompatibility stress, which increases the strength of the field, and, as a result, composites. But many of the factors reduce the stress on composite materials (compared to the mixing rule) [10][11][12][13][14]. Therefore, depending on the overall effect of all the factors involved, the behavior of the composite form change will be different. This non-conformance stress also depends on the temperature at which the composite cools [15][16][17]. It should be borne in mind that although the thermal inconsistency stress will increase the strength, this increase is not very high in composite particles. The clustering of particles and the presence of imperfections due to the presence of particles are among the factors influencing this issue [18][19][20][21][22].
The main strengths of the nano composites include the strength of the Orowan, the strength due to thinning, the strength of the solid solution, and the strength of the dislocation [23][24][25][26][27]. The linear summation of these phrases can be used to predict the yield stress, and the result will appear as follows.
σ composite = ∆σ Orwan + ∆σ grain + ∆σ solution + ∆σ dislocation (1) Orowan strengthening mechanism results from collision of dislocated and diffused particles. When the composite tolerates the load, plastic form change occurs in the material [28][29][30]. Nano-size and hard ceramic particles act as obstacles to near-particle and in-situ dislocations [31][32][33]. This strength factor increases with increasing ceramic phase volume fraction. The following statement is used to express the Orowan mechanism: (2) where G is the shear field modulus, b the Burgers vector, υ is the Poisson ratio, λ is the distance between particles, and D is the average diameter of the particles [34][35][36][37]. The distance between particles (λ) can be replaced by the volume fraction (V f ) and average particle diameter by the following equation [38,39]: The second phase particles also act as non-homogeneous nucleation catalyst for the aluminum matrix. This strengthening effect increases by increasing the volume fraction and decreasing the particle size in a constant volume.
Using the Hall-Petch equation, the fine-graining strength effect can be defined as follows [40][41][42][43]: Here σ grain is the contribution of the grains to the yield stress, σ 0 is the friction coefficient, d is the diameter of the grain and k is constant [44,45].
Therefore, the strength of the resulting fine-grained material can be obtained by the following equation in the nano sized composites: In the case of dissolved atom with a very different size in the field, there is a type of disproportionate strain region around the atom, which in colliding with dislocation can act as barrier to its movement [46][47][48][49]. The analysis of the collision between dislocations and the strain field of these particles can be represented by the following equation: Here G is the elastic shear modulus, x f is the fraction of the external atoms concentration, and ε is the fraction of the difference between the atoms and the atoms dissolved in the diameters [50,51].
Because of the difference in the thermal expansion coefficient of the field phase and the reinforcing phase, dislocations occur in the matrix during freezing [52,53]. Higher dislocation density increases the strength of the field. Factors such as thermal expansion coefficient, particle size and volume fraction, and strength of the field are effective in producing dislocation. The effect of the strengthening dislocations in nano composites can be expressed by the following equation [54][55][56]: Here, A is the total surface area of the particles, G is the shear modulus of the field, b the Burgers vector, Δρ is the increase in the density of dislocations in the field due to the presence of particles [57,58]. In this study, the effect of casting type on the mechanical properties of A356-Al 2 O 3 nanocomposites will be investigated.

Research Method
Aluminum was used as a metal matrix composite material from A356 alloy with 7% Si and 0.3% Mg. Alumina powder with different dimensions and percentage by weight was used as secondary composite phase particles. In the step model, the melting of the alloy was carried out in a resistance furnace and in a graphite crucible, and the discharge temperature was considered to be 720 ° C. After degassing with flux and loading the reinforcing particles at 720 ° C, the melt was added to the melt and mixed.
Then the melt was poured into the sand mold, and at the end, the model and the power were taken out of the mold and separated.
A rectangular cube was used for compo casting with a squeeze casting technique. About 1 kg of melt was required at each pouring. Alloy melting was carried out in a resistance furnace and in a graphite crucible and the temperature was considered to be about 720 ° C. After degassing with flux and melting at 720 ° C, the reinforcing particles were added to the melt.
When the mixture reached the desired temperature, it was poured into a metal stainless steel, and immediately pushed by a weighbridge. At the end, the model was removed from the mold and the center of the model was examined. In this study, an electromagnetic stirrer was used to determine the best distribution of particles. For the melting and preparation of the melt required for use in an electromagnetic stirrer, 1 kg melt was placed inside a graphite crucible and the crucible was transferred to the resistance furnace with its charge. The furnace temperature was adjusted to 720 ° C. After reaching this temperature, and after about half an hour, the crucible was

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Materials and Energy Research Center (MERC), Tehran, Iran
Department of mechanical engineering, University of Iowa, Iowa City, IA,

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