Effect of Ultrasonic Treatment Parameters on Microstructural and Mechanical Properties of A356 Aluminum Alloy

Because of the properties such as low density, corrosion resistance, good strength and castability, aluminum (Al) alloys have been the most popular materials and found wide range of applications for decades. Components made of Al alloys are generally critical parts of aerospace and automobile industries in terms of high safety requirements. Therefore, the mechanical properties of Al alloys have to meet strict specifications and the keys to obtain them are the manufacturing and treatment processes [1-5] Pores in a cast may serve as crack initiators and affect the mechanical properties negatively [6]. Catastrophic failure is inevitable for a component with high porosity. It is clear that porosity of the cast is the major problem due to high solubility of hydrogen in molten aluminum [7-9]. Because of the reaction between aluminum and air humidity (Equation 1), an oxide film is formed and entrained into the melt. It makes two non-wetting surfaces, known as bifilm, as shown in Fig. 1 [10-12].


Introduction
Because of the properties such as low density, corrosion resistance, good strength and castability, aluminum (Al) alloys have been the most popular materials and found wide range of applications for decades. Components made of Al alloys are generally critical parts of aerospace and automobile industries in terms of high safety requirements. Therefore, the mechanical properties of Al alloys have to meet strict specifications and the keys to obtain them are the manufacturing and treatment processes [1][2][3][4][5] Pores in a cast may serve as crack initiators and affect the mechanical properties negatively [6]. Catastrophic failure is inevitable for a component with high porosity. It is clear that porosity of the cast is the major problem due to high solubility of hydrogen in molten aluminum [7][8][9]. Because of the reaction between aluminum and air humidity (Equation 1), an oxide film is formed and entrained into the melt. It makes two non-wetting surfaces, known as bifilm, as shown in Fig. 1 [10][11][12].
Therefore, degassing process and grain refinement treatment are the most important operations affecting the performance of casting process [1]. There are various processes to reduce hydrogen amount in molten aluminum, such as; hexachloroethane (C2Cl6) pellets, vacuum degassing [7] and ultrasonic degassing [13][14][15]. The first mentioned degassing process is harmful for the environment [16] due to the release of dangerous Cl2 gases into the atmosphere [17]. On the other hand, the vacuum degassing method is a high cost method for the initial installation [7]. But, ultrasonic degassing is an environmentally clean technique [6]. In the ultrasonic degassing methods, high intensity

Abstract
The aim of this study is to investigate the effect of ultrasonic treatment on the microstructural and mechanical properties of A356 aluminum alloy. Ultrasonic treatment was applied to 2 kg and 4 kg molten metals at different melting temperatures (700 o C, 720 o C and 740 o C) for different durations (0, 60, 180 and 300 seconds). Bifilm indices of the samples were also measured after all parameters were tested. The ultimate tensile strength (UTS) and elongation values were used to calculate the Quality Index (QI) of the samples. According to the results, the amount of liquid metal directly affects the the ultrasonic intensity that is produced during the treatment. Maximum UTS and elongation results were determined at 720 o C during 300 seconds for 4 kg casting. Minimum bifilm density value was measured at the same ultrasonic temperature and time. 235 vibration is applied to molten aluminum to generate oscillating pressure fields which create large number of small cavities in the melt. These numerous cavities promote the hydrogen diffusion from molten aluminum to the developed bubbles which coagulate and float to melt surface [18][19][20]. In the literature, there are various studies investigating the effects of parameters of ultrasonic treatment on microstructure and mechanical properties of casting alloys. Xu et al [7] investigated the degassing effect of ultrasonic treatment at different temperature (620, 660, 700 and 740°C) for different amounts of molten A356 aluminum alloy (0.2, 0.6 and 2 kg). They only used reduce pressure test (RPT) to evaluate the density of samples. They deduced that ultrasonic treatment has significant effect on degassing process especially at the higher temperatures such as between 700 and 740 ℃. And additionally, the ultrasonic process was more effective on the smallest amount of melt. Jia et al [21] studied the effect of ultrasonic treatment on the grain refinement of A356 aluminum alloy. Tests were performed on 3 kg of molten alloy at 750°C for 15 min. To analyze the efficiency of ultrasonic vibration, they prepared samples with and without ultrasonic treatment. And they concluded that UTS, YS and elongation values increased from 228 MPa, 180 MPa and 4% to 190 MPa, 250 MPa and 4.9%, respectively. Li et al [22] investigated the influence of ultrasonic treatment time (0, 60, 120, 180, 240 s) and the cooling condition on the gas content of aluminum alloy through the ingot density. In all studies, the results are evaluated by measuring gas content and density by using RPT test method.
In addition to these related literatures, in this study the effects of ultrasonic treatment are investigated at different temperatures with different duration. The higher amount of molten metal was also used in the experimental studies and the effects of all parameters on microstrucure and mechanical properties are examined together. Bifilm analysis was conducted and the relationship between bifilm content and mechanical properties was discussed. Ultimate tensile strength and elongation values were used to calculate the Quality Index (QI) of A356 alloy as shown in the Equation 2 [22,23,36].
Abbreviations denoted by QI, UTS and K represent quality index, ultimate tensile strength and constant (150 MPa for A356), respectively.
QI is demanded by engineers to analyze if mechanical properties of alloys are suitable with working condition or not, especially in automotive and aerospace industry [24]. So this index can be used to evaluate alloys properties.

Experimental Procedure
Chemical analysis of the ingots used in this study was obtained with a commercial optical emission spectrometer. The chemical composition of A356 alloy ingots is given in Table 1. Table 1. Chemical composition of A356 alloy Two different crucibles were prepared with 2 kg and 4 kg of A356 ingot pieces to determine the effect of charge amount to the performance of ultrasonic degassing treatment. For each casting process, ingot pieces were weighed and held in a graphite crucible and melted in the electric furnace at different temperature (700, 720 and 740°C). After melting, a Rtul model ultrasonic equipment (Fig. 2) which consists of an ultrasonic generator, a transducer, a booster and a horn is submerged into the crucible for degassing process. This equipment is capable of converting 3 kW of electric energy at a constant resonant frequency of 19.8 kHz.

Fig. 2. Ultrasonic degassing equipment
Degassing process is performed at temperatures mentioned above during 60, 180 and 300 seconds. Casting samples without degassing were also prepared for comparison. Molten metal was poured into a bifilm mold (Fig. 3a) and a permanent mold (Fig. 3b)

Results and Discussion
Macrographs of the samples for 2 kg and 4 kg charged castings are given in Fig. 5 and Fig. 6.
According to the macrostructure it can be said that grain size is getting smaller by increasing ultrasonic treatment duration. Linear intercept method was used to measure the grain size of samples. The grain sizes of the samples produced with 2 kg at 740˚C were measured as 1940,6 µm and 1125,6 µm for the samples without ultrasonic application and for 300 seconds ultrasonic application, respectively. The grain sizes of the samples produced with 4 kg at 740˚C were measured as 2062,6 µm and 906,2 µm for the samples without ultrasonic application and for 300 seconds ultrasonic application, respectively. Jian et al [25] investigated the effect of different types of power ultrasound on solidification of A356 alloy as continuous, intermittent and isothermal processing. They declared that the average grain size reduces by intermittent acoustic vibration. Because while the latent heat is being released during decreasing the melt temperature, ultrasonic vibration assists to prevent the local heating up and allow for occurring heterogeneous nucleation [26]. So, ultrasonic vibration is firstly to generate the initial crystallites and activation of potential nucleation sites in melt volume [27]. Reduction of grain size is described by two mechanisms that cause cavitation-enhanced nucleation and cavitation-induced dendrite-fragmentation [28,29]. Also cavitation promotes to wet of non-metallic inclusions and decrease porosities [30]. The grain size of the samples which were ultrasonically treated at 700˚C and 740˚C for 300 seconds are 861,1µm and 1125,6 µm, respectively. This results show that low temperature has an additional effect on ultrasonic treatment in reducing grain size.
Micrographs of the samples for 2 kg and 4 kg castings are given in Fig. 7 and 8. According to the microstructural examination and some studies in the literature, it can be said that porosity decreases by increasing the ultrasonic treatment duration [20,31]. It can be clearly seen in Fig. 7 that dendirites are getting smaller with decreasing temperature and increasing duration. This can be explained by the inverse relationship between temperature and viscosity. It can be calculated theoretically by using Arrhenius as Equation 3 [32].
η0: is a constant for a given liquid E: activation energy for viscous flow of the liquid R: universal gas constant T: absolute temperature In the literature, Dispinar has detailed how bifilm index could be calculated [9][10][11][12][33][34][35]. In this study, in order to determine the correlation between bifilm and mechanical test results, bifilm density was taken into account. Equation 4 was used to calculate bifilm density: In Fig. 9, the change of the bifilm density with the ultrasonic treatment duration for 2 kg and 4 kg casting is given respectively. According to Fig. 9a, it can be seen that the bifilm density values vary irregularly. Since the amount of liquid metal in 2 kg casting is small, it fluctuates during ultrasonic treatment. In Fig. 9b, the bifilm density values decrease at 720°C and 740°C whereas these values increase at 700°C with increasing ultrasonic treatment duration (from 0 sec to 300 sec). The change in ultimate tensile strength (UTS) values is irregular as well as bifilm density values for 2 kg casting (Fig. 10a). At 720°C and 740°C UTS values increase with ultrasonic treatment duration for 4 kg casting (Fig. 10b). When Fig. 9 and 10 are evaluated it can be said that there is an inverse relationship between bifilm density and UTS. This results are supported by the earlier researches [9,11]. All the test results are considered to determine the main effect of temperature, amount of molten metal and ultrasonic treatment duration on UTS. "Main effect plot" in Minitab is generally used to examine differences between level means for one or more factors. The effects of three different parameter (melt temperature, amount of aluminium and ultrasonic application time) on UTS value are taken into consideration in the current study. According to the graphic in Fig. 10c, it can be said that decreasing temperature increases the UTS value. Additionally, the amount of molten metal has the most important effect on UTS results. Porosity density values are higher for 2 kg compared to 4 kg casting.
Because the amount of molten Al metal is low, for 2 kg casting molten metal is fluctuated by ultrasonic vibration and causes porosity formation. Also, with the increase in ultrasonic treatment duration, the inclusions at the bottom of the slag mix back into the melt even though the surface does not excessively fluctuate and the slag is not broken during the mixing process. For 4 kg, this situation is not seen. It can be seen in Fig. 9 that bifilm value of the sample taken from a 2 kg casting mold is higher than that taken from a 4 kg casting mold. Mechanical test results affected negatively due to being of bifilm as discussed previously. The second significant effect is casting temperature and the last one is ultrasonic treatment duration.   In Fig. 11, the change of elongation with time is given. The elongation value reaches its maximum value 7.59% at 720°C for 2 kg casting. For 4 kg casting, maximum elongation is 9.07% at 720°C for 300 seconds ultrasonic time. The results show that the increase in bifilm density leads to decrease in UTS and elongation [11,37].
Both ultimate tensile strength and elongation results were taken into account for the calculation of quality index (QI). These values are given in Fig. 12. It can be seen that ultrasonic treatment duration and temperature affect the mechanical test results significantly. The maximum values for UTS and elongation were reached at 720oC after 300 seconds for 4 kg casting.

Conclusions
The results of the study can be summarized as follows:  According to the macrostructures, grain size of the sample is getting smaller by increasing ultrasonic treatment duration at all temperature for both 2 kg and 4 kg casting samples.  Microstructural investigation of the 4 kg casting samples showed that porosity decreases by increasing the ultrasonic treatment duration. But the result is not clear for 2 kg casting samples.  Individual standard deviations are used to calculate the intervals.