A356 cast alloy with different addition of scrap (0, 20, 40, 60, 80, 100 %) were investigated in this study to determine the effect of scrap content on the tensile properties. The composition of A356 cast alloy with different addition of scrap and A356 alloy in ASTM were shown in Table 1. Scrap used in the study has same chemical composition as A356 primary alloy and produced in the shaving process of precision manufacturing process. Before the casting process, cleaning with surfactants and heat treatment at 500 °C for 3 hours was conducted to eliminate the cutting fluids on the surfaces of machining chip. Aluminum scraps were re-melted in graphite crucible furnace with A356 primary ingots at 750 °C to obtained molten aluminum and injected into mold.

In order to evaluate the tensile properties A356 cast alloy with different addition of scrap, sub-sized test specimens based on ASTM E8 were manufactured and tensile tests were conducted. The cross-head speed was set to 1 mm/min, and the test was repeated five times per experimental condition using UNIVERSAL TEST MACHINE KDMT-156.

Reduced pressure test (RPT) was performed to evaluate the melt quality. RPT applies a much lower pressure than atmospheric pressure to solidify the sample and enlarge the hydrogen in porosity inside the sample by nearly 100 times. This study conducted reduced pressure test to measure porosity area fraction. RPT samples were solidified at 0.8 atm for 240 seconds.

JmatPro^® software was used to analyze the thermodynamics of phase formation in A356 alloys. It was calculated based on the A356 chemical composition in ASTM, the temperature range was set from 25 °C to 750 °C. The phases formed in the simulation were compared with those actually shown in the A356 cast alloy with scraps.

All test specimens were polished from 200 to 2400-grit silicon carbide paper, 0.25 μm diamond suspension, and 0.04 μm colloidal suspension. Microstructure and fracture surfaces were observed using Optical microscope (OM, eclipse LV150N, Nikon) and field emission scanning electron microscopy (FE-SEM, MIRA3, Tescan) with Energy-dispersive X-ray spectroscopy (EDS, EDAX).

Figure 1 showed the microstructure of A356 cast alloy with the different addition of scrap using optical microscope. As shown in Figure 2, SDAS was observed to increase with scrap content increased. A356 cast alloy without scrap addition had smallest SDAS value of 26.21 μm. On the other hand, A356 alloy with 100 % scrap had the largest value of 36.34 μm, showing a difference of approximately 10 μm compared to the condition without scrap addition.

Figure 1: 
The microstructure of A356 cast alloy (a) without scrap, with the addition of (b) 20 % scrap, (c) 40 % scrap, (d) 60 % scrap, (e) 80% scrap, (f) 100 % scrap

Figure 2: 
The results of secondary dendrite arm spacing measurements of A356 cast alloy

Figure 3 showed the result of thermodynamic simulation using JmatPro^® software. As shown in Figure 3(a), α-Al matrix starts to form around 640 °C and eutectic Si phase begins to form around 560 °C. Figure 3(b) showed that the amount of α-Al₈Fe₂Si is estimated to be the highest based on the amount produced, and then decreases in the order of Al₅Cu₂Mg₈Si₆, β-Al₅FeSi, Mg₂Si, and Al₃M(M = transition element). Unlike the results of the analytical program, it is generally known that A356 alloys have no Al₅Cu₂Mg₈Si₆ phases with low Cu content of lower than 0.1 [16]-[17]. π-Al₈Mg₃FeSi appears to be produced near 500 °C, but dissipated at all around 400 °C.

Figure 3: 
(a) Mole fraction of different phases as a temperature function in the Al-7Si-0.3Mg-0.2Fe-0.1Ti-0.1Mn-0.1Zn-0.1Cu system. (b) Detail view except for Liquid, α-Al and Si

Figure 4 shows the microstructures, and Table 2 shows the intermetallic compound composition determined using FE-SEM and EDS. The microstructures observed were composed of an α-Al matrix, eutectic Si, β-Al₅FeSi, π-Al₈Mg₃FeSi₆, and α-Al₈Fe₂Si. The composition of the scrap content in Table 1 is thought to have no significant effect on the formation of intermetallic compounds. There were no significant differences in the intermetallic compounds of the A356 cast alloy with different scrap fractions. The β-Al₅FeSi and α-Al₈Fe₂Si observed in the microstructures are consistent with the thermodynamic simulation results. Meanwhile, in the case of π-Al₈Mg₃FeSi₆, the thermodynamic analysis of the A356 alloy showed that it was extinguished near 400 °C, but it was observed in the SEM analysis. When the Cu content was very low, the Al-Si-Mg system alloys form π-Al₈Mg₃FeSi₆ instead of Al₂Cu or Al₅Cu₂Mg₈Si₆ [18]. The intermetallic compounds in all conditions were identified to be the same as those of the commercial A356 cast alloy.

Figure 4: 
SEM microstructure of the A356 cast alloy with the addition of scrap, and the numbers correspond to the qualitative analysis in Table 2

The bifilms, produced by the entrainment of oxide, are nucleation sites for pores that allow the dissolved hydrogen to escape into the gap between the double oxide films [19]. As a result, with an increase in the oxide film entrainment quantity, the amount of bifilm produced also increases, resulting in an increased porosity. This phenomenon is illustrated in Figure 5, which shows a sample subjected to an RPT and the porosity area fraction for different scrap ratios.

Figure 5: 
(a) The picture of reduced pressure test samples of A356 cast alloys, (b) the results of porosity area fraction measurements of A356 cast alloy with the addition of scrap

A higher scrap content was observed to increase the size and number of pores. Without scrap addition, a 0.78% porosity area fraction was found, and it increased to 0.86% as the scrap content increased to 20%. The difference in the porosity area fraction between the case of no scrap addition and 20% scrap addition is 0.08% and is considered very small. When the scrap content increased to more than 40%, the porosity increased significantly, measuring 5.97% under scrap-only casting conditions.

Figure 6 shows tensile test results of A356 cast alloy with different addition of scrap. As scrap content increased, the average tensile strength, average yield strength, and average elongation tended to gradually decrease. The highest average tensile strength is 140.16 MPa, which is shown in the condition of scrap 20 % addition and has a small difference of about 8 MPa compared to the no scrap addition. As mentioned above, the two conditions showed a slight difference of 0.08 % when measuring the porosity area fraction, with relatively low porosity quantity compared to other conditions. Therefore, when the scrap content is less than 20 %, the amount of pores produced is similar to the condition without scraps, and the tensile properties are not significantly different. The average elongation was also found to have the highest value in a test piece with a scrap content of 20 %, as was the tensile strength. For the mean yield strength, the highest value was 103.15 MPa under the condition of no scrap addition, and then slowly decreased to an average value of 72.10 MPa under the condition of 100 % scrap content. When scrap content is greater than 40%, tensile properties have been deteriorated due to a rapid increase in porosity area fraction, but it is also believed that increased SDAS will have an effect on tensile properties.

Figure 6: 
(a) Stress-strain curves for A356 cast alloy with different addition of scrap, (b)Tensile strength, yield strength, and elongation of A356 cast alloy with the addition of scrap

The results of the analysis of the fracture surfaces of the tensile test specimens of A356 cast alloy with different addition of scrap are shown in Figure 7. Dimple which is the trace of ductile fracture and porosities were observed in all conditions of fracture surfaces. Thus, it can be seen that the porosities acted as a stress concentrator, affecting the propagation of the crack. Under the condition of casting with 100 % scrap, traces of very large pores were observed on the fracture surfaces. The oxide film on the scrap surface may have caused the formation of Bifilm and increased the amount of hydrogen gas incorporation, resulting in a sharp decrease in tensile strength.

Figure 7: 
Image of porosity(highlighted by white line) on fracture surfaces: A356 cast alloy (a) without scrap, with the addition of (b) 20 % scrap, (c) 40 % scrap, (d) 60 % scrap, (e) 80 % scrap, (f) 100 % scrap

An oxide analysis of the porosity surfaces of the fracture surface using FE-SEM and EDS is shown in Figure 8. Figure 8(a) is a microstructure of a porosity on fracture surface, which is thought to be gas porosity with no dendrite observed. The gas porosity has been reported to have a relatively smooth surface compared to the shrinkage porosity by inflating and forming the gap as hydrogen gas escapes through the wetted side of the bifilm [20]. Analysis of spherical particles shown in Figure 8(b) suggests that MgO was produced. Al₂O₃ forming bifilm combines with Mg over time when alloys in liquid phase and transformed to MgAl₂O₄, which can then undergo phase transformation with MgO [21]. Figure 8(d) is a dendrite observed inside the pores. Given that dendrite is observed on the surface of the pores, it is believed that liquid Al is cooled and contracted, resulting in shrinkage porosity. As the solidification contraction of the aluminum progresses, the volume of the bifilm expands rapidly, the oxide film on the surface is torn, and it is buried in the dendrite side [22]. As shown in Figure 8(e), traces of oxide film were observed on the dendrite surface of the shrinkage porosity.

Figure 8: 
Oxides on the porosity surface: (a) porosity on fracture surface, (b) magnification of the P1 in (a), (c) qualitative analysis of the intermetallic phase corresponding to the point in (b), (d) dendrites of shrinkage porosity surface, (e) qualitative analysis of the intermetallic phase corresponding to the point in (d).