EFFECT OF POURING TEMPERATURE AND DEGASSING ON THE CASTING QUALITY OF Al 6061: EXPERIMENTAL AND NUMERICAL STUDY

– As one of important components of an airplane, body of airplane is required to have high value of strength to weight ratio. In this study, transient heat transfer of Al 6061 in a sand casting process was investigated both experimentally and numerically. The effects of different pouring temperatures (700, 720 and 740 °C) and presence of thin film and H2 inclusions are considered in the present study. Composition, XRD, metallography and tensile strength tests have been carried out to examine the casting product quality, before and after degassing, a process to remove the inclusions from the cast. Correspondingly, heat transfer simulations were carried out by taking into account the variation of pouring temperatures and the presence of inclusions. From the present experimental and numerical study, it was found that: (i) Degassing enhanced significantly the strength of Al 6061 product. The highest tensile strength value has been found to be 64.30 MPa, related to the pouring temperature of 700 °C with degassing, while the lowest one is 35.85 MPa associated with the pouring temperature of 700 °C without degassing . (ii) Pouring temperature did not affect significantly to the strength of Al 6061 product, especially when degassing process was carried out, and (iii) The presence of thin film and hydrogen gas inclusions affected the cooling rate of the metal slab. Overall, the cooling of the metal slab with thin film inclusion became slower, while the cooling of the metal slab with hydrogen gas inclusions became faster.


Introduction
Aluminum 6061 is a main component for an airplane body. The material has a strength of 128 MPa for as-cast and up to 310 MPa for T-6 type (American Society for Testing and Materials, 2002;Totten and MacKenzie, 2003). In this study, both experimental and numerical study on sand casting process of Al 6061 was carried out. In particular, transient heat transfer of Al 6061 the process is investigated. The effects of different pouring temperatures (700, 720 and 740 °C) and presence of thin film and H2 inclusions are considered in the present study. Composition, XRD, metallography and tensile strength tests have been carried out to examine the casting product quality, before and after degassing, a process to remove the inclusions from the cast. Correspondingly, heat transfer simulations were carried out by taking into account the variation of pouring temperatures and the presence of inclusions.

Material
Raw materials Al ingot of 99.9% from PT Inalum, Mg ingot of 90% from PT Pinjaya Logam and Al-7.92%Si from PT Pinjaya Logam were used in this research study. The compositions are:

Casting Process and Testing
Electric furnace was used for the casting process. Pouring temperatures were varied as: 700 o C, 720 o C and 740 o C for both degassing (D) and non-degassing (ND). Based upon the design for composition as shown in Table 1, the materials were melted and poured into a sand casting mold to cast testing specimens. The mold was green sand. Optical Emission Spectroscopy (OES) in the Laboratory of Surabaya Shipbuilding State Polytechnic (PPNS) was used for the identification of chemical composition. Tensile testing was carried out in the Department of Naval Architecture, ITS Surabaya based on the ASTM B 557M standard (American Society for Testing and Materials, 2002). In addition, to identify phase transformation in the alloy after the process, XRD testing was carried out in the Division of Materials Characterization, ITS Surabaya by using PAN Analytical XRD machine. Filament of Cu is used with the current I = 30mA and voltage V = 40kV. Further, metallography was carried for microstructure observation by using an optical microscope type OLYMPUS BX51M-RF in the Laboratory of Metallurgy, Department of Materials and Metallurgical Engineering, ITS Surabaya.

Heat Transfer Simulation
For the heat transfer simulation, geometry and meshing of the casting mold, including the shapes of thin film and hydrogen inclusion, were prepared by using ANSYS Mechanical APDL 18.1. Open molding was chosen in this study. SOLID278 element (brick8node278) was chosen for the thermal simulation in order to obtain transient temperature history and distribution during the casting process (Firdaus et al., 2016;Anwar et al., 2020).

Al 6061 Composition
As mentioned previously, casting process in this study used raw materials Al ingot of 99.9%, Mg ingot of 90% and Al-7.92%Si. The actual chemical composition of Al 6061 obtained from the casting is shown in Table 2. It is observed that there is a discrepancy between the design and actual chemical composition. It is due to oxidation process where Mg and Al react into MgO and Al 2 O 3 . Based upon Ellingham diagram, the oxidation products will form slag (Shamsuddin, 2016). The slag can be removed through degassing and skimming processes (Gerrard, 2014).

XRD Result
Phase transformation in Al 6061 was observed through X-ray diffraction. The XRD results can be seen in Fig. 4. Peaks corresponding to alpha Al (α) can be observed for both Al 6061 ND and Al 6061 D, in which Mg 2 Si is still dissolved in the Al crystal lattice structure. The Mg 2 Si precipitate will be commonly formed when Al 6061 undergoes age hardening (Avner, 1974). The phase formed is Al with FCC crystal structure.

Al 6061 Microstructure
Microstructure of Al 6061 was observed by using metallography technique following ASM Vol 9 (ASM International, 2004). The etching solution has a composition of 190 ml aquades, 3 ml HCl, 5 ml HNO 3 and 2 ml HF and given to the specimen by using immerse method for 10-20 seconds. The microstructures of Al 6061 are shown in Fig. 5.  and with degassing (c), at magnification of 100x. It is observed that the product of Al 6061 without degassing has defect in the form of oxide film. This is due to a reaction between aluminium and oxygen during the casting process (Eisaabadi B. et al., 2012). On the other hand, the product of Al 6061 followed by degassing did not show any defect in the microstructure as slag can be removed through the process. It is also noted that both Al 6061 ND and Al 6061 D possesses the same phase i.e. α-Al confirmed from the XRD results.

Tensile Test of Al 6061
Effects of pouring temperature and degassing to the value of ultimate tensile strength (UTS) of the products were also examined in this study. Metal cylinder specimens following ASTM B 557M were used (American Society for Testing and Materials, 2002). The tensile test results are shown in Tabel 3.  Table 3, it can be seen that the Al 6061 product obtained by using pouring temperature of 700 o C followed with degassing has the highest UTS value, while its counterpart has the lowest UTS value. The process of degassing has indeed increased the strength of material by removing inclusions/slags in the molten metal (Rao, 1999). On the other hand, while the UTS value is affected by the pouring temperature in the casting process without degassing, it is not the case in the casting process with degassing i.e. the increase of pouring temperature did not always yield better UTS values.

Results of Heat Transfer Simulation
Temperature history and distribution in the metal slab were obtained from the finite element (FE) simulation. In the heat transfer simulation, the final time was set to be 60 minutes (3600 s). The value of convection coefficient was taken as 11.45 W/m.K (Pariona and Mossi, 2005;Yogatama et al., 2020). The variation of pouring temperature was 700 o C, 720 o C, and 740 o C, respectively, which was the same as in the experimental study. Bulk/surrounding/environment temperature was set to be 30 o C. Figs. 6-14 show the metal slab during cooling. As can be observed, cooling firstly starts at molten metal parts which are in contact with the sand mold. Also, the metal slab with thin film inclusion experienced slower cooling process. On the other hand, the metal slab with hydrogen inclusion experienced faster cooling process. It is seen that cooling rate at the beginning of cooling process took place with higher rate than that of subsequent times. Afterward, the cooling rate became slower having the temperature differences in the slab became lower as well due to the decrease of temperature.
For clarity of presentation, temperature history and distribution inside the thin film inclusion only are shown in Figs. 15-17. It is observed that the thin film experienced fast cooling rate so as to its temperature has already reached room temperature after 900 s. On the other hand, as can be seen in Figs. 18-20 the hydrogen inclusions had lower cooling rate compared with the thin film. At 900 s, their temperature still have not reached the room temperature yet, although comparable with the slab temperature. It is important to note that in the present study the number of hydrogen inclusions have not been checked for their adequacy and distributions throughout the metal slab. It is interesting to note that overall the presence of thin film and hydrogen gas inclusions affected the cooling rate of the metal slab, as shown in Figs. 9-14.  Fig. 21 shows cooling curve obtained from the simulation and experimental results, respectively. It is noted that the curves are extracted from the same point in the metal slab. It is shown that the simulated cooling rate is faster than that of the experimentation. It is probably due to the sand mold quality which affects heat transfer during the cooling process (Rao, 1999).

Cooling Curve
Investigation of inverse problems related to the casting process, in particular by using FEM  or other numerical techniques such as meshless Hidayat et al., 2017) and artificial neural networks (Hidayat, 2015) would be interesting as subjects of further research study.  MPa (pouring temperature of 700 o C), 39.57 MPa (pouring temperature of 720 o C) and 47.68 MPa (pouring temperature of 740 o C), respectively. Hence, the highest tensile strength value has been found to be 64.30 MPa, related to the pouring temperature of 700 °C with degassing, while the lowest one is 35.85 MPa associated with the pouring temperature of 700 °C without degassing. ii) Pouring temperature did not affect significantly to the strength of Al 6061 product, especially when degassing process was carried out. iii) The presence of thin film and hydrogen gas inclusions affected the cooling rate of the metal slab. Overall, the cooling of the metal slab with thin film inclusion became slower, while the cooling of the metal slab with hydrogen gas inclusions became faster.