Effects of the Solid Solution Treatment Conditions and Casting Methods on Mechanical Properties of Al-Si-Cu Based Alloys

Al-Si-Cu계 합금의 주조법과 용체화처리 조건이 기계적 특성변화에 미치는 영향

Abstract

In this study, the effects of two different casting methods (gravity casting and, diecasting) and various solid-solution conditions on the mechanical properties of ASC (Al-10.5wt%Si-1.75wt%Cu) and ALDC12 (Al-10.3wt%Si-1.72wt%Cu-0.76wt%Fe-0.28wt% Mn-0.32wt%Mg-0.9wt%Zn) alloys were investigated. A thermodynamic solidification analysis program (PANDAT) was used to predict the liquidus, solidus, and phases of the used alloys. In the results of an XRD analysis, ${\beta}$-AlFeSi peaks were observed only in the ALDC12 alloy regardless of the casting method or SST (solid-solution treatment) conditions. However, according to the results of a FE-SEM observation, both ${\theta}(Al_2Cu)$ and ${\beta}$-AlFeSi were found to exist besides ${\alpha}$-Al and eutectic Si in the gravity-casted ASC alloy at $500^{\circ}C$ after a SST of 120min. The ${\alpha}$-AlFeSi and ${\beta}$-AlFeSi phases including the eutectic phases were also found to exist in the ALDC12 alloy. The results of a microstructural observation and analyses by XRD, FE-SEM and EDS were in good agreement with the PANDAT results. The gravity-casted ALDC12 and ASC specimens showed the highest Y.S. and UTS values after aging for three hours at $180^{\circ}C$ after a SST at $500^{\circ}C$ for 30min. At longer solid-solution treatment times at $500^{\circ}C$ in the gravity-casted ALDC12 and ASC specimens, the elongations of the ASC alloys increased, whereas they decreased slightly in the ALDC12 alloys.

Fig. 1. Photo of the diecast ALDC12 alloy heat treated for various time at high temperatures.

Fig. 2. Graph shows the solid fractions of three phases along the temperature drop in ASC alloy during solidification.

Fig. 3. XRD results of the gravity casted ASC alloys (left) and ALDC12 alloys (right) with various solid solution times.

Fig. 4. XRD results of the die casted ASC alloys (left) and ALDC12 alloys (right) with various solid solution times.

Fig. 5. OM micro structures of the gravity casted for ASC and ALDC12 alloys. SST means solid solution treatment time at 500℃.

Fig. 6. OM micro structures of the die casted for ASC and ALDC12 alloys. SST means solid solution treatment time at 500℃.

Fig. 7. FE-SEM photographs and EDS Spectrums of the gravity casted ALDC12 alloy. (a): eutectic Si, (b): Al matrix and (c): α-AlFe(Mn)Si.

Fig. 8. FE-SEM photographs and EDS Spectrums of the gravity casted ALDC12 alloy. (a),(c): α-AlFe(Mn)Si, (b),(d): Al2CuMgSi.

Fig. 9. FE-SEM micro structures of the die casted for ASC and ALDC12 alloys. SST means solid solution treatment time at 500℃.

Fig. 10. FE-SEM micro structures of the die casted for ASC and ALDC12 alloys. SST means solid solution treatment time at 500℃.

Fig. 11. Results of the relationships between the hardness and the solid solution treatment conditions for the gravity casted ASC and ALDC12 alloys.

Fig. 12. Results of the relationships between the tensile properties and the solid solution treatment conditions for the gravity casted and the diecasted ASC and ALDC12 alloys respectively.

Table 1. Chemical compositions of the ASC and ALDC12 alloys.

Table 2. Thermodynamic solidification analysis of ASC alloy by PANDAT.

Table 3. Thermodynamic solidification analysis of ALDC12 alloy by PANDAT.