TY - THES

T1 - Solidification microstructure control in Al-7Si-0.4Mg (wt. %) alloys for improving fatigue properties

AU - Razavi, Seyedfarzin

N1 - no embargo

PY - 2022

Y1 - 2022

N2 - Al7Si0.4Mg-based alloys (i.e., A356) have been widely used in aerospace and automotive industries due to their excellent castability and high performance. More than 95% of foundry alloys are based on AlSiMg-based alloys. However, some aspects of the solidification microstructure still need to be controlled to improve further mechanical properties, particularly dynamic mechanical properties, including fatigue properties. Significantly, the grain refinement practice using chemical addition (Al-5Ti-1B) is well established for Al-Si-Mg based wrought alloys (i.e., 6082, 6061), however, just in case Al-Si-Mg based casting alloys, the addition of Al-5Ti-B grain refiner to Al-Si-Mg based casting alloys with Si content above three wt.% is not adequate. A so-called Si poisoning occurs, which is believed to be due to the twofold: (i) the chemical reaction between Ti and Si and thereby the formation of the AlSiTi phase, consuming the free Ti and reducing the growth restriction caused by free Ti, and (ii) the segregation of Si on the basal plane of TiB2 nucleation particle and thereby poisoning the TiB2 nucleation particle. Reducing or avoiding Si poisoning is one of the most critical issues for the advancement of Al-Si-Mg-based casting alloys. In this master thesis, we focus on the solidification microstructure control via four methodologies: (i) the long impeller degassing with Ar (about 20 mins) until the density index is down to 1.5, (ii) the grain refinement by NbB2 and AlB2 together with Zr or Ti, (iii) the modification of eutectic Si by Sr, and (iv) optimized solution treatment (540 °C for 6 h) and subsequent aged treatment (180 °C for 4 h). The obtained results demonstrate that (i) when the density index is less than 1.5, the formation of pores or porosities can be reduced significantly. (ii) the addition of NbB2 and AlB2 together with Zr into Al7Si0.4Mg based alloys is not so effective as the addition of NbB2 and AlB2 together with Ti into Al7Si0.4Mg based alloys, which can refine the grain size down to 300 µm. (iii) together with the addition of NbB2 and AlB2 together with Ti or Zr, the further addition of Sr into Al7Si0.4Mg based alloys can modify the eutectic Si well. (iv) After optimized solution treatment and aged treatment, not only the statistic tensile properties but also the dynamic fatigue properties can be significantly improved. This investigation provides a practical methodology to control the solidification microstructure of Al7Si0.4Mg- based alloys and thereby improve dynamic fatigue properties, which is very helpful for the broader application of Al7Si0.4Mg-based alloys.

AB - Al7Si0.4Mg-based alloys (i.e., A356) have been widely used in aerospace and automotive industries due to their excellent castability and high performance. More than 95% of foundry alloys are based on AlSiMg-based alloys. However, some aspects of the solidification microstructure still need to be controlled to improve further mechanical properties, particularly dynamic mechanical properties, including fatigue properties. Significantly, the grain refinement practice using chemical addition (Al-5Ti-1B) is well established for Al-Si-Mg based wrought alloys (i.e., 6082, 6061), however, just in case Al-Si-Mg based casting alloys, the addition of Al-5Ti-B grain refiner to Al-Si-Mg based casting alloys with Si content above three wt.% is not adequate. A so-called Si poisoning occurs, which is believed to be due to the twofold: (i) the chemical reaction between Ti and Si and thereby the formation of the AlSiTi phase, consuming the free Ti and reducing the growth restriction caused by free Ti, and (ii) the segregation of Si on the basal plane of TiB2 nucleation particle and thereby poisoning the TiB2 nucleation particle. Reducing or avoiding Si poisoning is one of the most critical issues for the advancement of Al-Si-Mg-based casting alloys. In this master thesis, we focus on the solidification microstructure control via four methodologies: (i) the long impeller degassing with Ar (about 20 mins) until the density index is down to 1.5, (ii) the grain refinement by NbB2 and AlB2 together with Zr or Ti, (iii) the modification of eutectic Si by Sr, and (iv) optimized solution treatment (540 °C for 6 h) and subsequent aged treatment (180 °C for 4 h). The obtained results demonstrate that (i) when the density index is less than 1.5, the formation of pores or porosities can be reduced significantly. (ii) the addition of NbB2 and AlB2 together with Zr into Al7Si0.4Mg based alloys is not so effective as the addition of NbB2 and AlB2 together with Ti into Al7Si0.4Mg based alloys, which can refine the grain size down to 300 µm. (iii) together with the addition of NbB2 and AlB2 together with Ti or Zr, the further addition of Sr into Al7Si0.4Mg based alloys can modify the eutectic Si well. (iv) After optimized solution treatment and aged treatment, not only the statistic tensile properties but also the dynamic fatigue properties can be significantly improved. This investigation provides a practical methodology to control the solidification microstructure of Al7Si0.4Mg- based alloys and thereby improve dynamic fatigue properties, which is very helpful for the broader application of Al7Si0.4Mg-based alloys.

KW - Al-7Si-0.4Mg

KW - Erstarrung

KW - mechanische Eigenschaften

KW - Kornfeinung

KW - Thermische Analyse

KW - Modifikation.

KW - Al-7Si-0.4Mg

KW - solidification

KW - mechanical properties

KW - grain refinement

KW - thermal analysis

KW - Modification

M3 - Master's Thesis

ER -