Investigation of Solidification and Precipitation Behavior of Si-Modified 7075 Aluminum Alloy Fabricated by Laser-Based Powder Bed Fusion

Fabrication of high strength aluminum alloys using laser-based powder bed fusion (L-PBF) encounters challenges, including the occurrence of solidification cracks and the loss of volatile elements, such as Zn and Mg. The current work developed a Si-modified Al7075 alloy aiming at introducing eutectic phases to mitigate the solidification cracking during L-PBF. Based on Kou's model, the addition of Zn and Mg decreased the crack susceptibility from 6504 degrees C to 5966 degrees C, and the addition of 3.74 wt pct of Si further decreased the crack susceptibility to 3960 degrees C. The Al7075 alloy fabricated by L-PBF exhibited a large amount of solidification cracks extending throughout the sample. Crack-free samples with a relative density of 99.94 pct, as inspected by X-ray microcomputed tomography, were achieved for the developed Si-modified Al7075 alloy. The microstructure showed a transition from a coarse columnar microstructure to a refined mixed columnar + equiaxed microstructure after alloy modification, with a concomitant grain size reduction from 59.0 +/- 42.2 to 15.0 +/- 9.4 mu m. Moreover, Si, Mg2Si, and Al2Cu phases were detected in the Si-modified Al7075 alloy. After a direct ageing heat treatment, the Si-modified Al7075 alloy showed minimal age hardening effect with a peak hardness of (146 +/- 3 Hv).

Automated summary

The study developed a Si-modified Al7075 alloy to reduce solidification cracking during L-PBF, resulting in crack-free samples with improved microstructure and grain size. The addition of Si, Mg2Si, and Al2Cu phases in the modified alloy contributed to the enhanced mechanical properties and age hardening resistance.