Origin of Aging-Induced Embrittlement in Shear-Driven Additive Manufacturing of Aerospace Aluminum 

Jake K. Yoder†, Abhishek Rastogi, Hang Z. Yu*

Department of Materials Science and Engineering, Virginia Tech, USA

† Current address: MELD Manufacturing Corporation, USA

* Corresponding author: hangyu@vt.edu

 

Abstract

High-performance aerospace-grade aluminum alloys, prone to hot cracking, are challenging to print using beam-based additive manufacturing. Solid-state additive manufacturing methods like additive friction stir deposition have occasionally achieved wrought or forged-like tensile properties in 7xxx aerospace aluminum with the appropriate temper. However, embrittlement often occurs after aging, with its origins and solutions remaining unclear. Here, we present an in-depth study on additive friction stir deposition and post-deposition aging of an aerospace Al-Zn-Mg-Cu-Cr alloy (AA7075), elucidating the mechanism of aging-induced embrittlement and validating possible mitigation strategies. Through microstructural comparisons of as-printed, solution-treated, and peak-aged conditions, we show that the intense shear during deposition can promote heterogeneous nucleation of Cu-rich precipitates on dispersoids near grain boundaries, replacing the conventional strengthening phase, MgZn2. The change in dissolution temperature and shear-induced mixing with dispersoids result in these precipitates remaining partially undissolved after solution treatment, leading to solute-enriched grain boundaries. Upon aging, therefore, precipitates with a high number density form a continuous path along grain boundaries; combined with the fine grain structure resulting from dynamic recrystallization, this leads to brittle behavior in tension.  With the embrittlement mechanism elucidated, we propose two mitigation strategies: (1) reducing shear during deposition through tool geometry control and (2) eliminating dispersoids through composition control. Both strategies have been proven to achieve wrought-standard peak-aged tensile strength and ductility in AA7075. Notably, the first strategy uniquely preserves the alloy chemistry while surpassing all previously reported additive manufacturing results.

Keywords:

Aerospace aluminum; additive manufacturing; shear-induced phenomenon; precipitate evolution; strength and ductility