Quench sensitivity of toughness in an Al alloy: Direct observation and analysis of failure initiation at the precipitate-free zone

Abstract

Analysis of toughness in 6156 Al-Mg-Si-Cu sheet has been performed using enhanced Kahn tear tests on samples quenched at different rates. Crack initiation energies were hardly affected by changing water quench temperature from 20°C to 60°C; however a significant reduction was evident on air cooling. Crack propagation energy was reduced for both 60°C water quenched and air cooled materials. Observation of failure initiation through synchrotron radiation computed tomography (SRCT), for the 60°C water quenched material revealed failure ahead of the crack tip of grain boundaries oriented at 45° to the main loading axis and crack "tongues" extending into the material ahead of the main crack. Failure was predominantly intergranular. Fractographic assessment revealed predominantly voiding and shear decohesion in the 20°C water quenched material. With the aid of the new findings past models on the influence of precipitate free zone parameters on toughness have been revised.

Effects of Microalloying Tin and Combined Addition of Silver and Tin on the Formation of Precipitate Free Zones and Mechanical Properties in Al-Zn-Mg Alloys

“…Research into the characterization of PFZs is essential because they are believed to affect greatly the mechanical properties of these alloys. [1][2][3][4][5][6] Our work has also revealed that presence of PFZ adversely affects the fracture properties of the alloys by correlating mechanical properties to microstructual parameters. [7][8][9] It is known that small additions of various alloying elements (microalloying element) often change the morphology, spatial distribution and size of precipitates.…”

Quench sensitivity of Al–Mg–Si alloys: A model for linear cooling and strengthening

“…In heat treatable Al-based alloys precipitation hardening is the dominant strengthening mechanism. For most commonly used alloys such as the Al-Mg-Si (6xxx) and the Al-Zn-Mg-(Cu) (7xxx) alloys, age hardening response can be seriously affected by the cooling rate from solution annealing (e. g. [1][2][3][4][5][6][7][8][9]; and also toughness can be reduced due to reduced cooling rate [10]. To achieve optimal mechanical properties, precipitation during quenching must be fully suppressed, and this is achieved only if the alloy is cooled with the upper critical cooling rate or faster (e. g. [4,8,7]).…”

Ciclo térmico y soldabilidad de las aleaciones de aluminio

“…La tendencia a la formación de precipitados no endurecidos es lo que se conoce como enfriamiento de sensibilización (Cavazos y Colás, 2003;Morgeneyer et al, 2008;Tiryakioğlu y Shuey, 2010). Examinados los efectos del enfriamiento sobre la variación que presenta el límite elástico de una aleación, esta variación es un indicador de la sensibilización del enfriamiento, conforme es mayor la velocidad de enfriamiento el límite elástico de la aleación va a ser menor.…”