A Study of the Quench Sensitivity of 6061-T6 and 6069-T6 Aluminum Alloys

The purpose of this study is to investigate the quench sensitivity of the mechanical properties of 6061 and 6069 aluminum alloys. The relationship between mechanical properties and quench delay time at various temperatures between 200–500°C was determined. It was concluded that the 6069-T6 was somewhat more quench sensitive than 6061, which may be consistent with the composition difference. This study also provides increased data on the quench sensitivity of the traditional alloy, 6061-T6. 1. Introduction The mechanical properties of the relatively new 6xxx series alloy, 6069, were extensively discussed by the authors in [1–3]. The objective of this study was to determine the quench sensitivity of the new alloy 6069, especially as compared to the traditional 6061 alloy. That is, on rapidly cooling from the solution annealing temperature by quenching, any reduction in the cooling rate translates to longer times at intermediate temperatures where “uncontrolled” nucleation can occur and lead to lower T6 properties subsequent to aging. The mechanical properties response varies for a given, decreased, cooling rate depending on the alloy composition. Gullotti et al. [4] and others [5, 6] found that for the 6xxx alloys, those that had higher Mg, Si, Mn, Cr, and Zr were more likely to have relatively accelerated Mg2Si precipitation leading to diminished T6 mechanical properties. Mondolfo [7] reported that Cu increases quench sensitivity, but Zoller et al. [5] found that Cu actually alleviates quench sensitivity somewhat. The alloy 6069 has been demonstrated to have superior T6 fatigue, tensile, and fracture toughness properties over 6061 [1, 2]. However, the improved properties are provided in association with alloy additions Mg, Si, Cr (and Cu for which the effect is uncertain) which may render 6069 more quench sensitive. Thus, this investigation assessed the quench sensitivity of 6061 and 6069, both prepared identically from extruded air slip direct chill casting (Air-Slipor ASDC). Both were solution treated at the same temperature and “quenched” into salt baths at various temperatures for various times followed by a water quench. The times at temperatures for a fixed deterioration (e.g., 5%) of T6 tensile (yield stress and ultimate tensile stress) were determined. 2. Experimental Methods The 6061 and 6069 aluminum alloys used in this study were extruded at Anodizing Inc. (Portland, Ore) from Air-Slip Direct Chill Cast (ASDC) ingots provided by Northwest Aluminum Company. 6061 ingot was extruded into solid flat bar with a thickness of 9.53？mm and width of