Suppressing negative natural aging effect in automotive AlMgSiCu alloys via Sn microalloying

Abstract

Although Sn has been established as an effective microalloying element for suppressing the negative natural aging (NA) effect in Al–Mg–Si alloys, its potential to mitigate the negative NA effect in Al–Mg–Si–Cu alloys remains to be confirmed. This study systematically investigated the role of Sn in the NA of Al–Mg–Si–Cu alloys through hardness measurements, differential scanning calorimetry, and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. Our results demonstrate that the addition of Sn significantly suppresses the adverse impact of NA on the peak-aged hardening capacity during subsequent artificial aging and substantially alleviates early-stage hardening kinetics degradation. Our findings suggest that Sn modifies the nature of the NA clusters in Al–Mg–Si–Cu alloys. A significant proportion of NA clusters in the Sn-added alloy effectively served as heterogeneous nucleation sites for strengthening the precipitates during artificial aging, thereby preserving the precipitate nucleation rates and preventing coarsening at the peak-aging stage. Atomic-resolution energy-dispersive X-ray spectroscopy revealed preferential occupation of Si atomic sites by Sn atoms within the β′ and C/Q′ phases. This investigation provides critical theoretical insights for optimizing alloy design in automotive-body aluminum applications.

1. Introduction

The 6xxx series Al–Mg–Si(–Cu) alloys have been extensively utilized as lightweight automotive body panels owing to their good corrosion resistance, excellent formability, and high strength-to-weight ratios [1–2]. In practical automotive production, these alloys undergo a paint-bake (PB) process at approximately 180°C for 30 min (equivalent to a short-term artificial aging (AA) process) to achieve desired strength levels [3]. Cu is added to automotive-grade 6000 series alloys because of its ability to accelerate the PB hardening response [4–12]. The substantial strength enhancement during AA originates from the precipitation of numerous nanosized semi-coherent metastable precipitates. The generally accepted precipitation sequence in Al–Mg–Si alloys can be described as follows: supersaturated solid solution (SSSS) → atomic clusters → Guinier–Preston (GP) zones/pre-β″ → β″ → β′ (/B′/U1/U2) → β [13–18]. Among all types of precipitates, the needle-like β″ phase and its precursors are recognized as the most effective for strengthening the alloys. Cu addition alters the precipitation sequence to SSSS → solute clusters → GP zones → β″, L, QP, QC → β′, Q′ → Q [11–12]. The co-precipitation of Cu-containing lath-like L, QP, and QC phases with a β″ phase is considered the origin of enhanced aging response and elevated peak hardness in Al–Mg–Si–Cu alloys.

In industrial practice, Al–Mg–Si(–Cu) alloys inevitably undergo natural aging (NA) prior to AA. NA not only deteriorates formability but also generally exerts a negative or detrimental effect on subsequent precipitation hardening during AA [19–22]. Consequently, NA behavior and its negative impact on AA in Al–Mg–Si(–Cu) alloys have attracted extensive research. Numerous studies have demonstrated that the negative NA effect correlates strongly with the NA duration and alloy composition (e.g., Mg/Si ratio and Mg + Si content) [23–25]. However, the fundamental mechanisms underlying the negative NA effect remain incompletely understood, primarily because of the intricate interplay between NA clusters and quenched-in vacancies—microstructural features that are challenging to characterize precisely. The NA clusters may reduce the vacancy concentration in the Al matrix by entrapping quenched-in vacancies, thereby retarding solute diffusion and suppressing precipitate development [26]. Concurrently, NA clusters reduce the supersaturation of solute atoms in the matrix while failing to provide favorable nucleation sites for the strengthening of precipitates upon AA [25,27]. Early atom probe tomography studies revealed that NA clusters exhibit Mg/Si ratios deviating significantly from 1, impeding their transformation into the β″ phase [28–29]. Recent annular dark-field scanning transmission electron microscopy (ADF-STEM) investigations confirmed substantial structural discrepancies between NA clusters and the β″ phase, explaining their transformation resistance [30].

Recent research has indicated that Sn microalloying effectively suppresses NA in Al–Mg–Si alloys through strong vacancy trapping, thereby inhibiting NA cluster formation [31–36]. Our recent study revealed that Sn addition fundamentally alters the precipitation behavior of Al–Mg–Si alloys [37].