Atomic-scale simulations and experimental insights into the effect of precipitates on the hemming performance of 6xxx Al alloys

Abstract

Al–Mg–Si alloys are widely employed in automotive vehicles; however, challenges such as cracking often arise during the hemming process (180° bending). Based on the molecular dynamics simulations and experiments, this study investigated the effects of the size and number of MgSi(Fe) clusters on the mechanical properties of 6xxx Al alloys. The results showed that medium-sized MgSi clusters (containing 10–19 atoms) at the grain boundaries (GBs) enhanced the strength of the GBs, effectively inhibiting crack initiation and significantly suppressing intergranular cracking. In addition, the ductility and brittleness of the model with the Fe-containing phase were significantly affected by the Si/Fe atomic ratio (~0.71). Tensile experiments confirmed that the failure morphology exhibited a distinct brittle fracture when the Si/Fe atomic ratio of the phase was 0.90. The pre-aging treatment promoted the dispersion of solute atoms, thereby reducing the yield strength of the AA6016 Al alloy to ~120 MPa, which improved its hemming performance. Furthermore, pre-aging facilitated the generation of finer Mg–Si phases at the GBs during bake hardening.

1. Introduction

Heat-treatable 6xxx Al alloys, whose main alloying elements are Mg and Si, are used in automotive body panels owing to their high specific strength and good formability, which satisfy the environmental protection, economic, and lightweight requirements of the automobile industry [1–3]. Moreover, automotive 6xxx alloys can tolerate certain amounts of Fe (<0.5wt%), which is beneficial for the utilization of recycled Al [4–5]. Automotive structures involve the bending of thick-gauge sheets wherein the outer and inner body panels are joined via a hemming process (180° bending) [6]. During fabrication, cracking or tearing stripes appear on the convex surface of the sheet after intense strain localization [7–9].

The defect-free manufacturing of body panels during stamping and hemming requires satisfactory formability and low yield strength (YS) [10]. It has been reported that the bendability or hemming performance of 6111 Al alloys deteriorates with an increasing YS in 6111 Al alloys [11]. For 6xxx Al alloys, the aging process has a significant impact on the strength, which is mainly related to the nature of the clusters (size, composition, and number density) [12]. For instance, Li et al. [13] reported that the formation of MgSi clusters was promoted during preaging (PA) treatment regardless of the alloy composition. After PA treatment, the YS of the 6082 Al alloys was only 105 MPa, while a subsequent one-month natural aging (NA) and 4 h artificial aging significantly increased the YS to 279 MPa [14].

It is well established that the precipitates or clusters that affect the YS are the preferred sites for void formation and crack propagation during hemming [15]. When the precipitates are located at the grain boundaries (GBs), including the Mg2Si and Al(Fe,Mn)Si phases, they can reduce the cohesive stress of the GBs and promote crack initiation and intergranular cracking (IGC) [16]. Specifically, Weng et al. [17] found that shear bands were generated during the hemming process in predeformed Al–Mg–Si–Cu alloys, which resulted in the fragmentation of Fe intermetallic particles. However, Trink et al. [5] reported that the addition of Fe and Mn could adjust the Si-containing precipitates in AA6016 Al alloys, and that their elongation reached ~30%. The above studies focused on the effects of alloy composition and heat treatment processes on the microstructure and hemming properties of 6xxx Al alloys.

Subsequently, surface baking results in a pronounced hardening effect. A low baking temperature (80°C for 1 h) could result in the transformation of the MgSi cluster into β″-MgSi precipitates, which has been shown to improve the elongation of AA6082 Al sheets [18]. Therefore, heat treatments influence the microstructure of Al alloys; however, the underlying mechanisms of the influences of different clusters on the mechanical properties at the nanoscale has yet to be conclusively elucidated.

It is difficult for macroexperiments to directly observe the dynamic emergence of cracks and their propagation at the atomic scale, which can now be elucidated through computational modeling [19–22]. With advances in molecular dynamics (MD) simulations, the deformation of polycrystals and dislocation/defect evolution in single/polycrystals have been widely studied [23–24]. For instance, Xue et al. [25] analyzed the effects of solute clusters and precipitates on the strength and ductility of Al–Si–Mg–Cu alloys. Moreover, Wu et al. [26] also reported the dislocation activity in Al–Si eutectics utilizing MD simulations and found that a nano-crack was nucleated at the in...