Effect of Room-Temperature Pre-rolling and Pre-cryorolling on Natural Aging and Bake Hardening Response of an Al–Mg–Si Alloy

ORIGINAL RESEARCH ARTICLE Effect of Room-Temperature Pre-rolling and Pre-cryorolling on Natural Aging and Bake Hardening Response of an Al–Mg–Si Alloy JIANRUI XING, GANG LEI, YAFEI WANG, LAXMAN BHATTA, CHARLIE KONG, and HAILIANG YU The auto industry makes extensive use of Al–Mg–Si alloys. This study investigated the effect of pre-cryorolling and room-temperature pre-rolling on the natural aging and bake hardening response of Al–0.92Mg–0.48Si alloy. The mechanical properties were analyzed using microhardness, tensile, and Erichsen cupping test. Optical microscope, transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry were used to examine the microstructure of samples. The molecular dynamics simulation was also employed to study the dislocation evolution in samples deformed at room and cryogenic temperature. The results show that both room-temperature pre-rolling and pre-cryorolling introduced immediately after solid solution treatment can effectively inhibit the adverse effects of natural aging, and promote the precipitation of strengthening phase during paint baking, leading to an improved bake hardening response. Compared with the room-temperature pre-rolling, pre-cryorolling can further improve the bake hardening response because of higher dislocation density. The results of this study indicate that pre-cryorolling with a reduction of 15 pct is the most appropriate pre-deformation procedure for this alloy, both in terms of formability and bake hardening response. JIANRUI XING, GANG LEI, YAFEI WANG, and HAILIANG YU are with the Light Alloys Research Institute, Central South University, Changsha 410083, P.R. China and also with the State Key Laboratory of Precision Manufacturing for Extreme Service Performance, Central South University, Changsha 410083, P.R. China. Contact e-mails: ; LAXMAN BHATTA is with the Advanced Materials Processing and Analysis Lab, Oregon State University, Corvallis 97330. CHARLIE KONG is with the Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia. Manuscript submitted April 24, 2023; accepted July 16, 2023. Article published online August 1, 2023 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 54A, OCTOBER 2023—3709 https://doi.org/10.1007/s11661-023-07150-5  The Minerals, Metals & Materials Society and ASM International 2023 I. INTRODUCTION THE automotive industry has extensively used Al–Mg–Si alloys as a significant lightweight material due to the alloy’s low density, medium and high strength, excellent formability, and excellent welding performance.[1–3] The formation of dispersed nanoscale precipitates during aging at a high temperature can significantly improve the strength and plasticity. The precipitation sequence of Al–Mg–Si alloys is as follows: supersaturated solid solution (SSSS)—clusters—GP zone—b¢¢ phase—b’ phase—b phase (Mg2Si).[4] In the process of automobile component production, this kind of high temperature aging is applied through the paint baking (PB). The bake hardening response (BHR) caused by metastable precipitated phases (mainly b¢¢ phase) is crucial to the service performance of coatings and components.[5] Natural aging (NA) cannot be avoided between the solid solution treatment and the forming process in Al–Mg–Si alloy plates. Due to thermodynamic instability, solute atoms within the supersaturated solid solution precipitate spontaneously at this stage, increasing initial strength and decreasing formability.[6] During the NA process, a large number of solute atoms are consumed, which hinders the precipitation of subsequent PB. Therefore, the negative effect of NA significantly reduces the BHR during paint baking, even softening the Al–Mg–Si alloys.[7] In addition, the short heat duration (20 to 30 minutes) during PB cannot give full play to the strengthening potential of the alloy, resulting 3710—VOLUME 54A, OCTOBER 2023 in a lower yield strength compared to the peak aging state. Some researchers reported that pre-deformation could improve the BHR.[8,9] Pre-deformation has great potential to promote the precipitation kinetics of several alloys, including Al–Mg–Si,[10] Al–Mg–Si–Cu–Zn,[11] Al–Cu–Li,[12] and Al–Mg–Cu.[13] The introduction of dislocations through pre-deformation plays an essential role in controlling precipitation mechanisms. Specifically, for Al–Mg–Si alloys, dislocations can act as sinks for vacancies, inhibiting clustering during room temperature storage and reducing the negative effect of NA.[14] Additionally, dislocations can provide heterogeneous nucleation sites for the GP zones, promoting their transformation into the b¢¢ phase, thus improving precipitation kinetics.[15] Yin et al.[16] studied the effects of tensile pre-deformation on artificial aging hardening behavior of an Al–Mg–Si–Cu–Zn alloy. They found that 5 pct pre-tensile strain before aging significantly increased the peak aging hardness. Jia et al.[17] reported that pre-deformation of AA6022 at 443 K effectively solved the high T4 hardness caused by traditional room temperature pre-deformation treatment and provided better BHR. Dislocation and cluster (2) introduced by high-temperature pre-deformation not only inhibited the negative effect of NA, promoted b¢¢ phase precipitation, but also reduced work hardening effect through dynamic recovery. Pre-rolling is also an important pre-deformation technique. Yuan et al.[18] investigated the effect of pre-rolling on the precipitation behavior and mechanical METALLURGICAL AND MATERIALS TRANSACTIONS A properties of Al–Mg–Si–Cu–Zn alloys with different Mg/Si ratios and Cu addition amounts, and discovered that pre-rolling effectively accelerated the precipitation kinetics of the alloys. Serizawa et al.[19] found that room-temperature pre-rolling with a 5 pct reduction increased the dislocation density inside the material, and the nanoclusters along the dislocation direction preferentially transformed into b¢¢ phase, which led to the rapid growth of b¢¢ phase, and effectively enhanced the BHR. However, even after PB, the yield strength of these alloys is still insufficient to meet application requirements, so the BHR of these alloys is expected to be enhanced further.[20] Moreover, the primary objective of pre-deformation is to introduce dislocation in the material. There are reports that deformation at cryogenic temperature can produce high dislocation density, which has been confirmed in Al,[21] Cu,[22] Ti alloys,[23] and multilayer composites.[24] However, there have been no reports about the effect of cryogenic pre-deformation on the BHR of Al–Mg–Si alloys until now. Currently, the majority of researches only focus on the mechanical properties of materials before and after PB[25] or disregards the influence of the NA stage.[11,18] There are few reports on the mechanical properties of materials throughout the entire solid solution treatment to bake hardening procedure. In this work, we applied pre-rollin (...truncated)