Effect of annealing on interface microstructures and tensile properties of rolled Al/Mg/Al tri-layer clad sheets

Abstract

The aim of this study was to identify the effect of post-roll annealing on the interface microstructure and tensile properties of 1060 pure Al/Mg–0.2Al–1.75Mn–0.75Ce alloy/1060 pure Al tri-layer clad sheets fabricated by a combined hot and cold rolling process. There were no interface reaction phases in the as-rolled clad sheet, and the clad sheet annealed at 200 °C for 2 h. On annealing at and above 250 °C, reaction phases of Al₃Mg₂ on the Al side and Al₁₂Mg₁₇ on the Mg side were observed to form at the interface. The thickness of interface intermetallic compounds increased markedly with increasing annealing temperature, and exhibited a linear increase with the square root of annealing time, which could be well predicted via diffusion mechanisms. With increasing annealing temperature, the ultimate tensile strength first increased from 200 °C to 250 °C, where a maximum value of about 160 MPa was achieved, then decreased. While the yield strength decreased linearly, the elongation increased significantly with increasing annealing temperature. On annealing at and below 250 °C, interface debonding was absent during the tensile tests, while partial debonding occurred via a distinctive mode of zig–zag multiple cracking or fragmentation in the intermetallic compounds in the clad sheets annealed at or above 300 °C.

Introduction

Due to the pressing environmental challenges and rising energy prices faced by the transportation industry, lightweighting of materials and structures is being considered as a key strategy for improving the fuel economy and lowering anthropogenic environment-damaging emissions [1], [2], [3], [4], [5], [6], [7]. Of the prevalent materials for the next-generation transportation vehicles in the automotive and aerospace industries, Mg alloys represent a lucrative option for the weight reduction owing to their low density and high strength-to-weight ratio [3], [8], [9], [10], [11], [12], [13]. Despite the ever-increasing demand for lightweight vehicles, the use of Mg alloys especially in the structural applications has been limited due to the concerns about poor corrosion resistance, poor formability, and high production cost compared to Al alloys. The most serious barrier to the application of Mg alloy would be its poor corrosion resistance [14], [15], [16], [17]. Various studies have been performed to improve the corrosion resistance of Mg alloys, e.g., by rare-earth alloying addition [18], [19], but only marginal improvement was achieved. Another way of overcoming the problem is to clad Mg alloy sheet with pure Al layer by rolling or other joining methods, since pure Al offers superior corrosion resistance while maintaining low density and high strength of the Al–Mg alloy core [20].

There are many fabrication processes to clad Al layer on a Mg alloy sheet, such as twin-roll casting [21], [22], [23], [24], explosive bonding [25], friction stir welding [26], [27], [28], [29], and diffusion bonding [30], [31]. Roll cladding or bonding, a solid-state welding technique in which the clad layer is bonded to the core layer by rolling under pressure at ambient or elevated temperatures, has also been applied to manufacture clad sheets, e.g., Cu/Al clad sheet and Al/Cu/Al laminated composites [23], [24]. Considering the simplicity, applicability and cost, hot or cold roll cladding has been widely used for making multi-layered sheet metals in recent years [32], [33], [34]. In the present study, a combination of a hot and cold rolling process to clad 1060 pure Al on a newly developed rare-earth containing Mg alloy were conducted to manufacture Al/Mg/Al tri-layered clad sheets.

The performance of an Al/Mg/Al clad sheet is mainly governed by the formation of strong metallurgical bond across the interface between Mg and Al. Post-roll bonding process such as annealing may result in the growth of interface layer and in turn affect the bonding strength and the relevant mechanical properties of the Al/Mg/Al clad sheets. Therefore, a good understanding of interface diffusion layer is necessary to optimize and improve the mechanical properties of clad sheets. There are a few studies on the effect of annealing on the interface microstructure and mechanical properties of Al/Mg alloy clad sheets. Matsumoto et al. [35] reported that with increasing annealing temperature above 150 °C the interface layer thickness increased, the yield stress decreased, and the elongation to failure increased in an Al/Mg–Li alloy clad plate fabricated by cold rolling. Lee et al. [36] showed that with increasing annealing time the interface layer thickness increased, and the optimum mechanical properties could be achieved after annealing at 300 °C for 3 h of STS-Al–Mg clad sheet fabricated by warm rolling. However, to the best of the authors' knowledge, no studies are seen on an Al/Mg/Al clad sheet fabricated by a combined hot and cold rolling process, while a Mg/Al multilayer fabricated by cold roll bonding at ambient temperature was reported [34]. It is unclear what types of interlayer would be generated due to annealing, how the annealing temperature and time affect the interlayer growth and the related strength, and in which mode the failure occurs. The purpose of this study was, therefore, to identify the relationship among the post-roll bonding process variables, such as annealing temperature and time, the evolution of interface microstructure, and the mechanical properties of the Al/Mg/Al clad sheets fabricated by a combined hot and cold rolling process.