Strengthening and ductilization mechanisms of friction stir processed cast Mg–Al–Zn alloy

Abstract

AZ91 alloy, the most widely used Mg casting alloy, exhibits low strength/ductility and weak energy absorption, which is a function of its large grain size and the presence of a coarse and continuous network of β-Mg₁₇Al₁₂ intermetallic compounds. This work demonstrated that friction stir processing (FSP) enables enhancement of strength and energy absorption capability of AZ91 alloy. The influence of FSP treatment on various potential strengthening mechanisms, including grain boundary, solid solution, and sub-micron particle strengthening mechanisms, was studied. It is identified that the grain boundary strengthening plays a significant contribution to the strength of the FSP treated AZ91. FSP treatment altered the failure mechanism of the alloy from brittle cleavage-dominant mode to ductile dimple-dominant mode, which can increase the potential of cast Mg alloys to use in the safety-critical application. The significant improvement in energy absorption capability is a function of grain refining and the formation of ultra-fine sub-micron Mg₁₇Al₁₂ particles. The trends in mechanical properties of AZ91 treated by various severe plastic deformation approaches are disused.

Introduction

Magnesium is the lightest structural metal, 4.5 times lighter than steel, 1.7 times lighter than aluminum, and even slightly lighter than carbon fibers [1]. Therefore, magnesium and its alloys are critical materials to address the growing global demands for energy-saving and CO₂ emissions reduction for all transportation applications, especially for automotive and aerospace industries [2,3]. However, low ductility and strength are the critical bottlenecks for their wide applications in safety-critical applications [4].

Due to low formability of Mg alloys at room temperature which arises from the development of strong basal texture during rolling (thermomechanical treatment) and a limited number of available deformation modes due to magnesium having hexagonal close-packed (HCP) structure, cast Mg alloys especially Mg–Al–Zn alloy often have to be used [5]. However, the ductility of the cast Mg–Al–Zn alloys (e.g., AZ91) is limited due to the coarse grain structure, formation of coarse eutectic micro-constituents and the presence of interdendritic micro-porosity [6]. Severe plastic deformation (SPD) techniques are viable approaches to improve the strength and ductility of Mg alloys. The conventional severe plastic deformation, such as Equal Channel Angular Pressing (ECAP ([7], Extrusion [8], Accumulative Roll Bonding (ARB) [9], Differential Speed Rolling (DSR) [9] have been successfully employed for grain refining of Mg alloys. Due to the low formability of Mg alloys at room temperature, SPD techniques are generally to be applied at high temperatures (i.e., 200–450 °C) [7,[10], [11], [12]]. For example, it is shown that ECAP of AZ91 at room temperature resulted in cracking and segmentation [12].

Friction stir processing (FSP) has been proven to be an effective pathway to refine the alloy microstructure [[13], [14], [15], [16], [17]], providing more intense plastic deformation as well as higher strain rates than other SPD methods. In this process, the heat needed for plastic deformation of the alloy is obtained via frictional heating between tool and workpiece as well as heating associated with the material plastic deformation. Therefore, FSP does not need to preheat the workpiece, and the process can be performed at room temperature. In this paper, the effect of FSP treatment on the microstructural features and tensile properties of as-cast AZ91 Mg alloy, as the most widely used Mg casting alloy, are reported. The strengthening and ductilization mechanisms of the FSP treated AZ91 are highlighted. The achieved mechanical properties are compared with those obtained in previous works on FSP treatment of AZ91 alloy. The trends in tensile properties of AZ91 treated by various severe plastic deformation approaches are reviewed. The effect of grain refinement associated with the SPD techniques on the strength, ductility, and energy absorption of AZ91is discussed.