Issue 38

A. Gryguc et al, Frattura ed Integrità Strutturale, 38 (2016) 251-258; DOI: 10.3221/IGF-ESIS.38.34 252 I NTRODUCTION here has been a significant amount of technology development in the past quarter century which has focused on developing lightweight vehicles to address the need for both better fuel economy and decreased emissions in the transportation industry [1]. As the lightest structural metallic material, magnesium and its alloys have shown to be promising candidates for use in vehicle components which are susceptible to fatigue fracture, such as suspension control arms [2]. However, the strong crystallographic texture that can form during processing and anisotropic mechanical properties leads to low ductility at room temperature which limits the uses of wrought magnesium alloys. Numerous studies [3, 4] have been performed with the aim of weakening the crystallographic texture which reduces the anisotropy in mechanical properties. The most commonly reported mechanism for texture weakening of magnesium alloy is introducing rare earth materials. The addition of Y, Nb, Gd, Ce, Ng etc. in magnesium alloys randomizes the texture during hot deformation processes like extrusion, forging and rolling, which causes a reduction of crystallographic texture intensity and activates the basal slip system [4]. However, the cost associated with rare earth elements significantly limits their application. Recently, Sarker et al. [3] proposes multidirectional compression as a method for randomizing the texture and improving the tension-compression asymmetry in magnesium. Though the use of rare earth alloying elements have been shown to weaken texture, it has not been established whether modified texture improves fatigue performance of Mg alloys. A goal of the current work is to better understand the mechanisms affecting fatigue performance of non-rare earth Mg alloys, including both the grain structure and texture. Although studies utilizing stress controlled fatigue methods provide important information into the design of engineering components [5], most of the published researches were focused on the tension-compression properties and fatigue behavior of cast, extruded, rolled or forged wrought magnesium alloys. For instance, Somekawa et al. [6] studied the fully reversed stress controlled cyclic and compression testing and observed twinning behavior in extruded magnesium alloy. Yin et al. [7] found that under strain controlled cyclic testing the stable crack propagation zone is characterized by a lamellar structure resulting from twinning, whereas the fracture zone has a dimpled morphology resulting from slip. Other studies [8] [9] [10] also investigated the effect of sample direction on fatigue crack growth and propagation in magnesium alloys. More detailed studies [11-18] regarding multiaxial cyclic response, failure mechanism and fatigue life modeling also indicate analogous effects of mechanical processing on the texture evolution, monotonic and cyclic responses. The study of the stress controlled fatigue resistance of extruded then forged magnesium alloy is very limited in the literature, especially in those alloys suitable for automotive structural applications. Specifically, it is not clear how changes in the microstructure and texture of extruded, then forged AZ31B magnesium alloy affect the fatigue performance. Therefore, the aim of this study was to discuss the sensitivity of forging on both tensile and fatigue properties of AZ31B extruded magnesium alloy. Another objective is to discuss the influence of forging on the microstructure and texture evolution. M ATERIAL AND EXPERIMENTS he material used in this investigation was commercially available Mg-Al-Zn magnesium alloy AZ31B. The chemical composition of this alloy in mass percentage is shown below in Tab. 1. The material was received in the form of an extruded billet of diameter 88.9mm in the as-fabricated condition. Al Zn Mn Al 2.5-3.5 2.5-3.5 2.5-3.5 2.5-3.5 Table 1 : Table of AZ31B alloy chemistry in wt. % Forging trials were conducted at CanmetMATERIALS using AZ31B extruded feedstock of the aforementioned diameter which was cut into 88.9 mm lengths. All tests were carried out on a 500 Ton hydraulic press with an upper and lower platen (die) which were both flat. The billet and tooling were heated separately to a temperature of 400°C. The orientation of the billet to the press was such that the extrusion direction was along the direction of the press stroke (i.e. direction of forging was coincident to extrusion direction of the billet). Forging was carried out at two different rates (10 and 100 mm/min with engineering strain rates of about 0.002 and 0.02 s 1 subsequently referred to as sample S1 and S2, respectively). The as-extruded material was forged to a height of 13 mm corresponding to 85% compressive engineering T T