PSI - Issue 25

A. Gryguć et al. / Procedia Structural Integrity 25 (2020) 486– 495 Andrew Gryuc/ Structural Integrity Procedia 00 (2019) 000–000

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Keywords: Magnesium;; Multiaxial Fatigue; Cracking Orientation; Forging; AZ80

1. Introduction The impetus for utilization of Mg alloys for lightweighting initiatives is strong as their low density, high specific strength and excellent machinability make them ideal candidates for a variety of applications. Recently wrought varieties of Mg alloys have presented themselves as ideal candidates for structural components as they have superior mechanical properties are less prone to manufacturing defects, such as those due to casting. The specific fatigue strength of Mg alloys is also a very attractive for lightweighting of components which are exposed to cyclic loading. Die-forging of components is another manufacturing method which produces properties and microstructure comparable or superior to other wrought varieties of Mg with the added benefit of achieving a near net shape final component which requires minimal post processing. As a result of this, several pioneering studies have been carried out surrounding the forging of Mg alloys[1]–[6], which focused on the feasibility of the physical manufacturing process. More recently, several studies have been done on closed-die forging of Mg alloys to produce near net shape components with high quality and excellent properties [7]–[22] Characterization of the fatigue properties have of AZ80 Mg alloy have been explored as it has good forgability, high aluminum content and superior strength and fatigue performance when compared to other commercially available Mg alloys [23]–[30]. This work aims to further explore the complexities of the fatigue behaviour by investigating the combined influence of shear and tensile loading upon the multiaxial fatigue behaviour and cracking orientation of forged AZ80 Mg alloy. 2. Experimental Setup The material utilized in this investigation was commercially available AZ80 Mg extruded billet that was then forged using a closed die forging technique to produce I-beam cross-section geometry. Details regarding sample extraction from the forged billet are illustrated in previous work by Gryguć et al. [28]. A detailed presentation of the experimental procedure, monotonic & cyclic characterization and multiaxial fatigue life modelling of forged AZ80 Mg can also be found in ref [28]. The extruded material had chemical composition of 8.0 ± 0.2% aluminum content, with other elements as per ASTM B91-12 standard. The material was received from Magnesium Elektron North America Inc. in the as-fabricated condition. The dimension of the extruded billet was a diameter of 63.5mm and a length of 1000 mm. The forging was conducted at CanmetMATERIALS (Hamilton, Canada) using the billets having dimension of ∅ 63.5 mm, 65-mm long. The billet and tooling were heated separately to 250 °C for sufficient time to allow any thermal gradients to decay. The orientation of the billet to the press was such that the radial direction was along the direction of the press stroke (i.e. the direction of forging was parallel to the radial direction of the billet). Forging was carried out in a single step at an initial displacement rate of 20 mm/sec. Strain controlled fatigue tests were carried out as per previous work by Gryguć et al. [28]. Five different strain paths were investigated, (A) pure axial, (B) pure shear, (C) bi-axial proportional, (D) bi-axial, non-proportional, 45 ° out-of phase and finally (E) bi-axial non-proportional 90 ° out-of-phase. The axial fatigue tests were performed as per ASTM E606 in an ambient environment using an MTS 810 Servo-Hydraulic test machine and shear/biaxial using an Instron 8874 test machine operating in strain control mode at a frequency ranges of 0.05–2 Hz, and 1–30 Hz in force control mode depending on the strain amplitude to maintain an approximately consistent loading rate between all tests. During strain control, the max strain rate averaged over the gauge section of measurement was ∼ 1.5E−2 sec−1 for the pure axial and torsional tests, and ∼ 3.2 E−3 sec−1 for the biaxial tests. For the purely axial tests, the strain was measured throughout the first 10,000 cycles using an MTS 632.26 extensometer with an 8-mm gauge and travel of±1.2-mm until stabilization of the cyclic hysteresis loop was achieved. Similarly, for the pure shear and biaxial tests, the strain was measured using an Epsilon biaxial extensometer (model 3550) with axial and shear ranges of±5% and±3° mounted on the specimen’s gauge length. The tests were conducted at a zero mean strain (i.e., R L = −1, fully reversed strain cycle) and strain amplitudes ranging from 0.32% to 2.0%. The failure criteria for the fatigue tests was considered to be a 50% reduction in the peak tensile axial stress or a 50% reduction in peak or valley shear stress. The quantitative fracture surface images were obtained using a Keyence VHX-7000.