Papers by Keyword: Cast Magnesium Alloy

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Authors: I. Basu, J.T. Wood, Jonathan P. Weiler
Abstract: In this study, commercial AM60B magnesium alloy was studied under different solidification conditions to understand the influence of cooling rate, thermal gradient, growth velocity, Niyama criterion, solidification time and mold dimensions on microstructural features such as secondary and tertiary dendrite arm spacing, grain size, porosity, pore shape and size, local morphological and phase variations. Porosity, grain size and dendrite arm spacing were measured as functions of the process variables. It was realized that the process of mold filling and solidification are simultaneous in nature and they significantly affect the microstructure development trends and its dependency on the process parameters. A significant effect, of the above mentioned, was found on the obtained porosity values and their variation along the casting. The results clearly indicate that rate of filling, nature of flow of liquid and shape of the mold greatly affect the solidification process and thereby the microstructure.
Authors: Xi Shu Wang, Jing Hong Fan, Bi Sheng Wu, Ying Li
Abstract: To study the fatigue microcrack initiation and propagation behaviors of cast magnesium alloys, the small fatigue crack propagation tests were carried out using the in-situ observation with scanning electron microscope (SEM). All initiations and propagations of fatigue small cracks focused on effects of the interaction of artificial two small holes, which there are the different distances and alignments of two small holes. The results indicate that the fatigue small cracks of cast magnesium alloys occurred mainly at the defects or root of notch but the early stage crack propagations were influenced on the distance and alignment between two small holes. For cast AM50 and AM60B alloys, the fatigue small cracking prior to occurred at the weak dendrite boundary and had some concomitances such as the plastic deformation on surface of α-Mg phase. For AZ91 alloy, the fatigue cracking characterization depended mainly on the brittle properties of β-Mg17Al12 phase, which the multi cracks occurred at the boundaries of β-Mg17Al12 phase. The effect of notch on the fatigue cracking behavior becomes weaker when the radius of notch is over 3-4 times than that of average α-Mg grain size. The fatigue crack propagation behaviors varied with the different arrangements of two small holes. The effects of distance and alignment of two small holes on the fatigue crack propagation behaviors are also obvious.
Authors: Richard Johnson
Authors: Tang Li, Qing Yuan Wang, Q.F. Dou, Chong Wang, M.R. Sriraman
Abstract: Very high cycle fatigue (VHCF) properties of high-pressure die cast Magnesium alloy AZ91HP have been investigated. Ultrasonic fatigue tests up to 109 cycles were conducted at the loading frequency of 20 kHz, under R=-1 condition and in ambient air. The experimental results show that specimens fail even after 107 cycles although the scatter seems to be large probably due to the presence of materials defects. However, there seems to be a fatigue limit at about 109 cycles. The fractures contain typical brittle features, with the fatigue cracks seen to initiate from the porosity in the material, either from the surface or beneath.
Authors: Jonathan P. Weiler, J.T. Wood, I. Basu
Abstract: Gravity step-casting experiments were performed to investigate process-structure-property relationships in three different die-cast magnesium alloys – AM60, AZ91 and AE44. The step-cast mold was instrumented to capture temperature profiles of the solidification of molten magnesium. This paper investigates the structure-property relationships of these magnesium alloys, specifically the dependence of the fracture properties upon the porosity that forms during the casting process. Sixteen tensile specimens were cut from the step-casting perpendicular to the solidification front, for each alloy examined. Correlations from X-ray tomography data were used to estimate the maximum area fraction of porosity from the average volumetric porosity in the specimens, assuming a typical size and spatial distribution of porosity. This relationship can be used in the absence of more accurate measure of porosity (i.e. serial sectioning, computed x-ray tomography). A failure model for die-cast alloys – which depends upon the strain-hardening coefficient and the maximum area fraction of porosity in the specimen – was used to predict fracture strains for each specimen. The experimental tensile elongation of each specimen was compared with predicted values. The resulting mechanical properties determined from these cast magnesium alloys will be used to develop process-structure-property relationships.
Authors: Q.Y. Wang, Hong Yan Zhang, S.R. Sriraman, S.L. Liu
Abstract: Magnesium alloys, on account of their lightweight, find useful applications in the automotive sector. During service, they experience very high number of fatigue cycles. Therefore, the understanding of their long life fatigue behavior becomes extremely important. This is possible by using ultrasonic fatigue testing, which is the only feasible way of doing it. In this study, the two such alloys viz. AE42 and AM60 has been investigated for their long life fatigue characteristics under fully reversed loading conditions, using a piezoelectric fatigue testing machine operating at a frequency of 20 kHz. The S-N data does not reach a horizontal asymptote at 107 cycles in either of the alloys. However, the alloy AM60 seems to show a fatigue limit at about at 109 cycles. The fractures examined by scanning electron microscopy (SEM) were found to be brittle in character. In very high cycle fatigue conditions, the crack was found to initiate from the specimen subsurface.
Authors: K. Maniraj, A.K. Lakshminarayanan
Abstract: An attempt is made to mitigate wear in cast magnesium alloy by incorporating flyash reinforcement on the surface through friction stir processing. Wear resistance is evaluated using pin-on-disc wear testing. Effect of dry sliding wear process parameters such as speed, time and distance were studied by using design of experiments. Empirical relationships were established between the process parameters and wear resistance of base metal as well as surface composite. Further, wear map is constructed using graphical optimization technique, which can be used to predict the lower and upper bound of wear rate for the both base metal and friction stir processed surface composites under dry sliding condition.
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