Experimental and numerical analyses of magnesium alloy hot workability (original) (raw)
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Hot workability analysis of extruded AZ magnesium alloys with processing maps
Materials Science and Engineering: A, 2010
The processing conditions applied to deform wrought magnesium alloys greatly affect their final microstructure and mechanical properties. Defining an applicable processing window in terms of the ranges of temperatures and strain rates to achieve a homogeneous microstructure is a prerequisite for process optimization, since magnesium alloys show a peculiar deformation behaviour. By using trustworthy input data from uniaxial hot compression tests, processing maps were constructed for the alloys AZ41, AZ61 and AZ80. These maps clearly indicate the conditions leading to dynamically recrystallized homogeneous microstructures after hot deformation, as verified by analysing the microstructures after hot deformation under different conditions. It is found that a strain rate of 5 s −1 and a temperature of 375 • C for AZ41 and 325 • C for both AZ61 and AZ80 would result in dynamically recrystallized homogeneous microstructures.
Advanced Materials Research, 2012
The effects of heat treatment on the microstructure, tensile property and fracture behavior of as-extruded AZ91 magnesium alloy were studied with OM and SEM. The results show that the grains of as-cast AZ91 alloy are refined by hot extrusion due to dynamic recrystallization, and the mechanical properties are improved obviously. The ductility is significantly enhanced after solution treatment of the as-extruded AZ91 alloy, tensile strength is almost the same as before, and hardness is significantly reduced after solution treatment and artificial aging treatment. The tensile strength reduced and the ductility is significantly enhanced of as-extruded after annealing processes. The fracture surface of as-extruded AZ91 magnesium alloy has the mixed modes of ductile and brittle characteristics. But after T6 or annealing treatment, the dimple number increases evidently.
Hot deformation behavior of AZ91 magnesium alloy in temperature ranging from 350°C to 425°C
Transactions of Nonferrous Metals Society of China, 2012
The flow behavior and microstructure evolution of AZ91 magnesium alloy during a thermomechanical process, hot compression test， was investigated. The specimens were hot compressed at a temperature ranging from 350 °C to 425 °C and at strain rate of 0.1 s −1 to the strains of 0.3, 0.5 and peak. Microstructural evolutions were studied using optical and scanning electron microscopes. The results show that during the compression process, the recrystallized grains nucleate along the pre-existing grain boundaries. The amount of dynamically recrystallized grains is increased with strain in a sigmoid scheme followed by Avrami equation. The size of dynamically recrystallized grains also increases at the beginning and decreases after reaching the maximum value.
Analysis of high-temperature deformation and microstructure of an AZ31 magnesium alloy
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2007
High-temperature plastic deformation and dynamic recrystallization of AZ31 extruded (EX) and heat treated (FA) alloy was investigated in the temperature range between 200 and 400 • C. High-temperature straining resulted in partial dynamic recrystallization above 250 • C; in the EX alloy recrystallization was complete at 300 • C, while a moderate grain growth was observed at 400 • C. The peak flow stress dependence on temperature and strain rate are described by means of the conventional sinh equation; the calculation of the activation energy for high temperature in the whole range of temperature deformation gives Q = 155 kJ/mol, i.e. a value that was reasonably close but higher than the activation energy for self diffusion in Mg. The microstructure resulting from high-temperature straining was found to be substantially different in EX and FA alloys; in particular, the EX alloy was characterized by a lower flow stress, a higher ductility and by a finer size of the dynamically recrystallized grains. These results are then discussed on the basis of the "necklace" mechanism of dynamic recrystallization.
A study of the hot and cold deformation of twin-roll cast magnesium alloy AZ31
Http Dx Doi Org 10 1080 14786435 2013 853884, 2014
Recent advances in twin-roll casting (TRC) technology of magnesium have demonstrated the feasibility of producing magnesium sheets in the range of widths needed for automotive applications. However, challenges in the areas of manufacturing, material processing and modelling need to be resolved in order to fully utilize magnesium alloys. Despite the limited formability of magnesium alloys at room temperature due to their hexagonal close-packed crystalline structure, studies have shown that the formability of magnesium alloys can be significantly improved by processing the material at elevated temperatures and by modifying their microstructure to increase ductility. Such improvements can potentially be achieved by processes such as superplastic forming along with manufacturing techniques such as TRC. In this work, we investigate the superplastic behaviour of twin-roll cast AZ31 through mechanical testing, microstructure characterization and computational modelling. Validated by the experimental results, a novel continuum dislocation dynamics-based constitutive model is developed and coupled with viscoplastic self-consistent model to simulate the deformation behaviour. The model integrates the main microstructural features such as dislocation densities, grain shape and grain orientations within a self-consistent viscoplasticity theory with internal variables. Simulations of the deformation process at room temperature show large activity of the basal and prismatic systems at the early stages of deformation and increasing activity of pyramidal systems due to twinning at the later stages. The predicted texture at room temperature is consistent with the experimental results. Using appropriate model parameters at high temperatures, the stress-strain relationship can be described accurately over the range of low strain rates.
Materials Science and Engineering: A, 2013
Hot workability of Zr alloy has been investigated by means of hot compression test using Gleeble-3800. Hot compression test was performed in the temperature and strain rate range of 700 to 925 o C and 0.01-10s-1 , respectively. Deformation behavior was characterized with the help of processing maps using Dynamic Material Model (DMM). From the power dissipation map peak efficiency of 48-55% was obtained at temperature of around 750 o C and strain rate of 1×10-2 s-1 , in (α +β) phase field. High efficiency suggests dynamic recrystallization and therefore is preferred processing condition. A domain of unstable flow was obtained at temperature range 700-730°C and strain rate range of 8×10-2-1s-1 in which instability parameter is negative. This region should be avoided during hot working. Effect of Cu in addition to Zr-2.5Nb also studied by power dissipation and instability map in this present study.
HIGH TEMPERATURE PLASTIC DEFORMATION OF A HEAT-TREATED AZ31 MAGNESIUM ALLOY
High-temperature plastic deformation and dynamic recrystallization were investigated in an extruded and heat-treated AZ31alloy in the temperature range between 200 and 400°C. High-temperature straining resulted in partial dynamic recrystallization above 250°C; at 400°C recrystallization was complete and a moderate grain growth was observed. The peak flow stress dependence on temperature and strain rate was described by means of the conventional sinh equation; calculation of the activation energy for high temperature in the whole range of temperature deformation gave Q=155 kJ/mol, i.e. a value that is reasonably close to, but greater than, the activation energy for self-diffusion in Mg. When the data obtained at the lowest temperature were excluded from the calculation, the activation energy increased to 180 kJ/mol. This difference in the activation energy value can be explained by the occurrence of dynamic recrystallization in the high-temperature regime; this observation was substantially confirmed by the plots of strain-hardening rate as a function of stress that were used to identify the onset of dynamic recrystallization.
Mechanical properties of magnesium alloy AZ91 at elevated temperatures
2006
AbstrAct Purpose: Purpose of this article is to extend a complex evaluation of magnesium alloys which requires very often knowledge of elastic-plastic properties at elevated temperatures. These properties are connected with microstructure that is influenced by metallurgical and technological factors and conditions of exploitation. Design/methodology/approach: Methodology Testing of magnesium alloys was based on tensile test in dependence on temperature. The methods of the light microscopy and SEM for metallographic and fracture analyses of alloys after testing were used. Findings: Objective of this work consisted in determination of changes of elastic-plastic properties of magnesium alloy AZ91 in dependence on temperature, including investigation of fracture characteristics. It was confirmed that during heating at chosen temperatures there occurs partial dissolution of minority phases. Homogenisation of microstructure is, however, accompanied by simultaneous forming of inter-granular non-integrities, which is unfavourable from the viewpoint of strength and plastic properties, especially at higher temperatures. Failure occurs practically at all temperatures basically by inter-crystalline splitting along the boundaries of original dendrites. At temperature testing near melting point of alloy the interdendrite areas melting were observed. Research limitations/implications: The experiment was limited by occurrence a void in cast alloys. Practical implications: The results may be utilized for a relation between plastic and strength properties of the investigated material in process of manufacturing. Originality/value: These results contribute to complex evaluation of properties magnesium alloys at higher temperatures namely for explanation of fracture mechanism near the melting point.
Metals
Hot extruded (EX) AZ61 magnesium alloy was processed by the twist channel angular pressing (TCAP) method, which combines equal channel angular pressing (ECAP) and twist extrusion (TE) processes and significantly improves the efficiency of the grain refinement process. Both the initial hot extruded AZ61 alloy and the alloy after completion of TCAP processing were examined by using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) and their corresponding micro-tensile testing (M-TT) and hardness testing at room temperature. The results showed that the microstructure of hot extruded alloy was refined well by TCAP due to dynamic recrystallization (DRX) caused by TCAP. The tensile properties, investigated by micro-tensile testing (M-TT), of the AZ61 alloy were significantly improved due to refined microstructure. The highest tensile properties including YS of 240.8 MPa, UTS of 343.6 MPa and elon...
Hot compression behavior of the AZ91 magnesium alloy produced by high pressure die casting
Journal of Materials Processing Technology, 2007
The hot deformation behavior of a Mg-9Al-1Zn alloy produced by high pressure die casting has been investigated by means of compression tests in the temperature and strain rate ranges of 125-300 • C and 1.6 × 10 −5 to 10 −1 s −1 , respectively. The samples were deformed in the high pressure die casting state or after an exposure at 415 • C for 2 h to evaluate any different response to deformation. A premature fracture at 45 • respect to the compression axis was observed in samples strained at low temperatures and high strain rates. The stress-strain curves are explained in terms of microstructure evolution, with emphasis on the eutectic phase and on dynamic precipitation phenomena occurring at low and high temperature of deformation. Constitutive parameters were evaluated in order to better understand the microstructure evolution. Light and scanning electron microscopy have been performed to correlate the microstructure to the deformation parameters in all the tested conditions.