Extruded AZ31 Magnesium Alloy Research Articles

Influence of Heat Treatment on Low-Cycle Fatigue Behavior of AZ61 Magnesium Alloy

In order to identify the influence of solid solution, aging and solid solution plus aging treatments on the low-cycle fatigue behavior of the extruded AZ61 magnesium alloy, the low-cycle fatigue tests were performed at room temperature for the extruded AZ61 magnesium alloy with different treating states. The results indicate that the cyclic stress response behavior of the extruded AZ61 magnesium alloy exhibits both cyclic strain hardening and stability. The solid solution, aging and solid solution plus aging treatments tend to decrease the cyclic deformation resistance of the extruded AZ61 alloy in most conditions. The solid solution treatment can enhance the fatigue lives of the extruded AZ61 alloy at medium total strain amplitudes. In addition, the aging treatment can prolong the low-cycle fatigue lives of the AZ61 alloy at most total strain amplitudes, while the case for the solid solution plus aging treatment is just contrary. For the extruded AZ61 alloy with different treating states, a linear relationship between cyclic stress amplitude and plastic strain amplitude is noted.

Effect of twinning, slip, and inclusions on the fatigue anisotropy of extrusion-textured AZ61 magnesium alloy

In this study, experiments were conducted to quantify structure-property relations with respect to fatigue of an extruded AZ61 magnesium alloy using a MultiStage Fatigue (MSF) model. Experiments were conducted in the extruded and transverse directions under low and high cycle strain control fatigue conditions. The cyclic behavior of this alloy displayed varying degrees of twinning and slip depending on the strain amplitude as observed in the hysteresis loops of both directions. Under low cyclic conditions, asymmetrical stress strain response was observed for both orientations. However, systematic stabilization of the hysteresis occurred by half-life due to subsequent twinning and detwinning mechanisms. In addition, under high cycle fatigue, pseudo-elasticity was observed at the first and at half-life cycles. Structure-property relations were quantified by examining the fracture surfaces of the fatigued specimens using a scanning electron microscope. In terms of crack incubation, fatigue cracks were found to initiate from intermetallic particles (inclusions) that were typically larger than the mean size. Quantified sources of fatigue crack incubation, microstructurally small cracks, and cyclic stress–strain behavior were correlated to the MSF model. Based on the specific material parameters, the MSF model was able to predict the difference in the strain-life results of the AZ61 magnesium alloy in the extruded and extruded transverse directions including the scatter of the experimental results. Finally, the MSF model revealed that the inclusion size was more important in determining the fatigue life than the anisotropic effects from the texture, yield, and work hardening.

Deformation Heterogeneity on the Cross-Sectional Planes of a Magnesium Alloy Processed by High-Pressure Torsion

Disks of an extruded AZ31 magnesium alloy were processed by high-pressure torsion (HPT) at a pressure of 6.0 GPa through 1/4, 1, and 5 turns either at room temperature (296 K (23 °C)) or at an elevated temperature of 463 K (190 °C). The cross-sectional planes of the disks were examined after processing, and it is shown that there is a high level of heterogeneity throughout all of the samples. This heterogeneity is revealed through the nature of the flow patterns, through the distributions of grain sizes, and by comprehensive microhardness measurements. The results demonstrate that it is possible to double the strength of the alloy in some areas of the disk after processing through 5 turns at room temperature.

Structure–property quantification of corrosion pitting under immersion and salt-spray environments on an extruded AZ61 magnesium alloy

Extruded AZ61 magnesium coupons were exposed to immersion and cyclical salt spray environments over 60 h in order to characterize their corrosion rates. The characteristics of general corrosion, pitting corrosion, and intergranular corrosion were quantified at various intervals. General corrosion was more prevalent on the immersion surface. In addition, more pits formed on the immersion surface due the continuous exposure to water and chloride ions. However, the pits on the salt spray surface showed larger surface areas, larger volumes, and covered more area on the micrographs as compared to the pits on the immersion surface, due to the dried pit debris that trapped chloride ions within the pits.

Enhanced very high cycle fatigue performance of extruded Mg–12Gd–3Y–0.5Zr magnesium alloy

Very high-cycle fatigue behaviors of extruded Mg–12Gd–3Y–0.5Zr (GW123k) magnesium alloy have been investigated and compared to that of conventional extruded AZ31 magnesium alloy. Typical post-fatigue microstructure and surface morphology features are presented for both the GW123k and the AZ31 alloy in order to stand out the uniqueness of GW123k alloy. Respective fatigue damage mechanisms for both alloys were also proposed in accordance. It is found that GW123k alloy contains a large amount of precipitated particles and possesses a relatively weak texture, which give rise to its much relieved tension–compression yield asymmetry and enhanced fatigue failure resistance. The much homogenized deformation mechanism in GW123k alloy is considered the underline reason for the improvement of material's fatigue performance.

Low-Cycle Fatigue Properties of Extruded AZ31 Magnesium Alloy

Fully reversed tension-compression low-cycle fatigue tests controlled by both of total strain and stress were conducted on extruded AZ31 magnesium alloy using smooth round bar specimens in ambient atmosphere at room temperature. The stress-strain hysteresis loops exhibited a characteristic asymmetric shape with a positive mean stress during total strain-controlled fatigue cycle, in comparison with those with a compressive mean strain under stress-controlled fatigue test. The mean stress was associated with the ratio of compressive yield stress and tensile one in the material, which resulted in mechanical twinning in the compressive phase. The fatigue lives obtained from both controlled experiments were nicely described by the Coffin-Manson relationship. From the plastic strain range-life curves, fatigue life under total strain-controlled fatigue test was shorter than that under stress-controlled one because of an effect of tensile mean stress. Fatigue criteria considering the mean stress effect previously proposed were evaluated in terms of fatigue life predictions based on the experimental results. Also, an energy-based model taken into account of plastic and elastic energy density was discussed to predict the fatigue lives obtained from total strain- and stress-controlled fatigue experiments.

Dynamic Friction Properties and Microstructural Evolution in AZ31 Magnesium Alloy at Elevated Temperature during Ring Compression Test

The dynamic friction properties of the extruded AZ31 magnesium alloy with the initial average grain size of 15 μm were investigated by the ring compression test at 473 and 523 K and in a strain rate range from 1.0×10−2 to 3.0 s−1. Two types of the tool, WC-Co tool (WC) and WC-Co coated with diamond like carbon tool (DLC) were used. At 523 K, few differences in terms of the friction coefficient were observed due to the difference with or without DLC. At 473 K, the friction coefficient for the sample deformed by DLC tool was smaller than that done by WC tool. The investigation of the texture near the surface of the tested work pieces with different tools reveals that the integration degree of the grains within 10 degree from 〈0001〉 direction to compressive axis in the sample deformed by the DLC tool was smaller than that done by WC tool. It was concluded that the larger friction could enhance alignment of the planes perpendicular to the compressive direction to the basal plane even if under same testing condition.

Microstructural development of the hot extruded magnesium alloy AZ31 under cyclic testing conditions

Abstract A study of the internal strain (stress) evolution during uni-axial cyclic deformation along the prior extrusion axis with fully reversed total constant strain amplitudes εA (0.5% < εA < 5%) was investigated by using in-situ high energy synchrotron X-ray diffraction. The deformation is dominated by {1012} 〈1011〉 twinning and detwinning mechanisms within the textured hot extruded magnesium alloy AZ31. The results show a strong load partitioning between the internal stresses of (1010) and (1120) parent grains and of the (0002) twinned daughter grains that are relaxed. Following the evolution of the twinning/detwinning behavior and the hkil dependent microstresses as a function of cycling numbers, two different regimes were observed whereby it was found that the transition between these regimes is marked by an applied strain amplitude of eA = 0.625%.

Microstructure and mechanical properties of differently extruded AZ31 magnesium alloy

Abstract Profiles of the magnesium alloy AZ31 were produced by conventional indirect extrusion and by indirect extrusion into counterpressure with variations of the billet temperature TB from 200 °C up to 350 °C. Microstructural examination by optical microscopy showed that for extrusion trials carried out with billet temperatures TB ≤ 250 °C fine dynamically recrystallized grains are found while elongated grains of the former cast structure remain. Above TB ≥ 300 °C most grains show higher dynamic recrystallization kinetics and additional static recrystallization and increase to grain sizes of d = 30 – 50 μm. At TB ≤ 250 °C the typical extrusion fiber texture is found, at TB ≥ 300 °C a double fiber texture is formed due to static recrystallization. The results of the quasistatic mechanical tests are due to these differences in the microstructure.

Analyses of deformation twinning in the extruded magnesium alloy AZ31 after compressive and cyclic loading

The influence of different loading conditions on the microstructural development of extruded magnesium alloy AZ31 was investigated by optical microscopy and electron backscattered diffraction. Extruded magnesium profiles exhibit a significant asymmetry in the mechanical properties, due to the low activation energy of the extension twinning system \( \left\{ {10\overline{1} 2} \right\}\langle {10\overline{1} 1}\rangle,\) when compressing along the extrusion direction. For the analyses of this twinning system, compression tests with different applied strains 0.4 ≤ e ≤ 11% were performed for two extrusion products exhibiting different microstructures. The main deformation mechanisms during cyclic loading are the formation of extension twins during compression and the detwinning during subsequent tensile loading. The strain-controlled fatigue tests were carried out with applied strain amplitudes 0.3 ≤ eA ≤ 5%. The tests were stopped at characteristic numbers of cycles N in the tensile or compression maximum of the hysteresis loop. The microstructural investigations deliver information about the type of twinning and the size, shape, local distribution, and volume fraction of twins as a function of the plastic deformation. These results will be discussed with regard to the microstructure of the initial state material and to the applied load.

Microstructural evolution during the annealing of an extruded AZ31 magnesium alloy

Microstructural evolution during the annealing of AZ31 extruded rod at 400 °C has been examined by employing neutron diffraction and electron backscatter diffraction (EBSD). In the as-extruded bar, equiaxed grains smaller than 5 μm and large elongated grains having significant degrees of internal misorientation are oriented mainly with < 1 0 1 ¯ 0 > parallel to the extrusion direction. Rapid grain growth occurs within the 180 s annealing period at 400 °C at the expense of the small grains with the internal orientation gradient as driving force. After short time annealing, small equiaxed grains are formed inside the large elongated grains, and grains having < 1 1 2 ¯ 0 > parallel to the extrusion direction show preferred growth. As a result, a transition of the main texture component to the < 1 1 2 ¯ 0 > component occurs after annealing for 1800 s at 400 °C.

Fatigue properties of the hot extruded magnesium alloy AZ31

The fatigue properties of differently extruded magnesium alloy AZ31 and hence different microstructures were investigated during total strain and stress-controlled fatigue tests with different applied strain and stress amplitudes, respectively. Extruded magnesium profiles exhibit a significant asymmetry in the quasi-static mechanical properties under tension and compression, due to the low activation energy for the { 1 0 1 ¯ 2 } 〈 1 0 1 ¯ 1 〉 extension twinning. The asymmetry of the tension–compression yield strength leads to a sigmoidal shape of the hysteresis loops under cyclic loading conditions which is reduced if the tension–compression asymmetry is reduced. Moreover the evolution of stress and strain amplitudes, the plastic strain amplitudes and the mean stress and strain were analyzed. The Manson–Coffin and the Basquin approaches were used to describe the lifetime of the differently extruded products.

Corrosion fatigue behavior of conversion coated AZ61 magnesium alloy

The corrosion fatigue behavior of conversion coated extruded AZ61 magnesium alloy has been investigated under two corrosive environments: (a) high humidity environment and (b) 5wt.% NaCl environment. It was found that under a low humidity environment without corrosive attack, the fatigue limit of coated material was almost the same as that of the bulk material under the same environment: the reduction rate of fatigue limit was about 3%. Under corrosive environments, i.e., under high humidity and 5wt.% NaCl environment, the application of conversion coating improved fatigue limit compared to that of bulk material under the same environment. However, when compared to the fatigue limit of bulk material under low humidity environment, the fatigue limit of coated specimen was reduced: the reduction rates were about 11% and 63% under high humidity environment and 5wt.% NaCl environment, respectively. Fracture surface observations of the coated specimens under high humidity environment showed no existence of corrosion pit in the crack nucleation region, which indicating that the conversion coating not perfectly but significantly protects the substrate material from the attack of high humidity environment, whereas under 5wt.% NaCl environment corrosion pits were observed at the crack nucleation region, which indicates that the conversion coating could not protect the substrate material from the attack of 5wt.% NaCl environment. Therefore, it is suggested that the conversion coating can work to a certain degree of protection from corrosive attack of high humid environment but not enough for perfect protection from the attack of 5wt.% NaCl environment.

Development of Twin Fractions and Microstresses of the Hot Extruded Magnesium Alloy AZ31 under Cyclic Loading Conditions

For investigating the twinning and detwinning behaviour under cyclic loading conditions in-situ stress measurements with different applied strain amplitudes were performed at the beamline 7T-MPW at BESSY II. Intensity measurements of the (10-10), (11-20) and (0002) reflections served for the determination of the twinned volume fraction. The measurements of the (hkil) dependent strains and stresses gave information on the load partitioning between different grain orientations. The intensity measurements performed in the compression regime showed that the evolution of the (0002) reflection intensity depends on the strain amplitude. For strain amplitudes ≥ 0.75 % an increase in intensity is observed whereas for smaller strain amplitudes the (0002) intensity decreases with increasing cycle numbers. In the tensile regime an increase in the (0002) reflection intensity is found for the whole range of applied strain amplitudes.

Effect of Tool Materials on Dynamic Friction Characteristics and Microstructural Evolution at Elevated Temperature in Extruded AZ31 Magnesium Alloy

The effect of the coating materials of the tools on the dynamic friction characteristics and microstructural evolution with straining was investigated by the ring-typed compressive test at a temperature of 473 K and at a strain rate of 10 � 2 s � 1 in the extruded AZ31 magnesium alloy with an average grain size of 15 mm. The values of the dynamically recrystallized grain size in the samples compressed up to about 45% by using the WC-Co tools with and without the DLC coating were 3.8 mm and 4.8 mm, respectively. The dominant deformation mechanism under all the testing condition for the present alloy was the climb-controlled dislocation creep. The dynamic friction coefficient, m value, for the sample compressed by the WC-Co tool was higher than that done by the DLC tool, even if though there was identical tendency in stress level. The integration degree of the grains within 10 degree from h0001i direction to compressive axis in the sample compressed by the WC-Co tool was larger than that done by the DLC tool. It was concluded that the higher m value could enhance an alignment of the planes perpendicular to the compressive direction to the basal plane even if under same testing condition. [doi:10.2320/matertrans.MBW200914]

320 Influence of Mechanical Surface Treatment on Fatigue Behaviour of extruded AZ61 Magnesium alloy

In the present study, fatigue tests were carried out to understand the influence of mechanical surface treatment on fatigue behavior of extruded AZ61 magnesium alloy at a stress ratio of -1 and frequency of 20 Hz under ambient environment. The mechanical surface treatments used in the present study were: (a) Shot-peening, (b) Micro-peening and (c) Barrel processing. From the results it was found that fatigue strength of shot-peened specimen degraded by about 30% compared to the bulk specimen. Cracks were nucleated from dents on the surface induced by shot-peening, which then propagated until final failure. On the contrary, both micro-peened and barrel processed specimens showed the same level of improvement in fatigue strength, which was about 15% higher than the bulk specimen. Though cracks were nucleated from the specimen surface with relatively small defects, the compressive residual stress induced near surface retarded the crack propagation, which was dominant for the improved fatigue strength.

圧縮ねじり加工したAZ61マグネシウム合金の微視組織に及ぼす回転回数の影響

Grain refinement and texture control in magnesium alloys are an effective way of improving mechanical properties. We have investigated the possibility of microstructure control of metal materials by compressive torsion processing. This process can apply a large strain to a cylindrical work piece by simultaneous compressive and torsional loadings without changing the shape of the work piece. It is expected that not only grain refinement but also texture control can be achieved. In the present work, we applied this process to the microstructure improvement of extruded AZ61 magnesium alloy. The effect of rotation number on microstructure control was investigated. The microstructure of each specimen after processing was analyzed quantitatively and the distribution of grain size was determined. In addition, texture was measured by X-ray diffraction analysis. Grains of several micrometers were observed in the specimens under all conditions. The grains in the specimen processed with a large rotation number were refined homogeneously over the cylindrical work piece. Most basal planes, which were initially oriented parallel to the cylindrical axis, were distributed nearly perpendicular to the axis, the compressive direction, after processing.

J0301-1-3 AZ31マグネシウム合金押出し材の比例ひずみ経路二軸圧縮における成形限界([J0301-1]工業材料の変形特性とそのモデル化(1))

J0301-1-3 AZ31マグネシウム合金押出し材の比例ひずみ経路二軸圧縮における成形限界([J0301-1]工業材料の変形特性とそのモデル化(1))

Investigation on Dynamic Friction Properties of Extruded AZ31 Magnesium Alloy Using by Ring Upsetting Method

The dynamic friction properties of the extruded AZ31 magnesium alloy of the grain size of 20 mm were investigated by ring upsetting method test at 523, 548 and 573 K at strain rate of 1:0 � 10 � 2 s � 1 , where all the initial testing conditions were the climb-controlled dislocation creep. The MoS2 lubricant maintained lower dynamically friction coefficient (m value) than the oil lubricant. The difference in m values between machined surface and polished surface was unclear. The m values for WC-Co and diamond like carbon (DLC) tools were similar in MoS2 lubricant. The m values for DLC tool were lower than those for the WC-Co tool in the oil lubricant. The extruded direction influenced to the friction properties. The aspect ratio of the inner diameter on 90 � to extruded direction after testing was almost isotropic; on the other hand, the anisotropy occurred on 0� and 45� . The extent of anisotropy at 548 K was the highest, although the lower temperature, the higher the critical shear stress of non-basal plane. The condition at 523 K, where the fine grain sizes less than 3 mm could be obtained by dynamic recrystallization during deformation, is suitable temperature to make superplasticity at the given strain rate. [doi:10.2320/matertrans.P-M2010811]

Texture evolution of extruded AZ31 magnesium alloy sheets

The evolution of texture during the annealing and hot rolling process of extruded AZ31 magnesium alloy sheets was studied. There are two kinds of texture components in the extruded AZ31 sheets. One is {0002}<10 ▪0> and the other is {10 ▪0}<11 ▪0>. The {0002}<10 ▪0>component predominates. After annealing at 723 K for 3 h, both {0002}<10 ▪0> and {10 ▪0}<11 ▪0> components are strengthened moderately. This indicates that grains with both two components mentioned above grow faster than those with other orientations. The {10 ▪0}<11 ▪0> component disappears and the intensity of {0002}<10 ▪0> component decreases significantly after hot rolling with a 30% reduction at 623 K. This is mainly attributed to rotational dynamic recrystallization (RDX) during the hot rolling.