Mechanical Properties Of AZ91 Alloy Research Articles

Microstructures and the improved mechanical properties of AZ91 alloy by incorporating Ti-6Al-4V particles

Magnesium (Mg) based composites are often difficult to achieve the trade-off between strength and ductility. The titanium (Ti) metal particles, which show better wettability and ductility than ceramic particles, are selected as reinforcements to simultaneously enhance the strength and plasticity of Mg alloys. In this work, the TC4 (Ti-6Al-4V) particles (2, 4 and 6 vol %) reinforced AZ91 composites were successfully fabricated by the mechanical and ultrasound-assisted stirring method. By incorporating 4 vol% TC4 particles, the composite reached the optimum yield stress, ultimate tensile stress, and elongation of 257 MPa, 370 MPa and 18.7%, respectively. The microstructural characterizations revealed that the fine and coarse grains were observed in the AZ91 and TC4/AZ91 composites. The dynamic recrystallization behavior was retarded in the TC4/AZ91 composites, which was rooted in the TC4 particles and fine Mg17Al12 phase. Transmission electron microscopy (TEM) showed that the interfacial product at TC4/Mg interface was Al3Ti phase. Based on the quantitative calculations, the strengthening effects of the 4TC4/AZ91 composite were mainly the coefficient of thermal expansion strengthening, grain refinement strengthening and the deformable TC4 particles.

Investigations on Microstructure and Mechanical Properties of AZ91 Alloy Processed by Single Pass Rolling with Varied Rolling Reductions

In this study, single pass rolling (SPR) with varied rolling reductions was conducted on pre-homogenized AZ91 alloys (H alloys) and ECAP-processed AZ91 alloys (HE alloys). The effects of rolling reduction on the microstructures and mechanical properties of HR alloys (pre-homogenized and rolled) and HER alloys (ECAP-processed and rolled) were investigated. The results showed that the HER alloys possessed much finer microstructures and superior mechanical properties than the HR alloys, which were significantly influenced by the rolling reduction. The microstructures of the HER alloys became bimodal, and the strength and elongation markedly improved with the increase in the rolling reduction. When the rolling reduction reached 70%, the HER alloys acquired a typical bimodal structure, contributing to their excellent comprehensive mechanical properties.

Enhanced grain refinement and mechanical properties of AZ91 alloy by a novel Al-5.1 V-2.3B refiner containing VB2 and VB particles

Addition of specialized master alloys containing effective nucleation substrates has become the most common practice in the casting industries. In this study, a novel Al-5.1V-2.3B grain refiner containing VB2 and VB particles was designed. The growth mechanisms of VB2 and VB particles in grain refiner followed the principle of atomic adsorption and growing layer by layer based on the fundamental crystal growth theories. Grain refinement and mechanical properties of AZ91 alloy induced by the grain refiner and subsequent extrusion were investigated. The average grain size of the as-cast AZ91 alloy decreased from 217 μm to 78 μm with the optimal addition of 1 wt% Al-5.1V-2.3B grain refiner, and the UTS, YS and EL of the as-cast AZ91 alloy were improved by 27.6 %, 42.0 % and 43.0 %, respectively. Mechanical properties of the hot extruded samples were further enhanced due to the grain refinement, weakened texture and dislocation strengthening. The matching relationships between VB2, VB particles and α-Mg were characterized with the assistance of the high-resolution transmission electron microscope and edge-to-edge models. There were good matching relationships between VB, VB2 particles and α-Mg, as [001] / [1 2¯1 0] in (110) / (10 1¯0) and [0001] / [1 2¯1 0] in (10 1¯0) / (0002), which were the key for grain refinement.

Study of Influence of Aluminum Nitride Nanoparticles on the Structure, Phase Composition and Mechanical Properties of AZ91 Alloy

In this work, magnesium-based composites were obtained by shock-wave compaction of a powder mixture of Mg-5 wt.% AlN at a shock-wave pressure of 2 GPa. Their microstructure was investigated and the phase composition was determined, from which it follows that the nanoparticles retain their phase composition and are uniformly distributed in the magnesium matrix. The materials obtained by shock-wave compaction were used as master alloys for the production of magnesium alloys by die casting. The amount of aluminum nitride nanoparticles in the AZ91 magnesium alloy was 0.5 wt.%. Studies of the microstructure of the magnesium alloys showed a decrease in the average grain size of the magnesium matrix from 610 to 420 μm. Studies of mechanical properties have shown that the introduction of aluminum nitride nanoparticles increases the yield strength from 55 to 119 MPa, the tensile strength from 122 to 171 MPa and the plasticity from 4 to 6.5%, respectively. The effect of nanoparticles on the fracture behavior of the magnesium alloy under tension was determined.

Enhanced Grain Refinement and Mechanical Properties of Az91 Alloy by a Novel Al-5.1v-2.3b Refiner Containing Vb2 and Vb Particles

Enhanced Grain Refinement and Mechanical Properties of Az91 Alloy by a Novel Al-5.1v-2.3b Refiner Containing Vb2 and Vb Particles

Effect of traverse and rotational speeds on microstructure, texture, and mechanical properties of friction stir processed AZ91 alloy

In the current work, the influences of both traverse and rotational speeds of friction stir processing (FSP) on the microstructural evolution, crystallographic texture, and mechanical properties of AZ91 alloy were investigated. The tool traverse and rotational speeds used in the present study were 20, 40, 60 mm/min, and 400, 800, 1200 rpm, respectively. The average grain size significantly decreased from 61.6 μm in the as-cast condition to below 10 μm in the FSP-processed samples owing to the occurrence of dynamic recrystallization (DRX). By increasing the tool traverse speed and decreasing the rotational speed, the grain size reduced. The grain boundaries pinned by fine β-Mg17Al12 particles, retarding the grain growth. The fraction of the β-Mg17Al12 remarkably decreased after the FSP owing to fragmentation and dissolution. The high strain rate of the 800–40 and 1200–40 samples coupled with their higher peak temperatures increased the dissolution of β-Mg17Al12 during FSP. It was found that a texture transition from (10.1) pyramidal to (00.2) basal occurred by decreasing the traverse speed and increasing the rotational speed. The highest microhardness, which was equal to 104.4 HV, belongs to the 800–20 sample due to its very fine grains. When the rotational speed increased from 400 to 1200 rpm, the ultimate tensile strength (UTS) improved from 236.8 to 267.1 MPa, the total elongation (TE) decreased from 7.5% to 6.6%, and the energy absorption (EA) improved from 13.1 to 14.2 J/cm3. These results were due to the higher accumulated strain, the larger solid solution strengthening, the lower aspect ratio of β-Mg17Al12, stronger {0002} texture, and the lower content of β-Mg17Al12 of the 1200–400 sample. The UTS and TE of the 1200–40 sample showed an enhancement of 96.4% in UTS, 26.9% in TE, and 132.8% in EA as compared to the as-cast AZ91. Up to the true strain of 0.05, with increasing the rotational speed, the strain hardening rate increased owing to decreasing the fraction of grain boundaries. After the true strain of 0.05, the strain hardening rate reduced by increasing the rotational speed due to strengthening the {0002} texture. The fractured surface of the 800–20 sample had the largest number and smallest size of dimples as compared to the other FSP-processed samples owing to its lowest grain size. Finally, the strong {0002} basal texture in the 800–40, 1200–40, and 800–20 samples led to tear ridges aligning with the normal direction of fractured surfaces.

Achieving single-pass high-reduction rolling and enhanced mechanical properties of AZ91 alloy by RD-ECAP pre-processing

It is widely recognized that rolling Mg alloys is difficult due to their poor ductility. Conventional rolling of Mg alloys often needs to be conducted at high temperatures with multi-pass small thickness reduction, and the strong basal texture often leads to poor ductility and plate cracking. Herein, we conducted a single-pass high-reduction rolling (SPHRR) on AZ91 alloy after pre-homogenization (PH) and RD-ECAP pre-processing (PH + ECAP). Results shows that the AZ91 rolled plate processed by SPHRR (PH + ECAP + R) exhibits a good combination of high tensile strength (ultimate strength of ~420 MPa, yield strength of ~335 MPa) and high ductility (elongation-to-failure of ~19%), together with significantly reduced edge cracks. Microstructure examinations revealed that the improved mechanical property is mainly attributed to a typical bimodal grain structure featured with relatively high Schmidt factors of basal slips (SF basal ). The fine-grain structure produced by the high-pass RD-ECAP pre-processing contributes to the improvement of formability, provides favourable conditions for dynamic recrystallization (DRX) in further rolling and also promotes the formation of the bimodal structure. Overall, the high-pass RD-ECAP is applicable to improve plastic formability of hard-to-deform materials like Mg alloys, and thus has a promising prospect for industrial applications.

The Effect of Rapid Deformation Process to Improve Creep and Tensile Resistance of AZ91 Magnesium Alloy Plates

Demand for increasing strength to weight ratio, elimination of electromagnetic waves, and vibration damping has led to the wide application of magnesium-base alloys such as AZ91 in various industries like aerospace, military, vehicle, and shipbuilding. However, because of the unstable secondary particles and casting defects located on the rough grain boundaries and in the dendritic regions, due to sliding of the grain boundary, the creep resistance and tensile strength of Mg alloys at high temperatures reduce. To improve the high-temperature properties, rapid deformation processes such as friction stir processing can be employed. In this study, the influence of multi-pass friction stir processing on microhardness, tensile, and creep behavior of AZ91 at several temperatures from 25 to 210 °C has been studied. Optical microscopy and scanning electron micrograph were used to study the microstructure of the cast and processed samples and Clemex commercial software was used for grain size measurement. The experimental results indicated that at room temperature, the microhardness, tensile, and creep strength of the processed samples as compared to the unprocessed ones increased by 23, 29 and 38%, respectively. In addition, after multi-pass friction stir processing, the tensile and creep strength of the samples at 210 °C increased by 31 and 47%, respectively. Also, the average grain size of the multi-passed friction stir processed AZ91 alloy decreased by 88%. The maximum ultimate tensile strength of 276 MPa was obtained at the tool rotational speed of 1200 r/min, the traverse speed of 60 mm/min, and the tool tilt angle of 3°. The empirical results indicated that this rapid deformation process can be useful in enhancing the mechanical properties of AZ91 alloy at high temperatures.

Influence of Electrostatic Field on the Microstructure and Mechanical Properties of AZ91 Mg Alloy during Ageing Treatment

The effects of electrostatic field on the microstructure and mechanical properties of AZ91 alloy during ageing treatment are studied. It is shown that electrostatic field can modify the precipitate distributions. With the help of electrostatic field (intensity 4 kV cm−1), the continuous precipitate (CP) is refined, whereas the discontinuous precipitate (DP) is suppressed to some extent. The tensile yield strength of peak‐aged AZ91 alloy is improved from 156 to 169 MPa. The mechanism by which the electrostatic field modifies the precipitate distributions is discussed via the resistivity data combined with the classical theory of vacancy‐dominant diffusion. It is shown that the electrostatic field probably enhances the bulk diffusion and then improves the competitiveness of CP against DP.

Effect of vibration on machining and mechanical properties of AZ91 alloy during FSP: modeling and experiments

The objective of the research is to determine the effect of friction stir processing (FSP) parameters and vibration on the machining behavior of AZ91 alloy. The article displays the state of the art of AZ91 alloy machinability including chip formation, chip morphology, and cutting forces. In this method, the vibration was applied to the process line through the fixture under the workpiece during the FSP. The three-dimensional finite element (FE) modeling was applied for simulating the small-hole drilling process of AZ91 alloy. The large deformation of work and the chip formation in drilling process is realized by incorporating Johnson-Cook material constitutive model and material failure criterion. It was concluded that the microstructure from 150 (μm) for base metal was modified to 28 (μm) and 16 (μm) for FSP and FSVP samples, respectively. It has been indicated that mechanical properties increase as vibration applied during the FSP. The vibration improves adiabatic shearing and thus leads to more discontinuous chips. To enhance the mechanical and machining properties of processed zone the friction stir vibration process as an “easy to apply” FSP method is recommended for different industries.

Effect of Extrusion Temperature on Mechanical Properties of AZ91 Alloy in Terms of Microstructure and Texture Development

This study investigates the effect of extrusion temperature on room temperature tensile properties of as-extruded AZ91 plates regarding microstructure and texture development. Forward extrusion was performed at 300, 350 and 400 °C with an extrusion ratio of 25:1. Yield strength rather increases from 212 to 224 MPa as extrusion temperature rises, despite grain growth from 11.9 to 41.2 µm. To understand the underlying mechanisms of this unusual hardening, the effects of microstructural features are discussed with emphasis on grain size, precipitates, stored strain energy and texture. Microscopic analyses and simulation results reveal that grain refinement, continuous precipitates and stored strain have no decisive effect on yield strength. Development of stronger basal texture leads to an increase in yield strength. Texture strengthening makes basal slip more difficult to be activated when loading along the extrusion direction. Based on this, stronger basal texture plays a more dominant role in increasing yield strength.

Effect of multi-pass friction stir processing on thermal distribution and mechanical properties of AZ91

In this paper, the effect of multi-pass friction stir processing on mechanical properties of AZ91 alloy has been studied. For this purpose, the microhardness, tensile, and creep tests were conducted at several temperatures. Optical microscopy and scanning electron micrograph were used to study the microstructure of the processed samples. The experimental results indicated that at room temperature, the microhardness, tensile, and creep strength of the processed samples as compared to the unprocessed ones increased by 23%, 29%, and 38%, respectively. Also, after friction stir processing, the tensile and creep strength of the samples at 210 °C increased by 31% and 47%. In addition, a three-dimensional model was developed to simulate two-pass friction stir processing using ABAQUS/Explicit software. This model involved the Johnson-Cook models for defining material behavior during the process and identifying the fracture criterion. To control the mesh distortion during consecutive passes, the Arbitrary Lagrangian-Eulerian technique was used. Using the developed model, the peak temperature, thermal distribution, and residual stress field during multi-pass friction stir processing on AZ91 have been studied. The empirical results indicated the beneficial influence of the multi-pass friction stir processing on the microstructure and high-temperature mechanical properties of AZ91 alloy.

Feasibility of grain refinement method for AZ91 alloy using commercial Al-SiC composite

SiC is well-known as an effective carbon inoculation material for grain refinement of Mg-Al-based alloys. In this study, grain refinement ability of stir-casted Al-SiC composite for Mg-Al-based alloy was investigated to confirm the feasibility of industrial application. Commercialized Al-20 vol.-% SiC composite ingot was used in AZ91 alloy as a grain refiner, and grain size of AZ91 alloy dramatically reduce from 650 to 74 µm and become uniform with 0.7 wt-% Al-SiC composite addition. And acicular type Al-Mn phase existed at the grain boundaries are changed to polygonal type and positioned inside grains after grain refinement. These morphological changes of small grain size and uniform second phases contributed to the excellent mechanical properties of AZ91 alloy.

Microstructure Evolution and Mechanical Properties of the AZ91 Magnesium Alloy with Sr and Ti Additions in the As-Cast and As-Aged Conditions

Effect of different amounts of Sr and Ti additions with T6 heat treatment on microstructure and mechanical properties of the AZ91 alloy was investigated. The microstructure of the AZ91 alloy is consisted of the α-Mg phase surrounded by the semicontinuous network of β-Mg17Al12 intermetallics. The average grain size and volume fraction of β phase decreased up to 1 wt.% Sr and 0.1 wt.% Ti additions. Exceeding Ti additions led to increase in both grain size and volume fraction. It was found that the grain refinement mechanism of AZ91 alloys with Sr additions was grain restriction factor. Also, 0.1 wt.% Ti addition could refine the grain structure of the AZ91 alloy by the formation of Al3Ti particles that act as nucleation sites for the α-Mg phase. Furthermore, adding 1 wt.% Sr and 0.1 wt.% Ti enhanced the hardness values for both cast and aged AZ91 alloy. Moreover, ultimate tensile strength and tensile elongation values reached their maximum amounts with the 0.5 wt.% Sr and 0.1 wt.% Ti additions and more Sr and Ti additions deteriorated the mechanical properties of cast and aged AZ91 alloys. The fracture surface characterizations had good relations with the tensile properties. The fracture modes were cleavage and quasi-cleavage in the cast and aged AZ91 alloys. Furthermore, the optimum levels of Sr and Ti additions altered the fracture modes to more quasi-cleavage fractures.

Managing strength and ductility in AZ91 magnesium alloy through ECAP combined with prior and post aging treatment

In the present work, we firstly and comparatively investigated the microstructure evolution and mechanical properties of AZ91 alloy processed by equal channel angular pressing (ECAP) combined with prior and post aging treatment. Both processes induce homogenous microstructures consisting of fine grains modified by uniformly dispersed spherical precipitates. The prior aging treatment facilitates a well-developed dynamic recrystallized microstructure with low dislocation density, and finer and more precipitates, while both processes induce comparable grain size. Thus, the prior aging treatment benefits to the occurrence of dynamic recrystallization during subsequent ECAP processing. Furthermore, both the strength and ductility of AZ91 alloy are enhanced and flexibly managed through ECAP with a combination of prior or post aging treatment. The post aging treatment is of benefit to the improvement in yield strength, while the prior aging treatment facilitates the more significant ductility enhancement.

Effect of β–Mg17Al12 phase on microstructure, texture and mechanical properties of AZ91 alloy processed by asymmetric hot rolling

In this research, the microstructure, texture, and mechanical properties of AZ91 magnesium alloy during asymmetric hot rolling were investigated. Optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), hardness, and tensile tests were used. The results showed that the coarse and continuous β–Mg17Al12 phase strongly affected on twinning and basal texture. The β phase suppressed the nucleation of extension twins. There was a strong [0001]||RD texture with the intensity of 5.1⨭R in the as-cast AZ91 sample. With increasing the rolling reduction, the overall texture intensity decreased. This was due to the presence of both continuous β phase along the grain boundaries and initial [0001]||RD texture in the as-cast AZ91 alloy. In addition, there was a texture transition from [0001]||RD to [101¯0]||RD and [112¯0]||RD during 15% thickness reduction. With increasing the strain, the average grain size increased owing to the dissolution of the β phase in the matrix during the pre-heat treatment before rolling and also the occurrence of no recrystallization during hot rolling. It was found that the as-cast and 15% hot-rolled samples have the highest homogeneity and the 8% hot-rolled sample has the maximum inhomogeneity factor (20.87). The hardness and tensile tests indicated that with increasing the deformation, owing to the strengthening of {0002} basal texture and increasing the fraction of twins, the strength increased and the ductility decreased. Due to the weak bond between the β phase and matrix, the micro-crack started from the α/β interfaces and extended. The fracture mode of all samples was the mixture of brittle and ductile. However, with increasing the rolling reduction, the fraction of brittle fracture increased due to the accumulation of twins during deformation.

Microstructures and mechanical properties of AZ91 alloys prepared by multi-pass friction stir processing

Abstract

Effects of homogenization time on aging behavior and mechanical properties of AZ91 alloy

This study demonstrates that the holding time of homogenization treatment conducted before aging strongly affects the precipitation behavior of AZ91 alloy during aging and its mechanical properties after aging. When homogenization is conducted for a relatively short time of 8h, obvious Al solute segregation is still observed near the grain boundaries and in the interdendritic region because of insufficient diffusion of the solute atoms. This locally high Al concentration accelerates discontinuous precipitation at the grain boundaries and continuous precipitation in the interdendritic region in the early stage of aging treatment. However, the growth of formed discontinuous precipitates (DPs) is prematurely terminated on account of a decrease in the driving force for the growth of DPs owing to the presence of α-Mg dendrites with a low Al concentration and promotion of the formation of continuous precipitates (CPs). As a result, the microstructures of the AZ91 samples homogenized for 8h and 96h are obviously different after aging for 2h, but they become similar after peak aging for 24h. The hardness and tensile strength of the two samples with different homogenization times are nearly identical in the homogenized and peak-aged states. However, in the 2-h-aged state, the hardness and strength of the sample homogenized for 8h are higher than those of the sample homogenized for 96h, which is mainly attributed to the presence of a larger amount of hard DPs in the former sample. The tensile strength and ductility of the two samples under the homogenized, 2-h-aged, and peak-aged conditions and the causes of their differences under these conditions are discussed in detail.

High Strength of Mg-9%Al-1%Zn Alloys Achieved by Severe Working

To obtain the excellent mechanical properties of AZ91 magnesium alloy (Mg-8.9%Al-0.6%Zn-0.2%Mn), the microstructural characteristics of AZ91 alloys are modified by various forging and heating processes. High tensile properties (ultimate tensile strength σ UTS = 420 MPa and fracture strain e f = 3%) are obtained for the alloy made by the following process: solution treatment (ST) at 410 °C for 24 h plus water quenching, multidirectional forging (MDF) with 5% strain applied in 15 forgings at room temperature, and warm unidirectional forging (WUF) at a forging rate of 75% at 225 °C. The high tensile strength is a reflection of improved microstructural characteristics, namely a fine α-Mg phase and a high dislocation density. Moreover, brittle β-phase is significantly attributed to the mechanical properties of AZ91 alloy. Because of the severe deformation undergone by the alloy during the MDF process, the solution treatment is important to achieve high ductility with low internal strain, i.e., normalization. In fact, the e f value for the ST sample is as high as 10%, leading to severe work hardening during the tensile test, with deformation twins and slips. The WUF process is conducted immediately after the sample has been heated to 225 °C, for less than 5 min, to avoid material softening. A relatively high tensile strength (σ UTS = 305 MPa) is also achieved using the WUF process (with a forging rate of 75% at 200 °C) after the ST and aging process (200 °C for 12 h) although low ductility is found (e f = 0.7%), with hard and brittle β-phases being precipitated around the grain boundaries.

Improvement in extrudability and mechanical properties of AZ91 alloy through extrusion with artificial cooling

This study demonstrates that the application of artificial water cooling during extrusion effectively increases the extrudability of the AZ91 alloy and significantly improves the mechanical properties of the extruded AZ91 alloy. The artificial cooling dramatically reduces the actual temperature of the deformation zone, which results in an increase in the maximum exit speed at which the alloy is extrudable without the occurrence of hot cracking from 4.5m/min to 7.5m/min. It also promotes dynamic recrystallization and precipitation behaviors during extrusion, which leads to a reduction in grain size and an increase in the amount of fine Mg17Al12 precipitates. As a result, for the AZ91 alloy extruded at an exit speed of 1.5m/min, the tensile and compressive yield strengths improve significantly by 51MPa and 114MPa, respectively, and its tension–compression yield asymmetry reduces from 0.73 to 1.02 owing to the refinement of the grain size by artificial cooling. In addition, the AZ91 alloy extruded at an exit speed of 7.5m/min with artificial cooling exhibits a finer grain structure than and superior mechanical properties to the AZ91 alloy extruded at a slower exit speed of 4.5m/min without artificial cooling. This result indicates that the application of artificial cooling can simultaneously improve the maximum extrusion speed and the tensile and compressive properties of Mg alloys.