Grain refinement of AZ91 alloy by introducing ultrasonic vibration during solidification

Ultrasonic vibration was introduced into the solidification of AZ91 alloy. Various microstructures were produced in this alloy using ultrasonic vibrations at different temperatures of the melt. The coarse dendrite microstructures were obtained with ultrasonic vibrations at temperatures below the liquidus temperature. The fine uniform grains were achieved under ultrasonic vibrations during the nucleation stage, which was mainly attributed to the cavitation and the acoustic flow induced by the ultrasonic vibration.     

Grain refinement of DC cast AZ91D Mg alloy by intensive melt shearing

Abstract

Melt conditioning by advanced shear technology (MCAST) is a new process for microstructural refinement of both cast and wrought magnesium alloys. Melt conditioned direct chill (MCDC) casting combines the MCAST process with conventional direct chill (DC) casting. In the present work, melt conditioning has been combined with permanent mould casting to simulate the production of DC cast AZ91D billets and slabs. The results show that the MCDC process can achieve significantly finer grain size and more uniform microstructure than conventional DC process for both billets and slabs. Grain refinement in the MCDC process is due to the fine and well dispersed oxide particles produced after processing in the MCAST unit.     

Effects of Ca addition on the microstructure and mechanical properties of AZ91magnesium alloy

The effects of Ca addition on the microstructure and mechanical properties of AZ91 magnesium alloy have been studied. The results show that the Ca addition can refine the microstructure, reduce the quantity of Mg₁₇Al₁₂ phase, and form new Al₂Ca phase in AZ91 magnesium alloy. With the Ca addition, the tensile strength and elongation of AZ91magnesium alloy at ambient temperature are reduced, whereas Ca addition confers elevated temperature strengthening on AZ91 magnesium alloy. The tensile strength at 150°C increases with increasing Ca content. The impact toughness of AZ91magnesium alloy increases, and then declines as the Ca content increases. The tensile and impact fractographs exhibit intergranular fracture features, Ca addition changes the pattern and quantity of tearing ridge, with radial or parallel tearing ridge increasing, tensile strength, elongation and impact toughness reduce.     

Microstructure and mechanical properties of spray-formed AZ91 magnesium alloy

The AZ91 magnesium alloy, preformed with complete shape, has been prepared using spray forming technology under a protective atmosphere. The microstructure and mechanical properties have been investigated. Initially, a homogeneous and equiaxed-grain structure with average grain size of 20 μm was obtained. The tendency for segregation of the divorced eutectic β(Mg₁₇Al₁₂) phase towards the grain boundary was greatly reduced. Further grain refinement was attributed to dynamic recrystallization during extrusion processing. When solution treated at 415 °C and aged at 175 °C, two kinds of β(Mg₁₇Al₁₂) precipitates are formed: the majority are lamellar discontinuous precipitates, in addition to a small amount of dispersed continuous precipitates. The average tensile ultimate and yield strength of the spray-formed and extruded AZ91 magnesium alloy samples were 435 MPa and 360 MPa with a room temperature elongation of 9.2%, indicating an enhanced combination of toughness and strength.     

Microstructure and mechanical properties of rheo-diecast AZ91D magnesium alloy

A new semisolid metal processing technology, rheo-diecasting (RDC) has been developed for production of Mg-alloy components with high integrity. The RDC process innovatively combines the dispersive mixing power of the twin-screw mechanism for creation of high quality semisolid slurry and the high efficiency, low cost nature of the high pressure die casting (HPDC) process for component shaping. AZ91D Mg-alloy was used to optimise the RDC process and to establish its advantages over both the HPDC process and other existing semisolid processing techniques. In this paper we present the RDC process for processing Mg-alloys and the resulting microstructure and mechanical properties of RDC AZ91D alloy. The solidification behaviour of the Mg-alloys in the RDC process and the co-relationships between microstructure and mechanical properties of the RDC AZ91D alloy are discussed. It was found that the RDC process is capable of producing Mg-alloy samples with close-to-zero porosity and a fine, uniform microstructure throughout the entire sample irrespective of the section thickness. Compared with those obtained by other existing processing techniques, the RDC samples have substantially improved or equivalent mechanical properties, with the tensile elongation showing more than 100% improvement.     

Grain refinement of AZ91D by spinning spray of carbon dioxide gas and its effect on mechanical property

Abstract

Grain refinement of AZ91D Mg alloy by a spinning spray of Ar+CO₂ mixture gas was investigated. The results have shown that the grain size was refined significantly and effectively by this processing after 15–30 min treatment, but further extension of the treatment did not refine the grain markedly. It was shown that this processing has an 'aging behaviour', which means that the refining efficiency declines with the extension of holding time at melting temperature. Although the cooling rate affected the final grain size of AZ91D sample refined by the processing, it did not change the aging behaviour. The effective time is 45–60 min. It was analysed that Al₄C₃ could act as nucleation site of the primary Mg, which contributes to the grain refinement. Mechanical property testing demonstrated that refinement by the Ar+CO₂ processing significantly refined the grain size of the as cast AZ91D samples produced by sand and permanent cast and in turn enhanced the strength and improved the elongation.     

Damping properties of porous AZ91 magnesium alloy reinforced with copper particles

Abstract

The well distributed open porous AZ91 magnesium alloy reinforced with copper particles was successfully prepared through powder metallurgy route based on space holder method. Its damping properties were characterised by internal friction and the internal friction measurement in a temperature range from room temperature to 100°C was performed by a multifunction internal friction apparatus. Experimental results revealed that the damping capacity of the porous AZ91 magnesium alloy was increased as a result of the addition of copper particle and the increase was further enhanced with increasing volume fraction of copper particle. Attempt is performed to correlate the increase in damping with the microstructural change arising due to the addition of copper particles. It was suggested that the increase should be related to the appearance of plastic zone and thus an increase in dislocation density at the matrix particles interfaces.     

Improvement in rollability of AZ91 magnesium alloy by carbon addition

The present study aims to investigate the effect of carbon addition on the hot rolling behavior of as-cast AZ91 alloy. The AZ91 and C-added AZ91 alloys were subjected to hot rolling at 400 °C with a reduction of 30% per one pass. The as-cast C-added AZ91 alloy with very fine equi-axed grains of approximately 75 μm exhibited excellent hot rollability compared to as-cast AZ91 alloy with coarse dendrite structure, although the final grain size of the rolled C-added AZ91 alloy sheet was slightly larger than that of the rolled AZ91 alloy sheet. The side-crack occurrence on the surface during hot-rolling is mainly affected by the existence of twin boundary and the area fraction of grain boundaries. Based on the results, the improvement in rollability of the C-added AZ91 alloy is attributed to fine equi-axed grains and the polygonal Al₈Mn₅ phase located inside grains, which can homogeneously distribute and effectively absorb strain energy and prohibit crack growth.     

Mechanical and fracture properties of an AZ91 Magnesium alloy reinforced by Si and SiC particles

A commercial AZ91 magnesium alloy (nominal composition Mg–9%Al; 1%Zn; 0.3%Mn, balance Mg in weight percent) reinforced with SiC particles and modified by the addition of Si has been used in this study. Formation of an “in situ” composite (Mg–Mg₂Si) results in strong bonding between Mg₂Si and the matrix interface. Samples were deformed in compression in the temperature interval from room temperature up to 300 °C. Stress relaxation tests were performed with the aim to reveal the thermally activated processes. Reinforcing effect of SiC and Mg₂Si particles decreases with increasing temperature. The estimated values of the activation volume as well as the activation enthalpy indicate that the main thermally activated process is connected with a rapid decrease of the internal stress. Fracture properties were studied in impact tests at various temperatures. A ductility enhancement was found at 200 °C and temperatures above 200 °C.     

Microstructure evolution and mechanical properties of a submerged friction-stir-processed AZ91 magnesium alloy

Journal of Materials Science - AZ91 casting alloy is subjected to friction stir processing (FSP) in air (NFSP) and under water (SFSP). The thermal histories of the two FSP procedures are measured,...     

Grain refinement of AM60B magnesium alloy by SiC particles

AM60B alloy has been refined by SiC particles and the corresponding refining mechanism has been mainly discussed. The results indicate that the addition of 0.2 wt% SiC particles in form of mixture with Mg powder decreases the grain size from 317 μm of the not refined alloy to 46 μm. The decrease of β phase and formation of Mg₂Si and Al₄C₃ phases well demonstrate that the reactions of 3SiC + 4Al = Al₄C₃ + 3Si and 2Si + Mg = Mg₂Si occur during refining treatment. In addition, the crystal nuclei are composed of two kinds of elements, Al and C. All of these imply that the formed Al₄C₃ particles are the actual heterogeneous nucleation substrates.     

Grain refinement and mechanical behavior of a magnesium alloy processed by ECAP

Equal-channel angular pressing (ECAP) is an effective tool for refining the grain structure of magnesium alloys and improving the ductility at moderate temperatures. However, grain refinement in these alloys differs from other metals because new grains are formed along the boundaries of the initial structure and these newly formed grains slowly spread to consume the interiors of the larger grains in subsequent passes. A model is presented for grain refinement in magnesium alloys processed by ECAP based on the principles of dynamic recrystallization where new fine grains are formed along the initial boundaries and along twin boundaries. This model provides an explanation for a wide range of experimental data and introduces the concept of grain size engineering for achieving selected material properties in magnesium alloys.     

Microstructures and mechanical properties of AZ91 alloy with combined additions of Ca and Si

Ca and Si additions to AZ91 alloy have been investigated and the results show that addition of Ca or Ca combined with Si resulted in the refinement of the as-cast microstructure, increase the thermal stability of β phase and the inhibition of discontinuous precipitations. Small amounts of Ca added to the AZ91 alloy mainly dissolved into β phase and raised the thermal stability of the phase, thus strengthening the alloy at elevated temperatures. Combined additions of Si with Ca to the AZ91 alloy were more effective on increasing the tensile strength at both ambient and elevated temperatures. The creep resistance of the alloy was also improved significantly in the alloy with Ca and Si additions. The creep rate of the alloy containing 0.3% Ca and 0.6% Si, tested at 473 °K and 50 MPa, was one order of magnitude lower than that of the base alloy (without Ca and Si addition). The mechanism of mechanical properties improvement caused by Ca and Si was also discussed.     

Grain refinement of AZ31 magnesium alloy by electromagnetic stirring under effect of grain-refiner

The effects of electromagnetic stirring and Al₄C₃ grain refiner on the grain refinement of semi-continuously cast AZ31 magnesium alloy were discussed in this investigation. The results indicate that electromagnetic stirring has an effective refining effect on the grain size of AZ31 magnesium alloy under the effect of Al₄C₃ grain refiner. Electromagnetic stirring can ‘activate’ the Al₄C₃ particles, resulting in more heterogeneous nucleation sites for the primary α-Mg grains. But, longer holding time can ‘deactivate’ the Al₄C₃ particles and poison the grain refining effect.     

添加剂对镁合金微弧氧化膜性能的影响

在偏铝酸盐一六偏磷酸盐体系中对镁合金进行微弧氧化处理,研究了金属盐缓蚀剂钨酸盐、多元醇或酸等添加剂对氧化膜性能的影响.通过正交实验优化和对微弧氧化膜结构,成分及性能的测试评价,得到了性能较好的微弧氧化电解液配方.SEM检测发现,复合添加剂(NaaEDTAlg/L,CH3(CH2)11SO3Na0.5g/L)通过抑制破坏性的微弧,能促进成膜、降低起弧电压,得到结构更加均匀和完整的陶瓷涂层;XRD检测表明,膜层的主要成分是MgO、Mg3(PO4)2,MgAl2O4和AlPO4等化合物;中性盐水腐蚀测试表明,陶瓷膜在48 h内具有较平缓的腐蚀速度;EIS和动电位极化曲线表明,在电解液中加入复合添加剂使镁合金试样微弧氧化膜的耐腐蚀性能得到了很大的提高.     

Improvement of microstructure and mechanical properties of AZ91/SiC composite by mechanical alloying

AZ91 magnesium alloy reinforced with SiC particulates was fabricated via powder metallurgy technique as well as mechanical alloying process where a planetary ball mill was employed. Microstructure and mechanical properties of the fabricated AZ91 composites had been evaluated. Microstructural study showed that grain size of the material was refined and SiC particulates were well distributed after mechanical alloying. Mechanical tests of the composite showed an enhanced yield and ultimate tensile strengths for the mechanically alloyed samples compared with those prepared via the powder metallurgical route.     

Grain refinement in as-cast AZ80 Mg alloy under large strain deformation

Grain refinement in as-cast AZ80 magnesium alloy under large strain deformation was studied by hot multiple forging (MF). The results show that during the deformation there exists a critical strain controlling the degree of the homogeneity, which is in the range of 2–2.4. A homogeneous microstructure with fine dynamic recrystallized grains can be attained when the applied strain exceeds the critical strain and after that, it is difficult to get more grain refinement further. A main characteristic of microstructure evolution is directly associated with grain splitting due to the formation of microbands that develop in various directions. Such microbands intersect each other during hot MF, resulting in continuous subdivision of coarse grains into misoriented fine domains. Further deformation leads to increase in the number and misorientation of these boundaries and finally almost full development of fine equiaxed grains in high strain. New grains are concluded to be evolved by a kind of continuous reaction, that is essentially similar to continuous dynamic recrystallization.     

Microstructure investigation and first-principle analysis of die-cast AZ91 alloy with calcium addition

In order to get improved mechanical properties of die-cast AZ91 alloy under elevated temperatures, Ca element was added as a cost-effective alloying constituent. It appeared that minor Ca addition less than 0.5 wt% would result in no apparent change in microstructure, but the tensile strength at elevated temperatures was improved considerably. When increasing Ca addition to more than 1.0 wt%, Al₂Ca phase will precipitate during solidification, no Mg₂Ca phase was discovered. Homogeneous microstructure and high temperature stability in tensile strength of die-cast AZ91 alloy with Ca addition was mainly attributed to the precipitation of Al₂Ca phase, which considerably refined the bulky β-Mg₁₇Al₁₂ phase distributed originally at the grain boundaries of die-cast AZ91 alloy with no Ca addition. The priority of Al₂Ca phase compared to Mg₂Ca phase in precipitation sequence was verified by first-principle calculation of their cohesive energy and formation enthalpy, and can also be associated with more bounding electrons between Al and Ca atoms.     

Microstructure and mechanical behaviour of semi-solid die-casting AZ91D magnesium alloy

Microstructures and mechanical properties of an AZ91D magnesium alloy prepared with semi-solid die-casting (SSDC) were characterized in as-cast conditions. The SSDC alloy exhibits a unique microstructure featuring primary α-Mg globules uniformly distributed in the matrix of fine secondary α-Mg grains and β-Mg₁₇Al₁₂ intermetallic. High ultimate tensile strength and elongation have been achieved before fracture. Observations on the vertical-section microstructure of the fractured sample by scanning electron microscopy (SEM) show that the crack mainly originated from the brittle fracture of the eutectic phase causes the interface decohesion of the ductile Mg phase, making fracture a rather critical event. Before that, the deformation of ductile α-Mg phase in the matrix as well as the “pulling out” of primary α-Mg phase combines to provide the SSDC alloy a certain strain.     

Microstructures and mechanical properties in dissimilar AZ91/AZ31 spot welds

The stir zone microstructures and mechanical properties of dissimilar AZ91/AZ31 friction stir spot welds made using different tool designs and tool rotational speed settings are investigated. Intermingled AZ91 and AZ31 lamellae are formed in the stir zones of dissimilar spot welds made using threaded, three-flat/0.7 mm/threaded and three-flat/no-thread tools and tool rotational speeds ranging from 1500 to 3000 rpm. The intermingled lamellae have chemical compositions, which are similar to those of the upper and lower sheets in the dissimilar sandwich. The flats on the rotating tool facilitate the downward transfer of upper and lower sheet materials in the location close to the pin periphery and therefore intermingled AZ91 and AZ31 lamellae are formed in the stir zones of dissimilar spot welds produced using a three-flat tool without a thread.The distance (Y) from the tip of the hook region to the keyhole periphery has a dominant influence on the mechanical properties of dissimilar AZ91/AZ31 spot welds, since the hook regions are curved inwards towards the axis of the rotating tool. The highest failure load properties and largest Y-values are found in dissimilar spot welds made using threaded and three-flat/0.7 mm/threaded tools and tool rotational speeds from 1500 to 3000 rpm. Dissimilar spot welds made using a rotational speed of 1000 rpm have the smallest Y-values and the lowest failure load properties.