•Ultrasound enlarges the diving depth of argon bubble up to 3.7 cm in the water.•Argon can be broken into bubbles with minimal size of 0.1 cm by ultrasound.•Microporosities are much eliminated by ultrasonic argon degassing.•Ultrasound is the key to achieve efficient degassing and microstructure refinement.In this work, the role of ultrasound in hydrogen removal and microstructure refinement by the ultrasonic argon degassing has been fully investigated by the experimental work in water and AZ91-0.4Ca magnesium melt, respectively. Ultrasound is able to break up argon gas into numbers of small bubbles and drive them diving deeply to the bottom of water, which are responsible for the efficient degassing regime of ultrasonic argon process. The argon flowrate plays a dominant role in promoting hydrogen removal effect. Meanwhile, the increasing argon flowrate can suppress the microstructure refinement, due to the subdued ultrasonic cavitation under a large argon flowrate. Mechanical properties of AZ91-0.4Ca alloy can be much promoted by the ultrasonic argon degassing process. Ultrasound is the key to achieve not only efficient degassing regime, but also microstructure refinement as well as mechanical properties promotion.

Microstructural evolution, textural development and strengthening mechanism of α, α+β and β structure Mg-Li alloys during extruding and annealing process were investigated. It showed that, with increasing Li content, the α-Mg matrix was transformed gradually to the β-Li matrix. In the three alloys, for α-Mg structure, the c-axis of close-packed plane of α-Mg was inclined perpendicular to the extrusion direction after extruding. However, for β-Li structure, the c-axis of close-packed plane of β-Li was inclined parallel to the extrusion direction after extruding. During annealing, a new predominance of {11−20} recrystallization texture was observed in α-Mg matrix. However, no recrystallization texture was achieved in β-Li matrix. Additionally, a solid solution strengthening during annealing could be observed apparently in the β-Li matrix. To study and quantify the strengthening behavior, the relationship models between ultimate tensile strength (UTS), yield strength (YS), hardness (HV) and AlLi volume fraction (VF) were defined using mathematical modeling and regression analysis. It showed that the effect of VF on UTS was much more than that of others. In LAZ832-0.2Y, the fitting models for UTS on VF and UTS on YS revealed the relationship of quadratic function. However, in LAZ1132-0.2Y, the fitting models for them exhibited the linear relationship.

The microstructural evolution and mechanical properties of α phase and α+β phase and β phase Mg-Li wrought alloys after heat treatment were investigated. The results showed that, in extruded Mg-5Li-3Al-2Zn-0.2Y, no precipitation was observed in the range of 220–320 °C for 12 h. In the extruded Mg-8Li-3Al-2Zn-0.2Y and Mg-11Li-3Al-2Zn-0.2Y alloys, a precipitation behavior was occurred in β phase after heat treatment. However, in the extruded Mg-11Li-3Al-0.2Y and Mg-11Li-3Al-0.2Ce alloys, there existed a transformation between the precipitation and the solid solution in β phase. In addition, the mechanical properties of the alloys were influenced remarkably by the precipitation and solid solution.

Microstructures and tensile properties of as-cast and as-extruded AZ91 magnesium alloys with individual and combined additions of Ca, Sm, and La elements are investigated. The results show that Al2Ca, Al2Sm, and Al11La3 new phases form after adding Ca, Sm, La elements, decreasing the amount of Mg17Al12 phases and refining the microstructures. Microstructures of as-cast and as-extruded alloys with combined additions are significant refined. The Al2Ca and Al11La3 intermetallic compounds are crushed into granules because of severe deformation during hot extrusion, while the Al2Sm intermetallic compounds are not. Tensile tests results at room temperature indicate that individual additions of Ca, Sm, and La elements could increase the elongation of as-extruded alloys, and tensile tests results at 150 °C indicate that individual additions of Sm and La elements could increase the ultimate tensile strength and yield strength of as-extruded alloys. AZ91–0.3La alloy exhibits the best comprehensive tensile properties both at room temperature and 150 °C. However, combined additions in AZ91 alloys leads to coarseness and aggregation of Al2Sm phases, resulting in slightly decline of tensile properties both at room temperature and 150 °C.

•Alloying species and addition have strong influence on hydrogen level of magnesium.•Rare earth additions sorely raise hydrogen level due to the formed cuboid hydrides.•Hydrogen gas pores deteriorates the mechanical properties of Mg alloys.•Additional hydrides put Mg-RE alloys almost free of gas porosities.This work investigates the hydrogen states and their corresponding effects on the mechanical properties of several kinds of magnesium alloys. Magnesium alloys can have tens to near hundreds microgram/gram of hydrogen, depending on the alloying element species and additions. Pure Mg has a hydrogen content of 12.7 μg/g. Al and Zn have a limited influence on hydrogen content, and the addition of rare earth elements Y, Gd and Nd significantly increases the hydrogen content to as high as 95 μg/g. Excess hydrogen causes highly alloyed magnesium alloys to suffer from gas porosity problems and deterioration of mechanical properties. The mechanical properties of as-cast AZ91-0.4Ca alloy can be promoted after the removal of hydrogen. The ultimate strength can be improved from 95 MPa to 150 MPa. The rare earth containing magnesium alloys are less likely to suffer gas porosity because excess hydrogen is incorporated into the cuboid-shaped rare earth hydrides. Additional hydride particles can form around the grain boundaries of the Mg-RE alloys during the homogenization. The formed hydrides should be mainly due to the strong hydrogen bonding by the rare earth atoms.Download high-res image (295KB)Download full-size image

The effects of the extrusion speed on the microstructures, mechanical properties and aging hardening behaviors of Mg9Gd3Y1.5Zn0.8Zr alloy have been investigated. The microstructure evolution during hot extrusion has also been discussed. The microstructures of the extruded alloys mainly consist of the equiaxed dynamically recrystallized grains and fiber-like long period stacking ordered (LPSO) phase. The increasing extrusion speed results in grain coarsening and the mechanical properties deterioration. Meanwhile, it could also retard the aging hardening response. The extrudability of the investigated alloy is limited. Cracks nucleate and grow up rapidly when the extrusion speed is over 0.3 m/min. It should be closely related to the large amount of LPSO phase and the unique microstructure evolution. The rapidly precipitating fine lamella during extrusion could pin the dislocations and grain boundaries effectively. It could strengthen the alloy but also limit the extrudability of the investigated alloy.

The microstructural evolution and a precipitation strengthening behavior of Mg-xLi-3Al-2Zn-0.2Y (x=5, 8, 11) by extruding and annealing process were investigated. Results show that when annealing at 300 °C for 6 h and 12 h and 24 h, the MgAlLi2 phase decomposes and vanishes from the matrices, meanwhile, recrystallization behavior of α and β grains can be observed. After annealing at 250 °C, in the Mg-8Li-3Al-2Zn-0.2Y and Mg-11Li-3Al-2Zn-0.2Y, a great deal of intermetallic compounds (1.8–2.5 µm) extensively precipitate from β phase. In contrast, no evident precipitation behavior happens in the Mg-5Li-3Al-2Zn-0.2Y. The large number of dispersive precipitates in β phase significantly enhance the strength of the alloys.

Four Mg–Gd–Y–Nd–Zr alloys were prepared by mold casting to investigate the effects of Nd/Gd ratios on microstructures and mechanical properties. The as-cast alloys mainly consist of α-Mg and β-Mg5(GdYNd). Volume fractions of the second phase increase and grains were slightly refined with the rising Nd/Gd ratio, when the alloying addition is equal. Meanwhile, fibers of second phase also increase in the extruded alloys when the Nd/Gd value increases. However, the Nd/Gd ratio could hardly influence the mechanical properties of the extruded alloys. The aging hardening response of the extruded alloy differs due to different Nd/Gd ratios. The potential mechanisms have also been discussed in detail.

The grain refinement and mechanical properties of AZ31 alloy with ZnO additions were investigated using suction casting method. ZnO addition could provide a more pronounced grain refinement and strengthening effects to the AZ31 alloy than Zn solute addition did. The grain size were well refined from 325 to 220 μm with 1.3 wt% ZnO addition. The grain refining mechanism of ZnO particles (~200 nm) mainly arose from the transformed Zn solute restricting the grain growth and the micro convection by the reaction enthalpy clearing solute suppressed nucleation effects. Meanwhile, the ZnO addition provided uniform solute field and steadly ensured a fine-grain and uniform second phase distribution. With 1.3 wt% ZnO addition, the ultimate tensile strength and yielding tensile strength of suction cast AZ31 alloy were as high as 205 and 66 MPa, respectively. The refining and strengthening mechanism have been discussed in detail.

•Hydrogen removal of magnesium was first attempted using ultrasonic argon degassing.•Mechanical properties of the degassed AZ91 alloys are much improved.•Ultrasonic argon degassing could both remove hydrogen and refine structures.•The mechanism for ultrasonic argon degassing has been discussed in detail.Argon degassing, ultrasonic degassing and a novel ultrasonic argon degassing treatment were applied for the hydrogen removal of AZ91 magnesium alloy. The hydrogen concentration, microstructures and mechanical properties have also been investigated. AZ91 alloys contains a high hydrogen concentration. The mechanical properties of the as-cast alloy are much improved using degassing process, which should be mainly attributed to the hydrogen removal. Among the three degassing process, the ultrasonic argon treatment is a high efficient process both for hydrogen removal and microstructure refining. One hand, ultrasonic wave could break up the purged argon bubble to improve the degassing efficiency of these bubbles. On the other hand, ultrasound could also generate many cavitation bubbles in the melt, which should account for the microstructure refinement. The ultrasonic argon treatment involves dynamics between the ascending argon bubbles and ultrasonic effects, such as cavitation and streaming, etc.

•Multi-pass severe rolling with vertical rolling was examined on coarse-grained AZ31B.•Pre-vertical rolling at 100 °C sharply restrain edge crack of rough-rolled sheets.•Pre-vertical rolling significantly change the texture of rough-rolled sheets.•The edge-crack level increases with increased vertical rolling temperature above 100 °C.•The vertical-rolling effect on properties depends on finish rolling temperature.AZ31B alloy was subjected to vertical rolling at various temperatures prior to multi-pass severe rolling processing including initial rolling including one 80% reduction pass and finish rolling at 300 °C and 350 °C, respectively. The depth and number of edge crack, microstructure evolution and tensile properties were examined. The results indicate that pre-vertical rolling at low temperature before severe rolling can significantly restrain edge crack, change relative frequency distribution of edge-crack depth, increase microstructure homogeneity and sharply change the intensity and distribution of basal texture of initial-rolled sheets. The level of edge crack increases with increased vertical rolling temperature above 100 °C mostly due to the combination of shear band density, microstructure homogeneity, grain size and texture of rolled sheets. Compared with conventional rolling, the effect of vertical rolling on final mechanical properties depends on the finish rolling temperature due to the combination of shear bands, twins and grain size. The variation trend of mechanical properties with increased vertical rolling temperature is also sensitive to finish rolling temperature. For as-rolled sheets, the severe rolling route with vertical rolling at 100 °C and finish rolling at 300 °C should be required.

The flow pattern and temperature field of direct chill (DC) casting of magnesium alloys with and without electromagnetic (EM) fields were studied by numerical simulation. The results reveal that the vortex agitation produced by EM field reduces the temperature gradient and shallows liquid sump depth during DC casting, and the surface quality of the billet could be improved owing to the lower contact pressure between the solidification skull and graphite annulus affected by Lorentz force. Lower AC frequency enhances the depth of penetration of electromagnetic field and higher AC current intensity intensifies vortex agitation. Casting velocity influences the flow pattern and temperature field as well as the valid operation time of the EM field.

•Qualitative and quantitative analysis on the temperature distribution along the thickness direction was performed.•Microstructure characteristics during the flat rolling with different process parameters were compared and analyzed.•Mathematical modeling for the maximum surface-temperature drop and center-temperature rise was carried out.•Proportion coefficient of chilling layer relative to the average thickness in the rolling zone was defined.•Theoretical equation of temperature rise due to plastic deformation was reasonably simplified for AZ31B Mg alloys.The present study aimed to investigate the temperature distribution in AZ31B Mg alloy plate along the thickness direction during the single-pass flat rolling. The detection and tracking of the plate temperature were carried out under different rolling conditions. Due to the influence of the complex heat transfer factors during rolling, the uneven degree of the temperature distribution was different, which was also reflected in microstructure characteristics between different metal layers. The temperature and microstructure both exhibited great similarity between the center and 1/4·H in the rolling due to the main effect of heat generation due to plastic work, while drastic difference between the surface and the center and 1/4·H due to the surface chilling effect. The maximum surface-temperature drop and center-temperature rise through rolling, regarded as the rolling exit temperature-distribution status, both presented mathematical relationships with respect to rolling process parameters. During the modeling process of the maximum surface-temperature drop, the proportion coefficient of chilling layer was identified for AZ31B Mg alloys, and it depends on the work roll temperature. In addition, during the modeling process of the maximum center-temperature rise, the theoretical equation of temperature rise due to plastic deformation could be reasonably simplified for AZ31B Mg alloys.Download high-res image (465KB)Download full-size image