气体雾化过程中涉及颗粒形貌和空心颗粒形成的金属液滴变形和破碎数值模拟

采用考虑液滴冷却和凝固的流体体积(VOF)方法模拟金属液滴在气体雾化过程中的变形和破碎。建立典型粉末形貌和液滴破碎行为之间的相关性,以指导球形粉末颗粒的制备。结果显示,当增加液滴的气动力与黏性力的比值时,球形颗粒的形成得到加强;然而,当减小这个比值时,预期的液滴破碎模型将被改变或仅仅只发生液滴的变形。从空心颗粒的形成和演变过程的数值模拟中观察到几种典型的情况,例如,开放的空心液膜的形成、液膜封闭、气泡的离心和气泡的脱离。通过提高气体速度或液滴温度,实现较高的非平衡拉普拉斯压力或较低的粘性力,促使气泡从液滴内部分离。

Semi-solid processing of aluminum and magnesium alloys:Status,opportunity,and challenge in China

Owing to its low cost,short process and low energy consumption,semi-solid processing(SSP)of aluminum(Al)and magnesium(Mg)alloys has been considered as a competitive approach to fabricate complicated components with excellent performance.Over the past decade,significant progress has been achieved in deeply understanding the SSP process,the microstructure and performance of the fabricated components in China.This paper starts with a retrospective overview of some common slurry preparation methods,followed by presenting the performance and the underlying mechanisms of SSP fabricated alloys.Then,the mainstream opinions on the microstructure evolution and rheological flow behavior of semi-solid slurry are discussed.Subsequently,the general situation and some recent examples of industrial applications of SSP are presented.Finally,special attention is paid to the unresolved issues and the future directions in SSP of Al and Mg alloys in China.

Numerical investigation on flow process of liquid metals in melt delivery nozzle during gas atomization process for fine metal powder production

Based on volume of fluid(VoF)interface capturing method and shear-stress transport(SST)k-ω turbulence model,numerical simulation was performed to reveal the flow mechanism of metal melts in melt delivery nozzle(MDN)during gas atomization(GA)process.The experimental validation indicated that the numerical models could give a reasonable prediction on the melt flow process in the MDN.With the decrease of the MDN inner-diameter,the melt flow resistance increased for both molten aluminum and iron,especially achieving an order of 10^(2) kPa in the case of the MDN inner-diameter≤1 mm.Based on the conventional GA process,the positive pressure was imposed on the viscous aluminum alloy melt to overcome its flow resistance in the MDN,thus producing powders under different MDN inner-diameters.When the MDN inner-diameter was reduced from 4 to 2 mm,the yield of fine powder(<150μm)soared from 54.7%to 94.2%.The surface quality of powders has also been improved when using a smaller inner-diameter MDN.

Characterization of cooling rate and microstructure of Cu Sn melt droplet in drop on demand process

Different sized single droplets of Cu-6%Sn alloy were prepared by drop on demand(DOD)technique.The secondarydendrite arm spacing was measured and correlated with the droplet cooling rate by a semi-empirical formula.The microstructure ofdroplets was observed by optical microscopy(OM)and electro backscatter diffraction(EBSD).The dendrite feature of singledroplets depends on solidification rate,cooling medium and flight distance.When droplets collide with each other at temperaturesbetween solidus and liquidus,the dendrites and grains are refined obviously possibly because the collision enhances the heat transfer.The cooling rate of colliding droplets is estimated to be more than4×104K/s based on a Newton’s cooling model.The dendritesgrow along the colliding direction because of the temperature gradient induced by the internal flow inside the droplets.

Corrigendum to “Characterization of cooling rate and microstructure of CuSn melt droplet in drop on demand process” [Trans. Nonferrous Met. Soc. China 27 (2017), 1636.1644]

In the printed contribution some basic modeling equations for the droplet behavior have not been given correctly. The authors apologize for this error. The computational model used to derive the results discussed in the paper is based on a thermal and a kinetic model for single droplet behavior in flight. The corrections are as follows.

Mg-6Zn-3Sn合金的均匀化热处理及其对显微组织和力学性能的影响

利用DSC、OM、XRD、SEM、TEM、维氏硬度计和万能试验机,研究Mg-6Zn-3Sn合金均匀化过程中的显微组织和力学性能演变,分析合金过烧的原因和扩散动力学,确定合金的均匀化制度。结果表明,单级均匀化处理后,Mg_(2)Zn_(3)相分解,同时伴随着Mg_(2)Sn相的析出。当均匀化温度上升到350℃时,Mg_(2)Zn_(3)相导致合金的过烧。合金合适的均匀化制度为(335℃,24 h)+(400℃,6 h)。在双级均匀化过程中,合金的硬度持续下降,力学性能呈现先升后降的趋势,这与Zn和Sn原子的固溶和Mg_(2)Sn相的析出有关。

Dislocation configuration evolution during extension twinning and its influence on precipitation behavior in AZ80 wrought magnesium alloy

Thermomechanical treatment T10(extension twinning+aging treatment)can largely enhance the precipitation strengthening effect of magnesium alloys.In this study,dislocation structure evolution and precipitation behavior during T10 treatment of an AZ80 extruded bar were analyzed mainly by two-beam diffraction in TEM.At a compressive strain of 1%in the extrusion direction(ED),a typical dislocation configuration,including basal I1 stacking faults(SFs)and<c+a>dislocations,has been established in extension twins.As the strain reaches 7%,the volume fraction of extension twins increases to more than 90%at which point high dense I1 SFs and<c+a>dislocations occur.After aging for 2 h at 150℃for the 7%strained sample,masses of basal I1 SFs and<c+a>dislocations remain in the extension twins and can act as effective nucleation sites and solute fast-diffusion channels for continuous precipitates.Consequently,the precipitates in extension twins become highly dense.

Microstructure and thermophysical properties of Mg-2Zn-xCu alloys

The microstructure and thermophysical properties of Mg-2 Zn-x Cu alloys(x=0.5, 1.0 and 1.5, at.%) were investigated through microstructural and thermophysical characterization, heat treatment, and first-principles calculations. It was found that the addition of Cu had influence on the microstructure and thermophysical properties of the alloy. As the Cu content increased, the content of the MgCuZn phase increased in the as-cast alloys along with the electrical and thermal conductivities. After solution treatment, the eutectic structure partially decomposed and Zn atoms dissolved into the matrix, leading to the decrease in both the electrical and thermal conductivities of the alloy. During the early stages of the aging treatment, solute atoms precipitated from the matrix, thus increasing the electrical conductivity of the alloy. After aging for 24 h, the thermal conductivity of Mg-2 Zn-1.5 Cu alloy reached the maximum of 147.1 W/(m·K). The thermostable MgCuZn phases were responsible for increasing the electrical and thermal conductivities. Smaller amounts of Zn atoms dissolved in the matrix resulted in smaller lattice distortion and higher conductivities. The first-principles calculations findings also proved that the MgCuZn phases had very high conductance.

Microstructure and corrosion resistance of bone-implanted Mg-Zn-Ca-Sr alloy under different cooling methods

The cooling gradient of Mg-3 Zn-1 Ca-0.5 Sr alloy in cast ingots under different cooling methods(air cooling,warm-water cooling and ice-water-mixture cooling) was examined and the effect of cooling rate on the structure and corrosion properties was studied.The microstructure of the alloy was composed of α-Mg,Ca_(2) Mg_(6) Zn_(3) and Mg_(17)Sr_(2) phases.As the solidification cooling rate increased,the grain was refined,Zn and Sr were less segregated,the distributions of Zn and Sr were more uniform,and corrosion rate was found to first increase and then decrease;this contradicts the findings of recent research.With cooling rate increasing,the number of corroded microcouples comprising second phase and α-Mg increases.More α-Mg participates in corrosion,leading to a layered and deep corrosion pit and an increased corrosion rate.However,as the microstructure became sufficiently dense,the corroded structure protected the deep α-Mg from participating in corrosion,thus reducing the corrosion rate.

Microstructure and texture evolution of Mg–7Y–1Nd–0.5Zr alloy sheets with different rolling temperatures

The extruded Mg-7Y-1Nd-0.5Zr(wt%)alloy were performed to the same strain hot rolling with different temperatures.The microstructure and texture evolution of the sheets were investigated by optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),and electron back-scattered diffraction(EBSD).The results indicate that the microstructure becomes homogeneous after hot rolling process and precipitated phase distributes at grain boundaries along rolling direction.With the increase in rolling temperature,the grains of sheet grow up.The sheet rolling at 400℃is composed of recrystallization grains,the necklace of the precipitated phase in the grain boundaries and excessive dislocations.The structure of necklace of the precipitated phase is fcc structure with lattice constant of a=0.75 nm.With rolling temperatures increasing from 400 to 450℃,the content of recrystallized grains in volume fraction with relatively random orientations increases significantly.Compared with the rolling process at 400℃,the amount of precipitated phases is reduced at the grain boundary,and the precipitated phase begins to appear in the grain interior when rolling at 450℃.The structure of the precipitated phase is fcc with lattice constant of a=2.22 nm.The recrystallization grains begin to grow in the rolling process at 500℃.The basal texture is obviously produced during the rolling process at 400 and 450℃;however,the basal texture is weak in the rolling process at 500℃.

Microstructure and phase transformation of as-cast and annealed Mg–4Zn–1Y alloy containing quasi-crystal phase

By means of optical microscope （OM）, X-ray diffraction （XRD）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM） analyses, the microstructures of as-cast and heat-treated Mg--4Zn-IY （wt%） alloy containing quasi-crystal phase were studied. The microstructure of the as-cast alloy consists of a-Mg solid solution grains, intermetallic particles and eutectic phases （W-phase and 1-phase）, and huge grains with seri- ous dendritic segregation are clearly observed. After heat treatment, phase transformation and dissolution occur in the alloy and many phases remain. When the alloy was treated above 410 ~C, the eutectic phases transform into spherical shape as the I-phase turns to W-phase. After heat treatment for long time, the alloy is over burnt and the W-phase decomposes to Mg-Y binary phase.

Homogenization heat treatment of Mg-7.0 wt%Y-1.0 wt%Nd-0.5 wt%Zr alloy

The microstructures and mechanical properties of Mg-7.0 wt%Y-1.0 wt%Nd-0.5 wt%Zr magnesium alloy were investigated both in as-cast condition and after homogenization heat treatment by differential scanning calorimetry(DSC),optical microscopy(OM),X-ray diffraction(XRD),scanning electron microscopy(SEM)and hardness measurement.The results indicate that the ascast alloy consists ofα-Mg matrix,Mg24 Y5 and Mg41 Nd5 phases,which are eutectic phases(cubic Y-rich phase).With the increase in homogenization temperature and time,the Mg24 Y5 and Mg41 Nd5 phases are completely dissolved into matrix,and only yttrium of intermetallic compounds leaves around boundary.After homogenization heat treatment,the elements distributed uniformly and the grains grow up not obviously,only yttrium element of intermetallic compounds left around boundary.The optimum homogenization condition is at 537℃for 16 h.The mechanical properties are improved after homogenization,with tensile strength ofσb=181 MPa,yielding strength ofσ(0.2)=144 MPa and elongation ofδ=5.5%,which are better than those of as-cast alloy.

Corrosion behaviors for peak-aged Mg-7Gd-5Y-lNd-0.5Zr alloys with oxide films

The corrosion behaviors of T5 （225 ℃, 6.5 h） and T6 （460 ℃, 2 h ＋ 225 ℃, 12 h） peak-aged Mg-7Gd- 5Y-1Nd-0.5Zr alloys with oxide films were investigated by optical microscope （OM）, scanning electron microscopy （SEM）, and energy-dispersive spectroscopy （EDS）. The weight loss rates and electrochemical tests were also analyzed. The thicknesses of T5 and T6 oxide films are roughly 0.6 and 1.0 μm, respectively. The components of oxide films mainly consist of O, Mg, Y, Nd, and Gd, and the T6 oxide film results in surfaces with larger peaks than T5 oxide film. In addition, Y, Nd, and Gd peaks are all higher than those of Mg-7Gd-5Y- 1Nd-0.5Zr alloys, but Mg peak is consistently far below than that of the alloys. The specimens could be arranged in de- creasing order of corrosion rates and corrosion current densi- ties： T6 oxide film 〈 T5 oxide film 〈 T6 without oxide film 〈 T5 without oxide film. The oxide films are compact to increase the corrosion resistance for Mg-7Gd-5Y-1Nd-0.5Zr alloys, which will provide a guiding insight into the corrosion and protection of Mg-RE alloys in atmospheric environments.