Effect of addition of minor amounts of Sb and Gd on hot tearing susceptibility of Mg-5Al-3Ca alloy

The effects of addition of minor amount of(0.5 wt.%) antimony(Sb) or gadolinium(Gd) and combined addition of Sb and Gd(0.5 wt.%,respectively) on the hot tearing susceptibility(HTS) of Mg-5Al-3Ca alloy were investigated experimentally using a “T-shaped” hot tearing measuring system. Various solidification parameters of the alloys were measured and calculated through thermal analysis experiments. The microstructure, grain size, and morphology of the crack zone were characterized by scanning electron microscopy and electron backscatter diffraction, and the crystal phases of the alloys were analyzed by X-ray diffraction and energy-dispersive X-ray spectroscopy. The results showed that the addition of 0.5 wt.% Gd resulted in the increase in the vulnerable temperature range(Tv) and reduced the eutectic structure content that could participate in feeding, thereby improving the HTS of the alloy. However, addition of 0.5 wt.% Sb or combined addition of Gd and Sb(0.5 wt.%, respectively) to the Mg-5Al-3Ca alloy shortened the Tvand improved the skeleton strength of the alloy, thereby reducing HTS. Moreover, significantly refined structure of Mg-5Al-3Ca-0.5Gd-0.5Sb alloy improved the feeding ability of the eutectic structure, thus the alloy exhibited the lowest HTS.

Microstructure of Mg-8Zn-4Al-1Ca aged alloy

The microstructure of Mg-8Zn-4Al-1Ca aged alloy was investigated by TEM and HRTEM. The results show that the hardening produced in the Mg-8Zn-4Al-1Ca alloy is considerably higher than that in the Mg-8Zn-4A1 alloy. A dense dispersion of disc-like Ca2Mg6Zn3 precipitates are formed in Mg-8Zn-4Al-1Ca alloy aged at 160 ℃ for 16 h. In addition, the lattice distortions, honeycomb-looking Moir＆#233; fringes, edge dislocations and dislocation loop also exist in the microstructure. The precipitates of alloy aged at 160 ℃ for 48 h are coarse disc-like and fine dispersed grainy. When the alloy is subjected to aging at 160 ℃ for 227 h, the microstructure consists of numerous MgZn2 precipitates and Ca2Mg6Zn3 precipitates. All the analyses show that Ca is a particularly effective trace addition in improving the age-hardening and postponing the formation of MgZn2 precipitates in Mg-8Zn-4Al alloy aged at 160 ℃.

Effects of cooling rate on bio-corrosion resistance and mechanical properties of Mg-1Zn-0.5Ca casting alloy

Mg?1Zn?0.5Ca alloys were prepared by traditional steel mould casting and water-cooled copper mould injection casting at higher cooling rate. Microstructure, mechanical properties and bio-corrosion resistance of two alloys were contrastively investigated. Grain size reduces remarkably and microstructure becomes homogenous when raising cooling rate. The bio-corrosion behaviour in 3.5% sodium chloride solution (3.5% NaCl) and Hank’s solution at 37°C was investigated using electrochemical polarization measurement and the results indicate that the alloy prepared at higher cooling rates has better corrosion resistance in both types of solution. Further mass loss immersion test in Hank’s solution reveals the same result. The reason of corrosion resistance improvement is that raising cooling rate brings about homogeneous microstructure, which leads to micro-galvanic corrosion alleviation. The tensile test results show that yield strength, ultimate tensile strength and elongation are improved by raising cooling rate and the improvement is mainly due to grain refinement.

Effect of ultrasonic melt treatment on degassing of Mg-6Zn-1Ca alloy

The effect of ultrasonic power and treatment time on degassing of Mg-6Zn-1Ca alloy was studied in this paper. The degassing effect was characterized by measuring densities of ingots. The results show that proper ultrasonic treatment can remove hydrogen from the melt of the Mg-6Zn-1Ca alloy. The ultrasonic degassing effect is closely related to the ultrasonic power density and treatment time. The degassing efficiency increases with an increase in ultrasonic power density when the melt is treated at 690 °C for 120 s, reaching its highest value at 1.2 W·cm-3. When the power density is 1.2 W·cm-3, with an increase in ultrasonic treatment time, the degassing efficiency increases at first, reaches its peak value at 120 s, then decreases as the ultrasonic treatment is further prolonged. In this experiment, the optimum degassing effect with an efficiency of 67.5 % is obtained by ultrasonic treatment with the power density of 1.2 W·cm-3 for 120 s. The maximum density of ingot can be increased from 1.8069 g·cm-3 to 1.8146 g·cm-3(increased by 0.43%).

Effects of calcium addition on as-cast microstructure and mechanical properties of Mg-5Zn-5Sn alloy

The effects of Ca addition on the as-cast microstructure and mechanical properties of the Mg-5Zn-5Sn (mass fraction,%) alloy were investigated.The results indicate that an addition of 0.5%-1.5% (mass fraction) Ca to the Mg-5Zn-5Sn alloy not only refines the as-cast microstructure of the alloy but also causes the formation of the primary and/or eutectic CaMgSn phases with high thermal stability;an increase in Ca amount from 0.5% to 1.5% (mass fraction) increases the amount and size of the CaMgSn phase.In addition,Ca addition to the Mg-5Zn-5Sn alloy improves not only the tensile properties at room temperature and 150 ℃ but also the creep properties.Among the Ca-containing Mg-5Zn-5Sn alloys,the one added 0.5% (mass fraction) Ca obtains the optimum ultimate tensile strength and elongation at room temperature and 150 ℃,however,the alloy added 1.5% (mass fraction) Ca exhibits the optimum yield strength and creep properties.

Microstructures and properties in surface layers of Mg-6Zn-1Ca magnesium alloy laser-clad with Al-Si powders

Laser surface cladding with Al-Si powders was applied to a Mg-6Zn-1Ca magnesium alloy to improve its surface properties.The microstructure,phase components and chemical compositions of the laser-clad layer were analyzed by using X-ray diffractometry(XRD),scanning electron microscopy(SEM)and energy dispersive spectrometry(EDS).The results show that the clad layer mainly consists ofα-Mg,Mg2Si dendrites,Mg17Al12and Al3Mg2phases.Owing to the formation of Mg2Si,Mg17Al12and Al3Mg2intermetallic compounds in the melted region and grain refinement,the microhardness of the clad layer(HV0.025310)is about5times higher than that of the substrate(HV0.02554).Besides,corrosion tests in the NaCl(3.5%,mass fraction)water solution show that the corrosion potential is increased from-1574.6mV for the untreated sample to-128.7mV for the laser-clad sample,while the corrosion current density is reduced from170.1to6.7μA/cm2.These results reveal that improved corrosion resistance and increased hardness of the Mg-6Zn-1Ca alloy can be both achieved after laser cladding with Al-Si powders.

The Solidification Behavior of Mg-9Al-2Ca Alloy under Furnace Cooling

To understand the solidification pathway and microstructure evolution of Mg-9Al-2Ca alloy,the cooling curve of the alloy solidified under furnace cooling was measured and the water-quenched samples were observed.The experimental results show that the matrix phase of α-Mg dendrites is first generated at 596℃ during the solidification process,then the eutectic phases of Al_(2)Ca and Mg_(17)Al_(12) are formed at 518 and 447℃,respectively,and the solidification is terminated at 436℃.In the process of solidification,the seaweed dendrites of α-Mg get coarser and are gradually transformed into the global dendrites;besides,the secondary dendrite arms spacing(SDAS)of α-Mg as well as the solid fraction are both increased,while the increasing rate of SDAS of α-Mg and the solid fraction in the temperature region of 600-550℃ is faster than that in the temperature region of 550-436℃.And a power function relationship can be used to illustrate the change of the SDAS and the solid fraction with the temperature of solidification.

医用Mg-1.0Ca合金微弧氧化膜生物相容性研究

使用微弧氧化技术对医用Mg-1.0Ca合金进行表面改性,采用扫描电镜(SEM)、X射线衍射(XRD)、析氢实验以及MTT法表征微弧氧化膜的性能。扫描电镜和XRD分析表明,氧化膜是一种典型的多孔结构,含有氧化镁、硅酸镁和硅酸钙等。在模拟体液中浸泡72 h后,氧化膜中钙和磷的含量显著增加,表明氧化膜具有很好的生物活性。析氢实验表明,微弧氧化能大大提高Mg-1.0Ca合金的耐蚀性。Mg-1.0Ca合金氧化前后的细胞毒性均为1级,表明基体以及氧化试样均具有良好的生物相容性。

Microstructure and Corrosion Behavior of the As-Extruded Mg-4Zn-2Gd-0.5Ca Alloy

In order to study the corrosion resistance of extruded magnesium alloys,the Mg-4Zn-2Gd-0.5Ca alloy was extruded at the speed of 0.01-0.1 mm/s with the temperature of 280-360℃in present study.Hot extrusion results show that the volume fraction of precipitates(Vpre),VDRX(the dynamic recrystallization rate) and the average size of DRXed grain(dDRX) decrease with the decrease in extrusion speed,and the corrosion rate of the alloy also shows a downward trend.On the contrary,the values of Vpre,VDRX and dDRX increase with the increase in extrusion temperature,and the corrosion resistance of Mg-4Zn-2Gd-0.5Ca alloy decreases.When the extrusion speed is 0.01 mm/s and the extrusion temperature is 280℃,the alloy has the best corrosion resistance.The corrosion of extruded Mg-4Zn-2Gd-0.5Ca alloy occurs preferentially on the magnesium matrix around W and I phases in the DRXed zone.With the further corrosion,the corrosion continues to spread along the phase,and the corrosion area gradually increases.Galvanic corrosion plays a leading role in the corrosion process.Moreover,there are a large number of basal plane textures in the unDRXed region,which is conducive to improving the corrosion resistance of magnesium alloys.In addition,the decrease in grain size also makes the corrosion of magnesium alloy more uniform.

Microstructural control and hardening response of Mg–6Zn–0.5Er–0.5Ca alloy

The effects of heat treatment on microstructures and hardening response of Mg-6Zn4）.5Er4）.5Ca （wt%） alloy were investigated by optical microscope （OM）, scanning elec- tron microscope （SEM）, and transmission electron microscope （TEM） in this paper. The results show that the Mg（Zn4）.5Er- 0.5Ca alloy contains Mg3Zn6Erx quasicrystalline phase （I- phase） and Ca2Mg6Zn3 phase under as-cast condition. Most of the Ca2Mg6Zn3 phases and I-phases dissolve into matrix during heat treatment at 475 ℃ for 5 h. After the as-solution alloy was aged at 175 ℃ for 36 h, a large amount of MgZn2 precipitate with several nanometers precipitate. It is suggested that the trace addition of Ca results in refining the size of the precipitate, and the presence of the nanoscale MgZn2 phase is the main factor to improve the peak-aged hardness greatly to 87 HV, which in- creases about 40 % compared with that of as-cast alloy.