热处理对喷射沉积Al-27%Si合金显微组织及力学性能的影响

采用喷射沉积和热挤压致密化相结合的方法制备Al-27%Si合金,研究热处理对合金的组织及性能的影响。利用金相显微镜、扫描电镜和MTS-858型疲劳试验机等手段对Al-27%Si合金喷射沉积挤压态和热处理态的显微组织和力学性能进行观察与测试。结果表明:喷射沉积Al-27%Si合金组织细小均匀,初晶Si均匀弥散地分布在连续Al基体中,无针状共晶硅存在;采用合理的热挤压致密化工艺可以显著消除材料中的孔隙,致密度达到99.5%;热处理能够进一步改善材料的组织,但会降低材料的屈服强度。

喷射沉积Al-27%Si合金的半固态挤压成形

研究喷射沉积 Al-27%Si 合金的半固态挤压成形工艺及其对合金组织与性能的影响。结果表明：喷射沉积Al-27%Si合金在600℃下二次加热10-12 min后，合金液相体积分数适中，Si相尺寸相对细小，形貌为近球形，适合于半固态成形。经600℃二次加热10-12 min后，进行半固态挤压能消除喷射沉积合金中的孔隙，Si相比挤压前更加均匀细小，挤压棒材具有良好的表面质量和均匀的微观组织。经600℃二次加热12 min后，半固态挤压的合金可达到最高的相对密度（99.5%）、抗拉强度（195 MPa）和伸长率（6.7%）。

循环热处理对喷射沉积Al-27%Si合金组织和性能的影响

采用喷射沉积技术制备Al-27%Si(质量分数)合金,研究循环热处理对其组织、残余应力和力学性能的影响。采用Ansys有限元模拟结合X射线检测评估Al-27%Si合金残余热应力的分布和大小。结果表明,喷射沉积Al-27%Si合金由均匀细小的初晶Si和α-Al基体组成,初晶Si的尺寸随热循环温度升高和次数增加而增大。Ansys有限元模拟结果显示喷射沉积Al-27%Si合金增强体Si颗粒受径向拉应力,Al基体受径向压应力,平均残余热应力为121 MPa,小于X射线法测的平均残余应力131.7 MPa。Al-27%Si合金屈服强度与残余热应力、Si相形貌和大小有关。循环热处理后,当Si相形貌和尺寸相当时,材料的屈服强度随残余热应力的降低而升高;当Si相尺寸随热循环增大时,在残余应力相当的情况下材料的屈服强度降低。

Effects of ZrO_2 Nanoparticles on the Microstructure and Thermal-protective Properties of PEO Coating on Al-12.5%Si Alloy

PEO ceramic coatings including ZrO_2-Al_2O_3-SiO_2 in three phases were prepared on an Al-12.5%Si alloy in electrolyte solutions containing ZrO_2 nanoparticles. The microstructures and phases of the coatings were analyzed by SEM and XRD, and the heat insulation performance and the thermal shock resistance of the coatings were investigated. The compactness of the coating increased significantly and the hindrance of the Si element on plasma electrolytic oxidation process was effectively weakened. The growth rate of the coating was improved substantially with the addition of ZrO_2 nanoparticles. The PEO ceramic coatings are primarily composed of SiO_2 and high temperature steady phases such as a-Al_2O_3 and c-ZrO_2. Both the content of c-ZrO_2 and the heat-insulating property of the coating increased significantly. The ceramic coatings with special microstructure and composition formed in the solutions containing ZrO_2 nanoparticles possess excellent heat insulation performance and thermal shock resistance.

MICROSTRUCTURE AND MACROSEGREGATION OF Al-7%Si ALLOY SOLIDIFIED UNDER COMPLEX EFFECTS OF ELECTROMAGNETIC AND CENTRIFUGAL FORCES

Macrosegregations and microstructures of Al-7%Si alloy solidified under complex of fects of magnetic field and centrifugal forces are studied by means of a set of selfdesigned electromagnetic centrifugal casting (EMCC) device. It is shown that electromagnetic field (EMF) has an important effect on the macrosegregation of centrifugal casting specimen of Al-7%Si alloy in two respects: one is that there exists always a kind of convection in the liquid in front of the S/L interface caused by effect ofthe electromagnetic force; the other is that different atomic clusters of solidparticles with different physical characteristics are subjected to quite different electromagnetic (Lorentz) force. Therefore, their movements get changed. In addition, the formation process of a complex band structure consisting of primary α-Al dendrites and (α-Al+β-Si) eutectics in hypoeutectic Al-Si alloys during EMCC and the effect of EMF are discussed.

Structural evolution of spray-formed Al-70wt.%Si alloys during iso-thermal holding in the semi-solid state

The grain growth behavior of spray-formed Al-70wt.%Si alloys was studied in the semi-solid state. The specimens were isothermally heat-treated at various temperatures between the solidus and liquidus of Al-Si alloys and then quenched in water. The microstructure of reheated specimens was characterized using optical and scanning electron microscopies. The isothermal holding experiment was carried out to investigate grain growth behavior as a function of holding time and temperature in the semi-solid state. The coarsening mechanism and the effect of porosity on microstructure were also studied.

STRUCTURAL TRANSFORMATION AND ITS REVERSIBILITY IN LIQUID Al-13%Si ALLOY AT HIGH TEMPERATURES

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Modification and aging precipitation behavior of hypereutectic AI-21wt.%Si alloy treated by P+Ce combination

In the present study, the tested hypereutectic Al-21wt.%Si alloys were prepared by modifying the melt using different proportions of P and Ce, and then applying T6 heat treatment. The modification effects and mechanism of P＋Ce complex modifier on the Si phase of hypereutectic Al-21wt.%Si alloy were studied, and the aging precipitation behavior after modification was characterized by means of tensile strength measurement, OM, SEM and TEM analysis. The results show that the massive primary silicon phase particles are significantly refined after modification, while the needle-like eutectic silicon crystals become fibrous and short. It was found that the mechanism of phosphorus modification on the primary silicon can be attributed to heterogeneous nucleation of AlP, while the modification mechanism of Ce can be explained by adsorbing-twinning theory. In the aged microstructure of the modified hypereutectic Al-21wt.%Si alloy, there existed some strengthening phases such as AI4Cu9, Al2Cu, AlCu3, and Al57Mn12. The P＋Ce complex modifier not only affected the size of primary silicon and eutectic silicon, but also the aging behavior of alloys under the heat treatment process. When Al-21wt.%Si alloy was modified using 0.08%wt.P ＋ 0.6wt.% Ce, the aging precipitates were dispersed uniformly in the alloy, and its mechanical properties at room and elevated temperatures are optimized （Rm = 287.6 MPa at RT, Rm = 210 MPa at 300 ℃）.

冷却速度对气雾化非平衡Al-27%Si合金粉末组织形貌特征的影响

采用快速凝固气体雾化技术制备Al-27%Si合金粉末,利用对流传热原理计算合金粉末的冷却速度,采用扫描电镜（SEM）观察分析不同粒度合金粉末的形貌。结果表明：快速凝固气体雾化的冷却速度介于103-106 K/s之间,对合金粉末形貌组织有较大影响。该合金粉末中细小初晶Si相和共晶Si相均匀弥散分布在α-Al基体中,且Si颗粒尺寸随粉末尺寸减小而减小。通过测定合金粉末二次枝晶间距,结合冷却速度与二次枝晶间距之间的经验公式,能较好地预测实验结果。

快速凝固过共晶铝硅合金粉末的形貌与显微组织

采用快速凝固气体雾化工艺制备Al-27%Si合金粉末,研究合金粉末形貌和热处理对组织的影响,利用X射线衍射仪和显微硬度计等对Al-27%Si合金粉末中硅相的析出与长大进行表征。结果表明:快速凝固Al-27%Si合金粉末由基体α(Al)相、块状β-Si相和枝晶状共晶硅相组成;经热处理后,β-Si相发生粗化,共晶硅相逐渐转变为块状相且发生粗化;在500℃下加热保温后,α(Al)相和β-Si相的衍射峰强度随时间延长而增大,α(Al)相衍射峰向低角度偏移;合金粉末显微硬度在500℃下加热,初期有下降趋势,但随时间延长,不再下降,而是维持相对稳定。