Fabrication of AA7005/TiB2-B4C surface composite by friction stir processing: Evaluation of ballistic behaviour

The present work aims to enhance the ballistic resistance of AA7005 alloy by incorporating the TiB₂ and B₄C ceramic reinforcement particles. Surface composites with different weight fractions of TiB₂ and B₄C particles were processed by friction stir processing. Micro-hardness and depth of penetration tests were carried out to evaluate the ballistic properties of the surface composites. The surface hardness of the composite was found to be nearly 70 HV higher than base alloy. The depth of penetration of the steel projectile was 20–26 mm in the composites as compared to 37 mm in the base alloy. Ballistic mass efficiency factor of the surface composite was found to be 1.6 times higher than base alloy. This is mainly attributed to the dispersion strengthening from the reinforcement particles.     

Formation of adiabatic shearing band for high-strength Ti-5553 alloy: A dramatic thermoplastic microstructural evolution

By using split Hopkinson pressure bar, optical microscopy and electronic microscopy, we investigate the influence of initial microstructures on the adiabatic shear behavior of high-strength Ti–5Al–5V–5Mo–3Cr (Ti-5553) alloy with lamellar microstructure and bimodal microstructure. Lamellar alloy tends to form adiabatic shearing band (ASB) at low compression strain, while bimodal alloy is considerably ASB-resistant. Comparing with the initial microstructure of Ti-5553 alloy, we find that the microstructure of the ASB changes dramatically. Adiabatic shear of lamellar Ti-5553 alloy not only results in the formation of recrystallized β nano-grains within the ASB, but also leads to the chemical redistribution of the alloying elements such as Al, V, Cr and Mo. As a result, the alloying elements distribute evenly in the ASB. In contrast, the dramatic adiabatic shear of bimodal alloy might give rise to the complete lamination of the globular primary α grain and the equiaxial prior β grain, which is accompanied by the dynamic recrystallization of α lamellae and β lamellae. As a result, ASB of bimodal alloy is composed of α/β nano-multilayers. Chemical redistribution does not occur in ASB of bimodal alloy. Bimodal Ti-5553 alloy should be a promising candidate for high performance armors with high mass efficiency due to the processes high dynamic flow stress and excellent ASB-resistance.     

Effect of metal powders on explosion of fuel-air explosives with delayed secondary igniters

In order to improve the energy level of fuel air explosive(FAE) with delayed secondary igniters, high energetic metal powders were added to liquid fuels mainly composed of ether and isopropyl nitrate. Metal powders' explosive properties and reaction mechanisms in FAE were studied by high-speed video, pressure test system, and infrared thermal imager. The results show that compared with pure liquid fuels, the shock wave overpressure, maximum surface fireball temperature and high temperature duration of the mixture were significantly increased after adding high energetic metal powder. The overpressure values of the liquid-solid mixture at all measuring points were higher than that of the pure liquid fuels. And the maximum temperature of the fireball was up to 1700 ℃, which was higher than that of the pure liquid fuels. After replacing 30％of aluminum powder with boron or magnesium hydride, the shock wave pressure of the mixture was further increased. The high heat of combustion of boron and the hydrogen released by magnesium hydride could effectively increase the blast effect of the mixture. The improvement of the explosion performance of boron was better than magnesium hydride. It shows that adding high energetic metal powder to liquid fuels can effectively improve the explosion performance of FAE.     

关于S147 铝合金形变时效强化研究

本文研究了S147铝合金板材在形变时效处理各工艺阶段中的组织与性能。用扫描电镜观察分析了材料的断口形貌,并用薄膜试样在透射电镜下分析研究了该合金形变时效处理后的微观亚结构及过渡相析出物特征。结果表明,最佳形变时效处理可显著提高S147铝合金的强度及塑韧性,并能改善其过渡相析出过程及特征。     

Micromechanical simulation and experimental investigation of aluminum-based nanocomposites

Ceramic reinforced metal matrix nanocomposites are widely used in aerospace and auto industries due to their enhanced mechanical and physical properties. In this research, we investigate the mechanical properties of aluminum/Nano-silica composites through experiments and simulations. Aluminum/Nano-silica composite samples with different weight percentages of silica nanoparticles are prepared via powder metallurgy. In this method, Nano-silica and aluminum powders are mixed and compressed in a mold, followed by sintering at high temperatures. Uniaxial tensile testing of the nanocomposite samples shows that adding one percent of Nano-silica causes a considerable increase in mechanical properties of nanocomposite compared to pure aluminum. A computational micromechanical model, based on a representative volume element of aluminum/silica nanocomposite, is developed in a commercial finite element software. The model employs an elastoplastic material model along with a ductile damage model for aluminum matrix and linear elastic model for nano-silica particles. Via careful determination of model parameters from the experimental results of pure aluminum samples prepared by powder metallurgy, the proposed computational model has shown satisfactory agreement with experiments. The validated computational model can be used to perform a parametric study to optimize the micro-structure of nanocomposite for enhanced mechanical properties.     

大容量非晶合金中频变压器的噪声抑制技术

从噪声源和噪声传播途径两个方面讨论了大容量非晶合金中频变压器的噪声抑制措施:通过对比两电平与三电平移相全桥逆变电路输出电压的总谐波畸变率确定直流变换器中逆变电路的工作点,从源头上抑制噪声;通过增加内衬吸音海绵的隔音柜阻碍空气噪声传播,并通过自行研制的一台大容量直流变换器进行了试验验证.结果表明:该措施能够很好地抑制中频变压器噪声.