Dissimilar friction stir spot welding of AA2024-T3/AA7075-T6 aluminum alloys under different welding parameters and media

This paper studies the friction stir spot welding of AA2024-T3/AA7075-T6 Al alloys in the ambient and underwater environments by clarifying the nugget features,microstructure,fracture and mechanical properties of the joints.The results show that the water-cooling medium exhibits a significant heat absorption capacity in the AA2024-T3/AA7075-T6 welded joint.Nugget features such as stir zone width,circular imprints,average grain sizes,and angular inter-material hooking are reduced by the water-cooling effect in the joints.Narrower whitish(intercalated structures)bands are formed in the under-water joints while Mg2Si and Al2CuMg precipitates are formed in the ambient and the underwater welded joints respectively.An increase in tool rotational speed(600-1400 rpm)and plunge depth(0.1-0.5 mm)increases the tensile-shear force of the welded AA2024-T3/AA7075-T6 joints in both the ambient and underwater environments.The maximum tensile-shear forces of 5900 N and 6700 N were obtained in the ambient and the underwater welds respectively.     

Influence of magnetically constricted arc traverse speed (MCATS) on tensile properties and microstructural characteristics of welded Inconel 718 alloy sheets

The magnetically constricted arc technique was implemented to mitigate the heat input related metal-lurgical problems in Gas Tungsten Arc Welding (GTAW) of Inconel 718 alloy particularly Nb segregation and subsequent laves phase evolution in fusion zone. This paper reports the direct effect of magnetically constricted arc traverse speed (MCATS) on bead profile, tensile properties and microstructural evolution of Inconel 718 alloy sheets joined by Gas Tungsten Constricted Arc Welding (GTCAW) process. The mechanism amenable for the microstructural modification and corresponding influence on the tensile properties of joints is investigated both in qualitative and quantitative manner related to the mechanics of arc constriction and pulsing. It is correlated to the solidification conditions during welding. The relationship between MCATS and Arc Constriction Current (ACC) was derived. Its interaction effect on the magnetic arc constriction and joint performance was analysed. Results showed that the joints fabricated using CATS of 70 mm/min exhibited superior tensile properties (98.39% of base metal strength with 31.50% elongation). It is attributed to the grain refinement in fusion zone microstructure leading to the evolution of finer, discrete laves phase in interdendritic areas.     

基于双特征参数解的直杆弹性动力后屈曲研究

利用差分方法求解动力后屈曲非线性方程解,研究了弹性直杆的2类轴向碰撞屈曲问题.将双特征参数解得出的含有小幅值参数的初始动力屈曲模态作为非线性后屈曲解的初始条件.理论计算的结果与文献中的实验数据达到了很好的一致,由此验证了双特征参数方法的正确性.研究结果还揭示了碰撞过程中屈曲变形扩展和发展的机理,以及轴向应力波和屈曲变形的相互作用规律.     

轴向应力波作用下的圆柱壳弹塑性后屈曲

为揭示柱壳屈曲变形的发展机理及屈曲波与弹塑性应力波耦合传播间的关系,运用非线性显示动力学有限元分析方法对承受轴向高速冲击载荷作用下柱壳的弹塑性后屈曲过程进行了研究。分析结果表明:对于柱壳在轴向高速冲击下的弹塑性轴对称动力屈曲,主要屈曲变形发生在柱壳的两端,屈曲变形的产生及发展与应力波在壳中的传播过程密切相关;塑性应力波在整个屈曲过程中起了绝对支配的作用;屈曲波的产生及发展,都与塑性波的传播及反射有关;弹性应力波由于在整个屈曲过程中持续的时间极短,处于次要的地位。     

列车用复合材料阻燃性能研究

不同类别的阻燃剂配合使用能产生协效作用,大大提高阻燃效果。在甲基丙烯酸类不饱和聚酯树脂9001基体中,添加微囊化红磷/氢氧化铝/三氧化二锑/甲基膦酸二甲酯(MRP/Al(OH)3/Sb2O3/DMMP)阻燃剂体系,对其树脂体系固化物及玻璃纤维织物复合材料的力学性能和阻燃性能进行了实验研究,提出了一种有望用于列车复合材料大尺寸构件制造的性能优异、价格低廉的新体系。结果表明,当质量添加比例分别为12%MRP、50%Al(OH)3、2%Sb2O3时,树脂体系室温粘度100mPa.s左右,凝胶时间超过80min,适用于RTM和VIMP等大尺寸构件成型工艺;复合材料拉伸强度215.4MPa,弯曲强度177.15MPa,拉伸模量13.85GPa,弯曲模量13.36GPa,氧指数39.7。     

硬质聚氨酯泡沫塑料填充薄壁钢管横向压缩吸能特性分析

为研究硬质聚氨酯泡沫塑料填充薄壁钢管后的吸能特性,对单钢管、钢管嵌套系统及填充硬质聚氨酯泡沫塑料后的钢管进行横向静力压缩试验,并对试验结果进行对比分析。结果表明:硬质聚氨酯泡沫塑料填充管比空钢管的吸能效果有明显提高,其中双钢管嵌套系统填充硬质聚氨酯泡沫塑料后吸能效果提高最明显,三钢管嵌套系统吸收外来的能量值最大。薄壁钢管填充硬质聚氨酯泡沫塑料是一种性能优良的吸能装置,在防护工程中可作为隔爆抗侵彻材料。     

MoDTP作锂基润滑脂添加剂的摩擦学性能研究

考察二烷基二硫代磷酸氧钼(Mo DTP)作锂基润滑脂添加剂时的摩擦学性能,并与传统添加剂二硫化钼(Mo S2)进行对比;采用扫描电子显微镜(SEM)及能量色散X射线分析仪(EDX)对长磨后钢球磨斑表面形貌及元素进行观测和分析,探讨Mo DTP的润滑作用机理。结果表明:在不同荷载下,Mo DTP作添加剂比Mo S2具有更好的减摩抗磨性能和更高的PB值,但PD值较低;摩擦过程中Mo DTP与摩擦副表面发生反应,生成了摩擦学性能显著的润滑层。     

量子环流理论研究富勒烯分子的磁学性质

在原子轨道线性组合方法中引进规范变换,推导出在外加恒定磁场下分子中的电子环流表达式,应用该理论导出了分子磁化率的一般公式,并对Ｃ６０和Ｃ７０分子中π电子的磁化率进行了计算。     

zn及Zn／Si02复合添加剂的摩擦学性能

实验室自制了锂基润滑脂,通过四球试验研究zn作为锂基润滑脂添加剂在不同工况条件的摩擦学性能,利用光学显微镜和EDX分析了磨斑表面形貌及元素组成,探讨了zn的润滑作用机理,并对zn与SiO2不同复配方案的摩擦学性能进行了研究。结果表明：在低载荷条件,zn能更显著地提高锂基润滑脂的抗磨性能;在中高载荷条件,zn同时具有减摩抗磨性能,但对Pn值的提高没有太大帮助。EDX能谱分析表明磨斑表面存在特征zn元素,说明zn能够在摩擦表面沉积,形成摩擦学性能显著的表面润滑层;同时,zn与硬质颗粒SiO2在适宜的复配比例时具有一定的协同效应,能提高锂基润滑脂的摩擦学性能．     

先驱体转化致密碳化硅纳米复合材料的制备及其热电性能

以聚碳硅烷和锑改性聚硅烷为先驱体,利用先驱体转化Si C材料的富余自由碳高温石墨化的微观结构演变特点,采用热压烧结、先驱体浸渍-裂解法以及退火工艺制备出先驱体转化Si C纳米复合材料。采用SEM、TEM、XRD和Raman等测试手段表征和分析了相组成和微观结构,讨论了样品的热导率、电导率和塞贝克系数等热电参数随温度变化关系。研究表明,所得致密Si C纳米复合材料为n型热电材料。由于纳米石墨的作用,材料热导率抑制在4~8W/(m·K)范围。1600℃退火处理能够降低热导率,同时提高电导率和塞贝克系数绝对值,使先驱体转化法得到的Si C纳米复合材料无量纲热电优值ZT达到0.0028(650℃),高于其他已报道的致密Si C/C复合材料和纳米复合材料体系。