Comparative study on local and global mechanical properties of 2024 T351, 2024 T6 and 5251 O friction stir welds

The effect of the microstructure heterogeneity on the global and local tensile properties of friction stir welded joints in 5251 (O temper) and AA2024 (T351 and T6 tempers) aluminium alloys has been investigated. Micro-tensile tests parallel to the welding direction have been carried out in the regions representative of the main microstructural zones. The digital image correlation technique (DIC) has been used during transverse tensile tests for mapping the strain distribution and to determine the local stress–strain curves. A 3-D finite elements model has been developed to predict the weld behaviour from the tensile curves of the individual regions of the weld.The tensile properties of the 5251 O weld are relatively homogeneous leading to high ductility and fracture in the base material. In contrast, the tensile properties of the various regions of the 2024 T351 and 2024 T6 welds are very heterogeneous and essentially controlled by the state of precipitation. The thermo-mechanically affected zone is the weakest region where the strain localises during a transverse tensile test. The 2024 T6 base material is stronger than the 2024 T351 alloy, leading to a more pronounced strain localisation during transverse tensile tests and a lower overall ductility. Local tensile data obtained by strain mapping are in good agreement with the curves obtained by micro-tensile tests, and these results can be safely used in a finite elements model to predict the behaviour of the overall weld assembly.     

Design of Fe-based nanocrystalline alloys with superior magnetization and manufacturability

Fe-based alloys with a nanocrystalline-amorphous nanostructure exhibit superior soft-magnetic performances; however they generally suffer from the low magnetization because of their heavy doping for an acceptable manufacturability. In this study, we proposed a revolutionary nanostructure-construction concept based on preforming dense nuclei in the melt-quenching process with a critical cooling-rate and refining the nano-structure via transient metalloid-rich interfaces. A novel alloy composition of Fe_85.5B₁₀Si₂P₂C_0.5 was developed via our multi-metalloid stabilization and critical formability strategies by using a total of only 4.6 wt. % light metalloids. This unique alloy design effort leads to unprecedented magnetic properties with the super-high B_s of 1.87 T and µ_e of 1.0–2.5 × 10⁴, which outperform all commercial counterparts and have a high potential to substitute for commercial bulk Si-steels currently used for soft-magnetic applications. This hetero-structuring and lean-alloying strategy provides a paradigm for the next-generation of magnetic materials.     

The use of a single-fibre fragmentation test to study environmental durability of interfaces/interphases between DGEBA/mPDA epoxy and glass fibre: the effect of moisture

The influence of environmental exposure, both thermal and hydrothermal, on the average critical length/diameter (L_c/D) ratio of fibre fragments, as measured by the single-fibre fragmentation test, has been investigated. The increase in _c/D for hydrothermally treated samples indicated that degradation of the interfacial properties was dominant, while L_c/D for the thermally treated samples showed no change. Apparently, degradation of both the interfacial shear strength and the fibres occurred under hydrothermal conditions. Analysis of the distilled water in which the hydrothermally treated samples had been soaked detected the presence of ions from E-glass. Furthermore, fragmentation of the fibre after hydrothermal treatment reached its limit (saturation) at a lower value of applied strain than did fibres after exposure to the dry or thermal environments. Changes in strength at the interface and in the fibre appear to be the major factors influencing the L_c/D values on hydrothermal exposure.     

基于递推最小二乘法的SCARA机器人动力学参数辨识研究

为了提高SCARA机器人在工业场合中的工作精度,对其动力学模型进行分析是有效途径之一。建立机器人拉格朗日动力学方程并进行线性化处理,得到一组关节力矩和待辨识参数的线性表达式。选取有限项傅里叶级数作为激励轨迹模型,并使机器人启停时关节角速度和角加速度为零,确保机器人运行平稳。再将线性表达式中观测矩阵的条件数作为优化指标,并结合MATLAB优化工具箱,获得关节最优激励轨迹系数。最后通过递推最小二乘法获得待辨识参数,并代入关节力矩表达式中,并与实际采集力矩值进行比较,确定两者变化趋势。结果表明:实验能够获得较为理想的效果,经过辨识计算所得关节力矩可用作相关领域的动力学控制。     

Impact Toughness of Heat-Affected Zones of 11Cr Heat-Resistant Steels

Aiming at the requirements of structural steel in Gen-IV nuclear reactor, the high-chromium martensitic heat-resistant steels containing 10–12% chromium were developed. The toughness of heat-affected zones(HAZs) is one of the important factors for evaluating the weldability of steels. In this paper, the simulated HAZs were fabricated using tempered SIMP steels. The effects of microstructures on the impact toughness of materials were analyzed using Vickers hardness tester, optical microscope, transmission electron microscope. Experimental results demonstrated that the HAZs of weldment were poor in toughness, much lower than that of the base metal. However, after experiencing post-weld heat treatment, the toughness of the HAZs increased greatly. The toughness became better in terms of CG-HAZ, FG-HAZ and IC-HAZ for the two steels, regardless of as-welded or after PWHT. Compared with SIMP7 steel, chemical compositions, such as C, Si, Mn and Cr, were adjusted to a lower content; the toughness of base metal and simulated HAZs was better in the case of SIMP11. The conjunct roles of dislocation density and carbon contents retained in the martensite led to poor impact toughness of the aswelded HAZs, because dislocations and carbon atoms affected the inner stresses within lattices.     

炭黑及丙烯腈含量对丁腈橡胶磨损机理的影响

丁腈橡胶是一种重要的螺杆泵定子材料.以不同炭黑及丙烯腈含量的丁腈橡胶与45#钢组成的摩擦副为研究对象,采用MPV-600型环-块摩擦磨损试验机,在干摩擦和原油润滑条件下进行摩擦磨损试验,对丁腈橡胶磨损量及摩擦系数进行分析,初步探讨了其磨损机理.从试验结果可以看出,干摩擦条件下丁腈橡胶的磨损量随着载荷的增大而增大,炭黑和丙烯腈含量的增加使丁腈橡胶的耐磨性增强.原油介质下的磨损规律与干摩擦基本相同,丁腈橡胶磨损表面平整,磨损量很小,且原油介质的润滑和冷却作用使橡胶的摩擦系数降低.     

Fe-2.2%Si钢在不同气氛下的高温氧化行为

为了探究硅元素、加热工艺参数(气氛、温度以及时间)对Fe-2.2%Si钢表面氧化铁皮形成过程的复合作用及影响机理,利用热重分析仪(TGA)研究了Fe-2.2%Si钢在600～1 150℃干燥空气和水蒸气条件下的高温氧化行为,利用试验得到的氧化增重数据计算出了氧化速率常数和氧化激活能,并建立了氧化动力学模型。此外,还明确了在2种气氛下氧化铁皮的生长机制与结瘤机理。试验结果表明,在2种不同氧化气氛下得到的氧化增重曲线均符合抛物线规律。当氧化气氛中含有水蒸气时,试验钢的氧化速率提高,氧化激活能降低,导致了试验钢的氧化增重增加。在2种氧化气氛下,试验钢的氧化产物均由氧化铁皮和氧化铁皮与基体界面处的富硅层组成,但在水蒸气下得到的试验钢的富硅层疏松多孔,且氧化铁皮中出现大量的孔洞以及裂纹。其氧化机制主要是氧化铁皮生长过程的生长应力以及水蒸气的存在导致生成的H₂向外扩散,造成氧化铁皮出现大量孔洞和裂纹,为氧化介质的扩散提供了通道,有利于气相物质扩散,进而促进了试验钢的氧化行为。此外,由于在氧化铁皮与基体界面形成的富硅层消耗了基体中的硅元素,导致基体表层出现贫硅区,氧化介质可以通...     

Hydrogen porosity in directionally solidified aluminium–copper alloys: a mathematical model

A combined continuum and stochastic model of diffusion-controlled growth was developed to simulate the formation of porosity during the solidification of aluminium alloys. The whole population of pores was tracked, rather than just the average values. A finite difference solution of the diffusion equations was used, combined with a stochastic model of nucleation. The growth of each individual pore was simulated, assuming the shape to be spherical until it impinged on the developing dendrites, at which point the growth was modelled as a hemispherically capped segmented cone, with the growth radius limited by the liquid space between the dendrites. A previously published model by one of the authors was used to predict the dendritic spacing as a function of the thermal conditions.The model was compared with in situ observations of the formation of porosity during the solidification of aluminium–copper alloys, where the size, distribution and morphological evolution of pores were measured as a function of temperature/time. The predicted development of the porosity, including the distribution in size and morphology, compared well with that observed experimentally. The qualitative agreement between the model predictions and experimental results supports the hypothesis that the effect of hydrogen and its diffusion must be incorporated into any accurate model of pore formation in aluminium alloys.