Non-keyhole Friction Stir Welding for 6061-T6 Aluminum Alloy

A novel non-keyhole friction stir welding technique was proposed to weld the butt joint of 6061-T6 aluminum alloy with the thickness of 6 mm. A sound joint was obtained by this technique, simultaneously eliminating the flash, shoulder mark and keyhole defects. The sleeve directly affected zone (SDAZ) and the sleeve indirectly affected zone (SIAZ) were divided into the joint according to the plunging position of the hollow sleeve. The lack of root penetration defect was avoided when the plunging depth of the hollow sleeve was only 4.2 mm, because the hollow part inside the sleeve improved the material flow below the sleeve. An S-shaped line was left at the SIAZ, and the height of it had the minimum value of 1.47 mm at 20 mm/min. Whether the failure location of the joint was in SIAZ/SDAZ or the heat-affected zone (HAZ) depended on the height and bonding strength of the S-shaped line. The joint fracture location changed from the SIAZ/SDAZ at 35 mm/min to the HAZ at 20 and 30 mm/min. The maximum tensile strength of 224.3 MPa was obtained at 30 mm/min which was 73.7% of that of the base material. The fracture surface morphology exhibited the typical ductile fracture.     

Alkali catalyzed liquefaction of corncob in supercritical ethanol–water

Corncob liquefaction in supercritical ethanol–water was performed with and without the addition of an alkali catalyst by direct addition or biomass impregnation in a 250-cm³ batch reactor. The effects of temperature, solvent and alkali addition on the biomass conversion level and oil yield were investigated to find the optimum condition. For non-catalytic liquefaction using a 1:1 (v/v) ethanol: water ratio, a maximum oil yield and conversion level of 49.0% and 93.4%, respectively, were obtained at 340 °C. For alkali catalytic liquefaction, the oil yield with KOH addition (57.5%) was higher than that from KOH-impregnated corncob liquefaction (43.3%). The oil from liquefaction with KOH addition had higher heating value (26.7–35.3 MJ kg⁻¹) than the corncob (19.1 MJ kg⁻¹). The dominant components of the obtained oil were found by GC/MS analysis to be aldehyde, ester, phenol derivatives and aromatic compounds.     

Fatigue performance of Friction Stir Welded titanium structural joints

The Friction Stir Welding process for producing corner and T-joints out of 6 mm Ti–6Al–4V was developed in this effort using previous work on butt weld joints as a starting point. A limited number of corner joints were also subjected to a bending fatigue test to preliminarily assess the applicability of the process in producing fatigue critical structures. These results were also compared to predictions made by applying stress concentration factors to previously generated uniaxial butt joint test data. While additional testing is still required to obtain a higher degree of confidence in the conclusions of this study, it was found that the performance of these corner joints in fatigue could be compared to butt joint data when a geometrically based stress concentration factor is applied. Furthermore, these welded joints possessed equivalent fatigue performance relative to identical test specimens machined from wrought product forms, both bar and extrusion. Thus, from the perspective of fatigue design, this study has shown that Friction Stir Welding is able to produce structures with the same performance as currently made from wrought materials.     

Molecular dynamics simulation on the tribology properties of two hard nanoparticles (diamond and silicon dioxide) confined by two iron blocks

The tribology behaviors of diamond and silicon dioxide (SiO₂) nanoparticles were examined via molecular dynamics simulations; four cases were simulated. At low velocity and low load, the nanoparticles separated the two blocks from each other and acted as ball-bearings. The plastic deformation, temperature distribution, and friction force were all improved due to the action of the nanoparticles. However, the crushing of the SiO₂ nanoparticles was accompanied by deformation-induced loss of the rolling effect, when the load was increased. Without nanoparticles, a transfer layer formed at high velocity and low load. The two nanoparticles provided support for a certain duration. However, at high velocity and high load, the support effect of these nanoparticles was lost in a short sliding time.     

一种无指导的子主题挖掘方法

为了解决用户查询经常存在表意模糊或歧义性等问题,明确用户的查询意图,该文提出了一种无指导的子主题挖掘方法。该方法首先在检索结果文档集中利用ATF × PDF模型挖掘候选主题词;其次,为保证子主题的多样性,该文基于HowNet语义相似度方法对候选主题词进行了层次聚类分析,进而得到潜在主题;最后,利用LCS算法生成多样性子主题。实验结果显示,系统平均D#-nDCG@10达到0.573,结果说明该方法在明确查询主题表意方面取得了较好效果。     

Densification,microstructure, elastic and mechanical properties of reactive hot-pressed ZrB2–ZrC–Zr cermets

In this study, cermets composed of zirconium diboride and zirconium carbide with intergranular zirconium were sintered by reactive hot-pressing. Relative density exceeding 97% was obtained for the sintered cermets having four distinct compositions varying in concentration of excess Zr. Their densification behaviour was examined by monitoring displacement during sintering. The microstructure was characterized by scanning electron microscopy and X-ray diffraction, and the elastic and mechanical properties were evaluated at room temperature. The effects of Zr concentration on the densification and mechanical properties were assessed. The ZrB₂ and ZrC micron-grains coarsened with increasing amount of Zr starting material. In addition, the cermets exhibited high flexural strength (546–890 MPa) and fracture toughness (6.63–10.24 MPa m^1/2), which simultaneously increased with increasing Zr concentration. However, the elastic moduli and hardness (11–18 GPa) decreased with increasing Zr. The shear modulus and Young's modulus were in the range of 150–190 GPa and 360–440 GPa, respectively.     

Sc添加对铸造Al-3Cu-1Li合金在酸性NaCl溶液中腐蚀行为的影响

在2 mol/L NaCl酸性溶液中,研究少量Sc对铸态Al-3Cu-1Li合金腐蚀行为的影响。通过XRD对铸态Al-3Cu-1Li-xSc（质量分数）合金进行相分析,用FE-SEM/EDS和TEM观察了Al-3Cu-1Li合金和Al-3Cu-1Li-0.5Sc合金的显微组织;采用极化曲线、电化学阻抗谱（EIS）、电化学噪声（EN）和浸泡法对合金的腐蚀行为进行了研究,并探讨腐蚀机制。结果表明：含Sc的Al-3Cu-1Li合金晶内富Cu球形相减少,晶界析出W相。随Sc含量的增加,合金腐蚀反应的活化能降低,阴极析氢速率增加,自腐蚀电流密度增加,合金的耐蚀性降低。添加Sc后,合金在空气中形成的表面膜保护作用减弱至消失,使含Sc合金的阻抗谱低频感抗弧逐渐消失。在EN测量期间,不含Sc和含0.1%Sc的合金表面处于“膜破裂-再钝化”状态;含0.3%Sc和0.5%Sc的合金发生严重的局部腐蚀（晶间腐蚀）,且与W相有关。     

质子酸处理二维碳化钛/碳纳米管复合材料的电磁屏蔽性能与机械性能

通过质子酸胶体处理,实现了碳化钛Ti₃C₂T_x的制备。此外,使用单壁碳纳米管(SWCNT)作为增强成分,提升了质子酸处理碳化钛(H-MXene)的机械性能——不仅保持了电磁屏蔽性能,而且将拉伸性能提升了近400%。结果表明,H-MXene和碳纳米管具有作为高机械性能电磁(EMI)屏蔽复合材料的潜力。     

工艺参数对钛合金激光熔覆 CBN 涂层几何形貌的影响

目的获得制备形貌较佳的CBN激光熔覆层的工艺参数。方法以CBN粉末为熔覆材料,在TC11钛合金表面制备CBN熔覆层。设计正交试验,利用金相法检测熔覆层的几何形貌参数,研究工艺参数(激光功率、扫描速度、离焦量、预置层厚度)对涂层几何形貌的影响规律。结果随着激光功率、扫描速度、离焦量和预置层厚度的增大,熔覆层宽度、高度以及熔池深度都发生相应的改变。其中扫描速度对熔覆层形貌的影响最大,其次为激光功率和预置层厚度,离焦量的影响最小。随着激光功率增大,熔覆层宽度先增大后减小,熔覆层高度逐渐降低,熔池深度逐渐增大。扫描速度、离焦量和预置层厚度的增加都导致熔覆层宽高和熔池深度的减小。结论最优的工艺参数为:激光功率1400W,扫描速度4mm/s,离焦量35mm,预置层厚度0.4mm。     

基于切削声信号与优化SVM的刀具磨损状态监测

提出了一种利用切削声实现刀具磨损状态多特征监测的方法。根据经验模态分解与Hilbert变换理论,提取切削声信号的内禀模态能量与不同频段的Hilbert谱能量作为监测信号的备选特征。采用支持向量机作为分类器,针对备选特征的有效筛选问题,利用多种群遗传算法对分类器的输入特征进行了优化,剔除备选特征中的干扰特征,利用多种群遗传算法对分类器的模型参数进行了优化。利用优化后的分类器对测试样本进行分类,并与优化前的分类结果进行了对比。结果表明,优化后分类器的分类性能得到了明显提升,该方法可以对刀具磨损状态进行有效识别。