New insights into microstructural changes during transient liquid phase bonding of GTD-111 superalloy

The effects of joining temperature (T_J) and time (t_J) on microstructure of the transient liquid phase (TLP) bonding of GTD-111 superalloy were investigated. The bonding process was applied using BNi-3 filler at temperatures of 1080, 1120, and 1160 °C for isothermal solidification time of 195, 135, and 90 min, respectively. Homogenization heat treatment was also applied to all of the joints. The results show that intermetallic and eutectic compounds such as Ni-rich borides, Ni−B−Si ternary compound and eutectic-γ continuously are formed in the joint region during cooling. By increasing t_J, intermetallic phases are firstly reduced and eventually eliminated and isothermal solidification is completed as well. With the increase of the holding time at all of the three bonding temperatures, the thickness of the athermally solidified zone (ASZ) and the volume fraction of precipitates in the bonding area decrease and the width of the diffusion affected zone (DAZ) increases. Similar results are also obtained by increasing T_J from 1080 to 1160 °C at t_J=90 min. Furthermore, increasing the T_J from 1080 to 1160 °C leads to the faster elimination of intermetallic phases from the ASZ. However, these phases are again observed in the joint region at 1180 °C. It is observed that by increasing the bonding temperature, the bonding width and the rate of dissolution of the base metal increase. Based on these results, increasing the homogenization time from 180 to 300 min leads to the elimination of boride precipitates in the DAZ and a high uniformity of the concentration of alloying elements in the joint region and the base metal.     

超高强7055铝合金铸锭均匀化工艺优化

采用光学显微镜、扫描电镜、差热分析及透射电镜等手段,对一种航空用超高强7055铝合金大尺寸铸锭的均匀化工艺进行分析与优化。提出分级提高均匀化温度的方法,在确保不发生过烧的情况下,最大限度改善铸锭质量。结果显示:低温预均匀化时,铸锭中析出大量的Al₃Zr相,能够大幅度降低对后续挤压变形的再结晶比例;对铸锭采用400 ℃×4 h+465 ℃×16 h+474 ℃×8 h的分级均匀化处理工艺,能够显著消除铸锭中低熔点相的含量,提高铸锭质量,同时相比于原工艺节省约13 h。新的均匀化处理工艺,不仅提高了生产效率,还能够为后续工艺提供良好的铸锭。     

Computational continua for linear elastic heterogeneous solids on unstructured finite element meshes

The computational continua framework, which is a variant of higher‐order computational homogenization theories that is free of scale separation, does not require higher‐order finite element continuity, and is free of higher‐order boundary conditions, has been generalized to unstructured meshes. The salient features of the proposed generalization are (i) a nonlocal quadrature scheme for distorted elements that accounts for unit cell distortion in the parent element domain and (ii) an approximate variant of the nonlocal quadrature that eliminates the cost of computing positions of the quadrature points in the preprocessing stage. The performance of the computational continua framework on unstructured meshes has been compared to the first‐order homogenization theory and the direct numerical simulation.     

Multi-Scale Analysis of Fretting Fatigue in Heterogeneous Materials Using Computational Homogenization

This paper deals with modeling of the phenomenon of fretting fatigue in heterogeneous materials using the multi-scale computational homogenization technique and finite element analysis (FEA). The heterogeneous material for the specimens consists of a single hole model (25% void/cell, 16% void/cell and 10% void/cell) and a four-hole model (25% void/cell). Using a representative volume element (RVE), we try to produce the equivalent homogenized properties and work on a homogeneous specimen for the study of fretting fatigue. Next, the fretting fatigue contact problem is performed for 3 new cases of models that consist of a homogeneous and a heterogeneous part (single hole cell) in the contact area. The aim is to analyze the normal and shear stresses of these models and compare them with the results of the corresponding heterogeneous models based on the Direct Numerical Simulation (DNS) method. Finally, by comparing the computational time and % deviations, we draw conclusions about the reliability and effectiveness of the proposed method.     

6061铝合金均匀化过程中AlMnSi弥散颗粒的析出尺寸对再结晶行为的影响

利用力学性能测试、光学显微镜、扫描电镜、透射电镜和能谱分析技术等手段研究了均匀化过程中AlMnSi弥散颗粒的析出尺寸对6061铝合金再结晶行为的影响。结果表明:AlMnSi弥散颗粒在均匀化过程中沿着铝基体[001]晶带轴析出长大,晶体结构为体心立方结构。随着均匀化温度的升高,弥散颗粒的析出尺寸逐渐增大,当AlMnSi弥散颗粒平均尺寸由0.07 μm增长到0.42 μm,最大尺寸由0.21 μm增大到1.12 μm时,力学性能则是先升高后降低。6061铝合金T6态组织仍为变形组织;AlMnSi弥散颗粒尺寸增大到1.12 μm,6061合金T6态组织中出现粗大再结晶,力学性能急剧降低。     

Multistage homogenization process of aluminum alloy 2618

Single-, two-, and three-stage homogenization treatments of heat-resistant alloy 2618 were conducted in this study. Results reveal a low melting point Al₂CuMg phase and high melting point Al₂Cu phase in the as-cast aluminum alloy 2618. After single-stage homogenization at 495 °C for 10 h, the Al₂CuMg phase dissolves completely, but the Al₂Cu phase cannot be completely dissolved even once the homogenization time is prolonged to 18 h. After the alloy 2618 are homogenized using two stages: 495 °C for 10 h and 520 °C for 8 h, a portion of the Al₂Cu phase remains in the alloy. The Al₂Cu phase remains undissolved even after prolonged time. After the two-stage homogenization treatment at 495 °C for 10 h and 540 °C for 5 h, the high melting point Al₂Cu phase completely dissolves but overburn occurs. After the alloy 2618 are homogenized using three stages at 495 °C for 10 h, 520 °C for 5 h, and 540 °C for 3 h, the Al₂Cu phase completely dissolves and no overburn is detected. The three-stage homogenization treatment is an effective method for dissolving the high melting point Al₂Cu phase in the alloy 2618 and increasing their overburn temperature and solid solution temperature.     

学科专业目录刍议

学科专业目录制度作为一项教育基本制度，在我国学科发展和人才培养方面发挥着不可替代的作用。但每次目录调整总会产生“专业设置滞后”“同质化培养”“教育资源被稀释”等负面影响。究其原因，一是目录调整周期过长，不能即时响应社会发展；二是执行主体的决定权有限，自主性和积极性受到抑制。为此，主管部门应该不断深化学科目录管理机制改革，加大“放、管、服”力度，将学科专业的设置、审核和“增撤”等决定权更多地赋予高校这个执行主体。     

热处理对GWZK94合金微观组织与力学性能的影响

为了改善铸态GWZK94合金微观组织的不均匀性，使用电阻加热炉在温度505-520 ℃范围内保温8-20 h进行热处理实验。本文采用光学显微镜（OM）,差示扫描量热仪（DSC），X射线衍射仪（XRD），扫描电子显微镜（SEM），能谱仪（EDS）,电子背散射衍射技术（EBSD），万能试验机和维氏硬度计进行微观组织演变及力学性能分析。铸态合金组织主要包括树枝状α-Mg基体，包含亚稳态层错（SFs）的片层结构，共晶相Mg24(Gd, Y, Zn)5，块状长周期有序堆垛结构(LPSO, Mg12(Gd, Y) Zn)和少量的富稀土相。在均匀化处理过程中，片层结构和共晶相Mg24(Gd, Y, Zn)5逐渐溶于基体中，同时块状LPSO相体积分数逐渐减小并伴随有片层状LPSO相像晶粒内部生长，颗粒状相在晶界附近析出。加热温度为520 ℃时出现复熔三角晶界，说明此时发生了合金的过烧现象。经过均匀化处理后，合金的极限抗拉强度（UTS）和合金屈服强度（TYS）表现出了与组织演变规律相同的变化趋势，同属得到了较为均匀的硬度分布情况。最佳的均匀化制度为515 ℃/16 h.     

A CFD-based design scheme for the perforated distributor with the control of radial flow

In the present study, flow homogenization by distributors in chemical apparatus is studied as a process of flow control and its mechanism is reconsidered from the model of resistance to the model of radial flow. This process is composed of four consecutive behaviors: the generation, distribution, conversion of the radial flow and the momentum transfer of axial flow. Based on these flow behaviors, the novel distributor is designed as the combination of perforated plate in the center area and vertical guiding baffles around. Taking the wire-screen catalytic reactor as a case study, numerical simulation is employed to optimize the structure of distributor and a CFD-based design scheme called “flow field analysis scheme” is proposed. Numerical simulation is conducted in the apparatus with a diffuser (inlet D₀ = 500 mm, main part D₁ = 3,000 mm) under the gas velocity of 3.6 m/s (corresponding Re ≈ 12,000). The numerical results from optimized distributor show that compared with the traditional perforated plate, the flow field adjusted by the novel distributor can achieve a better flow uniformity with lower energy consumption. The theoretical analysis and numerical results are also validated and proved by the experimental results.     

应用PMO提高连铸坯生产拉速及成分均匀性

为了实现电炉厂提产增效的目标,研究了脉冲磁致振荡(PMO)在提高铸坯生产拉速中的应用潜能。结果表明,优化PMO处理参数可以适应生产拉速提高的要求,并改善铸坯的中心偏析和宏观偏析。当连铸拉速提至1.05 m/min,PMO脉冲频率为1.16K_f Hz,峰值电压为1.00K_iV处理参数(K_f和K_i分别为仪器的放电频率和电压峰值)时, 铸坯横断面碳偏析指数可控制在1±0.05内;当连铸拉速提至1.15 m/min,PMO脉冲频率为1.28K_fHz,峰值电压为1.00K_iV处理参数时,铸坯横断面碳偏析指数可控制在1±0.07内。试验证明,在高拉速条件下通过保证单位时间内对钢液施加的脉冲能量可以使连铸坯质量得到有效改善。