Interfacial structure and joint strengthening in arc brazed galvanized steels with copper based filler

1 Introduction Arc brazing is a brazing method that the specimens are heated by arc[1]. It is the generic terms of GMA brazing and GTA brazing[2], and has both the characteristics of brazing and welding. Compared with traditional arc fusion welding, the s…     

Ti40Zr25Ni15Cu20非晶钎料钎焊Si3N4陶瓷的界面结构及强度

采用Ti40Zr25Ni15Cu20非晶钎料进行了Si3N4陶瓷真空钎焊连接,利用SEM、EDX等微观分析手段,研究了钎焊界面的微观结构,得出界面反应层有两部分组成,接头界面微观结构为Si3N4/TiN/Ti-Si,Zr-Si化合物/钎缝中心;在相同钎焊工艺条件下,研究对比了晶态和非晶态钎料钎焊接头的强度,发现非晶态钎料钎焊的接头强度大大超过用晶态钎料钎焊的接头.     

Ti/Fe EUTECTIC REACTION PROCESS

To obtain satisfactory joining between titanium alloys and stainless steel, it is of great significance to study the process of Ti-Fe eutectic reaction. The preliminary studies about the liquid forming, spreading and the structure of the contact reaction between pure Ti and pure Fe are carried out at 1 120℃for different holding times in high-frequency induction heated vacuum furnace. The results show that for Fe(particle-like)/Ti(plate-like) contact reaction couples, owing to Fe's higher density than titanium's, the Fe/Ti eutectic reaction develops in the thickness direction of plate-like Ti, named "deposition effect". Then the eutectic reaction can be realized without the inter-diffusion between Ti and Fe atoms, so the formation rate of the liquid phase is very quick. For Ti (particle-like)/ Fe (plate-like) contact reaction couples, the eutectic reaction has a strong direction, i.e. the eutectic liquid is formed mostly at the Ti side, which leads to much dissolution of Ti particle and very little dissolution of the Fe base metal. It can be seen that the diffusion rate of Fe in Ti determines the whole process of the eutectic reaction, behaved as that the dissolution rate of Ti (particle-like)/ Fe (plate-like) is much lower than that of Fe(particle-like)/Ti(plate-like), and the dissolution rate of Ti obeys the square root law. Whatever means the Ti, Fe are assembled by, only dissolution process can be founded, the spread phenomena is unobvious, which is concerned with the formation of the intermetallic compounds and the "deposition effect" of Fe.     

Effect of nano-Al2O3 reinforcement on the microstructure and reliability of Sn–3.0Ag–0.5Cu solder joints

Nanoparticle reinforced lead-free solder has previously been studied by several investigators, but few studies have evaluated its reliability. In this study, resistor chip (RC) micro joints were soldered using nano-Al₂O₃ particle reinforced Sn–Ag–Cu solder paste. The microstructure and reliability of RC micro joints having different nano-Al₂O₃ contents (0, 0.25, 0.5 and 1.0 wt%) were investigated in detail. More than 40 solder joints for each condition were made and examined in order to achieve reliable data. The results indicated that nano-Al₂O₃ particles refined the β-Sn grain size and enlarged the eutectic area of the micro solder joints. Those nanoparticles also reduced the IMC thickness of the Ni-solder and Cu-solder interfaces. Those effects can be attributed to the poor-wetting behavior of nano-Al₂O₃ particles. The nano-Al₂O₃ reinforcement mainly enhanced the reliability of the micro solder joints, but did not affect the strength of as-soldered joints obviously. The improvement of reliability was proportional to the nano-Al₂O₃ content. The microstructure and fracture analysis indicated that the reinforcement and stability of Ni-IMC and Cu-IMC interfaces accounted for better reliability.     

Physicochemical and thermo physical characterization of CaO–CaF2–SiO2 and CaO–TiO2–SiO2 based electrode coating for offshore welds

This paper investigates the physicochemical and thermo-physical properties of CaO–CaF₂–SiO₂ and CaO–TiO₂–SiO₂ based electrode coating for welding offshore structures. Twenty-one electrode coating compositions have been formulated using extreme vertices design method. The coating was crushed to powder form. The powder was characterized for weight loss, density, specific heat, enthalpy, thermal conductivity, diffusivity, and specific heat. Coating's structural analysis was done using X-Ray Diffraction and Fourier transformation. X-Ray Fluorescence, Thermogravimetric Analyzer, and Hot disc have been used to characterize the coating mixture. The regression analysis has been used to study the effect of individual constituents and their binary, tertiary interactions on the properties. The obtained output of properties has been optimized using multi-response optimization.     

Electronic transport in composites of graphite oxide with carbon nanotubes

We show that the presence of electrically insulating graphite oxide (GO) within a single wall carbon nanotube (SWCNT) network strongly enhances electrical conductivity, whereas reduced graphite oxide, even though electrically conductive, suppresses electrical conductivity within a composite network with SWCNTs. Measurements of Young’s modulus and of Raman spectra strongly support our interpretation of the “indirect” role of the oxide groups, present in GO within the SWCNT-GO composite, through electronic doping of metallic SWCNTs.     

Fast pressureless solid-state reaction sintering of highly infrared transparent MgAlON ceramics from MgAl2O4 and AlON powders by two-step heating

A two-step heating strategy was proposed to fabricate transparent MgAlON ceramics by solid-state reaction of MgAl₂O₄ and AlON powders via pressureless sintering. By dwelling 60 min at 1700 ℃ followed by 150 min at 1880 ℃, highly infrared transparent MgAlON ceramics with transmittance up to 80.4 % were successfully fast prepared. The phase transformation and microstructure evolution during heating from 1400 ℃ to 1800 ℃ and dwelling at 1700 ℃ for 0–90 min was thoroughly studied to reveal the solid-state reaction and densification mechanism of MgAlON by two-step heating. Surprisingly, it was found that the grown grains could break during dwelling at 1700 ℃. This secondary massive fragmentation of grown grains resulted in the minimized grain size and improved moveability of grains, which in turn prompted fast and high densification with pore free in the following sintering step. The grain breakage at 1700 ℃ could be attributed to the decomposition of AlON and formation of MgAlON.     

Multi-objective optimization of shield construction parameters based on random forests and NSGA-II

Metro shield construction will inevitably cause changes in the stress and strain state of the surrounding soil, resulting in stratum deformation and surface settlement (SS), which will seriously endanger the safety of nearby buildings, roads and underground pipe networks. Therefore, in the design and construction stage, optimizing the shield construction parameters (SCP) is the key to reducing the SS rate and increasing the safe driving speed (DS). However, optimization of existing SCP are challenged by the need to construct a unified multiobjective model for optimization that are efficient, convenient, and widely applicable. This paper innovatively proposes a hybrid intelligence framework that combines random forest (RF) and non-dominant classification genetic algorithm II (NSGA-II), which overcomes the shortcomings of time-consuming and high cost for the establishment and verification of traditional prediction models. First, RF is used to rank the importance of 10 influencing factors, and the nonlinear mapping relationship between the main SCP and the two objectives is constructed as the fitness function of the NSGA-II algorithm. Second, a multiobjective optimization framework for RF-NSGA-II is established, based on which the optimal Pareto front is calculated, and reasonable optimized control ranges for the SCP are obtained. Finally, a case study in the Wuhan Rail Transit Line 6 project is examined. The results show that the SS is reduced by 12.5% and the DS is increased by 2.5% with the proposed framework. Meanwhile, the prediction results are compared with the back-propagation neural network (BPNN), support vector machine (SVM), and gradient boosting decision tree (GBDT). The findings indicate that the RF-NSGA-II framework can not only meet the requirements of SS and DS calculation, but also used as a support tool for real-time optimization and control of SCP.     

SnAgCu/SnAgCuCe焊点的显微组织与性能

针对SnAgCu和SnAgCuCe两种无铅焊点,研究焊点内部组织、力学性能及热疲劳特性。研究结果表明,稀土元素Ce的加入可以提高焊点的力学性能,稀土元素的添加可以使SnAgCu焊点拉伸力提高近12.7%。稀土元素的添加细化SnAgCu基体组织,同时减小金属间化合物颗粒(Cu6Sn5和Ag3Sn)的尺寸,这是SnAgCuCe焊点力学性能提高的主要原因。拉伸断裂后的扫描电镜分析表明,两种焊点的断裂呈现明显的韧性断裂特征。另外在温度循环载荷下,稀土元素可以显著提高SnAgCu焊点的疲劳寿命。基于有限元模拟发现SnAgCuCe的抗蠕变性能显著高于SnAgCu焊点。     

Comparative studies on microelectronic reliability issue of Sn whisker growth in Sn-0.3Ag-0.7Cu-1Pr solder under different environments

In this study, comparative studies on Sn whisker growth in Sn-0.3Ag-0.7Cu-1Pr solder under different environments were conducted to investigate factors like ambient temperature, oxygen level, and 3.5 wt% NaCl solution on whisker growth. The experimental results revealed that ambient temperature and oxygen level are two important factors that could determine the oxidation rate of PrSn₃ phase, thus indirectly affecting the growth rate of Sn whiskers. In addition, mechanisms of whisker growth under these three environments were established from the perspective of atom diffusion based on the “compressive stress-induced” theory. Although whiskers under different environments were all squeezed out from Pr oxides (hydroxides), the forms of their driving forces were different. For whiskers squeezed out in air whether at room temperature or 150 °C, the driving force is the compressive stress produced by lattice expansion due to the oxidation of PrSn₃ phase. The representative example was whiskers' growth at 150 °C, which could be simplified as three stages: (1) squeezing out, (2) cracking and (3) bursting out. For whisker growth in 3.5 wt% NaCl solution, the driving force for much fewer whiskers' growth was proposed to come from lateral stress provided by interfacial IMC layer growth. Moreover, Sn nanoparticles and their agglomerations were also found to form under the driving force of the potential difference between Sn atoms and Sn crystals. Their morphologies could also be affected by factors of ambient temperature, oxygen level and Cl⁻ ions in corrosive liquid.