Study on crosslinking of polymers and joining mechanism of SiC ceramic

ABSTRACT

Based on crosslinking of polymers with different vinyl contents at low temperature and pyrolysis of the polymer at high temperature, joining temperature and impregnation cycles of an SiC joint were discussed. Polyvinylphenylsiloxane as the polymer with active groups of Si–OH and CH=CH₂ by crosslinking enhances ceramic yield and thermal stability. The microstructure of the polymer changes from amorphous ceramic into grains of SiO₂ and SiC, grains can dispersion strengthening enhances strength of joint layer. Shear strength of SiC joints reaches the maximum at 1200°C. A relatively good interface between the SiC substrate and pyrolysis product of the polymer is formed, but there exist loose cracks or voids in the joint layer which affect the shear strength of the joint. The shear strength of the SiC joint reaches 69.2?MPa through seven times of vacuum impregnation/pyrolysis enhancement. According to the microstructure and properties of the SiC joint, the pyrolysis mechanism of the joining layer as part of the joint by using polyvinylphenylsiloxane is explained.     

放电等离子技术连接铬酸镧陶瓷

铬酸镧陶瓷脆性高、难以变形,使其在加工上存在难度。利用陶瓷连接技术可以将小部件连接为大部件,实现低成本制造形状复杂的大型陶瓷构件。以铬酸镧的前驱体料浆作为中间层材料,利用放电等离子技术进行了铬酸镧陶瓷的连接,测定了连接件的连接强度,并借助SEM、EDS等手段分析了连接温度及保温时间对连接件强度的影响。结果表明,连接温度为1400℃时,连接件可以获得最高的连接强度,连接过程中母材中出现第二相Cr4O5。     

Dissimilar Joining of AISI 304L/St37 Steels by TIG Welding Process

In present work the weldings of an austenitic stainless steel （AISI 304L） and a ferritic carbon steel （St37） were conducted by tungsten inert gas （TIG） welding process using four different austenitic filler metals, namely ER308L, ER309L, ER316L and ER310. Microstructure characteristics and mechanical properties of the weldments were studied using optical and scanning electron microscopy, ferrit-ometry, hardness, tensile and impact tests. The ferrite number （_N-~） of the weldments made by different electrodes varies between 0.5 and 9.5. It was found that the increase in amount of delta ferrite in the microstructure of the weld metals, causes the decrease of the impact toughness of the weldments. It seems that using ER309L and ER316L electrodes can provide a good combination between the mechanical and metallurgical properties of the joint in AISI 304L/St37 dissimilar welding.     

Joining of SiC by Al infiltrated TiC tape: Effect of joining parameters on the microstructure and mechanical properties

SiC ceramics were successfully joined by Al infiltrated TiC tapes at 900-1100 °C for 0.5-2 h in vacuum. Phase constituents, microstructure and mechanical strength of the prepared SiC joints were characterized. The prepared SiC joints display dense interlayer and crack-free interface. The interlayer primarily consists of TiC and Al phases, together with small amount of TiAl₃ and trace of Al₄C₃. With increasing the joining temperature or time, the interface layer either thickens or grows to multiple layers. The bending strengths of the SiC joints are higher than 190 MPa as bonded at present conditions, and are closely related with the property of interface and interlayer.     

重型钢丝缠绕预应力剖分-坎合结构概述

重型整体结构转变成螺栓预应力剖分-组合结构再发展到钢丝缠绕预应力剖分-坎合结构是科学、技术、工程发展的结果,预应力技术是推动这一发展的关键因素.从预紧件的许用应力、被预紧件的应力波动比、预紧系数、预紧刚度、剩余顶应力、疲劳抗力、技术经济等多个角度系统分析这几种结构的优缺点和应用背景.着重指出重型螺栓预应力剖分-组合结构的原理性缺点,由于预紧件许用应力的限制,在重型和超重型结构应用中难以获得高的力学指标,预紧件须占据很大的空间且无法对被预紧件进行法向应力保护.详尽对比分析钢丝缠绕预应力剖分-坎合结构力学特性及应用于重型结构的优势,可提供高可靠性预紧力,被预紧件上的剩余压应力与预应力之比可达到50%左右,对防止子件间剪切错移十分有利.通过给出重型预紧结构设计参数优选表,为重型预应力结构的设计提供了科学的参考.     

An integrated assembly method of sandwich structured ceramic matrix composites

Sandwich structured composites have been widely studied and applied at ambient temperature in aeronautical, automobile and naval applications. For high temperature applications, an integrated ceramic sandwich structure could take advantage of multiple functions such as skin stiffness and core insulation. For thermo-structural applications, skins must be made of ceramic matrix composites (CMC) because of their strength, their resistance to high temperatures (beyond 1000 °C), and their low densities. Concerning foam cores, some carbides (e.g. SiC) are, for their outstanding thermo-mechanical properties, the most appropriate. These foams can withstand long oxidative exposing conditions with low material degradation. This paper presents an assembly method of SiC based sandwich structured CMC. It is performed during sandwich manufacturing in an integrated fashion and allows the production of complex shapes at low costs. Produced flat sandwich panels, characterized by three point bending tests, showed a marked toughening behaviour.     

Process robustness of single lap ultrasonic welding of thin, dissimilar materials

Fusion welding processes, such as resistance welding and laser welding, face difficulties in welding thin layers of dissimilar materials. Ultrasonic welding overcomes many of these difficulties by using high frequency vibration and pressure to input energy into the affected area to create a solid state weld. This paper presents a process robustness study of ultrasonic welding of thin metal sheets. Quality of the welded joints is evaluated based on mechanical tests and the quality criterion is then applied to evaluate the weldability. These results were used to determine both the optimal weld parameters and the robust operating range.     

Laser direct joining of AZ91D thixomolded Mg alloy and amorphous polyethylene terephthalate

Lap joints between AZ91D thixomolded Mg alloy and amorphous polyethylene terephthalate (PET) were produced by direct irradiation of high power diode laser beam from either plastic or metal side. Joints with strength higher than that of PET could be successfully produced. Joining mechanism involves the generation of gas bubbles in a narrow region inside PET specimen adjacent to the interface. The pressure induced by expansion of these bubbles secures tight bonding in the micro size between AZ91D and PET specimens. Discrete bubbles morphology associated with metal-side laser-irradiation promoted higher joint strength in comparison with networked wormhole morphology in the case of plastic-side laser-irradiation. The presence of pre-made pits on the AZ91D specimen surface proved to be effective to the improvement in the performance of plastic-side laser-irradiated joints.     

Combustion joinining of carbon/carbon composites by a reactive mixture of titanium and mechanically activated nickel/aluminum powders

Combustion joining of carbon/carbon (C/C) composites using a mixture of titanium and mechanically activated Ni/Al powders as a reactive medium is reported. A minimum preheating of the sample stack to 630 K is required to initiate the joining process. A robust crack- and pore-free joint layer (∼75−100 μm in thickness), which is composed of NiAl_x and TiC_y(O_z) phases, is produced. Tensile-strength testing of the joined C/C composites shows that the fracture does not occur along the joint layer.     

Thermal characterisation of ceramic/metal joining techniques for fusion applications using X-ray tomography

This work investigates the thermal performance of four novel CFC–Cu joining techniques. Two involve direct casting and brazing of Cu onto a chromium modified CFC surface, the other two pre-coat a brazing alloy with chromium using galvanisation and sputtering processes. The chromium carbide layer at the interface has been shown to improve adhesion. Thermal conductivity across the join interface was measured by laser flash analysis. X-ray tomography was performed to investigate micro-structures that might influence the thermal behaviour. It was found that thermal conductivity varied by up to 72%. Quantification of the X-ray tomography data showed that the dominant feature in reducing thermal conductivity was the lateral spread of voids at the interface. Correlations were made to estimate the extent of this effect.