Papers by Keyword: Void

Abstract: This paper proposes, for the first time, that the gas pressure loading path significantly influences the weld interface ratio of SPF/DB four-layer sandwich structures. Based on a gas pressure loading curve incorporating back pressure, experimental verification and analysis were conducted. Ultrasonic C-scanning, metallographic examination, and scanning electron microscopy (SEM) were employed to observe and analyze unwelded defects, elucidating their causes and formation mechanisms. Key process parameters—including back pressure time, back pressure value, and gas inlet delay time—were extracted and defined. The influence of these parameters on the weld interface ratio of four-layer sandwich structures was systematically investigated. Finally, with the objectives of eliminating surface grooves and achieving a high weld interface ratio, reference ranges for these process parameters are provided.

Abstract: 3C-SiC films have been grown on [100] n-doped Si substrates in a horizontal cold wall CVD reactor. Without the use of plasma enhancement, the precursors silane and propane are used to deposit silicon carbide films at T < 1200°C. The structure of the grown films has been investigated via FESEM, XRD and Raman spectroscopy. It has been found that the growth rates are between 200 and 300 nm/h. Additionally, structural analysis give evidence of polycrystalline phases. Reasons for that could be insufficient cracking of the precursors and homogenous nucleation of Si species in the gas phase.

Abstract: In general, a flux is used to braze a copper alloy. In many cases, when the molten brazing filler metal spreads in the set joint gap, vaporised flux and its residue are produced, and defects (mainly voids) are formed. Voids, which are formed on the brazed layer, cause deterioration in the strength and other properties. However, with conventional evaluation methods (e.g. ultrasonic or X-ray radiography tests), the behaviour of the molten brazing filler metal during the brazing process cannot be visually observed from the outside of the joint. Therefore, the void formation process cannot be clarified. To improve the quality of the brazed layer, it is necessary to elucidate the mechanism of void formation. The purpose of this study is to observe the behaviour of the molten brazing filler metal by performing an X-ray radiography test at the same time as brazing and to study how to reduce voids. In this study, a brass specimen was brazed with a Cu–P-based brazing filler metal. The specimen was brazed by heating in an electric furnace, and the specimen was irradiated with X-rays. The state where the molten brazing filler metal spread into the gap was photographed as the transmission image. Thereafter, the behaviour of the molten brazing filler metal was analysed.

Abstract: Automobiles are equipped with EGR (Exhaust Gas Recirculation) coolers to improve fuel economy and exhaust gas suppression performance. Inside the EGR cooler, the moisture in the gas is condensed by cooling the hot exhaust gas. This condensed water is highly corrosive because sulfur oxides dissolve. Therefore, stainless steel and Ni-based brazing metal having excellent corrosion resistance are used for the EGR cooler.Until now, stainless steel has been brazed under a vacuum atmosphere. However, there are increasing opportunities to braze stainless steel in an inert atmosphere gas at atmosphere for cost reduction and mass production. In this case, a paste-type brazing filler metal consisted of a powder brazing filler metal and a binder is used. As is well known, a debinding process that volatilizes the binder is needed. From previous research in this laboratory, it is clarified that the binder causes voids. In addition, it is said that the size and location of voids generated at the brazed joint affect the product performance. On the other hand, the detailed investigation about the influence which the installation position of a paste type brazing filler metal on the void formation process has not yet been made. Therefore, in this study, the arrangement method and influence on heating rate and debinder temperature on void formation were investigated by X-ray CT.

Abstract: It is well known that magnesium (Mg) shows anisotropic fatigue behavior. However, the fatigue mechanisms have yet to be elucidated. The relationships between crystal orientations and crack initiation behavior in Mg single crystals were investigated by uniaxial tension-compression fatigue tests. Three types of round-bar specimens were prepared. The lording direction of AD, BC and EF specimen were [110], [100] and [0001], respectively. Fatigue tests were carried out with the stress ratio R=-1 and the frequency of 10Hz at room temperature in laboratory air. At stress amplitude (σ_a) over 40 MPa, fatigue lives of BC specimen and EF specimen were the longest and shortest. However, at σ_a =20 MPa, the fatigue life of all specimens were almost the same. It was found that fatigue lives of Mg single crystals strongly depend on crystal orientations and stress.

Abstract: In this work we have studied the carbonization of 3C-SiC on misoriented Si substrates, using different thermal ramp rates and shapes. We observed that the heating rate (°C/sec) from carbonization temperature to film growth temperature plays a major role in controlling the void density. Moreover, void formation can be eliminated by the introduction of silane at different temperatures during the heating ramp. The studies were performed on a small research reactor and the results were successfully transferred to a production scale reactor, aimed to the production of 3C-SiC power devices manufactured on 100 and 150 mm Si substrates.

Abstract: As the world is moving towards green manufacturing, there is an increasing demand for the use of clean and environmentally friendly binder systems in metal injection moulding (MIM) industry. One example of these developed binders is polyethylene glycol (PEG) - polymethyl methacrylate (PMMA) based system. We have systematically evaluated and optimized this binder system, and reported some interesting new results. In this article, we reported the effect of PEG molecular weight on rheological properties of the feedstock and its water debinding behaviour. We also investigated the effects of different surfactants on MIM feedstock rheological and mechanical properties, and identified a potential surfactant that enhances compatibility between the binder components and metal powders. Furthermore, we reported an interesting problem – ‘voids formation’, which is associated with PEG crystallization. To minimize this void formation a crystallization inhibitor is incorporated in the PEG/PMMA system, thereby eliminating the void formation while maintaining the clean nature of this system. This paper is concluded with some new thoughts with regard to binder design.

Abstract: On the basis of the GPR method principle, data acquisition and processing, this paper mainly discusses two-dimensional GPR profiles and three-dimensional GPR images through GPR detection example about the voids of highway bridge head subgrade and the grouting distribution. Three-dimensional images accurately and directly evaluate the grouting effect. Thus it illustrates that GPR used to detect the voids of highway bridge head roadbed and the grouting distribution is an effective method.

Abstract: Elastic fields, generating by defects of the structure, influence the diffusion processes. It leads to the alteration of the phase transformation kinetic. One of the chief aims of our work is to obtain general equations for the diffusion fluxes under strain that give the possibility for using these equations at low temperatures, as in this case the strain influence on the diffusion fluxes is manifested in maximal degree. Our approach takes into consideration, that the strains can alter the surrounding atom configuration near the jumping one and consequently the local magnitude of the activation barrier and a rate of atom jump. The rates of atom jumps in different directions define the flux density of the vacancies. Now we take into account, that strain values are different in the saddle point and in the rest atom position, in differ from our consideration that was done by us earlier. As a result in the development of our approach the general equations for the vacancy fluxes are obtained for fcc and bcc metals. In our paper we discuss the main features of the theory of diffusion under stress and its applications. In particular we examine how elastic stress, arising from nanovoids, influence the diffusion vacancy fluxes and the growth rate of voids in metals.

Abstract: In order to research and master the effect of surface topography on average film thickness between the end faces for mechanical seals, average film thickness fractal model between the end faces for mechanical seal was established using fractal parameters charactering surface topography characteristics, based on contact fractal model of the end face for mechanical seal, and solving micro-void volume. Effect of surface topography fractal parameters on average film thickness between the end faces for B104a-70 mechanical seal was analyzed by theoretical calculation. The results showed that fractal dimension D and characteristic length scale G of the end face had a great influence on the average film thickness h₀, and h₀ decreased with the increased of D or decreased of G; h₀ decreased rapidly with the increased of D or with the decreased of G when the end face was coarser, however, it decreased slowly with the increased of D or with the decreased of G when the end face was smoother. In normal working parameters and surface topography fractal parameters range, average film thickness between end faces was in the range of 0.27~1.7μm. Studying on the effect of the change of surface topography on average film thickness has an important significance on the predicting of operating characteristics of end faces during actual operating and the design of end faces for contact mechanical seal.