Microstructural Characteristics And Transformation Mechanism In Dissimilar Metal Welds Of Composite Parts

Along with the development of new materials, new technologies and new equipment, the dissimilar metal welding is widely used in variant engineer machinery. This kind of joints not only can fully utilize the excellent properties of different materials and meet the actual usage, but also can save costs and make great economic profits. The service life of composite parts depends greatly upon the properties of welded joint and welding conditions. Hence, it is very important to study of formation mechanism and transformation of the microstructures. However, there are still many features not clear. In the dissimilar metal joint, there is a transition region (or weld bond) which is a heterogeneous region in microstructures, compositions, and physical and mechanical properties. These characteristics determine the service life of the joint. Hence, the study of weld bond becomes a very important project. In the present work, the microstructure, chemical composition, Fractography and mechanical property, and their transformations and behaves under high temperature service in the different kind of dissimilar metal welded joints were examined systematically by using scanning electron microscopy(SEM), transmission electron microscopy(TEM), optical microscope(OM), mechanical tests and theoretical calculation, techniques.This dissertation consists of nine chapters. Chapter One is an introduction, in which the origin and supporter of the project, significance and the major author's work are introduced briefly. The development and present research situation in the world about the dissimilar steel welding, non-ferrous metal — steel friction welding and non-ferrous metal — steel explosive welding etc. are reviewed. The further development of these research are also discussed.Welding and electron microscopy are two different branch of science subjects. The task of the present research work is to study the microstructure and its transformation and behavior of the welded joints by using mainly electron microscope techniques. In order to reflect this combination, in Chapter Two the fundamental knowledge of dissimilar welding and electron microscopy are introduced briefly.The selection of the typical dissimilar welded joints and their specimen preparation of electron microscopes are two important prerequisites for the researching of microstructure and its transformation. In Chapter Three, the welding materials, welding condition, post-welding heat treatment, high temperature service etc. of the joints are presented. The preparation procedure of the TEM foil specimen and the non-standard Charpy impact specimen of the welded joint which were firstly used in this work were also given in this Chapter in detail.In Chapter Four, the microstructures and its transformation in as-welded and high temperature treated states were analyzed. The microstructural variation around theweld bond, microstructural changes under high temperature treatment or service, weld interface, deformation twin and low-melted eutectic film in the austenitic weld metal zone etc. were observed directly and firstly in the different characteristic zone of the welded joint in the present work by using TEM mainly. The effect of carbon content of the base metal on the welded bond structure, formation mechanism of the "austenitic-rich zone" and "hydrogen-induced disbonding" etc. were also studied systematically. In Chapter Five, the diffusion process of substitutional atom, such as Cr, Ni etc. and interstitial atom, such as carbon(C) etc. was calculated theoretically and examined experimentally. The diffusion formulas were derived under the assumptions which were more appropriated to the practical conditions. In Chapter six, the morphology of the fracture surface of the dissimilar austenitic-pearlitic steel welded joints, the effect of the base metal carbon content and post-welded heat treatment on the fractography of the bond region were analyzed systematically by using our non-standard Charpy impact samples.In Chapter Seven, the microstructure feature and its relationship with the welding conditions and mechanical properties of the aluminum based MMC — AISI304 austenitic stainless steel friction joints were examined. In the joint zone, the direct TEM observation newly found that there was an 1nm micro-crystal oxide Fe(AI, Cr)2O4or FeO.(AI,Cr)2O3 layer, in addition to the well known FeAI3 intermetallic layer. Moreover, the deformation phenomena in the side of the stainless steel was also observed. This deformation was produced mainly by twinning and slipping. The morphology and width of the deformation regions depended upon the friction torque and welding conditions.In Chapter Eight, the microstructures and fractography of the joint zones in an explosive welded Al — Ti — Steel plate system were studied. The direct TEM observation found the transition zones in the bonded zone, and there are obvious interfaces between the transition zones and between transition zone and base metal. In the Al — Ti bonded zone, the transition zone is an Al+Ti mixture. In the Ti — Steel bonded zone, the transition zone is composed of the FeTi intermetallic layer and an amorphous layer. The morphology and size of the transition zones depend upon the diffusion of alloy elements and cooling rate during explosive welding.Chapter Nine is the conclusions of all the research work mentioned in this dissertation. The last part of the dissertation lists author's research projects and published papers finished during the doctorate study period.