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In situ microstructure observation techniques in welding

In arc welding and laser welding, the materials of weld metal are heated and melted, which pass through a liquid phase and then solidify before phase transformation and precipitation occur. It is also well known that although the heat-affected zone is not heated to melting point, it is heated to a high temperature and then phase transformation and precipitation also occur there. A range of methods have been considered to examine the microstructural change of the metal during welding. In many of these, the metal is heated to a certain temperature and then cooled rapidly from the specified temperature, and the microstructure is frozen at the state at this temperature and this microstructure is examined at room temperature. In the tin freezing method, for example, the welding bead is frozen with liquid tin during welding, and the microstructural change mechanism, which is controlled by the dispersion of solid solution elements, is halted. Observation of the solidified microstructure is possible by analysing the frozen microstructure. However, freezing must be repeated over a range of temperature, which requires a considerable amount of work. In order to meet the requirement to observe and evaluate microstructural changes during welding, in this article, we describe the ‘in situ observation’ method for microstructural changes during welding developed by our group – .