Abstract
Summary The macroscopic solidification structures developed in the weld metal of aluminium alloys are generally columnar crystals and equiaxed dendrites. The development of these macrostructures depends on the welding conditions. Feathery crystals are sometimes developed during low-speed GTA (TIG) welding. This paper describes an investigation of the characteristics of feathery crystals by scanning acoustic microscopy (SAM) and transmission electron microscopy (TEM). SAM enables structures to be observed without etching. The growth features of feathery crystals are clarified by observations made normal and parallel to the welding direction. Measurements of the velocity of the elastic surface wave of feathery crystals by SAM with an anisotropic acoustic lens show the velocity to be around 100 m/sec faster parallel to the welding direction than normal to it, confirming that the feathery crystals are acoustically anisotropic. Feathery crystals are also observed in electron beam welds by SAM, although it is not generally easy to observe the macrostructures of these welds because of the beam width being narrow. The diffraction patterns and TEM observations of the feathery crystals show them to be twinned crystals with the (111) plane as their twinning plane.
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