ＳＡＭ及びＴＥＭによるアルミニウム合金の溶接金属中に形成される羽毛状晶の検討

Abstract

Macrostructures which are usually developed near the center of weld metal of aluminum alloys are columnar crystals and equiaxed dendrites. The development of the macrostructures depends on the welding conditions. A feathery crystal is sometimes developed when welded by GTA in a low welding speed. In this study, we investigate characteristics of the feathery crystal using both a scanning acoustic microscope (SAM) and a transmission electron microscope (TEM). It is possible to observe structures without etching by SAM. By observing normal and parallel section to the welding direction, we made clear the growth feature of the feathery crystal. When we measure the velocity of elastic surface wave of the feathery crystal using SAM with an anisotropic acoustic lens, the velocity is about 100 m/s faster in parallel to the welding direction than in normal direction. That is, the feathery crystal is acoustically anisotropic. We could observe the feathery crystal even in electron beam welds using SAM. It is not generally easy to observe macrostructure of electron beam welds, because the bead width is narrow. Moreover, we confirm that the feathery crystal is a twin by analyzing electron diffraction patterns obtained using TEM.