Mechanism of Weld Solidification and Behavior of its Structure (Report 8)

Growth behavior of subgrains and competitive growth of columnar grains in weld metal

Abstract

Investigations were made on the growth behavior of subgrains which had been developed in the weld metal of 99.93%- and 1050-aluminum single crystal sheets. Besides, the mechanism of the competitive growth of the columnar crystals was made clear. Conclusions obtained are as follows:
1. Subgrains which had been developed in the weld metal of 99.93%- and 1050-aluminum single crystal sheets grew in the direction of ‹100›. This growth driection, however, was shifted a little in the direction of heat flow when the directions of ‹100› and the heat flow differed to some extent. This phenomenon was obviously observed in 99.93%-aluminum which had less amount of solute than 1050-aluminum. 2. Solidification structure in 99.93%-aluminum single crystal sheet is cellular when the direction of ‹100› almost agrees with the direction of the heat flow. On the other hand, when the directions of ‹100› and the heat flow differ to some extent the subgrains branched out in the direction of heat flow and the solidification structure changes to cellular dendrite.
3. When the direction of ‹100› inclines against the specimen surface the subgrains does not grow parallel to the specimen surface but grow in the direction of this ‹100›.
4. The competitive growth occurs when two or more crystals, which exist in the base metal side of the fusion boundary, grow in the weld metal. That is, the crystal whose ‹100› direction almost agrees with the direction of the heat flow grows easily, but the crystal whose ‹100› direction does not agree with the direction of the heat flow does not grow so easily as the former crystal and the crystal finally disappears on the way.