溶接凝固機構とその組織の特性(第7報)

Abstract

The changes and the construction mechanism of solidification structure were investigated for the weld metal which had been welded with a moving arc heat source. The materials used were aluminum sheets with different chemical compositions. The conclusions obtained are as follows:(1) The nominal growth rate in the weld metal increases generally as the inward growth proceeds, while the temperature gradient decreases.(2) When the welding is performed in a condition that the bead width is almost the same, the values of the temperature gradient decrease as the welding speed increases. Accordingly the extent of the constitutional supercooling zone in the weld metal increases as the inward growth proceeds and besides the welding speed increases.(3) The columnar crystals develop in the weld metal and under certain circumstances the equiaxed dendrites develop near the center of the weld metal.(4) In the 99.96%- and 99.93%-aluminum sheets, the planar growth is observed near the fusion boundary and afterwards the cellular subgrain is observed. In this case the equiaxed dendrites do not develop.(5) In the commercially pure aluminum sheets, the solidification structure changes from planar to cellular subgrain and cellular dendrite as the inward growth proceeds. The equiaxed dendrites develop near the center of the weld metal when the welding is performed in a high speed.(6) In aluminum alloy sheets, the cellular dendrite is observed even near the fusion boundary and afterwards a lot of equiaxed dendrites develop.(7) The changes of the solidification structure shown in (4), (5) and (6) can be accounted for in terms of the phenomenon of the constitutional supercooling.(8) When the welding is performed in a condition that the bead width is almost the same, it becomes easy for the equiaxed dendrites to develop in the weld metal as the welding speed increases. This phe-nomenon will be accounted for in terms of the extent of the constitutional supercooling zone.