Crystallographic Investigation for Origination of New Columnar Crystal in Aluminum Weld Metal Using Single Crystal Sheet

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By X-ray diffraction technique the conditions for origination and the crystallographic characteristics of the new columnar crystal (that is, stray crystal) in TIG weld bead have been investigated using single crystals of 1S commercially pure and 99.93% pure aluminum 1 mm thick sheets. Conclusions obtained are as follows : (1) Origination of new columnar crystal which was called "stray crystal" by authors was confirmed by means of TIG bead-on-plate welding on a single crystal of commercially pure aluminum sheet. (2) The stray crystal originates when the direction of the maximum temperature gradient at the solid-liquid interface of the columnar crystal growing from the fusion boundary has deviated from the nearest <100> direction by 40 to 50°. (3) The origination of the stray crystal is seen to be related with the purity of the material. Namely the decrease of the purity in the base metal generally urges the origination of stray crystals in the weld bead. (4) A <100> direction of the stray crystal usually coincides with the direction of the maximum temperature gradient at the originating location. Moreover the subgrains inside each stray crystal are parallel to a <100> direction.
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