Change of Weld Solideification Structure of Commercially Pure Aluminum after Annealing at High Temperature

Abstract

Inside of columnar crystals which are developed in the weld metal of commercially pure aluminum subgrains are generally observed. Besides, Al-Fe compound is developed along boundaries of the subgrains. These subgrains would affect the mechanical properties and corrosion resistivity of the weld metal. In this report the investigation was made on the change of weld solidification structure when the weld metal was annealed at high temperature which had been as-welded, tensile strained or cold rolled.
Macrostructures after annealing were grouped into following three classes: (i) macrostructure similar to that as-welded, (ii) base metal recrystallises and in the weld metal recrystallisation occurs partially only near the fusion boundary, (iii) recrystallisation occurs all over the weld metal. On the other hand, no considerable change was observed in features of the subgrains in the weld metal. That is, it would be concluded that the subgrains are considerably stable thermally. In the columnar crystal zone of aluminum weld metal fiber texture is developed whose fiber axis is [100]. This fiber texture was also observed after the annealing in the area where the columnar crystals were observed in the weld metal, but disappeared in the area where recrystallization occurred.
Then, the intensity distributions of FeKa radiation in the weld metal were measured with electron probe microanalyser. Each maximum in the intensity distribution curves corresponded to the part which was observed black in the microstructure. Electron micrographs showed that in the weld metal which was annealed at high temperature there existed both compounds and dislocations along the boundaries of the subgrains. This phenomenon is similar to that which was observed in the as-welded metal. When the weld metals which had been tensile strained or cold rolled were annealed the boundaries of the subgrains became discrete and rod- or granular-shaped compounds were also observed inside of the subgrains. These compounds found to be A1₃Fe by selected area diffraction.