A method for studying weld fusion boundary microstructure evolution in aluminum alloys

Aluminum alloys may exhibit a variety of microstructures within the fusion zone adjacent to the fusion boundary. Under conventional weld solidificatio n conditions, epitaxial nucleation occurs off grains in the heat-affected z one (HAZ) and solidification proceeds along preferred growth directions, in some aluminum alloys, such as those containing Li and Zr, a nondendritic e quiaxed grain zone (EQZ) has been observed along the fusion boundary that d oes not nucleate epitaxially from the HAZ substrate. The EQZ has been the s ubject of considerable study because of its susceptibility to cracking duri ng initial fabrication and repair. The motivation of this investigation was to develop a technique that would allow the nature and evolution of the fu sion boundary to be studied under controlled thermal conditions. A melting technique was developed to simulate the fusion boundary of alumin um alloys using the Gleeble(R) thermal simulator. Using a steel sleeve to c ontain the aluminum, samples were heated to incremental temperatures above the solidus temperature of a number of alloys. In Alloy 2195, a 4Cu-1Li all oy, an EQZ could be formed by heating in the temperature range approximatel y from 630-640 degrees C. At temperatures above 640 degrees C, solidificati on occurred by the normal epitaxial nucleation and growth mechanism. Fusion boundary behavior was also studied in Alloys 5454-H34, 6061-T6 and 2219-T8 . Nucleation in these alloys was observed to be epitaxial. Details of the t echnique and its effectiveness for performing controlled melting experiment s at incremental temperatures above the solidus are described.