Influence of grain size on mechanical properties of aluminum GTA weld metal

Abstract

Grain refinement is an important possibility to enhance the mechanical properties, such as strength, ductility, and toughness, of aluminum weld metal. In this study, grain refinement was achieved through the addition of commercial grain refiner AlTi5B1 to GTA (gas tungsten arc) weld metal of the aluminum alloys 1050A (Al 99.5) and 5083 (Al Mg4.5Mn0.7). The grain refiner additions led to a significant reduction of the weld metal mean grain size (Alloy 1050A: 86%, Alloy 5083: 44%) with a change in grain shape from columnar to equiaxed. In hardness tests, no relationship between hardness and weld metal mean grain size was found. Tensile tests showed for Alloy 5083 that the weld metal’s ductility can be increased through grain refinement. No improvement in weld metal strength (i.e., yield strength and ultimate tensile strength) was observed. Furthermore, tear tests with notched specimens revealed that the resistance against initiation and propagation of cracks in the weld metal can be enhanced through grain refinement. This increase in toughness was found to depend strongly on the alloying composition and hence on the size, distribution, and chemical composition of the intermetallic phases. Accordingly, the toughness was observed to increase clearly by grain refinement in weld metal of commercially pure Al (Alloy 1050A). In Alloy 5083 weld metal, the toughness was not improved through grain refinement, likely because of a semi-continuous network of brittle intermetallic phases that facilitate crack propagation.