Keizo Namba

Weld-Solidified Structure of Commercial 5083 Aluminum Alloy and its Mechanical Behavior

Experiments were conducted to study the structural features of weld-solidified metal in commercial 5083 aluminum alloy and its influence on the fracture behaviors.The shape of weld bead was considerably influenced by welding variables and heat input. In these behaviors, height of reinforcement was influenced by the heat input but depth of penetration was done mainly by the welding current, while width of bead depended on the both of them.Structures of common 5083 alloy welds containing no refining elements consisted of fine columnar, coarse columnar, granular and feather grains. In these structures, the volume fractions of fine columnar and feather grains were increased with heat input but those of coarse columnar and granular grains were decreased reversely. The width of columnar grain had a tendency to increase with heat input. On the other hand, 5083 alloy welds containing small amounts of Ti-B showed the fine granular structure all over the fusion zone.When weld metals having fine columnar, coarse columnar and granular grains were stretched, crack occurred preferentially at the grain boundary of coarse columnar structure. In weld metals having taining feather grains, however, it was important to consider the relationship between the growth direction of feather grain and the stretching direction. Crack occurred preferentially at feather grain when stretched transversely to growth direction of feather grain but it did not occur in this way when stretched parallelly.

Effects of distribution of solute elements on mechanical properties of aluminum alloy welds

Experiments were conducted to study the effects of distribution of solute elements on the mechanical properties of solidified weld metal in Al-4.5%Mg and Al-4%Zn-2%Mg alloys.The mechanical properties of welds were more deteriorated with the increase of welding heat input. After homogenizing, tensile strength and hardness of the deposited metal were increased on the whole; but Charpy impact value only showed a reverse result. However, both of the tensile strength and impact value still depended on the initial welding heat input or dendrite cell size even after homogenizing, but the hardness did not.After the above treatment, principal solute elements such as Mg and Zn were almost homogenized, which led to the vanishing of their segregations on the dendrite cell boundaries; but their concentrations in the cell matrix were increased. However, the distribution of impurities of Fe and Si (particularly, Fe) remained in as welded state even after homogenzing.It was clear from the above facts that the hardness of welds was markedly affected by the dispersion of the 2nd phases containing Mg or Zn on grain and dendrite cell boundaries and their concentrations in the cell matrix. Whereas, the tensile property and Charpy impact value, which will lead to rupture, would be affected by the distribution of Fe and Si as well as the above two factors.The above facts were proved by the experimental results that the mechanical properties of welds and permanent mould castings were deteriorated with the increase of Fe and Si. The network of the 2nd phases containing Fe and Si on grain and dendrite cell boundaries seemed to promote the crack propagation in rupture.However, the effects of Fe and Si were observed in only Al-4.5% Mg and Al-4%Zn-2%Mg alloys containing considerable amounts of Mg and (or) Zn as strengthening elements. Whereas, these effects were not observed in commercially pure aluminum, in which Fe and Si rather act as strengthening elements for mechanical properties.