Structural changes in weld bead in TIG welded Zn–22Al–2Cu alloy plates after tension

Abstract

By utilising the tungsten inert gas welding process, plates of Zn–22Al–2Cu (wt-%) alloy were welded. The weld bead was tested under tension in the transverse direction in order to study the structural changes. Once the tension samples were fractured, the weld bead and the contiguous region were analysed by means of X-ray diffraction. It was found that the applied load through the parent metal is responsible for the transformations in the crystalline phases inside the weld bead. From spectrum analysis, the aluminium rich solid solution, the zinc rich solid solution and the (CuZn₄) phases resulting from tension shifted to the low 2θ position. To explain these transformations, the reactions and were considered. To supply the necessary thermal energy so that these reactions happen, the hypothesis that the friction in the slip planes at the moment of deformation it produced the required heat for atomic diffusion is established. Finally, with the aid of scanning electron microscope, the morphology of the contiguous region to the fracture was observed. Near the fracture line, the lamellas inside the two-phase dendrites showed a tendency to be guided in parallel address to that of the tension load. In addition, as a result of the heating process, a coarsening of these lamellas can be observed. In a practical sense, the observed results are points to favour in the selection of this alloy for structures since no thermal treatments are needed for its stabilisation after welding.

Keywords:

• Zn–22Al–2Cu alloy plates
• TIG welding process
• Fracture
• Phase transformation
• Two-phase dendrites

The authors express their gratefulness to Mr Manuel Aguilar Franco (X-ray Diffraction Laboratory, IF-UNAM) and to Mrs Alicia del Real López (Electron Microscopy Laboratory, CFATA-UNAM) for their help in the experimental work.