Texture, residual stress and mechanical properties of 7039-T6 thick plate Al alloy with MIG-welded laminar tearing

Abstract

7039 Al alloys are widely used in armor vehicles, given the material’s high specific strength and fracture toughness. However, laminar tearing in the thickness plane of the base metal (BM), specifically in the normal direction (ND) and rolling direction (RD) plane, was occasionally observed after the welding of thick plates, resulting in premature material failure. A vertically metal-inert gas (MIG)-welded laminar tearing component of a 30 mm thick plate was analyzed to determine the factors associated with this phenomenon. The texture, residual stress, microhardness, and tensile properties were also investigated. The results indicated that the crack extended along the RD as a transcrystalline fracture and terminated at the BM. The grains near the crack grew preferentially in the (001) crystal direction. Furthermore, the tensile strength (83 MPa) and elongation (6.8%) in the RD were relatively higher than those in the ND. In particular, the primary factors for crack initiation include stronger texture, higher dislocation density, increased Al₇Cu₂Fe phases, lower proportion of small-angle grain boundaries, and varying grain sizes in different regions, leading to the fragile microstructure. The higher residual stress of the BM promotes the formation and extension of cracks. The restraining force due to fixation and welding shrinkage force transformed the crack into laminar tearing. Preventive measures of laminar tearing were also proposed.

Keywords

metal-inert gas welded / Al alloy thick plate / laminar tearing / texture / residual stress / mechanical properties

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