Microstructure Characterization Of 7N01S Aluminum Alloy Welded Joint And Its Mechanical Properties

The medium-strength weldable Al-Zn-Mg aluminum alloy have a good comprehensive performance, but in the process of welding, Partially melted zone or multi-layer welding are prone to the obvious phenomenon of grain boundary liquation. The grain boundary liquation may result in the production of liquation cracking under the action of tensile stress, which caused the deterioration of mechanical properties of welded joint.This research adopted normal welded and multiple thermal cycle test on domestic 7N01S-T5 by the melting polar gas shielded welding (MTG) and made use of microstructure observation and micro area composition analysis method such as OM, SEM, SEM-EBSD, SEM-EDS and so on. This research mainly focused on the grain boundary liquation and its formation mechanism in partially melted zone. The study found:1. The microstructure of 7N01S-T5 aluminum alloy welded joint:Fusion zone (FZ) was the typical casting microstructure;Partially melted zone (PMZ) was the small recrystallization grain; Heat affected zone (HAZ) retained its parent metal fibrous tissue, but the grain obviously grew up; The partially melted zone which was experienced multiple thermal cycle test broadened obviously.2. The second phase particles in HAZ were consistent with elements in parent metal. The second phase particles included white AlFeSiMn and black MgZn phase which were distributed in grain boundary. The atomic ratio of black Mg/Zn phase was 1.11.3. The result of Micro hardness distribution measurement showed that hardness in FZ was low and had a significant rise in PMZ. The maximum hardness was in the quenching area; Heat affected zone existed obvious softening region. Its boundary near FZ moved in the direction of FZ with the increase of welding thermal cycle time. The reason was that the precipitated phase particles in this area gathered and grew up in the welding process4. Brittle fracture of the fracture characteristic was obvious in the fracture of unqualified tensile properties specimen of joints by macro and micro fracture analyses. The results of observation on fracture profile microstructure showed that the side edge of fracture on the grain boundary was continuous distribution of low melting second phase and in the process of stretching cracking easily along the grain boundary nucleation and extensions.5. The results of the EBSD analysis in fracture profile edge and PMZ showed that the grain boundary with continuous distribution of the second low melting second phase was random large Angle grain boundary; the results of EDS composition test showed that low melting phase was MgZn phase.