Research On The Grain Boundary Liquation In The Heat Affected Zone Of 7N01 Aluminum Alloy Welded Joints

Generally, liquation along the grain boundary (GB) always occurs in a very narrow region immediately outside the weld fusion zone extending to heat affected zone during fusion welding of aluminum alloys. The GB liquation in aluminum welds can result in liquid cracking, which makes ductility loss after welding. The weldable Al-Zn-Mg alloys with medium-strength have good comprehensive performance are now widely used in many welded fabrications such as rail transport vehicles, armored vehicles, cryogenic pressure vessels, etc.. However, the occurrence of liquid cracking in the heat affected zone during welding limited the application of welded Al-Zn-Mg alloy component.In this study, domestic-made A7N01S-T5 aluminum alloy profiles were used, and the ordinary butt welds and multiple repairing welds were made by gas metal arc welding using filler metal ER5356. The Al-matrix and second phase particles in different regions of the welds were characterized by microstructure observation and local composition analysis methods such as OM, SEM, SEM-EBSD, TEM and SEM-EDS. Based on the above results, the characterization and formation mechanism of GB liquation located in the HAZ was systematically studied and discussed. It is found that,1. The 7N01S-T5 alloy profiles were characterized as typical hot deformed microstructure, as the deformed ?-Al grains tended to elongate parallel to the extrusion direction. Equiaxed grains with grain size of 5?m distributed in the deformed Al-matrix. A certain amount of second phase particles which were confirmed as MgZn particles with Zn/Mg atomic ratio of about 1.39 distributed along the grain boundaries of Al-matrix.2. The recrystallization and grain growth significantly occurred in the HAZ of the welds, the mean size of equiaxed grains is about 30?m. Aggregation and coarsening of MgZn particles distributed along the grain boundaries in HAZ were obvious. The Zn/Mg atomic ratio of MgZn phase is about 1.8, transiting to equilibrium phase MgZn2.3. Liquation of low melting point constituents (mainly the MgZn phase) distributed along the GB occurred in the HAZ closely near the FZ. Strains originated from the contraction of weld metal solidification are possibly the reason caused development of liquation cracking from GB liquation during welding.4. Multiple repairing welding aggravates the GB liquation. The mean lengths of liquation cracks observed in the HAZ of the welds made by ordinary welding, one-pass-repair-welding, two-pass-repair-welding and three-pass-repair-welding are 100?m,200?m,263?m and 265?m, respectively.