Research on Microstructure and Mechanical Properties of Square Wave Alternating Current Tungsten Inert Gas Welded Joints in 2219-T6 Aluminum Alloy

Square wave alternating current (AC) tungsten inert gas (TIG) welding possesses great advantages in welding stability, quality, flexibility, and cost in manufacturing thin-walled propellant storage tanks with 2219-T6 aluminum alloy. To solve the welding engineering problem of 4 mm-thick plates made by square wave AC TIG welding, we conducted process optimization experiments and performed common microstructure analysis and mechanical property testing. Tensile strength and microhardness reaching 63.8 and 83.1% of the base material appear in the optimal weld. The dendrite morphology, grain size, and eutectic content of all welds were compared and analyzed with optical microscopic observation, image segmentation, and grayscale statistics. The obtained quantitative results interpret the macroscopic properties of the optimal weld from a microscopic perspective. Shallow and uniform equiaxed dimples, evident tearing ridges, and second-phase particles were detected in the tensile fractures. Reticular, granular, and lath eutectic (alpha(Al) + CuAl2) were found in the interior and boundaries of dendritic solid solution (alpha(Al)). Unexpectedly, the epitaxial growth of the columnar grains was inconspicuous or absent in all welds. Uniform and fine equiaxed grains occupy almost all welds of different process parameters, while the G/R ratio of these welds reaches 15-23 degrees C center dot s/mm boolean AND 2, under which the equiaxed crystals should not have formed. We attributed the grain refinement in these welds to the secondary phase particles in base metal. They are broken into fragments during the multi-pass rolling process of plates, remain semi-dissolved under a fast cooling rate, and provide adequate heterogeneous nuclei for melt solidification. A further SEM and EDS test to base metal and secondary phase particles confirmed our findings and speculations. These results suggest that in the molten pool with sufficient heterogeneous nucleation, the mechanical blocking effect of heterogeneous nucleation may interfere with the thermodynamic effect of welding heat input and speed on grain structure.

Automated summary

Square wave alternating current tungsten inert gas welding is advantageous in manufacturing aluminum alloy storage tanks. Through process optimization experiments and analysis, we found that the optimal weld has similar tensile strength and microhardness to the base material. The presence of uniform and fine grains and secondary phase particles in the welds was attributed to the breaking of particles during the rolling process, providing heterogeneous nuclei for solidification.