Research On The Microstructure And Stress Evolution Of Arc Weld Joint Of Thick Aluminum Alloy Plates

7A52 aluminum alloy is a kind of high-strength weldable aluminum alloy. Ithas been widely used in aircraft structures, equipment vehicles and high-speed trainstructures. The alloy will play a more and more important role in elevating thelightweight level of equipments. This dissertation was set under the background ofwelding maintenance of vehicle and thick plate structures. Our work focused on themulti-pass welding process of equipment thick aluminum alloy, the study ofmicrostructure and evaluation of the mechanical properties of weld joint. Narrowgap MIG and TIG welding processes were developed. Differences inmicro-structure and mechanical properties of the welded joints were studied. Theevolution of the stress field in the joints obtained by the two welding processeswere revealed.The UV bevel for narrow gap welding was designed by experimental method.The numerical method was adopted as conductive theory to design the bottom Vbevel when welding thick aluminum alloy of 20 mm thickness by TIG process. Eachwelding processes was developed separately.The microstructure of different characteristic local areas of welded joints wasstudied. The micro-structure of the center of weld seam was equiaxed grain, eachside of which was columnar grain. For the former, more continuous stripprecipitates were found intergranularly, while the density of spherical precipitationswas decreased intragranularly. That resulted in the lower strength and hardness ofthe equiaxed grain zone in the weld center. Recrystallization transformation tookplace in the heat affected zone(HAZ), thus microstructure of banded structureclipping equiaxed grains was formed. Accumulated distribution of Si and thecontinuous distribution of Al2 Mg Cu phase in the HAZ played the role of crackingsources giving birth to local brittle fracture in the joint. New strengthening phasewas adopted into the seam by element redistribution. Hexagonal structured η phase(Mg Zn2) was thus obtained intragranularly with a novel non-continuous distributionof Al3Mg2 phase intergranularly. The weld seam was strengthened as a result of thewelding process. The η phase was generated by the way of crystallization andprecipitation in the seam and HAZ, respectively, resulting in the obvious differencein dimension.Linear regressive analysis was used to obtain the parameters and rectifyingcoefficient in the cylindrical Gauss heat source model for narrow gap MIG welding.Modified Arrhenius constitutive equations for the welded parent metal and fillermaterial were established according to the high temperature tensile test results. Withregard to TIG and narrow gap MIG welding method, BP-neural network model wasestablished. The heat source model parameters were set as input layer while thedifference between the predicted value by finite element method(FEM) and themeasured value was set as the output layer. For TIG welding, the double ellipsoidheat source parameters were optimized by fuzzy computation method under thecondition of backing welding, filler welding and cover welding, respectively. Fornarrow gap MIG welding, the double ellipsoid heat source parameters wereoptimized by fuzzy computation method under the condition of filler welding andcover welding. Specific heat capacity and thermal conductivity was linearly fitted,respectively. Equivalent specific heat capacity method was used for the treatment oflatent heat. Then the finite element method(FEM) model was established fornumerical simulation of welding thick aluminum alloy plates.The evolution of stress field under both kinds of welding processes wasrevealed. The blind hole method and X-Ray test results were used as the evaluationcriterion. It was found that the main differences between the evolution laws werethe transformation process of stress state. Moreover, two stages were necessary forthe formation of compressive stress including the release of tensile stress duringheating and the accumulation of compressive strain during cooling. With regard tothe narrow gap MIG weld joint, the transverse tensile stress in the back surface ofthe weld seam became steady before dropping to zero and stayed tensile. However,the transverse tensile stress transformed to compressive stress before going steadyin the back surface of the TIG weld seam. The prediction of stress field and themicrostructure of the joint was used for revealing the initiation mechanism of cracksin the joint.The compact properties of the joints obtained with both welding methods werestudied. It was found by the compact tests of the joints obtained by both weldingprocesses that the impact toughness of both joints was equal to each other.Maximum deformation was found for the seam area during initiation of cracks, thusthe impact toughness for the seam area was maximum. The toughness of the moltenzone and the HAZ varied little for the samples from different thickness position.The displacement could be improved, thus the crack formation energy wasincreased by by narrow gap welding. The impact absorption energy was increasedby 13.3%. Numerical simulation method was adopted to study the impactmechanical behavior of full-thickness joint. It was found that highest stressconcentration was located at the weld toe. When the joint was impacted verticallyon the up surface, stress concentration was higher in the area close to the back-sidetoe of the joint.