Investigation On Flow Behavior Of Liquid Metal In CMT Fusion-brazing Of 6061-T6 Aluminum Alloy/304 Stainless Steel

Adoption of aluminum/steel heterogeneous metal structure can effectively achieve the goal of structural lightweight,and then achieve the purpose of energy saving and emission reduction,and has a wide range of application prospects in automotive,aerospace,petroleum equipment,shipbuilding and other fields.However,due to the great difference between the thermophysical properties of aluminum alloy and steel,brittle intermetallic compounds are easily generated in the welded joint,which seriously affects the comprehensive properties of the joint.CMT welding process has the advantages of adjustable heat input and stable welding process,which can effectively inhibit the formation of brittle-hard phase and welding defects,and is suitable for the connection between aluminum alloy and steel dissimilar metals.In this paper,the research program combining welding experiment and numerical simulation is used to study the technology of 6061-T6 aluminum alloy/304 stainless steel CMT fusion-brazing welding process and simulate and analyze the temperature field and flow field,which provides sufficient basis for improving the weld forming and joint quality,and has important theoretical and engineering practical value.Aluminum alloy/steel fusion-brazing process test was carried out with 6061-T6 aluminum alloy and 304 stainless steel with 2 mm thickness as the base material and ER4043 aluminum-silicon welding wire as the filling metal to explore the influence of welding parameters on the macroscopic morphology and microstructure of the joint.The results show that the best weld formation can be obtained only under the condition of moderate welding heat input,and 1.57k J/cm is the best heat input.The microstructure of the weld zone is composed ofα-Al equiaxed grains and Al-Si eutectic precipitated along the intergranular.The microstructure of the weld side near the reaction layer of the interface zone and the microstructure of the fusion zone are columnar grains with the same growth direction,and both grow perpendicular to their respective boundaries towards the weld center.The reaction layer in the interface zone of the joint with a small welding heat input only consists of Al₈Fe₂Si.When the welding heat input increases to a certain extent,the reaction layer turns into a double-layer structure,consisting of Al₈Fe₂Si near the weld side and Fe₄Al₁₃ near the stainless steel side.The mechanical properties and corrosion properties of the joint were tested.It is found that the joint with smaller heat input is broken in the bonding layer,while the joint with larger heat input is broken in the heat affected zone on the aluminum side,which proves that the double-layer structure is more stable.The joint with 1.57 k J/cm has the highest tensile strength,which is 213.94 MPa,which is up to 75%of the strength of the aluminum base metal.During the welding process,the heat affected zone on the aluminum side occurs obvious"softening"phenomenon,which leads to the lowest microhardness value in this zone,while the weld zone is larger than the base metal of aluminum alloy.The hardness value of the reaction layer in the interface zone is the highest because of the formation of brittle and hard intermetallic compounds.The joint with small welding heat input is more corrosion-resistant than the joint with large welding heat input.In addition,the corrosion performance of the joint is better in neutral seawater solution,while the corrosion performance decreases obviously in weak acid solution,and can not even reach the engineering standard.Software such as Solid Works and Mesh were used to complete the solid modeling and mesh division.The files were imported into FLUENT to load the welding mobile heat source and each driving force source.After all the Settings were completed,the numerical simulation of the welding temperature field and the pool flow field was carried out.The flow behavior of liquid metal in the molten pool can be obtained by analyzing the nephogram of different sections of molten pool flow field in the quasi-steady state stage.According to the nephogram of the upper surface of aluminum base metal,it can be found that there are multiple circulations with different directions in the molten pool,which is beneficial to increase the melting width.It can be found from the longitudinal section nephogram that the liquid metal on the surface of the molten pool flows from the center to the edge of both sides under the main action of surface tension,and the liquid metal under the arc flows from the top to the bottom,forming multiple circulation which can increase both the width and the depth of the molten pool.It can be found from the cross section nephogram that there are mainly two circulations with different sizes and directions in the molten pool,which are beneficial to the spread of liquid metal on the stainless steel.In the quasi-steady state stage,the maximum flow velocity of the liquid in the molten pool is basically maintained at about 0.32 m/s,which appears on the surface of the molten pool,and the reason is mainly due to the effect of surface tension.Temperature field results show that the temperature rising in the welding temperature increase quickly,the molten pool volume increasing,while 2.85 s entered into the phase of quasi steady state,since molten pool size remains the same,basic peak temperature is maintained at about 1430 K,the cooling stage of weldment cooling rate slows down,in 1.05after welding molten pool in liquid metal solidification completely.In addition,due to the obvious differences in thermophysical properties of the two base metals,the temperature field nephogram presents an asymmetric distribution,and the temperature gradient on the aluminum side is smaller than that on the steel side.Compared with the actual welding results,it is found that increasing the welding heat input can effectively promote the expansion of the weld pool in three-dimensional size,but too large or too small heat input is not conducive to the formation of continuous and effective weld.