Study On Corrosion Resistance Of High-strength Aluminum Alloy RFSSW Joint

The high-strength aluminum alloy of 2A12-T42 has the advantages of light weight and high specific strength.It is widely used in the aerospace field.At present,the alloy is mainly connected by riveting,but the riveting technology increased the quality of the components and the cost is high.Refilled friction stir spot welding?RFSSW?is a new type of solid spot welding technology based on friction stir welding.It has the advantages of high bonding strength and low cost.It is expected to replace the riveting process.Based on the analysis of the microstructure and mechanical properties of 1mm+1mm 2A12-T42aluminum alloy RFSSW joints,This paper studied the corrosion resistance of RFSSW joint in EXCO solution and the stress corrosion cracking tendency in 3.5wt.%NaCl solution.RFSSW joint can be divided into five regions:shoulder affect zone?SAZ?,pin affect zone zone?PAZ?,thermomechanical affect zone?TMAZ?,heat affect zone?HAZ?,and base metal?BM?.The main precipitation phase of 2A12-T42 aluminum alloy was S phase?Al₂CuMg?,and it also contained a small amount of impurity phase Al₇Cu₂Fe.Due to the action of heat,the microstructure of HAZ was coarsened,and the precipitation phase had a tendency to aggregate.Due to thermomechanical action,SAZ and PAZ grains were refined,the precipitated phase was broken,and the chemical composition was uniform.TMAZ was plastically deformed on the action of shearing force,the grains were elongated,and the precipitated phase was melted.The size of the precipitated phase was smaller than that of HAZ.The difference of microstructure resulted in the different hardness values in different regions of the joint.The hardness value of HAZ was the lowest.The hardness values of PAZ and SAZ reached or even exceeded the hardness of the base metal.With the increase of rotation speed and plunge depth,the hardness of joint decreased gradually.The immersion test showed that as the rotation speed increased,the corrosion resistance of the RFSSW joint gradually increased.And as plunge depth was increased,the corrosion resistance inside the joint was gradually reduced.but the influence of plunge depth on the corrosion resistance of the joint was smaller than the rotating speed.The RFSSW joint first appeared pitting corrosion in EXCO solution.Pitting corrosion mainly originated from S phase,mainly because of the difference in self-corrosion potential between the precipitation phase and the matrix.The corrosion resistance of each region of the joint from good to bad was:SAZ>PAZ>BM>TMAZ>HAZ.The stress corrosion cracking test showed that the stress corrosion sensitivity of the RFSSW joint in 3.5wt.%NaCl solution was higher than that of air at a strain rate of 0.0025mm/min,and the maximum force loss was 20.77%.As the rotation speed increased,the ability of joints to resist stress corrosion cracking increased.With the increase of plunge depth,the resistance of the joint to stress corrosion cracking gradually decreased.The fracture locations of the joints were located at the edge of the joints under different parameters.This mainly due to the large stress concentration at the edge of the joint.The micro fracture morphology showed that the fracture morphology in air was mainly ductile-brittle mixed fracture,and mainly brittle in 3.5wt.%NaCl solution.