INVESTIGATION ON MECHANICAL AND STRESS CORROSION CRACKING PROPERTIES OF WEAKNESS ZONE IN FRICTION STIR WELDED 2219-T8 Al ALLOY
Ju KANG1,2,Jichao LI2,Zhicao FENG2,Guisheng ZOU1,Guoqing WANG4,Aiping WU1,3()
1 Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China 2 Fontana Corrosion Center, The Ohio State University, Columbus, Ohio 43210-1185, USA 3 State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China 4 China Academy of Launch Vehicle Technology, Beijing 100076, China
Cite this article:
Ju KANG,Jichao LI,Zhicao FENG,Guisheng ZOU,Guoqing WANG,Aiping WU. INVESTIGATION ON MECHANICAL AND STRESS CORROSION CRACKING PROPERTIES OF WEAKNESS ZONE IN FRICTION STIR WELDED 2219-T8 Al ALLOY. Acta Metall Sin, 2016, 52(1): 60-70.
Al alloy 2219 (AA2219) is widely used in the aerospace industry, and friction stir welding (FSW) is an ideal method to join it. The ultimate tensile strength of an FSW AA2219-T8 joint can be as high as 344 MPa which is significantly higher than that welded by other methods such as gas tungsten arc welding. However, the thermo-mechanically affected zone (TMAZ) in the FSW joints of AA2219-T6/T8 is a weakness zone of mechanical property and is susceptible to stress corrosion cracking (SCC), but the reasons are not been well understood. In this work, the mechanical and electrochemical properties of different zones in AA2219-T8 joints obtained by the FSW method were studied. The welding thermal cycles during welding were measured using an array of type K thermocouples. During the tensile process of the joints, digital image correlation (DIC) technique and high speed video technique were employed to investigate the deformational behavior and fracture pathway of the TMAZ, respectively. A microcell method was used to study the micro-electrochemical characteristics of the joints with and without stress. The results showed that the minimum strength located at a position where the weighted strengthening effects of both thermal cycles and stir action were the weakest. The DIC results revealed that the deformation concentrated mainly in the TMAZ during the tensile tests. However, due to the different restraints from the nugget zone (NZ) led to a large strain in the root side than that in the crown side. This made the root side susceptible to cracks initiation. In situ tensile testing indicated that cracks occurred only in the TMAZ at 190 MPa, indicating that the protective surface films in the TMAZ were more prone to crack than those in other zones of the joint. This led the TMAZ to be the weakest zone to pitting corrosion in an aggressive environment. Once pits generate in the TMAZ, the local stress will concentrate near the tip of the pitting, resulting in failure.
Fig.1 Schematic of test positions in the joints (RD—rolling direction, L—longitudinal, S—short transverse, T—transverse, AS—advancing side, RS—retreating side, DIC—digital image correlation)
Fig.2 Schematics of samples used for electrochemical experiments by microcell (unit: mm)
Fig.3 Schematic of microcell (a) and a 0.8 mm diameter tip for microcell test (b)
Fig.4 Schematic of modified ASTM G49 jig used to apply a constant stress at microcell test
Fig.5 Engineering stress-engineering strain curves for the base material (BM) and friction stir welding (FSW) joint of 2219-T8 Al alloy
Fig.6 Microhardness distributions of an FSW joint of 2219-T8 Al alloy (Inset shows a macroscopic metallographic image of the cross section of the joint)
Fig.7 DIC results for strain fields under different stresses of an FSW joint of 2219-T8 Al alloy
Fig.8 Engineering strain-engineering stress curve for the TMAZ at the position in Fig.2b
Fig.9 Fracture location and necking phenomenon in a fractured joint
Fig.10 Fracture process of a joint
Fig.11 Micro-electrochemical characteristics of different zones in the FSW 2219-T8 Al alloy joints with and without an applied stress (Microcell capillary 0.8 mm diameter tip, 0.5 mol/L NaCl, E—potential, i—current density)
Fig.12 OM images of the different zones in an FSW joint of 2219-T8 Al alloy before in situ tensile testing
Fig.13 Transgranular and intergranular cracks in the TMAZ after tensile testing at 191 MPa
Fig.14 OM image of fractured transverse section of an FSW joint of 2219-T8 Al alloy after in situ tensile testing
Fig.15 Schematic of crown side and root side in the joint (Lt—width of crown side in the NZ, Lb—width of root side in the NZ)
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