Local Liquation Phenomenon and Its Effect on Mechanical Properties of Joint in Friction Stir Welded 2219 Al Alloy
Ju KANG1,2,Suying LIANG3,Aiping WU1(),Quan LI1,4,Guoqing WANG5
1 State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China 2 State Grid Jibei Electric Power Co. Ltd. Research Institute,North China Electric Power Research Institute Co. Ltd., Beijing 100045, China 3 Shougang Institute of Technology, Beijing 100144, China 4 Capital Aerospace Machinery Company, Beijing 100076, China 5 China Academy of Launch Vehicle Technology, Beijing 100076, China
Cite this article:
Ju KANG,Suying LIANG,Aiping WU,Quan LI,Guoqing WANG. Local Liquation Phenomenon and Its Effect on Mechanical Properties of Joint in Friction Stir Welded 2219 Al Alloy. Acta Metall Sin, 2017, 53(3): 358-368.
Al alloy 2219 (AA2219) exhibits excellent mechanical properties in a wide temperature range from -250 ℃ to 250 °C, indicating great potential for application in aerospace structures. Compared to fusion welding, friction stir welding (FSW) could significantly improve mechanical properties of the AA2219 joints. Since invented by the welding institute (TWI) of UK in 1991, FSW has been treated as a solid-state joining technique by the commercial companies, which has been in an agreement in most scientific researchers. However, recently a controversy that has been raised over the viewpoint that FSW is a strict solid-state process, and some observations of liquation have been reported, especially in the stir zone of friction stir spot welding (FSSW) joint. However, the phenomenon of liquation in FSW AA2219 joints has not been reported previously. Therefore, the aim of this work is to reveal the evidence of local liquation during FSW AA2219-T8 and its effect on mechanical properties of the joints. In this work, AA2219-T8 plates (8 mm thick) were friction stir welded at a welding speed of 180 mm/min and a rotation speed of 800 r/min using a welding tool with threaded pin. Heat treatment and thermal simulation experiments were carried out to contrast the characteristics of the local liquation regions. A Vickers microhardness testing machine and an in situ SEM imaging tensile test facility were employed to study the effect of local liquation on mechanical properties of the joints. The results showed that the microstructures in the local liquation regions were divorced eutectic, and its formation was related to the coupled thermal-mechanical interaction during the FSW process. In the FSW process, the local high temperature led to constitutional liquation. During the cooling period, the semisolid mixtures decomposed into α(Al) matrices and θ (Al2Cu) particles under stir and material flow actions. The liquation regions had a lower value of hardness than the normal regions in the nugget zone (NZ), making the liquidation region susceptible to cracks initiation and decreasing the ultimate tensile strength and elongation for a local liquation region contained NZ sample. However, the negative effect of local liquation regions on the mechanical properties of the FSW AA2219-T8 joint was less than that of the thermo-mechanically affected zone (TMAZ), since the local liquation regions were only localized and tiny fractions being in the NZ, whereas the TMAZ was whole softened.
Fig.1 Schematic of the sampling positions in the joint (RD—rolling direction, L—longitudinal, T—transverse, S—short transverse, AS—advancing side, RS—retreating side)
Fig.2 Dimensions of in situ SEM imaging tensile test specimen for No.1 (unit: mm. No.2 specimen had a gauge length of 10 mm)
Fig.3 BSE images showing the size and distribution of secondary phase particles in base material (BM) (a) and nugget zone (NZ) (b) of an AA2219-T8 FSW joint
Fig.4 Local liquation in the NZ of an AA2219-T8 FSW joint (a) overall image showing the location of liquation in the cross section of the weld (b) magnified image of the rectangle region in Fig.4a (arrows show the eutectic films) (c) OM image of the rectangle region in Fig.4b (The micro-indentations showing the locations of micro- hardness test) (d) SEM image of the rectangle region in Fig.4b (The points of M1~M4 showing the locations of EDS) (e) magnified image of the rectangle region in Fig.4d
Fig.5 Dark field OM image showing the local liquation regions in the NZ subsurface
Fig.6 Morphologies of solidification microstructure showing the coupled eutectic of AA2219-T8 after heat treatment (a) and thermal simulation by Gleeble (b)
Fig.7 Engineering stress-engineering strain curves for the normal specimen (NZ-1) and local liquation regions contained specimen (NZ-2) (Inset shows the Portevin-Le Chatelier (PLC) effect)
Fig.8 Crack initiation in NZ-1 and NZ-2 specimens (a) in situ SEM image showing the obvious cracks initiation in the θ particles at 227 MPa (b) in situ SEM image showing the cracks (as shown by the arrows) in an un-etched NZ-2 specimen at 213 MPa (c) OM image showing the intergranular cracks (as shown by the arrows) in the etched NZ-2 specimen after 225 MPa
Fig.9 Crack initiation and propagation in NZ specimen during in situ fatigue test (a) a typical crack in the NZ specimen after 1001 cyc at a maximum stress of 200 MPa (b) evolution of the traced crack in Fig.9a after 23342 cyc at a maximum stress of 230 MPa (c) another crack resulting in failure after 23342 cyc at a maximum stress of 230 MPa
Fig.10 Engineering stress-engineering strain curve (a) and crack initiation and propagation (b~i) of the FSW-Joint specimen (a) engineering stress-engineering strain curve and magnified graph of the indicated section showing the PLC effect (inset) (b) transgranular cracks (as shown by arrow) in the TMAZ at 190 MPa (c) intergranular cracks (as shown by arrows) in the TMAZ at 190 MPa (d, e) cracks propagation along the grain boundaries (f) final form of the cracks before fracture at the TMAZ (arrows show the pathway of crack) (g) multi-cracks appear in the NZ after failure (h) magnified image of the selected region in Fig.10g showing the intergranular cracks (i) magnified image of the selected region in Fig.10h showing the intergranular rupture
Fig.11 Fracture surfaces of the NZ-1 and NZ-2 (a) dimple rupture showing plastic fracture features of the NZ-1
Fig.12 Fracture surfaces of the in situ fatigue test specimen (a) overall image showing crack source and crack growth regions (b) magnified image of the rectangle labeled b in Fig.12a showing the intergranular rupture (c) magnified image of the rectangle labeled c in Fig.12a showing the brittle fatigue striations (d) magnified image of the rectangle labeled d in Fig.12a showing the plastic fatigue striations
Fig.13 Ultimate tensile strength and elongation of the NZ-1, NZ-2 and FSW-Joint samples, respectively
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