Effect of static annealing on superplastic behavior of a friction stir welded Ti-6Al-4V alloy joint and microstructural evolution during deformation

doi:10.1016/j.jmst.2022.05.006

Abstract

Abstract:

Structural integration is one of the most critical developing directions in the modern aerospace field, in which large-scale complex components of Ti alloys are proposed to be fabricated via the method of welding + superplastic forming. However, the undesired strain localization appeared during superplastic deformation of the entire joint has largely hindered the development of this method. In our study, a combination process of friction stir welding (FSW) + static annealing + superplastic deformation was first time proposed to eliminate severe local deformation. To achieve this result, a fully fine lamellar structure was obtained in the nugget zone (NZ) via FSW, which was totally different from the mill-annealed structure in the base material (BM). After annealing at 900 °C for 180 min, the BM and NZ then exhibited the similar elongation of > 500% and similar flow stress at 900 °C, 3 × 10⁻³ s⁻¹, which was the precondition for achieving uniform superplastic deformation in the entire joint. Moreover, the different microstructures in the BM and NZ tended to become the similar equiaxed structure after deformation, which was the result of different microstructural evolution mechanisms in the NZ and BM. For the NZ, there was a static and dynamic spheroidization of the fully lamellar structure during the process, which could largely reduce the flow softening of the fully lamellar structure. For the BM, a new view of “Langdon-CRSS theory” (CRSS, critical resolved shear stress) was proposed to describe the fragmentation of the coarse equiaxed structure, which established the relationship between grain boundary sliding and intragranular deformation during deformation.

Key words: Friction stir welding, Titanium alloys, Superplasticity, Static annealing, Microstructure

C.L. Jia, L.H. Wu, P. Xue, D.R. Ni, B.L. Xiao, Z.Y. Ma. Effect of static annealing on superplastic behavior of a friction stir welded Ti-6Al-4V alloy joint and microstructural evolution during deformation[J]. J. Mater. Sci. Technol., 2022, 130: 112-123.

Figures/Tables 15

Fig. 1. (a) Schematic of friction stir welding process and sample positions, (b) heating process during superplastic tensile test.

Fig. 2. (a) Cross-section of friction stir welded Ti-6Al-4V joint, and transmission electron microscope (TEM) images of: (b) base material (BM) and (c, d) nugget zone (NZ), and (e) high resolution transmission electron microscope (HR-TEM) image for marked lamellar structure in (d).

Fig. 3. Scanning electron microscope (SEM) images of (a, b) BM and (c, d) NZ after annealing at 900 °C for (a, c) 5 min, (b, d) 180 min, respectively.

Fig. 4. Superplastic flow stress of BM and NZ after annealing at 900 °C for 5, 60 and 180 min: (a) true stress-strain curves of BM and NZ at 900 °C, 3 × 10?3 s?1, (b) variation of flow stress at true strain of 0.1 with strain rate from 1 × 10?4 s?1 to 1 × 10?2 s?1. The flow stress for the BM and NZ at 1 × 10?3 s?1 in (b) has been reported in Ref. [25].

Fig. 5. (a) Variation of initial flow stress and average grain size with annealing time, and (b) macroscopic tensile specimens after fracture at 900 °C, 3 × 10?3 s?1 for BM and NZ after annealing for different times.

Fig. 6. Inverse pole figure (IPF) maps of (a-c) BM and (d-f) NZ at engineering strain of (a, d) 0%, (b, e) 40% and (c, f) 200% after annealing for 5 min before deformation, respectively.

Fig. 7. IPF maps of (a-c) BM and (d-f) NZ at engineering strain of (a, d) 0%, (b, e) 40% and (c, f) 200% after annealing for 180 min before deformation, respectively.

Fig. 8. Variation of (a) average grain size for BM and NZ, and (b) aspect ratio and spheroidization fraction for lamellar structure in NZ with engineering strain.

Fig. 9. Schmid factor evolution for basal slip and prismatic slip of BM at engineering strain of (a, d) 0%, (b, e) 40% and (c, f) 200% after annealing for 180 min before deformation, respectively.

Fig. 10. Schmid factor evolution for basal slip and prismatic slip of NZ at engineering strain of (a, d) 0%, (b, e) 40% and (c, f) 200% after annealing for 180 min before deformation, respectively.

Fig. 11. Average Schmid factor evolution along tensile direction for (a) basal and (b) prismatic slip of BM and NZ during superplastic deformation.

Fig. 12. <0002> pole figure for a distribution of Schmid factor of the basal slip in left and texture in right for (a-c) BM-5 and (d-f) BM-180 and at an engineering strain of (a, d) 0%, (b, e) 40% and (c, f) 200% during superplastic deformation.

Fig. 13. <0002> pole figure for a distribution of Schmid factor for the basal slip in left and texture in right for (a-c) NZ-5 and (d-f) NZ-180 and at an engineering strain of (a, d) 0%, (b, e) 40% and (c, f) 200% during superplastic deformation.

Fig. 14. Schematic of microstructural evolution in BM during annealing and subsequent deformation: (a) initial microstructure for BM, (b) grain coarsening for BM after static annealing, (c) intragranular deformation occurred in coarse grains with lower CRSS, (d) coarse grains occurred recrystallization to be smaller ones.

Fig. 15. Schematic of spheroidization process of lamellar structure in NZ during annealing and subsequent deformation: (a) fully lamellar structure in NZ, (b) static spheroidization for lamellar structure after static annealing, (c) dynamic spheroidization process for lamellar structure, (d) spheroidized grains after deformation.

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