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J. Mater. Sci. Technol.  2018, Vol. 34 Issue (1): 73-91    DOI: 10.1016/j.jmst.2017.11.041
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Friction stir welding of high-strength aerospace aluminum alloy and application in rocket tank manufacturing
Guoqing Wanga*(), Yanhua Zhaob, Yunfei Haob
a China Academy of Launch Vehicle Technology, Beijing 100076, China
b Capital Aerospace Machinery Company, Beijing 100076, China
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Abstract  

Friction stir welding (FSW) has been widely adopted in aerospace industry for fabricating high-strength aluminum alloy structures, such as large volume fuel tanks, due to its exceptional advantages including low distortion, less defects and high mechanical properties of the joint. This article systematically reviews the key technical issues in producing large capacity aluminum alloy fuel tanks by using FSW, including tool design, FSW process optimization, nondestructive testing (NDT) techniques and defect repairing techniques, etc. To fulfill the requirements of Chinese aerospace industry, constant-force FSW, retractable tool FSW, lock joint FSW, on-line NDT and solid-state equal-strength FSW techniques, as well as a complete set of aerospace aluminum FSW equipment, have been successfully developed. All these techniques have been engineered and validated in rocket tanks, which enormously improved the fabrication ability of Chinese aerospace industry.

Key words:  Fuel tank      High-strength aluminum alloy      Friction stir welding      Application     
Received:  23 May 2017     
Corresponding Authors:  Wang Guoqing     E-mail:  wanggq@spacechina.com

Cite this article: 

Guoqing Wang, Yanhua Zhao, Yunfei Hao. Friction stir welding of high-strength aerospace aluminum alloy and application in rocket tank manufacturing. J. Mater. Sci. Technol., 2018, 34(1): 73-91.

URL: 

https://www.jmst.org/EN/10.1016/j.jmst.2017.11.041     OR     https://www.jmst.org/EN/Y2018/V34/I1/73

Fig. 1.  FSW process with tilt angle tool [12].
Fig. 2.  The first generation of FSW tool: (a) Concave shoulder plus Threads pin, (b) Concave shoulder plus Threads & Flutes pin.
Fig. 3.  The second generation of FSW tool with plane shoulder plus Archimedean spiral: (a) top view, (b) main view.
Fig. 4.  The third generation of FSW tool with convex cone/convex sphere shoulder plus Archimedean spiral pin: (a) isometric side view, (b) sectional view, (c) a tool picture.
Fig. 5.  Influence of tilt angle of conventional tool on mechanical properties of FSW AA2219 joints.
Fig. 6.  Influence of distance between pin and backing plate on tensile strength of FSW AA2219 joints.
Fig. 7.  Optimized FSW process window with tool rotation rate and traveling speed for AA2219.
Geometry of tool Tilt angle (°) Tensile strength (MPa) Elongation (%)
Concave shoulder 2.5 330-345 5.5-7.0
Convex sphere shoulder 0 320-335 4.5-8.5
Convex sphere shoulder 1 325-345 4.5-8.0
Table 1  Tensile strength of FSW AA2219 joints with various welding tools.
Thickness (mm) Tensile strength (MPa) Elongation (%)
3.0 340 5.0
4.0 345 5.5
5.5 345 6.0
6.0 345 6.0
8.0 335 4.5
Table 2  Mechanical properties of FSW AA2219 joints with different thicknesses.
Fig. 8.  Impact of butt gap on FSW joint performance (8 mm thick plate AA2219).
Fig. 9.  Impact of thickness difference on joint performance (8 mm thick plate AA2219).
Fig. 10.  Impact of tool deviation on joint performance (8 mm thick plate AA2219).
Fig. 11.  Typical joint and joint line defects of FSW.
Fig. 12.  Result comparison by PAUT and metallographical inspection: (a) result from PAUT, (b) result from metallographical inspection.
Fig. 13.  Result comparison from PAUT and metallographical inspection: (a) result from PAUT, (b) result from metallographical inspection.
Fig. 14.  Result comparison by PAUT and metallographical inspection: (a) result from PAUT, (b) result from metallographical inspection.
Defect type Repair method
Void, surface furrow, LOP repetitive FSW
Small size of defect, without high request for joint performance Manual fusion welding
Keyhole type Fusion filling/solid-state filling plus FSW
Table 3  Repair welding schemes for different defects of tank welds.
Mechanical properties of re-welded joint Repetitive times
0 1 2 3
Tensile strength (MPa) 340 343.3 341.7 338.3
Elongation (%) 6.0 5.67 6.33 7.0
Table 4  Impact of welding times on mechanical properties of 6 mm thick FSW AA2219 joint.
Fig. 15.  Metallographic diagram of repaired FSW AA2219 joints by fusion welding [20].
Fig. 16.  Repair design with fusion filling + FSW: (a) Defect position, (b) Filling defect, (c) One-time repair [21].
Fig. 17.  Macrostructure of FSW AA2219 joint produced by fusion filling and FSW repairing [20].
Fig. 18.  Profiles of precipitated phases of repaired AA2219 joint [20]: (a) base material, (b) stirred zone, (c) TMAZ, (d) HAZ.
Tool type Structural parameter of shoulder Structural parameter of pin
Configuration Diameter (mm) Configuration Root diameter (mm) Length (mm)
Regular tool Inside recess with 7° of slope 18 Tapered tread at 22° 6.0 5.8
Repair tool Inside recess with 10° of slope;
+2 concentric circles
20 Tapered thread at 20°;
+3 slopes
7.0 5.7
Table 5  Structural design of different FSW tools for regular welding and repairing application.
Fig. 19.  Macroscopic profile of FSW repaired joint for keyhole: (a) un-repaired FSW joint, (b) joint repaired by solid-state filling plus FSW.
Sample type No. Tensile strength (MPa) Elongation (%) Average Strength factor
One-time defect-less sample 1 340 7.0 343.3/7.7 78.0
2 345 8.0
3 345 8.0
Second-time welding repair sample 1 335 8.0 341.7/8.3 77.7
2 340 8.5
3 345 8.5
Keyhole repair sample 1 345 9.0 338.3/9.0 76.89
2 345 9.0
3 340 9.0
Table 6  Tensile properties of repaired AA2219 joint at different positions.
Fig. 20.  Schematic diagram for structure and main welds of launch vehicle tank.
Fig. 21.  Typical control modes in engineering application of FSW: (a) Constant-force mode, (b) Constant-displacement mode.
Fig. 22.  Schematic diagram of constant-force control unit.
Fig. 23.  Correlation between tool plunge depth and welding force and tensile strength with various thicknesses of AA2219 plates: (a) 4 mm, (b) 5 mm, (c) 6 mm.
No. Threshold value of force adjustment Joint appearance
1 2% Frequent adjustment, repetitive vibration of tool, rough appearance of joint
2 3% Slightly frequent adjustment, better appearance of joint
3 4% Good effect of adjustment, smooth appearance
4 5% Slightly hysteretic adjustment, slow adjustment of tool
5 6% Hysteretic adjustment, slow adjustment of tool, weak effect of constant force
Table 7  Correlation between force fluctuation and joint appearance.
Fig. 24.  Correlation between force and time during the welding of ellipsoid tank dome.
Gore No. Max. tensile strength (MPa) Min. tensile strength (MPa) Average tensile strength (MPa)
1 365 350 360
2 365 335 340
3 365 345 355
4 360 340 350
5 360 335 350
Table 8  Mechanical properties of variable-curvature FSW AA2219 joints of tank dome.
Fig. 25.  FSW tool with retractable pin for circumferential weld: (a) shoulder, (b) pin, (c) assembled tool, (d) real object of retractable tool.
Fig. 26.  Working principle of retractable FSW.
Fig. 27.  Retractable track of pin.
Fig. 28.  Transverse cross-section of retractable FSW AA2219 joint at different retraction positions: (a) retraction starting, (b) retraction to 25% of pin, (c) retraction to 50% of pin, (d) retraction to 75% of pin, (e) retraction ending.
Fig. 29.  Effect of retraction speed on mechanical properties of FSW AA2219 joints.
Fig. 30.  Elimination of keyhole.
Fig. 31.  Schematic diagram of lock joint employed in FSW process.
Fig. 32.  “Hook” defect in FSW lap joints [21].
Fig. 33.  Macroscopic profiles of FSW AA2219 lock joints with different pin lengths and AS locations as well as bending and deformation of lap joint [24]. (pin length was respectively 6.0 mm, 6.5 mm and 7.0 mm; the lock joint was that when short section and Y ring were respectively located on AS).
Fig. 34.  Effect of FSW pin length and AS location on tensile strength of FSW AA2219 lock joint [23]: (a) tensile strength at 20 °C, (b) tensile strength at -196 °C.
Fig. 35.  Schematic drawing of pin clamping fixture.
Fig. 36.  Automatic on-line PAUT for tank welding with non-planar path.
Fig. 37.  Schematic drawing of friction push plug welding.
Fig. 38.  Schematic drawing of friction pull plug welding.
Fig. 39.  Appearance of friction push plug weld sample: (a) Top view, (b) Bottom view.
Fig. 40.  Appearance of friction pull plug weld sample: (a) Top view, (b) Bottom view.
Fig. 41.  Metallographic structure of FPW repaired FSW AA2219 joint: (a) macroscopic profile, (b) microstructure.
Fig. 42.  Friction plug welding of tank sample: (a) FPW for Φ5 m barrel, (b) Local diagram for repair welding.
Fig. 43.  Fracture locations of friction pull plug repaired FSW joint [7].
Fig. 44.  Machine of friction pull plug repair.
Fig. 45.  Longitudinal FSW equipment of tank section.
Fig. 46.  FSW equipment of tank dome.
Fig. 47.  Circumferential FSW equipment for tank.
Fig. 48.  Application of FSW in longitudinal weld of tank section: (a) Φ3350 section, (b) Φ5000 section.
Fig. 49.  Application of FSW on Φ3350 dome.
Fig. 50.  Application of FSW on Φ 3350 tank.
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