Radial Additive Friction Stir Repairing of Mechanical Hole Out of Dimension Tolerance of AZ31 Magnesium Alloy Assisted by Stationary Shoulder: Process and Mechanical Properties

doi:10.1007/s40195-021-01223-3

Abstract

Abstract:

Radial additive friction stir repairing (R-AFSR) assisted by stationary shoulder was put forward in the present study, which can be employed to repair the mechanical hole out of dimension tolerance of AZ31 magnesium alloy sheet. The results show that the stationary shoulder has sealed-barrier, heat-sink and extra-forging effects. The heat-sink effect improves the microstructure uniformity along the stir zone (SZ) thickness and the surface appearance of repaired hole, and the sealed-barrier and extra-forging effects eliminate the super-fine grain band in the SZ. Therefore, these three effects improve the formation quality of repaired region, thereby enhancing the mechanical properties of repaired mechanical hole compared with conventional R-AFSR. The tensile and compressive shear strengths of the repaired hole by stationary shoulder R-AFSR both increase first and then decrease when the rotating speed changes from 1200 to 1800 rpm, and these maximum values, respectively, reach 190 ± 3 MPa and 64.5 ± 2 MPa at 1400 rpm. The addition of stationary shoulder during R-AFSR can obtain a higher-quality repaired hole and broaden the repairing process window.

Key words: Stationary shoulder, Radial additive friction stir repairing, Mechanical hole out of dimension tolerance, AZ31 magnesium alloy, Microstructure, Mechanical properties

Ying-Ying Zuo, Hua Liu, Peng Gong, Shu-De Ji, Bao-Sheng Wu. Radial Additive Friction Stir Repairing of Mechanical Hole Out of Dimension Tolerance of AZ31 Magnesium Alloy Assisted by Stationary Shoulder: Process and Mechanical Properties[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(10): 1345-1360.

Figures/Tables 20

Fig. 1 Sketch maps of SSR-AFSR process: a drilling stage, b filling stage, c repairing stage, d retracting stage

Fig. 2 Drilling and rotating tools used at a drilling and b repairing stages; c stationary shoulder tool system

Table 1 Chemical compositions and mechanical properties of AZ31 magnesium alloy (wt%)

Chemical compositions (mass %)							Mechanical properties
Al	Mn	Zn	Cu	Si	Fe	Mg	Tensile strength (MPa)	Elongation (%)	Hardness (Hv)
3.01	0.46	1.11	0.003	0.015	0.001	Bal.	240	12	58

Fig. 3 Sketch maps of samples: a tensile test, b compressive shear test

Fig. 4 Sketch map of compression-shear device and the actual process of compression shear test

Fig. 5 Model used in simulation: a rotating tool, b stationary shoulder, c mesh generation

Fig. 6 Specific heat and thermal conductivity of AZ31 magnesium alloy [22]

Fig. 7 Surface morphologies of repaired region with stationary shoulder at 1400 rpm: a after drilling, b after filling and c after retracting; d surface morphology after retracting of conventional R-AFSR

Fig. 8 Micrographs of cross sections of repaired holes: a 1200 rpm, b 1400 rpm, c 1600 rpm, and d 1800 rpm under SSR-AFSR; e 1400 rpm and f 1600 rpm under conventional R-AFSR [20]

Fig. 9 Microstructures of typical zones: a BM; b HAZ, and c TMAZ at 1400 rpm marked in Fig. 8b; d HAZ at 1400 rpm marked in Fig. 8e

Fig. 10 Microstructures of typical areas in SZ: a A1 and b B1 marked in Fig. 8b; c A, d B marked in Fig. 8e

Fig. 11 Bonding area and super-fine grain band under conventional R-AFSR: a area 1 marked in Fig. 8e and area 2 marked in Fig. 8f

Fig. 12 a Force analysis; material flow behavior of b cross section, c horizontal section with 1 mm away from the top surface of sheet

Fig. 13 Sketch maps of the interface formation: a conventional R-AFSR, b SSR-AFSR

Fig. 14 Microhardness testing results under SSR-AFSR: a sketch map of tested points, b actual tested points, c microhardness distribution of repaired region at 1400 rpm, d microhardness distributions under different rotating speeds

Fig. 15 Tensile test results of the repaired hole: a tensile strength, b stress-strain curves

Fig. 16 Compressive shear test results of the repaired hole: a compressive shear strength, b load-displacement curves

Fig. 17 Fracture paths of tensile samples: a 1400 rpm and b 1600 rpm under SSR-AFSR; c 1400 rpm, d 1600 rpm under conventional R-AFSR [20]

Fig. 18 Fracture surface micrographs of tensile samples near the FM/BM interface: a 1400 rpm and b 1600 rpm under SSR-AFSR; c 1400 rpm, d 1600 rpm under conventional R-AFSR [20]

Fig. 19 Fracture surface micrographs of compressive shear samples at 1400 rpm: a SSR-AFSR, b conventional R-AFSR

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