Influence of texture distribution in magnesium welds on their non-uniform mechanical behavior: A CPFEM study

doi:10.1016/j.jmst.2020.01.035

Abstract

Abstract:

Recent studies indicate that the texture distribution in friction stir welded (FSW) Mg alloys can be tailored and hence improve the joint performance. In this work, a crystal plasticity finite element modeling (CPFEM) was performed to understand the effects of texture distribution in stir zone (SZ) on the non-uniform plastic deformation and fracture localization. In total, six kinds of observed or purposely tilted texture distributions were modelled. The “concave-convex” appearance, as commonly observed in the tensile sample, was successfully simulated. It reveals that the mirror-symmetrical distribution of basal planes in the region of easy to activate basal slip (EABS) determined the “concave-convex” appearance in SZ-center. The asymmetrical appearance exchanged on plane A and plane B when the directions of basal planes were switched in the two EABS regions. Furthermore, the asymmetrical feature of plastic deformation was changed with varying the texture distribution in SZ. The “embossed” feature became more obvious in SZ-center first, and then gradually weakened with the c-axis rotated away from the weld plate plane. Severe necking was successfully simulated in SZ-center of FSW-H joint and in SZ-side of FSW-L joint. That might determine the observed fracture morphology. We believe that this simulation study is helpful for further improving the performance of FSW Mg joints.

Key words: Friction stir welding, Magnesium alloy, CPFEM, Non-uniform deformation, Fracture

Weijie Ren, Dejia Liu, Qing Liu, Renlong Xin. Influence of texture distribution in magnesium welds on their non-uniform mechanical behavior: A CPFEM study[J]. J. Mater. Sci. Technol., 2020, 46: 168-176.

Figures/Tables 12

Fig. 1. Non-uniform deformation and fracture features in a tensile sample of FSW Mg joints [26].

Fig. 2. Finite element model of Mg joints for the tensile test (a); texture distributions in various regions of FSW-H joint (b) and FSW-L joint (c); (0001) pole figure showing the six kinds of observed or purposely tilted texture distributions used in the model (d).

Table 1 Hardening parameters used for CPFEM simulations.

Mode	τ₀ (MPa)	τ_s (MPa)	h₀ (MPa)	A^th1	A^th2
Basal < a>	10	50	80	-	-
Prismatic < a>	70	200	130	-	-
Pyramidal < c+a>	121	128	50	-	-
Extension twin	28	30	10	0.42	0.4

Fig. 3. Measured (symbols) and simulated (lines) true stress-strain curves of the two kinds of Mg joints under the uniaxial tension along TD. The insets show the SF maps for {10-12} extension twinning in the transition region between TMAZ and SZ-side.

Fig. 4. EBSD orientation maps of the transition region between TMAZ and SZ-side on AS and RS after 5% deformation: (a) FSW-H joint and (b) FSW-L joint. The twin volume fractions, measured and simulated {0001} pole figures of each region were displayed nearby the respective EBSD maps.

Fig. 5. Relative activities of deformation modes in various regions of Mg joints on AS: (a) FSW-H joint and (b) FSW-L joint. Twin volume fraction is also simulated in some representative regions.

Fig. 6. Distribution of the displacement along z-direction (UZZ) after the simulated tension deformation to certain strains and the observed morphologies of fractured samples [29]: (a) FSW-H joint and (b) FSW-L joint. As indicated, the two joints fractured in different strains during the experimental tests.

Fig. 7. Distribution of the simulated displacement along y-direction (Uyy): (a) top surface and (b) bottom surface of FSW-H joint; (c) top surface and (d) bottom surface of FSW-L joint. The fracture features of FSW-H joint (e) and FSW-L joint (f) are displayed under the simulation results [29].

Fig. 8. The depth in WD along Path 1 (Plane A) (a) and Path 2 (Plane B) (b) for the samples modelled with different texture distributions (numbered in 1-6).

Fig. 9. Distribution of strain components after 8% strain for (a) FSW-H joint and (b) FSW-L joint.

Fig. 10. Schematic illustration of the distribution of basal planes in a FSW Mg joint (a) and the shear of basal planes in EABS during the tensile test (b)-(c); (d) (0001) projection showing the simulated trends of grain rotation in EABS and SZ-side. H and L in (d) indicate FSW-H joint and FSW-L joint, respectively.

Fig. 11. The “embossed” feature in FSW-H joint after 8% strain: (a) all deformation modes as described in section 3 are allowed, (b) prismatic slip was prohibited in SZ-center, (c) basal slip was prohibited in EABS, (d) all deformation modes are allowed but exchanging basal plane directions between the left and right EABS regions.

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