Friction stir spot welding of SPCC low carbon steel plates at extremely low welding temperature

doi:10.1016/j.jmst.2018.11.011

Abstract

Abstract:

Friction stir spot welding (FSSW) was applied to 2.0 mm thick steel plate cold-rolled commercial (SPCC) low carbon steel plates at a very low rotation speed that ranged from 5 to 50 rpm, which was much lower than that generally used for the conventional FSSW technique. Due to the very low heat input, the welding processes could therefore be completed at a peak welding temperature below 160 °C. As a result, a significantly refined microstructure with an average grain size of about 0.41 μm was formed in the stir zone of the joints and the J1{0-11}<-211> and J2{1-10}<-1-12> shear textures were the dominant components, which are different from the D1{11-2}<111> and D2{-1-12}<111> shear textures formed in the conventional FSSW joints. In addition, no heat affected zone could be detected along the cross-sectional plane of the joints. Although a few void-like non-bonded areas were still observed along the interface between the upper and lower steel plates, the shear tensile loads of the joints increased to about 10.0 kN when welded at a condition of 8 t, 20 rpm and 30 s, and the joints fractured through the plug failure mode.

Key words: Friction stir spot welding, Dynamic recrystallization, Microstructure, Steel

Y.F. Sun, H. Fujii, Y. Sato, Y. Morisada. Friction stir spot welding of SPCC low carbon steel plates at extremely low welding temperature[J]. J. Mater. Sci. Technol., 2019, 35(5): 733-741.

Figures/Tables 14

Table 1 Composition and mechanical properties of SPCC steel.

Chemical compositions (wt%)						Mechanical properties
C	Si	Mn	P	S	Fe	Tensile strength (MPa)	Hardness (HV)
0.04	0.01	0.16	0.014	0.006	Bal.	312	110

Fig. 1 (a) Configuration of LT-FSSW and (b) appearance of rotating tool.

Fig. 2 (a) OM image and (b) orientation color map of BM.

Fig. 3 Temperature profile during conventional FSSW and LT-FSSW processes measured by a thermal digital camera.

Fig. 4 (a) OM image showing cross-sectional microstructure of LT-FSSW joints, (b, c) SEM images showing center and edge microstructure along interfaces of joint and (d) enlarged SEM image showing voids formed at interface.

Fig. 5 EBSD map showing microstructure at different positions along interface: (a, b) center and side part of LT-FSSW joint, respectively; (c, d) center and side part of conventional FSSW joint, respectively (ND: normal direction; TD: transverse direction).

Fig. 6 (a) Grain size distribution and (b) misorientation distribution of center and side part of SZ for LT-FSSW and conventional FSSW joints.

Fig. 7 Texture formed in center and side part of SZ for (a) LT-FSSW and (b) conventional FSSW joints (Imax: maximum intensity; SPN: shear plane normal; SD: shear direction).

Fig. 8 EBSD measurement on (a) outside part and (b) center part of lower side of joints.

Fig. 9 Vickers-hardness profile on cross-sections of (a) LT-FSSW joint and (b) conventional FSSW joint.

Fig. 10 Shear tensile loads of LT-FSSW joints and conventional FSSW joints welded at different rotation speeds.

Fig. 11 Photos showing (a) appearance of tensile specimen and (b) two types of fracture modes of LT-FSSW joints.

Fig. 12 Effect of rotation speed on shear tensile load of the joints welded at applied loads of (a) 6?t, (b) 7?t and (c) 8?t.

Fig. 13 Effect of applied load on shear tensile load of joints welded at 10?rpm and 30?s.

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