Microstructural Evolution and Mechanical Behavior of Friction-Stir-Welded DP1180 Advanced Ultrahigh Strength Steel

doi:10.1007/s40195-019-00949-5

Abstract

Abstract:

Friction stir lap welding of a DP1180 advanced ultrahigh strength steel was successfully carried out by using three welding tools with different pin lengths. The effects of the welding heat input and material flow on the microstructure evolution of the joints were analyzed in detail. The relationship between pin length and mechanical properties of lap joints was studied. The results showed that the peak temperatures of all joints exceeded A_c3, and martensite phases with similar morphologies were formed in the stir zones. These martensite retained good toughness due to the self-tempering effect. The formation of ferrite and tempered martensite was the main reason for the hardness reduction in heat-affected zone. The mechanical properties of the lap joints were determined by loading mode, features of lap interface and the joint defects. When the stir pin was inserted into the lower sheet with a depth of 0.4 mm, the lap joint exhibited the maximum tensile strength of 12.4 kN.

Key words: Advanced ultrahigh strength steel, Friction stir welding, Microstructure, Mechanical behavior

Z. W. Wang, G. M. Xie, D. Wang, H. Zhang, D. R. Ni, P. Xue, B. L. Xiao, Z. Y. Ma. Microstructural Evolution and Mechanical Behavior of Friction-Stir-Welded DP1180 Advanced Ultrahigh Strength Steel[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(1): 58-66.

Figures/Tables 14

Table 1 Chemical composition of DP1180 steel (wt%)

C	Mn	Si	Cr	Mo	Cu	Al	S	P	Fe
0.18	2.4	0.60	0.02	0.01	0.02	0.05	0.005	0.01	Bal.

Table 2 Experimental welding parameters

d (mm)	ω (rpm)	ν (mm/min)	D (mm)	l (mm)
0.2				1.37
0.4	400	150	0.18	1.57
0.8				1.97

Fig. 1 Schematic diagrams of two kinds of FSW processes: a AS-loaded, b RS-loaded

Fig. 2 Surface morphologies of FSW joints at different inserted depths of the stir pin: a 0.2 mm, b 0.4 mm and c 0.8 mm

Fig. 3 Cross-sectional OM macrographs of FSW joints at different inserted depths of the stir pin: a, d 0.2 mm, b 0.4 mm, c, e 0.8 mm

Fig. 4 OM and SEM images in different regions with a 0.4 mm inserted depth joint: a the top sheet on the AS, b PM, c SZ, d HAZ1, e HAZ2, f HAZ3

Fig. 5 SEM images showing the SZ microstructures at inserted depths of a 0.2 mm, b 0.8 mm

Fig. 6 Hardness distribution along a TD of 0.4 mm inserted depth joint, b ND of all joints with different inserted depths

Fig. 7 Tensile shear properties of joints with different welding parameters

Fig. 8 OM images of fracture locations of tensile shear specimens with different welding parameters

Fig. 9 SEM micrographs of the fracture surfaces of typical tensile shear specimens: a-c 0.2 AS-loaded, d 0.4 RS-loaded

Fig. 10 Sketch phase diagram of Fe-Fe3C

Fig. 11 Material flow model during FSW process of a 0.2 mm, b 0.4 mm, c 0.8 mm inserted depth joints

Fig. 12 Stress distribution in two kinds of tensile shear specimens: a AS-loaded, b RS-loaded

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