Influence of Pin Offset on Microstructure and Mechanical Properties of Friction Stir Welded Mg/Ti Dissimilar Alloys

doi:10.1007/s40195-019-00899-y

Abstract

Abstract:

The influences of pin offset on the formation, microstructure and mechanical properties of friction stir welded joint of Ti6Al4V and AZ31B Mg dissimilar alloys were investigated. The results show that sound joints are obtained at different offsets. With the offset decreasing from 2.5 to 2.1 mm, the number of Ti alloy fragments is increased, and the stir zone (SZ) is enlarged and the grains in SZ become coarser. A hook-like structure is formed at the Mg/Ti interface and its length is increased with the decrease in pin offset. The Al element has an enrichment trend at the Ti alloy side near the Mg/Ti interface when the offset is decreased, which is beneficial to the bonding of the interface. An Al-rich layer with a thickness of 3-5 μm forms at the offset of 2.1 mm. All the joints fracture at the interface and present a mixed ductile-and-brittle fracture mode. The joint tensile strength is increased with the offset decreasing from 2.5 to 2.1 mm, and the maximum tensile strength of 175 MPa is acquired at the offset of 2.1 mm.

Key words: Mg/Ti dissimilar alloys, Friction stir welding, Pin offset, Mechanical properties, Microstructure

Qi Song, Zhong-Wei Ma, Shu-De Ji, Qing-Hua Li, Liu-Fang Wang, Rui Li. Influence of Pin Offset on Microstructure and Mechanical Properties of Friction Stir Welded Mg/Ti Dissimilar Alloys[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(10): 1261-1268.

Figures/Tables 9

Table 1 Chemical compositions of AZ31B and Ti6Al4V alloys (wt%)

Materials	Chemistry compositions
AZ31B	Al	Zn	Mn	Fe	Si	Mg
AZ31B	3.00	0.88	0.88	0.018	0.015	Bal.
Ti6Al4V	Al	V	Fe	C	N	Ti
Ti6Al4V	6.15	4.00	0.50	0.10	0.05	Bal.

Fig. 1 a Schematic of the FSW process; b size of tensile specimen

Fig. 2 Cross sections of the joints at different offsets: a 2.5 mm, b 2.3 mm, c 2.1 mm; d-f enlarged views marked in Fig. 2a-c

Fig. 3 Microstructures of a AZ31B alloy BM and SZs at offsets of b 2.5 mm, c 2.3 mm, d 2.1 mm

Fig. 4 Distributions of Al element near joining interfaces at offsets of a 2.5 mm, b 2.3 mm, c 2.1 mm; EDS results of d Point 1, e Point 2, f Point 3

Fig. 5 Microhardness profiles of the joint cross sections at offsets of 2.5 mm, 2.3 mm and 2.1 mm

Fig. 6 Tensile strengths of joints at different offsets

Fig. 7 Fracture position of the joint at 2.1 mm offset

Fig. 8 Fracture surface morphologies of joint at 2.1 mm offset: a-d regions A-D marked in Fig. 7; e, f enlarged views marked in Fig. 8a

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