Microstructure and Mechanical Properties of Dissimilar Double-Side Friction Stir Welds Between Medium-Thick 6061-T6 Aluminum and Pure Copper Plates

doi:10.1007/s40195-022-01436-0

Abstract

Abstract:

It is difficult to achieve Al/Cu dissimilar welds with good mechanical properties for medium-thick plates due to the inherent high heat generation rate at the shoulder-workpiece contact interface in conventional friction stir welding. Thus, double-side friction stir welding is innovatively applied to join 12-mm medium-thick 6061-T6 aluminum alloy and pure copper dissimilar plates, and the effect of welding speeds on the joint microstructure and mechanical properties of Al/Cu welds is systematically analyzed. It reveals that a sound Al/Cu joint without macroscopic defects can be achieved when the welding speed is lower than 180 mm/min, while a nonuniform relatively thick intermetallic compound (IMC) layer is formed at the Al/Cu interface, resulting in lots of local microcracks within the first-pass weld under the plunging force of the tool during friction stir welding of the second-pass, and seriously deteriorates the mechanical properties of the joint. With the increase of welding speed to more than 300 mm/min void defects appear in the joint, but the joint properties are still better than the welds performed at low welding speed conditions since a continuous uniform thin IMCs layer is formed at the Al/Cu interface. The maximum tensile strength and elongation of Al/Cu weld are, respectively, 135.11 MPa and 6.06%, which is achieved at the welding speed of 400 mm/min. In addition, due to the influence of welding distortion of the first-pass weld, the second-pass weld is more prone to form void defects than the first-pass weld when the same plunge depth is applied on both sides. The double-side friction stir welding is proved to be a good method for dissimilar welding of medium-thick Al/Cu plates.

Key words: Double-side friction stir welding, Medium-thick plate, Dissimilar welding, Microstructures, Intermetallic compounds, Mechanical properties

J.X. Tang, L. Shi, C.S. Wu, M.X. Wu, S. Gao. Microstructure and Mechanical Properties of Dissimilar Double-Side Friction Stir Welds Between Medium-Thick 6061-T6 Aluminum and Pure Copper Plates[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(12): 2027-2046.

Figures/Tables 24

Table 1 Chemical compositions of 6061-T6 aluminum and pure copper (wt%)

Materials	Al	Cu	Mg	Si	Fe	Zn	Mn	Cr
6061-T6 aluminum	Bal.	0.275	0.882	0.582	0.536	0.572	0.166	0.083
Pure copper	0.005	Bal.	-	0.009	0.005	0.036	-	0.002

Table 2 Mechanical properties of base metals

Materials	Ultimate tensile strength (MPa)	Elongation (%)	Hardness (HV)
6061-T6 aluminum	368	13	115
Pure copper	245	14	98

Fig. 1 Schematic drawing of double-side friction stir dissimilar welding of Al to Cu

Fig. 2 a Schematic diagram of selection position of the tensile specimens and metallographic samples in Al/Cu welds, b dimension of tensile specimen, c schematic drawing of cross-section metallographic in Al/Cu weld

Fig. 3 Surface morphologies of the Al/Cu joints at different welding speeds

Fig. 4 Macrographs of the cross section of the Al/Cu joints at different welding speeds: a 90 mm/min, b 180 mm/min, c 300 mm/min, d 400 mm/min, e 500 mm/min

Fig. 5 SEM backscattered electron images (BEI) of the stir zone at different welding speeds: a 180 mm/min, b 400 mm/min, c 500 mm/min

Fig. 6 a Microstructure of Al/Cu joints at the welding speed of 180 mm/min at region d in Fig. 5a; b Al/Cu strips structures; c Cu particles; d Cu bulks; e-j corresponding EDS mapping of regions b-d

Table 3 EDS point scanning results are marked in Figs. 6 and 7

Point number	Al (at.%)	Cu (at.%)	Possible phases
1	53.8	46.2	AlCu
2	66.1	33.9	Al₂Cu
3	76.3	23.7	Al₂Cu
4	65.4	34.6	Al₂Cu
5	73.4	26.6	Al₂Cu
6	65.1	34.9	Al₂Cu
7	56.8	43.2	AlCu
8	50.6	49.4	AlCu
9	74.7	25.3	Al₂Cu
10	31.4	68.6	Al₄Cu₉
11	58	42	Al₂Cu
12	66.3	33.7	Al₂Cu
13	55.6	44.4	AlCu
14	65.7	34.3	Al₂Cu
15	70.9	29.1	Al₂Cu
16	48.5	51.5	AlCu
17	66.8	33.2	Al₂Cu
18	62.5	37.5	Al₂Cu
19	59.9	40.1	Al₂Cu
20	51.6	48.4	AlCu
21	66.7	33.3	Al₂Cu
22	69.2	30.8	Al₂Cu

Fig. 7 Microstructures of Al/Cu joints at the welding speed of 180 mm/min a at region e in Fig. 5a; b at region f in Fig. 5a; e at region g in Fig. 5a; c, d enlarged areas corresponding to the red circles in b; f enlarged area corresponding to the red circle in e

Fig. 8 Microstructures of Al/Cu joints at the welding speed of 400 mm/min: a, c enlarged areas corresponding to the red circle in b; b SEM at region h in Fig. 5a; d-i EDS mapping of a-c regions

Table 4 EDS point scan results for the points marked in Figs. 8 and 9

Point number	Al (at.%)	Cu (at.%)	Possible phases
1	94.6	5.4	Al(Cu)
2	68.1	31.9	Al₂Cu
3	64.9	35.1	Al₂Cu
4	27.6	72.4	Al₄Cu₉
5	97.5	2.5	Al(Cu)
6	60.8	39.2	Al₂Cu
7	52	48	AlCu
8	68.2	31.8	Al₂Cu
9	69.8	30.2	Al₂Cu
10	58	42	AlCu
11	71.4	28.6	Al₂Cu
12	65.8	34.2	Al₂Cu
13	78.3	21.7	Al₂Cu
14	78.4	21.6	Al₂Cu
15	95.5	4.5	Al(Cu)
16	54	46	AlCu
17	50.9	49.1	AlCu
18	67.8	32.2	Al₂Cu
19	59.6	40.4	Al₂Cu

Fig. 9 a Microstructure of Al/Cu joints at region i in Fig. 5b for the welding speed of 400 mm/min; b magnified area of the red circle in a; c microstructures at region j in Fig. 5b; d magnified area of the red circle in c; e microstructures at region k in Fig. 5b; f EDS mapping of Fig. 9e

Fig. 10 Microstructures of Al/Cu joints at the welding speed of 500 mm/min: a at region m in Fig. 5c; b at region n in Fig. 5c; c-e the magnified area of the red circle in b; f-k corresponding EDS mapping of regions c-e

Fig. 11 Microstructures of the joint for the welding speed of 500 mm/min: a at region o in Fig. 5c; b, c magnified area of the red circle in Fig. 9a; d at region p in Fig. 5c

Table 5 EDS point scanning results for the points marked in Fig. 11

Point number	Al (at.%)	Cu (at.%)	Possible phases
1	53.2	46.8	AlCu
2	62.2	37.8	Al₂Cu
3	78.2	21.8	Al₂Cu
4	79.3	20.7	Al₂Cu
5	79.9	20.1	Al₂Cu
6	69.7	30.3	Al₂Cu
7	53.8	46.2	AlCu
8	63	37	Al₂Cu

Fig. 12 a Transverse cross section of Al/Cu weld at the welding speed of 180 mm/min with six locations F1-F6; and b-n corresponding SEM and EDS line scan profiles at locations F1-F6, respectively

Table 6 EDS point scanning results for the points marked in Fig. 12

Point number	Al (at.%)	Cu (at.%)	Possible phases
1	51.1	48.9	AlCu
2	67.9	32.1	Al₂Cu
3	68.3	31.7	Al₂Cu
4	7.1	92.9	Cu(Al)
5	39.7	60.3	Al₄Cu₉
6	65.1	34.9	Al₂Cu
7	41.1	58.9	Al₄Cu₉
8	64.9	35.1	Al₂Cu

Fig. 13 a Transverse cross section of Al/Cu weld at the welding speed of 400 mm/min with six locations F1-F6; and b-n corresponding SEM and EDS line scan profiles at locations F1-F6, respectively

Table 7 EDS point scan results for the points marked in Fig. 13

Point number	Al (at.%)	Cu (at.%)	Possible phases
1	62	38	Al₂Cu
2	74.3	25.7	Al₂Cu
3	62.7	37.3	Al₂Cu
4	79.9	20.1	Al₂Cu
5	64.9	35.1	Al₂Cu
6	76.4	23.6	Al₂Cu
7	59	41	Al₂Cu
8	77.8	22.2	Al₂Cu
9	51.1	48.9	AlCu
10	68.5	31.5	Al₂Cu

Fig. 14 Microhardness distributions of the weld at different welding speeds: a diagram of locations for hardness measurement, b 180 mm/min, c 400 mm/min, d 500 mm/min

Fig. 15 Tensile strength and elongation of the joints at different welding speeds

Fig. 16 Fracture locations of Al/Cu weld joints at different welding speeds: a 180 mm/min, b 400 mm/min, c 500 mm/min

Fig. 17 SEM images of fracture surfaces at the Al alloy side of Al/Cu joints for different welding speeds

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