Effect of Shoulder Diameter on the Force Generation, Microstructure and Mechanical Properties of Friction Stir-Brazed Aluminium Alloy and Copper with Ultrahigh Rotation Speed

doi:10.1007/s40195-019-00961-9

Abstract

Abstract:

Friction stir brazing with ultrahigh rotation speed was applied to 6061 aluminium alloy-pure copper lap joints with the aid of zinc foil. The effects of different shoulder diameters from 7 to 15 mm on the microstructure and mechanical properties of Al/Cu FSB joints were investigated along with the temperature and resistance of the friction tool. The oscillation of forward resistance and lateral force was related to the flow of the plastic metal and contributed to obtain a good appearance during the welding process. From the appearance of the welded joints, it was obvious that the phase difference between the forward resistance and lateral force had a significant influence on the joint characteristics. Obvious scale-like ripples appeared on the weld area when a sharp angle in the phase difference curve existed. Additionally, with a lower axial force and oscillation assistance, a satisfactory joint could be obtained. The results of the shear strength of the brazed joint showed that the shoulder with a 12 mm diameter yielded the highest shear strength. Meanwhile, the zinc foil in the middle melted completely and formed finely dispersed CuZn₅ Al-Zn eutectic structures at the Al-Cu interface.

Key words: Friction stir brazing, Ultrahigh rotation speed, Zn foil, Forward resistance, Lateral force

Yang Zhou, Shujin Chen, Di Wang, Ruifeng Li, Bin Liu, Juan Pu, Zhidong Yang. Effect of Shoulder Diameter on the Force Generation, Microstructure and Mechanical Properties of Friction Stir-Brazed Aluminium Alloy and Copper with Ultrahigh Rotation Speed[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(1): 154-164.

Figures/Tables 17

Fig. 1 High-speed friction stir welding equipment. FSW machine a, detection device b, detection principle of forward resistance c, detection principle of lateral force d

Fig. 2 Schematic of the FSW process a, position of the shear test sample, which are also temperature detection points measured via thermocouple b

Table 1 Base material chemical composition (wt%)

Material	Al	Cu	Zn	Si	Fe	Mn	Mg	Cr
6061-T6	Bal.	0.28	0.05	0.62	0.5	0.1	1	0.19

Table 2 Welding parameters and their levels

Factor	Parameters	Level 1	Level 2	Level 3	Level 4
A	Shoulder parameter (mm)	7	10	12	15
B	Rotation speed (rpm)	8000	10,000	12,000	14,000
C	Travel speed (mm/min)	50	100	150	200
D	Plunge depth (mm)	0.05	0.1	0.15	0.2

Table 3 Experimental layout using the L16 (45) orthogonal array

Group	Factor A	Factor B	Factor C	Factor D	Error	STS (MPa)
1	1	1	1	1	1	0
2	1	2	2	2	2	0
3	1	3	3	3	3	0
4	1	4	4	4	4	0
5	2	1	2	3	4	10.46
6	2	2	1	4	3	11.77
7	2	3	4	1	2	0
8	2	4	3	2	1	9.34
9	3	1	3	4	2	15.68
10	3	2	4	3	1	15.43
11	3	3	1	2	4	17.12
12	3	4	2	1	3	13.96
13	4	1	4	2	3	7.01
14	4	2	3	1	4	7.33
15	4	3	2	4	1	9.68
16	4	4	1	3	2	8.13

Table 4 Results of range analysis of the orthogonal test (MPa)

	A	B	C	D
$\overline{{T_{i1} }}$	0	8.292	9.259	5.323
$\overline{{T_{i2} }}$	7.897	8.636	8.528	8.369
$\overline{{T_{i3} }}$	15.550	6.701	8.091	8.510
$\overline{{T_{i4} }}$	8.043	7.861	5.611	9.287
R_i	15.550	1.935	3.648	3.964

Fig. 3 Temperature histories at one point a and axial force histories for the whole process b

Fig. 4 Distribution of the forward resistance and lateral force a and variation in detail b

Fig. 5 Resultant force histories for the whole process a and the average of the resultant force for different tool diameters b

Fig. 6 Weld surfaces and phase difference curves for different tool diameters

Fig. 7 Macrostructures of the cross sections of pin-less FSW with a Zn metal filler produced

Fig. 8 Tensile test results of the average tensile strength a and depictions of the two fracture types b

Fig. 9 Overall images of the Cu side of the fracture surface for different shoulder diameters: a D7, b D10, c D12, d D15

Fig. 10 SEM images of the Al/Zn/Cu interlayer produced by FSB under different shoulder diameters: a D7; b D10; c D12, d D15

Table 5 Element compositions and possible phase constitutions of the regions marked in Fig. 10

Sample	Regions	at.%	Phase	Oscillation amplitude
Sample	Regions	at.%	Phase	Axial	Resultant force
D7	A	33.35Al-77.65Zn	Al-Zn eutectic	0.15	0.008
	B	100Zn	Pure Zn
	C	19.74Cu-80.26Zn	CuZn₅
D10	D	39.63Al-10.27Cu-50.1Zn	Al-Zn eutectic	0.08	0.008
D10	E	7.52Al-20.74Cu-71.74Zn	CuZn₅	0.08	0.008
D12	F	83.54Al-1.84Cu-14.63Zn	α-Al	0.088	0.008
	G	6.32Al-22.14Cu-71.53Zn	CuZn₅
	H	22.26Al-12.44Cu-65.3Zn	Al-Zn eutectic
	I	3.47Al-19.59Cu-76.94Zn	CuZn₅
D15	J	37.7Al-12.2Cu-50Zn	Al-Zn eutectic	0.09	0.008
	K	67.26Al-31.77Cu-0.97Zn	Al₂Cu
	L	46.6-48.62Cu-4.78Zn	Al_4.2Cu_3.2Zn_0.7

Fig. 11 XRD patterns detected from the shear fracture surface of the copper side and the aluminium side after a shear test: a D12, b D15

Fig. 12 SEM images of the Cu side of the fractured surface: a D7, b D10, c D12, d D15

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