Rapid Ultrasonic-Assisted Soldering of AZ31B Mg Alloy/6061 Al Alloy with Low-Melting-Point Sn-xZn Solders Without Flux in Air

doi:10.1007/s40195-018-0749-x

Abstract

Abstract:

A novel ultrasonic-assisted low-temperature soldering was developed to join AZ31B Mg alloy and 6061 Al alloy with a series of Sn-xZn solders. The average maximum shear strength of the joints reaches up to 87.5 MPa at soldering temperature of 300 °C under ultrasonic assistance for only 5 s using Sn-20Zn solder. The fracture path propagates completely in the soldering seam. The results indicate that the microjet generated by ultrasonic pressure in liquid solder could strike and splinter the Mg₂Sn intermetallic compounds into small pieces, which contributes to the enhancement of the joint strength. In addition, the primary Al(Zn) solid solution phase formed during cooling stage could also strengthen the joint due to the prevention of microcracks propagation.

Key words: Ultrasonic-assisted soldering, Ultrasonic effect mechanism, Microjet, Intermetallic compound distribution, Solid solution, Joint strengthening

Zhi-Wei Lai, Zhe-Yuan Huang, Chuan Pan, Hui-Qiao Du, Xiao-Guang Chen, Lei Liu, Wei-Ming Long, Gui-Sheng Zou. Rapid Ultrasonic-Assisted Soldering of AZ31B Mg Alloy/6061 Al Alloy with Low-Melting-Point Sn-xZn Solders Without Flux in Air[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(3): 332-342.

Figures/Tables 14

Table 1 Chemical compositions of AZ31B Mg alloy and 6061 Al alloy (wt%)

Base metal	Al	Si	Zn	Mn	Mg
AZ31	2.5-3.5	0.1	0.5-1.5	0.2-0.5	Bal.
6061	98	0.5	0.0005	0.0038	0.97

Fig. 1 Schematic diagrams of ultrasonic-assisted soldering process a and shear strength test b

Fig. 2 Schematic diagrams of technological process parameters for different Zn contents of Sn-xZn (x = 0, 9, 20, 30, wt%) solders a and varied ultrasonic assistance time using Sn-20Zn solder b

Fig. 3 Microstructures of joint with different Zn contents: a pure Sn (260 °C); b Sn-9Zn (230 °C); c Sn-20Zn (300 °C); d Sn-30Zn (340 °C)

Fig. 4 Shear strength of Mg/Al alloy joint with a series of Sn-xZn solders

Fig. 5 Microstructures of joints with Sn-20Zn interlayer soldering under different ultrasonic assistance time: a 1 s; b 3 s; c 5 s; d 10 s; e 15 s

Fig. 6 Diagrams of acoustic cavitation effect: a formation process of acoustic bubble; b compression process of acoustic bubble; c distortion process of acoustic bubble; d collapse process of acoustic bubble

Fig. 7 Magnified microstructures of joints at Mg a-c and Al d-f alloys side interface with ultrasonic assistance time of 1 s a, d, 5 s b, e and 15 s c, f

Table 2 EDS results taken from possible phases (at.%)

Possible phase	Mg	Al	Sn	Zn
Sn	4.0	4.2	88.9	2.9
η-Zn	1.09	1.91	3.43	93.57
Mg₂Sn	66.9	1.7	27.9	3.5
Al(Zn) precipitated phase	0.8	87.9	00.4	10.6
Al(Zn) solid solution phase	0.88	92.23	1.2	6.22

Fig. 8 Joint fracture paths under varied ultrasonic assistance time: a 1 s; b 5 s; c 15 s

Fig. 9 Secondary electron a, c, e and backscattering electron b, d, f images of Sn-20Zn soldered joint fracture surface with ultrasonic assistance time of 1 s a, b, 5 s c, d; 15 s e, f

Fig. 10 a Joint shear strength and b XRD patterns of joint fracture surface with different ultrasonic assistance time

Fig. 11 Morphologies of Al(Zn) phase distribution a and microcrack propagation behavior b

Fig. 12 Joint formation model of ultrasonic-assisted soldering: a initial state of solders and base materials; b disintegration of layered Mg2Sn IMCs by microjet; c dispersive distribution of blocky Mg2Sn IMCs under ultrasound; d secondary crystallization of Mg2Sn IMCs

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