Evolution of Microstructure, Mechanical Properties and Corrosion Resistance of Ultrasonic Assisted Welded-Brazed Mg/Ti Joint

doi:10.1016/j.jmst.2016.08.029

Abstract

Abstract:

Ultrasonic vibration with different powers from 0 kW to 1.6 kW was applied during the tungsten inert gas welding-brazing of Mg/Ti. The microstructures, mechanical properties and corrosion resistance of the ultrasonic assisted tungsten inert gas (U-TIG) welded-brazed Mg/Ti joint were characterized. The results showed that, without being subjected to ultrasonic vibration, coarse columnar α-Mg grains occurred in the fusion zone of Mg/Ti joint. However, with ultrasonic power of 1.2 kW, the average grain size of columnar α-Mg grains was refined from 200 μm to about 50 μm and the tensile strength of joints increased ~18% up to 228 MPa. Besides, high fraction of grain boundaries was introduced by grain refinement, contributing to improve the corrosion resistance in two ways: (i) accelerating the formation of Mg(OH)₂ protective layer and (ii) reducing the mismatch and disorder between Mg(OH)₂ protective layer and Mg alloy surface.

Key words: Magnesium alloy, Titanium alloy, Microstructure, Mechanical properties, Corrosion resistance, Ultrasonic vibration

Xu Chuan,Sheng Guangmin,Cao Xuezhen,Yuan Xinjian. Evolution of Microstructure, Mechanical Properties and Corrosion Resistance of Ultrasonic Assisted Welded-Brazed Mg/Ti Joint[J]. J. Mater. Sci. Technol., 2016, 32(12): 1253-1259.

Figures/Tables 14

Table 1. Chemical composition of base metals (wt%)

Base metal	Al	Zn	Mn	Fe	V	Mg	Ti
AZ31B	2.5-3.5	0.5-1.5	0.2-0.6	0.005	-	Bal.	-
Ti-6Al-4V	5.5-6.5	-	-	0.3	3.5-4.5	-	Bal.

Fig. 1. Schematic illustration of ultrasonic assisted tungsten inert gas welding-brazing process.

Table 2. Experimental parameters used in the U-TIG process

Welding parameters	Value
Ultrasonic output power, P	0-1.6 kW
Ultrasonic vibration frequency, F	20 kHz
Welding current, I	40-80 A
Welding speed, V	0.2 m/min
Wire feed speed, V_w	0.6 m/min
Flow rate of shielding gas Ar	10 L/min
Welding arc length, L	2 mm

Fig. 2. Schematic illustration of the tensile test specimen.

Fig. 3. Schematic illustration of the process to measure the rate of hydrogen evolved.

Fig. 4. Typical image of the cross-section of Mg/Ti joint.

Fig. 5. Typical (a, b) SEM images and (c, d) EDS line scan results of interfacial zone of Mg/Ti joints: (a, c) with current of 60 A and without ultrasonic power; (b, d) with current of 60 A and ultrasonic power of 1.2 kW.

Fig. 6. EBSD maps of (a) AZ31B base metal and fusion zone of Mg/Ti joints with ultrasonic power of (b) 0 kW, (c) 0.4 kW, (d) 0.8 kW, (e) 1.2 kW, (f) 1.6 kW.

Fig. 7. Tensile strength of Mg/Ti joints: (a) joints welded with current of 60 A and various ultrasonic powers; (b) joints welded with ultrasonic power of 1.2 kW and various welding current.

Fig. 8. Typical image of the fracture location of Mg/Ti joints with ultrasonic power of (a) 0 kW, (b) 1.2 kW.

Fig. 9. Hydrogen evolution rate and weight loss: (a) volume of hydrogen evolved for different samples during 12 h SSIT in 3.5 wt% NaCl solution; (b) weight loss and hydrogen evolution rates of different samples in 3.5 wt% NaCl solution for 12 h.

Fig. 10. Corrosion graphs of cross-sectional zone of Mg/Ti joints welded without and with ultrasonic power and after SSIT for 6 and 20 min: (a) without ultrasonic power, 6 min SSIT; (b) with ultrasonic power, 6 min SSIT; (c) without ultrasonic power, 20 min SSIT; (d) with ultrasonic power, 20 min SSIT; (e) zone A in Fig. 10(d); (f) zone B in Fig. 10(d).

Fig. 11. Potentiodynamic polarization curves of different samples in 3.5 wt% NaCl solution.

Table 3. Corrosion parameters of different samples in 3.5 wt% NaCl solution

Sample	E_corr (V)	i_corr (A/cm²)
Type 1	-1.443	1.254 × 10^-3
Type 2	-1.461	2.531 × 10^-4
AZ31B	-1.485	1.475 × 10^-4

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