Ultrasonic vibration assisted tungsten inert gas welding of dissimilar magnesium alloys

Ultrasonic vibration assisted tungsten inert gas welding of dissimilar magnesium alloys

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Ultrasonic vibration assisted tungsten inert gas welding of dissimilar magnesium alloys Fangzhou Yang a,b , Jie Zhou a,∗ , Rongrong Ding a a b

College of Materials Science and Engineering, Chongqing University, Chongqing, 400030, China The School of Robot Engineering and Mechanical - Electrical Engineering, Chongqing University of Arts and Sciences, Yongchuan, 402168, China

a r t i c l e

i n f o

Article history: Received 11 May 2017 Received in revised form 8 July 2017 Accepted 16 July 2017 Available online xxx Keywords: Ultrasonic vibration assist Magnesium alloy TIG welding Microstructure Mechanical property

a b s t r a c t The effects of ultrasonic vibration assisted (UVA) treatment on the microstructures and mechanical properties of MB3/AZ31 dissimilar magnesium (Mg) alloy joints were studied by microstructural characterization, micro-hardness testing and tensile testing. Results indicate that the welding pores are eliminated and coarse ␣-Mg grains of fusion zone are refined to 26 ␮m, owing to the acoustic streaming effect and cavitation effect induced by the UVA treatment with an optimal ultrasonic power of 1.0 kW. In addition, Mg17 Al12 precipitation phases are fine and uniformly distributed in the whole fusion zone of weldment. Micro-hardness of fusion zone of the Mg alloy joints increases to 53.5 HV after UVA process, and the maximum tensile strength with optimized UVA treatment increases to 263 MPa, which leads to fracture occurrence in the Mg alloy base plate. Eventually, it is experimentally demonstrated that robust MB3/AZ31 Mg alloy joints can be obtained by UVA process. © 2018 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

1. Introduction Recently, under such a great pressure to reduce exhaust gas emissions from vehicles, railroad and air transportations, Mg alloys are currently receiving extensive attention owing to the excellent comprehensive performances, e.g., high specific strength, low density, outstanding castability and recycling ability [1–3]. In order to expand the application of Mg alloys with desired mechanical features, reliable joining methods were investigated in recent years [4,5]. At present, a series of joining techniques such as friction stir welding [6], diffusion bonding [7], laser welding [8] and transient liquid phase bonding [9] have been developed to achieve the reliable joining of dissimilar Mg alloys. However, weldments appearances are circumscribed by these welding technologies and the practical application is restricted owing to the high cost and low production capacity. Tungsten inert gas (TIG) welding, as an efficient bonding technique, is characterized of flexibility, high productivity and reliable welding quality [10,11]. In our previous studies, TIG welding process was investigated to join dissimilar Mg alloys [ref]. However, the success of joining dissimilar Mg alloys was

∗ Corresponding author. E-mail address: cqu [email protected] (J. Zhou).

limited because of the severe grain growth coarsening of welding seam of weldments. In order to surmount above defects, grain refinement of Mg alloy joints is usually prepared by adding alloy elements, e.g., Sr, C, Sb and rare earth elements [12]. Though the addition of these elements can refine the microstructure, a series of problems are encountered at the same time. Hence a new way of grain refinement of Mg alloys joints without other elements addition is necessary. Dynamic grain refinement means, e.g., ultrasonic vibration assisted (UVA) treatment, has been studied in the solidification of Mg alloys [12–15]. As ultrasonic vibration leads to a welding pool, cavitation phenomenon is motivated ultrasonically, which results in a great instantaneous pressure and temperature fluctuations in the welding pool [16]. Heterogeneous nucleation was induced accordingly for grain refinement. Furthermore, with the aid of ultrasonic vibration, spiral vortex occurs and leads to the acoustic streaming effect, which increases the cooling speed and decreases the temperature gradient of the welding pool, hence restrains the grain growth of Mg alloys. The existing literature shows that UVA welding technology was developed to join Al alloys and stainless steel parts [17,18]. Results reveal that ultrasonic vibration encourages grain refinement of welding pool of the weldments and bonding strength of these weldments with UVA treatment increases significantly. Consequently, previous research reveals a promising effect of UVA treatment on

https://doi.org/10.1016/j.jmst.2018.06.009 1005-0302/© 2018 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

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

Table 2 Welding parameters applied in the present study.

Alloys

Al

Zn

Mn

Fe

Si

Mg

Welding parameters

Values

AZ31 MB3 Wire

2.9 4.8 2.8

0.88 0.92 0.79

0.41 0.49 0.32

0.01 0.05 ..

0.05 0.10 0.04

Bal. Bal. Bal.

Welding current, I (A) Welding speed, v (m min−1 ) Filler feed speed, vw (m min−1 ) Flow rate of protective gas (L min−1 ) Diameter of tungsten electrode (mm) Arc voltage (V)

70–110 0.2 0.5 9 2.5 17

Fig. 2. Schematic illustration of a representative tensile testing specimen.

Fig. 1. Schematic illustration of UVA TIG welding.

3. Results 3.1. Cross-sectional macrostructures

the microstructure refinement and bonding strength improvement of the weldments [ref]. However, the applications of UVA treatment on Mg alloys weldments have been rarely investigated. Hence, the effects of UVA treatment on microstructure and bonding strength of Mg alloy weldments are studied by the experimental observations in this study.

2. Materials and methods Mg alloys AZ31 and MB3 plates with dimensions of 50 mm × 70 mm × 2 mm were used as parent materials for welding experiments. Filler wire was Mg alloy AZ31with 1.2 mm in diameter. Table 1 shows the chemical composition of the parent metals and filler wire. Before welding, surface of the parent metals was cleaned with absolute ethanol to remove greasy contaminations and ground with emery paper to remove surface oxides. A TIG welding machine (YC-300WP5HGN) was applied during the welding experiments of dissimilar Mg alloys. As shown in Fig. 1, ultrasonic vibration was produced by an ultrasonic generator with maximum 20 ␮m output amplitude and 20 kHz vibration frequency and subsequently imported into the molten bath by the variable amplitude bar. And the vibration direction was perpendicular to the parent metals. The main joining parameters are shown in Table 2. After welding, microstructures of AZ31/MB3 weldments were analyzed by electron backscatter diffraction (EBSD, JSM-7800 F, and JEOL). The specimens for EBSD analysis were electrochemically polished with a voltage of 20 V in an AC2 polishing solution at 20 ◦ C. The tensile testing specimens with a width of 10 mm were cut from the Mg alloy weldments, as shown in Fig. 2. Tensile tests were carried out on a universal tensile testing machine (AG-X, SHIMADZU) and the tensile direction was perpendicular to the welding seams. Micro-hardness tests on cross-section of the weldments were performed on a Vickers hardness tester (MH-5 L, China) with a load of 500 g, a holding period of 10 s and a step size of 50 ␮m. Besides, the cross-sectional macrostructures and fracture characteristics of Mg alloy weldments were observed by scanning electron microscopy (SEM, VAGA 2 LMH, and TESCAN).

Fig. 3 shows the typical cross-sectional macrostructures of Mg alloy AZ31/MB3 joints welded with different currents. It can be found that a large number of pores occur at the welding seam with the increasing current. This phenomenon can be attributed to the extreme increment in temperature of welding pool and the evaporable character of Mg alloy under the currents of 100 A and 110 A. Figs. 4 and 5 show the typical cross-sectional macrostructures of joints welded with the UVA treatment. As shown in Fig. 4, with an ultrasonic power of 1.0 kW, the number density of welding pores significantly decreases (compared with that in Fig. 3) indicating that reliable joining of Mg alloys was obtained with the above aided welding process. The improvement in cross-sectional macrostructures is mainly ascribed to the cavitation effect and acoustic streaming effect induced by the UVA process, which would be analyzed in the following discussion part. Fig. 5 shows the crosssectional macrostructures of joints welded with different ultrasonic power. Note that the number of pores decreases with increasing ultrasonic power from 0.5 to 1.0 kW. However, when ultrasonic power further increases to 1.5 kW, irregular pores occur in the welding seam, attributed to the extremely unstable solidification process of joining. Similar results were reported by Xu et al. [10]. 3.2. Microstructures Fig. 6 shows the typical microstructures of Mg alloy joints welded with and without the UVA treatment. In the present work, the average grain sizes of joints were estimated by a line intercept technique on EBSD graphs. It can be found that, without any UVA treatment, the coarse Mg grains with an average grain size of 38 ␮m were formed in the welding seam of joints. Fig. 6(b–d) is the microstructures of Mg alloy joint welded with UVA treatment. As illustrated in Fig. 6(b), the coarse grain area of joint was slightly refined with an ultrasonic power of 0.5 kW. Furthermore, with the ultrasonic power increased to 1.0 kW, the coarse grains were refined to about 26 ␮m. The above results indicate that a fine uniform welding pool was obtained with UVA treatment. As shown in Fig. 6(d), the average grain size of welding pool varied slightly with the further increasing ultrasonic power to 1.5 kW.

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Fig. 3. Typical cross-sectional macrostructures of AZ31/MB3 weldments under different current: a–e joints welded with current of 70, 80, 90, 100 and 110 A, respectively.

Fig. 4. Typical cross-sectional macrostructures of AZ31/MB3 weldments under optimized ultrasonic power of 1.0 kW and different currents: (a–e) joints welded with current of 70, 80, 90, 100 and 110 A, respectively.

Fig. 5. Typical cross-sectional macrostructures of AZ31/MB3 weldments under welding current of 110 A and different ultrasonic power: (a–d) joints welded with ultrasonic power of 0, 0.5, 1.0 and 1.5 kW, respectively.

Fig. 6. Typical microstructures of FZ of Mg alloy weldments with welding current of 90 A and various ultrasonic vibration power: (a–d) with ultrasonic power of 0, 0.5, 1.0 and 1.5 kW, respectively.

The above data indicate that with UVA treatment, the AZ31/MB3 weldments with fine uniform grains was obtained, which is attributed to the improvement in bonding strength by a fine-grain strengthening mechanism. Fig. 7 shows the precipitation phases distribution of FZ of Mg alloy weldments welded with and without UVA treatment. As shown in Fig. 7(a), some continuous Mg17 Al12 precipitation phases

occurred during the traditional TIG welding process. As a brittle phase [15], Mg17 Al12 has deleterious effects on the joining strength of joints. However, with UVA treatment, Mg17 Al12 precipitation phases were uniformly distributed in the entire FZ of the weldment, as shown in Fig. 7(b). Previous studies [19,20] show that when ultrasonic vibration is applied in the initial solidification of alloy melt, cavitation effect and acoustic streaming effect usually occur and

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Fig. 7. Typical microstructures of FZ of Mg alloy weldments welded with current of 90 A: (a) without UVA treatment, (b) with UVA treatment (with ultrasonic power of 1.0 kW).

Fig. 8. Typical microshardness distribution of AZ31/MB3 Mg alloy joint with welding current of 90 A and ultrasonic vibration power of 1.0 kW: (a) and (b) MB3 side and AZ31 side of joint along the horizontal direction, (c) along the vertical direction.

result in the fluid flow. With the aid of fluid flow, primary Mg17 Al12 dendrites were broken due to the force on the arm, which provides an artificial source of sufficient nuclei. Under the fluid flows, the fragmented dendrite arms were rapidly carried away from their mother precipitate and grew as a new precipitate. Consequently, the microstructures with fine uniform precipitation phases were obtained with UVA treatment.

where  0 and ˛HP are the material constants, dg is the average crystallite size of Mg alloy. According to Eq. (1), dg of FZ decreases with UVA treatment, which contributes to the increases in microhardness. However, the UVA has no effect on microstructure of HAZ. That is to say, microhardness of HAZ does not vary during the UVA process, as shown in Fig. 8.

3.3. Microhardness

3.4. Mechanical properties

Fig. 8 shows the typical microhardness distribution of the welding seam with UVA treatment. As shown in Fig. 8, the average microhardness of both AZ31 side and MB3 side of the weldments welded without UVA decrease from the base metal (BM) approaching the heat affected zone (HAZ) and fusion zone (FZ). Eventually, microhardness of FZ of weldments is 51.3 HV, which is the weakest zone of AZ31/MB3 weldment. It is worth noting that the microhardness of weldment welded with UVA treatment increases compared to that without UVA. Finally, microhardness of optimized FZ is 53.5 HV. Table 3 shows the effect of UVA parameters on microhardness distribution of AZ31/MB3 joints. It can be found that the average microhardness of FZ increases with an increase in ultrasonic power, but still lower than that of Mg alloy BM. On the other hand, average microhardness of HAZ and BM of both AZ31 side and MB3 side remains notably with the variation of ultrasonic power. According to open literature [21,22], microhardness distribution of alloy increases with a decreases in grain size and microhardness values is aligned with the Hall-Petch relationship:

Fig. 9(a) shows tensile strength of AZ31/MB3 weldments welded under the optimal current of 90 A and different ultrasonic vibration power. The results reveal that mechanical properties of Mg alloy joints welded under the ultrasonic power of 0.5 kW slightly increased, owing to slightly refinement of welding pool, as indicated in Fig. 6(b). Nevertheless, as the ultrasonic vibration power increased to 1.0 kW, the reliable joining of AZ31/MB3 was achieved and the maximum bonding strength is 263 MPa, owing to the significant grain refinement. Hence, it can be concluded that the sound AZ31/MB3 weldments were obtained by the UVA process. However, as the ultrasonic vibration power further increased to 1.5 kW, the mechanical properties of joints decreased in spite of effective grain refining, attributed to the extremely unstable solidification process of joints. Fig. 9(b) shows bonding strength of AZ31/MB3 weldments welded under the ultrasonic vibration power of 1.0 kW and different current. It is worth noting that bonding strength of Mg alloys joints welded under the current of 70 A decreased after UVA treatment. With the low heat input (70 A), some cracks were formed in the AZ31/MB3 joints owing to deficient heat input, which tended to rapidly expand during the UVA process and led to the decrease

-1/2

HP =0 + ˛HP dg

(1)

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Table 3 Average microhardness of AZ31/MB3 Mg alloy joints with welding current of 90 A and various ultrasonic power. UVA parameter (kW)

0 0.5 1.0 1.5

BM (HV)

HAZ (HV)

FZ (HV)

AZ31 side

MB3 side

AZ31 side

MB3 side

56.2 56.5 56.3 56.3

57.3 57.2 57.3 57.1

47.7 47.6 48.0 47.8

48.1 48.0 48.3 48.2

51.3 52.0 53.5 53.7

Fig. 9. Bonding strength of Mg alloy weldments: (a) Mg alloy weldments welded under the same current of 90 A and different ultrasonic vibration powers, (b) Mg alloy weldments welded with the ultrasonic power of 1.0 kW and different welding currents.

of bonding strength. Nevertheless, under the current of 80–110 A, bonding strength of AZ31/MB3 weldments increased apparently with UVA treatment. 3.5. Fracture characteristics Fig. 10 illustrates the typical fracture characteristics of Mg alloy AZ31/MB3 weldments welded with and without UVA treatment (1.0 kW). As indicated in Fig. 10(a), cracks tended to initiate and propagate throughout the fusion zone of weldments owing to the coarse Mg grains of fusion zone. Fig. 10(c) and (e) shows the corresponding joint fracture appearances. Besides, welding pores form during TIG welding process without UVA. These pores acted as initial fracture sources and resulted in the deterioration of bonding strength of Mg alloy weldments. Nevertheless, fracture occurred at the parent plate of AZ31/MB3 instead of the fusion zone, as illustrated in Fig. 10b. Results reveal that the fusion zone of AZ31/MB3 weldments was no longer the weak zone, which is in accordance with the microstructure characteristics of FZ. Fig. 10(d) and (f) shows the fracture appearances of the joints. It can be found that plastic fracture characteristics occur at the fracture surface of optimal weldment, which indicates the weldment suffers ductile fracture during tensile testing. Fig. 11 illustrates XRD testing results of typical Mg alloy joints fracture surfaces. As shown in Fig. 11(a), Mg and Mg17 Al12 phases were confirmed at the fracture surface of joint without UVA process, indicating fracture did not occur at the FZ of joint [23]. On the other hand, only Mg phase was detected at the fracture surface of joint with UVA treatment, which is agreement with Mg alloy base metal.

Fig. 10. Typical fracture characteristics of Mg alloy joints with and without UVA treatment (both with a welding current of 90 A): (a), (c) and (e) without UVA treatment; (b), (d) and (f) with UVA treatment.

4. Discussion Existing literature reveal that microstructure of materials depends on both the nucleation condition and subsequent growth stage [14]. As the ultrasonic field propagates in the welding pool and acoustic pressure exceeds the cavitation threshold, which can generate an ultrasonic cavitation effect. Numerous micro-cavities are

generated with the aid of alternating expanding and compressing pressure fluctuation [24]. Subsequently, these cavities expand rapidly, clog and burst in the welding pool, which results in a instant

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Fig. 11. Typical XRD patterns of fracture surfaces of Mg alloy joints: (a) without UVA treatment, (b) with UVA treatment.

local high pressure. The maximum instant pressure is expressed as [16]: Pmax =

P0 4-4/3

R

max

Rmin

3 (2)

where P0 is the static pressure for the Al fusant, Rmax is the maximum expanding radius of the cavitation bubbles, and Rmin is the minimum clogging radius. When the cavitation bubbles collapse under the compression stress of sound waves, tiny particles and energy of the collapse produced by collapsing cavity is transformed into pressure pulsing up to 1000 MPa and into cumulative juts up to 100 m/s [14]. With the assistance of instant high pressure pulses, the primary coarse Mg grains are broken and fragmented because of the force, which acts as the artificial sources of efficient nuclei. Eventually, AZ31/MB3 weldment with refined grains is obtained with UVA process during the nucleation stage, as illustrated in Fig. 6. Furthermore, during the UVA welding of AZ31/MB3, the viscous force and sound wave of welding pool interacted with each other, which resulted in sonic press gradient Then, the sonic press gradient is beneficial to the formation of spiral vortex and results in acoustic streaming effects: stirring and vibration [16]. As a result, the temperature gradient decreases and the cooling speed of molten bath increases owing to the acoustic streaming effects. Eventually, coarse Mg grains are eliminated during the grains growth stage, which is ascribed to further sufficient grain refining. In conclusion, with the UVA treatment applied in the TIG welding process, nucleation rate and cooling rate of welding pool increase simultaneously, derived from the sufficient refinement of coarse Mg grains. It is worth noting that, the phenomenon of cavitation effect and acoustic streaming effect induced high speed flow in the melt also helps to reduce the concentration gradient around the growing particles and restrain the formation of welding pores, which contributes to the further increase in joining strength. 5. Conclusions This study investigated microstructures and bonding strength of AZ31/MB3 weldments welded with UVA treatment. Main conclusions are as follows: 1 Welding pores were eliminated and coarse ␣-Mg grains of fusion zone were refined to 26 ␮m, owing to the cavitation effect and acoustic streaming effect induced by UVA treatment. 2 With UVA treatment, Mg17 Al12 precipitation phases were refined and uniformly distributed in the whole fusion zone of weldment.

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