Behaviors and effects of Zn coating on welding-brazing process of Al-Steel and Mg-steel dissimilar metals

Behaviors and effects of Zn coating on welding-brazing process of Al-Steel and Mg-steel dissimilar metals

Journal of Manufacturing Processes 31 (2018) 674–688 Contents lists available at ScienceDirect Journal of Manufacturing Processes journal homepage: ...

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Journal of Manufacturing Processes 31 (2018) 674–688

Contents lists available at ScienceDirect

Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro

Behaviors and effects of Zn coating on welding-brazing process of Al-Steel and Mg-steel dissimilar metals R. Cao ∗ , J.H. Chang, Q. Huang, X.B. Zhang, Y.J. Yan, J.H. Chen State Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals, Lanzhou University of Technology, Langongping 287 Road, Qilihe District, 730050, China

a r t i c l e

i n f o

Article history: Received 14 November 2017 Received in revised form 26 December 2017 Accepted 2 January 2018 Keywords: Welding-brazing Magnesium/aluminum alloy Zinc coating

a b s t r a c t The behaviors and effects of Zn-coating on the welding-brazing processes of steel with aluminum and magnesium were investigated by Cold Metal Transfer (CMT) arc. The behaviors of Zn-coating at the very beginning of burning and during the spreading of the liquid aluminum and liquid magnesium droplet over the steel plate were observed respectively. The effects of Zn-coating on the wetting process of liquid droplets and on the fracture of welding-brazing joints were analyzed. The effects of Zn-coating on joining mechanisms and tensile load of Al-steel joints and Mg-steel joint were compared and analyzed. For Alsteel, Mg-steel, some common behaviors can be found, the main effect of Zn is its evaporation which decreases the temperature at the interface to prevent steel from melting and makes the steel to expose the fresh metallic surface which is much easier to be spread by a liquid droplet than an oxidized surface. Zn-coating improves the wettability of liquid Al or liquid Mg over the steel surface. However, some effects of zinc coating on wetting behavior, joining mechanisms and tensile load of Al-steel joints and Mg-steel joint are still different. For Al-steel arc, Zn-rich triangle zone is not formed by driving the liquid Zn from inside and accumulated at the toe, rather it is formed by the diffusion of the liquid Zn from outside Znenriched periphery into the droplet. In the case of Mg-steel arc, the Zn-rich zone forms partly due to the accumulation of Zn driven from the center during the Zn evaporation. For Al-steel joint, Zinc coating has not reacted to form the interface layer between Al weld metal and steel, however, zinc coating reacted to form the interface layer between Mg weld metal and steel. Zinc coating improved the strength of Al-steel joints, but, it is not beneficial to the strength of Mg-steel joint. © 2018 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

1. Introduction Welding-brazing joints of aluminum to galvanized steel can be successfully performed by laser welding [1–4] and arc welding [5–12], and popularly used in recent automotive manufacturing. Some studies have found that long wetting length and smaller wetting angle is an important factor to achieve good mechanical properties of the joint of dissimilar materials [1,2]. The wetting length has a strong correlation with the temperature field on the coated steel surface and the location of the melting isothermal [3,4]. These studies have found that Zn coating has a crucial effect on joining Al to steel [5–7]. Following viewpoints of the effects of Zncoating on the welding-brazing of Al to galvanized steel are well recognized: (1) Al-steel cannot be firmly joined by welding-brazing

∗ Corresponding author. E-mail addresses: [email protected] (R. Cao), [email protected] (J.H. Chang), [email protected] (Q. Huang), [email protected] (X.B. Zhang), [email protected] (Y.J. Yan), [email protected] (J.H. Chen). URL: http://mailto:[email protected] (X.B. Zhang).

process without Zn-coating on steel plate [5–8]; (2) Zn enhances the wetting and spreading behavior of a liquid Al droplet on surface of galvanized steel [5–7]; (3) A liquid Zn-rich triangle region is formed at the toe of the liquid Al bead [6] which wets the steel plate well with a contact angle less than 15o [4–8,13–15]; (4) The Zn-rich area is formed by driving the melting Zn outward on the plate to accumulate at the toe of the liquid bead [13]; (5) This Znrich region leads the liquid Al bead to further spread forward. (6) In the case of Mg-steel the Zn-coating improves both the wettability and tensile-shear strength of the welding-brazing joint [16]. Although above studies have revealed the effects of Zinc-coating on welding-brazing of Al to galvanized steel joints and Mg-steels joints from various aspects, the dynamic processes, joining mechanisms and tensile strength of Zinc-coating on Mg-steel joints and Al-steel joints have not systematically compared and investigated. In this work, some new phenomena were revealed, some viewpoints were corrected and the additive effects of Zn-coating on wetting of liquid Al droplet and Mg droplet were revealed by investigating the behavior of Zn-coating at the very beginning (0.1s to

https://doi.org/10.1016/j.jmapro.2018.01.001 1526-6125/© 2018 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

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Fig. 1. (a) Zn-coating on the galvanized steel plate, (b) XRD analysis of zinc coating in Fig.1(a), (c) EDS line scanning for the corresponding position in (a), (d) area distribution of Fe element, (e) area distribution of Zn element, (f) area distribution of Al element.

Table 1 Nominal chemical compositions of hot dipping galvanized steel (weight %). Material

C

Si

Mn

P

S

Fe

Hot dipping Zn-coating (g/m2 )

Galvanized steel

0.01

0.01

0.39

0.30

0.025

balance

60

Table 2 Nominal chemical composition of aluminum alloy sheet (weight %). Material

Si

Fe

Cu

Mn

Mg

Zn

Cr

Ti

Al

6061

0.4-0.8

0.70

0.15-0.4

0.15

0.8-1.2

0.25

0.04-0.35

0.15

balance

Table 3 Nominal chemical compositions of aluminum alloy filler wire (weight %). Material

Si

Fe

Cu

Mn

Mg

Zn

Ti

Al

4043

4.5-6.0

0.80

0.30

0.05

0.05

0.10

0.20

balance

Table 4 Nominal chemical composition of Mg alloy sheet (weight %). Element

Al

Zn

Mn

Fe

Si

Cu

Ni

Mg

AZ31B

3.18

1.02

0.34

0.002

0.022

0.0021

0.00085

balance

0.9s) of arc burning and focusing the study on the dynamic process of formation of the Zn-rich triangle at the liquid Al droplet toe. Based on the observations, the behaviors of Zn-coating under cold metal transfer (CMT) are clarified. By comparing the weldingbrazing joints of Al alloy or Mg alloy with the galvanized steel plate and the bare steel plate, the mechanisms of the effects of Zn-coating on the wettability of liquid Al and Mg on steel plates were analyzed and the effects on the joint fracture were elucidated.

2. Experimental 2.1. Materials The nominal chemical compositions of the galvanized steel sheet, Al sheet, Al filler wire, Mg sheet and Mg filler wire are listed in Tables 1–5.

Table 5 Nominal chemical compositions of Mg alloy filler wire (weight %). Element

Al

Fe

Cu

Zn

Mn

Ni

Si

C

Mg

other

AZ61

5.8-7.2

≤0.005

≤0.05

0.4-1.5

0.15-0.5

≤0.005

≤0.05



balance

≤0.3

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Fig. 2. Schematic diagrams of experimental devices (a) CMT arc burning on a galvanized plate (b) lap joint of Mg or Al to steel (c) plug welding of Mg to steel.

Fig. 3. (a) The typical cross sectional structures of a welding-brazing joint between Al and steel, (b) the details of Zn-rich zone at the periphery of weld, (c) the microstructures of the transition interface zone, (d) EDS line scanning of the transition interface zone along yellow line in (c).

Fig. 1 displays the Zn-coating on the galvanized steel plate with X-ray diffraction (XRD) analysis result and the EDS line scanning map and area scanning map. Combining EDS analysis result that the compositions of the coating is composed of 96.8–97.7% Zn element and small Al element, and XRD analysis result in Fig. 1(b), it is seen that the coating of galvanized steel was mainly composed

of Zn solid solutions. As shown in Fig. 1(b), (d) and (e), Zn is the major element in the coating layer that had a thickness of approximately 10 ␮m. At the interface, an appreciable amount of Al is observed (Fig.1(c) and (f)), which is considered as added Al during hot dipping for preventing zinc from directly reacting with Fe. Thus, the microstructure adjacent to the interface is not only pure Zn-

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Fig. 4. (a) The typical cross sectional structures of a welding-brazing joint between Mg and galvanized steel, (b) the microstructures of transition interface zone, (c) EDS line scanning of the transition interface zone along the black line in (b), (d) the details of the Zn-rich zone.

rich solid solution but also ultrathin Fe-Al IMCs (Fig. 1(f)) resulted from the hot dip galvanization process, which is consistent with the results shown in Ref. [16].

2.2. Experimental processes The schematic diagram of experimental device for investigating the behavior of Zn coating at very beginning of arc burning is depicted in Fig. 2(a). An Al alloy 4043 wire with 1.2mm in diameter or Mg alloy wire with 1.6mm in diameter were molten under Cold metal transfer (CMT) arc on a galvanized steel plate or bare steel plate with 1 mm in thickness. For studying the dynamic behavior, the arc is burned for designated periods from 0.1 s to 0.9 s at an interval of 0.1 s. The welding regimes used for Al wire on steel sheet by CMT arc are Vfeed =1.2m/min, I = 19 A, U = 12.3 V; Vfeed =6.0 m/min, I = 136 A, U = 17.7 V, and that used for Mg wire on steel sheet are Vfeed =2.4m/min, I = 27A, U = 8.8V.

Fig. 2(b) and (c) display the lap configuration and the plug configuration for CMT arc welding-brazing processes, respectively. After welding, the surface of arc-burned areas and the cross sections of welding-brazing joints were observed by scanning electron microscope (Quanta 450 FEG (Field Emission Gun)), and the interface were characterized by EDS line scanning and point analyses. The joint strengths were tested by tensile-shear experiments. 3. Results and discussion 3.1. Behavior of Zn coating 3.1.1. General description of the behavior of Zn coating Fig. 3(a) illustrates the cross sectional structures of a weldingbrazing joint between Al and galvanized steel. The partly molten Al sheet and the molten filler metal together formed the welding bead which extending from the molten Al, smoothly covers the galvanized steel plate. A Zn-rich triangle zone (Fig. 3(a)) at the front

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Fig. 5. Appearance of Al molten metals on the galvanized steel surface by various arc burning periods for various welding speeds.

Fig. 6. Appearance of Mg molten metals on the galvanized steel surface by various arc burning periods.

toe of the weld is observed and its magnified details are shown in Fig. 3(b). The microstructures of the brazing transition interface between Al and steel and its EDS line scanning map are exhibited in Fig. 3(c) and (d) separately. These figures demonstrate that a FeAl layer exists on the Fe surface in the transition zone. It is worth noting that no Zn trace is found in the brazing interface. Fig. 4(a) displays the cross section of a welding-brazing joint between Mg and galvanized steel. The microstructures of the brazing transition interface zone and the EDS line scanning map of the transition interface zone are shown in Fig. 4(b) and (c). The Zn-rich zone is observed and illustrated in Fig. 4(d). As shown by Fig. 4(b) and the EDS line scanning map (Fig. 4(c)), opposed to the Al-steel joint, the Zn element remains and shows that a Fe-Al layer covered by a Mg-Zn layer are found in the brazing interface of Mg-galvanized steel joint. 3.1.2. Dynamic behavior of Zn-coating under CMT arc To reveal the dynamic behavior of Zn-coating under arc, the CMT spot welding experiments on galvanized steel sheet by Al wire and Mg wire were done by burning the arc for various designated short periods. Figs. 5 and 6 show the appearance of Al molten metal and Mg molten metal on galvanized steel sheets by burning the arc for various designated short periods respectively. From Figs. 5 and 6, it is seen that the regular weld spots can be formed only for Mg wire. To know the dynamic behavior of Zn-coating under arc, the Zn distribution on the surface and cross sections of molten metals was analyzed by SEM and area scanning and line scanning of EDS. The results of burning arc for several designated short periods are shown in Fig. 7–12, shows the appearance and the EDS area

scanning map for various elements on the surface of the galvanized sheet for molten Al metal after 0.1s arc burning. To be surprising, as seen in the EDS map of Zn element, only after 0.1 s, the arc burned off almost all of the Zn-coating. Fig. 8 shows the appearance of the area after 0.1 s arc burning and the corresponding EDS line scanning map showing the element distributions, specially showing the Zn distributions (in Fig. 8(c)). Fig. 9 depicts the appearances and EDS area scanning maps of Zn on surface of galvanized steel plate after various arc burning periods (a) 0.2s, (b) 0.3s, (c) 0.5s, (d) 0.8s. As revealed by Figs. 7–9, after a very short period (less than 1 s) arc burning, only negligible Zn remains on the burned area of the galvanized steel sheet surface. This phenomenon is consistent with the observed fact in Fig. 3 that in the transition interface only FeAl layer exists without Zn trace. It is also revealed by Figs.7–9 that a high Zn content region surrounds the Zn burned off area, which builds a Zn-enriched periphery. Fig. 10 shows the appearance and the EDS area scanning map for various elements on the surface of the galvanized sheet with molten Mg metal drop after 0.1s arc burning. In the EDS map of elements Zn, Mg and Fe, the purple region in the middle part of the Mg element map (Fig.10(c)) is actually the zone of molten Mg metal drop, the shape of which is corresponded to the black region in the middle part of the Fe element map (Fig.10(e)). The red region surrounding the molten Mg metal in Fe element map (Fig.10(e)) demonstrates the region of the Zn coating burned by the arc. It is consistent with the outer circular region of the Zn-coating burned by the arc in Zn element map (Fig.10(d)). Combining the Mg, Fe, and Zn element maps, it is found that the region of the arc burned Zncoating (surrounded by yellow circle in Fig.10(d) and (e) is larger

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Fig. 7. Apearence and EDS area scanning map of molten Al metal on the galvanized steel surface after 0.1s arc burning.

Fig. 8. (a) Appearance of area of molten Al metal on the galvanized steel surface after 0.1s arc burning; (b) EDS linear scanning map showing the element distributions for the red line in (a), (c) EDS linear scanning map showing the Zn distribution.

than that of the molten Mg metal (surrounded by white circle in Fig.10(d) and (e). Fig. 11, shows the appearance of arc burned Zn-coating region after 0.1 s arc burning with Mg filler wire (Fig. 11(a–d)) and the corresponding EDS line scanning map (Fig.11(e)) showing the element distributions and specially showing the Zn distributions. From Fig. 11, some Zn elements are still retained on the arc burned Zn coating region marked by points 1 and 2 in Fig. 11(a). By EDS point analysis, region 1 and region 2 contain ␣-Fe solid solution, Mg-Zn-Al compounds (Fig.11(b) and (c)). These compounds may be the reacted products of Fe element of exposed steel with Mg and Al of the molten Mg metal, and retained Zn coating, and local nonmolten Fe-Al IMC on the original galvanized steel sheet. Although there are some retained Zn on the arc burned zone, from line scanning analysis in Fig. 11(e), the content of element Zn is much less than that of original Zn coating surface. Fig. 12 depicts the appearances and EDS area scanning maps of element Zn on surface of galvanized steel plate with Mg molten

metal drop after various arc burning periods (a) 0.2s, (b) 0.3s, (c) 0.5s, (d) 0.8s. The black circle in the figures of the left column of Fig.12 are the image of Mg molten drop. The red areas surrounding the drop in the right column figures of Fig.12 show the Fe element. This area demonstrates the Zn-burned out area. The green areas surrounding the drop in the figures of middle column of Fig.12 are the Zn element map outside the arc burned Zn region. Because this area locates outside the red area, it means that in the red arc burned area only minor Zn remains. The firures in the left column of Fig. 12 reveal that with the increase of the welding time, the diameter of weld drop increases, but the area of the arc burned Zn region keeps almost the same, as showm by the red circular area in the right column of Fig. 12. The corresponding result is shown in Fig. 13. It means that once the arc is actioned on the gavanized steel surface, the area of arc-burned Zn coating is immediately formed. With the increase of the welding time, the diameter and the volume of the molten metal increase, but the area of the arc burned Zn region keeps almost the same.

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Fig. 9. Appearances and EDS area scanning maps of Zn on surface of molten Al metal on galvanized steel plate after various arc burning times (a) 0.2s, (b) 0.3s, (c) 0.5s, (d) 0.8s.

Fig. 10. Apearence and EDS area scanning map of molten Mg metal on the galvanized steel surface after 0.1s arc burning.

3.1.3. Formation of the Zn-rich region As shown in Fig. 3, a Zn-rich triangle zone exists at the front toe of the Al weld, which is previously considered as the results of accumulation of the Zn driven from the center of the weld. From Fig.7–9 it is obviously that the negligible Zn content in the arc-burned area is apparently insufficient to be driven to form this Zn-rich region. In Fig. 14, a dynamic process of the formation of Zn-rich triangle region by diffusion of Zn from the Zn-enriched peripheral area is revealed. As seen in Fig. 14, the Zn content at the Al droplet toe is very low at the moment after 0.1s arc-burning (Fig. 14(a)). Because the droplet toe is contiguous to a Zn-enriched periphery, the Zn diffuses into the droplet toe from this neighboring area. With increasing of the

duration, the Zn content in the weld toe increases (Fig. 14(b–e)) and reaches a high value after 0.9 s arc burning (Fig. 14(f)). Thus it is clearly revealed that the Zn-rich triangle zone is not formed by driving the liquid Zn from inside and accumulated at the toe, rather it is formed by the diffusion of the liquid Zn from outside Zn-enriched periphery into the droplet. Because the Zn-rich triangle zone is formed later that the drop, it is not the driving force to spread the main body of the metal drop. Fig. 15 presents an EDS area scanning map of Zn element which shows the distribution of Zn element in the Mg bead and with two short green lines at left and right end of the Mg molten toes show the retained Zn on the surface of the galvanized steel sheet. From

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Fig. 11. (a) Appearance of area of molten Mg metal on the galvanized steel surface after 0.1s arc burning, (b) magnified feature of point 1 in Fig. 11(a), (c) magnified feature of point 2 in Fig. 11(a), (d) magnified feature of point 3 in Fig.11(a), (e) EDS linear scanning map showing the element distributions for the red line in (a).

Fig. 15 it is seen that in the Mg bead there is almost not Zn element, however at the Mg molten toe on the galvanized steel, at the welding time of 0.3s, a Zn-enriched zone is formed, as shown in Fig.16. By this phenomenon the joining mechanism of Mg-steel weldingbrazing joint is presented as shown in Fig. 17 [17]. Fig. 17(a) gives a general view of a cross sectional structure of a plug welding joint between Mg and galvanized steel sheet. Fig. 17(b) exhibits the micro-feature at the end of steel sheet (shown by the red squad in the inset) where the sheet is directly under the arc. In this figure, at the brazing interface, only Fe3 Al exists without Zn layer. Fig. 17(c) exhibits the micro-feature at the point, a little distance from the arc (shown by the red squad in the inset), where a layer of Mg-Zn IMC exists between Fe-Al and the weld metal. It means that unlike the case of Al-galvanized steel brazing, a part of Zn-coating remains at

the interface. In Fig.17(d) a Zn-rich zone is observed at the weld toe (shown by the red squad in the inset), which is an extension of a thick Zn-rich layer under the bead. It is considered that because the arc temperature between Mg-steel is lower than that between Al-steel, the Zn-coating has not fully been burned off. Based on the observation that the Zn layer thickness increases with the distance from the arc center to the periphery, it is inferred that different from the case of Al-steel arc, in the case of Mg-steel arc, the Zn-rich zone forms partly due to the accumulation of Zn driven from the center during the Zn evaporation. 3.2. Effect of Zn-coating 3.2.1. Effect of Zn-coating on wetting of the liquid Al droplet

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Fig. 12. Appearances and EDS area scanning maps of Zn for molten Mg metal on surface of galvanized steel plate after various arc burning times (a) 0.2s, (b) 0.3s, (c) 0.5s, (d) 0.8s.

3.2.1.1. Effect of Zn-coating on wettability of Al bead and Mg bead on the galvanized steel. In the case of welding-brazing joint of Al-steel, two weldingbrazing joints are displayed in Ref. [6]. For Al-galvanized steel joint, the contact angle between Al bead and the galvanized steel is around 35◦ . For Al-bare steel joint, the contact angle between Al bead and bare-steel is around 55◦ . This observation identifies the well-known fact that the Zn-coating improves the wettability of liquid Al over the steel surface. It is consistent with Refs. [13–16].

Fig. 18 exhibits two Mg-steel welding-brazing joints. The macroscopic configuration in Fig. 18(a) showing the contact angle between Mg bead and the galvanized steel is around 50◦ , and the contact angle between Mg bead and bare-steel is around 95◦ as shown in Fig. 18(b). This observation also demonstrates the improvement of wettability by Zn-coating. Fig. 19 shows the evolution of the contact angles between liquid Al and the galvanized steel sheet with increasing the time of arc burning. As seen in Fig. 19, the contact angle decreases from 62◦

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Fig. 13. Relationships between the area and time of the arc burning for molten Mg metal on the galvanized steel surface.

Fig. 14. Process of formation of the Zn-rich triangle region by diffision of Zn from the peripheral area for Al molten metal on the galvanized steel surface.

at 0.2s to a value scattering between 42◦ and 31◦ in a period from 0.3s to 0.8s. An interesting phenomenon (seen in the figure at 0.8s) that while the contact angle of main droplet keeps 31◦ , a small protrusion extends toward the periphery with a contact angle of 15◦ . Comparing with Fig.14, this protrusion is coincided with the formation of the Zn-rich triangle. Although in Fig. 19 the contact angle decreases to as low as 15◦ in the Zn-rich triangular zone, which identifies the benefit effect of Zn on the wettability of Al on steel, yet this effect enhancing wettability only appears at a high content (around 22%) of Zn, and it expands only a small wetting length (around 300 ␮m) at the droplet toe and does not affects the spreading of the main body of the drop. The essential effect of Zn rather presents in that the apparent contact angle of the main body of the droplet with Zn coating keeps around 35◦ , considered as an intrinsic wetting angle of liquid Al on the galvanized steel, which is obviously improved from that of 55◦ on the bare steel. The

wetting angle of Mg molten toe on the galvanized steel is about 66◦ (Fig.18(a)). However the wetting angle of Mg molten toe on the bare steel is higher than 90◦ (Fig.18(b)). That is, the Zn-coating also improves the wetting for Mg melt drop. The micromechanism is analyzed as follows. 3.2.1.2. Micromechanism of improvement of wettability of Zn-coating on the galvanized steel. Taking account of the heavy loss of Zn-coating after very short time arc burning, the decrease of the contact angle from 62◦ to 42◦ and then keep 35◦ during welding-brazing process is considered to be caused by several effects. The following effects are revealed: (1) The weight effect, the weight of the metal drop acts as a part causing the expansion of the wetting area and reducing the contact angle from 62◦ to 42◦ .

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Fig. 15. Zn area scanning map showing distribution of Zn element on cross section of Mg molten toe on the galvanized steel.

Fig. 16. Zinc-enriched zone of Mg weld on steel sheet.

(2) The main effect of Zn coating is caused by its evaporation which exposed the fresh surface of solid Fe which makes the liquid Al keeping an intrinsic contact angle of 35◦ and being much easier to spread over the fresh Fe surface than having a contact angle of 55◦ on an oxidized surface of bare steel sheet. For Mg molten toe on steel surface in Fig. 18, the apparent contact angle of the main body of the droplet on the galvanized steel keeps around 65◦ , which is obviously improved from that of 105◦ on the bare steel. The wettability was improved by the alleviated effect and enhanced interfacial reactivity by Zn evaporation. (3) The evaporation of Zn-coating decreases the temperature of interface and prevents the Fe from melting. Experimental results found that for the bare steel sheet without heat dissipation by Zn evaporation, under the arc, the temperature of the sheet surface is higher which causes a partial melting of the

sheet as seen in Fig. 20. As soon as the Fe is molten it promptly reacts with Al forming multi-IMC of Al-Fe (Fig. 21), which prohibits the liquid Al from spreading. The formed thicker multi-Al-Fe IMC layer causes the fracture of interface as shown in Fig. 20 which is detected in the fracture surface in Fig. 21.

3.2.2. Effect of Zn-coating on joint strength 3.2.2.1. Welding-brazing Al-steel joint. To further reveal the effect of Zn-coating on the joint strength, the corresponded result is referred from Ref. [6]. The strength that between the Al bead to steel sheet without Zn-coating reach zero. For Al-galvanized steel joint, with the effect of the Zn-coating, the ultimate load can reach 5KN, about half value of Al plate with 1 mm in thickness and 25 mm in width [14].

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Fig. 17. Micro-behavior of Zn-coating at the brazing interface between Mg and steel, (a) A general view of a cross section of a plug welding joint between Mg and steel plates, (b) a point at the end of steel plate where the plate is directly under the arc, (c) a point, a little distance from the arc, (d) a point at the weld toe, the positions of three points are noted by a red squad in the inset.

Fig. 18. The macro-configuration of weld bead showing the contact angle of Mg-steel joint (a) Mg-galvanized steel welding-brazing bead (b) Mg-bare steel welding-brazing bead.

3.2.2.2. Welding-brazing Mg-steel joint. Fig. 22 plots the ultimate loads of Mg-galvanized steel joint and Mg-bare steel joint with increasing CMT arc current [18]. As seen in Fig. 22, the ultimate loads firstly increase with arc current and

then decrease at a certain current value. This is due to the brazing areas increase with heat input until the steel sheet is excessively penetrated.

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Fig. 19. The evolution of the contact angle for Al molten toe on the galvanized steel surface with increasing the time after arc burning.

Fig. 20. Cross section of Al bead on bare steel plate showing a partial melting (marked by a black arrow) of the steel surface.

Fig. 21. (a) Fractography of Al-bare steel brazing transition interface; (b) XRD map for the phases in the fracture surface.

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Fig. 22. The ultimate load versus CMT arc current for Mg-galvanized steel plate and Mg-bare steel plate.

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1 At very beginning of the arc burning the most Zn-coating on a steel plate is burned off, only neglected Zn remains. 2 A triangle Zn-rich zone at a brazing Al bead toe exists which is not formed by driving the molten Zn on the steel plate to the liquid bead periphery, rather it is formed by the diffusion of the liquid Zn from outside Zn-enriched periphery into the liquid bead and accumulation at the toe. 3 The wettability of Al and Mg liquids in the Zn-rich zone on steel plate is improved. The total wettability of the main body of the liquid Al droplet is not improved by the Zn rich zone at the bead toe, which only makes the bead toe to further spread to very short distance. 4 The main effect of Zn is its evaporation which decreases the surface temperature then prevents the Fe from melting. 5 The evaporation of Zn-coating makes a fresh Fe surface exposed to the liquid Al bead, which makes the liquid Al drop keeping an intrinsic contact angle of 35◦ and being much easier to spread than that a contact angle of 55◦ on an oxidized surface of bare steel plate. 6 For Al-galvanized steel joint the endurable ultimate load can reach 5KN about half value of Al plate with 1mm in thickness and 25mm in width. However without Zn-coating both the strengths that between weld bead/Al and that between weld bead/steel reach zero. 7 For Mg-steel welding-brazing joint a converse event is that the endurable ultimate load of the Mg/bare steel joint is higher than that of the Mg/galvanized steel joint. Acknowledgement This study was financially supported by National Nature Science Foundation of China (Nos. 51675255, 51761027 and 51265028), The Program of Innovation Groups of Basic Research of Gansu Province (17JR5RA107) and The Foundation of Collaborative Innovation Teams in College of Gansu Province (2017C-07).

Fig. 23. A fissure in Mg weld bead induced by penetration of low melting point eutectic ␣−Mg+MgZn in Mg-galvanized steel welding-brazing joint.

The surprising event is that although the wettability is worse, the ultimate load of the Mg-bare steel joint is higher than that of the Mg-galvanized steel joint. The reason that causes the lower resistant load of the Mg-galvanized steel joint is manifested by Fig. 23. In Fig. 23, a crack in Mg weld bead was induced by penetration of low melting point eutectic ␣−Mg+MgZn (melting point is 340 ◦ C) in Mg-galvanized steel welding-brazing joint. The detailed explanation is referred to Ref. [8]. The fracture in Mg-galvanized steel welding-brazing joint occurs in the interface between ␣−Mg+MgZn and Fe/Al IMC. Thus, the low melting point and brittle eutectic ␣−Mg+MgZn which is produced by the Zn-coating and the Mg liquid cause the lower resistant load by both inducing micro-fissure and creating a brittle interface with the Fe/Al IMC phase. For Mgbare steel welding-brazing joint, only Fe/Al IMC phases and ␣−Mg are detected. Due to the limited thickness of the Fe/Al layer the endurable load keeps an acceptable lever (higher than 6KN). In conclusion the Zn-coating does not benefit the strength of Mg-steel joints.

4. Summary Based on above CMT arc welding-brazing experiments of Alsteel and Mg-steel, the following ideas can be summarized:

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