Study on the weld joint of Mg alloy and steel by laser-GTA hybrid welding

Study on the weld joint of Mg alloy and steel by laser-GTA hybrid welding

M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 7 9–1 2 8 4 Study on the weld joint of Mg alloy and steel by laser-GTA hybrid welding L.M...

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M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 7 9–1 2 8 4

Study on the weld joint of Mg alloy and steel by laser-GTA hybrid welding L.M. Liu⁎, X. Zhao State Key Laboratory of Materials Modifications & School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, People's of Republic of China

AR TIC LE D ATA

ABSTR ACT

Article history:

A laser-GTA hybrid welding technique was chosen to study the weld of the dissimilar alloys

Received 25 April 2007

of AZ31B Mg alloy and 304 steel. A lap joint was formed between the two. The weld

Received in revised form

penetration, which determines the mechanical properties of the welded joints, depends on

10 October 2007

the laser power. A transition zone formed at the interface of the Mg–Fe during laser-GTA

Accepted 25 October 2007

hybrid welding and Mg element diffused into the Fe matrix by forming oxides and reacting in the transition. During tensile testing, the joints fractured at the interface between the Mg

Keywords:

alloy and the steel. Metallic oxides produced at the Mg–Fe interface were the reason for the

Laser-GTA hybrid welding

poor mechanical properties of the weld joints.

Magnesium alloy

© 2007 Elsevier Inc. All rights reserved.

Steel Interface of Mg–Fe Oxide

1.

Introduction

Magnesium alloys are the structural alloys with the lowest density. They offer the highest potential for saving weight in all areas where moving components are in use. Besides using Mg alloys in power train applications, the use of Mg for the chassis and the interior of a car is the aim of much research and development [1–5]. It is well known that steels are the most common materials of modern industry. Joining Mg alloys to steel efficiently will widen the application of magnesium alloys. Joining these two kinds of metals by friction stir welding method (FSW) has been investigated [6]. However, it is known that the FSW is difficult to use widely. So far, no reports have been published on joining these two metals by fusion welding, which is the most common method to join metals. The melting points of Mg and Fe are 649 °C and about 1538 °C, respectively. The tremendous difference in the melting points

between these two kinds of metals causes the difficulty in melting them at the same time during fusion welding. The maximum solid solubility of Fe in Mg is 0.00041 at.% Fe, and the Mg concentration at the eutectic point is estimated to be less than 0.008 at.%. There is also clear evidence that magnesium and steel do not mix in the liquid state at ambient pressure [7]. Moreover, they do not react with each other. Therefore, joining Mg alloys to steels by conventional fusion welding processes is almost impossible. laser-GTA hybrid welding is a modern welding technique with high efficiency. The laser beam should make Mg alloy and steel melt simultaneously because of its high energy intensity. And due to its fast stir action in the molten pool, the molten Mg alloy and steel should be well mixed[8–11]. Thus, this process provides the potential for joining Mg and Fe. In the present paper, laser-GTA hybrid welding technique was chosen to study the weld of the dissimilar alloys of AZ31B magnesium alloy and 304 steel, and a lap weld was obtained. The microstructures of the welded joints were observed using

⁎ Corresponding author. Tel./fax: +86 411 84707817. E-mail address: [email protected] (L.M. Liu). 1044-5803/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.10.012

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Table 1 – Chemical composition of AZ31B magnesium alloy (mass fraction, %) Al 2.5–3.5

Zn

Mn

Si

Mg

0.5–1.5

0.2–0.5

0.1

Balance

2 – electron Chemical composition 304a electron steel (mass a Table scanning microscope (SEM)ofand probe fraction, %) C

Mn

Si

Cr

Ni

Mo

Cu

N

Fe

0.03

1.7

0.6

18.5

8.5

0.2

0.2

0.05

Balance

microanalysis (EPMA), and the interface of Mg-to-Fe joint was analyzed to determine the weldability of Mg-to-Fe.

2.

Experimental

Plates of AZ31B Mg alloy (90 mm × 50 mm × 1.7 mm) and 304 steel (90 mm × 50 mm × 1.2 mm) were used in the experiments. Their chemical compositions are given in Tables 1 and 2. Fig. 1 is a schematic diagram of laser-GTA hybrid welding set-up. The maximum power of the YAG laser machine was 400 W. The diameter of the tungsten electrode was 3.2 mm, the angle between the electrode and the welding direction was 50°, and the argon shield flow was 0.5 L/s. The welding parameters are shown in Table 3. Before welding, the surfaces of the plates were treated. After burnishing, the surfaces of the AZ31B Mg alloy plate were washed with acetone and the upper surface of the 304 steel plates was washed with dilute sulfuric acid. After welding, the microstructures and elemental distribution in the Mg–Fe joints were determined using a JSM-5600LV scanning electronic microscope (SEM) and a EPMA-1600 electron probe microanalysis (EPMA), respectively. The welded joints were analyzed using energy-dispersive X-ray spectroscopy (EDS). Shear tests were carried out on a test machine at a cross-head velocity of 5 × 10− 2 mm/s.

3.

Results and Analysis

Table 3 – Laser-GTA hybrid welding parameters Welding parameters Laser power (W) Welding speed v (mm/s) TIG welding current I (A) Distance from laser beam spot to TIG arc DLA (mm) Laser focus position Z (mm) Laser impulse frequency f (Hz)

plate and steel plate were melted simultaneously by the laser beam. The Mg alloy melted under the laser power and arc power, while the steel melted only under the laser power. The steel was not placed on top because it needs more power to melt, which may cause the Mg to vaporize. Fig. 2 shows the weld appearance after a run at a speed of 15 mm/s. Good welding shape occurred, but the Mg alloy appeared to collapse at the beginning of the weld joint, due to high laser power at the beginning of the welding process. Fig. 3 shows a cross section of the Mg–Fe lap joint. The weld shows features typical of laser-GTA hybrid welding. Fig. 4 shows a vertical section of the Mg–Fe lap joint. The morphology of the Mg-to-Fe joint was approximately uniform for the pulsed laser condition. In order to analyze further the Mg–Fe interface, examination of the welded joint was carried out by scanning electron microscope at low magnification, see Fig. 5 EDS showed that Fe existed in the Mg matrix , as indicated by the arrow labelled A. Clearly, the molten Fe flowed to the molten welding pool and mixed with the molten Mg due to the stirring action of the laser-GTA hybrid welding. Fig. 6 shows a schematic diagram of the set-up for the shear test. The welded joints were pulled to fracture. It was observed that the joint fracture occurred at the interface of the Mg–Fe. The steel melted only under the laser power. Therefore, the weld penetration which determines the mechanical properties of the lapped joints of Mg-to-Fe is mainly influenced by the laser power. The tensile strength of the joint was 90 MPa which represented an average of 5 specimens. Fig. 7 shows the effect of laser power on the strength of weld bead. It can be seen that the tensile strength of the joint increased rapidly as the laser power increased. The fracture surface between the Mg alloy and steel was analyzed, using EDS, see Fig. 8. The results showed that many metallic oxides existed on the

From Fig. 1, it can be seen that the Mg plate is on the top and the steel plate is on the bottom. During welding, the Mg alloy

Fig. 1 – Schematic diagram of laser-GTA hybrid welding.

400 15 80 1 1 39

Fig. 2 – Weld appearance (P = 400 W, I = 80 A, DLA = 1 mm, Z = 1 mm, v = 15 mm/s).

M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 7 9–1 2 8 4

Fig. 3 – Optical micrograph of the cross section of the weld of a Mg alloy to steel.

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Fig. 6 – Test specimen for shear strength of the Mg–Fe lap joint.

electron (SE) image of the microstructure. It was evident that there is a transition zone indicated by the arrow A in Fe matrix, with a range of about 10 ~ 20 μm. The transition zone consists of

Fig. 4 – Optical micrograph of a vertical section through the weld of a Mg alloy to steel.

fracture surface. The main oxides were MgO, and Fe2O3, but Al2O3, and ZnO were also formed at the interface during the welding process. Clearly, the interface between Mg alloy and the steel was heavily oxidized during the welding process. Fig. 9 shows further analysis of the interface of the Mg–Fe after the laser-GTA hybrid welding. Fig. 9(a) is a secondary

Fig. 5 – SEM image of the microstructure of the welded Mg–Fe joint.

Fig. 7 – Effect of laser power on strength of weld bead (DLA 1 mm, welding speed 15 mm/s, welding current 80 A).

Fig. 8 – Energy-dispersive X-ray spectrum from the fracture surface of the Fe–Mg joint.

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Fig. 9 – SE image and X-ray maps of the interface of Mg–Fe after laser-GTA hybrid welding (a) SE image, (b) Fe mapping, (c) Mg mapping, (d) O mapping, (e) Cr mapping (P = 400 W, I = 80 A, DLA = 1 mm, v = 15 mm/s, Z = 0.5 mm).

M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 2 7 9–1 2 8 4

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Fig. 10 – Element analysis across the Mg–Fe interface from EPMA (P = 400 W, I = 80 A, DLA = 1 mm, v = 15 mm/s, Z = 0.5 mm).

coarse bulk grains. A weld defect indicated by the arrow B was present in the Mg matrix. Fig. 9(b–e) are elemental mappings of the interface using EDS. Fig. 9(b) shows Fe EDS mapping where it was found that Fe elements was not present in the Mg matrix area, and that the Fe content was lower in the transition zone than in the Fe matrix area. Mg EDS mapping showed that Mg was present in the Fe matrix, and that Mg element was present in the transition zone. EDS mapping of oxygen [Fig. 9(d)] showed that not only was the Mg alloy oxidized, but that the steel was also oxidized. Oxygen was mainly in the transition zone. Cr EDS mapping showed that it was only present in the Fe (Fig. 9e). Steel and magnesium are insoluble in each other and there is no reaction with them. It seems that Mg alloy recrystallizes at the Mg–Fe interface. But Fig. 9 shows that a transition zone existed in the Fe matrix, and that Mg element is present in the transition zone. It is clear that Mg diffused into the Fe matrix during the welding process. Fig. 10 shows a line scan across the Mg–Fe interface using EPMA. The Fe content was lower in the transition zone than that in the Fe matrix. The O content was very high in the transition zone, but much less in the other areas. Mg was low in the transition zone, and declined similarly to O. The change of Cr content followed that of Fe in the Fe matrix. The potential interactions between the Cr element and the Mg element are important because 304 steel also contains large amounts of Cr. The results showed that the Cr concentration profile changed in the same way as the Fe, but opposite to that of the Mg profile. The Cr element existed only in Fe matrix. Thus, the influence of the Cr can be neglected. The process of forming the transition zone could be considered to occur as follows: the magnesium alloy and the

steel both melt during the laser-GTA hybrid welding. The melting point of Fe is 1538 °C, but the boiling point of Mg is lower at 1090 °C. Therefore, the high temperature at the interface of the Fe matrix could cause the liquid magnesium alloy to evaporate. The unprotected steel was oxidized. MgO and Fe2O3 were both found, as shown in Fig. 8. It is known that the binding energy with O of Mg is much stronger than that with Fe. Thus, a reducing reaction would occur between the Mg alloy of liquid and Fe2O3 in the weld. Also, Fe2O3 could diffuse into the MgO [12,13]. It can be deduced that the cross diffusion between Mg and Fe element occurred by forming oxides and reacting during the laser-GTA hybrid welding. Some Mg diffused into the Fe matrix and formed the transition zone, as shown in Fig. 9(a). In the lap welding process, the interface between the Mg and Fe was difficult to protect. Hence, the interface was heavily oxidized. Thus, the joints fractured at the Mg–Fe interface. The brittle metallic oxides were probably the main cause of the low shear strength of the joints.

4.

Summary and Conclusion

The following conclusions can be drawn from the investigations conducted on the laser-GTA hybrid welding of AZ31B magnesium alloy and 304 steel: • A lapped weld joint between the Mg alloy and the steel could be produced. • A transition zone was formed at the Mg–Fe interface and Mg diffused into the Fe matrix by forming oxides and reacting in the transition zone.

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• Upon shear, the joints fractured at the interface between the Mg alloy and the steel. The metallic oxides produced at the Mg–Fe interface appear to be the cause of the poor mechanical properties of the weld joints.

Acknowledgements The authors gratefully acknowledge the sponsorship from the Program for New Century Excellent Talents in University under the project NCET-04-0271. This work was financially supported by the high technology support program of China (No.2006BAE04B05).

REFERENCES [1] Roberts CS. Magnesium and its alloys. New York: John Wiley and Sons; 1960. [2] Mordike BL, Ebert T. Magnesium properties—applications—potential. Mater Sci Eng 2001; A302:37–45. [3] Sun Z, Pan D. Comparative evaluation of tungsten inert gas and laser welding of AZ31 magnesium alloy. Wei J Sci Technol Weld Join 2002;7:343–51. [4] Asahina T, Tokisue H. Electron bean weldability of pure magnesium and AZ31 magnesium alloy. Mater Trans 2001;42:235–2355.

[5] Munitz A, Cotler C, Stern A, Kohn G. Mechanical properties and microstructure of gas tungsten arc welded magnesium AZ91D plates. Mater Sci Eng 2001;A302:68–73. [6] Takehiko W, Kazuhiko K, Ronbunshul YG. Solid state welding of steel and magnesium alloy using a rotating pin. Q J Japan Weld Soc 2006;2:108–15. [7] Mao HK, Bell PM. Equations of state of MgO and Fe under static pressure conditions. J Geophys Res 1979:4533–6. [8] Liu LM, Wang SX, Zhu ML. Study on TIG welding of dissimilar Mg alloy and copper with Fe as interlayer. Sci Technol Weld Join 2006;11:523–5. [9] Liu LM, Liu XJ, Liu SH. Microstructure of laser-arc hybrid welds of dissimilar magnesium alloy and aluminum alloy with Ce as interlayer. Scr Mater 2006;4:383–6. [10] Liu LM, Hao XF, Song G. A new laser-arc hybrid welding technique based on energy conservation. Mater Trans 2006;6:1611–4. [11] Liu LM, Mao YG, Song G. Effect of heat input on microstructure and properties of welded joint in magnesium alloy AZ31B. Trans Non Metal Soc 2004;14:88–92. [12] Pierre D, Viala JC, Peronnet M. Interface reactions between mid steel and liquid Mg–Mn alloys. Mater Sci Eng 2003; A349:256–64. [13] Vassent JL, Marty A, Gilles B. Thermodynamic analysis of molecular beam epitaxy of MgO. J Cryst Growth 2000;219:444–50.