Formation and Disappearance of Pores in Plasma Arc Weld Bonding Process of Magnesium Alloy

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1 Materials Transactions, Vol. 50, No. 7 (2009) pp to 1654 Special Issue on New Functions and Properties of Engineering Materials Created by Designing and Processing #2009 The Japan Institute of Metals Formation and Disappearance of Pores in Plasma Arc Weld Bonding Process of Magnesium Alloy Liming Liu* and Jianbo Jiang School of Materials Science and Engineering, Dalian University of Technology, Dalian , P. R. China A new welding technology called plasma arc weld bonding was designed by combining the plasma arc welding and adhesive bonding process in the lap welding of magnesium alloy. During the plasma arc weld bonding process, the major difficulty was the presence of porosity in the welding joint. This paper analyzed the formation mechanism of pores and the effect of welding parameters on pores behaviors during plasma arc weld bonding process of magnesium alloy by optical microscopy and electron probe microanalysis. The results showed that it easily formed a lot of pores in joint because of the existence of adhesive layer. The decomposition of adhesive in both the sides of welding seam was the main cause for the formation of pores. The regular-shape pores were formed by CO and CO 2, and the anomalous-shape pores were formed by the low molecular weight hydrocarbons. The pores behaviors were affected evidently by the heat input, and the favorable joint could be obtained when the heat input was about 396 kj/m. [doi: /matertrans.mf200919] (Received January 19, 2009; Accepted April 1, 2009; Published June 3, 2009) Keywords: plasma arc weld bonding, pores behaviors, magnesium alloy 1. Introduction Table 1 Chemical compositions of AZ31B, mass%. Plasma arc weld bonding (PAWB) process, a new welding technology, was presented recently. It was put forward as a hybrid of plasma arc welding (PAW) and the adhesive bonding process. PAW offered significant advantages over conventional gas tungsten arc welding (GTAW) in terms of penetration depth, joint preparation and thermal distortion. 1,2) Although its energy was less dense than laser beam welding (LBW) and electron beam welding (EBW), PAW was more cost effective and more tolerant of joint preparation. 3,4) In addition, PAW technique was suitable to weld the structural components which were difficult to be welded on the backside. In adhesive bonding process, because of the existence of the adhesive layer, the fatigue resistance and the corrosion resistance of the welding joint could be improved. 5) Moreover, upon loading, there was a more uniform distribution of stress over the bonded area, which would increase the properties of the welding joint. 6,7) To combine the advantages of PAW and the adhesive bonding process, a new welding technology namely PAWB process was designed in which the plasma arc welding was conducted when an adhesive layer existed in the interface of the sheets. In the previous experiments, PAWB have been used to join Mg alloy successfully. 8,9) It was found that the existence of the adhesive layer played an important role in PAWB process. However, the existence of the adhesive layer had not only the advantages but also some disadvantages. During welding process, the adhesive would decompose and produce a mass of decomposition products. It was benefit to form pores in PAWB joint, and then decrease the properties of the welding joint. The aim of this paper was to study the porosity of welding joint in PAWB process. A series of the PAWB experiments was conducted in which the two join methods were coupled in one process. The formation mechanism of pores and the effect of welding parameters on pores behaviors were analyzed. *Corresponding author, liulm@dlut.edu.cn Mg Al Zn Mn Si Fe Cu Others AZ31B Bal >0:2 <0:10 >0:03 <0: Experimental Procedure AZ31B extrusive plates with dimensions of 250 mm 100 mm 2:5 mm were used in this study. Its chemical composition in weight percentages was given in Table 1. The adhesive (Terokal 4555B) used in this experiment was a kind of structural epoxy adhesive, which would decompose above 230 C. The composition of the epoxy adhesive was 10 30% Epoxy Nitrile Rubber Amine Adduct, 10 30% Bisphenol A-Epichlorohydrin polymer, 10 30% Reaction Product of Epichlorohydrin and Bisphenol F, 1 10%Cashew, nutshell liq., glycidyl ethers, 1 10%Calcium carbonate, 1 10%Biphenol Resin, 1 10% Barium metaborate and 1 10% Clay (mass%). And its decomposition products were carbon monoxide, carbon dioxide and/or low molecular weight hydrocarbons. The LHMfE-315 plasma arc welding equipment was used in experiment. Before welding, the surfaces of specimens were prepared by grinding with carborundum paper to remove oxides. Acetone was used to remove grease. The lap joint was chosen in experiments as shown in Fig. 1. The adhesive was coated on the overlap area of Mg alloy sheets with a thickness of 0.1 mm. A plasma arc welding torch with 4 mm, W- 2%ThO 2 electrode was used. Pure argon was used as the shielding gas and plasma gas. The PAWB welding was conducted with keyhole mode and variable polarity mode. The TDS 1002 digital storage oscilloscope was used to record the arc voltage variation during the welding process. After welding, the specimens were cured under fixed stress, the temperature was ramped up from room temperature at a speed of 5 C/min, and the cure cycle was 30 min at 175 C. The pores behaviors in PAWB joint were analyzed by optical microscopy. The elements on the inwall of pores were investigated by electron probe microanalysis (EPMA).

2 1650 L. Liu and J. Jiang Table 2 Welding parameters of PAWB process. Base metal Plasma arc Optimal welding parameters Welding current I/A 180 Welding speed V/mmmin Plasma gas flow rate Q/Lmin Shielding gas flow rate Q/Lmin 1 20 Arc longer L/mm 1 Fig. 1 Adhesive layer Schematic of PAWB process. Pores with anomalous shape Biggish elliptical pores 250µm Micropores 100µm Fig. 2 Biggish elliptical pores in cross section of PAWB joint, the anomalous-shape pores and micropores. 3. Results and Discussions 3.1 Porosity of PAWB joint Figure 2 showed the cross section of PAWB joints. It was found that there were a lot of pores in the PAWB joint and most of pores distributed in the upper part of PAWB joint. Four biggish elliptical pores with major axis of about 3 mm grew from the upper part to the lower part of PAWB joint. Some lesser elliptical pores with 1 mm major axis and circular pores with 0.3 mm diameter distributed around the biggish elliptical pores. It was seen that the inwall of these regular-shape pores was smooth. Figure 2 showed some anomalous-shape pores at the edge of upper molten pool of PAWB joint, such as star-shaped and polygonal pores. It was seen that the inwall of the anomalous-shape pores was rugged. Besides, the micropores with diameters of about 10 mm were also observed at the edge of bottom molten pool of PAWB joint, as shown in Fig. 2. During the welding process, the keyhole mode and the variable polarity mode were adopted, which were helpful to release the gases and clear the inclusions. However, it easily found a lot of pores in PAWB joint yet. During PAWB process, the existence of the adhesive layer was helpful to form pores in welding joint. The adhesive would combust and gasify acutely by the effect of plasma arc. And then a mass of the decomposition products, such as gas and low molecular weight hydrocarbons, were produced which easily formed pores in welding joint. The elements on the inwall of pores were analyzed by EPMA, and the results were shown in Table 2. It was found that the elements on the inwall of all kinds of pores were Mg, C and O elements mainly. On the inwall of biggish elliptical pores (shown as Fig. 2), C and O content was up to 10 and 16.5%, respectively. The C and O content on the inwall of micropore (shown as Fig. 2) was up to 6.5 and 8.9%, respectively. And the C contented on the inwall of anomalous-shape pore (shown as Fig. 2) was up to 96.75%. It was concluded that the formation of the pores had a close relationship with the C and O elements. In addition, it was known that the decomposition of the adhesive were carbon monoxide, carbon dioxide and/or low molecular weight hydrocarbons. Therefore, the C and O elements on the inwall

3 Formation and Disappearance of Pores in Plasma Arc Weld Bonding Process of Magnesium Alloy 1651 (d) (e) (f) Area of pores in cross section of PAWB joint Melting width at the bottom of upper sheets Area, S/mm Melting width, W/mm Plasma gas flow rate, Q/L min -1 Fig. 3 Cross section of PAWB joint with different flow rate of plasma gas: 1.4 L/min; 1.6 L/min; 1.8 L/min; (d) 2.0 L/min; (e) 2.2 L/min, and pores area and L with different flow rate of plasma gas. of pores should be come from the adhesive, and the decomposition of the adhesive was the reason for the porosity of PAWB joint. Based on the mol rate between C and O, the gases in bigger elliptical pores and micropores should be composed of CO and CO 2, and their mol ratio was about 2:3. In the anomalous-shape pore, it was considered to be the low molecular weight hydrocarbons based on the C element content. 3.2 Effect of welding parameters on the pores During PAWB process, the decomposition of the adhesive was inevitable by the effect of the plasma arc. The bigger quantity of adhesive layer decomposed, the more quantity of decomposition products were formatted. It easily known that the quantity of decomposition adhesive layer was proportional to molten width (W) at the bottom of upper sheet. Decreasing quantity of decomposition adhesive was considered as one method to reduce the quantity of pores. With the increasing of the flow rate of plasma gas, the constriction of the plasma arc was increased, and W could be reduced. Furthermore, it was well known that increasing the heat input could increased the cooling time of the melt pool, which was helpful to release gases. Therefore, increasing the welding current and decrease the welding speed were considered as another one method to reduce the quantity of pores in PAWB joint. Analyzing the effect of the parameters on the pores behaviors and making them to be matched was the key to avoid the porosity of PAWB joint Flow rate of plasma gas With different flow rate of plasma gas, the pores behaviors and the change of W were investigated, as shown in Fig. 3. It was found that with the increasing of the flow rate of plasma gas, W decreased. The size of the pores reduced, and the shape of pores was changed to slender. However, the amount of the pores increased, the total area of pores in cross section of PAWB joints was almost not changed. With the increasing of the flow rate of plasma gas, the plasma arc constricted, and the arc force increased. It could reduce W and increase the stir to the molten pool. The biggish pores were break up, and the coalescence of the lesser pores was baffled. Therefore, it was concluded that the flow rate of plasma gas mainly affected the shape and size of the pores, and did not affect total quantity of pores in PAWB joint greatly Welding current Figure 4 showed the pores behaviors and change of W with different welding current. It was seen that with the

4 1652 L. Liu and J. Jiang (d) (e) (f) Area of pores in cross section of PAWB joint Melting width at the bottom of upper sheets Area, S/mm 2 Melting width, W/mm Welding current, I/A Fig. 4 Cross section of PAWB joint with different welding current: 130 A; 140 A; 150 A; (d) 160 A; (e) 170 A, and pores area and L with different welding current (f). increasing of the welding current, W increased. The amount of the pores reduced. The lesser pores coalesced into the biggish pores, then the pores moved to the upper part of PAWB joint, and released from the PAWB joint at last. The total area of the pores in cross section of PAWB joints reduced rapidly. With the increasing of the welding current, the heat input increased. Although W increased, the solidification time of the molten pool increased. It made the releasing of gas more easily. Therefore, it was concluded that the welding current affected the size, amount and distribution of the pores. With the increasing of welding current, the total quantity of the pores decreased Welding speed With different welding speed, pores behaviors and the change of W were observed, as shown in Fig. 5. It was found that with the decrease of the welding speed, W increased. The lesser pores coalesced to the biggish pores, the amount of the pore reduced. The total area of pores in cross section of PAWB joint reduced quickly. The effect of the welding speed on pores behavior was opposite to the welding current. With the decrease of welding speed, the heat input increased. Although W increased, the solidification time of the molten pool increased. It made the releasing of gases more easily. Therefore, it was concluded that the welding speed affected the size, amount and distribution of the pores. With decrease of welding speed, the total quantity of the pores decreased. 3.3 The remedy for eliminating pores In PAWB process, decomposition of adhesive in both the sides of welding seam was considered as the main cause for the porosity of PAWB joint. It easily known that the adhesive in the area of welding seam instant decomposed completely because of the high temperature of plasma arc during PAWB process. And under the action of the arc force, most of decomposition products released from molten pool along the keyhole with the plasma arc, only a few of decomposition products could followed with the melt metal of molten pool. During PAWB process, however, the adhesive in both the sides of welding seam also decomposed and produced a mass of decomposition products, such as CO and CO 2. During the solidification process of molten pool, CO and CO 2 could join the molten pool along the interface between the specimens, as shown in Fig. 4(d) and Fig. 5(e). If there were not enough time for the gases to release from the molten pool, the gases

5 Formation and Disappearance of Pores in Plasma Arc Weld Bonding Process of Magnesium Alloy 1653 (d) (e) (f) Area of pores in cross section of PAWB joint Melting width at the bottom of upper sheets Area, S/mm 2 Melting width, W/mm Welding speed, V/mm min -1 Fig. 5 Cross section of welding joint with different welding speed: 425 mm/min; 400 mm/min; 375 mm/min; (d) 350 mm/min; (e) 325 mm/min, and pores area and L with different welding speed (f). would form the pores at last. Therefore, the decomposition of adhesive in both the sides of welding seam was considered as the main cause for the porosity of PAWB joint. In previous experiments, it was known that the flow rate of plasma gas almost not affected the total quantity of the pores in PAWB joint. However, with the increasing of the welding current or decrease of the welding speed, the total quantity of the pores in PAWB joint decreased quickly. In PAWB process, the heat input J (kj/m) could be expressed as J ¼ P V ¼ UI ð1þ V where P was the power of plasma arc (kw), U was the output voltage (V), I was the output current, V was the welding speed (m/s). In the previous experiments, the output voltage was about 11 V by the observed. Base on the previous experiments, the relation between the total area of the pores in cross section of PAWB joint and the heat input was established, as shown in Fig. 6. It was found that with the increasing of the heat input, the total area of the pores reduced quickly. The total area of the pores in cross section of PAWB joint showed a linear relation to the heat input. The function of the relationship between the total area of the pores and heat input could be expressed as a simple equation Area, S/mm 2 Fig y = x Heat input, J/kJ m -1 Relation between the total area of the pores and the heat input. S ¼ 43:023 0:1151J where S was the total area of the pores (mm 2 ) in the cross section of PAWB joint and J was the heat input (kj/m). Based on the equation, it could be worked out that there would be no pores when J was above kj/m. In the experiment, under the condition of forming a good welding seam, the welding parameters were chosen base on the eq. (1) and eq. (2). It was found that a good welding seam was obtained when the welding parameters were chosen as ð2þ

6 1654 L. Liu and J. Jiang Fig. 7 shown in Table 2, namely the heat input was 396 kj/m. Figure 7 showed the cross section of the welding joint. It could be seen that there was no pores in the welding joint. 4. Conclusions Cross section of welding joint by the optimal welding parameters. During PAWB process, it easily formed a mass of pores because of the decomposition of the adhesive. The major axis of pores was from several microns to several millimeters. The regular-shape pores, such as circular and elliptical pores, were formed by CO and CO 2. The anomalous-shape pores, such as star-shaped and polygonal pores, were formed by low molecular weight hydrocarbons. With the increasing of the follow rate of plasma gas, the size of pores reduced and the amount of pores increased. The shape of pores changed to slender. The total area of pores in cross section of PAWB joint was almost not changed. With the increasing of welding current or decrease of welding speed, the lesser pores coalesced to biggish pores, then released from molten pool. The total area of pores in cross section of PAWB joint reduced quickly. It was a linear relation to the heat input. Based on this, a good welding seam without porosity was obtained when the heat input was 396 KJ/m. Acknowledgements The authors gratefully acknowledge the sponsorship from the high technology support program of China (No. 2006BAE04B05). REFERENCES 1) E. Craig: Weld. J. 67 (1988) ) M. Tomsic and S. Barhorst: Weld. J. 63 (1984) ) Welding Handbook: Vol. 2: Welding Processes. 8th edition, ed. by R. L. O Brien (American Welding Society, Miami, Fla, 1991). 4) Y. F. Hsu and B. Rubinsky: Int. J. Heat Mass Trans. 31 (1988) ) Adhesion and Adhesives Technology: An Introduction ed. by A. V. Pocius (Chapter I), Second Edition, (America, 2002). 6) W. Leahy, V. Baron, M. Buggy, T. Young, A. Mas and F. Schue: J. Adhesion. 77 (2001) ) S. J. Shaw and D. A. Tod: Mater. World 2 (1994) ) J. Jiang and Z. Zhang: J. Alloy. Compd. 466 (2008) ) L. Liu and J. Jiang: J. Mater. Proc. Technol. 209 (2009)