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Strengthening effect of nickel and copper interlayers on hybrid laser-TIG welded joints between magnesium alloy and mild steel
Materials and Design 31 (2010) 3960–3963
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Strengthening effect of nickel and copper interlayers on hybrid laser-TIG welded joints between magnesium alloy and mild steel Liming Liu *, Xiaodong Qi Institute of Welding Technology, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 7 December 2009 Accepted 22 March 2010 Available online 27 March 2010
a b s t r a c t AZ31B magnesium alloy and Q235 mild steel were lap joined with Ni and Cu interlayers using hybrid laser-TIG welding technique. Microstructure and mechanical properties of joints were examined. The results showed that the shear strength of Cu-added joint was a little higher than that of Ni-added joint, and the strength of both joints exceeded that of base material AZ31B Mg alloy. Microstructure in fusion zone of the both joints was in great difference and the comparison of microhardness profile in the fusion zone indicated that the morphology and distribution of intermediate phases played a vital role in strengthening the joints, and the strengthening effect of them on Cu-added joint was better than that on Ni-added joint. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Recently, with a rise of advanced welding techniques, dissimilar materials’ welding, especially the welding of those completely different materials has been paid more attention. For instance, the welding of Al and Ti alloys [1], Al alloy and pure Cu [2], Al alloy and steel [3,4], Mg and Ti alloys [5], alumina and mild steel [6], etc. As Al element can interact with Ti, Cu and Fe elements, the interactions between them ensured that they could be joined in the form of intermediate phases or solid solutions. However, as for the elements without any interaction according to binary diagrams, joining them together would face a big challenge. Therefore, the addition of intermediate element or interlayer into joint may be a feasible solution for them. The application of interlayers to diffusion bonding and brazing is quite extensive and the joint properties are usually obtained substantial increase. Torun [7] reported that a good bonding was attained when Ag interlayer was added into the diffusion bonded joint of AZ91 Mg alloy. Uzunov [8] investigated that under appropriate welding temperatures and duration; the addition of Cu interlayer into the diffusion joint of Ti alloys showed that the composition and structure of intermediate phase changed without lowering joint strength and ductility. Zhu [9] studied that after the addition of Mo net interlayers into the Al2O3/Nb brazed joints; thermal shock resistance of the joints was improved considerably. Accordingly, the addition of interlayer into joints could not only actualize the bonding of dissimilar materials in quite large physical and chemical difference, but also achieve great enhancement of joint properties. * Corresponding author. Tel./fax: +86 0411 84707817. E-mail address: [email protected] (L. Liu). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.03.039
Magnesium and iron have hexagonal close-packed and cubic body centre structures, respectively. They do not form solid solution or intermediate phase according to the Mg–Fe binary diagram. Thus it is difficult to obtain an effective joint for them. In the welding process, a type of interlayer which could interact with both Mg and Fe elements should be selected. Ni and Cu interlayers satisfy that condition according to their respective binary diagrams with Mg and Fe. In present paper, hybrid laser-TIG welding technique was used, as it combined the unique features of laser and tungsten inert gas (TIG) welding technique [10,11], which are higher energy density, welding speed of laser and larger heat input of TIG. The aim of the study is to investigate which kind of interlayer, Ni or Cu could be more qualified for the advancement of Mg alloy/steel joint performances from the view of microstructure and mechanical properties.
2. Experimental procedure The materials used in the experiment are AZ31B Mg alloy with composition of Mg–3Al–1Zn–0.2Mn–0.1Si (wt.%), and Q235 mild steel with composition of Fe–0.2C–0.3Si–0.7Mn (wt.%). The sizes of them are 60 80 2 mm and 60 80 1.2 mm, respectively. Before welding, they were degreased and ground by acetone and abrasive papers. Lap joint configuration was adopted in terms of their properties. Ni and Cu interlayers with 99.9% purity and 0.1 mm thick were set between the two plates. After welding, the weldment was cut into specimens with dimension shown in Fig. 1. Supporting plates were added to the ends of the specimen to keep the joint interface parallel to the load direction. The joint shear strength was calculated according to the equation rb_shear = F/S//, where F and rb_shear are the load and the ultimate
L. Liu, X. Qi / Materials and Design 31 (2010) 3960–3963
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Fig. 1. Sketch of tensile shear test specimen (mm).
tensile shear strength (UTSS), respectively; S// is a rectangular bonding area at the joint interface, which can be evaluated according to the size of fracture surface in Fig. 2a and b. And the strength was an average of at least three specimens. Fracture surface was observed macroscopically and microscopically. Cross-sections of the specimen were etched by Nital’s (4% HNO3 ethanol solution) reagent for 15–20 s, and were also observed by optical microscopy (OM) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). Microhardness was performed on base material and fusion zone of Mg alloy side with Vickers hardness tester at a load of 200 gf and a dwell time of 15 s. 3. Results Both Ni and Cu interlayers were attached to AZ31B Mg alloy intimately shown in Fig. 2a and b. Fig. 2c is an OM image of recombined specimen after tensile shear test and is a fracture feature of both Ni- and Cu-added joints, indicating that the fracture usually occurs at such locations between interlayers and steel. The respective UTSS of the joint is shown in Fig. 3. 160 MPa is the UTSS of base material AZ31B Mg alloy. It can be seen that the UTSS of Cu-added joint which is 170 MPa is a little larger than that of Ni-added one that is 166 MPa. Further inspection was performed on the fracture surfaces shown in Fig. 4. It could be seen that the fracture surface of Ni-added joint displays bigger torn arrises, larger smooth areas and many small pits, which is quasi-cleavage fracture, suggesting that the joint owns certain plasticity. However, the fracture surface of Cu-added joint shows a large number of fine arrises and pits without smooth areas or big arrises, indicating that the plasticity of Cu-added joint is better than that of Ni-added one. Transverse sections of Ni- and Cu-added joints are shown in Fig. 5. Some bigger particles could be seen in fusion zone (FZ). It could also be seen that a transitional zone (TZ) is also presented
Fig. 3. UTSS of corresponding specimens from Fig. 2a and b.
along the edge of weld pool and it bonded well with base material steel and interlayer. The circle regions in Fig. 5a and b are prone to produce stress concentration leading to fracture that is the same as the fractural pattern shown in Fig. 2c. Fig. 5c and d show details of region A and B in the FZ. The fine white dots shown in Fig. 5c distribute dispersively and contain 59.7% Mg and 40.3% Ni (wt.) according to EDS analysis at point C, while discontinuously reticular structures in Fig. 5d distribute uniformly in the FZ and a further magnification on one of the branches presents a delicate fine stick structure which contains 52.4% Mg and 47.6% Cu (wt.) at position D. Microhardness measurements were carried out to examine the roles of intermediate phases played in the FZ. Due to the fracture location shown in Fig. 2c and in order to facilitate the hardness comparison of FZ with base material AZ31B Mg alloy, indentations were performed along the line in the sketch of cross-sectioned joint shown in Fig. 6. The hardness profile in region I and II corresponds to the test regions in the joint. It can be seen from the profile of Ni-added joint that there is no big difference in between the base material Mg alloy with approximate 53 Hv and the FZ with the maximum 59 Hv, whereas the profile in the FZ of Cu-added joint shows noticeable difference from that in the base material Mg alloy. And the microhardness in region I of Cu-added joint is higher than that of Ni-added joint generally, indicating that the mosophase with reticulated structure of the FZ is the dominant strengthening phase in Cu-added joint, and the influence of the phase on the joint strength is positive, which means that the intermediate phase contributes to the improvement of joint shear strength.
Fig. 2. Fracture location of joints: (a) Ni-added joint, (b) Cu-added joint, (c) fracture location.
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Fig. 4. Fracture surface of joints: (a) Ni-added joints, (b) Cu-added joints.
Fig. 5. SEM images of transverse sections of the joints and the FZ: (a) Ni-added joint, (b) Cu-added joints, (c) from region A of (a), (d) from region B of (b).
Fig. 6. Microhardness profile on weld cross-sections of joints.
4. Discussion The addition of Ni and Cu interlayers into lap joint of AZ31B Mg alloy to Q235 steel improved the joint shear strength significantly,
because the UTSS of both joints exceeded that of base material AZ31B Mg alloy, which is 160 MPa. Some common features are as follows. First, TZ was formed along the edge of weld pool on steel side in both Ni- and Cu-added joints. Therefore, they especially the part of the TZ in circle regions shown in Fig. 5a and b could wrap around the FZ and avoid it suffering external load directly during tensile shear test, which is significant for the enhancement of joint shear strength. Second, the interaction of Ni and that of Cu with both Mg and Fe elements determined that Ni and Cu interlayers could be fit for joining AZ31B Mg alloy and steel together. On the basis of their binary diagrams, intermediate phases can be formed between Mg and Ni or Cu, and solid solution of Ni or Cu in Fe could also be generated. Accordingly, the fracture mode of both types of joints shown in Fig. 2 is identical, and Ni and Cu elements can be seen as visual bridges that joined the Mg alloy and steel together. Nevertheless, the difference in joint strengthening effect is conspicuous. It is well known that mechanical properties of joints are closely related to the morphology and distribution of microstructures [12–14], especially to that of intermediate phase in FZ of joint. The intermediate phases in the present study are mainly Mg2Ni and Mg2Cu inferred from the EDS analysis which is consis-
L. Liu, X. Qi / Materials and Design 31 (2010) 3960–3963
tent with our previous works [15,16]. In view of the hardness profile of Ni-added joint in Fig. 6, the distribution and morphology of the intermediate phase in Ni-added joint (see Fig. 5c) indicates that strengthening effect of Mg2Ni phase on the joint was obscure. However, considering the hardness profile in Cu-added joint and the joint UTSS shown in Fig. 3, the morphology and distribution of the mosophase Mg2Cu in the FZ shown in Fig. 5d suggests that a strengthening effect of the intermediate phase Mg2Cu on the joint was remarkable which could also be seen from comparison of joint shear strength in Fig. 3. From the above analysis, it can be seen that the production of intermediate phase in joints is related closely to the element selection in joining process. And the characterization of mosophase is another important factor that affects joint properties [17,18], which can provide an available guidance for the selection of interlayer to join together Mg alloy and steel. Therefore, in view of the strengthening mechanism of the joints, Cu interlayer could be more eligible for lap joining of AZ31B Mg alloy to Q235 steel.
5. Conclusions With the addition of Ni and Cu interlayers, AZ31B Mg alloy and Q235 steel were successfully lap joined by hybrid laser-TIG welding technique. The joint shear strength could exceed that of base material AZ31B Mg alloy. And though the strengthening effect of joints is similar, the strengthening mechanism is dissimilar a little. The morphology of intermediate phase in the FZ is a key factor that determines the Mg alloy/steel joint shear strength under the same fracture mode of joints. The addition of Cu interlayer into the joints is superior to that of Ni interlayer in shear strength.
Acknowledgments The authors gratefully appreciate the sponsorship supported by National Natural Science Foundation of China (No. 50675028) and Supported by Research Fund for the Doctoral Program of Higher Education of China (No. 20070141031).
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References [1] Chen YC, Nakata K. Microstructural characterization and mechanical properties in friction stir welding of aluminum and titanium dissimilar alloys. Mater Design 2009;30:469–74. [2] Abdollah-Zadeh A, Saeid T, Sazgari B. Microstructural and mechanical properties of friction stir welded aluminum/copper lap joints. J Alloy Compd 2008;460:535–8. [3] Lee WB, Schmuecker M, Mercardo UA, Biallas G, Jung SB. Interfacial reaction in steel–aluminum joints made by friction stir welding. Scripta Mater 2006;55:355–8. [4] Lee KJ, Kumai SJ, Arai T, Aizawa T. Interfacial microstructure and strength of steel/aluminum alloy lap joint fabricated by magnetic pressure seam welding. Mater Sci Eng A-Struct 2007;471:95–101. [5] Aonuma M, Nakata K. Effect of alloying elements on interface microstructure of Mg–Al–Zn magnesium alloys and titanium joint by friction stir welding. Mater Sci Eng B-Solid 2009;161:46–9. [6] Noh MZ, Hussain LB, Ahmad ZA. Alumina–mild steel friction welded at lower rotational speed. J Mater Process Technol 2008;204:279–83. [7] Torun O, Karabulut A, Baksan B, Celikyürek I. Diffusion bonding of AZ91 using a silver interlayer. Mater Design 2008;29:2043–6. [8] Uzunov TD, Stojanov SP, Lambov SI. Contact-reactive welding of titanium via a copper layer. Vacuum 1999;52:365–8. [9] Zhu DY, Ma ML, Jin ZH, Wang YL. The effect of molybdenum net interlayer on thermal shock resistance of Al2O3/Nb brazed joint. J Mater Process Technol 1999;96:19–21. [10] Liu LM, Song G, Liang GL. Pore formation during hybrid laser-tungsten inert gas arc welding of magnesium alloy AZ31B-mechanism and remedy. Mater Sci Eng A-Struct 2005;390:76–80. [11] Liu LM, Wang JF, Song G. Hybrid laser–TIG welding, laser beam welding and gas tungsten arc welding of AZ31B magnesium alloy. Mater Sci Eng A-Struct 2004;381:129–33. [12] Yang MB, Pan FS. Effects of Sn addition on as-cast microstructure, mechanical properties and casting fluidity of ZA84 magnesium alloy. Mater Design 2010;31:68–75. [13] Avazkonandeh-Gharavol MH, Haddad-Sabzevar M, Haerian A. Effect of copper content on the microstructure and mechanical properties of multipass MMA, low alloy steel weld metal deposits. Mater Design 2009;30:1902–12. [14] Meysami AH, Ghasemzadeh R, Seyedein SH, Aboutalebi MR. An investigation on the microstructure and mechanical properties of direct-quenched and tempered AISI 4140 steel. Mater Design 2010;31:1570–5. [15] Qi XD, Song G. Interfacial structure of the joints between magnesium alloy and mild steel with nickel as interlayer by hybrid laser-TIG welding. Mater Design 2010;31:605–9. [16] Liu LM, Qi XD. Effects of copper addition on microstructure and strength of the hybrid laser-TIG welded joints between magnesium alloy and mild steel. J Mater Sci 2009;44:5725–31. [17] Kong JH, Zhen L, Guo B, Li PH, Wang AH, Xie CS. Influence of Mo content on microstructure and mechanical properties of high strength pipeline steel. Mater Design 2004;25:723–8. [18] Wang W, Yan W, Zhu L, Hua P, Shan YY, Yang K. Relation among rolling parameters, microstructures and mechanical properties in an acicular ferrite pipeline steel. Mater Design 2009;30:3436–43.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Strengthening effect of nickel and copper interlayers on hybrid laser-TIG welded joints between magnesium alloy and mild steel Liming Liu *, Xiaodong Qi Institute of Welding Technology, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 7 December 2009 Accepted 22 March 2010 Available online 27 March 2010
a b s t r a c t AZ31B magnesium alloy and Q235 mild steel were lap joined with Ni and Cu interlayers using hybrid laser-TIG welding technique. Microstructure and mechanical properties of joints were examined. The results showed that the shear strength of Cu-added joint was a little higher than that of Ni-added joint, and the strength of both joints exceeded that of base material AZ31B Mg alloy. Microstructure in fusion zone of the both joints was in great difference and the comparison of microhardness profile in the fusion zone indicated that the morphology and distribution of intermediate phases played a vital role in strengthening the joints, and the strengthening effect of them on Cu-added joint was better than that on Ni-added joint. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Recently, with a rise of advanced welding techniques, dissimilar materials’ welding, especially the welding of those completely different materials has been paid more attention. For instance, the welding of Al and Ti alloys [1], Al alloy and pure Cu [2], Al alloy and steel [3,4], Mg and Ti alloys [5], alumina and mild steel [6], etc. As Al element can interact with Ti, Cu and Fe elements, the interactions between them ensured that they could be joined in the form of intermediate phases or solid solutions. However, as for the elements without any interaction according to binary diagrams, joining them together would face a big challenge. Therefore, the addition of intermediate element or interlayer into joint may be a feasible solution for them. The application of interlayers to diffusion bonding and brazing is quite extensive and the joint properties are usually obtained substantial increase. Torun [7] reported that a good bonding was attained when Ag interlayer was added into the diffusion bonded joint of AZ91 Mg alloy. Uzunov [8] investigated that under appropriate welding temperatures and duration; the addition of Cu interlayer into the diffusion joint of Ti alloys showed that the composition and structure of intermediate phase changed without lowering joint strength and ductility. Zhu [9] studied that after the addition of Mo net interlayers into the Al2O3/Nb brazed joints; thermal shock resistance of the joints was improved considerably. Accordingly, the addition of interlayer into joints could not only actualize the bonding of dissimilar materials in quite large physical and chemical difference, but also achieve great enhancement of joint properties. * Corresponding author. Tel./fax: +86 0411 84707817. E-mail address: [email protected] (L. Liu). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.03.039
Magnesium and iron have hexagonal close-packed and cubic body centre structures, respectively. They do not form solid solution or intermediate phase according to the Mg–Fe binary diagram. Thus it is difficult to obtain an effective joint for them. In the welding process, a type of interlayer which could interact with both Mg and Fe elements should be selected. Ni and Cu interlayers satisfy that condition according to their respective binary diagrams with Mg and Fe. In present paper, hybrid laser-TIG welding technique was used, as it combined the unique features of laser and tungsten inert gas (TIG) welding technique [10,11], which are higher energy density, welding speed of laser and larger heat input of TIG. The aim of the study is to investigate which kind of interlayer, Ni or Cu could be more qualified for the advancement of Mg alloy/steel joint performances from the view of microstructure and mechanical properties.
2. Experimental procedure The materials used in the experiment are AZ31B Mg alloy with composition of Mg–3Al–1Zn–0.2Mn–0.1Si (wt.%), and Q235 mild steel with composition of Fe–0.2C–0.3Si–0.7Mn (wt.%). The sizes of them are 60 80 2 mm and 60 80 1.2 mm, respectively. Before welding, they were degreased and ground by acetone and abrasive papers. Lap joint configuration was adopted in terms of their properties. Ni and Cu interlayers with 99.9% purity and 0.1 mm thick were set between the two plates. After welding, the weldment was cut into specimens with dimension shown in Fig. 1. Supporting plates were added to the ends of the specimen to keep the joint interface parallel to the load direction. The joint shear strength was calculated according to the equation rb_shear = F/S//, where F and rb_shear are the load and the ultimate
L. Liu, X. Qi / Materials and Design 31 (2010) 3960–3963
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Fig. 1. Sketch of tensile shear test specimen (mm).
tensile shear strength (UTSS), respectively; S// is a rectangular bonding area at the joint interface, which can be evaluated according to the size of fracture surface in Fig. 2a and b. And the strength was an average of at least three specimens. Fracture surface was observed macroscopically and microscopically. Cross-sections of the specimen were etched by Nital’s (4% HNO3 ethanol solution) reagent for 15–20 s, and were also observed by optical microscopy (OM) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). Microhardness was performed on base material and fusion zone of Mg alloy side with Vickers hardness tester at a load of 200 gf and a dwell time of 15 s. 3. Results Both Ni and Cu interlayers were attached to AZ31B Mg alloy intimately shown in Fig. 2a and b. Fig. 2c is an OM image of recombined specimen after tensile shear test and is a fracture feature of both Ni- and Cu-added joints, indicating that the fracture usually occurs at such locations between interlayers and steel. The respective UTSS of the joint is shown in Fig. 3. 160 MPa is the UTSS of base material AZ31B Mg alloy. It can be seen that the UTSS of Cu-added joint which is 170 MPa is a little larger than that of Ni-added one that is 166 MPa. Further inspection was performed on the fracture surfaces shown in Fig. 4. It could be seen that the fracture surface of Ni-added joint displays bigger torn arrises, larger smooth areas and many small pits, which is quasi-cleavage fracture, suggesting that the joint owns certain plasticity. However, the fracture surface of Cu-added joint shows a large number of fine arrises and pits without smooth areas or big arrises, indicating that the plasticity of Cu-added joint is better than that of Ni-added one. Transverse sections of Ni- and Cu-added joints are shown in Fig. 5. Some bigger particles could be seen in fusion zone (FZ). It could also be seen that a transitional zone (TZ) is also presented
Fig. 3. UTSS of corresponding specimens from Fig. 2a and b.
along the edge of weld pool and it bonded well with base material steel and interlayer. The circle regions in Fig. 5a and b are prone to produce stress concentration leading to fracture that is the same as the fractural pattern shown in Fig. 2c. Fig. 5c and d show details of region A and B in the FZ. The fine white dots shown in Fig. 5c distribute dispersively and contain 59.7% Mg and 40.3% Ni (wt.) according to EDS analysis at point C, while discontinuously reticular structures in Fig. 5d distribute uniformly in the FZ and a further magnification on one of the branches presents a delicate fine stick structure which contains 52.4% Mg and 47.6% Cu (wt.) at position D. Microhardness measurements were carried out to examine the roles of intermediate phases played in the FZ. Due to the fracture location shown in Fig. 2c and in order to facilitate the hardness comparison of FZ with base material AZ31B Mg alloy, indentations were performed along the line in the sketch of cross-sectioned joint shown in Fig. 6. The hardness profile in region I and II corresponds to the test regions in the joint. It can be seen from the profile of Ni-added joint that there is no big difference in between the base material Mg alloy with approximate 53 Hv and the FZ with the maximum 59 Hv, whereas the profile in the FZ of Cu-added joint shows noticeable difference from that in the base material Mg alloy. And the microhardness in region I of Cu-added joint is higher than that of Ni-added joint generally, indicating that the mosophase with reticulated structure of the FZ is the dominant strengthening phase in Cu-added joint, and the influence of the phase on the joint strength is positive, which means that the intermediate phase contributes to the improvement of joint shear strength.
Fig. 2. Fracture location of joints: (a) Ni-added joint, (b) Cu-added joint, (c) fracture location.
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Fig. 4. Fracture surface of joints: (a) Ni-added joints, (b) Cu-added joints.
Fig. 5. SEM images of transverse sections of the joints and the FZ: (a) Ni-added joint, (b) Cu-added joints, (c) from region A of (a), (d) from region B of (b).
Fig. 6. Microhardness profile on weld cross-sections of joints.
4. Discussion The addition of Ni and Cu interlayers into lap joint of AZ31B Mg alloy to Q235 steel improved the joint shear strength significantly,
because the UTSS of both joints exceeded that of base material AZ31B Mg alloy, which is 160 MPa. Some common features are as follows. First, TZ was formed along the edge of weld pool on steel side in both Ni- and Cu-added joints. Therefore, they especially the part of the TZ in circle regions shown in Fig. 5a and b could wrap around the FZ and avoid it suffering external load directly during tensile shear test, which is significant for the enhancement of joint shear strength. Second, the interaction of Ni and that of Cu with both Mg and Fe elements determined that Ni and Cu interlayers could be fit for joining AZ31B Mg alloy and steel together. On the basis of their binary diagrams, intermediate phases can be formed between Mg and Ni or Cu, and solid solution of Ni or Cu in Fe could also be generated. Accordingly, the fracture mode of both types of joints shown in Fig. 2 is identical, and Ni and Cu elements can be seen as visual bridges that joined the Mg alloy and steel together. Nevertheless, the difference in joint strengthening effect is conspicuous. It is well known that mechanical properties of joints are closely related to the morphology and distribution of microstructures [12–14], especially to that of intermediate phase in FZ of joint. The intermediate phases in the present study are mainly Mg2Ni and Mg2Cu inferred from the EDS analysis which is consis-
L. Liu, X. Qi / Materials and Design 31 (2010) 3960–3963
tent with our previous works [15,16]. In view of the hardness profile of Ni-added joint in Fig. 6, the distribution and morphology of the intermediate phase in Ni-added joint (see Fig. 5c) indicates that strengthening effect of Mg2Ni phase on the joint was obscure. However, considering the hardness profile in Cu-added joint and the joint UTSS shown in Fig. 3, the morphology and distribution of the mosophase Mg2Cu in the FZ shown in Fig. 5d suggests that a strengthening effect of the intermediate phase Mg2Cu on the joint was remarkable which could also be seen from comparison of joint shear strength in Fig. 3. From the above analysis, it can be seen that the production of intermediate phase in joints is related closely to the element selection in joining process. And the characterization of mosophase is another important factor that affects joint properties [17,18], which can provide an available guidance for the selection of interlayer to join together Mg alloy and steel. Therefore, in view of the strengthening mechanism of the joints, Cu interlayer could be more eligible for lap joining of AZ31B Mg alloy to Q235 steel.
5. Conclusions With the addition of Ni and Cu interlayers, AZ31B Mg alloy and Q235 steel were successfully lap joined by hybrid laser-TIG welding technique. The joint shear strength could exceed that of base material AZ31B Mg alloy. And though the strengthening effect of joints is similar, the strengthening mechanism is dissimilar a little. The morphology of intermediate phase in the FZ is a key factor that determines the Mg alloy/steel joint shear strength under the same fracture mode of joints. The addition of Cu interlayer into the joints is superior to that of Ni interlayer in shear strength.
Acknowledgments The authors gratefully appreciate the sponsorship supported by National Natural Science Foundation of China (No. 50675028) and Supported by Research Fund for the Doctoral Program of Higher Education of China (No. 20070141031).
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References [1] Chen YC, Nakata K. Microstructural characterization and mechanical properties in friction stir welding of aluminum and titanium dissimilar alloys. Mater Design 2009;30:469–74. [2] Abdollah-Zadeh A, Saeid T, Sazgari B. Microstructural and mechanical properties of friction stir welded aluminum/copper lap joints. J Alloy Compd 2008;460:535–8. [3] Lee WB, Schmuecker M, Mercardo UA, Biallas G, Jung SB. Interfacial reaction in steel–aluminum joints made by friction stir welding. Scripta Mater 2006;55:355–8. [4] Lee KJ, Kumai SJ, Arai T, Aizawa T. Interfacial microstructure and strength of steel/aluminum alloy lap joint fabricated by magnetic pressure seam welding. Mater Sci Eng A-Struct 2007;471:95–101. [5] Aonuma M, Nakata K. Effect of alloying elements on interface microstructure of Mg–Al–Zn magnesium alloys and titanium joint by friction stir welding. Mater Sci Eng B-Solid 2009;161:46–9. [6] Noh MZ, Hussain LB, Ahmad ZA. Alumina–mild steel friction welded at lower rotational speed. J Mater Process Technol 2008;204:279–83. [7] Torun O, Karabulut A, Baksan B, Celikyürek I. Diffusion bonding of AZ91 using a silver interlayer. Mater Design 2008;29:2043–6. [8] Uzunov TD, Stojanov SP, Lambov SI. Contact-reactive welding of titanium via a copper layer. Vacuum 1999;52:365–8. [9] Zhu DY, Ma ML, Jin ZH, Wang YL. The effect of molybdenum net interlayer on thermal shock resistance of Al2O3/Nb brazed joint. J Mater Process Technol 1999;96:19–21. [10] Liu LM, Song G, Liang GL. Pore formation during hybrid laser-tungsten inert gas arc welding of magnesium alloy AZ31B-mechanism and remedy. Mater Sci Eng A-Struct 2005;390:76–80. [11] Liu LM, Wang JF, Song G. Hybrid laser–TIG welding, laser beam welding and gas tungsten arc welding of AZ31B magnesium alloy. Mater Sci Eng A-Struct 2004;381:129–33. [12] Yang MB, Pan FS. Effects of Sn addition on as-cast microstructure, mechanical properties and casting fluidity of ZA84 magnesium alloy. Mater Design 2010;31:68–75. [13] Avazkonandeh-Gharavol MH, Haddad-Sabzevar M, Haerian A. Effect of copper content on the microstructure and mechanical properties of multipass MMA, low alloy steel weld metal deposits. Mater Design 2009;30:1902–12. [14] Meysami AH, Ghasemzadeh R, Seyedein SH, Aboutalebi MR. An investigation on the microstructure and mechanical properties of direct-quenched and tempered AISI 4140 steel. Mater Design 2010;31:1570–5. [15] Qi XD, Song G. Interfacial structure of the joints between magnesium alloy and mild steel with nickel as interlayer by hybrid laser-TIG welding. Mater Design 2010;31:605–9. [16] Liu LM, Qi XD. Effects of copper addition on microstructure and strength of the hybrid laser-TIG welded joints between magnesium alloy and mild steel. J Mater Sci 2009;44:5725–31. [17] Kong JH, Zhen L, Guo B, Li PH, Wang AH, Xie CS. Influence of Mo content on microstructure and mechanical properties of high strength pipeline steel. Mater Design 2004;25:723–8. [18] Wang W, Yan W, Zhu L, Hua P, Shan YY, Yang K. Relation among rolling parameters, microstructures and mechanical properties in an acicular ferrite pipeline steel. Mater Design 2009;30:3436–43.