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Effect of repair welding on microstructure and mechanical properties of 7N01 aluminum alloy MIG welded joint
Journal of Manufacturing Processes 54 (2020) 80–88
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Effect of repair welding on microstructure and mechanical properties of 7N01 aluminum alloy MIG welded joint
T
Shuai Lia,b, Honggang Dongc,*, Xingxing Wanga, Zhongying Liua, Zhaojun Tana,*, Linjian Shangguana, Quanbin Lud, Sujuan Zhongd a
School of Mechanical Engineering, North China University of Water Resources and Electric Power, Zhengzhou 450045, China New Energy Materials and Technology Institute Ltd. of Dalian University of Technology, Qingdao 266200, China c School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China d State Key Laboratory of Advanced Brazing Filler Metals & Technology, Zhengzhou Research Institute of Mechanical Engineering, Zhengzhou 450001, China b
ARTICLE INFO
ABSTRACT
Keywords: 7N01 aluminum alloy Repair welding Microstructure Mechanical properties
The effect of repair welding on the microstructure and mechanical properties of the welded joint was investigated. The results show that the difference of microstructure in different locations of the original welded joint (OWJ) is mainly related to the production process. The AT5 side is an extruded profile, exhibiting equiaxed grain characteristics, while the AT4 side is a rolled sheet, displaying flat long grain. The grain boundary orientation difference in different positions of repair welded joint (RWJ) is various from that of OWJ, especially the change of proportion of small angle grain boundary in AT4-HAZ II is large. The tensile strength of the OWJ and RWJ are 283 MPa and 280 MPa, respectively, and the corresponding welded joint coefficients are 78.6 % and 77.8 %, respectively, which can fulfill the demands of practical engineering applications. Due to the existence of pores and inclusions, the crack of OWJ originates in the junction of HAZ I near the 7N01-T4 side, then expands along the welded zone, while the RWJ fails in the junction of the original weld and the weld passes.
1. Introduction Environmental pollution and energy consumption have become a big problem in today's society [1]. And the lightweight of structure materials is one of the effective measures to achieve energy saving and emission reduction. The aluminum alloys have been widely applied in aerospace and transportation field because of the advantages of low density, high specific strength, good corrosion resistance and high recovery rate [2–4]. It can be divided into aging strengthened and nonaging strengthened aluminum alloy and the Al-Zn-Mg series alloy belong to aging strengthened aluminum alloy. The Al-Zn-Mg series alloy are usually joined by welding method in the industrial production condition, such as Tungsten inert gas welding (TIG), Metal inert-gas welding (MIG), laser welding (LW) and Friction stir welding (FSW), etc. Among various welding methods, MIG welding is commonly applied for welding different dimensional structures because of the relatively easy operation and better economy. Since the
⁎
welded joint need to be tested after welding or service for a period of time. Then the repair welding method will be carried out to lower the cost when the problem of excessive pores and welding cracks are found. For the Al-Zn-Mg series alloy, the properties are mainly related to the thermal cycles. The defects such as pores and crack can be eliminated by the means of repair welding, however the extra thermal cycles easily cause new problems to the reliability of welded joint. Qin [5] studied the effect of repair welding on the microstructure of Al-Zn-Mg alloy MIG weld joint and found out that the width of fine grain area near the fusion line of the plate increased, and the recovery and recrystallization occurred in the grain structure of heat-affected zones. Dong [6] pointed out that the repeated repair welding will increase the degree of grain boundary liquefaction of 7N01 aluminum alloy. Yan et al. [7] investigated the influence of repair welding on the mechanical properties of welded joint and found out that the yield strength decreased from 209.3 MPa to 118.8 MPa, the fracture toughness of the joint in various areas also decreased slightly. Liang et al. [8] reported that the hot
Corresponding authors. E-mail addresses: [email protected] (H. Dong), [email protected] (Z. Tan).
https://doi.org/10.1016/j.jmapro.2020.03.009 Received 14 November 2019; Received in revised form 20 February 2020; Accepted 3 March 2020 1526-6125/ © 2020 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 54 (2020) 80–88
S. Li, et al.
Fig. 1. The schematic diagram of MIG welded joint. Table 3 The abbreviations of specimens in this work. Abbreviations
Notes
AT5-BM AT5-HAZ AT5-HAZ WZ AT4-HAZ AT4-HAZ AT4-BM
7N01 aluminum alloy with T5 condition The location II of heat affected zone on 7N01-T5 side The location I of heat affected zone on 7N01-T5 side Welded zone The location I of heat affected zone on 7N01-T4 side The location II of heat affected zone on 7N01-T4 side 7N01 aluminum alloy with T4 condition
II I I II
Fig. 2. The flow chart of repair welding (a) OWJ; (b) groove machining; (c) RWJ. Table 1 Element content of 7N01 aluminum alloys and filler metal (wt.%). Element
Zn
Mg
Cr
Cu
Ti
Mn
Zr
Fe
Si
V
Al
ER5356 7N01-T5 7N01-T4
0.10 4.48 4.76
4.50 1.55 1.20
0.15 0.23 0.11
0.10 0.11 0.01
0.14 0.05 0.04
0.10 0.29 0.42
/ 0.18 0.07
/ 0.13 0.11
/ 0.05 0.04
/ 0.01 0.01
Bal. Bal. Bal.
Fig. 3. Sizes of welded joint tensile specimens (Unit, mm).
It is frequently necessary to join different types of aluminum alloys to fulfill the requirements of special properties [4,10,11]. However, the aluminum alloys have different response mechanisms to welding thermal cycles due to the differences in strengthening mechanism and the initial aging conditions, and usually the heat-affected zones of welded joint will soften in different degrees [4,10,11]. At present, the investigations on repair welding mainly focus on the same kind of aluminum alloy, and this work aims to illuminate the influence of repair welding on the microstructure and mechanical properties of the 7N01-T5/7N01-T4 aluminum alloy MIG welded joint.
Table 2 Welding parameters of MIG welded joint. No.
Welding speed (mm/s)
Voltage (V)
Current (A)
1 2 3 4
8.5 8.5 7.5 6.5
27 27 27 28
290 290 290 290
2. Experiment method
cracking trend of 7N01 aluminum alloy repair welded joint reduced by the method of cold metal transfer (CMT). Johnson et al. [9] studied the effect of repetitious repair welding on the properties of AA7020 aluminum alloy TIG welded joint and the results indicated that the size of the heat-affected zone increased from 30 mm to 210 mm after four times repair welding.
2.1. Experimental materials and welding process The base metals of 7N01-T4 and 7N01-T5 aluminum alloy were butt welded through MIG weld, and the schematic diagram of welding structure is displayed in Fig. 1. The flow chart of repair welding is
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Fig. 4. EBSD images of original MIG welded joint (a) AT5-BM; (b) AT5-HAZ II; (c) AT5-WZ; (d) AT4-WZ; (e) AT4-HAZ I; (f) AT4-HAZ II.
Fig. 5. Pole figure of original MIG welded joint in different locations (a) AT5-BM; (b) AT5-HAZ; (c) WZ; (d) AT4-HAZ; (e) AT4-BM.
displayed in Fig. 2, the repair welding groove is prepared by machining and the repair welding parameters are the same to the original welding process, as listed in Table 2. A T5 state means heat treatment of aluminum alloys which is cooled rapidly after high temperature molding process and then artificially aged. The T4 state means the aluminum alloys are subjected to solution heat treatment and then naturally aging. The chemical composition of materials in this research are illustrated in Table 1. The welding groove is 70° and the filler metal is ER5356, and the detailed welding parameters are illustrated in Table 2. The highpurify argon gas (99.999 %) is used as the shield gas. The acronyms were self-defined to stand for different locations in welded joint for convenience in this paper, as listed in Table 3.
2.2. Mechanical properties testing The specimens of mechanical properties testing are prepared according to the standard of GB/T 228.1–2010, as shown in Fig. 3. The mechanical properties were conducted on DNS100 equipment with tensile speed of 5 mm/min. 2.3. Microstructure observation The specimens was finely ground to 2000# by SiC sandpaper carefully and then polished with diamond paste. After polishing, the samples were etched with mixed acid (2 mL HF + 3 mL HCl+5 mL HNO3+190 mL H2O) with the time of 45 s. The microstructure of
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Fig. 6. Boundary misorientation distribution of original MIG welded joint on AT5 side. (a) AT5-BM; (b) AT5-HAZ II; (c) AT5-HAZ I; (d) WZ.
welded joint was observed by Leica MEF4 metallographic microscope. The fracture characteristics of welded joint were observed on Zeiss supra55 scanning electron microscope (SEM). Electron backscattered diffraction (EBSD) experiment was performed to analyze the microstructure evolution of welded joint. EBSD samples were grinded and polished carefully, then electrolytic polished with 30 vol.% nitric acidmethyl alcohol solution at −30 °C and the electro-polishing time is 10−20 s with the voltage of 15 V. The EBSD samples were washed in alcohol immediately after electro-polishing and the residual corrosive liquid was removed by ultrasonic cleaning machine. Finally, the sample was taken out and dried with a hairdryer.
consists of elongated grains, attributing to rolling process during production, as shown in Fig. 4d-f. The pole figure of OWJ in different locations is shown in Fig. 5. The maximum orientation density value in AT5-BM is 19.03 and the weighted distribution of polar projection points on the equatorial plane of the pole is concentrated, which means typical texture feature, as shown in Fig. 5a. The texture feature in AT5-HAZ becomes weaker with the maximum orientation density value of 10.66 compared to the location of AT5-BM, as shown in Fig. 5b. It can be concluded that the AT5 side shows typical texture characteristic. The main reason is likely related to the extrusion during process. The maximum orientation density value of WZ is 2.4, which is associated with the cast structure, as shown in Fig. 5c. The AT4 side also shows texture features and the maximum orientation density value in the AT4-HAZ and AT4-BM is 7.98 and 16.55, respectively, caused by the rolling process, as shown in Fig. 5d-e. It should be noted that the maximum orientation density value in HAZ adjacent to WZ decreases compared to the corresponding base metals. It is well known that the grain orientation of Al-Zn-Mg alloys show a certain degree of regularity after extrusion or rolling process, namely texture feature. The microstructure and properties of Al-Zn-Mg alloys will change due to extrusion or rolling process, meanwhile, the density
3. Results and discussion 3.1. Microstructure of welded joint Fig. 4 shows the EBSD images of OWJ. The AT5 side consists of typical exquiaxed grains, which has no obvious change after welding, as shown in Fig. 4a-c. The fine grains appear in the fusion zone of A in Fig. 4c, caused by recrystallization during welding. The welded zone shows typical cast structure with dendritic growth. The AT4 side
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Fig. 7. Boundary misorientation distribution of original MIG welded joint on AT5 side. (a) AT4-BM; (b) AT4-HAZ II; (c) AT4-HAZ I.
of vacancy and dislocation increases, making alloy in a thermodynamic instability state. Consequently, the HAZs on both sides of the WZ undergo partial recovery due to the thermal cycles during welding, then results in the change of the texture characteristics of welded joint. Fig. 6 shows the distribution of orientation difference of OWJ on AT5 side. It can be seen that the proportion of grains with different grain boundary angle ranges of θ≤10°,10° < θ≤15° and θ > 15° in the AT5-BM zone are 52.9 %, 7.6 %, and 39.5 %, respectively, and the AT5BM zone is dominated by low angle grain boundaries (θ≤15°). On the AT5 side, the proportion of the small angle grains increases gradually with the decrease of distance to the center of WZ. Additionally, it can be seen that the WZ is dominated by high-angle grain boundaries with a ratio of 60.4 %, as shown in Fig. 6d. The boundary misorientation distribution is mainly associated with different thermal cycles during welding, especially the peak temperature that can be obtained. The difference of boundary misorientation distribution between the WZ and AT5 side is attributed to the initial state [4]. The temperature range in the AT5-HAZ II and AT5-HAZ I of welded joint are 150−300 °C and 300-400 °C, respectively according to Rosenthal model [4,14]. Then the
recrystallization occurred for the 7N01 aluminum alloy during welding, which results in the change of boundary misorientation distribution. The AT4-BM zone mainly consists of low-angle grain boundaries with the percent of 88.5 % (θ < 15°), as shown in Fig. 7a. However, the distribution of orientation difference in the AT4-HAZ II changes obviously compared to AT4-BM with high-angle grain ratio of 75.5 %. This phenomenon is attributed to the thermal cycles in AT4-HAZ II during welding. The EBSD images of RWJ is illustrated in Fig. 8. The fine-grained region of fusion zone becomes wider, while the grain size of the HAZ adjacent to WZ has no obvious change after repair welding compared to the results in Figs. 4 and 8. Fig. 9 shows the distribution of orientation difference of RWJ, the proportion of the small angle grains in AT4-HAZ I and AT4-HAZ II increases from 71.9 % and 24.5 % to 77.9 % and 60.7 %, respectively. The corresponding proportion in the AT5-HAZ II and AT5-HAZ I of the small angle grains changes from 67.5 % and 71.0 % to 64.5 % and 78.6 %, respectively.
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Fig. 8. EBSD images of repair MIG welded joint (a) AT4-HAZII; (b) AT4-WZ; (c)WZ; (d) AT5-WZ; (e) AT4-HAZ II.
3.2. Mechanical properties
specifically, the failure of the RWJ occurs in the junction of original weld pass and repair weld pass, which is mainly related to the existence of pores and inclusions. Fig. 13 shows the fracture morphology of welded joint. There are obvious defect areas (pores and inclusions) in the fracture surface of OWJ, as displayed in Fig. 13(a).The gully-like features appear due to the connection of the pores in the OWJ, and the spherical particles in the gully are grains [4], as displayed in Fig. 13(b). Similar features occur in the RWJ, as illustrated in Fig. 13(c)-(d). The above phenomenon is mainly likely related to the appearance of pores in the welded joint. The typical casting structure is formed at the welded zone due to the uneven distribution of solute elements and the non-uniform recrystallization during the rapid cooling process in the weld pool [13]. The loose structure at welded zone often results in the appearance of cavities, which in turn reduces the elongation and mechanical properties of the welded joint. As for the welded zone, the ER5356 filler wire belongs to non-aging strengthened aluminum alloy and the main strengthening mechanism is solid solution strengthening, and the hardness is the lowest. Therefore, the welded joint often fails at the welded zone. Additionally, the gas porosity and inclusions are formed in the welded zone due to the existence of oxygen and hydrogen coming from base metals, welding consumables (shielding gas and flux) and air during welding. Then the existence of inclusions and gas porosity
The mechanical properties of welded joint are shown in Table 4, the tensile strength of OWJ and RWJ is 283 MPa and 280 MPa, respectively and the corresponding strength coefficient is 78.6 % and 77.8 %, respectively, which can fulfill the demands of engineering application [12]. Fig. 10 displays the cross-section images of the OWJ after failure. The crack appears in the junction of the AT5 side and the welded zone and the fracture surface on the AT4 side exists defects, as illustrated in Fig. 10b and c, respectively. The SEM pictures of RWJ is displayed in Fig. 11. The results show that the pores appear in the junction of the HAZ I and welded zone. It is well known that the pores and inclusions are often the origin of crack in welded joint. Consequently, it is speculated the fracture of OWJ originates in the junction of different weld passes or the confluence of HAZ I and weld pass, then expands along the welded zone. The images of the welded joint after fracture are obtained to analyze the fracture path further, as shown in Fig. 11(c)-(d). The SEM pictures shows that the failure of OWJ initiates from the junction of weld passes and AT4-HAZ I, then expands along the welded zone. The welded joint after fracture are examined to investigate the fracture path further, as illustrated in Fig. 12. It can be seen that the OWJ fails at the welded zone near AT4 side, as illustrated in Fig. 12 (a)(b). The RWJ fails at welded zone, as shown in Fig. 12(c)-(d). More
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Fig. 9. Boundary misorientation distribution of RWJ. (a) AT4-HAZ II; (b) AT4-HAZ I; (c) AT5-HAZ I; (d) AT5-HAZ II. Table 4 Mechanical properties of base alloys and weldment. Specimens
Elongation (%)
Tensile strength (MPa)
BM(AT5) BM(AT4) Welded joint Repair welded joint
20.6 24.3 9.0 8.2
360 432 283 280
results in the reduction of strength of welded joint [14]. Besides, the existence of inclusions in welded joint is likely related to the incomplete welding slag cleaning during multiple-pass welding or repair welding. 4. Conclusions 1) The difference in microstructure of both sides of the welded zone for the original welded joint is mainly attributed to the preparation process. The AT5 side is an extruded profile, exhibiting equiaxed grain characteristics, while the AT4 side is a rolled sheet, displaying flat long grain. The difference of grain boundary orientation in different positions of repair welded joint is different from that of the
Fig. 10. Images of fracture morphology of the OWJ (a) Macromorphology of the welded joint; (b) crack on AT5 side; (c) SEM images of fracture morphology of welded joint.
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Fig. 11. SEM images of OWJ. (a) near AT5 side;(b)near AT4 side; (c) welded zone; (d) confluence of AT4 side and welded zone.
Fig. 12. The fracture path of welded joint (a) image of OWJ before fracture; (b) picture of OWJ after fracture; (c) image of RWJ before fracture; (d) picture of RWJ after fracture.
original joint, especially the change of proportion of small angle grain boundary in AT4-HAZ II is large. 2) The tensile strength of original welded joint and repair welded joint is 283 MPa and 280 MPa, respectively and the corresponding strength coefficient is 78.6 % and 77.8 %, respectively, which can fulfill the demand of engineering application. 3) Due to the existence of pores and inclusions, the fracture of origin welded joint initiates in the junction of HAZ I near the 7N01-T4 side,
then expands along the welded zone, while the repair welded joint fails in the junction of the original weld and the weld pass. Funding This research was funded by Research Foundation for High-level talent Scholars in North China University of Water Resources and Electric Power (No. 201811034) and Open Fund of National Joint 87
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Fig. 13. The fracture morphology of welded joint(a)fracture features of OWJ; (b) the magnified fracture morphology image of OWJ; (c) fracture features of RWJ; (d) the magnified fracture features image of RWJ.
Engineering Research Center for abrasion control and molding of metal materials (No.HKDNM2019020).
2015. [6] Dong WK. Research on the grain boundary liquation in the heat affected zone of 7N01 aluminum alloy welded joint. Master′s Degree. Shen Yang: Northeastern University; 2013. [7] Yan ZJ, Chen SX, Shang Z. Property of repair welding joint of A7N01 aluminium alloy. Trans China Weld Inst 2014;35:51–4. [8] Liang ZM, Li YB, Zhao SS. Multiple repair welding of 7N01 aluminum alloy with pulsed MIG and DC CMT welding. Trans China Weld Inst 2014;35:27–31. [9] Maya-Johnson S, Santa JF, Mejía OL, Aristizábal S, Ospina S, Cortés PA, et al. Effect of the number of welding repairs with GTAW on the mechanical behavior of AA7020 aluminum alloy welded joints. Metall Mater Trans B 2015;46:2332–9. [10] Wang X, Mao S, Chen P, Liu Y, Ning J, Li H, et al. Evolution of microstructure and mechanical properties of a dissimilar aluminium alloy weldment. Mater Des 2016;90:230–7. [11] Chen Y, Wang H, Li H, Wang X, Ding H, Zhao J, et al. Investigation into the dissimilar friction stir welding of AA5052 and AA6061 aluminum alloys using pineccentric stir tool. Metals 2019;9:718. [12] ISO 15614-2:2005. Specification and qualification of welding procedures for metallic materials-welding procedure test-Part 2: arc welding of aluminium and its alloys. 2005. [13] Peng X, Cao X, Xu G, Deng Y, Tang L, Yin Z. Mechanical Properties, Corrosion behavior, and microstructures of a MIG-Welded 7020 Al alloy. J Mater Eng Perform 2016;25:1028–40. [14] Kou S. Welding metallurgy. 2rd ed. Hoboken, United States of America: A John Wiley & Sons, Inc.; 2003. p. 49–51.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors alone are responsible for the content and writing of this article. References [1] Li S, Dong HG, Wang X, Liu Z. Quenching sensitivity of Al-Zn-Mg alloy after nonisothermal heat treatment. Materials 2019;12:1595. [2] Li S, Dong HG, Li P, Chen S. Effect of repetitious non-isothermal heat treatment on corrosion behavior of Al-Zn-Mg alloy. Corros Sci 2018;131:278–89. [3] Li S, Guo D, Dong HG. Effect of flame rectification on corrosion property of Al–Zn–Mg alloy. Trans Nonferrous Met Soc China 2017;27:250–7. [4] Li S, Dong HG, Shi L, Li P, Ye F. Corrosion behavior and mechanical properties of Al–Zn–Mg aluminum alloy weld. Corros Sci 2017;123:243–55. [5] Qin F. The research of fatigue performance and microstructure of Al–Zn–Mg alloy weld joints used in high speed train. Master′s Degree. Chang Sha: Hunan University;
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Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Effect of repair welding on microstructure and mechanical properties of 7N01 aluminum alloy MIG welded joint
T
Shuai Lia,b, Honggang Dongc,*, Xingxing Wanga, Zhongying Liua, Zhaojun Tana,*, Linjian Shangguana, Quanbin Lud, Sujuan Zhongd a
School of Mechanical Engineering, North China University of Water Resources and Electric Power, Zhengzhou 450045, China New Energy Materials and Technology Institute Ltd. of Dalian University of Technology, Qingdao 266200, China c School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China d State Key Laboratory of Advanced Brazing Filler Metals & Technology, Zhengzhou Research Institute of Mechanical Engineering, Zhengzhou 450001, China b
ARTICLE INFO
ABSTRACT
Keywords: 7N01 aluminum alloy Repair welding Microstructure Mechanical properties
The effect of repair welding on the microstructure and mechanical properties of the welded joint was investigated. The results show that the difference of microstructure in different locations of the original welded joint (OWJ) is mainly related to the production process. The AT5 side is an extruded profile, exhibiting equiaxed grain characteristics, while the AT4 side is a rolled sheet, displaying flat long grain. The grain boundary orientation difference in different positions of repair welded joint (RWJ) is various from that of OWJ, especially the change of proportion of small angle grain boundary in AT4-HAZ II is large. The tensile strength of the OWJ and RWJ are 283 MPa and 280 MPa, respectively, and the corresponding welded joint coefficients are 78.6 % and 77.8 %, respectively, which can fulfill the demands of practical engineering applications. Due to the existence of pores and inclusions, the crack of OWJ originates in the junction of HAZ I near the 7N01-T4 side, then expands along the welded zone, while the RWJ fails in the junction of the original weld and the weld passes.
1. Introduction Environmental pollution and energy consumption have become a big problem in today's society [1]. And the lightweight of structure materials is one of the effective measures to achieve energy saving and emission reduction. The aluminum alloys have been widely applied in aerospace and transportation field because of the advantages of low density, high specific strength, good corrosion resistance and high recovery rate [2–4]. It can be divided into aging strengthened and nonaging strengthened aluminum alloy and the Al-Zn-Mg series alloy belong to aging strengthened aluminum alloy. The Al-Zn-Mg series alloy are usually joined by welding method in the industrial production condition, such as Tungsten inert gas welding (TIG), Metal inert-gas welding (MIG), laser welding (LW) and Friction stir welding (FSW), etc. Among various welding methods, MIG welding is commonly applied for welding different dimensional structures because of the relatively easy operation and better economy. Since the
⁎
welded joint need to be tested after welding or service for a period of time. Then the repair welding method will be carried out to lower the cost when the problem of excessive pores and welding cracks are found. For the Al-Zn-Mg series alloy, the properties are mainly related to the thermal cycles. The defects such as pores and crack can be eliminated by the means of repair welding, however the extra thermal cycles easily cause new problems to the reliability of welded joint. Qin [5] studied the effect of repair welding on the microstructure of Al-Zn-Mg alloy MIG weld joint and found out that the width of fine grain area near the fusion line of the plate increased, and the recovery and recrystallization occurred in the grain structure of heat-affected zones. Dong [6] pointed out that the repeated repair welding will increase the degree of grain boundary liquefaction of 7N01 aluminum alloy. Yan et al. [7] investigated the influence of repair welding on the mechanical properties of welded joint and found out that the yield strength decreased from 209.3 MPa to 118.8 MPa, the fracture toughness of the joint in various areas also decreased slightly. Liang et al. [8] reported that the hot
Corresponding authors. E-mail addresses: [email protected] (H. Dong), [email protected] (Z. Tan).
https://doi.org/10.1016/j.jmapro.2020.03.009 Received 14 November 2019; Received in revised form 20 February 2020; Accepted 3 March 2020 1526-6125/ © 2020 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 54 (2020) 80–88
S. Li, et al.
Fig. 1. The schematic diagram of MIG welded joint. Table 3 The abbreviations of specimens in this work. Abbreviations
Notes
AT5-BM AT5-HAZ AT5-HAZ WZ AT4-HAZ AT4-HAZ AT4-BM
7N01 aluminum alloy with T5 condition The location II of heat affected zone on 7N01-T5 side The location I of heat affected zone on 7N01-T5 side Welded zone The location I of heat affected zone on 7N01-T4 side The location II of heat affected zone on 7N01-T4 side 7N01 aluminum alloy with T4 condition
II I I II
Fig. 2. The flow chart of repair welding (a) OWJ; (b) groove machining; (c) RWJ. Table 1 Element content of 7N01 aluminum alloys and filler metal (wt.%). Element
Zn
Mg
Cr
Cu
Ti
Mn
Zr
Fe
Si
V
Al
ER5356 7N01-T5 7N01-T4
0.10 4.48 4.76
4.50 1.55 1.20
0.15 0.23 0.11
0.10 0.11 0.01
0.14 0.05 0.04
0.10 0.29 0.42
/ 0.18 0.07
/ 0.13 0.11
/ 0.05 0.04
/ 0.01 0.01
Bal. Bal. Bal.
Fig. 3. Sizes of welded joint tensile specimens (Unit, mm).
It is frequently necessary to join different types of aluminum alloys to fulfill the requirements of special properties [4,10,11]. However, the aluminum alloys have different response mechanisms to welding thermal cycles due to the differences in strengthening mechanism and the initial aging conditions, and usually the heat-affected zones of welded joint will soften in different degrees [4,10,11]. At present, the investigations on repair welding mainly focus on the same kind of aluminum alloy, and this work aims to illuminate the influence of repair welding on the microstructure and mechanical properties of the 7N01-T5/7N01-T4 aluminum alloy MIG welded joint.
Table 2 Welding parameters of MIG welded joint. No.
Welding speed (mm/s)
Voltage (V)
Current (A)
1 2 3 4
8.5 8.5 7.5 6.5
27 27 27 28
290 290 290 290
2. Experiment method
cracking trend of 7N01 aluminum alloy repair welded joint reduced by the method of cold metal transfer (CMT). Johnson et al. [9] studied the effect of repetitious repair welding on the properties of AA7020 aluminum alloy TIG welded joint and the results indicated that the size of the heat-affected zone increased from 30 mm to 210 mm after four times repair welding.
2.1. Experimental materials and welding process The base metals of 7N01-T4 and 7N01-T5 aluminum alloy were butt welded through MIG weld, and the schematic diagram of welding structure is displayed in Fig. 1. The flow chart of repair welding is
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Fig. 4. EBSD images of original MIG welded joint (a) AT5-BM; (b) AT5-HAZ II; (c) AT5-WZ; (d) AT4-WZ; (e) AT4-HAZ I; (f) AT4-HAZ II.
Fig. 5. Pole figure of original MIG welded joint in different locations (a) AT5-BM; (b) AT5-HAZ; (c) WZ; (d) AT4-HAZ; (e) AT4-BM.
displayed in Fig. 2, the repair welding groove is prepared by machining and the repair welding parameters are the same to the original welding process, as listed in Table 2. A T5 state means heat treatment of aluminum alloys which is cooled rapidly after high temperature molding process and then artificially aged. The T4 state means the aluminum alloys are subjected to solution heat treatment and then naturally aging. The chemical composition of materials in this research are illustrated in Table 1. The welding groove is 70° and the filler metal is ER5356, and the detailed welding parameters are illustrated in Table 2. The highpurify argon gas (99.999 %) is used as the shield gas. The acronyms were self-defined to stand for different locations in welded joint for convenience in this paper, as listed in Table 3.
2.2. Mechanical properties testing The specimens of mechanical properties testing are prepared according to the standard of GB/T 228.1–2010, as shown in Fig. 3. The mechanical properties were conducted on DNS100 equipment with tensile speed of 5 mm/min. 2.3. Microstructure observation The specimens was finely ground to 2000# by SiC sandpaper carefully and then polished with diamond paste. After polishing, the samples were etched with mixed acid (2 mL HF + 3 mL HCl+5 mL HNO3+190 mL H2O) with the time of 45 s. The microstructure of
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Fig. 6. Boundary misorientation distribution of original MIG welded joint on AT5 side. (a) AT5-BM; (b) AT5-HAZ II; (c) AT5-HAZ I; (d) WZ.
welded joint was observed by Leica MEF4 metallographic microscope. The fracture characteristics of welded joint were observed on Zeiss supra55 scanning electron microscope (SEM). Electron backscattered diffraction (EBSD) experiment was performed to analyze the microstructure evolution of welded joint. EBSD samples were grinded and polished carefully, then electrolytic polished with 30 vol.% nitric acidmethyl alcohol solution at −30 °C and the electro-polishing time is 10−20 s with the voltage of 15 V. The EBSD samples were washed in alcohol immediately after electro-polishing and the residual corrosive liquid was removed by ultrasonic cleaning machine. Finally, the sample was taken out and dried with a hairdryer.
consists of elongated grains, attributing to rolling process during production, as shown in Fig. 4d-f. The pole figure of OWJ in different locations is shown in Fig. 5. The maximum orientation density value in AT5-BM is 19.03 and the weighted distribution of polar projection points on the equatorial plane of the pole is concentrated, which means typical texture feature, as shown in Fig. 5a. The texture feature in AT5-HAZ becomes weaker with the maximum orientation density value of 10.66 compared to the location of AT5-BM, as shown in Fig. 5b. It can be concluded that the AT5 side shows typical texture characteristic. The main reason is likely related to the extrusion during process. The maximum orientation density value of WZ is 2.4, which is associated with the cast structure, as shown in Fig. 5c. The AT4 side also shows texture features and the maximum orientation density value in the AT4-HAZ and AT4-BM is 7.98 and 16.55, respectively, caused by the rolling process, as shown in Fig. 5d-e. It should be noted that the maximum orientation density value in HAZ adjacent to WZ decreases compared to the corresponding base metals. It is well known that the grain orientation of Al-Zn-Mg alloys show a certain degree of regularity after extrusion or rolling process, namely texture feature. The microstructure and properties of Al-Zn-Mg alloys will change due to extrusion or rolling process, meanwhile, the density
3. Results and discussion 3.1. Microstructure of welded joint Fig. 4 shows the EBSD images of OWJ. The AT5 side consists of typical exquiaxed grains, which has no obvious change after welding, as shown in Fig. 4a-c. The fine grains appear in the fusion zone of A in Fig. 4c, caused by recrystallization during welding. The welded zone shows typical cast structure with dendritic growth. The AT4 side
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Fig. 7. Boundary misorientation distribution of original MIG welded joint on AT5 side. (a) AT4-BM; (b) AT4-HAZ II; (c) AT4-HAZ I.
of vacancy and dislocation increases, making alloy in a thermodynamic instability state. Consequently, the HAZs on both sides of the WZ undergo partial recovery due to the thermal cycles during welding, then results in the change of the texture characteristics of welded joint. Fig. 6 shows the distribution of orientation difference of OWJ on AT5 side. It can be seen that the proportion of grains with different grain boundary angle ranges of θ≤10°,10° < θ≤15° and θ > 15° in the AT5-BM zone are 52.9 %, 7.6 %, and 39.5 %, respectively, and the AT5BM zone is dominated by low angle grain boundaries (θ≤15°). On the AT5 side, the proportion of the small angle grains increases gradually with the decrease of distance to the center of WZ. Additionally, it can be seen that the WZ is dominated by high-angle grain boundaries with a ratio of 60.4 %, as shown in Fig. 6d. The boundary misorientation distribution is mainly associated with different thermal cycles during welding, especially the peak temperature that can be obtained. The difference of boundary misorientation distribution between the WZ and AT5 side is attributed to the initial state [4]. The temperature range in the AT5-HAZ II and AT5-HAZ I of welded joint are 150−300 °C and 300-400 °C, respectively according to Rosenthal model [4,14]. Then the
recrystallization occurred for the 7N01 aluminum alloy during welding, which results in the change of boundary misorientation distribution. The AT4-BM zone mainly consists of low-angle grain boundaries with the percent of 88.5 % (θ < 15°), as shown in Fig. 7a. However, the distribution of orientation difference in the AT4-HAZ II changes obviously compared to AT4-BM with high-angle grain ratio of 75.5 %. This phenomenon is attributed to the thermal cycles in AT4-HAZ II during welding. The EBSD images of RWJ is illustrated in Fig. 8. The fine-grained region of fusion zone becomes wider, while the grain size of the HAZ adjacent to WZ has no obvious change after repair welding compared to the results in Figs. 4 and 8. Fig. 9 shows the distribution of orientation difference of RWJ, the proportion of the small angle grains in AT4-HAZ I and AT4-HAZ II increases from 71.9 % and 24.5 % to 77.9 % and 60.7 %, respectively. The corresponding proportion in the AT5-HAZ II and AT5-HAZ I of the small angle grains changes from 67.5 % and 71.0 % to 64.5 % and 78.6 %, respectively.
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Fig. 8. EBSD images of repair MIG welded joint (a) AT4-HAZII; (b) AT4-WZ; (c)WZ; (d) AT5-WZ; (e) AT4-HAZ II.
3.2. Mechanical properties
specifically, the failure of the RWJ occurs in the junction of original weld pass and repair weld pass, which is mainly related to the existence of pores and inclusions. Fig. 13 shows the fracture morphology of welded joint. There are obvious defect areas (pores and inclusions) in the fracture surface of OWJ, as displayed in Fig. 13(a).The gully-like features appear due to the connection of the pores in the OWJ, and the spherical particles in the gully are grains [4], as displayed in Fig. 13(b). Similar features occur in the RWJ, as illustrated in Fig. 13(c)-(d). The above phenomenon is mainly likely related to the appearance of pores in the welded joint. The typical casting structure is formed at the welded zone due to the uneven distribution of solute elements and the non-uniform recrystallization during the rapid cooling process in the weld pool [13]. The loose structure at welded zone often results in the appearance of cavities, which in turn reduces the elongation and mechanical properties of the welded joint. As for the welded zone, the ER5356 filler wire belongs to non-aging strengthened aluminum alloy and the main strengthening mechanism is solid solution strengthening, and the hardness is the lowest. Therefore, the welded joint often fails at the welded zone. Additionally, the gas porosity and inclusions are formed in the welded zone due to the existence of oxygen and hydrogen coming from base metals, welding consumables (shielding gas and flux) and air during welding. Then the existence of inclusions and gas porosity
The mechanical properties of welded joint are shown in Table 4, the tensile strength of OWJ and RWJ is 283 MPa and 280 MPa, respectively and the corresponding strength coefficient is 78.6 % and 77.8 %, respectively, which can fulfill the demands of engineering application [12]. Fig. 10 displays the cross-section images of the OWJ after failure. The crack appears in the junction of the AT5 side and the welded zone and the fracture surface on the AT4 side exists defects, as illustrated in Fig. 10b and c, respectively. The SEM pictures of RWJ is displayed in Fig. 11. The results show that the pores appear in the junction of the HAZ I and welded zone. It is well known that the pores and inclusions are often the origin of crack in welded joint. Consequently, it is speculated the fracture of OWJ originates in the junction of different weld passes or the confluence of HAZ I and weld pass, then expands along the welded zone. The images of the welded joint after fracture are obtained to analyze the fracture path further, as shown in Fig. 11(c)-(d). The SEM pictures shows that the failure of OWJ initiates from the junction of weld passes and AT4-HAZ I, then expands along the welded zone. The welded joint after fracture are examined to investigate the fracture path further, as illustrated in Fig. 12. It can be seen that the OWJ fails at the welded zone near AT4 side, as illustrated in Fig. 12 (a)(b). The RWJ fails at welded zone, as shown in Fig. 12(c)-(d). More
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Fig. 9. Boundary misorientation distribution of RWJ. (a) AT4-HAZ II; (b) AT4-HAZ I; (c) AT5-HAZ I; (d) AT5-HAZ II. Table 4 Mechanical properties of base alloys and weldment. Specimens
Elongation (%)
Tensile strength (MPa)
BM(AT5) BM(AT4) Welded joint Repair welded joint
20.6 24.3 9.0 8.2
360 432 283 280
results in the reduction of strength of welded joint [14]. Besides, the existence of inclusions in welded joint is likely related to the incomplete welding slag cleaning during multiple-pass welding or repair welding. 4. Conclusions 1) The difference in microstructure of both sides of the welded zone for the original welded joint is mainly attributed to the preparation process. The AT5 side is an extruded profile, exhibiting equiaxed grain characteristics, while the AT4 side is a rolled sheet, displaying flat long grain. The difference of grain boundary orientation in different positions of repair welded joint is different from that of the
Fig. 10. Images of fracture morphology of the OWJ (a) Macromorphology of the welded joint; (b) crack on AT5 side; (c) SEM images of fracture morphology of welded joint.
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Fig. 11. SEM images of OWJ. (a) near AT5 side;(b)near AT4 side; (c) welded zone; (d) confluence of AT4 side and welded zone.
Fig. 12. The fracture path of welded joint (a) image of OWJ before fracture; (b) picture of OWJ after fracture; (c) image of RWJ before fracture; (d) picture of RWJ after fracture.
original joint, especially the change of proportion of small angle grain boundary in AT4-HAZ II is large. 2) The tensile strength of original welded joint and repair welded joint is 283 MPa and 280 MPa, respectively and the corresponding strength coefficient is 78.6 % and 77.8 %, respectively, which can fulfill the demand of engineering application. 3) Due to the existence of pores and inclusions, the fracture of origin welded joint initiates in the junction of HAZ I near the 7N01-T4 side,
then expands along the welded zone, while the repair welded joint fails in the junction of the original weld and the weld pass. Funding This research was funded by Research Foundation for High-level talent Scholars in North China University of Water Resources and Electric Power (No. 201811034) and Open Fund of National Joint 87
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Fig. 13. The fracture morphology of welded joint(a)fracture features of OWJ; (b) the magnified fracture morphology image of OWJ; (c) fracture features of RWJ; (d) the magnified fracture features image of RWJ.
Engineering Research Center for abrasion control and molding of metal materials (No.HKDNM2019020).
2015. [6] Dong WK. Research on the grain boundary liquation in the heat affected zone of 7N01 aluminum alloy welded joint. Master′s Degree. Shen Yang: Northeastern University; 2013. [7] Yan ZJ, Chen SX, Shang Z. Property of repair welding joint of A7N01 aluminium alloy. Trans China Weld Inst 2014;35:51–4. [8] Liang ZM, Li YB, Zhao SS. Multiple repair welding of 7N01 aluminum alloy with pulsed MIG and DC CMT welding. Trans China Weld Inst 2014;35:27–31. [9] Maya-Johnson S, Santa JF, Mejía OL, Aristizábal S, Ospina S, Cortés PA, et al. Effect of the number of welding repairs with GTAW on the mechanical behavior of AA7020 aluminum alloy welded joints. Metall Mater Trans B 2015;46:2332–9. [10] Wang X, Mao S, Chen P, Liu Y, Ning J, Li H, et al. Evolution of microstructure and mechanical properties of a dissimilar aluminium alloy weldment. Mater Des 2016;90:230–7. [11] Chen Y, Wang H, Li H, Wang X, Ding H, Zhao J, et al. Investigation into the dissimilar friction stir welding of AA5052 and AA6061 aluminum alloys using pineccentric stir tool. Metals 2019;9:718. [12] ISO 15614-2:2005. Specification and qualification of welding procedures for metallic materials-welding procedure test-Part 2: arc welding of aluminium and its alloys. 2005. [13] Peng X, Cao X, Xu G, Deng Y, Tang L, Yin Z. Mechanical Properties, Corrosion behavior, and microstructures of a MIG-Welded 7020 Al alloy. J Mater Eng Perform 2016;25:1028–40. [14] Kou S. Welding metallurgy. 2rd ed. Hoboken, United States of America: A John Wiley & Sons, Inc.; 2003. p. 49–51.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors alone are responsible for the content and writing of this article. References [1] Li S, Dong HG, Wang X, Liu Z. Quenching sensitivity of Al-Zn-Mg alloy after nonisothermal heat treatment. Materials 2019;12:1595. [2] Li S, Dong HG, Li P, Chen S. Effect of repetitious non-isothermal heat treatment on corrosion behavior of Al-Zn-Mg alloy. Corros Sci 2018;131:278–89. [3] Li S, Guo D, Dong HG. Effect of flame rectification on corrosion property of Al–Zn–Mg alloy. Trans Nonferrous Met Soc China 2017;27:250–7. [4] Li S, Dong HG, Shi L, Li P, Ye F. Corrosion behavior and mechanical properties of Al–Zn–Mg aluminum alloy weld. Corros Sci 2017;123:243–55. [5] Qin F. The research of fatigue performance and microstructure of Al–Zn–Mg alloy weld joints used in high speed train. Master′s Degree. Chang Sha: Hunan University;
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