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Effect of cold sprayed Al coating on mechanical property and corrosion behavior of friction stir welded AA2024-T351 joint
Materials and Design 65 (2015) 757–761
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effect of cold sprayed Al coating on mechanical property and corrosion behavior of friction stir welded AA2024-T351 joint W.Y. Li ⇑, R.R. Jiang, C.J. Huang, Z.H. Zhang, Y. Feng State Key Laboratory of Solidification Processing, Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, Xi’an 710072, Shaanxi, PR China
a r t i c l e
i n f o
Article history: Received 26 August 2014 Accepted 1 October 2014
a b s t r a c t An aluminum coating was deposited on the surface of a friction stir welded 2024-T351 aluminum alloy joint via cold spraying for corrosion protection. The results show that a relatively dense coating could be formed except some pores on the coating surface layer under certain spray conditions, and the bonding between the coating and the weld surface seems good. Immersion tests in the exfoliation solution confirm that the presence of pure Al coating significantly decreases the corrosion attack. The most important finding is that the tensile properties of friction stir welds are improved after coating deposition. Taking into account the increased microhardness near the joint surface, the shot-peening effect and the release of residual stresses during particles deposition contribute to the improvement of joint tensile strength. Therefore, the combination of these two solid-state processing techniques will generate synergistic effects on the joint performance in service. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Friction stir welding (FSW) is an innovative solid state process invented by The Welding Institute, with which many difficult-toweld aluminum alloys, such as 2xxx and 7xxx aluminum alloys, can be joined [1]. Now it has been widely used in many industries, such as aerospace, aviation and shipbuilding. There have been a number of reports highlighting the changes of microstructural and mechanical properties due to the plastic deformation and frictional heat associated with FSW. However, in some cases where an aggressive environment is encounted, the corrosion performance of joints should be taken into consideration. Some studies about corrosion and stress corrosion cracking resistance of welds were performed [1]. The severe thermomechanical processing conditions during FSW alters the grain structure and the precipitation distribution, which as a result changes the corrosion susceptibility of welds [2,3]. It has been found that the weld nugget (WN) is very susceptible to corrosion [1,4–7]. The corrosion performance of welds could be improved by the application of several treatments during or after the welding process. During the welding, rapid cooling with water circulating through the anvil or water sprayed on the tool can decrease the extent of the attacked regions [3]. Paglia et al. [10,11] found that the short time post-weld heat treatment (PWHT) of FSW 7075 joint ⇑ Corresponding author at: School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, PR China. Tel.: +86 29 88495226. E-mail address: [email protected] (W.Y. Li). http://dx.doi.org/10.1016/j.matdes.2014.10.007 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved.
could increase the sizes of the intragranular and grain boundary precipitates. The size of the precipitate-free zone also increased, thus the corrosion properties were improved. An increase in corrosion resistance was also achieved for alloys 2024 [12], 2050 [13] and 6061 [14] by the use of PWHT. In addition, the surface modification techniques can also be used to protect the joints. Microarc oxidation coatings have been successfully applied to FSW 5083, 2219 and 7180 joints subjected to immersion corrosion [8,9]. Padovani et al. studied the effect of an Excimer laser treatment on the corrosion resistance of FSW 2024 [15] and 7449 [16] joints. Their results showed a decrease in anodic and cathodic reactivity in the weld region was achieved due to the formation of a 3–5 lm thick corrosion resistant layer. In our recent study, a promising coating technique, cold spraying (CS), was implemented to protect the joints. CS is also a solid-state particle deposition process, where powder particles are accelerated to high velocity by a high-speed gas stream, producing a metallurgical bond upon impacting on a substrate and forming a dense coating without harmful solidification defects [17–19]. CS has been used for surface protection of magnesium alloys [20] and aluminum alloys [21–23]. To date, only Trahan [24] made a presentation at ITSC’2013 to show the protection of FSW 7075 joints by CS Al7075, Al5083 or pure Al coatings. The results indicate that CS is a promising method to protect FSW joints against corrosion. In this work, pure Al coating was cold sprayed on the surface of FSW 2024 joints to investigate its effect on the properties of joints,
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including the corrosion resistance tested via the ASTM G34 corrosion test. 2. Experimental procedures 3 mm thick 2024-T351 plates with aluminum cladding were used as base metal. Two plates were butt-welded by FSW at a rotation speed of 600 rpm and a welding speed of 200 mm/min. Before spraying, flash of joints have been removed. Pure Al coating was deposited by the cold spray system developed in Xi’an Jiaotong University, China. The feedstock was a gas-atomized Al powder with spherical morphology ( 250 mesh). After several trials on spray conditions, nitrogen was used as the driving gas at a temperature of 300 °C and pressure of 2.8 MPa. The traverse speed of spray gun was 60 mm/s and the standoff distance was 30 mm. The powder feeder speed was 2.5 rad/min. The transverse cross-section of the weld was optically imaged after polishing and etching with Dix-Keller’s reagent. Microhardness measurements were performed across the polished crosssection. Tensile strength tests were conducted at room temperature with a crosshead speed of 1 mm/s. The dimensions of tensile specimens are shown in Fig. 1. The tensile tests have been repeated three times for both the coated joint and the uncoated joint. The corrosion tests consisted of immersing uncoated joints and coated joints in an exfoliation corrosion (EXCO) solution for 5 h according to the ASTM: G34-01 standard. The surface morphology was observed using scanning electron microscope (SEM) methods. 3. Results and discussion 3.1. Characteristics of CS Al coating and as-welded joint Fig. 2 shows the cross-section of friction stir welded joint. It is can be seen that the joint can be divided into four characteristic zones, base material (BM), weld nugget (WN), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). The BM is consisted of coarse grains elongated along the rolling direction. The grain structure within the WN is consisted of fine equiaxed recrystallized grains, largely due to the intensive thermo-mechanical effects during the welding process. Grains in TMAZ are slightly elongated and bent along the rotating direction. They all keep the pancaked shape of the original grains, which is indicative of the less
Fig. 1. Schematic illustration of tensile specimen cutting and dimensions of the tensile test specimen.
Fig. 2. Cross-section of the as-welded joint.
severe strain gradient experienced by the metal compared with that in the WN. Within HAZ, grains retain the structure of the BM. A 90–110 lm thick Al coating was obtained on the surface of the joint. The slightly etched cross-section of the cold sprayed Al coating is shown in Fig. 3 (high magnification of the region marked by red rectangle in Fig. 2). It is clearly seen from Fig. 3a that a relatively dense coating was obtained after careful selection of spray conditions. In the inner region of the coating, there is no evidence of pores and the coating is well bonded with the joint surface, which means the coating may be impermeable to solution. However, in the top region of the coating some pores can be observed (Fig. 3b), which is a normal phenomenon because of less deformation [25]. These observations indicate that subsequent incoming particles have a shot-peening effect on the substrate and the previously deposited particles. The average microhardness of Al coating reaches 53.2 ± 6.7 HV0.05.
3.2. Effect of CS Al coating on mechanical properties of welds The first interesting finding is that the microhardness of WN near the top surface was improved. The measurement of hardness at the region about 40 lm below the joint top surface shows that the average microhardness of WN near the top surface of the coated joint is 144.2 ± 6.1 HV0.2, which is obviously higher than that of the uncoated joint, 127.3 ± 4.3 HV0.2. This may result from the aforementioned shot-peening effect. During cold spraying, the surface layer of the joint undergoes an impact effect of high velocity particles, i.e. the shot-peening effect, especially from the unbonded particles. As shown in Fig. 4, after being coated, the secondary phase particles in the WN near the top surface becomes finer and the fraction seems higher. Thus the microhardness of WN near the top surface of the coated joint increased. Moreover, the release of residual stresses of joints during particle deposition may also contribute to the hardness improvement. The second interesting finding is that the tensile properties of joints were also improved after coating deposition. The average ultimate tensile strength (UTS) of the coated joint is 357.8 ± 3.6 MPa while that of the uncoated joint is only 333.0 ± 5.8 MPa. Moreover, the average elongation of the coated joint is 3.8% while that of the uncoated joint is only 3.0%. It is clear that the tensile properties of friction stir welds are improved by the cold sprayed Al coating. The average UTS and average elongation of coated specimens increase by 7.4% and 25.6%, respectively. All fracture of tensile samples falls into the WN. Tensile properties and fracture locations of the joints are, to a large extent, dependent on the welding defects, residual stresses and hardness distributions of joints [26–28]. When a joint is free of defects, tensile properties of the joint only depend on residual stresses and microhardness distributions of the joint. For both of the coated and uncoated joints, the microhardness reached a minimum in the WN. Besides, residual stresses analyses of FSW joints indicated that residual stresses of the WN are in tension [28,29]. Consequently, both kinds of joints fractured in the WN, the weakest region of joint. While the shot-peening effect of CS helps to enhance the WN of the coated joint by increasing the microhardness and releasing the residual
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Fig. 3. Cross-sectional microstructure of Al coating: (a) inner, and (b) top.
Fig. 4. The change of microstructure in the WN before (a) and after (b) being coated.
tensile stresses. Thus the coated joints show higher UTS and elongation. 3.3. Effect of CS Al coating on corrosion performance of welds The surface morphologies of the uncoated joint after immersion in EXCO solution are shown in Fig. 5. After immersion, the WN and the TMAZ have been attacked seriously, which is consistent with other investigations [4,30,31]. Small grains in the WN and deformed grains in the TMAZ increase grain boundaries are thermodynamically susceptible to corrosion. Besides, aluminum cladding has been damaged during FSW. Then those regions are easily attacked. There are no significant changes in surface morphologies of the BM and the HAZ, except that surfaces of the BM and HAZ become lusterless with white Al2O3 films becoming more obvious. Fig. 6 shows SEM micrographs of the uncoated joint after immersion taken from the WN and TMAZ. The WN has been homogenously corroded, and exfoliation corrosion is found in the TMAZ. The homogenous microstructure with fine equiaxed recrystallized grains in the WN contributes to its homogenous corrosion features.
Whereas grains in TMAZ hold their original pancaked shape. In exfoliation solution, corrosion in this alloy starts in the form of pitting. Then pits grow deeper and begin to become connected by a network of intergranular corrosion paths. Finally, corrosion spreads beneath the surface and causes exfoliation of surface layers by the corrosion products’ forces. Thus more elongated microstructures could generate higher force [32]. As a result, the TMAZ shows typical features of exfoliation corrosion other than homogenous corrosion. The surface morphologies of the coated joint before and after immersion are shown in Fig. 7. The surface of the Al coating appears to change a little after immersion (Fig. 7a and b). According to Tao et al. [33] and Liu et al. [34] the cathodic current density of cold sprayed Al coating and its open-circuit potential are high, which result in high corrosion resistance. When observed at high magnification by SEM (Fig. 7c and d), the coating surface has been obviously corroded. The corrosion starts from the interfaces between the deposited particles causing the separation of particles. After some time for corrosion, the coating thickness decreases and the arc corrugation on the weld surface becomes a little distinct again after immersion (Fig. 7b). Even so, the coating offers excel-
Fig. 5. Photos of the uncoated joint before (a) and after (b) immersion.
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Fig. 6. SEM micrographs of the surface of the uncoated joint after immersion: (a) WN, and (b) TMAZ.
Fig. 7. Photos of the coated joint surface before (a) and after (b) immersion, and the corresponding SEM micrographs (c and d).
Fig. 8. UTS and elongation of joints before and after immersion: (a) coated, and (b) uncoated.
lent corrosion protection for joints which are not attacked by the corrodent. Tensile tests of the joints after corrosion provide further support for the above finding. After immersion, the average UTS of a coated joint is 351.8 ± 3.3 MPa and the average elongation is 3.7%, as shown in Fig. 8a. The average UTS and elongation of coated joint only decrease by 1.7% and 3.2%, respectively. It is obvious that
the tensile properties of coated joint are barely lowered because of the excellent protection of cold sprayed Al coating. Although the Al coating has been peeled during the tensile test, no corrosion features can be found along the fracture path. However the average UTS and elongation of the uncoated joint after immersion are 304.6 ± 4.1 MPa and 2.5%, respectively, decreasing by 8.5% and 17.9%, respectively compared with the as-welded specimens.
W.Y. Li et al. / Materials and Design 65 (2015) 757–761
Therefore, the CS Al coating not only protects the joint, but also maintains the strength of joints. 4. Conclusions This study innovatively applied CS to FSW to improve the corrosion resistance of the FSWed joints, while the mechanical properties of joints were found to be also improved. The major conclusions arising out of the present study are as follows: (1) A relatively dense coating could be formed on the weld surface without pores or cracks, and well bonded to the weld by the use of CS with appropriate spray parameters. (2) The mechanical properties of friction stir welds are improved by the cold sprayed Al coating. The microhardness of the WN and TMAZ near the top surface is increased. The average UTS and elongation of coated specimens increase by 7.4% and 25.6%, respectively. (3) The presence of the Al coating offers excellent corrosion protection for welds, while without protection joints are terribly corroded. Besides, the strength of joints with coating protection is hardly lowered.
Acknowledgements The authors would like to thank for financial support from the Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (131052) and the 111 Project (B08040). References [1] Mishra RS, Ma ZY. Friction stir welding and processing. Mater Sci Eng R 2005;50:1–78. [2] Threadgill PL, Leonard AJ, Shercliff HR, Withers PJ. Friction stir welding of aluminium alloys. Int Mater Rev 2009;54:49–93. [3] Paglia CS, Buchheit RG. A look in the corrosion of aluminum alloy friction stir welds. Scripta Mater 2008;58:383–7. [4] Proton V, Alexis J, Andrieu E, Delfosse J, Lafont MC, Blanc C. Characterisation and understanding of the corrosion behaviour of the nugget in a 2050 aluminium alloy Friction Stir Welding joint. Corros Sci 2013;73:130–42. [5] Xu W, Liu J, Zhu H. Pitting corrosion of friction stir welded aluminum alloy thick plate in alkaline chloride solution. Electrochim Acta 2010;55:2918–23. [6] Kang J, Fu RD, Luan GH, Dong CL, He M. In-situ investigation on the pitting corrosion behavior of friction stir welded joint of AA2024-T3 aluminium alloy. Corros Sci 2010;52:620–6. [7] Bousquet E, Poulon-Quintin A, Puiggali M, Devos O, Touzet M. Relationship between microstructure, microhardness and corrosion sensitivity of an AA 2024-T3 friction stir welded joint. Corros Sci 2011;53:3026–34. [8] Lu L, Xue WB, Jin XY, Du JC, Li YL. Influence of microarc oxidation surface treatment on the tensile properties of aluminum alloy. Trans Mater Heat Treat 2011;32:2–5. [9] Rao KP, Janaki Ram GD, Stucker BE. Improvement in corrosion resistance of friction stir welded aluminum alloys with micro arc oxidation coatings. Scripta Mater 2008;58:998–1001. [10] Paglia CS, Jata KV, Buchheit RG. The influence of artificial aging on the microstructure, mechanical properties, corrosion, and environmental cracking susceptibility of a 7075 friction-stir-weld. Mater Corros 2007;58:737–50.
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[11] Paglia CS, Buchheit RG. The time–temperature–corrosion susceptibility in a 7050-T7451 friction stir weld. Mater Sci Eng A 2008;492:250–4. [12] Widener CA. Evaluation of post-weld heat treatments for corrosion protection in friction stir welded 2024 and 7075 aluminum alloys. Wichita State University; 2005. [13] Proton V, Alexis J, Andrieu E, Blanc C, Delfosse Jr, et al. Influence of postwelding heat treatment on the corrosion behavior of a 2050-T3 aluminum– copper–lithium alloy friction stir welding joint. J Electrochem Soc 2011;158:139–47. [14] Nikseresht Z, Karimzadeh F, Golozar MA, Heidarbeigy M. Effect of heat treatment on microstructure and corrosion behavior of Al6061 alloy weldment. Mater Des 2010;31:2643–8. [15] Padovani C, Davenport AJ, Connolly BJ, Williams SW, Siggs E, Groso A, et al. Corrosion protection of AA2024-T351 friction stir welds by laser surface melting with Excimer laser. Corros Eng Sci Technol 2012;47:188–202. [16] Padovani C, Davenport AJ, Connolly BJ, Williams SW, Siggs E, Groso A, et al. Corrosion protection of AA7449-T7951 friction stir welds by laser surface melting with an Excimer laser. Corros Sci 2011;53:3956–69. [17] Spencer K, Fabijanic DM, Zhang MX. The use of Al–Al2O3 cold spray coatings to improve the surface properties of magnesium alloys. Surf Coat Technol 2009;204:336–44. [18] Li WY, Zhang G, Liao HL, Coddet C. Characterizations of cold sprayed TiN particle reinforced Al2319 composite coating. J Mater Process Technol 2008;202:508–13. [19] Sansoucy E, Marcoux P, Ajdelsztajn L, Jodoin B. Properties of SiC-reinforced aluminum alloy coatings produced by the cold gas dynamic spraying process. Surf Coat Technol 2008;202:3988–96. [20] Irissou E, Legoux JG, Arsenault B, Moreau C. Investigation of Al–Al2O3 cold spray coating formation and properties. J Therm Spray Technol 2007;16:661–8. [21] Stoltenhoff T, Kreye H, Richter HJ. An analysis of the cold spray process and its coatings. J Therm Spray Technol 2002;11:542–50. [22] Spencer K, Zhang MX. Heat treatment of cold spray coatings to form protective intermetallic layers. Scripta Mater 2009;61:44–7. [23] Rech S, Trentin A, Vezzù S, Legoux JG, Irissou E, Guagliano M. Influence of preheated Al 6061 substrate temperature on the residual stresses of multipass Al coatings deposited by cold spray. J Therm Spray Technol 2010;20:243–51. [24] Trahan P. Corrosion protection of friction stir welded Al 7075 panels using cold gas dynamic spray, In: Presentation to 2013 international thermal spray, Busan, Republic of Korea. [25] Li WY, Zhang C, Wang HT, Guo XP, Liao HL, Li CJ, et al. Significant influences of metal reactivity and oxide film of powder particles on coating deposition characteristics in cold spraying. Appl Surf Sci 2007;253:3557–62. [26] Liu HJ, Fujii H, Maeda M, Nogi K. Tensile properties and fracture locations of friction-stir-welded joints of 2017-T351 aluminum alloy. J Mater Process Technol 2003;142:692–6. [27] Aydın H, Bayram A, Ug˘uz A, Akay KS. Tensile properties of friction stir welded joints of 2024 aluminum alloys in different heat-treated-state. Mater Des 2009;30:2211–21. [28] Peel M, Steuwer A, Preuss M, Withers PJ. Microstructure, mechanical properties and residual stresses as a function of welding speed in aluminium AA5083 friction stir welds. Acta Mater 2003;51:4791–801. [29] Reynolds AP, Tang W, Gnaupel-Herold T, Prask H. Structure, properties, and residual stress of 304L stainless steel friction stir welds. Scripta Mater 2003;48:1289–94. [30] Jariyaboon M, Davenport AJ, Ambat R, Connolly BJ, Williams SW, Price DA. The effect of welding parameters on the corrosion behaviour of friction stir welded AA2024-T351. Corros Sci 2007;49:877–909. [31] Wadeson DA, Zhou X, Thompson GE, Skeldon P, Oosterkamp LD, Scamans G. Corrosion behaviour of friction stir welded AA7108 T79 aluminium alloy. Corros Sci 2006;48:887–97. [32] Kelly DJ, Robinson MJ. Influence of heat treatment and grain shape on exfoliation corrosion of Al–Li alloy 8090. Corrosion 1993;49:787–95. [33] Tao YS, Xiong TY, Sun C, Kong LY, Cui XY, Li TF, et al. Microstructure and corrosion performance of a cold sprayed aluminium coating on AZ91D magnesium alloy. Corros Sci 2010;52:3191–7. [34] Liu DX, Shi ZC, Zhang XY, Sun LF, Zhi H, Tang ZH. Structure and anti-corrosion properties of cold sprayed Al coatings on ZM6 magnesium alloy. Mater Eng 2012;12:50–4 [in Chinese].
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effect of cold sprayed Al coating on mechanical property and corrosion behavior of friction stir welded AA2024-T351 joint W.Y. Li ⇑, R.R. Jiang, C.J. Huang, Z.H. Zhang, Y. Feng State Key Laboratory of Solidification Processing, Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, Xi’an 710072, Shaanxi, PR China
a r t i c l e
i n f o
Article history: Received 26 August 2014 Accepted 1 October 2014
a b s t r a c t An aluminum coating was deposited on the surface of a friction stir welded 2024-T351 aluminum alloy joint via cold spraying for corrosion protection. The results show that a relatively dense coating could be formed except some pores on the coating surface layer under certain spray conditions, and the bonding between the coating and the weld surface seems good. Immersion tests in the exfoliation solution confirm that the presence of pure Al coating significantly decreases the corrosion attack. The most important finding is that the tensile properties of friction stir welds are improved after coating deposition. Taking into account the increased microhardness near the joint surface, the shot-peening effect and the release of residual stresses during particles deposition contribute to the improvement of joint tensile strength. Therefore, the combination of these two solid-state processing techniques will generate synergistic effects on the joint performance in service. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Friction stir welding (FSW) is an innovative solid state process invented by The Welding Institute, with which many difficult-toweld aluminum alloys, such as 2xxx and 7xxx aluminum alloys, can be joined [1]. Now it has been widely used in many industries, such as aerospace, aviation and shipbuilding. There have been a number of reports highlighting the changes of microstructural and mechanical properties due to the plastic deformation and frictional heat associated with FSW. However, in some cases where an aggressive environment is encounted, the corrosion performance of joints should be taken into consideration. Some studies about corrosion and stress corrosion cracking resistance of welds were performed [1]. The severe thermomechanical processing conditions during FSW alters the grain structure and the precipitation distribution, which as a result changes the corrosion susceptibility of welds [2,3]. It has been found that the weld nugget (WN) is very susceptible to corrosion [1,4–7]. The corrosion performance of welds could be improved by the application of several treatments during or after the welding process. During the welding, rapid cooling with water circulating through the anvil or water sprayed on the tool can decrease the extent of the attacked regions [3]. Paglia et al. [10,11] found that the short time post-weld heat treatment (PWHT) of FSW 7075 joint ⇑ Corresponding author at: School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, PR China. Tel.: +86 29 88495226. E-mail address: [email protected] (W.Y. Li). http://dx.doi.org/10.1016/j.matdes.2014.10.007 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved.
could increase the sizes of the intragranular and grain boundary precipitates. The size of the precipitate-free zone also increased, thus the corrosion properties were improved. An increase in corrosion resistance was also achieved for alloys 2024 [12], 2050 [13] and 6061 [14] by the use of PWHT. In addition, the surface modification techniques can also be used to protect the joints. Microarc oxidation coatings have been successfully applied to FSW 5083, 2219 and 7180 joints subjected to immersion corrosion [8,9]. Padovani et al. studied the effect of an Excimer laser treatment on the corrosion resistance of FSW 2024 [15] and 7449 [16] joints. Their results showed a decrease in anodic and cathodic reactivity in the weld region was achieved due to the formation of a 3–5 lm thick corrosion resistant layer. In our recent study, a promising coating technique, cold spraying (CS), was implemented to protect the joints. CS is also a solid-state particle deposition process, where powder particles are accelerated to high velocity by a high-speed gas stream, producing a metallurgical bond upon impacting on a substrate and forming a dense coating without harmful solidification defects [17–19]. CS has been used for surface protection of magnesium alloys [20] and aluminum alloys [21–23]. To date, only Trahan [24] made a presentation at ITSC’2013 to show the protection of FSW 7075 joints by CS Al7075, Al5083 or pure Al coatings. The results indicate that CS is a promising method to protect FSW joints against corrosion. In this work, pure Al coating was cold sprayed on the surface of FSW 2024 joints to investigate its effect on the properties of joints,
758
W.Y. Li et al. / Materials and Design 65 (2015) 757–761
including the corrosion resistance tested via the ASTM G34 corrosion test. 2. Experimental procedures 3 mm thick 2024-T351 plates with aluminum cladding were used as base metal. Two plates were butt-welded by FSW at a rotation speed of 600 rpm and a welding speed of 200 mm/min. Before spraying, flash of joints have been removed. Pure Al coating was deposited by the cold spray system developed in Xi’an Jiaotong University, China. The feedstock was a gas-atomized Al powder with spherical morphology ( 250 mesh). After several trials on spray conditions, nitrogen was used as the driving gas at a temperature of 300 °C and pressure of 2.8 MPa. The traverse speed of spray gun was 60 mm/s and the standoff distance was 30 mm. The powder feeder speed was 2.5 rad/min. The transverse cross-section of the weld was optically imaged after polishing and etching with Dix-Keller’s reagent. Microhardness measurements were performed across the polished crosssection. Tensile strength tests were conducted at room temperature with a crosshead speed of 1 mm/s. The dimensions of tensile specimens are shown in Fig. 1. The tensile tests have been repeated three times for both the coated joint and the uncoated joint. The corrosion tests consisted of immersing uncoated joints and coated joints in an exfoliation corrosion (EXCO) solution for 5 h according to the ASTM: G34-01 standard. The surface morphology was observed using scanning electron microscope (SEM) methods. 3. Results and discussion 3.1. Characteristics of CS Al coating and as-welded joint Fig. 2 shows the cross-section of friction stir welded joint. It is can be seen that the joint can be divided into four characteristic zones, base material (BM), weld nugget (WN), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). The BM is consisted of coarse grains elongated along the rolling direction. The grain structure within the WN is consisted of fine equiaxed recrystallized grains, largely due to the intensive thermo-mechanical effects during the welding process. Grains in TMAZ are slightly elongated and bent along the rotating direction. They all keep the pancaked shape of the original grains, which is indicative of the less
Fig. 1. Schematic illustration of tensile specimen cutting and dimensions of the tensile test specimen.
Fig. 2. Cross-section of the as-welded joint.
severe strain gradient experienced by the metal compared with that in the WN. Within HAZ, grains retain the structure of the BM. A 90–110 lm thick Al coating was obtained on the surface of the joint. The slightly etched cross-section of the cold sprayed Al coating is shown in Fig. 3 (high magnification of the region marked by red rectangle in Fig. 2). It is clearly seen from Fig. 3a that a relatively dense coating was obtained after careful selection of spray conditions. In the inner region of the coating, there is no evidence of pores and the coating is well bonded with the joint surface, which means the coating may be impermeable to solution. However, in the top region of the coating some pores can be observed (Fig. 3b), which is a normal phenomenon because of less deformation [25]. These observations indicate that subsequent incoming particles have a shot-peening effect on the substrate and the previously deposited particles. The average microhardness of Al coating reaches 53.2 ± 6.7 HV0.05.
3.2. Effect of CS Al coating on mechanical properties of welds The first interesting finding is that the microhardness of WN near the top surface was improved. The measurement of hardness at the region about 40 lm below the joint top surface shows that the average microhardness of WN near the top surface of the coated joint is 144.2 ± 6.1 HV0.2, which is obviously higher than that of the uncoated joint, 127.3 ± 4.3 HV0.2. This may result from the aforementioned shot-peening effect. During cold spraying, the surface layer of the joint undergoes an impact effect of high velocity particles, i.e. the shot-peening effect, especially from the unbonded particles. As shown in Fig. 4, after being coated, the secondary phase particles in the WN near the top surface becomes finer and the fraction seems higher. Thus the microhardness of WN near the top surface of the coated joint increased. Moreover, the release of residual stresses of joints during particle deposition may also contribute to the hardness improvement. The second interesting finding is that the tensile properties of joints were also improved after coating deposition. The average ultimate tensile strength (UTS) of the coated joint is 357.8 ± 3.6 MPa while that of the uncoated joint is only 333.0 ± 5.8 MPa. Moreover, the average elongation of the coated joint is 3.8% while that of the uncoated joint is only 3.0%. It is clear that the tensile properties of friction stir welds are improved by the cold sprayed Al coating. The average UTS and average elongation of coated specimens increase by 7.4% and 25.6%, respectively. All fracture of tensile samples falls into the WN. Tensile properties and fracture locations of the joints are, to a large extent, dependent on the welding defects, residual stresses and hardness distributions of joints [26–28]. When a joint is free of defects, tensile properties of the joint only depend on residual stresses and microhardness distributions of the joint. For both of the coated and uncoated joints, the microhardness reached a minimum in the WN. Besides, residual stresses analyses of FSW joints indicated that residual stresses of the WN are in tension [28,29]. Consequently, both kinds of joints fractured in the WN, the weakest region of joint. While the shot-peening effect of CS helps to enhance the WN of the coated joint by increasing the microhardness and releasing the residual
W.Y. Li et al. / Materials and Design 65 (2015) 757–761
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Fig. 3. Cross-sectional microstructure of Al coating: (a) inner, and (b) top.
Fig. 4. The change of microstructure in the WN before (a) and after (b) being coated.
tensile stresses. Thus the coated joints show higher UTS and elongation. 3.3. Effect of CS Al coating on corrosion performance of welds The surface morphologies of the uncoated joint after immersion in EXCO solution are shown in Fig. 5. After immersion, the WN and the TMAZ have been attacked seriously, which is consistent with other investigations [4,30,31]. Small grains in the WN and deformed grains in the TMAZ increase grain boundaries are thermodynamically susceptible to corrosion. Besides, aluminum cladding has been damaged during FSW. Then those regions are easily attacked. There are no significant changes in surface morphologies of the BM and the HAZ, except that surfaces of the BM and HAZ become lusterless with white Al2O3 films becoming more obvious. Fig. 6 shows SEM micrographs of the uncoated joint after immersion taken from the WN and TMAZ. The WN has been homogenously corroded, and exfoliation corrosion is found in the TMAZ. The homogenous microstructure with fine equiaxed recrystallized grains in the WN contributes to its homogenous corrosion features.
Whereas grains in TMAZ hold their original pancaked shape. In exfoliation solution, corrosion in this alloy starts in the form of pitting. Then pits grow deeper and begin to become connected by a network of intergranular corrosion paths. Finally, corrosion spreads beneath the surface and causes exfoliation of surface layers by the corrosion products’ forces. Thus more elongated microstructures could generate higher force [32]. As a result, the TMAZ shows typical features of exfoliation corrosion other than homogenous corrosion. The surface morphologies of the coated joint before and after immersion are shown in Fig. 7. The surface of the Al coating appears to change a little after immersion (Fig. 7a and b). According to Tao et al. [33] and Liu et al. [34] the cathodic current density of cold sprayed Al coating and its open-circuit potential are high, which result in high corrosion resistance. When observed at high magnification by SEM (Fig. 7c and d), the coating surface has been obviously corroded. The corrosion starts from the interfaces between the deposited particles causing the separation of particles. After some time for corrosion, the coating thickness decreases and the arc corrugation on the weld surface becomes a little distinct again after immersion (Fig. 7b). Even so, the coating offers excel-
Fig. 5. Photos of the uncoated joint before (a) and after (b) immersion.
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Fig. 6. SEM micrographs of the surface of the uncoated joint after immersion: (a) WN, and (b) TMAZ.
Fig. 7. Photos of the coated joint surface before (a) and after (b) immersion, and the corresponding SEM micrographs (c and d).
Fig. 8. UTS and elongation of joints before and after immersion: (a) coated, and (b) uncoated.
lent corrosion protection for joints which are not attacked by the corrodent. Tensile tests of the joints after corrosion provide further support for the above finding. After immersion, the average UTS of a coated joint is 351.8 ± 3.3 MPa and the average elongation is 3.7%, as shown in Fig. 8a. The average UTS and elongation of coated joint only decrease by 1.7% and 3.2%, respectively. It is obvious that
the tensile properties of coated joint are barely lowered because of the excellent protection of cold sprayed Al coating. Although the Al coating has been peeled during the tensile test, no corrosion features can be found along the fracture path. However the average UTS and elongation of the uncoated joint after immersion are 304.6 ± 4.1 MPa and 2.5%, respectively, decreasing by 8.5% and 17.9%, respectively compared with the as-welded specimens.
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Therefore, the CS Al coating not only protects the joint, but also maintains the strength of joints. 4. Conclusions This study innovatively applied CS to FSW to improve the corrosion resistance of the FSWed joints, while the mechanical properties of joints were found to be also improved. The major conclusions arising out of the present study are as follows: (1) A relatively dense coating could be formed on the weld surface without pores or cracks, and well bonded to the weld by the use of CS with appropriate spray parameters. (2) The mechanical properties of friction stir welds are improved by the cold sprayed Al coating. The microhardness of the WN and TMAZ near the top surface is increased. The average UTS and elongation of coated specimens increase by 7.4% and 25.6%, respectively. (3) The presence of the Al coating offers excellent corrosion protection for welds, while without protection joints are terribly corroded. Besides, the strength of joints with coating protection is hardly lowered.
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