- Email: [email protected]
Mg laminates
International Journal of Adhesion and Adhesives 87 (2018) 98–104
Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Effect of electrolyte composition ratio of micro-arc oxidation on interlaminar strength of CFRP/Mg laminates
T
⁎
Siqiang Suna, Yingcai Panb, Guoqing Wua, , Xin Lina a b
School of Materials Science and Engineering, Beihang University, Beijing 100191, China Beijing Aerospace Unmanned Vehicles System Engineering Research Institute, Beijing 100094, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fiber metal laminates Surface treatment by chemical solutions Peel Adhesion by mechanical interlocking
In this study, the micro-arc oxidation (MAO) processes in different electrolyte composition ratio were carried out on magnesium alloy sheets and then the carbon fiber reinforced polymer (CFRP)/magnesium alloys laminates were manufactured by using a cure process. The Mode I (peel) interlaminar strength was measured to systematically investigate the effects of the electrolyte concentration on interfacial bonding strength and failure mode of CFRP/Mg laminates in consideration of surface microstructures and roughness. The results show that with the increase of KOH in electrolytes solutions, the Mode I (peel) strength of FMLs performs sharply decreasing due to the decrease of roughness in local regions and the emergence of interspace between magnesium and epoxy. In addition, with the rise of KF in electrolytes solutions, the Mode I (peel) strength of FMLs declines rapidly on account of the sharply weakening of electro-chemical reactions, which causes the decrease of surface roughness. The specimen exhibits the optimal Mode I interlaminar strength of 2.04 N/mm when the KOH and KF concentration is 7.28 g/L and 2.9 g/L, respectively.
1. Introduction Fiber metal laminates (FMLs) consisting of alternate metal sheets and thin composites layers is an advanced aircraft material. By integrating the superior fatigue performance and high strength of fiber reinforced polymer composites with the ductility of metal alloys, FMLs have excellent fatigue crack resistance, high impact strength, high strength-to-weight ratio and high stiffness-to-weight ratio [1,2]. Due to the lots of advantages aforementioned, FMLs have been widely applied to the aerospace field, such as for the upper fuselage structure of the Airbus A380 and the cargo door of the Boeing C-17 [3]. In the past decades, the researches in the domain of FMLs have mainly concentrated on aramid fiber reinforced aluminium laminate (ARALL), glass fiber reinforced aluminium laminate (GLARE), carbon fiber reinforced aluminium laminate (CARALL)., carbon fiber reinforced titanium laminate (TiGr) and so on [4–6]. With the increasingly request of reduction of fuel consumption in the aerospace and transportation sector, magnesium alloys have drawn growing attention for its low density [7]. Thus, there are increasingly studies having been carried out to research and explore the properties and applicability of FMLs based on magnesium alloys. It is reported that fiber reinforced magnesium alloy laminates have low weight and cost, high strength to weight ratio, good vibration reducing performance and electromagnetic shielding ⁎
Corresponding author. E-mail address: [email protected] (G. Wu).
https://doi.org/10.1016/j.ijadhadh.2018.09.013 Accepted 4 September 2018 Available online 26 September 2018 0143-7496/ © 2018 Elsevier Ltd. All rights reserved.
capability[1,7–9]. For the carbon fiber reinforced magnesium alloy laminates, the quality of interfacial bonding has been a crucial problem [10], which may easily cause delamination at the interface resulting in reduction of overall strength and stiffness of FMLs [11]. To enhance the interlaminar strength, many surface modification techniques have been used, such as chemical conversion [12], electrochemistry [13] and laser treatment [14]. The MAO method stands out as an eco-friendly plasma-assisted electrolytic technique capable of obtaining highly stable ceramic coatings with improved hardness, adhesion, corrosion and wear resistance compared to the coatings obtained by other electrolytic methods, such as anodising [15]. In addition, the treatment time of MAO is so short that its treatment efficiency is greatly high compared with laser treatment. Nowadays, this method has been developed to be an environmentally friendly process by replacing electrolyte or improving the process [16]. In previous study, our laboratory research group had demonstrated that micro-arc oxidation using the electrolyte of KOH and KF can effectively improve the Mode I (peel) interlaminar strength of laminates [17]. However, the study is only limited to research the influences of MAO electrolyte composition on the interlaminar strength of CFRP/Mg laminates. Therefore, it is worth making a further study of the influence of the other process parameters on the interfacial bonding strength. But the MAO process involves many different parameters, so
International Journal of Adhesion and Adhesives 87 (2018) 98–104
S. Sun et al.
that it is necessary to pay attention of research to the most important factors, such electrolytes concentration, voltage, treatment time, etc. The concentration of the electrolyte is an important parameter in deciding the discharge characteristics and quality of MAO coating, therefore it is investigated in this paper. In this study, the MAO processes in different electrolyte composition ratio were carried out on magnesium alloy sheets and then CFRP/Mg laminates were manufactured by using a cure process. The Mode I (peel) interlaminar strength was measured to systematically investigate the effects of the micro-arc oxidation treatment for different electrolyte composition ratio on interfacial bonding strength of CFRP/Mg laminates in consideration of surface microstructures and roughness. 2. Materials and methods 2.1. Materials
Fig. 1. The curing process flow diagram.
A magnesium alloy sheets (AZ31) of 0.3 mm thickness and carbon fiber reinforced polymer (CFRP) of 0.1 mm thickness were utilized to prepare the fiber metal laminates. The CFRP prepregs were produced by using unidirectional carbon fiber (65 Vol. %, type T700) and epoxy (type 648). The lay-up of the FMLs was composed of three layer of magnesium sheet and two layer of CFRP with [0°/0°] stacking sequence.
test method utilizing a tensile tester according to ASTM D 3167 [19]. The dimension of peel testing samples was 15 mm × 150 mm × 1.5 mm and the tensile loading rate was 50 mm/min. Finally, the peeling force was recorded and the tests were repeated three times for each kind of FMLs. The Mode I (peel) interlaminar strength is calculated by peeling force divided the sample width.
2.2. Surface treatment procedures
2.5. Surface characterization
The AZ31 magnesium alloy sheets were cut into 170 mm × 90 mm panels and abraded smoothly with #600 and #1200 grit sandpaper following rinse with ethyl alcohol. Then they were treated with microarc oxidation technique in different electrolyte solutions by employing an inverter pulsed power supply. The AZ31 magnesium alloy specimens and stainless steel panels were respectively used as anode and cathode, whose distance was approximately 8 cm. Based on the results of Pan [17], KOH and KF are chosen as the electrolyte. Narayanan [18] reported that the oxide coating prepared using 1.5 M KOH possess a coarser structure with larger oxide nodules and the presence of KF will result in the formation of coatings with larger pore diameter. Thus, in this paper, the content of KOH and KF in electrolyte solution is increased to study their influence on interfacial bonding strength of CFRP/Mg laminates. Table 1 shows the detailed parameters of the MAO process, where duty ratio is defined as the ratio of the on time to the whole period (on time + off time).
The surface topography of the treated magnesium alloys and the cross-section microstructure of the FMLs were analysed employing the scanning electron microscopy (Phenom™ Pro and S4800-SEM). The surface roughness was measured using surface roughness tester (SRA-2) with a 1μm diameter stylus tip. The specimen size was 15 mm × 100 mm and the sampling length was 20 mm. The roughness measurements were repeated three times to acquire more accurate results. 3. Results and discussion 3.1. Surface characteristics of magnesium alloy Fig. 2 presents the surface SEM images of magnesium alloys after different treatments. As shown in Fig. 2(a), the surface of MAO-2:1 sample remains polished morphology with alternant distribution of long striped ridges and grooves. It indicates that high KF content in electrolyte solution for MAO process is inhibiting to generate a normal micro-arc oxidation reaction. As the KF content decreases by a half, the sample of MAO-1:1 exhibits violent electrochemical dissolution on the magnesium film layer, which produces a multitude of irregularly shaped ridges and grooves, showing a typical etchpit-like structure. This surface characteristic can be explained by the non-uniform of extent of electrochemical dissolution of the film layer caused by the tensile stresses in the oxide film, which provides a preferential reaction zone in the crevice created by tensile stresses [17]. In addition, it is interesting to note that the local regions of magnesium surface appear a little deep and large pits and holes. The reason for this phenomenon may be the existence of uneven distribution of elements, which lead to local fast and severe electrochemical
2.3. Hot pressing moulding The magnesium alloy sheets with electrochemical surface treatment and CFRP were placed in a mould with dimensions of 170 mm × 270 mm, and then they were subjected to a cure process using a hot-press machine. The curing process flow diagram is shown in Fig. 1. Finally, the FMLs were fabricated and used for the following test. 2.4. Mode I (peel) interlaminar strength tests The measurements of Mode I (peel) interlamintar strength between the magnesium alloys and the CFRP were carried out by a roller peel Table 1 The parameters of the MAO process. Sample label
Electrolyte
Voltage(V)
Frequency (Hz)
Duty ratio
Treatment time (min)
MAO-2:1 MAO-1:1 MAO-1:2 MAO-1:3
5.8 g/L 2.9 g/L 2.9 g/L 2.9 g/L
230
500
30%
3
KF + 7.28 g/L KOH KF + 7.28 g/L KOH KF + 14.56 g/L KOH KF + 21.84 g/L KOH
99
International Journal of Adhesion and Adhesives 87 (2018) 98–104
S. Sun et al.
Fig. 2. Surface morphology of magnesium alloys after different treatment: (a) MAO-2:1, (b) MAO-1:1, (c) MAO-1:2, (d) MAO-1:3.
100
International Journal of Adhesion and Adhesives 87 (2018) 98–104
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Fig. 3. Surface profiles and roughness of magnesium alloys treated in different electrolyte solutions.
Fig. 4. The cross-sectional morphology of the FMLs after treatment in different electrolyte solutions：(a) MAO-2:1, (b) MAO-1:1, (c) MAO-1:2, (d) MAO-1:3.
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dissolution. When the KOH content is doubled, the surface micro topography is partly similar to that of MAO-1:1. But there are more and deeper pits and holes on magnesium surface and the amount and size of ridges and grooves in the region around those giant holes reduce. The difference in extent of electrochemical dissolution between districts is increasingly large for MAO-1:3 where the KOH content is tripled compared with that of MAO-1:1. Specifically, approximated 40% of the magnesium alloy surface appears giant pits and holes with greater depth and width, while other area exhibits slight electrochemical dissolution with smoother surface in visual inspection. It can be attributed to the increase of hydroxyl ion in solutions of electrolytes, which accelerates the electrochemical dissolution of local area. Fig. 3 presents the surface profile and the results of surface roughness analysis of magnesium alloys sheets after treatment. It can be seen that as the content of KOH in solutions of electrolytes increases, the peaks and valleys of profile curve gradually increase and become bigger, while the rest seems increasingly to be smooth. As an integrated result, the Ra (the arithmetic mean deviation of the surface height from the mean line through the profile) of MAO-1:2 is close to that of MAO1:1, namely 2.054 μm and 2.227 μm respectively. However, the Rmax (the height difference between the highest point and the lowest point of the profile) and Rv (the height of the lowest valley of the profile) of MAO-1:2 are respectively increased by 12.3% and 29.1% compared with that of MAO-1:1. As seen in Fig. 3, the surface profile of MAO-1:3 presents some extremely high peaks and deep valleys due to the existence of greatly large holes and pits on the surface. As a result, the Rmax and Rv show a highly great increase compared with that of MAO1:1, namely 122.2% and 117.5%. On the contrary, with the increase of KF in electrolytes solutions, the surface profile seems extremely smooth and the Ra of MAO-2:1 (0.265 μm) reduces by 88.1% when compared with that of MAO-1:1. These results are greatly consistent with the characteristic of magnesium surface microstructure after treatment in different electrolytes solutions.
Fig. 5. Load–displacement curve from peel test of FMLs treated in different electrolyte solutions.
3.2. Interlaminar characteristics of the FMLs Fig. 6. The surface roughness of treated magnesium sheets and Mode I (peel) interlaminar strength of FMLs treated in different electrolyte solutions.
Fig. 4 displays the cross-sectional morphology of the four FMLs where the darker layers are CFRP and the bright layers are magnesium alloys. It is obvious that some deep and irregular holes and pits were formed on the surface of the latter three samples and the depth of deepest holes, in turn, increases from 33.0 μm to 138.4 μm. These holes and pits would provide an increase in surface area and enhance, to some extent, mechanical interlocking between magnesium alloy layer and CFRP layer. It can be seen that the epoxy resin seeping from the CFRP prepregs flowed into the micro pits and holes on the magnesium surface during the curing process, which indicates that the magnesium has a great combination with the CFRP. However, for the sample of MAO-1:2 and MAO-1:3, some holes are so deep that the epoxy resin is insufficient to completely fill them, leaving air-gap between the CFRP layers and the magnesium layers. These gaps will weaken the interface bonding and adversely affect the interlaminar strength of FMLs.
Fig. 6 presents the mean values of Mode I (peel) interlaminar strength of FMLs treated in different electrolyte solutions. It can be seen that the electrolyte solutions have a significant effect on the peel strength of FMLs. Compared with MAO-1:1(2.04 N/mm), the peel strength of MAO-2:1 (0.41 N/mm) decreases by 79.9%, which indicates that with the increase of the content of KF in electrolyte solutions, the peel strength of FMLs decreases. This is mainly because the high content of KF in electrolyte solutions will restrain the electrochemical dissolution forming a relatively smooth coating on the magnesium surface during the MAO, thereby causing the decrease of interlaminar strength of FMLs. In addition, as the increases of KOH in electrolyte solutions the peel strength of FMLs is also in decline. However, it is interesting to note that when the KOH content is double in electrolyte solutions, the peel strength of MAO-1:2 (1.32 N/mm) sharply decreases by 35.3% compared with that of MAO-1:1(2.04 N/mm), while the surface roughness shows a little reduction. When keeping increasing the KOH content, the peel strength of MAO-1:3 (0.97 N/mm) remains decreasing by 26.5%, but the surface roughness drastically increases by 60.9% compared with that of MAO-1:2. The main reason for this phenomenon is that the difference of electrochemical dissolution between districts is gradually accelerated with increase of KOH in solutions of electrolytes, which causes the deep dissolved area becomes increasingly deeper and bigger, and the shallow dissolved area is getting more flat and smaller. Therefore, the interfacial adhesion between magnesium and epoxy becomes gradually weaker. In addition, as seen on the Fig. 4, some of these deep pits and holes in the deep dissolved area are difficult
3.3. Mode I (peel) strength of the FMLs The experimentally determined peel force versus displacement curves of FMLs treated in different electrolyte solutions are reported in Fig. 5. It can be seen that the load of all the FMLs increases with the increase of displacement, and then they reach the maximum and keep a dynamic stability when the crack initiates and propagates in interlaminar. It should be noted that the curve of MAO-1:3 performs more fluctuates than others at the relatively stable stage. This can be attributed to the great nonuniformity of surface topography of MAO-1:2 and MAO-1:3. Pulling out the epoxy from those holes on the magnesium surface or the fracture of epoxy requires more force, which will lead to the increase of load. Once the force is released, the load will have a great decrease. 102
International Journal of Adhesion and Adhesives 87 (2018) 98–104
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Fig. 7. The SEM images of cross-sectional and detached Mg surface of the four FMLs after peel tests: (a) MAO-2:1, (b) MAO-1:1, (c) MAO-1:2, (d) MAO-1:3.
to be fully filled with epoxy resin during the cure process so that there are many air-gaps between magnesium and epoxy leading to failed bonding. Hence, the FMLs exhibit a continuous decrease of peel strength, although the surface roughness is increased due to these deep pits and holes. Fig. 7 shows the SEM images of cross-sectional and detached Mg surface of the four FMLs after peel tests. It can be seen that the crack propagation mainly occurs at the CFRP/magnesium interface under the peeling load and the resin (black) can be observed on detached Mg surface, which indicates that there are two general types of failure modes, namely the adhesive failure between epoxy and magnesium alloys, as well as the cohesive failure. The cohesive failure may be attributed to the presence of deep grooves, pits and holes on the magnesium surface, which leads to the great difficulty for embedded epoxy to pull out. As a result, the epoxy is cleaved and the peel force goes up. For the sample of MAO-1:1, most area of the detached surface is occupied by the fractured epoxy, so the peel strength is the biggest among the four FMLs. Similarly, the proportion of the fractured epoxy for MAO-1:2 and MAO-1:3 is gradually declined, hence their peel strength also decreases in turn. It can be speculated that the cohesive failure has played a significant roles in the peeling failure of FMLs. The adhesive failure mainly occurs in the smooth areas where the epoxy cannot be locked by the microstructure of magnesium alloys surface. Compared with the cohesive failure, it has relatively small contribution to the peel force. Therefore, the peel strength of MAO-2:1 shows a great decrease.
(3) The failure mode of samples mainly consists of cohesive failure and adhesive failure. To be specific, the areas with large grooves, pits and holes primarily show cohesive failure while the smooth areas principally perform adhesive failure. Acknowledgments This paper is financially supported by the National Natural Science Foundation of China (50901005) and the Aeronautics Fundamental Science Foundation of China (2010ZF51068). References [1] Asaee Z, Shadlou S, Taheri F. Low-velocity impact response of fiberglass/magnesium FMLs with a new 3D fiberglass fabric. Compos Struct 2015;122:155–65. [2] Pawar OA, Gaikhe YS, Tewari A, Sundaram R, Joshi SS. Analysis of hole quality in drilling GLARE fiber metal laminates. Compos Struct 2015;123:350–65. [3] Sinmazçelik T, Avcu E, Bora MÖ, Çoban O. A review: fibre metal laminates, background, bonding types and applied test methods. Mater Des 2011;32(7):3671–85. [4] Zhang X, Zhang Y, Ma QY, Dai Y, Hu FP, Wei GB, et al. Effect of surface treatment on the corrosion properties of magnesium-based fibre metal laminate. Appl Surf Sci 2017;396:1264–72. [5] Burianek DA, Spearing SM. Delamination growth from face sheet seams in cross-ply titanium/graphite hybrid laminates. Compos Sci Technol 2001;61(2):261–9. [6] Burianek DA, Spearing SM. Fatigue damage in titanium–graphite hybrid laminates. Compos Sci Technol 2002;62(5):607–17. [7] Múgica JI, Aretxabaleta L, Ulacia I, Aurrekoetxea J. Impact characterization of thermoformable fibre metal laminates of 2024–T3 aluminium and AZ31B–H24 magnesium based on self-reinforced polypropylene. Compos: Part A 2014;61:67–75. [8] Pan YC, Wu GQ, Huang Z, Wu X, Liu YH, Ye HJ. Corrosion behaviour of carbon fibre reinforced polymer/magnesium alloy hybrid laminates. Corros Sci 2017;115:152–8. [9] Pärnänen T, Alderliesten RC, Rans C, Brander T, Saarela O. Applicability of AZ31B–H24 magnesium infibre metal laminates–an experimental impact research. Compos: Part A 2012;43:1578–86. [10] Pan YC, Wu GQ, Huang Z, Zhang ZK, Ye HJ, Li MY, Ji SD. Improvement in interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates by laser ablation. Mater Lett 2017;207:4–7. [11] Ning H, Weng S, Hu N, Yan C, Liu J, Yao J, et al. Mode-II interlaminar fracture toughness of GFRP/Al laminates improved by surface modified VGCF interleaves. Compos: Part B 2017;114:365–72. [12] Liu Z, Sun R, Mao Z, Wang PC. Effects of phosphate pretreatment and hothumid environmental exposure on static strength of adhesive-bonded magnesium AZ31 sheets. Surf Coat Technol 2012;206(16):3517–25. [13] Gao HT, Zhang M, Yang X, Huang P, Xu KW. Effect of Na2SiO3 solution concentration of micro-arc oxidation process on lap-shear strength of adhesive-bonded magnesium alloys. Appl Surf Sci 2014;314:447–52.
4. Conclusions (1) As the content of KOH in electrolytes solutions increases, the difference of electrochemical dissolution between districts is gradually accelerated and the surface roughness of magnesium alloys firstly shows a little reduction and then increases sharply. The increase in KOH exerts an adverse effect on interfacial adhesion and causes the sharp decline in the Mode I (peel) strength of FMLs. (2) Increasing KF in electrolyte solution for MAO process is inhibiting to generate a normal micro-arc oxidation reaction, which leads to the decrease of surface roughness and Mode I (peel) strength of FMLs. 103
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[17] Pan YC, Wu X, Huang Z, Wu GQ, Sun SQ, Ye HJ, et al. A new approach to enhancing interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates. Compos: Part A 2018;105:78–86. [18] Narayanan TSNS, Park IS, Lee MH. Strategies to improve the corrosion resistance of microarc oxidation (MAO) coated magnesium alloys for degradable implants: prospects and challenges. Prog Mater Sci 2014;60:1–71. [19] Pan YC, Wu GQ, Huang Z, Li MY, Ji SD, Zhang ZK. Effect of surface roughness on interlaminar peel and shear strength of CFRP/Mg laminates. Int J Adhes Adhes 2017;79:1–7.
[14] Alfano M, Lubineau G, Furgiuele F, Paulino GH. Study on the role of laser surface irradiation on damage and decohesion of Al/epoxy joints. Int J Adhes Adhes 2012;39:33–41. [15] Mingo B, Arrabal R, Mohedano M, Llamazares Y, Matykina E, Yerokhina A, Pardobet A. Influence of sealing post-treatments on the corrosion resistance of PEO coated AZ91 magnesium alloy. Appl Surf Sci 2018;433:653–67. [16] Park SY, Choi WJ, Choi HS, Kwon H, Kim SH. Recent trends in surface treatment technologies for airframe adhesive bonding processing: a review (1995–2008). J Adhes 2010;86:192–221. [Recent Trends in Surface Treatment Technologies for Airframe Adhesive].
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Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Effect of electrolyte composition ratio of micro-arc oxidation on interlaminar strength of CFRP/Mg laminates
T
⁎
Siqiang Suna, Yingcai Panb, Guoqing Wua, , Xin Lina a b
School of Materials Science and Engineering, Beihang University, Beijing 100191, China Beijing Aerospace Unmanned Vehicles System Engineering Research Institute, Beijing 100094, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fiber metal laminates Surface treatment by chemical solutions Peel Adhesion by mechanical interlocking
In this study, the micro-arc oxidation (MAO) processes in different electrolyte composition ratio were carried out on magnesium alloy sheets and then the carbon fiber reinforced polymer (CFRP)/magnesium alloys laminates were manufactured by using a cure process. The Mode I (peel) interlaminar strength was measured to systematically investigate the effects of the electrolyte concentration on interfacial bonding strength and failure mode of CFRP/Mg laminates in consideration of surface microstructures and roughness. The results show that with the increase of KOH in electrolytes solutions, the Mode I (peel) strength of FMLs performs sharply decreasing due to the decrease of roughness in local regions and the emergence of interspace between magnesium and epoxy. In addition, with the rise of KF in electrolytes solutions, the Mode I (peel) strength of FMLs declines rapidly on account of the sharply weakening of electro-chemical reactions, which causes the decrease of surface roughness. The specimen exhibits the optimal Mode I interlaminar strength of 2.04 N/mm when the KOH and KF concentration is 7.28 g/L and 2.9 g/L, respectively.
1. Introduction Fiber metal laminates (FMLs) consisting of alternate metal sheets and thin composites layers is an advanced aircraft material. By integrating the superior fatigue performance and high strength of fiber reinforced polymer composites with the ductility of metal alloys, FMLs have excellent fatigue crack resistance, high impact strength, high strength-to-weight ratio and high stiffness-to-weight ratio [1,2]. Due to the lots of advantages aforementioned, FMLs have been widely applied to the aerospace field, such as for the upper fuselage structure of the Airbus A380 and the cargo door of the Boeing C-17 [3]. In the past decades, the researches in the domain of FMLs have mainly concentrated on aramid fiber reinforced aluminium laminate (ARALL), glass fiber reinforced aluminium laminate (GLARE), carbon fiber reinforced aluminium laminate (CARALL)., carbon fiber reinforced titanium laminate (TiGr) and so on [4–6]. With the increasingly request of reduction of fuel consumption in the aerospace and transportation sector, magnesium alloys have drawn growing attention for its low density [7]. Thus, there are increasingly studies having been carried out to research and explore the properties and applicability of FMLs based on magnesium alloys. It is reported that fiber reinforced magnesium alloy laminates have low weight and cost, high strength to weight ratio, good vibration reducing performance and electromagnetic shielding ⁎
Corresponding author. E-mail address: [email protected] (G. Wu).
https://doi.org/10.1016/j.ijadhadh.2018.09.013 Accepted 4 September 2018 Available online 26 September 2018 0143-7496/ © 2018 Elsevier Ltd. All rights reserved.
capability[1,7–9]. For the carbon fiber reinforced magnesium alloy laminates, the quality of interfacial bonding has been a crucial problem [10], which may easily cause delamination at the interface resulting in reduction of overall strength and stiffness of FMLs [11]. To enhance the interlaminar strength, many surface modification techniques have been used, such as chemical conversion [12], electrochemistry [13] and laser treatment [14]. The MAO method stands out as an eco-friendly plasma-assisted electrolytic technique capable of obtaining highly stable ceramic coatings with improved hardness, adhesion, corrosion and wear resistance compared to the coatings obtained by other electrolytic methods, such as anodising [15]. In addition, the treatment time of MAO is so short that its treatment efficiency is greatly high compared with laser treatment. Nowadays, this method has been developed to be an environmentally friendly process by replacing electrolyte or improving the process [16]. In previous study, our laboratory research group had demonstrated that micro-arc oxidation using the electrolyte of KOH and KF can effectively improve the Mode I (peel) interlaminar strength of laminates [17]. However, the study is only limited to research the influences of MAO electrolyte composition on the interlaminar strength of CFRP/Mg laminates. Therefore, it is worth making a further study of the influence of the other process parameters on the interfacial bonding strength. But the MAO process involves many different parameters, so
International Journal of Adhesion and Adhesives 87 (2018) 98–104
S. Sun et al.
that it is necessary to pay attention of research to the most important factors, such electrolytes concentration, voltage, treatment time, etc. The concentration of the electrolyte is an important parameter in deciding the discharge characteristics and quality of MAO coating, therefore it is investigated in this paper. In this study, the MAO processes in different electrolyte composition ratio were carried out on magnesium alloy sheets and then CFRP/Mg laminates were manufactured by using a cure process. The Mode I (peel) interlaminar strength was measured to systematically investigate the effects of the micro-arc oxidation treatment for different electrolyte composition ratio on interfacial bonding strength of CFRP/Mg laminates in consideration of surface microstructures and roughness. 2. Materials and methods 2.1. Materials
Fig. 1. The curing process flow diagram.
A magnesium alloy sheets (AZ31) of 0.3 mm thickness and carbon fiber reinforced polymer (CFRP) of 0.1 mm thickness were utilized to prepare the fiber metal laminates. The CFRP prepregs were produced by using unidirectional carbon fiber (65 Vol. %, type T700) and epoxy (type 648). The lay-up of the FMLs was composed of three layer of magnesium sheet and two layer of CFRP with [0°/0°] stacking sequence.
test method utilizing a tensile tester according to ASTM D 3167 [19]. The dimension of peel testing samples was 15 mm × 150 mm × 1.5 mm and the tensile loading rate was 50 mm/min. Finally, the peeling force was recorded and the tests were repeated three times for each kind of FMLs. The Mode I (peel) interlaminar strength is calculated by peeling force divided the sample width.
2.2. Surface treatment procedures
2.5. Surface characterization
The AZ31 magnesium alloy sheets were cut into 170 mm × 90 mm panels and abraded smoothly with #600 and #1200 grit sandpaper following rinse with ethyl alcohol. Then they were treated with microarc oxidation technique in different electrolyte solutions by employing an inverter pulsed power supply. The AZ31 magnesium alloy specimens and stainless steel panels were respectively used as anode and cathode, whose distance was approximately 8 cm. Based on the results of Pan [17], KOH and KF are chosen as the electrolyte. Narayanan [18] reported that the oxide coating prepared using 1.5 M KOH possess a coarser structure with larger oxide nodules and the presence of KF will result in the formation of coatings with larger pore diameter. Thus, in this paper, the content of KOH and KF in electrolyte solution is increased to study their influence on interfacial bonding strength of CFRP/Mg laminates. Table 1 shows the detailed parameters of the MAO process, where duty ratio is defined as the ratio of the on time to the whole period (on time + off time).
The surface topography of the treated magnesium alloys and the cross-section microstructure of the FMLs were analysed employing the scanning electron microscopy (Phenom™ Pro and S4800-SEM). The surface roughness was measured using surface roughness tester (SRA-2) with a 1μm diameter stylus tip. The specimen size was 15 mm × 100 mm and the sampling length was 20 mm. The roughness measurements were repeated three times to acquire more accurate results. 3. Results and discussion 3.1. Surface characteristics of magnesium alloy Fig. 2 presents the surface SEM images of magnesium alloys after different treatments. As shown in Fig. 2(a), the surface of MAO-2:1 sample remains polished morphology with alternant distribution of long striped ridges and grooves. It indicates that high KF content in electrolyte solution for MAO process is inhibiting to generate a normal micro-arc oxidation reaction. As the KF content decreases by a half, the sample of MAO-1:1 exhibits violent electrochemical dissolution on the magnesium film layer, which produces a multitude of irregularly shaped ridges and grooves, showing a typical etchpit-like structure. This surface characteristic can be explained by the non-uniform of extent of electrochemical dissolution of the film layer caused by the tensile stresses in the oxide film, which provides a preferential reaction zone in the crevice created by tensile stresses [17]. In addition, it is interesting to note that the local regions of magnesium surface appear a little deep and large pits and holes. The reason for this phenomenon may be the existence of uneven distribution of elements, which lead to local fast and severe electrochemical
2.3. Hot pressing moulding The magnesium alloy sheets with electrochemical surface treatment and CFRP were placed in a mould with dimensions of 170 mm × 270 mm, and then they were subjected to a cure process using a hot-press machine. The curing process flow diagram is shown in Fig. 1. Finally, the FMLs were fabricated and used for the following test. 2.4. Mode I (peel) interlaminar strength tests The measurements of Mode I (peel) interlamintar strength between the magnesium alloys and the CFRP were carried out by a roller peel Table 1 The parameters of the MAO process. Sample label
Electrolyte
Voltage(V)
Frequency (Hz)
Duty ratio
Treatment time (min)
MAO-2:1 MAO-1:1 MAO-1:2 MAO-1:3
5.8 g/L 2.9 g/L 2.9 g/L 2.9 g/L
230
500
30%
3
KF + 7.28 g/L KOH KF + 7.28 g/L KOH KF + 14.56 g/L KOH KF + 21.84 g/L KOH
99
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Fig. 2. Surface morphology of magnesium alloys after different treatment: (a) MAO-2:1, (b) MAO-1:1, (c) MAO-1:2, (d) MAO-1:3.
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Fig. 3. Surface profiles and roughness of magnesium alloys treated in different electrolyte solutions.
Fig. 4. The cross-sectional morphology of the FMLs after treatment in different electrolyte solutions：(a) MAO-2:1, (b) MAO-1:1, (c) MAO-1:2, (d) MAO-1:3.
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dissolution. When the KOH content is doubled, the surface micro topography is partly similar to that of MAO-1:1. But there are more and deeper pits and holes on magnesium surface and the amount and size of ridges and grooves in the region around those giant holes reduce. The difference in extent of electrochemical dissolution between districts is increasingly large for MAO-1:3 where the KOH content is tripled compared with that of MAO-1:1. Specifically, approximated 40% of the magnesium alloy surface appears giant pits and holes with greater depth and width, while other area exhibits slight electrochemical dissolution with smoother surface in visual inspection. It can be attributed to the increase of hydroxyl ion in solutions of electrolytes, which accelerates the electrochemical dissolution of local area. Fig. 3 presents the surface profile and the results of surface roughness analysis of magnesium alloys sheets after treatment. It can be seen that as the content of KOH in solutions of electrolytes increases, the peaks and valleys of profile curve gradually increase and become bigger, while the rest seems increasingly to be smooth. As an integrated result, the Ra (the arithmetic mean deviation of the surface height from the mean line through the profile) of MAO-1:2 is close to that of MAO1:1, namely 2.054 μm and 2.227 μm respectively. However, the Rmax (the height difference between the highest point and the lowest point of the profile) and Rv (the height of the lowest valley of the profile) of MAO-1:2 are respectively increased by 12.3% and 29.1% compared with that of MAO-1:1. As seen in Fig. 3, the surface profile of MAO-1:3 presents some extremely high peaks and deep valleys due to the existence of greatly large holes and pits on the surface. As a result, the Rmax and Rv show a highly great increase compared with that of MAO1:1, namely 122.2% and 117.5%. On the contrary, with the increase of KF in electrolytes solutions, the surface profile seems extremely smooth and the Ra of MAO-2:1 (0.265 μm) reduces by 88.1% when compared with that of MAO-1:1. These results are greatly consistent with the characteristic of magnesium surface microstructure after treatment in different electrolytes solutions.
Fig. 5. Load–displacement curve from peel test of FMLs treated in different electrolyte solutions.
3.2. Interlaminar characteristics of the FMLs Fig. 6. The surface roughness of treated magnesium sheets and Mode I (peel) interlaminar strength of FMLs treated in different electrolyte solutions.
Fig. 4 displays the cross-sectional morphology of the four FMLs where the darker layers are CFRP and the bright layers are magnesium alloys. It is obvious that some deep and irregular holes and pits were formed on the surface of the latter three samples and the depth of deepest holes, in turn, increases from 33.0 μm to 138.4 μm. These holes and pits would provide an increase in surface area and enhance, to some extent, mechanical interlocking between magnesium alloy layer and CFRP layer. It can be seen that the epoxy resin seeping from the CFRP prepregs flowed into the micro pits and holes on the magnesium surface during the curing process, which indicates that the magnesium has a great combination with the CFRP. However, for the sample of MAO-1:2 and MAO-1:3, some holes are so deep that the epoxy resin is insufficient to completely fill them, leaving air-gap between the CFRP layers and the magnesium layers. These gaps will weaken the interface bonding and adversely affect the interlaminar strength of FMLs.
Fig. 6 presents the mean values of Mode I (peel) interlaminar strength of FMLs treated in different electrolyte solutions. It can be seen that the electrolyte solutions have a significant effect on the peel strength of FMLs. Compared with MAO-1:1(2.04 N/mm), the peel strength of MAO-2:1 (0.41 N/mm) decreases by 79.9%, which indicates that with the increase of the content of KF in electrolyte solutions, the peel strength of FMLs decreases. This is mainly because the high content of KF in electrolyte solutions will restrain the electrochemical dissolution forming a relatively smooth coating on the magnesium surface during the MAO, thereby causing the decrease of interlaminar strength of FMLs. In addition, as the increases of KOH in electrolyte solutions the peel strength of FMLs is also in decline. However, it is interesting to note that when the KOH content is double in electrolyte solutions, the peel strength of MAO-1:2 (1.32 N/mm) sharply decreases by 35.3% compared with that of MAO-1:1(2.04 N/mm), while the surface roughness shows a little reduction. When keeping increasing the KOH content, the peel strength of MAO-1:3 (0.97 N/mm) remains decreasing by 26.5%, but the surface roughness drastically increases by 60.9% compared with that of MAO-1:2. The main reason for this phenomenon is that the difference of electrochemical dissolution between districts is gradually accelerated with increase of KOH in solutions of electrolytes, which causes the deep dissolved area becomes increasingly deeper and bigger, and the shallow dissolved area is getting more flat and smaller. Therefore, the interfacial adhesion between magnesium and epoxy becomes gradually weaker. In addition, as seen on the Fig. 4, some of these deep pits and holes in the deep dissolved area are difficult
3.3. Mode I (peel) strength of the FMLs The experimentally determined peel force versus displacement curves of FMLs treated in different electrolyte solutions are reported in Fig. 5. It can be seen that the load of all the FMLs increases with the increase of displacement, and then they reach the maximum and keep a dynamic stability when the crack initiates and propagates in interlaminar. It should be noted that the curve of MAO-1:3 performs more fluctuates than others at the relatively stable stage. This can be attributed to the great nonuniformity of surface topography of MAO-1:2 and MAO-1:3. Pulling out the epoxy from those holes on the magnesium surface or the fracture of epoxy requires more force, which will lead to the increase of load. Once the force is released, the load will have a great decrease. 102
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Fig. 7. The SEM images of cross-sectional and detached Mg surface of the four FMLs after peel tests: (a) MAO-2:1, (b) MAO-1:1, (c) MAO-1:2, (d) MAO-1:3.
to be fully filled with epoxy resin during the cure process so that there are many air-gaps between magnesium and epoxy leading to failed bonding. Hence, the FMLs exhibit a continuous decrease of peel strength, although the surface roughness is increased due to these deep pits and holes. Fig. 7 shows the SEM images of cross-sectional and detached Mg surface of the four FMLs after peel tests. It can be seen that the crack propagation mainly occurs at the CFRP/magnesium interface under the peeling load and the resin (black) can be observed on detached Mg surface, which indicates that there are two general types of failure modes, namely the adhesive failure between epoxy and magnesium alloys, as well as the cohesive failure. The cohesive failure may be attributed to the presence of deep grooves, pits and holes on the magnesium surface, which leads to the great difficulty for embedded epoxy to pull out. As a result, the epoxy is cleaved and the peel force goes up. For the sample of MAO-1:1, most area of the detached surface is occupied by the fractured epoxy, so the peel strength is the biggest among the four FMLs. Similarly, the proportion of the fractured epoxy for MAO-1:2 and MAO-1:3 is gradually declined, hence their peel strength also decreases in turn. It can be speculated that the cohesive failure has played a significant roles in the peeling failure of FMLs. The adhesive failure mainly occurs in the smooth areas where the epoxy cannot be locked by the microstructure of magnesium alloys surface. Compared with the cohesive failure, it has relatively small contribution to the peel force. Therefore, the peel strength of MAO-2:1 shows a great decrease.
(3) The failure mode of samples mainly consists of cohesive failure and adhesive failure. To be specific, the areas with large grooves, pits and holes primarily show cohesive failure while the smooth areas principally perform adhesive failure. Acknowledgments This paper is financially supported by the National Natural Science Foundation of China (50901005) and the Aeronautics Fundamental Science Foundation of China (2010ZF51068). References [1] Asaee Z, Shadlou S, Taheri F. Low-velocity impact response of fiberglass/magnesium FMLs with a new 3D fiberglass fabric. Compos Struct 2015;122:155–65. [2] Pawar OA, Gaikhe YS, Tewari A, Sundaram R, Joshi SS. Analysis of hole quality in drilling GLARE fiber metal laminates. Compos Struct 2015;123:350–65. [3] Sinmazçelik T, Avcu E, Bora MÖ, Çoban O. A review: fibre metal laminates, background, bonding types and applied test methods. Mater Des 2011;32(7):3671–85. [4] Zhang X, Zhang Y, Ma QY, Dai Y, Hu FP, Wei GB, et al. Effect of surface treatment on the corrosion properties of magnesium-based fibre metal laminate. Appl Surf Sci 2017;396:1264–72. [5] Burianek DA, Spearing SM. Delamination growth from face sheet seams in cross-ply titanium/graphite hybrid laminates. Compos Sci Technol 2001;61(2):261–9. [6] Burianek DA, Spearing SM. Fatigue damage in titanium–graphite hybrid laminates. Compos Sci Technol 2002;62(5):607–17. [7] Múgica JI, Aretxabaleta L, Ulacia I, Aurrekoetxea J. Impact characterization of thermoformable fibre metal laminates of 2024–T3 aluminium and AZ31B–H24 magnesium based on self-reinforced polypropylene. Compos: Part A 2014;61:67–75. [8] Pan YC, Wu GQ, Huang Z, Wu X, Liu YH, Ye HJ. Corrosion behaviour of carbon fibre reinforced polymer/magnesium alloy hybrid laminates. Corros Sci 2017;115:152–8. [9] Pärnänen T, Alderliesten RC, Rans C, Brander T, Saarela O. Applicability of AZ31B–H24 magnesium infibre metal laminates–an experimental impact research. Compos: Part A 2012;43:1578–86. [10] Pan YC, Wu GQ, Huang Z, Zhang ZK, Ye HJ, Li MY, Ji SD. Improvement in interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates by laser ablation. Mater Lett 2017;207:4–7. [11] Ning H, Weng S, Hu N, Yan C, Liu J, Yao J, et al. Mode-II interlaminar fracture toughness of GFRP/Al laminates improved by surface modified VGCF interleaves. Compos: Part B 2017;114:365–72. [12] Liu Z, Sun R, Mao Z, Wang PC. Effects of phosphate pretreatment and hothumid environmental exposure on static strength of adhesive-bonded magnesium AZ31 sheets. Surf Coat Technol 2012;206(16):3517–25. [13] Gao HT, Zhang M, Yang X, Huang P, Xu KW. Effect of Na2SiO3 solution concentration of micro-arc oxidation process on lap-shear strength of adhesive-bonded magnesium alloys. Appl Surf Sci 2014;314:447–52.
4. Conclusions (1) As the content of KOH in electrolytes solutions increases, the difference of electrochemical dissolution between districts is gradually accelerated and the surface roughness of magnesium alloys firstly shows a little reduction and then increases sharply. The increase in KOH exerts an adverse effect on interfacial adhesion and causes the sharp decline in the Mode I (peel) strength of FMLs. (2) Increasing KF in electrolyte solution for MAO process is inhibiting to generate a normal micro-arc oxidation reaction, which leads to the decrease of surface roughness and Mode I (peel) strength of FMLs. 103
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[17] Pan YC, Wu X, Huang Z, Wu GQ, Sun SQ, Ye HJ, et al. A new approach to enhancing interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates. Compos: Part A 2018;105:78–86. [18] Narayanan TSNS, Park IS, Lee MH. Strategies to improve the corrosion resistance of microarc oxidation (MAO) coated magnesium alloys for degradable implants: prospects and challenges. Prog Mater Sci 2014;60:1–71. [19] Pan YC, Wu GQ, Huang Z, Li MY, Ji SD, Zhang ZK. Effect of surface roughness on interlaminar peel and shear strength of CFRP/Mg laminates. Int J Adhes Adhes 2017;79:1–7.
[14] Alfano M, Lubineau G, Furgiuele F, Paulino GH. Study on the role of laser surface irradiation on damage and decohesion of Al/epoxy joints. Int J Adhes Adhes 2012;39:33–41. [15] Mingo B, Arrabal R, Mohedano M, Llamazares Y, Matykina E, Yerokhina A, Pardobet A. Influence of sealing post-treatments on the corrosion resistance of PEO coated AZ91 magnesium alloy. Appl Surf Sci 2018;433:653–67. [16] Park SY, Choi WJ, Choi HS, Kwon H, Kim SH. Recent trends in surface treatment technologies for airframe adhesive bonding processing: a review (1995–2008). J Adhes 2010;86:192–221. [Recent Trends in Surface Treatment Technologies for Airframe Adhesive].
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