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Effect of anodizing on pulsed Nd:YAG laser joining of polyethylene terephthalate (PET) and aluminium alloy (A5052)
Materials and Design 37 (2012) 410–415
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of anodizing on pulsed Nd:YAG laser joining of polyethylene terephthalate (PET) and aluminium alloy (A5052) Farazila Yusof a,c,⇑, Miyashita Yukio b, Mutoh Yoshiharu b, Mohd Hamdi Abdul Shukor a,c a
Department of Engineering Design and Manufacture, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of System Safety, Nagaoka University of Technology, 940-2188 Nagaoka, Japan c Center of Advanced Manufacturing & Material Processing (AMMP Centre), University of Malaya, 50603 Kuala Lumpur, Malaysia b
a r t i c l e
i n f o
Article history: Received 29 November 2011 Accepted 4 January 2012 Available online 18 January 2012 Keywords: A. Multi-materials C. Surface treatment D. Welding
a b s t r a c t A series of laser joining experiments between polyethylene terephthalate (PET) and aluminium alloy (A5052) were conducted to investigate the effect of anodizing on A5052 surface on dissimilar materials used in joining. In this study, PET/A5052 joints with anodized A5052 surface exhibited greater shear strength compared to PET/A5052 joints without anodizing. The shear strength of the joints was increased with increasing of heat input and pulse duration. This indicates that the anodizing process could improve shear strength of the laser joining specimens. Significant molten pools were formed in both PET/A5052 (anodized) and PET/A5052 (as-received) joints except for PET/A5052 (as-received) sample joined at the lowest heat input and pulse duration. For the test results from laser joining under different pulse duration at the constant heat input, it was shown that joining behaviour was dominantly controlled by heat input and not by pulse duration. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Demand on dissimilar material joints have been increasing from the viewpoint of design requirements, energy consumption, environmental concerns, high performance, cost saving, etc. For example, unique dissimilar materials when combined between polymers and metals would provide advantages in numerous engineering applications. Metals have common properties such as high strength, excellent heat and electrical conductivity. On the other hand polymers offer excellent corrosion resistance, insulation, and are lightweight. Dissimilar materials joint between polymer and metal can be obtained from both the two different properties at the same time. However, joining between polymer and metal by common welding procedures is normally difficult due to incompatible structural, physical and chemical properties. Adhesives and screws are commonly applied in joining between polymer and metal. Amancio-Filho and dos Santos [1] reviewed the recent developments of polymer–metal hybrid joining using various joining techniques such as adhesive bonding, mechanical fastening, and welding. Elena and Lazar [2] reported the use of adhesive bonding for joining aluminium and polytetrafluoroethylene (PTFE). They have found that the bonding is only achieved after ⇑ Corresponding author at: Department of Engineering Design and Manufacture, University of Malaya, 50603 Kuala Lumpur, Malaysia. Tel.: +60 3 79677633; fax: +60 3 79675330. E-mail address: [email protected] (F. Yusof). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2012.01.006
treating a PTFE surface with natrium. In some other report, Fink et al. [3] have conducted an experimental investigation on bolted joints of carbon-fibre reinforced epoxy (CRFP) and titanium. Although the authors have reported an increase in joining strength, some drawbacks such as long processing time, high production costs and limitations of shape and size were identified. For solving these problems and obtaining high quality joints, laser joining has been considered as one of the promising candidates. In addition, laser transmission welding of polymer is widely applied in many engineering applications [4–9]. A few researchers have also successfully joined polymer and metal using laser transmission welding method. For example, the effect of laser parameters on dissimilar materials joints between polymers (polyimide (KaptonÒ FN) and also TeflonÒ FEP (fluorinated ethylene propylene)) and titanium (Ti) was investigated by Georgiev et al. [10]. KaptonÒ FN/Ti and TeflonÒ FEP/Ti microjoints were fabricated by using focused infrared laser irradiation. The bond strengths for the KaptonÒ FN/Ti and TeflonÒ FEP/Ti joints were found to be 3.32 and 8.48 MPa, respectively. They have also reported that chemical interaction, Ti–F bond was formed in the KaptonÒ FN/Ti joint. The feasibility of laser transmission joining between PET and titanium has been studied by Wang et al. [11]. The authors also discovered the occurrence of Ti–C bond at the joined interface, which may influence the mechanical strength of the joints. The utilization of high power diode laser in the joining of AZ91D with polyethylene terephthalate was reported by Wahba et al. [12]. The authors claimed that the joining mechanism is due to generation of gas bubbles
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F. Yusof et al. / Materials and Design 37 (2012) 410–415
inside the polymer region. These bubbles are expanded during laser joining process and pushed molten polymer to stick on metal surface. Although progressive research work to improve strength of polymer and metal joint have been carried out, the effect of anodizing on the joining surface for laser joining between polymer and metal have still not yet been explored. In the previous research work, the authors have successfully joined polyethylene terephthalate (PET) and aluminium alloy (A5052) using laser transmission welding technique. However, shear strength obtained for PET/ A5052 joint was slightly lower than that for PET/SUS304 joint. In the present study, the anodizing treatment was applied to the joining surface of A5052 sheets for improving shear strength of the PET/A5052 dissimilar materials joint. After laser joining under various welding conditions was performed, joining behaviour and shear strength were investigated in detail and the effectiveness of anodizing was discussed.
Table 1 Laser joining parameters under various heat input and pulse duration. Condition
Heat input E (J)
Pulse duration, t (ms)
A B C D
5.9 18.6 34.9 55.8
5 10 15 20
Table 2 Laser joining parameters for varied pulse duration under constant heat input. Condition
Heat input E (J)
Pulse duration, t (ms)
E F
18.5 18.5
10 20
2. Experimental procedure
Laser beam
2.1. Experimental setup
Clamping plate
PET
The materials used were aluminium alloy (A5052) sheet with thickness of 1 mm and polyethylene terephthalate (PET) sheet with thickness of 0.5 mm. Rectangular specimens with sizes of 20 mm 10 mm were cut from the sheets. Two kinds of A5052 surfaces were prepared: one was as-received (hereafter noted as A5052 (as-received)), and the other was anodized (noted as A5052 (anodized)). The anodizing process of A5052 specimen was accomplished through alumite treatment and comprising the following steps. First the A5052 specimens were cleaned using acetone inside the ultrasonic cleaner in order to remove oil and dirt. Then the specimens were immersed in a sulphuric acid electrolytic bath for 30 min. The temperature of electrolytic solution was controlled at 23°C and voltage during the treatment process was fixed to 12 V. After 30 min, the A5052 specimens were rinsed using distilled water. After that the A5052 specimens were dipped into hot water, which contained a chemical solution to seal the pores. The A5052 surface was finally washed by distilled water and dried by blowing air. Thickness of anodizing layer on the A5052 surface was about 10 lm, as shown in Fig. 1. Laser joining experiments were conducted in ambient air using a pulsed Nd:YAG (k = 1064 nm) laser machine (Toshiba LAY-822H YAG) with fibre-optic beam delivery system and adjustable focal distance. This machine has a maximum crest value of 338 V for laser pumping. The heat input was varied up to 65 J and pulse duration was also varied from 1 to 20 ms. Four levels of heat input were adopted as indicated in Table 1. The lowest heat input was about 6 J and the other levels were as follows: about 18 J (=three times of 6 J), about 36 J (=six times of 6 J) and about 54 J (=nine times of 6 J). In these experiments, the pulse duration was also changed with
Anodizing layer
Substrate (A5052)
A5052 (as-received and anodized)
Base block
Thermocouple Fig. 2. Schematic diagram showing the laser transmission joining experiment.
changing heat input. Therefore, to investigate the effect of pulse duration on shear strength, the experiments with various pulses duration under the constant heat input was also executed. The laser joining conditions for these experiments are listed in Table 2. Lap joining experiments with single pulse were performed in this study. Schematic diagram of laser joining experiment is shown in Fig. 2. Clamping plate was placed on the top of PET and tightly clamped to the base plate. Thus gap between PET and A5052 could be minimized. Focal distance was adjusted into a just focus position at the interface and spot size was about 200 lm. Tensile shear test of the spot welded specimens was carried out using a tensile test machine at crosshead speed of 0.5 mm/min. 2.2. Temperature measurement During laser irradiation, heat will be mainly absorbed into A5052 side by heat conduction or convection. Thus the absorbed heat during laser irradiation can be estimated by measuring the temperature near the joined region. In this study, the temperature was measured using a thermocouple, which was embedded inside a small hole in the A5052 (near welding region) as shown in Fig. 2. A small blind hole of 1 mm diameter and depth of 0.8 mm was drilled on the bottom part of A5052 specimen. Then a K-type thermocouple with a wire diameter of 0.2 mm was attached inside the hole by using a resistance spot welding machine. The temperature vs time diagram during laser joining process was recorded by a computer. 3. Results and discussion 3.1. Shear strength evaluation
Fig. 1. SEM image of anodizing layer on the A5052 surface.
The experimental results for the relationship between heat input (E) and nominal joined area is shown in Fig. 3. The nominal
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10
8
14 PET/A5052 (as-received) PET/A5052 (anodized)
12
t = pulse duration
7
t = 20 ms
6 5
t = 15 ms t = 10 ms
4 3
t = 5 ms
2
Shear strength, MPa
Nominal joined area, mm2
9
PET/A5052 (as-received) PET/A5052 (anodized) t = pulse duration t = 20 ms
10 8 t = 15 ms
6 t = 5 ms
t = 10 ms
4 2
1 0
0
5.9
18.6
34.9
55.8
5.9
18.6
Heat input, J Fig. 3. Relationship between heat input and nominal joined area.
joined area corresponds to the heat affected area at the joined interface. These areas were measured at the interface of joined specimens by observation through the transparent PET side using an optical microscope. It was found from the figure that the nominal joined area was enlarged with increasing heat input and pulse duration. The increase of joined area was almost 25% when the heat input increased by 50%. These results were expected since high heat input would promote more heat generation in the surrounding region of the laser irradiated area and thus spread the joining area. In addition, Ghorbel et al. [13] also found that by increasing irradiation time, larger and deeper weld seem could occur which due to higher energy deposited and enhanced diffusion phenomenon. It was also observed that the joined areas for PET/A5052 (anodized) joints were broader than those for PET/A5052 (as-received) joints by about 23%. According to Xie and Kar [14], the oxidized surface exhibited low reflectivity of laser beam and the reflectivity was decreased to about 35%, depending on the surface oxidation time. This happens due to modification of the texture and crystallographic of the A5052 surface when anodizing layer is employed. Higher laser energy absorption was observed for the anodized surface as compared to as-received surface. The shear strength of the PET/A5052 (anodized and as-received) joints was adopted through the ratio of tensile shear failure load and nominal joined area. The relationship between heat input and shear strength is summarized in Fig. 4. As shown from the figure, the PET/A5052 (anodized) joint showed 36% higher shear strength compared to the PET/A5052 (as-received) joint. It was noticed that the shear strength tended to increase with increasing heat input. It is believed that the heat input would give significant effect on the shear strength in the laser joining experiment. Additional experiments with various pulse durations (10 and 20 ms) under the constant heat input (18.5 J) were performed to confirm whether the pulse duration could influence shear strength. The results are shown in Fig. 5. As shown from the figure, the nominal joined area and the shear strength were almost constant regardless of pulse duration for both the PET/A5052 (as-received) joint and the PET/A5052 (anodized) joint. Lower reflectivity of the anodized surface would contribute to a larger welded area and higher shear strength for the PET/A5052 (anodized) joint as compared to the PET/A5052 (as-received) joint. These results suggested that pulse duration would have insignificant effect on nominal joining area and shear strength for both the PET/A5052 (anodized) joint and the PET/A5052 (as-received) joint. According to Tzeng [15,16], pulse duration would have minimal effect on weldability of the joint (in terms of weld dimension and quality). This probably happens due to short range of pulse duration available for the YAG laser controller system (0.1–20 ms). Thus, it can be
34.9
55.8
Heat input, J Fig. 4. Effect of heat input on shear strength for PET/A5052 (as-received) and PET/ A5052 (anodized) joints.
Nominal joined area (mm2) Shear strength(MPa)
PET/A5052 (as-received) PET/A5052 (anodized) PET/A5052 (as-received) PET/A5052 (anodized)
Pulse duration, ms Fig. 5. Effect of different pulse durations under constant heat input on welded area and shear strength.
concluded that heat input is a dominant parameter for controlling the joining characteristics. In addition, the anodizing surface would contribute better joining characteristics in terms of joined area and shear strength in the PET/A5052 dissimilar materials joint. Cross-sectional observations of selected joining interface of the PET/A5052 (as-received) and PET/A5052 (anodized) joints are shown in Fig. 6. It can be recognized from Fig. 6a that no significant molten pool in A5052 side was observed for PET/A5052 (as-received) joint welded at E = 5.9 J and t = 5 ms, although a PET melt layer with a thickness of approximately 20 lm was visible. On the other hand for the same laser parameters, a small molten pool was formed in the PET/A5052 (anodized) joint, as shown in Fig. 6b. Substantial amount of voids or bubbles were formed in PET side near the joining interface (Fig. 6c and d). The occurrence of these bubbles seemed more significant in PET/A5052 (anodized) joint compared to the PET/A5052 (as-received) joints.
3.2. Factors might influencing shear strength 3.2.1. Molten pool depth (dmp) In the laser welding process, the formation of molten pool is depended on the heat input and heat input distribution. It is believed that the molten pool might affect the joining shear strength. Fig. 7
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F. Yusof et al. / Materials and Design 37 (2012) 410–415
PET/A5052 (as-received)
PET/A5052 (anodized)
(a)
(b)
(c)
(d)
Fig. 6. Cross-sectional observations of PET/A5052 (as-received) and PET/A5052 (anodized) joints, laser joining parameters for (a) and (b) E = 5.9 J, t = 5 ms; (c) and (d) E = 55.8 J, t = 20 ms.
0.3
t = pulse duration 0.2 t = 20 ms
t = 15 ms
0.1
A : E = 5.9 J , t = 5 ms B : E = 18.6 J , t = 10 ms C : E = 24.9 J , t = 15 ms D : E = 55.8 J , t = 20 ms
dmp
PET/A5052 (as-received) PET/A5052 (anodized)
Temperature, °C
Molten pool depth (mm)
PET/A5052 (as-received) PET/A5052 (anodized)
t = 10 ms t = 5 ms
0 5.9
18.6
34.9
55.8
Heat input, J Fig. 7. Depth of molten pool for different heat input.
Time, x10-2 s shows the summary of the molten pool depth for different heat input. From the observation of joined interface, molten pools were found for both joints, with anodized and as-received surfaces except for the PET/A5052 (as-received) joint welded under low heat input and pulse duration (E = 5.9 J, t = 5 ms). It was also noticed, deeper molten pool depth was formed with increasing heat input (Fig. 7). Deeper molten pools were observed in the case of PET/ A5052 (anodized) compared to PET/A5052 (as-received) joints and the shear strength was rather high for the anodized surface compared to the as-received surface. It is surmised that anodized layer will absorb substantial amount of laser energy and created deeper molten pool. Similar finding was found by Huntington and Eagar [17]. They reported that anodized aluminium resulted in deeper keyhole compared to aluminium without anodizing. They also mentioned, anodized aluminium had 20–25% more absorption coefficient of laser light compared to the aluminium without anodizing. Substantial heat energy has been absorbed by
Fig. 8. Temperature distributions for specimen joined at various heat inputs and pulse durations for PET/A5052 (as-received) and PET/A5052 (anodized) joints.
anodized surface and being used to form deeper molten pool. The evidence of these is shown in the result of temperature measurement near joining interface (Figs. 8 and 9). Further discussion on this is written in the next section. In the previous work also, the authors [18] had mentioned the effect of deeper molten pool, which have affect, the shear strength. Shear failure resistance was increased with deeper molten pool and stronger joints would be obtained.
3.2.2. Bubble formation As determined from Fig. 6, circular voids or bubbles were formed in the vicinity of the joined interface in the PET side. It was noticed, the size and density of the voids significantly
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F. Yusof et al. / Materials and Design 37 (2012) 410–415
Temperature, °C
PET/A5052 (anodized) E : E = 18.5 J , t = 10 ms F : E = 18.5 J , t = 20 ms
4% even heat input increment was 30%. This difference may result from non-linear phenomenon during laser irradiation process such as plasma formation, materials evaporation, and melt convection between surface temperature and absorbed heat. Relationship between temperature and time for the PET/A5052 (anodized) joints at different duration times under the same heat input is shown in Fig. 8. As seen from the figure, the temperature–time curves almost coincident each other, which is consistent with the results shown in Fig. 5.
E F
Time, x10-2 s Fig. 9. Temperature evolutions for specimens welded at various pulse duration and fixed heat input for PET/A5052 joint with anodizing layer.
depended on laser parameters. Low heat input produce less bubbles formation at the interface for both the PET/A5052 (anodized) and PET/A5052 (as-received) joints. On the other hand, plenty bubbles were observed when higher heat input was applied, which much more significant in the PET/A5052 (anodized) joint. The size and amount of bubbles might have a significant influence on shear strength. Katayama and Kawahito [19] reported that formation of bubbles during laser welding would improve bonding strength between polymer and metal, since the bubble could have high inner pressure points within the polymer and simultaneously push the molten polymer to stick onto the metal surface. This implies that the bubble formation may play an important role in shear strength. Unfortunately, excessive bubbles may tend to decrease the shear strength as reported by Farazila et al. [18], Niwa et al. [20], and Amanat et al. [21]. It is recommended that the bubble formation should be minimized in order to obtain good joining quality.
3.3. Temperature measurements near joining interface Temperature near joining interface was measured in situ during the laser welding process for extending the explanation regarding molten pool and bubble formation. Relationship between temperature and time is shown in Fig. 8. As noticed from the figure, the maximum (peak) temperature (Tmax) increased with an increase in heat input for both the PET/A5052 (as-received) joint and the PET/A5052 (anodized) joint. It could be also found that Tmax for the PET/A5052 (anodized) joint was higher than that for the PET/ A5052 (as-received) joint. This result clearly indicates the phenomena happen during laser welding process for anodized and nonanodized surface. The anodized surface absorbed more laser energy if compared to as-received surface. This result was also consistent with the result plotted in Fig. 7, where the molten pool for the PET/A5052 (anodized) joint was larger than that for the PET/ A5052 (as-received) joint. In the case of B and C welding conditions (E = 18.6 J and 34.9 J), the molten pool formation in A5052 side was observed even though the peak temperature measured was lower than the melting temperature of A5052. This would result from the fact that the measuring point was 200 lm from the joining interface. It was also discovered that when heat input increased by 20–60%, Tmax increased by about 35–70%. This result is contrast to the findings by Doubenskaia and Smurov [22] who have conducted surface temperature measurements during pulsed laser irradiation using a pyrometer. They claimed that the percentage increment of Tmax was relatively low: the increment was about
4. Conclusions Effect of anodizing on shear strength of dissimilar materials joint between PET (polymer) and A5052 (aluminium alloy) was investigated. The main conclusions obtained are summarized as follows. The shear strength for the joined specimen with anodized joining surface (PET/A5052 (anodized) joint) was significantly higher than that for the joined specimen with as-received joining surface (PET/A5052 (as-received) joint). The shear strength increased with increasing heat input and pulse duration. From the result of the samples joined at different pulse duration under constant heat input, it was found that it was not pulse duration but heat input dominantly that influenced the welded area and shear strength. Depth of molten pool would influence the shear strength due to resistance to shear deformation. Bubbles formed near joining interface in PET side may contribute to higher shear strength since inner pressure will push the molten polymer onto the metal. However, excessive bubble formation will degrade the shear strength.
Acknowledgements High Impact Grant (HIR-MOHE-D000001-16001) from the Ministry of Higher Education Malaysia supported a part of this work. The authors are also indebted Professor Emeritus Dr. K.T. Joseph, University of Malaya, for his important suggestions to improve the language in the manuscript.
References [1] Amancio-Filho ST, dos Santos JF. Joining of polymers and polymers–metal hybrid structures: recent developments and trends. Polym Eng Sci 2009;49:1416–76. [2] Elena D, Lazar A. Adhesive bonding between aluminium and polytetrafluoroethylene. J Mater Process Technol 2003;144–143:191–4. [3] Fink A, Camanho PP, Andres JM, Pfeiffer E, Obst A. Hybrid CFRP/titanium bolted joints: performance assessment and application to a spacecraft payload adaptor. Compos Sci Technol 2010;70:305–17. [4] Potente H, Becker F, Fiegler G, Korte J. Investigations towards application of a new technique on laser transmission welding. Welding World 2001;45:15–20. [5] Jones I. Laser welding for plastic components. Assembly Autom 2002;22:129–35. [6] Nakamura H, Masaki T. Plastic welding with diode lasers. J Jpn Weld Soc 2003;72(3):125–36. [7] Brian GB. Welding of plastics with high power diode lasers. Ind Robot: Int J 2004;31:30–3. [8] Boehm AJ, Edison M. Laser transmission welding of plastic in medical device manufacturing – how to choose the right laser additive. Joi Med Plast Weld Fail Prevent 2005:69–71. [9] Edmund H, Wolf-Martin H, Reinhart P, Fahri S. Laser transm joi microtechnol. Microsyst Technol 2006:12632–9. [10] Georgiev GL, Sultana T, Baird RJ, Auner G, Newaz G, Patwa R, et al. Laser bonding and characterization of KaptonÒ FN/Ti and TeflonÒ FEP/Ti systems. J Mater Sci 2009;44:882–8. [11] Wang X, Li P, Xu ZK, Song XH, Liu HX. Laser transmission joint between PET and titanium for biomedical application. J Mater Process Technol 2010;210:1767–71. [12] Wahba M, Kawahito Y, Katayama S. Laser direct joining of AZ91D thixomolded Mg alloy and amorphous polyethylene terephthalate. J Mater Process Technol 2011;211:1166–74.
F. Yusof et al. / Materials and Design 37 (2012) 410–415 [13] Ghorbel E, Casalino G, Abed S. Laser diode transmission welding of polypropylene: geometrical and microstructure characterisation of weld. Mater Design 2009;30:2745–51. [14] Xie J, Kar A. Laser welding of thin sheet steel with surface oxidation. Weld Res Suppl 1999:343–8. [15] Tzeng Y-F. Pulsed Nd:YAG laser seam welding of zinc-coated steel. Weld Res Suppl 1999:238-s–44-s. [16] Tzeng Y-F. Effect of operating parameters on surface quality for the pulsed laser welding of zinc-coated steel. J Mater Process Technol 2000;100: 163–70. [17] Huntington CA, Eagar TW. Laser welding of aluminium and aluminium alloys. Weld Res Suppl 1993:105-s–7-s.
415
[18] Farazila Y, Miyashita Y, Hua W, Mutoh Y, Otsuka Y. YAG laser spot welding of PET and metallic materials. J Laser Micro/Nanoeng 2011;6:69–74. [19] Katayama S, Kawahito Y. Laser direct joining of metal and plastic. Scr Mater 2008;59:1247–50. [20] Niwa Y, Kawahito Y, Kubota S, Katayama S. Evolution of LAMP joining to dissimilar metal welding. In: Proceedings of 27th international congress on applications of lasers & electro-optics (ICALEO 2008). p. 311–7. [21] Amanat N, Chaminade C, Grace J, Mc Kenzie DR, James NL. Transmission laser welding of amorphous and semi-crystalline poly-ether–ether–ketone for applications in the medical industry. Mater Des 2010;31:4823–30. [22] Doubenskaia M, Smurov I. Surface temperature evolution in pulsed laser action of millisecond range. Appl Surf Sci 2006;252:4472–6.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of anodizing on pulsed Nd:YAG laser joining of polyethylene terephthalate (PET) and aluminium alloy (A5052) Farazila Yusof a,c,⇑, Miyashita Yukio b, Mutoh Yoshiharu b, Mohd Hamdi Abdul Shukor a,c a
Department of Engineering Design and Manufacture, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of System Safety, Nagaoka University of Technology, 940-2188 Nagaoka, Japan c Center of Advanced Manufacturing & Material Processing (AMMP Centre), University of Malaya, 50603 Kuala Lumpur, Malaysia b
a r t i c l e
i n f o
Article history: Received 29 November 2011 Accepted 4 January 2012 Available online 18 January 2012 Keywords: A. Multi-materials C. Surface treatment D. Welding
a b s t r a c t A series of laser joining experiments between polyethylene terephthalate (PET) and aluminium alloy (A5052) were conducted to investigate the effect of anodizing on A5052 surface on dissimilar materials used in joining. In this study, PET/A5052 joints with anodized A5052 surface exhibited greater shear strength compared to PET/A5052 joints without anodizing. The shear strength of the joints was increased with increasing of heat input and pulse duration. This indicates that the anodizing process could improve shear strength of the laser joining specimens. Significant molten pools were formed in both PET/A5052 (anodized) and PET/A5052 (as-received) joints except for PET/A5052 (as-received) sample joined at the lowest heat input and pulse duration. For the test results from laser joining under different pulse duration at the constant heat input, it was shown that joining behaviour was dominantly controlled by heat input and not by pulse duration. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Demand on dissimilar material joints have been increasing from the viewpoint of design requirements, energy consumption, environmental concerns, high performance, cost saving, etc. For example, unique dissimilar materials when combined between polymers and metals would provide advantages in numerous engineering applications. Metals have common properties such as high strength, excellent heat and electrical conductivity. On the other hand polymers offer excellent corrosion resistance, insulation, and are lightweight. Dissimilar materials joint between polymer and metal can be obtained from both the two different properties at the same time. However, joining between polymer and metal by common welding procedures is normally difficult due to incompatible structural, physical and chemical properties. Adhesives and screws are commonly applied in joining between polymer and metal. Amancio-Filho and dos Santos [1] reviewed the recent developments of polymer–metal hybrid joining using various joining techniques such as adhesive bonding, mechanical fastening, and welding. Elena and Lazar [2] reported the use of adhesive bonding for joining aluminium and polytetrafluoroethylene (PTFE). They have found that the bonding is only achieved after ⇑ Corresponding author at: Department of Engineering Design and Manufacture, University of Malaya, 50603 Kuala Lumpur, Malaysia. Tel.: +60 3 79677633; fax: +60 3 79675330. E-mail address: [email protected] (F. Yusof). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2012.01.006
treating a PTFE surface with natrium. In some other report, Fink et al. [3] have conducted an experimental investigation on bolted joints of carbon-fibre reinforced epoxy (CRFP) and titanium. Although the authors have reported an increase in joining strength, some drawbacks such as long processing time, high production costs and limitations of shape and size were identified. For solving these problems and obtaining high quality joints, laser joining has been considered as one of the promising candidates. In addition, laser transmission welding of polymer is widely applied in many engineering applications [4–9]. A few researchers have also successfully joined polymer and metal using laser transmission welding method. For example, the effect of laser parameters on dissimilar materials joints between polymers (polyimide (KaptonÒ FN) and also TeflonÒ FEP (fluorinated ethylene propylene)) and titanium (Ti) was investigated by Georgiev et al. [10]. KaptonÒ FN/Ti and TeflonÒ FEP/Ti microjoints were fabricated by using focused infrared laser irradiation. The bond strengths for the KaptonÒ FN/Ti and TeflonÒ FEP/Ti joints were found to be 3.32 and 8.48 MPa, respectively. They have also reported that chemical interaction, Ti–F bond was formed in the KaptonÒ FN/Ti joint. The feasibility of laser transmission joining between PET and titanium has been studied by Wang et al. [11]. The authors also discovered the occurrence of Ti–C bond at the joined interface, which may influence the mechanical strength of the joints. The utilization of high power diode laser in the joining of AZ91D with polyethylene terephthalate was reported by Wahba et al. [12]. The authors claimed that the joining mechanism is due to generation of gas bubbles
411
F. Yusof et al. / Materials and Design 37 (2012) 410–415
inside the polymer region. These bubbles are expanded during laser joining process and pushed molten polymer to stick on metal surface. Although progressive research work to improve strength of polymer and metal joint have been carried out, the effect of anodizing on the joining surface for laser joining between polymer and metal have still not yet been explored. In the previous research work, the authors have successfully joined polyethylene terephthalate (PET) and aluminium alloy (A5052) using laser transmission welding technique. However, shear strength obtained for PET/ A5052 joint was slightly lower than that for PET/SUS304 joint. In the present study, the anodizing treatment was applied to the joining surface of A5052 sheets for improving shear strength of the PET/A5052 dissimilar materials joint. After laser joining under various welding conditions was performed, joining behaviour and shear strength were investigated in detail and the effectiveness of anodizing was discussed.
Table 1 Laser joining parameters under various heat input and pulse duration. Condition
Heat input E (J)
Pulse duration, t (ms)
A B C D
5.9 18.6 34.9 55.8
5 10 15 20
Table 2 Laser joining parameters for varied pulse duration under constant heat input. Condition
Heat input E (J)
Pulse duration, t (ms)
E F
18.5 18.5
10 20
2. Experimental procedure
Laser beam
2.1. Experimental setup
Clamping plate
PET
The materials used were aluminium alloy (A5052) sheet with thickness of 1 mm and polyethylene terephthalate (PET) sheet with thickness of 0.5 mm. Rectangular specimens with sizes of 20 mm 10 mm were cut from the sheets. Two kinds of A5052 surfaces were prepared: one was as-received (hereafter noted as A5052 (as-received)), and the other was anodized (noted as A5052 (anodized)). The anodizing process of A5052 specimen was accomplished through alumite treatment and comprising the following steps. First the A5052 specimens were cleaned using acetone inside the ultrasonic cleaner in order to remove oil and dirt. Then the specimens were immersed in a sulphuric acid electrolytic bath for 30 min. The temperature of electrolytic solution was controlled at 23°C and voltage during the treatment process was fixed to 12 V. After 30 min, the A5052 specimens were rinsed using distilled water. After that the A5052 specimens were dipped into hot water, which contained a chemical solution to seal the pores. The A5052 surface was finally washed by distilled water and dried by blowing air. Thickness of anodizing layer on the A5052 surface was about 10 lm, as shown in Fig. 1. Laser joining experiments were conducted in ambient air using a pulsed Nd:YAG (k = 1064 nm) laser machine (Toshiba LAY-822H YAG) with fibre-optic beam delivery system and adjustable focal distance. This machine has a maximum crest value of 338 V for laser pumping. The heat input was varied up to 65 J and pulse duration was also varied from 1 to 20 ms. Four levels of heat input were adopted as indicated in Table 1. The lowest heat input was about 6 J and the other levels were as follows: about 18 J (=three times of 6 J), about 36 J (=six times of 6 J) and about 54 J (=nine times of 6 J). In these experiments, the pulse duration was also changed with
Anodizing layer
Substrate (A5052)
A5052 (as-received and anodized)
Base block
Thermocouple Fig. 2. Schematic diagram showing the laser transmission joining experiment.
changing heat input. Therefore, to investigate the effect of pulse duration on shear strength, the experiments with various pulses duration under the constant heat input was also executed. The laser joining conditions for these experiments are listed in Table 2. Lap joining experiments with single pulse were performed in this study. Schematic diagram of laser joining experiment is shown in Fig. 2. Clamping plate was placed on the top of PET and tightly clamped to the base plate. Thus gap between PET and A5052 could be minimized. Focal distance was adjusted into a just focus position at the interface and spot size was about 200 lm. Tensile shear test of the spot welded specimens was carried out using a tensile test machine at crosshead speed of 0.5 mm/min. 2.2. Temperature measurement During laser irradiation, heat will be mainly absorbed into A5052 side by heat conduction or convection. Thus the absorbed heat during laser irradiation can be estimated by measuring the temperature near the joined region. In this study, the temperature was measured using a thermocouple, which was embedded inside a small hole in the A5052 (near welding region) as shown in Fig. 2. A small blind hole of 1 mm diameter and depth of 0.8 mm was drilled on the bottom part of A5052 specimen. Then a K-type thermocouple with a wire diameter of 0.2 mm was attached inside the hole by using a resistance spot welding machine. The temperature vs time diagram during laser joining process was recorded by a computer. 3. Results and discussion 3.1. Shear strength evaluation
Fig. 1. SEM image of anodizing layer on the A5052 surface.
The experimental results for the relationship between heat input (E) and nominal joined area is shown in Fig. 3. The nominal
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10
8
14 PET/A5052 (as-received) PET/A5052 (anodized)
12
t = pulse duration
7
t = 20 ms
6 5
t = 15 ms t = 10 ms
4 3
t = 5 ms
2
Shear strength, MPa
Nominal joined area, mm2
9
PET/A5052 (as-received) PET/A5052 (anodized) t = pulse duration t = 20 ms
10 8 t = 15 ms
6 t = 5 ms
t = 10 ms
4 2
1 0
0
5.9
18.6
34.9
55.8
5.9
18.6
Heat input, J Fig. 3. Relationship between heat input and nominal joined area.
joined area corresponds to the heat affected area at the joined interface. These areas were measured at the interface of joined specimens by observation through the transparent PET side using an optical microscope. It was found from the figure that the nominal joined area was enlarged with increasing heat input and pulse duration. The increase of joined area was almost 25% when the heat input increased by 50%. These results were expected since high heat input would promote more heat generation in the surrounding region of the laser irradiated area and thus spread the joining area. In addition, Ghorbel et al. [13] also found that by increasing irradiation time, larger and deeper weld seem could occur which due to higher energy deposited and enhanced diffusion phenomenon. It was also observed that the joined areas for PET/A5052 (anodized) joints were broader than those for PET/A5052 (as-received) joints by about 23%. According to Xie and Kar [14], the oxidized surface exhibited low reflectivity of laser beam and the reflectivity was decreased to about 35%, depending on the surface oxidation time. This happens due to modification of the texture and crystallographic of the A5052 surface when anodizing layer is employed. Higher laser energy absorption was observed for the anodized surface as compared to as-received surface. The shear strength of the PET/A5052 (anodized and as-received) joints was adopted through the ratio of tensile shear failure load and nominal joined area. The relationship between heat input and shear strength is summarized in Fig. 4. As shown from the figure, the PET/A5052 (anodized) joint showed 36% higher shear strength compared to the PET/A5052 (as-received) joint. It was noticed that the shear strength tended to increase with increasing heat input. It is believed that the heat input would give significant effect on the shear strength in the laser joining experiment. Additional experiments with various pulse durations (10 and 20 ms) under the constant heat input (18.5 J) were performed to confirm whether the pulse duration could influence shear strength. The results are shown in Fig. 5. As shown from the figure, the nominal joined area and the shear strength were almost constant regardless of pulse duration for both the PET/A5052 (as-received) joint and the PET/A5052 (anodized) joint. Lower reflectivity of the anodized surface would contribute to a larger welded area and higher shear strength for the PET/A5052 (anodized) joint as compared to the PET/A5052 (as-received) joint. These results suggested that pulse duration would have insignificant effect on nominal joining area and shear strength for both the PET/A5052 (anodized) joint and the PET/A5052 (as-received) joint. According to Tzeng [15,16], pulse duration would have minimal effect on weldability of the joint (in terms of weld dimension and quality). This probably happens due to short range of pulse duration available for the YAG laser controller system (0.1–20 ms). Thus, it can be
34.9
55.8
Heat input, J Fig. 4. Effect of heat input on shear strength for PET/A5052 (as-received) and PET/ A5052 (anodized) joints.
Nominal joined area (mm2) Shear strength(MPa)
PET/A5052 (as-received) PET/A5052 (anodized) PET/A5052 (as-received) PET/A5052 (anodized)
Pulse duration, ms Fig. 5. Effect of different pulse durations under constant heat input on welded area and shear strength.
concluded that heat input is a dominant parameter for controlling the joining characteristics. In addition, the anodizing surface would contribute better joining characteristics in terms of joined area and shear strength in the PET/A5052 dissimilar materials joint. Cross-sectional observations of selected joining interface of the PET/A5052 (as-received) and PET/A5052 (anodized) joints are shown in Fig. 6. It can be recognized from Fig. 6a that no significant molten pool in A5052 side was observed for PET/A5052 (as-received) joint welded at E = 5.9 J and t = 5 ms, although a PET melt layer with a thickness of approximately 20 lm was visible. On the other hand for the same laser parameters, a small molten pool was formed in the PET/A5052 (anodized) joint, as shown in Fig. 6b. Substantial amount of voids or bubbles were formed in PET side near the joining interface (Fig. 6c and d). The occurrence of these bubbles seemed more significant in PET/A5052 (anodized) joint compared to the PET/A5052 (as-received) joints.
3.2. Factors might influencing shear strength 3.2.1. Molten pool depth (dmp) In the laser welding process, the formation of molten pool is depended on the heat input and heat input distribution. It is believed that the molten pool might affect the joining shear strength. Fig. 7
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PET/A5052 (as-received)
PET/A5052 (anodized)
(a)
(b)
(c)
(d)
Fig. 6. Cross-sectional observations of PET/A5052 (as-received) and PET/A5052 (anodized) joints, laser joining parameters for (a) and (b) E = 5.9 J, t = 5 ms; (c) and (d) E = 55.8 J, t = 20 ms.
0.3
t = pulse duration 0.2 t = 20 ms
t = 15 ms
0.1
A : E = 5.9 J , t = 5 ms B : E = 18.6 J , t = 10 ms C : E = 24.9 J , t = 15 ms D : E = 55.8 J , t = 20 ms
dmp
PET/A5052 (as-received) PET/A5052 (anodized)
Temperature, °C
Molten pool depth (mm)
PET/A5052 (as-received) PET/A5052 (anodized)
t = 10 ms t = 5 ms
0 5.9
18.6
34.9
55.8
Heat input, J Fig. 7. Depth of molten pool for different heat input.
Time, x10-2 s shows the summary of the molten pool depth for different heat input. From the observation of joined interface, molten pools were found for both joints, with anodized and as-received surfaces except for the PET/A5052 (as-received) joint welded under low heat input and pulse duration (E = 5.9 J, t = 5 ms). It was also noticed, deeper molten pool depth was formed with increasing heat input (Fig. 7). Deeper molten pools were observed in the case of PET/ A5052 (anodized) compared to PET/A5052 (as-received) joints and the shear strength was rather high for the anodized surface compared to the as-received surface. It is surmised that anodized layer will absorb substantial amount of laser energy and created deeper molten pool. Similar finding was found by Huntington and Eagar [17]. They reported that anodized aluminium resulted in deeper keyhole compared to aluminium without anodizing. They also mentioned, anodized aluminium had 20–25% more absorption coefficient of laser light compared to the aluminium without anodizing. Substantial heat energy has been absorbed by
Fig. 8. Temperature distributions for specimen joined at various heat inputs and pulse durations for PET/A5052 (as-received) and PET/A5052 (anodized) joints.
anodized surface and being used to form deeper molten pool. The evidence of these is shown in the result of temperature measurement near joining interface (Figs. 8 and 9). Further discussion on this is written in the next section. In the previous work also, the authors [18] had mentioned the effect of deeper molten pool, which have affect, the shear strength. Shear failure resistance was increased with deeper molten pool and stronger joints would be obtained.
3.2.2. Bubble formation As determined from Fig. 6, circular voids or bubbles were formed in the vicinity of the joined interface in the PET side. It was noticed, the size and density of the voids significantly
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Temperature, °C
PET/A5052 (anodized) E : E = 18.5 J , t = 10 ms F : E = 18.5 J , t = 20 ms
4% even heat input increment was 30%. This difference may result from non-linear phenomenon during laser irradiation process such as plasma formation, materials evaporation, and melt convection between surface temperature and absorbed heat. Relationship between temperature and time for the PET/A5052 (anodized) joints at different duration times under the same heat input is shown in Fig. 8. As seen from the figure, the temperature–time curves almost coincident each other, which is consistent with the results shown in Fig. 5.
E F
Time, x10-2 s Fig. 9. Temperature evolutions for specimens welded at various pulse duration and fixed heat input for PET/A5052 joint with anodizing layer.
depended on laser parameters. Low heat input produce less bubbles formation at the interface for both the PET/A5052 (anodized) and PET/A5052 (as-received) joints. On the other hand, plenty bubbles were observed when higher heat input was applied, which much more significant in the PET/A5052 (anodized) joint. The size and amount of bubbles might have a significant influence on shear strength. Katayama and Kawahito [19] reported that formation of bubbles during laser welding would improve bonding strength between polymer and metal, since the bubble could have high inner pressure points within the polymer and simultaneously push the molten polymer to stick onto the metal surface. This implies that the bubble formation may play an important role in shear strength. Unfortunately, excessive bubbles may tend to decrease the shear strength as reported by Farazila et al. [18], Niwa et al. [20], and Amanat et al. [21]. It is recommended that the bubble formation should be minimized in order to obtain good joining quality.
3.3. Temperature measurements near joining interface Temperature near joining interface was measured in situ during the laser welding process for extending the explanation regarding molten pool and bubble formation. Relationship between temperature and time is shown in Fig. 8. As noticed from the figure, the maximum (peak) temperature (Tmax) increased with an increase in heat input for both the PET/A5052 (as-received) joint and the PET/A5052 (anodized) joint. It could be also found that Tmax for the PET/A5052 (anodized) joint was higher than that for the PET/ A5052 (as-received) joint. This result clearly indicates the phenomena happen during laser welding process for anodized and nonanodized surface. The anodized surface absorbed more laser energy if compared to as-received surface. This result was also consistent with the result plotted in Fig. 7, where the molten pool for the PET/A5052 (anodized) joint was larger than that for the PET/ A5052 (as-received) joint. In the case of B and C welding conditions (E = 18.6 J and 34.9 J), the molten pool formation in A5052 side was observed even though the peak temperature measured was lower than the melting temperature of A5052. This would result from the fact that the measuring point was 200 lm from the joining interface. It was also discovered that when heat input increased by 20–60%, Tmax increased by about 35–70%. This result is contrast to the findings by Doubenskaia and Smurov [22] who have conducted surface temperature measurements during pulsed laser irradiation using a pyrometer. They claimed that the percentage increment of Tmax was relatively low: the increment was about
4. Conclusions Effect of anodizing on shear strength of dissimilar materials joint between PET (polymer) and A5052 (aluminium alloy) was investigated. The main conclusions obtained are summarized as follows. The shear strength for the joined specimen with anodized joining surface (PET/A5052 (anodized) joint) was significantly higher than that for the joined specimen with as-received joining surface (PET/A5052 (as-received) joint). The shear strength increased with increasing heat input and pulse duration. From the result of the samples joined at different pulse duration under constant heat input, it was found that it was not pulse duration but heat input dominantly that influenced the welded area and shear strength. Depth of molten pool would influence the shear strength due to resistance to shear deformation. Bubbles formed near joining interface in PET side may contribute to higher shear strength since inner pressure will push the molten polymer onto the metal. However, excessive bubble formation will degrade the shear strength.
Acknowledgements High Impact Grant (HIR-MOHE-D000001-16001) from the Ministry of Higher Education Malaysia supported a part of this work. The authors are also indebted Professor Emeritus Dr. K.T. Joseph, University of Malaya, for his important suggestions to improve the language in the manuscript.
References [1] Amancio-Filho ST, dos Santos JF. Joining of polymers and polymers–metal hybrid structures: recent developments and trends. Polym Eng Sci 2009;49:1416–76. [2] Elena D, Lazar A. Adhesive bonding between aluminium and polytetrafluoroethylene. J Mater Process Technol 2003;144–143:191–4. [3] Fink A, Camanho PP, Andres JM, Pfeiffer E, Obst A. Hybrid CFRP/titanium bolted joints: performance assessment and application to a spacecraft payload adaptor. Compos Sci Technol 2010;70:305–17. [4] Potente H, Becker F, Fiegler G, Korte J. Investigations towards application of a new technique on laser transmission welding. Welding World 2001;45:15–20. [5] Jones I. Laser welding for plastic components. Assembly Autom 2002;22:129–35. [6] Nakamura H, Masaki T. Plastic welding with diode lasers. J Jpn Weld Soc 2003;72(3):125–36. [7] Brian GB. Welding of plastics with high power diode lasers. Ind Robot: Int J 2004;31:30–3. [8] Boehm AJ, Edison M. Laser transmission welding of plastic in medical device manufacturing – how to choose the right laser additive. Joi Med Plast Weld Fail Prevent 2005:69–71. [9] Edmund H, Wolf-Martin H, Reinhart P, Fahri S. Laser transm joi microtechnol. Microsyst Technol 2006:12632–9. [10] Georgiev GL, Sultana T, Baird RJ, Auner G, Newaz G, Patwa R, et al. Laser bonding and characterization of KaptonÒ FN/Ti and TeflonÒ FEP/Ti systems. J Mater Sci 2009;44:882–8. [11] Wang X, Li P, Xu ZK, Song XH, Liu HX. Laser transmission joint between PET and titanium for biomedical application. J Mater Process Technol 2010;210:1767–71. [12] Wahba M, Kawahito Y, Katayama S. Laser direct joining of AZ91D thixomolded Mg alloy and amorphous polyethylene terephthalate. J Mater Process Technol 2011;211:1166–74.
F. Yusof et al. / Materials and Design 37 (2012) 410–415 [13] Ghorbel E, Casalino G, Abed S. Laser diode transmission welding of polypropylene: geometrical and microstructure characterisation of weld. Mater Design 2009;30:2745–51. [14] Xie J, Kar A. Laser welding of thin sheet steel with surface oxidation. Weld Res Suppl 1999:343–8. [15] Tzeng Y-F. Pulsed Nd:YAG laser seam welding of zinc-coated steel. Weld Res Suppl 1999:238-s–44-s. [16] Tzeng Y-F. Effect of operating parameters on surface quality for the pulsed laser welding of zinc-coated steel. J Mater Process Technol 2000;100: 163–70. [17] Huntington CA, Eagar TW. Laser welding of aluminium and aluminium alloys. Weld Res Suppl 1993:105-s–7-s.
415
[18] Farazila Y, Miyashita Y, Hua W, Mutoh Y, Otsuka Y. YAG laser spot welding of PET and metallic materials. J Laser Micro/Nanoeng 2011;6:69–74. [19] Katayama S, Kawahito Y. Laser direct joining of metal and plastic. Scr Mater 2008;59:1247–50. [20] Niwa Y, Kawahito Y, Kubota S, Katayama S. Evolution of LAMP joining to dissimilar metal welding. In: Proceedings of 27th international congress on applications of lasers & electro-optics (ICALEO 2008). p. 311–7. [21] Amanat N, Chaminade C, Grace J, Mc Kenzie DR, James NL. Transmission laser welding of amorphous and semi-crystalline poly-ether–ether–ketone for applications in the medical industry. Mater Des 2010;31:4823–30. [22] Doubenskaia M, Smurov I. Surface temperature evolution in pulsed laser action of millisecond range. Appl Surf Sci 2006;252:4472–6.