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Experimental and simulative investigation of the effects of laser-structured metal surface on metal-polymer direct joining
Journal Pre-proof Experimental and simulative investigation of the effects of laser-structured metal surface on metal-polymer direct joining Kakeru Enami, Fuminobu Kimura, Keisuke Yokoyama, Takeshi Murakami, Yusuke Kajihara PII:
S0141-6359(19)30915-8
DOI:
https://doi.org/10.1016/j.precisioneng.2019.12.011
Reference:
PRE 7082
To appear in:
Precision Engineering
Received Date: 16 June 2019 Revised Date:
9 November 2019
Accepted Date: 28 November 2019
Please cite this article as: Enami K, Kimura F, Yokoyama K, Murakami T, Kajihara Y, Experimental and simulative investigation of the effects of laser-structured metal surface on metal-polymer direct joining, Precision Engineering (2020), doi: https://doi.org/10.1016/j.precisioneng.2019.12.011. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Experimental and simulative investigation of the effects of laser-structured metal surface on metal-polymer direct joining
Kakeru Enamia*, Fuminobu Kimurab, Keisuke Yokoyamaa, Takeshi Murakamia and Yusuke Kajiharab
a
Core Technology R & D Center, NSK Ltd., 1-5-50 Kugenumashinmei, Fujisawa-Shi, Kanagawa
251-8501, Japan b
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505,
Japan
*
Corresponding author: [email protected]
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Abstract Direct joining of metal and polymer consists of surface treatment and injection molding techniques. This study employed a laser machining as the surface treatment to form periodic dimples on a joining surface. We experimentally investigated the effect of surface structures on joining strength under a constant molding condition; we focused on the diameter, aspect ratio and periodicity of the dimple, and the surface texture on its inner wall. Furthermore, we analyzed the effect of the surface texture on the inner wall of the dimple on the joining strength with finite element method. We found that the joining strength became higher with increasing number of dimples when total machined area was same. Another finding was that the joining strength increased with increasing aspect ratio and was saturated above the aspect ratio of 0.6. Furthermore, the experimental and simulated results strongly suggested that roughness on the inner wall of the dimple played an important role to enhance the joining strength. The findings of this study will promote the optimization of the metal surface structure for the metal-polymer direct joining.
Keywords Metal-polymer direct joining, Injection molding, Laser-structuring, Finite element analysis
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1. Introduction Metal materials are widely used for structural components in an automotive industry due to their high strength, high stiffness, and high thermal and heat conductivities. From the view point of the weight saving, many concerns still remain because of their high specific gravities. In order to reduce weight further, metal materials are recently replaced by polymer materials. However, there are some problems to be overcome. Since the mechanical properties of polymer materials are inferior to those of metal materials, polymer materials can not totally replace metal materials. Therefore, joining techniques of dissimilar materials are indispensable because composite materials inevitably have joining points. Conventional methods to obtain composites of metal and polymer parts are adhesive bonding or mechanical fastening using bolts or screws [1-3]. These methods often lead to an increase in manufacturing processes. Thus, direct joining techniques between metal and polymer parts without any extra parts are strongly required [4]. Many researchers have recently been studying the direct joining techniques and proposed various methods. One of the metal-polymer direct joining techniques is an injection-molded direct joining (IMDJ). Figure 1 illustrates a schematic of IMDJ. This joining technique consists of a combination of a surface treatment and an injection molding. The joining process is as follows: (i) forming micro/nano structures on the metal part, (ii) injection molding after the surface-treated metal part is fixed in a mold, and (iii) molten polymer fills into the structures and polymer is cooled. Thus, metal and polymer parts are strongly joined mainly via mechanical interlocking. This study focuses on IMDJ since it allows flexible composite structures with high throughput. Although the IMDJ technique is quite promising for weight saving, it is not widely employed in the automotive industry. This is mainly due to its low reliability which comes from the followings: (i) the optimum joining condition is not clear and (ii) the joining mechanism is not clear. To improve the reliability, it is important to understand how the surface structures and molding conditions affect the joining of metal and polymer parts. The surface treatment method is classified into two methods as a dry processing [5-10] and a wet processing [11-16]. As for the dry processing, laser machining forms surface structures with the size of several dozen micrometers [5-7] and abrasive blasting forms the ones with the size of several micrometers [8-10]. As for wet processing, a chemical etching called NMT (nano molding technology) method forms nanoscale porous structures [11]. A surface treatment by anodization which forms nanoscale porous layers on aluminum alloys is also used for the IMDJ [16]. Both dry and wet methods achieve the strong joint. When we focus on understanding the relationship between the surface structure and joining strength, well-defined surface structure is much more appropriate. Wet processing forms complex surface structures like an undercut-shaped, whereas dry processing forms geometrically well-defined structures.
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The controlled structures promote a better understanding of the joining mechanism. Therefore, we utilize a laser machining technique which is one of the dry processing techniques. Laser machining enables to geometrically control surface structures by adjusting laser machining conditions such as a laser power, a number of laser shots, and a machining periodicity. In addition, laser machines have a compatibility with a manufacturing process since they do not need any equipment for chemicals. This study employs a picosecond laser, a nanosecond laser and a continuous wave (CW) laser because these laser machines are widely used in the industry. It is known that ultra-short pulse laser forms self-organized periodic patterns [17-19]. The structure can contribute to the enhancement of the joining strength. Thus, the picosecond pulse laser is used. Some research groups investigated the effect of the periodicity and depth of the laser-structured surface on the joining strength [5-7]. However, other parameters including the machining width, aspect ratio and surface texture on the machined area are not fully investigated. These structures will also affect the mechanical interlocking between metal and polymer. To elucidate the relationship between the surface structures and the joining strength, a numerical analysis using finite element method (FEM) is also effective. Engelmann et al. investigated how the joining strength was influenced by parameters such as the amount and geometries of the structures under different load directions [20]. They revealed that the parameters affected the stress acting in the FEM model. However, they did not go into the details how the stress was affected by the parameters. In this study, we formed periodic dimples on metal plates by laser machining. We focused on the following surface parameters: the diameter, aspect ratio, periodicity of the dimple, and the surface texture on its inner wall. Changing the diameter of the dimple changes the maximum number of dimples which can be formed on the joining area. The smaller the dimple diameter is, the larger the number of the dimple is on the joining area. The number of dimples can affect the stress distribution on the joining area. The role of the aspect ratio has been investigated using abrasive blasted metals by Kajihara et al. They reported that the surface structure with the aspect ratio over a certain value contributed to the joining [10]. The periodicity of each dimple is related to the number of dimples on the joining area. The larger number of dimples should enhance the load-bearing capacity [7]. Furthermore, the surface texture on the inner wall of the dimple can disperse the stress in the polymer filled into the dimple. Thus, these parameters are expected to affect the joining strength. The aim of this study is to reveal the effects of surface structures on the joining strength. Investigating the relationship between the parameters mentioned above and joining strength offers further knowledge for the optimization of the surface structures. To change the surface texture on the inner wall of the dimple, we used different oscillation types of laser machines. After treating the metal surface with each
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laser machine, we manufactured lap joint specimens under a constant molding condition and then carried out tensile-shear tests. We also investigated the effect of the surface texture on the inner wall of the dimple on the joining strength with FEM. With the experimental and FEM analyses, we discussed how the surface structures affected the joining strength.
2. Experimental 2.1 Laser machining In this study, we used the picosecond laser, the nanosecond laser, and the CW laser to form periodic dimples at vertexes of a square lattice on the metal plates. The dimple structure enables to easily analyze the effects of its diameter, aspect ratio, and periodicity on the joining strength since the geometries are well defined. In the laser machining process, we controlled pulse width τ, wavelength λ, spot diameter φspot, peak power Ppeak, number of laser shots Nshot, and periodicity of the dimple, dperiod. The diameter of the dimple is controlled with Ppeak and φspot, while the depth of the dimple is controlled with Nshot. The laser beam was irradiated at the same point repeatedly to form the dimple in the depth direction. Metal plates were made of aluminum alloy, A5052. The tensile strength of A5052 was 260 MPa [21]. Prior to the injection molding, the metal plates were cleaned with acetone ultrasonically.
2.2 Direct joining by injection molding We manufactured a lap joint specimen to evaluate the joining strength of metal and polymer parts. Figure 2 illustrates the geometry of the specimen based on the ISO 19095-2 [22]. The joining area of metal and polymer parts is 5 mm × 10 mm. Figure 3 shows a schematic illustration of a mold used to manufacture the lap joint specimen. After the metal plate is set in the mold, molten polymer is injected into the mold. The mold has pressure and temperature sensors near the joining area for realtime monitoring of the parameters. To enhance a replication of the polymer to the dimple, we conducted the injection molding under an over-packing condition: the volume of injected polymer was larger than the mold cavity value. Therefore, the polymer pressure rose immediately after the molten polymer reached the end of the mold cavity. The peak value of the pressure, which we described packing pressure, was controlled by changing the volume of injected polymer. The larger the volume of injected polymer became, the higher the packing pressure became. Molding conditions are summarized in Table 1. In this study, the molding conditions are constant because we focus on the effect of the diameter, aspect ratio, periodicity of the dimple, and the surface texture on the inner wall of the dimple on the joining strength. The polymer material was polybutylene terephthalate (PBT) containing 30 wt% of glass fibers and its tensile strength was 120 MPa. Polymer
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pellets were dried at 130 °C for three hours or more before the injection molding. After the injection molding, the joining specimens were annealed at 130 °C for four hours to remove residual stresses.
2.3 Evaluation of joining strength We carried out tensile shear tests to evaluate the joining strength of the lap joint specimen. In this study, we utilized an original tensile tester [23] to correctly evaluate the tensile shear strength without unwanted deformation. The tensile tester applied a displacement to the lap joint specimen at a constant speed of 1 mm/min and recorded a load during the tensile test. The joining strength is the maximum load divided by the joining area (5 mm × 10 mm).
3. Experimental results 3.1 Effect of dimple diameter We formed dimples with the diameter of 40 and 80 µm on different metal plates and investigated the effect of dimple diameter on the joining strength. The nanosecond laser was used under the conditions listed in Table 2. Conditions A and B are for the diameter of 40 and 80 µm, respectively. To compare only the effect of different dimple diameter, the total machined area should be similar. Figure 4 shows a schematic illustration of metal plates with the same total machined area of dimples. Here the total machined area of dimples is defined as a summation of a project area of each dimple on the joining surface; that is, it is calculated by Ndimple πφ2dimple ⁄4, where Ndimple and φdimple are the number of dimples and the diameter of the dimples, respectively. The total machined area is equalized by controlling Ndimple and φdimple. The number of dimples is determined by the periodicity of the dimple, dperiod. The aspect ratio of the dimple is its depth, hdimple divided by the diameter, φdimple. The aspect ratio of the dimple was 1.1 for both conditions A and B. Figure 5 shows scanning electron microscope (SEM) images of the dimple. The inner wall of the dimple has micro roughness. The surface texture on the inner wall of the dimple may affect the joining strength resulting from the mechanical interlocking between the surface texture and polymer. The detail of the effect is investigated in section 3.3. Figure 6 shows a relationship between the joining strength and total machined area. When the total machined areas with different dimple diameters are almost the same, the joining strength with the smaller diameter is higher by approximately 10 percent. Observations of the fracture surface of the metal plate revealed that the polymer remained in the dimples. Therefore, the strength of the polymer itself affected the joining strength. Since the joining strength is higher when dimples are formed with smaller diameter, the stress acting in the polymer is lower in the case of the dimple with the smaller diameter. When the
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dimple has the smaller diameter under the above condition, the number of the dimples on the joining area is larger. It is considered that the load transferred to each dimple decreases when the joining area has a large number of dimples by forming the dimple with the smaller diameter. As a consequence, the stress acting in the polymer can decrease.
3.2 Effect of aspect ratio of dimple We formed dimples with different aspect ratios using the nanosecond laser under conditions shown in Table 3. The diameter was 59 µm, 69 µm, 80 µm, and 74 µm, and the aspect ratio was 0.19, 0.60, 1.1, and 1.9 when the number of the laser shots was 1, 5, 10, and 15, respectively. Figure 7 shows a relationship between the aspect ratio and the joining strength. The joined specimen with the aspect ratio of 0.19 was broken while the specimen was being set to the tensile tester. In this case, the joining strength was defined as 0 MPa. The aspect ratio shown in the Fig. 7 is based on the dimensions of the dimple formed on the metal plate. In Fig. 7, the joining strength rises with increasing aspect ratio and is saturated over the aspect ratio of 0.6. One possible reason of the saturation is that the polymer partially fills into the dimples and the amount of polymer in the dimples no longer increases with the aspect ratio of over 0.6. To evaluate the filling depth, cross sectional observations of each sample was conducted. Figure 8 shows micrographs of the cross section of the joining specimen. The aspect ratio of the dimple is 0.6 and 1.9. The micrograph shows that the polymer fully filles into the dimple even though the aspect ratio is over 0.6. Thus, Fig. 7 indicates that the aspect ratio of below 0.6 has a positive correlation with the joining strength, whereas the aspect ratio of over 0.6 does not affect the joining strength even when polymer fully fills into the dimple. The mechanical interlocking between the metal and polymer at the bottom of the dimple might have small effects against the applied load when the aspect ratio is higher than a certain value.
3.3 Effect of surface texture of inner wall of dimple The dimple formed using the nanosecond laser showed the micro roughness on the inner wall of the dimple. We assume that the surface texture on the inner wall of the dimple affects the joining strength as well as the diameter and aspect ratio. Here we investigate how they affect the joining strength. To fabricate various types of the surface textures, we utilize different types of laser sources. The difference is an oscillation mode, which causes the difference in machining properties.
3.3.1 Observation of dimple
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Table 4 shows laser machining conditions. We formed periodic dimples on the joining surface using the picosecond laser, nanosecond laser, and CW laser. The diameter and aspect ratio are as follows: 20 µm and 1.5 against the dimple machined with the picosecond laser, 80 µm and 1.1 against the dimple machined with the nanosecond laser, and 300 µm and 0.9 against the dimple machined with the CW laser. Figures 9 and 10 show SEM images of the dimples machined with the picosecond laser and CW laser, respectively. The SEM images of the dimple machined with the nanosecond laser are shown in Fig. 5. The pulse laser forms fine structure on the inner wall of the dimple as well as large dimples. The geometry of the fine structure depends on the laser. The picosecond laser forms finer structure with a periodicity of several hundred nanometers, while the nanosecond laser forms the rougher structure with micro-scale. The dimple formed with the CW laser has a smooth surface on its inner wall as shown in Fig. 10. The difference in the oscillation mode made a clear difference in the surface texture. Many papers report that an ultrashort pulse laser induces self-organized periodic patterns whose periodicity is close to the wavelength of the laser beam or smaller than half of its wavelength [17-19]. Weck et al. report that the wall of the hole drilled with the femtosecond laser shows fine ripples whose orientation depends on the laser polarization [24]. The picosecond laser used in this study induces the fine periodic patterns which is attributed to the excitation of surface plasmon polaritons. The nanosecond laser produces the dimples by irradiating the laser beam repeatedly at the same point. The machining process of the nanosecond laser is dominated by heat conduction, melting, evaporation, and plasma formation whereas the ultrashort pulse laser offers non-thermal ablation [25]. Melting and resolidification of the metal material produce the micro roughness on the inner wall of the dimple [26]. The machining with the CW laser is based on the continuous irradiation of the laser beam. Thus, during the laser irradiation, the material keeps molten. A surface tension of the molten material might form the smooth surface.
3.3.2 Results of tensile tests Figure 11 shows typical load-displacement curves of the joining specimens before the fracture. The joining specimen whose metal surface is machined with the picosecond laser supports the highest load under the condition of the same displacement. Figure 12 shows a relationship between the joining strength and the total machined area. Cross sectional observations revealed that the polymer fully filled into the dimples. In Fig. 12, the joining strength increases with increasing total machined area for all laser sources. The total machined area is determined by the number of dimples on the joining surface when the same laser source is used. Under the larger total machined area, the mechanical interlocking by dimples is enhanced due to its higher density, which leads to the higher joining strength. This tendency is the same in other previous studies [5,
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7]. Here, we compare the results based on the difference in surface texture on the inner wall of the dimple. First note that a dimple machined with each laser has the different diameter and aspect ratio. In section 3.1, the joining strength increases with decreasing diameter of the dimple when the machined area is the same. According to Figs. 6 and 12, the difference other than the diameter of the dimple contributes much more to the joining strength than the diameter. Moreover, the result obtained in section 3.2 is that the joining strength is saturated when the aspect ratio is over 0.6. The dimple machined in this section has the aspect ratio of more than 0.9, which suggests that the difference in the aspect ratio can be ignored. Therefore, the effect of the inner wall of the dimple is dominant on the joining strength in Fig. 12. Figure 12 indicates that the dimple whose inner wall has fine structure enhances the joining strength compared to the dimple with the smooth texture on its inner wall. The roughness is induced by the pulse laser and the smoothness is induced by the CW laser. Comparing the picosecond and nanosecond laser, the finer roughness induced by the picosecond one leads to the higher joining strength. The reason is discussed in Chapter 4. Figure 13 shows micrographs of fracture surfaces of metal plates. The fracture mode is affected by the surface texture of the dimple. Polymer is broken remaining on the joining surface or in the dimples in the case of the pulse laser. The specimen machined with the picosecond laser exhibits a cohesive failure. On the other hand, polymer is partially pulled out of the dimples when the dimples are machined with the CW laser. It confirms that not only the aspect ratio and diameter of the dimple but the roughness on the inner wall of the dimple and its size affect the joining strength. The roughness on the inner wall of the dimple is considered to enhance the mechanical interlocking and prevent the polymer from being pulled out of the dimple. The enhancement of the mechanical interlocking causes the difference in the load-displacement property, especially the slope of the load-displacement curve, as shown in Fig. 11. The picosecond laser forms the optimum surface structure in this study, resulting in the highest joining strength (18 MPa). Here, we compare the joining strength and the shear strength of the matrix polymer. The tensile strength of the polymer is 120 MPa. According to the yield criterion theory [27], the shear strength is 1⁄√3 times the tensile strength. The shear strength is calculated as 69 MPa, which means that the maximum joining strength is lower than the shear strength of the polymer itself. One possible reason is the stress concentration at the edge of the dimple. Applied load is not uniformly distributed on the joining area. Another reason can be the fiber distribution. Fiber distribution in the polymer affects its mechanical properties. The joining strength is also affected by the fiber distribution [13]. Fiber distribution should be optimized to further enhance the joining strength.
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4. Finite element analysis The difference in the laser machines causes the difference in the surface texture of the dimple. The pulse laser forms fine structure on the inner wall of the dimple whereas the CW laser forms the smooth surface. Here, we investigate the effect of the surface texture on the inner wall of the dimple on the joining strength with FEM using ANSYS 16.2 (ANSYS Inc.).
4.1 Analysis model Figure 14 shows a schematic illustration of an analysis model. A joining surface is 1 mm × 1 mm and has a dimple modeled as a truncated cone. A diameter of the dimple on the joining surface is 0.5 mm and its aspect ratio is 1.0. The value of the diameter in the model does not accord with that in the experiment since we confirmed that the difference in diameter from the experiment did not affect the evaluation of the simulation. Another parameter of the dimple is a slope angle. It is set to 75 ° since the slope angle of the dimple with the aspect ratio of 1.0 was estimated to be approximately 75 ° from cross sectional observations. As boundary conditions, a metal model is fixed and a tensile load of 1 N is applied to a side surface of the polymer model in the right direction. A top surface of the polymer model is elastic-supported with 10 N/mm to stabilize the analysis. Friction is applied to the interface between the metal and polymer models. The coefficient of friction is set to 0.5 to obtain the stability of the analysis. Young’s modulus and Poisson’s ratio of the metal model are 71 GPa and 0.33, respectively. Young’s modulus and Poisson’s ratio of the polymer model are 2.7 GPa and 0.35, respectively. The analysis focuses on the presence of the roughness on the inner wall of the dimple. To investigate the role of the roughness on the joining, the roughness on the inner wall of the dimple is simplified as a square wave structure as shown in Fig. 15. Width w and height h of the rectangle are 0.025 mm and 0.0125 mm, respectively. A periodicity of the square wave structure p is from 0.05 mm to 0.15 mm. It is set every 0.025 mm. The periodicity is varied to deepen the understanding how the number of structures affects the joining strength. When the periodicity is the smallest, the ratio of the diameter to the periodicity corresponds to that of the diameter to the roughness size on the inner wall of the dimple machined with the nanosecond laser.
4.2 Analysis results Material is broken when the stress generated in it exceeds its strength. We focus on the principal stress generated in the polymer model. Figure 16 shows a stress distribution of a center section of the polymer model. The section is parallel to the direction of the load, which is applied in the right. Figure 16(a)
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shows a result on the model with the square wave structure with the smallest periodicity, while Fig. 16(b) shows that with smooth surface. In Fig. 16, maximum principal stress is generated at the edge of the dimple in all of the FEM models due to stress concentration. This means that the origin of the failure is the edge, not the polymer mechanically interlocked with the metal model at the square wave structure. A comparison between Figs. 16(a) and 16(b) indicates that the principal stress is widely distributed in the case of the model having the square wave structure. Figure 17 shows a relationship between the periodicity of the square wave structure and the maximum principal stress in the polymer model. A dotted line indicates the principal stress in the case of the dimple model whose inner wall is smooth (the model of the dimple machined with the CW laser). The principal stress of the model with the square wave structure is lower than the dotted line. The principal stress increases with increasing periodicity and it saturates when the periodicity is over 0.1 mm.
5. Discussion The principal stress in the polymer model is different depending on the periodicity of the square wave structure although the same load is applied to each FEM model. As shown in Fig. 17, the maximum principal stress in the polymer model decreases with decreasing periodicity of the square wave structure on the dimple. Here, we discuss how the fine structure on the inner wall on the dimple affects the principal stress. We consider that the mechanical interlocking via the fine structure can increase the contact area between the metal and polymer models. Increase in the contact area can disperse the applied load widely around the interface, which leads to the decrease in the principal stress. To verify the assumption, we investigated the contact area via FEM simulation. The contact area was calculated as follows: first, the finite elements was selected that had contacts between the metal and polymer models. Next, the surface area of each selected element was summed. Figure 18 shows a relationship between the periodicity of the square wave structure and the contact area when the tensile load of 1 N is applied. A dotted line in Fig. 18 indicates the contact area of the dimple model having the smooth surface on its inner wall (the model of the dimple machined with the CW laser). It should be noted that the contact area is not equal to the surface area of the dimple model. The reason is that gaps between the metal and polymer models arise since the load gives the displacement to the polymer in the right direction. Figure 18 demonstrates that the contact area of the model with the square wave structure is larger than that without the square wave structure. The contact area increases with decreasing periodicity of the square wave structure. In order to investigate how much the applied load is dispersed, we focus on a contact pressure on the interface between the metal and polymer models. The contact pressure should reflect the load which is
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supported on the interface. The contact between the metal and polymer models give rise to contact force which acts on the surface of each element on the interface. The contact pressure was calculated by dividing the contact force on each contact element by its surface area. The contact pressure has a distribution depending on the contact position. Here, we focus on the maximum contact pressure since it should be lower when the applied load is widely dispersed on the interface. Figure 19 shows a relationship between the maximum contact pressure and the maximum principal stress. From Figs 18 and 19, the smaller periodicity of the square wave structure increases the contact area between metal and polymer models, leading to the lower contact pressure between them. As a result, the principal stress in the polymer model decreases when the periodicity of the square wave structure is smaller. This supports the assumption described above. In Fig. 17, the principal stress is saturated when the periodicity is over 0.1 mm. This is because the real contact area is almost the same and the dispersion of the load in the polymer filled into the dimple does not change with the periodicity over 0.1 mm. These findings suggest that it is important to form the roughness on the inner wall of the dimple so that the contact area and the contact pressure become lower. Moreover, forming the roughness with the small periodicity is effective to archive the strong joint.
6. Summary and future work This paper aimed to reveal the effects of the surface structures on the metal-polymer joining strength in IMDJ method. We focused on how the geometry of the dimple affected the joining strength. First, we experimentally investigated the effect of the diameter and aspect ratio of the dimple on the joining strength using the nanosecond laser. Second, we focused on the periodicity of the dimple and the surface texture on the inner wall of the dimple, and experimentally investigated the effects of them on the joining strength. The pulse laser formed the fine structure on the inner wall of the dimple. The finer roughness was formed by the picosecond laser compared with the structure formed by the nanosecond laser. The CW laser formed the smooth surface on the inner wall of the dimple. Finally, we investigated how the surface texture on the inner wall of the dimple affected the joining strength using FEM. The roughness on the dimple was simplified as the square wave structure. The following results were obtained in this study. The joining strength became higher with higher number and smaller diameter of dimples when the total machined area of dimples was almost the same. The joining strength increased and was saturated with increasing aspect ratio of the dimple. The joining strength increased with decreasing periodicity of the dimple. The dimple whose inner wall had roughness enhanced the joining strength compared to the dimple with
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the smooth texture. The fine roughness induced by the picosecond laser exhibited the highest joining strength. In FEM, the principal stress decreased with decreasing periodicity of the square wave structure on the inner wall of the dimple model. Toward the clarification of the joining mechanism, it is indispensable to investigate the correlation between surface structure and joining strength. An interesting finding of the effect of the surface structure is that the fine structure on the inner wall of the dimple plays an important role to enhance the joining strength. The FEM results quantitatively support the experimental results. Future work includes the finite element analysis on the effects of the diameter and aspect ratio of the dimple on the joining strength for the further understanding of the joining mechanism. In addition, we will investigate the effects of the molding conditions on the joining strength.
Acknowledgment This work was supported by Foundation for the Promotion of Industrial Science, Japan.
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[21] JIS H 4000. Aluminium and aluminium alloy sheets, strips and plates. Japanese Industrial Standard; 2014 (in Japanese). [22] ISO 19095-2. Plastics -- evaluation of the adhesion interface performance in plastic-metal assemblies -- part 2: test specimens. International Organization for Standardization; 2015. [23] Kadoya S, Kimura F, Kajihara Y. Tester for tensile shear evaluation of metal–polymer single lap joints. Precis Eng 2018;54:321–6. [24] Weck A, Crawford THR, Wilkinson DS, Haugen HK, Preston JS. Ripple formation during deep hole drilling in copper with ultrashort laser pulses. Appl Phys A Mater Sci Process 2007;89:1001–3. [25] Leitz KH, Redlingshofer B, Reg Y, Otto A, Schmidt M. Metal ablation with short and ultrashort laser pulses. Phys Procedia 2011;12:230–8. [26] Weck A, Crawford THR, Wilkinson DS, Haugen HK, Preston JS. Laser drilling of high aspect ratio holes in copper with femtosecond, picosecond and nanosecond pulses. Appl Phys A Mater Sci Process 2008;90(3):537–43. [27] Timoshenko S, Young DH. Elements of strength of materials. 4th ed., Maruzen Asian ed. Van Nostrand, Maruzen; 1964.
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Figure and table captions Fig. 1 Schematic illustration of direct joining. Fig. 2 Lap joint specimen. Fig. 3 Schematic illustration of the mold. Fig. 4 Schematic illustration of metal plates with the same total machined area. The total machined area is equalized by controlling the number of dimples, Ndimple and the diameter of the dimples, φdimple. Fig. 5 SEM images of the dimple machined with nanosecond laser. Fig. 6 Relationship between joining strength and total machined area. Dimples with the diameter of 40 µm and 80 µm were formed. Fig. 7 Relationship between joining strength and aspect ratio. Fig. 8 Micrographs of cross section of joining specimen: aspect ratio is (a) 0.6 and (b) 1.9. Fig. 9 SEM images of the dimple machined with picosecond laser. Fig. 10 SEM images of the dimple machined with CW laser. Fig. 11 Typical load-displacement curves before the fracture. Fig. 12 Joining strength with various laser machines: φ and α indicate the diameter and aspect ratio of the dimple, respectively. Fig. 13 Fracture surfaces of metal plates machined with (a) picosecond laser, (b) nanosecond laser, and (c) CW laser. Fig. 14 3D FEM model and its cross section. Fig. 15 Schematic illustration of dimple with roughness on its inner wall. Fig. 16 Results of stress distribution in polymer model coupled with (a) dimple with square wave structure and (b) dimple without square wave structure. Fig. 17 Relationship between principal stress acting in polymer and periodicity of square wave structure. Fig. 18 Relationship between contact area and periodicity of square wave structure. Fig. 19 Relationship between principal stress acting in polymer and contact pressure. The values in the legend indicate the periodicity of the square wave structure on the inner wall of the dimple model. Smooth surface indicates the dimple model without the square wave structure. Table 1 Molding conditions. Table 2 Laser machining conditions. The effect of the diameter of the dimple was investigated. Table 3 Laser machining conditions. The effect of the aspect ratio of the dimple was investigated. Table 4 Laser machining conditions of each laser machine.
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Table 1 Molding conditions. Packing pressure Holding pressure and time Injection speed Cylinder temperature Mold temperature
90 MPa 60 MPa × 8 s 100 mm/s 270 ºC 140 ºC
Table 2 Laser machining conditions. The effect of the diameter of the dimple was investigated. Condition A Condition B -9 τ, s 100 × 10 100 × 10-9 λ, nm 1064 1064 φspot, μm 36 59 Ppeak, W 2.0 × 103 10 × 103 Nshot 5 10 dperiod, μm 50, 65, 80, 100 82, 113, 150, 188
Table 3 Laser machining conditions. The effect of the aspect ratio of the dimple was investigated. τ, s 100 × 10-9 λ, nm 1064 φspot, μm 59 Ppeak, W 10 × 103 Nshot 1, 5, 10, 15 dperiod, μm 113
Table 4 Laser machining conditions of each laser machine. Laser ps laser ns laser CW laser -12 -9 τ, s 10 × 10 100 × 10 λ, nm 515 1064 1030 φspot, μm 16 59 170 Ppeak, W 2.3 × 106 10 × 103 3.0 × 103 Nshot 100 10 1 dperiod, μm 30, 40, 50 113, 150, 188 400, 500, 600
Polymer Surface-treated metal
Surface-treated metal
Fig. 1 Schematic illustration of direct joining.
Polymer Surface-treated metal
Metal
45
10
18
5
Polymer
t = 1.5 45
Fig. 2 Lap joint specimen.
t=3
10
Cavity
5
50
Thermo sensor
45
Pressure sensor Metal piece
18
Fig. 3 Schematic illustration of the mold.
Aspect ratio = hdimple / φdimple
hdimple, a
φdimple, a
Metal
dperiod, a
Ndimple, b
hdimple, b
Ndimple, a φdimple, b
dperiod, b
Metal
Fig. 4 Schematic illustration of metal plates with the same total machined area. The total machined area is equalized by controlling the number of dimples, Ndimple and the diameter of the dimples, φdimple.
Magnified image of A A
20 μm
Fig. 5 SEM images of the dimple machined with nanosecond laser.
4 μm
25
Strength [MPa]
20 15 10 Diameter: 40 µm Diameter: 80 µm
5 0 0
10
20
30
40
50
2
Total machined area [mm ]
Fig. 6 Relationship between joining strength and total machined area. Dimples with the diameter of 40 μm and 80 μm were formed.
25
Strength [MPa]
20 15 10 5 0 0
0.5
1 Aspect ratio
Fig. 7 Relationship between joining strength and aspect ratio.
1.5
2
(a)
(b)
Polymer
Metal
Polymer
50 μm
Metal
50 μm
Fig. 8 Micrographs of cross section of joining specimen: aspect ratio is (a) 0.6 and (b) 1.9.
Magnified image of B B
5 μm
Fig. 9 SEM images of the dimple machined with picosecond laser.
1 μm
Magnified image of C
C
50 μm
Fig. 10 SEM images of the dimple machined with CW laser.
5 μm
1
Load [kN]
0.8
0.6
0.4
Picosecond laser Nanosecond laser CW laser
0.2
0 0
0.1
0.2
0.3
Displacement [mm]
Fig. 11 Typical load-displacement curves before the fracture.
0.4
0.5
Picosecond laser Nanosecond laser CW laser 25 20 µm 1.5
Strength [MPa]
20
80 µm 1.1
15 10
300 µm 0.9
5 0 0
10
20
30 2
Total machined area [mm ]
Fig. 12 Joining strength with various laser machines: φ and α indicate the diameter and aspect ratio of the dimple, respectively.
Overall of joining area
Magnified image of joining area
(a)
Remained polymer
Joining area Remained polymer 2 mm
Glass fiber
200 μm
Remained polymer in the dimple
(b)
Joining area
200 μm
2 mm
Polymer is pulled out.
(c)
Joining area
2 mm
Remained polymer in the dimple 200 μm
Fig. 13 Fracture surfaces of metal plates machined with (a) picosecond laser, (b) nanosecond laser, and (c) CW laser.
Polymer
1
Load
0.5
1
75 °
Metal
Metal 1
Fig. 14 3D FEM model and its cross section.
1
h
Metal
Fig. 15 Schematic illustration of dimple with roughness on its inner wall.
Principal stress
(a)
Tensile direction ⇒
(b)
Tensile direction ⇒
45 MPa
-10 MPa
Fig. 16 Results of stress distribution in polymer model coupled with (a) dimple with square wave
structure and (b) dimple without square wave structure.
60 Smooth surface Principal stress [MPa]
50 40 30 20 10 0 0
0.05
0.1 Periodicity [mm]
0.15
0.2
Fig. 17 Relationship between principal stress acting in polymer and periodicity of square wave structure.
2
Contact area [mm ]
0.8
0.6
0.4
0.2 Smooth surface 0 0
0.05
0.1 Periodicity [mm]
0.15
0.2
Fig. 18 Relationship between contact area and periodicity of square wave structure.
0.05 mm 0.075 mm 0.1 mm
0.125 mm 0.15 mm Smooth surface
60
Principal stress [MPa]
50 40 30 20 10 0 0
10
20 30 Contact pressure [MPa]
40
50
Fig. 19 Relationship between principal stress acting in polymer and contact pressure. The values in the legend indicate the periodicity of the square wave structure on the inner wall of the dimple model. Smooth surface indicates the dimple model without the square wave structure.
Highlights Direct joining of polymer to metal plate having periodic dimples formed by laser. The joining strength depends on the diameter, aspect ratio and periodicity of the dimple. Roughness on the inner wall of the dimple enhances the joining strength. FEM results show that roughness on the inner wall of the dimple reduces principal stress acting in polymer.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0141-6359(19)30915-8
DOI:
https://doi.org/10.1016/j.precisioneng.2019.12.011
Reference:
PRE 7082
To appear in:
Precision Engineering
Received Date: 16 June 2019 Revised Date:
9 November 2019
Accepted Date: 28 November 2019
Please cite this article as: Enami K, Kimura F, Yokoyama K, Murakami T, Kajihara Y, Experimental and simulative investigation of the effects of laser-structured metal surface on metal-polymer direct joining, Precision Engineering (2020), doi: https://doi.org/10.1016/j.precisioneng.2019.12.011. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Experimental and simulative investigation of the effects of laser-structured metal surface on metal-polymer direct joining
Kakeru Enamia*, Fuminobu Kimurab, Keisuke Yokoyamaa, Takeshi Murakamia and Yusuke Kajiharab
a
Core Technology R & D Center, NSK Ltd., 1-5-50 Kugenumashinmei, Fujisawa-Shi, Kanagawa
251-8501, Japan b
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505,
Japan
*
Corresponding author: [email protected]
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Abstract Direct joining of metal and polymer consists of surface treatment and injection molding techniques. This study employed a laser machining as the surface treatment to form periodic dimples on a joining surface. We experimentally investigated the effect of surface structures on joining strength under a constant molding condition; we focused on the diameter, aspect ratio and periodicity of the dimple, and the surface texture on its inner wall. Furthermore, we analyzed the effect of the surface texture on the inner wall of the dimple on the joining strength with finite element method. We found that the joining strength became higher with increasing number of dimples when total machined area was same. Another finding was that the joining strength increased with increasing aspect ratio and was saturated above the aspect ratio of 0.6. Furthermore, the experimental and simulated results strongly suggested that roughness on the inner wall of the dimple played an important role to enhance the joining strength. The findings of this study will promote the optimization of the metal surface structure for the metal-polymer direct joining.
Keywords Metal-polymer direct joining, Injection molding, Laser-structuring, Finite element analysis
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1. Introduction Metal materials are widely used for structural components in an automotive industry due to their high strength, high stiffness, and high thermal and heat conductivities. From the view point of the weight saving, many concerns still remain because of their high specific gravities. In order to reduce weight further, metal materials are recently replaced by polymer materials. However, there are some problems to be overcome. Since the mechanical properties of polymer materials are inferior to those of metal materials, polymer materials can not totally replace metal materials. Therefore, joining techniques of dissimilar materials are indispensable because composite materials inevitably have joining points. Conventional methods to obtain composites of metal and polymer parts are adhesive bonding or mechanical fastening using bolts or screws [1-3]. These methods often lead to an increase in manufacturing processes. Thus, direct joining techniques between metal and polymer parts without any extra parts are strongly required [4]. Many researchers have recently been studying the direct joining techniques and proposed various methods. One of the metal-polymer direct joining techniques is an injection-molded direct joining (IMDJ). Figure 1 illustrates a schematic of IMDJ. This joining technique consists of a combination of a surface treatment and an injection molding. The joining process is as follows: (i) forming micro/nano structures on the metal part, (ii) injection molding after the surface-treated metal part is fixed in a mold, and (iii) molten polymer fills into the structures and polymer is cooled. Thus, metal and polymer parts are strongly joined mainly via mechanical interlocking. This study focuses on IMDJ since it allows flexible composite structures with high throughput. Although the IMDJ technique is quite promising for weight saving, it is not widely employed in the automotive industry. This is mainly due to its low reliability which comes from the followings: (i) the optimum joining condition is not clear and (ii) the joining mechanism is not clear. To improve the reliability, it is important to understand how the surface structures and molding conditions affect the joining of metal and polymer parts. The surface treatment method is classified into two methods as a dry processing [5-10] and a wet processing [11-16]. As for the dry processing, laser machining forms surface structures with the size of several dozen micrometers [5-7] and abrasive blasting forms the ones with the size of several micrometers [8-10]. As for wet processing, a chemical etching called NMT (nano molding technology) method forms nanoscale porous structures [11]. A surface treatment by anodization which forms nanoscale porous layers on aluminum alloys is also used for the IMDJ [16]. Both dry and wet methods achieve the strong joint. When we focus on understanding the relationship between the surface structure and joining strength, well-defined surface structure is much more appropriate. Wet processing forms complex surface structures like an undercut-shaped, whereas dry processing forms geometrically well-defined structures.
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The controlled structures promote a better understanding of the joining mechanism. Therefore, we utilize a laser machining technique which is one of the dry processing techniques. Laser machining enables to geometrically control surface structures by adjusting laser machining conditions such as a laser power, a number of laser shots, and a machining periodicity. In addition, laser machines have a compatibility with a manufacturing process since they do not need any equipment for chemicals. This study employs a picosecond laser, a nanosecond laser and a continuous wave (CW) laser because these laser machines are widely used in the industry. It is known that ultra-short pulse laser forms self-organized periodic patterns [17-19]. The structure can contribute to the enhancement of the joining strength. Thus, the picosecond pulse laser is used. Some research groups investigated the effect of the periodicity and depth of the laser-structured surface on the joining strength [5-7]. However, other parameters including the machining width, aspect ratio and surface texture on the machined area are not fully investigated. These structures will also affect the mechanical interlocking between metal and polymer. To elucidate the relationship between the surface structures and the joining strength, a numerical analysis using finite element method (FEM) is also effective. Engelmann et al. investigated how the joining strength was influenced by parameters such as the amount and geometries of the structures under different load directions [20]. They revealed that the parameters affected the stress acting in the FEM model. However, they did not go into the details how the stress was affected by the parameters. In this study, we formed periodic dimples on metal plates by laser machining. We focused on the following surface parameters: the diameter, aspect ratio, periodicity of the dimple, and the surface texture on its inner wall. Changing the diameter of the dimple changes the maximum number of dimples which can be formed on the joining area. The smaller the dimple diameter is, the larger the number of the dimple is on the joining area. The number of dimples can affect the stress distribution on the joining area. The role of the aspect ratio has been investigated using abrasive blasted metals by Kajihara et al. They reported that the surface structure with the aspect ratio over a certain value contributed to the joining [10]. The periodicity of each dimple is related to the number of dimples on the joining area. The larger number of dimples should enhance the load-bearing capacity [7]. Furthermore, the surface texture on the inner wall of the dimple can disperse the stress in the polymer filled into the dimple. Thus, these parameters are expected to affect the joining strength. The aim of this study is to reveal the effects of surface structures on the joining strength. Investigating the relationship between the parameters mentioned above and joining strength offers further knowledge for the optimization of the surface structures. To change the surface texture on the inner wall of the dimple, we used different oscillation types of laser machines. After treating the metal surface with each
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laser machine, we manufactured lap joint specimens under a constant molding condition and then carried out tensile-shear tests. We also investigated the effect of the surface texture on the inner wall of the dimple on the joining strength with FEM. With the experimental and FEM analyses, we discussed how the surface structures affected the joining strength.
2. Experimental 2.1 Laser machining In this study, we used the picosecond laser, the nanosecond laser, and the CW laser to form periodic dimples at vertexes of a square lattice on the metal plates. The dimple structure enables to easily analyze the effects of its diameter, aspect ratio, and periodicity on the joining strength since the geometries are well defined. In the laser machining process, we controlled pulse width τ, wavelength λ, spot diameter φspot, peak power Ppeak, number of laser shots Nshot, and periodicity of the dimple, dperiod. The diameter of the dimple is controlled with Ppeak and φspot, while the depth of the dimple is controlled with Nshot. The laser beam was irradiated at the same point repeatedly to form the dimple in the depth direction. Metal plates were made of aluminum alloy, A5052. The tensile strength of A5052 was 260 MPa [21]. Prior to the injection molding, the metal plates were cleaned with acetone ultrasonically.
2.2 Direct joining by injection molding We manufactured a lap joint specimen to evaluate the joining strength of metal and polymer parts. Figure 2 illustrates the geometry of the specimen based on the ISO 19095-2 [22]. The joining area of metal and polymer parts is 5 mm × 10 mm. Figure 3 shows a schematic illustration of a mold used to manufacture the lap joint specimen. After the metal plate is set in the mold, molten polymer is injected into the mold. The mold has pressure and temperature sensors near the joining area for realtime monitoring of the parameters. To enhance a replication of the polymer to the dimple, we conducted the injection molding under an over-packing condition: the volume of injected polymer was larger than the mold cavity value. Therefore, the polymer pressure rose immediately after the molten polymer reached the end of the mold cavity. The peak value of the pressure, which we described packing pressure, was controlled by changing the volume of injected polymer. The larger the volume of injected polymer became, the higher the packing pressure became. Molding conditions are summarized in Table 1. In this study, the molding conditions are constant because we focus on the effect of the diameter, aspect ratio, periodicity of the dimple, and the surface texture on the inner wall of the dimple on the joining strength. The polymer material was polybutylene terephthalate (PBT) containing 30 wt% of glass fibers and its tensile strength was 120 MPa. Polymer
5
pellets were dried at 130 °C for three hours or more before the injection molding. After the injection molding, the joining specimens were annealed at 130 °C for four hours to remove residual stresses.
2.3 Evaluation of joining strength We carried out tensile shear tests to evaluate the joining strength of the lap joint specimen. In this study, we utilized an original tensile tester [23] to correctly evaluate the tensile shear strength without unwanted deformation. The tensile tester applied a displacement to the lap joint specimen at a constant speed of 1 mm/min and recorded a load during the tensile test. The joining strength is the maximum load divided by the joining area (5 mm × 10 mm).
3. Experimental results 3.1 Effect of dimple diameter We formed dimples with the diameter of 40 and 80 µm on different metal plates and investigated the effect of dimple diameter on the joining strength. The nanosecond laser was used under the conditions listed in Table 2. Conditions A and B are for the diameter of 40 and 80 µm, respectively. To compare only the effect of different dimple diameter, the total machined area should be similar. Figure 4 shows a schematic illustration of metal plates with the same total machined area of dimples. Here the total machined area of dimples is defined as a summation of a project area of each dimple on the joining surface; that is, it is calculated by Ndimple πφ2dimple ⁄4, where Ndimple and φdimple are the number of dimples and the diameter of the dimples, respectively. The total machined area is equalized by controlling Ndimple and φdimple. The number of dimples is determined by the periodicity of the dimple, dperiod. The aspect ratio of the dimple is its depth, hdimple divided by the diameter, φdimple. The aspect ratio of the dimple was 1.1 for both conditions A and B. Figure 5 shows scanning electron microscope (SEM) images of the dimple. The inner wall of the dimple has micro roughness. The surface texture on the inner wall of the dimple may affect the joining strength resulting from the mechanical interlocking between the surface texture and polymer. The detail of the effect is investigated in section 3.3. Figure 6 shows a relationship between the joining strength and total machined area. When the total machined areas with different dimple diameters are almost the same, the joining strength with the smaller diameter is higher by approximately 10 percent. Observations of the fracture surface of the metal plate revealed that the polymer remained in the dimples. Therefore, the strength of the polymer itself affected the joining strength. Since the joining strength is higher when dimples are formed with smaller diameter, the stress acting in the polymer is lower in the case of the dimple with the smaller diameter. When the
6
dimple has the smaller diameter under the above condition, the number of the dimples on the joining area is larger. It is considered that the load transferred to each dimple decreases when the joining area has a large number of dimples by forming the dimple with the smaller diameter. As a consequence, the stress acting in the polymer can decrease.
3.2 Effect of aspect ratio of dimple We formed dimples with different aspect ratios using the nanosecond laser under conditions shown in Table 3. The diameter was 59 µm, 69 µm, 80 µm, and 74 µm, and the aspect ratio was 0.19, 0.60, 1.1, and 1.9 when the number of the laser shots was 1, 5, 10, and 15, respectively. Figure 7 shows a relationship between the aspect ratio and the joining strength. The joined specimen with the aspect ratio of 0.19 was broken while the specimen was being set to the tensile tester. In this case, the joining strength was defined as 0 MPa. The aspect ratio shown in the Fig. 7 is based on the dimensions of the dimple formed on the metal plate. In Fig. 7, the joining strength rises with increasing aspect ratio and is saturated over the aspect ratio of 0.6. One possible reason of the saturation is that the polymer partially fills into the dimples and the amount of polymer in the dimples no longer increases with the aspect ratio of over 0.6. To evaluate the filling depth, cross sectional observations of each sample was conducted. Figure 8 shows micrographs of the cross section of the joining specimen. The aspect ratio of the dimple is 0.6 and 1.9. The micrograph shows that the polymer fully filles into the dimple even though the aspect ratio is over 0.6. Thus, Fig. 7 indicates that the aspect ratio of below 0.6 has a positive correlation with the joining strength, whereas the aspect ratio of over 0.6 does not affect the joining strength even when polymer fully fills into the dimple. The mechanical interlocking between the metal and polymer at the bottom of the dimple might have small effects against the applied load when the aspect ratio is higher than a certain value.
3.3 Effect of surface texture of inner wall of dimple The dimple formed using the nanosecond laser showed the micro roughness on the inner wall of the dimple. We assume that the surface texture on the inner wall of the dimple affects the joining strength as well as the diameter and aspect ratio. Here we investigate how they affect the joining strength. To fabricate various types of the surface textures, we utilize different types of laser sources. The difference is an oscillation mode, which causes the difference in machining properties.
3.3.1 Observation of dimple
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Table 4 shows laser machining conditions. We formed periodic dimples on the joining surface using the picosecond laser, nanosecond laser, and CW laser. The diameter and aspect ratio are as follows: 20 µm and 1.5 against the dimple machined with the picosecond laser, 80 µm and 1.1 against the dimple machined with the nanosecond laser, and 300 µm and 0.9 against the dimple machined with the CW laser. Figures 9 and 10 show SEM images of the dimples machined with the picosecond laser and CW laser, respectively. The SEM images of the dimple machined with the nanosecond laser are shown in Fig. 5. The pulse laser forms fine structure on the inner wall of the dimple as well as large dimples. The geometry of the fine structure depends on the laser. The picosecond laser forms finer structure with a periodicity of several hundred nanometers, while the nanosecond laser forms the rougher structure with micro-scale. The dimple formed with the CW laser has a smooth surface on its inner wall as shown in Fig. 10. The difference in the oscillation mode made a clear difference in the surface texture. Many papers report that an ultrashort pulse laser induces self-organized periodic patterns whose periodicity is close to the wavelength of the laser beam or smaller than half of its wavelength [17-19]. Weck et al. report that the wall of the hole drilled with the femtosecond laser shows fine ripples whose orientation depends on the laser polarization [24]. The picosecond laser used in this study induces the fine periodic patterns which is attributed to the excitation of surface plasmon polaritons. The nanosecond laser produces the dimples by irradiating the laser beam repeatedly at the same point. The machining process of the nanosecond laser is dominated by heat conduction, melting, evaporation, and plasma formation whereas the ultrashort pulse laser offers non-thermal ablation [25]. Melting and resolidification of the metal material produce the micro roughness on the inner wall of the dimple [26]. The machining with the CW laser is based on the continuous irradiation of the laser beam. Thus, during the laser irradiation, the material keeps molten. A surface tension of the molten material might form the smooth surface.
3.3.2 Results of tensile tests Figure 11 shows typical load-displacement curves of the joining specimens before the fracture. The joining specimen whose metal surface is machined with the picosecond laser supports the highest load under the condition of the same displacement. Figure 12 shows a relationship between the joining strength and the total machined area. Cross sectional observations revealed that the polymer fully filled into the dimples. In Fig. 12, the joining strength increases with increasing total machined area for all laser sources. The total machined area is determined by the number of dimples on the joining surface when the same laser source is used. Under the larger total machined area, the mechanical interlocking by dimples is enhanced due to its higher density, which leads to the higher joining strength. This tendency is the same in other previous studies [5,
8
7]. Here, we compare the results based on the difference in surface texture on the inner wall of the dimple. First note that a dimple machined with each laser has the different diameter and aspect ratio. In section 3.1, the joining strength increases with decreasing diameter of the dimple when the machined area is the same. According to Figs. 6 and 12, the difference other than the diameter of the dimple contributes much more to the joining strength than the diameter. Moreover, the result obtained in section 3.2 is that the joining strength is saturated when the aspect ratio is over 0.6. The dimple machined in this section has the aspect ratio of more than 0.9, which suggests that the difference in the aspect ratio can be ignored. Therefore, the effect of the inner wall of the dimple is dominant on the joining strength in Fig. 12. Figure 12 indicates that the dimple whose inner wall has fine structure enhances the joining strength compared to the dimple with the smooth texture on its inner wall. The roughness is induced by the pulse laser and the smoothness is induced by the CW laser. Comparing the picosecond and nanosecond laser, the finer roughness induced by the picosecond one leads to the higher joining strength. The reason is discussed in Chapter 4. Figure 13 shows micrographs of fracture surfaces of metal plates. The fracture mode is affected by the surface texture of the dimple. Polymer is broken remaining on the joining surface or in the dimples in the case of the pulse laser. The specimen machined with the picosecond laser exhibits a cohesive failure. On the other hand, polymer is partially pulled out of the dimples when the dimples are machined with the CW laser. It confirms that not only the aspect ratio and diameter of the dimple but the roughness on the inner wall of the dimple and its size affect the joining strength. The roughness on the inner wall of the dimple is considered to enhance the mechanical interlocking and prevent the polymer from being pulled out of the dimple. The enhancement of the mechanical interlocking causes the difference in the load-displacement property, especially the slope of the load-displacement curve, as shown in Fig. 11. The picosecond laser forms the optimum surface structure in this study, resulting in the highest joining strength (18 MPa). Here, we compare the joining strength and the shear strength of the matrix polymer. The tensile strength of the polymer is 120 MPa. According to the yield criterion theory [27], the shear strength is 1⁄√3 times the tensile strength. The shear strength is calculated as 69 MPa, which means that the maximum joining strength is lower than the shear strength of the polymer itself. One possible reason is the stress concentration at the edge of the dimple. Applied load is not uniformly distributed on the joining area. Another reason can be the fiber distribution. Fiber distribution in the polymer affects its mechanical properties. The joining strength is also affected by the fiber distribution [13]. Fiber distribution should be optimized to further enhance the joining strength.
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4. Finite element analysis The difference in the laser machines causes the difference in the surface texture of the dimple. The pulse laser forms fine structure on the inner wall of the dimple whereas the CW laser forms the smooth surface. Here, we investigate the effect of the surface texture on the inner wall of the dimple on the joining strength with FEM using ANSYS 16.2 (ANSYS Inc.).
4.1 Analysis model Figure 14 shows a schematic illustration of an analysis model. A joining surface is 1 mm × 1 mm and has a dimple modeled as a truncated cone. A diameter of the dimple on the joining surface is 0.5 mm and its aspect ratio is 1.0. The value of the diameter in the model does not accord with that in the experiment since we confirmed that the difference in diameter from the experiment did not affect the evaluation of the simulation. Another parameter of the dimple is a slope angle. It is set to 75 ° since the slope angle of the dimple with the aspect ratio of 1.0 was estimated to be approximately 75 ° from cross sectional observations. As boundary conditions, a metal model is fixed and a tensile load of 1 N is applied to a side surface of the polymer model in the right direction. A top surface of the polymer model is elastic-supported with 10 N/mm to stabilize the analysis. Friction is applied to the interface between the metal and polymer models. The coefficient of friction is set to 0.5 to obtain the stability of the analysis. Young’s modulus and Poisson’s ratio of the metal model are 71 GPa and 0.33, respectively. Young’s modulus and Poisson’s ratio of the polymer model are 2.7 GPa and 0.35, respectively. The analysis focuses on the presence of the roughness on the inner wall of the dimple. To investigate the role of the roughness on the joining, the roughness on the inner wall of the dimple is simplified as a square wave structure as shown in Fig. 15. Width w and height h of the rectangle are 0.025 mm and 0.0125 mm, respectively. A periodicity of the square wave structure p is from 0.05 mm to 0.15 mm. It is set every 0.025 mm. The periodicity is varied to deepen the understanding how the number of structures affects the joining strength. When the periodicity is the smallest, the ratio of the diameter to the periodicity corresponds to that of the diameter to the roughness size on the inner wall of the dimple machined with the nanosecond laser.
4.2 Analysis results Material is broken when the stress generated in it exceeds its strength. We focus on the principal stress generated in the polymer model. Figure 16 shows a stress distribution of a center section of the polymer model. The section is parallel to the direction of the load, which is applied in the right. Figure 16(a)
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shows a result on the model with the square wave structure with the smallest periodicity, while Fig. 16(b) shows that with smooth surface. In Fig. 16, maximum principal stress is generated at the edge of the dimple in all of the FEM models due to stress concentration. This means that the origin of the failure is the edge, not the polymer mechanically interlocked with the metal model at the square wave structure. A comparison between Figs. 16(a) and 16(b) indicates that the principal stress is widely distributed in the case of the model having the square wave structure. Figure 17 shows a relationship between the periodicity of the square wave structure and the maximum principal stress in the polymer model. A dotted line indicates the principal stress in the case of the dimple model whose inner wall is smooth (the model of the dimple machined with the CW laser). The principal stress of the model with the square wave structure is lower than the dotted line. The principal stress increases with increasing periodicity and it saturates when the periodicity is over 0.1 mm.
5. Discussion The principal stress in the polymer model is different depending on the periodicity of the square wave structure although the same load is applied to each FEM model. As shown in Fig. 17, the maximum principal stress in the polymer model decreases with decreasing periodicity of the square wave structure on the dimple. Here, we discuss how the fine structure on the inner wall on the dimple affects the principal stress. We consider that the mechanical interlocking via the fine structure can increase the contact area between the metal and polymer models. Increase in the contact area can disperse the applied load widely around the interface, which leads to the decrease in the principal stress. To verify the assumption, we investigated the contact area via FEM simulation. The contact area was calculated as follows: first, the finite elements was selected that had contacts between the metal and polymer models. Next, the surface area of each selected element was summed. Figure 18 shows a relationship between the periodicity of the square wave structure and the contact area when the tensile load of 1 N is applied. A dotted line in Fig. 18 indicates the contact area of the dimple model having the smooth surface on its inner wall (the model of the dimple machined with the CW laser). It should be noted that the contact area is not equal to the surface area of the dimple model. The reason is that gaps between the metal and polymer models arise since the load gives the displacement to the polymer in the right direction. Figure 18 demonstrates that the contact area of the model with the square wave structure is larger than that without the square wave structure. The contact area increases with decreasing periodicity of the square wave structure. In order to investigate how much the applied load is dispersed, we focus on a contact pressure on the interface between the metal and polymer models. The contact pressure should reflect the load which is
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supported on the interface. The contact between the metal and polymer models give rise to contact force which acts on the surface of each element on the interface. The contact pressure was calculated by dividing the contact force on each contact element by its surface area. The contact pressure has a distribution depending on the contact position. Here, we focus on the maximum contact pressure since it should be lower when the applied load is widely dispersed on the interface. Figure 19 shows a relationship between the maximum contact pressure and the maximum principal stress. From Figs 18 and 19, the smaller periodicity of the square wave structure increases the contact area between metal and polymer models, leading to the lower contact pressure between them. As a result, the principal stress in the polymer model decreases when the periodicity of the square wave structure is smaller. This supports the assumption described above. In Fig. 17, the principal stress is saturated when the periodicity is over 0.1 mm. This is because the real contact area is almost the same and the dispersion of the load in the polymer filled into the dimple does not change with the periodicity over 0.1 mm. These findings suggest that it is important to form the roughness on the inner wall of the dimple so that the contact area and the contact pressure become lower. Moreover, forming the roughness with the small periodicity is effective to archive the strong joint.
6. Summary and future work This paper aimed to reveal the effects of the surface structures on the metal-polymer joining strength in IMDJ method. We focused on how the geometry of the dimple affected the joining strength. First, we experimentally investigated the effect of the diameter and aspect ratio of the dimple on the joining strength using the nanosecond laser. Second, we focused on the periodicity of the dimple and the surface texture on the inner wall of the dimple, and experimentally investigated the effects of them on the joining strength. The pulse laser formed the fine structure on the inner wall of the dimple. The finer roughness was formed by the picosecond laser compared with the structure formed by the nanosecond laser. The CW laser formed the smooth surface on the inner wall of the dimple. Finally, we investigated how the surface texture on the inner wall of the dimple affected the joining strength using FEM. The roughness on the dimple was simplified as the square wave structure. The following results were obtained in this study. The joining strength became higher with higher number and smaller diameter of dimples when the total machined area of dimples was almost the same. The joining strength increased and was saturated with increasing aspect ratio of the dimple. The joining strength increased with decreasing periodicity of the dimple. The dimple whose inner wall had roughness enhanced the joining strength compared to the dimple with
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the smooth texture. The fine roughness induced by the picosecond laser exhibited the highest joining strength. In FEM, the principal stress decreased with decreasing periodicity of the square wave structure on the inner wall of the dimple model. Toward the clarification of the joining mechanism, it is indispensable to investigate the correlation between surface structure and joining strength. An interesting finding of the effect of the surface structure is that the fine structure on the inner wall of the dimple plays an important role to enhance the joining strength. The FEM results quantitatively support the experimental results. Future work includes the finite element analysis on the effects of the diameter and aspect ratio of the dimple on the joining strength for the further understanding of the joining mechanism. In addition, we will investigate the effects of the molding conditions on the joining strength.
Acknowledgment This work was supported by Foundation for the Promotion of Industrial Science, Japan.
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Figure and table captions Fig. 1 Schematic illustration of direct joining. Fig. 2 Lap joint specimen. Fig. 3 Schematic illustration of the mold. Fig. 4 Schematic illustration of metal plates with the same total machined area. The total machined area is equalized by controlling the number of dimples, Ndimple and the diameter of the dimples, φdimple. Fig. 5 SEM images of the dimple machined with nanosecond laser. Fig. 6 Relationship between joining strength and total machined area. Dimples with the diameter of 40 µm and 80 µm were formed. Fig. 7 Relationship between joining strength and aspect ratio. Fig. 8 Micrographs of cross section of joining specimen: aspect ratio is (a) 0.6 and (b) 1.9. Fig. 9 SEM images of the dimple machined with picosecond laser. Fig. 10 SEM images of the dimple machined with CW laser. Fig. 11 Typical load-displacement curves before the fracture. Fig. 12 Joining strength with various laser machines: φ and α indicate the diameter and aspect ratio of the dimple, respectively. Fig. 13 Fracture surfaces of metal plates machined with (a) picosecond laser, (b) nanosecond laser, and (c) CW laser. Fig. 14 3D FEM model and its cross section. Fig. 15 Schematic illustration of dimple with roughness on its inner wall. Fig. 16 Results of stress distribution in polymer model coupled with (a) dimple with square wave structure and (b) dimple without square wave structure. Fig. 17 Relationship between principal stress acting in polymer and periodicity of square wave structure. Fig. 18 Relationship between contact area and periodicity of square wave structure. Fig. 19 Relationship between principal stress acting in polymer and contact pressure. The values in the legend indicate the periodicity of the square wave structure on the inner wall of the dimple model. Smooth surface indicates the dimple model without the square wave structure. Table 1 Molding conditions. Table 2 Laser machining conditions. The effect of the diameter of the dimple was investigated. Table 3 Laser machining conditions. The effect of the aspect ratio of the dimple was investigated. Table 4 Laser machining conditions of each laser machine.
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Table 1 Molding conditions. Packing pressure Holding pressure and time Injection speed Cylinder temperature Mold temperature
90 MPa 60 MPa × 8 s 100 mm/s 270 ºC 140 ºC
Table 2 Laser machining conditions. The effect of the diameter of the dimple was investigated. Condition A Condition B -9 τ, s 100 × 10 100 × 10-9 λ, nm 1064 1064 φspot, μm 36 59 Ppeak, W 2.0 × 103 10 × 103 Nshot 5 10 dperiod, μm 50, 65, 80, 100 82, 113, 150, 188
Table 3 Laser machining conditions. The effect of the aspect ratio of the dimple was investigated. τ, s 100 × 10-9 λ, nm 1064 φspot, μm 59 Ppeak, W 10 × 103 Nshot 1, 5, 10, 15 dperiod, μm 113
Table 4 Laser machining conditions of each laser machine. Laser ps laser ns laser CW laser -12 -9 τ, s 10 × 10 100 × 10 λ, nm 515 1064 1030 φspot, μm 16 59 170 Ppeak, W 2.3 × 106 10 × 103 3.0 × 103 Nshot 100 10 1 dperiod, μm 30, 40, 50 113, 150, 188 400, 500, 600
Polymer Surface-treated metal
Surface-treated metal
Fig. 1 Schematic illustration of direct joining.
Polymer Surface-treated metal
Metal
45
10
18
5
Polymer
t = 1.5 45
Fig. 2 Lap joint specimen.
t=3
10
Cavity
5
50
Thermo sensor
45
Pressure sensor Metal piece
18
Fig. 3 Schematic illustration of the mold.
Aspect ratio = hdimple / φdimple
hdimple, a
φdimple, a
Metal
dperiod, a
Ndimple, b
hdimple, b
Ndimple, a φdimple, b
dperiod, b
Metal
Fig. 4 Schematic illustration of metal plates with the same total machined area. The total machined area is equalized by controlling the number of dimples, Ndimple and the diameter of the dimples, φdimple.
Magnified image of A A
20 μm
Fig. 5 SEM images of the dimple machined with nanosecond laser.
4 μm
25
Strength [MPa]
20 15 10 Diameter: 40 µm Diameter: 80 µm
5 0 0
10
20
30
40
50
2
Total machined area [mm ]
Fig. 6 Relationship between joining strength and total machined area. Dimples with the diameter of 40 μm and 80 μm were formed.
25
Strength [MPa]
20 15 10 5 0 0
0.5
1 Aspect ratio
Fig. 7 Relationship between joining strength and aspect ratio.
1.5
2
(a)
(b)
Polymer
Metal
Polymer
50 μm
Metal
50 μm
Fig. 8 Micrographs of cross section of joining specimen: aspect ratio is (a) 0.6 and (b) 1.9.
Magnified image of B B
5 μm
Fig. 9 SEM images of the dimple machined with picosecond laser.
1 μm
Magnified image of C
C
50 μm
Fig. 10 SEM images of the dimple machined with CW laser.
5 μm
1
Load [kN]
0.8
0.6
0.4
Picosecond laser Nanosecond laser CW laser
0.2
0 0
0.1
0.2
0.3
Displacement [mm]
Fig. 11 Typical load-displacement curves before the fracture.
0.4
0.5
Picosecond laser Nanosecond laser CW laser 25 20 µm 1.5
Strength [MPa]
20
80 µm 1.1
15 10
300 µm 0.9
5 0 0
10
20
30 2
Total machined area [mm ]
Fig. 12 Joining strength with various laser machines: φ and α indicate the diameter and aspect ratio of the dimple, respectively.
Overall of joining area
Magnified image of joining area
(a)
Remained polymer
Joining area Remained polymer 2 mm
Glass fiber
200 μm
Remained polymer in the dimple
(b)
Joining area
200 μm
2 mm
Polymer is pulled out.
(c)
Joining area
2 mm
Remained polymer in the dimple 200 μm
Fig. 13 Fracture surfaces of metal plates machined with (a) picosecond laser, (b) nanosecond laser, and (c) CW laser.
Polymer
1
Load
0.5
1
75 °
Metal
Metal 1
Fig. 14 3D FEM model and its cross section.
1
h
Metal
Fig. 15 Schematic illustration of dimple with roughness on its inner wall.
Principal stress
(a)
Tensile direction ⇒
(b)
Tensile direction ⇒
45 MPa
-10 MPa
Fig. 16 Results of stress distribution in polymer model coupled with (a) dimple with square wave
structure and (b) dimple without square wave structure.
60 Smooth surface Principal stress [MPa]
50 40 30 20 10 0 0
0.05
0.1 Periodicity [mm]
0.15
0.2
Fig. 17 Relationship between principal stress acting in polymer and periodicity of square wave structure.
2
Contact area [mm ]
0.8
0.6
0.4
0.2 Smooth surface 0 0
0.05
0.1 Periodicity [mm]
0.15
0.2
Fig. 18 Relationship between contact area and periodicity of square wave structure.
0.05 mm 0.075 mm 0.1 mm
0.125 mm 0.15 mm Smooth surface
60
Principal stress [MPa]
50 40 30 20 10 0 0
10
20 30 Contact pressure [MPa]
40
50
Fig. 19 Relationship between principal stress acting in polymer and contact pressure. The values in the legend indicate the periodicity of the square wave structure on the inner wall of the dimple model. Smooth surface indicates the dimple model without the square wave structure.
Highlights Direct joining of polymer to metal plate having periodic dimples formed by laser. The joining strength depends on the diameter, aspect ratio and periodicity of the dimple. Roughness on the inner wall of the dimple enhances the joining strength. FEM results show that roughness on the inner wall of the dimple reduces principal stress acting in polymer.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: