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Production of polymer-metal hybrids using extrusion-based additive manufacturing and electrochemically treated aluminum
Journal Pre-proof Production of polymer-metal hybrids using extrusion-based Additive Manufacturing and electrochemically treated aluminum Sebastian Hertle (Conceptualization) (Methodology) (Formal analysis) (Investigation) (Writing - original draft) (Visualization), Tobias Kleffel (Conceptualization) (Writing - review and editing), Andreas W¨orz (Writing - review and editing), Dietmar Drummer (Supervision)
PII:
S2214-8604(19)32003-2
DOI:
https://doi.org/10.1016/j.addma.2020.101135
Reference:
ADDMA 101135
To appear in:
Additive Manufacturing
Received Date:
25 October 2019
Revised Date:
17 January 2020
Accepted Date:
14 February 2020
Please cite this article as: Hertle S, Kleffel T, W¨orz A, Drummer D, Production of polymer-metal hybrids using extrusion-based Additive Manufacturing and electrochemically treated aluminum, Additive Manufacturing (2020), doi: https://doi.org/10.1016/j.addma.2020.101135
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Production of polymer-metal hybrids using extrusion-based Additive Manufacturing and electrochemically treated aluminum
Sebastian Hertle*, Tobias Kleffel, Andreas Wörz, Dietmar Drummer ––––––––– Institute of Polymer Technology (LKT), Friedrich-Alexander University Erlangen-Nürnberg, Am Weichselgarten 9, 91058 Erlangen, Germany E-Mail: [email protected] –––––––––
Electrochemical microstructuring of aluminum enables the production of polymer-metal hybrids by means of Material Extrusion without the need of coatings The contact temperature between the metal sheet and the deposited polymer significantly influences the resulting component behavior A consolidation roll improves the filling of microstructures for low contact temperatures
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Highlights
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Abstract
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The development towards higher individualization and functional density pushes the need towards a flexible production of multi-material and lightweight components. In this paper, extrusion based additive manufacturing was used to produce polymer-metal hybrids with polypropylene and aluminum alloy. For this purpose, a screw-driven extruder on a six-axis robot was used. Due to the adhesion incompatibility of polypropylene and untreated metals, the surface of the aluminum sheets was electrochemically microstructured. The investigations show that this enables a mechanically stressable joint through the filling of the surface microstructures with polymer. Investigations on lap shear joints reveal a distinct influence of the contact temperature between the polymer and metal onto the lap shear strength. A sufficient contact temperature is required for filling surface microstructures. Thus, increased metal and extrusion temperatures favor higher strengths. Furthermore, the use of a consolidation roll shows beneficial influences in lower temperature ranges due to the application of higher pressures during the polymer strand deposition.
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Keywords: Material Extrusion; polymer-metal-hybrid; surface microstructure; lap shear strength; consolidation
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1.
Introduction and state of the art
The manufacturing industry is confronted with fundamental changes in society and economy. In addition to shorter development times and a globalized, highly fluctuating market, the demand for individualized products is rising [1]. This sets new challenges for the manufacturers, who need highly flexible and reconfigurable technologies [2]. Due to the layerwise process without molds, additive manufacturing (AM) enables the manufacturing of highly individualized components while also maintaining short manufacturing lead times [3].
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Additionally, the use of polymer-metal hybrid structures is of high importance for many applications, such as lightweight construction [4].Various and approved process strategies to produce polymer-metal hybrids, e. g. in-mold assembly (IMA) and post molding assembly (PMA) exist. IMA combines the assembly of the polymer and metal component with the injection molding of the polymer component. For this purpose, the metal profile is placed into the injection mold and overmolded with the polymer component. In PMA processes, the polymer component is injection molded and joined with the metal component in a separate assembly process, e. g. by hot stacking. However, these process strategies have disadvantages with regard to flexibility due to the necessary and often costly tools. This applies especially, if a component customization is requested. A promising approach for individualized polymermetal hybrids is the production of the metal components through conventional processes, followed by an individualization via additive manufacturing. This means that no cost-driving injection molding tools for IMA or fixtures for PMA are necessary, and different individual requirements can be addressed flexibly. As an additive process, melt extrusion manufacturing seems to be the most suitable process, as it is easy to stop and inserts or other components can be integrated [5]. Curved slicing [6] and curved layer melt extrusion manufacturing [7] enable the deposition along the topology of a shaped surface. Thus, functionalization and individualization can be carried out for a component that has already been formed and is close to its final shape. In additive melt extrusion manufacturing, which is synonymous with fused filament fabrication (FFF) and fused layer modeling (FLM), a plasticized polymer is deposited via a nozzle along a predetermined path. The path is generated directly from the CAD data.
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The challenge in the connection of thermoplastics with untreated metals is the adhesion incompatibility of these two materials. For this reason, common connection technologies use form fit, force fit or adhesive promoters as an intermediate layer, which are compatible to the used materials [8]. In IMA and PMA processes, form fit and force fit can be easily combined by means of cutouts or beadings in the metal component, which serve as anchorage points for the polymer component [9]. These cutouts or beadings can be manufactured during the form process of the metal component, which is usually a deep drawing process. However, the usage of such structures has major disadvantages. Around the anchorage points is a high stress concentration while the stress of the metal component is reduced through cutouts. In order to avoid these disadvantages, adhesive promoters on the surface of the metal component can be used, which enable a full-faced joining [10]. However, the adhesive promoter must always be adapted to the used polymer and metal and the application requires at least one additional process step.
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Another possibility to achieve a full-faced and mechanically stressable joint is the use of surface microstructures on the metal component. The prerequisite is the filling of these surface microstructures with polymer to generate undercuts. The surface microstructures can be produced e. g. by pickling [11], corundum blasting [12], laser treatment [13], or electrochemical treatment [14]. This strategy shows some advantages concerning tightness [15] and also the achievable joint quality, for both laser [16] and electrochemically [17] produced surface microstructures. In particular, laser structuring and electrochemical processes show great potential regarding the produced surface microstructures, because higher degrees of interlocking perpendicular to the joining surface are possible, which enables higher loads as described in [17]. In principle, electrochemical treatment can be applied to all metals with a suitable grain structure. The electrochemical surface microstructuring has already been reported for different materials, e. g. aluminum alloys [17, 18], titanium and zinc alloys [18] as well as steel [19, 20]. Polymer-metal hybrids with AM methods are a relatively new research area, where only little is published yet. Although, Fischer et al. [5] explain that functionalization by means of additive melt extrusion manufacturing is possible, no approach to this is shown. The production of steel polyamide 6 (PA6) hybrids with unidirectional (UD) oriented carbon fiber (CF) reinforced PA6 with varying surface preparation and laser angles was investigated by Stokes-Griffin et al. in [21] for the similar process of tape laying. While almost no influence of the laser angle was detected, the surface preparation showed a huge influence. Without a pre-treatment of the steel surface, no bond with the reinforced PA6 could be achieved for used single lap joints. For grit blasted surfaces an initial bond could be formed, which failed during the placement of further tapes. The placement onto PA6 coated steel surfaces was -2-
successful and achieved high lap shear strengths reaching values up to 30.7 N·mm-2 [21]. In further investigations, the same authors detected an increase in tensile shear strength with an increasing first ply placement rate for a steel PA6 CF hybrid [22]. It was concluded that this is attributed to the higher temperature in the contact zone during placement. Falck et al. showed in [23] for the first time the production of polymer-aluminum hybrids using FFF. To produce single lap joints, sandblasted aluminum sheets were coated in a first step with the subsequently used polymer. In a second step, the polymer component of the single lap joint was built up on the surface using the FFF process. Two polymers were used, on the one hand acrylic butadiene styrene (ABS) and on the other hand PA6. In case of the PA6aluminum hybrid, the polymer component was built using unidirectional carbon fiber reinforced filaments, which resulted in a smaller difference in stiffness between the polymer and metal component and reduced shrinkage compared to the use of pure PA6. In the following lap shear tests, ultimate lap shear strengths of 5.3 N·mm-2 for ABS-aluminum and 21.9 N·mm-2 for PA6 CF-aluminum were achieved. In a further study with ABS and ABS coated aluminum sheets, the same authors found that higher printing temperatures, lower layer heights, higher deposition speeds, and higher coating thicknesses led to higher lap shear strengths [24]. It was concluded that this is attributed to a smaller number of pores and a better interdiffusion between the individual strands.
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Finally, the state of the art can be summarized as follows. To produce full-faced polymer-metal joints, adhesive promoters and surface microstructures can be used. However, current investigations on the additive build-up of polymers on metals have always used coated metal sheets to enable a stressable joint. For the use of conventional FFF systems, the size of the components is currently limited due to the restricted envelope space available. In addition, due to the small nozzle diameter, long production times are required for the manufacturing of large-volume components.
Experimental Methods
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For these reasons, this study focuses on the manufacturing of polymer-metal hybrids by means of robotbased additive melt extrusion manufacturing on uncoated, electrochemically micro structured aluminum sheets. The effects of surface texture, extrusion temperature, substrate temperature and a consolidation roll on the bond strength and fracture behavior are presented using single lap shear tests and microscopy.
2.1 Materials
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For all experiments, a commercially available homopolymer polypropylene PP (Sabic Europe Sabic® PP 505P) was used as thermoplastic material. It is a multi-purpose grade for fiber, film and tape extrusion. The characteristic values are shown in Table 1. As metal component, 20 mm x 80 mm hotrolled sheets made of aluminum wrought alloy EN AW-5754 (AlMg3) with a thickness of 1 mm were used.
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2.2 Micro structuring of the aluminum sheets
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Surface microstructures with undercuts perpendicular to the joining surface were generated on the surface of the aluminum sheets enabling mechanical interlocking between the polymer and aluminum sheet. For the micro structuring, an electrochemical treatment was applied, using hydrochloric acid (HCl) with a concentration of 5 % as electrolyte. For this purpose, the experimental setup as described in [14] and schematically shown in Figure 1 was used. The laboratory glass was filled with 800 ml electrolyte. An AlMg3 plate was used as cathode and the aluminum sheet as anode. The anode and cathode were connected with 6 A to a direct current power supply (Voltcraft DPPS-32-30, Conrad Electronic SE, Hirschau, Germany). A magnetic mixer (RET-G, Janke & Kunkel GmbH & Co. KG, Staufen, Germany) constantly circulates the electrolyte to keep it homogenous. As the electrochemical treatment is an exothermic reaction, two cooling circuits were used to keep the temperature constant to achieve comparable surface microstructures on the aluminum sheets. In the primary cooling circuit, a peristaltic pump (Boxer 9700 Table Top Dispenser, Boxer GmbH, Ottobeuren, Germany) was used to circulate the electrolyte from the laboratory glass into a heat exchanger and back to the laboratory glass. In the secondary cooling circuit, a cryostat (Lauda Ecoline RE 112, Lauda Dr. R. Wobser GmbH & Co. KG, Lauda-Königshofen, Germany) was used to pump water with a temperature of 23 °C inside the heat exchanger.
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Figure 1: Experimental setup (schematically) for the electrochemical treatment of the metal sheets in accordance with [14]
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Before the electrochemical treatment, each aluminum sheet was cleaned with isopropyl alcohol in an ultrasonic bath for 1 min and remaining isopropyl alcohol on the metal surface was vaporized at ambient conditions. 2.3 Additive melt extrusion manufacturing of single lap shear specimens
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For the reproducible melt extrusion manufacturing of single lap shear specimens, a screw-driven hand welding extruder (WEGENER International GmbH, Eschweiler, Germany) was mounted on a 6-axis robot Kuka KR 15/2 (Kuka Roboter GmbH, Augsburg, Germany). The diameter of the extruder screw was 16 mm with a length/diameter-ratio of 10/1 and a nozzle diameter of 2 mm. A hot air fan type HL 1910 E (STEINEL Vertrieb GmbH, Herzebrock-Clarholz, Germany) was mounted in such way that the hot air heated the metal component directly in front of the nozzle to a surface temperature of 180 °C before the deposition of strands. Additionally, an aluminum consolidation roll with a diameter of 50 mm was mounted on the robot 70 mm behind the nozzle in order to enable a consolidation of the deposited material strands. Next to the robot, a table with a heating bed, which enables surface temperatures of the metal component of up to 120 °C, was used as building platform. The ambient temperature was approximately 23 °C and no additional envelope or closed chamber was used. The experimental setup is schematically shown in Figure 2.
Figure 2: Schematic illustration of the experimental setup for the robot-based additive melt extrusion manufacturing At the beginning of each manufacturing cycle, an aluminum sheet was positioned on the heating bed. On the aluminum sheet surface, a polyimide film was fixed in order to limit the overlapping area to 20 mm x 10 mm, (Figure 3 a). Furthermore, thermocouples (type K, Ø 0.2 mm) were placed on the aluminum sheet surface to adjust and control the so-called substrate temperature. The deposition process was started after the aluminum surface reached a nearly constant temperature (approximately ± 2 °C).
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Figure 3: AlMg3 sheets prepared with polyimide film (a), deposited PP strands for lap shear test generation (b) and setup during the deposition of the strands (c)
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2.4 Test methods
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During the extrusion, the mass rate was kept constant at a value of 20.5 g∙min-1. The deposition speed was 40 mm∙s-1, the air gap to the substrate was 1 mm and there was no overlapping between the deposited strands lying next to each other. The deposited path is depicted in Figure 3 (b). An untreated AlMg3 plate was positioned on the heating bed as distance element (Figure 3 (c)) to ensure a constant distance between the nozzle and the layup surface. In order to allow the clamping, the deposited strands above the aluminum sheet were cut off approx. 10 mm behind the joining surface with a band saw. For the investigation of the influence of the extrusion and substrate temperature and thus the contact conditions, single lap shear specimens were produced in which these parameters were varied according to Table 2. During the variation of one parameter, the other parameter was set to a constant value, which is denoted through a bold font in Table 2. Furthermore, single lap shear specimens for these parameter combinations were produced with and without a consolidation roll in order to analyze, if a pressure application improves the filling of the surface microstructures and therefore the bonding strength. For this purpose, the consolidation roll was fixed at the relevant specimens in such a way that the deposited strand was consolidated to a height of 2 mm.
Confocal laser scanning microscopy
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For the surface characterization of the hot rolled and electrochemically treated aluminum sheets, a confocal laser scanning microscope type VK-X1000 (Keyence Corporation, Osaka, Japan) was used. The area under consideration was 2 x 2 mm2 in the middle of the joining zone, Figure 3 a. All measurements were conducted with laser light and an optical magnification of 240. For the description of the surface, the arithmetical mean height Sa (Equation (1)) and maximum height Sz (Equation (2)) were determined as defined in DIN EN ISO 25178-2. 𝑆𝑎 =
1 ∬|𝑧(𝑥, 𝑦)|𝑑𝑥 𝑑𝑦 𝐴
(1)
𝑆𝑧 = 𝑆𝑝 + 𝑆𝑣
(2)
A describes the definition range, z(x, y) is the height of the surface at the position x and y, Sp is the maximum height of the peak in the definition range and Sv is the maximum height of the valley in the definition range. In addition, the surface topology was visualized in order to evaluate the surface structures with regard to their distribution. Rheological analyses
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As it can be assumed that the rheological behavior has a major influence on the filling of the surface microstructures, the shear viscosity, storage and loss modulus were determined using a rheometer HAAKE Mars (Thermo Fisher Scientific, Waltham, USA) with a parallel plate setup. For this purpose, cylinders with a diameter of 24 mm and a height of 1 mm were used as specimen. During the test, the deformation was set to 0.5 % and the cooling rate to 2 K∙min-1. The frequency of the oscillatory measurement was set to 1 Hz, because it can be assumed that the material does not experience any further shearing in the contact area after the strand deposition. Contact temperature calculation and measurement As an indication of the current temperature between the newly deposited polymer strand and the AlMg3 surface in time of contact, a simplified calculation of the contact temperature was used. The contact temperature TC thereby is the average value of the substrate temperature TS and the extrusion temperature TE (Equation (3)). 𝑇𝐶 =
𝑇𝑆 + 𝑇𝐸 2
(3)
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Additionally, experimental investigations performed with a thermocouple (type K, Ø 0.2 mm) were conducted to validate the contact temperature calculation experimentally. The thermocouples were positioned on the joining surface of the micro structured aluminum sheets. A repetition number of three was used for each process setting and the experimentally measured peak temperature was evaluated. Reflected light microscopy
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In addition to the laser scanning microscopy, cross sections of the middle of the joining zone of the single lap shear specimens in yz-plane (Figure 4 (a)) were analyzed by reflected light microscopy to investigate undercuts perpendicular to the surface and the filling of the surface microstructures with polymer. The cross sections were cut out with a water-cooled saw. Afterwards, they were embedded in a cold-curing resin, grinded and polished. The wet grinding was performed stepwise until a grit level of 2500 was reached. The grinding paper was wetted with water all the time. For the following polishing step, a diamond suspension was used. The polishing steps were carried out up to a diamond grain size of 1 µm. For the following analysis, a microscope type Axio Imager M2m (Carl Zeiss AG, Oberkochen, Germany) was used. Single lap shear test
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For the investigation of the lap shear strength, single lap shear tests according to EN 1465 with a free clamping length of 75 mm and a test speed of 1 mm∙min-1 were conducted using an universal testing machine type Zwick 1484 (Zwick Roell GmbH & Co. KG, Ulm, Germany). The used test setup is schematically shown in Figure 4 (b). All tests were carried out with standard atmosphere (23 °C and 50 % humidity) according to DIN EN ISO 291.
Figure 4: Position of the cross sections for microscopic investigations (a) and test setup for mechanical testing of the lap shear test specimens (b) Scanning electron microscopy -6-
After the single lap shear test, the fractured surfaces were analyzed using a scanning electron microscope (SEM) type Zeiss Ultra Plus (Carl Zeiss AG, Oberkochen, Germany). Additionally, the surface energy dispersive X-ray (EDX) spectroscopy was used to detect polymer residuals on the aluminum surface and vice versa. Aluminum as main indicator of the AlMg3 sheet and carbon as main indicator of the polymer were differentiated in the applied color mapping.
3.
Results and discussion
Surface characterization
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The surface topology of the untreated and electrochemically treated aluminum sheets are shown in Figure 5 (a). For the planar surface roughness of the aluminum sheets, a significant increase can be achieved by electrochemical treatment (Figure 5 (b)). The average value of the mean arithmetic height Sa of the investigated surface increases from 0.6 µm to 6.1 µm, and the maximum height Sz from 13.1 µm to 64.5 µm due to the electrochemical treatment. The original surface topography of the hot-rolled aluminum sheet disappears completely and a fissured surface develops. Thus, the electrochemically treated sheets have a high unevenness compared to the hot-rolled aluminum sheets and cavities can be seen on the surface.
Figure 5: Exemplary surface topology of an untreated and electrochemically treated aluminum sheet (a) and the roughness parameters Sa and Sz of such surfaces (b)
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Rheological behavior
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In Figure 6, the complex viscosity, storage and loss modulus are shown for the temperature range of 120 °C to 200 °C. It can be seen that a decrease in temperature below 163 °C leads to a higher increase of the storage modulus compared to the loss modulus. Thus, a shift of the material behavior from predominantly viscous to predominantly elastic behavior takes place [25]. For the filling of the surface microstructures it can be concluded that the contact temperature, which results from the interaction of the metal surface and polymer heat balance, should be higher than the temperature at the intersection between the curves of the storage and loss modulus. As the cool down behavior after the first contact depends significantly on the heat transfer inside the metal component, the metal temperature should be as high as possible, but must also allow a consolidation of the polymer for the deposition of further strands. In addition, the extrusion temperature should also be high for enabling a low viscosity for the filling of the surface microstructures.
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Figure 6: Complex viscosity, storage and loss modulus of the used polypropylene, measured with oscillatory rotational rheometry (1 Hz, 0,5 % strain) during cooling (2 K∙min-1) Contact temperature
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The measurement of the surface temperature with thermocouples shows that the simplified calculation of the contact temperature is valid for the substrate temperatures of 80 °C and 120 °C (Figure 7). Only small temperature differences of ± 3 °C occur. However, this approach is not valid for the high substrate temperature of 180 °C. The measured values are at least 10 °C lower than the calculated values. This means that rapid cooling takes place. As aluminum has a significantly lower heat capacity than polypropylene in the relevant temperature range and only the substrate surface of the 1 mm thick aluminum sheet was heated to 180 °C by means of hot air, the temperature decreases quickly. Considering the contact temperatures and viscosity behavior of the material, a substrate temperature of 80 °C seems unfavorable. The resulting contact temperature is below the intersection of loss and storage modulus and thus, a higher elastic material behavior may counteract the filling of surface microstructures. Moreover, the viscosity for the measured contact temperature shows the highest value of 5.5·103 Pa·s. This means that the flowability is the lowest and therefore the filling is expected to be the worst. For a substrate temperature of 120 °C, the contact temperature increases almost linearly with increasing extrusion temperature. At the same time, the viscosity as well as loss and storage modulus decrease at the interface of aluminum and polymer. Thus, the process setting of 180 °C substrate temperature and 230 °C extrusion temperature should be the most favorable for the filling of surface microstructures, as the contact temperature shows the highest value.
Figure 7: Calculated and measured contact temperatures for the different process settings Reflected light microscopy Due to the vertical direction of view, the confocal laser scanning pictures do not resolve the undercuts caused by the electrochemical treatment. However, the exemplary micrographs of the cross sections in Figure 8 (b) and Figure 9 show that undercuts are present for the electrochemically treated aluminum -8-
sheets. As expected, the samples failed as no connection was reached for the untreated sample in any case during processing without a consolidation roll.
Figure 8:
Characteristic micrographs of the polymer-metal interface in dependence of the substrate temperature ((a) 80 °C and (b) 180 °C) and an extrusion temperature of 230 °C, exemplarily without consolidation roll
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For a substrate temperature of 80 °C (Figure 8 (a)), the processing was not successful and no bonding occurred between the aluminum and the used polypropylene. However, aluminum particles can be detected in the micrographs of the deposited strands, which were in contact with the aluminum surface. Due to the high light reflection of aluminum, those particles shine through the surrounding polymer. The contacting surface line is rather smooth for a substrate temperature of 80 °C. This indicates a fast cooling, which prevents the polymer to cover the fissured aluminum surface and confirms the assumption that the aluminum temperature should be high due to its high heat conduction and the resulting contact temperature.
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For the other substrate temperatures of 120 °C and 180 °C, the polymer strands cover the surface of the aluminum sheets and fill the structures. As there is no difference detectable on the micrographs between the substrate temperatures of 120 °C and 180 °C, only the higher temperature is illustrated exemplarily in Figure 8 (b). Though hardly visible, some voids can be detected at the interface between polypropylene and aluminum, which may act as weak spots during mechanical load. The voids occur near the surface, regardless of the size of the surface microstructure. It can be assumed that air is trapped during the strand deposition between the aluminum sheet and polypropylene strand. Due to the filling of even very small surface microstructures, the enclosed air cannot be released and remains as voids. An equivalent optical result is obtained when using a consolidation roll.
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For the process variation of the extrusion temperature, the use of a consolidation roll leads to a contrary result for the lowest extrusion temperature of 210 °C. While the preparation by means of sawing led to a separation of the deposited strands from the aluminum surface without the use of a consolidation roll, the strands deposited with consolidation roll did not separate. Figure 9 (a) shows a characteristic micrograph of the polymer-metal interface of a sample, which was produced without consolidation roll. A large number of aluminum particles can be detected in the deposited strands. Moreover, the surface line of the polymer appears rough in the cross section. Thus, the polypropylene seems to have a sufficient flowability for a partial filling during the contact with the aluminum substrate. A reduced filling of the surface microstructures, especially the undercuts, might be a reason for the premature failure during the preparation. By using a consolidation roll, the surface microstructures of the substrate surface are filled almost completely (Figure 9 (b)). The results for the higher extrusion temperatures of 230 °C and 250 °C show a similar behavior regardless of the use of a consolidation roll.
Figure 9:
Characteristic micrographs of the polymer-metal interface of samples which were produced with an extrusion temperature of 210 °C, a substrate temperature of 120 °C with no consolidation roll (a) and with consolidation roll (b) -9-
The microscopic investigations and present contact temperatures show that the selected process settings cover the critical range concerning viscoelastic material behavior and filling of surface microstructures with polymer. For the measured contact temperature of 155 °C (80 °C substrate and 230 °C extrusion temperature, Figure 8 (a)), predominantly elastic material behavior is present and neither the surface microstructures are filled nor the rough surface of the substrate can be reproduced on the polymer surface. The filling of surface microstructures changes already for a contact temperature of 168 °C (120 °C substrate and 210 °C extrusion temperature. Here, the surface structures are reproduced on the polymer surface due to the material’s change to a slightly predominant viscous behavior with a lower viscosity (Figure 9 (a)). By using the consolidation roll in order to apply an additional pressure, the microstructures can be filled to a large range (Figure 9 (b)). Hence, the consolidation roll influences the bond between the electrochemically treated aluminum and polymer positively. However, a consolidation roll is not mandatory as the results with higher contact temperatures show. For the exemplarily chosen contact temperature of 194 °C (180 °C substrate and 230 °C extrusion temperature, Figure 8 (b)), even small surface microstructures are filled. This can be attributed to the even lower viscosities and a more predominantly viscous material behavior, but also to a capillary effect, which has to occur to fill the very small surface microstructures with their undercuts.
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Single lap shear tests
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Although the micrographs do not show significant differences for the examined cross sections, an increase in substrate temperature results in higher ultimate lap shear strengths (ULSS) and higher elongations at failure, Figure 10 (a). The ULSS increases from an unmeasurable value (80 °C) to (5.05 ±0.57) N·mm-2 (120 °C), and to (5.55 ±0.38) N·mm-2 (180 °C) without the use of a consolidation roll. This is consistent with the results of the color mapping of the SEM analysis (Figure 11). A rising proportion of carbon (colored red) on the aluminum surface (colored green) occurs for higher substrate temperatures. This means that more polymer residuals remain after the lap shear tests, which indicate a better mechanical interlocking. One specimen produced with a substrate temperature of 180 °C and no consolidation roll did not fail in the joining surface but in the deposited polymer. For the samples produced by using a consolidation roll one sample failed outside the joining surface in the polymer for a substrate temperature of 120 °C and two samples for 180 °C.
Figure 10: ULSS and elongation at failure for the variation of the substrate temperature (a) and for the
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variation of extrusion temperature (b);*for all test specimens a failure of the polymer component occurred
The ULSS tends to decrease by using a consolidation roll. At the same time, the elongation at failure does not decrease. As there is also no significant difference between the fractured surfaces for the use of a consolidation roll, Figure 11 (d) - (f), the reduced height of the deposited strands might explain the lower ULSS. Due to the compaction with the consolidation roll, the height of the deposited strands decrease from an average value of 3.7 mm to 2.1 mm. During testing, the applied tensile force leads to an additional bending moment, which is caused by eccentricity of the sample geometry. Tensile force and bending moment form the applied load on the sample. The composition of the applied load depends on the height, stiffness and overlap length of the joining components. Smaller heights lead to a reduction of the bending moment. This implies that the transferable load would be higher for the use of a consolidation roll due to the decreasing eccentricity. However, this is only valid for joints with balanced stiffness. As the stiffness of aluminum (about 70000 N/mm²) is much higher than the one of polypropylene (1500 N/mm²), an imbalanced stiffness occurs for the same joining partner height. A - 10 -
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reduction in height causes a higher deflection of the polymer due to a decrease in the area moment of inertia. This leads to higher stresses at the overlapping ends and an additional peel force. In the case of the bending moment, the height is linearly included in the calculation, while the height is included to the power of three for the calculation of the area moment of inertia. This means the effect of the reduction of the height will have a stronger influence on the mechanical tests. A higher bending occurs due to a lower moment of inertia, which is additionally superimposed by the imbalanced stiffness. Thus, smaller heights due to the use of a consolidation roll can explain the tendency to smaller ULSS.
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Figure 11: SEM scans of the fractured surfaces on the substrate and EDX spectroscopy color mapping in dependence of the substrate temperature and an extrusion temperature of 230 °C; (a) - (c) no consolidation roll, (d) - (f) with consolidation roll; green color indicates aluminum, red color indicates carbon
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For the extrusion temperature variation, a similar behavior can be seen for the ULSS, Figure 10 (b). In general, higher extrusion temperatures lead to an increase in the maximum stress and elongation at break. However, there are two points to consider. Firstly, there is a higher ULSS for the use of a consolidation roll at 210 °C than without a roll. Furthermore, none of the test specimens fails when using a consolidation roll with an extrusion temperature of 250 °C in the joining surface. Here, the deposited strands consistently fail and a significantly higher elongation occurs due to yielding of the polypropylene.
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In consequence, higher extrusion temperatures lead to an increase in mechanical performance of the polymer-metal hybrid structure, which is supported by the rheological investigations. Besides the lower viscosity with higher extrusion temperature, also the cooling time rises due to the higher contact temperature and thermal mass. This means, especially in the contact area, that lower viscosities are present for a longer time, so that flow processes take place more easily and faster. The fracture surfaces of the tested specimens are a further indication of this (Figure 12). The amount of carbon (colored red), which indicates polypropylene residuals, rises with an increasing extrusion temperature without a consolidation roll, Figure 12 (a) - (c).
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Figure 12: SEM scans of the fractured surfaces on the aluminum sheet and EDX spectroscopy color
mapping in dependence of the extrusion temperature and a substrate temperature of 120 °C; (a) - (c) no consolidation roll, (d) and (e) with consolidation roll; green color indicates aluminum, red color indicates carbon
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A different behavior occurs for the use of a consolidation roll. Using a consolidation roll, a high ULSS is already detected for a temperature of 210 °C. This differs from the higher temperature of 230 °C only in a significantly higher standard deviation. While for the temperature of 230 °C the ULSS values are in the range of 3.26 N·mm-2 to 4.51 N·mm-2, the values determined for 210 °C are in a much wider range. The lowest ULSS is 3.02 N·mm-2 and the highest is 5.5 N·mm-2, being almost in the range of the extrusion temperature of 250 °C (5.61 N·mm-2 to 6.66 N·mm-2) with consolidation roll. The specimen produced with 250 °C and consolidation roll failed exclusively in the polymer outside the joining surface.
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The mechanical results correlate well with the amount of visible polymer residuals on the aluminum surface. Both temperatures of 210 °C (Figure 12 (d)) and 230 °C (Figure 12 (e)) show a comparable appearance. Due to the fact that all specimens for an extrusion temperature of 250 °C failed outside the joining surface, no scanning electron micrographs were prepared. It is to be assumed that a high filling of the surface microstructures is present, so the occurring loads can be transmitted. Comparable to the variation of the substrate temperature, the ULSS are higher without the use of a consolidation roll. Likewise, the layer height shows higher values resulting in higher area moment of inertia, which has to be considered.
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However, this does not apply to the process setting of 210 °C. The layer height is 2.3 mm with the use of a consolidation roll. This results in a lower area moment of inertia but significantly higher lap shear strengths occur than without the use of a consolidation roll at 210 °C. Considering the contact temperature, it decreases with decreasing extrusion temperature. In case of an extrusion temperature of 210 °C and a substrate temperature of 120 °C the contact temperature is 165 °C. Consequently, an approximation to the intersection of the storage G’ and loss modulus G’’ takes place and a rather viscous material behavior is present over a short period. If the material reacts predominantly elastic to deformations, flow processes are more severely restricted and the filling of the structures with polymer is not possible or cannot be completed without an additional pressure. However, even a small amount of pressure, as applied by the consolidation roll, seems to be sufficient to overcome these filling limitations. A more viscous behavior also explains the rise in ULSS with increasing contact temperature as Figure 13 illustrates. The lower the viscosity, the better the filling of the surface microstructures, which is needed for the mechanical interlocking. Nevertheless, not only the contact temperature is of importance, but also the thermal properties of the polymer and metal. The highest lap shear strength does not occur for the highest measured contact temperature of 194 °C with a substrate temperature of 180 °C and an extrusion temperature of 230 °C, but for the measured contact temperature of 185 °C with a substrate temperature of 120 °C and an extrusion temperature of 250 °C. Since aluminum has a higher thermal conductivity than polypropylene, a quicker decrease of the contact temperature for the extrusion temperature of 230 °C and the substrate temperature of 180 °C than for the extrusion temperature of 250 °C and the substrate temperature of 120 °C may explain the differences in ULSS. Furthermore, there are generally lower viscosities in the deposited strand for extrusion temperature of 250 °C - 12 -
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compared to 230 °C. Although the performed mechanical tests do not indicate an increase in contact area due to the use of a consolidation roll, this is very likely and supported by the consideration of the ULSS in relation to the area moment of inertia. This results in higher values for the use of a consolidation roll and an increasing trend for increasing contact temperatures. Accordingly, the geometric boundary conditions play an important role and will therefore be considered in detail in further investigations.
Figure 13: ULSS of the different process settings in dependence of the calculated contact temperature between the substrate temperature and extrusion temperature
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Conclusions
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The ultimate lap shear strengths are clearly dependent on the substrate and extrusion temperature. Higher substrate and extrusion temperatures lead to higher contact temperatures at the joining surface and lower viscosities. Thus, the filling of surface microstructures is promoted which results in higher ULSS values. For the filling of the surface microstructures, a sufficient contact temperature is required. With a simplified calculation of the contact temperature between the polymer and metal sheet in combination with the intersection of the storage and loss modulus, an estimation for the minimum required contact temperature can be made. It is shown that the contact temperature should be higher than the temperature at the intersection of the storage and loss modulus, so that the material shows a predominantly viscous behavior. Using a consolidation roll does not necessarily improve the ULSS, although the ULSS generally shows lower values in comparison to no consolidation roll. As scanning electron and light microscopic investigations do not show a lower filling of the surface microstructures, the reason for lower mechanical properties can be most likely attributed to the reduction of layer height. Through the height reduction, the resistance against the bending moment is lower, which leads to a higher bending during the single lap shear test. However, taking the present area moment of inertia into account, the use of a consolidation roll can be beneficial. In case of low contact temperatures, the consolidation roll shows a beneficial influence. For the lowest investigated extrusion temperature of 210 °C, the resulting contact temperature is close to the intersection of the storage and loss modulus. The pressure applied by the consolidation roll is sufficient to fill the surface microstructures with polymer and to counteract its elastic deformations.
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This work investigated the manufacturing of polymer-metal hybrids by means of extrusion based additive manufacturing of polypropylene on aluminum sheets with electrochemically manufactured surface microstructures. For this, substrate and extrusion temperatures were varied and the influence of a consolidation roll was investigated. It could be demonstrated that a coating of the metal is not necessarily needed to reach a stressable joint. Based on the results, the following statements can be drawn:
Notwithstanding, these discussed results and dependencies are determined for a material combination of polypropylene and aluminum with lap shear joints. A transfer to other types of load, material - 13 -
combinations, and the dependency of the calculated contact temperature with the predominant material behavior might be limited. For this purpose, future investigations should also consider same layer heights, different geometric boundary conditions, other types of load, e.g. single rib specimens with a load transmission perpendicular to the joining surface, and a transfer to other polymers. Furthermore, the interactions that occur between component geometry, mechanical properties and the use of the consolidation roll must be further analyzed. To evaluate the influence of thermal mass and different heat capacities, thermal simulations and models offer a promising approach.
References
[7] [8] [9] [10] [11] [12] [13] [14] [15]
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[16]
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[6]
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[5]
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[4]
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[3]
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[2]
F.T. Piller, C. Weller, R. Kleer, Business Models with Additive Manufacturing—Opportunities and Challenges from the Perspective of Economics and Management, in: C. Brecher (Ed.), Advances in Production Technology, Springer International Publishing, Cham, 2015, pp. 39-48. Y. Koren, M. Shpitalni, P. Gu, S.J. Hu, Product Design for Mass-Individualization, Procedia CIRP 36 (2015) 64-71. M. Attaran, The rise of 3-D printing: The advantages of additive manufacturing over traditional manufacturing, Business Horizons 60(5) (2017) 677-688. S.T. Amancio Filho, L.-A. Blaga, Joining of polymer-metal hybrid structures: principles and applications, John Wiley & Sons2018. A. Fischer, S. Rommel, T. Bauernhansl, New Fiber Matrix Process with 3D Fiber Printer – A Strategic In-process Integration of Endless Fibers Using Fused Deposition Modeling (FDM), in: G. Kovács, D. Kochan (Eds.), Digital Product and Process Development Systems, Springer Berlin Heidelberg2013, pp. 167-175. D. Chakraborty, B.A. Reddy, A.R. Choudhury, Extruder path generation for Curved Layer Fused Deposition Modeling, Computer-Aided Design 40(2) (2008) 235-243. R.J.A. Allen, R.S. Trask, An experimental demonstration of effective Curved Layer Fused Filament Fabrication utilising a parallel deposition robot, Additive Manufacturing 8 (2015) 78-87. G. Erhard, Desigining with plastics, Carl Hanser Verlag, München, 2006. S.T. Amancio-Filho, J.F. dos Santos, Joining of polymers and polymer–metal hybrid structures: Recent developments and trends, Polymer Engineering and Science 49(8) (2009) 1461-1476. J. Vittinghoff, D. Drummer, Investigating the full-faced joining of polymer-metal hybrid structures, Annual Technical Conference - ANTEC, Conference Proceedings, 2014, pp. 1795-1800. P. Mitschang, R. Velthuis, S. Emrich, M. Kopnarski, Induction Heated Joining of Aluminum and Carbon Fiber Reinforced Nylon 66, 22(6) (2009) 767-801. G. Lucchetta, F. Marinello, P.F. Bariani, Aluminum sheet surface roughness correlation with adhesion in polymer metal hybrid overmolding, CIRP Annals 60(1) (2011) 559-562. C. Spadaro, C. Sunseri, C. Dispenza, Laser surface treatments for adhesion improvement of aluminium alloys structural joints, Radiat Phys Chem 76(8) (2007) 1441-1446. T. Kleffel, D. Drummer, Electrochemical treatment of metal inserts for subsequent assembly injection molding of tight electronic systems, Journal of Polymer Engineering, 2018, p. 675. A. Heckert, C. Singer, M.F. Zaeh, R. Daub, T. Zeilinger, Gas-tight Thermally Joined Metalthermoplastic Connections by Pulsed Laser Surface Pre-treatment, Physics Procedia 83 (2016) 1083-1093. A. Heckert, M.F. Zaeh, Laser Surface Pre-treatment of Aluminium for Hybrid Joints with Glass Fibre Reinforced Thermoplastics, Physics Procedia 56 (2014) 1171-1181. T. Kleffel, D. Drummer, Investigating the suitability of roughness parameters to assess the bond strength of polymer-metal hybrid structures with mechanical adhesion, Composites Part B: Engineering 117 (2017) 20-25. M. Baytekin-Gerngross, M.D. Gerngross, J. Carstensen, R. Adelung, Making metal surfaces strong, resistant, and multifunctional by nanoscale-sculpturing, Nanoscale Horizons 1(6) (2016) 467-472. M. Stöver, M. Renke-Gluszko, T. Schratzenstaller, J. Will, N. Klink, B. Behnisch, A. Kastrati, R. Wessely, J. Hausleiter, A. Schömig, E. Wintermantel, Microstructuring of stainless steel implants by electrochemical etching, J Mater Sci 41(17) (2006) 5569-5575. T. Haisch, E. Mittemeijer, J.W. Schultze, Electrochemical machining of the steel 100Cr6 in aqueous NaCl and NaNO3 solutions: microstructure of surface films formed by carbides, Electrochimica Acta 47(1) (2001) 235-241.
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[17]
[18] [19]
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[21] C.M. Stokes-Griffin, S. Ehard, A. Kollmannsberger, P. Compston, K. Drechsler, A laser tape placement process for selective reinforcement of steel with CF/PA6 composites: Effects of surface preparation and laser angle, Materials & Design 116 (2017) 545-553. [22] C.M. Stokes-Griffin, A. Kollmannsberger, S. Ehard, P. Compston, K. Drechsler, Manufacture of steel–CF/PA6 hybrids in a laser tape placement process: Effect of first-ply placement rate on thermal history and lap shear strength, Composites Part A: Applied Science and Manufacturing 111 (2018) 42-53. [23] R. Falck, S.M. Goushegir, J.F. dos Santos, S.T. Amancio-Filho, AddJoining: A novel additive manufacturing approach for layered metal-polymer hybrid structures, Materials Letters 217 (2018) 211-214. [24] R. Falck, J.F. dos Santos, S.T. Amancio-Filho, Microstructure and Mechanical Performance of Additively Manufactured Aluminum 2024-T3/Acrylonitrile Butadiene Styrene Hybrid Joints Using an AddJoining Technique, Materials 12(6) (2019) 864. [25] T. Mezger, The rheology handbook, 4th Edition ed., Vincentz Network, Hannover, 2014.
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Table 1: Characteristic values of the used polypropylene Parameters Unit
Standard
Value
cm³∙10-1∙min-1
ISO 1133
2.9 ± 0.05
°C
ISO 11357
159
°C
ISO 11357
116
Thermal conductivity+
W∙m-1∙K-1
ISO 8302
0.22
Specific heat capacity+
J∙kg-1∙K-1
ISO 11357
1700
Density+
kg∙m-3
ISO 1183
905
Melt flow rate (230 °C, 2.16
kg∙min-1)
Peak melting temperature# Peak crystallization
#Heating
temperature#
rate / cooling rate = 10 K·min-1 to datasheet
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+According
Table 2: Varied process parameters / conditions for the production of PP-AlMg3 single lap shear specimens. The bold font denotes the value, which was applied during the variation of the other parameters Parameter / conditions Unit Value °C
210
Substrate temperature
°C
80
Distance consolidation roll to substrate
mm
230
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Extrusion temperature
120
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∞
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250 180
2
PII:
S2214-8604(19)32003-2
DOI:
https://doi.org/10.1016/j.addma.2020.101135
Reference:
ADDMA 101135
To appear in:
Additive Manufacturing
Received Date:
25 October 2019
Revised Date:
17 January 2020
Accepted Date:
14 February 2020
Please cite this article as: Hertle S, Kleffel T, W¨orz A, Drummer D, Production of polymer-metal hybrids using extrusion-based Additive Manufacturing and electrochemically treated aluminum, Additive Manufacturing (2020), doi: https://doi.org/10.1016/j.addma.2020.101135
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Production of polymer-metal hybrids using extrusion-based Additive Manufacturing and electrochemically treated aluminum
Sebastian Hertle*, Tobias Kleffel, Andreas Wörz, Dietmar Drummer ––––––––– Institute of Polymer Technology (LKT), Friedrich-Alexander University Erlangen-Nürnberg, Am Weichselgarten 9, 91058 Erlangen, Germany E-Mail: [email protected] –––––––––
Electrochemical microstructuring of aluminum enables the production of polymer-metal hybrids by means of Material Extrusion without the need of coatings The contact temperature between the metal sheet and the deposited polymer significantly influences the resulting component behavior A consolidation roll improves the filling of microstructures for low contact temperatures
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Highlights
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Abstract
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The development towards higher individualization and functional density pushes the need towards a flexible production of multi-material and lightweight components. In this paper, extrusion based additive manufacturing was used to produce polymer-metal hybrids with polypropylene and aluminum alloy. For this purpose, a screw-driven extruder on a six-axis robot was used. Due to the adhesion incompatibility of polypropylene and untreated metals, the surface of the aluminum sheets was electrochemically microstructured. The investigations show that this enables a mechanically stressable joint through the filling of the surface microstructures with polymer. Investigations on lap shear joints reveal a distinct influence of the contact temperature between the polymer and metal onto the lap shear strength. A sufficient contact temperature is required for filling surface microstructures. Thus, increased metal and extrusion temperatures favor higher strengths. Furthermore, the use of a consolidation roll shows beneficial influences in lower temperature ranges due to the application of higher pressures during the polymer strand deposition.
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Keywords: Material Extrusion; polymer-metal-hybrid; surface microstructure; lap shear strength; consolidation
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1.
Introduction and state of the art
The manufacturing industry is confronted with fundamental changes in society and economy. In addition to shorter development times and a globalized, highly fluctuating market, the demand for individualized products is rising [1]. This sets new challenges for the manufacturers, who need highly flexible and reconfigurable technologies [2]. Due to the layerwise process without molds, additive manufacturing (AM) enables the manufacturing of highly individualized components while also maintaining short manufacturing lead times [3].
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Additionally, the use of polymer-metal hybrid structures is of high importance for many applications, such as lightweight construction [4].Various and approved process strategies to produce polymer-metal hybrids, e. g. in-mold assembly (IMA) and post molding assembly (PMA) exist. IMA combines the assembly of the polymer and metal component with the injection molding of the polymer component. For this purpose, the metal profile is placed into the injection mold and overmolded with the polymer component. In PMA processes, the polymer component is injection molded and joined with the metal component in a separate assembly process, e. g. by hot stacking. However, these process strategies have disadvantages with regard to flexibility due to the necessary and often costly tools. This applies especially, if a component customization is requested. A promising approach for individualized polymermetal hybrids is the production of the metal components through conventional processes, followed by an individualization via additive manufacturing. This means that no cost-driving injection molding tools for IMA or fixtures for PMA are necessary, and different individual requirements can be addressed flexibly. As an additive process, melt extrusion manufacturing seems to be the most suitable process, as it is easy to stop and inserts or other components can be integrated [5]. Curved slicing [6] and curved layer melt extrusion manufacturing [7] enable the deposition along the topology of a shaped surface. Thus, functionalization and individualization can be carried out for a component that has already been formed and is close to its final shape. In additive melt extrusion manufacturing, which is synonymous with fused filament fabrication (FFF) and fused layer modeling (FLM), a plasticized polymer is deposited via a nozzle along a predetermined path. The path is generated directly from the CAD data.
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The challenge in the connection of thermoplastics with untreated metals is the adhesion incompatibility of these two materials. For this reason, common connection technologies use form fit, force fit or adhesive promoters as an intermediate layer, which are compatible to the used materials [8]. In IMA and PMA processes, form fit and force fit can be easily combined by means of cutouts or beadings in the metal component, which serve as anchorage points for the polymer component [9]. These cutouts or beadings can be manufactured during the form process of the metal component, which is usually a deep drawing process. However, the usage of such structures has major disadvantages. Around the anchorage points is a high stress concentration while the stress of the metal component is reduced through cutouts. In order to avoid these disadvantages, adhesive promoters on the surface of the metal component can be used, which enable a full-faced joining [10]. However, the adhesive promoter must always be adapted to the used polymer and metal and the application requires at least one additional process step.
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Another possibility to achieve a full-faced and mechanically stressable joint is the use of surface microstructures on the metal component. The prerequisite is the filling of these surface microstructures with polymer to generate undercuts. The surface microstructures can be produced e. g. by pickling [11], corundum blasting [12], laser treatment [13], or electrochemical treatment [14]. This strategy shows some advantages concerning tightness [15] and also the achievable joint quality, for both laser [16] and electrochemically [17] produced surface microstructures. In particular, laser structuring and electrochemical processes show great potential regarding the produced surface microstructures, because higher degrees of interlocking perpendicular to the joining surface are possible, which enables higher loads as described in [17]. In principle, electrochemical treatment can be applied to all metals with a suitable grain structure. The electrochemical surface microstructuring has already been reported for different materials, e. g. aluminum alloys [17, 18], titanium and zinc alloys [18] as well as steel [19, 20]. Polymer-metal hybrids with AM methods are a relatively new research area, where only little is published yet. Although, Fischer et al. [5] explain that functionalization by means of additive melt extrusion manufacturing is possible, no approach to this is shown. The production of steel polyamide 6 (PA6) hybrids with unidirectional (UD) oriented carbon fiber (CF) reinforced PA6 with varying surface preparation and laser angles was investigated by Stokes-Griffin et al. in [21] for the similar process of tape laying. While almost no influence of the laser angle was detected, the surface preparation showed a huge influence. Without a pre-treatment of the steel surface, no bond with the reinforced PA6 could be achieved for used single lap joints. For grit blasted surfaces an initial bond could be formed, which failed during the placement of further tapes. The placement onto PA6 coated steel surfaces was -2-
successful and achieved high lap shear strengths reaching values up to 30.7 N·mm-2 [21]. In further investigations, the same authors detected an increase in tensile shear strength with an increasing first ply placement rate for a steel PA6 CF hybrid [22]. It was concluded that this is attributed to the higher temperature in the contact zone during placement. Falck et al. showed in [23] for the first time the production of polymer-aluminum hybrids using FFF. To produce single lap joints, sandblasted aluminum sheets were coated in a first step with the subsequently used polymer. In a second step, the polymer component of the single lap joint was built up on the surface using the FFF process. Two polymers were used, on the one hand acrylic butadiene styrene (ABS) and on the other hand PA6. In case of the PA6aluminum hybrid, the polymer component was built using unidirectional carbon fiber reinforced filaments, which resulted in a smaller difference in stiffness between the polymer and metal component and reduced shrinkage compared to the use of pure PA6. In the following lap shear tests, ultimate lap shear strengths of 5.3 N·mm-2 for ABS-aluminum and 21.9 N·mm-2 for PA6 CF-aluminum were achieved. In a further study with ABS and ABS coated aluminum sheets, the same authors found that higher printing temperatures, lower layer heights, higher deposition speeds, and higher coating thicknesses led to higher lap shear strengths [24]. It was concluded that this is attributed to a smaller number of pores and a better interdiffusion between the individual strands.
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Finally, the state of the art can be summarized as follows. To produce full-faced polymer-metal joints, adhesive promoters and surface microstructures can be used. However, current investigations on the additive build-up of polymers on metals have always used coated metal sheets to enable a stressable joint. For the use of conventional FFF systems, the size of the components is currently limited due to the restricted envelope space available. In addition, due to the small nozzle diameter, long production times are required for the manufacturing of large-volume components.
Experimental Methods
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For these reasons, this study focuses on the manufacturing of polymer-metal hybrids by means of robotbased additive melt extrusion manufacturing on uncoated, electrochemically micro structured aluminum sheets. The effects of surface texture, extrusion temperature, substrate temperature and a consolidation roll on the bond strength and fracture behavior are presented using single lap shear tests and microscopy.
2.1 Materials
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For all experiments, a commercially available homopolymer polypropylene PP (Sabic Europe Sabic® PP 505P) was used as thermoplastic material. It is a multi-purpose grade for fiber, film and tape extrusion. The characteristic values are shown in Table 1. As metal component, 20 mm x 80 mm hotrolled sheets made of aluminum wrought alloy EN AW-5754 (AlMg3) with a thickness of 1 mm were used.
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2.2 Micro structuring of the aluminum sheets
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Surface microstructures with undercuts perpendicular to the joining surface were generated on the surface of the aluminum sheets enabling mechanical interlocking between the polymer and aluminum sheet. For the micro structuring, an electrochemical treatment was applied, using hydrochloric acid (HCl) with a concentration of 5 % as electrolyte. For this purpose, the experimental setup as described in [14] and schematically shown in Figure 1 was used. The laboratory glass was filled with 800 ml electrolyte. An AlMg3 plate was used as cathode and the aluminum sheet as anode. The anode and cathode were connected with 6 A to a direct current power supply (Voltcraft DPPS-32-30, Conrad Electronic SE, Hirschau, Germany). A magnetic mixer (RET-G, Janke & Kunkel GmbH & Co. KG, Staufen, Germany) constantly circulates the electrolyte to keep it homogenous. As the electrochemical treatment is an exothermic reaction, two cooling circuits were used to keep the temperature constant to achieve comparable surface microstructures on the aluminum sheets. In the primary cooling circuit, a peristaltic pump (Boxer 9700 Table Top Dispenser, Boxer GmbH, Ottobeuren, Germany) was used to circulate the electrolyte from the laboratory glass into a heat exchanger and back to the laboratory glass. In the secondary cooling circuit, a cryostat (Lauda Ecoline RE 112, Lauda Dr. R. Wobser GmbH & Co. KG, Lauda-Königshofen, Germany) was used to pump water with a temperature of 23 °C inside the heat exchanger.
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Figure 1: Experimental setup (schematically) for the electrochemical treatment of the metal sheets in accordance with [14]
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Before the electrochemical treatment, each aluminum sheet was cleaned with isopropyl alcohol in an ultrasonic bath for 1 min and remaining isopropyl alcohol on the metal surface was vaporized at ambient conditions. 2.3 Additive melt extrusion manufacturing of single lap shear specimens
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For the reproducible melt extrusion manufacturing of single lap shear specimens, a screw-driven hand welding extruder (WEGENER International GmbH, Eschweiler, Germany) was mounted on a 6-axis robot Kuka KR 15/2 (Kuka Roboter GmbH, Augsburg, Germany). The diameter of the extruder screw was 16 mm with a length/diameter-ratio of 10/1 and a nozzle diameter of 2 mm. A hot air fan type HL 1910 E (STEINEL Vertrieb GmbH, Herzebrock-Clarholz, Germany) was mounted in such way that the hot air heated the metal component directly in front of the nozzle to a surface temperature of 180 °C before the deposition of strands. Additionally, an aluminum consolidation roll with a diameter of 50 mm was mounted on the robot 70 mm behind the nozzle in order to enable a consolidation of the deposited material strands. Next to the robot, a table with a heating bed, which enables surface temperatures of the metal component of up to 120 °C, was used as building platform. The ambient temperature was approximately 23 °C and no additional envelope or closed chamber was used. The experimental setup is schematically shown in Figure 2.
Figure 2: Schematic illustration of the experimental setup for the robot-based additive melt extrusion manufacturing At the beginning of each manufacturing cycle, an aluminum sheet was positioned on the heating bed. On the aluminum sheet surface, a polyimide film was fixed in order to limit the overlapping area to 20 mm x 10 mm, (Figure 3 a). Furthermore, thermocouples (type K, Ø 0.2 mm) were placed on the aluminum sheet surface to adjust and control the so-called substrate temperature. The deposition process was started after the aluminum surface reached a nearly constant temperature (approximately ± 2 °C).
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Figure 3: AlMg3 sheets prepared with polyimide film (a), deposited PP strands for lap shear test generation (b) and setup during the deposition of the strands (c)
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During the extrusion, the mass rate was kept constant at a value of 20.5 g∙min-1. The deposition speed was 40 mm∙s-1, the air gap to the substrate was 1 mm and there was no overlapping between the deposited strands lying next to each other. The deposited path is depicted in Figure 3 (b). An untreated AlMg3 plate was positioned on the heating bed as distance element (Figure 3 (c)) to ensure a constant distance between the nozzle and the layup surface. In order to allow the clamping, the deposited strands above the aluminum sheet were cut off approx. 10 mm behind the joining surface with a band saw. For the investigation of the influence of the extrusion and substrate temperature and thus the contact conditions, single lap shear specimens were produced in which these parameters were varied according to Table 2. During the variation of one parameter, the other parameter was set to a constant value, which is denoted through a bold font in Table 2. Furthermore, single lap shear specimens for these parameter combinations were produced with and without a consolidation roll in order to analyze, if a pressure application improves the filling of the surface microstructures and therefore the bonding strength. For this purpose, the consolidation roll was fixed at the relevant specimens in such a way that the deposited strand was consolidated to a height of 2 mm.
Confocal laser scanning microscopy
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For the surface characterization of the hot rolled and electrochemically treated aluminum sheets, a confocal laser scanning microscope type VK-X1000 (Keyence Corporation, Osaka, Japan) was used. The area under consideration was 2 x 2 mm2 in the middle of the joining zone, Figure 3 a. All measurements were conducted with laser light and an optical magnification of 240. For the description of the surface, the arithmetical mean height Sa (Equation (1)) and maximum height Sz (Equation (2)) were determined as defined in DIN EN ISO 25178-2. 𝑆𝑎 =
1 ∬|𝑧(𝑥, 𝑦)|𝑑𝑥 𝑑𝑦 𝐴
(1)
𝑆𝑧 = 𝑆𝑝 + 𝑆𝑣
(2)
A describes the definition range, z(x, y) is the height of the surface at the position x and y, Sp is the maximum height of the peak in the definition range and Sv is the maximum height of the valley in the definition range. In addition, the surface topology was visualized in order to evaluate the surface structures with regard to their distribution. Rheological analyses
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As it can be assumed that the rheological behavior has a major influence on the filling of the surface microstructures, the shear viscosity, storage and loss modulus were determined using a rheometer HAAKE Mars (Thermo Fisher Scientific, Waltham, USA) with a parallel plate setup. For this purpose, cylinders with a diameter of 24 mm and a height of 1 mm were used as specimen. During the test, the deformation was set to 0.5 % and the cooling rate to 2 K∙min-1. The frequency of the oscillatory measurement was set to 1 Hz, because it can be assumed that the material does not experience any further shearing in the contact area after the strand deposition. Contact temperature calculation and measurement As an indication of the current temperature between the newly deposited polymer strand and the AlMg3 surface in time of contact, a simplified calculation of the contact temperature was used. The contact temperature TC thereby is the average value of the substrate temperature TS and the extrusion temperature TE (Equation (3)). 𝑇𝐶 =
𝑇𝑆 + 𝑇𝐸 2
(3)
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Additionally, experimental investigations performed with a thermocouple (type K, Ø 0.2 mm) were conducted to validate the contact temperature calculation experimentally. The thermocouples were positioned on the joining surface of the micro structured aluminum sheets. A repetition number of three was used for each process setting and the experimentally measured peak temperature was evaluated. Reflected light microscopy
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In addition to the laser scanning microscopy, cross sections of the middle of the joining zone of the single lap shear specimens in yz-plane (Figure 4 (a)) were analyzed by reflected light microscopy to investigate undercuts perpendicular to the surface and the filling of the surface microstructures with polymer. The cross sections were cut out with a water-cooled saw. Afterwards, they were embedded in a cold-curing resin, grinded and polished. The wet grinding was performed stepwise until a grit level of 2500 was reached. The grinding paper was wetted with water all the time. For the following polishing step, a diamond suspension was used. The polishing steps were carried out up to a diamond grain size of 1 µm. For the following analysis, a microscope type Axio Imager M2m (Carl Zeiss AG, Oberkochen, Germany) was used. Single lap shear test
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For the investigation of the lap shear strength, single lap shear tests according to EN 1465 with a free clamping length of 75 mm and a test speed of 1 mm∙min-1 were conducted using an universal testing machine type Zwick 1484 (Zwick Roell GmbH & Co. KG, Ulm, Germany). The used test setup is schematically shown in Figure 4 (b). All tests were carried out with standard atmosphere (23 °C and 50 % humidity) according to DIN EN ISO 291.
Figure 4: Position of the cross sections for microscopic investigations (a) and test setup for mechanical testing of the lap shear test specimens (b) Scanning electron microscopy -6-
After the single lap shear test, the fractured surfaces were analyzed using a scanning electron microscope (SEM) type Zeiss Ultra Plus (Carl Zeiss AG, Oberkochen, Germany). Additionally, the surface energy dispersive X-ray (EDX) spectroscopy was used to detect polymer residuals on the aluminum surface and vice versa. Aluminum as main indicator of the AlMg3 sheet and carbon as main indicator of the polymer were differentiated in the applied color mapping.
3.
Results and discussion
Surface characterization
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The surface topology of the untreated and electrochemically treated aluminum sheets are shown in Figure 5 (a). For the planar surface roughness of the aluminum sheets, a significant increase can be achieved by electrochemical treatment (Figure 5 (b)). The average value of the mean arithmetic height Sa of the investigated surface increases from 0.6 µm to 6.1 µm, and the maximum height Sz from 13.1 µm to 64.5 µm due to the electrochemical treatment. The original surface topography of the hot-rolled aluminum sheet disappears completely and a fissured surface develops. Thus, the electrochemically treated sheets have a high unevenness compared to the hot-rolled aluminum sheets and cavities can be seen on the surface.
Figure 5: Exemplary surface topology of an untreated and electrochemically treated aluminum sheet (a) and the roughness parameters Sa and Sz of such surfaces (b)
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Rheological behavior
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In Figure 6, the complex viscosity, storage and loss modulus are shown for the temperature range of 120 °C to 200 °C. It can be seen that a decrease in temperature below 163 °C leads to a higher increase of the storage modulus compared to the loss modulus. Thus, a shift of the material behavior from predominantly viscous to predominantly elastic behavior takes place [25]. For the filling of the surface microstructures it can be concluded that the contact temperature, which results from the interaction of the metal surface and polymer heat balance, should be higher than the temperature at the intersection between the curves of the storage and loss modulus. As the cool down behavior after the first contact depends significantly on the heat transfer inside the metal component, the metal temperature should be as high as possible, but must also allow a consolidation of the polymer for the deposition of further strands. In addition, the extrusion temperature should also be high for enabling a low viscosity for the filling of the surface microstructures.
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Figure 6: Complex viscosity, storage and loss modulus of the used polypropylene, measured with oscillatory rotational rheometry (1 Hz, 0,5 % strain) during cooling (2 K∙min-1) Contact temperature
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The measurement of the surface temperature with thermocouples shows that the simplified calculation of the contact temperature is valid for the substrate temperatures of 80 °C and 120 °C (Figure 7). Only small temperature differences of ± 3 °C occur. However, this approach is not valid for the high substrate temperature of 180 °C. The measured values are at least 10 °C lower than the calculated values. This means that rapid cooling takes place. As aluminum has a significantly lower heat capacity than polypropylene in the relevant temperature range and only the substrate surface of the 1 mm thick aluminum sheet was heated to 180 °C by means of hot air, the temperature decreases quickly. Considering the contact temperatures and viscosity behavior of the material, a substrate temperature of 80 °C seems unfavorable. The resulting contact temperature is below the intersection of loss and storage modulus and thus, a higher elastic material behavior may counteract the filling of surface microstructures. Moreover, the viscosity for the measured contact temperature shows the highest value of 5.5·103 Pa·s. This means that the flowability is the lowest and therefore the filling is expected to be the worst. For a substrate temperature of 120 °C, the contact temperature increases almost linearly with increasing extrusion temperature. At the same time, the viscosity as well as loss and storage modulus decrease at the interface of aluminum and polymer. Thus, the process setting of 180 °C substrate temperature and 230 °C extrusion temperature should be the most favorable for the filling of surface microstructures, as the contact temperature shows the highest value.
Figure 7: Calculated and measured contact temperatures for the different process settings Reflected light microscopy Due to the vertical direction of view, the confocal laser scanning pictures do not resolve the undercuts caused by the electrochemical treatment. However, the exemplary micrographs of the cross sections in Figure 8 (b) and Figure 9 show that undercuts are present for the electrochemically treated aluminum -8-
sheets. As expected, the samples failed as no connection was reached for the untreated sample in any case during processing without a consolidation roll.
Figure 8:
Characteristic micrographs of the polymer-metal interface in dependence of the substrate temperature ((a) 80 °C and (b) 180 °C) and an extrusion temperature of 230 °C, exemplarily without consolidation roll
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For a substrate temperature of 80 °C (Figure 8 (a)), the processing was not successful and no bonding occurred between the aluminum and the used polypropylene. However, aluminum particles can be detected in the micrographs of the deposited strands, which were in contact with the aluminum surface. Due to the high light reflection of aluminum, those particles shine through the surrounding polymer. The contacting surface line is rather smooth for a substrate temperature of 80 °C. This indicates a fast cooling, which prevents the polymer to cover the fissured aluminum surface and confirms the assumption that the aluminum temperature should be high due to its high heat conduction and the resulting contact temperature.
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For the other substrate temperatures of 120 °C and 180 °C, the polymer strands cover the surface of the aluminum sheets and fill the structures. As there is no difference detectable on the micrographs between the substrate temperatures of 120 °C and 180 °C, only the higher temperature is illustrated exemplarily in Figure 8 (b). Though hardly visible, some voids can be detected at the interface between polypropylene and aluminum, which may act as weak spots during mechanical load. The voids occur near the surface, regardless of the size of the surface microstructure. It can be assumed that air is trapped during the strand deposition between the aluminum sheet and polypropylene strand. Due to the filling of even very small surface microstructures, the enclosed air cannot be released and remains as voids. An equivalent optical result is obtained when using a consolidation roll.
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For the process variation of the extrusion temperature, the use of a consolidation roll leads to a contrary result for the lowest extrusion temperature of 210 °C. While the preparation by means of sawing led to a separation of the deposited strands from the aluminum surface without the use of a consolidation roll, the strands deposited with consolidation roll did not separate. Figure 9 (a) shows a characteristic micrograph of the polymer-metal interface of a sample, which was produced without consolidation roll. A large number of aluminum particles can be detected in the deposited strands. Moreover, the surface line of the polymer appears rough in the cross section. Thus, the polypropylene seems to have a sufficient flowability for a partial filling during the contact with the aluminum substrate. A reduced filling of the surface microstructures, especially the undercuts, might be a reason for the premature failure during the preparation. By using a consolidation roll, the surface microstructures of the substrate surface are filled almost completely (Figure 9 (b)). The results for the higher extrusion temperatures of 230 °C and 250 °C show a similar behavior regardless of the use of a consolidation roll.
Figure 9:
Characteristic micrographs of the polymer-metal interface of samples which were produced with an extrusion temperature of 210 °C, a substrate temperature of 120 °C with no consolidation roll (a) and with consolidation roll (b) -9-
The microscopic investigations and present contact temperatures show that the selected process settings cover the critical range concerning viscoelastic material behavior and filling of surface microstructures with polymer. For the measured contact temperature of 155 °C (80 °C substrate and 230 °C extrusion temperature, Figure 8 (a)), predominantly elastic material behavior is present and neither the surface microstructures are filled nor the rough surface of the substrate can be reproduced on the polymer surface. The filling of surface microstructures changes already for a contact temperature of 168 °C (120 °C substrate and 210 °C extrusion temperature. Here, the surface structures are reproduced on the polymer surface due to the material’s change to a slightly predominant viscous behavior with a lower viscosity (Figure 9 (a)). By using the consolidation roll in order to apply an additional pressure, the microstructures can be filled to a large range (Figure 9 (b)). Hence, the consolidation roll influences the bond between the electrochemically treated aluminum and polymer positively. However, a consolidation roll is not mandatory as the results with higher contact temperatures show. For the exemplarily chosen contact temperature of 194 °C (180 °C substrate and 230 °C extrusion temperature, Figure 8 (b)), even small surface microstructures are filled. This can be attributed to the even lower viscosities and a more predominantly viscous material behavior, but also to a capillary effect, which has to occur to fill the very small surface microstructures with their undercuts.
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Single lap shear tests
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Although the micrographs do not show significant differences for the examined cross sections, an increase in substrate temperature results in higher ultimate lap shear strengths (ULSS) and higher elongations at failure, Figure 10 (a). The ULSS increases from an unmeasurable value (80 °C) to (5.05 ±0.57) N·mm-2 (120 °C), and to (5.55 ±0.38) N·mm-2 (180 °C) without the use of a consolidation roll. This is consistent with the results of the color mapping of the SEM analysis (Figure 11). A rising proportion of carbon (colored red) on the aluminum surface (colored green) occurs for higher substrate temperatures. This means that more polymer residuals remain after the lap shear tests, which indicate a better mechanical interlocking. One specimen produced with a substrate temperature of 180 °C and no consolidation roll did not fail in the joining surface but in the deposited polymer. For the samples produced by using a consolidation roll one sample failed outside the joining surface in the polymer for a substrate temperature of 120 °C and two samples for 180 °C.
Figure 10: ULSS and elongation at failure for the variation of the substrate temperature (a) and for the
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variation of extrusion temperature (b);*for all test specimens a failure of the polymer component occurred
The ULSS tends to decrease by using a consolidation roll. At the same time, the elongation at failure does not decrease. As there is also no significant difference between the fractured surfaces for the use of a consolidation roll, Figure 11 (d) - (f), the reduced height of the deposited strands might explain the lower ULSS. Due to the compaction with the consolidation roll, the height of the deposited strands decrease from an average value of 3.7 mm to 2.1 mm. During testing, the applied tensile force leads to an additional bending moment, which is caused by eccentricity of the sample geometry. Tensile force and bending moment form the applied load on the sample. The composition of the applied load depends on the height, stiffness and overlap length of the joining components. Smaller heights lead to a reduction of the bending moment. This implies that the transferable load would be higher for the use of a consolidation roll due to the decreasing eccentricity. However, this is only valid for joints with balanced stiffness. As the stiffness of aluminum (about 70000 N/mm²) is much higher than the one of polypropylene (1500 N/mm²), an imbalanced stiffness occurs for the same joining partner height. A - 10 -
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reduction in height causes a higher deflection of the polymer due to a decrease in the area moment of inertia. This leads to higher stresses at the overlapping ends and an additional peel force. In the case of the bending moment, the height is linearly included in the calculation, while the height is included to the power of three for the calculation of the area moment of inertia. This means the effect of the reduction of the height will have a stronger influence on the mechanical tests. A higher bending occurs due to a lower moment of inertia, which is additionally superimposed by the imbalanced stiffness. Thus, smaller heights due to the use of a consolidation roll can explain the tendency to smaller ULSS.
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Figure 11: SEM scans of the fractured surfaces on the substrate and EDX spectroscopy color mapping in dependence of the substrate temperature and an extrusion temperature of 230 °C; (a) - (c) no consolidation roll, (d) - (f) with consolidation roll; green color indicates aluminum, red color indicates carbon
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For the extrusion temperature variation, a similar behavior can be seen for the ULSS, Figure 10 (b). In general, higher extrusion temperatures lead to an increase in the maximum stress and elongation at break. However, there are two points to consider. Firstly, there is a higher ULSS for the use of a consolidation roll at 210 °C than without a roll. Furthermore, none of the test specimens fails when using a consolidation roll with an extrusion temperature of 250 °C in the joining surface. Here, the deposited strands consistently fail and a significantly higher elongation occurs due to yielding of the polypropylene.
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In consequence, higher extrusion temperatures lead to an increase in mechanical performance of the polymer-metal hybrid structure, which is supported by the rheological investigations. Besides the lower viscosity with higher extrusion temperature, also the cooling time rises due to the higher contact temperature and thermal mass. This means, especially in the contact area, that lower viscosities are present for a longer time, so that flow processes take place more easily and faster. The fracture surfaces of the tested specimens are a further indication of this (Figure 12). The amount of carbon (colored red), which indicates polypropylene residuals, rises with an increasing extrusion temperature without a consolidation roll, Figure 12 (a) - (c).
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Figure 12: SEM scans of the fractured surfaces on the aluminum sheet and EDX spectroscopy color
mapping in dependence of the extrusion temperature and a substrate temperature of 120 °C; (a) - (c) no consolidation roll, (d) and (e) with consolidation roll; green color indicates aluminum, red color indicates carbon
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A different behavior occurs for the use of a consolidation roll. Using a consolidation roll, a high ULSS is already detected for a temperature of 210 °C. This differs from the higher temperature of 230 °C only in a significantly higher standard deviation. While for the temperature of 230 °C the ULSS values are in the range of 3.26 N·mm-2 to 4.51 N·mm-2, the values determined for 210 °C are in a much wider range. The lowest ULSS is 3.02 N·mm-2 and the highest is 5.5 N·mm-2, being almost in the range of the extrusion temperature of 250 °C (5.61 N·mm-2 to 6.66 N·mm-2) with consolidation roll. The specimen produced with 250 °C and consolidation roll failed exclusively in the polymer outside the joining surface.
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The mechanical results correlate well with the amount of visible polymer residuals on the aluminum surface. Both temperatures of 210 °C (Figure 12 (d)) and 230 °C (Figure 12 (e)) show a comparable appearance. Due to the fact that all specimens for an extrusion temperature of 250 °C failed outside the joining surface, no scanning electron micrographs were prepared. It is to be assumed that a high filling of the surface microstructures is present, so the occurring loads can be transmitted. Comparable to the variation of the substrate temperature, the ULSS are higher without the use of a consolidation roll. Likewise, the layer height shows higher values resulting in higher area moment of inertia, which has to be considered.
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However, this does not apply to the process setting of 210 °C. The layer height is 2.3 mm with the use of a consolidation roll. This results in a lower area moment of inertia but significantly higher lap shear strengths occur than without the use of a consolidation roll at 210 °C. Considering the contact temperature, it decreases with decreasing extrusion temperature. In case of an extrusion temperature of 210 °C and a substrate temperature of 120 °C the contact temperature is 165 °C. Consequently, an approximation to the intersection of the storage G’ and loss modulus G’’ takes place and a rather viscous material behavior is present over a short period. If the material reacts predominantly elastic to deformations, flow processes are more severely restricted and the filling of the structures with polymer is not possible or cannot be completed without an additional pressure. However, even a small amount of pressure, as applied by the consolidation roll, seems to be sufficient to overcome these filling limitations. A more viscous behavior also explains the rise in ULSS with increasing contact temperature as Figure 13 illustrates. The lower the viscosity, the better the filling of the surface microstructures, which is needed for the mechanical interlocking. Nevertheless, not only the contact temperature is of importance, but also the thermal properties of the polymer and metal. The highest lap shear strength does not occur for the highest measured contact temperature of 194 °C with a substrate temperature of 180 °C and an extrusion temperature of 230 °C, but for the measured contact temperature of 185 °C with a substrate temperature of 120 °C and an extrusion temperature of 250 °C. Since aluminum has a higher thermal conductivity than polypropylene, a quicker decrease of the contact temperature for the extrusion temperature of 230 °C and the substrate temperature of 180 °C than for the extrusion temperature of 250 °C and the substrate temperature of 120 °C may explain the differences in ULSS. Furthermore, there are generally lower viscosities in the deposited strand for extrusion temperature of 250 °C - 12 -
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compared to 230 °C. Although the performed mechanical tests do not indicate an increase in contact area due to the use of a consolidation roll, this is very likely and supported by the consideration of the ULSS in relation to the area moment of inertia. This results in higher values for the use of a consolidation roll and an increasing trend for increasing contact temperatures. Accordingly, the geometric boundary conditions play an important role and will therefore be considered in detail in further investigations.
Figure 13: ULSS of the different process settings in dependence of the calculated contact temperature between the substrate temperature and extrusion temperature
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Conclusions
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The ultimate lap shear strengths are clearly dependent on the substrate and extrusion temperature. Higher substrate and extrusion temperatures lead to higher contact temperatures at the joining surface and lower viscosities. Thus, the filling of surface microstructures is promoted which results in higher ULSS values. For the filling of the surface microstructures, a sufficient contact temperature is required. With a simplified calculation of the contact temperature between the polymer and metal sheet in combination with the intersection of the storage and loss modulus, an estimation for the minimum required contact temperature can be made. It is shown that the contact temperature should be higher than the temperature at the intersection of the storage and loss modulus, so that the material shows a predominantly viscous behavior. Using a consolidation roll does not necessarily improve the ULSS, although the ULSS generally shows lower values in comparison to no consolidation roll. As scanning electron and light microscopic investigations do not show a lower filling of the surface microstructures, the reason for lower mechanical properties can be most likely attributed to the reduction of layer height. Through the height reduction, the resistance against the bending moment is lower, which leads to a higher bending during the single lap shear test. However, taking the present area moment of inertia into account, the use of a consolidation roll can be beneficial. In case of low contact temperatures, the consolidation roll shows a beneficial influence. For the lowest investigated extrusion temperature of 210 °C, the resulting contact temperature is close to the intersection of the storage and loss modulus. The pressure applied by the consolidation roll is sufficient to fill the surface microstructures with polymer and to counteract its elastic deformations.
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This work investigated the manufacturing of polymer-metal hybrids by means of extrusion based additive manufacturing of polypropylene on aluminum sheets with electrochemically manufactured surface microstructures. For this, substrate and extrusion temperatures were varied and the influence of a consolidation roll was investigated. It could be demonstrated that a coating of the metal is not necessarily needed to reach a stressable joint. Based on the results, the following statements can be drawn:
Notwithstanding, these discussed results and dependencies are determined for a material combination of polypropylene and aluminum with lap shear joints. A transfer to other types of load, material - 13 -
combinations, and the dependency of the calculated contact temperature with the predominant material behavior might be limited. For this purpose, future investigations should also consider same layer heights, different geometric boundary conditions, other types of load, e.g. single rib specimens with a load transmission perpendicular to the joining surface, and a transfer to other polymers. Furthermore, the interactions that occur between component geometry, mechanical properties and the use of the consolidation roll must be further analyzed. To evaluate the influence of thermal mass and different heat capacities, thermal simulations and models offer a promising approach.
References
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Table 1: Characteristic values of the used polypropylene Parameters Unit
Standard
Value
cm³∙10-1∙min-1
ISO 1133
2.9 ± 0.05
°C
ISO 11357
159
°C
ISO 11357
116
Thermal conductivity+
W∙m-1∙K-1
ISO 8302
0.22
Specific heat capacity+
J∙kg-1∙K-1
ISO 11357
1700
Density+
kg∙m-3
ISO 1183
905
Melt flow rate (230 °C, 2.16
kg∙min-1)
Peak melting temperature# Peak crystallization
#Heating
temperature#
rate / cooling rate = 10 K·min-1 to datasheet
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+According
Table 2: Varied process parameters / conditions for the production of PP-AlMg3 single lap shear specimens. The bold font denotes the value, which was applied during the variation of the other parameters Parameter / conditions Unit Value °C
210
Substrate temperature
°C
80
Distance consolidation roll to substrate
mm
230
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Extrusion temperature
120
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250 180
2