High-tensile joints of continuously fusion bonded hybrid structures

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High-tensile joints of continuously fusion bonded hybrid structures ⁎

Tobias Reincke , Sven Hartwig, Klaus Dilger Technische Universität Braunschweig, Institute of Joining and Welding, Langer Kamp 8, 38106 Braunschweig, Germany

A R T I C L E I N F O

A B S T R A C T

Keywords: Hybrid structures CFR-TP Climbing drum peel test Primer

Tailored hybrid structures manufactured within a continuous production process, for instance in a continuous roll forming line, offer a high potential for the automotive industry, due to its weight reduction compared to pure metal parts. Therefore, the hybrid roll formed parts consist of steel sheets reinforced by carbon fibre reinforced thermoplastic tapes (CFR-TP). Both materials are joined by fusion bonding whereby the surface of the thermoplastic matrix is melted on the steel surface. The pre-treatment of the steel with a primer, customized for joining of steel and CFR-TP, is a promising approach, due to its easy implementation into continuous coil-coating processes. Major challenges consist in achieving sufficient joint strength as well as developing of a testing method for the evaluation of the adhesion between both materials. Relating to joint strength, various process parameters (i.e. temperature of steel and CFR-TP) are examined. The fracture pattern are analysed optically by microscopy to detect possible primer damage. Furthermore, the evaluation is carried out by mechanical testing of peel specimens within climbing drum peel test to determine the production-related influence in a continuous process. The examinations show the usability of climbing drum peel test and the increase of joint strength with higher energy input.

1. Introduction and state of the art Automotive serial production requires lightweight solutions manufactured within low cycle times and production costs. In consideration of multi-materials structures, innovative manufacturing and joining technologies have to be developed. Roll forming of steel sheets reinforced by carbon fibre reinforced thermoplastic tapes combines the approach of lightweight and high-volume production. Along with recyclability and decoupling of shaping and forming process, the application of a thermoplastic matrix enables reduction of process steps, due to no application of additional adhesive. Joining of steel and fibre reinforced thermoplastics can be carried out by fusion bonding. After melting of the surface-near area of a thermoplastic matrix, the metal surface is wetted and a hybrid joint is created by solidification of the matrix. However, integration of fusion bonding processes in automotive high-volume production is still and especially challenging because of interactions between both materials, especially corrosion as well as hygrothermal stresses and strains. The state of the art focuses on commonly applied testing methods for CFR-metal joints as well as on the influence of process parameters and surface pre-treatments on fusion bonded CFR-TP-metal joints. Lap shear tests were mainly used for testing of fusion bonded CFRTP-metal structures [1–3]. Peel tests for multi-material structures

⁎

mostly focus on investigations of adhesives [4] or fibre metal laminates [5,6] and vary with regard to fixture configuration. Exemplary used peel tests for multi-material structures are roller peel test [4,7–10], 90° degree peel test [11,12], T-Peel test [13] fixed arm peel test [14,15], mandrel peel test [5,16–18] or climbing drum peel test [19]. In consideration of fusion bonded structures, roller peel tests of DC01 steel and carbon fibre reinforced Polyamide 6 (CFR-PA6) tape were carried out [3,9,10]. In addition, relating to mandrel peel tests, rigid adherend of titanium and flexible adherend of carbon fibre reinforced Polyetherketoneketone (PEKK) were tested [18]. Peel tests with a flexible carbon fibre reinforced plastic (CFRP)-adherend could be critical. The material is bent with a defined curvature during peel test [20] which can result in fibre breakage [17,21,22]. Previous investigations showed breakage of CFR-PA6 tape during roller peel test by using SACO® (sandblast coating) pre-treatment for the steel adherend [10]. Most of the investigations in literature focus on the influence of various process parameters and were carried out by sequential fusion bonding processes. Various studies consider hybrid joints of metals consisting of aluminium or steel and glass fibre or carbon fibre reinforced thermoplastics [1,3,9,23,24]. The Influence of steel and CFRPA6 temperatures on peel resistance of continuously manufactured joints was investigated with regard to separately heated DC01 steel and

Corresponding author. E-mail address: [email protected] (T. Reincke).

https://doi.org/10.1016/j.compstruct.2017.12.027 Received 30 November 2017; Accepted 11 December 2017 0263-8223/ © 2018 Elsevier Ltd. All rights reserved.

Please cite this article as: Reincke, T., Composite Structures (2018), https://doi.org/10.1016/j.compstruct.2017.12.027

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CFR-PA6 [10].The influence of surface iron oxides on the adhesion between CFR-PA6 and DC01 steel was also examined [3]. In consideration of the influence of various pre-treatments on the adhesion between continuously joined DC01 steel and CFR-PA6, mechanical pre-treatments of steel, i.e. low or high pressure blasting, or pre-treatment of CFR-PA6 by plasma were investigated [9,10]. The most promising approach was the pre-treatment of the steel surface by SACO®, which combines mechanical pre-treatment by high pressure blasting with application of a primer. However, surface pre-treatment for high tensile joints was only examined to a limited extent within continuously manufactured fusion bonded structures. In contrast to that, investigations with sequential processes already considered high pressure blasting [1], sand blasting [25], plasma cleaning [1], pickling [26,27], laser structuring [24] or application of primers [28,29]. Furthermore, pre-treatment of PA6 composites by plasma was investigated [30]. In summary, most of the examinations with regard to fusion bonding technology only focus on roller peel or mandrel peel tests. The investigations relating to continuous fusion bonding processes mainly consider joining of mutual heated hybrid materials or focus on basic research of the process without achieving sufficient joint strength for application in automotive components. Especially, the integration of surface pre-treatments in continuous fusion bonding process without lowering processing speeds and evaluation of these high tensile hybrid joints were not investigated. Hence, the aim of the paper was to close this gap by manufacturing fusion bonded hybrid joints with sufficient joint strength and modifying existing test methods to examine these multi-material structures.

Table 1 Material properties of DC01, CFR-TP and primer. Metal adherend

DC01

HX340 LAD Z100 MB

Yield strength [MPa] Tensile strength [MPa] Thickness [mm] Coating Coating thickness [g/m2]

Max. 280 270 – 410 1.00 None –

340–420 410–510 1.00 Galvanized 100

CFR-TP adherend

Carbon fibre reinforced thermoplastic tape

Matrix material Tensile strength [MPa] Tensile modulus [GPa] Tensile strain to fail [%] Melting temperature [°C] Glass transition temperature [°C] Density [g/cm3] Thickness [mm] Fibre volume ratio [%]

Polyamide 6 1909 100 1.76 220 47 1.45 0.13 48

Primer

Vestamelt® Hylink

Base Melting temperature [°C] Beginning curing temperature [°C] Density [g/cm3]

Copolyamide and curing agent 135 160 1.0–1.3

by SACO® with DELO-SACO® Plus [31] from DELO Industrie Klebstoffe GmbH & Co. KGaA (Windach, Germany) was used for reference with primer application and was also carried out manually comparing to the high pressure blasting process. No additional primer was applied after coating by SACO® process. Due to the aim of an integration of the pre-treatment in continuous fusion bonding processes, the applied primer had to be adaptable into small or large scale production. The selected primer Vestamelt® Hylink offers this possibility and can be integrated into the manufacturing of steel by using a coil coating process. Within this investigation, the primer was available in powdered form. As a consequence, the powder was applied on cleaned DC01 and HX340 steel surface and was joined in a press at a temperature of 150 °C for 2 min [32]. Curing of primer begins at a temperature of 160 °C with the result that the primer is still able to cure within the following continuous fusion bonding process. The primer thickness on the steel surface was approximately 0.08 mm and was kept constant within this investigation.

2. Materials and methods This paper examined fusion bonded multi-material structures consisting of steel with an applied primer and CFR-TP. The objective of this work was the continuous manufacturing of CFR-TP-steel structures and to characterize the fusion bond with an adequate testing method in dependency of process parameters, especially steel and CFR-TP temperature, as well as surface condition. Therefore, steel surface was pretreated by selected primer. Furthermore, climbing drum peel test was adapted for the evaluation of the adhesion between steel and CFR-TP. 2.1. Materials applied The material properties of the investigated steel and CFR-TP as well as applied primer are shown in Table 1. The CFR-PA6 as well as the primer Vestamelt® Hylink (VM) from Evonik Industries AG (Marl, Germany) were not dried or conditioned prior to joining process. On the one hand, manufacturing and testing were carried out at relative humidity of 50% and constant temperature of 23 °C to prevent influence of changing temperature and moisture content. On the other hand, objective of the developed process was the realistic and cost-efficient simulation of production of hybrid structures. Moisture content of Polyamide 6 matrix of 3.11% was measured after drying at a temperature of 80 °C for 7 days and afterwards storing at climatic conditions (23 °C, 50%) for 7 days. The steel was cleaned by wiping the surface with n-Heptane to remove any present contamination and afterwards dried for at least 10 min. The pre-treatment was carried out according to Chapter 2.2 and in case of a mechanical pre-treatment again cleaned with n-Heptane. In addition, remaining particles on mechanically pre-treated samples were removed by oil- and water free compressed air.

2.3. Continuously manufactured fusion bonded multi-material structures The continuous production focuses on heating of both materials as well as subsequent fusion bonding which are important process steps in a continuous manufacturing process of multi-material structures. Both materials had to be heated to a defined temperature for subsequent fusion bonding process. Therefore, steel with applied primer was heated by induction and CFR-PA6 by infrared heating. The CFR-PA6 could be heated from the upper side because of its low thickness of 0.13 mm. Furthermore, the upper aluminium roll was coated with a polytetrafluorethylene tape to prevent an adhesion between upper roll and heated CFR-PA6. 2.4. Test methods applied The determination and evaluation of mechanical properties of multi-material joints produced within continuous fusion bonding process requires adequate testing methods. The approach presented within this paper was the application and modification of commonly used and standardized test methods for adhesive bonds. Therefore, testing of the interface of the CFR-TP-steel specimens was carried out by climbing drum peel test which is derived from the DIN EN ISO 2243-3 for sandwich testing and was modified for testing of CFR-TP-steel samples.

2.2. Surface pre-treatment The reference steel surface without primer application has been pretreated mechanically by high pressure blasting with white corundum with a grain size in the range of 210 µm to 300 µm. The pre-treatment 2

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Fig. 1. Sample geometry [a], evaluation area [b] and climbing drum peel test [c]

hybrid structures into interfacial failure (AF), cohesion failure of CFRPA6 (CF) as well as substrate close cohesion failure (SCF) [10]

Furthermore, CFR-TP was investigated by a thermogravimetric analysis to determine the beginning of degradation. Due to different melting and decomposition temperatures, applied primer was also examined by thermogravimetric analysis. With focus on determining possible degradation within continuous fusion bonding process, surfaces of steel specimens with applied primer were investigated for different steel temperatures.

2.4.2. Analytical tests The thermogravimetric analysis was performed with a TG 209 F1 Libra® (Erich NETZSCH GmbH & Co. Holding KG, Selb, Germany). The heat flow during heating of powdered primer as well as CFR-PA6 was measured by varying the heating rate in the range of 10 K/min up to 50 K/min from a starting temperature of 25 °C up to 500 °C and ambient pressure. The examination of steel surface with applied primer was carried out with digital microscope Keyence VHX 2000 (Keyence Corporation, Osaka, Japan).

2.4.1. Climbing drum peel test The peel test was selected due to its suitability for comparing various pre-treatments and the evaluation of adhesion between steel and primer as well as CFR-PA6 and primer in dependency of joining parameter (i.e. temperature of steel or CFR-PA6). In comparison with i.e. lap shear tests, peel tests are more suitable, due to the occurring line load within the test, and thus lead to higher sensitivity of this test for adhesion defects [33]. In addition, peel tests are more preferable for testing of samples produced in continuous fusion bonding process described in Chapter 2.3. The geometry of CFR-PA6-steel samples for climbing drum peel tests is shown in Fig. 1a whereas the configuration of the test is shown in Fig. 1c. The CFR-PA6-steel specimens consisted of a rigid adherend of steel with applied primer and a flexible adherend of CFR-PA6. The CFR-PA6 was fixed to climbing drum and peeled of the steel with the fibre orientation of 0° in the longitudinal direction of the sample. The mechanical tests to measure peel resistance were performed with a universal testing machine zwicki Z1.0 with a load cell of 1000 N (Zwick GmbH & Co. KG, Ulm, Germany) and a testing speed of 100 mm/min. Due to induction induced heat accumulation at the ends of the steel adherends, an evaluation area of the multi-material specimens was selected according to Fig. 1b. Within this evaluation area, temperature only varies approximately ± 5 °C. The evaluation of fracture pattern is derived from evaluation of adhesives (DIN EN ISO 10,365) and can be divided with regard to

2.5. Test parameter Relating to the continuous fusion bonding process, processing speed (10 mm/s) as well as joining pressure (8.38 N/mm2) was kept constant for all climbing drum peel samples. For each of the described roller peel tests at least five specimens were tested. The test matrix for climbing drum peel test examinations is shown in Table 2. The first step consisted in the examination of the influence of CFRPA6 temperature at constant steel temperature of 255 °C. This steel temperature was selected due to promising results in previous investigations without primer [9,10]. Concerning cooling occurring in the area between inductor and joining area, the steel had to be heated to higher temperatures to achieve the targeted temperature within the joining area. The second step consisted in the investigation of interaction of the influence of steel and CFR-PA6 temperature. Therefore, the steel temperature was varied in the range of 235–275 °C and the CFR-PA6 temperature was considered in the range of 203–263 °C. The third step consisted in combination of higher pressure blasting and application of primer to investigate further potential of steel

Table 2 Test matrix climbing drum peel test examinations. Aim of analyses

Tsteel [°C]

Influence of DC01 and CFR-PA6 temperature Influence of HX340 and CFR-PA6 temperature References Influence of DC01 and CFR-PA6 temperature

235, 255, 255, 255,

255, 275 275 275 275

TCFR-PA6 [°C]

Surface condition CFR-PA6

Surface condition steel

203, 229, 263 263 263 263

Cleaned Cleaned Cleaned Cleaned

Vestamelt® Hylink Vestamelt® Hylink SACO®, high pressure blasted High pressure blasted + Vestamelt® Hylink

3

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surface modification prior to joining process. To compare and classify the experimental results, two references were investigated. On the one hand, reference samples pre-treated with high pressure blasting and thus without primer were manufactured. On the other hand, reference specimens with a combined mechanical and chemical pre-treatment by SACO® were investigated. In addition, HX340 with galvanized coating was examined considering corrosion protection and transferability of the results of uncoated to galvanized steel.

Table 4 Maximum temperatures interface between steel and primer. DC01 Joining temperature steel [°C] Temperature steel under inductor [°C]

156

185

200

221

235

255

275

295

171

207

224

250

263

298

316

340

3. Results 10 mm prior to joining area and results in increased heating rate of CFR-PA6 up to joining area. Beginning from TIR+S by contact of both materials, CFR-PA6 is additionally heated by conduction of pre-heated steel. A higher temperature of CFR-PA6 compared to steel would result in a cooling rate of CFR-PA6 up to joining area. The joining temperature TJ is most important for adhesion between both materials. Due to short contact time of approximately 0.3 s between joining rolls and CFR-PA6, time above melting temperature Tm (PA6) within this example is only 1.0 s and in case of lower CFR-PA6 temperature before joining area even lower. The contact between joining rolls leads to cooling within interface (temperature after joining TR). Following increase in temperature up to TC can be attributed to stored heat within both materials. Afterwards, cooling of multi-material specimens takes place. The targeted joining temperature of steel TSteel of 255 °C could not be measured because of insufficient measurement frequency.

Hereafter, the results of temperature measurements and analytic tests are presented. In addition, investigations considering influence of temperatures of steel and CFR-PA6 as well as influence of steel surface condition on joint strength are shown. 3.1. Temperature measurement In consideration of the evaluation of heating and subsequent joining of both materials, determination of process temperatures is very important for process comprehension. The steel surface was coated with graphite spray prior to measuring temperatures with an infrared camera FLIR SC655 (FLIR Systems Inc., Wilsonville, Oregon, United States). The infrared camera was positioned between inductor and joining rolls. To calculate temperatures under inductor as well as joining rolls, extrapolation was applied. In addition, infrared camera was positioned behind joining rolls to measure steel surface temperature after contact with joining rolls. Type K thermocouples, which were bonded by Polyimide tape on CFR-PA6 surface, were used to measure CFR-PA6 temperature within pre-heating process. In addition, these thermocouples were also used to measure the temperature within the interface of CFR-PA6 and steel within joining area. The measured temperatures on steel surfaces prior as well as subsequent to joining process in the range of 221 °C and 275 °C are shown in Table 3. In addition, heating and cooling rates within continuous process are listed. In consideration of applying a primer on steel surface prior to manufacturing process, maximum temperatures in the interface between steel and primer were calculated in Table 4 by extrapolating measured temperatures between inductor and joining rolls. Higher joining temperatures require increased temperatures under the inductor. Due to distance of 142 mm between middle of inductor and joining area, primer is exposed to higher temperatures for approximately 14 s. An exemplary temperature curve of infrared heated CFR-PA6 on 225 °C with the targeted steel temperature of 255 °C is shown in Fig. 2a. Within the temperature curve contact to guiding wire, which results in decrease of temperature for short period, is observable. This wire is used to compare different temperature measurements and to define the distance to joining area of 55 mm which implicates process time of 5 s. Therefore, temperature Tw by contact to the wire is important within this process. Due to geometrical reasons of joining rolls and material thicknesses, contact between steel and CFR-PA6 begins approximately

3.2. Analytical tests The thermogravimetric analysis was applied for evaluation of decomposition start of Polyamide 6 matrix as well as primer VM in dependency of atmosphere and heating rate. The measured temperature onsets of decomposition Onset Td are shown in Table 5 for heating rates in the range of 10 °C/min and 50 °C/min and for oxygen (O2) as well as nitrogen (N2) atmosphere for CFR-PA6. Higher heating rates lead to increased Onset Td, due to lower energy input per time and lower melt kinetic. The oxygen atmosphere also results in decomposition at lower temperatures because of oxidation reaction of the Polyamide 6 matrix. In comparison to CFR-PA6 copolyamide based VM has decreased Onset Td, due to chemical composition of primer. Therefore, decomposition of primer is more critical as decomposition of CFR-PA6 within the process. Continuous fusion bonding process relates in quite high heating rates with the result that temperatures above 300 °C seem to be critical. Optical microscopy of steel surfaces with applied primer focused on indications for decomposition of primer or possible effects of processing as kinetic reactions. Steel surfaces with primer were induction heated without subsequent temperatures of CFR-PA6 in the range of 221 °C and 295 °C respectively maximum process temperatures between 250 °C and 340 °C and shown in Fig. 3. Increasing temperature leads to significant change within primer appearance. Temperatures of 235 °C (250 °C) result in transformations within primer whereas temperatures of 255 °C (290 °C) effect in white boundaries along to these changes. Further increase in temperature results in higher incidence of these effects. In addition, temperatures of 295 °C (340 °C) lead to yellow discoloration within primer which can be considered as decomposition of primer.

Table 3 Temperature overview steel. DC01 Joining temperature [°C] Start temperature [°C] Phase 1 – heating [°C/s] Temperature under inductor [°C/s] Phase 2 – compensation [°C/s] Phase 3 – joining [°C/s] Temperature after contact [°C/s] Phase 4 – cooling [°C/s]

221 23 +150 250 −1 −240 149 −2

235 23 +160 263 −2 −258 158 −2

255 23 +179 298 −2 −275 172 −3

275 23 +195 316 −2 −294 187 −3

3.3. Influence of steel and CFR-PA6 temperature Considering adhesion between steel and primer as well as primer and CFR-PA6, influence of steel and CFR-PA6 temperature on peel resistance is shown in Fig. 4. All investigated multi-material specimens showed no indications of discoloration of primer within evaluation area. Steel temperatures of 4

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Fig. 2. Exemplary CF-PA6 temperature curve of process [a] and test setup with IR-emitters [b]

Table 5 Results thermogravimetric analysis.

CFR-PA6 CFR-PA6 VM

Heating rate [°C/min]

10

20

50

Onset Td [°C] O2 Onset Td [°C] N2 Onset Td [°C] O2

312

322

356

396

421

439

271

284

303

235 °C in combination with CFR-PA6 temperatures up to 229 °C do not lead to sufficient peel resistance whereas totally molten CFR-PA6 results in significant increase in peel resistance of 1.06 ± 0.18 N/mm. However, all these specimens showed interfacial failure between steel and primer whereas samples with lower CFR-PA6 temperatures in some areas additionally fail between primer and CFR-PA6. Increase in steel temperature on 255 °C resp. 275 °C results in higher peel resistance for different CFR-PA6 temperatures. On the one hand, no difference between both steel temperatures is identifiable for CFR-PA6 temperature of 203 °C resp. 229 °C with highest peel resistance of 0.60 ± 0.14 N/ mm. In comparison with steel temperature of 235 °C all samples show interfacial failure. On the other hand, steel temperature of 275 °C in

Fig. 4. Influence of steel and CFR-PA6 temperature on peel resistance.

combination with CFR-PA6 temperature of 263 °C achieve highest peel resistance of 1.85 ± 0.23 N/mm within this test series. The fracture pattern can be identified as interfacial failure between steel and primer. In summary, increased energy input due to higher steel temperature Fig. 3. Microscopy of steel surfaces with applied primer in dependency of temperature.

221 °C (250 °C)

235 °C (263 °C)

255 °C (290 °C)

275 °C (316 °C)

295 °C (340 °C)

295 °C (340 °C) 5

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Table 6 Roughness’s pre-treated steel surfaces. Steel

Surface pre-treatment

Roughness Ra [µm]

DC01 DC01 DC01 HX340

Cleaned High pressure blasting SACO® Cleaned

0.99 2.59 1.75 1.70

± ± ± ±

Rz [µm] 0.05 0.20 0.19 0.11

4.82 ± 0.33 16.82 ± 1.27 12.35 ± 1.72 7.34 ± 0.42

Highest peel resistance can be achieved by combining primer application and mechanical pre-treatment. 4. Discussion This discussion focuses on adhesion between applied materials in dependency of manufacturing process and surface condition of steel. The analytical tests show temperature induced material alterations of primer VM in terms of segregated transformations as well as discoloration at temperatures above 275 °C. Amounts as well as size of segregations increase with higher energy input due to increased steel temperatures. In addition, primer is affected by high heating rates (up to 195 °C/s) in combination with short heating time above melting temperature of approximately 18.5 s. Due to heating of primer by conduction of steel, especially the interface between steel and primer is affected by temperature. The base of primer consists of a copolyamide as well as a curing agent. On the one hand, a possible explanation for segregation formation could be the cross linking reaction within primer. As a result, condensation products of crosslinking reaction could lead to segregation formation. Furthermore, energy input is not enough to cure primer entirely. On the other hand, transformation could be caused by high heating rates in combination with high temperatures, which lead to the accumulation of moisture or gas volatiles within primer. Steel temperatures in joining area above 275 °C lead to discoloration of primer VM in terms of yellow boundaries along to segregated transformations. Due to pre-heating of the steel on 295 °C within joining area, primer is affected by temperatures up to 340 °C. The steel temperature of 275 °C within joining area with a maximum process temperature of 316 °C does not lead to discoloration effects. Thermogravimetric analysis show temperature onset of decomposition of 303 °C with heating rate of 50 °C/s. In addition, higher heating rates result in increased temperature onset of decomposition. Considering discoloration as a sign of decomposition, results of thermogravimetric analysis are transferable to continuous fusion bonding process with heating rates up to 195 °C/s. Adhesion between primer and steel can be improved by application of higher steel temperatures which can be observed independently of CFR-PA6 temperature. On the one hand, higher steel temperatures lead to increased energy input and lower viscosity of primer as well as CFRPA6. Lower viscosity results in better wetting of steel surface and enables more interactions between materials which improve adhesion between steel surface and primer as well as primer and CFR-PA6. However, primer VM was applied in a press prior to joining process by application of pressure and temperature. As a consequence, steel surface including various cavities should be wetted totally by primer. On the other hand, amount and size of segregated transformations within primer increase with higher steel temperatures. Considering these as indicator of curing reaction, adhesion between steel surface and primer can be improved due to more interactions between both materials. Cross-linking of primer (starting at 160 °C) also leads to covalent bonds between primer and PA6 matrix of CFR-PA6 which also increases joint strength. In addition, higher steel temperatures influence curing kinetics of primer. Due to higher energy input, reaction kinetic is accelerated. Nevertheless, reaction time is limited within this continuous fusion bonding process. Therefore, all specimens show interfacial

Fig. 5. Influence of steel and CFR-PA6 temperature on peel resistance.

lead to increased adhesion between primer and steel. In addition, totally molten CFR-PA6 improves adhesion between primer and CFR-PA6 which also results in higher peel resistance. 3.4. Influence of surface condition of steel The results of the influence of various steel surface conditions on peel resistance are shown in Fig. 5. The CFR-PA6 temperature of 263 °C was kept constant during this test series. Investigated multi-material specimens with primer VM applied on DC01 achieve peel resistance of 1.26 ± 0.22 N/mm for steel temperature of 255 °C and peel resistance of 1.85 ± 0.23 N/mm for steel temperature of 275 °C (see Chapter 3.3). High pressure blasted DC01 leads to a decrease in peel resistance compared to application of VM primer. Steel temperature of 275 °C results in higher peel resistance of 0.28 ± 0.05 N/mm compared to steel temperature of 255 °C whereas fracture pattern shows complete interfacial failure between steel surface and CFR-PA6 for both steel temperatures. Application of combined mechanical and chemical pre-treatment by SACO® leads to peel resistances comparable to high pressure blasted samples. Highest peel resistance of 0.31 ± 0.07 N/mm is achieved with steel temperature of 275 °C with interfacial failure between steel and primer. Pre-treatment of higher pressure blasting prior to application of primer VM results in highest peel resistance of 1.90 ± 0.49 N/mm for steel temperature of 255 °C as well for steel temperature of 275 °C with peel resistance of 3.15 ± 0.39 N/mm. The fracture pattern of specimens manufactured at steel temperature of 275 °C shows complete interfacial failure between primer and CFR-PA6 which is entirely different to all other investigated surface conditions. Galvanized HX340 steel surface with steel temperature of 275 °C leads to peel resistance of 1.83 ± 0.40 N/mm, which is comparable to uncoated DC01 steels. However, steel temperature of 255 °C results in higher peel resistance compared to uncoated DC01 steel. The fracture patterns of these specimens show interfacial failure between steel and primer VM. The surface condition of the steel, especially macroscopic roughness, has to be considered for the comparison of measured peel resistances. Therefore, roughness of different steel surfaces is shown in Table 6. Pre-treatment by high pressure blasting leads to significant rougher structure (Rz = 16.82 ± 1.27 µm) compared to cleaned DC01 (Rz = 4.82 ± 0.33 µm) or cleaned HX340 (Rz = 7.34 ± 0.42 µm). The steel surface pre-treated by SACO® (Rz = 12.35 ± 1.72 µm) has rougher surface compared to cleaned steel surfaces but significant lower roughness compared to high pressure blasted steel surfaces. In summary, condition of steel surface has significant influence on adhesion between steel and primer respectively primer and CFR-PA6. 6

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the fracture pattern at a steel temperature of 275 °C, adhesion failure between primer and CFR-PA6 is identifiable which is caused by high adhesion in the interface between steel and primer. As a result, too less bonds between PA6-matrix and primer are formed during joining process. This can be explained by short joining time of 0.3 s as well as short time above melting temperature within process which is not enough to create a total firmly bonded joint between CFR-PA6 and primer. Higher peel resistance could be expected by thinner coatings in the range up to 10 µm as well as designed systems for selected material combination of steel and CFR-PA6, i.e. two-layer primer films. These designed systems should enable the formation of durable covalent bonds within joining times less than 1 s. Mechanical adhesion of steel specimens pre-treated by SACO®, due to higher roughness (Rz = 12.35 ± 1.72 µm) because of more mechanical interlockings, and specific adhesion, due to higher surface area between steel and organofunctional primer, is increased. In addition, specific adhesion of primer to CFR-PA6 is improved. However, peel resistance is comparable to high pressure blasted multi-material samples because of lower roughness, which lead to decreased mechanical and specific adhesion compared to high pressure blasted specimens. Considering the classification of investigated results, these multimaterial specimens are only comparable to samples tested with equal test method and adherends [10]. De Freitas et al. [41] examined influence of different rigid and flexible adherends within roller peel test. On the one hand, peel resistance of 11 N/mm for aluminium adherends bonded by epoxy based film adhesive was achieved whereas fracture pattern was cohesion failure of adhesive. On the other hand, same epoxy based adhesive have decreased peel resistance of 0.68 N/mm with shares of adhesion as well as cohesion failure of adhesive by application of flexible adherend consisting of carbon fibre reinforced plastics and rigid adherend consisting of aluminium. Therefore, peel resistance varies in dependency of flexible adherend but this does not mandatory mean poor adhesion between applied materials [4]. The developed climbing drum test is suitable to evaluate the adhesion in the interfaces of multi-material specimens. In contrast to i.e. roller peel tests, no tape breakage of flexible adherend consisting of CFR-PA6, especially by application of primer, was identifiable. However, tested specimens do not show cohesive failure of primer or CFR-PA6. As a result, application of this test method has to be proved for peel resistance up to 10 N/mm or even higher. In summary, application of selected primer is limited in terms of steel temperature. Higher steel as well as CFR-PA6 temperature improve adhesion between steel and primer respectively primer and CFRPA6. In addition, steel surface condition has significant influence on adhesion. The climbing drum peel test is suitable for the evaluation of adhesion within interfaces of multi-material specimens.

failure between steel and primer. Significant increase in peel resistance could be achieved by using CFR-PA6 temperature of 263 °C. In contrast to other applied CFR-PA6 temperatures, PA6 matrix is not molten only in the interface between steel and CFR-PA6 and viscosity is considerably lower. As a result, functional carboxamides groups within PA6 matrix are more reactive. The reactivity also has to be discussed with regard to joining time. Joining time within continuous fusion process shown in Chapter 3.1 between primer and CFR-PA6 is approximately 1 s whereas pressure only is applied for 0.3 s. As a consequence, reaction time and formation of covalent bonds between primer and CFR-PA6 is limited and depending of steel and CFR-PA6 temperature. Surface condition of steel is important relating to adhesion between steel and primer or steel and CFR-PA6 due to interaction between various materials. In addition, influences of mechanical and specific adhesion in dependency of surface condition have to be considered. The effect of mechanical interlocking on adhesion is controversy discussed in literature [34–36]. Discussion focuses especially on the fact if mechanical interlocking provides adhesion or if increased surface area because of higher roughness is responsible for adhesion [34]. Surface irregularities and load direction are important with regard to mechanical interlocking [35,37]. In addition, molten thermoplastic has to fill surface irregularities completely to enable mechanical interlocking [38]. Therefore, properties of thermoplastic as i.e. viscosity and shape of pores have to be considered [39,40]. Higher roughness mostly correlates with increasing amount of mechanical interlockings. High pressure blasting of steel in Fig. 6b lead to higher roughness (Rz = 16.82 ± 1.27 µm) which results in increased mechanical adhesion. Therefore, the primer has to completely impregnate all pores and irregularities on metal surface [38] which can be demonstrated by considering the displayed microsections. In addition, specific adhesion is increased due to more surface area between both materials which results in more interactions between steel and primer or steel and CFR-PA6. However, peel resistance of high pressure blasted steel at steel temperature of 275 °C is much lower (85%) compared to application of primer VM on cleaned surfaces. Due to minor roughness of cleaned steel surfaces (Rz = 4.82 ± 0.33 µm for DC01 and Rz = 7.34 ± 0.42 µm for HX340) in Fig. 6a (DC01) and Fig. 6c (HX340), mechanical interlocking as well as surface area is lower. Therefore, increase in peel resistance is caused by higher specific adhesion between primer and steel. In addition, cross-linking of primer can create coordinate bonds between steel and primer. Galvanized steel in Fig. 6c achieves peel resistance comparable to uncoated DC01 steel both steel temperatures. However, comparison of both steel surfaces could only be carried out with regard to comparable roughness. Nevertheless, zinc oxide surface has different centres for coordination compared to iron oxide which can influence adhesion between CFRPA6 and steel surfaces. The pre-treatment of high pressure blasting prior to primer application in Fig. 6b combines these mentioned effects on adhesion between steel and primer and therefore show highest peel resistance. Concerning

[a]

5. Conclusion Within this paper, climbing drum peel test was proved adequate to evaluate adhesion of continuously fusion bonded multi-material

[b]

[c]

Fig. 6. Microsections CFR-PA6-primer-steel specimens with steel temperature of 275 °C: cleaned steel [a], high pressure blasted steel [b] and galvanized steel c]

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specimens. Using this test method, influence of CFR-PA6 and steel temperature with applied primer on steel as well as influence of surface condition of steel was investigated. In addition, joint strength could be increased with higher energy input, especially by increasing the steel temperature. The presented test setup enabled continuous fusion process for CFRPA6-steel samples as well as multi-material specimens consisting of CFR-PA6 and steel with applied primer. The process window of primer application within manufacturing process is limited by steel temperature whereas higher steel temperatures result in transformations within primer. These transformations can be explained by cross-linking reactions within material. In addition, higher steel temperatures as well as molten CFR-PA6 are required for durable multi-material specimens. These results are transferable to various steel surface conditions. However, adhesion varies significantly depending on surface condition whereas pre-treatment including primer between steel and CFR-PA6 are recommendable and show potential for the integration in continuous roll forming line. Further examinations have to focus on primer application in terms of thinner primer layer and application of coil coated primer. In addition, two layer primer films could be an alternative considering the approach of multi-material components to enable better adhesion in the interfaces to both materials. The extension of experimental setup with regard to subsequent pressure unit has to be investigated. Furthermore, influence of joining time on adhesion between selected materials could be examined by varying processing speed of continuous fusion bonding process. The discoloration of primer should be examined with combined scanning electron microscopy and energy dispersive X-ray spectroscopy to investigate interfaces between primer and steel as well CFRPA6 and steel.

[9] [10]

[11]

[12] [13]

[14]

[15]

[16]

[17] [18]

[19] [20] [21] [22]

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Acknowledgements

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This research and development project TRoPHy2 (02PQ5135) is funded by the German Federal Ministry of Education and Research (BMBF) within the Forschungscampus “Open Hybrid LabFactory” and managed by the Project Management Agency Karlsruhe (PTKA). The authors would like to thank the partners of the participating research institute of the German Aerospace Center as well as the project partner data M Sheet Metal Solutions GmbH, EDAG GmbH & Co. KGaA, Salzgitter Mannesmann Forschung GmbH and Volkswagen AG Nutzfahrzeuge. In addition, the authors would like to thank the Evonik Industries AG (Marl, Germany) for the material supply and the involved students, especially Mr. Max Waller. The authors are responsible for the contents of this publication.

[25] [26] [27] [28]

[29]

[30]

[31]

References

[32] [33]

[1] Mitschang P, Velthuis R, Emrich S, Kopnarski M. Induction heated joining of aluminum and carbon fiber reinforced Nylon 66. J Thermoplast Compos Mater 2009;22(6):767–801. [2] Ageorges C, Ye L, Hou M. Advances in fusion bonding techniques for joining thermoplastic matrix composites: A review. Compos A Appl Sci Manuf 2001;32(6):839–57. [3] Reincke T, Kreling S, Dilger K. The production-related influence of iron oxides on steel surfaces on the adhesion of fusion-bonded hybrid structures. 1st ed.Silva Lucas, editor. Materials Design and Applications Springer international PU; 2017. p. 363–76. [4] de Freitas ST, Sinke J. Test method to assess interface adhesion in composite bonding. Appl Adhes Sci 2015;3(1):2. [5] Kawashita LF, Kinloch AJ, Moore DR, Williams JG. A critical investigation of the use of a mandrel peel method for the determination of adhesive fracture toughness of metal-polymer laminates. Eng Fract Mech 2006;73(16):2304–23. [6] Kawashita LF, Moore DR, Williams JG. Comparison of peel tests for metal-polymer laminates for aerospace applications. J Adhes 2005;81(6):561–86. [7] Teixeira de Freitas S, Banea MD, Budhe S, de Barros S. Interface adhesion assessment of composite-to-metal bonded joints under salt spray conditions using peel tests. Compos Struct 2017;164:68–75. [8] Barros S de, Freitas S de, Banea MD, Budhe S, Arouche, M.M. et al. Adhesion quality

[34] [35] [36] [37] [38] [39]

[40]

[41]

8

of steel-CFRP interface bonding: 2016. In: Brazilian Conference on Composite Materials, editor. Proceedings of the Brazilian Conference on Composite Materials; 2016. Reincke T, Kreling S, Dilger K. Roll-forming of multi-material composites. J Adv Mater 2016;3:161–72. Reincke T, Kreling S, Dilger K. Adhesion of continuously manufactured fusion bonded multi-material structures consisting of steel and carbon fibre reinforced Polyamide 6. Int J Adhes Adhes 2017;79:73–82. Moore DR, Williams JG. A protocol for determination of the adhesive fracture toughness of flexible laminates by peel testing: fixed arm and T-peel methods. ESIS Protocol 2010. Mohammed IK, Kinloch AJ, Charalambides MN. Modelling the peeling behavior of soft adhesives. Procedia Struct Integrity 2016;2:326–33. Padhye N, Parks DM, Slocum AH, Trout BL. Enhancing the performance of the Tpeel test for thin and flexible adhered laminates. Rev Sci Instrum 2016;87(8):85111. Moore DR, Williams JG. Peel testing of flexible laminates. Fracture Mechanics Testing Methods for Polymers, Adhesives and Composites Elsevier; 2001. p. 203–23. Nase M, Langer B, Grellmann W. Bruchmechanische Kennwertermittlung im TPeeltest und im Fixed-Arm Peeltest. In: Frenz H, Grellmann W, editors. Herausforderungen neuer Werkstoffe an die Forschung und Werkstoffprüfung: Tagung Werkstoffprüfung. Berlin: DVM; 2008. p. 223–8. Kawashita LF, Moore DR, Williams JG. The development of a mandrel peel test for the measurement of adhesive fracture toughness of epoxy-metal laminates. J Adhes 2010;80(3):147–67. Grouve W, Warnet LL, Akkerman R. Critical assessment of the mandrel peel test for fiber reinforced thermoplastic laminates. Eng Fract Mech 2013;101:96–108. Su Y, de Rooij M, Grouve W, Warnet L. Characterisation of metal–thermoplastic composite hybrid joints by means of a mandrel peel test. Compos B Eng 2016;95:293–300. Khan S. Bonding of sandwich structures – the facesheet/honeycomb interface – a phenomenological study. Baltimore, MD; 2007. Kinloch AJ, Lau CC, Williams JG. The peeling of flexible laminates. Int J Fract 1994;66(1):45–70. Thouless MD, Yang QD. A parametric study of the peel test. Int J Adhes Adhes 2008;28(4–5):176–84. Kawashita LF, Moore DR, Williams JG. Analysis of peel arm curvature for the determination of fracture toughness in metal-polymer laminates. J Mater Sci 2005;40(17):4541–8. Heckert A, Zaeh MF. Laser surface pre-treatment of aluminum for hybrid joints with glass fiber reinforced thermoplastics. J. Laser Appl. 2015;27(S2):S29005. Amend P, Pfindel S, Schmidt M. Thermal joining of thermoplastic metal hybrids by means of mono- and polychromatic radiation. Physics Procedia 2013;41:98–105. Bergmann JP, Stambke M. Potential of laser-manufactured polymer-metal hybrid joints. Physics Procedia 2012;39:84–91. Didi M, Mitschang P. Diskontinuierliches Induktionsschweißen von CF/PEEK und CF/PA66 mit Aluminium. Augsburg; 2011. Chan-Park MB, Ngew HS, Yip D, Er C, Zee SW. Heating methods for bonding thermoplastics to aluminum alloy. J Adv Mater 2001;33(4):52–61. Grujicic M, Sellappan V, Omar MA, Seyr N, Obieglo A, Erdmann M, et al. An overview of the polymer-to-metal direct-adhesion hybrid technologies for loadbearing automotive components. J Mater Process Technol 2008;197(1–3):363–73. Arnold JR, Sanders D, Belevou DL, Martinelli AA, Gaskin G. A study of titanium surface pretreatments for bonding with polyimide and epoxy adhesive. USA: Orlando; 1997. Schäfer J, Hofmann T, Holtmannspötter J, Frauenhofer M, von Czarnecki J, Gudladt H-J. Atmospheric-pressure plasma treatment of polyamide 6 composites for bonding with polyurethane. J Adhes Sci Technol 2014;29(17):1807–19. DELO Industrie Klebstoffe GmbH & Co. KGaA. Technical Information DELO-SACO® PLUS(08.17 (Revision 9)); 2016. Risthaus M, Wönicker H-J. Reaktive Schmelzklebstoffe enthaltende Hybridbauteile (EP 1 808 468 A2); 2007. Habenicht G. Kleben: Grundlagen, Technologien, Anwendungen. 6th ed. Berlin, Heidelberg: Springer, Berlin Heidelberg; 2009. Awaja F, Gilbert M, Kelly G, Fox B, Pigram PJ. Adhesion of polymers. Prog Polym Sci 2009;34(9):948–68. Baldan A. Adhesion phenomena in bonded joints. Int J Adhes Adhes 2012;38:95–116. Kinloch AJ. The science of adhesion. J Mater Sci 1980;15(9):2141–66. van der Leeden MC, Frens G. Surface properties of plastic materials in relation to their adhering performance. Adv. Eng. Mater. 2002;4(5):280–9. Digby RP, Packham DE. Pretreatment of aluminium: topography, surface chemistry and adhesive bond durability. Int J Adhes Adhes 1995;15(2):61–71. Maeva E, Severina I, Bondarenko S, Chapman G, O'Neill B, Severin F, et al. Acoustical methods for the investigation of adhesively bonded structures: A review. Can J Phys 2004;82(12):981–1025. Packham DE. The Adhesion of Polymers to Metals: The Role of Surface Topography. In: Mittal KL, editor. Adhesion Aspects of Polymeric Coatings. Boston, MA: Springer, US; 1983. p. 19–44. de Freitas ST, Sinke J. Adhesion properties of bonded composite-to-aluminium joints using peel tests. J Adhes 2014;90(5–6):511–25.