1. Introduction
The development of new lightweight solutions for structural parts of future vehicles helps to enable the widespread use of electric powertrain concepts and increases the effectivity of fuel-based solutions. One promising strategy for lightweight construction is the replacement of metal parts by hybrid structures consisting of metallic and fiber reinforced plastic components. This multi-material approach combines the high strength and ductility of the metal with the low density, high strength and stiffness of the fiber-reinforced composite material. The joint interface between the metal and polymer component is, however, susceptible to failure caused by surface contaminations like e.g. oils
[1], which can lead to weakening of the bonding. A pulsed laser surface pretreatment of the fiber reinforced plastic
[2,3] and the metal, or the metal alone
[4,5,6,7], prior to adhesion, has proven to be an effective and reproducible method to counter this problem and increase the mechanical strength of the hybrid joint. However, the findings of Schanz et al. also revealed that the pretreatment parameters have to be chosen carefully to promote enhanced mechanical strength and prevent a loss of mechanical strength by corrosion processes
[8].
In prior studies on metal-polymer joints that were adhesively bonded after laser surface pretreatments, it was commonly assumed that besides the removal of surface contaminations, the surface enlargement and the morphology of the micro- and nanostructures generated by the laser were responsible for a further increase of mechanical strength and resistance against hydrothermal aging of the joints
[9,10,11,12]. Trauth et al. suggested that the surface enlargement also plays an important role in the enhancement of the interaction of the bonding partners in hybrid titanium – self-reinforced PLA joints
[7]. Furthermore, Akman et al. found that laser-pretreated aluminum – CFRP joints with the deepest craters provided the highest single-lap shear (SLS) strengths
[6]. Ostapiuk et al. concluded that the morphology of the surface micro- and nano-sized features also plays an important role for the resulting mechanical strength of anodized AW 2024-T3 – CFRP or glass-fiber reinforced plastic hybrids
[13].
In a previous study we characterized the laser-generated surface structures on AW 6082-T6 specimen that were adhesively bonded with the epoxy adhesive E320. The laser-induced surface enlargement on the micro- and nanoscale, as well as the depth of the ablated craters on the surface were determined with a digital image analysis approach for scanning electron microscope (SEM) images and a laser scanning microscope (LSM). It was concluded that surface structures leading to a high micro- and nano-surface enlargement, combined with deep melt craters and undercut structures, promote high SLS strengths and a high resistance against hydrothermal aging
[14].
In order to investigate the influence of the surface morphology on the mechanical performance and aging resistance of AW 6082-T6 – CFRP hybrid joints, the SLS strengths of hybrid specimens before and after 7 days of hydrothermal aging are determined. The AW 6082-T6 surfaces are pretreated with the same laser parameter sets that have been investigated in the previous study
[14] so that the influence of the surface enlargement, crater depth and undercut structures on the SLS strength of hybrid specimens can be compared directly with AW 6082-T6 – E320 adhesive joints. Furthermore, stereomicroscopic and SEM analyses of the fracture surfaces are performed in order to investigate the influence of the laser-generated surface structures on the failure patterns of hybrid and adhesively bonded metal SLS specimens. These experiments allow revealing similarities and differences in the structure-property relations of metal-polymer and hybrid metal-CFRP adhesive joints with the same polymer acting as adhesive and matrix material.
4. Discussion
The results show the strong dependence between SLS strengths and laser process settings for hybrid metal-CFRP specimens with pretreated metal surfaces. In general, for all laser parameter sets the pretreatment enhances the mechanical strength of the unaged hybrid SLS specimens compared to the untreated reference. The joint strength of the latter specimens was insufficient even for mounting in the testing setup.
Nevertheless, a difference of ≥ 30 MPa between the SLS results for the different groups of pretreatment parameter sets is found that points to differences in terms of highly and less pronounced surface structures. The results can be split into three groups:
- I.
Pretreatments R1 to R3 and R17 with mean SLS strengths of > 40 MPa before and > 35 MPa after hydrothermal aging,
- II.
R18 and R19, which lead to SLS strengths up to 10 MPa before and after hydrothermal aging, and
- III.
R34 to 36 specimens with less than 10 MPa before and negligible SLS strengths after hydrothermal aging.
Fracture surfaces of specimens from the third group were the only ones that suggested the formation of aluminum hydroxides after the aging step of these untreated samples (
Figure 6 (c)) [
14].
The SLS strengths of AW 6082-T6 – CFRP specimens from group I are similar to the SLS strengths of laser-pretreated AW 6082-T6 – E320 joints with different adhesives [
12,
14]. However, SLS strengths of more than 50 MPa that were obtained in case of the metal-polymer joints could not be reached for the AW 6082-T6 – CFRP joints. Since the chemical composition of laser-pretreated surface and the morphology of the generated surface structures must be expected to be comparable to those from the previous study [
12,
14], the differences must be related to the CFRP joining partner. The general trend of the resulting SLS strengths for different laser pretreatment parameter sets for hybrid AW 6082-T6 – CFRP specimens is found to be similar to that of the AW 6082-T6 – E320 specimens.
The best performing pretreatment parameter sets of the metal adhesive bonds are also the most suitable parameter sets for the pretreatment of hybrid specimens (
Figure 10). However, there is a much larger difference between the best- and less performing pretreatment parameter sets in case of the metal-CFRP joints: The laser-pretreated AW 6082-T6 – E320 SLS specimens all present mean SLS strengths of more than 30 MPa in the unaged state and more than 25 MPa after hydrothermal aging, while the SLS strength of the R18-19 and R34-36 metal-CFRP hybrid specimens drops to SLS strengths < 10 MPa. Furthermore, even the untreated AW6082-T6 – E320 bonded specimens show a mean SLS strength of ~30 MPa in the unaged and ~15 MPa in the aged state, while all untreated hybrid specimens, aged and unaged, failed before testing (
Figure 10).
Figure 10.
Mean SLS strengths of the single-lap shear tests performed with hybrid AW 6082-T6 – CFRP specimens and with AW 6082-T6 – E320 metal adhesive joints from [
13] for comparison.
Figure 10.
Mean SLS strengths of the single-lap shear tests performed with hybrid AW 6082-T6 – CFRP specimens and with AW 6082-T6 – E320 metal adhesive joints from [
13] for comparison.
In contrast to the metal-polymer adhesive bonds, differences in the coefficients of thermal expansion of the metal and CFRP component induce intrinsic thermal residual stresses in the production process that lead to a visible elastic deformation of the specimens. In all untreated and in the aged specimens pretreated with the laser parameter sets of R34–R36, these stresses weaken the bonding state to an extent at which very small additional loads (i.e., sample mounting) lead to a complete failure at the interface. For all of these specimens, the resulting micro- and nano-surface enlargement is low. For pretreated surfaces with a high micro- and nano-surface enlargement (R1–R3, R17), i.e. also with a higher amount of chemical bonds between the metal oxide surface and the polymer, the residual stresses can be compensated well (e.g., possibly due to the higher number density of specific chemical bonds or due to slightly higher bond strengths), which seems also to reduce the damage by hydrothermal aging.
Da Silva et al. found that the thickness of the adhesive layer has an influence on the resulting SLS strength of metal-polymer joints, which increases with a decreasing thickness of the adhesive layer [
20,
21]. In a metal – FRP joints, the entire FRP matrix thickness can in first approximation be considered as adhesive layer. However, this is not correct as finite-element simulations show [
22,
23] since the embedded fibers of course affect the load distribution differently than an unperturbed matrix.
The influence of the laser-generated surface structures on the SLS strength before and after hydrothermal aging is obvious. The surface enlargement on a micro- and nanoscale, the crater depth and microstructural features like undercut surface structures, which have been found to correlate with the resulting SLS strength of AW 6082-T6 – E320 adhesive joints [
14], also play a similar role for the metal-CFRP joints. Those features are known to affect the mechanical strength of metal-composite hybrids [
6,
7,
13], which agrees with this study. The first group (R1-3, R17) of pretreatment parameter sets, which produce surface structures that lead to median micro-surface enlargement values of ~350%-560%, median nano-surface enlargement values of ~1400%-2700% and crater depths of ~11-32 µm [
14], result in the highest SLS strengths of the hybrid specimens before and after hydrothermal aging.
A drop of the median micro-surface enlargement to values < 60% and the median nano-surface enlargement to values below 900% [
14] from samples of the second and third groups (R18-19 and R34-36) leads to much weaker SLS strengths. Especially for the micro- and nano-surface enlargement apparently a certain threshold needs to be surpassed in order to achieve high SLS strengths for unaged and hydrothermally aged metal-CFRP joints. The lower micro- and nano-surface enlargement of the R18-19 metal-CFRP specimens seems to be partly compensated by undercut structures [
14]. The decreasing trend of SLS strengths with increasing rank of the laser parameter set is rather continuous for the metal-polymer joints and shows no large drop from one set to the other as is the case between R17 and R18 for the metal-CFRP specimens (
Figure 10). Since the chemistry of the polymer matrix and the chemistry of the amorphous metal oxide films generated by the same laser pretreatments are generally identical (covalent and ionic bonds as well as physiochemical interactions [
24]) for the metal-polymer and the metal-CFRP hybrid joints, these differences cannot simply be attributed to a qualitative change in the chemical bonding or the interlocking contribution with undercuts. This further emphasizes the importance of the surface enlargement along with the crater depth and the presence of undercut structures for high strength, aging-resistant hybrid joints. This hybrid metal-CFRP joints are found to be even more sensitive to changes of these surface features than the corresponding metal-polymer joints.
The laser-generated surface structures also determine the type of failure for the SLS specimens. If the surface enlargement is high and the ablated craters are deep, a complete or at least more than 90% cohesive failure is achieved for unaged hybrid specimens (
Figure 4). If the surface enlargement is low and the craters are flat, the fracture surfaces reveal mainly adhesive failure with less than 10% CFRP remaining on the laser-structured surface of unaged specimens. Hydrothermal aging leads to a shift from mainly cohesive to a mix of cohesive and adhesive failure or completely adhesive failure (
Figure 4 and
Figure 6). The decrease of remaining CFRP on the fracture surface of aged specimens correlates with the loss of SLS strength. An exception is represented by the parameter set of R3: Two of the three aged R3 samples show mainly adhesive or pseudo-adhesive failure but the testing of all three specimens resulted in SLS strengths of more than 30 MPa. Nevertheless, thicker layers of the matrix polymer are still encountered over a large surface area of the R3 samples’ fracture surfaces.
The shift towards adhesive failure after hydrothermal aging is also seen on the fracture surfaces of AW 6082-T6 – E320 metal-polymer joints. The polymer layer on the laser-structured surface gets thinner and smaller, and unconnected polymer layers vanish completely. For the SLS specimens, this shift correlates as well with the decrease of the mean SLS strength (
Figure 10). However, the decrease of SLS strength is less pronounced compared to the hybrid specimens. Specimens that present mainly adhesive failure (e.g., R36 specimens) still reach mean SLS strengths of more than 25 MPa before and about 15 MPa after aging instead of failing before testing.
Author Contributions
Conceptualization, J.F., M.L. and J.H.; methodology, J.F., I.L., S.W., A.D.; validation, J.F., M.L, J.H.; formal analysis, J.F, I.L..; investigation, J.F, I.L..; resources, J.H., F.W., T.T.; data curation, J.F.; writing—original draft preparation J.F.; writing—review and editing, J.H., M.L, A.D., S.W., F.W., T.T.; visualization, J.F.; supervision, J.H., F.W., T.T.; project administration, J.F., A.D., S.W.; funding acquisition, J.H., F.W., T.T. All authors have read and agreed to the published version of the manuscript.