Forming Epoxy Coatings on Laser-Engraved Surface of Aluminum Alloy to Reinforce the Bonding Joint with a Carbon Fiber Composite

Abstract

This study designed laser engraving and resin pre-coating (RPC) treatments on an aluminum alloy (AA) surface to construct through-the-thickness “epoxy pins” for improving the bonding strength with carbon fiber reinforced polymer (CFRP). A laser engraving treatment was used to create a pitted structure on the AA surface; higher wettability was acquired and greater vertical spaces were formed to impregnate epoxy resin, resulting in stronger mechanical interlocking. The RPC technique was further used to guide high-viscosity epoxy resin into pits to form the epoxy coatings and to minimize defects between the resin and the substrate. The bonding strength of the specimen treated with both laser engraving with a unit dimension of 0.3 mm and RPC increased up to 227.1% in comparison with that of the base. The failure modes of the hybrid composites changed from the debonding failure of the AA surface to the delamination-dominated failure of the laminated CFRP composites. It was confirmed that laser engraving is a feasible and effective method when combined with RPC for treating AAs to improve the bonding strength of AA-CFRP composites, which provides a reference for preparing high-performance hybrid composites with metals.

1. Introduction

Carbon fiber reinforced polymers (CFRPs) exhibit many advantages, including high specific strength, stiffness, and fatigue resistance, making them materials commonly selected for numerous applications, especially in some cutting-edge fields that prioritize lightweight design [1,2,3,4,5]. The advantages of the light weight and high strength of CFRPs are particularly evident in the aviation field. Using CFRPs to semi-replace or fully replace structural parts can decrease the weight of civil aircraft so that their basic fuel consumption and pollution emissions are reduced [6,7]. As the main metal or alloy used in most civil aircraft, aluminum alloy (AA) is usually connected with CFRP to manufacture lightweight and high-performance structural components. An efficient connection between them is very critical for ensuring the stability of the structural strength and the safety of their practical applications. The traditional mechanical fastening method exhibits relatively poor corrosion resistance and incident stress concentration at the connection point, and it causes the discontinuity in fibers due to undesirable machining damage [8,9]. Welding can be used for connecting similar AAs or AA with other metals, but the generated high temperature is not good for laminated epoxy-based CFRP composites and even causes deterioration of the properties [10,11]. In order to not damage dissimilar substrates, adhesive bonding seems to be a better alternative for the connection in AA and CFRP composites [12]. Indeed, adhesive bonding has been employed in industry to manufacture some complex heterostructures and has been studied by many experts to improve the process techniques and parameters to strengthen bonding joints [13,14,15]. However, creating the bonding interface with less defects and stronger mechanical interlocking is still a research problem, and a simpler, more innovative, and more effective method needs to developed.

As is well known, an as-received AA surface has a passive oxide layer that is not beneficial in the formation of strong intermolecular forces with epoxy adhesive, and the relatively neat surface is not conducive to constructing mechanical interlocking along the bonding interface either. Therefore, AA surface modification is the central issue in manufacturing stronger adhesive bonding joints, and some surface treatments should be applied to the AA surface to provide better surface conditions for adhesive bonding. Creating some pores, channels, cavities, or protrusion is an effective solution that has been confirmed in previous publications [16,17,18], and those mechanical, chemical, and electrochemical treatments can be adopted. Sun et al. [19] and Wang et al. [20] used simple grinding and sandblasting to create micropores and channels that allowed epoxy resin to enter, and the contact area between the epoxy and substrate could also be increased. Hu et al. [21] prepared a 10 wt% NaOH solution to etch the AA surface for forming uneven grooves. Cheng et al. [22,23] and Cui et al. [24] utilized anodic oxidation and micro-arc oxidation to treat the AA surface to prepare channels and pores, which could significantly enhance the surface roughness, hardness, corrosion resistance, and wettability of AAs. The above-mentioned studies focused on adhesive bonding strength, and various reinforcing effects were exhibited. However, those surface treatments are not highly controlled with regard to the surface morphologies of the engraved pit shape and unit dimension.

Laser engraving technology [25,26,27] is commonly used for etching various types of metals, polymers, and inorganic nonmetallic materials; it is very effective in creating a desired structure with the designed shape and dimensions, especially for aluminum substrates, to reinforce the bonding strength of an adhesive joint. Once the laser jet contacts the AA substrate, heat is produced, and the temperature sharply increases, causing the alloy to melt. The liquid aluminum quickly solidifies to form specific patterns when exposed to air, as shown in Figure 1. It is helpful to increase the contact area of the epoxy adhesive and substrate, and the bonding between the formed oxides and polymers can be reinforced with the help of secondary bonds (hydrogen bonds) [28]. Therefore, laser engraving has also been employed by many researchers to treat AA surfaces for adhesive joint toughening. Temesi et al. [29] discovered that aluminum–polypropylene joints were stronger than joints without laser treatment because the grooves created by engraving could provide a better mechanical interlocking effect. Schricker et al. [30] also demonstrated that increasing the joint area and roughness using laser-engraved grooves in aluminum alloy specimens significantly improved the strength of the joint.

Further surface improvements or modifications are necessary to enhance the physical bonds between a treated substrate and an epoxy adhesive, as proven in many publications [31,32]. Resin pre-coating (RPC) is an effective technology for reducing micro-scale void defects and strengthening vertical mechanical interlocking by guiding high-viscosity epoxy (without hardener) into pores, improving the wettability between a substrate and an epoxy adhesive (with hardener) and constructing fiber bridging (if reinforcing fibers are introduced) [33,34].

In this study, simple laser engraving and RPC were applied to treat an AA surface to increase the contact area, improve the wettability, and form mechanical interlocking, to prepare a stronger adhesive joint of an AA-CFRP composite. Single-lap shear tests were performed to verify the influence of the combined treatments on bonding strength. The failure modes were studied to analyze the reinforcing mechanisms. The surface microstructure, pore distribution, chemical component, and wettability of laser-engraved AA were tested.

2. Composites’ Preparation and Characterization

2.1. Major Raw Materials and Preparation of RPC Solution and Epoxy Adhesive

The major raw materials for preparing AA-CFRP composites included AA plates, CFRP panels, epoxy resin, hardener, and acetone; their relevant parameters are listed in

Table 1. Major parameters of main starting materials and equipment.

Materials/Equipment	Special Features and Models	Origins
Aluminum alloy	AA plates (3 mm in thickness, 25.4 mm in width, 101.6 mm in length)	Dongguan Jiejin Metal Materials Co., Ltd., Dongguan, China
CFRP panel	3K twill weave CFRP panel (3 mm in thickness, 25.4 mm in width, 101.6 mm in length)	Carbonwiz Technology Co., Ltd., Shenzhen, China
Epoxy resin	Bisphenol A epichlorohydrin epoxy resin	Huntsman Advanced Chemical Materials Co., Ltd., Guangzhou, China
Hardener	Triethylenetetramine hardener	Huntsman Advanced Chemical Materials Co., Ltd., Guangzhou, China
Acetone	AR, toxic, boiling point 56 °C	Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China
Laser engraving machine	JL-F30	Liaocheng Jiuling Laser Equipment Co., Ltd., Liaocheng, China

in detail. RPC solution was prepared using acetone to disperse the commercially purchased epoxy resin that had a mass proportion of 10 wt%, and it was used as the interface modifier to obtain a stronger bonding joint. The epoxy adhesive was obtained via evenly mixing 50 vol% epoxy resin and 50 vol% hardener, which was used to bond AA substrates and CFRP panels.

2.2. Manufacturing of Composite Materials

The preparation processes of AA-CFRP composites included substrate surface treatment, RPC treatment, and adhesive bonding, as shown in Figure 2.

The AA was immersed in a beaker filled with acetone and cleaned by ultrasonic waves for 10 min. The laser processing parameters were set to 80 mm/s velocity, 24 W power, and 30 kHz frequency. The AAs were placed on the table of the laser engraving machine and were engraved into solid squares with different side lengths (0.1, 0.3, 0.6, and 0.9 mm), and each specimen with each pattern was engraved once. The cross-ply [0/90]_10s CFRP panels were treated using a DL6391 handheld electric grinder. The ground CFRP sheets were ultrasonically cleaned in acetone for 10 min to remove residual dust particles from the surface.

The treated AA specimens were immersed in RPC solution for 10 s after laser engraving and were then taken out; the epoxy coatings formed once the acetone evaporated. The ground CFRP sheets were also immersed in the RPC solution for preparing epoxy coatings.

The epoxy adhesive was processed for the epoxy bonding of AA-CFRP composites. All the parameters, including the bonding area thickness, complied with ASTM D5868 [31], as illustrated in Figure 2. After an initial 12 h curing period, all specimens were put in a drying oven for 72 h at 60 °C to be cured. A total of 5 groups of specimens were prepared with various surface treatments, as listed in

Table 2. AA-CFRP composites with different surface treatment combinations on AA substrates and CFRP panels.

Group	AA Substrate Treatment	CFRP Treatment	Amount
C a	Acetone ultrasonic cleaning	Acetone ultrasonic cleaning	5
LE-0.1 b	Laser engraving (a = 0.1 mm) + RPC	Sanding + RPC	5
LE-0.3	Laser engraving (a = 0.3 mm) + RPC	Sanding + RPC	5
LE-0.6	Laser engraving (a = 0.6 mm) + RPC	Sanding + RPC	5
LE-0.9	Laser engraving (a = 0.9 mm) + RPC	Sanding + RPC	5

^a C: Control group, both AA and CFRP substrates were treated using acetone ultrasonic cleaning. ^b LE-0.1: The AA substrates were laser engraved in the shape of a square with the side length of 0.1 mm, and RPC was then applied.

.

2.3. Test and Characterization Methods

The cross-sectional morphology of the AA following laser engraving was viewed under a metallographic microscope (WMJ-9590 metallographic microscope, Shanghai Wu Mor Optical Instrument Co., Ltd., Shanghai, China), and the efficiency of the RPC method in improving the bonding interface was confirmed. A scanning electron microscope (SEM, Hitachi SU8600, Tokyo, Japan) was used to observe the micro-morphology of the treated AA surface at a voltage of 3.0 kV.

Grazing incidence X-ray diffraction (GIXRD, Malvern Panalytical B.V., Almelo, The Netherlands) was used to analyze the surface chemical components of the AAs after laser engraving. Scanning was performed with Cu Kα radiation (wavelength λ = 1.54 Å) at a grazing incidence angle of 1° between 20° and 80° with a step increment of 0.04°.

Contact angles (CAs) between the AA surface and ultrapure water or 1-bromonaphthalene were tested with a Krüss DSA30 droplet shape analyzer (Krüss GmbH, Göttingen, Germany) using the sessile drop method. The droplet size was 2.5 μL. Three measurements were conducted for each surface condition. The surface free energies (SFEs) of AA specimens were calculated using the Owens–Wendt–Rabel–Kaelble (OWRK) method (1), which divides the SFE of a solid into dispersive and polar components. Water and 1-bromonaphthalene were chosen as the test liquids. The SFE values and parameters are shown in

Table 3. Main surface roughness parameters of the laser-engraved AA specimens.

Treatments	Ra (μm)	Rz (μm)	Depth of Pit Gap (μm)
LE-0.1	5.1	21.4	49.7
LE-0.3	5.6	25.9	176.2
LE-0.6	6.3	26.4	108.3
LE-0.9	6.2	29.5	36.4

.

$${\gamma }_s={\gamma }_s^D+{\gamma }_s^P$$

(1)

where ${\gamma }_s$ is the SFE of the solid; ${\gamma }_s^D$ is the dispersion component of the solid’s SFE; ${\gamma }_s^P$ is the polar component of the solid’s SFE. The values of ${\gamma }_s^D$ and ${\gamma }_s^P$ w calculated from Equations (2) and (3).

$${\gamma }_{lw}\left(1+\mathit{cos}{\theta }_w\right)=2{\left({\gamma }_s^D{\gamma }_{lw}^D\right)}^{{\displaystyle \frac{1}{2}}}+2{\left({\gamma }_s^P{\gamma }_{lw}^P\right)}^{{\displaystyle \frac{1}{2}}}$$

(2)

$${\gamma }_{lb}\left(1+\mathit{cos}{\theta }_b\right)=2{\left({\gamma }_s^D{\gamma }_{lb}^D\right)}^{{\displaystyle \frac{1}{2}}}+2{\left({\gamma }_s^P{\gamma }_{lb}^P\right)}^{{\displaystyle \frac{1}{2}}}$$

(3)

where ${\gamma }_{lw}$ is the SFE of water; ${\theta }_w$ is the measured CA of water; ${\gamma }_{lw}^D$ is the dispersion component of water’s SFE; ${\gamma }_{lw}^P$ is the polar component of water’s SFE; ${\gamma }_{lb}$ is the SFE of 1-bromonaphthalene; ${\theta }_b$ is the measured CA of 1-bromonaphthalene; ${\gamma }_{lb}^D$ is the dispersion component of 1-bromonaphthalene’s SFE; ${\gamma }_{lb}^P$ is the polar component of 1-bromonaphthalene’s SFE.

The 3D surface roughness was measured using an optical 3D surface profilometer (Chotest Technology Inc., Shenzhen, China. Measurements were made in the x direction and y direction on an area of 5.51 × 7.34 mm². The 3D images of material surfaces were processed and analyzed by system software to obtain area roughness parameters.

The single-lap shear tests of the composites were performed using a WANCE ETM105D universal testing machine (Shenzhen WANCE Testing Machine Co., Ltd., Shenzhen, China) with a 150 kN load cell. Tension was applied to the specimens at a constant rate of 2 mm/min. A total of 5 specimens were tested for each group. The bonding strength of the composites was obtained via dividing the maximum load by the area of the bonded area.

3. Results and Discussion

3.1. Surface Morphologies of AA Substrates after LE Treatment

The front and cross-section morphologies of the AAs treated with laser engraving are displayed in Figure 3. As the dimensions of the laser-engraved pattern decreased, more square pits were prepared on the AA surface at the same magnification, as shown in Figure 3a,c,e,g. Relatively more regular patterns are demonstrated in Figure 4, where those laser-engraved pits are more visible for the specimens with engraving side lengths of 0.1 mm or 0.3 mm. Figure 3b,d,f,h exhibit the cross-section morphologies of the AAs after RPC treatment, and their surfaces were adhered using epoxy adhesive. Changes in the dimensions of the square pits were discovered, and it was also observed that the epoxy adhesive could effectively enter the square pits. The void defects at the root of the square pits were reduced and through-the-thickness epoxy pins were constructed to strengthen the mechanical interlocking with the help of the RPC technique.

Notably, the boundary of the square pits seemed became less obvious with increasing engraving unit dimension, as shown in Figure 3f,h, which might indicate that the engraving depth was reduced at the same engraving power when the engraving unit area was greater. It should also be emphasized that the melted product during laser engraving flowed into the original square pits, causing an uneven bottom wall pattern, or flowed into adjacent square pits to weaken the pit boundary. Therefore, indistinct square pit boundaries formed for engraving units of 0.6 and 0.9 mm, and these morphologies would be detrimental to the strength of adhesive bonding joints.

3.2. Chemical Component Analysis of AA Substrates

Figure 5 shows the grazing incidence X-ray diffraction analysis of the AAs after the laser engraving treatment. The characteristic diffusion peaks at 38.4°, 44.7°, 65.1°, and 78.2° on all the laser-engraved specimens were caused by the corresponding crystalline planes of Al (111), Al (200), Al (220), and Al (311) [22,36]. The characteristic peaks matched those of the only-acetone-cleaned specimen, and the relative intensities of each crystallographic plane with different engraving conditions presented minor changes. These results illustrated that the surface chemical components of the specimens after the laser treatment did not change because the AA was completely un-ionized at low laser energy densities. At this time, the main physical process that took place was a transformation from the condensed phase to the gas phase, including melting and evaporation [37,38].

3.3. Surface Roughness of AA Substrates after LE Treatment

The three-dimensional surface height profiles of laser-engraved specimens are shown in Figure 6, and their surface roughness parameters are listed in Table 3. Ra is the surface arithmetical mean roughness, and Rz is the maximum height. It can be clearly observed that three-dimensional surface height profiles were very consistent with the surface topography in Figure 3a,c,e,g. Their Ra values ranged from 5.1 μm to 6.3 μm, and specimens with an engraving unit of 0.3 mm exhibited a greater pit gap depth, and this result is also revealed by the cross-section morphology in Figure 3d, which contributed to constructing the through-the-thickness epoxy pins. These rough surfaces might have increased the contact area between the epoxy adhesive and AA as well as improve the surface wettability to improve adhesive bonding.

3.4. Contact Angles of AA Substrates after Acetone Cleaning and LE Treatment

The spreading ability of a liquid on the surface of an AA substrate is reflected by surface wettability. Figure 7 displays the static contact angles of the AAs with various surface treatments. The acetone-cleaned specimens had large contact angles of 60.3 ± 0.5° and 24.8 ± 0.93°, as shown in Figure 7a,f. The contact angles of various laser-engraved AAs notably decreased to near 0° for both water and 1-bromonaphthalene because the droplets swiftly entered the square laser-engraved pits during the test.

The surface free energies were calculated from the contact angles of the AAs with various surface treatments using the Owens–Wendt–Rabel–Kaelble method, and the results are given in

Table 4. Surface free energies (mJ/m²) obtained using OWRK method.

Treatment Conditions	${\mathit{\gamma }}_{\mathit{s}}^{\mathit{D}}$	${\mathit{\gamma }}_{\mathit{s}}^{\mathit{P}}$	${\mathit{\gamma }}_{\mathit{s}}$
Acetone ultrasonic cleaning	40.4	12.0	52.4
LE-0.1	44.4	34.1	78.5
LE-0.3	44.4	34.1	78.5
LE-0.6	44.4	34.1	78.5
LE-0.9	44.4	34.1	78.5

. The results showed that the acetone-cleaned specimens’ surfaces had the lowest surface free energy value, and the surface free energies value of all the laser-engraved specimens was 78.5 mJ/m² because their CAs tested using the two liquids were near 0°. Therefore, it could be concluded that the interfacial wettability of the AA had been effectively improved by the laser engraving treatment.

3.5. Bonding Strength of AA-CFRP Composites

Figure 8a displays the single-lap shear test results of the AA-CFRP adhesive joints, where all curves have linear and nonlinear regions. Because of the initial gaps between the fixture components, the curves were nonlinear at the beginning of the loading process. The loads increased linearly as a result of the elastic deformation of the specimens. The specimen reached the yield limit and converted into the plastic deformation stage as the loads gradually increased nonlinearly. Structural failure finally occurred once the peak loads were reached.

Figure 8b shows the bonding strength of the adhesive joints under different treatment conditions, and the detailed bonding strengths are listed in

Table 5. Single-lap shear test results of AA-CFRP composites.

Specimens	Average Pmax/N	Standard Deviation/N	Bonding Strength/MPa	Standard Deviation/MPa
C	4090.3	592.2	6.3	0.9
LE-0.1	11,699.3	619.3	18.1	1.0
LE-0.3	13,377.4	560.9	20.7	0.9
LE-0.6	10,312.8	418.9	16.0	0.7
LE-0.9	9213.6	541.6	14.3	0.8

. The acetone-cleaned specimens had a bonding strength of 6.3 MPa, which was much lower than the bonding strengths of the specimens following laser engraving and RPC treatments. This indicated that the previously mentioned treatments increased the strength of the AA-CFRP adhesive joints. The highest bonding strength was found for the specimens engraved with a pit side length of 0.3 mm, which was 227.1% higher than that of the only-acetone-cleaned specimens. It could be concluded that greater engraving unit dimensions were not conducive to bonding strength development.

3.6. Failure Modes of AA-CFRP Composites

The variations in the bonding strength produced by laser engraving with different parameters were mainly revealed by the failure morphologies of the AA-CFRP composites, as shown in Figure 9. Figure 9a displays that there was no epoxy adhesive residue on the surface of the AA substrate for the only-acetone-cleaned specimens, illustrating typical debonding failure. This failure mode was effectively improved by applying laser engraving and RPC treatments to the AA surface. When larger unit dimensions of the laser-engraved square pits were used, it was very difficult to form sufficiently strong mechanical interlocking at the AA/epoxy adhesive interface due to the lack of pores and the vertical distribution of the epoxy in the pores, and it was prone to causing mixed cohesive and adhesive failure in the specimens with side lengths of 0.6 and 0.9 mm. As the pit dimensions decreased, the pores were constructed by the larger number of smaller-size pits, creating the vertical space required for the epoxy adhesive to form quasi-vertical epoxy pins. The gradually increasing mechanical interlocking reinforced the AA/epoxy interface to cause mixed cohesive failure or delamination failure on the AA or CFRP substrates, as shown in Figure 9b.

Stronger adhesive bonding interfaces were successfully formed once the delamination damage of the laminated CFRP composite was caused because the interfacial bonding strengths between the epoxy adhesive and substrates were even higher than the interlaminar strength of the CFRP [22]. In the specimens with side lengths of 0.3 mm, a large amount of CFRP interlayer fibers peeled off, and delamination-dominated failure was very obvious, which explained why it had the greatest bonding strength.

3.7. Reinforcement Mechanism Analysis

It was critical to construct through-the-thickness epoxy pins to strengthen the failure resistance to achieve a stronger adhesive bonding joint. If there was a lack of mechanical interlocking in the adhesive layer, as shown in Figure 10a, and the structure of the original passivation oxide film of the AA substrates caused weak intermolecular forces, it was not hard for a low load to damage the AA/epoxy adhesive interface [13,39], explaining the lower bonding strength of only-acetone-cleaned specimens. The laser engraving treatment prepared square pits on the AA surface and damaged the original passivation oxide film; the contact area and wettability were improved significantly, and the modified AA surface was prone to impregnating by the epoxy adhesive [35]. Void defects should be considered because it was difficult for the high-viscosity epoxy adhesive to enter the pits and form better bonds with the substrates. It was necessary to use RPC to further treat the AA surface to achieve a voidless interface by guiding the diluted epoxy into the pits before the adhesive process to produce stronger adhesive bonding joints [22,24,40]. It should be stressed that the pores could be prepared using a small engraving unit even if a rough surface could also be formed with large engraving units, and the through-the-thickness epoxy pins were easily constructed between the narrow pore walls, as depicted in Figure 10b,c. These through-the-thickness epoxy pins contributed to stronger mechanical interlocking and naturally toughened the AA/epoxy interface.

4. Conclusions

This study focused on using laser engraving to treat AA surfaces to strengthen the adhesive bonding between AA and CFRP. The AA surfaces were laser engraved with varying pattern dimensions to prepare distinct surface morphologies. The CFRP panels were ground to ensure failure would not occur at the CFRP/epoxy interfaces so that the influence of laser engraving parameters on the AA surfaces could be investigated. To enhance the bonding contact, RPC techniques were further applied. A few significant conclusions can be highlighted, as follows:

(1) The combined treatment with laser engraving and RPC was very beneficial in the construction of through-the-thickness epoxy pins for improving the bonding strength of the AA-CFRP composite. The highest bonding strength of the LE-0.3 specimen was 227.1% greater than that of the only-acetone-cleaned specimen.

(2) Stronger mechanical interlocking could be created as the small engraving unit contributed to forming micropores to contain epoxy adhesive in the vertical space. The failure mode changed from adhesive failure of the AA/epoxy interface to the delamination-dominated failure of the CFRP substrate.

(3) Laser engraving was a treatment method that was controlled in shape and dimensions, and it was found to be an effective and feasible method of modifying the AA to improve the bonding strength of the AA-CFRP composite, which could provide a reference for preparing other high-performance hybrid composites with metals.