Mechanical Investigation of Recyclability for Sustainable Use of Laser-Based Metal–Polymer Joints

Abstract

Metal–plastic hybrid components combine the strength of metal with the low density of plastic. Due to weight reduction, these components are becoming increasingly important. To reduce the need for raw materials, processes for the recyclability of hybrid compounds are being investigated to reuse the metal part. The aim of this research is to characterize the mechanical bond strength after laser-based cleaning and reuse of the metal component. For this purpose, laser radiation is used to introduce microstructures into the metal surface. Afterwards, the polymer is joined to the metal component with laser radiation. As a reference of the initial mechanical bond strength, the joined samples are examined in a tensile testing machine. The polymer residues remaining in the structured metal surface are removed with different laser-based cleaning strategies. The metal is used again to generate another hybrid joined sample with a new polymer component. The results of the subsequent tests in the tensile testing machine are used for a detailed analysis of the reusability. As a result of this investigation, the laser-cleaned specimens showed significant improvements in bond strength compared to the uncleaned specimens. The process of laser-based cleaning for the reuse of the metallic part of hybrid joined components provides a fundamental procedure for improving the circular economy. In the future, this study should be validated in subsequent investigations on realistic components with complex geometries.

1. Introduction

Weight reduction through innovative lightweight construction is an important change, particularly in the automotive sector, to reduce fuel consumption [1]. One way of reducing weight is multi-material construction. This involves using different materials, such as metal and polymer, and the right material, depending on the load. On one hand, the high specific rigidity, the simple shaping and, at the same time, the low weight of the polymer are utilized. On the other hand, the properties of polymers can be combined with the high load-bearing capacity and temperature stability of metals [2]. Joining these two materials directly is challenging due to their different chemical and physical material properties. Conventional joining methods, such as adhesive bonding, require complex process control and long curing times and are often difficult to detach [3]. Screw connections require additional elements and cause high notch effects as well as weak points in the material [4]. A reliable joining process for aluminum and fiber-reinforced polymers is a laser-based, two-step process in which the metal component is first microstructured, and then, the polymer part is thermally joined [5]. How such a laser-based hybrid joint can be separated residue-free by the end of the product life cycle and how the components can be reused, as required by the EU Commission for future joints, are remaining questions [6]. Since a residue-free separation of the structured metal from the polymer part can be realized using a laser, initial approaches to the recyclability of metal–plastic hybrid components are examined in this article. The focus is on the laser-based cleaning process to reuse the metal component. Process chains for recycling and reusing the polymer part already exist and are not being investigated further [7,8,9].

Many publications on laser-based structuring of metals have been published to date. Various surface structures applied with laser radiation were investigated and then joined with a polymer component [3,5,10,11,12,13]. Investigations were conducted on the wettability of the microstructures, which provide information on the subsequent joining behavior with plastics [1,3]. Furthermore, the joining of metal and polymer with laser radiation and the influences on the mechanical strength have already been examined [13,14,15,16]. The different joining mechanisms, such as adhesive bonding and induction joining, were compared with laser-based joining regarding their mechanical strength [5,17]. Laser cleaning of metal surfaces has been investigated in the field of adhesive bonding [18]. The cleaning process was used to prepare the surface before bonding [19]. Laser cleaning was also used for pre-treatment for welding two polymers [20] or metals [21,22,23]. In addition, the influence of a laser-cleaning process when joining metal and polymer with laser radiation was investigated [24,25]. However, the pre-treatment with laser cleaning is not a focus of this article.

It is not known yet to what extent the structures in the metal surface can be cleaned with laser radiation after separation of metal and polymer to subsequently reuse the metal component. The effects of laser-based cleaning on the mechanical strength of the hybrid joint have not yet been investigated. This leads to the central research questions:

(1)

Can the polymer residues in the cavities of the structured metal surface be removed by using laser radiation?

(2)

How high are the achievable strengths after laser-based cleaning compared to uncleaned samples?

The purpose of this article is to gain initial insights into laser cleaning and to find parameters that will remove the polymer from the cavities while the structured metal remains undamaged. In addition to the mechanical analysis, optical analyses provide information about possible residual particles that remain in the cavities after cleaning. Figure 1 provides an overview of the planned research concept.

After the laser-based structuring of the metal surface and the thermal joining of the metal with the polymer, the first separation of the two components takes place in the tensile testing machine. The initial strength of the joint is documented. This is followed by laser-based cleaning of the metal surface and a control group of uncleaned specimens. The recycled (laser-cleaned and uncleaned) metal component is then rejoined with a new polymer part and separated again in the tensile testing machine. The second separation allows for a comparison between the cleaned and uncleaned samples.

2. Materials and Methods

To produce laser-based metal–polymer joints, the first step is to structure the surface of the metal component. For this purpose, a 2 kW fiber laser from Coherent (Santa Clara, CA, USA) of the type Rofin Laser FL 020 C with a wavelength of 1070 nm is used. The optical system consists of a scanner from Scanlab (Puchheim, Germany) of the type intelliSCAN SE20, which provides the deflection of the laser beam in the x and y direction. Attached to the scanner is an F-Theta lens from Sill Optics (Wendelstein, Germany), type Q 340/20 (S4LFT1330/328), which maps the focus position into a flat field. The setup results in a focus diameter of the laser beam of 38 µm. For structuring the metal component, the laser beam is focused on the surface of the specimen. The metal is locally heated with the laser radiation, melts and vaporizes. The vapor pressure expels the melt and partially solidifies it at the neck of the forming cavity. By deflecting the laser beam through mirrors in the scanner and guiding it over the metal surface, trench-like cavities are formed. By passing the laser beam several times on the same spot, the structure depth can be increased, and an undercut can be formed by the material, solidifying at the structure neck. Figure 2 illustrates the formation of the cavities.

Due to the widespread use of aluminum as a material in lightweight construction, the materials to be used are determined in advance tests. The aluminum EN AW-5754 is selected, which is used in all the examinations of this study. The thickness of the aluminum is 2 mm. The chosen polymer is polypropylene with a glass fiber content of 30% by volume and a thickness of 3 mm. The second polymer used in this study is polyamide 6 with a thickness of 2 mm. The materials used and the physical properties are illustrated in

Table 1. Properties of the materials used.

Aluminum EN AW-5754:
Property	Value	Unit
Tensile strength	245–290	MPa
Tensile modulus	68	GPa
Thermal conductivity	147	W/mK
Temper type	H22	-
Absorption of laser radiation (900–1080 nm)	4–14	%
Polypropylene with 30% Glass Fiber:
Property	Value	Unit
Tensile strength (Yield)	85	MPa
Tensile modulus	6.5	GPa
Density	1.15	g/cm3
Melting point	165	°C
Absorption of laser radiation (900–1080 nm)	95	%
Polyamide 6:
Property	Value	Unit
Tensile strength (Yield)	78	MPa
Tensile modulus	3.3	MPa
Density	1.14	g/cm3
Melting point	221	°C
Absorption of laser radiation (900–1080 nm)	6–12	%

[26,27,28].

In the preliminary tests, experiments are first conducted on the size and parameterization of the structured metal surface on tensile shear specimens. For this purpose, linear structures (transverse and longitudinal to the tensile direction) as well as crossed structures (0° and 45° starting angle) are investigated. The different structures are illustrated in Figure 3.

It was determined that a large joining surface leads to cohesive failure of the polymer. This is not desired for the further investigations, since in subsequent steps, a cleaning of the polymer residues that remain in the cavities should be investigated. As a result, the joining surface was reduced in its size to cause complete detachment of the polymer, leaving only the residues in the cavities.

Since crossed structuring at 0° and 45° starting angles showed the best results in terms of remaining polymer parts in the cavities, this arrangement of the structures is chosen for the tensile shear specimens of this study. The structure distance is held constant at SA = 200 µm. For the investigation of the head tension tests, the structure arrangement of 0° is chosen, as the angle is less important for this load case. In total, two different laser parameters are used for the head tensile tests. These two parameters show different cavity shapes within the cross sections of the preliminary investigations. For the tensile shear specimens, a laser power of P = 1.1 kW with N = 3 passes is selected. These parameters are also used for the head tensile specimens and extended with another set: P = 1.3 kW with N = 2 passes. The scanner speed is left constant at v = 20 m/s for the structuring of the metal sample in this investigation.

Table 2. Experimental plan of the investigations.

Shear Stress Specimen:
#	Laser power	Passes	Scanner Speed	Structuring Parameter	Cleaning
1.1	P = 1.1 kW	N = 3	v = 20 m/s	SA = 200 µm, 45° crossed	No
1.2	P = 1.1 kW	N = 3	v = 20 m/s	SA = 200 µm, 45° crossed	Yes
1.3	P = 1.1 kW	N = 3	v = 20 m/s	SA = 200 µm, 0° crossed	No
1.4	P = 1.1 kW	N = 3	v = 20 m/s	SA = 200 µm, 0° crossed	Yes
Normal Stress Specimen:
#	Laser power	Passes	Scanner Speed	Structuring Parameter	Cleaning
2.1	P = 1.3 kW	N = 2	v = 20 m/s	SA = 200 µm, 0° crossed	No
2.2	P = 1.3 kW	N = 2	v = 20 m/s	SA = 200 µm, 0° crossed	Yes
2.3	P = 1.1 kW	N = 3	v = 20 m/s	SA = 200 µm, 0° crossed	No
2.4	P = 1.1 kW	N = 3	v = 20 m/s	SA = 200 µm, 0° crossed	Yes

summarizes the parameters determined during the preliminary tests for this study.

The reason for choosing two different laser parameters for the head tensile specimens is the different cavity shapes in the metal. A laser power of P = 1.1 kW with N = 3 passes and a scanner speed of v = 20 m/s results in cavities which are u-shaped, whereas the parameters listed in Table 2 at 2.1 and 2.2 produce undercut cavities with a narrow neck. The influence of the cavity shape on the laser-based cleaning as well as on the mechanical properties will also be analyzed as part of this investigation.

The joining process is carried out by using a diode laser from Laserline (Mülheim-Kärlich, Germany) of the type LDM 3000-100 with a wavelength range from 900 to 1080 nm. The fiber of the laser is attached to a variable optic, which allows for the laser beam to be adjusted in a variable rectangular shape in the processing plane. This optic is produced by the company Laserline with the designation OTZ-5 VR Zoom. The rectangular spot size is adjustable between 6 mm and 80 mm in the x and y directions, depending on the focusing lens mounted on the optic. The focusing lens has a focal length of f = 250 mm. For the joining process, the size of the laser beam in the working plane is always selected to continuously irradiate the entire structured surface. An illuminated area of 10 × 10 mm² is selected for the tensile shear specimens, and an illuminated area of 2 × 24 mm² is selected for the head tensile specimens. A positioning aid is built for the joining process of both specimen arrangements, which ensures correct and reproducible insertion of the aluminum and polymer components. The aluminum is fixed above the polymer, and both components are pressed together. An illustration of the specimen geometry and direction of irradiation during joining is given in Figure 4.

The correct application of pressure to the aluminum and polymer components is very important while joining. The structured surface of the metal must be in contact with the plastic permanently. The joining force guarantees no gap between the two materials. The laser heats the aluminum component on the opposite side of the structures, and the plastic begins to melt because of the heat conduction. Driven by the joining pressure, the plastic begins to flow into the structured cavities. After the joining process is complete, the plastic solidifies in the cavities of the aluminum, and a form-fitting bond is created between the two materials. The surface pressure is set via a manometer during the joining process and kept constant at p = 6.3 N/mm² for both joining surfaces of the two specimen geometries. Figure 5 shows the two laser parameters used during the joining process.

After joining, the product life-cycle follows for real components. The separation of the materials and the reuse take place at the end of the life-cycle. The samples analyzed in this study are separated in a tensile testing machine directly after the joining process. This type of separation provides information on the bond strength and serves as a reference for subsequent tests. The tensile testing machine used is the Z100 type from Zwick Roell (Ulm, Germany), which is equipped with a load cell up to 10 kN. While the tensile shear specimens can be clamped in the machine’s pneumatic fixtures, a tool is built for the head tensile specimens to ensure that the specimen can be held. The two specimen shapes are chosen to test two different load cases (shear and tensile) on the cavities during this study.

After the separation with the tensile testing machine, one half of the specimens is cleaned with laser radiation and compared with the other half, the uncleaned specimens. The Laserline diode laser LDM 3000-100 is again used to clean the samples. For this, the samples are placed in a fixture under the laser, and the structured surface of the aluminum is irradiated with a laser beam. Preliminary tests have shown that the process window for cleaning is not large. If too little heat is applied with the laser, no cleaning effect occurs. If too much heat is absorbed by the aluminum specimen, it begins to melt or changes its shape. The laser parameters that turned out to be suitable for cleaning the tensile shear specimens and the head tensile specimens are shown in Figure 6.

The laser-cleaning parameters correspond to a roof profile. After a short heat-up time of 750 ms, the laser power is kept constant over a period of 1200 ms until the laser power drops to 0 W within 750 ms. The short heating and cooling times ensure a more uniform temperature profile during the cleaning process. After half of the specimens have been cleaned, all specimens are joined again with a new plastic component. The second joining process is carried out analogously to the first joining process. In the second joining step, polyamide is joined in addition to polypropylene. After the second joining, the subsequent testing with the tensile testing machine allows for comparisons of the mechanical properties of the cleaned and uncleaned specimens as well as conclusions about the forces of the initial reference specimen. To better evaluate the qualitative analysis of the cleaning influence, the samples are optically analyzed at different points in the process chain. For this purpose, cross-sections and images with a laser-scanning microscope (LSM) are taken.

3. Results and Discussion

The mechanical investigations for the tensile shear tests are first carried out on test specimens joined with polypropylene with a glass fiber content of 30% in both joining steps. The results of the tensile tests are shown in Figure 7.

On the left of the diagram, the results for the 45° structuring are shown. The initially joined specimen with polypropylene GF 30 shows the highest tensile shear strengths with an average of 14.9 MPa until it breaks. The uncleaned and laser-cleaned specimens with 45° structuring have similar tensile shear strengths with averages of 13.9 MPa and 14.0 MPa, respectively. This happens through the residual plastic remaining in the cavities after the first separation of aluminum and polypropylene. The remaining plastic residues in the surface are welded to the new plastic when the uncleaned specimens are rejoined. As a result, the tensile shear strength is on the same level as the laser-cleaned specimen. The cavities of the cleaned specimen are predominantly free of the plastic residues, so the new plastic can flow into the cleaned cavities during the joining process. The use of the same two types of plastics in both joining steps leads to the results of the reference specimen: a hybrid specimen of aluminum and polypropylene with plastic-filled cavities in the metal surface. The deviations in tensile shear strength between the reference specimen and the two specimens used a second time are within the standard deviation. Nevertheless, a tendency can be observed. It can be explained by a non-optimal bonding of the plastic residues, which also seem to be present in the cleaned samples, to the new plastic. The results of the 45° structuring coincide with those of the 0° structuring in the right area of the diagram; the reference also breaks under the highest tensile stresses with an average of 14.3 MPa. In the second tensile shear test, the uncleaned and cleaned specimens show the same strengths until fracture. Overall, the tensile shear forces of the 0° texturing are slightly below those of the 45° texturing but are within the tolerance.

The influence of welding the remaining residues of polypropylene GF 30 in the cavities with the new plastic to be joined should be prevented in further investigations. By using a plastic that, due to its highly polar properties, cannot be welded to the non-polar polypropylene, the influence of the uncleaned metal sample can be shown. As soon as the aluminum and the polypropylene are separated in the tensile shear test, most of the cavities in the metal surface remain filled with residual polypropylene. As soon as this sample is used uncleaned for rejoining with a polar polymer, the new polymer can neither flow into the cavities and produce a form-fit bond nor be welded to the residual polypropylene in the cavities. A comparison of uncleaned samples with laser-cleaned samples, in which the polypropylene is vaporized from the cavities, will be possible due to the different polymers. Polyamide 6 is chosen as the polar polymer for further investigations. An overview of the results, in which polypropylene is used in the first joining step and polyamide in the second joining step, is shown in Figure 8.

A distinction is made between the 45° and the 0° structure arrangement in each case with and without laser-based cleaning. The uncleaned specimens with a 45° structure arrangement show an average maximum strength of 5.6 MPa in the tensile shear test. The cleaned specimens with the same structure arrangement have an average bond strength of 9.1 MPa. Even though the spread of the cleaned specimens is large, the tendency shows an increase in mechanical strengths after the laser-based cleaning. Similar results are shown for the 0° structure arrangement. The uncleaned samples have a maximum strength of 5.6 MPa in the tensile shear test, and the cleaned samples have an average of 9.9 MPa. These results show that laser-based cleaning has cleaned a large part of the cavities from polypropylene and that laser-based cleaning of the cavities is successful. A comparison with the original tensile shear strength of 14.9 MPa for the 45° structured specimens and 14.3 MPa for the 0° structured specimens joined with polypropylene is not clearly possible due to the different polymers. The two polymers have different material characteristics, which have an influence on the tensile shear strength. The previous Figure 7 shows that there still seem to be residues of the polypropylene in the cavities after cleaning, which causes a lower bond strength. Cross-sections of the samples and images taken with the laser-scanning microscope provide information about the possible polymer residues. These will be evaluated later in this investigation.

In addition to the analysis of the tensile shear strength, they are also analyzed by means of normal tensile tests. The reason for this is the different loading direction of the cavities during separation. In the tensile shear test specimens, the mechanical separation of the two materials leads to shearing of the plastic component on the aluminum surface. In the case of shear tensile tests, the cavities filled with plastic are loaded transversely to their height. In normal tensile tests, the plastic component is loaded in a transverse direction to the structured metal surface. The plastic is removed vertically from the cavities. To investigate the influence of the different loading directions on the effects of laser cleaning, this load case is investigated by using T-shaped specimens. Again, the initial specimens for reference are joined with polypropylene, whereas polyamide is used in the second joining step to prevent the two polymers from sticking together due to residual particles in the cavities. The results of the head tensile tests are shown in Figure 9.

The left section of the diagram shows the results for a laser power of P = 1.3 kW and N = 2 passes of the laser at a constant speed of v = 20 m/s. The reference samples result in an average force of 6.3 MPa for normal load direction. The uncleaned specimens show an average force of 4.3 MPa, whereas the cleaned samples have an increased normal tensile force with an average of 5.0 MPa. The results are consistent with those from the tensile shear test; laser-based cleaning increases the bond strength between aluminum and polyamide compared to uncleaned specimens. Although the error bars overlap in a large area, this result is recognizable. In Figure 9, no distinction is made between the arrangement of the structures for the normal tensile tests, as was previously the case for the tensile shear tests with 45° and 0° structure arrangements. Instead, a distinction is made between the laser power and the number of passes of the laser in Figure 9. The specimens on the right of the diagram are structured with a power of P = 1.1 kW and N = 3 passes. The different laser parameters lead to different cavity geometries. The two resulting cavity shapes are shown in the upper part of the image. The left cavity resembles an open hole (u-shaped) with vertical outer walls and a rounded bottom. The right cavity is narrower and deeper in the metal and has an undercut shape. This undercut shape has a major influence on the load case investigated here. As the force is applied vertical to the surface, the undercut prevents the plastic from pulling out of the cavities. At the same time, the smaller width of the cavity in the neck area leads to a smaller cross-section of the plastic, which tears off there. The results of the normal tensile tests on the right of Figure 9 show a higher bond strength with 9.3 MPa on average for the undercut cavity shape. The subsequent cleaning of the metal component before rejoining leads to an increased bond strength of 7.0 MPa in comparison to the uncleaned samples with an average of 5.4 MPa. Due to the undercut shape, more plastic remains in the cavities after the first separation than with a wide, vertical cavity. Furthermore, despite the undercut cavity and the associated tendency to shadow the laser during cleaning, laser-based cleaning is similarly effective as with the open cavity on the left. With the open cavity, 79% of the original bond strength is achieved in the second joining step after laser-based cleaning and joining with two different polymers. For the undercut cavities, 75% of the strength of the reference sample is achieved by using two different polymers in each joining step.

The quantitative results of the tensile tests are extended with cross-sections and images taken with the laser-scanning microscope. They are intended to provide further information about the cavities in the individual stages of the investigation. For this purpose, random samples are taken at the various steps of the process chain and examined. The investigated steps are assigned alphabetically for clear classification: (a) directly after the first structuring of the metal; (b) after the first joining of the structured metal component with polypropylene; (c) after separation in the tensile testing machine without laser-based cleaning; (d) after separation in the tensile testing machine with laser-based cleaning; (e) uncleaned sample after the second joining with polyamide; and (f) cleaned sample after the second joining with polyamide. Figure 10 provides an overview of the different cross-sections.

The structured aluminum surface is shown in the lower part of each image, and the plastic component (if present) is shown above it. In the first cross-section (Figure 10a), the laser-based structuring creates cavities that are approximately 150 µm deep and have an opening width of 70 µm. The second cross-section (Figure 10b) shows the hybrid joint made of structured aluminum with cavities that are filled with polypropylene. The glass fibers (30%) are visible. However, the plastic, which is inside the cavities, contains hardly any glass fibers. The reason for this is that the glass fibers are surrounded by a protective matrix material made of polypropylene, which flows into the cavities first during joining. The sample (Figure 10c) has already been separated using tensile shear testing. The cavities are still filled with polypropylene, and the plastic part above the structured surface has been sheared off. The material in the upper part of this image is the embedding material used to prepare the sample for the cross-section. The shadow at the fracture edge and the different pattern of the materials show the difference to the polypropylene. In the cross-section at the bottom left (Figure 10d), the shown cavities are after laser-based cleaning. It appears that a few polypropylene residues remain in the cavities. The laser heats the metal surface so that a major part of the plastic melts and evaporates. However, the cross-section only shows four of many cavities in one section plane. For a more precise evaluation, an attempt is made to examine a larger part of the specimen using surface images. These images provide information about a larger cleaned area. The LSM images are shown below. A cross-section of an uncleaned sample that is rejoined with new polyamide is shown at the bottom center of Figure 10e. The cavities are still filled with polypropylene due to the lack of cleaning. Although the polyamide is melted onto the aluminum, it cannot flow into the cavities and create a form-fit bond. Due to the material properties of polypropylene and polyamide, there is no bond between the two plastics by mixing the two materials in the melt. The newly joined polyamide is only attached to the irregularities of the aluminum surface in the area between the cavities. The result is shown in the previously investigated tensile tests in which the uncleaned and newly joined samples showed a significant reduction in the bond strength compared to the laser-cleaned samples. A laser-cleaned sample that is rejoined with polyamide is shown on the bottom right (Figure 10f). The cavities are filled with polyamide, which were laser cleaned from the polypropylene residues. The form closure between the metal and plastic increases the bond strength, which was displayed in the tensile tests.

Further images are taken with the LSM to carry out a full-surface examination of the cleaned surface. These images are intended to extend the results from the previous two-dimensional cross-sections and provide information about the height profile of the cavities. Figure 11 shows three images taken with the laser-scanning microscope.

The figure on the left (Figure 11a) shows the surface directly after structuring a crossed pattern. Due to the processing strategy, there are twice the number of passes at the crossing points. Therefore, a deeper structuring can be found there. The middle image (Figure 11b) shows an aluminum surface joined and separated with polypropylene. The rectangular residues of the unprocessed surface are visible. The cavities are filled with polypropylene, which has been sheared off. In some cases, small burrs of plastic protrude from the surface. Furthermore, the plastic has been torn out of the cavities in some places, resulting in a depression. This indentation does not show the original cavity depth. It can be concluded that some of the polypropylene is still adhering to the lower area of the cavity. The right-hand image (Figure 11c) shows the laser-cleaned surface of the aluminum. The initial structure pattern is visible. The cavities have gained depth because of the cleaning process, which vaporizes the plastic residues. The full depth of the original cavities has not yet been achieved. Due to the residual particles of plastic that are still on the ground of the cavities after cleaning, the potential for a form-fit bond between the two joining partners is not available in the second joining step compared to the initial one. This shows that further investigations into laser cleaning are necessary.

4. Summary and Outlook

The investigations of the laser-based cleaning of microstructures for reusing the metal component showed promising results. The investigation of the tensile shear specimens showed stresses of 14.9 MPa for a 45°-angled structured surface and 14.3 MPa for a 0°-angled structured surface for the initial reference specimen made of aluminum and polypropylene with a glass fiber content of 30%. Both the uncleaned specimens and the laser-cleaned specimens showed similar failure forces to the reference after rejoining with polypropylene. The forces are less than 10% below the respective reference for both structural arrangements (45° and 0°). The reason for this is that the cavities of the cleaned samples are removed of polypropylene during the cleaning process, and the polypropylene can flow into the open cavities during the rejoining process to create a form-fit bond. In the uncleaned samples, the cavities are still filled with the remains of the old polypropylene at the start of the joining process. The new joining process welds the old polypropylene to the sample to be joined in the area close to the surface and forms a new form-fit bond. To investigate the influence of laser-based cleaning, polyamide 6 is, therefore, used in further tests in the second step, as this cannot be welded with the polypropylene due to its different polarity. The results showed clear differences between the uncleaned and laser-cleaned samples for both structural arrangements. On average, the tensile shear strength of the uncleaned samples is 5.6 MPa with a 45° structural arrangement and a 0° structural arrangement. Laser-based cleaning increased the breaking strength to 9.1 MPa for the 45° structural arrangement and 9.9 MPa for the 0° structural arrangement. A clear deviation due to the different orientation of the structure (45° and 0°) could not be determined. Similar results are also obtained in the head tensile test. Two different cavity shapes, which are created by using different laser parameters, are examined in detail. Both cavity shapes showed an increase in the bond strength between aluminum and polyamide. For the vertical cavities, cleaning led to an increase in bond strength of 16%, and for the undercut cavities, laser cleaning led to an increase of 30%. The optical analyses of various samples in cross-section and using a laser-scanning microscope clarified the previous results. The cavities of the separated metal component are still filled with polypropylene. The polymer shears off in the neck area of the cavity. In the uncleaned samples, the broken polypropylene offers the possibility of preventing a newly joined plastic component from shearing off. Depending on the direction of loading, this influence has a significantly greater effect on the tensile shear specimens than on the tensile head specimens. Furthermore, the optical analyses showed that plastic is removed from the cavities in the laser-cleaned specimens, and, thus, retroactively to the research question, laser cleaning of the cavities in the metal component is possible. Nevertheless, residual pieces of the initially joined plastic still appear to remain in the lower area of the cavities after cleaning.

The main conclusions of this investigation are the following:

• When using the same polymer in both joining steps, similar strengths are achieved with and without laser cleaning, as in the reference specimen;
• If polypropylene is used in the first joining step and polyamide in the second joining step, over 69% (shear strength) and over 75% (normal strength) of the initial reference strength are achieved;
• The microscope examinations revealed polymer residues at the base of the cavities after cleaning, which are responsible for a reduction in the strength compared to the initial reference.

The results of this work provide optimization for improving laser cleaning, which could provide significant insights with other laser beam sources and cleaning parameters as well as with different materials. Furthermore, temperature measurements on the surface during the cleaning process are also planned.