Abstract
Lightweight construction is a central technology in today’s industrial production. One way to achieve the climate goals is the production of hybrid compounds of metal and plastic. The manufacturing process for these hybrid parts can be divided into in-mold assembly and postmold assembly. The postmold assembly includes thermal joining by laser, which is applied in the context of this paper. For the investigations, four plastics (MABS, PA6.6-GF35, PP, and PC), which differ in their properties, and three metals (unalloyed steel, stainless steel, and aluminum) are combined and analyzed. These materials have been used, since they have a huge significance in the automotive industry. Preliminary studies showed that an adhesive bond between the two materials is achieved using metal with a structured surface. According to these studies, three structuring processes for metals (selective laser melting (SLM), NRX, and a welded metallic tissue) are tested. The quality of the material/structure combinations is tested in tensile-shear-tests, microscopy images, and alternating climate tests. Compounds with SLM-Structure achieve highest strength, while compounds with aluminum are much more complex to manufacture.
1. Introduction
Lightweight construction is one of the key technologies of the 21st century. In addition to the substitution of metals by lighter materials, lightweight construction includes also a selective combination of different materials in order to combine their advantages. One of these combinations is the so-called hybrid compound of metal and plastics. Examples can be found in the automotive sector in the front end of the Ford Focus or the pedal block of the Mercedes-Benz C-Class. The use of hybrid compounds in these areas leads to a cost and weight reduction as well as an increase of the functional integration.
The production of hybrid parts is carried out by either in-mold assembly or postmold assembly. Postmold assembly involves, beside mechanical joining and bonding, also thermal joining. Thermal joining includes joining of metal-plastics-hybrid compounds by heat contact, ultrasonic induction, and laser radiation and was part of several studies [1–18]. The studies identified some limitations and problems of the processes. For example, ultrasonic welding has low flexibility of the sample geometry as well as a small joining area [2]. In contrast, the challenges of induction welding are primarily in the process layout and control concerning the easily influenced magnetic field and extremely high heating rates [2].
Metal-plastics-hybrid joining by laser radiation is not industrialized yet, because basic conditions like long-term strength or the process control have not been experienced deeply enough.
The thermal joining by using laser radiation has been studied in different combinations. During the joining of laser-transparent or partially laser-transparent plastics with metals, the metal component is used as a laser beam absorber. In this way, the plastic is locally heated above the melting point and holding forces are formed between the joining partners. The high flexibility in terms of the joining geometry as well as the extremely short cycle time qualifies this process for an application in the medium- and large-scale production. The thermal joining with laser radiation has been investigated in different combinations of materials, such as Polyethylene terephthalate (PET) with steel [4, 19, 20] or Polyamide 6 (PA6) or Polyamide 66 (PA66) with unalloyed steels [21, 22]. Under the influence of external forces, the contact between the joining partners affects the wetting of the metal surface. However, if transparency is not given, the metal is heated by direct irradiation [12, 22–29].
Bauernhuber and Markovits have been concerned with the joining of PMMA and steel by using pulsed Nd:YAG-laser. On the one hand, the influence of process parameters (heating time, laser setting, surface roughness, and joining pressure) on the weld strength has been investigated. On the other hand, the temperature distribution was determined during different heating conditions. Thus, higher surface roughness, due to mechanical anchors, results in a higher weld strength. During the joining process, there was blistering in the material which weakens the weld but can be controlled via the joining pressure [21, 22].
Schricker et al. used present results of PA66 and EN AW 6082 to transfer these to macroscopic structures in order to find the optimal alignment angles for different structures (drilled holes and grooves). The investigation shows that the determined shear force is independent from structure (geometry and alignment angle). However, the alignment angle depends on the relative structure; for example, the alignment angle for grooves is −45° [30].
Previous investigations [4, 12, 19–31] show that the type of surface structure of the metallic joining partner has an important influence on the bond strength. The highest bond strength was achieved using structures with undercuts. So far, such structure could only be applied by laser technology [32].
In order to investigate the applicability and requirements of laser joining of hybrid compounds, bonded samples are examined in strength tests as well as alternating climate tests. Furthermore, the bond strength was investigated regarding metallic material and their surface structure. For this purpose, four plastics and three metals with three different surface structures were investigated. The plastics used are methyl-methacrylate-acrylonitrile-butadiene-styrene (MABS), Polyamide 6.6 with 35 wt.-% glass fibers (PA6.6-GF35), polypropylene (PP), and polycarbonate (PC). They differ from each other in optical, rheological, thermal, and mechanical properties. In addition, three different metallic materials (unalloyed steel, stainless steel, and aluminum) were utilized. The surface structure was applied by using three common technologies: SLM-Process, NRX-Technology, and welding of a metallic tissue.
2. Test Procedure
2.1. Sample Geometry
The plastics sample with a basic size of mm2 has stepped jumps from 1 mm in thickness direction. The thickness of the steps extends from 1 mm to 4 mm. The metallic joining partner is a cuboid with the dimensions 80 25 3 mm3. The structured area of the sample starts at one sample edge and extends over the whole width with a length of 12.5 mm, making a joining area of 25 12.5 mm2 available. Figure 1 shows the geometry of both samples.
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2.2. Joining with Laser
During laser joining of hybrid compounds, the metal absorbs the irradiated laser energy and converts it into heat. The laser radiation can be applied in two ways. In the first method, the polymer sample is positioned above the metal sample. Laser radiation transmits through the polymer before it is absorbed in the metal (Figure 2).
The heating is carried out by thermal conduction and radiation in the joining zone.
In the second variant, the metal is placed above the polymer sample and is directly irradiated by the laser. The generated heat is transported by thermal conduction through the complete thickness of the metal sample into the joining zone. This type of heating requires more time and energy than the first method and is therefore not preferred. However, the first variant is only possible for plastics with high transmission properties. Otherwise the laser radiation is already converted into heat in the polymer and cannot expand into the joining zone. The direct radiation of the metal is only used for material combinations which are not joinable with the transmission technique.
In this study, the system was configured to heat the joining zone homogeneously in short time. For the experiments, Leister Technologies AG, Kägiswil, Switzerland, provided a diode laser source (LineBeam) that distributes a homogeneous line direct out of the diode stack with an optical power of maximum 600 W. In order to prevent back reflection of radiation into the laser module, the LineBeam is assembled under an angle of 4°.
The line laser is moved across the joining area at a constant joining pressure of 0.4 MPa. For homogeneous melting of the plastics, the laser is moved across twice. The first crossing preheats the metal, while the second crossing heats to the final joining temperature. In order to prevent the plastics from damage due to higher temperatures, the temperature may be monitored using a pyrometer. Heating of the metal until the melting temperature of the plastics is reached ensures that the melted plastics can flow into the cavities of the structure. In this way, “mechanical interlocking” occurs during cooling. The cooling takes place at a constant joining pressure as long as all melt is solidified inside the joining area.
2.3. SLM-Process
The selective laser melting- (SLM-) Process belongs to the group of additive manufacturing. During the manufacturing process, a powder material is applied on a base sheet, which is positioned on a lowerable working table. By laser radiation, according to CAD parameters, the powder is melted locally. After fully generating structure on one layer, the working table is moved down by a layer thickness. These steps are repeated until the desired structure (Figure 3) is obtained [32, 33].
2.4. NRX-Technology
The NRX-Technology is a mechanical method which is protected by Nucap Industries Inc., Toronto, Canada. The surface structure is created with a chisel tipped roller. When the chisel tipped roller is in contact with the metal plate, small hooks are dislodged from the plate. However, the caused hooks are still connected to the plate. The size of the resulting hooks (Figure 4) depends on the size of the chisels.
2.5. Welding a Metallic Tissue on Metal
The welding of a metallic tissue also belongs to the group of surface structures, but the surface of the metal is untreated. The base material and the material of the metallic tissue are the same. The undercuts of the metallic joining partner are produced via the welding of a metallic tissue (Figure 5). The used tissue has a wire size of 0.2–0.3 mm and a mesh size of 0.4–0.5 mm. The welding is executed over the entire width of the sample, in a way that the tissue does not relieve from the base material. The additional stripes welded on the side are necessary for a stronger bond between the tissue and base material but may cause notch effects.
2.6. Tensile-Shear-Tests
The joining tests are evaluated using two techniques, a tensile-shear-test according to DIN ISO 527 [34] and microscopy images. The microscopy images should illustrate both the connection between plastics and metal and the filling of the structures in the joining area. The metal-plastics-hybrid joining is based solely on adhesion and form closure. Tensile tests showed that the joining strength of the tested combinations reaches a plateau over a wide range of energy input. Too much energy leads to visual destruction of the plastic material, before the strength may decrease. The reason is that the polymeric joining partner will not be damaged or degraded at the optimized joining temperature [35]. Overall, achieving a breaking force that exceeds the basic material strength of the respective polymers is desirable and implies a cohesive failure of the compound, since an adhesive failure means a failure in the boundary layer itself.
2.7. Alternating Climate Test
With the alternating climate test, the climate stability of the compounds is determined. For this purpose, compounds with a good bonding strength are examined in an alternating climate cycle according to DIN IEC 60068-2-38 [36]. The chronological sequence of the cycle, which is repeated six times, is shown in Figure 6. After the alternating climate test, the weathered compounds are subjected in a tensile-shear-test in accordance with DIN ISO 527, too [34]. The influence of the alternating climate tests on the bonding strength is obtained through a comparison with results of the unweathered compounds.
3. Discussion of the Results
3.1. Optical Properties
The transmission technique requires high transmittance of the plastic as well as a high absorption factor of the metal. In contrast, the direct irradiation of the metal is independent from the plastics optical properties. In this case, only a high absorption factor is required. The influence of the reflection factor can be neglected.
The optical properties of the samples are determined using a UV-VIS Spectrometer type Lambda 1050 by Perkin Elmer Corporation, Waltham, USA. The results of the optical properties show that the amorphous thermoplastics PC and MABS have transmittance of around 90% at the relevant wavelength of 940 nm. Therefore, the amorphous thermoplastics require a lower laser power in order to achieve a connection between the two joining partners than the two semicrystalline polymers PA6.6-GF35 and PP (Table 1). The semicrystalline thermoplastics have significantly lower transmittance due to the refraction of light at the crystalline structures as well as the glass fibers in PA6.6-GF35.
The highest absorption factor of the metals, independently of the structure, has unalloyed steel, while aluminum provides the lowest. A comparison of the absorption factors in dependency of the various structures shows that the welded metallic tissue reaches the highest value (Table 2).
3.2. Results for Compounds with MABS
According to preliminary test, compounds with MABS can be produced with the transmission technique. However, the maximum joining power depends on the metallic joining partner.
Furthermore, a parameter combination to join aluminum with welded tissue and MABS could not be discovered. The plastic sample simply pyrolyzes before joining.
3.2.1. NRX-Structure
Metal samples with NRX-Structure create a cohesive failure when a complete connection between the samples is given. Up to a joining energy of 3.0 kJ the samples fail adhesively, afterwards cohesively (Figure 7). This shows that the compound of plastic and metal can absorb higher forces than the plastic sample itself. The required energy for a complete connection of the plastics to the metal differs considerably between the metals. For example, aluminum needs a joining energy three times higher than both stainless steel and unalloyed steel (10 kJ).
3.2.2. SLM-Structure
The results of compounds with MABS and SLM-Structure provide the same effect as MABS and NRX-Structure. The change from adhesive to cohesive fracture is slightly above the NRX-Structures. Similar to the NRX-Structure is the transition (2.3 kN) between adhesive and cohesive failure for stainless steel and unalloyed steel by applying the same energy input (2 kJ).
3.2.3. Welded Metallic Tissue
The strength behavior of MABS and metal samples, which are occupied with a welded metallic tissue, differs from the other two structures. The breaking force increases with increasing joining energy until the transition between cohesive and adhesive failure is reached. As soon as the decomposition temperature is achieved, the breaking force decreases instead of reaching a limiting value (Figure 8).
Since the surface of the metallic tissue is reticular, the plastic material has uneven contact. In consequence, plastic sample heats at different rates. At places with contact, the plastic heats conductively. The other areas heat by convection, which requires both more time and higher temperature (see Figure 5). But higher temperature leads to decomposition of the plastics material in the conductively heated areas.
A precise classification between the two fractures cannot be determined with these samples. One reason is the sharp-edged additional material that causes notch effects. Another reason is the impact of the initiating decomposition of the plastic to the material strength.
3.2.4. Alternating Climate Test
After execution of the alternating climate tests, a dependency of the metallic joining partner and structure on the strength behavior has been discovered. The alternating climate test has no influence on the strength of the compounds with SLM-Structure. The measured strengths before and after the alternating climate tests are the same.
Samples with welded tissue react similarly if the compound breaks cohesively in unweathered condition. If it breaks adhesively, the strength decreases. The fact that compounds are destroyed cohesively independently of weathering proves that the alternating climate test has no influence on the basic material strength of MABS.
Regardless of the metallic material, a loss of strength is discovered by compounds with NRX-Structure after the alternating climate test. A reason may be the different thermal expansion of plastics and metal, which loosen the connection and lead to a less strong joint. This only appears with the NRX-Structure, since this structure can only absorb forces in one direction. With the other structures, which can absorb forces in all three directions, the influence of the thermal expansion can be neglected, as long as the structures are filled completely.
3.3. Results for Compounds with PA6.6-GF35
Similar to MABS, PA6.6-GF35 can be welded using the transmission technique. Due to the significantly higher melting temperature ( = 260°C) of PA6.6-GF35 in contrast to MABS ( = 110°C), a connection of PA6.6-GF35 and aluminum with NRX-Structure cannot be realized. But since internal absorption is higher, the PA6.6-GF35 heats up while the laser radiates through the material and can be joined to the aluminum with welded tissue sample.
3.3.1. NRX-Structure
The combination of PA6.6-GF35 with NRX-structured metal samples shows a nearly linearly increasing breakout force that grows with higher joining energy. Because of the higher material strength [6], even with complete connection of the polymer to the metal sample, no cohesive fracture can be achieved. Therefore, a maximum achievable breakout force can be determined (Figure 9).
3.3.2. SLM-Structure
The joining of metals with SLM-Structures and PA6.6-GF35 is not constant, since the inhomogeneous distribution of the glass fibers complicates the production of a reproducible connection between the joining partners. An inhomogeneous distribution of glass fibers could apply locally higher forces to the SLM-Structure, which results in a failure of the structure. Figure 10 shows the microscopic images of PA6.6-GF35 with SLM-structured stainless steel. As illustrated, an increase of the joining energy by 1 kJ is sufficient to complete the connection between metal and polymer, in which case it comes to a cohesive failure. Although the left picture already shows a “mechanical interlocking,” a residual amount of bubbles is visible. An increase of joining energy ensures a decrease of viscosity of the polymer melt. However, the melt can flow to the bottom of the metal structure and the bearable breaking force of the connection is able to achieve higher value than the material itself.
3.3.3. Welded Metallic Tissue
Similar to the other two structures, an increase of joining energy results in an increase of the bearable force until the cohesive fracture behavior or the strength limit of the tissue is reached.
The limit between a tear-up of the metal tissue and a cohesive failure of the sample depends on the loadable joining area. Above a specific loadable joining area, the required breaking force is so high that the basis material strength of the aluminum tissue is exceeded and the samples fail significantly earlier in the tissue.
3.3.4. Alternating Climate Test
The alternating climate tests result in a decrease of strength in the hybrid compounds with PA6.6-GF35 as polymeric joining partner (Figure 11).
In general, the decrease of the strength can be explained by water absorption of the polymer during the alternating climate test. Water absorption of PA6.6-GF35 may achieve 3.5% by weight, which reduces the material strength.
An influence of the alternating climate tests on the strength of stainless steel with SLM-Structure can only be observed by high energy input. In comparison, the aluminum with welded tissue samples does not decrease in mechanical strength, independently of climate tests. This is due to the fact that the compounds already break through rupture of the tissue at low forces, even in the unweathered condition.
3.4. Results for Compounds with Polypropylene (PP)
In comparison to amorphous thermoplastics, the transmittance of semicrystalline thermoplastics such as PP or PA6.6-GF35 is significantly lower (Table 1). However, the transmittance is high enough for laser transmission joining. One advantage of PP is that it can be joined with all combinations of metal, independently of the structure. This behavior is based on the low melting temperature ( = 163°C) as well as the low melt viscosity of PP [7].
3.4.1. NRX-Structure
Independent of the used metal with NRX-Structure, a combination with PP can reach the transition to a cohesive failure of the compound. This is caused by the small melting section as well as the good flowability of the polypropylene melt [8]. At a force of 1.3 kN, which corresponds to a material tensile strength of 21.5 MPa, the transition from the adhesive to the cohesive failure occurs.
According to already mentioned reasons, this transition is way lower when using unalloyed steel compared to aluminum. Using aluminum, temperatures are reached, where the viscosity of the polymer melt is low enough to achieve a complete connection through “mechanical interlocking” of the polymer to the metal.
3.4.2. SLM-Structure
In the case of SLM-Structures, the development of the tear force is dependent on the metal. Furthermore, there is no exact transition from adhesive to cohesive failure, since the samples cannot be joined below an energy input of 1.4 kJ. Above this level, all samples break cohesively.
3.4.3. Welded Metallic Tissue
In contrast to other polymers, PP can be connected with every welded tissue structure. Thereby, a cohesive sample failure can be reached for every material. The needed force for breaking the samples cohesively is approximately 0.9 kN.
During the joining process, the polymer material flows into the subjected area of the tissue due to the low melt viscosity. Therefore, the sample cross section is reduced in the area of the tissue. At this point (thinnest sample cross section), the sample fails cohesively (Figure 12). With this structuring, the limit of the mechanical strength from adhesive to cohesive break is approximately 24 MPa.
3.4.4. Alternating Climate Test
The alternating climate tests have only an influence on the combination of PP with unalloyed steel, which has the NRX-Structure or the welded tissue.
Compared to the unweathered compounds, tensile strength is lower. This can be argued by the corrosion of the metallic welding partner in the weld seam. Corrosion implements tensions, which reduce the bonding strength. This fact does not occur at the unalloyed steel samples with SLM-Structure, since the material of the applied structure does not match the basis material. Furthermore, the polymer mainly connects with the applied material, so that the corrosion of the basis material does not affect the sample strength. Figure 13 shows the influence to the bonding force of unalloyed steel and stainless steel with a NRX-structured surface. The decrease of the strength of the weathered compound with PC and unalloyed steel might be a result of thermal decomposition. When the energy input is too high, thermoplastic materials can disintegrate. This might have happened with the energy input of 3.5 kJ.
3.5. Results for Compounds with Polycarbonate (PC)
In contrast to the other polymers, PC is not joined with the transmission technique, because first trials showed a trend for combustion using this method. In general, PC may carbonize applying to much heat locally. In particular in the contact areas, the metal is getting so hot applying laser radiation that PC starts to carbonize. This ends in a dark layer on top of the plastic surface (Figure 14), which increases the absorption coefficient of the PC. Anyway, the PC sample immediately heats up above decomposition temperature and combusts.
For this reason, the PC is joined to the metallic partner by direct irradiation. For this purpose a temperature above the melting temperature ( = 148°C) has to be reached inside the joining zone. Since the heat penetrates through the whole metallic sample, the required welding energy is considerably higher than in the transmission joining process.
3.5.1. NRX-Structure
The breaking force of the PC samples with NRX-structured metals rises with increasing energy input up to a maximum amount. The value of stainless steel sample is approximately 3400 N, while unalloyed steel is about 3000 N. Independent from the used metal, this maximum is reached at an energy of 6.7 kJ (Figure 15). At this breaking force, the stainless steel samples may break both ways cohesively or adhesively.
The difference in the strength of the used metals may be determined by dirt on the surface of the unalloyed steel samples. Due to this layer of dirt, the PC sample can slip off the basis sheet of the metal sample and fail at low forces adhesively.
3.5.2. SLM-Structure
The compounds of PC and metal samples with SLM-Structure have strength maxima which are only 0.3 kN higher than the strength maxima of the compounds with NRX-Structure. In contrast to the NRX-Structure, the needed energy input for reaching these strength maxima with unalloyed steel samples is 8.5 kJ, while the welding energy for stainless steel is the same as with the NRX-Structure (6.7 kJ). At these energies, the transition from adhesive to cohesive failure occurs. Due to the undercuts of the SLM-Structure, the layer of dirt does not have an influence on the bonding strength, since the undercuts result in a kind of “mechanical interlocking.”
3.5.3. Welded Metallic Tissue
In comparison to the other two structures, a considerable lower energy input is needed to reach the transition from adhesive to cohesive material failure. With unalloyed steel, this transition is reached at a joining energy of 5.1 kJ, while stainless steel requires 6.1 kJ for the transition.
Furthermore, the breaking force decreases after the transition from adhesive to cohesive failure with rising energy input (Figure 16). After reaching the transition, the material is damaged through the increase of the joining temperature to temperatures above the decomposition temperature.
3.5.4. Alternating Climate Test
Similar to the compounds with PA6.6-GF35, the alternating climate test has an influence on the mechanical strength in the hybrid compounds with PC as polymeric joining partner. The breaking force of the combination of unalloyed steel with SLM-Structure (Figure 17) is reduced. It is remarkable that the decrease of the breaking force increases with rising energy input.
The fundamental decrease of the sample strength as well as its dependence on the joining temperature can be explained by a degradation of the molecular weight of the PC during the joining process [37]. Here, beginning polymer degradation can be assumed.
4. Conclusion and Outlook
The studies for the production of metal-plastics-hybrid compounds presented in this paper could be evaluated using several important test criteria. Aside from the achievement of strength where the compounds show a cohesive failure, the climate stability has been an important test criterion.
On behalf of the polymers, PP and MABS especially were able to fulfill these requirements. For PA6.6-GF35 and PC, which cannot be joined using the transmission technique, the alternating climate test influences the material strength negatively.
The investigation of the metallic joining partner shows that the production of a compound with aluminum is significantly more complex, due to its optical and thermal properties. In comparison, unalloyed steel and stainless steel show similar properties during the joining process. Anyway, they are significantly more suitable for the production of metal-plastics-hybrid compounds. The disadvantage of unalloyed steel is the corrosive behavior, which can lead to weakening of the hybrid compound with changing climate.
The observation of the different structures showed that the best results can be achieved with an SLM-Structure, whereas the results with an NRX-Structure are the worst. Using the SLM-Structure, strengths above the material strength were achieved for all used polymers. The climate changing tests resulted in a decrease of the strength of compounds with SLM-Structure as well, but only due to the effect of material strength degradation.
Further studies should especially work on improved process monitoring. Furthermore, laser optics should be developed specifically for this joining process. In this way, one can ensure that the joining zone is heated optimally and evenly. In addition, it will be discussed in the future, how, and with which processes, possible samples can be structured more efficiently.
Competing Interests
The authors declare that there are no competing interests regarding the publication of this paper.