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, Volume 11, Issue 4, pp 16–21 | Cite as

Laser Structuring of Metal Surfaces for Hybrid Joints

  • Dominik Spancken
  • Kira van der Straeten
  • Jonas Beck
  • Norbert Stötzner
Cover Story Metals
  • 246 Downloads

There is a need to join metals and plastics together in numerous lightweight structures. Engineers from Fraunhofer LBF, Fraunhofer ILT, Siegfried Hofmann and Weber Fibertech structure the surfaces of metallic joining partners before the molten mass envelopes them as inlays in the molding process.

Cost-efficient Laser Processing

Using hybrid materials from dissimilar material combinations is gaining popularity in safety-relevant and functional components. Glass fiber-reinforced polyamides and high-alloy steel and aluminum materials are used particularly in highly stressed component areas in the automotive industry. With conventional joining technologies for connecting metal and plastic parts (e.g. bolting, riveting, welding or gluing one is bound to run into technical and economic constraints.

Laser technology is key from the technical and economic perspective when it comes to large-scale production. Lasering allows contact-free processing with high process speeds and abundant design freedom. New beam guidance and processing concepts pave the way for using the laser as a tool for structuring the metallic joint partners and processing the edges of fiber-composite and metal components to join them thermally.

Laser beams create microstructures with indentation on the metal surface.

Technology

The metal surface is first modified in a two-step process, Figure 1. To this end, the laser beam is used to create microstructures on the metal surface in the form of indentations that allow the plastic to grip the metal surface. This is followed by the actual joining process, where molten plastic flows into the created structures and solidifies.
Figure 1

Process structure for producing hybrid connections made from metal and plastic (© Fraunhofer LBF)

Two distinct approaches are currently used to structure the surface, differing in source of the beam, processing strategy and resultant structural geometry. Differences emerge, for example, between sponge-like, self-organized microstructures, as in Figure 2, which cover the entire surface, and linear structures, as in Figure 3, which can be arranged in any structural patterns. Both structural approaches end up enlarging the surface and have indented geometry.
Figure 2

Conical protrusions on steel (REM photo) (top), formation of CLPs on steel with different numbers of passes (bottom) (© Fraunhofer ILT)

Figure 3

Linear structuring, here in a cross arrangement with a structural distance of 500 μm (© Fraunhofer LBF)

During the subsequent joining process, molten plastic flows into the indentations, linking the two joining partners via a mechanical grip. Heat conduction or heat radiation methods can be used for the thermal joining process. In conventional production processes like injection-molding, pressing, or resin-transfer molding (RTM), the structured metallic joining partners can be inserted and joined directly in the respective production process.

From an economic perspective, the most interesting production and manufacturing processes for hybrid connections using short glass fiber-reinforced polyamide and common metal alloys are injection in an injection molding process or pressing process and thermal joining using laser beams.

Sponge-like Microstructures

To create a sponge-like surface with self-organized microstructures, so-called Cone-Like Protrusions (CLP), Figure 2 (top), the entire metal surface is ablated layer by layer using ultra-short laser pulses. After only a few passes, the physical interaction between laser pulses and material results in conical protrusions and holes that spread out further as ablation continues, until the entire surface is covered. This growth process depends on the material used. With standard steel varieties, the microstructures are fully formed after 20 to 30 passes. Figure 2 (bottom) shows the growth of CLP structures, which depends on the number of passes. The individual structures are 50 to 100 μm deep and 20 to 40 μm wide.

Indented Line Structures

A laser source with high beam quality is used to create line structures with indented geometry; this allows the laser output power to be focused on a very small area. A scanner system with a very high deflection speed is used to move the laser beam in the prescribed pattern over the work item. The high intensity causes part of the metal to vaporize, while the resultant sublimation pressure presses the molten mass from the base to the edge of the structure, where it partially solidifies. Indentation occurs when this process is repeated a number of times. Compared with CLP structuring, the line structures are somewhat coarser. On a laboratory scale however, this structuring method is far faster to process than CLP structuring.

Validation Tests

In practice, hybrid connections are usually subject to multiaxial, in-phase or phase-shifted loads. Defining characteristic multiaxial load values and their use in assessment methods to ensure safe operation is a major challenge. A test specimen was developed for this purpose, Figure 4, that could be used to investigate multiaxial load states such as tensile and torsional stresses as well as how they overlap.
Figure 4

Test specimen for determining the mechanical characteristic values of the hybrid connection: metal (top) and thermoplastic (bottom) (© Fraunhofer LBF)

Using the test specimen, the factors that influence strength in the injection- molding process were determined in a process parameter validation step using static tensile tests. The process factors of holding pressure, inlay temperature and structuring method were also analyzed. The plastic used is a polyamide 66 with 25 % glass fibers by weight (PA66 GF25) and the metallic joining partner is machinery steel (E335). In the joining zone, only the matrix material penetrates the indentations. During the investigations undertaken to date, it was not possible to establish gripping or penetration on the part of the glass fibers.

Figure 5 shows how CLP structuring exhibits greater break stress than line structuring. Greater holding pressure and a higher inlay temperature help bolster the static break stress. Overall, the static breaking strength shows little scattering, indicating stable process parameters in the joining process for the two hybrid joining methods investigated. It is also apparent that thermal joining via lasering can achieve greater breaking stress than with injection molding.
Figure 5

Validation of the process factors holding pressure, inlay temperature and structuring arrangement for injection molding (© Fraunhofer LBF)

Microscopic examinations of the fracture surfaces revealed that, unlike injection molding, with thermal welding using lasers, the molten mass completely penetrates the indentations and remains tightly gripped there after the break. This is attributable to the thermal gradients found along the boundary surface between metal and plastic with injection molding. With thermal welding using lasers, the electromagnetic radiation is absorbed by the metallic joining partner, whereupon the metal heats up. Thermal conduction causes the plastic joining partner to heat to melting point, whereupon it flows into the indentation structures. In the case of injection molding, the temperature gradient between the inlay and molten mass precludes the molten mass penetrating the indentations, since it solidifies prematurely.

Figure 6 compares the tensile and shear strengths of the CLP structuring feasible for production processes using thermal welding using lasers, pressing and injection molding. The highest tensile and shear strengths are achieved with pressing. In the case of pressing, variothermal tool and inlay temperature control at 185 °C and a polyamide 6 with 40 % glass fibers by weight (PA6 GF40) were used. Despite the presence of temperature gradients, the static strength achieved with pressing exceeds that of thermal welding using lasers. This is due to a more even and homogeneous application of energy and joining pressure with extrusion.
Figure 6

Comparison of joint seam strength for thermal joining using lasering, extrusion and injection molding (© Fraunhofer LBF)

Internal Pressure Resistance

One particular field of application for the technology is its use under internal pressure loads or in applications where a leakproof seal is required. In this case, the technology competes with gluing and conventional joining methods. Burst pressure strength was established with the test specimen using a special test bench. This is 41 bar with thermal joining using lasering and 45 bar with extrusion. Based on experimental S/N curves, an internal pressure load capacity of 30 bar for 106 load changes applies.

When joining thermally using lasering, the molten mass completely penetrates the indentations.

Roof Bows

The technology analyzed with the test specimen was implemented in a roof bow, Figure 7, based on an original part from a BMW 7 Series vehicle. The roof bow comprises a fiber glass-reinforced plastic brace joined to two metallic connection plates that serve as connecting elements to the body. The plastic and metal are joined using form locking and adhesion instead of gluing and riveting as previously.

The roof bow was designed as an open profile, allowing it to be manufactured in a pressing production process. The pre- structured metal inlays were preheated in an oven and inserted into the variothermal, close-contoured, heated pressing tool at a temperature of 200 °C. Local UD tapes were inserted into the middle and peripheral belts to increase stiffness and secured in place using vacuum nozzles. A semi-finished product is extruded from a twin-screw extruder and inserted into the pressing tool, then the mold is closed. The roof element is ejected after a cycle time of 70 s, which also includes variothermal heating and cooling over the pressing tool and in the area around the joint. The advantage of this concept is that it saves 70 % of the process cycle time and one additional processing step while halving the raw material costs.
Figure 7

Demonstrator roof bow: the connecting plates at the side are connected to the glass-fiber reinforced plastic brace during extrusion (© Fraunhofer LBF)

Integration into Existing Processes

A key feature of the technology is that the structured inlay parts can easily be integrated into conventional and existing production processes. The range of available structuring methods opens the way for choosing between the technical benefit of stiffness characteristics or the economic benefit of expedited processing.

The investigation of internal pressure resistance revealed that the hybrid connection provides static burst pressure of up to 45 bar, and an internal pressure load capacity of 30 bar for a cyclic load of 106 load changes. This result makes the technology ideal for use in applications exposed to internal pressures. It is also suited for use with environmental influences like moisture, or to keep dirt away from sensitive, encased components.

Savings included 70 % on the process cycle time, one additional process step and half the raw material costs.

The strength of the hybrid connection is significantly influenced by temperature gradients, the homogeneity of heat and joining pressure input along the boundary surface. The temperature gradient is lowest when using thermal welding by laser. Here, however, there is still a need to technically optimize the process with regard to heat and pressure input along the boundary surface. If this technology is to be used in an injection molding or pressing process, it is advisable to arrange for variothermal and close- contoured tool heating and preheating of the inlays to minimize temperature gradients and internal stress.

Copyright information

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2018

Authors and Affiliations

  • Dominik Spancken
    • 1
  • Kira van der Straeten
    • 2
  • Jonas Beck
    • 3
  • Norbert Stötzner
    • 4
  1. 1.Experimental Operational Stability AssessmentFraunhofer Institute for Structural Durability and System Reliability LBFDarmstadtGermany
  2. 2.Plastics Processing teamFraunhofer Institute for Laser TechnologyAachenGermany
  3. 3.Research & DevelopmentWerkzeugbau Siegfried Hofmann GmbHLichtenfelsGermany
  4. 4.Weber Fibertech GmbHMarkdorfGermany

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