Laser Structuring of Metal Surfaces for Hybrid Joints
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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.
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.
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.
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.
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.
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.
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.
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.