Materials Science Forum Vols. 825-826

Abstract: Multi-Material Design has been identified to be an important enabler for lightweight structures, especially with regards to the goals for the large-scale implementation of e-mobility concepts. A novel 3D-Hybrid technology has been developed to combine the advantages of metal and fibre-reinforced thermoplastics in one structural part. This leads to significant weight reduction in combination with an increase in functionality. Additionally, the amount of single parts can be reduced; these factors combined make the technology competitive with conventional steel-sheet design. Investigations on basic profiles showed the feasibility of the technology in single stage production processes and proved the superior performance of the structure compared to conventional design. Finally, a B-pillar demonstration structure was produced in a highly automated process and investigated in side-impact related component tests.

Abstract: A significant advantage of continuous-fiber-reinforced thermoplastics (composite sheets) compared with sheet steel or aluminum is that the forming process can be integrated directly into the injection molding process. To do so, the composite sheet is heated by contact or infrared heating to a suitable temperature, then formed with the closing stroke of the injection molding tool, and subsequently back molded. In this way it is possible to achieve a high level of functional integration, because functional elements such as snap hooks and clips can be directly integrated as part of the molded component without the need for any additional process steps after finishing the part.This study investigates the influence of key material properties on the strength of the bond between such a composite sheet and the back-molded thermoplastic component, determined with a peel test.

Abstract: In order to exploit full potential of hybrid materials, it is necessary to develop optimized load-dependent component designs, new manufacturing processes and joining technologies. Structural integrity concerning the interfaces between the single materials of the hybrid component poses a key factor to success. In this case, adhesion often constitutes the limiting factor for the maximum transferable load. In this investigation, a load-oriented innovative concept to increase the structural integrity of hybrid plastic-metal parts was developed. Local mechanical undercuts on the metal surface were created to generate an additional mechanical interlocking effect between the join partners. The aim is to find the best surface structure geometry to enhance mechanical bonding. Therefore, metal samples were structured by a new process and transferred to hybrid specimens by injection molding. For comparison, specimens with adhesive bonding (epoxy resin) of metal and plastic were prepared. The join partners aluminum AlCuMg1-2017 and PA6 as well as PA6GF30 were investigated. The evaluation of an increase in the structural integrity was determined using tensile tests. A significant improvement in joint strength compared with direct joining using adhesive bonding was achieved.

Abstract: Carbon fiber reinforced plastic (CFRP) was integrated with steel fibers in order to improve the toughness and to enhance the structural integrity during crash. An epoxy system with internal mold release was chosen as the matrix system. The surface modification of steel fibers was done by sandblasting and twisting in order to improve the fiber-matrix adhesion through mechanical interlocking mechanism. The pull-out test of surface modified steel fiber doubled the adhesive strength. The steel fiber integration increased the maximum bending stress of the composites up to 20% whereas the elongation at break reduced to 2.3%. The energy dissipation factor of the steel fiber integrated CFRPs was also reduced compared to CFRPs without steel fiber. An increase in fracture toughness was observed for the CFRPs with steel fibers that amounts to 17 J.

Abstract: Continually increasing exhaust emission standards for automobiles and an increasing environmental awareness push design engineers to develop new constructive and material concepts. So-called sandwich panels, consisting of stiff facings and light-weight cores, offer the possibility to combine properties of different materials synergistically. When processing large quantities, as is the case in the automotive industry commonly used manufacturing processes for cutting sandwich panels, like sawing or milling, are not applicable. A common manufacturing process to cut metal sheets in high quantities is shear cutting. However, pre-trials of shear cutting of sandwich panels have shown that it is not possible to achieve flawless cutting surfaces with current process layouts. Characteristic types of failure like high bending of the facings, delamination effects, burr formation and an undefined cracking of the core material were ascertained. Thus, in this study, the influence of cutting parameters, such as the clearance and the punch diameter, on these types of failure is examined. Five different clearances between 0.025 mm and 0.4 mm with two punch diameters, 8 mm and 32 mm, were investigated. In order to compare the influence of different materials, three commercially available sandwich panels were studied. The chosen sandwich panels differ both in the face sheet thickness and the core material. Finally, the shear cutting force is measured to identify a possible correlation between the cutting force and the face bending. As a result, optimal clearances to minimize the face bending are derived. Additionally, the influence of the core stiffness on the cutting force is determined.

Abstract: In this study, aluminum sheet metal reinforced magnesium structures have been manufactured by high pressure die casting (HPDC). Selected interfaces of the hybrid structures were analyzed before and after exposure to corrosive environments. The characterization of the as cast bounding surfaces of aluminum sheets and magnesium cast alloys was carried out to quantify the appearance of crevices, which are significantly influencing the extent of the corrosive attack. Depending on the geometrical design of local bonding areas, the observed interface conditions varied from defect-free form closure to crevice widths beyond 35 μm. A minor percentage of the analyzed segments revealed areas of local metallic continuity, detected as intermetallic phases Al₃Mg₂ and Al₁₂Mg₁₇. In order to evaluate acting corrosion mechanisms, hybrid samples featuring the material combinations EN AW 5083 + AZ91 HP and EN AW 6082 + AM50 HP were subjected to immersion tests using 0.1M NaCl solution at a pH of 7.5. The results showed a strong influence by the spread of the potential difference. Alternating corrosion tests (VDA 621-415) were applied to prove effectiveness of cathodic dip coatings (CDP) and wax sealing on standard profile structures, since the uncoated Al-Mg samples sustained severe corrosion damages immediately.

Abstract: The Development of Fiber Reinforced Plastics (FRP) offers a great opportunity for applications in automobile industry, aeronautics and consumer goods to achieve light weight structures. However, the connection technology between FRP and mainly metallic based structures is the key to use the full potential of the FRP. Out of this motivation recent developments address this aspect.Using the powder metallurgical approach to generate a metal/ FRP connection module by spark plasma sintering a great variety is possible by integration of different metal and/ or fiber components. In this work aluminum and stainless steel was chosen for the upper and lower metallic side. The fibers integrated into the metal were glass, basalt and carbon fiber in one layer, two layer and mixed layer configuration. To connect the sintered module to greater CF weaves an infiltration process with a room temperature curing resin was used in a modified vacuum infusion (MVI) setup. In not optimized configuration the shear test after infiltration indicated an initial value for module shear strength above 20 MPa which can be enhanced in future developments by optimized armor between the upper and lower metal side and the number of integrated fiber layers of the connection module. A model is predicted to calculate the module shear strength in sintered state by multiplication of the armor area with the shear strength of the armor material. First experiments additionally show the possibility to weld the connection module directly to metallic structures.

Abstract: Composite extruded, unidirectionally spring steel wire reinforced profiles have great potential for lightweight applications. Joining of these profiles represents a difficult challenge due to the different material properties, which eliminate fusion welding as a possible joining method. Friction stir welding (FSW) has recently been used to join these profiles, the disadvantage of which is that the reinforcement elements may be bent and fragmented during welding leading to drastically reduced mechanical properties compared to non-joined profiles and even to the unreinforced matrix material. In order to minimize the disturbing influences of the reinforcement elements on the joining strength, the weld axis was shifted to the retreating side, reducing wire-pin contact at the advancing side. As a second remedial measure an insert made of matrix material was placed between the joining partners before joining in order to reduce the contact on both sides. In-situ tensile tests within an X-ray micro computed tomograph (µCT) showed that shifting the weld axis could not improve the joint quality significantly. However, the use of an insert improves the offset yield strength (R_p0.2) by about 50 %.

Abstract: Hybrid components made of metals and fibre reinforced plastics (FRP) need new joining technologies which are suitable for all used materials. At the Technische Universität Chemnitz, Spin-Blind-Riveting (SBR) was developed, a hybrid process which combines blind-riveting, flow drilling and hole moulding. SBR is suitable for FRP, metals and has no need of predrilling. In this paper different material combinations made of aluminium, magnesium and FRP are joint by SBR. Mechanical tests show higher strength than conventional blind-riveting and macro sections show only slight damage to the FRP. It can be said that SBR is a high promising technology to join hybrid components.

Abstract: Hybrid material systems are designed by the specific combination of different materials. As a result, expanded property profiles can be achieved, which would not be possible with monolithic material solutions. For lightweight, high strength and high rigidity, complex shaped structural components, which are used in the automotive industry and in aerospace, hybrid material systems offer an outstanding potential. A comprehensive understanding regarding the interaction of the individual components of the hybrid material is of great importance for a more efficient design of future structures. In this work, existing hybrid solutions for industrial applications and those, which are subject of current research, are analyzed and categorized first. Intrinsic and extrinsic material combinations are considered at different levels, ranging from hybrid laminates on shell level to complex hybrid structures on component level. Based on the situation analysis, different hybrid solutions are evaluated and compared considering the requirements of the automotive industry. Furthermore, the associated physical mechanisms which are responsible for the respective property profile are considered and explained systematically. The long-term objective of the work is to establish a methodology to derive the necessary physical mechanisms and, based on that, to derive optimal hybrid solutions for desired property profiles.