Advanced Materials Research Vol. 907

Abstract: The Accumulative Roll Bonding (ARB) process enables the manufacturing of high strength sheet metals with outstanding mechanical properties by repeated rolling. However, the significant increase in strength leads to loss in ductility, especially regarding aluminum alloys of the 6000 series. The low formability obviously limits the implementation of these sheet products for formed components in automotive applications. To enhance formability, a local short term heat treatment according to the Tailored Heat Treated Blanks technology is used. For the finite element based design of forming operations accurate information about the plastic behavior of these tailored materials is required. Therefore, different stress - strain paths are considered using the tensile test and the layer compression test. In this context, heat treated and non-heat treated specimens out of ARB processed AA6016 were tested at room temperature. With the experimental results true stress strain curves and yield loci determined from different criteria and represented in a principal stress state were established. Regarding the experimental setup of the ARB process, an upscaling is essential for the production of sufficiently large strips to cut out blanks for the forming of components such as B-pillars. However, this requires the adaptation of the different process steps of the ARB process. In this context, the surface treatment before rolling of such large sheets is investigated, since it is particularly relevant for obtaining a strong bonding between the sheets. Another aspect is the investigation of the rolling process using the finite element analysis. In this regard, a thermal mechanical coupled simulation model of the roll bonding operation will be developed for the evaluation of different material combinations, different process temperatures and varying roller geometries. These investigations will enable the production of lightweight automotive components made of ARB processed high strength aluminum sheet metal with tailored properties.

Abstract: Internally pressurized components in hydraulic systems are subjected to high mechanical stresses. In case of dynamic pressure profiles this may lead to fatigue and hence a limited lifetime. This is particularly the case for fuel injection systems in combustion engines. Components of diesel injection systems in automobiles are popular examples for these demands. They have to withstand pressures of 2,200 bar and higher for at least 250,000 km. The increasing usage of high-strength materials and higher wall thicknesses will lead to a dead end as the weight and the complex manufacturing will tie up costs and resources. Autofrettage is a manufacturing process with high potential for the lightweight design of highly stressed hydraulic components. By considering the same wall thickness and applying optimal parameters, the fatigue strength may be increased by a factor of 3.5. If transferred to lightweight concepts wall thickness reductions as well as cost and resource savings by more than 45 % may be realized. However, from the manufacturing perspective the Autofrettage process poses some challenges. This paper presents results from Finite Element simulations and experiments and discusses the interaction between manufacturing processes with respect to residual stresses and deformations. The scientific findings may be used to tear down barriers in the application of Autofrettage and to optimize process chain layouts. It also serves to make a significant contribution to weight reduction in car manufacturing and other high performance hydraulic applications. Abbreviations: AF : Autofrettage; AFM : Abrasive Flow Machining; ECM : Electro-Chemical Machining; FEA : Finite Element Analysis; K-ratio : outer to inner radius ratio; L = length of the cylinder (mm); pAF : Autofrettage pressure (bar); pWP : working pressure (bar); piY : pressure to initiate yielding at the bore (bar); Ra : roughness average (μm); Rz : average maximum height of the roughness profile (μm); RPM : Revolutions Per Minute (1/min.); ri : inner radius (mm); ro : outer radius (mm); ρ : density (kg/dm3); σVM : von Mises equivalent stress (MPa); σy : yield stress (MPa); σt : tensile stress (MPa); σY : yield strength (MPa); SF : Safety Factor;

Abstract: Conventional strips can be converted into tailored strips by further processing such as rolling and welding. Tailored strips have a thickness or thickness distribution which is designed according to the expected loads. A new approach for the production of tailored strips is the twin roll casting of profiled strips. This technology combines the advantages of direct strip casting and the production of steel strip with an optimized cross section. In this paper the achievable process limits regarding the geometry of tailored strips with varying thickness in the cross section made by strip profile rolling, twin roll casting and welding are discussed and compared. Furthermore, experiments to demonstrate the suitability of twin roll casting to produce tailored strips made of AISI 304 stainless steel are treated. A selected tailored strip geometry of 150 x 1.5 mm2 (width x thickness) with a difference in strip thickness of 33% over the width was cast.

Abstract: At present, guards on woodworking machine tools have to undergo an impact test with a 100 g projectile and an impact speed of 70 m/s to protect the operator against breaking tools. At the moment, only sheet metal housings and plastic elements of polycarbonate as well as particular safety curtain systems meet these requirements regarding the retention capacity of machine enclosures for woodworking centres according to EN 848-3. In context of this work, a machine enclosure made out of lightweight materials was developed as alternative to the conventional sheet metal housings. A solution for safety curtains that passes easy over the workpieces without damaging the edges was also worked out. With regard to rigid guards, many different kinds of material combinations were examined concerning retention capacity and acoustic behaviour. In addition to the experimental tests, a FEM model was developed representing the impact behaviour of the composite based. Finally, a lightweight enclosure prototype was designed and examined with regard to the retention capacity at critical spots and of sound emission.

Abstract: The machining process of carbon fiber reinforced plastics is very complex due to its inhomogeneous material structure and anisotropic properties. Experimental investigation can be very demanding and time consuming because of the different fiber/matrix compositions and the influence of the fiber volume content which needs to be considered. In this work milling of unidirectional CFRP was simulated using two different material models with implicit and explicit description of fiber and matrix. The objective is to improve the knowledge of the physical mechanism of the cutting process and to predict the cutting forces and surface damage. For this purpose machining of unidirectional CFRP with fiber orientations 90°, 0°, +45° and-45° was simulated, verified by experiments. A significant influence of the fiber orientation on all determined variables was found. The cutting mechanism is dominated by matrix crushing for the fiber orientations 90°, 0° and +45°. Surface damage is caused by either matrix cracking and/or failure of fiber/matrix interface and is occurring in orientations 90° and-45° only. Additionally the evolution of a saw-tooth shaped surface could be identified for the-45° orientation.

Abstract: The economic machining of materials used in the automotive and aeronautical industries, such as aluminium silicon alloys is often not possible without the use of superhard tools. CVD diamond coated tools have demonstrated their suitability for these applications in the past, however, the insufficient coating adhesion and thus tool failure remains an issue to date. Within the work presented here, two cemented carbide types were studied as substrates for CVD diamond coatings. Milling and turning tests were undertaken in order to assess the coating adhesion of the diamond tools. Furthermore, residual stress analysis was undertaken with the aim of understanding the impact of the coating and substrate residual stresses on the tool’s process performance.

Abstract: Production technologies often turn out to be a limiting factor for the geometrical freedom in part design. By the use of Additive Layer Manufacturing processes, the existing restrictions can be negotiated due to their generative character. To exploit their whole capabilities, new approaches in part design have to be applied. One of these numerous possibilities is mesoscopic lightweight design, like for example by the use of lattice structures instead of massive material accumulations. Currently, these structures have a periodic build-up, which leads to unfavourable stress states like bending loads in the single strut elements. By an adaption of the course of the structure to the flux of force inside a part, predominantly push and pull forces appear inside the struts, which is very positive for the structures lightweight performance. To prove the capability of this optimization approach, Finite-Element-Analyses have been executed for periodic and for flux of force adapted lattice structures. Thus, the great potential of this optimization method has been shown.

Abstract: Modern lightweight structures containing hybrid materials allow an improvement of the weight-specific properties. However, to exploit the potential as far as possible novel joint concepts are necessary, enabling an economic structure manufacturing. The DFG-researcher group Schwarz-Silber (FOR 1224) at the University of Bremen aims to explore and develop interface structures for advanced FRP-Al compounds. Considering textile, welding and casting techniques novel joint concepts are under development, in five interdisciplinary projects. Within their work the researcher group focuses on three concepts realizing the transition structures: the usage of wires (titanium), foils (titanium) and fibres (glass fibre) as transition elements between CFRP and aluminium. Typical examples for such hybrid structures can be found in products from the aerospace industry (e.g. hull segments), the car industry (e.g. CFRP roof structures), but also in general mechanical engineering (e.g. rotor blade elements). In this paper, the joint configuration based on titanium wires and a laser beam conduction welding process will be presented. As beam source a lamp pumped Nd:YAG laser (HL4006D) was used. First specimens obtained will be discussed with respect to their properties. It will be shown that the novel approach is in principle suitable to produce load-bearing CFRP-aluminium structures. The wire concept represents a parallel arrangement of miniaturized loop connections. It is characterized by joining a CF-Ti-textile to an aluminium sheet. A carbon fibre loop is threaded through a titanium wire loop by textile technologies on one side. On the side opposite to the CF, the titanium wire loops of the CF-Ti-textile are joined to an aluminium component by welding or casting. A double-sided laser beam heat conduction welding process was applied, for both concepts. During processing, the laser beam was travels along the aluminium edge. The titanium-aluminium structure is welded in two steps. During the first step (i.e. the first weld pass) the aluminium and titanium are heated by the defocused laser beam simultaneously on both sides. An aluminium melt pool is formed, supported by the action of gravity and a certain amount of pre-heating of the titanium-wire or the titanium-foils by the laser beam and by heat conduction through the aluminium melt pool. In the second, immediately subsequent step (i.e. the second weld pass), due to a pre-heating of the materials by the first pass and an increased heat transfer between both materials, a complete wetting of the titanium structures in the joining zone is achieved.

Abstract: In forming technology, uncertainty can arouse from fluctuations in demand scenarios on one hand and in properties of semi-finished parts on the other. These technologies are usually characterized by a high productivity in mass production. However, high development efforts and investment costs for processes and machines lead to a rigid product and process spectrum. One approach to encounter these uncertainties is the introduction of flexibility into forming technologies by enlarging the number of degrees of freedom without drastically reducing productivity. The 3D Servo Press fulfils the mentioned requirements by exceeding free ram motion of conventional servo presses by two rotational ram DoFs. The adaptive control system coordinates the machine motion and controls product properties by model-based algorithms. Possibilities of this approach are demonstrated in a free bending process of a heat dissipater, resulting in uniform product quality despite variations in material, sheet thickness and desired geometry.