Key Engineering Materials Vols. 410-411

Abstract: The need of light weight construction for high efficient vehicles leads to the use of new materials like aluminium and magnesium alloys or high strength and ultra high strength steels. At elevated temperatures the formability of steel increases as the flow stresses decrease. Forming high complex geometries like chassis components or components of the exhaust system of vehicles can be done by hydroforming. The hydroforming process by oils is limited to temperatures of approximately 300 °C and brings disadvantages of possible leakage and fouling. Using granular material like small ceramic beads as medium could be an approach for hydroforming of ultra high strength steels like MS W1200 and CP W800 at temperatures up to 600 °C. The material properties of granular material are in some points similar to solid bodies, in other points similar to liquids. For understanding and simulation of the behaviour of the medium a basic characterisation of ceramic beads with different ball diameters is necessary. Powder mechanics and soil engineering give ideas for experimental setups. For the conversion of these approaches on the one hand the behaviour of the ceramic beads itself has to be characterized, on the other hand the contact between a blank and the beads have to be investigated. For the tests three different kinds of spheres with a diameter between 63 microns and 850 microns are used. In unidirectional compression test compressibility, pressure distribution in compression direction and transversal compression direction and the effect of bead fracture are investigated. The tests are carried out at different compression velocities and for multiple compressions. For determination of friction coefficients between blank and beads and determination of shear stress in bulk under compression a modified Jenike-Shear-Cell for use in universal testing machines with the possibility of hydraulic compression of the beads is built up. The gained data can be used for material modelling in ABAQUS using Mohr-Coulomb or Drucker-Prager model.

Abstract: The hydroforming of sheet metals belongs to deep drawing processes, where the forming capacitiy is performed by a pressure medium. This process is especially suitable for parts with a non-rotationally symmetric geometry. Light-weight structures require particularly weight optimized geometries with a possibility to attach joints or secondary shape elements. Weight and strength optimised structures often use stringer elements to fulfill this requirement. They are used in flat plates or in single curved bodies. The assembling of the sheet stringers is often costly in terms of time and money. Hence, the hydroforming process of those sheet stringers offers a new process chain for the manufacturing of complex, multi-curved hollow parts with pre-attached stiffeners. Thereby, tools and process control strategies are similar to the conventional hydroforming process of blank sheets. This paper describes the hydroforming process of laser welded sheet stringers. It gives a first outline of the potential and the limitations of the new process chain. Blank holder force and the fluid pressure are the main parameters during the hydroforming process.

Abstract: To simplify the recycling of automobiles, aluminium alloy sheets were joined by means of a self-piercing rivet. Although steel rivets used in conventional self-pierce riveting are removed from the aluminium alloy sheets in recycling, the removal is not required for aluminium alloy rivets. The joined sheets with the rivets are directly melted due to the same material, aluminium. For the joining of aluminium alloy sheet by the aluminium rivets, the joinability was improved by the designed shape of the rivet and die. To pierce the upper sheet, the diameter and edge angle of the rivet are modified. The diameter of the depth and the cavity of the die were also designed from trial and error using finite element simulation. The effectiveness of the designed rivet and die were evaluated from an experiment on riveting. The aluminium alloy sheets were joined by the optimised aluminium alloy self-piercing rivet and their effectiveness measured.

Abstract: The laser weldability of austenitic stainless steel (ASS) is good because of the material’s high absorptivity and favourable microstructure. There can be a slight possibility of solidification cracking at high welding speeds and low Crekv/Niekv ratios. Test welds were welded with a Yb:YAG disc laser. The test material was 3.2 mm EN 1.4404 2H C700 type stainless steel plate which was work hardened by cold rolling. The test materials were welded with different heat inputs ranging from 0.024 kJ/mm to 0.12 kJ/mm and with 300 mm and 200 mm focal lengths. The weld seams were square-groove welded as butt weld without filler material. The edges of the groove were made by mechanical or laser cutting. The hardness profiles from cross-sections of the welds were measured with a Vickers microhardness tester using 200 g weight. The mechanical properties were tested with tensile tests. The welds were classified with radiographic verification by an accredited laboratory. A number of the welds were fatigue tested with a bending fatigue tester. The mechanical properties (Rp 0.2%, Rm) of the laser welds were almost the same as in the base material except at the highest heat input. In the radiographic classification, the welds which were welded to the laser-cut edge were classified as class B (accepted). The other welds were classified as class D or C (rejected). The main reasons for the rejection of welds made on mechanically cut edges were lack of penetration or undercut of the weld. A problem with mechanically cut edges, and hence the welds, is that they can be non-square and bent edge. Fatigue tests and tensile tests gave no evidence of solidification cracking in the microstructure of the solidified parts of the welds.

Abstract: A process innovation is proposed by the authors to weld aluminum alloy tubes by means of a high power diode laser. In order to make the temperature profile uniform along the entire welding line, multiple passes of the laser source along the welding path can be performed at very high scan speeds. In the current study, this effect is achieved by focusing the standstill laser spot on the external surface of the aluminum alloy tubes which were put in rotation at high speed. The tubes were clamped together by using a threaded rod passing inside the tubes. Experimental tests were performed by changing the aluminum alloy heat treatment (6060 T1 and 6060 T5), the length of the samples to weld (80 and 100 mm), and the laser power (ranging from 800 to 950 W). The outer diameter of the tube was 10 mm, the thickness was 1 mm, and the rotational speed was kept constant at 1000 rpm. The welding time was monitored during the tests and ranged from 30 to 100 s in dependence on the material and process parameters. Specimens were extracted from the joints to perform tensile tests so as to measure the tensile strength. In the best condition, a high tensile strength was obtained (about 140 MPa).

Abstract: The increased application of lightweight materials, such as aluminium has triggered many investigations into new joining techniques for aluminium alloys. The Resistance Spot Welding concept for aluminium has always attracted many researchers from different organizations. Self-piercing riveting is the major production process used to join aluminium sheet body structures for the automotive industry. Mean while, interest in laser welding and application as a candidate for joining aluminium is also growing. These key technologies for aluminium assembly are therefore being investigated by the research team at the University Of Warwick. The paper reported here looks at the feasibility of each process for joining thin gauge aluminium and compares the mechanical behavior for each joint type. The results suggested that all three joining technologies are feasible for joining aluminum and their mechanical behaviour is strongly dependent on the selection of process parameters

Abstract: Friction stir welding is a solid state joining process which enables most metals and alloys to be welded without fusion temperatures occurring. The centre of the weld is comprised of dynamically recrystallized material which is beneficial for superplastic deformation. The high temperatures involved with conventional fusion welding techniques disrupt the delicate microstructure of superplastic materials. Superplasticity is heavily reliant on a small grained equiaxed matrix structure pinned by a fine distribution of hard second phase particles to inhibit grain growth during forming of the material. During the superplastic forming operation, the heavily strained structure present in the parent material undergoes a transformation from a banded structure comprising of very long, thin grains to fine equiaxed grains through various static and dynamic recrystallization mechanisms. During forming of friction stir welded materials the stability of the weld region has been investigated. Grain growth is more apparent in the strain hardening of AA5083 due to the relatively small amounts of strengthening precipitates. This material statically recrystallizes during the preheat stage of the superplastic forming process, the grains then begin to grow during a dynamic recovery process becoming far too large to allow superplastic deformation. AA2004 is specially designed superplastic forming (SPF) alloy which contains a large amount of Zr for grain stability. This alloying element is preferentially distributed along the grain boundaries which prevents grain growth during SPF. During the forming process the AA2004 dynamically recrystallizes; disruption to the parent material structure causes discontinuous dynamic recrystallization that results in a heterogeneous structure and makes the material prone to abnormal grain growth. The weld regions of FSWs in AA5083 and AA4004 have been shown to exhibit AGG in weld nugget and shoulder influenced regions. The stability of the superplastic material is reliant on their strengthening precipitates. The Zr in the AA2004 is a much more effective precipitate for maintaining stability.

Abstract: The effect of varying certain geometric and material property parameter on the stiffness of bonded joints has been investigated for both single overlap joints and for coach (T peel) joints. An understanding of bonded joint behaviour under load has been gained from this parametric study and this insight has been used to fashion some design guidelines aimed at optimising these parameters to increase joint stiffness. To develop these design guidelines, laboratory tensile testing of single overlap joints and coach joints, manufactured from steel and aluminium and bonded with either an epoxide or a methacrylate adhesive, was utilised to establish joint stiffness. For single overlap joints, joint stiffness was found to be sensitive to changes in adherend thickness, adherend’s Young’s modulus, bondline thickness and overlap length but indifferent to changes in adhesive material properties, unsupported length and joint width. Coach joints, stiffness was found to be sensitive to changes in adherend material, adherend thickness, fill ratio and outer radius and adhesive material. It must be recognised that the Design of Experiments technique, used to analyse the coach joint samples may have masked some of the effects caused by changes to joint parameters.

Abstract: This paper describes a new development in incremental, robot-based sheet metal forming (Roboforming). Roboforming is a dieless sheet metal forming process which ensures cost-effective manufacturing of prototype parts and small batches. Its principle is based on flexible shaping by means of a freely programmable path synchronous movement of two industrial robots driving work-piece independent forming tools. The final shape is produced by the incremental inward motion of the forming tool in depth direction and its movement along the contour in lateral direction on a heli-cal path. The supporting tool, with its simple geometry, holds the sheet on the backside by moving syn¬chronously along the outer contour, at constant depth. In this way no special dies are needed. For mil¬ling machines, which are used in numerous incremental forming approaches, CAD/CAM inter¬faces exist for generating necessary tool paths. For industrial robots only a few simple solutions emerge, which do not have the potential of classical CAD/CAM interfaces and are unusable for co-operating robot systems. While the two coupled robot programs can be programmed manually for simple geometries, this approach does not work for complex geometries. In this paper a further de-velopment in robot programming systems is presented that is now able to derive helical tool paths from any CAD file and generate two cooperating programs for the forming and the sup¬porting tools. The helixes pitch is variable and dependent on the geometry’s wall angle. To increase the part accu¬racy a process database is used, that stores relevant information about the process pa¬rameters, sensor data and used equipment. Based on this information strategies for increasing the part accuracy can be applied.

Abstract: Build-to-order was once the only way in which products were made, but limited the market to only the rich buyers. Mass production contributed to a wider access to products, however with losses in individuality. Finally, mass customization aimed at holding out the promise of both, and “lean” concepts helped to (partly) make it a reality. However, the world has changed significantly since the first introduction of “lean” – especially in the most recent years. European companies are facing a growing international competition in volume markets due to the increasing economical and technical emancipation of low labour cost countries. While multinational enterprises are shifting their manufacturing activities to Far East to keep competitive in terms of costs, small and medium sized companies often have to leave their traditional market segments and retreat into niches. However, smaller production lot sizes and the increasing complexity of product programmes require innovative manufacturing strategies. According to several studies and empirical proves, less than 0,5% of a company’s production lead time is value adding, the bigger part of it is dedicated to waiting, handling and internal transport. This paper presents a new approach for the design of lean manufacturing support systems in make-to-order production systems that have to deal with a huge variety of product types and with high variations in demand. A special focus is given to the design of manual material handling and transport (MMHT) solutions. With the help of axiomatic design principles, a tree of design parameters is derived and translated into generally applicable design rules. With the help of a practical example from make-to-order industry, the validity of the methodology is illustrated.