Key Engineering Materials Vol. 716

Abstract: The feasibility of combining spinning, shear forming and flow forming processes has been demonstrated through manufacturing of a representative of a hub component using industrial scale hybrid-forming machine available at the Advanced Forming Research Centre (AFRC). The manufacturing cycle consisted of single to multiple passes of shear forming, spinning and flow forming. The research has proven that the spinning, shear forming and flow forming can be combined using a single machine with a single set of tooling and single process cycle. Circumferential and axial cracking was observed in initial set of trials which were eliminated using a series of experiments. The methodology that was used in these series of trials to remove the cracks/defects that may occur during forming of such component is presented here.

Abstract: The effect of the hole edge shape formed by a punching process including thickening on the fatigue strength of the hole of ultra-high strength steel sheets was investigated. The bending and tensile fatigue tests of the thickened hole edge were performed. For the bending fatigue strength, the high height of hole edge was effective because of high bending stiffness. Although the high tensile stiffness was important for the high tensile fatigue strength, the shape of hole edge was alternative. To increase both the bending and tensile fatigue strengths, a compressive process in rollover of hole edge was added for the thickened sheet having a high bending fatigue strength. It was found that the thickened and compressed hole edge of ultra-high strength steel sheet was effective in improving both the bending and tensile fatigue strengths.

Abstract: The influence of burnishing conditions on the burnishing force in the inclined roller burnishing method, which was developed by the authors, is investigated. The wear behaviors of non-coated and TiN- and Diamond-Like-Carbon-coated rollers and the effect of the coated roller on the inclined roller burnishing were also investigated. A round bar of carbon steel and an aluminum-based alloy were used as the workpiece material. The burnishing force was measured by a strain-gauge type 3-component dynamometer. The burnishing force component, which acts in the circumferential direction of the workpiece, increased with increasing inclination angle of the roller. Improvement of the tool life of the roller was obtained with a TiN-coated roller on which nitriding treatment of the base roller was performed prior to TiN coating. A satisfactory burnished surface was obtained by burnishing with the DLC-coated roller.

Abstract: The economic application of various cutting processes in product development can lead to significant forming problems when changing from prototypes to serial parts. Every cutting process causes characteristic effects on the resulting edge and consequently specific edge cracking sensitivities. For exterior trim components of premium car models various bright finishing alloys are used but were not yet focus of scientific research. Within this study influences of laser beam cutting and machining on the edge cracking sensitivity of the alloys Al99.7Mg0.8Cu and EN AW-5505 are analyzed by executing hole expanding tests according to ISO 16630 enhanced with an offline evaluation method. To assess possible impacts on the resulting hole expansion ratios material and edge characterizations are conducted. Generated knowledge can support reducing forming problems occurring while changing from prototypes to serial parts.

Abstract: Shearing is the cutting of a sheet material with a punch and a die. The cut surface obtained by shearing is composed of rollover, a sheared surface, a fracture surface and burr. It has been reported that rollover is formed by the insufficiency of material and material flow in the lateral direction. However, the rollover in the sheet material on the die and that in the sheet material under the punch have not been evaluated individually in previous studies. In this report, the relationship between the material flow and the formation of rollover in the sheet material on the die is discussed on the basis of experimental results obtained by image processing. The relationship between the material flow and the formation of rollover in the sheet material under the punch also is discussed. Double-sided shearing with a counter punch was carried out. The deformation of the sheet material was observed through reinforced glass using a high-speed CCD camera. The image processing was carried out to investigate the material flow. The results were as follows. When the clearance is small, the material flow in the clearance is toward the sheet material on the die. When the clearance is large, the material flow in the clearance is toward the sheet material under the punch. The area of rollover in the sheet material on the die is equal to the sum of the amount of material flow in the clearance and that in the lateral direction. On the other hand, the area of rollover in the sheet material under the punch is equal to the sum of the amount of material flow in the lateral direction under the punch, that in the clearance and the insufficiency of material. These findings are useful for considering the relationship between the material flow and the formation of rollover in the case that the rollover in the sheet material on the die and that in the sheet material under the punch are evaluated individually.

Abstract: Joining by forming is an attractive assembly method for mass produced parts due to easiness in execution, no heat effect, various material combinations etc. and industrial applications have been increased in electric and automotive industries. In this paper, a disc plate and stepped shaft is assembled by forming. Assembling is achieved by indenting the disc with a stepped and grooved shaft up to a certain depth. Through the experiments using an aluminum disc and a steel shaft, the influence of the overlap length and penetrating depth of the stepped section on joining force, axial strength and dimensional change of the disc plate are investigated. Disassembling tests revealed that the joint by the proposed method had sufficient axial strength if the groove of the shaft was completely filled.

Abstract: In this study, unalloyed titanium (Gr.1) and deoxidized high phosphorus copper plates were joined through explosive welding process. Different stand-off distances were used to investigate their influence on the microstructure and mechanical properties of the clad fabricated at the same amount of explosive charge. Microstructures near-the-interface were examined with the use of scanning electron microscope equipped with energy dispersive X-ray spectrometer and then microhardness measurements were carried out on the clad. Microstructure examinations showed that with increasing stand-off distance the length and amplitude of the waviness and the quantity of melted zones in areas near-the-interface increase. The inclusions of the melted zones are formed behind the wave crests on titanium side or within the wave vortex. Microhardness measurements indicate a significant increase of both plates microhardness, especially near-the-interface.

Abstract: New steel grades for forged components are designed to meet the requirements of the automotive industry in order to obtain excellent strength and toughness behavior as well as a high endurance limit. Beside precipitation hardened ferritic-pearlitic steels, bainitic steels have gained more and more importance. Basic considerations on the alloy design (C-, Si-, Cr-; B-content) are done using JMatPro-Calculations and by some experimental trials. Using the thermomechanical testing system Gleeble 3800, various cooling strategies have been applied and the kinetics of the bainite formation has been measured at different holding temperatures and times. A detailed microstructural characterization has been done with relation to the mechanical properties. The isothermal tests are compared to continuous cooling situations. Finally, forging trials are performed to find out the most suitable and robust production schedule to be used in practice. The actual findings support the increasing use of bainitic steels for forged parts, especially regarding material saving, independence of cross section and good fatigue performance.

Abstract: Common joining methods used in automotive industry are welding, adhesive bonding, friction stir welding, mechanical fastening, self-piercing rivets, mechanical clinching and so on, for multi-material designed automotive bodies. Among different joining methods, mechanical clinching which achieves geometrical interlocking by plastic deformation has several advantages such as no need of additional joining elements and fast joining. But mechanical clinching is difficult to join a ductile material with a high-strength or low ductility material. Therefore the hole clinching as a new mechanical clinching process has been proposed to join these material combinations. In the hole clinching process, as punch force is applied to a upper sheet (a ductile material), it is indented into hole of lower sheet (low ductile material) on die and then interlocked by plastic deformation. It is very important for a successful design of hole clinching to predict the failure mode such as neck fracture and button separation and the strength of hole-clinched joint. For this an analytical approach was carried out for the hole clinching process of Al6061 and DP980. Tool geometry used in hole clinching was designed by the predicted failure mode. Preliminary finite element simulation was performed to validate the geometrical interlocking and joinability. The predicted failure mode and strength were verified by the results of cross tension test.

Abstract: In hot stamping of thin quenchable steel sheet, local thinning, springback and hardness of stamped parts were investigated. The sheets having 0.6, 1.0 and 1.6 mm in thickness were heated at 900 °C by a furnace and hot-stamped into a hat shape. Local thinning around the bottom corner of the hat-shaped part for a thickness of 0.6 mm was considerably remarkable in comparison with those above 1.0 mm in thickness. Local thinning was relieved for a transfer time of 15 s from the furnace because of the uniformity of the temperature distribution. In addition, the springback was prevented above 5s.