Key Engineering Materials Vols. 622-623

Abstract: During the bending of tubes, specific geometric deviations from the desired shape geometry occur. These deformations comprise cross-sectional deformation, wall-thinning of the outer arc and wall-thickening of the inner arc. During the bending of parts with small bend factors, geometric deviations on the inner arc in form of waves and wrinkles may arise as other typical quality criteria. A quantitative evaluation of those deviations has yet to be defined. Currently, only an instruction for measuring the height of the wrinkles at the inner arc according to DIN EN 13480-4 is known. In this standard, only the two highest wrinkles and the valley in between are included. The characteristics of the wrinkle, especially the flank angle and the rise of deformation are disregarded, although they are responsible for the failure of the bent part. Tool damages can also occur. A development of an evaluation factor to assess deformations in the bow area for bent parts is presented in this paper. In addition, it will be possible to quantify geometric deviations in the bow area. By using the newly developed evaluation factor for geometrical deformations in the bow area, the determination of the quality of the bent part should become more reproducible.

Abstract: A hinge bracket is usually produced by bending of a sheet panel or welding of a hollow bar into a sheet panel. The hinge bracket that is made by bending or welding, however, does not have sufficient durability due to the stress concentration on the bended region or low corrosion resistance of the welded region. In order to make hinge bracket with high durability, a bracket is needed to be produced by forming process. In this study, a thickening process of a sheet panel for the hinge bracket is investigated. The maximum thickening limit in one stage was determined for various aspect ratios of specimen. Finally, the optimum multi-stage forming process was designed for hinge bracket forming and the process was verified by numerical experiment.

Abstract: An analytical method suitable for modelling the creep-age forming (CAF) of doubly curved aluminium plates is presented. This new mechanics model combines an efficient numerical integration method with a robust set of CAF constitutive equations that has been experimentally validated with AA7055 at a CAF condition. Corresponding finite element simulations show good agreement with the analytical results. Using the validated analytical model to investigate a three-stage CAF process (loading, creep-ageing, and unloading), through-thickness strain distributions are studied for aluminium plates that have been subjected to different creep-ageing time. A creep activation point (CAP) is revealed and a correlation is found between the two parameters introduced in this work – the normalised location of CAP, zCAP and normalised bend ratio, β. More specifically, a linear relationship is found between β and zCAP, which indicates that the onset of creep strain, and hence the sizes of the pure elastic core and the outer creep region, can be readily predictable within the limits of study. It is now possible to simulate two-dimensional bending CAF processes for aluminium alloys that have non-spherical precipitates.

Abstract: In this study, the authors employ a multi-point die-support structure to hold the upper die for deep drawings in order to adjust the distribution of the blank holding force (BHF) so as to eliminate wrinkles. The developed multi-point support structure has 12 support cells (support units) between the upper die and the outer slide of a double-action press; the cells are metal cylinders working as springs. The support unit has a ball contact at the interface with the upper die, and the interface freely rotates and slides horizontally. The support unit has strain gauges on the side surface, and the bearing load at each unit can be determined, as well as the elastic deformation. The bearing load distribution is observed through a trial blow, and then the support units are manually relocated to better distribute the supporting points to create the appropriate BHF distribution. To demonstrate the efficiency of the suggested structure, the authors perform deep drawing with off-centered setting of a blank to create wrinkles intentionally. They then employ the multi-point die-support system, relocate the support units, and eliminate wrinkles.

Abstract: A new strategy termed selective heating and press hardening, for hot stamping of boron steel parts with tailored properties is proposed in this paper. Feasibility studies were carried out through a specially designed experimental programme. The main aim was to validate the strategy and demonstrate its potential for structural optimisation. In the work, a lab-scale demonstrator part was designed, and relevant manufacturing and property-assessment processes were defined. A heating technique and selective-heating rigs were designed to enable certain microstructural distributions in blanks to be obtained. A hot stamping tool set was designed for forming and quenching the parts. Demonstrator parts of full martensite phase, full initial phase, and differentially graded microstructures have been formed with high dimensional quality. Hardness testing and three point bending tests were conducted to assess the microstructure distribution and load bearing performance of the as-formed parts, respectively. The feasibility of the concept has been validated by the testing results.

Abstract: A hat channel, also called a furring channel, is a channel with a bottom horizontal web and two vertical flanges, as well as an outward lip that is fabricated by roll forming. When the channel is cut off at a specified length, the edge of the product will change due to the release of residual stress, and this change is generally called cut end deformation. The cut end deformation of channel steel was investigated via experiment and three-dimensional finite-element simulation. The effect of initial thickness on the cut end deformation of hat channel steel was studied. For hat channel steel, the deformations at the front end and back end increase when the sheet metal thickness increases. However, the influence of initial thickness on the cut end deformation of hat-shape channel steel is small.

Abstract: For the process modeling, material properties such as anisotropy values at different orientation are very important. The most commonly used method for the determining of the anisotropy values is the tensile test that is performed for samples are prepared at different orientation. Besides the tensile test, the anisotropy parameters of TRIP 800 steel are measured with the hole-expansion test. The effects of the test methods on the yield surfaces are determined for using two different anisotropic yield criteria are Hill-48 and Barlat-89. Results illustrated that a significant difference is observed in the yield surfaces for the two test methods. The material under biaxial deformation is deformed plastically faster than the uniaxial deformation mode.

Abstract: Under stretch-bending conditions where the curvature is relatively large, the plane stress assumption leads to a significant inaccuracy in forming limit prediction. In processes where a contact pressure acts on the material combined with a small punch radius it is observed by FE simulations that a significant thickness stress gradient develops. This gradient influences the materials plastic flow behavior through thickness and hence affects the cross-sectional stability of the sheet. Based on this observation a theoretical study has been performed in order to quantify the effects of different process and material parameters on the formability of the sheet. Furthermore, it is observed numerically and experimentally that during stretch-bending the cross-sectional stability is not lost instantaneously but gradually over time. Therefore, the concept of FLC proves to be insufficient as to judge acceptability of the deformed product. Another approach is proposed where an instability criterion is developed that also accounts for the partial loss of stability of the cross-section. The criterion is implemented in FE environment to be used in Shell type elements. The results are compared with stretch-bending experiments on selected HSS and AHSS.

Abstract: This paper presents experimental and numerical investigations on the effect of ultrasonic vibration on cylindrical cup drawing processes of a cold rolled steel sheet (SPCC). An experimental apparatus to superimpose high frequency oscillation on deep drawing processes was constructed by installing ultrasonic vibration generators consist of piezoelectric transducer and resonator to the die. Conventional and vibration-assisted cylindrical deep drawing tests were carried out for various drawing ratios, and the limiting drawing ratio (LDR) was compared. In order to evaluate the contribution of ultrasonic vibration to the reduction of friction between tools and a material quantitatively, finite element analyses were carried out. Through a series of parametric analyses, friction coefficients which minimize the differences of punch load histories between the experiment and simulation were determined. The results showed that the application of ultrasonic vibration make for improving LDR by reducing the friction between tools and the material, effectively.

Abstract: Hot stamping has gained increasing importance in the last years due to the introduction of High Strength Steels (HSS) to improve the strength-to-mass ratio of stamped components. Despite the advantages in terms of load decrease, springback reduction and increased formability, the elevated temperatures the tools are subjected to may determine severe thermal mechanical cycling, increased oxidation and wear, which influence the tools service life and the quality of the produced parts. In addition, the frictional behaviour is also changing with temperature, thus affecting the performance of the forming operation itself. In this paper a novel experimental apparatus suitable for reciprocating sliding wear tests at elevated temperatures is presented. It consists of a linear sliding guideway connected to an electrical actuator and equipped with a heating plate to heat metal sheets. A solid frame embeds a screw device used to apply normal load. Thermocouples placed both on the plate and on sheet sample are used to control temperature during the test. The machine is also equipped with two load cells to record the normal and the tamgential loads. The 22MnB5 high strength steel was chosen as reference material for the machine testing. The results showed the capability of the new equipment and the good stability of the mechanical and thermal condition during testing.