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Hot stamping is an advanced manufacturing process in automobile manufacturing. Different from the traditional forming process in room temperature, the strain hardening, temperature, strain rate and microstructure of the material have great influence on the mechanics and failure behavior of the high strength steel. The traditional theoretical forming limit prediction method based on room temperature is not effective enough to predict the formability of parts under hot stamping condition. A forming limit prediction model for the high temperature forming process of 22MnB5 is obtained based on Norton-Hoff constitutive model, BBC 2005 yield criterion, and M-K model. The accuracy of the prediction model is verified by comparing with the experimental data in the literature. This model can be used as the forming limit prediction criterion for high strength steel in hot stamping process. The characteristics of forming limit curves under different conditions are obtained and the effect of yield criterion, temperature, and strain rate are summarized. [ABSTRACT FROM AUTHOR]

In micro-scaled plastic deformation, the deformation capability of sheet metals is quite different from those in macro-scale. Compared with the conventional sheets, the ratio of free surface roughness to the thickness of thin sheet metal is more obvious. The so-called free surface roughening phenomenon and size effect may affect the micro-scale forming limit curve (μ-FLC) of sheet metal. To explore how the interaction of free surface roughening and size effect affects the μ-FLC of thin sheet metal, the original M-K model was modified by introducing free surface roughening. The continuous geometrical imperfection calculated by the original M-K model was compared with the experimental surface irregularity. In addition, the effect of grain size on the μ-FLC was also analyzed. Through the comparison between the predicted μ-FLCs and the experimental ones, the modified M-K model is proven to be able to model the decrease of μ-FLCs caused by the geometry and grain size effects, which reveals that the modified M-K model is more suitable for the prediction of μ-FLCs by considering the combination of size effect and free surface roughening. [ABSTRACT FROM AUTHOR]

Predicting and controlling the forming defects in transitional region are two key problems needed to be solved in the isothermal local loading forming of Ti-alloy rib-web component. To this end, the authors have conducted systematic works, which are reviewed here. First, according to the structural features of large-scale rib-web component, the finite element (FE) model of transitional region was established and validated by physical experiment. Then, the material flow and deformation inhomogeneity of transitional region are quantitatively studied by the FE model. In addition, the formation mechanisms of forming defects were revealed. Subsequently, an adaptive folding index was proposed to measure the severity of folding defect, based on which a quick prediction model of the folding defect was developed. Finally, the forming limit considering the forming defects in transitional region was determined by the stepwise searching method combined with the defect prediction model. [ABSTRACT FROM AUTHOR]

To improve both forming limit and forming accuracy in friction stir incremental forming, multistage forming was carried out. JIS: A5052-H34 aluminum alloy sheets with a size of 100 mm × 100 mm × 0.5 mm were employed for specimens. The sheets were formed into a frustum of conical shape and forming limit was evaluated by the formable minimum wall angle without fracture. The formable minimum wall angle by single- and two-stage friction stir incremental forming was 70 ° and 72 °, respectively. When the wall angle was set to 70 ° and the sheets were formed by single- and two-stage forming, the formed shape by two-stage forming was closer to the objective shape than that by single-stage forming. The deformation of the sheet was shearing in single-stage forming, but that was two axis stretching in two-stage forming [ABSTRACT FROM AUTHOR]

The material characterisation in sheet metal forming is significantly improved with the introduction of optical measurement systems. The optical measurement is based on the digital image correlation (DIC) technique, which provides accurate strain measurement of the local strain distribution on specimens. The changes of the stochastic distribution due to the forming are therefore used in order to measure displacements and deformations. An acknowledged application of the DIC technique is the evaluation of the forming limit curve (FLC) for sheet metals. Typically, the FLC is evaluated by conducting Nakajima tests. The Nakajima test is a stretch forming-based test and allows the analysis of stretching at different stress conditions. A previous investigation based on pattern recognition on Nakajima tests has shown that for a conventional deep drawing steel DC04 the pattern evolution on the strain distribution during the test can be examined in order to predict the material failure. However, even if the investigation shows promising results, the correspondence between pattern on the surface and changes in the material structure is missing. The aim of the present work is an analysis of the surface structure of Nakajima tests at different stress states and drawing depths. The correlation between surface modifications and failure behaviour is conducted on the deep drawing steel DC04. The scanning electron microscope (SEM) (Merlin Zeiss GmbH) is employed in order to investigate the evolution of the surface at different strain conditions. The results of the surface and the metallographic investigation are finally compared with the forming limit prediction of the FLC and the failure behaviour at different stress conditions is discussed. [ABSTRACT FROM AUTHOR]

In the household appliance industry piecework coating of body components has been successively substituted by usage of coil-coated sheets. For components with low strains or components not visible for the customer and therefore with no requirements concerning the visual impression these organic-coated materials bear a proper possibility for cost savings. But besides that, for high quality household appliances there are high visual requirements on the front panels. Already the gloss reduction of the coating layer due to higher strains restricts the application for front panels in most cases. From an economic point of view the application of coil-coated sheets even for these components is highly demanded. For an acceptable substitution the resulting paintwork properties after the forming process should be able to forecast within numerical simulations. Therefore the forming-induced gloss reduction of the respective coated material has to be determined in a proper manner. This paper deals with analyses concerning the forming-induced and strain-state-depending gloss reduction of a coil-coated material commercially used for white goods components. The investigations include measurements of strains and gloss by using an in situ approach. For that purpose, material specimens are formed using a modified experimental setup referring to the Marciniak -Test (ISO 12004-2). A fundamental advantage of the in situ measurements is a minor demand of experiments together with a high resolution of the measured variables. In conclusion, based on the experimental results a conceivable forming limit criterion is defined and integrated in an exemplarily simulation model. The front panel of a front-loading washing machine is used as a practice-oriented application example. Using of the pursued approach improper gloss reductions on the formed panel can be predicted within forming simulation. [ABSTRACT FROM AUTHOR]

Synchronous multi pass spinning is a metal spinning method that can form asymmetric shapes with a noncircular bottom and vertical walls from a metal sheet. In this method, the tool trajectory is calculated by linear interpolation between the mandrel shape and the blank disk shape along a pass set. Here, the pass set corresponds to the tool trajectory in conventional spinning. The aim of this study is to examine the effect of a pass set on the formability of a circular cup shape and a rectangular box shape. We have experimentally examined the formability using various pass sets made by simple rule. The angle growth of the rotational pass set, the incremental movement of the translational pass set, and the nominal product height are varied for comparison in the experiments. [ABSTRACT FROM AUTHOR]

In order to improve the mold filling in molding of aluminum foams, a molding process by using a rotating mold is proposed. Aluminum powder mixed with a foaming agent is formed to a precursor by hot powder extrusion and extruded into the mold through the die heated to a temperature higher than the melting point, and then the mold is filled with the aluminum foam. When the diameter of a cylindrical mold is large, the influence of gravity is significant and the mold is not filled successfully. In the proposed process, the aluminum foam is extruded into the rotating mold of large diameter and the shape of foam is improved. However, the effect of the mold rotation on the mold filling is limited. [ABSTRACT FROM AUTHOR]

An incremental sheet forming system with dynamic local heating by laser irradiation was developed to form magnesium alloy sheet which is typical lightweight hard-to-form sheet metal. In this process, tool contact area of a blank sheet is locally heated by a moving laser beam spot to increase the material ductility and decrease the material strength dynamically and partially. Incremental forming experiments of AZ31 magnesium alloy sheet was carried out under several laser irradiation conditions and forming speeds using the developed forming system. The experimental results show that the formability of AZ31 sheets increased remarkably with increasing laser power. Defocusing distance of laser beam and forming speed also affected the formability. The results also indicate that residual stress of the formed product was markedly reduced to almost zero by local heating. [ABSTRACT FROM AUTHOR]