Local Formability Research Articles

Edge crack sensitivity of lightweight materials under different load conditions

This study addresses the analysis of edge crack sensitivity of DP800 steel and AA5182 aluminum alloy in dependency of punching and machining operation as well as load case of subsequent forming. The inserting of a round hole by punching with defined punch-to- die-clearance, milling and drilling is compared. Subsequent forming is performed by standardized hole expansion test and by Nakajima-tests with three different specimen geometries. Local strain distribution at the surface for Nakajima-tests is measured by optical strain measurement technique and investigated in order to evaluate local deformation before failure. Additionally, resulting hole expansion ratio λ is determined. Significant higher X as well as local strain values εmax are achieved by machined holes. This is directly coupled to higher local formability and stretchability for both materials. Furthermore, the load condition has a strong impact on the edge crack sensitivity of the material. Prior failure is observed with changing stress conditions using different specimen geometries also influencing the reachable maximum failure strain. Higher edge crack sensitivity is observed for DP800, which is in good accordance to the material properties in terms of ductility and strength. These data in dependency of the process parameter can be used for the design of automotive components.

Investigation on Shearing and Local Formability of Hot-Rolled High-Strength Plates

In order to evaluate the shearing quality, the material inhomogeneity through thickness after shearing is introduced by the authors. This study investigates the shearing and local formability of hot-rolled high-strength steel (HSS) plate, which is generally exploited for the manufacturing of the beam of heavy trucks. Various kinds of plates with different thicknesses and strengths are used to figure out the effect of material properties on the shearing quality. Both the shear surface morphology and microhardness distribution of the sheared edge are considered for evaluating the influence of the sheared-edge quality on local formability during the following forming process. Vickers hardness tests are conducted to analyze the microhardness distribution on the shear surface, which is proved to have significant effect on the local formability of the sheared edge. Furthermore, two kinds of bending tests and simulation are employed to study the edge cracking phenomenon, and the results indicate that the junctional zone of burnished zone and fracture zone, which is defined as peak hardness zone (PHZ), has a significant impact on major strain distribution on shear surface in the side bending test and this region is the main cause of edge cracking in normal bending test.

The effects of property differences in multiphase sheet steels on local formability

As the automotive industry continues to push towards higher strength materials, lower weight, and reduced costs, high strength ferrous alloys have provided a viable solution. However, as strength levels increase, formability concerns, specifically in regards to local formability at areas of significant geometrical change, have arisen. One of the contributing factors to reduced formability limits has been identified as localized variations in material microstructure. In contrast to single phase steels used in the past, many of the current alloys currently being pursued utilize complex, multiphase microstructures. This presents new and interesting challenges for the current steel producers and consumers. This paper investigates the influences of systematic differences in phase properties on local formability by use of unique testing techniques including nanoindentation to determine phase hardness, and a bending under tension test to determine formability limits. It was found that increased local formability limits were observed with decreased hardness ratio between the hardest and the softest phase in the microstructure.

Edge cracking mechanism in two dual-phase advanced high strength steels

Abstract One of the manufacturing issues restricting the applications of advanced high-strength steels is edge cracking during forming. Computer simulations using the conventional forming limit curve (FLC) as a failure criterion often fail to predict edge cracking. A new failure criterion is needed to assess the edge stretchability in simulations and applications, subsequently understanding the fracture mechanism in the microstructural scale is a prerequisite. In this study, the edge fracture mechanisms of two selected dual-phase steels (designated as DP980 and IBF980) with identical chemistry but different edge cracking behaviors are studied by controlled edge tension tests and scanning electron microscopy (SEM). The results show that the edge cracking behaviors are essentially fractures propagated from pre-existing microcracks introduced by the shearing process. The dominant edge fracture mechanism is decohesion between the martensite and ferrite phases. This study employs the interfacial strength between ferrite and martensite as an index to predict the material edge cracking resistance. The interfacial strength is calculated to be 1070 MPa for IBF980 and 854 MPa for DP980 using void-nucleation models. This result indicates that IBF980 has a higher resistance to edge cracking and subsequently a higher local formability, which is consistent with the higher hole expansion ratio values observed. Refining the martensite particle size and minimizing banded martensite structures are effective approaches to increase the local formability.

Development of an Empirical Model to Characterize Fracture Behavior During Forming of Advanced High Strength Steels Under Bending Dominated Conditions

Higher strength advanced high-strength steels (AHSS) such as DP780 and DP980 are more susceptible to fractures at bend radii during press stampings in comparison with more ductile low carbon sheet steels used by the automotive industry. Most research work to develop predictive guidelines for preventing failures at bend radii have centered on determining critical R/t ratios to avoid failures caused by bending. In this paper, results from bending tests with and without applied tension conducted on a number of AHSS steel lots to generate different conditions for fracture are presented. For bending tests with applied tension, measures of overall formability as a function of R/t ratio of the punch are presented. Consistent with other studies reported in literature, the overall formability was found to increase with increasing R/t ratio reaching saturation for higher R/t ratios. In addition, local formability was determined for all the bending tests by measuring the thickness strains at failure using an optical microscope. It was observed that the thickness strain at failure was dependent on the R/t ratio and the loading mode. Examination of fracture surfaces from the different tests using an SEM reveals that fracture initiation occurs primarily at the ferrite/martensite interphase boundary. To analyze the local loading conditions leading to fracture, 2D finite element analyses (FEA) of the different bending tests using ABAQUS standard were conducted. Results of the FEA were analyzed, and a parameter describing bending dominance in a stamping process was isolated. An empirical fracture criterion relating the thickness strain at fracture as a function of this parameter was developed. Implications of the generated results and their applications for part design and evaluation of stamping feasibility are also discussed.

Influence of Manufacturing Processes and Microstructures on the Performance and Manufacturability of Advanced High Strength Steels

Advanced high strength steels (AHSS) are performance-based steel grades and their global material properties can be achieved with various steel chemistries and manufacturing processes, leading to various microstructures. In this paper, we investigate the influence of the manufacturing process and the resulting microstructure difference on the overall mechanical properties, as well as the local formability behaviors of AHSS. For this purpose, we first examined the basic material properties and the transformation kinetics of three different commercial transformation induced plasticity (TRIP) 800 steels under different testing temperatures. The experimental results show that the mechanical and microstructural properties of the TRIP 800 steels significantly depend on the thermomechanical processing parameters employed in making these steels. Next, we examined the local formability of two commercial dual phase (DP) 980 steels which exhibit noticeably different formability during the stamping process. Microstructure-based finite element analyses are carried out to simulate the localized deformation process with the two DP 980 microstructures, and the results suggest that the possible reason for the difference in formability lies in the morphology of the hard martensite phase in the DP microstructure. The results of this study suggest that a set of updated material acceptance and screening criteria is needed to better quantify and ensure the manufacturability of AHSS.

Enhancing dual phase steel formability by diode laser heat treatment

This paper studies the effect of different typologies of surface treatment by diode laser on the local formability of dual phase steel. An analytical thermal model allowed the temperature and cooling rate curves to be predicted and used to select the process parameter conditions. Microhardness measurements, microstructure observations, mechanical tensile test, and Erichsen cup test allowed the positive effect of the heat treatment on dual phase steel formability to be quantified.