A Modified Mroz Model for Springback Prediction

A new anisotropic material hardening model is introduced in this study for springback simulation. It is modified from the Mroz multi-yield surface hardening model and incorporated more realistic Bauschinger effect for cyclic loading and anisotropic yield surfaces for sheet metals. The model is targeted for sheet metal forming simulations where the accurate springback predictions are important, and where materials have more rapid hardening characteristics and ability to sustain higher stresses such as so-called advanced high-strength steels (AHSS). The constitutive integration algorithm is derived and it is numerically implemented in the commercial FEA code via a user-material subroutine. The new model is applied to a U-channel forming test with DP600 steel. Experiments are conducted and springback results are compared with numerical prediction to demonstrate the new model’s effectiveness. This article was presented at Materials Science & Technology 2006, Innovations in Metal Forming symposium held in Cincinnati, OH, October 15–19, 2006.     

Electromagnetic impulse calibration of high strength sheet metal structures

This study focused on using electromagnetically generated impulses to correct dimensional errors resulting from springback on two different geometric features. Modification or correction of a corner feature is considered first. A simple linear actuator is used to correct springback on a simply bent high strength aluminum alloy (AA 5754) and a high strength steel alloy (DP 600). It is shown to be possible to fully correct for springback using a net-shape die with both materials with higher electromagnetic discharge energies being required for the steel. The second geometric feature considered is a sidewall curl ‘defect’ that is the result of bending and unbending U-Channel drawing. A serpentine actuator and net-shape die were shown effective at correcting for this defect in both DP 600 and TRIP 700 high strength steels. A somewhat higher forming energy is required for effective shape correction in the higher strength TRIP 700 alloy. A detailed mechanistic understanding of springback correction remains elusive, but these results are consistent with several other studies in the literature that show this is an effective means to control springback.     

Effect of niobium on the microstructure and mechanical properties of hot rolled microalloyed steels after recrystallization-controlled rolling

Microalloyed steels with increased strength and ductility are of considerable interest for use in the ‘as-hotrolled’ condition. However, there is a lack of information on their microstructural characteristics and mechanical properties. Seven different microalloyed steels with variable Nb and C content were evaluated in this work. First, characterization of the microstructure by optical and scanning and transmission electron microscopy was performed. Different microstructural constituents and grain size distributions were observed, and three different groups of precipitates were identified. For all steels, tensile tests were performed and ductile-to-brittle transition temperatures were determined. Finally, the complex interplay between microstructural features and mechanical properties was analyzed to determine structure-property relations for the steels under evaluation.     

Effect of normal anisotropy on springback

Share of high and advanced high strength steels in automobile is increasing, however, such steels generally have poor formability and high amount of springback. One of the focus areas of research in high strength automotive steel is to increase the normal anisotropy to get better formability. Effect of strength and process parameters on springback has been studied by many researchers but that of anisotropy has not been studied by many. In the present work the effect of anisotropy on springback is predicted using finite element analysis for the benchmark problem of Numisheet-2005 [2005 Numisheet Benchmark 2, Springback prediction of a cross member, Proceedings of the 6th International Conference and Workshop on Numerical Simulation of 3D Sheet Metal Forming Processes, Detroit, USA, August 15–19, 2005]. An analytical model is developed to cross check the trends predicted from the finite element analysis. The effective stress has not been treated as a constant and the radial stress is considered in the present model. Both the models (FE and analytical) predict that higher anisotropy, in general, gives higher springback. Finite element analysis of the problem shows that springback is minimum for an isotropic material.     

高强钢材料性能对汽车零件扭曲回弹的影响

高强度钢板材料冲压性能的波动对冲压成形后零件精度的影响较大,主要表现在成形性能不稳定和回弹波动较大。文章侧重材料性能波动对冲压件扭曲回弹的影响,结合某汽车车身高强钢零件的冲压结果,借助有限元仿真工具,比较实验与仿真结果,在结果基本一致的基础上,研究高强度钢板的屈服强度、应变硬化指数、摩擦系数及板料厚度波动对该零件扭曲回弹影响规律。应用科学实验设计方法,考察了主要因素的影响规律,得到高强钢材料性能参数影响板料扭曲回弹的相关结论,从降低回弹波动及扭曲对制造精度的影响出发,提出了材料性能参数优化选取原则。     

基于胀弯成形的高强度钢板回弹特性实验与分析

高强度钢板是汽车车身结构件广泛应用的材料之一,对比普通碳钢,高强度钢板具有较高的强度、较高的屈强比、较低的延伸率等,使用过程中容易产生开裂、起皱和回弹等缺陷,特别是回弹问题严重影响了零件的尺寸精度。文章以胀弯成形为基础,通过MTS试验机进行成形实验,研究了低合金高强钢B410LA、双相钢DP500在不同冲压参数、模具几何参数等条件下成形后的回弹影响规律,并通过显著性分析得出,胀弯角度对侧壁卷曲的影响不显著,拉延筋凸筋的位置对冲压回弹有较大的影响。     

The role of electrochemical factors in the initial stage of fatigue-corrosion destruction of stainless steels

The corrosion-fatigue destruction of stainless steels of different classes is studied. It is shown that a decrease in their endurance limit in chloride media is determined by their structure, rather than chemical composition and strength. Based on the electrochemical studies of cyclic-deformed steel samples, their additional deformation dissolution during the period of cracks initiation was calculated. An electrochemical criterion of the susceptibility of corrosion-resistant steels to a corrosion fatigue is suggested and its characteristic analytical formula is derived. The criterion is based on the mutual correlation between the metals’ relative deformation-dissolution at the strain equal to the fatigue limit on one hand and the degree of decrease in their endurance under the effect of environment on the other. The occurrence of physical limit of the corrosion fatigue for passive metals is demonstrated. A nondestructive method of determining the corrosion fatigue limit of corrosion-resistant steels, based on the change in the sign of polarizing current is developed, and accelerates the tests by a factor of 10–15. Original Russian Text ? V.I. Pokhmursky, M.S. Khoma, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 3, pp. 268–275.     

The development of alumina-forming austenitic stainless steels for high-temperature structural use

A new family of alumina-forming austenitic stainless steels is under development at Oak Ridge National Laboratory for structural use in aggressive oxidizing environments at 600–900°C. Data obtained to date indicate the potential to achieve superior oxidation resistance compared to conventional Cr₂O₃-forming iron-and nickel-based heat-resistant alloys, with creep strength comparable to state-of-the-art advanced austenitic stainless steels. A preliminary assessment also indicated that the newly developed alloys are amenable to welding. Details of the alloy design approach and composition-microstructure-property relationships are presented. Author’s Note: Part of this research summary is based on a recent review paper (see Reference 1) and findings first reported in References 2–7.     

Springback characterization and behaviors of high-strength Ti–3Al–2.5V tube in cold rotary draw bending

Stress-relieved Ti–3Al–2.5V bent tube in hydraulic bleeding systems improves the overall performance of advanced aircraft and spacecraft due to its unique high specific strength. However, the high ratio of yield strength to Young's modulus may induce significant elastic recovery after unloading. The precision bending of the high strength Ti-tube (HSTT) depends on the understanding of the springback features and mechanisms. Using the plasticity deformation theory, the explicit/implicit 3D-FE and the physical experiments, the springback behaviors of the HSTT under multi-die constrained cold rotary draw bending (RDB) are addressed. The results show that: 1) The elastic recovery of the HSTT should be characterized by the significant angular springback, the radius growth and the sectional springback; Both the angular and radius springback should be compensated, while the sectional one decreases the cross-section flattening; 2) Among multiple parameters, both the material properties (Young's modulus, strength coefficient and anisotropy exponent) and the geometrical dimensions (bending angle and bending radius) dominate the unloading; Both the angular and radius springback values decrease with the smaller bending radii; The angular springback increases linearly with the larger bending angles, while the radius growth fluctuates little with the increasing of the bending angles at the later bending stages; Both the springback values of the HSTT are far larger than the ones of the 5052O Al-alloy tube and the 1Cr18Ni9Ti tube; The maximum variations of the angular and radius springback with changing of the processing parameters are 78% and 62.5% less than the maximum ones under different material properties and geometrical ones, respectively. 3) A two level springback compensation methodology is proposed to achieve the precision bending in terms of both springback angle and radius.     

Modeling of phase transformation behavior in hot-deformed and continuously cooled C-Mn steels

Computer models of phase transformation from austenite to ferrite, austenite to pearlite, and austenite to bainite in hot-deformed carbon-manganese steels during continuous cooling were established on the basis of Cahn’s transformation theory, thermal-dilatometric experiments, and thermodynamic calculations. These models showed good agreement with results measured from pilot hot rolling experiments.     

TRIP钢薄板冲压的回弹研究

冲压过程中回弹模拟的精度依赖于准确的材料模型。对于TRIP钢,传统的有限元因为没有考虑TRIP效应对材料弹塑性性能的影响而对回弹预测的精度很低。该文中首先建立耦合TRIP效应和它对材料内部变量的影响的弹塑性本构模型,该模型能够很好的描述冲压过程中的相变规律和弹性模量随相变的变化情况。该模型的仿真结果与传统仿真方法相比,与实验结果更相符。     

Development of a new model for plane strain bending and springback analysis

A new mathematical model is presented for plane strain bending and springback analysis in sheet metal forming. This model combines effects associated with bending and stretching, considers stress and strain distributions and different thickness variations in the thickness direction, and takes force equilibrium into account. An elastic-plastic material model and Hill’s nonquadratic yield function are incorporated in the model. The model is used to obtain force, bending moment, and springback curvature. A typical two-dimensional draw bending part is divided into five regions along the strip, and the forces and moments acting on each region and the deformation history of each region are examined. Three different methods are applied to the two-dimensional draw bending problems: the first using the new model, the second using the new model but also including a kinematic directional hardening material model to consider the bending and unbending deformation in the wall, and the third using membrane theory plus bending strain. Results from these methods, including those from the recent benchmark program, are compared. University of Michigan, Dept. of Mechanical Engineering and Applied Mechanics, Ann Arbor, Mi 48109, USA.     

Springback evaluation of friction stir welded TWB automotive sheets

Springback behavior of automotive friction stir welded TWB (tailor welded blank) sheets was experimentally investigated and the springback prediction capability of the constitutive law was numerically validated. Four automotive sheets, aluminum alloy 6111-T4, 5083-H18, 5083-O and dual-phase DP590 steel sheets, each having one or two different thicknesses, were considered. To represent mechanical properties, the modified Chaboche type combined isotropic-kinematic hardening law was utilized along with the non-quadratic orthogonal anisotropic yield function, Yld2000-2d, while the anisotropy of the weld zone was ignored for simplicity. For numerical simulations, mechanical properties previously characterized [1] were applied. For validation purposes, three springback tests including the unconstrained cylindrical bending, 2-D draw bending and OSU draw-bend tests were carried out. The numerical method performed reasonably well in analyzing all verification tests and it was confirmed that the springback of TWB as well as of base samples is significantly affected by the ratio of the yield stress with respect to Young’s modulus and thickness.     

Effects of Deformation and Phase Transformation Microstructures on Springback Behavior and Biocompatibility in β-Type Ti-15Mo Alloy

This study examined the mechanical properties, springback behavior from three-point bending loading-unloading tests and biocompatibility from human osteoblast cell adhesion and proliferation experiments in Ti-15Mo alloy with different microstructures. The springback ratio increased after the appearance of deformation microstructures including {332} < 113 > twins and dislocations, due to the increased bending strength and unchanged Young’s modulus. By contrast, the change in springback ratio was dependent on the competing effect of the simultaneous increase in bending strength and Young’s modulus after phase transformation, namely, the isothermal ω-phase formation. Good cell adhesion and proliferation were observed on the alloy surface, and they were not significantly affected by the deformation twins, dislocations and isothermal ω-phase. The diversity of deformation and phase transformation microstructures made it possible to control the springback behavior effectively while keeping the biocompatibility of the alloy as an implant rod used for spinal fixation devices.     

Thermo-mechanical coupled simulation of hot stamping components for process design

At present, hot stamping represents an innovative manufacturing process for forming of high strength steels, implying a sheet at austenite temperature being rapidly cooled down and formed into a die at the same time (quenching). This forming process is used for the manufacturing of automobile structural components with a strength of up to 1,500 MPa, thus enabling extensive cost savings and good crash performance. Better formability at elevated temperatures and lower springback are further advantages of parts formed by hot stamping. The Finite Element Analysis is an essential precondition for a good process design including all process parameters. This paper presents the finite element simulation of a hot stamping process by means of experimentally calculated material data and describes a number of procedures for the simulation of hot stamping, aiming at a notable decrease in computation time. For a faster calculation thermal and mechanical phenomena are decoupled in two simulation programs. The investigations presented in this paper are kindly supported by the German Research Foundation (DFG)—Research Unit FOR 552.     

变压边力下高强度钢板的回弹研究

回弹是影响板料成形精度的缺陷之一,特别是对高强度钢板控制回弹是板料成形中研究的重要课题。通过对高强度钢板的回弹仿真,证明合适的变压边力能够减小成形后的回弹,并通过实验验证了这一结论的正确性。     

Metallurgical stability and the fracture behavior of ferritic stainless steels

The influence of aluminum on the thermal stability of ferritic stainless steels has been investigated using two commercial alloys— Armco Type 18SR and AISI430. Two reaction stages have been detected in these alloys during aging at 475 °; each stage is accompanied by changes in the hardness, yield strength, strain-hardening exponent, and elongation to fracture. The initial stage is attributed to the precipitation of carbide and nitride particles and the second stage to the precipitation of the chromium- rich a’ phase. The 430 alloy exhibits more pronounced changes than 18SR during the first stage due to the higher concentration of interstitials retained in solution after quenching. The effects of the second- stage aging reaction are detected after shorter aging times in the 18SR alloy and are more pronounced than in the 430 alloy, consistent with the influence of aluminum on the coherency strains associated with a’ precipitation. The fracture mechanism in both alloys changes from ductile dimples in the solution- treated and quenched condition to a mix of ductile dimples, intergranular fracture, and transgranular cleavage with increased aging times. Longitudinal cracking at the grain boundaries precedes failure of the aged alloys in tension; it is attributed to the combined effects of void initiation at fine grain boundary precipitates, a’ embrittlement that limits localized plasticity, and the transverse stress components resulting from triaxiality after the onset of necking.     

Springback prediction of TWIP automotive sheets

In an effort to reduce the weight of vehicles, automotive companies are replacing conventional steel parts with light weight alloys and/or with advanced high strength steels (AHSS) such as dual-phase (DP), twinning induced plasticity (TWIP), and transformation induced plasticity (TRIP) steels. The main objective of this work is to experimentally and numerically evaluate the macro-performance of the automotive TWIP sheet in conjunction with springback. In order to characterize the mechanical properties, simple tension and tension-compression tests were performed to determine anisotropic properties, as well as the Bauschinger, transient, and permanent softening behaviors during reverse loading. For numerical simulations, the anisotropic yield function Yld2000-2d was utilized along with the combined isotropic-kinematic hardening law based on the modified Chaboche model. Springback verification was performed for the unconstrained cylindrical bending and 2D draw bending tests.     

高强钢精密冲压件回弹量预测及控制研究

运用Dynaform软件建立高强钢板DP590的U形件有限元模型,进行了恒定压变力、恒定冲压速度条件下的回弹预测,并通过相同条件下的实验验证了回弹预测的准确性。为实现高强钢精密冲压件回弹的智能控制,进行了变冲压速度和变压边力条件下的U形件的拉深试验。其试验结果表明,采用变压力技术比变冲压速度技术更能有效地控制高强钢精密冲压件的回弹。     

Effect of the carbide phase on the tribological properties of high-manganese antiferromagnetic austenitic steels alloyed with vanadium and molybdenum

Effect of special carbides (VC, M ₆C, Mo₂C) on the wear resistance and friction coefficient of austenitic stable (M _s below −196°C) antiferromagnetic (T _N = 40–60°C) steels 80G20F2, 80G20M2, and 80G20F2M2 has been studied. The structure and the effective strength (microhardness H _surf, shear resistance τ) of the surface layer of these steels have been studied using optical and electron microscopy. It has been shown that the presence of coarse particles of primary special carbides in the steels 80G20F2, 80G20M2, and 80G20F2M2 quenched from 1150°C decreases the effective strength and the resistance to adhesive and abrasive wear of these materials. This is caused by the negative effect of carbide particles on the toughness of steels and by a decrease in the carbon content in austenite due to a partial binding of carbon into the above-mentioned carbides. The aging of quenched steels under conditions providing the maximum hardness (650°C for 10 h) exerts a substantial positive effect on the parameters of the effective strength (H _surf, τ) of the surface layer and, correspondingly, on the resistance of steels to various types of wear (abrasive, adhesive, and caused by the boundary friction). The maximum positive effect of aging on the wear resistance is observed upon adhesive wear of the steels under consideration. Upon friction with enhanced sliding velocities (to 4 m/s) under conditions of intense (to 500–600°C) friction-induced heating, the 80G20F2, 80G20M2, and, especially, 80G20F2M2 steels subjected to quenching and aging substantially exceed the 110G13 (Hadfield) steel in their tribological properties. This is due to the presence in these steels of a favorable combination of high effective strength and friction heat resistance of the surface layer, which result from the presence of a large amount of special carbides in these steels and from a high degree of alloying of the matrix of these steels by vanadium and molybdenum. In the process of friction, there are formed nanocrystalline austenitic structures possessing high effective strength and wear resistance on the wear surface of these steels.