Induction Heat Treatment of Sheet‐Bulk Metal‐Formed Parts Assisted by Water–Air Spray Cooling

In order to produce components with massive secondary functional elements from sheet metal bulk forming operations, termed sheet‐bulk metal forming, can be applied. Owing to high, three‐dimensional stress and strain states present during sheet‐bulk metal forming, ductile damage occurs in the form of micro‐voids. Depending on the material flow properties, tensile residual stresses can also be present in the components' formed functional elements. During service, the components are subjected to cyclic loading via these functional elements, and tensile residual stresses exert an unfavorable influence on crack initiation and crack growth, and therefore on the fatigue life. Following the forming process, temperature and microstructurally related compressive residual stresses can be induced by local heat treating of the surface. These residual stresses can counteract potential crack initiation on the surface or in the subsurface regions. In the present study, the adjustability of the residual stress state is investigated using a workpiece manufactured by orbital cold‐forming, which possesses an accumulation of material in its edge region. Based on residual stress measurements in the workpiece's edge region using x‐ray diffractometry, it is possible to verify the compressive residual stresses adjusted by varying the cooling conditions.     

Identification of Rheological Parameters on the Basis of Various Types of Compression and Tension Tests

The objective of the project was to determine flow stress on the basis of various plastometric tests. The experiments used uniaxial compression, ring compression, and plane strain compression for two sizes of samples and tensile tests. The material was carbon‐manganese steel, and all the tests were performed at three temperatures (900, 1000, and 1100°C) and at three strain rates for each temperature (0.1, 1, and 10 s^?1). Inverse analysis was applied to the interpretation of the results of all compression tests. The flow stresses obtained from various compression tests were compared resulting in the following observations: consistent results between the tests were observed for low values of the Zener‐Hollomon parameter, but some discrepancies appeared for larger values of this parameter. The sensitivity of the results of inverse analysis with respect to the friction factor was investigated next, and it was concluded that the flow stress determined from ring compression showed the largest sensitivity to friction. This sensitivity was lower for uniaxial compression and plane strain compression of small samples, and no sensitivity was observed for plane strain compression of large samples. Finally, the simulations of the tensile tests were performed using the rheological models determined in compression, and reasonably good results were obtained.     

Investigation of Hot Deformation Behavior of Duplex Stainless Steel Grade 2507

Recently, duplex stainless steels (DSSs) are being increasingly employed in chemical, petro-chemical, nuclear, and energy industries due to the excellent combination of high strength and corrosion resistance. Better understanding of deformation behavior and microstructure evolution of the material under hot working process is significant for achieving desired mechanical properties. In this work, plastic flow curves and microstructure development of the DSS grade 2507 were investigated. Cylindrical specimens were subjected to hot compression tests for different elevated temperatures and strain rates by a deformation dilatometer. It was found that stress–strain responses of the examined steel strongly depended on the forming rate and temperature. The flow stresses increased with higher strain rates and lower temperatures. Subsequently, predictions of the obtained stress–strain curves were done according to the Zener–Hollomon equation. Determination of material parameters for the constitutive model was presented. It was shown that the calculated flow curves agreed well with the experimental results. Additionally, metallographic examinations of hot compressed samples were performed by optical microscope using color tint etching. Area based phase fractions of the existing phases were determined for each forming condition. Hardness of the specimens was measured and discussed with the resulted microstructures. The proposed flow stress model can be used to design and optimize manufacturing process at elevated temperatures for the DSS.     

Improvement of accuracy in determining flow stress in hot upsetting tests

Knowledge of correct flow stress curves of steel at high temperatures is of essential importance for reliable plasto-mechanical simulations in materials processing and for an effective planning and designing of industrial hot forming schedules. Error considerations of flow curve determinations allow an estimation of the quality of flow curves. The experiments are performed on a computer controlled servo-hydraulic test maschine. Flow stress errors are calculated as a function of geometry deviations (due to thermal expansion, scaling, predeformation and inaccuracies in specimen geometry), temperature and strain rate deviations. The influence of friction and initial microstructure on flow curves have to be considered. Metal flow can be improved by using a suitable specimen geometry and lubricants. Thermal treatment before testing has to provide a microstructure, similar to the real structure of the material. Different initial microstructures (from varying thermal treatments) influence the overall magnitude of the flow stress curve. Using the flow stress data in a rolling simulation programme the effects on forming loads and torques are analysed.     

Experimental Methodology for Obtaining the Flow Curve of Sheet Materials in a Wide Range of Strains

An experimental approach for determining the stress–strain curve over a large range of strains through tensile test is introduced. The novel aspect of the proposed approach is to apply different degrees of cold working on the sheet metal specimens before tensile test. By adding the pre‐strain derived from cold working to the strain from tensile test, the corresponding stress–strain curves can be shifted to large strains. Since the variation trend of these curves is in high coincidence with the results from compression test, it is convinced that the stress–strain curve of tensile test over a large range of strains can be experimentally extrapolated based on such an approach, especially aiming at medium‐thick sheet metal. Based on the tensile test results, five different extrapolation models were evaluated with respect to different sampling dataset. It was revealed that the true stress–plastic strain curve over a given strain range could be approximated well by the extrapolation models of Ludwik, Ghosh, and Hocket‐Sherby based on sampling data points of standard tensile test combined with a prescribed data point from tensile test after cold working.     

Yield Locus of a Cold Rolled and Solution Treated Nimonic-263 Alloy Sheet,Determined Using Asymmetrical Knoop Micro Hardness Indenter

The Von Mises or perhaps the Tresca criterion is adequate for predicting the onset of yielding under combined stress loading for isotropic materials. However, this prediction is not so straight forward for anisotropic materials in which the magnitude of tensile and compressive yield stresses are different along the different orientations. Such directionality in yield behavior of anisotropic materials may strongly depend on the nature and degree of crystallographic texture. Texture driven yield surfaces are the representatives of the states of plane stress (tension–tension, tension–compression and compression–compression) in thin wall tube by externally applied forces and internal pressure. The applications of yield surfaces are far too wide and particularly have direct relevance to the metal forming, such as cold rolling. A simple procedure based on Knoop microhardness measurements has been proposed in the literature for determining the plane stress yield surface of the sheet materials. In the present study, an attempt has been made to describe this technique and the procedure to determine the yield locus using Knoop microhardness indenter and compare the yield surface anisotropy determined with the experimentally observed tensile properties in the in-plane directions and thus establish the usefulness of this technique in case of a cold rolled and solution treated Nimonic-263 alloy sheet of 1.0 mm thickness in solution treated condition.     

A New Shear Test for Sheet Metal Characterization

In order to predict plastic material behavior for sheet forming processes by finite element simulation, shear tests are useful to identify material parameters. Since the existing shear test setups have certain disadvantages, a new twin bridge torsion shear test is proposed. Stress and strain calculation is derived from the presented geometrical features. The clamping situation and the shear gauge dimensions are investigated to evaluate the quality of the obtained flow curves. It is shown that this test specimen is suitable to determine anisotropic yield behavior and to characterize prestrained specimen, for instance due to cold rolling, when the yield locus is shifted by a backstress tensor.     

采用铁钴镍合金夹层的316L/EH40复合板界面特征

 为了提高带夹层不锈钢复合板层间真空度,提出采用熔融态金属制备夹层的方法。基于真空吸铸成型原理,选取等原子比配比的铁、钴、镍合金作为夹层材料,加热至熔融状态,在真空压力差作用下向复合坯料充型夹层。采用数值模拟方法确定充型工艺参数,对充型样件进行热轧成形试验,通过光学显微镜、扫描电子显微镜等仪器表征分析了复合板的界面形貌特征。结果表明,夹层填充完整且充型率达到100%,拉伸强度和拉剪强度分别为490和319 MPa,铁钴镍合金与不锈钢和碳钢结合界面平直,复合状态良好且洁净无氧化物,热轧后夹层厚度大于碳和铬元素的扩散距离,能够避免铬的碳化物生成。     

Simulation of the forming of mash seam welded sheets

The paper presents results on the optimization of the process of mash seam welding for improving the formability of the joined sheet metal semi-products. An integrated concept including the welding, smoothing and heat treatment of mash welded sheets has been developed. As an example, fatigue tests and the simulation of the forming behaviour of a mash seam welded tensile specimen simulated by the finite element method (FEM) is discussed. The determination of the material properties of the seam which, so far, have been determined experimentally is thus considerably simplified.     

Mechanical Properties,Microstructural Evolution,and the Effect of Friction on the Plastic Flow of the AISI 321 Austenitic Stainless Steel Tube During Cold Pilgering: An Experimental and Simulation Analysis

The mechanical properties, microstructural evolution, and the effect of friction on the plastic flow of the AISI 321 austenitic stainless steel (ASS) tube were investigated during the cold pilger process. The elastic–plastic behavior of as-received tube was simulated by the Johnson-cook model. The model parameters were obtained by the compression and tensile tests. The mechanical properties of the material were examined by the tensile and microhardness tests. Based on the ring compression test results, three different situations for friction conditions were considered. The stress and strain states at these conditions were examined for the outer and inner surfaces of the tube. Numerical evaluation of plastic shearing due to the cold pilgering of the AISI321 tube was performed. The X-ray diffraction, ferrite scope tests, and optical microscope were also used for the microstructure evaluation and verification of the simulation results, respectively. The Latham-Cockcraft damage was calculated for different conditions by introducing the new subroutine, showing that it was strongly dependent on the friction conditions and the shear strain, ε_zr, respectively. It was shown that the friction coefficient of 0.3 resulted in the minimum damage of the tube and the different friction conditions between the two surfaces of the tube enhanced the damage function. Also, the strain induced ?-martensite increased the work-hardening capacity and affected the ductility of the tube.     

金属薄板面内压剪变形的损伤断裂行为

相变诱导塑性钢（TRansformation induced plasticity, TRIP）作为常用的先进高强钢在汽车等交通工具的轻量化方面有广泛的应用前景。而对于其复杂零件的成形过程,韧性断裂是不可忽视的问题之一。本文针对现有实验装置不易诱发薄板承受面内压剪时断裂失效,从而无法研究板料负应力三轴度区间断裂行为的问题,以高强钢TRIP800薄板为研究对象,设计了可在单向试验机完成压剪实验的试样和夹具。通过调整夹具旋转角度和试样装夹位置可以实现同一种试样在广泛的负应力三轴度范围内进行压剪断裂分析。基于ABAQUS/Explicit平台建立了三个典型加载方向20°、30°和45°对应的压剪过程有限元模型,分析表明:三种情况的试样局部变形区域的应力三轴度都小于0且断裂点的应力三轴度低至?0.485,验证了设计的装置可实现负应力三轴度区间的断裂失效分析,同时基于MMC断裂准则分析了不同应力状态的初始损伤情况及损伤扩展路径。      

热轧中厚板超快速冷却过程温度场数值模拟

 超快速冷却工艺作为热轧钢板生产的核心技术,对改善板材产品组织形态、提升产品性能具有重要意义。在中厚钢板的超快速冷却过程中,心部与表面之间的冷却速度差异使得钢板在厚度方向上形成内外温度差,而超快速冷却中钢板表面的换热机制较为复杂,两者综合提升了中厚板冷却机制的界定难度。为提升中厚板超快冷模型计算精度,完善其换热体系,建立了中厚钢板轧后超快速冷却过程中等效换热系数反求法的数学模型。该模型依托离散解析法,基于导热微分方程及物体正规阶段的状态特点,将求得的超越方程根转化为等效换热系数,并将此作为超快冷温度场模型的边界条件。在此基础上,构建了超快速冷却温度场仿真模型,验证了20 mm钢板超快速冷却机制下的温度场。结果表明,等效换热系数反求法的数学模型能够适用于中厚钢板的超快冷工艺。     

中厚板减薄过程残余应力的演变行为

 针对中厚板使用过程中的变形行为，研究了中厚板因减薄引起横向残余应力重新分布规律。基于剥层法理论建立了中厚板减薄过程中残余应力分布模型和挠曲变形模型，应用有限元分析法模拟了厚板减薄过程。对比分析了残余应力分布形式和中厚板挠曲变形程度的计算结果和有限元仿真模拟结果，验证了两种分析方法的可行性，并进一步分析了应力分布状态及厚板减薄方式对薄板减薄过程变形的影响。结果表明，两种分析结果都能反映中厚板减薄变形特征，但有限元仿真模拟方法能够随薄板减薄而改变中厚板约束状态，结果更为准确；中厚板内部原始残余应力分布状态及使用过程减薄方式对其减薄过程变形有重要影响，为中厚板的合理生产设计和使用提供理论依据。     

An Enhanced Herschel–Bulkley Model for Thixotropic Flow Behavior of Semisolid Steel Alloys

An enhanced model based on Herschel–Bulkley model has been proposed. Modifications are based on the attempts to overcome the limitations of the previously used models. The proposed model predicts thixotropic time-dependent flow behavior at medium shear rates, while reaching toward Newtonian viscosity at higher shear rates, which is a typical behavior of semisolid metal slurries. Rheology tests on M2 high-speed steel were then conducted using a self-developed high-temperature Searle-type rheometer for three different solid fractions. All of the tests were performed in an electric furnace with an argon-controlled atmosphere. Two series of experiments were performed to evaluate the rheological behavior of the material: steady-state flow stress experiments to determine the equilibrium flow curves, and step-change of shear-rate experiments to determine the time-dependent characteristics of the material. The model parameters were then derived using the experimental results and calibrated by curve fitting of experimental data. The model was then linked to a commercial CFD software, and simulation of the process was conducted to evaluate the model. The results show that the model fits well with the experimental data and is capable of simulating a wide range of shear rates compared with typical Herschel–Bulkley model.     

Rapid Finite Element Analysis of Bulk Metal Forming Process Based on Deformation Theory

 The one-step finite element method (FEM), based on plastic deformation theory, has been widely used to simulate sheet metal forming processes, but its application in bulk metal forming simulation has been seldom investigated, because of the complexity involved. In this paper, a bulk metal forming process is analyzed by using a rapid finite element simulation method based on deformation theory. The material is assumed to be rigid-plastic, strain hardening. The constitutive relationship between stress and total strain is adopted, whereas the incompressible condition is enforced by penalty function. The geometrical non-linearity in large plastic deformation is taken into consideration. Furthermore, the force boundary condition is treated by a simplified equivalent approach, considering the contact history. Based on constraint variational principle, the deformation finite element method is proposed. The one-step forward simulation of axisymmetric upsetting process is performed by this method. The results are compared with those obtained by the traditional incremental FEM to verify the feasibility of the proposed method.     

Comparative Study on Failure Prediction in Warm Forming Processes of Mg Alloy Sheet by the FEM and Ductile Fracture Criteria

An important concern in metal forming is whether the desired deformation can be accomplished without any failure of the material, even at elevated temperatures. This paper describes the utilization of ductile fracture criteria in conjunction with the finite element (FE) method for predicting the onset of fracture in warm metal working processes of magnesium alloy sheets. The uniaxial tensile tests of AZ31 alloy sheets with a thickness of 3 mm and FE simulations were performed to calculate the critical damage values for three kinds of ductile fracture criteria. The critical damage values for each criterion were expressed as the function of strain rate at various temperatures. In order to find out the best criterion for failure prediction, Erichsen cupping tests under isothermal conditions were carried out at various temperatures and punch velocities. Based on the plastic deformation histories obtained from FE analysis of the Erichsen cupping tests and the critical damage value curves, the initiation time and location of fracture were predicted under bi-axial tensile conditions. As a result, Cockcroft–Latham’s criterion showed good agreement with the experiments.     

Influence of Pre-Deformation on Springback Effect of Advanced High Strength Steel

In sheet metal forming process of automotive components,the springback effect is significant,in particular for Advanced High Strength Steels (AHSS),for example the Dual Phase (DP) steel.Most of construction parts of modern vehicles have very complex shapes and therefore multi-step procedures are necessary to form such a part.Steel sheets,which firstly undergo pre-deformation,can show considerable change in mechanical behavior during the forming process.However,at present there are limited sufficient data concerning pre-deformation effect on the springback available.In this work,a study of influences of different pre-strain levels on the springback of steel sheet made of AHSS materials has been carried out.The sheet specimens were firstly pre-stretched on a tensile testing machine and the pre-strain values were calculated based on the engineering strain.Furthermore,the steel sheets prepared parallel,transverse,and 45° to the rolling direction have been investigated.A modified U-shape forming was used to evaluate the degree of springback of the steel sheets under various conditions.In parallel,FE simulation of the U-shape forming was performed.Both isotropic model using stress-strain responses from tensile test of specimens with different directions and anisotropic Hill’s 48 model have been applied.The experimental results are compared with the sheet metal forming FE simulations.The primarily aim is to basically understand the springback mechanism by means of the simple models.And finally,conclusions with regard to the springback modeling will be presented.     

Modelling of flow curves for hot deformation

A new mathematical model for hot deformation flow curves is presented. The developed expression can be used for austenite or ferrite deformation, for monotonically increasing flow curves as well as for those with a maximum or with oscillating shape. By experimental determination of 4 flow curves, the parameters for the flow curve field can be calculated. Experiments have been carried out using aluminium- killed mild steels in hot compression tests with a range of deformation temperature between 700 and 1250°C and of strain rate between 0.1 and 20 s^-1. The accuracy of the new approach and the ability to meet the different flow curve shapes proved to be very good.     

碳及碳锰锅炉钢板在变形过程中的再结晶与时效硬化

通过对一起高压锅炉母材裂纹的实例分析,并针对碳及碳锰钢板的形变冷、热加工过程与钢材组织和性能的关系,从理论和实际两方面讨论了碳锰锅炉钢板在冷、热变形过程中的再结晶与时效硬化。通过对碳锰类钢板的加热、变形和温度的合理控制,使这类钢板的热塑性变形与金属的再结晶相结合,以获得细小的晶粒组织,使钢板具有优异的综合力学性能,从而避免因变形过程中的时效脆性使材料损坏。