A constitutive model for spring-back prediction in which the change of Young's modulus with plastic deformation is considered

In order to improve the prediction capability of spring-back in the computational analysis of sheet metal forming processes, a stress–strain constitutive formulation of non-linear combined hardening rule has been proposed in this paper according to non-linear kinematic hardening theory of Lemaitre and Chaboche and Hill's 1948 anisotropic yielding function. Traditionally, Young's modulus is considered as a constant in engineering application and numerical simulation. In fact, it decreases with plastic deformation. So the effect of the change of Young's modulus with plastic strain on spring-back is considered in the constitutive model. The algorithm of stress update is elastic prediction, plastic correcting and radial returning. Numerical results and experimental results show that the proposed constitutive model significantly improves the prediction accuracy of spring-back.     

Microstructure and creep strength of welds in advanced ferritic power plant steels

Abstract

The microstructure and creep strength of simulated heat affected zone (HAZ) specimens and welded joints have been investigated for advanced 9-12%Cr steels in order to understand the mechanisms responsible for Type IV cracks and to improve the creep strength of welded joints at high temperature. The creep and creep rupture tests were carried out at 650° C (923 K) for up to about 10⁴ h. The creep crack growth tests were also carried out for welded joints, base metal and simulated HAZ specimens using the CT specimens. The creep rupture time of simulated HAZ specimens has its minimum after heating to A_C3 temperature, which produces fine-grained martensitic microstructure. Decreasing the width of HAZ by means of electron beam (EB) welding is effective for the extension of creep life but the brittle Type IV fracture appears even in the EB welded joints at low stress and long time conditions. Most of the welded joint specimens were fractured in fine-grained HAZ and resulted in shorter creep life than those of base metals as a result of the formation of creep voids and cracks. It should also be noted that in the fine-grained zone, the recovery of martensitic microstructure during creep is inhomogeneous as shown by the formation of coarse subgrains in the region of fine subgrains. Using a specially designed FEM code for Type IV crack growth behaviour, the vacancy diffusion under multi-axial stress conditions of welded joints in HAZ is analysed. The effect of creep ductility and void formation ahead of the crack tip on creep crack growth rate is successfully simulated.     

Electrohydraulic trimming of advanced and ultra high strength steels

Electrohydraulic trimming (EHT) is a novel method of trimming sheet metal panels and is based upon the electro-hydraulic effect: a complex phenomenon related to the discharge of high voltage electrical current through a liquid. In EHT, electrical energy is stored in a bank of capacitors and is converted into kinetic energy within the liquid and the sheet metal blank by rapidly discharging the stored energy across a pair of electrodes submerged in a fluid. The objective of this paper is to describe the newly developed EHT process, to report the results of early proof-of-concept experiments, and to provide an explanation for the observed results through the use of a numerical modeling technique developed as a part of this work. The key innovation behind the EHT concept is the chamber design, which consists of a narrow fluid channel positioned directly below a sharp trim steel. The narrow channel can transmit fluid pressure very efficiently, and the design has enough inherent flexibility such that it can be used to cut straight lines and can also be applied to more complex curvature. The new, channel chamber design concept was successfully demonstrated for electrohydraulic trimming of Advanced and Ultra High Strength Steels, including DP500 at 0.65 mm thick, DP590 at 1.0 mm thick, DP980 at 1.0 mm and 1.4 mm thick, and AISI 4130 steel at 2.0 mm thick. Separation of the offal from the part initiates in the area of the blank directly above the discharge channel. Further separation along the remainder of the trim line is influenced by a number of factors, including the strength and thickness of the sheet material, the mass density of the sheet material, and the propagation of pressure waves along the fluid channel and their reflection from the walls at the ends of the fluid channel. A numerical model was developed which is able to predict the offal separation mechanism and the sequence of offal separation during electrohydraulic trimming. The developed model incorporates several individual models into one integrated simulation, including models for the plasma channel, the liquid within the fluid channel, the steel chamber and trim blade, and the deformable blank.     

Weldability of advanced high strength steels using an Yb:YAG disk laser

In this paper, weldability results of 1030 nm, 6 kW Yb:YAG disk laser welding of various combinations of advanced high strength steels (transformation induced plasticity steel, dual phase steel and boron steel) of 1-2 mm thickness are presented. Weldability is expressed in terms of penetration, weld profile, weld defects, microhardness and melting efficiency. Full penetration keyhole welds with hour-glass shape were produced. Microhardness measurements indicated a substantial increase in hardness in the weld zones, attesting the superiority of laser welding. Analyses revealed that the typical melting efficiency is on the order of 50-70% for full penetration welding. Tensile test results confirmed the high quality of the welds obtained.     

基于Yoshida-Uemori硬化模型的高强钢冲压件回弹预测研究

基于LS-DYNA软件对高强钢薄板零件的冲压成形进行仿真分析与回弹预测,研究了不同材料硬化模型对回弹预测精度的影响。在仿真分析中应用各向同性硬化模型和考虑包申格效应的Yoshida-Uemori随动硬化模型,将模拟回弹值同试验回弹量进行比较,验证了Yoshida-Uemori材料模型在高强钢回弹预测中的可靠性,为实际生产中模具设计及修正提供理论指导。     

应用本构模型和神经网络模型预测铝/镁基纳米复合材料的高温流变行为(英文)

为了预测Al/Mg基纳米复合材料的高温流变行为,在不同的应变速率(0.01-1.0s-)和温度(523,623和1723K)的条件下进行热压缩试验,利用所得到的应力-应变数据,开发了本构模型,比如一般流动方程。阿累尼乌斯双曲模型、Johnson-Cook(JC)和改性的Zerilli-Armstrong(ZA)模型及人工神经网络(ANN)模型。通过使用统计参数,例如均方根误差(RMSE)、回归系数(R2)、平均相对误差(MRE)和分散指数(Is),比较了人工神经网络和不同的本构模型。结果表明,人工神经网络模型对AA5083-2%TiC复合材料的热变形流动应力的评估准确性更高。     

Determination of forces in high speed blanking using FEM and experiments

The increasing demand for micro-formed and stamped parts such as connectors in the electronic industry is forcing manufacturers to push the speed limit of conventional press technologies to improve throughput. Designing dies/tooling for higher speeds and obtaining extended tool life requires a thorough understanding of the process. This paper discusses an experimental study of the interaction between punch, stripper plate and sheet material at various blanking velocities up to 1600 mm/s. The effect of velocity on punching force is also studied. A methodology to obtain high strain and strain rate dependent material flow stress data using blanking test and finite element modelling is presented.     

Evaluation of stamping lubricants in forming advanced high strength steels (AHSS) using deep drawing and ironing tests

In forming AHSS, the lubricant must reduce the friction between die and sheet as well as the effect of heat generated from deformation and friction, especially in forming at high stroking rates. In this study, the effectiveness of stamping lubricants was evaluated by using the deep drawing and ironing tests. Various stamping lubricants were tested in forming of DP590 GA round cup samples. In these tests, the performance of lubricants was ranked via evaluation criteria that include punch force and the geometry of tested specimens. Deep drawing tests were conducted at two different blank holder forces, BHF (30 and 70 ton) at a constant ram speed (70 mm/s). The ironing tests were conducted to evaluate the performance of lubricants at higher tool–workpiece interface pressure than that is present in deep drawing. Polymer-based thin film lubricants with pressure additives (e.g. Lubricants A and B) were more effective than other lubricants as shown by the force (e.g. maximum punch force and applicable BHF without cup fracture) and geometry indicators (e.g. draw-in length, flange perimeter and sidewall thinning).The pressure and temperature distributions at the die–sheet interface were predicted by FE simulation of deep drawing and ironing tests. As expected, the value of interface pressure and temperature were maximum at the die corner radius.     

Mechanical and fatigue behaviour of laser and resistance spot welds in advanced high strength steels

Abstract

A study was carried out on laser and resistance spot welds in overlapped sheets of dual phase advanced high strength steel (DP780) and deep drawing steel (DC04) of 2˙0 mm in thickness. The aim of the study was to investigate the fatigue performance of these joints under tensile shear loading as well as the monotonic performance for applications in the automotive industry. The mechanical properties, failure behaviour and fatigue life analyses of spot welds in similar and dissimilar joints were investigated by experimental and numerical methods. The structural stress concept was used to describe the fatigue lives of spot welded specimens. The results revealed different failure types with different fatigue behaviours for laser and resistance spot welds under the application of cyclic loads at 'high load' and 'low load' levels.     

2024Al高温高应变率下动态塑性本构关系的实验研究

运用SHPB装置,文章对2024Al在不同温度和不同应变率条件下的动态力学行为开展了系列的实验研究,基于Johnson-Cook(JC)本构模型,通过实验数据拟合得到了相应的材料模型参量,从而建立了2024Al的动态塑性本构关系。为进一步验证该本构关系,基于有限元方法,对常温下应变率为700s-1的SHPB实验进行了数值模拟,模拟计算结果与实验结果相吻合,表明所得到的模型可以很好地描述该材料在高温高应变率下的动态塑性力学行为。     

先进高强钢电阻点焊接头的断裂模式分析与预测

研究了先进高强钢(advanced high strength steel, AHSS)两层板电阻点焊接头的断裂模式,不同的断裂模式会影响点焊接头断裂时的机理、力学性能及断裂位置,基于不同组合下的临界熔核尺寸、最大载荷、断口宏观形貌、初始断裂位置、宏观金相组织以及微观硬度曲线等试验结果,阐明了板材厚度和板材强度两类因素对于断裂模式的影响规律. 结果表明,板材强度因素会直接影响断裂模式、初始断裂位置以及最大载荷;板材厚度因素影响断裂模式但不改变初始断裂位置及最大载荷. 临界熔核尺寸的影响因素有板材厚度、板材强度、熔核中缺陷以及拔出断裂位置距熔合线的距离. 在此基础上,文中提出了临界熔核尺寸(D_CR)的预测模型及预测方法,该方法与试验值符合较好,为实际工业应用中的临界熔核尺寸判定提供了理论依据.     

高强韧Ti6246合金热变形行为及应变补偿型本构模型

在温度为1123~1423 K,应变速率为0.01~10 s-1条件下,对Ti-6Al-2Sn-4Zr-6Mo(Ti6246)合金进行高温热压缩试验。研究温度、应变速率和应变对Ti6246合金高温流变应力的影响规律,建立了该合金考虑应变补偿的Arrhenius本构模型,同时获得了热激活能Q和本构模型中材料参数对应变的响应规律。将模型计算结果与热压缩试验值进行对比发现,预测结果较为准确,其相关系数(R)及平均相对误差(AARE)分别为0.9984和1.71%,表明该合金热变形过程中的流变应力可用构建的应变补偿Arrhenius本构模型来描述。     

QP980-DP980异种先进高强钢激光焊接头微观组织及力学性能

采用光纤激光焊实现了异种汽车用先进高强钢QP980与DP980的拼焊连接,对异种焊接接头的微观组织、硬度分布进行了观察和测试,对焊接接头的拉伸性能进行了测试,对断口形貌进行了观察和分析。结果表明,焊缝区域的组织全为马氏体组织,且硬度最高(535 HV);两侧焊接热影响区均可分为完全相变区、不完全相变区和回火区三个区域。两侧热影响区中的完全相变区由于冷却速度快全部为马氏体组织,硬度提高;不完全相变区部分形成马氏体,成为马氏体和铁素体的混合区,回火区由于回火马氏体的出现使其硬度下降。QP980侧的回火区由回火马氏体、铁素体和残余奥氏体组成;DP980侧的回火区由回火马氏体和铁素体组成。接头拉伸断裂发生在DP980侧热影响区,抗拉强度达到DP980母材的98．9%,断后伸长率约为DP980母材的70．9%,断裂模式为韧性断裂。     

Evaluation of tool materials, coatings and lubricants in forming galvanized advanced high strength steels (AHSS)

The major objective of this study is to establish guidelines to select the optimum combination of die materials, coatings and lubricants in stamping galvanized AHSS (DP600, TRIP780 and DP980) for automotive structural parts. For this purpose, Finite Element Analysis (FEA) and various tribotests, e.g. Twist Compression Test (TCT), Deep Drawing Test (DDT) and Strip Drawing Test (SDT), were used. The results of this study helped to determine the critical interface pressure and temperature that initiate lubricant failure and galling in forming galvanized AHSS for a given die material and coating.     

高强7A62铝合金动态力学响应及其J-C本构关系

利用万能试验机和Hopkinson拉杆(SHTB)对7A62铝合金进行了准静态和动态拉伸性能测试,研究该合金室温及高温塑性流动应力动态响应特征,结合OM、SEM、TEM、DSC、LFA等测试对该合金的物理性能及原始微观组织进行分析。结果表明:7A62铝合金在大量高密度细小沉淀析出物粒子及亚微米级高熔点平衡析出粒子复合强化作用下,准静态屈服强度可达608 MPa。在室温动态变形过程中,该合金应变率强化效应显著。随着应变率高于684 s^-1时,合金屈服强度对应变率敏感性显著增强。在1100 s^-1、25~500℃条件下,合金表现出温度敏感性的沉淀强化相回溶及动态再结晶的热软化效应,500℃高温动态屈服强度超过200 MPa。在动态力学性能变化规律的基础上,建立了7A62铝合金的Johnson-Cook(J-C)本构模型。     

成形和淬火工艺对高强度热成形零件硬度-强度的影响

通过热成形高温平板分区冷却系列试验,以淬火前后热成形材料力学性能为评价指标,结合正交试验分析方法研究了热成形工艺条件下保压时间、压强和板料厚度对热成形零件硬度强度的影响规律。采用统计学3σ法则对优化工艺下热成形零件力学性能进行分析,建立了基于柔性指数模型的热成形钢板强度硬度非线性回归方程和热成形钢板强度-硬度等级评估区间图。并基于优化条件下的U型热冲压实验验证了该模型的正确性。结果表明:建立的性能评价标准可为热成形零件的力学性能预测和在线质量检测控制提供重要指导依据。     

Evaluation of susceptibility of high strength steels to delayed fracture by using cyclic corrosion test and slow strain rate test

To evaluate susceptibilities of high strength steels to delayed fracture, slow strain rate tests (SSRT) of notched bar specimens of AISI 4135 with tensile strengths of 1300 and 1500 MPa and boron-bearing steel with 1300 MPa have been performed after cyclic corrosion test (CCT). During SSRT the humidity around the specimen was kept high to keep absorbed diffusible hydrogen. The fracture stresses of AISI 4135 steels decreased with increment of diffusible hydrogen content which increased with CCT cycles. Their delayed fracture susceptibilities could be successfully evaluated in consideration of both influence of hydrogen content on mechanical property and hydrogen entry.     

The high temperature flow behavior modeling of NiTi shape memory alloy employing phenomenological and physical based constitutive models: A comparative study

The present study was conducted to examine the constitutive behavior of a near equiatomic Nickel Titanium shape memory alloy through modified Zerilli–Armstrong and Arrhenius type models. The acquired flow stress data from isothermal hot compression tests in a temperature range of 700–1100 °C and under the strain rates of 0.001–0.1 s⁻¹ was employed to calculate the material constants thereby properly establishing the corresponding constitutive equations. Furthermore, a comparative study has been made on the capability of the aforementioned models to predict the high temperature flow behavior of the adapted shape memory alloy. This was performed by comparing the prediction relative errors, average absolute relative error and correlation coefficient. The results show that Arrhenius type equation predicts the flow behavior more accurately than the Zerilli–Armstrong one. However, the latter requires lesser material constants and running time to be established.     

Similar and dissimilar resistance spot welding of advanced high strength steels: welding and heat treatment procedures,structure and mechanical properties

Similar and dissimilar combinations of a 1000 MPa galvanised dual phase (DP) steel and a 980 MPa transformation induced plasticity (TRIP) steel were resistance spot welded under different welding and heat treatment parameters. The microstructure and mechanical properties of spot welds were evaluated using metallographic technique, microhardness and tensile shear tests. The results showed that the tendency to fail in the pullout mode increased in the order of DP/DP, TRIP/TRIP and DP/TRIP welds, which was caused by the different hardness distribution, carbon equivalent and susceptibility to shrinkage void formation of spot welds for different combinations. In the study of the effects of heat treatments on the DP/TRIP welds, the pre-heating procedure improved the splash of welding to some extent. When the cooling time was larger than or equal to 1000 ms, the post-heating procedure improved the mechanical properties of spot welds owing to the temper of spot weld microstructure.     

不同成分体系罩退高强IF钢的组织与性能

研究了不同成分体系含P高强IF钢在罩式退火条件下的析出物、组织与性能差异。结果表明,微合金元素Nb和Ti是影响试验钢析出物、组织和性能的关键因素。添加Ti的Ti-HIF和Nb+Ti-HIF两种成分体系的试验钢FeTiP析出物数量多,r值较差;添加Nb可以细化钢板组织,使钢板析出强化、细晶强化、固溶强化效果更显著,有利于提高抗拉强度;Nb-HIF成分体系试验钢性能最优,适合罩式退火工艺条件下生产。