Deep drawing behavior of twinning-induced plasticity-cored three-layer steel sheet

Herein, we report the results of our experimental and computational investigation of the deep drawing behavior of twinning-induced plasticity (TWIP)-cored three-layer steel sheet. Various directional tensile tests for monolayer sheets of the TWIP and low-carbon (LC) steels were performed for evaluation of flow curves and anisotropic coefficients, which are used as input data of the finite element analyses for deep drawing of the monolayer and three-layer sheets. The experimental deep drawing behaviors of the layered sheets of three thickness ratios, and of the monolayer sheets, are computationally analyzed in terms of earring and variations of local thickness along the positions in the drawn cups. It was found that the three-layer sheets are more planar-isotropic in the sense of earring than the monolayer sheets, even more than the TWIP steel sheet. The thickness uniformity along the initial radial direction in the layered sheets is in between the heterogeneous TWIP steel and the relatively homogeneous LC steel. The present results shed light on the future direction of development of steel sheets with planar-isotropic and homogeneous deep-drawing characteristics.     

Benefits of stress superposition in combined bending-linear flow splitting process

Linear flow splitting is a multi-station sheet-bulk metal forming process which allows continuous production of bifurcated profiles without joining, lamination or external heating of sheet metal. This process induces high hydrostatic stresses in the forming zone which leads to an elevated formability of the workpiece material. The aim of this research is to bend linear flow split profiles in transverse direction in a continuous manner. This is achieved by combining the linear flow splitting process with a continuous bending process. An analytical and a numerical model are described in this paper which predict bending moments for different radii. Results from both models are validated with experimental results. It is found that combining the linear flow splitting with a bending process leads to a severe reduction in the bending moments due to superposition of stresses in the forming zone. The superposition maintains the cross sectional shape of the bent profiles.     

铝/钢薄板无铆连接接头成形过程及形貌控制

目的 研究铝/钢薄板无铆连接过程中接头形貌的影响因素。方法 采用有限元模拟方法分析了铝/钢薄板在平底模具下接头形貌的成形过程。结合变形区域的金属流动情况,解释了互锁结构的形成机理,并分析了不同工艺参数对接头形貌的影响。结果 接头互锁结构主要是依靠铝板填充钢板的凹陷部位而形成的,抑制钢板与模具接触一侧的金属流动有助于接头底部和侧壁的钢板拉薄,进而形成内部凹陷,促进互锁结构的形成。摩擦因数对接头形貌参数影响较大,增大摩擦因数可显著提高接头的互锁量。结论 冲头半径、冲头圆角、凹模深度以及摩擦因数对颈厚值和互锁量均有显著影响。通过控制这些影响因素,可以得到良好的接头。此外,接头的失效形式以铝板颈厚较薄处的剪断失效为主,因此对于采用铝上钢下的无铆连接,保证颈厚值相对于互锁量更加重要。     

Modeling of resistance spot welding of multiple stacks of steel sheets

Weld quality is a major challenge for resistance spot welding of multiple stacks of steel sheets. Because of the differences in mechanical and physical properties of steel sheets and the sheet gage variation, the contact state between sheets and welding current flow throughout the stack joint is complicated. As a result, discrepant weld sizes at the faying interfaces become an issue. In this study, a coupled thermal–mechanical/thermal–electrical incremental model has been developed to reasonably predict the weld nugget formation process of resistance spot welding of a sheet stack made of 0.6 mm thick galvanized SAE1004+1.8 mm thick galvanized SAE1004+1.4 mm thick galvanized dual-phase (DP600) steel using published thermal, electrical, and mechanical properties. It was found that the weld nugget on the faying interface of DP600 forms earlier than that on the other interface, which agrees well with the experimental results. Based on the coupled model, the effects of the sheet gage combination and steel grade combination were examined. The results show that, for a multiple stacks of steel sheets SAE1004 + SAE1004 + DP600, the critical ratio of sheet thickness between the top and bottom sheets is approximately 1:3. The model could provide an important guidance in the selection of the welding variables, sheet gage and steel grade to meet the weld quality of steel component.     

基于数字散斑应变测量法的薄板各向异性力学性能研究

系统地说明了采用数字散斑相关法研究薄板各向异性的实验方法和数据处理方法,进而对SPCC钢板和AA6061铝板的各向异性及其演化规律进行了研究。结果表明:散斑应变测量法是一种获取薄板力学性能的有效手段,其最大优点在于能够获得变形过程中的整体应变场,这是研究复杂加载条件下材料力学性能的关键;对于SPCC钢板,其流动应力的各向异性并不严重,但全量和增量形式的Lankford系数(r值和r′值)均表现出了明显的各向异性,且其值随着变形的增加而逐渐降低,这与传统的采用引伸计进行应变测量时只能获得恒定的Lankford系数不同;对于AA6061铝板,其流动应力和r值的各向异性均不明显,但与轧制方向成不同角度试样的伸长率表现出了明显的差异,并且流动应力的加工硬化速率和r′值在拉伸真实应变处于0.15～0.20时出现了剧烈的波动;随着变形的增加,两种薄板应变的各向异性都逐渐增强,SPCC钢板增强得更为明显。     

Stress gradient due to incremental forming of bonded metallic laminates

The residual stress and associated gradient can affect the performance of a material/component during service. Sheet-metal in incremental sheet forming (ISF) due to missing back support may experience high stress gradient across the thickness. The current work is aimed at experimentally analyzing the through-thickness stress gradient in the Cu/steel bonded laminates after ISF deformation. It is found that ISF induces compressive stress gradient, which can be a way greater (about 18 times) than that the rolling process induces in the parent laminates while bonding, specifically when the deformation angle is high. Further, the tool imposes more stress gradient (1–50% depending on the forming conditions) in its motion direction than that in the transverse (or stretching) direction. Moreover, un-strained Cu/steel laminated sheet experiences higher (25%–68%) gradient than that the pre-strained/rolled sheet endures. Regarding the role of technological parameters, high angle, small tool, average step-size and spindle rotation, and low flow-stress induce high stress gradient. The tension tests of the ISFed samples reveal that the post-ISF tensile strength of laminated sheet increases as the stress gradient increases, thus showing a direct relationship between stress gradient and strain hardening in ISF. Finally, models are proposed to predict the stress gradient in the ISFed Cu/steel components.     

Modelling of electro hydraulic free and die forming of sheet steels

Electro hydraulic forming of a range of different sheet steels was studied experimentally and with finite element methods. Four carbon and stainless sheet materials were studied. In this paper we present results on a mild steel (IF210), two high strength steels (DPX800 and TRIP700) and one stainless steel (1.4509). The flow properties of the materials were evaluated at a range of strain rates up to 1000/s. These were typical strain rates in the FE simulations. The flow properties were characterized with the Johnson Cook model. Electro hydraulic forming trails were performed with a chamber of water with a pair of electrodes on one side of the sheet. In one case free forming was performed and in the other case forming was performed into a truncated conical die. Geometrical shapes and strain distributions were evaluated after forming. A finite element model was formulated in ABAQUS explicit. The model takes the chamber filled with water into account and the effect of the electrical discharge is modeled as a pressure wave originating from the location of the electrodes. The sheet is given the properties defined by the Johnson Cook model and stiff tools are used. The forming of the sheet is described including rebound effects at the tools. The model shows satisfactory results in relation to the experimental trials regarding both shape and strains of the pressed sheets.     

Large-strain Bauschinger effect in austenitic stainless steel sheet

Large-strain Bauschinger effect in cold-rolled austenitic stainless steel sheet is investigated after large amounts of prestrain. The material is prestrained in uniaxial tension, and the tensile properties of the prestrained material are measured in different angles with respect to the prestraining direction. By comparing the differences in the yield stresses in different orientations, the effect of prestraining on material anisotropy is studied. The method is applied to AISI 304-type stainless steel sheet. The test results are analyzed using a combined isotropic–kinematic hardening model. The results indicate that this kind of material shows a considerable Bauschinger effect. Transient and permanent softening is observed in the experiments. The experimental Young's modulus also seems to decrease with prestrain.     

Analysis of micro/mesoscale sheet stamping processes based on crystalline plasticity model

A Taylor-type crystalline plasticity model, implemented into commercial finite element analysis software, is coded as a subroutine to investigate the behavior of a stainless steel sheet with a body-centered cubic structure in this study. Thickness variations of the sheet are examined under the microgroove formation procedures. Effects of the spatial distribution of crystallographic orientations and orientation assignment approach adopted in the simulations on the thickness distribution over the sheet are demonstrated. Numerical results, based on a sheet with textured orientations, are in good agreement with the associated experimental measurements reported in the literature.     

渐进成形A3003铝板减薄带分析及数值模拟研究

研究渐进成形过程中板料减薄带的变化,可以提供合理的加工参数,提高板料的成形性能和加工利用率,减少零件破裂失效.基于渐进成形过程中金属板料轮廓的变化与理想情况下轮廓的区别,对渐进成形初始成形阶段A3003铝板减薄带的产生原因和剪切力的变化过程进行了理论分析,并通过有限元模拟分别从未变形区金属板料的长度和强度两个角度对板料渐进成形过程中未变形区下沉的影响,以及成形角度和杨氏模量对变形区回弹的影响两个方面,对减薄带的产生原因进行研究.结果表明:板料未变形区的下沉和变形区的回弹使板料在初始加工阶段形成一段平缓区域,工具头在平缓区域的变形性质发生了变化,平缓区域发生剪切变形导致了板料在初始加工阶段形成了减薄带;渐进成形时减小板料未变形区的长度,增大板料与垂直方向的角度可以一定程度上阻碍减薄带的产生,模拟结果与理论分析相符合.     

Fatigue fracture behaviour of spot welded B1500HS steel under tensile‐shear load

The microstructural characterizations, micro‐hardness measurements and fatigue tests of B1500HS steel spot welded tensile‐shear specimens were performed. The high hardness values of base material (470 HV) and nugget (515 HV) are closely related to the dominant formation of martensitic microstructures, while the occurrence of soft zone is the result of the formation of ferrite phases in inter‐critical heat‐affected zone (HAZ), as well as martensite tempering in sub‐critical HAZ. The fatigue failure modes involve the fracture along the circumference or along the direction of width. The fatigue property of spot welded B1500HS is found to be better than that of spot welded M190 because of the thicker sheet and suitable nugget size, which follows the standard rule of 5t^0.5, where t is the sheet thickness.     

Magnetic Properties and Workability of 6.5% Si Steel Sheet Manufactured by Siliconizing Process

1. IntroductionIt is well known that increasing the St content improves the soft magnetic properties of St steel sheet.Steel sheet containing about 6.5% St exhibits excelledssoft magnetic properties. However, increasing the St.content makes the steel brittl.11]. A steel containingmore than 4.5% St has no elongation at room temperature, and a thin sheet cannot be made by coldrolling.Recently, the siliconning method to manufacture6.5% St steel sheet has been developedl2]. As a result,6.5% St…     

Material characterization in flexible flow splitting

There is a significant demand on flexibility in production processes, regarding freedom of part design and freedom of process design. The forming process linear flow splitting satisfies these demands by enriching the semi‐finished product sheet metal with geometric features. Additional flanges and bifurcations in integral style expand the range of component applications. By the further development of flexible flow splitting, these flanges can now be produced along a non‐linear bifurcation line and the application can be expanded on products with varying cross‐section. The continuous steady‐state process linear flow splitting is transferred into a flexible forming process that is characterized by non‐steady states and measurands. Methods of materials characterization are applied to investigate, if those non‐steady properties, that are visible in process characteristics and part geometry, can also be identified in the mechanical behavior.     

Design method for TRIP-aided multiphase steel based on a microstructure-based modelling for transformation-induced plasticity and mechanically induced martensitic transformation

In recent years, for automotive applications, the need for new advanced high-strength sheet steels (AHSSs) with high ductility has rapidly increased. This is mainly related to the need for more fuel-efficient (and therefore more environmentally friendly) cars, and the increasing consumer demand for safer vehicles. In this research, the transformation-induced plasticity (TRIP) that accompanies the mechanically induced martensitic transformation (MIMT) in TRIP-aided multiphase steel was analyzed. The analysis was performed using a computational model that takes the ductile fracture during tensile deformation into account. The TRIP and MIMT phenomena were calculated using the concept of variant selection, which is based on the Kurdjumov–Sachs (K–S) orientation relationship. To consider the localization of the plastic flow in the deforming material, the increase in void nucleation due to the martensitic transformation and the void growth based on the yield criterion for porous material were studied. The feasibility of the extra advanced high-strength sheet steel (X-AHSS) was assessed by analyzing the results obtained using various initial volume fractions and various stabilities of the retained austenite in the TRIP-aided multiphase steel. Subsequently, the optimum volume fraction and stability of the retained austenite in the TRIP-aided multiphase steel could be determined.     

Einfluss von Temperatur und Vorverformung auf das plastische Werkstoffverhalten von modernen Karosseriestählen

Influence of temperature and prestraining on the plastic material behaviour of modern sheet steels for autobody applications Within the scope of a common research project of the automotive and steel industry, characteristic values describing the plastical behaviour of 20 sheet steels have been determined. In detail, quasistatic tensile tests at the testing temperatures ‐40 °C, 23 °C and 100 °C were carried out to obtain flow curves for the as delivered materials as well as for steels after a defined prestraining or heat treatment. Additionally, sheet metal testing led to forming limit diagrams and limiting drawing ratios including the working ranges for deep drawing. The results of the tensile tests showed significant differences between steel groups with regard to their strain hardening behaviour, which can be described by the ratio of yield and tensile strength R_p0,2/R_m or the Θ_IV‐value, and their temperature sensitivity. Within one steel group, consisting of steels with similar strain hardening behaviour, it might be possible to determine flow curves of one steel in a defined condition in order to calculate the flow curves of other steels with different strength. An advantage would be a lesser number of experimental tests which have to be performed in order to supply reliable input data for numerical material and component modelling.     

Fatigue strength of self‐piercing riveted joints in lap‐shear specimens of aluminium and steel sheets

The self‐piercing riveting (SPR) process is gaining popularity because of its many advantages. This study investigated the fatigue strength of SPR joints in tensile‐shear specimens with dissimilar Al‐5052 and steel sheets. A structural analysis of the specimen was conducted. For this specimen, the upper steel sheet withstood applied load in a monotonic test and played a major role in the low‐cycle region. In the high‐cycle region, however, the harder surface of the upper steel sheet reduced the fatigue strength by enhancing fretting crack initiation on the opposite softer aluminium surface. Therefore, the fatigue endurance of the specimen was reduced. The fatigue endurance of a SPR joint with the combination of steel and aluminium sheets was found to be governed by the strength of the lower sheet, which is more vulnerable to the applied loading. Thus, it is desirable to use a stronger metal sheet as the lower sheet with regard to the fatigue performance. Scanning acoustic microscopy was effectively used to reveal and prove the formation and growth of subsurface cracks in SPR joints. The structural stress can predict the fatigue lifetimes of the SPR joint specimens within a factor of three.     

Research on the influence of the band edge processing on the process of linear flow splitting

Sheet metal is available in a wide range of properties and material composition. There are standardized quality characteristics for mechanical and geometrical properties in the plane of the sheet. However, the band edges are influenced by the correspondent metal working process and the properties may vary or be unknown. The forming zone of the manufacturing process linear flow splitting is located at the band edge. Thus, the knowledge of probable influences of metal working induced band edge properties on the process of linear flow splitting is essential and this knowledge will be presented in the following article.     

Manufacturing‐induced material properties of linear flow split and linear bend split profiles

In this work the two massive forming processes linear flow splitting and linear bend splitting, which generate profiles from sheet metal, are evaluated with respect to characteristic manufacturing‐induced material properties of the produced parts. Resulting microstructural features such as grain size and shape as well as crystallographic textures are linked to mechanical properties such as strength, ductility and anisotropic elasticity and general rules for their evolution are defined. Residual stress distributions are detailed and discussed with regard to the causing geometrical and forming process related aspects. The aim of this paper is to give a comprehensive overview of the properties of profiles produced by linear flow splitting and linear bend splitting and to illustrate general rules for their evolution in order to provide guidelines for an optimized product development process which allows a beneficial use of the manufacturing‐induced properties.     

Welding of aluminum alloy to zinc coated steel by cold metal transfer

Introducing the aluminum alloy into the steel body structure allows the reduction in the vehicle weight and improves the fuel efficiency. However, it is a still great challenge to weld aluminum alloy to steel due to their differences in the physical, mechanical and metallurgical properties. In this study, aluminum alloy 6061-T6 was welded with zinc coated low carbon steel by cold metal transfer (CMT). Effects of the pre-setting gap at the interface of aluminum alloy sheet and steel sheet as well as the offset distance of the electrode torch from the aluminum alloy sheet edge on the weld qualities were investigated. The tensile shear tests were carried out to evaluate the mechanical property of the welds. In addition, optical micrograph, scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS) were used to analyze the weld microstructure. Experimental results indicated that the intermetallic layer thickness in the CMT welds was well controlled below the 10 μm, which facilitates the achievement of relatively high weld strength. Furthermore, a pre-setting gap and an appropriate post-weld heat treatment can improve the weld strength. However, the weld strength was decreased by increasing the offset distance of arc torch. In addition, the pre-setting gap also affects the intermetallic layer morphology. The formation of brittle Al_xFe_y could be suppressed by the presence of the remained zinc in the steel side.     

电机定子硅钢片磁致伸缩效应引起的振动

为了研究磁致伸缩效应对电机定子硅钢片的影响,建立磁-机械耦合数值模型,研究了在磁致伸缩作用下电机定子的振动情况,得到电机定子硅钢片的振动主要为供电频率的一倍频与二倍频。设计了一种仿真实验模型,模拟电机中的磁场走向,线圈与无取向硅钢片不接触,这样无取向硅钢片上只存在由变化磁场导致的磁致伸缩力;通过改变硅钢片的位置,分析磁场不同走向下磁致伸缩效应对无取向硅钢片应力特性的影响,得到无取向硅钢片主要振动频率为供电频率的一倍频和二倍频,且硅钢片处于不同位置时影响其振动的主要频率不同;在磁路发生偏转时造成无取向硅钢片在供电频率一倍频处振动明显。同时设计了实验,测量变化磁场下无取向硅钢片上的振动信号,对仿真结果进行验证,与仿真结果相符合。