高强度双相钢板拉弯成形断裂的微观组织形貌分析

引入拉弯比来表征高强度双相钢板在不同凸模圆角半径下的拉弯应力状态,根据不同的拉弯比判定不同的成形断裂模式,在剪切断裂试验平台上进行双相钢板的拉弯试验,并对断裂试样的微观组织形貌进行分析.结果表明:高强度双相钢的剪切断裂属于韧性断裂;凸模圆角半径越小及板料强度越高,板料越容易发生剪切断裂.     

1 000 MPa级双相钢薄板极限成形性能

基于圆筒拉深试验,获得了1 000 MPa级先进高强度钢拉深时破裂的临界点和极限拉延比(LDR).研究了Hill、Barlat和BBC 3种不同屈服准则的数值仿真效果.结果表明：1 000 MPa级双相钢LDR值为2.07,且Barlat屈服准则模拟结果与实际试验相符.对断口进行扫锚电镜（SEM）观测,发现1 000 MPa级双相钢有别于传统板材,其拉深断裂模式为沿板材厚向45°的韧性切向断裂,无明显颈缩现象.     

凹模温度对AZ31B镁合金板材冲压成形性能的影响

采用Nakazima半球凸模胀形法获取不同凹模温度下AZ31B镁合金板材的成形极限曲线(FLC),研究了温冲压过程中凹模温度对镁合金板材成形性能的影响.运用有限元法对等双拉试样进行热-力耦合模拟,得到不同凹模温度下的温度场,分析AZ31B镁合金板材与凹模在热传递过程中的热-力耦合关系.另外,通过试制汽车行李箱铰链支架盖板零件,验证了实际工况下凹模温度对AZ31B镁合金板材成形极限的影响.结果表明:凹模温度的降低,会显著改变AZ31B镁合金薄板成形时的温度梯度分布,造成材料成形极限的下降以及破裂位置的改变;不同凹模温度下所得FLC的模拟值与其实验值相符.     

基于韧性断裂准则的AZ31B镁合金板材成形极限预测

结合损伤起始判据和损伤演化准则,建立了完整的韧性断裂准则,基于ABAQUS中韧性损伤材料模型对AZ31B镁合金板材成形极限进行了预测。通过拟合单向拉伸应力应变曲线得到材料本构模型及损伤演化参数,建立了板材的Nakazima半球形凸模胀形有限元仿真模型,再基于韧性断裂准则预测了AZ31B镁合金板材室温下的成形极限,并分析了不同板材断裂失效判据对成形极限的影响。研究结果表明,基于所建立的韧性断裂准则,并以损伤演化过程中应变路径转变作为断裂失效判据,可以较准确地预测镁合金板材成形极限,得到的成形极限图与实验结果吻合较好。     

激光拼焊板破裂判据的建立及成形极限预测

为准确判断差厚高强钢激光拼焊板何时、何处发生破裂,在失效行为研究的基础上建立一种新的破裂判据.该破裂判据为:在建立拼焊板左半边成形极限图时,凸模载荷达到最大值时拼焊板发生破裂;在建立拼焊板右半边成形极限图时,拼焊板应变路径向平面应变状态漂移时破裂发生.基于该破裂判据,结合对高强钢拼焊板成形极限试验的仿真,建立其成形极限图.预测结果与试验数据吻合较好,验证了拼焊板破裂判据的正确性和有效性.基于建立的破裂判据,结合对拼焊板成形极限试验的有限元仿真,能够准确、快速获得差厚高强钢激光拼焊板的成形极限图.     

工艺参数对AA5754铝合金温成形回弹的影响

以圆筒拉深件切环实验为基础,采用正交试验设计与数值模拟相结合的方法,对温成形过程中铝合金圆筒拉深件在不同工艺参数下的回弹特性进行研究,建立铝合金温成形的有限元模型.基于正交试验对圆筒拉深件进行数值模拟,研究板料成形初始温度、冲压速度、摩擦系数、压边力、凸模圆角半径、凹模圆角半径及凸凹模间隙对制件回弹的影响,并确立优化工艺参数组合.研究结果表明:从工艺参数对制件回弹影响的显著水平来看,板料成形初始温度最高,增加成形初始温度可显著减小制件回弹;压边力次之,其后依次为凹模圆角半径、凸凹模间隙及摩擦系数,而冲压速度和凸模圆角半径低至几乎可以忽略.     

一种板材小圆角胀压复合成形工艺解析

针对薄壁板材零件小圆角特征成形制造难的问题,提出了一种新型胀压复合成形工艺.其关键工艺参数为:预成形高度、预成形凹圆角大小和终成形胀形压力与背压凸模运行速度匹配关系.预成形高度决定了终成形小圆角的材料储备,预成形凹圆角的最佳值为充液拉深时凸模圆角可取的最小值,通过理论分析给出了预成形高度和预成形凹圆角的计算方法.建立了胀压复合成形过程力学模型,通过应力状态分析给出了不同胀形压力与背压凸模运行速度匹配关系下坯料圆角区变形状况.同时基于有限元模拟和工艺试验,研究了预成形高度和终成形胀形压力与背压匹配路径对试验件成形质量的影响,验证了理论分析的准确性,并证明了该新工艺的适用性.     

扁袋笼顶盖冲压模设计与成形分析

在扁袋笼顶盖零件工艺分析的基础上,得出其优化工艺为落料-浅拉深(小于内壁高)-冲孔-内翻边,重点介绍落料拉深复合模的设计和三维建模.为了保证扁袋笼顶盖拉深成功,采用Dynaform软件模拟了扁袋笼顶盖的成形过程,对凸模圆角、凹模圆角和压边力工艺参数进行优化,并得到了其成形极限和减薄量图.结果显示,当凸模圆角5 mm、凹模圆角6.5 mm、压边力为50 kN时成形效果较好,无明显缺陷.     

各向异性GTN损伤模型及其在板料成形中的应用

基于Hill的二次各向异性弹塑性屈服准则（Hill’48）,发展了可以用来描述各向异性多孔延性材料损伤演化发展的GTN（Gurson—Tvergaard—Needleman）弹塑性细观损伤本构模型,将材料的塑性各向异性行为和损伤发展有机地耦合起来．分别采用基于Mises的各向同性GTN弹塑性损伤模型、基于Hill’48的各向异性GTN弹塑性损伤模型和传统的Mises弹塑性模型,对NUMISHEET’2002中考题圆杯深拉深成形工艺进行了数值模拟,对板料成形过程中损伤的累积发展及其对材料性能的影响进行了分析．结果表明：延性板料的各向异性行为对其宏观力学性能和损伤演化都有影响;GTN模型中的损伤变量反映了成形历史的影响,为复杂加载路径下的成形极限判据的提出提供了理论依据．     

1 200 MPa级超高强度钢板冲压成形性能及其断裂机理的实验研究

通过圆筒拉深试验研究了1 200 MPa级超高强度双相钢板（DP1200）和纯马氏体钢板（M1200）的极限拉深成形性能,采用扫描电子显微镜观察分析了钢板的微观组织形貌与结构,探讨了其微观组织结构与断裂机理的关系.结果表明：DP1200和M1200的极限拉延比分别为2.03和1.99,其单向拉伸性能不能准确地反映钢板材料在复杂应力状态下的成形性能;2种钢板的断裂模式均为断口分布着大量韧窝的韧性断裂,但其裂纹扩展机理不同.DP1200中的裂纹是以沿铁素体/马氏体相界为主,兼有穿过马氏体相并扩展的失效模式;而M1200钢中的裂纹是沿马氏体晶界扩展的失效模式.     

先进高强度双相钢汽车板剪切断裂实验

通过翻边实验和槽型件实验,研究了先进高强度双相(DP)钢在小半径拉弯成形中的断裂特性.翻边实验表明,高强度DP钢强度级别越高越容易发生剪切断裂.槽型件实验与仿真表明,高强度DP钢的断裂特性与压边力大小关系密切,压边力大则弯曲圆角处容易发生剪切断裂;压边力小则易在侧壁上发生颈缩断裂.先进高强度DP钢最终的断裂模式是剪切断裂和颈缩断裂相互竞争的结果,任一断裂条件先达到则板料发生该种断裂.     

Failure analysis of AZ31 magnesium alloy sheets based on the extended GTN damage model

Based on the Gurson-Tvergaard-Needleman (GTN) model and Hill’s quadratic anisotropic yield criterion, a combined experimental-numerical study on fracture initiation in the process of thermal stamping of Mg alloy AZ31 sheets was carried out. The aim is to predict the formability of thermal stamping of the Mg alloy sheets at different temperatures. The presented theoretical framework was implemented into a VUMAT subroutine for ABAQUS/EXPLICIT. Internal damage evolution due to void growth and coalescence developed at different temperatures in the Mg alloy sheets was observed by scanning electron microscopy (SEM). Moreover, the thermal effects on the void growth, coalescence, and fracture behavior of the Mg alloy sheets were analyzed by the extended GTN model and forming limit diagrams (FLD). Parameters employed in the GTN model were determined from tensile tests and numerical iterative computation. The distribution of major and minor principal strains in the specimens was determined from the numerical results. Therefore, the corresponding forming limit diagrams at different stress levels and temperatures were drawn. The comparison between the predicted forming limits and the experimental data shows a good agreement.     

一种针对颈缩现象的高强钢板材力学行为研究方法

为表征一种双相钢板材的塑性力学行为,对其进行了4种不同应变率下的单向拉伸试验,结果表明颈缩现象存在于各应变率下的试验中. 为解决颈缩现象中三向应力状态给常规公式计算所得强化曲线带来不可靠性的问题,使用了一种试验与仿真相结合的逆向方法. 首先在试验所得曲线的基础上判断颈缩起始点,并根据该点信息使用Ludwik准则对颈缩后曲线进行外插;再将外插曲线应用于仿真,通过仿真与试验结果的对比优化外插曲线. 将该方法应用于所研究的双相钢材料后,获得了能够准确表征单向拉伸过程的材料曲线.     

边部冲裁对双相钢拉伸性能影响的实验研究

为研究边部冲裁对双相钢(DP590、DP780和DP980)拉伸性能的影响,采用冲裁和线切割2种方法制备实验试样.基于MTS793材料试验机,使用非接触式视频应变测量方法,在常温下完成静拉伸实验.通过材料的力学性能曲线,分析原始标距对拉伸性能的影响以及不同断面质量下拉伸试样的力学性能响应、颈缩过程与断口形貌.结果表明:原始标距减小,基本不影响屈服强度、抗拉强度和最大力非比例伸长率,而断裂总伸长率显著增大;冲裁试样与线切割试样在拉伸颈缩段存在较大差别,断裂总伸长率低于线切割试样,且随着冲裁间隙的增大而减小;DP590和DP780冲裁试样在非均匀颈缩和横向裂纹出现后,呈现折线状断口,而DP980冲裁试样则与线切割试样的剪切滑移型断口类似,颈缩时产生显著的剪切带.     

应力-应变曲线形式对铝合金板料成形极限的影响

为使板料的成形性分析更符合实际,以Marciniak-Kuczynski(M-K)理论为基础,提出了采用应力-应变测量数据预测铝合金板料成形极限的方法.分别采用应力-应变测量数据、幂指数以及多项式拟合曲线三种应力-应变形式对6016-T4和7075-T6铝合金板料的成形极限进行研究.通过单向拉伸实验得到材料的应力-应变数据,构建了不同形式的曲线模型,计算了相应的极限应变,并绘制了板料成形极限曲线.将理论上预测的FLC与胀形实验结果进行对比,结果表明,采用应力-应变测量数据对板料成形极限的预测最好,而常用的幂指数拟合曲线得到的预测结果与实验存在一定的偏差.     

Effect of hot-dip galvanizing processes on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel

A C-Mn dual-phase steel was soaked at 800℃ for 90 s and then either rapidly cooled to 450℃ and held for 30 s (process A) or rapidly cooled to 350℃ and then reheated to 450℃ (process B) to simulate the hot-dip galvanizing process. The influence of the hot-dip galvanizing process on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel (DP600) was investigated using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile tests. The results showed that, in the case of process A, the microstructure of DP600 was composed of ferrite, martensite, and a small amount of bainite. The granular bainite was formed in the hot-dip galvanizing stage, and martensite islands were formed in the final cooling stage after hot-dip galvanizing. By contrast, in the case of process B, the microstructure of the DP600 was composed of ferrite, martensite, bainite, and cementite. In addition, compared with the yield strength (YS) of the DP600 annealed by process A, that for the DP600 annealed by process B increased by approximately 50 MPa because of the tempering of the martensite formed during rapid cooling. The work-hardening coefficient (n value) of the DP600 steel annealed by process B clearly decreased because the increase of the YS affected the computation result for the n value. However, the ultimate tensile strength (UTS) and elongation (A₈₀) of the DP600 annealed by process B exhibited less variation compared with those of the DP600 annealed by process A. Therefore, DP600 with excellent comprehensive mechanical properties (YS=362 MPa, UTS=638 MPa, A₈₀=24.3%, n=0.17) was obtained via process A.     

Effects of overaging temperature on the microstructure and properties of 600 MPa cold-rolled dual-phase steel

C–Mn steels prepared by annealing at 800°C for 120 s and overaging at 250–400°C were subjected to pre-straining (2%) and baking treatments (170°C for 20 min) to measure their bake-hardening (BH2) values. The effects of overaging temperature on the microstructure, mechanical properties, and BH2 behavior of 600 MPa cold-rolled dual-phase (DP) steel were investigated by optical microscopy, scanning electron microscopy, and tensile tests. The results indicated that the martensite morphology exhibited less variation when the DP steel was overaged at 250–350°C. However, when the DP steel was overaged at 400°C, numerous non-martensite and carbide particles formed and yield-point elongation was observed in the tensile curve. When the overaging temperature was increased from 250 to 400°C, the yield strength increased from 272 to 317 MPa, the tensile strength decreased from 643 to 574 MPa, and the elongation increased from 27.8% to 30.6%. Furthermore, with an increase in overaging temperature from 250 to 400°C, the BH2 value initially increases and then decreases. The maximum BH2 value of 83 MPa was observed for the specimen overaged at 350°C.