Effects of Pre-Strain on Bake Hardenability and Precipitation Behavior of Al-Mg-Si Automotive Body Sheets

The study investigates the effects of pre-strain on the bake hardenability and precipitation behavior of Al-Mg-Si automotive body sheets. The scanning electron microscopy, transmission electron microscopy, tensile test, Vickers hardness test, and differential scanning calorimetry were conducted for the purpose. It was found that the pre-strain treatment partially inhibits the natural aging hardening effect but cannot completely eliminate it. The pre-straining significantly enhances the bake hardening effect, with the 5% pre-strain sample showing the highest increase in yield strength and hardness. The formation of fine β" precipitates and dislocation structures contribute to the observed strengthening. Additionally, the study highlights the importance of optimizing pre-strain levels to achieve the best balance between strength and ductility in bake-hardened aluminum alloys.

A pre-strain strategy of current collectors for suppressing electrode debonding in lithium-ion batteries

The interfacial debonding between the active layer and the current collector has been recognized as a critical mechanism for battery fading,and thus has attracted great efforts focused on the related analyses.However,much still remains to be studied regarding practical methods for suppressing electrode debonding,especially from the perspective of mechanics.In this paper,a pre-strain strategy of current collectors to alleviate electrode debonding is proposed.An analytical model for a symmetric electrode with a deformable and limited-thickness current collector is developed to analyze the debonding behavior involving both a pre-strain of the current collector and an eigen-strain of the active layers.The results reveal that the well-designed pre-strain can significantly delay the debonding onset(by up to 100%)and considerably reduce the debonding size.The critical values of the pre-strain are identified,and the pre-strain design principles are also provided.Based on these findings,this work sheds light on the mechanical design to suppress electrode degradation.

Effects of high temperature pre-straining on natural aging and bake hardening response of Al-Mg-Si alloys

The influences of high temperature pre-straining (HT-PS) on the natural aging and bake hardening of Al?Mg?Si alloys were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. The results show that pre-straining at 170 °C immediately after quenching can effectively resolve the rather high T4 temper hardness caused by the conventional room temperature (RT) pre-straining treatment, and give a better bake hardening response (BHR) after paint-bake cycle. HT-PS 7% at 170 °C for 10 min is chosen as the optimum process as it provides lower T4 temper hardness and better BHR. The simultaneous introduction of dislocations and Cluster (2) can significantly suppress the natural aging and promote the precipitation of β″ phase, and reduce the effects of deformation hardening by dynamic recovery.

Effects of pre-strain and baking parameters on the microstructure and bake-hardening behavior of dual-phase steel

In a typical process, C-Mn steel was annealed at 800℃ for 180 s, and then cooled rapidly to obtain the ferrite-martensite microstructure. After pre-straining, the specimens were baked and the corresponding bake-hardening （BH） values were determined as a function of pre-strain, baking temperature, and baking time. The influences ofpre-strain, baking temperature and baking time on the microstructure evolution and bake-hardening behavior of the dual-phase steel were investigated systematically. It was found that the BH value apparently increased with an increase in pre-strain in the range from 0 to 1%; however, increasing pre-strain from 1% to 8% led to a decrease in the BH value. Furthermore, an increase in baking temperature favored a gradual improvement in the BH value because of the formation of Cottrell atmosphere and the precipitation of carbides in both the ferrite and martensite phases. The BH value reached a maximum of 110 MPa at a baking temperature of 300℃. Moreover, the BH value enhanced significantly with increasing baking time from 10 to 100 min.

Effect of pre-straining on structure and formation mechanism of precipitates in Al−Mg−Si−Cu alloy

The effect of pre-straining on the structure and formation mechanism of precipitates in an Al−Mg−Si−Cu alloy was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM).Elongated and string-like precipitates are formed along the dislocations in the pre-strained Al−Mg−Si−Cu alloy.The precipitates formed along the dislocations exhibit three features:non-periodic atomic arrangement within the precipitate;Cu segregation occurring at the precipitate/α(Al)interface;different orientations presented in one individual precipitate.Four different formation mechanisms of these heterogeneous precipitates were proposed as follows:elongated precipitates are formed independently in the dislocation;string-like precipitates are formed directly along the dislocations;different precipitates encounter to form string-like precipitates;precipitates are connected by other phases or solute enrichment regions.These different formation mechanisms are responsible for forming different atomic structures and morphologies of precipitates.

Transverse relaxation characteristic and stress relaxation model considering molecular chains of HTPB coating based on pre-strained thermal aging

In order to accurately describe the transverse relaxation characteristic and stress relaxation modulus of HTPB coating during pre-strain thermal aging process,a one month thermal aging test was carried out at70C with pre-strain of 0%,3%,6%and 9%,respectively.The low-field1 H NMR and stress relaxation modulus tests were carried out for HTPB coating at different aging stages.The stress relaxation model considering the molecular chains was proposed according to the changes of crosslinking chain and dangling chain of HTPB coating during pre-strain aging.The results showed that with the increase of aging time,the decay rate of transverse relaxation curve became faster,the transverse relaxation time decreased,the value of combined parameter q Mrl increased,the proportion of crosslinking chain decreased,while the proportion of dangling chain increased.Moreover,the stress relaxation modulus increased,the crosslinking network structure of HTPB coating became denser and the degree of crosslinking increased.At the initial aging stage,the pre-strain will destroy the crosslinking network structure of HTPB coating to a certain extent.With the increase of aging time,the effect of pre-strain will gradually weaken and the influence of aging on materials will gradually increase.The correlations between the stress relaxation model considering the molecular chains and the test results were more than 0.9950,which can accurately describe the stress relaxation modulus of HTPB coating during the pre-strain thermal aging process.

Effect of Pre-strain on the Transformation and Recovery Behaviours of a Ti_(44)Ni_(47)Nb_9 Shape Memory Alloy

The recovery strain, stress and transformation temperature of different pre-strained specimens of Ti44Ni47Nb9 were investigated by tensile test and electrical resistance measurement. The results indicated that pre-strain increases the reverse martensitic transformation temperature (A ' (s)) and hysteresis (A(s) - M-s). The recovery strain and stress are higher if the specimens are pre-strained between M-s and A(s) temperature than outside this temperature range. There exists an optimal pre-strain value, about 10%, at which the specimen exhibits maximum recovery strain and stress.

MARTENSITE AND REVERSE TRANSFORMATION IN PRE-STRAINED TiNi SHAPE MEMORY ALLOY THIN FILM

The effect of pre-strain on phase transformation of TiNi shape memory alloy film was studied by differential scanning calorimeter measurement (DSC). Compared with un-defarmed TiNi film, the reverse transformation of pre-strained specimens was elevated to a higher temperature on the first heating, but martensite and reverse transformation on subsequent thermal cycles occurred at a lower temperature. The evolution of transformation behavior in pre-strained TiNi film was related to the change of elastic strain energy, irreversible energy and internal stress field.

Effect of pre-strain on microstructural evolution during thermal exposure of single crystal superalloy CMSX-4

Compressive and tensile pre-strains at room temperature were induced in the second generation single crystal superalloy CMSX-4 fully heat treated.Both compressive and tensile pre-strain generated shear bands in the single crystal CMSX-4.Thermal exposure for 10 h at 950 ℃ on the pre-strained single crystal CMSX-4 developed preferential coarsening of γ＇ particles along the shear bands.γ＇ particles on the shear band gradually invaded γ channel.Finally,the γ channels along the shear band disappeared.TCP-like particles appeared at the expense of γ channel disappearance.However,thermal exposure for 10 h of the normal single crystal CMSX-4 did not develop TCP precipitation or preferential coarsening of γ＇ particles.Thermal exposure for 100 h of pre-strained CMSX-4 developed preferential coarsening of γ＇ and TCP phase particles along the shear bands as well as coarsening of γ＇ particles in the matrix.

Effect of pre-strain on fatigue crack growth of 2E12 aluminum alloy

The influences of pre-strain on the mechanical property and fatigue crack growth of 2E12 aluminum alloy were evaluated by SEM,TEM,mechanical property and fatigue tests. The axial fatigue tests were conducted under a constant amplitude sinusoidal wave loading at stress ratio of 0.1 in laboratory air and salt fog at room temperature. The results show that the yield stress of pre strain material is higher than that of the material without undergoing pre-strain,but pre-strain can not make the increase of the growth rate of fatigue crack. Fatigue crack growth rates of the alloy in salt fog are higher than those in air. The increased fatigue crack growth of the alloy in a given environment and more brittle striations can be observed in salt fog.

Improved creep aging response in Al-Cu alloy by applying pre-aging before pre-straining

The improvement of post-form properties without compromising creep formability has been a critical issue in creep age forming of aluminum alloy component. A pretreatment process incorporating artificial pre-aging at 165 °C for 6 h/12 h/24 h followed by pre-strain(3%–9%)has been developed. This method not only evidently improves the strength but also accelerates the creep deformation during creep aging of an Al-Cu alloy. A strength increase of 50 MPa with a slight decrease of ductility relative to the 9% pre-strained alloy is acquired in the alloy artificially pre-aged for 24 h regardless of the pre-strain level(3%–9%). Artificial aging for 24 h prior to 3%pre-strain enables an increase of creep strain by 30%. The creep strain in the alloy artificially preaged for 24 h and pre-strained by 6% is comparable to that in the alloy pre-strained by 9%. The strength and ductility in the alloy artificially pre-aged for 6 h/12 h and pre-strained by 3% are even slightly higher than those in the alloy purely pre-strained by 9%. The characterizations by transmission electron microscopy reveal that pre-aging at 165 °C could promote the accumulation of dislocations during pre-straining due to the pinning effect of pre-existing Guinier-Preston zones(GP zones)/θ’’phases and thus expedite the creep deformation in respect to the pure pre-straining treatment. The enhanced precipitation of θ’phases at these pinned dislocations contributes to the improved strength after creep aging. The results demonstrate applying artificial pre-aging before pre-straining is an efficient strategy to elevate the creep aging response in Al alloys.

Interactions between Pre-strain and Dislocation Structures and Its Effect on the Hydrogen Trapping Behaviors

This work investigated the effect of pre-strain and microstructures and their interactions on hydrogen trapping behaviors in case of 1-GPa high-strength martensitic steel Fe-0.05C-0.30Si-1.10Mn-3.50Ni-0.53Cr-0.50Mo-0.03 V(wt%).We found that the trapped reversible and trapped irreversible hydrogen contents increased significantly after applying a pre-strain of 5%,with an increase in the trapped reversible hydrogen content from 0.6 ppm in the original sample to 2.1 ppm.The hydrogen desorption activation energy also showed a slight increase.The microstructural evolution revealed that the concomitant dislocation cell-twin duplex microstructure with high-density tangled dislocations after pre-strain substantially increased the trapped reversible hydrogen contents.Additionally,the tangled dislocations pinned by the nanoprecipitates acted as deep irreversible hydrogen traps,increasing the trapped hydrogen at high temperatures after applying 5%pre-strain.These findings provide an expanded understanding of the hydrogen trapping behaviors of pre-strained microstructures.

预应变SMA⁃TMD对钢框架结构抗震性能影响研究

针对钢结构在地震激励下加速度响应过大的问题,介绍了一种将形状记忆合金(SMA)应用到调谐质量阻尼器(TMD)装置中的解决方案,利用SMA的特性实现有效的自复位和稳定的阻尼,将预应变SMA棒应用到TMD中,提高装置的阻尼性能。通过试验测试及有限元模拟,研究安装施加预应变SMA-TMD的钢框架抗震性能。结果表明:预应变SMA棒不仅有助于自复位,而且还能大大提高SMA-TMD的等效黏滞阻尼比;经过优化的SMA-TMD装置可以大大降低钢框架的地震动响应,并加快地震能量消耗。

DP900钢的成形性及预应变对烘烤硬化行为的影响

对汽车结构件用DP900钢的成形极限图进行了测试和研究,并运用数学模型进行了预测分析。利用内耗仪和透射电子显微镜从微观角度研究了预应变对烘烤硬化值的影响。结果表明:DP900钢室温下的力学性能为屈服强度604 MPa,抗拉强度935 MPa,均匀伸长率达到14%,断后伸长率为19%。冲压成形过程中,成形极限FLD0为0.245,基于Hollomon方程建立的成形极限数学模型的预测误差在0.2%~13.5%,平均误差为5.87%。预应变量的大小对BH值有显著的影响。随预应变增大,BH值先升高后降低,预应变为7%时达到最高值45 MPa。内耗峰值变化趋势与BH值一致。

预应变对S30408不锈钢力学性能和马氏体相变的影响

为了探究不同预应变量下S30408不锈钢力学性能的变化规律,课题组在室温下对S30408不锈钢开展不同预应变量下的单轴拉伸试验,分析了预应变对其力学性能及马氏体相变的影响规律,在此基础上建立了考虑预应变影响的修正Ludwigson模型,并结合基于Johnson-Cook的脆化模型合理预测预应变后材料力学性能。结果表明:随着预应变量的增加,S30408不锈钢的屈服强度明显增加,抗拉强度缓慢增加,断后延伸率降低;利用金相和X射线衍射试验获取不同预应变量下微观组织和析出相成分,发现在预应变量大于9%后,S30408不锈钢开始逐渐出现形变诱导的马氏体相变。课题组的研究实现了不同预应变量下的S30408不锈钢应力-应变关系的合理评价。

开挖前抽水条件下基坑围挡两侧非极限土压力计算模型

基坑开挖前的坑内抽水可引起围挡发生厘米级的侧移,但现行基坑设计理论仅提出了基于弹性支点法的土方开挖诱发围挡受力变形的计算方法,却并未提出如何计算基坑开挖前抽水引起的围挡侧移。若要沿用弹性支点法进行抽水侧移的计算,则如何确定抽水过程中围挡两侧土压力分布成为关键。为了提出基坑开挖前抽水条件下围挡两侧土压力的计算模型,首先提出了基坑开挖前抽水过程中围挡两侧土体受扰动区形态与范围;然后探明了扰动区内土体应变分布规律并提出土体应变分布模式;进一步地,建立了土体应变与围挡侧移的数学方程,并结合土体应力−应变方程最终构建了开挖前抽水条件下考虑基坑围挡侧移影响的土压力计算模型。所提出的土压力模型能够反映基坑开挖前抽水过程中围挡侧移对土压力的非线性影响,公式形式简单,参数取值方便,可为后续预测抽水引起的基坑围挡侧移提供重要基础。

预变形对2195铝锂合金拉伸力学性能的影响

通过硬度测试、室温拉伸测试、电子背散射衍射分析(EBSD)、透射电子显微分析(TEM)以及数字图像相关(DIC)等手段,研究不同拉伸预变形量对2195铝锂合金在155℃时效后拉伸力学性能的影响。结果表明:拉伸预变形量为2%、4%、6%、8%的合金经过155℃时效后,其抗拉强度分别为560.4 MPa、570.8 MPa、573.6 MPa、575.9 MPa。随着预变形量的增大,合金强度不断提高,这是由于位错是强化相T1相的有利形核点位,拉伸预变形量的增大使得位错密度增加,从而使得T1相数量密度增加。但随着拉伸预变形量的增大,拉伸预变形对合金的强度提升幅度逐渐减小,这主要是位错堆积缠结,析出相形核点重叠导致的。随着拉伸预变形量从2%增大到8%,合金的伸长率从9.6%下降到6.4%。通过DIC观察,拉伸预变形量的增大会使得拉伸过程中的应变集中现象提前出现,颈缩稳定性有所降低。这是由于较大的拉伸预变形量会使得合金在预变形阶段的应变分布不均匀,合金内出现破碎的变形组织晶粒。这些变形组织晶粒与周围其他晶粒组织的晶粒取向不同,更有利于微裂纹的萌生和扩展。4%的拉伸预变形量可以使2195铝锂合金达到最佳的强塑性匹配,此时抗拉强度为570.8 MPa、屈服强度为543.4 MPa、伸长率为8.7%。

Effects of Combined Pre-straining and Pre-aging on Natural Aging and Bakehardening Response of an Al-Mg-Si Alloy

For a series of Al-0.34／%Mg-1.05／%Si-0.08／%Cu alloys, the effects of pre-treatment on natural aging, bake hardening response and plasticity were investigated using Vickers hardness test and tensile test. Differential scanning calorimetry （DSC） analysis was conducted to reveal corresponding precipitation mechanisms. The results showed that pre-straining and pre-aging alone couldn＇t completely suppress natural aging and improve bake hardening response simultaneously. The sensitivity of the pre-straining and pre-aging on the mechanical properties was evaluated. It was found that pre-aging was the main factor which greatly improved bake hardening response and pre-treatment had a weak influence on plasticity in pre-treated samples. DSC analysis showed that the dissolution trough of clusters disappeared in pre-treated samples, both β”and β＇ precipitation peak shifted to lower temperature and were reduced in pre-strained and pre-aged samples. It was concluded that the formation of Mg/Si clusters was replaced by the accelerated precipitation ofβ” andβ＇ phases, which caused the suppression of natural aging and the improvement of bake hardening response （BHR）.

Dynamic cavitations in incompressible hyperelastic pre-strained circular sheets

A class of dynamic cavitations is examined for an isotropic incompressible hyperelastic circular sheet under a pre-strained state caused by an initially applied finite radial tension.The solutions that describe the radially symmetric motion of the pre-strained sheet are obtained.The conditions of cavitated bifurcation that describe cavity formation and motion with time at the axial line of the pre-strained sheet are proposed,that is to say,a circular cavity will form if the suddenly applied radial tensile load exceeds a certain critical value;dynamically,it is proved that the formed cavity will present a nonlinearly periodic oscillation,which is essentially different from the singular periodic oscillation of the formed cavity in an incompressible hyperelastic solid sphere.Numerical simulations show the effects of prescribed radial tension,material parameter and tensile load on critical ten-sile load describing cavity formation and periodic oscillation of the pre-strained circular sheet.

预变形对Q&P钢微观组织及烘烤硬化性能的影响

分析了Q&P钢在不同预变形(0%、3%、6%和9%)条件下的烘烤硬化行为。采用扫描电镜、X射线衍射仪(XRD)和电子背散射衍射(EBSD)等表征了试验钢在预变形和烘烤过程中的微观组织演变。结果表明:随着预变形量的增加,试验Q&P钢的烘烤硬化(BH)值先增加后减小,预变形量为6%时,BH值最大,为144 MPa。此外,在预变形较小时,晶粒尺寸是影响残留奥氏体稳定性的主要因素,马氏体转变优先发生在尺寸较大的残留奥氏体当中;随着预变形量的增加,大尺寸残留奥氏体消耗殆尽,在剩余残留奥氏体尺寸相差不大的情况下,其所在位置及其周围组织分布决定了残留奥氏体的稳定性。最后,烘烤过程中,会造成残留奥氏体数量的少量减少,这也会影响后续变形过程中相变诱导塑性效应的发生,不利于材料的整体塑性。