长轴类零件开式挤压工艺应用及成形极限研究

长轴类零件开式挤压失稳包括弹性失稳和塑性失稳。将压杆失稳原理应用于开式挤压,得到了开式挤压弹性失稳的临界长径比及临界挤压应力的计算方法,建立了开式挤压工艺弹性稳定性校核模型,提出了提高抵抗弹性失稳能力的措施,如降低挤压应力、采用夹持器支承等方法。分析塑性失稳的原因,是零件的传力段的应力达到材料的屈服应力,而在入模口处出现局部镦粗。用上限法计算了圆截面件开式挤压应力,并绘制成可方便查阅的线图。提出了挤压变形程度、凹模半锥角、润滑状态及摩擦因子等是影响塑性失稳成形极限的主要因素,并结合计算线图给出了各影响因素的合理范围。以汽车传动花键轴开式挤压为例,应用失稳理论对挤压工艺参数的合理制定进行指导。     

Analysis of plastic instability during sandwich sheet rolling

In this study, plastic instability occurring in the hard inner layer during symmetrical sandwich sheet rolling was investigated. Two criteria for diffusion necking and local necking under a tri-axial stress state were developed to predict the occurrence of plastic instability in the hard inner layer during sandwich sheet rolling. Stress states in the inner hard layer at the exit of the roll-gap, where plastic instability is most likely to occur, were employed to formulate the plastic instability criteria. The effects of various rolling conditions such as the thickness ratio of sheets, yield shear stress ratio of sheets, etc. upon the critical thickness reduction, under which plastic instability can be avoided, were discussed systematically. Furthermore, experiments on sandwich sheet rolling were also conducted by employing aluminum and mild steel as layers of sandwich sheets. It was found that the theoretical predictions of the critical reduction were in good agreement with the experimental measurements. Throughout the study, it is clear that the proposed plastic instability criteria are able to offer useful knowledge in designing the pass-schedule of sandwich sheet rolling processes.     

Tube joining by asymmetric plastic instability

This paper explores inclined, out-of-plane, plastic instability waves in thin-walled tubes subjected to axial compression and the factors affecting its development and propagation. Explanations are given for the experimental observations and finite element predictions of plastic flow and compression load in a wide range of process parameters. Basic design guidelines are provided, making use of plastic instability waves to produce tube attachments in situations where the axis of the branch tube is inclined to the axis of the main body tube.The overall content of the paper widens and enhances previous work published by the authors in perpendicular tube attachments by providing a complete characterization of the process feasibility window for inclined tube attachments. The new developments in asymmetric plastic instability are a step forward in tube joining because their offer an effective and environmentally friendly solution for producing complex tube attachments in situations where there are little or no-alternatives of utilizing conventional processes involving welding and brazing.     

方形轧件在椭圆孔的塑性失稳分析

针对35CrMo合金冷锻钢粗轧过程出现的表面褶皱，采用非线性有限元分析方法， 分析不同鼓形率轧件在椭圆孔中的轧制过程，从变形的角度分析了轧件表面由于塑性失稳产生褶皱的原因及位置。结果表明，方形轧件在椭圆孔中轧制时，轧件角部区域塑性变形剧烈，塑性应变能密度分布出现峰值，最易产生塑性失稳而形成褶皱。增大方形轧件的鼓形率，可减小塑性应变能密度分布峰值，降低轧件表面发生褶皱缺陷的概率。     

Plastic instability in polycrystalline metals after low temperature irradiation

The dose dependence of plastic instability behavior has been investigated for polycrystalline metals after neutron irradiation at low temperatures (<200 °C). The analyzed materials consist of 10 body-centered cubic (bcc), 7 face-centered cubic (fcc), and 2 hexagonal close packed (hcp) metals. It was found that the metals after irradiation showed necking at yield when the yield stress exceeded the true plastic instability stress, σ_IS, for the unirradiated material. It was also shown that σ_IS was almost independent of dose below a critical dose. The critical dose is called the dose to plastic instability at yield, D_C, because at higher doses the material shows necking at yield. The D_C values ranged from 0.002 to 0.2 dpa for bcc and hcp metals, except for a high purity iron, that had a D_C value of 6 dpa; whereas the fcc metals gave generally high values ranging from 0.1 to 40 dpa. It is attempted to explain the dose independence of the plastic instability stress by a straightforward shifting of tensile curves by the appropriate strain corresponding to the radiation-induced increase in yield stress. The dose independence of strain-hardening behavior suggests that radiation-induced defects and deformation-produced dislocations give similar net strain-hardening effects.     

5052铝合金板材磁脉冲动态拉伸塑性失稳分析

为揭示磁脉冲成形的增塑机制,采用理论分析与微观组织观察相结合的方法对5052铝合金板材磁脉冲动态拉伸过程中动态成形行为和塑性失稳机制进行了系统研究.结果表明,惯性力在动态成形中起主要作用,惯性力对试样的结构失稳具有抑制作用,从而使试样的塑性提高并产生分散失稳;5052铝合金动态成形和准静态成形的成形性质相似,不会产生特殊的组织结构,塑性变形机制均为位错滑移机制;准静态成形过程以均匀单系位错滑移为主,断裂伴随着位错的缠结和攀移;而动态成形过程中,位错滑移趋于多系开动,在大面积区域出现明显的交滑移现象,且滑移带较准静态成形时窄且密,位错组态更均匀;动态成形的多系滑移和位错均化作用可在比准静态成形高的多的塑性应变水平下形成,从而使材料表现出较高的塑性和强度.     

INVESTIGATION OF THE EFFECT OF TEMPERATURES ON TENSILE LOAD DROPS OF TITANIUM ALLOY

1.IntroductionLowtemperaturesmaysignificantlyaffectthemechanica1propertiesofmaterials-Basedonthelowtemperaturestrellgth-toughnesscharacteristics,cry0genicstructuralmaterial8weredividedint0threeclassesbyMorrisll].Thefirstclassincludesmaterialsthatundergoaductile-brittletransitionatlowtemperatures,thesecondclassthatremainsductileatalltemperaturesandthethirdthemetastablealloyswhoseductilityandtoughnessarelargelydeterminedbylowtemperaturephaJsetransf0rmations.Severalillterestingmechanicalphenomen…     

Effect of deformation geometry on the intermittent plastic flow associated with the Portevin–Le Chatelier effect

The unstable plastic flow caused by the Portevin–Le Chatelier effect is studied experimentally. Non-local effects due to strain heterogeneity are addressed by comparing the statistical and multifractal properties of tension and compression deformation curves of an AlMg alloy. Power-law statistical distributions and multifractal scaling are found in certain experimental conditions, and provide a quantitative characterization of the plastic-flow intermittency. The analysis reveals distinct temporal patterns associated with stress serrations in tension and compression, reflecting distinct modes of the collective dislocation dynamics. In contrast, the onset of instability follows similar relationships in both loading conditions. This similarity justifies the conventional approach to modeling the loss of stability of a uniform plastic flow as a local property, governed by the homogenized behavior of the dislocation ensemble.     

气体温度对冷喷涂Ni粒子结合与变形行为的影响

利用SEM对冷喷涂工艺中不同气体温度下Ni粒子碰撞Cu合金基板后的结合及变形形貌进行了表征.结果表明,随着气体温度的升高,粒子扁平率呈渐缓趋势增加,射流面积增大,粒子的结合率提高.变形粒子内存在剪切失稳薄层、强塑性变形区和低塑性变形区等3个变形程度不同的区域.粒子与基板的结合质量主要受剪切失稳薄层控制,而变形后粒子的形状主要受塑性变形区控制.证实了绝热剪切失稳是Ni粒子与Cu合金基板结合的主要机制.     

Comprehensive study of aluminium alloy plates under compression conditions

Aluminium alloys have been vital for the progress of many technologies during the last decades. In particular, most stiffeners in aerospace structures are composed of aluminium panels often solicited with elastic and plastic bucking under particular boundary and loading conditions. The purpose of this paper is to enhance the difficulties encountered to predict the incipient elastic–plastic buckling for thin aluminium plates under combined compressive loads.The used methodology was an analytic non-linear approach, validated further with an experimental investigation. In fact, the instability of thin elastic–plastic rectangular panels made of 2024 T45 alloys is analyzed. In this study, general concept of the Von Kaman's equation with a set of trigonometric and harmonic functions was used in the analytic model. A methodology to develop this kind of analytic resolution is pointed out and has been illustrated for a set of variables. This paper presents a stable and low-cost analytic solution to deal with instability phenomenon in elastic and plastic range for the design of light alloy aluminium plates. This approach was applied to assess the conditions for which plastic buckling phenomenon can happen when particularly thin aluminium panels are used. This latter, can be implemented in finite element standard codes.