Enhancement of Strength–Ductility Balance of the Laser Melting Deposited 12CrNi2 Alloy Steel Via Multi-step Quenching Treatment

A ferrite–austenite 12CrNi2 alloy steel additively manufactured by laser melting deposition(LMD) was heat treated by direct quenching(DQ) and tempering inter-critical quenching(TIQ) at 800 ℃ for enhancing its strength–ductility balance. Both heat-treated alloy steels have the martensite–ferrite dual-phase(DP) microstructures. The volume fractions of martensite in the two treated alloy steels are nearly similar(～ 85 vol%), while the sizes of the prior austenitic grain for martensite are different. The martensite-dominated DP microstructure resulted in an obvious improvement in strength–ductility balance of the alloy steel. Compared with the DQ treatment, the multi-step TIQ treatment caused the strength–ductility balance of the alloy steel to be enhanced due to its higher total elongation. The better ductility of the TIQ-treated alloy steel can be attributed to the optimization of the microstructure. The preferred orientation of ferritic grain in the as-deposited alloy steel which was adverse to plastic deformation through dislocation slip was eliminated via the multi-step TIQ treatment. Moreover, the TIQ treatment promoted the formation of finer-grained martensite with larger areas of grain boundaries and twinning boundaries which resulted in the enhancement of the coordinated deformability of the martensite with the ferrite.     

Axial plastic collapse behavior of plates

In the present study, the collapse behavior of a plate subjected to axial compression is investigated parametrically using the finite element method. It is shown that the axial collapse of the plate, which has various hardening characteristics, is controlled by two dominant factors: the stress limitation of the material and the limitation of in-plane lateral deformation at the side edges of the plate. Then, a simplified collapse condition, which corresponds to two collapse types, is derived based on the effective width concept and the plastic theory, and the validity of the collapse condition is then verified by the comparison of numerical results obtained under various material and geometric properties. Furthermore, a method for predicting the collapse stress is proposed based on the present collapse condition.     

Micromechanical modeling of the work-hardening behavior of single- and dual-phase steels under two-stage loading paths

Work-hardening behavior of single-phase steel and dual-phase steel which is made of hard martensite surrounded by soft ferrite is analyzed by using an elastoplastic crystal plasticity model in conjunction with the incremental self-consistent model. Two-stage loading paths consisting of uniaxial tension, unloading and subsequent uniaxial tension/compression for various directions are applied. Bauschinger effect and transitional re-yielding behavior, which depends on the direction of the second loading path, are predicted and analyzed with respect to the distribution of the residual resolved shear stresses within the material. These features, which are caused by the inhomogeneity of the residual stress field, are especially pronounced in the case of the dual-phase steel because of the strong mechanical contrast between ferrite and martensite phases.     

7050-T7451铝合金干切削表面完整性研究

旨在探究干式切削条件下切削参数对7050-T7451铝合金表面完整性的影响规律,基于单因素面铣削实验,得到了切削参数对切削力、工件表面形貌、加工硬化和残余应力的影响规律。结果表明:切削三要素对切削力和工件表面粗糙度有着明显的影响,在切削速度较低时,X向切削力略微增大,而切削速度由500 m/min变化到1000 m/min时,X向切削力逐渐较小,随后呈增大的变化趋势,切削力与切削深度、进给量呈正相关关系。较高的切削速度和较小的进给量可以改善表面粗糙度,切削深度对表面粗糙度影响较小;加工硬化随切削速度与进给量的增大呈先增大后减小的变化趋势,而加工硬化程度与切削深度呈负相关关系;残余应力随切削参数的改变呈“勺”形分布,切削速度与进给量对残余应力的影响较大,且表层残余压应力的最大值基本在0.05~0.2 mm,而亚表层残余拉应力最大值在0.25~0.4 mm。     

工件表面层加工硬化深度的数值分析

根据金属塑性变形理论，提出了在低速正交金属切削中工件表面层加工硬化深度的预报准则。讨论了切削变形区内的弹塑性应力场，建立了剪切平面边界载荷的分布方程，并通过有限元法模拟已加工表面的形成过程，计算了工件表面层的应力和硬化深度。数值分析与试验研究相一致。     

固着磨料高速研磨工件表面硬化规律的研究

为研究固着磨料高速研磨工件表面硬度的变化规律,对加工零件的硬化现象与研磨工艺参数之间的关系进行了研究。结果表明,磨料粒度的增加,并未引起工件表面硬化程度的明显变化;随着研磨压力和速度的增加,工件表面硬化程度变大。分析认为,研磨后工件表面发生位错增殖,从而导致位错密度增加;同时由于位错运动导致的位错交割,是导致工件表面硬化的主要原因。该工艺具有节约磨料、环境污染小、加工工艺简单、加工质量好、加工效率高等优点。     

Effects of age heat treatment and thermomechanical processing on microstructure and mechanical behavior of LAZ1010 Mg alloy

An Mg–Li–Al–Zn (designated as LAZ1010) alloy containing about 10 wt% of Li has been prepared by melting and solidification in a carbon steel crucible, and extruded at a billet preheating temperature of 200 °C with an extrusion ratio of approximately 29. Effects of age heat treatments and thermomechanical processing on microstructures and mechanical properties were performed in this study. Hardness, optical microscopy, X-ray diffraction studies, and tensile testes were carried out to explore the variations in microstructures and mechanical behaviors during processing. The results showed that LAZ1010 alloy presented age hardening effect at temperatures below 50 °C. Rapid decrease in hardness with aging temperature at intermediate temperatures should be resulted from the transformation of θ phase into the equilibrium phase AlLi. Kocks–Mecking type plots were used to illustrate different stages of work hardening of the cold rolled specimens. The results indicated that cold rolled LAZ1010 alloy showed stage III and stage IV work hardening behaviors.     

板的轴向塑性破坏性能研究

采用有限元法对板在轴压作用下的破坏性能进行了分析。在板的轴向破坏中,部分硬化参数由两个主要因素控制:材料的极限应力和板边缘的极限平面内侧向变形。然后,基于有效宽度理论和有效板理论,对两类破坏形式推导出一个简化的破坏条件,并且通过与各种材料和几何特性下的数值分析结果进行对比验证了这个破坏条件的有效性。此外,基于本文所提议的破坏条件提出了一个预测极限应力的方法。     

Application of method of fundamental solutions for elasto-plastic torsion of prismatic rods

In this paper torsion of prismatic bars considering elastic-plastic material behavior is studied. Based on the Saint-Venant displacement assumption and deformation theory of plasticity for stress-strain relation the boundary value non-linear problem for stress function is formulated. The purpose of our paper is application of method fundamental solution (MFS) and radial basic function (RBF) for solution of this problem. The non-linear torsion problem in plastic region is solved by means of the Picard iteration. Proposed algorithm is based on solution of the linear Poisson equation on each iteration steps.     

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy was studied. The results show that there are obvious yield points on true stress–true strain curves of annealing structures, then a stress drop occurs. The curves show linear work-softening after yielding at annealing temperature of 720–780 °C and linear work-hardening at annealing temperature of 800–840 °C. Elastic energy stored in the α-phase is dramatically released after plastic deformation of the β-phase, which leads to the stress drop.