On a plastic instability criterion for ultra-large radial-axial ring rolling process with four guide rolls

A dynamic mechanical model is proposed to describe the complexing actions of all the rolls on the ring during the ultra-large radial-axial ring rolling(RARR)process with four guide rolls.Based on the model,the calculation models for bending moment and normal stress at any section of the ring are deduced by force method.If the maximum section bending normal stress exceeds the yield stress of the ring materials,the ring will be distorted thus leading to the instability of the RARR process.According to this,a plastic instability criterion for the ultra-large RARR process with four guide rolls is developed,based on which a mathematical model to calculate the critical guide force for avoiding plastic instability of ring is obtained.The influence rule of the position of guide roll on the dangerous ring section of plastic instability is revealed,from which it is found the dangerous ring section mainly appears at the radial and axial deformation regions and the contact positions of the guide rolls and ring.The optimized layout of guide roll around the ring in favor of stability is determined to be about a1=61°and a2=119°.The plastic instability criterion is proven to be reliable from the aspects of the critical guide force,the section bending moment and normal stress and the dangerous ring section of plastic instability.Intelligent simulation case studies for the RARR process of ultra-large aluminum alloy ring indicate that the stable forming of the process can be effectively realized by regulating the guide force based on the plastic instability criterion.This work could provide a valuable guidance for the control of guide rolls and the optimization of the ultra-large RARR process with four guide rolls.

From n-alkane to polyacetylene on Cu(110):Linkage modulation in chain growth

Direct coupling or transformation of inert alkanes based on the selective C-H activation is of great importance for both chemistry and chemical engineering.Here,we report the coupling of polyenes that are transformed from n-dotriacontane(n-C_(22)H_(66))through on-surface cascade dehydrogenation on Cu(110)surface,leading to the formation of polyacetylene(PA).Three distinct linkages have been resolved by scanning tunneling microscope(STM)and noncontact atomic force microscope(nc-AFM).Apart from the α-type linkage which is the stemless coupling of the terminal C-C double bond in trans-configuration,β-and γ-type linkages appear as knots or defects which are,in fact,the C-C couplings in cis-configurations.Interestingly,the“defects”can be effectively suppressed by adjusting the surface coverage,thus making it of general interest for uniform structure modulation.

表面在位化学与液相化学反应的关联和差别

作为一门化学与物理交叉的新兴学科,表面在位化学(on-surface chemistry)自诞生以来一直处于表面科学的研究前沿.表面在位化学反应既依托于传统溶液反应,又与传统液相化学反应有着明显的区别.传统溶液反应中的诸多经验、理论在表面在位化学反应中依旧适用,很多经典的溶液反应也能在表面上得以实现.然而,利用表面限域效应可以引发一系列在传统溶液反应中难以实现的,具有高精度、高选择性的化学反应,这使得表面在位化学在功能性大分子设计、合成化学以及新材料制备中显示出十分诱人的潜能.本文总结了一些具有代表性的表面在位化学反应,通过比较其与传统溶液反应路径和产物上的差别,归纳了表面在位化学的特点,并对这种"新型"反应手段未来将会面临到的机遇与挑战做出展望.

A novel unified model predicting flow stress and grain size evolutions during hot working of non-uniform as-cast 42CrMo billets

The cast preformed forming process(CPFP) is increasingly considered and applied in the metal forming industries due to its short process, low cost, and environmental friendliness, especially in the aerospace field. However, how to establish a unified model of a non-uniform as-cast billet depicting the flow stress and microstructure evolution behaviors during hot working is the key to microstructure prediction and parameter optimization of the CPFP. In this work, hot compression tests are performed using a non-uniform as-cast 42 CrMo billet at 1123–1423 K and 0.01–1sà1. The effect laws of the non-uniform state of the as-cast billet with different initial grain sizes on the flow stress and microstructure are revealed deeply. Based on experimental results, a unified model of flow stress and grain size evolutions is developed by the internal variable modeling method. Verified results show that the model can well describe the responses of the flow stress and microstructure to deformation conditions and initial grain sizes. To further evaluate its reliability, the unified model is applied to FE simulation of the cast preformed ring rolling process.The predictions of the rolling force and grain size indicate that it could well describe the flow stress and microstructure evolutions during the process.

Spherical quantum dot in modified Kratzer-Coulomb potential:study of optical rectification coefficients

We consider the effects of quantum dot radius,confinement potential depth and controllable effective mass on the optical rectification coefficient(ORC)in spherical quantum dots,which is confined with Modified Kratzer-Coulomb Potential(MKCP).Using the Nikiforov-Uvarov method and compact density matrix theory,the ground state energy,ORC and wave function of electrons under the combined action of many factors are calculated.The results show that they affect the optical rectification response from different angles,including the position of peak and formant.

The second harmonic generation in GaAs/GaAlAs spherical quantum dots under Woods-Saxon plus attractive inversely quadratic potential

We examine the profile of second harmonic generation(SHG)for GaAs/GaAlAs spherical quantum dots(QDs)of Woods-Saxon(WS)plus attractive inversely quadratic(AIQ)potential under the joint influence of additional factors(pressure and temperature)and structural parameters(strengths and radius).The energies and wave functions in GaAs/GaAlAs spherical QDs under WS-AIQ limiting potential are calculated using the parametric Nikiforov-Uvarov(NU)method.Depending on the calculated energies and corresponding wave functions,the SHG coefficient is examined by the iterative procedure in the density matrix method for this system.Finally,the calculated results display that a strong SHG coefficient response,and red shift or blue shift energy can be acquired by adjusting parameters.