Optimization of friction stir welding parameters for improved corrosion resistance of AA2219 aluminum alloy joints

The aluminium alloy AA2219(Al—Cu—Mg alloy) is widely used in the fabrication of lightweight structures with high strength-to-weight ratio and good corrosion resistance.Welding is main fabrication method of AA2219 alloy for manufacturing various engineering components.Friction stir welding(FSW) is a recently developed solid state welding process to overcome the problems encountered in fusion welding.This process uses a non-consumable tool to generate frictional heat on the abutting surfaces.The welding parameters,such as tool pin profile,rotational speed,welding speed and axial force,play major role in determining the microstructure and corrosion resistance of welded joint.The main objective of this work is to develop a mathematical model to predict the corrosion resistance of friction stir welded AA2219 aluminium alloy by incorporating FSW process parameters.In this work a central composite design with four factors and five levels has been used to minimize the experimental conditions.Dynamic polarization testing was carried out to determine critical pitting potential in millivolt,which is a criteria for measuring corrosion resistance and the data was used in model.Further the response surface method(RSM) was used to develop the model.The developed mathematical model was optimized using the simulated annealing algorithm optimizing technique to maximize the corrosion resistance of the friction stir welded AA2219 aluminium alloy joints.     

Effects Of Subjective Factors On Second Language Learnig(L2L)

<正>In order to promote the activeness and initiative of learners and allow them to take theirlearning advantages,a teacher should be fully aware of the psychological process and models ofL2L．For no matter how well learners can command a language,their subjective incentives play adecisive role,that is the case particularly for non—Eniglish majors,who are always exposed tocrowded,serious and teacher—centred learning contexts.     

Interfacial reinforcement of core-shell HMX@energetic polymer composites featuring enhanced thermal and safety performance

The weak interface interaction and solid-solid phase transition have long been a conundrum for 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane(HMX)-based polymer-bonded explosives(PBX). A two-step strategy that involves the pretreatment of HMX to endow —OH groups on the surface via polyalcohol bonding agent modification and in situ coating with nitrate ester-containing polymer, was proposed to address the problem. Two types of energetic polyether—glycidyl azide polymer(GAP) and nitrate modified G...     

Cold Flow Experiments and Acoustic Investigation on Pressure Oscillation Induced by Flow Instability

Pressure oscillation in solid rocket motor is believed to be the results of the interaction between the flow instability and the acoustics of combustion chamber.Several reasonable and necessary hypothesizes are given to establish an equation to describe this coupling.A cold flow motor called CVS60D(corner vortex shedding 60°)was designed to study the flow-acoustic coupling based on theoretical analysis.Experimental investigations were carried out to determine the acoustics of CVS60D.Corner vortex shedding is generated at the backward facing step which is designed similar to the geometry of the motor with finocyl propellant after the burnout of its fins.A pintle was used to modify the velocity in the duct to change the frequency of vortex shedding.It is found that large amplitude pressure oscillation occurs when the pintle moves to a range of specific position,which indicates that the frequency of vortex shedding is close to one order of acoustic modes of combustion chamber.The amplitude of pressure oscillation changes as the pintle moves.     

The Two Techniques of O. Henry''''s

O.Henry is the founder of American modern short story, What makes his stories enjoy most popularity is his own styles——(1)Reasonable artistic exaggreration. (2)Unexpected ending. In this article I emphatically comment its value in the two stories: The Romance of a Busy Broker and The Last Leaf. Up to Now there is still much in Henry's method that we can make use of beneficially.     

当代军语快译通24

B allistic m issile——any m issile whichdoes notrely upon aerodynam ic surfaces to producelift and consequently follow s a ballistic trajectoryw hen thrustis term inated.弹道导弹不依靠空气动力表面产生升力,且在推力中断后沿一定惯性弹道飞行的导弹。B allistics——the science orartthatdeals with the m o-tion,behavior,appearance,orm odification ofm issiles orothervehicles acted upon by propellants,w ind,gravity,tem perature,orany otherm odifying substance,condition,orforce.弹道学研究处理基于推力、风…     

Al/Hf ratio-dependent mechanisms of microstructure and mechanical property of nearly fully dense Al—Hf reactive material

This study proposed three types of Al—Hf reactive materials with particle size ratios(a), which were almost completely dense(porosity of <5.40%) owing to their preparation using hot-pressing technology.Microstructure characteristics and phase composition were analyzed, and the influence of particle size ratios on dynamic mechanical behavior and damage mechanism were investigated. The prepared sample with a = 0.1 exhibited continuous wrapping of the Hf phase by the Al phase. Hf—Hf contact(cont...     

Ballistic performances of the hourglass lattice sandwich structures under high-velocity fragments

In this paper, the numerical simulation method is used to study the ballistic performances of hourglass lattice sandwich structures with the same mass under the vertical incidence of fragments. Attention is paid to elucidating the influences of rod cross-section dimensions, structure height, structure layer, and rod inclination angle on the deformation mode, ballistic performances, and ability to change the ballistic direction of fragments. The results show that the ballistic performances of hou...     

当代军语快译通（37）

column form a tion—(*)A formation inwhich elements are placed one behind the other.纵队队形一种一个人排在另一个人身后的队形。combat are a—A restricted area(air,land,orsea)that is established to prevent or minimizemutual interference between friendly forcesengaged in combat operations.战斗地区为使参战友邻部队互相不受干扰或相互干扰最小而划分的(空中、地面或海上)地区。c om ba t forces—those forces whoseprimary missions are to participate in combat.战斗部队基本任务为参战的部队。Com ba t inte lli…     

当代军语快译通

Base development——theacquisition,development,expansion,improvement,andconstruction and/or replacement of the facilities andresources of an area or location to support forcesemployed in military operations or deployed inaccordance with strategic plans.基地发展一个地方或地点的设施和资源的获取、发展、扩充、改进、构成和置换或用以支持在军事行动中使用的部队或按战略计划要求部署的部队。Ba s e of ope ra tions——an areaor facility from which a military force begins itsoffensive operation,to which it f…     

An Aid to Adults'learning English Words

<正>With the implementation of the policy of oponing to the outside,more and more people inChina have come to realize the importance of learnin and mastering a foreign 1anguage,English in particular,as an instrument of communicating with people from other countriesand.more often,of seeking for information contained in myriads of foreing publications soas to keep themselves posted in and keep up with the latest advences in the world.     

H_∞-Mixed Sensitivity Design of Roll Channel of Bank-to-Turn Autopilot for Small Diameter Bomb

According to requirements of the bank-to-turn (BTT) control for a small diameter bomb (SDB), the robust design problem for the roll autopilot was studied by H∞-mixed sensitivity control method. A roll channel dynamics model was established. Considering the couple between the yaw and roll channel as uncertain disturbance, the roll autopilot was designed using dual-loop scheme which takes a linear quadratic regulator (LQR) as inner-loop, to ensure the control effect of the certain part in model, and an H∞-mixed sensitivity control as outer-loop, to restrain coupling disturbance and strengthen the system's robust performance. The dynamic tracking performance and the robustness for the parameter disturbance of the roll controller were analyzed. The simulated results show that the roll control system functions better and robustly.     

strengthened so as to spur the postgraduate education to transform

ative expansion to qualitative improvement.Secondly,key construction plan of higher education,focusing on "hardware" as well as "software" construction,should be human-oriented.Thirdly,doctoral supervisors to instruct postgraduates should be encouraged.Fourthly,postgraduates should be encouraged to write dissertation in the light of their supervisors' research subjects.Fifthly,to eliminate the regional differences in postgra     

Words and Their Connotations

<正>A distinctiona is often made in talking about the meaning of a word between its denotation and its connotation;both of them concern the relation of a word to the world.I am going to considerone fundamental aspect about word meaning:the connotation of word.     

Reheat effect on the improvement in efficiency of the turbine driven by pulse detonation

Due to the strong unsteadiness of pulse detonation, large flow losses are generated when the detonation wave interacts with the turbine blades, resulting in low turbine efficiency. Considering that the flow losses are dissipated into the gas as heat energy, some of them can be recycled during the expansion process in subsequent stages by the reheat effect, which should be helpful to improve the detonationdriven turbine efficiency. Taking this into account, this paper developed a numerical model ...     

当代军语快译通(2)

access to classified information - Theability and opportunity to obtain knowledge of classified information. Persons have access to classified information if they are permitted to gain knowledge of the information or if they are in a place where they would be expected to gain such knowledge. Persons do not have access to classified information by being in a place where classified information is kept if security measures prevent them from gaining knowledge of the information.     

An impact sensitivity assessment method of spacecraft based on virtual exterior wall

The impact sensitivity assessment of spacecraft is to obtain the probability of spacecraft encountering the OD/M(orbital debris or meteoroid), which is a prerequisite for survivability assessment of on-orbit spacecraft. An impact sensitivity assessment method of spacecraft based on virtual exterior wall was proposed to improve the computational efficiency. This method eliminates determination of the outermost surface elements of the spacecraft before generating the debris rays, which are assumed...     

Fabrication and characterization of multi-scale coated boron powders with improved combustion performance: A brief review

Boron has high mass and volume calorific values, but it is difficult to ignite and has low combustion efficiency. This literature review summarizes the strategies that are used to solve the above-mentioned problems, which include coatings of boron by using fluoride compounds, energetic composites, metal fuels, and metal oxides. Coating techniques include recrystallization, dual-solvent, phase transfer, electrospinning, etc. As one of the effective coating agents, the fluorine compounds can react...     

当代军语快译通

A ssum ed azim uth——The as-sum ption of azim uth origins as a field expedientuntil the required data are available.假定方位角在得到准确数据之前,为方便战场行动而假设的方位角。A ssum ed grid——A grid constructed using anarbitrary scale superim posed on a m ap;chart,or photo-graph for use in point designation w ithout regard to actualgeographic location.假定方格在地图、图表或者照片上根据任意比例分层次建立的表格,其目的是淡化实际地理情况的制约,便于图上对点作业。A ssum ption——A supposition on …     

Tuning the particle size and morphology of high energetic material nanocrystals

Morphology controlled synthesis of nanoparticles of powerful high energetic compounds(HECs) such as l,3,5-trinitro-l,3,5-triazinane(RDX) and 1,3,5,7-tetranitro-l,3,5,7-tetrazocane(HMX) were achieved by a simple solvent—antisolvent interaction(SAI) method at 70 ℃.The effects of different solvents on particle size and morphology of the prepared nano-HECs were studied systematically.Particle size and morphology of the nano-HECs was characterized using field emission scanning electron microscopy(FE-SEM) imaging.X-ray diffraction(XRD) and Fourier transform infrared(FTIR) spectroscopy studies revealed that RDX and HMX were precipitated in their most stable polymorphic forms,i.e.a and P,respectively.Thermogravimetric analysis coupled with differential scanning calorimetry(TGA-DSC) studies showed that the thermal response of the nanoparticles was similar to the respective raw-HECs.HEC nanoparticles with spherical and rod shaped morphology were observed under different solvent conditions.The mean particle size also varied considerably with the use of different solvents.