大型三塔斜拉桥铝合金试验模型的研制

介绍了研制的岳阳洞庭湖大桥 (三塔双索面 PC斜拉桥 )全桥静动力试验模型 ,重点介绍了该模型的结构设计及关键工艺。该特大型铝合金试验模型在国内外实属罕见 ,制造难度较大 ,目前已顺利完成了全部静动力试验 ,取得了令人满意的结果。

基于ABAQUS和FRANC3D的铝合金裂纹扩展试验研究

本文以铝合金试样作为研究对象,采用有限元仿真软件ABAQUS与裂纹扩展软件FRANC3D相结合的方法,进行铝合金的疲劳裂纹扩展试验仿真。首先,通过疲劳裂纹扩展试验及光学显微镜的裂纹形貌研究,获得疲劳裂纹扩展次数及裂纹扩展形貌;其次在有限元仿真平台ABAQUS中建立试验静力学模型;最后基于裂纹扩展软件FRANC3D预制初始裂纹,采用奇异性网格划分,并采用最大周向力准则进行裂纹扩展仿真,以及借助Paris公式计算剩余寿命。结果发现:基于ABAQUS和FRANC3D的铝合金裂纹扩展试验仿真与现实裂纹扩展试验结果有很好的一致性;并通过计算应力强度因子判别此裂纹扩展为I型(张开型)裂纹,从而得到裂纹更清晰有效的演化过程。

双层桥面三桁刚性悬索加劲钢桁梁桥全桥试验模型

为了研究中国首座双层桥面三桁刚性悬索加劲钢桁梁桥——东江大桥的空间受力行为,并检验施工方案,针对其结构独特、受力复杂、施工难度大的特点,进行了全桥缩尺模型试验。详细介绍了东江大桥模型试验的目的、模型总体构造、截面设计原则、连接构造、施工过程模拟、支座设计以及试验加载等。从模型设计可以看出:该模型制造难度较大;研制的模型能够满足利用试验模拟施工全过程以及对结构局部进行测试的要求。

Mg-Nd-Gd-Zn-Zr镁合金铸造组织与力学性能

采用正交试验方法,通过砂型铸造制备9种不同成分的Mg-Nd-Gd-Zn-Zr系镁合金。研究该系列镁合金的铸造组织和室温力学性能,并通过对力学性能试验数据的分析,研究主要合金化稀土元素Gd和Nd的作用。研究发现:该系列镁合金铸态组织为α-Mg基体和Mg12Nd化合物。经过固溶处理后,铸态组织中晶界上的化合物大部分溶入基体,但在晶界上还有一些颗粒状的化合物。Gd含量越高,合金的室温抗拉强度、屈服强度和延伸率就越高。Nd含量越高,抗拉强度和屈服强度也越好,但延伸率在Nd含量超过2水平(2.85%)后会降低。抗拉强度和屈服强度受Nd含量的影响最大,Gd含量的影响次之。Zn含量越高屈服强度越高,但抗拉强度和延伸率降低,其中延伸率受Zn含量的影响最大。

Effect of stacking fault energy on mechanical properties of ultrafine-grain Cu and Cu-Al alloy processed by cold-rolling

Cu,Cu-2.2%Al and Cu-4.5%Al with stacking fault energies（SFE） of 78,35 and 7 mJ/m2 respectively were processed by cold-rolling（CR） at liquid nitrogen temperature（77 K） after hot-rolling.X-ray diffraction measurements indicate that a decrease in SFE leads to a decrease in crystallite size but increase in microstrain,dislocation and twin densities of the CR processed samples.Tensile tests at room temperature indicate that as the stacking fault energy decreases,the strength and ductility increase.The results indicate that decreasing stacking fault energy is an optimum method to improve the ductility without loss of strength.

CUTTING FORCES FOR HIGH-SPEED DRILLING OF COMPOSITES

The thrust and the torque of various carbide drills are studied for the high-speed drilling of fiber reinforced epoxy composites. The orthogonal experiment is carried out with different feed speeds at high rotation speed. Experimental results show that the spindle rotation speed is the most influential factor. The thrust andthe torque decrease under the condition of high rotation rate. With the decrease of the feed speed, the thrust and the torque decrease. But the effect of the feed speed is less than that of the spindle rotation rate. Moreover, the effect of drill materials on the thrust and the torque is more notable than that of the drill geometries and the feed speed. The thrust is greatly affected by the feed speed while the torque is obviously affected by drill geometries.

IMPACT-SLIDING WEAR OF MATERIALS

A simple impact-sliding wear test rig is designed for studying the wear behavior between solid materials on a repetitive normal impact accompanied with the tangential sliding. The test rig consists of a cantilever beam forced by the dynamoelectric vibration exciter and a rotational shaft driven by a spindle. It has a widely adjustable range of testing parameters, including the impact frequency, the impact load and the sliding velocity. The avail- able maximum impact frequency, impact load and sliding velocity are 100 Hz, 200 N and 4.52 m/s, respectively. To evaluate the capability of the test rig, tests are carried out and the impact load is measured. Results show that the test rig has the good repeatability under the same test conditions and the repeatable error is less than 7%. Furthermore, non-destructive examination results by the mass loss method, two-dimensional profilometry and the scanning electron microscopy （SEM） show that the test rig can meet the demands for the impact-sliding wear.

Effects of additives on corrosion and wear resistance of micro-arc oxidation coatings on TiAl alloy

Ceramic coating was deposited on TiAl alloy substrate by micro-arc oxidation(MAO)in a silicate-aluminate electrolyte solution with additives including sodium citrate,graphite and sodium tungstate.The microstructures and compositions were analyzed by SEM,EDX and XRD.The corrosion and wear properties of the coatings were investigated by potentiodynamic polarization and ball-on-disc wear test,respectively.The results show that the MAO coatings consist of WO3,Ti2O3,graphite and Al2O3 besides Al2TiO5 and Al2SiO5.With additives in the electrolyte,the working voltage at the micro-arc discharge stage decreases,and the ceramic coating gets smoother and more compact.The corrosion current density of MAO coating is much lower than that of TiAl substrate.It can be reduced from 9.81×10-8A/cm 2to 3.02×10-10A/cm 2 .The MAO coatings composed of hard Al2O3,WO3 and Ti2O3 obviously improve the wear resistance of TiAl alloy.The wear rate is-3.27×10-7g/(N·m).

New extrusion process of Mg alloy automobile wheels

The recent research and development of forged magnesium road wheel were reviewed.Methods of flow-forming,spin forging of manufacturing a forged magnesium alloy wheel were introduced.A new extrusion method was investigated especially. Extrusion from hollow billet was proposed in order to enhance the strength of spoke portion and reduce the maximum forming load. By means of the developed technique,the one-piece Mg wheels were produced successfully by extrusion from AZ80+alloy.At the same time,the existing problems on the research and development of forged magnesium road wheel were analyzed.The impact testing,radial fatigue testing and bending fatigue testing results show that AZ80+wheel can meet application requirement in automobile industry.

Warm hydroforming of magnesium alloy tube with large expansion ratio

Process of warm tube hydroforming was experimentally investigated for forming an AZ31B magnesium alloy tubular part with a large expansion ratio. Effects of temperature on the mechanical properties and formability were studied by uniaxial tensile test and hydraulic bulge test. Total elongation increases with temperature up to 250℃, but uniform elongation and maximum expansion ratio get the highest value at 175℃. Different axial feeding amounts were applied in experiments to determine the reasonable loading path. A preform with useful wrinkles was then realized and the tubular part with an expansion ratio of 50% was formed. Finally, mechanical condition to produce useful wrinkles is deduced and the result illustrates that useful wrinkles are easier to be obtained for tube with higher strain hardening coefficient value and tubular part with smaller expansion ratio.

Microstructural evolution and creep properties of Mg-4Sn alloys by addition of calcium up to 4 wt.%

The effects of addition of calcium up to 4 wt.% on the microstructure and creep properties of Mg-4 Sn alloys were investigated by the impression creep test. Impression creep tests were performed in temperature range between 445 and 475 K under normalized stresses σ/G(where σ is the stress;G is the shear modulus) between 0.0225 and 0.035. Optical microscopy and scanning electron microscopy were used to study the microstructure of samples. It is observed that the addition of Ca more than 2 wt.% suppresses less stable Mg Sn2 phase, and instead forms more thermally stable phases of Ca-Mg-Sn and Mg2 Ca at the grain boundaries which improve the creep resistance of Mg-4 Sn alloys. According to the stress exponents(6.04<n<6.89) and activation energies(101.37 k J/mol<Q<113.8 k J/mol) which were obtained from the impression creep tests, it is concluded that the pipe diffusion climb controlled dislocation creep is the dominant creep mechanism.

Influence of process parameters on deep drawing of AA6111 aluminum alloy at elevated temperatures

To gain a deep insight into the hot drawing process of aluminum alloy sheet, simulations of cylindrical cup drawing at elevated temperatures were carried out with experimental validation. The influence of four important process parameters, namely,punch velocity, blank holder force(BHF), friction coefficient and initial forming temperature of blank on drawing characteristics(i.e.minimum thickness and thickness deviation) was investigated with the help of design of experiments(DOE), analysis of variance(ANOVA) and analysis of mean(ANOM). Based on the results of ANOVA, it is shown that the blank holder force has the greatest influence on minimum thickness. The importance of punch velocity for thickness deviation is 44.35% followed by BHF of 24.88%,friction coefficient of 15.77% and initial forming temperature of blank of 14.995%. After determining the significance of each factor on forming characteristics, how the individual parameter affects characteristics was further analyzed by ANOM.

Effect of tool plunge depth on joint formation and mechanical performance of friction stir forming joints made between AA 5052-H32 and AA 6061-T6 sheet metals

The potential of friction stir forming for joining dissimilar grades of aluminum alloys namely,AA 5052-H32 and AA 6061-T6,was investigated.Study on the effect of tool plunge depth revealed that,lap shear load of 7.16 kN and cross-tensile load of 3.51 kN,recorded at medium tool plunge depth range from 0.5 to 0.7 mm,measured using a universal testing machine,were much larger than those of friction stir welded and friction stir spot welded joints fabricated on the same materials.Joint macrostructure observed with optical microscope revealed that joints were strengthened either by mechanical pin interlocking or by metallurgical bonding.The effect of tool plunge depth on the stir zone formation and the influence of frictional heat flux on the lower sheet were revealed through the microhardness measurement using Vickers hardness tester.Morphological studies revealed that tool plunge depth has a significant influence on the pin formation and the geometric features,generated in these joints.Occurrence of various failure modes such as pin pull-out,pin shear,partial bond delamination,and tear-off,were governed by the formation of critical weak zones at various tool plunge depths.

Formability analysis of dissimilar tailor welded blanks welded with different tool pin profiles

An attempt has been made to study the tailor welded blanks （TWBs） made by friction stir welding （FSW） with five different tool pin profiles. The formability of welded blanks was evaluated at constant FSW process parameters. It has been found that the welded blanks with square pin profile tool produced better formability properties as compared to other tool pin profiles. It was found that the pulsating action of the square pin profile is said to be the reason for this improvement. A microstructural evaluation performed on the blanks revealed that the stir zone created by taper cylindrical and stepped cylindrical tools was less homogeneous than that of the square tool, resulting in less formability. The limiting dome height （LDH） of square pin profile tool was found to be 14 mm while those of taper cylindrical and stepped cylindrical tools are 12.5 mm and 12.4 mm, respectively.

Effect of extrusion on properties of Al-based composite

An aluminium alloy and its composite with dispersed SiC particles made by liquid metallurgy route were extruded under optimized conditions.The properties were characterized in terms of microstructure,hardness and sliding wear behaviour and then compared between the extruded and cast alloys and composites,in order to understand the benefits of composite and extrusion on the alloy.It was observed that composites drastically increased the hardness and the extruded composites further increased this value.The advantage of composites was realized in sliding wear tests.

Hot deformation behavior of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy with acicular microstructure

The characteristics of hot deformation of an α＋β titanium alloy Ti-6.5Al-3.5Mo-1.5Zr-0.3Si with acicular microstructure were studied using isothermal hot compressive tests in a strain rate range of 0.01-10 s^-1 at 860-1 100 ℃. The true stress-tree strain curves of alloy hot-compressed in the α＋β region exhibit a peak stress followed by continuous flow softening; whereas in the β region, the flow stress attains a steady-state regime. At a strain rate of 10 s^-1 and in a wide temperature range, the alloy exhibits plastic flow instability. According to the kinetic rate equation, the apparent activation energies are estimated to be about 633 kJ/mol in the α＋β region and 281 kJ/mol in the β region, respectively. The processing maps show a domain of the globularization process of a colony structure and α dynamic recrystallization in the temperature range of 860-960 ℃ with a peak efficiency of about 60%, and a domain of β dynamic recrystallization in the β region with a peak efficiency of 80%.

Void damage evolution of LF6 aluminum alloy welded joints under external load and thermal cycling conditions

Based on the simulated aerospace thermal cycling tests,the effect of thermal cycle on the void damage evolution mechanism of LF6 aluminum alloy welded joint was investigated.The results show that micro-voids form around the second phase particles under the thermal cycling tests.The thermal stress coupled with external stress leads to dislocations pile-up around the particles,and when the dislocation density reaches a certain degree,the stress concentration will exceed the bonding strength at the interface between particles and matrix,resulting in the formation of micro-cracks.The numerical simulation is successfully implemented with the finite element to describe the void damage evolution of the welded joint under thermal cycling conditions.

Deformation Behavior of Hot Isostatic Pressing FGH96 Superalloy

The deformation behavior of hot isostatic pressing （HIP） FGH96 superalloy was characterized in the temperature range of 1000-1100 ℃ and strain rate range of 0. 001-0. 1 s^-1 using hot compression testing. The flow curves of HIP FGH96, superalloy during hot deformation was analyzed systematically. The results show that deformation temperature, strain rate and strain are the main influence factors on flow stress of HIP FGH96 superalloy during hot deformation. The flow stress displays a peak at a critical strain and then decreases with further increase in strain. For a given strain, the flow stress decreases with the increase of deformation temperature, and increases with the increase of strain rate. A mathematical model of these flow curves was established through regression analysis and taking the strain as a modification factor. The calculated stress values agree well with the experimental values.

Microstructure and mechanical properties of as-cast Mg-8Sn-1Al-1Zn-xNi alloys

Mg-8Sn-1Al-1Zn-xNi(x=0.5%, 1.0%, 1.5%, 2.0%, mass fraction) alloys were designed and prepared. The microstructures and the mechanical properties were studied by using optical microscope, scanning electronic microscope, energy dispersive X-ray spectroscope, X-ray diffraction and a standard universal testing machine. The results show that the microstructure of Ni-containing alloys consist of α-Mg, Mg2 Sn, β-Mg-Ni-Al and γ-AlNi phases. No β-Mg-Ni-Al phase was observed in TAZ811-2.0Ni alloy due to its 1:1 atomic ratio of Ni/Al. The addition of Ni refines the α-Mg dendrites and suppresses the formation of coarse Mg2 Sn phase. The tensile properties results show that the TAZ811-0.5Ni alloy presented the best mechanical properties, which is due to the rod-like β-Mg-Ni-Al phase, refined α-Mg dendrites and Mg2 Sn phase, as well as γ-AlNi phase. The tensile fracture mechanism transits from cleavage to quasi-cleavage fracture with the increasing Ni addition.

Effect of heating-rate on failure temperature of pre-loaded magnesium alloy

The response and failure of magnesium alloy AZ31 specimens subjected to different pre-loaded-stress levels and heating rates were investigated with a Gleeble-1500 thermo-mechanical material testing system.It is found that the increases of either pre-loaded stresses or heating-rates decrease the failure temperatures of the specimens.The metallographs of the tested specimens were also observed.It is shown that the high heating-rate may cause stronger local thermal inconsistency,which remarkably increases the microdefects and reduces the macroscopic mechanical properties of the material.