近β型钛合金线性摩擦焊接头变形行为及组织演变的热物理模拟

为了更好地理解近β型钛合金线性摩擦焊(LFW)接头的界面连接行为,采用热压缩物理模拟的方法,以TB2近β型钛合金为研究材料,设计符合LFW工艺特点的帽形热压缩试样,在不同下压量、变形温度及应变速率条件下进行热压缩试验,研究热压缩接头的变形行为及组织演变。结果表明,热压缩过程中热压缩接头连接区(BZ)的峰值剪切应力随变形温度的降低及应变速率的增加而增大,接头BZ的宽度随变形温度和应变速率的增加而减小。接头BZ边缘由沿剪切方向变形的晶粒组成,接头BZ中心由细小的等轴再结晶晶粒组成。接头的动态再结晶机制为连续动态再结晶(CDRX),且CDRX的程度随下压量、变形温度及应变速率的增加而增大。此外,在界面剪切力的作用下接头BZ边缘形成{112}[111]织构,接头BZ中心形成{110}[001]织构。

Local inhomogeneity of mechanical properties in stir zone of friction stir welded AA1050 aluminum alloy

The local inhomogeneity of the stir zone in friction stir welded face-centered cubic metal was investigated,which has multiple activated slip systems during plastic deformation,by selecting commercial AA1050 aluminum alloy as an ideal experimental material.The local inhomogeneity was evaluated by uniaxial tensile tests using small samples with a 1 mm gauge length.The corresponding microstructural parameters such as grain size,misorientation angle distribution,and micro-texture,were quantified by the backscattered electron diffraction technique.A comprehensive model was used to reveal the microstructure−mechanical property relationship.The experimental results showed that the uniaxial tensile property changes significantly across the weld.The maximum ultimate tensile strength(UTS)occurred in the center of the stir zone,which was 99.0 MPa.The weakest regions were located at the two sides of the stir zone.The largest difference value in UTS reached 14.9 MPa,accounting for 15%of the maximum UTS.The analysis on the structure−mechanical property relationship suggests that the micro-texture change with the location formed during the rotational material flow is the main reason for the local inhomogeneity.

Microstructure and properties of probeless friction stir spot welding of AZ31 magnesium alloy joints

The AZ31 magnesium alloy with a thickness of 1.8 mm was welded by the probeless friction stir spot welding process without Zn interlayer.The influence of process parameters on joint microstructure and mechanical properties was investigated by using different rotating speeds and dwell time.Microstructure of joints is divided into three regions:stir zone,thermomechanically-affected zone and heat-affected zone.With the increase of rotation speed and dwell time,the depth of stir zone gradually increases,and hook defects extend from the interface of two plates to the surface of the upper plate.The tensile shear strength of joints and two fracture modes(shear fracture and plug fracture)are closely related to hook defects.The maximum tensile shear strength of the joint is 4.22 kN when rotation speed and dwell time are 1180 r/min and 9 s,respectively.Microhardness value and its fluctuation in upper sheet are evidently higher than those of the lower sheet.

Impact of travel speed on the microstructure and mechanical properties of adjustable-gap bobbin-tool friction stir welded Al–Mg joints

The butt welds of 4-mm thick 5A06 aluminum alloy plates were produced by adjustable-gap bobbin-tool friction stir travel with travel speeds of 200, 300, and 400 mm/min in this study.The microstructure was studied using optical microscopy and electron backscatter diffraction(EBSD).Tensile tests and microhardness measurements were performed to identify the effect of the travel speed on the joint mechanical properties.Sound joints were obtained at 200 mm/min while voids were present at different positions of the joints as the travel speed increased.The EBSD results show that the grain size, high angle grain boundaries, and density of geometrically necessary dislocations in different regions of the joint vary depending on the recovery and recrystallization behavior.Specific attention was given to the relationship between the local microstructure and mechanical properties.Microhardness measurements show that the average hardness of the stir zone(SZ) was greater than that of the base material, which was only affected slightly by the travel speed.The tensile strength of the joint decreased with increasing travel speed and the maximal strength efficiency reached 99%.

Effect of Kinetic Alfvén Waves on Electron Transport in an Ion-Scale Flux Rope

At the Earth＇s magnetopause, the electron transport due to kinetic Alfvén waves（KAWs） is investigated in an ion-scale flux rope by the Magnetospheric Multiscale mission. Clear electron dropout around 90° pitch angle is observed throughout the flux rope, where intense KAWs are identified. The KAWs can effectively trap electrons by the wave parallel electric field and the magnetic mirror force, allowing electrons to undergo Landau resonance and be transported into more field-aligned directions. The pitch angle range for the trapped electrons is estimated from the wave analysis, which is in good agreement with direct pitch angle measurements of the electron distributions. The newly formed beam-like electron distribution is unstable and excites whistler waves,as revealed in the observations. We suggest that KAWs could be responsible for the plasma depletion inside a flux rope by this transport process, and thus be responsible for the formation of a typical flux rope.

Base material location dependence of corrosion response in friction-stir-welded dissimilar 2024-to-5083 aluminum alloy joints

The effects of the base material(BM)location on the mechanical properties and the exfoliation corrosion performance of friction-stir-welded(FSWed)dissimilar 2024-to-5083 aluminum alloy joints were investigated.Scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),tensile tests and electrochemical experiments were conducted.The results revealed that the BM location had little effect on the tensile properties of the joints.The grain orientation spread(GOS)value of 2024 alloy side was lower than that of 5083 alloy side.Intergranular corrosion occurred mainly on the 2024 alloy side,while the grain interior of the 5083 alloy side was corroded due to the higher GOS value and dislocation density.The FSWed dissimilar joints with a superior exfoliation corrosion resistance could be achieved when the 5083 aluminum alloy with better corrosion performance was positioned on the retreating side.

Microstructures and microhardness for sheets and TIG welded joints of TA15 alloy using friction stir spot processing

Effects of friction stir spot processing(FSSP)on the microstructures and microhardness of tungsten inert gas(TIG)welded TA15titanium alloy joints were investigated.The macro/micro structural observation and microhardness evaluation of the TA15alloy sheets and TA15TIG welded joints were carried out using optical microscope and microhardness tests.The results show that FSSP effectively improves the microstructure and increases the microhardness of the TA15sheets.As for the TIG welded joints,FSSP also effectively improves the microstructure of joints.And the average microhardness value in weld nugget zone is improved significantly,while a small increase of this value in heat affected zone is observed.The hardness in stirring zone is significantly higher than that in the base metal.Two peak values of hardness appear along the width direction in stirring zone.After FSSP,the average hardness of the weld zone of TA15TIG welded joint is significantly higher than that before FSSP.Under the present process parameters,both the surface oxidation in TA15sheets and in TIG welded joints after FSSP are not evident,while the surface forms the bright white layer,which is composed of a great multitude of fine grains.

Effect of heat treatment on the microstructure and mechanical properties of structural steel–mild steel composite plates fabricated by explosion welding

Two dissimilar steel plates,structural steel and mild steel,were joined by explosion welding to form a composite.The composite was then heat-treated by quenching at 840℃ for 30 min followed by tempering at 200℃ for 3 h.The microstructure was investigated under an optical microscope and a scanning electron microscope.The mechanical properties were measured using Vickers microhardness and Charpy impact tests.The results show a deformed interface with typical wave features at the welding zone,but no defects were observed.Moreover,the ferrite in the parent plate in the weld zone was elongated due to the strong plastic deformation from the explosion.After heat treatment,the hardness of the flyer plate(structural steel)was over HV0.2800,while that of the parent plate(mild steel)was HV0.2200.The increase in hardness was due to the presence of martensite.Moreover,the average impact energy was increased from 18.5 to 44.0 J following heat treatment;this is because of the formation of recrystallized grains at the weld interface,which is due to the dynamic recovery and local recrystallization,and the strong elemental diffusion that occurred between the two plates.

The effects of forging pressure and temperature field on residual stresses in linear friction welded Ti6Al4V joints

Linear friction welding （LFW）, as a solid state joining process, has been developed to manufacture and repair blisks in aeroengines. The residual stresses after welding may greatly influence the performance of the welded components. In this paper, the distribution of residual stresses in Ti6Al4V joints after LFW was inves- tigated with numerical simulations. The effects of applied forging pressure and temperature field at the end of the oscillating stages on the residual stresses within the joints were investigated. The results show that, the residual tensile stresses at the welded interface in the y-direction are the largest, while the largest compressive stresses being present at the flash root in the z-direction. Furthermore, the forging pressure and temperature field at the end of the oscillating stages strongly affect the magnitude of the residual stresses. The larger forging pressure produced lower residual stresses in the weld plane in all three directions （x-, y-, and z-directions）. Larger variance, a, which decides the Gaussian distribution of the temperature field, also yields lower residual stresses. There is good agreement between simulation results and experimental data.