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。

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.

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.

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.