Dynamic globularization kinetics during hot working of TA15 titanium alloy with colony microstructure

The dynamic globularization kinetics of TA15（Ti-6Al-2Zr-1Mo-1V） titanium alloy with a colony α microstructure during deformation at temperature range of 860-940 ℃ and strain rate range of 0.01-10 s-1 was quantitatively studied through isothermal compression tests.It is found that the dynamic globularization kinetics and the kinetics rate of TA15 are sensitive to deformation parameters.The dynamic globularized fraction increases with increasing strain,temperature but decreasing strain rate.The variation of globularized fraction with strain approximately follows an Avrami type equation.Using the Avrami type equation,the initiation and completion strains for dynamic globularization of TA15 were predicted to be 0.34-0.59 and 3.40-6.80.The kinetics rate of dynamic globularization increases with strain at first,then decreases.The peak value of kinetics rate,which corresponds to 20%-33% globularization fraction,increases with increasing temperature and decreasing strain rate.

Residual elastic stress strain field and geometrically necessary dislocation density distribution around nano-indentation in TA15 titanium alloy

Nanoindentation and high resolution electron backscatter diffraction（EBSD） were combined to examine the elastic modulus and hardness of α and β phases,anisotropy in residual elastic stress strain fields and distributions of geometrically necessary dislocation（GND） density around the indentations within TA15 titanium alloy.The nano-indention tests were conducted on α and β phases,respectively.The residual stress strain fields surrounding the indentation were calculated through crosscorrelation method from recorded patterns.The GND density distribution around the indentation was calculated based on the strain gradient theories to reveal the micro-mechanism of plastic deformation.The results indicate that the elastic modulus and hardness for α p hase are 129.05 GPas and 6.44 GPa,while for β phase,their values are 109.80 GPa and 4.29 GPa,respectively.The residual Mises stress distribution around the indentation is relatively heterogeneous and significantly influenced by neighboring soft β phase.The region with low residual stress around the indentation is accompanied with markedly high a type and prismatic-GND density.

Simulated and experimental investigation on discontinuous dynamic recrystallization of a near-α TA15 titanium alloy during isothermal hot compression in βsingle-phase field

A cellular automaton（CA） modeling of discontinuous dynamic recrystallization（DDRX） of a near-α Ti-6Al-2Zr-1Mo-1V（TA15） isothermally compressed in the β single phase field was presented.In the CA model,nucleation of the β-DDRX and the growth of recrystallized grains（re-grains） were considered and visibly simulated by the CA model.The driving force of re-grain growth was provided by dislocation density accumulating around the grain boundaries.To verify the CA model,the predicted flow stress by the CA model was compared with the experimental data.The comparison showed that the average relative errors were10.2%,10.1%and 6%,respectively,at 1.0,0.1 and 0.01 s^-1 of 1020 ℃,and were 10.2%,11.35%and 7.5%,respectively,at 1.0,0.1and 0.01 s^-1 of 1050 ℃.The CA model was further applied to predicting the average growth rate,average re-grain size and recrystallization kinetics.The simulated results showed that the average growth rate increases with the increasing strain rate or temperature,while the re-grain size increases with the decreasing strain rate;the volume fraction of recrystallization decreases with the increasing strain rate or decreasing temperature.

Morphology development of elongated α phases in hot working of large-scale titanium alloy plate

The role of subtransus hot working on microstructure morphology of TA15 titanium alloy plate with elongatedαphases was studied by quantitative metallography on different sections. The results show that the microstructure morphology is mainly affected by loading direction. When the sample is compressed along normal direction, microstructure on the section vertical to normal direction has equiaxed primaryαphase but microstructure on the section vertical to rolling direction has strip primaryαphase with long axis along tangential direction. When the sample is compressed along rolling direction, microstructure on the section vertical to normal direction has strip primaryαphase elongated along tangential direction but microstructure on the section vertical to rolling direction consists of strip and irregular broad-band primaryαphase. The strip primaryαphase aspect ratio is smaller at lower temperature due to the dynamic break-down ofαphase. The difference on primaryαphase aspect ratio between different sections decreases after compression along distinct directions in two loading passes, suggesting the improvement of equiaxity of primaryαphase.

Deformation banding in β working of two-phase TA15 titanium alloy

To study deformation banding inβworking of TA15titanium alloy,hot simulation compression experiments were carried out on a Gleeble3500thermal simulator,and the microstructure was investigated by optical microscopy(OM)and electron backscattered diffraction(EBSD).It is found that inβworking of TA15titanium alloy,deformation banding is still an important grain refinement mechanism up to temperature as high as0.7Tm(Tm is the melting temperature).Boundaries of deformation bands(DBBs)may be sharp or diffusive.Sharp DBBs retard discontinuous dynamic recrystallization(DDRX)by prohibiting nucleation,while the diffusive ones are sources of continuous dynamic recrystallization(CDRX).Deformation banding is more significant at high strain rate and large initial grain size.The average width of grain subdivisions is sensitive to strain rate but less affected by temperature and initial grain size.Multi-directional forging which produces crossing DDBs is potential to refine microstructure of small-size forgings.

Hot spinning of cylindrical workpieces of TA15 titanium alloy

In order to form large-diameter thin-wall cylindrical workpieces of TA15 titanium alloy,tube hot spinning experiments of the alloy were conducted on a CNC hot spinning machine.The causes of some forming defects occuring in hot spinning,such as crack,pileup,bulge and corrugation,were analyzed and the corresponding measures were put forward to avoid spinning defects,based on which a proper process scheme of hot spinning of TA15 alloy was obtained and the large-diameter and thin-walled cylindrical workpieces were formed with good quality.The results show that spinning temperature has distinct influence on forming quality of spun workpieces.The range of spinning temperature determines the spinnability of titanium alloy and the ununiformity of temperature distribution near the deformation zone leads to the formation of bulge.The reasonable heating method is that the deforming region is heated to the optimum temperature range of 600-700 ℃,the deformed region is heated continuously and a certain length of undeformed region is preheated.With the thickness-to-diameter ratio(t/D) of spun workpiece reducing to certain value(t/D<1%),surface bulge and corrugation is rather easier to come into being,which could be controlled through restraining diameter growth and employing smaller reduction rate and lower temperature in the optimum spinning temperature range.

Flow behavior and microstructure evolution during hot compression of TA15 titanium alloy

The samples of TA15 titanium alloy were hot compressed in the temperature range of 550-1 000℃at constant strain rate from 0.01 s-1 to 1.0 s-1.The flow behavior and microstructural evolution during hot deformation of TA15 alloy were investigated, based on which the hot working parameters of TA15 alloy were selected. The results show that with the increase of deformation temperature and decrease of stain rate, the flow stress decreases gradually, but the magnitude of stress drop varies with the increase of temperature in different temperature intervals. According to the flow stress and deformation microstructure, the deformation behavior can be classified into three types as working hardening(550-600℃,α+βphase), dynamic recrystallization (650-900℃,α+βphase) and dynamic recovery(950-1 000℃,βphase). The main softening mechanism is dynamic recrystallization(DRX) ofαphase inα+βphase zone and dynamic recovery(DRV) ofβphase inβphase zone. As the stain rate decreases dynamic recrystallization ofαphase proceeds more adequately inα+βzone and theβsubgrains of dynamic recovery have the tendency to grow inβzone. The reasonable temperature for warm forming of TA15 alloy is in the range of 600-700℃, which has been verified by warm spinning experiment of tube workpieces.

Microstructure and defect of titanium alloy electron beam deep penetration welded joint

The microstructure, phase composition and cold shut defect of thick titanium alloy electron beam welded joint were studied. The results showed that the microstructure of weld zone was composed of α′ phase; the heat affected zone was divided into fine-grained zone and coarse-grained zone, the microstructure of fine-grained zone was primary α phase ＋ β phase ＋ equiaxed α phase, and the microstructure of coarse-grained zone was primary α phase ＋ acicular α′ phase; the microstructure of base metal zone basically consisted of primary α phase, and a small amount of residual β phase sprinkled. The forming. reason of cold shut was analyzed, and the precaution of cold shut was proposed.

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.

TA15钛合金的相变、组织与拉伸性能

研究TA15合金在4个典型温度1020、1000、900、800℃以及水淬、空冷和炉冷3种冷却方式下的相变、组织与性能的关系。结果表明:随着冷却速度的降低,1020℃热处理合金的显微组织由马氏体α'相向针状(α+β)相和片状(α+β)相转变,970℃及900℃热处理后则由初生α-马氏体α′相向初生α+针状(α+β)和等轴α+晶界β演变,800℃热处理后只有α和β两相组织;在1020～900℃热处理后,TA15合金的室温和高温强度随冷却速度降低有明显下降,冷却速度相同时,强度随温度升高而提高,而在800℃热处理后的强度和塑性与冷却方式无关。

热处理工艺对TA15钛合金冲击性能的影响

热处理工艺是调整TA15钛合金显微组织和力学性能的重要手段之一。以往文献较多地研究了热处理制度对TA15钛合金拉伸性能的影响。研究了退火温度对TA15钛合金锻件、棒材显微组织和冲击性能的影响,目的是通过改变退火温度调整合金的显微组织参量,改善TA15钛合金冲击性能。其结果表明:提高退火温度可以增加初生α相尺寸和次生α相片层厚度,从而提高合金的冲击性能。特别是对小规格锻件,退火温度从810℃提高到850℃,冲击值可提高10 J.cm-2以上。

基于BP神经网络的TA15钛合金热变形工艺-性能预报

将经过热约束变形的TA15钛合金进行力学性能测试,获得了工艺(温度、应变、应变速率及冷却方式)、性能(抗拉强度、延伸率)参数数据,利用BP人工神经网络建立起其间的关系网络模型。研究表明:所建立的网络可以很好地反映出本材料的工艺-性能之间的关系并且具有一定的精度,网络模型可以用来预测不同变形条件下TA15钛合金的性能。

预热对激光沉积修复TA15钛合金形貌尺寸和组织的影响

采用感应加热装置实现激光沉积修复基材的预热,完成基材预热下的激光沉积修复试验,研究不同基材预热温度对试样形貌尺寸、显微组织及硬度的影响规律。结果表明:基材预热后的沉积试样表面更加平整,试样熔宽和熔高有所增大;随着预热温度的升高,沉积试样底部的柱状晶越粗壮、尺寸越短,顶部的枝晶数量越少,显微组织中片层α组织越粗大;且沉积试样显微硬度轻微降低。基材预热可明显改善沉积质量。

TA15钛合金大型整框锻造成形数值模拟与实验研究

应用有限元软件DEFORM-3D对TA15钛合金大型整框零件锻造成型过程进行数值模拟分析,确定了合理的坯料形状尺寸,选择了合理的模锻工艺参数,形成了可行的整框模锻件的模锻工艺。对数值模拟结果进行了验证性生产试验。结果表明,采用模拟方法所确定的坯料所生产出的终锻件成形质量良好、工艺可行、模具设计合理,整框锻件产品性能稳定可靠。

TA15钛合金超高频氩弧焊工艺试验研究

针对TA15钛合金材料进行了超高频脉冲钨极氩弧焊接工艺试验。采用金相试验方法和力学性能试验对超高频焊接和常规焊接工艺下的显微组织和性能进行对比。结果表明,采用超高频氩弧焊的焊缝宽度明显比常规氩弧焊窄,晶粒尺寸减小30%以上;α相片层厚度要厚于常规焊,且分布更规律、更有序。超高频脉冲焊接接头抗拉强度与常规氩弧焊基本相当,塑性优于常规氩弧焊,其疲劳性能也高于常规焊。

超声冲击对TA15钛合金焊接接头的影响

研究了TA15钛合金氩弧焊焊接接头超声冲击前后的组织及力学性能,并对接头拉伸断口进行了观察。结果表明,焊接后焊缝区和热影响区的组织与母材的组织差别很大,表现为魏氏组织特征;超声冲击前后母材和接头的组织均变化不大。冲击处理使焊缝区和母材区的强度和伸长率均有所增加。冲击前后的焊缝及母材的室温拉伸断口均属于韧窝型断口。冲击后接头的表面和断面显微硬度均得到了提高。

不同等温锻TA15钛合金试验件的组织与力学性能研究

研究了不同等温锻工艺对TA15等温锻试验件组织和力学性能的影响规律。结果表明,先采用950℃等温锻造、空冷,然后再进行975℃等温锻、快冷工艺,试验件中初生α相含量约15%,且还有细小的β转变基体和薄片状α此种组织在不降低塑性下具有较高的室温和高温强度,横、纵向性能相差也较小,完全满足锻件要求,故选用该工艺作为等温锻工艺。

TA15钛合金电子束焊缝形貌及其组织研究

采用不同工艺参数对TA15钛合金进行电子束焊接,通过观察焊缝形貌、测量其形状参数研究了焊接接头形貌的变化规律,并分析了焊接接头组织。结果表明:增大电子束流时,熔深、半熔深熔宽、焊缝宽度都增大,焊缝横截面的形状从"钉形"转变为"钟罩形";增大焊接速度,对焊缝形状影响不大,熔深、半熔深熔宽及焊缝宽度均减小,深宽比先增大后减小;聚焦电流的增大对熔深作用较明显,半熔深熔宽及焊缝宽度变化不大;扫描幅值的增加使焊缝熔深减小,半熔深熔宽增大。靠近母材的热影响区组织与母材组织相近,主要由初生等轴状α相及转变β相组成,并出现针状(α+β);靠近熔合线的组织由α相和针状(α+β)相构成,并出现α'马氏体;熔合区组织由α'组成,熔合线周围柱状晶垂直于焊缝中心生长,并在焊缝中心形成单列或多列的等轴状晶。

TA15钛合金板材冷折弯成形的有限元模拟及实验研究

为了进一步研究TA15钛合金板材的冷折弯成形规律,使用有限元模拟成形软件PamStamp2G对其成形过程进行了模拟,分析了凸模行程、凹模跨度、凸模圆角对板材折弯后回弹的影响规律,并对TA15板材折弯的极限角度和回弹角度进行了研究。通过对比模拟结果与实验数据,发现两者趋势基本一致。这验证了PamStamp2G有限元模拟技术在TA15板材冷折弯成形过程中的有效性和可行性。

轧制工艺对TA15钛合金薄板组织及力学性能的影响

研究了不同轧制工艺对TA15钛合金薄板显微组织和力学性能的影响。结果表明,经均匀化处理在α＋β相区换向包覆叠轧轧制后可以获得晶粒尺寸均匀细小、力学性能均匀的TA15钛合金薄板。