Stress-strain characteristics of linear friction welding of TC11 and TC17 alloys

Abstract Transient stress and strain fields of dissimilar titanium alloys （TCll and TC17 ） joint during linear friction welding （ LFW） were investigated by a two-dimensional model with ABAQUS/Explicit. The results showed that in the X-axis, the maximum compressive stress of 850 MPa occurred in the center zone of friction interface , and the maximum tensile stress of 190 MPa distributed at the flash; in the Y-axis, the maximum compressive stress of 1 261 MPa located at the junction region between the welding fixture and edge of the specimen, and the maximum tensile stress of 320 MPa distributed in the connecting portion between the flash and edge of the specimen. In addition, areas of plastic strain increased gradually during welding process. In the X-axis, tensile strain mainly existed at the heads of the specimens; in the Y-axis, compressive strain mainly occurred at the heads of the specimens.

β锻TC17钛合金的高周疲劳研究

对β锻的TC17合金的高周疲劳性能进行了研究,获得了光滑试样的应力疲劳寿命曲线(S-N曲线)。结果表明,β锻的网篮组织具有较高的高周疲劳性能,疲劳极限达到544 MPa。对疲劳断口和裂纹扩展路径进行了分析,发现网篮组织的高周疲劳断口具有裂纹扩展路径曲折、二次裂纹较多的特征。原始β晶界的粗大α相是显微组织中的"弱区",疲劳裂纹易沿粗大α相萌生和扩展。

TC17钛合金热变形过程中片状组织演变规律

通过等温锻造试验和有限元模拟,研究了TC17钛合金在α+β两相区变形过程中热加工工艺参数对显微组织演变的影响规律。通过组织观察分析发现:随着变形程度和变形温度的增加,TC17钛合金中的片层组织逐渐向球化组织转变。变形量对片状组织的球化起决定作用,当变形量为小于20%时,仅有少数片状α相发生了弯折或扭曲,球化现象不明显;随着变形量的增加,片状组织被不同程度的弯曲、破碎,球化程度随着变形量的增加逐渐变大。变形温度对球化过程也起一定的作用,随着变形温度的升高,球化效果越来越明显,这与较高的变形温度会提高位错或原子的迁移能力使片状组织有足够的能量通过界面迁移实现断裂、球化有关。

TC17钛合金片层组织动态球化的神经网络预测模型

在Gleeble-1500热模拟试验机上通过热压缩试验研究具有初始片层组织的TC17钛合金在变形温度为780～860℃、应变速率为0.001～10 s^(-1)、变形量为15%～75%范围内的组织演变,定量分析热变形参数对片层组织动态球化过程的影响。采用结合贝叶斯归一化算法的BP人工神经网络,建立TC17钛合金片层组织动态球化演变的预测模型,误差分析表明模型精度较好。

固溶处理对TC17合金组织与性能的影响

对厚度为8.0mm的TC17合金固溶处理温度、时间、冷却方式进行了研究,讨论了热处理工艺参数、组织与性能之间的关系,为该合金板材选择合适的热处理制度提供依据。

激光冲击强化对TC17硬度和拉伸性能的影响

采用脉冲能量为7 J的纳秒激光对TC17钛合金进行激光冲击强化处理。采用扫描电镜(SEM)、透射电镜(TEM)对TC17激光冲击的微观组织进行观察,并分别采用显微硬度仪、电子万能材料试验机对材料的显微硬度、拉伸性能进行测试。结果表明,激光冲击强化后,在材料表面形成微塑性变形,在冲击区域近表面形成高密度位错。与基体材料相比,冲击区域的显微硬度提高50 HV,抗拉强度提高56 MPa,屈服强度降低11 MPa。

Influence of heat treatments on microstructure and mechanical properties of laser additive manufacturing Ti-5Al-2Sn-2Zr-4Mo-4Cr titanium alloy

The effect of heat treatments on laser additive manufacturing(LAM)Ti-5Al-2Sn-2Zr-4Mo-4Cr titanium alloy(TC17)was studied aiming to optimize its microstructure and mechanical properties.The as-deposited sample exhibits features of a mixed priorβgrain structure consisting of equiaxed and columnar grains,intragranular ultra-fineαlaths and numerous continuous grain boundaryα(αGB).After being pre-annealed inα+βregion(840°C)and standard solution and aging treated,the continuousαGB becomes coarser and the precipitate free zone(PFZ)nearby theαGB transforms into a zone filled with ultra-fine secondaryα(αS)but no primaryα(αP).When pre-annealed in singleβregion(910°C),allαphases transform intoβphase and the alloying elements distribute uniformly near the grain boundary.DiscontinuousαGB and uniform mixture ofαP andαS near grain boundary form after subsequent solution and aging treatment.The two heat treatments can improve the tensile mechanical properties of LAM TC17to satisfy the aviation standard for TC17.

High-temperature failure mechanism and defect sensitivity of TC17 titanium alloy in high cycle fatigue

Crack initiation is an essential stage of fatigue process due to its direct effect on fatigue failure.However,for titanium alloys in high-temperature high cycle fatigue(HCF),the crack initiation mechanisms remain unclear and the understanding for the defect sensitivity is also lacking.In this study,a series of fatigue tests and multi-scale microstructure characterizations were conducted to explore the high-temperature failure mechanism,and the coupled effect of temperature and defect on TC17 titanium alloy in HCF.It was found that an oxygen-rich layer(ORL)was produced at specimen surface at elevated temperatures,and brittle fracture of ORL at surface played a critical role for surface crack initiation in HCF.Besides,internal crack initiation with nanograins at high temperatures was a novel finding for the titanium alloy.Based on energy dispersive spectroscopy,electron backscatter diffraction and transmission electron microscope characterizations,the competition between surface and internal crack initiations at high temperatures was related to ORL at surface and dislocation resistance in inner microstructure.The fatigue strengths of smooth specimens decreased at elevated temperatures due to the lower dislocation resistance.While the fatigue strengths of the specimens with defect were not very sensitive to the temperatures.Finally,a fatigue strength model considering the coupled effect of temperature and defect was proposed for TC17titanium alloy.

Effects of pretreatment and HVOF sprayed cermet coating on fatigue properties of TC21 titanium alloy

The effects of grit blasting (GB),shot peening (SP) pretreatment and high velocity oxygen fuel(HVOF) sprayed WC-17Co cermet coating on the fatigue properties of the novel ultra-high strength TC21 titanium alloy were investigated with a rotating bending fatigue test machine.The basic properties and surface integrity of the coating were investigated by using X-ray diffraction (XRD),surface roughness meter,microscopic hardness tester,scanning electron microscopy (SEM) and X-ray stress test instruments.The results showed that the residual compressive stress could be introduced into the surface of TC21 alloy by GB and SP pretreatment,and that HVOF sprayed WC-17Co coating was compactly bonded with TC21 alloy substrate and it significantly improved the surface hardness of the substrate.However,there was a certain residual tensile stress in the sublayer of the coating.SP could significantly increase fatigue resistance of TC21 alloy due to the surface residual compressive stress.There was no significant effect on fatigue resistance by GB treatment due to the offset between the influence of surface residual compressive stress and the surface notch effect.The fatigue resistance of TC21 alloy was significantly reduced by HVOF WC-17Co coating prepared with GB pretreatment.This could be attributed to the relaxation of surface residual compressive stress during the HVOF heating process,pore defects and residual tensile stress distribution in the WC-17Co coating with low toughness,and the surface notch induced by GB.The fatigue resistance of TC21 alloy was slightly reduced by HVOF WC-17Co coating prepared with SP pretreatment.This was attributed to the offset between the advantage effect by SP and disadvantageous effect of WC-17Co coating and high temperature factor during the HVOF process.

魏氏组织TC17钛合金绝热剪切带(ASB)的组织与织构

利用热模拟试验机对原始组织为魏氏组织的TC17钛合金进行了热压缩试验,采用电子背散射衍射技术(EBSD)研究了不同变形参数对魏氏组织TC17钛合金材料显微组织与结构的影响。结果表明:在700℃以上进行热压缩变形时,魏氏组织TC17钛合金的材料均发生绝热剪切行为,所形成的ASB中心区域组织主要由动态再结晶形成的等轴状β相晶粒和少量α相组成。随变形温度升高,残余α相略有增加,β相晶粒尺寸增大。不同变形参数对于ASB中心区域β相的取向分布影响甚微,在β相中主要形成高斯织构。同时绝热剪切敏感性的研究表明,魏氏组织TC17钛合金具有较高的绝热剪切敏感性,且随变形温度升高,绝热剪切敏感性增强。

TC17钛合金高温变形行为研究

采用Gleeble 3500D热模拟试验机对TC17钛合金进行了高温压缩试验。其变形温度为973～1223 K,应变速率为0.001～10 s^(-1),应变0.9。结果表明:TC17钛合金高温流变应力对应变速率和变形温度非常敏感。在温度为1123,1183和1223 K,应变速率为10 s^(-1)时,TC17钛合金的流动应力出现了明显的应力不连续屈服现象。利用Zener-Holloman参数建立了TC17钛合金的高温本构方程,与试验结果对比表明:该方程可以准确地描述TC17钛合金的的高温流动行为。基于动态模型,建立了TC17钛合金的热加工图,并结合微观组织分析验证了加工图的准确性。

基于计算机有限元分析的激光冲击强化钛合金疲劳性能研究

基于限元分析软件ABAQUS和MSC.Fatigue,建立了激光冲击TC17钛合金标准紧凑拉伸试样及其疲劳裂纹扩展的有限元分析模型。对不同区域下激光冲击强化TC17钛合金后的残余应力分布及疲劳裂纹扩展性能进行分析,进而探讨了残余应力场对疲劳裂纹扩展的影响。结果表明,激光冲击TC17钛合金经后,试样上下表面处理区域均呈现压应力分布,最大残余压应力达-473 MPa,残余压应力层深度达0.76 mm,同时,表面残余压应力随激光功率密度和冲击区域的增大,逐渐增大并达到饱和。相对于未冲击件,激光冲击使TC17试样疲劳寿命大幅延长,疲劳裂纹扩展速率显著降低;且随冲击区域的增大,疲劳寿命不断延长,表明激光冲击诱导的残余应力场对TC17钛合金疲劳裂纹扩展具有较好的抑制作用。

Uncovering the hierarchical clusters in the heat-affected zone of an electron beam weldedα/βtitanium alloy joint

Using high-resolution transmission Kikuchi diffraction(TKD)and transmission electron microscopy(TEM),we examined the hierarchical clusters that form in situ in the heat-affected zone(HAZ),which are com-monly referred to as“ghost”structures,of bimodal titanium alloy Ti-5Al-2Sn-2Zr-4Mo-4Cr(wt%,TC17).The ghost structures are enriched with Al elements but poor in Mo and Cr compared to the surroundingβmatrix.TKD results show that the ghost structure in middle-HAZ mainly consists ofα_(L)laths with a high-angle grain boundary,which exhibits the classic Burgers orientation relationship(BOR)with the host matrix,while it encircles theα_(P)grains in far-HAZ.And the ghost structure is evidenced to form via in-complete martensitic transformation.TEM results further confirm that the ghost structure is composed ofαL and tinyβ_(L)laths with BOR,with the former being enriched with Al and poor with Cr and Mo,while the latter is the opposite.Interestingly,twoα_(L)variant clusters with a check-mark morphology are fre-quently observed viewed along[0001]_(αL)//[110]_(βL)directions,which are dominated by the crystallographic and geometrical relationships betweenαandβphases.Based on the microstructural characterization,it is hypothesized that the ghost structure is transformed from the initialα_(P)phase,due to the coupling ef-fect of high thermal stress(which induces the formation of a large number of dislocations)and element diffusion caused by sudden temperature increase and plunge cooling in the HAZ during the welding pro-cess.