【作者】 芦伟；

【导师】 雷永平；

【作者基本信息】 北京工业大学 ， 材料加工工程， 2013， 博士

【摘要】 TC4-DT钛合金具有高比强度、高比刚度、良好的耐热性、抗腐蚀性和损伤容限性能，广泛应用于飞机重要承力框梁结构制造。钛合金材料的热物理性能和电子束高能密度焊接的特殊性，导致钛合金电子束焊接接头形成大梯度残余应力和大梯度组织和力学性能不均匀性等问题。这些问题对钛合金整体焊接结构的断裂行为有重要影响。本文分析了厚壁TC4-DT钛合金电子束焊接接头的显微组织、焊缝几何特征、接头力学性能不均匀性及断裂行为，揭示了焊缝几何和力学不均匀性对断裂行为和失效评定图的影响。同时，对实际接头进行了断裂评定，探讨了不同评定级别对断裂评定结果的影响。通过上述研究给出厚壁钛合金电子束焊接接头断裂安全评定的工程简化方法。对两种典型焊缝几何的焊接接头进行了显微组织观察和力学性能测试，发现厚壁钛合金电子束焊接接头沿横向和焊缝深度方向显微组织与力学性能不均匀。TC4-DT钛合金母材显微组织为等轴状初生α相和层片状(α+β)所构成的典型双态组织。根据焊接热循环作用下热影响区的β转变温度，热影响区显微组织可分为两部分：近焊缝热影响区为少量等轴初生α+针状马氏体组织；近母材热影响区为等轴初生α+含针状α的转变β组织。焊缝区组织为粗大β柱状晶和分布在柱状晶内呈网篮状排列的针状马氏体。钛马氏体的形成使焊缝金属和热影响区的显微硬度、屈服强度和抗拉强度高于母材，塑性低于母材，焊接接头为高匹配。沿焊缝深度方向，焊缝区β柱状晶和马氏体晶粒尺寸变小，导致焊缝金属的硬度和强度增大。沿焊缝深度方向焊缝宽度变化较小的I形焊缝的力学不均匀性小于焊缝宽度变化较大的Y形焊缝。加入纯钛填充金属使厚壁TC4-DT电子束焊接接头焊缝区的显微组织和力学性能发生明显变化。焊缝中心区强化合金元素Al和V的含量明显降低，显微组织主要由塑性和韧性较好的针状α相组成，避免了马氏体组织生成。焊缝金属的屈服强度和抗拉强度均低于母材，焊接接头为焊缝强度低匹配。对母材、无填充金属接头和填充纯钛金属接头的断裂韧性进行测试，研究了接头不均匀对断裂韧性的影响，并分析断口形貌，揭示了接头的断裂性质。发现母材韧性较好，断裂韧性大于100MPa·m1/2。高匹配接头的断裂韧性降低，而填充纯钛金属后低匹配接头的断裂韧性明显改善，接近母材断裂韧性。这表明钛合金电子束焊接接头的断裂韧性主要取决于其显微组织形态，接头不均匀性对断裂韧性的影响主要表现为组织不均匀性的影响。另外，母材、无填充金属接头和加入填充金属接头在断裂韧性测试中均发生延性断裂。根据TC4-DT钛合金母材和电子束全焊缝金属拉伸试验结果，研究了母材和焊缝金属硬度与拉伸性能的关系。发现TC4-DT钛合金及其电子束焊缝的显微硬度与强度线性相关；应变硬化系数与屈强比线性相关。通过回归分析，建立了显微硬度与强度以及应变硬化系数和屈强比之间的工程经验关系式。当拉伸性能未知时，可以根据焊接接头的显微硬度估算出接头各区域的拉伸性能。这解决了钛合金电子束焊接接头拉伸性能数据测试困难的问题，为钛合金结构断裂评定提供了性能预测关系。通过有限元模拟和理论分析，研究了平面应力和平面应变状态单边裂纹焊接板的极限载荷，提出了相应的极限载荷解法。考虑到电子束焊接接头的尺寸特征，给出平面应变和平面应力状态下估算单边裂纹电子束焊接板极限载荷的简化方法：平面应变状态下，高匹配和轻微低匹配接头的极限载荷可以用母材的极限载荷替代；对严重低匹配接头而言，只有ψ大于10时，焊接接头的极限载荷可以用母材的极限载荷替代。平面应力状态下，高匹配焊接接头的极限载荷可以用母材的极限载荷替代；低匹配接头的极限载荷可用焊缝金属的极限载荷近似替代。另外，将实际厚壁钛合金电子束焊接接头焊缝几何表征为Y形，通过研究焊缝几何参量对接头极限载荷的影响，建立了电子束焊接接头焊缝几何简化方法：实际焊接接头断裂安全评定时，可将Y形焊缝几何简化为宽度2Heq的等效矩形。如果裂纹尖端位于倒梯形段，对高匹配焊接接头，Heq取最小焊缝半宽Hmin，对低匹配接头，Heq取最大焊缝半宽Hmax；当裂纹尖端位于矩形段时，对高匹配接头和低匹配接头，Heq均取最小焊缝半宽Hmin。研究了IIW FFS方法对钛合金电子束焊接接头断裂安全评定的适用性，讨论了厚壁TC4-DT电子束焊接接头用第1级评定的可能性和边界条件。发现IIW FFS方法中依据经验公式确定失效评定线的标准评定级别适用于钛合金电子束焊接接头的断裂评定。电子束焊接接头焊缝较窄，当强度失配程度不超过30%时，用母材性能进行评价仍能得到合理的结果。与传统熔焊相比，电子束焊接接头用第1级评定的边界条件可从δM=10%扩宽到δM=30%。另外，用IIW FFS方法对实际接头进行断裂评定，发现电子束焊缝强度失配对断裂评定的影响很小，高的评定级别并没明显的优越性。因此，断裂评定时可将焊接接头看做均质结构，用第1级评定可得到足够准确的结果。更多还原

【Abstract】 TC4-DT titanium alloy is widely applied in siginificant load bearing frame orgrider structues of aircraft due to its good properties, such as high specific strength,low specific gravity, high heat and corrosive resistance, and well damage tolerantproperties. Thermal physical properties of titanium alloy and characteristic of electronbeam welding as high energy density welding may result in some problems for thickelectron beam welded joints, such as large gradient residual stress, microstructure andmechanical heterogeneity. The problem may siginificantly affect fracture behavior ofthe overall titanium welded structures. In this work, microstructure, weld geometriccharacteristic, mechanical heterogeneity and fracture behavior of thick electron beamwelded TC4-DT joint were analyzed, and the effect of weld geometry and mechanicalheterogeity on fracture behavior and failure assessment diagram were revealed.Meanwhile, fracture assessment of actual thick electron beam welded TC4-DT jointwas performed. The effect of various options on assessment results was discussed.Based on the research results, engineering simplified recommendations were given forthe fracture assessment of electron beam welded thick-walled titanium alloy joints.Microstructure was observed and tensile peoperties were tested for electron beamwelded joint with two kinds of typical weld geometry. It is found that microstructureand mechanical properties of the thick electron beam welded TC4-DT joint areheterogeneous along weld transverse and weld depth. The microstructure of TC4-DTalloy has a typical bimodal structure, which consists of primary α phase and thelamellar (α+β) structures. Based on the β phase transus temperature during weldingthermal cycle, microstructure of heat affected zone can divide into two regions. Theheat affected zone near weld metal is composed of acicular martensite and a smallportion of primary α phase. The heat affected zone near base metal consists of primaryα phase and transformed β containing aciculate α. Microstructure of weld metal iscomposed of course β columnar grain and basketweave martensite distributed inside βcolumnar grain. The formation of titanium martensite makes the weld metal and heataffected zone exhibit increased yield strength, ultimate tensile strength andmicrohardness, while decreased ductility and toughness as compared with the basemetal. The joints are weld strength overmatching. In addition, the grain size of βcolumnar grain and martensite decreases along the weld depth, which result inincreased microhardness and strength of weld metal along the weld depth. Themechanical heterogeneity of I shaped welds with small weld width change along welddepth is smaller than Y shaped welds with high weld width change. The addition ofpure titanium filler metal makes microstructure and mechanical of weld metal change obviously. Thus, the content of strengthening alloy element Al and V of weld metaldecreases. Microstructure is changed to acicular α phase with high ductile andtoughness without the formation of martensite. Meanwhile，yiled strength andultimate tensile strength are lower than those of base metal, and the joints are strengthundermatching.Fracture toughness tests were carried out for base metal, electron beam weldedjoints and pure titanium added joints to study the effect of mechanical heterogeinityon the fracture toughness. Meanwhile, fractography was observed to reveal fracturefeature of the welded joint. The results indicate that the fracture toughness of the basemetal is relatively high, which is up to100MPa·m1/2. Fracture toughness ofovermatching electron beam welded joints decreases as compared with base metal.However, the fracture toughness of undermatching joints with pure titanium fillermetal is obviously improved approaching that of base metal. It is concluded thatfracture toughness of electron beam welded joint is much dependent on themicrostructure. The effect of joint heterogeneity on fracture toughness is mainlyheterogenous in microstructure. In addition, the ductile fracture for base metal,welded joint and pure titanium added welded joint are founded during fracturetoughness tests.Correlation between tensile properties and hardness of electron beam weldedtitanium joints was investigated according to the tensile tests results of TC4-DT basemetal and all weld metal. It is found that strength and microhardness are linear related,as well as the strain hardening coefficient and ratio of yield strength to ultimate tensilestrength is linear related. Base on the regression analysis, linear correlations amongthe yield strength, ultimate tensile strength and microhardness are established,meanwhile, an empirical correlation between strain hardening coefficient and ratio ofyield strength to ultimate tensile strength is proposed. Based on the correlations, thetensile properties of various region of welded joint can be predicted from themicrohardness measurement of welded joints when tensile properties are unknown.This work solves the difficulty of tensile properties testing of electron beam weldedtitanium joints, and gives the predicted correlations for fracture assessment oftitanium structure.The plain stain and plain stress mismatching limit loads for single-edged crackwelded plate in tension (SEC (T)) were investigated by finite element simulation andtheoretical analysis. Special limit load solutions were then proposed. Moreover,considering the dimension features of electron beam welded joints, simplifiedengineering recommendations were given for evaluating the plain strain and plainstress limit loads of electron beam welded structures with SEC (T). Under the plainstrain condition, the mismatch limit loads of overmatching and slightly undermatchingjoints can be obtained assuming that the components are wholly made of base metal, while the mismatch limit loads of extreme undermatching joint can be obtainedassuming that the components are wholly made of base metal only for the structure ofψ>10. Under the plain stress condition, the mismatch limit loads of overmatchingjoints can be obtained assuming that the components are wholly made of base metal,where the mismatch limit loads of undermatching joint can be obtained assuming thatthe components are wholly made of weld metal. In addition, the weld geometry ofactual electron beam welded joint was characterized as Y shaped, and then a methodfor simplifying weld shape method was established by studing the effect of weldgeometry related parameters on limit load. It is considered that weld geometry can besimplified as a rectangle with weld width as2Heqfor the fracture assessment of actualelectron beam welded joints. When the crack tip lies in the inverted trapezoid,minmum weld half-width Hminis chosen as Heqfor overmatching joint, and maxmumweld half-width Hmaxchosen for undermatching joint. When the crack tip lies in therectangle, minmum weld half-width Hminis chosen as Heqfor both overmatching andundermatching joint.The applicability of IIW FFS procedure for the thick electron beam weldedtitanium alloy joint was validated, and the possibility and critical condition δMofoption1for the fracture assessment of thick electron beam welded TC4-DT joint assdiscussed. It is found that for the standard options of IIW FFS procedure, the failureassessment lines eatabilished by empirical equation, are suitable for the fractureassessment of electeon beam welded titanium joints. As the weld width is rathernarrow for electron beam welded joint, the welded joint can be assessed to achievereasonable results by the base metal properties when the weld strength mismatching isno more than30%. Compared to convetional fusion welded joints, the criticalcondition using option1is expanded from δM=10%to δM=30%for electron beamwelded joint. In addition, fracture assessment of actual thick electron beam weldedtitanium joint using the IIW FFS procedure was performed. It is found that the effectof strength mismatching of electron beam welded joint on fracture assement isrelatively small. There are no potential advantages of applying higher options ormismatch options. Thus, the present welded joints can be treated as homogeneousstructure during the fracture assessment, and standard option1can be used to achieveaccurate enough results.更多还原