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    • Study on the Microstructure and Properties of Ti60/TC17 Dissimilar Titanium Alloy Inertia Friction Welded Joints

    • GAO Shan

      1 ,

      YUAN Mingqiang

      23
    • Vol. 53, Issue 8, Pages: 115-121(2023)   
    • DOI: 10.7512/j.issn.1001-2303.2023.08.15     

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  • GAO Shan, YUAN Mingqiang.Study on the Microstructure and Properties of Ti60/TC17 Dissimilar Titanium Alloy Inertia Friction Welded Joints[J].Electric Welding Machine, 2023, 53(8): 115-121. DOI: 10.7512/j.issn.1001-2303.2023.08.15.
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    Abstract

    This article mainly uses inertial friction welding technology to connect Ti60/TC17 alloy, and analyzes the microstructure and properties of the welded/heat treated joints; During the weld process, there was viscous flow of metal droplets, and the metal materials on both sides fused well. The morphology at the welded joints presented an irregular curve shape, with a reduction of 4.2 mm and an axial misalignment of less than 0.3 mm; Dynamic recrystallization occurred on both sides of the weld interface, resulted in a higher microhardness values in the weld zone, and a lower microhardness value was further away from the weld; In the welded state, the structure of the weld was composed of acicular α slats and equiaxed grains, the heat engine affected zone on both sides had obvious streamline morphology; After heat treatment, the weld was needle shaped α flat noodles refinement, secondary α the phase increases, the recrystallization grain boundaries was obvious, and the streamline morphology of the heat affected zone on both sides disappeared; Through joint tensile performance test, the tensile fracture positions of the welded/heat treated specimens at room temperature occurred on the TC17 side, and the fracture morphology showed typical cleavage fracture. In the welded state, the average tensile strength was 978.5 MPa, reaching 92.06% of the base metal. After heat treatment, the average tensile strength was 1 076.5 MPa, an increase of 98 MPa, exceeding the tensile strength of the base metal. This indicates that heat treatment was beneficial for releasing residual stress in the organization and was easy to precipitate needle like structures α Phase, thereby improving the mechanical properties of the welded joint.

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    Keywords

    titanium alloy; inertia friction welded; dissimilar material; dissimilar joints; mechanical properties

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    0 前言

    钛合金具有高强度质量比、优异的疲劳和良好的耐腐蚀性能,已经被广泛地应用于航空发动机的关键零部件

    1-2。近些年来,由于航空航天的技术不断突破,高温钛合金作为航空发动机风扇、压气机盘件和大截面锻件的重要材料已经受到高度重视,其中压气机盘件是航空发动机零部件的重点3。Ti60合金是一种新型的近α型高温钛合金,服役温度在600 ℃下还具有高蠕变抗力、疲劳强度和损伤容限性能良好,是先进航空发动机中高温部件的候选材料4。TC17是一种富β稳定元素的α-β型两相钛合金,具有高强度、良好的韧性、高可靠性、低成本等优点,且此材料长时间服役于450 ℃,具有优异的高温力学性能5。为了提高压气机的推重比,对压气机结构和材料提出更为严格的要求,单一材料已经不能满足需求,双合金新材料研究迫在眉睫。
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    目前发动机压气机组件连接多采用惯性摩擦焊或电子束焊接

    6-7,而惯性摩擦焊与电子束焊接相比,具有工艺参数少、稳定性高、热输入少、焊缝区间窄、接头力学性能高等优点8-9,在工业领域内有着巨大的潜力,很多产品应用于航空航天领域10。科研工作者在钛合金焊接接头的工艺、组织等方面展开了深入研究,乌彦全等11研究Ti-6Al-4V型钛合金惯性摩擦焊接头焊态/热处理态组织特征及性能,发现焊态下由板条状马氏体α'相+晶界片状αp相+亚稳态β相经过热处理后转变成为晶界片状αp相+晶内片状αs+β相,进而改变接头的力学性能。陈超等12采用电子束焊连接Ti60和TC17,研究了异种接头的微观结构特征、硬度、拉伸和高周疲劳行为。针对Ti60和TC17钛合金惯性摩擦焊接头焊态/热处理态组织特征及性能还未见报道。因此,本文针对Ti60/TC17进行惯性摩擦焊接试验,旨在分析焊态和热处理态状态下Ti60/TC17惯性摩擦焊接头的微观组织及其对力学性能的影响规律,为工程应用提供理论依据。
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    1 试验材料和方法

    试验用TC17合金棒材,直径60 mm,长98 mm,名义成分为Ti-5Al-2Sn-2Zr-4Cr-4Mo。试验用Ti60合金棒材,焊接直径60 mm,长61.6 mm,名义成分为Ti-5.5Al-4Sn-3.5Zr-1Mo-0.4Si-0.85Nd,Ti60母材组织如图1所示,为双态组织,初生等轴α相少于α+β层片状结构;TC17母材组织如图2所示,为典型的篮网组织,在层片状α相之间,分布着大量的次生α相,这种次生α相是在时效过程中由初生α相和亚稳态β相分解得到的,次生α相是TC17钛合金的强化相,次生α相的大小直接影响材料的力学性能

    5。两种材料的母材力学性能如图3所示。
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    图1  Ti60母材组织

    Fig.1  Structure of Ti60 base metal

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    图2  TC17母材组织

    Fig.2  Structure of TC17 base metal

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    图3  母材力学性能

    Fig.3  Mechanical properties of base metal

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    试验设备选用哈尔滨焊接研究所有限公司自主研制的HWI-IFW-130型轴/径向惯性摩擦焊机,最大焊接力为1 300 kN,极限转速为850 r/min。焊前对焊口使用乙醇溶液擦拭,去除表面杂质。焊接工艺参数如表1所示。总能量计算公式见式(1)式(2),焊接后将试样飞边进行车削,同时采取荧光探伤的方式进行检测。

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    E=0.5IW2
    (1)
    W=2πN60
    (2)

    式中 E为总能量(单位:J);I为转动惯量(单位:kg·m2);W为角速度(单位:rad);N为转速(单位:r/min)。

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    表1  Ti60TC17 试验件焊接工艺参数
    Table 1  Welding process parameters of Ti60 and TC17 test pieces
    惯量/(kg·m2转速/(r·min-1顶锻压力/MPa角速度/rad总能量/J
    280 400 70 41.89 245 643.48
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    2 试验结果与分析

    2.1 Ti60/TC17惯性摩擦焊接过程及焊接接头宏观形貌

    Ti60和TC17试件的焊接过程如图4所示,焊接时间共30 s。两种材料相互摩擦,总共经历四个阶段,分别为摩擦阶段→粘塑性阶段→冷却阶段→成型阶段。图4a为摩擦阶段,当主轴飞轮转速达到400 r/min时,尾座立即施加70 MPa顶锻力,两种材料迅速开始摩擦,并产生大量的飞溅金属。图4b为两种材料摩擦接触10 s,明显观察到两种材料发生熔合,产生暗黄色光芒,且有粘流态金属滴落。图4c4d为冷却过程,在摩擦界面焊缝处的金属开始向着两侧外卷边,形貌较粗糙。

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    图4  Ti60TC17惯性摩擦焊焊接过程

    Fig.4  Welding process of Ti60/TC17 inertia friction welded

    图5为Ti60和TC17惯性摩擦焊焊接接头试件的宏观形貌,左侧为Ti60棒料,右侧为TC17棒料(螺纹是固定工件与尾座夹持所用)。由图可知,两侧飞边量适中,飞边形貌呈曲线状。为了进一步检测两种材料焊接完成后是否存在缺陷,立即将试件置于立式车床上,将飞边去除后采取荧光检测,结果发现两种材料熔合较好,未有缺陷产生。最后测量试件整体长度,缩短量为4.2 mm,轴向错边小于0.3 mm。全部完成后对试件进行退火处理(630 ℃保温3 h空冷)。

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    图5  Ti60TC17惯性摩擦焊接头

    Fig.5  Ti60/TC17 inertia friction welded joint

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    2.2 Ti60TC17惯性摩擦焊接头焊态/热处理态显微组织特征

    2.2.1 焊态/热处理态接头宏观组织形貌

    图6为Ti60和TC17钛合金惯性摩擦焊接头低倍显微组织形貌。Ti60/TC17惯性摩擦焊接头主要由融合区(FZ)、热影响区(HAZ)和母材(BM)三部分组成。由图可知,在焊缝区域内,有明显的熔合边界,在两种材料焊缝区域内未发现金属间化合物层,焊缝形貌成形较好。在焊态接头近Ti60侧的焊缝区域,全部为β层片状结构,近TC17一侧发现等轴晶粒产生,晶粒在0.1 mm左右。在经过焊后热处理后,发现近Ti60侧的焊缝晶界边缘处析出少量次生α相,晶粒大小未发生明显变化。由于两种材料耐腐蚀程度不同,在近TC17侧焊缝区晶粒变小,颜色较深,局部组织模糊。

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    图6  宏观接头金相

    Fig.6  Macroscopic joint metallographic diagram

    2.2.2 焊态/热处理态接头微观组织形貌

    为了进一步观察微观焊缝处的微观结构,焊态/热处理态接头焊缝处微观组织SEM图如图7所示。由图7a可以观察到焊缝表面处熔合较好,焊缝区组织发生动态再结晶,由生成的针状α板条和等轴晶粒组成,表明焊接过程中摩擦界面温度高于β相转变温度,促进了α→β相转变,又因晶界处具有较高能量容易满足固态相变的能量起伏,高温β相在冷却过程中α相优先在β晶界上析出,形核长大成平行排列的全针状α马氏体结构

    13,在近Ti60侧有流线形貌存在,分析为在受到热-力耦合作用下,组织发生塑性变形。如图7b所示,在经过热处理后,近Ti60侧流线形貌消失,通过与焊态焊缝区组织对比,α板条细化,次生α相明显增多,次生α相作为材料基体的强化相,其数量直接影响材料的性能,且焊缝中由于成分不均匀而形成的点状及条状迷糊影像基本消除,周军等人14在研究中也发现类似的现象。
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    图7  焊接接头微观SEM

    Fig.7  Microscopic SEM image of welded joint

    TC17和Ti60热机影响区组织形貌如图8所示,由于TC17和Ti60合金的化学成分不同,在受到剧烈的挤压变形作用下,两侧的组织有明显的流线形貌。图8a为焊态TC17,在摩擦热的作用下,当温度达到1 000 ℃以上,时效过程中形成的细小短棒状α相完全溶解,亚稳定β相析出细小的α相弥散分布在β相基体上。图8b为焊态Ti60,全针状马氏体相互交织在一起,未发现固定的晶界。经过热处理后,两种材料的流线形貌消失,经再结晶形成的晶界变得明显,近Ti60侧析出粗棒状α相。

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    图8  焊态/热处理态热机影响区

    Fig.8  Heat affected zone in welded/heat treated state

    2.3 Ti60TC17惯性摩擦焊接头拉伸试验和断口分析

    2.3.1 接头力学性能测试

    为更加深入地研究界面处两侧显微硬度的差异,采取显微硬度仪分别对Ti60和TC17侧进行显微硬度测试,如图9所示,根据曲线走势可分为3个区域:焊缝区、弱化区、母材区。由图可知,焊态下,原始Ti60/TC17母材硬度分别为341 HV、334.2 HV,在焊缝处观察到接头的最高硬度344.6 HV。经退火处理后,Ti60、TC17母材硬度分别为344 HV、440 HV,焊缝处接头的最高硬度为437.8 HV,硬度得到大幅度提升。分析为在经历过热力耦合作用以后,焊缝的亚稳态β相发生时效分解,形成全针状α相和细小等轴晶。在经历热处理后,亚稳定的β相析出次生α相弥散分布在基体上,且次生α相变细,变多,故显微硬度较高。热影响区因其受热循环作用,温度远小于焊缝界面,原有针状α相受热长大,导致显微硬度下降,整个弱化区间约为2 mm,但此区域显微硬度仍高于母材。

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    图9  焊态/热处理态接头显微硬度

    Fig.9  Microhardness of welded/heat treated joint

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    为对比焊态/热处理态试样的拉伸性能,接头断裂情况如图10所示,焊接接头均断裂于TC17一侧。具体拉伸性能如图11所示,热处理后拉伸强度有明显的提升,平均抗拉强度为1 076.5 MPa,平均屈服强度为1 022.5 MPa,均超过母材强度。根据前文分析可知,热处理后有利于将组织内的残余应力释放,同时易于析出针状α相。

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    图10  拉伸断裂试样

    Fig.10  Tensile fracture specimen

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    图11  焊态/热处理态接头拉伸性能测试

    Fig.11  Tensile performance testing of welded/heat treated joint

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    2.3.2 断口形貌分析

    由于焊态/热处理态的断口形貌差异较小,所以取热处理态试样(2-1)进行微观分析如图12所示。从断口宏观形貌来看(见图12a),未发现明显的缩颈现象,其主要分为b、c两个区域,首先,b区域中韧窝区域很少,塑性变形区域小、微观晶粒形貌呈河流花样,并存在大量高密度短而弯曲的撕裂棱线条。在c区域发现多处灰色片状形貌,此区域塑性变形组织少于b区域,且明显有大晶粒出现,这证明此区域起裂速度较快。综上所述,当试件在轴向力的加大作用下,裂纹从边缘位置向内部迅速扩展,由于扩展速度较快,中心区域来不及发生塑性变形,整体形貌呈撕裂状,为典型的解理断裂。

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    图12  拉伸试样断口表征

    Fig.12  Fracture characterization of tensile specimens

    3 结论

    针对Ti60+TC17钛合金惯性摩擦焊接头焊态/热处理态组织特征及性能进行研究,现得出以下结论:

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    (1)在惯性摩擦焊的热-力耦合作用下,焊态焊缝区组织由细棒状α板条和等轴晶β相组成,在两侧热机影响区存在流线形貌,近TC17侧存在粗棒状α相,近Ti60侧形成α+β相层片状分布,且α相明显被拉长。热处理后,焊缝区再结晶的晶界明显,针状α板条增多,两侧热机影响区的流线形貌消失。

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    (2)焊态下,Ti60/TC17母材显微硬度分别为341 HV、334.2 HV,焊缝处显微硬度最高344.6 HV,热处理后,两侧材料流线形貌消失,Ti60母材显微硬度未发生变化,但TC17侧母材得到了强化,焊缝处α板条细化,次生α相明显增多,晶粒越细小,越能阻碍位错运动,故其硬度越高。

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    (3)通过对比焊态/热处理态拉伸性能,最高抗拉强度为1 079 MPa,已超过母材,延伸率达6%,多数断于TC17一侧,从裂纹扩展途径上来看,属于解理断裂。

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