惯性摩擦焊接作为一种先进固相连接技术，具有高效、节能及环保等显著优势，能有效避免高温合金焊接过程中常见的气孔及裂纹缺陷[4,5]。在航空发动机转子等关键部件设计中，为了兼顾结构轻量化和高可靠性要求，异质高温合金的惯性摩擦焊接工艺得到广泛应用[6,7]。Senkov等[8]研究了低固溶温度高熔点(LSHR)和Mar-M247镍基高温合金在惯性摩擦焊中的焊接行为，发现焊接界面的温度接近或超过γ′相溶解温度，高温合金经历剧烈的摩擦生热和塑性变形，导致焊接界面附近温度接近或超过材料的再结晶温度，引发动态再结晶并形成细晶层。此外，Huang等[9]研究了惯性摩擦焊IN718镍基高温合金的剧烈塑性变形过程，发现该过程引入了大量位错和亚结构。在高温阶段(如顶锻保压或冷却过程)，上述变形储能驱动再结晶过程，显著改变了接头各区域的微观组织[10]。在这一动态再结晶过程中，镍基高温合金因其较低的堆垛层错能，易于通过晶界迁移或位错重组形成退火孪晶[11]。这是由于在低层错能材料中，孪晶界面的形成能较低，使孪生成为比传统晶界迁移更有利的晶界结构调整方式[12]，这一机制普遍存在于fcc金属的退火孪晶形成过程中。
传统的IN718镍基高温合金广泛应用于航空发动机涡轮盘等关键部件，其在650 ℃服役温度以下具有优异的抗拉强度和蠕变性能，但高温强度和抗氧化能力相对有限[13]。GH4065A作为一种新型镍基高温合金，相较于传统的IN718高温合金，在高温环境下展现出更优异的蠕变强度、抗氧化性和热疲劳性能，在更高温度和应力等极端服役工况下具有明显的优势[13,14]。目前，国内外学者已针对GH4065A和IN718高温合金的惯性摩擦焊接工艺已开展了较为系统的研究，主要包括GH4065A同质组配[15,16]、IN718同质组配[17,18]、IN718/720Li异质组配[19]及IN718/FGH96异质组配[20~22]的惯性摩擦焊接工艺优化以及微观组织演变机理分析。然而，鲜有针对GH4065A/IN718异质组配高温合金惯性摩擦焊接工艺的报道，且焊后热处理对接头界面微观组织演变机理的影响尚不明晰，这在一定程度上限制了该材料组配在航空发动机关键部件中的工程化应用。
本工作以GH4065A和IN718异质高温合金作为焊接母材，采用惯性摩擦焊接技术成功制备了高质量焊接接头，并对其进行焊后热处理。系统研究了焊后热处理对接头界面微观组织演变的影响规律，建立了焊后热处理工艺与接头微观组织演变的映射关系，以期为推动航空发动机关键部件的高性能制造提供理论依据和技术支撑。
1 实验方法
采用GH4065A和IN718异质高温合金进行惯性摩擦焊接实验，其主要化学成分如表1所示。采用HWI-IFW-377轴向/径向惯性摩擦焊机进行焊接，其最大轴向力为3600 kN，最高转速为600 r/min。焊接工艺参数为：初始主轴转速300 r/min，转动惯量800 kg·m2，顶锻力300 MPa。焊接完成后，采用精密车削工艺去除接头飞边，并随后对焊件进行标准焊后热处理。焊后热处理工艺制度为：将焊件以10 ℃/min的升温速率加热至760 ℃，保温6 h后随炉冷却至室温。
表1 GH4065A和IN718高温合金的化学成分 (mass fraction / %)
Table 1
| Alloy | C | Cr | Co | W | Mo | Al | Ti | Nb | Zr | B | Fe | Ni |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| GH4065A | 0.011 | 16.00 | 13.0 | 4.0 | 4.00 | 2.10 | 3.70 | 0.70 | 0.05 | 0.015 | 1.00 | Bal. |
| IN718 | 0.010 | 19.39 | - | - | 3.05 | 0.53 | 1.02 | 5.21 | - | - | 16.69 | Bal. |
通过多尺度表征技术系统研究了焊后热处理对GH4065A/IN718异质高温合金接头界面微观组织的影响规律。采用电火花线切割技术沿GH4065A/IN718异质高温合金焊缝横截面切取微观组织分析试样。利用SiC砂纸对试样进行逐级研磨至2000号，并采用W1.5金刚石抛光膏进行抛光处理，随后采用50 g CuCl2 + 250 mL HCl + 250 mL C2H5OH溶液对试样进行化学腐蚀，制备出光学显微镜(OM)试样；采用150 mL H3PO4 + 10 mL H2SO4 + 15 g CrO3溶液对试样进行电解腐蚀以及电解抛光，制备出扫描电子显微镜(SEM)试样。采用DMi8 OM对接头焊缝微观组织形貌进行观察。采用JXA-8530F Plus电子探针显微分析仪(EPMA)对接头界面进行成分分析。采用配有电子背散射衍射(EBSD)系统的IT800-SHL型场发射SEM分析接头的强化相形貌和分布规律，接头轴向各区域在剧烈热力耦合作用下的动态再结晶机制，以及各区域晶粒尺寸和形态。采用JEM-2100 F高分辨透射电子显微镜(TEM)对焊接接头界面处的成分分布、晶格类型等进行精细化表征，以揭示焊后热处理与界面微观组织演变的映射关系。
采用HV-1000Z型Vickers显微硬度计进行显微硬度测试，载荷为200 g，保载时间为10 s。依据标准GB/T 228—2002，采用Instron 5982型万能材料试验机进行焊接接头的室温和高温拉伸测试，拉伸速率为0.5 mm/min。
2 实验结果与讨论
2.1 金相组织分析
异质合金惯性摩擦焊接头的界面质量直接决定了接头整体结构的服役可靠性。图1为焊态和热处理态接头沿轴向各区域显微组织的OM像。可见，GH4065A/IN718异质合金接头界面较为平滑且组织均匀致密，界面结合质量较好，无明显的孔洞或裂纹等缺陷。这表明焊接过程具有充足的热输入，且材料发生了充分的塑性流动和扩散，形成了良好的冶金结合。GH4065A/IN718异质高温合金接头沿轴向的主要特征区域为焊核区(weld zone，WZ)和热机械影响区(thermo-mechanically affected zone，TMAZ)。焊态接头WZ处于惯性摩擦焊接头的中心区域，该区域显微组织发生了剧烈且充分的动态再结晶，形成细小均匀的等轴晶组织，经过后续的热处理过程，热处理态WZ晶粒的分布和形貌与焊态相似，并未发生明显改变。焊接过程中摩擦界面及附近区域产生了较高的剪切塑性变形，促使大量的亚晶粒形成并作为潜在的再结晶形核点。由于焊接温度高于IN718和GH4065A高温合金的再结晶温度，因此进一步促进了形核及周围组织的再结晶过程。由于惯性摩擦焊具有升温快和高温持续时间短等特性，晶粒的长大受到很大程度的制约，因此整个焊接过程是一个完全动态再结晶过程，WZ处的晶粒转变成了细小的再结晶晶粒。焊态和热处理态接头TMAZ中晶粒呈粗晶与细晶共存的分布特征，其受到热-力耦合作用的影响仅次于WZ，部分变形晶粒在高温与机械双重作用下发生动态再结晶，由于此区域晶粒的变形量远小于WZ，因此该区域组织的再结晶程度相对较低。
图1
图1
焊态和热处理态接头沿轴向各区域显微组织的OM像
Fig.1
Low (a, e) and high (b-d, f-h) magnified OM images of welded joint (a-d) and post-weld heat treated joint (e-h) (The black and yellow arrows in Figs.1a and e denote the widths of the weld zone (WZ) and thermo-mechanically affected zone (TMAZ) on IN718 side, respectively) (b, f) WZ (c, g) TMAZ on GH4065A side (d, h) TMAZ on IN718 side
经焊后热处理后，接头WZ以及IN718和GH4065A侧TMAZ的宽度均有所增大。这主要是由于焊后热处理促进了WZ和TMAZ中冷变形组织的再结晶[24]，尤其是在TMAZ，由于该区域在焊接过程中产生了较大的塑性变形，应变能积累较高，焊后热处理时更容易发生再结晶。焊后热处理过程中，再结晶晶粒逐渐长大，且TMAZ向低能量区域扩展，从而使得TMAZ的范围扩大。此外，热处理驱动了合金元素的扩散和成分均匀化，削弱了由焊接导致的成分梯度。二者共同导致WZ/TMAZ与母材间的显微组织差异减小，原焊接态下清晰的显微组织边界变得模糊，从而表现为WZ和TMAZ的宽度增加。
2.2 强化相形貌及分布
图2为焊态和热处理态接头沿轴向各区域微观组织的SEM像。由图可见，在焊态接头中，GH4065A侧TMAZ呈现出较为均匀、致密的微观结构。IN718/GH4065A接头界面平整，表明两种合金在惯性摩擦焊过程中实现了良好结合。在焊态接头WZ的细晶组织中，基本未观察到强化相析出。GH4065A侧TMAZ中有颗粒状γ′强化相弥散分布，IN718侧TMAZ中则以尺寸较小的γ′′相为主，同时部分区域存在由γ′′相转变而来的短棒状δ相。此外，两侧TMAZ中均有碳化物弥散分布于基体。上述组织特征与焊接过程的热循环密切相关，惯性摩擦焊过程具有快速、瞬时和局部加热的特点，焊接后接头区热量通过对流与传导迅速散失，温度骤降至强化相析出温度以下。WZ最高温度可达约1300 ℃，高于γ′′、γ′等强化相的固溶温度范围(1110~1120 ℃)，导致该区域中的强化相完全固溶于基体，因此在焊态下WZ细晶组织中基本未见强化相析出。而TMAZ受到的高温影响相对较弱，热暴露时间较短，致使该区域仅部分强化相溶解于基体。其中，GH4065A侧中的γ′相以颗粒状保留，IN718侧则保留较多细小γ′′相，部分γ′′相在高温下转变为短棒状δ相，较大尺寸δ相的存在可能对接头力学性能产生不利影响[25]。由于惯性摩擦焊属于固态焊接工艺，界面区不会出现熔化和再凝固所形成的铸态组织，而是通过摩擦和塑性流动形成致密结合区。材料之间的冶金结合主要依靠扩散和再结晶实现，因此接头质量通常较高。在此过程中，TMAZ表现为带状分布的微观组织特征。这是由于焊接时界面区在摩擦和高压作用下发生剧烈塑性流动，导致材料局部形成变形带。这些变形带在宏观金相组织中呈现为较为规则的条带状结构，直观反映出该区域在焊接过程中经历了显著的高塑性变形。
图2
图2
焊态和热处理态接头沿轴向各区域微观组织的SEM像
Fig.2
SEM images of various axial regions in welded joint (a-c) and post-weld heat treated joint (d-f) (a, d) WZ (b, e) TMAZ on GH4065A side (c, f) TMAZ on IN718 side
如图2d所示，热处理态接头WZ仍未观察到明显的析出相，这是由于析出相尺寸较小，难以通过分辨率相对较低的SEM分辨。如图2e和f所示，热处理后，GH4065A和IN718侧的碳化物尺寸及分布未发生明显变化，但γʹ相和δ相的形态和分布发生了一定变化。GH4065A侧的γ′相主要呈现颗粒状，尺寸小于焊态接头中的γ′相，说明在热处理过程中γ′相发生了重熔并析出，且γʹ相的生长受到一定的控制。IN718侧均匀分布着较多的短棒状δ相，其尺寸与焊态相比明显减小，细小的短棒状δ相有助于力学性能的提升[26]。随着与WZ中心的距离增加，强化相的尺寸和体积分数均逐渐增加。热处理过程中，WZ和TMAZ中的强化相形成元素从过饱和基体中析出，相较于焊态接头，热处理态接头中强化相的体积分数增加，且平均尺寸变大，第二相强化作用增强[27]。
图3为焊态及热处理态接头界面的SEM像和EPMA像。由图可见，在焊态接头中，界面区域的元素扩散层宽度明显小于热处理态。接头左侧为GH4065A合金，含有固溶态的Co、W等合金元素，其主要强化相为γ′相(Ni3(Al, Ti))；右侧为IN718合金，含有固溶态的Fe等元素，其主要强化相为γ′′相(Ni3Nb)。经过热处理后，界面两侧的材料发生了更充分的互扩散和混合，元素迁移距离显著增加。这种充分的元素互扩散有助于消除界面成分突变，从而增强界面的冶金结合强度。
图3
图3
焊态和热处理态接头界面的SEM像和EPMA像
Fig.3
SEM images and EPMA elemental distribution maps of interface in welded joint (a) and post-weld heat treated joint (b)
2.3 动态再结晶机制
图4为焊态GH4065A/IN718异质接头沿轴向各区域的EBSD分析结果。如图4a~d所示，焊态下WZ的晶粒均为细小的等轴晶，平均晶粒尺寸为2.93 μm，大于GH4065A侧和IN718侧TMAZ的晶粒尺寸，这一差异主要归因于WZ所经历的完全动态再结晶过程，在高温和较大应变作用下，WZ获得了充分的热力学驱动力，使得原始晶粒完全重组，并通过晶界迁移和晶粒吞并机制形成均匀细小的等轴晶组织，同时较高的热输入与相对较低的应变速率为晶粒的充分长大提供了条件，最终表现出相对TMAZ更大的平均晶粒尺寸。如图4e~l所示，GH4065A侧和IN718侧TMAZ的平均晶粒尺寸分别为1.85和2.21 μm，且TMAZ区域晶粒形态复杂、取向差异明显，部分区域晶粒较为粗大，另一部分区域则呈现细小晶粒，这种非均匀组织是不完全动态再结晶与应变局部化共同作用的结果[28]，由于TMAZ在焊接过程中经历的温度较低但应变速率较高，其再结晶过程受到抑制，仅在高应变累积区域发生动态再结晶形成细晶，而相邻区域则保留部分原始粗晶，同时塑性变形引起的晶格旋转与位错重排导致明显的晶粒取向梯度，反映了该区域强烈的热‑力耦合效应及非均匀组织演化特征。
图4
图4
焊态接头沿轴向各区域的EBSD分析
Fig.4
Grain size and kernel average misorientation (KAM) distributions (a, e, i), inverse pole figures (IPFs) (b, f, j), KAM maps (c, g, k), and misorientation angle distributions and grain boundary maps (insets) (d, h, l) of various axial regions in welded joint (davg—average grain diameter, KAMavg—average KAM, Std. Dev.—standard deviation, fLAGB—fraction of low angle grain boundary, fHAGB—fraction of high angle grain boundary) (a-d) WZ (e-h) TMAZ on GH4065A side (i-l) TMAZ on IN718 side
如图4d、h和l所示，WZ中大角度晶界(HAGB，取向差角θ > 15°)的占比为87.23%，略高于GH4065A侧TMAZ (83.23%)和IN718侧TMAZ (78.59%)。上述差异与各区域的再结晶程度及热暴露历史密切相关，在高温长时间作用下，WZ中小角度晶界(LAGB，2° < θ ≤ 15°)通过位错重组和晶界迁移逐渐转变为HAGB，提高了晶界的结构稳定性和力学兼容性；而TMAZ因热输入较低、再结晶不完全，保留了较多LAGB，这些晶界往往与亚晶结构和高位错密度区域相关联，可能成为微裂纹萌生和扩展的潜在位置。综上所述，WZ因经历完全再结晶和充分的微观应变释放而具有更均匀、稳定的显微组织，有利于提高接头的整体力学性能和服役可靠性；TMAZ则因组织非均匀性、较高的局部应变以及较多LAGB的存在，可能成为接头力学性能的薄弱区域，尤其在动态载荷或高温环境下其性能退化风险更为显著。
图5为热处理态接头沿轴向各区域的EBSD分析。由图可知，热处理后，接头各区域晶粒尺寸均有所增加，其中，WZ的晶粒尺寸仍为最大，WZ、GH4065A侧TMAZ和IN718侧TMAZ的晶粒尺寸分别达到3.31、3.20和2.68 μm，这是由于热处理为晶界迁移提供了必要的热激活能，从而驱动了晶粒长大。KAM统计结果表明，与焊态接头相比，热处理态接头WZ的KAM值仍然为0.37°，这是因为焊态接头WZ中应力较小，因此热处理对其KAM影响较小；GH4065A侧和IN718侧TMAZ的KAM值分别降低至0.41°和0.46°，这表明在热处理后上述两个区域的应力集中现象得到充分缓解，残余应力被消除。此外，热处理后WZ和IN718侧TMAZ中的HAGB占比均有小幅增加，而在GH4065A侧TMAZ，HAGB占比略有减小。总体来看，热处理对这三个区域HAGB占比的影响较小。
图5
图5
热处理态接头沿轴向各区域的EBSD分析
Fig.5
Grain size and KAM distributions (a, e, i), IPFs (b, f, j), KAM maps (c, g, k), and misorientation angle distributions and grain boundary maps (insets) (d, h, l) of various axial regions in post-weld heat treated joint (a-d) WZ (e-h) TMAZ on GH4065A side (i-l) TMAZ on IN718 side
2.4 位错及孪晶演变机理
采用TEM对焊态下WZ的位错、孪晶以及强化相等微观组织进行分析，结果如图6所示。焊态接头中晶界及晶内存在大量的位错，部分位错互相缠结(图6a和b)，表明WZ中存在少量的应力集中。图6c和d分别为典型区域的高分辨TEM (HRTEM)像及选区电子衍射(SAED)花样。由图可知，WZ未析出强化相，WZ的SAED花样中仅能观察到基体的衍射斑点。这是由于在惯性摩擦焊过程中，WZ因剧烈的塑性变形和摩擦作用而显著升温，其峰值温度超过强化相的溶解温度，导致强化相完全固溶于基体。此外，WZ中存在部分退火孪晶，孪晶周围分布着较多的位错，如图6e~h所示。这是由于惯性摩擦焊的热作用过程持续时间短，并且在空气中冷却；此外，Ni基体本身层错能较低，位错难以交滑移，因此倾向于通过孪生机制协调塑性变形[29,30]，因此，再结晶时层错容易扩展为退火孪晶。在冷却过程中形成的孪晶有助于释放焊接过程中WZ产生的高温残余应力，从而起到应力松弛的作用。
图6
图6
焊态接头焊核区(WZ)的TEM分析
Fig.6
TEM analyses of WZ in welded joint
(a) bright field (BF) TEM image
(b, e) BF TEM images of distributions of dislocation (b) and twin (e)
(c) high-resolution TEM (HRTEM) image
(d, h) selected area electron diffraction (SAED) patterns of matrix (d) and twin (h)
(f, g) dark field (DF) TEM images of twins observed in different directions (aʹ and bʹ represent matrix and twin, respectively)
图7为焊后热处理态接头WZ的TEM分析。经过热处理后，接头的WZ处仍保留部分孪晶(图7a)，这是由于在热处理温度下，孪晶界迁移或晶粒吞并过程未能充分进行。值得注意的是，WZ中析出了大量细小的γ′/γ′′强化相(图7b~d)，γ′相和γ′′相分别作为GH4065A和IN718合金的主要强化相，能够显著增强焊接接头的力学性能。这是由于γ′/γ′′相与γ基体之间存在共格关系，这种共格界面处的弹性应变场会阻碍位错运动，提高位错切割γ′/γ′′颗粒所需的应力[31]。此外，在拉伸、弯曲等机械变形过程中，位错需反复切割细小的γ′/γ′′强化相，显著增加变形阻力，提升接头强度。如图7e和f所示，热处理后接头的WZ出现了大量层错，这是由于位错在切割γ′/γ′′强化相的过程中会产生堆垛层错[32]，这同时也会增加位错运动的阻力，从而对接头的WZ起到强化作用。
图7
图7
焊后热处理态接头WZ的TEM分析
Fig.7
TEM analyses of WZ in post-weld heat treated joint (a, d) low (a) and high (d) magnified BF TEM images (b, e) HRTEM images of γ′/γ′′ distribution (b) and stacking faults (e) (c, f) SAED patterns of γ′/γ′′ (c) and stacking faults (f)
2.5 力学性能
如图8a所示，在室温下，焊态接头的整体抗拉强度和屈服强度分别为1340和1067 MPa，断后伸长率为15.0%，这得益于惯性摩擦焊的固相连接特性避免了熔焊缺陷，同时剧烈的塑性变形引发动态再结晶，形成细晶强化组织；结合图6和7的TEM分析可知，焊态WZ中存在大量相互缠结的位错及退火孪晶，但强化相完全固溶于基体，未析出γ′/γ′′相，因此其强度主要来源于细晶强化、加工硬化及孪晶对残余应力的松弛作用。经焊后热处理后，接头在室温下的整体强度略有增加，这是由于热处理后WZ析出了大量细小的γ′/γ′′强化相，这些共格析出相通过阻碍位错运动显著增强了接头强度，同时位错切割析出相过程中产生的层错进一步增加了变形阻力，从而弥补了因位错密度下降可能带来的强度损失。如图8b所示，焊态接头的高温(650 ℃)屈服强度为920 MPa，略低于热处理态接头的925 MPa，这是由于焊态WZ缺乏强化相钉扎位错，高温下位错更易滑移和攀移，导致强度略有下降；而热处理态接头中细小的γ′/γ′′强化相在高温下仍能有效阻碍位错运动，同时高温促进了动态回复与再结晶，使断后伸长率从室温的15.9%提高至21.7%，展现出良好的高温塑性与强度匹配。如图8c和d所示，接头WZ处的显微硬度从GH4065A侧向IN718侧呈逐渐下降趋势，这是由于高温下GH4065A侧强化相的稳定性更好，在焊接过程中，强化相溶解比例较IN718侧更少。经焊后热处理后，各区域的硬度分布趋于均匀，尤其是WZ区域硬度显著提升，这是由于γ′/γ′′强化相的大量析出增强了沉淀强化效果，同时再结晶进一步消除残余应力，使得硬度整体分布更加平缓，接头力学性能的一致性得到改善。
图8
图8
焊态和热处理态接头的强度和硬度
Fig.8
Mechanical properties of welded joint and post-weld heat treated joint (Insets in Figs.8c and d are schematics of sampling position for hardness testing)
(a) room temperature strength
(b) high temperature (650 oC) strength
(c, d) hardnesses of welded joint (c) and post-weld heat treated joint (d)
3 结论
(1) 采用惯性摩擦焊成功实现了GH4065A/IN718异质高温合金的高质量连接，接头界面结合良好，无宏观缺陷。焊接过程中的热-力耦合作用使WZ发生完全动态再结晶，形成细小的等轴晶；GH4065A侧和IN718侧TMAZ则呈现粗晶与细晶共存的不均匀组织。焊后热处理进一步促进再结晶和晶界迁移，使WZ与TMAZ区域宽度显著增加，整体微观组织得到改善。
(2) 焊接热循环与热处理显著影响强化相的分布与形态。焊态接头WZ中的强化相完全溶解，TMAZ中强化相部分溶解。热处理后，强化相重新析出，GH4065A侧TMAZ中γ′相呈均匀颗粒状分布，IN718侧TMAZ中δ相尺寸有所减小，碳化物在两侧TMAZ中均弥散存在，整体组织更加均匀致密。
(3) 焊态接头的再结晶行为受热-力耦合作用控制，WZ形成细小等轴晶，TMAZ晶粒进一步细化。热处理后，晶粒略有长大，WZ的KAM值变化不明显，而TMAZ的KAM值显著降低，表明焊接应力集中得到缓解，各区域HAGB比例整体呈现升高趋势。
(4) 焊态接头WZ内存在大量位错缠结及局部应力集中，强化相完全溶解并形成部分退火孪晶，有助于释放残余应力。热处理后，WZ中析出细小γ′/γ′′强化相，这些析出相通过阻碍位错运动并诱导堆垛层错形成，进一步增加变形阻力，从而提升了接头的整体力学性能。
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Knowledge of process-structure-property relationships to engineer better heat treatments for laser powder bed fusion additive manufactured Inconel 718
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