GH4099合金是一种以Ni为基体，用W、Mo、Cr等合金元素固溶强化、以Al、Ti元素为时效强化元素的典型时效强化型镍基高温合金。这种合金的化学成分与前苏联的ЭП693合金相近，由fcc结构的γ相基体和γ′ (Ni3(Al, Ti))和碳化物等析出相组成[1,2]。GH4099具有较高的热强性，可在低于900℃的温度长期使用，其最高工作温度为1000℃。这种合金的组织稳定，具有良好的热加工成形和焊接性能，可用于制造航空航天发动机的热端部件，如涡轮盘、发动机轴及航天发动机加力燃烧室筒体等[3,4]。燃烧室薄壁筒体长期在高温高负荷条件下工作，必须具有足够高的强度和刚度以免于高温变形。航天发动机零部件锻造成形的周期长、加工难度大，而传统工艺不能满足其几何精度和设计自由度要求[5,6]，因此进行一体化设计需要低成本高效的成形方案。
粉末热等静压(Hot isostatic pressing，HIP)工艺可用于制造形状复杂、显微组织均匀和力学性能优异的零件[9]，在航空航天等领域得到了广泛的应用[10,11]。例如，采用粉末热等静压工艺可加工Rene 95[10]、Astroloy[12]、EP741NP[10]和Inconel 718[13]和制造多种发动机部件[14]。从2003年起中国科学院金属研究所开展粉末近净成形技术研究，制造的粉末冶金氢泵叶轮等复杂构件已经得到应用。GH4099合金可满足航天发动机的高性能要求，中国已经针对这个体系的粉末热等静压成形机理展开了研究。薄壁筒体的最小壁厚约为15.9 mm，最大壁厚约为25.2 mm，因壁厚变化范围较大和加工精度要求较高，进行传统加工难度较高。鉴于此，本文采用等离子旋转电极雾化法(PREP)和无坩埚感应熔炼超声气体雾化法(EIGA)制备洁净GH4099预合金粉末，根据其相转变规律设计热等静压制度，研究热等静压温度对GH4099合金的组织和性能的影响。同时采用有限元仿真技术辅助包套设计并预测粉末压坯的收缩规律，制造GH4099粉末冶金大型薄壁筒体。
1 实验方法
GH4099母合金采用相同批次制粉电极，采用EIGA和PREP法分别制备了GH4099预合金粉末。用ICP 7300 DV型等离子发射光谱仪测试了预合金粉末的化学成分，用ONH836型氧氮氢分析仪测试了制粉电极和预合金粉末中H、O和N的含量，其主要化学成分列于表1。
表1 GH4099预合金粉末的主要化学成分
Table 1
| Al | Ti | Cr | Co | Mo | W | H | N | O | Ni | |
|---|---|---|---|---|---|---|---|---|---|---|
| Master Alloy | 2.07 | 1.28 | 18.46 | 6.34 | 4.13 | 6.18 | 0.00012 | 0.0009 | 0.0006 | Bal. |
| PREP | 2.22 | 1.27 | 18.64 | 6.76 | 3.85 | 5.71 | 0.00022 | 0.0014 | 0.0039 | Bal. |
| EIGA | 2.16 | 1.29 | 18.67 | 6.95 | 3.91 | 5.95 | 0.00024 | 0.0015 | 0.0049 | Bal. |
用Mastersizer 2000型激光粒度仪测试粉末的粒度分布，用S-3400N型扫描电镜(SEM)观察预合金粉末的表面形貌。为了表征GH4099粉末的氧化膜厚度，使用Escalab Xi+型多功能表面分析系统通过X射线电子能谱分析(XPS)检测粉末表面和深度方向的元素分布，溅射速率为0.1 nm/s，溅射时间300 s，溅射深度为30 nm。用差示扫描量热法(Differential scanning calorimetry，DSC)和STA449F3型超高温综合热分析仪测量了GH4099粉末样品的相转变温度，升温速率为10℃/min。
用Shimadzu型拉伸实验机测试室温拉伸及900℃高温拉伸性能，试样屈服前为应变速率控制，屈服后为位移控制。室温拉伸试样屈服前的应变速率为0.00025/s，屈服后的位移速率为12.1 mm/min；高温拉伸试样屈服前的应变速率为0.00007/s，屈服后的位移速率为3.0 mm/min。用VersaXRM-500型X射线三维成像系统表征制备的GH4099合金内的孔隙缺陷。将金相样品打磨、抛光和腐蚀。腐蚀液的配比为：35 g氯化铁，100 mL盐酸，40 mL乙醇，10 mL氢氟酸和40 mL去离子水。将腐蚀液配好后静置0.5 h。用TESCAN MIRA3型扫描电子显微镜观察合金的微观组织和断口的形貌。
2 结果和讨论
2.1 预合金粉末的形貌和粒度分布
图1
图1
GH4099预合金粉末的形貌
Fig.1
Morphologies of GH4099 pre-alloyed powders (a) PREP, (b) EIGA
预合金粉末的工艺性能特别是粒度分布，直接影响其在构件不同部位的振实密度[15]。粉末的装填密度影响后续的热等静压致密化行为、显微组织和合金内孔隙的分布。图2给出了用激光粒度仪测试的粉末的粒度分布。图2中的D50 (D50表示粉末粒度累计到50%所对应的等效粒径，其物理意义表示大于该等效粒径的颗粒各占50%，通常用来表示粉末的平均粒度)分别为58 μm与52 μm。两种粉末的平均粒度接近，与EIGA粉末相比，PREP粉末的粒度更集中。这种大颗粒和小颗粒混合分布的粉末(106 μm以下的全粒度粉末)有利于粉末的填充和振实以及后续的热等静压致密化。相对于增材制造工艺对细粉末的需求，热等静压工艺选用的粉末粒度分布更宽，提高了材料的利用率。
图2
图2
GH4099预合金粉末的粒度分布
Fig.2
Particle size distribution of GH4099 pre-alloyed powders
在用两种方法制备预合金粉末的过程中，粉末物理吸附或化学反应吸附氧元素，在粉末表面生成的氧化层影响粉末颗粒间的结合。可用X射线电子能谱分析(XPS)粉末表面以及深度方向元素的分布，进而了解预合金粉末表面氧化层的厚度。图3给出了EIGA和PREP两种预合金粉末表面Ni 2p的XPS深度分析，图中的黑色虚线为Ni 2p的金属态峰所在位置，红色虚线为Ni 2p的氧化态峰所在位置。结果表明，随着溅射时间的增加Ni 2p氧化态峰的强度降低，金属态峰的强度提高。溅射一定时间后氧化态峰消失，表明溅射深度超过了氧化层的厚度。用EIGA法制备预合金粉末溅射120 s时氧化态峰消失，表明氧化层的厚度约为12 nm。同理，从图3b中观察到用PREP法制备的预合金粉末氧化层厚度约为6 nm。用PREP法制备的预合金粉末中氧化层的厚度较小，表明PREP粉末氧化程度更低。这个结果，与表1中用氧氮氢测试仪测定的PREP粉末的氧含量比EIGA粉末的氧含量低的结果一致。
图3
图3
GH4099预合金粉末的XPS谱
Fig.3
XPS patterns of GH4099 pre-alloyed powders (a) EIGA, (b) PREP
在热等静压过程中，高温高压Ar气经包套传导压力将粉末颗粒挤压紧密，粉末颗粒表面氧富集生成坚硬的氧化薄膜，氧化膜破碎后元素在粉末中扩散形成致密合金。较薄的氧化膜在热等静压致密化过程极易破碎而使粉末颗粒间结合强度提高，进而提高合金的力学性能。用PREP法制备的预合金粉末粒度分布更集中，填充密度更高，表面氧化层更薄，有利于合金致密化。同时，用PREP法不会出现用EIGA法制备时的空心粉。因此，本文优选用PREP制粉工艺制备GH4099高温合金粉末。
预合金粉末热等静压致密化，主要发生在热等静压的升温升压阶段[14]。热等静压是实现致密化的关键，热等静压温度是最重要的参数。选择热等静压温度，须考虑材料的相转变温度。在用热等静压工艺制备高温合金粉末过程中，高温高压的Ar气经模具将压力传导给GH4099合金粉末，粉末间的挤压使氧化膜破碎，因此热等静压温度不应高于固液转变温度。图4给出了GH4099预合金粉末的DSC曲线，将曲线外推得到合金的熔化温度为1346℃。在粉末热等静压致密化成形工艺中，热等静压温度的选择与材料的种类、晶体类型以及强化相的溶解温度密切相关，通常采用的热压温度为(0.80~0.85)Tm[16]。因此，根据DSC分析结果并结合γ′相的析出温度(720~950℃[17])和完全溶解温度(约1160℃[1])，确定热等静压温度范围为1165~1230℃。
图4
图4
GH4099预合金粉末的升温DSC曲线
Fig.4
DSC curves of GH4099 pre-alloyed powders during the heating stage
2.2 GH4099合金的热等静压制度
依据GH4099预合金粉末的热转变特征优选GH4099预合金粉末热等静压制度，本文选择的三种热等静压工艺制度列于表2。
表2 GH4099合金粉末的热等静压制度
Table 2
| Preparation process | Temperature | Pressure | Holding time | Cooling type |
|---|---|---|---|---|
| HIP1 | 1165oC | 150 MPa | 4 h | Furnace cooling |
| HIP2 | 1200oC | 150 MPa | 4 h | Furnace cooling |
| HIP3 | 1230oC | 150 MPa | 4 h | Furnace cooling |
2.2.1 GH4099合金的组织结构
用SEM表征用三种热等静压制度制备的样品的组织状态。图5a给出了用HIP1制度成形的GH4099合金基体的组织，图5b给出了用HIP1制度成形的合金晶界处组织的高倍图像，图5，d分别给出了用HIP2和HIP3制度制备的基体组织。对组织的观察发现，在用HIP1、HIP2制备的试样的随机视场中可观察到基体中有较多的网状PPBs。随着HIP温度的提高合金晶界处的沉淀相由连续分布转变为断续状分布，粉末间的结合程度有所提高。图5b的EDS结果表明，与基体相比，沉淀相的C、Ti含量较高，分析结果表明其为典型的MC碳化物。碳化物是合金晶界与原始颗粒边界的主要组成部分，但是其体积分数较小，无法用XRD等手段分析。
图5
图5
GH4099合金的SEM照片
Fig.5
SEM images of GH4099 alloys (a, b) HIP1, (c) HIP2, (d) HIP3
MC碳化物是粉末冶金成形的高温合金中常见的沉淀相，FGH4097合金中的MC碳化物呈块状，分布在晶内和晶界，其中M主要为Nb、Ti、Hf等元素[24]。Inconel718合金中的MC碳化物，主要成分为Ti、Nb等元素[25]。为了进一步确认GH4099合金基体的原始颗粒边界与晶界处碳化物的结构和成分，用透射电镜通过电子衍射和能谱分析了在1230℃热等静压成形的GH4099合金粉末原始颗粒边界处碳化物。探测图6a中的基体区域，其电子衍射斑点如图6b所示。从图6b可见，在{100}和{111}晶面上未出现超点阵，由此可确定图6a中颗粒1处为不含第二相的面心立方MC型碳化物。结合前文的SEM结果和图6c中的EDS表征，确认粉末原始颗粒边界与晶界上的析出相主要是富含Ti、C的MC型碳化物。
图6
图6
在HIP3条件下GH4099合金中PPBS处碳化物颗粒的TEM形貌
Fig.6
Carbide at the PPBs in GH4099 alloys under HIP3. (a) TEM images, (b) Electron diffraction pattern and (c) EDS spectrum of particle 1 in (a)
2.2.2 GH4099合金的孔隙
图7
图7
GH4099合金内部显微孔隙的大小和分布
Fig.7
Micro-CT images of GH4099 alloys (a) HIP1, (b) HIP3, and (c) column comparison chart
2.2.3 热等静压温度对GH4099合金拉伸性能的影响
图8给出了三种用HIP制度制备的GH4099合金样品的拉伸性能。可以看出，随着热等静压温度在γ′相完全溶解温度与固相线之间的变化(1165℃到1230℃)，在室温拉伸实验条件下，热等静压温度对GH4099合金的室温拉伸性能没有显著的影响。
图8
图8
GH4099合金在不同温度下的拉伸性能
Fig.8
Tensile properties at room and elevated temperature of as-HIPed GH4099 alloys (a, b) room temperature and (c, d) 900oC
900℃是GH4099合金的长期服役温度。在900℃实验条件下可观察到，随着热等静压温度的提高GH4099合金的抗拉强度和屈服强度都略有提高。抗拉强度从330 MPa提高到了430 MPa，延伸率从14.5%显著提升到18.5%。图9给出了上述样品在900℃拉伸断口的SEM形貌。
图9
图9
不同热等静压温度下的GH4099合金900℃拉伸断口
Fig.9
Tensile fracture surface of GH4099 alloys at 900oC (a) HIP1, (b) HIP2, (c) HIP3
图9给出了热等静压致密化的GH4099合金试样的宏观拉伸形貌，未观测到宏观塑性变形；小平面之间的塑性变形以撕裂方式相连接，在断口未观测到脆性解理断口的河流花纹。这表明，PREP成形的GH4099合金试样的拉伸断裂方式为准解理断裂，其断口呈现出解理断裂形貌，断口表面的韧窝较少，表明拉伸塑性较高。裂纹主要沿PPBs扩展，粉末颗粒表面的碳化物与金属基体的塑性不同，其弹性模量也与基体不同，因此在拉伸过程中受力时两者的变形不协调，在PPBs表面形成孔洞进而成为裂纹源。在本文的实验中，随着热等静压温度的提高粉末原始颗粒边界逐渐消融，粉末之间的结合程度提高，在宏观上表现为延伸率的提高。还能观测到，用HIP3制度制备的试样其拉伸断口没有明显的颗粒脱粘，表明粉末的结合程度较高。以上结果表明，用HIP3制度制备的GH4099合金试样的拉伸性能显著优于HIP1和HIP2。
2.3 GH4099合金薄壁筒体的成形
选用PREP法制备的GH4099粉末为原材料，按照1230℃/150 MPa/4 h的热等静压制度制造薄壁筒体。粉末在包套内充分震动，振实密度可达到致密体的65%~69%。在热等静压致密化过程中粉末/包套体的体收缩超过30%。如此大的致密化变形收缩量提高了控制粉末构件尺寸的难度[8]。用有限元模拟可提高效率优化出最佳的实验方案，可对粉末热等静压成形过程进行有限元模拟。
基于连续介质模型的有限元方法进行热等静压的数值模拟，使用ABAQUS软件自带的多孔金属模型，对GH4099粉末压坯的热等静压致密化过程包套/模具的尺寸收缩进行模拟。该零件为轴对称回转体，为了减少计算工作量和提高计算效率，构建了二维轴对称网格模型。
本文采用的有限元本构模型为Gurson模型，包含材料的弹塑性过程，屈服准则为
其中cosh(x)为双曲余弦函数，q1、q2、q3为参数，由Tvergaard[29]引入，q和p分别为Mises等效应力和静水应力的绝对值，f为孔隙率。相对密度趋于1(f = 0)时该屈服条件收敛于Mises屈服准则形式。有限元的应力积分求解过程基于完全隐式的返回映射算法，Aravas[30]推导了该模型的一致切线模量。有限元模拟前后处理过程在Abaqus/CAE界面下实现，分析过程使用Abaqus/Standard求解器。根据薄壁筒体的对称性特点，取筒体的1/4为有限元模拟的研究对象，设置周期对称边界。基体材料的初始相对密度设定为0.68，q1、q2、q3取值为1.6、1、2.5[31]，温度与压力初始值设置为23℃、0.1 MP，二者演化过程同热等静压制度保持一致。图10给出了有限元模拟的薄壁筒体尺寸收缩情况，其中粉末体共设置58746个四面体单元。
图10
图10
有限元模拟薄壁筒体收缩前后尺寸的对比
Fig.10
Predicted size of thin-walled cylinder before (a) and after (b) shrinkage by FEM
采用包套热等静压工艺制备的GH4099粉末合金薄壁筒体，其截面模型的示意图如图11a所示。薄壁筒体设计高度为550 mm，沿高度方向筒体的直径从上到下有显著的变化，最大直径约为520 mm，最小直径约为440 mm，筒体上不同高度的直径均有差异，因此成形难度较大。与实际薄壁筒体热等静压成形后关键部位的截面收缩对比，如图11b所示，分别在上下端面和中部共取6个特征点进行尺寸对比。薄壁筒体实际尺寸与模拟尺寸的对比，列于表3。以关键部位01处(是薄壁筒体与其他结构的连接处)GH4099薄壁筒体顶端的内径值为例，实际测量尺寸为450.51 mm，模拟尺寸为465.48 mm，模拟偏差为3.2%，可见模拟结果与实测结果吻合良好。其中筒体底端外径06处的偏差较大，推断是筒体底部与地面摩擦或粉末粒度偏析所致。根据表3中的数据，实际尺寸与模拟计算值的整体尺寸偏差小于8%，关键尺寸相对偏差小于5%。用粉末冶金热等静压制备复杂或者大型构件不可能实现净成形，为了精准控制构件的关键尺寸可降低部分非关键部位尺寸的精度。
图11
图11
薄壁筒体实物模型图和薄壁筒体截面模拟与实测值的相对位置关系
Fig.11
Physical model diagram of thin-walled cylinder (a) and relative positional relationship between simulated and measured values of thin-walled cylinder cross-section (b)
表3 薄壁筒体特定位置尺寸的实测值与模拟值的对比
Table 3
| Position | Actual size / mm | Simulated size / mm | Deviation / mm | Relative error / % |
|---|---|---|---|---|
| 01 | 450.51 | 465.48 | 14.97 | 3.2 |
| 02 | 479.49 | 497.52 | 18.03 | 3.6 |
| 03 | 443.90 | 462.56 | 18.66 | 4.0 |
| 04 | 476.77 | 496.88 | 20.11 | 4.0 |
| 05 | 447.58 | 455.08 | 7.50 | 1.6 |
| 06 | 480.32 | 521.03 | 40.71 | 7.8 |
进行有限元模拟辅助包套设计，可预测热等静压成形后的GH4099薄壁筒体的尺寸，指导大尺寸复杂薄壁件整体成形制备和控制成形尺寸。用X射线对薄壁筒体内部质量检验的结果表明，没有出现孔洞、夹杂和裂纹等缺陷。采用荧光检验了粉末制件的表面质量，结果表明没有出现裂纹、未充填满、穿透性缺陷等。对粉末制件的目视检查表明，没有出现明显的缺陷、飞边、毛刺和模具残留。
3 结论
(1) PREP法比EIGA法更适用于热等静压成形，用此法制备的GH4099粉末球形度更好，卫星球较少，粉末氧化程度更低。
(2) 选择1165℃~1230℃内较高的1230℃能更好地实现GH4099粉末的致密化，成形后的GH4099合金力学性能最优。
(3) 用有限元模拟可指导设计GH4099薄壁筒体的模具，关键尺寸的模拟结果与实测值的偏差小于5%，制备出的GH4099薄壁筒体复杂件没有出现冶金缺陷。
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