C/C和C/SiC复合材料与金属钎焊接头界面组织与性能研究

IMR OpenIR

	C/C和C/SiC复合材料与金属钎焊接头界面组织与性能研究
	沈元勋
学位类型	博士
导师	张劲松
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料加工工程
关键词	复合材料钎焊中间层显微组织力学性能残余应力 Composites Brazing Interlayer Microstructure Mechanical Properties Residual Stress
摘要	"本文针对C/C、C/SiC与高温合金、钛合金和铜合金的钎焊连接进行研究。通过以下措施制备高强度陶瓷与金属接头：采用活性元素Ti和Cr金属化改善钎料的润湿性；采用低膨胀中间层配合高柔性钎料缓解残余应力；在钎焊界面构建具有强烈钉扎增强作用的指接界面结构。以溅射Ti膜和TiH2粉为活性元素，分别采用Ag28Cu, Ag9Pd9Ga和Cu3.5-Si钎料钎焊制备了C/C与高温合金接头，C/SiC与钛合金接头和C/C与铜合金接头。结果表明活性元素Ti明显促进钎料在复合材料润湿性。钎焊过程中Ti与陶瓷基体发生化学反应生成碳化物和硅化物反应层。界面反应层既提高了钎料润湿性，也形成较强的界面结合。为满足接头的高温应用，采用AgPd钎料钎焊C/C与高温合金接头和C/SiC与高温合金接头。对复合材料进行铬金属化处理，表面形成的铬化反应层明显促进钎料润湿性。发现C/SiC表面铬化反应层并不能有效阻止合金元素与SiC的恶性反应，继而采用气相化学沉积法渗碳改性后再铬化处理可有效解决这一问题。以上两种改性方法均能实现钎料在毛化复合材料表面较好的润湿，且钎料在激光孔针内填充饱满，界面无明显缺陷，结合完好。采用中间层制备的接头强度明显提高，其增强作用在于对接头残余应力的缓解。中间层对残余应力的缓解体现在两个方面：一是有效缓解了复合材料与金属之间的线膨胀系数不匹配。Al2O3、Mo、Nb和4J33线膨胀系数介于复合材料和金属之间，接头形成线膨胀系数梯度过渡的界面结构，有效减小了残余应力。另一方面，采用中间层接头焊缝组织有效改善，焊缝基体为均匀、塑性较好的固溶体组织，通过其弹塑性变形有效缓解了残余应力。实验结果表明采用Al2O3中间层的C/C与高温合金接头和C/SiC与钛合金接头强度分别由未采用中间层时的16 MPa和21 MPa明显提高至32 MPa和63 MPa。与中间层相比，指接界面结构对接头增强作用更加显著。以Al2O3为中间层的毛化C/C与高温合金接头和毛化C/SiC与钛合金接头强度分别达到73 MPa和103 MPa。其增强机理主要为：一是增大接头连接面积。与平面接头相比，激光针孔被钎料填充，使连接面积明显增大；二是钎料针的强烈钉扎作用。钎焊后钎料针紧密镶嵌于复合材料基体，较强的界面结合在加载过程中起到强烈的钉扎作用从而提高接头强度。断口分析表明毛化接头断裂过程中裂纹发生明显偏转远离钎焊界面，部分偏转进入复合材料基体造成复合材料纤维粘撕断裂，部分偏转穿过钎料针造成钎料针断裂甚至发生塑性变形，同时提高接头韧性。此外，指接结构明显提高接头抗热震性能。采用ABAQUS有限元对接头残余应力场进行模拟。结果表明中间层对缓解接头残余应力效果显著。对于C/C与高温合金接头，主要破坏应力为C/C钎焊界面剪应力，最大剪应力约120 MPa（Ag28Cu钎焊），出现在距自由外表面0.5mm处。采用中间层后剪应力显著降低至97MPa，且转移至距外表面0.1~0.2mm处。中间层厚度变化对剪应力变化影响不大。对于毛化C/C接头，钎料针根部出现较为严重的剪应力集中，达到140~150 MPa，说明接头容易从钎料针根部发生断裂，与实际断裂模式吻合。对于C/SiC与Ti55接头，接头外表面陶瓷近钎焊界面产生较为严重的拉应力集中现象。钎料柔塑性对减小残余应力效果明显，采用低屈服强度的AgPd钎料接头最大拉应力由Ag28Cu钎焊时的329 MPa迅速降低至95.7 MPa。配合中间层后进一步降低至88.8 MPa。采用毛化C/SiC后接头最大拉应力转移至C/SiC母材距钎焊界面0.8mm处，达到275 MPa，远大于平面接头拉应力峰值88.8 MPa。模拟结果与本文制备接头的实际断裂形式均吻合较好。 "
其他摘要	"C/C composites and C/SiC composites have excellent mechanical properties at elevated temperature, which make their attractive materials for thermal structural applications in aerospace, brakes and nuclear reactors. Many such applications require joining these composites to various metals. In this paper, we joined the C/C and C/SiC composites to nickel-based superalloy, titanium alloy and copper alloy using brazing method. There are two main problems in the manufacturing of these joints. One is the wettability of the brazes on the composites; the other is the large mismatch between their coefficients of the thermal expansion (CTE), which induces great residual stresses during cooling process. In order to overcome these problems, several effective solutions were proposed. First, active elements Ti and/or Cr metallization process was used to promote the wettability of the brazes. Second, interlayers with low CTE were inserted between the composites and metals to mitigate the CTE mismatch between them. In addition, ductile brazes were utilized to accommodate the residual stresses through their plastic deformation. Third, a novel strengthening method by construction of a zig-zag interfacial structure between composites and braze was proposed. In this paper, C/C-superalloy joint, C/SiC-Ti alloy joint and C/C-Cu alloy joint were brazed using Ag28Cu, Ag9Pd9Ga and Cu-3.5Si braze respectively. The results showed that the use of active element Ti could obviously promote the wettability of brazes on the composite substrates. During the brazing, the active Ti reacted with the composites to form reaction layer consisting of carbide and/or silicide at the interface. Thus, the wettability of the brazes had been improved through the formation of the reaction layer and thus a strong interfacial bond of the brazes on the composites was obtained. In order to achieve high temperature applications, AgPd braze was used to join C/C to superalloy and C/SiC to superalloy. For the purpose, the composites were metallized by treating them in Cr powders and a layered structure reaction layers consisting of chromium carbides and/or chromium silicides were obtained on the composites surfaces. The formation of the reaction layer obviously improved the wettability of the AgPd braze. In addition, it was found that the reaction layer on C/SiC surface could not prevent the diffusion and reaction of the alloying elements with SiC. Furthermore, prior to the metallization process, a thin carbon layer was deposited on the machined C/SiC surface by CVI process. The carbon layer combined with the chromium carbides layer acted as diffusion barrier at the joining interface. Consequently, the reactions between alloying elements and SiC were effectively suppressed. For both of the composites modified by Ti and Cr, it should be noted that all the laser-machined holes had been completely filled with the braze alloy. The joints revealed intimately contacted interface and were free of cracks and voids. The joint with an interlayer exhibited higher strength than that without interlayer. The improvement of joint strength is due to the reduced residual stresses by using an interlayer. There are two main aspects that may be responsible for the decreased residual stresses. On one hand, the use of interlayer favours gradual transition of CTE from the composites to metals. The interlayer materials (Al2O3, Mo, Nb, 4J33) have the medium CTE. Thus, the use of interlayer benefits the mitigation of CTE mismatch between the composites and metals and effectively decreases the stress gradient in joint. On the other hand, the interlayer exhibits positive effect on the microstructures of the joint. The joint with interlayer has homogeneous and ductile microstructure which will benefit the stress accommodation via its plastic deformation. For example, the C/C-superalloy joint and C/SiC-Ti55 joint with Al2O3 interlayer exhibited bending strength as high as 32MPa and 64MPa respectively, while those joints without interlayer got strength as low as 16MPa and 21MPa respectively. Furthermore, the proposed zig-zag interfacial structure exhibited more obvious strengthening effect on the joint strength than that of the interlayer. The strength of the C/C-superalloy joint and C/SiC-Ti55 joint with machined composites using Al2O3 interlayer was dramatically enhanced to 73MPa and 103.7MPa, respectively. The factors that have induced mechanical strengthening of the zig-zag interfacial structure may include the enlarged joining area and strong pinning effect of the braze spikes. The micro-machined composites surface directly resulted in a larger joining area than that of a flat joint by creating a 3-D transition region between the composites and braze after the holes were fully filled with braze. Moreover, the dispersing braze spikes into the composites obviously strengthened the joint through the pinning effect. The fracture micrographs show that the crack paths were well moved from the composites/braze interface and deflected into the composites or cross the braze spikes, leading to a large amount of composites matrix attaching on the metal side of the fracture surface, pulling out and breaking of the braze spikes. Theses factors also enhance the toughness of the joint. In addition, the joints with a zig-zag interfacial structure showed excellent thermal resistance during the thermal shock tests. Non-linear finite element code ABAQUS was employed to evaluate the distribution of residual stresses in the composites-metal joint. The results showed that the use of interlayer significantly decreased the residual stresses in joint. For the C/C-superalloy joint brazed by Ag28Cu braze without interlayer, the most destructive residual stress was the shear stress (τxy) at C/C/braze interface at the location about 0.5mm from the free surface, which was approximately 120MPa. For the joint with interlayer, the maximum τxy decreased to 97MPa and appeared at location closer to the free edge. Besides, the change of the thickness of the interlayer did not show great influence on the maximum τxy. In the case of C/C-superalloy joint with micro-machined C/C, great τxy as high as 140~150MPa was concentrated at the root segment of the braze spikes, which indicated that the joint was prone to crack across the braze spikes. For the C/SiC-Ti55 joint, large axial tensile stresses (σyy) were concentrated in the corner of the composites region near the joining interface. It was also found that the ductile braze with low yield strength resulted in significant stress reductions. The maximum σyy of the joint brazed by AgPd braze sharply decreased down to 95.7MPa comparing to that of the joint brazed by AgCu braze (329MPa). In particular, the maximum σyy was further reduced to 88.8MPa for the joint using interlayer. For the joint using micro-machined C/SiC, a great concentration of σyy up to 275MPa presented in composites at free surface at locations about 0.8mm from the joining interface. The high residual stresses zones obtained through the simulation fitted well with the fractographic observation of the joints with the zig-zag interfacial structure. "
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64465
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	沈元勋. C/C和C/SiC复合材料与金属钎焊接头界面组织与性能研究[D]. 北京. 中国科学院金属研究所,2012.