表面金属热渗辅助钎焊C/SiC-Nb接头界面增强机制

Microstructure improvement of C/SiC-Nb brazing joints reinforced with heat penetration

针对碳纤维增强碳化硅复合材料(C/SiC)与Nb金属钎焊接头存在残余应力高导致连接强度下降的难题,借助反应扩散机制对C/SiC复合材料表面进行改性,成功制备了纤维增强热渗区,降低了钎焊接头的残余应力,实现了界面结构强化,获得了高质量钎焊连接接头.本研究选用的热渗金属Ni-Cr-Si合金可在C/SiC复合材料表面充分润湿,为扩散的快速充分进行奠定了基础;之后探究了热渗工艺对热渗深度及界面结构的影响规律,实现了任意深度热渗区的可控制备.用AgCuTi钎料对热渗处理后的C/SiC复合材料与Nb进行了钎焊连接,钎焊接头的典型界面结构可表示为:C/SiC/Cf+Ni_(2)Si+Cr_(3)Ni_(2)Si+Cr_(23)C_(6)/Ag(s,s)+(Cu,Ni)/Nb.本研究从残余应力的调控与界面结构改善所引导的第二相强化等方面,详细阐述了热渗工艺对接头的界面强化机制.热渗区热膨胀系数介于复合材料与钎料之间,形成属性梯度过渡,有效缓解了接头高残余应力.热渗区钎料的连接由Cu、Ni互扩散实现,替代原位脆性化合物反应层;碳纤维与弥散分布碳化物起到的第二相增强作用,有效提高区域断裂强度,裂纹断裂路径也随之延长,接头强度获得进一步提升.在热渗深度为77μm时,接头平均抗剪切强度可提升至115.2 MPa,相比未热渗接头提高了36%.

Brazing C/SiC composites and Nb is always trouble with high residual stress owing to high property mismatch,especially in terms of the coefficient of thermal expansion.It weakens the joining strength and reduces the joining quality due to welding defects.The traditional method tends to modify the brazing alloy to overcome the problem of property mismatch by introducing reinforcement or interlayer.However,the application of these methods was limited by the low addition and poor distribution,thus inhibiting the development of brazing composites and alloy,as well as the use of composites.An increase in aircraft speed and size requires the composite-alloy joint to exhibit higher joining strength.To achieve this,heat penetration was employed to modify the composite surface of brazing C/SiC and Nb with the AgCuTi brazing alloy.It aims to provide a new idea for the design of the composite-alloy joint.In this study,the microstructure,residual stress distribution,and reinforced mechanism of the joints were investigated in detail.Ni-Cr-Si alloy was introduced to penetrate due to the good reactive diffusion of Ni with SiC.The Ni_(2)Si+C phase was in situ synthesized by replacing the SiC matrix and constructing the penetration zone using carbon fibers.The high strength and elasticity of the carbon fiber dispersed in the penetration zone increase the mechanical properties and decrease the brittleness.Moreover,the wettability of Ni-Cr-Si alloy on C/SiC composites improved as the Cr content increased.The ideal content of penetrating alloy was confirmed as Ni10Si15Cr(wt%).To reduce the brittleness of the composite-alloy joint,such content needs to be removed,so that bonding between alloys can be achieved.The typical microstructure of penetrated joints was C/SiC/Cf+Ni_(2)Si+Cr_(3)Ni_(2)Si+Cr_(23)C_(6)/Ag(s,s)+(Cu,Ni)/Nb.The Cr elements were slightly diffused into the penetrating zone,thus reinforcing the bonding between carbon phases and Ni-Si compounds.The reaction layer of the compounds on C/SiC composites was replaced with the fiber reinforced penetration zone.The depth of this zone directly influenced the joining strength.To investigate in detail the penetration parameters,the penetration depth was optionally and homogeneously controlled.Accurate regulation of the penetration depth is beneficial for various application conditions.In addition,the reinforcing mechanism of penetration focused on the reduction of residual stress and enhancement of the C/SiC-alloy interface.In the penetration zone,the coefficient of thermal expansion gradually changed from C/SiC to AgCuTi alloy,which reduced the residual stress.The stress concentration of the C/SiC interface in the original joint was also eliminated.The fiber bundles supported the residual stress to prevent the appearance of cracks.Consequently,the residual stress in the brazing seam was reduced.The carbon fibers and dispersed phase enhanced the fracture strength and toughness of the penetrating zone.Fracture occurred in the penetration zone rather than in the reaction layer,with breaking and pulling out of fibers.A longer fracture path requires more energy for crack propagation.All these improvements contributed to the joining quality.The average shear strength increased from 85.5 to 115.2 MPa,with a penetrating depth of 77μm.The penetration treatment of C/SiC composites provides a new method of brazing composites to alloy.Such method is suitable for different brazing alloys and various brazing requirements.Moreover,it enables the surface modification of composites.Thus,it is important to pay attention to the crucial effect of fiber reinforcement on composites.This research aimed to efficiently expand the application of composites and dissimilar joints.