Microstructure Evolution Of Super-Ni/NiCr Laminated Composite And CR18-NI8Steel High Temperature Brazed Joint

Super-Ni/NiCr laminated composite is a newly developed material composed of super-Ni cover layer and Ni80-Cr20powder by vacuum pressing. Super-Ni cover layer is featured with good toughness, oxidation resistance and corrosion resistance at elevated high temperature. Super-Ni cover layer is able to suppress crack propagation in NiCr base layer, which will prevent instantaneous destruction of the components. Super-Ni/NiCr laminated composite is an attractive candidate in aerospace, energy and power industries due to its low density, good oxidation resistance and corrosion resistance. Two main difficulties in the welding of Super-Ni/NiCr laminated composite were identified. First, both Super-Ni cover layer and NiCr base layer should have good combination with weld metal. Second, it is important to keep the structure integrity of Super-Ni/NiCr laminated composite after welding. Therefore, welding is the key problem impeding the engineering application of Super-Ni/NiCr laminated composite.The joints of Super-Ni/NiCr laminated composite to Cr18-Ni8steel were established by vacuum brazing technology using Ni-Cr-P and Ni-Cr-Si-B nickel-based filler metal. Good combination between Super-Ni/NiCr laminated composite and Cr18-Ni8steel was prepared at980～1060℃and1040～1120℃for20min under a vacuum of1.33×10-4～1.33×10-5Pa. Shear strength of the brazed joints reached143MPa and195MPa. Microstructure, elemental distribution, phase constitution and shear strength were investigated by means of scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electromechanical universal testing machine. The relationship between brazing parameters, microstructure and fracture mechanism was proposed. The diffusion-solidification behavior of the brazed joint was revealed.The brazed region was mainly composed of γ-Ni solid solution, eutectic of γ-Ni(P) and M3P, and some FeNi3when Super-Ni/NiCr laminated composite and Cr18-Ni8steel was brazed with Ni-Cr-P filler metal. P diffused into Super-Ni cover layer along grain boundaries leading to the precipitation of Ni-P eutectic. There was only solution reaction without brittle reaction layer between Ni-Cr-P filler metal and NiCr base layer.When Super-Ni/NiCr laminated composite and Cr18-Ni8steel was brazed with Ni-Cr-Si-B filler metal, element B was the key factor controlling the solidification and interface reaction. When the brazing temperature was1040℃, the brazed region constituted of γ-Ni, Ni3Si, Ni3B, CrB particles. And the Ni3Si dispersed in y-Ni. When the brazing temperature was1060～1100℃, the brazed region constituted of γ-Ni, Ni3Si, Ni3B, blocky CrB and Ni6Si2B. When the brazing temperature was1120℃, the brazed region constituted of γ-Ni, Ni3Si, Ni3B. The athermally solidified stage was concluded as following:â‘ the residual luquid solidified into y-Ni solid solution and the residual luquid L1was rich in B;â‘¡Ni-B binary eutectic reaction:L1â†'γ-Ni+Ni3B and the residual luquid L2was rich in Cr;â‘¢Ni-Cr-B ternary eutectic reaction:L2â†'γ-Ni+Ni3B+CrB and the residual luquid L3was rich in Si;â‘£Ni-Si-B ternary eutectic reaction:L3â†'γ-Ni+Ni3B+Ni6Si2B.γ-Ni solid solution forming in the isothermally solidified stage prevented the infiltration of liquid filler metal into porosities of NiCr base layer, which decreased the combination area between the brazed region and NiCr base layer. The shear strength test and fracture morphology analysis was conducted. The Super-Ni/NiCr laminate/Cr18-Ni8brazed joint fractured along the Ni-P eutectic in the brazed region when brazed with Ni-Cr-P filler metal. When brazed with Ni-Cr-Si-B filler metal, the brazed region of Super-Ni cover layer fractured along the Ni3B interface displaying intergranular brittle fracture. The brazed region of NiCr base layer fractured along a mixed path along eutectic in brazed region and NiCr base layer near the interface.