Preparation Of TiB₂-TiC-Ni Cermet By Field-Activated And Pressure-Assisted Combustion Synthesis And Weldability With Ti

In this paper, TiB2-TiC-Ni cermets were prepared via in-situ synthesis based on the Ti-B4C-Ni reaction system by utilizing the field-activated and pressure-assisted combustion synthesis technology (FAPACS), and by adding TiAl intermetallic compound layer to achieve the metallurgial diffusion bonding of TiB2-TiC-Ni cermets and Ti in one step. FAPACS is used to realize the synthesis of the materials quickly under the condition of multifields including electric field, temperature field and mechanical field, which is considered as a fast, energy-saving and environmental method to prepare materials. Compared with hot-pressing and hot isostatic pressing method, FAPACS possesses a series of advantages, such as low energy and high efficiency, strong activation, the products with high density, easy to control the parametres and a wide range of sintering materials system, which has been achieved rapidly development in the field of synthesizing new materials.In this study, Ti-Al and Ti-B4C-Ni reaction system were used as research, powders with different ratios were passed mechanical alloying. The synthesis mechansim and transformation rule of the powders were analyzed in the process of mechanical alloying, the reaction mechanism between Ti and B4C was investiagated at the condition of adding Ni binding or without adding Ni binding, separately. Then the microstructure, mirohardness, wear resistance, density and fracture toughness of the TiB2-TiC-Ni cermets with different Ni ratio under different preparation parameters were tested, and the solidification mechanism for the TiB2-TiC hypoeutectic was also dissussed. Finally, the synthesis of TiB2-TiC-Ni cermets and connection with Ti were reliazed by FAPACS in one step.The microstructure and properties of the joining were tested and analyzed, the influence of the different current or different pressure on the interfical microstructure and mechnical properties of the joining of Ti/TiAl/TiB2-TiC-Ni were also studied.The results show that, the powders can be significantly refined by mechanical alloying and a small amount of product phase can be synthesizd, such as Ti19Al6, TiAl3, TiB2 and TiC, and so on.This maybe have an important impact on the subsequent reaction process.The microstructure of TiB2-TiC-Ni cermet is homogeneous distribution and TiB2 particles appears in the form of rod-like or hexagonal, while TiC is elongated squared or irregular, and the ratio is about 2:1. The dark TiB2 grains and light gray TiC grains staggered distribution in the white Ni matrix. The maximum microhardness of TiB2-TiC-Ni cermet prepared by FAPACS is about 2760HV, and the wear resistance is superior to the specimens prepared by hot pressing, about 2 times, but significantly higher than the wear-resistant alloy steel and W18Cr4V. The wear resistance increases with the reduction of Ni content in the cermet and the synthesis current in sintering process. When the synthesis current is 1500A, the composition is 75% (Ti+B4C)+25% Ni and load is 15N, the wear is only 1.2000mg. The density and fracture toughness of the samples sintering by FAPACS are significantly higher than the samples prepared by hot-pressing, when the Ni content reached 50%, TiB2-TiC composite ceramics have the greatest density, reaching about 5.937g/cm3. When the synthesis current is 1400A and the pressure is 60MPa, the fracture toughness of the sample has reached 10.82 MPa·m1/2,which the Ni content is 25%.The reaction mechanism between Ti and B4C is changed when Ni binder is added, which will affect the following solidification behavior of TiB2-TiC composite ceramic. In the FAPACS process, the grains growth morphology of 66.7% mol TiB2-TiC is hypoeutectic and it's unique, the first eutectic phase TiB2 and secondary phase TiC staggeres distribution in the white Ni matrix, the microstructure of eutectic (TiB2+TiC) is nanoscale which appears at the boundaries of TiB2 and TiC.The microstructure and mechanical properties tests of the diffusion bonding joint between Ti and TiB2-TiC-Ni composites indicate that the gradient cermet gets connection with Ti very well used the TiAl intermetallic compound as the intermediate layer. The interface among every layer forms intensive metallurgical combination, and the diffusion layer appears which is about 25μm and 100μm width, respectively. The microhardness of the joint gradually increases from the Ti substrate to the layer C. The measured maxminum values for the surface composite layer rechches 2760HV.The shear strength of interface is determined by the different welding current and different pressure, when the current is 1400A and the pressure is 30MPa, the shear strength is relatively stable, which is about 55MPa.When the current is 1500A and the pressure is 60MPa, the shear strength reaches the maximum value which is about 85.88Mpa. No matter the size of current and pressure, the fracture occurres at the interface between TiB2-TiC-Ni cermet and TiAl intermetallic compound layer, which indicates the interface is a weakness zone.