Research Progress of γ-Free Layer Gradient Cemented Carbide AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY22030030

Chinese Library Classification Number:TG135.5

Citation Information:

Cemented carbide is a composite material prepared by powder metallurgy with refractory metal compounds（WC，TiC，TaC，NbC，etc.）as matrix and transition metal elements （Co，Fe，Ni，etc.） as binder phase. It is widely used in the fields of machine tool cutting tools，mining and rock drilling tools，tensile die and wear-resistant parts. Coated gradient cemented carbide is the most commonly used cutting tool and plays an important role in precision machining. When used in combination with TiC，TiN，Ti （C，N） and Al₂O₃ coatings，cracks often occur between coating and matrix due to the different thermal expansion coefficients，which affects the cutting performance and service life of the tool. The binder-rich and γ-free layer of the gradient cemented carbide can improve the toughness of the alloy surface，reduce the crack generation and prevent its propagation. In this paper，the formation mechanism and preparation method of γ-free layer gradient cemented carbide are sketched，the application and development of phase diagram calculation method on the simulation of gradient layer formation are summarized，and the effects of powder size，sintering process and alloy compositions （including nitrogen source，binder phase and secondary carbide） on gradient layer formation and alloy properties are described in detail. The future research direction of gradient cemented carbide with high-performance coating has also been prospected. The formation of the gradient layer is based on the strong thermodynamic coupling between Ti and N atoms. During the sintering process，denitrification occurs at the surface of the cemented carbide. Ti （C，N） phase in the surface layer dissolves continuously into the liquid binder phase along with the outward diffusion of N and the inward diffusion of Ti through the liquid binder phase，resulting in the formation of the γ-free surface gradient layer. The formation of the gradient layer is affected by many factors. CALPHAD method can accurately describe the phase diagram thermodynamics and diffusion dynamics information of the multi-component material system under different alloy compositions and process conditions based on the Gibbs free energy and atomic mobility parameters of each phase. The evolution law of the microstructure and its control mechanism can be studied effectively. Combined with the hardness and fracture toughness model，the hardness and fracture toughness of the gradient layer can be also predicted. The accuracy and reliability of the simulation on the gradient layer formation are mainly affected by three aspects，that is，the thermodynamic database，the diffusion kinetic database and the labyrinth factor characterizing effective diffusion coefficient. Many efforts have been made on their improvement to make the simulation more accurate. However，there still exist some limitations in material design only using CALPHAD approach. The labyrinth factor，as a key factor affecting the simulation results，is mainly obtained by experience at present. The diffusion couple test or tracer diffusion test can be designed to obtain the reliable labyrinth factor. In the development and research of high-performance gradient cemented carbide，the finer powder size of the raw materials has been utilized to prepare ultrafine gradient cemented carbide. A thicker gradient layer and higher hardness are obtained，but the fracture toughness is reduced. The traditional two-step sintering process system has been maturely applied to the preparation of graded cemented carbide，but some new preparation processes are still in the preliminary stage of exploration. The use of new nitrogen sources，binder phases and secondary carbides has proved to be an effective means to change the gradient layer structure or improve the properties of the alloy. Besides TiN and Ti （C，N），more new nitrogen sources are being applied，and external nitrogen sources and nitrogen-free graded cemented carbides are also being studied gradually. New binder phases such as intermetallic compounds and high entropy alloys have been successfully applied to homogeneous cemented carbide and cermet，but there is little research on graded cemented carbide，and more attempts should be made in the future. The strengthening mechanism of new secondary carbides needs to be further explored. the evaluation of the service performance of the coating tool is limited for the gradient cemented carbides as the matrix with the new nitrogen sources，binder phases and secondary carbides. In the development of the new gradient cemented carbide，the combination with the coating and the cutting performance of the new gradient cemented carbide should be paid attention to in addition to the gradient layer and mechanical properties. Multi-scale material computational simulation and material genetic engineering methods have been successfully applied in steel materials，aluminum alloys and other alloy systems，greatly accelerating the material research and development cycle. In the future，methods such as multi-scale material computational simulation and material genetic engineering can be focused on to greatly accelerate the material development cycle. The multi-scale simulation of graded cemented carbide can be carried out to reveal the formation and control mechanism of the gradient layer structure from different layers. Based on material genome engineering，high-throughput experiments and high-throughput calculations can be carried out，and the big-data platform for graded cemented carbides can be established to realize the rapid screening and development of newly graded cemented carbides in terms of composition，process and properties.