Study On The Ptlp Diffusion Bonding Process Of Si₃N₄ Ceramic/Ht250 Cast Iron

Ht250 cast iron is widely used in hydraulic parts,cylinder blocks,pump bodies,compressors and other pressure and leakage resistant parts because of its low casting cost and good cutting performance.Si₃N₄ceramic have excellent properties such as high hardness,high strength,low coefficient of thermal expansion,good oxidation resistance and corrosion resistance.It is very suitable for thermal insulation parts of diesel engine.Through the connection of Si₃N₄ceramic/Ht250 cast iron,the problem that Si₃N₄ceramic are difficult to process and shape due to poor plasticity can be well solved.Ti/Cu/Ti is used as the intermediate layer for PTLP diffusion connection between Si₃N₄ceramics and Ht250 cast iron.It mainly relies on Ti and Cu to form eutectic liquid phase at high temperature.With the mutual diffusion between elements,the liquid phase zone advances to both sides and spreads freely on the surface of ceramics and cast iron.The active element Ti reacts with Si₃N₄on the ceramic surface to form Ti N and various Ti-Si phases,and reacts with graphite and Fe on the surface of Ht250 cast iron to form Ti C,Ti Fe.With the isothermal solidification process in the liquid phase region,closely bonded joint structures are formed at the joints on both sides.When the welding temperature is 1000℃and the holding time is 1H,the maximum shear strength of the joint is 97MPa.However,with the further increase of temperature or holding time,the residual thermal stress at the joint increases,and a variety of intermetallic compounds gather and grow at the joint interface.These brittle phases lead to the embrittlement of the interlayer material,the ability to relieve stress decreases,and the joint strength decreases rapidly,only 47MPa-73MPa.Ti/Cu/Kovar/Cu/Ti is used as the interlayer for partial transient liquid phase（PTLP）diffusion connection between Si₃N₄ceramic and Ht250 cast iron.It mainly relies on Ti/Cu to form eutectic liquid phase at high temperature.With the mutual diffusion of elements,the liquid phase advances to both sides until Ti and Cu are completely dissolved.Compared with Ti/Cu/Ti intermediate layer,the difference is that in addition to the reaction of Ti element on the surface of Si₃N₄ceramic and Ht250 cast iron,Ti element also diffuses to the surface of Kovar alloy through Ti-Cu eutectic liquid phase area.Ti element reacts with Fe and Ni element at the interface of Kovar alloy,forming Ti Ni₃,Ti Fe and other phases,which makes the composition of the whole joint more uniform.It alleviates the problem of aggregation of a large number of Ti-Cu compounds near the Si₃N₄ceramic interface,and finally forms a tightly bonded joint with the isothermal solidification process of liquid phase area.With Ti/Cu/Kovar/Cu/Ti as interlayer,Si₃N₄ceramic and Ht250 cast iron are connected by partial instantaneous liquid phase（PTLP）diffusion at different welding temperatures and holding times.With the increase of temperature or holding time,the composition transition of each interface reaction layer in the joint was more uniform.The shear strength of the joint reached the highest value of 112 MPa when the welding temperature was 1000℃,holding time was 1h and welding pressure was 2MPa.Compared with Ti/Cu/Ti as interlayer,the joint strength is increased by 15.5%.However,with the further increase of temperature or holding time,the residual thermal stress at the joint increases,and a variety of intermetallic compounds gather and grow at the joint interface.These brittle phases reduce the plasticity of the interlayer material,reducing the ability to relieve the stress in the joint,and the joint strength decreases to 64MPa-89MPa rapidly.