Microstructure And Properties Of Ni-Al Based Composite Coating On TC21 Titanium Alloy By Laser Cladding

Titanium alloys have been widely used in aerospace,petrochemical,biomedical,marine engineering and other fields due to the excellent mechanical and functional properties,however,the disadvantages of poor surface performance limit their deeper applications.Ni-Al based intermetallic compounds are ideal surface coating materials because of their excellent mechanical properties,high temperature oxidation resistance and wear resistance.As one of the most effective surface modification technologies,laser cladding has many advantages such as fast cooling rate,dense microstructure,firm metallurgical bonding and high efficiency.Therefore,using laser cladding technology to prepare Ni-Al based composite coatings is an effective method to improve the surface comprehensive performance of titanium alloys.In this paper,mixed powders with different content of Ni,Al,Cr,CeO₂,and WS₂ were selected as cladding materials to prepare Ni-Al based composite coating on the surface of TC21titanium alloy by laser cladding.Using X-ray diffractometer,scanning electron microscope and other equipments to analyze the microstructure of the cladding layer,the microhardness distribution and wear resistance were tested by micro hardness tester and CFT-I material surface performance comprehensive tester,respectively.This paper aims to improve the surface properties of TC21 titanium alloy by studying the effects of Cr content on the microstructure and properties of the Ni-Al based composite cladding layer,and then studying the effects of CeO₂ and WS₂ content on the microstructure of Ni-Al based composite cladding layer on this basis.The experimental results of the influence of Cr content on the microstructure and properties of Ni-Al based composite coatings indicate that theα-Cr precipitated in the cladding layer after adding Cr element,and the microstructure is mainly composed of Ni（Al,Ti）and its surrounding network distribution intercrystalline structure TiNi,α-Cr and other phases.The microhardness of the cladding layers increased to about 2 times of the substrate.Cr element is conducive to improving the fracture toughness and wear resistance of Ni-Al based composite coating.When the addition content of Cr is 20 at.%,the friction coefficient of the cladding layer is the lowest and most stable,the wear resistance is about 2.95 times of TC21 titanium alloy substrate.The experimental results of the influence of CeO₂ and WS₂ content on the microstructure of Ni-Al based composite coatings show that the phase composition of the cladding layer is more diverse,and new phases such as metal oxides Al₂O₃,TiO,NiO,intermetalic CeNi₅ and ceramic TiN_0.3 appeared after CeO₂ is dropped.CeO₂ can refine the crystal grains,purify the microstructure,adjust the chemical composition of Ni（Al,Ti）,and promote the generation of ceramic reinforced phases such as TiN_0.3 by adsorbing impurity elements in the molten pool.What’s more,TiS self-lubricating phase were generated in-situ andα-W precipitated in the cladding layer after WS₂ adding into the cladding material,and Ni（Al,Ti）/α-Cr eutectic structure appeared in the upper part of the cladding layers at the same time.In addition,the number and size of TiS,α-W increase with increase of WS₂,but the relative content of Ni（Al,Ti）/α-Cr eutectic structure decrease gradually.