NiTi形状记忆合金电弧熔融涂覆及微连接机理

Arc-fused Coating Process and Micro-joining Mechanism of NiTi Shape Memory Alloys

NiTi形状记忆合金作为表面涂覆材料,有利于提高构件的耐磨和抗氧化性能。基于非熔化极氩弧焊(Tungsten inert gas welding,TIG)电弧熔融涂覆技术,分别采用常规直流、超高频脉冲(Ultra high-frequency pulsed,UHFP)电流两种模式,在TA1纯钛板表面沉积3层NiTi涂层。基于有限体积法,利用FLUENT建立计算流体力学(Computational fluid dynamics,CFD)仿真模型开展熔融涂覆工艺及微连接机理研究,模拟对比了两种电流模式下的熔池传热传质、NiTi熔滴过渡、熔覆涂层及微连接界面的Ni元素传输和分布规律,数值计算和试验结果吻合较好。结果表明,涂层主要由NiTi(B19’)和NiTi_(2)相组成,其中近等原子比的NiTi占主导地位;直流模式下涂层主要为粗糙的球状晶粒,而超高频模式下涂层为细化的树状晶粒,且超高频电流有助于熔池振动和元素均匀分布,并有效提升成形质量。

NiTi Shape memory alloys(SMAs)is one of the most promising coating materials to improve the wear and oxidation resistance of the base components.In the study,the conventional direct current and ultra high-frequency pulsed(UHFP)current were respectively utilized to deposit 3-layer NiTi-coating on TA1 pure titanium substrates with tungsten inert gas welding(TIG)arc-fused coating technology.Using FLUENT software,a computational fluid dynamics(CFD)simulation model was established to analyze the coating process and micro-joining mechanism.The multiphysics,NiTi droplet transfer,Ni element transport and distribution in the micro interlayers were numerical analyzed under UHFP current compared with the direct current case.The numerical and experimental results agree well.It can be concluded that the 3-layer coatings are mainly composed of B19’NiTi martensites and NiTi_(2) phases,in which NiTi with nearly equal atomic ratio is dominant.The coating of the direct current case is mainly composed of coarse globular grains,while refined into smaller dendrites once UHFP current is used.In addition,the UHFP current is beneficial to the vibration of the molten pool,which promotes the uniform distribution of Ni element and effectively improves the forming quality of the coatings.