Research On Formation Mechanisms And Properties In Wire Arc Additive Manufacturing Of NiTi Shape Memory Alloys

NiTi shape memory alloys(SMAs)have been widely used in aerospace and other industries due to their excellent properties such as superelasticity(SE),shape memory effect(SME),biocompatibility,etc.However,the traditional manufacturing methods and laser/electron beam based additive manufacturing(AM)techniques have difficulties in fabricating large-sized and complex NiTi SMAs parts and will introduce impurities or porosity defects,which results in poor overall performance.Wire arc additive manufacturing(WAAM)based on gas tungsten arc welding(GTAW),has advantages such as high efficiency,low cost and low porosity,making it an ideal method for fabricating NiTi SMAs.Currently,the research on GTAW-WAAM of NiTi SMAs is in its early stages,and the fabricated parts have poor ductility and do not exhibit functional properties such as superelasticity.Moreover,poor metallurgical bonding is also obsearved between the WAAMed NiTi SMAs and dissimilar substrates,making it difficult to meet the requirements of practical application.In this dissertation,the formation mechanism and properties under different droplet transfer and current modes are analyzed during GTAW-WAAM of NiTi SMAs.NiTi parts with highly ductile and excellent superelastic are successfully fabricated,and sound metallurgical bonding effect between the deposits and dissimilar substrate is achieved.Based on the results of the dynamic monitoring and material characterization experiments,the computational fluid dynamics(CFD)models were established to reveal the mechanisms of molten pool dynamics,microstructure evolution,and solute diffusion.The results can be concluded as follows:(1)The relationship between the height of NiTi wire(h),with a range of 0.5-2.0 mm,and the droplet transfer modes is established.The formation mechanism and properties under different droplet transfer modes are then revealed.The results indicate that as the NiTi wire height increases from 0.5 mm to 0.75 mm,the droplet transfer mode changes from the liquid bridge transfer(LBT)to free droplet transfer(FDT)mode.Compared to the FDT mode,the LBT mode shows improvements in the macroscopic morphology,grain size,and mechanical properties of the deposited samples.(2)A coupled CFD_c-CFD_i model with high precision and efficiency is established to investigate the characteristics of the molten pool dynamics,microstructure evolution under different droplet transfer modes.The results indicate that,compared to the FDT mode,the LBT mode exhibits a quasi-steady state of the molten pool with slower fluid flow.The temperature gradient(G)and the grain growth rate(R)of the molten pool determine the grain distributions in the deposits.The equiaxed and columnar grains are formed at the top and bottom of the deposits,respectively.(3)A WAAM method for NiTi SMAs based on the direct current superimposed with ultra-high frequency pulsed(SUHFP)current under frequency of 20 k Hz is proposed.Based on the results of material characterizations and numerical simulations,the grain refinement mechanism during SUHFP-WAAM is revealed.The study demonstrates that compared to the direct current(DC)mode,the SUHFP current promotes the molten pool vibration and grain fragmentation on the solidification front to achieve the fine-grain strengthening.The samples in SUHFP mode exhibit good superelasticity at room temperature,and the fatigue properties are improved compared to the DC-WAAMed samples.(4)The metallurgical bonding effect and solute diffusion behavior during NiTi SMAs WAAM based on the dissimilar TA1 titanium alloy substrate are investigated.The strengthening mechanism of the interface structure between the solute mixing zone and the substrate was revealed during SUHFP-WAAM.The results indicate that compared to the DC mode,the molten pool vibration facilitates the bubble escape,uniform solute diffusion,and grain refinement on the solidification front,which enhances the metallurgical bonding based on the dissimilar substrate.