Microstructure And Martensitic Transformation Of Selective Laser Melted TiNi Alloys Processed By Electropulsing

Selective laser melting（SLM）overcomes the difficulty that traditional processing can only produce the components of TiNi alloys with simple geometric shapes,while retaining excellent properties like shape memory effect and superelasticity.However,TiNi alloys fabricated by SLM still need post heat-treatment to eliminate the problems,like defects,uneven microstructure.Electropulsing can overcome these problems by rapidly reaching high temperatures,improving the microstructure and properties.In this paper,SLM-TiNi alloy with near equal atomic composition was selected as the target material.Three different initial states of alloy,namely,as-fabricated state,solution-treated state,and aged state,weretreated by electropulsing.Phase constituent was determined by X-ray diffraction（XRD）.Microstructure was studied by optical microscopy observation and transmission electron microscope（TEM）,martensitic transformation was studied by differential scanning calorimetry（DSC）and shape memory effect and microhardness were studied using a dynamic thermomechanical analyzer（DMA）and a microhardness tester,respectively.As the current intensity or applied time increases,the energy barrier of precipitation reduces due to the thermal and non-thermal effects of electropulsing,resulting in the precipitation in the as-fabricated alloy dissolving and then precipitation at a temperature lower than the common dissolution temperature.At a current intensity of 420 A,recrystallization occurs in the solution-treated sample after electropulsing.After electropulsing at 420 A for 7 s,the grain size reduces from 34μm to 17μm.As the applied time increases,the lenticular Ti₃Ni₄phase in the aged sample dissolves and then forms with a uniform distribution.As the current intensity or applied time increases,the martensitic transformation temperature of the as-fabricated sample decreases first and then increases.During the cooling process,the as-fabricated sample with 15 s electropulsing and the aged samples with 1 s and 7s electropulsing exhibit a multi-step transformation in the way of combination of B2→R→B19’and B2→B19’,which is related to the distribution of precipitates.In the solution-treated samples,a two-step transformation of B2→B19’occurs in the electropulsed samples for 3 s,15s,and 30 s,which may be caused by the effect of electropulsing treatment on the atomic diffusion and the inhomogeneous composition.After electropulsing,both the as-fabricated and the aged samples exhibit better shape memory effects.After electropulsing,the reversible strain increases,and the irreversible strain keeps almost constant in the as-fabricated sample.However,the reversible strain decreases slightly,but the irreversible strain significantly decreases in the aged sample treated by electropulsing.After electropulsing,the microhardness of samples increases in comparison with that of the different initial samples,and the value first increases and then decreases with the increase of applied time.The above results establish the relationship of electropulsing treatment,microstructure,martensitic transformation and functional properties of SLM-TiNi alloys and may provide a reference for the development of the additive manufactured TiNi alloy.