Laser Powder Feeding Additive Manufacturing Of NiTi Shape Memory Alloy Based On Ceramic Inoculant

NiTi shape memory alloy is a typical smart material,which is widely used in many fields such as medicine and aerospace.In order to enable the laser additive manufacturing of NiTishape memory alloys to achieve large-scale production in practical applications,and to promote the application of 4D printing of shape memory alloys.In this project,the method of adding fine ceramic inoculant to NiTishape memory alloy is used to provide a heterogeneous nucleation core inside the sample during laser coaxial powder feeding additive manufacturing,which promotes the nucleation of grains;Avoid defects such as large columnar crystals,thermal cracks and pores inside the sample due to high temperature gradient,repeated melting and thermal cycling of laser additive manufacturing.The effects of ceramic inoculant content,laser power and scanning rate on the forming quality,microstructure,phase composition,phase transition temperature,room temperature tensile,compressive and tensile fracture morphology and cross-sectional microhardness of composite alloy samples were studied.The test results obtained are as follows:The mismatch degree was calculated by Bramfitt theory,and TiC and TiB₂ceramic particles with a small mismatch degree between the lattice structure and NiTialloy were selected as inoculants.The low-energy ball milling process is as follows:the ball milling ratio is ball:material=1.5:1,and the ball is milled for 8 hours.The ceramic particles can evenly adhere to the surface of the NiTishape memory alloy spherical powder,and the spherical powder has a complete shape and is not broken.The process parameters of laser coaxial powder feeding additive manufacturing were optimized by single-pass test.The optimized parameters are:laser power 1000,1250 w,scanning speed 300,400 mm/min,powder disk speed 2 r/min(5 g/min),spot diameter 3 mm,powder feeding gas flow 5L/min,high-purity argon gas(12 L/min);The forming effect of the sample is good,and there are few defects such as cracks and pores.The composite powder as-deposited sample has columnar crystals along the manufacturing height of the thin-walled part,and with the increase of the ceramic inoculant,when the TiC content is 0.6 wt%,the coarse columnar crystals can be effectively suppressed and improve the degree of uniformity of the structure compared with 0.3 wt%TiC.The XRD results of phase analysis of the samples at room temperature show that all samples have both B2 austenite phase and B19'martensite phase.Field emission electron scanning microscope EDS line scanning results show that during the laser additive manufacturing process of NiTishape memory alloy,the Tielement of the substrate diffuses to the bottom of the sample,resulting in a decrease in the Ni/Ticontent ratio and an increase in the phase transition temperature of the sample;The phase transition temperature at the top of the as-deposited sample is about 20°C,and the phase transition temperature at the bottom is about 60°C.DSC test of pre-alloyed powder shows that with the increase and decrease of temperature,the endothermic/exothermic peaks of the curve are obvious,because the mixed alloy powder has uniform composition and good heat transfer.Compared with NiTi50 powder,the composite powder added with TiC and TiB₂ceramic particles has higher latent heat of transformation whether it is inverse martensitic transformation during heating or martensitic transformation during cooling.At the same time,all pre-alloyed powders and most of the as-deposited samples have one phase transition peak during heating,namely martensite?austenite phase,while there are two phase transition peaks during cooling,austenite?R phase,and then from R phase?martensite phase.Through the microhardness test of the as-deposited cross-section,it is found that when the laser power remains unchanged,the microhardness of the alloy increases with the increase of the scanning speed,and the maximum value can reach 808.27 HV.Through the room temperature tensile test,it is found that the fracture mode of all samples is brittle fracture,the maximum tensile strength along the scanning speed direction can reach 809.27 MPa,and the maximum elongation can reach 7.801%.Through the room temperature compression test,it is found that the compressive deformation perpendicular to the scanning speed direction is higher than that in the parallel direction,and the maximum compressive strength reaches 1619.16 MPa;The test results show that when the laser power and the addition of ceramic particles are kept unchanged,with the increase of the scanning speed,the yield strength and compressive strength of the sample perpendicular to the scanning speed direction both show an increasing trend,and the strain also increases.