The Investigation Of Ti-Ni Alloy Fabricated By Additive Manufacturing Technology

Ti-Ni alloys have two difference phases including austenite(B2)stable at high temperature and martensite(B19’)stable at low temperature,three different crystal structures including twinning martensite,detwinning martensite and austenite,and achieves the shape memory effect or super-elasticity through six transformation modes between the above three different crystal structures.Moreover,they also have excellent comprehensive mechanical properties,corrosion resistance and biocompatibility,which make them widely used in industrial,medical and daily life.But the difficulties existing in the preparation and machining process limit the further application for Ti-Ni alloys.The development of additive manufacturing technology(AM)has provided another method to fabricate complex shape and structure of Ti-Ni alloy and expanded the application range of Ti-Ni alloys.However,the researchs on AM of Ti-Ni alloys is still in the infancy stage,so it is particularly important to study the effect of AM parameters on the properties of equiatomic Ti-Ni alloys.Firstly,the effect of selective laser melting(SLM)parameters on the properties of solid and porous Ti-Ni alloy and the improvement of post heat treatment were studied.The results show that Ti-Ni pre-alloyed powders with a particle sizes ranging among 15-60 μm are used for SLM,and the laser power,scan spacing and layer thickness were kept constant at 200 W,100μm and 50 μm respectively,with laser scanning speed for 1000 mm/s.Under,the input laser energy density of 40 J/mm3,the relative density of solid sample can reach 99.5%.The sample has a higher compressive fracture strength and a lower compressive fracture strain than the conventional casting samples.The changing of laser scanning speeds can cause different types of defects inside the samples,but it has little effect on the phase composition,phase transformation behavior and micro-hardness of SLM Ti-Ni alloy.The hot isostatic pressing(HIP)was used to improve the mechanical properties of tensile bars fabricated by the optimal SLM parameters,for which through the elimination of lattice defects and uniform of microstructure under high temperature with the elimination of metallurgical defects and uniform of Ti2Ni phase under high pressure to improve for 67.3%and 23.2%on ultimate tensile stress and elongation,respectively.The solid sample fabricated by SLM are mainly composed of B19’ martensite,but the faster cooling rate during AM process is allowed the B2 phase stable at room temperature,and the special thermal cycling,a larger number of dislocations and the formation of a stress field by Ti2Ni phase at grain boundaries could form the R phase transformation.It results in the transformation temperature of As lower than Ms.The formation of Ti2Ni on the SLM samples increases the Ni content in the matrix,and the effect of formation of Ti2Ni phase is greater than the effect of evaporation of Ni during the SLM process to make the transformation temperatures lower than the Ti-Ni pre-alloyed powders.Apart from this,the HIP has no effect on the phase composition of the SLM Ti-Ni tensile bars,but the high temperature and high pressure can affect the phase transformation temperatures,and the two effects are all not as good as the formation of Ti2Ni phase during the furnace cooling process.There are only one transformation peak during the cooling and heating process of Ti-Ni porous structure samples fabricated by SLM,and the transformation temperatures are also lower than the Ti-Ni pre-alloyed powder with the As lower than Ms.The compressive strength,elastic modulus and fatigue properties are all decreased with the increasing of porosity.The phase transformation during the fatigue process makes the hysteresis line to a hysteresis loops and extend the fatigue initiation stage,which could work together with the secondary and deflection of cracks let the normalized ration between text stress with yield stress reach 0.24-0.33 for the fatigue life reach to 106.The fatigue failure of the porous samples is composed of cyclic creep and fatigue damage,and cyclic creep becomes the main cause of fatigue failure.The effects of electron beam melting(EBM)parameters on the properties of solid equiatomic Ti-Ni alloy were investigated,and the preheating temperature was determined by the effect of heat treatment on the properties of pre-alloyed powder used for EBM.The results show that the pre-alloyed powder with a size from 53 to 106 μm is different from that used for SLM,the number of satellite balls on the surface increases,and the surface of some larger sized powders has a slight coating effect.Due to the heterogeneous of the composition,the introduction of internal stress with dislocations and difference grain sizes make the EBM used per-alloyed powder have multi-step transformation peaks,which is different from SLM used powder.The transformation temperatures as including Mf and Af for the pre-alloyed used for EBM are increased with the increasing of the heat treatment temperatures until 650℃.Based on the sintered state of the pre-alloyed powder under different heat treatment temperatures with phase transformation behavior,the optimal preheating temperature for EBM is determined to be 750 ℃。The Ti-Ni samples had a high relative density reached 97%for fabricated by different combinations of focus offset(FO)and speed function(SF)in the selected range.The corresponding phase transformation temperatures for all the Ti-Ni samples fabricated by EBM were higher than the pre-alloyed Ti-Ni powder,due to the effect of evaporation of Ti element higher than the formation of Ti-rich Ti2Ni phase during the quickly melting and solidification process.On the other hand,the EBM manufacturing parameters of FO and SF had limited influence on the phase contents,phase transformation temperatures and Vickers hardness.Due to the feature of the EBM fabricating method,the different types defect would be introduced in the Ti-Ni solid samples.Though all the samples had similar high relative density,the performance of the compression behavior were shown great difference,and the crack defect had the larger affect than the gas and lack-of fusion porosities on the compression fracture stress and strain.