Study On Microstructure And Corrosion Properties Of Medical Cobalt-based Alloy By Laser Direct Metal Deposition

With the continuous progress of medical technology,medical implants are more and more popular,Co-based alloys have good elastic modulus and wear resistance and corrosion resistance,while in the solution of liquid to maintain a stable passive state,so it becomes the one of preferred material of medical implants.However,the traditional manufacturing technology restricts the development of medical implants due to the long processing cycle,many defects,and individual customization difficulties.Therefore,a new technology is needed to overcome the above difficulties and accelerate the development of medical implants.As a rapid manufacturing technology,Direct Direct Deposition(DMD)technology is a material manufacturing key technology and personalized "3D printing"-one of the important symbols of the "Third Industrial Revolution".The technology can combination computer control technology and laser rapid melting technology,and really overcome the traditional technology in the processing cycle is long,difficult to process complex parts,customization difficulties and other difficulties,so as to provide a new medical implant materials preparation technology.In this study,the effects of different molding process parameters on the microstructure and corrosion resistance of Co-based alloys were investigated.The samples were characterized by optical microscopy(OM),scanning electron microscopy(SEM),electron probe microanalysis(EPMA),transmission electron microscopy(TEM),X-ray diffraction(XRD),ion-elution,X-ray photoelectron spectroscopy(XPS)and Polarization curve.Thin walls and blocks with good formability,no obvious cracks,can be directly machined.The typical morphologies of the samples with different process parameters are that there are a lot of equiaxed grains at the upper part and two kinds of grain forms at the bottom of the sedimentary layer,which are the columnar grains distributed in the lower part of the sedimentary layer and equiaxed grains distributed in the upper part,but equiaxed grains also through the multi-layer dendrite be found.With the increase of the ratio of energy,The volume fraction of the columnar crystal increases and the equiaxed crystal fraction decreases,both of them exhibit competitive growth mechanism When the mass ratio is between 0 and 1000J/g,the grain size increases rapidly with the increase of the mass to energy ratio.When the ratio of mass to energy is more than 1000J/g,the size of the grain size changes smoothly.The grain boundary tends to be rich Mo-poor Co,while the grain boundary is rich Co-depleted Mo.Laser direct deposition of samples formed by a large number of face-centered cubic y-Co and a small amount of close-packed hexagonal σ-Co phase composition.In the Hanks’ solution and simulated acid saliva,the maximum amount of Cr ion elution were 0.604 mg/L and 1.620 mg/L and the maximum amount of Cr ion elution was 0.018 mg/L and 0.003 mg/L,which were lower than those of the national standard Dissolution of biomedical materials,will not have an impact on the human body.The corrosion rate of samples immersed in Hanks’ solution(pH=7.4)increased with the increasing grain size,a large amount of Cl-in the solution forms unstable complexes with the metal on the surface,which hinders the formation of the passivation film.OH-in the solution reacts with the surface of the immersion metal to form CoO,Co3O4 and CoOOH,Co(OH)2 is decomposed to form CoO;the other is Co(OH)2 and OH-continue to form Co3O4 and CoOOH and other substances,Cr products Cr2O3.The corrosion rate of samples immersed in simulated acid saliva(pH=5.0)decreased with the increasing grain size.While a large amount of H+ in the solution reacts directly with the metal surface,hindering the formation of the passive film,after the immersion of the surface of the sample due to the role of H+ only the presence of Co and Cr2O3.The corrosion rate of the samples immersed in the two solutions increased with time,and the corrosion potential Ecorr of the direct deposition is higher than that of the casting and the corrosion current Icorr is smaller than that of the casting,indicating that the corrosion performance is better than that of the casting.