Research On The Forming Processes And Propertiesin Selective Laser Melting Of Medical Alloy Powders

Selective Laser Melting (SLM) is one of the most potential Rapid Prototyping and Manufacturing technologies and as one of the most promising areas of development. It is based on the principle of additive manufacturing and by using a high-energy laser to melt metal powder directly, it can manufacture metal parts which are nearly full density and have high-performance, especially for low-volume, personalized metal manufacture artificial body repair. However, metal forming properties of artificial prostheses tend to have strict requirements, at present, domestic medical technology and performance SLM forming alloy powder has not been systematically studied. Therefore, this paper focuses on the forming process, forming mechanism, microstructure and mechanical properties characterization and regulation of the SLM forming medical alloy powders, and the manufacture of denture has also been researched. The main content of research in the paper is as following:(1) The forming process window, microstructure and mechanical properties of the316L stainless steel powders in SLM process were systematically studied. For the316L stainless steel powders with average particle size of approximately30μm, the increasing of scan speed, the lower energy density of the single track is needed, but the minimum of laser power required for totally melting the powers is gradually increased. It’s found that the laser power has important effects on the scanning pitch. The more of the laser power and the larger of the optimum scan pitch is needed. In the paper, the optimum scanning pitch is0.06mm. The microstructure has obvious epitaxial growth characteristics and the grain is mainly composed by the afterbirth-like crystal with diameter of about0.5μm. Through the analysis, it’s found that the [001] direction is the preferred growth direction of the grain and a certain angle with maximum temperature gradient. At room temperature, the tensile strength can reach up to795MPa, but the elongation is only18.6%. This indicates that the mechanical properties have high strength and low ductility. By the solution treatment, the grain size is changed and the mechanical properties of parts are adjusted with it. The experimental result shows that with the increasing of solution temperature, the grain size becomes larger and the tensile strength decreases slowly, but the elongation increased significantly. When the heat treatment temperature is1050℃, the tensile strength decreases5%but the elongation increase up to62%. The study also found that the size of the error of the parts have a random distribution characteristics and is mainly distributed in the horizontal direction. By adjusting the process parameters the accuracy of manufacturing can be improved in SLM processing.(2) The forming processes window, microstructure and mechanical properties of the Ti6A14V alloy powders in SLM process were systematically studied. For the Ti6A14V alloy powders with average particle size of approximately70μm, the forming process window is very narrow. The minimum laser power is80W for completely melted the powders. When the scan speed is faster than300mm/s, some large powders will can’t be completely melted. Under the optimal process parameters, the relative density can reach99%. In SLM process, the extremely fast cooling rate lead to the microstructure is mainly composited by needle martensite, which contains large amounts of a phase and primary β phase. The martensitic transformation leads to unconspicuousnt laser track boundaries. Compared with the forging Ti6A14V alloys, the horizontal tensile properties (yield strength of1204MPa, tensile strength of1346MPa, and elongation of11.4%) are better than it, and tensile properties in the vertical direction (yield strength of1116MPa, tensile strength of1201MPa) can meet the requirements, but elongation of9.88%is slightly smaller. Because of the hard brittle martensite, the tensile fracture mechanism is mainly the quasi-cleavage fracture at room temperature. In SLM process, a large amout of residual stress stains in the parts because of the high temperature gradients, even can cause cold crack. The anisotropic mechanical properties are caused by the fibrous texture and the <1000> is the preferred growth direction. By adjusting the annealing process, the microstructure can be improved. The stress relief annealing can eliminate residual stress and improve the tensile strength; The recrystallization annealing can eliminate the anisotropic mechanical properties, but it will reduce the tensile strength; The grains will overgrowth by treated with the fully annealed and lead the mechanical properties to worse. Eventually, by adjusting the scanning spacing of the porous Ti6A14V implant, the pores diameter in range of200～450μm can be gained. The yield strength and Young’s modulus were be designed in the range of467-862MPa and16～85GPa, independently. The intrinsic relation between mechanical properties and scan line spacing is set up via the formulas. The major failure mechanism of the porous structure is the adiabatic shear band (ASB) fracture and there is no significant influence of the scan line spacing on the failure mechanism.(3) The forming process window, microstructure and mechanical properties of the Co-Cr alloy powders in SLM process were systematically studied. For the Co-Cr alloy powders with particle diameter of about20μm have a wide processes window. When laser power between80～120W and the scanning speed in the range of300-500mm/s, smooth surface can be gained in the experiment and the optimal scan spacing is of0.04mm. The microstructure of SLM-made Co-Cr alloy is mainly composed by the CoCrMo austenite substrate and the diffused filamentous carbide.The carbide concentration in the molten pool on the grain boundary. The carbide enhances the hardness (476±6HV) and makes it slightly higher than the conventional forged Co-Cr alloy. The tensile properties also exhibit anisotropy at room temperature. The yield strength in the horizontal direction is of1142MPa, a tensile strength is of1465MPa and an elongation is of7.6%. Hoever, in the vertical direction, the yield strength is of1002MPa, a tensile strengthis of1428MPa and an elongation is of10.5%. The tensile strength is similar to forging parts and the elongation is similar to casting parts. The EBSD results shows that the parts have asimilarfibrous texture in [001] directions. On the basis of the previous studies, the three-dimensional porous Co-Cr alloy implants are prepared.(4) The key forming processes and metal-ceramic bond strength of SLM-made denture were systematically studied. This method also is successfully verified can be used in clinical application. The study find that the outer diameter and the sport spacing of the tubular support are the key factors for SLM-made dentures. The result shows that the outer diameter of1.0mm and sport spacing of0.8mm can satisfy the denture requirements. In order to gain better contact between the sports and the dental surface, the sport must carve into the dentures. For the porcelain fused to metal (PFM) restorations, the special surface character of SLM technology can enhance the mechanical lock between the metal and ceramic layers and this can significantly improve the metal-ceramic bond strength. For the powders with average particle size of70μm, the incompletely raw powders adhered to the surface is the special surface characteristic. In this condition, the bond strength can reach116MPa, more than3times compared with the international ISO9693(minimum value is of25MPa) standard. For the powders with average particle size of20μm, the characteristic surface is the uneven surface formed by the overlap track each other. The metal-ceramic bond strength can reach49.2MPa. The result found that the fire temperature influence the thickness of transition layer, which affects the metal-ceramic bond strength. In the study, the optimum fire temperature is930℃and the thickness of the interface is about2μm. Meanwhile, the metal-ceramic bond strength has minimal system errors in the optimum fire temperature. The experiment found that the surface treatment of polish+sand blast is better for the bond strength (49.23MPa) than just polish (40.27MPa) or just sand blasting (39.47MPa) the surface. This mainly because of the surface treatment of polish+sand blast not only reducing the metal particles adhered on the surface and which can reduce the incidence of defects such as porosity but also still retains the special characters of SLM process. So this surface treatment method can enhance the bond strength by improving the mechanical lock effect. Finally, the author successfully made dentures suing SLM technology for patients and the SLM-made denture good cooperate with belt type.In summary, the paper focuses on the forming processes window, microstructure and mechanical properties of medical alloy powders (including316L stainless steel, Co-Cr alloy and Ti6A14V alloy) in SLM processing. By fabricating Co-Cr alloy dentures, the SLM technique is verified feasibility in clinical application. The study provides a theoretical and experimental guidance for medical alloys formed by SLM technique.