Metal 3D printing technology is a novel process technology that uses computer-aided 3D design as a blueprint. Through layered discretization of components and CNC forming systems, it employs laser sintering, laser cladding, or plasma rapid deposition processes to create three-dimensional metal entities. A 3D printer divides the 3D CAD model into several layers, sintering or bonding 3D printing metal atomized sphere powder or specialized high-end 3D printing Ni-Ti alloy wire and other materials together. These layers are then stacked layer by layer, with different ring shapes added one by one, until a three-dimensional solid is finally printed.
Shape memory alloy materials can remember their original shape after low-temperature deformation; this phenomenon is called alloy shape memory. Shape memory alloys undergo a structural transformation during the transition from a stable high-temperature austenitic state to a stable low-temperature martensite state. The phase transformation temperature is 20~80℃, and the deformation temperature is 0~5℃, which can be adjusted through slight changes in composition and heat treatment. Among them, Ti-56Ni, Ti-31Ni, Ti54-57Ni alloys are widely used in the medical field. For example, biomedical surgical implants—artificial bone joints, skull, sacrum, ribs, hip bones, patellar claws, encircling bone plates, hip nails, dental bones, vascular stents, and other biomaterials. Medical devices and surgical implants have strict requirements for 3D printing materials. Considering the physical properties and biocompatibility of materials in terms of biomechanics, the main requirements that their performance must meet are: excellent corrosion resistance; (3) excellent biocompatibility, bioadhesion, and bone fusion; (4) excellent mechanical properties, high strength, high stability, high fatigue strength, high tensile strength, and low elastic modulus. Under normal circumstances, 3D printing porous surfaces can reduce the elastic modulus of materials; (5) It has good processing performance, is environmentally friendly, non-toxic, has good deformability, is fast, quick to form, has strong applicability, low cost, and uniform chemical composition. Due to the special performance requirements of Ni-Ti shape memory atomized spherical powder and filament, its chemical composition (mass fraction) is designed as follows: Ni, 54.5%~57.0%; Ti is sponge titanium of grade 0 or above; C, ≤ 0.05%; H, ≤ 0.050%; N, ≤ 0.050%; Co, ≤ 0.050%; Cu, ≤ 0.010%; Cr, ≤ 0.010%; Nb, ≤ 0.025%; Fe, ≤ 0.050%. The vacuum induction furnace melting of Ni-Ti shape memory alloy material introduced in this article fully meets the requirements of biomedical applications for material performance. For nickel-titanium shape memory alloy atomized spherical powder and high-end Ni-Ti shape memory alloy wire, different vertical development fields have emerged according to the different application directions of their 3D printing materials. For example, 3D printed vascular self-expanding stents are used for permanent implants in the cardiovascular system, which are implanted through a catheter and unfold according to shape memory; 3D printed Ni-Ti shape memory alloy materials are used to print skulls, cephalic bones, plates, rib joints, etc. The key technology for preparing 3D printed high-end Ni-Ti shape memory alloy wire by vacuum induction furnace melting method is the ratio of nickel-titanium alloy elements, and the control of impurity elements oxygen, nitrogen, hydrogen and carbon is particularly important. A water-cooled copper crucible is used to obtain an ingot with less pollution and uniform chemical composition under the action of electromagnetic eddy current stirring. The ingot is hot-rolled into coils with a diameter of ϕ 6~8 mm at a hot rolling temperature of 800~900 ℃, and then forged at a rotary forging temperature of 700~850 ℃. After cold drawing, intermediate annealing is performed every 10% of the wire processed, at a temperature of 700~850 ℃. Nickel-titanium shape memory alloys have good hot working properties and can be processed by forging, extrusion, hot rolling, rotary forging, and drawing to obtain wires of various specifications. High-end Ni-Ti shape memory alloy wires with a diameter of ϕ 1.0~3.5 mm are 3D printed.
Using a plasma rapid deposition 3D printer, the alloy wire is fed to a plasma nozzle cooled by an argon gas hood. The alloy wire melts in an argon atmosphere, and after digitally controlled layer-by-layer printing, solid forms such as the skull, sternum, and ribs can be printed. A small amount of grinding is then performed to create the finished product.
3D printing Ni-Ti shape memory alloy atomized spherical powder was prepared by vacuum induction furnace melting method. A 50~500 kg vacuum induction furnace was used for melting, and gas atomization and water atomization methods were employed. The atomized spherical metal powder can be used in metal 3D printers for laser cladding and plasma rapid deposition processes. The quality standards of the prepared 3D printing atomized spherical powder were based on national standards for similar materials. Currently, no national standards for the quality of shape memory alloy atomized spherical powder have been established. The particle size, shape, and purity of the powder are mainly determined according to customer requirements. Particle size is generally divided into four levels: 50~1000 μm is coarse powder; 10~50 μm is fine powder; 0.5~10 μm is micro powder; <0.5 μm is ultrafine powder. The chemical composition must be uniform, without chemical segregation, and the physical properties must be excellent. Because Ni-Ti shape memory alloys have the best shape memory performance and are widely used, even small differences in the alloying elements will significantly cause changes in shape memory performance and mechanical properties. Therefore, the control of the ratio of alloying elements Ni and Ti and impurity elements O, N, H, and C during the material preparation process is particularly important. The commonly used ratio has a Ni content of 54.5% to 57.0%. Changes in the element content in Ti-Ni alloys will cause changes in the phase transformation temperature. When the N content (atomic fraction) changes by 0.1%, the As point will change by about 10 to 20 K. The properties of impurity element O are similar to those of N. The design composition requires an O content (mass fraction) ≤ 0.050%. An increase in O content will cause a decrease in phase transformation temperature, a decrease in shape memory performance, and a deterioration in mechanical properties. Meanwhile, the carbon (C) content must be controlled within a certain range, requiring a C content (mass fraction) ≤ 0.050%. It is important to note that C easily infiltrates during smelting, therefore extra care must be taken when using graphite crucibles. If the impurity elements O, N, H, and C are not properly controlled, the processed wires and atomized spherical powders will experience hot brittleness, fracture, hydrogen embrittlement, reduced plasticity, and easy segregation of chemical composition, and may even cause the alloy material to lose its shape memory properties.
