Thixotropic mixing and 3D printing of biodegradable alloys for medical implant applications

Biodegradable alloys such as Magnesium-Zinc (Mg-Zn) and Zinc-Iron (Zn-Fe) are new emerging materials for medical implant devices. Unlike traditional medical implant metals such as stainless steel, titanium, cobalt chromium, etc., that require high corrosion resistance in the human body, biodegradable alloys have unique degradation characteristics, and their produced implants can dissolve completely once the bone and tissue returns to their original normal functions. Major metal additive manufacturing (AM) systems, including powder-based and wire-feed systems, have been developed for more than 30 years. However, these technologies are still constrained by high cost and inefficient printing. Direct printing of molten alloys for freeform fabrication, on the other hand, has not yet been accomplished. To overcome these constraints, a novel material processing and printing method: thixotropic metal 3D printing was studied in this research. The new method converts a biodegradable alloy into a two-phase, thixotropic slurry with a finely dispersed globular morphology, and an extrusion-based 3D printing system was designed to fabricate desired metal components. By manipulating thixotropic properties of alloys, the investigated Zinc-based and Magnesium-based alloys exhibit "paste-like" material behavior below their melting point. In addition, thixotropic processed alloys received more homogenous structure than raw materials. It has been found that the average hardness loss for processed thixotropic alloys in the test was less than 10% compared with their raw samples. In the printability test, a sectional heating control system was developed in the printing process to provide precise temperature control. The thixotropic processed alloy was reheated in the reservoir and maintained a stable liquid fraction of 0.6 at the nozzle outlet that is within the printable range of 0.4 to 0.6 explored by our simulation. Besides critical printing parameters were analyzed to improve material printability, it has been tested that thixotropic alloys can be printed under 1.5mm, 1.0mm, and 0.8mm diameter nozzles and with the configurations of extrusion speed, nozzle-substrate distance, and platform moving speed, the printing resolution reached 1.01mm using a 0.8mm nozzle, which can potentially produce desired medical implants.