Study On Process And Microstructure And Properties Of ZK61 Magnesium Alloy By Selective Laser Melting

Magnesium and magnesium alloys are very similar to human bones due to their density and modulus.They have high specific strength and can be naturally degraded by metabolism after being ingested into the human body.They are more suitable as bone implant materials than traditional metal materials.Selective Laser Melting（SLM）is an additive manufacturing technology that uses a high-energy laser beam as a heat source to form near-compact and complex parts by melting fine metal powders in a selected layer.Due to its high precision and short cycle,the SLM process is particularly suitable for customizing medical artificial bone implants.At present,there are many researches about SLM on stainless steel,titanium and its alloys,cobalt-chromium alloys and other biomedical metals,but few involvs magnesium and its alloys.For this reason,pure magnesium and ZK61 magnesium alloy powders were used as raw materials to explore the process conditions of magnesium alloy In this paper,and the phase structure and mechanical properties of the SLM samples were studied in detail.The main research work and conclusions are summarized as follows:（1）The process conditions and laser parameters of SLM forming ZK61 magnesium alloy were studied.The SLM forming process window of pure magnesium is:laser power 80W-140 W,scanning speed 100 mm/s-500 mm/s,filling pitch 70μm,layer thickness 40μm,In this window,pure magnesium cladding monolayer morphology clear,smooth and continuous without overlapping microcracks.the magnesium alloy substrate with similar composition must be used when forming ZK61 magnesium alloy under the SLM process.The molten metal of the laser is well wetted with the substrate,no brittle phase and inclusions are generated at the interface,so that excessive splashing and warping cracking are well avoided.Preheating of the substrate during the SLM forming process can uniformly dissipate the sample,effectively improve the flatness of the forming surface,and facilitate the smooth processing.The SLM forming process parameter window of ZK61 magnesium alloy is:laser power 50 W-80 W,scanning speed 100 mm/s-200 mm/s,filling pitch 60μm,layer thickness 40μm.A slight laser energy density is likely to cause excessive ablation of the molten metal and a little higher Scanning speed may Cause serious volatile powder recoil for its Low boiling point and density,and both will cause spheroidization.The optimal process parameters are laser power 60 W,scanning speed 100mm/s,filling pitch 60μm,layer thickness 40μm,the relative density of forming sample under which is 99.27%,almost without micropores and microcracks.（2）The phase composition,microstructure and mechanical properties of SLM formed ZK61 magnesium alloy were studied.Due to the influence of extreme temperature gradient and cooling rate,the grain size of SLM-formed ZK61 magnesium alloy is about 25 times smaller than that of homogenous cast alloy.In the extremely short crystallization process,the solute is too late to redistribute,and thus a new phaseβ’(Mg_0.97Zn_0.03)which is close to the original component ratio is formed after solidification,replacing theβphase in the conventional cast ZK61 magnesium alloy.In addition,under the combined influence of fine grain strengthening and loss ofβ-brittle hard phase,the microhardness of SLM state ZK61alloy reaches up to 93±10 HV,which is 32%higher than the as-cast state and 29%higher than the as-rolled state.Different from the 45°shear failure of rolled sample,a small amount of micropores and microcracks in the SLM-formed ZK61 magnesium alloy are gradually expanded under pressure and eventually become mixed fracture.The alloy has a compressive strength of 286 MPa,equivalent to the casting level.The modulus of elasticity is 4.8 GPa,which meets the mechanical properties of bone implant materials.