Experimental study on the collapse behavior of graded Ti-6Al-4V micro-lattice structures printed by selective laser melting under high speed impact

Functionally graded Ti-6Al-4V micro-lattice structures were manufactured by selective laser melting (SLM) method. By controlling the cell size along the building direction, two density distribution modes including step-wise gradient and continuous gradient were designed. The density distributions of the specimens were evaluated according to the geometric model reconstructed from the X-ray tomography images. Afterwards, the high speed impact experiments on the graded specimens were conducted by direct Hopkinson Pressure Bar (DHPB) system. Through the high speed photography and digital image correlation (DIC) method, the effects of gradient on the global deformation evolution of the graded Ti-6Al-4V micro-lattice structures under high velocity loading were revealed. Meanwhile, cell assembly based 3D mesoscopic finite element (FE) models were respectively created based on perfect beam elements and X-ray tomography. Accordingly, the dynamic response of the 3D-printed graded structures were simulated by LSDYNA. A more detail vision to the local deformation of the specimens under impact loading was provided. Finally, the protective abilities of the two graded structures under high speed collision were compared and analyzed. It was concluded that the continuously graded micro-lattice structures with negative gradient could provide better protection for the object behind, while the stepwise graded structures might lead to secondary shock which should be avoided.