Defects and Microstructure of Be-Al Alloy Prepared by Laser Additive Manufacturing
Be-Al alloy was prepared by laser selective melting.The forming defects,microstructure,and hardness of the Be-Al alloy were analyzed under low or high laser power with two scanning strategies of layer by layer unidirectional line scanning and 67° interlayer staggered scanning.The results show that the primary forming defects in the Be-Al alloy include pores,micropores,hot cracks,and poor fusion.The pores are small in size and few in number,thus having little effect on the properties of the alloy.The micropores can be eliminated by using the 67° interlayer staggered scanning and increasing the laser power.The hot cracks are mainly caused by the tensile stress exerted on the liquid film in the long channels between the columnar structures at the center of the molten pool at the end of solidification,extending in the subse-quent layer-by-layer deposition process.And the opening width of the hot cracks increases with the increase of energy density.The molten pool is affected by multiple thermal cycles when it is remelted by the sur-rounding newly scanned molten pool with a high lap rate,leading to a steady-state transition of Be-Al two-phase diffusion,dendrite growth of Be phase,and difussion of Al phase to cause microvoids that result in poor fusion.The microstructure of the Be-Al alloy prepared by additive manufacturing mainly consists of Al phase,Be phase,and some metastable microstructure,in which the Be phase and metastable microstruc-ture collectively cause a significant residual stress in the alloy.Additionally,while the orientation of the Be phase base plane(0001)in the alloy has a certain preferential distribution,it also exhibits randomness.In contrast,the direction of Al phase(100)has an obvious preferential orientation along the scanning direc-tion of the molten pool,resulting in the anisotropy of the alloy microstructure.Meanwhile,the hardness test along the scanning direction of the molten pool reveals that the Be-Al alloy exhibits higher microhard-ness due to its fine microstructure and a large number of metastable microstructure.
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