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Refining microstructure is an important issue for laser additive manufacturing (LAM) of titanium alloy. In the present work, the microstructures of LAM-fabricated Ti6Al4V alloy were refined using a low energy density with the combination of a small spot diameter, a low laser power, and a high scanning speed. The microstructure, hardness, wear performance, and molten pool thermal behavior of LAM-fabricated Ti6Al4V coatings were studied. The results show that the grain sizes of both prior β and α phases are strongly dependent on the cooling rate of the molten pool. The fine prior β grains and submicron-scale acicular α phases were obtained under a low energy density of 75 J mm−2 due to the high cooling rate of the molten pool. In addition, the as-fabricated Ti6Al4V sample with submicron-scale acicular α phase showed a very high hardness of 7.43 GPa, a high elastic modulus of 133.6 GPa, and a low coefficient of friction of 0.48. This work provides a good method for improving the microstructure and mechanical performance of LAM-fabricated Ti6Al4V alloy.

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The 2xxx series Al alloys have been widely used in aerospace industry owing to their high strength, good plasticity and superior formability. To ensure a good control of shape, the quenched alloy sheets require a small pre-deformation before artificial aging. However, this pre-deformation considerably deteriorates the mechanical strength of the Al-3.0Cu-1.8Mg-0.5Si (wt%) alloys due to the formation of unfavorable large-sized precipitates at dislocations. To tackle this issue, we designed a pre-aging process prior to the pre-deformation. The thermal-mechanical treatment, involving pre-aging, pre-deformation and subsequent aging, markedly enhanced the ultimate tensile strength up to 521 MPa compared to that (448 MPa) of the alloy without pre-aging. Microstructure characterization revealed that the fine precipitates (~ 2 nm) with a uniform dispersion were promoted within the Al matrix, which in turn partly suppressed the formation of the unfavorable large-sized precipitate (~ 100 nm). Our findings provide a new clue for designing stronger Al alloys with age-hardenability.

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