Formation and control of martensite in Ti-6Al-4V alloy produced by selective laser melting

The morphology and substructure of alpha' martensite in Ti-6Al-4V cuboid samples produced by selective laser melting (SLM) have been investigated to explore the formation mechanism and control method of a' martensite. Results showed that the microstructures of SLMed Ti-6Al-4V samples are composed of a typical hierarchical martensite structure with a high density of dislocations and twins, including primary, secondary, tertiary and quartic alpha' martensites within columnar prior beta grains. Also, the formation and evolution mechanisms of such hierarchical martensites are proposed based on heat history of SLM, material characteristics and substructure features. Besides, based on quantitative metallography, the lengths of major and minor axes as well as aspect ratio of primary and secondary alpha' martensites are mainly in the range of 10-70 mu m, 1.0-2.0 mu m and 5-40, respectively. The martensitic sizes increase with hatch spacing, but firstly increase and then decrease with the increase of scanning velocity from 600 to 1100 mm/s. So, martensitic size can be controlled by adjusting the SLM processing parameters. (C) 2016 Elsevier Ltd. All rights reserved.