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The microstructure of Ti-6.5Al-2Zr-1Mo-1 V (TA15) alloy fabricated by laser powder bed fusion (LPBF) is characterized by the ultrafine α′ martensite with a high density of dislocations and twins, which is distinct to that of conventionally forged TA15 alloy. The α′ martensite in the as-built alloy transforms to stable α phase when annealing below the β transus, and meanwhile the dislocation density decreases and the α lamellae coarsen with increasing of annealing temperature. An excellent synergy of strength and ductility was achieved in the sample after designated heat treatments, which is better than that of the forged counterpart at both room temperature and 500 °C. In-situ tensile test at 500 °C demonstrated that the plasticity of as-built TA15 alloy was dominated by the formation of microscale shear bands (MSBs) between the α′ lamellae, which results in pronounced strain localization and limited tensile ductility. In the sample with coarsened α lamellae, it was found that the activation of pyramidal slip system at 500 °C effectively alleviated the strain localization and cracking tendency during mechanical loading, which is essential to achieve a desired balance of strength and ductility. These findings are helpful to promote the practical applications of LPBF near-α titanium alloys.

The non-uniform mixture microstructure is always revealed in laser welds of Al–Si coated hot stamping steel. It is very necessary to understand aluminum-rich segregation on phase morphology evolution. At same time the uniform and high aluminum welds were tried to obtain by replacing keyhole mode with conduction mode. The uniform fusion zones contained 3–6 wt.% Al, higher than normal keyhole welds. The weld microstructure was mixed of delta ferrite, lath martensite and few discussed needle phases with different morphology. Lower Al region revealed high fraction of island martensite, when aluminum content w(Al) > 5 wt.%, the needle phase was precipitated from the matrix of delta ferrite. High resolution transmission electron microscopy confirmed it long periodical stacking ordered structure in body-cubic crystal matrix.