Effects of process parameters on porosity in laser powder bed fusion revealed by X-ray tomography

This paper reports on X-ray tomography of a series of coupon samples (5 mm cubes) produced under different process parameters, for laser powder bed fusion of Ti6Al4V. Different process parameters result in different pore formation mechanisms, each with characteristic pore sizes, shapes and locations within the 5 mm cube samples. While keyhole pores, lack of fusion pores and metallurgical pores have been previously identified and illustrated using X-ray tomography, this work extends beyond prior work to show how each of these not only exist in extreme situations but how they vary in size and shape in the transition regimes. It is shown how keyhole mode porosity increases gradually with increasing power, and how this depends on the scan speed. Similarly, lack of fusion pores are shown to occur following scan tracks in situations of poor hatch overlap, or a similar but different distribution of lack of fusion porosity due to large layer height spacing, showing respectively vertical and horizontal lack of fusion pore morphologies. Increased spacing between hatch scan tracks and contour scan tracks is demonstrated to form a near-surface porosity similar to that previously reported for slowing at the end of scan tracks which can cause keyhole mode porosity. Insights from 3D images allow improvements in parameter choices for optimized density of parts produced by laser powder bed fusion, and generally allow a better understanding of the porosity present in additively manufactured parts.