A new approach to quantify powder's bed surface roughness in additive manufacturing

The powder bed surface roughness plays an important role in the quality of the final parts fabricated using powder bed Additive Manufacturing processes. This paper reports the development and application of a new technique and prototype instrument which can be used to simulate the creation of a powder bed and reveal the surface properties of that bed. The tester was successfully used to study powder bed surface roughness, but can also be used to evaluate powder bed relative packing fraction, change in particle size and shape over the build plate and electrostatic charges over the powder bed. The focus of this paper is on the developments of metrics to define powder bed surface roughness. In this investigation, six plastic and two metal powders, as well as two recoater blade shape, were used. Two gap sizes of the recoaters were tested, namely two and five times higher than the powder D90. A novel shadowgraphy technique based on illuminating the surface with low-angle collimated light and analysing the image was employed to quantify powder bed surface roughness. Two main metrics to quantify the surface roughness of the powder bed were found to be useful. The first metric is amplitude of variation in surface height which is defined as the difference between surface height at each data point and the average height over 100 points. The second metric is wavelength of roughness which is calculated as the average horizontal distance between positive peaks, averaged over 100 data points. The surface roughness results showed that the shadowgraphy technique as well as the metrics developed by this method to quantify powder bed surface roughness, are capable of quantifying and distinguishing various powder bed features (and therefore potential quality failures in the bed). These metrics successfully captured the impact of different spreading variables; i.e. recoater shape, gap size and particularly the powder flow functions on the powder bed surface roughness.