Characterization of surface roughness and subsurface pores and their effect on corrosion in 3D-printed AlSi10Mg

Abstract

Denel Dynamics, a South African armaments development and manufacturing company, is interested in introducing additive manufacturing (AM) in the fabrication of missile components. The practicality of using AM for industrial applications, considering its unique challenges, was questioned. To investigate this, defects (surface roughness and porosity) associated with AlSi10Mg parts produced by laser powder bed fusion were characterized. In this case study, 40 rectangular, 30 mm × 20 mm × 4 mm samples, 20 of which were 'smooth' (3.7 μm Ra) and 20 'rough' (7.3 μm Ra), were analysed. The surface roughness, porosity volume percentage, and the porosity type were characterized using stereo, optical, and scanning electron microscopy as well as nano-CT scans. The pores were observed to be located in the subsurface of the samples. The smooth and rough samples were found to have subsurface pores mainly positioned between 100-650 μm and 220-600 μm from the surface, respectively. To improve the surface roughness, the samples were polished by centrifugal barrel finishing (CBF). Smooth (polished) and unpolished samples were corroded using potentiodynamic tests in a 3.5% NaCl solution. The smooth, polished samples were found to undergo more corrosion than the unpolished samples. This unexpected result could be explained by the subsurface pores of the polished samples being exposed after surface layers were removed during CBF. The location of porosity in 3D printed samples is therefore of high importance when surface polishing is done before exposure to a corrosive environment. Even though CBF decreases the surface roughness, subsurface pores that are exposed during polishing are detrimental to pitting corrosion resistance. Laser shock peening, which has been found to successfully close pores as deep as 700 μm without compromising the surface roughness, is suggested as a possible solution.