Surface engineering on additive manufactured 316L stainless steel for marine transportation

Abstract

Maritime activities provide great economic value to Malta as they supply food, energy, and commodities. Marine vessels are constantly subjected to adverse weather conditions, which result in wear and tear. Additive manufacturing (AM) could be of advantage to the maritime industry. AM produces parts rapidly, according to the required shape, thus lowering the lead time. However, such parts are subject to high porosity and poor surface roughness. Shot peening (SP), physical vapour deposition (PVD) for coating deposition and a combination of these could be applied. The SP reduces crack propagation, as the crack needs to overcome this compressive stress generated before continuing in its propagation and the PVD coating protects the material against corrosion. The hypothesis for this study is that additively manufactured 316L SS can be enhanced with the addition of surface engineering treatments to make it better suited for the marine environment and conditions. In this research, SP and a TiAlCuN coating were applied individually and also together, to form a hybrid treatment, to study their effects on selective laser melted 316L stainless steel (SS). Impact tests were performed on as-printed 316L SS, obtaining a maximum load of 15.23 ± 0.10 kN and a total of 75.08 ± 2.00 J absorbed energy. Tensile tests were also performed on the as-printed material, acquiring a Young’s Modulus of 155 ± 7 GPa, yield strength of 493 ± 5 MPa, yield strength of 644 ± 5 MPa and elongation of 41 ± 2 %. Furthermore, material characterisation techniques were performed to study the surface. The microhardness tests executed showed that SP generated a 40% increase and after carrying out roughness tests, a 50% decrease in surface roughness of the SP was detected over the as-printed samples. Residual stress measurements via X-ray diffraction (XRD) were also done. These indicated that SP generated 589 MPa of surface residual compressive stresses, while the combination of SP and coating deposition produced 693 MPa of surface residual compressive stresses, highlighting that the effect of the combined treatments could be highly beneficial. Corrosion tests and scratch tests were also carried out. The sample with the combination of both treatments showed the best corrosion resistance when performing cyclic polarisation tests in substitute ocean water, while the printed and coated sample showed the optimal adhesion properties.