Corrosion and Cavitation Erosion Resistance of 316L Stainless Steels Produced by Laser Metal Deposition

Corrosion and cavitation erosion are important indicators for evaluating the performance and reliability of hydraulic machinery. Laser metal deposition (LMD), as an important technique for both surface modification and complex component fabrication, is proven to be effective in enhancing the mechanical properties of materials. In this study, 316L stainless steel (316L SS) samples were fabricated using LMD and the effects of laser power, scanning strategy, surface remelting, and build direction on the electrochemical corrosion and cavitation erosion resistance of the LMD-produced samples were systematically studied. The obtained results were compared with those of a wrought counterpart. The corrosion resistance of the LMD-produced samples in a 3.5%NaCl solution was tested via open-circuit potential measurement and potentiodynamic polarization tests. Also, the cavitation erosion resistance of the LMD-produced samples was studied according to different process parameters. The microstructure of the forged 316L SS sample was characterized with uniformly distributed equiaxed grains, whereas the LMD-produced samples exhibited a process-dependent nonequilibrium microstructure consisting of high-/low-angle grain boundaries, tortuous grains, cellular/dendritic substructures, and processing-related defects. The grain size of the LMD-produced 316L SS sample was much larger than that of the forged 316L SS. By increasing the laser power or changing the sample from horizontally built to vertically built, both the grain size and dendritic arm spacing of the material tended to increase. However, when surface remelting and the 90 degrees-rotation scanning strategy were adopted, the changes in the grain size and dendritic arm spacing of the material were obviously different. Results of a microhardness test showed that the dendritic arm spacing can better match the microhardness evolution than the grain size. This microstructural difference also led to a significantly different electrochemical corrosion and cavitation erosion performance from that of the forged 316L SS. Results of an electrochemical corrosion test showed that the corrosion resistance of the LMD-produced 316L SS sample was much better than that of the forged 316L SS, i.e., the polarization resistance (R-p) of the LMD produced 316L SS sample under different processing increased by about 2-98 times, while the corrosion current density (i(corr)) decreased by one to two orders of magnitude. The test results of an ultrasonic vibration cavitation system showed that the cavitation erosion resistance of the LMD-produced 316L SS sample was better than that of the forged 316L SS. However, stress concentration may be induced in local areas such as pores and grain boundaries, which, in turn, facilitate preferentially cavitation damage in these areas. Also, protrusion topography appeared, and gradually disappeared to form a large number of dimples in the subsequent cavitation erosion process. The cavitation erosion resistance of the material mainly depended on its local mechanical properties. The microhardness test results showed that the hardness of the LMD-produced 316L SS sample was significantly higher than that of the forged sample, so its cavitation erosion resistance was significantly improved. However, because of the heterogeneous microstructure and process-related pore defects formed in the LMD-produced samples, the microhardness contour exhibited a spatially nonuniform distribution characteristic; hence, the surface morphology of the LMD-produced 316LSS sample was seriously eroded in some local areas after cavitation.