Investigations on laser nitriding with trochoidal irradiation for enhancing corrosion resistance of laser-wire directed energy deposited low carbon steel

Abstract

This study presents the process improvement in high-temperature surface nitriding using a lower pulsed laser beam and a trochoidal laser irradiation strategy. The trochoidal irradiation strategy is implemented for nitriding process improvement on heterogeneous laser-wire directed energy deposited (LWDED) low-carbon steel in a nitrogen gas atmosphere. The trochoidal path improves nitriding by overcoming the beam overlapping effect and uniform heating during laser interaction. The process mechanism reveals the petal-like structure on the processed surface, which creates a large surface area. This large surface area helps the nitrogen atom trap during surface melting to form a nitride surface. The intermediate diffusivity from a pulsed laser enabled the controlled nitride growth. It facilitated localized diffusion and the precipitation of the uniform Fe-nitride layer. The EIS analysis showed that the sample nitride at a laser frequency of 100 Hz has the best corrosion resistance in a 3.5 % NaCl solution. The higher-order Fe-nitride formation at 100 Hz frequency is retained after the corrosion test. This finding highlights the superior stability and durability of the 100 Hz laser frequency nitriding compared to the other samples. The effect of pulse frequency and trochoidal path on the formation of Fe-nitride is thoroughly investigated before and after electrolytic corrosion test using advanced characterization techniques like scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and electrochemical analysis. This study demonstrates the effectiveness of pulsed laser nitriding and a trochoidal strategy for enhancing corrosion resistance in additively manufactured parts.