Effects of Different Scan Speeds on Microstructural and Corrosion Properties of Additively Manufactured HSLA Steels in 3.5% NaCl Solution

Abstract

In automotive applications, high-strength low alloy (HSLA) steels are providing enhanced material properties, due to the high yield strength, less fragility, and lower weight, which also promotes lower fuel consumption. HSLA steels contain a small amount of carbon (under 0.2%) and also contain small amounts of alloying elements such as copper, nickel, niobium, vanadium, chromium, molybdenum and zirconium. This eliminates the toughness reducing effect of a pearlitic volume fraction, yet maintains and increases the material's strength by refining the grain size. Therefore, HSLA steels are used for structures intended to handle large amounts of stress or that need a good strength-to-weight ratio. Traditional manufacturing methods for steel (e.g., blast furnace or electric arc furnace methods) fail to provide the necessary optimization for best output in terms of performance and application. The quality of molten steel is greatly affected by scrap steel. The smelting period is longer, and the power consumption is large. In addition, these processes allow more impurities into the molten steel, which compromises the quality of the final product. However, additive manufacturing (AM) can can be used to fabricate HSLA steel components without these drawbacks. In addition, AM can provide a relatively faster process with low manufacturing costs, in comparison with traditional processing methods. Although AM has several benefits, studies shows that AM processes can result in HSLA steels with microstructural defects, such as non-homogeneity, internal cavities, inclusions, and impurities. Consequently, these microstructural features have a significant affect on the corrosion properties of AM parts as corrosion tends to initiate in defective regions. The AM processing parameters directly impact the microstructure of the fabricated part. Hence, it is important to understand the relationship between the AM processing parameters on resulting microstructural features. The present work evaluated the electrochemical corrosion properties of HSLA steels fabricated by AM via selective laser melting (SLM) under different processing conditions. The goal of the work was to investigate the role of microstructure on electrochemical corrosion in high-strength low alloy steels due to SLM processing conditions.Two types of HSLA steels, Fe 367 and Fe 398, were fabricated by AM via (SLM). Fe 398 differs from Fe 367 as it contains molybdenum and nickel. Each type was fabricated at a laser power of 100W, and scan speed of 600, 800, 100 and 1200 mm/s. The samples were exposed to 3.5% NaCl for 15 days, individually. To acquire the electrochemical corrosion data and to perform analysis, a Gamry reference 600 potentiostat/galvanostat was used. The electrochemical data were obtained by collecting the impedance spectra, and measuring the polarization resistance every 5 days. On day 15, cyclic polarization data was collected for each sample. These measurements helped to identify the localized corrosion as well as provide detailed information about the corrosion properties, such as passive layer growth, initiation and secession of pitting, and corrosion rate. The topography of the materials was observed by SEM before and after the corrosion tests. Energy Dispersive Spectroscopy (EDS) was performed on the samples to identify the chemical elements. The surface roughness was observed through confocal microscope.The Confocal and SEM images showed the change in surface microstructure and topographical properties of the samples before and after the corrosion testing. The difference in laser power and scan speed affected the microstructure and corrosion properties of the materials. As the samples were manufactured at same laser power, the scan speed was responsible for different topographical and corrosion behavior. Samples manufactured at lower scan speed showed less flaws on the surface of the materials than the samples manufactured at higher scan speed. Therefore, both Fe 398 and Fe 367 showed better surface topography at 600 mm/s and 800 mm/s. They also demonstrated significantly lower corrosion rate than the other samples. EDS identified chemical oxides and chlorides which were formed during the corrosion test. Overall, this work demonstrated that Fe 398 shows better microstructural and corrosion properties than Fe 367 at lower scan speed.