Microstructure and Selective Corrosion of Alloy 625 Obtained by Means of Laser Powder Bed Fusion

Abstract

The effect of microstructure on the susceptibility to selective corrosion of Alloy 625 produced by laser powder bed fusion (LPBF) process was investigated through intergranular corrosion tests according to ASTM G28 standard. The effect of heat treatment on selective corrosion susceptibility was also evaluated. The behavior was compared to commercial hot-worked, heat treated Grade 1 Alloy 625. The morphology of attack after boiling ferric sulfate-sulfuric acid test according to ASTM G28 standard is less penetrating for LPBF 625 alloy compared to hot-worked and heat-treated alloy both in as-built condition and after heat treatment. The different attack morphology can be ascribed to the oversaturation of the alloying elements in the nickel austenitic matrix obtained due to the very high cooling rate. On as-built specimens, a shallow selective attack of the border of the melt pools was observed, which disappeared after the heat treatment. The results confirmed similar intergranular corrosion susceptibility, but different corrosion morphologies were detected. The results are discussed in relation to the unique microstructures of LPBF manufactured alloys.

Stress Corrosion Cracking of Additively Manufactured Alloy 625

“…The main process parameters were a laser power of 195 W, a scanning speed of 1200 mm/s, a hatching distance of 0.09 mm and a layer thickness of 0.02 mm combined with a scanning strategy with rotation of 67 • between consecutive layers. The selected parameters resulted in a high densification level of the samples close to a relative density of 100%, as reported in previous studies [23,39]. Two different metallurgical conditions were investigated: (i) un-treated alloy with the characteristic microstructure obtained at the end of LPBF (named UT-LPBF), and (ii) heat treated at 980 • C for 32 min (grade 1 [33]) followed by water quenching (named HT-LPBF specimens).…”

“…The resistance to pitting of Alloy 625 is very high, much higher than the traditional austenitic stainless steels. In fact, electrochemical results, already published by the present authors elsewhere, demonstrated that the alloy produced by laser powder bed fusion was not susceptible to pitting in either 1 M H 2 SO 4 at 40 • C, or neutral or acidified NaCl 35 g/L at 40 • C, and had a crevice corrosion resistance slightly higher than that of traditional wrought material; these results were also confirmed via critical crevice temperature (CCT) tests [23,24]. Post manufacturing heat treatment can also have an impact on corrosion resistance; however, the alloy may become susceptible to intergranular corrosion if subjected to improper solubilization treatment [25]; in fact, the formation of carbides and secondary phases can affect localized corrosion resistance and environment assisted cracking susceptibility.…”

“…It could be postulated that hydrogen diffusion through the UT-LPBF specimens during pre-charge should be more efficient than that on HT-LPBF specimens owing to the finer microstructure. In a previous work by the present authors, it was observed that heat treatment at 980 • C for 32 dissolved the melt pool microstructure obtained by the LPBF process, producing some niobium carbide formation without appreciable recrystallization [23]. For these reasons, no appreciable differences were observed in the SSRT at 3.15 × 10 −6 s −1 and scathodic polarization.…”

In Situ Alloying of a Modified Inconel 625 via Laser Powder Bed Fusion: Microstructure and Mechanical Properties

“…Among the Ni-based superalloys, IN625 is one of the most popular materials thanks to its intrinsic high oxidation and corrosion resistance in aggressive environments associated with good mechanical properties at elevated temperatures [8][9][10][11]. Moreover, this material exhibits high weldability, allowing the production of parts with low residual defects through LPBF [5,12].…”

Scanning Electrochemical Microscopy (SECM) Investigation of 3D Printed Parts Produced by CMT Welding Technology

“…The microstructures formed during preparation seriously can affect the corrosion resistance of alloys . Cabrini et al investigated the susceptibility to intergranular corrosion of Alloy 625 produced by LPBF (Laser Powder Bed Fusion) method .…”