Features of the structure of gradient layers «steel - Inconel - steel», obtained by laser direct metal deposition

Introduction. Traditionally, the most common technology for producing parts from nickel alloys involves casting followed by heat treatment to achieve the required phase composition. Significant disadvantages of this method include the segregation of chemical elements, the presence of large undesirable inclusions such as Laves phase and eutectic structures, and the non-uniform distribution of strengthening phases throughout the workpiece cross-section. At the same time, many complex-shaped parts are assembled into a single combined structure using welding. An analysis of the hardening characteristics of nickel alloys and the products derived from them suggests that additive manufacturing techniques are a promising approach for fabricating such workpieces. The structure and phase composition of the material volumes formed via layer-by-layer deposition will differ significantly from those obtained by conventional methods. In the case of producing combined structures using additive methods, identifying the patterns of structure and phase composition formation becomes an even more complex challenge. Therefore, the purpose of this work is to identify the structural features of “steel - nickel alloy – steel” gradient layers fabricated by direct metal deposition. The study examines dissimilar joints produced using the “Welding and Surfacing Complex based on a Multi-Coordinate Arm and a Fiber Laser” at the S.A. Khristianovich Institute of Theoretical and Applied Mechanics of the Siberian Branch of the Russian Academy of Sciences, employing direct metal deposition technology. Research methods. A Carl Zeiss Axio Imager A1m light microscope and a Carl Zeiss EVO 50 XVP scanning electron microscope, equipped with an INCA X-Act energy-dispersive X-ray spectroscopy (EDS) attachment, were utilized for microstructural investigations of the fabricated layers. Phase composition analysis of the samples was performed using an ARL X'TRA X-ray diffractometer. Microhardness testing was conducted using a Wolpert Group 402 MVD Vickers hardness tester. Results and discussion. It was observed that the maximum layer height (up to 7 mm) was achieved when implementing the following parameters: 1,000 W laser power with a scanning speed of 35 mm/s, and 1,500 W laser power with a scanning speed of 15 mm/s. In the first case, minimal material mixing at the fusion boundary was noted. In all fabricated compositions, defects in the form of unmelted powder particles were observed, as well as cracks in the first steel layers. During the deposition of Inconel 625 onto 316L stainless steel, the transition zone exhibited solidification modes consistent with the formation of iron-based alloys, specifically FA (ferrite-austenite), AF (austenite-ferrite), and A (austenite) sequentially. When depositing 316L stainless steel onto Inconel 625, the transition zone exhibited a solidification mode characterized by the formation of only the austenite phase. The microhardness values were found to be 230 ±15 HV for 316L stainless steel and 298 ± 20 HV for Inconel 625.