Welding Studies and Characterisation of Additively Manufactured LPBF Maraging Steel

Abstract

Additive manufacturing (AM) comes under the category of advanced manufacturing techniques that enables the manufacture of complex shaped components with reduction in multi-part assemblies, production lead times and weight. Maraging steel is a strategic material for manufacturing of components such as rocket motor casings, bulkheads etc. in defence and aerospace sectors. Laser Powder Bed Fusion (LPBF) AM technique has been explored in fabrication of Maraging steel components for end-use applications. In many applications, additively manufactured maraging steel parts are required to be welded to conventional material and it is important to understand weldability of these materials and their characteristics to ensure good bonding between the parts. It is also necessary to assess how welding process may affect the microstructure and consequently the mechanical properties of the AM maraging steel. In the present study, welding of AM maraging steel AM300 with conventional MDN250 was explored. With the available optimized parameters, maraging steel plates (160x100x6mm³) were additively manufactured at low porosity without any defects of soot and spatter. The effect of heat treatment conditions on the volume fraction of reverted austenite in AM300 was also studied to arrive at an appropriate condition before carrying out the welding of AM300 plates. XRD and EBSD analysis revealed the formation of very fine reverted austenite in the as-deposited (AD) and Direct-aged (DA) conditions at the cell boundaries. Specimens when subjected to solution-treated and aged (STA) condition had almost eliminated the formation of reverted austenite at room temperature. Thus, the AM processed plates were subjected to solution treatment before carrying out the TIG welding of AM300 to MDN250 plates using W2 filler. Weldments of AM300-W2-MDN250 showed the formation of Fusion zone (FZ) and dark band Heat affected Zones (HAZ) on both the sides of FZ. Weld specimens subjected to ageing times at 490°C for 3.5hrs and 6hrs have shown similar average hardness values in AM300, FZ and MDN250 as 700HV, 675HV and 650HV respectively. Tensile strength and %El of as-welded, aged (3.5hrs) and aged (6hrs) specimens were evaluated to be 925MPa, 2.7%; 1730MPa, 2.4%; 1850MPa, 1.4% respectively. The tensile strength of AM300-W2-MDN250 weldment aged to 3.5hrs is found to be higher than that of conventional MDN250 weldment, but with about 60% reduction in ductility. However, higher weld strength being the main criteria, the joining of AM300 to MDN250 can be considered as a viable option for relevant applications.