Laser Powder Bed Fabrication of Nickel‐Base Superalloys: Influence of Parameters; Characterisation, Quantification and Mitigation of Cracking

Abstract

The use of a selective laser melting (SLM) powder-bed method to manufacture Ni-based superalloys components provides an economic approach for low production run components that operate under a high-temperature and stress environment. A major concern with the SLM of precipitation hardenable Ni-based superalloys is their high susceptibility to cracking, which has been heavily documented in the field of welding. Weld cracking may occur either during processing (hot cracking, liquation cracking and ductility-dip cracking) or during the post weld heat-treatment stage (strain-age cracking). Due to the complex thermal history of SLM fabricated material there is the potential for all of these mechanisms to be active. In this investigation, cuboidal coupons of the Ni-based superalloy CM247LC were fabricated by the SLM of argon gas atomised powder. Parametric studies were performed to investigate the influence of the process parameters (laser scan speed, power and scan spacing) on the cracking density and morphology through conducting a stereological study of scanning electron microscope (SEM) micrographs. Further microstructural evidence is presented, illustrating the different crack morphologies observed as well as suggesting the responsible mechanisms. Finally a postfabrication Hot Isostatic Pressing (HIP) treatment was performed to investigate its utility in ‘healing’ the internal cracks, and providing a route to retro-fix the cracking problem in the heat treatment stage of production. The findings highlight the need for process models of the SLM method in order to understand the thermal history and the laser fabricated structures observed. Introduction The discipline of additive layer manufacture (ALM) has been steadily growing since the 1980’s and now encompasses a wide variety of technologies. They all share the common feature of producing a three dimensional shape by combining two dimensional ‘slices’ of a predetermined thickness. In recent years, ALM technologies have been developed to push the field forward from ‘rapid-prototyping’ towards ‘rapid-manufacturing’ and the production of fully dense and functional metallic components. In terms of laser fabrication there are now two key technologies for the rapid manufacture of fully-dense metallic components; Direct Laser Fabrication (DLF or any ‘Blown Powder’ system) and ‘Selective Laser Melting (SLM) Powder-Bed’ manufacturing. Comprehensive reviews of the different ALM methods can be found elsewhere [1-4]. SLM powder-bed technology has attracted the interest of aerospace manufacturers for several key reasons including: The elimination of the need the expensive tooling associated with forging and investment casting; the immediate recyclability of the unused metal powder, and the significant reduction in the ‘design-to-component’ time thus allowing for actual physical testing and many design iterations. The present study aims to assess the microstructural and physical characteristics of the SLM fabricated Ni-based superalloy for high-temperature application. Weld Cracking Laser fabrication process can be considered analogous to a continuous laser welding process. Due to this, an alloy weldability can be used as an indication as to its processability by SLM. Figure 1 [5] shows a number of typical Ni-based superalloys plotted according to their Al and Ti contents (γ′ forming elements). The alloys lying above the dotted line show a high volume fraction of the γ′ phase and are typically considered unweldable due to their cracking susceptibility. This relationship between the cracking susceptibility and γ′ fraction is attributed to the precipitation hardening that occurs within the aging temperature of the alloy; reheating the material to within this region (either in the welding process or as part of a post-weld heat treatment (PWHT)) results in hardening accompanied by a reduction in ductility leaving the material prone to cracking [5]. A review of the relevant literature highlighted four potential cracking mechanisms associated with welding and reheating of Ni-based superalloys, which are: Figure 1. Plot showing increasing cracking susceptibility with γ′ forming elements (Al and Ti) [5]. Alloys lying above the dotted line are particularly susceptible to cracking during welding or PWHT. Solidification Cracking Also referred to as ‘Hot-Tearing’, is reported to occur within the solidifying melt pool (or the mushy zone) where the material is in