Two-step heat treatment for laser powder bed fusion of a nickel-based superalloy with simultaneously enhanced tensile strength and ductility

Abstract

Nickel-based superalloys show severe cracking tendency during laser powder bed fusion (LPBF), which hinders their widespread applications in aerospace. A two-step heat treatment, including hot isostatic pressing (HIP) and solid solution heat treatment (SSHT), was proposed to obtain crack-free LPBF nickel-based superalloy components with a supersaturated solid solution of alloying elements for desirable mechanical performance. The HIP process aimed to annihilate microcracks, and the subsequent SSHT focused on modifying the microstructure and improving the solid solution extent of alloying elements. The pore-and-microcrack-containing defects with a volume fraction of 0.96% in the LPBF samples were transformed to pore-dominated defects with a volume fraction of 0.08% after the HIP process. After the SSHT, it was not observed the reappearance of the previously coalesced microcracks, but the porosity volume fraction showed a slight rebound to 0.11% due to the coarsening or regrowth of the pores. The tensile strength and elongation at break of HIP + SSHT samples printed along the horizontal plane at room temperature were 3.6% and 113.5% higher than those of as-fabricated ones. An 11.9% and 410.0% improvement in tensile strength and ductility at 900 ℃ was achieved after the two-step treatment. The development of the microstructure after the HIP and SSHT, involving sub-grains, dislocation networks, carbide precipitates, and grains, was revealed systematically. The correlation between the microstructure and tensile properties was unveiled in depth. This work is anticipated to provide an efficient route with excellent industrial applicability for LPBF superalloy components to mitigate microcracks and acquire attractive mechanical properties.

Introduction

Nickel-based superalloys have been widely accepted as the preferred materials for crucial hot end components in aerospace engines due to their superior thermal stability and thermal oxidation resistance [1], [2], [3]. The rapid and cost-effective manufacture of these nickel-based superalloy components with geometrical complexities and reliable mechanical properties is the Achilles’ heel for their industrial application. For instance, the manufacture of a jet engine nickel-based superalloy turbine blade with conformal cooling channels requires a complicated processing route — ceramic mold preparation, investment casting, high-temperature heat treatment, and precision machining [4]. Each procedure has to be completed under strict process control and monitoring with substantial material waste and high rejection rates, making only ~10% of superalloy feedstock end up as the end-use products.

Laser powder bed fusion (LPBF), commonly known as selective laser melting, a promising metal processing technique in the additive manufacturing (AM) family [3], [5], [6]. It can theoretically create arbitrarily complex geometries by selectively melting powder-form raw materials using a bottom-up manner with the assistance of lasers. The simplified manufacturing mode, complex structure manufacturing capability, and minimum material waster render LPBF to become an ideal alternative for the fabrication of metallic components with geometrical complexity in the aerospace domain. General Electrical (GE) employed the LPBF process for the mass production of fuel nozzles for their next-generation LEAP jet engines [7]. The component number of the printed nozzle was decreased from 18 to a whole piece, which contributed to the nozzle with a 25% reduction in weight, a five-fold increase in durability, and a ~15% improvement in fuel efficiency compared with the predecessor.

Unfortunately, the widespread application of LPBF nickel-based components in aerospace is obstructed by unavoidable microcracking defects with lengths of 10–200 µm that seriously degenerate the mechanical performance of the components [8]. Generally, the high cracking susceptibility for the LPBF process of nickel superalloys is mainly associated with three different mechanisms: solidification cracking, liquation cracking, and cold cracking [9], [10]. The solidification cracking arises from rapid cooling/solidification during LPBF, which results in the trapping of liquid between already solidified dendrites. These weak mushy regions are apt to rupture and tear, then creating jagged solidification cracks under the stimulation of excessive residual stress [11]. Low-melting-point phases like carbides resulting from constitutional undercooling form in grain-boundary regions; when the laser energy penetrated from the current layer to the previously solidified layers, these phases were remelted to be liquid at grain boundaries, then leading to liquation cracking by contracting material elsewhere and pulling apart the weakened grain boundaries [12], [13]. Cold cracking is typically attributed to excessive residual stresses due to the combination of large temperature gradients, rapid melting/solidification, and complex thermal cycling in LPBF when residual stresses exceed the ultimate tensile strength of printed materials [10]. As a result, it is difficult to eliminate the microcracks completely through printing parameter optimization.

Recently, many efforts have been carried out to suppress the initiation and propagation of microcracks by the alloying modification of nickel-based superalloys. Han et al. introduced 1 wt% TiC nanoparticles into Hastelloy X superalloy for reducing the alloy’s susceptibility to hot cracking and improving its heterogeneous nucleation ability [14]. The microcrack density in the TiC-modified samples was decreased to 0.14% from 0.65% relative to the LPBF process of the original alloy, and a ~100 MPa improvement in tensile strength at room temperature was achieved. Nevertheless, the addition of ceramic particles is highly likely to sacrifice the high-temperature tensile and fatigue properties due to the distinct differences in physicochemical properties between the TiC and nickel-based alloy. Harrison et al. proposed to alleviate the microcracking via a trace increase in solid solution elements (Co, W, and Mo) and a small reduction in tramp elements (Mn and C), which were beneficial to enhancing the thermal shock resistance of Hastelloy X [8]. A 65% decrease in microcracking density can be realized in the LPBF modified alloy, but it was accompanied by a dramatic reduction in ductility at the service temperature of 760 ℃. These findings imply that the fundamental theory on composition modification to eliminate or relieve the microcracking defects for LPBF nickel-based superalloy components is immature and imperfect currently. More importantly, the existing nickel-based superalloys were designed for traditional processing means rather than LPBF, which caused that the designers did not consider the unique processing features of LPBF. The compositions that are not the most proper for LPBF also make the alloying modification tougher.

Hot isostatic pressing (HIP), which leverages the combination of high temperature and pressure to close pores or cracks in the printed components, provides another effective and easily manipulated solution to mitigate the cracking for LPBF components in industrial applications [15], [16]. Montero-Sistiaga et al. conducted a HIP post-treatment for an LPBF Hastelloy X to close all cracks [17]. Unexpectedly, the yield stress of the HIP-treated Hastelloy X coupons showed a slight decrease compared to that of as-fabricated ones. Similar decrease tendencies were also found in the reports of Tomus et al. and Han et al. [18], [19]. The decline in strength was derived from grain coarsening, dislocation networks disappearance, and carbide enrichment at grain boundaries after HIP. This is suggested that a further heat treatment is required to modify the inferior microstructure to acquire desirable mechanical properties.

In this study, a two-step heat treatment, including HIP and solid solution heat treatment (SSHT), was proposed to obtain LPBF nickel-based superalloy components with simultaneously improved tensile strength and ductility. The first-step heat treatment HIP aims to annihilate microcracks for ductility improvement, and the subsequent SSHT focuses on eliminating carbides at grain boundaries and improving the solid solution extent of alloying elements to reinforce the strength. A solid solution nickel-based superalloy GH3536, which had an identical alloying composition with Hastelloy X, was selected as the experimental material. The optimized LPBF processing window was established for the GH3536 alloy. The development of element segregation, grains, carbides, and microcracks in the printed GH3536 samples subjected to the HIP and SSHT processes was unveiled in depth. The relationship between the microstructure and tensile properties at room temperature (20 °C) and high-temperature (900 °C) was uncovered in detail. The mechanism for ductility-dip cracking (DDC) phenomena that were present in the high-temperature tensile measurements was systematically analyzed.