https://research.birmingham.ac.uk/portal/files/39642722/Wang_et_al_Microstructure_and_yield_strength_Acta_Materialia_2017.pdf

Microstructure and yield strength of SLM-fabricated CM247LC Ni-Superalloy

Alloys-by-design: Application to new superalloys for additive manufacturing

Characterization of heat affected zone liquation cracking in laser additive manufacturing of Inconel 718

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

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

Inconel 738 is one of the widely used nickel-based superalloys in high-temperature applications, especially in land-based and aerospace gas turbine engines. This paper reports the feasibility of direct laser metal deposition (LMD) of Inconel 738. Cracks evolved during deposition at the substrate/deposit interface and within the deposit along high angle grain boundary for scanning speed of 6 and 12 mm/s due to the intense residual stress and incipient melting. Results showed liquation cracking due to low melting crack boundary γ′ and significant micro-segregation of Al and Ti along the crack boundaries. By maximizing the energy density and by reducing the scanning speed to 3 mm/s, crack-free single wall specimens were successfully manufactured. Microstructural evolution of primary, secondary, grain boundary γ′, MC carbides, and M2B borides in the as-deposited and heat-treat specimens are discussed. Mechanical properties and microstructural development were investigated using tensile testing and scanning electron microscopy. Energy dispersive spectroscopy confirmed significant micro-segregation on various elements along the interdendritic and grain boundaries. X-ray diffraction validated the presence of the observed carbides and boride in the as-deposited and heat-treated samples.

Direct laser metal deposition of Inconel 738

The laser welding of a directionally solidified INCONEL 738 superalloy (DS IN738) was investigated. Microstructural analysis of the fusion zone (FZ) and the heat-affected zone (HAZ) revealed that cracking occurred mainly in HAZ with only some cracks being extended into the FZ. However, the frequently observed centerline cracking in FZ of DS and single crystal (SX) alloys did not occur, which was attributed to the presence of negligible volume fraction of γ-γ′ eutectic in FZ. Constitutional liquation of secondary solidification constituents (MC carbides, M3B2 borides, M2SC sulphocarbides, and γ-γ′ eutectic) and γ′ precipitate particles was found to be the major cause of grain boundary liquation and the resultant intergranular microfissuring in the HAZ. The extent of HAZ microfissuring was, however, observed to be smaller in samples welded along the transverse direction (perpendicular to solidification direction) than when welding was done along the longitudinal direction (solidification direction). Nonetheless, more severe HAZ cracking occurred in samples of similarly welded conventionally cast (CC) alloy, and the present results indicate that the severity of HAZ liquation cracking in IN738 superalloy can be reduced by using a DS version of the alloy.

Microstructural Analysis of Laser-Beam-Welded Directionally Solidified INCONEL 738

Microstructural study of laser-beam-welded IN 738 superalloy was carefully performed to better understand the causes of heat-affected zone (HAZ) cracking and to determine an improved approach of alleviating the weldability problem. The HAZ cracks in the alloy were intergranular liquation cracks that resulted from the liquation reaction of both secondary solidification products (MC carbides and γ-γ′ eutectic) and solid-state reaction products (γ′ particles) present in the preweld material. In contrast to the expectation based on Chadwick’s equation, a reduction of grain boundary liquid film thickness did not produce a decrease in HAZ cracking owing to increased base alloy hardness that accompanied a preweld heat treatment designed to reduce the intergranular liquation. Moreover, a major factor limiting the effectiveness of an existing preweld heat treatment with low base alloy hardness in reducing HAZ cracking was found to be the formation of intergranular M5B3 boride particles during the heat treatment. These borides can widen the HAZ brittle temperature range (BTR) during weld cooling and increase the propensity for cracking. Based on the results, a new preweld heat treatment that induces a moderate hardness and precludes grain boundary boride formation was found and was shown to produce a significant reduction in HAZ cracking in the welded alloy compared to the most effective pre-existing preweld heat treatment.

Improvement in Laser Weldability of INCONEL 738 Superalloy through Microstructural Modification

The microstructural response of directionally solidified Rene 80 (DS Rene 80) superalloy to gas-tungsten-arc (GTA) welding was investigated. Rapid heating during welding resulted in a significant grain-boundary liquation of solid-state reaction product γ′ precipitates, intergranular elemental segregation induced M5B3 borides, and secondary solidification constituents MC carbides and sulfocarbides, which were all present in the preweld heat-treated alloy. Liquation of these particles embrittled the grain boundaries in the heat-affected zone (HAZ) and caused microfissuring along the liquated grain boundaries. Nevertheless, contrary to the generally observed increase in HAZ cracking in superalloys with an increase in Ti and Al concentration, due to increase in the alloy’s hardness, significantly reduced cracking was observed in DS Rene 80 compared to the conventionally cast IN738 welded under the same conditions, despite its hardness being higher than that of IN738. This was related to the nature of base-metal grain- boundary intersections at the fusion-zone boundary in these materials.

Microstructural Response of Directionally Solidified René 80 Superalloy to Gas-Tungsten Arc Welding

The heat affected zone (HAZ) cracking behaviour in a laser beam directionally solidified (DS) Rene 80 nickel based superalloy subjected to preweld heat treatments was studied. The HAZ cracks in the alloy are grain boundary liquation cracks caused by liquation reaction of both non-equilibrium secondary solidification product, MC carbides and equilibrium solid state reaction product, γ′ precipitates. In contrast to theoretical prediction based a preweld heat treatment that reduced grain boundary liquid film thickness did not result in a lower HAZ cracking, which can be related to concomitant reduction in the ability of the base alloy to relax welding stress. In addition, formation of intergranular M5B3 boride particles in preweld alloy appeared to have aided cracking susceptibility by lowering grain boundary liquation temperature and widening the brittle temperature range in the HAZ during cooling. Based on the analysis of the results, application of a new preweld heat treatment that prevents the fo...

On preventing HAZ cracking in laser welded DS Rene 80 superalloy

The microstructure of directionally solidified Rene 80 (DS Rene 80) superalloy in the standard solution heat treated condition was examined. Sub-solidus incipient melting was observed, which was found to be caused by the liquation of terminal solidification reaction products present in front of the γ–γ′ eutectic in the as-cast alloy. Based on the differential scanning calorimetry (DSC) measurements, and microstructural examination of heat-treated and water-quenched specimens, sub-solidus incipient melting in the alloy is likely to occur below 1,160 °C, and this is a key factor to be considered in the development of a suitable solution heat treatment scheme for DS Rene 80.

R.K. Sidhu

Papers

Microstructural Analysis of Laser-Beam-Welded Directionally Solidified INCONEL 738

Improvement in Laser Weldability of INCONEL 738 Superalloy through Microstructural Modification

Microstructural Response of Directionally Solidified René 80 Superalloy to Gas-Tungsten Arc Welding

On preventing HAZ cracking in laser welded DS Rene 80 superalloy

Sub-solidus melting of directionally solidified Rene 80 superalloy during solution heat treatment