Sunday, July 14, 2024

Development Of Nickel-Base Superalloy & Components For Aeroengine Applications

By I.Balasundar, T. Raghu & G. Madhusudhan Reddy 

Under service conditions, the components made out of the material are expected to have high strength exhibiting greater resistance to deformation and damage while the same material is expected to show better ductility or workability during manufacturing. Tailoring the microstructure of the material conducive for thermomechanical processing while manufacturing and restoring its properties for actual application has always been a challenge to material developers.

Nickel (Ni) base superalloys are extensively used as stators and rotors in various aero engines.  As a rotor, the material is used in the form of turbine discs, blades, shafts etc.  Directionally solidified or single crystal turbine blades are generally cast using investment casting processes for these high temperature applications. Turbine discs for aeroengine application are generally manufactured either using the conventional melting + ingot casting + thermomechanical processing (MC-TMP) route or powder processing-hot Isostatic pressing (PM-HIP) and/or thermo-mechanical processing (PM-TMP) route. Most often, the MC-TMP route is followed because of economic viability and processing capabilities. Development of high strength Ni base superalloys through the MC-TMP route is always a challenging task. 

Nickel base superalloy DMR SN 742 is a medium alloyed material that is used as high pressure compressor (HPC) and turbine (HPT) rotors in a variety of aero engines. Like many Ni base superalloys, the gamma prime  precipitates strengthen the matrix gamma  phase in DMR SN 742 as well. In addition to γ′ precipitates, carbides & nitrides of varying composition and morphology are also present in the material. The morphology and topology of various phases present in the material individually and synergistically affect the thermo-mechanical processing behaviour of the material which in turn influences the static and dynamic mechanical properties that can be achieved. 

The technology for casting the ingots, converting the as-cast ingots into airworthy bar stock and converting the bar stock into critical class-I aero-engine components out of the material was not available in the country. To achieve self-reliance in this technology, the Defence Metallurgical Research Laboratory (DMRL) of DRDO started work towards “Indigenization of nickel-base superalloy and establishing forging technology to produce turbine discs for aero-engine”. 

Double or triple melted large as-cast ingots subjected to appropriate thermal treatment are required to be subjected to primary thermo-mechanical processing (TMP) such as sequential cogging or extrusion at high temperatures above and/or below the gamma Prime solvus to break the cast structure, improve the soundness and integrity of the material which can be then be used as a bar stock for further secondary thermo-mechanical processing to manufacture the required components viz. turbine discs. Appropriate control of processing (thermo-mechanical and thermal treatment) parameters and the resulting microstructure at each and every stage of material development (ingot casting, primary and secondary thermo-mechanical processing) has to be carried out not only to maximise the yield but also to achieve the desired properties in the material. 

In order to develop Nickel base superalloy DMR SN 742 material (bar stock), initial laboratory scale melts were utilised. The effect of thermal exposure on the microstructure of the material was evaluated and a suitable thermal treatment required for achieving better high temperature workability was established. The hot workability of the thermally treated as-cast material was evaluated over a wide range of temperature and strain rate by carrying out isothermal hot compression tests and an optimum thermomechanical processing window for the material was identified. The kinetics of microstructure evolved was also evaluated and ascertained to identify the critical and   total amount of deformation that is required to break and refine the microstructure of the material that provides better workability during thermomechanical processing.  

Double melted (Vacuum induction melting + Vacuum arc remelting) ingots of  400 mm diameter were manufactured at MIDHANI Hyderabad. The know-how and know-why generated on the lab-scale material was successfully used to identify an appropriate primary processing (cogging) cycle for converting the industrial scale material into airworthy bar stock of required dimensions. The entire thermo-mechanical processing sequence has been designed in such a way that the same can be implemented by any forging industry once the technology is transferred by DRDO.   

By integrating science and knowledge based tools with advanced forging techniques, the nickel base superalloy DMR SN 742 bar stock was successfully converted into high pressure turbine (HPT) disc forgings under near isothermal conditions in single stage at DMRL using the unique 2000 MT forge press. Further, a two-stage heat treatment cycle required for achieving the desired microstructure that provides the required static and dynamic mechanical properties was identified. The first batch of HPT disc forgings produced were heat treated and subjected to mechanical property evaluation as per the airworthiness requirements. The mechanical properties of the component met the stipulated requirements. The forged HPT rotors  machined to sonic shape  are being evaluated rigorously for qualification.  

It may be noted that this is the first cast & wrought nickel-base superalloy where DRDO has been involved from melting to component development. The scientific knowledge developed on lab-scale material has been successfully implemented on industrial scale material to realise bar stock (material) and the component (HPT rotor). The entire development activity (both bar stock and rotor component) is carried out in-house by DRDO. 

DRDO (DMRL, GTRE, CEMILAC, MIDHANI & DGAQA) is currently working to expedite and ensure that this crucial technology is type certified towards achieving self-reliance. The technology for manufacturing the bar stock and component has been designed in such a way that it can be implemented by any forging industry having suitable infrastructure. 

Further, the methodology adopted here is generic in nature and the same can be suitably adapted to manufacture complex aero-engine components by fine-tuning the parameters which would result in significant foreign exchange savings and also towards achieving Aatma Nirbhar in this crucial technology.  

About The Authors  

The authors are DMRL (DRDO) scientists. Dr I. Balasundar is project leader, Dr T. Raghu is Associate Director, & Dr G. Madhusudhan Reddy is Director, DMRL

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