The aim of the proposed project was to develop castable steel alloys for aero engine applications and to refine the process to be able to cast thin-walled, light-weight and durable cast steel components. Today, cast titanium components is the predominant choice over alternatives like nickel base alloys and stainless steels for critical components in the aero engine. The main downside of using cast titanium components is the additional cost accompanied with the material. Using a technically equivalent, but cheaper construction material leaves more economical headroom for using higher performing light-weight materials in other part of the air craft, thereby making it possible to manufacture lighter air crafts producing less emission during its active usage cycle.
In the proposed project the possibility to cast thin-walled cast steel components was investigated using an iterative process of performing casting trials and tuning simulation parameters for simplified geometries. After a successful methodology had been developed and validated, complex aerospace-engine components were cast and simulated where section sizes down to 1 mm are combined with thicker sections. In this work the steel alloy 17-4PH was investigated. This alloy is a common engineering material and is used in cast components today. Another important part of the proposed work was the investigation of the possibility to cast steel alloys which are not commonly cast today. These alloys possess material properties that make the suitable for aerospace applications, but are today only produced as forged components or sheet metal. The above mentioned tasks were performed in collaboration between the research institutes Swerea SWECAST and FRI, Foundry Research Institute together with the investment casting foundry TPC Components AB. The participating institutes play active roles in European research networks, which provided efficient communication paths for dissemination of the project results.
The overall object in LEAN, Development of light-weight steel castings for efficient aircraft engines, was to develop existing investment casting manufacturing methods to enable replacement of more expensive materials with thinner steel components for aircraft engines. The work was divided between research institutes and industry. Swerea SWECAST, the Swedish foundry research institute, which has a long tradition in collaboration with the industry in research projects was the responsible project leader and leader in one of the work packages (Minimize section thickness of steel alloy 17-4PH). The Polish Foundry Research Institute, which has advance equipment for material science and material data investigations, was the leader of the second work package (Castability limits for other steel alloys). The foundry TPC Components AB conducted casting trials and contributed with their wide knowledge of investment casting.
Casting trials have been performed in order to investigate the influence of different process parameters governing the fluidity of thin walled investment castings. The alloy used was CbCu7-1, i.e. the cast analogy of the stainless precipitation-hardening steel 17-4PH. Two levels of geometry complexity were used as well as top- and bottom gated casting systems. In the first trial, a simple slightly curved blade was cast with blade thicknesses ranging from 0.7-2.0 mm. Pouring temperature, shell temperature and blade thickness were variables in these trials. In the second trial, some features were added to the blade as well as a textured surface on one side to improve castability of a subset of the blades. Pouring temperature, pressure height and blade thickness were chosen as variables in these trials. As expected, a rather large variation in fluidity was observed. It was shown that the top gated casting system showed an overall improved fluidity compared to the bottom gated casting system for the simpler geometry. Blade thickness and pouring temperature were shown to have the greatest impact on fluidity. Adding some features to the simple geometry drastically decreased the differences between the filling systems. Whereas the top filling system still showed to be dependent on process parameters, the bottom filling system showed low dependency of the selected parameters. Using a one side textured blade with thickness of 1.3 and 1.5 mm was comparable with 1.5 and 2.0 mm flat castings thus reducing weight of the thinnest sections of a steel casting. Predictions of miss-runs with simulations were shown to be in good agreement with experiments and gave valuable insight to problems in the casting trials. Differences in porosity levels were seen between the top- and bottom gated casting systems, where the former showed a larger amount of porosity. Tensile testing of the thin blades was performed and all samples had yield- and tensile strengths within specifications. However, some specimens had an area reduction below the minimum value. This could readily be explained by the occurrence of shrinkage porosities.
Besides work performed on fluidity of the cast analogy of 17-4PH, a number of other alloys not commonly used for castings today were evaluated in terms of their fluidity and were compared to 17-4PH. Before casting trials, measurements and calculations of liquidus and solidus temperatures were performed. It was shown that JETHETE 152M had the best fluidity followed by Custom 465, L0H12N4M and 17-4PH. CSS 42L and PH13-8M ranked worst in the fluidity comparison and were therefore excluded from further investigations. Mechanical testing at both ambient and elevated temperatures was performed and it was shown from that all alloys met the demands of tensile strength. However, JETHETE 152M was later excluded due to its corrosion properties. In the corrosion test, at 400 degrees Celsius, for 100 hours with salt spray fog, it was determined that the 17-4PH and L0H12N4M showed similar corrosion rate with minimal differences. Wettability test performed on two different shell systems with 17-4PH showed that the shell/alloy system is important to consider during filling of thin sections. However, after a thorough consideration, taking into account other aspects, such as stiffness, weld ability and machinability, it was concluded that the cast analogy of 17-4PH was the most suitable alloy. Therefore, this alloy was used in the casting trials of a demonstrator. It was demonstrated during the casting trials that filling of a wide flat section with a thickness below 2 mm is hard to achieve.