Life cycle analysis results for engine blisk LCA

Nomenclature

3D

= three-dimensional;

CML

= Centrum voor Millikunde Leiden;

CMM

= coordinate measuring machine;

CO₂

= carbon dioxide;

DALYs

= disability-adjusted life years;

DTU

= Danmarks tekniske universitet;

ECM

= Electro chemical machining;

EBC

= environmental barrier coatings;

Eq.

= equivalent;

FPI

= fluorescent penetrant inspection;

GEO

= geographical correlation;

IBR

= integrally bladed rotor;

K_A

= imputed depreciation;

K_E

= energy costs;

K_F

= manufacturing costs;

K_I

= maintenance costs;

K_LH

= labour costs per hour;

K_M

= machine costs;

K_MH

= machine hour rate;

K_R

= room costs;

K_W

= tool costs;

K_Z

= imputed interest;

LCA

= life cycle assessment;

LCI

= life cycle inventory;

LCIA

= life cycle impact assessment;

Ni

= nickel; and

Ti

= titanium.

Introduction

Global air traffic has doubled approximately every 15–20 years and is expected to continue, albeit with some limitations because of the COVID-19 pandemic (Pearce, 2020). The European Union’s Flightpath 2050 roadmap and the Clean Sky program (Clean Sky 2 JU, 2015; Clean Sky 2 Joint Undertaking, 2017) are currently driving technological advancements in the aviation sector. Resource-efficient manufacturing beyond the operational phase is crucial for the overall ecological evaluation, and the construction of aeroengine components is of great significance as engine design affects the entire life cycle. Integrally manufactured compressor rotors, also known as Blisks/integrally bladed rotors (IBRs), are one of the most demanding components to manufacture in aeroengines. Complex shapes, tight tolerances and tough materials like titanium and nickel-based alloys require intricate manufacturing process chains. The environmental impact of such process chains can be analysed through life cycle assessment (LCA) according to ISO 14040/14044, the scientific standard for evaluating the environmental impact of product systems. LCA outlines a process for “compiling and assessing the input and output flows and the potential environmental impacts of a product system during its life cycle” [International Organization for Standardization (ISO), 2020]. The procedure comprises the phases (Figure 1):

• Goal and scope.

  In this first step, the focus of the study and the regarded use-cases are described. This includes a definition of the component in scope, the processes that are being compared and analysed, as well as the functional unit.

• Life cycle inventory.

  The second step includes the acquisition of life cycle inventory (LCI) data. These resemble the input and output flows of each individual process step including mass and energy flows (electrical energy, tools used, etc.).

• Life cycle impact assessment: (focus of this paper).

  In the third step, the actual environmental impact of each process is calculated based on specific indicators, such as global warming potential (GWP), water pollution and toxicities. This step is conducted within software environments such as OpenLCA or GaBi (Ganzheitliche Bilanzierung, engl.: holistic balancing) (Lüdemann, 2014).

• Interpretation: (focus of this paper).

  This step is included in all three previous steps and requires domain-specific knowledge and technical expertise to transfer the findings and conclude overall results.

The following literature search provides an overview of existing LCA studies in the context of aviation. Several studies have dealt with the topic and carried out environmental impact assessments in a wide range of applications.

In 2013, Atılgan et al. conducted an environmental impact assessment for a turbo-prop engine using an exergo-analysis. The related environmental impact of each component of the engine was first calculated, and then an individual environmental performance was assigned to each component using the exergo rate. The study shows that the compressor, combustion chamber, gas generator turbine, power turbine and exhaust nozzle create different percentages of the total environmental impact of the engine. The exergy-based analysis is suggested as a valuable supplement to conventional analysis for getting more accurate results. However, no in-depth assessment of particular engine components was conducted here (Atılgan et al., 2013).

Vinodh et al. published an LCA for a turbine blade. The article presents an LCA of a turbine blade used in aircraft engine systems to determine its environmental impact. The study uses the ISO 14040 standard and the Eco Indicator method to assess the environmental profile of the turbine blade. The results show that plating has the highest impact, followed by raw materials. Laser machining causes a high environmental impact, and efforts are being made to reduce this impact by adopting alternate eco-friendly machining processes. The article concludes that there is a vital need to perform environmental analysis of aircraft components to develop eco-friendly aircraft products, and more studies could be conducted on the LCA of complex aircraft products in the future (Vinodh et al., 2017).

Musacchio et al. conducted a study to identify and define key environmental performance factors (KEPF) for gas turbine eco-design and production through LCA. The study aimed to develop a comprehensive framework for evaluating the environmental impact of gas turbine production and identifying opportunities for improving its sustainability. The article discusses the need for environmental impact assessment of new products, particularly in the oil and gas and power generation markets and emphasizes the importance of considering environmental impact aspects from the early stages of product development. The assessment is based on a Cradle-to-Gate LCA and is combined with case-specific qualitative and quantitative information such as material selection, manufacturing processes, mass quantity and coatings to provide environmental assessments. The authors present a case study of LCA applied to a heavy-duty gas turbine to outline the relative weight of each KEPF. The authors stress the importance of reducing the carbon footprint (CF) of products while maintaining thermodynamic efficiency and highlight the potential for using upgraded biogas instead of fossil-origin natural gas to reduce CF (Musacchio et al., 2020). Musacchio et al. also presented a comprehensive study on the LCA of gas turbine components, with a focus on the decarbonization of materials and machining processes. The authors conducted a general analysis of the environmental impacts of gas turbine components, from the extraction of raw materials to the end-of-life stage. The authors found that machining operations contribute to the CF of gas turbine components without, however, transparently outlining the impact of specific core engine parts like Blisks/IBRs or referencing any significant LCI data (Musacchio et al., 2021).

Previous works on Blisk-LCA from our scientific working department focused on very basic preliminary results from small-scale Blisk by Bergs et al. (2021) and a collaboration with DTU Copenhagen analysed potential benefits of Ti-Blisk recycling without going into technical detail and in-depth analysis (Rupcic et al., 2022). An in-depth assessment of full-size Blisk manufacturing is the focus of this study.

2. Method – life cycle assessment goal and scope

An investigation framework and a target must be defined in a first step regarding the product to be investigated as a functional unit or the processes to be analysed. The goal and scope of this study have been defined in the first paper titled “Geometry Model for Future Blisk LCA” (Fricke et al., 2021b), whereas the definition and acquisition of LCI data have been described in the second paper titled “Life Cycle Inventories for Engine Blisk LCA” (Fricke et al., 2022). As continuation for the two previous papers, this paper describes the results of the life cycle impact assessment (LCIA) after the acquisition of LCI data of different Blisk manufacturing scenarios, which will be explained in the next sections.

3. Results and analysis – life cycle impact assessment results and interpretation

Based on the quantified LCI data, various indicators are calculated to compare the environmental impact of Blisk manufacturing Scenarios #1 and #2. These indicators are taken from the EF3.0 impact category group. A representative set of these indicators – with significant results – is presented in this paper. In the following, the LCIA results are presented as depicted in Figure 8. While comparing both scenarios, the result is normalized to the higher value; therefore, the value of 100% means that the corresponding scenario has the higher value of the two in the respective indicator. It is noticed that Scenario #1 (titanium Blisk, milling) has lower values in all indicators than Scenario #2 (nickel Blisk, milling) to a considerable extent, because in almost all indicators the value of Scenario #1 is even less than half of Scenario #2. Firstly, this can be explained by the more complex and resource-consuming raw material generation process for Inconel 718, which contains more elements (around 15 individual metals, including chromium and molybdenum) compared with Ti6Al4V (three individual metals). Furthermore, the machining processes (turning and milling) of Inconel 718 are also more complex and resource-consuming than those of Ti6Al4V, as indicated by the LCI data (Figure 5), which ultimately leads to the higher CO2 footprint of nickel Blisk manufacturing compared to the titanium Blisk.

As a deep dive, the specific individual flows and their impact on GWP are analysed regarding Scenario #1 (the titanium Blisk) and Scenario #2 (the nickel Blisk), respectively. In Figures 9 and 10, the absolute (left) and relative (right) shares of elementary flows in the indicator GWP of both Scenarios #1 and #2 are presented. The results show that both scenarios have similar relative contributions in the impact category GWP, with raw material production being the highest contributors in the CO₂ footprint, constituting almost half (45%–49%) of the total calculated value. However, this analysis did not take recycling into consideration and depending on the alloy composition, the data situation is relatively untransparent and often heavily dependent on literature data. Electricity consumption is the second highest influence, comprising up to 40% of the total GWP. For the CO₂ footprint, the influence of tungsten carbide and cobalt through the tool usage is the third highest, which is up to 10%. Other minor influences are contributed through lubricants (around 2%).

Besides the “basic material and energy flows”, the results according to process level are also investigated. Figure 11 shows the relative shares of main process flow in GWP of Scenario #1 (left) and Scenario #2 (right). Corresponding to the result of the basic material flows, the raw material generation has the highest influence on the CO₂ footprint. A higher recycling rate as well as the application of additive manufacturing processes for the raw part can also possibly address the significant impact of material generation on the ecological footprint. Furthermore, it is revealed that the central machining processes (milling and turning) also have a high impact on the ecological footprints in both scenarios, which form around 30%–36% of the total GWP. This significant influence of milling and turning processes can be addressed, for instance, by developing more efficient processes (improved machining strategies, e.g. tool selection) and adaptation of the process chain (substitution of processes, e.g. milling replaced with ECM), which will be addressed in future studies.

It is worth noting that while raw material generation has the highest influence in terms of GWP, this is not always the case in the other impact indicators. An example of this is presented in Figure 12, which shows the absolute and relative shares of basic flows for the indicator Freshwater Aquatic Ecotoxicity, respectively. Here, cobalt has the highest impact, at around 50% of the total value. Cobalt is solely used in the milling tools of the machine process. A substitution of the tools or the process in general could potentially improve the overall eco-footprint in these categories. The same reasoning is true for tungsten carbide (ca. 10%), which makes up 90% of the milling tool’s weight composition.

As an interim conclusion, it can be stated that a major contributor regarding the ecological footprint – specifically the CO2 footprint – during Blisk manufacturing can be associated with the raw material generation. A reduction of this impact can be addressed by introducing alternative raw material generation processes such as additive manufacturing and increasing recycling and repair rates. On the contrary, the ecological impact generated through the machining processes cannot be neglected. An optimization of machining strategies based on alternative tool and technology selections has the potential to reduce the overall environmental footprint.

4. Discussion & summary

In this paper, the results of LCIA for two Blisk manufacturing scenarios (milled titanium Blisk and milled nickel Blisk) are presented and discussed. Based on the standardised approach of ISO 14040/44, the first two steps of goal and scope definition and LCI data acquisition were presented based on the detailed description of previous papers (Fricke et al., 2021b; Fricke et al., 2022).

Based on these findings An LCIA was conducted and various EF3.0 impact indicators were calculated using the software GaBi, with primary focus in this investigation on the impact indicator GWP. The results show, that the titanium Blisk generates an overall lower ecological impact compared with the nickel Blisk in all EF3.0 impact categories. This is due to the fact that the raw material impact, machining time and tool wear are higher during the nickel Blisk manufacturing. An in-depth assessment of the particular impact of specific material and energy flows reveals that a main driver of specifically the CF of both components comes from the raw material (if recycling is not included).

The large Impact on the ecological footprint during material generation can be reduced by higher recycling rates of metal chips and other wastes, as well as alternative raw material generations such as additive manufacturing. On the main process level, the central machining processes (milling and turning) also have significant influences on the ecological footprints. This can be addressed by developing more efficient processes (improved machining strategies, e.g. tool selection), the use of “greener materials” (e.g. less cobalt and tungsten in milling tools) and adaptation of the process chain (substitution of processes, e.g. milling replaced with ECM). The LCC assessment comes to similar conclusions. The manufacturing of the nickel Blisk results in higher costs, which are mainly driven by raw material and milling tool purchases, as well as longer machining times that result in higher costs of electricity and labour. Future investigations will focus on the third manufacturing scenario (nickel Blisk with ECM) as well as variations of the milling processes to minimise the negative impacts during the machining processes.