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Contenido archivado el 2024-06-18

Direct Manufacturing of stator vanes through electron beam melting

Final Report Summary - AEROBEAM (Direct Manufacturing of stator vanes through electron beam melting)

Executive Summary:
Electron beam melting (EBM) process is identified to as potential candidate but is not yet fully understood and controlled because of its lack of maturity. One important advantage of this process is that the powder particles not affected by the heat source can be recycled for further fabrications, meaning that only the quantity of material required to build up the parts is used in contrast to machining where up to 80% of material is removed away (reducing the buy to fly ratio). Case studies show that the waste of raw material is reduced by up to 40% when using Additive Manufacturing (AM) technologies – such as EBM – instead of subtractive (machining) technologies. It is due to the capacity of additive technologies to build designs that are not viable for conventional processes, reducing significantly the use of raw material.
In the EBM process the powder is distributed in layers over the building platform. This powder is initially preheated and heated every layer by the electron beam in order to maintain a working temperature around 650º C. In every layer, the electron beam melts also the corresponding slice of part till all layers are completed and part manufactured. EBM produces Near-net-shape parts making possible to save material (in contrast to machining), especially with very complex and lightweight geometries for the aeronautical sector. However, one of the major issues for repeatability of mechanical properties in Ti parts made on EBM is the reliability of recycled powder. Some 95-98% of powder that is not melted can be used again after sieving. However, different factors must be controlled to be able to rely on recycled powder:
1. Chamber temperature., since during building, powder is submitted to high build chamber temperature (around 650ºC in case of Ti64) which changes chemical composition due to evaporation of some elements (mainly Aluminium).
2. Oxygen pickup, since an important source of oxygen in Ti powder is the humidity that sticks onto build chamber walls. The oxygen and hydrogen pickup may increase material fragility, but also affects powder flowability.
3. Particle size and shape. After removing parts from build chamber, it is common to perform blasting in the Powder Recovery System (PRS) using pressurized air with Ti64 particles. The collision of projected particles with sintered bed causes particles to deform and break, which might significantly change their morphology.
For the aerospace parts, the reduction of the raw material consumption is essential, but mechanical properties must be guaranteed. One of the key issues is to guarantee that the recycled powder is comparable in all above mentioned aspects to fresh powder. Also, it must be clearly verified that the differences that may appear between fresh and recycled powder do not interfere in mechanical performance of functional parts built of recycled powder. Based on this context, this project is aimed at investigating the mechanical properties of aeronautical Ti6Al4V stator vanes elaborated by Electron Beam Melting. These stator vanes will be compared, in terms of geometry, surface roughness and mechanical properties, to the stator vanes manufactured by selective laser melting in previous CS-RTD calls. This is particularly interesting because it is known that EBM has higher building rate than SLM although it has lower surface quality and lower quality of details. Cylindrical mechanical specimens (tensile and fatigue) will be manufactured with both fresh powder and recycled powder, in order to assess the mechanical properties of Ti6Al4V material elaborated by EBM. Finally, this task will enable to determine the limit of use of the recycled Ti6Al4V atomized powder associated with the EBM process. Therefore, the constraints will be defined to reduce as much as possible the raw material consumption to produce aeronautical components but without putting at risk mechanical performance of these parts.
Project Context and Objectives:
The main objectives of this project were to determineif the powder properties change and to what extent throughout the consecutive recycling of powder and to see if there is a detrimental effect on the mechanical properties when powder is recycled. Also, the standard processing parameters have been confirmed valid since the solid material obtained has been proved in compliance with the ASM4911 norm.

The work in this project has been organized in 7 work packages. Five work packages are of RTD nature, WP6 is Project Management package and WP7 is Exploitation and Dissemination. The RTD packages and all related tasks has been led by AIMME with the participation of CEIT. The RTD packages were organized in a way that ensures progress from processing parameters until final product to be certified. This way WP1 refers to the optimization of build parameters, WP2 deals with characterization of processed material, WP3 deals with characterization of recycled powder and WP4 deals with characterization of material processed out of recycled powder. The completion of these four packages will ensure that the stator vanes built in WP5 are completely reliable regarding aeronautical requirements and can be equally used no matter if vanes are built from fresh or recycled powder as long as the powder is below its maximum level of recyclability.

WP1

WP1 is the package in which the EBM processing parameters have been optimized. The work in this package is similar to what is commonly done when new powder is developed for EBM. A series of flat samples has been built and metallographic study has been performed each time the parameters are modified. Parameters such as line offset, line order, focus offset and speed function are most commonly changed, but some other has been considered as well. The main objective of the parameters optimization was to obtain a fully dense material as understood by aeronautical norms. In other words - low porosity content (<0.5%), low defects size (<300µm) and absence of cracks and distortions. That was clearly observed and verified by the metallographic studies which confirmed that the metallurgy of EBM processed Ti64 is excellent. In addition, the metallographic study revealed the grain size of 50 micrometers of width and grain orientation in build direction which indicates clearly the material behavior. However, some samples have been tested on compression in order to relate the yield strength to metallographic observations showing good results.

WP2

WP2 consisted in fabrication of 2 batches of 30 mechanical specimens using fresh powder, with and without posterior HIP treatment. A proper build layout has been presented and horizontal (15) and vertical (15) specimens has been built. In WP2 it has been twice – once to be HIP-ed and once without HIP.

WP3

WP3 dealt with the determination of maximum level of recyclability of Ti64 powder. AIMME’s strategy for this task was to manufacture in 16 consecutive builds of mechanical cylindrical specimens (this time higher since the facilities of AIMME demanded it) using the same powder recycled from build to build and using optimized EBM process parameters (WP1). In each build with reused powder, six mechanical specimens have been manufactured in 2 building directions so that tensile tests could be performed 3 times per direction to obtain reliable material properties.
The remaining sintered powder around parts and the powder stored in hoppers was reused for further EBM fabrications. In terms of powder recyclability 3 properties of the powder have been checked:
• Chemical composition: Reused powder is affected by the heat of the EBM process. The heat developed in the process could cause the evaporation of some alloying elements, i.e. aluminum content due to its low melting point.
• Oxygen pickup: Powder absorbs O2 from moisture present in the ambient due to its high surface. The content of O2 in the Ti6Al4V alloy is limited by standards; therefore this factor could set a limit in the number of uses of the powder.
• Particle size, shape and powder flowability. After removing parts from build chamber, it is common to perform blasting in the PRS using pressurized air with Ti64 particles. The projection of air charged with particles dissolves the bed of semi-sintered powder that surrounds the part. However, the collision of projected particles with sintered bed causes particles to deform and break, which might significantly change their morphology and thus their flowability.
Powder analyses, including chemical, particle and oxygen pick-up analyses, has been performed after each build in order to establish trend lines of chemical content and powder particle size and shape, as well as the tendency of oxygen content in powder. Due to above mentioned changes in powder, it is not recommendable to mix fresh and recycled powder since it won’t be possible to distinguish clearly the trend lines and make subsequent deductions. This is why a clear control of nº of recycling has been kept during the process. In addition, tensile tests has been performed to the last batch with correct powder properties.
The results showed that the level of recyclability of Ti64 in EBM is set to 12, since the powder exceeded the level of oxygen at that point. The tensile test showed that the mechanical properties of the test bars were in compliance with the ASM4911.

WP4

WP4 dealt with the fabrication of one batch of 30 mechanical specimens on EBM using recycled powder, with a posterior HIP treatment. These specimens has been built out of recycled powder that was proven to be valid for reuse after the study of WP3.

WP5

WP5 consisted in the fabrication of 20 stator vanes on EBM, with posterior HIP treatment. Following the plan of previous work packages, 10 stator vanes has been manufactured using fresh-powder and the other 10 stator vanes with recycled-powder. Topic Manager is in charge of its characterization.

Project Results:
The foreground information, generated in the AEROBEAM project, is mainly defined by the project deliverables and can be summarized in three main results:

1. The first result is particular for aeronautical sector which is the knowledge of optimal process parameters for obtaining of microstructure suitable for aeronautical use, in particular in stator vanes, as described in the corresponding norm AMS 4911M-Ti6Al4V;
2. The second result is of more general character and represents the testing results of mechanical behavior of test specimens built after different number of powder cycles. This includes the High Cycle Fatigue test results and tensile tests at elevated temperature of test specimens with and without HIP made of fresh and recycled powder, as well as tensile tests after each powder cycle (up to 16).
3. Other more general result, which is the study of the tendency of variation of chemical composition of powder, its morphology, particles size and shape in relation to a suitable microstructure. This study will determine the maximum number of builds that the powder can be used with a guarantee that the processed material will have the same microstructure that ensures the aeronautical performance. This study is said to be general because it can be extrapolated to other industrial sectores.

Potential Impact:
Positive outcome of this project may have three types of impact:

• Reduction of buy to fly ratio with all positive aspects on lower transport costs and lower environmental impact;
• Extension of the results to other materials processed on EBM as well as other sectors of application;
• Fostering the expansion of European technology and industry.

Reduction of buy to fly ratio

The design and AM of components will also have a great impact on the energy consumption of produces during the use-phase of the product life-cycle. This will be of particular importance to the transport sector. Savings in CO2 emissions through the use of optimal designs to lightweight vehicles can reduce the fuel consumption of vehicles particularly those used in air transport. Reducing the weight of a long range aircraft by 100kg results in a 1.3 MtCO2e saving over the lifetime of the aircraft, the equivalent of saving $2.5 million worth of fuel. EBM-manufactured Ti products qualification will greatly contribute to this impact.

Extension of the results to other materials and sectors

For the time being, there are four commercially available materials for EBM technology: three types of Titanium alloys (Titanium Grade 2, Titanium 64 and Titanium 64 ELI) and Cobalt Chromium. Yet, there are plenty of materials that are under independent development and will be soon available. For example, recently at AIMME new processing parameters for Ti67 have been developed for biomedical use. As previously mentioned, Italian company Avio developed processing parameters for γ-TiAl in cooperation with Arcam. In other words, EBM technology is a promising and perspective technology for high added value aerospace parts.

Although it is true that the processing parameters are different from material to material, certain conclusions will be obtained from studies that will be performed in this project that can be extrapolated to future developments. The particle size (45-108µm) and type (spherical) used in EBM powders are constant no matter what material is processed. Therefore, the results of particle size distribution, particle shape, number of satellites, etc. after recycling and reuse, which are very valuable for getting conclusions on powder flowability and raking density, can be used in future studies. Also, there are several materials which principal alloying element is Aluminum, the element which is most likely to evaporate during EBM processing due to its low melting point. The results we get in present project can be indicative for other powders as well.

On the other hand, chemical composition and state of particles after processing are also of huge importance in other sectors of applications. One of the strongholds of EBM technology is biomedical sector. So as to be applied for medical implants, processed Ti alloys must be subdued to a strict control of chemical contents. Hence, it is essential to have control on evaporation and be able to predict the behavior of recycled powder. Also, the particle size and shape might have certain influence is capturing oxygen from humidity. The formation of titanium oxide due to increased oxygen content has negative influence on biological behavior of implants.

Fostering European industry and technology

AM is strongly growing in Europe since the number of installations has tripled in just 7 years (2000-2007). It is to be noted that Europe emerges in the market of AM technologies producers. The most relevant ALM technologies for metal processing are European brand: German (Concept Laser, EOS, SLM Solutions, ReaLizer), English (Renishaw), Swedish (Arcam) and French (Phenix Systems) companies.
On the other hand, as potential AM users, several markets have been identified (Vision on 2020 for Rapid Manufacturing) as the most relevant:
• Medicine: scaffolds and intelligent implants, artificial organs, bioactive bone, skin tissue, prosthetics, surgical tools, hearing aids, lab-on-chip, orthotics
• Consumer customized products: helmets, shoes, jewellery, furniture, toys
• Automotive: functional test products, (driver adapted) interiors, limited edition cars, connectors, mechanical devices, aerodynamic parts, engine parts
• Aerospace: spare parts in space, all low volume products, customized pilot equipment, cabin interior, air ducts
• High end equipment: complex and high performance parts, energy appliances, electronics, electrical engineering, packaging, cutting tools, batteries/fuel cells, sensors, etc.

Mass production is almost completely shifted to Third World countries. The only opportunity left for regions with strong technological background is to specialize in high value added products such as aerospace parts. However, uncertainties are present in emerging technologies such as EBM due to its lack of maturity. These facts are still keeping it out of some crucial industrial sectors. It is very important that the technology gains in understanding of physical phenomena that govern the process since it is the only way to be able to rely on it for crucial applications.

It is worthy of mentioning that in other regions like China and India, there are developments of technologies similar to EBM since they are getting closer to the European level of technological advance. So as to get involved in new sectors before the emerging countries do, it is necessary to perform certification of new emerging technologies which can make the difference. This way, strongholds in high-tech sectors of application such as aeronautical sector can be established. This is definitely the principal aim of this project – establish EBM as a commonly used technology in aeronautical sector.

List of Websites:
The website of the project is aerobeam.wordpress.es.

The relevant contact details are: