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:

• 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).
• 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.
• 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 aimed at investigating the mechanical properties of aeronautical Ti6Al4V stator vanes elaborated by Electron Beam Melting. These stator vanes were 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 was 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) were manufactured with both fresh powder and recycled powder, in order to assess the mechanical properties of Ti6Al4V material elaborated by EBM. Finally, this task enabled to determine the limit of use of the recycled Ti6Al4V atomized powder associated with the EBM process. Therefore, the constraints were 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.