circular manufacturing

Sustainability Snapshots: Circular manufacturing

Sustainable aerospace manufacturing is key in adressing 1/3 of the aircraft carbon footprint. Circular manufacturing in particular can be a game-changer in reducing waste, minimizing demand for materials and energy, and improving resource efficiency. The third edition of Sustainability Snapshots investigates circular manufacturing of aircraft, and how SUSTAINair will contribute in advancing this vital solution for circular aviation. 

Current state-of-play of aerospace manufacturing

Historically, aerospace manufacturing has used raw materials as inputs and produced outputs such as landfill waste or materials recycled into other industries. But in this process, little has been brought back so far into the aerospace production chain itself. Furthermore, the initial extraction of raw materials (such as e.g. titanium, aluminum) has been one of the biggest sources of carbon emissions during production. Reducing material demand is thus the lever with the largest potential to reduce not only energy needs, but also to decrease negative environmental impacts of the industry (e.g. use of scarce natural resources). Moreover, the aerospace manufacturing processes themselves can be made vastly more efficient. Metal aerospace components e.g. are often manufactured by following a subtractive process. This process tends to produce parts with high buy-to-fly ratios, and are also dependent on scrap recycling to avoid waste.    

There are two key strategies to reduce the raw material required in the manufacturing of new aerospace components. Firstly, by lowering the manufacturing buy-to-fly ratio. The buy-to-fly ratio refers to the weight of raw material required compared to the weight of the final part. Secondly,  by optimizing topological design to increase the structural efficiency of components. The objective is to deposit material only where it is required, thus producing so-called near-net shape parts. This reduces not only waste in the manufacturing process, but also reduces overall demand for materials and energy.

Shifting traditional aerospace manufacturing to circular manufacturing requires a multitude of changes in the sector. First and foremost, it comes down to using less (raw) materials, while keeping the materials used for as long as possible in a continuous, virtuous loop. In addition to this, it is about harnessing new advanced (digital) technologies to simplify aerospace manufacturing processes, such as additive manufacturing, digital twin and improved joining & welding techniques. And finally, it is about making logistics more efficient, such as through localizing production and envisioning manufacturing facilities as closed-loop systems. 

circular manufacturing sustainair
A finite element model of CFRP/Titanium single-lap shear joints reinforced with additively manufactured pins by SUSTAINair partners DLR & INVENT (© TU Delft)

Additive manufacturing as a key enabler for circularity

Additive manufacturing in particular is explored by aviation and aerospace sectors as promising solution for circular manufacturing. In a nutshell, additive manufacturing – also known as 3D printing – builds up components through an addition of material to create the final product. Where previously parts would be created by chipping away at raw materials, this manufacturing solution builds in an additive and not a subtractive way. This means that e.g. powder or molten material is laid down only where it is needed. Additionally, additive manufacturing can use even recycled material in this regard, thereby massively reducing waste in the production process. 

Additive manufacturing includes a plethora of methods, including e.g. laser-directed energy deposition (LDED), wire-arc additive manufacturing (WAAM), high pressure die casting (HPDC) or laser powder bed fusion (LPBF). For metallic parts, methods like LDED and LPBF are considered by the aerospace industry as among the most mature to manufacture and repair parts. HPDC has been especially used so far as a proven, efficient manufacturing method in the automotive industry, and is slowly making its way into the aerospace industry as well. The WAAM method is also becoming more attractive to aerospace engineers, to aircraft parts such as the fuselage in a faster, cheaper and safer manner. While these methods differ from each other, they all ultimately play a role in bringing the circular economy to aviation and aerospace sectors – by reducing and reusing materials, minimizing the energy consumed, optimizing the structural efficiency of designs and by manufacturing parts where no additional tooling is required.    

circular manufacturing sustainair 2
Example of a casted part using the high pressure die casting manufacturing process at SUSTAINair coordinator LKR (© AIT/Zinner)

How SUSTAINair will contribute to circular manufacturing

SUSTAINair will focus in its circular manufacturing research on several aspects. Firstly, the project will explore the differences in joining techniques and resulting overall quality of the different joints, applied to the material combinations under research. Secondly, the aspects including the joining for production of assemblies will be investigated. Thirdly, SUSTAINair will concentrate on improving conditions and economics for repair and EOL (End of Life), disassembly and recycling. SUSTAINair aims to achieve this by applying multiple circular design parameters:

  • Casting process development for near-net shape metal test elements (additive manufacturing)
  • Joining of metal test elements
  • Joining of composite with metal test elements
  • Joining of thermoplastic 1st and 2nd life composite test elements
  • Joining of morphing elastomeric materials

In its approach to circular manufacturing for aircraft, SUSTAINair will incorporate three key enabling technologies for (circular) manufacturing. Firstly, it will employ online process monitoring of induction and conduction welding of thermoplastic composite structures. Secondly, it will work on advanced near-net shape manufacturing (additive manufacturing) of nano-eutectic aluminium alloys providing repairable, laser-welded structural assemblies. Thirdly, the project will design combined nano- and macro-structures for joint interfaces of dissimilar materials.

Throughout its research on circular manufacturing, SUSTAINair will establish a comprehensive database with test results for the several joining techniques, material combinations and sensors. This will be done for both new and recycled material, to allow for comparison of the techniques and quick referencing for individual joint properties.