Project Outline

Laser energy can be used to (re)melt small amounts of material in a precise manner using a minimal heat input. Through advances in computer technology, a first laser-additive, rapid manufacturing process emerged in 1987, in the form of stereo-lithography, whereby parts were produced layer-by-layer by polymerising a plastic monomer. A metallic powder-variant of this concept was only developed in recent years, with its simplest form, laser cladding (developed long before stereo-lithography), applied in a single layer to enhance the properties of a material surface. Moving from single-layer deposits to multi-layer deposits created new opportunities, beside surface treatment, including repair and rapid prototyping. Although extensively used for prototyping, the direct application of this concept in manufacturing is growing, but currently limited.

The (laser) powder bed concept (Figure 1) was operates by scanning a focused laser beam across a bed of powder, locally fusing the powder in a pre-determined pattern. Upon completion of the first scan, a new layer of powder is applied on top of it and the process is repeated. These steps are repeated, building up the part layer-by-layer. The current accuracy of this technique, depending on the size and the complexity of the structure ranges between 50 to 250µm, with a productivity of 3 to 7 cm³ of powder fused per hour.

Laser Metal Deposition operates by locally melting a substrate, or underlying layer, and introduces additional material into the melt pool. There are two LMD processes considered in the MERLIN project.

  1. LMD-p makes use of metal powder to clad conformal surfaces and to build up self supporting 3D structures (Figure 2). The powder, delivered through a nozzle into a molten pool created on a substrate by a transient laser beam, is used to form the material layer. By building up a series of layers, each one being fusion bonded to the underlying layer, allows surface features and 3D geometry to be realised.
  2. LMD-w operates in a similar way to LMD-p but instead of blowing powder into the melt volume, LMD-w introduces wire from a continuous spool feeder (Figure 3).

In both LMD-p and LMD-w the laser, powder feed nozzle and wire feed gun can be manipulated in 3D space using a gantry system or industrial manipulator. Current LMD-p technology is capable of building walled features as small/thin as 200-300µm, at a productivity of up to 10 cm³ of powder per hour. Conversely, LMD-w technology has much higher productivity but at the expense of resolution (minimum wall thickness approximately 4-5mm).

Laser based additive techniques will be at the core of the development in MERLIN because of the nature of the materials to be processed and the parts required by aero engines. Laser techniques provide both low heat input, accurate and small feature creation, and good surface finish, which cannot be delivered by electron beam and arc based additive techniques. The deposition rates involved in laser based techniques can be less than those using other heat sources, therefore achieving further step change improvements in the productivity of Laser Metal Deposition (LMD-p and LMD-w) and Selective Laser Melting (SLM) are objectives of MERLIN.

Figure 1 Selective Laser Melting (SLM) concept

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Figure 2 Laser Metal Deposition with powder (LMD-p) concept

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Figure 3 Laser metal deposition with wire (LMD-w) concept

Currently the OEMs involved in civil aerospace jet engines have come to tolerate low material usage ratios, which would not be considered viable in other industry sectors. Given the environmental pressures that will increasingly be placed on the industry, this approach is not sustainable in the long term. The latest designs of large high thrust aero engines (see Figure 4) can weigh around 7 tonnes, however, to create the parts to enable such an engine to be assembled 28 tonnes of high specification, high environmental impact material needs to be processed (Data provided by Rolls-Royce Plc.). This is an overall buy-to-fly ratio, a term used to describe the ratio between the amount of material bought to make the part and the amount of material in the final part itself, of 4:1. This typically results in the creation of 21 tonnes of high impact waste material, much of which is highly toxic and carcinogenic. Many individual parts in aero engines have buy-to-fly ratios substantially higher than 4:1, with 10:1 and even 20:1 not uncommon. See Figure 5 for an example of a bladed disk or blisk. The billet used to create such a part may weigh in excess of 800kg with the final part being just 90kg, after all the material has been machined away (Data Provided by Rolls-Royce).

It is forecast that over the next twenty years 24,300 new passenger and freight aircraft will be delivered to support worldwide demand (Wood D sourced from Airbus figures). This will result in a requirement for approx. 60,750 engine deliveries. This will result in an estimated total wastage of approx. 1.28Mn tonnes of high performance material (assuming above figures of 21 tonnes wasted per engine). Such material has often gone through high environmental impact processing in its creation, involving excessive use of energy, water and chemicals. The cost implications are also excessive. The materials involved have an estimated average value of €10,000 per tonne giving an overall estimated cost of 12.8 Bn€ in waste material. The vast majority of this material cannot be recycled, or is recycled far down the value stream. Recycling of swarf is seen by many as being 'eco friendly'. However, the environmental cost of recycling the waste is of the same order as manufacturing the original material. AM techniques have the potential to approach zero waste through close to 100% material utilisation and the use of recycling within the processes. This results in a reduction in emissions because over 75% less raw material is consumed.

AM is also a key enabler for design or topology optimisation because the additive nature of the techniques allows very complex parts to be created. This can give freedoms to lightweight components and to design for performance rather than emphasis on the method of manufacture.

Cutaway diagram of Trent 1000 jet engine
Figure 4 Schematic of a Rolls-Royce aero engine (Courtesy Rolls-Royce Plc.)

 

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Figure 5 Image of a Rolls-Royce Blisk, or bladed disk (Courtesy Rolls-Royce Plc.)