Additive layer manufacture expands options for airframes

The Welding Engineering Research Centre at Cranfield University has been examining new opportunities in additive layer manufacturing using arc + wire over the last few years. Simon Lott takes a look at the progress it has made.

Additive layer manufacturing is currently a hot topic in advanced engineering circles thanks to recent progress


in various laser + powder deposition methods, but researchers at Cranfield have been developing and championing


additive layer manufacture using arc + wire systems since the mid-nineties.


 


Initially approached by Rolls-Royce over a decade ago for aero engine applications, researchers at the university initially developed the wire + arc deposition process to examine the use of Inconel, titanium, aluminium and various nickel alloys. Progress made during this time led to the development of a 9-axis gantry-type machine within a full production facility by the engine OEM.





From engines to airframes





Since then, with demand from major civil aerospace manufacturers, the focus has shifted to airframes. Although


laser and powder methods are useful for certain applications such as rapid prototyping or for small highly complex parts, the technology is limited by its speed and the size of component it can accurately manufacture. In contrast, the processes being developed at Cranfield are designed for high deposition rates.




To put this difference into context, the centre is currently targeting a deposition rate of 10kg an hour for titanium,


compared with a typical 0.1kg using laser + powder methods, which can also potentially carry the risk of the material not being fully consolidated if fusion has not occurred between grains. Additive arc + wire systems are also capable of producing parts several metres in size and simplify the process of producing single piece linear intersections.

All in one





One of the main projects at the Welding Engineering Research Centre, RUAM (Ready to Use Additive Manufacturing), takes the technology one step further. The £2 million, four year programme began in 2007 with funding from both the University's Innovative Manufacturing Research Centre and the industry, from which it has 15 partners and end users committed. The idea is to simplify the process of complete product production by including shape measurement and grinding within a single one-hit additive manufacturing system comprising of a fully integrated robotic machine. The system would also be available at a competitive cost through the combination of these processes.




Additive layer manufacturing offers several advantages for certain structural airframe components such as a vast reduction in material wastage, especially when producing many heterogeneous parts, and the ability to produce a great variety of part designs for prototype work quickly.




There is also the key benefit that it allows the consideration of unconventional designs that otherwise would not be practical because of manufacturing or cost constraints due to, for example, complex or unusual geometries, bringing with it many different opportunities and challenges.




One such example is the development of a corrugated structure from titanium for potential use in wing support structures from 500mm by 500mm titanium plate, researched by PhD student Panos Kazanas. After evaluating the buckling behaviour of a series of novel designs through finite element analysis and establishing reference loads, it was concluded that a design comprising of a plate with curved stiffeners 50mm high and attached in two directions gave the best result for both uni-axial and biaxial loads. Such a design, with its otherwise difficult to machine intersections presents no such problems for the additive method and does not suffer from the potential weaknesses that may result from conventionally fastened assemblies. However, the quality of the part in the first place is also limited by the efficiency and reliability of the weld process. Thus, the RUAM project is also about ensuring repeatable process to required standards, and to this end progress is ongoing.

From prototype to process





Now halfway through the project, the centre's key objective is to identify and establish design guidelines which will eventually result in a standard handbook consisting of specific routines. Through this research, the Welding Engineering Research Centre is also in the process of optimising welding parameters by simulating and minimising part distortion and developing an interface for generating robot codes from a CAD model for the complete RUAM process, ultimately resulting in a prototype software tool. Through this empirically developed process model, all the operator would have to do is input the necessary geometries and leave the machine to do the rest.




RUAM has clear targets in terms of bringing the technology to market, although there is still much work to do to produce the data required. Currently, due to the commercial availability of systems, laser + powder processes are the methods of choice for additive manufacturing amongst the OEMs, so part of the centre's goal is to overhaul that position.




The second concentrates on production systems, with the next phase being the production of a pre-prototype machine and improving the management of residual stresses (i.e. heat from the beam oxidising inside the material and producing faults). There are also several variations on the essential process being developed. For example, a multi-wire tandem torch comprising two heads will give users the ability to mix any proportion of elements to produce various alloys, with tests into popular compounds such as Al-Cu-Mg well underway. Commercially there is much interest in the potential of aluminium deposition, and multi-wire technology will be essential to its acceptance, making alloys simpler to obtain and produce.




There is also a variation on MIG welding where the wire is moved in and out of the weld pool, which vastly reduces the risk of spatter. In this method, the molten metal moves


into the pool by surface tension, which as an additional benefit, improves the initial surface finish. Most metals can be manipulated in this way although further research is still


concentrated on developing titanium applications to utilise the metal's very high surface tension properties.




Despite many applications still being at a preliminary stage, it would seem that the research being done at The Welding Engineering Research Centre holds much potential.

www.cranfield.ac.uk/sas/welding