Additive manufacturing is empowering designers with a whole new approach - particularly to the design of parts where complex voids and internal features have historically led to restrictions in materials or method of manufacture, long lead times and heavy overheads. The challenge of designing 'what's not there' - often the starting point for many projects - can now be addressed by designing for a process which creates hidden features and complex internal geometries as a by-product of its bottom up build style.
The original additive manufacturing (AM) processes were developed using CO2 lasers for the production of pseudo plastic parts using photo cure resins or sintered polymers. AM in metal became a possibility as ytterbium infrared fibre lasers were developed with the ability to melt metal. Selective Laser Melting or SLM, the metal AM process being developed by MTT Technologies, builds parts from a range of fine metal powders that are fully melted by a laser in a tightly controlled atmosphere. It uses an industry standard 3D CAD file in.stl format, from which the system's software creates a file that can be read by the machine in 2D layers. Once the file is loaded, the dense metal part is built layer by layer in thicknesses from 20-100microns.
Medical sector
SLM is already widely used commercially in the dental industry for the creation of patient specific crowns and bridging units. In the medical sector it has been used to build bespoke as well as non-patient specific medical implants, but the most exciting research is leading towards the creation of implants which incorporate a surface which encourages osseointegration. Currently these surfaces have to be post applied, but SLM has the imminent potential to produce implants with an integral surface into which bone will grow.
For the aerospace sector, 360mm rocket propulsion test parts are being produced as part of a development project, between MTT and a major US aerospace manufacturer, using an extended SLM250 machine. At the University of Liverpool the worlds largest SLM machine, the SLM500 with a 500mm (x,y,z) part building envelope is being developed to overcome the perceived productivity issues with the technology. The process is also being used to trial the production of other lightweight aircraft parts. While certification of the process for use in the industry is progressing, manufacturers are striving to speed up the process in order to address economic issues. MTT for example is working on the commercialization of higher power lasers in order to increase productivity rates.
Waste handling
The SLM process uses a range of gas atomised metallic powders. The first to be qualified were Stainless Steel (316L and 17-4 PH) and Cobalt Chrome. MTT then met additional challenges in the development of special powder and waste handling systems to enable the safe processing of reactive materials such as Titanium and Aluminium. Materials nearing the end of process qualification include Nickel alloys such as Inconel and Hastalloy X and Tool Steel (H13).
MTT uses a unique method to ensure the quality of its SLM build atmosphere - a crucial factor in any metals AM process if part quality and mechanical performance is to be optimised. First a vacuum is created to remove all the air from the build chamber and material hopper. Then the chamber is refilled with high purity Argon. During the build process the atmosphere is always maintained at below 1000ppm oxygen but normally runs much lower at around 50ppm or below. Gas consumption by MTT's equipment is very low compared to other AM systems, at around 30-50litres per hour for the SLM250 and SLM125.
Processing parameters
Development work being carried out at various universities on the design and production of heat exchangers demonstrates the elaborate geometries the SLM process is capable of building. The Welding Institute in Sheffield (TWI) was approached by Rolls Royce to develop a heat exchanger using SLM for an aerospace application. TWI experimented using different geometries and created highly efficient heat exchangers that have the potential to take up significantly less space whilst still maintaining performance. The parts were built in Titanium Al6 V4 using MTT's SLM250 system with TWI developing new processing parameters specifically for the application.
There are limitations to metal AM processes. Clearly they are not suited to building large areas of dense material. Processes such as SLM work best for producing small, complex geometries and structures such as heat exchangers and medical implants; hidden internal features such as conformal cooling channels and for the production of parts in noble materials and alloys which are difficult to machine and hazardous to cast.
Today metal AM processes are still very applications based. But I predict the future of the technology will be driven by designers themselves as they come to understand its advantages as well as its limitations, and start to design parts with AM in mind as the manufacturing methodology. These systems are the machine tools of the future and in terms of potential market place it's very much a global one. Wherever a machine tool is deployed today represents a potential opportunity for AM to deliver enhanced performance through increased design freedom.l
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Robin Weston is Group Marketing Manager, MTT Technologies Group, Stone, Staffordshire, UK. www.mtt-group.com
