Additive manufacturing - a design-driven manufacturing process
The last decade has seen drastic improvements into the capabilities of the freeform fabrication machines used for additive manufacturing (AM) with particular emphasis in the mechanical properties and density of AM produced metallic components, enabling AM to become an established manufacturing methodology.
Additive Manufacturing (AM) is a relatively new concept in which materials are bonded together to create three-dimensional components with the aid of Computer Aided Design (CAD) data. Components are completed in a layer-wise fashion, in which successive cross-sections are combined to build models from the base upwards. Comparatively, traditional machining processes are termed ‘Subtractive Manufacturing’ (SM), as material is removed from the bulk material in order to create a component.
AM is largely synonymous with rapid prototyping or rapid manufacturing, which was initially created to aid in prototype design. With the aid of AM, prototypes of components can be built or changed relatively quickly and without the need for additional machines or machining processes. The biomedical industry has invested heavily in AM technologies with much research being undertaken into titanium alloy implants created via AM. Electron Beam Melting (EBM) and the Selective Laser Sintering (SLS) technologies have emerged as major methodologies for the additive manufacturing of metallic components.
Electron Beam Melting (EBM) is a highly competitive AM technique for use with metallic powders. In this technology, a high powered electron beam generates the energy needed to fuse metallic powders. The electron beam is controlled using a series of electromagnetic coils and is directed onto a build table. The kinetic energy of the electrons is transformed into heat energy upon impact with the powder, melting the powder locally around the beam spot. A computer package controls the movement of the directional coil to create cross-sections according to CAD data. A powder rake sweeps a fresh powder layer over the previously built layer and the process continues. Selective Laser Sintering (SLS) is the term given to several techniques which can be differentiated from one another by the degree of melting which occurs at the laser focal point. SLS uses a high energy laser to bind small particles of plastic, metal, or glass in successive cross-sections. The substrate in SLS is in a powder form similar to that of the EBM process. As each layer is traced out using the high energy laser, the build platform lowers and a fresh layer of powder is spread across the work piece. The process is characterized by short laser-powder interaction times resulting in the development of steep thermal gradients, rapid solidification, and fast cooling. Powders employed are typically 200microns in size and, in high quality applications typically less than 50 to 100 microns, to facilitate the firm bonding to the melted layer and thus avoid laminations or poor bonding. Typical applications are in the medical prosthesis field and aerospace industries. The size constraints of machines generally preclude the use of AM techniques is for the manufacture of large components.
The last decade has seen drastic improvements into the capabilities of the freeform fabrication machines used for AM manufacturing, with particular emphasis in the mechanical properties and density of AM produced metallic components, rapidly enabling AM to become an established manufacturing methodology. However, there are still issues with the porosity of materials, residual stresses which cause distortion of the components and material anisotropy. Such anisotropy results in mechanical properties (and especially toughness) varying in the planes perpendicular to and in the build direction. This anisotropy is compounded by residual stresses that are introduced by thermal gradients developed during cooling of regions in the newly deposited material. Although AM technologies are without doubt extremely progressive, these issues currently still limit the performance of components in comparison to similar parts made using conventional machining and forging techniques where the material properties can be controlled more easily and beneficial flow patterns can be introduced.
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Published in Technical Tips by Origen Engineering Solutions on 1 December 2018