FEATURED ARTICLE 3D PRINTING BY CARL HAUSER Introduction
An additive manufacturing method developed by TWI within the framework of an EU-funded project could drastically reduce component manufacturing times.
TWI engineers have been using laser metal deposition (LMD) to produce net shape thin-walled engine casings, aiming to reduce the environmental impact of civil aerospace manufacturing.
In LMD, a weld track is formed using metal powder as a filler material which is fed, through a coaxial nozzle, to a melt pool created by a focused high-power laser beam. An inert gas carrier transports and focuses the powder into a small area in the vicinity of the laser beam focus (powder-gas beam focus). By traversing both the nozzle and laser, a new material layer develops with good precision and user-defined properties. The application of multi-layering techniques allows 3D structures to be created directly from a CAD model without the need of additional tooling. Historically, coatings and 3D objects deposited by LMD tend to be considered as near net shape.
The focus of the study was an axis-symmetric cylindrical casing with a maximum diameter of 300 mm, a wall thickness of 0.8 mm and a height of 88 mm (see Figure 1). The component is traditionally manufactured from a nickel alloy (Inconel 718), forming a complex geometrical topography requiring specialist tooling, all of which absorbs significant resource (six months lead time) and generates a large amount of waste material when manufactured.
From Design to Manufacture Two years of development and six months of demonstration
activity, led by the team at TWI’s Technology Centre in South Yorkshire, concluded the validation of CAM-style software tools created as a plug in to TWI’s ToolCLAD software: a software package being developed at TWI specifically for the LMD CAD- to-part-manufacturing process. The plug in maps a five-axis vector toolpath with deposition parameters to guide a three-axis coaxial LMD nozzle across a moving substrate manipulated by a two-axis CNC rotary table, creating a novel method of LMD manufacturing.
With precise synchronisation of the movements of rotation and tilt of the substrate with incremental movements of the coaxial nozzle (predominantly in the +Z direction), a
continuous
spiralling weld track can be deposited or ‘grown,’ layer on layer, from out of the substrate. The helical multi-layering technique allows a thin-walled 3D contour to form, which accurately follows the changing directions of the original CAD surface profile (STL file). The process is analogous to a clay pot forming on a potter’s wheel. By allowing the substrate to control movement, rather than traversing the nozzle around a circular path, gives a consistent and regular weld track, and therefore, a good surface finish. Furthermore, the tipping of the substrate to axially align the orientation of the growing wall with the cladding nozzle allows overhanging features to be created without the need to build additional support structures.
A key innovation was the development and use of an adaptive slicing algorithm which automatically varies the numerical slice height (lead distance or pitch) between each helical revolution of the calculated tool path. The magnitude of the change is governed by the orientation of the facet (triangle) normal at the required slice height within the STL CAD model. However,
Of Net Shape Geometries by Laser Metal Deposition
Figure 1. 6 LIATODAY FOCUS: INDUSTRIAL AM JULY/AUGUST 2016
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