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Additive manufacturing – Joule Printing

additive manufacturing patent© D Young & Co

Additive manufacturing techniques involve adhesive bonding followed by sintering, laser sintering and molten metal deposition. Sintering in adhesive bonding processes increases the complexity and time required to produce parts.

In laser sintering processes, avoiding unwanted sintering of particles adjacent to the area heated by the laser requires use of metals with poor heat conduction. Further, the preparation of metal powder is associated to high costs and any excess material may not be readily reusable.

In molten metal deposition techniques, the resolution achieved by spraying metal can be poor. Printing speeds are slow as underlying layers, heated to extremely high temperatures by plasma or electric arc, must cool before being built upon. Additional machining steps may further slow the overall process.

A novel additive manufacturing technique for the fabrication of metallic structures that does not use metal powders as the raw material, and does not generate excessive heat nor involve any timeconsuming and uneconomical sintering steps, is possible by a process recently patented in the UK by Digital Alloys Inc, as patent GB 2549653, titled Additive manufacturing of metallic structures.

This process relates to Digital Alloys’ Joule PrintingTM technology. It leverages knowledge from gas metals arc welding, resistive spot welding, and computer-aided manufacturing technologies. It involves the simultaneous positioning and instant melting of a metal wire by Joule heating – resistance heating – and enables the controlled deposition, at desired locations, of beads of metal fused together in a layer-by-layer build-up of fully dense 3D metallic structures.

The process comprises a mechanical gantry (105) capable of motion in one or more of five or six axes of control via one or more actuators (110). Using a computational representation of a 3D object, a wire feeder (115) – that continuously feeds metal wire (120) from a source (125) – positions a metal wire in contact with a desired printing (melting) location on a support – a metallic baseplate (130) controlled via an actuator (135) or a layer previously deposited over the support.

An electrical power supply (140) provides a current that flows through the metal wire and the support – and any layer previously deposited over the support – subjecting the metal wire to Joule heating. The heat generated is determined by the amount of current passed, the contact resistance and the duration of the application of current. Sufficient heat melts a portion of the wire enabling the controlled deposition of a molten droplet – or ‘particle’ – and for its fusion to adjacent particles. This process is repeated to form additional layers to build a 3D metallic structure.

Optionally, the gantry, the wire feeder, the mechanical actuators 110 and 135, the power level and the duration of the application of current, are controlled via a controller (145). A system memory (150) stores computational representations of the 3D structures, and a 3D rendering module (155) assigns properties to certain sections of the part based on deposition parameters determined using CAD software. A system bus (165) is used to couple various system components including the system memory to a processing unit (160).

With the contact resistance calculated dynamically, the current may be precisely controlled to ensure the exact amount of heat is applied to achieve the desired deposition parameters and/or particle characteristics. The deposition parameters may be varied to determine the degree of fusion between particles i.e. if heat is increased, fusion between particles will be greater and porosity will be lower. Porosity may be further controlled by altering the spacing between adjacent depositions.

Teachings of the disclosure further include a mechanical wheel or an optical system to track use of the wire, a gas flowed over at least the tip of the wire during deposition to prevent oxidation and/ or control the cooling rate of the particle, a sacrificial raft structure printed on the baseplate before the part to facilitate its removal after printing, and selecting wires of different diameters and/or retracting the wire or feeding additional wire during deposition, while the particle is still molten, to control its morphology.

Read the full patent here

This article first appeared in the June 2019 issue of Materials World, the member magazine of the Institute of Materials, Minerals and Mining.