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Post-processing operations

Post-processing of DMLS parts are required to impart the targeted material, mechanical and thermal properties, improved surface integrity and dimensional tolerance.

Several post-processing steps are carried out on DMLS built components, as detailed below:

DMLS parts can be brushed, tapped, shaken, vibrated or vacuumed to remove loose powder from the build platform. This can then be sieved and returned to the machine for the next build.

Blasting or peening can be performed on a single part or a whole platform of parts. The blasting media depends on the material being built.
Blasting abrasives can be used to clean the parts and remove and loose powder particles attached to the surface.

Often a secondary blasting unit is used with a finer grade of abrasive or shot peening to further smooth the surfaces.

Heat treatment is performed to reduce the internal residual stresses built up during DMLS.

This is usually done before the parts are removed from the platform, as the support structures can hold the parts in position while they are attached to the platform, then during part/support removal stresses can cause the parts to distort during wire cutting.

The heat treatment procedure helps to reduce the anisotropy (different material behaviour depending on the direction) due to the layer by layer fabrication.

Finally, the heat treatment is necessary to obtain sufficient material properties, mechanical or thermal properties.

DMLS supports are usually hollow or thin structures, built with teeth attaching the support to the part.

Removal consists of two phases, i.e. removing the supports and parts from the platform and removing the part from the supports.

This can be done using several methods such as EDM wire cutting, manual removal, band sawing or machining.

DMLS parts are generally very dense but not 100%.

The pores found in AM parts tend to be smaller than those found in cast parts, and may be smaller than one layer thickness.

Internal porosity can be decreased or removed by using a Hot Isostatic Pressing (HIP) process after the parts have been built and removed from the platform.

This process also reduces the grain size and enhances the material ductility.

Any DMLS parts can be machined to achieve the targeted surface quality.

Typical machining processes include milling (in recessed areas at different grades) and grinding (on flat or external surfaces only but gives a finer finish than milling).

Sometimes a standard blasted or shot peened finish is required following machining.

If further finishing or polishing is required, this can be either done automatically (e.g. disc finishing, drag finishing, stream finishing, vibratory finishing, barrel finishing, pulse finishing, electropolishing, laser polishing etc) or manually (e.g. hand polishing using polishing mops, wheels, hand held tools etc.).

Maestro use several post processing operations

As the AM fabricated parts typically suffer from several major drawbacks such as high surface roughness (Ra=~5-7 μm), stair-step effects on surfaces, balling, adverse residual stresses and poor dimensional accuracy, the objective of MAESTRO is to use several post-processing operations in order to realise the required functional characteristics of the component.

Although conventional milling remains one of the prominent post-processing operations to add micro/meso-scale feature, the process is time consuming and also suffer from the problem of the breakage of micro-tools.

DMLS part: half base material and half machined.
SEM micrograph showing a laser polished SS316L surface against the parent surface
SS316L base material
Laser polished SS316L

On the contrary, one of the viable technologies to carry out post-processing at a much faster speed is laser polishing. The technology has been continuously developed for over 20 years and has been shown to be capable in improving the surface morphologies on certain materials such as diamond, glass and metallics including Ti-6Al-4V, steel and aluminium.

The proposed laser polishing operation in MAESTRO is capable of rendering the surface roughness of the DMLS parts (stainless steels, aluminium, titanium alloys etc.) below 0.5 μm. By optimising the laser operating parameters and using an inert gas environment it is possible to achieve an oxidation free surface with an improvement in the roughness value by ~90%.

3D lattice structure with 150 µm strut size as fabricated from aluminium using DMLS
3D lattice structure with 150 µm strut size after fully transformed to alumina

DMLS and post-processing heat treatment

Another post-processing operation in MAESTRO will involve the development and fabrication of complex ceramic structures with high resolution (<100 µm).

Some attempts were made to produce ceramic structures with the aid of a binder. However, the complexity of the components produced using this approach is very limited.

Powder metallurgy and 3D printing have also been used to produce ceramic components.
The resolution resulting from these manufacturing approaches is typically over 300 µm and the corresponding strength and surface finish are usually poor.

In MAESTRO, novel complex ceramic structures with high resolution, high structure integrity and high surface finish will be produced using DMLS and post-processing heat treatment.

An aluminium preform with engineered properties will be fabricated using DMLS, which then will be transformed to alumina using a novel heat treatment cycle.

The temperature, time and oxygen level will be controlled to allow the aluminum preform to fully transform to Al2O3 without compromising the surface quality and structure integrity.