Commercial viability of laser-based 3D printing and its general acceptance as a valid production technique resides in how it can limit the need for post--surface or -heat treatments. Surface roughness, with its capability to affect air flows and aerodynamics is a critical factor in aerospace or medical applications. The efficient function of high value components, such as combustion chambers in turbine engines, relies on high quality and controllable surface finish.
Producing high quality components with low as-built top surface roughness (<2/3µm) will facilitate surface processing or avoid it altogether.
In powder bed fusion technology, such as selective laser melting (SLM), the finish quality of surfaces facing upwards (referred to as up-skin) relies on a simple laser-processing concept known as laser polishing.
| Yes, in SLM, you already know laser energy melts the powder which then re-solidifies to form the final solid product. Specific building laser parameters are required to achieve suitable core mechanical properties for the finished component. | Used over 3 or 4 upper layers, specific ‘polishing’ parameters tend to smoothen out the top surface roughness by a factor 3x to 4x. |
Laser polishing applies to the 3 or 4 outer, upper layers of the component and requires different parameters, and usually higher energy densities, to create a smooth(er) surface finish. The possibility to fine tune the laser parameters allocated to these outer layers relies entirely on the flexibility of the machine control software to conditionally assign parameters to individual layers. For instance, it becomes possible to ascribe each layer a different processing parameters depending on whether this layer is an outer layer, in a core area, facing upwards (top surface) or downwards (bottom surface) etc… This flexibility is very machine-dependent as each SLM machine supplier has its own proprietary machine controls and software commands.
SEM of CO2 laser polishing of 420 stainless steel–40 wt.% bronze indirect-SLS slab, 50x.
Generally, laser polishing of metals relies on either matter ablation or remelting and can be achieved with pulsed (nanosecond) or continuous wave laser sources. Laser (re)melting, unlike abrasive polishing techniques, is based on redistributing matter (to even it out) using surface tensions naturally arising in melt pools: a thin surface layer of the workpiece is treated and the subsequent smoothing of the surface roughness occurs.
Pulsed laser radiation with pulse durations of several 100ns is used for metallic surfaces with a small initial surface roughness, e.g. after grinding. In SLM, as the initial roughness is higher, continuous laser radiation fitted in SLM machines is perfectly suited. In this case, the remelting depth is >100μm for continuous laser radiation.
The melting effect was found also useful for sealing of micropores and cracks and removing of surface scratches.
Schematic of laser polishing by remelting a thin surface layer with continuous laser radiation.
Obviously if/when the parameters used to create optimum optimal density and mechanical properties at high build rate happen to give rise to a low surface roughness, you won’t need to bother with this. Realistically, chances are you will!
 Without built-in laser polishing parameters... |  With built-in laser polishing parameters... |
Unlike abrasive polishing techniques, laser polishing/remelting can only enhances the previous layer surface roughness as the melt is redistributed to fill micro-valleys and to smoothen micro-peaks. Used over 3 or 4 layers, specific ‘polishing/smoothening’ parameters tend to smoothen out the top surface roughness by a factor 3 to 4. A good rule of thumb is to use laser energy 2x or 3x higher than that used for the core component. We tend to keep same laser power and hatching distance and decrease the scanning speed by 4. Even if instabilities arise (most probably at the boundaries between power and remelted material), they won’t have time to build up and jam the recoater and ruin your build.
Give it a shot and let us know how it goes in the comments below!