What actually really happens in a Laser Powder Bed Fusion process?
Updated: Jun 9
I remember the first time I came in contact with the L-PBF process was during my master's studies at the university. It took me a while to understand that the laser selectively remelts powder based on a contour that we had previously defined in the form of a 3D CAD model. Further jobs were initially not that interesting, as I mostly stood at the post-processing table and removed the support structures.
Preparation of first test series
After showing perseverance in support removal and continuing to enjoy the technology, I was able to plan my first series of tests. A few cubes placed over the build plate should not be a problem for now. However, I was taught better and had to find out that densities below 95% can also be produced in the L-PBF without any problems. The results of the polish preparation and microstructure analysis were correspondingly disappointing.
From a "filter structure" to the first success
After a brief consultation with my mentor at that time, the next steps were obvious: I should take another detailed look at the process, check the position of the test specimens on the build plate, and further vary the energy parameters. After that, I started to look at the process in depth again.
Laser on - Welding - Welding spatters - Welding vapor - Laser off
It became more and more clear to me that the welding spatter is a crucial indicator of the stability of the molten pool. The height and intensity are key factors here.
Low height of the welding spatter and a wobbling molten pool:
Are indicators of insufficient energy density. The weld pool is unstable and the spatters land directly on the previously exposed surface, which also reduces the density (Lack of fusion) and results in lower mechanical properties. An energy density that is too low can also lead to process failure, since elevations on the exposed surface due to spatters mean that homogeneous coating application can no longer be guaranteed.
Stable weld pool with high weld spatter:
This is exactly what we want to achieve during process development. Through homogeneous melting, we achieve high densities with reproducible mechanical properties. The welding spatters are either directly extracted by the gas flow or land outside the previously exposed structure in the powder bed. This allows us to achieve a homogeneous deposit after exposure, which has a positive influence on our process stability.
The welding vapor can also be used as an indicator of the stability of the process. If it has a uniform profile and is continuously extracted by the process gas flow into the outlet nozzle, we can speak of a homogeneous shielding gas flow. Especially with multilaser machines (4-12 lasers), the vapor plays another crucial role, since we want to prevent one laser (x) starts exposing in the welding vapor of the other laser (y).
As you can see, L-PBF is not just "plug and print". The process is the key factor to be able to scale up later and to set up a larger plant for production (up to series production) of additive components. If you understand the process, you can also influence the quality of the components in a targeted manner.
I am looking forward to your opinion,