Under high-volume melting, rapid processing conditions, spatter formation seems to be part and parcel of powder-bed laser-based fusion technology. Three types of spatter particles have been identified:
How common is it
How important is it?
Why should I care?
What are the physical mechanisms responsible
A. Recoil-pressure driven spatter
B. Jet-stream induced spatter or entrainment-driven spatter
Although the physics involved in jet-stream spatter is somewhat specific to PBFAM technology, the effect is akin to a cyclone developing above the ocean where water heats up and vapor-saturated hot air rises up forming an upward vertical jet stream, causing a low pressure region in the evaporation zone that pulls in the surrounding environment and transfer it to the rising heated air toward the cooler atmosphere.
In the PBFAM, the jet size is comparable to the particle size, which allows us to observe entrainment of single particles. Under certain processing conditions, the metal vapor flux from the melt pool induces an inward gas flow that pulls the particle along the vapor flow direction .
- pulled into the melt pool and become subsumed,
- travel toward the vapor jet but miss the laser beam and eject as cold particles
- or travel toward the vapor jet, intersect the laser beam leading to rapid heating and ultimate ejection as hot, bright particles that are observed on the macroscopic scale as molten spatter or sparks.
- when it occurs between two solid particles, collision generates a large momentum transfer that changes the course of the particles involved.
- when it occurs between two hot droplets, the molten material merges to form a large droplet that cools down into a large particle redeposited onto the powder bed and that can create lack-of-fusion defects .
The entrainment velocity is also inversely proportional to particle density. For instance, dense materials spatter such as stainless steel has lower speed than less dense materials such as titanium.
Entrained particles typically have sizes in the range of powder bed particle sizes, however, there are cases where the particles are larger.
What’s the dominant regime?
There are currently 2 identified spatter formation mechanisms: a) recoil pressure and b) jet stream formation.
The entrainment process leads to both local denudation and the distribution of molten droplets across the powder bed, each potentially sources of defects in PBFAM.
There are 3 types of spatter particles: i) cold particles, ii) hot particles and iii) resolidified particles.
The observed ratio of these different spatter particles is dependent of laser processing parameters and powder characteristics.
Spatter ejection trajectory is dependent of laser processing parameters, powder flowability, material density as well as inert gas flow direction and amplitude across the bed.
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 Sonny Ly, Alexander M. Rubenchik, Saad A. Khairallah, Gabe Guss & Manyalibo J. Matthews, Scientific RepoRts | 7: 4085 | DOI:10.1038/s41598-017-04237-z