In ‘Dynamics of pore formation during laser powder bed fusion additive manufacturing,’ US researchers continue to improve on 3D printing, exploring how to prevent pores from forming during laser powder bed fusion. As LPBF continues to become more popular in metal additive manufacturing processes, users seek better quality and less headaches in production, inspiring the research team to improve geometric quality of melt tracks and production overall.
As is the case with many different types of 3D printing, LPBF is a powerful technology still left highly unexplored within industry due to trepidation about quality in parts, and especially integrity of mechanical properties. Unpredictability in both thermal history and material solidification have given way to doubts and worry over potential defects and resulting instability.
Keyhole pores have been a common problem, caused by superfluous energy in the melt pool. The pores degrade mechanical properties and can have a negative impact on parts created during the LPBF process. Temperature issues were a major focus in the study.
“To improve the confidence in components built by LPBF, a greater understanding of laser–metal interaction in this extreme thermal regime and its correlation with defect generation during the LPBF process is required,” state the researchers.
The team took X-rays to examine the printing process further, attempting to get a front-row seat look at pore formation. Tests with a titanium alloy showed pores forming at laser turn points, allowing the team to begin formulating a solution to reduce defects in parts, and increase the credibility of LPBF as a technology, with X-ray imaging serving as an effective new way to explore issues during LPBF.
With the turn point being a major focus, the researchers noted that it increases due to laser power, regardless of steady-state scan speed. They also discovered that pores always form within 200 µm of the turn point. Pores closes to the turn point were also the deepest.
“Inspection of an X-ray image time series captured at each respective processing condition reveals that pores form very quickly on time scales comparable to the sampling rate of our measurement (50 µs).”
In exploring depression depth further, the researchers found that the highest amount of vapor depression post-turn occurred due to heat buildup. This was a result of the ‘long dwell time’ of the near-stationary laser.
“When the depression exceeds a depth on the order of 100 µm the deep keyhole regime is entered and a dramatic increase in the absorption of the laser power is realized due to multiple interactions between the melt pool and reflected laser,” stated the researchers.
When surface temperatures are lower, the melt pool tension increases—causing complete collapse of the depression, with pores trapped when the material solidifies rapidly. When the laser scan is maximized, pores are created with the vapor depression transitioning into a deep keyhole regime. As the walls collapse rapidly, pores are formed. The researchers raise the question of turning off the laser at the turn point, but they decided it was not viable due to previous studies where such action ended in pore formation.
The researcher’s pore mitigation strategy was used to stop pores from forming at the turn point by ‘removing the rapid variation in depression depth inherent in the unmitigated case.’ This also refined the geometric tolerance of the tracks by eliminating problems with overhearing.
“Conceptually similar strategies should be applicable to any abrupt laser on/off points during LPBF. The successful mitigation strategy presented here illustrates the potential of in situ X-ray measurements coupled with high fidelity modeling for driving process improvements and paves the way to increasing the quality of LPBF-built components,” concluded the researchers.
This is just one of many recent studies in improving metal 3D printing processes, from finding ways to make additive manufacturing more affordable to using high entropy alloys, and even re-use powders. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.[Source / Images: Dynamics of pore formation during laser powder bed fusion additive manufacturing]
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