Plasma evolution in laser powder bed fusion using a double pulse format: Time-resolved measurements and physics-based modeling

Abstract

A novel laser powder bed fusion (LPBF) process utilizing a special double-pulse format was previously proposed. In this process, typically two different types of laser pulses are fired alternatively in time: the low-intensity “sintering laser pulses” intended to melt and coalesce particles, followed by the high-intensity “pressing laser pulse(s)” intended to induce plasma to generate high pressure onto the powder bed to suppress balling and enhance the densification of the sintered material. Laser-induced plasma plays a key role in the novel double-pulse LPBF process, but has not been studied sufficiently. Some critical questions remain to be better answered, such as: what is the minimum plasma-induced pressure needed to effectively suppress balling and how do the “sintering pulses” influence the plasma evolution? This paper reports a model-experiment integrative study of the plasma in DP-LMS, which is seldom reported in a paper in literature to the authors’ knowledge. The plasma evolution is observed in-situ with a high temporal resolution using an intensified CCD (ICCD) camera. The optical emission spectrum (OES) of the plasma is measured and the plasma temperature is deduced from the OES. A physics-based model for the plasma is developed by combining multiple modules. Under the investigated conditions, the model predictions show acceptable agreements with the measured plasma temperature and top front propagation. Utilizing the model, it has been found that under the conditions studied, the minimum plasma-induced pressure required to effectively suppress balling is approximately the pressure making the Weber number exceed ∼1 for the approximately analogized process of a droplet impacting a solid surface. The effect of the “sintering pulses” also influences the evolution of the plasma and obviously increase the plasma-induced pressure on the powder bed surface.