Analysis of melt ejection during long pulsed laser drilling

In pulsed laser drilling, melt ejection greatly influences the keyhole shape and its quality as well, but its mechanism has not been well understood. In this paper, numerical simulation and experimental investigations based on 304 stainless steel and aluminum targets are performed to study the effects of material parameters on melt ejection. The numerical method is employed to predict the temperatures, velocity fields in the solid, liquid, and vapour front, and melt pool dynamics of targets as well. The experimental methods include the shadow-graphic technique, weight method, and optical microscope imaging, which are applied to real-time observations of melt ejection phenomena, measurements of collected melt and changes of target mass, observations of surface morphology and the cross-section of the keyhole, respectively. Numerical and experimental results show that the metallic material with high thermal diffusivity like aluminum is prone to have a thick liquid zone and a large quantity of melt ejection. Additionally, to the best of our knowledge, the liquid zone is used to illustrate the relations between melt ejection and material thermal diffusivity for the first time. The research result in this paper is useful for manufacturing optimization and quality control in laser-material interaction.

HOLE PROFILE AND TUNNELLING EFFECT IN LASER DRILLING

A theoretical model was developed to describe blind hole profiles in Nd :YAG laser dril- ling. In most cases, the drilled holes resemble a spindle-like shape as is predicted by the theorctical model. Derived from the model, an interesting and useful phenomenon: the so-called tunnelling ef- fect, by which cylindrical holes with minimal tapering together with a relatively small heat affected zone can be achieved. The mechanism and the drilling conditions to realize the tunnelling effect are analyzed and experimental evidences are provided.

Process in laser drilling of deep microholes without taper on metal materials

Deep microhole machining is currently a prominent research area within the aerospace field,encompassing blade film cooling and fuel injection control technologies.However,taper defects in metal materials may lead to performance degradation or even structural damage over a component’s lifetime.Trepanning and helical drilling,facilitated by ultrashort pulse lasers,have proven more suitable for achieving high-precision,deep holes in metal materials.Nonetheless,excessive repetition rates can also result in severe thermal damage.Various methods are commonly employed for controlling taper,including parameter optimization,assistance,and secondary modification.Tilted laser beam drilling is widely utilized and has been integrated into relevant machining systems for commercial applications.Typical deep microholes include film cooling holes and injection microholes.Laser drilling is a potential machining method for new materials in the aerospace field.Although laser drilling processing has been studied,numerous related scientific challenges and technical difficulties must be addressed before practical implementation.

A Comparison in Laser Precision Drilling of Stainless Steel 304 with Nanosecond and Picosecond Laser Pulses

Precision drilling with picosecond laser has been advocated to significantly improve the quality of micro-holes with reduced recast layer thickness and almost no heat affected zone.However,a detailed comparison between nanosecond and picosecond laser drilling techniques has rarely been reported in previous research.In the present study,a series of micro-holes are manufactured on stainless steel 304 using a nanosecond and a picosecond laser drilling system,respectively.The quality of the micro-holes,e.g.,recast layer,micro-crack,circularity,and conicity,etc,is evaluated by employing an optical microscope,an optical interferometer,and a scanning electron microscope.Additionally,the micro-structure of the samples between the edges of the micro-holes and the parent material is compared following etching treatment.The researching results show that a great amount of spattering material accumulated at the entrance ends of the nanosecond laser drilled micro-holes.The formation of a recast layer with a thickness of;5μm is detected on the side walls,associated with initiation of micro-cracks.Tapering phenomenon is also observed and the circularity of the micro-holes is rather poor.With regard to the micro-holes drilled by picosecond laser,the entrance ends,the exit ends,and the side walls are quite smooth without accumulation of spattering material,formation of recast layer and micro-cracks.The circularity of the micro-holes is fairly good without observation of tapering phenomenon.Furthermore,there is no obvious difference as for the micro-structure between the edges of the micro-holes and the parent material.This study proposes a picosecond laser helical drilling technique which can be used for effective manufacturing of high quality micro-holes.

Study on Laser Drilling in Rare Earth Magnets

A Nd/YAG pulse laser is used to drill Sm Co and Nd Fe B permanent magnetic rotor The experimental studies and analysis on the morphology and the phase composition of the drilled rotor are described In the centre of the drilled rotor, there is a hole whose diameter is roughly equal to the girdle size of the Gauss laser beam The hole wall is composed of rapidly fused amorphous matter, whose morphology and composition are obviously different from that before hole drilling The grain in heat affected zone has been fined, but the composition has not changed The experimental results show that the reasonable selection of laser parameters according to physical properties of the materials is necessary in order to improve product quality and production efficiency They also show that the lower laser power, the narrower pulse and the mulitiple drilling are advantageous to the processing for hard brittle materials like rare earth magnets