Technical Research Of Picosecond Laser Micro-drilling On Metals

Laser drilling has been extensively used due to high efficiency, non-contactmachining, local machining, flexibility, etc. Compared with nanosecond laser (10-9s),picosecond laser (10-12s) has small heat effect and it can acquire excellent machiningquality comparable with femtosecond laser (10-15s) as well as simple structure andgood reliable and stable performance. Picosecond laser has become research spotlightin the field of micro-machining.This paper carries out picosecond micro-drilling by self-developed all-solid kHzlarge pulse energy picosecond laser. The experimental materials are metals especiallytitanium alloy which is mostly used in aviation. The main contents include thefollowing:(1) Make an introduction of the properties of laser beam， and analyze themechanism of interaction between laser and metals as well as picosecond pulseablation mechanism based on one dimension double-temperature equation.(2) The influence of polarization on the hole formation is investigated. Two metals ofdifferent properties---stainless steel and copper are chosen to conduct the experiment,however, phenomena are different from reported before. Polarization state has greatinfluence on exit shape of stainless steel, while has no influence on the copper. Thisdissimilarity is explained in detail. The major factor that determines the influence ofpolarization on the hole roundness is discussed.(3) Heat effect of percussion drilling and helical drilling on1mm thick titanium alloyis studied. The thickness of recast layer and heat affected zone (HAZ) are observedand measured. The average thickness of recast layer is20μm by percussion drilling,while by helical drilling, the average thickness of recast layer is10μm. In addition,different cutting speeds are studied to reduce the recast layer. Compared with lowcutting speed, the inner wall processed by high machining speed is quite smooth. In acertain range, fast machining speed could basically eliminate recast layer. What’smore, whether by percussion drilling or helical drilling, the organization around thehole is in accordance with the parent material, this illustrates that no HAZ is generatedon when machined with laser pulse of15ps.(4) Process tests by percussion drilling, trepanning and helical drilling are conducted.Morphology, taper and roundness of the hole by different processing methods are watched and measured. A hole with aspect ratio up to13:1is acquired by percussiondrilling, pinhole arrays of300μm are achieved by trepanning, and a small taper of0.34ois obtained by helical drilling. In addition, surface roughness of different drillingmethods is measured, and the relationship between surface roughness and recast layeris analyzed. When thick recast layer exists, the roughness is2.08μm, when the recastlayer is thin, the roughness is0.97μm, and when there is no recast layer, the inner wallis smooth and the roughness is nearly0.39μm. Analyze different advantages andcharacters of each method as well as optimal process parameters, providing reliablebasis for micro-drilling.