Understanding the self-limiting effect in picosecond laser single and multiple parallel pass drilling/machining of CFRP composite and mild steel

• The paper provides the first theoretical explanation of the self-limiting effect in high power picosecond laser machining.
• The paper unveils a new machining mechanism involved in parallel passes picosecond laser machining of composites and metals.

The availability of ultra-short laser pulse sources such as picosecond lasers makes it possible to improve the quality (e.g. heat affected zone < 10 μm) in laser machining engineering materials such as metals and carbon fibre reinforced polymer (CFRP) composites. However, the processing rate and the machining depths of such lasers are very low and thus demands better understanding of process limitations and machining strategy. This paper identifies a self-limiting effect in high power picosecond laser machining and compares single and multiple parallel pass strategies during drilling/machining of CFRP composite and mild steel sheets. Electromagnetic (EM) simulation was performed to understand the taper formations and the self-limiting effects in ps-pulsed laser drilling and to determine the maximum cutting/drilling depths. The work shows that the machining depth and material removal rate were significantly improved by increasing the number of parallel passes under the same energy delivery and machining time. A new machining mechanism is proposed. This applies to both the CFRP and the mild steel investigated.

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