Model-based Quality Optimisation for Laser Cutting under Transient Conditions

Quality improvements in laser cutting of mild steel have been achieved by a newly developed model-based optimisation strategy. The specific aims of such efforts are to assure quality of cut right up to pre-cut sections and boundaries, which are inherent in intricate part programs. Such boundaries frustrate heat transfer and result in bulk heating of the workpiece. This in turn leads to a degradation of the cut quality. In order to focus on the .development of such optimisation, constant thermo-physical properties and two-dimensional transient heat conduction is simply assumed. The model is therefore valid for the thin plate laser cutting case, where temperatures are approximately homogeneous with workpiece depth. Close inspection of the laserworkpiece interaction zone reveals that the cutting front exhibits dynamic behaviour, and such mobility plays a significant role in temperature determination. Non-linear parameter adaption profiles are generated via the optimisation strategy in order to stabilise cutting front temperatures. Currently, extensive trial-and-error based experimentation is needed in order to improve cut quality in such regions. Thus modelbased optimisation has the added benefit of reducing this time-exhaustive step whilst leading to an optimal solution. Experimental results are presented and it is demonstrated that such process manipulation can lead to significant quality improvements. NOMENCLATURE amu b Bi Cv D H hf hn atomic mass units kerf width mesh-size biot number heat capacity workpiece thickness net energy input forced convective heat transfer coefficient natural or free convective heat transfer coefficient 29 ~ ~ & ~ '" :;; .... ~ .. '" ? ti " il ~ I • ~~ ~~ ;t t1 ii ii 11 >1 ~ ~~. ;;c ' _,