Nanosecond Yb fibre laser milling of aluminium: effect of process parameters on the achievable surface finish and machining efficiency

Abstract

Pulsed ytterbium-doped fibre lasers based on a master oscillator power amplifier (MOPA) architecture possess attractive characteristics over their Q-switched diode-pumped solid-state counterparts. These include a relatively low cost of ownership and a flexible operating window with respect to the pulse duration, shape and repetition rate. For micro machining applications, given this inherent large processing window available with respect to the pulse characteristics, the effect of process parameters on particular machining outcomes needs to be investigated systematically. In this context, this paper considers the effect of a number of factors on the achievable material removal rate and surface roughness when processing aluminium with such fibre laser system operating at 1,064-nm wavelength and having built-in capabilities for selecting the duration of the delivered pulses in the nanosecond regime. The process parameters under investigation in this study were the pulse duration and repetition frequency, the pulse overlap, the scanning strategy and the distance between linear machined tracks. Initial results showed that, for pulse durations comprised between 25 and 140 ns, the specific frequency at which both the highest energy and average power are delivered leads to the maximum material removal rate (MRR) achievable and to high values of surface roughness. Following this, a design of experiments was conducted for a given pulse length with the aim of identifying an optimum combination of parameters with respect to the attained surface finish while operating at the frequency resulting in the highest MRR. This optimisation study resulted in a 60-% decrease in the achieved surface roughness and also showed that the distance between machined tracks had the highest influence on the surface finish.