Process parameter dependencies of continuous and pulsed laser modes on surface polishing of additive manufactured aluminium AlSi10Mg parts

Abstract

Laser powder bed fusion is a well‐established 3D printing technique for metal alloys, but exhibits a poor surface quality. Laser polishing provides the possibility of a fast contact‐free and fully‐automatable surface treatment. This paper deals with the experimental investigation of laser polishing of laser powder bed fusion parts made of aluminium AlSi10Mg. Laser polishing is done with a 4 kW solid state disc laser in combination with a multi‐axis system and a one dimensional scanner optic. The laser is operated at continuous and pulsed operation mode. The parameter study reveals a high dependency of the achievable roughness on the laser beam intensity, the track and pulse overlap, the energy density and the number of polishing passes and polishing directions. Pulsed laser polishing mode with up to four passes from different directions revealed the lowest surface roughness of 0.14 μm Ra. With respect to the initial average surface roughness of Ra = 8.03 μm a reduction of the surface roughness of greater than 98 % could be achieved. Polishing with continuous laser radiation at one polishing pass resulted in Ra = 0.23 μm at an area rate of 20 cm2/min. Laser polishing using four passes achieved a further improvement up to Ra = 0.14 μm.

Laser Polishing of Additive Manufactured Aluminium Parts by Modulated Laser Power

“…Polishing with continuous laser radiation at a wavelength of 1.03 µm was found to be best with a laser beam intensity of 1057 W/mm 2 and an energy density of 42 J/mm 2 . Thereby a roughness improvement of Ra = 0.23 µm with single polishing, and 0.14 µm with four times polishing at an area rate of 5 cm 2 /min is reached [45]. Polishing with a CO2 laser (10.6 µm) the highest roughness improvement of 85% from Sa = 22.3 µm to 7.9 µm with the highest tested energy density of 330 J/mm 2 is reached [46].…”

“…The laser power of the modulated laser emission was fixed to 1700 W, which was found to be suitable for pulsed mode laser polishing on additive manufactured L-PBF aluminium surfaces [45]. The resulting peak power, average laser power and constant minimum laser power of the modulated laser emission are adapted, by adjusting the modulation time, see Figures 7 and 8.…”

“…A latency time of 30 s between polishing of two fields was considered to prevent a significant rise in temperature of the aluminium substrate. The investigations with modulated laser power emission are executed with a defocused laser beam diameter d s of 1298 µm (focal position 12 mm), which was measured by a focus monitor system FM+ from Primes and a constant peak to peak overlap PPOav and track overlap TO, which were found to be suitable for pulsed mode laser polishing at additive manufactured AlSi10Mg [45]. Detailed constant process parameters can be taken from Table 4.…”

“…To prevent this, the pendulum axis is sectioned into 15 segments with adjustable laser power. Therefore, a progressive reduction of the constant pump module laser power P L of 1700 W in two steps and of 200 W at both turning points is defined, which was found to be suitable for laser polishing with pulsed and continuous laser radiation in [45], see Figure 9.…”

“…Laser polishing of L-PBF-Aluminium AlSi10Mg parts with pulsed and continuous laser radiation can reach high roughness reduction rates up to 98% Ra [44][45][46][47][48]. At a pulse duration of 0.3 ms, a beam intensity of 1285 W/mm 2 and an energy density of 76.5 J/mm 2 the highest roughness improvement from Ra = 7.9 µm to Ra = 0.66 µm on vertical built samples is reached [45]. Multiple laser polishing with up to four scanning passes from different direction offers a further reduction to Ra = 0.14 µm at an area rate of 1 cm 2 /min.…”