Laser polishing of ground aluminum surfaces with high energy continuous wave laser

Abstract

Conventional polishing of components is a time- and labor-intensive process. Moreover, manual polishing mostly depends on the processor's abilities. In accordance with these and additional difficulties, there is a strong need for automatable and efficient variants. For this purpose, polishing by means of laser beam, which enables a contact free and automatable processing of surfaces, provides a promising alternative. This paper investigates the laser polishing of the aluminum alloy AlSi9MnMg. Aluminum poses a particular challenge for laser polishing, due to high level of reflection, high thermal conductivity, and high coefficient of thermal expansion. For the investigations, a disk laser with a maximum power of 4000 W was used. The processing took place with pulsed wave laser beam at a pulse frequency of 1000 Hz and a minimal pulse duration of 0.3 ms as well as a continuous wave laser beam. The beam guidance was done by 1D-scanner optics in a highly pure inert gas atmosphere. The samples were first generated by vacural pressure and gravity die casting and afterward (belt) ground with a grain size of mesh 180 resulting in a roughness of the initial surfaces from Ra = 1 μm to Ra = 4 μm. A cleaning of the surfaces by means of short pulsed wave laser beam with ns pulse duration followed. The samples were analyzed quantitatively by perthometer for determining surface characteristics Ra, Rz, and Rt according to EN ISO 4288 as well as qualitatively using a white light interferometer and a microscope. First of all, the influence of the manufacturing process of the components on the laser polishing was investigated. A dependence is detected on laser polishing with pulsed and continuous wave laser beam, resulting in more favorable pressure die casted samples compared with gravity die casted samples. Both laser processing variants lead to a reduction of roughness of the initial surfaces in the range of Ra = 2.17 μm to Ra = 2.34 μm to a minimum roughness reached by laser polishing with pulsed wave laser beams of Ra = 0.19 μm and with continuous wave to a minimum of Ra = 0.15 μm. The achieved area rates at a continuous wave mode ranged between 20 and 60 cm2/min, whereas pulsed wave laser beams achieved only 5.5 cm2/min due to hardware-specific limits of the used system.