Influence of the beam oscillation pattern and oscillation frequency on the temperature field in laser brazing with keyho
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ORIGINAL ARTICLE
Influence of the beam oscillation pattern and oscillation frequency on the temperature field in laser brazing with keyhole formation I. Henze 1
&
P. Woizeschke 1
Received: 30 April 2020 / Accepted: 16 September 2020 / Published online: 4 October 2020 # The Author(s) 2020
Abstract Laser keyhole brazing is an opportunity to increase the process efficiency in laser brazing processes. Using small spot sizes increases the intensity and leads to the formation of a vapour capillary (keyhole) in the brazing material when a material-specific threshold value is exceeded. Due to multiple reflections/absorptions of the laser beam in the keyhole, the process efficiency increases in comparison with conventional brazing processes with single Fresnel absorption on the surface, especially when using high-reflectivity braze materials, such as aluminium-based or copper-based alloys. The energy must be distributed adequately by applying beam oscillation transversal to the brazing direction. In laser brazing processes, the temperature field in the interface between brazing and substrate material is a major factor. To analyse the effect of beam oscillation, it is assumed in this study that the temperature distribution at the surface of the melt pool is a suitable approximation for the temperature distribution at the interface to the substrate. Two key parameters are defined to quantify the temperature field referring to the homogeneity: the temporal local temperature-time curve and the temperature distribution transverse to the brazing direction. While the oscillation frequency influences the first mentioned parameter by decreasing the time interval between the local laser passes, the oscillation pattern affects the second parameter by adjusting the local actual beam velocity and its consistency. Keywords Joining . Laser brazing . Beam oscillation . Temperature measurement
1 Introduction Laser brazing offers the ability to join different materials with good visual quality. The joint is achieved by the wetting of a preheated substrate material with a molten brazing material. Conventional laser brazing processes use large spot sizes of several millimetres to realise the process, based on a single Fresnel absorption of the laser beam [1]. Frequently used brazing materials are aluminium and copper. These materials are highly reflective, leading to a low absorption of the laser beam and low process efficiencies [2]. A concept derived from laser welding offers the opportunity to increase the absorption, whereby decreasing the laser spot leads to an intensity that is high enough for the local evaporation of the metal (threshold intensity). A vapour
* I. Henze [email protected] 1
BIAS—Bremer Institut für angewandte Strahltechnik GmbH, Klagenfurter Str. 5, 28359 Bremen, Germany
capillary called a keyhole is formed, in which the laser beam is absorbed multiple times, thereby increasing the efficiency [3]. The energy input into the material is done localised in the keyhole and distributed by heat conduction and convection [4].
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