Experimental and Numerical Investigation of an Overheated Aluminum Droplet Wetting a Zinc-Coated Steel Surface

Marius Gatzen,Tim Radel,Claus Thomy,Peer Woizeschke,Frank Vollertsen

doi:10.3390/met7120535

Marius Gatzen, Tim Radel + Show 3 more

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https://doi.org/10.3390/met7120535

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Abstract

Wetting steel surfaces with liquid aluminum without the use of flux can be enabled by the presence of a zinc-coating. The mechanisms behind this effect are not yet fully understood. Research results on single aluminum droplets falling on commercial galvanized steel substrates revealed the good wetting capability of zinc coatings independently from the coating type. The final wetting angle and length are apparently linked to the time where zinc is liquefied during its contact with the overheated aluminum melt. This led to the assumption that the interaction is basically a fluid dynamic effect of liquid aluminum getting locally alloyed by zinc. A numerical model was developed to describe the transient behavior of droplet movement and mixing with the liquefied zinc layer to understand the spreading dynamics. The simulations reveal a displacement of the molten zinc after the impact of the droplet, which ultimately leads to an accumulation of zinc in the outer weld toe after solidification. The simulation approach neglects the effect of evaporating zinc, resulting in a slight overestimation of the final droplet width. However, in terms of spreading initiation during the first milliseconds, the simulation is in good correlation with experimental observations and demonstrates the reason for the good wetting in the presence of zinc coatings.

Highlights

Understanding the wetting behavior of liquefied metal on solid surfaces of other metallic substrates is essential; for example, for laser brazing processes as well as for the thermal joining of dissimilar materials like aluminum and galvanized steel
This led to the assumption that the interaction is basically a fluid dynamic effect of liquid aluminum getting locally alloyed by zinc
We demonstrated the spreading behavior of individual over-heated aluminum droplets on zinc-coated steel surfaces at room temperature both experimentally and numerically

Summary

Introduction

Understanding the wetting behavior of liquefied metal on solid surfaces of other metallic substrates is essential; for example, for laser brazing processes as well as for the thermal joining of dissimilar materials like aluminum and galvanized steel. In case of a novel approach called laser keyhole brazing, overheated braze material must wet substrate surfaces which are not heated directly by the laser irradiation; see [1]. The amount of absorbed energy can be increased up to 93% by changing the welding mode from heat conduction welding to deep penetration welding with keyhole (vapor capillary) formation, as shown for example in [3]. The absorption of laser energy in keyhole welding depends, among other parameters, significantly on the keyhole shape (depth, diameter, and curvature). The keyhole formation and its shape depend on laser process parameters

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