Steinbeis experts develop laser coating technique by using vibrations
It’s important for components subject to severe wear and tear to be given hard surfaces. This is something metallic materials usually don’t have, so to strengthen surfaces, weld overlaying is used to add an extra layer and provide more protection. Typically a material like tungsten carbide is used for this overlaying process. It is embedded as isolated hard particles into a tough nickel/boron/silicon matrix material. The high density of tungsten carbide comes hand in hand with a high mass and this makes weld overlaying more difficult. Gravity causes hard material particles to sink within the molten matrix and this has a negative impact on the wear protection offered by the coating. Strong protection is only possible if the hard tungsten carbide particles stay near the surface. The experts at the Steinbeis Innovation Center for Intelligent Functional Materials, Welding and Joining Techniques, Implementation have now found a solution to this problem.
As part of a collaborative project with a partner from industry, Lunovu Integrated Laser Solutions from Herzogenrath, the Steinbeis team has been testing the use of vibrations in laser weld overlaying processes. The aim of the joint development project was to see if mechanical vibrations can adapt the flow dynamics of molten coating layers and stop tungsten carbide particles sinking away from the surface. Ultimately, it should be possible to add a protective coating with evenly spread tungsten carbide.
With laser weld overlaying, the material that is going to be added to the surface starts as a mixed powder (60% nickel/boron/silicon, 40% tungsten carbide). The powder is fed onto workpieces using argon, which shields it from the air and allows the powder to melt under a laser beam. The project partners integrated a vibration monitoring device into the process to make it possible to control the frequency of linear oscillations along the base material and melting area.
Tests showed that vibrations have a clear influence on flow dynamics during weld overlaying. The experts discovered that the distribution of the tungsten carbide was particularly uniform at low frequencies with high oscillations and as a result, this prevented sinking. The microscopic image below shows an example of a coating layer with a welding process frequency of 100 hertz:
The two following images show a comparison between three overlapping welding layers. In the first image, no vibrations were applied and as a result there was marked sinking of the tungsten carbide within each of the three weld passes. In the second image the distribution is homogeneous. The welding process was subjected to vibrations with a frequency of 50 hertz:
Measuring the hardness of layer surfaces made it possible to improve protection and confirmed that the new welds were working. This meant the project team had achieved their development goal and it is now possible to improve the wear properties of welded overlay materials containing tungsten carbide. A number of options for successfully transferring the vibration process to large, complex components have already been worked out and tested.
Associate Professor Dr. habil. Khaled Alaluss, Martin Sandig, Prof. Dr.-Ing. Gunnar Bürkner
Steinbeis Innovation Center Intelligent Functional Materials, Welding and Joining Techniques, Implementation (Dresden)