Influence on the microstructure of laser beam welds of high-strength steels
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Boron-alloyed high-strength steels are challenging for modern joining methods such as laser welding as they have a protective AlSi-layer which can reduce the strength qualities of the weld if not removed beforehand. To counteract this decrease in quality, innovative and application-specific solutions for the production of the desired weld seam structure sought for.
Pressure-hardened Steels for Car Bodies
The application of piezo shaker offers an economic alternative.
The application of higher strength steels is especially important for elements which need to be especially strong as the material thickness which would otherwise be necessary can be reduced, thus, effectively saving weight. Additionally, high-strength steels show a higher resistance to deformation in crash situations which improves the quality of the components [1, 2].
Modern joining methods such as laser beam welding are often used to join these components as they have a high process speed, can induce energy precisely and, as a consequence, expose the components to little stress through heat. Laser beam welding is especially used for the joining of higher strength steels. However, the supreme qualities of these steels are still reduced locally through this welding method . As a result, innovative and application-specific solutions have to be developed which limit the decrease in quality and can be adapted to the specific needs of the component’s design. This paper would like to address the question as to how ultrasound influences the different chemical and physical properties and layers of various materials.
Innovative Method for the Coupling of Structure-borne Sound
As the demands on the quality of the weld seam increase, various solutions are approached in order to meet the demands. One promising method to increase the mechanical-technological properties of the weld is the integration of sound stimulation into the welding process. This method, which was adapted from foundry technology, positively influences the weld seam by mechanically inducing vibrations. This can have different effects on the melt which can then also influence each other strongly. These induced dynamics can have mechanical, metallurgical, as well as fluid and thermodynamic effects which have partly been proven in experimental and theoretical research projects. Primarily, ultrasound supported welding can cause acoustic cavitations and currents as well as high temperature gradients. Whether these effects occur depends on the material, the manner in which the stimulation is created as well as further framework conditions of the induction of vibration. All in all, these factors determine the kind of oscillation, its intensity and effect. The expansion of ultrasound waves in the melt causes local pressure and temperature changes due to the forming of compression and decompression areas. These pressure areas change constantly and cause cavities in the melt which then lead to mechanical effects such as flow turbulences. This, in turn, creates a higher amount of dendrite shears and nucleation during the soli dification process, which ultimately leads to a finer microstructure in the weld seam [3, 4, 5].
Analysis of the Vibration Stimulation
In dependence to the frequency and amplitude, differently strong deformation gradients of the component’s surfaces can be detected. The snapshots shown in Figure 4 exemplarily show the vibration amplitudes (in grey levels) during stimulation with a frequency of 12.5 kHz (left) and a frequency of 20.5 kHz (right). In both cases, the amplitude was kept at the level of 5 V. When the frequency was low, only little deformation on the surface of the component in the area of the joining zone could be registered. Stimulation in the ultrasound area (> 20 kHz), on the other hand, showed a clear stimulation of the joining partners over the entire length of the weld seam. The shearographic measurements shown in this paper were taken of samples of boron-alloyed Q & T steel (22MnB5) with a material thickness of 1.2mm and a sample size of 100 mm x 80 mm, welded with butt joints. The best suited stimulation parameters can vary strongly, depending on the material, dimensions and material thickness. A further influencing factor are the positions and amount of the clamping devices. In this case, the clamping devices were deliberately positioned with sufficient distance to the joining zone in order keep the influence of the devices as low as possible. However, further tests at the department (not published so far) have shown that the position of the clamping devices and the resulting possible effect should always be taken into account.
Influencing the Forming of a Microstructure in the Weld
In comparison to the weld welded without vibration stimulation, the micro-section of the weld welded with ultrasound had a slightly broader weld seam, Figure 5 (right). Both samples still show the AlSi-layer in the base material which had not been removed before welding. However, the mirco-sections do not allow an exact statement as to whether aluminum and silicon have accumulated within the weld metal or whether these elements have dispersed into the joining zone.
In comparison to Figure 6, which shows a large accumulation of aluminum elements in the area of the fusion line, the EDX mapping of this sample which was laser welded with ultrasound, shows a much more homogeneous distribution of aluminum in the entire cross-section of the weld seam. Further measurements in the area of the base material, fusion line and weld meatal were performed to achieve a direct comparison with the sample welded without ultrasound. The acceleration voltage of 20 kV was the same as in the prior EDX measurements.
A method to increase the mechanical-technological properties is the integration of sound stimulation into the welding process.
The results show that the aluminum accumulation was clearly reduced to 1.75 percentage by weight in the fusion line (measurement point P2).
Laser beam welding of boron-alloyed Q & T steels highly challenges users such as the automotive industry because of the AlSi protection layer which is necessary for pressure hardening. In order to generate a weld which is strong enough, the AlSi-layer often needs to be removed elaborately from the fusion zone in a pre-welding stage. Here, the application of piezo shaker offers an economic alternative. Superimposing the welding process with mechanically induced vibration allows welding with the protective layer and leads to a much more homogeneous distribution of the elements aluminum and silicon in the weld metal, as could be proven in this paper. Additionally, sufficiently strong ultrasound support can influence the nucleation and the later dendrite growth and, thus, foster a fine microstructure . Subsequently, this allows to further improve the deformation behaviour during a later hot forming process. |