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, Volume 10, Issue 4, pp 40–45 | Cite as

Influence on the microstructure of laser beam welds of high-strength steels

  • Stephan Völkers
  • Vladislav Somonov
  • Stefan Böhm
  • Thomas Niendorf
Production Welding
  • 256 Downloads

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

In order to meet the increasing demands for resource preservation and the reduction of the environmental impact, the automotive industry has invested much effort to reduce the fuel consumption and emissions of the new car models. A measure which can reduce the energy consumption by about 30 % and therefore has the greatest potential, are light weight constructions. Here, the car body is the focus as it is the biggest connected element of a car and constitutes about one quarter of the entire weight. As a result, the reduction of weight in this area clearly reduces the consumption of fossil fuels and also the amount of CO2 emissions. However, the requirements for crash safety, comfort and functionality have increased. This has led to an increasing use of high-performance materials such as pressure-hardened steels (22MnB5) in order to meet both goals, Figure 1.
Figure 1

Integration of pressure-hardened elements into the car boy (© Audi)

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 [2]. 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].

Generally, the sound waves are generated and coupled using sonotrodes, which are normally used for ultrasound welding. However, the Department for Cutting and Joining Manufacturing Processes (tff) wants to address an alternative approach to the generation and coupling of structure-bourne sound in order to integrate the influence and analysis of occurrences within the joining area into the process. Piezo shakers which function similarly to sonotrodes, but are built relatively compactly, have already been used for some time by the department tff for shearographic systems in the field of non-destructive testing methods. This is an optical method for the areal recording and graphic depiction of deformation gradients on the surface of components. Here, single images of different stress situations can be recorded and superimposed in order to form an interferogram with the aid of special software, Figure 2. The piezo shakers cause the deformation on the surface of the component which couple mechanical vibration into the surface of the component in a fashion analogous to that of sonotrodes. The shakers at the department tff can achieve frequencies of up to 30 kHz and amplitudes of up to 5 V, thus, the maxi-mum deflection of the piezo elements is 6 μm. Here, the maximum force which is brought upon the components is 5 kN. Depending on how the parameter combinations are set, vibrations up to the area of ultrasound can be achieved.
Figure 2

Schematic depiction of the vibration analysis with shearography (© tff)

Analysis of the Vibration Stimulation

Before the actual welding process can start, the best suited parameter combinations for the stimulation of vibration in the specific material, with regard to its thickness and geometry, need to be identified. For this purpose, the shearographic system of the department can simply be integrated into the laser welding unit, Figure 3.
Figure 3

Test set-up for the vibration analysis within the laser welding unit, shown from the side (left) and from the perspective of the shearographic camera (right) (© tff)

The shearographic camera is positioned over the joining area with a stand so that the angle only differs slightly from the vertical angle. As can be seen in the Figure, the piezo shakers are positioned at both sides of the joining area. They are fixated with suction cups onto the surface of the component. Then, the measurement is started and the entire possible frequency range between 0 and 30 kHz is run through in form of a frequency sweep. Here, the amplitude is kept the same. The effects of these mechanical vibrations can be viewed in real time on the measuring computer. The collected data can then help to define the suited stimulation parameters for the welding process and the best distance of the shakers to the joining zone, Figure 4.
Figure 4

Visualisation of the vibration of the component material 22MnB5 in butt joint with vibration in the range of ultrasound F = 12.5 kHz (left) and ultrasound area F = 20.5 kHz (right) (© tff)

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 order to determine the influence of the vibration behaviour on the welding process, the welding parameters, such as laser power and feed rate, were kept constant during the tests. The selection of the vibration parameters for the shakers was based on the performed vibration analysis. No additional materials were used for the tests, as this could have caused unwanted alloying during the welding process which could then inhibit the precise analysis of the influence of the vibration stimulation. Optical light analysis of the weld showed only very slight differences between the laser welded weld processed with and without vibration stimulation. The micro-sections in Figure 5 exemplarily show these results.
Figure 5

Results of the laser welding process of untempered thin metal sheets, 22MnB5+AlSi, with butt joints without ultrasound (left) and with ultrasound (F = 20.5 kHz; amplitude = 5 V) (right)(© tff)

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.

For this reason, the micro-sections were additionally examined with a scanning electron microscope (SEM), using the secondary electron contrast (SE). In order to characterise the composition of the ele-ments in smaller areas of the micro-sections, energy-dispersive X-ray spectroscopy (EDX) was performed. First, the chemical composition of the sample weld which was welded without vibration was analysed, Figure 6.
Figure 6

SE-image and EDX-measurement of the micro-section of a conventionally laser welded sample without ultrasound (left) and the determined local chemical composition, using EXW (right) (© IfW-MW)

The chemical composition was determined, as shown in the SE-images in Figure 6 (left), for the base material, respectively, the heat-affected zone (HAZ), the fusion line and the weld metal. The fusion line constitutes the boundary between the weld metal, where the fusion partners melt, and the HAZ, where the fusion partners are merely heated up, but still below the liquidus temperature. The analysis clearly showed that there were amounts of aluminum in the fusion line and weld metal. However, the percentual amount of aluminum was higher in the area of the fusion line — 2.87 percentage by weight — than in the weld metal. As aluminum increases the probability for the formation of brittle intermetallic phases, which can then lead to cracking in the weld seam when press hardened (e.g. tailored blanks), this can cause cracking in the weld seam and the subsequent failure of the component. To demonstrate the accumulation of aluminum in the area of the fusion line, the SE-Image was superimposed with the mapping of the distribution of the aluminum amounts as recorded with EDX. This process is demonstrated Figure 7.
Figure 7

SE-image with superimposed Al-accumulation of a conventional laser weld with ultrasound (left) and the determined local chemical composition as analysed with EDX (right) (©IfW-MW)

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).

Conclusion

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 [6]. Subsequently, this allows to further improve the deformation behaviour during a later hot forming process. |

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Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Stephan Völkers
    • 1
  • Vladislav Somonov
    • 2
  • Stefan Böhm
    • 1
  • Thomas Niendorf
    • 1
  1. 1.University of KasselWiesbadenGermany
  2. 2.State Polytechnical UniversitySt. PetersburgRussia

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