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COMPARISON OF 22MNB5-STEEL WITH AND WITHOUT ALSI-COATING DURING LASER HYBRID ARC WELDING

P. Norman1*, G. Wiklund1, P. Janiak2, N. Malmberg2, A. F. H. Kaplan1

1 2

Luleå University of Technology, TVM Department, Luleå, Sweden Swerea KIMAB AB, Stockholm, Sweden

Abstract

22MnB5/Usibor1500 is a hot-dip aluminized manganese-boron steel grade that is used within the car manufacturing industry and as lasers are increasingly used for welding of the car body the amount of research concerning laser welding of this material follows. For thicker material the laser hybrid arc welding process has grown to a well-known tool to increase productivity, but for thinner materials the advantages are less obvious. In the present research the melt pool and wire drop transfer was observed with a high speed camera. Differences between welding, with and without the coating were clearly visible and are described and discussed in the paper. The resulting welds were examined microscopically and chemically to unveil the composition of the material. It is aimed to avoid possible brittle inter-metallic phases directly during the process, hence by skipping the usual additional manufacturing step of removing the AlSicoating in advance. Keywords: laser, hybrid, arc, welding, coating, AlSi, steel

1

Introduction

Lightweight structures, good forming properties and, last but not least, high performance in terms of crash behavior whenever the key focus is on safety, are vital aspects in the production of high-grade components for the car body shell. If these properties can be combined with a scale-resistant coating which will prevent oxidization of the surface in the tooling units, and therefore avoiding soiling of the tools, this can be achieved by using a manganese-boron steel alloy with a 25µm thick aluminium-silicon coating. This material, Table 1, is not only used for supporting components in motor vehicles, but also as vital safety components such as lateral impact protection, door sills, and the A and B pillars. Table 1: Chemical composition of 22MnB5[1], average value is shown [ %]

22MnB5 C 0,225 Si 0,25 Mn 1,25 Cr 0,155 P <0,025 S <0,008 B 0,0035 Al >0,015 Ti 0,035

The coating consists of a Fe- Al- Si alloy diffusion layer and an aluminium- silicon layer. The contents in the surface layer after heating to 950°C for 30 seconds, is shown in Fig. 1, [2].

Fig. 1: Chemical content of surface coating, measured from top surface, into material For the base material the cooling rate after heating to the correct temperature is the important factor. According to Merkleinet[3] the critical cooling rate is 27°/s, see Fig. 2. The cooling rate is however controlled by the process when performing the hot stamping.

Fig. 2: TTT-diagram for the steel grade 22MnB5[3]

With a standard laser welding process serious dilution occurs in the molten zone leading to a creation of intermetallic phases and modification of cooling diagrams in the area that produce ferrite, both visible as white areas in the fusion zone. During tensile tests on the weld, the failure can occur along these white areas, the tensile strength of the weld is decreased by these intermetallic phases. There has been extensive research performed within the area of presshardenable boron steels [4-10]. The research ranges from the numerical simulations performed by Hein et al [4] and Engels et al [10], where they simulate both the part and the process itself, or investigations of the properties of the material during tensile testing, Merklein et al[5], or changes in the actual production process to optimize it[6-9].

2

Experimental setup

The setup consists of a 15kW Yb-fibre laser, this is combined with a MAG torch to form a laser hybrid arc welding process. The laser is leading during welding. The process zone images are captured with a High-speed camera at 3000fps aimed from the side at an angle of 45°. The area is illuminated by a pulsed laser of 500W, Fig. 3, synchronized with the camera shutter.

High-speed camera Laser optics Illumination-laser

MAG-torch

Work-table Fig. 3: Experimental setup of the Hybrid Laser Arc Welding. The material welded is 22MnB5, a press hardenable boron steel, Table 1, with and without coating of AlSi, the material is called Usibor with the coating. After the welding the steel sheets are quenched and tempered to allow the sheets to harden to its final state. The following results are from the welding itself and do not take into account any affects that the process of quenching and tempering may have on the final product as this is controlled by the individual process at the manufacturing site. The results focus on altering the oxide formation on the meltpool surface as these affect the meltpool stirring.

3 Results and discussion

In the following a series of figures are shown to visualize the effects of changed parameters on the meltpool surface-oxide formation. In Fig. 4 the pieces are welded without any surface coating and the oxide free meltpool surface is seen as a totally black surface. The edge of the meltpool is affected by the MAG-arc and forms a branch like structure depending on the surface topology and the surfaces ability to conduct the current from the MAG-torch.

Fig. 4: Steel grade 22MnB5 without surface coating If the material is switched to the coated version the process zone appears totally different, Fig. 5. The parameters of the laser and the MAG-arc are held constant. The meltpool is covered by an oxide that to different extents are blended in the weld zone. Droplet Keyhole Wire

Meltpool

Base material

Weld direction

Fig. 5: Usibor1500 (22MnB5 with AlSi coating), low laser power, moderate welding speed, moderate wirefeed The oxide formation is then tried to be modified by changing the welding parameters. In Fig. 6 the laser power is increased and the wire and welding speed is decreased. The change in the parameters has the effect that the oxide layer appears thinner, or more dispersed. This effect is probably dependant on the mixing/stirring of the meltpool by the increased heat input.

Fig. 6: Usibor1500, moderate laser power, low welding speed, low wirefeed In Fig. 7, the heat input is lowered by increasing the welding speed and keeping the laser power constant, at the same time the wirefeed is increased. The result is that the oxide free part of the meltpool increases and the mixing of the oxides around the keyhole decreases.

Fig. 7: Usibor1500, moderate laser power, moderate welding speed, moderate/high wirefeed By changing the wire-material composition the hypothesis is that the oxides will be formed differently and that the aggregation of ferrite or intermetallic phases will be minimized. In Fig.8, the effect of the wire change is seen as a dark edge between the meltpool and unmelted base material.

Fig. 8: Usibor1500, moderate laser power, moderate welding speed, moderate/high wirefeed

The macrographs that is to be performed will unveil the composition of the weldzone. The size of the oxide free area around the keyhole is also kept constant. In Fig 9, the wirefeed is increased to try to increase the dilution of the surface oxides. This increase in the material flow into the meltpool has the effect that the wire droplets stays ontop of the oxide surface and only part of the wire droplet volume is diluted in the weld.

Fig. 9: Usibor1500, moderate laser power, moderate welding speed, high wirefeed If the wire feed, weld speed and laser power is decreased the same as in Fig. 9 happens, Fig. 10. The heat input is kept constant.

Fig. 10: Usibor1500, low laser power, low welding speed, high wirefeed The heat input can be increased in respect to the setting in Fig. 8, without affecting the surface appearance, Fig 11.

Fig. 11: Usibor1500, moderate laser power, low welding speed, low wirefeed One trial has also been performed where the MAG is leading, Fig. 12. The same surface appearance where obtained with these setting.

Fig. 12: Usibor1500, low laser power, low welding speed, high wirefeed The first wire-material composition, Fig. 5-7, is not sensitive to the laser power(LP)/arc power(AP) ratio. To facilitate the droplet dilution into the meltpool when using the second wire-material composition, Fig. 8-12, the LP/AP ratio must be kept above 2.5 as shown in Fig. 8 and Fig.11, as the oxide surface behaves differently than with the first wire-material. If the correct ratio cannot be obtained the MAG-torch can be setup as leading and hence giving a good weld with the ratio set to approximately 1, Fig. 12.

4

Conclusions

For the tested parameters in the selected material and combinations of process parameters the following conclusions can be drawn: The wire material composition affects the oxide formation in a way that the surface tension or meltpool stirring is altered The LP/AP ratio affects the droplet dilution so when the ratio is to low the dilution is less good, and droplets from MAG are not completely diluted into the weld The laser-MAG setup affects the droplet dilution/oxide formation in a way, so when the laser is leading, the meltpool surface shows a higher surface tension.

5

Acknowledgements

The authors would like to thank the VINNOVA foundation and their FFI programme for the funding of this research.

6

[1] [2]

References

N.N.: USIBOR 1500 precoated, Arcelor, 2003. M. Suehiro, K. Kusumi, T. Miyakoshi, J. Maki, M. Ohogami, Properties of aluminium-coated steels for hot-forming, Nippon Steel Technical Report No. 88, July 2003. M. Merklein et al., Investigation of the thermo-mechanical properties of hot stamping steels, Journal of Materials Processing Technology, 117, 2006, 452 P. Hein, R. Kefferstein, and Y. Dahan,"Hot Stamping of USIBOR 1500P: Part and Process Analysis Based on Numerical Simulation," in proceedings from The International Conference "New Development in Sheet Metal Forming Technology," Stuttgart, Germany, 2006, pp. 163-175. M. Merklein, J. Lechler, and M. Geiger, "Characterisation of the Flow Properties of the Quenchenable Ultra High Strength Steel 22MnB5," Annals of the CIRP, Vol. 55 (2006), Kobe, Japan. Vierstraete, R.; Duque Munera, D.; Pinard, F.; Pic, A.: Development of adapted laser weld procedure for high strength press-hardenable boron steel Usibor®1500P and its applications. 67th LMPC, December 11th, 2006, Tokyo, Japan Pic, A.; Cretteur, L.; Schmit, F. et al.: Innovative Hot-Stamped Laser-Welded Blank Solutions for Weight Savings and Improved Crashworthiness. Sheet Metal Welding Conference XIII, May 14-16, 2008, Livonia, United States Pic, A.; Duque Munera, D.; Cretteur, L.; Schmit, F.; Pinard, F.: Innovative warmumgeformte Lösungen aus Tailored Blanks. In: stahl und eisen 128 (2008), p. 59-66 Ehling, W.; Cretteur, L.; Pic, A.; Vierstraete, R.; Yin, Q.: Development of a laser decoating process for fully functional Al-Si coated press hardened steel laser welded blank solutions. Fifth International WLT-Conference on Lasers in Manufacturing, June 2009, Munich, Germany

[3] [4]

[5]

[6]

[7]

[8]

[9]

[10]

H. Engels, O. Schalmin, and C. Müller-Bollenhagen, "Controlling and Monitoring of the Hot-Stamping Process of Boron-Alloyed Heat-Treated Steels," in proceedings from The International Conference "New Development in Sheet Metal Forming Technology," Stuttgart, Germany, 2006, pp. 135-150.

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