Physics Procedia 39 ( 2012 ) 33 – 42

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbHdoi: 10.1016/j.phpro.2012.10.011

LANE 2012

Experimental investigation of the influence of edge morphology in high power fiber laser welding

Mikhail Sokolova, , Antti Salminena,b aLaboratory of Welding Technology and Laser Processing, Lappeenranta University of Technology, Tuotantokatu 2, Lappeenranta,

53850 Finland bMachine Technology Centre Turku Ltd, Lemminkaisenkatu 28, Turku, 20520 Finland

Abstract

The objective of the research is to investigate and compare the quality of welded samples of two low alloyed steels, S355 and St 3 with various edge preparations in butt joint configuration. Welding was performed with high power fiber lasers with a wavelength of 1070 nm at power levels of 10 and 14 kW. Joint edge surface roughness and the air gap between the plates had a significant effect on penetration depth. Optimum roughness levels and recommended increase in the air gap between the plates to provide maximum penetration depth are presented. © 2011 Published by Elsevier Ltd. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH Keywords: laser welding; fiber laser; weld penetration; edge morphology; absorptance

1. Introduction

Recent developments in the field of high power fiber lasers (HPFL) have led to renewed interest in thick section laser welding. A number of studies have shown that HPFL offer the potential for high-speed processing of different kinds of metals of different sheet thickness. This potential can be realized in areas where flexibility, accuracy, cost and time reductions, automation and DFMA-integration are important factors: shipbuilding, aerospace engineering, armored vehicles, and many novel applications with potential benefits to a range of industries [1-2].

Laser welding research to date has tended to focus on optimizing beam and mechanical parameters and

relatively little attention has been paid to surface preparation of the joint edges [3-10]. The rule off thumb

* Corresponding author. Tel.: +358-451-189-808 ; fax: +358-56-243-082 . E-mail address: mikhail.sokolov@lut.fi .

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34 Mikhail Sokolov and Antti Salminen / Physics Procedia 39 ( 2012 ) 33 – 42

seems to be no air gap and fine machined edges are required for the laser welding. Several studies have produced estimates of the influence of edge morphology in laser butt joint welding, but there are still insufficient data for reliable estimation of the influence of such factors as edge surface roughness level and the air gap between the plates [11-13]. In addition, international [14] and European standards [15] do not state these parameters as critically affecting the quality of the weld.

The issue has grown in importance in light of recent research [16] that revealed that the absorption of

structural steels in butt joint laser welding has a significant dependence on the edge surface roughness at Fig. 1, in butt joint laser beam welding at edge surface

roughness levels of Ra 1.6 – Ra a unacceptable quality.

Fig. 1. Energy absorption level at different roughness levels, St 3, t = 20 mm, PL = 10 kW, VW = 0.8 m/min, fpp = -4 mm, fC = 150 mm, fF = 250 mm, dOF = 360 mm [16]

The purpose of this paper is to evaluate and validate the effect of surface roughness and air gap in joint between the plates in laser butt joint welding of structural steels with high power fiber lasers at power levels of 10 and 14 kW. Based on previous studies it is justifiable to claim that absorption of the laser beam to process will differ at Ra 0.8-in changes in weld quality and weld cross section dimensions.

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Nomenclature

dK working fiber core diameter,

dOF

fC collimation length, mm

fF focusing length, mm

fpp focal point position, mm

PL laser power, kW

t material thickness, mm

VW welding speed, m/min

2. Experimental

2.1. Material properties

Welding with HPFL IPG YLR 15000 and IPG YLS 10000 was performed on structural 20 mm thickness St 3 GOST 380-94 [17] and 15 mm thickness S355 EN 10025-2 [18] steel plates, and their weld penetration depths were compared. Using a water jet cutting machine, plates of 20 mm thickness were cut from a root side into test pieces, and then processed to the desired roughness level with a milling machine. The surface roughness of the joint edges was measured with a contact roughness measuring device Taylor-Hobson Surtronic 10 Ra with a measuring range of 0.1-40 μm. Roughness was measured on the surface prepared for welding according to EN 10049 [19]. Two variants of the setup for laser welding were used: a butt joint and a butt joint with a pre-set air gap. The air gap in the latter case was created with use of 0.2 mm carbon steel 1.1274 foil strips. Fig. 2 illustrates the setup.

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Fig. 2. Welding setup: (a) butt joint laser welding; (b) butt joint laser welding with a pre set air gap

Randomly chosen pairs of S355 and St 3 samples were tested for chemical analysis. The results are shown in Table 1.

Table 1. Alloying composition of St 3 and S355 structural steels

Chemical composition, wt% C Si Mn P S Cr Ni Cu Al N

St 3 0.16 0.19 0.44 0.01 0.01 0.03 0.01 0.02 0.03 0.01

S355 0.18 0.37 1.39 0.02 0.01 0.03 0.01 0.02 0.04 0.01

2.2. Design of experiments

The experiment plan was divided into two sets of experiments. In the first set of experiments was performed in the welding laboratory of Saint-Petersburg State Polytechnic University (SPbSPU), Russia, using an IPG YLR 15000 fiber laser at 14 kW power level. Steel St3 was welded in two setups to estimate the influence of the surface roughness level and additional gap between the plates on the penetration depth and weld quality. One variable at a time (OVAT) experimental design was used.

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The second set of tests was based on the results and analysis of the previous set of experiments and

was performed in Machine Technology Centre Turku Ltd (KTK), Finland, using an IPG YLS 10000 fiber laser at 10 kW power level. Three welding setups were used: butt-joint and butt joint with a pre set air gap of 0.05 and 0.01 mm. S355 steel samples were machined in pairs to achieve different roughness levels based on the two-level factorial experimental design with four replication points. The purpose of the experiments was to estimate the combined effect of the surface roughness level and air gap between the plates. The process and laser parameters for the sets of experiments were based on earlier studies and are listed in Table 2.

Table 2. Process parameters and variables

Variables Units Levels

Set 1: SPbSPU

Set 2: KTK

Laser power kW 14 10

Welding speed m/min 2 2

Focal point position mm -7.5 -4

Material, Thickness mm St3, 20 S355, 15

Shielding gas, flow l/min Ar, 20 none

Collimation lens focal length mm 140 200

Focusing lens focal length mm 400 300

Focal point diameter 570 300

3. Results and Discussion

Welding and optics parameters were constant for all experiments in a set and are specified in the captions of the figures. Macrograph photos for each roughness level are included in the separate figure, each with a millimeter scale enclosed. The quality of the welded samples was evaluated according to ISO 13919-1 [20] standard with quality levels: D – moderate; C – intermediate; B – stringent.

The results for the first set of experiments for both the butt joint setup and the butt joint with a pre set

air gap set up are shown in Fig. 3. Macrographs are shown in Fig. 4.

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Fig. 3. Penetration depth at different roughness and air gap levels (Set 1): St 3, t = 20 mm, PL = 14 kW, VW = 2 m/min, fpp = -7.5 mm, fC = 140 mm, fF = 400 mm, dOF = 570 mm

Fig. 4. Macrographs of weld samples at different roughness and air gap levels (Set 1): St 3, t = 20 mm, PL = 14 kW, VW = 2 m/min, fpp = -7.5 mm, fC = 140 mm, fF = 400 mm, dOF = 570 mm

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The results show minor changes in the penetration depth or the geometry form of the weld at the Ra

3.2- steel strip (foil) to increase the gap between the plates does not change the trend at these roughness levels. At was recorded for the case of the pre set air gap. A maximum penetration level of 15.05 mm was achieved

eased to 18.30 mm. In both cases the welds were of the confident stringent B level for partial penetration with no critical imperfections.

The results for the second set of experiments for are shown in Fig. 5. Macrographs of the samples

marked 1, 2 and 3 on Fig. 5 are shown in Fig. 6.

Fig. 5. Penetration depth at different roughness and air gap levels (Set 2): S355, t = 15 mm, PL = 15 kW, VW = 2 m/min, fpp = -4 mm, fC = 200 mm, fF = 300 mm, dOF = 300 mm

In Fig. 5 there is a clear trend of increasing in penetration depth at the surface edge roughness levels Ra 4.8 – A positive correlation was found between additional air gap increase up to 0.1 mm and increase in the penetration depth at different roughness levels.

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Fig. 6. Macrographs of weld samples at different roughness and air gap levels (Set 2): 1. Ra = = 0.1 mm

Full penetration welds were achieved only at roughness levels of with additional air gap of 0.1 mm with additional gap of 0.05 mm. However, according to ISO 13919-1 cracks with more than 1 mm2 crack area are not permitted. The only weld sample of acceptable quality D – moderate from the set 2 is shown on Fig. 6 marked by number 1. The analysis of the macrographs showed that hot cracking was detected in all samples. It can be explained by non-optimal choice of the welding parameters.

Comparison of the penetration depth and the surface roughness shows a strong correlation between the

two variables. The results from the set 1 can be compared with the results in set 2 which shows that surface edge preparation to the roughness level of about with increased air gap gives a deeper penetration level and better weld quality. These results therefore need to be interpreted with caution. The air gap between the pieces was not accurately measured and only the dimensions of the added steel strips were recorded. Since the measurements carried out with mechanical measuring device are not very precise, the results may vary also in case of using another model of roughness measuring device. In future investigations it might be possible to use areal roughness parameters (Sa) which are defined in EN ISO 25178-6 [21].

4. Conclusions

In this study, laser welding of structural steels S355 and St 3 with high power fiber lasers achieved the stated objective of revealing the optimum edge preparation setup for butt joint laser beam welding. The

increased air gap of 0.1-0.2 mm gives a deeper penetration level and better weld quality. Full penetration welding at edge surface roughness levels higher than Ra An implication of these findings is that both optimum edge surface roughness levels and increased air gap between the plates in butt joint laser welding should be taken into account at the stage of product design. It is, however, important to recognize the limitations of the findings of this study: namely, the welding

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setups, equipment and the materials used in the experiments. It seems that the formation of keyhole is easier in case of small air gap in joint improving the utilization of laser power. Based on current knowledge, further experimental investigations are needed to ascertain the phenomena underlying this correlation. Investigation of the correlation between edge surface preparation methods and weld quality in welding of other materials is recommended.

Acknowledgements

The authors wish to express their gratitude to Fimecc Oy and the Finnish Funding Agency for Technology and Innovation for funding the project via the Trilaser project of the innovation and network research program. The authors want to thank Mr. Esa Lappalainen from Lappeenranta University of Technology for his help during the second set of experiments and performing the metallography.

References

[1] Quintino, L.; Costa, A.; Miranda, R.; Yapp, D.; Kumar, V.; Kong, C. J.: Welding with high power fiber lasers - A preliminary study. Materials & Design, 28 (2007).

[2] Ueda, K.; Liu, A.: Future of High-Power Fiber Lasers. Laser Physics, 8 (1998). [3] Kinoshita, K.; Mizutani, M.; Kawahito, Y.; Katayama, S.: Phenomena Of Welding With High-Power Fiber Laser. Proc.

of 25th International Congress on Applications of Lasers & Electro-Optics (2006). [4] Katayama, S.; Kawahito Y.; Kinoshita K.; Matsumoto N.; Mizutani M.: Weld penetration and phenomena in 10 kW fiber

laser welding. Proc. of 26th International Congress on Applications of Lasers & Electro-Optics (2007). [5] Salminen, A.; Lehtinen, J.; Harkko, P.: The effect of welding parameters on keyhole and melt pool behavior during laser

welding with high power fiber laser. Proc. of 27th International Congress on Applications of Lasers & Electro-Optics (2008).

[6] Salminen, A.; Piili, H.: The effect of parameters on process behavior and efficiency in fiber laser welding. Proc. IIW International Conference on Advances in Welding and Allied Technologies (2009).

[7] Salminen, A.; Purtonen, T.: The effect of welding parameters on keyhole and melt pool dimensions and behavior during fiber laser welding. Proc. of 12th Nordic Conference on Laser Material Processing (2009).

[8] Kaplan, A.; Wiklund, G.: Advanced welding analysis methods applied to heavy section welding with a 15 kW fiber laser. International Conference on Welding of the IIW, 53 (2009).

[9] Katayama, S.; Kawahito, Y.; Mizutani, M.: Elucidation of laser welding phenomena and factors affecting weld penetration and welding defects. Physics Procedia, 5 (2010).

[10] Sokolov, M.; Salminen, A.; Kuznetsov, M.; Tsibulskiy, I.: Laser welding and weld analysis of thick section S355 structural steel. Materials & Design, 32 (2011).

[11] Arata, Y.; Miyamoto, I.: Some Fundamental Properties of Hogh Power Laser Beam as a Heat Source (Report 2). Transactions of the Japan Welding Society. 3 (1972).

[12] Covelli, L.; Jovane, F.; De lorio, L.; Tagliaferri, V.: Laser Welding of Stainless Steel: Influence of the Edges Morphology. CIRP Annals - Manufacturing Technology, 37 (1988).

[13] Ricciardi, G.; Cantello, M.: Laser Material Interaction: Absorption Coefficient in Welding and Surface Treatment. CIRP Annals - Manufacturing Technology. 43 (1994).

[14] EN ISO 15609-4. Specification and qualification of welding procedures for metallic materials - Welding procedure specification. Part 4: Laser beam welding (2009).

[15] EN 1011-6. Welding - Recommendation for welding of metallic materials. Part 6: Laser beam welding (2005).

42 Mikhail Sokolov and Antti Salminen / Physics Procedia 39 ( 2012 ) 33 – 42

[16] Sokolov, M.; Salminen, A.; Somonov, V.; Kaplan, A.: Laser welding of structural steels: Influence of the edge roughness

level. Optics & Laser Technology, 44 (2012). [17] GOST 380-94. Common quality carbon steel. Grades (1994) (in Russian). [18] EN 10025-2. European structural steel standard (2004). [19] EN 10049. Measurement of roughness average. Ra and peak count RPc on metallic flat products (2005). [20] EN ISO 13919-1. Welding. Electron and laser-beam welded joints. Guidance on quality levels for imperfections. Part 1:

Steel (1996). [21] EN ISO 25178-6. Geometrical product specifications (GPS). Surface texture: areal. Part 6: Classifications of methods for

measuring surface texture (2010).