Materials and Design 32 (2011) 5127–5131

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Materials and Design

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Technical Report

Laser welding and weld hardness analysis of thick section S355 structural steel

Mikhail Sokolov a,⇑, Antti Salminen a, Mikhail Kuznetsov b, Igor Tsibulskiy b

a Laboratory of Welding Technology and Laser Processing, Lappeenranta University of Technology, Finlandb Institute of Laser and Welding Technologies, Saint-Petersburg State Polytechnical University, Russia

a r t i c l e i n f o

Article history:Received 15 March 2011Accepted 31 May 2011Available online 6 June 2011

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.05.053

⇑ Corresponding author. Tel.: +358 4511 89808; faxE-mail addresses: mikhail.s.sokolov@gmail.com (M

lut.fi (A. Salminen), kuznetsov_mich@ltc.ru (M(I. Tsibulskiy).

a b s t r a c t

Laser welding is becoming increasingly common and has been found to be of particular interest in thewelding of various steel structures. A new generation of high power lasers has entered the market duringrecent years. These lasers have thus far mostly been used for welding of thin sheet. However, with theavailability of higher power, such lasers have possible application to thick section welding. This studyinvestigates the performance and potential of deep penetration laser welding of S355 EN 10025 struc-tural steel of 20 and 25 mm thickness with a high power fiber laser at power levels of 12–30 kW. Visualexaminations of the macrographs and hardness tests of all welded specimens were made. Quality win-dows were drawn based on the results of the experiments. Preferable welding parameters are formedbased on the experimental study. The results of the hardness test show that surface hardness level ofthe weld is up to 2.5 times higher than the surface hardness of the base material.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, considerable interest has been shown in thefield of materials science in possible benefits arising from the useof high-power fiber lasers (HPFL) for materials processing. Thenew lasers allow laser processing to expand the scope of its use,overcoming some previous limitations. Currently used mostly inwelding of thin sheets, due to vast demand from the car manufac-turing industry, new lasers have great potential in other sectors ofindustry, too. Compared to old generation lasers, such as CO2

lasers, new lasers have better absorption, better beam qualityand better electrical efficiency. Furthermore, they are not as sensi-tive to metal vapor on top of the keyhole as CO2 lasers. Detailedinformation about the actual performance of these lasers is,however, still lacking [1–3].

Compared to traditional arc welding methods, laser weldinggives a narrower heat-affected zone, due to the lower energy inputapplied per unit length, reducing workpiece thermal distortions.Furthermore, the welding process is always automated, whichreduces the necessity of reworking [4]. Manufacturing industryhas long need for an easily-automated, configuration-independent,fast and reliable joining method for thick section welding [5].

In the laser welding process, absorption of the laser radiationcauses local heating of the surface layer of the material. Theabsorption of such energy (>106 W/cm2) leads not only to meltingof the material, but also to evaporation of the material. As the laser

ll rights reserved.

: +358 5624 3082.. Sokolov), antti.salminen@

. Kuznetsov), igor@ltc.ru

beam focusing point moves along the joint, a keyhole is traversedthrough the material with the molten walls sealing up behind it[6–8]. After passage of the laser beam, the molten material coolsdown because of heat transfer from the liquid metal to the not-melted material. High cooling rates are achieved in this coolingprocess, causing significant changes in the material properties ofthe weld and in heat affected zone (HAZ), for example, changesin surface hardness, impact strength, and tensile strength [9]. Theextremely high rate of cooling of the weld causes hardening ofthe material in the HAZ. The main parameters influencing the cool-ing cycle are welding speed and the laser output power level. Laserbeam welding has lower line energy than laser arc hybrid weldingor arc welding processes and higher cooling rates (2000–3000�/s)[10]. Around the fusion zone, where the cooling rate is highest, alarge increase in the hardness has been recorded [11,12].

Excessive weld hardness is an undesirable quality. One possibleway of reducing the weld hardness level is the use of preheatingtechniques to lower the cooling rate, as typically found with arcwelding. Several articles [13–16] have indicated that the use ofpreheating up to 120–160 �C decreases the maximum materialhardness in the weld zone by 20% compared to maximum hardnesswithout preheating. However, preheating typically requires severalwork phases and increases the complexity of the welding processconsiderably.

This paper investigates thick section laser welding of S355structural steel at high welding speeds. The purpose of the exper-imental study is to identify the processing parameters of theautogenous laser welding in a form of recommendations for themanufacturing industry. However, achieving maximum weldingspeed that gives an acceptable weld without imperfections doesnot mean that the weld will have acceptable mechanical

5128 M. Sokolov et al. / Materials and Design 32 (2011) 5127–5131

properties. Hardness tests were carried out to evaluate the level ofchanges in the material structure of the weld and the heat affectedzone. Trends in surface hardness depending on the changes ofwelding parameters are identified.

Fig. 1. Thick section laser welding setup: (1) Welding head (Note: The MIG/MAGwelding torch was not used during the experiments), (2) welding table and fixingequipment, (3) shielding gas systems.

Table 2Ytterbium fiber laser IPG YLS-30000 specification.

Characteristics Typical value

Parameters of the welding laser

2. Experimental procedure

2.1. Experimental equipment and materials

The aim of this study was to find out the best combination oflaser welding parameters and equipment setup leading to high-quality full-penetration welds with a minimum amount of welddefects in butt-joints of thick steel plates.

Welding with a high-power fiber laser (HPFL) was performed onS355 EN 10025-2 structural steel with four variables: laser power,welding speed, focal point position, and material thickness. Struc-tural steel grade S355 EN 10025 is used in a great number of struc-tural manufacturing processes, from offshore structures andmining equipment to wind mill tower components. Two plates ofS355 20 mm and 25 mm thickness were cut by water jet cuttinginto test pieces of 200 mm � 75 mm in size. The chemical compo-sition and mechanical properties of the steel are given in Table 1.

The laser welding head was mounted on an industrial robot (seeFig. 1), which moved the laser welding head over the clampedsheets at the desired welding speed. Before each test, sand blastingwas used to clean the edges of the test pieces. The samples weretightly fixed flat on the jig such that the welding optics was per-pendicular to the direction of the welding and the workpiece sur-face. Argon with flow rate of 20 l/min was used as a shielding gas,delivered to the weld via a copper tube. Laser parameters are listedin Table 2.

Operation mode Continuous wave (CW) modeNominal output power (kW) 30Emission wavelength (nm) 1070Beam parameter product (mm�mrad) 10.5Working fiber core diameter (lm) 200

Parameters of the welding opticsCollimation lens focal length (mm) 140Focusing lens focal length (mm) 300Focal point diameter (lm) 420Fiber diameter (lm) 200

Table 3Process variables.

Equipment Variables Units Levels

IPG YLR 30000 Laser power kW 12, 15, 20–30Welding speed m/min 1.6, 1.8–3Focal point position mm �10, �9 to +0Material thickness mm 20/25Shielding gas, flow l/min Ar, 20 constant

2.2. Experimental design

The laser power was tested at five levels and the welding speedwas set to achieve the highest value for each thickness. In view ofthe numerous parameters affecting the laser welding process, aprocedure of trial and error was used to choose a range of param-eters that could produce an acceptable weld at the highest weldingspeed possible for each power level and material thickness. First,two preliminary experiments were done for a material thicknessof 20 mm to ascertain the optimum power level and focal positionfor full penetration with a fixed welding speed of 2 m/min. After vi-sual examination of the weld and macrographs of the preliminarytest welds, an experiment was done with the best parameters toconfirm that chosen parameters result in a full penetration weldof acceptable quality. The acceptance criteria in visual inspectionof the tested sample were a smooth and plain face and root sideof the weld seam, a minimal amount of spatter, no visual cracksor other defects listed in EN ISO 13919-1, and full visible penetra-tion on the root side.

Process variables are given in Table 3.Cross-sections of all welded samples were visually examined

using the Lumenera ‘‘Infinity 3’’ digital camera and related soft-ware. Conclusions were made based on the EN ISO 13919-1 stan-dard. Vickers hardness macro-hardness tests (EN 895) were

Table 1Structural steel S355 EN 10025 nominal chemical composition and physical properties.

C max Si Mn S max P max Cr max Mo max

0.14 0.15/0.55 1.00/1.65 0.01 0.02 0.25 0.08Thickness (mm) Minimum yield strength (MPa) Tensile str

t 6 25 355 470–630

performed across the fusion zone, HAZ and the base metal. Mar-co-hardness tests were done at room temperature in three lines:at the middle of the sample, 2 mm from the root and 2 mm from

Nb max V max Ti max Ni max Cu max Al total N max

0.04 0.06 0.025 0.5 0.3 0.015/0.055 0.01ength (MPa)

M. Sokolov et al. / Materials and Design 32 (2011) 5127–5131 5129

the top of the weld. The distance between the measurements was0.3 mm. The purpose of the hardness tests was to determine the le-vel of change in the material structure.

The correlation between consumed energy and changes inmaterial properties was also investigated. Laser line energy (EL)was calculated based on the equation:

EL ¼ PL=Vw ð1Þ

where PL is laser output power level (kW) and Vw is welding speed(m/min).

Fig. 3. Weld quality window for steel S355, 25 mm thickness.

Table 4Maximum welding speeds for S355 structural steel.

No. Materialthickness (mm)

Laser power(kW)

Welding speed(m/min)

Focal pointposition (mm)

1 20 15 1.8 �7.52 20 20 2.4 �7.53 25 25 1.8 �12.54 25 30 2.4 �15

3. Results and discussion

3.1. Visual inspection

The results of the tests are presented separately for each mate-rial thickness as a welding speed to laser power plot (Figs. 2 and 3).Acceptable results are marked with circles. Results with differentimperfections are marked with crosses, squares and triangles forcut through, incomplete penetration and other imperfection,respectively. The focal point position is marked for experimentswhich were repeated with the same laser power level and weldingspeed but at different focal point position.

The main focus of the study was to determine the maximumachievable welding speed with appropriate weld quality for S355steel of 20 and 25 mm thickness with laser power levels 15, 20,25 and 30 kW. Recorded variables were: laser power, weldingspeed, and focal point position. Summarized results are presentedin Table 4.

3.2. Hardness tests

For 20 mm thickness samples the average weld hardness valuedecreases when the welding line energy increases. This trend canbe explained by the increase in the cooling rate of the moltenmaterial with the decrease in the line energy, which causesformation of a more solid structure. Investigation of 25 mm thicksamples shows no significant changes in weld hardness at line

Fig. 2. Weld quality window for steel S355, 20 mm thickness.

energy levels from 11.2 to 16.7 J/mm. Fig. 4 presents the resultsas average weld hardness to line energy.

Increasing the welding speed from 2.4 m/min to 2.8 m/min(17%) at constant laser power level and focal point position (fpp),shown in Figs. 5 and 6, causes an increase in the average weldhardness of 8.6%, from 435.2 to 472.8, and a decrease in the aver-age HAZ width by 10.2%, from 1.96 mm to 1.76 mm.

Increasing the laser power from 20 kW to 25 kW at constantwelding speed and focal point position (see Figs. 5 and 7) gives adecrease in the average weld hardness by 0.02%, from 435.2 to425.5, and an increase in the average HAZ width by 11.2%, from1.96 mm to 2.18 mm.

In order to get a full penetration of 25 mm thickness at 2.4 m/min speed an increase in laser power is needed. As it is shown inFig. 8 the hardness level significantly increases at the middle andat the root. This is caused by deepening of the fpp and the increaseof the cooling rate at those levels.

Comparison of the hardness curves at different welding param-eters shows that a reduction in the laser welding speed decreasesthe difference in material hardness between the weld and the basematerial. The same dependence was discovered also in thick

Fig. 4. Weld average macro-hardness, S355 20 and 25 mm thickness.

Fig. 5. Hardness curves, S355, 20 mm, PL = 20 kW, Vw = 2.4 m/min, fpp = �7.5 mm, EL = 8.4 J/mm, average weld hardness = 435.2 HV5, average HAZ width = 1.96 mm.

Fig. 6. Hardness curves, S355, 20 mm, PL = 20 kW, Vw = 2.4 m/min, fpp = �7.5 mm, EL = 7.2 J/mm, average weld hardness = 472.8 HV5, average HAZ width = 1.76 mm.

Fig. 7. Hardness curves, S355, 20 mm, PL = 25 kW, Vw = 2.4 m/min, fpp = �7.5 mm, EL = 10.5 J/mm, average weld hardness = 425.5 HV5, average HAZ width = 2.18 mm.

Fig. 8. Hardness curves, S355, 25 mm, PL = 30 kW, Vw = 2.4 m/min, fpp = �15 mm, EL = 12.5 J/mm, average weld hardness = 493.5 HV5, average HAZ width = 1.78 mm.

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section welding of heavily doped cold-resistant steel [17] and instainless steels [18,19]. Increase of the welding speed causes anincrease in weld solidification growth rate and cooling rate.According to [10] high hardness level (>450 HV) indicated at theborders of the HAZ may be explained by changing the solidificationmode at high welding speeds (Vw P 2 m/min) towards binatemode that causes the formation of the acicular ferrite. In generalcase the exposure time of the maximum heating temperature ef-fects on the material structure and the grain size in these zones.Temperature field transient simulation models may be used toevaluate the exposure time and the temperatures of the thermalcycle in the HAZ [20,21].

The average hardness level of 430 HV is too high for many weld-ing applications. The acceptable level of hardness for structuralsteels is considered to be less then 350 HV [22]. To achieve thelowest weld hardness level, equipment configurations should aimto achieve the lowest welding speeds that give welds of acceptablequality at the relevant laser power level or by using preheatingtechniques as described in [15,16]. Another possible techniquefor lowering the weld hardness is using a dual-beam laser welding.The comparison of laser welding of a hot-rolled AISI 4140 steel(base hardness about 300 HV) without any preheating and withthe 800 W laser preheating recorded a decrease in the weld hard-ness from 588 HV to 506 HV [23].

4. Conclusion

This study of laser welding of structural steels with high-powerfiber laser of 30 kW maximum power level was able to ascertainmaximum welding speeds for S355 structural steel of 20 and 25material thickness at different laser power levels with narrowwelds of a good visual appearance. The main findings can be sum-marized as follows:

(1) Laser welding with a high-power fiber laser of 20 mm S355steel welded at speeds of 1.8 and 2.4 m/min requires 15 and20 kW laser power respectively at the focal point position of�7.5 mm. 25 mm S355 welded at speeds of 1.8 and 2.4 m/min requires 25 and 30 kW laser power at the focal pointpositions of �12.5 and �15 mm respectively.

(2) The hardness of S355 steel tends to increase to 450–500 HVin the HAZ when the base material hardness level is190–200 HV.

(3) Increase in the welding speed results in an increase in theaverage weld hardness and decrease in the average HAZwidth. Increase in the laser power gives a decrease in theaverage weld hardness and increase in the average heatHAZ width.

(4) The hardness of the weld tends to reduce with increasingline energy for material of thickness 20 mm. For 25 mmthickness increase in the line energy does not show signifi-cant changes in the weld hardness; average weld hardnessremains at the same level – 480 HV5.

(5) The difference between the base material hardness and theweld hardness may be reduced by decreasing the weldingspeed or by the use of preheating techniques.

Acknowledgments

The authors are grateful to Fimecc Oy and the Finnish FundingAgency for Technology and Innovation for funding via the Trilaser

project of the Innovation and Network Research Program. Theauthors also want to thank Mr. Berthold Kessler and Mr. KarstenKlinker from IPG Laser GmbH for assistance with the organizationof the experiments and Mr. Antti Heikkinen of LappeenrantaUniversity of Technology for performing the metallography.

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