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STRUCTURE AND CORROSION BEHAVIOR OF LASER-WELDED STAINLESS STEELS USING A REMOTE SCANNER LASER

N.V. Benohanian1, A.M. El-Aziz1, J. Drechsel2, H. Exner2

1 German University in Cairo, Materials Engineering Department, 11835 New Cairo, Egypt 2 Hochschule Mittweida (FH), Technikumplatz 17, D-09648 Mittweida

An der Hochschule Mittweida wurde das Hochgeschwindigkeits-Scannerschweißen als neue Methode zum verschweißen von Blechen größerer Dicke untersucht. Die Versuche wurden 2 Stählen, einem austenitischen Edelstahl X1 NiCrMoCuN 25-20-6 (1.4529) und dem Super-Duplex Edelstahl X2CrNiMoCuWN25-7-4 (1.4501), in den Materialstärken von 6 mm, 8 mm und 10 mm durchgeführt. Die Versuchsauswertung umfasste die mechanische Prüfung, die optische Mikroskopie sowie die elektrochemische Korrosionsprüfung. Die jeweils 3 Materialdicken wurden in der mechanischen Prüfung sowie in der Schweißkornausbildung verglichen. Die Optische Mikroskopie deckte Typ A-Verfestigung im 1.4529 und deutliche Mikrostrukturänderungen innerhalb des Duplex Edelstahls sowie viele Schweißdefekte auf. Die Härteprüfung ergab eine Härtezunahme zur Schweißnahtmitte hin und Unterschiede bezüglich der Härte bei den verschiedenen Dicken artgleicher Materialien. Die Zugprüfung ergab, dass die Schweißnaht fast so hohe Festigkeiten wie das Grundmaterial erreichen kann, wenn sie fehlerfrei geschweißt ist. Bei der Korrosionsrate zeigte sich, dass geschweißtes Material empfindlicher gegen Kor-rosion und Lochfraß ist.

High-speed remote scanner laser-welding is a relatively new technique that is being tried out at the Mittweida Laser institute for welding stainless steel sheets of high thicknesses. The used materials were 6, 8 and 10 mm thick. Two different materials were tested: X1 NiCrMoCuN 25-20-6 (1.4529) super-austenitic stainless steel (ASS) and X2CrNiMoCuWN25-7-4 (1.4501) duplex stainless steel (DSS). The work involved mechanical testing using (tensile and hardness). Optical microscopy was employed to investigate the phase changes and weld defects inside the materials. Electrochemical corrosion testing was used to evaluate the corrosion behaviour of the weld zone. The three different thicknesses were compared during mechanical testing and weld bead examination. Microstructure investigation revealed the formation of finer microstructures with differ-ent grain boundaries due to the melting and the resolidification of the base metal and extensive microstructure changes inside the materials. Many weld defects were also observed. Hardness measurements revealed that hardness increases towards the centre and differences in the hardness are observed in similar materials with different thicknesses. Tensile testing revealed that the weld has the potential to have very high strength approaching that of the original material. Measuring corrosion rates shows that the welded material is more susceptible to corrosion and to pitting in all cases, due to the formation of ferrite as the primary phase in duplex stainless steel and compositional variations due to melting and resolidification. 1. Introduction

High-speed remote scanner laser welding on large thicknesses is a relatively new topic; so it is important that maximum penetration is made inside these thick-nesses with minimum distortion to obtain optimum results. Laser beam welding provides weldments with minimum distortion and minimum heat-affected zones [1, 2].

Therefore, to ensure that we obtain high-quality welds in high thicknesses, the welding is done using keyhole welding. Keyhole welding or deep-penetration welding is a technique in which the laser beam pene-trates partially or completely through a workpiece form-ing a keyhole; so as the laser beam progresses, mol-ten metal fills in behind the hole to form the weld bead; which results in a homogenous weld [3, 4]. When high laser beam power densities are achieved, deep-penetration fusion welding is accomplished by a key-hole energy transfer mechanism.

The Remote Scanner Welding System is a high-speed system used to create multiple laser stitch-welds, combined with very short non-productive weld-ing times, making this technique a primary competitor for Resistance Spot welding. [5]

Scanner technology is a third alternative to moving either the machining head or the workpiece; the laser beam is deflected and positioned by one or two galva-nometrically moved rotating mirrors, focusing is usually

effected via an optimized lens system. In our case, for the remote welding, the focusing is usually positioned in front of the scanner head [6, 7].

High-performance lasers with a power in excess of 3 kW and high beam quality are required to obtain a dependable Remote Welding System. [8] In the auto-motive industry the laser technology was implemented to increase the productivity, strength of constructions and new designs [9, 10]. First applications with long welds on the car body are now followed by a replace-ment of resistive spot welds by short laser welded stitches with the advantage of shorter processing times. [11, 12]

Fig. 1 Working area of the laser scanner system (Courtesy of Trumpf Lasertechnik)

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The mechanical properties and corrosion resistance of laser-welded stainless steels may be deteriorated due to micro-segregation, unfavourable phase content, presence of porosities, solidification cracking, micro-fissures and loss of materials by vaporization. In the present work, the microstructure, mechanical and cor-rosion behaviour are studied. As well the comparison between the base metal and the weld zone are dis-cussed taking in account the effect of large thick-nesses. 2. Experimental procedures The chemical composition of the materials is shown in Table 1. Duplex stainless steel 4501 (DSS) and 4529 austenitic stainless steel (ASS) are used [13].

Table 1 Chemical composition of the materials C Cr Ni Mo N

ASS 4529 0.01 20 25 6 0.2 DSS 4501 0.02 25 7 4 0.27

Material preparation The materials were received as 400 mm x 100 mm

plates; so to create work-pieces of suitable dimen-sions, they had to be cut carefully. A suitable cut-off wheel (54A25 cut-off wheel) was used with water-cooling to avoid any change in the metallurgical struc-ture of the materials and then the sheets were clamped mechanically to provide a straight edge suitable for welding. For the tensile tests, 100 mm x 25 mm pieces were cut and the welding edges were prepared by cutting off the rough edges to provide us with suitable edges which help in providing deep welding penetration and mini-mum distortion. After preparing these pieces, welding was done to provide us pieces with approximately 200 mm x 25 mm dimensions. For every material and every thickness 6 welded specimens were prepared and 2 un-welded specimens were prepared.

The welding of stainless steel sheets of such great thicknesses was a very tricky procedure; literature was not very helpful in providing us accurate weld speeds to perform a very sound weld. Therefore, we took in our account the trails. After several trials on each ma-terial and thickness, we reached a weld speed that provided us with a weld that looked sound upon visual inspection.

The welding equipment was used a high-power 3kW Ytterbium Fiberlaser Remote Scanner Laser, the shielding gas used was Argon which flowed during the welding to remove the plasma. Different welding speeds (Table 2) were used for each material of the same thickness to provide us with as much beam penetration as possible, with minimum thermal distor-tion. For all welds, no filler metals were used.

Table 2 Welding speeds Material type Thickness Weld speed

6 mm 4 m/min 8 mm 2.5 m/min DSS 4501

10 mm 1.5 m/min 6 mm 4 m/min 8 mm 2.5 m/min ASS 4529

10 mm 1.5 m/min

Microstructure investigation

The laser-welded specimens were sectioned, pol-ished and etched. The microstructure and phases in the weld line were analyzed by optical microscopy. The weld defects inside the weld profile were also exam-ined to determine if improvements can be made inside the weld in the future which will minimize weld defects in the future, such as incomplete penetration, porosity and cracking. Mechanical testing Hardness test

The hardness of the laser-welded specimens was determined using a Vickers micro-hardness tester and the measurements were made using DIN standards [14]. The load applied was 300 g and the loading time was 15 s. All the specimens were ground to provide a smooth surface for the test. Tensile test

Tensile tests on welded and unwelded specimens were performed to observe the mechanical perform-ance of the welded specimens. The observed property in this test was the Ultimate Tensile Strength, i.e. the magnitude of stress during which complete failure of the material will occur [15]. To obtain the geometry that was according to ASTM E8 Standard [16] that is of the shape seen in the figure below, the material was milled using CNC machining. The tensile test was carried out under these condi-tions: Strain rate from 0% to 0.2% = 4 mm/min Strain rate from 0.2% to failure= 10 mm/min Corrosion behaviour

Even though a great number of experiments have been done that is related to stainless steel weldments, it was felt that there is a need to examine the effects of such high-speed welding on the corrosion behaviour of the material [17, 18]. The instrument used was a PGZ-100 Voltalab potentiostat. The specimen is covered with an epoxy and left to harden. To avoid any changes in the corrosion reading due to surface mate-rials roughness [19], the materials are ground using silicon carbide paper up to grade 1200, and then pol-ished with diamond pastes 9, 3 and 1μm, respectively.. Electrochemical measurements of the base metal and the welded zones were carried out in N2-purged 3.5% NaCl solution; all potentials are measured relative to a saturated calomel electrode (SCE) at 25oC.

Open-circuit potential (OCP) for every speci-men, the OCP was performed for 60 minutes.

Potential cyclic voltammetry (PCV) is per-formed at 300 mV below the OCP of the mate-rial until a suitable voltage with a scan rate of 2 mV/s.

The corrosion rate is obtained using the Tafel method.

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3. Results and discussion

Optical microscopy - Weld defects

From the figures below (Fig. 2-6), it is observed that there are different types of defects present in the welds which should be taken into consideration. The most common defect is incomplete penetration of the weld bead inside the material with such great thicknesses; this might be due to the reflection of the incoming laser off the surface of the material. There are techniques to decrease the defects. Some techniques are recom-mended that might decrease the reflectivity of the laser off the surface of the material which we did not use, but we might apply later, such as coating the weld surface with thin absorptive coating such as black ox-ide that will decrease the surface reflection and aid in the fusion process without changing the microstruc-tures and properties of the materials. Other techniques could include increasing the power input if it is possible to compensate for the high reflectivity of the material or increasing the surface roughness of the material [20, 21].

Fig. 2 4529 ASS 6 mm incomplete penetration

Fig. 3 4529 ASS 8 mm underfill

Fig. 4 4501 DSS 6 mm incomplete penetration

Fig. 5 4529 DSS 8 mm porosity

Fig. 6 4529 DSS 10 mm incomplete penetration

Microstructure investigation

A complete austenitic structure is observed in figure 8. In figure 9, a clear difference is seen between the base metal and the weld metal, where the weld metal also has a more refined structure and smaller grain size due to the heat input from welding. An interesting phenomenon is observed during solidification called

"fully austenitic" solidification or Type A solidification; where in the microstructure of the weld lines cells and dendrites are shown. In theory, this solidification oc-curs in austenitic alloys due to the segregation of alloy-ing and impure elements that occur during solidification and the relatively low diffusivity of these elements at higher temperatures which preserves the separation profile during solidification. The solidification occurs as primary austenite and the structure remains fully aus-tenitic upon cooling to room temperature. The solidifi-cation substructure is apparent as cells and dendrites. In all thicknesses, Type A solidification is apparent. Furthermore, in the figure 10, after using a larger mag-nification, the different sub-grain boundaries are ob-served: where an example of each of migrated grain boundary (MGB), solidification grain boundary (SGB) and solidification sub-grain boundary (SSGB) are marked [22].

Fig. 8 Base metal of ASS 4529

Fig. 9 Weld line and base metal of ASS 4529

Fig. 10 Weld line of ASS 4529

In figure 11, the image of the base metal of the duplex stainless steel is seen. It contains 2 different phases, δ-ferrite and γ-austenite. The dark grey regions are

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ferrite and the white gray regions are austenite. In fig-ure 12, a dramatic change is observed in the micro-structure of the material where larger grains are formed and the ferrite becomes the major phase inside the material. The 2 subsequent phases are marked in fig-ure 13.The increase in the δ-ferrite phase can also be observed in other sources during autogenous welding of other duplex stainless steels [23, 24]. The distur-bance of the ferrite/ austenite phase balance in the weld metal might be remedied via the use of welding consumables having a ‘more austenitic’ composition, and/or the use of a shielding gas containing an appro-priate amount of N2, which is an austenite-promoting element [25]. However, there is a higher probability of forming intermetallic precipitates and nitrides in the weld metal, both of which would decrease the corro-sion resistance [26, 27].

Fig. 11 Base metal of DSS 4501

Fig. 12 Weld line and base metal of DSS 4501

Fig. 13 Weld line of DSS 4501

Hardness testing The hardness measurements in fig. 14 and 15 showed that the hardness increases towards the weld line until it is at its maximum at the centre of the weld. The phenomenon may be explained by a reason for each material. The first reason might be due to the grain refinement and the formation of smaller grains, which means more grain boundaries. These grain boundaries provide more strength to the material which causes an increase of the hardness. This reason would explain the austenitic stainless steel alloy 4529 as it experienced grain refinement in the weld. How-ever, it would not explain the very large hardness in-crease inside the 4501 duplex stainless steel, because larger grains were formed inside the duplex stainless steel.

-10 -5 0 5 10240

260

280

300

320

340

360

380

400

420

Har

dne

ss (

HV

300)

Distance from centre (mm)

4501 6 mm 4501 8 mm 4501 10 mm

Fig. 14 Hardness measurements of DSS 4501

-10 -5 0 5 10180

190

200

210

220

230

240

250

260

Har

dnes

s (H

V 300)

Distance from the centre (mm)

4529 6 mm 4529 8 mm 4529 10 mm

Fig. 15 Hardness measurements of ASS 4529

The reason in the 4501 DSS is the increase of the levels of δ-ferrite inside the weld line. The δ-ferrite phase has a higher hardness than the austenite phase, so if the percentage of the ferrite increases inside the weld, the hardness would also increase. [28,29]. Since, a very large increase in the ferrite is observed, where the primary phase becomes the ferrite phase, so a very large increase in the hardness is observed reaching as much as 54% more than the original hardness (Fig. 14). Another interesting phenomenon that was observed was, as the thickness of the material used increased, the overall hardness of the material decreases. This

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was observed in all three materials. Tensile testing of the materials Austenitic stainless steel ASS 4529 (a) 6 mm thickness Table 4 Tensile strength of ASS 4529, 6 mm

Material type σmax Welded 1 279.86 Welded 2 252.09 Welded 3 689.88 Welded 4 298.84 Welded 5 89.54 Welded 6 305.9

UW1 773.24 UW2 760.84

The average strength of each weld was: σmax,welded= 319.35 MPa σmax,unwelded= 767.04 MPa One of the results obtained were very poor (89.54), also the majority of the other results might not seem very impressive. However, one of the results (689.88) is very desirable. (b) 8 mm thickness Tab. 5 Tensile strength of ASS 4529, 8 mm

Material type σmax Welded 1 628 Welded 2 596.95 Welded 3 601.74 Welded 4 593.26 Welded 5 601.1

UW1 693.47 UW2 700.23

The average strength of each weld was: σmax,welded= 604.21 MPa σmax,unwelded= 696.85 MPa The results from these samples were very good, pro-viding us with results that were better than the other specimens. All the welds that were performed had very high tensile strengths that were not very far from the tensile strength of the unwelded specimens. (c) 10 mm thickness Table 6 Tensile strength of ASS 4529, 10 mm

Material type σmax Linear 1 351.31 Linear 2 422.8 Linear 3 635.78 Linear 4 341.08 Linear 5 299.49 Linear 6 342.9

UW1 726.25 UW2 736.09

The average strength of each weld was: σmax,welded= 398.89 MPa σmax,unwelded= 731.17 MPa Only one of the welds performed well (635.78 MPa). The rest of the results were not outstanding. However, they were similar to each other.

Duplex stainless steel DSS 4501 (a) 6 mm thickness Table 7 Tensile strength of DSS 4501, 6mm

Material type σmax Weld 1 279.77 Weld 2 306.58 Weld 3 795.19 Weld 4 693.38 Weld 5 454.22 UW1 840.06 UW2 837.35

The average strength of each weld was: σmax,welded= 505.828 MPa σmax,unwelded= 838.71 MPa We got 2 very good breaking points one of them ap-proaching the tensile strength of the unwelded speci-mens (795.2 & 693.4 MPa). The rest were mediocre results. (b) 8 mm thickness Table 8 Tensile strength of DSS 4501, 8 mm

Material type σmax Linear 1 302.05 Linear 2 251.17 Linear 3 85.4 Linear 4 151.5 Linear 5 404 Linear 6 758.8

UW1 844.73 UW2 842.72

The average strength of each weld was: σmax,welded= 325.49 MPa σmax,unwelded= 843.73 MPa Only one of the above results stands out (758.8 MPa) as an excellent results, the rest were not very convinc-ing, with two of them giving us very poor numbers (85.4 and 151.5 MPa), the other 3 (251.2, 404 & 302.5 MPa) were mediocre. (c) 10 mm thickness Table 9 Tensile strength of DSS 4501, 10 mm

Material type σmax Weld 1 574.11 Weld 2 764.97 Weld 3 227.39 Weld 4 755.38 Weld 5 702.9 Weld 6 299.7

σmax,average= 554.08 MPa The 4501 unwelded specimens were not tested be-cause the force required to reach the tensile strength of the welded specimens exceeds the maximum force of the machine itself. However, by utilizing the tensile strength of the unwelded specimens of the 6 mm and 8 mm thicknesses, and comparing them with the above data, we find that except for the 2 poor numbers ob-tained (227.4 and 299.7), we got one mediocre number (574.11 MPa) and 3 excellent numbers (702.9,764.97 and 755.38 MPa). Tensile testing of all the materials showed that even though some of the welds were performed very well, which gave us strengths that were close to the original

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material, other specimens did not perform very well. This was due to a variety of reasons. The main reason would be the weld defects found inside the weld lines; the pores & cracks would create stress concentrations that would cause an increase in the cracking and would substantially weaken the materials, causing the materials to break at a point greatly below the original material. Furthermore, the incomplete penetration weakens the material completely as the welding area becomes less than the original specimen size. Upon visual inspection of the tested materials, the materials that performed best were the materials that had the fewest imperfections. Therefore, to decide which materials had the more precise welding condi-tions, we see which specimens had the highest aver-age ultimate strength. The materials that performed the best were the 8 mm 4529 austenitic stainless steel and the 10 mm 4501 duplex stainless steel. Even though the data was not perfect, the welding conditions seem to be the more precise. The thickness did not seem to have significant effect on the tensile strength. How-ever, if we overcome the defects perhaps the differ-ences may become more apparent. Electrochemical testing (a) Austenitic stainless steel ASS 4529

0 500 1000 1500 2000 2500 3000 3500-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

Unwelded

Welded

Pot

ent

ial v

s. S

CE

(V

)

Time (s) Fig. 16 OCP of ASS 4529 The open-circuit potential (OCP) gives us information about the dynamic behaviour of the passive oxide layer film, whereas the more positive the value of the OCP, the more noble the specimen becomes [30]. As seen from Fig. 16, the OCP in 3.5% NaCl of the laser-welded is more positive than the unwelded one. This means that the welded material has a higher passivity than the unwelded material. From Fig. 17, there is no pitting was observed in both the materials (welded and unwelded). Even upon vis-ual inspection, the surface was still smooth without pitting; this is due to the high molybdenum content inside the material which would cause the material to require more rigorous conditions to initiate pitting. However, even though there is no pitting initiated, we can assume that the welded specimen is more suscep-tible to pitting corrosion because it is observed that the current density in the material is higher at each voltage [30].

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-0.00010

-0.00008

-0.00006

-0.00004

-0.00002

0.00000

0.00002Welded

Unwelded

Cur

ren

t de

nsity

(A

/cm

2 )

Potential vs. SCE (V)

Fig. 17 PCV of ASS 4529 The calculated corrosion rates from Tafel's method are summarized in Table 10. Table 10 Corrosion rates (nm/Y) of ASS 4529

Condition Corrosion rate (nm/Y) Unwelded 680.3 Welded 792.6

The results obtained in table 10, show that regardless the condition, this material has extremely corrosion resistant to the 3.5% NaCl solution due to the very low corrosion rate. However, it can also be observed that in the welded condition, this material has a higher cor-rosion rate than the unwelded condition. (b) Duplex stainless steel DSS 4501

0 500 1000 1500 2000 2500 3000 3500-0.70-0.65-0.60-0.55-0.50-0.45-0.40-0.35-0.30-0.25-0.20-0.15-0.10-0.050.000.050.100.150.200.250.30

Unwelded

Welded

Po

tent

ial v

s. S

CE

(V

)

Time (s)

Fig. 18 OCP of DSS 4501 In Fig. 18, it is clear that the laser-welded material has higher OCP values than the unwelded material. This means that the welded material has a more passive behaviour in 3.5% NaCl.

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-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6-0.002

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

Unwelded

Welded

Cu

rren

t d

ensi

ty (A

/cm

2 )

Potential vs. SCE (V) Fig. 19 PCV of DSS 4501 Once again there is no pitting observed for DSS 4501 (Fig. 19). This is also due to the very high molybdenum content. However, the higher current density in the welded material suggests that it would be more sus-ceptible to pitting corrosion compared to the unwelded material. Table 11 Corrosion rates (µm/Year) of DSS 4501

Condition Corrosion rate Unwelded 17.06 µm/Year Welded 3.663 mm/Year

As clear in Table 1, the corrosion rate of the un-welded stainless steel is much lower than of the welded stainless steel. In the duplex 4501 stainless steel and the austenitic 4529 stainless steel, it is observed that the welded materials have a higher passivity than the unwelded materials inside the NaCl solution The change in corrosion behavior due to laser weld-ing could arise from different causes depending on the material. It could be due to the harmful effect of δ-ferrite on the pitting corrosion resistance or due to the galvanic effect existing between γ-austenite and δ-ferrite [31]. In fusion welding, solidification from the melt pool in general results in local compositional variations, which would in turn result in less stable passive film and hence lower corrosion resistance [32]. The compositional heterogeneity in the weld metal could arise from three main causes: microsegregation during weld metal solidification, element partition in solid-state transformation from ferrite to austenite, and precipitation of intermetallic phases, carbides and ni-trides, leading to the formation of Cr-depleted regions [33, 34]. In ASS 4529 the corrosion rate is seen to be incredi-bly low, this is because the material is successfully designed to withstand harsh conditions. However, it can still be seen that the welded material has a slightly higher corrosion rate than the unwelded material. After welding, the welded material still withstands the solu-tion quite well. As for the pitting, no pitting was observed for both materials, this might be due to the high Mo content in both materials. However, the higher current densities in the welded material suggest that at later voltages, the welded material is more susceptible to pitting cor-

rosion. In the 4501 duplex stainless steel a large difference between the corrosion rate of the welded and un-welded material is observed, where the unwelded ma-terial has a corrosion rate of 17.06 µm/Year, while the welded material has a corrosion rate of 3.663 mm/Year. The difference is astonishingly large. The impairment in the corrosion resistance of the laser welds might be attributed to microsegregation, and also to unfavorable ferrite/austenite phase content [35]; as it is observed that in the duplex stainless steel, the primary phase becomes the ferrite phase as observed in Figure 13, this might cause the large increase in the corrosion rate of the material. 4. Conclusion

1. Optical microscopy reveals that ASS 4529 under-goes type A solidification and experience grain re-finement. DSS 4501 experiences grain growth and it is observed that the ferrite becomes the primary phase.

2. It is observed that there are many weld defects that need to be overcome to provide a very sound weld, the defects include: cracking, porosity, incomplete penetration and incomplete fusion in the weld walls.

3. Hardness tests reveal that the hardness in the ma-terial increases from the base metal to the centre of the weld, reaching the maximum at the centre. The hardness increase in the duplex stainless steel was very high. It was also revealed that as the material thickness increases, the maximum hardness de-creases.

4. Tensile testing revealed that the best specimens welded were ASS 8 mm and DSS 10 mm No con-nection could be found between the thickness of the material and the tensile strength.

5. Passivity is higher in welded materials of DSS 4501 and ASS 4529 stainless steels.

6. It is a general trend that the welded materials have a higher corrosion rate than the unwelded materi-als.

7. No pitting could be observed in ASS 4529 and DSS 4501 stainless steels. The welded material is more susceptible to pitting than the unwelded material.

8. Further work is required to improve the welding procedure.

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