Laser welding of dissimilar materials

Laser welding of dissimilar materials

Available online at www.sciencedirect.com ScienceDirect Materials Today: Proceedings 19 (2019) 1066–1072 www.materialstoday.com/proceedings BraMat ...

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Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 19 (2019) 1066–1072

www.materialstoday.com/proceedings

BraMat 2019

Laser welding of dissimilar materials Elena Manuela Stanciu, Alexandru Pascu*, Ionut Claudiu Roata, Catalin Croitoru, Mircea Horia Tierean Transilvania University of Brasov, Materials Engineering and Welding Department, 29 Eroilor Blvd., 500036, Brasov, Romania

Abstract This study addresses to the laser welding with active flux (SiO2 + Poly (vinyl alcohol)) for improving the geometrical profile and microstructure of the dissimilar carbon steel – stainless steel joints. The experimental tests were carried out using a TRUMPH 556 pulsed laser and a Precitec YW50 welding head manipulated by a robotic arm. AISI 321 and S235 plates with the dimensions of 50X100X0.5 mm were used as welding specimens. Several fusion lines were made in order to determine the optimal SiO2 / Poly mixing ratio and to highlight the influence of the SiO2 flux on the microstructure. Addition of the active flux results in a more stable welding process and with less plasma plume and a narrower weld bead profile. The samples have been analyzed by optical and electron microscopy showing that SiO2 addition lead to formation of mixed morphology of lathy and skeletal ferrite in the weld zone. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 11th International Conference on Materials Science & Engineering, BraMat 2019 Keywords: laser welding; stainless steel; dissimilar; SiO2.

1. Introduction Over the past years, the laser welding technology, besides the cutting, cladding or the laser marking processes became a standard in joining of metallic materials. The laser technology presents several clear advantages such us high speed, precision, low HAZ and the non-contact feature compared with the conventional welding techniques.

* Corresponding author. Tel.: +4 0722723154 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 11th International Conference on Materials Science & Engineering, BraMat 2019

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Nowadays, the laser welding technology can be successfully used for automatized welding of different types of steel and nonferrous alloys in a similar of dissimilar joint configurations [1-3]. Dissimilar welding become a new tool for engineers to design components or structures that require joining of dissimilar alloys or materials. Nowadays, the laser welding can be used for joining totally different materials (steels with polymers [4, 5]) or ferrous with non-ferrous alloys (ie. pure copper to stainless steel 6, 7]) alloys with similar morphologies and melting temperatures like carbon steel to stainless steel [8-10]. The dissimilar welding of carbon steel with stainless steel, also known as black and white joint [8], benefits from all the advantages offered by the laser technology being successfully used in various industrial applications. This type of heterogenous welding are investigated and reported in the literature by Naffakh [12], Khan [13], Mishra [14] revealing good results in term of mechanical proprieties of dissimilar joints. M.P. Prabakaran et al. [15] optimize the process parameters for stainless steel AISI316/AISI1018 low carbon steel laser welding. Improved tensile strength was attended after a post weld heat treatment at 860 °C and 960 °C due to the elimination of chromium carbide. Zhou et al. [16] investigate the weld bead microstructure of nickel-based superalloy (C-276) and austenitic stainless steel (304) laser-welded joint and reveals that element distribution is inhomogeneous in case of dissimilar materials due to the rapid cooling of the molten pool. The weld bead is characterized by insula and peninsula distribution of different elements and morphologies. The stainless steel – carbon steel joints are now used in various fields of industry and is still the subject of ongoing research projects. The laser is the key technology for obtaining high quality heterogenous welding. Even so, a major concern is this research direction is the low penetration depth due to the limited power of laser generators. This study aims to improve the welding bead profile by adding an active flux with the role of increase the laser absorption and to promote the formation ferrite during the solidification of the weld bead. 2. Experimental 2.1. Materials The experimental tests have been conducted on 50x100x0.5 plain sheets of carbon steel (S235) and stainless steel AISI 321. The chemical composition of both materials is presented in the table 1. Table 1. Chemical composition of the carbon steel and stainless steel. Material

Element wt. (%) C%

Si %

Mn %

P%

S%

Cr %

Ni %

Mo %

Ti %

S235

0.099

0.168

0.51

0.0033

0.0069

0.043

0.041

0.0024

-

AISI 321

0.08

1

2

0.045

0.03

18

11

-

0.15

The active flux was prepared by mixing SiO2 with Polyvinyl alcohol in a ratio of 15 %. The side of each plate was immersed in the active flux and then air dried at room temperature for 2 hours. 2.2. Experimental set-up The experimental welding tests were carried out using a pulsed laser, namely TRUMPH TruePulse 556 and a welding module PRECITEC YW 50 with a focal distance of 200 mm. Positioning of the welding had was accomplished by a robotic cell composed from CLOOS 7 axes robot and two axes positioning table. A special designed clamping device was used for positioning and fastening of the test plates in a butt joint geometry. Argon with purity of 99.99 % was used for shielding the top and the root of the weld bead. The weld bead root was shielded trough a channel machined into the clamping device as illustrated in figure 1a. Prior the welding tests, several fusion lines have been produced on a 1.5 mm AISI 321 stainless steel in order to determine the

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influence of the SiO2 mixing ratio and the process parameters domain. The preliminary tests show that laser power can be varied between 2000 and 2500 W at a fixed frequency of 150 Hz.

Fig. 1. Set-up used for the experimental tests, a) fastening device for welding of carbon steel - stainless steel, b) stainless steel plate with 15 % SiO2 and 30% SiO2 active fluxes.

The parameters summarized in the table 2 have been chosen for the final tests in order to emphasize the influence of the active flux. Increasing of the welding speed was necessary to not over-melt the weld bead. Table 2. Laser welding parameters

PARAMETERS

UNITS

Laser power Frequency Pulse duration Welding speed

W Hz ms cm/min

SAMPLE 1 – without active flux

2 – with SiO2 active flux

2300 150 1 73

2000 150 1 90

The samples have been cut using a hydraulic machine and prepared for investigation by polishing and electrochemical etching in 10% oxalic acid aqueous solution. The specimens have been analyzed using a LEICA DM ILM LED inverted optical microscope and a SEM: Quanta FEG 250, FEI (The Netherlands) at 30 kV. 3. Results and discussion The SiO2 is an oxidizing type flux and is acceptor of free oxide ions, O2-. The activator flux produces increased levels of oxygen and silicon into the weld bead. Certain amount of oxygen will form oxide particle that can promote the formation of acicular ferrite with benefits on welding toughness [17] but in the same time too much oxygen presents numerous down backs onto the mechanical proprieties and cracking susceptibility of the weld bead. Similar, too much silicon into the weld bead can harden the metal matrix lowering the toughness of the welded joint. Due to the high temperature obtained during laser welding the SiO2 will decompose as [17]:

E.M. Stanciu et al. / Materials Today: Proceedings 19 (2019) 1066–1072 ଵ

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(1)

Two mixing ratios of SiO2 / polyvinyl alcohol (15% and 30% wt SiO2) have been used to determine the optimal composition of the activator flux. The aim is to increase the penetration depth but not to oversaturate the melted bath with oxidizing elements. Also, the stability of the welding process and the plasma plume is a key factor that is influence the quality of the welding. Therefore, preliminary tests have been realized on 1.5 mm stainless steel plate. The plate was covered with the 15% and 30 % SiO2 active flux and then five fusion lines were made to determinate the influence of the active flux, as illustrated in figure 2. The fusion lines were made with fixed speed (80 cm/min), pulse duration (1 ms) and frequency (120 Hz) and increased power from 1500 to 3500 W in 500 W increment (line 1 - 1500 W, line 2 - 2000 W, line 3 - 2500 W, line 4 - 3000 W and line 5 – 3500 W). Lines/samples 1 and 2 will not be further discussed due to low melting/no melting of the surface.

Fig. 2. Fusion lines realized on stainless steel plate with different content of the SiO2 flux.

Fig. 3. Cross-section of the fusion lines realized with 15% and 30% SiO2 flux (3, 4 and 5 reference sample without flux, sample 3-15, 4-14 and 5-15 with 15% SiO2, sample 3-30, 4-30 and 5-30 with 30% SiO2).

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It can be clearly seen in figure 3 that active flux influences the geometry profile and the microstructure of the melt area. The active flux improves the laser absorption and create a more key-hole profile of the melt zone, even if the melt depth is smaller compared with the reference samples. In the same time, the SiO2 modify the solidification pattern and promote the formation of ferrite phase (fig. 4b), vermicular and lathy ferrite, instead of austenite phase. The fusion zone of the reference sample (fig. 4a) is characterized by an austenitic morphology with austenite grains growth from the unmelted austenite grains of the substrate. The growth direction is aligned with the thermal gradient produced by the laser. The microstructure of the fusion zones obtained with SiO2 addition exhibits a mixed morphology of lathy and skeletal ferrite, visible as darker structures in figure 4b. An acicular ferrite is desirable in case of stainless-steel welding (similar or dissimilar configuration) due to the improvement of toughness. Analyzing the data from figure 3 resulted that a higher content of SiO2 can reduce the melt depth of the fusion line. Therefore, the active flux with 15 % SiO2 can provide better results in term of weld depth and microstructure formation.

Fig. 4. Image of the reference fusion line (sample 5) and the fusion line with 15% SiO2 addition (sample 5-15).

Analyzing the data from preliminary tests have been determined the optimal parameters for dissimilar welding of stainless-steel carbon steel as presented in table 2. Figure 5 show the laser weld joints made with and without the active flux. The weld bead and heat affected zone is smaller in case of the sample welded with 15 % SiO2 active flux.

Fig. 5. Image of the dissimilar laser welding with and without the SiO2 flux; SS-stainless steel, CS-carbon steel.

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Fig. 6. Macro image of the weld bead cross-section; a) reference sample, b) sample obtained with the addition of SiO2 flux.

A complete penetration of thin sheets butt welds is required for obtaining the best tensile behavior. As shown in figure 6, full penetration has been obtained in case of CS-SS laser welding with and without the addition of SiO2. The analyses of the geometrical profile of the weld bead shows that SiO2 flux decrease the heat affected zone of the weld joint on the stainless steel - carbon steel side. Moreover, by using the 15% SiO2 flux a decreasing of the laser power with almost 18 % is possible. Full penetration welding with narrower with and smaller heat affected zone was obtained in case of SiO2 active flux addition.

Fig. 7. Macro image of the weld bead sample with the addition of SiO2 flux.

In Figure 7b one can observe the distinct darker regions that indicate a large amount of ferrite transferred from the carbon steel, besides whiter areas with a high content of nickel-chromium. The ferrite branches enveloped by the austenite matrix are presented in the high magnification SEM from figure 8. The microstructure changes from fine equiaxed ferrite in the middle of the weld bead to an oriented skeletal ferrite morphology at the interface with the carbon steel.

Fig. 8. Electron microscopy of the weld bead near the stainless steel side, with (a) and without SiO2 (b).

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4. Conclusions The effects of the SiO2 active flux during the laser processing of stainless steel and dissimilar welding of stainless steel – carbon steel have been studied in this research. Dissimilar thin sheets of 0.5 mm thickness comprising of stainless steel and carbon steel could be successfully welded by laser and an active flux. A reduction with almost 18% of the laser power can be achieved by using the 15% SiO2 active flux for dissimilar laser welding. The results show that using a proper ratio between laser power and welding speed ensure a pore and crack free dissimilar welding of carbon steel and welding steel. The SS-CS weld bead microstructure is composed of strong non-homogeneity islands rich in vermicular and lathy ferrite alternating with areas of mixed austenitic-ferrite microstructures. The microstructure of the fusion zone is characterized by lathy ferrite enveloped by the austenite matrix on the stainless-steel side and by a large area of undiluted ferrite near the interface with the carbon steel. Acknowledgements We hereby acknowledge the structural founds project PRO-DD (POS-CCE, O.2.2.1., ID 123, SMIS 2637, ctr. No 11/2009) for providing the infrastructure used in this work. The characterization of the samples was supported by the Transilvania University of Brasov scholarship for international mobilities. References [1] F. Khodabakhshi, L.H. Shah, A.P. Gerlich, Optics & Laser Technology 112 (2019) 349-362. [2] S. Gao, Yuntao Li, L. Yang, W. Qiu, Materials Science and Engineering: A 720 (2018) 117-129. [3] Y. Ai, X. Shao, P. Jiang, P. Li, Y. Liu, W. Liu, Optics and Lasers in Engineering 86 (2016) 62-74. [4] J.P. Bergmann, M. Stambke, Phys. Procedia 39 (2012) 84-9. [5] A. Roesner, S. Scheik, A. Olowinsky, A. Gillner, U. Reisgen, M. Schleser, Phys. Procedia 12 (2) (2011) 373-380. [6] Y. Meng, X. Li, M. Gao, X. Zeng, Optics & Laser Technology 111 (2019) 140-145. [7] A. Mannucci, I. Tomashchuk, V. Vignal, P. Sallamand, M. Duband, Procedia CIRP 74 (2018) 450-455. [8] A. Pascu, E.M Stanciu, I. Voiculescu, M.H. Tierean, I.C. Roata, J.L. Ocana, Materials and Manufacturing Processes 31 (3) (2016) 311–318. [9] E.M. Stanciu, A. Pascu, M.H. Tierean, I.C. Roata, I. Voiculescu, I. Hulka, C. Croitoru, Technical Gazette 25 (2) (2018) 344-349. [10] E.M. Stanciu, A. Pascu, I.C. Roata, Advanced Materials Research 1029 (2014) 134-139. [11] D. Iordachescu, E. Scutelnicu, M. Iordachescu, A. Valiente, J. R. Hervias, J. L. Ocaña, Welding in the World 55 (2011) 2-11. [12] H. Naffakh, M. Shamanian, F. Ashrafizadeh, Journal of Materials Processing Technology 209 (2009) 3628-3639. [13] M.M.A. Khan, L. Romoli, M. Fiasch, G. Dini, F. Sarri, Journal of Materials Processing Technology 4 (2012) 856-867. [14] D. Mishra, M.K. Vignesh, B. Ganesh Raj, P. Srungavarapu, K.D. Ramkumar, N. Arivazhagan, S. Narayanan, Procedia Engineering 75 (2014) 24-28. [15] M.P. Prabakarana, G.R. Kannanb, Optics and Laser Technology 112 (2019) 314–322. [16] S. Zhou, D. Chai, J. Yu, G. Ma, D. Wu, Journal of Manufacturing Processes 25 (2017) 220-226. [17] Sindo Kou, Welding Metallurgy Second Edition, John Wiley & Sons, Inc., Hoboken, New Jersey (2003).