Effect of active flux addition on laser welding of austenitic stainless steel

Effect of active flux addition on laser welding of austenitic stainless steel R. Kaul*1, P. Ganesh1, N. Singh2, R. Jagdheesh1, M. S. Bhagat1, H. Kumar1, P. Tiwari3, H. S. Vora2 and A. K. Nath1 The use of active flux in tungsten inert gas (TIG) welding is known to increase its weld depth. The present paper involves study of active flux laser beam welding (ALBW) of austenitic stainless steel sheets with respect to its effect on plasma plume, microstructure and mechanical properties of the resultant weldments. ALBW performed with SiO2 as the flux significantly modified shape of the fusion zone (FZ) to produce narrower and deeper welds. Plasma plume associated with the process was considerably smaller and of lower intensity than that produced during bead on plate laser beam welding (LBW). Flux addition during LBW produced thin and rough weld bead associated with humping. The development of such a weld bead is cause by reversal in the direction of Marangoni flow by oxygen induced inversion of surface tension gradient, widely fluctuating plasma plume and presence of oxides on the weld pool surface preventing free flow of the melt. Active flux laser weldments exhibited lower ductility than that of bead on plate laser weldments. Keywords: Laser welding, Active flux, Stainless steel, Plasma, Spectroscopy, Marangoni flow, Humping

Introduction It is know that addition of small amount of active flux during tungsten inert gas (TIG) welding can significantly increase its depth of penetration. The modified process is referred to as active flux TIG welding or ‘ATIG’.1–3 Increased depth of ‘ATIG’ welds is attributed to significant arc focusing and reversal of melt convection owing to inversion of surface tension gradient.4 The first effect is based on the effects of vapour from the flux, which usually contains oxygen and fluorine. Such gases are known to attach electrons and the resultant negative ions, formed at the periphery of the arc, have lower mobility than electrons. Consequently, for a given current, it generates higher current density at the centre of the arc resulting in increased weld depth.5–7 SiO2 with fine particle size has been found to be an effective flux for deep penetration TIG welding of austenitic stainless steel and aluminium alloys.7,8 Besides flux, small additions of oxygen or carbon dioxide in Ar shielding gas also significantly modify weld shape from shallow wide shape to a deep narrow weld shape.2,9 Carbon dioxide can be used for both plasma suppression and shielding in keyhole laser beam welding (LBW) of low carbon and alloy steels. For modest penetrations, carbon dioxide can sometimes yield a more favourable bead profile, particularly if small amount of helium is 1

Industrial CO2 Laser Section, 2Laser Systems Engineering Division Synchrotron Utilization & Materials Science Division, Raja Ramanna Centre for Advanced Technology, PO CAT, Indore 452 013 India 3

*Corresponding author, email [email protected]

ß 2007 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 18 July 2006; accepted 7 September 2006 DOI 10.1179/174329307X159793

added.10 Addition of few per cent of oxygen with helium shroud gas has been found useful in reducing electron density of the plasma plume generated during LBW of aluminum.11 It has been reported that by using active gas mixtures (argon with oxygen and/or carbon dioxide), in place of conventional shielding gases (argon, helium, nitrogen), power limits of a 1 kW Nd:YAG LBW unit can be extended, in terms of greater weld depth and faster welding speed.12 Naito et al. found that the penetration and the shape of Nd:YAG laser welds are influenced by volume of oxygen in the ambient atmosphere and pronounced ‘nail head’, obtained while welding with argon, disappeared when LBW is performed in an atmosphere of higher oxygen content, as well as in air.13 In spite of large literature available on ATIG welding, very little work has been reported till date on the effect of active flux addition during LBW. Heiple et al. obtained .200% increase in depth/width ratio of defocused laser weld of stainless steel by adding 66 ppm of surface active element selenium in the weld pool.14 Ding et al. reported that both surface activating flux and surface active element S produce fantastic effects on the YAG laser weld shape in terms of increased weld penetration and depth/width ratio.15 Cretteur et al. developed flux paste for CO2 LBW of 6061 aluminium alloys.16 Su et al. reported that CO2 LBW of AISI 304 stainless steel sheets with active flux resulted in increased weld depth through decrease in electron density of the plasma plume. In contrast, active flux had little effect on the depth of Nd:YAG laser welds.17 The work, however, did not address effect of flux addition on the microstructure and mechanical

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properties of the resultant weldments. The present work has been undertaken with the objective of studying active flux laser beam welding (ALBW) of AISI 304 stainless steel and to compare microstructural and mechanical properties of resultant weldments with those made without flux. It also involved study of flux induced changes in the characteristics of plasma plume.

Background In deep penetration keyhole LBW, incident laser power density is high enough to cause evaporation and even ionisation of the material to produce plasma. The high vapour pressure of the evaporated material pushes molten material downwards, thus producing deeper welds. Metallic plasma coming out of the keyhole causes ionisation of the shroud gas. Plasma, owing to large number of free electrons, effectively absorbs laser beam (LB). In addition, plasma also affects considerable defocusing of the incident LB.18 During LBW, plasma plume fluctuates in intensity and in height over the substrate being welded which adds to keyhole instability thereby inducing fluctuations in weld depth and entrapment of porosities in the fusion zone (FZ). Helium, because of its high ionisation potential and resistance to breakdown, is preferred over argon as the shroud gas to suppress plasma and to produce enhanced uniformity in weld depth and weld bead profile.19 The convection in laser weld pool is largely controlled by surface tension forces. The direction of convective (or Marangoni) flow in the weld pool is governed by the sign of surface tension gradient dc/dT (usually ,0). Higher surface tension of cooler melt towards fusion boundary (with respect to centre of the weld pool) drives outward flow of the melt. However, a positive value of dc/dt reverses melt flow to produce narrower and deeper welds. Surface active agents such as O, S, Se and Te modify dc/dT at the weld pool surface.20 S of 0.05 wt-% in steel completely inverts dc/dT.21 The concentration of S and O needed to invert dc/dt has been found to be within the range of what many specifications tolerate.22 Too high or too low O content in the weld pool does not increase depth/width ratio of TIG weld. Presence of O over a critical value (70 wt-% ppm) alters dc/dT on the weld pool surface and hence changes Marangoni flow. However, at higher O content (.6000 vol.-% ppm), a thick oxide layer is formed on the pool surface that retards oxygen conveyance to the weld pool and prevents free melt flow.2

Effect of active flux addition on laser welding of austenitic stainless steel

expensive ZnSe lens from possible spatter from the substrate being welded. Before taking LBW runs, power of unfocused LB was measured with a hand held laser power meter immediately ahead of a 45u mirror.

Specimen preparation and LBW Laser welding experiments were performed on sheets (dimensions: 100675 mm) of AISI 304. The chemical compositions of stainless steel sheets with 2.5 and 6 mm in thickness were Fe–0.07C–19.0Cr–8.00Ni–1.2Mn–0.4Si and Fe–0.07C–18.4Cr–8.00Ni–1.3Mn–0.8Si–0.06Mo respectively. Before LBW, surfaces of AISI 304 specimens were ground with 180 grit emery papers and cleaned with acetone. Cleaned specimens were partly flux coated so that the effect of flux addition could be obtained in the same weld bead while avoiding possible variation in laser power during LBW made with and without flux. Flux coatings were manually applied in the form of a paste made with polyvinyl alcohol (PVA) and water (the method is frequently adopted for effecting laser surface alloying24). Flux coated specimens were baked in an oven at 423 K for 3 h to drive out moisture and volatile constituents, if any, from the coating. The amount of flux coated on stainless steel sheets was 18–22 mg cm22 while coating thickness was typically 1 mm. Bead-onplate LBW involved scanning the surface of the substrate with a focused LB, while using argon as the shroud gas. Since the binder PVA boils at a low temperature of 501 K (Ref. 25) and rapidly decomposes above 523 K (Ref. 26), the binder and its decomposition products are likely to leave the surface of the substrate well before initiation of surface melting. Hence, presence of PVA in the flux coating is unlikely to affect the process of LBW. For determining effect of flux addition on partial penetration conduction limited weld and partial and full penetration welds involving keyhole formation, a preliminary laser welding exercise was performed to identify process parameters to obtain desired welds. In order to record flux induced change in weld depth, if any, particular emphasis was placed in obtaining partial penetration laser welds while full penetration laser welds were required for preparing specimens for mechanical testing. Experimental parameters identified for the main experiments were that the laser power is 1.1–3.5 kW and the welding speed is 10– 20 mm s21. In the rest of the present paper bead-onplate ‘laser welded’ and ‘active flux laser welded’ specimens are referred as ‘LW’ and ‘ALW’ ‘respectively’.

Techniques used for characterisation

Experimental Experimental set-up Laser welding experiments were performed with an indigenously developed 10 kW continuous wave CO2 laser.23 Laser welding experimental set-up consisted of laser system, integrated with a beam delivery system and a computer controlled 3 axis workstation. Laser beam, emanating out of the laser system, was folded with a 45u plane gold coated copper mirror and subsequently focused with a 127 mm focal length planoconvex ZnSe lens, housed in a water cooled copper nozzle. Bead-onplate LBW involved scanning the surface of AISI 304 substrate with a focused multimode LB. The working distance between the nozzle and the substrate at the focal plane was 14 mm. During the course of LBW, argon gas was flown through the nozzle for protecting

Laser welded specimens were characterised by optical macroscopy, microstructural examination, microhardness measurement, guided bend tests and tensile tests. Laser produced plasma was characterised by real time plasma monitoring and spectroscopy. Plasma observation involved its imaging on a 12’’ colour charge coupled device (CCD) camera and captured images were transferred and stored in a computer using a frame grabber card. On the other hand, plasma plume emission spectra were recorded online with a compact spectrograph ‘USB 2000’ from M/s. Ocean Optics. The instrument consisted of a spectrometer complete with 2048 element silicon CCD array. It had 600 lines/mm holographic grating positioned such that CCD array observed a wavelength range 178–877 nm. The CCD detector had a minimum integration time of 1 ms. For


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1 Images (CCD) of plasma plume during laser welding of partly flux coated stainless steel sheet

plasma spectroscopy, plume emissions were coupled to the input slit of the spectrometer through an optical fibre and the recorded spectra were analysed with an indigenously developed software Promise.27,28 The emitted plasma spectra were captured at regular intervals and at the end of the process captured spectra are stored in the dynamically allocated memory of the computer as an image named as a composite picture. In this resultant composite image, referred as time integrated spectral band (TISB) in the following of the present paper, each spectrum is stored as an image line.

Results Online plasma monitoring An important observation of LBW of partly flux coated AISI 304 sheets was an abrupt drop in intensity and size of plasma plume as focused LB passed from bare to flux coated surface. On the bare surface, large intense white plasma plume core was surrounded by diffused bluish and orange coloured radiation. Plasma plume always remained in contact with the surface being welded. On the other hand, ALBW was marked with significantly reduced size and intensity of white plasma core with more dominant orange background radiation. During ALBW, plume intensity widely fluctuated and at times central plasma core almost disappeared leaving behind orange background radiation. Figure 1 presents images of plasma plume during LBW of a 2.5 mm thick partly flux coated Stainless Steel (SS) specimen.

Macroscopic examination Macroscopic examination of laser weldments showed that uniform and smooth weld bead on bare surface transformed into a rougher bead associated with humping on the flux coated surface. In contrast to LBW, ALBW produced thinner bead with longitudinal variation in weld bead width, as shown in Fig. 2.

Metallographic examination ALBW significantly modified shape of the FZ. These welds were narrower and deeper than their LW counterparts. The effect of flux addition was more apparent in effective reduction in weld bead width rather than increase in weld depth. Flux induced change in the FZ was more marked in welds involving keyhole formation, although conduction limited laser welds also carried signatures of flux addition on its shape (Fig. 3a). The most apparent difference in the case of deeper ALW welds was transformation of a typical ‘wine glass’ shaped FZs into a more uniformly tapered ‘V-shaped’ FZs, as shown in Fig. 3b and c. In the 6 mm thick sheet,

FZ in ALW specimens gradually changed from ‘V’ to ‘U’ shaped with reduction in welding speed at constant laser power (Figs. 3c and 4). Figure 5 shows increase in weld depth with flux addition on the longitudinal crosssection of a laser weld in partly flux coated AISI 304 sheet. The details of experimental laser welds, as presented in Table 1, includes: (i) dimensions of FZs along with associated aspect ratios (weld depth/weld width) (ii) relative laser absorption parameter, A is melted volume per unit weld length (mm3)/incident laser energy per unit length (kJ) (iii) incident laser energy per unit welded area E – inverse of welding efficiency. As seen from Table 1, weld aspect ratio registered a significant increase from conduction limited to keyhole welds. In most of the experimental laser welds, flux addition is marked with increase in weld aspect ratio. The increase in weld aspect ratio is largely caused by decrease in weld bead width rather than increase in weld penetration. However, in the case of laser welds made in 6 mm thick stainless steel sheets at 3.5 kW laser power and 7 mm s21 welding speed (representing experimental weld with highest laser energy input) flux addition during LBW failed to cause noticeable change in the width and depth of the resultant weld, thereby yielding no change in weld aspect ratio. In 2.5 mm thick stainless steel sheet, conduction limited weld carried significantly lower overall welding efficiency (higher E value) than the welds made in keyhole mode. However, the efficiency of laser welds made near the threshold of key holing effects (partial keyhole weld in 2.5 mm thick sheet) was lower than full penetration weld made with higher incident laser power density. The reason for this observation is attributed to more efficient laser absorption A in the keyhole LBW mode. As compared with laser weld made near the threshold of keyhole welding, full penetration weld made at higher incident laser power density was associated with stronger laser absorption owing to enhanced multireflections from the walls of the keyhole. In 6 mm thick stainless steel sheets, reduction in welding speed was associated with progressive reduction in welding efficiency (increasing value of E). The drop in welding efficiency with increase in laser energy input (caused by decrease in welding speed) should have been brought about either by enhanced blocking/defocusing of incident laser radiation by plasma plume (also reflected in lower values of relative laser absorption parameter A at slow welding speed of 7 mm s21) or decrease in weld aspect ratio at slower welding speed.

2 Macroscopic view of laser weld bead profile on 2.5 mm thick partly flux coated stainless steel sheet


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a 2.5 mm thick, laser power 1.1 kW, welding speed 10 mm s21; b 2.5 mm thick, laser power 2 kW, welding speed 20 mm s21; c 6 mm thick, laser power 3.5 kW, welding speed 17 mm s21 3 Comparison of fusion zones of LW and ALW specimens with different thickness

Except in the case of partial keyhole laser weld in 2.5 mm thick stainless steel sheet, where flux addition caused ,60% increase in laser absorption over that of bead-on-plate LW specimen, the change in average laser absorption caused by flux addition was not very significant. Likewise, laser welding efficiency also remained largely unchanged by flux addition, except in the case of partial keyhole laser weld in 2.5 mm thick stainless steel sheet where ALBW carried ,40% increase in welding efficiency over that of LBW. Metallographic examination of laser weldments did not reveal any cracks in any of the specimens examined, although some isolated porosities were found in both kinds of welds. No noticeable inclusions were noticed in the FZs of ALW specimens. Fusion zones of both LW and ALW specimens exhibited similar cellular/dendritic microstructure with primary ferrite mode of solidification. The degree of microstructural refinement was largely similar, indicating similar cooling rate conditions existing during two kinds of welding. Both kinds of welds carried similar d ferrite contents of 9–10%.

Figure 6 compares FZ microstructures of LW and ALW specimens. Energy dispersive spectroscopic (EDS) analysis performed with scanning electron microscopy (SEM) showed that chemical analysis of FZ remained largely unchanged by flux addition. However, a closer look at EDS spectra showed that CrLa peak for FZ of ALW specimen carried greater eccentric broadening towards lower energy side than those of the substrate and FZ of LW specimen, as shown in Fig. 7. In view of close proximity of CrLa line (0.5729 keV) to OKa line (0.5249 keV), the eccentric broadening of the spectral line could have been caused by increase in O concentration. Estimation of chemical composition, by including O as an alloying element, showed O concentrations (in wt-%) in substrate, FZ of LW and FZ of ALW specimens as 1.55–2.67, 3.12–5.6 and 5.2–10.09 respectively. Although O concentration, so obtained, carried considerable error, the estimated values do indicate that flux addition during LBW resulted in increased O concentration in the FZ with respect to FZ of LW


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a LW; b ALW 4 Comparison of fusion zones of 6 mm thick specimens: laser power is 3.5 kW; welding speed is 7 mm s21

specimen which, in turn, carried higher O content than the substrate.

Microhardness measurement Microhardness profiles were recorded across the width and depth of the FZs of laser welded specimens with a load of 0.981 N. Microhardness profiles recorded along the width of the FZs values were taken close to the top

5 Longitudinal cross-section of laser weld in partly flux coated AISI 304 sheet

Table 1 Details of laser welds

Laser welding parameters* Sheet thickness CL{ P: 1.1 kW V: 10 mm s21 PKH1 P: 1.5 kW V: 10 mm s21 FKH" P: 2.0 kW V: 20 mm s21 Sheet thickness PKH1 P: 3.5 kW V: 17 mm s21 PKH1 P: 3.5 kW V: 10 mm s21 PKH1 P: 3.5 kW V: 7 mm s21

Width w and depth d, mm

Weld aspect ratio d/w LW (ALW)

Rel. laser absorption parameter, A{

Laser energy per unit welded area E (5 P/vd), kJ mm22

LW

ALW

LW

ALW

LW

ALW

LW

ALW

w: 1.03 d: 0.53

0.90 0.63

0.51

0.70

3.2

4.2

0.21

0.17

w: 2.02 d: 1.44

1.68 2.13

0.71

1.27

10.1

16.3

0.104

0.070

w: 2.6 d: 2.54

1.84 2.64

0.98

1.43

33.3

30.5

0.039

0.038

w: 4.18 d: 4.15

3.17 4.46

0.99

1.41

44

39.1

0.051

0.047

w: 4.47 d: 4.67

3.60 4.96

1.04

1.38

32

35.1

0.075

0.071

w: 5.88 d: 5.21

5.72 5.01

0.89

0.88

35.2

34.1

0.101

0.105

is 2.5 mm

is 6 mm

*

P: laser power, V: welding speed. A is melt volume per unit weld length (mm3)/incident laser energy per unit weld length (kJ). CL: conduction limited weld. 1 PKH: partial penetration keyhole weld. " FKH: full penetration keyhole weld. { {


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6 Fusion zone microstructures of 2.5 mm thick LW and ALW specimens: laser power is 2 kW; welding speed is 20 mm s21

surface. Figure 8 presents microhardness profiles across the FZs of 2.5 and 6 mm thick laser weldments. As seen from Fig. 8a, in 2.5 mm thick stainless steel specimen welded with 1.5 kW laser power at a speed of 10 mm s21, ALW specimens exhibited higher hardness than LW specimens while for welds made with laser power of 2 kW at a welding speed of 20 mm s21, the FZ of ALW specimen was relatively softer than that of corresponding LW specimen. This difference was caused by relative softening of the FZ of ALW specimen as laser power was increased from 1.5 to 2 kW. In 6 mm thick AISI 304 weldments made at 3.5 kW laser power at welding speeds of 10 and 17 mm s21, FZs of ALW specimens were relatively softer than their ‘LW’ counterparts. However, in laser weldments made with 3.5 kW laser power at a slower welding speed of 7 mm s21, FZs of LW and ALW specimens exhibited largely similar microhardness values near the surface. On the other hand, depth profile of microhardness revealed that along

the depth of the weld, FZ of this ALW specimen was relatively softer than its LW counterpart.

Tensile Testing Tensile tests were performed on 2.5 mm thick LW and ALW specimens, made as per ‘Boiler and pressure vessel code, section 9’.29 Laser welding parameters used for preparation of tensile test specimens were: laser power was 2 kW; scan speed was 20 mm s21. The specimens carried welds at the centre of their gauge lengths. Unlike LW specimens that failed in the base metal, ALW specimens suffered failure in the weld with lower ductility. These specimens yielded at a lower stress followed by higher rate of strain hardening than those experienced by LW specimens. Both LW and ALW specimens carried largely similar tensile strengths. Figure 9 presents stress–strain curves of laser weldments while results of tensile tests are summarised in Table 2. Fracture surfaces of both kinds of specimens carried equiaxed dimples. However, ALW specimens displayed considerably finer dimples than those exhibited by LW specimens, as shown in Fig. 10.

Guided bend ductility testing For comparing ductility of LW and ALW specimens, guided longitudinal face bend tests were conducted on 2.5 mm thick laser weldments, as per ASTM E190-80.30 Laser welding parameters used for specimen preparation were the same as those used for the preparation of tensile test specimens. Convex surface of bent LW specimens did not exhibit any defects while bent ALW specimens carried fine microfissures along columnar grains normal to applied bending stress, indicating relatively lower ductility of ALW specimens with respect to LW specimens. Figure 11 presents macroscopic views of convex surfaces of bend tested LW and ALW specimens. Cross-sectional examination of ALW specimen at the suspected defect site did not reveal any defect, implying that the defects noticed on the bent surface were superficial in nature.

Online plasma spectroscopy

7 Changing shape of EDS CrLa line in substrate (BM), FZs of LW and ALW specimens

Plasma emissions were largely confined in 230–605 nm (ultraviolet to orange) spectral range. The most intense emissions were in 478–548 nm range (blue to yellowish green), followed in intensity by the emissions in the spectral range 230–290 nm (ultraviolet) and 400–479 nm


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8 Microhardness profiles along depth and width of FZs of a 2.5 and b 6 mm thick LW and ALW specimens

(violet-blue) respectively. In addition, low intensity emissions were also recorded in 548–605 nm range (yellowish green-orange). Plume spectra carried peaks for Fe(I), Cr(I), Mn(I), Ni(I) and Cr(II), indicating presence of vaporised atoms of Fe, Cr, Mn and Ni along with Crz ions in the plume. No peak from the gaseous elements was recorded. Figure 12 compares plasma emission spectra obtained during LBW and ALBW. The results of plasma spectroscopy are presented in the form of a time integrated spectral band (TISB), as shown in Fig. 13. The horizontal and vertical axes of TISB represent wavelength and time (starting from top) respectively. In other words, a horizontal line on TISB represents an emission spectrum at a particular moment during LBW while a vertical line on TISB shows

temporal intensity variation of a particular wavelength. Relative brightness distribution in TISB reflects relative intensities of various spectral emissions with time. The spectrum shown at the bottom of TISB represents emission spectrum at a particular moment (in the wavelength range 243–551 nm, corresponding to 170– 1026 pixel shown in the figure) corresponding to the position of curser on TISB. On the other hand, the profile presented on the right side of TISB shows temporal intensity variation of a particular wavelength during LBW. Closely spaced alternate bright and dark horizontal bands in TISB show regular temporal fluctuations in the intensity of various wavelengths. Abrupt drop in the brightness in the lower part of TISB (also reflected in the intensity profile of 357.94 nm


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Dl1=2 ~2W (ne =1016 )0A

(1) 23

9 Stress–strain curves of LW and ALW specimens: solid and broken lines represent stress–strain curves of LW and ALW specimens respectively

spectral line on the right side of TISB) coincides with flux introduction. The drop in intensity was not confined to some specific spectral lines. In stead, all the spectral lines suffered significant drop in intensity with flux addition. Addition of flux did not introduce any new peak in the spectrum. Electron temperature measurement

Electron temperature of the plasma plume was determined by assuming plasma to be in a state of local thermal equilibrium. The Boltzman plot method was adopted for electron temperature estimation.31–34 Three different Fe(I) spectral lines, namely 527.0357, 532.418 and 540.577 nm, were selected for this purpose. Electron temperature of plasma remained largely unaffected by flux addition. Plasma plume produced during LBW, across the whole range of experimental parameters, carried similar values of electron temperature (