Materials Transactions, Vol. 48, No. 2 (2007) pp. 219 to 226 #2007 The Japan Institute of Metals

Effects of Different Shielding Gases and Power Waveforms on Penetration Characteristics and Porosity Formation in Laser of Inconel 690 Alloy

Tsung-Yuan Kuo1;* and Yong-Ding Lin2

1Department of Mechanical Engineering, Southern Taiwan University of Technology, Tainan 710, Taiwan, R.O. China 2Department of Mechanical Engineering, Nan Jeon Institute of Technology, Tainan 737, Taiwan, R.O. China

A high power Nd-YAG laser was used to perform bead-on-plate (BOP) welding on high viscosity Inconel 690 alloy plates of 3 mm in thickness with three different shielding gases (Ar, He, and N2). Adopting a rectangular laser power waveform, four different peak-base power differentials (P) were applied with a constant average power of 1.5 kW. A comprehensive investigation was performed into the influences of the shielding gas, the flow rate, and the value of P on the characteristics of the resulting welds, including the weld morphology, the penetration depth, the plume volume, and the porosity formation. The results showed that the weld penetration depth, the depth-to-width ratio, the weld surface roughness and the degree of weld spattering all increased with increasing P. The choice of shielding gas had a significant effect on the porosity ratio (Pr) of the weld. The weld formed under Ar shielding had the highest Pr, while that formed under N2 shielding had the lowest. Under He shielding, the gas flow rate had a significant effect on the porosity ratio. However, under the higher density gases of Ar and N2, the porosity appeared to be insensitive to the flow rate. Finally, an increased P yielded a significant reduction in Pr for the welds with higher porosity. [doi:10.2320/matertrans.48.219]

(Received May 30, 2006; Accepted November 29, 2006; Published January 25, 2007) Keywords: Inconel 690, Nd-YAG(neodymium: yttrium-aluminum-garnet) laser, welding, plume, porosity, shielding gases

1. Introduction output power waveform play an important role in minimiz- ing the weld porosity and increasing the penetration Due to their excellent corrosion resistance and favorable depth.4,8,12–18) The high chromium and low carbon content high-temperature mechanical properties, nickel-based alloys of Inconel 690 alloy provide an enhanced corrosion resist- are commonly used to fabricate the heat exchange tubes in ance and render this alloy a suitable candidate for the repair nuclear power plants. Although the heat exchangers in a of damaged heat exchange tubes. This study performs a series nuclear power plant are typically designed to operate for 40 of BOP welding trials on Inconel 690 using a high-power Nd- years or more, the tubes within them frequently show signs of YAG laser. Welding is performed using four different levels damage before the end of their service lives due to severe of P (with a constant average power of 1.5 kW) and three oxidation and the corrosive conditions in which they operate. different shielding gases, i.e. Ar, He and N2. The respective These tubes are generally repaired using some form of influences of the shielding gas, the flow rate, and the level of plugging or sleeving procedure carried out using a welding P on the weld morphology, plume volume, penetration process.1–6) However, the sluggish weld metal of nickel- characteristics and porosity formation are systematically based alloys does not flow out or wet to the same extent as examined. or weld metals.7) Therefore, the finished welds commonly exhibit defects such as porosity or 2. Experimental Procedure solidification cracking. Furthermore, carrying out the repair task is not only difficult because of the confined space in the The Inconel 690 base metal was supplied by Sumitomo heat exchanger, but also dangerous due to the radioactive Metal Technology (Hyogo, Japan) in plate form. Prior to environment. delivery, the alloy was solution heat treated at 1050C for 5 The Nd-YAG laser beam can be transmitted by a fiber minutes and then quenched in water. The chemical compo- optic and the welding process performed by a robotic system. sition of the base metal is summarized in Table 1. The alloy Therefore, Nd-YAG laser welding is a potential candidate for plates were machined into welding specimens of dimensions the repair task discussed above.2–4,8) Compared with conven- 80 80 3 mm. tional processes, the higher power density of Welding was performed using a 2.5 kW Nd-YAG laser increases the depth to width (D=W) ratio system (Rofin-Sinar CW025), operating at a mean power of of the bead, lowers the heat input, and results in a more rapid 1.5 kW and using a rectangular output power waveform. By solidification. These welding characteristics are beneficial gradually reducing the value of P (P ¼ Pp Pb, where since they minimize distortion and yield excellent mechan- Pp is the peak power and Pb the base power), the output ical properties.5,6,9,10) It has also been shown that laser power mode was changed progressively from a pulsed wave welding suppresses solidification cracking of nickel-based (PW) to a continuous wave (CW). As shown in Fig. 1, four alloys.11) However, relatively little literature is available different power modes were employed, namely PW1 (P ¼ concerning the porosity formation and penetration character- 2234 W), PW2 (P ¼ 1572 W), PW3 (P ¼ 858 W), and istics of laser welded high viscosity metals. CW (P ¼ 0 W). The corresponding peak to base power In laser welding, the nature of the shielding gas and the ratios, Pp=Pb, reduced from an initial value of 6.8 to a final value of 1. The frequency and duty of the rectangular *Corresponding author, E-mail: [email protected] modulated beam were maintained at 100 Hz and 50%, 220 T.-Y. Kuo and Y.-D. Lin

Table 1 Compositions of Inconel 690 (mass-%).

Ni Cr Fe C Si Mn P S Cu Co Ti Inconel 690 59.05 29.45 Bal. 0.019 0.33 0.29 0.01 0.001 0.02 0.014 0.28

10° W W / / Laser Beam P P PP PP Welding Direction Power,

Power, 1500 1500 Nozzle Shielding Gas Pb 4

Pb Workingpiece Moving 25 ° 5 10 15 Time, t / ms 5 10 15 Time, t / ms (a) PW1 (b) PW2

Groove 10 W W / /

P P Unit : mm

PP Fig. 2 Experimental setup for Nd-YAG laser welding.

Power, 1500 Power, 1500

Pb

Pr ¼ðAp=Aw 100%Þ, where Ap is the total area of porosity 5 10 15 5 10 15 Time, t / ms Time, t / ms and Aw is the weld area. The porosity measurements were (c) PW3 (d) CW conducted by using an image inspection system to examine CCD digital images of three longitudinal metallographic Wave Peak power Base power ∆ − P= Pp Pb P /P sections (length ¼ 10 mm) extracted from parallel positions modes P , (W) P , (W) p b p b (W) of the weld located 0.2 mm to the left of the FZ centre, in the PW12617 383 2234 6.8 centre of the FZ, and 0.2 mm to the right of the FZ, PW22286 714 1572 3.2 PW31929 1071 858 1.8 respectively. CW 1500 1500 0 1.0 3. Results and Discussions Fig. 1 Laser power output modes and P level: (a) PW1, (b) PW2, (c) PW3, and (d) CW. (mean power 1500 W). 3.1 Weld morphology Figure 3 shows the surface morphologies and cross-sec- tional photos of weld beads produced under various values of respectively. The laser beam was transferred to the work- P using different shielding gases and flow rates. It is station via an optical fiber of diameter 600 mm. The focal observed that in every case, the weld becomes rougher and length for welding was 120 mm, corresponding to a spot size the weld spatter effect more pronounced as P increases. of 600 mm on the upper surface of the welding specimen. A higher P causes a more abrupt pressure change in the Figure 2 presents a schematic illustration of the laser welding weld pool as a result of the more pronounced periodic pulse process. A constant welding speed of 1500 mm/min was used effect of the output power.8,10,19) The enhanced pulse effect throughout the welding trials. Three different shielding gases, increases the agitation of the molten pool and causes the 20) namely Ar, He and N2, were used to provide face shielding. keyhole to become unstable and to collapse. Therefore, the The shielding gas nozzle had a diameter of 4 mm and was surface of the weld becomes increasingly rough as P arranged at an angle of 25 to the workpiece. The shielding increases. Spatter is generally the result of molten metal gas was delivered with flow rates of 10 L min1, 16 L min1 being carried by the evaporating metal as it is ejected from and 22 L min1, respectively. To prevent the risk of back the keyhole of the weld pool. As P increases, the energy reflection, with the associated risk of accidental damage to input to the workpiece during the Pp stage increases. This the optics, the laser beam was orientated at a forward angle of promotes the evaporation of metal from the bottom tip of the 10 relative to an imaginary line normal to the workpiece keyhole,17) and hence results in a greater degree of weld surface. A high-speed video camera system with a frame spatter. It has also been reported that the degree of spattering acquisition rate of 1000 frames per second was utilized to is directly related to the viscosity of the molten metal.21) It is observe the laser plume behavior under different welding thought that spattering is affected by the flow of the conditions. vaporized metal from the keyhole and by the surface tension In calculating the D=W ratio, D is the actual penetration gradient forces of the molten metal, which in turn depend on depth of the weld and W is the average width of the weld in the characteristics of the alloying elements (e.g. the vapor- the region of the fusion zone (FZ) extending from 0.2 mm ization temperature, the vapour pressure, and the surface below the cap to 0.2 mm above the root. Meanwhile, the activity). percentage of porosity, Pr, in the weld is evaluated as Effects of Different Shielding Gases and Power Waveforms on Penetration Characteristics and Porosity Formation 221

10 L/min 16 L/min 22 L/min

PW1

PW2

PW3

CW 3 mm

(a) Ar

10 L/min 16 L/min 22 L/min

PW1

PW2

PW3

CW 3 mm

(b) He

10 L/min 16 L/min 22 L/min

PW1

PW2

PW3

CW 3 mm

(c) N2

Fig. 3 Effects of shielding gas and flow rate on appearance, morphology and weld spatter of weld for various power waveforms: (a) Ar, (b) He, and (c) N2 (travel speed: 1500 mm/min, average output power: 1.5 kW).

3.2 Depth of penetration and D=W ratio a Nd:YAG laser yielded similar findings. Their results Figure 4 shows the effects of the shielding gas and the P indicated that the frequency of the Nd:YAG laser beam is level on the penetration depth. It is apparent that the depth of higher than that of the laser plume, with the result that the penetration increases with increasing P. Furthermore, for a Nd:YAG laser energy is not absorbed in the laser plume. given set of welding parameters, similar penetration shapes In essence, a pulsed welding process is similar to a drilling and depths are obtained irrespective of the shielding gas process since each pulse makes its own keyhole. A high value employed. of P is associated with a high peak power Pp (high pulse In CO2 laser welding, the plume attenuates the laser beam level). Therefore, the laser beam concentrates more energy and reduces the amount of beam energy transferred to the on the workpiece during the Pp stage and produces an workpiece, thereby reducing the penetration depth.22–24) increased penetration depth as a result. Additionally, a higher However, in Nd:YAG laser welding, laser absorption in the value of P causes a greater agitation of the molten pool due plume is very weak,25) and therefore the plume volume has a to the enhanced periodic pulse effect and increases the lesser influence on the penetration depth. Consequently, the evaporation of metal from the bottom tip of the keyhole.17) penetration depths identified in the present study are very Both effects increase the fluidity of the molten metal and the similar despite the fact that the plumes formed under the transition of thermal energy and therefore increase the three shielding gases are quite different in size (as shown penetration depth. Fig. 6 in next section 3.3). A study by Takano, et al.8) into the Figure 5 illustrates the relationship between the D=W ratio dissimilar lap welding of Inconel 690 with Inconel 600 using and P for various flow rates and shielding gases. The D=W 222 T.-Y. Kuo and Y.-D. Lin

3.0 4 2.8 (a) Ar (a) 10 L/min

/mm 2.6 3.5 D 2.4 2.2 2.0 3 D/W 1.8 1.6 2.5 1.4

Penetration depth, 1.2 Ar He N2 PW1 PW2 PW3 CW 1.0 2 10 16 22 PW1 PW2 PW3 CW -1 Flow rate, F /L·min Wave modes (a) Ar (a) 10 L/min

3.0 4 (b) 16 L/min 2.8 (b) He

/mm 2.6 3.5 D 2.4 2.2 2.0 3 D/W 1.8 1.6 2.5 1.4 Ar He N2

Penetration depth, 1.2 PW1 PW2 1.0 2 10 16 22 PW1 PW2 PW3 CW Flow rate, F /L·min-1 Wave modes (b) He (b) 16 L/min 4 3.0 (c) 22 L/min 2.8 (c) N2 /mm 2.6 3.5 D 2.4 2.2 2.0 3 D/W 1.8

1.6 2.5 1.4 Penetration depth, 1.2 Ar He N2 PW1 PW2 PW3 CW 1.0 2 10 16 22 PW1 PW2 PW3 CW Flow rate, F /L·min-1 Wave modes (c) 22 L/min (c) N2 Fig. 5 Variation of D=W ratio with power waveform for different shielding Fig. 4 Penetration depth for weld beads as function of shielding flow rate gases and flow rates: (a) 10 L/min, (b) 16 L/min, and (c) 22 L/min under different P levels and shielding gases: (a) Ar, (b) He, and (c) N2. (1500 mm/min, 1.5 kW). ratio increases with increasing P because a higher value of (10 ms). The results are presented in Fig. 6. Note that the 5 P increases the penetration depth under the same average images of the Pb and Pp stages, respectively, are very similar, power. The results of Fig. 5 indicate that the shielding gas and hence Fig. 6 shows only one image frame for each stage. and flow rate have no significant effect on the D=W ratio. For each shielding gas, the plume volume is small during the low power stage, Pb, since the laser power density is low. 3.3 Plume formation and volume During the high power stage, Pp, the maximum plume The relationship between the shape of the plume and the volume is generated under Ar shielding, while the lowest power waveform was recorded for each of the shielding gases volume is produced under He. As P (Pp) reduces, the plume in a series of images acquired at a rate of 10 frames per cycle volume decreases slightly. In high power CO2 laser welding, Effects of Different Shielding Gases and Power Waveforms on Penetration Characteristics and Porosity Formation 223

PW1 PW2 PW3 CW Pp Pb Pp Pb Pp Pb

Ar

He

N2 1cm

Fig. 6 High-speed observations of laser plume during laser welding process, showing relationship between power waveform and shape of plume under Ar, He or N2 shielding (16 L/min).

Table 2 Properties of different shielding gases.

Shielding Dissociation potential (First) Ionization potential Molecular weight Gas (WmK1) (eV) (eV) (g mole1) Ar 0.0159 — 15.7 39.94 He 0.1381 — 24.5 4.00 N ¼ 14:5 N2 0.0234 9.8 (N2 ! N þ N) 28.02 N2 ¼ 15:5 laser-induced plume (plasma) is generated, composed of metallic plasma and gas plasma.14,26) Metallic plasma, which 10 L/min 16 L/min 22 L/min is ejected from the keyhole and remains attached to it, is caused by the ionization of the vaporized metal by the laser beam. Meanwhile, gas plasma is formed by the ionization of Ar the atoms or molecules of the shielding gas by the laser beam and appears to be attached both to the metallic plasma and the workpiece. In the current case, however, the wavelength of the Nd:YAG laser is a tenth that of the CO2 laser. As a result, during laser welding, the ionization behaviors of the shield- He ing gas and the base metal under the influence of laser radiation are conspicuously different. Consequently, the composition of the laser-induced plume is also different. Since the wavelength of the Nd:YAG laser is shorter, the shielding gas is not easily ionized during welding process. 27) Greses, et al. also reported that the ionization potential of N2 the shielding gas does not play an important role in Nd:YAG 1cm laser welding. Although the vaporized materials from the workpiece generally have a lower ionization potential than the shielding gas, the amount of metallic plasma induced Fig. 7 High-speed observations of laser plume during laser welding during the Nd:YAG laser welding process is generally very process, showing relationship between shielding gas flow rate and shape of little or no. Matsunawa et al.28) and Lacroix, et al.29) have plume under Ar, He or N2 shielding (PW1, Pp stage). also indicated that the vapour ejected from the keyhole under high energy densities in Nd:YAG laser welding is a high- temperature thermally excited gas rather than a partially shielding gas rather than by its ionization potential. When He ionized plasma. Therefore, the plume formed in the Nd:YAG is used as the shielding gas, its high thermal conductivity (see laser welding process consists predominantly of un-ionized Table 2) causes the heat to be rapidly conducted away from vaporized materials from the base metal and excited hot gas. the plume with the result that the plume volume is reduced Since the shielding gas is not ionized in the Nd:YAG laser (see Fig. 6). Additionally, the higher the shielding gas flow welding process, the volume and morphology of the plume rate, the more pronounced the cooling effect of the plume and are determined by the thermal conductivity and density of the hence the smaller the plume volume, as shown in Fig. 7. 224 T.-Y. Kuo and Y.-D. Lin

(a) Ar (b) He (c) N2

Fig. 8 Porosity in longitudinal sections of weld beads under various shielding gases and power waveforms: (a) Ar, (b) He, and (c) N2 (16 L/min).

Figure 7 also shows the effect of the shielding gas flow rate the shielding gas (e.g. the type of shielding gas),14,18) the on the blow-away plume. Under Ar shielding, the blow-away shielding conditions,16,32–34) the vapour pressure of the plume is the most obvious of the three gases at a high flow metal,35,36) the viscosity of the molten metal21) and the rate due to its higher density, which results in a much higher dynamics of the keyhole during the welding process.10,37) The dynamic pressure compared to other gases flowing at the floatability of the bubbles is affected by the characteristics of 38) same velocity. Under N2 shielding, the blow-away plume at a the output power (e.g. the power waveform), the surface high flow rate is still large, but is reduced slightly compared tension of the molten pool,8) and the direction of the molten to that of Ar due to its smaller mass. Finally, under He metal flow (Marangoni flow).35,39,40) Therefore, the content of shielding, there is no appreciable change in the blow-away the weld porosity depends on the factors which trap the plume with increasing flow rate due to its low density. bubbles and enhance the bubble floatability. Additionally, the plume images (Fig. 6) also show that the Regarding the effects of the shielding gas on the porosity spatter will increase with the rise of P, and lead to more ratio, Pr, the effect of gas plasma pressure on the keyhole spatter on the weld surface (sec. 3.1). With the same level of need not be considered because gas plasma is not generated P, different shielding gases would induce the various under any of the three shielding gases used in the current Nd- volume of plume. Because the plume didn’t contain the laser- YAG laser welding process. This is in direct contrast to the induced plasma, its energy absorption on the laser beam phenomenon observed in CO2 laser welding. In Nd:YAG would not change with its blocking volume in the laser beam. laser welding, under an identical mean power and power Thus, different shielding gases would not vary the penetra- wave form, the amount of the vaporized materials and excited tion depth under the laser radiation (sec. 3.2). hot gas in the keyhole is similar under the different shielding gases. Therefore, the level of porosity under different 3.4 Analysis of porosity formation shielding gases is related to the amount of shielding gas Figure 8 presents a series of photographs of longitudinal blown into the keyhole and the suppression effect by the sections of the various welds extracted from the vicinity of shielding gas of the vaporized materials ejected from the the weld centers. It can be seen that the porosity of the weld keyhole. The higher the density of the shielding gas, and the varies with the shielding gas. Specifically, Ar shielding higher its momentum, the greater the amount of shielding gas results in the highest weld porosity, and N2 the lowest. and vaporized materials trapped within the keyhole. The Furthermore, as shown in Fig. 9, Pr decreases with increasing increased amount of shielding gas and vaporized materials at P under Ar or He shielding, i.e. from 5.5% (CW) to 2.5% the solidifying front results in a higher porosity. The density (PW1) and from 3% (CW) to 1.5% (PW1), respectively. of the He shielding gas is comparatively low. Consequently, However, under N2 shielding, the variation in porosity is very the amount of shielding gas and vaporized materials trapped slight, i.e. 0:1 0:6%. within the keyhole is less than that in Ar shielding and hence Some or all of the bubbles in the weld pool, mainly the porosity of the weld is reduced. The porosity of the weld metallic vapour and shielding gas and sometimes entranced produced under N2 shielding is the lowest of the three welds. air, are trapped by the instability and collapse of the keyhole It has been suggested that the surface tension of the molten 14,30,31) during welding, causing porosity in the finished weld. pool is lower under N2 shielding and hence the bubbles are The content of the trapped bubbles depends on the nature of more easily able to escape the weld pool.8) Additionally, it Effects of Different Shielding Gases and Power Waveforms on Penetration Characteristics and Porosity Formation 225

6.5 the entrapment of shielding gas in the keyhole and suppress 10 L/min 16 L/min 22 L/min (a) Ar 6 the evaporation of metallic vapour, leading to increased 5.5 porosity. However, as P (Pp level) increases, the pulse 5 effect of the laser power on the molten pool also increases 4.5 and the evaporation of molten metal from the bottom tip of 4 17) 3.5 the keyhole is enhanced. Both factors enhance the flow of 3 the molten metal (carrying the bubbles) and promote the 2.5 ability of the bubbles to float out of the molten pool 2 surface.19) In other words, on the one hand, an increased P Porosity ratio (%) 1.5 traps more bubbles, while on the other, it enhances the ability 1 of these bubbles to escape from the molten weld pool. The 0.5 present results for high-viscosity Inconel 690 welds show that 0 P decreases with increasing P. Therefore, it appears that PW1 PW2 PW3 CW r Wave modes the enhanced ability of the bubbles to escape from the weld pool under a high P more than compensates for the higher (a) Ar tendency of bubbles to become entrapped. Previous stud- 21,38,45,46) 6.5 ies of the relationship between P and porosity 10 L/min 16 L/min 22 L/min 6 (b) He formation in metals of different viscosities welded using a 5.5 Nd-YAG laser revealed that as P increases, the Pr of 5 aluminum alloys increases,45,46) while that of stainless steel 4.5 decreases slightly38) and that of Inconel 690 decreases more 4 significantly.21) Therefore, it is apparent that the reduction in 3.5 3 Pr associated with increasing P for high-viscosity materials 2.5 is more significant than that for low-viscosity materials 2 (viscosity: Inconel 690 > stainless steel > aluminum al- Porosity ratio (%) 1.5 loys). It may be suggested that as P increases, the 1 increasing level of keyhole instability in high viscosity 0.5 materials is less than that in low viscosity materials.36) In 0 other words, the amount of metallic vapour and shielding gas PW1 PW2 PW3 CW trapped in the weld pool during laser welding is less in high Wave modes viscosity materials than in low viscosity materials. Addition- (b) He ally, as P increases, the enhancement in the floatability of

6.5 the bubbles in the molten metal in high-viscosity materials is 16 L/min 22 L/min (c) N 2 6 10 L/min lower than that in low-viscosity materials. The effect of 5.5 increasing viscosity in reducing the level of bubble flow is 5 less than the effect of increasing P in enhancing the 4.5 floatability of the bubbles. As a consequence, the degree of 4 porosity reduction in high viscosity materials is greater than 3.5 that in low viscosity materials. Furthermore, for welds with 3 low porosity, the increased pulse effect at higher levels of P 2.5 21) 2 yields only a marginal reduction in porosity. Therefore, the Porosity ratio (%) 1.5 Pr reduction with increasing P in the specimens welded 1 under He shielding is less than that observed under Ar. 0.5 Meanwhile, under N2 shielding, increasing P has no 0 discernible effect in reducing the specimen porosity. PW1 PW2 PW3 CW Figure 9 also shows the effect of the shielding gas flow rate Wave modes on Pr. It is observed that the flow rate has a significant effect (c) N2 on the porosity when welding is performed under He shielding. Furthermore, it is apparent that a lower flow rate Fig. 9 Influence of P level and shielding gas flow rate on porosity tends to increase the porosity. The shielding gas density plays formation ratio P of weld beads under various shielding gases: (a) Ar, (b) r a key role in determining the shielding efficiency of the weld He, and (c) N2. pool against the ambient atmosphere. Under He shielding, a low flow rate provides a poorer protection of the weld pool than a higher flow rate due to its low molecular weight has been reported that the high solubility of N in high Cr weld (Table 2). Therefore, the atmosphere around the molten weld metals ensures an effective reduction in porosity forma- pool readily invades the pool and results in a higher porosity. 19,41,42) tion. For shielding gases with a higher gas density, i.e. Ar and N2, Regarding the influence of the power waveform on Pr, the current flow rates provide an adequate protection of the 22,36,43,44) previous studies have shown that keyhole instability molten pool and therefore Pr is comparatively insensitive to increases with increasing P. This instability may increase changes in the flow rate. 226 T.-Y. Kuo and Y.-D. Lin

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