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The influence of process parameters on porosity formation in hybrid LASER-GMA welding of AA6082 aluminum alloy
Optics & Laser Technology 44 (2012) 1485–1490
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
The influence of process parameters on porosity formation in hybrid LASER-GMA welding of AA6082 aluminum alloy Alessandro Ascari a,n, Alessandro Fortunato b, Leonardo Orazi c, Giampaolo Campana b a
DIEM – Department of Mechanical Constructions Engineering, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy DIEM – Department of Mechanical Constructions Engineering, University of Bologna, 40136 Bologna, Italy c DISMI – Department of Sciences and Methods for Engineering, University of Modena and Reggio Emilia, 42100 Reggio Emilia, Italy b
a r t i c l e i n f o
abstract
Article history: Received 28 October 2011 Received in revised form 10 December 2011 Accepted 10 December 2011 Available online 27 December 2011
This paper deals with an experimental campaign carried out on AA6082 8 mm thick plates in order to investigate the role of process parameters on porosity formation in hybrid LASER-GMA welding. Bead on plate weldments were obtained on the above mentioned aluminum alloy considering the variation of the following process parameters: GMAW current (120 and 180 A for short-arc mode, 90 and 130 A for pulsed-arc mode), arc transfer mode (short-arc and pulsed-arc) and mutual distance between arc and LASER sources (0, 3 and 6 mm). Porosities occurring in the fused zone were observed by means of X-ray inspection and measured exploiting an image analysis software. In order to understand the possible correlation between process parameters and porosity formation an analysis of variance statistical approach was exploited. The obtained results pointed out that GMAW current is significant on porosity formation, while the distance between the sources do not affect this aspect. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Porosity Hybrid welding X-ray
1. Introduction Hybrid LASER-GMA welding techniques are receiving a growing industrial attention due to the synergic effect achieved by coupling two technologically different welding methods together. The presence, in fact, of a high energy density source, such as the LASER one and of an arc source with a filler material, such as the GMA one, allows to set up a well defined process, characterized by its own peculiar characteristics. These characteristics are the result of the complementarity of the two coupled techniques and derive from the fact that in hybrid processes the main positive aspects of one involved technology allow to override the main drawbacks of the other and vice versa. In particular the most relevant benefits of hybrid LASER-GMA welding technique are high process speed, very good bridging ability, high bead penetration, possibility to modify the bead metallurgy and possibility to deal with variable gaps, misalignments and chamfers. Typical applications of hybrid LASER-GMA welding concern shipbuilding, automotive, pipeline constructions and, more in general, but joints with high thickness sheets [1] where the main difficulty is to deal with variable gaps between the sheets and to achieve single pass and high speed weldings in order to maximize productivity. On the other hand, considering the availability of modern high beam quality and fiber-deliverable LASER sources,
n
Corresponding author. Tel.: þ39 051 2090494; fax: þ 39 051 2090484. E-mail address: [email protected] (A. Ascari).
0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.12.014
such as Nd:YAG disk and fiber ones, the application of hybrid LASER-GMA welding also on low thickness sheets is becoming very common in many industrial applications [2]. According to the above mentioned considerations, the combination of two technologically different energy sources leads to several undoubted advantages, but, on the other hand, it also determines the occurrence of several technological problems which have to be faced. Some of these problems concern the stability and the repeatability of the process, mainly related to the shielding gas flux and composition [3–8], drops deposition [9,10], arc current [11,12] sources position and mutual interaction [13–15] and porosity formation [16]. In particular, mechanisms and physical phenomena related to the porosity formation are not fully understood yet even if they drastically influence the mechanical resistance especially in aluminum welding. In [16] it was first observed and described the phenomena of bubble formation in AISI 304 stainless steel and AA5052 aluminum alloy for hybrid GTA-YAG and GMA-YAG LASER welding under different process conditions and it was pointed out that arc current is the main factor in bubble formation. According to the previous assumptions, this paper investigates the influence of arc welding parameters such as arc current and transfer mode and of the mutual distance between arc andLASER sources on the number and on the extension of the porosity generated during the hybrid welding process. The experiments were carried out on a AA6082 aluminum alloy exploiting a CO2 hybrid LASER-GMA welding technique. The test specimens were analyzed by means of X-ray inspection and the porosities were counted and measured exploiting computer image analysis. The porosity related
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A. Ascari et al. / Optics & Laser Technology 44 (2012) 1485–1490
parameters were then analyzed by means of analysis of variance (ANOVA) statistical techniques in order to understand the actual correlation between process parameters and output ones and to find eventual mathematical models expressing their relationships.
Laser beam GMAW Torch Torch Inclination
Workpiece
2. Experimental setup The experimental equipment exploited in this campaign consisted of an El.En. FAF 3 kW continuous wave CO2 LASER source, equipped by a three axes CNC cell, and by a Selco 380 A synergic pulsed GMAW generator. The LASER beam was focused by means of a copper parabolic mirror with a focal length of 200 mm which allowed a minimum spot diameter of 500 mm. The sources were coupled together by means of a Binzel automatic torch fixed on the vertical (Z) axis of the CNC cell. The welding direction was set to LASER leading as presented in Fig. 1. Every experimental trial was carried out in a bead on plate configuration on AA6082 8 mm thick plates fixed on the X–Y table of the CNC cell. The obtained weld beads were straight linear ones characterized by a length equal to 100 mm. The LASER beam focal position was constantly kept on the upper surface of the workpiece. During the experimental campaign the parameters given in Table 1 were kept constant while arc current and distance between the two energy sources were varied in accordance to Table 2. The distance between the heat sources and the inclination of the torch were defined as shown in Fig. 2. In particular two levels for the arc current, combined with three levels for the sources distance, were investigated for both pulsed and short-arc GMAW configuration. Finally, three repetitions were performed for every welding configuration in order to evaluate the repeatability of the process. The gas mixture composition reported in Table 1
Laser beam
Base Material
GMAW Torch
Keyhole
Arc
D
GMAW Wire
Fig. 2. Definition of D (distance between the heat sources) and torch inclination.
Table 3 X-ray inspection parameters. Focal distance
Power
Current
Exposure
700 mm
55 kW
3 mA
60 s
Porosities
Fig. 3. Radiography showing a bead affected by a large number of porosities.
represents a good trade off between arc stability and plasma formation during welding as reported in [5]. The shielding gas was supplied through the GMA torch. According to the previous descriptions the whole experiment consisted of 36 experimental trials: (2 levels for the arc current), (3 levels for the distance between the heat sources), (3 repetitions for every trial) ¼18 samples for short-arc mode and other 18 samples for pulsedarc mode.
Bead Welding direction
3. Experimental results
Fig. 1. Hybrid welding setup.
Table 1 Constant parameters. LASER power Gas mixture Gas flow Torch inclination Welding speed Wire type Wire diameter
3 kW 60% He, 37% Ar and 3% O2 17 l/min 651 0.6 m/min Al–Mg ER5356 1.2 mm
Table 2 Variable parameters. Current – short-arc Current – pulsed-arc Distance (D)
120 and 180 A 90 and 130 A 0, 3 and 6 mm
The specimens obtained during the experimental phase were radiographed by means of a General Electric ERESCO 42MF portable equipment. The parameters used during this inspection are summarized in Table 3. Every X-ray plate was then digitally acquired by means of a double face scanning equipment in order to allow a computerized image analysis. Figs. 3 and 4 show two examples of digitized X-ray plates. By means of the image analysis software MediaCybernetics ImagePro Plus 6.0 the bubbles could be detected, counted and measured in order to characterize the porosities of the obtained welding specimens. The mentioned software, in fact, can be conveniently tuned in order to apply a so-called ‘‘segmentation’’ on the picture which allows to separate single objects (bubbles in this case) from the background of the image and to subsequently count and measure them in terms of absolute number and total area. Fig. 5 shows the objects detection phase in the mentioned software: the green objects (indicated by the arrow) were classified as porosities, while the red and white ones are classified as dust, granulometry and imperfections of the X-ray plates according to the thresholds set in the segmentation phase. Every specimen was radiographed
A. Ascari et al. / Optics & Laser Technology 44 (2012) 1485–1490
1487
P(%)
0.75 0.50
0.25 0.00
Fig. 4. Radiography showing a bead affected by a negligible number of porosities.
0 2 Dis tan ce
Green object Red and white objects
180 160
[m 4 m]
140 6
120
]
t [A
ren
Cur
Fig. 6. 3D plot of the correlation surface between the output Pð%Þ and the process parameters (short-arc). The red dots correspond to the actual measurements.
P(%)
1.5 1.0 0.5 0.0
Dis
Fig. 5. Detection and measurement of porosities by means of the Image Pro Plus software.
on a plane parallel to the upper surface of the welded plates and no particular preparation was necessary, such as polishing, etching, etc. In order to conveniently characterize and define the results of the specimens analysis two different output parameters were defined. The first one concerned the relative porosity and it was defined according to Eq. (1), where Sp is the total extension of the bubbles detected by the image analysis (in terms of mm2 ) and Sb is the total extension of the specific weld bead. Sp Sb
Sp Np
[m
120
4 m]
105
6
90
A]
ent [
Curr
ð1Þ
This parameter takes into account the degree of porosity referred to the total surface covered by the weld bead and, thus, it is independent from the absolute dimensions of the bead itself. The second output parameter taken into consideration was the average bubble area described in Eq. (2), where Np is the total number of porosities detected on a specific weld bead. Sm ¼
135
2 tan ce
Fig. 7. 3D plot of the correlation surface between the output Pð%Þ and the process parameters (pulsed-arc). The red dots correspond to the actual measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ð2Þ
The actual measurements, in terms of Pð%Þ and Sm, are represented by the red dots in Figs. 6–9. The second phase of the analysis of the results was aimed at defining the actual significance effect of the varied process parameters (welding current, mutual distance between the sources and arc transfer mode) on the output parameters Pð%Þ and Sm. In order to achieve this goal a two-ways ANOVA was conducted on the whole experiment by
5000 Sm [µm2]
Pð%Þ ¼
0
4000 3000 2000
180
0 2 tan
160
Dis
4 mm ]
ce [
140 6
120
ent
r Cur
[A]
Fig. 8. 3D plot of the correlation surface between the output Sm and the process parameters (short-arc). The red dots correspond to the actual measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 5 ANOVA results for Pð%Þ (pulsed-arc).
Sm [µm2]
3000 2500
Source
DF
SS
MS
F
P
Current (A) Distance (mm) Interaction Error Total
1 2 2 12 17
0.09921 0.04461 0.37773 1.95325 2.47481
0.099214 0.022306 0.188865 0.162771
0.61 0.14 1.16
0.450 0.873 0.346
2000 1500 0
135 2 tan
Dis
120
ce
4 [m m]
105 6
90
A]
t[ rren
Cu
Fig. 9. 3D plot of the correlation surface between the output Sm and the process parameters (pulsed-arc). The red dots correspond to the actual measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 6 ANOVA results for Sm (short-arc). Source
DF
SS
MS
F
P
Current (A) Distance (mm) Interaction Error Total
1 2 2 12 17
6 026 390 71 311 3 169 765 4 228 547 13 496 013
6 026 390 35 656 1 584 883 352 379
17.10 0.10 4.50
0.001 0.905 0.035
Table 7 ANOVA results for Sm (pulsed-arc).
Table 4 ANOVA results for Pð%Þ (short-arc). Source
DF
SS
MS
F
P
Current (A) Distance (mm) Interaction Error Total
1 2 2 12 17
0.205926 0.085182 0.038928 0.529912 0.859948
0.205926 0.042591 0.019464 0.044159
4.66 0.96 0.44
0.052 0.409 0.654
Source
DF
SS
MS
F
P
Current (A) Distance (mm) Interaction Error Total
1 2 2 12 17
1 847 850 486 663 131 465 2 173 009 4 638 987
1 847 850 243 331 65 732 181 084
10.20 1.34 0.36
0.008 0.297 0.703
value, if the null hypothesis is true. A commonly used cut-off value for the p-value is about 0.05. means of the software Minitab 16.0. Thanks to this approach a statistical judgement could be assessed concerning the fact that the variation of porosity from specimen to specimen can be ascribed only to the natural variability and casuality of the experiment or to a specific influence of the varied process parameters. The results of the ANOVA performed on the first output parameter is summarized in Tables 4 and 5. The meaning of the parameters used in the ANOVA technique is described as follows: 1. DF (Degrees of Freedom): If the factor A is characterized by a levels and the factor B is characterized by b levels the DF value is calculated as follows: DFðAÞ ¼ a1, DFðBÞ ¼ b1, DFðABÞ ¼ ða1Þðb1Þ, DFðErrorÞ ¼ abðn1Þ and Total ¼ abn1, where n is the total number of observations. 2. SS (Sum of Squared distances): Considering the two factors A and B, SS(A) and SS(B) are the amount of variation of the estimated factor level mean around the overall mean, SS(Total) is the total variation in the data and SS(Error) is the amount of variation of the observations from their fitted values. 3. MS (Mean Squares): Considering the previously mentioned parameters the MS values can be calculated as follows: FðAÞ ¼ MSðAÞ=MSðErrorÞ, FðBÞ ¼ MSðBÞ=MSðErrorÞ, FðABÞ ¼ MSðABÞ=MS ðErrorÞ and MSðErrorÞ ¼ SSðErrorÞ=DFðErrorÞ. 4. F is the test parameter used to determine whether the interaction and main effects of the factors are significant and it is calculated as follows: MSðAÞ ¼ SSðAÞ=DFðAÞ, MSðBÞ ¼ SSðBÞ=DFðBÞ, MSðABÞ ¼ SSðABÞ=DFðABÞ and MSðErrorÞ ¼ SS ðErrorÞ=DFðErrorÞ. Larger values of F support rejecting the null hypothesis that there is not a significant effect of the selected factor. 5. P (p-value): The p-value is the probability of obtaining a test statistic that is at least as extreme as the actual calculated
Figs. 6 and 7 show the 3D interpolation surfaces expressing the output Pð%Þ as a function of the welding current and of the mutual distance between the sources. The results of the ANOVA performed on the second output parameter is summarized in Tables 6 and 7. The confidence level for both the experiments was set to 95% and, according to this, the critical value for the Fisher distribution is F ð0:05,3;12Þ ¼ 3:49.
4. Discussion According to the analysis of the results described in the previous paragraph it is evident that arc transfer mode affects the significance of process parameters on the formation of porosities in hybrid LASER-GMA welding of aluminum alloys. Considering Table 4, in fact, it can be stated that the influence of welding current on the value of Pð%Þ is statistically significant as F Current 4 3:49, while the distance between the sources is not statistically significant as F Distance o 3:49. Moreover, according to Figs. 6 and 10, it can be noticed that porosity tends to increase when welding current increases, while the process tends to become less repeatable as the variance of the output parameter Pð%Þ becomes larger for high currents. Considering the significance of the effect of welding current on the porosity, a simple linear regression model can be applied in order to calculate the relationship between Pð%Þ and the current (I) itself, as shown in the following equation: Pð%Þ ¼ 0:1602 þ 0:003565I
ð3Þ
The regression model proposed can be calculated as y ¼ b0 þ b1 x where y is the response, x is the predictor and b0
A. Ascari et al. / Optics & Laser Technology 44 (2012) 1485–1490
Interval Plot of P(%) - Short-arc 95% CI for the Mean
1.5
Interval Plot of Sm - Short-arc 95% CI for the Mean 7000 6000
1.0
5000 Sm [µm2]
P (%)
1489
0.5
4000 3000 2000
0.0
1000 -0.5 Distance [mm] Current [A]
0
3 120
6
0
3 180
0 Distance [mm] Current [A]
6
Fig. 10. Variance of Pð%Þ with respect to process parameters (short-arc). The red dots correspond to the actual measurements, the blue dots correspond to the mean values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
6
0
3 180
6
Interval Plot of Sm - Pulsed-arc 95% CI for the Mean
5000 4000 Sm [µm2]
2.0 1.5 P (%)
3 120
Fig. 12. Variance of Sm with respect to process parameters (short-arc). The red dots correspond to the actual measurements, the blue dots correspond to the mean values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Interval Plot of P(%) - Pulsed-arc 95% CI for the Mean
2.5
0
1.0 0.5
3000 2000 1000
0.0 0 Distance [mm] Current [A]
-0.5 Distance [mm] Current [A]
0
3 90
6
0
3 130
6
Fig. 11. Variance of Pð%Þ with respect to process parameters (pulsed-arc). The red dots correspond to the actual measurements, the blue dots correspond to the mean values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and b1 are computed as in Eqs. (4) and (5), where x is the mean predictor, y is the mean response, xi is the i-th predictor value and yi is the i-th response value: b0 ¼ yb1 x P b1 ¼
ðxi xÞðyi yÞ P ðxi xÞ2
ð4Þ ð5Þ
On the other hand, considering Table 5, it is clear that both the varied parameters are not statistically influent on the porosity when the arc transfer mode is set on pulsed-arc as the respective F values are both lower than 3.49. This means that the so called ‘‘null hypothesis’’ cannot be rejected and that the variation of Pð%Þ throughout the experiment is due to casuality and not to the actual variation of the considered process parameters. This result is also confirmed by the graph in Fig. 11 which shows a very large variance of the results both for high and low values of the welding current. Considering the second output parameter, Sm, the situation slightly changes. In fact the results of the ANOVA shown in Table 6 demonstrate that, in this case, not only welding current is statistically significant on the average bubble dimension ðF Current 4 3:49Þ, but also the interaction between current and distance is influent on the process as F Interaction 4 3:49. The distance between the sources still
0
3 90
6
0
3 130
6
Fig. 13. Variance of Sm with respect to process parameters (pulsed-arc). The red dots correspond to the actual measurements, the blue dots correspond to the mean values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
has no effect on the output parameter even in this case. Considering the graph in Figs. 8 and 12 it can be noticed that the variance of results tends to increase a little as welding current increases, while its behavior with respect to the distance between the sources is completely different moving from low currents to high ones. The linear regression model relating Sm to the variation of welding current can be expressed as in the following equation: Sm ¼ 551:9þ 19:29I
ð6Þ
When arc transfer mode switches to pulsed-arc the situation is radically different: the effect of welding current on the average bubble dimension Sm is significant, as shown in Table 7. In this case, in fact, F Current 43:49. Considering Figs. 9 and 13 it is clear that average bubble dimension and process variance tend to increase if welding current increases. The linear regression model relating Sm to welding current is shown in the following equation: Sm ¼ 340:2 þ 16:2I
ð7Þ
5. Conclusions This paper reports an analysis regarding the effect of process parameters on porosity formation in hybrid LASER-GMA welding
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of AA6082 aluminum alloys. The effect of GMAW current, mutual distance between the sources and arc transfer mode is studied. In particular the degree of porosity per unit area of the weld bead ðPð%ÞÞ and the average pore dimension (Sm) were chosen as output parameters and by means of ANOVA a statistical judgement on their variation was assessed as follows: 1. The influence of GMAW current is statistically significant on Pð%Þ only in short-arc transfer mode. 2. The influence of GMAW current is statistically significant on Sm both in short-arc and pulsed-arc transfer modes. 3. The influence of the distance between the sources is never statistically significant both on Pð%Þ and on Sm.
References [1] Dilthey U, Olshock S. Modern beam welding technologies in advanced pipe manufacturing. In: Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 163–9. [2] Staufer H. Laser hybrid welding and laser brazing: state of the art in technology and practice by the examples of the Audi A8 and VW Phaeton. In: Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 203–8. [3] Chung B, Rhee S, Lee C. The effect of shielding gas types on CO2 laser tailored blank weldability of low carbon automotive galvanized steel. Materials Science and Engineering A 1999;272(2):357–62. [4] Tani G, Campana G, Fortunato A, Ascari A. The influence of shielding gas in hybrid laser-mig welding. Applied Surface Science 2007;253:8050–3.
[5] Tani G, Ascari A, Campana G, Fortunato A. A study on shielding gas contamination in laser welding of non-ferrous alloys. Applied Surface Science 2007;254:904–7. [6] Campana G, Ascari A, Fortunato A, Tani G. Hybrid laser-MIG welding of aluminum alloys: the influence of shielding gases. Applied Surface Science 2009;255:5588–90. [7] Grevey D, Sallamand P, Cicala E, Ignat S. Gas protection optimization during ND: YAG laser welding. Optics and Laser Technology 2005;37(8):647–51. [8] Wang H, Shi Y, Gong S, Duan A. Effect of assist gas flow on the gas shielding during laser deep penetration welding. Journal of Materials Processing Technology 2007;184(1):379–85. [9] Campana G, Fortunato A, Ascari A, Tani G, Tomesani L. The influence of arc transfer mode in hybrid laser-MIG welding. Journal of Materials Processing Technology 2007;191:111–3. [10] Fellman A, Salminen A, Kujanpaa V. A study of the molten filler material movements during co2 laser-MAG hybrid welding. In: Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 171–6. [11] Rayes ME, Walz C, Sepold G. The influence of various hybrid welding parameters on bead geometry. Welding Journal 2004:147–53. [12] Fuhrmann C, Petring D, Poprawe R. Recent results on high power CO2 and ND:YAG laser-arc welding of thick steel plates. In: Editor A, Editor B, editors. Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 185–92. [13] Katsuna M, Chen L. Interaction of both plasmas in CO2 laser-mag hybrid welding of carbon steel. In: Proceedings of SPIE, vol. 4831; 2003. p. 341–6. [14] Ancona A, Sibillano T, Tricarico L, Spina R, Lugara P, Basile G, et al. Comparison of two different nozzles for laser beam welding of AA5083 aluminium alloy. Journal of Materials Processing Technology 2005;164: 971–7. [15] Park Y, Kim H, Lee Y, Kim J, Shin J, Ha S, et al., Study on the welding variables according to offset and separation using hybrid laser welding. In: Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 199–202. [16] Katayama, S., Naito, Y., Uchiumi, S., Mizutani, M., Penetration and porosity prevention mechanism in laser-arc hybrid welding. In: Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 193–8.
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
The influence of process parameters on porosity formation in hybrid LASER-GMA welding of AA6082 aluminum alloy Alessandro Ascari a,n, Alessandro Fortunato b, Leonardo Orazi c, Giampaolo Campana b a
DIEM – Department of Mechanical Constructions Engineering, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy DIEM – Department of Mechanical Constructions Engineering, University of Bologna, 40136 Bologna, Italy c DISMI – Department of Sciences and Methods for Engineering, University of Modena and Reggio Emilia, 42100 Reggio Emilia, Italy b
a r t i c l e i n f o
abstract
Article history: Received 28 October 2011 Received in revised form 10 December 2011 Accepted 10 December 2011 Available online 27 December 2011
This paper deals with an experimental campaign carried out on AA6082 8 mm thick plates in order to investigate the role of process parameters on porosity formation in hybrid LASER-GMA welding. Bead on plate weldments were obtained on the above mentioned aluminum alloy considering the variation of the following process parameters: GMAW current (120 and 180 A for short-arc mode, 90 and 130 A for pulsed-arc mode), arc transfer mode (short-arc and pulsed-arc) and mutual distance between arc and LASER sources (0, 3 and 6 mm). Porosities occurring in the fused zone were observed by means of X-ray inspection and measured exploiting an image analysis software. In order to understand the possible correlation between process parameters and porosity formation an analysis of variance statistical approach was exploited. The obtained results pointed out that GMAW current is significant on porosity formation, while the distance between the sources do not affect this aspect. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Porosity Hybrid welding X-ray
1. Introduction Hybrid LASER-GMA welding techniques are receiving a growing industrial attention due to the synergic effect achieved by coupling two technologically different welding methods together. The presence, in fact, of a high energy density source, such as the LASER one and of an arc source with a filler material, such as the GMA one, allows to set up a well defined process, characterized by its own peculiar characteristics. These characteristics are the result of the complementarity of the two coupled techniques and derive from the fact that in hybrid processes the main positive aspects of one involved technology allow to override the main drawbacks of the other and vice versa. In particular the most relevant benefits of hybrid LASER-GMA welding technique are high process speed, very good bridging ability, high bead penetration, possibility to modify the bead metallurgy and possibility to deal with variable gaps, misalignments and chamfers. Typical applications of hybrid LASER-GMA welding concern shipbuilding, automotive, pipeline constructions and, more in general, but joints with high thickness sheets [1] where the main difficulty is to deal with variable gaps between the sheets and to achieve single pass and high speed weldings in order to maximize productivity. On the other hand, considering the availability of modern high beam quality and fiber-deliverable LASER sources,
n
Corresponding author. Tel.: þ39 051 2090494; fax: þ 39 051 2090484. E-mail address: [email protected] (A. Ascari).
0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.12.014
such as Nd:YAG disk and fiber ones, the application of hybrid LASER-GMA welding also on low thickness sheets is becoming very common in many industrial applications [2]. According to the above mentioned considerations, the combination of two technologically different energy sources leads to several undoubted advantages, but, on the other hand, it also determines the occurrence of several technological problems which have to be faced. Some of these problems concern the stability and the repeatability of the process, mainly related to the shielding gas flux and composition [3–8], drops deposition [9,10], arc current [11,12] sources position and mutual interaction [13–15] and porosity formation [16]. In particular, mechanisms and physical phenomena related to the porosity formation are not fully understood yet even if they drastically influence the mechanical resistance especially in aluminum welding. In [16] it was first observed and described the phenomena of bubble formation in AISI 304 stainless steel and AA5052 aluminum alloy for hybrid GTA-YAG and GMA-YAG LASER welding under different process conditions and it was pointed out that arc current is the main factor in bubble formation. According to the previous assumptions, this paper investigates the influence of arc welding parameters such as arc current and transfer mode and of the mutual distance between arc andLASER sources on the number and on the extension of the porosity generated during the hybrid welding process. The experiments were carried out on a AA6082 aluminum alloy exploiting a CO2 hybrid LASER-GMA welding technique. The test specimens were analyzed by means of X-ray inspection and the porosities were counted and measured exploiting computer image analysis. The porosity related
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A. Ascari et al. / Optics & Laser Technology 44 (2012) 1485–1490
parameters were then analyzed by means of analysis of variance (ANOVA) statistical techniques in order to understand the actual correlation between process parameters and output ones and to find eventual mathematical models expressing their relationships.
Laser beam GMAW Torch Torch Inclination
Workpiece
2. Experimental setup The experimental equipment exploited in this campaign consisted of an El.En. FAF 3 kW continuous wave CO2 LASER source, equipped by a three axes CNC cell, and by a Selco 380 A synergic pulsed GMAW generator. The LASER beam was focused by means of a copper parabolic mirror with a focal length of 200 mm which allowed a minimum spot diameter of 500 mm. The sources were coupled together by means of a Binzel automatic torch fixed on the vertical (Z) axis of the CNC cell. The welding direction was set to LASER leading as presented in Fig. 1. Every experimental trial was carried out in a bead on plate configuration on AA6082 8 mm thick plates fixed on the X–Y table of the CNC cell. The obtained weld beads were straight linear ones characterized by a length equal to 100 mm. The LASER beam focal position was constantly kept on the upper surface of the workpiece. During the experimental campaign the parameters given in Table 1 were kept constant while arc current and distance between the two energy sources were varied in accordance to Table 2. The distance between the heat sources and the inclination of the torch were defined as shown in Fig. 2. In particular two levels for the arc current, combined with three levels for the sources distance, were investigated for both pulsed and short-arc GMAW configuration. Finally, three repetitions were performed for every welding configuration in order to evaluate the repeatability of the process. The gas mixture composition reported in Table 1
Laser beam
Base Material
GMAW Torch
Keyhole
Arc
D
GMAW Wire
Fig. 2. Definition of D (distance between the heat sources) and torch inclination.
Table 3 X-ray inspection parameters. Focal distance
Power
Current
Exposure
700 mm
55 kW
3 mA
60 s
Porosities
Fig. 3. Radiography showing a bead affected by a large number of porosities.
represents a good trade off between arc stability and plasma formation during welding as reported in [5]. The shielding gas was supplied through the GMA torch. According to the previous descriptions the whole experiment consisted of 36 experimental trials: (2 levels for the arc current), (3 levels for the distance between the heat sources), (3 repetitions for every trial) ¼18 samples for short-arc mode and other 18 samples for pulsedarc mode.
Bead Welding direction
3. Experimental results
Fig. 1. Hybrid welding setup.
Table 1 Constant parameters. LASER power Gas mixture Gas flow Torch inclination Welding speed Wire type Wire diameter
3 kW 60% He, 37% Ar and 3% O2 17 l/min 651 0.6 m/min Al–Mg ER5356 1.2 mm
Table 2 Variable parameters. Current – short-arc Current – pulsed-arc Distance (D)
120 and 180 A 90 and 130 A 0, 3 and 6 mm
The specimens obtained during the experimental phase were radiographed by means of a General Electric ERESCO 42MF portable equipment. The parameters used during this inspection are summarized in Table 3. Every X-ray plate was then digitally acquired by means of a double face scanning equipment in order to allow a computerized image analysis. Figs. 3 and 4 show two examples of digitized X-ray plates. By means of the image analysis software MediaCybernetics ImagePro Plus 6.0 the bubbles could be detected, counted and measured in order to characterize the porosities of the obtained welding specimens. The mentioned software, in fact, can be conveniently tuned in order to apply a so-called ‘‘segmentation’’ on the picture which allows to separate single objects (bubbles in this case) from the background of the image and to subsequently count and measure them in terms of absolute number and total area. Fig. 5 shows the objects detection phase in the mentioned software: the green objects (indicated by the arrow) were classified as porosities, while the red and white ones are classified as dust, granulometry and imperfections of the X-ray plates according to the thresholds set in the segmentation phase. Every specimen was radiographed
A. Ascari et al. / Optics & Laser Technology 44 (2012) 1485–1490
1487
P(%)
0.75 0.50
0.25 0.00
Fig. 4. Radiography showing a bead affected by a negligible number of porosities.
0 2 Dis tan ce
Green object Red and white objects
180 160
[m 4 m]
140 6
120
]
t [A
ren
Cur
Fig. 6. 3D plot of the correlation surface between the output Pð%Þ and the process parameters (short-arc). The red dots correspond to the actual measurements.
P(%)
1.5 1.0 0.5 0.0
Dis
Fig. 5. Detection and measurement of porosities by means of the Image Pro Plus software.
on a plane parallel to the upper surface of the welded plates and no particular preparation was necessary, such as polishing, etching, etc. In order to conveniently characterize and define the results of the specimens analysis two different output parameters were defined. The first one concerned the relative porosity and it was defined according to Eq. (1), where Sp is the total extension of the bubbles detected by the image analysis (in terms of mm2 ) and Sb is the total extension of the specific weld bead. Sp Sb
Sp Np
[m
120
4 m]
105
6
90
A]
ent [
Curr
ð1Þ
This parameter takes into account the degree of porosity referred to the total surface covered by the weld bead and, thus, it is independent from the absolute dimensions of the bead itself. The second output parameter taken into consideration was the average bubble area described in Eq. (2), where Np is the total number of porosities detected on a specific weld bead. Sm ¼
135
2 tan ce
Fig. 7. 3D plot of the correlation surface between the output Pð%Þ and the process parameters (pulsed-arc). The red dots correspond to the actual measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ð2Þ
The actual measurements, in terms of Pð%Þ and Sm, are represented by the red dots in Figs. 6–9. The second phase of the analysis of the results was aimed at defining the actual significance effect of the varied process parameters (welding current, mutual distance between the sources and arc transfer mode) on the output parameters Pð%Þ and Sm. In order to achieve this goal a two-ways ANOVA was conducted on the whole experiment by
5000 Sm [µm2]
Pð%Þ ¼
0
4000 3000 2000
180
0 2 tan
160
Dis
4 mm ]
ce [
140 6
120
ent
r Cur
[A]
Fig. 8. 3D plot of the correlation surface between the output Sm and the process parameters (short-arc). The red dots correspond to the actual measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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A. Ascari et al. / Optics & Laser Technology 44 (2012) 1485–1490
Table 5 ANOVA results for Pð%Þ (pulsed-arc).
Sm [µm2]
3000 2500
Source
DF
SS
MS
F
P
Current (A) Distance (mm) Interaction Error Total
1 2 2 12 17
0.09921 0.04461 0.37773 1.95325 2.47481
0.099214 0.022306 0.188865 0.162771
0.61 0.14 1.16
0.450 0.873 0.346
2000 1500 0
135 2 tan
Dis
120
ce
4 [m m]
105 6
90
A]
t[ rren
Cu
Fig. 9. 3D plot of the correlation surface between the output Sm and the process parameters (pulsed-arc). The red dots correspond to the actual measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 6 ANOVA results for Sm (short-arc). Source
DF
SS
MS
F
P
Current (A) Distance (mm) Interaction Error Total
1 2 2 12 17
6 026 390 71 311 3 169 765 4 228 547 13 496 013
6 026 390 35 656 1 584 883 352 379
17.10 0.10 4.50
0.001 0.905 0.035
Table 7 ANOVA results for Sm (pulsed-arc).
Table 4 ANOVA results for Pð%Þ (short-arc). Source
DF
SS
MS
F
P
Current (A) Distance (mm) Interaction Error Total
1 2 2 12 17
0.205926 0.085182 0.038928 0.529912 0.859948
0.205926 0.042591 0.019464 0.044159
4.66 0.96 0.44
0.052 0.409 0.654
Source
DF
SS
MS
F
P
Current (A) Distance (mm) Interaction Error Total
1 2 2 12 17
1 847 850 486 663 131 465 2 173 009 4 638 987
1 847 850 243 331 65 732 181 084
10.20 1.34 0.36
0.008 0.297 0.703
value, if the null hypothesis is true. A commonly used cut-off value for the p-value is about 0.05. means of the software Minitab 16.0. Thanks to this approach a statistical judgement could be assessed concerning the fact that the variation of porosity from specimen to specimen can be ascribed only to the natural variability and casuality of the experiment or to a specific influence of the varied process parameters. The results of the ANOVA performed on the first output parameter is summarized in Tables 4 and 5. The meaning of the parameters used in the ANOVA technique is described as follows: 1. DF (Degrees of Freedom): If the factor A is characterized by a levels and the factor B is characterized by b levels the DF value is calculated as follows: DFðAÞ ¼ a1, DFðBÞ ¼ b1, DFðABÞ ¼ ða1Þðb1Þ, DFðErrorÞ ¼ abðn1Þ and Total ¼ abn1, where n is the total number of observations. 2. SS (Sum of Squared distances): Considering the two factors A and B, SS(A) and SS(B) are the amount of variation of the estimated factor level mean around the overall mean, SS(Total) is the total variation in the data and SS(Error) is the amount of variation of the observations from their fitted values. 3. MS (Mean Squares): Considering the previously mentioned parameters the MS values can be calculated as follows: FðAÞ ¼ MSðAÞ=MSðErrorÞ, FðBÞ ¼ MSðBÞ=MSðErrorÞ, FðABÞ ¼ MSðABÞ=MS ðErrorÞ and MSðErrorÞ ¼ SSðErrorÞ=DFðErrorÞ. 4. F is the test parameter used to determine whether the interaction and main effects of the factors are significant and it is calculated as follows: MSðAÞ ¼ SSðAÞ=DFðAÞ, MSðBÞ ¼ SSðBÞ=DFðBÞ, MSðABÞ ¼ SSðABÞ=DFðABÞ and MSðErrorÞ ¼ SS ðErrorÞ=DFðErrorÞ. Larger values of F support rejecting the null hypothesis that there is not a significant effect of the selected factor. 5. P (p-value): The p-value is the probability of obtaining a test statistic that is at least as extreme as the actual calculated
Figs. 6 and 7 show the 3D interpolation surfaces expressing the output Pð%Þ as a function of the welding current and of the mutual distance between the sources. The results of the ANOVA performed on the second output parameter is summarized in Tables 6 and 7. The confidence level for both the experiments was set to 95% and, according to this, the critical value for the Fisher distribution is F ð0:05,3;12Þ ¼ 3:49.
4. Discussion According to the analysis of the results described in the previous paragraph it is evident that arc transfer mode affects the significance of process parameters on the formation of porosities in hybrid LASER-GMA welding of aluminum alloys. Considering Table 4, in fact, it can be stated that the influence of welding current on the value of Pð%Þ is statistically significant as F Current 4 3:49, while the distance between the sources is not statistically significant as F Distance o 3:49. Moreover, according to Figs. 6 and 10, it can be noticed that porosity tends to increase when welding current increases, while the process tends to become less repeatable as the variance of the output parameter Pð%Þ becomes larger for high currents. Considering the significance of the effect of welding current on the porosity, a simple linear regression model can be applied in order to calculate the relationship between Pð%Þ and the current (I) itself, as shown in the following equation: Pð%Þ ¼ 0:1602 þ 0:003565I
ð3Þ
The regression model proposed can be calculated as y ¼ b0 þ b1 x where y is the response, x is the predictor and b0
A. Ascari et al. / Optics & Laser Technology 44 (2012) 1485–1490
Interval Plot of P(%) - Short-arc 95% CI for the Mean
1.5
Interval Plot of Sm - Short-arc 95% CI for the Mean 7000 6000
1.0
5000 Sm [µm2]
P (%)
1489
0.5
4000 3000 2000
0.0
1000 -0.5 Distance [mm] Current [A]
0
3 120
6
0
3 180
0 Distance [mm] Current [A]
6
Fig. 10. Variance of Pð%Þ with respect to process parameters (short-arc). The red dots correspond to the actual measurements, the blue dots correspond to the mean values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
6
0
3 180
6
Interval Plot of Sm - Pulsed-arc 95% CI for the Mean
5000 4000 Sm [µm2]
2.0 1.5 P (%)
3 120
Fig. 12. Variance of Sm with respect to process parameters (short-arc). The red dots correspond to the actual measurements, the blue dots correspond to the mean values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Interval Plot of P(%) - Pulsed-arc 95% CI for the Mean
2.5
0
1.0 0.5
3000 2000 1000
0.0 0 Distance [mm] Current [A]
-0.5 Distance [mm] Current [A]
0
3 90
6
0
3 130
6
Fig. 11. Variance of Pð%Þ with respect to process parameters (pulsed-arc). The red dots correspond to the actual measurements, the blue dots correspond to the mean values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and b1 are computed as in Eqs. (4) and (5), where x is the mean predictor, y is the mean response, xi is the i-th predictor value and yi is the i-th response value: b0 ¼ yb1 x P b1 ¼
ðxi xÞðyi yÞ P ðxi xÞ2
ð4Þ ð5Þ
On the other hand, considering Table 5, it is clear that both the varied parameters are not statistically influent on the porosity when the arc transfer mode is set on pulsed-arc as the respective F values are both lower than 3.49. This means that the so called ‘‘null hypothesis’’ cannot be rejected and that the variation of Pð%Þ throughout the experiment is due to casuality and not to the actual variation of the considered process parameters. This result is also confirmed by the graph in Fig. 11 which shows a very large variance of the results both for high and low values of the welding current. Considering the second output parameter, Sm, the situation slightly changes. In fact the results of the ANOVA shown in Table 6 demonstrate that, in this case, not only welding current is statistically significant on the average bubble dimension ðF Current 4 3:49Þ, but also the interaction between current and distance is influent on the process as F Interaction 4 3:49. The distance between the sources still
0
3 90
6
0
3 130
6
Fig. 13. Variance of Sm with respect to process parameters (pulsed-arc). The red dots correspond to the actual measurements, the blue dots correspond to the mean values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
has no effect on the output parameter even in this case. Considering the graph in Figs. 8 and 12 it can be noticed that the variance of results tends to increase a little as welding current increases, while its behavior with respect to the distance between the sources is completely different moving from low currents to high ones. The linear regression model relating Sm to the variation of welding current can be expressed as in the following equation: Sm ¼ 551:9þ 19:29I
ð6Þ
When arc transfer mode switches to pulsed-arc the situation is radically different: the effect of welding current on the average bubble dimension Sm is significant, as shown in Table 7. In this case, in fact, F Current 43:49. Considering Figs. 9 and 13 it is clear that average bubble dimension and process variance tend to increase if welding current increases. The linear regression model relating Sm to welding current is shown in the following equation: Sm ¼ 340:2 þ 16:2I
ð7Þ
5. Conclusions This paper reports an analysis regarding the effect of process parameters on porosity formation in hybrid LASER-GMA welding
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of AA6082 aluminum alloys. The effect of GMAW current, mutual distance between the sources and arc transfer mode is studied. In particular the degree of porosity per unit area of the weld bead ðPð%ÞÞ and the average pore dimension (Sm) were chosen as output parameters and by means of ANOVA a statistical judgement on their variation was assessed as follows: 1. The influence of GMAW current is statistically significant on Pð%Þ only in short-arc transfer mode. 2. The influence of GMAW current is statistically significant on Sm both in short-arc and pulsed-arc transfer modes. 3. The influence of the distance between the sources is never statistically significant both on Pð%Þ and on Sm.
References [1] Dilthey U, Olshock S. Modern beam welding technologies in advanced pipe manufacturing. In: Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 163–9. [2] Staufer H. Laser hybrid welding and laser brazing: state of the art in technology and practice by the examples of the Audi A8 and VW Phaeton. In: Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 203–8. [3] Chung B, Rhee S, Lee C. The effect of shielding gas types on CO2 laser tailored blank weldability of low carbon automotive galvanized steel. Materials Science and Engineering A 1999;272(2):357–62. [4] Tani G, Campana G, Fortunato A, Ascari A. The influence of shielding gas in hybrid laser-mig welding. Applied Surface Science 2007;253:8050–3.
[5] Tani G, Ascari A, Campana G, Fortunato A. A study on shielding gas contamination in laser welding of non-ferrous alloys. Applied Surface Science 2007;254:904–7. [6] Campana G, Ascari A, Fortunato A, Tani G. Hybrid laser-MIG welding of aluminum alloys: the influence of shielding gases. Applied Surface Science 2009;255:5588–90. [7] Grevey D, Sallamand P, Cicala E, Ignat S. Gas protection optimization during ND: YAG laser welding. Optics and Laser Technology 2005;37(8):647–51. [8] Wang H, Shi Y, Gong S, Duan A. Effect of assist gas flow on the gas shielding during laser deep penetration welding. Journal of Materials Processing Technology 2007;184(1):379–85. [9] Campana G, Fortunato A, Ascari A, Tani G, Tomesani L. The influence of arc transfer mode in hybrid laser-MIG welding. Journal of Materials Processing Technology 2007;191:111–3. [10] Fellman A, Salminen A, Kujanpaa V. A study of the molten filler material movements during co2 laser-MAG hybrid welding. In: Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 171–6. [11] Rayes ME, Walz C, Sepold G. The influence of various hybrid welding parameters on bead geometry. Welding Journal 2004:147–53. [12] Fuhrmann C, Petring D, Poprawe R. Recent results on high power CO2 and ND:YAG laser-arc welding of thick steel plates. In: Editor A, Editor B, editors. Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 185–92. [13] Katsuna M, Chen L. Interaction of both plasmas in CO2 laser-mag hybrid welding of carbon steel. In: Proceedings of SPIE, vol. 4831; 2003. p. 341–6. [14] Ancona A, Sibillano T, Tricarico L, Spina R, Lugara P, Basile G, et al. Comparison of two different nozzles for laser beam welding of AA5083 aluminium alloy. Journal of Materials Processing Technology 2005;164: 971–7. [15] Park Y, Kim H, Lee Y, Kim J, Shin J, Ha S, et al., Study on the welding variables according to offset and separation using hybrid laser welding. In: Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 199–202. [16] Katayama, S., Naito, Y., Uchiumi, S., Mizutani, M., Penetration and porosity prevention mechanism in laser-arc hybrid welding. In: Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 193–8.