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Auger electron spectroscopy on a laser-welded 6061 Al alloy
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June 1994
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Material Letters 20 ( 1994) 1 1- 17
Auger electron spectroscopy on a laser-welded 606 1 Al alloy Paolo Delogu, Sebastian0 Tosto ENEA CRE Casaccia. via Anguillarese 301. 00060 Rome, Italy Received 6 January 1994; in final form 20 March 1994; accepted 22 March I994
Abstract Auger spectroscopy has been utilized to investigate the mechanisms of formation of porosity in a laser-welded 6061 Al alloy. The analyses carried out on the internal surface of the porosity of samples cleaved under ultra high vacuum have shown evidence for preferential enrichment of volatile impurities.
1. Introduction
The welding of Al alloys is a difficult task because of the tendency to form porosity during solidification of the molten pool [ 11. It is widely recognized that the occurrence of porosity is mainly due to the strong dependence of the solubility of hydrogen in liquid aluminium on temperature: values of 0.7 to 50 cm3/ 100 g are reported in the literature in the range 6602000°C [ 21. This seventy-fold increase in the temperature range typically found during plasma or arc welding causes first solution of hydrogen in the weld pool and then coalescence of gas bubbles during cooling and solidification. Such an effect becomes particularly critical in the case of laser or electron beam welding, where the possible escape of gas bubbles is prevented by high cooling rates. Such effects are found also in several Al alloys [ 3 1. Hydrogen comes usually from the thermal decomposition of pollutants deposited and adsorbed on the surface of the alloy during manufacturing (for instance, from rolling oils or hydrocarbons). In any case, the detrimental effect of porosity on the mechanical properties of joints [4] prevents advanced applications of the Al alloys. Such problems have been effectively found also during our treatments of welding of the 6061 Al alloy for aero-
nautical components. The purpose of the present paper is: (i) to show some preliminary results of the AES analysis of a CO1 laser-welded 6061 alloy and (ii) to provide information to better understand the mechanisms of formation of porosity during laser irradiation.
2. Experimental Thin sheets of 6061 Al-Mg-Si commercial alloy with dimensions 30 x 100 mm and 2.3 mm thick have been butt welded, scanning the laser beam with a constant velocity along the longest dimension. The nominal composition of the alloy was: 1.O%Mg, 0.6% Si, 0.28% Cu, 0.2% Cr, Al balance. Laser processing conditions were as follows: beam power 2 kW, travel speed 2 m/min, beam radius ~0.4 mm. Cross sections of the welded samples were polished and etched according to the usual metallographic procedures and observed by a scanning electron microscope (SEM). Pores have been found in the weld bead. Fig. 1 shows in detail one of them. Auger electron spectroscopy (AES) has also been carried out utilizing a PHI 600 spectrometer to investigate in particular the chemical composition of the internal surface of the cavities,
0167-577x/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0167-577x(94)00062-R
P. Deiogu,S. Tosto/M~ter~~~Letters20 (1994) I l-i 7
Fig. 1. SEM image of the internal surface of a pore in the weld bead.
Fig. 2. Secondary electron image of a cavity in the laser-welded zone. Sample fractured under ultra high vacuum.
revealed by cleavage of the weld bead at the liquid nitrogen temperature under ultra high vacuum (UHV). To this purpose, some specimens were cut normally to the weld line and machined to obtain notched rods 3.2 cm long with a diameter of 2.3 mm. The notch enabled a brittle fracture to be induced just in the welded zone. Semi-quantitative point analyses and Auger mapping have been carried out to compare the chemical composition of the internal surface of the pores statistically present on the fracture plane and that of the grain boundaries in the laser affected zone. The accelerating voltage of the electron probe was 5 kV for the Auger maps and 10 kV for the point analyses. The diameter of the beam was about 1000 A. The energy resolution at constant Al?/E ratio of the CMA was 0.3%. The residual pressure in the specimen chamber was 2 x IO- **Torr. As an exampie of the results obtained, Fig. 2 shows a secondary electron image of a cavity obtained with the Auger spectrometer on a fractured specimen. Figs. 3a and 3b report the Auger analyses in points 1 and 2 of Fig. 2. Elements like S, P, Cl and K are present only on the internal surface of the cavity. Moreover, also a considerable Mg enrichment was found in the cavities. Fig. 4 shows another cavity where an inclusion is also present. The Auger maps of 0, N and K in the cavity of Fig. 4 are shown in Figs. 5-7. Fig. 8 reports a detail of the Auger peaks of Al in the points indicated in Fig. 4.
3. Discussion The chemical composition of the internal surface of the cavities was found to be very different from that of the bulk of the weld bead cleaved under UHV. It is apparent comparing the spectra of Figs. 3a and 3b referred to Fig. 2. As concerns Al, two Auger peaks at 68 and 57 eV were found within the cavities and in the inclusions inside them, whilst only the peak at 68 eV was found on the cleaved surface. A typical Auger spectrum of this element is reported in Fig. 8. The LMM peak at 68 eV is due to metallic Al. The chemical shift of the peak at 57 eV shows that also bound Al is present within the cavities. The main peak of Al oxide is reported in the literature at 5 1 eV; this suggests that the peak at 57 eV of Fig. 8 is due presumably to some (Al, N, 0) compound or to a nonequilibrium phase whose identification is in progress. In effect, Figs. 5 and 6 show that both 0 and N are preferentially deposited on the internal surface of the cavity of Fig. 4. Also interesting is the case of Mg. The intensity of the Auger peak of this element in the bulk of the molten zone is very small with respect to that of Al, in agreement with the fact that its nominal content in the ahoy is about 1%. Being the full scale of the Auger spectrum no~alized to the most intense Al peak, Mg is clearly visible only in case of local preferential enrichment. Fig. 9 compares the peaks of Mg in the bulk of the molten zone and within the cavity. In the former case (point 2 ), the position of the KLL peak
P. Delogu,S. Tosto/MaterialsLetters20 (1994) 1I- I? ZUR’fE ‘f
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Fig. 3. Auger analysis in (a) point 1 of Fig. 2, and (b) point 2 of Fig. 2.
( 1186 eV) agrees with that reported in the literature for the metal phase. A considerable enrichment is found within the cavity, where the peak is shifted towards a lower kinetic energy ( 1180 eV), close to the value reported in the literature for MgO, 1I74 eV. In another point of the cavity, point 3, both peaks are present and appear convoluted. This suggests therefore that again a non-stoichiometric oxide, whose identi~cation is in progress, has been formed during melting. In any case, regardless of the chemical na-
ture of the phases formed, it is remarkable that the Mg enrichment and the impurities are found systematically within the cavities and not on the grain boundaries evidenced by the UHV cleavage. This suggests that these elements were evaporated and deposited during the growth of the bubble. To summarize, the experimental results show that: (i) the boiling of volatile elements including those present as impurities in the alloy, is involved in the mechanism of formation of the cavity; (ii) the pref-
14
P. Delogu, S. Tosto /Materials Letters 20 (I 994) I I- I7
Fig. 4. Auger image of a cavity in the fracture plane of the laserwelded sample cleaved under ultra high vacuum. Point 1: internal surface of the cavity. Point 2: cleaved welded alloy. Point 3: inclusion within the cavity.
Fig. 6. Auger map of nitrogen corresponding to Fig. 4.
Fig. 7. Auger map of potassium corresponding to Fig. 4.
Fig. 5. Auger map of oxygen corresponding to Fig. 4.
erential enrichment of these elements is a specific feature of the pores and not of the grain boundaries. The composition fluctuations in the liquid phase and the microsegregation of alloying elements and impurities in the fusion zone are well-known phenomena [4] and can explain the present results. Then, the mechanism of formation of bubbles does not involve only hydrogen or residual moisture on the surfaces to be welded; rather, it is more complex than so far believed because of the uncontrolled introduction of impurities during the industrial manufacturing of the alloy. There are at least five main reasons to stress
the importance of the local anomalous chemical composition: ( 1) It is known that the solubility of hydrogen in liquid Al as a function of temperature depends on the local chemical composition; for instance, the presence of Cu reduces the solubility [ 51 whilst Mg increases it [ 6 1. No literature data have been found about the specific effect of elements like K or P on the solubility of hydrogen in liquid aluminium. However, it is likely to be expected that also these impurities affect the dynamical equilibrium between the amount of hydrogen in solution in the liquid phase and that coalesced to form the bubble. (2) Particles of A&O3have been found often within the cavities. This agrees with the idea that heteroge-
15
P. Delogu, S. Tosto /Materials Letters 20 (1994) 11-l 7 oxide
-_3
l3
40
metal
60
50 KINETIC
ENERGY
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Fig. 8. Detail of the Auger peaks of Al in points 1, 2 and 3 of Fig. 4.
neous nucleation is the main mechanism responsible for the formation of the bubbles. In effect, it is reported in the literature that homogeneous nucleation of embryonic bubbles is difficult [ 5 1. A reason preventing spontaneous nucleation is the surface tension of the liquid Al, i.e. the energy barrier required to separate two layers of liquid. The gas pressure to grow the bubble is given in fact by P= Phydr-I-20/r , where r is the radius of the bubble, o the surface tension and Phydrthe room pressure plus that due to the liquid column above the bubble. Assuming that Phydr is equal to the ambient pressure with a good approximation, the capillary term is negligible only at large values of r, but plays a major role during the early stages of formation and growth of the bubble. How-
1150
1160
1170
KINETIC
1190
ENERGY
1190
1200
(eV)
Fig. 9. Detail of the Auger peaks of Mg in points 1, 2 and 3 of Fig. 4.
ever, it is known that the presence of impurities lowers the surface tension of a liquid metal [ 7 ] and thus also the energy per unit surface required to form a bubble. Then, the present results suggest that the doping of the layers immediately near to the internal surface of the bubbles makes more probable the formation of cavities also by homogeneous nucleation. In effect, this agrees with the fact that several cavities obtained by UHV cleavage were found free of oxide particles, as shown for instance in Fig. 2. (3) It is widely accepted that surface-tension driven mass flow under temperature gradients is the main mechanism responsible for convection and remixing in the molten pool. In effect, the capillary and buoyancy forces enable a homogeneous chemical composition of the liquid phase. Clearly, the pres-
16
P. Deiogu,S. Tosto /materials Letters 20 (1994) I l-l 7
ence of surface active impurities reduces the capillary flow and thus the efficiency of remixing [ 71. Therefore, it is reasonable to expect that the local enrichment of elements like P or S acts as a self-inhibiting factor in the homogenisation of chemical composition. This explains the presence of these zones in spite of the strong fluid dynamics of the laser welding process. (4) The partial pressure of the volatile impurities contributes itself, together with that of hydrogen, to grow the bubble. According to the phase diagram of Al-H [ 8 1, about 2 cm3 of H are in solution in 100 g of liquid Al at 1000 K under the pressure of 1 bar. It corresponds to an order of magnitude of 10 ppm of H2 in Al. This order of magnitude corresponds to the typical content of impurities in the commercial Al alloys commonly available. This shows that even traces of volatile elements evaporating under laser irradiation can produce an effect comparable with that of hydrogen, so far considered the only gas controlling the growth of the bubble. Of course, a synergic effect of both is to be expected in the weld pool. (5) Mg is a volatile element. It is reported in the literature that small concentrations of this element cause a rise in the vapor pressure of the molten alloy and stabilize the keyhole [ 91; moreover, Cieslak [ lo] and Schauer [ 111 have found that increasing the Mg content decreases the temperature at the keyhole surface. This explains the experimental fact reported in Ref. [9] that the weld depth of Al alloys increases with the Mg content, the heat input per unit length being the same. Therefore, it is to be expected that the local depletion of this element by oxide formation perturbes the stability of the keyhole and the weld depth. Moreover, the presence of a metal phase on the inner surface of the cavity shows that also the partial pressure of Mg vapour contributes to the formation of the pores. The presence of porosities has been found also during the CO2 laser remelting of a plasma-sprayed Ni coating on a 22 19 Al alloy, containing about 6 wt% of Cu as main alloy element [ 121. Also in this case, the cleavage under high vacuum of laser-treated samples enabled a comparison of the chemical composition in the bulk of alloyed zone and in the pores formed in it. The experimental results, in particular the presence of elements such as K, Cl and 0 on the surface of the pores, were analogous to those de-
scribed in the present paper, in spite of the different treatment conditions and chemical composition of the alloys. This suggests that the segregation and coalescence of volatile impurities under laser irradiation of the Al alloys must be regarded as a systematic consequence of the surface pollution rather than as a peculiarity of either the welding treatment or the surface processing.
The results of the present research show that the deformation of porosity during laser welding of a 606 1 Al alloy is controlled not only by the presence of gases (mainly hydrogen and moisture) dissolved in the molten pool but also by the amount of volatile elements and impurities introduced in the alloy probably during manufacturing. In effect, the discussion carried out shows that also the impurities control the mechanisms of formation of porosities during laser welding of the alloy. The activity presently in progress is aimed to investigate in a more systematic way these problems by means of Auger spectroscopy and electron microscopy.
Acknowledgement Thanks are due to Dr. M. Corchia for stimulating discussions.
References [ 1] R.C. Andrew and G.A. Chadwick, Welding Res. Abroad XXIV (1978) 20.
[ 21 W. Tuttle, Welding J., February 1991, p, 43. [ 31 S. Kou, Welding Res. Council Bull., No. 320, December 1986.
[ 41 S. Kou, Welding metallurgy (Wiley, New York, 1988) pp. 74ff.
[ 51J.H. Devletian and W.E. Wood, Welding Res. Council Bull. No. 290, Suppl., December 1983. [6] R.J. Shore and R.B. McCauley, Welding J., July 1970, p. 311. [ 71 J.F. Lancaster, Physics of welding (Pergamon Press, Oxford, 1984) p. 36. [ 8 ] T.B. Massalsky, Binary phase diagrams (American Society for Metals, Metals Park, Ohio, 1986) p. 118.
P. Delogu, S. Tosto/Materials Letters20 (1994) 11-l 7 [9] J. Rapp, C. Glumann, F. Dausinger and H. Hugel, in: 5th International Conference on Welding and Melting by Electron and Laser Beams “CISFEL”, 14-18 June, 1993 (Institut de Soudure, CEA and TIW, La Baule) pp. 275272. [lo] M.J. Cieslak and P.W. Feuersbach, Metall. Trans. B 19 (1988) 319.
1-l
[ 111 B.A. Schauer, W.H. Giedt and SM. Shintaku, Welding Res. Suppl., May 1978, p. 127.
[ 121 M. Corchia, P. Delogu, R. Giorgi, S. Tosto, P. Antona and S. Appiano, in: Proceedings of the International Conference ISATA, Florence, June 1988, ed. J. Soliman.
K
June 1994
$1
B
4._
ELWVlF,K
Material Letters 20 ( 1994) 1 1- 17
Auger electron spectroscopy on a laser-welded 606 1 Al alloy Paolo Delogu, Sebastian0 Tosto ENEA CRE Casaccia. via Anguillarese 301. 00060 Rome, Italy Received 6 January 1994; in final form 20 March 1994; accepted 22 March I994
Abstract Auger spectroscopy has been utilized to investigate the mechanisms of formation of porosity in a laser-welded 6061 Al alloy. The analyses carried out on the internal surface of the porosity of samples cleaved under ultra high vacuum have shown evidence for preferential enrichment of volatile impurities.
1. Introduction
The welding of Al alloys is a difficult task because of the tendency to form porosity during solidification of the molten pool [ 11. It is widely recognized that the occurrence of porosity is mainly due to the strong dependence of the solubility of hydrogen in liquid aluminium on temperature: values of 0.7 to 50 cm3/ 100 g are reported in the literature in the range 6602000°C [ 21. This seventy-fold increase in the temperature range typically found during plasma or arc welding causes first solution of hydrogen in the weld pool and then coalescence of gas bubbles during cooling and solidification. Such an effect becomes particularly critical in the case of laser or electron beam welding, where the possible escape of gas bubbles is prevented by high cooling rates. Such effects are found also in several Al alloys [ 3 1. Hydrogen comes usually from the thermal decomposition of pollutants deposited and adsorbed on the surface of the alloy during manufacturing (for instance, from rolling oils or hydrocarbons). In any case, the detrimental effect of porosity on the mechanical properties of joints [4] prevents advanced applications of the Al alloys. Such problems have been effectively found also during our treatments of welding of the 6061 Al alloy for aero-
nautical components. The purpose of the present paper is: (i) to show some preliminary results of the AES analysis of a CO1 laser-welded 6061 alloy and (ii) to provide information to better understand the mechanisms of formation of porosity during laser irradiation.
2. Experimental Thin sheets of 6061 Al-Mg-Si commercial alloy with dimensions 30 x 100 mm and 2.3 mm thick have been butt welded, scanning the laser beam with a constant velocity along the longest dimension. The nominal composition of the alloy was: 1.O%Mg, 0.6% Si, 0.28% Cu, 0.2% Cr, Al balance. Laser processing conditions were as follows: beam power 2 kW, travel speed 2 m/min, beam radius ~0.4 mm. Cross sections of the welded samples were polished and etched according to the usual metallographic procedures and observed by a scanning electron microscope (SEM). Pores have been found in the weld bead. Fig. 1 shows in detail one of them. Auger electron spectroscopy (AES) has also been carried out utilizing a PHI 600 spectrometer to investigate in particular the chemical composition of the internal surface of the cavities,
0167-577x/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0167-577x(94)00062-R
P. Deiogu,S. Tosto/M~ter~~~Letters20 (1994) I l-i 7
Fig. 1. SEM image of the internal surface of a pore in the weld bead.
Fig. 2. Secondary electron image of a cavity in the laser-welded zone. Sample fractured under ultra high vacuum.
revealed by cleavage of the weld bead at the liquid nitrogen temperature under ultra high vacuum (UHV). To this purpose, some specimens were cut normally to the weld line and machined to obtain notched rods 3.2 cm long with a diameter of 2.3 mm. The notch enabled a brittle fracture to be induced just in the welded zone. Semi-quantitative point analyses and Auger mapping have been carried out to compare the chemical composition of the internal surface of the pores statistically present on the fracture plane and that of the grain boundaries in the laser affected zone. The accelerating voltage of the electron probe was 5 kV for the Auger maps and 10 kV for the point analyses. The diameter of the beam was about 1000 A. The energy resolution at constant Al?/E ratio of the CMA was 0.3%. The residual pressure in the specimen chamber was 2 x IO- **Torr. As an exampie of the results obtained, Fig. 2 shows a secondary electron image of a cavity obtained with the Auger spectrometer on a fractured specimen. Figs. 3a and 3b report the Auger analyses in points 1 and 2 of Fig. 2. Elements like S, P, Cl and K are present only on the internal surface of the cavity. Moreover, also a considerable Mg enrichment was found in the cavities. Fig. 4 shows another cavity where an inclusion is also present. The Auger maps of 0, N and K in the cavity of Fig. 4 are shown in Figs. 5-7. Fig. 8 reports a detail of the Auger peaks of Al in the points indicated in Fig. 4.
3. Discussion The chemical composition of the internal surface of the cavities was found to be very different from that of the bulk of the weld bead cleaved under UHV. It is apparent comparing the spectra of Figs. 3a and 3b referred to Fig. 2. As concerns Al, two Auger peaks at 68 and 57 eV were found within the cavities and in the inclusions inside them, whilst only the peak at 68 eV was found on the cleaved surface. A typical Auger spectrum of this element is reported in Fig. 8. The LMM peak at 68 eV is due to metallic Al. The chemical shift of the peak at 57 eV shows that also bound Al is present within the cavities. The main peak of Al oxide is reported in the literature at 5 1 eV; this suggests that the peak at 57 eV of Fig. 8 is due presumably to some (Al, N, 0) compound or to a nonequilibrium phase whose identification is in progress. In effect, Figs. 5 and 6 show that both 0 and N are preferentially deposited on the internal surface of the cavity of Fig. 4. Also interesting is the case of Mg. The intensity of the Auger peak of this element in the bulk of the molten zone is very small with respect to that of Al, in agreement with the fact that its nominal content in the ahoy is about 1%. Being the full scale of the Auger spectrum no~alized to the most intense Al peak, Mg is clearly visible only in case of local preferential enrichment. Fig. 9 compares the peaks of Mg in the bulk of the molten zone and within the cavity. In the former case (point 2 ), the position of the KLL peak
P. Delogu,S. Tosto/MaterialsLetters20 (1994) 1I- I? ZUR’fE ‘f
~~-9.41~5,
.oon
m1=15.38
PTSI
wf3w87
13
m.el
P-C
.. _~.-_-v-+-.i .’ “. _--...a‘-,._,i _T_.,. . tt . : ..__....i_ -..
:
i
. .. .__.. Porn1
,
I
..-...--.
:
_. +:::::::::t+-+++:::::::::-;:::: +-+ 488 588 688 30 8 KINETIC
ENERGY,
788
808
988
1898
EV
6
e 188
288
388
4ee KINETIC
see 688 EHERSY, EY
788
888
988
1aae
Fig. 3. Auger analysis in (a) point 1 of Fig. 2, and (b) point 2 of Fig. 2.
( 1186 eV) agrees with that reported in the literature for the metal phase. A considerable enrichment is found within the cavity, where the peak is shifted towards a lower kinetic energy ( 1180 eV), close to the value reported in the literature for MgO, 1I74 eV. In another point of the cavity, point 3, both peaks are present and appear convoluted. This suggests therefore that again a non-stoichiometric oxide, whose identi~cation is in progress, has been formed during melting. In any case, regardless of the chemical na-
ture of the phases formed, it is remarkable that the Mg enrichment and the impurities are found systematically within the cavities and not on the grain boundaries evidenced by the UHV cleavage. This suggests that these elements were evaporated and deposited during the growth of the bubble. To summarize, the experimental results show that: (i) the boiling of volatile elements including those present as impurities in the alloy, is involved in the mechanism of formation of the cavity; (ii) the pref-
14
P. Delogu, S. Tosto /Materials Letters 20 (I 994) I I- I7
Fig. 4. Auger image of a cavity in the fracture plane of the laserwelded sample cleaved under ultra high vacuum. Point 1: internal surface of the cavity. Point 2: cleaved welded alloy. Point 3: inclusion within the cavity.
Fig. 6. Auger map of nitrogen corresponding to Fig. 4.
Fig. 7. Auger map of potassium corresponding to Fig. 4.
Fig. 5. Auger map of oxygen corresponding to Fig. 4.
erential enrichment of these elements is a specific feature of the pores and not of the grain boundaries. The composition fluctuations in the liquid phase and the microsegregation of alloying elements and impurities in the fusion zone are well-known phenomena [4] and can explain the present results. Then, the mechanism of formation of bubbles does not involve only hydrogen or residual moisture on the surfaces to be welded; rather, it is more complex than so far believed because of the uncontrolled introduction of impurities during the industrial manufacturing of the alloy. There are at least five main reasons to stress
the importance of the local anomalous chemical composition: ( 1) It is known that the solubility of hydrogen in liquid Al as a function of temperature depends on the local chemical composition; for instance, the presence of Cu reduces the solubility [ 51 whilst Mg increases it [ 6 1. No literature data have been found about the specific effect of elements like K or P on the solubility of hydrogen in liquid aluminium. However, it is likely to be expected that also these impurities affect the dynamical equilibrium between the amount of hydrogen in solution in the liquid phase and that coalesced to form the bubble. (2) Particles of A&O3have been found often within the cavities. This agrees with the idea that heteroge-
15
P. Delogu, S. Tosto /Materials Letters 20 (1994) 11-l 7 oxide
-_3
l3
40
metal
60
50 KINETIC
ENERGY
70
I
80
IEvl
Fig. 8. Detail of the Auger peaks of Al in points 1, 2 and 3 of Fig. 4.
neous nucleation is the main mechanism responsible for the formation of the bubbles. In effect, it is reported in the literature that homogeneous nucleation of embryonic bubbles is difficult [ 5 1. A reason preventing spontaneous nucleation is the surface tension of the liquid Al, i.e. the energy barrier required to separate two layers of liquid. The gas pressure to grow the bubble is given in fact by P= Phydr-I-20/r , where r is the radius of the bubble, o the surface tension and Phydrthe room pressure plus that due to the liquid column above the bubble. Assuming that Phydr is equal to the ambient pressure with a good approximation, the capillary term is negligible only at large values of r, but plays a major role during the early stages of formation and growth of the bubble. How-
1150
1160
1170
KINETIC
1190
ENERGY
1190
1200
(eV)
Fig. 9. Detail of the Auger peaks of Mg in points 1, 2 and 3 of Fig. 4.
ever, it is known that the presence of impurities lowers the surface tension of a liquid metal [ 7 ] and thus also the energy per unit surface required to form a bubble. Then, the present results suggest that the doping of the layers immediately near to the internal surface of the bubbles makes more probable the formation of cavities also by homogeneous nucleation. In effect, this agrees with the fact that several cavities obtained by UHV cleavage were found free of oxide particles, as shown for instance in Fig. 2. (3) It is widely accepted that surface-tension driven mass flow under temperature gradients is the main mechanism responsible for convection and remixing in the molten pool. In effect, the capillary and buoyancy forces enable a homogeneous chemical composition of the liquid phase. Clearly, the pres-
16
P. Deiogu,S. Tosto /materials Letters 20 (1994) I l-l 7
ence of surface active impurities reduces the capillary flow and thus the efficiency of remixing [ 71. Therefore, it is reasonable to expect that the local enrichment of elements like P or S acts as a self-inhibiting factor in the homogenisation of chemical composition. This explains the presence of these zones in spite of the strong fluid dynamics of the laser welding process. (4) The partial pressure of the volatile impurities contributes itself, together with that of hydrogen, to grow the bubble. According to the phase diagram of Al-H [ 8 1, about 2 cm3 of H are in solution in 100 g of liquid Al at 1000 K under the pressure of 1 bar. It corresponds to an order of magnitude of 10 ppm of H2 in Al. This order of magnitude corresponds to the typical content of impurities in the commercial Al alloys commonly available. This shows that even traces of volatile elements evaporating under laser irradiation can produce an effect comparable with that of hydrogen, so far considered the only gas controlling the growth of the bubble. Of course, a synergic effect of both is to be expected in the weld pool. (5) Mg is a volatile element. It is reported in the literature that small concentrations of this element cause a rise in the vapor pressure of the molten alloy and stabilize the keyhole [ 91; moreover, Cieslak [ lo] and Schauer [ 111 have found that increasing the Mg content decreases the temperature at the keyhole surface. This explains the experimental fact reported in Ref. [9] that the weld depth of Al alloys increases with the Mg content, the heat input per unit length being the same. Therefore, it is to be expected that the local depletion of this element by oxide formation perturbes the stability of the keyhole and the weld depth. Moreover, the presence of a metal phase on the inner surface of the cavity shows that also the partial pressure of Mg vapour contributes to the formation of the pores. The presence of porosities has been found also during the CO2 laser remelting of a plasma-sprayed Ni coating on a 22 19 Al alloy, containing about 6 wt% of Cu as main alloy element [ 121. Also in this case, the cleavage under high vacuum of laser-treated samples enabled a comparison of the chemical composition in the bulk of alloyed zone and in the pores formed in it. The experimental results, in particular the presence of elements such as K, Cl and 0 on the surface of the pores, were analogous to those de-
scribed in the present paper, in spite of the different treatment conditions and chemical composition of the alloys. This suggests that the segregation and coalescence of volatile impurities under laser irradiation of the Al alloys must be regarded as a systematic consequence of the surface pollution rather than as a peculiarity of either the welding treatment or the surface processing.
The results of the present research show that the deformation of porosity during laser welding of a 606 1 Al alloy is controlled not only by the presence of gases (mainly hydrogen and moisture) dissolved in the molten pool but also by the amount of volatile elements and impurities introduced in the alloy probably during manufacturing. In effect, the discussion carried out shows that also the impurities control the mechanisms of formation of porosities during laser welding of the alloy. The activity presently in progress is aimed to investigate in a more systematic way these problems by means of Auger spectroscopy and electron microscopy.
Acknowledgement Thanks are due to Dr. M. Corchia for stimulating discussions.
References [ 1] R.C. Andrew and G.A. Chadwick, Welding Res. Abroad XXIV (1978) 20.
[ 21 W. Tuttle, Welding J., February 1991, p, 43. [ 31 S. Kou, Welding Res. Council Bull., No. 320, December 1986.
[ 41 S. Kou, Welding metallurgy (Wiley, New York, 1988) pp. 74ff.
[ 51J.H. Devletian and W.E. Wood, Welding Res. Council Bull. No. 290, Suppl., December 1983. [6] R.J. Shore and R.B. McCauley, Welding J., July 1970, p. 311. [ 71 J.F. Lancaster, Physics of welding (Pergamon Press, Oxford, 1984) p. 36. [ 8 ] T.B. Massalsky, Binary phase diagrams (American Society for Metals, Metals Park, Ohio, 1986) p. 118.
P. Delogu, S. Tosto/Materials Letters20 (1994) 11-l 7 [9] J. Rapp, C. Glumann, F. Dausinger and H. Hugel, in: 5th International Conference on Welding and Melting by Electron and Laser Beams “CISFEL”, 14-18 June, 1993 (Institut de Soudure, CEA and TIW, La Baule) pp. 275272. [lo] M.J. Cieslak and P.W. Feuersbach, Metall. Trans. B 19 (1988) 319.
1-l
[ 111 B.A. Schauer, W.H. Giedt and SM. Shintaku, Welding Res. Suppl., May 1978, p. 127.
[ 121 M. Corchia, P. Delogu, R. Giorgi, S. Tosto, P. Antona and S. Appiano, in: Proceedings of the International Conference ISATA, Florence, June 1988, ed. J. Soliman.