CO2 Laser Beam Welding of 6061-T6 Aluminum Alloy Thin ... Laser beam welding is an attractive welding process for age-hardened aluminum alloys, because ...
CO2 Laser Beam Welding of 6061-T6 Aluminum Alloy Thin Plate AKIO HIROSE, HIROTAKA TODAKA, and KOJIRO F. KOBAYASHI Laser beam welding is an attractive welding process for age-hardened aluminum alloys, because its low heat input minimizes the width of weld fusion and heat-affected zones (HAZs). In the present work, 1-mm-thick age-hardened Al-Mg-Si alloy, 6061-T6, plates were welded with full penetration using a 2.5-kW CO2 laser. Fractions of porosity in the fusion zones were less than 0.05 pct in beadon-plate welding and less than 0.2 pct in butt welding with polishing the groove surface before welding. The width of a softened region in the-laser beam welds was less than 1/4 times that of a tungsten inert gas (TIG) weld. The softened region is caused by reversion of strengthening b" (Mg2Si) precipitates due to weld heat input. The hardness values of the softened region in the laser beam welds were almost fully recovered to that of the base metal after an artificial aging treatment at 448 K for 28.8 ks without solution annealing, whereas those in the TIG weld were not recovered in a partly reverted region. Both the bead-on-plate weld and the butt weld after the postweld artificial aging treatment had almost equivalent tensile strengths to that of the base plate.
I.
INTRODUCTION
LASER beam welding is characterized by its high energy density and flexibility and has obvious advantages, such as high speed, low distortion, and ease of automation. These properties make laser beam welding suitable for welding aluminum alloys. In particular, since its low overall heat input minimizes weld fusion zone and heat-affected zone (HAZ), there is an advantage in applying laser beam welding to age-hardened aluminum alloys, which have a problem of softening of the HAZ.[1,2,3] However, difficulties of laser beam welding of aluminum alloys lie in their high reflectivities to laser beam, porosity formation, and vaporization of magnesium and zinc in the weld metal.[4–9] The possible sources of porosity have been reported to be hydrogen, metal vapor, and shielding gas.[7,8,9] It has also been reported that fully penetrated welds have less porosity than partly penetrated ones.[7] The extent of vaporization of magnesium decreases with increasing welding speed.[6] Although the causes of the problems arising in laser beam welding of aluminum alloys have been investigated, these problems have not been completely overcome. In spite of the difficulties, laser beam welding is considered to have a great advantage, particularly in fully penetrated single-pass welds of age-hardened aluminum alloys with high welding speed. In the present work, laser beam welding was applied to thin plates of age-hardened Al-Mg-Si alloy (6061-T6) using a 2.5-kW CO2 laser. Bead-on-plate and butt welds with full penetration were performed, and formation of porosity in the weld metal was investigated. Softening occurs in the welds of the age-hardened 6061-T6. Thus, aging after welding was carried out to recover the hardness of 6061-T6 laser AKIO HIROSE, Associate Professor, and KOJIRO F. KOBAYASHI, Professor, are with the Department of Manufacturing Science, Graduate School of Engineering, Osaka University, Osaka 565, Japan HIROTAKA TODAKA, formerly Student, Graduate School of Engineering, Osaka University, is Engineer with Toyota Motor Corporation, Nagoya, 471, Japan Manuscript submitted January 1, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
beam welds, and its effect was compared with that in TIG welds to reveal an advantage of laser beam welding. Tensile strengths of the welded joints were measured. II.
EXPERIMENTAL PROCEDURE
The aluminum alloy studied in the present experiment is a 1-mm-thick plate of 6061-T6, whose chemical composition is listed in Table I. Bead-on-plate welding and butt welding were performed using a 2.5-kW CO2 laser. Dimensions of specimens were 1 3 100 3 100 mm and 1 3 100 3 50 mm for bead-on-plate welding and butt welding, respectively. Immediately before welding, the surfaces of the specimens were finished with a stainless steel wire brush and cleaned with acetone before and after the brushing. In butt welding, two different methods for finishing the groove surface were employed. One was cleaning the surfaces only with acetone without the brushing and the other was polishing the surfaces with 0.2-mm alumina powder followed by acetone cleaning. Aluminum alloys are generally difficult to weld using a CO2 laser because of their high reflectivities. To reduce the reflectivities, surface treatments such as graphite coating and surface oxidation have been applied.[10,11] However, these surface treatments may cause porosity, contamination, and cracking in weld metal. It is known that once a keyhole is created, the absorptivity to laser beam significantly increases.[12] In the present work, the surface of the first 10 mm of the weld line, with 100 mm total length, was coated with graphite. This treatment assists the formation of a keyhole at the start of welding and does not contaminate the rest of the weld bead. The process parameters of the laser beam welding are listed in Table II. In butt welding, two specimens of 1 3 100 3 50 mm were square butt welded without filler metal. For all welds, the workpiece was tilted at 12 deg to prevent any reflection of the beam from re-entering the focusing optics. The TIG welding with conditions shown in Table III was also performed as a reference process for the laser beam welding. VOLUME 28A, DECEMBER 1997—2657
Table I.
Chemical Composition of 6061-T6 Aluminum Alloy Used Chemical Composition (Wt Pct)
Si 0.66 Table II.
Fe 0.25
Cu 0.31
Mn 0.08
Mg 0.99
Cr 0.16
Zn 0.01
Process Parameters of CO2 Laser Beam Welding
Power Beam mode Focal position Shielding gas Nozzle Underbead Welding speed
2.5 kW multimode just at the specimen surface Ar 1.00 3 1024m3/s Ar 66.7 to 167 mm/s
Table III.
Condition of TIG Welding
Welding Current
AC 40A
Shielding Nozzle Underbead Welding speed
Ar 1.67 3 1024m3/s Ar 3.33 to 6.67 mm/s
Since porosity in fusion zones of the laser beam welding was quite small in size, less than 50 mm in most cases, it could not be effectively detected with radiographic inspection. Therefore, formation of porosity in the fusion zones was investigated by measuring area fractions of porosity on cross sections of fusion zones. Four different cross sections for each weld bead were exposed, polished, and etched with Keller’s reagent. The diameter and number of pores were measured in the cross sections with an optical microscope using a magnification of 400. Thus, total area of porosity was obtained and divided by the total area of the fusion zones of the four cross sections to give area fraction of porosity. After welding, many of the samples were subjected to natural aging or artificial aging treatment prior to hardness testing. The natural aging treatments were made at room temperature with varying aging times up to 18.1 Ms (5040 hours). The artificial aging treatments were made at 448 K with varying aging times from 7.2 ks (2 hours) to 28.8 ks (8 hours). After either welding or welding followed by the aging treatments, micro-Vickers hardness measurements were performed on etched cross sections of the welds with a load of 1.96 N. Microstructures of the etched cross sections were observed using an optical microscope and a scanning electron microscope (SEM). Tensile tests were performed using samples after either laser beam welding with a welding speed of 133 mm/s or welding followed by an artificial aging treatment for 28.8 ks with a crosshead speed of 1.7 3 1022 mm/s. The shape of the tensile specimen is shown in Figure 1. The surface of the specimen was not finished before the test. III.
Fig. 1—Shape of tensile specimen.
RESULTS AND DISCUSSION
A. Bead Shape and Microstructure Bead widths of the laser beam bead-on-plate welds are plotted against welding speed in Figure 2. While bead 2658—VOLUME 28A, DECEMBER 1997
Fig. 2—Effect of welding speed on bead width of laser beam bead-onplate weld.
widths of the back side of the welds were somewhat narrower than those of the top side of the welds, full penetration welds could be made even at a welding speed of 167 mm/s in the present condition. On the contrary, in TIG welding, full penetration welds could be achieved only at welding speeds of 5 mm/s or less with a welding current of 40 A. Cross sections of a typical laser beam bead-on-plate weld and the TIG bead-on-plate weld are shown in Figures 3(a) and (b), respectively. The laser beam welds included no cracks and a very small amount of porosity, whose area fractions were less than 0.05 pct for all welding speeds. The bead width of the TIG weld was approximately 2 times that of the laser beam weld. Figures 4(a) and (b) show detailed microstructures of the fusion zones of the laser beam weld and TIG weld observed with SEM. As shown in Figure 4(a), a fine cellular-dendritic solidification structure was seen in the fusion zone of the laser beam weld. The solidification structure of the TIG weld, as shown in Figure 4(b), was significantly coarser than that of the laser beam weld. This difference in the solidification structures is thought to be caused by the difference in cooling rates in the fusion zones. The relationship between dendrite arm spacing (DAS) and cooling rate at solidification temperature has been reported to obey the following equation for an Al-Cu alloy:[13] METALLURGICAL AND MATERIALS TRANSACTIONS A
(a)
(b) (a)
Fig. 3—Cross sections of bead-on-plate welds of (a) laser beam welding (2.5 kW, 133 mm/s) and (b) TIG welding (40 A, 5 mm/s).
d 5 70C20.39 R
[1]
where d 5 DAS (mm) and CR 5 cooling rate (K/s). A similar equation for Al-Mg-Si alloy has not been reported. Here, applying Eq. [1] to the 6061 alloy, we estimated the cooling rates in the fusion zones of the laser beam weld and TIG weld. The DAS values and the estimated cooling rates are summarized in Table IV. The cooling rate at the solidification temperature in the laser beam welding was estimated to be more than 5 times that in the TIG welding. B. Hardness and Strength Figures 5(a) and (b) show hardness profiles obtained from the laser beam and TIG bead-on-plate welds in the as-welded condition and after natural aging. In the aswelded condition, the hardness measurements were made within 10 hours after welding. In this condition (Figure 5 (a)), a softening occurred in a region up to approximately 2 mm from the bead center. This is caused by reversion (dissolution) of strengthening b" (Mg2Si) precipitates due to the heat input of welding.[1,2,3] The minimum hardness value in a fully reverted region, up to approximately 0.5 mm from the fusion line, was 50 to 55 Hv. After the natural aging, the hardness of the fully reverted region was recovered to approximately 70 Hv, and the hardness recover due to the natural aging was completed in 605 ks (168 hours) after welding. The unrecovered hardness drop in the partly reverted region was not seen in the laser beam welding after the natural aging. On the contrary, as shown in Figure 5(b), the width of a fully reverted region in the TIG weld was approximately 4 times that in the laser beam weld, and an unrecovered hardness drop was clearly seen in a region of 3 to 4 mm from bead center. The hardness drop is caused by insufficient solute contents in the matrix of this region METALLURGICAL AND MATERIALS TRANSACTIONS A
(b) Fig. 4—Secondary electron images of weld fusion zones of (a) laser beam welding (2.5 kW, 133 mm/s) and (b) TIG welding (40 A, 5 mm/s).
Table IV.
Laser beam welding TIG welding
DAS and Estimated Cooling Rates DAS (mm)
Cooling Rate (K/s)
2.09 4.07
7883 1475
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(a)
(b) Fig. 5—Hardness profiles obtained from (a) laser beam (2.5 kW, 133 mm/s) and (b) TIG (40 A, 5 mm/s) bead-on-plate welds in as-welded condition and after natural aging.
Fig. 6—Hardness profiles of laser beam welds (2.5 kW, 133 mm/s) subjected to aging treatment at 448 K with varying aging time. 2660—VOLUME 28A, DECEMBER 1997
Fig. 7—Average hardness value of fusion zone and HAZ plotted against aging time at 448 K.
due to precipitation of nonhardening b' and/or b phases during weld thermal cycle.[2,3] The above results mean that the region where hardness is not recovered after natural aging is unmeasurably narrow in the laser beam welds because of a low heat input of this process. Thus, the laser beam welding could achieve narrower softened region and higher minimum hardness level in the welds of the 6061T6 after natural aging in comparison with the TIG welding. However, even in the laser beam welds, the hardness values of the fusion zone and the HAZ were not recovered to that of the base 6061-T6 alloy after natural aging. The HAZ needs to be artificially aged to better recover its hardness. Artificial aging treatments at 448 K were performed on laser beam welds with varying aging times. Figure 6 shows hardness profiles of the laser beam welds subjected to the aging treatments. Hardness values of the softened regions of the fusion zones and the HAZ increased with aging time. Unrecovered regions in hardness were not seen in the HAZ. Figure 7 shows average hardness values of the fusion zones and the HAZ plotted against aging time. The hardness value of the softened region was almost fully recovered to that of the base alloy after a aging treatment at 448 K for 28.8 ks. Thus, it is found that the postweld aging treatment without solution annealing can recover the hardness of the softened regions of the laser beam welds of the 6061-T6 thin plates. On the contrary, a TIG weld subjected to the aging treatment at 448 K for 28.8 ks after welding had an appreciable hardness drop in a region between 4 to 7 mm from bead center, as shown in Figure 8. This region is thought to be the partly reverted region, where contents of solute elements in the matrix were insufficient for precipitation of strengthening b" (Mg2Si) phase because of precipitation of nonhardening b' and/or b phases during weld thermal cycle.[2,3] Therefore, the TIG weld will require a solution annealing treatment before artificial aging to fully recover the HAZ hardness, unlike the laser beam welds. Figure 9 shows tensile strengths of the base metal and the laser beam bead-on-plate welds before and after the aging treatment. The as-welded specimen fractured at the softened region in the HAZ and had a considerably lower strength than that of the base metal. As expected from the hardness profile in Figure 7, the tensile strength of the specimen subjected to the aging treatment after welding was METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 8—Hardness profiles of TIG welds (40 A, 5 mm/s) in as-welded condition and after aging at 448 K for 28.8 ks.
Fig. 9—Tensile strengths of base 6061-T6 alloy and laser beam beadon-plate weld joints (2.5 kW, 133 mm/s) before and after aging at 448 K for 28.8 ks.
(a)
(b) Fig. 10—Cross sections of laser beam butt welds: (a) 2.5 kW, 100 mm/s and (b) 2.5 kW, 167 mm/s.
almost equivalent to that of the base plate. However, the fractured position was in the fusion zone, whose hardness values were slightly lower than those of the HAZ (Figure 6). This may be caused by slight vaporization and/or segregation of magnesium in the fusion zone. However, from the preceding results, butt welding without filler metal is considered to be possible. C. Butt Welding Butt welds were successfully made with welding speeds of 100 to 167 mm/s. Typical cross sections of the welds were shown in Figure 10. No cracks or large porosity were observed in the fusion zones. Figure 11 shows area fractions of porosity in the fusion zones plotted against the welding speed for bead-on-plate welding and butt welding. Figure 11 also represents an effect of the methods of finishing the groove surfaces on the formation of porosity in butt welding. The butt weld with an as-cut groove surface cleaned with acetone included significantly greater porosity than the bead-on-plate welds. The formation of porosity may be caused by moisture that is contained in air trapped METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 11—Area fractions of porosity in fusion zones plotted against welding speed for laser beam bead-on-plate welding and butt welding. VOLUME 28A, DECEMBER 1997—2661
in the joint gap and in oxide film formed on the groove surface. It was thought that the acetone cleaning alone could not effectively eliminate the moisture. In the butt welds with polished groove surface, fractions of porosity decreased to less than 0.2 pct. Polishing the groove surface may effectively eliminate the adsorbed layer and oxide film and reduce the joint gap owing to the smooth groove faces. These effects may cause the reduction of the fractions of porosity. To reduce porosity, finishing the groove surface is thought to be more significant in laser welds than in TIG welds, because laser welds have a higher ratio of groove surface area to molten metal volume and are subjected to rapid solidification. Although the butt welds with polished groove surface had higher fractions of porosity than the bead-on-plate welds, the values were less than 0.2 pct and will have no significantly detrimental effects on mechanical properties of the welds. The butt weld subjected to the postweld aging treatment had almost equivalent tensile strength, 290 MPa, to those of the base metal and the beadon-plate weld. Thus, laser beam welding could achieve nonfiller butt welds of the 6061-T6 thin plates, which had almost equivalent joint strengths to the base plate strength after the artificial aging without solution annealing.
IV.
CONCLUSIONS
Fully penetrated laser beam welds of 1-mm-thick agehardened Al-Mg-Si alloy, 6061-T6, thin plates were performed using a 2.5-kW CO2 laser. The properties of the welds were investigated and compared to those of TIG welds. The results obtained are as follows. 1. In the laser beam welding, full penetration welds could be achieved at a welding speed of 167 mm/s, and the bead width was approximately half of that of the TIG weld. The fusion zones of the laser beam bead-on-plate welds included no cracks and less porosity, less than 0.05 pct in area fraction, and had a fine cellular-dendritic solidification structure due to rapid cooling. 2. Although softening caused by reversion of strengthening b" (Mg2Si) precipitates occurred in the fusion zones and
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the HAZ of the laser beam welds, the width was less than 1/4 times that of the TIG weld. 3. The hardness values of the softened regions in the laser beam welds were almost fully recovered to that of the base metal after an artificial aging treatment at 448 K for 28.8 ks without solution annealing, whereas those in the TIG weld were not recovered in a partly reverted region. The tensile strength of the laser beam bead-onplate weld subjected to the artificial aging treatment was almost equivalent to that of the base plate. 4. Laser beam butt welds without filler metal were successfully made, and area fractions of porosity in the fusion zones were reduced to less than 0.2 pct by polishing the groove surface before welding. The tensile strength of the butt weld was almost equivalent to that of the base plate. REFERENCES 1. T. Enjo and T. Kuroda: Trans. JWRI, 1982, vol. 11, pp. 61-66. 2. O.R. Myhr and Ø. Grong: Acta Metall. Mater., 1991, vol. 39, pp. 2693-2702. 3. O.R. Myhr and Ø. Grong: Acta Metall. Mater., 1991, vol. 39, pp. 2703-08. 4. D.M. Roesslar: The Industrial Laser Annual Handbook, Penn Well Books, Tulsa, OK, 1986, pp. 16-30. 5. D.W. Moon and E.A. Metzbower: Weld. J., 1983, vol. 62, pp. 53s58s. 6. M.J. Cieslak and P.W. Fuerschbach: Metall. Trans. B, 1988, vol. 19B, pp. 319-29. 7. S. Katayama and C.D. Lundin: J. Light Met. Weld. Const., 1991, vol. 29, pp. 295-307. 8. H. Simidzu, F. Yoshino, S. Katayama, and A. Matsunawa: Proc. LAMP ’92, Nagoya, June 1992, High Temperature Society of Japan, Osaka, pp. 511-16. 9. I. Jones, S. Riches, J.W. Yoon, and E.R. Wallach: Proc. LAMP ’92, Nagoya, June 1992, High Temperature Society of Japan, Osaka, pp. 523-33. 10. Y. Arata and I. Miyamoto: Trans. JWRI, 1974, vol. 3, pp. 1-20. 11. I. Masumoto, M. Kutsuna, and J. Suzuki: Proc. 5th Int. Symp. of JWS, Tokyo, Apr. 1990, JWS, Tokyo, pp. 23-28. 12. I. Miyamoto, H. Maruo, and Y. Arata: Proc. Int. Conf. on Welding Research in the 1980’s, 1980, Welding Research Institute of Osaka Univ., Osaka, p. 103-108. 13. T.F. Bower, H.D. Brody, and M.C. Fleming: Trans. TMS-AIME, 1966, vol. 236, pp. 624-34.
METALLURGICAL AND MATERIALS TRANSACTIONS A
