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Tensile fracture behavior in CO2 laser beam welds of 7075-T6 aluminum alloy
Materials & Design Materials and Design 25 (2004) 573–577 www.elsevier.com/locate/matdes
Tensile fracture behavior in CO2 laser beam welds of 7075-T6 aluminum alloy Cheng Liu, D.O. Northwood 1, S.D. Bhole
*
Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, Canada M5B 2K3 Received 3 September 2003; received in revised form 18 February 2004; accepted 18 February 2004
Abstract CO2 laser beam (LB) welding is conducted on 7075-T6 aluminum alloy sheets at two different welding speeds and compared with gas tungsten arc (GTA) welding. The mechanical and microstructural characteristics of the welds are evaluated using tensile tests, hardness tests, optical microscopy and energy dispersive X-ray spectroscopy (EDS). Results indicate that both the hardness and tensile strength of LB welds are higher than those of GTA welds. It is shown that the hardness value of the softened region in LB welds is in the fusion zone (FZ), whereas that in the heat affected zone (HAZ) of the GTA weld. Tensile strengths of LB welds after the post-weld artificial aging treatment at 120 C for 26 h are improved but cannot be equivalent to those of base metal because the softened FZ of the LB weld is not completely recovered after post-weld artificial treatment. 2004 Elsevier Ltd. All rights reserved. Keywords: Heat treatments (C); Welding (D); Mechanical (E)
1. Introduction The aluminum alloy Al–Zn–Mg–Cu 7075 is used in various auto body and aerospace applications because of its high specific strength, low quench sensitivity, wide range of solution heat treatment temperatures and rapid natural aging characteristics [1–3]. At present, the alloy is joined by mechanical fastening method owing to the tendency for cracking in the weld metal and softening in the heat affected zone (HAZ) by conventional welding methods [4]. There are some serious disadvantages in mechanical fastening that hinder the successful use of the alloy such as slow assembly, stress-raisers and galvanic corrosion. Much more emphasis has been placed on quality welding aluminum alloys because of their growing importance in industrial applications. Compared to conventional welding techniques, laser beam (LB) welding is suitable for welding aluminum alloys, with the characteristics of high welding speeds, high flexibility of the *
1
Corresponding author. E-mail address: [email protected] (S.D. Bhole). Now at University of Windsor, Windsor, Canada.
0261-3069/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2004.02.017
laser beam tool, high energy density of the beam and low distortion [5–7]. In particular, there is an advantage in applying LB welding to age-hardened aluminum alloys since its low overall heat input minimizes the size of the weld fusion zone (FZ) and HAZ. Although many studies have been carried out to modify the conditions of LB welding process and post-weld heat treatment, most of them have been limited to certain aluminum alloys including Al–Li–Cu alloy, Al–Mg–Mn alloy, Al– Mg–Si–Cu alloy and Al–Mg–Si alloy [8–13]. There have been few extensive investigations of the mechanical properties and the fracture behavior of welds in Al–Zn– Mg–Cu 7075 alloy. In some investigations, 7075 aluminum alloy is recognized as an aluminum alloy that cannot be recommended for welding because of the very high vapor pressures of Zn and Mg [10,15]. The loss of both alloying elements reduces the effect of hardening and results in the reduction of tensile strength and ductility [16,17]. However, Hirano et al. [18] and Adair [19] developed the CO2 laser technology for welding 7075 aluminum alloys which produced consistently, high quality welds. The present work describes the application of LB welding to plates of age-hardened 7075-T6 aluminum
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C. Liu et al. / Materials and Design 25 (2004) 573–577
Table 1 Chemical composition of 7075-T6 alloy (wt%) Zn
Mg
Cu
Fe
Si
Mn
Cr
Ti
Al
5.1
2.1
1.2
0.5
0.4
0.3
0.18
0.15
Bal.
alloy using a CO2 laser. By comparing with gas tungsten arc (GTA) welding, the advantages of LB welding are assessed by tests of hardness distribution and tensile strength. Also, the effect of the artificial aging treatment on the LB welds is discussed.
Weld bead
R12.5
12.5mm
20 mm
50.8mm
Fig. 1. Schematic diagram of tensile specimen.
2. Experimental The chemical composition of the 7075 aluminum alloy used for this study is given in Table 1. Plates of the aluminum alloy, with a thickness of 9.5 mm, were used in the peak-strength T6 heat treatment condition. Before welding, prior surface preparation of the plates was done with a steel brush followed by light sanding with 400 grit SiC paper and degreasing with acetone. Bead-on-plate welding was performed using a 6-kW CO2 laser. The process parameters of the LB welding are listed in Table 2. The GTA welding given in Table 3 was also performed as a reference process for the LB welding. Microstructures of the welds were observed by optical microscopy. The constituent elements of second phase particles were determined by energy-dispersive X-ray microanalysis (EDS). After welding, an artificial aging treatment was carried out at 120 C for 26 h on welded specimens. The tensile tests were carried out according to ASTM standards (B-557 M-94) at a strain rate of 1.67 10 4 s 1 on a materials test system (MTS) servo-hydraulic testing machine. The final test result was the average of
three tests. The shape of tensile samples is shown in Fig. 1. Micro-Vickers hardness measurements were conducted on the etched cross-section of the welds with a load of 1 kgf holding for 15 min.
3. Results and discussion Fig. 2 exhibits an elongated matrix grain morphology in the parent Al 7075-T6 plate. The typical constituents of the particles are shown by EDS, Fig. 3. It is evident from Fig. 3 that the particles are enriched in Al, Cu, Fe, and Mg, indicating the presence of the isomorphous phases MgZn2 and MgAlCu as strengthening precipitates in 7075 aluminum alloy [20,21]. The microstructures of FZ in the LB-2 weld and the GTA weld are shown in Fig. 4. A fine cellular-dendritic solidification structure is observed throughout most of the FZ in LB, see Fig. 4(a). Compared to the LB weld, a significantly coarser cellular-dendritic solidification structure is found in the GTA weld, as shown in Fig. 4(b).
Table 2 Welding process parameters of CO2 LB welding
Power (kW) Speed (mm/min) Shield gas The flow rate of shield gas (l/min) Focal position
LB-1 (low)
LB-2 (high)
6 1016 Helium 70
6 635 Helium 70
At the specimen surface
At the specimen surface
Table 3 Welding process parameters of GTA welding Current (A) Voltage (V) Speed (mm/min) Shield gas The flow rate of shield gas (l/min)
78 75 159 Mixture of argon and helium 20
Fig. 2. Microstructure of parent metal – Al 7075 plate (·108).
C. Liu et al. / Materials and Design 25 (2004) 573–577 Table 4 The location of tensile fracture in welded specimen
Intensity, I
Al Ka
ZnLa MgKa FeKa 1.3
575
2.6
3.9
5.2
6.5
Welding
Specimen
Fracture location (as-welded)
Fracture location (after artificial age)
LB-1
1 2 3
FZ FZ FZ
FZ FZ FZ
LB-2
1 2 3
FZ FZ FZ
FZ FZ FZ
GTA
1 2 3
HAZ HAZ HAZ
HAZ HAZ HAZ
CuKa
7.8
9.1
Energy, keV
Fig. 3. EDS of the particles in Fig. 2.
200 Hardness of BM
190
Vickers hardness, Hv
180 170 160 150 140 130 120 110 100
FZ -5
HAZ
GTA weld LB-1 weld LB-2 weld
0 5 10 Distance from fusion boundary, mm
15
Fig. 5. Hardness distribution of welds.
Fig. 4. Microstructure of weld fusion zones (·108): (a) LB-1 weld; (b) GTA weld.
Table 4 indicates that the tensile fracture of all LB welded specimens takes place in the FZ region whereas the fracture position of GTA welds is in the HAZ. The same tensile fracture behavior is obtained in the welds after the artificial ageing treatment. This implies that the tensile strength of FZ is lower than that of HAZ and BM in the LB welded specimens. This result is totally different from the investigation of Akio Hirose, etc. [13]. They found that the LB welded specimens of 6061-T6 alloy fractured in the HAZ, which is the softened region in weld comparing to the FZ and base metal. This be-
havior can be explained by an analysis of hardness distribution along the welds. The typical results of hardness tests in the different welds are summarized in Fig. 5. The main point to note here is that the hardness within the FZ for both LB-1 and LB-2 welds is considerably lowers than that of HAZ or BM. According to Baker [22], the tensile strength decreases with decreasing hardness reducing since they both are indicators of a material’ resistance to plastic deformation. Thus, the FZ region is the weakest area in the tensile test and much easier to fracture. It is also seen from Fig. 5 that the tensile fracture is obtained in the HAZ for GTA welds because of the lowest hardness value in this region. One reason that hardness of FZ is lower than that of HAZ in LB welds is due to the vaporization of alloying elements because of the high melt pool temperature generated. The measurement of the solute chemistry within the cellular-dendritic solidification structure in the FZ of the LB-2 welds is given in Fig. 6. It is clear that the more serious elemental loss of Zn and Mg is obtained in the FZ. Zn and Mg, as main elements added to aluminum, possess lower boiling points and much higher vapor pressure than the other elements, see Fig. 7 [15].
576
C. Liu et al. / Materials and Design 25 (2004) 573–577 200
Al Ka
Hardness of BM
Intensity, I
190
MgKa ZnLa 1.3
CuKa
FeKa 2.6
3.9
5.2
6.5
7.8
9.1
Enegy, keV
Fig. 6. EDS of FZ in LB-2 welds.
Vickers hardness, Hv
180 170 160 150 140 130 120
Vapour Pressure, mm Hg
110
1000 100 Zn 10 Mg 1 0.1 Li 0.01 0.001 0.0001 0.00001 1E-6 1E-7 1E-8 Al 1E-9 800 1000
100
FZ -5
HAZ 0 5 10 Distance from fusion boundary, mm
GTA weld LB-1 weld LB-2 weld
15
Fig. 8. Hardness distribution of welds after artificial aging treatment at 120 C for 26 h.
Mn
Ni Fe Cu
1200
1400
Si
1600
1800
2000
Temperature, K Fig. 7. Equilibrium vapor pressure as a function of temperature for various elements [15].
Fig. 4(a) shows that negligible precipitation is obtained in the dendritic core region of the as-welded fusion zone. It is demonstrated that the segregation of alloy elements due to non-equilibrium solidification occurred and resulted in the lack of precipitation strengthening in FZ, according to the investigation of Hou and Baeslack [23]. It is impossible to completely evaporate the alloy elements during the LB welding for a short time. So the solute segregation commonly takes place in the fusion zone of aluminum alloy welds. In such fusion zones, Cu, Mg and Zn segregated along cellular boundaries, which resulted in the formation of low-melting-point eutectic products [14,24]. For example, Cu contents in the dendritic core regions were much lower than the normal content. On the other hand, the Cu contents exceeded the normal composition in the region within approximately 0.5 lm from the eutectics. Other alloy elements such as Li, Mg, Zr, etc. have same trend in LB welds [23]. Hence, the softening of FZ region occurs owing to the evaporation of two elements and loss of the precipitating strengthening due to the segregation, since the LB creates a high temperature increase in FZ. When the weld is loaded in tension, the FZ constitutes the weakest area where all the strain concentrates. As a result, tensile failure is obtained in the FZ. The hardness drop in the HAZ of the GTA weld is caused by insufficient
strengthening in the matrix of this region due to precipitation of non-hardening phases during weld thermal cycle [25]. A harder FZ of the LB welds than that of the GTA weld is attributed to a finer solidification microstructure which is caused by the much faster cooling rates at the solidification temperature in the FZ of LB welding than in the GTA welding. The reason that the hardness of LB-1 is slightly higher than that of LB-2 is due to the difference of welding speeds. Venkat et al. [26] reported that the lower the LB welding speed, the more the Mg loss from the FZ. It can be seen from Fig. 8 (compared to Fig. 5) that the hardness of all welds increases after a 120 C for 26 h post-weld aging treatment. However, the hardness profiles and the softened regions have not changed. The hardness of FZ cannot fully recover to that of the base metal after an artificial aging treatment. The FZ is still the weakest area for LB welds, so as is the HAZ for the GTA weld. Hou and Baeslack [23] considered that the non-uniform distribution of strengthening precipitates in FZ during post-weld aging treatment precluded hardening and resulted in relatively soft FZ of LB welds. For GTA weld, the contents of solute elements in the HAZ were insufficient for precipitation strengthening during aging treatment [13]. The tensile strength results of as-welded welds and welds after artificial aging are shown in Fig. 9. By comparing the tensile values of different welds, one finds that the tensile strength of as-welded welds and welds after artificial aging may be ranked in the following sequence: LB-1 > LB-2 > GTA: The above result can be explained by the hardness distribution in Figs. 5 and 8. Also, it is not surprising to see from Table 4 that all the tensile specimens fracture in the FZ of LB welds after artificial aging. The artificial aging
C. Liu et al. / Materials and Design 25 (2004) 573–577 as-welded
600 Tensile Strength, MPa
after artifical aging
500 400 300 200 100 0 Basemetal
GTA
LB-2
LB-1
Fig. 9. Tensile strength of base 7075-T6 alloy, GTA weld, LB-1 weld and LB-2 weld.
as a post-weld treatment can increase the tensile strength of LB welds, although tensile strengths of welds are lower compared to the base metal because of FZ softening of welds.
4. Conclusions In LB welded 7075-T6 aluminum alloy, the tensile fracture occurred in the FZ rather than in the HAZ. The FZ softening is attributed to the evaporation of Mg and Zn and loss of the precipitating strengthening due to the segregation. The hardness and tensile strength of LB welds are higher than those of GTA welds. The hardness value of the softened FZ regions in the LB welds is partially recovered after the post-weld artificial aging treatment at 120 C for 26 h. The tensile strength of LB welds subjected to the artificial aging treatment is still lower than that of the base metal.
Acknowledgements The financial support of the Natural Sciences and Engineering Research Council of Canada through Research Grant No. OGP0023234 is gratefully acknowledged. The work reported in this paper is based on graduate research of Warren Beveridge and Santosh Kamat at the University of Regina. References [1] Totten GE, Webster GM, Bates CE. Cooling curve and quench factor characterization of 2024 and 7075 aluminum bar stock quenched in type 1 polymer quenchants. Heat Transfer Res 1998;29:163. [2] Kandil HM, Salama SF, Naggar AA. Mechanical and natural aging pretreatment of age hardenable 7075 aluminum alloy. J Eng Appl Sci 1999;46:65. [3] Sang-Yong L, Jung-Hwan L, Young-Seon L. Characterization of Al 7075 alloys after cold working and heating in the semi-solid temperature range. J Mater Process Technol 2001;111:42.
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[4] Dickerson P, Irving B. Welding aluminum: Its not as difficult as it sounds. Welding J 1992;(April):45. [5] Sakamoto H, Shibata K, Dausinger F. Laser welding of different aluminum alloys. In: Proceedings of the Laser Materials Processing Symposium, Oct 25–29 1992, Orlando, USA, 1993, p. 523. [6] Schellhorn M. Application of a high-power CO laser in aluminum welding. In: Proceedings of SPIE – The International Society for Optical Engineering, Fifth Int. Conference on Industrial Lasers and Laser Applications ’95, Bellingham, June 1995, USA, 1996, p. 287. [7] Shida T, Hirokawa M, Sato S. CO2 laser welding of aluminum alloys (No. 1) – welding of aluminum alloys using CO2 laser beam in combination with MIG arc. Welding Res Abroad 1997;43:36. [8] Molian PA, Srivatan TS. Weldability of Al–Li–Cu alloy 2090T8E431 using lasers. Aluminum 1990;66:69. [9] Molian PA, Srivatsan TS. Laser-beam weld microstructures and properties of aluminum–lithium alloy 2090. Mater Lett 1990;9:245. [10] Katayama S, Lundin CD. Laser welding of 5456 aluminum alloy. J Light Met Construct 1991;29:295. [11] Zhao H, Debroy T. Weld metal composition change during conduction mode laser welding of aluminum alloy 5182. Metall Mater Trans B 2001;32:163. [12] Lee MF, Huang JC, Ho NJ. Microstructural and mechanical characterization of laser-beam welding of a 8090 Al–Li thin sheet. J Mater Sci 1996;31:1455. [13] Hirose A, Todaka H, Kobayashi KF. CO2 laser beam welding of 6061-T6 aluminum alloy thin plate. Metall Mater Trans A 1997;28A:2657. [14] Meyer BC, Doyen H, Emanowski D, Tempus G, Hirsch T, Mayer P. Dispersoid-free zones in the heat-affected zone of aluminum alloy welds. Metall Mater Trans A 2000;31A:1453. [15] Lida T, Guthrie RIL. The physical properties of liquid metals. Oxford: Clarendon Press; 1988. [16] Lawrence FV, Munse WH. Effects of porosity on tensile properties of 5083 and 6061 aluminum alloys weldments. Welding Res Counc Bull 1973:7. [17] Katayama S, Matsunawa A, Kojima K. CO2 laser weldability of aluminum alloys (2nd report): Defect formation conditions and causes. Welding Int 1998;12:44. [18] Hirano M, Kobayasi K, Tonda H. Effect of the additional element on the weldability of high-strength Al–Zn–Mg alloy. J Soc Mater Sci 2000;49:92 (in Japanese). [19] Adair R. Welding aluminium alloys with CO2 lasers. Focus on Laser 1994;24:96. [20] Wert GW. Effect of friction stir welding on microstructure of 7075 aluminum. Scr Mater 1981;15:455. [21] Rhodes CG, Mahoney MW, Bingel WH, Spuring RA, Banpton CC. Effect of friction stir welding on microstructure of 7075 aluminum. Scr Mater 1997;36:69. [22] Baker H. Properties and selection: Nonferrous alloys and pure metals. In: Metals Handbook, vol. 2. American Society of Metals; 1979. p. 327. [23] Hou KH, Baeslack WA. Characterization of the heat-affected zone in gas tungsten arc welded aluminum alloy 2195-T8. J Mater Sci Lett 1996;15:239. [24] Yamaguchi, Katoh M, Nishio K, Fukami K. Hardness distribution of aluminum alloy A5052 welded with YAG laser. Quart J Japan Welding Soc 2001;19:114. [25] Hirose A, Todaka H, Yamaoka H, Kurosawa N, Kobayashi KF. Quantitative evaluation of softened regions in weld heataffected zones of 6061-T6 aluminum alloy-characterizing of the laser beam welding process. Metall Mater Trans A 1999;30A:2115. [26] Venkat S, Albright CE, Ramasamy S, Hurley JP. CO2 laser beam welding of aluminum 5754-O and 6111-T4 alloys. Welding J 1997;76:275s.
Tensile fracture behavior in CO2 laser beam welds of 7075-T6 aluminum alloy Cheng Liu, D.O. Northwood 1, S.D. Bhole
*
Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, Canada M5B 2K3 Received 3 September 2003; received in revised form 18 February 2004; accepted 18 February 2004
Abstract CO2 laser beam (LB) welding is conducted on 7075-T6 aluminum alloy sheets at two different welding speeds and compared with gas tungsten arc (GTA) welding. The mechanical and microstructural characteristics of the welds are evaluated using tensile tests, hardness tests, optical microscopy and energy dispersive X-ray spectroscopy (EDS). Results indicate that both the hardness and tensile strength of LB welds are higher than those of GTA welds. It is shown that the hardness value of the softened region in LB welds is in the fusion zone (FZ), whereas that in the heat affected zone (HAZ) of the GTA weld. Tensile strengths of LB welds after the post-weld artificial aging treatment at 120 C for 26 h are improved but cannot be equivalent to those of base metal because the softened FZ of the LB weld is not completely recovered after post-weld artificial treatment. 2004 Elsevier Ltd. All rights reserved. Keywords: Heat treatments (C); Welding (D); Mechanical (E)
1. Introduction The aluminum alloy Al–Zn–Mg–Cu 7075 is used in various auto body and aerospace applications because of its high specific strength, low quench sensitivity, wide range of solution heat treatment temperatures and rapid natural aging characteristics [1–3]. At present, the alloy is joined by mechanical fastening method owing to the tendency for cracking in the weld metal and softening in the heat affected zone (HAZ) by conventional welding methods [4]. There are some serious disadvantages in mechanical fastening that hinder the successful use of the alloy such as slow assembly, stress-raisers and galvanic corrosion. Much more emphasis has been placed on quality welding aluminum alloys because of their growing importance in industrial applications. Compared to conventional welding techniques, laser beam (LB) welding is suitable for welding aluminum alloys, with the characteristics of high welding speeds, high flexibility of the *
1
Corresponding author. E-mail address: [email protected] (S.D. Bhole). Now at University of Windsor, Windsor, Canada.
0261-3069/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2004.02.017
laser beam tool, high energy density of the beam and low distortion [5–7]. In particular, there is an advantage in applying LB welding to age-hardened aluminum alloys since its low overall heat input minimizes the size of the weld fusion zone (FZ) and HAZ. Although many studies have been carried out to modify the conditions of LB welding process and post-weld heat treatment, most of them have been limited to certain aluminum alloys including Al–Li–Cu alloy, Al–Mg–Mn alloy, Al– Mg–Si–Cu alloy and Al–Mg–Si alloy [8–13]. There have been few extensive investigations of the mechanical properties and the fracture behavior of welds in Al–Zn– Mg–Cu 7075 alloy. In some investigations, 7075 aluminum alloy is recognized as an aluminum alloy that cannot be recommended for welding because of the very high vapor pressures of Zn and Mg [10,15]. The loss of both alloying elements reduces the effect of hardening and results in the reduction of tensile strength and ductility [16,17]. However, Hirano et al. [18] and Adair [19] developed the CO2 laser technology for welding 7075 aluminum alloys which produced consistently, high quality welds. The present work describes the application of LB welding to plates of age-hardened 7075-T6 aluminum
574
C. Liu et al. / Materials and Design 25 (2004) 573–577
Table 1 Chemical composition of 7075-T6 alloy (wt%) Zn
Mg
Cu
Fe
Si
Mn
Cr
Ti
Al
5.1
2.1
1.2
0.5
0.4
0.3
0.18
0.15
Bal.
alloy using a CO2 laser. By comparing with gas tungsten arc (GTA) welding, the advantages of LB welding are assessed by tests of hardness distribution and tensile strength. Also, the effect of the artificial aging treatment on the LB welds is discussed.
Weld bead
R12.5
12.5mm
20 mm
50.8mm
Fig. 1. Schematic diagram of tensile specimen.
2. Experimental The chemical composition of the 7075 aluminum alloy used for this study is given in Table 1. Plates of the aluminum alloy, with a thickness of 9.5 mm, were used in the peak-strength T6 heat treatment condition. Before welding, prior surface preparation of the plates was done with a steel brush followed by light sanding with 400 grit SiC paper and degreasing with acetone. Bead-on-plate welding was performed using a 6-kW CO2 laser. The process parameters of the LB welding are listed in Table 2. The GTA welding given in Table 3 was also performed as a reference process for the LB welding. Microstructures of the welds were observed by optical microscopy. The constituent elements of second phase particles were determined by energy-dispersive X-ray microanalysis (EDS). After welding, an artificial aging treatment was carried out at 120 C for 26 h on welded specimens. The tensile tests were carried out according to ASTM standards (B-557 M-94) at a strain rate of 1.67 10 4 s 1 on a materials test system (MTS) servo-hydraulic testing machine. The final test result was the average of
three tests. The shape of tensile samples is shown in Fig. 1. Micro-Vickers hardness measurements were conducted on the etched cross-section of the welds with a load of 1 kgf holding for 15 min.
3. Results and discussion Fig. 2 exhibits an elongated matrix grain morphology in the parent Al 7075-T6 plate. The typical constituents of the particles are shown by EDS, Fig. 3. It is evident from Fig. 3 that the particles are enriched in Al, Cu, Fe, and Mg, indicating the presence of the isomorphous phases MgZn2 and MgAlCu as strengthening precipitates in 7075 aluminum alloy [20,21]. The microstructures of FZ in the LB-2 weld and the GTA weld are shown in Fig. 4. A fine cellular-dendritic solidification structure is observed throughout most of the FZ in LB, see Fig. 4(a). Compared to the LB weld, a significantly coarser cellular-dendritic solidification structure is found in the GTA weld, as shown in Fig. 4(b).
Table 2 Welding process parameters of CO2 LB welding
Power (kW) Speed (mm/min) Shield gas The flow rate of shield gas (l/min) Focal position
LB-1 (low)
LB-2 (high)
6 1016 Helium 70
6 635 Helium 70
At the specimen surface
At the specimen surface
Table 3 Welding process parameters of GTA welding Current (A) Voltage (V) Speed (mm/min) Shield gas The flow rate of shield gas (l/min)
78 75 159 Mixture of argon and helium 20
Fig. 2. Microstructure of parent metal – Al 7075 plate (·108).
C. Liu et al. / Materials and Design 25 (2004) 573–577 Table 4 The location of tensile fracture in welded specimen
Intensity, I
Al Ka
ZnLa MgKa FeKa 1.3
575
2.6
3.9
5.2
6.5
Welding
Specimen
Fracture location (as-welded)
Fracture location (after artificial age)
LB-1
1 2 3
FZ FZ FZ
FZ FZ FZ
LB-2
1 2 3
FZ FZ FZ
FZ FZ FZ
GTA
1 2 3
HAZ HAZ HAZ
HAZ HAZ HAZ
CuKa
7.8
9.1
Energy, keV
Fig. 3. EDS of the particles in Fig. 2.
200 Hardness of BM
190
Vickers hardness, Hv
180 170 160 150 140 130 120 110 100
FZ -5
HAZ
GTA weld LB-1 weld LB-2 weld
0 5 10 Distance from fusion boundary, mm
15
Fig. 5. Hardness distribution of welds.
Fig. 4. Microstructure of weld fusion zones (·108): (a) LB-1 weld; (b) GTA weld.
Table 4 indicates that the tensile fracture of all LB welded specimens takes place in the FZ region whereas the fracture position of GTA welds is in the HAZ. The same tensile fracture behavior is obtained in the welds after the artificial ageing treatment. This implies that the tensile strength of FZ is lower than that of HAZ and BM in the LB welded specimens. This result is totally different from the investigation of Akio Hirose, etc. [13]. They found that the LB welded specimens of 6061-T6 alloy fractured in the HAZ, which is the softened region in weld comparing to the FZ and base metal. This be-
havior can be explained by an analysis of hardness distribution along the welds. The typical results of hardness tests in the different welds are summarized in Fig. 5. The main point to note here is that the hardness within the FZ for both LB-1 and LB-2 welds is considerably lowers than that of HAZ or BM. According to Baker [22], the tensile strength decreases with decreasing hardness reducing since they both are indicators of a material’ resistance to plastic deformation. Thus, the FZ region is the weakest area in the tensile test and much easier to fracture. It is also seen from Fig. 5 that the tensile fracture is obtained in the HAZ for GTA welds because of the lowest hardness value in this region. One reason that hardness of FZ is lower than that of HAZ in LB welds is due to the vaporization of alloying elements because of the high melt pool temperature generated. The measurement of the solute chemistry within the cellular-dendritic solidification structure in the FZ of the LB-2 welds is given in Fig. 6. It is clear that the more serious elemental loss of Zn and Mg is obtained in the FZ. Zn and Mg, as main elements added to aluminum, possess lower boiling points and much higher vapor pressure than the other elements, see Fig. 7 [15].
576
C. Liu et al. / Materials and Design 25 (2004) 573–577 200
Al Ka
Hardness of BM
Intensity, I
190
MgKa ZnLa 1.3
CuKa
FeKa 2.6
3.9
5.2
6.5
7.8
9.1
Enegy, keV
Fig. 6. EDS of FZ in LB-2 welds.
Vickers hardness, Hv
180 170 160 150 140 130 120
Vapour Pressure, mm Hg
110
1000 100 Zn 10 Mg 1 0.1 Li 0.01 0.001 0.0001 0.00001 1E-6 1E-7 1E-8 Al 1E-9 800 1000
100
FZ -5
HAZ 0 5 10 Distance from fusion boundary, mm
GTA weld LB-1 weld LB-2 weld
15
Fig. 8. Hardness distribution of welds after artificial aging treatment at 120 C for 26 h.
Mn
Ni Fe Cu
1200
1400
Si
1600
1800
2000
Temperature, K Fig. 7. Equilibrium vapor pressure as a function of temperature for various elements [15].
Fig. 4(a) shows that negligible precipitation is obtained in the dendritic core region of the as-welded fusion zone. It is demonstrated that the segregation of alloy elements due to non-equilibrium solidification occurred and resulted in the lack of precipitation strengthening in FZ, according to the investigation of Hou and Baeslack [23]. It is impossible to completely evaporate the alloy elements during the LB welding for a short time. So the solute segregation commonly takes place in the fusion zone of aluminum alloy welds. In such fusion zones, Cu, Mg and Zn segregated along cellular boundaries, which resulted in the formation of low-melting-point eutectic products [14,24]. For example, Cu contents in the dendritic core regions were much lower than the normal content. On the other hand, the Cu contents exceeded the normal composition in the region within approximately 0.5 lm from the eutectics. Other alloy elements such as Li, Mg, Zr, etc. have same trend in LB welds [23]. Hence, the softening of FZ region occurs owing to the evaporation of two elements and loss of the precipitating strengthening due to the segregation, since the LB creates a high temperature increase in FZ. When the weld is loaded in tension, the FZ constitutes the weakest area where all the strain concentrates. As a result, tensile failure is obtained in the FZ. The hardness drop in the HAZ of the GTA weld is caused by insufficient
strengthening in the matrix of this region due to precipitation of non-hardening phases during weld thermal cycle [25]. A harder FZ of the LB welds than that of the GTA weld is attributed to a finer solidification microstructure which is caused by the much faster cooling rates at the solidification temperature in the FZ of LB welding than in the GTA welding. The reason that the hardness of LB-1 is slightly higher than that of LB-2 is due to the difference of welding speeds. Venkat et al. [26] reported that the lower the LB welding speed, the more the Mg loss from the FZ. It can be seen from Fig. 8 (compared to Fig. 5) that the hardness of all welds increases after a 120 C for 26 h post-weld aging treatment. However, the hardness profiles and the softened regions have not changed. The hardness of FZ cannot fully recover to that of the base metal after an artificial aging treatment. The FZ is still the weakest area for LB welds, so as is the HAZ for the GTA weld. Hou and Baeslack [23] considered that the non-uniform distribution of strengthening precipitates in FZ during post-weld aging treatment precluded hardening and resulted in relatively soft FZ of LB welds. For GTA weld, the contents of solute elements in the HAZ were insufficient for precipitation strengthening during aging treatment [13]. The tensile strength results of as-welded welds and welds after artificial aging are shown in Fig. 9. By comparing the tensile values of different welds, one finds that the tensile strength of as-welded welds and welds after artificial aging may be ranked in the following sequence: LB-1 > LB-2 > GTA: The above result can be explained by the hardness distribution in Figs. 5 and 8. Also, it is not surprising to see from Table 4 that all the tensile specimens fracture in the FZ of LB welds after artificial aging. The artificial aging
C. Liu et al. / Materials and Design 25 (2004) 573–577 as-welded
600 Tensile Strength, MPa
after artifical aging
500 400 300 200 100 0 Basemetal
GTA
LB-2
LB-1
Fig. 9. Tensile strength of base 7075-T6 alloy, GTA weld, LB-1 weld and LB-2 weld.
as a post-weld treatment can increase the tensile strength of LB welds, although tensile strengths of welds are lower compared to the base metal because of FZ softening of welds.
4. Conclusions In LB welded 7075-T6 aluminum alloy, the tensile fracture occurred in the FZ rather than in the HAZ. The FZ softening is attributed to the evaporation of Mg and Zn and loss of the precipitating strengthening due to the segregation. The hardness and tensile strength of LB welds are higher than those of GTA welds. The hardness value of the softened FZ regions in the LB welds is partially recovered after the post-weld artificial aging treatment at 120 C for 26 h. The tensile strength of LB welds subjected to the artificial aging treatment is still lower than that of the base metal.
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