Surface and microstructural characterization of laser beam welds in an

Surface and microstructural characterization of laser beam welds in an aluminum alloy & ntara P. A. P. Nascente,a) C. Bolfarini, C. L. Benassi, and N. G. Alca ˜ o Carlos, Departamento de Engenharia de Materiais, Universidade Federal de Sa ˜ o Carlos, SP, Brazil Centro de Caracterizac¸˜ao e Desenvolvimento de Materiais, Sa

J. F. Santos GKSS Forschungszentrum Geesthacht GmbH, Max Plank Strasse, D-21 502, Geesthacht, Germany

共Received 22 November 2001; accepted 29 April 2002兲 The weldability of aluminum alloys is one of the main requirements to be considered for their application in the automotive and aerospace industry. In this work, the weld joints of an AA7020 alloy obtained by laser process were characterized by scanning electron microscopy, energy-dispersive x-ray spectrometry, wavelength-dispersive x-ray spectrometry, and x-ray photoelectron spectroscopy. The laser process can be described as a high-speed solidification process. The solidification rate can be estimated from the correlation among several solidification rates and the corresponding dendritic/cellular spacings, and the value calculated for this rate was 3.0⫻103 K/s. The inclusions present in the base metal were identified as (Cr,Fe) 4 Si4 Al13 , which were not observed in the fused zone. The fused zone microstructure showed two distinct phases: an aluminum solid solution and fine precipitates of MgZn2 . The transition from the base metal to the fused zone presented a narrow heat-affected zone. A strong depletion of Zn was observed, and this does not influence the hot tearing susceptibility since hot cracks were not observed in the weld zone. © 2002 American Vacuum Society. 关DOI: 10.1116/1.1487868兴

I. INTRODUCTION 3

Aluminum has a very low density (2,7 Mg/m ), good corrosion resistance in most environments, and some of its alloys, such as the heat treatable series 2XXX共Al–Cu–Mg兲 and 7XXX共Al–Zn–Mg–Cr兲, can exceed structural steels in strength. As a consequence, they are used in a wide range of engineering applications, which include aerospace, automobile, transportation, shipbuilding, tankage, and piping industries.1 From an application point of view, the weldability of aluminum alloys is a key property to be considered and factors that may have an influence on it are 共i兲 the formation of a very stable aluminum oxide layer, 共ii兲 high coefficient of thermal expansion and thermal conductivity, 共iii兲 wide solidification–temperature range, 共iv兲 hydrogen contamination, and 共v兲 high reflectivity, which leads to difficulties in laser beam welding.2 The aluminum oxide layer forms immediately on the surface upon exposure to air and must be removed prior to welding to avoid reduction in ductility, lack of fusion, and possibly weld cracking. Thus during the welding, a protective atmosphere of an inert gas is used to prevent reoxidation in all welding processes, including laser beam.3 The combination of high coefficient of thermal expansion and high thermal conductivity may cause considerable distortion of aluminum alloys during welding. This can be minimized by high welding speeds such as those delivered by using a laser beam, which normally produces low distortion in the welds.4 The relative wide solidification–temperature range combined with the high coefficient of thermal expansion and a兲

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solidification shrinkage of aluminum alloys, particularly the heat treatable alloys, make them susceptible to cracking. High restraint conditions, which cause high tensile stress in the weld joint, increase the potential for hot cracking and can be avoided by proper joint design. By laser beam welding, hot cracking is not expected to be a major difficulty. However, this process can result in cracking when magnesium and zinc, high-vapor-pressure alloying elements present in the 7XXX alloys, are boiled off.5,6 Pores may be formed in aluminum alloys when hydrogen gas is entrapped during solidification because aluminum displays an abrupt drop in its hydrogen solubility when going from liquid to solid. Porosity is often encountered in laser beam welding as well as in conventional processes. In practice, a certain amount of porosity can be allowed in aluminum alloy welds depending on the quality requirements of the component. To minimize this problem, filler wire and shielding gas should be moisture-free in laser beam welding.7 There is an increasing interest in applying laser beam welding to aluminum alloys since it offers advantages such as high welding speeds and low distortion. However, difficulties are encountered due to the high reflectivity 共or low absorption兲 of the laser and high thermal conductivity of the aluminum alloys. These problems can be overcome by using recent developed high power, pulsed Nd:YAG laser welding systems producing an average power of 2 MW.7 The increased available power allows deeper penetration in materials of high thermal diffusivity and reflectivity such as aluminum alloys. Proper control of the duty cycle can eliminate hot cracking and porosity as well. Otherwise, the highenergy power of these systems may affect the weld pool

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Nascente et al.: Surface and microstructural characterization of welds

chemistry by evaporating alloying elements that have a high vapor pressure. This phenomenon can affect the chemical and mechanical properties of the weld joints.2,7,8 From a mechanical property perspective, due to the strong relationship between chemistry and strengthening, composition modifications may adversely affect the resultant properties in these alloys. The purpose of this study was to characterize the microstructure and surface of laser beam welds in aluminum alloy 7020 by scanning electron microscopy 共SEM兲, energydispersive x-ray spectrometry 共EDS兲, wavelength-dispersive x-ray spectrometry 共WDS兲, and x-ray photoelectron spectroscopy 共XPS兲. II. EXPERIMENTAL PROCEDURES The aluminum alloy investigated in this study was the 7020, with the following chemical composition 共wt %兲 determined by WDS: 4.50% Zn, 1.40% Mg, 0.15% Cr, 0.09% Mn, 0.66% Fe, 0.25% Si, and the remainder Al. Zinc and magnesium are the main alloying elements, while chromium is a minor one which controls the grain growth during thermomechanical processing of the alloy. Iron, manganese, and silicon are impurities normally present in the aluminum alloys. The dimensions of the plates to be welded were 150 ⫻350 mm with a thickness of 3 mm, and are referred to in this article as the base metal. Autogenous butt welds were made with a single pass. A Lumonics 200 MW pulsed Nd:YAG laser was employed in this study. The welds were made with a pulsed laser at an average power of 1.9 kW, frequency of 100 Hz, and travel speed of 0.4 m/min. Helium gas was used to shield the molten weld pool. The effects of the different cooling rates on the microstructure of aluminum alloy 7020 was investigated by melting and casting it into a cast iron chill mold with an wedgelike geometry. This mold had an aperture thickness varying from 1 to 26 mm with an aperture angle of 8°. Three type K thermocouples were inserted in the internal aperture of the chill mold and connected to a data acquisition system. These thermocouples registered the temperature versus time curves, allowing for the calculation of the cooling rates during the solidification, which were then correlated with the measured dentritic/cellular spacing values. The alloy was melted in an induction furnace and cast directly in the chill mold. The molten alloy received an adequate treatment to remove gas, and it remained liquid for the shortest possible time to avoid losing the alloy elements.2 Another point was obtained for the abovementioned curve by melting the alloy and slowly solidifying inside the furnace. After the solidification, the samples were prepared for metalographic observation by optical microscopy. A dendritic form is characterized in terms of the primary 共dendrite trunk兲 spacing and the secondary 共dendrite arm兲 spacing. These secondary arms grow perpendicularly to the primary trunk in the case of a cubic crystal. In this study, the secondary dendritic arm spacing values were measured near the regions where the thermocouple tips were located, permitting the correlation between the cooling JVST A - Vacuum, Surfaces, and Films

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rate and the microstructure. This correlation was later used in order to estimate the cooling rates prevailing during the solidification of the welding pool. The SEM microstructural analyses were carried out using a LEICA/Cambridge Stereoscan 440 microscope equipped with EDS and WDS. The XPS analyses were performed under ultrahigh vacuum 共low 10⫺7 Pa range兲 employing a Kratos XSAM HS spectrometer. Both Mg and Al K ␣ radiations were used as x-ray sources, with a power given by an emission of 15 mA and a voltage of 15 kV. The high-resolution spectra were obtained with analyzer pass energy of 20 eV. Argon ion sputtering 共kinetic energy of 2 keV, partial pressure of 1.3 ⫻10⫺5 Pa兲 was used to clean the samples. The binding energies were referenced to the adventitious carbon 1s line set at 284.8 eV. The Shirley background and a least-square routine were applied. The sensitivity factors for quantitative analysis were referenced to F 1s⫽1.0. III. RESULTS AND DISCUSSION The different solidification rates applied to the alloy resulted in distinct microstructures. A dendritic/cellular microstructure was observed for the weld joint, which comprises the fused and resolidified region of the two plates being welded. Only dendritic microstructures were observed for the samples solidified in the cast iron chill mold and inside the furnace. Assumed that the very thin dendritic cellular spacing and the secondary dendritic arm spacing are equivalent, we have derived the following relationship between the cooling rates and the dentritic/cellular spacing values: ln ␭⫽3.553– 0.287 ln R sol ,

共1兲

where ␭ is the interdendritic/cellular spacing and R sol is the solidification rate. The interdendritic/cellular spacing of ⬃3.5 ␮ m measured in the weld joint gives rise to a solidification rate value of 3.0⫻103 K/s. This value is similar to those encountered by other authors7,9 for 2XXX alloys, which have a chemical composition close to the alloy used in this work. A SEM photomicrograph of a typical constituent phase, about 30 ␮m long, in the 7XXX base metal is shown in Fig. 1. ED spectra of this phase indicate the presence of the (Cr,Fe) 4 Si4 Al13 complex.9 The base metal, heat-affected zone 共HAZ兲, and fused zone are shown in Fig. 2. It can be seen that the HAZ, about 25 ␮m wide, was recrystallized due to the temperature increase caused by the laser beam. The observed microstructure for the fused zone presents a dendritic/cellular growth with an intermetallic phase formed by segregation during the solidification and absence of cracks and voids. This power beam welding generates a high temperature gradient between the fused zone and the base metal, resulting in a high cooling rate in the welding joint. Figure 3 shows in detail the intermetallic phase, which was identified by EDS as a Mg–Zn compound, probably MgZn2 , with a small amount of silicon. Moreover, it is possible to observe in the photomicrograph that the majority of the coarse phase present in the base

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FIG. 1. SEM photomicrograph of a typical constituent phase, about 30 ␮m long, in the base metal.

metal 共Fig. 2兲 is melted and its constituents 共Fe and Cr兲 are incorporated into the aluminum solid solution. This is a consequence of the high cooling rate prevailing in the weld zone during the solidification. WDS analysis was performed in the cross section center of the weld zone and presented the following values 共wt %兲: 3.08% Zn, 1.35% Mg, 0.15% Cr, 0.09% Mn, 0.66% Fe, 0.25% Si, and remainder Al, which indicate a strong depletion of Zn, a high vapor pressure element. In the specific case of this alloy, the Zn depletion did not seem to influence the hot tearing susceptibility since hot cracks were not observed in the weld zone, as could be the case for other alloys. Jennings et al.10 established that the hot-tearing tendency of Al– Mg, Al–Si, and Al–Mg–Si is a strong function of composition and decreases when the alloying element contents increases. The mechanical properties could also be affected by Zn depletion. In the case of Al–Zn alloys, strengthening is achieved by precipitation hardening of a semicoherent MgZn2 phase. A stoichiometric ratio of Zn to Mg greater than 2 is desired to optimize the mechanical properties11 and

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FIG. 3. SEM photomicrograph presenting the dendritic/cellular microstructure of the weld joint.

the observed Zn depletion would be expected to adversely affect the tensile properties. Surface analyses by XPS were conducted to obtain chemical state and semiquantitative information for three samples: 共a兲 base metal, 共b兲 upper face of the weld joint, and 共c兲 lower face of the weld joint. Besides Al; Zn, Si, Mg, and O were detected in all three samples; Ca, in samples 共a兲 and 共b兲; Cr and Ni, only in sample 共a兲. Calcium and nickel are impurities incorporated during the electrolysis processing of aluminum. Both are present in very low amount, undetectable by WDS. The metal core level lines indicate that the surfaces of both the base metal and the fused zone were oxidized. Argon ion sputtering significantly removed the oxide layers. Each Al 2 p spectrum obtained after sputtering has two main components: one at a binding energy of ⬃72.5 eV is associated with metallic aluminum, and a broad one at higher binding energies is attributed to oxidized aluminum. For the three analyzed samples, the Zn 2p 3/2 binding energies are in the range of 1021.3–1021.7 eV, while the Si 2p binding energies are at about 103 eV. Comparative atomic ratios of Zn/Al and Si/Al indicate a slight enrichment of Zn and a slight depletion of Si in the weld joint surface. Regarding the concentration of Zn, the WDS and XPS results indicate depletion in the weld bulk and a surface enrichment. The bulk composition is more important than the surface one for mechanical properties. IV. CONCLUSIONS

FIG. 2. SEM photomicrograph of the base metal 共left兲, heat-affected zone 共center兲, and fused zone 共right兲. J. Vac. Sci. Technol. A, Vol. 20, No. 4, JulÕAug 2002

The fused zone microstructure presents a dendritic/ cellular growth with an intermetallic phase formed by segregation during the solidification and absence of cracks and voids. The thermal cycle caused by the high energetic process generates a high temperature gradient between the fused zone and the base metal, resulting in a high cooling rate in the welding joint. This rate was calculated to be 3.0 ⫻103 K/s, based on dendrite spacing measurements. The intermetallic phase is probably MgZn2 , with a small amount of silicon. The majority of the coarse constituent phase present in the base metal is melted. Fe and Cr are incorporated into the aluminum solid solution, a consequence of the high cool-

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ing rate in the weld zone during the solidification. A strong depletion of Zn was observed, and this does not influence the hot tearing susceptibility since hot cracks were not observed in the weld zone. ACKNOWLEDGMENTS This work was supported by CNPQ, CAPES, and FAPESP of Brazil. The authors would like to thank H.C. Amorim and C.R. Chinaglia for their assistance in some of the experiments. 1

ASM Handbook Properties and Selection: Nonferrous Alloys and SpecialPurpose Materials 共ASM International, 1993兲, Vol. 2.

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M. J. Cieslak and P. W. Fuerschbach, Metall. Trans. B 19, 319 共1988兲. J. L. Murphy, R. A. Huber, and W. E. Lever, Weld. J. 共Miami兲 69, S125 共1990兲. 4 D. W. Moon and E. A. Metzbower, Weld. J. 共Miami兲 62, S53 共1983兲. 5 T. W. Clyne and G. J. Davies, Proceedings of International Conference on Solidification 共1979兲, p. 275. 6 Y. Arata, K. Matsuta, K. Nakata, and I. Sasaki, Trans. JWRI 5, 53 共1976兲. 7 J. O. Milewski, G. K. Lewis, and J. E. Wittig, Weld. J. 共Miami兲 72, S341 共1993兲. 8 P. B. Dickerson and B. Irving, Weld. J. 共Miami兲 71, 45 共1992兲. 9 A. Munitz, Metall. Trans. B 16, 149 共1985兲. 10 P. H. Jennings, A. R. Singer, and W. J. Pumphrey, J. Inst. Met. 74, 275 共1978兲. 11 L. F. Mondolfo, Aluminium Alloys 共Pergamon, London, 1976兲. 2 3