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Effects of zinc on the laser welding of an aluminum alloy and galvanized steel
Accepted Manuscript Title: Effects of zinc on the laser welding of an aluminum alloy and galvanized steel Author: Liu Jia Jiang Shichun Shi Yan Ni Cong Chen junke Huang Genzhe PII: DOI: Reference:
S0924-0136(15)00183-1 http://dx.doi.org/doi:10.1016/j.jmatprotec.2015.04.017 PROTEC 14388
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
21-1-2015 13-4-2015 15-4-2015
Please cite this article as: Jia, L., Shichun, J., Yan, S., Cong, N., junke, C., Genzhe, H.,Effects of zinc on the laser welding of an aluminum alloy and galvanized steel, Journal of Materials Processing Technology (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.04.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights: 1. We show a new Al-Fe welding mode, laser beam welding with filler powder. 2. Filling powder can improve the energy utilization rate in the fusion-brazing process. 3. The zinc coated layer can cut down the brazing temperature from 1535.0℃ to 660.4℃. 4. The intermetallic compound layer was made up of Fe2Al5Zn0.4 phase. 5. The formation of Fe2Al5Zn0.4 phase is because zinc atoms substitute iron atoms.
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Effects of zinc on the laser welding of an aluminum alloy and galvanized steel Liu Jia1, Jiang Shichun, Shi Yan, Ni Cong, Chen junke, Huang Genzhe School of Electromechanical Engineering, Changchun University of Science and Technology,
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Changchun 130022, Jilin, China ABSTRACT
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The fusion-brazing connection of the dissimilar metals 5052 aluminum alloy/ST07Z galvanized steel was achieved using a Nd:YAG laser, and the effects of the zinc layer on the forming properties of the
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fusion-brazing bead and the growth behaviors of the intermetallic compound (IMC) were
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investigated. The results indicate that the galvanized layer can particularly change the initial conditions of the brazing, reducing the brazing temperature from near the melting point of Fe
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(1535.0℃) to that of Al (660.4℃). The IMCs can transform from the Fe2Al5 and FeAl3 phases, which were observed in the interface of the aluminum/non-galvanized steel pair to the Fe2Al5Zn0.4
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phase, which was observed in the interface of the aluminum/galvanized steel pair; this phenomenon
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occurs when zinc atoms are substituted for iron atoms during the formation of the IMCs. Keywords: Laser fusion-brazing; Aluminum alloy; Galvanized steel; Zinc coated layer; Intermetallic
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compound
1. Introduction
The demand for the dissimilar material joining of aluminum to steel has increased rapidly in manufacturing industries because the application of light aluminum alloys in welding structures produce significant weight reductions and energy savings (Miller et al., 2000). However, it remains difficult to weld dissimilar materials, such as aluminum and steel, due to the large differences in thermal-physical properties between the two materials, particularly their melting points, thermal conductivities and thermal expansion coefficients (Li and Sun, 2011). These differences in material properties result in large stresses and the formation of brittle intermetallic compound (IMC) that can 1
Corresponding author: Jia LIU, Tel/Fax: +86-431-85582207; Email: [email protected] 2
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induce cracking in the weld and therefore reduce the weld strength (David et al., 2014). These problems can be overcome by controlling the diffusion process (Schubert et al., 2001). For this reason, laser welding is considered to join dissimilar materials due to its high heating and cooling
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rates (Sierra et al., 2007). The IMC thickness was significantly decreased by using a continuous wave/pulse wave (CW/PW) dual beam YAG laser heat source compared to a single continuous wave
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YAG laser, and a root-shaped structure, which could improve the strength of the joints, was formed in the welding process (Shi et al., 2010). A two-pass laser welding method was used to join
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galvanized high-strength steel (DP590) to an aluminum alloy (AA6061) to increase working speed
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and a joint with a mechanical resistance of 158 N/mm was obtained under a preheating scanning speed of 50 mm/s and a welding speed of 100 mm/s (Ma et al., 2014). However, in the keyhole laser
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welding process using a single-beam or a dual-beam laser, strong mixing of the keyhole could produce non-uniform IMCs in the aluminum/steel interface, thus severely deteriorating the fatigue
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life of the joints (Liu et al., 2011).
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Recently, laser fusion-brazing, which combines the advantages of fusion welding and brazing by utilizing fusion welding on the aluminum side and brazing on the steel side, has become an attracting
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research focus due to its low heat input that could effectively suppress the generation of brittle intermetallic phases and reduce the thermal stresses in the joint. A laser fusion-brazing with filler materials of zinc and aluminum alloy has been succeeded in joining an aluminum alloy (A6016) to galvanized steel (AISI 1020) without flux. However, the joints usually failed in two positions: the heat affected zone (HAZ) of the aluminum alloy and the base steel sheet with a mechanical resistance of more than 200 N/mm (Rodriguez et al., 2005). A large number of laser fusion-brazing results show that the zinc coating layer can act as flux to enhance wetting behavior and increase the joint strength (Saida et al., 2008). The strength of an aluminum/galvanized steel joint is more than two times the strength of an aluminum/non-galvanized steel when flux is used, and the strength of an aluminum/galvanized steel joint without flux lies between the above two types of joint strength
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(Peyre et al., 2007). Conversely, to improve corrosion resistance, low-carbon deep-drawing steel sheets are typically hot-dipped or electro galvanized with zinc or zinc alloys in a continuous process in the automotive
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industry (Katundi et al., 2010). Although many scholars have conducted welding experiments on galvanized steels and aluminum alloys and found that the galvanized layer improves the wettability
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of liquid aluminum on the surface of the solid steel, they have not analyzed how the galvanized layer participates in the aluminum/galvanized steel welding process. Therefore, it is necessary to study the
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application of lightweight structures in vehicle production.
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effects of zinc on the laser welding of an aluminum alloy and a galvanized steel to describe the
The purpose of this study is to investigate the effects of zinc on the surface of solid steel on the
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wetting of liquid aluminum and the growing of IMCs. A three-dimensional (3D) finite element model of the laser welding of aluminum to galvanized steel has been developed to calculate the temperature
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distribution of the welded joint to support the analysis of the forming process of IMCs. A series of
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experiments, including the laser welding of aluminum to galvanized steel without a filler metal, the laser welding of aluminum to galvanized steel with pure Al filler powders and the laser welding of
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aluminum to non-galvanized steel with pure Al filler powders, are carefully performed. 2. Materials and Methods
The experimental materials consisted of 70-110 μm diameter pure Al powders, 1.5 mm thick A5052-H34 Al alloy sheets, and 0.8 mm thick ST07Z hot-dip galvanized steel sheets with a 10 μm thick Zn layer on both sides of the steel. Before welding, all sheets were cleaned with acetone. Particularly, the ST07Z galvanized steel sheet surfaces subjected to laser welding were dissolved in 1:9 hydrochloric acid inhibited with 5g/L sodium arsenite to remove the zinc coating layer. Chemical compositions of A5052-H34 Al alloy and ST07Z hot-dip galvanized steel are tabulated in Table 1. The physical properties of Fe, Al and Zn are listed in Table 2 (Brandes and Brook, 1992). Table 1 Chemical compositions (mass %) of A5052-H34 Al alloy and ST07Z galvanized steel 4
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ST07Z galvanized steel ≤0.01 ≤0.10 ≤0.30 ≤0.025 ≤0.020 ≤0.10 ≤0.10 Balance ≥0.015
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A5052-H34 Al alloy C Si ≤0.25 Mn ≤0.10 P S Mg 2.2~2.8 Cr 0.15~0.35 Cu ≤0.10 Ti Nb Zn ≤0.10 Fe 0~0.40 Al Balance
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Table 2 Physical properties of Fe, Al and Zn Fe Al Zn 1535.0 660.4 419.5 Melting point (℃) 2750 2467 908 Boiling point (℃) Density (g·cm-3) 7.87 2.70 7.14 -1 -1 Specific heat (J·kg ·K ) 444 900 389 Thermal conductivity (W·m-1·K-1) 73.3 238 113 Coefficient of expansion (10-6K-1) 12.2 23.9 31 The aluminum alloy was placed on top of the galvanized steel, and then the lap joint was placed at
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an angle of 60 º relative to the laser beam and feed head (Fig. 1a and 1b). A 4 kW continuous wave
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(CW) Nd:YAG laser (HL4006D, TRUMPF GmbH) was applied in this work. The beam traveled to
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the target through a 600 μm diameter fiber-optic cable. The parts to be joined were moved by a numerically controlled 6-axis robot (KR 30HA, KUKA GmbH). An optical system combining a collimating lens with a 200 mm focal length and a focusing lens with a 200 mm focal length was placed between the end of the cable and the target. The diameter of the beam site was theoretically 600 μm at the focal point but was brought out of focus to produce a larger spot of approximately 2 mm in diameter on the work piece. This entailed a slight loss in the distribution of energy compared with that at the focal plane. A 3D graph of the spatial distribution of the energy of the laser spot in this setup is shown in Fig. 1c. A multiple layer structure paraxial powder feeding head was used during the laser fusion-brazing process with the filler powder process, and shielding gas at a constant flow rate of 15 L/min on the outermost layer was used to protect the processing zone and control the plasma during the welding. To make the powder only enter the laser irradiation region, the angle 5
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between the laser beam and the feeding head was first set to 30º, and then the distance between the laser beam and powder feeding head was adjusted incrementally to achieve the desired results, as
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shown in Fig. 1c.
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Fig. 1. Schematic diagram of laser welding procedures with different views: (a) three-dimensional (3D) stereogram; (b) front elevation; and (c) right elevation
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Table 3 Laser welding conditions Processing parameters Without filler powder With filler powder Laser power, P (W) 2400-2800 1750-2750 Defocusing distance, Δf (mm) 12 12 Travelling velocity, v (m/min) 1.0 1.0 Powder feeding speed, fp (g/min) 0 4.53 0 5 Carrier gas flow rate, fg (L/min) Ar shielding gas flow rate, fAr (L/min) 15 15 Laser power was chosen as a shared variable in the laser welding processes with and without filler powder. Specific parameters are listed in Table 3. After welding, inspections of the microstructure and elementary analysis of the welded joint were performed by scanning electron microscopy (SEM, type of Zeiss EVO18) and electron probe micro analysis (EPMA). Welds were in turn sectioned, mounted, ground, polished with a diamond slurry to a 3 μm surface finish and etched with Nital acid (3% HNO3 ethanol solution) and Keller’s reagent (1.0 vol.% HF, 1.5 vol.% HCl and 2.5 vol.% HNO3 in deionized water) for the Zn-coated steel and Al alloy, respectively, for further evaluations. To further observe the IMCs/steel substrate interface and analyze the growth process of the IMC layer, 6
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solutions of 0.5 vol.% HF and 50 vol.% HNO3 in deionized water were applied to dissolve the aluminide layer and steel substrate, respectively, after laser welding with filler powder. 3. Modeling of the Dissimilar Metal Welding Process
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A 3D finite element model was developed to accurately capture the temperature fields both metals during the dissimilar metal welding process. The commercial software SYSWELD was used to
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perform the thermal calculations during welding. The thermal physical properties of the base materials of both A5052 Al alloy and ST07Z galvanized steel are shown in Tables 4 and 5,
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respectively.
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Table 4 Thermal and physical properties of A5052 Al alloy Temperature Thermal Conductivity Specific Heat Density (J·kg-1·K-1) (kg·mm-3) (W·mm-1·K-1) (℃) 20 0.1376 911 2.680E-06 200 0.1547 1002 2.664E-06 500 0.175 1157 2.584E-06 800 0.195 1275 2.361E-06 1000 0.202 1320 2.337E-06
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Table 5 Thermal and physical properties of ST07Z galvanized steel Temperature Thermal Conductivity Specific Heat Density (W·mm-1·K-1) (J·kg-1·K-1) (kg·mm-3) (℃) 20 0.068 444 7.850E-06 200 0.059 550 7.800E-06 400 0.047 610 7.730E-06 600 0.036 710 7.653E-06 800 0.029 865 7.566E-06 900 0.027 565 7.519E-06 1450 0.033 630 7.261E-06 2500 0.033 707 6.940E-06 The theoretical background for the welding simulation is described below. The equations used by SYSWELD during the thermal analysis are described briefly (ESI GmbH, 2008). The transient heat transfer equation for welding is given by:
ρc
G ∂T ( x, y, z, t ) = −∇ ⋅ q ( x, y, z, t ) + Q ( x, y, z, t ) ∂t
(1)
where ρ is the density of the materials, c is the specific heat capacity, T is the current temperature,
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G q is the heat flux vector, Q is the internal heat generation rate, x, y and z are the coordinates in the reference system, t is the time, and ∇ is the spatial gradient operator. The non-linear isotropic Fourier heat flux constitutive equation is described by:
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G q = −k ∇T where k is the temperature dependent thermal conductivity.
(2)
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Due to the melting of the aluminum alloy and non-forming of the keyhole in the laser
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fusion-brazing process, the heat from the moving welding laser is applied as a volumetric heat source with a double ellipsoidal distribution, as proposed by Goldak (Goldak et al., 1984). The front half of
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the source is the quadrant of one ellipsoidal source, and the rear half is the quadrant of another ellipsoid. In this model, the fractions ff and fr of the heat deposited in the front and rear quadrants are
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required, where ff + fr = 2.
The power density distribution inside the front quadrant becomes:
abcπ π
e −3 x
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2 / a 2 −3 y 2 / b2 −3⎣⎡ z + v (τ −t )⎦⎤ / c
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(3)
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q ( x, y , z, t ) =
Similarly, for the rear quadrant of the source, the power density distribution inside the ellipsoid
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becomes:
q ( x, y , z, t ) =
6 3 f rQ −3 x2 / a 2 −3 y 2 / b2 −3⎣⎡ z + v(τ −t )⎦⎤2 / c2 e e e abcπ π
(4)
To perform a welding simulation, a reference line, welding trajectory line, start and end nodes and the start elements are required. Based on the welding velocity and the length of the molten zone along with the welding trajectories, the parameters a, b, c are automatically generated by SYSWELD. The total absorbed power must then be fitted to an intensity function during the thermal analysis. Both the thermal radiation and heat transfer onto the weld surface are assumed. Radiation losses dominate at higher temperatures near and in the weld zone, and convection losses dominate at lower temperatures away from the weld zone (Radaj, 1992). For the radiative and convective boundary 8
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conditions, a combined heat transfer coefficient was calculated from the relationship:
H = ε C0 (Ts + T f ) (Ts 2 + T f 2 ) + H 0
(5)
where ε is the emissivity or degree of blackness of the surface of the body, C0 is the
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blackbody radiation coefficient, Ts is the temperature of the skin of the materials, Tf is the environmental temperature, and H0 is convection losses coefficient. Values of 0.8 and 25 were
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assumed for ε and H0, respectively, as recommended for hot rolled steel.
Laser welding with filler powder
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Laser welding without filler powder
Fig. 2. Surface morphologies of the joints produced by laser welding without filler powder (welding parameters: v=1.0m/min, Δf=12mm, fAr=15L/min) and with filler powder (welding parameters: v=1.0m/min, Δf=12mm, fp=4.53g/min, fg=5L/min, fAr=15L/min) at different laser powers
4. Results and Discussion
4.1 Forming properties of the fusion-brazing bead Surface morphologies of the joints from laser welding with and without filler powder under different laser powers are shown in Fig. 2. In process without filler powder, the time interval, which is measured from the start of welding until liquid aluminum begins to spread into the solid steel surface, gradually shortens with the increase of laser power. However, the time interval is not reduced to zero until laser power increases to 2800W. Conversely, in the processes with filler power, liquid aluminum is found to spread into the solid steel surface at the start of laser welding at low 9
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laser powers. It is generally thought that this is because the demand for liquid aluminum spreading in galvanized steel surface is related to the temperature of the interface between aluminum and galvanized layer and must be no less than the Al-Zn eutectic temperature (382℃) (Лякишев, 2009).
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The laser absorption rate on the surface of aluminum alloy is low, and filler powder can remarkably improve it (Zuo, 2002). Therefore, laser welding without filler powder required additional time to
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accumulate energy, while laser welding with filler powder can reach the accumulation process in a
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relatively short time.
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Fig. 3. Surface morphologies of the joints produced by laser welding an aluminum alloy to non-galvanized steel with filler powder
Fig. 4. Cross-section morphologies of the joints from laser welding an aluminum alloy to non-galvanized steel (welding parameters: P=2250W, v=1.0m/min, Δf=12mm, fp=4.53g/min, fg=5L/min, fAr=15L/min) with filler powder in different positions (SEM): (a) the melting zone, as shown by Line A in Fig. 3; and (b) no melting zone, as shown by Line B in Fig. 3 Although the filler powder can improve the laser absorption rate on the aluminum alloy surface and shorten the time for accumulation energy in the laser fusion-brazing process, laser welding with filler powder is unable to achieve the effective connection of the aluminum alloy and the non-galvanized steel using different laser powers. Fig. 3 is a typical surface morphology of the joints from laser welding an aluminum alloy to a non-galvanized steel with filler powder. In the process of 10
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laser welding an aluminum alloy to a non-galvanized steel with filler powder, the liquid aluminum cannot wet into the solid steel surface until the steel melts, as shown in Fig. 4. Similar to the laser welding of an aluminum alloy to galvanized steel without filler powder, the aluminum wetting zone
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is expanded by an increase in laser power. However, steel melting and the large differences in thermal and physical properties between the two materials can result in great stress and a large
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number of brittle IMCs. Therefore, cracks and damage occur in the IMCs, as shown in Fig. 5. The chemical compositions of Points 1 to 7 in Fig. 5 are listed in Table 6. Significant and random
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variations of chemical compositions between Points 1 and 7 were also obtained. According to the
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atomic percentage of aluminum, Points 1 and 7 are located in the base metals of the aluminum and steel, respectively. It can be inferred that Al rich phases are observed at the other points, where the
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atom percentage of aluminum is between 63.74% and 79.18%. According to the Al-Fe phase diagram (Massalski and Okamoto, 1990), the atom percentage of aluminum of the Al rich phases of FeAl2,
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Fe2Al5 and FeAl3 are 67.1%-67.9%, 70.1%-72.6% and 74.3%-76.1%, respectively. However,
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because the FeAl2 phase is a metastable phase that cannot exist after welding, only the Fe2Al5 and FeAl3 phases can be formed at the Al-Fe interface. Because the Gibbs free-energy change of the
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formation of the Fe2Al5 phase is smaller than that of FeAl3 (Shi et al., 2013), the Fe2Al5 phase is formed earlier than FeAl3. Thus, as the temperature decreases, the solubility of iron in the molten aluminum decreases gradually; iron atoms are thus precipitated by the acicular FeAl3 phase on the side of the Al (Fig. 5b). Therefore, Al+FeAl3, Fe2Al5 and Fe+Fe2Al5 phases are observed at Points 2 to 6, respectively, as listed in Table 6.
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Fig. 5. Partial view of the melting zone in the Fig. 4a (SEM): (a) the all melting zone; (b) the aluminum side of the melting zone, as shown in Fig. 5, Zone A; and (c) the steel side of the melting zone, as shown in Fig. 5, Zone B
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Table 6 Chemical compositions of Points 1 to 7 in Fig. 5 Chemical compositions (at. %) Point Phase Al Fe 1 98.53 1.47 Al 2 79.18 20.82 Al+FeAl3 3 72.59 27.41 Fe2Al5 4 72.31 27.69 Fe2Al5 5 69.20 30.80 Fe2Al5+Fe 6 63.74 36.26 Fe2Al5+Fe 7 0.78 99.22 Fe
4.2 Growth behaviors of IMC
The cross-section morphologies of the joints from the laser welding of the aluminum alloy to the galvanized steel is shown in Fig. 6 with filler powder under a laser power of 2250W, a travelling velocity of 1.0m/min, a defocusing distance of 12mm, a powder feeding speed of 4.53g/min, a carrier gas flow rate of 5L/min, and a shielding gas flow rate of 15L/min. The interface of the aluminum/steel is straight, and no significant erosion is observed. A dendrite microstructure is 12
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observed at the edge of the filler layer, and there is no a clear interface reaction layer at the aluminum/steel interface, as shown in Fig. 7. Based on the analysis of the element distribution in the region, it is clear that a large amount of zinc appears in the edge of filler layer, and a dividing line
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between the iron and aluminum is present at the aluminum/steel interface. Thus, it is proved that the thickness of the interface reaction layer is significantly thinner in the zinc rich zone. The laminar and
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acicular IMCs, which are similar to the IMCs in the joint produced by laser welding of an aluminum alloy to non-galvanized steel with filler powder, are observed in the center of the welded joint, as
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shown in Fig. 8. Through the analysis of the element distribution in the region, it can be determined
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that the iron and aluminum are concentrated at the interface reaction layer, and a small quantity of residual zinc is detected on the side of the steel interface. However, as a result of the identification of
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the production phase by X-ray diffraction patterns, it is found that this layer consisted of a Fe-Al intermetallic compound (Fe2Al5Zn0.4), as shown in Fig. 9. Because the Fe2Al5Zn0.4 phase is not a
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brittle phase (Nishimoto et al., 2008), which is different from the Al-rich phases of FeAl3 and Fe2Al5
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obtained in the laser welding of an aluminum alloy to non-galvanized steel with filler powder.
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Therefore, no cracks are found in the IMC layer.
Fig. 6. Cross-section morphologies of the joints from laser welding an aluminum alloy to galvanized steel (welding parameters: P=2250W, v=1.0m/min, Δf=12mm, fp=4.53g/min, fg=5L/min, fAr=15L/min) with filler powder (SEM)
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Fig. 7. Microstructure and elementary distribution in the Zn-rich zone (as shown in Fig. 6, Zone A) analyzed by EPMA
Fig. 8. Microstructure and elementary distribution in the center zone (as shown in Fig. 6, Zone B) analyzed by EPMA
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5000 ♦ Al ◊ Fe2Al5Zn0.4
♦
3000
2000
1000
0
◊
◊ ◊
20
30
♦ ◊
♦
◊ ◊
40
50
◊ ♦
◊
60
70
2 Theta (Deg.)
80
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◊ ♦ ◊
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Intensity (CPS)
4000
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Fig. 9. X-ray diffraction pattern of the reaction layer at the galvanized steel/brazed metal interface In Al-Fe laser welding, the atomic diffusion of Al and Fe across the interface is the rate-limiting
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factor for the growth of IMCs due to the unmelted steel; the materials’ thickness as a function of time can be expressed by the following equation (Jindal and Srivastava, 2008): (5)
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x = Dt
where x (μm), D (μm2·s-1), and t (s) are the IMC’s thickness, growth rate constant, and aging time,
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respectively. The growth rate constant D is related to temperature and can be expressed as a simple
(6)
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⎛ Q ⎞ D = D0 exp ⎜ − ⎟ ⎝ RT ⎠
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Arrhenius relationship (Ji and Xue, 2013):
where D, D0, Q, R, and T are the growth rate constant, the frequency factor, the activation energy for the growth of the interfacial IMC layer, the gas constant (8.314 kJ·mol-1·K-1), and the temperature in Kelvin (K), respectively. From Eq. (5) and Eq. (6), it is found that the thicker the interface reaction layer is, the higher the temperature and the longer the time of the brazing required are. Based on the temperature field of laser welding an aluminum alloy to galvanized steel with filler powder by numerical simulation, it can be found that the liquid residence time and the peak temperature of the Al/Fe interface increase from the weld toe to the center, as shown in Fig. 10. According to the results of the temperature distribution, it can be predicted that the thicknesses of the ICMs increase gradually with the change of the regions from the Zn-rich zone to the center of the joint. Fig. 11 15
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shows the morphologies of the IMCs in different regions of the dissimilar metal joint of aluminum alloy and galvanized steel from laser welding with filler powder; the accuracy of the prediction is
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thus proved.
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1200 (c)
Point A Point B
1100
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900 800 700 600
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Temperature (oC)
1000
500 400
200
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300 2.4
3.6
4.8 Times (s)
6.0
7.2
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Fig. 10. Temperature field results and thermal cycling curves of laser welding of an aluminum alloy to galvanized steel (welding parameters: P=2250W, v=1.0m/min, Δf=12mm, fp=4.53g/min, fg=5L/min, fAr=15L/min) with filler powder: (a) temperature field on the surface of the specimens; (b) temperature field on the cross-section of the specimens; and (c) thermal cycling curves at Points A and B, as shown in Fig. 10b
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ip t cr us an M d te Ac ce p Fig. 11. IMCs in different regions of the dissimilar metal joint of aluminum alloy and galvanized steel from laser welding with filler powder Due to high heating and cooling rates in the laser welding process, the different morphologies of
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the IMCs that are obtained in the different zones from the Zn-rich zone to the center of the joint can be regarded as the morphologies of the different steps of the IMCs forming process in the center of
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the joint.
Fig. 12 Schematic illustration of the formation process of ICMs At the beginning of welding, the galvanized layer is melted by liquid aluminum because the melting point of aluminum is higher than that of zinc. Then, a portion of solute zinc atoms are dissolved in the molten aluminum to form an Al-Zn liquid solution; other zinc atoms are then pushed 18
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to accumulate at the weld toe and form a Zn-rich zone because the density of aluminum is much less than the density of zinc, and the electron affinity of Al-Fe is larger than that of Al-Zn (Zhao et al., 2006), as shown in Fig. 12a and Fig. 12b. When the liquid aluminum contacts the solid steel, iron
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atoms begin to diffuse into the liquid aluminum and separate from the liquid aluminum as an Al-rich phase at the interface of the solid steel and liquid aluminum when the concentration of iron in the
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liquid aluminum reaches a certain value. As the Gibbs free-energy changes because the formation of Fe2Al5 is smaller than that of FeAl3 (Shi et al., 2013), the Fe2Al5 phase is formed earlier than the
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FeAl3 phase, as shown in Fig. 12c. The formation of the continuous Fe2Al5 phase layer can hinder
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iron atom diffusion into the liquid aluminum due to a small diffusion coefficient of iron atoms in the Fe2Al5 phase (Chen and Nakata, 2008). Then, as the temperature decreases, the solubility of iron in
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the molten aluminum decreases gradually, and iron atoms are precipitated by the acicular Al-rich intermetallic phase, FeAl3, on the side of the Al, as shown in Fig. 12d and Fig. 12e. Because
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the zinc atoms in the liquid aluminum can substitute iron atoms to join to the formation of the IMCs
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(Zhao et al., 2006), the intermetallic phase is Fe2Al5Zn0.4 at the interface of the aluminum alloy and the galvanized steel in the joint of dissimilar metals after welding, as shown in Fig. 12f.
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The primary problems in laser welding aluminum alloy to steel are the low energy utilization rate of the laser beam and the poor wettability of molten aluminum onto the surface of the solid steel. Due to the high reflectivity rate of the laser on the surface of the base metals such as aluminum alloy and steel (Kawahito et al., 2011), a high laser power is usually required in laser fusion-brazing process of an aluminum alloy and galvanized steel. Because of the large surface area of the powders and the change of the laser reflectivity type from specular to diffuse reflection with the joining powders (Zuo, 2002), filling powder is used to improve the energy utilization rate of laser beam in laser welding process and has been shown to be an effective method (Fig. 2). Because the melting point of Al (660.4 ℃) is higher than that of Zn (419.5 ℃) and significantly lower than that of Fe (1535.0 ℃) (Brandes and Brook, 1992), the Zn-coated layer melts while the 19
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steel is unable to be melted upon contact with the molten aluminum. Therefore, molten aluminum can spread at the beginning of laser welding aluminum alloy/galvanized steel (Fig. 2) but cannot spread until the steel is melted in the laser welding aluminum alloy/non-galvanized steel process (Fig.
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3 and Fig. 4) due to the poor wettability of aluminum onto the steel surface. Thus, the galvanized layer can change the initial temperature of the brazing from near the melting point of Fe (1535.0℃)
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to that of Al (660.4℃).
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5. Conclusions
In this study, the laser fusion-brazing of an aluminum alloy to galvanized and non-galvanized
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steels was performed. The resulting formations and microstructure features of the fusion-braze bead were specifically assessed. Based on this study, several conclusions are summarized as follows:
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(1) Based on the comparison of the results produced by laser welding an aluminum alloy to galvanized steel with and without a filler powder, it was found that filler powders could improve
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the energy utilization rate to reduce laser power requirements in the laser fusion-brazing process.
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(2) According to the comparative results of laser welding with filler powder to join an aluminum alloy and a galvanized or non-galvanized steel, it was found that the zinc coated layer produced a
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good wetting effect by changing the types of the materials that come into contact with the liquid aluminum, reducing the brazing temperature from near the melting point of Fe (1535.0℃) to that of Al (660.4℃).
(3) X-ray diffraction analysis results reveal that the IMC of Fe2Al5Zn0.4, a ductile and tough phase, is formed at the galvanized steel/filler layer interface. It is shown that zinc acts critically as flux and is also involved in metallurgical reactions. (4) The forming process of the Fe2Al5Zn0.4 phase is firstly forming from the Al-rich phases of Fe2Al5 and FeAl3; then, Zn atoms slowly diffuse into the Al-Fe phases to substitute for iron atoms.
Acknowledgments 20
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The authors would like to thank the Natural Science Foundation of Jilin Province, China for financially supporting this research under Contract No. 201115156.
References
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Brandes, E. A., Brook, G. B., 1992. Smithells metals reference book (seventh edition). Butterworth Heinemann.
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Chen, Y.C., Nakata, K., 2008. Effect of the Surface State of Steel on the Microstructure and Mechanical Properties of Dissimilar Metal Lap Joints of Aluminum and Steel by Friction Stir
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Welding. Metallurgical and Materials Transactions. 39(8), 1985-1992.
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David, A.K., PSM Team, 2014. Study of metallurgic and mechanical properties of laser welded heterogeneous joints between DP600 galvanized steel and aluminum 6082. Materials and Design. 54,
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184-195. ESI GmbH, 2008. SYSWELD welding handbook.
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Goldak, J., Chakravarti, A., Bibby, A., 1984. A new finite element model for welding heat sources.
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Metallurgical and Materials Transactions. 15(2), 299-305. Jindal, V., Srivastava, V. C., 2008. Growth of intermetallic layer at roll bonded IF-steel/aluminum
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interface. Journal of Materials Processing Technology. 195(1-3), 88-93. Ji, F., Xue, S.B., 2013. Growth behaviors of intermetallic compound layers in Cu/Al joints brazed with Zn-22Al and Zn-22Al-0.05Ce filler metals. Materials and Design. 51, 907-915. Katundi, D., Tosun, B.A., Bayraktar, E., Toueix, D., 2010. Corrosion behavior of the welded steel sheets used in automotive industry. Journal of Achievements in Materials and Manufacturing Engineering. 38(2), 146-153.
Kawahito, Y., Matsumoto, N., Abe, Y., Katayama, S., 2011. Relationship of laser absorption to keyhole behavior in high power fiber laser welding of stainless steel and aluminum alloy. Journal of Materials Processing Technology. 211(10), 1563-1568. Li, L.B., Sun Y.F., 2011. Handbook of physical properties of metal materials. China Machine Press.
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(In Chinese) Liu, B., He, G.Q., Jiang, X.S., Zhu, M.H., 2011. Review on effect factor and research method of material fatigue life. Materials Review. 25(5), 103-106.
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Ma, J.J., Harooni, M., Carlson, B., Kovacevic, R., 2014. Dissimilar joining of galvanized high strength steel to aluminum alloy in a zero-gap lap joint configuration by two-pass laser welding.
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Materials and Design. 58, 390-401.
Massalski, T.B., Okamoto, H., 1990. Binary alloy phase diagrams, vol. 3, 2nd ed. Materials Park
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(OH): ASM International.
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Miller, W.S., Zhuang, L., Bottema, J., Wittebrood, A.J., Smet, P. D., Haszler, A., Vieregge, A., 2000. Recent development in aluminum alloys for the automotive industry. Materials Science and
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Engineering A. 280(1), 37-49.
Nishimoto, K., Harano, T., Okumoto, Y., Atagi, K., Fujii, H., Katayama, S., 2008. Mechanical
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properties of laser-pressure-welded joint between dissimilar galvannealed steel and pure aluminum.
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Quarterly Journal of the Japan Welding Society. 26(2), 174-180. Peyre, P., Sierra, G., Deschaux, B.F., Stuart, D., Fras, G., 2007. Generation of aluminum steel joints
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with laser-induced reactive wetting. Materials Science and Engineering A. 444, 327-338. Radaj, D., 1992. Heat effects of welding, temperature field, residual stress and distortions. Springer. Rodriguez, L., Mathieu, A., Langlade, C., Vannes, A., Grevey, D., Mattei, S., 2005. Laser brazing of steel aluminum assembly. Laser in Engineering. 5-6, 413-425. Saida, K., Ohnishi, H., Nishimoto, K., 2008. Fluxless laser brazing of aluminum alloy to galvanized steel using a tandem beam-dissimilar laser brazing of aluminum alloy and steels. Japan Welding Society. 26(3), 235-241. Schubert, E., Klassen, M., Zerner, I., Walz, C., Sepold, G., 2001. Light-weight structures produced by laser beam joining for future applications in automobile and aerospace industry. Journal of Materials Processing Technology. 115, 2-8.
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Shi, Y., Zhang, H., Takehiro, W., Tang J.G, 2010. CW/PW dual-beam YAG laser welding of steel/aluminum alloy sheets. Optics and Lasers in Engineering. 48, 732-736. Shi, Y., He, C.C., Huang, J.K., Fan, D., 2013. Thermodynamic analysis of the forming of
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intermetallic compounds on aluminum-steel welding interface. Journal of Lanzhou University of Technology. 39(4), 5-7. (In Chinese)
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Sierra, G., Peyre, P., Deschaux, B.F., Stuart, D., Fras, G., 2007. Steel to aluminum key-hole laser welding. Material Science and Engineering A. 447, 197-208.
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Zhao, L., Sun, Y., Meng, X.F., Li, Y.G., Song, Q.L., 2006. First-principles studies of the effects of Zn
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on Fe/Al interface. http://www.paper.edu.cn/releasepaper/content/200612-328. (In Chinese) Zuo, T.C., 2002. Laser materials processing of high strength aluminum alloys. National Defense
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Industry Press. 150-159. (In Chinese)
Лякишев, Н. П., 2009. Handbook of binary alloy phase diagram. Chemical Industry Press. (In
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Chinese)
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S0924-0136(15)00183-1 http://dx.doi.org/doi:10.1016/j.jmatprotec.2015.04.017 PROTEC 14388
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
21-1-2015 13-4-2015 15-4-2015
Please cite this article as: Jia, L., Shichun, J., Yan, S., Cong, N., junke, C., Genzhe, H.,Effects of zinc on the laser welding of an aluminum alloy and galvanized steel, Journal of Materials Processing Technology (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.04.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights: 1. We show a new Al-Fe welding mode, laser beam welding with filler powder. 2. Filling powder can improve the energy utilization rate in the fusion-brazing process. 3. The zinc coated layer can cut down the brazing temperature from 1535.0℃ to 660.4℃. 4. The intermetallic compound layer was made up of Fe2Al5Zn0.4 phase. 5. The formation of Fe2Al5Zn0.4 phase is because zinc atoms substitute iron atoms.
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Effects of zinc on the laser welding of an aluminum alloy and galvanized steel Liu Jia1, Jiang Shichun, Shi Yan, Ni Cong, Chen junke, Huang Genzhe School of Electromechanical Engineering, Changchun University of Science and Technology,
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Changchun 130022, Jilin, China ABSTRACT
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The fusion-brazing connection of the dissimilar metals 5052 aluminum alloy/ST07Z galvanized steel was achieved using a Nd:YAG laser, and the effects of the zinc layer on the forming properties of the
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fusion-brazing bead and the growth behaviors of the intermetallic compound (IMC) were
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investigated. The results indicate that the galvanized layer can particularly change the initial conditions of the brazing, reducing the brazing temperature from near the melting point of Fe
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(1535.0℃) to that of Al (660.4℃). The IMCs can transform from the Fe2Al5 and FeAl3 phases, which were observed in the interface of the aluminum/non-galvanized steel pair to the Fe2Al5Zn0.4
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phase, which was observed in the interface of the aluminum/galvanized steel pair; this phenomenon
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occurs when zinc atoms are substituted for iron atoms during the formation of the IMCs. Keywords: Laser fusion-brazing; Aluminum alloy; Galvanized steel; Zinc coated layer; Intermetallic
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compound
1. Introduction
The demand for the dissimilar material joining of aluminum to steel has increased rapidly in manufacturing industries because the application of light aluminum alloys in welding structures produce significant weight reductions and energy savings (Miller et al., 2000). However, it remains difficult to weld dissimilar materials, such as aluminum and steel, due to the large differences in thermal-physical properties between the two materials, particularly their melting points, thermal conductivities and thermal expansion coefficients (Li and Sun, 2011). These differences in material properties result in large stresses and the formation of brittle intermetallic compound (IMC) that can 1
Corresponding author: Jia LIU, Tel/Fax: +86-431-85582207; Email: [email protected] 2
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induce cracking in the weld and therefore reduce the weld strength (David et al., 2014). These problems can be overcome by controlling the diffusion process (Schubert et al., 2001). For this reason, laser welding is considered to join dissimilar materials due to its high heating and cooling
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rates (Sierra et al., 2007). The IMC thickness was significantly decreased by using a continuous wave/pulse wave (CW/PW) dual beam YAG laser heat source compared to a single continuous wave
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YAG laser, and a root-shaped structure, which could improve the strength of the joints, was formed in the welding process (Shi et al., 2010). A two-pass laser welding method was used to join
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galvanized high-strength steel (DP590) to an aluminum alloy (AA6061) to increase working speed
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and a joint with a mechanical resistance of 158 N/mm was obtained under a preheating scanning speed of 50 mm/s and a welding speed of 100 mm/s (Ma et al., 2014). However, in the keyhole laser
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welding process using a single-beam or a dual-beam laser, strong mixing of the keyhole could produce non-uniform IMCs in the aluminum/steel interface, thus severely deteriorating the fatigue
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life of the joints (Liu et al., 2011).
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Recently, laser fusion-brazing, which combines the advantages of fusion welding and brazing by utilizing fusion welding on the aluminum side and brazing on the steel side, has become an attracting
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research focus due to its low heat input that could effectively suppress the generation of brittle intermetallic phases and reduce the thermal stresses in the joint. A laser fusion-brazing with filler materials of zinc and aluminum alloy has been succeeded in joining an aluminum alloy (A6016) to galvanized steel (AISI 1020) without flux. However, the joints usually failed in two positions: the heat affected zone (HAZ) of the aluminum alloy and the base steel sheet with a mechanical resistance of more than 200 N/mm (Rodriguez et al., 2005). A large number of laser fusion-brazing results show that the zinc coating layer can act as flux to enhance wetting behavior and increase the joint strength (Saida et al., 2008). The strength of an aluminum/galvanized steel joint is more than two times the strength of an aluminum/non-galvanized steel when flux is used, and the strength of an aluminum/galvanized steel joint without flux lies between the above two types of joint strength
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(Peyre et al., 2007). Conversely, to improve corrosion resistance, low-carbon deep-drawing steel sheets are typically hot-dipped or electro galvanized with zinc or zinc alloys in a continuous process in the automotive
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industry (Katundi et al., 2010). Although many scholars have conducted welding experiments on galvanized steels and aluminum alloys and found that the galvanized layer improves the wettability
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of liquid aluminum on the surface of the solid steel, they have not analyzed how the galvanized layer participates in the aluminum/galvanized steel welding process. Therefore, it is necessary to study the
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application of lightweight structures in vehicle production.
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effects of zinc on the laser welding of an aluminum alloy and a galvanized steel to describe the
The purpose of this study is to investigate the effects of zinc on the surface of solid steel on the
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wetting of liquid aluminum and the growing of IMCs. A three-dimensional (3D) finite element model of the laser welding of aluminum to galvanized steel has been developed to calculate the temperature
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distribution of the welded joint to support the analysis of the forming process of IMCs. A series of
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experiments, including the laser welding of aluminum to galvanized steel without a filler metal, the laser welding of aluminum to galvanized steel with pure Al filler powders and the laser welding of
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aluminum to non-galvanized steel with pure Al filler powders, are carefully performed. 2. Materials and Methods
The experimental materials consisted of 70-110 μm diameter pure Al powders, 1.5 mm thick A5052-H34 Al alloy sheets, and 0.8 mm thick ST07Z hot-dip galvanized steel sheets with a 10 μm thick Zn layer on both sides of the steel. Before welding, all sheets were cleaned with acetone. Particularly, the ST07Z galvanized steel sheet surfaces subjected to laser welding were dissolved in 1:9 hydrochloric acid inhibited with 5g/L sodium arsenite to remove the zinc coating layer. Chemical compositions of A5052-H34 Al alloy and ST07Z hot-dip galvanized steel are tabulated in Table 1. The physical properties of Fe, Al and Zn are listed in Table 2 (Brandes and Brook, 1992). Table 1 Chemical compositions (mass %) of A5052-H34 Al alloy and ST07Z galvanized steel 4
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ST07Z galvanized steel ≤0.01 ≤0.10 ≤0.30 ≤0.025 ≤0.020 ≤0.10 ≤0.10 Balance ≥0.015
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A5052-H34 Al alloy C Si ≤0.25 Mn ≤0.10 P S Mg 2.2~2.8 Cr 0.15~0.35 Cu ≤0.10 Ti Nb Zn ≤0.10 Fe 0~0.40 Al Balance
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Table 2 Physical properties of Fe, Al and Zn Fe Al Zn 1535.0 660.4 419.5 Melting point (℃) 2750 2467 908 Boiling point (℃) Density (g·cm-3) 7.87 2.70 7.14 -1 -1 Specific heat (J·kg ·K ) 444 900 389 Thermal conductivity (W·m-1·K-1) 73.3 238 113 Coefficient of expansion (10-6K-1) 12.2 23.9 31 The aluminum alloy was placed on top of the galvanized steel, and then the lap joint was placed at
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an angle of 60 º relative to the laser beam and feed head (Fig. 1a and 1b). A 4 kW continuous wave
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(CW) Nd:YAG laser (HL4006D, TRUMPF GmbH) was applied in this work. The beam traveled to
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the target through a 600 μm diameter fiber-optic cable. The parts to be joined were moved by a numerically controlled 6-axis robot (KR 30HA, KUKA GmbH). An optical system combining a collimating lens with a 200 mm focal length and a focusing lens with a 200 mm focal length was placed between the end of the cable and the target. The diameter of the beam site was theoretically 600 μm at the focal point but was brought out of focus to produce a larger spot of approximately 2 mm in diameter on the work piece. This entailed a slight loss in the distribution of energy compared with that at the focal plane. A 3D graph of the spatial distribution of the energy of the laser spot in this setup is shown in Fig. 1c. A multiple layer structure paraxial powder feeding head was used during the laser fusion-brazing process with the filler powder process, and shielding gas at a constant flow rate of 15 L/min on the outermost layer was used to protect the processing zone and control the plasma during the welding. To make the powder only enter the laser irradiation region, the angle 5
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between the laser beam and the feeding head was first set to 30º, and then the distance between the laser beam and powder feeding head was adjusted incrementally to achieve the desired results, as
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shown in Fig. 1c.
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Fig. 1. Schematic diagram of laser welding procedures with different views: (a) three-dimensional (3D) stereogram; (b) front elevation; and (c) right elevation
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Table 3 Laser welding conditions Processing parameters Without filler powder With filler powder Laser power, P (W) 2400-2800 1750-2750 Defocusing distance, Δf (mm) 12 12 Travelling velocity, v (m/min) 1.0 1.0 Powder feeding speed, fp (g/min) 0 4.53 0 5 Carrier gas flow rate, fg (L/min) Ar shielding gas flow rate, fAr (L/min) 15 15 Laser power was chosen as a shared variable in the laser welding processes with and without filler powder. Specific parameters are listed in Table 3. After welding, inspections of the microstructure and elementary analysis of the welded joint were performed by scanning electron microscopy (SEM, type of Zeiss EVO18) and electron probe micro analysis (EPMA). Welds were in turn sectioned, mounted, ground, polished with a diamond slurry to a 3 μm surface finish and etched with Nital acid (3% HNO3 ethanol solution) and Keller’s reagent (1.0 vol.% HF, 1.5 vol.% HCl and 2.5 vol.% HNO3 in deionized water) for the Zn-coated steel and Al alloy, respectively, for further evaluations. To further observe the IMCs/steel substrate interface and analyze the growth process of the IMC layer, 6
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solutions of 0.5 vol.% HF and 50 vol.% HNO3 in deionized water were applied to dissolve the aluminide layer and steel substrate, respectively, after laser welding with filler powder. 3. Modeling of the Dissimilar Metal Welding Process
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A 3D finite element model was developed to accurately capture the temperature fields both metals during the dissimilar metal welding process. The commercial software SYSWELD was used to
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perform the thermal calculations during welding. The thermal physical properties of the base materials of both A5052 Al alloy and ST07Z galvanized steel are shown in Tables 4 and 5,
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respectively.
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Table 4 Thermal and physical properties of A5052 Al alloy Temperature Thermal Conductivity Specific Heat Density (J·kg-1·K-1) (kg·mm-3) (W·mm-1·K-1) (℃) 20 0.1376 911 2.680E-06 200 0.1547 1002 2.664E-06 500 0.175 1157 2.584E-06 800 0.195 1275 2.361E-06 1000 0.202 1320 2.337E-06
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Table 5 Thermal and physical properties of ST07Z galvanized steel Temperature Thermal Conductivity Specific Heat Density (W·mm-1·K-1) (J·kg-1·K-1) (kg·mm-3) (℃) 20 0.068 444 7.850E-06 200 0.059 550 7.800E-06 400 0.047 610 7.730E-06 600 0.036 710 7.653E-06 800 0.029 865 7.566E-06 900 0.027 565 7.519E-06 1450 0.033 630 7.261E-06 2500 0.033 707 6.940E-06 The theoretical background for the welding simulation is described below. The equations used by SYSWELD during the thermal analysis are described briefly (ESI GmbH, 2008). The transient heat transfer equation for welding is given by:
ρc
G ∂T ( x, y, z, t ) = −∇ ⋅ q ( x, y, z, t ) + Q ( x, y, z, t ) ∂t
(1)
where ρ is the density of the materials, c is the specific heat capacity, T is the current temperature,
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G q is the heat flux vector, Q is the internal heat generation rate, x, y and z are the coordinates in the reference system, t is the time, and ∇ is the spatial gradient operator. The non-linear isotropic Fourier heat flux constitutive equation is described by:
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G q = −k ∇T where k is the temperature dependent thermal conductivity.
(2)
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Due to the melting of the aluminum alloy and non-forming of the keyhole in the laser
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fusion-brazing process, the heat from the moving welding laser is applied as a volumetric heat source with a double ellipsoidal distribution, as proposed by Goldak (Goldak et al., 1984). The front half of
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the source is the quadrant of one ellipsoidal source, and the rear half is the quadrant of another ellipsoid. In this model, the fractions ff and fr of the heat deposited in the front and rear quadrants are
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required, where ff + fr = 2.
The power density distribution inside the front quadrant becomes:
abcπ π
e −3 x
2
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2 / a 2 −3 y 2 / b2 −3⎣⎡ z + v (τ −t )⎦⎤ / c
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(3)
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q ( x, y , z, t ) =
Similarly, for the rear quadrant of the source, the power density distribution inside the ellipsoid
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becomes:
q ( x, y , z, t ) =
6 3 f rQ −3 x2 / a 2 −3 y 2 / b2 −3⎣⎡ z + v(τ −t )⎦⎤2 / c2 e e e abcπ π
(4)
To perform a welding simulation, a reference line, welding trajectory line, start and end nodes and the start elements are required. Based on the welding velocity and the length of the molten zone along with the welding trajectories, the parameters a, b, c are automatically generated by SYSWELD. The total absorbed power must then be fitted to an intensity function during the thermal analysis. Both the thermal radiation and heat transfer onto the weld surface are assumed. Radiation losses dominate at higher temperatures near and in the weld zone, and convection losses dominate at lower temperatures away from the weld zone (Radaj, 1992). For the radiative and convective boundary 8
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conditions, a combined heat transfer coefficient was calculated from the relationship:
H = ε C0 (Ts + T f ) (Ts 2 + T f 2 ) + H 0
(5)
where ε is the emissivity or degree of blackness of the surface of the body, C0 is the
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blackbody radiation coefficient, Ts is the temperature of the skin of the materials, Tf is the environmental temperature, and H0 is convection losses coefficient. Values of 0.8 and 25 were
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assumed for ε and H0, respectively, as recommended for hot rolled steel.
Laser welding with filler powder
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Laser welding without filler powder
Fig. 2. Surface morphologies of the joints produced by laser welding without filler powder (welding parameters: v=1.0m/min, Δf=12mm, fAr=15L/min) and with filler powder (welding parameters: v=1.0m/min, Δf=12mm, fp=4.53g/min, fg=5L/min, fAr=15L/min) at different laser powers
4. Results and Discussion
4.1 Forming properties of the fusion-brazing bead Surface morphologies of the joints from laser welding with and without filler powder under different laser powers are shown in Fig. 2. In process without filler powder, the time interval, which is measured from the start of welding until liquid aluminum begins to spread into the solid steel surface, gradually shortens with the increase of laser power. However, the time interval is not reduced to zero until laser power increases to 2800W. Conversely, in the processes with filler power, liquid aluminum is found to spread into the solid steel surface at the start of laser welding at low 9
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laser powers. It is generally thought that this is because the demand for liquid aluminum spreading in galvanized steel surface is related to the temperature of the interface between aluminum and galvanized layer and must be no less than the Al-Zn eutectic temperature (382℃) (Лякишев, 2009).
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The laser absorption rate on the surface of aluminum alloy is low, and filler powder can remarkably improve it (Zuo, 2002). Therefore, laser welding without filler powder required additional time to
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accumulate energy, while laser welding with filler powder can reach the accumulation process in a
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Fig. 3. Surface morphologies of the joints produced by laser welding an aluminum alloy to non-galvanized steel with filler powder
Fig. 4. Cross-section morphologies of the joints from laser welding an aluminum alloy to non-galvanized steel (welding parameters: P=2250W, v=1.0m/min, Δf=12mm, fp=4.53g/min, fg=5L/min, fAr=15L/min) with filler powder in different positions (SEM): (a) the melting zone, as shown by Line A in Fig. 3; and (b) no melting zone, as shown by Line B in Fig. 3 Although the filler powder can improve the laser absorption rate on the aluminum alloy surface and shorten the time for accumulation energy in the laser fusion-brazing process, laser welding with filler powder is unable to achieve the effective connection of the aluminum alloy and the non-galvanized steel using different laser powers. Fig. 3 is a typical surface morphology of the joints from laser welding an aluminum alloy to a non-galvanized steel with filler powder. In the process of 10
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laser welding an aluminum alloy to a non-galvanized steel with filler powder, the liquid aluminum cannot wet into the solid steel surface until the steel melts, as shown in Fig. 4. Similar to the laser welding of an aluminum alloy to galvanized steel without filler powder, the aluminum wetting zone
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is expanded by an increase in laser power. However, steel melting and the large differences in thermal and physical properties between the two materials can result in great stress and a large
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number of brittle IMCs. Therefore, cracks and damage occur in the IMCs, as shown in Fig. 5. The chemical compositions of Points 1 to 7 in Fig. 5 are listed in Table 6. Significant and random
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variations of chemical compositions between Points 1 and 7 were also obtained. According to the
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atomic percentage of aluminum, Points 1 and 7 are located in the base metals of the aluminum and steel, respectively. It can be inferred that Al rich phases are observed at the other points, where the
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atom percentage of aluminum is between 63.74% and 79.18%. According to the Al-Fe phase diagram (Massalski and Okamoto, 1990), the atom percentage of aluminum of the Al rich phases of FeAl2,
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Fe2Al5 and FeAl3 are 67.1%-67.9%, 70.1%-72.6% and 74.3%-76.1%, respectively. However,
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because the FeAl2 phase is a metastable phase that cannot exist after welding, only the Fe2Al5 and FeAl3 phases can be formed at the Al-Fe interface. Because the Gibbs free-energy change of the
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formation of the Fe2Al5 phase is smaller than that of FeAl3 (Shi et al., 2013), the Fe2Al5 phase is formed earlier than FeAl3. Thus, as the temperature decreases, the solubility of iron in the molten aluminum decreases gradually; iron atoms are thus precipitated by the acicular FeAl3 phase on the side of the Al (Fig. 5b). Therefore, Al+FeAl3, Fe2Al5 and Fe+Fe2Al5 phases are observed at Points 2 to 6, respectively, as listed in Table 6.
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Fig. 5. Partial view of the melting zone in the Fig. 4a (SEM): (a) the all melting zone; (b) the aluminum side of the melting zone, as shown in Fig. 5, Zone A; and (c) the steel side of the melting zone, as shown in Fig. 5, Zone B
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Table 6 Chemical compositions of Points 1 to 7 in Fig. 5 Chemical compositions (at. %) Point Phase Al Fe 1 98.53 1.47 Al 2 79.18 20.82 Al+FeAl3 3 72.59 27.41 Fe2Al5 4 72.31 27.69 Fe2Al5 5 69.20 30.80 Fe2Al5+Fe 6 63.74 36.26 Fe2Al5+Fe 7 0.78 99.22 Fe
4.2 Growth behaviors of IMC
The cross-section morphologies of the joints from the laser welding of the aluminum alloy to the galvanized steel is shown in Fig. 6 with filler powder under a laser power of 2250W, a travelling velocity of 1.0m/min, a defocusing distance of 12mm, a powder feeding speed of 4.53g/min, a carrier gas flow rate of 5L/min, and a shielding gas flow rate of 15L/min. The interface of the aluminum/steel is straight, and no significant erosion is observed. A dendrite microstructure is 12
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observed at the edge of the filler layer, and there is no a clear interface reaction layer at the aluminum/steel interface, as shown in Fig. 7. Based on the analysis of the element distribution in the region, it is clear that a large amount of zinc appears in the edge of filler layer, and a dividing line
ip t
between the iron and aluminum is present at the aluminum/steel interface. Thus, it is proved that the thickness of the interface reaction layer is significantly thinner in the zinc rich zone. The laminar and
cr
acicular IMCs, which are similar to the IMCs in the joint produced by laser welding of an aluminum alloy to non-galvanized steel with filler powder, are observed in the center of the welded joint, as
us
shown in Fig. 8. Through the analysis of the element distribution in the region, it can be determined
an
that the iron and aluminum are concentrated at the interface reaction layer, and a small quantity of residual zinc is detected on the side of the steel interface. However, as a result of the identification of
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the production phase by X-ray diffraction patterns, it is found that this layer consisted of a Fe-Al intermetallic compound (Fe2Al5Zn0.4), as shown in Fig. 9. Because the Fe2Al5Zn0.4 phase is not a
d
brittle phase (Nishimoto et al., 2008), which is different from the Al-rich phases of FeAl3 and Fe2Al5
te
obtained in the laser welding of an aluminum alloy to non-galvanized steel with filler powder.
Ac ce p
Therefore, no cracks are found in the IMC layer.
Fig. 6. Cross-section morphologies of the joints from laser welding an aluminum alloy to galvanized steel (welding parameters: P=2250W, v=1.0m/min, Δf=12mm, fp=4.53g/min, fg=5L/min, fAr=15L/min) with filler powder (SEM)
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ip t cr us an
Ac ce p
te
d
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Fig. 7. Microstructure and elementary distribution in the Zn-rich zone (as shown in Fig. 6, Zone A) analyzed by EPMA
Fig. 8. Microstructure and elementary distribution in the center zone (as shown in Fig. 6, Zone B) analyzed by EPMA
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5000 ♦ Al ◊ Fe2Al5Zn0.4
♦
3000
2000
1000
0
◊
◊ ◊
20
30
♦ ◊
♦
◊ ◊
40
50
◊ ♦
◊
60
70
2 Theta (Deg.)
80
ip t
◊ ♦ ◊
cr
Intensity (CPS)
4000
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Fig. 9. X-ray diffraction pattern of the reaction layer at the galvanized steel/brazed metal interface In Al-Fe laser welding, the atomic diffusion of Al and Fe across the interface is the rate-limiting
an
factor for the growth of IMCs due to the unmelted steel; the materials’ thickness as a function of time can be expressed by the following equation (Jindal and Srivastava, 2008): (5)
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x = Dt
where x (μm), D (μm2·s-1), and t (s) are the IMC’s thickness, growth rate constant, and aging time,
d
respectively. The growth rate constant D is related to temperature and can be expressed as a simple
(6)
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⎛ Q ⎞ D = D0 exp ⎜ − ⎟ ⎝ RT ⎠
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Arrhenius relationship (Ji and Xue, 2013):
where D, D0, Q, R, and T are the growth rate constant, the frequency factor, the activation energy for the growth of the interfacial IMC layer, the gas constant (8.314 kJ·mol-1·K-1), and the temperature in Kelvin (K), respectively. From Eq. (5) and Eq. (6), it is found that the thicker the interface reaction layer is, the higher the temperature and the longer the time of the brazing required are. Based on the temperature field of laser welding an aluminum alloy to galvanized steel with filler powder by numerical simulation, it can be found that the liquid residence time and the peak temperature of the Al/Fe interface increase from the weld toe to the center, as shown in Fig. 10. According to the results of the temperature distribution, it can be predicted that the thicknesses of the ICMs increase gradually with the change of the regions from the Zn-rich zone to the center of the joint. Fig. 11 15
Page 16 of 24
shows the morphologies of the IMCs in different regions of the dissimilar metal joint of aluminum alloy and galvanized steel from laser welding with filler powder; the accuracy of the prediction is
cr
ip t
thus proved.
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1200 (c)
Point A Point B
1100
an
900 800 700 600
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Temperature (oC)
1000
500 400
200
d
300 2.4
3.6
4.8 Times (s)
6.0
7.2
Ac ce p
te
Fig. 10. Temperature field results and thermal cycling curves of laser welding of an aluminum alloy to galvanized steel (welding parameters: P=2250W, v=1.0m/min, Δf=12mm, fp=4.53g/min, fg=5L/min, fAr=15L/min) with filler powder: (a) temperature field on the surface of the specimens; (b) temperature field on the cross-section of the specimens; and (c) thermal cycling curves at Points A and B, as shown in Fig. 10b
16
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ip t cr us an M d te Ac ce p Fig. 11. IMCs in different regions of the dissimilar metal joint of aluminum alloy and galvanized steel from laser welding with filler powder Due to high heating and cooling rates in the laser welding process, the different morphologies of
17
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the IMCs that are obtained in the different zones from the Zn-rich zone to the center of the joint can be regarded as the morphologies of the different steps of the IMCs forming process in the center of
Ac ce p
te
d
M
an
us
cr
ip t
the joint.
Fig. 12 Schematic illustration of the formation process of ICMs At the beginning of welding, the galvanized layer is melted by liquid aluminum because the melting point of aluminum is higher than that of zinc. Then, a portion of solute zinc atoms are dissolved in the molten aluminum to form an Al-Zn liquid solution; other zinc atoms are then pushed 18
Page 19 of 24
to accumulate at the weld toe and form a Zn-rich zone because the density of aluminum is much less than the density of zinc, and the electron affinity of Al-Fe is larger than that of Al-Zn (Zhao et al., 2006), as shown in Fig. 12a and Fig. 12b. When the liquid aluminum contacts the solid steel, iron
ip t
atoms begin to diffuse into the liquid aluminum and separate from the liquid aluminum as an Al-rich phase at the interface of the solid steel and liquid aluminum when the concentration of iron in the
cr
liquid aluminum reaches a certain value. As the Gibbs free-energy changes because the formation of Fe2Al5 is smaller than that of FeAl3 (Shi et al., 2013), the Fe2Al5 phase is formed earlier than the
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FeAl3 phase, as shown in Fig. 12c. The formation of the continuous Fe2Al5 phase layer can hinder
an
iron atom diffusion into the liquid aluminum due to a small diffusion coefficient of iron atoms in the Fe2Al5 phase (Chen and Nakata, 2008). Then, as the temperature decreases, the solubility of iron in
M
the molten aluminum decreases gradually, and iron atoms are precipitated by the acicular Al-rich intermetallic phase, FeAl3, on the side of the Al, as shown in Fig. 12d and Fig. 12e. Because
d
the zinc atoms in the liquid aluminum can substitute iron atoms to join to the formation of the IMCs
te
(Zhao et al., 2006), the intermetallic phase is Fe2Al5Zn0.4 at the interface of the aluminum alloy and the galvanized steel in the joint of dissimilar metals after welding, as shown in Fig. 12f.
Ac ce p
The primary problems in laser welding aluminum alloy to steel are the low energy utilization rate of the laser beam and the poor wettability of molten aluminum onto the surface of the solid steel. Due to the high reflectivity rate of the laser on the surface of the base metals such as aluminum alloy and steel (Kawahito et al., 2011), a high laser power is usually required in laser fusion-brazing process of an aluminum alloy and galvanized steel. Because of the large surface area of the powders and the change of the laser reflectivity type from specular to diffuse reflection with the joining powders (Zuo, 2002), filling powder is used to improve the energy utilization rate of laser beam in laser welding process and has been shown to be an effective method (Fig. 2). Because the melting point of Al (660.4 ℃) is higher than that of Zn (419.5 ℃) and significantly lower than that of Fe (1535.0 ℃) (Brandes and Brook, 1992), the Zn-coated layer melts while the 19
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steel is unable to be melted upon contact with the molten aluminum. Therefore, molten aluminum can spread at the beginning of laser welding aluminum alloy/galvanized steel (Fig. 2) but cannot spread until the steel is melted in the laser welding aluminum alloy/non-galvanized steel process (Fig.
ip t
3 and Fig. 4) due to the poor wettability of aluminum onto the steel surface. Thus, the galvanized layer can change the initial temperature of the brazing from near the melting point of Fe (1535.0℃)
cr
to that of Al (660.4℃).
us
5. Conclusions
In this study, the laser fusion-brazing of an aluminum alloy to galvanized and non-galvanized
an
steels was performed. The resulting formations and microstructure features of the fusion-braze bead were specifically assessed. Based on this study, several conclusions are summarized as follows:
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(1) Based on the comparison of the results produced by laser welding an aluminum alloy to galvanized steel with and without a filler powder, it was found that filler powders could improve
d
the energy utilization rate to reduce laser power requirements in the laser fusion-brazing process.
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(2) According to the comparative results of laser welding with filler powder to join an aluminum alloy and a galvanized or non-galvanized steel, it was found that the zinc coated layer produced a
Ac ce p
good wetting effect by changing the types of the materials that come into contact with the liquid aluminum, reducing the brazing temperature from near the melting point of Fe (1535.0℃) to that of Al (660.4℃).
(3) X-ray diffraction analysis results reveal that the IMC of Fe2Al5Zn0.4, a ductile and tough phase, is formed at the galvanized steel/filler layer interface. It is shown that zinc acts critically as flux and is also involved in metallurgical reactions. (4) The forming process of the Fe2Al5Zn0.4 phase is firstly forming from the Al-rich phases of Fe2Al5 and FeAl3; then, Zn atoms slowly diffuse into the Al-Fe phases to substitute for iron atoms.
Acknowledgments 20
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The authors would like to thank the Natural Science Foundation of Jilin Province, China for financially supporting this research under Contract No. 201115156.
References
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Brandes, E. A., Brook, G. B., 1992. Smithells metals reference book (seventh edition). Butterworth Heinemann.
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Chen, Y.C., Nakata, K., 2008. Effect of the Surface State of Steel on the Microstructure and Mechanical Properties of Dissimilar Metal Lap Joints of Aluminum and Steel by Friction Stir
us
Welding. Metallurgical and Materials Transactions. 39(8), 1985-1992.
an
David, A.K., PSM Team, 2014. Study of metallurgic and mechanical properties of laser welded heterogeneous joints between DP600 galvanized steel and aluminum 6082. Materials and Design. 54,
M
184-195. ESI GmbH, 2008. SYSWELD welding handbook.
d
Goldak, J., Chakravarti, A., Bibby, A., 1984. A new finite element model for welding heat sources.
te
Metallurgical and Materials Transactions. 15(2), 299-305. Jindal, V., Srivastava, V. C., 2008. Growth of intermetallic layer at roll bonded IF-steel/aluminum
Ac ce p
interface. Journal of Materials Processing Technology. 195(1-3), 88-93. Ji, F., Xue, S.B., 2013. Growth behaviors of intermetallic compound layers in Cu/Al joints brazed with Zn-22Al and Zn-22Al-0.05Ce filler metals. Materials and Design. 51, 907-915. Katundi, D., Tosun, B.A., Bayraktar, E., Toueix, D., 2010. Corrosion behavior of the welded steel sheets used in automotive industry. Journal of Achievements in Materials and Manufacturing Engineering. 38(2), 146-153.
Kawahito, Y., Matsumoto, N., Abe, Y., Katayama, S., 2011. Relationship of laser absorption to keyhole behavior in high power fiber laser welding of stainless steel and aluminum alloy. Journal of Materials Processing Technology. 211(10), 1563-1568. Li, L.B., Sun Y.F., 2011. Handbook of physical properties of metal materials. China Machine Press.
21
Page 22 of 24
(In Chinese) Liu, B., He, G.Q., Jiang, X.S., Zhu, M.H., 2011. Review on effect factor and research method of material fatigue life. Materials Review. 25(5), 103-106.
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Ma, J.J., Harooni, M., Carlson, B., Kovacevic, R., 2014. Dissimilar joining of galvanized high strength steel to aluminum alloy in a zero-gap lap joint configuration by two-pass laser welding.
cr
Materials and Design. 58, 390-401.
Massalski, T.B., Okamoto, H., 1990. Binary alloy phase diagrams, vol. 3, 2nd ed. Materials Park
us
(OH): ASM International.
an
Miller, W.S., Zhuang, L., Bottema, J., Wittebrood, A.J., Smet, P. D., Haszler, A., Vieregge, A., 2000. Recent development in aluminum alloys for the automotive industry. Materials Science and
M
Engineering A. 280(1), 37-49.
Nishimoto, K., Harano, T., Okumoto, Y., Atagi, K., Fujii, H., Katayama, S., 2008. Mechanical
d
properties of laser-pressure-welded joint between dissimilar galvannealed steel and pure aluminum.
te
Quarterly Journal of the Japan Welding Society. 26(2), 174-180. Peyre, P., Sierra, G., Deschaux, B.F., Stuart, D., Fras, G., 2007. Generation of aluminum steel joints
Ac ce p
with laser-induced reactive wetting. Materials Science and Engineering A. 444, 327-338. Radaj, D., 1992. Heat effects of welding, temperature field, residual stress and distortions. Springer. Rodriguez, L., Mathieu, A., Langlade, C., Vannes, A., Grevey, D., Mattei, S., 2005. Laser brazing of steel aluminum assembly. Laser in Engineering. 5-6, 413-425. Saida, K., Ohnishi, H., Nishimoto, K., 2008. Fluxless laser brazing of aluminum alloy to galvanized steel using a tandem beam-dissimilar laser brazing of aluminum alloy and steels. Japan Welding Society. 26(3), 235-241. Schubert, E., Klassen, M., Zerner, I., Walz, C., Sepold, G., 2001. Light-weight structures produced by laser beam joining for future applications in automobile and aerospace industry. Journal of Materials Processing Technology. 115, 2-8.
22
Page 23 of 24
Shi, Y., Zhang, H., Takehiro, W., Tang J.G, 2010. CW/PW dual-beam YAG laser welding of steel/aluminum alloy sheets. Optics and Lasers in Engineering. 48, 732-736. Shi, Y., He, C.C., Huang, J.K., Fan, D., 2013. Thermodynamic analysis of the forming of
ip t
intermetallic compounds on aluminum-steel welding interface. Journal of Lanzhou University of Technology. 39(4), 5-7. (In Chinese)
cr
Sierra, G., Peyre, P., Deschaux, B.F., Stuart, D., Fras, G., 2007. Steel to aluminum key-hole laser welding. Material Science and Engineering A. 447, 197-208.
us
Zhao, L., Sun, Y., Meng, X.F., Li, Y.G., Song, Q.L., 2006. First-principles studies of the effects of Zn
an
on Fe/Al interface. http://www.paper.edu.cn/releasepaper/content/200612-328. (In Chinese) Zuo, T.C., 2002. Laser materials processing of high strength aluminum alloys. National Defense
M
Industry Press. 150-159. (In Chinese)
Лякишев, Н. П., 2009. Handbook of binary alloy phase diagram. Chemical Industry Press. (In
Ac ce p
te
d
Chinese)
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