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steel cold metal transfer joints
Journal of Materials Processing Tech. 272 (2019) 40–46
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Heat input, intermetallic compounds and mechanical properties of Al/steel cold metal transfer joints ⁎
T
⁎
Jin Yanga, , Anming Hub, Yulong Lic, , Peilei Zhanga, Dulal Chandra Sahad, Zhishui Yua a
School of Materials Engineering, Shanghai University of Engineering Science, Shanghai, 201620, China Beijing Engineering Researching Center of Laser Technology, Beijing, 100124, China c Key Lab for Robot and Welding Automation of Jiangxi Province, Mechanical and Electrical Engineering School, Nanchang University, Nanchang, 330031, China d Centre for Advanced Materials Joining, Department of Mechanical & Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1 Canada b
A R T I C LE I N FO
A B S T R A C T
Associate Editor: J.E. Kinder
Cold metal transfer welding/brazing of AA5754 aluminum alloy and Q235 low carbon steel was performed in a lap joint configuration using an AlSi12 filler wire. The effect of heat input on intermetallic compounds (IMC) formations and mechanical properties has been investigated. At a low heat input of 157 J/mm, a layer of IMC, with a composition of Al7.2Fe1.8Si was generated at the fusion zone/steel interface. Low interfacial energy and good interfacial bonding between fusion zone and Al7.2Fe1.8Si led increased joint strength. An additional Fe (Al,Si)3 IMC formed at the interface in conjunction to Al7.2Fe1.8Si at a high heat input of 201 J/mm. The joint strength was significantly decreased and attributed to a high interfacial energy and poor interfacial bonding between Al7.2Fe1.8Si and Fe(Al,Si)3. The results showed that the interfacial energy at interphase boundaries had remarkable effect on the joint strength.
Keywords: Aluminum and steel joint CMT welding/brazing Intermetallic compounds Shear strength Interplanar mismatch
1. Introduction Application of multi-materials in the automotive industry is one of the effective ways of reducing both fuel consumption and greenhouse gas emission. A proper joining of dissimilar materials (for example, Al to steel) by a welding method is difficult to achieve due to large differences in their physical and chemical properties. In literature, various welding technologies have been reported. For example, fusion welding techniques, including laser welding and electron beam welding, have been used to join Al and steel, where the joining is achieved by the mixing and inter-diffusion between molten Al and steel. Because of the low solubility of Fe in Al, a thick and brittle intermetallic compounds (IMC) is readily generated at the weld interface which can deteriorate the load bearing capacity of the weld, as reported by Torkamany et al. (2010). In addition, the joints may also produce some weld defects such as shrinkage voids, porosities and solidification cracking, etc. (Dinda et al. (2019)). To overcome the complexities in fusion welding, solid-state welding techniques, such as diffusion bonding, ultrasonic spot welding, friction stir welding and vaporizing foil actuator welding, have also been explored in the past. The solid-state welds were achieved by the interdiffusion of Fe and Al atoms under high pressure, low temperature or
⁎
their combinations. However, the aforementioned methods either requires a longer processing time (e.g., diffusion bonding) or specific workpiece geometries (e.g., friction stir welding). To overcome these limitations, a combined welding/brazing technique has been developed for Al/steel joining. The technique has a dual characteristic as it creates a fusion weld at Al side and a brazed joint at steel side. In the welding/ brazing application, laser, arc, and their combinations have usually been used as main heat sources. Investigations by Xia et al. (2018) and Yang et al. (2015) showed that welding/brazing technology offers a great potential for dissimilar joining of Al to steel. One of the advanced arc welding/brazing technologies is cold metal transfer (CMT) welding/ brazing, which has been used in Al/steel joining due to its low heat input, small deformation and free spattering. In the investigation of CMT welding/brazing of Al to boron steel, Cao et al. (2014) pointed out that the Al/galvanized steel joints possessed higher tensile strength than that of other types of coated/uncoated steels (e.g., uncoated, Al-Si coated, and galvannealed). It was reported that the interfacial layer thickness of CMT Al/steel joint was ranging from 2 to 5 μm. In another study of CMT welding/brazing of Al to steel by Cao et al. (2013), they optimized the welding process parameters by Taguchi method. They also pointed out that the degradation of aluminum heat-affected-zone and thickness of the IMC was minimized by controlling the heat input
Corresponding authors. E-mail addresses: [email protected] (J. Yang), [email protected] (Y. Li).
https://doi.org/10.1016/j.jmatprotec.2019.05.004 Received 24 October 2018; Received in revised form 2 May 2019; Accepted 6 May 2019 Available online 06 May 2019 0924-0136/ © 2019 Elsevier B.V. All rights reserved.
Journal of Materials Processing Tech. 272 (2019) 40–46
J. Yang, et al.
Table 1 Chemical compositions of the materials in wt.%. Materials
Mg
Zn
Q235 steel 5754 Al ER4043
– 2.6-3.6 0.1
– 0.2 0.01
Ti
Si
Cr
Mn
Ti
Fe
S
C
Al
– 0.15 –
0.22 0.4 4.5-6.0
– 0.3 –
0.60 0.5 –
– 0.15 –
Bal. 0.4 0.17
0.02 – –
0.18 – –
0.45 Bal. Bal.
within 100–200 J/mm. Although it is well-established relationship between heat input, interfacial IMC thickness and joint mechanical properties in laser Al/steel welds as presented by Pardal et al. (2014) and Wang et al. (2017), this relationship is still not conclusive in CMT Al/steel joining. Therefore, this paper aims to study the influence of heat input on interfacial IMC and joint strength for CMT Al/steel joints.
Table 2 The optimized welding parameters used for welding/brazing.
2. Experimental 2.1. Materials
Welding parameters
Optimized range
Values
Current (I) [A] Voltage (U) [V] Peed (Ws) [mm s−1] Wire feed speed (Wf) [m min−1] Calculated heat input (K) [J mm−1]
67-69 10.9-11.8 3-7 4
69 11.3 7 4
68 11.8 6
67 11.7 5
69 11.2 4
68 11.4 3
–
111
129
157
201
260
A 2.0 mm thick AA5754 aluminum alloy and a 1.8 mm thick Q235 low carbon steel sheets were used. An Al-Si based alloy (ER4043) with a diameter of 1.2 mm was selected as a filler metal. The chemical compositions of the materials are given in Table 1. In order to prevent oxidation, a brazing flux QJ201 was used.
dispersive X-ray spectrometry (EDS). The X-ray diffraction (XRD) analysis was performed on a Panalytical X’Pert X-ray diffractometer (CuKα, Holland).
2.2. CMT welding/brazing process
Rectangular tensile-shear coupons were cut by a wire saw from the joints with the dimension of 20 mm × 160 mm. The joint tensile-shear testing was performed at a cross-head speed of 1.0 mm/s. The microhardness measurements were carried out using an automated computerized hardness tester with a 25 g load and 15 s dwell time.
2.4. Mechanical testing
The sheets were machined as rectangular strips of 100 mm × 150 mm. An integrated ABB six-axis robot with a Fronius arc welding system (CMT 4000 Advanced) was used for welding/brazing application. Prior to the welding/brazing, the sheets were cleaned with acetone to remove oil and debris. The aluminum sheet was placed on top of the steel sheet in a lap joint configuration with an overlap distance of 15 mm. The schematic diagram of the joint configuration is illustrated in Fig. 1. Preliminary experiments were conducted to optimize the process parameters (Table 2). The heat input was calculated by using Eq. (1)
K=
IU Ws
3. Results 3.1. Metallography Fig. 2 shows weld appearance and cross sections of CMT Al/steel joints with different heat inputs. An acceptable surface appearance without spattering and undercut was achieved. An uneven weld toe with rough surface was observed when heat input was less than 157 J/ mm due to the poor wettability of the molten filler metal. The wettability was gradually improved with more stable and uniform surface appearance at a higher heat input. To validate the statement, quantitative analysis was performed. The molten filler metal’s wettingspreading ability was evaluated by measuring the wetting angle and brazed width (Fig. 2). As the heat input increased from 111 J/mm to 260 J/mm, the wetting angle gradually decreased from 102 ± 9° to 66 ± 7°, while the brazed size progressively became wider from 3.8 ± 0.2 mm to 10.0 ± 0.7 mm. The decreased wetting angle and increased brazed width is an indication of improved wetting and spreading ability of molten filler metal. Fig. 3 shows the cross-section of the joints with different heat inputs. A layer of IMC was observed at the interface between fusion zone and steel. The measured thickness of the reaction layers with varied heat inputs were shown in Fig. 2. The increase of reaction layer thickness with heat input was related to the formation and growth of IMC, i.e., time- and temperature- controlled diffusion process. Similar results were reported by Li et al. (2018) for dissimilar Al/steel joints made by laser welding/brazing. It is clear that only one reaction layer was observed in Fig. 3 (a–c); while, some fine dark features were generated within the reaction layer from the steel side as shown in Fig. 3(d–e). It was indicated that a new phase was formed at the fusion zone/steel interface, which was confirmed by SEM analysis and discussed in a later section. Since the phase constituents were identical with only minor differences when heat input
(1)
where, I, U and Ws were welding current, welding voltage, and welding speed, respectively. 2.3. Microstructure characterization The samples were prepared using standard grinding and polishing method followed by etching with Keller’s reagent to reveal the microstructure. The microstructure characterization was carried out by optical microscope (OM, model: Olympus 4XCJZ) and scanning electron microscope (SEM, model: Hitachi S3400-N) with EDAX Genesis energy-
Fig. 1. Schematic diagram of the joint configuration of CMT welding/brazing. 41
Journal of Materials Processing Tech. 272 (2019) 40–46
J. Yang, et al.
Fig. 2. Macrostructure, wetting angle, brazed width, thickness of reaction layer of the CMT Al/steel joints with different heat inputs.
fusion zone and steel substrate were about 66 HV and 144 HV, respectively. In all cases, the hardness at the fusion zone/steel interface sharply increased over 350 HV. The increase of the microhardness was attributed to the hard and brittle nature of Fe-Al and Fe-Al-Si IMCs. The joints were all fractured at the fusion zone/steel interface after tensile-shear testing. Thus, the joint strength was characterized by shear strength using the following equations:
was less than 157 J/mm (single-phase reaction layer) or larger than 201 J/mm (dual-phase reaction layer), the discussion on microstructure was focused in the joints obtained at heat inputs 157 J/mm and 201 J/ mm, denoting as low and high heat input, respectively. Fig. 4(a) represents the SEM image of typical IMC layer with low heat input (157 J/mm). A single-phase reaction layer composed of serrated IMC was formed at the fusion zone/steel interface. Based on EDS analysis, the IMC layer (P1) composed of 71.5 at. % Al, 9.2 at. % Si and 19.3 at. % Fe (Table 3). According to the Fe-Al-Si ternary phase diagram, the IMC layer was identified as Al7.2Fe1.8Si. However, at high heat input (201 J/mm), a dual-phase reaction layer was observed at the fusion zone/steel interface (Fig. 4(b)). Adjacent to the fusion zone, a scallop-like IMC was obvious, which had the chemical composition of 73.2 Al at. %, 17.8 Fe at. % and 9.0 Si at. % (Table 3). Thus, the IMC was identified as Al7.2Fe1.8Si. Adjacent to the steel side, a newly formed needle-like IMC with a composition of 71.6 Al at. %, 23.2 Fe at. % and 5.6 Si at. % (P3) was observed. The possible phase of the IMC was identified as Fe(Al,Si)3. This kind of dual-phase IMCs layer, consisted of Al7.2Fe1.8Si and Fe(Al,Si)3, was also reported by Song et al. (2009) in tungsten inert gas welded Al/steel joints.
τ = F/A
(1)
A = Wb*L
(2)
where, τ was the joint shear strength, F was the joint fracture load, A was the interface bonding area, Wb was brazed width (shown in Fig. 2), L was the specimen width (20 mm). The plot of measured joint shear strength and interfacial layer thickness as a function of heat input are shown in Fig. 6. The joint shear strength first slightly decreased from about 65 MPa to 54 MPa when heat input increased from 111 J/mm to 157 J/mm, and then it dropped to around 28 MPa as heat input exceeded 201 J/mm. On the other hand, a nearly linear correlation between interfacial layer thickness and heat input was identified. Thus, the remarkable decrease in the joint strength was little correlated with the interfacial layer’s thickness variation. Besides, at low heat input, the fracture location was in the interface between fusion zone and Al7.2Fe1.8Si as shown in Fig. 6. At a high heat input, the fracture location changed to the Al7.2Fe1.8Si /Fe(Al,Si)3 interface. Hence, it implies a
3.2. Mechanical properties Fig. 5 shows microhardness profile across the fusion zone/steel interface produced at different heat inputs. The hardness values of the
Fig. 3. Optical microscopy images of the fusion zone/steel interface with different heat inputs: (a) 111 J/mm, (b) 129 J/mm, (c) 157 J/mm, (d) 201 J/mm, and (e) 260 J/mm. 42
Journal of Materials Processing Tech. 272 (2019) 40–46
J. Yang, et al.
Fig. 4. SEM images of the fusion zone/steel interfacial region in the CMT Al/steel joints with different heat inputs: (a) low heat input, and (b) high heat input.
strong correlation between the joint shear strength and joint failure location which will be discussed in the later section.
Table 3 EDS analysis of the marked zones in Fig.4 and Fig.7 in at.%. No.
Al
Fe
Si
Possible phase
P1 P2 P3 A1 A2 A3 A4
71.5 73.2 71.6 90.1 78.2 74.2 70.0
19.3 17.8 23.2 1.1 13.5 18.4 25.7
9.2 9.0 5.2 8.8 8.3 7.4 4.3
Al7.2Fe1.8Si Al7.2Fe1.8Si Fe(Al,Si)3 α-Al + Al-Si eutectic Al7.2Fe1.8Si Al7.2Fe1.8Si Fe(Al,Si)3
3.3. Fractography Fig. 7 shows the fracture surface of the joint at a low heat input. At the fusion zone side, the typical cast structure with lots of dendrites was observed (Fig. 7(a)). The average chemical composition of the fracture surface was 90.1 Al at. %, 1.1 Fe at. % and 8.8 Si at. % (Table 3). Thus, it was identified as α-Al and Al-Si eutectic. At the steel side, a smooth plane with some terrace-like structures was obvious. Based on the EDS analysis, the IMC at the fracture surface was identified as Al7.2Fe1.8Si (Table 3). According to the morphology of the fracture surfaces, it suggested that a brittle fracture propagated along the fusion zone/ Al7.2Fe1.8Si interface, which was further confirmed by XRD analysis as shown in Fig. 7(b) and Fig. 7(d). Fig. 8 presents the fracture surface of the joint at a high heat input. At the fusion zone side, a smooth plane with some step-like structures was observed in Fig. 8(a). It was identified as Al7.2Fe1.8Si (Table 3). At the steel side, a relatively smooth surface consisting of Fe(Al,Si)3 phase was confirmed (Table 3). Therefore, it was concluded that a brittle failure occurred at the interphase boundary between Al7.2Fe1.8Si and Fe (Al,Si)3. The phase constitutions at the fracture surface were also confirmed by XRD analysis as shown in Fig. 8(b) and Fig. 8(d). 4. Discussion It has been identified that the phase constituents at the fusion zone/ steel interface in CMT Al/steel joints vary with the heat input, viz., Al7.2Fe1.8Si with low heat input and Al7.2Fe1.8Si + Fe(Al,Si)3 with high heat input. The formation mechanism of phase constituents can be understood from the thermodynamics analysis. It is well-known that Gibbs free energy ΔG is a criterion for chemical reaction to occur. At ΔG < 0 condition, the chemical reaction occurs spontaneously. Among all possible reactions, the reaction with the lowest ΔG tends to occur first. In addition, the IMC formed with lower ΔG is more stable. According to classical thermodynamics, ΔG in standard conditions can be expressed as:
Fig. 5. Microhardness profile across the fusion zone/steel interface of CMT Al/ steel joints with different heat inputs.
0 0 ΔG 0 = ΔH298 − TΔS298
ΔG 0
(3) 0 ΔH298
0 ΔS298
is Gibbs energy change, is enthalpy change, is where, entropy change, T is temperature. Besides, the element composition is a well-known factor to determine the chemical reaction. Previous study by Mei et al. (2013) on laser-arc hybrid welding of Al to steel using an Al-Si filler wire shows that the interfacial reaction is caused by the diffusion of Fe atom to molten Al-Si (fusion zone). Besides, Al and Si atoms are enough for the interfacial reaction and the key factor for the reaction is Gibbs energy of Fe atom per molar. Based on related investigations, the calculated Gibbs energy functions of Al7.2Fe1.8Si and Fe(Al,Si)3 are listed below:
Fig. 6. The plot of joint shear strength and interfacial layer thickness as a function of heat input. The blue dashed rectangles highlight the joint fracture locations (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
0 ΔGAl7.2Fe1.8Si = −147677.5 + 47.80T
43
(4)
Journal of Materials Processing Tech. 272 (2019) 40–46
J. Yang, et al.
Fig. 7. SEM images and XRD patterns of fracture surfaces of the joint with low heat input: (a–b) fusion zone side, (c–d) steel side. 0 ΔGFe(Al,Si)3 = −142770.0 + 50.8T
Al7.2Fe1.8Si; while, at a high heat input, sufficient diffusion of Fe promotes the formation of Fe(Al,Si)3 and Al7.2Fe1.8Si. With the increase of heat input, the interfacial phase constituent changes from Al7.2Fe1.8Si to Al7.2Fe1.8Si + Fe(Al,Si)3 which lead to the remarkable decrease in the joint shear strength. It is suggested that the formation of Fe(Al,Si)3 contributes to the decreased strength (Fig. 6). Because Fe(Al,Si)3 alters the interfacial structure from steel/ Al7.2Fe1.8Si/fusion zone to steel/Al7.2Fe1.8Si/Fe(Al,Si)3/fusion zone,
(5)
The plot of calculated ΔG as a function of temperature ranging from 700 to 1200 K is shown in Fig. 9. It is found that 0 0 ΔGAl7.2Fe1.8Si < ΔGFe(Al,Si)3 , indicating that Al7.2Fe1.8Si is more stable than Fe(Al,Si)3 hence more likely to form. Thus, the evolution of interfacial IMC with heat input is proposed as follows: at a low heat input, combining with the intrinsic low heat input of CMT technology, the diffusion of Fe is strictly limited, which only allows the formation of
Fig. 8. SEM images and XRD patterns of the fracture surfaces of the joint with high heat input: (a–b) fusion zone side, (c–d) steel side. 44
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Table 5 Calculated interplanar spacing mismatch at the interphase boundary between αAl and Al7.2Fe1.8Si. Matching planes
α-Al interplanar spacing (Å)
Al7.2Fe1.8Si interplanar spacing (Å)
Interplanar mismatch (%)
{111} {110} {100} {111} {110} {100} {111} {110} {100}
2.338 2.862 4.049 2.338 2.862 4.049 2.338 2.862 4.049
13.118 13.118 13.118 10.889 10.889 10.889 10.745 10.745 10.745
82.2 78.2 69.1 78.5 73.7 62.8 78.2 73.9 62.3
α −Al //{0002} Al7.2Fe1.8Si α −Al //{0002} Al7.2Fe1.8Si α −Al //{0002} Al7.2Fe1.8Si
¯ 1} Al7.2Fe1.8Si α −Al //{101 ¯ 1} Al7.2Fe1.8Si α −Al //{101 ¯ 1} Al7.2Fe1.8Si α −Al //{101 ¯ 0} Al7.2Fe1.8Si α −Al //{101 ¯ 0} Al7.2Fe1.8Si α −Al //{101 ¯ 0} Al7.2Fe1.8Si α −Al //{101
Fig. 9. Calculated Gibbs formation energy for Fe(Al,Si)3 and Al7.2Fe1.8Si IMCs.
interplanar spacing d along (hkl) planes is expressed using the equation as proposed by Cullity (2001):
and the appearance of a new interface, i.e., Al7.2Fe1.8Si/Fe(Al,Si)3, is likely to be responsible for the remarkable decrease of joint strength. This statement can be supported by the fractography in Figs. 7 and 8. At a low heat input, the joint fracture occurs at the fusion zone/Al7.2Fe1.8Si interface, while it occurs at the Al7.2Fe1.8Si/Fe(Al,Si)3 interface at a high heat input. It infers that the bonding strength at the Al7.2Fe1.8Si/ fusion zone interface is higher than that of Al7.2Fe1.8Si/Fe(Al,Si)3 interface. It is well-established that interphase bonding strength strongly correlates to the interplanar mismatch at the interphase boundary. For example, in the investigation of resistance spot welding of Mg alloy to Zn-coated steel, Liu et al. (2011) demonstrated a nanocale transitional Fe2Al5 layer significantly improved the joint strength, without which it was difficult to join owing to original sharp interface and high interfacial mismatch. By using the edge-to-edge matching model proposed by Zhang and Kelly (2005), Nasiri (2013) revealed that interplanar mismatch determined interfacial energy at the interphase boundary which in turn affected interphase metallurgical bonding strength in laser Mg/steel joints with Ni, Zn and Sn transitional layers. The above transitional layers play the key role of the interplanar mismatch reduction. Normally, the matching planes at interphase boundaries are the close-packed or nearly close-packed planes. In the present study, the metallurgical bonding strength at the interphase boundary is semiquantitatively evaluated by comparing the interplanar mismatch at the interphase boundaries. As HCP phase, Al7.2Fe1.8Si has the lattice constant of aAl7.2Fe1.8Si = 12.406 Å and cAl7.2Fe1.8Si = 26.236 Å. Fe(Al,Si)3 has a monoclinic crystal structure with the lattice constant of bFe(Al,Si)3 = 8.083 Å, cFe(Al,Si)3 = 12.476 Å, aFe(Al,Si)3 = 15.489 Å, β = 107.7°. As the main constituent phase in fusion zone, α-Al is a FCC phase with the lattice constant of aα-Al =4.050 Å. The close-packed planes of FCC and HCP are displayed in Table 4. The close-packed planes of monoclinic Fe(Al,Si)3 are identified by looking at the powder X-ray diffraction intensities, which are {332}, {025} and {620}. The
k 2sin2β 2hlcosβ 1 1 ⎛ h2 l2 ⎞ = + 2 + 2 − ⎜ 2 ⎟ 2 2 d sin β ⎝ aFe(Al,Si)3 aFe(Al,Si)3 c Fe(Al,Si)3 ⎠ bFe(Al,Si)3 cFe(Al,Si)3 (6) where, h, k, l are miler indices of a crystallographic plane, d is the interplanar spacing along the plane (hkl). Tables 5 and 6 summarize the interplanar mismatch of all possible matching planes at the interphase boundaries between α-Al and Al7.2Fe1.8Si as well as Al7.2Fe1.8Si and Fe(Al,Si)3. The interplanar mismatch at the interphase boundary between α-Al and Al7.2Fe1.8Si ranges from 62.3% to 82.2%, while it is up from 411.7 % to 539.9% at the Fe (Al,Si)3/Al7.2Fe1.8Si interface. Both the interplanar mismatches are larger than 10%, indicating the formation of noncoherent interfaces. According to the Hooke’s law:s
Eε =
where, Eε , G, ν, ε0 , and f(y) are free (strain) energy density, shear modulus, Poisson's ratio of the reaction product, lattice mismatch, and the unit step function, respectively. The equation suggests that the free (strain) energy is always positive and proportional to the lattice mismatch. The increase of free (strain) energy will increase the total interfacial energy as stated by Fredriksson and Åkerlind (2012). Thus, a high interplanar mismatch leads to a high interfacial energy, which in turn results in a poor interfacial bonding. Hence, at macroscale, α-Al/ Al7.2Fe1.8Si interfaces exhibit much higher bonding strength than that of Fe(Al,Si)3/Al7.2Fe1.8Si interfaces.
Table 6 Calculated interplanar spacing mismatch at the interphase boundary between Al7.2Fe1.8Si and Fe(Al,Si)3.
Table 4 The close-packed planes and their interplanar spacings for HCP and FCC crystal structures. Crystal structure *
HCP (aH, cH)
*
FCC (aF)
Close-packed plane
Interplanar spacing
{0002} {101¯1}
cH /2
G 2 2 ε0 f (y ) 1−v
aH cH 3 2 + 3a2 4cH H
Matching planes
Al7.2Fe1.8Si interplanar spacing (Å)
Fe(Al,Si)3 interplanar spacing (Å)
Interplanar mismatch (%)
{0002} {101¯1}
Al7.2Fe1.8Si //{332} Fe(Al,Si)3
13.118
2.09
527.7
Al7.2Fe1.8Si //{332} Fe(Al,Si)3
10.889
2.09
421.0
{101¯0}
Al7.2Fe1.8Si //{332} Fe(Al,Si)3
10.745
2.09
414.1
{0002} {101¯1}
Al7.2Fe1.8Si //{025} Fe(Al,Si)3
13.118
2.05
539.9
Al7.2Fe1.8Si //{025} Fe(Al,Si)3
10.889
2.05
431.2
{101¯0} {111}
3
3 aF
{101¯0}
Al7.2Fe1.8Si //{025} Fe(Al,Si)3
10.745
2.05
424.1
{110}
2
2 aF
Al7.2Fe1.8Si //{620} Fe(Al,Si)3
13.118
2.10
524.7
{100}
aF
{0002} {101¯1}
Al7.2Fe1.8Si /{620} Fe(Al,Si)3
10.889
2.10
418.5
{101¯0}
Al7.2Fe1.8Si //{620} Fe(Al,Si)3
10.745
2.10
411.7
3 aH /2
* Lattice parameters. 45
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5. Conclusions
proofreading. References
1 At a low heat input not higher than 157 J/mm, a single-phase reaction layer, composed of Al7.2Fe1.8Si, is formed at the fusion zone/ steel interface. At a high heat input not lower than 201 J/mm, the interfacial microstructure is changed to a combination of Fe(Al,Si)3 and Al7.2Fe1.8Si. 2 The fracture locations vary with the heat inputs: the fusion zone/ Al7.2Fe1.8Si interface with low heat input, while interphase boundary between Al7.2Fe1.8Si and Fe(Al,Si)3 with high heat input. 3 Interfacial lattice mismatch has a significant influence on the joint strength: relatively low interfacial interplanar mismatch is obtained at the Al7.2Fe1.8Si/fusion zone interface making a good interfacial bonding and high joint strength (˜54 MPa); but it decreases to 28 MPa due to relatively high interfacial interplanar mismatch and poor bonding at the Fe(Al,Si)3/Al7.2Fe1.8Si interface. 4 Comparing to the wettability of filler metal, the thickness and phase constituent of reaction layer have more pronounced influences on the joint strength. The heat input should be kept as low as possible to maintain a thin and single-phase (Al7.2Fe1.8Si) reaction layer at the fusion zone/steel interface to achieve a good interfacial bonding and a sound joint.
Cao, R., Yu, G., Chen, J.H., Wang, P.H., 2013. Cold metal transfer joining aluminum alloys-to-galvanized mild steel. J. Mater. Process. Technol. 213, 1753–1763. Cao, R., Sun, J.H., Chen, J.H., Wang, P.C., 2014. Weldability of CMT joining of AA6061T6 to boron steels with various coatings. Weld. J. 93, 193S–204S. Cullity, B.D., 2001. Elements of X-Ray Diffraction, third ed. Wesley Publishing Company, Massachusetts. Dinda, S.K., Jana, S., Roy, G.G., Srirangam, P., 2019. Effect of beam oscillation on porosity and intermetallics of electron beam welded DP600-steel to Al 5754-alloy. J. Mater. Process.Technol. 265, 191–200. Fredriksson, H., Åkerlind, U., 2012. Solidification and Crystallization Processing in Metals and Alloys. John Wiley & Sons. Li, L., Xia, H., Tan, C., Ma, N., 2018. Effect of groove shape on laser welding-brazing Al to steel. J. Mater. Process. Technol. 252, 573–581. Liu, L., Xiao, L., Feng, J., Li, L., Esmaeili, S., Zhou, Y., 2011. Bonding of immiscible Mg and Fe by coated nanoscale Fe2Al5 transition layer. Scripta Mater. 65, 982–985. Mei, S.W., Gao, M., Yan, J., Zhang, C., Li, G., Zeng, X.Y., 2013. Interface properties and thermodynamic analysis of laser–arc hybrid welded Al/steel joint. Sci. Technol. Weld. Joining 18, 293–300. Nasiri, A.M., 2013. Laser Brazing of Magnesium to Steel Sheet. Pardal, G., Meco, S., Ganguly, S., Williams, S., Prangnell, P., 2014. Dissimilar metal laser spot joining of steel to aluminium in conduction mode. Int. J. Adv. Manuf. Technol. 73, 365–373. Song, J.L., Lin, S.B., Yang, C.L., Fan, C.L., 2009. Effects of Si additions on intermetallic compound layer of aluminum–steel TIG welding–brazing joint. J. Alloys Compd. 488, 217–222. Torkamany, M.J., Tahamtan, S., Sabbaghzadeh, J., 2010. Dissimilar welding of carbon steel to 5754 aluminum alloy by Nd:YAG pulsed laser. Mater. Des. 31, 458–465. Wang, C., Cui, L., Mi, G., Jiang, P., Shao, X., Rong, Y., 2017. The influence of heat input on microstructure and mechanical properties for dissimilar welding of galvanized steel to 6061 aluminum alloy in a zero-gap lap joint configuration. J. Alloys Compd. 726, 556–566. Xia, H., Zhao, X., Tan, C., Chen, B., Song, X., Li, L., 2018. Effect of Si content on the interfacial reactions in laser welded-brazed Al/steel dissimilar butted joint. J. Mater. Process. Technol. 258, 9–21. Yang, J., Li, Y., Zhang, H., Guo, W., Weckman, D., Zhou, N., 2015. Dissimilar laser Welding/Brazing of 5754 aluminum alloy to DP 980 steel: mechanical properties and interfacial microstructure. Metall. Mater. Trans A. 46, 5149–5157. Zhang, M.X., Kelly, P.M., 2005. Edge-to-edge matching and its applications: part II. Application to Mg-Al, Mg-Y and Mg-Mn alloys. Acta Mater. 53, 1085–1096.
Acknowledgements Financial supports of the National Natural Science Foundation of China (51805315, 51665038 and 51775091), Beijing Engineering Researching Center of Laser Technology (BG0046-2018-06), the Academic and Technical Leaders Founding Project of Major Disciplines of Jiangxi Province (2018) and the Jiangxi Science Fund for Distinguished Young Scholars (2018ACB21015), and the Talent Program of Shanghai University of Engineering Science (2018RC452018) are gratefully acknowledged. The authors would like to thank Dr. Denzel Bridges from Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee for valuable
46
Contents lists available at ScienceDirect
Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Heat input, intermetallic compounds and mechanical properties of Al/steel cold metal transfer joints ⁎
T
⁎
Jin Yanga, , Anming Hub, Yulong Lic, , Peilei Zhanga, Dulal Chandra Sahad, Zhishui Yua a
School of Materials Engineering, Shanghai University of Engineering Science, Shanghai, 201620, China Beijing Engineering Researching Center of Laser Technology, Beijing, 100124, China c Key Lab for Robot and Welding Automation of Jiangxi Province, Mechanical and Electrical Engineering School, Nanchang University, Nanchang, 330031, China d Centre for Advanced Materials Joining, Department of Mechanical & Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1 Canada b
A R T I C LE I N FO
A B S T R A C T
Associate Editor: J.E. Kinder
Cold metal transfer welding/brazing of AA5754 aluminum alloy and Q235 low carbon steel was performed in a lap joint configuration using an AlSi12 filler wire. The effect of heat input on intermetallic compounds (IMC) formations and mechanical properties has been investigated. At a low heat input of 157 J/mm, a layer of IMC, with a composition of Al7.2Fe1.8Si was generated at the fusion zone/steel interface. Low interfacial energy and good interfacial bonding between fusion zone and Al7.2Fe1.8Si led increased joint strength. An additional Fe (Al,Si)3 IMC formed at the interface in conjunction to Al7.2Fe1.8Si at a high heat input of 201 J/mm. The joint strength was significantly decreased and attributed to a high interfacial energy and poor interfacial bonding between Al7.2Fe1.8Si and Fe(Al,Si)3. The results showed that the interfacial energy at interphase boundaries had remarkable effect on the joint strength.
Keywords: Aluminum and steel joint CMT welding/brazing Intermetallic compounds Shear strength Interplanar mismatch
1. Introduction Application of multi-materials in the automotive industry is one of the effective ways of reducing both fuel consumption and greenhouse gas emission. A proper joining of dissimilar materials (for example, Al to steel) by a welding method is difficult to achieve due to large differences in their physical and chemical properties. In literature, various welding technologies have been reported. For example, fusion welding techniques, including laser welding and electron beam welding, have been used to join Al and steel, where the joining is achieved by the mixing and inter-diffusion between molten Al and steel. Because of the low solubility of Fe in Al, a thick and brittle intermetallic compounds (IMC) is readily generated at the weld interface which can deteriorate the load bearing capacity of the weld, as reported by Torkamany et al. (2010). In addition, the joints may also produce some weld defects such as shrinkage voids, porosities and solidification cracking, etc. (Dinda et al. (2019)). To overcome the complexities in fusion welding, solid-state welding techniques, such as diffusion bonding, ultrasonic spot welding, friction stir welding and vaporizing foil actuator welding, have also been explored in the past. The solid-state welds were achieved by the interdiffusion of Fe and Al atoms under high pressure, low temperature or
⁎
their combinations. However, the aforementioned methods either requires a longer processing time (e.g., diffusion bonding) or specific workpiece geometries (e.g., friction stir welding). To overcome these limitations, a combined welding/brazing technique has been developed for Al/steel joining. The technique has a dual characteristic as it creates a fusion weld at Al side and a brazed joint at steel side. In the welding/ brazing application, laser, arc, and their combinations have usually been used as main heat sources. Investigations by Xia et al. (2018) and Yang et al. (2015) showed that welding/brazing technology offers a great potential for dissimilar joining of Al to steel. One of the advanced arc welding/brazing technologies is cold metal transfer (CMT) welding/ brazing, which has been used in Al/steel joining due to its low heat input, small deformation and free spattering. In the investigation of CMT welding/brazing of Al to boron steel, Cao et al. (2014) pointed out that the Al/galvanized steel joints possessed higher tensile strength than that of other types of coated/uncoated steels (e.g., uncoated, Al-Si coated, and galvannealed). It was reported that the interfacial layer thickness of CMT Al/steel joint was ranging from 2 to 5 μm. In another study of CMT welding/brazing of Al to steel by Cao et al. (2013), they optimized the welding process parameters by Taguchi method. They also pointed out that the degradation of aluminum heat-affected-zone and thickness of the IMC was minimized by controlling the heat input
Corresponding authors. E-mail addresses: [email protected] (J. Yang), [email protected] (Y. Li).
https://doi.org/10.1016/j.jmatprotec.2019.05.004 Received 24 October 2018; Received in revised form 2 May 2019; Accepted 6 May 2019 Available online 06 May 2019 0924-0136/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Chemical compositions of the materials in wt.%. Materials
Mg
Zn
Q235 steel 5754 Al ER4043
– 2.6-3.6 0.1
– 0.2 0.01
Ti
Si
Cr
Mn
Ti
Fe
S
C
Al
– 0.15 –
0.22 0.4 4.5-6.0
– 0.3 –
0.60 0.5 –
– 0.15 –
Bal. 0.4 0.17
0.02 – –
0.18 – –
0.45 Bal. Bal.
within 100–200 J/mm. Although it is well-established relationship between heat input, interfacial IMC thickness and joint mechanical properties in laser Al/steel welds as presented by Pardal et al. (2014) and Wang et al. (2017), this relationship is still not conclusive in CMT Al/steel joining. Therefore, this paper aims to study the influence of heat input on interfacial IMC and joint strength for CMT Al/steel joints.
Table 2 The optimized welding parameters used for welding/brazing.
2. Experimental 2.1. Materials
Welding parameters
Optimized range
Values
Current (I) [A] Voltage (U) [V] Peed (Ws) [mm s−1] Wire feed speed (Wf) [m min−1] Calculated heat input (K) [J mm−1]
67-69 10.9-11.8 3-7 4
69 11.3 7 4
68 11.8 6
67 11.7 5
69 11.2 4
68 11.4 3
–
111
129
157
201
260
A 2.0 mm thick AA5754 aluminum alloy and a 1.8 mm thick Q235 low carbon steel sheets were used. An Al-Si based alloy (ER4043) with a diameter of 1.2 mm was selected as a filler metal. The chemical compositions of the materials are given in Table 1. In order to prevent oxidation, a brazing flux QJ201 was used.
dispersive X-ray spectrometry (EDS). The X-ray diffraction (XRD) analysis was performed on a Panalytical X’Pert X-ray diffractometer (CuKα, Holland).
2.2. CMT welding/brazing process
Rectangular tensile-shear coupons were cut by a wire saw from the joints with the dimension of 20 mm × 160 mm. The joint tensile-shear testing was performed at a cross-head speed of 1.0 mm/s. The microhardness measurements were carried out using an automated computerized hardness tester with a 25 g load and 15 s dwell time.
2.4. Mechanical testing
The sheets were machined as rectangular strips of 100 mm × 150 mm. An integrated ABB six-axis robot with a Fronius arc welding system (CMT 4000 Advanced) was used for welding/brazing application. Prior to the welding/brazing, the sheets were cleaned with acetone to remove oil and debris. The aluminum sheet was placed on top of the steel sheet in a lap joint configuration with an overlap distance of 15 mm. The schematic diagram of the joint configuration is illustrated in Fig. 1. Preliminary experiments were conducted to optimize the process parameters (Table 2). The heat input was calculated by using Eq. (1)
K=
IU Ws
3. Results 3.1. Metallography Fig. 2 shows weld appearance and cross sections of CMT Al/steel joints with different heat inputs. An acceptable surface appearance without spattering and undercut was achieved. An uneven weld toe with rough surface was observed when heat input was less than 157 J/ mm due to the poor wettability of the molten filler metal. The wettability was gradually improved with more stable and uniform surface appearance at a higher heat input. To validate the statement, quantitative analysis was performed. The molten filler metal’s wettingspreading ability was evaluated by measuring the wetting angle and brazed width (Fig. 2). As the heat input increased from 111 J/mm to 260 J/mm, the wetting angle gradually decreased from 102 ± 9° to 66 ± 7°, while the brazed size progressively became wider from 3.8 ± 0.2 mm to 10.0 ± 0.7 mm. The decreased wetting angle and increased brazed width is an indication of improved wetting and spreading ability of molten filler metal. Fig. 3 shows the cross-section of the joints with different heat inputs. A layer of IMC was observed at the interface between fusion zone and steel. The measured thickness of the reaction layers with varied heat inputs were shown in Fig. 2. The increase of reaction layer thickness with heat input was related to the formation and growth of IMC, i.e., time- and temperature- controlled diffusion process. Similar results were reported by Li et al. (2018) for dissimilar Al/steel joints made by laser welding/brazing. It is clear that only one reaction layer was observed in Fig. 3 (a–c); while, some fine dark features were generated within the reaction layer from the steel side as shown in Fig. 3(d–e). It was indicated that a new phase was formed at the fusion zone/steel interface, which was confirmed by SEM analysis and discussed in a later section. Since the phase constituents were identical with only minor differences when heat input
(1)
where, I, U and Ws were welding current, welding voltage, and welding speed, respectively. 2.3. Microstructure characterization The samples were prepared using standard grinding and polishing method followed by etching with Keller’s reagent to reveal the microstructure. The microstructure characterization was carried out by optical microscope (OM, model: Olympus 4XCJZ) and scanning electron microscope (SEM, model: Hitachi S3400-N) with EDAX Genesis energy-
Fig. 1. Schematic diagram of the joint configuration of CMT welding/brazing. 41
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Fig. 2. Macrostructure, wetting angle, brazed width, thickness of reaction layer of the CMT Al/steel joints with different heat inputs.
fusion zone and steel substrate were about 66 HV and 144 HV, respectively. In all cases, the hardness at the fusion zone/steel interface sharply increased over 350 HV. The increase of the microhardness was attributed to the hard and brittle nature of Fe-Al and Fe-Al-Si IMCs. The joints were all fractured at the fusion zone/steel interface after tensile-shear testing. Thus, the joint strength was characterized by shear strength using the following equations:
was less than 157 J/mm (single-phase reaction layer) or larger than 201 J/mm (dual-phase reaction layer), the discussion on microstructure was focused in the joints obtained at heat inputs 157 J/mm and 201 J/ mm, denoting as low and high heat input, respectively. Fig. 4(a) represents the SEM image of typical IMC layer with low heat input (157 J/mm). A single-phase reaction layer composed of serrated IMC was formed at the fusion zone/steel interface. Based on EDS analysis, the IMC layer (P1) composed of 71.5 at. % Al, 9.2 at. % Si and 19.3 at. % Fe (Table 3). According to the Fe-Al-Si ternary phase diagram, the IMC layer was identified as Al7.2Fe1.8Si. However, at high heat input (201 J/mm), a dual-phase reaction layer was observed at the fusion zone/steel interface (Fig. 4(b)). Adjacent to the fusion zone, a scallop-like IMC was obvious, which had the chemical composition of 73.2 Al at. %, 17.8 Fe at. % and 9.0 Si at. % (Table 3). Thus, the IMC was identified as Al7.2Fe1.8Si. Adjacent to the steel side, a newly formed needle-like IMC with a composition of 71.6 Al at. %, 23.2 Fe at. % and 5.6 Si at. % (P3) was observed. The possible phase of the IMC was identified as Fe(Al,Si)3. This kind of dual-phase IMCs layer, consisted of Al7.2Fe1.8Si and Fe(Al,Si)3, was also reported by Song et al. (2009) in tungsten inert gas welded Al/steel joints.
τ = F/A
(1)
A = Wb*L
(2)
where, τ was the joint shear strength, F was the joint fracture load, A was the interface bonding area, Wb was brazed width (shown in Fig. 2), L was the specimen width (20 mm). The plot of measured joint shear strength and interfacial layer thickness as a function of heat input are shown in Fig. 6. The joint shear strength first slightly decreased from about 65 MPa to 54 MPa when heat input increased from 111 J/mm to 157 J/mm, and then it dropped to around 28 MPa as heat input exceeded 201 J/mm. On the other hand, a nearly linear correlation between interfacial layer thickness and heat input was identified. Thus, the remarkable decrease in the joint strength was little correlated with the interfacial layer’s thickness variation. Besides, at low heat input, the fracture location was in the interface between fusion zone and Al7.2Fe1.8Si as shown in Fig. 6. At a high heat input, the fracture location changed to the Al7.2Fe1.8Si /Fe(Al,Si)3 interface. Hence, it implies a
3.2. Mechanical properties Fig. 5 shows microhardness profile across the fusion zone/steel interface produced at different heat inputs. The hardness values of the
Fig. 3. Optical microscopy images of the fusion zone/steel interface with different heat inputs: (a) 111 J/mm, (b) 129 J/mm, (c) 157 J/mm, (d) 201 J/mm, and (e) 260 J/mm. 42
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Fig. 4. SEM images of the fusion zone/steel interfacial region in the CMT Al/steel joints with different heat inputs: (a) low heat input, and (b) high heat input.
strong correlation between the joint shear strength and joint failure location which will be discussed in the later section.
Table 3 EDS analysis of the marked zones in Fig.4 and Fig.7 in at.%. No.
Al
Fe
Si
Possible phase
P1 P2 P3 A1 A2 A3 A4
71.5 73.2 71.6 90.1 78.2 74.2 70.0
19.3 17.8 23.2 1.1 13.5 18.4 25.7
9.2 9.0 5.2 8.8 8.3 7.4 4.3
Al7.2Fe1.8Si Al7.2Fe1.8Si Fe(Al,Si)3 α-Al + Al-Si eutectic Al7.2Fe1.8Si Al7.2Fe1.8Si Fe(Al,Si)3
3.3. Fractography Fig. 7 shows the fracture surface of the joint at a low heat input. At the fusion zone side, the typical cast structure with lots of dendrites was observed (Fig. 7(a)). The average chemical composition of the fracture surface was 90.1 Al at. %, 1.1 Fe at. % and 8.8 Si at. % (Table 3). Thus, it was identified as α-Al and Al-Si eutectic. At the steel side, a smooth plane with some terrace-like structures was obvious. Based on the EDS analysis, the IMC at the fracture surface was identified as Al7.2Fe1.8Si (Table 3). According to the morphology of the fracture surfaces, it suggested that a brittle fracture propagated along the fusion zone/ Al7.2Fe1.8Si interface, which was further confirmed by XRD analysis as shown in Fig. 7(b) and Fig. 7(d). Fig. 8 presents the fracture surface of the joint at a high heat input. At the fusion zone side, a smooth plane with some step-like structures was observed in Fig. 8(a). It was identified as Al7.2Fe1.8Si (Table 3). At the steel side, a relatively smooth surface consisting of Fe(Al,Si)3 phase was confirmed (Table 3). Therefore, it was concluded that a brittle failure occurred at the interphase boundary between Al7.2Fe1.8Si and Fe (Al,Si)3. The phase constitutions at the fracture surface were also confirmed by XRD analysis as shown in Fig. 8(b) and Fig. 8(d). 4. Discussion It has been identified that the phase constituents at the fusion zone/ steel interface in CMT Al/steel joints vary with the heat input, viz., Al7.2Fe1.8Si with low heat input and Al7.2Fe1.8Si + Fe(Al,Si)3 with high heat input. The formation mechanism of phase constituents can be understood from the thermodynamics analysis. It is well-known that Gibbs free energy ΔG is a criterion for chemical reaction to occur. At ΔG < 0 condition, the chemical reaction occurs spontaneously. Among all possible reactions, the reaction with the lowest ΔG tends to occur first. In addition, the IMC formed with lower ΔG is more stable. According to classical thermodynamics, ΔG in standard conditions can be expressed as:
Fig. 5. Microhardness profile across the fusion zone/steel interface of CMT Al/ steel joints with different heat inputs.
0 0 ΔG 0 = ΔH298 − TΔS298
ΔG 0
(3) 0 ΔH298
0 ΔS298
is Gibbs energy change, is enthalpy change, is where, entropy change, T is temperature. Besides, the element composition is a well-known factor to determine the chemical reaction. Previous study by Mei et al. (2013) on laser-arc hybrid welding of Al to steel using an Al-Si filler wire shows that the interfacial reaction is caused by the diffusion of Fe atom to molten Al-Si (fusion zone). Besides, Al and Si atoms are enough for the interfacial reaction and the key factor for the reaction is Gibbs energy of Fe atom per molar. Based on related investigations, the calculated Gibbs energy functions of Al7.2Fe1.8Si and Fe(Al,Si)3 are listed below:
Fig. 6. The plot of joint shear strength and interfacial layer thickness as a function of heat input. The blue dashed rectangles highlight the joint fracture locations (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
0 ΔGAl7.2Fe1.8Si = −147677.5 + 47.80T
43
(4)
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Fig. 7. SEM images and XRD patterns of fracture surfaces of the joint with low heat input: (a–b) fusion zone side, (c–d) steel side. 0 ΔGFe(Al,Si)3 = −142770.0 + 50.8T
Al7.2Fe1.8Si; while, at a high heat input, sufficient diffusion of Fe promotes the formation of Fe(Al,Si)3 and Al7.2Fe1.8Si. With the increase of heat input, the interfacial phase constituent changes from Al7.2Fe1.8Si to Al7.2Fe1.8Si + Fe(Al,Si)3 which lead to the remarkable decrease in the joint shear strength. It is suggested that the formation of Fe(Al,Si)3 contributes to the decreased strength (Fig. 6). Because Fe(Al,Si)3 alters the interfacial structure from steel/ Al7.2Fe1.8Si/fusion zone to steel/Al7.2Fe1.8Si/Fe(Al,Si)3/fusion zone,
(5)
The plot of calculated ΔG as a function of temperature ranging from 700 to 1200 K is shown in Fig. 9. It is found that 0 0 ΔGAl7.2Fe1.8Si < ΔGFe(Al,Si)3 , indicating that Al7.2Fe1.8Si is more stable than Fe(Al,Si)3 hence more likely to form. Thus, the evolution of interfacial IMC with heat input is proposed as follows: at a low heat input, combining with the intrinsic low heat input of CMT technology, the diffusion of Fe is strictly limited, which only allows the formation of
Fig. 8. SEM images and XRD patterns of the fracture surfaces of the joint with high heat input: (a–b) fusion zone side, (c–d) steel side. 44
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Table 5 Calculated interplanar spacing mismatch at the interphase boundary between αAl and Al7.2Fe1.8Si. Matching planes
α-Al interplanar spacing (Å)
Al7.2Fe1.8Si interplanar spacing (Å)
Interplanar mismatch (%)
{111} {110} {100} {111} {110} {100} {111} {110} {100}
2.338 2.862 4.049 2.338 2.862 4.049 2.338 2.862 4.049
13.118 13.118 13.118 10.889 10.889 10.889 10.745 10.745 10.745
82.2 78.2 69.1 78.5 73.7 62.8 78.2 73.9 62.3
α −Al //{0002} Al7.2Fe1.8Si α −Al //{0002} Al7.2Fe1.8Si α −Al //{0002} Al7.2Fe1.8Si
¯ 1} Al7.2Fe1.8Si α −Al //{101 ¯ 1} Al7.2Fe1.8Si α −Al //{101 ¯ 1} Al7.2Fe1.8Si α −Al //{101 ¯ 0} Al7.2Fe1.8Si α −Al //{101 ¯ 0} Al7.2Fe1.8Si α −Al //{101 ¯ 0} Al7.2Fe1.8Si α −Al //{101
Fig. 9. Calculated Gibbs formation energy for Fe(Al,Si)3 and Al7.2Fe1.8Si IMCs.
interplanar spacing d along (hkl) planes is expressed using the equation as proposed by Cullity (2001):
and the appearance of a new interface, i.e., Al7.2Fe1.8Si/Fe(Al,Si)3, is likely to be responsible for the remarkable decrease of joint strength. This statement can be supported by the fractography in Figs. 7 and 8. At a low heat input, the joint fracture occurs at the fusion zone/Al7.2Fe1.8Si interface, while it occurs at the Al7.2Fe1.8Si/Fe(Al,Si)3 interface at a high heat input. It infers that the bonding strength at the Al7.2Fe1.8Si/ fusion zone interface is higher than that of Al7.2Fe1.8Si/Fe(Al,Si)3 interface. It is well-established that interphase bonding strength strongly correlates to the interplanar mismatch at the interphase boundary. For example, in the investigation of resistance spot welding of Mg alloy to Zn-coated steel, Liu et al. (2011) demonstrated a nanocale transitional Fe2Al5 layer significantly improved the joint strength, without which it was difficult to join owing to original sharp interface and high interfacial mismatch. By using the edge-to-edge matching model proposed by Zhang and Kelly (2005), Nasiri (2013) revealed that interplanar mismatch determined interfacial energy at the interphase boundary which in turn affected interphase metallurgical bonding strength in laser Mg/steel joints with Ni, Zn and Sn transitional layers. The above transitional layers play the key role of the interplanar mismatch reduction. Normally, the matching planes at interphase boundaries are the close-packed or nearly close-packed planes. In the present study, the metallurgical bonding strength at the interphase boundary is semiquantitatively evaluated by comparing the interplanar mismatch at the interphase boundaries. As HCP phase, Al7.2Fe1.8Si has the lattice constant of aAl7.2Fe1.8Si = 12.406 Å and cAl7.2Fe1.8Si = 26.236 Å. Fe(Al,Si)3 has a monoclinic crystal structure with the lattice constant of bFe(Al,Si)3 = 8.083 Å, cFe(Al,Si)3 = 12.476 Å, aFe(Al,Si)3 = 15.489 Å, β = 107.7°. As the main constituent phase in fusion zone, α-Al is a FCC phase with the lattice constant of aα-Al =4.050 Å. The close-packed planes of FCC and HCP are displayed in Table 4. The close-packed planes of monoclinic Fe(Al,Si)3 are identified by looking at the powder X-ray diffraction intensities, which are {332}, {025} and {620}. The
k 2sin2β 2hlcosβ 1 1 ⎛ h2 l2 ⎞ = + 2 + 2 − ⎜ 2 ⎟ 2 2 d sin β ⎝ aFe(Al,Si)3 aFe(Al,Si)3 c Fe(Al,Si)3 ⎠ bFe(Al,Si)3 cFe(Al,Si)3 (6) where, h, k, l are miler indices of a crystallographic plane, d is the interplanar spacing along the plane (hkl). Tables 5 and 6 summarize the interplanar mismatch of all possible matching planes at the interphase boundaries between α-Al and Al7.2Fe1.8Si as well as Al7.2Fe1.8Si and Fe(Al,Si)3. The interplanar mismatch at the interphase boundary between α-Al and Al7.2Fe1.8Si ranges from 62.3% to 82.2%, while it is up from 411.7 % to 539.9% at the Fe (Al,Si)3/Al7.2Fe1.8Si interface. Both the interplanar mismatches are larger than 10%, indicating the formation of noncoherent interfaces. According to the Hooke’s law:s
Eε =
where, Eε , G, ν, ε0 , and f(y) are free (strain) energy density, shear modulus, Poisson's ratio of the reaction product, lattice mismatch, and the unit step function, respectively. The equation suggests that the free (strain) energy is always positive and proportional to the lattice mismatch. The increase of free (strain) energy will increase the total interfacial energy as stated by Fredriksson and Åkerlind (2012). Thus, a high interplanar mismatch leads to a high interfacial energy, which in turn results in a poor interfacial bonding. Hence, at macroscale, α-Al/ Al7.2Fe1.8Si interfaces exhibit much higher bonding strength than that of Fe(Al,Si)3/Al7.2Fe1.8Si interfaces.
Table 6 Calculated interplanar spacing mismatch at the interphase boundary between Al7.2Fe1.8Si and Fe(Al,Si)3.
Table 4 The close-packed planes and their interplanar spacings for HCP and FCC crystal structures. Crystal structure *
HCP (aH, cH)
*
FCC (aF)
Close-packed plane
Interplanar spacing
{0002} {101¯1}
cH /2
G 2 2 ε0 f (y ) 1−v
aH cH 3 2 + 3a2 4cH H
Matching planes
Al7.2Fe1.8Si interplanar spacing (Å)
Fe(Al,Si)3 interplanar spacing (Å)
Interplanar mismatch (%)
{0002} {101¯1}
Al7.2Fe1.8Si //{332} Fe(Al,Si)3
13.118
2.09
527.7
Al7.2Fe1.8Si //{332} Fe(Al,Si)3
10.889
2.09
421.0
{101¯0}
Al7.2Fe1.8Si //{332} Fe(Al,Si)3
10.745
2.09
414.1
{0002} {101¯1}
Al7.2Fe1.8Si //{025} Fe(Al,Si)3
13.118
2.05
539.9
Al7.2Fe1.8Si //{025} Fe(Al,Si)3
10.889
2.05
431.2
{101¯0} {111}
3
3 aF
{101¯0}
Al7.2Fe1.8Si //{025} Fe(Al,Si)3
10.745
2.05
424.1
{110}
2
2 aF
Al7.2Fe1.8Si //{620} Fe(Al,Si)3
13.118
2.10
524.7
{100}
aF
{0002} {101¯1}
Al7.2Fe1.8Si /{620} Fe(Al,Si)3
10.889
2.10
418.5
{101¯0}
Al7.2Fe1.8Si //{620} Fe(Al,Si)3
10.745
2.10
411.7
3 aH /2
* Lattice parameters. 45
Journal of Materials Processing Tech. 272 (2019) 40–46
J. Yang, et al.
5. Conclusions
proofreading. References
1 At a low heat input not higher than 157 J/mm, a single-phase reaction layer, composed of Al7.2Fe1.8Si, is formed at the fusion zone/ steel interface. At a high heat input not lower than 201 J/mm, the interfacial microstructure is changed to a combination of Fe(Al,Si)3 and Al7.2Fe1.8Si. 2 The fracture locations vary with the heat inputs: the fusion zone/ Al7.2Fe1.8Si interface with low heat input, while interphase boundary between Al7.2Fe1.8Si and Fe(Al,Si)3 with high heat input. 3 Interfacial lattice mismatch has a significant influence on the joint strength: relatively low interfacial interplanar mismatch is obtained at the Al7.2Fe1.8Si/fusion zone interface making a good interfacial bonding and high joint strength (˜54 MPa); but it decreases to 28 MPa due to relatively high interfacial interplanar mismatch and poor bonding at the Fe(Al,Si)3/Al7.2Fe1.8Si interface. 4 Comparing to the wettability of filler metal, the thickness and phase constituent of reaction layer have more pronounced influences on the joint strength. The heat input should be kept as low as possible to maintain a thin and single-phase (Al7.2Fe1.8Si) reaction layer at the fusion zone/steel interface to achieve a good interfacial bonding and a sound joint.
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Acknowledgements Financial supports of the National Natural Science Foundation of China (51805315, 51665038 and 51775091), Beijing Engineering Researching Center of Laser Technology (BG0046-2018-06), the Academic and Technical Leaders Founding Project of Major Disciplines of Jiangxi Province (2018) and the Jiangxi Science Fund for Distinguished Young Scholars (2018ACB21015), and the Talent Program of Shanghai University of Engineering Science (2018RC452018) are gratefully acknowledged. The authors would like to thank Dr. Denzel Bridges from Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee for valuable
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