Effect of Filler-Wire Composition on Microstructure and Properties of Al/Steel-Welded Joints by Laser Welding–Brazing

Abstract

Laser welding–brazing experiments with 6061 aluminum alloy and Q235 steel were conducted using AlSi12 and ZnAl22 as filler metals. The macroscopic morphologies and microstructures of welding–brazing joints were analyzed by optical microscopy (OM) and scanning electron microscopy (SEM). The effects of Si and Zn elements in filler wires on the macroscopic morphologies, interfacial microstructures, tensile properties, and corrosion resistances of joints were studied. The results show that the wettability of ZnAl22-filler wire was better than that of AlSi12-filler wire. The fusion zone of the joint welded with AlSi12-filler wire consisted of α–Al solid solution and Al–Si eutectic mixture, while the interfacial reaction zone was composed of η-Fe₂(Al, Si)₅ and τ₅-Fe_1.8Al_7.2Si. The fusion zone of the joint formed using AlZn22-filler wire consisted of α–Al solid solution, η-Zn solid solution, and ZnAl eutectic structure. The joint welded with AlSi12-filler wire showed better tensile strength and corrosion resistance compared to that welded with ZnAl22-filler wire.

1. Introduction

Al–steel joining is widely utilized in the automotive and aerospace industries due to its significant weight-saving advantages [1,2]. Due to the significant differences in thermal conductivity, coefficient of linear expansion, weldability, etc. between Al and steel, which are dissimilar metals, defects such as pores and cracks tend to occur in the weld zone. This results in a weld seam that has inferior mechanical properties, making the welding of Al to steel a current research hotspot [3,4,5]. Laser welding is an automated welding process that utilizes a laser as the heat source; it is characterized by its higher welding speed, reduced post-welding deformation, and more accurate keyhole positioning [6,7,8]. Owing to the significant disparity in melting points between Al and steel, simultaneous melting and adequate elemental diffusion cannot be achieved. To address these challenges, scholars have integrated laser technology with brazing, resulting in the innovative laser welding–brazing process [9]. The laser welding–brazing process not only solves the issue of excessively rapid Al melting but also enhances the spreading performance of Al on the steel surface.

Due to the differences in physical and chemical properties between Al and steel, welding these dissimilar materials can lead to the formation of a considerable amount of brittle intermetallic compounds (IMC), which can greatly compromise the performance of the welded joint [10,11,12]. Numerous studies have demonstrated that the addition of various elements to the Al/steel-welding process can effectively impede the diffusion metallurgical reaction between Al and Fe [13,14,15,16], thus enhancing the mechanical properties of the welded joint. Dong et al. [13] examined the influence of Si, Cu, and Zn as alloying elements on the microstructure and tensile strength of lap joints between Al and steel using gas tungsten arc welding with various filler wires. The results showed that adding Si to the weld suppressed Fe diffusion from the base metal, reduced IMC thickness, and improved joint tensile strength. Yang et al. [14] found that adding alloying elements such as Si and Zn into pure Al to form Al–Si or Al–Zn filler wires can increase the fracture load of Al/steel joints. Yu et al. [15] explored the welding characteristics of Al–5052/Q235 galvanized steel joints using various filler wires, including pure Al, AlSi5, AlMg5, AlCu6, and ZnAl2, through a laser welding–brazing process. They found that the Si element markedly inhibits the formation of IMCs and that the joints welded with AlSi5 filler wire demonstrated superior load-bearing and fracture strengths compared to other joints. Further investigation showed that adding Si changed the interface microstructure from brittle Fe–Al IMCs to ductile Fe-Al–Si IMCs, inhibiting crack propagation and ensuring sufficient joint strength [16].

Control of heat input during Al/steel welding is crucial for reducing the formation of brittle Al–Fe IMCs [17]. Su et al. [18] found that the fracture load of laser fusion-welded Al/steel joints initially increased and subsequently decreased as the laser power increased. These changes can be primarily attributed to the alterations in interfacial phase morphology and bonding length induced by variations in laser power. Yu et al. [19] showed that unbeveled Al/steel joints can be effectively welded using the laser-arc hybrid welding–brazing process. The tensile strength of the joint initially increased and then decreased as the welding speed or wire feed speed increased. Narsimhachary et al. [20] indicated that the welding performance and corrosion resistance of laser welded–brazed Al/steel joints are influenced by the wire-deposition rate. Li et al. [21] examined the influence of groove shape on the formation performance of Al/steel joints produced through the laser welding–brazing process. The results showed that the half-V-shaped groove had the smallest temperature gradient across the steel interface, leading to the most uniform IMC thickness and highest tensile strength compared to square and half-Y-shaped grooves. Additionally, Li et al. [22] improved the reliability of laser welded–brazed Al/steel joints by using a novel surface-texturing technique that modifies reaction behavior and strain-stress distribution, thus enhancing line load capacity. Jiang et al. [23] achieved reliable joining between Al and steel through the application of a porous high-entropy alloy coating. In this case, the results indicated that the joint wettability improved as the defocused distance decreased, with optimal wettability achieved at 0 mm defocused distance, which yielded a wetting length of 5.57 mm and a contact angle of 59.8°. There is evidence suggesting that using HEAs as a transient layer or filler metal can help reduce the formation of brittle IMCs caused by reactions between Al and steel [24,25]. This, in turn, enhances the strength of welded joints.

In addition, Mohammadpour et al. [26] demonstrated the superiority of the double-beam laser welding–brazing process in joining dissimilar Al–6022 and galvanized steels. Yuan et al. [27] employed dual-beam laser welding–brazing to regulate the wetting angle, wetting distance, and the thickness of the IMC layer, thereby achieving successful formation of Al/steel lap joints without the use of filler material. Xia et al. [28] investigated weld formation via laser welding–brazing between Al and steel using single laser beams, as well as cross and in-line dual laser beams. The results demonstrated that the dual-laser-beam technique positively contributed to reducing the IMC thickness and promoting a more uniform interfacial IMC morphology. Moreover, the interface exhibiting the most homogeneous IMC and the joint demonstrating the highest tensile strength were achieved using the cross dual laser beams. Meng et al. [29] used a laser-beam-oscillation technique to optimize the uniformity of the IMC layer. The results indicated that as the optimized laser offset increased from 0.4 to 0.8 mm, the IMC thickness variance from top to bottom decreased from 105.14 to 0.0022, enhancing the ultimate tensile strength of the joint from 77 to 160 MPa.

It can be seen that the addition of alloy elements or modifications to the welding process can lead to changes in the joint’s performance. Therefore, this study conducted experimental investigations on the laser welding–brazing of 6061 aluminum alloy and Q235 steel utilizing AlSi12- and ZnAl22-filler wires. The influence of Si and Zn elements on the macroscopic morphology, microstructural constitution, mechanical properties, and corrosion resistance of the welded joints was investigated.

2. Materials and Methods

6061 aluminum alloy and Q235 mild steel with the same dimensions (150 mm × 80 mm × 2 mm) were used as base metals in this study. The filler wires were a 1.6 mm diameter AlSi12-filler wire and a ZnAl22-filler wire with a flux composition of No-clock flux (97% KAlF4, 3% CsAlF4). The chemical compositions of the base metal and welding material are presented in

Table 1. Chemical compositions and properties of base metals and filler wires.

Materials	Composition (wt. %)								Tensile Strength (MPa)
6061 aluminum alloy	Al	Si	Fe	Mn		Mg	Cu	Zn	290
	Bal.	0.8	0.7	0.15		0.8	0.2	0.2
Q235 steel	C	Si	Mn	S		P	Cr	Fe	375
	0.2	0.2	0.2	0.03		0.03	0.70	Bal.
AlSi12-filler wire	Si	Cu	Zn	Mg		Mn	Fe	Al	210
	12.15	0.0057	0.0048	0.0002		0.075	0.209	Bal.
ZnAl22-filler wire	Zn				Al				210
	78				22
Flux	KAlF4				CsAlF4				/
	97				3

.

Laser welding–brazing was performed on 6061 aluminum alloy and steel using AlSi12-filler wire and ZnAl22-filler wire, respectively. Figure 1 illustrates the schematic diagram of the laser-brazing process. The base metal was assembled in the form of a butt joint, and a copper substrate was placed under the base material. By utilizing the high thermal conductivity of copper, the heat generated during welding can be eliminated, thereby reducing the growth of IMCs. The laser was tilted 10° away from the vertical direction to prevent laser reflection from damaging the laser head. The specific parameters of the welding process are shown in

Table 2. Laser welding–brazing parameters.

Process Parameters	Value
Laser power (P)	2800 W
Wire-feeding speed (mm·${\mathrm{m}\mathrm{i}\mathrm{n}}^{-1})$	2.9 m/min
Welding speed	420 mm/min
V-shaped groove on the steel side	45°
Defocused distance	30 mm
Shielding gas flow rate	15 L/min

.

Metallographic samples and tensile specimens from the weld seams were extracted perpendicular to the welding direction. After grinding, polishing, and etching, the morphologies of the cross-sectional microstructures of the steel/welded seam interfaces were characterized by scanning electron microscopy (SEM) and the chemical compositions were identified by energy dispersive spectrometry (EDS). The tensile properties of Al/steel joints were evaluated by preparing three specimens for each joint and determining the average test results. Additionally, electrochemical and X-ray photoelectron spectroscopy (XPS) analyses were conducted on metallographic specimens to investigate their corrosion resistance.

3. Results and Discussion

3.1. The Macroscopic Morphology of Different Al/Steel Joints

Figure 2 depicts the macroscopic morphology of Al/steel welded joints achieved by employing AlSi12- and ZnAl22-filler wires. As shown in Figure 2a, the front surface of the joint welded with AlSi12-filler wire exhibited a fish-scale-like weld seam, indicating good formation. However, the formation of the back surface was suboptimal, with welding defects such as a biting edge observed. Additionally, the wettability of the back surface was also unsatisfactory. Figure 2b shows the macroscopic morphology of the joint welded with ZnAl22-filler wire. Despite the favorable wettability and spreadability of the joint, the morphology suggests that the formation of the joint was suboptimal. The front surface of the weld seam did not exhibit any fish-scale-like patterns. Moreover, a large number of slag inclusions were generated on the surface of the joint. The wetting angles and weld reinforcements of the joints formed utilizing AlSi12- and ZnAl22-filler materials were measured as 44.5°, 1.76 mm and 28.2°, 1.21 mm, respectively. The wettability of the joint formed using ZnAl22-filler wire was significantly enhanced compared to that of the joint made with AlSi12-filler wire, resulting in a more continuous formation at the back. However, the overall formation of the joint prepared with Zn-based filler material was inferior to that of the joint made with Al-based filler material, which could significantly impact the mechanical properties of the welded joint. In fact, the spattering of ZnAl22-filler wire during welding contributed to the formation of welding slag, which adhered to the surface of the welded joint. Furthermore, the melting point of ZnAl22 (420–480 °C) is much lower than that of AlSi12 (595–625 °C), resulting in a higher heat input during welding with ZnAl22-filler wire, which led to the collapse of the weld joint.

3.2. Interfacial Microstructure of Al/Steel Joints

Figure 3 shows the SEM image of the interfacial microstructure of the brazing zone at the steel-side interface. The EDS results for different points (point 1–6, as shown in Figure 3) within the IMC layer are presented in

Table 3. The EDS results for points 1–6 (see Figure 3).

Filler Wires	Points	Composition (at. %)				Possible Phases
		Al	Si	Fe	Zn
AlSi12	1	55.62	8.39	36.00	/	η-${\mathrm{F}\mathrm{e}}_2{(\mathrm{A}\mathrm{l},\mathrm{S}\mathrm{i})}_5$
	2	74.71	9.40	13.89	/	${\mathsf{\tau }}_5$-${\mathrm{F}\mathrm{e}}_{1.8}{\mathrm{A}\mathrm{l}}_{7.2}\mathrm{S}\mathrm{i}$
	3	73.97	23.65	2.38	/	Al–Si eutectic
ZnAl22	4	39.32	/	36.73	23.95	${\mathrm{F}\mathrm{e}}_2{\mathrm{A}\mathrm{l}}_{5-\mathrm{x}}{\mathrm{Z}\mathrm{n}}_{\mathrm{x}}$ + Fe${\mathrm{Z}\mathrm{n}}_{10}$
	5	61.90	/	26.70	11.41	${\mathrm{F}\mathrm{e}}_2{\mathrm{A}\mathrm{l}}_{5-\mathrm{x}}{\mathrm{Z}\mathrm{n}}_{\mathrm{x}}$
	6	63.23	/	24.89	11.88	${\mathrm{F}\mathrm{e}}_2{\mathrm{A}\mathrm{l}}_{5-\mathrm{x}}{\mathrm{Z}\mathrm{n}}_{\mathrm{x}}$

. The IMC layer near the steel matrix in the joint using AlSi12-filler wire, identified as τ₅-Fe_1.8Al_7.2Si, exhibited a zigzag-like morphology (referred to as I layer), while the IMC layer near the weld displayed a continuous planar shape and was identified as η-Fe₂(Al, Si)₅ (referred to as II layer), as shown in Figure 3a. The IMC layer formed in the joint using ZnAl22-filler wire exhibited a continuous planar shape (identified as Fe2Al5-xZnx) near the weld, while a zigzag-like layer (identified as FeZn₁₀) was present closer to the steel matrix, as shown in Figure 3b.

Figure 4 shows the EDS line-scanning results for brazing zones within joints made with AlSi12- and ZnAl22-filler wires. The thickness of the IMC layer formed in the joint welded with AlSi12-filler wire was 2.06 μm, as shown in Figure 4a,b. Variation in the content of Al, Fe, and Si elements can be observed in the transition from the steel-side base metal to the weld, as depicted in Figure 4b. The concentration of Al decreased progressively with increasing distance from the steel side, while the concentration of Fe exhibited a gradual increase. Segregation of Si elements was observed at the interface between the steel and the weld seam. The IMC layer formed within the joint using ZnAl22-filler wire is depicted in Figure 4c and exhibited a thickness of 6.32 μm. The concentration of Al in the IMC layer exhibited an initial increase, which was followed by a subsequent decrease. The Fe content gradually decreased from left to right, reaching a certain value before further declining upon achieving dynamic stability. Additionally, the segregation of Si elements was also observed at the interface on the steel side. It can be seen that, in comparison with Zn, Si can better inhibit the growth of IMC, a finding in agreement with the results reported in previous studies [10,12].

3.3. Mechanical Properties of Al/Steel Joints

Figure 5 illustrates the tensile strength and displacement curves of the joints welded using AlSi12- and ZnAl22-filler wires. The tensile strength of the joint welded with AlSi12-filler wire was observed to be 203 MPa, with a corresponding tensile displacement of 5.8 mm. The tensile strength of the joint using ZnAl22-filler wire was 186 MPa, and the tensile displacement measured 4.5 mm. Despite the superior wetting properties exhibited by ZnAl22-filler wire, its mechanical characteristics were found to be less favorable compared to those of AlSi12-filler wire. The reason is that Si can diffuse into the vacancies of Fe2Al5, thereby reducing the direct reaction between Fe and Al and decreasing the formation of IMCs such as brittle Fe2Al5 [16]. The addition of Si can also decrease the activation energy required for Fe to diffuse in Al, facilitating the diffusion of Fe. The affinity of Si for Al is greater than that of Fe, facilitating the formation of Al–Fe–Si IMC. According to the Al–Zn phase diagram, Al and Zn can dissolve in each other indefinitely and exhibit a strong ability to interact with one another. Additionally, Zn can form Al–Fe–Zn IMCs with Fe and Al, as well as an Al–Zn solid solution [15]. The mechanical properties of the joint, however, were compromised as a result of the excessive thickness of the IMC layer. However, an excessive amount of Zn will lead to the formation of an overly thick IMC interface layer, thereby degrading the mechanical properties of the joint.

3.4. The Corrosion Property of Al/Steel Joints

The open-circuit voltage-variation diagram of the joints welded with AlSi12- and ZnAl22-filler wire is illustrated in Figure 6. The open-circuit potentials of the joints welded with AlSi12- and ZnAl22-filler wire are presented in

Table 4. Open-circuit potential of joints welded with AlSi12- and ZnAl22-filler wire.

Joints Welded with Different Wires	Open-Circuit Potential E/V
AlSi12	−0.730
ZnAl22	−1.055

. The open-circuit potential of the joint welded with AlSi12-filler wire was −0.730 V, while that of the joint welded with ZnAl22-filler wire was −1.055 V. The higher open-circuit potential indicates that the joint welded with AlSi12 filler exhibited improved electrochemical stability and a reduced susceptibility to corrosion. The weld seam of the joint welded with AlSi12-filler wire primarily consisted of α–Al and Al–Si eutectic, facilitating the formation of a compact aluminum oxide film during the corrosion. The dense aluminum-containing oxide film covering the weld surface could play a role in isolating the corrosive liquid, resulting in increased potential and passivation of corrosion resistance. The dense oxide film containing aluminum on the surface of the weld could effectively isolate corrosive liquids, leading to an increased potential and enhanced resistance to corrosion.

Figure 7 shows the alternating current (AC) impedance spectroscopy of the joints welded with AlSi12 and ZnAl22-filler wires. The AC impedance spectra of both joints exhibit a semicircular shape, as illustrated in Figure 7a. The radius of the AC impedance spectrum was larger for the AlSi12-welded joint than for the ZnAl22-welded joint, indicating that the AlSi12-welded joint exhibited better corrosion resistance. The thickness of Al/steel interface layer obtained from ZnAl22 welding was much higher than that obtained with AlSi12 welding. The excessive formation of IMC can result in galvanic corrosion at the joint, which may further compromise its resistance to corrosion. Sravanthi et al. [30] also found that the formation of IMC enhanced the galvanic corrosion of an Al/steel welded joint and significantly reduced the corrosion resistance of the hybrid structure. In addition, the ZnAl22-welded joint also displayed a linear tail at an angle of 45°. The appearance of such a curve on the Nyquist plot signifies an inhibition of diffusion of a corrosive substance towards the interface between the matrix and film. This also indicates that the corrosion layer on the joint surface was dense enough to prevent diffusion from occurring.

Figure 8 shows the polarization curves of the joints welded with AlSi12- and ZnAl22-filler wires.

Table 5. The self-corrosion current densities and the self-corrosion potentials of tested joints.

Joints Welded with Different Wires	Ecorr/V	Icorr/(A·cm−2)
AlSi12	−0.93788	3.3578 × 10−5
ZnAl22	−1.0536	1.7668 × 10−4

shows the self-corrosion current density and self-corrosion potential of the test joints, as calculated using Tafel extrapolation [31]. The self-corrosion potential of the joint welded with AlSi12-filler wire (E_{corr–AlSi12}) was −0.93788 V, while that of the joint welded with ZnAl22-filler wire (E_{corr–ZnAl22}) was −1.0536 V. The self-corrosion current density of the joint welded with AlSi12-filler wire (I_{corr–AlSi12}) was 3.3578 × 10⁻⁵ A·cm⁻², and the self-corrosion current density of the joint welded with ZnAl22-filler wire (I_{corr–ZnAl22}) was 1.7668 × 10⁻⁴ A·cm⁻². A smaller self-corrosion current means slower corrosion, while a higher self-corrosion potential indicates stronger corrosion resistance [32]. E_{corr–AlSi12} was greater than E_{corr–ZnAl22}, indicating that the joint welded with ZnAl22-filler wire was more susceptible to self-corrosion behavior. Additionally, the higher self-corrosion current density indicates that the joint welded with AlZn22-filler wire would corrode faster and had poorer corrosion resistance. Therefore, the joints welded using AlSi12-filler wire exhibited superior corrosion resistance.

Figure 9 and Figure 10 present the XPS spectra of the passivation films formed on the surfaces of the joints welded using AlSi12- and ZnAl22-filler wires, respectively. As illustrated in Figure 9a, spectrum peaks corresponding to the elements Fe, O, C, Si, and Al were observed. It can be inferred that the passivation film was mainly constituted by elements Fe, Al, Si, and O. Meanwhile, the spectrum peaks shown in Figure 10a indicate that the passivation film that developed on the surface of the joint welded with the ZnAl22-filler wire was primarily composed of Fe, Al, Zn, and O. The iron profiles displayed in Figure 9b and Figure 10b suggest that Fe²⁺ and Fe³⁺ are the main forms of iron in the passive films. The fine spectra of the Al2p spectrum of both joints are shown in Figure 9b and Figure 10b, where two peaks in the Al2p orbitals can be observed. Al easily reacts with O to form Al2O3, enhancing the corrosion resistance of the Al/steel joint [33,34,35]. Figure 9d and Figure 10d show the fitting results of the Si2p and Zn2p orbitals for the joints welded with Al–Si12 and Zn–Al22-filler wire, respectively. This indicates that silicon oxides and zinc oxides formed in the passive films of different joints. Moreover, in conjunction with the aforementioned results (see Figure 8), these results show that the addition of the Si element is more effective in enhancing corrosion resistance.

4. Conclusions

In this study, the effects of the alloying elements Si and Zn on weld morphology, microstructural constitution, mechanical property, and corrosion resistance were systematically investigated. The main conclusions can be summarized as follows:

(1)

The wetting properties of ZnAl22-filler wire were better than those of AlSi12-filler wire, but the overall aesthetics of the welded joint were not as good as those obtained with AlSi12.

(2)

Both Si and Zn elements can inhibit the formation of IMC. The thickness of the IMC in the joint welded with AlSi12-filler wire was 2.06 μm, while that in the joint welded with ZnAl22-filler wire was 6.32 μm.

(3)

The tensile strength of the joint welded with AlSi12-filler wire was 203 MPa, and the displacement measured 5.8 mm. The joint welded with ZnAl22-filler wire had a tensile strength of 186 MPa, with a displacement measuring 4.5 mm.

(4)

The joint welded with AlSi12-filler wire exhibited better corrosion resistance compared to the joint welded with ZnAl22-filler wire.