Microstructure and Mechanical Properties of 4343/3003/6111/3003 Four-Layer Al Clad Sheets Subjected to Different Conditions

Abstract

To meet the lightweight demands of automobiles, Al composite sheets require excellent mechanical properties under the condition of minimal thickness after high-temperature brazing processing. Generally, the standard used Al brazing sheets have a low strength before and after brazing. To overcome this issue, this work develops a novel 4343/3003/6111/3003 four-layer Al clad sheet, whose microstructure and mechanical properties are systematically investigated. The results show the observable fibrous microstructure with elongated grains parallel to the rolling direction in the developed four-layer Al clad sheet of the cold-rolled and annealed states. After brazing, this fibrous microstructure transforms into coarse equiaxed grains. In addition, the 4343 layer is the brazing layer. Si is mainly distributed in the 4343 layer of the cold-rolled Al clad sheets, whereas Si penetrates into the core layer along the grain boundaries after brazing. The cold-rolled samples present a certain brittleness from fracture morphology, whereas the final annealed ones show a typical ductile fracture. Meanwhile, the typical intergranular fracture is visible after brazing. The mechanical properties of the Al clad sheets are improved after brazing, with an increase of 76% in tensile strength and 62% in yield strength, compared with the final annealed ones. The elongation is increased by 29% compared with that of the cold-rolled ones. This study provides a theoretical basis for further improvement of the strength of aluminum honeycomb panels.

1. Introduction

Multi-layer aluminum brazing sheets are generally composed of a support layer (core layer) and a brazing layer (clad layer), and they have been widely used in the automotive industry due to their advantages of low density, a high thermal conductivity, good brazeability, and affordability [1,2,3]. For example, heat exchangers in automotive manufacturing are commonly fabricated by bonding multi-layer aluminum brazing sheets and aluminum tubes [4]. The lightweight trend of automobiles calls for a decrease in the thickness of aluminum brazing sheets [5,6,7,8,9]. However, the thinner aluminum brazing sheets are prone to collapse during brazing. A large number of studies have been conducted to improve the high-temperature deformation resistance of aluminum brazing sheets by increasing the sagging resistance and some other mechanisms. Zhao et al. [10] studied the effects of the annealing process on the Si diffusion and the sagging resistance of 4343/3003/4343 three-layer Al clad sheet. The results showed that Si diffusion was the major factor influencing the sagging resistance. Lee and his colleagues [11,12] investigated the sagging behavior of three-layer Al clad sheets, revealing that the sagging resistance of the Al clad sheet was determined by both the intermediate annealing temperature and the reduction rate of final cold rolling. Kim et al. [13] and Tu et al. [14] found that coarse grains could significantly reduce the number of grain boundaries, thereby improving the sagging resistance. Shin et al. [15] found that the addition of Ti could improve the sagging resistance of 4343/3003/4343 aluminum alloy clad sheets as the core layer strengthening effect was greater than the softening effect during the brazing process.

Many studies have focused on the sagging resistance and the related mechanisms of three-layer Al composite sheets. However, a significant increase in the bearing capacity of Al composite sheets is required to avoid brazing-induced collapse deformation. In addition, Al brazing sheets are often subjected to specific loads. Therefore, there is an urgent need to clarify the variations in the strength of the Al brazing sheets before and after brazing.

The strength of three-layer Al brazing sheets is typically quite low regardless of the brazing. Recently, a novel 4343/3003/6111/3003 four-layer Al clad sheet has been developed with a four-layer structure comprising a clad layer (AISI 4343 alloy), two protective layers (AISI 3003 alloy), and a core layer (6111 alloy). The microstructural and mechanical properties of each layer are closely related to the strength of the brazed Al composite sheets. In this architecture, the 4343 alloy and 6111 alloy act as the solder metal and the support layer, respectively. The outer 3003 alloy prevents the core layer from corrosion, restricting the diffusion of Si particles to the core materials during the brazing. As a consequence, the strength of this four-layer Al brazing sheet (4343/3003/6111/3003) is increased by 50% relative to the three-layer Al brazing sheet. However, the effects of the processing conditions on the mechanical properties and microstructure of the 4343/3003/6111/3003 four-layer Al clad sheet have yet to be studied systematically.

In this work, the Si diffusion behavior, fracture morphology, and strength of 4343/3003/6111/3003 Al clad sheets before and after brazing are investigated by tensile test, optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) observations. The effects of the microstructure, especially the secondary phase and Si diffusion, on the strength are discussed before and after brazing. Overall, this study provides a theoretical basis for the application of the 4343/3003/6111/3003 four-layer Al composite clad sheet.

2. Materials and Methods

2.1. Materials Preparation

Cold-rolled 4343/3003/6111/3003 four-layer Al clad sheet was manufactured by China Northeast Light Alloy Corporation. The chemical composition of the clad sheet is listed in

Table 1. Chemical composition of 4343/3003/6111/3003 Al clad sheet (wt.%).

Material	Mass Fraction
	Si	Fe	Cu	Mn	Mg	Zn	Ti	Al
4343	8.0	0.25	0.05	0.05	0.03	-	-	Bal.
3003	0.6	0.7	0.10	1.2	-	0.10	-	Bal.
6111	1.1	<0.4	0.9	0.3	0.8	<0.15	<0.10	Bal.

. A schematic illustration of the four-layer Al composite sheet is given in Figure 1. The cladding ratio (4343/3003/6111/3003) is 10%, 5%, 77% and 8% for 4343, 3003, 6111 and 3003 Al alloy, respectively. During the brazing process, the clad layer melts and forms a joint upon cooling and solidification at the connection of the radiating fin to the Al tube [16].

2.2. Experimental Procedure

Annealing experiments unveiled that the strength of the developed 6111 alloy is the highest under the final annealing process at 330 °C/3 h, reaching 190 MPa. After brazing, the strength of the 6111 alloy treated by further aging strengthening at 180 °C/8 h can reach 326 Mpa. The schematic illustration of the experiment is shown in Figure 2. Firstly, the cold-rolled 4343/3003/6111/3003 Al clad sheet was annealed for 3 h at 330 °C (annealed state), then held at 610 °C for 10 min to simulate the brazing environment (in nitrogen). Simulated brazing refers to the brazing heat treatment, which simulates the actual brazing process conditions. The purpose is to detect whether the alloy can meet the performance requirements of actual brazing after simulated brazing. The simulated brazing experiment was carried out in a resistance furnace (Shangcheng Instrument, Shaoxing, China), which had been held at 610 °C for 20 min to minimize the temperature change. After simulated brazing, the furnace door was immediately opened to cool down to 400 °C, followed by an air cooling outside the stove to obtain the brazed state specimens. Finally, the brazed samples were put into a DHG-9070A electrothermal blast drying oven (Lichen Technology Instrument Co., Ltd., Shanghai, China) and aged at 180 °C for 8 h (aged state).

2.3. Test Method

According to GB/T228-2002, the cold-rolled Al clad sheet was made into tensile specimens of the shape and size as shown in Figure 3. The yield strength, tensile strength, and elongation of tensile samples under different processing conditions were tested on an electronic universal testing machine (CSS-44100) (Jinan Jindier Experimental Machine Co., Ltd., China) at a rate of 0.57 mm/min at room temperature. Three specimens were prepared for each experiment. Finally, the average value was taken to represent the tensile property value.

Metallographic samples with a length of 10 mm (RD) were cut from specimens in four different states and inlaid into a cylindrical shape. The observation surface was an RD-ND cross-section. Samples were grounded mechanically with the SiC sandpaper and then polished with a 0.5 μm diamond polish and soapy water. Then, the pieces were electro-polished (at 12 V/0.2 mA) with a perchloric acid + alcohol (1:9). Microstructures in different conditions were observed with an OLYMPUS BX51M optical microscopy (OM) (Olympus Corporation of Japan) and a Zeiss EVO MA10 scanning electron microscope (SEM) (Karl Zeiss Company, Germany), equipped with an Oxford X-MaxN energy dispersive spectroscope (EDS). Transmission electron microscopy (TEM) was performed on TecnaiG220 (FEI, Hillsboro, OR, USA) with an acceleration voltage of 200 kV. The grounded sample’s thickness of about 80 µm was punched into a small wafer with a diameter of 3 mm, and the burrs were gently grounded away with metallographic sandpaper, followed by electropolishing in a solution containing 30% nitric acid and 70% methanol between −20 °C to −35 °C in liquid nitrogen. After the double spraying, the samples were washed with ethanol and then dried to remove the dual spraying candle solution.

3. Results and Discussion

3.1. Microstructures under the Different States of 4343/3003/6111/3003 Al Clad Sheets

Figure 4 shows the microstructure of the Al clad sheet before brazing, indicating many Si particles distributed in the clad layer. A representative SEM image of the specimen is given in Figure 5. Microstructures of 4343/3003/6111/3003 Al clad sheets in different conditions are shown in Figure 6. As indicated in Figure 6a,b, a fibrous structure with elongated grains parallel to the rolling direction is observed on the aluminum clad sheet in the cold-rolled and annealed state, and the interface between each alloy layer is discernible. The elongated grain increases in the annealed state (as shown in Figure 6b). Figure 6c,d reveals the microstructure of the Al clad sheet in the brazed state, coarse equiaxed grains formed in the core layer, with an average size of 110 ± 10 μm. The interface between the 4343 and 3003 layers is clearly distinguishable.

SEM images of 4343/3003/6111/3003 clad sheets under different conditions are shown in Figure 7. In Figure 7a,b, the boundary between the 3003 and 6111 layers can be distinguished. A mass of dispersed particles can be observed in the 6111 layer, as shown in Figure 7a. After annealing, the distribution of dispersed particles becomes more uniform in Figure 7b. As shown in Figure 5c and Figure 7c, after 10 min of simulated brazing at 610 °C, the fiber microstructure disappears and gradually converts to the recrystallized microstructure. At this time, Si diffuses along the grain boundary and gathers into short strips as indicated by the red arrow in Figure 7c,d. After the aging treatment (as shown in Figure 7d), the amount of dispersed Si is decreased.

3.2. Evolution of the Secondary Phase in the Core Layer

In Al-Mg-Si-Cu alloys with excess Si, the precipitation follows the sequence [17]:

SSSS → Atomic clusters → GP zones → β″ → β′, Q′→ Q, β, Si

The strengthening phases of Al-Mg-Si-Cu Al alloy are mainly Mg₂Si and AlMgSi (Cu) [18]. Figure 8 shows the SEM images of the Al clad sheet in the cold-rolled state. Combined with the core layer element mappings and EDS spectral analysis, it can be inferred that the white particles are AlFeSi(Mn,Cu) phase (blue box in Figure 8a) and the other black particles are Mg₂Si phase (red box in Figure 8a). They are mainly distributed on or near the grain boundaries. As shown in Figure 7a and Figure 8, the average size of the AlFeSi (Mn,Cu) dispersed phase is about 4.1 μm, which inhibits recrystallization and becomes the nucleation core of strengthening phase precipitation. The Mg₂Si phase, with an average length of about 400 nm, is the main strengthening phase. For the microstructure of the core layer in the annealed state (Figure 7b), it is found that the number of AlFeSi (Mn,Cu) dispersed phase particles increases, and the average particle size is 5.8 μm with a rise of 41% compared with that of the cold-rolled sheet (Figure 7a,b). As shown in Figure 7b, the black Mg₂Si phase decreases, indicating that Mg₂Si particles are partially dissolved into the aluminum matrix during final annealing.

As shown in Figure 9, before the TEM characterization, the brazed and aged test samples were obtained in the red box (core layer). TEM-EDX analysis of precipitates from Figure 10b,c corresponds to the red arrows 1 and 2 in Figure 10a, respectively. One can see that there exist two phases in the brazing process, namely the Mg₂Si (needle-like phase) and AlMgSiCu phase (lath-like Q′ phase). The average length of Mg₂Si is about 500 nm, and the length for the Q′ phase is about 350 nm. After the aging treatment, the enhanced phase gradually evolved into the Q′ phase (Figure 11). Figure 12 shows the evolution of strengthening phases in the core (6111) of Al clad sheets at different states. There are mainly needle-like Mg₂Si strengthening phases at the cold-rolled state. After annealing, the needle-like Mg₂Si phase becomes larger. There are two strengthening phases after brazing, namely the Mg₂Si phase and Q′(AlMgSiCu) phase. After aging, the Mg₂Si phase gradually evolves into the Q′ phase.

Ding et al. [19] discussed the composition of Al, Mg, Si, and Cu forming the Q phase. The composition of Al₃Cu₂Mg₉Si₇ (x = 3) is widely accepted [19,20]. Phase particles nucleate and grow in the matrix in the form of a strip or rod. Some phase particles aggregate at grain boundaries [21,22,23,24,25]. The Q phase exists widely in 2xxx series Al-Cu-Mg-Si alloy and 6xxx series Al-Mg-Si-Cu alloy. After heat treatment, the Q phase is formed, significantly improving the aging strength of alloys [20,25,26]. In the condition of the aged state, it is indicated that the strengthening phase gradually transforms into the Q′ phase.

The core material (6111) of the four-layer composite sheet accounts for about 77 % of the matrix, which plays a significant supporting role in the composite aluminum foil. It is necessary to have a high strength to improve the collapse resistance in the condition of high temperature. The Cu content in the 6111 alloy ranges from 0.5 to 1.0. Q′ phases forming in Cu-containing alloy have better high temperature softening resistance and higher thermal stability [25]. As exhibited in

Table 2. Mechanical properties of 4343/3003/6111/3003 Al clad sheet in different conditions.

Specimen	Elongation (%)	Yield Strength (Mpa)	Tensile Strength (Mpa)
cold-rolled state	17	152	214
annealed state	31	59	131
brazed state	22	103	211
aged state	20	117	235
without brazing	33	55	116

, the tensile strength and yield strength improve after brazing due to the formation of the Q′ phases.

3.3. Evolution of Si Diffusion in Different Conditions

The results from SEM and elemental mapping analysis of the 4343/3003/6111/3003 clad sheet in different conditions are shown in Figure 13, and the distribution of Si in each layer before and after brazing can be observed. Some white secondary phases in the form of points before brazing can be seen, and they gradually become some shot lines along the grain boundary after brazing. The cold-rolled and annealed samples before brazing are shown in Figure 13(a1,b1), and Si is mainly distributed in the 4343 layer. In addition, Si has a tendency to diffuse at the annealed state (see Figure 13(b1)). As shown in Figure 13(c1), Si in the 4343 layer penetrates into the core layer along the grain boundary after brazing, so that Si at the grain boundary of the core layer tends to be connected.

The annealed samples are fibrous (Figure 6b), without recrystallization in the recovery stage. Because the stacking fault energy of aluminum is high, the work hardening is reduced, and the plasticity is improved in the recovery stage. After being brazed, the subgrains of the samples disappeared, and only the intact grain boundaries with high angles existed (Figure 6c,d). The overall grain boundary area in the material is reduced, which reduces the diffusion channel of solder elements and the sliding interface. The number of secondary phase-distributed grain boundaries increases, which strengthens the pinning effect of the secondary phase on grain boundaries. Thus, the slip deformation along the grain boundaries becomes difficult, and the strength of the material is improved.

3.4. Mechanical Properties and Fracture Morphology of 4343/3003/6111/3003 Al Clad Sheets

The measured yield strength, tensile strength, and elongation are shown in Table 2. It can be seen that the elongation of the cold-rolled is relatively lower, and the tensile strength and yield strength are relatively higher. In the condition of the annealed state, the tensile strength and yield strength decrease. In contrast, the elongation increases by about 82% compared with the cold-rolled sample. After brazing, the tensile strength and yield strength increase by 76% and 62%, respectively, compared with the annealed samples. The elongation is higher compared with that of the cold-rolled sample. Only after final annealing and aging (without brazing), the tensile strength and yield strength of the sample are the lowest, as shown in the following table.

Figure 14 shows the fracture morphology of the 4343/3003/ 6111/3003 Al cad sheet in different states. The fracture morphology of the four-layer Al composite sheet before and after brazing is quite different. As shown in Figure 14a, the fracture morphology of the cold-rolled sample is relatively brighter, with shallow dimples distributed sporadically. In addition, there exist some “<” shaped tearing edges, as well as apparent cracks in the fracture morphology resulting in a brittle state. Figure 14b shows flat, dense, and deeper dimples in the fracture morphology of the annealed samples, indicating a typical ductile fracture. The number of dimples for the annealed samples (Figure 14b) is much more than that of the cold-rolled samples (Figure 14a). This phenomenon is consistent with the results in Table 2. In the cold-rolled state, due to the dislocation strengthening of materials, more stress concentration appears, indicating more dislocation. In the tensile process, dislocation pile-up groups are formed at grain boundaries, phase boundaries, and defects, which induce the initiation and growth of micropores at the stress concentration [27]. In the annealed state, the stress concentration and dislocation density are decreased, and dislocation accumulation groups are reduced. Therefore, dislocation strengthening is weakened.

As shown in Figure 14c,d, the tensile fractograph of the brazed and aged specimens presents a typical intergranular fracture with rock sugar shapes, and the grain boundary interfaces are smooth surfaces. After brazing, complete recrystallization occurs. Si of the clad layer diffuses along the grain boundaries of recrystallized grains during brazing. Therefore, the grain boundary is fragile and intergranular fracture occurs. The crack length in Figure 14d is longer than that in Figure 14c, which has severe brittleness and lower elongation. During brazing, the filler alloy 4343 of Si diffuses to the core material along the grain boundary, and a new Al-Si eutectic is formed at the grain boundary. Under the action of external forces, these Al-Si eutectics will directly bear loads, which is easy to fracture and form cracks, so these cracks propagate along the grain boundary, resulting in the fracture of materials. The Si of the filler alloy diffuses to the core, leading to an increase of Si content in the core and a decrease of high-temperature strength. However, coarse recrystallized grains and dispersed phases in the core are conducive to improving the high-temperature strength of Al clad sheets. According to the tensile test data (see Table 2), the tensile strength and yield strength of the specimen after brazing have been significantly improved. This implies that the effect of Si erosion on the four-layer composite sheet is less than that of the coarse recrystallized grain and dispersion phases.

4. Conclusions

Based on our work, we can make the following conclusions:

(1)

A fibrous structure with elongated grains parallel to the rolling direction is observed on the cold-rolled Al clad sheet and annealed state. After brazing, fiber microstructure disappears and gradually converts to recrystallized microstructure. The microstructure of the Al clad sheet of the core layer is coarse equiaxed crystal, with an average size of 110 ± 10 μm.

(2)

Before brazing, the tensile strength and yield strength of Al Clad Sheets are 1301 and 59 MPa, respectively, and the elongation reaches 31%. After brazing, the corresponding values are 211 MPa, 103 MPa, and 22%, respectively. The mechanical properties and elongation before and after brazing meet the requirements of the material.

(3)

The number of dimples of annealed samples is much more than that of cold-rolled models, and they are relatively deeper, indicating that the annealed sample has better toughness. The fracture morphology of the brazed and aged samples is a typical intergranular fracture, and the cracks of the aged state are longer and more brittle than those of the brazed state.

(4)

The Si of the filler alloy diffuses to the core, resulting in an increase in the Si content of the core and a decrease in the high-temperature strength. The tensile strength and yield strength of the specimen after brazing have significantly been improved. This implies that the effect of Si erosion on the four-layer composite sheet is less than that of the coarse recrystallized grain and dispersion phases.