Microstructure and Performance of Al-Coating on AZ31 Prepared by Pack-Cementation with Different Heating Methods

Abstract

In this study, Al-containing coatings were prepared on the surface of AZ31 magnesium alloy by pack-cementation through box-type furnace heating (BFH) and induction heating (IH) methods. Phases, microstructure, composition, and performance were characterized by X-ray diffraction technique (XRD), secondary electron imaging (SEI), backscattered electron imaging (BSEI), and energy dispersive spectroscopy (EDS), the Vickers hardness test and potentiodynamic polarisation test, respectively. The results show that the heating method has a significant impact on the phases, microstructure, thickness, and performance of the coatings. Both aluminized layers are relatively flat and dense, and no obvious second phase is observed. The thickness of the aluminized layer of the IH sample is much larger than that of the BFH sample because the diffusion rate of IH is greater than that of BFH. Both aluminized layers are composed of an outermost layer of β-Mg₂Al₃ and an inner layer of γ-Mg₁₇Al₁₂ near the side of the substrate. The evolution of different heating methods is discussed. The microhardness and corrosion behavior of the aluminized coatings were also investigated and discussed. The results indicate that the hardness and corrosion resistance of the IH diffusion sample is better than that of the BFH, and this is related to the content of the intermetallic compound phase.

1. Introduction

Magnesium alloys are the lightest metallic structural materials with high specific strength, good thermal conductivity, good machinability, and high recycling potential. They are widely used in aerospace, the automotive industry, electronic products, and other fields [1,2,3,4]. However, the poor surface performance of magnesium alloy, including low electrode potential and weak corrosion resistance, greatly restricts its application [5,6]. To expand the application of magnesium alloys, surface modification processes such as plasma electrolytic oxidation [7], physical vapor deposition [8], thermal spraying technology [9], chemical conversion coatings [10], and laser surface treatment [11] have been applied to improve the surface properties of magnesium alloy.

Compared with other processes, diffusion of alloying elements on the surface of the material is a series of very effective surface protection methods, which can improve the overall performance of the material’s surface hardness and corrosion resistance without damaging its original performance [12,13,14,15]. Among these methods, pack-cementation is widely applied to the surface diffusion process of metals and alloys due to its simple operation and affordability [16]. For magnesium alloys, the formation of Al–Mg or Zn–Mg intermetallic compounds on the surface of the magnesium alloy substrate can improve the corrosion resistance of the magnesium and the magnesium alloy parts [17]. Among the two compounds, the formation of Al–Mg on the surface of magnesium alloy is a more efficient and practical treatment method [18]. Shigematsu et al. [19] prepared an Al-rich layer on AZ91D using Al powder and ZrO powder as diffusion sources and obtained an Al–Mg intermetallic compound at a temperature of 450 °C; this is considered to be slightly too high, as this temperature will cause the magnesium alloy matrix to become partially melted. Therefore, temperature reduction in magnesium alloy production and improvement in the diffusion efficiency of the coating are the focus of current research.

Induction heating (IH) diffusion is regarded as one of the most promising methods, thanks to its simple application and good reproducibility compared with other processes, such as box-type furnace heating (BFH). Studies have shown that using the molten-salt system (AlCl₃-NaCl) as the diffusion source can effectively reduce the diffusion temperature [18,20]. Therefore, it is meaningful to study the use of IH to further enhance the performance of coatings prepared by the molten-salt system. In this study, in order to improve the surface properties of the magnesium alloy, AlCl₃-NaCl was used as a diffusion source. AZ31 magnesium alloy, as base material, was packed aluminizing through BFH and IH. Then, the microstructures, formation, and properties of the two coatings were investigated, and properties including microhardness and corrosion resistance were studied and compared.

2. Experiments and Methods

2.1. Sample Preparation

AZ31 magnesium alloy with a chemical composition (wt%) of 3.05% Al, 1.05% Zn, 0.42% Mn, 0.04% Si, 0.01% Cu, 0.003% Fe, 0.001% Ni, and Mg in balance was selected as the raw material. Before aluminizing, the base material was cut to a gauge dimension of 25 × 20 × 7 mm. Then, the specimens were ground by SiC papers to 3000 grit and ultrasonically cleaned in acetone solution.

The powder composition for aluminizing was 50% AlCl₃ (molar ratio, and hereafter) as Al feedstock, 50% NaCl as the activator. These two powders form a molten salt system [21]. The diameter of all powders was less than 75 μm. The coating samples were prepared through box-type furnace and induction furnace heating, and named BFH and IH, respectively, based on the different heating methods. Prior to the aluminizing treatment, the mixed powders were thoroughly stirred, then put into a ceramic crucible (30 mL) and sealed with the mixture of refractory and sodium silicate. Then, the aluminizing treatment of both the BFH and IH samples was performed in a heating furnace at 400 °C for 1 h. After pack-cementation, all the samples were naturally cooled to room temperature in the furnace.

2.2. Characterization and Performance Test Methods

The phase components of coatings were identified by a Empyrean Series 2 X-ray diffraction instrument (PANalytical B.V., Almelo, The Netherlands) with Cu Kα radiation. The parameters were chosen as follows: voltage 40 kV, current 0.4 mA, angles range 20~90°, and step size 0.013°. Cross-sectional microstructures and morphologies of tempered and coated samples were observed using secondary electron imaging (SEI) and backscattering electron imaging (BSEI) detectors installed in a Zeiss Sigma HD field emission gun scanning electron microscope (Zeiss, Dresden, Germany). The AZtech Max2 energy dispersive spectroscopy (Oxford Instruments, London, UK), equipped in the FEG-SEM, was employed to characterize the distribution of various elements. Before microstructure characterization, the samples were ground by SiC papers to 3000 grit and then corroded with magnesium alloy etching solution (mixing equal amounts of ethanol, glacial acetic acid, and deionized water, and then adding picric acid until saturated).

The microhardness of the coated samples was determined by a HVS-1000Z automatic location table digital microhardness tester (Shanghai CSOIF Co., Ltd., Shanghai, China) equipped with a quadrangular pyramid indenter, using the parameters of load 0.5 N and a holding time of 10 s. The potentiodynamic polarization curves were tested in a 3.5% NaCl solution, using a Reference 3000 Gamry instrument (Warminster, Pennsyivania, PA, USA) to evaluate the corrosion behavior of the coated samples. Saturated calomel, platinum, and sample were used as the reference electrode, auxiliary electrode, and working electrode, respectively. The dynamic potential range was −1.8 to −1.0 V, and the scanning speed was 2 mV/s.

3. Results and Discussion

3.1. Phase Analysis

The XRD patterns of uncoated and coated samples are shown in Figure 1. The International Diffraction Data Center (ICDD) database was used to determine the phases. The Mg phase can be seen clearly on the XRD pattern of the as-received sample. After aluminizing treatment, the intensity of the diffraction peaks of Mg substrate is significantly reduced compared with the as-received sample. The peaks of Mg₁₇Al₁₂ and Mg₂Al₃ phases are observed both in the BFH and IH samples.

During the diffusion-reaction process, due to the difference in the stability of the concentration maintained between the packed powders and the matrix, the Al atoms with a certain activity of aluminizing agent atoms diffuse into the magnesium matrix [21]. As the diffusion reaction continues, the concentration of Al atoms continues to increase. According to the Al–Mg binary phase diagram [22], γ-Mg₁₇Al₁₂ will first form on the surface after the Al concentration exceeds its solid solubility, 12.7 wt%Al in Mg. When the concentration of Al in the surface layer continues to increase and exceeds 43.2 wt% Al, the β-Mg₂Al₃ phase will form. According to the content of the Al element, it can be judged that the β-Mg₂Al_3, with more Al, is closer to the outside of the coating, whereas the γ-Mg₁₇Al₁₂, with less Al, is closer to the substrate. It can also be seen from Figure 1, that the intensity of the diffraction peaks of Mg has been significantly reduced after aluminizing, which was caused by the recovery recrystallization experienced by the magnesium alloy in the furnace [23].

3.2. Microstructure Characteristics

Figure 2 shows the cross-sectional microstructure of, and distribution of the elements in, the BFH sample. From Figure 2a, it can be seen that the average thickness of the Al-coating is less than 20 µm and the aluminized layer is relatively uniform. No obvious second phases were observed on the coating. The EDS line scan results in Figure 2b are the distribution of Mg and Al in the BFH sample along the arrow direction in Figure 2a. Within the thickness of the aluminized layer, the two elements of Mg and Al always existed together, whereas the concentration of Al atoms decreased along the depth direction, and the content of the Mg element was always higher than that of Al. This indicates that there is a phenomenon of mutual diffusion between the aluminized layer and the matrix.

Figure 2c shows the results of the EDS surface scan in Figure 2a. The element distribution in the aluminized layer and matrix can be observed. It can be seen that the Al-rich phase of the aluminized layer can be divided into two layers, the one closer to the outside is brighter, and the layer closer to the Mg substrate is darker. The results of the point scan in Figure 2d show that the content of the Al element decreases with the depth of the aluminized layer towards the matrix, and the atomic percentage of the Al element in the matrix (point A3) is only 6.03%. By analyzing and comparing the ratio of the amount of Mg and Al element substances in point A1 and point A2, combined with the above-mentioned XRD phase analysis and Al–Mg binary phase diagram [24], it can be judged that point A1 is the Mg₂Al₃ phase, and point A2 is Mg₁₇Al₁₂ phase. This adds further evidence that the aluminized layer of the BFH sample is composed of the outermost β-Mg₂Al₃ layer and the innermost γ-Mg₁₇Al₁₂ layer. Compared with other types of coatings on Mg alloy, such as PEO coatings, the pack-cementation coatings are dense and without the need for a subsequent processing or addition of borosilicate glass to the base electrolyte to seal the pores in the coatings [7].

Figure 3 shows the cross-sectional microstructure of, and distribution of the elements in, the IH sample. It can be seen that there is an obvious diffusion layer on the outside of the AZ31 magnesium alloy matrix. The structure of the aluminized layer is similar to that of the BFH sample. There is no obvious boundary between the aluminized layer and the matrix, which means that the combination of the aluminized layer and the matrix is relatively tight.

The heat source used in IH is of the internal type; therefore, the temperature gradient during Al diffusion is opposite to that of BFH. The temperature of the IF sample surface is always higher than that of the diffusing agent, which is extremely favorable for the generation, adsorption, and diffusion of active Al atoms. It can be seen that the thickness of the aluminized layer of the IH sample is about 35 µm, according to Figure 3a,b, which is much larger than that of the BFH sample. This shows that the diffusion rate of IH is greater than that of BFH. In addition, Figure 3c,d shows that the proportion of the β-Mg₂Al₃ layer in the aluminized layer is greater than that in BFH sample. That is to say, in the IH sample, more β-Mg₂Al₃ is formed, which is due to the higher degree of diffusion alloying in the IA-1 sample [25].

3.3. Formation of the Al–Mg Coatings

Figure 4a shows the thickness of the aluminized layer of the two specimens. The aluminized layer thickness of the IH sample is about 35 μm, which is nearly twice that of the BFH sample (18 μm). The reason for this phenomenon is the heating rate diffusion coefficient difference between IH and BFH [26]. Typically, the diffusion coefficient can be calculated based on diffusion distance, from the equation [27]:

$$\mathrm{D}=\frac{{\mathrm{d}}^2}{4\mathrm{t}}$$

(1)

where D is the diffusion coefficient, d is the diffusion distance (the thickness of Al-coating in this study) and t is the heating time. Thus, the diffusion coefficient of the BFH sample is 3.15 × 10⁻¹² m²/s, whereas that of IH is 5.10 × 10⁻¹² m²/s. The improvement in the diffusion efficiency of the IH sample is possibly related to atom electromigration, because of the electromagnetic effect in the induction heating furnace [26,28].

Figure 5 shows the formation of the aluminized layer structure. In this experiment, the embedded powder formed a molten salt system [29]. At lower temperatures, the high activity of Al in the molten salt helped the formation of aluminum alloy coatings. In addition, in this study, the Al element exists in the form of an active Al ion [21]. For the molten salt system, AlCl₃ will firstly react with NaCl to form NaAlCl₄ [21,29]. Then the Mg matrix may undergo the following reaction with the molten salt to generate active Al atoms.

3Mg + 2NaAlCl₄ → 2Al + 3MgCl₂ + 2NaCl

(2)

According to the thermodynamic data [21], calculation of the Gibbs free energy from the equation (2) above, the variation formula ∆G = −461997 + 5.071t can be derived. Moreover, as the ∆G in this study (at the temperature (t) of 400 ℃) is far less than zero, the reaction of Equation (2) can occur spontaneously. Figure 5a shows the mutual diffusion process of the aluminizing agent powder and AZ31 magnesium alloy matrix. When aluminized at 400 °C, the Al ions are activated and diffuse between the filled Al powder and the magnesium matrix under the action of the chemical concentration gradient.

After the concentration of Al ion in magnesium exceeds the solid solution limit in the magnesium matrix, a layer of γ-Mg₁₇Al₁₂ will be formed (Figure 5b). As the concentration of diffusion Al continues to increase, β-Mg₂Al₃ will be formed in the outmost layer (Figure 5c). The formation process of the IH sample is the same as that of the BFH sample, but the difference is that the IH process has higher diffusion efficiency than that of BFH [26]. Therefore, the thickness of the IH sample aluminized layer is larger and the concentration of Al is higher than that of the BFH sample. Figure 5d shows Al/Mg ratio along the scanning line in the IH and BFH samples. It can be seen that the penetration depth and the concentration within the magnesium matrix are completely different due to the difference in the diffusion coefficient.

3.4. Performance

3.4.1. Microhardness

Figure 6 is the cross-sectional hardness distribution diagram of the samples from different heating methods. As shown in the figure, the hardness value of the BFH aluminized layer is about 94 HV, and the hardness of the IH aluminized layer is about 170 HV. The hardness of the aluminized layer of the two samples is much higher than the hardness value of the matrix (about 57 HV). This is because there are a large number of Al–Mg intermetallic compounds in the diffusion layer, and the Mg–Al intermetallic compound is composed of strong covalent bonds instead of metal bonds, which can significantly increase the hardness of the diffusion layer [30,31]. The hardness value is determined mainly by the kind and content of the intermetallic compound phase. The hardness of the aluminized layer of the IH sample is much higher than that of the BFH sample because a higher Al concentration leads to a higher content of β-Mg₂Al₃.

3.4.2. Polarization Test

Figure 7 shows the dynamic potential polarization curves of AZ31 magnesium alloy before and after aluminizing. It can be seen that the polarization behavior of the alloy changes greatly. The polarization curve is composed of cathode hydrogen evolution and anode solution. The anode curve after aluminum plating has no obvious passivation. Since the self-corrosion potentials of Mg₂Al₃ and Mg₁₇Al₁₂ are higher than that of the Mg matrix [32], after aluminizing treatment, the self-corrosion potentials all became more positive, indicating that aluminizing treatment has a certain inhibitory effect on the anode reaction [33,34].

Table 1. Corrosion potential (Ecorr), corrosion current density (Icorr), and average corrosion rate (CR).

Samples	Ecorr (v)	Icorr (A/cm2)	CR (mm/y)
As-received	−1.42	2.63 × 10−4	5.7654
IH	−1.37	6.00 × 10−6	0.1315
BHF	−1.38	4.24 × 10−5	0.9689

shows the corresponding corrosion potential, corrosion current density, and corrosion rate of the samples in Figure 7, where the corrosion rate (CR) is calculated by Faraday’s equation [35].

The current corrosion data table shows that the self-corrosion current of the two samples has been reduced by more than an order of magnitude, and that the CR is significantly reduced, indicating that the corrosion resistance after aluminizing has been improved. Studies have shown that the corrosion resistance, in a sequence from high to low, proceeds from Mg₂Al₃, to Mg₁₇Al₁₂, and then matrix, so the presence of Mg₁₇Al₁₂ and Mg₂Al₃ can greatly delay and prevent the interaction between solution and matrix, and improve the corrosion resistance of magnesium alloy in Cl- environments [30]. This further illustrates that the Al–Mg coating can improve the corrosion resistance of AZ31 magnesium alloy, whereas the corrosion resistance of the IH sample is higher than that of the BFH sample.

4. Conclusions

Aluminized coatings were prepared on the surface of AZ31 magnesium alloy by pack-cementation with different heating methods. The phase composition, microstructure, performance, and formation were studied and analyzed. The main conclusions are as follows:

• Both aluminized layers are relatively flat and dense, and no obvious second phase is observed. The coating is composed of the outermost β-Mg₂Al₃ layer and the γ-Mg₁₇Al₁₂ layer near the inner side of the substrate.
• The diffusion efficiency of IH aluminizing is significantly higher than that of the BFH, and the coating thickness of the IH sample is twice that of the BFH sample.
• Both coatings can effectively improve the surface hardness and corrosion resistance of the magnesium alloy. The hardness and corrosion resistance of the IH diffusion sample is better than that of the BFH, which is due to the content of the intermetallic compound phase.