Novel multilayer Mg–Al intermetallic coating for corrosion protection of magnesium alloy by molten salts treatment

A novel multilayer Mg–Al intermetallic coating on the magnesium alloy was obtained by AlCl₃–NaCl molten salt bath treatment. The molten salt was treated at 400 °C, which is lower than the treatment temperature of solid diffusion Al powder. The thick Al₁₂Mg₁₇, Al_0.58Mg_0.42 and Al₃Mg₂ multilayer Mg–Al intermetallic coating forms on the magnesium alloy. The corrosion resistance of AZ91D alloy with and without coating by multilayer of Mg-Al intermetallic compound was evaluated by electrochemical impedance spectroscopy measurements in 3.5% (mass fraction) NaCl solution. The polarization resistance value of the multilayer coating on the magnesium alloy by molten salt bath treatment is greater than that of the uncoated one, which is attributed to the homogenously distributed intermetallic phases.     

Corrosion properties of surface-modified AZ91D magnesium alloy

Samples of AZ91D magnesium alloy were dipped into AlCl₃–NaCl molten salt at different temperatures between 250 °C and 400 °C for 28800 s. The thickness of the alloying layer is increased with the rise of the treatment temperatures. The coating was mainly composed of Al₁₂Mg₁₇ and Al₃Mg₂ intermetallic compounds. The corrosion resistance of the coating which is obtained at 300 °C for 28800 s is the best. When the treatment temperature is higher than 300 °C, some cracks developed in the alloying layers. The cracks were resulted from the thermal stress due to the different thermal expansion coefficient of the AZ91D substrate and the alloying coating during the rapid cooling process.     

Effect of heat treatment on the intermetallic layer of cold sprayed aluminum coatings on magnesium alloy

Dense and thick pure aluminum coatings were deposited on AZ91D-T4 magnesium substrates using the cold spray process. Heat treatments of the as-sprayed samples were carried out at 400 °C using different holding times. The feedstock powder, substrate and coating microstructures were examined using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) as well as Vickers microhardness analysis. The results demonstrate that aluminum coatings having dense and uniform microstructure can be deposited successfully using a relatively large feedstock powder. It has been identified that the intermetallics Al₃Mg₂ (γ phase) and Mg₁₇Al₁₂ (β phase) were formed at the coating/substrate interface during heat treatment. The growth rate of these intermetallics follows the parabolic law and the γ phase has a higher growth rate than the β phase. The thickness of the Mg₁₇Al₁₂ and Al₃Mg₂ intermetallic layers has reached 83 μm and 149 μm, respectively. This result is almost 45% higher than what has been reported in the literature so far. This is attributed to the fact that T4 instead of as cast Mg alloy was used as substrate. In the T4 state, the Al concentration in the Mg matrix is higher, and thus intermetallic growth is faster as less enrichment is required to reach the critical level for intermetallic formation in the substrate. The AZ91D-T4 magnesium substrate contains single α phase with fine clusters/GP-zones which is considered beneficial for the intermetallic formation as well as the intimate contact between the coating/substrate interface and the deformed particles within the coating.     

Influence of microstructure on corrosion properties of multilayer Mg–Al intermetallic compound coating

A multilayer Mg–Al intermetallic coating was fabricated on the Mg alloy in molten salts at 400 °C with treatment time range from 2 to 8 h. The coating consists of a single Al₁₂Mg₁₇ intermetallic layer or Al₁₂Mg₁₇, Al_0.58Mg_0.42 and Al₃Mg₂ intermetallic layers. The corrosion resistance of the coating which is obtained at 400 °C for 2 h is the best. When the treatment time is higher than 2 h, some cracks developed in the layers. The cracks were resulted from the thermal stress due to the different thermal expansion coefficient of the substrate and the intermetallic layer during the rapid cooling process.     

Corrosion behaviour of magnesium/aluminium alloys in 3.5 wt.% NaCl

Corrosion behaviour of commercial magnesium/aluminium alloys (AZ31, AZ80 and AZ91D) was investigated by electrochemical and gravimetric tests in 3.5 wt.% NaCl at 25 °C. Corrosion products were analysed by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. Corrosion damage was mainly caused by formation of a Mg(OH)₂ corrosion layer. AZ80 and AZ91D alloys revealed the highest corrosion resistance. The relatively fine β-phase (Mg₁₇Al₁₂) network and the aluminium enrichment produced on the corroded surface were the key factors limiting progression of the corrosion attack. Preferential attack was located at the matrix/β-phase and matrix/MnAl intermetallic compounds interfaces.     

Broad-beam laser cladding of Al-Si alloy coating on AZ91HP magnesium alloy

In the present study, an attempt was made to improve the wear resistance and the corrosion resistance of AZ91HP magnesium alloy by laser cladding Al-Si eutectic alloy. The results showed that the clad layer mainly consisted of Mg₂Si, Mg₁₇Al₁₂ and Mg₂Al₃ phases. The microstructure of the bonding zone changed from columnar grains to equiaxial grains along the direction of heat-flow. The heat-affected zone consisted of α-Mg and α-Mg + β-Mg₁₇Al₁₂ eutectic. The formation of multiple Mg intermetallic compounds allowed the clad layer to exhibit higher hardness, better wear resistance and corrosion resistance.     

Microstructure and corrosion behaviour of Al2O3–13 wt-% TiO2 laser alloyed magnesium alloys

Nanostructured Al₂O₃–13?wt-% TiO₂ was prepared on AZ91D magnesium alloy surface by laser surface alloying to improve its corrosion resistance. The microstructure of the laser surface alloyed specimens before and after corrosion tests was characterised by scanning electron microscope and optical microscope (OM). The phase and element composition were investigated by X-ray diffractometer and energy-dispersive spectrometry. An electrochemical workstation was used to evaluate the corrosion behaviour of the specimens. Results showed that the laser surface alloyed layer was primarily composed of Mg and Mg₁₇Al₁₂. Al₂O₃ and TiO₂ existed in the form of agglomerated particles. The corrosion resistance was improved after laser surface alloying.     

Heat-treatment induced material property variations of Al-coated Mg alloy prepared in aluminum chloride/1-ethyl-3-methylimidazolium chloride ionic liquid

An Al coating film, electrodeposited on a Mg alloy from aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl₃-EMIC) ionic liquid, effectively prevents the substrate from rapid corrosion in a hostile environment. The thickness of the Al film can be easily determined by controlling the total cathodic charge applied, because the current efficiency of the electrodeposition reaction is close to 100%. Heat treatment at 450 °C under an argon atmosphere for 10 min causes an inter-diffusion at the Al/Mg interface, optimizing the protective performance of the coating film. Prolonging heating leads to a Mg₁₇Al₁₂ intermetallic phase and a Mg solid solution phase to be formed at the expense of the deposited Al film. This phase transformation gives rise to a degradation in the corrosion resistance of the Al-coated sample.     

Effect of cerium addition on microstructure and corrosion resistance of die cast AZ91 magnesium alloy

A novel AZ91 Ce containing magnesium alloy characterized by excellent corrosion resistance is fabricated by adding rare earth Ce (cerium) in the form of a Mg‐Ce master alloy. The metallographic investigation shows that Ce added to AZ91 can obviously decrease the size of β‐Mg₁₇Al₁₂ and forms Al₁₁Ce₃ intermetallic compounds in the shape of fine needles. The corrosion tests and electrochemical measurements indicate that the corrosion resistance of AZ91 Ce containing magnesium alloy is obviously higher than that of AZ91. Furthermore, increasing the content of Ce in the magnesium alloy can further enhance the corrosion resistance. X‐ray photoelectron spectroscopy (XPS) reveals that Ce can be incorporated into corrosion products in the form of CeO₂ in the course of corrosion. Based on the preliminary analysis, the addition of Ce can improve the corrosion resistance of AZ91 by decreasing the size of β‐Mg₁₇Al₁₂ and enhancing the protective effectiveness of corrosion products.     

Laser cladding of eutectic-based Ti-Ni-Al alloy coating on magnesium surface

A Ti_70.3Ni_22.2Al_7.5 alloy, optimized from a basic binary eutectic Ti₇₆Ni₂₄ alloyed with different amounts of Al, was laser-clad on AZ91HP magnesium alloy. The coating mainly consists of β-Ti solid solution and Ti₂Ni intermetallic compound resulting in high hardness, good wear resistance and corrosion resistance. The interface between the clad layer and the substrate has a good metallurgical bonding.     

AZ31B镁合金表面电镀铝锰合金的耐蚀性

在镁合金AZ31B表面通过预镀锌处理后采用无机熔盐电沉积铝锰合金。使用SEM、EDX和XRD分析镀层的表面形貌、成分和组织,采用动电位极化曲线及表面显微硬度测量考察了镀层对镁合金耐蚀耐磨性的影响。结果表明,熔盐成分、电流密度和熔体温度等典型工艺参数对铝锰合金镀层的形貌、成分和组织都具有重要的影响,进而影响了镀层的耐蚀性。镁合金电镀铝锰合金后,腐蚀电位有很大的提高, 而腐蚀电流密度大幅度的下降;同时铝锰合金镀层表现出很高的硬度,显著的提高了镁合金的耐蚀耐磨性。     

Surface alloying of AZ91E alloy by Al–Zn packed powder diffusion coating

A new surface coating technique, namely packed powder diffusion coating (PPDC), for AZ91E magnesium alloy is reported. This new technique uses a powder mixture of aluminium and zinc as diffusion source and produces uniform and thick coatings at temperatures below 420 °C. Experimental results showed that zinc in the powder mixture significantly promotes the formation of intermetallic layers on the surface of the magnesium alloy at process temperatures between 350 °C and 413 °C, which is more than 50 °C lower than the previously reported processes. Depending on the temperature and the Zn-content in the powder, X-ray diffraction analysis identified three intermetallic phases and Mg(Al, Zn) solid solution that consist of the surface alloyed layer. The intermetallic compounds are τ-Mg₃₂(Al,Zn)₄₉, φ-Al₅Mg₁₁Zn₄ and β-Mg₁₇(Al,Zn)₁₂. The hardness of the over 500 μm thick surface alloyed layers is up to four times higher than the substrate. Both the β-Mg₁₇(Al,Zn)₁₂ phase and the τ-Mg₃₂(Al,Zn)₄₉ phase show one to two order magnitude higher corrosion resistance than the α-phase (solid solution) and the φ-Al₅Mg₁₁Zn₄ phase in 5% NaCl solution. A process parameter window for the layer thickness as well as a schematic model for the formation of the layer is proposed. The PPDC process is a promising technique that provides effective protection of AZ91E alloy from both wear and corrosion.     

镁合金表面新颖多层的耐腐蚀Mg-Al金属间化合物涂层(英文)

通过AlCl3-NaCl熔盐扩散表面改性,在镁合金表面制备多层的Mg-Al金属间化合物层。熔盐扩散的处理温度为400 °C,此温度低于纯铝粉扩散的处理温度。熔盐扩散处理后,在镁合金表面形成 Al12Mg17、Al0.58Mg0.42和Al3Mg2多相层的金属间化合物涂层。通过电化学阻抗实验将表面改性和未经表面改性的镁合金的耐蚀性进行比较,发现镁合金表面经过熔盐扩散处理的极化电阻远大于未经表面改性镁合金的。这是因为在镁合金表面形成均匀的多相金属间化合物层。     

Hot rolling characteristics of spray-formed AZ91 magnesium alloy

AZ91 magnesium alloy was prepared by spray forming. The spray-deposited alloy was subsequently hot-rolled with a 80% reduction at 350℃. The microstructural features of the as-spray-deposited and hot-rolled alloy were examined by optical microscopy, scanning electron microscopy and X-ray diffractometry. The results show that the spray-formed AZ91 magnesium alloy has, compared with the as-cast ingot, a finer microstructure with less interrnetallic phase Mg17Al12 dispersed in the matrix due to fast cooling and solidification rates of spray forming process, and, therefore showing excellent workability. It can be hot-rolled with nearly 20% reduction for one pass at lower temperatures （330-360℃）, and the total reduction can reach 50% prior to annealing. After proper thermo-mechanical treatment, the spray-formed AZ91 magnesium alloy exhibits outstanding mechanical properties.     

Effects of low temperature thermal treatment on zinc and/or tin plated coatings of AZ91D magnesium alloy

A new method was investigated to obtain composite coatings on the AZ91D magnesium alloy by electrodeposition and low temperature thermal treatment. Zinc and tin were introduced to AZ91D Mg alloy surface by electroplating firstly. And a succedent thermal treatment was carried out at 190 ± 10 °C for 12 h. The surface and cross-section morphologies of the plated coatings with and without thermal treatment were studied by scanning electron microscopy (SEM). And the microstructure was determined by X-ray diffraction (XRD). The results reveal that it was difficult to obtain good adhesion plated Sn coating but easy to get well-adherent plated Zn coating. And the thermal treatment promoted the formation of Mg₂Sn in the plated Sn coating and the recrystallization in the plated Zn coating. The plated double Zn-Sn coating owned good adhesion and uniform surface. Furthermore, when the plated double Zn-Sn coating was treated at 190 ± 10 °C for 12 h, a three-layer structure coating was formed due to the diffusion of tin. The results of the anodic polarization behaviors in 5 wt.% NaCl solution show that the three-layer structure coating could provide better protection for AZ91D substrate than the plated Zn-Sn coating.     

Mg/Al脉冲加压扩散连接接头显微组织及力学性能

采用脉冲加压扩散连接工艺,实现了AZ31镁合金与5083铝合金的连接.借助扫描电镜、EDS、X射线衍射仪和显微硬度计等手段对接头的显微组织及力学性能进行了研究.结果表明,接头有镁合金基体、冶金反应层、扩散层和铝合金基体组成.焊缝中形成了Mg₂Al₃,AlMg和Al_0.56Mg_0.44金属间化合物.接头最高硬度值达3300 MPa.随着保温扩散时间的延长,接头的抗拉强度出现了先升高后降低的现象,最高接头强度达46 MPa,在断口中发现了部分韧窝,断口属于韧性和准解理混合断口.在镁合金和铝合金两侧,硬度变化区域出现不对称现象.     

AZ91镁合金的挤压和析出硬化行为(英文)

挤压比为4:1,将铸态AZ91镁合金分别在250,300和350℃下进行挤压,随后进行析出硬化处理(T6)。经过热挤压和析出硬化处理后,铸态AZ91镁合金中粗大的和偏析Mg17Al12析出相被细化并均匀分布在α-镁基体中。在不同的挤压温度下合金中发生了部分或全部动态再结晶。经挤压后,该合金的极限抗拉强度从铸态的190MPa增加到570MPa。AZ91镁合金的时效硬化特征与晶粒尺寸有关。在250、300和350℃下以4:1的挤压比挤压该合金后,获得峰值硬度的时效时间分别为35、30和20h。SEM观察到在AZ91基体中存在均匀细小的Mg17Al12析出相。     

Microstructure and mechanical properties of friction stir lap welded Mg/Al joint assisted by stationary shoulder

Using magnesium alloy as upper sheet, 3 mm-thick AZ31 magnesium alloy and 6061 aluminum alloy were joined using friction stir lap welding assisted by stationary shoulder. The effects of tool rotating speed on cross-sections, microstructure and mechanical properties of Mg/Al lap joints were mainly discussed. Results showed that stationary shoulder contributed to joint formation, by which stir zones (SZ) were characterized by big onion rings after welding. Because of the big forging force exerted by stationary shoulder, the upper region of hook was well bonded. SZ showed much higher hardness because of intermetallic compounds (IMCs). The bonding conditions at the base material (BM)/SZ interface at advancing side and the hook region played important roles on joint lap shear properties. The X-ray diffraction pattern analysis revealed that the main IMCs were Al₃Mg₂ and Al₁₂Mg₁₇.     

Analysis of the electrochemical reaction behavior of alloy AZ91 by EIS technique in H3PO4/KOH buffered K2SO4 solutions

The EIS technique was used to analyze the electrochemical reaction behavior of Alloy AZ91 in H₃PO₄/KOH buffered K₂SO₄ solution at pH 7. The corrosion resistance of Alloy AZ91 was directly related with the stability of Al₂O₃ · xH₂O rich part of the composite oxide/hydroxide layer on the alloy surface. The break down of the oxide layer was estimated to occur mainly on the matrix solid solution phase in Alloy AZ91. The mf capacitive loop arose from the relaxation of mass transport in the solid oxide phase in the presence of Al₂O₃ · xH₂O rich part and from Mg⁺ ion concentration within the broken area in the absence of Al₂O₃ · xH₂O rich part in the composite oxide structure on the alloy surface. The lf inductive loop had tendency of disappear when the dissolution rate of the alloy decreased as a result of the formation of the protective oxide layer.     

Enhanced corrosion performance of magnesium phosphate conversion coating on AZ31 magnesium alloy

Magnesium phosphate conversion coating (MPCC) was fabricated on AZ31 magnesium alloy for corrosion protection by immersion treatment in a simple MPCC solution containing Mg²⁺ and PO^3?₄ ions. The MPCC on AZ31 Mg alloy showed micro-cracks structure and a uniform thickness with the thickness of about 2.5 µm after 20 min of phosphating treatment. The composition analyzed by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed that the coating consisted of magnesium phosphate and magnesium hydroxide/oxide compounds. The MPCC showed a significant protective effect on AZ31 Mg alloy. The corrosion current of MPCC was reduced to about 3% of that of the uncoated surface and the time for the deterioration process during immersion in 0.5 mol/L NaCl solution improved from about 10 min to about 24 h.