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.     

Fabrication and performance evaluation of dissimilar magnesium–aluminium alloy multi-seam friction stir clad joints

This investigation is aimed to establish empirical relationships between continuous multi-seam friction stir cladding process parameters (i.e., rotational speed, welding speed and shoulder overlap ratio) and the quality characteristics (bond tensile strength, shear strength and corrosion) of dissimilar magnesium–aluminium alloy clad joints. The influence of considered process parameters on the clad properties was reported. Furthermore, multi-criterion optimization procedure was used to obtain ideal processing conditions, which can yield higher interface strength and lower corrosion rate of fabricated composite plate. Results indicate that, the aluminium-rich thin continuous layer, Mg-rich irregular shaped regions consists of Al₃Mg₂ and Al₁₂Mg₁₇ intermetallic compounds and nature of mechanical interlocking has great influence on the joint interface strength. On the other hand, the corrosion resistance of the clad joints is greatly affected by the amount of magnesium mixed with top aluminium sheet during friction stirring. Also, bend testing shows that, the cladded joints exhibit excellent ductility.     

Microstructure and corrosion resistance of magnesium alloy ZE41 with laser surface cladding by Al-Si powder

The microstructure and corrosion behavior of commercial alloy ZE41 modified by surface laser cladding with Al-Si powder mixture was studied by SEM, TEM, X-ray diffraction and electrochemical methods. The coating is composed of an Al-Mg matrix and dendrite precipitates of Mg₂Si. In function of the laser speed, the matrix is formed by a Mg solid solution in Al or by the intermetallic phase Mg₁₇Al₁₂. The presence of different matrixes is responsible for galvanic corrosion and decrease of corrosion resistance in interfacial area between coats. Isolated samples of the bulk coatings material showed similar corrosion potentials inspite of different matrix composition. This interpreted in terms of a mechanism involving two steps: (1) an initial dissolution of anodic Mg₂Si particles followed by (2) pitting in the formed crevices. The proposed mechanism corresponds well with the experimental observations and the mechanisms of localized corrosion observed for aluminium alloys in the chloride media described in the literature. Improved corrosion resistance can be achieved by the microstructure homogenization through the optimization of laser parameters and/or following heat treatment.     

AZ33M镁合金激光熔覆Al-Si涂层的组织与性能

利用激光熔覆技术在AZ33M镁合金表面制备了Al-Si涂层,通过采用腐蚀电化学测试结合X射线衍射仪(XRD)、扫描电镜(SEM)及显微硬度计等对熔覆层微观组织和性能进行了表征。结果表明,熔覆层主要由Mg和Mg₁₇Al₁₂、Mg₂Si及Mg₂Al₃相组成。熔覆层显微组织由柱状树枝晶和方向各异的树枝晶组成。由于第二相强化和细晶强化等原因,制备的Al-Si涂层相比镁合金基体具有更高的硬度。熔覆层的自腐蚀电位相比基体提高了约400 mV,自腐蚀电流降低了一个数量级,熔覆层的耐蚀性明显优于基体镁合金。     

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.     

Effect of ageing treatment on microstructures,mechanical properties and corrosion behavior of Mg-Zn-RE-Zr alloy micro-alloyed with Ca and Sr

Effects of ageing treatment on the microstructures, mechanical properties and corrosion behavior of the Mg-4.2Zn-1.7RE-0.8Zr-xCa-ySr [x=0, 0.2 (wt.%), y=0, 0.1, 0.2, 0.4 (wt.%)] alloys were investigated. Results showed that Ca or/and Sr additions promoted the precipitation hardening behavior of Mg-4.2Zn-1.7RE-0.8Zr alloy and shortened the time to reaching peak hardness from 13 h to 12 h. The maximum hardness of 77.1±0.6 HV for the peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy was obtained. The microstructures of peak-aged alloys mainly consist of α-Mg phase, Mg₅₁Zn₂₀ phase and ternary T-phase. The Zn-Zr phase is formed within the α-Mg matrix, and the Mg₂Ca phase is formed near T-phase due to the enrichment of Ca in front of the solid-liquid interface. Furthermore, fine short rod-shaped β′₁ phase is precipitated within the α-Mg matrix in the peak-aged condition. The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy exhibits optimal mechanical properties with an ultimate tensile strength of 208 MPa, yield strength of 150 MPa and elongation of 3.5%, which is mainly attributed to precipitation strengthening. In addition, corrosion properties of experimental alloys in the 3.5wt.% NaCl solution were studied by the electrochemical tests, weight loss, hydrogen evolution measurement and corrosion morphology observation. The results suggest that peak-aged alloys show reduced corrosion rates compared with the as-cast alloys, and minor additions of Ca and/or Sr improve the corrosion resistance of the Mg-4.2Zn-1.7RE-0.8Zr alloy. The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy possesses the best corrosion resistance, which is mainly due to the continuous and compact barrier wall constructed by the homogeneous and continuous second phases.

     

添加RE和Ti元素对Zn-2.5Al-3Mg合金微观结构和耐腐蚀性能的影响

采用XRD、SEM、TEM和XPS等研究了RE和Ti元素对Zn-2.5Al-3Mg合金微观结构和耐蚀性的影响。结果表明,Zn-2.5Al-3Mg合金的微观结构由富Zn相、二元共晶（Zn-MgZn₂/Mg₂Zn₁₁）和三元共晶（Zn/Al/Mg₂Zn₁₁）组成,而含有RE和Ti元素的合金中出现了新相(Ce_1-xLa_x)Zn₁₁和Al₂Ti。电化学阻抗谱表明,相对于Zn-2.5Al-3Mg合金,Zn-2.5Al-3Mg-0.1RE-0.2Ti合金的耐蚀性得到了显著的提高。XPS分析结果表明,RE元素的添加促进腐蚀产物Zn₅(CO₃)₂(OH)₆和MgAl₂O₄的形成,而RE和Ti元素的同时添加促进腐蚀产物 Zn₅(CO₃)₂(OH)₆、ZnAl₂O₄和MgAl₂O₄的形成,且都抑制了疏松多孔ZnO的生成。Zn₅(CO₃)₂(OH)₆、ZnAl₂O₄和MgAl₂O₄能够很好地粘附在试样表面,提供一层致密的保护层,从而提高Zn-2.5Al-3Mg合金的耐腐蚀性。     

Laser (Nd:YAG) cladding of AZ91D magnesium alloys with Al + Si + Al2O3

The laser surface cladding of AZ91D magnesium alloy with Al + Si + Al₂O₃ powders (in the wt.% ratio of 7:1:2) was investigated. Laser processing was carried out with a 5 kW Nd:YAG laser. The microstructure and phase analyses of the surface cladding layer were carried out; furthermore, the morphology of the cladding zone, volume fraction and distribution of the Si and Al₂O₃ particles in the laser surface cladding layer were also investigated. The average microhardness of the surface layer was measured in detail as a function of laser parameters. Microhardness of the surface layer was significantly improved to as high as 210 HV_0.05 as compared to 60–70 HV_0.05 of the AZ91D substrate, and the average microhardness of the cladding layer of the composite surfaced specimen decreases with increasing of laser power. However, the rate of decrease is faster at lower power. Though the average microhardness of the surface cladding layer remains constant, there is a little fluctuation in the reading in some regions possibly, because of the random distribution of hard particles in the cladding layer. Finally, the proper processing parameter ranges for formation of a homogeneous microstructure and enhancement of microhardness for laser surface cladding of AZ91D with Al + Si + Al₂O₃ were established.     

Microstructure characteristics of Ti3Al/TiC ceramic layer deposited by laser cladding

Al + TiC laser cladding coatings were prepared on Ti-6Al-4V alloy by CO₂ laser cladding technique. The microstructure, micro-hardness and phase constitutes of the laser cladding layer were investigated by means of scanning electron microscope (SEM), X-ray diffraction (XRD) and microsclermeter. The results indicated that the laser cladding layer solidified into the fine microstructure rapidly, and TiC hard phase was dispersived in the cladding layer. When the mass percent of TiC was 40%, the micro-hardness (1100HV_0.2-1250HV_0.2) of Al + TiC cladding layer was 3 times more than that of the Ti-6Al-4V alloy substrate (350-370HV_0.2). The cladding layer mainly consisted of α-Ti (Al), β-Al (Ti), Ti₃Al, TiAl, Al₃Ti and TiC phase. There phases were beneficial to improve the hardness and wear resistance of the cladding layer.     

Characteristics of plasma electrolytic oxide coatings on Mg-Al-Zn alloy prepared by powder metallurgy

Mg-6 wt.%Al-1 wt.%Zn alloy powders were produced by gas atomization, and subsequently compacted and sintered under various conditions of temperature, time, and pressure. The bulk Mg-6 wt.%Al-1 wt.%Zn alloy was coated by the plasma electrolytic oxidation (PEO) method. The optimum condition of compaction and sintering for PEO coatings was established based on the investigation of microstructure, microhardness, and corrosion properties of coatings which were compared to those of cast Mg-6 wt.%Al alloy. The coatings on Mg-6 wt.%Al and Mg-6 wt.%Al-1 wt.%Zn alloys consisted of MgO, MgAl₂O₄, and Mg₂SiO₄. The Mg-6 wt.%Al-1 wt.%Zn alloy compacted at room temperature for 10 min and sintered at 893 K for 3 h showed the most porous and nonuniform coating layer because the coatings had grown through grain boundaries that resulted from poor bonding between powder particles in the substrate. However, the coated Mg-6 wt.%Al-1 wt.%Zn alloy hot-compacted at 593 K for 10 min had the thickest coating layer and the highest microhardness. In addition, it demonstrated the best corrosion resistance as verified by polarization curves in 3.5% NaCl solution.     

Localized Corrosion Behavior of Al-Si-Mg Alloys Used for Fabrication of Aluminum Matrix Composites

The relationship between microstructure and localized corrosion behavior in neutral aerated chloride solutions was investigated with SEM/EDAX, conventional electrochemical techniques, and with scanning Kelvin probe force microscopy (SKPFM) for two custom-made alloys with Si/Mg molar ratios of 0.12 and 0.49. In this order, Al₃Fe, Al₃Mg₂, and Mg₂Si intermetallics were identified in the first alloy and Al(FeMn)Si and Mg₂Si particles in the second one. Anodic polarization curves and corrosion morphology showed that the alloy with higher Si/Mg molar ratio exhibited a better corrosion performance and evidence was shown that it had a more corrosion-resistant passive film. The corrosion process for both alloys in aerated 0.1 M NaCl solutions was localized around the Fe-rich intermetallics. They acted as local cathodes and produced dissolution of the aluminum matrix surrounding such particles. Mg₂Si and Al₃Mg₂ exhibited anodic behavior. SKPFM was successfully used to map the Volta potential distribution of main intermetallics. The localized corrosion behavior was correlated with a large Volta potential difference between the Fe-rich intermetallics and the matrix. After immersion in the chloride solution, such Volta potential difference decreased.     

Thermodynamic calculation of high zinc-containing Al-Zn-Mg-Cu alloy

Phase fraction and solidification path of high Zn-containing Al-Zn-Mg-Cu series aluminum alloy were calculated by calculation of phase diagram (CALPHAD) method. Microstructure and phases of Al-9.2Zn-1.7Mg-2.3Cu alloy were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The calculation results show that η(MgZn₂) phase is influenced by Zn and Mg. Mass fractions of η(MgZn₂) in Al-xZn-1.7Mg-2.3Cu are 10.0%, 9.8% and 9.2% for x=9.6, 9.4, 8.8 (mass fraction, %), respectively. The intervals of Mg composition were achieved for θ(Al₂Cu)+η(MgZn₂), S(Al₂CuMg)+η(MgZn₂) and θ(Al₂Cu)+S(Al₂CuMg)+η(MgZn₂) phase regions. Al₃Zr, α(Al), Al₁₃Fe₄, η(MgZn₂), α-AlFeSi, Al₇Cu₂Fe, θ(Al₂Cu), Al₅Cu₂Mg₈Si₆ precipitate in sequence by no-equilibrium calculation. The SEM and XRD analyses reveal that α(Al), η(MgZn₂), Mg(Al,Cu,Zn)₂, θ(Al₂Cu) and Al₇Cu₂Fe phases are discovered in Al-9.2Zn-1.7Mg-2.3Cu alloy. The thermodynamic calculation can be used to predict the major phases present in experiment.     

Ca Addition Effects on the Microstructure,Tensile and Corrosion Properties of Mg Matrix Alloy Containing 8 wt.% Mg<Subscript>2</Subscript>Si

The microstructure, tensile properties and corrosion behavior of the Mg-8 wt.% Mg₂Si-x%Ca alloy have been studied by the use of optical microscopy, scanning electron microscopy equipped with energy-dispersive spectroscopy, x-ray diffraction, standard tensile testing, polarization test and electrochemical impedance spectroscopy (EIS) measurements. Microstructural studies indicated that Ca modifies both primary and eutectic Mg₂Si phase. It was found that the average size of primary Mg₂Si particles is about 60 μm, which is dropped by about 82% in the alloy containing 0.05 wt.% Ca. By the addition of different Ca contents, Ca-rich intermetallics (i.e., CaSi₂ and CaMgSi) were formed. The modification mechanism of adding Ca during solidification was found to be due to the strong effect of CaMgSi phase as a heterogonous nucleation site, apart from CaSi₂ which was reported before, for Mg₂Si intermetallics. Tensile testing results ascertained that Ca addition enhances both ultimate tensile strength (UTS) and elongation values. The optimum amount of Ca was found to be 0.1 wt.%, which improved UTS and elongation values from about 130 MPa and 2% to 165 MPa and 5.5%, whereas more Ca addition (i.e., 3 wt.%) reduced the tensile properties of the alloy to about 105 MPa and 1.8%, which can be due to the formation of CaMgSi intermetallics with deteriorating needle-like morphology. Polarization and EIS tests also showed that the Mg-3%Si-0.5%Ca alloy pronounces as the best anti-corrosion alloy. Nevertheless, further added Ca (up to 3 wt.%) deteriorated the corrosion resistance due to predominance of worse galvanic coupling effect stemmed from the presence of stronger CaMgSi cathode in comparison with Mg₂Si. With higher Ca additions, an adverse effect was seen on corrosion resistance of the Mg-3%Si alloy, as a result of forming a weak film on the alloy specimen surface.     

激光熔覆Ni2Si／Ni3Si2金属硅化物合金涂层显微组织与耐蚀性

以Ni76Si24（质量百分数）合金粉末为原料，利用激光熔覆技术在A3钢表面制得了组织由条件Ni2Si初生相及少量Ni2Si/Ni3Si2共晶组成的新型金属硅化物合金涂层，分析涂层显微组织并测定其在0．5mol/1 H2SO4水溶液及不同浓度NaCl水溶液中的阳极极化曲线，结果表明激光熔覆Ni2Si/Ni3Si2金属硅化物合金涂层表面平整，组织细小，与基体为完全冶金结合，同时由于涂层的组织组成相Ni3Si2本身均具有极好的耐蚀性并具有快速凝固细小均匀的显微组织，该激光熔覆Ni2Si/Ni3Si2金属硅化物合金涂层在0．5mol/l H2SO4及3．5％NaCl水溶液中均具有优良的耐蚀性能。     

Effect of solid solution treatment on in vitro degradation rate of as-extruded Mg-Zn-Ag alloys

The degradation behaviors of the as-extruded and solution treated Mg-3Zn-xAg (x=0, 1, 3, mass fraction, %) alloys, as well as as-extruded pure Mg, have been investigated by immersion tests in simulated body fluid (SBF) at 37 °C. The as-extruded Mg-Zn(-Ag) alloys contained Mg₅₁Zn₂₀ and Ag₁₇Mg₅₄. While the quasi-single phase Mg-Zn(-Ag) alloys were obtained by solution treatment at 400 °C for 8 h. The quasi-single phase Mg-Zn(-Ag) alloys showed lower degradation rate and more homogeneous degradation than corresponding as-extruded Mg alloys. Degradation rate of solid-solution treated Mg-3Zn-1Ag and Mg-3Zn-3Ag was approximately half that of corresponding as-extruded Mg alloy. Moreover, the degradation rate of solid-solution treated Mg-3Zn and Mg-3Zn-1Ag was equivalent to that of as-extruded pure Mg. However, heterogeneous degradation also occurred in quasi-single phase Mg-Zn-Ag alloys, compared to pure Mg. So, preparing complete single-phase Mg alloys could be a potential and feasible way to improve the corrosion resistance.     

AZ31B镁合金表面火焰喷涂Al-Mg_2Si涂层的耐腐蚀性能

为提高AZ31B镁合金表面的耐腐蚀性能,用火焰喷涂方法在镁合金表面制备Al-Mg_2Si复合涂层。采用XRD、SEM和EDS分析涂层的物相组成、微观组织及元素分布;通过电化学试验测试样品在3.5%NaCl溶液中的腐蚀电位、腐蚀电流密度;通过3.5%NaCl溶液浸泡试验测试样品的腐蚀速率;并测试涂层的显微硬度。结果表明:涂层中的主要物相有Mg_2Si、Al,组织比较致密,元素分布均匀。Tafel极化曲线测试表明,Al-Mg_2Si涂层样品与AZ31B镁合金样品相比腐蚀电位从-1.489 V正移到-1.366 V,腐蚀电流密度从2.817×10~(-3) A/cm~2降低到1.198×10~(-3) A/cm~2。浸泡试验结果表明,喷涂Al-Mg_2Si的镁合金的腐蚀速率明显低于没有喷涂的镁合金。显微硬度测试表明,涂层的显微硬度集中分布在259~308 HV0.05之间,镁合金为50~60 HV0.05。因此在AZ31B镁合金表面火焰喷涂Al-Mg_2Si涂层可以提高其耐腐蚀性能,表面硬度显著提高。     

Investigation on microstructure characterizations and phase compositions of rapidly solidification/powder metallurgy Mg-6 wt.% Zn-5 wt.% Ce-1.5 wt.% Ca alloy

Rapidly solidification/powder metallurgy (RS/PM) Mg-6Zn-5Ce-1.5Ca (wt.%) alloy in form of rods has been prepared by hot extrusion with RS powders, produced via atomizing the alloy melt and subsequent splat-quenching on the water-cooled copper twin-rollers. Microstructure characterizations and phase compositions of the alloy have been investigated. The results showed that the RS/PM Mg-6Zn-5Ce-1.5Ca alloy was the characteristics of very fine grains with the size ranging from 200 to 650 nm and was composed of α-Mg, Mg_xZn_yCe_z phase with a few Ca (about 1 at.%) shortened as the Mg_xZn_yCe_z-(Ca) phase, and a small quantity of Mg₅₁Zn₂₀ phases. The melting point of the Mg_xZn_yCe_z-(Ca) phase in the alloy was a little higher than that of the Mg_xZn_yCe_z phase in RS/PM Mg-6Zn-5Ce alloy, which may be due to the dissolving of Ca in it. Moreover, the atomic percentage ratio of Zn to Ce in the Mg_xZn_yCe_z-(Ca) phase around the grain boundary was close to 1.5.     

Microstructure and properties of laser cladding FeCrBSi composite powder coatings with higher Cr content

FeCrBSi alloy powder with higher Cr content was used for laser cladding by employing a 3 kW solid-state laser. Ni- and Fe-based alloy powders, which were more resistant to cracking, were added into FeCrBSi alloy powder with higher Cr content to increase the ductile phases, lower thermal expansion coefficient, and reduce the crack sensitivity of the cladding layer. FeCrBSi alloy powder with higher Cr content combined Ni- and Fe-based alloy powder were cladded on the substrates, which yield two different phases. The hard phases of the cladding layer were mainly composed of carbide phase M₂₃C₆, and the ductile phases which played a lubrication function in the cladding layer were mainly composed of austenite γ-Fe and γ-Ni. The ductile phases increased by adding Ni- and Fe-based alloy powder into FeCrBSi alloy powder with higher Cr content, and the hard phases became sparser relatively. Smooth cladding layers, which were free of macroscopic pores, cracks and void between the adjacent tracks, were achieved. Therefore, the toughness of the cladding layer was improved, and the crack tendency was reduced. Three kinds of composite powder were obtained. The composition and morphology of the cladding layer were analyzed, and the microhardness between the hard phases and the ductile phases was compared. The average microhardnesses of the three cladding layers varied from HV_0.2 760 to HV_0.2 950.