Effect of heat diffusion on properties of zinc-aluminum coating on AZ91D magnesium alloys

A protecting zinc and aluminum coating on the surface of AZ91D magnesium alloys was obtained by thermal spraying to improve the corrosion and wear resistance performances. In order to enhance the combination between magnesium alloy matrix and zinc and aluminum coating, the sample was heat-treated at 300℃for 2 h, then, the cross-section patterns, XRD pattern, micro-hardness, wear and corrosion resistance abilities were researched. The results indicate that the interface between the coating and substrate is metallurgical bond, and a transitional fusion layer is formed by diffusion. The micro-scale abrasion test and polarization test in 3% NaCl solution show that the diffusion-treated specimen has better wear and corrosion resistance performances in comparison with the undiffusion-treated and substrate magnesium alloys; in addition, it has relatively higher micro-hardness than the undiffusion-treated magnesium alloys.

Microstructural analyses of aluminum–magnesium–silicon alloys welded by pulsed Nd: YAG laser welding

Revealing grains and very fine dendrites in a solidified weld metal of aluminum–magnesium–silicon alloys is difficult and thus,there is no evidence to validate the micro-and meso-scale physical models for hot cracks. In this research, the effect of preheating on the microstructure and hot crack creation in the pulsed laser welding of AA 6061 was investigated by an optical microscope and field emission electron microscopy. Etching was carried out in the gas phase using fresh Keller’s reagent for 600 s. The results showed that the grain size of the weld metal was proportional to the grain size of the base metal and was independent of the preheating temperature. Hot cracks passed the grain boundaries of the weld and the base metal. Lower solidification rates in the preheated samples led to coarser arm spacing;therefore, a lower cooling rate. Despite the results predicted by the micro and meso-scale models, lower cooling rates resulted in increased hot cracks. The cracks could grow in the weld metal after solidification;therefore, hot cracks were larger than predicted by the hot crack prediction models.

Galvanic Corrosion of Magnesium Alloy and Aluminum Alloy by Kelvin Probe

Galvanic corrosion on samples of AZ91D magnesium alloy coupled with 2A12 aluminum alloy during neutral salt spray test was investigated.The variations of the surface potential were measured using scanning kelvin probe（SKP）.The results showed that galvanic effect on the corrosion of AZ91D magnesium alloy is closely related to the potential difference between the anodic and cathodic materials.In the initial period,corrosion only occurred in a narrow area at the coupling interface because of the limited distance galvanic current.Then,the corrosion rate of 2A12 aluminum alloy was accelerated due to its poor stability in strong alkali environment,which was attributed to the strong alkalization caused by the corrosion of AZ91D magnesium alloy.With the increase of the potential of 2A12 aluminum alloy as a result of the continuous covering of corrosion products,the potential difference between the two materials was enlarged,which enhanced the galvanic corrosion.

Effect of Cu addition on microstructure and properties of Mg-10Zn-5Al-0.1Sb high zinc magnesium alloy

To improve the strength,hardness and heat resistance of Mg-Zn based alloys,the effects of Cu addition on the as-cast microstructure and mechanical properties of Mg-10Zn-5Al-0.1Sb high zinc magnesium alloy were investigated by means of Brinell hardness measurement,scanning electron microscopy (SEM),energy dispersive spectroscopy (EDS),XRD and tensile tests at room and elevated temperatures.The results show that the microstructure of as-cast Mg-10Zn-5Al-0.1Sb alloy is composed of α-Mg,t-Mg32(Al,Zn)49,φ-Al2Mg5Zn2 and Mg3Sb2 phases.The morphologies of these phases in the Cu-containing alloys change from semi-continuous long strip to black herringbone as well as particle-like shapes with increasing Cu content.When the addition of Cu is over 1.0wt.%,the formation of a new thermally-stable Mg2Cu phase can be observed.The Brinell hardness,room temperature and elevated temperature strengths firstly increase and then decrease as the Cu content increases.Among the Cu-containing alloys,the alloy with the addition of 2.0wt.% Cu exhibits the optimum mechanical properties.Its hardness and strengths at room and elevated temperatures are 79.35 HB,190MPa and 160MPa,which are increased by 9.65%,21.1% and 14.3%,respectively compared with those of the Cu-free one.After T6 heat treatment,the strengths at room and elevated temperatures are improved by 20% and 10%,respectively compared with those of the as-cast alloy.This research results provide a new way for strengthening of magnesium alloys at room and elevated temperatures,and a method of producing thermally-stable Mg-10Zn-5Al based high zinc magnesium alloys.

EIS Study on Pitting Corrosion of AA5083-H321 Aluminum-Magnesium Alloy in Stagnant 3.5% NaCl Solution

In this research, EIS （electrochemical impedance spectroscopy） technique was utilized to study the pitting corrosion behaviour of AA5083-H321 aluminum-magnesium alloy in 3.5% NaCl solution. Impedance spectra were obtained during 240 h of exposure of the sample to the test solution. The surface and cross-section of the samples were studied by scanning electron microscopy （SEM） and EDAX （energy dispersive analysis of X-ray） analysis. The results indicated that as the resistance of the passive layer on intermetallic particles is very small, this parameter on the sample surface layers is controlled by that of pure passive layer. However, the capacitors in the proposed equivalent circuit are replaced with the constant phase elements （CPE）, due to non-uniformity and occurrence of pitting corrosion on the surface. The outward diffusion of Al^＋3 ions through the passive layer and the thickening of this layer cause the impedance decrease in the first 24 h and increase afterwards. The detachment of intermetallic particles from some of pits and the accumulation of the corrosion products inside some others are factors that prevents the continuation of cathodic reactions on the top of the intermetallic particles.

Effect of Sb on microstructure and mechanical properties in Mg-10Zn-5Al high zinc magnesium alloys

In view of high strength and poor impact toughness of the high-zinc magnesium alloys,the method of micro-alloying was adopted to improve the impact-toughness of Mg-10Zn-5Al high zinc magnesium alloys by adjusting the addition of Sb.The results show that:the dystectic Mg3Sb2 particles formed by Sb and Mg are distributed in the front of dendrite,restrain the growth of the secondary phase and then change the morphology of the secondary phases,which refines the matrix in some extent and improves the impact-toughness of the alloy significantly.But with further increase of Sb addition,Mg3Sb2 particles congregate and form needle-like Mg3Sb2 phase,which dissevers the matrix and leads to the reduction of the strength and impact-toughness of the alloys.Hence,the mechanical prosperities of ZA105 high zinc magnesium alloy are significantly improved with appropriate Sb addition,which provides a new way to extend the application fields of high zinc magnesium alloy.

Role of Chloride Ion and Dissolved Oxygen in Electrochemical Corrosion of AA5083-H321 Aluminum-Magnesium Alloy in NaCl Solutions under Flow Conditions

Flow-induced corrosion consists electrochemical and mechanical components. The present paper has to assessed the role of chloride ion and dissolved oxygen in the electrochemical component of flow induced corrosion for AA5083-H321 aluminum-magnesium alloy which is extensively used in the construction of high-speed boats, submarines, hovercrafts, and desalination systems, in NaCI solutions. Electrochemical tests were carried out at flow velocities of 0, ：2, 5, 7 and 10 m/s, in aerated and deaerated NaCI solutions with different sodium chloride concentrations. The results showed that the high rate of oxygen reduction under hydrodynamic conditions causes an increase in the density of pits on the surface. The increase of chloride ions concentration under flow conditions accelerates the rate of anodic reactions, but have no influence on the cathodic reactions. Thus, in the current work, it was found that under flow conditions, due to the elimination of corrosion products inside the pits, corrosion resistance of the alloy is increased.

Peculiarities of forming diffusion bimetallic joints of aluminum foam with a monolithic magnesium alloy

The work is carried out to determine an optimal method to obtain the welded bimetallic joints of monolithic Mg-alloy with porous Al-alloy using gallium as chemical activator and heating up to 300 ℃ by two different methods:long-term in vacuum oven and short-term without vacuum by passing of low voltage current.There is no microstructure change in Al-foam but indentation test records the negligible reduction of the mechanical properties.SEM showed the crystallization of two types of Mg_(5)Ga_(2) and Mg_(2)Ga inter-metallic phases in the wavy uneven diffusion zone on Mg-alloy side with significant increase of micro-hardness and Young’s modulus.The narrow depth of the diffusion zone takes place in joints by short-term heating,so this method is more applicable for welding of monolithic and porous alloys at chemical activation using gallium.

Friction self-piercing riveting(F-SPR)of aluminum alloy to magnesium alloy using a flat die

Friction self-piercing riveting(F-SPR)process based on a pip die has been invented to solve the cracking problems in riveting high-strength and low-ductility light metals,such as magnesium alloys,cast aluminum,and 7 series aluminum alloys.In this paper,in order to solve quality issues caused by the misalignment between rivet and pip-die in F-SPR,a flat-die based F-SPR process was proposed and employed to join 1.27 mm-thick AA6061-T6 to 3 mm-thick AZ31B.The results indicate that a 1.0 mm die distance is effective to avoid rivet upset and insufficient flaring.As the feed rate increases,the heat input in the whole process decreases,resulting in a larger riveting force,which in turn increases both the bottom thickness and interlock amount.Besides,solid-state bonding,including Al-Mg intermetallic compounds(IMCs),Al-Mg mechanical mixture,and Al-Fe atom interdiffusion was observed at the joint interfaces.The upper Al layer was softened,but the lower Mg layer was hardened,and both sheets exhibited a narrowed affected region with the increase of feed rate,while the rivet hardness shows no obvious change.Three fracture modes appeared accompanying the variations in lap-shear strength and energy absorption as the feed rate increased from 2 mm/s to 8 mm/s.Finally,the F-SPR process using a flat die was compared to those using a pip die and a flat bottom die to show the advantage of flat die on coping with the misalignment problem.

Dissimilar friction stir welding between 5052 aluminum alloy and AZ31 magnesium alloy

Dissimilar friction stir welding between 5052 Al alloy and AZ31 Mg alloy with the plate thickness of 6 mm was investigated.Sound weld was obtained at rotation speed of 600 r/min and welding speed of 40 mm/min.Compared with the base materials,the microstructure of the stir zone is greatly refined.Complex flow pattern characterized by intercalation lamellae is formed in the stir zone.Microhardness measurement of the dissimilar welds presents an uneven distribution due to the complicated microstructure of the weld,and the maximum value of microhardness in the stir zone is twice higher than that of the base materials. The tensile fracture position locates at the advancing side(aluminum side),where the hardness distribution of weld shows a sharp decrease from the stir zone to 5052 base material.

Phase selection of ternary intermetallic compounds during solidification of high zinc magnesium alloy

The phase selection of ternary intermetallic compound τphase (Mg32(Al, Zn)49 ) and φ phase (Al2Mg5Zn2) in high zinc magnesium alloys was studied by using scanning electron microscope, X-ray diffractometer and differential scanning calorimeter, etc. The results indicate that, when adding element Si in Mg-8Zn-4Al-0.3Mn (ZA84) alloy, φ phase is promoted, whereas τ phase is inhibited. The Chinese script-type Mg2Si and matrix microstructure are greatly refined, the formation of τ phase is facilitated and φ phase is restrained when modifier Al-AlP master alloy is added in ZA84 alloy containing Si. The kinetics study of phase selection indicates that there is a critical degree of undercooling of the melt. If the undercooling exceeds the critical value, τ phase preferentially forms while φ phase is restrained; otherwise, φ phase preferentially forms while τ phase is restrained.

Aluminum-zinc alloy squeeze casting technological parameters optimization based on PSO and ANN

This paper presents a kind of ZA27 squeeze casting process parameter optimization method using artificial neural network (ANN) combined with the particle swarm optimizer (PSO). Regarding the test data as samples and using neural network create ZA27 squeeze casting process parameters and mechanical properties of nonlinear mapping model. Using PSO optimize the model and obtain the optimum value of the process parameters. Make full use of the non-neural network mapping capabilities and PSO global optimization capability. The network uses the radial direction primary function neural network,using the clustering and gradient method to make use of network learning,in order to enhance the generalization ability of the network. PSO takes dynamic changing inertia weights to accelerate the convergence speed and avoid a local minimum.

Formation of Aluminum-magnesium Alloy Cup by Hydrodynamic Deep Drawing with Twin-loading Paths

In order to overcome the limitation of hydro-rim deep drawing, a new process of hydrodynamic deep drawing （HDD） with independent radial hydraulic pressure was proposed. By employing the dynamic explicit analytical software ETA/DynaformS.5 which is based on LS-DYNA3D, the effects of independent radia! hydraulic pressure on the stress, strain and the sheet-thickness of aluminum-magnesium cylindrical cup with a hemispherical bottom were analyzed by numerical simulation. The feature of stress distribution is that there exists a stress-dividing circle in the flange, and the radius of dividing circle was determined by theoretical analysis and stimulation. The experimental results indicate that the reasonable match of independent radial hydraulic pressure and liquid chamber pressure can effectively reduce the thinning at the bottom of hemisphere, decrease the radial stress-strain, and improve the drawing limit of aiuminum-magnesium alloy cylindrical cup.

Effect of magnesium on the aluminothermic reduction rate of zinc oxide obtained from spent alkaline battery anodes for the preparation of Al–Zn–Mg alloys

The aluminothermic reduction of zinc oxide（ZnO） from alkaline battery anodes using molten Al may be a good option for the elaboration of secondary 7000-series alloys. This process is affected by the initial content of Mg within molten Al, which decreases the surface tension of the molten metal and conversely increases the wettability of ZnO particles. The effect of initial Mg concentration on the aluminothermic reduction rate of ZnO was analyzed at the following values： 0.90wt%, 1.20wt%, 4.00t%, 4.25wt%, and 4.40wt%. The ZnO particles were incorporated by mechanical agitation using a graphite paddle inside a bath of molten Al maintained at a constant temperature of 1123 K and at a constant agitation speed of 250 r/min, the treatment time was 240 min and the ZnO particle size was 450？500 mesh. The results show an increase in Zn concentration in the prepared alloys up to 5.43wt% for the highest initial concentration of Mg. The reaction products obtained were characterized by scanning electron microscopy and X-ray diffraction, and the efficiency of the reaction was measured on the basis of the different concentrations of Mg studied.

WEAR PERFORMANCE AND MECHANISM OF ZINC-ALUMINUM（ZA27）ALLOY

ZA27 alloy has the best performance and the widest applications in high aluminum zinc based die casting alloy series. One of its main applica-tions is used as abrasion resistant alloy,instead of nonferrous alloys such as copper alloy.The frictional wear p

Fatigue property comparison of TIG welded joints of magnesium alloy and aluminum alloy

The fatigue properties of the TIG welded joints of both AZ31B magnesium alloy and 5A06 aluminum alloy were investigated. The four types of welded joints were used in fatigue tests, such us butt joints, transverse cross joints, fillet joints and lateral connecting joints. The fatigue strengths at 2 × 10^6 cycles of the four welded joints of AZ31B magnesium alloy are 39. 0 MPa, 24. 4 MPa, 32. 1 MPa and 24. 2 MPa, which are 55. 0% , 42. 2%, 78. 0% and 50. 2% of that of 5A06 aluminum alloy, respectively. The fatigue strength levels at slope m = 3 of the aluminum alloy＇ s welded joints are mostly higher than the FAT recommended by the International Institute of Welding （ HW） , while those of the magnesium alloy＇ s welded joints are all lower than the FAT. It is indicated that the FAT of magnesium alloy＇ s welded joints should be established as early as possible in order to be applied in the design of magnesium alloy＇ s welded structures.

Effect of magnesium in aluminum alloys on characteristics of microarc oxidation coatings

Microarc oxidation(MAO) coatings were prepared on the surface of aluminum alloys with different contents of magnesium. The morphologies and surface roughness of the coatings were characterized by Confocal laser scanning microscopy(CLSM). Phase and chemical composition of the MAO coatings were analyzed by X-ray diffractometry(XRD) and X-ray photoelectron spectroscopy(XPS). The experimental results show that the coatings formed on different substrates have two-layer morphologies and are mainly composed of Al2O3 and Al-Si-O phases. In addition, the content of Al2O3 increases with increasing the content of magnesium. XPS results prove that magnesium from substrate indeed participates in the MAO process and is incorporated into the coating in the form of MgO. The coating formed on Al-3Mg substrate has the smallest mass loss and the lowest friction coefficient of 0.17-0.19.

Tsinghua University Tsinghua-TOYO Research and Development Center of Magnesium and Aluminum Alloys Processing Technology

Tsinghua-TOYO Research and Development Center of Magnesium and Aluminum Alloys Processing Technology was officially established between Tsinghua University and TOYO Machinery & Metal Co. on Feb. 26, 2002, which is a non-independent legal research organization located in Department of Mechanical Engineering of Tsinghua University. The center was equipped with one set of 650t automatic magnesium and aluminum alloys diecasting machine and necessary accessories including the melting furnaces for magnesium alloys and aluminum alloys, an accurate magnesium pump to transfer the magnesium alloys to the shot sleeve of the die casting machine, a die temperature controller, and data logging systems for cavity pressure and die temperature distributions, etc. The center is aimed to the research and development of magnesium and aluminum die casting process and their related technologies, and the main research contents include:

Electroplating zinc transition layer for electroless nickel plating on AM60 magnesium alloys

Electroplating zinc coating as transition layer of electroless nickel plating on AM60 magnesium alloys was investigated. The zinc film can be deposited in a pyrophosphate bath at 50-60℃under current density of 0.5-1.5 A/dm2. A new fore treatment technology was applied by acid cleaning with a solution containing molybdate and phosphorous acid, by alkaline cleaning in a bath containing molybdate and sodium hydroxide. The subsequent electroless plating was carried out in nickel sulfate bath. The SEM observation shows that the deposition is uniform and compact. The polarization curve measurements show that the corrosion potential of the zinc plating in 3.5% NaCl is about -1.3 V(vs SCE) which is noble than that of magnesium substrate. The zinc electroplating can be applied as the pretreatment process for electroless nickel plating on magnesium alloys.