Preparation and Corrosion Resistance of Rare Earth Conversion Coatings on AZ91 Magnesium Alloy

The feasibility of forming Ce-containing rare earth conversion coating (short for RECCs),which is nonpoisonous and green to the environment,onto the AZ91 magnesium alloy surface in order to increase the corrosion resistance was studied. And the optimum technological conditions, such as the appropriate concentrations of the components in the solution,temperature and duration of the coatings formation were also settled. The protection of conversion coatings on magnesium alloy surface was evaluated by moisture/heating test, anodie polarization, etc.     

Preparation and Corrosion Resistance of Rare Earth Conversion Coatings on AZ91 Magnesium Alloy

Magnesiumalloysarewidelyusedinweaponin dustry,spacenavigationandhavegoodpotentialof applicationanddevelopmentindailycommunication facilitiesduetotheirpropertiesoflightweightandhighstrength[1].Appliedasstructuralmaterials,mag nesiumalloysmayeasilycrackduet…     

Analysis of Microstructure Characteristics and Precipitation Behavior＇of Automobile Beam Steels Produced by Compact Strip Production

The microstructure characteristics and precipitation behavior of automobile beam steels produced by compact strip production (CSP) were investigated by use of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy. The result shows that the final microstructure is mainly composed of polygonal ferrite and small amount of pearlite, and the average ferrite grain size is about 3—6 μm. Small amount addition Ti to aluminium-killed steel can help to refine the microstructure and improve the mechanical properties. A large number of fine precipitates have been observed in automobile beam steels. The mean particle size is about 10—30 nm. Remarkable strengthening and grain refinement can be obtained by these nano-particles.     

Nonuniform Recrystallization and Growth Behavior of β Grains Dominated by Grain Misorientation and Interfacial Energy in Metastable β Titanium Alloy

Metallurgical and Materials Transactions A - Metastable β titanium alloys usually exhibit nonuniform β grain growth behavior under β solution treatment, resulting in “black...     

Temperature,Stress and Distortion of Ti–6Al–4V Alloy Low-Temperature Friction Stir Welding Assisted by Trailing Intensive Cooling

The low-temperature friction stir welding (FSW) in which peak temperature is lower than the transus temperature of β phase was achieved using rotational speed of 100 rpm and welding speed of 30 mm/min. Trailing intensive cooling with liquid nitrogen was successfully applied to FSW under the low-temperature welding conditions. Comparisons of the temperature field, plastic strain, residual stress and welding distortion between intensive and conventional cooling were investigated by experiment and simulation. Results reveal that trailing intensive cooling is attributed to shrink high-temperature area and reduce the value of peak temperature and plastic strain. Longitudinal residual stress presents M shape, and the reduction of maximum tensile residual stress reaches 4.8%. The welding distortion shows an anti-saddle shape, and the decrement of welding distortion in transverse direction is 34.5%.     

Mechanical Strength and Failure Characteristics of Cast Mg-9 pctAl-1 pctZn Alloys Produced by a Heated-Mold Continuous Casting Process: Tensile Properties

The mechanical properties and failure characteristics of a cast Mg alloy (AZ91: Mg-Al_8.9-Zn_0.6-Mn_0.2) produced by a heated-mold continuous casting process (HMC) are investigated. In a modification of the original HMC process, the cooling of the liquid alloy by direct water spray is carried out in an atmosphere of high-purity argon gas. The HMC-AZ91 alloy exhibits excellent mechanical properties (high strength and high ductility) that are about twice as high as those for the same alloy produced by conventional gravity casting. The increased material strength and ductility of the HMC sample are attributed to nanoscale and microscale microstructural characteristics. The fine grains and tiny spherical eutectic structures (e.g., Mg₁₇Al₁₂ and Al₆Mn) distributed randomly in the matrix of the HMC alloy result in resistance to dislocation movement, leading to high tensile strength. Basal slip on (0001) planes in the relatively organized crystal orientation of the HMC alloy, as well as grain boundary sliding through tiny spherical eutectic structures, results in high ductility. Details of the failure mechanism under static loading in the HMC alloy are also discussed using failure models.     

Effect of Pb Addition on the Discontinuous and Continuous Mg17Al12-β Precipitate During Solidification of AZ91 Magnesium Alloy

The purpose of this study is to investigate the effect of varying Pb additions on the precipitation sequence of the as-cast and aged AZ91 alloy. The amount of discontinuous precipitate gets reduced by increasing the Pb addition. The phase fraction of as-cast and aged alloys demonstrates that by Pb addition Mg₁₇Al₁₂ phase is reduced to 37.7 and 38.7 % respectively.     

Relationship Between the 3D Porosity and β-Phase Distributions and the Mechanical Properties of a High Pressure Die Cast AZ91 Mg Alloy

Currently, most magnesium lightweight components are fabricated by casting as this process is cost effective and allows forming parts with complex geometries and weak textures. However, cast microstructures are known to be heterogeneous and contain unpredictable porosity distributions, which give rise to a large variability in the mechanical properties. This work constitutes an attempt to correlate the microstructure and the mechanical behavior of a high pressure die cast (HPDC) Mg AZ91 alloy, aimed at facilitating process optimization. We have built a stairway-shaped die to fabricate alloy sections with different thicknesses and, thus, with a range of microstructures. The grain size distributions and the content of β-phase (Mg₁₇Al₁₂) were characterized by optical and electron microscopy techniques as well as by electron backscatter diffraction (EBSD). The bulk porosity distribution was measured by 3D computed X-ray microtomography. It was found that the through-thickness microhardness distribution is mostly related to the local area fraction of the β-phase and to the local area fraction of the pores. We correlate the tensile yield strength to the average pore size and the fracture strength and elongation to the bulk porosity volume fraction. We propose that this empirical approach might be extended to the estimation of mechanical properties in other HPDC Mg alloys.     

Formation of Grain Boundary <Emphasis Type="Italic">α</Emphasis> in <Emphasis Type="Italic">β</Emphasis> Ti Alloys: Its Role in Deformation and Fracture Behavior of These Alloys

Beta-Ti alloys contain sufficient concentrations of β stabilizing alloy additions to permit retention of the metastable β phase after cooling to room temperature. Decomposition of the metastable β phase results in the formation of several possible phases, at least two of which are metastable. Concurrently, equilibrium α phase often forms first by heterogeneous nucleation at the α grain boundaries with an accompanying precipitate free zone observed adjacent to the grain boundary α. The grain boundary regions are softer than the precipitation hardened matrix. As a consequence, fracture follows the prior β grain boundaries, especially in high-strength conditions. This fracture mode results in low tensile ductility and/or fracture toughness. This article will describe methods of minimizing or eliminating grain boundary α formation by using metastable transition precipitates to nucleate α more rapidly. The effects on fracture behavior also will be described.