Analysis of the hot deformation of ZK60 magnesium alloy

Hot deformation of cast-homogenized and extruded(in both the extrusion and transverse directions)ZK60 magnesium alloy was conducted using the Gleeble®3500 thermal-mechanical simulation testing system.A new approach to model the high temperature constitutive behavior of the alloy was done using two well-known equations(i.e.hyperbolic sine and Ludwig equations).For this approach,the deformation conditions were divided into regimes of low and high temperature and strain rate(four regimes).Constitutive model development was conducted in each regime and the material parameters(P)were evaluated as strain,strain rate and temperature-dependent variables;P(ε,ε,T).Using this approach,the flow curves were predicted with high accuracy relative to the experimental measurements.Moreover,detailed information on the evolution of hot deformation activation energy was obtained using the modified hyperbolic sine model.Using the modified Ludwig equation,details of strain hardening and strain rate sensitivity of the ZK60 material during hot deformation were obtained.

Efects of low frequency electromagnetic ield on surface quality, microstructure and hot-tearing tendency of semi-continuous casting ZK60 magnesium alloy billets

The low frequency electromagnetic field was applied during direct chill(DC) semi-continuous casting of the ZK60 magnesium alloy billets. Effects of low frequency electromagnetic field on surface quality, microstructure and hot-tearing tendency of Φ500 mm ZK60 magnesium alloy billets were investigated. The results showed that with the application of the low frequency electromagnetic field, the surface quality of the ZK60 magnesium alloy billets is markedly improved and the depth of cold fold is decreased. The microstructure of the billets is also significantly refined. Besides, the distribution of the grain size is relatively uniform from the billet surface towards its center, where the average grain size is 42 μm at surface and 50 μm at center. It also shows that the hot-tearing tendency of DC semi-continuous casting ZK60 magnesium alloy billets is significantly reduced under low frequency electromagnetic field.

Microstructural and mechanical response of ZK60 magnesium alloy subjected to radial forging

Radial forging(RF)is an economical manufacturing forging process,in which four dies arranged radially around the workpiece simultaneously act on the workpiece with high-frequency radial movement.In this study,a ZK60 magnesium alloy step-shaft bar was processed under different accumulated strains by RF at350℃.The deformation behavior,microstructure evolution,and mechanical responses of this bar were systematically investigated via numerical simulations and experiments.At the early deformation stage of forging,the material undergoes pronounced grain refinement but an inhomogeneous grain structure is formed due to the strain gradient along the radial direction.The grains in different radial parts were gradually refined by increasing the RF pass,resulting in a bimodal grained structure comprising coarse(~14.1μm)and fine(~2.3μm)grains.With the RF pass increased,the initial micro-sizeβ-phases were gradually crushed and dissolved into the matrix mostly,eventually evolving to form a higher area fraction of nano-sized Zn2 Zr spheroidal particles uniformly distributed through the grain interior.The texture changed as the RF strain increased,with the c-axes of most of the deformed grains rotating in the RD.Additionally,excellent mechanical properties including higher values of tensile strengths and ductility were attained after the three RFed passes,compared to the as-received sample.

Research of growth mechanism of ceramic coatings fabricated by micro-arc oxidation on magnesium alloys at high current mode

Micro-arc oxidation(MAO)coatings of ZK60 magnesium alloys were formed in a self-developed dual electrolyte composed of sodium silicate and phosphate at the high constant current of 1.8 A(15 A/dm^(2)).The MAO process and growth mechanism were investigated by scanning electron microscopy(SEM)coupled with an energy dispersive spectrometer(EDS),confocal laser scanning microscopy and X-ray diffraction(XRD).The results indicate that the growth process of MAO coating mainly goes through“forming→puncturing→rapid growth of micro-arc oxidation→large arc discharge→self-repairing”.The coating grows inward and outward at the same time in the initial stage,but outward growth of the coating is dominant later.Mg,Mg_(2)SiO_(4) and MgO are the main phases of ceramic coating.

Effects of Bi on the microstructure and mechanical property of ZK60 alloy

Microstructures and phase compositions of as-cast and extruded ZK60-xBi(x=0-1.64)alloys were investigated.Meanwhile,the tensile mechanical property and hardness were tested.With increasing the Bi content,the as-cast microstructure is first refined obviously,and then becomes coarse slightly.New small block compound which is rich in Zr,Zn,Bi and poor in Mg increases gradually,and MgZn_(2) phase decreases gradually.The second phase mainly precipitates along the grain boundary.The as-cast tensile mechanical property is first enhanced obviously,where the tensile strengthσb,yield strengthσ0.2 and elongationδcan reach 265 MPa,151 MPa and 13.5%for ZK60-0.23Bi alloy,respectively,then remains the high value for ZK60-(0.37-1.09)Bi alloys,and finally decreases obviously for ZK60-1.64Bi alloy.After hot extrusion,the obvious dynamic recrystallization occurs.Broken block compound distributes along the extrusion direction by zonal shape.The average grain size can reach only 4-6μm.The extruded tensile mechanical property is enhanced significantly,where σ_(b),σ_(0.2) and δ are at the range of 345-360 MPa,285-300 MPa and 15.5-19.5%,respectively.Extruded tensile fracture exhibits a typical character of ductile fracture.

Property and microstructure analysis of ZK60 alloy superplastic sheet

It was investigated that the superplastic mechanical properties of fine-grained ZK60 magnesium alloy sheets at the temperature range of 200-420 ℃ and strain rate range of 5.56 × 10-4 -5.56 ×10-2 s-1 by tensile tests.And the microstructure evolution during the superplastic deformation of ZK60 magnesium alloy was examined by metallurgical microscope and transmission electronic microscope (TEM).The results showed that fine-grained ZK60 magnesium alloy starts to exhibit superplasticity from 250 ℃ and the maximum elongation is about 1106% at 400 ℃ and 5.56 × 10-4 s-1.The strain rate sensitivity is significantly enhanced with the increase of temperature and with the decrease of strain rate.The predominate superplastic mechanism of ZK60 magnesium alloy is grain boundary slide (GBS) at the temperature range of 300-400 ℃.The grains of ZK60 alloy remain equaxial after superplastic deformation,and dynamic continuous recrystallization (DCRX) is an important softening mechanism and grain stability mechanism during the superplastic deformation of the alloy.The curved grain boundaries and crumpled bands at grain boundaries after deformation prove GBS generates during superplastic deformation of ZK60 magnesium alloy.

Effect of Y addition on microstructure and corrosion behavior of extruded Mg-Zn-Nd-Zr alloy

In this study,the effect of Y addition(0,0.5,1 and 2 wt%)on microstructure and corrosion properties of Mg-6Zn-0.5Zr-1 Nd(wt%)alloy was investigated.The alloys were produced by low-pressure die casting method and extruded at 300℃and 400℃after homogenization treatment at 400℃for 24 h.The results showed that the as-cast microstructure of the alloy with no Y addition consisted ofα-Mg,Mg-Zn binary and Mg-Zn-Nd ternary phases.With increasing Y additions,the average grain size showed a substantial decrease and two kinds of ternary Mg-Zn-Y ternary phases,designated as I-phase(Mg3Zn6Y)and W-phase(Mg3Zn3Y2)were formed.Homogenization treatment resulted in a partial dissolution of second phase particles.Extrusion process gave rise to a remarkable grain refinement due to the DRX mechanism.The extruded alloys with no Y addition exhibited poor corrosion resistance due to the strong micro galvanic coupling effect.Y additions up to 1 wt%improved the corrosion resistance due to the formation of finer grains,fine and uniform distribution of second phase particles and more stable oxide film.