Dingfei Zhang

Microstructure Evolution and Mechanical Properties of Mg-x%Zn-1%Mn(x=4, 5, 6, 7, 8, 9) Wrought Magnesium Alloys

The roles of Zn content and thermo-mechanical treatment in affecting microstructures and mechanical properties of Mg-x%Zn-1%Mn (mass fraction, x=4, 5, 6, 7, 8, 9) wrought Mg alloys were investigated. The microstructure was extremely refined by dynamic recrystallization (DRC) during extrusion. With increasing Zn content, the DRC grains tended to grow up, at the same time, more second phase streamlines would be present, which restricted the further growing. During solution treatment, the DRC grains would rapidly grow up; however, higher Zn content could hinder the grain boundary expanding, which results in finer ultimate grains. MgZn2 dispersoid particles which are coherent with the matrix would precipitate from the supersaturated solid solution during the one-step aging process, and nano-sized GP zones formed during the pre-aging stage of the two-step aging provide a huge amount of effective nuclei for the MgZn2 phases formed in the second stage, which makes the MgZn2 particles much finer and more dispersed. The mechanical properties of as-extruded samples were not so sensitive to the variation of Zn content, the tensile strength fluctuates between 300 and 320 MPa, and the elongation maintains a high value between 11% and 14%. The strength of aged samples rises as a parabolic curve with increasing Zn content, specifically, the tensile strength of one-step aged samples rises from 278 to 374 MPa, and that of two-step aged ones rises from 284 to 378 MPa, yet the elongation of all aged samples is below 8%. When Zn content exceeds its solid solution limit in Mg-Zn system (6.2%, mass fraction), the strength rises slowly but the elongation deteriorates sharply, so a Mg-Zn-Mn alloy with 6% Zn possesses the best mechanical properties, that is, the tensile strengths after one-and two-step aging are 352 and 366 MPa, respectively, and the corresponding elongations are 7.98% and 5.2%, respectively.

Effects of Ce addition on microstructure and mechanical properties of Mg-6Zn-1Mn alloy

The effects of Ce addition on the microstructure of Mg-6Zn-1Mn alloy during casting, homogenization, hot extrusion, T4, T6 and T4+two-step aging were investigated. The mechanical properties of alloys with and without Ce were compared. The results showed that Ce had an obvious effect on the microstructure of ZM61-0.5Ce alloy by restricting the occurrence of dynamic recrystallization and restraining the grain growth during extrusion and heat treatment subsequently. A new binary phase Mg12Ce was identified in ZM61-0.5Ce alloy, which distributed at grain boundaries and was broken to small particles distributed at grain boundaries along extrusion direction during extrusion. The mechanical properties of as-extruded ZM61-0.5Ce alloy were improved with the addition of Ce. The improved tensile properties of as-extruded ZM61-0.5Ce alloy were due to the finer grain sizes as compared to ZM61 alloy. However, the UTS and YS decreased severely and the elongation increased when ZM61-0.5Ce was treated by T6 and T4+two-step aging. Brittle Mg12Ce phase, which was distributed at the grain boundary areas and cannot dissolve into the Mg matrix after solution treatment, became crack source under tensile stress.

Numerical and physical simulation of new SPD method combining extrusion and equal channel angular pressing for AZ31 magnesium alloy

Abstract A new serve plastic deformation (SPD) including initial forward extrusion and subsequent shearing process (ES) was proposed. The influence of the ES forming on the grain refinement of the microstructure was researched. The components of ES forming die were manufactured and installed to Gleeble1500D thermo-mechanical simulator. The microstructure observations were carried out on the as-extruded rods (as-received) and ES formed rods. From the simulation results, ES forming can increase the cumulative strain enormously and the volume fraction of dynamic recrystallization. From the physical modeling results, the microstructures can be refined.

Prediction of edge cracks and plastic-damage analysis of Mg alloy sheet in rolling

Abstract A thermal-mechanical-damage coupled finite elements model was established to investigate temperature changes, edge cracks and rolling force during rolling of magnesium alloy sheet. A cuneal sheet was also adopted to study the influence of reduction on temperature, damage and rolling force. The results show that with increasing the reduction, the rolling force increases, and the temperature of the Mg sheet decreases. Edge cracks occur when the reduction is above 51.6%, with the damage value of above 0.49. The plastic-damage in Mg sheet rolling is a result of hole development, shearing deformation and accumulative plastic strain.

Die structure optimization of equal channel angular extrusion for AZ31 magnesium alloy based on finite element method

Abstract Three-dimensional(3D) geometric models with different corner angles (90° and 120°) and with or without inner round fillets in the bottom die were designed. Some important process parameters were regarded as the calculation conditions used in DEFORM TM -3D software, such as stress—strain data of compression test for AZ31 magnesium, temperatures of die and billet, and friction coefficient. Influence of friction coefficient on deformation process was discussed. The results show that reasonable lubrication condition is important to plastic deformation. The change characteristics for distributions of effective stress and strain during an equal channel angular extrusion (ECAE) process with inner angle of 90° and without fillets at outer corner were described. Inhomogeneity index ( C ) was defined and deformation heterogeneity of ECAE was analyzed from the simulation and experiment results. The deformation homogeneity caused by fillets at outer corner increased compared with the die without fillets. The cumulated maximum strains decrease with increasing the fillets of outer corner in ECAE die and the inner corner angle. The analysis results show that better structures of ECAE die including appropriate outer corner fillet and the inner corner angle of 90° for the die can improve the strain and ensure plastic deformation homogenization to a certain extent. The required extrusion force drops with increasing the fillet made at outer corner in ECAE die. It is demonstrated that the prediction results are in good agreement with experiments and the theoretical calculation and the research conclusions in literatures.

Grain refinement in AZ31 magnesium alloy rod fabricated by extrusion-shearing severe plastic deformation process

A new severe plastic deformation (SPD) method that is extrusion-shearing (ES), which includes initial forward extrusion and shearing process subsequently, was developed to fabricate the fine grained AZ31 Mg alloys. The components of ES die were manufactured and installed to gleeble1500D thermo-mechanical simulator. Microstructure observations were carried out in different positions of ES formed rods. The results show that homogeneous microstructures with mean grain size of 2 μm are obtained at lower temperature as the accumulated true strain is 2.44. Occurring of continuous dynamic recrystallization (DRX) is the main reason for grain refinement during ES process. The experimental results show that the ES process effectively refines the grains of AZ31 magnesium. The production results of ES extrusion with industrial extruder under different extrusion conditions show that the ES extrusion can be applied in large-scale industry.

Analysis of the cracks formation on surface of extruded magnesium rod based on numerical modeling and experimental verification

To reduce the surface cracks of extrusion rod for AZ31 magnesium caused by nonhomogeneous metal flow in extrusion process, 3D computer finite element (FE) simulations of extruding a wrought magnesium alloy AZ31 into rods have been performed and the results have been verified in extrusion experiments under identical conditions. The tendency to generate the dead zone is decreased by employing the die angle 60° at the cone-shaped die comparing with the die angle 180°. The surface additional tensile stresses of the rod at the die exit are decreased greatly so that the surface cracks caused are avoided by using the die angle 60°. The extrusion die with die angle 180° would increase the higher temperature rise and possibility of crack formation on the rod surface that caused by die angle 60° and temperature rise decrease tensile strength of the AZ31 rod. The experimental results show that die angle 180° could cause continuous cracks on the surface of the extruded rod. The extrusion force required is reduced approximately 15 ton by employing the die angle 60°. Theoretical results obtained by the Deform™-3D simulation agreed well with the experiments. The obtained results provide the fundamental and also practical guidelines for the design and correction of dies to produce magnesium rod with good surface quality.

Microstructure and tensile properties of as extruded and as aged Mg–Al–Zn–Mn–Sn alloy

Abstract The effect of 0–4 wt-% Sn addition on the microstructure and tensile properties of AZ80 alloys was investigated. The results indicated that Mg2Sn particles were barely formed during the extrusion process until the content of Sn is >2 wt-%. The dislocation density in alloys after extrusion declined with the addition of Sn due to the promotion of dynamic recrystallisation after adding Sn. In aging treatment, Mg17Al12 precipitates were promoted by Sn and the phases distributed uniformly at low density level of dislocation. The AZ80-2 wt-% Sn alloy possessed the excellent tensile properties in as extruded and as aged state.

Effects of heat treatment on microstructure and mechanical properties of ZK60 Mg alloy

The microstructure and mechanical properties of ZK60 Mg alloy were investigated under different solution treatments and artificial aging conditions. When as-cast ZK60 alloy was solution treated at 400℃ for 10 h and artificially aged at 150℃, the volume fraction of precipitates increased with the aging time up to 30 h. When the as-cast ZK60 alloy was solution treated at 400℃ for 10 h and artificially aged at 200℃ for 15-20 h, the volume fraction of precipitates reached a peak value. Tensile test at room temperature showed that a high density of the second phase precipitates was beneficial to improving the strength and elongation. Solution treatment at 400℃ for 10 h and artificial aging at 150℃ for 30 h is considered the optimum heat treatment condition to obtain a good combination of strength and ductility.

Influence of pre-deformation on age-hardening response and mechanical properties of extruded Mg–6%Zn–1%Mn alloy

Abstract The effect of cold plastic deformation between solution treatment and artificial aging on the age-hardening response and mechanical properties of alloy was investigated by micro-hardness test, tensile test, optical microscopy (OM) and TEM observation. After solution treatment at 420 °C for 1 h, three kinds of pre-deformation strains, i.e. 0, 5% and 10%, were applied to extruded ZM61 bars. Age-hardening curves show that pre-deformation can significantly accelerate the precipitation kinetics and increase peak-hardness value; however, as pre-deformation strain rises from 5% to 10%, there is no gain in peak hardness value. The room temperature (RT) tensile properties demonstrate that increasing the pre-deformation degree can enhance the yield strength (YS) and ultimate tensile strength (UTS) but moderately reduce elongation (EL); furthermore, the enhancement of YS is larger than that of UTS. No twin can be observed in 5% pre-deformed microstructure; however, a large number of twins are activated after 10% pre-deformation. The peak-aged TEM microstructure shows that pre-deformation can increase the number density of rod-shaped β 1 ′ precipitates which play a key role in strengthening ZM61 alloy.