Influence of aging on microstructure and mechanical properties of AZ80 and ZK60 magnesium alloys

The effect of aging on the microstructure and mechanical properties of AZ80 and ZK60 wrought magnesium alloys was studied with optical microscope and mechanical testers. The results demonstrate that both the tensile strength and elongation of AZ80 alloy increase firstly and then decrease as the aging temperature rises, the peak values appear when the aging temperature is 170 ℃ The hardness of ZK60 alloy increases firstly and then decreases as the aging temperature rises, and the hardness reaches its peak value at 170 ℃. However, the toughness of the alloy is just the opposite. Moreover, ZK60 alloy has good performances in both impact toughness and other mechanical properties at the aging temperature from 140 ℃ to 200 ℃.

Flow behavior and microstructure of ZK60 magnesium alloy compressed at high strain rate

Flow behavior and microstructure of a homogenized ZK60 magnesium alloy were investigated during compression in the temperature range of 250-400 ℃ and the strain rate range of 0.1-50 s^-1. The results showed that dynamic recrystallization （DRX） developed mainly at grain boundaries at lower strain rate （0.1-1 s^-1）, while in the case of higher strain rate （10-50 s^-1）, DRX occurred extensively both at twins and grain boundaries at all temperature range, especially at temperature lower than 350 ℃, which resulted in a more homogeneous microstructure than that under other deformation conditions. The DRX extent determines the hot workability of the workpiece, therefore, hot deformation at the strain rate of 10-50 s^-1 and in the temperature range of 250-350 ℃ was desirable for ZK60 alloy. Twin induced DRX during high strain rate compression included three steps. Firstly, twins with high dislocation subdivided the initial grain, then dislocation arrays subdivided the twins into subgrains, and after that DRX took place with a further increase of strain.

Microstructures and mechanical properties of laser welded wrought fine-grained ZK60 magnesium alloy

Fine-grained ZK60 magnesium alloy sheets of 2.0 mm in thickness were successfully joined by laser beam welding （LBW）. The effects of welding parameters including laser power and welding speed on the microstructures and mechanical properties of the joints were investigated. A sound bead, with the ultimate tensile strength （UTS） of 300 MPa and elongation of 12.0%, up to 92.5% and 65% of those of the base metal, respectively, is obtained with the optimized welding parameters. No liquation cracking is visible in the partially melted zone （PMZ） owing to the inhibitory action of the fine dispersed precipitates and the fine-grained microstructure in the as-rolled magnesium alloy sheets. The fusion zone （FZ） is featured with the equiaxed dendritic grains of the average grain size about 8 μm, which are similar to those in the heat affected zone （HAZ）, and this contributes to the relatively high joint efficiency.

Effects of Gd on microstructure and mechanical property of ZK60 magnesium alloy

Microstructures and phase compositions of as-cast and extruded ZK60-xGd （x=0-4） alloys were investigated. Meanwhile, the tensile mechanical property was tested. With increasing the Gd content, as-cast microstructure is refined gradually. Mg-Zn-Gd new phase increases gradually, while MgZn2 phase decreases gradually to disappear. The second phase tends to distribute along grain boundary by continuous network. As-cast tensile mechanical property is reduced slightly at ambient temperature when the Gd content does not exceed 2.98%. After extrusion by extrusion ratio of 40 and extrusion temperature of 593 K, microstructure is refined further with decreasing the average grain size to 2 μm for ZK60-2.98Gd alloy. Broken second phase distributes along the extrusion direction by zonal shape. Extruded tensile mechanical property is enhanced significantly. Tensile strength values at 298 and 473 K increase gradually from 355 and 120 MPa for ZK60 alloy to 380 and 164 MPa for ZK60-2.98Gd alloy, respectively. Extruded tensile fractures exhibit a typical character of ductile fracture.

Microstructure evolution and mechanical properties of ZK60 magnesium alloy produced by SSTT and RAP route in semi-solid state

The microstructure evolution and mechanical properties of a ZK60 magnesium alloy produced by the semi-solid thermal transformation （SSTT） route and the recrystallization and partial melting （RAP） route were studied, respectively. The microstructure evolution during partial remelting was studied at different temperatures for different time. The tensile mechanical properties of thixoformed components by the two routes at room temperature were examined. The results show that coalescence is dominant in the SSTT alloy and Ostwald ripening is dominant in the RAP alloy. Compared with the SSTT route, the RAP route can produce finer semi-solid microstructure under the similar isothermal holding condition. The microstructure of the RAP alloy is much more spheroidized compared with the SSTT alloy. Thixoforming for the ZK60 magnesium alloy produced by the SSTT and RAP route results in successful filling of the die, and the thixoforming process improves the mechanical properties of ZK60 magnesium alloy. The RAP alloy shows significantly advantageous mechanical properties over that of the SSTT alloy.

Characterization of ceramic coating on ZK60 magnesium alloy prepared in a dual electrolyte system by micro-arc oxidation

Micro-arc oxidation （MAO） process was cartied out in an optimized dual electrolyte system to fabricate a compact, smooth, and corrosion resistant coating on ZK60 Mg alloy. The microstructural characteristics of coating were investigated by scanning electron microscopy （SEM） coupled with an energy dispersive spectrometer （EDS） and X-ray diffraction （XRD）. Test of mass loss was conducted at a 3.5 % NaCl solution to assess the resistance to corrosion. The bonding strength between the coating and ZK60 substrate was evaluated using scratch experiment. The results reveal that MgA1204 and MgO are the main phases of ceramic coating obtained in the dual electrolyte system. The corrosion rate of coating prepared in the optimized dual electrolyte is only 0.0061 g.m-2.h-1, which demonstrates excellent corrosion resistance. This is mainly due to the compact, uniform coating with high bonding strength.

Influence of impurities on microstructure and mechanical properties of ZK60 magnesium alloy

The influence of impurity content on the microstructure and mechanical properties of ZK60 magnesium alloys was investigated by optical microscopy,scanning electron microscopy and tensile test.ZK60 alloys were prepared by changing holding time of alloy melt during semi-continuous casting in order to control the content of impurity elements.The alloy with lower purity content is found to have less second precipitates and larger grain size in the as-cast state.However,in the as-extruded state,reducing impurities brings about a decrease in grain size and an increase in yield strength from 244 MPa to 268 MPa,while the elongations in the as-extruded alloys with different contents of impurities are almost the same.After T5 treatment,impurity content is found to have more obvious effect on the yield strength of ZK60 alloy.The yield strength of ZK60-45 alloys with low impurity content is increased up to 295 MPa after T5 treatment.

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.

Influence of impurities on damping properties of ZK60 magnesium alloy

The influence of impurities on damping capacities of ZK60 magnesium alloys in the as-cast,as-extruded and T4-treated states was investigated by dynamically mechanical analyzer at room temperature.Granato and Lucke dislocation pinning model was employed to explain damping properties of the alloys.It is found that reducing impurity content can decrease the amount of second-phase particles,increase grain size and improve damping capacity of the as-cast alloy slightly.The as-extruded alloy with lower impurity content is found to possess obviously higher damping capacity in the relatively high strain region than that with higher impurity concentration,which appears to originate mainly from different dislocation characteristics.The variation tendency of damping property with change of impurity content after solution-treatment is also similar to that in the as-extruded and as-cast states. Meanwhile,the purification of the alloy results in an evident improvement in tensile yield strength in the as-extruded state.

Plastic deformation behavior of ZK60 magnesium alloy with addition of neodymium

The hot deformation simulation of a ZK60 magnesiuln alloy at different temperatures from 373 to 673 K and different strain rates of 0.1, 0.01 and 0.002 s^-1 was studied by using the Gleebe-1500 simulator. The plastic deformation behavior was measured and the deformation activation energy was calculated. The microstructures of ZK60 magnesium alloy with an addition of neodymium during the deformation process were observed by using Polyvar-MET optical microscope and Tecnai G^2 20 TEM. The results show that the working hardening, the dynamic recovery and the dynamic recrystallization occur during the plastic deformation process at different temperatures and strain rates. The dynamic recrystallization starts when the temperature is over 473 K and the DRX grain size after hot deformation is only 5-10 μm. So the refined grains improve both the tensile strength and the elongation of alloys at room temperature. Neodymium is added into the alloy and a precipitate phase Mg12Nd that impedes the movement of dislocations is formed, which benefits to increasing mechanical properties of ZK60 magnesium alloy.

Hot deformation behavior and processing map of squeeze cast ZK60 magnesium alloy

The deformation behavior of squeeze cast ZK60 magnesium alloy was investigated by compressive tests conducted at temperatures of 250-450℃and strain rates of 0.001-10 s-1 with Gleeble—1500D thermal simulator system. The hot deformation behavior of squeeze cast ZK60 magnesium alloy was characterized using processing map developed on the basis of the dynamic materials model. The processing map gives safe 'processing windows' in which the processes of dynamic recovery and dynamic recrystallization occur. It reveals that the dynamic recrystallization domain occurs at 375℃and strain rate of 0.001 s-1,and its power dissipation efficiency approximately corresponds to 36%, which should be considered the optimum parameters for hot working of squeeze cast ZK60 magnesium alloy. The variation of the instability parameterξ(ε) with temperature and strain rate constitutes an instability map, which is used for delineating the region of flow instability. The material exhibits flow instability which should be avoided in mechanical processing.

Microstructures and properties of reciprocating extruded as-cast ZK60 magnesium alloy

As-cast d 50 mm ZK60 magnesium alloys were reciprocating extruded for different passes under(335±5)℃. The microstructures and properties of the alloys were studied under different reciprocating extrusion passes. The results show that,reciprocating extruded microstructures were refined and uniformed effectively,in compared with as-cast condition. For 2 passes,the grain size is about 7 μm. After 4 passes,the grain size is about 6 μm. During reciprocating extrusion,refined as-cast eutectic and reinforcement phases in dispersed particles are uniformly distributed on matrix,and their size is less than 50 nm. The tensile strength,yield strength and the elongation of 4 passes reciprocating extruded as-cast ZK60 magnesium alloy are 311 MPa,214 MPa and 17.93%,respectively. The thermal expansion coefficient of RE-4-EX-ZK60-CT magnesium alloy is the lowest. More or less extrusion passes can not decrease the value of the thermal expansion coefficient.

轧后退火时间对轧制ZK60镁合金显微组织及力学和阻尼性能的影响

对大应变轧制态ZK60镁合金进行350℃退火处理,研究了退火时间(0~3 h)对其显微组织、阻尼性能和力学性能的影响。结果表明:随着轧后退火时间由0延长到3 h,ZK60镁合金发生静态再结晶的程度增大,平均晶粒尺寸由6.86μm增大至23.35μm,位错密度由3.5391×10^(15) m^(-2)减小至1.3368×10^(15) m^(-2),(0002)基面织构强度由12.782弱化至1.715,抗拉强度由309 MPa降低到291 MPa,断后伸长率由22.60%增大到28.60%。在应变振幅小于0.01时,轧后退火处理对合金阻尼性能的影响较小;在应变振幅大于0.01时,随着退火时间的延长,轧后退火态合金的阻尼性能提升。

稀土Ce变质ZK60变形镁合金的微观组织及其机制研究

在ZK60变形镁合金基体中加入了不同质量分数的稀土元素Ce。借助金相、SEM、EDS、XRD研究稀土Ce变质ZK60变形镁合金微观组织演变,探讨变质机理。结果表明,当Ce从0%增加到0.9wt%时,合金晶粒尺寸逐渐减小,尺寸由88μm减小到51μm,抗拉强度由104 MPa增加到160 MPa,伸长率从6%增加到7%;当Ce增加到1.2wt%时,晶粒尺寸反而增大到61μm,出现过变质现象;MgZn2可作为α-Mg的异质形核核心;Ce、Mg与Zn结合成Mg42Zn53Ce5化合物钉扎在晶界处,有利于减小晶粒尺寸。

Effects of Cu addition on microstructure and mechanical properties of as-cast magnesium alloy ZK60

The effects of Cu addition on the microstructure and mechanical properties of the as-cast magnesium alloy ZK60 were investigated with optical microscope, SEM, TEM, XRD, EPMA and tensile tester. The mechanism by which the mechanical properties are affected by Cu addition was discussed. The results show that Cu can effectively eliminate the intragranular solute segregations in the alloy, and the grain size of the alloy is decreased considerably with increasing the Cu amount. A ternary eutectic phase MgZnCu with a face-centered cubic structure is identified in the Cu-bearing alloys, which predominantly distributes at the grain boundary and acts as the nucleation sites of microcracks during the plastic deformation process. It is also found that the tensile properties of the alloy firstly increase by the trace addition of 0.5%-1%Cu and then decrease by a further addition up to 2.0%.

ZK60镁合金搅拌摩擦焊接头疲劳裂纹扩展分析

利用龙门式FSW设备完成ZK60板材的对焊,通过实验测试研究接头疲劳裂纹扩展性能。研究结果表明:试样表面形成了许多微小的裂纹结构。处于初期疲劳断裂阶段时,在疲劳断口区域形成了裂纹扩展小刻面。裂纹到达稳定扩展阶段后形成扇形扩断口形貌特征,并且在稳定扩展区中形成了河流状花样,生成了众多解理面与解理台阶。疲劳焊接接头出现晶粒位向变化而发生协调变形的情况。当焊核区发生疲劳断裂后,将引起再结晶晶粒数量的显著减小。该研究有助于分析焊接头的力学性能,为设计优异的焊接头期待理论支撑意义。

超声表面滚压工艺对固溶态ZK60镁合金硬度梯度及显微组织的影响

采用超声表面滚压工艺,通过粗糙度测试、显微组织观察、硬度测试等方法,研究了不同滚压静压力和滚压道次对固溶态ZK60镁合金硬度梯度及显微组织的影响。结果表明:固溶态ZK60镁合金材料通过超声滚压后,样品获得了显微梯度组织和对应的梯度硬度变化;随着超声滚压的静压力(150、250、350 N)和滚压道次(6、9、12、15和18)的不断增大,样品的表层显微硬度值与梯度强化层深度也随之增大,表层的晶粒得到一定程度的细化;同时,表层显微组织形成的流变层更加明显,样品的表面完整性更好,样品的粗造度值最小为Ra=0.135μm,表层硬度值最大为130 HV。

Weakened anisotropy of mechanical properties in rolled ZK60 magnesium alloy sheets with elevated deformation temperature

The rolling direction（RD） and the transverse direction（TD） samples were obtained from an as-rolled ZK60 magnesium alloy sheet with strong anisotropy of initial texture and their mechanical properties were tested at various deformation temperatures. Meanwhile, the microstructure and texture of these samples after fracture were investigated. Results revealed that a higher flow stress along the RD than that along the TD at room temperature were ascribed to the strong anisotropy of transitional texture, and this texture effect was remarkably weakened with the increase of deformation temperature. Deformation structure was dominant at 100℃, and was replaced by dynamic recrystallization structure when the deformation temperature increased to 200℃ and 300℃. The texture presented a strong texture（transitional texture in the RD sample and basal texture in the TD sample） at 100℃, but its intensity visibly decreased and texture components became more disperse at 200℃ and 300℃. These microstructure and texture results were employed in conjunction with calculated results to argue that raising deformation temperature could increase the activity of non-basal slip by tailoring the relative critical resolved shear stress of each deformation mode and finally result in low texture effect on mechanical anisotropy.