Thixotropic behavior of semi-solid AZ91D magnesium alloy

The thixotropie behavior of semi-solid AZ91D slurry was studied through a Couette type viseometer.The results show that the apparent viscosity of semi-solid AZ91D magnesium alloy slurry increases after being isothermally held, but the apparent viscosity quickly falls down to a steady state value after being stirred again and it takes on a sharp shear-thinning behavior. With the same shearing rate and the rest time increasing, the steady apparent viscosity increases because of the agglomeration of the solid particles, and the time required for the slurry to reach the steady state also becomes longer. If the solid fraction increases, it takes longer time for the slurry to reach the steady apparent viscosity with the same shearing rate and the same rest time. If the solid fraction and the rest time are the same, but the shearing rate rises, it takes shorter time for the slurry to reach the steady apparent viscosity and the final steady apparent viscosity also decreases.     

半固态AZ91D镁合金的触变性

利用Couette型同轴双桶流变仪，对等温搅拌和重熔加热的半固态AZ91D镁合金浆料的触变性能进行了研究．实验结果表明：经过连续降温和等温搅拌，半固态浆料的表观粘度逐渐降低，并达到一个稳态值；此时，若停止搅拌剪切，并经过某一时段的等温静置，再次在某一剪切速率下搅拌时，半固态浆料的表观粘度会突然升高，然后再迅速降低，最后到达一个稳态值，呈现出典型的剪切变稀特性；延长静置时间、或增加固相分数、或降低剪切速率，再次剪切搅拌时，半固态浆料到达稳态表观粘度的时间随之增长，同时稳态表观粘度值也相应增大；重熔加热后的半固态浆料的稳态表观粘度比相同固相分数的等温搅拌的半固态浆料的稳态表观粘度低；随重熔加热保温时间的延长，半固态浆料的稳态表观粘度逐渐降低，当稳态表观粘度达到最低值后，随重熔加热保温时间的延长，稳态表观粘度又逐渐增大．     

连续冷却条件下半固态AZ91D镁合金组织及流变特性

在自行设计的Couette型同轴双桶流变仪上对半固态AZ91D镁合金浆料的流变性能以及组织进行了研究。结果表明，连续冷却条件下，连续搅拌半固态AZ91D镁合金浆料的表面粘度随固相分数的增大先缓慢而后迅速增大。此外，冷却速率和剪切速率也都显著影响半固态AZ91D镁合金浆料的表面粘度，降低冷却速率和增大剪切速率，都将导致表观粘度下降，显微组织观察表明，降低冷却速率和增大剪切速率都有利于固相颗粒形态向非枝晶形态转变，从而导致表观粘度的降低。     

半固态废弃铝硅合金的稳态流变性能的研究

利用同轴双筒流变仪研究了以废旧铝料为原料的半固态铝硅合金浆料的稳态流变性能.结果表明,半固态铝硅合金浆料的稳态表观黏度随固相分数的增加而增加,但随剪切速率的增大而减小,并得出稳态表观黏度的公式来描述固相率、剪切速率对稳态表观黏度的影响.     

Rheological behavior of semi-solid 7075 duminum alloy at steady state

The further application of semi-solid processing lies in the in-depth fundamental study like rheological behavior. In this research, the apparent viscosity of the semi-solid slurry of 7075 alloy was measured using a Couette type viscometer. The effects of solid fraction and shearing rate on the apparent viscosity of this alloy were investigated under different processing conditions. It can be seen that the apparent viscosity increases with an increase in the solid fraction from 10% to 50% （temperature 620 ℃ to 630 ℃） at steady state. When the solid fraction was fixed, the apparent viscosity can be decreased by altering the shearing rate from 61.235 s-1 to 489.88 s-1 at steady state. An empirical equation that shows the effects of solid fraction and shearing rate on the apparent viscosity is fitted： ηα = [0.871 - 0.00849. γ0.74924]. exp（3.7311, fs） . The microstructure of quenched samples was examined to understand the alloy＇s rheological behavior.     

半固态AZ91D镁合金压铸过程的数值模拟

采用机械搅拌方式研究了半固态AZ91D合金的流变特性,并根据实验数据,拟合出剪切速率为238s-1时,半固态AZ91D合金的固相体积分数和表观粘度之间的关系式。从流体力学的角度,采用数值技术对半固态AZ91D镁合金的压铸过程进行了研究。数值模拟结果表明,半固态浆料温度为550℃、模具温度为300℃、压铸速度为1m/s时,该文所设计的压铸件的压铸过程能很好地进行。同时,该研究工作能为分析压铸成形过程提供有益的信息。     

超声原位Al2Y /AZ91镁基复合材料流变模型研究

利用高能超声法原位制备Al2Y /AZ91镁基复合材料半固态浆料,对其半固态表观粘度进行测量,实验表明：半固态Al2Y /AZ91镁基复合材料表观粘度随固相分数和Al2Y的体积分数的增加而增大,半固态表观粘度与固相分数之间关系按指数规律变化,在相同的实验条件下,半固态表观粘度随超声功率增大而减小。在对实验数据拟合的基础上,建立了半固态Al2Y /AZ91镁基复合材料的流变模型。     

半固态A356铝合金的稳态流变性能

采用Couette型同轴双筒流变仪对半固态A356铝合金的等温稳态表观粘度进行了测试．结果表明;半固态A356合金的稳态表观粘度（ηa）随着固相分数（fs）的增加而增加,但随剪切速率（γ）的增大而减小,当,fs达到某一临界值之后,ηa迅速增加．可用方程ηa=2．79exp（6．27fa）γ^-0.7描述fs,γ对ηa的影响．     

半固态AlSi6Mg2铝合金的触变性

利用Couette型同轴圆桶流变仪研究了等温搅拌的半固态AlSi6Mg2铝合金浆料的触变性能。试验结果表明：经过连续冷却和等温搅拌，半固态AlSi6Mg2铝合金浆料的表观粘度逐渐降低，并达到一个稳态值；此时，若停止搅拌并保温静置一段时间，又以某一剪切速率搅拌，半固态AlSi6Mg2铝合金浆料的表观粘度会突然增加，然后再迅速降低，最后达到一个稳态值，呈现出典型的剪切变稀特性；静置时间越长，合金浆料在开始剪切瞬间的表观粘度越大，同时达到稳态所需时间也越长，其稳态表观粘度值也略有提高；等温搅拌过程中突然增加剪切速率，半固态AlSi6Mg2铝合金浆料的表观粘度会突然降低，并逐步达到稳态；当剪切速率突然降低时，合金浆料的表观粘度会突然增加，然后逐步达到稳态。而且，剪切速率突然增加之后达到稳态所需要的时间要比剪切速率突然降低之后达到稳态所需要的时间短得多。     

半固态Al-6.5Si合金的流变特性

用半固态金属流变模型研究了Al-6.5si(质量分数，％)体系的稳态和暂态流变行为．分析表明，半固态金属的流变行为与其微结构有很强的关联，在固相体积分数一定的情况下，体系的表观粘度完全由其微结构所决定，而剪切速率和剪切时间对表观粘度的影响是通过改变体系的微结构来实现的．稳态粘度随固相体积分数的增加而增加、随剪切速率的增加而减小，这与文献中的实验结果相符合；体系的解聚过程比聚集过程快两个数量级．在各种形变条件下所出现的滞后环的大小由上升时间、静置时间、原初剪切速率、最大剪切速率等因素决定．     

AZ91D镁合金半固态浆料制备的研究

采用AZ91D镁合金为试验原料，用自行研制的新型半固态浆料设备及直接流变成形装置为试验设备，研究不同温度、剪切速率下半固态组织的变化规律，并与常规铸造组织相比较。研究表明：适量提高剪切速率或降低温度，半固态组织的固相颗粒会变得细小、均匀和圆整。同时，对新型装置制备半固态组织的机理进行了研究。     

半固态Mg2Si/AM60复合材料的流变特性研究

利用半固态成形技术与原位反应相结合,用机械搅拌实验装置对半固态Mg2Si/AM60浆料进行剪切实验,研究了固相分数、剪切速率和剪切时间等工艺参数对半固态Mg2Si/AM60复合材料流变特性的影响.结果表明,表观粘度随Si含量及固相分数的增加而增加,随剪切速率的增大和剪切时间的延长而降低,剪切时间达到300 s后,其表观粘度不再随剪切时间的延长而继续减小,达到一个稳态值.     

半固态金属流变仪的研制

根据同轴圆筒旋转法粘度测量原理自行研制了1台半固态金属流变仪，并通过测量甘油的粘度来检验流变仪的测量精度。在3个不同温度下测得的甘油绝对粘度值与标准值接近。用该台流变仪研究半固态A356铝合金的流变特性表明：半固态浆料稳态表观粘度随着剪切速率的增大而下降，且浆料温度越低，伪塑性越明显；当固相率小于10％时，在高剪切速率下浆料呈现出牛顿流体的特征；在连续冷却条件下，半固态浆料的表观粘度随着温度的降低而先缓慢后迅速增大，冷却速度显著影响半固态合金的表观粘度。     

热压制工艺回收AZ91D镁合金屑的半固态组织演变(英文)

以从机械加工现场直接收集的AZ91D镁合金屑料为原料,采用热压制工艺再生制备半固态加工所需的坯料,研究二次加热过程中的半固态组织演变。结果表明,半固态组织演变可分为三个阶段:Mg17Al12相的溶解和Al原子的扩散,高溶质含量区域的熔化和固相颗粒的生成,固相颗粒的球化和长大。同时,在固相颗粒内部形成大量的嵌入式液滴。这种嵌入式液滴的数量和尺寸随着保温时间的变化而变化。随着保温时间的延长,固、液两相最终达到动态平衡,固相率不再发生变化。由于界面能降低和界面张力的作用使固相颗粒尺寸逐渐增大,并且越来越圆整。     

Microstructural evolution and tensile mechanical properties of thixoformed AZ91D magnesium alloy with the addition of yttrium

The microstructure evolution of AZ91D magnesium alloy in the semi-solid state has been proposed or reported in previous literature. However, no detailed investigation has been conducted regarding the relationship between the microstructure and tensile mechanical properties of the thixoformed AZ91D magnesium alloy. In this paper, the microstructure of AZ91D alloy with the addition of yttrium was produced by the semi-solid thermal transformation (SSTT) route and the strain-induced melt activation (SIMA) route, respectively. Isothermal holding experiments investigated grain coarsening and the degree of spheroidization as a function of holding time in the semi-solid state. The SSTT route and the SIMA route were used to obtain the semi-solid feedstock for thixoforming. The results show that solid particles of the SSTT alloy are spheroidized to some extent but the previous irregular shape is still obvious in some of them. While the SIMA alloy exhibits ideal, fine microstructure, in which completely spheroidized solid particles contain little entrapped liquid. The microstructure of the SSTT alloy is less spheroidized compared with the SIMA alloy under the similar isothermal holding condition. As the holding time increases, the mean solid particle size of the SSTT alloy decreases initially, then increases, while the mean solid particle size of the SIMA alloy increases monotonously at 560 °C. Compared with the SSTT alloy, the SIMA alloy obtains finer grains under the similar isothermal holding condition. The mechanical properties of the thixoformed AZ91D alloy with the addition of yttrium produced by the SIMA route are better than those of the thixoformed alloy produced by the SSTT route. The ultimate tensile strength, yield strength and elongation for the thixoformed alloy produced by the SIMA route are 303.1 MPa, 147.6 MPa and 13.27%, respectively. The tensile properties for the AZ91D alloy with the addition of yttrium thixoformed from starting material produced by the SIMA route are better than those of the AZ91D alloy with the addition of yttrium thixoformed from starting material produced by the SSTT route.     

半固态条件下丁二腈-水合金的稳态流变行为

采用Searle型同轴双筒剪切流变仪考察了由类金属丁二腈(SCN)和水配制的SCN-5at.%Wat.透明合金在半固态条件下的稳态流变行为。SCN-5at.%Wat.合金的稳态表观粘度表现出类似半固态金属浆料的剪切稀化现象。当冷却速率较高时,SCN-5at.%Wat.合金的稳态表观粘度随初始剪切速率的提高而减小;而当冷却速率较低时,其随初始剪切速率的提高略有增大。当初始剪切速率较低时,SCN-5at.%Wat.合金的稳态表观粘度随冷却速率的提高而增大;而当初始剪切速率较高时,其随冷却速率的提高,在较高的终了温度下表现为减小,在较低的终了温度下反而略有增大。另外,在较低初始剪切速率和较高冷却速率下,SCN-5at.%Wat.合金的稳态表观粘度总体较高。     

Effects of SiCp on microstructures of semi-solid extruded AZ91D magnesium alloys in recrystallization process

AZ91D and 8.5vol.% SiC_p/AZ91D magnesium matrix composites were fabricated by a semi-solid extruded processing method,and treated with solution and aging heat treatment.The effects of SiC_p on the microstructures of the semi-solid extruded AZ91D magnesium alloy during recrystallization were studied by observing and analyzing the microstructure evolution during extrusions and heat treatments.The results show that the addition of SiC_p inhibits the dynamic recrystallization of AZ91D during the semi-solid extrusion with only 26% of the volume fraction of recrystallization.Furthermore,the addition of SiC_p refines the sizes of grains and second phases,and upgrades the volume fraction of second phase.After solution and aging treatment,the recrystallization continues,and the addition of SiC_p promotes the recrystallization and the recrystallized microstructure is much more stable.Meanwhile,the sizes of grains and second phases continue to be refined,and the volume fraction of second phases continues to increase.     

添加0.5%富铈混合稀土AZ91D镁合金半固态组织的形成

半固态浆料的固相颗粒尺寸、形态和分布主要取决于熔化过程中液相的形成与演化过程。采用添加0.5%富铈混合稀土来改善AZ91D镁合金的铸态组织,研究在半固态等温热处理中的组织演变以及非枝晶组织制备与控制的机理。结果表明:稀土合金化处理可促进初生相在等温热处理过程中由枝晶向粒状晶的转变,可获得更加细小、均匀的球状固相颗粒,并且其粗化速度较慢。半固态等温热处理过程中,整个系统处于熔化和结晶的动态平衡,铸态组织中枝晶根部高溶质浓度区或系统的温度、浓度起伏是固相颗粒内液相形成的内在和外在条件。     

半固态A356铝合金流变特性研究

用自行研制的同轴圆柱旋转流变仪对半固态A356铝合金浆料的流变特性及组织演变进行研究.结果表明:等温剪切时,浆料稳态表观粘度随着剪切速率的增大而下降,半固态A356铝合金具有明显的触变性.在连续冷却条件下,半固态浆料的表观粘度随着温度的降低而先缓慢后迅速增大,冷却速度显著影响半固态A356铝合金的表观粘度,降低冷却速度,由于搅拌充分而导致表观粘度明显下降.显微结构分析也可以反映出剪切速率、温度和冷却速度对表观粘度的影响规律.     

搅拌作用下半固态AZ91D合金组织的数学描述

分析了搅拌速度和二次冷却速度对半固态AZ91D合金组织平均尺寸和形状因子的影响,将搅拌速度与二次冷却速度作为凝固组织演变的主要因素。分析发现,增加搅拌速度对细化初生相颗粒、提高颗粒圆整度是有利的;而提高冷却速度虽有利于细化晶粒,但不利于提高颗粒圆整度。应用多元非线性回归方法描述凝固组织随搅拌速度与二次凝固速率的关系建立了相关的本构方程,为获取理想凝固组织提供理论参考。