AZ91镁合金半固态凝固组织特征的研究

将经过压缩形变的AZ91镁合金坯料加热到半固态温度,等温处理后挤入模具,分析了合金加热与凝固组织的变化过程.结果表明:形变AZ91镁合金在二次加热过程中出现晶内"小液池"特征,其组织与晶界处相同,均为(α β)共晶体.     

脉冲电场下制备AZ91D镁合金部分重熔过程的组织演变

研究利用低压脉冲电场技术制备的AZ91D镁合金在部分重熔过程中的组织演变,考察加热温度和保温时间对不同脉冲电压制备的AZ91D合金初生相形态和尺寸的影响。结果表明:经低压脉冲电场处理后,将AZ91D镁合金进行部分重熔可得到非枝晶的半固态合金,随坯料制备的脉冲电压增加,重熔时初生相变得圆整且尺寸减小,但制备坯料的脉冲电压过高,重熔时初生相尺寸有所增加;脉冲电场处理后的AZ91D镁合金坯料经适当的部分重熔处理可获得具备触变过程所需要的组织状态。     

加热工艺对AZ91D镁合金部分重熔组织的影响

提出半固态金属坯料先在液相线以上温度适当加热再降低温度至两相区温度继续等温保温的二次加热工艺。采用该工艺对晶粒细化AZ91D镁合金坯料进行部分重熔,研究了其组织演变规律,并与等温二次加热工艺进行比较。结果表明,与等温二次加热工艺相比,坯料先在液相线以上温度适当加热再降低温度至两相区温度继续保温,坯料重熔速度明显加快,相同加热时间时,晶粒更加细小和圆整。组织演变机理分析表明,加快液相形成速度可适当抑制晶粒的合并,降低晶粒长大速度,并促进晶粒球化。     

二次加热过程中半固态AZ31镁合金的显微组织演变

利用波浪形倾斜板振动技术制备AZ31镁合金半固态坯料,获得较为理想的球形或近球形晶粒组织。结果表明:随二次加热温度的升高和保温时间的延长,半固态组织中的液相体积分数增大,固相逐渐长大并球化;AZ31镁合金580℃和610℃时二次加热组织均不适合半固态触变成形;适合触变成形的二次加热最优工艺为590℃保温40～60 min、或者600℃保温30 min;此条件下获得的平均晶粒直径为58～61μm,固相率为87%(体积分数)左右。晶格扩散机制对二次加热原子扩散起主导作用,是造成合金固相颗粒尺寸变化的根本原因;固液界面张力是造成颗粒形状球形或近球形变化的重要原因。     

形变镁合金半固态加热过程中的再结晶及晶粒球状化

实验研究了应变诱发熔化激活法(SIMA)制备AZ91D合金半固态坯料过程中的再结晶现象及其对晶粒球化的作用。结果发现：形变AZ91D镁合金半固态熔化过程中，无论加热速度快慢，组织都要发生再结晶，演变为等轴晶粒。加热速度仅影响到等轴晶粒的均匀性；再结晶是形变组织球化过程的必要条件。快速升温到半固态温度时，再结晶几乎与球状化过程同时进行。     

双层金属管用半固态坯料制备及二次加热

采用机械搅拌的方法制备半固态浆料,利用专门的制坯模按照预定尺寸制得能够使用于挤压成形双层金属管的半固态AZ91镁合金棒料和A356铝合金坯料,研究制备工艺以及二次加热温度及保温时间对半固态坯料微观组织的影响.通过组织分析,对双层金属管用AZ91镁合金坯料和A356铝合金坯料的触变性进行了研究.结果表明,双层金属管用AZ91镁合金坯料最佳尺寸为24 mm,二次加热温度为560 ℃,保温时间为21 min;A356铝合金环状坯料最佳尺寸壁厚为8 mm,二次加热温度为600 ℃,保温时间为20 min时,此时能得到适合于进行半固态触变成形的球化组织.     

两相区铸造AZ91D半固态坯料的部分重熔工艺与组织演变

对在两相区铸造进行半固态制浆所得AZ91D镁合金坯料的部分重熔工艺条件及组织演变进行了研究. 结果表明 AZ91D镁合金在两相区温度(585 ℃)静置30 min且在水冷铁模中浇注的两相区铸造半固态坯料, 在575 ℃下保温30～45 min进行部分重熔时, 其组织具有固相颗粒球化程度高、颗粒细小且粒度均匀、有效液相体积分数高等特点; 坯料组织演变初始化阶段、球化阶段和集聚合并长大等3阶段部分相互重叠. 对特定原始半固态坯料, 改变部分重熔温度, 可缩短重叠区, 扩大球化区, 实现坯料重熔进程及组织形态的优化, 从而增加工艺可控性和提高坯料的触变性.     

AZ91D镁合金波浪型倾斜板振动技术触变成形

采用自制的波浪型倾斜板振动装置对AZ91D镁合金半固态坯料的制备及触变成形进行研究。结果表明,在倾角为45？,振幅1.45 mm,浇注温度630-650℃的条件下,采用波浪型倾斜板振动技术可以制备出组织优良的半固态AZ91D镁合金坯料,坯料由细小的等轴晶组成,并在二次加热温度为575℃、保温时间30-60 min的条件下球化理想。通过触变锻压,在模具的预热温度为400-450℃时,制备出表面光洁、组织优良的成品制件。触变锻压时,液相流动是主要的变形方式。制品上部由于液相偏聚,硬度较低;下部由于固相的微塑性变形,硬度较高。     

加热温度对AZ91D镁合金部分重熔组织的影响

分别在固液两相区580 ℃和600 ℃、620 ℃对晶粒细化AZ91D镁合金坯料进行等温部分重熔,利用光学显微镜,研究了加热温度对坯料部分重熔组织的影响.结果表明,加热温度越高,坯料重熔速度越快,重熔后晶粒越细小.组织演变机理分析表明,提高加热温度,加快液相形成速度对晶粒合并长大具有一定的抑制作用,有利于细化部分重熔晶粒组织.     

Sr细化AZ91D合金部分重熔过程的组织演变

研究了添加微量Sr细化的AZ91D镁合金部分重熔过程中的组织演变.考察了加热温度和等温时间对合金初生相形态和尺寸的影响.结果表明:添加质量分数为0.1%的Sr,在610℃浇注的AZ91D镁合金部分重熔后,可以获得初生相为球状或粒状的非枝晶半同态合金,基本具备触变过程所需要的组织状态;等温时间一定时,随着重熔温度提高,半固态组织球化过程加快,但加热温度过高,试样表面有大量液相渗出,并在自重作用下严重变形;当重熔温度一定时,随等温时间延长,初生相变得愈加膪l整,但等温时间过长,初生相有长大倾向.     

SEMI-SOLID MICROSTRUCTURE AND ITS EVOLUTION OF WROUGHT ALUMINUM ALLOY BY DIRECTLY HEATING-ISOTHERMAL TREATMENT

Microstructure evolution of wrought aluminum alloy extruded rods and the mechanism of liquid phase formation during reheating were investigated. And the relation between the volume fraction of liquid phase and the recrystallization microstructure was proposed. The results show that increase in reheating temperature and time can augment the volume fraction of liquid phase and accelerate the grain spheroidization, as a result of which the requirement of semi-solid forming can be satisfied. Due to the higher aberration energy of grain boundary, the melting point is lowered as a result of the easy diffusion of atoms. At higher reheating temperature the grain boundary melts, the growth of the recrystallized grain is inhibited and the grain is refined. The composition of the low melt-point phase along the recrystallized grains was determined using EDS. It can be seen from the experimental results that when the extrusion rod of the wrought aluminum alloy is reheated at 610℃ for 20min, perfect fine equiaxial grains can be obtained, the average grain size is about 66.34μm and the volume fraction of solid phase is about 68%.     

原位内生TiB_2/Al-4Cu复合材料半固态二次加热组织演化

对原位内生TiB2/Al-4Cu复合材料半固态坯料进行二次加热,利用光学显微镜,图像分析仪等手段,对坯料二次加热微观组织的演化进行了研究。结果表明,随着加热温度的升高和保温时间的延长,液相分数增加,α(Al)晶粒发生了长大和圆整化。TiB2/Al-4Cu复合材料合适的半固态重熔参数为:加热温度570～600℃,保温时间小于10min。组织演化机制分析表明,二次加热初期,液相少,晶粒主要通过快速合并长大。随着加热温度的升高和保温时间的延长,液相增加,晶粒主要通过原子扩散缓慢长大并发生球化。     

Influence of Liquid Film Characteristics on Hot Cracking Initiation in Al-Cu Alloys at the End of Solidification

In this study, Al-4Cu alloy specimens with spherical grains and liquid films were obtained by isothermal reheating treatment. The hot cracking of the solidification process was determined using a modified constrained rod casting experimental apparatus, and the effect of liquid film characteristics at the end of solidification on hot cracking initiation of Al-4Cu alloys was systematically investigated by combining molecular dynamics simulations and other methods. With the extension of soaking time, the liquid fraction (liquid film fraction at the end of solidification) and grain shape factor increased with higher isothermal reheating temperatures. Additionally, the widened filling channel decreased the hot cracking initiation temperature and the critical hot cracking shrinkage stress was found to increase, thus reducing the hot cracking severity in Al-4Cu alloys. Molecular dynamics simulations revealed that with the extension of soaking time, the composition of the liquid film changed at different isothermal reheating temperatures, but the short-range structure and atomic ordering of the liquid film remained the same. The activity of the liquid film increased in equilibrium, leading to a decrease in viscosity and an increase in fluidity, which contributed to the filling behaviour. After isothermal reheating at 640 °C for 60 min, the liquid fraction reached the maximum, and the viscosity of the liquid film was the minimum. In addition, almost no hot cracks were found.     

W—Ni—Fe纳米晶粉末的流变特性

研究了W-Ni-Fe纳米晶粉末的注射成形（MIM）。纳米晶粉末采用机械合金化（MA）的方法制备,然后将这种纳米晶粉末与蜡基粘剂混合以制备出喂料。讨论了MA球磨时间、纳米粉末的体积装载量和实验温度以喂流料流变行为的影响。结果表明,随球磨时间增加,喂料粘度降低,粘度对剪切速率的敏感性也降低,因此,随球磨时间增加,喂料的流动性和成形性变好,随粉末装载量增加,喂料粘度呈非线性增加,喂料粘度与粉末装载量的关系为:η=η0A[1-（ΦΦm）]∧-n,其中n=0.68,粉末经球磨后,粘度变化随温度和剪切速率的变化不大。因此,注射温度和注射速度的变化对MTM产品的质量影响不大。     

Anomalous reheating behavior of SiCp/Al composite with high volume fraction reinforcement

The reheating behavior of 50 vol.% SiCp/Al squeeze casting composite was investigated at temperatures ranging from 600°C to 900°C using XRD and SEM techniques from the microstructural point of view. It was found that SiCp/Al composite could hold its original shape while being reheated at temperatures elevated even far above the melting temperature of pure Al. The high volume fraction of SiC reinforcement, which would restrict the fluidity of molten Al matrix and the reconfiguration of SiC particles during the reheating of SiCp/Al composite, was thought to be responsible for the “remelting resistance” of the SiCp/Al composite. The extent of the reaction between the SiC particles and molten Al was found to increase with increased reheating temperature. From the viewpoint of controlling the formation of aluminum carbide, reheating temperature either for recycling or for remelting processing of the SiCp/Al composite, a temperature lower than 750°C would be better. Despite its being unfavorable to remelting or recycling processing, the remelting resistance of the SiCp/Al composite with high volume fraction reinforcement is attractive for thermal function and high temperature applications.     

AZ61合金半固态二次加热工艺及组织演变

研究了应力诱发熔体激活法(SIMA)制备的AZ61镁合金半固态坯料在二次加热时加热温度和保温时间对其组织的影响,研究表明,二次加热初期半固态组织首先熔合合并,随着保温时间延长,晶粒逐渐长大和球化,液相份数增加;保温温度越高,晶粒长大和球化速度加快。在592℃加热、保温20min~40min,可以获得均匀、圆整的半固态组织,晶粒大小为80μm~90μm,液相率为40%~42%。高于597℃时,试样重熔过程中易发生严重变形。     

半固态5083铝合金的二次加热组织与触变轧制

采用大挤压比热挤压预变形的SIMA法制备了5083铝合金半固态坯料,研究了在不同加热温度和保温时间条件下二次加热重熔组织的演变规律,以及不同工艺参数对一道次触变轧制后带材力学性能的影响.结果表明,在二次加热过程中,晶粒形状和液相率主要受加热温度影响,而受加热保温时间的影响不大.在一道次触变轧制中,当二次加热温度为600℃,轧制变形量为60％时,可以获得抗拉强度为260.93MPa,伸长率为26.81％的较好综合力学性能的带材.经40％变形量二次冷轧后,带材的抗拉强度提高了70MPa.结合拉伸断口的宏观和微观形貌分析,可知带材的断裂方式为微孔聚集型的韧性断裂.     

半固态铝合金重熔组织演变的研究

通过自行研制开发的新型半固态连续机械搅拌设备,制备了半固态铝合金,并对半固态坯料在半固态温度区间重熔加热,研究不同重熔温度、时间下半固态组织的变化规律.研究表明:保温温度越高,晶粒长大和球化速度加快,保温时间越短;随着保温时间延长,晶粒逐渐长大和球化,液相份数增加.半固态铝合金Y112重熔加热适宜温度区间为565～575℃.     

Semisolid forging electronic packaging shell with silicon carbon-reinforced copper composites

To fabricate electronic packaging shell of copper-matrix composite with characteristics of high thermal conductivity and low thermal expansion coefficient, semisolid forming technology, and powder metallurgy was combined. Conventional mechanical mixing of Cu and SiC could have insufficient wettability, and a new method of semisolid processing was introduced for billets preparation. The SiC/Cu composites were first prepared by PM, and then, semisolid reheating was performed for the successive semisolid forging. Composite billets with SiC 35% volume fraction were compacted and sintered pressurelessly, microstructure analysis showed that the composites prepared by PM had high density, and the combination between SiC particles and Cu-alloy was good. Semisolid reheating was the crucial factor in determining the microstructure and thixotropic property of the billet. An optimised reheating strategy was proposed: temperature 1,025 °C and holding time 5 min.