Influence of Processing Parameters on Microstructural Evolution and Tensile Properties for 7075 Al Alloy Prepared by an ECAPBased SIMA Process

A modified strain-induced melt activation(SIMA) process consisting of homogenization, equal-channel angular pressing(ECAP) and subsequent heating to the semisolid temperatures was introduced to prepare the 7075 aluminum alloy with superior thixotropic behaviors. The effects of both the homogenization and the number of ECAP passes, as well as the isothermal temperatures on the microstructural evolution, were investigated. The results indicate that ideal microstructure wherein fine and globular solid grains surrounded by uniform liquid films can be achieved through ECAP deformation–recrystallization mechanism. Increasing the number of ECAP passes accelerates the recrystallization of strained grains,thus reducing the average grain size and improving the grain sphericity. Moreover, higher holding temperatures and prolonged soaking time can improve the growth of the solid grains. Two main coarsening mechanisms, viz. coalescence and Ostwald ripening, contribute to the growth of the solid grains simultaneously and independently. The tensile strength of the 7075 alloys after four-pass ECAP-based SIMA and T6 heat treatment is relatively lower than the as-received billet,while the elongation of SIMA processed samples is much higher than that of as-received ones. Increasing the number of ECAP passes improves the tensile strength for alloys with and without T6 treatment due to the fine grain strengthening mechanism.     

等温处理对挤压态AZ80A镁合金半固态组织的影响

采用应变诱发熔化激活法(SIMA)工艺制备了AZ80A镁合金半固态坯料,研究了保温温度和保温时间对半固态组织的影响。结果表明:随着保温温度的升高和保温时间的增加,AZ80A镁合金的平均晶粒尺寸与液相率都呈上升趋势,形状因子呈先增大后减小的趋势。半固态组织由α-Mg晶粒、Al、Zn元素富集形成的晶界处液相和晶内“小液池”组成,其组织演变分为初始晶粒合并长大,晶粒球化、彼此分离,最终合并粗化3个阶段。采用该种方法制备AZ80A镁合金半固态坯料时合适的保温温度为550 ℃、保温时间为45 min,此时半固态组织的平均晶粒尺寸、形状因子和液相率分别为89 μm、0.795和26.7%。     

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。     

Microstructural evolution of equal channel angular pressed AZ91D magnesium alloy during semi-solid isothermal heat treatment

The microstructural evolution of AZ91D magnesium alloy processed by equal channel angular pressing during isothermal heat treatment at 570℃ was investigated. The results indicated that the equal channel angular pressing followed by semi-solid isothermal heat treatment was an effective method to prepare semisolid nondendritic slurry of AZ91D magnesium alloy. During this process, its microstructure change underwent four stages, the initial coarsening stage, the structure separation stage, the spheroidization stage and the final coarsening stage. The microstructural spheroidization effect was the best after being heated for 15 min for the alloy pressed for four passes, and the grain size was the smallest. With the further increase of heating time, the grain size and shape factor increased. When the heating time was kept constant, the grain size and shape factor decreased with the increase of pressing passes.     

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。     

Microstructure evolution of semi-solid SiCp/AZ91D nanocomposite during isothermal heat treatment process

ABSTRACT

The microstructure evolution of semi-solid SiC_p/AZ91D nanocomposite during isothermal heat treatment process in the mushy-zone was investigated. The results indicate that the nano-size SiC particles in composite are distributed uniformly and the grains are refined significantly by the addition of nano-size SiC particles. The semi-solid microstructure evolution experiences four stages during isothermal heat treatment process: the initial coarsening, structural separation, spheroidization and final coarsening. The grain size of the primary α-Mg phase decreases with the increasing of holding temperature. With the prolongation of holding time, the grain size of the primary α-Mg phase decreases at first and then increases. The optimum isothermal heat treatment parameter is 575℃ for 30min, under which the average grain diameter is 58μm and shape factor is 1.25.     

等温热处理对AZ91D+Ce镁合金半固态组织的影响

研究了等温热处理对AZ91D+Ce镁合金半固态组织的影响,获得了较理想的球状或类球状晶粒组织。结果表明,在等温热处理的过程中,加入稀土Ce会阻碍原子向固相粒子聚集和结合,抑制固相颗粒的长大,形成细小圆整的半固态组织。随等温热处理温度的升高,原子活动能力增强,熔化速度加快,液相量增加,固相颗粒尺寸先减小后增大。在等温初始阶段,熔化对初生固相颗粒尺寸起决定作用,使得颗粒尺寸减小。但是,随等温时间的进一步增加,由于合并粗化和Ostwald熟化的作用,固相颗粒开始长大。     

Liquid formation and microstructural evolution during re-heating and partial melting of an extruded A356 aluminium alloy

Thixoforming processes require feedstock having a non-dendritic, equiaxed microstructure, which in some cases can be obtained by a deformation–recrystallisation technique followed by partial melting. From the study of an extruded, re-heated and partially re-melted A356 alloy, a number of phenomena were observed and analysed: grain and particle growth, texture and low angle grain boundary formation, primary phase coarsening during isothermal holding in the semi-solid state and uneven distribution of Si particles and liquid phase. Both grain growth (solid state) and particle growth (semi-solid state) obey a R³ against time relationship. It was also found that microstructural coarsening follows an Ostwald ripening type of growth and that coalescence was also present, although exerting a minor role. Finally, phenomena such as the evolution with time of the proportion of low angle grain boundaries, and the relationship between recrystallisation, the onset of liquid formation and the coarse Si particle precipitation and dissolution are presented and discussed.     

Effects of holding temperature and time on semi-solid isothermal heat-treated microstructure of ZA84 magnesium alloy

The feasibility of fabricating ZA84 magnesium alloy with non-dendritic microstructure by a semi-solid isothermal heat treatment process and the effects of holding temperature and time on the semi-solid isothermal heat-treated microstructure of the alloy were investigated. The results indicate that it is possible to produce ZA84 alloy with non-dendritic microstructure by suitable semi-solid isothermal heat treatment. After being treated at 560-575℃ for 120min, ZA84 magnesium alloy can obtain a non-dendritic microstructure with 14.2%-25.6% liquid fraction and an average size of 56-65μm of the unmelted primary solid particles. With the increasing holding time from 30 to 120min or holding temperature from 560 to 575℃, the average size of unmelted primary solid particles decreases and globular tendency becomes more obvious. Under the experimental condition, the microstructural evolution of ZA84 alloy during semi-solid isothermal treatment is mainly composed of three stages of initial coarsening. structulseparation and spheroidization. The subsequent coarsening of spheroidal grains is not observed.     

Recycling of AZ91 Mg alloy through consolidation of machined chips by extrusion and ECAP

AZ91 Mg alloy recycled by a solid state process and equal channel angular pressing (ECAP) exhibited a superior strength. The mechanical properties of AZ91 Mg alloy recycled from machined chips by extrusion at 623 K and ECAP at 573 K and 623 K were compared with those of the reference alloy which was produced from an as-received AZ91 Mg alloy block under the same conditions as the recycled alloy. The recycled specimens show a higher strength at room temperature than the reference alloy. The improvement of the tensile properties is attributed not only to the small grain size, but also to the dispersed oxide contaminants.     

等通道挤压对AZ80镁合金的组织和织构的影响

研究了AZ80镁合金在300℃下经A路径等通道挤压后显微组织和织构的演变规律,揭示了该应变路径下织构特征形成的内在机制,并讨论了ECAP对第二相析出的影响。结果表明:材料经ECAP后最终获得3~4μm的等轴晶,并显著促进了粒状Mg17Al12相的连续析出,但增加挤压道次晶粒细化效果减弱,粒子粗化;挤压初始可获得基面法线向挤压方向偏转的倾斜织构与基面织构共存的织构特征,在剪切面倾斜度效应和通道间隙的影响下,随着挤压道次的增加,倾斜织构的偏转程度减小,最终获得单一的基面织构。     

Al2O3sf/AZ91D-Y镁基复合材料二次重熔及等温压缩研究

为了推动半固态加工在镁基复合材料成形中的应用,采用液态浸渗法制备出体积分数为10%的Al2O3sf/AZ91D-Y镁基复合材料,并采用等径道角挤压对镁基复合材料进行了形变诱导。再对镁基复合材料进行了二次重熔,并采用等温压缩实验对镁基复合材料在半固态下的力学性能进行了研究。研究表明：在550℃和560℃时延长保温时间有利于组织的球化,在560℃比550℃时,更加能促进晶粒的结晶球化;在相同的应变速率下,压缩变形时的峰值应力随着加热温度升高而降低;在相同的加热温度下,应变速率越大,峰值应力越大。     

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.     

Mechanical Behavior and Microstructural Development of 6063-T1 Aluminum Alloy Processed by Equal-Channel Angular Pressing (ECAP): Pass Number Influence

Equal-channel angular pressing (ECAP) is a very interesting method for modifying the microstructure in producing ultrafine-grained materials and nanomaterials. It consists mainly of pressing test samples through a die containing two channels that are equal in cross section and intersect at a certain angle. As a result of pressing, the sample theoretically deforms by simple shear and retains the same cross-sectional area to allow repeat pressing for several cycles. A 6063-T1 aluminum alloy was investigated in this study. The specimens were processed for up to nine passes (one, three, six, and nine ECAP passes, respectively) using a die channel angle of 90°. After ECAP, the samples were cut from each specimen and prepared for metallographic analysis and mechanical testing. The microstructures of the ECAP treated and as-received material were investigated using both optical microscopy and scanning electron microscopy. All samples (ECAP processed and as-received) were mechanically investigated in compression tests. Ultimate compressive strength, yield strength, and compression modulus were obtained. Also, all ECAP processed specimens were investigated for microhardness and compared with the as-received material.     

Thermomechanicai coupling simulation and experimental study in the isothermal ECAP processing of Ti-6Al-4V alloy

The thermomechanical coupling simulation of the isothermal equal channel angular pressing(ECAP)of Ti-6Al-4V alloy was conducted.The effect of processing parameters,ECAP pass number and the residual billet on the effective strain,stress and temperature distribution was investigated.Based on the coupling simulation results,it is found that the shear factor,ram speed,deformation temperature,channel intersection angle and residual billet significantly affect the ECAP deformation behaviors.Meanwhile,the experimental study of the isothermal ECAP process of Ti-6Al-4V alloy using route C,in which the repeated rotation angle around the longitudinal billet axis before reinsertion in channel intersection angle of 120°.Furthermore,a large amount of recrystallization occurs and some prior α phase grains grow in the post-ECAP process of Ti-6Al-4V alloy.The yield strength of post-ECAP Ti-6Al-4V alloy increases compared with that of as-received Ti-6Al-4V alloy.     

LY11合金的半固态成形研究

研究了变形温度和应变速率对半固态和供应态LY11合金峰值流动应力、稳态流动应力以及微观组织的影响.由试验结果可知:变形温度对LY11合金的峰值应力影响显著,对稳态应力影响较小.应变速率对峰值应力的影响与变形温度有关.变形温度对微观组织的影响是:随着变形温度的升高,半固态LY11合金的近球状晶粒不断长大,供应态LY11合金的不规则细长晶粒逐渐向近球状转变,这种转变有助于改善材料的流动性能.     

SiC_P/AZ61复合材料半固态触变成形的数值模拟

采用等温热处理法制备SiCP/AZ61复合材料半固态坯料。运用所建立的高固相SiCP/AZ61复合材料本构模型,对其触变成形过程进行数值模拟,得到成形过程中的应力和应变分布,并对比了触变成形和常规成形的成形特点。结果表明,SiCP/AZ61复合材料触变成形具有变形抗力小、应力分布均匀的特点。通过对比试验结果和模拟结果可知,两者吻合较好,所获结果可指导镁基复合材料触变成形工艺实践。     

SiC颗粒、保温时间对SiC_P/AZ61复合材料半固态组织的影响

研究SiC颗粒、保温时间对SiCP/AZ61复合材料半固态组织的影响,并探讨复合材料等温过程中半固态组织演变机理。结果表明,SiCP/AZ61复合材料在温度595℃,不同保温时间(0min～90min)下,其组织的演变过程为,枝晶臂合并→大块状组织→晶界处局部熔化分离→晶粒组织球化→球状组织缓慢长大。在温度595℃,保温30min～60min时,SiCP/AZ61复合材料可以获得最佳的半固态组织;与AZ61基体合金相比,由于SiC颗粒的加入,使得SiCP/AZ61复合材料在等温热处理过程中的半固态组织更为细小,并且随着SiC颗粒体积分数增加,其半固态组织中球状颗粒的尺寸越小。     

Microstructure development and thixoextrusion of magnesium alloy prepared by repetitive upsetting-extrusion

Thixoextrusion involves processing alloys with a spheroidal microstructure in the semi-solid state. Before thixoextrusion, repetitive upsetting-extrusion (RUE) is introduced into the strain induced metal activation (SIMA) process to predeform AZ80 magnesium alloy. Microstructure evolution of RUE formed AZ80 magnesium alloy during partial remelting is studied at temperatures for times. Tensile mechanical properties of thixoextruded components are determined and compared with those of AZ80 magnesium alloy thixoextruded from starting material produced by casting. The results show that with increasing number of RUE passes solid grain size decreases and the rate of liquation is improved. Prolonged holding time results in grain coarsening and the improvement of degree of spheroidization. The variation of the solid grains with holding time obeys the Lifshitz, Slyozov and Wagner law. Increasing the heating temperature is favorable for the formation of spheroidal solid grains. The tensile properties for AZ80 magnesium alloy thixoextruded from starting material produced by RUE are better than those of AZ80 magnesium alloy thixoextruded from starting material produced by casting.     

Evolution and distribution of Al2Sm phase in as-extruded AZ61–xSm magnesium alloys during semi-solid isothermal heat-treatment

The evolution and distribution of Al₂Sm phase in as-extruded AZ61–xSm (x=0, 1.5, 2.0 and 2.5, mass fraction, %) magnesium alloys during semi-solid isothermal heat treatment were investigated. The results showed that when as-extruded AZ61 magnesium alloys were modified with Sm, the smaller and rounder grains were obtained during semi-solid isothermal heat treatment. When the Sm content is 2.0% (mass fraction), the average size of the globular grains reached the smallest value of 90 μm. Although a few Al₂Sm particles existed in the α-Mg grains, most of Al₂Sm particles solidified at the edge of the globular grains with the width of ~20 μm. These phenomena are mainly attributed to the forces acting on Al₂Sm particles in front of the solid–liquid interface, leading to Al₂Sm particles accumulating at the solid–liquid interface and then solidifying at the edge of the globular grains in the quenching process.