Microstructural evolution and mechanical properties of back-extruded Al 7075 alloy in the semi-solid state

The thixoforming process is a new method for manufacturing complicated and net shape components through which high strength materials can be formed more easily. In this study 7075 Al alloy which has low extrudability has been thixoformed by backward extrusion process. The recrystallisation and partial melting (RAP) route was used to obtain the semi-solid feedstocks for thixoforming. Microstructural evolution during partial remelting was studied at temperatures for times. Results showed that a fine and globular microstructure can be obtained by the RAP route. The results showed that high semisolid isothermal temperature would increase the liquid volume fraction and accelerate the spherical processing of the solid particles. Furthermore at long holding time, the globular grains coarsened slightly and the average grains size are increased. The experimental results showed that when the semisolid billet is hold at 580°C with the holding time, less than 30 min, the microstructure of the billet is composed of spherical grains and remnant liquids, the average grain size are smaller than 100 μm. So the remelted billet is suitable for thixoforming. In this paper, a back-extruding of 7075 Al alloy with a high solid fraction in the semi-solid state at 580°C for 10 min was performed. Mechanical properties of thixoformed components at room temperature were examined. Tempering treatment T6 has been applied after thixoforming to investigate the effects of heat treatment on mechanical properties of thixoformed parts. The tensile properties and low hardness values in the as-thixoformed 7075 Al alloy were improved by subsequent heat treatment. Post-forming heat treatment is one of the key parameters for improving the mechanical properties of thixoformed parts.     

Microstructural evolution and semisolid forming of SiC particulate reinforced AZ91HP magnesium composites

Abstract

The aim of this paper is to report the experimental results concerning the microstructural evolution and formability of semisolid forming SiC particulate reinforced AZ91HP Mg composites. The composite materials were produced in the form of cylindrical billets by a rotating cylinder method. The microstructural evolution was characterised by conventional parameters (liquid fraction, average size of globules, and number of globules per unit area) as functions of temperature and isothermal holding time in the semisolid state. In addition, special attention was paid to the shapes of the globules and the liquid droplets entrained within the globules and to the variation of hardness with regard to microstructural evolution. Finally, the formability of the composites was evaluated through thixocasting trials, and then correlated with the effective liquid fraction of the composites.     

大塑性变形的AM60镁合金半固态等温处理研究

为了制备晶粒细小且球化程度高的的AM60镁合金半固态坯料,对铸态和等径道角挤压态的AM60镁合金半固态等温处理过程进行了研究.借助金相显微镜对AM60镁合金铸坯和等径道角挤压后的铸坯在半固态等温处理中的微观组织演变进行了观察.研究结果表明:对于AM60镁合金,直接等温处理获得的半固坯晶粒很粗大,其平均晶粒尺寸都在100μm以上,晶粒球化效果不理想,很难获得合格的半固态坯;新SIMA法是一种非常理想制备AM60镁合金半固态坯的方法,利用该方法制备的AM60半固态坯的微观组织晶粒十分细小,平均晶粒尺寸在8～22μm,晶粒球化程度高;随着保温时间的延长,新SIMA法制备的AM60半固态坯的微观组织出现长大现象;随着等温处理温度的升高,固相晶粒的平均尺寸先增加后减小,晶粒球化程度越来越高.     

Partial melting behavior and thixoforming properties of extruded magnesium alloy AZ91 with and without addition of SiC particles with a volume fraction of 15%

A series of reheating-isothermal holding experiments and compression tests were conducted on pristine magnesium alloy AZ91 extruded by equal channel angular extrusion(ECAE) and Si C particles(a volume fraction of 15%) reinforced AZ91 composite(AZ91-SiC_p) by regular extrusion. Dissolution of eutectic compounds and partial melting of the α-Mg matrix occurred during the reheating of these materials. Spherical semisolid slurries of these materials were obtained when the reheating temperature and isothermal holding time were 550?C and 20 s, respectively. The presence of SiC_p in AZ91-Si Cpnot only caused lower liquid fractions of semisolid slurries but also resulted in higher values of flow stress during semisolid compression tests. Both AZ91 alloy and AZ91-Si Cpcomposite exhibited better thixoforming properties at high temperatures. Segregation of Si Cpdid not occur during thixoforming of AZ91-Si Cpcomposite after an isothermal holding at semisolid temperatures for 20 s.     

镁合金ZK60-RE半固态坯的微观组织演变

采用常规铸造法和等径道角挤压分别制备了镁合金ZK60-RE半固态坯;用金相显微镜研究了2种半固态坯料在等温热处理过程中的微观组织演变.结果表明:与传统铸造方法制备的半固态坯相比,采用等径道角挤压制备的半固态坯的晶粒细小、圆整,适合于半固态成形.在等温热处理过程中,2种坯料晶粒粗化的机制是合并长大和Ostwald长大.铸...     

022Cr25Ni7Mo4N双相不锈钢等温处理中的组织演变EI北大核心CSCD

研究经1100℃等温处理2~20 h后022Cr25Ni7Mo4N双相不锈钢的显微组织演变。观察钢中奥氏体晶粒形态变化并对其尺寸进行定量表征,测量铁素体/奥氏体两相中的元素含量变化,并探讨组织演变对实验钢中铁素体相体积分数的各向异性和低温冲击韧性的影响。结果表明:随着保温时间的延长,奥氏体晶粒发生聚集、长大、粗化现象,并伴随显著的晶粒形态变化,a/b值≥4.0时细长棒状晶粒体积分数从近20%骤降至5%以下,a/b值介于1.0~1.9的等轴晶粒体积分数显著上升的同时,尺寸≥20μm的晶粒体积分数快速增加。保温时间的延长使得Mo,Cr元素进一步向铁素体相扩散、富集,并提高铁素体相抗点蚀当量(pitting resistance equivalent number,PREN)值。细长棒状奥氏体晶粒比例的显著下降,是奥氏体体积分数各向异性改善和实验钢低温冲击韧性提高的主要原因。     

Coarsening of equiaxed microstructure in the semisolid state of aluminum 7075 alloy through SIMA processing

In the present study, the coarsening mechanism of equiaxed grains in the semisolid state of aluminum 7075 alloy, treated via strain induced melting activation process, was investigated. The kinetics of equiaxed grain growth in the semisolid state of the experimental alloy was determined. The results revealed that when the holding temperature increased, the coarsening rate constant (K) showed a precipitously increasing character in the range of 590–610 °C. This was attributed to the extensive effect of the coalescence mechanism on the grain growth at the high solid fractions. By further increasing the holding temperature to 620 and 625 °C (increasing the liquid fraction), the effect of coalescence on the grain growth appeared to be weakened, that is, although there was a slight decrease at 620 °C, a gently increasing character could be generally supposed. Severe segregation of Zn and Cu alloying elements at grain boundaries and intragranular droplets was detected at 620 and 625 °C after 15 and 10 min, respectively.     

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Abstract

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (～20 μm) and a coarse average grain size (～260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ～534°C.     

Al_2O_(3P)/Al-Cu复合材料的二次加热组织演变

采用原位反应近液相线铸造方法制备Al2O3P/Al-Cu复合材料,对其进行二次加热,研究晶粒的形貌演变和长大规律。用光学显微镜观察组织结构,应用Image Pro Plus软件测量并统计出平均晶粒尺寸及合金液相体积分数,并与理论计算数值进行比较。结果表明,在590℃保温10~60 min后,不含Al2O3颗粒的Al-6.8%Cu基体合金平均晶粒尺寸为89~132μm,液相体积分数为14%~26.8%,而3.6 wt%Al2O3P/Al-6.8%Cu复合材料的平均晶粒尺寸为73~107μm,液相体积分数为11.6%~20.9%。说明Al2O3颗粒在合金的二次加热过程中对晶粒长大行为及液相体积分数的增长均有明显的抑制作用,从而为优化半固态组织提供了一种新思路。     

3A21铝合金半固态坯料制备工艺研究

以3A21铝合金为研究对象,将等径角挤压工艺与等温处理工艺相结合,从实验角度研究其中的工艺参数对半固态组织尺寸形貌的影响。采用Bc路径进行ECAP3道次处理,然后在660℃下保温20,25,30min,得到27个试样的半固态组织。结果表明,随着挤压道次的增加,晶粒尺寸减小,变形更加均匀,并且累积变形能的增多,也为后续半固态等温处理提供了更好的应变诱导条件;保温时间越长,半固态组织演化越完全,晶粒球化越完整,但晶粒尺寸会随着保温时间的延长而长大。最后得出最佳工艺参数匹配:室温下沿Bc路径等径角挤压3道次,660℃下保温25min;最终半固态坯料显微组织的平均等积圆直径d=83.7μm,平均形状系数Fc=0.84。     

Effect of Prior Austenite Grain Size on the Morphology and Mechanical Properties of Martensite in Medium Carbon Steel

In industrial application,unintentional manufacturing line troubles often consequence in heating raw materials excessively,in terms of either time or temperature.One of the effects of such occurrence is a product with a variation of prior austenite grain size,even if after the heat treatment the end result is the same,martensite.The variation of the prior austenite grain size is believed to vary the end results of the martensite.This undesirable variation includes the variation of fatigue resistance,impact strength,yield strength,hardness,etc.This research studies the effect of the prior austenite grain size on the morphology of the martensite microstructure.The results show that within the typical industrial application of temperature and holding time set up,as holding time or the temperature increases,the prior austenite average diameter increases.The block and packet sizes in the martensite also increase.The variation of mechanical property dependence on the grain size is indeed due to the different characteristics reflected in the martensite morphology.With respect to the same area,smaller grain has more blocks and packets,which agrees with higher dislocation density verified with transmission electron microscopic evaluation.     

Effects of pulsed magnetic field on microstructures and morphology of the primary phase in semisolid A356 Al slurry

A novel technique for introducing pulsed magnetic field (PMF) during isothermal holding period of A356 Al alloy has been employed to prepare semisolid slurry. The effects of vibration frequency of PMF on morphology of the primary phase in semisolid A356 Al alloy slurry were studied, and some characteristic parameters characterized the morphology and grain size of the primary phase were obtained. The results show that the primary α-Al particles become smaller and rounder with the increase of vibration frequency. The slurry with primary α-Al particle average diameter of about 79 μm and average shape factor of about 0.58 can be prepared under the action of a PMF at 10 Hz and 600 °C for 4 min. It is feasible to use PMF processing to prepare semisolid alloy slurry, because of its strong forced convection within the whole bulk melt.     

X120管线钢奥氏体长大规律研究

研究了不同保温温度和等温时间对奥氏体晶粒长大规律的影响,结果表明:随加热温度的升高和保温时间的延长,奥氏体晶粒尺寸逐渐增加。当温度低于1150℃时,奥氏体晶粒尺寸长大缓慢;加热温度超过1150℃后,晶粒尺寸长大速度显著增加,建立描述X120管线钢奥氏体晶粒长大动力学模型为:D6.1-D06.1=2.08×1027t exp(-327028RT),拟合结果良好。     

Viscosity and structure variations of Al–Si alloy in the semi-solid state

The viscosity and microstructure evolution of A356 Al–Si alloy in the semi-solid state were investigated. During continuous cooling, the viscosity increases slowly at the onset of the semi-solid region, and starts to increase abruptly after reaching a critical temperature. The higher the shear rate, the lower is the critical temperature. During isothermal stirring, the viscosity decreases and the sphericity increases as the shear rate increases. The grain size decreases and the sphericity increases as the shear rate increases for short stirring times. The viscosity increases as the temperature decreases during isothermal stirring. The spheroidizing effect is not significant for short stirring time. During isothermal stirring, the viscosity increases as the rest time increases. Both the grain size and sphericity increase as the rest time, following stirring, increases. The coarsening exponent is 3.2, suggesting that the coarsening is controlled by the volume diffusion of the grains. During isothermal stirring, the viscosity decreases as the stirring time increases. The sphericity increases and the grain size decreases as the stirring time increases. We propose that there is a critical stirring time, above which the grain size increases with stirring time, but below which the grain size decreases with stirring time. This revised version was published online in November 2006 with corrections to the Cover Date.     

盾构刀具用5Cr5MoSiV1钢的奥氏体晶粒长大规律研究

研究了盾构刀具用5Cr5MoSiV1钢淬火时在不同的奥氏体化温度和保温时间下的奥氏体晶粒长大规律.结果表明:在1000～1150℃加热时,试验钢奥氏体的平均晶粒尺寸D-随加热温度T的升高而增大,且二者间的定量关系近似服从Arrhenius关系,合适的奥氏体化温度应选在1000℃左右;该钢在1050℃等温加热时,奥氏体晶粒随保温时间的延长近似呈抛物线关系长大,等温长大指数η接近1/2.     

Segregation of Liquid Phase in the Extruded Semisolid Metal

The influence of stress and strain on the solid/liquid segregation formed in deformed semisolid metal was studied. The compression experiment at a low constant strain rate was conducted on a Gleeble 1500 thermo-mechanical simulator.The stress and strain fields of the samples were calculated by finite element method. The experiment results and the modeling results prove that the hydrostatic pressure gradient is the driving force for solid/liquid segregation. The segregated liquid phase usually agglomerates in the micropores and shear bands are created by sliding of equiaxed dendrite grains. The sliding among the grains is influenced by the solid fraction and grain size of the semisolid slurry,so as the solid fraction and grain size change, the morphology of the segregated liquid changes.     

高碳钢奥氏体晶粒长大的预测

借助于高温共聚焦显微镜(CLSM)、透射电镜(TEM)研究含Ti钢和无Ti钢的奥氏体晶粒长大行为。试样在1123~1473K之间保温60min时测量一系列温度下不同保温时间的奥氏体晶粒尺寸。结果表明:两种钢奥氏体晶粒尺寸随着温度的上升而增大;另外,两种钢奥氏体晶粒尺寸随时间的延长而长大,并符合抛物线方程。并且,观察到了第二相粒子,用第二相粒子的熟化公式和体积公式分别计算两种钢的含Ti粒子尺寸与体积分数。同时,采用修正的Gladman公式预测两种钢的奥氏体晶粒长大,实验结果和预测结果吻合较好。     

Quantitative analysis of the microstructure of transitional region under multi-heat isothermal local loading forming of TA15 titanium alloy

In this paper, an analogue experiment was carried out to study the effect of processing parameters including deformation temperature, deformation degree, cooling mode and loading pass on the microstructure of transitional region under isothermal local loading forming of TA15 titanium alloy. The volume fraction, grain size and aspect ratio of primary α phase of transitional region were quantitatively characterized. It is found that deformation temperature and deformation degree also have interaction on the microstructure evolution of transitional region under isothermal local loading forming. At a certain deformation degree, primary α grain size increases first and then decreases with increasing temperature. However, primary α grain size varies little with deformation degree at higher temperature (in upper two phase region) but increases firstly and then decreases with deformation degree at lower temperature (in lower two phase region). Primary α aspect ratio increases with deformation degree at lower temperature but varies little at higher temperature. The morphology of transformed structure in β matrix is greatly influenced by deformation temperature and less influenced by deformation degree under air-cooling. The precipitated Widmanstatten α phase in β matrix is in lamellar form and arranges in colonies under air-cooling, but it is in thinner acicular form and distributes disorderly under water quenching. Loading pass has little influence on the morphology of microstructure.     

Steady State Rheological Characteristic of Semisolid Magnesium Alloy

Isothermal compressive experiments at different temperatures, strain rates and holding time for semisolid AZ91D, Zr modified AZ91D and MB15 alloy with higher solid volume fraction were carried out by using Gleeble-1500D simulator and the true stress-strain curves were given directly. The relationship of apparent viscosity vs temperature, shear rate and holding time of the three kinds of semi-solid magnesium alloys, as well as isothermal steady state rheological characteristic and mechanical behavior were studied. The results show that the three magnesium alloys had the characteristic of shear-thinning. The rheological characteristic of the semi-solid MB15 is different from that of semi-solid AZ91D. The semi-solid MB15 has higher apparent viscosity and deformation resistance.     

Prediction model for the austenite grain growth in a hot rolled dual phase steel

The main purpose of this work is to develop a pragmatic model to predict isothermal austenite grain growth in a hot rolled dual phase steel. Austenite grain growth kinetics have been studied in different heating conditions, involving soaking time and soaking temperature as well as heating rate. The contribution of the initial grain size to the time exponent in Beck equation was analyzed mathematically. The time exponent and initial grain size were also quantitatively described by models proposed. It was found that the time exponent value which had a wide range increased when the temperature decreased. No meaningful activation energy can be obtained for the grain growth process when the time exponent varies. The initial grain size increases with the soaking temperature. The model predictions present a good agreement with experimental austenite grain growth data.