Effects of moulding sands and wall thickness on microstructure and mechanical properties of Sr-modified A356 aluminum casting alloy

The effects of different cooling conditions on the mechanical properties and microstructures of a Sr-modified A356 （Al-7Si-0.3Mg） aluminum casting alloy were comparatively investigated using three moulding sands including quartz, alumina and chromite into multi-step blocks. The results show that the mechanical properties and microstructures using chromite sand are the best. As the cooling speed increases, the dendrite arm spacing （DAS） decreases significantly and the mechanical properties are improved, and the elongation is more sensitive to the cooling speed as compared with the tensile strength. The increase of the properties is primarily attributed to the decrease of the DAS and the increase of the free strontium atoms in the matrix. In particular, the regression models for predicting both the tensile strength and the elongation for Sr-modified A356 aluminum casting alloy were established based on the experimental data.

Influence of serpentine channel pouring process parameters on semi-solid A356 aluminum alloy slurry

Semi-solid A356 aluminum alloy slurry was prepared by using serpentine channel pouring process, and the influences of the channel diameters and pouring temperatures on the semi-solid A356 aluminum alloy slurry were investigated. The experimental results show that when the channel diameter is 20 and 25 mm, respectively, and the pouring temperature is 640-680 ℃, the average diameter of primary α（Al） grains in the prepared A356 aluminum alloy slurry is 50-75 and 55-78 μm, respectively, and the average shape factor of primary α（Al） grains is 0.89-0.76 and 0.86-0.72, respectively. With the decline in the pouring temperature, the microstructure of semi-solid A356 aluminum alloy slurry is more desirable and a serpentine channel with smaller diameter is also advantageous to the microstructure imProvement. During the preparation of semi-solid A356 aluminum alloy slurry, a large number of nuclei can be produced by the chilling effect of the serpentine channel, and owing to the combined effect of the chilled nuclei separation and melt self-stirring, primary α（Al） nuclei can be multiplied and spheroidized finally.

Gating system optimization of low pressure casting A356 aluminum alloy intake manifold based on numerical simulation

To eliminate the shrinkage porosity in low pressure casting of an A356 aluminum alloy intake manifold casting, numerical simulation on fi lling and solidifi cation processes of the casting was carried out using the ProCAST software. The gating system of the casting is optimized according to the simulation results. Results show that when the gating system consists of only one sprue, the fi lling of the molten metal is not stable; and the casting does not follow the sequence solidifi cation, and many shrinkage porosities are observed through the casting. After the gating system is improved by adding one runner and two in-gates, the fi lling time is prolonged from 4.0 s to 4.5 s, the fi lling of molten metal becomes stable, but this casting does not follow the sequence solidifi cation either. Some shrinkage porosity is also observed in the hot spots of the casting. When the gating system was further improved by adding risers and chill to the hot spots of the casting, the shrinkage porosity defects were eliminated completely. Finally, by using the optimized gating system the A356 aluminum alloy intake manifold casting with integrated shape and smooth surface as well as dense microstructure was successfully produced.

Effect of various melt and heat treatment conditions on impact toughness of A356 aluminum alloy

The microstructure and impact behavior of A356 aluminum alloy were studied after melt treatment processes of grain refinement and modification under both non-heat treated and T6 heat treated conditions. The modification and grain refinement were done with the addition of Al-10%Sr and Al-5Ti-1B master alloys, respectively. All casting parameters were kept constant in order to focus on the influence of mentioned treatments. The results indicate that the eutectic silicon morphology is the main parameter to control the impact behavior of alloy. Consequently, the individual grain refinement of as-cast alloy does not improve the impact toughness as the modification does. While, simultaneous grain refinement and modification provide higher impact toughness in comparison with individual treatments. T6 heat treatment of the alloy improves the impact toughness under all melt-treated conditions. This is related to the further modification of eutectic silicon particles. To verify the results and clarify the mechanisms, three-point bending test and fractography were used to interpret the improvement of impact toughness of the alloy.

Taper barrel rheomoulding process for semi-solid slurry preparation and microstructure evolution of A356 aluminum alloy

The self-developed taper barrel rheomoulding (TBR) machine for light alloy semi-solid slurry preparation was introduced.The semi-solid slurry was obtained from the intense shearing turbulence of the alloy melt in the cause of solidification, which was further caused by the relative rotation of the internal and external taper barrel whose surface contained wale and groove.The heat transmission model of TBR process, the flow rules and the shearing model of the alloy melt were deduced.Taking A365 as experimental material, the microstructure evolution rules under different slurry preparation processes were analyzed.The results show that decreasing the pouring temperature of A365 alloy melt properly or increasing the shearing rate helps to obtain ideal semi-solid microstructure with the primary particle size of about 70 μm and the shape factor of above 0.8.

Filling ability of semi-solid A356 aluminum alloy slurry in rheo-diecasting

The effects of slurry temperature, injection pressure, and piston velocity on the rheo-filling ability of semisolid A356 alloys were studied by the reho-diecasting methods. The results show that the slurry temperature of the semi-solid A356 aluminum alloy has an important effect on the filling ability; the higher the slurry temperature, the better is the filling ability, and the appropriate slurry temperature for rheo-filling is in the range of 585-595℃. The injection pressure also has a great effect on the filling ability, and it is appropriate to the rheo-filling when the injection pressure is in the range of 15-25 MPa. The piston velocity also has a great effect on the filling ability, and it is appropriate to the rheo-filling when the piston velocity is in the range of 0.072-0.12 m/s. The filling ability of the slurry prepared by low superheat pouring with weak electromagnetic stirring is very good and the microstructural distribution in the rheo-formed die castings is homogeneous, which is advantageous to the high quality die casting. 2008 University of Science and Technology Beijing. All rights reserved.

Numerical simulation on the rheo-diecasting of the semi-solid A356 aluminum alloy

The effect of injection pressure, piston velocity, and the forming temperature of semisolid slurry on the filling behavior of the semi-solid A356 aluminum alloy was investigated by simulation methods. The simulation results show that these processing parameters have an important effect on the filling behavior of the semi-solid A356 aluminum alloy. The slurry flows steadily in the cavity when the injection pressure, the piston velocity, and the forming temperature are low, but it is prone to turbulent flow when the injection pressure, the piston velocity, and the forming temperature are much higher. Therefore it is necessary to determine the appropriate processing parameters to get a steady flow of the slurry in the cavity.

Influence of melt pouring temperature and plate inclination on solidification and microstructure of A356 aluminum alloy produced using oblique plate

The preparation of semisolid slurry of A356 aluminum alloy using an oblique plate was investigated. A356 alloy melt undergoes partial solidification when it flows down on an oblique plate cooled from underneath by counter flowing water. It results in continuous formation of columnar dendrites on plate wall. Due to forced convection, these dendrites are sheared off into equiaxed/fragmented grains and then washed away continuously to produce semisolid slurry at plate exit. Melt pouring temperature provides required condition of solidification whereas plate inclination enables necessary shear for producing semisolid slurry of desired quality. Slurry obtained was solidified in metal mould to produce semisolid-cast billets of desired microstructure. Furthermore, semisolid-cast billets were heat treated to improve surface quality. Microstructures of both semisolid-cast and heat-treated billets were analyzed. Effects of melt pouring temperature and plate inclination on solidification and microstructure of billets produced using oblique plate were described. The investigations involved four different melt pouring temperatures (620, 625, 630 and 635 °C) associated with four different plate inclinations (30°, 45°, 60° and 75°). Melt pouring temperature of 625 °C with plate inclination of 60° shows fine and globular microstructures and it is the optimum.

Rheological behavior,microstructure and hardness of A356 aluminum alloy in semisolid state using backward extrusion process

The influence of temperature on the flow behavior and rheological characteristics of an A356 alloy in the semi-solid state was investigated using backward extrusion process.Experiments were performed at 5 temperatures and 4 different wall thicknesses.Viscosities were determined using the force-displacement graphs obtained form back extrusion tests.As observed experimentally,at a constant temperature,the increase of shear rate results in the decrease of alloy viscosity exponentially.Raising the temperature increases the liquid fraction hence reduces the semi-solid alloy viscosity.Metallographic and image analyses show that,because of low forming speed,liquid has time to escape from solid phase forward the sample wall.This condition is the main reason for the segregation phenomenon seen in the base and walls.Vickers hardness test on samples reveals that the hardness increases with the decrease of temperature and wall thickness.

(Al_(63)Cu_(25)Fe_(12))_p/A356 aluminum alloy composites prepared by spray co-deposition

A novel Al-based composite material (Al63Cu25Fe12)p/A356 was prepared by spray co-deposition. It is revealed that the reinforcement of Al63Cu25Fe12 quasicrystalline particles disperses uniformly in the composite with small crystalline grain structure of about 10μm. The reaction between the Al63Cu25Fe12 quasicrystalline particle and the matrix metal is constrained or depressed because of the high cooling velocity in the process of spray co-deposition. Compared with the composite reinforced by non-continuous ceramics particle, the aging harden behavior of the composites of (Al63Cu25Fe12)p/A356 has outstanding characteristics with less time on aging peak happening and with higher hardening rate. The mechanical properties of the composites evidently enhance except plastic strain.

TiO_(2)变质对A356铝合金微观组织的影响

以TiO_(2)粉末为变质细化剂,研究了其对A356铝合金预处理后α-Al相和共晶Si的细化变质作用。结果表明,当TiO_(2)的最佳添加量为0.5wt%时,α-Al晶粒的二次枝晶间距可降至20μm,共晶Si组织呈短棒状和球状;此时,合金的布氏硬度较未变质前提高了25%,达到67.9 HBW。分析认为,铝合金熔体与TiO_(2)原位生成γ-Al_(2)O_(3)同时细化变质了α-Al相和共晶Si相,协同γ-Al_(2)O_(3)的颗粒增强作用共同提高了材料的性能。

脉冲磁场孕育处理下A356铝合金凝固组织的演变规律

基于自行设计的磁场晶粒细化装置,研究在不同频率的脉冲磁场孕育处理下A356铝合金凝固组织的演变规律,利用蔡司显微镜(OM)、场发射扫描电镜(FESEM)、X射线衍射仪(XRD)、差示扫描量热仪(DSC)等测试方法对A356铝合金的凝固组织进行分析。结果表明:脉冲磁场孕育处理能降低形核激活能与临界形核半径,影响液相线以上原子团的演变规律,使得原子团的尺寸减小,数目增加,并在更小的临界尺度上形成大量稳定晶核,从而导致A356铝合金凝固组织中初生α-Al相和共晶Si相显著细化;其中频率为30 Hz的脉冲磁场细化效果最佳,与未处理相比,试样初生α-Al晶粒平均尺寸下降39%,共晶Si平均尺寸下降60%。同时α-Al晶粒磁各向异性产生的磁各向异性能与磁转矩导致α-Al晶粒发生旋转,使得晶粒沿(200)面择优生长。

A356铝合金套筒低压铸造工艺参数的优化

为了探究最优的A356铝合金套筒低压铸造工艺参数组合,本文综合运用了铸造数值模拟、响应面分析法及遗传算法,以最小缩孔缩松体积和最低二次枝晶臂间距值为优化目标,对上模具预热温度、侧模具预热温度、下模具预热温度和浇注温度进行匹配及优化。结果表明,优化的工艺参数组合为上模具预热温度307℃,侧模具预热温度为330℃,下模具预热温度为394℃,浇注温度为701℃。上述优化参数组合能有效提高铸件的成型质量及力学性能。

稀土Y及Al-5Ti-B细化剂复合添加对A356铝合金组织和力学性能的影响

用Al-20Y变质剂和Al-5Ti-B细化剂处理A356铝合金熔体,研究稀土Y与Al-5Ti-B复合添加对A356铝合金组织和力学性能的影响。结果表明:稀土Y和Al-5Ti-B复合添加能提高形核率、细化α-Al晶粒尺寸和改变共晶硅形貌,共晶硅的形貌由针状和板片状变为短杆状和颗粒状,使铸造铝合金的力学性能提高。添加稀土Y和Al-5Ti-B的A356铸造铝合金的抗拉强度、屈服强度和延伸率分别为146 MPa、98 MPa和4.0%。

A356铝合金的表面改性与耐磨及耐蚀性能研究

为了提升A356铝合金的表面耐磨耐蚀性能,采用等离子喷涂的方法在A356铝合金基体表面制备了4种高熵合金涂层(FeCoCrNi和FeCoCrNi+Mn/Cu/Ti),对比分析了各等离子喷涂涂层的物相组成、显微形貌、硬度、耐磨和耐蚀性能。结果表明4种等离子喷涂涂层中都可见面心立方固溶体和氧化物的衍射峰。在FeCoCrNi中加入Cu/Ti后,涂层中还出现了Cu/Ti的衍射峰。4种等离子喷涂涂层厚度都介于210~300μm,涂层与基体界面以机械结合为主。4种等离子喷涂涂层的硬度、平均摩擦系数都高于A356铝合金基体,磨损率从低至高顺序为:FeCoCrNi(涂层Ⅰ)<FeCoCrNiTi(涂层Ⅳ)<FeCoCrNiMn(涂层Ⅱ)<FeCoCrNiCu(涂层Ⅲ)<基体。相较A356铝合金基体,涂层的腐蚀电位发生正向移动,腐蚀电流密度和点蚀电位减小,Nyquist圆弧半径、阻抗模值|Z|0.01 Hz和电荷转移电阻Rct增大。4种等离子喷涂涂层的耐蚀性能都优于A356铝合金基体,且涂层Ⅳ的耐腐蚀性能最好。

TiC添加对A356铝合金组织 与力学性能的影响研究

采用搅拌熔铸法,制备了组织结构均匀的TiC/A356铝合金,研究了TiC含量对A356铝合金的物相组成、微观组织和力学性能的影响规律。结果表明,随着TiC含量的增加,TiC/A356铝合金的拉伸强度和伸长率呈先上升后降低的规律,维氏硬度则持续上升。当TiC含量为3wt%时,合金的综合性能最佳,其极限抗拉强度为197.1 MPa、伸长率为9.10%、维氏硬度为84.7 HV。此外,还分析了TiC复合强化A356合金的作用机理模型。

A356铝合金热处理工艺的数值模拟

基于Abaqus有限元分析软件,研究A356铝合金经热处理后的温度变化规律以及残余应力的大小和分布情况,并对A356铝合金进行拉伸模拟,将得到的拉伸模拟结果与实际拉伸试验结果进行对比分析。结果表明,固溶处理可以有效消除材料表面的残余应力,尤其是可以基本消除合金棱边处的残余应力,而淬火后合金的残余应力反而变大。经过时效处理后合金表面的残余应力明显减小,能有效避免裂纹的产生,在固溶温度为555℃时铝合金的残余应力最小,约为77 MPa,此时A356铝合金的抗拉强度最大,约为300 MPa,与拉伸试验结果符合。采用有限元分析软件对A356铝合金的热处理进行模拟分析,验证最佳固溶温度并计算出最小残余应力,对抗拉强度进行预测,为工程结构应用可提供一定参考。

基于多目标粒子群算法的A356铝合金蠕变本构模型的构建

在150,250,300℃温度和20~150 MPa应力下,对A356铝合金进行单向拉伸蠕变试验,研究了其蠕变特性;采用多目标粒子群算法得到Norton-Bailey幂律模型、Garofalo双曲正弦模型和基于变形机制的真应力蠕变模型的参数,并采用这3种模型对A356铝合金的最小蠕变速率-应力曲线进行预测,分析3种模型的预测精度。结果表明:A356铝合金的最小蠕变速率与试验温度和应力成正相关,但温度对最小蠕变速率的影响程度大于应力;基于变形机制的真应力蠕变模型具有最高的预测精度,对最小蠕变速率的预测结果中90%数据点均位于2倍误差带内。结合多目标粒子群算法得到的基于变形机制的真应力蠕变模型可以用来准确模拟宽幅温度和应力范围下A356铝合金的蠕变特性。

固溶处理和时效处理对A356铝合金性能的影响及较优处理温度确定

目的揭示A356铸造铝合金不同固溶处理温度与合金硬度之间的关系,详细分析A356铸造铝合金在不同温度时效处理过程中原子尺度下组织结构的演变规律。方法采用4种固溶处理工艺(525℃×5 h、550℃×5 h、575℃×5 h和600℃×5 h),并借助金相显微镜、扫描电子显微镜(Scanning Electron Microscope)、能谱仪(Energy Dispersive Spectrometry)、高分辨透射电子显微镜(High-Resolution Transmission Electron Microscopy)和背散射电子显微技术(Electron Backscatter Diffraction)等现代分析检测手段对汽车转向节用A356铸造铝合金进行微观组织分析。结果在550℃下进行5 h的固溶处理后,α相中溶入了较多的Mg2Si和Si晶体,出现固溶强化现象,A356铝合金的硬度达到93HV。随着时效温度提高至200℃,α相中的硅原子和镁原子发生扩散现象,并在间隙、位错处汇聚,导致系统能量下降,回火硬度急剧上升,硬度也相应增加,材料的维氏强度达到了峰值113HV。随着人工时效处理温度继续升高,硬度呈现出持续变弱的态势,表明合金内部的共格关系逐渐遭到破坏,析出强化效果减弱。结论A356铝合金在550℃固溶处理5 h后进行200℃时效处理,硬度达到峰值113HV,随后时效温度升高,硬度逐渐减弱,表明共格关系逐渐遭到破坏,析出强化效果减弱;经550℃固溶处理后,200℃时效1 h可获取A356铝合金的最佳性能,为获取高质量的A356汽车转向节铸件提供了重要的科学依据。

精炼工艺对A356铝合金熔体渣含量及性能的影响

采用Prefil-Footprinter铝渣检测仪、室温拉伸机、金相显微镜等多种检测分析手段,研究了精炼工艺对A356合金熔体渣含量及力学性能的影响。研究结果显示,加入精炼剂量为铝液总量的0.15%,以20 L/min流量通入高纯度氩气15 min,静置20 min后进行检测,此时A356合金熔体中渣含量最低,且力学性能最高。因此该精炼工艺将对现场A356铝合金锭熔炼和生产具有一定的指导意义。