冷却速率对A356铝合金半固态浆料凝固组织的影响

通过将半固态浆料浇注到不同冷却速率的模具中,研究冷却速率对半固态浆料凝固组织的影响。结果表明,铸件的凝固组织受冷却速率的影响较大,随着冷却速率的降低,晶粒的形状逐渐向近球形演化,同时伴随着晶粒尺寸的长大。综合考虑凝固组织中的晶粒尺寸和形状的变化规律,当冷却速率为0.40~0.31℃/s时,所得凝固组织最为理想。     

高密度脉冲电流对A356铝合金凝固组织的影响

研究了高密度脉冲电流对A356铝合金凝固组织的影响，实验中对液相线以下溶体的凝固过程在不同阶段进行电脉冲处理。对比实验结果表明：经过电脉冲处理后，A356的合金凝固组织得到明显细化；在同样的电脉冲条件下，凝固组织细化效果与电脉冲放电开始时间有关，凝固过程中放电开始时间越早，凝固组织细化效果越明显；反之，凝固组织细化效果较差；同时，脉冲充电电压越高，凝固组织细化程度越大。定量计算了脉冲电流产生的电磁力并分析了脉冲电流影响凝固组织的作用机制。     

高密度脉冲电流对A356铝合金低温熔体凝固组织的影响

研究了高密度脉冲电流的A356铝合金低温熔体凝固组织的影响。实验中对680和630℃的低温熔体施加脉冲电流处理。对比试验和凝固曲线测试结果表明。低温熔体中施加脉冲电流有利于增大凝固过冷度,提高冷却速率。缩短凝固时间。细化凝固组织。同时,经脉冲电流处理后,随熔体的保温时间延长。凝固组织逐渐粗化。分析认为,低温熔体结构比凝固过程本身更能影响最终凝固组织。     

稀土元素变质A356铝合金的研究进展

总结了目前国内外采用稀土对A356铝合金变质的研究进展,归纳了稀土元素对A356铝合金的变质效果。综合目前的研究结果,结合商业化A356铝合金应用的实际情况,认为采用DOE成分设计方法对相对价廉的二元、甚至多元稀土或稀土与其他元素的复合变质体系进行成分设计是有前景且符合实际的。     

TiO2变质对A356铝合金微观组织的影响

以TiO₂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TiO₂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TiO₂原位生成γ-Al₂O₃同时细化变质了α-Al相和共晶Si相,协同γ-Al₂O₃的颗粒增强作用共同提高了材料的性能。     

稀土La对半固态A356铝合金凝固组织的影响

利用Al-La稀土中间合金对液态A356铝合金进行了细化处理,并用低温浇注技术制备了半固态A356铝合金浆料,研究了细化处理对所制备半固态A356铝合金的初生α-Al相形貌和尺寸的影响。结果表明,细化处理的A356铝合金经低温浇注可制备具有颗粒状和蔷薇状初生α-Al相的半固态浆料,稀土La可显著改善半固态A356铝合金中初生α-Al相的晶粒尺寸和颗粒形貌。探讨了稀土La对半固态A356铝合金的初生α-Al相细化机理。     

摆动振荡对A356铝合金半固态组织的影响

提出一种制备半固态浆料的新方法-摆动振荡法.采用该方法制备了A356铝合金的浆料.结果表明:摆动振荡可有效地防止稳定凝固壳的形成,可制备出组织优良的半固态浆料,其初生α(Al)的等效直径在60 μm以下,形状因子可达0.8以上,特别地,可以大幅度提高浇注温度.     

熔化和热处理条件对A356铝合金冲击韧性的影响（英文）

研究A356铝合金在非热处理和T6热处理条件下经熔化处理过程中的晶粒细化和改性后的显微组织和冲击性能。通过添加Al-10%Sr和Al-5Ti-1B中间合金分别实现改性和晶粒细化。为了重点突出上述处理的影响,所有的铸造参数都保持不变。结果表明:共晶硅形貌是控制合金冲击行为的主要参数。因此,铸态合金的晶粒细化不能像改性一样改善其冲击韧性。然而,相对于单个处理,同时进行晶粒细化和改性能提供更高的冲击韧性。合金的T6热处理在所有的熔化处理条件下都改善了冲击韧性,这与共晶硅颗粒的进一步改性有关。为证明这些结果并阐明其机制,使用三点弯曲实验和金属断面的显微观察来阐述合金冲击韧性的改善。     

冷却速度对A356铝合金铸件组织影响的定量分析

围绕A356铝合金金属型重力铸造成形试样的微观组织形貌与铸造条件之间的关系,通过组织分析和冷却速度的计算机模拟,研究了共晶含量与冷却速度之间的定量关系。结果表明,随着冷却速度的提高,凝固组织中的共晶含量增多,而且初生相组织的形态也由粗大的树枝晶逐步向蔷薇状枝晶演变。通过计算机模拟可较为准确地了解铸件各部位的冷却情况,并为预测铸件不同位置的组织和性能提供参考,为在工业生产中研发结构复杂、形状各异的零件奠定必要的基础。     

冷却速度对A356铝合金变质效果的影响

采用不同铸型的阶梯形试样,分别添加量为0．04％的Al—10％Sr中间合金和1％的三元钠盐变质剂对A356熔体进行变质处理。研究了冷却速度对A356合金变质效果的影响和这两种变质剂对冷却速度的敏感性。结果表明：冷却速度越大,枝晶间距越小,变质效果越好;Na变质对于冷却速度的敏感性高于Sr。     

不同工艺条件下A356合金的力学性能、微观组织和热疲劳特性研究

研究了不同工艺条件下A356微观组织、力学性能和热疲劳特性。结果表明:细化变质+微合金化+T6工艺的A356合金力学性能、热疲劳性能最优。热疲劳裂纹萌生期为沿晶生长,扩展期为沿晶+穿晶混合生长。裂纹扩展速率呈先增加后减缓的趋势,未脱落的Al_2Cu相、裂纹分叉与二次裂纹的产生均会减缓裂纹扩展速率。循环初期形成的氧化膜可抵消部分拉应力,阻碍氧化腐蚀;后期氧化膜遭到破坏,氧化腐蚀加剧,裂纹扩展加速。裂纹生长过程:位错滑移→空位→微坑→微裂纹→尖端闭合、张开→扩展。     

Microstructure and Mechanical Properties of X80 Pipeline Steel Joints by Friction Stir Welding Under Various Cooling Conditions

X80 pipeline steel plates were friction stir welded(FSW) under air, water, liquid CO_2 + water, and liquid CO_2 cooling conditions, producing defect-free welds. The microstructural evolution and mechanical properties of these FSW joints were studied. Coarse granular bainite was observed in the nugget zone(NZ) under air cooling, and lath bainite and lath martensite increased signifi cantly as the cooling medium temperature reduced. In particular, under the liquid CO_2 cooling condition, a dual phase structure of lath martensite and fi ne ferrite appeared in the NZ. Compared to the case under air cooling, a strong shear texture was identifi ed in the NZs under other rapid cooling conditions, because the partial deformation at elevated temperature was retained through higher cooling rates. Under liquid CO_2 cooling, the highest transverse tensile strength and elongation of the joint reached 92% and 82% of those of the basal metal(BM), respectively, due to the weak tempering softening. A maximum impact energy of up to 93% of that of the BM was obtained in the NZ under liquid CO_2 cooling, which was attributed to the operation of the dual phase of lath martensite and fi ne ferrite.     

不同热变形条件下体育器械用TC11合金的组织性能研究

以体育器械用TC11钛合金为研究对象,研究了不同的变形温度和变形速率对合金组织与性能的影响。结果表明,随着变形温度的升高,当应变速率分别为0.010和0.001 s-1时,α相的面积分数增加;除应变速率为0.001 s-1时,在其余应变速率下,β相的动态再结晶晶粒尺寸随变形温度的升高而增大。     

Thermal analysis as a microstructure prediction tool for A356 aluminium parts solidified under various cooling conditions

Thermal analysis technique has been used for a long time,in both ferrous and nonferrous industries for evaluating the metallurgical quality of the liquid metal before casting.However,obtaining a proper microstructure in a standard cup does not ensure that the microstructure is correct in real parts which may solidify at very different cooling rates.For this study,alloy A356 with different metal quality in terms of modification and grain refinement was tested.Different cooling rates were obtained by using cylindrical test samples with various diameters cast in sand and metallic moulds.The correlation between microstructure features such as grain size,modification rate and secondary dendrite arm spacing (SDAS) measured in the standard thermal analysis cup with those obtained in the cylindrical test parts has been investigated.Thus,knowing the thermal modulus and the mould type it is possible to establish the required grain size and modification rate in the standard cup in order to get a desired structure in a real part.Corrective actions can then be taken in order to improve the metallurgical quality before casting the part.     

Modeling and Simulation of Microstructure Formation for Porosity Prediction in Thermal Barrier Coatings Under Air Plasma Spraying Condition

Effective physical and mechanical properties of thermal barrier coatings are strongly dependent on the coating microstructure. The main objective of this study is the coating porosity prediction during the coating formation by simulation. For this purpose, two simulation approaches are presented. The first model takes into account physical impact, deformation, and overlying of powder particles on the solid substrate. Therefore, computational fluid dynamics and the volume of fluid method for this model were used. In the second approach, a faster and therefore more efficient model was developed, hence it was strongly simplified to simulate the formation of coatings and their microstructure. The splat formation was handled by calculating the flattening degree as a function of the Reynolds number. The disc-shaped particles were discretized by cuboids. The neighboring cuboids are moveable against each other at their contact areas. The displacement of those depends on material properties and the Reynolds number as well. Both approaches for predicting the microstructure were mutually compared.     

Microstructural Evolution of Creep-Induced Cavities and Casting Porosities for a Damaged Ni-based Superalloy Under Various Hot Isostatic Pressing Conditions

The microstructural evolution of casting porosities and creep-induced cavities for a damaged nickel-based superalloy under different hot isostatic pressing(HIP) conditions was investigated in order to understand the effects of HIP parameters on the healing behavior of micropores. A number of small-sized creep cavities formed during long-term service and large-sized porosities formed during the casting process were observed. These microdefects were partially healed after treated at high temperature of 1100 °C combined with 150 MPa pressure for 2 h, together with the formation of the socalled concentrically oriented c0 rafting structure. When HIP temperature was increased to 1150 and 1175 °C, both the amount and the size of the microdefects were decreased. The concentrically oriented c0 rafting around creep cavities became more remarkable, and the primary c0 denuded zone was also formed between the raft structure and the cavity.Energy-dispersive X-ray spectroscopy analysis revealed that the c matrix solute atoms diffused toward the cavity under the concentration gradient, whereas the c0-forming elements diffused in a negative direction. When increasing HIP temperature up to 1200 °C, the micropores were hardly observed, indicating that both casting porosities and creep-induced cavities had almost been healed. Meanwhile, the c0 rafting structure disappeared since HIP temperature was beyond the c0 solvus temperature. It is revealed by the experimental results that the atomic diffusion could mainly dominate the healing process of micropores.