Solidification behaviour of Al-7 %Si-Mg casting alloys

The effects of Mg content and cooling rate on the solidification behaviour of Al-7% Si-Mg(mass fraction)casting alloys have been investigated using differential scanning calorimetry, differential thermal analysis and microscopy. The Mg contents were selected as respectively 0.00%, 0.35% and 0. 70% (mass fraction). DTA curves of Al-7%Si-0.55%Mg(mass fraction) alloy at various cooling rates were accomplished and the alloy melt was cast in different cooling rates. The results indicate that increasing Mg content can lower the liquidus and binary Al-Si eutectic transformation temperatures. Large Fe-rich π-phases (AlsFeMg3Si6) are found in the 0.70% Mg alloys together with some small β-phases (Al5FeSi) ; in contrast, only β-phases are observed in the 0.35% Mg alloys. The test results of the Al-7%Si-0.55% Mg alloys identify that the liquidus and binary Al-Si eutectic transformation temperatures decrease, and the quantity of ternary Al-Si-Mg2 Si eutectic phase decreases as the cooling rate increases.     

不同相组成铸态Mg-xLi-1Al合金的阻尼性能(英文)

采用真空熔炼法制备三种不同相态的Mg-xLi-1Al三元合金,并使用光学显微镜、X射线衍射分析及动态力学分析仪研究其显微组织及阻尼性能。结果表明:由于锂含量的增加,合金的相结构发生转变且出现新的阻尼机制,其中BCC结构的出现使阻尼性能显著提高,同时弹性模量的减小也使临界应变振幅值降低。两相共存状态的合金随温度变化表现出较高的阻尼现象,并在低温与高温区均出现两个较为明显的峰值,其中低温区的P1峰是位错阻尼峰,高温区的P2峰是晶界阻尼峰。     

不同冷却速率热处理对热轧316L相转变的影响

本文采用高温激光共聚焦显微镜、纳米压痕和EBSD,研究不同冷却速率下热轧316L不锈钢微结构、织构和相转变。结果表明：316L热轧板中第二相呈长条状,硬度达到13.48 Gpa,内部相结构复杂,由奥氏体、铁素体和σ (Sigma)相混合组成。原位观察发现457.01s时温度达到1065.4℃,第二相中的铁素体和Sigma相向奥氏体转变;10min短暂均匀化后,原第二相处奥氏体晶粒仍富集Cr、Mo元素,降温后奥氏体向铁素体/Sigma相转变,仍呈长条状。不同冷却速率冷却后,初始形变织构弱化,转变为随机织构。冷却速度越慢,合金均匀化程度越高,奥氏体百分含量越高。原第二相长条位置奥氏体晶粒,Cr、Mo元素富集, 500℃/min 时全转变为铁素体,100℃/min时出现少量Sigma相,12℃/min冷速时,大部分铁素体转变成Sigma相。     

Effects of cooling rates on microporosity in DC casting Al-Li alloy

During the direct chill(DC)casting process,primary cooling from the mold and bottom block,and secondary cooling from the waterjets produce a concave solid shell.The depth of this liquid pocket and mushy zone not only depends on the solidification range of the alloy but also the boundary conditions such as cooling rates.Al-Li alloys solidify in a long solidification range increasing the susceptibility of porosity nucleation in the semi-solid region.In this study,the effects of cooling rate on the porosity formation were quantified for the large ingot casting using X-ray computed tomography(XCT).By characterizing pore size distributions at four different cooling conditions,the correlation between the mechanical properties at both room and high temperatures and the microstructure features was identified.The constitutive equations were constructed.It is found that increasing the cooling rate reduces the grain size,increases the number density of micropores,and minimizes the number of large pores,thereby improving the mechanical performance.Therefore,long mushy zones and deep liquid pockets in Al-Li alloys can be effectively controlled by controlling the boundary conditions of the DC casting solidification process,thereby obtaining castings with excellent mechanical properties.     

Effect of eutectic phase on damping and mechanical properties of as-cast Mg-Ni hypoeutectic alloys

Dynamic mechanical analysis (DMA) was applied to systematically investigate the low frequency damping properties of as-cast hypoeutectic Mg-Ni alloys. The results show that the as-cast hypoeutectic Mg-Ni alloys exhibit high damping capacities. The strain amplitude dependent damping curve has its own special characteristic, in which the damping is strongly related to the strain amplitude. The effect of the eutectic phase on damping and the mechanical properties of as-cast hypoeutectic Mg-Ni alloys were also discussed in detail.     

Liquid phase separation of Cu-Cr alloys during rapid cooling

1 Introduction Cu-Cr alloys containing 20%- 50%Cr have been widely investigated because they are the most important contact materials used in high voltage vacuum switches. It has been well known that the smaller the Cr particles in Cu-Cr alloys are, the b…     

热处理对不同冷速Al合金共晶相形貌及含量的影响

选取成分为Al-5.17Cu-2.63Si(合金A)、Al-4.29Cu-1.09Mg(合金B)和Al-2.09Si-1.66Mg(合金C)三元铝合金,分别进行不同冷速下的凝固实验。对比研究不同凝固速率下得到的共晶相形貌与含量在接近各合金体系三元共晶温度下热处理前后的变化行为。结果表明:同一合金冷速越慢,得到的原始组织二次枝晶间距越大,合金A的石墨型、砂型、保温型冷却组织的二次枝晶间距分别为24.17、63.32和99.88μm,合金B的二次枝晶间距分别为24.35、82.78和139.42μm。均匀化热处理的热扩散过程可以明显地溶解非平衡共晶相,由于原始组织的尺度不同,共晶相所处的溶解阶段与溶解程度不同。合金A的石墨型、砂型、保温型组织热处理后与热处理前的共晶相含量比值分别为0.44、0.49和0.68,合金B的共晶相含量比值分别为0.084、0.30和0.38。     

Solute segregation in an Al2O3f／Al-4.5Cu alloy composite during solidification at different cooling rates

An Al2O3f/Al-4.5Cu composite was made by squeeze casting. The solute segregation in the composite at different cooling rates was studied. The results indicate that the primary crystal of AI-4.5Cu alloy nucleates and grows in the interstice between fibers. The fiber surface cannot serve as site for the heterogeneous nucleation of a primary dendrite. There exists an increasing Cu concentration gradient from the center of the interstice between fibers to the interface or the grain boundaries. There are the eutectic phases around the fibers. The solute segregation in the matrix increases with the cooling rate rising. The amount of eutectic phases followed by imbalance crystalline can be numerically calculated with Clyne-Kurz formula.     

Effect of weld cooling rate on Laves phase formation in Inconel 718 fusion zone

Inconel 718 (2 mm thick) was welded using argon and helium gas shielded tungsten arc welding process with a filler metal. Both constant current and compound current pulse modes were applied and the cooling rates calculated. The dependence of Laves phase formation, dendrite arm spacing and niobium segregation ratios in fusion zone on the nature of shielding gases and current was studied. The maximum instantaneous weld cooling rate was achieved for the combination of Helium shielding gas and compound current pulse mode. This ultimately resulted in reduction of laves phase, segregation of niobium and dendrite arm spacing in the fusion zone.     

A rationale for the quality index of Al-Si-Mg casting alloys

The quality index introduced on empirical grounds by Drouzy, Jacob and Richard¹ is widely used by the casting industry as a tool for predicting the effect of solidification rate, chemical additions and heat treatment on the ductility and tensile strength of Al-7Si-Mg casting alloys. A drawback of the approach is its lack of physical justification. A simple analytical model is used to derive the quality index charts for A1-7Si-0.4 Mg alloys and to provide a physical meaning to the parameters involved. It is shown that the quality index relates to the fraction of uniform deformation available to the material. The application of the concept to Cu-containing alloys is also discussed.     

A rationale for the quality index of Al-Si-Mg casting alloys

The quality index introduced on empirical grounds by Drouzy, Jacob and Richard¹ is widely used by the casting industry as a tool for predicting the effect of solidification rate, chemical additions and heat treatment on the ductility and tensile strength of Al-7Si-Mg casting alloys. A drawback of the approach is its lack of physical justification. A simple analytical model is used to derive the quality index charts for A1-7Si-0.4 Mg alloys and to provide a physical meaning to the parameters involved. It is shown that the quality index relates to the fraction of uniform deformation available to the material. The application of the concept to Cu-containing alloys is also discussed.     

相组成对Cr-Nb合金高温氧化行为的影响

采用机械活金化 热压烧结法制备5种不同相组成的Cr-Nb合金,研究相组成对Cr-Nb合金在950～1200℃空气中氧化行为的影响。结果表明,Cr相能显著增加NbCr2合金950℃的抗氧化性能;而Nb相不利于合金高温抗氧化性的提高,甚至发生灾难性氧化。SEM和XRD分析显示,单相Cr-2.5Nb合金发生了Cr的外氧化,只形成单一的Cr2O3膜;而双相Cr-18.5Nb合金和单相Laves相NbCr2合金均发生Cr的外氧化和内氧化,形成两层结构的氧化膜。但随着氧化温度增加到1200℃,由于Cr2O3的挥发,导致Cr-Nb合金高温抗氧化性变差。因此,为满足实际高温应用要求,对富软第二相Cr或Nb的NbCr2基合金实施相应的表面防护是必须的。     

铸态Ti-Sc二元合金的组织特征、相结构和显微硬度

采用真空电弧熔炼技术制备了4种成分的Ti-Sc二元合金,并借助金相显微镜(OM)、扫描电子显微镜(SEM)、能谱分析(EDS)、X射线衍射(XRD)技术及显微硬度测试,对合金的组织形貌、相成分以及不同形貌相的显微硬度进行了分析.结果表明:合金铸态组织形貌为螺旋状、团簇状片层、双相组织;金属Sc对合金的组织有着明显的细化作用,在合金中Sc与Ti形成了固溶体,不同形貌的相,其显微硬度差别较大;当Sc含量为2%～3%时,Ti-Sc二元合金的显微硬度达到最高值,Sc含量高于2%～3%时,合金基体硬度高于片层组织硬度,Sc含量低于2%～3%时,合金基体硬度低于片层组织硬度,少量Sc对合金性能有所提高,对α相片层组织有强化作用.     

低碳低合金高强钢的连续转变行为及其相变模型

用DIL805型热膨胀仪研究了低碳低合金高强钢(HSLA)奥氏体连续冷却过程的相变规律,用膨胀法结合金相法建立了实验钢奥氏体的连续冷却转变曲线(CCT),随着冷却速度的增加,实验钢的组织分别发生了铁素体转变、珠光体转变、贝氏体转变和马氏体转变。建立了相变点-冷却速度以及相变量-冷却速度之间的数学方程,并回归计算了拟合度较高的相变模型。结果表明,计算值与试验值之间能很好的吻合,证明了该相变模型的可行性。     

Dendrite coherency point of A357 alloys

The dendrite coherency point was determined for A357 alloys using double-thermocouples technique. The effects of the cooling rate and different treatments such as grain refinement, modification, and superheat on the dendrite coherency point were investigated. The results show that the increase of the cooling rate and refinement significantly postpones the coherency point of A357 alloys. The morphology of silicon phase is modified, which slightly leads to lower solid fraction （fs） at the coherency point. But the fs^coh value decreases after superheat treatment at 850 ℃ for 30 min. The grain size plays a decisive role in the fs^coh value ofA357 alloys. The smaller the grain size is, the more the fs^coh value increases, which is beneficial to improving the final property of this alloy, while the secondary dendrite arm spacing and modification have almost no influence on fs^coh.     

Effects of silicon,magnesium and strontium content on the qualities of Al-Si-Mg alloys

According to the specification of AA standard, the magnesium content of 356.1 alloy ranges from 0.25 to 0.45%. In producing Al-Si-Mg alloy the strontium content for the modified 356 (Al-7%Mg) alloy might range from 0.005 to 0.02%. Therefore, 356 alloys might be produced with different percentages of Mg and/or Sr. The effects of changes in Si content (from 6.6% to 10.8% in gravity casting and 6.5% to 10.3% in squeeze casting), Mg content (from 0.36% to 0.48% in gravity casting and 0.3% to 0.44% in squeeze casting), and Sr content (from 0.0007% to 0.0158% in gravity casting and 0.0015% to 0.04% in squeeze casting) on the microstructures, density, mechanical properties and strength of different Al-Si-Mg alloys were fully investigated and discussed. Different melts were poured in the ASTM B108 permanent mould and dies in a vertical squeeze machine to produce bar castings. These bar castings were then machined and the mechanical properties tested. Experimental results showed that if a high strength and a high elongation were desired in a squeeze casting after T6 treatment, an increase in Si and Sr content to 9.9% and 0.019% (Mg at 0.3%) would lead to 280 MPa strength, 12% elongation and Weibull modulus of 34 in reliability of strength. If both strength and reliability were desired in a squeeze casting after T6 treatment, an increase in Si content to 10.3% (Sr at 0.0015% and Mg at 0.3%) would result in 294 MPa strength, 8.7% elongation and a great Weibull modulus of 67. In gravity casting, after T6 treatment, increasing Si content to 10.4–10.55% and Sr content in the range of 0.0007–0.0208% and 0.35% Mg could develop 282–284 MPa strength at about 6.6% elongation with Weibull modulus of around 31. In Al-7Si-Mg alloy, increasing Mg content from 0.26% to 0.48% decreased the tested elongations of both as-cast and heat treated squeeze castings and gravity castings.     

基于相变与回弹的热冲压成形冷却过程控制

回弹是影响高强度板热冲压成形精度的主要因素,通过22MnB5硼镁合金板热冲压成形实验,考察了冷却速率对回弹和相变的影响,提出了热冲压成形模具冷却系统设计相关的技术问题。研究表明,只有当冷却速度达到或超过临界冷却速度时,奥氏体才能直接转变为均匀马氏体;在模具结构、冷却系统、冷却介质等因素确定的情况下,可以通过控制冷却系统的水流速度,实现对最佳冷却速度的控制,消除热冲压成形中变形和回弹等缺陷,提高高强度板热冲压成形质量。