ZL201铝合金半固态流变压铸组织与性能

研究了ZL201铝合金半固态流变压铸组织与性能,包括半固态浆料的制备、压铸成形和固溶、时效处理。结果表明:在ZL201铝合金液相线温度附近施加交变电磁场,能够获得均匀、细小、近球形非枝晶组织的半固态浆料;半固态浆料经压铸成形后,零件具有等轴或蔷薇状晶粒组织;经T5热处理后,ZL201铝合金的抗拉强度为253N/mm2,伸长率为7%。流变压铸件的组织和性能优于半固态触变压铸件的。     

Shear bands of as-cast and semi-solid Ti48Zr27Cu6Nb5Be14 bulk metallic glass matrix composites

The as-cast Ti₄₈Zr₂₇Cu₆Nb₅Be₁₄bulk metallic glass matrix composites(BMGMCs)were fabricated using a copper mold suction casting method.Then,the semi-solid BMGMC samples were obtained following an isothermal treatment(heating at 900°C for 10 min,then cooling with water).The microstructure and compression property were investigated by scanning electronic microscopy(SEM)and universal mechanical tester.As a result of the isothermal treatment,the crystal shapes change from fine,granular,and dendritic to spherical or vermicular,and the average crystal size of the as-cast and semi-solid samples is 2.2μm and 18.1μm,respectively.The plasticity increases from 5.31%in the as-cast to 10.23%in the semi-solid samples,with an increase of 92.66%.The shear bands from different areas of the side surfaces of as-cast and semisolid compression fracture samples were observed.The characteristic changes of multiplicity,bend,branch and intersection of shear bands in different areas indicate that the deformation of as-cast and semi-solid samples is non-uniform during compression.It is found that poor plasticity of the as-cast samples or good plasticity of the semi-solid samples are reflected by characteristics of the shear bands.The semi-solid isothermal treatment improves the plasticity by forming large crystals which can block the expansion of shear bands and promote the multiplicity of shear bands.     

Nanocrystalline refractory metals for extreme condition applications

For the last decade, there has been research aimed at engineering plastic instability into the deformation behavior of body centered cubic (b.c.c.) metals. At dynamic strain rates, the adiabatic shear band deformation mode has been shown to improve the performance of kinetic energy penetrator materials. However, for some b.c.c. metals the transition to localized plastic deformation dominates at all strain rates. This limits the traditional engineering properties (e.g., ductility and toughness) and feasibility of incorporation into a long rod penetrator system. Recently, we demonstrated that nanocrystalline tantalum shows significant promise as it deforms via adiabatic shear bands in dynamic compression but shows significant tensile elongation in quasi-static deformation.     

镁合金热变形下变形带的形貌和晶体学特征

对不同温度单向压缩下AZ31镁合金不均匀形变组织的形貌和晶体学特征进行了研究.结果显示:形变组织具有很强的温度和应变敏感性;250℃时,晶粒内在变形初期出现大量的{1012}拉伸孪晶和少数{1011}压缩孪晶,随着应变量的加大,拉伸孪晶因相同取向孪晶的合并而急剧减少,而压缩孪晶明显变粗,数量也有所增加;300℃以上时,非基面滑移被激活后,出现了与压缩轴基本垂直的扭折带,其晶体学方向垂直于(0001)基面,扭折带两侧的主滑移系都为(0001)基面滑移,变形初期扭折带界面取向差为2°—6°,随着变形量的增加,扭折带密度加大;温度升高至400℃时,扭折界面发生明显的弯曲.对扭折带和其他变形带的特征进行了对比考察.     

Conception of tooling adapted to thixoforging of high solid fraction hot-crack-sensitive aluminium alloys

Thixoforging is a type of semi-solid metal processing at high solid fraction (0.7<?_s<1), which involves the processing of alloys in the semi-solid state. Tooling has to be adapted to this particular process to benefit shear thinning and thixotropic behaviour of such semi-solid material. Tooling parameters, such as the forming speed and tool temperature, have to be accurately controlled because of their influence on thermal exchanges between material flow and tool. These thermal exchanges influence the high-cracking tendency and the rheology of the semi-solid material during forming, which affects parts properties and therefore their quality. Extrusion tests show how thermal exchanges influence quality of thixoforged parts made of 7075 aluminium alloys at high solid fraction by modifying process parameters like forming speed, tool temperature and tool thermal protector. Thus an optimum in terms of thermal exchanges has to be found between surface quality and mechanical properties of the part. A direct application is the evaluation of surface quality of thixoforged thin wall parts made of 7075 aluminium alloy.     

Development of aluminum rheo-extrusion process using semi-solid slurry at low solid fraction

An aluminum extrusion process is mainly used to fabricate long tubes, beams and rods for various applications. However, this process has a high production cost due to the need for investment of high-pressure machinery. The objective of this work is to develop a new semi-solid extrusion process using semi-solid slurry at low solid fractions. A laboratory extrusion system was used to fabricate aluminum rods with the diameter of 12 mm. The semi-solid metal process used in this study was the gas induced semi-solid (GISS) technique. To study the feasibility of the GISS extrusion process, the effects of extrusion parameters such as plunger speed and solid fraction on the extrudability, microstructure, and mechanical properties of extruded samples were investigated. The results show that the plunger speed and solid fraction of the semi-solid metal need to be carefully controlled to produce complete extruded parts.     

半固态等温热处理对ZA12合金组织和性能的影响

将初生相为枝晶形态的常规ＺＡ１２合金铸锭重新加热到半固态进行等温热处理，可获得具有良好触变性的非枝晶组织合金。文中对半固态热处理工艺参数与显微组织之间的关系进行了研究，并对铸态和经过半固态热处理的两种材料的半固态形性能以及成形制品的力学行为进行了试验对比。结果表明，经适当工艺热处理后，ＺＡ１２合金可以实现半固态成形，不仅成形抗力低，液相偏析少，且提高了制品的力学性能。     

Effect of alloying elements on structure and mechanical properties of semi-solid processed cast iron

Semi-solid metal processing of alloys is one of the key technologies for producing advanced materials. Through semi-solid processing, it is possible to produce high quality cast components from grey cast iron. A series of experiments were carried out to clarify the effect of the alloying elements copper, chromium, molybdenum, and nickel on the properties of cast iron. A comparison was made of the microstructure and mechanical properties in semi-solid processed cast iron and ordinary cast iron. This showed that an increase in the level of alloying elements in cast iron gave a higher level of hardness. However, the tensile strength of alloyed semi-solid cast iron did not exceed that of grey iron, for every composition, as a result of the characteristic microstructure produced by semi-solid processing. Thus, the alloying elements had little effect on the tensile strength of semi-solid cast iron, but did have an effect on elongation. The tensile strength of semi-solid cast iron, however, still depends on the cooling rate.     

半固态流变铸造镁合金浆料的组织特征

采用自制的镁合金半固态成形机组,研究半固态浆料的微观组织特征.结果表明,浆料的铜模激冷凝固得到的微观组织既不同于半固态压铸成形之后得到的近球状组织,也不同于镁合金液态压铸的细小枝晶组织,他是一种玫瑰状组织.压铸过程中对浆料的二次搅拌剪切作用可以促进近球状非枝晶组织的形成.     

Non-dendritic structural 7075 aluminum alloy byliquidus cast and its semi-solid compression behavior

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压铸机压射冲头的改进

针对球墨铸铁压射冲头在压铸过程中的损耗量大、更换频繁的问题,进行了材质和制造工艺的改进,改进后的压射冲头的高温强度、硬度和耐磨性优于球墨铸铁压射冲头,其寿命比球墨铸铁压射冲头提高了50倍左右.     

Fabrication and microstructure evolution of semi-solid LY11 alloy by SIMA

For semi-solid metal forming, it is essential to fabricate the semi-solid materials with spheroidal microstructure. Among several fabrication techniques of the semi-solid materials, (strain-induced melt activation (SIMA) is an ideal candidate with the advantages of simplicity and low equipment costs. In this paper, the microstructure evolution of LY11 alloy (approximately corresponding to ASTM 2017) was investigated in the SIMA process, which had two essential stages: deformation and isothermal heat treatment. The deformation stage was conducted using a CSS-1100C material testing machine and the isothermal heat treatment stage was performed in a resistance furnace. Different levels of deformation temperatures, ram velocities, isothermal temperatures, and holding times were used in this investigation. The microstructure of LY11 alloy was observed by a NEOPHOT-1 optical microscope. The results indicated that the processing parameters must be selected properly to obtain the fine, uniform and spheroidal microstructure by SIMA. The deformation-recrystallization mechanism for microstructure evolution in SIMA process was supported by experimental evidence.     

Effects of nanocrystalline and ultrafine grain sizes on constitutive behavior and shear bands in iron

The mechanical behaviors of consolidated iron with average grain sizes from tens of nanometers to tens of microns have been systematically studied under uniaxial compression over a wide range of strain rates. In addition to the well-known strengthening due to grain size refinement, grain size dependence is observed for several other key properties of plastic deformation. In contrast with conventional coarse-grained Fe, high-strength nanocrystalline and submicron-grained Fe exhibit diminished effective strain rate sensitivity of the flow stress. The observed reduction in effective rate sensitivity is shown to be a natural consequence of low-temperature plastic deformation mechanisms in bcc metals through the application of a constitutive model for the behavior of bcc Fe in this strain rate and temperature regime. The deformation mode also changes, with shear localization replacing uniform deformation as the dominant deformation mode from the onset of plastic deformation at both low and high strain rates. The evolution and multiplication of shear bands have been monitored as a function of plastic strain. The grain size dependence is discussed with respect to possible enhanced propensity for plastic instabilities at small grain sizes.     

金属材料半固态凝固及成形技术进展

简要介绍了金属材料半固态凝固及成形技术发展的历史和现状,重点介绍了近年开发的几种金属材料半固态凝固制备技术的代表性方法与特点、铝合金和镁合金等轻金属材料半固态凝固及流变成形的组织与性能、高熔点合金材料半固态凝固制备及成形技术的进展以及半固态凝固成形技术的应用现状,最后对金属材料半固态凝固及成形技术的进展状况及发展前景作了简单的总结和展望。     

半固态Al-6.5Si合金的流变特性

用半固态金属流变模型研究了Al-6.5si(质量分数，％)体系的稳态和暂态流变行为．分析表明，半固态金属的流变行为与其微结构有很强的关联，在固相体积分数一定的情况下，体系的表观粘度完全由其微结构所决定，而剪切速率和剪切时间对表观粘度的影响是通过改变体系的微结构来实现的．稳态粘度随固相体积分数的增加而增加、随剪切速率的增加而减小，这与文献中的实验结果相符合；体系的解聚过程比聚集过程快两个数量级．在各种形变条件下所出现的滞后环的大小由上升时间、静置时间、原初剪切速率、最大剪切速率等因素决定．     

Moderation of the recrystallization texture by nucleation at copper-type shear bands in Al-1Mg

The critical dependency of copper type shear band formation on deformation variables in Al-1Mg has been described (Acta mater., 49 (2001) 2739). The influence of copper type shear bands on recrystallization behaviour during post-deformation annealing in Al-1Mg is now discussed. Local orientation measurements and orientation images of grains nucleating at shear bands have been made using the electron back-scatter diffraction (EBSD) technique. Bulk recrystallization texture is investigated from X-ray diffraction (XRD) measurements. Shear bands are shown to be potent nucleation sites during annealing and a weak, almost random, texture associated with nucleation at shear bands has been inferred. Most significantly, shear band formation is found to have a pronounced moderating influence on the strength of the potentially dominant cube component of the recrystallization texture.     

Effect of room-temperature compression on microstructure of ductile cast iron subjected to hot plastic deformation

The change in the microstructure of ductile cast iron subjected to hot plastic deformation has been investigated after the fracture of the samples induced by compression (upset forging) at room temperature. It has been shown that compression-induced tangential stresses cause shear deformation, which results in the shear fracture of test samples at an angle of 40°–50° to the longitudinal axis of a sample. It has been established that the fracture is accompanied by the formation of a narrow zone of severe plastic deformation of ductile cast iron, which is located on both sides of the major fracture. In this zone, the initial microstructure undergoes significant changes due to the plastic flow of the matrix and graphite inclusions.     

Mechanisms of high-strain-rate deformation and fracture of fine-grained unalloyed uranium upon explosive loading

The study of the microstructure and properties of quasi-statically extruded fine-grained uranium before and after explosive loading has shown that the main mechanism of its high-strain-rate deformation is multiple twinning. The nucleation of spall and shear microdamages upon explosive loading occurs at oxycarbonitride inclusions, just as in the coarse-grained cast metal upon the analogous regime of explosive loading. The absence of an increase in the spall strength of uranium upon a decrease in the initial grain size by a factor of 100–250 is related to the presence of a significant amount of impurities (to 1.5 ± 0.2 vol %) in the material.     

A macro–micro-multi-level modeling scheme to study the effect of particle distribution on wrap-bendability of AA5754 sheet alloys

A two-level finite element model to simulate wrap bending of flat sheet metal has been developed. Firstly, a sample sized plane strain analysis is performed and the location of most severe deformation is identified. Then a plane strain sub-model incorporating microstructural input, such as second phase particles at this location, is performed. The results show that models with particles distributed primarily in stringers exhibit lower bendability than those containing a random distribution of particles. This is consistent with the difference in bendability observed between continuous cast (CC) and direct chill cast (DC) AA5754 alloys which have characteristic stringer and random particle distributions. The morphology of cracking in the model with stringers is similar to that observed in AA5754 CC sheets. The crack initiates at the surface between particles or near particles and propagates along local shear bands to other stringers in the bulk or jumps from one stringer to another.     

Semi-solid squeeze casting of ignition-proof mg-ca-zr system alloys

The production method of the near-net shape molded Mg-Ca-Zr alloys by the devised semi-solid squeeze casting was established and the fabrication factors for sound products were also investigated. The Mg-Ca-Zr alloys having an ignition-proof property and very fine spherical structure provide high production safety and good fluidity for quality near-net shape products in semi-solid state processing. The orifice with a diameter of 30 mm and a plunger speed of 80 mm/s contribute successfully to infiltrate the semi-solid state prema-terial into the designed mold perfectly within the limited infiltration time. The tensile strengths and elongations of the ignition-proof Mg-Ca-Zr and Mg-Zn-Ca-Zr alloys are improved by applying the high pressure of 100 MPa, indicating that pressure more than 100 MPa is also an important fabrication factor.