Modeling high-temperature stress-strain behavior of cast aluminum alloys

A modified two-state-variable unified constitutive model is presented to model the high-temperature stress-strain behavior of a 319 cast aluminum alloy with a T7 heat treatment. A systematic method is outlined, with which one can determine the material parameters used in the experimentally based model. The microstructural processes affecting the material behavior were identified using transmission electron microscopy and were consequently correlated to the model parameters. The stress-strain behavior was found to be dominated by the decomposition of the metastable θ′ precipitates within the dendrites and the subsequent coarsening of the θ phase, which was manifested through remarkable softening with cycling and time. The model was found to accurately simulate experimental stress-strain behavior such as strain-rate sensitivity, cyclic softening, aging effects, transient material behavior, and stress relaxation, in addition to capturing the main deformation mechanisms and microstructural changes as a function of temperature and inelastic strain rate.     

三峡闸门铸钢轨道超声波探伤

介绍了闸门铸钢轨道超声波探伤方法与应用技术,分析了铸钢轨道探伤的适应条件和灵敏度调整的方法,阐述了铸钢轨道探伤应关注的重点.     

Al-Zn-Mg-based cast alloys having high mechanical properties

The effect of alloying elements and heat treatment on the structure and properties of cast alloys based on the Al-Zn-Mg system and containing more than 9% (Zn + Mg) is analyzed. The optimum chemical composition of an alloy from this system is determined, and the heat-treatment conditions required for improving the mechanical properties of the alloy are found.     

Development of the manufacture of billets based on high-strength aluminum alloys

When pressure is applied upon casting as a factor of external impact on melt, the problems related mainly to filling of molds are solved; however, some casting defects cannot be avoided. The experimental results demonstrate that complete compensation of shrinkage under pressure can be achieved by compressing of casting by 8–10% prior to beginning of solidification and by 2–3% during the transition of a metal from the liquid to the solid state. It is mentioned that the procedure based on compressing a liquid metal can be efficiently applied for manufacture of high-strength aluminum alloy castings. The selection of engineering parameters is substantiated. Examples of castings made of V95 alloy according to the developed procedure are given. In addition, the article discusses the problems related to designing of engineering and special-purpose equipment, software, and control automation.     

Oxide films, pores and the fatigue lives of cast aluminum alloys

In the absence of gross defects such as cold shuts, the fatigue properties of castings are largely determined by the sizes of microstructural defects, particularly pores and oxide films. In contrast, the effects of grain size, second-phase particles, and nonmetallic inclusions are insignificant. The authors review the fatigue properties of castings made by gravity die casting, sand casting, lost-foam casting, squeeze casting, and semisolid casting, and compare A356/357 alloys with 319-type alloys. The application of fracture mechanics enables the properties to be rationalized in terms of the defects that are characteristic of each casting process, noting both the sizes and types of defect. The differences in the properties of castings are entirely attributed to their different defect populations. No single process is inherently superior. For defects of the same size (in terms of projected area normal to the loading direction), oxide films are less detrimental to fatigue life than pores. Areas of current controversy are highlighted and suggestions for further work are made. This article is based on a presentation made in the John Campbell Symposium on Shape Casting, held during the TMS Annual Meeting, February 13–17, 2005, in San Francisco, CA.     

A statistically significant experimental technique for investigating microsegregation in cast alloys

A simple, statistically significant experimental technique was developed to investigate microsegregation in a model cast Al-4.5 wt pct Cu alloy. This technique involves systematic composition measurements of the primary alloy phase of polished samples by use of the electron microprobe. The microprobe data were processed to obtain the detailed compositionvs fraction solid profile, which was an estimation of microsegregation of solute element in the sample. The number of data collected directly affected the statistical significance of the results. The results suggested that measurements of the order of 100 are adequate for obtaining statistically significant solute distribution profiles and, hence, for quantitatively determining the severity of microsegregation.     

Microstructure and mechanical properties of squeeze cast and semi-solid cast Mg–Al alloys

Magnesium–aluminium castings produced by means of squeeze casting, new rheocasting and thixocasting have been investigated. These casting processes provide very different microstructures consisting of α-Mg and β-Mg₁₇Al₁₂. The shape and distribution of the brittle β-Mg₁₇Al₁₂ has a large influence on mechanical properties. Isolated particles of β-phase in squeeze cast components are less detrimental to ductility than the continuous β-phase network found in semi-solid processed parts. A heat treatment results in complete dissolution of β-Mg₁₇Al₁₂ and accounts for significant improvements of ductility and fracture toughness. Crack propagation in solution heat treated Mg–Al castings is associated with extensive twinning.     

泡沫钛及其合金制备方法研究进展

对泡沫金属钛及其合金的制备技术进行了总结,根据泡沫金属内部气孔的结构不同,探讨了开孔和闭孔泡沫钛的制备方法及原理.重点介绍了混合粉末烧结法与气体卷入技术.分析了各种方法的优缺点和制备泡沫钛及其合金的可行性.     

A new quality index for characterizing aluminum cast alloys with regard to aircraft structure design requirements

A new quality index Q _D for characterizing aluminum cast alloys with regard to their suitability for use in damage-tolerant aircraft structures is introduced. The proposed quality index involves the material properties yield strength R _p and strain energy density W. The yield strength accounts for the region of allowable service stresses and strain energy density accounts for tensile ductility and fracture toughness. Both properties, R _p and W, are evaluated from the tensile test. The index Q _D was used to evaluate the quality of the cast aluminum alloys A357, A224, and 7475 and rank them accordingly. The evaluation results were compared against the results obtained by using the classical quality index currently involved by the industry to characterize the quality of aluminum cast alloys.     

Effects of process-control agents on mechanical alloying of nanostructured aluminum alloys

The effects of the process-control agents (PCAs) stearic acid and methanol on the mechanical alloying (MA) of a nanostructured aluminum alloy (Al₉₃Fe₃Ti₂Cr₂) have been investigated. The dependency of the powder-particle sizes, grain sizes, atomic-level strains, lattice parameters, formation of solid solutions, and microstructural evolution of the aluminum alloy on the types of PCAs and their concentrations have been studied using a variety of analytical instruments including X-ray diffraction scanning electron microscopy, and transmission electron microscopy. The results clearly indicate that prevention of excessive cold welding of Al particles can be achieved by the addition of a PCA at the expense of reductions in the grain size, formation rate of solid solutions, and rate of microstructural refinement, all of which are desired in MA of the Al alloy. Furthermore, a PCA that is more effective in preventing excessive cold welding will also impose more hindrance to the MA process. These phenomena have been discussed in the light of the adsorption of the PCA on the metal surface and the lubricating function of the PCA.     

Hot plane strain compression testing of aluminum alloys by channel-die compression

A thoroughly tested, high-temperature channel-die compression (CDC) rig is described for simulating hot plane strain compression of metallic alloys up to 500 °C. The equipment is currently used to characterize the flow stress and microstructure evolution in hot-rolled Al alloys. It has been validated by several tests involving (1) metallographic analysis of deformed samples; (2) flow stress comparisons with the same, or similar alloys deformed in conventional uniaxial or plane strain compression; and (3) microstructure and texture measurements. The use of modern lubricants enables one to obtain accurate flow stresses and true plane strain deformations that are homogeneous over 80 pct of the sample. The equipment also features rapid heating and cooling systems to minimize thermally-induced microstructure changes. Some results on high-temperature slip systems, hot deformation textures, and microstructures, and the behavior of constituent particles are outlined to illustrate the advantages of the technique.     

Influence of grain refinement on some mechanical properties of non ferrous cast alloys

The influence of the grain refining method on some mechanical properties of non ferrous casting, by analyzing published data, is presented. Grain refining methods are grouped into: cooling rate control; nucleant agents addition; and mechanical agitation. Grain size decrease influence on the reported mechanical properties can be grouped into: decrease; improvement; and no observed influence. This analysis shows that there is a relationship between the grain refining method and the mechanical behavior of the studied alloys.     

Development of RuAl-based cast alloys

Heterophase RuAl-based alloys with a β-RuAl + (1–20) vol % ɛ-Ru structure and alloyed with chromium, titanium, and hafnium are produced by vacuum arc melting. The effect of the method of preparing charge materials on their behavior during alloy formation is studied. The effect of a structure on the deformability of the alloys at room temperature is estimated. All alloys exhibit ductility and can be deformed by upsetting at a strain higher than 10–12%. The effect of deformation by upsetting at 800°C and subsequent heat treatment on the structure and properties of the alloys is investigated. The high-temperature strengths of RuAl-, TiAl-, Ni₃Al-, and NiAl-based alloys are compared by measuring their hot hardnesses at temperatures up to 1100°C. The high-temperature strength characteristics of the RuAl-based alloys are higher than those of the Ni3Al-, TiAl-, and NiAl-based alloys over the entire temperature range under study; at temperatures ≥900°C, the hardness of ruthenium monoaluminide is higher than those of the other alloys by a factor of 2–4.     

Study of the phase compositions of Al-Se-Ge alloys for designing solders for aluminum alloys

Alloys based on the Al-Si-Ge system are studied in order to design solders with a liquidus temperature lower than 570°C. Electron-probe microanalysis, differential scanning calorimetry, and X-ray diffraction analysis are used to optimize the solder composition and the process of solder production. Quantitative phase analysis methods, which are based on an experimental determination of the lattice parameters of the phases and their diffraction line intensities, are developed for these alloys.     

回归再时效对超高强铝合金力学性能及组织的影响

在传统的回归再时效(retrogression and re-aging,RRA)工艺(峰时效)基础上降低预时效或再时效温度,对Fe和Si杂质含量高的超高强Al-Zn-Mg-Cu合金挤压棒材进行RRA处理,通过拉伸性能和疲劳性能测试以及扫描电镜和透射电镜观察,研究RRA工艺对合金力学性能与组织的影响。结果表明:降低预时效或再时效温度都可明显提高该合金的塑性和抗疲劳损伤性能,略微降低合金的抗拉强度。采用峰时效温度(120℃)RRA处理后的合金,晶内的主要析出相为尺寸较大的η′相,不能被位错切割,合金强度较高(674 MPa),但塑性和抗疲劳损伤性能差,伸长率为11.1%,最终应力强度因子幅值ΔK=26.8 MPa·m1/2;降低时效温度可增加析出相中GP区粒子的比例,减小η′相的尺寸,从而提高塑性变形能力以及抗疲劳损伤性能。     

Effect of lithium on the mechanical properties and microstructure of sic whisker reinforced aluminum alloys

Aluminum-silicon carbide whisker composites containing nominally 3 to 5 pct Li in the matrix alloys have been fabricated and tested. Tensile and compression tests have been conducted at room temperature, and compression creep tests have been conducted at elevated temperatures. Lithium additions were found to increase the strengthening effect of silicon carbide whiskers at room and elevated temperatures. Lithium also reduced the density of the composites and increased the elastic modulus. Transmission electron microscopy showed no obvious chemical reaction between the whiskers and the aluminum-lithium alloy matrix.