回收工艺对再生铝合金性能影响述评

铝合金因其密度小、强度高、耐腐蚀、加工性能好的优点,已经成为工业应用第二大金属材料。随着原铝产量下降和现役铝合金材料使用年限逐渐到期,具有节约矿产与能源、经济社会效益高、可持续发展等特点的再生铝产业逐渐受到关注。然而,经过回收再生的铝材由于成分混杂等原因,性能往往有所下降。因此提升再生铝合金性能的方法成为相关领域研究的重点。文中介绍了近年来国内外有关提升再生铝合金性能方法的研究进展,对废铝预处理、合金成分调配、熔体精炼、热加工工艺的发展进行了总结,概述了提升固态回收再生铝合金性能的最新研究进展,并对再生铝技术的发展提出展望。      

Preparation and properties of fine grain β- CuAlNi strain- memory alloys

A method has been developed to produce grain sizes as small as 5 μm in alloys of β-CuAlNi. The alloys were of eutectoid composition and a procedure was developed for determining the composition of a eutectoid alloy having any required value for transition temperature (M _s ). The thermo-mechanical treatment involved two sequential stages of warm rolling followed by recrystallization. The alloys produced were single phase β-type with no second phase being present. Characteristic two-stage stress-strain curves were obtained for most of the specimens. It was generally found that the tensile strength and strain to failure increased with decreasing grain size according to a Hall-Petch type relationship down to a grain size of 5 μm. A fracture strength of 1200 MPa and a fracture strain of 10 pct were obtained in the best alloy. It was found that the major recovery mode, whether pseudoelastic or strain-memory, did not have any significant effect on the total recovery obtained. Recovery properties were not affected significantly by decreasing grain size, and 86 pct recovery could still be obtained at a grain size of around 10 μm. Grain refinement improved the fatigue life considerably, possibly due to the high ultimate fracture stress and ductile fracture mode. A fatigue life of 275,000 cycles could be obtained for an applied stress of 330 MPa and a steady state strain of 0.7 pct. At fine-grain sizes most of the fractures were due to transgranular-type brittle fracture and micro void-type ductile fracture, depending on the alloy composition. It was suggested that the difference between the alloys was due to differences in oxygen segregation at the grain boundaries.     

Quantitative analysis of texture evolution in cold-rolled, continuous-cast AA 5xxx-series aluminum alloys

The texture evolution of continuous-cast AA 5xxx-series aluminum alloys during cold rolling was investigated by X-ray diffraction. Texture volume fractions were calculated by an improved integration method. The results show that the relationship between the texture volume fractions and true strain can be quantified by mathematical formulae. The effect of alloy composition as well as initial microstructure and texture on rolling-texture evolution can be evaluated by the k _i and n _i values in the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The k _i value reflects the rate of formation or disappearance of the texture components. The high-Fe AA 5754 aluminum alloy exhibits the lowest formation rate of the β fiber among the AA 5005, 5052, 5754, 5182, and high-Fe 5754 aluminum alloys.     

Magnetic investigation of the effect of small additions of cobalt and manganese on the martensite reversal in an Fe-Ni alloy

Data on the temperature and composition dependence of the magnetic moment and Curie temperature of several Fe-Ni-Co and Fe-Ni-Mn alloys have been obtained. The temperature dependence of the magnetization was obtained for each alloy from 298 to 873 K, following the magnetization change through the transformation from martensite to austenite. The effect of cobalt and manganese additions to an Fe-29.9 at. pct Ni alloy on the reverse transition temperature,A _s , the Curie temperature,T _c , and the saturation magnetization at absolute zero, ρ_so, has been determined, Values forA _s , T _c , and ρ_so were obtained by fitting a Brillouin function to the respective contributions of austenite and martensite to the total magnetization. This technique represents a very sensitive method of obtaining transition temperatures and the respective amounts of each phase present in the alloys. A theoretical prediction of ρ_so andT _c was in agreement with the experimentally determined values.     

Rapidly solidified P/M X2020 aluminum alloys

X2020 aluminum alloys were produced with variations in the Li/Cu ratio by the ultrasonic gas atomization process. In alloy 68 (Al-4.9Cu-l.2Li) and 69 (Al-4.4Cu-l.55Li) alloys, the Θ′ and T₁, phases are dominant with evidence of the T_B phase. In the 70 (Al-3.5Cu-2.8Li) alloy, the δ′ phase is dominant with a trace of T₁. It was found that Θ′ andT ₁ are effective strengtheners whereas δ′ provides excellent fatigue crack initiation resistance. Overall results indicate that the fracture behavior of three RS-PM X2020 alloys is closely related to alloy production route as well as to the phases present in the alloys. Formerly Research Assistant, Massachusetts Institute of Technology.     

Reactive phosphide inclusions in commercial ferrosilicon

The goal of this work was to determine the origin of phosphine gas (PH₃), which has been reported to be generated from wet, commercial ferrosilicon alloys containing ∼75 wt pct Si. Based on previous work, it is suspected that PH₃ evolves when phosphides present within the alloy react with atmospheric moisture or water. Reactive phosphides have been identified in synthetic ferrosilicon alloys, which contain higher amounts of phosphorus than are typically present in commercial alloys. Therefore, reactive phosphides in commercial FeSi75 alloys are expected to be important to the evolution of PH₃ from these alloys. To identify the role of reactive phosphides in the evolution of PH₃ from commercial FeSi75 alloys, the microstructures of four commercial and two synthetic FeSi75 ferrosilicon alloys were investigated. Reactive phosphides were observed in each of the commercial alloys and characterized with respect to composition, morphology, and location within the microstructure. The phosphides observed in all of the commercial alloys contained aluminum, calcium, and magnesium. The phosphides had inclusion-like morphologies and were located on the silicon/ζ (high-temperature FeSi₂) interfaces at microcracks. The microstructural features observed support the hypothesis that atmospheric moisture penetrates ferrosilicon, reacting with the phosphide inclusions to produce PH₃. A possible mechanism describing the spontaneous crumbling sometimes observed in ferrosilicon alloys is also presented.     

Microstructure and Corrosion Characterization of Squeeze Cast AM50 Magnesium Alloys

Squeeze casting of magnesium alloys potentially can be used in lightweight chassis components such as control arms and knuckles. This study documents the microstructural analysis and corrosion behavior of AM50 alloys squeeze cast at different pressures between 40 and 120 MPa and compares them with high-pressure die cast (HPDC) AM50 alloy castings and an AM50 squeeze cast prototype control arm. Although the corrosion rates of the squeeze cast samples are slightly higher than those observed for the HPDC AM50 alloy, the former does produce virtually porosity-free castings that are required for structural applications like control arms and wheels. This outcome is extremely encouraging as it provides an opportunity for additional alloy and process development by squeeze casting that has remained relatively unexplored for magnesium alloys compared with aluminum. Among the microstructural parameters analyzed, it seems that the β-phase interfacial area, indicating a greater degree of β network, leads to a lower corrosion rate. Weight loss was the better method for determining corrosion behavior in these alloys that contain a large fraction of second phase, which can cause perturbations to an overall uniform surface corrosion behavior.     

The effect of electron concentration on the thermodynamic properties of two alloy phases in the Al-Zn-Ag system

A reversible electrochemical cell was used to determine the thermodynamic activity of aluminum in a series of α-phase Al-Zn and Al-Zn-Ag alloys, and a dew-point technique was used to determine the thermodynamic activity of zinc in a series of ε-phase Ag-Zn-Al alloys. The compositions investigated for both systems included those at which previous authors had related the stability of the phase to electronic factors. The data are presented in the form of graphs showing the excess relative partial molal Gibbs free energy of aluminum, (Δ•G_A1 ^xs), and zinc, (Δ•G_Zn ^xs), as a function of both mole fraction(X _i ) and electron-to-atom ratio (e/a). The latter curves are interpreted to be indications that the effect of electron concentration upon alloy phase stability has been experimentally isolated. Equations are developed which predict the effects of ternary additions in these alloy systems. The rigid-band model is shown to be incapable of predicting the direction of the effect and an argument based upon electron screening of ion-ion interactions is offered. Formerly Graduate Students, Metallurgy Department, Washington State University, Pullman, Wash. This paper constituted a portion of the theses submitted by RONALD E. MILLER and JERRY L. STRAALSUND in partial fulfillment of the requirements of the degree of Doctor of Philosophy in Engineering Science at Washington State University.     

Activities of oxygen in liquid Cu-Sb and Cu-Ge alloys

In order to determine the activity coefficients of oxygen, γΩ in liquid Cu-Sb and Cu-Ge alloys at 1373 K as a function of alloy composition, the modified coulometric titrations, described previously, have been performed by using the galvanic cell: O in liquid Cu-Sb or Cu-Ge alloys/ZrO₂ (+CaO)/Air, Pt. A pronounced point of inflection in the In γΩvs alloy composition curve has been observed both for Cu-Sb and Cu-Ge alloys, as predicted by Jacob and Alcock’s quasichemical equation. The measured data itself, however, are significantly different from those predicted by their equation. The validity of Wagner’s solution model with one or two energy parameters has been also tested. YOSHIHIRO MATSUMURA, formerly a Graduate Student, Osaka University.     

Evaluation of Al-Ti-C Master Alloys as Grain Refiner for Aluminum and Its Alloys

Al-Ti-C master alloys have a great potential as efficient grain refiners for aluminum and its alloys. In the present work, the Al-Ti-C master alloys are synthesized via a relatively novel technique through the reaction of a compacted mixture of K₂TiF₆ and graphite with molten aluminum. The obtained alloys are examined using scanning electron microscopy (SEM), energy-dispersive spectroscopy, and X-ray diffraction (XRD) methods. The results indicate that the produced Al-Ti-C master alloys contain TiC and TiAl₃ particles within the aluminum matrix. Also, these alloys were evaluated using the KBI test mold. The results indicate that the produced Al-Ti-C master alloy is an efficient grain refiner for pure aluminum and its alloys compared with the Al-Ti-B one. The factors affecting the grain refinement of aluminum and its alloys are studied. The proper conditions for evaluating the efficiency of the produced Al-Ti-C master alloy to obtain a minimum grain size are as follows: temperature, 993 K (720 °C); holding time, 2 minutes; and (Ti/Al) weight ratio, 0.01 pct.     

Iron-rich intermetallic phases and their role in casting defect formation in hypoeutectic Al-Si alloys

Iron is the most common and detrimental impurity in aluminum casting alloys and has long been associated with an increase in casting defects. While the negative effects of iron are clear, the mechanism involved is not fully understood. It is generally believed to be associated with the formation of Fe-rich intermetallic phases. Many factors, including alloy composition, melt superheating, Sr modification, cooling rate, and oxide bifilms, could play a role. In the present investigation, the interactions between iron and each individual element commonly present in aluminum casting alloys, were investigated using a combination of thermal analysis and interrupted quenching tests. The Fe-rich intermetallic phases were characterized using optical microscope, scanning electron microscope, and electron probe microanalysis (EPMA), and the results were compared with the predictions by Thermocalc. It was found that increasing the iron content changes the precipitation sequence of the β phase, leading to the precipitation of coarse binary β platelets at a higher temperature. In contrast, manganese, silicon, and strontium appear to suppress the coarse binary β platelets, and Mn further promotes the formation of a more compact and less harmful α phase. They are therefore expected to reduce the negative effects of the β phase. While reported in the literature, no effect of P on the amount of β platelets was observed. Finally, attempts are made to correlate the Fe-rich intermetallic phases to the formation of casting defects. The role of the β phase as a nucleation site for eutectic Si and the role of the oxide bifilms and AlP as a heterogeneous substrate of Fe intermetallics are also discussed.     

Thermodynamics of phosphorus in molten Si-Fe and Si-Mn alloys

The thermodynamics of phosphorus in molten Si-Fe and Si-Mn alloys has been investigated at 1723 K by equilibrating the alloys in a controlled phosphorus partial pressure. The activity coefficient of phosphorus in each alloy shows a maximum value at a certain composition due to a strong interaction between silicon and iron and between silicon and manganese. Interaction coefficients between phosphorus and iron in molten silicon were found to be ε _P ^Fe =7.43 and ρ _P ^Fe =−16.4 (0≦X _Fe≦0.65), and those between phosphorus and manganese were ε _P ^Mn =12.0 and ρ _P ^Mn =−22.2 (0≦X _Mn≦0.5). Further discussion has revealed that the Si-Fe-P and Si-Mn-P systems approximately conform to a regular solution within the composition ranges investigated in the present work.     

Computer simulation of annealing and recovery effects on serrated flow in some Al-Mg alloys

Strain to the onset of serrated flow (ε _c ) remains an important measure of formability for aluminum alloys, especially in the 5XXX-series (Al-Mg) alloys, since serrated flow can signal undesirable surface appearance as well as premature fracture. For this reason, extensive experimental and theoretical work has been performed in order to identify the key parameters that affect the onset of serrated flow. In particular, it has been determined that thermal treatments applied after cold rolling can favorably impact the ε _c of an Al-Mg alloy (5182 can end stock). Recent developments of dislocation density-related constitutive modeling provide a unique opportunity to theoretically address this important problem. Using the Ananthakrishna model described and extensively tested previously, results are presented here to show that recovery processes (as simulated by a reduction in the initial density of dislocations) can cause significant increases in ε _c , in agreement with the aforementioned experimental results. Smaller initial dislocation densities can result in a considerable delay of serrated behavior. Using standard computational tools used in nonlinear dynamics (phase portraits and bifurcation diagrams), the influence of recovery (as mimicked by initial dislocation density changes) has also been examined on Luders behavior, and results which confirm previous semiquantitative considerations in the literature have been obtained. The model in its present form is limited to homogeneous straining and, thus, cannot treat the propagative instabilities often observed in experiments. Therefore, the next step in the theoretical development will be to include effects of nonhomogeneous strain.     

Selective oxidation and internal nitridation during high-temperature exposure of single-crystalline nickel-base superalloys

The process of internal nitridation of the three commercial single-crystalline nickel-base superalloys CMSX-2, CMSX-6, and SRR99 has been studied in air and oxygen-free nitrogen atmospheres at 800 °C to 1100 °C using thermogravimetric techniques supplemented by extensive microstructural examinations. Non-protective oxide formation, particularly cracking and spalling at edges or curved surfaces, enables nitrogen to penetrate into the alloy leading to the precipitation of stable Ti and Al nitrides. The high-temperature corrosion behavior of the superalloys studied is strongly affected by compositional differences between dendritic and interdendritic areas due to segregation resulting in an inhomogeneous internal precipitation zone. Furthermore, the stability of the strengthening γ′ phase (Ni₃(Al, Ti, Ta)) in front of the growing internal-nitridation zone was observed to depend clearly on the alloy composition. Therefore, the near-surface area of the alloys can be weakened by γ′ depletion and by embrittlement resulting from internal-nitride precipitation. The results obtained on the nickel-base superalloys are discussed, taking into account the results of a computer-based simulation of internal-corrosion processes. Furthermore, results on Ni-base model alloys of the system Ni-Cr-Al-Ti provided information on the role of the alloy composition. It was found that a higher Cr concentration seems to increase the nitrogen solubility and diffusion in Ni-base alloys.     

NiAl powder alloys: I. Production of NiAl powders

The influence of five methods of production of Ni₅₀Al₅₀ powder alloys on the processes occurring during reactive alloy formation of nickel monoaluminide during heating is considered. It is shown that, when powder mixtures obtained by agitation in ball mills and cladded composite powders with a low level of internal stresses are used, it is possible to produce a material with a nearly equilibrium phase composition in the course of reactive sintering due to an exothermic effect with the participation of a liquid phase (aluminum melt) in the reaction. The sintered material is porous and has an island structure. Mechanical alloying in a high-energy ball mill (attritor) results in the formation of layered Ni/Al granules with a developed interface and a high level of internal stresses and defects, which makes it possible to decrease the temperatures of initiation of reactive interaction by ∼300°C. This interaction develops in the solid phase according to a slow diffusive mechanism leading to the formation of intermediate nickel aluminides and hindering the achievement of equilibrium phase composition. The microingot granules (∼80 wt % particles 100–400 μm in size) produced by melt spraying by gases (N, Ar) has the composition of the melt, but grain boundaries are depleted of aluminum in comparison with the volume. The NiAl powders (∼90 wt % particles <40 μm in size) produced by combined hydride-calcium reduction are characterized by a highly homogeneous nickel and aluminum distribution, and their composition is close to equilibrium. These two types of powders are selected as the initial material for investigating the compacting and production of NiAl-based alloys.     

Fracture mechanics and surface chemistry studies of fatigue crack growth in an aluminum alloy

Fracture mechanics and surface chemistry studies were carried out to develop further understanding of the influence of water vapor on fatigue crack growth in aluminum alloys. The room temperature fatigue crack growth response was determined for 2219-T851 aluminum alloy exposed to water vapor at pressures from 1 to 30 Pa over a range of stress intensity factors (K). Data were also obtained in vacuum (at < 0.50 μPa), and dehumidified argon. The test results showed that, at a frequency of 5 Hz, the rate of crack growth is essentially unaffected by water vapor until a threshold pressure is reached. Above this threshold, the rates increased, reaching a maximum within one order of magnitude increase in vapor pressure. This maximum crack growth rate is equal to that obtained in air (40 to 60 pct relative humidity), distilled water and 3.5 pct NaCl solution on the same material. Parallel studies of the reactions of water vapor with fresh alloy surfaces (produced either byin situ impact fracture or by ion etching) were made by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The extent of surface reaction was monitored by changes in the oxygen AES and XPS signals. Correlation between the fatigue crack growth response and the surface reaction kinetics has been made, and is consistent with a transport-limited model for crack growth. The results also suggest that enhancement of fatigue crack growth by water vapor in the aluminum alloys occurs through a “hydrogen embrittle ment” mechanism.     

Grain-boundary character distribution in recrystallized L1<Subscript>2</Subscript> ordered intermetallic alloys

The grain-boundary character distribution (GBCD) of cold-rolled and, subsequently, recrystallized Co₃Ti and Ni₃(Si,Ti) ordered alloys with an L1₂ structure was studied by the electron backscattered diffraction (EBSD) method, in association with texture. For comparison, the GBCD of recrystallized pure copper and aluminum was also determined. The recrystallization textures of the Co₃Ti alloys as well as the Ni₃(Si,Ti) alloy were significantly weak and different from those of the pure copper and aluminum with a strong cube texture. The GBCD of the Co₃Ti alloys was characterized by a high frequency of Σ3 boundaries. On the other hand, the GBCD of the Ni₃(Si,Ti) alloy was characterized by a lower frequency of Σ3 and higher frequency of random (e.g., Σ>29) boundaries than that of the Co₃Ti alloys. However, the GBCDs of the Co₃Ti and Ni₃(Si,Ti) alloys were similar to each other and also quite similar to those of the pure copper and aluminum, when Σ3 boundaries are excluded from the GBCD. Based on these results, the formation mechanism responsible for the recrystallization textures and the grain-boundary structure and energy of the Co₃Ti and Ni₃(Si,Ti) alloys were discussed, in comparison with those of pure copper and aluminum.     

Morphology of y’ and y” precipitates and thermal stability of inconel 718 type alloys

The precipitation of the γ^’ (Ll₂) and γ^" (DO₂₂) phases has been studied in four alloys Fe-Ni-Cr-Ti-Al-Nb containing a higher Ti + Al/Nb ratio than that of the INCONEL 718 alloy. For these alloys, the precipitation microstructure varies rapidly with aging temperature and composition. Bct γ^"particles have always been found to precipitate on γ^’ phase. Moreover, by aging three alloys above a critical temperature, a “compact ntorphology” has been observed: cube-shaped γ^’ particles coated on their six faces with a shell of γ^" precipitate. This microstructure has proved to be very stable on prolonged aging. A thermal stability better than that encountered in nominal INCONEL 718 alloy can thus be achieved. The influence of composition and aging temperature on the conditions that bring about this “compact morphology” has been investigated. A minimal Ti + Al/Nb ratio between 0.9 and 1 has been determined, allowing the “compact morphology” to be obtained. This paper is based upon a thesis submitted by R. COZAR in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the University of Nancy.     

Cavitation erosion of copper and copper-based alloys

Cavitation erosion studies of five multiphase copper-based alloys for a screw propeller application and of three α-solution alloys of comparative purpose have been carried out to determine the resistance of the alloys to cavitation erosion and to understand the influence of the microstructure and/or composition of the alloys on the morphology of erosion damage. The experiments were conducted using a vibratory system. Erosion rates were measured, and the mechanism of damage under cavitation action was studied. No correlation between the cavitation erosion resistance (CER) and the mechanical properties of the alloys has been found. However, a good correlation between the resistance and work-hardenability, defined by means of stacking-fault energy (SFE), being a chemistry-dependent property, has been established for the α-solution alloys. A better CER can be expected for a lower SFE and higher work-hardenability. The resistance of the multiphase alloys in terms of chemical composition and mode of damage was discussed. An advantageous influence of nickel and aluminum and a low effectiveness of zinc on cavitation resistance have been outlined.     

On the stress exponent and the rate-controlling mechanism of high-temperature creep in some solid solutions

The internal stress, σ_i, and the effective-stress exponent of the dislocation velocity,m*, have been determined during creep of Fe-3.5 at. pct Mo alloy at 1123 K under 10.8 to 39.2 MN/m² and of Ni-10.3 at. pct W alloy at 1173 K under 19.6 to 88.2 MN/m². Both alloys have been classified among class I alloys under a certain condition including the present one, because the applied-stress exponent of the steady-state creep rates,n, is almost 3. Values of σ_i obtained by stress-transient dip test were small and almost independent of the applied stress, σ_c, in Fe-3.5 Mo alloy. On the other hand, in Ni-10.3 W alloy σ_i increased with increasing σ_c as in the case of many pure metals. The value ofm* obtained by analyzing stress-relaxation curves immediately after creep deformation was unity in Fe-3.5 Mo alloy, whereas in Ni-10.3 W alloy it was about 2.5. These results indicate that the rate-controlling mechanisms in creep are different from each other in these two alloys and that the classification according ton-value does not always coincide with the classification according to the rate-controlling mechanisms. It is concluded that the fact thatn ≃ 3 is not a sufficient evidence supporting that creep is controlled by one of microcreep mechanisms.