Definition of quality in cast aluminum alloys and its characterization with appropriate indices

In this paper, the issue of “quality” of cast aluminum alloys from various viewpoints is interpreted. Many methods to characterize the quality of materials are available; the methods used currently for the quality evaluation of cast aluminum alloys include nondestructive testing, characterization of the microstructure, and mechanical testing. With regard to mechanical testing, a number of quality indices have been devised to evaluate and characterize the quality of cast aluminum alloys. As these quality indices use different mechanical properties for the quality evaluation, they are expected to lead to different results. In this work, the application of proposed quality indices and their suitability is discussed for a number of situations, including minor variations in chemical composition, different solidification rate, solid solution and artificial aging heat treatments.     

Quality index and hot tearing susceptibility of Al–7Si–0.35Mg–xCu alloys

The effects of Cu addition (0.5%, 1%, 1.5%, 2%, and 3%, mass fraction) on the quality index (Q_i) and hot tearing susceptibility (HTS) of A356 alloy were investigated. According to the results, Cu addition up to 1.5% increases the Q_i by almost 10%, which seems to be due to its solid solution strengthening and dispersion hardening effect of Cu-rich Al₂Cu and AlMgCuSi compounds. However, further addition of Cu (up to 3%) decreases the Q_i by almost 12%, which is likely due to the reduction of tensile strength and elongation caused by increased volume fraction of brittle Cu-rich intermetallics and microporosities in the microstructure. It is also found that Cu increases the HTS of A356 alloy measured by constrained rod casting method. According to the thermal analysis results, Cu widens the solidification range of the alloy, which in turn, decreases its fluidity and increases the time period during which the mushy-state alloy is exposed to the hot tearing susceptible zone. SEM examination of the hot tear surfaces in high-Cu alloys also demonstrates their rough nature and the occurrence of interdendritic/intergranular microcracks as convincing evidences for the initiation of hot tears in the late stages of solidification in which there is not enough time for crack healing.     

Modeling of Alternative Compositions of Recycled Wrought Aluminum Alloys

Nowadays, a significant part of postconsumed wrought aluminum scrap is still used for the production of comparatively cheaper cast alloys, in that way losing an important part of the potential added value. The share of postconsumed scrap in wrought aluminum alloys could be increased either by sorting to fractions with the required chemical composition and/or by broadening the standard compositional tolerance limits of alloying elements. The first solution requires hand or automatic sorting of postconsumed scrap as alloys or groups of alloys to the degree of separation sufficient to enable the blending of standard compositions of wrought alloys; the second solution is much more radical, predicting changes in the existing standards for wrought aluminum alloys toward nonstandard alloys but yet having properties acceptable for customers. In this case, the degree of separation of incoming postconsumed scrap required is much less demanding. The model presented in this work enables the design of optimal (standard and nonstandard recycling-friendly) compositions and properties of wrought aluminum alloys with significantly increased amounts of postconsumed scrap. The following two routes were modeled in detail: (I) the blending of standard and nonstandard compositions of wrought aluminum alloys starting from postconsumed aluminum scrap sorted to various degrees simulated by the model and (II) changing the initial standard composition of wrought aluminum alloys to nonstandard “recycling-friendly” ones, with broader concentration tolerance limits of alloying elements and without influencing the selected alloy properties, specified in advance. The applied algorithms were found to be very useful in the industrial design of both procedures: (I) the computation of the required chemical composition of the scrap streams obtained by sorting (or, in other words, the postconsumed scrap sorting level), necessary for achieving the standard wrought alloy composition and (II) the transformation of standard to nonstandard (recycling-friendly) compositions with the key alloy properties (e.g., tensile strength and elongation) remaining the same.     

Numerical analysis of shear deformation localization using a double-variable damage model

A double-variable damage model was introduced into the constitutive equations to demonstrate the effect of the material damage for the isotropic elastic, hardening, and damage states, and for the isothermal process. The shear damage variable D _s and the bulk damage variable D _b may be, respectively, used to describe the effect of shear damage and bulk damage for material properties without the superfluous constraint, D _b=D _s, that is found in the single-variable damage model. The double-variable damage model was implemented to form the finite element code for analyzing the effect of shear damage and bulk damage. In this article, two numerical simulation examples were completed to model the whole process of initiation and propagation of shear bands in an aluminum alloy. The numerical computational results are coincident with the experimental results.     

Ni tracer diffusion in the B2-compound NiAl: influence of temperature and composition

The effect of composition and temperature on Ni bulk self-diffusion is investigated for nine different single crystalline NiAl alloys with well-defined compositions between 46.8 and 56.6 at.% Ni in the temperature range from 1050 to 1630K. The diffusion penetration profiles of Ni in NiAl were determined by applying two different techniques of profile detection. Radiotracer experiments have been carried out using the ⁶³Ni tracer, a serial sectioning technique, and sensitive liquid scintillation counting for the high temperature measurements, while at lower temperatures the diffusion profiles were analyzed by secondary ion mass spectrometry (SIMS) using the highly enriched stable isotope ⁶⁴Ni. In contrast to the literature data on Ni self-diffusion in NiAl alloys by Hancock and McDonnell [Phys. stat. sol. A4, (1971) 143], the present measurements show an unexpected concentration dependence of the Ni diffusion coefficients D with nearly constant diffusivities for stoichiometric and Al-rich alloys and increasing D values with increasing Ni content on the Ni-rich side of the NiAl composition range. The effective diffusion activation enthalpy Q is equal to about 3.0±0.07 eV for the Al-rich, stoichiometric, and slightly Ni-rich NiAl alloys, while for the compositions with larger Ni content a decrease of Q was observed with increasing Ni content, for example, Q=2.39 eV for the Ni_56.6Al_43.4 alloy. The present experimental results imply that mainly the same diffusion mechanism operates on both sides of stoichiometry in NiAl. This mechanism is identified with the triple defect mechanism. Its contribution is compositionally independent. The activation energy of Q=3.18 eV was calculated for the triple defect mechanism using empirical EAM potentials in agreement with the experimental data. The decrease of Q at large Ni concentrations on the Ni-rich side is explained by an additional contribution of the anti-structure bridge mechanism with the activation energy of Q=1.73 eV.     

The Effect of Small Amount of Titanium Addition on the Grain Refinement and Mechanical Properties of ZA48 Alloy

Low-titanium (Ti) aluminum alloys were prepared, and the effects of Ti elements on the microstructure, tensile property, and wear property of zinc-aluminum alloys were investigated. The addition of Ti is found to be effective in refining the grain size of the zinc-aluminum alloys. With a Ti content of 0.04 wt.%, the minimum grain size is achieved, and the tensile property of the test alloy reaches the maximum. Wear resistance is improved with decreased grain size. The grain-refining mechanism of the low-Ti aluminum alloys can be explained by the formation of Al₆₆Ti₂₅Zn₉ particles. These particles serve as the nucleation sites and effectively restrict α phase growth.     

Effect of the deviation of the chemical composition from the stoichiometric composition on the structural and phase transformations and properties of rapidly quenched Ti50 + xNi25 ? xCu25 alloys

Methods of X-ray diffraction, transmission and scanning electron microscopy, and electron diffraction have been used to study phase composition and structure of an almost stoichiometric alloy Ti₅₀Ni₂₅Cu₂₅. The alloys of the quasi-binary section TiNi-TiCu to be studied, which exhibit in the initial ascast state thermoelastic martensitic transformations B2 ↔ B19 and related shape-memory effects, have been produced by rapid quenching of the melt (melt spinning technique). The chemical composition of the Ti_{50 + x} Ni_{25 − x} Cu₂₅ alloys was varied with respect to titanium and nickel within x ≤ ±1% (from Ti₄₉Ni₂₆Cu₂₅ to Ti₅₁Ni₂₄Cu₂₅). It has been shown that the rapid quenching from the melt at a cooling rate of 106 K/s provides amorphization for all the alloys under consideration. Heating to 723 K or higher temperatures leads to the devitrification of the amorphous alloys with the formation of a polycrystalline structure of the B2 austenite. The mechanical properties of the alloys have been measured in the initial amorphous state and after subsequent heat treatment. It has been established that, depending on the degree of deviation of the alloy from the stoichiometric composition, which leads to solid solution decomposition in the process of nanocrystallization upon heat treatment, there occur regular changes in the mechanical properties and shape-memory effects of the alloys. The characteristic temperatures of the onset and finish of the process of crystallization from the amorphous and amorphous-crystalline states and the critical temperatures of the onset and finish of the forward and reverse thermoelastic martensitic transitions have been determined by measuring temperature dependences of the electrical resistivity of the alloys. The diagram of the dependence of the critical temperatures on the chemical composition of the alloy has been constructed.     

Relationship Between Mechanical Properties of Lead-Free Solders and Their Heat Treatment Parameters

The research was undertaken to establish mechanical properties of as-cast and heat-treated Sn-Zn-based alloys of binary and ternary systems as candidates for lead (Pb)-free solder materials for high-temperature applications. The heat treatment of as-cast alloys was made under different combinations of processing parameters (168 h/50 °C, 42 h/80 °C, and 24 h/110 °C). The systematic study of structure-property relationships in Sn-Zn, Sn-Zn-Ag, and Sn-Zn-Cu alloys containing the same amount of Zn (4.5, 9, 13.5 wt.%) and 1 wt.% of either Ag or Cu was conducted to identify the effects of chemical composition and heat treatment processing parameters on the alloy microstructure and mechanical behavior. Structural characterization was made using optical microscopy and scanning electron microscopy techniques coupled with EDS analysis. Mechanical properties (initial Young’s modulus E, ultimate tensile strength UTS, elastic limit R _0.05, yield point R _0.2, elongation A ₅, and necking Z) were determined by means of static tensile tests. All the examined Sn-Zn-based alloys have attractive combination of mechanical characteristics, especially tensile strength, having values higher than that of common leaded solders and their substitutes of Pb-free SAC family. The results obtained demonstrate that the Sn-Zn-based alloys present competitive Pb-free solder candidates for high-temperature applications.     

Electrodeposition of ternary Zn–Ni–Sn alloys from an ionic liquid based on choline chloride and their characterisation

Ternary Zn–Ni–Sn alloy coatings with a range of compositions were potentiostatically electrodeposited on steel substrates from a deep eutectic solvent-based electrolyte. The effect of electrodeposition potential on the morphology, chemical and phase compositions, and corrosion behaviour of the deposits was analysed. The co-deposition mechanism of Zn–Ni–Sn alloys was found to be normal whereby increasing the electrodeposition potential enhanced the ternary alloy Zn content (active element) and greatly suppressed the alloy Sn and Ni content (noble elements). The X-ray diffraction phase analyses showed that Ni in the deposits exists in the form of metal compounds including β-Ni₃Sn₂ as well as γ-NiZn₃. The improved corrosion resistance observed in all ternary alloys was attributed to their compact morphology, phase content and chemical composition. Comparison of corrosion performances shows that ternary Zn–Ni–Sn alloys are superior for sacrificial corrosion protection of steel metallic substrates compared to binary Zn–Sn and Zn–Ni alloys.     

Corrosion performance of magnesium alloys MEZ and AZ91

The corrosion performance of sand cast MEZ_S, zirconium-grain-refined MEZ_R, sand cast AZ91_S, and high pressure diecast AZ91_D magnesium alloys were evaluated by means of salt spray testing, optical metallography, hydrogen evolution, polarisation curve measurement and AC impedance spectroscopy. The results show that the corrosion resistance of the four alloys can be ranked in decreasing order as AZ91_D > AZ91_S ≈ MEZ_R > MEZ_S and that the intergranular phases and chemical composition of the matrix phase have a significant influence on the corrosion performance. Alloys with a finer grain size and higher aluminum or zirconium contents exhibit better corrosion resistance.     

Elemental and phase composition,morphology, and chemical features of the surface of Al–Ni Alloys in contact with liquid Ga–In eutectic

A set of aluminum–nickel alloys has been studied. The elemental composition of the samples has been determined by atomic emission and atomic absorption spectrometry. X-ray diffraction analysis has revealed that the alloying of the metals leads to the formation of Al₃Ni and Al₃Ni₂ intermetallic compounds, while a portion of Al remains in a metallic phase. The local chemical composition and surface morphology of the original alloys and the alloys activated with the liquid Ga–In eutectic have been studied by scanning electron microscopy and X-ray microanalysis. It has been shown that the original alloys are characterized by a pronounced morphological heterogeneity of interfacial regions in the near-surface layers. It has been found that the studied Al–Ni alloys are activated by the liquid Ga–In eutectic; however, one of the alloy components—the Al₃Ni intermetallic compound—does not undergo significant morphological and chemical changes in contact with the liquid eutectic.     

Effect of chloride ions on electrochemical properties of anodized <Emphasis Type="Italic">AV</Emphasis> and <Emphasis Type="Italic">D</Emphasis>16 aluminum alloys in aqueous and glycerin-containing aqueous sodium sulfate solutions

The effect of chloride ions (0.01 N NaCl) on the electrochemical properties of anodized (in chromic anhydride or sulfuric acid) AV and D16 aluminum alloys in aqueous sulfate (0.5% Na₂SO₄) and glycerin-containing aqueous sulfate (0.5% Na₂SO₄, 33% glycerin) solutions is studied. Depending on the conditions of anodizing and the composition of the alloy and environment, currents on the anodized alloys in the passive range are shown to be smaller by one to four orders of magnitude compared to those on nonanodized alloys. Anodizing increases the resistance of alloys against pitting corrosion. Alloys anodized in sulfuric acid and then treated in dichromate are not susceptible to pitting corrosion. Alloys anodized in chromic anhydride are less resistant against pitting.     

Effects of chemical composition on nanocrystallization kinetics, microstructure and magnetic properties of finemet-type amorphous alloys

In this study, the kinetics of nanocrystallization of amorphous Fe_73.5Si_13.5B₉Nb₃Cu₁ (F1) and Fe₇₇Si₁₁B₉Nb_2.4Cu_0.6 (F2) alloys is investigated. The microstructure and magnetic properties of the nanocrystalline alloys are compared. The crystallization temperature of F2 alloy is shifted towards lower temperatures with respect to F1. Thus, the crystalline volume fraction and the crystalline grain size at specific annealing temperature for the F2 alloy are higher than for the F1 alloy, accounting for the higher coercive force of F2 alloy with respect to the one of F1 alloy. According to isoconversional methods, the activation energy for crystallization is variable as a function of transformed fraction because of the continuous changes in chemical composition during the transformation. Mean values of 350 and 290 kJ/mol are obtained for F1 and F2, respectively. Microstructural observations confirm that minor changes in chemical composition affect the kinetics and final microstructure of the nanocrystalline alloy, that determine the observed magnetic properties.     

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.     

Effect of aluminum content on mechanical properties of hydrogenated Mg–Al magnesium alloys

The mechanical properties of hydrogenated Mg–Al magnesium alloys with various aluminum content were investigated. The ductility, yield strength (YS) and ultimate tensile strength (UTS) of the hydrogenated material decreased while the hardness increased with increasing the aluminum content. Microscopic observations of cross-sections of hydrogenated specimens with various Al content revealed that hydrogen cracks extended deeply as the Al content in the Mg–Al alloys increased. Moreover, X-ray diffraction (XRD) analysis revealed that MgH₂ and AlH₃ hydrides are formed during hydrogenation and were found to contribute to hydrogen embrittlement of Mg–Al alloys. However, the embrittled zone was observed to be larger at the fracture surface of Mg–15Al alloy than that of Mg–5Al alloy. Moreover, the fracture surface of Mg–30Al alloy exhibited completely brittle fracture after hydrogenation.     

Study of fatigue crack growth rate for austenitic Fe-Al-Mn alloys

A study was made of the crack growth rate (da/dN) versus stress-intensity variation ΔK behavior of Fe-Al-Mn alloys with different percentages of carbon, aluminum, and manganese at ambient temperature. The experimental results are described with respect to a Paris equation,da/dN = C(†K)ⁿ, where the exponent n, index for crack growth resistance of materials, was strongly influenced by alloy composition. It was found that higher manganese content provided better crack growth resistance, and that carbon and aluminum had an opposite effect. Scanning electron microscopy, x-ray diffraction, and mechanical properties evaluation were performed and correlated to the change of n values.     

Observations of nodule growth during the oxidation of pure binary iron-aluminum alloys

Oxidation studies were carried out in oxygen at 800°C, on a series of pure binary iron-based alloys with between 1.9 and 9.8 wt. % aluminum. The results are presented in conjunction with the existing literature and these permit the development of a classification of scale morphologies based on alloy composition. Alloys with less than about 2.4 wt. % aluminum form bulky stratified scales composed of Fe₂O₃ and Fe₃O₄ with FeAl₂O₄ and Al₂O₃ at the scale-metal interface. Alloys with between 2.4 and 6.9 wt. % form an external Al₂O₃ scale but this is interspersed with iron oxide nodules that penetrate the alloy substrate. Only alloys with greater than 6.9 wt. % aluminum form completely protective Al₂O₃ scales. Models based on oxide nucleation are presented for the growth of bulky scales and also the iron oxide nodules.     

A Modified Method to Estimate Fatigue Parameters of Wrought Aluminum Alloys

Several methods for estimating fatigue properties of wrought aluminum alloys from simple tensile data or hardness were discussed. Among them, Park-Song’s modified Mitchell’s method provided the best estimation results in low fatigue life regime. Roessle-Fatemi’s hardness method tends to be very erroneous in the present estimations. None of the investigated methods provide the satisfactory estimation results in the case of N _f > 3 × 10⁴ cycles. Besides, correlation between ultimate tensile strength and Brinell hardness was developed. Then, the modified Mitchell’s method utilizing the ultimate tensile strength predicted from Brinell hardness was proposed in this study and successfully applied to estimate fatigue properties for wrought aluminum alloys. This simple method requires only Brinell hardness and modulus of elasticity as inputs, both of which are either commonly available or easily measurable. Prediction capability of this method was evaluated for wrought aluminum alloys with hardness between 120 and 157 HB. Results show that the proposed method provides the best life predictions for wrought aluminum alloys.