3-D granular modeling and in situ X-ray tomographic imaging: A comparative study of hot tearing formation and semi-solid deformation in Al–Cu alloys

The mechanical behavior of partially solidified Al–Cu alloys is investigated to assess the influence of mushy zone deformation on hot tearing. For this purpose, the results of a semi-solid tensile test conducted in situ using X-ray microtomography are compared with the predictions of a coupled hydromechanical granular model in order to both validate the predictions of the model and explain the experimental observations. It is shown that hot tears initiate in the widest liquid channels connected to the free (oxidized) surfaces as long as there is contact between the intergranular liquid and the ambient air. The necking behavior is associated with the deformation-induced liquid pressure drop. Overall, the stresses predicted by the granular model under tensile and shear deformations agree well with the experimental data. Thus, the granular model achieves an important step in predicting hot tearing formation.     

Relationship between amounts of low-melting-point eutectics and hot tearing susceptibility of ternary Al−Cu−Mg alloys during solidification

A systematical study on the relationship between the amounts of different eutectic phases especially the low-melting-point (LMP) eutectics and the hot tearing susceptibility of ternary Al−Cu−Mg alloys during solidification was performed. By controlling the concentrations of major alloying elements (Cu, Mg), the amounts of LMP eutectics at the final stages of solidification were varied and the corresponding hot tearing susceptibility (HTS) was determined. The results showed that the Al−4.6Cu−0.4Mg (wt.%) alloy, which contained the smallest fraction of LMP eutectics among the investigated alloys, was observed to be the most susceptible to hot tearing. With the amount of total residual liquid being approximately the same in the alloys, the hot tearing resistance is considered to be closely related to the amounts of LMP eutectics. Specifically, the higher the amount of LMP eutectics was, the lower the HTS of the alloy was. Further, the potential mechanism of low HTS for alloys with high amounts of LMP eutectics among ternary Al−Cu−Mg alloys was discussed in terms of feeding ability and permeability as well as total viscosity evolution during solidification.     

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.     

A comparative study on corrosion of Mg–Al–Si alloys

Corrosion behavior of various Mg–Al–Si alloys (AS11, AS21, AS41, AS61 and AS91 series), cast under the same cooling conditions and controlled alloying composition, was investigated systematically. Optical microscopy and scanning electron microscopy were used for microstructural examinations. The corrosion behavior was evaluated by immersion tests and potentiodynamic polarization measurements in 3.5% NaCl solution. The results from both immersion tests and the potentiodynamic polarization measurements showed that marginal improvement in corrosion resistance was observed with 2.0% Al (mass fraction) containing alloy (AS21) whereas Al addition above 2.0% (AS41, AS61 and AS91) deteriorated the corrosion resistance which was attributed to β phase, acting as cathode, and the interruption of continuity of the oxide film on the surface of the alloys owing to coarsened β and Mg₂Si phases.     

A study on the application of ICA to GMAW

A novel method, independent component analysis （ ICA ） , is introduced to gas metal arc welding （GMAW） process monitoring. ICA was applied to arc signals, i. e. welding current and arc voltage, to remove the correlation between them and extract an independent component IC. Two series of robotic GMA W experiments were carried out to study the feasibility of ICA for online monitoring. It was found that IC put up an abnormity when there was a step disturbance in the welding process. Experimental results showed that the IC could be used as a state variable representing the process variation. By applying statistical process control （SPC） for the obtained IC, a burning-through defect was isolated from the normal operation. The comparison between ICA and principal component analysis （PCA） was also made for the processes, which led to an interesting result and was in need for further study.     

Reducing welding hot cracking of high-strength novel Al–Mg–Zn–Cu alloys based on the prediction of the T-shaped device

ABSTRACT

The higher hot cracking tendency during fusion welding in traditional high-strength 7000 series alloys has been an obstacle for its further application. In this study, the cracking susceptibility can be suppressed by fabricating Al–Mg–Zn–Cu alloys with Zn/Mg≤1 and Cu/Mg≤0.25 while simultaneously maintaining the high strength. A T-shaped device combined with non-equilibrium solidification is developed to simulate the solidification during fusion welding, and it is effective to predict the shrinkage load, temperature and solid fraction. The effect of solidification temperature range, the amount of eutectics at the terminal stage of solidification and the shrinkage load during solidification on the hot cracking susceptibility are discussed in detail.     

The in-situ Ti alloying of aluminum alloys and its application in A356 alloys

1. Introduction Titanium is one of the most effective grain refinement elements of aluminum alloys. The grain of aluminum alloys can be effectively refined if containing up to 0.2% titanium, which results in improvement of mechanical properties and performance [1-8]. Titanium is usually added into aluminum melt by melting Al-Ti or Al-Ti-B (Ti/B= 5:1) master alloy. This requires strict control in melting temperature and holding time, resulting in the complicacy to control the quality of allo…     

Microstructure and hot cracking susceptibility of high conductivity Al–Fe–Si alloys

Aluminium–silicon based casting alloys have been extensively utilised in various industrial applications, but their relatively low electrical and thermal conductivities make them unsuitable for high conductivity parts. In this research, Al–Fe–Si based high conductivity alloys containing limited silicon content were investigated. Al–0·5Fe–xSi alloys with silicon ranging from 0·5 to 2% showed significantly higher electrical conductivity than conventional Al–Si based alloys. The hot cracking susceptibility of Al–Fe–Si alloys became seriously high as the Si content increased up to 1·5%, then susceptibility rapidly reduced with the further increase in Si. The relationship between solidification characteristics and hot cracking susceptibility of Al–0·5Fe–xSi alloys was discussed based on the thermal and cooling curve analyses and microstructural observations.     

Effects of hot rolling on microstructure and properties of a 20 vol. % SiCp／Al composite

A 20 vol.% SiCp/Al composite was fabricated by squeeze casting, of which a new process for fabricating the preform was used by blending Al powder and SiC particulates with average diameters of 10 and 3.5μtm, respectively. The microstructure of the as-cast and the hot-rolled composite was investigated by using TEM, EDS, and SEM, and their tensile properties were measured at room temperature. The results show that the ultimate tensile strength and ultimate elongation of the hot-rolled composite are 80% and 140% higher than those of the as-cast one. The TEM observation result indicates that there are high density of dislocations and dislocation tangles in the hot-rolled composite. Al2O3 layers in the composite resuiting from the surface oxidation of the aluminum powders were damaged to spherical particles during hot rolling. All the results indicate that hot-rolling can improve the mechanical properties of the composite and, therefore, engineering components of the 20 vol.% SiCp/Al composite can be produced by squeeze casting followed by hot-rolling.     

Twinnability of Al–Mg alloys: A first-principles interpretation

Al–Mg alloys are considered to have potentials to form twins during deformation because Mg can reduce the intrinsic stacking fault energy γ_ISFE of Al. Nevertheless, twinning has rarely been found in Al–Mg alloys even subjected to various severe plastic deformation (SPD) techniques. In order to probe the twinning propensity of Al–Mg alloys, first-principles calculations were carried out to investigate the effects of Mg and vacancies on the generalized planar fault energy (GPFE) of Al. It is found that both Mg and vacancies exhibit a Suzuki segregation feature to the stacking fault, and have the influence of decreasing the γ_ISFE of Al. However, γ_ISFE does not decrease and the twinnability parameter τ_a of Al does not increase monotonically with increasing Mg concentration in the alloy. On the basis of τ_a evaluated from the calculated GPFE of Al–Mg alloys, we conclude that deformation twinning is difficult for Al–Mg alloys even with a high content of Mg. Besides, the decrease of γ_ISFE caused by the introduction of Mg and vacancies is supposed to have the effect of improving the work-hardening rate and facilitating the formation of band structures in Al–Mg alloys subjected to SPD.     

Effects of grain refiner additions (Zr,Ti–B) and of mould variables on hot tearing susceptibility of recently developed Al–2 wt-%Cu alloy

Abstract

The hot tearing susceptibility of the new Al–2 wt-%Cu based alloys prepared using Zr and Ti–B additions was tested using the constrained rod casting mould under different mould variables. The 206 alloy type was used to evaluate the results obtained from this new alloy. It was found that the hot tearing susceptibility of the alloys under investigation decreases proportionally as the mould temperature is increased; thus, the hot tearing susceptibility of the Al–2 wt-%Cu and 206 alloys decreases from 21 for both the alloys to 3 and 9 respectively as the mould temperature is increased from 250 to 450°C. This beneficial effect of elevated mould temperatures may be attributed to a reduction in the contraction strain rate and in the porosity level. Grain refinement additions of Ti–B or Zr–Ti–B enhance the hot tearing resistance of the Al–2 wt-%Cu to a significant level.     

A metallographic and quantitative analysis of the influence of stacking fault energy on shock-hardening in Cu and Cu–Al alloys

This paper deals with the mechanical behavior of Cu and solid–solution Cu–Al alloys that were shock-deformed to 10 and 35 GPa. All the shock-deformed materials showed shock-strengthening that was greater at higher shock pressure and decreased with decreasing stacking fault energy (SFE) at both shock pressures. In the literature, shock-strengthening has been qualitatively ascribed to a greater dislocation density and the formation of deformation twins without addressing the question as to why shock-strengthening is lower in low SFE materials. This question is addressed in the present work by quantifying the twin contribution to the total post-shock strength. The twin contribution was found to increase with decreasing SFE suggesting that the contribution of dislocations concurrently decreases. The stored energy of as-shock-deformed materials was measured and found to decrease with decreasing SFE implying a lower net stored dislocation density in the lower SFE alloys. It is suggested that a lower net stored dislocation density in low SFE alloys results in the observed lower shock strengthening.     

The thermodynamics of and strengthening due to co-clusters: General theory and application to the case of Al–Cu–Mg alloys

Co-clusters in ternary or higher order metallic alloys are metastable structures involving two or more distinct alloying atoms that retain the structure of the host lattice. A thermodynamic model based on a single interaction energy of dissimilar nearest neighbour interaction energy is presented, and a model for the strengthening due to these co-cluster dimers is derived. The model includes a new treatment of (short-range) order strengthening relevant to these co-clusters and further encompasses modulus hardening and chemical hardening. The models are tested against data on a wide range of Al–Cu–Mg alloys treated at temperatures between 20 and 220 °C. Both quantitative calorimetry data on the enthalpy change due to co-cluster formation and strengthening due to co-clusters is predicted well. It is shown that in general (short-range) order strengthening will be the main strengthening mechanism.     

Effect of Sr modification on microstructure and thermal conductivity of hypoeutectic Al−Si alloys

Trace amount of Sr (0.05 wt.%) was added into the hypoeutectic Al−Si (3−12 wt.% Si) alloys to modify their microstructure and improve thermal conductivity. The results showed that the thermal conductivity of hypoeutectic Al−Si alloys was improved by Sr modification, and the increment and increasing rate of the thermal conductivity gradually increased with Si content increasing. The improvement of thermal conductivity was primarily related to the morphology variation of eutectic Si phases. In Sr-modified Al−Si alloys, the morphology of eutectic Si phases was a mixed morphology of fiber structure and fine flaky structure, and the proportion of the fine flaky eutectic Si phases gradually decreased with Si content increasing. Under the Si content reaching 9 wt.%, the proportion of fine flaky eutectic Si phases was nearly negligible in Sr-modified alloys. Correspondingly, the increment and increasing rate of thermal conductivity of Sr-modified alloys reached the maximum and tended to be stable.     

Effect of composition and processing route on the wear behaviour of Al–Si alloys

The wear behaviour of five Al–Si alloys, with Si contents between 12% and 16% and different Cu and Ni additions, has been studied using pin-on-disc tests. The importance of alloying elements and alloy processing has been proven. New thixoformed microstructures showed better wear resistance than alloys produced in metallic moulds.     

The effects of Sc on the microstructure and mechanical properties of hypo-eutectic Al−Si alloys

The eutectic Si microstructure in Al-8.5wt.%Si alloy was changed from large flakes to fine lamellar when the Sc amount in the alloy reached 0.2 wt.%. 0.8wt.%Sc was optimal in terms of attaining the best modification effect. Study on the distribution of the modifiers and measurement of the surface tension of Al-8.5wt.%Si alloy melt with added Sr, Na, and Sc modifiers, respectively, reveals that Sc modifies eutectic Si by a decrease of surface tension, while Sr and Na modify eutectic Si mainly by an impurity-induced twinning mechanism. Al-8.5wt.%Si-0.4wt.%Sc alloy displayed approximately 50 and 70% increases in tensile strength and elongation, respectively, over Al-8.5wt.%Si alloy in the cast state. It also presented approximately 65 and 70% increases in tensile strength and elongation, respectively, over Al-8.5wt.%Si alloy at a ppt heat-treated state at 200°C for 3 h.     

Influence of Cr on the nucleation of primary Al and formation of twinned dendrites in Al–Zn–Cr alloys: Can icosahedral solid clusters play a role?

The equiaxed solidification of Al–20 wt.% Zn alloys revealed an unexpectedly large number of fine grains which are in a twin, or near-twin, relationship with their nearest neighbors when minute amounts of Cr (1000 ppm) are added to the melt. Several occurrences of neighboring grains sharing a nearly common 〈1 1 0〉 direction with a fivefold symmetry multi-twinning relationship have been found. These findings are a very strong indication that the primary face-centered cubic Al phase forms on either icosahedron quasicrystals or nuclei of the parent stable Al₄₅Cr₇ phase, which exhibits several fivefold symmetry building blocks in its large monoclinic unit cell. They are further supported by thermodynamic calculations and by grains sometimes exhibiting orientations compatible with the so-called interlocked icosahedron. These results are important, not only because they provide an explanation of the nucleation of twinned dendrites in Al alloys, a topic that has remained unclear over the past 60 years despite several recent investigations, but also because they identify a so far neglected nucleation mechanism in aluminum alloys, which could also apply to other metallic systems.     

Hereditary effect of Al-based modifiers and grain refiners on structure and properties of A356.2 alloys

The hereditary effect of Al-Ti, Al-Ti-B, Al-Sr master alloys on the structure and properties of A356.2 alloys was investigated, and comparison analysis between the master alloys used in the foundry industry and the fine-crystalline grain refiners produced by technologies of Samara State Technical University was conducted. The results show that less than 0.5% additions of FCR master alloys can promote 8%-20% in the elongation of as-cast A356.2 alloys. FCR additives are more efficient in comparison with conventional grain refiners and modifiers. Their effectiveness depends on their genetic effect of their finer structures.