Determining hot cracking index of Al–Mg–Zn casting alloys calculated using effective solidification range

The possibility of determining the hot cracking index using the calculated value of the effective solidification range is investigated for multicomponent cast aluminium alloys based on the Al–Mg–Zn system with Mn, Ni, Fe and Si additives. The upper limit of the effective solidification range was calculated as the temperature of formation of a 65?wt-% solid phase using the Sheil model. The linear relationship of the hot cracking index and the effective solidification range in the industrial and experimental multicomponent alloys based on the Al–Mg–(Zn) system is demonstrated.     

Effect of pressure on solidification cracking susceptibility of Al–Si alloys

ABSTRACT

An approach was developed to calculate the crack susceptibility under various levels of pressure, and the corresponding numerical method was presented. The binary Al–Si alloy system was selected for study because the effect of high pressure on its phase diagram has been reported. The results showed a higher pressure can lead to a higher crack susceptibility and shift the most crack susceptible composition to higher solute contents. It was found a higher pressure can increase the effect of back diffusion on the solidification path and hence the crack susceptibility. This study provides a new understanding of the effect of pressure on solidification cracking susceptibility and can be a relevant starting point for studying solidification cracking under high pressures.     

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.     

Modelling yield strength of heat treated Al–Si–Mg casting alloys

Abstract

A model for the yield strength of artificially aged Al–Si–Mg casting alloys has been developed. The model includes Mg concentrations between 0·2 and 0·6 wt-% and aging temperatures between 150 and 210°C. Spherical precipitates with the composition Mg₅Si₆, which grow by diffusion of Mg from the surrounding α-Al matrix, are assumed in the model. Nucleation is assumed to be instantaneous and growth of the precipitates is modelled using Fick’s second law and mass balance. When supersaturation is lost the continued precipitate growth is modelled using the Lifshitz–Slyozov–Wagner coarsening law. An average precipitate radius is calculated and a precipitate size distribution is introduced by using a relation between the average radius and its standard deviation. The strength contribution from precipitates is calculated using coherency strengthening and Orowan strengthening. The agreement between the model and experimental data is generally good; however, modelling the underaged condition needs further refinement.     

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.     

In situ observation of solidification phenomena in Al–Cu and Fe–Si–Al alloys

Abstract

Synchrotron radiation enables the observation of solidification in metallic alloys. In situ observations of solidification for Al–Cu alloys (5, 10 and 15 wt-%Cu) are reported. Nucleation and fragmentation of dendrite arms were often observed in the 15 and 10%Cu alloys when unidirectional solidification was performed from the planar interface. In contrast, nucleation and fragmentation were rarely observed in the 5%Cu alloys. The nucleation ahead of the solidifying front and the fragmentation in the mushy region strongly depended on alloy composition. This paper also presents in situ observation of solidification of Fe–10Si–0·5Al (at-%) alloys. The dendritic growth of δ-Fe was clearly observed using this technique. The development of X-ray imaging techniques enables the solidification of various conventional cast alloys such as Al, Ni and Fe alloys to be observed and will be increasingly used to investigate solidification phenomena.     

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.     

Modelling of microsegregation in ternary alloys: Application to the solidification of Al–Mg–Si

A fast numerical model has been developed for the quantitative prediction of microsegregation during solidification of ternary alloys. Considering a small volume of uniform temperature, the back-diffusion equations in the primary solid phase are solved in a 1-dimensional configuration using an implicit finite difference formulation with a Landau transformation onto a fixed [0,1] interval. The other phases which may precipitate during solidification are supposed to be stoichiometric and at equilibrium while the liquid is in a state of complete mixing. These calculations are coupled with phase diagram data through the use of mapping files: the liquidus surface, the monovariant lines and all the pertaining information are mapped through calls to Thermo-Calc [B. Sundman, B. Jansson and J. O. Andersson, CALPHAD, 9, 153 (1985)], prior to starting the microsegregation calculation itself. This very efficient microsegregation model can thus be coupled directly to macrosegregation computations performed at the scale of a whole casting: from the average enthalpy and concentrations variations computed at each mesh point of a casting during one time step, this microsegregation model is capable of predicting the variations of temperature, of the volume fractions of the various phases, of the liquid concentrations and of the average density. The efficiency of this coupling between microsegregation calculation and thermodynamic mapping files is demonstrated in the particular case of the Al–Mg–Si system.     

Effects of fluidised bed quenching on heat treating characteristics of cast Al–Si–Mg and Al–Si–Mg–Cu alloys

Abstract

The quench sensitivity of Al–Si–Mg (D357 unmodified and Sr modified), and Al–Si–Mg–-Cu (354 and 319 Sr modified) cast alloys was investigated using a fluidised bed (FB). The average cooling rate of castings in the fluidised bed is lower than those quenched in water; the cooling rate first increases to a certain maximum and then decreases during quenching. The change in the cooling rate during quenching in water was more drastic, where the cooling rate varied from 0 to ?80 K s^?1 in less than 8 s, as compared with those quenched in FB, where the cooling rate varied from 0 to ?14 K s^?1 in 18 s. The FB quenching resulted in the formation of several metastable phases in Al–Si–Mg–Cu alloys; in contrast, no such transformation was observed during water quenching. The T4 yield strength of the FB quenched alloys was greater than water quenched alloys owing to the formation of a greater volume fraction of metastable phases in the FB quenched alloys. The tensile properties of T6 treated alloys show that Al–Si–Mg alloys (both unmodified and Sr modified) are more quench sensitive than Al–Si–Mg–Cu alloys. The high quench sensitivity of the Al–Si–Mg alloys is because GP zones are not formed, whereas GP zones are formed during quenching of the Al–Si–Mg–Cu alloys as predicted by time temperature transformation and continuous cooling transformation) diagrams.     

Change in microstructure of Al–Si–Cu casting alloys during high temperature solution treatment

Abstract

The solution treatment in Al–Si system casting alloys is usually performed to obtain supersaturated solid solution and spheroidising Si particles. It can be inferred that a high temperature solution treatment enhances mechanical properties without any special apparatuses or techniques. However, it is well known that the solution treatment close to an eutectic temperature causes local melting. In this study, the change in microstructure of Al–Si–Cu casting alloys, which have been solution treated at temperatures ranging from 773 to 824 K, have been investigated from a viewpoint of Cu concentration and the distributions of micropores and locally melt regions due to eutectic reaction. Tensile and hardness tests were carried out to discuss the relationship between mechanical properties and microstructures. In addition to a surface observation, an internal microstructural observation was carried out using the high resolution X-ray computed tomography. The burnt regions during the high temperature solution treatment were identified to be Cu rich. Porosity increased with increasing the solution treatment temperature. The porosity in the sample solution treated above a binary eutectic temperature was confirmed to be >0˙2 vol.-%. The Cu concentration in the α-phase increased below the binary eutectic temperature.     

Directional solidification of TiAl–Si alloys using a polycrystalline seed

Directional solidification (DS) process of Ti–43Al–3Si alloys has been studied. Successful DS ingots having not only fully lamellar microstructure parallel to longitudinal axis but also rotated columnar grains with respect to longitudinal axis were obtained using a polycrystalline seed material. Successful seeding and growing require plane-front solidification condition during the entire DS process. Fracture toughness of the DS alloys were superior to the PST alloys, with a value of K_Q=21.7–31.7 MPa (m)^1/2 for the crack arrest/divide mode and K_Q=7.4–19.0 MPa (m)^1/2 for the short transverse mode. The orientation dependence of fracture toughness for the crack arrest/divide mode was improved in the DS alloy compared to the PST alloy.     

Effect of strontium on crystallization of Mg2Si phase in Al—Si—Mg casting alloys

Otical microscope and SEM were used to observe the changes of the microstructure of Al-11.6%Si-0.4% Mg alloys with varying strontium additions and the effect of strontium on the crystallization of Mg2Si phase was discussed.It is found that Mg2Si phase nucleastes on the the surfaces of the eutectic silicon flakes in the unfully modified alloys,growing as meshwork or bamboo-shoot shape,however,very few and fine Mg2Si particles phase are isolated at the boundaries of the eurectic cells in the fully modified alloys.Strontium has an important influence on the crystallization of Mg2Si phase in the Al-Si-Mg casting alloys and it is thought to be related to the increase of the amunt of dendritic αphase and the modifying degree of eutectic silicon phase.     

Stress-corrosion cracking of aged AlCuMg alloys in NaCl solution

Pitting potentials and stress corrosion life-times of AlCuMg alloys (mainly 2024 alloy) with various ageing structures have been measured in a de-aerated 1M NaCl solution under conditions of controlled potential. The aged alloy, which has the higher susceptibility to stress-corrosion cracking, showed two pitting potentials corresponding to pitting at the grain boundaries and within the grains. The susceptibility of the alloys to intergranular stress-corrosion cracking occurred at potentials above the pitting potential of the grain boundaries. The intergranular stress-corrosion cracking is caused not by the dissolution of the grain boundary precipitates (S phase) but by the pitting dissolution of the solute-denuded zones along the grain boundaries. Aspects of SCC in the alloys are similar to those in the Al-4%Cu alloy without Mg.     

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.     

Modelling of the age hardening behaviour of Al–Mg–Si alloys

In the present investigation a special control volume formulation of the classical precipitation model for coupled nucleation, growth and coarsening has been adopted to describe the evolution of the particle size distribution with time during thermal processing of Al–Mg–Si alloys. The analysis includes both isothermal and non-isothermal transformation behaviour. Well established dislocation theory is then used to evaluate the resulting change in hardness or yield strength at room temperature, based on a consideration of the intrinsic resistance to dislocation motion due to solute atoms and particles, respectively following heat treatment. The model is validated by comparison with experimental microstructure data obtained from transmission electron microscope examinations and hardness measurements, covering a broad range in the experimental conditions. It is concluded that the model is sufficiently relevant and comprehensive to be used as a tool for predicting the response of Al–Mg–Si alloys to thermal processing, and some examples are given towards the end.     

Mechanical properties of Al–Si–Cu alloys produced by the continuous casting process with heating process

The mechanical properties of aluminium alloys produced by the continuous cast process and heating process (heat-cast-sample) were investigated, where the aluminium alloys are heated continuously to high temperatures for 1 h immediately following heated mould continuous casting (HMC) and sand gravity casting (SGC). The material strength and ductility of the aluminium alloys were irregularly altered depending on the heating temperature. The mechanical properties decreased when the heating temperature increased to 400 °C and were then recovered when the temperature increased to 520 °C. However, these properties decreased again when heated to more than 540 °C. The mechanical properties of the HMC-heat-cast-sample showed overall higher than those for the SGC-sample. In addition to high tensile strength, high ductility was obtained for the HMC-520 °C samples compared with those for the as-cast-sample. Such changes were found to be directly attributable to the different severity of precipitate; moreover the crystal orientation was unchanged even after the heating process.     

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.     

Wetting of porous titanium carbonitride by Al–Mg–Si alloys

Wetting of porous TiC_0.17N_0.83 by six alloys from the Al–Mg–Si system (pure Al, pure Mg, Al–15 at.% Mg, Al–10 at.% Si, Mg–5 at.% Si, and Al–10 at.% Mg–10 at.% Si) in an argon atmosphere was studied using the sessile drop experiment. The contact angle of the liquid drops on TiC_0.17N_0.83 substrates was measured as a function of temperature. Aluminium, Al–10 at.% Si, and Al–10 at.% Mg–10 at.% Si did not wet TiC_0.17N_0.83 in the studied temperature range. Magnesium always wetted TiC_0.17N_0.83 with a minimum contact angle of ≈44° at 900°C, and alloying with Mg significantly lowered the contact angle of Al on TiCN. Alloying with Si deteriorated the wetting of TiCN by Mg. A comparative study between the systems was conducted, based on the results and on data available in the literature. The improvement of the wetting of TiCN by Al due to alloying with Mg can be explained by the segregation of Mg to the interface with TiCN, where it lowers the interface energy. The addition of Si to pure Mg or to Al–Mg results in an increase in the contact angle on TiCN.     

Study of prevention of solidification cracking in Al‐Mg‐Si alloy laser welds. CO2 laser welding of Al‐Mg‐Si alloys (1st report)

The choice of weld bead size in the case of welded cruciform joints can be problematic, especially when the sheets forming the joint are of differing thickness. Technological standards generally recommend a weld bead thickness less than the minimum thickness of the sheets to be joined, whereas structural standards do not envisage any dependency between joint static and fatigue strength and weld bead dimensions, unless these are so reduced as to lead to failure starting and propagating in the weld bead itself rather than the base metal plates. The scope of this study is the theoretical and experimental analysis of the change in fatigue strength with varying weld bead thickness and minimum welded plate thickness.