Search

• The effects low-temperature retrogression and re-aging treatments on the strength and electrical conductivity of 7050 aluminium alloy plate were studied. • Low retrogression temperature can delay or slow down precipitation and growth of precipitates, and help prolonging the retrogression times to more than ten hours. • Balancing strength and corrosion resistance (electrical conductivity) needs integrated optimization of microstructure and aging processes. The retrogression and re-aging (RRA) processes, aimed mainly at tailoring intergranular precipitates, could significantly improve the corrosion resistance (i.e., stress corrosion cracking resistance) without considerably decreasing the strength, which signifies that an efficient control of the size, distribution and evolution of intergranular and intragranular precipitates becomes critical for the integrated properties of the (mid-)thick high-strength Al alloy plates. Compared to RRA process with retrogression at 200 °C (T77), this study investigated the impact of a modified RRA process (MT77) with lower retrogression temperatures (155-175 °C) and first-stage under-aging on the properties of a high-strength AA7050 Al alloy, in combination with detailed precipitate characterization. The study showed that the strength/microhardness of the RRA-treated alloys decreased with raising retrogression temperature and/or prolonging retrogression time, along with the increased electrical conductivity. The rapid responsiveness of microstructure/property typical of retrogression at 200 °C was obviously postponed or decreased by using MT77 process with longer retrogression time that was more suitable for treating the (mid-)thick plates. On the other hand, higher retrogression temperature facilitated more intragranular η precipitates, coarse intergranular precipitates and wide precipitate free zones, which prominently increased the electrical conductivity alongside a considerable strength loss as compared to the MT77-treated alloys. With the preferred MT77 process, the high strength approaching T6 level as well as good corrosion resistance was achieved. However, though a relatively homogeneous through-thickness strength was obtained, some small discrepancies of properties between the central and surface areas of an 86-mm thick 7050 Al alloy plate were observed, possibly related to the quenching sensitivity. The precipitate evolution and mechanistic connection to the properties were discussed and reviewed for high-strength Al alloys along with suggestions for further RRA optimization. [ABSTRACT FROM AUTHOR]

Using the good criteria to predict hot tearing is very important during DC casting of aluminium alloys. Among all the hot tearing criteria, a fracture-mechanics based SKK criterion proposed by Suyitno et al. has made considerable improvements in the hot tearing prediction. However, its obtained hot tearing susceptibility (HTS) evolution during solidification is also not completely consistent with real industrial production circumstances, especially when approaching the solidus temperature. In this paper, some further modifications are made based on the SKK criterion to emphasise the important effect of solid bridging/grain coalescence on hot tear propagation. It is proved that the HTS evolution in freezing range predicted by the modified hot tearing criterion is in good agreement with casting practice.

Using the good criteria to predict hot tearing is very important during DC casting of aluminium alloys. Among all the hot tearing criteria, a fracture-mechanics based SKK criterion proposed by Suyitno et al. has made considerable improvements in the hot tearing prediction. However, its obtained hot tearing susceptibility (HTS) evolution during solidification is also not completely consistent with real industrial production circumstances, especially when approaching the solidus temperature. In this paper, some further modifications are made based on the SKK criterion to emphasise the important effect of solid bridging/grain coalescence on hot tear propagation. It is proved that the HTS evolution in freezing range predicted by the modified hot tearing criterion is in good agreement with casting practice.

In the present study, X-ray computer tomography is used to determine damage evolution in extrusion weld seams loaded close to failure for two comparable AlSiMg alloys with distinctively different grain structures. In the dispersoid free alloy, recrystallisation occurs upon extrusion and the weld seam has no effect on mechanical properties. No preferential void formation in the weld seam region was detected. In the non-recrystallising dispersoid rich alloy, the weld seam leads to a reduced ductility. There are no signs of internal void formation, and damage initiation was found to be close to the surface. It is argued that for the non-recrystallising alloy, the effect of the weld seam on the ductility is related to a sharp texture gradient across the weld seam.

Effects of upward or downward forced flow in the centre of the sump of an Al alloy billet on structure and macrosegregation were studied. The billet was cast at a pilot vertical direct chill caster and had dimensions of 195 mm diameter and 1400 mm length. Introduction of forced flow in the liquid part of the sump resulted in some changes in the structure and dramatic changes in macrosegregation. Upward flow coarsened the structure and increased negative centreline segregation. Downward flow made the structure finer and more uniform across the billet, and suppressed the macrosegregation. Strong downward flow produced positive macrosegregation in the billet. The experimental results are analysed using computer simulations of heat flow and solute transport, and discussed in terms of macrosegregation mechanisms.

The objective of the present work was to study the effects of forced flow on solute transport and associated phase composition in an Al – Mg – Si alloy. Possible solidification paths were calculated using Thermo-Calc from macrosegregation patterns obtained for the experimental alloy solidified in the melt velocity range between 0.14 m s-1and 0.44 m s-1. The comparison of the calculated and experimentally observed phase compositions showed good agreement. In order to reveal the microstructural features and to identify the phases present, metallographic analysis in combination with electron-probe analysis and X-ray measurements was extensively used.

Abstract: One of the major problems during direct chill (DC) casting is hot tearing. These tears initiate during solidification of the alloy and may run through the entire ingot. To study the hot tearing mechanism, tensile tests were carried out in semisolid state and at low strain rates, and crack propagation was studied in situ by scanning electron microscopy (SEM). These experimentally induced cracks were compared with hot tears developed in an AA5182 ingot during a casting trial in an industrial research facility. Similarities in the microstructure of the tensile test specimens and the hot tears indicate that hot tearing can be simulated by performing tensile tests at semisolid temperatures. The experimental data were compared with existing hot tearing models and it was concluded that the latter are restricted to relatively high liquid fractions because they do not take into account the existence of solid bridges in the crack.

Recent thermomechanical modeling to calculate the stress field in industrially direct-chill (DC) cast-aluminum slabs has been successful, but lack of material data limits the accuracy of these calculations. Therefore, the constitutive behavior of three aluminum alloys (AA1050, AA3104, and AA5182) was determined in the as-cast condition using tensile tests at low strain rates and from room temperature to solidus temperature. The parameters of two constitutive equations, the extended Ludwik equation and a combination of the Sellars-Tegart equation with a hardening law, were determined. In order to study the effect of recovery, the constitutive behavior after prestraining at higher temperatures was also investigated. To evaluate the quantified constitutive equations, tensile tests were performed simulating the deformation and cooling history experienced by the material during casting. It is concluded that both constitutive equations perform well, but the combined hardening-Sellars-Tegart (HST) equation has temperature-independent parameters, which makes it easier to implement in a DC casting model. Further, the deformation history of the ingot should be taken into account for accurate stress calculations.

Linear contraction in the solidification range is a principal characteristic for the understanding and modelling of hot tearing, macro-segregation and contraction behaviour.The developed technique allowed us to measure the linear contraction (shrinkage) in the solidification range of binary and commercial wrought aluminium alloys. It was shown that the linear shrinkage depends on the composition, as-cast structure, and applied force. The solidification behaviour and the volume solidification shrinkage were simulated. The linear contraction starts at temperatures close to the solidus of an alloy when the volume fraction of solid ranges from 85 to 95%.

Abstract: Mold filling was studied for vertical, thin, plate-shaped cavities, with the liquid entering via some vertical ingate system connected to the bottom. In this study, sand molds were filled up with molten aluminum and molten cast iron, while water was used to fill up perspex models. The front walls of the sand molds were replaced by glass plates to allow observations of temperature distributions and free-surface behavior of the melt during filling. Computational fluid dynamics (CFD) simulations of the filling process were carried out to study free-surface behavior, velocity patterns, and temperature distributions. Digital particle-image velocimetry (DPIV) was used to validate the computer simulations for water. Generally, visual observations of the molten liquids, CFD simulations, and DPIV results are in good agreement. Combining the three techniques has resulted in a better understanding as to how a plate-shaped cavity is filled up. The behavior of the free surface is different for water and the molten metals. An analysis of surface waves is presented that explains these differences. Current ideas as to the role of the Weber number must be rejected. Rather, instabilities are associated with low values of the Ohnesorge number, with surface tension providing the driving force for surface instabilities and with viscosity as the damping force.

Fluidity and castability studies were carried out for F3S.20S (Al-20SiCp) Duralcan composites with additional amounts of silicon. The fluidity measurements were carried out using the gravity casting method and the castability studies using Low Pressure Casting (LPC) as well as gravity pouring methods. It was observed that with both the gravity and LPC methods, the fluidity and castability were found to increase, with the increasing silicon levels (up to 20 wt%) present in the composites. The castability of the composite was increased with each increase in temperature for the range of temperatures studied (1003,1013 and 1033 K).

Aluminium-silicon alloys with compositions in the range 0 at% to 33.9 at % Si were rapidly quenched from the melt at cooling rates between 106 and 107 K sec-1 using the melt-spinning technique. The resulting ribbons were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction methods. Metastable solid solubilities of silicon in aluminium were determined from lattice parameter and DSC data. The values found were strongly dependent on specimen thickness and a maximum of about 5 at % Si was reached for an alloy composition of 15 at % Sl (maximal equilibrium solid solubility of silicon in aluminium is 1.58 at % Si). Discrepancies between published values of metastable silicon solid solubities were related to the interpretation of the lattice parameter data. Alloy composition was shown to determine the lattice parameter of the silicon-rich phase. The crystallite sizes and the lattice distortions in the aluminium-rich and silicon-rich phases were determined by X-ray diffraction line profile analysis. From the aluminiumrich phase only strain broadening was observed whereas the silicon-rich phase gave rise to both size and strain broadening. The origin of the lattice strains was discussed. Changes in solidification behaviour are reflected in the structure parameters measured.

Hypo- and hyper-eutectic AlSi alloys were rapidly quenched from the melt using the melt-spinning technique with two spinning velocities. Structural differences between the wheel (chill) and upper sides of the melt-spun ribbons were investigated by optical and scanning electron microscopy and X-ray diffraction methods (texture- and size-strain analyses). The Al-rich phase of the hypo-eutectic alloys was textured. The textures observed from both sides of the ribbons were different; in neither case was it of fibre type. For the larger spinning velocity applied, the structural imperfection of the wheel side was larger than that of the upper side for both the Al-rich and the Si-rich phases.

Melt spinning is successfully used for the preparation of a rapidly solidified SiC particle reinforced AlSi7Mg0.3 alloy. The composites are prepared by introducing SiC particles in a semi-solid matrix slurry (SiC volume fractions up to 0.15, particle size 10 or 20 µm). Duralcan material (SiC volume fraction 0.20, particle size 12 µm) was also used. After stirring in the semi-solid state the composites are heated above the liquidus temperature and subsequently melt-spun. Featureless, columnar and dendritic zones can be identified in the ribbons. A finer dendritic structure is found around the SiC particles. The SiC particles tend to segregate to the air side of the ribbons and the segregation effect is influenced by particle size and volume fraction. As interface velocities are higher than the critical velocities predicted by models on interface pushing, it is concluded that fluid flow in the melt puddle is responsible for the segregation effect.

A mathematical model to calculate the hot-cracking tendencies during DC casting is described. The model combines a new simplified thermal model for DC casting with the concept of solidification shrinkage not eliminated by afterfeeding (Feurer) and the concept of the critical time interval during solidification (Clyne and Davies). This model is available to calculate hot-cracking tendencies as a result of the effects of composition, casting rate, and ingot diameter. In spite of the absence of sufficient physical and solidification data, it is shown that there is a satisfactory degree of correlation between prediction and practical casting knowledge.

A series of batch-type stirring experiments have been performed to investigate the morphological changes in the growth of the primary solid phase in Al-6Cu(Al-6 wt% Cu), as a function of the cooling rate and the rotational speed of the stirrer. The experiments show, that the cell-spacings of primary particles in stircast microstructures increase as a result of stirring, when compared to secondary dendrite arm spacings in the unstirred alloy. This result can be explained regarding heat transport during solidification. It is suggested that the solid-liquid interface of floating crystals in bulk metal liquids is cellular.

A criterion of stircastability is formulated, which relates the specific process limits for semicontinuous stircasting (rheocasting or thixotropic slurry casting) to alloy properties, and which can be used to decide which alloy compositions can be continuously stircast in a given apparatus. It provides a better understanding of the casting rate behaviour of alloys in continuous stircasting. For a specific type of apparatus, described in this paper, it predicts an upper and a lower limit of the nominal alloy composition, for continuous stircasting of thixotropic slurries. In practice, this means that eutectic or close to eutectic alloys cannot be stircast in this apparatus, and that stircastability decreases toward smaller freezing ranges.

The conditions of solidification in a stirred bulk liquid are investigated to explain the non-dendritic microstructure of stircast alloys. A model of stirred solidification is presented, which allows a comparison of the solidification behaviour of metal alloys and organic analogues. This shows that nucleation and growth of solid in e bulk liquid is facilitated under the influence of stirring, provided the Prandtl-number is greater than unity. It is shown further that the solute gradient ahead of the solid-liquid interface of floating particles in a bulk liquid is reduced by the fluid flow. Combined with the thermal properties, and in analogy with the constitutional supercooling criterion, A is shown that solid growth in metals is likely to be cellular in an early stage of the solidification, In contrast, in stirred organic analogues, the solidification is dendritic in the early stage.

Mathematical and physical modelling have been used interactively to predict the behaviour of two liquid metals mixing in the nosetip of a Roll-casting machine. Attention has been given to the transient development of density driven backflows due to incomplete mixing and forward transport of fluid. The results are compared with temperature and composition data obtained from metallurgical trials.

In situ TiB2particle reinforced Al alloys are produced by reactive synthesis from elemental and prealloyed powders. The influence of 2124 alloying elements on the reactive synthesis is evaluated with a comparison of elemental AI, elemental AI-Cu mixture, and 2124 Al prealloyed powders as matrix materials. Experimental investigations by differential scanning calorimetry and dilatometry showed that the presence of Cu leads to an increase in the reaction rate during the formation of intermediate reaction products in comparison with the elemental Al matrix. X-ray diffraction of the reaction products showed a more complete conversion of the intermediate Al3Ti as a result of Cu addition. The Cu has no influence on the TiB2particle size, but the TiB2morphology changed from pure hexagonal to a more rounded morphology.

Cracking is one of the most critical defects that may occur during aluminum direct-chill (DC) casting. There are two types of cracking typical of DC casting: hot tearing and cold cracking. To study and predict such defects, currently we are using a process simulator, ALSIM. ALSIM is able to provide semi-quantitative predictions of hot tearing and cold cracking susceptibility. In this work, we performed benchmark tests using predictions of both types of cracks and experimental results of DC casting trials. The trials series resulted in billets with hot tearing as well as cold cracking. The model was also used to study the influence of several casting variables such as casting speed and inlet geometry with respect to the cracking susceptibility in the ingots. In this work, we found that the sump geometry was changed by the feeding scheme, which played an important role in hot tear occurrence. Moreover, increasing the casting speed also increased the hot tear and cold crack susceptibility. In addition, from the result of simulation, we also observed a phenomenon that supported the hypotheses of connection between hot tearing and cold cracking.

Controlling macrosegregation is one of the major challenges in direct-chill (DC) casting of aluminium alloys. In this paper, the effect of the inlet geometry (which influences the melt distribution) on macrosegregation during the DC casting of 7050 alloy billets was studied experimentally and by using 2D computer modelling. The ALSIM model was used to determine the temperature and flow patterns during DC casting. The results from the computer simulations show that the sump profiles and flow patterns in the billet are strongly influenced by the melt flow distribution determined by the inlet geometry. These observations were correlated to the actual macrosegregation patterns found in the as-cast billets produced by having two different inlet geometries. The macrosegregation analysis presented here may assist in determining the critical parameters to consider for improving the casting of 7XXX aluminium alloys.

Ultrasonic melt treatment (UST) is known to induce grain refinement in aluminum alloys, especially when transition metals like Zr and Ti are present. The refinement of primary intermetallics (e.g. Al3Zr, Al3(Zr,Ti)) caused by UST may influence the subsequent solidification process when intermetallics act as nucleation sites. In this paper, an Al-Ti-Zr alloy is used to analyse the effect of different ultrasonic intensities on the formation of these primary intermetallics. The possible nucleation behaviour of Al3Zr particles during UST is also discussed and an edge to edge matching model is used to make a preliminary analysis of lattice mismatch between aluminum oxide and Al3Zr phase. The experimental results show that when UST is applied, the Al3(Zr,Ti) particles might nucleate on aluminum oxides and remain fine during solidification.

Aluminum alloy 7050 is of interest for aerospace industries due to its superior mechanical properties. However, its inherent solidification behaviour may augment the accumulation of residual stresses due to uneven cooling conditions upon direct-chill (DC) casting. This can increase the propensity of cold cracking (CC), which is a potentially catastrophic phenomenon in casting ingots. To predict the outcome of the aluminum casting process, ALSIM software is utilised. This software has the capability to predict CC susceptibility during the casting process. However, at the moment, ALSIM lacks the information regarding material constitutive behaviour in the sub-solidus temperature range, which is considered important for studying CC phenomenon. At the moment, ALSIM only has a partial constitutive database for AA7050 and misses data, especially in the vicinity of non-equilibrium solidus (NES) point. The present work presents measurements of tensile constitutive parameters in the temperature range between 400 degC and NES, which is for this alloy defined as 465 degC. The mechanical behaviour is tested in a Gleeble 3800 thermo-mechanical simulator. Constitutive parameters such as stress-strain curves, strain-rate sensitivity and ductility of the alloy have been measured at different test temperatures. With these constitutive data, we expect to improve the accuracy of ALSIM simulations in terms of CC prediction, and gain more insight into the evolution of mechanical properties of AA7050 in the temperature nearby the NES.

Low Carbon (LC) steel is not expected to be sensitive to hot tearing and/or cracking while microalloyed steels are known for their high cracking sensitivity during continuous casting. Experience of the Direct Sheet Plant caster at Tata Steel in Ijmuiden (the Netherlands), seems to contradict this statement. It is observed that a LC steel grade has a high risk of cracking alias hot tearing, while a High Strength Low Alloyed (HSLA) steel has a very low cracking occurrence. Another HSLA steel grade, with a similar composition but less N and V is however very sensitive to hot tearing. An extreme crack results in a breakout. A previous statistical analysis of the breakout occurrence reveals a one and a half times higher possibility of a breakout for the HSLA grade compared to the LC grade. HSLA with extra N, V shows a four times smaller possibility of breakout than LC. This study assigns the unexpected effect of the chemical composition on the hot tearing sensitivity to the role of some alloying elements such as V and N as structure refiners.

Solidification structure of a High Strength Low Alloy (HSLA) steel, in terms of dendrite arm spacing distribution across the shell thickness, is studied in a breakout shell from a thin-slab caster at Tata Steel in IJmuiden. Columnar dendrites were found to be the predominant morphology throughout the shell with size variations across the shell thickness. Primary Dendrite Arm Spacing (PDAS) increases by increasing the distance from meniscus or slab surface. Subsequently, a model is proposed to describe the variation of the PDAS with the shell thickness (the distance from slab surface) under solidifiction conditions experienced in the primary cooling zone of thin-slab casting. The proposed relationship related the PDAS to the shell thickness and, hence, can be used as a tool for predicting solidifcation structure and optimizing the thin-slab casting of low alloy steels.