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The effect of Zn addition on the hot tearing susceptibilities of non-refined Al- xZn-2Mg-2Cu ( x = 2-12 wt pct) alloys was investigated via direct crack observations and load response measurements. The obtained experimental results were compared with the predictions made using a modified Rappaz-Drezet-Gremaud (RDG) hot tearing model. Both the minimum crack width and load at the non-equilibrium solidus (NES) temperature (which served as a good indicator of hot tearing response) were observed at a Zn concentration of approximately 4 wt pct, and the formation of cracks was highly correlated with the predictions made via the modified RDG hot tearing model (although the obtained relationship critically depended on the magnitude of fraction solid at which solid coalescence was expected to occur). Furthermore, it was confirmed from the load development pattern that the addition of Zn into the matrix of Al- xZn-2Mg-2Cu alloys promoted the formation of coalesced networks, which decreased their corresponding coalescence fraction solids. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Direct-chill (DC) casting billets of an AA7050 alloy produced with different melt feeding schemes and casting speeds were examined in order to reveal the effect of these factors on the evolution of microstructure. Experimental results show that grain size is strongly influenced by the casting speed. In addition, the distribution of grain sizes across the billet diameter is mostly determined by melt feeding scheme. Grains tend to coarsen towards the center of a billet cast with the semi-horizontal melt feeding, while upon vertical melt feeding the minimum grain size was observed in the center of the billet. Computer simulations were preformed to reveal sump profiles and flow patterns during casting under different melt feeding schemes and casting speeds. The results show that solidification front and velocity distribution of the melt in the liquid and slurry zones are very different under different melt feeding scheme. The final grain structure and the grain size distribution in a DC casting billet is a result of a combination of fragmentation effects in the slurry zone and the cooling rate in the solidification range. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Extensive research has been carried out previously in ultrasonic (cavitation) melt treatment. Such treatment can reduce grain size and improve material homogeneity. However, it is still not well understood which of the mechanisms is responsible for cavitation aided grain refinement. For better understanding of the processes which take place during solidification under the influence of ultrasonic vibrations, it is very important to consider the change in the cooling conditions during such treatment and thermal effects which are generated in the cavitation zone and in the bulk of liquid. The current paper presents the results of an experimental work on temperature effects in aluminium alloys and their impact on resultant microstructures. The results are discussed based on heat flow, nucleation and fragmentation during solidification. [ABSTRACT FROM AUTHOR]

Effects of upward or downward forced flow in the centre of the sump of a 195 mm Al-alloy billet on structure and macrosegregation were studied. 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 results are discussed in terms of macrosegregation mechanisms. [ABSTRACT FROM AUTHOR]

Uniaxial compression tests were performed to understand the solid-state bonding process that occurs in porthole dies during the manufacturing of hollow aluminum extrusions. Strain, strain rate, and temperature applied during the tests were those typically encountered during aluminum extrusion. For a good bond, strain was found to be the most influential parameter, along with bonding time. The effect of temperature on bonding depended on alloy composition. Bonding stress was less influential within the range investigated. Surface stretching was introduced to the AA6060, AA6082, and AA7020 aluminum alloys as a criterion to quantify the bonding phenomena and distinguish their bondabilities. [ABSTRACT FROM AUTHOR]

Direct-chill (DC) cast billets 192 mm in diameter of an Al-Cu-Mg alloy were examined in detail with the aim to reveal the effects of grain refining (GR) and casting speed on structure, “floating” grains, and centerline macrosegregation. Experimental results show that grain size and dendrite arm spacing (DAS) tend to coarsen toward the billet center with a local refinement DAS in the center. In GR billets, grain size does not change much with the cooling rate, casting speed, and grain refiner amount. Coarse-DAS (floating) grains are observed around the billet axis regardless of GR and the amount of Ti, though their amount is significantly higher in GR billets. Macrosegregation profiles show a negligible influence of GR, while the effect of casting speed is large. The concept of solute-depleted floating grains contributing to the centerline macrosegregation is substantiated by microsegregation measurements, which show that, independent of GR, coarse dendrite branches have a depleted concentration plateau in contrast to the fine dendrite arms. [ABSTRACT FROM AUTHOR]

The solidification of an Al-4.5 pct Cu alloy in a shallow cavity under conditions of forced flow was studied both by fluid-dynamics simulations with solidification included and by experiments. The variation in bulk-flow velocity and initial superheat dramatically changes the macro- and microstructure, promoting grain refinement, an equiaxed-to-columnar transition (ECT), the formation of peculiar grain and dendrite morphologies, etc. The solidification parameters during solidification in the shallow cavity under forced-flow conditions have been determined by computer simulations and partially compared with the experimental results. The interaction between flow vortices and the progressing solidification front and its effect on structure evolution have been analyzed. Finally, quantitative correlations between microstructure, solidification, and flow parameters have been established. [ABSTRACT FROM AUTHOR]

It is well known that in hypoeutectic Al–Si alloys addition of titanium boride refines primary aluminium and addition of strontium modifies eutectic silicon. In this work their individual and simultaneous effect on both features has been investigated quantitatively. On the basis of experimental observations and quantitative analysis, it was concluded that an addition of 0·002 wt-% titanium almost halves grain size of primary aluminium in an ingot of A356, cast in a bar-shaped copper mould. Combining 0·002 wt-% titanium (as Al–3Ti–B) with 0·02 wt-% strontium resulted in a decrease in this grain refinement. However, an addition of 0·02 wt-% strontium has a slight refinement effect on primary aluminium phase. In order to quantify the effect of addition on eutectic silicon readily, as-cast specimens were heat-treated. It was observed that the number of eutectic silicon particles in Sr-modified specimens increased in a higher level compared to a Sr + Ti treated specimen. It was also found that Ti slightly influences the size of eutectic silicon particles. [ABSTRACT FROM AUTHOR]

Abstract: Melt flow plays a significant role in structure formation and redistribution of solid phase in liquid during solidification processes. The effect of melt flow along the solidification front on macro- and microstructure of an aluminium alloy is investigated. An electromagnetic pump is used as a tool to create the controlled forced flow of liquid metal. The experiments are performed on an Al–4.5% Cu alloy in a wide range of melt flow velocities and temperatures. Simulations of melt flow and solidification using “computational fluid dynamics” software are applied to clarify the structure formation and to obtain the temperature gradient along the solidification front. It is observed that the constant melt flow influences the structure formation, growth direction and grain size during solidification, i.e. the grain morphology changes from equiaxed under no-flow conditions to columnar under constant flow. The effect of melt flow on dendrite arm spacing is more complex. [Copyright &y& Elsevier]

The structure and morphology changes during first minutes of isothermal holding of a binary Al–Cu alloy in the mushy state were studied by holding and quenching experiments. Additional experiments were done in a laser scanning confocal microscope in an attempt of in situ observation of grain coarsening, especially during the first stages of melting of the eutectic phase. [Copyright &y& Elsevier]

Longitudinal weld seams are an intrinsic feature in hollow extrusions produced with porthole dies. As these joins occur along the entire extruded length, it is desirable that these weld seams have a minimal impact on the structural integrity of the extrudate. In particular, defects associated with weld seam formation should be avoided. In this research, the occurrence of defects related to material flow inside the extrusion tooling is studied. In lab-scale experiments, EN AW-6060 and EN AW-6082 aluminium alloy billets are formed into strips by means of the direct hot extrusion process. By utilising model dies with an internal obstruction similar to the supports present in porthole dies, a strip with a central longitudinal weld seam is formed. The effects of different geometries of the weld-chamber and the processing conditions on the quality of the weld seam are investigated. Characterisation is performed through mechanical testing, focusing on the ability of the weld seam area to accommodate plastic deformation, and microstructural analysis provides insight into the defects related to unsound metal flow. Through computer simulations, conditions related to weld seam formation are modelled and correlated with the experimental results. The experimental results demonstrate that metal flow controlled by the die geometry causes defects leading to inferior mechanical performance of the extrudate. It is further argued that current weld seam formation criteria utilised in finite element modelling need enhancement to incorporate these flow related defects. [ABSTRACT FROM AUTHOR]

Abstract: Intergranular brittle fracture has been mainly observed and reported in steel alloys and precipitation hardened Al-alloys where intergranular precipitates cover a major fraction of the grain boundary area. 7xxx series aluminum alloys suffer from this problem in the as-cast condition when brittle intermetallics cover almost all the interdendritic spaces and grain boundaries. Brittle intermetallics formed during the non-equilibrium solidification upon direct chill (DC) casting provide favorable sites for crack initiation and propagation at different stages of solidification and cooling. Such micro-cracks may propagate rapidly in the presence of residual thermal stresses and lead to catastrophic failure of DC-cast billets that is called “cold cracking”. In this research work fractographic investigations were performed on as-cast AA7050 specimens failed during tensile tests. In-situ tensile tests were also performed using an Environmental Scanning Electron Microscope (ESEM) to follow the propagation path of cracks at critical temperatures. They revealed that the alloy loses its ductility below 300°C and becomes extremely vulnerable to cracking and failure. Comparison of the fracture surfaces of tensile samples with real cracked billets helps to understand the mechanisms of crack formation and catastrophic failure. [Copyright &y& Elsevier]

Abstract: The processing conditions applied to deform wrought magnesium alloys greatly affect their final microstructure and mechanical properties. Defining an applicable processing window in terms of the ranges of temperatures and strain rates to achieve a homogeneous microstructure is a prerequisite for process optimization, since magnesium alloys show a peculiar deformation behaviour. By using trustworthy input data from uniaxial hot compression tests, processing maps were constructed for the alloys AZ41, AZ61 and AZ80. These maps clearly indicate the conditions leading to dynamically recrystallized homogeneous microstructures after hot deformation, as verified by analysing the microstructures after hot deformation under different conditions. It is found that a strain rate of 5s−1 and a temperature of 375°C for AZ41 and 325°C for both AZ61 and AZ80 would result in dynamically recrystallized homogeneous microstructures. [Copyright &y& Elsevier]

The effects of permanent mold cast microstructure on the growth of anodic oxide layers on three different aluminum substrates (i.e. Al99.8, AlSi10, and AlSi10Cu3, wt.%) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), and laser scanning confocal microscopy (LSCM). The anodic oxidation was performed galvanostatically in 2.25 M H2SO4, at 0 °C. The oxide layers developed a microscale topography mainly determined by the morphology of aluminum grains and cells. A low amount of insoluble impurities, uniformly distributed, would contribute to the growth of oxide layers with minimum defects and uniform thickness on the pure aluminum substrate whereas for the binary and ternary systems, a fine cell structure and a modified morphology of Si particles would be favorable. The Al–Fe and Al–Fe–Si particles were occluded in the oxide layers next to Si particles, blocking locally the oxide growth whereas Al2Cu particles were preferentially oxidized. In addition, the presence of Si particles in the layer influenced pore morphology by development of deflected pores around the particles. [Copyright &y& Elsevier]

• The asymmetrical rolled 7050 aluminium alloy plate exhibits relatively lower fatigue life at low strain amplitudes compared to symmetrical rolled one. • The surface recrystallized grains and more indissoluble particles lead to a relatively short crack initiation stage for asymmetrical rolled plate. • The transgranular and/or intergranular fracture modes of fatigue crack was closely connected with through-thickness microstructures. • The fatigue fracture mechanism was closely related and discussed basing on the microstructure differences. The asymmetrical rolling (ASR) process with shear deformation is considered as a promising technology to adjust/improve through-thickness microstructure homogeneity and integrated properties of high-strength aluminium alloy plates. But the advantages may come with caveats that are the subject of our research. In this paper, the room-temperature low cycle fatigue properties and fracture behaviour of the ASR-ed AA7050 aluminium alloy plates are compared with the symmetrical rolling (SR) one. It is shown that after either type of rolling the plates exhibit similar low-cycle fatigue lives but the SR-ed one displays a better cyclic deformation ability and slightly higher fatigue lives at high strain amplitudes. It is demonstrated that the severe surface-localized deformation contributes to the formation of slip relief on the surface and subsequently initiates micro-cracks that are propagated via transgranular and/or intergranular fracture modes along with obvious fatigue striations. Recrystallised grains with coarse grain boundary precipitates and wide precipitate-free zones near the upper/bottom layers as well as numerous and larger secondary particles in the ASR-ed plates may cause early crack initiation for a short crack initiation life. However, the SR-ed plate with more frequent subgrains near surface layers, numerous fine subgrains and less indissoluble particles could possess better crack initiation/propagation resistance and cyclic loading behaviour. Fine subgrains with higher microhardness/strength can facilitate passing of dislocations or slip bands into adjacent grains so as to delay crack propagation such as via energy-intensified transgranular fracture for extending fatigue life. Properly balancing the through-thickness strain/deformation distribution and the formation of recrystallization/indissoluble particles via implementing a feasible ASR process becomes a critical issue to achieve fracture-resistant microstructures for high-strength aluminium alloy plates. The underlying causes/mechanisms regarding the differences of microstructures and mechanical behaviour are revealed and discussed based on modelling through-thickness temperature/strain distribution and detailed microstructure characterization. [ABSTRACT FROM AUTHOR]