Twin-roll Casting Research Articles

The influence of surface on direction of diffusion in Al-Fe clad material

Aluminium-steel clad composite was manufactured by twin-roll casting. An intermetallic layer of Al 5 Fe 2 and Al 13 Fe 4 formed at the interface upon annealing above 500 °C. During in-situ annealing in transmission electron microscope, the layer grew towards the steel side of the interface in tongue-like protrusions. A study of furnace-annealed samples revealed, that the bulk growth of the interface phase proceeds towards the aluminium side. The growth towards steel is a surface effect that takes place simultaneously with the bulk growth towards aluminium. At the beginning of the intermetallic layer formation diffusion of Fe into aluminium prevails, afterwards Al atoms diffuse throught the newly formed intermetallic layer towards steel and the whole interface shifts towards aluminium. The kinetics of growth of the intermetallic layer follows parabolic law in both cases, indicating that the growth is governed by diffusion. • In Al-steel clad material intermetallic layer grows towards Al in bulk material. • During in-situ heating in TEM layer grows towards steel. • Steel-facing layer is a surface phenomenon. • Growth of the intermetallic layer follows parabolic law.

Improving mechanical properties of a cast Al–Mg alloy with high Mg content by rapid solidification

Cast Al–Mg alloys with high Mg contents (>10 wt%) have received particular attention due to much lower density than those of other cast Al alloys. However, high amounts of coarse dendritic Al–Mg intermetallic phases substantially deteriorate both strength and ductility. This work aims to reduce amount and size of Al–Mg intermetallic phases by rapid solidification (RS) and to improve both strength and ductility in a cast Al-15 wt%Mg alloy. This paper reports a study on Al-15 wt%Mg alloy processed by a RS technique, namely twin-roll casting (TRC). The results of optical microscopy, electron backscatter diffraction and scanning electron microscopy analysis show refined Al grains and refined granular Al3Mg2 particles, and the results of X-ray diffraction analysis reveal the increased Mg solid solubility and correspondingly the decreased amount of Al3Mg2 phase, in the TRC Al-15 wt% alloy, as compared to the conventionally mold cast (CMC) counterpart. Consequently, the tensile testing results show 0.2% offset yield strength of 243 MPa and ultimate tensile strength of 355 MPa, accompanied by uniform elongation of 3.6%, in the TRC Al-15 wt% alloy, relative to premature fracture in the CMC counterpart. The microstructural features and the mechanisms underlying improved strength and ductility in the TRC Al-15 wt%Mg are analyzed and discussed.

An Overview on the Process Development and the Formation of Non-Dendritic Microstructure in Semi-Solid Processing of Metallic Materials

Semi-solid metal (SSM) processing has been an attractive method for manufacturing near-net-shape components with high integrity due to its distinct advantages over conventional forming technologies. SSM processing employs a mixture of solid phase and liquid metal slurries and/or non-dendritic feedstocks as starting materials for shaping. Since the original development from 1970s, a number of SSM processes have been developed for shaping components using the unique rheological and/or thixotropic properties of metal alloys in the semi-solid state, in which the globular solid particles of primary phase are dispersed into a liquid matrix. In this paper, the progress of the development of shaping technologies and the formation of non-dendritic microstructure in association with the scientific understanding of microstructural evolution of non-dendritic phase are reviewed, in which the emphasis includes the new development in rheomoulding, rheo-mixing, rheo/thixo-extrusion and semi-solid twin roll casting, on the top of traditional rheocasting, thixoforming and thixomoulding. The advanced microstructural control technologies and processing methods for different alloys are also compared. The mechanisms to form non-dendritic microstructures are summarised from the traditional understanding of mechanical shear/bending and dendrite multiplication to the spheroidal growth of primary phase under intensively forced convection. In particular, the formation of spheroidal multiple phases in eutectic alloys is summarised and discussed. The concluding remarks focus on the current challenges and developing trends of semi-solid processing.

Achieving high strength-ductility synergy through high-density coherent precipitation in twin-roll cast Al–Zn–Mg–Cu strips

Achieving dense coherent precipitates in 7xxx aluminum alloys during artificial aging to attain high strength-ductility synergy remains a challenging task as they are unstable and tend to dissolve or transform to the semi-coherent phase. Here, we successfully obtained high-density (1.32 × 1024 m−3) and fine-sized (∼4.1 nm) coherent precipitates in the twin-roll cast Al-6.3Zn-3.3Mg-1.8Cu (wt.%) alloys through pre-strain and subsequent artificial aging, i.e. strain aging. Therefore, the peak-aged sample exhibited a satisfactory tensile strength of ∼600 MPa and extraordinary fracture elongation of ∼15%. The inconspicuous interface between coherent precipitates and the Al matrix reduces the tendency of interfacial stress concentration, which ensures high room temperature ductility. The strengthening mechanism was calculated by the dislocation shearing model, indicating that coherent precipitates contribute ∼80% of the total yield strength. This work could provide a short-process and low-budget method for preparing high-performance 7xxx series alloy strips.

Temperature of Aluminum Alloy Strip During Casting Using a Vertical Type High Speed Twin Roll Caster

The temperature of a strip cast using a vertical type high-speed twin-roll caster was measured by inserting a k-type thermocouple into the strip. The effects of the roll speed and roll load on the cooling rate and temperature at the roll gap were investigated using two aluminum alloys: A383 and Al–4.8%Mg. The thickness of the thermocouple was 0.1 mm, and the sampling time was 0.01 s. The cooling rate increased and the temperature at the roll bite decreased with decreasing roll speed and increasing roll load. The positions of the liquidus and solidus lines were measured, and the reduction of the thickness of the strip was inferred from the roll gap at the solidus line.

Casting Process at Roll Bite in Strip Cast Using Vertical-Type High-Speed Twin-Roll Caster

The solidification process in strip casting using a vertical-type high-speed twin-roll caster was clarified by inserting a 0.1 mm diameter K-type thermocouple into a strip during casting. The roll speeds of the copper rolls were 10 m/min and 30 m/min, and the roll loads were 5 kN and 20 kN. Al-Si alloys with a Si content of 2%, 6%, and 12% were used to investigate the influence of latent heat and viscosity in the semisolid state. The strip temperature increased after exiting the rolls. This indicated that the inside of the strip had not completely solidified at the roll bite, and secondary cooling was necessary to rapidly solidify the inside of the strip. The distance from the roll bite to the solidification starting position ranged from 30 mm to 63 mm. The cooling rate and degree of supercooling increased as the Si content decreased. The cooling rate ranged from 400 °C/s to 4000 °C/s. The cooling rate and degree of supercooling decreased as the viscosity of the semisolid increased.

Study on surface wettability of nickel coating prepared by electrodeposition combined with chemical etching

Nickel coatings with different surface characteristics were prepared on copper substrate by electrodeposition. Subsequently, the as-coating surface was etched with a mixed solution of FeCl3, HCl and H2O2 at different times, to obtain different surface morphologies and special microstructures. The surface morphologies, element composition, roughness and liquid contact angle were characterized by scanning electron microscope, energy dispersive X-Ray spectroscopy, 3D digital microscope and contact angle measurement. The van Oss-Chaudhury-Good method was used to calculate the solid surface free energy and discuss the variation of surface wettability. The results show that etching time has a significant effect on surface morphology, roughness and wettability. After long-time physical aging, the multiple polygonal-layered architectures and larger-sized layered microstructure can maintain high surface free energy and keep the liquid metal in a good hydrophilic state, which provides a reference for optimizing the surface morphologies of the casting roll. This will improve the heat transfer efficiency and promote heterogeneous nucleation during twin-roll continuous casting.

Homogenization of twin-roll cast AA8079 aluminum alloy studied by in-situ TEM

AA8079 is a commonly used stock material for manufacturing thin packaging foils. The primary alloying elements Fe and Si can form binary and tertiary intermetallics. In-situ TEM simulating homogenization annealing process of the as-cast material was used to analyze the real-time changes of the shape, type, and distribution of these particles. They affect the mechanical properties of the final product and susceptibility of the material to the formation of pinholes and other macroscopic defects. Another set of as-cast samples were annealed in a regime simulating industrial treatment in combination with measurements of resistivity to validate the results of the in-situ experiment. The results show clear temperature intervals of recovery, matrix desaturation, and phase transformations occurring in several stages: spheroidization of the original particles above 450 °C, nucleation of new particles at 475 °C, particles coarsening above 525 °C, and an entire dissolution of the original particles above 550 °C.

Deformation twinning caused by warm rolling and secondary recrystallization in twin-roll strip casting Fe81Ga19 alloy

In the present work, the body-centered-cubic (BCC) Fe81Ga19 alloy was produced by twin-roll strip casting technology. The following warm rolling and slow heating heat treatment (from 800 ℃ to 1200 ℃, 20 ℃/h) was successfully applied to realize the secondary recrystallization of Goss grains. The magnetostrictive coefficient of the complete secondary recrystallization sheet was ~197 ppm. Deformation twins can be observed by electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) in warm-rolled sheets. An in-situ compression experiment showed that deformation twinning was related to grain orientation. The twins first formed in the grains with a higher Schmid factor and then appeared in the lower Schmid factor grains with the increase of deformation. Besides, deformation twinning can adjust the grain orientation and further activate dislocation slip. The unique Slip-Twinning-Shearing plastic deformation mechanism of BCC Fe-Ga alloy provides the possibility to overcome the intrinsic brittleness and produce sheets by rolling technology.

Effect of Σ3 grain boundaries on microstructure and properties of oriented Fe-6.5 wt%Si in twin-roll strip casting

Oriented Fe-6.5 wt%Si alloy with excellent magnetic properties was prepared by twin-roll strip casting (TRSC). We studied the abnormal Σ3 grain boundaries in the as-cast strip. The experimental results demonstrated that the Σ3 grain boundary (GB) in oriented Fe-6.5 wt%Si steel had a good thermal stability, which could refine the microstructure by inhibiting growth of structure in as-cast strip. Moreover, the Σ3 GB in Fe-6.5 wt%Si as-cast strip could improve the toughness of matrix and restrain the propagation of cracks, which was beneficial to its plastic working. The Σ3 GB was relatively stable during warm rolling and could be inherited partly after annealing. At the same time, the low migration rate of Σ3 GB in primary recrystallization structure could stabilize the matrix and help to obtain the abnormal growth of Goss grains. The finished Fe-6.5 wt%Si steel had excellent magnetic properties, B800 and P10/50 were 1.76 T and 0.59 W/kg, respectively.

Effect of erbium on microstructure and properties of AA6061 twin-roll casting sheet

In this study, the effect of adding trace Er element on the microstructure and mechanical properties of AA6061 twin-roll casting sheet was evaluated. An NF6–300 vertical twin-roll cast-rolling mill was used to prepare AA6061 twin-roll casting sheet with different Er contents, and the main phase morphology and formation mechanism of Er in the alloy were analyzed using microstructure observation, scanning electron microscopy, transmission electron microscopy, and other microstructure analysis techniques. The results show that the addition of a small amount of Er can form the primary Al3Er phase in the alloy. This phase structure becomes an excellent heterogeneous nucleation core of α-Al. This significantly affects the grain size of AA6061 TRC sheets. In addition, Er interacts with the Fe in the TRC sheet. Therefore, the iron-rich phase in the structure changes from coarse needle-like and skeletal shapes to smaller short rods and blocks, and micron-sized spheroids or lumps appear in the structure. Spheroid-like iron-rich phase contains Er. When the amount of Er addition exceeds 0.4 wt%, the number of rare-earth compounds and impurity phases in the structure increases significantly, and they mainly precipitate along the grain boundaries. This increases the strengthening effect of second phase and greatly reduces the plasticity of the material. When the amount of Er addition reaches 0.4 wt%, the comprehensive mechanical properties of TRC sheet are the best. At this time, the tensile strength, yield strength, and elongation are 160.14 MPa, 124.77 MPa, and 10.68%, respectively.

Optimization of Si Content to Inhibit Inhomogeneous Deformation in Al-Mg-Si Alloy Fabricated via Twin-Roll Casting

Herein, the effects of Si content on the microstructure evolution of Al-Mg-Si alloys during twin-roll casting (TRC) and subsequent heat treatment were characterized using scanning electron microscopy, emission electron probe micro analysis, electron backscatter diffraction, and transmission electron microscopy. The effects of insoluble Si particles, solute gradient, and recrystallization on the mechanical properties of the alloy in the T4P state were analyzed. An inhomogeneous deformation in the thickness direction of the TRC strip was observed during the pre-strain test. A premature local deformation at the element barren region (EBR) in the middle of the strip was considered to be the origin of the limited mechanical properties. By increasing the Si from 0.7 wt% to 1.1 wt%, the content and uniformity of the solute in the EBR can be effectively improved. The stronger work-hardening ability weakens the inhomogeneous deformation. Si addition significantly increased the number of insoluble Si particles during the heat treatment. The structure with a hard shell and soft core in the TRC strip significantly reduced the negative effect of insoluble Si particles on the mechanical properties. The tensile strength and uniform elongation of the strip increased from 159.44 MPa and 18.36% to 209.96 MPa and 29.791%, respectively.

Effect of test temperature on tensile behavior of Fe-6.5wt.%Si alloy as-cast strip

The non-oriented Fe-6.5wt.%Si alloy as-cast strip is prepared by the top side-pouring twin-roll casting (TSTRC) process and the solidification structure of the as-cast strip is a uniform fine equiaxed grain structure. This paper studied the mechanical properties, fracture morphology, ordered phase morphology, and types of Fe-6.5wt.%Si alloy as-cast strips at different temperatures. The results show that the yield and tensile strength of the as-cast strip decreased gradually with the increase of tensile temperature, and the elongation after fracture increased gradually. The temperature rise prevented the formation of cleavage cracks, and the temperature rise is an effective method to improve the plasticity of Fe-6.5wt.%Si alloys. Concurrently, the ambient temperature is the main factor determining the tensile mechanical properties of Fe-6.5wt.%Si alloy at medium temperature. When the stretching temperature exceeds 500 °C, the Fe-6.5wt.%Si alloy has apparent plastic deformation ability. The Fe-6.5wt.%Si alloy exhibits excellent elongation when the stretching temperature is 700 °C. The serration fluctuations that appear on the 400 °C stretch curve should be attributed to the Portevin-Le Chatelier (PLC) effect caused by the reordering of the antiphase domains and the deformation-induced disorder. The Fe-6.5wt.%Si alloy as-cast strip is directly subjected to warm-rolling without the hot-rolling. When the warm-rolling is performed at the temperature of 700 °C, there are almost no cracks at the edge of the plate, the edge cracks are effectively controlled, and the surface quality of the warm-rolled sheet is fine. The obtained 700 °C warm-rolled sheets were cold-rolled at room temperature, and after recrystallization annealing, the sample had good comprehensive magnetic properties. Suppose the TSTRC-warm rolling-cold rolling-annealing process prepares the Fe-6.5wt.%Si alloy cold-rolled sheet. In that case, the production efficiency will be significantly improved, the process flow will be shortened, and the goal of energy-saving and emission reduction will be achieved.

Influence of vortex effect on the central segregation defects of horizontal twin-roll casting Al–Mg–Mn–Zn–Cu–Cr alloy sheet

In this paper, the formation mechanism of the vortex zone at different low cast-rolling speed was investigated elaborately with the aid of simulating flow field in cast-rolling zone. Additionally, the Influence of vortex effect on the central segregation defects of horizontal twin-roll casting (TRC) Al–Mg–Mn–Zn–Cu–Cr alloy sheet was analyzed in detail. The concrete results revealed that the area of the vortex zone shows an upward trend with decreasing cast-rolling speed, whereas the grain aspect ratio in the central area exhibits the reduction trend. The macro-segregation ratio of different elements at the vertex of the curve were also diminished when the cast-rolling speed decreases, due to the vortex effect can improve the nucleation rate and uniform the temperature field. More importantly, the presence of obvious valleys on both sides of the macro-segregation ratio curve, which fully proves the existence of vortex zones in the cast-rolling zone of the D2 and D3 cast-rolled sheets. Moreover, the values, uniformity and consistency of the hardness were significantly improved, mainly result from the vortex effect which enhances solid solubility of the alloy and improves the microstructural uniformity of sheet. While, the intensities of the shear textures and rolling texture of the cast-rolled sheet was notably enhanced with the gradual reduction of cast-rolling speed. Ultimately, the high alloying Al–Mg–Mn–Zn–Cu–Cr alloy cast-rolled sheet with excellent comprehensive properties was obtained.

Effect of low cast-rolling speeds on the microstructure and mechanical properties of twin-roll casting high Mg AA5059 alloy sheets

This study was undertaken to investigate the effect of low cast-rolling speeds (<15 mm/s) on the microstructure and the corresponding mechanical properties of twin-roll casting (TRC) high Mg AA5059 Al alloy sheets by metallographic microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), differential scanning calorimetry (DSC), and tensile testing. When low cast-rolling speeds were applied, the central segregation defects and micro-segregation defects disappeared significantly and the number of non-equilibrium eutectic phases abated remarkably. Nevertheless, the secondary dendrite arm spacing (SDAS) slightly increased and the degree of grain deformation in the core region of the cast-rolled sheet increased. The melt in the casting-rolling zone is in contact with the rolls for a longer period of time and the melt is also subjected to increased rolling force that together cause changes in the shape and depth of the sump in the twin-roll casting (TRC). Under the action of the change in the shape and depth of the sump, the temperature gradient along the thickness direction and the concentration gradient of the solute at the tip of the dendrite were significantly reduced and the solid solubility of the alloy was greatly improved. Eventually, the comprehensive mechanical properties of the high Mg AA5059 alloy sheet were enhanced.

Microstructure and Properties after Friction Stir Processing of Twin-Roll Cast Al–Mn–Cu–Be Alloy

We studied the effect of friction stir processing (FSP) on the microstructure and properties of high-speed twin-roll cast strips made of an experimental Al–Mn–Cu–Be alloy. The samples were examined using light, scanning, and transmission electron microscopy, microchemical analysis, X-ray diffraction, and indentation testing. During FSP, the rotational speed varied, while other parameters remained constant. The uniformity of the microstructure increased with the growing rotational speed. In the stir zone, several processes took place, and the most important were: recrystallisation of the matrix grains, fragmentation of the primary intermetallic particles Al15Mn3Be2 and their more uniform distribution in the stir zone, fracture, and dispersion of the eutectic icosahedral quasicrystalline phase (IQC), transformation of tiny Al15Mn3Be2 and IQC particles into the τ1-Al26Mn6Cu4 phase and precipitation of Al–Mn–Cu precipitates. In the thermomechanically affected zone, new dislocations formed as well as dispersion of the IQC eutectic phase and recrystallisation of the matrix grains. In the heat-affected zone, dissolution of θ’-Al2Cu precipitates occurred. The hardness variation was not severe between the stir and heat-affected zones.

Effect of Casting Condition on Ripple Mark and Surface Crack of Roll-Cast Al-Mg Alloy

Effect of casting conditions on ripple mark and surface crack of as-cast Al-4.7%Mg strip was investigated. A vertical type high speed twin roll caster was used. A casting speed was 30 m/min. Casting conditions were tip-shape of a back-dam plate, gap between a roll and the back dam plate and texture on the roll-surface. The texture on the roll-surface was useful to reduce the ripple mark. In bending test conducted to investigate surface crack of as-cast strip, the texture on the roll surface was effective to the reduce the crack. The position of the crack was not coin side with the position of the ripple mark. This result shows that there is no correlation between the ripple mark and the ductility or the strength of as-cast strip. Small roll load was effective to reduce the crack and degree of the ripple mark. Effect of the roll load on the ripple mark was discussed including solidification delay and seepage of Magnesium.

Twin-Roll Casting of Al-4.8%Mg Alloy with Added Fe

Fe (0.2, 0.4, 0.6, and 0.8 %) was added to roll-cast Al-4.8%Mg alloy to create a model of recycled Al-4.8%Mg and its effect was investigated. An unequal-diameter twin-roll caster was used. The roll speed was 20 m/min. An as-cast strip was cold-rolled down to 1 mm and annealed. Its mechanical properties were then evaluated. The surface condition of the as-cast strip was not influenced by the Fe content. The thickness of the as-cast strip decreased with increasing Fe content. The tensile strength and 0.2% proof stress increased with Fe content up to 0.4%Fe and then decreased. When 0.8% Fe was added to Al-4.8%Mg, the tensile strength and 0.2% proof stress became higher than those for Al-Mg. Elongation and the limiting drawing ratio gradually decreased with increasing Fe content. When 0.8% Fe was added, elongation in the lateral direction was 19.6% and the limiting drawing ratio was 1.7.

Kirkendall Effect in Twin-Roll Cast AA 3003 Aluminum Alloy

The formation of an inhomogeneous structure with central segregation occurs in commercial twin-roll cast AA 3003 aluminum alloy. The segregations form as manganese, iron, and silicon-rich channels spread in the rolling direction. Diffusion of silicon occurs during annealing at 450 °C, and the formation and growth of voids due to the Kirkendall effect occur. The evolution of pores studied by scanning electron microscopy and X-ray computed tomography shows that pores are formed near original pure silicon clusters. Their coalescence and the formation of large voids in the central zone of the strip occur at longer annealing times.

Microstructure and properties of ultra-thick Fe-based metallic glass by twin-roll strip casting versus a traditional process

In this study, an ingenious twin-roll strip casting approach is described to process large-size Fe71Ni9P12B4.9Si2.5Cu0.6 amorphous ribbons. The 150 μm thick RT ribbon (novel process) characterized by a completely amorphous structure is 5 to 7 times thicker than the conventional Fe-based RS (conventional process) ribbon. The minimum cooling rates of the RS and RT ribbon were 1.78 × 106 and 5.58 × 105 K/s. The RT ribbon's excellent thermodynamic properties led to fine grain size and a high magnetic induction of 1.63 T. However, high internal stress led to a large magnetocrystalline anisotropy. The hardness of the two ribbons was similar at ∼1160 Hv. Moreover, the significant size effect led to a slight decrease in the fracture toughness of the RT ribbon. The impressive specifications, high impurity tolerance, and good properties of the RT ribbon render twin-roll strip casting promising for industrial applications.