Melt Conditioned Twin Roll Casting Research Articles

Mechanical Deformation and Heat Treatment Processing of Twin Roll Cast and Melt Processed Al-5 wt.% Mg Alloy Strips

Abstract Twin-roll casting (TRC) is a set up applied to manufacture sheets of aluminium alloys. In spite of huge potential, extreme centerline segregation emerges during casting. This segregation limits the range of alloys suitable for commercial applications. To enhance the quality of the TRC strips, another innovation– the melt-conditioning twin-roll casting (MCTRC) – has been used. Cold rolling followed by homogenization heat treatment of the MCTRC strip results in more uniform solute distribution across the thickness of the strip, leading to strips of higher quality as compared to the conventional TRC strip.

Fe-Rich Intermetallic Formation and Mechanical Properties of Recycled AA6111 Alloy Strips Produced by Melt Conditioning Twin Roll Casting

The impact of high shear melt conditioning on the morphology and distribution of Fe intermetallic compounds was investigated in the processing of recycled AA6111 alloy by the twin roll casting process.The optical and scanning electron microscope (SEM) micrograph studies were carried out on twin roll casting (TRC) and melt conditioning twin roll casting (MC-TRC) samples. The microstructural analysis showed fine and uniformly distributed Fe-rich intermetallic phases for MC-TRC strips compared with the TRC strips. There was a significant improvement in the mechanical properties of MC-TRC strips after applying melt conditioning. The as-cast MC-TRC samples showed 18.7%, 19.2% and 9.8% increases in elongation, yield strength and ultimate tensile strength, respectively, in the longitudinal direction, while in the transverse section, the MC-TRC samples showed 32.3%, 2.2% and 3.8% increases in elongation, yield strength and ultimate tensile strength, respectively.

Towards directly formable thin gauge AZ31 Mg alloy sheet production by melt conditioned twin roll casting

A novel melt conditioned twin roll casting (MCTRC) strategy with emphasis on solidification control has been investigated for producing thin gauge Mg alloy sheets ready for component forming, with the minimal requirement of down-stream processing. The performance of 1.5 mm thick AZ31 Mg alloy sheet is reported in this study to demonstrate the effect of the solidification control strategy on the microstructure and mechanical properties of the Mg alloy sheet produced by MCTRC process. The anisotropy of in-plane tensile properties of the AZ31 sheets is evaluated in association with characteristic features of solidification, deformation behaviour and texture evolution during the solidification control MCTRC process. The improved mechanical performance is attributed to the refined and uniform microstructure, reduced centre-line segregation and minimized deformation, which results in a weak basal texture.

An investigation of the cold rolled AZ31 alloy strip produced by MC-TRC process

Melt Conditioning Twin Roll Casting (MC-TRC) of magnesium AZ31 alloy sheets were subjected to cold rolling at room temperature. It was observed that 13.8% thickness reductions were obtained without any cracks. In the case of MC-TRC sample a strong texture was observed which suggested orientation of basal plane towards the rolling direction during the casting process and hence deformation at cold condition was possible. Detailed optical microscopic investigations were carried out for homogenized cold rolled MC-TRC samples. Heat treatment of MC-TRC sample was done at 400 °C and it was observed that fine equiaxed grain structure obtained in 12.5 min. Tensile testing at 200 °C of cold rolled heat treated (HTCR) MC-TRC samples were performed and compared to the commercially available AZ31 TRC samples.

Melt Conditioning Twin Roll Casting with Thermo-Mechanical Treatment of Recycled AA6111 Alloy

A high shear melt conditioning twin roll casting process with thermo-mechanical treatment was tested to increase the tolerance to high impurity levels in recycled aluminium alloy. Applying high shear melt conditioning prior to casting of recycled AA6111 alloy exhibits reduced centre-line segregation and a fine equiaxed grain structure with improved sheet quality. Homogenisation at 540°C for 8 h with annealing at 520°C for 10 min of the melt-conditioned twin roll casting strip (MC-TRC) showed a clear grain recrystallisation, which was better than the conventional twin roll casting TRC strip. The mechanical test showed a significant improvement in the hardness after applying high shear melt conditioning with thermo-mechanical treatment.

The impact of melt conditioning on microstructure, texture and ductility of twin roll cast aluminium alloy strips

Twin roll casting (TRC) is recognised as an effective processing route for producing low cost Al sheet. However, the quality of the Al alloys sheets produced by the TRC process is limited by the formation of centre-line segregation which prevents its application in a wide range of engineering sectors. To improve the quality of the TRC strips, a new technology, melt conditioning twin roll casting (MC-TRC) has been developed from the industrial point of view. Enhanced nucleation by melt conditioning favours the advance of an equiaxed solidification front which results in reduced sump depth and a refined, uniform, defect free microstructure. The results obtained show that the differences in microstructure and in the crystallographic texture have a significant impact on ductility (22% improvement) of the MC-TRC sheets compared to TRC sheets.

Microstructure and Texture Development during Melt Conditioned Twin Roll Casting and Downstream Processing in an AZ31 Magnesium Alloy

The novel melt conditioned twin roll casting (MCTRC) process, in which the melt is conditioned by intensive shearing prior to twin roll casting, has allowed magnesium sheets to be produced with a fine and uniform microstructure and substantially reduced segregations across the sheet thickness. It is thus possible to eliminate the extensive downstream processing via repetitive hot rolling, which is required after conventional twin roll casting, and to produce sheets to the required thickness for forming. The present work was conducted to study the feasibility of producing magnesium sheets ready for stamping by the MCTRC process, focusing on the development of microstructures and textures. An AZ31 magnesium alloy was used in the investigation and MCTRC experiments were carried out to produce sheets of 6 mm and 2.5 mm in thickness respectively. After MCTRC, the 6 mm sheet was processed following the conventional procedures via homogenization, hot rolling and annealing, whereas the 2.5 sheet was only homogenized. Experimental results showed that: 1) the as-cast microstructures for both sheets were similalr in terms of grain size and distribution and their texture intensity and components were also similar, being dominated by basal components with a small fraction of primatic components; 2) downstream processing by hot rolling substantially intensified the basal textures for the 6 mm sheet; 3) the 2.5 mm sheet subjected only to homogenization after casting showed a grain structure similar to that obtained after repetitive hot rolling and annealing with substantially weakened textures. Mechanisms of texture formation and development during MCTRC and downstream processing are discussed in the paper.

Influence of intensive melt shearing on subsequent hot rolling and the mechanical properties of twin roll cast AZ31 strips

In the present investigation, strips were produced by melt conditioned twin roll casting (MC-TRC) process. The strips were homogenized at 400C for one hour followed by hot rolling (400C) process. Severe deformation was applied to deform the strip up to 40% by single and two passes. Tensile tests at elevated temperature (200C) were conducted on TRC, MC-TRC, and hot rolled MC-TRC samples. Single pass hot rolled MC-TRC sample showed excellent tensile strength and ductility compared to the other samples. Detail microstructural studies revealed that fine grain structure can be obtained by giving maximum deformation during the hot rolling process and finer the grain size better is the tensile properties at warm forming temperature.

Melt Conditioned Twin Roll Casting (MC-TRC) for Aluminium Alloys

Twin roll casting (TRC) has been demonstrated to be an effective route for producing Al sheet at a reduced cost. However, the quality of the Al alloys sheets produced by the TRC process is limited by the formation of centreline segregation, which hampers the downstream processing, degrade the mechanical performance of the final rolled products, limit TRC to dilute alloys with a narrow freezing range, and hence prevent its application in a wide range of engineering sectors. To improve the quality of the TRC strips, a new technology, melt conditioning twin roll casting (MC-TRC) has been developed. It has been demonstrated that the MC-TRC process is capable of producing Al-alloy strips with minimal centerline segregation which is attributed to enhanced nucleation by well dispersed naturally occurring oxides in liquid Al alloys. This work aims to investigate the upper composition limit allowed for the MC-TRC process using the binary Al-Mg alloys as model alloys. The results showed that melt conditioning increases the range of alloys (solute content up to 7%) that can be twin roll cast without severe segregation.

Microstructural Evaluation during Melt Conditioned Twin Roll Casting (MC-TRC) of Al-Mg Binary Alloys

Twin-roll casting (TRC) is an established route to produce sheets of aluminium alloys. Despite enormous potential, severe centreline segregation arising during casting limits the extension of alloy range suitable for commercial applications. At given casting conditions, the centreline segregation in conventional twin-roll-cast strip increases as the solute content in the alloy (and hence freezing range) increases. To improve the quality of the TRC strips, a new technology, melt conditioning twin roll casting (MC-TRC) has been developed. Enhanced nucleation by melt conditioning favours the advance of an equiaxed solidification front during solidification. It has been demonstrated that the MC-TRC process is capable of producing high quality Al-alloy strips with minimal centreline segregation. In this paper we use binary Al-Mg as a model alloy to investigate the composition limit (the maximum Mg concentration) for the MC-TRC strip without centreline segregation.

Melt Conditioned Twin Roll Casting (MC-TRC) of Thin Mg-Alloy Strips for Direct Stamping of Mg Components

In this paper we introduce a novel process for the production of thin-walled magnesium components by direct stamping of twin roll cast thin Mg strips. In this process, the melt conditioned twin roll casting (MC-TRC) process is used to produce thin Mg strips (thickness <2 mm) which have a fine equiaxed grain structure and little basal texture and, more importantly, are free from centreline segregation. Such thin Mg strips can be used for thin-walled component production by direct stamping without any rolling. A major advantage of this process is that it circumvents the low formability problem inherently associated with Mg based alloys. In this paper, AZ31 alloy is used to demonstrate this new process. For both TRC and MC-TRC strips, we will analyze the microstructures, assess the mechanical performance at elevated temperatures and conduct hot stamping in the as-cast condition without any prior rolling.

Hot Cracking Susceptibility in the TIG Joint of AZ31 Mg-Alloy Plates Produced by the TRC Process with and without Intensive Melt Shearing

AZ31 Mg-alloy plates produced by both conventional twin roll casting (TRC) and the melt-conditioned TRC (MC-TRC) processes were used to compare the hot cracking susceptibility in the joints of one bead-on-plate TIG welds. The plates cast with melt shearing were employed as the welding wire. The results showed that the MC-TRC plate has higher liquefied cracking resistance in the heat affected zone (HAZ) than that of the TRC plate. The improved liquefied cracking resistance of the MC-TRC plate can be attributed to the well dispersed and uniformly distributed eutectic regions in the MC-TRC microstructure.

Microstructural evaluation of melt conditioned twin roll cast Al–Mg alloy

Aluminium alloy strip has been successfully cast using the melt conditioned twin roll casting (MC-TRC) process. The liquid metal to be strip cast was fed into a melt conditioner, where it was intensively sheared under conditions of high shear rate and high turbulence induced by corotating twin screws. Melt conditioning provides a melt which has a uniform temperature and chemical composition and well dispersed nuclei. The conditioned melt was then fed into a twin roll caster. The MC-TRC process produced high quality Al–Mg alloy strip with fine and uniformly distributed intermetallics throughout the cross-section with minimal centreline segregation compared to conventional TRC strip. The Erichsen height value of MC-TRC sheet was superior to that of TRC sheet. The lower formability of the TRC sheet is attributed to the severe centreline segregation.

Extremely fine and uniform microstructure of magnesium AZ91D alloy sheets produced by melt conditioned twin roll casting

Magnesium AZ91D alloy strips consisting of equiaxed grains with a mean size less than 100 μm were fabricated successfully by using the melt conditioned twin roll casting (MC-TRC) process. A melt conditioning by advanced shear technology (MCAST) process for conditioning liquid metals at temperatures either above or below the alloy liquidus using a high shear twin screw mechanism was combined with the twin roll casting (TRC) process to form an innovative technology, namely, the MC-TRC process for casting Al alloy and Mg alloy strips. During the MC-TRC process, liquid melt with a specified temperature is continuously fed into the MCAST machine. By intensive shearing under the high shear rate and high intensity of turbulence, the liquid is transformed into conditioned melt with uniform temperature and composition throughout the whole volume. The conditioned melt is then fed continuously into the twin roll caster for strip production. The experimental results show that the AZ91D MC-TRC strips with different thicknesses have a fine and uniform microstructure. It consists of equiaxed grains with a mean size of 60–70 μm, and also displays extremely uniform grain size and composition throughout the entire cross-section. Investigation also shows that the MC-TRC processes are extremely effective to reduce the formation of defects, particularly the formation of the central line segregations.

Twin Roll Casting and Melt Conditioned Twin-Roll Casting of Magnesium

Recently, BCAST at Brunel University has developed a MCAST (melt conditioning by advanced shear technology) process for conditioning liquid metal at temperature either above or bellow the alloy liquidus using a high shear twin-screw mechanism. The MCAST process has now been combined with the twin roll casting (TRC) process to form an innovative technology, namely, the melt conditioned twin roll casting (MC-TRC) process for casting Al-alloy and Mg-alloy strips. During the MC-TRC process, liquid alloy with a specified temperature is continuously fed into the MCAST machine. By intensive shearing under the high shear rate and high intensity of turbulence, the liquid is transformed into conditioned melt with uniform temperature and composition throughout the whole volume. The conditioned melt is then fed continuously into the twin-roll caster for strip production. The experimental results show that the AZ91D MC-TRC strips with different thicknesses have fine and uniform microstructure. The strip consists of equiaxed grains with a mean size of 60-70μm. The strip displays extremely uniform grain size and composition throughout the whole cross-section. Investigation also shows that both TRC and MC-TRC processes with reduced deformation are effective to reduce the formation of defects, particularly the formation of the central line segregations.