Melt conditioning by advanced shear technology (MCAST) for refining solidification microstructures

MCAST (melt conditioning by advanced shear technology) is a novel processing technology developed recently by BCAST at Brunel University for conditioning liquid metal prior to solidification processing. The MCAST process uses a twin screw mechanism to impose a high shear rate and a high intensity of turbulence to the liquid metal, so that the conditioned liquid metal has uniform temperature, uniform chemical composition and well-dispersed and completely wetted oxide particles with a fine size and a narrow size distribution. The microstructural refinement is achieved through an enhanced heterogeneous nucleation rate and an increased nuclei survival rate during the subsequent solidification processing. In this paper we present the MCAST process and its applications for microstructural refinement in both shape casting and continuous casting of light alloys

쌍롤박판주조법 중에 합금원소 첨가 및 공정 변수에 따른 알루미늄 합금 판재 내부 편석 거동 분석

학위논문 (석사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2019. 2. 신광선.Since Al has a low density and high specific strength along with sound corrosion resistance, the Al alloys have a great potential of application as structural materials in various industries. However, the high fabrication cost compared to conventional steel materials is the main obstacle for wider application. Twin roll casting (TRC) is a continuous casting method which allows the fabrication of strips directly from melt which helps to decrease the fabrication cost and increase the productivity. Despite all the advantages, the segregation forms during the TRC process is apparently harmful. Internal macro-segregation which is usually located in the center layer of the TRC strips cannot be completely removed by thermomechanical process after the initial solidification. Therefore, studies on the formation mechanism of different types of segregation and how to reduce segregation are still necessary. FEM simulation program DEFORM was used to predicted the solidification behavior during the twin-roll casting of Al alloys. The temperature and liquid fraction distribution of Al alloys with various Mg concentration from 0.5% to 5% were investigated based on the thermodynamic properties calculated using JMatPro program. The effect of casting speed on the temperature and liquid fraction distribution was discussed. Specific points along the thickness direction were selected to track the local strain and liquid fraction during the TRC process. According to the calculated results, freeze range increased and heat transfer coefficient decreased with the increasing Mg concentration which both indicated that more segregation can be expected from high Mg alloys. Furthermore, higher casting speed caused the less contacting time between the roll and melt which delayed the cooling process and resulted in more segregation during the TRC process. Besides, the center layer of the cross-section was calculated to have lower cooling rate and higher deformation while the side layer was confirmed with a higher cooling rate. As one of the most widely used alloying elements for Al alloys, the effect of Mg element on the macro-segregation behavior of twin-roll cast Al alloys was investigated. From experimental results, larger segregation area was found in samples with high Mg and higher casting speed which matched the simulation results. The types and shapes of internal macro-segregation changed with different casting speed. A vortex-shaped segregation was observed in low-speed samples while a banded structure with channel segregation was found in high-speed samples.알루미늄 합금은 낮은 밀도, 높은 강도 및 우수한 부식 저항성을 지니고 있어 구조재료로써 산업에 적용되고 있다. 하지만 철강 보다 비싼 생간 원가로 인하여 적용 범위가 국한된다. Twin-roll casting (TRC) 는 금속의 용탕 상태에서 직접 판재로 주조 가능한 연속 주조 공정이다. TRC 공정을 이용하여 원가 절감 및 생산 효율 향상 등 장점을 가지고 있지만 주조 과정에서 형성되는 중심부 편석 및 주조 결함은 최종 판재 품질에 매우 나쁜 영향을 준다. 특히 중심부 편석은 후속 열처리 및 소성가공 공정을 거쳐도 제거되지 않아 최종 품질에 안 좋은 영향을 끼쳐 편석 형태 및 편석 형성 원인을 규명하는 것은 매우 중요하다고 판단한다. FEM 전산모사 프로그램인 Deform을 이용하여 TRC 공정 과정에서 합금의 응고 거동을 분석하였으며 마그네슘 함량 0.5%에서 5.0%까지 변화 시켜 TRC 공정 중에서 온도 분포 및 액상 분율을 산출하였다. TRC 공정 중에서 주조속도의 영향을 조사하였으며 판재 두께 방향 각 지점의 액상 분율 및 변형 거동을 분석하였다. 전산모사 결과를 기반으로 TRC 공정을 이용하여 마그네슘 첨가량 및 공정 변수가 다른 판재를 제조하였다. 실험 결과에 의하면 편석 형태가 공정 변수에 따라 변화하고 낮은 주조 속도에서 Vortex-shape 형태를 가지고 있는 구조가 발견되었으며 이러한 구조는 후속 공정을 거치면 Super fine grain 구조가 발달되어 우수한 기계적 성질을 가진다. 반면에 높은 주조 속도로 제조한 판재에서 Banded 주조 및 Channel 편석이 복합적으로 존재하여 비교적으로 비균일한 Grain 구조 및 많은 편석 면적이 발견되었다. 또한, 상용 AA6016 합금을 대상으로 최적 공정 변수를 조사하였다. 13개 공정 변수를 조사한 결과를 기반으로 양산에 도움이 되는 Channel segregation free diagram을 도식화하였다.1. Introduction 1 1.1 Application of Al Strips 1 1.2 Al Twin-roll Casting 2 1.3 Surface and Internal Macro-segregation in Twin-roll Cast Aluminum Strips 4 1.4 Current Research Objectives 6 Bibliography 8 2. Experimental method 11 2.1 Twin-roll Caster in Current Study 11 2.2 Microstructure Observation 11 3. Thermo-dynamic and Finite Element Method simulation of the solidification behavior of TRC aluminum alloys 14 3.1 Introduction 14 3.2 Experimental 17 3.3 Results and discussions 20 3.3.1 Solidification behavior of Al-xMg binary alloys during the TRC process 20 3.3.2 FEM simulation of the solidification behavior during the TRC process with different Mg additions 26 3.3.3 FEM simulation of the solidification behavior during the TRC process of Al-4Mg alloy with different casting speeds 29 3.4 Conclusion 30 Bibliography 35 4. Effect of alloying elements and casting speed on internal macro-segregation of Al-Mg binary alloys during the TRC process 36 4.1 Introduction 36 4.2 Experimental 37 4.3 Results and discussions 41 4.3.1 Effect of alloying elements on the internal macro-segregation of TRC Al-xMg strips 41 4.3.2 Effect of casting speed on the internal macro-segregation of TRC Al-4Mg strips 42 4.3.3 Discussion of the formation mechanism of different segregation types and the effect of segregation behavior on final properties in current study 57 4.4 Conclusion 61 Bibliography 62 5. Effect of casting parameters on internal macro-segregation of commercial AA6016 alloy 64 5.1 Introduction 64 5.2 Experimental 65 5.3 Results and discussions 67 5.4 Conclusion 72 Bibliography 73 6. Final conclusion 75Maste

Heat flow control and segregation in directional solidification: Development of an experimental and theoretical basis for Bridgman-type growth experiments in a microgravity environment

Within the framework of the proposed research, emphasis was placed on application of magnetic fields to semiconductor growth systems. It was found that magnetic fields up to 3 kGauss do not affect the growth behavior nor the macro-segregation behavior in the system Ge(Ga). Applied fields are found to significantlty alter the radial dopant distribution, which is attributed to alterations in the spatial orientation of convective cells. Increasing the magnetic field to 30 kGauss is found to have a fundamental effect on dopant segregation. Emphasis is also placed on the potential of KC-135 flights for preliminary studies on the effects of reduced gravity environments on the wetting behavior of semiconductor systems in growth configuration. The limited number of experiments conducted does not allow any conclusions on the merits of KC-135 flights for semiconductor processing research

Magnetic field-assisted solidification of W319 Al alloy qualified by high-speed synchrotron tomography

Magnetic fields have been widely used to control solidification processes. Here, high-speed synchrotron X-ray tomography was used to study the effect of magnetic fields on solidification. We investigated vertically upward directional solidification of an Al-Si-Cu based W319 alloy without and with a transverse magnetic field of 0.5 T while the sample was rotating. The results revealed the strong effect of a magnetic field on both the primary α-Al phase and secondary β-Al5FeSi intermetallic compounds (IMCs). Without the magnetic field, coarse primary α-Al dendrites were observed with a large macro-segregation zone. When a magnetic field is imposed, much finer dendrites with smaller primary arm spacing were obtained, while macro-segregation was almost eliminated. Segregated solutes were pushed out of the fine dendrites and piled up slightly above the solid/liquid interface, leading to a gradient distribution of the secondary β-IMCs. This work demonstrates that rotating the sample under a transversal magnetic field is a simple yet effective method to homogenise the temperature and composition distributions, which can be used to control the primary phase and the distribution of iron-rich intermetallics during solidification

Effect of Gravity on the Macro-Segregation of Larger Steel Ingots

Upon the problem involved in the study of solidification and segregation of larger steel ingots numerous papers have hitherto been published. Nevertheless their nature is still vague from both the scientific and practical points of view. Previous studies have been devoted to the measurement of specific physical values of molten steel, the detailed observation of the sections of solidified ingots, simulat-ion experiments using different materials, experiments on the formation and change of non-metallic inclusions, etc. Isn't there any room for re-examination in these methods of study? Even if these items could be clarified in details, it would not give rise to a satisfactory solut-ion for the phenomenon of ''differential freezing" and to the reduction of segregation and other accompanied defects. We feel much necessity of thorough studies on the solidi-fication and segregation of large steel ingots, before we rush into the epoch of large scale vacuum casting of steel ingots

1D Cahn-Hilliard dynamics : coarsening and interrupted coarsening

Many systems exhibit a phase where the order parameter is spatially modulated. These patterns can be the result of a frustration caused by the competition between interaction forces with opposite effects. In all models with local interactions, these ordered phases disappear in the strong segregation regime (low temperature). It is expected however that these phases should persist in the case of long range interactions, which can't be correctly described by a Ginzburg-Landau type model with only a finite number of spatial derivatives of the order parameter. An alternative approach is to study the dynamics of the phase transition or pattern formation. While, in the usual process of Ostwald ripening, succession of doubling of the domain size leads to a total segregation, or macro-segregation, C. Misbah and P. Politi have shown that long-range interactions could cause an interruption of this coalescence process, stabilizing a pattern which then remains in a micro-structured state or super-crystal. We show that this is the case for a modified Cahn-Hilliard dynamics due to Oono which includes a non local term and which is particularly well suited to describe systems with a modulated phase

Dendrite growth direction measurements : understanding the solute advancement in continuous casting of steel

Maintaining competitiveness in steel manufacturing requires improving process efficiency and production volume whilst enhancing product quality and performance. This is particularly challenging for producing value-added advanced steel grades such as advanced high strength steels and electrical steels. These grades due to higher weight percentage of alloying elements cause difficulties in various stages of upstream and downstream processing, and this includes continuous casting, wherein high solute levels are critical towards macro-segregation. Interface growth direction in systems with more than one component is dictated by the solute profile ahead of the moving solidification front. Understanding the profile of growth direction with casting process parameters during the progress of casting will provide an important perspective towards reducing the macro-segregation in the cast product. In the present study, two steel slab samples from conventional slab caster under the influence of electromagnetic brake (EMBR) at Tata Steel in IJmuiden (The Netherlands) have been investigated for dendrite deflection measurements. The samples showed a transition zone where a change in the deflection behavior occurs. Also, the magnitude of the deflection angle decreases away from the slab surface. Correlating these experimental data with modeled fluid flow profile will help in improving the understanding of the dynamic nature of the solute advancement so that the casting parameters can be optimized to improve product quality

Twin roll casting and melt conditioned twin-roll casting of magnesium alloys

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

An Investigation of Manganese Macro segregation in Cast Dual Phase Steels using Micro X-ray Fluorescence Spectroscopy

Advanced High Strength Steels with a high manganese content offer an exceptional balance of strength and ductility. However, the occurrence of segregation in these steels, particularly at the centerline, leads to quality control issues in continuous casting. Segregation, which is a non-uniform distribution of manganese in the as-cast structure, originates during solidification. Moreover, segregation of manganese during the continuous casting process can lead to the formation of detrimental martensite bands in subsequent manufacturing operations. Traditionally, macro segregation, which occurs over large distances, has been measured using etching techniques that provide qualitative insights. As the attention on high Mn Advanced High Strength steels is increasing, it is imperative to develop more effective and quantitative methods of measurements. From literature, it is known that segregation is largely influenced by casting parameters such as casting speed and superheat. These parameters have a significant effect on the development and size of equiaxed and columnar zones in the as-cast structure. This research investigates the effects of casting parameters such as the casting speed and superheat, nominal manganese concentration, and spatial orientation on the macro segregation present in as-cast Dual Phase 600 steels. A new analysis technique, using micro X-ray fluorescence, was used to quantitatively measure manganese macro segregation in industrially cast steels and create metrics to assess the macro segregation in the as-cast structure. This technique was validated against a current quantitative method. To assess centerline segregation, four different metrics, using two distinct methods, were developed in this thesis. Each metric measures segregation in different ways, and ranges from calculating arithmetic means to determining segregation spatial sizes. The developed metrics shed light on the spatial behavior of manganese segregation as a function of the selected casting conditions and nominal manganese compositions