Published Article / The heat treatment cycles that are currently applied to semi-solid processed components are mostly those that are in use for traditional dendritic alloys. These heat treatments are not necessarily the optimum heat treatments for SSM processing. The T6 heat treatment of aluminium alloys is a relatively expensive process and a reduction in treatment times would be advantageous. In order to improve the T6 heat treatment cycle for rheocast alloy A356, the effect of various parameters on the quality index were investigated. These included solution treatment time, natural aging time, artificial aging temperature and artificial aging time.

<p>In the last forty years, most of researches in casting fields especially in semi-solid metal state were dedicated to find new ways to enable near net shaped casting of Al alloys to improve the product properties and decreases the product cost. The thixoforming and rheocasting processes are presented as a ways by which the microstructure of the alloys can be changed to non-dendritic microstructure leading to improve the mechanical properties by mitigating the defect associated with the dendritic microstructure. Unfortunately, these processes have proved to be capital cost prohibitive and complicated for commercial production. Further, near net shaped casting of Al wrought alloys along with the superior properties and performance of these alloys have been a challenge for conventional casting routes due to the main disadvantage of hot tearing or hot cracking during solidification, which renders the cast component ineffective. To overcome the disadvantages of thixoforming and rheocasting processes, Controlled diffusion solidification (CDS) process was innovated to enable casting aluminum alloys with a non-dendritic morphology of the primary Al phase in the resultant cast microstructure and thus alleviating the problem of hot tearing and obtaining a cost effective product with improved mechanical properties. The CDS is a simple process involving mixing of two precursor alloys of different thermal masses (temperature and solute) and subsequently cast the resultant mixture of the desired solute composition and temperature as a near net shaped cast product. The process lends itself to easy commercialization with a marginal capital cost required for set up such as the addition of an extra holding furnace. Further, the CDS process would prove itself to be unique in its ability to cast Al based wrought alloys into near net shaped components without additional processes and cost.</p><p>The CDS process has been proven to yield a cast product with a non-dendritic Al phase morphology and this dissertation presents the in-depth details and analysis of the various events occurring during the process to obtain a successful cast part. The process involves various inter-related events such as mixing, re-distribution of thermal field, redistribution of solute field, three types of nucleation events and growth of these different nuclei. Further the dissertation aims to present a study of the critical parameters such as temperatures of the two pre-cursor alloys, initial mass ratio of these alloys and the rate of mixing them on the effectiveness of the CDS process.</p> <p>The results from this study shows that mixing two precursor alloys to form the final desired alloy presents a natural environment for copious nucleation events aided by distribution of these nuclei by forced convection followed by the formation of unique cells in the resultant mixture (micro-scale) with significant thermal and solute gradients. The solidification in the CDS process is unique and different from conventional casting process in that initial growth of the nuclei takes place with the solute diffusing towards and temperature diffusing away from the solid/liquid interface which presents a favorable environment for a stable unperturbed growth of the solid/liquid interface resulting in a non-dendritic morphology of the primary AI phase.</p><p>The proposed events in the CDS process has been verified with a few Al based wrought alloys and organic alloy systems.</p> / Thesis / Doctor of Philosophy (PhD)

In several applications of casting, dendritic microstructure is not desirable as it results in poor mechanical properties. Enhancing fluid flow in the mushy zone by stirring is one of the means to suppress this dendritic growth. Strong fluid flow detaches the dendrites formed at the solid-liquid interface and carries them into the mould to form slurry. When this slurry solidifies, the microstructure is characterized by globular, non-dendritic primary phase particles, separated and enclosed by a near-eutectic lower-melting secondary phase. This property represents a great potential for further processing in semisolid forming (SSF) by various techniques such as pressure die casting and forging. Among all currently available methods, linear electromagnetic (EM) stirring is considered as one of the most suitable routes for large scale production of semisolid feed stock. One of the biggest advantages of EM stirring is that the stirring intensity and direction can be modulated externally and in a non-intrusive manner. With this viewpoint, the primary objective of the present research is to investigate rheocasting using linear electromagnetic stirring. A systematic development of a linear electromagnetic stirrer (LEMS) is the subject of the first part of the thesis. The LEMS consists of a set of six primary coils displaced in space. These coils are excited by a three-phase power supply to produce an axially travelling magnetic field. The metal to be stirred is placed in the annular space of the stirrer. The travelling field induces secondary current in the molten metal. The current and magnetic fields interact to generate a net mechanical force in the metal, commonly termed as the Lorentz force. The molten metal is stirred under the influence of this force. Two prototype stirrers, one for low melting alloys and the other for aluminium alloys are developed. The stirrers are characterized by measuring forces on low melting point alloy and on solid aluminum cylinders placed inside the annular space of the stirrer. As an outcome of these tests, a non-intrusive method of detecting stirring of liquid metal is developed. The development of a rheocasting mould for the LEMS forms the second part of the work presented in the thesis. The mould design and cooling arrangement are such that solidification in the mould is primarily unidirectional. Heat from the solidifying metal is extracted at the bottom of the mould, so that the axisymmetric EM stirring effectively shears the dendrites formed at the solid-liquid interface. The outer surface of the mould is cooled with water or air exiting from 64 jets, each of 4 mm diameter. Such an arrangement provides a high heat transfer coefficient and a wide range of cooling rate in the metal ranging from 0.01 to 10 K/s. Temperature is measured at various depths in the solidifying melt and at other key locations in the mould to assess the various heat transfer mechanisms. The results from the rheocasting experiments using the above mould and LEMS are presented in the third and final part of the thesis. Such studies are required for understanding the solidification process in presence of electromagnetic stirring and for highlighting the important issues connecting solidification, fluid flow, dendrite fragmentation and the resulting microstructure. A series of experiments are performed with A356 (Al-7Si-0.3Mg) alloy. Experiments are conducted with various combinations of operating parameters, and the resulting microstructures and cooling curves at various locations are examined. The key process parameters are stirring current, cooling rate, pouring temperature, and stirring current frequency. The parametric studies also include the case without EM stirring in which liquid aluminium is poured into the rheocast mould without powering the LEMS. It is found that stirring at high currents produces non-dendritic microstructures at all locations of the billet. For lower currents, however, dendritic microstructures are observed in regions outside the zone of active stirring. Stirring also enhances heat loss from the exposed top surface, leading to solid front advancement from the top as well. Without EM stirring, microstructures are found to be dendritic everywhere. The percentage of primary α-Al phase and its number density are found to increase with stirring intensity. With a decrease in cooling rate with air as the coolant, the average grain size of primary α-Al phase increases. Excitation frequency is found to be an important parameter, with lower frequencies generating a more uniform force field distribution, and higher frequencies enhancing induction heating. At higher frequencies, the effect of higher induction heating results in the formation of larger and coarser primary phase grains. This phenomenon has led to the development of a one-step process for rheocasting and heat treatment of billets.

Semi-Solid Metal (SSM) casting is a promising technology offering an opportunity to manufacture net-shape, complex geometry metal components in a single operation. However, the absence of foundry guidelines and limited design data for SSM casting makes it challenging to predict the performance of both process and components. The objective of this research was to develop and offer new solutions to material processing-related issues in the electronics industry. By investigating the opportunities afforded by the recently developed RheoMetalTM rheocasting process, a better understanding of the critical factors needed for an effective manufacturing process and optimised component characteristics was achieved. A study of the evolution of microstructure at different stages of the RheoMetalTM process demonstrated the influence of multistage solidification on the microstructural characteristics of the rheocast components. The microstructure of a slurry consists of the solute-lean and coarse globular α-Al particles with a uniform distribution of alloying elements, suspended in the liquid matrix. Additional solute-rich α-Al particles were identified as being a consequence of discrete nucleation events taking place after the initial slurry production. In the final components, macrosegregation was observed in the form of variations in the ratio of solute-lean coarse globular α-Al particles and solute-rich fine α-Al particles in both longitudinal and transverse directions. The relation between microstructural characteristics and material properties was established by determination of the local properties of a rheocast component. The fracture of a rheocast telecom component was strongly affected by microstructural inhomogeneity. In particular, macrosegregation in the form of liquid surface segregation bands and sub-surface pore bands strongly affected the fracture behaviour. Thermal conductivity measurements revealed that regions of the component with a high amount of solute-lean globular α-Al particles showed higher thermal conductivity. The effect of the local variation in thermal conductivity on the thermal performance of a large rheocast heatsink was evaluated by simulation. The results clearly show the importance of considering material inhomogeneity when creating a robust component design.

"Semi-solid metal (SSM) processing has emerged as a preferred manufacturing method due to the superior quality associated with semi-solid castings. In recent years, the driving force to reduce process cost has led to the development of a few rheocasting (also termed slurry-on-demand) processes. These include UBE’s New Rheocasting (NRC) process [1], Idra Prince’s Semi-Solid Rheocasting (SSR) process [2], and THT’s Sub-Liquidus Casting (SLC®) process [3]. A novel slurry-making SSM process developed at ACRC/MPI, termed the “Continuous Rheoconversion Process” (CRP), is a passive liquid mixing technique in which the nucleation and growth of the primary phase are controlled using a specially designed “reactor”. The reactor provides heat extraction, copious nucleation and forced convection during the initial stage of solidification, leading to the formation of thixotropic structures. In these studies, the critical issues/challenges to optimize the CRP for industrial applications have been addressed through validation experiments and pre-industrial trials."

Orientador: Maria Helena Robert / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-07-23T20:58:55Z (GMT). No. of bitstreams: 1 Bubenik_RubensLuiz_M.pdf: 9820006 bytes, checksum: 74e4259834492d5dafe6eb6b382dbc4f (MD5) Previous issue date: 1998 / Resumo: Este trabalho trata do projeto e construção de um equipamento capaz de alterar a morfologia da fase primária de ligas metálicas em solidificação, diferenciando-a das estruturas dendríticas produzidas na solidificação convencional. Neste processo uma força externa provoca movimentos no metal líquido com objetivo de alterar a morfologia de crescimento do sólido, resultando em estruturas pré-reofundidas. No equipamento idealizado é utilizado um campo magnético girante para fornecer a força necessária ao movimento. O trabalho envolve projeto, construção e montagem de circuitos elétricos, circuitos eletrônicos e dispositivos mecânicos, bem como testes de eficiência do equipamento na produção de estruturas pré-reofundidas da liga AA 2011. Para testes foram variados o tempo de agitação do líquido (230 e 300 s), e o modo de agitação (normal e com reversão), sendo mantidas constantes a potência (1040 W) e a taxa de resmamento ('20 GRAUS¿/min.). Os resultados obtidos atestam a eficiência do equipamento na obtenção de lingotes com macroestrutura equiaxial refinada, homogênea em todo o seu volume, e microestrutura apresentando dendritas fragmentadas, com fator de forma de 2 a 3 para todas as condições analisadas. Estes valores de fator de forma podem ser considerados excelentes para materiais a serem utilizados como matéria prima para a conformação no estado pastoso reofundido / Abstract: This work relates the design and development of an equipment capable of modifying the morphology of primary phase during solidification of metal alloys, differentiating such structures ftom those dendritic obtained in conventional solidification. In this process turbulence is imposed to the liquid metal by means of a external force with the purpose of modifying the morphology of the growing solid, resulting in pre-rheocast structures. In the idealized equipment a rotating magnetic field is used to provide the necessary stirring in the Liquid. This work involves design, building and assembling of the equipment, as well as preliminarytests to analyse the efficiency of the equipment to produce ingots of the alloy AA 2011 with pre-rheocast structures. Different stirring times (230 and 330 s) and type of movement (normal or reverted) were utilized in the tests, while power and freezing rate in the material were kept constant (1040 W and '20 DEGREES¿/min., respectively). Results showed the efficiency of the designed equipment in the production of ingots with fine, equiaxial and homogeneous macrostructure and microstructures presenting highly ftagmented dendrites, with values of morphology index varying ftom 2 to 3 for all operational conditions tested. Those values of morphology index can be considered excellent for a material to be utilized as raw material for forming in the semi-solid state or thixoforming / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

The main topic of this Master thesis is proposal of the streamline the production of the machined part of a handguard. The aim of the work is to evaluate the current machining process, to design possible innovation, to select the best way of construction of a design part. The conclusion will be devoted technical and economic evaluation in terms of time, cost and material consumption.

The car industry is constantly under pressure to be competitive in their product development which includes not only safety and increased performance, but also in their environmental impact. Volvo Cars strives to be more environmental and high performing in their product development, which of course includes manufacturing methods since these highly effect a components environmental impact and mechanical performance. To optimize these aspects of car components it is important to have good knowledge of current manufacturing processes and the possibilities and challenges that come with them and their relationship to already well-established processes. This report focuses on the relatively new RheoMetal-process (called rheocasting in this report), a process that uses a partly solidified slurry to fill the mold, and the conventional high pressure die casting (HPDC) which uses a liquid melt, and their differences in casting aluminum-alloys from both a process-and design perspective. The RheoMetal-process is also compared to a few other semi-solid processes on the market, thixocasting, GISS and SEED. This report is meant to work as a foundation for product developers at Volvo Cars to learn about rheocasting, and eventually use to make future choices for manufacturing processes for aluminum alloys. With a literature review, interviews with experts and professionals and analysis of casting simulations, information of the processes and their effect on material properties was collected and compiled. What could be concluded from these methods was that the slurry in the RheoMetal process has a globular microstructure, unlike the melt in HPDC which is dendritic. The globular microstructure in rheocasting is caused from shearing the slurry which makes it run more easily. The thicker consistency of the slurry makes the flow more laminar when filling a mold which makes for less air entrapment and defects in the material. The reduced number of defects opens the possibility of using T6 heat treatment on rheocasted components to reach improved mechanical properties, which would normally not be recommended for components made with HPDC. The globular microstructure makes the slurry flow easier which means, in combination with less defects, thinner components can be cast with similar strength as thicker components made with HPDC. This would contribute to weight reduction in a car, and in turn less fuel consumption. With rheocasting, a wider range of alloys can be used because of the ability of using non-eutectic alloys for the slurry. From research and analysis of simulations it could also be concluded that a combination of a high solid phase fraction and low gate velocity result in a more laminar flow and therefore fewer porosities in the final component.

"New, economical semi-solid metal (SSM) processes rely on forced convection during solidification to influence non-dendritic growth. The fundamental mechanisms that produce SSM microstructures in the presence of forced convection (due to fluid flow) are not fully understood. The objective of this work is to elucidate these mechanisms through the use of a new semi-solid slurry-making technique. Employing an apparatus developed at WPI, two alloy melts are mixed within a static reactor that induces convection and rapid cooling. Experiments carried out with this apparatus, named the “Continuous Rheoconversion Process” (CRP), result in globular semi-solid microstructures throughout a wide range of processing conditions. These conditions include the superheat in the melts before mixing, cooling rate of the slurry through the SSM range, and the presence or absence of inoculants in the melts. The results comprise repeatable sets of semi-solid microstructures having fine particle size and shape factors approaching unity. Even in the absence of melt inoculants, uniform distributions of α-Al particle sizes of about 60µm are attainable. Entrapped liquid is not present in the majority of the samples obtained with the CRP, and irregular particles that form in the process are of a limited distribution. Variation of slurry analysis methods indicates that these structures can be obtained consistently for both thixocasting and rheocasting applications. The design of the mixing reactor leads to turbulent fluid flow just as solidification commences. The results suggest that the following factors must be considered in identifying the mechanisms operating under the above conditions: copious nucleation of the primary phase; dispersion of nuclei throughout the bulk liquid; and inhibited remelting of nuclei due to temperature uniformity. In the CRP, these factors consistently lead to suppression of dendritic growth, significant grain refinement, and globular slurries. The exact fundamental mechanism leading to this effect is yet to be uncovered; however it is clear that temperature gradients ahead of the liquid are such that a cellular, non-dendritic morphology is the most stable growth form. Through further exploration of the process and identification of the operating mechanisms, future development of economical, continuous rheocasting methods will be facilitated."

Most aircraft component are currently being manufactured by machining, forging, welding and also assembling such parts. However, the possibilities of cutting cost from a single component has brought about a growing trend towards looking into casting as a possible option for manufacturing aircraft parts. This thesis was done at the request of Saab Avitronics. It evaluates the possibilities of one aircraft part, a chassis for an electronic unit that was first designed to be machined from a blank, to be cast. The thesis goes through the multifaceted tasks of product development. Casting process selection, cast alloy selection as well as geometry modification were some of these tasks that were performed in this thesis. It also evaluates the performances of chosen casting processes, the design of gating systems as well as various process parameters set, by simulating the casting processes. The alloy chosen was A356.0 with a T6 temper and the casting processes chosen were plaster mold casting and rheocasting. The geometry of the original chassis, which had very thin sections and undercuts which were complex to cast, was modified and made easier to cast with an acceptable slight increase of mass and size. The modification done on the geometry as well as the gating systems used had proven to be worthwhile, as the simulation of both process showed that such a part can be casted with no crucial defects foreseen. However, probable cavities might occur at the very tip of the chassis’s thin-fins – that it has for carrying away heat. Minor subsurface porosities might also be formed, which would not impair the function of the chassis. The modified chassis was made as close to as finished piece as possible, for the purpose of reducing machining costs. The cost of producing such a part by casting was also seen to be much less than machining it from blank. This could be taken as rationale for casting the chassis with thicker sections, to avoid problems that may arise in casting, and to subsequently machine these faces later, as it would still be cheaper than machining the chassis from a blank.