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Non-Dendritic Solidification Mechanisms of Light Alloys

#2851


A Profound Study of Non-Dendritic Solidification Mechanisms at Ultrasonic Cavitation Effect upon the Melt for Obtaining Ultimate Refined Structure in Light Alloy Ingots

Tech Area / Field

  • MAT-ALL/High Performance Metals and Alloys/Materials
  • CHE-THE/Physical and Theoretical Chemistry/Chemistry
  • MAN-MAT/Engineering Materials/Manufacturing Technology

Status
3 Approved without Funding

Registration date
01.08.2003

Leading Institute
Moscow State University / Department of Chemistry, Russia, Moscow

Supporting institutes

  • All-Russian Scientific Research Institute of Non-Organic Materials named after A. Bochvar, Russia, Moscow\nAll-Russian Institute of Light Alloys, Russia, Moscow

Collaborators

  • Eindhoven University of Technology, The Netherlands, Eindhoven\nUniversity of Idacho / Institute for Materials and Advanced Processes, USA, ID, Moscow\nCorus Research Development & Technology, The Netherlands, IJmuiden

Project summary

The main objective of the Project is in a profound experimental and theoretical investigation of mechanisms, peculiarities and regularities of non-dendritic structure formation in aluminum and magnesium alloy ingots when the melt solidification occurs under ultrasonic treatment (UST). The results of the study will create the scientifically grounded basis for subsequent development of industry applicable technologies for casting light alloy ingots with an ultimate refined non-dendritic structure and enhanced mechanical and technological specifications providing improved properties to the following wrought semi-products. In this respect the project objectives are referred to applied researches in the field of manufacturing technologies and high performance metals and alloys. Besides the high applied value the work will contribute to obtaining of a new fundamental knowledge in the field of physical and chemical mechanisms of non-equilibrium processes taking place at non-dendritic melt solidification under the UST external action. The project also has a great commercial potential, as far as its main issues may be used for creation of an industrially applicable technology which will be highly beneficial for light alloy metallurgy manufacturing aluminum and magnesium alloys with substantially enhanced service and technological properties for application in aircraft construction, automotive industry, shipbuilding, construction engineering and many other industries.

An ordinary dendritic grain structure consisting of grains with developed brunches formed during normal solidification of light alloy castings and ingots gives rise to significant deterioration of plastic properties and cracking of both large ingots of high strength aluminum and magnesium alloys and worked semi-products manufactured from them because the structure and properties of wrought products strongly depend on those of initial billets irrespective of the deformation method (closed die-forging, press forming or rolling). Refining of the cast grain structure enhances the properties of both the ingots and following worked semi-products. E.g., the ductility of sheets obtained from high strength aluminum alloy billets with non-dendritic structure consisting of globular grains without dendritic brunches can be several times higher compared to the materials manufactured from ordinary ones. The concentration of defects in ingots and worked products may be reduced about 10 times due to the cast structure refining. In addition to this advantages, the light alloy billets with non-dendritic structure have the unique property of a deformation thixotropy in a semi-solid state. The billets in the thixotropic state display an ability of a superplastic deformation. This feature opens principally new opportunities for fabrication of complex shape products with improved characteristics at lower power inputs. With regard to the above reasons, obtaining of ingots with ultimate refined non-dendritic structure free from defects is extremely important for manufacturing of high quality products and semis.

The main condition for forming of a globular non-dendritic grain structure consists in creation of an excess amount of nucleating centers in the melt before the solidification front. Any means would be appropriate for this and several practically applicable ways providing extraordinarily strong increase of the nucleating centers amount are known today. First, it is an addition of powerful grain refining modifiers, e.g. Sc + Zr to aluminum alloy composition. However, this method has a serious disadvantage because it is very sensitive to the alloy composition and solidification conditions. Besides, the addition of Sc strongly rises the product cost. The non-dendritic structure can be also formed at homogeneous melt solidification with high cooling rates (higher than 103 °C/s). The ultra high cooling rate creates melt overcooling before the solidification front and additional solidification centers appear from the matrix melt. As far as light alloy melts are lesser disposed to deep overcooling than, e.g. nickel alloys, the casting techniques with ultra high cooling rates are lesser interesting for us than traditional ones. An alternative approach to enhancement of the crystallizing nucleus concentration and refining grain structure of castings lies in application of external dynamic actions upon the melt during its solidification. Melt agitation or vibrating techniques based on heterogeneous crystallization mechanisms are sometimes used for this purpose at continuous casting with cooling rates up to 100 °C/s. A cavitation ultrasonic treatment (UST) of melts deserves special consideration among the dynamical methods increasing amount of solidification centers for refining grain structure of light alloy castings.

An acoustic cavitation created by sufficiently intensive ultrasound causes wetting of multiple uncontrolled impurities naturally available in real melts and involves them into solidification process as additional potential nucleating centers ensuring the cast structure refinement. Research works on application of the melts UST for refining the grain structure of light alloy ingots have been started more than thirty years ago and are being continued by Professor G.I.Eskin (Scientific manager of the suggested project) at VILS. The group of scientists including professor G.Eskin have filed the scientific discovery (the priority in the USSR of January 16, 1978; certificate No.271 issued in 1983) regarding the new mechanism of solidification of metallic materials under ultrasound action upon the melts. The research works have displayed that non-dendritic structure of ingots obtained due to UST of the melt enhances technological plasticity under casting and subsequent hot deformation. The refined structure of the ingots also improves the properties of the worked products fabricated from them. Great advantage of the UST method lies in the fact, that it can be easily incorporated into traditional casting technologies, does not require expensive modifiers and does not create additional environmental pollution compared to traditional casting processes. Despite the fact that large defect free pilot ingots (up to 1200 mm in diameter and 10 ton in weight) with non-dendritic structure have been successfully cast from commercial aluminum alloys with aid of the melt UST in combination with continuous casting, the technology still remains the laboratory one. The main reason of this lies in the lack of knowledge of the mechanisms responsible for the observed effects and solidification process itself.

There is a number of questions that need to be answered before an industrial application of this technique. These include:


1. What critical amount of nucleuses is required for the non-dendritic structure formation.
2. What influence the ultrasound intensity will have on the refinement efficiency.
3. Is it possible to increase the refining effect of ultrasound by applying it to the melt by an optimally designed transducer and its optimal positioning in the casting installation?
4. What influence has the melt flow rate and residence time of the metal in the zone of ultrasound on the efficiency of refinement.
5. How alloy dependant is the enhancement effect.
6. What grain refining additions (master alloy) could be used in conjunction with ultrasound as an enhancing refining agent.
7. How the properties of the worked products depend on the initial cast structure refined by the melt UST.

The answer of these and some other important questions needs an exhaustive investigation of the melt solidification process under ultrasound external action. The study is to be carried out by modern experimental methods of physical metallurgy and crystallography with use of the theory of physical chemistry of nonequilibrium processes and mathematical modelling of casting and deformation of ingots. An integrated study of the sort demands involving of scientists skilled in physics, chemistry and mathematics in addition to experts in casting and physical metallurgy of light alloys. Just such a team of scientists which meets the above mentioned demands including great experience in light alloy casting with UST application for grain refinement has been assembled from three different institutions by the managing group of the suggested project for solving the problem. The professional competence of the project team in the specific fields is confirmed by multiple publications on the topics, small part of which is enclosed as an example to the project proposal.

Experimental studies will be performed by casting and structure investigation of the ingots cast at the nucleating centers amount varied artificially by different means. The structure and morphology of obtained ingots and following worked semi-products will be investigated experimentally by various precision methods including quantitative optical microscopy, transmission and scanning electron microscopy, electron microprobe analysis, diffraction of electrons, X-rays and neutrons, which will provide quantitative structural information in both macroscopic and microscopic scales. Theoretical studies will be concentrated on the development of a physico-mathematical model of the solidification process taking into account its main controlled variables. The model will be based on the achievements of the theory of non-equilibrium processes and will consider early solidification stages until the appearance of morphologic instabilities causing the dendritic structure formation. Corresponding computer simulation program developed on the basis of this model will permit one to simulate the casting conditions including those with the melt UST and calculate the nucleation site growth conditions favorable for forming a globular non-dendritic structure as well as the expected parameters of the structure. The correspondence between cast billet structure and the structure and properties of the deformed worked semi-products obtained from them will be also studied by both experimentally and with aid of computer simulation of the non-dendritic structure evolution during plastic deformation of the billets. The correlation dependency of the worked product properties on the initial cast billet specifications will give one an opportunity to determine optimal parameters of the as-cast non-dendritic structure providing the most enhanced properties of the following worked semi-products. The complex of experimentally obtained results in combination with the physico-mathematical models developed within the project will permit us to create a reliable parametric description of the physical and chemical mechanisms operating at non-dendritic solidification of light alloys with the melt UST application and suggest a fundamentally new approach to casting of aluminum and magnesium alloy ingots with a predicted non-dendritic structure for manufacturing of worked semi-products with improved technologic and service properties by means of cold or hot rolling, hot or isothermic pressing and superplastic forming.

After completion of the project, its results may be used as a basis in developments of advanced industry-applicable technologies for continuous casting of aluminum and magnesium alloys with the melt UST application to produce ingots/castings having ultimate refined non-dendritic grain structure. Such a technology will practically eliminate cracking of large ingots at casting and provide them higher uniformity of longitudinal and transverse properties and their homogeneity over the ingot cross-section. The non-dendritic ingots are very attractive for manufacture of worked semi-products by means of both cold and hot plastic deformation methods including helical rolling and semi-solid (superplastic) deep deformation because the products will have increased and uniform strength in transverse and longitudinal directions and enhanced ductility. The main scientific issues of the project will be summarized in a number of scientific papers and reports and collective monograph “Non-dendritic solidification of light alloys” in order to make the results accessible to the audience interested in the efficient advanced casting technology development.

The proposed Project meets ISTC goals and objectives since it: provides weapons scientists and engineers, possessing knowledge and skills related to nuclear weapons and missiles, opportunities to redirect their talents to researches having great potential for peaceful branches of industry, supports basic research related to peaceful purposes, reduction of environmental pollution and energy consumption due to improvement of environmental safety of casting technologies by reduction or elimination of special modifiers which are widely used for obtaining non-dendritic light alloy ingots and because of the decrease of waste and rejected materials.

The foreign collaborators are prepared to take an active part in the Project through joint discussion of projects plans and evaluation of the results, share knowledge and experience in physical chemistry of Al-alloys and relevant intermetallic compounds, participation in some joint research works, collective scientific publications and probable patents with the project participants.


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