Aluminum Strip Casting for Thin aluminum Foil Production

The introduction of continuous casting of thin aluminum strip based on AA 1050 aluminum and AA 8011  aluminum  causes a fundamental change in the microstructure and therefore physical and chemical properties of the material when compared to conventional working of DC or semi-continuous cast slabs.

6082 aluminum strip
Aluminum Strip Casting for Thin aluminum Foil Production

One of the goals of rolling modernization program is to focus production on making converting, household and insulation foils. Foil is a high added-value product if manufactured from in-house raw material. The basic challenges that manufacturers wishes to deal with in the proposed project are the mastering new technology, use of continuous cast strip for making high-quality foil and broadening its marketing possibilities with final products of new quality.
At this stage of the new technology implementation and restructuring of the roll casting programmed it was realized that it would be absolutely essential
to develop and master new knowledge in the field of developing cast strip properties (roughness, oxide impurities and such);
to modified chemical composition of alloys for continuous casting and to unified various alloys into a few suitable for continuous casting, without affecting the properties of final products, which is extremely challenging in terms of technology;
to develop the microstructure, phase composition and formability of cast strip as well as final products (firstly 8-10 μm gauge insulating, converter and household foils); and
to continue development of new products/applications from cast strip.
The introduction of continuous casting of thin aluminum strip based on AA 1050 and AA 8011 causes a fundamental change in the microstructure and therefore physical and chemical properties of the material when compared to conventional working of DC or semi-continuous cast slabs.
During water-cooled rolling, due to the increased speed of crystallization, and simultaneous hot rolling in semi-solid state, the solid solution is strongly supersaturated and non-equilibrium intermetallic phases AlmFe and AlxFeySiy are precipitated. The composition, distribution, size, shape and fraction of the microstructural constituents have a crucial effect on the formability of these alloys and the final quality of foil.
The presence of fine-grain equilibrium oxide in the metal at the strip surface, the size and orientation solidification grains and the central segregations all affect grain size and macroporosity. Diffusion processes during crystallization and relaxation processes, i.e. the possibility of dislocation sliding during cold working, are much slower for continuous cast thin strip.
Understanding how the interrelationship between important alloying as well as trace elements (Si, Fe, Mn, Cr, Zn, Ti) in the based aluminum space lattice affects their solubility makes it possible to change and optimize the chemical composition of aluminum alloys 1050 and 8011, achieving in that way the standard and comparable quality of foil stock suitable for further downstream processing.
The project objective was to understand the processes and determine the evolution of microstructure of rapidly solidifying aluminum alloys (1050 and 8011) cast in new caster, in order to enable faster development and implementation of new alloys with the aim of expanding the range of products made from continuous cast thin strip. By knowing the effect of continuous casting process parameters, by optimizing and unifying alloys and understanding thermo mechanical processes we can expect to be able to change sufficiently the microstructure, chemical composition of the existing phases and thereby the technological properties and quality level of foil made from continuous cast strip in such a way as to get a favorable response in the market. The new knowledge gained about the material will furthermore serve as a basis for describing foil properties and their comparison with foil made from hot-rolled strip manufactured from conventional semi- continuously cast slabs.
Strip production on twin roll caster. Activities were divided into four phases, each phase corresponding to one of the casting widths (1050, 1150, 1600 and 1700 mm). Each phase covers: casting parameter definition, developing casting operator ability and autonomy to produce under stable casting conditions, improving cast strip geometry (longitudinal gauge variations over the coil length below +/-2%, longitudinal gauge variations over one roll revolution below +/-1%, cross profile between 0 and 1%, tilting between two edges limited to 1%), and improving cast strip surface quality, taking into account the feedback from the rolling mill regarding the end products (foil production). Casting parameters were defined for various widths, alloys and roll cambers.
Foil production from caster strip. For the whole foil production program (final gauge from 0,006 to 0,100 mm), some representative final thickness and widths were chosen. The suitability of the technological procedures for all foils in the thickness range from 0,006 to 0,100 mm was assessed based on the approval of the technology and process for foil production and on the approval of quality by customers for the chosen representative final thickness and width. Approval of caster strip quality and technology process for foil production was also performed for all chosen widths using alloys AA 1050, AA 1200, AA 8011 and AA 8079.
As-cast microstructure of strips was monitored in the longitudinal and the transverse cross section at the mid-with as well as at the mid- thickness of the coils AA 1200. It has been shown that selected casting parameters result in the production of a cast strip of good quality, with especially: very fine microstructure, typical of strip casting at 6 mm with adequate grain refiner addition, small centerline segregation, similar to that in the cast strip of competitors, no hard phases (that could lead to porosity problems), and no surface segregations.

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