Evaluation of Strip Rolling Directly from the Semi-Solid State
Antonio de Pádua Lima Filho
1,a
and Márcio Iuji Yamasaki
1,b
1
UNESP-São Paulo State University – Departamento de Engenharia Mecânica; Av. Brasil Centro
56; Ilha Solteira, SP, CEP 15385-000, Brazil.
a
padua@dem.feis.unesp.br,
b
miyamasaki@aluno.feis.unesp.br
Keywords: Thixorolling, Strip casting, Roll casting, Solidification range
Abstract
The aim of this work is to study the s olidification condit ions necessary to produce good quality/low
defect metal alloy strip when thixorolling directly from the semi-solid state. To facilitate the study
lead/tin alloys were chosen for their rel
atively low operating temperat ure. The objective is to
extrapolate these findings to the higher temperature aluminium alloys. Three alloys (70%Pb-
30%Sn, 60%Pb-40%Sn, 50%Pb-50%wtSn) were used particularly to study the influence of the
solidification interval. The equipment consists of a
two roll mill arranged as an upper and lower
roller, where both rollers are driven at a controlled speed. The lower roller is fed with semi solid
alloy through a ceramic nozzle attached to the lower end of a cooling slope. Several types of nozzle
and their posi
tion at the roller were tested. This produced different solidifications and consequently
different finis hed strip. The alloys were first cast and t hen poured onto the cooling slope through a
tundish in order to create a continuous l
aminar flow of slurry and uniformity of metal strip quality.
The pouring was tested at different posit ions along the slope. The cooling slope was coated with
colloidal graphite to promote a smooth slurry flow and avoid the pr ob lem of adherence
and
premature solidification. The metallic slurry not only cools along the slope but is also initially
super-cooled to a mush by the lower roller whilst at room temperatures, thus enabling thixorolling.
It was als
o found that the nozzle position could be adjusted to enable t he upper roller to also
contribute to the solidification of the metallic slurry. However the rollers and the cooling slope
naturally heat up. Temperature distribution in these zones
was analysed by means of three
thermocouples positioned along t he cooling slope and a fourth in the base of the semi solid pool
within the nozzle. The objective being to design an optimum pouring and cooling system. The
formed strip was cooled down to room temperatu
re with a shower of water. Microstructures of the
thixorolling process were analysed. The differences in solidification conditions resulted in differing
qualities of finished strip and corresponding defect types , all of which are a
serious quality issue for
the rolled product.
Introduction
The mechanical processing of metallic alloys in the mushy state (thixoforming) only began to be
used for industrial production after 10 years of research. This proce
ssing produces pieces in just
one st ep (single forming step). The mechanical properties of thixoformed pieces (for example,
tensile strength) are better than for cast processing [1].
The thixorolling of metallic strips from the semi-solid state has
been used over the last years
because of the potential of producing rolled strips directly from the liquid phase [2,3]. This
technique uses a cooling slope onto which is poured the cast metal which cools to a semi-solid
mush that feeds t
he rolling mill. The lower roll drags the metallic mush and is then formed by the
upper roll. There are a number of issues such as: relative limitations on type of alloys that can be
used, the necessary technology needed to cool the system,
and the number of variables involved,
that must be considered in order to obtain an acceptable product [2,3].
However, metallic slurry rolling is simpler and saves energy consumption, since there is no
moulding of ingots required and no need for t
he installation of a secondary rolling mill. In
comparison, metallic strips are obtained continuously from molten metal alloys using rapid
solidification and thixorolling and no further reduction of the strip it is necessary. With this
Solid State Phenomena Vols. 116-117 (2006) pp. 433-436
online at http://www.scientific.net
© (2006) Trans Tech Publications, Switzerland
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