Home > The effects of Mg, Cu and Ca on the rolling and drawing performance of 5xxx aluminum alloy wire
THE EFFECTS OF MG, CU AND CA ON THE ROLLING AND DRAWING PERFORMANCE OF 5XXX ALUMINUM ALLOY WIRE
ABSTRACT. In
the rolling and drawing procession of 5xxx series aluminum alloy wire,
chevron and breaking are problems more frequently met. Several failure
cases were analyzed, alkali element Ca worsens continuous casting and
rolling processions, Ca rich phase solidifies on the level control part
and cause problems. Mg micro-scale segregation causes local site hardened
and form chevrons on the surface of the wire, track amounts of Cu causes
wire brittle during drawing. Mg, Ca and Cu segregation is caused by
that not uniformly cooling during continuous casting.
Key words:
Aluminum, continuous casting, rolling, failure analysis
1. Introduction
Alkali elements,
Li, Na and Ca even in very low level, for example 2-5ppm, could greatly
affect the hot rolling performance of 5xxx series aluminum alloys. Sodium
and calcium caused the segregation to grain boundaries [1,2,3].
For sodium, even at 0.6ppm it caused 200°C to 400°C plastic property
drop a lot and caused chevron which worsen the draw-ability of the product,
for example, rod [4]. The impurity elements also could cause casting
problem for continuous casting and rolling processions. Several processions,
such as fluxing, have been developed for removing the impurity elements.
In practice, it gives some way to get more pure 5xxx series alloy product.
For continuous casting and rolling re-drawable rod, there are still
some another factors which cause some problem, for example, trace element
segregation, lubrication and internal stress. This paper is based on
several cases of failure during casting, rolling and cold drawing of
5xxx series aluminum alloy rod wire to investigate the element and their
distribution effects on the casting, rolling and drawing performances.
2. Experiments
and failure analysis
2.1 Effect of Ca on the continuous casting procedures
For the continuous
casting of aluminum alloy bar section area is about 50cm2,
the even level of molten metal in the tundish is required. The precipitation
of some solid phases on the tube surface through which the molten metal
flow over often cause the problem for the level control, for example,
the continuous bar broken or chevron and other defect on the final rod
product. For the failure case of the level control, some samples were
taken from the surface precipitation stuff of the tube and tundish and
were analyzed with scanning electronic microscope (SEM).
Fig 1. The precipitation stuff EDX analysis, the element count in the bright area. Fig 2. The precipitation stuff EDX analysis in the more dark area
The analysis results show that the coatings on the surface of tundish are made of Al-Mg and some other elements, including Ca-Ti-Cr-V. The coatings on the surface of collie hat are made of Al-Mg and, including Ca-Ti-Cr-V, but the concentrations of Ca and Ti of the surface precipitation stuff on the collie hat is more than that in the coating on the surface of tundish, fig 1 and 2. Ti and Cr are the elements for 5xxx series alloy and Ca and V are impurity elements for that kind of alloy. With the high melt point for all other three elements, Ca in fact promotes the process of solidification of the coatings. Because in the molten metal, Ca concentration is only 0.0009%, the concentration is not high enough for the Ca rich phase to precipitate according to the phase diagram of Al-Ca, Al-Cr, Al-Ti and Al-V. But because the materials of the refractory wall content some CaO, CaCO3, there may be the reactions happened between Ca in molten metal and refractory wall.
Ca++ + 2e Ca
The high concentration
of Ca caused the high trend of solidification of Ca rich phases on the
wall. Because of the coating with the precipitation of Ca, Ti, Cr and
V rich phases on the surface of collie hap and tundish, the inner diameter
of the tube was reduced and the level of the molten metal in the tundish
was not even enough to get a good solidification. This usually causes
bar broken. And also, Ti and V are all the elements to be easily
frozen to the wall of tundish and collie hat and form the frozen cores
in the procession of solidification.
With low Ca
level molten 5xxx series alloy, the trend to precipitate on the wall
of the tundish and collie hat is much low than that with some little
bit high Ca level. When the Ca concentration was controlled, the problems
were then fixed.
2.2 The
root cause of the surface scale generated during cold drawing of 5xxx
series aluminum alloy wire
2.2.1 Observations
Surface scale
is one type of failures during the cold drawing of aluminum alloy wire,
several cases were analyzed on 5xxx, 6xxx and 2xxx series alloys. Most
of them were connected with the bad distribution of the hardening elements
such as Mg, Cu, even with the chilly cold continuous cast and hot rolling
rod.
3 pieces of
6.40mm dia. 5356 wire samples were analyzed for the root cause of the
scales on the wire surface generated during drawing. Samples were
cut from 3 pieces of 6.40mm wire for SEM and microscope observation.
The samples for micro-structure observation were cross sectioned in
two directions, approximately vertical each other. The microscope
observations on cross sections show that there are no inclusions and
porosity were found, except the edge area. SEM observations on
cross sections also show that no obvious micro segregation was found.
The appearance of the scales is shown in fig 3. On the surface of the
sample, many small fissures were found. The problems are that
during the drawing process, there were some surface cracks generated
and formed scales on the surface.
The analysis
on the surface shows that many small cracks initiated at the site where
Mg concentration is more than the average and the color is more dark
than the matrix in the observation in the style of back scatter electronic
image. The second hard phase particles are intermetallic which are crystal
lattice match or semi-crystal lattice match to the matrix phase.
The boundary between the second hard phases and matrix usually the week
area, during cold drawing, there was not enough plastic for the coordinate
deformation and therefore formed the fissures.
As shown in
fig 4 and 5, some cracks just located on the boundary between second
phases and matrix and some big cracks are not connected to the segregation
of second phases, but they located just between the prolonged second
phase segregation stripes. Because from the samples some of the
surface cracks are between the stripes and not along the boundary between
the second phases and matrix, that means at least there are over one
mechanism the fissures initiated with.
Compare photographs
of element distribution, the contour of Mg distribution is partially
matching the contour of crack. This is indirect proof that some of the
cracks between the prolonged Mg rich second phase strips initiated at
the boundary of Mg rich phases and matrix.
The width of
the crack in the direction approximately vertical to the longitudinal
direction of the wire is about 360 um, and the distance between the
adjoined second phase segregation stripes is about 590um.
Fig
3. Appearance of the scales on 6.40mm wire.
Fig 4. Small cracks initiated at the site where Mg is rich.
Fig 5. 100X The cracks initiated
at the site where Mg is rich and Mg content in half qualitative
analysis is about 12%, more than the nominal composition -- Mg 5%.
2.2.2 Discussion on the fissure initiation on the wire for 5xxx series aluminum alloy
The microstructure of continuous casting and rolling rod is the mixture of the dynamic re-crystallized and deformed f.c.c structure. At the end of the rolling, the temperature of the rod is about 380°C, just a little bit above the re-crystallization point. Therefore during rolling all the Mg segregation and grains would be prolonged and the dynamic re-crystallization partially happened. So, the Mg segregation phase would cause some longitudinal defect but not vertical to the direction in macro scale.
In the following
cold drawing procession, there are two kinds of texture for cold drawing
aluminum alloy wire, internal and surface textures formed, and the intermetallics
phase in 5356 aluminum alloy, the major part of them are Al3Mg2
which is f.c.c. lattice structure as well, but the lattice distance
is big than that of matrix of 5356 alloy. During the rotation of the
crystal, it is easy cause some internal stress along the bound between
intermetallics phases and matrix. As shown in fig 3,4.
There are many micro scale fissures, but there are macro-scale fissures
that generated between the prolonged Mg rich phases. The reason for
that is the prolonged Mg rich phased stopped coordinated deformation
of the surface of the wire, some inclusions and Mg rich isolated phases
is the source site of the macro scale fissures. So, if we could decrease
Mg segregation for the cast bar, we could decrease the segregation of
rod therefore to decrease the trend of surface fissures.
2.3 The
effect of residue copper on the draw-ability of 5xxx series alloy wire
Several cases of failure analysis were carried on 5052 alloy rod. The breakage were collected from the fabrication of 0.64mmX9.5mm strip, cold drawing and rolling from 5052 9.5mm rod. Some similar phenomena happened during the wire drawing 5356 alloy as well. Other than the root cause of chevron and inclusion cluster defects, there are some other factors to cause the breaking. The statistics data shown that if the alloy made with the furnace some residue copper was there, the frequency of wire breaking is high. But if the alloy made with the furnace some residue Si there, the frequency of wire breaking is low.
Copper, 0.15
to 0.40% wt, in 6061 aluminum alloy is as an aging strengthening element.
Some research results [5] show that Cu has the hardening function for
that kind of alloys even in a low concentration. For 5xxx series alloy,
there are always some residue elements left in the alloy, for example
Si, Fe and Cu. In micro scale, there were some trends to form hard phases.
The breakage analysis shown that there are some fissures on the surface
of the 5xxx series alloy wire and the breaking usually happened in the
hardening element rich site.
Fig 6. The fissures on the surface of 5052 wire. Fig 7. The surface appearance of the fracture surface.
Fig 8. Back scatter electronic photo
of the fissures on the surface of 5052 wire.
The fracture
surface of the wire is in the shear style. There are some slide bands
along the longitudinal direction. The micro cracks are always
in the site where there are some Ca, Cr and Cu island-like phases. That
Cr and Cu segregated in micro scale and formed some hard phases which
peeled off caused some fissures during cold drawing.
3. Conclusions:
For continuous
casting and rolling processions, calcium promote the precipitation of
the hardener element to the surface of the level control part if the
refractory wall contents calcium, decrease calcium level in the molten
metal could get better level control. Magnesium macro scale and micro
scale segregation could cause surface fissure and worsen the surface
texture structure which could cause some cracks and scales during the
cold drawing process for 5xxx series aluminum wire. Residue copper in
a very low level can cause the breaking during cold drawing.
[1] K.Horikawa, S.Kuramoto and M.Kanno, Acta Mater., 49, 3981-3989 (2001).
[2] Guang-Hong Lu, Masanori Kohyama and Ryoichi Yamamoto, Mat. Res.Soc. Symp. Proc., 653, Z7.17.1-Z7.17.6(2001)
[3] Zi-kui Liu, Effect of impurities on the processing of aluminum alloys, Industrial Technologies Program, US Department of Energy Washington, D.C. 20585
[4] Research results in Kannno Lab., Japan, 2003
[5] M.Murayama, K.Hono, W.F. Miao, and D.E. laughlin, Met Trans, Volume 32A, 2001,pp239-246
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