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Materials Sciences and Applications, 2020, 11, 263-284
https://www.scirp.org/journal/msa
ISSN Online: 2153-1188
ISSN Print: 2153-117X
DOI:
10.4236/msa.2020.114018 Apr. 30, 2020 263 Materials Sciences and Applications
A Qualitative Study of the Influence of Grooved
Mold Surface Topography on the Formation of
Surface Marks on As-Cast Ingots of Aluminum
Alloy 3003
Prince N. Anyalebechi
School of Engineering, Grand Valley State University, Allendale, USA
Abstract
The effects of the wavelength and orientation of machined grooves on a mold
surface, casting speed, and melt superheat on the formation of surface marks
on as-cast ingots were studied with an immersion casting tester and copper
mold chill blocks. The mold surface topographies included a polished smooth
surface, and those with machined unidirectional parallel contoured grooves
oriented either parallel (vertical) or perpendicular (horizontal) to the casting
direction. The unidirectional grooves were 0.232 mm deep with wavelength
or spacing between 1 and 15 mm. The casting speed and melt superheat were
between 1 and 200 mm/s, and 10 and 50 K, respectively. Two primary types
of surface marks were observed on ingots cast with the copper mold with
smooth surface topography, namely the finer and closely spaced ripples (Type
I), and the widely spaced but coarser laps (Type II). The latter were more
prevalent at the higher casting speeds and melt superheats. Qualitatively,
formation of both types of surface marks on the as-cast ingots of the alumi-
num alloy 3003 appeared to be alleviated by increase in casting speed and
melt superheat, and by the use of molds with grooved surface topography. In
fact, casting with a mold surface with 1 mm spaced grooves that are perpen-
dicular to the casting direction eliminated the formation of surface marks at
casting speeds greater than 1 mm/s. It also improved the uniformity of the
ingot subsurface microstructure and eliminated the associated subsurface se-
gregation.
Keywords
Ripples, Laps, Grooved Mold Surface Topography, Casting, Aluminum Alloy,
Lap Formation
How to cite this paper:
Anyalebechi, P.N.
(20
20) A Qualitative Study of the Influence
of Grooved Mold Surface Topography on
the Formation of Surface Marks on As
-Cast
Ingots of Aluminum Alloy
3003.
Materials
Sciences
and
Applicati ons
,
11
, 263-284.
https://doi.org/10.4236/msa.2020.114018
Received:
February 29, 2020
Accepted:
April 27, 2020
Published:
April 30, 2020
Copyright
© 2020 by author(s) and
Scientific
Research Publishing Inc.
This
work is licensed under th e Creative
Commons
Attribution International
License
(CC BY 4.0).
http://creativecommons.org/licenses/by/4.0/
Open Access
P. N. Anyalebechi
DOI:
10.4236/msa.2020.114018 264 Materials Sciences and Applications
1. Introduction
Defective surface marks such as ripples and laps form on as-cast surfaces of alu-
minum and aluminum alloy ingots and on their continuously cast slabs and bars
[1]-[11]. These defects and their associated non-uniform subsurface micro-
structure make it difficult to produce surface-sensitive products such as alumi-
num foil, beverage can sheets, and aircraft and automotive body sheet from the
cast ingots without the expensive scalping operation [12]. This is despite im-
provement in ingot casting technology such as the development of electromag-
netic casting and low head composite mold casting processes. Scalping of
semi-continuously cast ingots and billets, and continuously cast bar surfaces is a
generally accepted practice in the aluminum industry [12]. It involves the re-
moval of 12.0 to 25 mm of the as-cast surface and subsurface of the cast ingots
and billets before subsequent fabrication of the ingots into different aluminum
products by forging, extrusion, and rolling. While scalping as much as 25 mm of
the surface of a large ingot may represent a very small loss relative to the total
volume of cast metal, removing the same depth of material from a small bar or
thin slab constitutes a prohibitively large fraction of scrap. Besides, scalping is an
extra process step that requires capital equipment. In fact, this extra process step
costs the aluminum industry more than ten million dollars a year and reduces
recovery.
Results of various experimental studies suggest that the formation of surface
defects on cast aluminum alloy products can be directly attributed to the
non-uniform rate of heat transfer at the mold-casting interface [13]-[20]. Similar
studies have been conducted on other types of metals and their alloys [21]-[26].
They suggest that under certain casting conditions, non-uniform heat transfer at
the early stages of the molten metal making contact with the mold surface causes
both lap and/or ripple formation on the as-cast surface. The rate and uniformity
of heat transfer in casting operations can be controlled by the manipulation of
different casting process variables such as molten metal temperature, nature and
method of mold cooling and/or insulation, mold material, and mold surface to-
pography. The use of machined grooved mold surface topography in casting op-
erations has been substantially investigated [27]-[42]. The surface topographies
that have been examined have varied from unidirectional grooves to discrete re-
cessions or cavities. The studies have yielded conflicting results and only three of
the studies [29] [30] [39] have been conducted on aluminum and its alloys. In
the three studies, the effects of process variables and mold surface topographical
parameters such as the wavelength (or pitch), depth, and orientation of the sur-
face topography were not investigated. In this study, the effects of the wave-
length and orientation of parallel contoured unidirectional grooves on a mold
surface on the formation of surface marks on as-cast ingots and attendant sub-
surface microstructure are carefully investigated.
2. Experimental Procedure
This study was conducted with a computer-controlled immersion casting tester
P. N. Anyalebechi
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10.4236/msa.2020.114018 265 Materials Sciences and Applications
[43] [44] [45] [46] and rectangular solid pure copper mold chill blocks with dif-
ferent surface topographies, namely a smooth and different unidirectional
grooved surface topographies. As described elsewhere [43] [44] [45] [46], the
immersion tester consisted of: 1) a chill block (mold) with the desired surface
topography, 2) a programmable servo motor with an electric cylinder actuator
designed to move the chill block (mold) in a controlled and repeatable manner
at speeds of up to 25 mm/s, 3) a system for measuring temperature as a function
of time at a fast rate, 4) an electric resistance furnace, and 5) a 254 mm diameter
graphite crucible coated with boron nitride for melting and holding the molten
metal. The mass of the aluminum alloy melt was 23.4 kg and its depth in the
crucible was 457.3 mm.
The copper chill blocks were 33 mm thick × 120 mm long × 100 mm wide.
The surface of the non-grooved chill block was polished whereas the contoured
groves on the grooved chill blocks were obtained by wire electric discharge ma-
chining (EDM). The grooves were unidirectional and oriented parallel or per-
pendicular to the casting direction (Figure 1). The peak-to-valley depth of the
grooves for each of the four mold surface topographies was 0.232 mm, but the
wavelengths (
i.e.
, crest-to-crest separation or spacing) were 1, 5, 10, and 15 mm.
The arithmetic average roughness for the blocks with vertical or horizontal
grooves with wavelength (spacing) of 1, 5, 10, and 15 mm was 84, 46, 32 and 15
µm, respectively. However, the arithmetic average roughness
(Ra)
of the surface
of the non-grooved mold chill block was 0.12 μm. Thermocouples were located
in the chill blocks at increasing distance (1 mm, 18 mm, and 33.5 mm) from the
polished or grooved surface in contact with the molten metal at 4, 20, 63 and 127
mm from the bottom.
The immersion shell casting testing involved the immersion of the solid cop-
per chill blocks with the desired surface topography into a large bath of molten
metal at a controlled rate and then reversing the mold to retrieve the planar solid
Figure 1. Photographs of the different grooved surface topographies of the copper mold
chill blocks (wavelength, λ, is spacing between grooves).
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shell (Figure 2). Two thin wire hangers attached to the top edges of the mold fa-
cilitated the extraction of the solidified ingot shell during emersion of the mold.
During emersion of the mold, most of the molten aluminum was drained from
the surface of the solidified shell, so the shape of the solid shell was deemed to be
a good representation of the morphology of the solidification growth front. To
maintain a unidirectional solidification, every part of the copper mold block,
except the machined surface, was covered with preheated 50 mm thick Marinite®
boards (Figure 2). This also ensured that for the duration of the immersion test,
the thermal resistance at the melt-mold interface was dominant.
Majority of the immersion tests were conducted over a casting speed range of
1 - 100 mm/s and at melt superheat 10, 25, and 50 K. However, a few immersion
tests were conducted at 0.5 and 200 mm/s. The range of casting speed investi-
gated in the study is within the range of casting speed of commercial ingot, bil-
let, and slab casting processes used in the aluminum industry [4]. At the end of
immersion, the chill block was held in the melt for approximately 2 s to allow for
sufficient shell growth, before it is removed (with the solidified shell) from the
melt at 254 mm/s. The total mold-melt contact time was 4 to 100 s, taking into
consideration the immersion and emersion speeds, and 2 s hold time.
Prior to immersion tests, the chemical compositions of solid samples of the
AA 3003 melt were determined by optical emission spectroscopy. On average
the chemical composition of the AA 3003 melt consisted of 1.20 wt% Mn, 0.57
wt% Fe, 0.24 wt% Si, 0.024 wt% Ti, 0.0006 wt% B, and the remainder was alu-
minum. Titanium and boron were deliberately added to the melt for grain re-
fining.
Characterization of the cast ingot shells involved the measurement of the dis-
tance between the surface marks and metallographic examination of the subsur-
face cast microstructure. The distance between the marks on the as-cast surface
was measured manually with a metallurgical microscope-based image analyzer.
Electron microprobe analysis was conducted on the subsurface microstructure
to determine the extent of inverse segregation (or liquation) as a function of the
Figure 2. Schematic of the immersion ingot shell casting apparatus.
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mold surface topography. The method of inverse problem of heat conduction
was used to determine the heat flux at the mold-melt interface based on the
temperature data obtained with the thermocouples located in the molten metal
and in the chill block.
3. Results
In this study, the quality of the as-cast chill surface of the shells was qualitatively
evaluated on the basis of the presence and characteristics of geometric surface
marks and defects such as ripples, wrinkles, and laps. These are periodic mark-
ings that form across the as-cast surface and perpendicular to the casting direc-
tion. They are known to form on the as-cast surfaces of ingots and continuous
cast bars and slabs [1] [2] [3] [4]. Ripple marks are fine grooves with smaller
wavelength or spacing. The wrinkles (also referred to as transverse depressions)
are concave in nature and are coarser and more widely spaced than the ripples
and laps. They are superimposed on the fine pattern of ripples. Lap marks are
smaller than wrinkles but they are coarser, deeper, and more widely spaced than
ripple marks [21] [22] [23]. Ripples, wrinkles, and laps are referred to as Types I,
II, and III marks, respectively [21] [23]. In majority of cases, the ripple marks are
prevalent through the laps and wrinkles. These terms are sometimes used inter-
changeably to describe surface marks on as-cast surfaces in the technical litera-
ture. Other terms used to describe these defects include folds, striations, bands,
and cold shuts. In this study, only two main groups of the surface marks were
observed—the finer closely spaced ripples (
i.e.
, Type I), and the coarser and
widely spaced wrinkles and laps (Types II and III), The latter will henceforth be
referred to as lap marks or Type II surface marks. It is noteworthy that, the very
small and finer ripples were also found between the coarser and more widely
spaced lap marks.
The macrophotographs of the as-cast chill surface (
i.e.
, mold-shell surface) of
solid shell castings produced in the immersion tests with are given in Figure 3
and Figure 4. They clearly show that at all of the casting speeds and melt super-
heat investigated, immersion casting with the copper mold block with smooth
surface topography produced shells with ripples and laps that are perpendicular
to the casting direction. The lap marks decreased in frequency and appeared to
be coarser and deeper with increase in casting speed (Figure 3) and melt super-
heat (Figure 4). In addition, the measured spacing between the laps appeared to
increase with increase in casting speed, especially from 25 to 50 mm/s (Figure
5). It also appeared to increase with increase in melt superheat especially at
casting speeds above 10 mm/s (Figure 6). In contrast, the spacing between the
ripples (the finer surface marks) appeared to decrease with increase in casting
speed and melt superheat. That is, the number of the ripples per unit distance
increased.
The as-cast chill surfaces of the shells produced with copper mold chill blocks
with grooves displayed the machined groove patterns on the mold chill blocks
(Figures 7-12). This is particularly the case with the shells produced with mold
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Figure 3. Effect of casting speed on the as-cast surface of ingot shells produced by im-
mersion of a copper block with machined smooth surface topography into an aluminum
alloy 3003 melt at a superheat of 25 K.
Figure 4. Effect of immersion (casting) speed and superheat on as-cast surface of ingot
shells produced by immersion of a copper chill block with machined smooth surface to-
pography into an aluminum alloy 3003 melt.
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Figure 5. Effects of casting speed and melt superheat on the spacing between surface lap
marks on the as-cast surface of AA 3003 ingot shells produced in casting immersion tests
with a copper chill mold block with a machined smooth surface topography.
Figure 6. Effects of casting speed and melt superheat on the spacing between surface lap
marks on the as-cast surface of aluminum alloy 3003 ingot shells produced in casting
immersion tests with a copper chill mold block with a smooth surface topography.
Figure 7. Effects of the wavelength of
vertical
grooves
on a copper mold surface and
casting speed on the as-cast surface quality of AA 3003 ingot shells at 25 K melt superheat.
P. N. Anyalebechi
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Figure 8. Effects of the wavelength of 0.232 mm deep
horizontal
grooves
on a copper
mold surface and casting speed on the as-cast surface quality of AA 3003 ingot shells at 25
K melt superheat.
Figure 9. Effects of melt superheat and the wavelength of 0.232 mm deep
vertical
grooves
on a copper mold surface on the as-cast surface quality of AA 3003 ingot shells cast at 50
mm/s.
Figure 10. Effects of melt superheat and the wavelength of 0.232 mm deep
grooves
on a
copper mold surface on the as-cast surface quality of AA 3003 ingot shells cast at 50
mm/s.
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Figure 11. Effect of immersion (casting) speed and superheat on as-cast surface of ingot
shells produced by immersion of a copper chill block with machined
vertical
grooves
,
0.232 mm deep and 1 mm apart, surface topography into an AA 3003 melt.
Figure 12. Effect of immersion (casting) speed and superheat on as-cast surface of ingot
shells produced by immersion of a copper chill block with machined horizontal grooves
(0.232 mm deep and 1 mm apart) surface topography into an AA 3003 melt.
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chill blocks with horizontal grooves that are perpendicular to the casting direc-
tion (Figure 8, Figure 10 and Figure 11). Most importantly, the prevalence of
ripples and laps was significantly reduced and in some cases completely elimi-
nated (Figure 11 and Figure 12). However, at 1 mm/s, the chill blocks with both
vertical and horizontal grooves produced shells with ripples and laps. In the
shells produced with the chills with vertical groove spacing between 5 and 15
mm, the number of laps decreased and the spacing between the laps increased
with increase in casting speed and melt superheat. However, the ingot shells
produced with the copper mold chill block with vertical grooves with 1 mm
spacing at casting speeds of 5 mm/s or more were free from laps. Interestingly, at
casting speeds of 5 mm/s or more, the ingot shells produced on chills with hori-
zontal grooves were free of laps. Unlike with the vertical grooves, copper mold
chill blocks with horizontal grooves, 5 mm or more apart, produced shells that
were free from laps (Figure 5). The pitch of the surface marks (
i.e.
, distance be-
tween them) on the shells solidified on the chill blocks with horizontal grooves
at 1 mm/s was practically equivalent to the spacing between the grooves.
In general, the subsurface microstructure of the shells solidified on the copper
chill with smooth surface topography was non-uniform and consisted of eutec-
tic-enriched segregation layer with signs of solid shell remelting-induced liqua-
tion (Figures 13-15). The subsurface microstructures of the shells solidified on
chills with grooved surface topography were comparatively more uniform than
the subsurface microstructure of the shells produced with the smooth mold sur-
face topography (Figure 14 and Figure 15). In general, however, there were
Figure 13. Photomicrographs showing the liquation (inverse segregation) associated with
the surface laps on ingot shells of AA 3003 produced by immersion of a copper chill block
with a machined smooth (non-grooved) surface topography into the melt at 50 mm/s and
25 K superheat (Etched in 0.5% HF for 12 s).
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Figure 14. Photomicrographs showing the effect of the wavelength of 0.232 mm deep
vertical
grooves
on a copper mold surface on the subsurface microstructure of AA 3003
shells produced by immersion of copper chill blocks at 50 mm/s and 25 K superheat
(Etched in 0.5% HF for 12 s).
Figure 15. Photomicrographs showing the effect of the wavelength of 0.232 mm deep
ho-
rizontal
grooves
on a copper mold surface on the subsurface microstructure of AA 3003
shells produced by immersion of copper chill blocks at 50 mm/s and 25 K superheat
(Etched in 0.5% HF for 12 s).
signs of surface segregation in the subsurface microstructures of shells solidified
on grooved mold chill blocks with a 15 mm wavelength. In contrast, the subsur-
face microstructures of the shells solidified on the chills with surface topography
grooves with 1 mm spacing were uniform and free of segregation (Figure 14 and
Figure 15). In fact, results of electron microprobe analysis of the subsurface mi-
crostructures presented in Figure 16 confirmed significant reduction in the se-
gregation of manganese in the shells solidified on chills with 1 mm grooved
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Figure 16. Results of electron microprobe analysis of the subsurface microstructures of
as-cast aluminum alloy 3003 shells cast at 50 mm/s and 25 K melt superheat showing the
effect of mold surface topography on the segregation of Mn.
surface topography. The orientation of the unidirectional grooves to the casting
direction (or molten metal surface) did not affect the subsurface microstructures
of the cast shells. There was no discernible difference between the subsurface
microstructures of shells solidified on vertical and horizontal grooved chills.
4. Discussion
It is apparent from the results that casting with molds with parallel contoured
unidirectional grooved surface topography can have profound effects on the
as-cast surface quality of an aluminum alloy cast product. The extent of the ef-
fects depends on the casting speed, melt superheat, and the wavelength and
orientation of the grooves to the casting direction.
The characteristics and prevalence of surface marks observed on the chill
as-cast surface of the ingot shells are similar to those of Types I, II, and possibly
III meniscus marks reported by Wray [21] from decanting tests of molten lead
solidified on a copper plate. It appears that the observed coarser and more wide-
ly spaced lap marks that formed at faster casting speed and higher melt super-
heat (Figure 3 and Figure 4) are Type II marks instead of Type III marks. Ac-
cording to Wray [21], Type III marks form at low casting speed and low degree
of melt superheat. The ripples and lap marks observed in this study are also sim-
ilar to those reported by Weirauch
et
al.
[39] [40] from immersion tests in mol-
ten pure aluminum and aluminum alloys, Ackerman
et
al.
[8], Jacobi and
Schwerdtfeger [23], and Saucedo
et
al.
[9]. The slightly downward bent corners
of the surface marks and the fact that they are perpendicular to the casting direc-
tion are consistent with the ripple and lap marks that form on commercial-size
ingots [1] [2] [3] and continuous cast products [4].
Ripple and lap marks are sometimes collectively referred to as meniscus marks
[9] [18] because they form at the meniscus or as cold shuts [13] [14] [29]. As
previously stated, for simplicity, the surface marks observed in this study will be
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collectively referred to as laps or lap marks. They form by a cyclic solidification
process that begins just after the initial contact between the molten metal and
the mold surface as a thin solid shell grows outward from the mold surface along
the liquid metal/vapor surface (meniscus) [8] [9] [17] [18] [20] [26]. The liquid
periodically overrides the solid shell and re-establishes fresh chill contact gene-
rating the lap marks. Lap marks are indicative of a meniscus instability. They are
apparently due to periodic wetting (or non-wetting), solidification of the curved
part (a convex surface) of the meniscus in contact with the mold, followed by
one or more of the following [8] [9] [17] [18] [20] [23] [26]: 1) flow of molten
metal over the partially solidified meniscus, 2) remelting of the solidified shell,
or 3) inward bending of the solid shell caused by shrinkage during and after so-
lidification allowing the melt to overflow. These events are apparently aided by
thermal distortion of the solidifying shell [21] [47] [48], oxide film on the mol-
ten metal surface that increases its surface tension [8], and/or and dynamic ef-
fects of the surface waves of the liquid [22]. The remelting or inward bending of
the solid shell allows the eutectic-enriched liquid to periodically overflow over
the partially solidified meniscus and/or bend the solid tip, thus forming the sur-
face depression characteristic of the defect [8] [9]. Formation of lap marks is ex-
acerbated by a heat imbalance at the meniscus curvature by either too much heat
being conducted away or insufficient heat supplied to the meniscus region. As a
result, the frequency (
i.e.
, number) of laps marks can be reduced by one or a
combination of the following: 1) reduction of the residence time of the melt in
the meniscus by increasing the casting rate, 2) reduction of the rate of heat ex-
traction by increasing the thermal resistance of the mold and the gap between
the mold and the solidifying shell in the meniscus region, and 3) delay of solidi-
fication by superheating the melt, preheating of the mold, insulation of the
mold, use of low thermal conductivity mold materials, and increasing the casting
speed.
4.1. Effect of Mold Surface Topography
In general, casting with the copper mold chill blocks with vertical and horizontal
grooves produced ingot shells that had significantly fewer lap marks than the
copper chill with the smooth surface topography (Figures 7-10). In fact, ingot
shells produced at casting speed faster than 1 mm/s with mold chill blocks with
horizontal grooves that are 1 mm or more apart were completely free from lap
marks (Figure 7). The observed alleviation of lap formation on the AA 3003
shells with grooved mold surface topography is consistent with that reported by
Irman [29]. However, a mold surface topography consisting of 0.1 mm deep ver-
tical grooves (parallel to the casting direction) that are 1 mm apart for narrow
freezing range alloys such as AA 3003 suggested by Irman [29] was not the most
effective. The observed amelioration of ripple and lap mark formation can be at-
tributed to three possible reasons, namely the effects of the grooves on: 1) the
rate of heat extraction in the meniscus region, 2) wetting of the mold chill sur-
face by the molten metal, and 3) wave motion of the meniscus. As described
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above, alleviation of the formation of lap marks requires the moderation of the
rate of heat extraction (and consequently rate of solidification) from the menis-
cus region. It therefore appears that the grooved copper mold chill blocks alle-
viated or eliminated lap formation on the as-cast surface of the ingot shells by
moderating the rate of heat extraction in the meniscus vicinity. That is, the
grooves reduce meniscus solidification. This is probably because mold surfaces
with grooves reportedly reduce heat flux or heat transfer coefficient or increase
thermal contact resistance at the mold-melt interface [32] [33]. This is due to the
trapped air in the grooves. On a smooth surface thermal contact resistance be-
tween the melt and the chill surface is low because little air is trapped in the sur-
face cavities. In fact, in this study, the local heat flux (calculated from the tem-
perature measurements) for the mold chill blocks with 1 mm spaced grooves is
less than that for the mold with a smooth surface topography (Figure 17). As
shown in Figure 18, they also exhibited the lowest maximum interfacial heat
(a)
(b)
Figure 17. Effect of groove wavelength on calculated interfacial heat flux during solidifi-
cation of AA 3003 on grooved copper chill blocks (5 mm/s and 25 K melt superheat). (a)
Molds with vertical grooves; (b) Molds with horizontal grooves.
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Figure 18. Effect of groove wavelength or spacing on maximum interfacial heat flux dur-
ing solidification of AA 3003 on grooved copper chill blocks (5 mm/s and 25 K melt su-
perheat).
flux during solidification of the shells. However, 5 - 15 mm spaced grooves on
the mold chill surface resulted in higher interfacial heat flux than that obtained
with a smooth mold surface topography. This is consistent with the reported ef-
fects of grooved mold surface topography on heat flux at metal-mold interface
during solidification by some investigators. For example, Murakami
et
al.
[34]
reported 10% - 30% increase in heat flux in the first 5 seconds of the solidifica-
tion of hypoperitectic steel on a water-cooled chill with 1 - 5 mm spaced vertical
grooves compared to that on a smooth copper mold. Bouchard
et
al.
[27] [28]
reported even higher increase in heat flux obtained in the early stages (<0.5 s) of
solidification of a copper alloy on a water-cooled sand blasted copper chills and
copper chills with 0.5 - 2 mm deep vertical grooves with 0.15 - 1 mm spacing.
This suggests that the observed alleviation of surface lap marks with grooved
mold surface topography is not entirely due to moderation of interfacial heat
transfer.
It is also conceivable that the grooves on the chill surface disrupt the meniscus
instability and the associated formation of the surface marks by enhancing the
uniformity of degree of wetting of the mold chill surface by the molten metal.
The replication of the main features of the grooved chill surface topographies
(especially the horizontal grooves) indicates good wetting of the crests of the
chill surface by the liquid metal (Figure 3 and Figure 4) and the non-wetting of
the grooves. The non-filling and non-wetting of the troughs of the grooves are
engendered by surface tension of the oxide-covered melt surface. Another possi-
ble reason for the alleviation of ripple and lap surface marks on the ingots with a
grooved mold surface topography is that the grooves disrupt the gravity-induced
waves on the surface of the molten metals which occur during the immersion
casting [22] [23].
Horizontal grooves with 5 mm or more spacing are more effective than vertic-
al grooves in precluding lap formation (Figure 11 and Figure 12). In fact, there
is a strong correspondence between the surface condition of the mold chill block
with horizontal grooves and that of its shells. This strongly suggests that better
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wetting conditions were present just before the AA 3003 alloy solidified against
the horizontally grooved mold surface. It is particularly noteworthy that, at the
slowest casting speed (1 mm/s) investigated, none of the grooved copper chills
could prevent the formation of laps on the as-cast surface of the shells. This im-
plies that grooved mold surfaces will be more effective in alleviating the forma-
tion of as-cast surface defects such as lap marks (or cold shuts) in continuous
casting than in ingot casting processes, with casting speed range of 50 - 230
mm/s and 0.5 - 1 mm/s, respectively [4].
4.2. Effects of Casting Speed
The observed effects of casting speed on finer and closely spaced ripple (Type I)
and the coarser and wider spaced lap (Type II) marks depend on the type of
mold surface topography. Formation of both types of surface marks on the ingot
shells solidified on the mold with a smooth topography are much more sensitive
to casting speed than the limited surface marks on the ingot shells solidified on
the mold with grooved surface topography (Figure 7 and Figure 8). For exam-
ple, it was found qualitatively that, with the exception of the ingot shells solidi-
fied on chill blocks with horizontal grooves, the spacing between the Type I sur-
face marks (
i.e.
, ripples) appeared to be independent of casting speed (Figure 3).
The effects of casting speed on the formation of surface marks, especially the
finer and closely spaced ripple (Type I) marks, have been studied quantitatively
by several investigators [8] [15] [16] [17] [20] [21] [22] [23] [24] [26] [39] [40].
The studies have involved careful quantitative profile measurement of ripple
spacing and groove depth as functions of casting speed and melt superheat. Most
of these investigators reported that increase in casting speed decreases the depth
of and spacing between surface marks [8] [15] [16] [17] [20] [21] [22] [23] [24]
[26], apparently improving the surface quality of the ingots. This is attributed to
its effect on the formation of the solid skin at the edge of the meniscus. Accord-
ing to Jacobi and Schwerstfeger [23], at faster casting speed (which is equivalent
to the rise of the meniscus) the tip of the skin may be located in the lower part or
below the curved edge of the meniscus. However, decreasing the casting speed
could cause the tip to move upwards and into the curvature, thus enhancing
melt overflow and exacerbating ripple formation. That is, the slower casting
speed allows the shell to solidify along the meniscus well away from the mold
wall. Interestingly, consistent with the results of this study, Stemple
et
al.
’s [22]
found the finer Type I marks, which they referred to as secondary ripples, to be
independent of casting speed. It is noteworthy that these fine and closely spaced
ripple (Type marks) run undisturbed through the coarser and widely spaced lap
(Type II) marks (Figure 4). This strongly suggests that the ripple marks formed
prior to the formation of the lap marks. The latter, as explained below, are said
to be engendered by thermal stresses in the solidifying shell.
However, in contrast to its effect on the Type I marks (ripples), increase in
casting speed appears to increase the spacing and depth of the more widely
spaced and coarser Type II (lap) marks (Figures 3-6). This result confirms
P. N. Anyalebechi
DOI:
10.4236/msa.2020.114018 279 Materials Sci ences and Applications
Wray’s [21] and Stemple
et
al.
’s [22] observations. Wray [21] reported that the
spacing between the more widely spaced lap marks, which he referred to as Type
II marks, increased with increase in casting speed. Wray [21] attributed the ob-
served contrasting effects of casting speed on the formation of ripples and lap
marks to the differences in the mechanisms for the formation of both types of
surface marks. According to Wray [18], the widely spaced Type II surface marks
are caused by distortion of the ingot shell engendered by the thermal stresses
developing in the shell during solidification. Type II marks apparently do not
form at the meniscus but at some distance below the meniscus line due to tem-
perature perturbations that cause uneven thickening of the shell. Thus, they are
associated with the non-uniform thickening or growth of the shell. Stemple
et
al.
[22] also found that the spacing between the more widely spaced clearly deli-
neated surface marks, which they referred to as primary surface marks, on bot-
tom-poured Sn-Pb ingots increased linearly with increase in casting speed. They
contend that surface marks (which they refer to as primary ripples) are caused
by surface waves, gravity induced meniscus motion, during filling of the mold.
On the basis of wave theory, the spacing between the surface marks can be
shown to be directly proportional to the casting speed [23].
Qualitatively, it is well known that surface ripple marks on castings can be
eliminated or reduced by eliminating the meniscus or by reducing heat extrac-
tion in the vicinity of the meniscus during the early stages of solidification [13]
[14] [49]. This can be accomplished by increasing casting speed, changing lubri-
cation practice, using higher frequencies of mold oscillation (in continuous
casting of steels), use of low thermal conductivity mold materials, high melt su-
perheating, etc. Increasing the casting speed reduces the residence time in the
meniscus in accordance with Equations (1) and (2):
rc
b
tV
=
(1)
where
tr
= residence time in the meniscus,
Vc
= the casting speed, and
b
= me-
niscus height which represents the vertical distance during which the mold-casting
heat exchange occurs in the meniscus region; it depends on the surface tension
and density of the molten alloy in accordance with the equation:
12
2
bg
γ
ρ
=
(2)
where
b
= meniscus height,
γ
= surface tension of the molten metal,
g
= accele-
ration due to gravity, and
ρ
= density of the molten metal.
It is also noteworthy that rapid changes in temperature gradient may exacer-
bate thermal distortion of the shell tip which may alter the rate of solidification
near the meniscus and ultimately the shape of the meniscus-based surface
marks. It is conceivable that there are two (or more) different mechanisms for
the formation of different types of surface marks and/or at different casting
speeds. At slow casting speeds, formation of surface marks is probably caused by
P. N. Anyalebechi
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10.4236/msa.2020.114018 280 Materials Sci ences and Applications
the overflow mechanism resulting in irregular pitch (spacing) and depth. In this
case, the casting rate is sufficiently slow that the shell has time to solidify along
the meniscus away from the mold or chill block wall. That is inward bending of
the solid shell which would allow the melt to overflow the solidified meniscus
near the mold wall and form a ripple [9] [17] [20]. At faster casting rates howev-
er, surface marks are probably formed by the folding mechanism [26].
4.3. Effect of Melt Superheat
Like the casting speed, the effects of melt superheat on ripple (Type I) and lap
(Type II) marks also depend on the type of mold surface topography. The sur-
face marks on the shells solidified on the mold with a smooth topography are
much more sensitive to melt superheat than the limited surface marks on the
shells solidified on the mold with grooved surface topography (Figure 9 and
Figure 10). Also, the melt superheat has contrasting effects on the narrowly
spaced Type I marks (ripple) and the widely spaced Type II (lap) marks. How-
ever, in general, the effects of melt superheat are not as pronounced as that of
casting speed. For example, increase in melt superheat appeared to decrease the
distance between the Type I marks, the ripples. In fact, qualitatively, at a casting
speed of 50 mm/s and 50 K melt superheat, it appears that the ripples are elimi-
nated, only the coarser and wider spaced lap marks are left (Figure 9 and Figure
10). The observed effects of melt superheat on Type I marks are consistent with
the reported effects of melt superheat on the formation of surface marks on the
as-cast surfaces of aluminum alloy ingots by several investigators [8] [9] [15]
[21] [22] [23] [24] [25]. For, example, qualitatively, Thornton [24] and Saucedo
et
al.
[9] reported that the “degree or severity of rippling” was reduced by in-
crease in melt superheat. Several investigators [8] [15] [21] [23] [25] quantita-
tively found that increase in melt superheat decreased the spacing and depth of
Type I (ripples). The effect of melt superheat on the finer Type I surface marks
(ripples) can be attributed to its effect on the rate of solidification or heat extrac-
tion at the meniscus region. Increase in melt superheat decreases the meniscus
solidification. At sufficiently high melt superheat and casting speed, which in
this study are 50 K and 50 mm/s, respectively, the fine ripples are completely
eliminated.
The effects of melt superheat on the widely spaced and coarser (Type II) lap
surface marks depend on the casting speed (Figure 6). At the fastest casting
speed, 50 mm/s, increase in melt superheat increased the measured average dis-
tance between the coarse lap (Type II) marks (Figure 6). This suggest that, like
the Type I marks, at the fastest casting speed, increase in melt superheat de-
creases the frequency and possibly severity of lap marks on ingot surfaces pro-
duced with a mold surface topography. This can be attributed to the decrease in
meniscus solidification and the overall decrease in solidification rate of the ingot
at higher melt superheats. According to Wray [21], the widely spaced and coars-
er Type II surface marks do not form at the meniscus but at some distance below
P. N. Anyalebechi
DOI:
10.4236/msa.2020.114018 281 Materials Sci ences and Applications
the meniscus line because of the distortion of the shell at the early stages of soli-
dification. The distortion of the shell is engendered by the thermally induced
mechanical stresses developing in the solidifying and thickening shell. The slow-
er rate of solidification is also expected to result in less distortion of the ingot
shells.
However, at slower casting speeds (10 and 25 mm/s), increase in melt super-
heat slightly decreased the distance between the widely spaced lap (Type II)
marks (Figure 6). These results are consistent with the observations by Stemple
et
al.
[22] and Jacobi and Schwerdtfeger [23]. Stemple
et
al.
[22] did not find any
significant effect of melt superheat on the spacing between what they referred to
as primary ripple marks. Whereas, Jacobi and Schwerdtfeger [23] found that in-
crease in melt superheat decreased the space between Type II surface marks
which they referred to as transverse depressions.
It is noteworthy, however, that solidification on grooved mold surface topo-
graphy either eliminates or significantly decreases the sensitivity of both ripple
and lap mark formation to melt superheat. As apparent in Figure 11 and Figure
12, ripples and lap marks are completely eliminated on cast shells solidified on
copper mold with 1 mm spaced vertical or horizontal molds at casting speed of
10 mm/s and above, and melt superheat of 10 K and above. The practical impli-
cations of these results are significant. They suggest that formation of ripples
and lap marks can be avoided in direct chill cast ingots and continuous cast slabs
and bars by using molds with 1 mm spaced grooves. There is also an indication
that these surface marks can be avoided with the grooved mold surface topogra-
phy in stationary bottom poured castings. Most importantly, the non-uniform
subsurface microstructural features of the cast ingots such as liquation associated
with the surface laps and ripples can also be alleviated by casting with molds
with grooved surface topography.
5. Conclusions
1) In general, casting of aluminum alloy 3003 with copper molds with vertical
or horizontal grooves produced ingots with significantly fewer ripple (Type I)
and lap (Type II) surface marks than casting with copper molds with a smooth
surface topography. This was particularly the case at casting speeds greater than
5 mm/s.
2) Increase in casting speed and melt superheat appeared to decrease the pro-
pensity for formation of ripple and lap surface marks on ingots produced with
copper molds with smooth surface topography.
3) The critical wavelength (spacing) for grooves on a grooved mold surface
topography for significant alleviation of lap surface mark formation during im-
mersion (or continuous) casting of aluminum alloy 3003 on a copper chill is 1
mm. This is independent of the orientation of the grooves to the casting direc-
tion.
4) At 5 mm or more spacing, grooves perpendicular to the casting direction
(horizontal) appear to be more effective than those that are parallel to the casting
P. N. Anyalebechi
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10.4236/msa.2020.114018 282 Materials Sci ences and Applications
direction (vertical) in alleviating the formation of ripple and lap surface marks
on cast ingots of aluminum alloy 3003.
5) Casting with a mold surface topography with 1 mm spaced grooves elimi-
nated the formation of subsurface liquation (inverse segregation) and engen-
dered a more uniform subsurface cast microstructure on ingots of aluminum al-
loy 3003.
Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this pa-
per.
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