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Abstract

The reduced solubility of hydrogen in molten aluminium is believed to be a key factor influencing the formation of gas porosity, which adversely affects the mechanical properties. In this study, two crucibles of AlSi10MnMg alloy were degassed using conventional rotary degassing and high shear melt conditioning (HSMC) respectively and then cast into tensile specimens using the high-pressure die casting (HPDC) process. An optimal holding time of 10 min was established for both processing techniques corresponding to reduced density index (DI) and reduced variation in tensile performance. After rotary degassing, DI values were found to increase with increasing holding times, rising to 4.1% after 70 min. For HSMC, a quasi-steady state was observed with a maximum DI value of 1.4% after 190 min. The pore size in HPDC cast specimens was observed to be considerably lower after degassing with the HSMC device compared with rotary degassing.
TECHNICAL ARTICLE
Effective Degassing for Reduced Variability in High-Pressure
Die Casting Performance
EWAN LORDAN ,
1,2
JAIME LAZARO-NEBREDA,
1,3
YIJIE ZHANG,
1,4
and ZHONGYUN FAN
1,5
1.—Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University, Uxbridge,
Middlesex UB83PH, UK. 2.—e-mail: ewan.lordan@brunel.ac.uk. 3.—e-mail: jaime.lazarone-
breda@brunel.ac.uk. 4.—e-mail: yijie.zhang@brunel.ac.uk. 5.—e-mail: zhongyun.fan@brunel.ac.uk
The reduced solubility of hydrogen in molten aluminium is believed to be a key
factor influencing the formation of gas porosity, which adversely affects the
mechanical properties. In this study, two crucibles of AlSi10MnMg alloy were
degassed using conventional rotary degassing and high shear melt condi-
tioning (HSMC) respectively and then cast into tensile specimens using the
high-pressure die casting (HPDC) process. An optimal holding time of 10 min
was established for both processing techniques corresponding to reduced
density index (DI) and reduced variation in tensile performance. After rotary
degassing, DI values were found to increase with increasing holding times,
rising to 4.1% after 70 min. For HSMC, a quasi-steady state was observed
with a maximum DI value of 1.4% after 190 min. The pore size in HPDC cast
specimens was observed to be considerably lower after degassing with the
HSMC device compared with rotary degassing.
INTRODUCTION
To address the CO
2
challenge whilst maintaining
customer appeal, car manufacturers are striving
towards a light-weighting solution to improve fuel
economy and reduce green gas emissions. This
increased interest in light-weighting materials has
led to heavy investment into the advancement of
high-pressure die casting (HPDC) technologies. A
major defect preventing the mass production of
structural components using the conventional
HPDC process is gas porosity, which adversely
affects the materials performance in both tensile
and fatigue properties, whilst also producing blis-
ters after subsequent heat treatment. The reduced
solubility of hydrogen in liquid aluminium is
believed to be a key factor influencing the nucle-
ation and growth of gas porosity, with many novel
processing techniques being developed to control the
hydrogen content within the liquid metal prior to
injection.
Rotary degassing is commonly used in foundries
to minimise hydrogen levels in aluminium alloy
melts and involves the bubbling of an inert purge
gas (usually argon) into the molten metal. In this
process, a smaller purge bubble equates to a higher
degassing efficiency; however it is difficult to obtain
a bubble diameter below 10 mm.
1
The process
therefore requires a considerably high argon flow
rate between 4 L/min and 10 L/min and a signifi-
cant processing time between 15 min and 30 min to
achieve an acceptable hydrogen level below
0.15 cm
3
/100 g.
2
Increased rotor speeds may result
in a reduced bubble size; however the increased
surface turbulence and vortex introduced at these
high speeds severely deteriorate the melt quality by
entraining harmful oxide films.
3
The degree of hydrogen supersaturation in the
liquid metal depends on the hydrogen concentra-
tion, silicon concentration and melt temperature.
When this supersaturation exceeds a threshold
value, a gas bubble will nucleate, with the diffusion
of hydrogen from the surrounding liquid resulting
in the subsequent growth of bubbles.
4
It has been
suggested that the pressures required for the clas-
sical heterogeneous nucleation of gas pores are
almost unattainable for aluminium alloys, prompt-
ing research into several non-classical nucleation
theories. Oxide films are believed to possess the
potential to nucleate gas porosity with minimal
JOM
https://doi.org/10.1007/s11837-018-3186-4
Ó2018 The Minerals, Metals & Materials Society
difficulty.
5
The diffusion of hydrogen into the air
gap between the unbonded oxides causes the oxide
film to unfurl, inflating to considerable size.
Previous studies on vane rheometry and direct
shear cells have suggested that the rheology of
equiaxed solidifying alloys can be interpreted as a
cohesionless compacted granular material,
6,7
exhibiting characteristics such as Reynolds’ dila-
tancy (volumetric expansion in response to shear)
and dilatant shear banding (regions of low crystal
packing density where deformation localises).
Adopting this framework, defect bands in high-
pressure die casting (HPDC) are believed to form
because of strain localisation in the solidifying
material.
811
The thicknesses of defect bands have
been found to be in the range of 7–18 mean grains
wide, which is in the signature range of 6–20 mean
grains wide for dilatant shear banding.
12
The low
crystal packing density of the band is expected to
result in positive macrosegregation providing suffi-
cient liquid can be drawn to the dilating band. When
insufficient liquid is drawn into the dilating band
the liquid pressure decreases, resulting in interden-
dritic porosity.
11
The Brunel Centre for Advanced Solidification
Technology (BCAST) has successfully developed an
alternative means of addressing these issues by the
deagglomeration and dispersion of harmful oxide
films and inclusions throughout the melt. This is
achieved by intensive melt shearing using a rotor–
stator-based high-shear-melt conditioning (HSMC)
device.
13,14
In this process, the melt is subjected to
high shear rates in the gap between the rotor and
stator as well as in the stator openings, resulting in
effective distributive and dispersive mixing. The
application of HSMC also acts to enhance nucle-
ation by the forced wetting of oxide films and is
known to reduce the size of porosity and defect
bands in HPDC components.
14,15
Degassing using
the HSMC device has demonstrated improved
degassing efficiency compared with rotary degas-
sing, primarily resulting from the effective disper-
sion of each individual inert purge bubble into many
fine bubbles, increasing the overall surface area of
bubbles throughout the melt. With this process a
bubble size below 1 mm can be achieved with a low
argon flow rate of around 0.1 L/min.
1
A key aspect of the degassing procedure is the
holding time, with an optimal value being such to
allow time for the inert purge bubbles to escape
through the melt surface, whilst also being suffi-
ciently low to minimise the extent of re-gassing. The
study aims to establish an optimal holding time for
both rotary degassing and degassing by HSMC. The
efficiency of both processing techniques will be
discussed in relation to the integrity of the casting
structure and the materials performance in tensile
loading. The cause of failure will be analysed, with
defects identified using optical microscopy and
scanning electron microscopy (SEM) equipped with
energy dispersive x-ray spectroscopy.
METHOD
Rheinfelden’s Silafont-36 (AlSi10MnMg) HPDC
aluminium alloy was used throughout this study,
with measured chemical composition 9.66 wt.% Si,
0.64 wt.% Mn, 0.34 wt.% Mg, 0.096 wt.% Fe,
0.127 wt.% Ti and 0.023 wt.% Sr. Two 40-kg cru-
cibles of Silafont-36 were melted in electric resis-
tance furnaces and held at 750°C for 30 min to
maintain a uniform composition distribution. In the
first crucible, the melt was degassed using a con-
ventional rotary degassing unit for 10 min with a
stirring speed of 350 rpm and an argon flow rate of
4 L/min, in accordance with common industrial
practice. The second crucible was degassed using
the HSMC device, which is described elsewhere.
1
This process involved two phases (1) degassing for
10 min with a rotor speed of 1500 rpm and a
considerably lower argon flow rate of 0.4 L/min
followed by (2) conditioning via intensive melt
shearing for an additional 20 min without argon
flow. The end of gas injection marks the zero point
for the subsequent holding/processing times dis-
cussed later in this article.
Following melt treatment, molten metal was
poured into the shot sleeve of a Frech 4500 kN
locking force cold chamber HPDC machine using a
transfer ladle. The temperatures of the melt, shot
sleeve and die cavity were maintained at 680°C,
180°C and 150°C respectively. The molten metal
was then injected into the die cavity at a slow shot
speed of 0.3 m s
1
and a filling speed of 3.6 m s
1
to
produce eight round tensile samples with a nominal
gauge diameter of /6.35 mm in accordance with
ASTM standards. The geometry of the mould used
to produce these tensile specimens is illustrated in
Fig. 1.
After degassing, the rotary degassing unit was
removed from the first crucible and the melt allowed
to settle. A shot was taken every 5 min from 0 min
to 25 min to observe the influence of the holding
time on the performance and variability of the cast
HPDC tensile specimens. After degassing by HSMC,
and while the melt was subjected to intensive
shearing, shots were taken every 5 min between
0 min and 25 min. The reduced pressure test (RPT)
is commonly used to access the influence of hydro-
gen on the formation of porosity and involves
solidifying the alloy in conical steel cups in air and
under reduced pressure (80 mbar for 4 min).
5
The
density of the RPT samples was derived using
Archimedes’ principle and the density index (DI)
calculated using (1), where D
air
and D
vac
are the
density of samples solidified in air and under partial
vacuum respectively.
1
DI ¼Dair Dvac
ðÞ=Dair ð1Þ
Tensile testing was carried out on an Instron 5500
universal electromechanical testing system with an
extensometer gauge length of 25 mm and a cross-
head speed of 1 mm/min. Tensile specimens were
Lordan, Lazaro-Nebreda, Zhang, and Fan
aged naturally in air for 24 h and tested in the as-
cast state at ambient temperature. Samples for
microstructural observation were taken from the
centre of the gauge length, acting perpendicular to
the tensile direction. Samples were ground and
polished to a 1-lm finish using standard metallo-
graphic techniques. To reveal the microstructure
samples were etched with Keller’s reagent (95%
H
2
O, 2.5% HNO
3
, 1.5% HCl, and 1% HF). The
porosity levels in the HPDC cast components and
from the RPT samples were observed via optical
microscopy (OM) and analysed using the Fiji image-
processing software package based on ImageJ.
16
After tensile testing, the fracture surfaces were
analysed using a tungsten filament SEM (LEO SEM
145VP; Carl Zeiss) equipped with energy-dispersive
x-ray spectroscopy to identify the cause of failure in
relation to the defect population.
RESULTS AND DISCUSSION
Density Index
As shown in Fig. 2, the DI values obtained from
the RPT samples rapidly decrease after degassing
commences, with minimum values of 2.6% and 1.0%
obtained after approximately 10 min holding time
for rotary degassing and 70 min for HSMC degas-
sing respectively. These two values are well below
the industrially accepted threshold of approxi-
mately 5% (0.15 cm
3
/100 g hydrogen). Upon reach-
ing this minimum, the DI values are observed to
increase for samples treated with rotary degassing
with increasing holding times, reaching values as
high as 4.1% after 70 min. This is likely caused by
the turbulence and vortex introduced during rotary
degassing, which disturbs the melt surface, entrain-
ing oxide films and accelerating re-gassing of
hydrogen from the newly exposed surface.
3
However
this increase is not observed when degassing using
the HSMC device. As shown in Fig. 2, the DI values
remain relatively constant, reaching a maximum of
1.4% after 190 min. This can be explained by three
main mechanisms: (1) the effective dispersion of the
inert purge bubbles, reducing disturbance of the
melt surface previously caused by the large argon
bubbles produced during rotary degassing, (2) the
HSMC device is designed to produce minimal
surface turbulence and vortex, decreasing the rate
of re-gassing, and (3) the forced wetting of inclu-
sions introduced by intensive shearing, removing
the potency of oxide films as favourable nuclei for
porosity and consequently enhancing the heteroge-
neous nucleation of solid during subsequent solid-
ification processing.
14
Observations of the RPT cross
sections show strong agreement with the trends in
DI after degassing by rotary degassing and HSMC.
Porosity was found to significantly decrease after
degassing commenced, subsequently increasing
with increased holding times for samples treated
with rotary degassing.
Mechanical Properties
Figure 3illustrates the average yield strength
(0.2% proof strength) and average elongation of
four HPDC tensile samples as a function of holding
time. The yield strength is shown to remain
relatively stable at approximately 160 MPa for
both processing techniques for the entirety of the
experiment. This value of 160 MPa is higher than
the yield strength of 141 MPa quoted by Rhein-
felden for a Mg content of 0.33 wt.%.
17
The vari-
ability in elongation for the HPDC cast tensile
specimens is shown to also reach a minimum at 10-
min holding time, corresponding to the DI values
obtained in Fig. 2. Dybalska et al.
18
established an
optimal mixing time of 4 min for 2.7 dm
3
of liquid
aluminium for effective liquid metal processing.
Once linearly scaled for a 40-kg crucible and a
rotor speed of 1500 rpm, an optimal mixing time of
approximately 44 min is predicted. This corre-
sponds very well with the results obtained in this
experiment. With increasing holding time, the
variation for both processing techniques also
increases. For rotary degassing this is likely
caused by the previously mentioned re-gassing
mechanisms. However, the observed increase in
variation at later HSMC holding times is unex-
pected and may result from disturbance of the
surface introduced whilst collecting samples or
from the formation of other underlying defects
during the HPDC process.
Fig. 1. Geometry of the casting used to produce tensile specimens.
Effective Degassing for Reduced Variability in High-Pressure Die Casting Performance
Porosity Distribution and Defect Bands
The HPDC samples for both processing tech-
niques exhibited considerable porosity, particularly
towards the centre of the casting (Fig. 4). After
degassing, gas porosity was significantly reduced;
however macropores resulting from solidification
shrinkage remained. Defect bands of positive
macrosegregation were observed in both samples
treated with rotary degassing and HSMC, located
towards the centre of the casting and following the
casting contour. As shown in Fig. 4, samples treated
with HSMC exhibit a reduced defect band thickness
compared with those treated with rotary degassing.
This has been attributed to the uniform distribution
of externally solidified crystals after the application
of intensive melt shearing, which in turn influences
the mush rheology during filling and subsequent
feeding of the casting.
11,14
Optical micrographs taken at higher magnifica-
tion from the centre of the cross section were used to
determine the gas pore density and average pore
size as shown in Table Iand Fig. 5. Within the Fiji
image processing software package based on Ima-
geJ, optical micrographs were binarised using an
appropriate threshold to differentiate pores from
the matrix, as shown in Fig. 5. Pores with circular-
ity above 0.5 were classified as gas porosity and
separated for further analysis (the number of these
gas pores is denoted N
p
). The average pore size (U)
was determined using the equivalent circle
Fig. 2. The effect of holding time on the density index of RPT samples.
Fig. 3. Average elongation, average yield strength and variability of tensile performance as a function of holding time.
Lordan, Lazaro-Nebreda, Zhang, and Fan
approach, using (2), and the pore density was
defined as the number of pores in the image divided
by the image area.
Area of pore ¼p/i
4;/¼X/i
Np
ð2Þ
Samples treated with HSMC exhibit a decreased
pore size and increased pore density compared with
those treated with rotary degassing. This is caused
by the effective dispersion of the argon purge
bubbles during HSMC, leading to a high density of
fine pores using a substantially reduced argon flow
rate compared with rotary degassing. For samples
treated with rotary degassing, the increased pore
size at 10 min holding time appears to contradict the
DI values observed in Fig. 2. Although the hydrogen
content might be at a minimum, the melt turbulence
introduced during rotary degassing entrains oxide
films from the surface, providing an initiation source
Fig. 4. Pore distribution (a, b) and defect bands (c, d) of samples taken from the gauge length perpendicular to the tensile direction. Showing
HPDC specimens treated with HSMC (left) and rotary degassing (right) for a holding time of 10 min.
Table I. Comparison between pore density and pore size for castings treated with HSMC and rotary
degassing at holding times of 0 min, 10 min and 30 min
0 min after degassing 10 min after degassing 30 min after degassing
Pore density N
p
/mm
2
Pore U
lm
Pore density
N
p
/mm
2
Pore U
lm
Pore density
N
p
/mm
2
Pore U
lm
HSMC degassing 434 ±4 3.1 ±0.8 495 ±4 4.8 ±03 436 ±14 3.8 ±0.4
Rotary degassing 205 ±24 4.3 ±0.3 230 ±19 5.1 ±1.7 341 ±66 5.3 ±0.4
All values were calculated using Fiji (ImageJ) using optical micrographs shown in Fig. 5.
Effective Degassing for Reduced Variability in High-Pressure Die Casting Performance
for porosity.
19
Whilst increased holding times pro-
vide sufficient time for the evacuation of hydrogen
through the surface, when the melt quality is poor
sufficient time is also provided for the diffusion of
hydrogen into the bifilm, causing it to unfurl. For
HSMC, the forced wetting of oxide films removes
their potency as nuclei for gas porosity; therefore the
sudden increase in porosity after 10 min shearing is
unexpected and likely results from the disturbance
of the melt surface during collection of the samples.
Fracture Analysis
Observation of fracture surfaces via SEM
equipped with energy-dispersive x-ray spectroscopy
was carried out to identify defects relating to
fracture. For the samples treated with HSMC at
10-min processing time, defects observed within the
fracture path mainly constituted large Fe-rich
intermetallic particles and smaller cubic-shaped
Mn-rich intermetallic particles (Fig. 6). For the
samples treated with rotary degassing, along with
the Mn-rich intermetallic particles previously men-
tioned, a large oxide film was also observed towards
the casting surface (Fig. 7). These large oxides were
likely entrained during rotary degassing and were
not found in the HSMC-treated samples because of
the effective deagglomeration of oxide films with the
application of intensive melt shearing.
14
The pres-
ence of these large oxide films will have a significant
impact on the variability of mechanical properties,
particularly elongation.
Fig. 5. Optical micrographs and corresponding binarised images taken from the centre of the HPDC samples used to quantify the pore density
and size for samples treated with HSMC for (a) 0 min holding and (b) 30 min holding and samples treated with rotary degassing for (c) 0 min
holding (d) 30 min holding.
Fig. 6. SEM micrograph showing Fe-rich and Mn-rich intermetallic
particle compounds on the fracture surface of an HPDC tensile
specimen treated with HSMC for 10 min holding. Values from the
energy-dispersive x-ray spectrum are shown in the tables.
Fig. 7. SEM micrograph showing an oxide film on the fracture
surface of an HPDC tensile specimen treated with rotary degassing
for 10 min holding. Values from the energy-dispersive x-ray
spectrum are shown in the table.
Lordan, Lazaro-Nebreda, Zhang, and Fan
CONCLUSION
Two 40-kg melts of Silafont-36 HPDC aluminium
alloy were degassed using rotary degassing and
HSMC respectively, with tensile samples produced
using the cold-chamber HPDC process. For both
melt treatments, an optimal holding time of 10 min
was established corresponding to minimum hydro-
gen content and minimum variation in the tensile
performance of the HPDC cast specimens. Upon
reaching this minimum, the DI and tensile variation
increases for the melt treated with rotary degassing,
believed to be caused by the surface turbulence and
vortex introduced during processing. For samples
treated with HSMC, the variation increases with
increasing holding time; however the DI values
remain in a quasi-steady state. This increase in
variation was linked to disturbance of the melt
surface during sample collection and the formation
of casting defects such as the Fe- and Mn-rich
intermetallic particles observed with SEM. Optical
microscopy revealed that the samples conditioned
using the HSMC device contained smaller pores and
a higher pore density compared with those treated
with rotary degassing, despite the use of a consid-
erably lower argon flow rate of 0.4 L/min. The
increased degassing efficiency observed for the
HSMC device results from the effective dispersion
of the large inert purge bubbles leading to many fine
pores. For rotary degassing, the pore size and
density were found to increase with prolonged
holding times, corresponding to the increased DI
values and the variation in tensile performance. For
the samples treated with rotary degassing, a large
oxide film was observed near to the casting surface,
which is likely to have been introduced during
degassing. For the HPDC samples treated with
HSMC no oxide films were observed.
ACKNOWLEDGEMENTS
The authors acknowledge the financial support
provided by the Engineering and Physical Sciences
Research Council (EPSRC). The authors sincerely
thank Dr. Kun Dou for producing simulations aid-
ing in the research.
REFERENCES
1. J.B. Patel, J.L. Nebreda, and Z. Fan, in Proceedings of 6th
Decenn. International Conference on Solidification Process-
ing (Old Windsor, 2017).
2. J. Davis, ASM Int. 3, 784 (1993).
3. D. Dispinar, S. Akhtar, A. Nordmark, M. Di Sabatino, and L.
Arnberg, Mater. Sci. Eng. A 527, 3719 (2010).
4. R.C. Atwood and P.D. Lee, Metall. Mater. Trans. B 33, 209
(2002).
5. D. Dispinar and J. Campbell, Int. J. Cast Met. Res. 17, 280
(2004).
6. A.K. Dahle and D.H. StJohn, Acta Mater. 47, 31 (1998).
7. C.M. Gourlay, B. Meylan, and A.K. Dahle, Acta Mater. 56,
3403 (2008).
8. A.K. Dahle, S. Sannes, D.H. St. John, and H. Westengen, J.
Light Met. 1, 99 (2001).
9. C.M. Gourlay and A.K. Dahle, Nature 445, 70 (2007).
10. C.M. Gourlay, H.I. Laukli, and A.K. Dahle, Metall. Mater.
Trans. A Phys. Metall. Mater. Sci. 35, 2881 (2004).
11. C.M. Gourlay, H.I. Laukli, and A.K. Dahle, Metall. Mater.
Trans. A Phys. Metall. Mater. Sci. 38, 1833 (2007).
12. S. Otarawanna, C.M. Gourlay, H.I. Laukli, and A.K. Dahle,
Mater. Charact. 60, 1432 (2009).
13. Z.Y. Fan, Y.B. Zuo, and B. Jiang, Mater. Sci. Forum 690, 141
(2011).
14. H.R. Kotadia, N. Hari Babu, H. Zhang, S. Arumuganathar,
and Z. Fan, Metall. Mater. Trans. A Phys. Metall. Mater.
Sci. 42, 1117 (2011).
15. J.B. Patel, X. Yang, C.L. Mendis, and Z. Fan, JOM 69, 1071
(2017).
16. Fiji/ImageJ, https://imagej.nih.gov/ij/. Last accessed 23 Aug
18.
17. Rheinhfelden, CAR DS-Mould Des. 1 (2015).
18. A. Dybalska, D. Eskin, and J.B. Patel, JOM 69, 720 (2017).
19. J. Campbell, Castings, 2nd ed. (Oxford: Butterworth-
Heinemann, 2003).
Effective Degassing for Reduced Variability in High-Pressure Die Casting Performance
... for the full recirculation regime [34,37]. For the case of the HSMC 90 mm diameter unit, when considering a gas flow of 0.5 L/min, the regime transitions are satisfied at around 350 rpm for flooded to loaded, at 650 rpm for loaded to dispersed, and at 1100 rpm for dispersion to recirculation (see Figure 2a), which are the conditions already used and validated in previous studies [42,43]. ...
... i.e., an A356/LM25-type alloy [24,32]. However, the technology has also been successfully tested on other aluminum alloys such as the A380/LM24 [42] and Silafont-36 (AlSi10MnMg) [43] HPDC alloys, the Zorba cast fraction scrap (~LM27) [44], the 7032 [31,45], 7075 [9,40], and 2024 [46] wrought alloys, and also for recycling the A20X alloy (Aeromet [47]) prepared from recovered scrap material. In all these cases, a significant reduction in hydrogen and in oxide bifilm content were found, when compared to conventional rotary degassing. ...
... From Figure 10, it can be observed how HSMC produces higher values of UTS and elongation, while the yield strength remains unaffected by the degassing method. Furthermore, HSMC promotes a reduced variability in the properties, a result that has been confirmed in various studies for different alloys and casting conditions [42,43]. [24]. ...
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This article provides an overview of high-pressure die casting (HPDC)-related research undertaken at the EPSRC Future LiME Hub between 2015–2022. The project aimed to identify the cause of variability in the tensile ductility of die-cast structures, and to develop novel processing techniques to address this issue. Variability in tensile ductility was related to the size of large pores and non-metallic inclusions. It was proposed that these non-metallic inclusions formed during the pyrolysis of commercial plunger lubricants in the shot sleeve, and that these large pores derived from dilatational strains introduced during semi-solid deformation. Processing parameters and die design were found to significantly influence the microstructure of die-cast products, and the subsequent variability in tensile ductility. To close, recent progress on the application of intensive melt shearing to HPDC is reviewed. Intensive melt shearing was found to induce significant grain refinement in both Al and Mg alloys due to the effective dispersion of native oxide particles, and the use of these particles as heterogeneous nucleation substrates. The presence of native oxide particles also enabled the use of novel heat treatment procedures that avoided conventional issues such as surface blistering and geometrical distortion.
... Then stabilization treatment was followed with holding times of 0 min (0minS), 10 min (10minS), 20 min (20minS), and 30 min (30minS). The GBF processing conditions were designed by consulting previous studies [5,14,[21][22][23][24][25]. The dross on the surface of the molten metal was removed after GBF and stabilization treatment. ...
... Considering the time required for gas and inclusions in the melt to float to the surface to complete the cleaning of the molten metal, the optimal time for GBF treatment will be longer than T m . The optimal GBF time of 10 min observed in this study is similar to results of previous studies [5,[22][23][24]. GBF time can also be considered as a proxy for the total amount of gas injection, which can be expressed as a process parameter of the amount of gas injected into a unit amount of molten metal. ...
... GBF time can also be considered as a proxy for the total amount of gas injection, which can be expressed as a process parameter of the amount of gas injected into a unit amount of molten metal. In this study, 10 min of GBF time is equivalent to a gas injection of 1 L/kg, which is similar to 1 L/kg [22] and 2.5 L/kg [35] of other papers. Considering the optimal stabilization holding time after GBF treatment, the degree of molten metal cleanliness declined as the stabilization time increased from 10 to 20 and 30 min, so this study suggests that 10 min is optimal. ...
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In this study, the optimal conditions of gas bubbling filtration (GBF) treatment for securing highly-clean molten Al-Si-Mg-Cu alloy were identified. The effects of GBF treatment time and stabilization time on the degree of molten metal cleanliness were examined by measuring melt quality parameters such as density index, bifilm index, porosity, and the amount of dissolved hydrogen [H]. A high melt quality was achieved when GBF treatment was performed on 10 kg melt for more than 10 min (i.e., 1 L gas/kg melt). However, as the stabilization holding time after GBF treatment increased to 10, 20, and 30 min, the melt quality degraded. GBF treatment for 30 min had a similar effect to treatment for 10 min, and the degree of deterioration of melt quality during the stabilization time was also similar. Considering the economics, 10 min GBF treatment and short holding time are required. Observations of the shape and volume of the largest pore suggested the cause of defect formation and confirmed that the volume of the largest pore can be used as an index of the melt quality.
... This inconsistency leads to high scrap rates, and increased safety factors for component design. Although the tensile ductility of die-castings has been linked to various casting defects-porosity [5][6][7][8], oxides [8,9], and sludge intermetallic particles [9][10][11] to name a few-the underlying cause of variability remains enigmatic. ...
... This inconsistency leads to high scrap rates, and increased safety factors for component design. Although the tensile ductility of die-castings has been linked to various casting defects-porosity [5][6][7][8], oxides [8,9], and sludge intermetallic particles [9][10][11] to name a few-the underlying cause of variability remains enigmatic. ...
... bulk porosity content [3,4,12,13], maximum pore size [5][6][7], grain size [13][14][15], eutectic fraction [3,4,10]). The second relates the scatter in tensile ductility to statistical variations in melt quality (e.g. chemical composition [11,16], gas content [8], number of inclusions in the melt [9]). The third considers the stochastic nature of fluid flow, and the subsequent encapsulation of air and oxides [17][18][19][20]. ...
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This article unmasks the probabilistic nature of high-pressure die casting; specifically, the cause of scatter in the tensile ductility of die-cast Al8Si0.4Mn0.3Mg (wt.%) alloy. Scatter in tensile ductility is related to the size of large pores and non-metallic inclusions. We propose that these non-metallic inclusions form during the pyrolysis of commercial plunger lubricants, and that these large pores derive from dilatational strains introduced during semi-solid deformation. The apparent randomness of pore formation is thus ascribed to the heterogeneous nature of the semi-solid network. Reducing heat loss in the shot chamber is shown to promote a more homogeneous grain structure, leading to a decrease in the maximum pore size from 1.32 mm to 0.37 mm, and an increase in the minimum tensile ductility from 6.8 % to 9.4 %.
... native MgO particles in Mg alloys [1]) creates copious nuclei for heterogeneous nucleation of solidification, which leads to grain refinement [2]. Breakup may also play a prominent role in die-casting processes where defect-forming suspensions, such as gas bubbles [3,4] and oxide films [5], are readily transported by the bulk-liquid flow. Such defects adversely affect the fracture properties of die-cast structures by introducing considerable scatter in the material response [6]. ...
... Once the cavity is full, a pressure of 30~100 MPa [12] is applied to the solidifying alloy to compress gaseous phases and to assist in the feeding of shrinkage strains. The shear rates during die-filling are of a similar magnitude to those found in melt-conditioning (10 5~1 0 6 s -1 [8]), where breakage has previously been evidenced [3,7,8]. This invites an obvious question: can fluid flow be manipulated in such a way as to promote breakage during the transportation of liquid metals? ...
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The breakup of agglomerates and bodies suspended in turbulent flows are important phenomena that influence many aspects of modern solidification processing. It is often assumed that breakup operates in high-pressure die casting, wherein molten metal is transported at high speed through a narrow orifice system. To test this assumption, X-ray tomography and electron backscatter diffraction mapping are used to characterise pores, inclusions, and primary α-Al grains in die-cast samples produced with different flow field intensities. Numerical simulations are performed in ProCAST (ESI Group) to quantify the three-dimensional flow fields and to relate the derived quantities to breakage. Increasing the dissipation rate of turbulent kinetic energy is shown to induce a refinement of both non-metallic inclusions and primary α-Al1 grains nucleated in the shot chamber, a phenomenon which is ascribed to breakage. Several breakup mechanisms are discussed, with emphasis on the role of fluid turbulence.
... However, no mechanical properties were reported in any of those previous studies. Recently, Zhang et al. (2018) found that the use of high shear melt conditioning after degassing reduces the variation of the mechanical properties of HPDC components based on the A380 alloy, via oxide dispersion and enhanced grain nucleation during solidification, and Lordan et al. (2019) found that it also helps reducing the size and amount of porosity and defects in the HPDC castings. However, no significant increment on the mechanical properties was observed in these studies because of the inevitable high level of defects originated during this casting process, as recently reported by Zhang et al. (2020). ...
... In the present study with lower rotor speed and lower Ar gas flow, there was no re-gassing observed up to 50 min after HSMC degassing, which is a significant improvement for the HSMC technology. Similar low density index and minimal re-gassing for long holding periods when using low gas flow rate have been also reported by Lordan et al. (2019) on larger melts, highlighting the reproducibility of the HSMC degassing results. That way, and provided the surface is not intensively disturbed, the processed melt by the HSMC technology can rest for longer after degassing with the guarantee that neither hydrogen content nor density index will increase rapidly and will not require covering fluxes as it is common practice after rotary degassing. ...
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The degassing of aluminium alloy melts is a crucial step in the production of high-quality casting products, as the presence of dissolved hydrogen and oxide bi-films is detrimental to the mechanical properties. Current rotary degassing techniques are effective, but they lack efficiency because of the high gas flow and long processing times required. This study aims to solve this problem by presenting an innovative rotor-stator degassing technology, that combines controlled inert gas injection with intensive melt shearing. It has been applied to the liquid metal treatment of an aluminium cast alloy to evaluate the effect on melt cleanliness, casting integrity and mechanical properties. The optimum conditions for an efficient bubble dispersion have been obtained by water modelling. The melt quality during and after degassing has been assessed by in-situ measurement of hydrogen concentration and by reduced pressure test sampling for oxide bi-films and porosity content evaluation. This new technology is faster, requires less gas flow consumption and produces higher melt quality than the existing degassing techniques, due to a characteristic combination of distributive and dispersive mixing flow. In addition, re-gassing is minimised, maintaining a high melt quality for longer time after processing. This results in castings with less defects and better mechanical properties. The improved degassing efficiency of this technology makes it an excellent alternative in industry to increase melt quality and casting productivity.
... The degassing process is also of great importance for the removal of oxide layers (so-called bifilms), which are largely responsible for the metallurgical quality of the liquid alloy [18][19][20][21][22]. In [23], it was shown that argon exerts the most favorable influence on the alloy's quality in this respect. ...
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The article is devoted to the optimization of the degassing parameters of the AlSi9Cu3(Fe) alloy. The alloy was degassed with a solid degasser (Ecosal) and nitrogen or argon. The variable parameters were time and type of degasser. The test castings were made in permanent molds with an internal diameter of 25 mm and a length of 150 mm. The effect of the degassing time and the amount of degasser on the mechanical properties, as well as the hydrogen content and density index were investigated. The ALU SPEED TESTER developed by FMA was used to test the hydrogen content and the density index. Magmasoft software was used to design the geometry of the test castings. A significant effect of the solid degasser and degassing time on both the density index and the hydrogen content was demonstrated. Replacing nitrogen with argon did not bring any significant improvement in the above-mentioned parameters. The effect of degassing parameters on the mechanical properties of the EN AC-46000 alloy was much less significant, but was still visible. The optimal degassing parameters needed to obtain the highest strength parameters of the EN AC-46000 gravity die castings were determined.
... Contrary, several researchers reported higher bifilm quantity after rotary degassing treatments, which is commonly attributed to the surface turbulence and vortex formation around the impeller shaft, as well as the free melt surface sloshing near the vessel sidewalls, which phenomena can be avoided by using optimal treatment parameters. [32][33][34][35] According to Campbell, [3] degassing treatments are only effective in removing relatively large-sized bifilms, while numerous small-sized bifilms can be introduced into the liquid metal during the treatments. This statement is in accordance with the findings of Lazaro-Nebreda et al., [36] Yorulmaz et al., [37] Dispinar and Campbell, [38] as well as Gyarmati et al. [39] As was highlighted by Cao and Campbell, [40] the purging gases, which are often considered inert, could contain trace oxygen and water vapor which can lead to the oxidation of the inner surface of the purging gas bubbles. ...
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Rotary degassing is one of the most frequently used melt treatment technologies used for processing liquid aluminum alloys. Despite this, the information available about the possible effects of this method on the double oxide- and nitride film (bifilm) content, especially when using different purging gases, is quite limited. For this reason, in this study, the effects of multiple rotary degassing treatments conducted with N 2 and Ar purging gases on the bifilm quantity of a casting aluminum alloy were compared. The characterization of the melt quality was realized by the computed tomographic (CT) analysis of reduced pressure test (RPT) specimens, image analysis, and scanning electron microscopy (SEM) of the fracture surfaces of K-mold samples. Based on the results, by the application of Ar as a purging gas, relatively low bifilm content can be achieved. On the other hand, while the use of N 2 leads to the formation of numerous small-sized nitride bifilms, which significantly increased the pore number density inside the RPT specimens. This can be associated with the nitride formation by the chemical reaction between the liquid aluminum alloy and the N 2 purging gas bubbles during the degassing treatments. Graphical abstract
... He et al. [94] suggested that Ma should be lower than 0.1, which can be achieved by specifying t in Eq. (15) to be close to 0.5. Finally, from the equation of state of an ideal gas, the fluid pressure field p(x i , t) can be derived from the fluid density field: ...
Article
Rheological transitions from suspension flow to granular deformation and shear cracking are investigated in equiaxed-globular semi-solid alloys by combining synchrotron radiography experiments with coupled lattice Boltzmann method, discrete element method (LBM-DEM) simulations. The experiments enabled a deformation mechanism map to be plotted as a function of solid fraction and shear rate, including a rate dependence for the transition from net-contraction to net-dilation, and for the initiation of shear cracking. The LBM-DEM simulations are in quantitative agreement with the experiments, both in terms of the strain fields in individual experiments and the deformation mechanism map from all experiments. The simulations are used to explore the factors affecting the shear rate dependence of the volumetric strain and transitions. The simulations further show that shear cracking is caused by a local liquid pressure drop due to unfed dilatancy, and the cracking location and its solid fraction and shear rate dependence were reproduced in the simulations using a criterion that cracking occurs when the local liquid pressure drops below a critical value.
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A fine and equiaxed solidification process delivers multidimensional benefits to Mg-alloys, such as improved castability, reduced casting defects, enhanced mechanical properties, increased corrosion resistance and potential for increased recycled contents. Despite extensive research on grain refinement of Mg-alloys in the last few decades, currently, there is no effective grain refiner available for refining Mg-Al alloys, and our current understanding of grain refining mechanisms is not adequate to facilitate the development of effective grain refiners. Under the EPSRC (UK) LiME Hub's research program, substantial advances have been made in understanding the early stages of solidification covering prenucleation, heterogeneous nucleation, grain initiation and grain refinement. In this paper, we provide a comprehensive overview of grain refinement of Mg-alloys by native MgO particles. We show that native MgO particles can be made available for effective grain refinement of Mg-alloys by intensive melt shearing regardless of the alloy compositions. More importantly, we demonstrate that (1) the addition of more potent exogenous particles will not be more effective than native MgO; and (2) MgO particles are difficult to be made more impotent for grain refinement through promoting explosive grain initiation. We suggest that the most effective approach to grain refinement of Mg-alloys is to make more native MgO particles available for grain refinement through dispersion, such as by intensive melt shearing.
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Casting is the first step toward the production of majority of metal products whether the final processing step is casting or other thermomechanical processes such as extrusion or forging. The high shear melt conditioning provides an easily adopted pathway to producing castings with a more uniform fine-grained microstructure along with a more uniform distribution of the chemical composition leading to fewer defects as a result of reduced shrinkage porosities and the presence of large oxide films through the microstructure. The effectiveness of high shear melt conditioning in improving the microstructure of processes used in industry illustrates the versatility of the high shear melt conditioning technology. The application of high shear process to direct chill and twin roll casting process is demonstrated with examples from magnesium melts.
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Melt shearing has been suggested to be an efficient means of structure refinement through oxide dispersion and fragmentation. One of the process parameters that needs to be optimized is the shearing time. In this paper, the effect of shearing time on alumina powder refinement was studied in a model system with water as a working fluid. The established time was taken as a first approximation for experiments with the liquid metals processing by a high shear device based on a rotor–stator technology. The water model findings were confirmed experimentally on liquid aluminum alloys, and indicate that the optimal time of mixing is equal to 4 min in fully agitated conditions for the volume of 2.7 dm3.
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Melt quality is crucial for both continuous and shape casting of light alloys. Gas, oxides and other inclusions in the melt usually deteriorate the quality of the casting products. Conventional refining techniques, such as filtration and rotary degassing, can refine the melt by removing the inclusions although they are costly and time-consuming. A new technology for liquid metal treatment through intensive melt shearing was developed recently to improve the melt quality prior to metal casting. The new technology uses a simple rotor-stator unit to provide intensive melt shearing, which disperses effectively the harmful inclusions into fine particles to enhance nucleation during the subsequent solidification processing. Experimental results have demonstrated that the high shear unit can be used for general melt treatment, physical grain refinement, degassing and preparation of metal matrix composites and semisolid slurries. In this paper we offer an overview of the high shear device and its application in processing light alloys.
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The decrease in the solubility of hydrogen in liquid aluminium with temperature has been believed to be a major source of porosity. Therefore, degassing treatment is carried out in foundries to decrease porosity in castings. However, it was shown that it is difficult to nucleate hydrogen porosity in the absence of bifilms where bifilms simply aid the growth of pores. For this purpose, several casting experiments were carried out with commercial A356 alloy. In the first series of testing, the melt was degassed and then upgassed to three different levels. In the second series, the melt was first upgassed and then degassed gradually. In another test, hydrogen level was kept constant and sample collection was carried out. In all the trials, 10 bars were cast into sand mould to produce cylindrical samples for tensile testing. A step mould was used to investigate the porosity distribution and tensile samples were also collected from the same casting. For each casting experiment, a reduced pressure test sample was taken to check metal quality by using bifilm index. It was found that the turbulence and vortex (i.e. increase in bifilm population) during rotary degassing has a more significant effect on mechanical properties and porosity than the hydrogen content.
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During the production of aluminium ingots and castings, the surface oxide on the liquid may be folded into the bulk liquid to produce crack-like defects (bifilms) that are extremely thin, but can be extensive, and so constitute seriously detrimental defects. In this work, it has been found that bifilms are the initiator and hydrogen is only a contributor to the porosity formation process. For the first time, evidence is presented for the contribution of air (or perhaps more strictly, residual nitrogen from air) as an additional gas, adding to hydrogen in pores in cast Al alloys. The discriminating use of the RPT clearly reveals the existence of bifilms, and the effect of hydrogen on porosity formation. However, it seems that the RPT is of little use to evaluate the hydrogen content of the alloy. To investigate these effects, two alloys were studied in laboratory experiments LM0 (99.5%Al), LM4 (Al– 5Si–3Cu) and one in an industrial environment LM24 (Al–8Si–3Cu–Fe).
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The effect of the processing temperature on the microstructural and mechanical properties of Al-Si (hypoeutectic) alloy solidified from intensively sheared liquid metal has been investigated systematically. Intensive shearing gives a significant refinement in grain size and intermetallic particle size. It also is observed that the morphology of intermetallics, defect bands, and microscopic defects in high-pressure die cast components are affected by intensive shearing the liquid metal. We attempt to discuss the possible mechanism for these effects.
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A computational model for the prediction of porosity due to dissolved hydrogen in binary aluminum-silicon alloys has been developed. The model combines the cellular automata technique for the simulation of the growth of the solid phase, the finite-difference technique for the simulation of diffusion of the dissolved species, and a quasi-equilibrium model for the growth of individual bubbles. The growth of the solid and gas phases is initiated by a stochastic nucleation model, depending upon the undercooling (for the solid) or the supersaturation ratio (for the gas). The results agree favorably with experiments. The low supersaturation values needed to simulate the experimental results are consistent with a nucleation mechanism of gas pockets entrained within the melt.
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