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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.
8–11
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.
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Effective Degassing for Reduced Variability in High-Pressure Die Casting Performance
