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A Comparison Between Semisolid Casting Methods for Aluminium Alloys

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Semisolid casting of aluminium alloys is growing. For magnesium alloys, Thixomoulding became the dominant process around the world. For aluminium processing, the situation is different as semisolid processing of aluminium is more technically challenging than for magnesium. Today three processes are leading the process implementation, The Gas-Induced Superheated-Slurry (GISS) method, the RheoMetal process and the Swirling Enthalpy Equilibration Device (SEED) process. These processes have all strengths and weaknesses and will fit a particular range of applications. The current paper aims at looking at the strengths and weaknesses of the processes to identify product types and niche applications for each process based on current applications and development directions taken for these processes.
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metals
Review
A Comparison Between Semisolid Casting Methods
for Aluminium Alloys
Anders E. W. Jarfors
Department of Materials and Manufacturing, School of Engineering, Jönköping University, Box 1026,
551 11 Jönköping, Sweden; anders.jarfors@ju.se
Received: 18 September 2020; Accepted: 8 October 2020; Published: 13 October 2020


Abstract:
Semisolid casting of aluminium alloys is growing. For magnesium alloys, Thixomoulding
became the dominant process around the world. For aluminium processing, the situation is dierent as
semisolid processing of aluminium is more technically challenging than for magnesium. Today three
processes are leading the process implementation, The Gas-Induced Superheated-Slurry (GISS)
method, the RheoMetal process and the Swirling Enthalpy Equilibration Device (SEED) process.
These processes have all strengths and weaknesses and will fit a particular range of applications.
The current paper aims at looking at the strengths and weaknesses of the processes to identify
product types and niche applications for each process based on current applications and development
directions taken for these processes.
Keywords:
semisolid casting; rheometal process; GISS process; SEED process; production; capability;
surface treatment; heat treatment; tool-life; productivity
1. Introduction
Semisolid casting has developed strongly over the years with many dierent processes being
developed. In general, all these processes can be divided into two main routes for semisolid casting,
thixocasting and rheocasting [
1
]. The principal dierence between these is that the Thixocasting
route involves the reheating a material into a semisolid state. Rheocasting, on the other hand,
utilises a standard melt, cooled into a semisolid state. Thixocasting gained, through Thixomoulding
a wide commercial implementation for Mg alloys. [
2
] Thixocasting requires special pretreatments of the
billets to be reheated and struggles with material and process cost for aluminium. Rheocasting does
not have the same diculties with process cost, not materials costs as standard alloys can be used
and are now expanding strongly for aluminium [
3
]. Driven by the needs to produce high quality,
lower costs parts for primarily the electronics and automotive industries, several dierent rheocasting
processes have been developed. Examples of these processes are the New Rheocasting process
(NRC) [
4
,
5
], Sub-Liquidus Casting process (SLC) [
6
,
7
], Semisolid Rheo Casting process (SSR) [
8
10
],
rheo-die casting [
7
], Gas-Induced Superheated-Slurry process (GISS) s [
11
], Rapid-S or the RheoMetal
process [
8
], Swirling Enthalpy Equilibration Device process (SEED) [
12
18
] and many more. The leading
processes with the strongest industrialisation are the GISS, RheoMetal and SEED processes [3].
The processes GISS, RheoMetal and SEED, can be dierentiated based on the solid fraction of
the solid fraction entering the shot sleeve/die cavity. GISS uses the lowest fraction and SEED uses the
highest. The quality of a slurry also depends on the morphology determine this. All these processes
are combined with High-Pressure Die-Casting (HPDC) processing. This also implies that all these
processes will be very similar to HPDC and can be combined with vacuum processing resulting in that
all three processes are highly capable.
In common for all three processes is that the primary phase has a decisive influence on the
mechanical properties of the casting. For Al-Si-Mg alloys, the mechanical properties are dominated by
Metals 2020,10, 1368; doi:10.3390/met10101368 www.mdpi.com/journal/metals
Metals 2020,10, 1368 2 of 14
the dissolved Mg content in the primary slurry particles. [
19
] This fact makes the fabrication of the
slurry particles very important for the component performance in many dierent aspects. The intricate
dierences in the slurry making process between GISS, RheoMetal and SEED processes make them
better for some applications and more dicult in others. Important to remember is that this is valid for
both physical properties such as thermal conductivity [
20
] and mechanical properties [
14
], making the
matching between component and process important.
The current paper aims to identify common traits, product types and niche applications for GISS,
RheoMetal and SEED processes based on current applications and development directions taken for
these processes.
2. Materials and Methods
The method used was a literature survey and interviews with the developers of the processes.
The features that were common for all three processes were identified to clarify the significant generic
benefits of semisolid casting. The individual characteristics were then used to identify the specific
strengths of each process in order to give and indications of the best choice of process for a particular
product group.
3. Results
The main common characteristics are collated, in Table 1, to provide an idea of the generic
benefits of semisolid casting based on the proven benefits of the processes discussed in this paper.
The specific capabilities are collated in Table 2to provide a foundation for the choice of processes based
on component requirements. The leading products in production are collated in Table 3. to give an
idea of what is commercialised.
Table 1. Collation of common traits [2125].
Common Element Comment
Tool life
Reduced thermal load on dies improve die-life with GISS and SEED
proving up to 4 times that of HPDC
Lubrication/release agent used GISS has proven reductions of 40%
Cycle time reduction GISS has proven cycle time reductions of 20% compared to an
HPDC cycle
Process yields
GISS shows from 30% to 5% and RheoMetal from scrap rates of 20%
to well below 1%.
Productivity
Many applications mean a change from gravity die casting or
low-pressure die casting to rheocasting with cycle time changes from
4–8 min to 1–2 min in Rheocasting as it is based on the HPDC cycle.
Productivity is also increased due to yield increase
Weight reduction The thin-walled capability allows for significant weight reduction
(Radio filter down to 72% of HPDC cast version)
Weldability Porosity reduction gives increased weldability
Heat treatment All processes have proven that F, T5 and T6 conditions are possible
Table 2. Collation of process capabilities [2125].
Capability Measure GISS RheoMetal SEED
Anodizing and surface
treatment
Colour anodising possible Thick anodising
layers possible Anodizing possible.
Anodizing of 7xxx, 6xxx alloys
and Al-Mg alloy
Fatigue resistance N/AExcellent with thick-walled
component
Excellent.
Ex. Up to 22% increase in fatigue
life (turbo impeller case)
Wall thickness
From >10 cm down to less
than 0.5 mm, most common
1–3 mm
From >10 cm down to less
than 0.35 mm, most common
2–3 mm
Down to 0.75 mm
Metals 2020,10, 1368 3 of 14
Table 2. Cont.
Capability Measure GISS RheoMetal SEED
Proven alloy capability
Casting alloys: A356, Al-Si7,
A380, A383, Silafont 36,
Magsimal 59, A390,
Pure Aluminum
Casting alloys: Al-8Si, A356,
A357, A319, Magsimal 59
Casting alloys: A356, A357,
319S, B206
Wrought alloys: 6063, 6061,
5082, 7075 Wrought alloys: 6082 Wrought alloys: 6061
Strength
Normal strength Softer as-cast condition
Excellent elongation Normal strength
Moderate T5 response Moderate T5 response Excellent Elongation
Excellent T6 response Excellent T6 Response Excellent T6 response
Other notable
achievements
Colour anodising to
thin-walled and thick-walled
components
Experimental casting tested
successfully down to 0.45% Si High-Solid fraction up to 50%
Used in gravity die casting
with a cycle time reduction of
up to 20%
Pressure tight castings
without impregnation
Process range from 2 kg to
18 kg slugs.
Pressure tight castings
without impregnation Use of sand cores Other alloys:
Improved thermal
conductivity by up to 15%
Improved thermal
conductivity by up to 17%
Duralcan composite
Flexibility to switch between
rheocasting and HPDC-
Suitable for 20–40 kg slurries
Excellent slurry homogeneity
due to secondary stirring
Flexibility to switch between
rheocasting and HPDC
Table 3. Collation of components in production [2125].
Application Area GISS RheoMetal SEED
Automotive
Auto gearbox Compressor parts Brackets
Brake system components
Cooling units for
power electronics
Control arm
Chain covers Engine bearing cap
Engine block Engine bracket
Oil pan Shock towers
Steering wheels Turbo impeller
Electronics
Handphone covers Heat sinks
Heat sinks
Hard disc drive housing
Radio filters 4G and 5G
Heat sinks
Radio filters 4G and 5G
Heavy Duty Truck
components Truck gearbox
CAB mounts Battery holder
Muer holders
Brake calliper
Brackets
Knuckle
Skeleton joint
Machinery Machine parts with
steel inserts
Marine application Sacrificial anode Winch housing
Medical components Prosthetics
Military components Cast 7075 composite
armour plate – –
Sports Bicycle components Bicycle components
Motocross frame structural
components (steering knuckle
and others)
Motorcycle parts Wheel knuckle (Quad)
3.1. GISS Technology
The Gas Induced Super-heated Slurry process or GISS process is the market leader with more
than 100 licensed units used for parts used in the automotive, heavy truck, military, electronics and
medical industries [21].
The GISS process is a process that oers a cost-eective, quick installation and is an entry-level
process with a low threshold for implementation of semisolid casting. The process steps in the GISS
process are as follows, Figure 1[17]:
1. The melt is ladled from the furnace with a 10 to 20 K superheat.
2.
A porous graphite body is immersed for 5–20 s with Nitrogen gas seeping through this porous
body. The gas bubbling results in local cooling and nucleation of the primary solid-phase to
Metals 2020,10, 1368 4 of 14
initialise the slurry formation. It should here be noted that the gas seeping out through the
graphite body hinders metal from sticking to its surface to facilitate a clean retraction of the rod
from the melt.
3.
As the graphite body was removed, the slurry precursor is directly poured into the shot sleeve
where the slurry forms and is immediately cast.
4. The ladle is cleaned returns to the processing step 1.
Metals 2020, 10, x FOR PEER REVIEW 5 of 15
Figure 1. The Gas-Induced Superheated-Slurry (GISS) process (courtesy GISSCO).
The primary characteristics of the process are that it runs on a relatively low fraction starting at
some 5% solid fraction in the ladle and following cooling in the shot sleeve typically resulting in 25%
to 30% solid fraction making is a low solid fraction process [21]. The lower solid fraction and rapid
cooling during filling together with a large number of nuclei generate conditions that do not give the
time nor promotes the formation of dendrites. The main control her is the time for the gas bubbling
that initiates the solid phase generation and thus acts as viscosity control and the gate speed through
the second phase speed in the HPDC machine. The ratio gate speed/gas bubbling tome is essentially
identical to the Reynolds number and thus directly related to the level of turbulence during the die
filling process. It is the possibility to control this that allows for the manufacturing of parts with
intricate shapes and thin-walled parts down to 0.5 mm wall thickness [22].
A wide range of materials is found in the applications ranging from conventional casting alloys
such as A356 to wrought material such as AA6061 and AA7075. In terms of post-processing, both T6
and anodising are possible depending on alloy choice.
The GISS process set-up and process characteristics are such that it can be directly implemented
into an existing conventional HPDC production. This provides a low implementation threshold and
also good exampled with the possibility for direct comparison and identification of the benefits of
semisolid casting to conventional HPDC casting. Noticeably, as claimed by all processes, is that the
thermal load on the die is reduced. The first and direct benefit is an increased die-life, as thermal
fatigue, and erosion dominate die life in HPDC casting. The introduction of the GISS process has
resulted in a die-life extension up to a factor of 4 compared to the corresponding HPDC process
[21,22]. This has resulted in that the die-life in the GISS process has exceeding 400k parts produced
in a single die. The combination of reduced heat input and a reduced fill speed furthermore allows
for a reduction of die lubrication/release agents with up to 40% [22]. Die spraying reduction will
directly reduce spray duration and will reduce cycle time. Additionally, as the slurry is semisolid,
the time for solidification in the biscuit is reduced, shortening the time to part ejection. Reduced
duration for the part ejection and spray duration has reduced cycle time with as much as 20% [21,22].
To achieve this, it is essential to manage slurry generations properly to ensure that there is no
infringement on From a practical standpoint this also involves planning robot actions so that there is
no waiting time due to the slurry preparation [22].
In terms of internal soundness, porosity level can be significantly reduced by the introduction
of the GISS process together with the optimisation of the lubrication spray cycle. The reduced
porosity allows for the delivery of parts in F, T5 and T6 states. Reduced porosity also enables
weldability as in HPDC the main hinder for welding is entrained gas in the die cavity during filling.
It is well-known that porosity is the main reason for rejection in HPDC processing; the use of the
GISS process has resulted in rejection reductions from 30% down to 5% [22].
Intrinsic advantages originating from the thin-wall capability is that parts can be redesigned for
semisolid casting weight reduction may be possible improving the sustainability of the produced
components with increased resource efficiency and reduced energy content. The increased internal
soundness allows for a process change from gravity die casting or low-pressure die casting to
semisolid casting resulting in cycle time reduction from typically 4–8 min down to 1–2 min [22].
Figure 1. The Gas-Induced Superheated-Slurry (GISS) process (courtesy GISSCO).
The primary characteristics of the process are that it runs on a relatively low fraction starting
at some 5% solid fraction in the ladle and following cooling in the shot sleeve typically resulting in
25% to 30% solid fraction making is a low solid fraction process [
21
]. The lower solid fraction and
rapid cooling during filling together with a large number of nuclei generate conditions that do not
give the time nor promotes the formation of dendrites. The main control her is the time for the gas
bubbling that initiates the solid phase generation and thus acts as viscosity control and the gate speed
through the second phase speed in the HPDC machine. The ratio gate speed/gas bubbling tome is
essentially identical to the Reynolds number and thus directly related to the level of turbulence during
the die filling process. It is the possibility to control this that allows for the manufacturing of parts
with intricate shapes and thin-walled parts down to 0.5 mm wall thickness [22].
A wide range of materials is found in the applications ranging from conventional casting alloys
such as A356 to wrought material such as AA6061 and AA7075. In terms of post-processing, both T6
and anodising are possible depending on alloy choice.
The GISS process set-up and process characteristics are such that it can be directly implemented
into an existing conventional HPDC production. This provides a low implementation threshold and
also good exampled with the possibility for direct comparison and identification of the benefits of
semisolid casting to conventional HPDC casting. Noticeably, as claimed by all processes, is that the
thermal load on the die is reduced. The first and direct benefit is an increased die-life, as thermal
fatigue, and erosion dominate die life in HPDC casting. The introduction of the GISS process has
resulted in a die-life extension up to a factor of 4 compared to the corresponding HPDC process [
21
,
22
].
This has resulted in that the die-life in the GISS process has exceeding 400k parts produced in a single
die. The combination of reduced heat input and a reduced fill speed furthermore allows for a reduction
of die lubrication/release agents with up to 40% [
22
]. Die spraying reduction will directly reduce spray
duration and will reduce cycle time. Additionally, as the slurry is semisolid, the time for solidification
in the biscuit is reduced, shortening the time to part ejection. Reduced duration for the part ejection
and spray duration has reduced cycle time with as much as 20% [
21
,
22
]. To achieve this, it is essential
to manage slurry generations properly to ensure that there is no infringement on From a practical
standpoint this also involves planning robot actions so that there is no waiting time due to the slurry
preparation [22].
In terms of internal soundness, porosity level can be significantly reduced by the introduction of
the GISS process together with the optimisation of the lubrication spray cycle. The reduced porosity
allows for the delivery of parts in F, T5 and T6 states. Reduced porosity also enables weldability as in
Metals 2020,10, 1368 5 of 14
HPDC the main hinder for welding is entrained gas in the die cavity during filling. It is well-known
that porosity is the main reason for rejection in HPDC processing; the use of the GISS process has
resulted in rejection reductions from 30% down to 5% [22].
Intrinsic advantages originating from the thin-wall capability is that parts can be redesigned for
semisolid casting weight reduction may be possible improving the sustainability of the produced
components with increased resource eciency and reduced energy content. The increased internal
soundness allows for a process change from gravity die casting or low-pressure die casting to semisolid
casting resulting in cycle time reduction from typically 4–8 min down to 1–2 min [22].
3.2. RheoMetal Process
The RheoMetal process, with more than 30 machines delivered, have primarily found applications
within heat sinks and Light Emitting Diode (LED) fittings but is now finding new applications within
the area of heavy trucks and automotive components [
23
,
24
]. There are also application examples
from marine equipment and sports equipment as well [
3
]. The main characteristic is that a thick high
solid fraction slurry is made in a very short time. The rapid slurry generation is enabled through the
use of melting consumable or Enthalpy Exchange Material (EEM) to cool the melt into the semisolid
region. The rapid slurry generation results in a highly non-equilibrium solidification in the slurry
processing and a hard to predict solid fraction, but once trimmed, it is very stable and repeatable [
23
].
The repeatability is due to that the balance is not temperature-controlled but rather mass-based.
The process steps are as follows, Figure 2:
1.
A preheated ladle is filled from the furnace with a melt with a superheat of typically 20
C,
but depending on the melt delivery set up.
2. Typically, 5–8% us teemed ofrom the ladle and cast around a steel rod into a cylindrical shape
and placed in a carousel. This material is to be used as EEM.
3.
A rod with an EEM previously cast (normally six cycles earlier as the carousel typically holds six
EEMs to allow degating and cooling), is immersed under rotation into the ladle with the remaining
melt. The rotation provides the required shear to turn the solidified particles non-dendritic.
The EEM is stirred until complete melting which typically takes 5–40 s. In newer systems,
a secondary stirring is added for improved slurry homogeneity.
4. The slurry in the ladle is directly poured into the shot sleeve and injected into the mould cavity.
5. The ladle is cleaned and returned to preheating and returns to the processing step 1.
6. The steel rod is cleaned and returned to the casting station for a new EEM casting in step 2.
Metals 2020, 10, x FOR PEER REVIEW 6 of 15
3.2. RheoMetal Process
The RheoMetal process, with more than 30 machines delivered, have primarily found
applications within heat sinks and Light Emitting Diode (LED) fittings but is now finding new
applications within the area of heavy trucks and automotive components [23,24]. There are also
application examples from marine equipment and sports equipment as well [3]. The main
characteristic is that a thick high solid fraction slurry is made in a very short time. The rapid slurry
generation is enabled through the use of melting consumable or Enthalpy Exchange Material (EEM)
to cool the melt into the semisolid region. The rapid slurry generation results in a highly non-
equilibrium solidification in the slurry processing and a hard to predict solid fraction, but once
trimmed, it is very stable and repeatable [23]. The repeatability is due to that the balance is not
temperature-controlled but rather mass-based. The process steps are as follows, Figure 2:
1. A preheated ladle is filled from the furnace with a melt with a superheat of typically 20 °C, but
depending on the melt delivery set up.
2. Typically, 5–8% us teemed off from the ladle and cast around a steel rod into a cylindrical shape
and placed in a carousel. This material is to be used as EEM.
3. A rod with an EEM previously cast (normally six cycles earlier as the carousel typically holds
six EEMs to allow degating and cooling), is immersed under rotation into the ladle with the
remaining melt. The rotation provides the required shear to turn the solidified particles non-
dendritic. The EEM is stirred until complete melting which typically takes 5–40 s. In newer
systems, a secondary stirring is added for improved slurry homogeneity.
4. The slurry in the ladle is directly poured into the shot sleeve and injected into the mould cavity.
5. The ladle is cleaned and returned to preheating and returns to the processing step 1).
6. The steel rod is cleaned and returned to the casting station for a new EEM casting in step 2).
Figure 2. The RheoMetal process (courtesy Comptech).
The RheoMetal process is classified as a high fraction semisolid casting process with its typically
30–45% solid phase in the slurry [26]. The normal slurry making time is within 20 s. This short
duration for the shearing will create a slurry with a solid phase is less globular compared to other
processes. For high strength and more demanding applications, the introduction of a short secondary
stirring has improved the slurry quality to generate very low levels of porosity. The reduced porosity
has also allowed for pressure-tight castings to be produced, eliminating the need for impregnation
[23].
Like all semisolid processes, thin-walled components are possible, and the RheoMetal process
has shown that 40 mm high walls down to 0.35 mm thicknesses can be produced industrially even
for as complex products as a radio filter [23]. Compared to the other processes, fatigue loaded thick-
walled components are also being produced with wall thicknesses above 10 cm being in production
[3]. Again, this was achieved by strong management of the slurry quality.
Figure 2. The RheoMetal process (courtesy Comptech).
Metals 2020,10, 1368 6 of 14
The RheoMetal process is classified as a high fraction semisolid casting process with its typically
30–45% solid phase in the slurry [
26
]. The normal slurry making time is within 20 s. This short duration
for the shearing will create a slurry with a solid phase is less globular compared to other processes.
For high strength and more demanding applications, the introduction of a short secondary stirring has
improved the slurry quality to generate very low levels of porosity. The reduced porosity has also
allowed for pressure-tight castings to be produced, eliminating the need for impregnation [23].
Like all semisolid processes, thin-walled components are possible, and the RheoMetal process has
shown that 40 mm high walls down to 0.35 mm thicknesses can be produced industrially even for as
complex products as a radio filter [
23
]. Compared to the other processes, fatigue loaded thick-walled
components are also being produced with wall thicknesses above 10 cm being in production [
3
]. Again,
this was achieved by strong management of the slurry quality.
The material variety for the RheoMetal process is comprehensive, and Al-8Si, A356, A357, A319,
Magsimal 59 have all been realised, as well as the wrought alloy 6082. Experimentally, alloys down to
0.45% Si have been cast successfully in full industrial-scale [23,24].
Critical understanding, for the RheoMetal process, are the consequences of the deviation from
equilibrium. The first issue is that as the EEM is immersed into the melt, a freeze-on layer forms [
26
28
].
This layer has a composition which is given by the composition of the solidus line in the phase diagram
at the slurry forming temperature. This temperature is often just a just 2–5
C below the liquidus of the
melt. The level of solutes in the slurry particles is significantly lower than for other processes and has
two direct consequences. Firstly, thermal conductivity is increased with up to 17% compared to the
same material cast using HPDC or gravity die-casting. [
20
,
29
,
30
] The downside is that yield strength is
commonly slightly lower than for HPDC casting, but ductility is improved [25,31].
In terms of die-life, the same abundance of die-life data does not exist for the RheoMetal process
as it does for the GISS process due to that most of the RheoMetals products have not been HPDC cast
before and direct comparison is not possible. The main reason for an increase die life is the intrinsic
heat in the slurry entering the die. The higher solid fraction RheoMetal process and the associated
lower amount of intrinsic heat suggest die-life should be at least that seen for the GISS process. Die-life
is also depending n part size and geometry as well as on the use of lubrication agents and the air
blowing cycles. Being able to reduce this also reduce the die cooling action driving the generation of
tensile stress in the surface, which also may aect the die life achievable.
The use of sand cores in conjunction with the RheoMetal process has been successfully tested but
not yet been fully utilised commercially [23].
3.3. SEED Process
The SEED process has shown many applications, but the market penetration is not clear.
High-performance heavy-duty components for the automotive, heavy truck and sports industries have
been targeted for the development of the SEED process. This type of application requires high-quality
slurry. To achieve the best possible quality, the duration for the slurry making process is significantly
longer for the SEED process compared to the GISS and RheoMetal processes. The longer duration of
the shearing phase gives SEED an advantage in the ability to generate a good quality slurry. The higher
slurry quality also enables for even higher levels of solid fraction than the GISS and RheoMetal
processes. The main process steps are, Figure 3[25]:
1. A clean slurry making container is filled from the furnace.
2.
The slurry making container with the melt is placed on an oscillating table to create the swirling
flow of the melt inside the container to provide the required shear in the melt to produce the
globular microstructure.
3.
Older systems had a draining stage to allow the high fraction solid slurry to be removed from the
container. In newer systems, this step is no longer needed making the processing time shorter
4.
The slurry is poured from the container into the shot sleeve, and cast using high pressure die
casting equipment.
Metals 2020,10, 1368 7 of 14
5. The ladle is cleaned and returned to preheating and returns to the processing step 1.
Figure 3. The Swirling Enthalpy Equilibration Device (SEED) process (Courtesy STAS).
As part of the process design, there is a relation between the shot weight and the wall thickness of
the container. The basic principle is that there should be a thermal equilibrium between the container
and the slurry and to some extent the process is not purely temperature controlled but also mass
controlled to provide a stable and repeatable process. The slurry making time rage from 100 s up to
160 s, hence at least three slurries need to be under processing in order to not interfere with the process
as common cycle times for an HPDC casting process is in the range between 30 s and 90 s [17,25].
The high solid fraction targets high-performance parts and not ultra-thin walls components.
The viscosity of a 50% solid fraction melt makes thin-walled casting, and the minimum wall thickness
for SEED is 0.75 mm, thicker than what the GISS and RheoMetal process have been used to produce.
The material variety for the SEED process is similar to both RheoMetal and the GISS process
but more focused towards alloys suitable for heavy-duty and high-performance application and as
such not as comprehensive. SEED is, on the other hand, the only process that has targeted B206,
which contains 5% Cu and is one of the highest-performing alloys that can be cast [
1
]. Unique to the
SEED process is also that it has been used to cast Duralcan composite material and that it also has
proven to improve fatigue performance with up to a 22% performance increase [25].
As for the RheoMetal process, existing data for die-life improvement is limited, but again as solid
fraction is higher in the SEED process than for the other processes, and in theory, it should produce the
highest die-life improvement provided that thermal fatigue is the limiting die life factor.
4. Discussion
4.1. Generic Features and Comparison to HPDC
The semisolid casting processes have recently started to expand after many years struggling to
find applications significantly. The reasons for the current changes are several, but perhaps the most
important change is due to that conventional HPDC processing has started to struggle to provide parts
with the required features and properties. New applications for automotive components have large
series, and permanent mould casting lacks sucient productivity and cost-eciency. The need for
improved performance has driven the implementation of several new processes for high integrity
castings [
32
,
33
]. It appears as the industry is at a pivoting point searching for a solution. The common
traits of the processes hold a promise of a solution to many of these diculties, Table 1.
All semisolid casting processes have in common that heat is removed from the melt and a solid
phase is precipitated. The first and most noticeable eect is that viscosity is increased in the presence
of the solid phase. The Reynolds number, Re, Equation (1) is a characteristic measure of the level of
turbulence, and an increased viscosity will result in a reduction of Re indicating a reduced level of
turbulence [34].
Re =
vρDH
µ(1)
Metals 2020,10, 1368 8 of 14
Here vis speed,
ρ
is density, D
H
is the hydraulic diameter/characteristic length of the system,
and
µ
is viscosity. In semisolid casting the increase viscosity id often also combined a reduction of the
injection speed, adding to the reduction of Re and turbulence. This was also the first main focus of
the research as it provides the core of the process control for the injection phase, solid fraction and
injection speed and also the primary contributor to the yield improvements compared to HPDC that
can be seen, Table 1.
The heat removal is managed slightly dierently in the processes. In the GISS process, this takes
place in the ladle but perhaps foremost in the shot sleeve where the solid phase fraction increases
significantly, In the RheoMetal process this occurs in the pouring ladle and the SEED process in the
unique preparation crucible. Independent of this, the first consequence is that the intrinsic heat of the
melt entering the mould cavity is significantly reduced and the thermal load of the mould materials is
reduced. Die-life will be inherently improved. Die-life extension up to four times, compared to HPDC,
is possible and that a die-life of more than 400,000 shots have been realised in production., Table 1.
The fact that the amount of heat entering the die is reduced will also reduce the die thermal
distortion. The part contraction will be similar as for the HPDC processing, but the reduced thermal
distortion in the die also allows for a reduced release agent usage due to a lesser amount of relative
motion between the mould and casting. Release agent usage reduction as large as 40% compared
to HPDC has been achieved [
22
]. This aects the die cavity atmosphere as the o-gassing from
die-spray residues will be less than otherwise can cause significant rejection rates [
35
]. This reduces
the available material for entrainment with or without a vacuum system during casting and reduces
the requirements on venting. Similarly, reduced spraying also allows for a reduction of cycle-time with
up to 20% compared to HPDC [
22
]. It should here be noted that the solid fraction does not strongly
aect the cycle time as the solidification time in most cases is significantly shorter than the spraying
cycle-time. The solid fraction is important only for systems with a large biscuit dimension. The thermal
management thus aects both the par rejection rate and cycle time indirectly, Table 1.
The increased viscosity and the reduction of release agent usage together reduced tool distortion
will create conditions for production with reduced rejections from porosity and fewer process
interruptions as the flash formation and sticking tendencies are reduced as well. The most important
part is also to realise that entrainment porosity will be located randomly and is hard to control.
The pressure inside the entrained gas is also of the same order of magnitude as the die cavity pressure
at the end of the intensification period. Removing these the only porosity that will remain is shrinkage
porosity. Shrinkage porosity has the advantage that is can be managed through part geometry design
and as such is manageable. It will also be reduced, as only 50 to 80% of the solidification takes place in
the die cavity, reducing the feeding requirement. It should here be noted that due to the presence of
the solid phase, feeding is more complicated and requires more research to be fully understood.
Porosity reduction will also have the most profound eect on part performance and post-processing
capability. Reduction of the entrained gas will allow for heat treatment and welding as it is the entrained
gas that causes the main issues for these processes with blistering and poor weldment quality.
The ability to fill combined with a high viscosity in a shear-thinning or even thixotropy allows for
greater variation in section thickness than for conventional HPDC casting. This is one of the main
issues for the electronics industry that is heavily depending on heatsinks with cooling fins. These fins
can be made thinner with optimised distancing using semisolid casting and was also among the first
industry sectors where this was used [
3
]. Sustainability of the casting process is thus also significantly
improved, from the process-yield increase, together with the possibility of part weight reduction that
together drives resource eciency. Besides, the reduction of release agent usage also results in reduced
use of silane, siloxane and resins.
4.2. Specific Capabilities
In terms of process capabilities, the dierent processes have dierent strengths making them
more suitable for dierent applications and easy to implement. The main dierence between the
Metals 2020,10, 1368 9 of 14
process resides in a specific manner the solid fraction in the slurry is generated with some very intricate
dierences between the processes and the resulting material characteristics and process capabilities.
The GISS process results in a slurry with a low fraction solid of the melt entering the mould cavity
(5–25%). The actual treatment results in approximately 5% solid phase that acts as seeds that will
aid nucleation during solidification in the shot sleeve and the die cavity. This results in relatively
well-rounded particles, but the low fraction will make the cooling rates and process conditions similar
to HPDC processing, compared to the other processes. This means that segregation patterns and
mechanical properties will be similar to the HPDC process materials except for a significant reduction
of porosity, Table 2.
The RheoMetal processing is generating a slurry rapidly with an extreme deviation from
equilibrium. In contrast to both the GISS and the SEED process, the RheoMetal process has an
element of Thixocasting included. The EEM that melts is not fully molten but equilibrated with
the other particles in the slurry. That means that the slurry consists of particles that originate from
both reheating and solidification, making the RheoMetal process a hybrid between Rheocasting and
Thixocasting. Another deviation is the dendritic freeze on layer formed on the EEM, that forms and
disintegrates during the slurry preparation. The primary deviation from equilibrium is chemistry-based.
Due to that, the majority of the solid phase is formed at very high temperature; the primary slurry
particles are lean in solutes. For an A356-type of alloy, the time to homogenise the Si and Mg content in
the slurry particle s of a 70
µ
m diameter is approximately 30 s [
28
]. This is of the same order of time as
it takes to make the slurry. For the RheoMetal process, it is dicult to accurately predict the amount of
solid phase produced due to the kinetics of the alloy and the processing system. Under production,
the shot temperatures vary less than 1
C, even though furnace temperature is much less controlled
than this. The solid fraction varies from 30–40% typically, and the flowability of the slurry limits the
upper limit due to that the particles are not as round as for GISS and SEED processes. It should here be
noted that the latest developments of the RheoMetal process also includes a secondary stirring step to
improve slurry homogeneity and particle roundness., Table 2.
The materials proceed using the SEED process will be closer to equilibrium as the duration of
the slurry making process is longer with the typical range of 100–160 s. The direct consequence of
this is the generation of well-rounded particles richer in solutes. The improved shape of the particles,
compared the RheoMetal process, allows the SEED process to run at higher solid fractions than the
other processes (35–50%) [31].
The deviation from equilibrium seen in firstly the RheoMetal process and to a lesser degree in the
GISS process with reduced amounts of dissolved solutes results in improved thermal conductivity.
In the RheoMetal process, an increase by as much as 17% compared to an HPDC cast material with the
same composition can be seen, Table 2.
The reduced porosity from primarily entrainment porosity allows for heat treatment. Since T6
treatments are possible, a high productivity alternative permanent mould casting is found resolving
productivity issues for high-performance components. All three processes do well in the T6 condition
in terms of mechanical response. In the as-cast and T5 condition, slight dierences are arising from
the slurry making process dierences. The RheoMetal process producing a primary phase low in
solutes gives a slightly lower strength but better ductility material compared to the GISS and SEED
processes. Similarly, the reduced amounts of solutes will also aect the T5 response, where a slightly
lower strength is to be expected in the RheoMetal process compared to GISS and SEED processes.
It should be noted that this is component dependent and will be depending on the eciency of the use
of water quenching on ejection for parts that should be T5 heat treated Table 2.
Fatigue performance has for long been the weak point of castings due to porosity, and only
permanent mould casting has had some success in this area. Semisolid processing is changing
this. GISS that operates at lower fractions solid compared to RheoMetal and SEED will be more
subjected to the existence of porosity. RheoMetal and SEED operate at a high level of solid-phase,
reducing entrainment porosity eectively and will thus have a better potential och achieving excellent
Metals 2020,10, 1368 10 of 14
fatigue resistance as porosity is the leading cause of fatigue crack initiation followed by oxides. That this
is the case can be seen in applications with RheoMetal being used in heavy truck components with
heavy fatigue loads and SEED with an impeller with 22% improvement in fatigue life compared to
HPDC, Tables 2and 3.
All processes are capable of casting wrought alloys for strength and also for the possibility to
anodise for protection and appearance. GISS has developed colour anodised motorbike brake callipers
in 7075. For the RheoMetal process, the choice of direction has been slightly dierent where work has
focused both to cast with reduces Si amounts in the material and to increase the anodising capability to
understand better the relationship between the base material and the quality of the anodised layer.
Inoculation and strontium treatment together with a high fraction solid allows for better anodising
outcome also for the Silicon alloyed materials, Table 2.
GISS is furthermore the only process capable of being used with permanent mould casting due to
the lower solid fraction that may be generated, Table 2.
4.3. Industrial Applications
GISS has found the broadest range of application and also has the most comprehensive range
of alloys tested with a proven capability. This has its foundation with the commercial success of the
GISS process. The SEED process has chosen a dierent set of alloys with copper-rich aluminium alloys
designed for strength and performance. This has been a strategic decision to focus on the automotive
industry and high-value components, Table 3.
Based on the current usage of the processes GISS has an excellent cover of the automotive process
with a slight dominance of passive components such as chain coves and oil pans, but also pressure-tight
application such as compressor housing. The RheoMetal process also achieves pressure tightness.
The SEED process, on the other hand, has more applications such as shock towers, control arms
and turbo impeller with components that are subjected to high dynamic loads. This implies that
for the automotive industry, the GISS and RheoMetal process have found more applications relying
on thin-walled capability with weight reduction as one feature as well as reduced porosity for
cost-eective impregnation free applications. The SEED process has targeted more critical high strength
and high-performance components. Herein lies also the alloy capability and the use of the B206 alloy,
Tables 2and 3.
For heavy-duty truck components, GISS relies on thin-walled and shape replication capability
through gearbox castings. SEED and RheoMetal have found application in more fatigue loaded
components. The RheoMetal process, in particular, has found application in thick-walled heavy-duty
fatigue loaded components replacing both cast irons and forged aluminium components, Table 3.
In terms of marine components and military components, very few applications have been
realised, with simple shape component realised by GISS and a slightly more complex shaped part by
the RheoMetal process to relieve die sticking and production problems associated with the Magsimal59
alloy and as such use the reduced intrinsic heat in the slurry, Table 3.
Electronics components often have limited mechanical properties requirements, and the focus
is more on low-cost and thermal conductivity requirements. High thermal conductivity ofter means
dicult to cast alloys, and thermal transfer means complicated shape driving the capability to cast
complex shapes using un-castable alloys where machining solutions are common. GISS having a short
run-in and introduction cycle have found a niche in the electronics industry with a reduction of process
yield improvements for the complex shape products. GISS is thus often used to improve existing
production issues. A slightly dierent approach and the benefit were found for the RheoMetal process.
The use of an EEM results in a higher slurry temperature and as a consequence making the primary
phase solute lean. This particular eect results in that the RheoMetal process improves thermal
conductivity, allowing an as-cast thermal conductivity that generally would require a heat treatment
with other casting processes. This attractive feature also has made a niche for the RheoMetal process
in electronics. The main application is, however, for new product projects. The SEED process is also
Metals 2020,10, 1368 11 of 14
capable, but due to a lesser focus on highly complex shaped electronics components and thin-walled
capability, SEED has fewer applications in this area. The longer slurry making sequence would also
cause a lesser deviation from equilibrium ant, thus not reap the same benefits of increase thermal
conductivity as seen in the RheoMetal process, Table 3.
In the medical component area, there are only parts produces by GISS as a lightweight prosthetics
focusing on internal soundness and weight without compromising part performance.
In the sports industry, bicycle components are the entry lever and motorbike components.
All processes are capable, but with the high-performance target of the SEED process, there are more
product examples for motorbike applications for SEED than for GISS and RheoMetal process. Here the
SEED process has seen applications in the most challenging high-performance application through
complex shape structural motorbike parts, Table 3.
The RheoMetal process is the only process that has found application in machinery manufacturing
where the thick-walled capability is used in an application where a steel insert was over-moulded,
Table 3.
5. Conclusions
The main conclusions drawn in this comparison can be made in the following areas.
Process dierences
Process capabilities
Application areas
5.1. Process Dierences
The dierence between the processes resides in the actual generation of the slurry particles as this
lays a foundation for the slurry characteristics. The first dierence is that the GISS process generates
only 5% solid phase in the ladle and the rest is a fast, dynamic process in the shot sleeve and die cavity
while RheoMetal and SEED directly create more solid phase in the ladle. The consequence is that GISS
generates a material more similar to HPDC processed material. This similarity also results in that the
transition from HPDC to semisolid processing is relatively quick and easy. The rich nucleation and
rapid cooling support the creation of relatively well-rounded particles.
RheoMetal and SEED generates a high fraction solid in the ladle/crucible, and the slurry properties
are changing less dynamically in the shot sleeve and are dominated by the conditions in the ladle.
The main dierence is the time to process the slurry. RheoMetal typically takes 10–30 s to make the
slurry while SEED takes 180 s. The primary particles in the RheoMetal are formed through kinetics,
and the solid phase is far from equilibrium compared to the SEED process. This makes the RheoMetal
processed slurry particles lean in solutes that alters thermal conductivity and heat treatment responses
compared to what is seen in the SEED process. The longer processing time for the SEED process allows
the generation of more well-rounded particles, allowing SEED to be operated at higher solid fractions
than GISS and SEED.
5.2. Process Capabilities
The process capability can be seen as (1) shape capability, (2) material properties capability
(3) productivity capability
Compared to HPDC, all processes have improved shape capability and especially thin-walled
capability. In actual thin-walled capability, the RheoMetal process has reached furthest with 0.35 mm
on a radio filter.
The materials property capability is a broad field and resides in both actual improvements in the
material properties as well as in alloy capability. GISS has the broadest range of proven capability and
is as such the most flexible in terms of choice. SEED has targeted high strength alloys and is as such,
the choice for strength if copper is acceptable as an alloying element.
Metals 2020,10, 1368 12 of 14
The most detrimental influence on mechanical properties ad part performance is porosity.
All processes deliver materials with improved soundness compared to HPDC Defects caused by gas
entrainment is reduced as the solid fraction of the slurry is increased. It should here be noted that
solid fraction is not the only factor dominating the entrainment defects, but other elements such as
the use of a vacuum during casting and the amount of release agent also have a strong influence
on the occurrence of entrainment defects. Shrinkage porosity is part geometry dependent, but the
presence of solid-phase will reduce the overall solidification shrinkage. It should also be noted that
just increasing the solid fraction may not always decrease porosity as solidification characteristic and
feeding resistance becomes important. Increase fraction will, however, always distribute porosity and
make pores smaller supporting increased fatigue resistance explaining the benefits seen in RheoMetal
and the SEED processes.
Productivity change compared to HPDC is challenging to measure, and the only bulk of data
existing originates from the GISS process where direct comparisons were made. Both cycle-time and
process yield improvements were realised. It is not clear if these can be realised to the same amounts for
the RheoMetal and SEED processes. This requires a reduction of the release agent usage. Increasing the
solid fraction means, however, that the heat received into the die is reduced and that should reduce
distortion and allow for a reduced release agent usage. In theory, this is possible but convincing
proof of actual achievement is missing. Die life data has a similar relationship to the amount of solid
fraction with an increasing fraction reducing the thermal load and as such, should improve die-life.
GISS has actual achievements recorded that can be compared. Again, in theory, this is also possible for
RheoMetal process and SEED process, but the actual convincing proof is still missing.
5.3. Application Areas
GISS has so far found the widest applications which likely is due to its ease of introduction. This is
also the likely reason for that there exist more direct comparisons between HPDC and GISS. RheoMetal
and SEED appear to be introduced to new projects where HPDC is unable to support the requirements
on the component.
GISS has been applied in almost all areas in terms of applications, but the dominant field is
within the electronics industry. This suggests that the main benefits of the GISS process reside in
productivity improvements and the capability to produce complex-shaped sound products eectively.
The immediate proven gains are found in rejection rates, cycle times, release agent usage and foremost
also in die-life improvements. The electronics industry also benefits from that the primary phase is
somewhat lean in solutes and can provide improved thermal conductivity of the material compared
to HPDC.
The RheoMetal process has two main areas where it is applied, electronics and in the heavy
truck component manufacturing. The electronics applications are found in China where it is the
complex-shape capability together with the significantly improved thermal conductivity, due to the
reduced amounts of dissolved solutes in the solid phase, that drives the application. The European
heavy truck industry uses the reduced porosity to make heavy sectioned components competing with
cast iron and forged aluminium component that are under fatigue load. These are in heat-treated
conditions and often a T5 condition. This has taken some development eort in process control and
timing. The T5 response in the RheoMetal process is more dicult compared to the other processes
due to the reduced amount of solutes in the slurry phase and is entirely dominated by the Mg content
in the slurry particles [31].
The SEED process is capable of producing heatsinks just as GISS and RheoMetal, but due to the
relatively high solid fraction used the extremely thin-wall capability has not been achieved. The longer
processing timed does not support the significant improvements in thermal conductivity seen in the
GISS and RheoMetal process. The focus has been to draw benefit from the high solid fraction and to
use this to cast high-performance alloys and to drive its implementation toward extremely demanding
parts. These parts are often complex shape, relatively thin-walled and will often require T6 heat
Metals 2020,10, 1368 13 of 14
treatment. The achievable mechanical properties are higher than what is seen for GISS and RheoMetal
using alloys such as B207.
Funding: This research was funded by the Knowledge Foundation, grant number 20170066.
Acknowledgments:
The author is deeply indebted to Jessada Wannasin with GISSCO, Pascal Cote with STAS and
to Magnus Wess
é
n, with the RheoMetal company as well as Per Jansson and Staan Zetterström with Comptech
and last but not least Chen Qiurong with the RheoComp Technology company, previously Fujian RheoMet Light
Metals, for providing information on applications and capabilities of their processes to make this work possible.
The acknowledgement also includes providing the illustrations used in the current paper.
Conflicts of Interest: The author declares no conflict of interest.
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... The SEED process uses an external cooling container and a swirling flow to generate a high-solid-fraction slurry taking up to 180 s. The RheoMetal process relies on a consumable rotation body or Enthalpy Exchange Material (EEM) that is inserted into the melt and cools the melt as it is heated up and partially melts and disintegrates [8]. These processes are market dominant because they offer an improved internal part quality with reduced rejection rates and better cost efficiency primarily due to a significantly extended die-life in which the solid fraction and rheological properties play a central role, and the solid fraction is one critical element [8]. ...
... The RheoMetal process relies on a consumable rotation body or Enthalpy Exchange Material (EEM) that is inserted into the melt and cools the melt as it is heated up and partially melts and disintegrates [8]. These processes are market dominant because they offer an improved internal part quality with reduced rejection rates and better cost efficiency primarily due to a significantly extended die-life in which the solid fraction and rheological properties play a central role, and the solid fraction is one critical element [8]. ...
... The RheoMetal process deviates significantly from equilibrium, as shown by Santos et al. [9]. This deviation is more significant for the RheoMetal process than for the SEED and GISS processes [8]. Santos et al. [9] found that for the RheoMetal process at the slurry forming temperature, the solid fraction in a conventional A356 alloy was 0.31 ± 0.04, whilst the equilibrium value was 0.23, calculated using ThermoCalc TM . ...
Article
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Aluminium can be essential in reducing climate impacts as weight reduction is critical. Rheocasting is getting more and more attention from the electronics and automotive industries. The solid fraction in Rheocasting determines the processing outcome. The RheoMetal process is one of the leading processes with the most significant deviation from equilibrium, making presetting the slurry-making parameters difficult. A deeper analysis of the physics of the solid fraction deviation from equilibrium is made based on literature data using a simplistic mathematical model. The developed model confirms that the process is far from equilibrium and that the growth conditions of the freeze-on layer on the cooling agent used in the process determine the slurry temperature and cause the formation of excess solid fraction.
... Although several different rheocasting processes have been developed (i.e. new rheocasting process (NRC) [2], sub-liquidus casting process (SLC) [3], semisolid rheocasting process (SSR) [4], gasinduced superheated-slurry process (GISS) [5], swirling enthalpy equilibration device process (SEED) [6]), rapid-S or the rheoMetal process [7]), maintaining the cleanliness of the metal in rheocasting is still challenging. In contrast, although thixocasting is costlier than rheocasting, thixocasting can produce the components with excellent functional and mechanical properties. ...
... A quantitative comparison of microstructure and casting defects between thixocasting and rheocasting of Al-Si-Mg alloys is still required. It should be noted that although a comparison between thixocasting and rheocasting by producing different parts has been made [7], a direct comparison between thixocasting and rheocasting by producing the same part is still needed. In this paper, a direct comparison between thixocasting and rheocasting of Al-7Si-0.6Mg ...
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In this paper, both thixocasting and rheocasting of Al-7Si-0.6Mg alloy (EN AC 42200) for the same part was performed. It was found that rheocasting of Al-7Si-0.6Mg alloy show a smaller primary Al grain size and significant improvement of cast defects compared with thixocasting of Al-7Si-0.6Mg alloy. This paper demonstrates that rheocasting of Al-7Si-0.6Mg alloy is more beneficial in terms of microstructure and cast defects compared with thixocasting of Al-7Si-0.6Mg alloy.
... By evacuating the gas from the mould cavity, gas pores can be avoided during filling of the mould and possible small pores can be compressed to microscopic dimensions during subsequent intensification [6]. As Jarfors [9] points out, semi-solid processing opens up new possibilities and therefore also requires new actions and adaptations to the new technologies. ...
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In the automotive industry, casting products produced by high pressure die casting are essential. Due to the higher mechanical demands on these castings, the technological requirements of the process are also increasing. Therefore, the control of the microstructure and the development of defects play a major role. High pressure die casting parts made of aluminium usually contain gas porosity due to gas compression during the filling process of the cavity and the intensification during solidification. The use of semi-solid casting thus opens new doors to fulfil promising future demands. In this study, the venting system was adapted to the Rheometal TM process of aluminium and designed in the form of gaps, thus ensuring better venting. Subsequently, the results obtained were compared with casting process simulations to highlight possible differences.
... On the other hand, the semisolid forming permits a reduction of the thermal load on the dies, improving die life, reducing the porosity and increasing the weldability [2]. Therefore, application areas for semisolid mainly involve automotive, electronics, military, sports, and medical components [3]. Among all the semisolid processes, Rheocasting enables the production of near-net shape parts that are Heat Treatable through the common aluminium heat treatments, such as T6 (solution temper followed by artificial ageing). ...
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Rheocasting is a semisolid casting process allowing to obtain near-net shape parts. Through the Rheocasting process, it is possible to achieve aluminium castings having a low grade of porosity if compared to traditional die-casting methods, encouraging the production of automotive frame parts. However, casting processes, as commonly known, may cause tensile residual stresses inside the parts. On the other hand, compressive stresses inside castings can significantly increase the life of components: residual compressive stresses increase the material's resistance by counteracting crack initiation and propagation. The cracks propagate when the material is under tensile stress, while the Rheocasting technique seems to promote compressive stresses inside the castings. This work aims to analyse an aluminium rheocasted frame component for race cars in both the as-cast and heat-treated conditions. First, the mechanical properties of the components were evaluated in terms of tensile tests and microhardness. Then, residual stresses were measured at specific points of the casting. Finally, the evolution of the residual stresses inside the component before and after heat treatment led to assessing the effect of the Rheocasting process condition and the heat treatment, proving the marked advantage of using such a technology.
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Al 7075 alloy was invented more than 70 years ago and numerous investigations are still ongoing to make it lucrative in several applications. The alloy's high strength and low weight are most attractive properties that contribute to its widespread use in transportation applications in line with the growing worries about global warming. Additionally, Al 7075 alloy is utilized in lightweight equipment and are being applied in aviation, energy and even in the medical field. Aluminum and its alloys are used in a wide range of application and different heat treatment conditions are used based on application. Aluminum alloy have special combination of characteristics and used as most adaptable materials, for various industrial applications. Heat treatments and alloying ingredients give aluminum its ideal characteristics based on specific use. This encourages the production of microscopic strong precipitates that impede the dislocations motion and enhance the material's mechanical characteristics, including strength and hardness. In this work modeling of the laser cutting of 7075 T6 Aluminium alloys are performed to see the stress distribution and HAZ for the laser cutting. Al 7075 T6 alloy is thermally treated at varying aging temperatures and holding times. The ultimate strength and hardness properties of the Al 7075 T6 alloy are assessed in relation to the heat treatment procedure.
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The present text is the second part of an editorial written for a Special Issue entitled Advances in Metal Casting Technology [...]
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The grain refinement mechanism of Al–Ti–C grain refiners used in the casting procedure of aluminum has been studied by first‐principles calculation. Considering the controversy over Al 3 Ti, TiC, or Al 3 Ti–TiC as nucleation sites, the adsorption behavior of possible nucleation surfaces for Al atoms, including adsorption energies and adsorption sites, as well as the interfacial properties of Al/Al 3 Ti/TiC are calculated and compared. In the results, it is shown that the Al 3 Ti(112) surface has a stronger adsorption capacity for Al than TiC. By using the edge‐to‐edge matching crystallographic model and the calculation of the adsorption energy, it is found that the monolayer Al 3 Ti(112) can be stabilized on the TiC(100) surface, and the combination of TiC and monolayer Al 3 Ti can significantly improve the adsorption capacity of Al. Interfacial electronic structure analysis shows stronger chemical bonding between Al and Ti at the Al/Al 3 Ti/TiC interface compared to Al/Al 3 Ti, which explains that the monolayer Al 3 Ti covering the TiC surface has the strongest adsorption capacity for Al atoms. Therefore, it is speculated that TiC with a monolayer of Al 3 Ti covering the surface will serve as a potential nucleation site during grain refinement, which will provide a theoretical reference for future research work.
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Aluminium semi-solid castings have gained increased attention due to their superior mechanical properties, lower porosity compared to conventional high pressure die cast material. These characteristics suggests that semi-solid casting should be suitable to produce thick-walled structural components, yet most successful applications of semisolid casting have been for thin-walled components. There is a lack of understanding on filling and feeding related defect formation for semi-solid castings with thick-walled cross-sections. In the current study an AlSi7Mg0.3 aluminium alloy was used to produce semi-solid castings with a wall thickness of 10mm using a Vertical High Pressure Die Casting machine. The RheoMetal TM process was used for slurry preparation. The primary solid α-Al fraction in the slurry was varied together with die temperature. The evaluation of the filling related events was made through interrupted shots, stopping the plunger at different positions. Microscopy of full castings and interrupted test samples were performed identifying the presence of surface segregation layer, shear bands, gas entrapment, shrinkage porosity as well as burst feeding.
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Magnesium and silicon concentrations in the interior of primary α-Al of Al-7Si-Mg alloys were studied at temperatures in the liquid-solid range and just after solidification was completed. Analysis of the results showed that the magnesium concentration in the interior of primary α-Al is very low in the temperatures range from the liquidus to the start of the Al-Si eutectic reaction. Formation of silicon-rich phases during eutectic reactions, such as eutectic silicon and β-Al5FeSi, phases trigger a significant increase in the magnesium concentration in the interior of primary α-Al, when sufficient time is allowed for solid-state diffusion to occur. When fast cooling rates are applied during the Al-Si eutectic reaction, most of the magnesium is retained in π-Al8FeMg3Si6 and Mg2Si phases formed during solidification. Semi-solid Al-7Si-Mg castings were produced with varying magnesium contents, and the mechanical properties were evaluated in the as-cast, T5 and T6 conditions. It was found that the 0.2% offset yield strength of the semi-solid Al-7Si-Mg castings in the T5 and T6 conditions increases linearly with the square root of the magnesium concentration in the interior of the α-Al globules formed during the slurry preparation process.
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Recent advances in rheocasting have resulted in significant expansion in the types of products currently in full commercial production. The current paper gives an overview of components in production in Europe and in China produced using the RheoMetal TM process, that has taken the lead in a strong drive towards new heavy-duty applications made from aluminium alloys. In China, the dominating applications are found in the telecom industry. The trend in Europe is more towards marine and automotive applications commonly in fatigue loaded applications. The reason for the choice of rheocasting for complicated shape thin-walled electronics components with requirements is dominated by process yield and by the ability to improve thermal conductivity. The heavy-duty truck chassis thick walled components target weight reduction through design and to sustain fatigue load normally requiring forged components. Common in all applications are seen in production yield, reduced tool wear and reduction of die soldering.
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Recent developments and applications of rheo-diecast components for transportation markets are described. The components have been produced using the SEED (Swirled Enthalpy Equilibrium Device) semi-solid process. The development process typically involves several steps, including (1) an evaluation (and modification) of the component design to ensure it is suitable for semi-solid die casting, (2) the use of flow and solidification simulation to optimize the layout of the gating system, and to ensure that the modified component design can be produced without filling turbulence and shrinkage porosity, (3) stress modeling to ensure that the redesigned component will meet performance targets, and (4) in-plant development of the optimum process parameters. This development process will be described in detail for several commercial components, including two brackets for trucking applications, and connectors used in the frame of an electric bus.
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In the past, there have been a lot of effort to solve gas and shrinkage porosity defects in die casting. The common solutions are vacuum technology, jet cooling technology, and application of squeeze pins. However, these solutions often increase the die casting production costs. A new solution that has recently been introduced worldwide is GISS Technology. This technology applies the superheated slurry casting process. Gas and shrinkage porosity defects can be reduced. Furthermore, the production costs are lowered due to die life extension, cycle time reduction, melting energy reduction, and lubrication usage reduction. This paper describes the principle of GISS Technology, and selected applications and case studies are also be presented.
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In the context of increasing needs for lightweighting vehicles, semi-solid casting of aluminium components is a proven route that can be efficiently applied for automotive parts. Although semi-solid forming has not yet reached the market penetration that suits its actual potential, it is currently and efficiently used in many applications around the world on a daily basis. An example of such will be shown. This paper presents a case study on the application of the SEED rheocasting technology for the casting of an engine bracket. The part is made of the widely used AlSi7Mg0.3 alloy and is heat treated in T6 condition to benefit from the enhanced mechanical properties made possible by semi-solid forming. Throughout the development phase, different aspects associated with semi-solid casting, such as slurry condition, gate design, mold filling behaviour, lubrication, blistering and others, were addressed successfully. In the final, the combination of the SEED technology with a thorough development process and the specific casting rules for semi-solid forming led to actual commercial production and contributed to weightsaving on the actual part as compared to a former design made from high pressure die casting.
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The melting sequence of the enthalpy exchange material (EEM) and formation of a slurry in the RheoMetal™ process was investigated. The EEM was extracted and quenched, together with a portion of the slurry at different processing times before complete melting. The EEM initially increased in size/diameter due to melt freezing onto its surface, forming a freeze-on layer. The initial growth of this layer was followed by a period of a constant diameter of the EEM with subsequent melting and decrease of diameter. Microstructural characterization of the size and morphology of different phases in the EEM and in the freeze-on layer was made. Dendritic equiaxed grains and eutectic regions containing Si particles and Cu-bearing particles and Fe-rich particles were observed in the as-cast EEM. The freeze-on layer consisted of dendritic aluminum tilted by about 30 deg in the upstream direction, caused by the rotation of the EEM. Energy dispersion spectroscopy analysis showed that the freeze-on layer had a composition corresponding to an alloy with higher melting point than the EEM and thus shielding the EEM from the surrounding melt. Microstructural changes in the EEM showed that temperature rapidly increased to 768 K (495 °C), indicated by incipient melting of the lowest temperature melting eutectic in triple junction grain boundary regions with Al2Cu and Al5Mg8Si6Cu2 phases present. As the EEM temperature increased further the binary Al-Si eutectic started to melt to form a region of a fully developed coherent mushy state. Experimental results and a thermal model indicated that as the dendrites spheroidized near to the interface at the EEM/freeze-on layer reached a mushy state with 25 pct solid fraction, coherency was lost and disintegration of the freeze-on layer took place. Subsequently, in the absence of the shielding effect from the freeze-on Layer, the EEM continued to disintegrate with a coherency limit of a solid fraction estimated to be 50 pct.
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Volume 2A is a practical guide to aluminum and its engineering applications. It begins with a review of temper designations and product forms and the underlying physical metallurgy of aluminum alloys. It then examines manufacturing practices and techniques, focusing in critical areas such as casting, metalworking, heat treating, machining and finishing, surface treatment, and joining. It describes melt and solidification processes, high-integrity die casting, forging, extrusion, powder and additive methods, and the production and use of aluminum foams. The volume also discusses quenching, anodizing, organic and conversion coating, brazing, and laser welding, and offers insights on process selection, quality, performance, service ability, and other product lifetime concerns. For information on the print version of Volume 2A, ISBN: 978-1-62708-207-5, follow this link.
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The growing demand for increasingly more cost and energy effective electronics components is a challenge for the manufacturing industry. To achieve higher thermal conductivity in telecom components, an aluminum alloy with a composition of Al-2Si-0.8Cu-0.8Fe-0.3Mn was created for rheocasting. Yield strength and thermal conductivity of the material were investigated in the as cast, T5 and T6 heat-treated conditions. The results showed that in the as-cast condition thermal conductivity of 168 W/mK and yield strength of 67 MPa was achieved at room temperature. A T5 treatment at 200°C and 250°C increased thermal conductivity to 174 W/mK and 182 W/mK, respectively, while only a slight increase in yield strength was observed. Moreover, a T6 treatment resulted in similar thermal conductivity as the T5 treatment at 250°C with no significant improvement in yield strength. Therefore, the T5 treatment at 250°C was suggested as an optimum condition for the current alloy composition.