Mechanism and application of a newly developed
pressure casting process: horizontal squeeze
casting
* Li Peijie
Born in 1962, Professor. He received his B.S., M.S. and Ph.D. degrees from Harbin Institute of Technology in 1984, 1989 and 1995, respectively. The topic of his Ph.D. dissertation is the heredity of liquid structure of aluminum alloy. After graduation, he worked at Harbin Institute of Technology until 2000, and since then has worked at Tsinghua University. Li Peijie is the director of National Centre of Novel Materials for International Research, Tsinghua Universtiy. His research interests mainly focus on the heredity of liquid structure of light metals, solidifying structure and performance control of light metals, and equipments for processing of aluminum and magnesium alloys. He is the author and co-author of more than 100 papers and 30 patents.
E-mail: lipj@mail.tsinghua.edu.cn
Received: 2014-05-27 Accepted: 2014-06-25
*Li Peijie1
, Huang Xiusong1
, He Liangju1
, Liu Xiangshang2
, and Wang Benci2
1. Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China; 2. L. K. Technology Shanghai Yi Da Machinery Co. Ltd., Shanghai 200000, China
Abstract:
Compared to traditional high-pressure die casting (HPDC), horizontal squeeze casting (HSC) is a more promising way to fabricate high-integrity castings, owing to a reduced number of gas and shrinkage porosities produced in the casting. In this paper, the differences between HSC and HPDC are assessed, through which it is shown that the cavity filling velocity and the size of the gating system to be the most notable differences. Equipment development and related applications are also reviewed. Furthermore, numerical simulation is used to analyze the three fundamental characteristics of HSC: slow cavity filling, squeeze feeding and slow sleeve filling. From this, a selection principle is given based on the three related critical casting parameters: cavity filling velocity, gate size and sleeve filling velocity. Finally, two specific applications of HSC are introduced, and the future direction of HSC development is discussed.Key words:
horizontal squeeze casting; high-integrity; cavity filling; squeeze feeding; sleeve fillingCLC numbers: TG249.2 Document code: A
Article ID: 1672-6421(2014)04-232-07
H
igh pressure die casting (HPDC) is widely used to produce complex and thin-walled light metal parts, especially in the automotive industry[1], due to its high productivity, good dimensional accuracy and surface quality of castings. The high cavityfilling velocity and thin gate used in HPDC, however, result in a large number of porosity defects in the casting, which limit its application in load-bearing parts. Thus, improving the internal quality of parts made by pressure casting has become a significant research
focus[2]. The recent application of high-integrity pressure casting processes such as
semi-solid die casting[3], vacuum-assisted HPDC[4], horizontal squeeze casting (HSC)[5] and
double control forming[6] has brought some hope to the idea of producing load-bearing
parts by pressure casting. Among these processes, HSC offers the advantage of both a low porosity and a fine microstructure, which is similar to the casting achieved with
traditional vertical squeeze casting (VSC)[7]. Although VSC is not sensitive to gas
entrapment and flow turbulence due to its filling sequence being from bottom to top,
it does have its disadvantages in terms of the large volume and complex operation required. In contrast, the use of a horizontal shot system with HSC means that the
equipment needed is much simpler and cheaper, and can be easily manufactured from
existing HPDC machinery[8]. Furthermore, recent reports of the successful fabrication
of thick-walled engine bracket mountings by HSC prove the potential of this process[9]
1 Differences between HSC and
HPDC
The process of HSC is similar to HPDC, in that the melt is
first poured into the shot sleeve, and then fills the shot sleeve and die cavity when the plunger advances. After filling, the
melt solidifies under the pressure from the plunger and the
shrinkage in the casting is fed by the alloy through the gate. However, there are two notable differences between HPDC and HSC processes. First of all, the cavity filling velocity in HPDC
is extremely high (more than 10 m∙s-1), so the highly turbulent
flow can become partially atomized [10]
. While in HSC, the melt
steadily fills the cavity at a low velocity (10-1
-100 m∙s-1) and
ideally from bottom to top, which prevents the entrapment of
gas and oxides[11]. This difference in filling velocity is shown in Fig. 1(a). The second difference lies in the fact that the gate size used in HPDC is typically quite small so as to facilitate
its quick removal for subsequent cleaning. As a result, feeding through the gate is weakened and a large number of shrinkage pores are generated in the casting. But, in HSC, both the
runner and gate are quite large, which helps to strengthen the feeding effect through the gate and ensure sound castings
being produced [11]. Figure 1(b) shows this difference in gating
system design.
Fig. 1: Filling velocities (a) and gating systems (b) of HPDC and HSC
Fig. 2: Toyo 3S die casting system[16]
2 Equipment and application
The most widely used squeeze casting machines were those
developed by Ube Industries, which includes the VSC
machines popularized in the 1970s[12] and the horizontal vertical die casting machines popularized in the 1980s[13].
Since the 1990s, HSC techniques were further developed, which, compared to HPDC, offer a strict limit on the gas entrapment created during the sleeve filling stage. Prince Machine Corporation adapted a full-sleeve technique to a
conventional HPDC machine, which uses a “cross chamber” to close the pour hole, allowing the shot sleeve to be totally
filled prior to injection, and thus the gas entrapment in the sleeve is totally avoided [14]. It was further proved by
full-sleeve technique that castability can be improved by as much
as 20% compared to conventional HPDC techniques [15]. There
is another method that can be taken to avoid gas entrapment,
and that is to develop a super-slow sleeve filling technique. For example, in the Toyo 3S die casting system (Fig. 2), the shot sleeve temperature is controlled to create a melt with good fluidity, then the melt is injected into the die cavity through
a multi-stage process in which the velocity can be as low as
0.05 m∙s-1 [16]. Other manufacturers have developed similar die
casting machines, such as the DEC150MT by Toshiba, which are also capable of achieving velocities as low as 0.05 m∙s-1 [17]
. The Chinese manufacturer L. K. Technology also offers HSC
machines ranging in size from 280 to 800 t (Fig. 3) that can not only achieve an injection velocity as low as 0.03 m∙s-1, but
also can be switched between HSC/HPDC functionality as
needed.
Several examples of components successfully fabricated by HSC have been reported since 2000, and it has been shown that castings made by the Toyo 3S die casting system can be heat treated[16]
. Furthermore, through careful die design and process parameter control, Youn et al. [9] were able to
use a sleeve filling velocity of 0.08 m∙s-1 with HSC for the
Air
Fixed half
Cavity Short ceramic sleeve
Ceramic ladle
Plunger High melt fraction Heating
unit Thermo
couple Partial squeeze Moving half
HSC
t Sleeve filling Cavity filling
v
0.1 m s· -1
HPDC HPDC
Die
HSC
Sleeve
Plunger Cavity
Table 1: Major parameters in simulation (filling velocity is expressed by plunger velocity) fabrication of an engine bracket mounting. Ji et al. [18]
further studied
the influence of casting parameters on the quality of ADC12 bars, and suggested that the filling velocity through the gate should be kept lower
than 0.6 m∙s-1.
3 Selection principles of casting
parameters
From the statement above, it is clear that the three main characteristics required in the design of a HSC casting process are slow cavity filling,
Fig. 4: Three-dimensional model for simulation
3.1 Slow cavity filling
With a high filling velocity, the melt surface easy enfolds or the exhaust exit is blocked by the melt, then the entrapment
of gas and oxides occur. In such situations, the cavity should
ideally be filled from bottom to top, which means the melt flows without jetting. It has been demonstrated that a critical velocity exists for almost all liquids, above which jetting of the
melt occurs and casting quality dramatically decreases[19]. This
critical velocity can be calculated by[19]
:
Vcrit = 2(γg/ρ)
1/4 (1)
where γ is surface tension, g is the acceleration of gravity and
ρ is the density of the melt. In the case of A356, when ρ is 2,420 kg∙m-3 and γ is 0.889 N∙m-1 [20], Vcrit is 0.49 m∙s
-1
.
Figure 5 shows the cavity filling process with three different plunger velocities. It can be seen from Fig. 5 that when the
plunger velocity is 0.1 m∙s-1, corresponding to the maximum
melt velocity of 0.38 m∙s-1 at the gate [Fig. 5(a)], the melt flows
from the bottom to the top and no jetting occurs. When the
plunger velocity is 0.4 m∙s-1, corresponding to the maximum
melt velocity of 1.07 m∙s-1 at the gate [Fig. 5(c)], jetting of the melt occurs, then the melt front drops and the surface enfolds, resulting in the entrapment of gas and oxides in the melt. When the plunger velocity is 0.13 m∙s-1, jetting is about
to occur, and the maximum melt velocity of 0.52 m∙s-1 at the
gate is considered to be the critical velocity [Fig. 5(b)], which is close to the preceding theoretical calculation. Although the
critical velocity varies with material and gate structure, it has
been suggested by Campbell to be close to 0.5 m∙s-1
in most engineering alloys[19]. Thus, the gate design and velocity setting
in HSC should ensure that the gate velocity is below 0.5 m∙s-1
. Moreover, this velocity is close to the turning point of casting quality of 0.6 m∙s-1 derived from experiments[18].
Fig. 3: 800 t S-type squeeze machine by L. K. Technology
squeeze feeding, and minimal gas entrapment in
the shot sleeve. The commercial software ProCAST was used to simulate the filling and solidification process of a thick-walled plate by HSC, and
the selection principles of these three critical parameters were discussed. The three-dimensional simulation model is shown in Fig. 4, which consists of a plate with a thickness of 10 mm, gating system and a shot sleeve with an internal diameter of 50 mm and a length of 300 mm. The die and casting
materials selected were H13 steel and A356
alloy, respectively, and the major parameters in
simulation are shown in Table 1.
Cavity
Melt Sleeve
Plunger
Constants Variables
Melt temperature Die temperature Initial melt fraction Cavity filling Gate thickness Sleeve filling
(°C) (°C) in sleeve (m∙s-1
) (mm) (m∙s-1
)
0.1, 0.13, 0.4 10 0.1
670 200 0.5 0.1 4, 7, 10 0.1
0.1 10
3.2 Squeeze feeding
The feeding mechanism in HSC can be expressed by the
Poiseuille equation[19]:
dp/dx = 8vη/πR4 (2)
where dp/dx is the pressure gradient, v is the interdendritic flow velocity, η is the viscosity, and R is the radius of the
channel. Using this equation, it is found that even with a
high pressure gradient, interdendritic flow stops when the
interdendritic channel is blocked. Consequently, it should be possible to achieve directional temperature field in
HSC. Figure 6 shows the temperature field associated with
solidification using a gate with a thickness of 4, 7, or 10 mm.
It shows that when the temperature of the 4 and 7 mm gates
[Figs. 6(a) and (b)] falls to the solidus, a large area of the
casting still remains above this temperature, resulting in a high
degree of shrinkage porosity in the final casting. However, when the gate thickness is 10 mm [Figs. 6(c)], which is equal
to the plate thickness, directional temperature field is achieved and the lower temperature of the casting allows it to be fed through the gate. Many factors influence the achievement of directional temperature field, such as sizes of the equivalent
hot spot of gate and casting, geometry of runner, temperature
field of die. Among them, sizes of equivalent hot spot of gate
and casting are the two most important parameters, which can
be approximately represented as the gate thickness and plate
thickness in this simulation. Thus, an approximate conclusion
that the gate thickness in HSC should be designed as large as
the thickness of the casting, is made.
Fig. 5: Cavity filling with plunger velocities of 0.1 (a), 0.13 (b) and 0.4 m∙s-1 (c)
Fig. 6: Temperature field during solidification with a gate thickness of 4 (a), 7 (b), and 10 mm (c)
Fig. 7: Critical velocity filling of a shot sleeve
3.3 Slow sleeve filling
Compared to vertical squeeze casting, it is quite difficult to totally eliminate gas entrapment during the horizontal
injection of HSC. Garber [21] developed a mathematical model
to describe the flow in the sleeve when the plunger advances (Fig. 7), indicating that the gas entrapment can be avoided if the melt advances with a stable front. If the plunger velocity is
higher than the critical velocity, the melt will reach the ceiling
of the sleeve and roll over, thereby leading to gas entrapment.
When the plunger velocity is lower than the critical velocity,
the melt will hit the far end of the sleeve and move backward, thereby blocking the gate and also causing gas entrapment to
occur. Tszeng and Chu[22] have proposed a simple equation to calculate the critical velocity, vc:
vc = 2((2gR)
1/2-(gh)1/2) (3)
where R is the internal radius of the shot sleeve and h is the initial height of the melt in the shot sleeve. Given an R of 25 mm and an h of 25 mm, the critical value vc is therefore
found as 0.41 m∙s-1.
The simulated results of the shot sleeve flow process with
different plunger velocities are shown in Fig. 8. It demonstrates that with a plunger velocity of 0.1 or 0.2 m∙s-1, the melt near
the plunger does not reach the ceiling of the sleeve. As a result, when the melt front reaches the far end of the sleeve it reflects back, blocking the gate and trapping the gas (indicated by the
Plunger Air Air
Melt R
Melt h
θ
Total time: 1.8750 s
Fluid velocity magnitude (m·s-1
) 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0
Total time: 1.8528 s
Fluid velocity magnitude (m·s-1)
0.6 0.54 0.48 0.42 0.36 0.3 0.24 0.18 0.06 0
Total time: 1.8375 s
Fluid velocity magnitude (m·s-1
) 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
(a) (b) (c)
Total time: 18.6213 s
Temperature (°C) 620 610 600 590 580 570 560 550 540 530 520
Tliq 616.0
Tsol 556.0
Total time: 15.8288 s
Temperature (°C) 620 610 600 590 580 570 560 550 540 530 520
Tliq 616.0
Tsol 556.0
Total time: 13.1011 s
Temperature (°C) 620 610 600 590 580 570 560 550 540 530 520
Tliq 616.0
Tsol 556.0
arrow) remaining in the sleeve. With the lower velocity of 0.1 m∙s-1, the gate is blocked until the sleeve filling
is almost finished, and thus only a small quantity of gas
is entrapped. When the plunger velocity is increased to 0.33 m∙s-1, a small quantity of gas is entrapped; when the plunger velocity is 0.4 m∙s-1, the melt front rolls over and
gas entrapment occurs; and yet, no rolling or reflecting occurs with an injection velocity of 0.38 m∙s-1. This
indicates that 0.38 m∙s-1 is the optimal value for sleeve
filling, which is close to the theoretical calculation given
earlier (0.41 m∙s-1). However, there may be errors between
this theoretical calculation and reality, which cannot be compensated for by experimental testing due to a lack of
validation methods. Most HSC manufacturers therefore
tend to adopt a super-slow velocity in sleeve filling, and it is suggested that it only needs to be sufficient to ensure that the melt has a good fluidity. A typical value would be in the
order of 0.1 m∙s-1.
4 Examples
Figure 9 shows an engine connecting rod that was fabricated by a L.
K. DCC280S machine. The design of this rod incorporated a large gate, with a plunger velocity of 0.1 m∙s-1 during the sleeve and
cavity filling stages. This casting could be heat treated, and there was no visible evidence of gas porosity in cut sections. Figure 10(a) shows an experimental motorcycle wheel, which was fabricated by
a L. K. DCC800S machine. In this instance, a large gate was used in order to achieve the required squeeze effect, resulting in several
shrinkage porosities in the wheel hub when a 20 mm gate diameter was used [Fig. 10(c)]. When the gate diameter was subsequently
enlarged to 30 mm, only a very small degree of porosity was
observed, as shown in Fig. 10(d). This demonstrates the importance
of gate size in ensuring the integrity of HSC castings. Furthermore, the fact that the gate is not positioned at the lowest point of the
casting means that a bottom-to-top filling sequence cannot be
achieved, which would also have an effect on the casting quality.
Fig. 8: Flowing process in sleeve with different plunger velocities
Fig. 9: Parameter curves (a) and casting of an engine connecting rod with gating system (b) fabricated by HSC
Fig. 10: Plan view (a) and section view (b) of a wheel fabricated by HSC, and porosity in casting with a
gate diameter of 20 mm (c) and 30 mm (d)
Fig. 11: Microstructure of gate
Fig. 12: Segregation bands in a cross-section of
wheel spoke
Investigation of the microstructure of the wheel revealed
clear segregation band containing a large volume of Al-Si eutectic. In Fig. 11, the three segregation bands located near the gate can be seen to be broad and diffuse along the direction of flow. It is suggested that these bands were formed by a dilatant
shear mechanism[23] and hot tearing mechanism[24] during
solidification. These bands contain a large amount of liquid phase during solidification, which accelerates the movement
of the central material and strengthens the feeding effect[25]
.
Segregation bands were also observed in a cross section of
the wheel spoke, as shown in Fig. 12, including a skin-related
band (i) and dilatant shear band (ii). The characteristics of segregation bands at other locations have also been studied in
the previous research [24]. It was found that segregation bands tend to appear near the concave corner rather than the convex corner of the cross section, and in the thin section rather than the thick section where the section thickness changes. It was
deduced that from the wall to the centre of the section, a first
decreasing and then increasing shear stress gradient forms in the solidifying alloy and when the shear stress reaches the critical value at one position, the grain network collapses and a
dilatant band forms by the dilatant shear mechanism[26]
.
5 Development directions
The fact that HSC has been successfully used to fabricate
several thick-walled parts indicates that it has further potential to produce high-integrity parts. However, as the development
of the HSC technique is still in its relative infancy, considerable work is still needed to extend the applicability of this method
to other components. First, the equipment used for HSC needs
to be further developed to incorporate the full-sleeve technique, a good combination of super-slow velocity and good fluidity in the sleeve, and die heating/cooling techniques. Secondly, the mechanism of the HSC process should be further studied, including the effects of flow type in the shot sleeve and cavity at slow velocity, solidification characteristics in the shot sleeve
during filling, and solidification characteristics in the cavity under high pressure. Of particular concern is the fact that
the segregation behavior in thick parts appears to be more
pronounced in HSC than HPDC, which could cause the casting to fail in service. It is therefore important to fully understand
this segregation behavior, and the mechanisms through which it occurs, and especially give a suitable mathematical description.
Finally, further study is also needed to ascertain the effect of
casting parameters on the quality of the final casting, including
surface quality, macro porosity and microstructure.
6 Conclusions
(1) Compared to HPDC, the HSC process has a lower filling
velocity and larger gate size, making it suitable for producing high-integrity castings. Two techniques have been developed for HSC, the full-sleeve technique and super-slow injection technique, both of which have been mainly aimed at minimizing gas entrapment in the shot sleeve. As a result, HSC has been successfully used to produce a bracket mounting and connecting rod.
( 2 ) T h r o u g h t h e o r e t i c a l c a l c u l a t i o n a n d n u m e r i c a l simulation, it was found that the gate velocity in HSC should
be below 0.5 m∙s-1, the gate thickness should be designed as
large as the thickness of the casting, and the filling velocity should be the minimum required to ensure the melt retains good fluidity in the sleeve (a typical value is 0.1 m∙s-1
).
(3) The fabrication of a motorcycle wheel by HSC revealed
that a large gate helps to eliminate shrinkage porosity in the
casting. However, segregation bands, which play a significant role in feeding, were observed in the wheel, and could cause
the casting to fail in service.
(4) The future development of HSC should be focused
on improving the equipment used, the understanding of the process itself, and studying the effect of casting parameters on
the final product quality.
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