IMPROVEMENT OF MECHANICAL PROPERTIES OF HPDC A356 ALLOY THROUGH MELT
QUENCHING PROCESS
Shouxun Ji, Wenchao Yang, Bo Jiang and Z. Fan
Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
Keywords: High pressure die casting, melt quenching, nucleation, microstructural evolution
Abstract
Melt quenched high pressure die casting (MQ-HPDC) is a new die
casting process developed recently for improving the casting
quality of the conventional high pressure die casting (HPDC)
process. In the MQ-HPDC process, an alloy melt with a specified
dose and superheat is quenched by directly pouring the alloy melt
into a preheated metallic container. The thermal mass and
preheating temperature of the container is selected so that the
alloy melt is quenched just below the alloy liquidus and
heterogeneous nucleation takes place during the melt quenching.
The quenched alloy melt is then fed immediately into the shot
sleeve for component casting. In this paper we present the MQHPDC process and the resultant microstructures and mechanical
properties of a MQ-HPDC A356 alloy.
Introduction
The cold chamber high pressure die casting (HPDC) process has
been widely used for manufacturing shaped castings of light
alloys due to its high efficiency, high consistency and low further
machining, low cost and versatility in component shapes.
However, this process still faces several technological challenges.
For example, die castings have relatively poor mechanical
performance due to porosity and other casting defects and the
formation of a large solid lump in the shot sleeve before die
filling. Major effort has been made over many years to overcome
these challenges, using semisolid metal (SSM) processing [1,2],
vacuum die casting [3,4] and squeeze casting [5,6].
Recently, we have developed a melt quenched high pressure die
casting (MQ-HPDC) process to overcome the problems associated
with conventional HPDC process, in particular, to reduce the
casting defects and the formation of the large solid lump in the
shot sleeve. This paper presents the MQ-HPDC process and the
microstructure and mechanical properties of A356 aluminum
alloy processed with the MQ-HPDC process.
The MQ-HPDC process
The MQ-HPDC process (Figure. 1) consists of three sequential
steps: melt quenching, melt dosing and high pressure die casting.
This process originated from the concept that large dendrites and
solidification in the shot sleeve could be reduced by quenching the
melt in a metallic container before being transferred into the shot
sleeve. For a given alloy melt with a specified superheat and dose,
the thermal mass and the preheating temperature of the metallic
container can be selected so that the alloy melt can be quenched to
a temperature close to the alloy liquidus for enhancing
heterogeneous nucleation without significant crystal growth. The
quenched alloy melt has the following characteristics:
(a) a temperature just a few degree below the alloy
liquidus;
(b) an almost isothermal temperature due to the severe
convection caused by melt pouring;
(c) copious nuclei due to significantly enhanced
heterogeneous nucleation during melt quenching.
If the nuclei are uniformly distributed throughout the entire
volume of the melt, the growth of such nuclei ensures a relatively
uniform melt temperature in the shot sleeve before filling the die
cavity. This will prevent the formation of the large dendrites and
the solidification in the shot sleeve will facilitate die filling and
will promote a more uniform solidification after die filling, and
therefore reduce the formation of casting defects.
The MQ-HPDC process requires the use of a number of
metallic containers. The number required for processing is
determined by the specified melt dose, cycle time and container
thermal mass. The container will be cooled naturally to the
temperature which equals the preheating temperature described
previously for a single container.
Step I: Quenching
ladler
Step II: Dosing
die
Step III: Shaping
container
Piston
container
Shot sleeve
die
Shot
sleeve
Figure 1. Schematic illustration of the melt quenched high
pressure die casting (MQ-HPDC) process.
Experimental
Commercial grade ingots of A356 alloy were used to demonstrate
the MQ-HPDC process. The A356 ingots were melted at 710°C in
a clay-graphite crucible using an electric resistance furnace. The
container used was made of stainless steel with dimensions (in
mm) as shown in the inset in Figure 2. The container was
preheated to the specified temperature in the furnace. The die
casting was carried out using a 4500kN cold chamber die casting
machine. The melt was manually dosed into the shot sleeve
followed by the standard HPDC process. In the MQ-HPDC
process, the melt was poured at 710°C into the preheated
container, and then transferred immediately to the shot sleeve
followed by a full automatic HPDC cycle. The HPDC die
produced three 6.35mm diameter tensile test bars and three square
bars (2x6.3mm2, 3x6.3mm2, 5x6.3mm2) in each shot. The die was
heated by the circulation of mineral oil at 250°C.
Tensile tests were conducted following ASTM standard B557,
using an Instron 5500 Universal Electromechanical Testing
Systems with a ±50kN load cell. All tensile tests were carried out
at ambient temperature with a 2mm/min crosshead speed. The
specimen for microstructural examination was cut from the
middle of the tensile test bar. Each specimen was examined using
a Zeiss optical microscope with quantitative metallography using
an AxioVision 4.3 Quantimet digital image analysis system.
700
Temperature (oC)
600
500
temperature between 200°C and 400oC. Formation of a solid lump
at the bottom of the shot sleeve prior to die filling is inevitable.
Such a solid piece is pushed and deformed by the moving piston
during the die filling, resulting in severe cracking in the solid.
Such cracked solid is usually found at the bottom of the biscuit
(Figure 3c) and occasionally also found at the top of the biscuit.
Except the cracked solid in the biscuit, the rest of the biscuits
(middle and the top section) exhibit a coarse dendrite structure as
shown in Figure 3a&b).
400
a
300
T1(ºC)
200
T2(ºC)
100
T3(ºC)
0
0
1
2
3
4
5
6
Time (s)
Figure 2. Temperature recorded by three thermo-couples in the
stainless steel container during quenching of the A356 melt. The
insert shows the dimensions of the container in mm and the
positions of the thermo-couples in the container. The temperature
of A356 melt was 650°C. The initial container temperature was
200°C.The liquidus temperature of A356 is 615ºC.
b
Results
Experiments were carried out to understand the thermal history of
the melt during melt quenching. Three thermocouples were fixed
in the metallic container at different locations prior to melt
pouring as shown in the insert in Figure. 2. The recorded
temperatures against the time during one of the quenching
experiments are shown in Figure. 2. At all the three locations the
temperature increased in a fraction of second to a maximum and
then leveled off to a temperature just below the alloy liquidus
(615°C). The slightly slower reaction of the thermal couple T2 is
mainly caused by the delayed emersion of T2 in the alloy melt. In
consideration of the above time delay, it should be noted that the
recorded temperatures reached the maximum value within 1s and
achieved an almost isothermal temperature within 2s. It needs to
emphasis that the initial temperature includes the responding time
of the thermocouple. However, from the time to reach the
maximum temperature and the pouring temperature, it is possible
to estimate the average cooling rate of the melt during melt
quenching. The results showed that the cooling rate during the
melt quenching was in the order of 102K/s, which is comparable
with that achieved in the shot sleeve during HPDC process. As
discussed in ref. [7], the cooling rate in the shot sleeve of die
caster is at a level of 100 K/s. The similar level of cooling rate
will provide similar undercooling for microstructural evolution. It
needs to emphasis that the operation from pouring liquid into the
container and then to the shot sleeve can results in the temperature
variation because of the heat loss and the multiple process steps.
However, the process time of melt quenching is very short and the
whole process can be automatically achieved during production.
Therefore, the benefits are obvious.
The microstructure of the biscuits (the material solidified in the
shot sleeve) was examined to understand the primary phase
formation in the shot sleeve prior to die filling. During the
conventional HPDC process, superheated alloy melt (about
700°C) is directly poured into the shot sleeve that is at a
c
200μm
Figure 3. Optical micrographs showing the microstructure of
A356 alloy biscuit (70mm) after HPDC process on a cross
section 15mm from the surface in touch with the piston, (a) top of
the section, (b) middle of the section, and (c) bottom of the
section.
a
a
b
b
c
Figure 5. Optical micrographs showing the microstructure of
A356 alloy (6.35mm bar) produced by HPDC process showing
(a) the overall microstructure and (b) the morphology of primary
α-Al phase.
200μm
Figure 4. Optical micrographs showing the microstructure of
A356 alloy biscuit (70mm) after MQ-HPDC process on a cross
section 15mm from the surface in touch with the piston, (a) top of
the section, (b) middle of the section and (c) bottom of the section.
However, although with the same condition, the biscuit
produced by the MQ-HPDC process has a different
microstructure: (1) the cracked solid region (Figure. 4a) is very
much reduced or perhaps even eliminated; (2) no cracked solid
was found at the top of the biscuit; (3) the dendrite size was
considerably smaller (Figure. 4a&b). This demonstrates that melt
quenching can significantly enhance heterogeneous nucleation
(smaller dendrite size) and reduce/eliminate solid formation in the
shot sleeve.
The microstructures of A356 Al-alloy processed by the
conventional HPDC process the MQ-HPDC and are shown in
Figures 5 and 6, respectively. The HPDC sample shows
fragmented dendrites with an elongated morphology and up to
100µm in size (Figure. 5a), fine spheroids of 8µm in average size
(Figure. 5b) and substantial amount of irregular shaped cracks
resultant from hot tearing during the final solidification in the die
cavity (Figures 5a&b). However, the microstructure of the MQHPDC sample is considerably different from that of the HPDC
sample. Figure 6 shows that the primary α-Al phase was
characterised by the co-existence of two types of globular
particles. The relatively large primary α-Al globules (Figure. 6a)
with an average size of 35µm were fragmented from the rosettes
formed in the shot sleeve, while the fine α-Al globules (Figure.
6b) with an average size of 7.5µm were formed in the die cavity.
The MQ-HPDC samples were much less cracked than the HPDC
samples, suggesting that there was much less hot tearing in the
MQ-HPDC process. For A356 alloy, the measured average
porosity level was 0.95% for the HPDC castings and 0.27% for
the MQ-HPDC samples. This indicates that MQ-HPDC could
produce castings with lower porosity and higher integration.
a
Discussion
The unique feature of the MQ-HPDC process is melt quenching
prior to conventional HPDC process. The high cooling rate
achieved by pouring superheated alloy melt into a relatively cold
metallic container results in a large thermal undercooling, which
allows an increased number of existing solid particles in the melt
to nucleate. However, the growth of such nuclei will be restricted
in the container due to lack of time and cooling capacity. The fast
growth of the nuclei in the alloy melt in the shot sleeve can
generate latent heat, which acts as a temperature regulator to
ensure a relatively uniform temperature field in the alloy melt, in
comparison of the situation in conventional HPDC process. The
simultaneous growth of increased number of primary grains
results in a relatively small growth of each primary grain and
therefore large dendrites are eliminated, which improve the die
filling and result in a reduction of casting defects and improve the
mechanical properties.
b
Summary
A melt quenched high pressure die casting (MQ-HPDC) process
has been developed to improve casting quality. In this process,
melt quenching by purpose designed metallic container is used to
enhance heterogeneous nucleation; and the latent heat released by
the growth of the increased number of nuclei in the shot sleeve is
used to achieve a relatively uniform melt temperature. This helps
to reduce the formation of large dendrites and solidification in the
shot sleeve, produces slurry with fine globules, and facilitates the
die filling process, reduces casting defects and improves casting
quality.
References
Figure. 6 Optical micrographs showing the microstructure of
A356 alloy (6.35mm bar) produced by MQ-HPDC process
showing (a) the overall microstructure and (b) the morphology of
primary α-Al phase.
The mechanical properties of A356 alloy processed by both MQHPDC and HPDC processes are shown in Table 1. Although both
yield strength and ultimate tensile strength (UTS) show little
difference between the two groups of samples, elongation of the
MQ-HPDC samples was significantly improved. The average
elongation was increased from 10.1% for HPDC samples to
12.6% for MQ-HPDC samples, a 25% increase.
Table 1. Tensile properties of A356 alloy produced by
HPDC and MQ-HPDC processes.
Yield
strength
(MPa)
Ultimate
tensile
strength
(MPa)
Elongation
(%)
HPDC
120.2±3.97
262.1±2.66
10.1±1.05
MQ-HPDC
120.6±3.56
262.3±2.93
12.6±1.16
[1] M. C. Flemings, Behavior of metal alloys in the semi-solid
state, Metall. Mater. Trans. A 22A (1991) 957-81.
[2] Z. Fan, Semisolid Metal Processing, Inter. Mater. Review,
47(2002) 49-85.
[3] X.P. Niua, B.H. Hua, I. Pinwilla, H. Lib, Vacuum assisted
high pressure die casting of aluminum alloys, J. Mater. Proc.
Tech. 105(2000)119–127.
[4] E.S Kim, K.H Lee, Y.H Moon, A feasibility study of the
partial squeeze and vacuum die casting process, J. Mater. Proc.
Tech. 105 (2000) 42–48.
[5] A. Maleki, B. Niroumand, A. Shafyei, Effects of squeeze
casting parameters on density, macrostructure and hardness of
LM13 alloy, Mater. Sci. Eng. A, 428 (2006) 135–140.
[6] M.R. Ghomashchi, A Vikhrov, Squeeze casting-an
overview, J. Mater. Proc. Tech., 101 (2000) 1–9.
[7] S.Ji, Y. Wang, D. Watson, Z. Fan, Microstructural Evolution
and Solidification Behavior of Al-Mg-Si Alloy in High-Pressure
Die Casting. Metall. Mater. Trans. A 44A (2013)3185-97.