Available via license: CC BY 4.0
Content may be subject to copyright.
ARCHIVES
of
F O U N D R Y E N G I N E E RING
10.24425/afe.2023.144289
Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences
ISSN (2299-2944)
Volume 2023
Issue 2/2023
10 – 14
2/2
© The Author(s) 2023. Open Access. This article is licensed under a Creative Commons Attribution 4.0 International
License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and
reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes were made.
Influence of Defects on Deformation
Behavior of High-Pressure Die-Casting
Magnesium Alloys
K. Braszczyńska-Malik
Czestochowa University of Technology, Faculty of Production Engineering and Materials Technology,
Department of Materials Engineering, 19 Armii Krajowej Ave., 42-200 Czestochowa, Poland
Corresponding address: e-mail: kacha@wip.pcz.pl
Received 13.06.2022; accepted in revised form 12.01.2023; available online 28.04.2023
Abstract
The results of investigations of defects in AME-series magnesium alloys produced by the high-pressure die-casting method are presented.
The analyzed magnesium alloys contain about 5 wt% aluminum and 1-5 wt% rare earth elements introduced in the form of mischmetal.
The casts were fabricated using a regular type cold-chamber high-pressure die-casting machine with a 3.2 MN locking force. The same
surfaces of the casts were analyzed before and after the three-point bending test in order to determine the influence of the gas and
shrinkage porosity on the deformation behavior of the alloys. The obtained results revealed that the most dangerous for the cast elements is
the shrinkage porosity, especially stretched in the direction perpendicular to the that of the tensile stress action. Additionally, the influence
of deformation twins arise in the dendrites of the primary (Mg) solid solution and its interaction on the cracking process was described.
Keywords: AME-series magnesium alloy, High-pressure die-casting, Gas and shrinkage porosity, Microstructure, Deformation twins
1. Introduction
The high-pressure die-casting (hpdc) method is very attractive
for the multi-serial production of thin-walled components of
complicated shapes with dimensional precision. This technology
is widespread especially for aluminum alloys; however, it is also
used to produce magnesium elements. Although magnesium
alloys require protective atmospheres during all casting processes,
they offer very good castability and properties like excellent flow
characteristics. On the other hand, magnesium alloys require a
shorter time to fill a die than aluminum due to the low heat
content. The higher flow speed of magnesium (characterized by
low density) is also caused by higher the inertia of this metal vs.
aluminum [1-5]. For these reasons, magnesium alloys involve
different injection parameters than aluminum alloys. The main
hpdc parameters like the temperature of the liquid metal and die,
plunger speed in the first and second stage or intensification
pressure directly influencing the solidification conditions of
magnesium alloys, determine the level of microstructure
refinement of the element and also the level of its porosity.
Especially low intensification pressure and simultaneously high
plunger speed in the first and second stage result in a high amount
of gases occluded during casting. Additionally, it should be noted
that other factors (which determine the manner of filling the
cavity of the mold with liquid metal) also directly affect the
quality of the casting. Among these factors the size of the element
and die design (i.e. design of gating system, size and shape of in-
gate, number, size and shape of venting channels and also volume
of the pressing chamber) can be distinguished. The quality of the
castings, including the level of internal porosity, is the main
aspect of the line design for hpdc. Nevertheless, it is well known
A R C HI V ES o f F O UN D R Y E N G I N EE R I NG V ol u m e 2 3 , I ss u e 2 / 2 02 3 , 1 0 - 14 11
that hpdc parts quite often exhibit a high level of porosity. Both
gas and shrinkage porosity in hpdc aluminum and magnesium
alloys were intensively investigated in works [6-13]. In the
present paper, investigations of the three-point bending surfaces
allowed the role of individual defects during deformations of
AME-series magnesium alloys to be assessed.
2. Experimental procedures
The AME-series magnesium alloys described in works [5, 10,
14-15], with 5.0 wt% aluminum and 1.0-5.0 wt% rare earth
elements (in the form of cerium-rich mischmetal) were chosen for
this study. Experimental casts were produced using a regular type
cold-chamber high-pressure die-casting machine with a 3.2 MN
locking force in the same condition. Samples with a length of 30
mm, height 5 mm and width 3 mm were deformed in a three-point
bending experiment (at ambient temperature). Before the
deformation, one side of samples were polished in the standard
manner (non-etched). The same areas of the samples before and
after the three-point bending test were investigated, which is
presented schematically in Fig. 1. Observations were performed
on the tension surface by means of light microscopes (Olympus
GX41 and GX51 with differential interface contrast). The
percentage of deformation was determined from the deflection.
After the above described experiment, a standard metallographic
technique was also repeated and samples were etched in a 1%
solution of nitrous acid in ethyl alcohol for about 60 s.
Fig. 1. A scheme of experiment
3. Results and discussion
Fig. 2 presents a typical microstructure of the AME-series
magnesium alloys casts obtained by means of the cold-chamber
die casting machine. A visible dendritic structure with a bimodal
primary dendrite size distribution is typical for a high-pressure
die-cast of these alloys. The microstructure of the investigated
AME-series alloys consisted mainly of primary (Mg) solid
solution dendrites (impoverished in alloying elements compared
to the phase diagram) and the Al11RE3 intermetallic phase. The
microstructure and mechanical properties of hpdc AME-series
magnesium alloys were described in detail in previous works [5,
10, 14-15].
Fig. 3 shows the changes due to the tensile stresses on the
surface of the AME501 alloy. The initial surface of sample visible
in Fig. 2a was characterized by the presence of gas and shrinkage
porosity. After 6% deformation the main crack began to spread
from the shrinkage pore marked as “y” (purple rectangle) in Fig.
3. It should be noted that this defect in the material was extended
in the direction perpendicular to the direction of the tensile stress.
During deformation, the visible porosity "opened" and then the
crack expanded. Contrary to this, the shrinkage pore marked as
“x” in Fig. 3 (navy blue rectangle) was arranged in a direction
parallel to the direction of the tensile stress. After deformation, no
changes in the shape or size of this defect were observed.
Similarly, no significant changes in the size or shape of the gas
pores were observed on the surface of the sample (some of them
were marked with red ovals). Additionally, the appearance of slip
bands in the form of characteristic lines was observed on the
surface of the sample. Some of them are marked with green
arrows.
Fig. 2. Microstructure of cold-chamber die-cast AME501 (a) and
AME505 (b) magnesium alloys; light microscopy
Fig. 3. Same hpdc AME501 alloy sample surface before (a) and
after 6% deformation (b); light microscopy
Similar phenomena were observed after 8% deformation. Fig.
4 illustrates changes on the sample surface caused by tensile
stresses. The main cracks were developed from the shrinkage
porosity arranged perpendicular to the direction of the tensile
stresses. Also in this case, the distribution and shape of the gas
pores did not significant change (some of them were marked with
red circles). The presented micrographs (Fig. 3 and 4) show that
the main places of crack initiation in the casting are shrinkage
12 A R C HI V ES o f F O UN D R Y E N G I N EE R I NG V ol u m e 2 3 , I ss u e 2 / 2 02 3 , 1 0 - 14
pores, especially perpendicular to the direction of the tensile
stresses. Similar conclusions were formulated by Li at al. [13]
after in situ observation of the tensile deformation of the hpdc
AM60B magnesium alloy. On the other hand, an analogical hpdc
AE44 magnesium alloy was investigated by Lee et al. [12]. They
concluded that the fraction path preferentially went through the
regions of both highly localized gas clusters and shrinkage pores.
The results presented in this study unequivocally indicate that
shrinkage porosity, which is very often less visible than gas
porosity during standard cast investigations, is more dangerous
and may be the main sites of fracture development.
Fig. 4. Same hpdc AME501 alloy sample surface before (a) and
after 8% deformation (b); light microscopy
It should also be noted that after the 8% deformation on the
surface devoid of porosity, the presence of such visible cracks
was not observed. Fig. 5 shows the surface without porosity in its
initial state and after 8% deformation. The action of stresses
caused plastic deformation of the surface without the formation of
distinct cracking paths, which were formed on the surface
containing shrinkage porosity at the same degree of deformation
(Fig. 4).
Detailed analyses of the surfaces of the samples after
deformation also revealed the presence of microcracks caused by
stress concentration in the places of the intersection of
deformation twins. It is well known that magnesium and its alloys
have a hexagonal closed packed crystallographic structure, which
due to the lack of a sufficient number of independent slip systems,
undergoes strong twinning during deformation at room
temperature. Twins are also the main microstructure defects of
magnesium alloys. Fig. 6 shows the relief on the deformed surface
resulting from the clearly visible deformation twins with the
lenticular shape characteristic of magnesium. The appearance of
cracks was observed at the intersection of the deformation twins.
Some of the cracks thus formed on the tension surface are marked
with black arrows. Deformation twins are formed in dendrites of
primary (Mg) solid solution and can extend through the entire
crystals. Their intersection points are favorable places for the
nucleation of cracks due to the accumulation of stresses.
Figs. 7 and 8 show the microstructure of the alloys after 8%
deformation (metallographic specimens made on tension
surfaces). For the investigated alloys, the presence of deformation
twins inside the primary (Mg) solid solution crystals was
revealed; however, especially large intersections of twins were
visible in the case of the alloy characterized by large (Mg) solid
solution dendrites (Fig. 7).
Fig. 5. Same hpdc AME505 alloy sample surface before (a) and
after 8% deformation (b); light microscopy
The presented results also indicate that in case of the
investigated magnesium alloys, an intensive reduction in the size
of primary (Mg) solid solution dendrites is also an important
factor influencing the properties of hpdc elements. This factor can
be influenced by both the chemical composition of the alloy and
the parameters of the casting process.
Fig. 6. Hpdc AME501 alloys sample surface after 8% deformation
(a) and higher magnification of area marked by navy blue square
(b); light microscopy with differential interference contrast
A R C HI V ES o f F O UN D R Y E N G I N EE R I NG V ol u m e 2 3 , I ss u e 2 / 2 02 3 , 1 0 - 14 13
Fig. 7. Microstructure of hpdc AME503 alloys sample after 8%
deformation; light microscopy with differential interference contrast
Fig. 8. Microstructure of hpdc AME505 alloys sample after 8%
deformation; light microscopy with differential interference contrast
4. Conclusions
In the presented paper, high-pressure die-cast AME-series
magnesium alloys after deformation by the three-point bending
method was studied. The main conclusions drawn are as follows:
1. Shrinkage porosity is more dangerous for hpdc elements put
into commission than gas porosity and can be the main sites
of fracture development during operation under stresses.
2. Shrinkage pores stretched in a direction perpendicular to the
direction of the tensile stress action are especially dangerous
for cast elements.
3. For magnesium alloys, a strong reduction in the size of
crystals of the primary (Mg) solid solution is especially
important due to the formation (inside them) of deformation
twins, the interaction of which are privileged places for the
nucleation of cracks.
References
[1] Dahle, A.K., Sannes, S., John, D.H. & Westengen, H.
(2001). Formation of defect bands in high pressure die cast
magnesium alloys. Journal of Light Metals. 1(2), 99-103.
https://doi.org/10.1016/S1471-5317(01)00002-5.
[2] Vogel, M., Kraft, O., Dehm, G. & Arzt, E. (2001). Quasi-
crystalline grain-boundary phase in the magnesium die-cast
alloy ZA85. Scripta Materialia. 45(5), 517-524.
https://doi.org/10.1016/S1359-6462(01)01052-1.
[3] Unigovski, Y.B. & Butman, E.M. (1999). Surface
morphology of a die-cast Mg alloy. Applied Surface Science.
153, 47-52. https://doi.org/10.1016/S0169-4332(99)00337-
2.
[4] Tong, K.S., Hu, B.H., Niu, X.P. & Pinwill, I. (2002). Cavity
pressure measurements and process monitoring for
magnesium die casting of a thin-wall hand-phone
component to improve quality. Journal of Materials
Processing Technology. 127(2), 238-241.
https://doi.org/10.1016/S0924-0136(02)00149-8.
[5] Braszczyńska-Malik, K.N. (2017). Effect of high-pressure
die casting on structure and properties of Mg-5Al-0.4Mn-
xRE (x = 1, 3 and 5 wt.%) experimental alloys. Journal of
Alloys and Compounds. 694, 841-84.
https://doi.org/10.1016/j.jallcom.2016.10.033.
[6] Blondheim, D. Jr. & Monroe, A. (2022). Macro porosity
formation: A study in high pressure die casting. Internation
Journal of Metalcasting. 16, 330-341.
https://doi.org/10.1007/s40962-021-00602-x.
[7] Li, X., Xiong, S.M. & Guo, Z. (2016). Improved mechanical
properties in vacuum-assist high pressure die casting of
AZ91 alloy. Journal of Materials Processing Technology.
231, 1-7. https://doi.org/10.1016/j.jmatprotec.2015.12.005.
[8] Lordan, E., Zhang, Y., Dou, K., Jacot, A., Trileroglou, Ch.,
Blake, P. & Fan, Z. (2021). On the probabilistic nature of
high-pressure die casting. Materials Science Engineering: A.
817, 141391, 1-8.
https://doi.org/10.1016/j.msea.2021.141391.
[9] Ignaszak, Z. & Hajkowski, J. (2015). Contribution to the
identification of porosity type in AlSiCu high-pressure-die-
castings by experimental and Virtual Way. Archives of
Foundry Engineering. 15(1), 143-151. DOI: 10.1515/afe-
2015-0026.
[10] Braszczyńska-Malik, K. & Malik, M.A. (2020). Impact
strength of AE-type alloys high pressure die castings.
Archives of Foundry Engineering. 20(3), 5-8.
DOI:10.24425/afe.2020.133321.
[11] Balasundaram, A. & Gokhale, A.M. (2001). Quantitative
characterization of spatial arrangement of shrinkage and gas
(air) pores in cast magnesium alloys. Materials
Characterisation. 46, 419-426.
https://doi.org/10.1016/S1044-5803(01)00141-3.
[12] Lee, S.G., Patel, G.R., Gokhale, A.M., Sareeranganathan, A.
& Horstemeyer, M.F. (2006). Quantitative fractographic
analysis of variability in the tensile ductility of high-pressure
die-cast AE44 Mg-alloy. Materials Science Engineering A,
427(1-2), 255-262. DOI: 10.1016/j.msea.2006.04.108.
[13] Li, X., Xiong, S.M. & Guo, Z. (2015). On the porosity
induced by externally solidified crystals in high-pressure
14 A R C HI V ES o f F O UN D R Y E N G I N EE R I NG V ol u m e 2 3 , I ss u e 2 / 2 02 3 , 1 0 - 14
die-casting of AM60B alloy and its effect on crack initiation
and propagation. Materials Science and Engineering A. 633,
35-41. https://doi.org/10.1016/j.msea.2015.02.078.
[14] Braszczyńska-Malik, K.N. & Grzybowska, A. (2016).
Influence of phase composition on microstructure and
properties of Mg-5Al-0.4Mn-xRE (x = 0, 3 and 5 wt.%)
alloys, Materials Characterization. 115, 14-22.
https://doi.org/10.1016/j.matchar.2016.03.014.
[15] Braszczyńska-Malik, K.N. (2014). Some mechanical
properties of experimental Mg-Al-Mn-RE alloy. Archives of
Foundry Engineering. 14(1), 13-16. DOI: 10.2478/afe-2014-
0003.
