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Microstructure of AM50 die casting magnesium alloy

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Andrzej Kielbus at Silesian University of Technology
Andrzej Kielbus
Tomasz Rzychoń at Silesian University of Technology
Tomasz Rzychoń
Cibis R
Cibis R
Cibis R
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Abstract

Purpose: AM50 magnesium alloy allows high-energy absorption and elongation at high strength and has goodcastability. It contains aluminum and manganese. Typically, it is used in automotive industry for steering wheels,dashboards and seat frames. The aim of this paper is to present the results of investigations on the microstructureof the AM50 magnesium alloy in an ingot condition and after hot chamber die casting.Design/methodology/approach: Die casting was carried out on 280 tone locking force hot-chamber die castingmachine. For the microstructure observation, a Olympus GX+70 metallographic microscope and a HITACHIS-3400N scanning electron microscope with a Thermo Noran EDS spectrometer equipped with SYSTEM SIXwere used.Findings: Based on the investigation carried out it was found that the AM50 magnesium alloy in as ingotcondition is characterized by a solid solution structure a with partially divorced eutectic (a + Mg17Al12) andprecipitates of Mn5Al8 phase. After hot chamber die casting is characterized by a solid solution structure awith fully divorced eutectic a + Mg17Al12. Moreover, the occurrence of Mn5Al8 phase and some shrinkageporosity has been proved.Research limitations/implications: Future researches should contain investigations of the influence of the diecasting process parameters on the microstructure and mechanical properties of AM50 magnesium.Practical implications: AM50 magnesium alloy can be cast with cold- and hot-chamber die casting machine.Results of investigation may be useful for preparing die casting technology of this alloy.Originality/value: The results of the researches make up a basis for the investigations of new magnesium alloysfor hot chamber die casting with addition of RE elements designed to exploitation in temperature to 175°C.
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© Copyright by International OCSCO World Press. All rights reserved. 2006
VOLUME 18
ISSUE 1-2
September–October
2006
Short paper 135
of Achievements in Materials
and Manufacturing Engineering
of Achievements in Materials
and Manufacturing Engineering
Microstructure of AM50 die casting
magnesium alloy
A. Kiełbus a,*, T. Rzychoń a, R. Cibis b
a Silesian University of Technology,
ul. Krasińskiego 8, 40-019 Katowice, Poland
b NTP CIBIS Sp. z o.o.
ul. Szkolna 15, 47-225 Kędzierzyn-Koźle, Poland
* Corresponding author: E-mail address: andrzej.kielbus@polsl.pl
Received 15.03.2006; accepted in revised form 30.04.2006
Materials

Purpose: AM50 magnesium alloy allows high-energy absorption and elongation at high strength and has good
castability. It contains aluminum and manganese. Typically, it is used in automotive industry for steering wheels,
dashboards and seat frames. The aim of this paper is to present the results of investigations on the microstructure
of the AM50 magnesium alloy in an ingot condition and after hot chamber die casting.
Design/methodology/approach: Die casting was carried out on 280 tone locking force hot-chamber die casting
machine. For the microstructure observation, a Olympus GX+70 metallographic microscope and a HITACHI
S-3400N scanning electron microscope with a Thermo Noran EDS spectrometer equipped with SYSTEM SIX
were used.
Findings: Based on the investigation carried out it was found that the AM50 magnesium alloy in as ingot
condition is characterized by a solid solution structure a with partially divorced eutectic (a + Mg17Al12) and
precipitates of Mn5Al8 phase. After hot chamber die casting is characterized by a solid solution structure a
with fully divorced eutectic a + Mg17Al12. Moreover, the occurrence of Mn5Al8 phase and some shrinkage
porosity has been proved.
Research limitations/implications: Future researches should contain investigations of the influence of the die
casting process parameters on the microstructure and mechanical properties of AM50 magnesium.
Practical implications: AM50 magnesium alloy can be cast with cold- and hot-chamber die casting machine.
Results of investigation may be useful for preparing die casting technology of this alloy.
Originality/value: The results of the researches make up a basis for the investigations of new magnesium alloys
for hot chamber die casting with addition of RE elements designed to exploitation in temperature to 175°C.
Keywords: Metallic alloys; Methodology of research; Electron microscopy; AM50 magnesium alloy
1. Introduction
Magnesium and its alloys are often used for many technical
applications, including the aerospace, automobile and motor
vehicle, metallurgical, chemical and electronical industries [1,2].
Magnesium alloys are not only very attractive materials for
producing very lightweight automobile components but offer the
designer many unique properties not found in other alloys [3].
They are characterised by low density: ~1,8 g/cm3, tensile
strength Rm=300÷350 MPa, elongation A5=20% and hardness
~100HB [4].
Most commercial magnesium alloys (AM50, AZ91) are based
on the magnesium-aluminium system. They contain aluminium,
manganese (AM50, AM60), and zinc, which allow obtaining
suitable properties. Aluminium enhances both tensile strength and

Short paper
136
Journal of Achievements in Materials and Manufacturing Engineering
A. Kiełbus, T. Rzychoń, R. Cibis
Volume 18 Issue 1-2 September–October 2006
hardness, and improves casting properties of an alloy. The best
ratio of mechanical to plastic properties is obtained with a 6% Al
content. Manganese does not cause any increase of tensile
strength, however, it does slightly increase the yield point. It also
brings about an increase of resistance to the action of sea water.
The quantity of manganese in magnesium alloys is limited by its
relatively low solubility in magnesium. Manganese content in
alloys with an Al addition does not exceed 0.3% and 1.5% in
alloys without Al addition. An addition of zinc in combination
with Al aims at improving tensile strength at a room temperature,
however 1% of Zn with a 7÷10% Al content in an alloy enhances
hot cracking [4, 5].
The castability of magnesium alloys of the AM-series using
the die-casting process is excellent. The good flow properties
allow the casting of thin-walled parts and costs are reduces due to
the fact that less material is needed. High pressure die casting is
the dominant process for the mass production of magnesium
components [6]
Hot chamber die casting means that the molten alloy is
transported directly from the furnace to the die via heated casting
equipment (transfer tube, gooseneck, nozzle). The melting and
gasting furnaces are directly adjoining parts of the hot chamber
die casting machine (Fig.1) [4,7,8].
Fig. 1. Schematic of the hot chamber die-casting process [7]
The maximum solubility of aluminium in magnesium at an
eutectic temperature (437°C) is 14%, whereas an eutectic mixture (D
+ Mg17Al12 intermetallic phase) occurs at ca. 33% Al content. The
content of aluminium in all industrial alloys of the AM series is not
higher than the boundary solubility of Al in Mg. The equilibrium
structure of these alloys is characterised by 100% presence of a solid
solution, whereas the unbalanced structure, additionally metastable
in casting alloys, shows the presence of an eutectic already at a 2%
Al content [9,10]. During die-casting solidification, the following
sequence occurs. Primary solid solution a fine grains are nucleated at
liquidus temperature. As the temperature is lowered, the time for
diffusion is too short to allow equilibrium solidification. This caused
a core structure, with an increasing concentration of aluminium
towards grain boundaries. Next, along the grain boundaries, a
divorced eutectic is formed [11-14].
2. Description of the work methodology,
and material for research
2.1. Material for research
The material for the research was a AM50 alloy in ingot
condition and after hot chamber die casting. The chemical
composition of the AM50 alloy is provided in Table 1.
Table 1.
Chemical composition of the AM50 alloy in wt.-%
Al Mn Zn Si Ni Cu Fe Be Mg
4.9 0.45 0.2 <0.05 <0.01 <0.008 <0.004 0.001 balance
2.2. Research methodology
Die casting was carried out on 280 tone locking force hot-
chamber die casting machine. Table 2 lists the process parameters
used for this work. Casting was undertaken at the NTP firm in
KĊdzierzyn-KoĨle, Poland.
Table 2.
Hot chamber die-casting process parameters
Parameter Value
Piston speed
Pressure
Casting temperature
Die temperature
2.5 m/s
100 bar
680 °C
170 °C
The samples for structural examination were ground,
mechanically polished and finally etched in a reagent containing
2g of oxalic acid and 100 ml of H2O.
For the microstructure observation, a Olympus GX+70
metallographic microscope and a HITACHI S-3400N scanning
electron microscope with a Thermo Noran EDS spectrometer
equipped with SYSTEM SIX were used.
3. Description of achieved results of own
researches
3.1. Microstructure of the AM50 ingot
The AM50 magnesium alloy in as ingot condition is
characterized by a solid solution structure D with partially
divorced eutectic D + Mg17Al12 and precipitates of Mn5Al8 phase
(Fig.2,3). The partially divorced eutectic is characterized by
“islands” of the D-Mg solid solution occur inside Mg17Al12 phase
precipitations (Fig.3) [9,15]. The divorced eutectic areas are
placed inside or on the grain boundaries.
The analysis of the chemical composition have shown
a change of the aluminum content from 5.69 at.-% in the solid

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137
Materials
Microstructure of AM50 die casting magnesium alloy
solution grain core, through 13.85 at.-% in an area located near
divorced eutectic regions, to 32.29 at.-% in the Mg17Al12 phase.
Fig. 2. LM microstructure of AM50 ingot
Fig. 3. SEM microstructure of AM50 ingot. Partially divorced
eutectic (D+Mg17Al12)
The precipitates containing manganese and aluminium
(Mn5Al8 phase) are characterised by a polygonal or an irregular
shape, they very often take the form of spines (Fig.4,5).
The microanalysis results of the chemical composition of
individual phases in AM50 alloy are shown in Table 3.
Table 3.
Chemical composition of identified in AM50 alloy phases.
Element [at.- %]
Phase Mg Al Si Mn
D 95,80 4,20 - -
Mg17Al12 64,88 34,59 0,53 -
Mn5Al8 24,75 46,25 0,71 28,28
The mapping of Mg, Al, Mn in the areas of partially divorced
eutectic and Mn-Al precipitates surrounded by the D solid
solution visible in the SE image can be seen in Figure 5.
Fig. 4. Mn5Al8 precipitates in ingot of AM50 alloy
SE Mg
Al Mn
Fig.5. The SE image and the distribution of Mg, Al, Mn in
microareas of AM50 alloy by means of SEM + EDS analysis
3.2. Microstructure of the AM50 alloy after
hot chamber die casting
The AM50 magnesium alloy after hot chamber die casting is
characterized by a solid solution structure D with fully divorced
eutectic D + Mg17Al12. The solid solution is characterized by very
small grains. They have almost spherical shape divided by fully
divorced eutectic regions (Fig.6). The fully divorced eutectic is
where the two eutectic phases are completely separate in the
microstructure [1]. In AM50 alloy after die casting precipitations
of Mg17Al12 phase are surrounded by D-Mg solid solution (Fig.7).


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138 READING DIRECT: www.journalamme.org
Journal of Achievements in Materials and Manufacturing Engineering Volume 18 Issue 1-2 September–October 2006
Fig. 6. LM picture of AM50 alloy after hot chamber die casting
The particles of Mg17Al12 phase (grey precipitates on Fig.7)
are between the D grains. Moreover, the occurrence of Mn5Al8
phase (white precipitates on Fig.7) and some shrinkage porosity
has been proved. The shrinkage porosity occurs between grains,
where during solidification, the last liquid remains. The Mn5Al8
precipitates are distributed inside and on the D solid solution grain
boundaries. The particles of Mg17Al12 phase are always much
larger than Mn5Al8 precipitates and are located only on grain
boundaries.
Fig. 7. SEM picture of AM50 alloy after hot chamber die casting
4. Conclusions
Based on the research results obtained, it has been found that:
1. The AM50 magnesium alloy in as ingot condition is
characterized by a solid solution structure D with partially
divorced eutectic (D + Mg17Al12) and precipitates of Mn5Al8
phase. The divorced eutectic areas and Mn5Al8 precipitates
are placed both inside and on the grain boundaries.
2. The AM50 magnesium alloy after hot chamber die casting is
characterized by a solid solution structure D with fully
divorced eutectic D + Mg17Al12. Moreover, the occurrence of
Mn5Al8 phase and some shrinkage porosity has been proved.
The particles of Mg17Al12 phase are always much larger than
Mn5Al8 precipitates and are located only on grain boundaries.
The Mn5Al8 precipitates are distributed both inside and on the
D solid solution grain boundaries.
Acknowledgements
This work was supported by the Polish Ministry of Education
and Science under the Grant No. 6 T08 2003 C/06325.
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Book
  • Jan 2021
This collection from the 12th International Conference on Magnesium Alloys and Their Applications (Mg 2021)—the longest running conference dedicated to the development of magnesium alloys—covers the breadth of magnesium research and development, from primary production to applications to end-of-life management. Authors from academia, government, and industry discuss new developments in magnesium alloys and share valuable insights. Topics in this volume include but are not limited to the following: • Primary production • Alloy development • Solidification and casting processes • Forming and thermo-mechanical processing • Other manufacturing process development (including joining and additive manufacturing) Corrosion and protection • Modeling and simulation • Structural, functional, biomedical, and energy applications • Advanced characterization and fundamental theories • Recycling and environmental issues
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RESEARCH STRATEGY FOR AM60 MAGNESIUM STEERING WHEEL
Article
Full-text available
Driven largely by the never ending quest for weight reduction to decrease fuel consumption and emission, the automotive industry in Korea is predicting significant growth in the use of magnesium alloys. About a half decade ago, the price of magnesium alloys was more than twice that of aluminum alloys on a weight basis in Korea. Currently, magnesium alloys cost about one and a half times that of aluminum alloys on a weight basis and thus the price of magnesium alloys is the same as or lower than that of aluminum alloys on a per volume basis. However, if considering performance of magnesium components (not specific mechanical properties of them) and recycling aspect of magnesium alloys, it is required to realize niche applications of magnesium alloys which meet the cost requirement on performance basis and/or offer more than weight reduction. There are many other factors that make magnesium a good choice: component consolidation, improved safety for driver and passengers, and improved noise vibration and harshness (NVH), to name a few. As one of these efforts to adopt magnesium alloys in automotive component, this paper describes the research strategy for high pressure die casting of AM60 magnesium steering wheel with emphasis on cost driving factors and necessities for cost reduction, explaining why AM60 magnesium alloy is chosen with design and die casting process optimization.
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Microstructural characterization of as-cast Co–B alloys
Article
Full-text available
  • Jan 2006
  • MATER CHARACT
This work presents results of microstructural characterization of as-cast Co–B alloys. Samples of different compositions were prepared by arc melting Co (min. 99.97%) and B (min. 99.5%) under argon atmosphere in a water-cooled copper crucible with non-consumable tungsten electrode and titanium getter. All samples were characterized by scanning electron microscopy (SEM) in back-scattered electron (BSE) mode, X-ray diffraction (XRD) and wavelength dispersive spectrometry (WDS). A good agreement is observed between the obtained microstructures and those expected by the currently accepted Co–B phase diagram.
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On the influence of process variables on the thermal conditions and properties of high pressure die-cast magnesium alloys
Article
  • Jan 1998
  • J MATER PROCESS TECH
The influence of pressure and velocity in high-pressure magnesium die casting on the thermal conditions and on the casting properties is studied. Specimens with the shape of a tensile test plate with a thickness of 12 mm and a length of 295 mm were cast using the alloys AM20HP, AM50HP, AS41, AE42, AZ91HP. Two gate velocities of the liquid metal 40 and 80 m s−1, were used for die filling and two pressures, 30 and 70 MPa, were applied to the metal during the solidification phase. Other processing conditions were kept constant. Temperature measurements at different positions in the die and at the metal/die interface were made during the operating cycle. Temperature distributions obtained from a simple two-dimensional numerical heat-flow model were found to agree generally with the measured values. The temperature distribution did not change significantly when varying the pressure and/or velocity. Calculated cooling curves for the alloy AZ91 indicate that the specimens solidified completely within 4.5 s. The measured bulk density of the casting was found to increase with velocity and/or pressure: Consequently, the tensile strength also increased. The density distribution along the specimen was examined, the density being found to decrease as the distance from the gate increases. The surface hardness for each alloy was generally similar at all positions in the test specimen and did not vary much with velocity or pressure. However, the surface hardness was always higher than the hardness inside the specimen, being due to the fine structure at the surface and the coarse structure inside the specimen resulting from different solidification times.
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Magnesium Alloys and Their Applications
Article
  • Jan 1994
  • MATER SCI TECH-LOND
The methods for producing magnesium are briefly described and its alloying behaviour is discussed with particular reference to the roles of individual alloying elements. Cast and wrought alloys are considered separately and particular attention is paid to microstructure–property relationships and corrosion behaviour. Alloys produced via new processing techniques are treated separately and mention is made of metal matrix composites and the use of rapid solidification to produce amorphous and other novel materials. Finally, current and future trends in the use of magnesium alloys are considered together with examples of recent applications.MST/2023
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Influence of Process Parameters on the Microstructure and Mechanical Properties of Magnesium Die Castings
Chapter
  • Apr 2005
The past decade has witnessed significant growth in the utilisation of magnesium alloys. This has been primarily driven by the concerted efforts of automotive manufacturers to reduce the weight of vehicles in order to meet stringent fuel economy standards. The most efficient method for the manufacture of magnesium parts is via high pressure die casting. It is a productive and cost-effective process for light metals that is capable of high surface quality and dimensional accuracy. Although die casting is an established process, the relationships that exist between the processing, microstructure and properties are inadequately understood. The present study identified process-microstructure-property relationships for three Mg-Al alloys (AZ91D, AM60B and AS21) through the development of a diverse range of microstructures via the manipulation of four processing variables, and the correlation of these resultant structures to the tensile properties. The processing variables investigated were gate velocity (Vgate), accumulator pressure (Pacc), die (Tdie) and casting temperatures (Tcast).
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Characterizing solidification by non-equilibrium thermal analysis
Article
  • Jan 2003
Research and development of Mg alloys depends on the metallurgist's understanding of and ability to control microstructures. In this research, equipment for non-equilibrium thermal analysis of Mg alloys has been created and tested using AZ91E. AZ91E was selected as a test case since it is among the most widely used and understood Mg alloys. Results from thermal analysis demonstrate the equipment's sensitivity and the technique's ability to detect many of the salient features of solidification. These results correlate well with existing data. Relative to other research equipment required for studying microstructural evolutions this non-equilibrium thermal analysis equipment is rapid, inexpensive, easy to maintain and operate, and surprisingly sensitive. Therefore, it should prove to be a valuable research tool for those people interested in Mg alloy development. Introduction The automotive industry is investigating the potential for replacing some parts historically manufactured from cast Al alloys with cast Mg alloys. As a result the reduction in vehicle weight will decrease environmental impact of vehicles while continuing to satisfy the competitive needs among brands. Unfortunately, few Mg alloys currently exist which meet the demands. Mg alloy development is in critical need if Mg is to successfully replace Al castings. However, development of new Mg alloys exhibiting properties competitive with Al alloys has been slow and academic participation limited. Knowledge of solidification and resultant microstructure is fundamental to the alloy development process. It is from the initial cast microstructure that all pursuant material properties must be developed. Controlling the as-solidified microstructure often affords the alloy designer the greatest influence over final alloy performance and should therefore be considered paramount to the intelligent, rapid design of improved alloys.
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Development of the as-cast microstructure in magnesium-aluminium alloys
Article
  • Feb 2001
  • J Light Met
This paper presents an overview of several projects undertaken at CAST to increase our understanding of the solidification characteristics of Mg–Al alloys. With the increased use of magnesium alloys, and with casting dominating as a production route, there is a need for a more comprehensive understanding of the mechanisms of solidification and defect formation to allow further optimisation of alloys and casting processes. The paper starts with considering the formation of the primary magnesium dendrites and the means for grain refinement of magnesium–aluminium alloys. The Mg–Al system is then shown to display a range of eutectic morphologies for increasing aluminium content, ranging from a divorced structure, through several intermediate structures, to a fully lamellar structure at the eutectic composition. The eutectic also influences discontinuous precipitation which occurs in the aluminium-rich regions of the magnesium phase. The paper concludes with a section on porosity formation as a function of aluminium content and an outline of the mechanism responsible for the formation of banded defects in magnesium alloys, particularly in products made in pressure assisted casting processes.
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The Role of the Magnesium Industry in Protecting the Environment
Article
  • Nov 2001
  • J MATER PROCESS TECH
This paper aims at reviewing and evaluating the prospects of magnesium use and applications in the transportation industry that can significantly contribute to the environmental conservation. This relates to the basic characteristics of magnesium being 35% lighter than aluminum, which is used as structural material for vehicles and aerospace applications. The lightness of structural magnesium components results in reducing the weight of transportation means and hence reducing the fuel consumption and CO2 emissions.In particular, this paper will introduce the current and future applications of magnesium in the transportation industry with special attention to the needs of alloy developments and advancement in production technologies.
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Research for a “new age of magnesium” in the automotive industry
Article
  • Nov 2001
  • J MATER PROCESS TECH
Motivation for (more) magnesium in the automotive industry — research strategies for bringing about a “new age of magnesium” by means of the vehicle modules drive train, interior, body and chassis — use of realised and potential future magnesium components, differentiated according to the time frame and conceivable likelihood of realisation — R&D activities for the implementation of the predicted use of magnesium illustrated by example components and projects.
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