Process-Structure Relationships of Magnesium Alloys

144 Pages·2017·2.46 MB·English

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Arindam Banerjee, The University of Western Ontario
Supervisor: Dr. J.T.Wood, The University of Western Ontario
A thesis submitted in partial fulfillment of the requirements for the Master of Engineering
Science degree in Mechanical and Materials Engineering
© Arindam Banerjee 2013
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Process-Structure Relationships of Magnesium Alloys

by

Arindam Banerjee

Graduate Program in Engineering Science

Department of Mechanical & Materials Engineering

A thesis submitted in partial fulfillment
of the requirements for the degree of
Master of Engineering Science

The School of Graduate and Postdoctoral Studies
The University of Western Ontario
London, Ontario, Canada

© A. Banerjee 2013
Abstract
This research study characterizes the effects of solidification conditions on the resulting
microstructure of an AM60B magnesium alloy during the solidification cycle of the casting.
Seventeen control points are chosen from different sections of an instrument panel beam
casting and its centerline coordinates are located. These control points locations are then used
by Meridian Lightweight Technology Inc. to run a simulation in a MAGMASoft casting
software to obtain temperature-time specific data.
An exact analytical solution to the Stefan problem is used to compute the one dimensional
heat transfer for a section of the casting and to calculate its temperature distribution along the
thickness, during solidification. Next, the cooling rates are calculated with respect to its
distance from the mold wall of the casting.
Finally a generalized relationship between the cooling rate and the grain size of different
casting processes is developed in the form of a mathematical model. This model is used in
concurrence with the analytical solution to determine the extent of skin thickness formed
during solidification and the grain size distribution in the section of a high pressure die-cast
instrument panel beam.
Keywords

H.P.D.C, Sand Casting, MAGMASoft, Solidification parameters, Cooling rate, Grain size,
Skin thickness, G.D∝Rn, One dimensional heat transfer, Stefan problem, grain size

distribution.
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Acknowledgments
First, I would like to thank Meridian Lightweight Technologies Inc., for granting access to
their materials, equipment and facilities in order to run the MAGMASoft simulations that
made this research work possible.
Second, this research project was also partly funded by AUTO21, Network of Centres of
Excellence, an automotive research and development program focusing on issues relating to
the automobile in the 21st century. AUTO21, a member of the Networks of Centres of
Excellence of Canada program is funded by the Natural Sciences and Engineering Research
Council (NSERC), the Social Science and Humanities Research Council (SSHRC) and
multiple industry and government partners.
Third, I would like to gratefully acknowledge the significant contributions of and Mr. Dong
Yin for the initial microstructural characterization of the magnesium die casting and Mr.
Indraniel Basu in performing the gravity sand casting experiments and providing invaluable
information and data for this research study.
Fourth, I would also like to acknowledge the exceptional contributions and guidance
provided by Dr. Jon Weiler, during the course of this study. I would also like to thank my
fellow magnesium group members Mr. Mehdi Farookhinijad and Mr. Pouya Sharifi for their
support and suggestions, while performing my experiments and my fellow lab members and
friends, Mr. Jamal Jamaloudin, Mr. Ariful Islam, and Mr. Babak Kamalizonouzi for their
corporation and support during the period of writing my thesis.
Finally, I would like to express my deepest of gratitude for my advisor, Dr. Jeffrey. T. Wood
for his tremendous patience, guidance and generosity over the tenure of this research. His
constant support and encouragement, both in my personal and professional life has been
unparalleled and is the reason behind the successful completion of this research.

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Dedications
I would like to dedicate this thesis to my beloved parents who have loved me unconditionally
and stood by me through thick and thin in my life. Without their support and encouragement

this wouldn’t have been possible. I also dedicate my thesis to my aunt; Mrs. Shikha
Battacharya, whose battle with lung cancer gives me courage and hope, and has taught me
never to give up in life.
I would like to thank my elder sister and her family for the love that they have showered
upon me in my life and for being there for me whenever needed.

Finally, I would like to thank my best friends Dr. Vishesh Vikas and Major Madhusudan
Nair for their support and words of encouragement, and for lifting me up at times when my
spirit was low. I would also like to thank my dear friend Andrew Sharpe, for his generous
support and for helping me through the toughest of times.
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Table of Contents
Abstract ............................................................................................................................... ii
Acknowledgments.............................................................................................................. iii
Dedications ........................................................................................................................ iv
Table of Contents ................................................................................................................ v
List of Tables ................................................................................................................... viii
List of Figures .................................................................................................................... ix
List of Appendices ........................................................................................................... xiii
Chapter 1 ............................................................................................................................. 1
1 INTRODUCTION ......................................................................................................... 1
Chapter 2 ............................................................................................................................. 4
2 LITERATURE RIVIEW ................................................................................................ 4
2.1 Magnesium alloys and their properties ................................................................... 4
2.1.1 Effect of alloying elements on magnesium ................................................. 5
2.2 Designation of magnesium alloys ........................................................................... 7
2.3 Magnesium-Aluminum system of alloys ................................................................ 8
2.4 Die Cast magnesium alloys AM60B..................................................................... 10
2.4.1 High Pressure Die-Casting ........................................................................ 11
2.4.2 Solidification Parameters .......................................................................... 13
2.5 Process – Structure Relationships ......................................................................... 16
2.5.1 Niyama Criterion ...................................................................................... 16
2.5.2 Solidification Microstructure .................................................................... 18
2.6 MAGMASoft Simulation Software ........................................................................ 30
2.7 Prior Experiments ................................................................................................. 31
2.7.1 Gravity Sand Casting ................................................................................ 32
v
2.7.2 Microstructural characterization of AM60B magnesium high-pressure-die-
casting ....................................................................................................... 36
3 Modelling Techniques .................................................................................................. 39
3.1.1 Material ..................................................................................................... 39
3.1.2 Specimen Mapping ................................................................................... 40
3.1.3 Specimen Characterization ....................................................................... 40
3.1.4 Control point selection .............................................................................. 42
3.2 Heat Transfer model ............................................................................................. 56
3.2.1 The Classical Stefan Problem: .................................................................. 56
3.2.2 One dimensional analytical solution for the solidification of liquid metal in
a finite slab: ............................................................................................... 58
4 RESULTS AND DISCUSSIONS ................................................................................ 72
4.1 Analysis of process variables ................................................................................ 72
4.1.1 Determination of the cooling rate “R” ...................................................... 72
4.1.2 Analysis of cooling rate “R” ..................................................................... 74
4.2 Determination of the constant surface temperature T for the exact analytical
0
solution to the one dimensional heat transfer problem: ........................................ 77
4.3 Determination of the Skin thickness of a section in the casting ........................... 82
4.4 Average skin and average core grain size measurements: .................................... 94
4.5 Summary ............................................................................................................. 103
5 CONCLUSION AND FUTURE WORK .................................................................. 104
5.1 Control point selection and analysis of MAGMASoft simulation data: ............. 104
5.2 Analysis of process variables .............................................................................. 104
5.3 Heat transfer model ............................................................................................. 105
5.4 Predicting the grain size distribution in a casting ............................................... 106
5.5 Future work ......................................................................................................... 107
6 REFERENCES ........................................................................................................... 108
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APPENDICES ................................................................................................................ 112
Curriculum Vitae ............................................................................................................ 130
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List of Tables
Table 2-1 Code letters used for magnesium alloys [2] ............................................................. 7
Table 2-2 Magnesium alloys, their properties and elongation [1, 5] ...................................... 10
Table 2-3 The chemical composition of AM60B in wt%. [5] ................................................ 11
Table 2-4 Grain diameter (G.D) in micro meters, gradient (G) in °C/mm and cooling rate (R)
in °C/s values at the specified distances from the tip of the casting………………………...34
Table 3-1 Data obtained for the sample positions across the casting with their average core
and skin grain size values in micro meters (µm). [6] .............................................................. 54
Table 4-1 Numerical cooling rate values (Rnum) for the 17 locations throughout the casting
along with their average core grain diameter values .............................................................. 73
Table 4-2 The 17 locations in the casting along with their calculated skin thickness values in
micro meters (µm) .................................................................................................................. 92
Table 4-3 Average skin and average core grain diameter values (a) calculated using Equation
4.2 and (b) measured experimentally by J.P.Weiler [27] ....................................................... 99

viii
List of Figures
Figure 2-1 Diagram for the Mg-Al phase system [3] ............................................................... 8
Figure 2-2 Depicts a micrograph of an Mg-15 wt% Al alloy with fully developed dendrites in
a permanent mold casting. (A) The dendrites have a six fold symmetric shape, (B) is the
secondary eutectic phase Mg17Al12 shown as the white phase and (C) the aluminum rich ... 9
Figure 2-3 (a) Image of a cold chamber H.P.D.C in production [47], (b) Schematic of a cold
chamber H.P.D.C. [48] ........................................................................................................... 12
Figure 2-4 Schematic illustration of the relationship between shrinkage porosity and the
Niyama criterion [8] ................................................................................................................ 17
Figure 2-5 Structural changes in the equilibrium solidification of Cu-40% Ni alloy [37] ..... 19
Figure 2-6 The structural changes in a Cu-40% Ni alloy during non-equilibrium cooling [37]
................................................................................................................................................. 21
Figure 2-7 Diagram for the Mg-Al phase system [3] ............................................................. 23
Figure 2-8 Illustrates the effect on the microstructures of Mg-Al alloys with increasing Al
content. [3] .............................................................................................................................. 24
Figure 2-9 Shows two images of hypoeutectic Mg-Al alloy. (a) Fully divorced and (b)
partially divorced eutectic morphologies in [3] ...................................................................... 27
Figure 2-10 The influence of Aluminum content, Zinc content and cooling rate on the
eutectic morphology of cast hypo-eutectic Mg-Al alloys [3] ................................................. 28
Figure 2-11 Two types of precipitation reactions in AZ91E sand cast alloy (a) Continuous
and (b) Discontinuous. [3] ...................................................................................................... 29
Figure 2-12 Illustration of the sand cast sample for the alloy of AM60B with the four regions
of different heights at 4, 8 and 12mm [6] ............................................................................... 33
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