Process and Tool Design for the High Integrity Die Casting of Aluminum And

Home , Chill (casting), Die casting, End mill

Process and Tool Design for the High Integrity Die Casting of Aluminum and

Magnesium Alloys

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in

the Graduate School of The Ohio State University

Varun Nandakumar

Graduate Program in Industrial and Systems Engineering

The Ohio State University

2014

Master's Examination Committee:

Dr. Jerald Brevick, Advisor

Dr. Alan Luo

Varun Nandakumar

2014

Abstract

The push for higher efficiency and lower emissions on present day automobiles have created a greater need for light alloys such as Aluminum and Magnesium to replace the customary steel parts in an automobile. A lot of these replaced parts are in structural load bearing components on a vehicle. This usually meant that processes such as the traditional High Pressure Die Casting would not be able to claim a stake in it due to its innate air and Hydrogen entrapment issues. But, with an overall higher maintenance of quality in the traditional process, it is possible to create higher integrity parts that are usable in structural applications. This thesis describes and tests some methods to improve the traditional High Pressure Die Casting process to enable it to produce higher integrity casts for Aluminum and Magnesium alloys. Methods to achieve a higher quality melt are researched and a rotary degasser is used for two experimental trials of similar Aluminum alloys. Similarly, methods to integrate vacuum into the existing setup are studied and the tooling part of the vacuum assist system with chill blocks is fully designed. The design of the chill blocks is carried out from the ground up using MAGMA and ANSYS simulation tools which were available at the Integrated Systems Engineering Lab. A designed experiment is completed to understand the effects of changing parameters on the design.

Finally, the optimum design is completed on 3D CAD software and then manufactured in house.

To Charuchechi

iii

Acknowledgments

To Dr. Brevick - my advisor, the best teacher I’ve had and possibly the most patient man

I will ever meet. I can’t even begin to describe how much I’ve enjoyed working under you. I have never come out of a discussion – be it in the hallway, on a drive or at your office, without learning something new. You have a knack of making any topic interesting and I’m going to miss popping into your room with the silliest question knowing that you will do your best to break it down for me. Thank you sir for everything.

To Martin Doyle from Gibbs Die Casting at Henderson, KY- Thank you for helping us understand vacuum die casting, giving us a tour of your facility and graciously providing us with tool bits to make the sealant cut outs.

To Charuchechi - Without your generosity I wouldn’t be here. Thank you for a second chance at life. To My Parents – It is a tremendous joy knowing that the two of you are constantly behind me. Thank you for always being there. To Deepta – Thanks for all the encouragement, the talks and help when I thought I couldn’t make it. To my Sister, friends and so many others who are in one way or the other responsible for my success, I am deeply humbled by all of your kindness.

Last but by no means least, to my colleague William Tullos - I’ve had a great time working with you the past year and a half, thank you for always responding to my email within minutes of receiving them and your constant backing.

Vita

March 19, 1987 ...... Born – Bangalore, India

2008 ...... B.E. Mechanical Engineering, Visvesvaraya

Technological University

Fall 2010 to present ...... M.S. Industrial and Systems Engineering,

The Ohio State University

May 2013 to present ...... Graduate Research Associate, Department

of Industrial and Systems Engineering, The

Ohio State University

Fields of Study

Major Field: Industrial and Systems Engineering

Table of Contents

Abstract ...... ii

Acknowledgments...... iv

Vita ...... v

Table of Contents ...... vi

List of Tables ...... xi

List of Figures ...... xii

Chapter 1: Introduction ...... 1

The High Pressure Die Casting Process ...... 1

Advantages and Limitations ...... 4

Types of High Pressure Die Casting ...... 5

The Cold Chamber Process ...... 6

The Hot Chamber Process ...... 7

High Integrity Processes...... 8

Applications of High Pressure Die Casting ...... 9

Automotive Applications ...... 10

Structural Automotive Applications ...... 13 vi

Light alloys used in Structural Die Casting ...... 14

Aluminum alloys ...... 14

Magnesium alloys ...... 15

Process consideration for structural alloys in the automotive industry ...... 16

Research Summary ...... 17

Objectives ...... 18

Approach ...... 18

Chapter 2: Dealing with porosity problems in HPDC ...... 20

Gas Porosity ...... 21

Hydrogen ...... 21

Air Entrapment ...... 21

Oil or water from Lubrication spray ...... 21

Solidification shrinkage...... 22

Porosity reduction and obtaining a high integrity cast ...... 22

Melt quality Improvements ...... 23

Degassing ...... 23

Passive Degassing...... 24

Liquid argon shield ...... 25

Gaseous argon shield ...... 25

vii

Chemical tablets ...... 26

Rotary Impeller Degassing ...... 27

Flux Induced Rotary degassing ...... 28

Choosing a Purge Gas...... 29

Additional and often overlooked Benefit of Degassing ...... 30

Reduced Pressure Test ...... 30

Improved process control ...... 31

Metallurgical Modification ...... 33

Vacuum Die Casting ...... 34

Advantages of Vacuum Die Casting ...... 37

Vacural Process ...... 38

High Q Process ...... 39

Valve based and valve-less systems ...... 41

Valve based ...... 41

Valve-less (Chill Block) ...... 41

Comparison ...... 44

Chapter 3: Melt Quality Improvement at OSU ...... 47

Equipment ...... 47

Furnace and Die casting machine ...... 47

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Degassing...... 48

Reduced Pressure & Archimedes density measurement ...... 49

Experiment and obtaining best operating conditions ...... 50

Operating Conditions ...... 51

Experiment...... 51

Results ...... 52

Chapter 4: Vacuum Capable Die Design at OSU ...... 55

Chill Block Design ...... 55

Existing Die dimensional study ...... 55

Chill Surface Profile geometry ...... 58

Gap thickness, material and Heat Transfer coefficients ...... 59

Magma Simulation ...... 61

Designed Experiments ...... 63

ANSYS® Simulations & Results of the Designed Experiment ...... 64

Analysis of Variance ...... 67

Die design ...... 71

Venting cut outs ...... 71

Sealant cut out ...... 72

Chapter 5: Conclusion and Future Work ...... 75

Future Work ...... 76

References ...... 77

Appendix A: Palmers degassing Chart ...... 81

Appendix B: A 380 Degassing ...... 82

Appendix C: Alloy B Degassing...... 83

Appendix D: Properties of H-13 Tool and Copper used in Ansys Simulaitons ...... 84

List of Tables

Table 1, Some Shot sleeve diameters and their corresponding critical slow shot speeds

(Anderson, 2004) ...... 32

Table 2, Summary of the Pros and Cons of a valve vs. a valve less design (the 3D valve is not considered) ...... 46

Table 3, Design for the 23 experiment ...... 64

Table 5, Results of the experiment (fill time and distances) highlighted values go beyond available length and can’t be used ...... 66

Table 6, ANOVA results...... 67

List of Figures

Figure 1. Relationship of various casting processes to pressure and velocity (Anderson,

2004) ...... 2

Figure 2. Steps in a HPDC process (NADCAdesign)...... 3

Figure 3, Advantages of High Pressure Die Casting (Twarog, 2011) ...... 5

Figure 4. Cold Chamber Process (Groover, 2007) ...... 6

Figure 5. The Hot Chamber Process (Groover, 2007) ...... 8

Figure 6, Graph howing the relative cost of a component manufactured by different casting processes (Kalpakjian, 2009) ...... 10

Figure 7, Magnesium Lincoln MKT lift gate (Die Casting Engineer, 2010) ...... 12

Figure 8, Upper Fire wall (European Aluminium Association) ...... 12

Figure 9, Audi Space frame, Showing Structural Die Castings in Red (Hartlieb, 2013) . 14

Figure 10, Fe rich needle like phase that causes low elongation and flow capabilities

(Hartlieb, 2013) ...... 15

Figure 11, Major sources of defects in a HPDC product (Twarog, 2011) ...... 20

Figure 12, Solubility of Hydrogen drops drastically as the aluminum solidifies, not so much a problem with Magnesium (Campbell, 2011) ...... 24

Figure 13, Protection of the surface of the melt by the addition of liquid Argon ...... 25

xii

Figure 14, Surface protection by delivery of argon gas parallel to the surface of the melt

...... 26

Figure 15, Efficiencies of three different techniques to degassing ...... 28

Figure 16, Coupling flux injection with rotor degassing (Neff, 2002) ...... 29

Figure 17, For a given size of a bubble, (a) larger oxides are easily floated out while (b) smaller ones follow the streamlines of the rising bubble and so do not collect the bubble

(Campbell, 2011) ...... 30

Figure 18 Vss- Critial Slow Shot Velocities (Anderson, 2004) ...... 32

Figure 19, Illustration showing Atomized flow that is typical in a conventional HPDC

(Vinarcik, 2002 ) ...... 33

Figure 20, Left - Close to eutectic with a small mushy zone, Right - bigger mushy zone

(NADCAdesign) ...... 34

Figure 21 Conventional Vacuum Die Casting Process (PFIEFER Vacuum, 2006) ...... 35

Figure 22, Difference time in vacuum application for Vacural vs. others (Jorstad, 2008 )

...... 37

Figure 23, Vacuural Process schematic (Jorstad, 2008 ) ...... 39

Figure 24, Residual gas content a seen on X-ray (Jorstad, 2008 ) ...... 40

Figure 25, Valve and Valve less technology for creating vacuum inside the die cavity .. 43

Figure 26, Erroneous Cavity vacuum reading in case of chill blocks (Bagnoud & Bigger,

2008) ...... 44

Figure 27, Comparison between Chill Blocks and Valves, (Adapted from (Bagnoud &

Bigger, 2008)) ...... 44

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Figure 28, 250 T Cold Chamber die casting Machine at OSU ...... 47

Figure 29, PYROTEK® STAR 300® Rotary Degasser ...... 48

Figure 30, Specially designed mounts on the furnace to receive the Rotary degasser ..... 49

Figure 31, Palmers® Reduced Pressure Tester ...... 50

Figure 33, Density measurement after degassing, yellow points indicate that degassing was done...... 53

Figure 34, Four Heat treated samples and one as cast on the extreme right, no blistering observed ...... 54

Figure 35, Ejector and cover die with inserts ...... 56

Figure 36, Available design space on the cover die half was 4X2X.5 in ...... 57

Figure 37, Outer envelope of chill block and initial concept design ...... 58

Figure 38, ISO view and section view of entire chill block assembly showing internal trapezoidal passages and required area for venting into vacuum tank...... 59

Figure 39, Initial geometry for chill profile with a cut away (.52X.40in) for consistent vent area ...... 60

Figure 40, MAGMA simulation Parameters ...... 62

Figure 41, Temperature and Flow lengths at an arbitrary 84% fill, Stars showing location of control points ...... 62

Figure 42, Boundary Conditions (BC1 and BC2) & Thermal loads for the 2D Model .... 66

Figure 43, Main effect plots for the factors ...... 68

Figure 44, Chill Block design process ...... 70

xiv

Figure 45, CAD model section view showing the vent passage in the ejector die and hole on the cover die ...... 71

Figure 46, Ejector die vent cutout area ...... 72

Figure 47, Design criteria for Sealant; to determine optimal sealing...... 73

Figure 48, Sealant Path on the cover die ...... 74

Chapter 1: Introduction

The High Pressure Die Casting Process

High Pressure Die Casting (HPDC) is a process in which molten metal is injected into a precisely dimensioned steel mold, within which pressure is maintained until solidification has been completed. The die casting accordingly reproduces, with high fidelity, the finest detail of the impression within which it was formed. The process has been called” the shortest distance between raw material and finished product” (Street, 1986).

The history of the HPDC process dates back to the mid-19th century when an increasing demand for printed matter caused many inventors to design and build mechanized typing machines that were originally invented by Guttenberg. This in turn entailed that a subset of the overall casting process be developed in response to the need for mass production of the typing machine. By the 1850s the casing machines had almost begun to take a shape of the modern day hot chamber machines: means of opening and closing a precisely machined metal die, injection of molten metal under pressure into the closed die and ejecting the chilled component after casting. When the need arose for mass production in other industries such as automotive, these existing machines were modified from producing typing press parts. A notable mention on the early adopters of this process included Charles Babbage who produced precision engineered die cast components for an early computer in 1898 (Street, 1986).

The main differentiating factor in HDPC to other casting process is the pressure and the velocity of the injected metal. As illustrated in Figure 1, the HPDC process is recognized by its high injection pressure and velocities.

Figure 1. Relationship of various casting processes to pressure and velocity (Anderson,

2004)

Die casting in North America typically applies to a process where pressure is used to push the molten metal into the mold. The terms Die casting and High Pressure Die

Casting are commonly used interchangeably. In HPDC, the pressures can be anywhere

2 between 3000 and 20000 psi with gate velocities as high as 1800 in/sec (NADCA). When you look at the market for Metal Molding processes, by far and away the HPDC process is the largest. For example, in the case of Aluminum castings, it accounts for over 2/3rd of the market share of castings produced in North America (NADCAdesign). Throughout the thesis, the focus will be on this critical casting process and unless otherwise stated, all casting related terms are to be associated to this process only.

Figure 2. Steps in a HPDC process (NADCAdesign)

Figure 2 shows the steps involved in a typical High Pressure Die casting process. They are as described on the following page.

1. Liquid Metal is ladled into the shot sleeve (the location of which decides on

further classification as mentioned later).

2. Liquid metal is injected into the cavity – this process of the plunger moving inside

the shot sleeve is called the ‘shot’ and is further broken down into two parts - the

slow shot and the fast shot (will be discussed in detail in the upcoming chapters).

3. The die is opened and the ejector pins push the casting out where usually a robotic

arm extracts the casting and places it in an appropriate location.

4. The die is sprayed with lubrication and is closed. The lubrication serves mainly

two purposes

a. To homogenize the die surface temperature.

b. To act as a parting agent allowing the part to be ejected easily.

Advantages and Limitations

A recent NADCA study (Twarog, 2011) found that Cost and Strength form the major resons for the choice of High Pressure Die castings. Owing to the quick processing times, this process is well suited to the needs of a fast paced manufacturing setup. With the extremely short freezing times, the microstructure developed(atleast on the surfece) is fine and hence contributes to the strength. The major advantages are illustrated in Figure

3. One other important advantage when compared to other casting proceses is that thin walls can be cast. The disadvantages are really down to the fact that only lower melting point metals can be die cast and that the part geometry must allow for removal from the die (undercuts if not avoidable must be manageable with cores). There is also inherent limitation of the conventional process to counter porosity. Apart from this, HPDC is a

4 very attractive process for large volume manufactirung and a lot of attention is being given at present to use it in ways that have not been explored before.

Figure 3. Advantages of High Pressure Die Casting (Twarog, 2011)

Types of High Pressure Die Casting

The HPDC process is divided into two main sub groups based on the location of the shot sleeve

The Cold Chamber Process

In the cold chamber process, the Shot sleeve (or chamber) is exposed to the atmosphere.

We pour liquid metal into the shot sleeve and a hydraulic activated metal plunger (or

Ram) is used to push the liquid metal into the die cavity. The alloys used in the Cold chamber process are all Aluminum alloys, some Magnesium and Zinc alloys. Since aluminum is aggressive towards iron, shot sleeves are used here. As the liquid metal hardly spends any time in the sleeve, this helps prevent too much interaction between the steel and liquid aluminum and keeps the dissolution to a minimum. Some of the magnesium alloys include AZ91D and AM50.

(Exposed to the Atmosphere)

Figure 4. Cold Chamber Process (Groover, 2007)

Hot chamber processes are usually smaller machines and for bigger parts Mg is cast on cold chamber machines. Zinc alloys such as ZA-12 and ZA-27 alloys are also cast on cold chamber machines since they have a very large aluminum alloying percentage.

About 80% of the die castings are produced using the Cold Chamber method. Chief advantages of this process are the possibility of complex parts, and application of higher pressures.

The Hot Chamber Process

However, with some alloys which have a lower melting point such as zinc alloys we take the injection system and immerse it into the furnace. i.e., we immerse the shot sleeve inside the molten metal. The shot sleeve is now called a “gooseneck” and this method has some advantages. It will not lose as much heat during the ladling process and we can have better metal temperature control, it tends to be faster since no time is wasted ladling

(automatic refilling of the plunger) and lastly, there is a lower chance of the metal oxidizing. The problem with the hot chamber process is that it cannot be used for certain alloys. Aluminums affinity for die steels will mean that it will dissolve the chamber if used here. A variation of the hot chamber die casting process is the miniature Die casting process where instead of two die halves, 3 or more dies are used. The Miniature Die casting is used for very small parts (example - metal headphone jack). Typical alloys used in this method are Zinc ZA-8 (low aluminum content ZAMAK alloys) and

Magnesium alloys when needed for smaller parts.

Shot sleeve immersed in the furnace

Figure 5. The Hot Chamber Process (Groover, 2007)

High Integrity Processes

Although the hot and cold chamber methods are the commonly used types of HPDC, because of the inherent limitations with the process - that of porosity due to solidification shrinkage and gas entrapment, it necessitates the development of alternative process that improve on the traditional. There are three such high integrity processes that have been successfully developed for commercial use in high volume production (Vinarcik, 2002

).They are Vacuum Die Casting, Squeeze Casting and Semi Solid casting. Vacuum diecasting is a process that uses a controlled vacuum to extract the gases from the cavity and runner system during shot injection. When utilized properly, this method has the ability 8 to remove upto 95% of the gas in the die cavity at 750mm of Hg (Vinarcik, 2002 ).

Squeeze casting is a type of HPDC which utilizes large gate areas and has a planar filling of the metal front (unlike atomized in the case of conventional HPDC). In case of semi sold metal casting, a partially liquid-solid mixture is injected into the die cavity and here too the fill front is planar. An ever present intensification pressure (because of large gates that take long to solidify) helps greatly in reducing the solidification shrinkage as well.

As with Squeeze cast, a benefit here because of the large amount of solid fraction present in the mixture is that the amount of solidification shrinkage is reduced greatly. Discussed in chapter two are some pertinent examples of how the cold chamber die casting process is modified using vacuum to overcome its biggest issue – porosity.

Applications of High Pressure Die Casting

From smaller, decorative items to large single piece casts used in automobiles as door panels etc., HPDC finds applications in numerous areas. As long as the volume is high and the initial cost can be amortized, there will always a way to justify the use of Die

Casting. Figure 5 shows the per component cost based break up to help with the decision making process.

Figure 6. Graph howing the relative cost of a component manufactured by different casting processes (Kalpakjian, 2009)

Automotive Applications

Most Aluminum housings for applications in transmission and driveline are produced by

HPDC (Lutsey, 2010). These have traditionally been so because Al is light and it is used on member that is not a safety critical load-bearing member – so the inherent issues with

HPDC, those of porosity and lower elongation do not factor as much. The ever growing trend in Die casting is the overall push of the automotive industry in making use of die

10 casting to substitute parts that were traditionally made using steel and cast irons with light alloys such as Al and Mg. This is as a direct consequence of stringent requirements for improved fuel economy and tighter emissions. This trend also helps from an environmental standpoint because of a reduction in energy usage that comes because of eliminating downstream operations on a net shape part. Several studies, as well as some automakers’ announced plans, indicate that mass-reduction technology with minimal additional manufacturing cost could achieve up to a 20% reduction in the mass of new vehicles in the 2015-2020 timeframe (Lutsey, 2010). This incremental mass-reduction approach would, in turn, result in a 12% to 16% reduction in CO2 emissions while maintaining constant vehicle size and performance (Lutsey, 2010). High Pressure Die

Casting Processes help act as an alternate and viable alternative to produce body components that are currently made using High Strength Steels by stamping and forging processes. Figure 7 shows a Lincoln MKT which is believed to be the largest magnesium automotive closure panel currently produced (54 x 52 inches). Switching to a die-casting allowed six parts to be integrated into one large die cast magnesium lift gate inner (Die

Casting Engineer, 2010). Figure 8 shows the front wall (upper firewall) is at present the largest aluminum cast component made in large series. The aluminum casting integrates six individual components into a single part using a High Pressure Vacuum Die Casting

Process (European Aluminium Association).

Figure 7. Magnesium Lincoln MKT lift gate (Die Casting Engineer, 2010)

Figure 8. Upper Fire wall (European Aluminium Association)

Structural Automotive Applications

Among the components produced in an automotive using lightweight alloys, Projects and applications for structural die castings, which are usually characterized by special alloys and sophisticated processes are on the rise (Hartlieb, 2013). In the 2012-2015 periods alone, a study by the Ducker World Wide LLC suggests that the over 50lbs increase in

Aluminum (Worldwide, Ducker, 2014) consumption for automobile components will be in body structures. Cast components have the advantage of specific redirection of forces and allow a variation of the local wall thicknesses according to the encountered loads.

Thus, it is possible to optimize topology and have local areas that can handle large stresses (European Aluminium Association). The term structural die castings not only includes automotive components such as A or B pillars, inner door pillars etc., but also include casts in other areas such as marine applications and motorcycle and other recreational vehicle frames. What they have in common is that they are all usually large and thin walled (2-3mm) and contain a complex geometry. The costs although high when compared to normal die-casting is attractive because structural die casts usually replace two or more stamped steel structures. The structural die-casts are not only lighter but also make sense from an economic stand point, not to mention the environmental stand point as with reducing the steps comes lower energy usage. (Hartlieb, 2013)

Figure 9. Audi Space frame, Showing Structural Die Castings in Red (Hartlieb, 2013)

Light alloys used in Structural Die Casting

Aluminum alloys

In typical HPDC processes, Aluminum is mainly chosen for its light weighting effect.

The problems of traditional casting alloys is they rely on high iron content to help with the die soldering (affinity of aluminum to the iron in the die steel can be compensated by adding reasonable amounts in the alloy itself) cause prevent traditional die casting alloys from attaining the elongations that are necessary to classify an alloy as structural. This is mainly because of the formation of needles that are rich in iron that cause a very negative impact to elongation as well as flow.

Figure 10. Fe rich needle like phase that causes low elongation and flow capabilities

(Hartlieb, 2013)

Structural aluminum alloys circumvent this problem by replacing the Iron with

Manganese for the die soldering resistance. Silafont TM, Aural TM, Calypso TM are all alloys along the same vein. Mercaalloy TM is another structural that was developed by

Mercury Marine that uses Strontium to prevent die soldering.

Magnesium alloys

Another important lightweight alloy competitor of aluminum is magnesium. Magnesium alloys have higher fluidity and are therefore well suited to manufacture thin-walled parts in very complex geometries by high pressure die-casting. They are 33% lighter than aluminum and are also easier to machine (Cole & Sherman, 1995). The most popular Die 15

Casting alloy, AZ91 is typically not usable in a structural application owing to the β -

Mg17Al12 precipitates that on the grain boundaries that act as a discontinuous phase. (Shi, et al., 2013). High-ductility AM50 or AM60 alloy which are preferred for crashworthy automotive applications have performed well in crash simulation tests and many vehicles with these materials have achieved a five-star crash rating (Luo, 2013). There is still limited data about these alloys’ failure mechanisms. It was found that the mechanism of failure is not folding like aluminum or steel but shear and it is not yet clear why (Luo,

2013). These alloys even though ductile do not respond to heat treatment. This lack of heat treatable alloys (age hardenable) is in fact is one of the bigger disadvantages of

Magnesium alloys compared to aluminum. Tin based ternary alloys (AT72) and Mg-RE systems are new alloy developments that are providing significant precipitation hardening

(increased strength) and also an increased creep strength with improved corrosion resistance to boot (Shi, et al., 2013) .

Process consideration for structural alloys in the automotive industry

Structural applications in an automobile are areas wherein the design of the component is based directly on the impact resistance. This is measured in terms of the amount of elongation that is possible in the component. Typically, to be deemed a structural (crash worthy) alloy, the rule of thumb is that the component has an elongation of 12% or above

(Franke, Dragulin, Zovi, & Casarotto, 2007). Therefore, when the alloy is being qualified as a structural alloy, it is extremely important that something external such as variation in the process does not cause failure prior to maximum yielding. In the case where these alloys are used in structural applications, highest quality melt, free from entrapments,

16 hydrogen and other gasses and the highest integrity die casting process (typically applying vacuum throughout the most or all of the shot), often followed by low distortion heat and surface treatments are necessary steps to help the product attain its required properties. Thus, it is of paramount importance that the entire process used be capable of producing parts of high integrity.

Research Summary

At the Ohio State University Die Casting lab, we have a 250 SC Buhler cold chamber high pressure die casting machine that is capable of Squeeze casting and High pressure die casting for both magnesium and aluminum alloys. The Overall objective of this research was to select and interject processes/systems into this existing cell that will enable the university lab to create defect free, high integrity die-castings. The current stationary gas fired furnace is has no means of maintaining or evaluating the melt quality.

The first part of this work deals with determining best practices for degassing aluminum alloys and evaluating the gas content level, select the best and implement them. Ancillary equipment such as Rotary degassers and Reduced Pressure Testers were acquired and the best practices for using them are recorded. An alternate system to check the density of the reduced pressure test sample based on the Archimedes principle was constructed and its use is also illustrated as a qualitative measurement device. Similarly, part two of this work deals with studying the various vacuum systems available that match our desired capabilities for magnesium vacuum die casting and redesign our tooling accordingly. A commercial die caster was benchmarked, finite element and finite difference analyses are used to validate the designs of requisite chill blocks which form an important part of the

17 vacuum system. Using CAD software, the entire vacuum system is designed and is ready for machining.

Objectives

1. Understand and develop a system to reduce the Hydrogen porosity in the melt.

2. Design a vacuum capable die that can create and sustain low levels of vacuum in

the die cavity.

Approach

1. Melt Quality

a. Study the various methodologies available to obtain a clean melt and

deploy the ones that promise the best compromise between cost and

efficiency.

b. Conduct an observational study on the results obtained from multiple

campaigns and obtain best operating conditions for the equipment.

2. Vacuum system

a. Benchmark a commercial die caster and study literature to understand the

various methods of creating vacuum in a die casting cavity.

b. Decide on the methodology to create the vacuum system from different

approaches based on cost, efficiency and ease of manufacture and

maintenance.

c. Conduct a Designed Experiment to determine the optimum factor levels

conditions for the variables in the chill block design. The following were

deemed as the most important factors each with two levels

i. Material – Cu and H13

ii. Gap – 20 and 40 thousandths of an inch

iii. Heat transfer coefficient – 120000 and 60000 in W/m2K d. Create the design on 3D CAD software. e. Validate the design using flow simulations.

Chapter 2: Dealing with porosity problems in HPDC

There are a few limitations with the HPDC process as is discussed in the previous section. However, a recent NADCA survey revealed that when it comes to the main concern for consumers of die cast products, porosity is single handedly the biggest contributor to defects (Twarog, 2011).

Figure 11. Major sources of defects in a HPDC product (Twarog, 2011)

Porosity in a high pressure die casting can be due to a manifold of reasons. The most common areas of porosity are due to Gas Porosity or due shrinkage pores caused because of insufficient metal to fill areas as the metal is solidifying.

Gas Porosity

All gas porosity can be traced down to one of three reasons (Vinarcik, 2002 )

Hydrogen

Aluminum absorbs and dissolves a lot more Hydrogen in the liquid state than in the solidified state. Therefore, when it changes state during solidification, all of the

Hydrogen that was earlier dissolved begins to show up in the solidified metal. Hydrogen porosity is not as much of an issue with Magnesium alloys during HPDC. The metal is usually metered via a pressure in a closed chamber since liquid Mg is highly susceptible to catching fire and the only literature available about Magnesium is by passing chlorine gas through the melt (Peters, 1964).

Air Entrapment

Air entrapment is an issue that is prevalent in any alloy cast using the cold chamber die casting machine. A precise control over shot parameters (mainly slow shot and fast shot velocities) is needed to ensure that no air entrapment occurs in the shot sleeve or in the cavity. This topic is covered in greater detail in future chapters.

Oil or water from Lubrication spray

There is a chance that the Lubrication or coolant spray that is usually added at the end of each casting cycle does not completely evaporate and gets trapped inside the solidifying

21 metal. Usually, the water disintegrates into hydrogen and this causes further hydrogen based porosity.

Solidification shrinkage

Solidification shrinkage porosity as the name suggests is formed during the solidification of the metal. For example, pure aluminum contracts by 6% when it changes from liquid to solid and often times, there isn’t enough liquid aluminum to feed this contraction. This results in an area that is left bereft of metal. Good thermal management is a tool that can be used to ensure solidification shrinkage occurs at locations that can be fed. The intensification pressures applied during the high pressure de casting cycle also aid significantly in its reduction. A quick observation of a cut section and one can easily identify the reasons for the porosity. Gas porosity is usually well formed and spherical

(forms when the metal is still liquid and has the ability to choose a shape of least energy).

On the other hand, shrinkage porosity is irregularly shaped as it is formed at the absolute end of solidification only due to the lack of metal to fill the necessary volume. The porosity formed due to lube/water entrapment is shiny and its shape more like porosity due to a gas than it is due to shrinkage (NADCAdesign).

Porosity reduction and obtaining a high integrity cast

One can think of four major ways to reduce porosity and increase the casting integrity.

a. Improve melt quality by reducing the entrapped Hydrogen and inclusions

b. Change process parameters to reduce air entrapment

c. Change the process to one of the higher integrity process mentioned earlier

d. Change the alloy chemistry to reduce porosity

Melt quality Improvements

One of the methods to reduce porosity is the elimination (or more likely, vast reduction) of it from the source – the melting/holding furnace. The following are some of the components that are commonly used to detect issues and improve melt quality in a modern day die casting facility.

1. Degassing

2. Improved Process control to reduce entrapped air

3. Metallurgical Modification

4. Vacuum Die Casting

Degassing

As mentioned earlier and illustrated in Figure 12, the entrapped Hydrogen plays a very critical role in the amount of porosity that is observed in the final cast part.

Figure 12. Solubility of Hydrogen drops drastically as the aluminum solidifies, not so much a problem with Magnesium (Campbell, 2011)

Any die caster who wants to ensure the best quality cast will have to ensure low levels of hydrogen in his melt. Several methods exist to reduce this hydrogen content. They are discussed as below. (Campbell, 2011)

Passive Degassing

By reducing the temperature of the furnace some of the hydrogen content can be made to escape into the atmosphere naturally. The method is simply based on the fact that H2 content in the melt along with that in the atmosphere is within tolerance levels for most products. 24

Liquid argon shield

For smaller melts, pouring of a stream of liquid argon helps to reduce the levels of H2.

The liquefied gas being heavier than air and lighter than the liquid metal forms a layer on the surface of the melt. In addition, the argon expands drastically to about 600 times its volume thereby carrying away surface emissions.

Figure 13. Protection of the surface of the melt by the addition of liquid Argon

Gaseous argon shield

Injecting an inert gas however increases the gas content by spreading an equal volume of air across the melt and equilibrating with it. Thus the environmental gas predominantly air gets introduced. To avoid the entrapment of air, the argon is injected tangential to the melt surface. It is circulated and contained within a cone to prevent entrainment of air.

The rate of gas usage can also be regulated by the swirling technique. The hot expanded gas is made to escape from the top of the cone. Such degassing methods require the

25 circulation of melt within a diffusion distance of the surface. Figure 14 illustrates protection of a surface by injection of argon parallel to it.

Figure 14. Surface protection by delivery of argon gas parallel to the surface of the melt

(Campbell, 2011)

Chemical tablets

The use of chemical tablets to degas small melt surfaces was among the first techniques.

Mostly tablets are composed of hexachloroethane (C2Cl6). The tablet forms aluminum chloride gas bubbles that absorb the hydrogen, which then rise slowly into the atmosphere. Static lances, wands or flux tubes also have been used for degassing.

However, these techniques are not as effective since the area to volume ratio of the gas bubbles is too large. It in turn causes the gas bubbles to rise too quickly to the melt

26 surface without enough time to collect the hydrogen. Although the technique of creating large bubbles was inefficient, it allowed for the environmental exposure of fresh melt every time a bubble burst. It enabled the melt equilibrate with the environment. Such degassing may be acceptable on a dry day but, on a damp day the melt may end up gaining hydrogen faster than losing it. The bursting of large bubbles caused additional complications through the entrainment of new oxides and chlorides. Dampness in the tablet facilitated the creation of oxides. Introduction of inert gases using the lances also produced mixed results.

Rotary Impeller Degassing

Many techniques rely on purge gasses to collect the dissolved hydrogen. Hydrogen degassing of aluminum works on the principle that the hydrogen gas will move from an area of high pressure (in the melt) to an area of low pressure (the inert gas). Here, purge gas is introduced to the melt through a rotating shaft and rotor. This increases surface- area-to-volume ratio of bubbles in the melt. A lot of smaller bubbles spend longer time inside the metal, allowing for a higher capability of collecting the hydrogen atoms present. The situation is further improved by introducing a porous refractory diffuser on the end of the lance. Figure 15 compares the relative degassing efficiency of the three techniques - lance, porous plug refractory and rotating shaft/rotor.

Figure 15. Efficiencies of three different techniques to degassing (Neff, 2002)

Flux Induced Rotary degassing

An improvement to the rotary degassing would be to combine a flux dispensing system with it. The rotary degassing will also help with the uniform distribution of the melt and help in cleaning up the metal from the dross that is always formed. An appropriate flux composition can treat this dross in its original place, not only reducing the dross volume but also substantially decreasing the metallic content of the dross from 85% to 30% or less. (Neff, 2002)

Figure 16. Coupling flux injection with rotor degassing (Neff, 2002)

Choosing a Purge Gas

There are noticeable changes in the kind of purge gas used in the degassing process.

1. Chlorine – Not in use any more because of the hazardous Hydrochloric acid that is

formed as a byproduct- much like the chemical tablets.

2. Nitrogen--It is the most commonly employed since it is the least expensive. It creates

’wet’ dross, one that is rich in metallic aluminum.

3. Argon—Even though it is far more expensive when compared with N2, produces

dross that is not high in metallic Aluminum. Being heavier and inert, it also provides

a protective cover over the melt during degassing.

Type of degassing and choice of purge gas selected are both important in the kind of

melt quality you will end up receiving. 29

Additional Benefit of Degassing

Along with the hydrogen, the purge gas particles while moving to the top of the melt usually carry along with them larger oxide bifilms whereas the smaller ones are left behind. Research has shown that that the smallest oxide that can be removed by a bubble is approximately the same size as the bubble, thus there are multiple advantages having a lot of smaller bubbles as is observed in the rotary degasser case. (Campbell, 2011)

Figure 17. For a given size of a bubble, (a) larger oxides are easily floated out while (b) smaller ones follow the streamlines of the rising bubble and so do not collect the bubble

(Campbell, 2011)

Reduced Pressure Test

The reduced pressure test is a testing methodology to check the usefulness of the previously discussed melt improvement methods. It is based on the principle that vacuum enlarges the hydrogen in the melt and this allows for a quick and easy reading of porosity in the melt. A preexisting chart helps to easily assess the melt quality even on the shop 30 floor. This setup supplies a simple, easy to use partial pressure test unit for this verification. To use, the operator warms a sample cup, dips the cup into the furnace for a metal sample, quickly places the sample into the vacuum chamber, and starts the vacuum pump. The aluminum solidifies under a vacuum causing any entrained hydrogen gas bubbles to expand greatly. (Neff, 2002) Once solid and cooled, the sample is cut in half and compared to the chart (appendix A) for gas level analysis.

Improved process control

In a cold chamber die casting machine, there are two main process parameters when it comes to the actual delivery of the metal from the shot that significantly affect porosity.

They are the slow and fast shot parameters

1. Slow Shot speed – This is speed at which the plunger moves initially after the metal is

poured into the sleeve. If the speed is too fast, air is entrapped inside the metal. If the

speed is too low, a wave is created inside the shot sleeve that again causes air

entrapment. This is as shown in Figure 18. Research by Dr. Lester Garber (Anderson,

2004) allows the die caster to select the precise slow show velocity called the Critical

low Shot Speed (Vss) that reduces air porosity. Table 1 lists an abridged version of his

research.

Figure 18. Vss- Critial Slow Shot Velocities (Anderson, 2004)

Table 1. Some Shot sleeve diameters and their corresponding critical slow shot speeds

(Anderson, 2004)

2. Fast shot speed – This is essentially the speed with which the metal enters the gate.

Unlike the other forms of casting wherein the preferred mode of metal flow front is

planar, studies have shown that the most effective method to fill a cavity in the High

Pressure Die Casting process is by Atomized flow where the velocities are anywhere

between 1020-1560 in/s while for magnesium it is 2040 – 2400 in/s) (Anderson,

2004). 32

Figure 19. Illustration showing Atomized flow that is typical in a conventional HPDC

(Vinarcik, 2002 )

The change from slow shot to fast shot is done at a point when the entire shot sleeve is full of liquid metal. There is also research suggesting that constantly accelerating to the slow shot speed from the initial zero speed gives better results as compared with directly starting at the prescribed slow shot speed. (Brevick, Duran, & Karni, 1991)

Metallurgical Modification

In an alloy, if one keeps the alloy composition closer to the eutectic, then it will solidify with a smaller mushy zone. This means that there is a small range of solidification and as and when solidification occurs, there is always liquid metal to fill the shrinkage created.

This will result in shrinkage in the last place to solidify and as long as one uses good thermal management practices and ensure the last place of solidification is a non-essential part such as the overflow, this change will help in getting better integrity casts.

Figure 20. Left - Close to eutectic with a small mushy zone, Right - bigger mushy zone

(NADCAdesign)

Vacuum Die Casting

As mentioned in the previous sections, the three major commercialized methods to improving the casting integrity are squeeze casting, semi sold casting and high vacuum process. This body of work is based on the last type and therefore the below discussion is limited to vacuum die casting processes.

Figure 21. Conventional Vacuum Die Casting Process (PFIEFER Vacuum, 2006)

It is useful to note here that Vacuum die casting processes do not have an additional mechanism to aid in the shrinkage porosity when compared with conventional cold chamber HPDC. Their major claim to fame is their ability to vastly reduce air entrapment in the cavity. The use of vacuum in the high pressure die casting process is an innovative development. By creating a lower than atmospheric pressure in the injection chamber and die cavity, the relative absence of air results in casting of better quality. Back pressures encountered by metal trying to fill the die cavity are also reduced.

Two types of vacuum systems: (Niu, Hu, Pinwill, & Li, 2000)

1. Complete vacuum system: here the whole die casting system, including the die,

injection chamber and furnace are completely sealed and evacuated during casting.

Although this system is able to achieve a higher vacuum level, it is relatively complex

with stringent requirements in the system sealing.

2. Vacuum assist system: Here, a vacuum valve is incorporated into a die to evacuate

the entrapped air in the cavity. More attention has been focused on this system

recently, as this system is relatively simple, cheap and requires little or no machine

modification as it is a standalone system.

Two of the main process used commercially in the United States is the VACURALTM

(complete vacuum system) and the High-QTM (Vacuum Assist). These can be also be classified on the amount of time that the vacuum exists during the shot as well as how the metal is ladled (Jorstad, 2008 ). As shown in Figure 22, the Vacural Process has the vacuum applied for the entire cycle while conventional High Vacuum Process such as the

High-Q process apply it only for a short period of time. It can also be seen that the amount of vacuum pulled is larger since in this case the metering is also done by means of the suction provided by it.

Figure 22. Difference time in vacuum application for Vacural vs. others (Jorstad, 2008 )

Advantages of Vacuum Die Casting

1. Enhanced metallurgical properties by significantly reducing entrapped air (one of the

major causes of porosity) causing overall improvement in part quality. Allows greater

ductility (crash-worthiness), Heat treatability (Most Commercial valve manufacturers

claim that 1/10th of atmospheric pressure as a vacuum level is sufficient for T6 and

weld capabilities) (Wyman, Brevick, Chu, & Altan, 1992)

2. Vacuum greatly reduces the back pressure and allows flow of metal into otherwise

inaccessible areas (Niu, Hu, Pinwill, & Li, 2000). This is especially beneficial in

cases of thin walled castings.

3. Reduced resistance to flow means lower melt temperatures can be used (Furnace

superheat can be lower) thus reducing energy costs. This also means lower velocities

can be used giving lower turbulence and consequently better surface finishes.

4. Using vacuum assist is claimed by some casters to compensate for minor design flaws

[13]. Therefore, it can help to rapidly introduce die designs into production without

having to fine tune the runner, gate, overflow and vent systems.

Vacural Process

This process was developed by Riter aluminum in Germany in the mid-19th century.

After passing multiple owners, the technology is currently owned by Oskar Frech GmbH.

In this process, the metal is drawn from a holding furnace into the shot sleeve by the vacuum suction created in the die cavity and shot sleeve. The amount of metal ladled is time controlled and after the requisite metal is pulled into the shot sleeve, the rest of the process follows a methodology similar to any high pressure die casting process. The plunger moves forward pushing the metal forward and there are large intensification pressures just like in the case of conventional die castings. Finally, the vacuum valve is shut off once the metal reaches a predefined level. Termed AVDC (Alcoa Vacuum Die

Casting), Alcoa also uses a very similar technology to produce high integrity castings.

Figure 23. Vacural Process schematic (Jorstad, 2008 )

High Q Process

The High Q process is the current name for a process developed and patented by Alcan in

1996. It is essentially an improvement over the minimum fill time technology developed by the Thurner family over 20 years ago. It is essentially a cold chamber HPDC with a vacuum pump connected to one end of the cavity. It employs a piezo electric control valve that can shut off over 12 times faster than a hydraulic valve and stay open longer to ensure longer vacuum suction and helps in obtaining vacuum levels that are about 20-50 mbar in the die cavity (Jorstad, 2008 ). Figure 24 shows how the High-Q cast helps with reducing porosity.

Figure 24. Residual gas content a seen on X-ray (Jorstad, 2008 )

One of the concerns with this type of vacuum die casting is that there won’t be a large enough vacuum created soon enough especially since the times for fill in many die castings will be very short (can be in the order of msec). Besides these two processes, there are also processes that are similar to the High-Q but use valve less systems to generate similar results. Lastly, there are also systems that besides pulling air from the cavity also remove air from the shot sleeve( Vacu2 method developed jointly by Pfeiffer

Vacuum GmbH and Gilmo N V) that are showing promising results (PFIEFER Vacuum,

2006).

Valve based and valve-less systems

To achieve the desired vacuum levels in any of the Vacuum systems requires an inlet to the vacuum pump from the die cavity through which the suction is felt. The types for this fall into two broad areas, Valve and Valve-less. This is illustrated in Figure 25 as a comparison between the types of vacuum suction.

Valve based

Valve Based systems, further broken down into mechanically actuated (by metal flow) or by Sensor actuation are typically valves that have a circular cross section for maximum efficiency and a way to shut the vacuum off before the liquid metal can get into the sensor system. Latest technologies allow very fast electro-pneumatic closure: about 15 ms (VDS Vacuum Diecasting Services S.A). Valve based systems have had a history of maintenance issues because of metal solidifying inside the actuators. This is still the case with mechanically actuated systems. However, in the sensor type, the valve closure is triggered by plunger position and one can typically expect robust performance from these valves.

Valve-less (Chill Block)

Valve less type of vacuum venting use a chill block to impede the metal flow and cause an automatic shutoff to the vacuum when the metal solidifies. It is similar to the mechanically actuated valve system in the sense that both use the metal flow to shut off the vacuum. In every other way, the chill block type is a very simple piece of equipment that has no moving parts. There classifications of the chill block are based on the geometry of the chill profile the metal is forced through before it solidifies. It can be

41 either trapezoidal or triangular. Research has suggested that there is very little to choose between the two profiles when it comes to evacuation efficiency (Wang, Gershenzon,

Nguyen, & Savage, 2007). The third type of chill block design is a 3D shape “CASTvac” that is registered to Dr. Wang of Wanda technologies (Wang, Savage, Rogers, & Nguyen,

2010). This method utilizes a novel idea of increasing the surface area for the chill (thus increasing evacuation efficiency) without having to sacrifice machine capability to increased projected area. This type of valve is almost 4 times more efficient as compared to a regular chill profile and is directly comparable to the mechanically actuated valve system. One of the major drawbacks of the Chill Block system is their evacuation efficiency. Studies have shown that a circular vent has over double the evacuation efficiency of a similar sized trapezoidal or triangular vent (Wang, Gershenzon, Nguyen,

& Savage, 2007). There is one other potential problem when it comes to Chill Blocks.

The vacuum measurement is often times not reflective of the levels attained in the die cavity. This is as shown in Figure 24. Once the metal (A) solidifies, the pressure reading is then reading directly from the vacuum tank and not the cavity (Bagnoud & Bigger,

2008). Since all of this happens in times very quickly in a HPDC, it is possible that the true cavity vacuum pressure is missed.

Figure 25. Valve and Valve less technology for creating vacuum inside the die cavity

Figure 26. Erroneous Cavity vacuum reading in case of chill blocks (Bagnoud & Bigger,

2008)

Comparison

Figure 27. Comparison between Chill Blocks and Valves, (Adapted from (Bagnoud &

Bigger, 2008))

Figure 27 shows the results of tests that were made for a 4 kg shot-weight part with an evacuation volume of 3 liters, both having the same critical evacuation section of 60 mm2. Chill vents took longer to evacuate and gave an erroneous vacuum level reading.

Table 2 summarizes the advantages and limitations of each type of system. One system not discussed here that has the potential to have the cake and eat it too is one that can have a chill block as well as a cheaper mechanical valve based system in parallel. The chill blocks major advantage besides the cost is their ability to pull a vacuum until the last part of the cavity is filled, while the mechanically actuated system although has good evacuating capabilities, has always been prone to metal getting stuck in the valve causing down time. By using a system that has both in parallel, you can get the best of both worlds – quick evacuation by the valve and complete evacuation by the chill block. This is the basic idea behind the Nippondenso system (Seizi Ikeya, 1986).

Table 2. Summary of the Pros and Cons of a valve vs. a valve less design (the 3D valve is not considered)

Chapter 3: Melt Quality Improvement at OSU

This chapter goes through the equipment required and records the operational methods adopted to improve the melt quality so as to produce the best quality of casting.

Equipment

Furnace and Die casting machine

The setup at the die casting lab already consisted of a 250 SC Buhler Cold chamber die casting machine, a gas fired furnace and some of the smaller ladling and material handling equipment.

Figure 28. 250 T Cold Chamber die casting Machine at OSU

Degassing

Degassing was an essential part of maintaining a good melt quality and a Rotary

Degasser (PYROTEK® STAR 300®) was chosen for this purpose. This unit used compressed air to drive the rotor. No flux was used and Argon was chosen as the purge gas.

Figure 29. PYROTEK® STAR 300® Rotary Degasser

The reason Argon is not used in the industry as compared to Nitrogen even though

Nitrogen produces wet dross is because of the prohibitive cost during daily production.

However, in our case, since we didn’t have that many production runs, the cost wasn’t a factor. Because of the length of the shaft (too long for our furnace crucible depth), we had to design and fabricate 3 special mounts to accept the 3 legs of the degassing unit.

This raised the unit high enough to avoid the rotor from hitting the crucible, and also located the degasser rotationally on the furnace deck (Figure 30).

Figure 30. Specially designed mounts on the furnace to receive the Rotary degasser

Reduced Pressure & Archimedes density measurement

A Reduced Pressure Testing device manufactured by Palmer was procured and used as the qualitative measurement system. As a supplementary measurement system, an

Archimedes style density measurement apparatus was constructed after buying the

49 necessary components and set up so that it could act as a quick measurement and act as a quantitative redundancy tool for the same measurement.

Figure 31. Palmers® Reduced Pressure Tester

Experiment and obtaining best operating conditions

Two campaigns were conducted with similar aluminum alloys to validate the use of the degasser and to determine and record the necessary operating conditions to obtain optimum results. The first experiment was conducted with A380 alloy and the second with a company-patented alloy whose chemistry and trade name shall not be disclosed because of legal obligations and shall be referred to as Alloy B. Nonetheless, since the alloys are similar and since changing of alloy type will not appreciably modify the gas/hydrogen content in an aluminum alloy, the individual results are comparable. The present two-trial comparison is useful as a first-hand observational study to understand some of the vital variables in degassing and their effect on metal porosity reduction. Trial

1 was conducted with A380 and Trial 2 with Alloy B. In both cases, equipment used were all of those discussed previously - The PYROTEK® STAR 300® Rotary Degasser,

Palmers® RPT apparatus and the Archimedes density test apparatus.

Operating Conditions

1. Degasser assembly was hung over the molten bath of metal in the furnace for a few

minutes before immersing into the metal before use (helps remove any entrapped

moisture in the Graphite lance & rotor by preheating).

2. Flow rate of 10-12 SCFH for the Argon prior to lowering it into the molten metal and

28 SCFH while in operation. If the star rotor and shaft are placed into the molten

metal before the argon is turned on, then the aluminum can enter, solidify and block

the argon path in the shaft, which reduces the efficiency.

3. Air compressor pressure between 5-10 psi. The resulting flow rate was adjusted so as

to not to cause vortexing, but at the same time, fast enough to allow for small bubbles

to come up onto the surface (bubbles seen as tiny openings of red molten metal on the

dross covered surface).

Experiment

In the case of A380, degassing was done for 5 minutes at a time. For Alloy B, the first degassing was carried out before the first shot for twenty minutes. 10 minutes into the operation a sample was collected to determine density using the reduced pressure tester.

Subsequently, measurements were taken at various durations during the entire casting operation. The melt was degassed once again mid-way through the operation for 17.5 minutes. This change in degassing times allows us to understand the effect of excessive

51 degassing. An important point to note is that in the case with Alloy A, the sponsor insisted we use Argon gas as a cover gas. A tube placed at an angle that was almost perpendicular to the melt surface made this possible.

Results

In both cases, each collected sample was cut, polished and photographed. These results can be viewed in Appendix B & C Figure 32 shows the comparison between the two trials. In the first case, each time the melt is degassed for 5 minutes and at the 15th minute of degassing which is 178 minutes into the campaign, a very good value for the metal density of 2.6 g/cc is observed. In comparison, the maximum value for density observed in case of the Alloy B is 2.52 g/cc, which is obtained after degassing for 17.5 minutes at

107 minutes after the start.

 The results suggest that the ideal time that the melt should be degassed is somewhere

between 10 and 15 minutes. Degassing between 10 - 15 minutes ensured that the melt

was hydrogen depleted (below 3 on the chart provided by the manufacturer –

Appendix A) for at least 20 minutes.

 One of the reasons for the lower value of density obtained with the second trial can be

attributed to the argon cover gas being sent out onto the surface. When the gas is sent

with a jet perpendicular (or close to perpendicular as in this case), the tendency of

pushing the surrounding atmospheric air into the melt is high. Since the air has water

vapor, this setup is very likely to increase the hydrogen content of the melt.

2.7

2.6

2.5

2.4

2.3

2.2 A 380 2.1

Density Density (g/cc) Alloy B 2

1.9

1.8

1.7 0.00 50.00 100.00 150.00 200.00 250.00 300.00 Time (Mins)

Figure 32. Density measurement after degassing, yellow points indicate that degassing was done.

In both cases the required number of tensile and fatigue samples (which was our die) were successfully cast and in the second case, the samples were solution heat treated by heating to 485 degree C for 3hrs and quenching in Room Temperature water, we observed that there was no blistering – meaning that the gas content was below a 10g/100 cc value (Speece-Moyer, 2008). Even though alloy B was not heat treatable in the sense that there would be no appreciable strength increase from this heat treatment, one can appreciate that it will still be annealed and softened thus allowing for greater elongation.

Figure 33 shows how a slight yellow shade develops on the Heat Treated samples and one can see that there has been no blistering on any of them.

Figure 33. Four Heat treated samples and one as cast on the extreme right, no blistering observed

Chapter 4: Vacuum Capable Die Design at OSU

This Chapter details the steps taken to convert the current OSU die to one that can pull and sustain a vacuum.

It was decided that a vacuum assist type system with chill blocks be used as the methodology for the vacuum. Since the cost involved in buying a chill vent was prohibitive under the budget of this project, it was decided that they be designed from ground up.

Chill Block Design

Existing Die dimensional study

To create a working design, first a dimensional study was carried out on the existing cover and ejector die halves to find the best possible location for the chill block. The current die setup at OSU is two halves of an 18X13 in die. The Cover Die is 2.88 in deep and Ejector Die 4.88 in. Inserts are 7.75X7.75 in and 2.13 in deep on the cover side and

1.69 in on the ejector side.

The design envelope for the chill block was restricted because of two die cooling water lines and a tapped hole for the eye hook that was very close to the die face and left only

3/4th of an inch (depth) to work with. Since it was required that a sealant tube pass through above the top of the chill block, and its width was 0.25 in, it was decided that

0.75 of an inch be left from the top surface of the die (so that there would be a minimum

0.125 of an inch between the sealant and the chill block/die top surface).

Figure 34. Ejector and cover die with inserts

Figure 35. Available design space on the cover die half was 4X2X.5 in

The width was chosen arbitrarily ensuring that it sufficiently covered the exit of the casting cavity. Figure 35 shows the available design space in the cover die. The space on the ejector side was not as constrained and within the same length and height, the depth could if necessary be up to 2 inches. This is because the eye hook for hoisting the ejector die is in the geometric center at a distance of 2.44 inches from the surface. Hence, it was decided that the chill block on the cover side be made with a base of 0.5 in and with the teeth sticking out into the receiver on the ejector side. Figure 35 shows the initial outer bound design using .51 inches depth on the cover half and 1 inch on the ejector half. The

0.01 inch extra is so that the chill blocks are slightly proud as compared to the die surface ensuring complete mating.

Teeth extending into the

ejector chill block half

Figure 36. Outer envelope of chill block and initial concept design

Chill Surface Profile geometry

As mentioned earlier, there can be two types of profiles for a chill vent block design

(three if you count the 3D CASTvacTM). As can be seen in Figure 38, the chosen profile is trapezoidal. From previous research (Wang, Gershenzon, Nguyen, & Savage, 2007) it is seen that both triangular and trapezoidal profiles are almost similar in their evacuation efficiencies. But for a given chill block height, it is easily observed that the trapezoidal profile has a greater channel length. And since in this case the length is limited to 2 inches, to increase chances of complete freezing in the short time frame, the trapezoidal profile was chosen.

Figure 37. ISO view and section view of entire chill block assembly showing internal trapezoidal passages and required area for venting into vacuum tank.

Gap thickness, material and Heat Transfer coefficients

Once the decision was made to use trapezoidal profiles, the design problem was to understand the effect of different gap thicknesses and material on the freezing times. It was also understood that an important consideration of the geometry design was to maintain the same vent area throughout until the vent reaches the vacuum suction inlet - through the die out to the vacuum tank. Otherwise, one can think of a scenario where an area other than the chill block channel becomes the bottleneck for the flow. Hence, worst case section geometry of .16 square inches (0.04 *4 in) was maintained throughout. This meant that only two complete trapezoidal ‘teeth’ was possible for reasonable ‘pitch’ and

‘depth’ (standard spur gear terminology). Other than this, the exact profile geometry was chosen attempting to maximize the amount of channel length all the while incorporating good DFM principles of keeping the geometry as square as possible (pitch = depth). This is as shown in Figure 38, allowing for a .40 X .52 in cut out for vent area consistency.

The 0.04 in gap shown here is varied in a designed experiment to identify its effect.

Upcoming sections elaborate on why 0.04 in was chosen.

Figure 38. Initial geometry for chill profile with a cut away (.52X.40in) for consistent vent area

Magma Simulation

At this point, a magma simulation was run to obtain the values of temperature and velocity at the inlet of the chill block. A 3D CAD geometry of the cavity as the metal after solidification would have created is modeled and used as the input for MAGMA.

Choosing the material as AZ91, and the Die material as H13, a flow simulation was run with the parameters calculated from the fill time formula by Wallace as 58s and the slow shot acceleration was timed by MAGMA when supplied with the shot sleeve length of

330 mm and pour hole distance from one end as 127 mm. The input parameters are as shown in Figure 39. Control points were used on the exit surfaces to capture the exit velocities and temperatures. Figure 40 shows the model from which the information was obtained and he location of the control points.

The exit temperature was found to be 909K and the velocity of the exiting metal was computed as 16.65m/s.

Figure 39. MAGMA simulation Parameters

Figure 40. Temperature and Flow lengths at an arbitrary 84% fill, Stars showing location of control points

Designed Experiments

With the knowledge of the velocities and the temperatures at locations which are essentially the entry points into the chill blocks, a two level designed experiment with the following three factors (2n with n=3) was run with the following factors and levels to determine the best design:

1. Gap thickness – 0.02 and 0.04 (in)

2. Material – H13 and Cu

3. Heat Transfer Coefficient – 60 and 120 (kW/m2K)

The reason for choosing gap thickness of 0.04 and 0.02 was rule of thumb suggestions from a die casting manufacturer that we benchmarked. A center point was added to see if there was curvature. The two materials, H13 and Cu were considered because our die was made of H13 and there was a Cu block in the lab which was left over from a previous experiment that was thought could be used if found advantageous (even though it seems plausible from a basic understanding of materials that it would be, it is always nice to have a designed experiment validate the same). Lastly, this simulation was carried out using the properties of magnesium AZ91. Even though the alloy of interest is the experimental AT72, at the time of writing this, all the properties necessary as an input for an Ansys thermal simulation are not available for this alloy. Hence with the consultation of the creator of the alloy, AZ91 was decided to be used as it has similar properties.

Nonetheless, the heat transfer coefficient is still a matter of some contention. Since it is a 63 value that is extremely dependent on the individual geometry, it was though as a good candidate for checking its effect. The values of 60 and 120 kW/m2K are based on values obtained from previous research (Hamasaiid, Dour, Dargusch, Loulou, Davidson, &

Savage, 2008). Since the design was created with center points, it was felt that using a lower Heat Transfer Coefficient and checking for its effect would be an interesting exercise. Hence 60 and 120 were chosen as the extreme points with a center point of

90kW/m2K. Table 3 shows the experimental design.

RunOrder CenterPt Blocks Material Gap x 10-3 in HTC(kW/m2K) 1 1 1 Cu 20 120000 2 1 1 Cu 20 60000 3 1 1 Cu 40 60000 4 1 1 H13 40 120000 5 1 1 H13 40 60000 6 0 1 H13 30 90000 7 1 1 Cu 40 120000 8 1 1 H13 20 60000 9 0 1 Cu 30 90000 10 1 1 H13 20 120000 Table 3. Design for the 23 experiment

ANSYS® Simulations & Results of the Designed Experiment

A total of ten simulations were carried out on ANSYS following the order of the designed experiment. It is of significance to note the methodology adopted here – it is

64 assumed that the Mg alloy has already filled the cavity at a temperature of 909K

(obtained from MAGMA flow simulation) and the time taken to reach a temperature below its solidus temperature of 743K is determined. For this simulation, the model is also simplified as a 2 dimensional model. This is deemed reasonable since the chill block depth is significantly larger when compared to the plane section geometry considered. A

Plane 77 element type was used and a purely thermal transient analysis was carried out with boundary conditions and loads as shown in Figure 42. As will be noticed, only a single tooth is considered for the simulation, this is a reasonable approximation in this case since the geometry is repeating and is done so as to save simulation time. The results that were obtained for the different cases are as shown in Table 4.

The total distance that is available for the metal to travel inside the chill block profile is

57.69mm. The approximate distance travelled is computed by multiplying the average velocity at the inlet of the chill block (obtained from flow simulations as 16.6m/s) to the fill times obtained from the simulation. The results are tabulated in Table 5; highlighted values are those with fill times greater than the available 57.69mm.

Figure 41. Boundary Conditions (BC1 and BC2) & Thermal loads for the 2D Model

Run Order Material Gap (x 10-3 in) HTC(W/m2K) Fill time (ms) Distance(mm) 1 Cu 20 120000 1.3 21.45 2 Cu 20 60000 1.4 23.1 3 Cu 40 60000 4.4 72.6 4 H13 40 120000 6 99 5 H13 40 60000 6.69 110.385 6 H13 30 90000 4.06 66.99 7 Cu 40 120000 4.06 66.99 8 H13 20 60000 2.39 39.435 9 Cu 30 90000 2.52 41.58 10 H13 20 120000 2.01 33.165 Table 4. Results of the experiment (fill time and distances) highlighted values go beyond available length and can’t be used

Analysis of Variance

An analysis of variance was carried out to determine the significant factors and if there was any interaction that was noteworthy between the factors. As can be observed from the ANOVA results in Table 6, from a purely statistical point of view, the Heat Transfer

Coefficient (HTC) is not significant at the 2% level (since the p value >.02). So one could also make the case that at these levels and for our geometry, the values of HTC chosen won’t affect the simulation results too much. Figure 41 shows the main effect plots for the experiment – the parallel line in case of the HTC visually illustrates that it is not an important factor. It is seen that copper shows a lower freezing time and a lower gap shows a smaller freezing time, which is in line with what is expected since a smaller gap and copper with its greater thermal conductivity will aid in heal removal.

Table 5. ANOVA results

But the result that the Heat Transfer Coefficient is not a significant factor is interesting and non-obvious point to come out as a result of this experimental trial. Also, it is seen that the only significant interaction is the Material*gap combination. Hence, after taking into consideration the results, the geometries used in run order 1, 2, 8, 9, 10 were the only ones possible. Among these, 1 and 2 were eliminated because they were over designed and we didn’t require the final design to have a fill distance of half of the length.

Figure 42. Main effect plots for the factors

Finally, among 8, 9 and 10, trial 9 (copper with a width of 30 X10-3) was chosen because

a. It was easier to manufacture a gap of 30 thousandths of an inch as compared to

20.

b. Copper as shown in the main effect plot was a better material choice

Figure 42 shows the entire process in a single flow chart. From initial brainstorming to the final manufactured product it was an 8 step process as described in detail above.

Figure 43. Chill Block design process

Die design

Venting cut outs

The cut outs for vacuum venting in the die were designed keeping in mind to maintain the minimum vent area of .16 sq. inches. The idea was to have the majority of the cut out for the vent passage in the ejector die and have only a hole in the cover die where the vacuum pipe shall be located. The thought behind locating the vacuum pipe and trap on the ejector side is to avoid motion each time the dies open in a cycle.

Figure 44. CAD model section view showing the vent passage in the ejector die and hole on the cover die 71

Figure 45. Ejector die vent cutout area

Sealant cut out

The sealant cut out on the die was based on the dimensions of the available sealant material and the end mill available to make the cut out. The end face mill bit and seal available to us was .25" and .26" diameter respectively. The decision to be made was what the distance 'a' be - directly controlled by dimensions 'b' and 'c' (width and depth of cut out) as shown in Figure 47.

Figure 46. Design criteria for Sealant; to determine optimal sealing.

Corresponding with the die caster who helped us out with the basic understanding of the vacuum system it was determined that the slot needs to be .250" wide and .220" deep.

This thickness of the seal being compressed between the two die half's was sufficient to ensure adequate sealing. The logic behind this rule of thumb being that if too much of the sealant stuck out, there is a chance that it would fall out during normal operation. But too little sticking out means that there is a chance that not enough sealing happens since the compression of the sealant that is above the surface is what causes the sealing action.

Figure 478 shows the path taken by the sealant. This was decided making sure that none of the sliding pins get in the way (there were some on the ejector side that also necessitated the exaggerated profile).

Figure 47. Sealant Path on the cover die

Chapter 5: Conclusion and Future Work

This work has attempted to integrate scattered knowledge in the die casting realm in order to create an overall framework to produce high integrity casts using Aluminum and Magnesium. Two of the major topics covered are the melt quality maintenance and

Vacuum die casting. Equipment such as Rotary degassers, Reduced Pressure Testers and

Archimedes based density devices were all used and the best practices and learning were recorded. There was some evidence to suggest that impinging a slow jet of Argon on the melt surface does more harm than good to the melt purity.

A vacuum die was designed that was capable of being machined into the existing die. Multiple methods were studied and a vacuum assist with chill blocks to vent were decided as the best chose given the constraints. The chill blocks were designed from the ground up using simulations on MAGMA, ANSYS and designing it on SOLIDWORKS.

A 23 designed experiment was run to understand the influence of three factors, Gap thickness, Material and Heat Transfer coefficient on the freezing time. ANOVA results made it clear that the HTC had little influence and that copper with a smaller gap have the best freezing times. This exercise helped validate the chill block design. The chill block since has been manufactured and the die is also in the process of being machined.

Future Work

1. Create the remainder of the vacuum assist system - the trap and the control system

2. Run trials with and without vacuum to see the effect on this die

3. The reason Ansys was used to predict the time for flow was because it was easier to

conduct trails at a rapid pace for a designed experiment. MAGMA has been used in

the industry to complete such simulations and it could be used here to validate the

approximations obtained via ANSYS.

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Appendix A: Palmers degassing Chart

Appendix B: A 380 Degassing

Sl No. Time State Image Density in g/cm3 Corressponding Palmers Chart image Values are- Aluminum comarative standard for gas percent surface area of porosity density 1 11:30 W/o Degas 1.8

Degassed 2 11:45 1.95 for 5 min

Degassed for 5 3 14:13 additional 2.42 mins (total10)

Degassed for 5 4 14:43 additional 2.6 mins (total15)

Stationary for 30 mins 5 15:15 after 2.57 degasing for 15 mins

Appendix C: Alloy B Degassing

Sl No. Time State Image Density in g/cm3 Corressponding Palmers Chart image

10 minutes 1 16:20 into 2 Degassing

After 20 2 16:40 minutes 2.49 degassing

3 16:50 2.49

4 17:40 2.44

After degassing for 5 18:07 2.52 an additional 17.5 min

6 19:55 2.56

Taken out 7 21:25 during 2.37 pigging

Appendix D: Properties of H-13 Tool and Copper used in Ansys Simulations

 Density, (Harvey 1982, 458). Density ( ) is assumed to be uniform throughout the

material and constant over the temperature range of interest. = constant.

= 7750 Kg/m3

 Specific Heat, (Harvey 1982, 458). Specific heat (c) is assumed to be constant over

the temperature range of interest. c = constant.

C = 460.53 J/kgK

 Thermal Conductivity, (Benedyk, Moracz, and Wallace 1970, 194). Thermal

conductivity (k) is assumed to be isotropic and a function of temperature only. k =

k(T).

k(311 K) = 32.2 W/m-k

k(588.7 K) = 30.4

k(922 K) = 29.1

Properties of Pure Copper (Cu 10100)

Density ( ) (ASM International Handbook Committee, 1990). Density is assumed to be uniform throughout the material and constant over the temperature range of interest. =

constant.

= 8940 Kg/m3

 Specific Heat (ASM International Handbook Committee, 1990). Specific heat (c) is

assumed to be constant over the temperature range of interest. c = constant.

C = 385 J/kg.K

 Poisson’s Ratio (Deform 2D). Poisson’s ratio ( ) is assumed constant over the

temperature range of interest. = constant.

= 0.34

 Thermal Conductivity (ASM International Handbook Committee, 1990). Thermal

conductivity (k) is assumed to be isotropic and constant over the temperature range of

interest.

k = 391 W/m-k