Direct-Chill Casting of Light Alloys
Science and Technology
by John Grandfield, D. G. Eskin, Ian Bainbridge
CONTENTS
PREFACE AND ACKNOWLEDGEMENTS
DIRECT-CHILL CASTING: HISTORICAL AND INDUSTRIAL PERSPECTIVE
- Industrial Perspective
- Historical Development
LIQUID METAL SUPPLY, ALLOY PREPARATION, AND MELT TRANSPORT
- Plant Layout, Metal Scheduling, and Liquid Supply
- Alloying Elements and Master Alloys
- Furnace Technology
- Mixing Technology
- Temperature Control
- Melt Transport to and from the Furnace
- Furnace Filling
- Scrap Charging and Melting
- Furnace Cleaning
- Molten Metal Transportation from Furnace to Caster
- Chemical Analysis
- Magnesium Melt Protection and Handling
- Safety
MELT REFINING AND IMPURITY CONTROL
- Impurity Sources
- Aluminium
- Magnesium
- Effect of Impurities
- Dissolved Hydrogen
- Dissolved Metallic Impurity Elements Including Alkali Metals
- Inclusions
- Impurity Removal
- Dissolved Metal Impurities
- Hydrogen Removal: Degassing
- Inclusion Load Minimisation
- Inclusion Removal
- Alkali Metal Removal
- Magnesium Flux Refining
- Flux-Free Refining of Magnesium
- Measurement of Impurities
- Inclusion Measurement
- Hydrogen Measurement
- Alkali Content Measurement
- Temperature Measurement
- System Layouts, Safety, and Cost Considerations
GRAIN REFINEMENT
- Historical Overview
- Fundamentals of Grain Refinement
- Mechanisms of Grain Refinement in Aluminium and Magnesium Alloys
- Grain Refinement through Phases Formed by Alloying Elements during Solidification
- Grain Refinement by Added Insoluble Particles
- Grain Refinement by Indigenous Insoluble Particles
- Grain Refinement by Multiplication of Solidification Sites
- Technology of Grain Refi nement in DC Casting
- Grain Refining of Aluminium Alloys by Al–Ti–B and Al–Ti–C Master Alloy Rods
- Grain Refi nement Using Master Alloys Added in the Furnace
- Addition of Grain Refiners as Salts, Fluxes, Compounds, and Gases
SOLIDIFICATION PHENOMENA AND CASTING DEFECTS
- Effect of Cooling Rate and Melt Temperature on Solidifi cation of Aluminium Alloys
- Microsegregation
- Effects of Process Parameters on the Dendrite Structure
- Effect of Process Parameters and Alloy Composition on the Occurrence of Specific Structure Defects
- Macrosegregation
- Mechanisms of Macrosegregation
- Effects of Process Parameters on Macrosegregation during DC Casting
- Effect of Composition on Macrosegregation: Macrosegregation in Commercial Alloys
- Hot Tearing
- Thermal Contraction during Solidifi cation
- Mechanical Properties in the Semi-Solid State
- Mechanisms and Criteria of Hot Tearing
- Application of Hot-Tearing Criteria to DC Casting of Light Alloys
- Effects of Process Parameters on Hot Tearing and Shape Distortions during DC Casting
- Cold Cracking
- Mechanical Properties of As-Cast Alloys and Mechanisms of Cold Cracking
- Cold-Cracking Criteria
- Methods to Prevent Cold Cracking
- Defects Related to the Technology of DC Casting
DC CASTING TECHNOLOGY AND OPERATION
- Introduction
- Mould Technology
- Mould Heat Transfer
- Water Cooling Heat Transfer
- Mould Design: General Development
- Electromagnetic DC Casting
- Extrusion Billet Mould Technology Variants and Evolution
- Gas-Pressurised Hot-Top Mould Operation
- Mould Dimensions
- Casting Parameters
- Rolling Slab Moulds and Cast Start Technology
- HDC Casting
- Lubrication and Mould Friction
- Other Equipment
- Mould Table
- Starting Head Base and Starting Heads
- Molten Metal Delivery to the Moulds
- Molten Metal Level Control
- Casting Machine
- Ancillary Equipment and Pit Engineering
- Water System
- General Description
- Water Requirements
- Control Systems
- General Requirements
- Automated Systems
- Equipment Failure-Related Defects
- Safety Considerations
POST-CASTING PROCESSING
- Introduction
- General
- Inspection and Sawing
- Homogenisation and Stress Relieving
- Sawing and Packaging
- Safety Issues
MODELLING AND SIMULATION
- Introduction and History
- Physical Modelling
- Flow Modelling
- Water Spray Heat Transfer
- Non-Dimensional Number Analysis
- Mathematical Modelling Methods
- Modelling Requirements
- Model Formulation
- Boundary Condition and Property Data
- Validation and Experimental Verification
- Post Processing
- Resources: People, Hardware, and Software
- Flow Modelling of Metal Delivery Systems
- Macrosegregation Modelling during DC Casting of Aluminium Alloys
- Background
- Example of Macrosegregation Simulation
- Stress and Cracking Modelling
- Hot Tearing during DC Casting
- Cold Cracking during DC Casting
- Modelling of Mould Processes
- Mould Distortion, Ingot Shape Modelling, and Control
- Air-Gap Formation and Surface Segregation
- Gas-Pressurised Mould Meniscus Modelling
- Modelling of Magnesium DC Casting
- Final Remarks on Application of Models
Appendix
- Analytical Solutions to DC Casting
ECONOMIC CONSIDERATIONS
- DC Product Markets and Margins
- Financial Measures
- Examples of the Application of Financial Measures
- Throughput, Audit, Key Performance Indicator (KPI), and Benchmarking Analysis
Some very usuful figures from this book on practical aluminium billet DC casting
Figure 2.2 – Typical tilting reverberatory furnace configuration,
with bottom-mounted magnetic stirrer [Furnace Engineering]
Figure 2.11 – Typical launder cross section showing some construction features.
An optional hinged cover lid is shown.
Figure 3.6 – Cross section through the filter box.
Figure 3.15 – General layout of furnaces and melt treatment units
in relation to casting machine.
Figure 3.16 – Automatic trough dam actuator.
Figure 5.2 – A schematic depiction of typical regions in the solidifying billet
with respect to the heat flow in the mould (primary cooling) and
below the mould (air gap and secondary cooling).
The transition region is subdivided to slurry and mushy zones
by the coherency isotherm.
Figure 5.24. Shape distortions typical of direct-chill cast ingots.
A view from the short side of the ingot.
Figure 6.1 – Schematic section of a VDC casting unit
showing each of the main components.
Figure 6.8 – Cross section of a hot-top mould
together with cast product and
the molten metal feed into the mould,
plus water cooling supply to the mould.
The mould shape is generally round, but may be oval.
Figure 6.11. Typical modern billet gas-pressurised hot-top mould
with carbon liner configuration.
(Hycast)
Figure 6.14 – Schematic of gas-pressurised mould
with gas pressure balancing the metal head pressure.
Figure 6.20 – The effect of a change in billet diameter
on the heat input into the mould for a constant casting speed.
Figure 6.28 – General arrangement of a tilting DC casting table.
Figure 6.31 – Cross section of billet mould with starting head
located in cast start position
showing starting head/mould gap and
starting head alignment system.
Figure 6.35 – Molten metal feed system to moulds for a hot-top integrated system.
Only one molten metal level control is required,
usually at the entry point to the mould table
in the central section of the molten metal distribution system.
