Mg Casting

OUTLINE
• Melt Preparation and Transfer
• Die Casting Technologies
MAGNESIUM IN METALLURGY
CASTING
Hot Chamber
Cold Chamber
Henry Hu, Ph.D.
• Thixomolding
Professor
Engineering Materials Program
Department of Mechanical, Automotive & Materials Engineering
University of Windsor
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Mg Alloy Phase Diagram
2
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Melting Furnace for Mg Alloys
Atomic % Aluminum
700
10
20
650
30
Typical Casting
Temperature Range
630 - 650 oC
600
Temperature C
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α + liquid
500
Die Casting Alloys
• AZ91D
• AM60B
•AM50A,
400
α+β
300
Alloy Composition 9.0% Al
200
0
10
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20
30
40
Weight % Al
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Crucible for Holding Liquid Mg
Charging Ingots Prior to Melting
Materials:
•
•
•
Cr-Mo Nickel Free Steel (life: 24 months)
Laminated Cr Steel outer Shell (heating)
Mild Steel Inner (life: 6-12 months)
All Mild Steel (life: 6 months)
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•
•
•
Clean
Dry
Storage:
- Dry to remove MgCO3 •H2O
MgO + CO2 + H2O -> MgCO3 •H2O
•
Preheated (> 150 oC)
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1
Prevention of Melt Burning /Oxidization
Temperature (C)
3000
Prevention of Melt Burning /Oxidization
2450
2500
2000
1107
1500
650
1000
660
500
0
Mg Melting Al Melting
Mg Boiling
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Al Boiling
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Prevention of Melt Burning /Oxidization
• Salt Flux
0.2% SF6 + CO or 0.5% SO2 + Dry Air
- No flux contamination
- Lower % of metal loss
- Non-toxic (SF6)
Problems: Flux comtamination, corrosion,
- Need crucible cover +
HCl fume, Cl2 pollution
gas mixing system
• Burning Inhibitors
- High concentration causes
corrosion problems
- Small quantity needed if well
(if not used properly)
distributed over the melt
e.g., 50% MgCl, 30% KCl and 20% NaCl
Disadvantages:
- Create strong, thin metallic film
8
Prevention of Melt Burning /Oxidization
• Protective Gases
Advantages:
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Addition of 0.0005% (5 ppm) Be
- Not efficient at very high T
- Green house effect
- Health concerns (SO2)
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Hot Chamber Die Casting
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Hot Chamber Die Casting
Cooling
Lines
Coolant In
Vacuum
Manifold
Goose Neck
Mold
Piston
Molten
Metal
Bath
Coolant
Out
Step 1. -Mold is closed.
Step 2. - Vacuum is applied to draw
liquid metal up goose neck.
Step 3. - Piston injects metal into mold.
Piston
Ejection
Pins
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Step 4. - Part solidifies.
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Step 5. - Vacuum is released, mold
opened and part ejected.
Step 6. - Feed gates are
machined off part.
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2
Hot Chamber Die Casting
Hot Chamber Die Casting- Vacuum
The hot chamber process is used extensively for
casting of smaller magnesium parts with shot weights
up to 2-3 kilograms.
Vacuum is used to a limited extent in hot chamber
magnesium die casting. The technical literature is
limited as to the effect of vacuum assistance on the
properties of magnesium die castings. Further
investigations are needed to document the extent to
which vacuum provides significant improvements of
properties in magnesium die castings.
This process is not used for aluminium parts. Static
metal pressures are usually less than in cold chamber
machines,typically in the range of 20-30 MPa (29004400 psi).
A typical wall thicknesses of castings is around 1.50
mm for the complex stiffener frame or could be less
than 1 mm for simple gemetries such as notebook
computer and cellular phone cases.
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Hot Chamber Die Casting- Applications
notebook computes
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Cold Chamber Die Casting – Cell Layout
Telecommunication
Power Tool
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www.cwmdiecast.com, www.lunt.com
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Cold Chamber Die Casting – Melt Transfer
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NADCA
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Cold Chamber Die Casting
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Die Casting:
Cold Chamber Die Casting
Magnesium vs. Aluminum
Coolant In
Cooling
Lines
Piston
Coolant Out
Disadvantages
• lower density for
weight reduction
(up to 50%)
• higher material cost (1.3
times in volume)
• faster shot speed
(up to 50%)
Shot Chamber
Step 1. - Metal mold is closed and
the injector shot-chamber is filled.
Advantages
Step 2. - New and re-mixed alloy
is injected into the mold.
• longer die life
(2-5 times)
• poorer high temperature
Properties
• lower elastic modulus
• more difficulties in scrap
recycling
• thinner wall casting
(1 - 2 mm)
Step 3. - Rapid solidification
of part under high pressure.
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• lower machining cost
(up to 40%)
Step 4. - Ejection of part & runner.
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Cold Chamber Die Casting- Machines
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Cold Chamber Die Casting – Die Design
In general, the cold chamber die casting process, can
be used for magnesium and aluminium alloys.
However, the lower heat content in magnesium
compared to aluminium is important to the die casting
process. To avoid solidification of the magnesium
alloy during die filling, a shorter fill time is required for
magnesium than for aluminium. For this reason, some
magnesium die casters specify machine designs with
maximum shot plunger speeds exceeding 10 m/s.
Static casting pressures are commonly in the range of
30- 70 MPa (4400-10000 psi). The locking force of the
machine holding the two die halves together
exceeding 4000 tons are commercially available.
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Cold Chamber Die Casting – Die Design
The die is a complex device that has to fulfill a multitude of functions. It
defines the general geometry of the part and has a strong influence on
the dimensional variations from shot to shot. The use of fixed or
moveable cores adds to the flexibility to cast complex, near net shapes.
The geometry of the runner and gate system determines the die filling
characteristics. The thermal conditions in the die determine the
solidification of each part and thereby the microstructure and quality.
Over a large number of shots, the heat transfer characteristics of the
die determine the attainable cycle time. The die is fitted with a system
to eject the part after solidification.
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www.hydro.com/magnesium/en/
Cold Chamber Die Casting – Die Materials
• steel resistant to thermal shock (commonly, H13 steel or a steel with
similar qualities)
• H13 premium quality steel is commonly supplied to the die manufacturers
in a soft annealed condition with spheroidized carbides to improve
machinability. After machining, the die cavity parts are hardened and
partially annealed to a hardness typically in the range of 46-48 HRC.
• Only the die cavity and special parts of the die need to be made of H13
steel. This usage typically corresponds to 20-25 % of the die weight. The
remaining parts of the die are made from mild steel and medium carbon
steel. Frequently, standardized unit dies are used, especially for smaller
castings with relatively simple geometries. Such unit dies consist of the
main frame of the die, including the ejector system.
• The die cavity inserts can be exchanged, and the same unit die can thus
be used for a number of different castings.
•Magnesium die casting alloys contain less heat per unit volume than
aluminium alloys, and the solubility of iron in the molten metal is very low.
This leads to a considerably longer lifetime for the dies used for
magnesium, a factor of two or more being quite common.
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www.hydro.com/magnesium/en/
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4
Cold Chamber Die Casting – Part Design
Cold Chamber Die Casting- Process
• Section thickness
- 1-4 mm due to excellent die filling characteristics
- Uniform to avoid local hot spots causing solidification shrinkage
- Gradual change in section thickness
• Easy die filling
PQ2 Diagram
- Round edges and cornersto facilitate smooth filling
• Rib for strengthening
- Strength and stiffen parts instead of increasing section thickness
•Local overheating
- Direct impact of molten metal at high speed causing local
overheating of the die especially at small protrusions
• Draft Angle
- Normally 2-5 degrees
- Possibly 1-3 degrees, or even zero draft due to low thermal contraction of Mg
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Cold Chamber Die Casting – Max Metal Pressure 1
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www.hydro.com/magnesium/en/
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Cold Chamber Die Casting – Max Metal Pressure 1
Metal Pressure, pm
The pressure required to
force molten metal
through the die’s gate
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NADCA
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Cold Chamber Die Casting – Max Metal Pressure 1
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Cold Chamber Die Casting – Max Metal Pressure 1
1
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5
Cold Chamber Die Casting – Max. Fill Rate
Cold Chamber Die Casting – Max. Fill Rate
2
2
1
2
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Cold Chamber Die Casting – Theoretical Fill Rate 3
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NADCA
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Cold Chamber Die Casting – Fill Time
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Cold Chamber Die Casting – Theoretical Fill Rate 3
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NADCA
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3
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Cold Chamber Die Casting –
Metal Pressures for Max and Min Gate Velocities 4 ,5
1
3
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2
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6
Cold Chamber Die Casting –
Cold Chamber Die Casting –
Metal Pressures for Max and Min Gate Velocities 4 ,5
Metal Pressures for Max and Min Gate Velocities 4 ,5
• 40 - 85 m/s (1600 - 3390 in/sec) for typical
1
Mg die castings
5
(Al: 25 -40 m/s, 1000 - 1600 in/sec
4
Zn: 40 – 55 m/s, 1600 – 2160 in/sec )
• 100 m/s (4000 in/sec) for thin-wall castings
• less than 30 m/s for casting with 4-5 mm
3
2
wall thickness
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NADCA
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Cold Chamber Die Casting – PQ2 Diagram
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NADCA
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Cold Chamber Die Casting – Gate Area
1
Ag =
(minimum)
cd
7
Q
p (2 g )
ρ
5
6
Q: volume flow rate
p: metal pressure
cd : constant (0.5)
g: gravity
4
3
2
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NADCA
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Cold Chamber Die Casting – Gating & Plunger
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Cold Chamber Die Casting – Gate Velocity
• 40 - 85 m/s for typical Mg casting
(Al: 25-40 m/s; Zn: 40 – 55 m/s)
• 100 m/s for thin-wall castings
• less than 30 m/s for casting with 4-5 mm
wall thickness
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www.hydro.com/magnesium/en/
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NADCA
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Global Die Cast Mg Applications
Company
Magnesium Instrument Panel
Applications
• Audi and GM
• AM60 and AM50 Alloys
• One
- piece die casting vs. 12 steel
stampings and plastic parts
• 30
- 50% weight reduction
• Reduced cost
• More styling flexibility
instrumental panels, clutch housing & piston, transmission stators,
glove box door, roof frame, seat frame, pedal bracket, steering column
bracket, road wheels, transfer case, valve cover, electric motor housing
GM
FORD
Chrysler
IP bracket, sun roof panel, pedal bracket, steering column bracket &
component, transfer case, valve cover, clutch housing, steering wheels
air bag housing, steering column bracket, valve cover, steering wheel,
instrumental panels, alternator bracket, front headlight retainer
Audi
instrumental panels, manual transmission housing
BMW
wheels, gearbox housing, valve cove
seat structure, instrumental panels, steering wheel
Fiat
Mercedes-Benz
seat frames, sun roof panel, steering column component intake
manifold
Honda
Oil pan, cylinder head cover, wheels, valve cover
Toyota
steering wheels, valve cover, bracket
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GM “W” Car Instrument Panel
GM “G” Van Instrument Panel
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Magnesium Seat Structure
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Alfa Romeo Seat Frames
• Fiat and Mercedes
• AM60 Alloy
• Total part count reduction
• 50% weight reduction
• Reduced assembly operations
• Dimensional accuracy
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Magnesium Oil Pan
Honda Engine Oil Pan
• Honda
• ACM522 Alloy
• 35% weight reduction
• Gasoline
- power hybrid car- Insight
• Best fuel consumption – 35 km/liter
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GM 4 Wheel Drive Transfer Case
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Potential Die Cast Mg Applications
• Transmission housing
• Road wheels
• Engine cradle
• Body panels (door and hood)
• Engine block
• Radiator support
• Knuckle
• Bumper reinforcement beam
• Oil pan
• Oil/water pump housing
• Pulley
• Break disk rotor and caliper
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Why Thixomolding?
Thixomolding
• Net shape metal part production
• Higher quality than diecast
• Increased design flexibility
• Reduced gas permeability
• Lower energy and operating costs than
diecasting
• Better environmental cleanliness
• Better worker safety
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9
Thixomolding
Mg Alloy Phase Diagram
Atomic % Aluminum
Normal Die Casting Temperatures > 700 C
700
10
20
650
Temperature C
30
Typical Thixomolding
Temperature Range
560 - 590 C
600
α + liquid
500
400
α+β
300
Alloy Composition 9.0% Al
200
560- 590 oC
0
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http://www.jsw.co.jp/en/mg_f/mg_indexe.htm
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565
- 585°C
175
- 250°C
700
- 1000 bar
100
- 250 rpm
400 cm/s
150
- 250 cm/s
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Thixomolded product
• Lower melting
• Fewer blow holes
• Less shrinkage
• Fewer hot cracks
• Smaller distortion
• Higher dimensional
temperature than
die
- casting
• Smooth melt flow
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http://www.jsw.co.jp/en/mg_f/mg_indexe.htm
Item
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Thixomolding
Thixomolding
vs.
H13 steel
High temperature tolerances
More robust design
Vacuum venting
Hot oil heated (250 oC)
Technical Thin wall Mold
Die Casting
59
Cold-Chamber
Molding temp. °C
590~610
630~650
680~700
Injection speed m/s
1~4
1~4
1~8
Injection pressure kgf/cm2
500~1200
250~350
400~700
Material
Chip
Ingot
Ingot
Project area at same clamping force
Medium
Large
Medium
Max. machine size
1600t
900t
4500t
Blow hole
Few
Small
Many
Surface defect
Few
Small
Many
Shrinkage crack
Few
Small
A few
Fluidity
Excellent
Good
Good
Surface roughness
Excellent
Good
Good
Flash
Small
Few
Much
Shrinkage
Few
Small
Many
Mold shrinkage Dimension accuracy
3.8~4.5/1000
Excellent
5~5.5/1000
Good
7~8/1000
Poor
Warp
Few
Small
Much
Mechanical properties
Excellent
Good
Good
Corrosion resistance
Good
Good
Poor
Shot cycle
1(Standard)
0.8
0.9
Material cost
1(Standard)
0.85
0.9
Material yield
1(Standard)
1
1.2
Die's life
1(Standard)
0.9
0.8
Safe operation
Excellent
Good
Poor
Protection gas
Ar
SF6
SF6
Nothing
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Montreal
Much
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Dross/Sludge
58
Die-Casting
Hot-Chamber
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accuracy
Mold Requirements
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40
30
Thixomolding process
Thixomolding
•
•
•
•
•
•
20
Weight % Al
Thixomolding
Typical Processing Parameters
• Barrel Temperature:
• Mold Temperature:
• Injection Pressure:
• Screw Rotation:
• Max. Linear Inj. Rate:
• Typical Inj. Rates:
10
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10
Thixomolding
Tensile Properties
Tensile properties (AM50A) vs. process parameters
Thixomolding vs. Cold Chamber Die Casting
Material
Process
Barrel
Temperature
(K)
Injection
velocitiy
(m/s)
Y.S.
(MPa)
U.T.S.
(MPa)
El.
(%)
AZ91D
Thixomolding
878
1.4
180
299
10
160
240
3
Die casting
AM60B
Thixomolding
893
1.4
148
278
19
Die casting
963
2.9
115
239
12
130
225
8
(Die casting)*
AM50A
Thixom olding
898
1.4
140
269
20
Die casting
963
2.9
112
232
13
125
210
10
157
249
9
140
215
6
(Die casting)*
AS41B
Thixom olding
903
1.7
(Die casting)*
* Values from the literature
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http://www.jsw.co.jp/en/mg_f/mg_indexe.htm
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Thixomolding – Tensile Properties
Liquid
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http://www.jsw.co.jp/en/mg_f/mg_indexe.htm
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Thixomolding – Applications
Digital Camcorder
Liquid
VL
•
•
•
PD1
Five components
0.8 to 1.0 mm thick
by Nifco, Japan
Solid
Dendrite
Solid
Dendrite
Conventional Dendrite
Formation in Cast Alloy
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Sony DCR- PC10
• Right side housing
• 0.8 to 1.0 mm thick
Thixotropic Structure of
Semi-solid Alloy Resulting
From Stirring
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Thixomolding – Applications
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Thixomolding – Applications
Notebook Computers
Mini-Disc Players
HP Sojourn
• base and LCD cover
• 1.0 mm thick
Panasonic SJ
- MJ5/7
• 0.4 to 0.6 mm thick
Toshiba
• Libretto: base and
LCD cover- 0.7 mm
• Portege
Sony MZ
- E50
• 0.6 mm
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Thixomolding – Applications
Thixomolding – Applications
Digital Cameras
Digital Projectors
Fuji DS
- 300
•
•
Three components
1.0 mm thick
EPSON
Powerlite 7300
VL-EF1
Sony VPL
- X600U
CV11
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Thixomolding – Applications
Cellular Telephones
Technical Challenges
for Magnesium Expansion
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Technical Challenges
Technical Challenges
Component Requirements
Manufacturing Issues
• Creep Resistant Alloys
• Mechanical Properties
• Interaction of Mechanical properties
with the Vehicle Environment
• Failure Analysis
• Surface Protection
• Corrosion & Erosion
• Composites
• Elimination of SF6 for Mg Melt Protection
• Recycling
• New Casting Processes
(Squeeze, Semi-solid, Vacuum)
• Techniques for Melt Cleanliness Evaluation
• Rapid Prototypes
• Joining
• Wrought Products
• Heat Treatment
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