Post on 26-Mar-2018
1
Extrusion Defects Fundamentals & Solutions for Optimum FinishColumbus, Ohio – May 16-17, 2017
Jerome Fourmann
© 2017 Rio Tinto. All rights reserved
© 2017 Rio Tinto. All rights reserved.
2AEC – Finishing Workshop 2017 2Extrusion Defects: Fundamentals & Solutions
Defects framework – 6xxx series aluminum alloys
Common Extrusion Alloys
6360 “high performance”
6063 “general purpose”
6063 “high strength” -T54, -T65
6005A
6061 / 6351
6082
Soft Alloys
Medium Strength Alloys
Propertiesincrease
Extrudabilitydecrease
2
© 2017 Rio Tinto. All rights reserved.
3AEC – Finishing Workshop 2017 3Extrusion Defects: Fundamentals & Solutions
Defects framework – Acceptance level
Basics
Excellence
Fair
• PRINTER COPIER TUBE
• COMPUTER
• MOBILE DEVICE CASE
• LUGGAGE COVER
• DECORATIVE PARTS
Surface critical
• SUNROOF RAILS
• SIDE TRIMS
• HEAT EXCHANGERS
• HEAT & FLUID TRANSFER
• HEAT SINKS
Roughness critical
• CRASH SYSTEM
• ABS HOUSING
• TRAILOR
• PLATFORM
• SUPERSTRUCTURE
• INDUSTRIAL
COMPONENT
• ENGINE PARTS
• ENGINE HOUSING
Mill Finish
images courtesy of Constellium
Roller aluminum tube
© 2017 Rio Tinto. All rights reserved.
4AEC – Finishing Workshop 2017 4Extrusion Defects: Fundamentals & Solutions
Defects classification in 6xxx aluminum extrusions
During Extrusion / Mill Finish
Pick-up
Speed Cracking
Die line
Front-end TW
Die streak
Longit. weld
Blister
Tearing
Micro die line
Back-end Coring
Snap mark
Hot spots
After Extrusion - Metallurgical
Coarse MgSi
Coarse grain
banding
Corrosion pitting
Spacer marking
Corrosion fretting
Inclusion cast-in
MgSi at grain
boundary
Orange peel
Corrosion fingerprint
Excess lubricant
Water stain
Inclusion extrusion process
After Anodization
Atmos. corrosion
Alkali corrosion
Spacer marking
White etch bloom
Dull finish
Inadequate rinse
Acid corrosion
Rinse water corrosion
Chloride contami.
Spangle
Coloring due to MgSi
Castle and moat
3
© 2017 Rio Tinto. All rights reserved.
5AEC – Finishing Workshop 2017 5Extrusion Defects: Fundamentals & Solutions
Defects occurring during extrusions / Mill Finish
During Extrusion / Mill Finish
Pick-up
Speed Cracking
Die line
Front-end TW
Die streak
Longit. weld
Blister
Tearing
Micro die line
Back-end Coring
Snap mark
Hot spots
© 2017 Rio Tinto. All rights reserved.
6AEC – Finishing Workshop 2017 6Extrusion Defects: Fundamentals & Solutions
Pick-up
During Extrusion / Mill Finish
Pick-up
Speed Cracking
Die line
Front-end TW
Die streak
Longit. weld
Blister
Tearing
Micro die line
Back-end Coring
Snap mark
Hot spots
Pick-up under paint layer
4
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7AEC – Finishing Workshop 2017 7Extrusion Defects: Fundamentals & Solutions
Pick-up – Topography tells it all: Cauliflower appearance
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8AEC – Finishing Workshop 2017 8Extrusion Defects: Fundamentals & Solutions
Pick-up – Topography tells it all: Comet tail appearance
Irregular surface appearance
Normal surface appearance
5
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9AEC – Finishing Workshop 2017 9Extrusion Defects: Fundamentals & Solutions
Pick-up – Mechanisms
Coating of aluminum oxide on the die bearing exit
Extrusion direction
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10AEC – Finishing Workshop 2017 10Extrusion Defects: Fundamentals & Solutions
Pick-up – Possible causes and prevention measures
• Exit temperature too high
– Optimize billet temperature
– Optimize extrusion speed
“Stay in your lane!”
6
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11AEC – Finishing Workshop 2017 11Extrusion Defects: Fundamentals & Solutions
Pick-up – Possible causes and prevention measures
• Deterioration of die bearing condition − Ensure correct re-nitriding to avoid premature wear of the bearing by flacking or cracking
− Correct die preheating to prevent excessive oxidation of the die bearing (no overheating)
− Die bearing too long or of incorrect contour, polishing, handling…
Pristine and fragile surface
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12AEC – Finishing Workshop 2017 12Extrusion Defects: Fundamentals & Solutions
Pick-up – Possible causes and prevention measures
• Extrusion environment Nitrogen shrouding reduces oxidation and
therefore pick-up
• Inferior billet quality– Un-homogenized or inadequate
homogenization (95% or more
alpha Fe broken up)
– High Fe content: < 0.18%
– Inclusions: use quality billet
– Billet skin: limit the number
of billets
7
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13AEC – Finishing Workshop 2017 13Extrusion Defects: Fundamentals & Solutions
Not always pick-up! – foreign material
The Energy Dispersive X-Ray analysis (EDX) on the painted longitudinal section across the defect revealed the presence of foreign material composed of Al, Mg, Ca, O. This is likely a
particle of clay, chalk or other pigment used to color the coating.
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14AEC – Finishing Workshop 2017 14Extrusion Defects: Fundamentals & Solutions
Not always pick-up! – various inclusions
CrCrSi FeMgC Fe
FeO
Al
0 1 2 3 4 5 6 7 8Pleine échelle 768 cps Curseur : 10.094 (0 cps )
Cl KNa KMg
O Cl
C
Al
0 1 2 3 4Pleine échelle 731 cps Curseur : 10.094 (0 cps )
Example of tool steel particles
Example of carbon base lubricant
8
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15AEC – Finishing Workshop 2017 15Extrusion Defects: Fundamentals & Solutions
Not always pick-up! – polishing compound
KK
CKS
O
Al
Si
0 1 2 3 4 5 6 7 8 9 10 11keV Pleine échelle 501 cps Curseur : -0.042 (176 cps )
Spectrum 2
Longitudinal polished section.
The anodic film that wets the particle indicates it laid on the surface before the anodizing process
Mainly silicon carbide and potassium particles compound particle caused by buffing operations
© 2017 Rio Tinto. All rights reserved.
16AEC – Finishing Workshop 2017 16Extrusion Defects: Fundamentals & Solutions
Blister
During Extrusion / Mill Finish
Pick-up
Speed Cracking
Die line
Front-end TW
Die streak
Longit. weld
Blister
Tearing
Micro die line
Back-end Coring
Snap mark
Hot spots
9
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17AEC – Finishing Workshop 2017 17Extrusion Defects: Fundamentals & Solutions
Blister
Blow hole
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18AEC – Finishing Workshop 2017 18Extrusion Defects: Fundamentals & Solutions
Blister – Possible causes and prevention measures
• Entrapment of air during billet upsetting
– Burp cycle not used
– Distortion of billet front surface
– Poor butt shearing
– Billet too long or 2 piece-billet
– Container liner worn out, misalignment
– Billet quality
• Entrapment of volatile liquid via back end of billet (minimum use of lubricants)
• Extrusion of billet skin
• Billet temperature too high (front-end should be hotter than back-end)
10
© 2017 Rio Tinto. All rights reserved.
19AEC – Finishing Workshop 2017 19Extrusion Defects: Fundamentals & Solutions
Blister – Possible causes and prevention measures
• Use burp cycle to release entrapped air
Trapped air from upsetting too fast - Upset-By-Distance vs. Pressure
Billet discards show presence of air?
© 2017 Rio Tinto. All rights reserved.
20AEC – Finishing Workshop 2017 20Extrusion Defects: Fundamentals & Solutions
Blister – Possible causes and prevention measures
• Incomplete Upset in the “die-end” of the Liner
Check container liner condition, press alignment
Sticky alloy, i.e. 3xxx
11
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21AEC – Finishing Workshop 2017 21Extrusion Defects: Fundamentals & Solutions
Speed cracking, tearing
During Extrusion / Mill Finish
Pick-up
Speed Cracking
Die line
Front-end TW
Die streak
Longit. weld
Blister
Tearing
Micro die line
Back-end Coring
Snap mark
Hot spots
© 2017 Rio Tinto. All rights reserved.
22AEC – Finishing Workshop 2017 22Extrusion Defects: Fundamentals & Solutions
Speed cracking, tearing
Delamination
Inner surface in hollow section
12
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23AEC – Finishing Workshop 2017 23Extrusion Defects: Fundamentals & Solutions
Speed cracking, tearing – Causes and prevention measures
• Excessive die exit temperature
Occurs near the alloy melting point
Ultimate limit is speed cracking
Tem
per
atu
re (
oF
)
SOLVUS
SOLIDUS
200
400
600
800
1000
1200
1400
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
6060 6063 6061
% Mg2Si
Solid solution
600
500
400
300
200
100
650
550
450
350
250
150
750
700
Tem
pera
ture
(o C
)
6360
© 2017 Rio Tinto. All rights reserved.
24AEC – Finishing Workshop 2017 24Extrusion Defects: Fundamentals & Solutions
Speed cracking, tearing – Causes and prevention measures
• Entrapment of billet skin in extrusions
• Check die design to ensure die cavity at least 20 mm away from container wall
• Check container alignment and liner condition (no excessive wear)
• Clean container regularly to prevent build-up of aluminium oxides
• Reduce container temperature to ensure billet to container temp differential more than 50oF
• Increase butt length
13
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25AEC – Finishing Workshop 2017 25Extrusion Defects: Fundamentals & Solutions
Not always Speed cracking, tearing ! – Coring
Caustic etched – White arrows indicate the location of the
longitudinal welds
Presence of transverse weld (red arrows) and coring(white arrows) associate with the longitudinal weld (yellow arrows)
Tearing is associated with coring
© 2017 Rio Tinto. All rights reserved.
26AEC – Finishing Workshop 2017 26Extrusion Defects: Fundamentals & Solutions
Not always Speed cracking, tearing ! – various inclusions
• Fragments (chips) of tool steel originated from die or cutting tool (scalping, shear, dummy block…)
14
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27AEC – Finishing Workshop 2017 27Extrusion Defects: Fundamentals & Solutions
Speed cracking, tearing – Causes and prevention measures
• Excessive die exit temperature Should not exceed 1020ºF (Use IR probe)
• Entrapment of billet skin in extrusions container alignment, liner condition, container 45ºF-70ºF colder than billet, adequate front & back-
end scrap…
• Fragments (chips) of tool steel originating from die or cutting tool (if billet scalping is used) Nitride layer, die condition, container liner, dummy block…
• Foreign materials such as Al oxides and other materials Clean container to prevent build-up of Al oxides, increase butt length…
© 2017 Rio Tinto. All rights reserved.
28AEC – Finishing Workshop 2017 28Extrusion Defects: Fundamentals & Solutions
Die lines
During Extrusion / Mill Finish
Pick-up
Speed Cracking
Die line
Front-end TW
Die streak
Longit. weld
Blister
Tearing
Micro die line
Back-end Coring
Snap mark
Hot spots
15
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29AEC – Finishing Workshop 2017 29Extrusion Defects: Fundamentals & Solutions
Die lines / Micro die lines / Streaks – Industry context
Common problem for extruders
Difficult to identify the root cause:
− Billet
− Extrusion process
− Finishing process
Streaking is revealed at last step of the process
• Many contributing factors from previous steps
• Evidence removed after anodizing
Need to understand why before we can know how
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30AEC – Finishing Workshop 2017 30Extrusion Defects: Fundamentals & Solutions
Die lines / Micro die lines / Streaks – Definitions
Aluminum
Mounting media
Die line: deep grove
Aluminum
Mounting media
Micro die line: fine lines Streaks: bands of dark or light colors
A
B
A
B
16
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31AEC – Finishing Workshop 2017 31Extrusion Defects: Fundamentals & Solutions
Die lines / Micro die lines – Mechanisms
Die line: deep grove
Micro die line: fine lines
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32AEC – Finishing Workshop 2017 32Extrusion Defects: Fundamentals & Solutions
Die lines – Possible causes and prevention measures
• Causes:
‒ Pitting in die bearing surface due to wash-out or spalling of the nitriding layer;
‒ Presence of large AlFeSi particles in the aluminium coating layer on die bearing surface;
‒ Build-up of oxides in bearing relief region;
‒ Large inclusions in billet;
• Countermeasures:
‒ Remove die from press for cleaning and bearing polishing
‒ Improve die preparation, die preheating, die nitriding and re-nitriding practices;
‒ Prevent inflow of billet skin;
‒ Review and adjust extrusion conditions;
‒ Use high quality billet;
17
© 2017 Rio Tinto. All rights reserved.
33AEC – Finishing Workshop 2017 33Extrusion Defects: Fundamentals & Solutions
Micro die lines – Mechanisms
Microscopic top and transverse views of a “streaky” as-extruded
surface considered as die lines
In streak Normal surface
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34AEC – Finishing Workshop 2017 34Extrusion Defects: Fundamentals & Solutions
Micro die lines – Possible causes and prevention measures
• Causes:
‒ Deteriorated die bearing surface condition resulting from scratching by AlFeSi particles in billet skin;
‒ Excessive build-up of an aluminium coating on the bearing surface due to high deformation heating in the localised area;
‒ Improper design of bearing length, pocket or feeder plate opening;
‒ Billet quality: surface contamination, thick segregation zone, inadequate homogenisation, high inclusion content;
• Countermeasures:
‒ Prevent inflow of billet skin;
‒ Improve die design, die correction and die maintenance;
‒ Use high quality billet;
18
© 2017 Rio Tinto. All rights reserved.
35AEC – Finishing Workshop 2017 35Extrusion Defects: Fundamentals & Solutions
Streaks – Mechanisms
TYPE ICaused by
contaminants
TYPE IICaused by die or profile design
TYPE IIICaused by
extrusion process
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36AEC – Finishing Workshop 2017 36Extrusion Defects: Fundamentals & Solutions
Streaks – Mechanisms
Textural streak anodizedSurface topography 0.2µmVariable grain size and attack
19
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37AEC – Finishing Workshop 2017 37Extrusion Defects: Fundamentals & Solutions
Streaks – Mechanisms
Non-uniform Metal Flow
Variation in Pressure andFriction Force on Die Bearing
Difference in Surface Morphology after etching
Difference inSurface Roughness & Flatness
Difference in Grain Orientation or Grain Size
Difference inLight Reflection
Streaks on Mill Finish Product
Difference inLight Reflection
Streaks on Anodised Product
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38AEC – Finishing Workshop 2017 38Extrusion Defects: Fundamentals & Solutions
Streaks – Causes and prevention measures• Always check die bearing first
‒ Bearing length:
Correctly designed to ensure uniform metal flow
‒ Bearing shape:
No rounded edge at entrance
Square in respect to the die face
‒ Bearing surface condition
Flat and free of pits and damage
‒ Bearing blending – smooth and no shape steps
Bearing Bearing
X
Pristine and fragile surface
20
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39AEC – Finishing Workshop 2017 39Extrusion Defects: Fundamentals & Solutions
Streaks – Causes and prevention measures• Balance metal flow
‒ Area with a screw:
Reduce bearing length at the tips of the screw, better from the die front side;
If the flow in the screw area is still not fast enough, increase flow resistance of the flat part of the profile (but ensure no steps in bearing);
‒ Area with an intersection:
Check metal flow of the vertical branch during die trial and adjust bearing length/angle accordingly;
‒ Area under a port
Reduce web height and increase welding chamber size;
Delay the flow under the port by using a pre-chamber or increasing bearing length;
‒ Reduce speed and increase temperature
(a) (b)
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40AEC – Finishing Workshop 2017 40Extrusion Defects: Fundamentals & Solutions
Streaks – Causes and prevention measures• Case studies:
Streak
Web intersection
Streaked region
Zone 3
Zone 1
Zone 2
Zone 1
69100
86
220
Solution: not over-restrict the flow of the vertical part near the intersection
5
5
5
54
4
44
5.5
2
< 3535
5
5
54 4
44
5.52
Original die design Modified die design
Solution: remove the groove on the mandrel to prevent “overfeeding”
21
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41AEC – Finishing Workshop 2017 41Extrusion Defects: Fundamentals & Solutions
Streaks – Causes and prevention measures
Contaminants that build up in dead metal zone
Dead Zone
Billet #
60
59
58
57
Die Face
Feeder Wall
Feeder
Die
Old metal
New metal
Core / mandrel
Rim of body
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42AEC – Finishing Workshop 2017 42Extrusion Defects: Fundamentals & Solutions
Streaks – Causes and prevention measures
Impact of bearing
• Bearing squareness, over polishing• Cleanliness between runs• Over-heated or soaked too long in the die oven• Burrs, wash-out…
• Nitride case quality
• Anything that causes a build-up on the bearing• Bearing change too abrupt, blending not smooth enough, matching to
section shape…
22
© 2017 Rio Tinto. All rights reserved.
43AEC – Finishing Workshop 2017 43Extrusion Defects: Fundamentals & Solutions
Streaks – Causes and prevention measures
Impact of billet
• Alloy 6061, 6005A, 6063, 6060, 6360• Internal structure after homogenization• Billet surface quality and external cleanliness
© 2017 Rio Tinto. All rights reserved.
44AEC – Finishing Workshop 2017 44Extrusion Defects: Fundamentals & Solutions
Streaks – Causes and prevention measures
Impact of extrusion practice
• Container too hot
• Press misalignment
• Excess skull build-up in container
BADNOT BAD
But not perfect
23
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45AEC – Finishing Workshop 2017 45Extrusion Defects: Fundamentals & Solutions
Defects occurring after extrusions / MetallurgicalAfter Extrusion - Metallurgical
Coarse MgSi
Coarse grain
banding
Corrosion pitting
Spacer marking
Corrosion fretting
Inclusion cast-in
MgSi at grain
boundary
Orange peel
Corrosion fingerprint
Excess lubricant
Water stain
Inclusion extrusion process
© 2017 Rio Tinto. All rights reserved.
46AEC – Finishing Workshop 2017 46Extrusion Defects: Fundamentals & Solutions
MgSi After Extrusion - Metallurgical
Coarse MgSi
Coarse grain
banding
Corrosion pitting
Spacer marking
Corrosion fretting
Inclusion cast-in
MgSi at grain
boundary
Orange peel
Corrosion fingerprint
Excess lubricant
Water stain
Inclusion extrusion process
MgSi image by High resolution transmission electronic microscope and atom-probe
24
© 2017 Rio Tinto. All rights reserved.
47AEC – Finishing Workshop 2017 47Extrusion Defects: Fundamentals & Solutions
MgSi - Where does MgSi formation take place?
Casting Homogenization
Ext
rusi
on Ageing
Temperature
Pre
heat
ing
Coo
ling
Time
Liquid
Solid,Mg & Sidissolved
Deformationtemperature
How muchMg & Siavailable ?
Cooling quickenough to preventcoarsening of MgSi?
Cooling quickenough to preventcoarsening of MgSi?
Natural ageinglong enough?
Exit temperaturehigh enough to dissolve MgSi?
Artificial ageingoptimised?
Preheatingfast enoughto prevent coarseningof MgSi?
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48AEC – Finishing Workshop 2017 48Extrusion Defects: Fundamentals & Solutions
MgSi - Aiming for the right condition!
Critical factor: temperature, temperature, temperature !
Large number of fine MgSi precipitates
Strength & properties of 6xxx extrusions=
25
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49AEC – Finishing Workshop 2017 49Extrusion Defects: Fundamentals & Solutions
MgSi – Coarse undissolved MgSi
Good sampleFine MgSi particles
Die exit temperature achieved
Bad sampleCoarse undissolved MgSi particles
Failure to achieve die exit temperature
i.e. Low hardness: Presence of undissolved MgSi particles oriented in the extrusion direction
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50AEC – Finishing Workshop 2017 50Extrusion Defects: Fundamentals & Solutions
MgSi – Coarse undissolved MgSi
Undissolved MgSi in an Extrusion
20m0.001”
Reasons include: - Extended billet preheat soak
- Failure to reach required die exit temperature
- Failure to reach required core temperature on heavy sections
- Low billet temperature
- Coarse homogenized structure
Recommended die exit temperatures:6060, 6360, 6063, 6463 at 930oF minimum6005A, 6061, 6082 at 950oF minimum
26
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51AEC – Finishing Workshop 2017 51Extrusion Defects: Fundamentals & Solutions
MgSi After Extrusion - Metallurgical
Coarse MgSi
Coarse grain
banding
Corrosion pitting
Spacer marking
Corrosion fretting
Inclusion cast-in
MgSi at grain
boundary
Orange peel
Corrosion fingerprint
Excess lubricant
Water stain
Inclusion extrusion process
1 nm
MgSi image by High resolution transmission electronic microscope and atom-probe
© 2017 Rio Tinto. All rights reserved.
52AEC – Finishing Workshop 2017 52Extrusion Defects: Fundamentals & Solutions
MgSi precipitation at the grain boundaries
i.e. Low Properties: Precipitates concentrated at grain
boundaries and more MgSi out of Solution
i.e. Good Properties: No precipitates at grain boundaries and good
distribution overall
27
© 2017 Rio Tinto. All rights reserved.
53AEC – Finishing Workshop 2017 53Extrusion Defects: Fundamentals & Solutions
MgSi precipitation at the grain boundaries
MgSi out of Solution and at the grain boundary
Reasons include: - Inadequate press quenching
- Premature ageing on run out
table (soft spots)
© 2017 Rio Tinto. All rights reserved.
54AEC – Finishing Workshop 2017 54Extrusion Defects: Fundamentals & Solutions
MgSi After Extrusion - Metallurgical
Coarse MgSi
Coarse grain
banding
Corrosion pitting
Spacer marking
Corrosion fretting
Inclusion cast-in
MgSi at grain
boundary
Orange peel
Corrosion fingerprint
Excess lubricant
Water stain
Inclusion extrusion process
28
© 2017 Rio Tinto. All rights reserved.
55AEC – Finishing Workshop 2017 55Extrusion Defects: Fundamentals & Solutions
Coarse grain banding (PCG) - Crack
Appearance of the cracked area
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56AEC – Finishing Workshop 2017 56Extrusion Defects: Fundamentals & Solutions
Coarse grain banding (PCG) - Crack
Bad partAppearance of the PCG area
Good partuniform grain
29
© 2017 Rio Tinto. All rights reserved.
57AEC – Finishing Workshop 2017 57Extrusion Defects: Fundamentals & Solutions
Coarse grain banding (PCG) – Causes and prevention measures
• Objective: Produce a fully Rx structure or a completely un-Rx structure
• Die conditions
– Bearing length and choke angle influence the PCG thickness in quenched profiles
– Extrusion speeds with acceptable PCG thickness can be several times higher with long choked bearings and in addition the onset of speed cracking can be delayed.
© 2017 Rio Tinto. All rights reserved.
58AEC – Finishing Workshop 2017 58Extrusion Defects: Fundamentals & Solutions
MgSi After Extrusion - Metallurgical
Coarse MgSi
Coarse grain
banding
Corrosion pitting
Spacer marking
Corrosion fretting
Inclusion cast-in
MgSi at grain
boundary
Orange peel
Corrosion fingerprint
Excess lubricant
Water stain
Inclusion extrusion process
30
© 2017 Rio Tinto. All rights reserved.
59AEC – Finishing Workshop 2017 59Extrusion Defects: Fundamentals & Solutions
MgSi precipitates at the grain boundaries: Orange Peel
Orange peel surface Normal surface
Mg2Si precipitate more visible. 43 HRFW
Mg2Si precipitate less visible.56 HRFW
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60AEC – Finishing Workshop 2017 60Extrusion Defects: Fundamentals & Solutions
Orange peel – Causes and prevention measures
Possible causes:
• Alloys metallurgy
– Alloys suitable to bending condition: not subject to coarse grain outer-band
• Extrusion conditions
– Slow billet preheating
– Exit temperature not high enough to dissolve MgSi
– Exit temperature too high favours re-crystallized outer-band
– Insufficient press quenching and avoid delay into the quench box
– Over stretching
– Ensure billet to container temperature differential more than 50oF
31
© 2017 Rio Tinto. All rights reserved.
61AEC – Finishing Workshop 2017 61Extrusion Defects: Fundamentals & Solutions
Defects occurring after anodizationAfter Anodization
Atmos. corrosion
Alkali corrosion
Spacer marking
White etch bloom
Dull finish
Inadequate rinse
Acid corrosion
Rinse water corrosion
Chloride contami.
Spangle
Coloring due to MgSi
Castle and moat
© 2017 Rio Tinto. All rights reserved.
62AEC – Finishing Workshop 2017 62Extrusion Defects: Fundamentals & Solutions
Spacer markAfter Anodization
Atmos. corrosion
Alkali corrosion
Spacer marking
White etch bloom
Dull finish
Inadequate rinse
Acid corrosion
Rinse water corrosion
Chloride contami.
Spangle
Coloring due to MgSi
Castle and moat
32
© 2017 Rio Tinto. All rights reserved.
63AEC – Finishing Workshop 2017 63Extrusion Defects: Fundamentals & Solutions
Spacer mark
Also known as hot spots, dull spots, or black spotting.
• Typically visible after anodization• Cool and reheat section creates the environment for a rapid precipitation of MgSi
Heavy density of MgSi precipitates in the defect
Low density of MgSi precipitates away from the
defect
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64AEC – Finishing Workshop 2017 64Extrusion Defects: Fundamentals & Solutions
Dull finishAfter Anodization
Atmos. corrosion
Alkali corrosion
Spacer marking
White etch bloom
Dull finish
Inadequate rinse
Acid corrosion
Rinse water corrosion
Chloride contami.
Spangle
Coloring due to MgSi
Castle and moat
33
© 2017 Rio Tinto. All rights reserved.
65AEC – Finishing Workshop 2017 65Extrusion Defects: Fundamentals & Solutions
Dullness Appearance
Dull Surface“Good” Surface
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66AEC – Finishing Workshop 2017 66Extrusion Defects: Fundamentals & Solutions
Dullness Appearance
Dull Sample – note precipitates at grain boundaries
Desirable Sample – less precipitates at grain boundaries and overall
34
© 2017 Rio Tinto. All rights reserved.
67AEC – Finishing Workshop 2017 67Extrusion Defects: Fundamentals & Solutions
Dullness Appearance
Dull Sample – note precipitates at grain boundaries
Desirable Sample – less precipitates at grain boundaries and overall
© 2017 Rio Tinto. All rights reserved.
68AEC – Finishing Workshop 2017 68Extrusion Defects: Fundamentals & Solutions
Color finishAfter Anodization
Atmos. corrosion
Alkali corrosion
Spacer marking
White etch bloom
Dull finish
Inadequate rinse
Acid corrosion
Rinse water corrosion
Chloride contami.
Spangle
Coloring due to MgSi
Castle and moat
35
© 2017 Rio Tinto. All rights reserved.
69AEC – Finishing Workshop 2017 69Extrusion Defects: Fundamentals & Solutions
Color finish
The anodic layer:• light sample: 16-18 µm thick • dark sample: 20-22 µm thick
AA6063 Dark LightSi 0.42 0.43Fe 0.22 0.21Cu 0.027 0.001Mn 0.064 0.032Mg 0.47 0.50Cr 0.017 0.000Ni 0.006 0.002Zn 0.009 0.002Ti 0.015 0.005
High MgSi precipitates concentration in the “bad” dark sample
Low MgSi precipitates concentration in the “good” light sample
bad
good
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70AEC – Finishing Workshop 2017 70Extrusion Defects: Fundamentals & Solutions
Color finish – Mechanisms pits & grooves during etching
Increased Fe content promotes matt finish
36
© 2017 Rio Tinto. All rights reserved.
71AEC – Finishing Workshop 2017 71Extrusion Defects: Fundamentals & Solutions
Color finish – Mechanisms pits & grooves during etching
Caustic etching is microstructure sensitive
© 2017 Rio Tinto. All rights reserved.
72AEC – Finishing Workshop 2017 72Extrusion Defects: Fundamentals & Solutions
Color finish – Mechanisms pits & grooves during etching
0
10
20
30
40
AA6063 Hi Cu Lo Fe Hi Zn AA6360
An
od
ised
Glo
ss 6
0°
Alloy
Acid etching minimizes sensitivity of alloy composition
Pilot Caustic etch
Commercial Caustic -10
Acid Etch
20 film
Caustic etching is microstructure sensitive
180s Acid Etch. Insensitive to microstructure
180s Acid + 40s NaOH. Attack microstructure
37
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73AEC – Finishing Workshop 2017 73Extrusion Defects: Fundamentals & Solutions
Dull and Color Appearance – Mechanisms
Dull / Color appearance
Diffusion of reflected light
Pits & grooves formed during alkaline etching
Pits in grain interior
Fe intermetallic Grain size
Profile Die
Grooves in grain boundaries
MgSi Grainorientation
Alloy
Whole process
MgSi
whole processHomo
Pits - Dissolution of aluminium matrix surrounding AlFeSi intermetallic particles
Grooves - Dissolution of MgSi precipitates in grain boundary
© 2017 Rio Tinto. All rights reserved.
74AEC – Finishing Workshop 2017 74Extrusion Defects: Fundamentals & Solutions
Dull and Color Appearance – Causes and prevention measures
Possible causes:
• Over-ageing
– Ageing conditions not optimised: can occur on purposely over-aged products
– Poor control of ageing oven: conduct yearly oven survey
• Extrusion conditions that lead to MgSi coarsening
– Slow billet preheating
– Exit temperature not high enough to dissolve MgSi
– Insufficient press quenching
• Poor billet quality
– High Fe content in alloy (i.e. secondary billet): < 0.18%
– Billet not or not adequately homogenized
– Slow post-homo cooling rate
38
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75AEC – Finishing Workshop 2017 75Extrusion Defects: Fundamentals & Solutions
SpangleAfter Anodization
Atmos. corrosion
Alkali corrosion
Spacer marking
White etch bloom
Dull finish
Inadequate rinse
Acid corrosion
Rinse water corrosion
Chloride contami.
Spangle
Coloring due to MgSi
Castle and moat
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76AEC – Finishing Workshop 2017 76Extrusion Defects: Fundamentals & Solutions
Spangle – “grainy” or “galvanized” surface
Normal surface
Spangle surfaceCoarse grain size
SEM - varying level of attackafter caustic etch
preferential grain attack
Irregular surfaceAnodic layer thickness 6μm
Irregular surfaceAnodic layer thickness 14μm
SEM - Step
39
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77AEC – Finishing Workshop 2017 77Extrusion Defects: Fundamentals & Solutions
Spangle – Causes and prevention measures
• Metal composition
– Presence of zinc as a contaminant in the alloy: Zn below 0.025%
• Extrusion - Metallurgy
– Hollow more prone than solids because of their generally coarser grain size.
– Coarse grain structure in the extrusion: composition, extrusion process
• Etching and anodizing conditions
– Presence of free zinc as a contaminant in the caustic etch solution: Zn below 6ppm
– Long life vs. recovery process: Crystallizers do not remove Zn along with the aluminum in recovery process and no additive as the filters can clog by the precipitate. Long-life etch the additive (usually sodium sulfide) precipitates the zinc (as zinc sulfide) and is skimmed off the top.
– Appropriate level of caustic soda: 50-60 g/l achieve the same etching as 100 g/l.
– High-speed or high etching temperature: 125-135 ˚F and extend time if needed
Extrusion Defects Fundamentals & Solutions for Optimum FinishColumbus, Ohio – May 16-17, 2017
Jerome Fourmann
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