1. Thermal Fatigue Studies Using HF Induction Heating of Die Materials for Light Metals Casting
S. Gulizia1,2, D. Jones1,2, M. Z. Jahedi1,2, & T. Kearney2 & P. Koltun1,3
1CAST CRC
2CSIRO Materials Science & Engineering
3CSIRO Process Science & Engineering
PRICM 7
August 2 – 6 1, 2010, QLD, Australia

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Presentation Outline
Introduction to Thermal Fatigue Failure in HPDC Tool Materials
What is thermal fatigue?
Mechanism of thermal fatigue failure in HPDC
Development of a New Thermal Fatigue Test Facility
30 KW induction furnace
Energy control
Automated and PLC control
Tool Materials & Experimental
Eight commercial ferrous & non-ferrous tool materials selected
Image analysis used to examine crack initiation and growth
Vickers hardness measurements
Results
Overall performance of ferrous and non-ferrous tool materials
Optical examination
Tool softening
Microstructure analysis
Summary
Overall thermal fatigue performance of tool materials after 6000 cycles
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3. What is Thermal Fatigue?
Tool Steel Die
Aluminium Alloy Casting
Thermal fatigue largest cause of die failure
Cracks can extend to water cooling lines
Leads to catastrophic failure
Casting require post treatment to remove
thermal fatigue marks
Photograph of Thermal Fatigue Damage to Die
Photograph of Thermal Fatigue Damage to Castings
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4. Mechanism of Thermal Fatigue Failure During HPDC
Thermal fatigue during HPDC is unlike isothermal fatigue
There are 3 primary sources of cracks; fatigue, oxidation and creep
Cracks form during cooling because the stress and strain are at their highest levels
Compression
Tension
Hysteresis Loop of heating and cooling during HPDC
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5. PC based Data Acquisition Unit & Control Software
Development of a Laboratory
Thermal Fatigue Test Facility
30 KW induction furnace
Previous TF test rigs used surface temperature to control cycle
Dipping samples into molten metal and cooling was also limited
A new test facility was needed that could heat samples using
The same power irrespective of material physical properties
Induction Coil
Signal Buffer & Scale
Power Meter
Power Transducer
RF Voltage Monitor
RF Current Monitor
Existing Circuitry
Convert to optical signal
Fibre Optic Link
Receiver & signal conditioner
PC based Data Acquisition Unit & Control Software
Voltage signal proportional to Heating Power
PLC
Covert Heating Power signal to total energy per cycle in Joules
Energy target Joules per cycle reached – start sample quench
Sample heat or quench
HF Power Out
Optical fibre link used to isolate data acquisition part from high noise HF fields associated with induction heating hardware
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6. New Thermal Fatigue Heating Methodology
Identical heating energy
for each TF cycle
Similar cycle times
for each material
Total energy control methodology subjects each sample to similar energy levels for each heating cycle
Cycle times are similar irrespective of material properties such as thermal conductivity
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7. Chemical Composition of Tool Materials
Thermal fatigue resistance of eight tool materials measured
Seven ferrous and one non-ferrous material tested
Most samples were heat treated and triple tempered according to industry standards (46-50HRC)
According to the manufacturer heat treatment was not required for the Tiamet material (39-41 HRC)
Manufacturer/Supplier
Designation C Mn Si Cr Ni Mo V Co Ti W
1. Swiss Steel $
Thyrotherm 2343 EFS Supra (H11)
0.38
1.0
5.3
1.3
0.4
2. Swiss Steel $
Thyrotherm 2367 EFS Supra
0.37
5.0
3.0
0.6
3. Swiss Steel $
Thyrotherm E38K
0.35
0.3
5.00
1.35
0.45
4. Bohler Uddeholm
Orvar Supreme (H13)
0.39
5.2
1.4
0.9
5. Bohler Uddeholm $$
Bohler W321 (H10A)
2.9
2.8
0.65
6. Bohler Uddeholm $$
QRO 90 Supreme
0.75
2.6
2.25
7. Wolfram Industries $$$
Triamet 5 95
8. Metso $$$
Marlok C1650
<0.008
<0.10
<0.30
14.0
4.5
10.5
0.2
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8. Experimental Methodology
Image analysis used to measure crack propagation
Crack length, crack width, crack density, and crack depth examined
Each material is subjected to a total six thousand thermal fatigue cycles and examined every 1000 cycles
The thermal profile for each cycle is designed to be similar to a industry HPDC cycle; heating to 640°C using 130 KJ induction heating and rapid cooling 500°C/sec with HPDC die lubricant
Vickers Hardness measurements of cross-sectioned samples after 6000 thermal fatigue cycles
Optical & metallurgical examination also conducted
Length
Width
Crack Density
Crack Length
Crack Width
Crack Depth
Crack Measurements
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Results
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10. Optical Examination after 6000 Thermal Fatigue Cycles
Thyrotherm 2343 EFS Supra
Thyrotherm 2367 EFS Supra
Thyrotherm E38K
Ovar Supreme
Bohler W321
QRO90 Supreme
Triamet
Marlok 1650
Cracks appear to be in an orderly pattern and run perpendicular to each other in the longitudinal and axial direction of the sample
eventually combining with each other to form major cracks
In the background of major cracks there is a finer network of cracks that is developing and its anticipated they will also develop
into major cracks with further thermal fatigue cycles
“pull-outs” have been observed as seen in HPDC dies used in production
Exponential crack propagation occurs once cracks have nucleated
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11. Image Analysis Measurements Crack Length
Width
Crack Density
Crack Length
Crack Width
Crack Depth
Crack Measurements
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12. Image Analysis Measurements Crack Width
Length
Width
Crack Density
Crack Length
Crack Width
Crack Depth
Crack Measurements
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13. Image Analysis Measurements Crack Density
Length
Width
Crack Density
Crack Length
Crack Width
Crack Depth
Crack Measurements
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Crack Depth
Length
Width
Crack Density
Crack Length
Crack Width
Crack Depth
Crack Measurements
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15. Softening of Tool Material
Significant softening observed for all ferrous tool materials ~100HV points
Softening contained to ~3 mm below the surface then gradually recovers to original hardness
Softening observed in production dies usually after >103 cycles
No considerable hardness drop with the Triamet™ material
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16. Microstructure Analysis
Oxide formation
Thermal fatigue
cracks
Optical image of etched Marlok 1650 cross-sectioned
sample showing higher degree of etched at surface indicating
softeneing
Optical image of cross-sectioned Thyrotherm E38K
showing oxide formation depth inside cracks
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17. Modelling Total Fatigue Life
Fatigue Damage Model
where sf'- fatigue strength coefficient; b - fatigue strength exponent; ef' - fatigue ductility coefficient; c - fatigue ductility exponent; E - elastic modulus
Oxidation Damage Model
where eo - threshold strain for oxide cracking; Hcr - constant related to critical oxide thickness; b - mechanical strain range exponent; b - thermal strain rate sensitivity exponent
Creep Damage Model
where DHcr- activation energy for creep; Acr - scaling constant for creep ; m - creep stress exponent; a1- stress state constant ; a2- hydrostatic stress sensitivity constant
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18. Modelling Total Fatigue Life (continue)
Oxidation Model
Creep Model
Heat Transfer Model
where , cp and are density, specific heat and thermal conductivity of the sample’s material, respectively; r and z – are axis of the along sample’s radius and height, respectively; T – is the temperature within the sample
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19. Material properties for TMF modelling
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Development of a Numerical TMF Modelling
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Development of a Numerical TMF Modelling
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Summary
Overall eight die materials were successfully tested for their ability to resist thermal fatigue cracking during HPDC
A new thermal fatigue test method has been successfully developed that enables ferrous and non-ferrous tool materials to be tested under identical thermal cyclic conditions similar to a HPDC cycle
Numerical model has been developed based on the triple mechanism of TFM life prediction.
Tungsten, Bohler W321 & Marlok C1650 tool materials containing cobalt performed better than die Bohler Supreme, QRO 90 and Thyrotherm die materials
Image analysis measurements showed the worst performing also have the widest and longest and generally deepest thermal fatigue cracks
Significant softening was observed on all ferrous tool materials of ~100HV points
Metallographic analysis of crack depth reveal oxide formation deep inside cracks
Triamet non-ferrous material did not suffer from ant thermal fatigue cracks after 6000 cycles
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