EFFECT%20OF%20DESIGN%20FACTORS%20ON%20THERMAL%20FATIGUE%20CRACKING%20OF%20DIE%20CASTING%20DIES

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NADCA Die Materials Committee Meeting
EFFECT OF DESIGN FACTORS ON THERMAL FATIGUE
CRACKING OF DIE CASTING DIES
John F. Wallace David Schwam Sebastian
Birceanu Case Western Reserve University
Rosemont, March 6, 2002
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EFFECT OF MAXIMUM TEMPERATURE ON THERMAL FATIGUE
DAMAGE OBJECTIVE Determine the effect of the
maximum temperature on the thermal fatigue
cracking at the corners of the 2x2x7 H13
specimen. APPROACH (1) Vary the immersion time
(5, 7, 9, 12 sec.), while the overall cycle time
remains the same (36 sec). (2) Vary the cooling
line diameter 1.5, 1.6, 1.7, 1.8), without
changing the cycle time (9 sec. immersion time,
36 sec overall cycle time.)
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(1) Variation in the immersion time
Up motion travel time Down motion
travel time 2sec
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Corner Temperature Measurement Setup
T/C Junction
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12 sec immersion time
9 sec immersion time
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CRTICAL TEMPERATURE
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12
9
12
9
7
5
7
12
5
7
9
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MICRO-HARDNESS MEASUREMENT

• The hardness is measured at the center of the
  specimen,beginning at
• 0.01 in (0.254 mm) from the edge.
• The next testing steps 0.02in (0.508 mm),
  0.04in (1.016 mm),
• 0.06in (1.524mm), 0.08in (2.032mm), 0.1
  (2.54mm), 0.15in , 0.2in
• then in 0.1in increments until no further
  variation of hardness occurrs

3.5 in(89 mm)
7 in(178 mm)
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Note Longer immersion times cause more severe
softening
5 sec
7 sec
9 sec
12 sec
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The loss in hardness is most severe at the corner
and becomes less severe further away
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Distribution of the Carbides in the Thermal
Fatigue Specimen
A mechanism of softening at the corners is
carbide coarsening
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MECHANISM OF THERMAL FATIGUE CRACK NUCLEATION
AND PROPAGATION

• Most new H13 die have sufficient strength to
  resist immediate
• formation of cracks.
• After being exposed to thermal fatigue cycling,
  the hot areas
• of the die will soften, thereby losing
  strength. When the fatigue
• strength of the steel drops below the operating
  stresses cracks
• will form and propagate.
• Crack propagation is gradual and controlled by
  the gradual
• softening that progresses with time deeper
  into the die.
• Note If the operating thermal stresses
  combined with stress
• concentration factors exceed the fatigue strength
  of the steel,
• fatigue cracks can propagate even w/o softening.

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(2) Variation of the cooling line diameter
D 1.5
D 1.6
D 1.8
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Experimental Data for Stress in the 1.5 Cooling
Line of 2x2x7 H13 Specimen
Stress psi
Time sec
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Details of Through Cracks on Sides of 2x2x7
Specimen (annealed off-center cooling line, 5000
cycles)
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Crack Distribution on Sides of 2x2x7 H13
Specimen (annealed 5000 cycles)
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Details of the Largest Cracks on Sides of 2x2x7
H13 Specimen (annealed 5000 cycles)
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• CONCLUSIONS
• Below a certain temperature threshold the
  thermal fatigue damage is minimal this
  observation applies to the ground 2x2x7 H13
  specimen tested to 15,000 cycles, in the absence
  of high stress concentrators.
• The thermal fatigue damage is mainly determined
  by the temperature-time cycle, the thermal
  stresses and the softening of the specimen.
• A longer dwell time at high temperature is more
  damaging than
• a short one. This is because of the accelerated
  softening effect at
• high temperature.

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• CONCLUSIONS (continued)
• Long dwell times at high temperature simulate die
  casting of large components, where the die
  surface is subjected to elevated temperature for
  longer periods of time.
• The experimental results demonstrate less thermal
  fatigue damage when the cooling line is closer to
  the surface and lowers the temperature.
• However, bringing the cooling lines closer to the
  surface may cause high hoop stresses in the
  cooling line and at the surface. This may
  increase the danger of gross cracking.

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METHODS OF KEEPING HOT SPOTS IN DIES
BELOW CRITICAL TEMPERATURE
1. Longer cycle time that allows die to cool -
slows production. 2. More insulating die
lubricants - slows production. 3. More water
spray - danger of thermal shock. 4. Die
materials with better heat diffusivity. 5.
Larger cooling lines drilled closer to hot spots
- accessibility. 6. Optimized use of Baffles and
Bubblers.
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EVALUATION OF BAFFLES AND BUBBLERS

• OBJECTIVE
• Compare the efficiency of commercially available
  baffles and
• bubblers in removing heat from hot spots.
• METHOD
• Use standard size OD/ Length H13 specimen inside
  furnace.
• Vary internal cooling line diameter and water
  flow rate.
• Use inter- changeable baffles and bubblers.
• Compare outlet water temperature and specimen
  temperature
• for constant inlet temperature.

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Set-up for Evaluation of Baffles and Bubblers
Water Outlet
Meter for Flow Rate and Temperature
Water Inlet
Flow
Specimen
Data Acquisition
Furnace
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Set-up for Evaluation of Baffles and Bubblers
Water Inlet
Water Outlet
Flow Meter
Furnace
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Schematic of Baffle-cooled specimen
Baffle
Water-out
Water-in
Water flow
Hole for Thermocouple
HOT SPOT!
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Schematic of Bubbler-Cooled Specimen
Water In
Bubbler
Water Out
Hole for Thermocouple
HOT SPOT!
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Furnace at 1800oF
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• INTERIM CONCLUSIONS
• For identical water flow rates, smaller diameter
  bubblers
• generate a higher flow velocity and are more
  efficient in
• cooling a localized hot spot.
• Baffles and bubblers can be used to reduce the
  local temperature
• of hot spots in the die below critical
  temperatures that
• accelerate soldering.
• Surgical needle-size bubblers are commercially
  available for
• cooling hard-to-access hot spots and thin
  sections.
• Further experiments are planned to compare the
  cooling
• efficiency of different designs of baffles and
  bubblers.