EFFECT OF DESIGN FACTORS ON THERMAL FATIGUE CRACKING OF DIE CASTING DIES John F. Wallace David Schwam Sebastian Birceanu Case Western Reserve University NADCA Die Materials Committee Meeting Rosemont, March 6, 2002

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.)

“Up” motion travel time + “Down” motion travel time ~ 2sec (1) Variation in the immersion time

3.5 in (89 mm) 0.1 in (2.54 mm) Corner Temperature Measurement Setup T/C Junction

12 sec immersion time 9 sec immersion time

CRTICAL TEMPERATURE

7 in(178 mm) 3.5 in(89 mm) 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 MICRO-HARDNESS MEASUREMENT

5 sec 7 sec 9 sec 12 sec Note: Longer immersion times cause more severe softening

The loss in hardness is most severe at the corner and becomes less severe further away

Distribution of the Carbides in the Thermal Fatigue Specimen A mechanism of softening at the corners is carbide coarsening

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.

D = 1.5 “D = 1.6 “D = 1.8 “ 1.5” 1.6” 1.8” (2) Variation of the cooling line diameter

1.5” 1.6” 1.8”

1.5” 1.6” 1.8”

1.5” 1.6” 1.8”

Stress [psi] Time [sec] Experimental Data for Stress in the 1.5 “Cooling Line of 2x2x7 H13 Specimen

Details of Through Cracks on Sides of 2x2x7” Specimen (annealed; off-center cooling line, 5000 cycles)

Crack Length [  m] Crack Distribution on Sides of 2x2x7” H13 Specimen (annealed; 5000 cycles)

Details of the Largest Cracks on Sides of 2x2x7” H13 Specimen (annealed; 5000 cycles )

CONCLUSIONS Below a certain temperature threshold the thermal fatigue damage is minimal; this observation applies to the ground 2”x2”x7” 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.

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.

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.

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.

Flow Furnace Water Inlet Water Outlet Meter for Flow Rate and Temperature Specimen Set-up for Evaluation of Baffles and Bubblers Data Acquisition

Set-up for Evaluation of Baffles and Bubblers Water Inlet Water Outlet Flow Meter Furnace

Hole for Thermocouple Baffle Water flow Water-in Water-out Schematic of Baffle-cooled specimen HOT SPOT!

Hole for Thermocouple Bubbler Water In Water Out Schematic of Bubbler-Cooled Specimen HOT SPOT!

Furnace at 1800 o F

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.