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

John F. WallaceDavid Schwam

Sebastian BirceanuCase 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 m

m)

0.1 in (2.54 mm)

Corner Temperature Measurement Setup

T/C Junction

The Maximum Temperature Cycle for 5 sec Immersion

0

200

400

600

800

1000

1200

0 6 12 18 24 30 36

Time [sec]

Te

mp

era

ture

[F

]

max 1008 F

The Maximum Temperature Cycle for 7 sec Immersion

0

200

400

600

800

1000

1200

0 6 12 18 24 30 36

Time [sec]

Te

mp

era

ture

[F

]

max 1130 F

Temperature Variation at the Corner of 2x2x7 H13 Specimen

The Maximum Temperature Cycle for 9 and 12 sec Immersion Time

0

200

400

600

800

1000

1200

1400

0 6 12 18 24 30Time [sec]

Tem

per

atu

re [

F]

12 sec immersion time

9 sec immersion time

Maximum Temperature Cycle for 9 and 12 sec Immersion - Close-Up

0

200

400

600

800

1000

1200

1400

3 5 7 9 11 13 15 17Time [sec]

Tem

per

atu

re [

F]

12 sec immersion

9 sec immersion

max 1212 F max 1228 F

Total Crack Area vs. Maximum Temperature at the Corner of 2x2x7 H13 Specimen For Different Immersion Times

1008 F

1228 F

1212 F

1130 F

0

20

40

60

80

100

120

140

160

180

1000 1050 1100 1150 1200 1250

Temperature [F]

To

tal C

rack

Are

a [x

10

6

m2]

Corner, 15000 cycles

5 sec 7 sec

9 sec

12 sec

CR

TIC

AL

TE

MP

ER

AT

UR

E

TOTAL CRACK AREA OF H13-45HRC5, 7, 9 and 12 sec immersion time

0 0.37 1.970.09 0.875.9

0.19 0.19

39.69

167.72

17.53

108.56

0

20

40

60

80

100

120

140

160

180

5000 10000 15000

Thermal Cycles

To

tal C

rac

k A

rea

(x

106 m

2)

H13-45 HRC/5 sec immersion time

H13-45 HRC/7 sec immersion time

H13-45 HRC/9 sec immersion time

H13-45 HRC/12 sec immersion time

2"X2"X7", WC7

AVERAGE MAX CRACK LENGTH OF H13-45HRC5, 7, 9 and 12 sec immersion time

0

2

4

6

8

10

12

14

16

18

20

5000 10000 15000

Thermal Cycles

Avera

ge M

ax C

rack L

en

gth

(x

100

m) H13-45HRC, 5 sec immersion time

H13-45HRC, 7 sec immersion time

H13-45HRC, 9 sec immesion time

H13-45HRC, 12 sec immersion time

2"X2"X7", WC7

7 9 12

12

12

9

9

77

5

5

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

Softening of the 2x2x7 H13 Specimen

15

18

21

24

27

30

33

36

39

42

45

2.54 5.08 7.62 10.16 12.7 15.24 17.78 20.32 22.86 25.4

Distance from the Corner [x100 m]

Ha

rdn

es

s H

RC

H13/45 HRC, 5 sec immersion

H13/45 HRC, 7 sec immersion

H13/45 HRC, 9 sec immersion

H13/45 HRC, 12 sec immersion

The hardness testing begins at the corner

5 sec

7 sec

9 sec

12 sec

Note: Longer immersion times cause more severe softening

Hardness Loss vs. Immersion Time for Different Distances from the Corner

0

5

10

15

20

25

3 4 5 6 7 8 9 10 11 12 13

Immersion Time [sec]

Har

dnes

s Lo

ss [H

RC

]

0.02"

0.06"

5 sec

7 sec

9 sec

12 sec

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 stressconcentration 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

TOTAL CRACK AREA of 2X2X7 H13 SpecimenDifferent Cooling Line Diameters

0.19

17.53

108.56

0.18

13.96

0.1813.81

35

79.44

0

20

40

60

80

100

120

140

160

180

5000 10000 15000

Number of Cycles

To

tal C

rack

Are

a (x

10

6

m2)

1.5" cooling line1.6" cooling line1.8" cooling line 2"X2"X7", WC7

1.5”

1.6”

1.8”

AVERAGE MAXIMUM CRACK LENGTH of 2x2x7 H13 Specimen - Different Cooling Line Diameters

0.85

5.25

12.5

0.75

5.25

12.25

0.75

5

8.25

0

2

4

6

8

10

12

14

16

18

5000 10000 15000

Number of Cycles

Av

era

ge

Ma

x C

rac

k L

en

gth

(x1

00

m

) 1.5" cooling line1.6" cooling line1.8" cooling line

2"X2"X7", WC7

1.5”

1.6”

1.8”

Hardness Variation for Different Cooling Line Diameters(15000 cycles)

20

25

30

35

40

45

50

55

2.54 7.62 12.7 17.78 22.86

Distance from the Corner [x100 m]

Ha

rdn

es

s H

RC

1.5" cooling line1.6" cooling line1.8" cooling line1.5”

1.6”

1.8”

0

2

4

6

8

10

12

14

900 950 1000 1050 1100

Corner Temperature [F]

Ave

rage

Max

imum

Cra

ck L

engt

h [x

100

m

]

1.8"

1.7"

1.6" 1.5"

15, 000 cycles

The Effect of Cooling Line Diameter on Average Maximum Crack Length

The larger the cooling line the more heat it removes, thus lowering the temperature, reducing softening and cracking

Str

ess

[psi

]

Time [sec]

Experimental Data for Stress in the 1.5 “Cooling Line of 2x2x7 H13 Specimen

0

20000

40000

60000

80000

100000

120000

36 72 108 144 180 216 252

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

Cra

ck L

engt

h [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 thana short one. This is because of the accelerated softening effect athigh 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 BELOWCRITICAL 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 InletWater Outlet

Flow Meter

Furnace

Hole for Thermocouple

Baffle

Water flow

Water-inWater-out

Schematic of Baffle-cooled specimen

HOT SPOT!

Hole for Thermocouple

BubblerWater In

Water Out

Schematic of Bubbler-Cooled Specimen

HOT SPOT!

"Hot Spot" Temperature for 0.2" ID Bubbler vs. 0.3" ID Bubbler

500

550

600

650

700

750

800

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Flow Rate [gal/min]

Tem

per

atu

re [

F]

0.3" ID

0 0.2" ID

Furnace at 1800oF

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.