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

Transcription

1 NADCA Die Materials Committee Meeting EFFECT OF DESIGN FACTORS ON THERMAL FATIGUE CRACKING OF DIE CASTING DIES John F. Wallace David Schwam Sebastian Birceanu Sun Feng Case Western Reserve University Cleveland, June 11, 2002

2 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.)

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

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

5 The Maximum Temperature Cycle for 5 sec Immersion 1200 max 1008 F 1000 Temperature [F] Time [sec]

6 The Maximum Temperature Cycle for 7 sec Immersion 1200 max 1130 F 1000 Temperature [F] Time [sec]

7 1400 Temperature Variation at the Corner of 2x2x7 H13 Specimen The Maximum Temperature Cycle for 9 and 12 sec Immersion Time Temperature [F] sec immersion time 9 sec immersion time Time [sec]

8 Maximum Temperature Cycle for 9 and 12 sec Immersion - Close-Up max 1212 F max 1228 F Temperature [F] sec immersion 9 sec immersion Time [sec]

9 Total Crack Area [x 10 6 µm 2 ] Total Crack Area vs. Maximum Temperature at the Corner of 2x2x7 H13 Specimen For Different Immersion Times Corner, cycles 5 sec 7 sec 1008 F CRTICAL TEMPERATURE 1130 F 9 sec 12 sec 1212 F 1228 F Temperature [F]

10 TOTAL CRACK AREA OF H13-45HRC 5, 7, 9 and 12 sec immersion time H13-45 HRC/5 sec immersion time 2"X2"X7", WC Total Crack Area (x 10 6 µm 2 ) H13-45 HRC/7 sec immersion time H13-45 HRC/9 sec immersion time H13-45 HRC/12 sec immersion time Thermal Cycles

11 AVERAGE MAX CRACK LENGTH OF H13-45HRC 5, 7, 9 and 12 sec immersion time Average Max Crack Length m) (x H13-45HRC, 5 sec immersion time H13-45HRC, 7 sec immersion time H13-45HRC, 9 sec immesion time H13-45HRC, 12 sec immersion time Thermal Cycles 2"X2"X7", WC

12 7 in(178 mm) 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)

13 Softening of the 2x2x7 H13 Specimen Hardness HRC The hardness testing begins at the corner H13/45 HRC, 5 sec immersion H13/45 HRC, 7 sec immersion H13/45 HRC, 9 sec immersion H13/45 HRC, 12 sec immersion Distance from the Corner [x100 µm]

14 Hardness Loss vs. Immersion Time for Different Distances from the Corner " 12 sec " 9 sec Hardness Loss [HRC] sec 7 sec Immersion Time [sec] The loss in hardness is most severe at the corner and becomes less severe further away

15 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 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.

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

17 Total Crack Area (x 10 6 µm 2 ) TOTAL CRACK AREA of 2X2X7 H13 Specimen Different Cooling Line Diameters 1.5" cooling line 1.6" cooling line 1.8" cooling line 2"X2"X7", WC Number of Cycles 35

18 Average Max Crack Length(x100 µm) AVERAGE MAXIMUM CRACK LENGTH of 2x2x7 H13 Specimen - Different Cooling Line Diameters 1.5" cooling line 1.6" cooling line 1.8" cooling line Number of Cycles 2"X2"X7", WC7 8.25

19 Hardness Variation for Different Cooling Line Diameters (15000 cycles) Hardness HRC " cooling line 1.6" cooling line 1.8" cooling line Distance from the Corner [x100 µm]

20 The Effect of Cooling Line Diameter on Average Maximum Crack Length Average Maximum Crack Length [x 100 µm] " 1.7" 1.6" 15, 000 cycles The larger the cooling line the more heat it removes, thus lowering the temperature, reducing softening and cracking " Corner Temperature [F]

21 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.

22 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.

23 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.

24 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 and molten aluminum. Vary internal cooling line diameter and water flow rate. Use inter- changeable baffles and bubblers. Compare outlet water temperature and specimen temperature for constant water inlet temperature at different flow rates.

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

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

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

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

29 "Hot Spot" Temperature for 0.2" ID Bubbler vs. 0.3" ID Bubbler 800 Furnace at 1800 o F " ID Temperature [F] " ID Flow Rate [gal/min]

30 The Effect of Flow Velocity through Tubes on the Heat Transfer Coefficient

31 Schematic of Bubbler/Specimen for Molten Al Bubbler Water Flow Specimen Hole for Thermocouple

32 Schematic of Experimental Rig Water Outlet Specimen Water Inlet Flow Meter Thermocouple Molten A356:1350 F Furnace Cycle time 25 seconds in 25 seconds out

33 Gap of Bubbler/Specimen φ 1.5 φ 9/16 Bubbler Gap Between Bubbler & Specimen Immersion Depth: 1 I.D. O.D. 1.5mm 3/8 5mm Specimen

34 Geometry Parameters for Bubbler & Specimen I.D. of Bubbler, [inch] O.D. of Bubbler, [inch] Wall Thickness of Bubbler, [inch] Inner Section Area of Bubbler, [inch 2 ] I.D. of Bubbler, [inch] O.D of Bubbler, [inch] Bubbler/Specimen Gap, [inch] Bubbler/Specimen Annular Area, [inch 2 ]

35 940 Effect of Water Flow Rate on "Hot-spot" Temperature in Dip in/out Experiments " Bubbler Temp. [F] " Bubbler Flow Rate [g/m] 0.17" Bubbler 0.307" Bubbler

36 Effect of Water Jet Velocity on "Hot-spot" Temperature 0.307" Bubbler Temperature [F] " Bubbler Water Jet Velocity [feet/second] 0.17" Bubbler 0.307" Bubbler

37 Depndency of Outlet Water Temperature on Flow Rate Outlet Water Temp. [F] Inlet water temperature is 53F Flow Rate [g/m] 0.17" Bubbler 0.214" Bubbler 0.307" Bubbler

38 The Relationship Between Outlet Cooling Water and Jet Flow Velocity Outlet Water Temperature [F] "Bubbler 0.307" Bubbler 0.214" Bubbler Jet Velocity [Feet/second] 0.17" Bubbler 0.214" Bubbler 0.307" Bubbler

39 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 and thermal fatigue cracking 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.