1. Optimization of Laser Deposited Rapid Tooling D. Schwam and Y. Wang Case Western Reserve University NADCA DMC, Chicago – Feb.17, 2010 1

2. 2 Outline Objectives-why laser deposit? The POM process Preliminary thermal fatigue results In-plant evaluation of cores at General Die Casters FEA optimization of laser deposited layer Conclusions

3. 3 Objectives – why laser deposit? Use of high thermal conductivity alloys in cores and inserts can accelerate heat extraction, lower surface temperature reduce soldering and shorten cycle time. Copper alloys have good thermal conductivity and are cost effective candidates for this application. However, copper alloys dissolve in molten aluminum. High temperature and high velocity molten metal flow exacerbate dissolution. A layer of laser deposited H13 over the copper can prevent dissolution.

4. Washout Damage at the Corners of Cu-Ni-Sn Thermal Fatigue Specimens ToughMet 3 ToughMet 2 0.5” Copper alloys dissolve in molten aluminum

5. Metal Mold Material Properties

6. Rapid Tooling by DMD *-Courtesy POM Direct Metal Deposition of H13 on Copper - the POM Method

7. H13 / H13 sample – as deposited/ finished H13 / Alloy 940 sample – as deposited/ finished

9. Evaluation of Laser Deposited Cores The core is surrounded by molten aluminum and has a record of overheating and soldering. Extracting heat more efficiently from the core can lower maximum temperature, prevent soldering and allow shorter cycle times. 9

10. Typical H13 core with cooling line 10

11. Composite Core -- H13 Deposited on Cu H13 Cu 11

12. 12 Thermal Profile of Cover Half Steel During Solidification Composite CoreH-13 Core At start of a cycle

13. Thermal Profile of Section Through Cover Half at Die Opening Composite CoreH-13 Core At 20 seconds Temperature of Composite Core is lower. 13

14. Temperature Advantage of the Copper Core Temperature of Composite Core is lower. 14

15. Creep Failure of in First Composite Core The core shows creep damage after 250 cycles due to insufficient stiffness and strength at high temperature. 15

16. Surface Cracks in the H13 Layer 16

17. Remedial Approaches Caves in Bulges out The distortion of the core seems to originate from insufficient strength and stiffness at the operating temperature. Anviloy and H13 cores do not suffer from this problem. Priority 1 - Increase strength: use core as deposited w/o tempering (downside-lower toughness). Priority 2 - Increase thickness of H13 layer(downside slows down heat transfer).

18. 18 Optimization of the Laser Deposited H13 The first core was tempered aggressively. The hardness of the laser deposited H13 was reduced from 51HRC to 40 HRC to improve toughness. However, this reduced the strength and caused excessive distortion. The core had to be removed after 250 shots. A new core was made with the laser deposited H13 in the as-deposited condition (no tempering) at 51HRC. This core has been in production at General Die Casters. So far it has accumulated 5,000 shots.

19. FEA optimization of the H13 laser deposited layer thickness The 3D Die Casting model was simplified to a 2D axi- symmetric FEA model and analyzed with commercial FEA software Abaqus. The accuracy of 2D Abaqus analysis was verified by comparing it to a 3D MagmaSoft simulation. Cyclic heat transfer and stress analysis was applied to study the failure mechanism of the composite core. Composite cores with different H13 layer thickness and hardness were compared, to optimize the design. 19

20. Finite Element Analysis Model Mold (H13) Casting (A389) Composite Core: Copper (394) Steel (H13) 2D Axi-symmetric model 20

21. Boundary Conditions Preheat (40 sec) Mold close, Metal Injection and Solidification (20 sec) Mold open, Ejection, air cooling (4 sec) Spray cooling (3 sec) Air cooling (13 sec) New cycle begins, each cycle takes 40 seconds. Simulation initial conditions Casting Alloy A389 Mold Material H13 Furnace Temperature 1350 o F Material plastic behavior strain rate independent isotropic hardening Process flow chart: Stress Strain 0 Yield strength Elongation Tensile strength Material stress-strain property definition: 21

22. Parameters studied Label of different designs Thickness of H13 in composite core Core0500.05 in Core1000.1 in Core1500.15 in Parameter 2 – Thickness of the H13 layer: Label of different hardness Yield strength at room temperature (ksi) UTS at room temperature (ksi) HR40153183 HR50239289 Parameter 1 – H13 hardness / strength: ( Ref: Philip D. Harvey, Engineering Properties of Steel, American Society for Metals, Metal Parks, Ohio, 1982, P457-462.) 22

23. Verification of Axi-symmetric FEA Heat Transfer Analysis* Maximum temperature difference is smaller than 20 o C. 23 * Abaqus compared to MagmaSoft

24. Thermo-mechanical Analysis for Core100 with H13-HR40 A B C Temperature and stress fields become stable after five cycles. Edge, 1” from corner 24

25. Thermo-mechanical Analysis for Core100 B I. Thermal gradient II. Thermal expansion mismatch III. Thermal gradient IV. Thermal expansion mismatch Dominant factors in deformation: 2 1 3 The thermal gradient at the surface is a key factor in deformation and failure. Normal stress along the hoop’direction is the largest. Temperature 25

26. Thermo-mechanical analysis findings The temperature and stress fields become stable in about five cycles. The thermal gradient at the surface is the key factor to deformation and failure. Normal stresses in the hoop direction are largest at the surface, and may cause surface cracks. The failure mechanism depends on the maximum stress and whether it exceeds the material strength at the operating temperature. 26

27. Type I Failure Mechanism – Creep Core100 H13-HR40/ Copper Core after 250 cycles When strength is low (ex. H13-HR40). σ yield << σ max ~ UTS Plastic deformation accumulates rapidly. 2 1 3 Buckling occurs due to plastic deformation. Improving material strength will prevent this failure mechanism. 27

28. Type II Failure - Low cycle fatigue When strength is high (ex. H13-HR50). σ yield <σ max < UTS Plastic deformation accumulates slowly. Radial surface cracks occur due to low cyclic fatigue. The number of cycles (N) to failure can be predicted by an empirical model: ΔSΔS 28

29. Factors that determine low cycle fatigue life ΔS=σ max -σ min ΔT 1 ΔT 2 Fatigue life N Stress variation ΔS Surface thermal gradient Temperature variation ΔT Core thermal conductivity α Higher conductivity for thinner H13 layer Not linear relationship to H13 thickness H13 layer thickness Detailed analysis 29

30. The Effect of layer thickness on the low cycle fatigue life of the composite core The design of composite core with 0.05” thick H13-HR50 layer offers: Lower max temperature; Longer fatigue life. Fatigue life for typical H13 Core 30

31. Summary A pre-requisite to prevent premature failure is sufficient strength to avoid creep. A 0.050” thick H13-HR50 layer provides a good balance of heat transfer and thermal fatigue resistance: – Maximizes heat flux, decreasing mold temperature, and shortening cycle time. – Reduces the thermal gradient, lowering maximum stress and extending core life. 31