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Stress Ratio Effects in Fatigue of Lost Foam Aluminum Alloy 356

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Lost foam casting is a highly versatile metalcasting process that offers significant benefits in terms of design flexibility, energy consumption, and environmental impact. In the present work, the fatigue behavior of lost foam cast aluminum alloy 356, in conditions T6 and T7, was investigated, under both zero and non-zero mean stress conditions, with either as-cast or machined surface finish. Scanning electron microscopy was used to identify and measure the defect from which fatigue fracture initiated. Based on the results, the applicability of nine different fatigue mean stress equations was compared. The widely-used Goodman equation was found to be highly non-conservative, while the Stulen, Topper-Sandor, and Walker equations performed reasonably well. Each of these three equations includes a material-dependent term for stress ratio sensitivity. The stress ratio sensitivity was found to be affected by heat treatment, with the T6 condition having greater sensitivity than the T7 condition. The surface condition (as-cast vs. machined) did not significantly affect the stress ratio sensitivity. The fatigue life of as-cast specimens was found to be approximately 60 – 70% lower than that of machined specimens at the same equivalent stress. This reduction could not be attributed to defect size alone, and may be due to the greater frequency of oxide films near the as-cast surface. Directions for future work, including improved testing methods and some possible methods of improving the properties of lost foam castings, are discussed.

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Stress Ratio Effects in Fatigue of Lost Foam Aluminum Alloy 356

  1. 1. Stress Ratio Effects in Fatigue of Lost Foam Aluminum Alloy 356 David E. Palmer, P.E. BRP – Marine Propulsion Systems Division, Sturtevant, WI
  2. 2. Introduction • Lost foam casting (LFC) is used to make outboard engine components (engine block, cylinder head, etc.) 2
  3. 3. Introduction • These components are used in fatigue, generally with non-zero mean stress max min   R 3
  4. 4. Introduction • Fatigue failures typically initiate from porosity or from the as-cast surface Fracture surfaceAs-cast surface Bead structure Fissure between foam beads Porosity 4
  5. 5. Problems • Large number of mean stress equations (Goodman, Soderberg, Walker, etc.) – which one to use? • Lack of published data on effect of as-cast surface on fatigue of LFCs • How to account for presence of porosity in LFCs? 5
  6. 6. Motivations 1. Provide as-cast mechanical property data for design engineers 2. Understand factors that influence fatigue of aluminum LFCs in order to find ways to make better castings 3. Gain insight into stress ratio sensitivity of materials 6
  7. 7. Objectives For LF aluminum alloy 356-T6 and 356-T7 with as-cast and machined surfaces: 1. Evaluate monotonic tensile properties 2. Generate S-N curves (R = –1, R = 0, R > 0) 3. Determine appropriate mean stress correction 4. Evaluate effect of defect size on fatigue life 7
  8. 8. Lost foam casting 8
  9. 9. Lost foam casting • Patterns made from expanded polystyrene (EPS) • Raw bead size 0.25 – 0.50 mm • Impregnated with 5 – 7% hexane (blowing agent) 9
  10. 10. Lost foam casting Poor pattern fusion can occur if beads are above Tg for insufficient time during molding process 10
  11. 11. Lost foam casting 11 • Assembled cluster is coated with refractory slurry • Coating may penetrate into gaps in foam beads
  12. 12. Lost foam casting 12 • Molten metal is poured directly into the EPS mold • As metal front advances, EPS degrades, melts, and vaporizes • LF mold filling is a highly complex process
  13. 13. 13 Lost foam casting Foam Coating Sand Metal Decomposition layer
  14. 14. Lost foam casting 14 Collapse mode: • Occurs when patterns have density gradients or poor fusion • Gaps between foam beads provide escape path for gas, resulting in low local pressures • Metal front advances in “fingers” • This mode results in fold defects as liquid pyrolysis products are trapped between metal fronts.
  15. 15. 15 Fatigue and mean stress
  16. 16. Fatigue and mean stress • There are a large number of equations that relate fatigue with mean stress (R ≠ –1) to an equivalent fully reversed stress (R = –1) • These include the Goodman, Soderberg, Morrow, Gerber, ASME-Elliptic, Smith- Watson-Topper, Stulen, Topper-Sandor, and Walker equations 16
  17. 17. Fatigue and mean stress 17 Goodman equation         u m a eq     1
  18. 18. 18 Fatigue and mean stress Soderberg equation         o m a eq     1
  19. 19. 19 Fatigue and mean stress Morrow equation            f m a eq     1
  20. 20. 20 Fatigue and mean stress Gerber equation 2 1         u m a eq    
  21. 21. 21 Fatigue and mean stress ASME-Elliptic equation 2 1         o m a eq    
  22. 22. 22 Fatigue and mean stress Smith-Watson-Topper equation aeq  max
  23. 23. 23 Fatigue and mean stress Stulen equation maeq A  • If A = σe / σu , this is equivalent to the Goodman equation; if A = σe / σo , it is equivalent to the Soderberg equation, etc. • Value of A must be determined from tests at different R ratios
  24. 24. 24 Fatigue and mean stress Topper-Sandor equation   maeq  • Power law relationship between σm and σeq • Value of α must be determined from tests at different R ratios
  25. 25. 25 Fatigue and mean stress Walker equation   aeq   1 max • If γ = 0.5 , this is equivalent to the Smith- Watson-Topper equation • According to Dowling, γ ≈ 0.45 for aluminum and 0.65 for steels • Value of γ must be determined from tests at different R ratios
  26. 26. 26 Aluminum alloy 356
  27. 27. Aluminum alloy 356-T6 27
  28. 28. 28 Aluminum alloy 356-T6
  29. 29. 29 Experimental
  30. 30. Experimental design 30 356-T6 As-cast 356-T7 As-cast 356-T6 Machined 356-T7 Machined Tension testing: 5 specimens each Fatigue testing: 15 specimens R = -1 15 specimens R = 0 15 specimens R > 0 SEM porosity measurements
  31. 31. Sample preparation: machined 31
  32. 32. Sample preparation: as-cast 32
  33. 33. Pattern fusion testing • Pattern permeability apparatus developed at University of Alabama-Birmingham (UAB) • Measures air flow rate when 21 kPa vacuum is applied to surface of foam pattern • Used to evaluate pattern fusion for as-cast specimens 33
  34. 34. Pattern permeability 34 Average: 4.3 cm/s Standard deviation: 2.1 cm/s
  35. 35. Tensile testing 35 • Performed per ASTM E8 • Constant displacement rate (5 mm/min.) • Specimen deflection measured with extensometer
  36. 36. Stress-strain curves 36 356-T6 machined 356-T6 as-cast
  37. 37. Stress-strain curves 37 356-T7 machined 356-T7 as-cast
  38. 38. Tensile fracture surfaces 38 356-T6 machined 356-T6 as-cast
  39. 39. Tensile fracture surfaces 39 356-T7 machined 356-T7 as-cast
  40. 40. Fatigue testing 40 • Performed per ASTM E466 • Tested in force control • Three different R-ratios (R = -1, R = 0, R > 0) • Six different load levels at each R-ratio • For R > 0 testing, σmax was held at 0.5σy while σmin was varied to produce R = 0.09, R = 0.26, R = 0.31, R = 0.40, R = 0.44, and R = 0.62 conditions
  41. 41. S-N curves 41 356-T6 Machined
  42. 42. S-N curves 42 356-T6 As-cast
  43. 43. S-N curves 43 356-T7 Machined
  44. 44. S-N curves 44 356-T7 As-cast
  45. 45. Fatigue fracture surfaces 45 356-T6 machined 356-T6 as-cast
  46. 46. Fatigue fracture surfaces 46 356-T7 machined 356-T7 as-cast
  47. 47. Weibull analysis 47
  48. 48. Weibull analysis 48 356-T7 As-cast B50 = 54.8 MPa B10 = 44.2 MPa α = 57.1 ß = 8.72
  49. 49. Weibull analysis 49
  50. 50. Critical pore size 50 Average area of critical pore 0.111 mm² (machined); 0.120 mm² (as-cast)
  51. 51. Effect of pore size on fatigue 51 Results for T6 and T7 lie along the same line
  52. 52. Effect of pore size on fatigue 52 Difference in fatigue life between as-cast and machined cannot be attributed to porosity
  53. 53. Folds in as-cast specimens 53
  54. 54. 54 Comparison of mean stress equations
  55. 55. Comparison of mean stress equations 55 1. Goodman 2. Soderberg 3. Morrow 4. Gerber 5. ASME-Elliptic 6. Smith-Watson-Topper 7. Stulen 8. Walker 9. Topper-Sandor
  56. 56. Comparison of mean stress equations 56 Error = Predicted life – actual life Actual life
  57. 57. Comparison of mean stress equations 57 Condition Surface Goodman Soderberg Morrow T6 Machined 254% 134% 1238% As-cast 343% 339% 1347% T7 Machined 323% 246% 1040% As-cast 202% 189% 748%
  58. 58. Comparison of mean stress equations 58 Condition Surface Gerber ASME- Elliptic SWT T6 Machined 1517% 1840% 18% As-cast 1947% 2592% 44% T7 Machined 1866% 2362% -23% As-cast 1091% 1370% -10%
  59. 59. Comparison of mean stress equations 59 = Predicted life – actual life Actual life Absolute error
  60. 60. 60 Comparison of mean stress equations Condition Surface Stulen Topper- Sandor Walker T6 Machined 47% 36% 33% As-cast 44% 38% 40% T7 Machined 40% 30% 34% As-cast 48% 42% 45%
  61. 61. 61 Comparison of mean stress equations 356-T7 Machined No mean stress correction
  62. 62. 62 Comparison of mean stress equations 356-T7 Machined Goodman correction
  63. 63. 63 Comparison of mean stress equations 356-T7 Machined ASME- Elliptic correction
  64. 64. 64 Comparison of mean stress equations 356-T7 Machined Walker correction
  65. 65. 65 Mean stress sensitivity parameters Condition Surface Stulen Topper- Sandor Walker T6 Machined 0.417 0.793 0.530 As-cast 0.454 0.803 0.563 T7 Machined 0.341 0.734 0.459 As-cast 0.372 0.749 0.480
  66. 66. Mean stress sensitivity 66 Kirby and Beevers (1971): In air: da/dN = f(ΔK, R) In vacuum: da/dN = f(ΔK) ONLY! Chalwa et al (2011): R-ratio effects increase with P(H2O)
  67. 67. Hypothesis: Greater mean stress sensitivity of 356-T6 compared to 356-T7 is due to greater oxidation rate on crack surface. 67 Mean stress sensitivity This hypothesis will be tested in future work.
  68. 68. 68 Conclusions
  69. 69. Conclusions 69 1. Lost foam 356-T6 and 356-T7 specimens with as-cast surface have significantly lower monotonic and fatigue properties compared to specimens with a machined surface.
  70. 70. Conclusions 70 2. Ranking of mean stress equations: Topper-Sandor Walker Stulen Smith-Watson-Topper Soderberg Goodman Morrow Gerber ASME-Elliptic Best Worst DO NOT USE (Tie) Best if no data for fit
  71. 71. Conclusions 71 3. Ranking of effects on fatigue of lost foam aluminum 356: As-cast surface Porosity Heat treatment> >
  72. 72. 72 Conclusions 4. Lost foam 356-T6 has greater stress ratio sensitivity than lost foam 356-T7.
  73. 73. 73 Future work
  74. 74. Future work • Investigate effect of pattern fusion: > 10 cm/s (“beady”) 1 – 10 cm/s (present work) < 0.5 cm/s (smooth) 74
  75. 75. Future work 75 • Measure crack growth rates (da/dN) for as-cast and machined specimens Hypothesis: Crack propagation is faster in as-cast specimens due to presence of folds
  76. 76. Future work 76 • Measure polarization resistance of lost foam 356-T6 and 356-T7 Hypothesis: Greater mean stress sensitivity of 356-T6 compared to 356-T7 is due to greater oxidation rate on crack surface
  77. 77. Future work 77 • Investigate effect of chills on as-cast properties of lost foam castings
  78. 78. Future work 78 • Investigate other possible means of improving properties of LF castings: Vibration during solidification Vacuum-assisted filling Solidification under pressure
  79. 79. Future work 79 • Fully-reversed four-point bending fatigue fixture
  80. 80. Future work 80 • Investigate environmental effects on fatigue of LF castings: Saltwater Water velocity Water temperature Galvanic potential
  81. 81. Acknowledgements UWM - Dr. Rohatgi, Dr. Venugopalan, Dr. El-Hajjar, Dr. Church, Betty Warras BRP - Glover Kerlin, Bill Barth, Jim Bonifield, Ken Chung, Matt Coyne, Todd Craft, Ben Jones, Mark Noble, Rich Smock, Karl Glinsner, Pete Lucier IIT - Dr. Sheldon Mostovoy Virginia Tech - Dr. Norman Dowling ASU - Dr. Nik Chawla UAB - Harry Littleton My family - Thanks for everything! 81

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