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Microstructure Analysis of Shot Peened Cast Ti-6Al-4V after Fatigue Fracture

Microstructure Analysis of Shot Peened Cast Ti-6Al-4V after Fatigue Fracture. By Chun Ling Huang Southern University and A&M College Baton Rouge, Louisiana 70813 ASEE Mid Atlantic Conference on April 12, 2003. Abstract.

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Microstructure Analysis of Shot Peened Cast Ti-6Al-4V after Fatigue Fracture

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  1. Microstructure Analysis of Shot Peened Cast Ti-6Al-4V after Fatigue Fracture By Chun Ling Huang Southern University and A&M College Baton Rouge, Louisiana 70813 ASEE Mid Atlantic Conference on April 12, 2003

  2. Abstract • Shot peening is a cold working process in which the surface of a part is bombarded with small spherical balls. • Cast Ti-6Al-4V samples were shot peened and compared to non-shot peened samples after both were subjected to a low-cycle fatigue test at a constant frequency of 10 Hz. • SEM examination of fractured specimen materials showed loss of voids, and deformed cleavage paths, an indication of crack initiation and consequent propagation, which resulted in higher fatigue strength of the shot-peened parts.

  3. Uses of Titanium Alloy • Aerospace applications • Lightest possible/withstand cylic fatigue stresses • Shot peening has long been used to improve… • Corrosion resistance, high temperature, high strength to weight ratio • Titanium is a brittle metal but alloy…

  4. What is Shot Peening? • A cold working process in which the surface of a part is bombarded with high velocity small spherical shots emanating from a nozzle. • The dimpled surface occurs as a result of the surface fibers material yielding in tension. • The resistance of the subsurface material that tries to restore the surface to its original shape creates a hemisphere of cold-worked material highly stressed in compression. • The thickness of this compressive layer is about 1 mm. • Nearly all fatigue and stress corrosion failures originate at the surface, compressive stresses by shot peening provide considerable increases in the fatigue and life of the part.

  5. What is Shot Peening? (Cont’d) • Max fatigue strength of titanium shot peened with steel balls occurred about 0.2 mmN. • Shot-peening may improve fatigue life by factor 2 up to 40 . • Microstructure of titanium alloys consists two types- primary alpha and 2ndary alpha . • A micro structural examination of titanium alloys will therefore provide useful information about material, processing, both melting and metalworking. • It seems obvious that the cold metal working process of shot peeing conceivably could be used as a method of improving the fatigue strength of cast titanium parts.

  6. Materials and Experiments • Materials: ASTM standards – cast Ti-6Al-4V dimensions=0.157”x 0.7874”x 0.5512” • Experimental procedure: 1. Fatigue test MTS® servo-hydraulic testing equipment. 2. Constant freq. 10 Hz and stress ratio of –1 to failure. 3. Data acquisition TEST STAR II® software/computer 4. Five coupons of both shot-peened and non shot-peened were respectively subjected to these reversed axial fatigue loading until fracture. 5. Coupons examined by Hitachi® Sacnning Electron Microscope (SEM).

  7. Experimental Result 1 • Top side view of non-shot peened

  8. Experimental Result 2 • Top side view of shot peened

  9. Experimental Result 3 • Side-edge view of non-shot peened

  10. Experimental Result 4 • Side-edge view of shot peened

  11. Conclusion • Most structural investigation of the fractured shot peened and non-shot peened cast Ti-6Al-4V from a low cycle fatigue tests revealed the cold-worked shot peened surface would improve the fatigue strength of the material. • It may be applied to design titanium alloys for maximizing this property.

  12. Questions?

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