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

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					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

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                         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.


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        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…


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           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.        EXIT




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




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Experimental Result 2
                                    • Top side
                                      view of shot
                                      peened




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




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Experimental Result 4
                                • Side-edge
                                  view of shot
                                  peened




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                 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.
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Questions?



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