Microstructure Analysis of Shot Peened
Cast Ti-6Al-4V after Fatigue Fracture
Chun Ling Huang
Southern University and A&M College
Baton Rouge, Louisiana 70813
ASEE Mid Atlantic Conference on April 12, 2003
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.
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…
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
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
• 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.
Materials and Experiments
Materials: ASTM standards – cast Ti-6Al-4V
dimensions=0.157”x 0.7874”x 0.5512”
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
Experimental Result 1
• Top side view
Experimental Result 2
• Top side
view of shot
Experimental Result 3
Experimental Result 4
view of shot
• 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.