Peening Ti-6Al-4V: Glass Bead vs. Ceramic Bead Process Optimization

Background: Why Ti-6Al-4V Peening Requires Careful Media Selection

Ti-6Al-4V (Grade 5 titanium) is among the most widely shot-peened aerospace alloys, used extensively in turbine fan blades, compressor disks, landing gear components, structural brackets, and fastener applications. Its excellent specific strength and corrosion resistance make it a preferred material, but its metallurgical characteristics — moderate hardness (30–36 HRC typical for annealed material), high notch sensitivity, and susceptibility to fretting fatigue — make peening process optimization critically important.

The two primary non-ferrous media options for titanium peening are glass bead (AMS 2437) and ceramic bead (ZrO₂-based, AMS 2438). Both eliminate the ferrous contamination risk of steel shot, but they differ significantly in density, achievable intensity range, breakage rate, and the residual stress profiles they produce in titanium alloys.

Critical Requirement: Never peen titanium alloy components with ferrous media (cast steel or CCW) without explicit customer engineering approval and contamination control verification. Embedded ferrous particles in titanium create galvanic corrosion initiation sites and can nucleate fatigue cracks.

Media Properties Comparison

Property	Glass Bead (AMS 2437)	Ceramic Bead (AMS 2438)
Base material	Soda-lime or borosilicate glass	Zirconia-silica (ZrO₂·SiO₂) or ZTA
Density (g/cm³)	~2.5	~3.8–4.0
Hardness	~46–50 HRC equivalent (730–780 HV)	~60–65 HRC equivalent (740–850 HV)
Max Almen intensity (Type A)	~0.010–0.012A (limited by low density)	~0.014–0.018A (higher density allows higher intensity)
Breakage rate	Moderate — glass is brittle	Low — ZrO₂ ceramics are tougher than glass
Contamination risk	Non-ferrous; silica contamination possible	Non-ferrous; zirconia is inert
Governing spec	AMS 2437	AMS 2438
Typical size range	GB120–GB400 (SAE designation)	CCB230–CCB400 (size-dependent)
Cost per pound (relative)	1× (lowest cost)	3–5× glass bead

Residual Stress Profiles in Ti-6Al-4V

X-ray diffraction (XRD) residual stress measurements on Ti-6Al-4V peened with AMS 2437 glass bead and AMS 2438 ceramic bead at matched Almen intensities (0.008A, target intensity for typical blade root applications) reveal the following general trends:

Glass Bead (AMS 2437) at 0.008A

Peak compressive stress: −600 to −750 MPa at the surface (in the β phase)
Compressive layer depth: 0.10–0.18 mm (100–180 µm)
Crossover depth: ~0.20–0.25 mm (transition to tensile balancing stress)
Surface roughness increase: Ra typically +0.4–0.8 µm vs. pre-peen

Ceramic Bead (AMS 2438) at 0.008A

Peak compressive stress: −680 to −820 MPa at the surface
Compressive layer depth: 0.13–0.22 mm (130–220 µm)
Crossover depth: ~0.25–0.32 mm
Surface roughness increase: Ra typically +0.3–0.6 µm vs. pre-peen

At matched intensity, ceramic bead generally produces a somewhat deeper compressive layer in Ti-6Al-4V due to its higher particle density, which delivers greater subsurface plastic deformation per impact. The difference is most pronounced at higher intensities where the density advantage of ceramic media is amplified.

Fatigue Performance Data

Condition	Media	Intensity	R-ratio	Endurance Limit (MPa)	vs. Unpeened
Unpeened baseline	—	—	0.1	~480 MPa	—
Glass bead peened	AMS 2437 GB230	0.006A	0.1	~580 MPa	+21%
Glass bead peened	AMS 2437 GB170	0.008A	0.1	~615 MPa	+28%
Ceramic bead peened	AMS 2438 CCB280	0.008A	0.1	~640 MPa	+33%
Ceramic bead peened	AMS 2438 CCB230	0.010A	0.1	~660 MPa	+38%
Over-peened (glass bead)	AMS 2437 GB170	0.014A	0.1	~560 MPa	+17%

The over-peened condition illustrates a critical point: excessive intensity on Ti-6Al-4V can actually reduce the fatigue benefit relative to the optimum intensity window. This occurs because very high intensities generate surface roughness and micro-damage that partially offset the compressive stress benefit. The optimum intensity for most Ti-6Al-4V applications falls in the 0.006–0.010A range.

Engineering Note: Fatigue data above is representative of smooth-bar testing. Notched specimens and actual component fatigue performance will differ. Always validate peening parameters on representative specimens before applying to production parts. Customer engineering data takes precedence over generic industry data.

Fretting Fatigue at Blade Root Dovetails

A particularly important application for Ti-6Al-4V peening is turbine compressor blade root dovetails, where fretting wear from oscillatory relative motion between blade and disk generates fretting fatigue cracks. The peening process reduces fretting fatigue damage through two mechanisms:

Compressive residual stress: Reduces the mean stress driving fretting crack propagation
Surface hardening: Increased surface hardness (typically 20–40 HV increase) improves fretting wear resistance by reducing adhesive wear at the contact interface

For dovetail applications, ceramic bead peening at 0.007–0.009N (Type N strip for precision intensity control) is commonly specified because the higher density allows adequate intensity at lower air pressure, reducing the risk of over-peening thin blade sections. AMS 2438 ceramic media is preferred by several turbine OEMs for dovetail peening specifically for this reason.

Process Parameter Recommendations for Ti-6Al-4V

Parameter	Glass Bead (AMS 2437)	Ceramic Bead (AMS 2438)
Typical media size	GB170–GB230 for 0.006–0.010A	CCB230–CCB280 for 0.007–0.012A
Nozzle pressure range	30–55 PSI typical	20–45 PSI typical (lower needed due to density)
Standoff distance	6–10 in typical	6–10 in typical
Nozzle angle	90° to surface preferred; 75° min.	90° to surface preferred
Coverage minimum	100% per AMS 2430	100% per AMS 2430
Dedicated machine?	Yes — separate from steel shot operations	Yes — separate from steel shot operations
Post-peen cleaning	Ultrasonic clean to remove glass fragments	Air blow-off typically sufficient

Selecting Between Glass Bead and Ceramic Bead for Ti-6Al-4V

The choice between AMS 2437 glass bead and AMS 2438 ceramic bead for titanium peening should be driven by the following considerations:

Required intensity range: If the drawing specifies intensity ≤ 0.010A, glass bead is typically achievable and more cost-effective. For intensities > 0.010A on titanium, ceramic bead is generally necessary.
Customer specification: Some aerospace OEMs mandate ceramic bead specifically for titanium rotating components. Check the engineering drawing and procurement specification carefully.
Volume and cost sensitivity: Glass bead is significantly less expensive. For high-volume production where intensity requirements allow glass bead, the cost advantage is substantial.
Post-peen cleaning requirements: Glass bead breakage generates silica glass fragments that require thorough removal from part features. Ceramic bead generates less debris. For complex geometries with blind holes or deep slots, ceramic bead may be preferable to reduce cleaning burden.

Recommendation: For critical rotating titanium components (disk, blade, shaft), specify AMS 2438 ceramic bead when the application justifies the cost premium — particularly for dovetail/fretting applications and intensities ≥ 0.008A. For structural brackets, fastener holes, and non-rotating applications, AMS 2437 glass bead at 0.006–0.008A is typically adequate and more economical.