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Shot Peening Titanium SLM Aerospace Parts: AMS 2430, Fatigue Life Improvement, and NADCAP Compliance

Shot peening is the primary fatigue life improvement process for titanium SLM aerospace components. The SLM build process introduces tensile residual stress at the surface — a direct fatigue liability. Shot peening reverses this with a compressive residual stress layer that raises the high-cycle fatigue limit toward and in some cases above the wrought material baseline. This guide covers the complete technical and compliance framework: residual stress mechanics, AMS 2430 requirements, ceramic shot selection, and NADCAP AC7117 audit criteria.

1. The Fatigue Problem in Titanium SLM and the Peening Solution

Titanium SLM components used in load-bearing aerospace structures operate under cyclic loading where fatigue is the dominant failure mode. The SLM build process introduces a specific fatigue liability: tensile residual stress at the part surface, arising from the steep thermal gradients and rapid solidification of laser powder bed fusion. Measured surface tensile residual stresses of 200–600 MPa are routinely reported for Ti-6Al-4V SLM, representing 20–60% of the material’s ultimate tensile strength applied as a pre-existing tensile load before any service loading begins. The result is a high-cycle fatigue (HCF) limit that can be 20–40% below the equivalent wrought Ti-6Al-4V baseline — a structural deficit incompatible with aerospace primary structure design margins.

Shot peening directly reverses this deficit. By bombarding the surface with high-velocity spherical media under defined process conditions, the process plastically deforms the surface layers and introduces compressive residual stress (CRS) extending 0.1–0.5 mm into Ti-6Al-4V. This compressive zone must be overcome by the applied tensile loading before fatigue crack nucleation can proceed at the surface, effectively raising the allowable cyclic stress amplitude for the same fatigue life requirement. Published fatigue life improvements for peened Ti-6Al-4V SLM components range from 30–100%+ over unpeened as-built surfaces, with higher-intensity peening protocols bringing the HCF limit close to the wrought material benchmark. This article is part of the series on abrasive finishing for titanium SLM parts.

2. Compressive Residual Stress: Mechanism and Measurement

The compressive residual stress induced by shot peening arises from plastic deformation of the surface and near-surface layers. Each spherical media particle impact deforms the titanium surface locally beyond its yield point, elongating the deformed material laterally. The surrounding elastic bulk constrains this elongation, generating biaxial compressive stress in the deformed zone. For Ti-6Al-4V, achievable CRS magnitudes are −400 to −800 MPa at the surface, transitioning through zero and into tensile stress at depths of 0.3–0.7 mm (equilibrium requires a compensating tensile zone deeper in the part).

CRS depth profiles are measured by X-ray diffraction (XRD) using the sin²ψ method, which relates measured lattice d-spacing changes to biaxial stress through elasticity theory. XRD measurements at the surface and at successive electropolished depths build the complete stress-vs-depth profile. For NADCAP-audited peening processes, XRD measurement on Almen coupon companion specimens or actual parts is a periodic documented requirement. The depth of the compressive zone (typically defined as the depth at which the CRS crosses zero) should exceed the crack nucleation depth for the component’s critical stress concentration features to be effective in fatigue life improvement.

3. AMS 2430: Intensity, Coverage, and Documentation

AMS 2430 (Shot Peening, Computer Controlled) is the aerospace standard that defines controlled shot peening for metal components. For titanium SLM aerospace parts, the key AMS 2430 requirements are:

• Almen intensity: Measured using calibrated steel strips (type A for most Ti-6Al-4V applications) clamped to an Almen block fixture before and after peening. The arc height difference in thousandths of an inch defines the intensity. Typical specifications for Ti-6Al-4V structural components: 6A–18A; for higher-stress applications (turbine attachments, wing spars), 14A–22A may be required. Intensity specification must be traceable to the design requirement via a CRS depth calibration study.
• Coverage: ≥98% of the specified surface area must show evidence of impact, verified by fluorescent tracer method or approved visual inspection under defined conditions. Over-peening (coverage >200%) is also controlled, as excessive impact reduces the compressive layer benefit by inducing competing tensile residual stress at the surface from over-work-hardening.
• Saturation curve: AMS 2430 requires establishing a saturation curve (Almen arc height vs. peening time) to confirm that the process intensity is stable and defined. The operational intensity must be at least 80% of the saturation value.
• Documentation: Every production peening run requires batch records documenting: Almen strip results, media batch/certification, machine settings (pressure, nozzle size, work distance, traverse speed), operator ID, and part identity. Records are retained for the component life cycle.

4. Media Selection: Ceramic Shot vs. Conditioned Glass vs. Steel

Ceramic ZrO₂ shot is the definitive choice for aerospace titanium SLM peening. It delivers the full Almen intensity range required for structural Ti-6Al-4V components, produces no iron contamination, maintains consistent spherical morphology through 2,000–4,000 blast cycles (vs. 400–800 for glass), and conforms to AMS 2431/7. S110–S170 ceramic shot covers the 6A–12A range typical of structural components; S230–S330 reaches 14A–20A for high-intensity applications. Conditioned glass beads (AMS 2431/6) are acceptable for lower-intensity applications where the maximum required intensity does not exceed 12A and iron contamination can be managed by equipment segregation and passivation. Cast steel shot introduces unacceptable iron contamination risk for titanium in corrosion-sensitive or medical-adjacent aerospace applications.

5. NADCAP AC7117: Audit Requirements and How to Qualify

NADCAP accreditation for shot peening (AC7117) is required by most major aerospace primes for flight-critical component special processes. The AC7117 checklist assesses six key areas that a qualifying facility must demonstrate:

• Equipment calibration: Blast pressure gauges, Almen strip gauges, nozzle orifice gauges, and work distance fixtures must be calibrated to traceable standards at defined intervals. Calibration records must be current and retrievable.
• Almen strip control: Strips must be sourced from NADCAP-approved suppliers, stored correctly (flat, no contact with ferrous surfaces), applied to the correct block configuration, and read on a calibrated gauge. Strip deflection results must be recorded with part/batch traceability.
• Coverage verification: The method (fluorescent tracer, visual) must be documented, and operators must demonstrate consistent application. For complex geometries, coverage on all specified surfaces must be demonstrably achievable.
• Media management: Media lot traceability, condition monitoring (hardness, size distribution, broken particle content), and replacement intervals must be defined and followed. Media condition records are a frequent NADCAP audit finding area.
• Operator qualification: Training, testing, and periodic re-qualification records for all peening operators must be maintained and current.
• Process change control: Any change to media, equipment, or parameters requires documented change control, engineering review, and re-qualification before implementation. Unauthorized deviations are a major finding in AC7117 audits.

Initial NADCAP audit preparation typically requires 6–12 months for a facility new to the process. Annual surveillance audits maintain accreditation status.

6. First-Article Qualification and Production Control

First-article qualification for a new titanium SLM component establishes the validated peening parameters for all subsequent production. The sequence: intensity–CRS calibration study (coupon peening trials at multiple Almen intensities, followed by XRD measurement to build the intensity–CRS depth curve for the specific material and geometry); fatigue life verification (peening specimens at the qualified intensity and testing to confirm the required HCF life improvement); and first-article hardware peening with complete documentation review before production release.

The qualified process specification defines all controlled parameters: media type and grade per AMS 2431, nominal Almen intensity and tolerance, coverage requirement, equipment model and configuration, nozzle size, work distance range, blast pressure range, and media recycling frequency. Any deviation from the qualified specification triggers a non-conformance and requires engineering disposition before affected parts can ship. The process specification document is maintained under document control and is the reference for all production peening runs of that part number.

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