# Example: Friction Stir Weld Selection for Marine Aluminum Joints (Manufacturing Engineering)

**Status:** Draft
**Phase:** The Bedrock Phase

## What This Example Demonstrates

Context record structure (OE-0003) applied to a manufacturing process selection decision, illustrating how physical test observations, trade-offs between joining methods, and production constraints are captured in a preserved engineering record (OE-0007).

## The Observation

Fatigue testing of gas metal arc welded joints in marine-grade aluminum alloy 5083-H321 showed a fatigue strength reduction factor of 0.45 at ten million cycles relative to base material, with weld toe stress concentration and heat-affected zone grain growth identified as the primary mechanisms. Salt spray corrosion testing revealed preferential galvanic corrosion at the weld fusion boundary due to microsegregation of magnesium-rich phases. Preliminary solid-state joining trials on the same material produced joints with a fatigue strength reduction factor of 0.78 and no detectable preferential corrosion in the weld zone.

## Engineering Translation

Joining process selection in structural manufacturing is governed by the interaction between the process mechanics and the material's metallurgical response. A fusion welding process introduces a liquid-to-solid transformation that creates microstructural gradients — heat-affected zones with altered grain structures and fusion boundaries with compositional segregation. These gradients become preferential sites for both fatigue crack initiation and electrochemical corrosion attack. A solid-state joining process avoids the liquid phase entirely, preserving the base material's microstructure and eliminating the fusion-boundary corrosion pathway. The engineering decision therefore hinges on whether the performance advantage of the solid-state process justifies the investment in specialized tooling and the operational constraints it introduces. This reasoning must be preserved because a future engineer evaluating a similar joint in a different alloy or environment needs to understand not just what was chosen but why the fusion-welding mechanisms were dominant failure drivers in this specific material system.

## Context Record

| Field | Content |
|---|---|
| **Decision** | Select friction stir welding (FSW) over gas metal arc welding (GMAW) and riveted construction for longitudinal stiffener-to-skin joints in a marine-grade aluminum alloy (5083-H321) structure. |
| **Observation** | (1) Fatigue testing of GMAW joints in 5083-H321 showed a fatigue strength reduction factor of 0.45 at 10⁷ cycles relative to the base material, attributed to weld toe stress concentration and heat-affected zone grain growth. (2) Corrosion testing per ASTM B117 (salt spray, 1000 hours) revealed preferential galvanic corrosion at the weld fusion boundary of GMAW joints due to microsegregation of magnesium-rich phases. (3) Preliminary FSW trials on 8mm 5083-H321 produced joints with fatigue strength reduction factor of 0.78 at 10⁷ cycles and no detectable corrosion preferential to the weld zone. |
| **Alternatives** | (A) Gas metal arc welding (GMAW) — rejected: 55% fatigue strength reduction relative to base material, heat-affected zone corrosion susceptibility, and distortion requiring post-weld rework on 40% of production joints. (B) Riveted construction — rejected: rivet holes act as stress concentrators (fatigue reduction factor 0.52 at 10⁷ cycles), added sealing requirements for watertightness, and 35% higher assembly labor. (C) Friction stir welding — selected. |
| **Constraints** | Joint must achieve fatigue life exceeding 10⁷ cycles at design stress range of 85 MPa. Structure must resist marine atmospheric corrosion for 25-year service life without coating in the weld zone. Maximum allowable distortion is 1.5mm over a 3m stiffener run. Production rate requires completion of one 3m longitudinal joint in under 15 minutes. Material is 8mm 5083-H321 aluminum alloy plate. |
| **Reasoning** | FSW is a solid-state joining process — the material does not melt, eliminating the fusion-zone microsegregation and heat-affected zone grain growth that drive both the fatigue reduction and corrosion susceptibility of GMAW. The observed fatigue strength of 0.78 (versus 0.45 for GMAW and 0.52 for riveted) directly translates to either a lighter structure for the same fatigue life or a longer service life for the same structural mass. The absence of a fusion zone eliminates the galvanic corrosion pathway entirely. The process is repeatable and produces consistent properties — reducing the 40% rework rate observed with GMAW to near zero in preliminary trials. |
| **Verification** | (1) Full-scale fatigue testing of a representative 3m stiffened panel (4 stiffeners, FSW-joined) survived 2 × 10⁷ cycles at design stress range with no crack initiation detected by eddy-current inspection at 1 × 10⁶ cycle intervals. (2) Salt spray testing (ASTM B117, 2000 hours) of FSW joints showed no preferential corrosion — corrosion rate in the weld zone was equivalent to base material (0.02 mm/year). (3) Dimensional inspection of 50 production joints showed maximum distortion of 0.8mm over 3m (within the 1.5mm constraint). (4) Cycle time measurement: 11 minutes per 3m joint at production parameters (within the 15-minute constraint). |
| **Lineage** | Builds on manufacturing process evaluation CR-MFG-2023-003 (initial screening of 6 joining methods for marine aluminum). Inherits the fatigue and corrosion test methodologies from CR-MFG-2022-011 (GMAW performance baseline study). |
| **Assumptions** | FSW tool life remains consistent across the 500-joint production run (tool wear was not observed in the 50-joint trial but has not been tested at full production volume). The 8mm plate thickness is within the reliable process window for the selected FSW tool geometry. No metallurgical anomalies (e.g., kissing bonds) exist below the resolution of the nondestructive inspection methods used (ultrasonic C-scan at 0.5mm resolution). Ambient temperature variations at the production facility (5–35°C) do not significantly affect joint properties. |
| **Open Questions** | What is the tool replacement interval at full production volume? Can FSW parameters be optimized further to approach base-material fatigue strength? How does FSW perform on T-joint and corner-joint geometries required elsewhere in the structure? |
| **Discipline-Specific Data** | Process parameters: rotational speed 400 RPM, traverse speed 250 mm/min, axial force 35 kN, tool shoulder diameter 25mm, pin diameter 8mm, pin length 7.5mm. |

## Self-Fading Assessment

This example builds a bridge from the abstract context record structure (OE-0003) to the specific challenge of manufacturing process selection, where the decision is driven by the physical mechanisms the process introduces into the material. The reader has crossed this bridge when they can examine any solid-state versus fusion joining comparison and immediately identify which fields of the context record carry the decisive engineering weight — in this case, the Observation field (mechanism-based performance data) and the Reasoning field (causal link between process physics and failure modes). Once that mapping is intuitive, this example becomes unnecessary.