Rotary Friction Welding (RFW) Upset Allowance & Flash Removal Vector Prepress Guide

Rotary Friction Welding (RFW) and Inertia Friction Welding (IFW) are 100% solid-state joining processes that produce forged, full-strength metallurgical bonds between identical or dissimilar cylindrical metals without flux, shielding gas, or melting. This technical guide outlines how to calculate axial burn-off allowances, design flash curl trap geometries, and prepare production-grade CAD/CAM DXF turning vector toolpaths.

Solid-State Welding Principles & Axial Shortening Compensation

Unlike fusion welding processes (MIG, TIG, Laser, or Electron Beam), Rotary Friction Welding operates entirely in the solid state below the melting temperature ($T_{\text{solidus}}$) of the parent metals. Frictional heat is generated at the planar rotating interface under axial contact pressure, plasticizing a narrow heat-affected zone (HAZ). Rapid braking followed by high-tonnage forging upset consolidates the joint, expelling impurities and oxides radially outward into a characteristic bilateral flash curl.

Because parent metal is expelled during the friction heating and forging phases, the initial workpiece blanks experience predictable axial shortening (burn-off). Prepress CAD/CAM engineers must add this exact burn-off allowance ($\Delta L_{\text{total}}$) into the unmachined blank billet drawings:

• Friction Burn-off Allowance ($\Delta L_{\text{fric}}$): The axial distance consumed during the rotating heating phase to reach forging temperature ($850^\circ\text{C}-1250^\circ\text{C}$).
• Forge Upset Shortening ($\Delta L_{\text{upset}}$): The rapid axial displacement produced during spindle arrest when peak forging pressure ($P_{\text{upset}} = 1.5 - 2.5 \times P_{\text{fric}}$) is applied.
• Total Part Shortening: $\Delta L_{\text{total}} = \Delta L_{\text{fric}} + \Delta L_{\text{upset}}$. Blanks must be dimensioned as $L_{\text{blank}} = L_{\text{finished}} + \Delta L_{\text{total}} + L_{\text{finish\_facing}}$.

Engineering Standards & Process Parameter Reference

CAM Flash Removal & Prepress DXF Toolpath Best Practices

• Root Stress Concentration Fillet: Never turn post-weld flash flush with a sharp 90° corner toolpath. Program a generous circular arc fillet ($R \ge 1.5 \times \text{wall thickness}$ or $R \ge 3.0\text{mm}$) to eliminate fatigue stress risers across the plasticized HAZ.
• Internal Flash Trap Reservoirs for Hollow Tubes: For sealed hollow tubes where internal ID machining is impossible after welding, design an internal counterbore flash trap pocket into the joint prep DXF to capture the inward curling flash curl without choking the internal bore flow.
• Bimetallic Asymmetric Flow Scaling: When friction welding hard superalloys to softer carbon steels, the softer alloy expels ~75% of the total burn-off. Scale the blank stock allocation proportionally to preserve required final component dimensions.
• Spindle Deceleration & Alignment Locking: If angular orientation between non-round features on mating shafts is required, utilize direct-drive servo-controlled friction welders with mechanical shot-pin detent locking upon spindle arrest.