Avionics Cold Plate: CNC and Friction Stir Welding Guide
As avionics modules continue to pack more processing power into smaller volumes, thermal management has become one of the most critical challenges in aerospace electronics design. The avionics cold plate — a liquid-cooled heat exchanger that removes waste heat from electronic components — has emerged as the preferred solution for high-power-density applications. This guide examines the complete manufacturing cycle for aerospace-grade liquid cold plates, from CNC channel milling through friction stir welding (FSW) sealing to final pressure testing.
Cold Plate Design Fundamentals
An avionics cold plate operates by circulating a coolant — typically a water-glycol mixture or dielectric fluid — through internal channels machined into a metal plate. Heat from mounted electronic components conducts through the plate material and is carried away by the flowing coolant. The efficiency of this process depends on channel geometry, material thermal conductivity, flow rate, and the quality of the interface between the cold plate and the electronics.
6061-T6 aluminum is the standard material for aerospace cold plates. Its thermal conductivity of 167 W/m·K provides effective heat spreading, while its yield strength of 276 MPa ensures structural integrity at operating pressures up to 1.5 MPa. The alloy's excellent machinability and weldability make it compatible with both CNC channel milling and friction stir welding.
| Parameter | Typical Value | Impact on Performance |
|---|---|---|
| Channel width | 3.0–8.0 mm | Wider channels reduce pressure drop; narrower channels increase heat transfer coefficient |
| Channel depth | 4.0–10.0 mm | Deeper channels increase coolant mass flow; shallower channels improve wall conduction |
| Channel pitch (center-to-center) | 5.0–12.0 mm | Tighter pitch increases surface area; wider pitch reduces weight and pressure drop |
| Wall thickness above channel | 1.0–2.5 mm | Thinner walls reduce thermal resistance; thicker walls improve structural margin |
| Coolant flow rate | 2.0–8.0 L/min | Higher flow improves heat rejection; lower flow reduces pump power requirements |
Typical cold plate dimensions for avionics LRUs range from 100 × 150 mm to 300 × 400 mm, with thickness between 12 mm and 25 mm. Thermal performance targets commonly specify a thermal resistance of 0.02–0.06 °C·cm²/W and a maximum pressure drop of 50–100 kPa at the design flow rate.
CNC Channel Milling for Coolant Flow Paths
The coolant channels in an avionics cold plate are machined using CNC milling, typically on a 3-axis or 4-axis vertical machining center. Channel geometry can range from simple parallel serpentine patterns to complex pin-fin or offset strip-fin arrays optimized for specific heat flux distributions.
The machining process begins with facing the blank plate to achieve an initial flatness of 0.10 mm or better. Channel roughing uses a 6–10 mm diameter end mill at 10,000–14,000 RPM with a feed rate of 1,500–2,500 mm/min. The finishing pass employs a 4–6 mm end mill with corner radius, running at 14,000–18,000 RPM with a 0.2–0.3 mm radial depth of cut to achieve a channel surface finish of Ra 1.6 μm or better.
| Channel Feature | Tool Type | Spindle Speed (RPM) | Feed Rate (mm/min) | Tolerance (mm) |
|---|---|---|---|---|
| Serpentine channel (rough) | Solid carbide, 2-flute, 8 mm | 10,000–12,000 | 1,500–2,000 | ±0.10 |
| Serpentine channel (finish) | Solid carbide, 4-flute, 6 mm | 14,000–16,000 | 1,200–1,600 | ±0.05 |
| Pin-fin array | Micro-grain carbide, 2 mm | 18,000–24,000 | 800–1,200 | ±0.03 |
| Inlet/outlet manifold | Solid carbide, 10 mm | 8,000–10,000 | 1,000–1,400 | ±0.05 |
Chip evacuation is critical during channel milling. Through-spindle coolant at 30–50 bar clears chips from deep channels and prevents recutting. For complex pin-fin arrays where coolant access is limited, compressed air blow-off cycles are programmed between finishing passes to clear the cavity.
Friction Stir Welding for Hermetic Sealing
After channel machining, the cold plate requires a cover plate to seal the coolant channels. Friction stir welding (FSW) has become the gold standard for this application, producing a fully hermetic joint without the porosity, distortion, or filler metal issues associated with conventional fusion welding.
In FSW, a rotating tool with a specially designed pin and shoulder is plunged into the joint line between the base plate and cover plate. The friction-generated heat plasticizes the material, and the rotating tool stirs the plasticized material together, creating a solid-state bond. For 6061 aluminum cold plates, typical FSW parameters include a tool rotation speed of 800–1,500 RPM, a traverse speed of 100–300 mm/min, and a forge force of 8–15 kN.
The primary advantages of FSW for cold plate sealing include:
- No filler metal — the bond is parent material only, eliminating galvanic corrosion risks
- Minimal distortion — the solid-state process produces significantly less heat-affected zone (HAZ) than laser or TIG welding
- Full penetration — FSW joints achieve 100% bond line integrity across the channel perimeter
- Process repeatability — CNC-controlled FSW machines produce consistent joint quality across production runs
Flatness Control and Post-Weld Machining
The finished avionics cold plate must provide a flat mounting surface for the electronic components to minimize thermal interface resistance. The specification typically requires overall flatness of 0.05 mm (0.002 in) across the entire plate surface after welding and machining.
FSW introduces residual stresses that can cause the cold plate to distort. Stress relieving at 250–300 °C for 2–4 hours after welding reduces these stresses by 50–70% before final machining. Post-stress-relief machining includes:
- Face milling of the mounting surface to final thickness, removing 0.3–0.5 mm of material
- Drilling and tapping of mounting holes for component attachment
- Machining of inlet and outlet ports for fluid connectors
Pressure Testing and Leak Detection
Every avionics cold plate must undergo rigorous pressure testing to verify the integrity of the welded joint and channel structure. The test protocol typically includes both proof pressure testing and leak detection:
| Test Type | Test Pressure (MPa) | Hold Time (min) | Acceptance Criteria | Standard |
|---|---|---|---|---|
| Proof pressure test | 2.25 (1.5× working) | 5 | No permanent deformation, no pressure drop > 1% | ASME B31.3 |
| Burst pressure test (design validation) | 3.75 (2.5× working) | 1 | No rupture or leakage | MIL-STD-810 |
| Helium leak test | 0.5 (pressurized channel) | 2 | Leak rate ≤ 1 × 10⁻⁶ atm·cc/s | MIL-STD-883 |
| Thermal cycle leak test | 1.5 (working) | 10 cycles -55/+85 °C | No measurable leak after cycling | DO-160 |
For production testing, pressure decay method at 1.5–2.0 MPa with a hold time of 60 seconds provides rapid screening. Any units showing pressure loss above the threshold are subjected to helium mass spectrometry to locate the leak path. Leak repair typically involves localized TIG welding or, for FSW joints, a secondary FSW pass over the affected area.
Surface Treatment and Final Assembly
The completed cold plate receives surface treatment to protect against corrosion and improve thermal interface performance. Chromate conversion coating per MIL-DTL-5541, Type I, Class 3 provides corrosion resistance with an electrical surface resistance below 10 mΩ — important if the cold plate is used as a grounding plane for electronics.
For improved thermal performance, the mounting surface may be machined to a flatness of 0.025 mm and specified at Ra 0.8 μm finish to minimize thermal interface material (TIM) bond line thickness. The mating surface of the electronics module is typically lapped to a similar finish. A TIM layer of 0.05–0.10 mm with a thermal conductivity of 3–5 W/m·K fills micro-scale gaps between the two surfaces.
Final assembly includes installing fluid connectors (typically MS flared or SAE O-ring boss types per AS5202), pressure testing the completed assembly, and marking with a laser-engraved serial number for traceability per AS9102. Each cold plate is shipped with a test report documenting flatness, pressure test results, flow versus pressure drop curve, and thermal resistance measured at the specified flow rate.
