Aerospace industry can save time and costs by selecting the right materials and processes earlier, say experts

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Issues with durability and strength of components are predominantly exposed at the metallic testing stage, and can often be avoided through collaboration with heat treatment experts at the design stage.  

This is the conclusion of aluminium heat treatment experts, Alloy Heat Treatment, following on from a successful collaboration with Thales. Thales, specialists in electronic on-board equipment and ground-based systems for navigation, air-traffic control and simulation enlisted the expertise of Alloy Heat Treatment to heat treat Elbow and Base Mounting parts used for the Sea King Mk7’s high tech Airborne Early Warning radar equipment, to improve properties at the metallics testing stage.

The radar is held onto the Sea King Mk7’s helicopter by the elbow. This is then attached to the base mounting part, which is then bolted onto the aircraft. The Sea King’s Mk7 high-tech radar is vital because it allows crews to combat military operations by detecting land and stationary movement. The boom assembly requires in service strength and structural resilience.

The Elbow and Base Mounting parts, were originally designed as castings, but required replacing due to some parts cracking in service. In order to stock a supply of spares, ready to replace a casting at short notice, Thales took the decision to machine the parts from a heat treated (T6) solid billet.

Generally, the effectiveness of the heat treatment of solid billet decreases from outside to centre, and parts machined from billets tend to be produced from the billet centre, which is the least sensitive to heat treatment, hence, has the lowest strength. Thales confirmed this to be the case, with scattering of proof (yield) strength values of 181 to 237 MPa, and hardness values of 81-90HV (Vickers Hardness), where proof stress values of 255MPa minimum and hardness values of 100HV minimum are expected. Thales, being aware of these issues, decided to re-heat treat the parts and nominated Alloy Heat Treatment.

The parts were supplied to AHT in near finished machine state, but had relatively heavy sectioned integral machining bosses which may have impacted on the effectiveness of the heat treatment and residual stress distribution. AHT recommended that heavily bossed areas be machined hollow to allow the parts to uniformly heat up and to uniformly fast cool (quench) during heat treatment. AHT also recommended that parts be quenched into polymer solution, to ensure dimensional stability and to minimise residual stress. Following re- heat treatment of the parts by AHT, metallics testing procedures confirmed that parts had responded as required, exceeding minimum T6 specification values, whilst maintaining dimensional tolerances.

Excessive machining allowance and large cross sectional changes give rise to reduced sensitivity to heat treatment and increased residual stress. Minimal machining allowances could result in parts being scrapped due to the parts not being able to retain dimensional stability during heat treatment.

Uses of polyalkylene glycol (polymer) solutions for quench media have become a necessity for heat treatment of parts that require close dimensional stability. Polyalylene glycol solutions deposit polymers on the parts surfaces during quenching, which slows down the rate of heat transfer during quench. No vapour / steam blankets are generated during the polymer quenching process.

Mark Percox, Technical Manager at Alloy Heat Treatment said: “AHT is presented with a manufacturing problem near to end of the manufacturing layout, which has resulted in re-design or re-processing, and in some cases, re-making of parts. Heat treatment is regarded by aircraft OEMs as a special process, and involving heat treatment experts in the material and process selection stage of design, will prove to have huge benefits.”