Lightweight plastics and ceramics may reduce the need for metals in next-generation aircraft. But aircraft and engine manufacturers today remain sharply focused on the quality and performance of the metalworking fluids permitted in their plants.
At the 4th Symposium on Metal Removal Fluids in
Boeing’s $100,000 Question
“How to get coolants qualified at Boeing has been a work in process for over 15 years,” said Senior Manufacturing Engineer Brian Hovik, with Boeing Research & Technology in
At Boeing, all parts machined from metals, composites and plastics must be machined using a Boeing-qualified fluid. “This protects fly-away parts from discrepancies caused by coolants and lubricants,” said Hovik, “and applies to all subcontractors as well.”
As part of the qualification process, Boeing can require the candidate product to undergo up to eight standard tests, most addressing corrosion. These are:
1. Weight change corrosion: Does the coolant etch or remove metal?
2. Stress corrosion: Does the coolant enable stress corrosion?
3.
4. Galvanic corrosion: Does coolant use promote galvanic corrosion?
5. Titanium compatibility: Does it stain, etch or contaminate titanium? (Chlorinated paraffins, for example, don’t work well with titanium, Hovik noted.)
6. Sealant compatibility: Does the coolant attack seals?
7. Paint compatibility: Does the coolant attack paint?
8. Micro-structure effects: Does the coolant create untempered martensite in heat-treated steels? If so, it could lead to unwanted brittleness.
The Usage Scheme
To determine which specific tests are required, Boeing looks at six separate usage areas where the manufactured parts may go. Usage area one applies to any parts that are painted or sealed that also need holes drilled for assembly. A fluid intended for this use would need to pass tests number one, six and seven.
Usage area two includes any low-alloy steel parts that are heat-treated after metal removal and before paint or sealant is applied. For this use, tests one, two and eight are required. “Chlorinated metalworking fluids historically do not pass the number eight micro-structural test,” Hovik noted.
Usage area three covers any parts that are machined with two or more dissimilar metals in contact with each other during the metal removal process. The fluid must not promote any type of corrosion, and the fluid residue left on the parts likewise must not create corrosion, said Hovik. This application requires passing tests one, two, four, five and eight.
Usage area four “was the ‘American Airlines’ usage area, but they no longer use shining aluminum planes,” said Hovik, so this usage area now simply addresses airplane exterior aluminum parts. To demonstrate that the fluid will not adversely affect the airplane skin, fluids for this application must pass tests three and seven.
The fifth usage area is any parts made from raw stock that don’t fall into any other area; they need only pass test one. And the sixth and last usage area is any raw stock titanium parts, requiring tests one and five.
But the eight tests are not Boeing’s only criteria, Hovik continued. He and his team will first test and evaluate a candidate fluid’s sump life, health and safety effects, ease of cleaning the fluid from parts, ease of separating spent fluid from water, machinability, foaming, biostability, and of course the purchase price.
The Qualification Process
Complete testing costs up to $100,000, paid entirely by the fluid supplier, and can take over a year, said Hovik, and coolant failure can happen at any time in any test. To determine if a candidate coolant is what Boeing wants, the company developed a standard qualification process.
A prospective supplier must first submit a package to Boeing that includes the basic make-up of the fluid, the materials it is intended to machine, its standard MSDS, and the results of a third-party machining test — the International Working Industrial Group (IWIG) TechSolve milling test. “The supplier pays for the cost of third-party testing, for comparison with other products,” said Hovik. Only if the IWIG milling test data shows the product is as good as or better than current coolants will the qualification process continue.
The next hurdle for a candidate fluid is full disclosure to Boeing’s toxicology department of the fluid’s composition, to review for adverse chemicals. Asked about prohibited chemicals, Hovik said Boeing does not make that information public. He added that Boeing also strongly prefers no biocides.
Then coolant samples are tested for waste-treatment analysis and tank cleanability. If the fluid cannot be wastetreated or its residue cannot be cleaned from parts, it is rejected. The next round of testing covers foaming and sump longevity. Only after clearing these steps is a fluid ready for the actual qualification corrosion and compatibility tests.
The supplier can contract with Boeing Technology Services or testing can be performed by an outside testing facility like TechSolve in
Adding tests two, four and six is necessary to qualify to machine heat-treated steel. And all the tests are mandatory if parts with paint and/or sealant are to be machined.
If the candidate fluid passes all its tests, it will be listed as an approved fluid for use by Boeing subcontractors. But use at Boeing itself requires yet more proof of performance. The fluid is subjected to a minimum 90-day trial at Boeing, with the supplier providing sufficient sample concentrate to supply at least two machines. The supplier must also educate Boeing’s operators, mechanics, managers and others about the maintenance and proper use of the product.
Only at the conclusion of this long and arduous process will the candidate fluid join the roughly 12 other new fluids approved each year to lubricate and cool the parts to build Boeing’s newest aircraft.
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