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I recently did some research on the effect of cooling on CFRP/Al7075 drilling. 7075 is used for aeroplane main wing upper skin, and the plates need to be overlayed and drilled for flush riveting.

But I read somewhere that liquids are forbidden for cooling operations on these multi layer plates. Is this true? If so, why are they forbidden? Do I have to change the subject of my research?

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    $\begingroup$ Welcome to SE.Aviation. As an engineer, I am intrigued by why cooling fluid on 7075 would be a bad thing. However, this question as asked is a materials/machining question, and so IMHO is out of scope for SE.Aviation. Maybe ask it on SE.Engineering, engineering.stackexchange.com? $\endgroup$ Sep 28 '21 at 17:37
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    $\begingroup$ @KennSebasta I think aircraft manufacturing techniques should be in scope. This is not some widely used material. $\endgroup$
    – Sanchises
    Sep 28 '21 at 18:53
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    $\begingroup$ A quick search suggests in contrary wet drilling is beneficial for health when drilling CFRP, preventing small particles to be inhaled. There are also liquid coolants optimized for these operations. Can you cite or detail a bit more the recommendation to not use liquid coolants? $\endgroup$
    – mins
    Sep 28 '21 at 23:19
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    $\begingroup$ @mins engineering.stackexchange.com/questions/47466/… $\endgroup$ Sep 28 '21 at 23:35
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    $\begingroup$ Hi Mohamad, perhaps best to add in the question that 7075 is used for aeroplane main wing upper skin, and that the plates need to be overplayed and drilled for flush riveting, so that the relevance to aircraft manufacturing becomes clear. $\endgroup$
    – Koyovis
    Sep 29 '21 at 7:04
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According to this article:

A Phased Approach to Optimized Robotic Assembly for the 777X

drilling multi-material composites does require some special trickery, especially when automated assembly is part of the process:

There were lessons learned relating to process capabilities whereby technologies were added or modified to prevent the need to return to hole locations for further processing. Additional cutting lubricants and methods were specified to allow layer by layer drilling optimization specifically targeted for one-up assembly.

The cooling/chip evacuation methods were varied during single hole drilling per current material layer of stack. The reason for this is explained on the document to be:

Critical for one-up assembly as the fastening immediately follows the drilling process, composite stacks are drilled dry and metallic substructure layers drilled with minimal air/oil mist. This keeps the prepared hole dry and clean. To keep the evacuation system clean from carbon build-up when drilling dry or with air/oil mist, a coolant jet was added downstream of the nose piece to enable the line to be washed.

So each material in the stack requires its own specific method for cooling and "chip" evacuation (drilling CFRP does not produce chips as they are traditionally considered in machining, but anyway).

Other articles suggest that using liquid coolants when drilling CFRP results in less tool wear and better finish, but the reason not to use liquids when drilling CFRP (or layers of it in a stack) in this specific case seems to deal with the requirement that the hole must be both clean and dry before fasteners can be installed. Plastic composites are always a little porous to some extent and drilling will slightly break up the surface, further making it more permeable. Coolant residue in a joint sealed by a fastener would elevate the risk of corrosion in the joint.

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First, the caveats, I'm not in the Aerospace industry, and I'm not knowledgeable about CFRP composites. Also, this article is about CFRP/Ti, not AL, but that said, I found the following:

The broader/commercial impact of this project will be the potential to provide an environmentally-benign coolant system for CFRP-Ti drilling applications. The aerospace industry is using more CFRP-Ti in aircraft components as it is more durable and lighter than aluminum or similar metals. However, CFRP-Ti is abrasive and difficult to drill, resulting in significant heat being generated from the force and the friction of drilling, which is detrimental to the CFRP base materials. The industry currently uses petroleum oil and water for cooling, which has following problems, (1) it does not provide adequate cooling; (2) it is not environmentally friendly and must be treated as hazardous waste; (3) it requires biocides to reduce bacteria; and (4) it is not conducive to a healthy and safe worker environment.

(The above is formatted straight from the original document)

It would appear that CFRP composite drilling, using water or petroleum-based coolants: the actual coolant effects are not enough, it creates hazardous waste, requires biocides, and are not healthy or safe for workers.

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  • $\begingroup$ Good answer, but conclusion does not make sense. Drilling lubrication fluid is common in metals to avoid overheating. One might focus on the potential effects of drilling fluids on CFRP. Need for biocides may be a bingo. Even if fluid just seeped into the matrix a little, it may compromise it enough to cause worries. $\endgroup$ Sep 29 '21 at 16:00
  • $\begingroup$ These properties are already found in usual steel cutting fluids, including microbial pollution, they are not a problem except the treatment cost. Of course if no fluid is efficient for CFRP this would be a good reason to not use them (but the metallic layer seem to require one). $\endgroup$
    – mins
    Sep 29 '21 at 21:29

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