The plant can't physically handle 640w of LED at this size and age. You want me to explain it? here it is.
umols/second (PPF or photosythetic photon flux) is a measurement of how much light that a plant can use a light produces. umols/m2/second (PPFD or photosynthetic photon flux density) is a measurement of how much light that a plant can use is hitting a certain spot at a given time. DLI (daily light integral) is the amount of plant usable light that hits a certain spot per day and is calculated using this formula: ppfd*3600*H/1,000,000. H being the number of hours the lights are on per day.
Most LED lights produce over 2 PPF/w so let's go with 1.75 to be on the low side. If you have 640w, that would come out to 1114 PPF. It doesn't seem like you have a tent so a lot of that light is wasted so let's say you only get 50% of that light which is also on the low side since your lights are only 1 ft away. at 1.5ftx1.5ft, 1114 umol and 50% usage, you will get an average ppfd of over 2600ppfd. Now just a quick side note, as a general rule, you should not go above 1000ppfd during flower when the plants need more light since they get less time with the lights on. It gets more complicated with supplemental CO2 but you don't have that. Ok so let's plug these numbers into our equation. 2600*3600*18(I assume)/1,000,000=168 DLI. If you are very skilled, you can grow under 60 DLI. Anything above that is pushing it. So now you understand why those lights can't be 80w each.
If you look back a few pages, I explained blurples. They aren't too bad but I don't like them and in my opinion, full spectrums are better. Not that you can't grow under blurples btw. I only switched to full spectrum because my blurple wasn't bright enough and I needed a new light so I went for full spec since I prefer it.
Go look at the stem in the pic I sent last page.
Edit: Never mind, you can't see it. Go look at it on my profile. A reminder, my plant is younger. More leaves means more photosynthesis and more photosynthesis means a thicker stalk.