Can i use 18w charger with ipad 6 and air 2 ?

You can safely use your Apple 18W Power Adapter (charger) with any of your iPads and iPhones.

If purchasing a new Power Adaptor, the small difference in cost will provide greater flexibility when used with newer devices. New iPad and iPhone models benefit from USB-PD fast charging support - this being the standard supported by Apple.

If your devices have a Lightning connector, to connect them to the 18W Power Adapter (that has a USB-C connection), you’ll need a Lightning to USB-C cable - such as this:

USB-C to Lightning Cable (1m)

https://store.apple.com/uk/xc/product/MX0K2ZM/A

It won’t, but you will not see any benefits from it.

Those iPads will just take as much power input at its designed regulated amount.

Neither the 2014 iPad Air 2 nor the 2018 6th gen iPad support quick charging with the Apple 18W power/charging adapter.

Just purchase the 12 W adapter instead.

Best of Luck to You!