Can i use a18 watt adapter to my iphone 7 plus???

To be more specific, you can use any power source made by Apple or a quality 3rd party manufacturer, but anything more than 12 watts will not charge an iPhone 7 any faster than a 12 watt iPad charger. Any charger that has matching cables will still be safe to use, however.

Fast charge your iPhone

Use fast charge with certain iPhone models. You can recharge your iPhone up to 50 % battery in around 30 minutes.

You can fast charge your iPhone 8 and later up to 50 % battery in around 30 minutes.

Fast charging works when you use an Apple USB-C to Lightning cable and one of these adapters:

• Apple 18W, 20W, 29W, 30W, 61W, 87W, or 96W USB-C Power Adapter
• A comparable third-party USB-C power adapter that supports USB Power Delivery (USB-PD)

If you're not sure which Apple Power Adapter you have, check the wattage on the top or bottom of the adapter:

Yes. But it may not fast charge but will surely charge your iPhone.

Sure. It will only supply 5V and won’t take the maximum current, but it should be safe. That should be the case with a USB-C to Lightning cable, or with a USB-A cable with a USB-C to USB-A adapter.