Yes you can solder 18650 cells to make your own battery pack. There's no need to buy an expensive welder. Here I show you how I made a 6 cell pack in series.

You'll need the following:
Soldering iron capable of 800-850° https://amzn.to/3aP4HJd or https://amzn.to/37SFZFI
Wide soldering tip 5mm https://amzn.to/2WSgbDC
60/40 rosin core solder https://amzn.to/38LekFQ
Flux Pen https://amzn.to/2X4NFyJ
Sanding Stick https://amzn.to/2M6vBBM
Tabbing, minimum 5mm https://amzn.to/3aWQrOi
Scissors https://amzn.to/3hsyDw2
Popsicle stick https://amzn.to/34O1O7G
Combination Square https://amzn.to/3rzuV8n
Medium viscosity super glue https://amzn.to/38F5uJX
Super glue accelerator https://amzn.to/2WT1yzG