$1.$ In diffusion welding, the process involves the use of $\_$ under high $\_$ and at elevated $\_\_\_$ to join materials without melting them, making it suitable for materials that are difficult to weld using conventional methods. $($Options: vacuum, pressure, temperature, electrical current, atmospheric conditions$)$

$2.$ Friction welding creates a strong joint by transforming mechanical $\_$ energy into heat through intense $\_$ at the interface of the two materials, typically used for joining $\_$ and $\_$ metals. $($Options: kinetic, rotational, frictional, dissimilar, similar, ferrous, non$-$ferrous$)$