 Like the first welding process , forge welding , some modern welding methods do not involve the melting of the materials being joined . One of the most popular , ultrasonic welding , is used to connect thin sheets or wires made of metal or thermoplastic by vibrating them at high frequency and under high pressure . The equipment and methods involved are similar to that of resistance welding , but instead of electric current , vibration provides energy input . Welding metals with this process does not involve melting the materials ; instead , the weld is formed by introducing mechanical vibrations horizontally under pressure . When welding plastics , the materials should have similar melting temperatures , and the vibrations are introduced vertically . Ultrasonic welding is commonly used for making electrical connections out of aluminum or copper , and it is also a very common polymer welding process . 
