The word welding often invokes visions of arcs and sparks flying while the welder is stooped over and peering through the welding visor.  However, there are many welding processes where there are no arcs and sparks; in fact, there is no melting at all!  The surfaces to be joined are brought together and the weld is formed without raising the interface temperature above melting point of either material.  If the material did not melt, should the joint be called a solid state weld?  Or should it be called a solid state bond?
 
Continuing with the theme from the previous newsletter (Fall 2009: Brazing or Soldering - What is in the name? ), I thought it would be interesting to delve into the distinction between welding and bonding.  As with any solid-state joining process, the key elements are identical - bring the two surfaces to be joined into intimate contact over a period of time.  Intimate contact can be produced with a combination of pressure, temperature, and/or other forms of activation to soften the parts including vibration, friction, extrusion, or ultrasonic energy.  Once the metal atoms from either side are in intimate contact, they form a "metallic" bond; this bond is same as the bond between any two adjacent metal atoms inside a grain or crystal of metal or even across grain boundaries.  Metals do not need any chemical reaction to form a bond.  At this stage, the joint can be defined as a solid-state bond.  What happens after this metallic bond forms is what distinguishes welding from bonding.  To form a weld, the original joint interface has to be metallurgically altered by a variety of solid-state phenomena including grain growth, recrystallization, and diffusion across the weld interface.  Given sufficient time and temperature, a solid-state bond can be transformed into a solid-state weld across the interface.
 
Is welding vs. bonding just a matter of semantics? Not so.  Choosing between welding and bonding will depend on the metals/alloys being joined.  If the bond is between dissimilar metals that could form brittle and detrimental intermetallics, the "welding" process should be stopped once a metallic bond is formed and not allowed to proceed to form a weld.  There are many processes in industry that are labeled as weld or bond but do not necessarily reflect what happens at the joint interface.  For example, ultrasonic "welding" actually produces a solid-state bond (irrespective of the mechanical distortion of the original joint interface) while diffusion bonding is actually a weld.  On close inspection, you will find that many resistance welds between conductive materials are actually bonds not welds.  Keep in mind that formation of a bond or weld does not reflect on the strength of the joint; either one, when properly designed, can be strong.