The Weld Nugget (Winter 2009) - A Newsletter to inform, educate, and entertain
Solid State Welding or Bonding?
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.
Weldbonding - An Oxymoron?
While searching the web for welding vs. bonding, I came across a process called Weldbonding; sure sounded like an oxymoron but apparently not. Weld bonding is a combination of resistance welding and adhesive bonding used to assemble sheet metal structures. Parts to be joined are assembled with adhesive paste in between the two. The assembly is then spot welded, at predetermined spacing between spots, through the adhesive; the welding force is high enough to displace the adhesive and form a strong weld. The welded assembly is then cured to form a strong adhesive bond. The process combines benefits of adhesive bonding with ability of resistance welds to provide peel strength. Resistance spot welds also provide part fixturing during curing. Greatest benefit could be achieved for Aluminum alloys where spot welds alone might not be sufficient. As always, while handling adhesive, one has to be aware of contamination of resistance welding electrodes and fumes generated during welding and bonding.
Source: The Welding Institute, Abbington, UK
Hydrogen - The Little Big Element
Of all the elements that cause trouble with welding, hydrogen is definitely the most insidious. It is the smallest element and can easily diffuse through the matrix. Hydrogen is typically not an impurity in the base metal but is absorbed into the weld from external sources such as organics or moisture in the shielding gas and/or from moisture on the part surface. Hydrogen cannot be detected by SEM analysis and the user is left to guess the root cause based on circumstantial evidence. Once dissolved into the weld, it can cause all sorts of problems. In aluminum welds, hydrogen has very low solubility in solid-state and forms fine bubbles in the fusion zone. In copper, it can react with oxygen to form water vapor and blistering. In metals such as titanium and zirconium, it can react to form brittle hydrides. In steels, it can form high pressure gas around defect sites and reduce weld ductility. Embrittlement can be delayed by up to 24 hours after welding because of the time required for diffusion. In welding of steels, pre- and post-weld heating can help in driving the hydrogen out of the matrix. However, for most metals and alloys, once hydrogen is in the weld, there is not much you can do. When it comes to this little big element, prevention is definitely better than any cure.