Fall 2022 Contents:                                                                                                Issue No. 56


Solid-State Welding

This nugget discusses type of bond formed in solid state welds.

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(Below is text of the article without figures; if you would like to download pdf copy with figures and tables, please click on the link above)


Joining two metal components to form a strong bond has been a human endeavor for many centuries.   Engineers and scientists have been very creative in forming new objects by joining two components togethers and testing the joint for performance which can include strength and hermeticity.   Joining processes such as soldering, brazing, and adhesives are easy to understand as an obvious third material is added to form a bond; similar situation exists in arc welding with filler wire.  However, direct metal joining is a bit of mystery, where a strong bond is interpreted to necessarily have some level of melting at the interface.  In this newsletter, we will review the mechanics of bond formation in metal joining without any melting and mixing, a notion that throws off many experienced welding engineers – especially those with a background in arc welding processes.


Let’s start with the basics: Why do metals bond?   For that we will have to start at the atomic level.  Metal atoms are composed of a positively charged nucleus and a cloud of negatively charged electrons around it.  Most metals are electrically conductive and have a few valence electrons – those that are free to be shared between other atoms.  When a bunch of metals atoms are close to each other, these valence electrons float around in a cloud that surrounds all the nuclei, leaving each atom with a slight positive charge which then attracts the electrons of the neighboring atoms producing a bond between adjacent atoms.  The bond thus formed is non-directional and produces attraction forces with all immediate neighbors – such a bond is called a metallic bond.   This is the bond that keeps adjacent metal atoms in a grain sticking to each other, and adjacent metals grains bonded to each other and not falling apart like grains of sand.  In contrast, ionic (e.g., NaCl) or covalent bonds (e.g., Si3N4) are directional and do not have free electrons, resulting in materials that are not electrically conductive and also quite brittle.


Two pieces of metal can be bonded together without producing any reaction or melting at the bond line.  For example, if you take a clean foil of soft gold and press it hard against a clean foil of soft copper and hold the assembly in place for some time, those two foils will develop a strong bond between them – without any fusion or reaction between gold and copper atoms.  Key is to have surfaces that are free of any contaminants or oxides, and soft enough that atoms on either side can be brought into intimate contact.  Such an experiment can be conducted in a lab, but in a practical sense will take too long to form a bond.  In real joining applications, energy and pressure are used to bring atoms on both sides into intimate contact to produce a bond in reasonable time, which can range from milli-seconds for resistance welding to a few hours for diffusion bonding.


Energy to produce a solid-state bond can be supplied in various forms including internal vibrations during ultrasonic welding, resistive heating during resistance welding, friction during friction welding, and an intense surface upheaval during explosion welding.  In processes such as explosion welding and friction-stir welding, the interface appears to be in turbulence with a lot of mechanical mixing at the interface.  Where as in resistance welding, a fairly smooth interface can be observed along the bond line, as seen in Figure 1.


 (download pdf document in link above to see figures)     

Figure 1. Weld section photos show vigorous mixing at the weld interface in friction stir welding (left) and clean weld interface in resistance welding (right).



One of the great benefits of solid-state welding is the ability to join dissimilar metals that would have otherwise cracked during fusion welding.  Welding aluminum-to-steel with explosion welding is a good example of a solid-state joint that can be used as a transition material that is weldable with fusion process to their respective alloys on either side.  Other dissimilar metal combinations commonly welded with solid-state welding include copper-aluminum, copper-steel, and dissimilar steels.  A dissimilar metal bond does increase the risk of interaction between the two components to form intermetallic compounds along the bond line; such intermetallic compounds are usually brittle and have lower electrically conductivity in comparison to the base metals.  The amount and distribution of intermetallics depends on temperature exposure during welding or during use of the welded components.  There is also a possibility of intermetallic growth associated with high current flow across the interface.


A primary requirement of the solid-state welding is that the surface should be clean of contaminants as well as oxides.   Contaminants are often present inadvertently on part surfaces as residuals from prior process such as metal forming (stamping, rolling, etc.), plating, and handling.  Processes such as resistance welding are a bit less sensitive as the heat during welding is able to burn off most of the organic contaminants.   Surface oxides can also affect the joining process.  Parts that have been exposed to environment for a long time or have an obvious layer of oxidation/discoloration should be cleaned and/or etched prior to welding.   Platings on the surface that are too hard should be avoided; on the other extreme, tin plating is bit too slippery and can interfere with ultrasonic welding.


Solid-state welding opens up a whole slew of options for joining metals, especially dissimilar metals and alloys that would be difficult to join with fusion welding processes.  A welding engineer would do well to review all options before committing to any specific process and design to meet functional performance requirements of the weld.