The Challenges of Welding Copper
Copper is a fantastic conductor of both electricity and heat. It’s soft and easily malleable, and visually an attractive material, making it a very useful metal in a wide variety of applications ranging from electronics to cookware. Copper melts at 1,984°F / 1,085°C. Its thermal conductivity is approximately 385.0 W/m-K.
Copper’s particular characteristics can make it a difficult material to weld. It is highly reflective of light, which makes laser welding copper extremely problematic, as much of the light (and energy) of the laser is reflected away from the weldment. Further, copper gets more reflective as it melts, requiring a ramping up of energy, which can easily overheat the material as it approaches its melting point. As copper is heated its ability to absorb heat changes, becoming highly absorptive at its melting point. It is a Catch-22 situation: copper needs a lot of energy to melt, but it is very easy to overheat it. Without very precise control of the energy flowing into a weld, it is likely that copper welds will be substandard with blow outs and spatters. Copper’s high thermal conductivity is also conducive to heat deformity and damage, making it very easy to warp or distort parts.
The thermo-dynamic and fluid-dynamic properties of a copper are such that smooth heating and cooling of the melt pool is difficult. Copper’s low viscosity melt pool is sensitive to rippling and movement, which causes issues because copper cools and solidifies very quickly, resulting in irregular weld morphologies and poor filling of weld gaps.
Copper welds are generally softer than the base material as copper is non-allotropic and phase transformations do not occur. Melted and then cooled, copper tend to solidify with a coarse microstructure that can be crack prone. Copper oxides exacerbate the problem, as oxygen reacts with hydrogen in the environment to produce steam, which can cause intercrystalline cracking. This sort of cracking can be mitgated by using oxygen-free copper (OFC) or oxygen-free high thermal conductivity copper (OFHC), but it points to a very basic problem that makes welding copper difficult: the presence of oxygen and other gases in the weld environment.