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.
Electron Beam Welding: a superior process for welding copper
In our experience, we’ve found that nothing welds copper better than an electron beam. Inherent to the electron beam welding process are several factors that effectively negate many problems.
First and foremost, an electron beam isn’t isn’t affected by copper’s reflectivity, so it couples to copper much more easily than a laser. Most of the problems caused by welding with a laser simply do not happen with an electron beam. Electron beam welders also have tremendous power output capabilities and are capable of easily delivering the amounts of energy required to melt copper. Further, the precise focusing and consistent energy control of automated or semi-automated EB welders mean that blow outs and slattering are controlled. Electron beam welding, when combined with CNC automation and tight parameter control, is highly repeatable process.
As noted above, oxygen and copper oxides are the main cause of cracking in copper welding. Electron beam welding, in all but a few very specialized circumstances, must happen in vacuum (10^-3 or greater), which eliminates oxygen and other gasses from the equation. Welding in a vacuum, coupled with properly prepared copper parts made of OFC/OFHC, mitigates many of the porosity and cracking problems that typically happen when copper is welded with other processes.
Material Preparation and Joint Preparation
Copper parts that are to be electron beam welded must be very clean, free from surface hydrocarbons and oxides. Copper should be carefully stored to prevent contamination, and carefully handled and prepped.
- Choose machining methods that don’t leave a ground up or smeared surface. Joints that aren’t clean require extra machining or filing before assembly and fixturing.
- Avoid grinding or sanding copper parts and joints. These processes can embed grit into the surface of the material which will contaminate the weld. Scotch Bright products should also be avoided with copper.
- Use acetone or alcohol-based solvents to clean copper parts of hydrocarbons and other surface contaminants.
- Use fresh, clean clothes or specialty wipes to clean copper surface prior to welding. When cleaning a surface with solvents, use clean cloth such as cheese cloth or paper towels. Avoid shop rags or other cleaning tools that might be contaminated.
- Blow debris off copper parts using gas, such as nitrogen or argon, instead shop air, which invariably contain hydrocarbon contaminates and moisture.
- If copper parts require abrasive cleaning, use stainless steel brushes and only do so after the parts have been thoroughly solvent cleaned to avoid driving hydrocarbons or other contaminants into the part’s surface.
- Avoid cross contamination always. Use cleaning tools for copper parts only on copper parts and not on any other metals.
- Maintain high standards of cleanliness when preparing copper for electron beam joining.
Weld joints should be tightly fitted in order to avoid mismatch and maintain gap tolerances. Weld fixtures are typically precisely made to ensure firm part hold and consistent positioning of the electron beam.
- Butt Joint:
A fit-up tolerance of 10% of the material thickness is desirable. Sheared edges are acceptable provided they are straight and square. Misalignment and out-of-flatness of parts should be less than 10% of the material thickness.
- Lap joint (burn-through or seam weld):
Air gaps between pieces to be Lap Joint welded severely limit weld penetration.
- Fillet Joint:
This joint configuration is especially suitable due to copper’s high shrinkage rate. Square edges and good fit-up are also necessary.