The Challenges of Welding Copper
Copper is used in a wide range of applications because it’s malleable and a great conductor of both electricity and heat. Its thermal conductivity is approximately 385.0 W/m-K and its melting point is 1,984°F / 1,085°C.
Copper is highly reflective of laser light, especially infrared lasers. Therefore, it takes a large amount of power to cause copper to couple. However, as its temperature rises so does its ability to absorb heat, and at its melting point, copper becomes highly absorptive and the chances of blow-outs and spattering drastically increase. Due to copper’s high thermal conductivity it is very easy to deform and damage a part by applying too much heat. Ways of avoiding this include using lasers with shorter wavelengths or of particular colors (green) as well as careful ramping of the laser power intensity.
A consistent weld seam requires the melt pool to be smooth and even as it solidifies. Copper, however, has a low viscosity melt pool – much lower than steel or aluminum – and is prone to rippling and movement. Copper also solidifies quickly, resulting in weld seams with an irregular morphology compared to other materials, such as steel, and poor filling of the weld gap. With copper, the laser itself causes waves and streams in the melt pool, which in turn cause turbulence throughout. At EB Industries we develop copper welds with a long, oval shaped melt pool such that turbulence diminishes in the rear of the pool before solidification. This is difficult to achieve and requires precise control of heat and feed speeds.
Copper weld seams are typically soft compared to the base material because copper is non-allotropic and phase transformations do not occur. Molten copper solidifies with a coarse microstructure that can be crack prone. The problem worsens depending on the amount of oxygen in the copper. Copper oxides can react with hydrogen to produce steam, which can cause intercrystalline cracking. Using oxygen-free copper (OFC) or oxygen-free high thermal conductivity copper (OFHC) can mitigate cracking. Careful use of cover gases and control of the weld environment can also help to mitigate cracking and increase the quality of the weld.
Material Preparation and Joint Preparation
Copper parts that are to be welded must be clean and free from surface hydrocarbons and oxides. The amount of pre-weld preparation is largely dependent on the condition of the copper parts to be welded. Conditions of the parts are often effected by the storage conditions and the cleanliness of the machine procedures used to make the part thus far. Special care in machining, cleaning, and assembly must be taken in order to maintain the condition of copper.
- Machining methods that leave a ground or smeared surface should be avoided. For example, a band saw will leave metal smeared along the path of the blade. This problem can be avoided by filing or machining the joint edge post cutting.
- Avoid grinding or sanding if possible, as these processes can result in embedding grit in the copper. Scotch Bright products are particularly unsuitable for working with copper that is to be welded. If grinding cannot be avoided, use a course disk.
- Hydrocarbon residue on copper parts can generally be removed using acetone or alcohol-based solvents. Avoid using chlorinated solvents in the welding area because they may form toxic gases when heated.
- When cleaning a surface with solvents, use clean cloths such as cheese cloths or paper towels. Do not use shop rags that may be contaminated with oil residue.
- Avoid using compressed shop air to blow off debris from the area of the joint. Compressed air contains moisture and oil contaminates. If a part must be blown off, use a bottled gas such as nitrogen or argon.
- Use a stainless steel wire brush to clean a joint only after solvent cleaning to avoid embedding hydrocarbons or other into the copper. The stainless steel brushes used should be strictly used on copper to avoid cross-contamination.
- Clean all wire brushes and cutting tools frequently.
Laser welding requires a precise joint in order to maintain a permissible gap and avoid mismatch. The quality of the weld fixturing is important so that the laser beam can be placed accurately. Laser welding and cutting are thus inherently machine guided processes – welding copper is not a hand-held sort of task.
- 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 and/or feed speed. For round welds in copper, no gap can be tolerated unless inert gas coverage can be maintained over the entire weld area.
- Fillet Joint:
This joint configuration is especially suitable due to copper’s high shrinkage rate. Square edges and good fit-up are also necessary.
Recommended Lasers for Copper Welding
The wavelength of the light emitted by the laser can have a big effect on welding efficiency and weld quality when it comes to working with copper. As an example, CO2 lasers couple better than Nd:YAG lasers, while disk lasers that operate in the green spectrum (approx. 515 nanometers) work extremely well with copper.
We generally use a continuous wave laser rather than a pulsed laser when working with copper. This means that the weld is basically a keyhole weld. We’ve found the steady state heat of continuous wave results in far less cracking when welding copper, whereas the constant heat/cool cycle of a pulse laser tends to exacerbate cracking.
The exception to this is when we weld beryllium copper: a pulsed laser can be used in this circumstance.
Cover Gases for Copper Welding
Argon is the preferred cover gas when laser welding copper. Argon is very dense, and in a class 4 laser environment it forces out the oxygen from the weld area, resulting in a pure, strong well with low levels of contaminates.