Laser Welding of Copper — Technology vs. Temperament
Contents:
Laser Copper Welding
Humankind has used copper on an industrial scale since the Bronze Age, so it might seem that its properties had long been thoroughly studied, leaving little surprise. By the end of the 18th century, copper was relegated to the dull role of a material for small coins and decorative items. It was practically pushed out of progress. However, electricity and aluminum twice overturned our understanding of the physical capabilities of the red metal—from a nearly forgotten source of luck, copper once again became almost the heart of the "New Civilization," as Herbert Wells described it. Even more remarkable is copper's fate over the past 50 years. Once iron and steel had destroyed copper in battle, now copper triumphs over iron in microelectronics.
Copper was forged, twisted, and pressed, but hardly ever properly melted, except when made into bronze. Iron and steel had pushed it out of everyday engineering use.
Early Electric Motors and Initial Challenges
Contrary to popular belief, steam engines in industry were replaced not by diesels, but by electric motors. Copper proved to be the ideal and accessible metal for the electrical industry in every respect—malleability, ductility, electrical and thermal conductivity. Its ductility and malleability drastically reduced the cost of electric motors, as copper revealed that electron movement occurs on the surface rather than throughout the conductor's thickness. Naturally, wire for electric motors had to be judged primarily by surface area rather than cross-sectional area. There was no longer a need to economize using inconvenient steel wire. Copper's ductility allowed a relatively expensive pound of metal to be rolled by hand into nearly half a mile of wire, almost like sewing thread. This primitive device lasted on electro-diesel submarines for almost fifty years.
Sailors discovered another interesting property of copper: a punch with a minimal tip diameter, applied with sufficient force, practically "solders" two thin copper plates together. In other words, copper's thermal conductivity, as a heavy metal, is good, but heat transfer is still somewhat slow. The impact area heats enough for surface intermixing of the two plates—a cheap and excellent method to join two copper wires "live."
However, despite all its advantages, copper has its drawbacks, mostly related to its chemical properties. For example, before the advent of laser welding, all fine copper work, especially soldering and welding, was practically a form of jewelry-making! As a rule, simple twisting and vise pressure were used for joining operations.
The “Copper + Gas” Problem
Among modern methods of joining copper parts, the simplest—gas, specifically acetylene welding—was the first to be rejected. Chemists were happy: copper is the best catalyst for acetylene polymerization. However, technologists had to avoid using pure copper almost everywhere except in electrical engineering. For example, simple acetylene pipelines are made from alloys with copper content not exceeding 64%.
Working with pure hydrogen also has issues: the gas itself is expensive and has long been replaced by arc welding with water splitting to produce "oxyhydrogen"—a mixture of hydrogen and oxygen. This setup is bulky, and the heat released during hydrogen combustion is so intense that precise fine operations become impossible.
Solution
The theoretical solution is quite simple: solid-state or fiber lasers. Arc welding is also effective, but due to the need for argon, it is too cumbersome and better suited for "more fundamental" tasks, such as working on transformer contacts. But this techno-harness does not fit every transformer!
Lasers have none of these problems—they are, first of all, portable. The laser beam is thin and transmits energy to the welding point almost without loss. Solid-state models have low efficiency and, accordingly, low energy output. However, even typical compact fiber lasers can reach 1.5 kW—enough to weld 5-mm steel sheets, but for copper, only sheets up to 1 mm thick. Need more? Switch to pulse mode: fiber lasers can increase the beam power up to 2.5 kW. And remember—this is just the power of a standard iron.
In fact, a fiber laser welding device is sufficient for any operation, from soldering computer board tracks to creating monumental statues, but you need to understand certain physical properties of copper, which will be discussed below.
Welding Electrical Equipment from Batteries to Terminals
Copper in normal atmospheric conditions does not form oxides. Sunlight is probably the least significant environmental factor that can affect the metal surface, as copper’s reflectivity reaches 0.95. But there is one interesting exception: the green light spectrum of 500–550 nm, which corresponds to the oscillations of electrons in the outer orbital of the Cu atom, is very actively absorbed by the metal as thermal energy. A copper handrail under the setting sun can easily burn your hand.
This property of copper has been utilized in modern industry, specifically in laser welding. It should be noted that typically powerful lasers operating in these ranges are handheld fiber models. This aligns with safety practices, as the green wavelength is close to the human eye’s "twilight vision" maximum. Gas lasers cannot produce a green beam with sufficient energy concentration to be visible even in daylight reflection. With solid-state fiber lasers, there is no such risk—the beam is visible even after multiple reflections. Clearly, special safety measures are unnecessary.
The absorption of green light is entirely natural—copper is called the "red metal" in many languages for a reason. However, the crucial point is not just the absorption of energy, but that the reflection coefficient of copper for green light starts at an extreme 0.95 at normal temperature and sharply drops to 0.5 near the melting point. Even more important is that Brownian motion in one of the heaviest industrial metals cannot keep up with the accelerating heating: the green laser can easily create a highly stable molten pool deep within the plate or between workpieces. This is often called a “reverse keyhole”—a deep, microscopic spherical analog of a “magma core,” like in the interior of a volcano.
Obviously, in the inverted keyhole, the molten metal has nowhere to splatter except back into the “reaction core,” maintaining the internal temperature of the weld zone and heating the surrounding metal mass. Notably, the green laser beam is indifferent to the copper surface condition: polished, oxidized, matte, or etched—all will be penetrated by the beam almost equally, with oxidized surfaces possibly slightly faster.
From the operator’s perspective, everything boils down to precise laser pulse control and workpiece temperature. Using inert gases for welding is completely unnecessary. The lack of additional complexities allows manual gun control—well before ejection, the glowing red overheated zone around the laser beam is clearly visible. In essence, the operation feels as “natural” as steering a car or piloting an airplane rudder.
Advantages and Disadvantages of Laser Copper Welding
It so happens that electrical engineering and electronics implicitly rely on lightweight materials. For example, batteries, accumulators, and electronic circuits are overloaded with heavy metals, which increases their weight. Copper has a high density but also a relatively high melting point, resistance to atmospheric oxygen, and excellent alloying properties, especially with aluminum. Such alloys almost retain copper’s conductivity, nearly reach aluminum’s low density, but still have peculiarities and tricky drawbacks, especially in welding, where processes occur in a second or fractions of a second.
These drawbacks are so varied that the only general solution is speed.
In "heavy industry," pure copper is used only in expensive batteries/accumulators and critical high-voltage control boards. For example, purified copper ore costs about 8–9 thousand dollars per ton without refining, while copper suitable for electronics costs at least 1.5 times more. The reason is simple: even a 0.015% aluminum impurity (most pure copper is obtained via salt hydrolysis, which inevitably involves aluminum) increases electrical resistivity from 0.0001 μΩ·m to 0.02 μΩ·m. Imagine a household 220~5 V transformer made with such wire suddenly heating up and burning your hand!
But there is another side of the coin. Other impurities or intentional interventions practically do not affect the electrical conductivity of copper alloys once cooled. It is straightforward to calculate the required cooling measures and alloy parameters to achieve the desired result. Each batch of electrolytic or chemical copper undergoes chromate analysis before being assigned the proper grade. Just as Muhammad did not create a 99% mountain, he assigns 99% to what deserves it. Aluminum also conducts electricity well and gains additional strength during alloying. There are so many copper-aluminum alloys that some of them do not even have “broad engineering” names.
First, note a huge advantage of copper: it dissolves in aluminum even in the solid phase, of course when heated. The maximum concentration of 5.65% is reached near the melting point and decreases with cooling. The intermetallic CuAl2, due to its molecular size, fits perfectly into both molten aluminum and copper, providing mechanical strength and enhancing temperature resistance by approximately +150–200 ℃.
But do not attempt to weld such an alloy by any method other than instantaneous ones! CuAl2 is very brittle and hard. Without a precisely focused laser or spot arc under general heating, the weld joint will simply tear. Arc welding requires experience and "intuition." Laser beams work better in this context.
It is simple: during the golden age of electrical engineering, this alloy was one of the best and cheapest, but its drawbacks quickly emerged, preventing industrial adoption. True copper-aluminum eutectic is achieved at 33% copper. The CuAl2 intermetallic, whose dendrites provided strength at 5–10% copper, began showing brittleness. Thin joints or contacts after repeated "heat-cool" cycles under load suddenly cracked and split. Even a successful 6% copper alloy quickly became unusable after several heat-cool cycles.
However, the problem had a silver lining: excellent copper solubility in aluminum combined with the relatively low melting point of the eutectic alloy (+548℃) allowed creating numerous interesting alloys with varying density, conductivity, and different strength/castability ratios.
Laser Copper Welding in Various Industries
Now you understand that copper welding is a delicate operation but does not require guru-level knowledge. It just requires attention.
First: when welding copper or almost any of its alloys, there is the problem of incomplete melting of impurities. Copper is chemically low-reactive, but metallic impurities readily form intermetallics with their own unique properties. Worse, impurity compounds create localized areas with distinct physical properties—higher brittleness and heat capacity.
This problem is easily solved: you need a powerful focused beam. It is also helpful to have the option of additional preheating. Clearly, a handheld laser gun or a finely controlled robot is ideal for this task.
Second: in electronics and, generally, in power relay engineering, thin copper or copper-aluminum contact plates often need to be joined. Here we face two challenges: residual deformation and heating. Both heat and temperature shifts can easily damage nearby electronic components, and in power circuits, heat and ceramics are entirely incompatible.
Again, the solution is simple: speed and a powerful focused beam. Copper transfers heat relatively slowly and cools quickly, so a fiber laser and a robotic manipulator are very helpful.
However, it is entirely possible to use a handheld fiber laser without additional machinery. The only thing to pay attention to is the initial preheating of the weld area. You need to create a "reverse keyhole," i.e., carefully burn a single point for a few seconds before moving to create the seam.
By the way, handheld fiber lasers have another great advantage—the gun, and the fiber itself, are very lightweight. For extremely precise welding, such as electronic components, you can use any lightweight and inexpensive robotic manipulator.
Third: there is another complex problem that the laser and manipulator may not solve immediately. Preliminary experiments are required. If two copper alloys or copper and an alloy differ significantly in copper content, thermal cracks are almost guaranteed, along with unpredictable jumps in electrical resistance later. You will need to rely on your judgment to evaluate welding success, but it is better to over-prepare: laser, robot, and additional preheating. Even better, if possible, practice a few times on scrap alloy pieces. With a handheld laser, a few minutes of preparation are enough for your "hand to feel it."
