Laser Welding of Aluminum — Modern Physics vs. Ancient Chemistry
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Laser Welding of Aluminum
Aluminum is an excellent material in all engineering fields, from mechanical engineering to construction: low density, very high bending strength, and good ductility, which prevents rupture under overload. However, technologists and machine operators often emphasize the metal's corrosion resistance when explaining the specific and costly methods for processing products made from its alloys. In most cases, they are mistaken, but somehow act correctly from a technological standpoint. It’s worth resolving this discrepancy, especially since civilization has already matured to laser welding of metals.
Characteristics of Aluminum and Its Alloys
In industry, all classical metal processing methods are used, and for turning, drilling, and milling, aluminum is one of the best materials in every respect. However, when it comes to welding, the physical and chemical properties of aluminum make the task expensive and energy-intensive.
Besides flexibility and ductility, aluminum has a major advantage: it melts at only 660℃. Pressure processing requires neither significant energy nor specially trained personnel. Sometimes slight preheating is needed, and an industrial workpiece can be shaped under moderate pressure of 500 MPa.
However, complex parts cannot be made using such simple methods. The metal's activity immediately makes conventional gas or arc welding problematic — the finest scale can be easily removed, but its tiny fragments inevitably compromise the weld’s density and quality. Moreover, aluminum at 400℃ becomes chemically active and begins absorbing light atoms and compounds, from oxygen to water vapor. Worst of all, it even absorbs inert helium in molten form! This makes using an inert gas stream quite challenging. The resulting weld is uneven, with numerous pores and inclusions of brittle aluminum oxide and gas bubbles mixed with scale fragments.
Another major drawback is aluminum’s excellent thermal conductivity. During conventional welding, most of the energy simply spreads throughout the welded part, while the local heating zone does not reach 670℃. Around the weld zone, an oxidized layer forms, which will likely need to be mechanically removed — for example, using a helium stream.
All these challenges practically rule out high-quality manual welding with gas or arc by a standard worker.
The only solution is very fast localized heating, possibly supported by an inert gas stream and strict temperature control of the weld area. This task is easily solved using an industrial laser.
The laser beam allows for extremely localized heating, instantly adjusting the energy level and even eliminating the need for inert gases. Chemical processes are tens of times slower than physical ones. Properly configured laser radiation can melt the edges of parts before absorption or oxidation begins. The speed and precision of the laser beam remove the need for special operator skills.
Types of Equipment for Laser Welding Aluminum
Of the four types of coherent light sources used in industry, two are widely used: solid-state and gas lasers. Another one catching up is the fiber laser. The operating principle is the same for all types: the active medium (artificial ruby or a gas mixture in a capsule) absorbs pump light; impurity atoms in the medium excite electrons from normal orbitals to higher ones. This excited state is temporary; electrons release energy as photons and return to their original orbitals. The medium is placed between two mirrors — one fully reflective and one partially transparent. Photons generated along the axis between mirrors are repeatedly reflected and "accumulate." Once a certain pumping power is reached, the photons pass through the partially transparent mirror.
At first glance, this seems like an overly complex way to generate light from another source. Classic solid-state lasers based on aluminum oxide with chromium impurities emit only about 2% of the absorbed pump energy. Gas lasers reach 15%. It seems wasteful, but the photon energy emitted by the impurity atom's electron behaves coherently with others. Coherent radiation is so energy-dense and easy to control with simple optics that a lens, barely able to burn an ant with sunlight, can melt steel with a laser beam.
Solid-state and gas lasers are now mostly used in laboratories. Industrial laser systems with up to 70% efficiency are already available.
Solid-State Laser
The artificial ruby crystal (aluminum oxide with chromium salts) heats up significantly during operation, affecting its optical parameters and efficiency. The mirrors are the polished ends of the crystal, so heating reduces the already low efficiency. A quality cooling system is essential.
- Stable wavelength / monochromatic
- Compact and simple optics
- Expensive
- Extremely low efficiency
Gas Laser
In a tube with carefully treated end mirrors, a gas analogue of artificial ruby contains a small amount of atoms with unfilled outer orbitals. Electrical discharge pumps the medium instead of light, which is much more efficient. Efficiency reaches 15%, but the device is proportionally larger. A decent cooling system is still required.
- Relatively inexpensive
- Better efficiency compared to solid-state laser
- Complex mirrors and lenses
- Wavelength variation makes perfect focusing difficult
Semiconductor Laser
The most efficient, but youngest and expensive type. Its principle is similar to the others, but the working atoms are concentrated in a micro-region. With many semiconductor pairs, industrial systems range from 1 to 40 kW, comparable to gas lasers.
- High power
- Compact
- Wear of the active semiconductor surface under industrial loads is not yet fully studied
Fiber Laser
This is a subtype of solid-state lasers with semiconductor characteristics. The active medium is a composite fiber with a core of optical quartz doped with rare-earth ions, similar to a solid-state ruby. Diodes with specific wavelengths are used for pumping — highly efficient and cheap. The medium extends into a fiber, minimizing energy loss. Industrial units reach 100 kW in modest sizes. Fixed wavelength allows perfect focusing.
The fiber length is usually 20–40 m, sometimes up to 100 m. The fiber cladding conducts pump light along the length, providing additional amplification of the laser beam.
- High power
- Compact
- Versatility
- Relatively inexpensive
- Durable
- Flexible fiber allows complex geometries to be processed without robots
While laser equipment is now advanced, aluminum itself presents challenges at both chemical and physical levels.
Properties of Aluminum
Chemical Activity
Technically complex and expensive methods for joining aluminum parts have a simple reason: aluminum is extremely reactive. Engineers often mention "corrosion resistance" — a misconception. Al reacts even with water, releasing tiny hydrogen bubbles. Similarly, it reacts readily with atmospheric oxygen. The result is unexpected: aluminum oxide and hydroxide form extremely dense surface films only a few atoms thick. These films are soluble only in strong acids and are very brittle. Scratch the layer, and the shiny metal beneath quickly oxidizes. Multiply aluminum's reactivity by welding temperatures — even aluminum foil can ignite from a match.
More dramatic examples exist, though rarely catastrophic: a plane scraping a concrete runway could catch fire in seconds. Fuel tanks in duralumin wings prevent such disasters: spilled fuel rapidly cools the aluminum surface by 200–400 ℃ within fractions of a second. This principle is also leveraged in laser welding, but intentionally.
Other "protective" properties include inertness in concentrated acids: oxidized aluminum compounds are dense, insoluble, and brittle, forming a protective nanoscale shell. Higher-positioned metals in the periodic table generally form more robust compounds.
Physical Properties
Aluminum benefits from well-documented mechanical properties. Its light atomic weight and low density give excellent ductility. Surprisingly, the tensile strength of pure Al exceeds that of pure nickel.
Historically, Galilei observed that bauxite ores were ductile, "contradicting expectations," and noted aluminum oxide adhesion. At high localized temperatures without oxygen, native aluminum could form in nuggets, always coated with a brittle oxide "scale" that shattered easily under slight pressure.
Another key physical property is the high reflectivity: 0.97! This means 97% of laser energy can reflect away, posing a safety hazard in certain laser welding setups. The oxide layer is so thin (~1/1000 of the laser wavelength) that it does not block the reflected beam.
Following basic safety rules ensures only parts, not people, are at risk. Typically, some parts may be lost during robot welding algorithm tuning. Never remove protective eyewear while the laser is on.
Welding Methods
Welding equipment can be divided into two main types: continuous and pulsed welding. Additionally, it can be classified as stationary or "handheld" operation.
Continuous Mode
This type of welding is intended for joining thick and massive workpieces. Due to aluminum's excellent thermal conductivity, it is necessary not only to heat the weld groove but also to maintain the molten pool so the metal does not absorb water vapor and gases from the air. Another issue is helium: despite being inert, aluminum absorbs it due to atomic size.
For high-quality welding, the front side of the seam is blown with helium at a pressure of 1.2–1.3 atm. Lower pressure won’t sufficiently displace oxygen or remove oxide deposits, while higher pressure overcools the weld zone. The back side can be blown with the cheaper argon gas.
Pulsed Welding
For spot welding, most difficulties are solved by the laser mode itself, except for one — the thickness of the parts or "penetration depth." Inert gas purging is usually unnecessary, but mechanical and sometimes chemical surface preparation is essential.
The principle is straightforward: a short, powerful pulse melts both edges of the parts, and the molten metal rapidly solidifies during the pulse pause. Aluminum oxide does not have time to form, resulting in an ideal seam. It is crucial to monitor the seam “settling”: for strong welds, the groove depth should not exceed 10% of the material thickness at the welding point.
Methods to Reduce Porosity in Laser Welding
Welding light metals comes with many challenges. Most of them are practically eliminated when using laser processing due to the very thin working zone, instant heating even with high thermal conductivity, and easy heat removal when necessary. However, there is a general problem related to the high speed of the operation and uneven heating—no matter how ductile the metal is, it still has a crystalline lattice, not clay. Any foreign particle drastically reduces strength and increases seam brittleness.
The key issue is that as aluminum approaches its melting temperature, it oxidizes very rapidly. The oxide itself melts unevenly at +2000 ℃. Not only is this film brittle, but its inclusion in the weld is unacceptable, and it also absorbs oxygen from the air like activated carbon.
For aluminum-magnesium alloys (the most common materials), the situation is even worse. Magnesium reacts readily with water at room temperature and oxidizes even more aggressively when heated, releasing hydrogen directly into the metal. Microbubbles in the weld are thus a major welding defect.
Additionally, aluminum's high thermal expansion leads to noticeable residual deformation after any heat treatment.
How to solve these issues?
Chemistry
This is the simplest and most universal method for all welding modes and laser types. Weld defects are often caused by impurities that don’t fit into the crystal lattice, so they must be removed.
First, machine oil is removed from the workpieces. Then, the weld joints are treated with 5% sodium hydroxide, followed by 30% nitric acid with 2% hydrofluoric acid. Finally, the parts are rinsed with water to remove aluminum and sodium nitrates.
Theoretically, treated surfaces can be welded within three days, but for high-quality pulsed welding, it’s best not to exceed one day. For general welding, this is a cheap and effective method, though insufficient for delicate or complex structures.
Laser Mode
About 3/4 of porosity issues can be addressed by careful laser parameter tuning. Aluminum has an interesting effect: its reflectivity is 0.97 at 600 ℃, but drops sharply when molten. The molten metal has a reflectivity below 0.5, even though most molten metals behave like mirrors. Molten solder reflects almost as poorly as mercury.
Properly timed pulses allow the weld to fuse before slow chemical reactions occur, resulting in an ideal seam. Overexposure, however, can completely ruin the weld.
This method works best with solid-state and fiber lasers due to their precise, short wavelengths. Gas lasers are more challenging and usually require inert gas shielding.
Inert Atmosphere
Helium and argon have long been used to shield welds from unwanted reactions and work well with gas or arc welding. Slight bulkiness does not interfere with laser setups.
Modern lasers can often operate without gas purging, especially with aluminum, though it is recommended for gas lasers. Interestingly, porosity differs significantly depending on the shielding gas. Helium surprisingly produces a weld about three times worse than argon. Too low a flow (10–12 L/min) foams the molten metal, while too high a flow encourages absorption of gas by aluminum, ruining the weld.
A cheaper alternative is nitrogen (or nitrogen-CO₂ mix), which produces superior welds under minimal thermal control. At ~25 L/min, the gas flow shields the weld, removes oxide, and even cools the welding head. Currently, pulsed fiber laser welding is the best choice for thin or strong aluminum welds, especially when avoiding hazardous chemicals and inert gas setups.
