Laser Welding of Aluminum and Aluminum Alloys: Part One

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Introduction to Laser Welding Technology

Laser energy is rapidly developing as a production process for welding and cutting metals. The word laser means "light amplification by stimulated emission of radiation." The light beam is highly collimated and can be focused to result in weld widths approximately three times the aluminum sheet thickness. Lasers are available in sizes up to 25 kilowatts power output, with lasers in the 3 kW to 10 kW range typically adequate for aluminum sheet thicknesses in automotive applications.

Laser welding stands at the frontier of welding technology, with keyhole welding adoption increasing across various manufacturing sectors. The capability to weld materials of varying thickness quickly and efficiently, while producing small heat-affected zones, continues to attract growing industrial interest in laser welding applications.

Laser Welding Process Fundamentals

However, keyhole welding (>10⁶W/cm²) presents challenges including instability, keyhole oscillation, and intermittent keyhole closure that often leads to porosity. In some alloys, high weld speeds and consequently high cooling rates can cause embrittlement in the weld or heat-affected zone. Laser conduction welding (LCW) (<10⁶W/cm²) offers comparatively stable operation and may provide an alternative means of welding traditionally difficult materials such as aluminum alloys.

Advances in laser keyhole welding and investigations into accompanying difficulties are well documented in literature. Analytical, numerical, and empirical studies have been undertaken to achieve better process understanding.

Of several laser types, those used for welding most commonly include solid-state Nd:YAG (Neodymium: Yttrium-Aluminum-Garnet) and gas CO₂ types. CO₂ gas lasers easily achieve power ranges suitable for welding aluminum body sheet gauges. Recent developments in fiber optics for Nd:YAG laser systems have made them suitable for welding thinner aluminum sheet gauges.

Aluminum Laser Welding Challenges

Upon laser weld pool formation in aluminum, the molten metal's shiny surface reflects much of the light beam energy. This energy loss and rapid heat dispersion in the workpiece due to aluminum's high conductivity require rather high power levels for welding.

Laser welding requires excellent joint fit-up regarding permissible gap and low mismatch levels. Good weld fixturing is necessary for accurate beam placement. Laser welding and cutting are therefore inherently machine-guided processes.

High energy density laser and electron beam welding characteristically produce deep, narrow weld beads. This bead forms through keyhole mode operation, where the keyhole cavity is produced by metal vaporization at power densities of 10⁵ watts/cm² or greater.

Laser vs. Electron Beam Welding Comparison

Significant differences exist between these processes: lasers heat with photons of approximately 0.1eV energy while electron beams use particles of 100,000eV energy. This results in a laser light beam that readily interacts with free electrons found in plasma. The plasma forms through metal surface vaporization, and this interaction defocuses part of the incident beam, producing a characteristic "nail-head" type weld.

Electron beam electrons, conversely, are too energetic for significant plasma deflection. Consequently, energy coupling is much more efficient using electron beam welding compared to laser welding.

Aluminum is among the easiest metals to penetrate with an electron beam but one of the most difficult to melt with a laser. Poor laser energy coupling results partly from high free electron density in the solid, making aluminum one of the best light reflectors. Additionally, many aluminum alloys contain magnesium or zinc, which vaporize easily and form plasma that blocks the incident beam.

Process Parameters and Surface Preparation

Previous investigations have demonstrated low power absorption, alloy compositional differences, and surface preparation importance when laser welding aluminum. Power absorption changes dramatically at times, producing an unstable process with poor penetration control and rough bead surfaces. The difference in absorbed power fraction is commonly attributed to metal melting.

Alloy composition and aluminum surface preparation significantly influence laser light absorption. Most laser conduction welding (LCW) research has focused on investigating the process to understand laser welding's complex nature. Low powers at focus have been mainly utilized, with most work concentrated on spot welding. High-power defocused laser beam investigations have been restricted to situations where focus distances ranged from –10mm to +10mm for both conduction and keyhole welding. This work examines workpiece distances below focus ranging from +5mm to +100mm, truly entering the conduction welding regime.

Advantages of Laser Welding Aluminum Alloys

Laser welding aluminum alloys offers numerous advantages: precise heat input, narrow weld beads, narrow heat-affected zones, minimal thermal distortion, elevated welding speeds on thin sections, and deep penetration on thick sections. These advantages drive increasing laser welding applications for aluminum alloys, particularly in the automobile sector for body and exterior paneling.

Although aluminum alloys are weldable by traditional fusion welding methods, lasers offer unique qualities making them ideal technology for joining aluminum alloys. Most strain-hardenable alloys (such as AA5182 and AA5754) can be laser welded autogenously, although filler metal can be introduced during laser welding to add reinforcement or improve joint strength and ductility if desired.

Hot Cracking Challenges and Solutions

Many heat-treatable alloys (such as AA6016) are susceptible to hot cracking during welding due to their chemical compositions and thermal strains induced during welding. To avoid hot cracking, filler metal is used to adjust weld bead composition beyond the crack-sensitive range.

Industrial experience shows that hot cracking can be avoided by adding eutectic alloys, such as Al-Si, to the weld. However, this leads to shorter solidification intervals. The filler wire technique has two major disadvantages. First, joint macroscopic properties change uncontrollably. Second, the laser beam's small size leads to occasional filler wire feeding directly into the beam, causing inconsistent penetration and weld-pool instability. For example, adding 4043 Al-Si alloy wire to 6061 alloy welds reduces hot cracking susceptibility, but both tensile and ultimate strengths are reduced by 50%. Therefore, process parameter optimization to reduce cracking sensitivity would be highly desirable for producing lightweight, high-strength components, especially for transport equipment industry.

Hot cracking susceptibility during welding is usually evaluated when strain or stress changes during the process. Examples include Sigmajig, Varestrain, and Houldcroft tests. These tests are performed in specific mechanical devices where loading or bending is applied. Major limitations of these methods include poor reproducibility due to complex mounting geometry and difficult result interpretation after testing.

Precise solidification process control reduces cracking sensitivity. Using pulsed Nd:YAG lasers, Katayama and collaborators developed a pulse shaping method where laser power continuously decreases with time to control solidification rate. These authors showed that pulse shaping could effectively reduce hot cracking in Al-Cu alloys. The method's major limitation is small processing velocity, as pulse length increases from 5 to 20 ms, welding speed reduces by a factor of four.

Continuous laser processing is desirable welding technique due to processing speed, weld quality, and mechanical joining properties. The Al-Cu system forms the base of commercial alloys particularly affected by hot cracking.

Joint Design Considerations

Three joint designs (butt, lap, and flange) appear best suited for laser welding.

• Butt Joints - Sheared edges are acceptable provided they are square and straight. A fit-up tolerance of 15% of material thickness is desirable. Misalignment and out-of-flatness of parts should be less than 25% of metal thickness.
• Lap Joints - Burn-through or seam weld type. Air gaps between pieces to be welded severely limit weld penetration and/or weld travel speed. For round welds in aluminum, no gap can be tolerated unless inert gas coverage can be maintained over the entire weld area.
• Flange Joints - This geometry is especially suited for aluminum because of aluminum's high shrinkage rate. Square edges and good fit-up are also necessary.

Advantages and Limitations

Laser welding, like most welding processes, has advantages and disadvantages.

Advantages include:

The laser beam, unlike other fusion processes, can be projected in air with no power loss, and no vacuum chamber is needed. The ease of beam transmission and directional control permits multi-station operation. Nearly 100% duty cycle is possible by switching the beam from station to station. The high energy density beam enables rapid welding speeds, narrow welds and narrow heat-affected zones, therefore providing good retention of metal properties and relatively low workpiece distortion.

Disadvantages include:

Most multi-kilowatt laser systems operate at only approximately 10 to 20% efficiency for converting electrical power into focused infrared laser beams. Precise fit-up (a fit-up tolerance of 15% of material thickness desirable) is necessary on butt and lap joints for good quality. The high cost of laser beam equipment ranges from approximately $30,000 to $1,000,000. This large capital investment requires high volume production or critical applications to justify the expenditure.