Laser Welding of Aluminum and Aluminum Alloys: Part One

In recent years there has been interest in the use of laser technology for the realization of corrosion-resistant surfaces on engineering alloys based on steel and this work has been reported in several review papers. More recently, there has been a growing interest in improving the corrosion performance of aluminum alloys by laser techniques similar to those used for steel. The techniques principally involved are laser surface melting (LSM) and laser surface alloying (LSA). A range of unique microstructures is produced by these techniques resulting from the non-equilibrium cooling conditions established when the relatively thin laser melted surface layer is allowed to resolidify in contact with the unaffected substrate which provides a large heat sink.

Laser energy is 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 about three times the aluminum sheet thickness. Lasers are available in sizes up to 25 kilowatts power output. Lasers in the size range 3 Kw to 10Kw are usually adequate for the aluminum sheet thicknesses automotive applications.

Laser welding is at the frontier of welding technology and the use of keyhole welding has been adopted increasingly by various sectors of the manufacturing industry. The possibility of welding materials of varying thickness quickly, efficiently, and with resultant small heat affected zones continues to attract more and more industrial interest in laser welding.

However, keyhole welding (>10⁶W/cm²) is not without its problems such as instability, keyhole oscillation, and intermittent closure of the keyhole that often leads to porosity. In some alloys, the high weld speed (and hence high rates of cooling) can lead to embrittlement in the weld or heat affected zone. Laser conduction welding (LCW) (<10⁶W/cm²) is comparatively stable and may offer an alternative means of welding traditionally difficult materials such as aluminum alloys.

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

Of the several types of Lasers those used for welding are most commonly the solid state Nd:YAG (Neodymium: Yttrium-Aluminum-Garnet) and the gas CO₂ types. The CO₂ gas lasers are easily capable of the power range suitable for welding aluminum body sheet gauges. Recent developments in the use of fiber optics in the Nd:YAG laser systems have made them suitable for welding the thinner sheet gauges of aluminum.

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

Laser welding requires good joint fit up in regard to permissible gap and a low level of mismatch. Good weld fixturing is necessary so that the throw beam can be placed accurately. Laser welding and cutting are thus inherently machine guided processes.

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

There are, however, significant differences between the two processes: lasers heat with photons of approximately 0.1eV energy while electron beams use particles of 100,000eV energy. These results in a beam of laser light which readily interacts with the free electrons found in the plasma. The plasma is formed by vaporization of the surface of the metal, and this interaction defocuses part of the incident beam producing a characteristic "nail-head" type of weld.

The electrons of an electron beam, on the other hand, are too energetic to be deflected significantly by the plasma. As a result, it is possible to couple the energy much more efficiently using electron beam welding compared to laser welding.

Aluminum is one of the easiest metals to penetrate with an electron beam. On the other hand, it is one of the most difficult to melt with a laser. The poor coupling of the laser energy is due in part to the high density of free electrons in the solid, making aluminum one of the best reflectors of light. In addition, many aluminum alloys contain magnesium or zinc, which are easily vaporized and thereby form a plasma that blocks the incident beam.

Previous investigations have shown low power absorption, alloy compositional differences and the importance of surface preparation when laser welding aluminum. The power absorption changes dramatically at times, producing an unstable process with poor penetration control and a rough bead surface. It is commonly believed that the difference in the fraction of absorbed power is caused by melting of the metal.

It is well known that alloy composition and surface preparation of aluminum influence the absorption of laser light. The emphasis of most of the work done so far in the area of laser conduction welding (LCW) has been the investigation of the process as a means of understanding the complex nature of laser welding as a whole. Low powers at the focus have been mainly utilized and most of the work has been in the area of spot welding. Investigations into the effects of high-power defocused laser beams have so far been restricted to situations where the distances from the focus have been in the range –10mm to +10mm for both conduction and keyhole welding. In this work the distances of the work piece below the focus are in the range of +5mm to +100mm which truly takes the welding into the conduction welding regime.

Laser welding aluminum alloys offers many advantages: precise heat input, narrow weld bead, narrow heat-affected zone, minimal thermal distortion, as well as elevated welding speeds on thin sections and deep penetration on thick sections. It is because of these advantages that the application of laser welding of aluminum alloys is increasing, such as in the automobile sector for body and exterior paneling.

Although aluminum alloys are wieldable by other more traditional fusion welding methods, the laser offers unique qualities that make it an ideal technology for joining aluminum alloys. Most strain-hardenable alloys (for example, AA5182 and AA5754) can be laser welded autogenously, although filler metal can be introduced during laser welding to add reinforcement or to improve the strength and ductility of the joint, if desired. Many heat-treatable alloys (for example, AA6016) are susceptible to hot cracking during welding, due both to their chemical compositions and the thermal strains induced in the metal during welding. To avoid hot cracking, a filler metal is used to adjust the weld bead composition beyond the crack-sensitive range.

Industrial experience has shown that hot cracking can be avoided by the addition of an eutectic alloy, such as AI-Si, to the weld. However, this leads to a shorter solidification interval. The filler wire technique has two major disadvantages. Firstly, the macroscopic properties of the joint will change in an uncontrolled way. Secondly, the small size of the laser beam leads to occasional feeding of filler wire directly into the beam, causing inconsistent penetration and weld-pool instability. For example, adding a 4043 AI-Si alloy wire to the 6061 alloy weld reduces hot cracking susceptibility, but both the tensile and ultimate strengths are reduced by 50%. Therefore an optimization of the process parameters in order to reduce cracking sensitivity would be highly desirable for the production of light weight high-strength components, especially for transport equipment industry.

Hot cracking susceptibility during welding is usually evaluated when the strain or stress is changed during the process. Some examples are the Sigmajig, Varestrain and Houldcroft tests. These tests are performed in a specific mechanical device where loading or bending is applied. The major limitations of these methods include poor reproducibility due to complex mounting geometry and difficult interpretation of the results after testing.

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

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

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

Butt Joints -sheared edges are acceptable provided they arc square and straight. A fit-up tolerance of 15% of the material thickness is desirable. Misalignment and out-of flatness of parts should be less than 25% of the metal thickness.

Lap joint -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 Joint- this geometry is especially suited for aluminum because of aluminum's high shrinkage rate. Square edge and good fit-up are also necessary.

Advantage and Limitations

Laser welding, like most welding processes, has its advantages as well as its disadvantages. Some of the advantages are:

• The laser beam, unlike other fusion processes, can be projected in air with no loss in power, and no vacuum chamber is needed.
• The case 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, and therefore good retention of metal properties and relatively low distortion of the work piece.

Some disadvantages are:

• Most of the multi-kilowatt laser systems operate at only about 10 to 20% efficiency for converting electrical power into a focused infrared laser beam.
• 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 would require high volume production or critical applications to justify the expenditure.