Laser Welding Aluminum

Laser beam welding is one of our most popular services for welding aluminum. The process is ideal for fast, clean welds. The heat affected zone is minimized and weld penetration can range up to 0.25” in aluminum. Laser beam welding can be used with crack sensitive materials, such as the 6000 series of aluminum alloys when combined with an appropriate filler material such as 4032 or 4047 aluminum. There are several different types of lasers that work well with aluminum, and often the use of a cover gas is prudent.

Pre-Weld Preparation

The amount of pre-weld preparation is largely dependent on the condition of the aluminum parts to be welded, and that is generally dependent on the storage conditions and the cleanliness of the machine procedures used to make the part thus far.

To avoid oxide films and hydrocarbon contamination, aluminum to be laser welded must be thoroughly cleaned. This is often achieved mechanically, using stainless steel wire brushes, grinding, filing or scraping to remove any oxides. Alternatively, there are chemical cleaning methods utilizing immersions in caustic solutions and water that are effective at removing aluminum oxide.

Hydrocarbon residue on aluminum 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. Hydrocarbon contamination must be removed before abrading the surface to remove aluminum oxide.

Joint Preparation

A very important aspect to welding aluminum is how the joint is fabricated. Special care in machining and assembly must be taken because aluminum is softer than most metals. Contaminants can easily be transferred to a part and then pushed under the surface of the joint.

• Avoid machining methods that leave a ground or smeared surface. We do not weld joints that are not properly machined and clean post cutting.
• Avoid grinding process if possible. If grinding cannot be avoided, use a coarse disk.
• When cleaning a surface with solvents, use clean cloth such as cheese cloth or paper towels.  Do not use shop rags that may be contaminated with oil residue. Precision parts should be handled wearing powder free, latex gloves, and cleaned using link free cotton swabs and delicate task wipes with the appropriate solvent
• 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.  Wire brushing prior may embed hydrocarbons and other contaminants in the aluminum.
• Always use new, or recently cleaned, stainless steel brushes to clean a joint.  Older brushes sitting around a work bench may contain oils and other contaminants.  Do not use brushes that have been used on other metals as metal flakes can be carried on the brush bristles, then embedded under the surface of the aluminum during brushing.
• Generally, surfaces that have been chemically etched, passivated or precision cleaned should not be wire brushed.
• Clean all wire brushes and cutting tools frequently.

Fixturing

Laser welding requires a fairly precise joint in order to maintain permissible gap and mismatch. Good weld fixturing is necessary so that the laser beam can be placed accurately. Laser welding and cutting are thus inherently machine guided processes.

Joint Types

• Butt Joint:
  • A fit-up tolerance of 15% 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 25% 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 aluminum, 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 aluminum’s high shrinkage rate.
  • Square edges and good fit-up are also necessary.

Recommended Lasers for Aluminum Welding

There are four main categories of lasers that are suitable for welding aluminum:

1. CO2
2. Nd:YAG (Neodymium: Yttrium-Aluminum-Garnet)
3. Fiber (generally Ytterbium doped)
4. Disk (Yb:YAG ytterbium)

All of these technologies are capable of producing high quality aluminum welds, and the method to be used is often dependent on operational costs rather than weld quality. However, each process has slightly different characteristics which can make some types of lasers preferable for certain applications, joint configurations, and aluminum alloy combinations.

Laser Welding Modes of Operation

Laser beam energy can be applied to the work piece either as a series of pulses, as a continuous beam, or in a laser stir weld configuration. The decision to use a particular method is dependent on the application, the properties of the materials, etc.

Pulsed Laser Welding

A pulsed laser is exactly that: the beam is switched on and off at a very high rate (10-1000 hz) such that the energy applied to the work piece is a series of separate bursts. Each pulse creates an area of melted material, the work piece is then moved slightly and another pulse is applied, resulting in a series of overlapping welds creating a continuous bead. Each weld area created by a pulse cools quickly, which minimizes the amount of heat in the surrounding material, which in turn limits how hot the part might become, which in turn minimizes melting and distortion of the part. Because of aluminum’s high thermal conductivity, a pulsed laser is generally the best way to laser weld aluminum when low thermal input is required.

Continuous Wave Laser Welding

Continuous wave laser welding is used for deep penetration welds, and is often referred to as keyhole welding. A steady beam of laser light is applied to the work piece, which is then moved beneath the beam. Material on the leading edge of the laser beam melts as the trailing edge cools. Continuous wave lasers typically feed at speeds of 25 to 100 inches per minute in order to not overheat the parts. Because heat is applied at a constant rate, and the part is not subject to the constant heating and cooling of a pulsed laser, continuous wave welding may be better suited for some of the more crack sensitive alloys of aluminum.

Laser Stir Welding

Laser welding aluminum without cracking is a constant challenge. The standard technique when welding crack prone alloys is to use a filler wire or shim made from a more weldable alloy (such as 4047) in order to achieve quality weld joint. For welding heat sensitive components, such as electronics housings, using filler materials and welding with a pulsed laser is indicated. However, for welds with deeper penetration in crack prone aluminum alloys, we’ve had a lot of success using our proprietary Laser Stir Welding technique.

Laser stir welding is a process in which a continuous beam laser is oscillated at a relatively high frequency, which causes a stirring action within the molten weld pool – hence the term “stir welding.” The result is a manipulation of the weld pool/vapor cavity, which changes some key characteristics of the weld.

Benefits:

• Laser Stir Welding results in largely defect free joints, with no hot cracking, porosity or solidification cracks.
• More precise control of the weld pool for increased keyhole stability.
• Improved control of the profile and geometry of the weld – as an example, joints can be designed with more width at the root of the weld, which can be very useful for Lap/Thru-/Blind welds.
• Weld profiles can be manipulated into asymmetry, such as increasing the penetration on one side of the weld joint.
• Patterns can be programmed to compensate for large gaps in weld joints and other potentially problematic weld geometry problems.
• Higher feed rates can be achieved than with pulsed laser techniques.
• No filler materials are required.

As stated earlier, cover gases are often required when laser welding aluminum. Choice of cover gas is generally dependent on the type of laser and its power rating as the use of the wrong cover gas can result in access plasma generation and/or changes to the properties of the welded materials. Generally, cover gasses are chosen on a per project basis, but a few general guidelines are:

• Argon: commonly used with Nd:YAG lasers, in order to minimize plasma generation, Argon should not be used with C02 lasers exceeding 3kW of power in order to minimize plasma generation.
• Helium: tends to suppress plasma generation, and since it is very light weight it can require a high flow rate, which can cause weld pool turbulence, which is undesirable.
• Argon-Helium Mixtures: generally recommended for most aluminum laser welding applications depending on laser power level.
• Argon-Oxygen Mixtures: can provide high efficiency and acceptable welding quality.
• Argon-Hydrogen Mixtures: can provide high efficiency and acceptable seam shape in welding of austenitic stainless steels. It should be considered that hydrogen may result in brittle behavior of ferrite steels! Gases and gas mixtures are supplied in cylinders.
• Nitrogen – C02 Mixtures: can produce acceptable welds although often the seam will be slightly oxidized.