The Challenges of Welding Aluminum
Aluminum has certain properties which make it more challenging to weld than other metals. Its relatively high thermal conductivity (approx. 209 W/m K) and low melting point (1,221°F/660.3°C) make it such that only fusion welding processes can be used to weld it.
Fusion welding processes, such as MIG, TIG, Laser, and Electron Beam, generate intense heat in small area to melt the material in the desired weld area. This small heat affected zone is essential as aluminum’s high thermal conductivity tends to result in heat traveling throughout the work piece, either melting too much material or deforming the entire part. The amount of heat applied and the location to which it is applied must be controlled very precisely. Manual welding processes, such as MIG and TIG, rely on operator skill and heat sinking to control these factors. Because aluminum doesn’t change in appearance as it approaches its melting point, welding processes which require visual judgment of material readiness can be unreliable. Automated methods, such as Laser, EB, and robotic welding, which use computers to control feed rate, power, and weld location, offer more precise weld quality.
Another challenge of welding aluminum involves the formation of oxide film on the work surface. The melting point of aluminum oxide is approximately 3x the melting point of pure aluminum, which can result in particles of aluminum oxide contaminating the weld and leading to porosity issues. In most cases, oxide film must be removed either by mechanical or chemical means prior to welding. Oxide films on aluminum can insulate and prevent grounding of the arc weld circuit when MIG and TIG welding in addition to the potential for impure welds. Aluminum oxide can also affect laser welding: oxide films can change the reflectivity of the parts surface, which negatively impacts the amount of laser energy making it to the base metal.
Hydrocarbon contamination of aluminum during storage and preparation of the material can cause problems when welding. Aluminum parts are frequently formed, sheared, sawed and machined prior to the welding operation. If a lubricant is used during any of these pre-weld operations, complete removal of the lubricant prior to welding is essential to avoid bad welds. Prudence dictates that aluminum parts which are to be welded should be pre-weld processed in a such a manner that minimal to no lubricants are used — sawing and machining of aluminum can often be performed dry.
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.
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.
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. Contaminates can easily be transferred to a part and then pushed under the surface of the joint.
- Machining methods that leave a ground or smeared surface should be avoided. For example, a band saw will leave metal smeared along the path of the blade. This problem can be overcome by filing or machining the joint edge post cutting.
- Avoid grinding process if possible. If grinding cannot be avoided, use a course 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.
- 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 imbed hydrocarbons and other contaminates in the aluminum.
- Always use a new or recently cleaned stainless steel brushes to clean a joint. Older brushes sitting around a work bench may contain oils and other contaminates. Do not use brushes that have been used on other metals as metal flakes can be carried on the brush bristles, then imbedded under the surface of the aluminum during brushing.
- Stainless steel wire brush any metal surface that has been etched. By-product residuals from etching can alter the chemical composition of the weld pool.
- Clean all wire brushes and cutting tools frequently.
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.
- 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.
Lasers for Aluminum Welding
Recommended Lasers for Aluminum Welding
There are four main categories of lasers that are suitable for welding aluminum:
- Nd:YAG (Neodymium: Yttrium-Aluminum-Garnet)
- Fiber (generally Ytterbium doped)
- 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.
Pulsed vs Continuous Wave Laser Welding
Laser beam energy can be applied to the work piece either as a series of pulses or a continuous beam, depending on the application, the materials, etc.
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 is 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 create 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 is analogous to 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.
Cover Gas Requirements for Aluminum Welding
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.
Other Aluminum Welding Processes
MIG/TIG vs. Automated Laser Welding
One of the big deciding points with choosing a welding process for aluminum involves determining whether an automated process is better than a manual process. The decision often comes down to trade-offs in cost versus quality. Automated processes generally are best suited for large quantities of parts requiring simple, fast, and repeatable welds. However, automation require parts fixturing as well as computer control to move the part and control the welding parameters. Manual welding is usually best suited for complex welding or welding of parts that are bulky and not easily moved. As way of a simple example, TIG welding generally operates at a feed rate of 10” per minute. Laser welding operates at 100” per minute — laser welding is much faster than TIG. A laser welder moves so quickly and has so much power that it cannot be controlled by a human hand: precision CNC control is required or some type of robotics, and this can be expensive.
Stir welding is a relatively new process. The basic idea is that the materials to be welded are fixtures together against a backplate. A rotating tool with a specific profile is then moved along the centerline of the weld, plasticizing the metals due to high pressure and friction. The result is an excellent weld, but the process is expensive and requires a lot of tooling and support technology. Laser stir welding adds a laser which heats the weld area before the stir tool comes in contact with it.