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.
Fusion welding processes, such as MIG, TIG, Laser, and Electron Beam, generate intense heat in a small area to melt the material. This small heat affected zone is essential as aluminum’s high thermal conductivity tends to result in heat traveling throughout the work piece, resulting in melting too much material or deforming the entire part. To avoid this, 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. Complicating matters, aluminum doesn’t change in appearance as it approaches its melting point, making welding processes which require visual judgment of material readiness unreliable. Automated methods, utilizing Laser and Electron Beam, which use computers to control feed rate, power, and weld location, offer far more precise and consistent weld quality.
Another challenge of welding aluminum involves the formation of oxide films 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, leading to porosity issues. In most cases, oxide films must be removed either by mechanical or chemical means prior to welding. Oxide films can also change the reflectivity of the part’s surface, which negatively impacts the amount of laser energy making it to the actual weldment.
Hydrocarbon contamination of aluminum during storage and preparation 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 — cutting and machining of aluminum should be performed dry, if possible, and if not, the parts must be thoroughly cleaned.
Laser Hermetic Sealing Aluminum
Laser hermetic sealing is one of the best ways to protect electronic components in harsh environments, and many packages involved in critical applications make use of aluminum. Aluminum is light, easy to form, and provides excellent electrical, physical and chemical protection. When combined with laser welded seals, the result is cost effective, lightweight and robust.
Laser hermetic seals are more durable than epoxy, soldered, and mechanical seals. Laser hermetic sealing can be used with crack sensitive materials, such as the 6000 series of aluminum alloys in combination with 4000 series aluminum alloys, commonly used for containers. The LHS is ideal for fast, clean welds with minimal distortion of the part: the heat affected zone is minimized and weld penetration can range up to 0.25” in aluminum, providing an exceptionally durable hermetic seal. Using a CNC laser allows for precise welding of complex container geometries. Coupled with a EOD and FOD safe environment, such as those found in our glovebox welding stations, the result is unbeatable for parts that can’t fail in a critical application.
Pre-weld preparation is largely dependent on the condition of the aluminum parts to be welded, and that is typically dependent on the storage conditions and the cleanliness of the machining procedures used.
- To avoid oxide films and hydrocarbon contamination, aluminum parts must be thoroughly cleaned. This is done 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 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.
- For parts containing electrically sensitive components, often pre-weld preparation must occur in our electrostatic discharge (ESD) safe room, following strict protocols to minimize stray electric charges from damaging components.
Aluminum parts destined for our glovebox environments typically go through additional processing, such as vacuum baking. This process occurs in a computer controlled vacuum oven that will result in the parts containing less than 1 ppm of oxygen and moisture. Vacuum baking can be done at various temperatures and durations to meet a wide range of requirements.
A very important aspect of the laser hermetic sealing process 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.
- When cleaning an aluminum surface using 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 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 for cleaning parts to be welded. 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.
- Clean all wire brushes and cutting tools frequently.
- Any metal surface that has been etched or chemically cleaned we do not wire-brush to preserve the precision cleaned surface and prevent scratches.
Laser hermetic sealing requires a precise joint to maintain permissible gap and mismatch. This reduces the risk of weld nonconformities and damage to internal components. In conjunction with a CNC guided laser, adequate fixturing is necessary to eliminate the risk of a displaced part and ensure the beam is placed precisely and accurately.
- 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 non-flat 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 Hermetically Sealing Aluminum
The main category of laser suitable for hermetic sealing is fiber optic. This technology is capable of producing high quality aluminum welds and is used in our glove box laser welders. In addition to fiber optic lasers, our gloveboxes feature computer-controlled movement and parameters, oxygen and moisture monitoring, and vacuum ovens. Our vacuum ovens have inert gas drying with purification and recycling capabilities, while being able to record data and produce gas analyses data. Supporting equipment includes laminar flow benches and exhaust hoods.
Laser beam energy can be applied to the work piece either as a series of pulses 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 Hermetic Seal
A pulsed laser utilizes a beam that is switched on and off at a very high rate (10-1000 hz), such that energy is applied to the work piece in 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 which create a continuous bead. Each weld area created by a pulse cools quickly and minimizes the amount of heat in the surrounding material.
Limiting the transfer of heat within the part reduces the risk of defunct electrical components and the distortion of the part’s configuration. Since aluminum has high thermal conductivity, a pulsed laser is generally the best way to laser seal aluminum.
Laser Stir Welding
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, 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 which changes some key characteristics of the weld.
- 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 over the 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.
- Filler material (wire or shim) is typically added manually prior to machine welding.
Cover Gas Requirements for Aluminum Welding
Cover gases are often required when hermetically sealing 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 excess 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 and can minimize plasma generation, Argon should not be used with C02 lasers exceeding 3kW of power to further reduce plasma generation.
- “90-10” Nitrogen–Helium: commonly used in housing assemblies, the helium tends to suppress plasma generation, but since it is very light weight, it can require a high flow rate to maintain coverage, which can cause weld pool turbulence, which is undesirable.
- Argon–Helium Mixtures: recommended for most general laser welded aluminum applications depending on laser power level.
- Nitrogen–C02 Mixtures: can produce acceptable welds although often the seam will be slightly oxidized.
Generalized Process for Hermetic Sealing an Aluminum Housing
A common housing-to-cover assembly utilizes 6061 (housing) and 4047 (cover) to hermetically seal a wide variety of components for many different applications.
At the start of the process, our qualified and highly trained operators ensure the proper calibration of all equipment utilized. ESD precautions are put in place per industry standard specifications and customer requirements before any handling of the parts occurs.
Once components are ready to be processed, a common step, but not required, is to perform a face down leak test, which verifies the integrity of ports found on the unit, such as feed-thrus or connectors, prior to welding. The units are then cleaned using acetone following proper procedure before the covers are manually laser tacked in place.
Depending on the requirements, the parts are then vacuum baked at high temperatures to mitigate oxygen and moisture. While this step can be lengthy, it greatly increases the chance of successfully sealed units. Once the bake is done and any pertinent data is recorded, the parts move into the welding area of the glovebox environment.
Using rigid welding fixtures, the parts are squared off to ensure a true and repeatable weld of all covers to the housings, hermetically sealing the parts. The units are further cleaned if required and visually inspected per industry standard specifications.
Depending on the requirement, parts may go through a series of inspections and tests post weld.
- Fine Leak Testing: Our glovebox welding environments contain a standard 10% Helium that EBI can adjust to a specific requirement, some of which is sealed into the part during welding. Fine leak testing can then detect leaks of that helium by utilizing a helium mass spectrometer in a special testing environment.
- Pressure Bombing: This step exposes the completed parts to helium for an extended period of time in a specialized test environment, which can locate leaks again using a mass spectrometer to detect larger leaks.
- Gross Leak Testing: Leaks which are too large to be detected using pressure bombing or fine leak testing can be subjected to gross leak testing. The parts in question are exposed to Fluorinert FC-72 under pressure. The parts are then submerged into a bath of FC-40, which is a similar liquid with a higher boiling point. The FC-40 is then heated, which causes the FC-72 to bubble out from any units with leaks, which is clearly visible to our operators.
All testing is done in accordance with MIL-STD 883 Method 1014, MIL-STD 202 Method 112, and MIL-STD 750 Method 1071.6.
Finally, the parts are cleaned and packaged carefully with ESD precautions still in place to ensure parts are returned safely to our customers or sent to the next link in the supply chain.