Aluminum Welding Challenges
Aluminum and Heat: a Problem for Welding
Aluminum has certain high thermal conductivity (approx. 209 W/m K) and low melting point (1,221°F/660.3°C), and because of this it’s very easy to crack or otherwise deform an aluminum part. In fact, the metallurgical characteristics of aluminum make it exceptionally difficult to weld with any method other than a fusion welding process.
Fusion welding, such as MIG, TIG, Laser, and Electron Beam, create welds by essentially melting the materials into each other by applying heat precisely to the selected weld area. Heat from the weld area spreads into the material surrounding it during the welding process, created what is termed a heat affected zone or “HAZ.” Too much heat flowing into the HAZ can create a host of problems such as part distortion, melting, cracking, and porosity. Precise control of heat is critical to accomplishing an acceptable weld. For MIG and TIG, the heat is controlled by only using highly trained, skilled personnel. The process can be automated but weld speed can be an issue.
Laser and EB welding have an advantage in that all parameters are typically controlled by CNC, allowing careful placement of precise amounts of power, resulting in much faster speeds. Welding speed for TIG is typically around 10 inches per minute (“IPM”). Laser and EB can easily move in excess of 100 IPM. The high thermal conductivity of aluminum can also significantly impede the depth of penetration of a weld. Both MIG and TIG rely on thermal conductivity down into the part from the point that the welding torch touches the part. Significant heat can be applied but it disperses in the part quickly. Laser can weld a bit deeper as the laser energy applied utilizes both radiant heat transfer as well as conduction. However, EB welding can create the deepest welds by far. EB relies on the kinetic energy of electrons to heat the part: the electrons penetrate the molecular lattice of the aluminum to a much deeper level before transferring their heat.
Aluminum Oxidation and porosity
Most metals oxidize, and aluminum is no different: it forms a thin layer of aluminum oxide when left in contact with oxygen. However, what makes aluminum oxide especially problematic is that its melting point is about 3x above the melting point of the pure aluminum surface it is on. Melting the metal results in contamination from the unmelted oxides, which build up to form bubbles of contaminant in the weld pool. This situation, called porosity, is a major issue with welding aluminum and can result in welds that might look fine from the outside, but internally are actually weak and breakage prone. Oxide films can also act as an insulator, which can cause improper grounding of the weld circuit in an a TIG or MIG welding situation.
Aluminum oxides can also change the light reflectivity of the weld surface, which can cause problems for laser welding. From a heat transfer perspective, EB welding is not as sensitive to oxides as other processes. Regardless of which welding technology is used, it is best practice to make sure oxides are removed prior to welding by a chemical means. Mechanically removing oxides should be avoided if possible. Wire brushing , grinding, or filing a part can drive contaminates beneath the surface of the part, and then when heat is applied these impurities can find their way into the weld pool.
Hydrocarbon contamination can be a significant problem when welding aluminum. First, the impurities can change the material properties of the weld pool. Second, when heated, hydrocarbons react by burning or exploding, and thus can cause melted aluminum to erupt out of the weld area. Like oxides, hydrocarbons can be forced beneath the aluminum’s surface by mechanical removal, so a chemical means should be used. Avoid using dirty shop rags or oily tools which can contaminate the part. If parts are leak checked, special care should be taken when using vacuum grease.
Certain types of aluminum forming and machining can also be very problematic for welding aluminum, particularly if oil is used the process. For example, spin forming can impregnate oil in the aluminum as the part is formed, and once in the material, no amount of cleaning will remove it. EB welding is just as susceptible to hydrocarbon contamination problems as other welding process. The fact that the parts are welded under vacuum does boil off some of the water based contaminates, however, most oil based contaminates will be unaffected. There is no substitute for thoroughly cleaning the welding area.
Electron Beam Welding Aluminum
The electron beam welding process is excellent for use with aluminum parts, resulting in clean, very pure welds. EB has a very precise and highly controllable heat affected zone, and weld penetration is better than any other welding process. The heat affected zone is very narrow with a weld penetration range up to 0.75” for a typical high voltage EB welding machine. Because of the unique way electron beam welding applies heat, it can weld certain crack sensitive aluminum alloys with ease, such as the 6000 series.
EB welds are also very pure due to the fact that the welding occurs under vacuum, and thus retain nearly 100% of the original strength of the material. Finally, electron beam welding must be almost fully automated, so parts can be processed repeatedly with great precision in terms of weld parameters.
Pre-weld preparation is dependent on the condition of the aluminum parts to be welded, how they were stored, how they were machined or pre-processed, etc. Aluminum to be electron beam welded is generally completely cleaned first using chemical cleaning methods to remove oxides and hydrocarbons, and then using using stainless steel wire brushes, grinding, filing or scraping. Chemical cleaning methods include immersion in caustic solutions and water to remove aluminum oxides and alcohol based solvents or acetone to remove hydrocarbon residues.
Because aluminum is a relatively soft metal, care must be taken when fabricating joints to be welding.
- Avoid machining methods that leave a ground or smeared surface. As an example, band sawing a part can leave metal fragments along the path of the blade. This problem can be overcome by filing or machining the joint edge post cutting.
- Avoid grinding aluminum if possible, and if it cannot be avoided, use a course disk.
- Clean surfaces using a fresh cloth such as cheese cloth or paper towels. Do not use shop rags that have been laying around and may be contaminated with oil residue or other hydrocarbons.
- Don’t use compressed shop air to blow off debris from the joint. Compressed shop air usually contains moisture and oil contaminates. Blow off parts using a bottled gas such as nitrogen or argon.
- Wire brush joint areas AFTER solvent cleaning, and use a stainless steel brush. Wire brushing before solvent cleaning can force contaminants into the weld area. Solvent clean first, wire brush after.
- Use new or recently cleaned stainless steel brushes only. Older brushes sitting around a work bench are often contaminated with hydrocarbons or other materials that can affect weld quality adversely.
- If you’re welding an etched surface, make sure you stainless steel wire brush it before further processing the part because by-products from etching can alter the chemical composition of the weld pool and weaken the joint drastically.
- Clean all wire brushes and cutting tools frequently and thoroughly.
An electron beam is generally very small and must be precisely positioned on the part to achieve the proper weld. X, Y, and Z dimensions are all critical as is the speed at which the part moves beneath the beam. An error in X or Y by a few thousandths of an inch can drastically change the characteristics of the weld. Additionally, the Z dimension is critical for EB welding. This is because the beam of electrons is focused at a particular point, much the way a magnifying glass focuses sunlight. Small errors in focus can have significant impact on weld penetration. For this reason, parts need to be fixtured using high precision tooling. Additionally, all movement must be precisely controlled by either precision electromechanical means or full CNC control. EB welding also occurs in a vacuum and therefore the movement of the part is further complicated. The good news is all the required precision makes the process very repeatable.
EB welding can weld all the typical joint types used in other forms of welding. However, because it is a fusion welding process, filler material is generally not used. This means care must be taken to make sure the parts fit precisely together: EB welding demands a precise joint in order to maintain permissible gap and mismatch. If a electron beam hits a gap, heat is not generated and the materials will not fuse.
- Butt Joint:
- Machined edges are preferred, but sheared edges are acceptable provided they are straight and square.
- Generally gaps need to be less than 0.003”
- Lap joint (burn-through or seam weld):
- Lap joints work great provided there is not a significant gap between the pieces being welded.
- Fillet Joint:
- EB welding is a fusion process, so filler material is not required. However, for fillet welding, extra material is needed in order to achieve a fillet of proper size. Therefore the mating parts should be designed with this in mind or else an under fill condition will occur.
- Square edges and good fit-up are also necessary.
Other Aluminum Welding Processes
Electron Beam Welding vs. Laser Beam Welding
Both electron beam and laser beam welding are excellent processes well-suited to welding aluminum. Electron beam welding produces the purest, strongest weld possible, along with the deepest weld penetrations possible, but because of the vacuum chamber requirements, the process is more complex to set-up and to run, and hence it can be less cost effective than laser beam welding. Laser welding, which doesn’t require a vacuum and instead can happen in open air with cover gases, is fundamentally an easier set-up and is typically capable of faster production rates.
Stir welding is a relatively new process. The basic idea is that the materials to be welded are fixtured together against a back plate. A rotating tool with a specific profile is then moved along the center line 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.