Aluminum Welding Challenges
Aluminum and Heat: a Problem for Welding
Aluminum has high thermal conductivity (approx. 209 W/m K) and a 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.