- Refractory nature of aluminium oxide
- Lack of color change as the metal approaches the welding temperature
- High heat conductivity
- High coeficient of thermal expansion
Aluminium Oxide.
Aluminium and oxygen have a strong affinity for each other, and aluminium oxide instantaneously begins to form on aluminium surfaces exposed to air. This oxide film is thin, transparent, tenacious, refractory, and has a melting temperature much higher than that of aluminium. An excessively thick oxide film or water staining can cause welding difficulties and affect weld quality and should be removed by wire brushing before welding. The normal oxide film is removed progressively or ruptured by the welding arc, and additional oxide must be prevented from forming on the molten metal surfaces under the arc if fusion is to occur.
A flux is not required in gas tungsten arc welding (GTAW) or in gas metal arc welding (GMAW). The electric arc in both process penetrates and breaks up the aluminium oxide film, which is not an electrical conductor. A regulated flow of inert gas (either argon or helium, or a combination of oxide without contaminating the metal and permits the welder to deposit filler metal with a high degree of control. Gas purging of the pipe or gas shielding of the weld root is not normally necessary for aluminium pipe welds.
No Color Change
there is no visible color change in aluminium as it is heated from room temperature to welding temperature. Therefore, the welder must pay careful attention to the area being heated to observe when melting begins and control the flow of metal when visible melting begins.
High Heat Conductivity
Since aluminium is an excellent heat conductor, a different welding technique is required from that for steel, which has lower heat conductivity. welding currents and welding speeds are generally higher used for aluminium. Stringer beads are generally used, although the final pass is sometimes welded with a weaving technique with GTAW.
High Coefficient of Expansion.
Aluminium expands about twice as much as steel for a given increase in temperature. This can be an advantage where the joint is loosely fitted in preparation for welding, since the joint will tend to close as the temperature rises. It can also be a disadvantage when joints are tightly fitted because expansion forces can increase the metal thickness at the joint. Measurements have shown that up to about 3mm (0.12in) shrinkage will occur when butt joints are welded in schedule 40 pipe in sizes through 324 DN (12NPS).
The gas metal arc welding process with its concentrated heat and its lower heat input generally causes less shrinkage than does the gas tingsten arc welding process.
Weldability of Aluminium Alloys.
Aluminium is alloyed with small amounts of other metals to develop specific characteristics such as a high strength. To the welder, aluminium and its weldable alloys appear much alike while being welded. During solidification, the contraction strains which occur may be sufficient to cause cracking unless proper procedures and filler metals are used.
The cracks may ocur either in the weld metal or in the heat-affected zone of the base metal . Commercially pure aluminium and the lower strength alloys (for example, 1100 and 3003) are not generally susceptible to cracking . However, the higher strength alloys such as 6061 or 6063 tend to be suceptible to hot cracking, especially if filler metal of the same composition as the base metal is used.
Welding these alloys autogenously (i.e., without fillr metal) will almost always resoult in cracking. For this reason, 6061 and 6063 alloys are welded with filler metals having enough silicon or magnesium, for example 4043 or 5356, to produce a crack resistan composition in the weld. It should be noted that the recommended filler metals given for a pipe in Table 2 of AWS D10.7 are in some cases different from those recommended for welding other structures.
Because pipe is often exposed to particularly corrosive environments, filler metal alloys have been selected to give best corrosion resistance. When filler metals diferenting in composition from the base metal are used to eliminate solidification cracking (as in the case of 6061 or 6063 alloy), the weld metal should consist of about 70 percent added filler metal diluted with not more than 30 percent base metal. This may require beveling the joint, increasing the root opening, or limiting the amount of base metal.