In metal alloys known some cracking mechanisms. Fortunately, aluminum alloys, many of them, for example, hydrogen rastreskivanie, which is also called "cold cracking", do not apply. Almost all the cracks in the welds at arc welding of aluminum alloys It arises due to hot cracking. Namely: cracks appear in the welded seam when it solidifies as a result of the effect of shrinkage stresses on its microstructure.
Effect of chemical composition of aluminum
to understand, why the aluminum weld seam cracks and how the choice of the welding alloy (welding rod) affects the tendency to cracking, It is very useful to look at the picture.This figure shows the effect of four different dopants – Si, With и Mg, and Mg and Si as Mg2Si - on aluminum alloy tendency to hot cracking during solidification. It is important to note, that all alloying elements influence the propensity for crack formation in a similar manner. With increasing concentration of the additive increases the tendency to cracking, It reaches a maximum and then decreases to a relatively low level of.
Most of the aluminum alloys, which are considered not to be welded without welding rod, They have a chemical composition of at least one dopant concentration near the peak with the tendency to cracking. alloys, that show a minor tendency to formation of cracks, is, They have a chemical composition, which is far from these peaks. Really, alloys tendency to cracking depends mainly only on their chemical composition.
Selection of welding material
Similarly the tendency to cracking during welding depends mainly on the chemical composition of the weld "bath". therefore, selection principle welding material is as follows.
1) For materials, which exhibit a low tendency to formation of cracks is necessary to apply a welding material having the same chemical composition.
2) For materials, which exhibit a high propensity to crack formation must be applied weld material, which greatly differs from them by chemical composition, so as, to try to get the chemical composition of the weld cracking away from the peak.
Compliance with this principle can be followed in the consideration of welding materials, which are used in welding 5xxx and 6xxx series alloys.
Welding of aluminum-magnesium alloys
Most 5xxx series alloys exhibit a low propensity to cracking of. They can be welded without any welding rod. Peak propensity to cracking is approximately 1,5 % Mg. All 5xxx series alloys, except alloy 5052 (AMg2,5 according to GOST 4784), It contains much more magnesium, than 1,5 % and are therefore away from cracking peak. However, the alloy 5052 lies directly on the peak of magnesium and cracking, really, It shows a relatively high propensity to cracking.
When choosing the material of the welding rod for alloys of the 5XXX series, the general rule (except for alloy 5052) is used as a welding alloy of the 5XXX series with a slightly higher Mg content, than in the base (weldable) alloy. alloy 5052 apply a welding alloy with much magnesium, such as 5356 from 5 % Mg. This gives a large "shear" magnesium content in the solidifying weld cracking away from the peak.
Welding alloys magnesium-aluminum-silicon
Most 6xxx series alloys - alloys Al-Mg-Si - are very sensitive to cracking. Really, their chemical composition as the time gets close to the peak, shown in the lower graph of figure. None of these alloys can not be welded without welding rod or a welding rod of the same chemical composition. If you try to do it, they will crack every time, if the welding seam does not exert significant compressive stresses, as it is done in the resistance spot welding.
Therefore, the choice of material for these welding prone to cracking 6xxx alloys is the use of welding alloys with a chemical composition, to obtain a weld with the chemical composition peak away from cracking. For this, use or welding alloys with a high content of Mg, for example, 5356, or with a high content of Si, usually – alloy 4043. For this reason, there is not a welding rod of alloy 6061. And if he was, and we would use it for welding alloy 6061, the weld will inevitably be cracked.
With welding rod only!
Important to remember, that most of the 6xxx series alloys can not be welded without welding rod. Moreover, even when welding with welding rod, if it fed in insufficient quantities, also may cause cracks.