2000 series aluminum alloys typically contain between 2-10% copper (see below tables for 2219 & 2204). Copper addition facilitates precipitation hardening of aluminum to reach strength levels comparable to steel. These are heat-treated Al-Cu alloys with tensile & yield strength of up to 70 & 50 Ksi respectively, which is absolutely amazing. But there is one big problem with these alloys; like most precipitation-hardened alloys, they are prone to cracking, especially with arc welding.
Hot cracking sensitivity (during welding) and stress corrosion cracking tendency (after welding) are the most common problems of welding this group of aluminum alloys.
Hot cracking sensitivity of the Al-Cu alloys peaks at around 3% Cu and then, it decreases to a relatively low level at 5% Cu content.
Those 2XXX alloys with higher Cu-content are considered to be resistant to hot cracking. Alloy 2219 is the most common in this group with 6.3% Cu and 0.2% Zirconium but with no Magnesium addition.
As you might have noticed, I underlined “no Mg addition” for describing 2219 alloy. This is important because Magnesium (Mg) causes weldability problems for Al-Cu alloys, even if they are in the safe zone of the crack sensitivity diagram. Alloy 2024 is a good example of this; it contains 4.5% Cu and 1.5% of Mg. To put it briefly, magnesium depresses the solidus temperature and widens the “mushy range” or “coherence range” during solidification which causes more segregation at the grain boundaries and increases the susceptibility to both types of cracking (Hot cracking & Stress Corrosion Cracking).
Using filler metals with very low Mg content is extremely important for welding of 2000 series aluminum alloys. In addition to that, zirconium & titanium additions to the filler metal reduce the crack sensitivity of the weld metal (Refer below for chemical composition of ER2319 filler wire).
And, never forget the golden rule of aluminum welding: Always try to keep the welding heat input as low as possible.