Aluminum Welding Guide

Aluminum base metals that are referred to as “non-weldable” are base metals that have an elevated solidification cracking tendency and in some cases have an increased susceptibility to stress corrosion cracking in the as-welded condition. These base metals are unsuitable for arc welding applications.

Some of the base metals that have these tendencies and susceptibilities are as follows:

6262 and 2011 (Used for screw machine stock applications.)

These two base metals have lead, bismuth, and/or tin added in small quantities to facilitate machinability. These small additions of low melting point metals seriously increase their solidification cracking tendency. Therefore, these base metals are typically mechanically fastened rather than welded.

2xxx series alloys containing, aluminum-copper-magnesium

(Used for aerospace and other high performance applications.) Examples: 2017 and 2024

These types of base metals can be susceptible to stress corrosion cracking and premature failure if arc welded. Note: There are other 2xxx series base metals such as 2219 which are aluminum-copper alloys with no magnesium added that are considered weldable.

7xxx series alloys, aluminum-zinc-copper-magnesium

(Used for aerospace and other high performance applications.) Examples: 7075, 7178, 7050, and 7150

These types of base metals can be susceptible to stress corrosion cracking and premature failure if arc welded. Note: There are other 7xxx series base metals such as 7005 which are aluminum-zinc-magnesium metals with no copper added that are considered weldable.

Explanation:

In the heat affected zone of the non-weldable 2xxx and 7xxx series alloys, low melting point elements are preferentially precipitated into the grain boundaries which lowers and widens the solidification temperature range of the grain boundary. Consequently, when arc welding these types of base metals, the grain boundaries become the last to solidify and can easily crack due to solidification shrinkage stresses. In addition, the difference in galvanic potential between the grain boundaries and the remainder of the grain structure in these alloys is increased, making them more susceptible to stress corrosion cracking. These base metals are typically mechanically fastened rather than arc welded.

Note: Some of these base metals are presently being welded with the friction stir welding (FSW) process, which operates at lower temperatures than arc welding and does not melt the base metal during welding thereby eliminating solidification problems.