Aluminum alloys are welded mainly arc welding in inert gases, non-consumable or consumable electrode, usually - argon arc welding. Most aluminum alloys are easy to weld. However, for some aluminum alloys, arc welding is never used.. Why? Let us briefly consider various series of wrought aluminum alloys in terms of their weldability..
Weldable aluminum alloys
Series 1ХХХ. Technically pure aluminum (not less 99 %). Applicable, primarily, as a conductor of electric current or for products with high corrosion resistance. All these alloys (aluminum grades) easy to weld. The alloy is most often used as a welding alloy 1100 (aluminum grade AD according to GOST 4784 on deformable aluminum alloys).
Series 3ХХХ. This series includes medium strength aluminum alloys, which are easy to form. Often used for heat exchangers and air conditioners. All these alloys easy to weld welding aluminum alloys 4043 or 5356 (analogues according to GOST 4784 - welding alloys SvAK5 and SvAMg5).
Series 4ХХХ. These aluminum alloys are commonly used as welding or brazing alloys.. However, sometimes they can also be used as weldable materials.. In this case, they are welded with an alloy 4043 (SvAK5).
Series 5ХХХ. This is a series of aluminum alloys mainly for high strength sheets and plates. All of them easy to weld using welding alloy 5356 (SvAMg5). For the toughest alloys, such as 5083 (AMg4.5), use alloys 5183 or 5556.
Series 6ХХХ. it - aluminum alloys, mainly, for extruded profiles, although they are also used for sheets and plates. They are prone to hot cracking when welding. However, with the proper technology, they all weld quite well with welding alloys 4043 and 5356.
Non-weldable aluminum alloys
And where are the famous high-strength aluminum alloys of the 2XXX and 7XXX series? Why not in the forefront of welding? Here's why!
Series 2XXX. These high-strength aerospace aluminum alloys ("duralumin") are used mainly in the form of sheets and plates.. Their chemical composition makes most of them non-arc weldable due to their high tendency to hot cracking.. The exception is alloys 2219 and 2519, which weld well using welding alloys 2319 or 4043. The weldability of these alloys gives an almost complete absence of magnesium in them.. The analogue of these two alloys is the domestic aluminum alloy D20, from which high-strength weldable plates are made. Popular for its high strength alloy 2024 (D16 according to GOST 4784) never welded arc welding, as it is extremely susceptible to hot cracking during welding.
Series 7ХХХ. This is also a series of high strength aerospace aluminum alloys. Like alloys of the 2XXX series, most of them not welded arc welding methods due to hot cracking and tendency to stress corrosion. Exceptions are alloys with a minimum copper content – less 0,1 %. These are alloys 7003 and 7005 (our 1915) for extruded profiles and alloy 7039 for sheets. All three alloys weld well using a welding alloy 5356.
Why don't they weld duralumin?
The main reason for hot cracking when welding "non-weldable" aluminum alloys of the 2XXX and 7XXX series is as follows. In the course of welding, in the seam zone - the heat-affected zone - along the grain boundaries, alloy components - eutectics and intermetallics - are separated – with a melting point below, than the base alloy. This reduces and extends the temperature range of solidification of grain boundaries. therefore, in arc welding of these types of alloys, the grain boundaries are solidified last and, therefore, easily cracked under the influence of shrinkage stresses. Little of, this leads to an increase in the galvanic potential difference between the grain boundaries and the rest of the grain structure, which makes grain boundaries more susceptible to stress corrosion.
When rivets are better than welding
For all these reasons, aluminum alloys of the 2XXX and 7XXX series more often connected mechanically, than welded. Sometimes contact welding is used, as well as friction welding. Friction welding "works" at significantly lower temperatures, than arc, does not melt the base metal and therefore eliminates problems, hardening.
Aluminum and Aluminum Alloys, J.R. Davis, Ed., 1996.
G. Mathers, The Welding Aluminium and its Alloys, Woodenhead Publishing Ltd, 2002.