Are 4xxx Series Alloys Heat Treatable? Discover the Truth

Heat Treatment Ability of the 4xxx Series Alloys

Q -  I recently received a letter asking for an explanation about the heat treatment ability of the  4xxx series alloys. The letter stated that there was a discrepancy in the way the writer was taught and an article he had read. He had been instructed that the 4xxx series alloys are non-heat treatable alloys only. However, the article said that some 4xxx series aluminum alloys could be heat treatable alloys. The writer was looking for clarification on the subject.

A -  I would like to address this issue and hope this will clear up any preconceived beliefs about the heat treatable and non-heat treatable 4xxx series aluminum alloys.

An Insight Into 4xxx Series Alloys

The 4xxx series alloys are most often encountered when they are being used as filler material within the welding or brazing processes.  The most common of these alloys are 4043 and 4047.  The 4043, which is a 5.0% Si (Silicon) alloy, is extensively used as a welding filler alloy. The 4047, which is a 12.0% Si alloy, is used for welding and brazing. Both of these 4xxx series alloys are non-heat treatable.  To be precise, these alloys alone will not respond to precipitation hardening.
Consideration may be required when using these non-heat treatable filler alloys if we intend to weld a heat treatable base material such as the 6xxx series alloys or one of the heat-treatable cast aluminum alloys such as 356.0 or 357.0, and then perform post weld heat treatment, such as solution heat treatment and/or artificial ageing.

Post-Weld Heat Treatment

Heat Treatable 4xxx Series - 6061-T6

The heat treatable 4xxx series aluminum alloys are most often encountered when  involved with the post weld heat treatment of the welded, heat treatable, aluminum base alloys. Typically, the heat-treatable base alloys, such as 6061-T6, lose a substantial proportion of their mechanical strength after welding. For example, 6061-T6 typically has 45,000-PSI tensile strength prior to welding and around 27,000-PSI in the as-welded condition. Consequently, on occasion, it may be desirable to perform post-weld heat treatment to return the mechanical strength to the manufactured component. When post-weld heat-treating, the filler alloy’s ability to respond to the heat treatment must be evaluated.

Non-Heat Treatable 4xxx Series - 4043 and 4047

It is important to understand that the common 4xxx series filler alloys, such as 4043 and 4047 are not heat treatable and these filler alloys may or may not respond to heat treatment when deposited as weld metal. Their response to post weld heat treatment is largely dependent on the welding procedure used and whether the filler alloy can obtain a sufficient amount of alloying elements from the base material during welding.  Consistent results after heat treatment may be difficult to achieve from the non-heat treatable filler alloys because of the number of variables within the welding procedure that affect the amount of base dilution during welding.

Key Points to Consider In Post Weld Heat Treatment

When performing post weld heat treatment on a welded heat treatable base alloy, it may be important to consider using a filler alloy that is heat treatable. The heat treatable filler alloy 4643 was designed for welding 6xxx series base alloys and developing high mechanical properties in the post-weld heat-treated condition. It was formulated by taking the well-known alloy 4043, reducing the silicon, and adding 0.10 to 0.30 percent magnesium. This modification to its chemistry produces a 4xxx series alloy, which will form the magnesium silicide compound required to respond to post weld heat treatment.

Other heat treatable 4xxx series filler alloys have been developed for some of the cast aluminum base alloys.  Alloys 356.0 and A356.0 (Al-Si-Mg) are used in large quantities for sand and permanent mold castings.  A variety of heat treatments can be used to produce the desired combinations of mechanical properties in these castings.  Alloy A356.0 has a lower iron content than 356.0, which, in effect, provides higher tensile properties in premium quality castings.  Alloy A356.0 filler wire is optimum for joining and repairing both 356.0 and A356.0 castings.  Alloy A356.0, when fabricated as a wrought wire product, is registered by the Aluminum Association as the heat treatable alloy 4010.  Filler alloy 4008 is also available; this is a tightly controlled chemistry version of the A356.0 (4010) alloy and is heat treatable.  The 4008 alloy is generally formulated to meet the requirements of AMS 4181 specification.

Aluminum casting alloys 355.0, A355.0 and C355.0 (Al-Si-Cu-Mg) are alloyed with copper to afford a greater response to heat treatment.  Alloy C355.0 contains a low amount of iron to create higher tensile properties in premium quality castings.  Aluminum filler alloy C355.0 was designed for joining and repairing 355.0, A355.0 and C355.0 castings.  Alloy A355.0, when fabricated as a wrought wire product, is registered by the Aluminum Association as heat treatable alloy 4009.

Summary:

Aluminum alloys within the 4xxx series group are both heat treatable and non-heat treatable alloys.  The primary difference is that the non-heat treatable alloys contain aluminum with silicon as the principal alloying element with no other elements added that could allow heat treatability.  The heat treatable 4xxx series alloys contain silicon and also controlled additions of magnesium and/or copper, which provide the material the opportunity to respond to, and increase its strength through, heat treatment.