A - To address your question appropriately, it is necessary to divide it into three separate sections: Aluminum base alloy strength: aluminum filler alloy strength and as-welded aluminum joint strength, and the AWS filler alloy classification system for aluminum.

Aluminum Base Alloy Strength

One of the most extensively used references for information regarding the mechanical properties of aluminum base alloys is Aluminum Standards and Data published by the Aluminum Association.  This manual contains useful information and data pertaining to chemical composition limits, mechanical and physical properties, tolerances and other characteristics of various aluminum and aluminum alloy wrought products.  The content of the manual is subject to periodic revision to include advances in production methods, to add data on new alloys and products and to delete those alloys that become inactive. 

The latest edition was published in 2003 and is comprised as follows.

The first three sections of the manual contain general information that is useful for comparing materials.  The typical properties and characteristics listed in these first three sections are not guaranteed and should not be used for design purposes.  The fourth section contains information relating to testing, inspection and identification. The fifth section lists the definitions of many terms used in the wrought aluminum industry. 

The remaining twelve sections of the manual consist of composition limits, mechanical property limits, dimensional tolerances and other data classified by product form.  Such products as sheet and plate, fin stock, foil, wire, rod and bar rolled or cold finished and extruded, tube and pipe, structural profiles channels and I-beams, forging stock, forgings and electric conductor bus bars.  The mechanical property and product dimensional tolerance limits in these sections are statistically based guaranteed limits and may be used  as  the basis of design.

Aluminum Filler Alloy Strength And As-Welded Aluminum Joint Strength

The strength of aluminum filler alloys and as-welded strength of arc-welded joints is an extremely broad subject; therefore, for this discussion, the focus will be transverse groove weld tensile strength and longitudinal and transverse shear strength of fillet welds. The transverse groove weld tensile strength is a property often used for evaluating the performance of groove welds when conducting welding procedure qualification tests.  By utilizing the relevant welding codes and standards, you will find the minimum allowable tensile strength values.  For example, the AWS D1.2 2003 Structural Welding Code, Aluminum has these values listed in its table 3.2.  The transverse tensile strength of an arc welded aluminum groove weld in the as-welded condition is contingent on the tensile strength of the heat-affected zone (HAZ) of the weld.  The HAZ is the area of base material immediately adjacent to the weld whose mechanical properties or microstructure transform by the heat of the welding.  In the non-heat treatable aluminum alloys, the HAZ is near the annealed condition of the base alloy.  In the heat treatable alloys, the HAZ will typically become partially annealed and over aged, depending on the original temper of the base alloy prior to welding and the heat input during welding.  The tensile strength values provided in the welding codes, such as AWS D1.2, are for test evaluation purposes and assists with the development and qualification of welding procedures.  These values are not intended to be used for design calculations.

Aluminum filler alloy manufacturers often publish the typical fillet weld shear strength values of the filler alloys that they supply These values are a comparison between the various filler alloys and are values that may be expected to be achieved if fillet weld shear strength testing is performed.  These values are not intended to be used for design calculations.

For design purposes, there are values for both groove weld tensile strength and fillet weld shear strength, which are published in the Aluminum Association’s Aluminum Design Manual, Part IA, section 7.   These values are strengths; they must be reduced by using an appropriate safety factor. The Aluminum Design Manual specifies a safety factor on welds of 1.95 for building type structures.  In terms of the AWS D1.2 Structural Welding Code – Aluminum, section 2.1 Structural Design,  “Welds shall be sized for strength requirements using the effective areas defined in section 2 of this code in conformance with the Aluminum Design Manual Specification for Aluminum Structures unless otherwise required by the contract documents.”

The AWS Filler Alloy Classification System For Aluminum

You are quite correct; the AWS filler alloy classification system for aluminum does not provide information relating to the strength of filler alloys.  To understand the reason why the aluminum filler alloy classification system is different from steel, it is necessary to consider the overall differences in filler alloy selection between these two materials.

When welding steel, using a filler alloy with very similar chemistry and very similar strength to that of the base alloy being welded is typical.  When welding aluminum there are often other variables, which may need further consideration.  Often, there are a number of filler alloys that may be used to join any given aluminum base alloy.  The selection of the most suitable filler alloy is usually dependent on the service conditions of the manufactured component.    In some cases, a trade off situation emerges, where it is required to choose between different characteristics of the completed weld in order of importance.  Some considerations for the selection of a filler alloy for aluminum that are different from that of steel are:

1. Ease of welding - this is the relative freedom from weld cracking and it is often based on the chemistry developed between the base alloy and filler alloy when combined in the weld.  Unlike steel, aluminum base alloys are often welded with filler alloys with a very different chemistry than that of the base alloy in order to avoid high crack sensitivity. 
2. Ductility – it is not uncommon to have the choice between two filler alloys that have substantially different ductility.
3. Corrosion Resistance – Because there is the possibility of using filler alloys with different chemistry than that of the base alloy, there is a possibility of a difference in solution potential between base alloy and filler alloy.
4. Sustained Temperature Service - The reaction of some filler alloy chemistries at sustained elevated temperature may promote premature component failure due to stress corrosion cracking.
5. Color Match - Base alloy and filler alloy color match after anodizing can be of major concern in some cosmetic applications.
6. Post Weld Heat Treatment - The ability of the filler alloy to respond to post weld heat treatment associated with filler alloy chemistry and joint design.  Some filler alloys will not respond to heat treatment, others will only respond favorably to heat treatment if they acquire sufficient amounts of alloying elements from the base material during welding, and others will  respond regardless of dilution.

It is apparent that the system used for the selection of filler alloys for aluminum welding is quite different to that of steel.  The filler alloy selection system for aluminum is based largely on chemistry issues.  The classification system for aluminum filler alloys is based on chemistry and not mechanical properties.  It is common practice to use a filler alloy selection chart that addresses the variables to consider when choosing the most suitable filler alloy for a particular aluminum welding application.

Summary:

There are published resources available for establishing the typical and design allowable strengths of the aluminum base alloys and there welded joints. 

Some publications mentioned above are:

Aluminum Standards and Data – From the Aluminum Association at  www.aluminum.org

AWS D1.2 2003 Structural Welding Code – Aluminum – From AWS at www.aws.org

Aluminum Design Manual Specification for Aluminum Structures at www.aluminum.org

If you would like a filler alloy selection chart, request it by email, and I will send you one free of charge.