Filler metal selection
Filler metal specifications are to be found in BS 2019 Part 4, although this will be replaced in the near future by a CEN specification. The BS specification lists 11 filler metal types in the 1XXX, 3XXX, 4XXX and 5XXX series and details the delivery conditions. BS 2901 does not include any filler metals capable of being age hardened. The American Welding Society has
Table 3.1 General guidance on filler metal selection
Table 3.2 Guidance on filler metal selection - dissimilar metal joints for specific alloys
published a similar specification, AWS A5.10 ‘Specification for Bare Aluminium and Aluminium Alloy Welding Electrodes and Rods’, which fulfils a similar role. This specification includes 15 separate filler metal compositions, comprising alloys in the 1XXX, 2XXX, 4XXX and 5XXX series. In addition there are five age-hardening filler metals designed for use in the welding of castings. AWS A5.10 also includes delivery conditions and the testing requirements for usability and soundness.
As mentioned earlier, filler metal selection is crucial to producing crack - free, optimum strength welded joints but there are other considerations that may need to be included when making the choice. Unlike selecting consumables for welding steel, where the composition of the filler metal generally matches that of the parent metal with respect to composition, mechanical properties, corrosion resistance and appearance, aluminium alloys are often welded with filler metals that do not match the parent metal in some or all of these properties. This presents the engineer with some problems when it comes to deciding on the optimum filler metal composition. In addition to strength and crack resistance the choice may also need to include colour match, corrosion resistance, response to anodising and
Table 3.3 Filler metal selection to achieve specific properties for the commoner structural alloys
creep strength. Guidance on suitable fillers can be found in Table 3.1, for specific alloys, in Table 3.2 and to achieve specific properties in some of the commoner structural alloys in Table 3.3. In Table 3.1 there are three recommendations based on the best strength, the upper figure; the highest crack resistance, the middle figure; and an acceptable alternative, the lower figure. Note that the alloys are arranged in families - for a recommendation on filler metal read directly across and down from the alloys of interest.
There are a number of specific points to be made to amplify the guidance given in Tables 3.1-3.3:
• When welding alloys containing more than 2% magnesium avoid the use of fillers containing silicon as the intermetallic compound magnesium silicide, Mg3Si, is formed. This embrittles the joint and can lead to failure in joints that are dynamically loaded. The converse is also true, that Mg3Si will be formed when welding alloys containing more than 2% silicon with 5XXX fillers.
• 5XXX filler metals with more than 5% Mg should be avoided if the service temperature exceeds 65 °C as Al2Mg is formed, which makes the alloy susceptible to stress corrosion. Filler metals such as 5454 or 5554 containing less than 3% Mg should be used.
• High-purity 5654 is preferred for the welding of high-purity aluminium in hydrogen peroxide service.
• 4643 may be used to weld the 6XXX alloys as the small amount of magnesium improves the response to solution treatment.
• The pure aluminium 1XXX alloys are very soft and wire feeding problems can be experienced.
• Low magnesium (<2%) 5XXX alloys such as 5251 may suffer hot cracking if matching composition fillers are used. Use Al-Mg5 type instead.
• When welding the 7XXX alloys 5039 filler metal may give more effective age hardening in low-dilution applications.
• 6XXX alloys exhibit solidification cracking if welded autogenously.
• Titanium and zirconium are sometimes added to filler metals to reduce the risk of weld metal hot cracking by means of grain refinement.
• 4047 may be used to prevent weld metal cracking in joints involving high dilution or restraint but remember the first point above.
• The 2XXX series of copper containing alloys were generally regarded as unweldable until the higher (>4%) copper alloys such as 2219 became available. If it is necessary to weld the lower copper-containing alloys then 4047 is the best choice as a filler metal.