The Best Filler Alloy for 6061-T6 Operating at 250°F

How to Choose the Best Filler Alloy for 6061-T6 Operating at 250°F

One of the key factors in welding is selecting the suitable filler alloy based on the requirement. Let us consider a case where the welder needs to select the most suitable filler alloy for a particular GMAW (Gas Metal Arc Welding) welded aluminum structure.

The Case

The welder is looking for some assistance with the selection of the most suitable filler alloy for a particular GMAW welded aluminum structure. The component is used for material handling manufactured from 6061-T6 in material thicknesses of 1/8 to 1/2 inch. The joint design is fillet weld.

Special Operating Considerations: The selected welding filler alloy should be suitable for sustained high-temperature application (250°F), provide good structural integrity, and suitable feedability characteristics. This component is to be used in the as-welded condition and will not undergo any post weld heat treatment.

The Solution

The sustained high-temperature application (above 150°F) requirement for this component is probably the main consideration and will most definitely influence the filler alloy selection. Also, as this component is intended to be used for handling material and the welds being used are fillet welds, you need to certainly consider the shear strength characteristics of the filler alloy.

The high service temperature requirement eliminates the use of any filler alloy with more than 3% Mg such as the frequently used 5356 and the higher strength filler alloys 5183 and 5556. This provides you with 4643, 4043, 4047, and 5554 as possible filler alloys for your application.

Best Filler Alloy for 6061-T6 Operating at 250°F

To find out the most appropriate filler alloy let us evaluate each of these four filler alloys based on their features and characteristics.

Considerations relating to four filler alloys that may be used for this application: 4643, 4043, 4047, and 5554.

Filler alloy ER4643

This filler alloy was developed for welding 6xxx series base alloys and developing high mechanical properties in the post-weld heat-treated condition. This alloy was produced by taking the well-known alloy 4043, reducing the silicon, and adding 0.10 to 0.30 percent magnesium, thus ensuring its ability to respond to post-weld heat treatment. This is a premium filler alloy that is priced for its specialized characteristics and would not normally be used unless post weld heat treating was being performed. As post weld heat treatment is not to be used, there is no reason to use this alloy.

Filler Alloy ER4043

While the 4043 filler alloy, a 5% Silicon alloy, is often used to weld 6061 base materials, it is not commonly used when the shear strength of the component is a predominant consideration. The 4043 filler alloy is a silicon-based filler alloy that is often used to take advantage of the element’s capability of promoting fluidity in aluminum. While groove welds can be made using this filler alloy that can characteristically pass minimum transverse tensile test requirements, this filler alloy has considerably lower shear strength when compared to the 5xxx series filler alloys.

Filler Alloy ER4047

Like the 4043 filler alloy, 4047 is a silicon-based alloy but has a much higher level of silicon, approximately 12%. The extra silicon within the alloy provides exceptional fluidity. This alloy, which is also registered as a brazing alloy, is most often used for leak-tight joints in thin-wall applications. The 4047 filler alloy has a very narrow freezing range and consequently is very resistant to solidification cracking. The 4043 and 4047 filler alloys have remarkably similar mechanical characteristics - 4047 having slightly higher tensile strength. Both 4043 and 4047 have lower ductility and lower shear strength properties than the 5xxx series filler alloys. They also possess lower column strength when compared to the 5xxx series filler alloys, and consequently, require extra care during feeding to minimize feedability problems. 

Filler Alloy ER5554

The 5554 filler alloy was designed to provide the improved strength and ductility characteristics of a magnesium-based filler alloy that was also suitable for elevated temperature applications. It contains 2.4% to 3.0% magnesium thereby providing an alloy that is not susceptible to stress corrosion cracking but has improved shear strength and improved ductility when compared to the 4xxx series filler alloys. The typical transverse shear strength of 4043 and 4047 is 15.0 ksi and the typical transverse shear strength of 5554 is 23 ksi, an increase of 65% (see Fig 1).

In addition to the improved mechanical properties of 5554 on the completed weld, we see an increase in the column strength of the wire. This characteristic can distinctly improve the wire feedability. The 5554 filler alloy will typically produce a weld that is not as cosmetically appealing as the 4xxx series filler alloys and has a slightly greater potential for producing “smut” (a black deposit of metal oxides that is not harmful to the weld).

Shear strength

Typical Shear Strength of fillet welds.

Conclusion

1. Filler alloy 4643 was designed for its post-weld heat treatment capabilities and is therefore not particularly suited for your application.
2. Filler alloys 4043 and 4047 have considerably lower shear strength values when compared to 5554.
3. Filler alloys 4043 and 4047 wire have considerably lower tensile (column) strength and require more care when feeding when compared to 5554.
4. Filler alloy 4047 may provide some advantages over 4043: enhanced fluidity, the possibility of faster welding speeds, and lower heat input. This filler alloy may be a consideration for thinner components and produces excellent leak-tight joints.

Filler alloy 5554 will typically provide welds of higher strength but with lower cosmetic appeal when compared to the 4xxx series filler alloys.