Which filler metal should you choose?
Comparing, contrasting 4XXX and 5XXX filler metals
6XXX series is a common base material used in aluminum applications and, generally speaking, its best filler metal match is either the 4XXX or 5XXX series. The filler metal you choose depends on how the finished weld will be used. Will it be exposed to sustained elevated temperatures? Are strength, ductility, and toughness key requirements? Determine which properties are most important for the completed weld and select the filler metal according to those priorities.
A 4943 filler can produce a weld with approximately 25 percent higher tensile strength and shear strength in the as-welded condition.
Did you know aluminum contributes to 8 percent of the Earth’s crust? Not only is this material part of the very ground we stand on, but it also makes up many of the things that surround us in our everyday lives.
Among its many benefits, aluminum has formability, toughness, corrosion resistance, light weight, strength, elasticity, and energy absorption properties. For example, in manufacturing semitrailers—an application in which light weighting is important—fabricating various parts from aluminum as opposed to mild steel can reduce weight significantly, such as 1,000 lbs. for a side wall, 187 lbs. for a rear door, 60 lbs. for a cab roof, and 30 lbs. per wheel.
This material, however, is not without its challenges when it comes to welding. Because of aluminum’s low melting temperature and high thermal conductivity, you must take extra care to prevent burn-through on thin gauges and to ensure adequate fusion or penetration on thicker gauges.
When the application calls for aluminum, 6XXX series base material is a common choice due to its versatility for many applications. One specific alloy, 6061-T6, is frequently used in applications such as shipbuilding, automotive manufacturing, and trailer construction.
So what type of filler metals should you use with 6XXX series aluminum? The answer is either 4XXX or 5XXX series filler metals. But keep in mind that the final use and requirements for the part you are welding are still the most important factors.
Common Obstacles With 6XXX Base Materials
The as-welded mechanical properties of 6XXX series base material make them more sensitive to different welding variables, such as heat input and joint design, compared to 5XXX series materials.
For example, 6XXX series base materials have 30 percent more thermal conductivity than 5XXX series base metals, making it more difficult to produce quality welds consistently. The higher thermal conductivity of 6XXX alloys requires a higher heat input to get the same penetration, which in turn can make the material more prone to distortion.
Using heat treatment with 6XXX series base metals is one way to address some of the material’s challenges. When 6XXX base materials are welded, the microstructure of the heat-affected zone (HAZ) is degraded, and mechanical properties can be reduced by 30 to 50 percent. If the material was a T6 temper before welding, it can be solution-heat-treated and aged after welding, which would restore it to a T6 temper.
Also, it is important to remove the oxide layer before welding any type of aluminum, including 6XXX series. Use a stainless steel brush designated for this sole purpose. Otherwise, the heat input required to penetrate the oxide layer during welding can burn through the base material.
Choosing the Right Filler Metal
When you are selecting a filler metal for 6XXX series aluminum, it’s critical to understand how the finished weld will be used. The desired outcome influences the choice. Will it be exposed to sustained elevated temperatures? Are strength, ductility, and toughness key requirements? Determine which properties are most important for the completed weld and select the filler metal according to those priorities.
When you are selecting a filler metal to use with 6XXX series aluminum, it’s critical to understand how the finished weld will be used. Filler metals in the 4XXX and 5XXX series are most commonly used to weld aluminum.
There is a give-and-take when choosing filler metals—certain filler metals provide specific properties in the final weld, such as high strength, crack resistance, or the ability to be anodized. A 4XXX series filler metal, for example, eliminates the ability for a color match after anodizing the material, but allows you to heat-treat the weld. Conversely, a 5XXX series filler metal allows for color matching after anodizing, but it does not allow for postweld heat treatment (with the exception of 5554).
Filler metals in the 4XXX and 5XXX series are most commonly used to weld 6XXX series aluminum. Consult an aluminum filler metal selector chart or guide when making the choice to determine how your filler metal selection can affect the following:
- Cracking sensitivity
- Strength
- Ductility
- Corrosion resistance
- Elevated-temperature service
- Color match after anodizing
- Postweld heat treat
- Toughness
4XXX Series Filler Metals. When selecting a 4XXX series filler metal for use with 6XXX aluminum, 4043 and 4943 are the most popular choices.
4043 filler metal has several benefits such as excellent crack and corrosion resistance, its ability to tolerate postweld heat treatment, its performance in high-temperature applications of 150 to 350 degrees F, and ductility. However, 4043 doesn’t color-match as well as 5XXX fillers do when the part is anodized after welding, and its strength is lower than that of 5XXX filler metals.
4943 filler metal has many of the same characteristics as 4043, but it responds to heat treatment better. The added magnesium in 4943 filler metal also provides higher repeatable strength without relying on dilution of the base material to create a sound weld.
The dilution process—in which elements in the base material combine with those in the filler metal—influences an aluminum weld’s final chemical composition and mechanical properties, including strength. While you can estimate dilution and the resulting weld strength, numerous operator variables, such as torch angle, travel speed, and amperage, affect penetration depth, which in turn affects dilution. As a result, you may not always get the estimated dilution per your calculations. Using 4943 filler metals can take the guesswork out of the question of strength.
With 4943 filler metal you can produce a weld with approximately 25 percent higher tensile strength and shear strength in the as-welded condition. This filler metal uses the same voltage and wire feed speed as 4043. It has high fluidity, a low shrinkage rate, and lower weld fume compared to 4043. It also can be used to weld 1XXX, 3XXX, and 5XXX base materials with less than 3 percent magnesium (such as 5052), as well as 6XXX base metals.
In general, 4XXX series filler metals produce less weld discoloration and smut compared to 5XXX series filler metals, which can help reduce the time and money you’d spend on postweld cleanup.
5XXX Series Filler Metals. There are numerous 5XXX filler metal options that you can use to weld 6XXX aluminum base material. Two common choices are 5356 and 5556.
When welding 6XXX series, the 5356 filler metal offers good cracking resistance and strength, excellent ductility, acceptable corrosion resistance, excellent color-matching capability after anodizing, and excellent toughness. Note that these filler metals cannot handle postweld heat treatment or high-temperature (150-350 degrees) applications.
Dilution, in which elements in the base material combine with those in the filler metal, plays a role in an aluminum weld’s final chemical composition and mechanical properties, including strength. The chart shows, from left to right, the least desirable dilution ratio, where problems could occur, to a better dilution ratio for a sounder weld.
5556 filler metals have similar cracking resistance, ductility, and corrosion resistance to 5356, but they also offer even higher strength. Like 5356 alloys, 5556 filler metals cannot tolerate high-temperature applications or postweld heat treatment, but they do color-match well after anodizing.
Another 5XXX series filler metal—5554—is the only 5XXX series filler that can tolerate high-temperature applications and postweld heat treatment, but it is not as widely used.
Finding the Best Match
In choosing a filler metal for aluminum, you will find there is no one-size-fits-all solution. The operating conditions and final use of the welded part are critical factors in making the right choice.
Consult with a trusted filler metal manufacturer or welding distributor for support and to determine which classification of filler metal is the best option.
While one filler metal may provide higher strength or toughness, another may provide better corrosion resistance or ductility. The goal is to choose an aluminum alloy that produces a weld most capable of meeting the requirements of the product and its intended use.
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