6 variables that affect aluminum filler alloy selection
May 15, 2009
Choosing the best filler metal for aluminum welding involves takinginto account the operating conditions of the finished weldedcomponent as well as six important variables that can affect theoperating condition.
The crack sensitivity curve shows the relative crack sensitivity of the most common weld metal chemistries that develop during welding.
Editor's Note: This article is adapted from Tony Anderson's presentation for the Aluminum Association Conference, May 5-6, 2009, Toronto, by the American Welding Society.
Filler alloy selection for welding aluminum is an essential part of the development and qualification of a suitable weld procedure specification (WPS). Choosing the most appropriate filler alloy for welding aluminum is based on the operating conditions of the finished welded component and a number of variables.
When selecting a filler alloy for welding aluminum, it's a good idea to compare the performance of each filler alloy against each of the six variables: ease of welding, strength of the welded joint, ductility, corrosion resistance, sustained-temperature service, and postweld heat treatment.
Ease of welding is often an important factor in filler alloy selection. Its significance is based on the filler/base alloy combination (chemistry) and its relative crack sensitivity.
To avoid chemistry in the weld that produces high crack sensitivity, be sure the mixture of the base alloy and filler alloy produces a low-crack-sensitivity material. Chemistry ranges that fall into the cracksensitive category are best understood by examining the information in Figure 1, which shows the crack sensitivity curves for the most common weld metal chemist-ries developed during welding. Some examples are:
The aluminum-silicon alloys (4xxx series) are seen predominantly as filler alloys and commonly contain 4.5 percent to 13 percent silicon. Silicon analuminum filler/base alloy mixture of 0.5 percent to 2 percent produces a crack-sensitive weld metal composition. A weld with this chemistry has a high probability of producing hot cracking problems. Exercise care when welding a 1xxx series (pure aluminum) base alloy with a 4xxx series filler alloy to prevent a weld metal chemistry mixture within this crack-sensitive range.
The aluminum-copper alloys (2xxx series) are heat-treatable, high-strength materials often used in specialized applications and exhibit a wide range of crack-sensitive characteristics. Some of these base alloys are not suitable for arc welding because of their sensitivity to hot cracking, but others are welded easily using the correct filler alloy and welding procedure.
The aluminum-magnesium alloys (5xxx series) have the highest strengths of the non-heat-treatable aluminum alloys and, for this reason, are very important for structural applications. In Figure 1, a magnesium content of 0.5 to 3.0 percent in aluminum produces a crack-sensitive composition. As a rule, the aluminum-magnesium base alloys with less than 2.5 percent magnesium content can be welded with either the 4xxx series or the 5xxx series filler alloys, depending on your weld performance requirements. The aluminum-magnesium base alloys with more than 2.5 percent magnesium typically cannot be welded successfully with the 4xxx series filler alloys because of problems associated with decreased ductility and increased crack sensitivity.
The aluminum-magnesium-silicon alloys (6xxx series) are heat-treatable alloys and contain about 1.0 percent Mg2Si. These alloys cannot be arc welded successfully without filler alloy. They can be welded with 4xxx series or 5xxx series filler alloys depending on weld performance requirements. It is very important to dilute the base material with sufficient filler alloy to reduce weld metal crack sensitivity and hot cracking problems (see Figure 2).
Groove Welds. Typically the heat-affected zone (HAZ) of a groove weld dictates the strength of the joint. Groove welds in non-heat-treatable alloys will result in complete annealing of the HAZ area adjacent to the weld. These alloys are annealed when they are heated between 600 and 700 degrees F for a short time. Therefore, the HAZ typically is the weakest area of the joint. The WPS qualification requirement for these alloys is based on the minimum tensile strength of the base alloy in its annealed condition.
Heat-treatable alloys must be heated for an extended period to become fully annealed. This does not occur during welding, and therefore the strength of the HAZ will be only partially annealed. However, even with the best welding procedures and relatively low heat input, these alloys experience a significant loss of strength after arc welding because of time and temperature. The faster the welding process and heat dissipation, the smaller the HAZ, and the higher the as-welded strength. Excessive preheating, lack of interpass cooling, and excessive heat input all increase peak temperature and time at that temperature, which result in lower strength in the HAZ (see Figure 3).
Fillet Welds. Unlike groove welds, fillet weld strength largely depends on the composition of the filler alloy. The joint strength of fillet welds is based on shear strength, which can be affected considerably by the filler alloy. The 4xxx series filler alloys have lower ductility and provide less shear strength in fillet-welded joints. The 5xxx series fillers typically have more ductility and can provide close to twice the shear strength of a 4xxx series filler alloy in some circumstances.
Tests have shown that a required shear strength value in a fillet weld in 6061 base alloy needs a 1/4-inch fillet weld with 5556 filler, compared to a 7/16-in. fillet with 4043 filler, to meet the same required shear strength. This can mean the difference between a one-run and a three-run fillet to achieve the same strength (see Figure 4).
Ductility is the ability of a material to flow plastically before fracturing. Fracture characteristics are described in terms of ability to undergo elastic stretching and plastic deformation in the presence of stress raisers (weld discontinuities).
Increased ductility ratings for a filler alloy indicate greater ability to deform plastically and to redistribute load, decreasing the crack propagation sensitivity. Ductility may be a consideration if forming is performed after welding or if the weld is subjected to impact loading. It is considered when conducting bend tests during procedure qualifications.
The 4xxx series filler alloys have relatively low ductility. This is addressed with special requirements within the code or standard relating to test sample thickness, bending radius, or material condition when bend testing is required.
Most unprotected aluminum base alloy and filler alloy combinations are satisfactory for general exposure to the atmosphere. If you use a dissimilar aluminum alloy combination for the base and filler when an electrolyte is present, galvanic action between the dissimilar compositions can occur.
The difference in alloy performance can vary based on the type of exposure. Filler alloy ratings typically are based on fresh- and salt-water exposure only. Corrosion resistance can be a complex subject. If weldments will be subjected to a highly corrosive environment, consult an engineer with experience in this field.
Stress corrosion cracking is a condition that can result in premature weld failure. This condition develops through magnesium segregation at the grain boundaries of the material. This will occur only in alloys with more than 3 percent magnesium when they are exposed to prolonged elevated temperature (above 150 degrees F).
The 5356, 5183, 5654, and 5556 filler alloys all contain more than 3 percent magnesium (5 percent), so they are not suitable for elevated-temperature service. Alloy 5554 has less than 3 percent magnesium and was developed for high-temperature applications.
Alloy 5554 is used for welding 5454 base alloy, which also is used for high-temperature applications. The Al-Si (4xxx series) filler alloys may be used for some high-service-temperature applications, depending on base alloy type and weld performance requirements.
The common heat-treatable base alloys, such as 6061-T6, lose a substantial amount 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. On occasion you may want to perform postweld heat treatment to return the mechanical strength to the manufactured component. When postweld heat treating, consider the filler alloy's ability to respond to the heat treatment.
Most commonly used filler alloys will not respond to postweld heat treatment without adequate dilution with the heat-treatable base alloy. This is not always easy to achieve and can be difficult to control consistently. For this reason, filler alloys have been developed to respond independently to heat treatment.
Filler alloy 4643, for example, was specifically designed for welding 6xxx series base alloys and producing high mechanical properties in the postweld heat-treated condition. It was developed by reducing the silicon in the well-known alloy 4043 and adding 0.10 percent to 0.30 percent magnesium. A number of other heat-treatable filler alloys are available for welding some of the 2xxx series alloys and some of the heat-treatable cast aluminum alloys.
Successfully selecting the best filler alloy can be achieved only after a full analysis of the many variables associated with welding aluminum components and their applications. Keep in mind the type and chemistry of the base material to be welded, as well as the welded components' performance requirements. You can use an aluminum filler alloy selection chart to assist in selecting the most appropriate filler alloy.