Selecting the Right Filler Alloy for Aluminum Welding

How to Choose the Best Filler Alloy for Aluminum Welding: A Comprehensive Guide

When considering the welding of aluminum alloys, and the development and qualification of welding procedures one of the main considerations must be filler alloy selection.  Typically there are a variety of filler alloys available that may be used to join any given base alloy.  However, there are numerous variables associated with the selection of the most suitable filler alloy to be used for any base alloy or alloy combination.  In some cases, we may find ourselves in a trade-off situation, where we need to choose between different characteristics of the completed weld in order of importance.

Some considerations for the selection of a filler alloy are typically ease of welding, the strength of welded joint, ductility, corrosion resistance, sustained temperature service, color match, and post-weld heat treatment.

AN INTRODUCTION TO FILLER ALLOY SELECTION

Typically, filler alloy selection for welding steel is based on matching the tensile strength of the base alloy alone. Aluminum, on the other hand, often has a variety of filler alloys, which can meet or exceed the tensile strength of the base material in the as-welded condition. The selection of a filler alloy for aluminum typically is based on other variables, which may be as important, or possibly more important than that of tensile strength. Choosing the correct filler alloy for aluminum is based on the operating conditions of the finished welded component and the consideration of several variables that can affect the operating condition of the weld. AlcoTec offers a filler alloy selection chart that compares the performance of each filler alloy against each of the following variables.

1. EASE OF WELDING

Ease of welding is often an important consideration in filler alloy selection. Its significance is based on the filler/base alloy combination (chemistry) and its relative crack sensitivity. For base alloys with a high susceptibility to hot cracking, such as many in the 2xxx series alloys, a 4xxx series filler may be chosen, such as 4145, which has an extremely low solidification temperature (970 degrees Fahrenheit). This low solidification temperature of the filler alloy ensures that the 4145 weld is the last area to solidify and thereby allows the base material to solidify completely and reach its maximum strength before the solidification/shrinkage stresses of the weld are applied.

By using a filler alloy such as 4047, which has a freezing range of around 10 degrees F, welds can be made that solidify quickly. This reduces the time that liquid metal is subjected to shrinkage during the solidification process.

2. CRITICAL CHEMISTRY RANGES

Critical chemistry ranges are best addressed by using the information in Fig 1, which shows the crack sensitivity curves for the most common weld metal chemistries developed during welding structural base alloy materials.

2.1 Aluminum/Silicon Alloys – 4xxx Series

The 4xxx series aluminum/silicon alloys are used predominately for filler alloys and are found as non-heat-treatable and heat-treatable alloys containing 4.5 to 13 percent silicon. Silicon in an aluminum filler/base alloy mixture of 0.5 to 2 percent produces a weld metal composition that is crack-sensitive. A weld with this chemistry usually will crack during solidification.

Care must be exercised if welding a 1xxx series (pure Al) aluminum base alloy with a 4xxx series (Al/Si) filler alloy to prevent a weld metal chemistry mixture within this crack-sensitive range.

2.2 Series Aluminum/Copper Alloys – 2xxx Series

Aluminum/copper alloys (2xxx series) are heat-treatable high-strength materials often used in specialized applications. They exhibit a wide range of crack-sensitive characteristics. Some base alloys are considered poor for arc welding because of their sensitivity to hot cracking, but others are welded easily using the correct filler alloy and procedure.

2.3 Aluminum/Magnesium Alloys – 5xxx Series

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. Magnesium (0.5 to 3.0 percent) in an aluminum weld produces a crack-sensitive weld metal composition. As a rule, the Al/Mg base alloys with less than 2.8 percent Mg content can be welded with either the Al/Si (4xxx series) or the Al/Mg (5xxx series) filler alloys dependent on weld performance requirements.

The Al/Mg base alloys with more than about 2.8 percent Mg typically cannot be welded successfully with the Al/Si (4xxx series) filler alloys because excessive amounts of magnesium silicide Mg2Si develop in the weld structure, decreasing ductility and increasing crack sensitivity.

2.4 Aluminum/Magnesium/Silicon Alloys – 6xxx Series

The aluminum/magnesium/silicon alloys (6xxx series) are heat treatable. The 6xxx series base alloys, typically containing around 1.0 percent magnesium silicide Mg2Si, cannot be arc welded successfully without filler alloy. These alloys can be welded with 4xxx series (Al/Si) or 5xxx series (Al/Mg) filler alloys depending on weld performance requirements. It is important to dilute the Mg2Si percentage in the base material with sufficient filler alloy to reduce weld metal crack sensitivity.

3. CRACK SENSITIVITY

When we consider the aluminum alloys that fall into this difficult-to-weld category, we can divide them into different groups. We will first consider the small selection of aluminum alloys, that are designed for machine-ability, not weldability.  Alloys such as 2011 and 6262, that contain 0.20-0.6, Bi, 0.20-0.6 Pb, and 0.40-0.7 Bi, 0.40-.07 Pb respectively.  The addition of these elements (Bismuth and Lead) to these materials greatly assists in chip formation in these free-machining alloys.  However, because of the low solidification temperatures of these elements, they can seriously reduce the ability to produce sound welds in these materials.

Several aluminum alloys are quite susceptible to hot cracking if arc welded, these alloys are usually heat-treatable and are most commonly found in the 2xxx series (Al-Cu) and 7xxx series (Al-Zn) groups of materials.

To understand why some of these alloys are unsuitable for arc welding (unweldable) we need to consider the reasons why some aluminum alloys can be more susceptible to hot cracking.

Hot cracking or solidification cracking occurs in aluminum welds when high levels of thermal stress and solidification shrinkage are present while the weld undergoes various degrees of solidification.  The hot cracking sensitivity of any aluminum alloy depends on the combination of mechanical, thermal, and metallurgical factors.

Several high-performance, heat-treatable aluminum alloys have been developed by combining various alloying elements to improve the material's mechanical properties.  In some cases, the blend of the required alloying elements has produced materials with high hot cracking sensitivity.

3.1 Coherence Range

Perhaps the most important factor affecting the hot crack sensitivity of aluminum welds is the temperature range of dendrite coherence and the type and amount of liquid available during the freezing process.  Coherence is when the dendrites begin to interlock with one another to the point that the melted material begins to form a mushy stage.

The coherence range is the temperature between the formation of coherent interlocking dendrites and the solidus temperature.  This could be referred to as the mushy range during solidification.  The wider the coherence range, the more likely hot cracking will occur because of the accumulating strain of solidification.

3.2 The 2xxx Series Alloys (Al-Cu)

Hot cracking sensitivity in the Al-Cu alloys increases as we add Cu up to approximately 3% Cu and then decreases to a relatively low level at 4.5% Cu and above.  Alloy 2219 with 6.3% Cu exhibits good resistance to hot cracking because of its relatively narrow coherence range.  Alloy 2024 contains approximately 4.5% Cu which may initially encourage us to suppose it would have relatively low crack sensitivity.  However, alloy 2024 also has a small amount of Magnesium (Mg).  The small amount of Mg in this alloy depresses the solidus temperature but that does not affect the coherence temperature, therefore the coherence range is extended and the hot cracking tendency is increased.

The problem to be considered when welding 2024 is that the heat of the welding operation will allow segregation of the alloying constituents at the grain boundaries and the presence of Mg, as stated above, will depress the solidus temperature.  Because these alloying constituents have lower melting phases, the stress of solidification may cause cracking at the grain boundaries and/or establish the condition within the material conducive to stress corrosion cracking later.  High heat input during welding, repeated weld passes, and larger weld sizes can all increase the grain boundary segregation problem (segregation is a time-temperature relationship) and subsequent cracking tendency.

3.3 The 7xxx Series Alloys (Al-Zn)

The 7xxx series of alloys can also be separated into two groups as far as weldability is concerned.  These are the Al-Zn-Mg and the Al-Zn-Mg-Cu types.

Al-Zn-Mg Alloys such as 7005 will resist hot cracking better and exhibit better joint performance than the Al-Zn-Mg-Cu alloys such as 7075. The Mg content in this group (Al-Zn-Mg) of alloys would generally increase the cracking sensitivity.  However, Zr is added to refine grain size, this effectively reduces the cracking tendency.  This alloy group is easily welded with high magnesium filler alloys such as 5356, which ensures the weld contains sufficient magnesium to prevent cracking.  Silicon-based filler alloys such as 4043 are not generally recommended for these alloys because the excess Si introduced by the filler alloy can result in the formation of excessive amounts of brittle Mg2Si particles in the weld.
Al-Zn-Mg-Cu Alloys such as 7075 have small amounts Cu added.  The small amounts of Cu, along with the Mg, extend the coherence range and therefore increase the crack sensitivity.  A similar situation can occur with these materials as with the 2024-type alloys. The stress of solidification may cause cracking at the grain boundaries and/or establish the condition within the material conducive to stress corrosion cracking later. 

It should be stressed that the problem of higher susceptibility to hot cracking from increasing the coherence range is not only confined to the welding of these more susceptible base alloys such as 2024 and 7075.  Crack sensitivity can be substantially increased when welding incompatible dissimilar base alloys (which are normally easily welded to themselves) and/or through selection of an incompatible filler alloy.  For example; by joining a perfectly weldable 2xxx series base alloy to a perfectly weldable 5xxx series base alloy, or by using a 5xxx series filler alloy to weld a 2xxx series base alloy, or a 2xxx series filler alloy on a 5xxx series base alloy, we can create the same scenario.   If we mix the high Cu and high Mg we can extend the coherence range and therefore increase the crack sensitivity.

4. GROOVE WELD TENSILE STRENGTH

Typically, the HAZ of the groove weld indicates the strength of the joint, and many filler alloys can satisfy this strength requirement. However, two factors should be considered when developing welding procedures for the nonheat-treatable and the heat-treatable alloys.
For non-heat-treatable alloys, the area adjacent to the weld will be annealed completely. These alloys are annealed by heating to 600 to 700 degrees F for a short time. The welding procedure has little effect on the transverse ultimate tensile strength of the groove weld because the annealed HAZ typically is the weakest area of the joint. Welding procedure qualification 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 a longer time to fully reduce its strength. This typically does not occur during welding, and the strength of the HAZ will only be reduced partially. The amount of strength loss is both time- and temperature-related in these alloys. The faster the welding process and heat dissipation, the smaller the HAZ will be and the higher the as-welded strength will be.
Excessive preheating, lack of interpass cooling, and excessive heat input all increase peak temperature and time at temperature. These factors alone or the use of too small a specimen to provide adequate heat sink can create overheating so that minimum strength values are not met.
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.

In structural applications, the selection between 5xxx series filler or 4xxx series filler may not be so significant regarding groove welds’ tensile strength. However, this may not be the case when considering the shear strength of fillet welds.

Typically, 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 required a 1/4-inch fillet weld with 5556 filler, compared to a 7/16 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.

5. DUCTILITY

Ductility is a property that describes 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 and thereby decrease 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 and 6xxx series base alloys have relatively low ductility. This is addressed with special requirements within the code or standard relating to test sample thickness, bending radius and/or material condition.

6. CORROSION RESISTANCE

Most unprotected aluminum base filler alloy combinations are satisfactory for general exposure to the atmosphere. When a dissimilar aluminum alloy combination of base and filler is used and 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. When welding in a specialized highly corrosive environment, it may be necessary to consult an engineer with experience within this field.

7. SERVICE TEMPERATURE

Stress corrosion cracking is a condition that can result in premature weld failure. This condition can be developed through magnesium segregation at the grain boundaries of the material. This typically will only occur in alloys of more than 3 percent magnesium when exposed to prolonged elevated temperature (above 150 degrees F).

The 5356, 5183, 5654, and 5556 filler alloys all contain more than 3 percent Mg (typically around 5 percent). Therefore, they are not suitable for elevated temperature service. Alloy 5554 has less than 3 percent Mg 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 service temperature applications depending on weld performance requirements.

8. POST-WELD HEAT TREATMENT

Typically, the common heat-treatable base alloys, such as 6061-T6, lose a substantial proportion 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.
Consequently, on occasion, it’s desirable to perform post-weld heat treatment to return the mechanical strength to the manufactured component. When post-weld heat treating, the filler alloy’s ability to respond to the heat treatment should be evaluated.

Most of the commonly used filler alloys will not respond to post-weld heat treatment without substantial 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 independently respond to heat treatment.

Filler alloy 4643 was developed for welding 6xxx series base alloys and developing high mechanical properties in the post-weld heat-treated condition. It was developed by taking the well-known alloy 4043, reducing the silicon, and adding 0.10 to 0.30 percent magnesium.
Filler alloy 5180 was developed for welding 7xxx series base alloys. It falls within the Al/Zn/Mg alloy family and responds well to post-weld thermal treatments. This alloy is used to weld 7005 bicycle frames and responds to heat treatment without dilution of the thin-walled tubing used for this high-performance application.

A number of other heat-treatable filler alloys are available for welding some of the heat treatable cast aluminum alloys.

CONCLUSIONS

Selecting the most suitable filler alloy can only be achieved successfully after a full analysis of the many variables associated with welding of aluminum components and their applications.  Firstly consideration must be given to the type and chemistry of the base material to be welded, and secondly consideration of the welded components performance requirements. Filler alloy selection for welding aluminum is an essential part of the development and qualification of a suitable welding procedure specification.

 

Fig 1.  Crack sensitivity of various elements in aluminum.
 

Fig 2.  Gas Metal Arc Welding on an aluminum wheel.
This application often uses a 5454 base material welded with an ER5554 filler alloy that is made specifically to match the base alloy content.

Fig 3.  These automotive components are often welded with 4047 filler alloy.  Taking advantage of this 12% silicon alloy's ability to produce welds that are extremely fluid and easily create leak-tight joints.  The 4047 filler alloy is also suitable for temperature applications.