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Aluminum Filler Alloy Selection

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Let's talk about aluminum welding and the phase of it that's essential to the making of good welds; the choice of filler metal, and the selection of the correct alloy to use as filler metal.

Are all aluminum alloys suitable as filler metal? Are all aluminum alloys weldable? What alloy characteristics should the welder look for? What pitfalls should he avoid? These are some of the questions we will answer as we consider the subject of "Alloy Selection."

Let's begin by stating that most aluminum alloys are weldable and that in choosing a filler alloy the alloys composition is much more important than its form. Other critical factors are: the end use of the weld and the performance expected of it. Also, although many alloy combinations can be welded using any of several filler alloys, only one filler alloy may be optimum for a specific application. In any event, determination of the best filler alloy to use in a given situation should be made only after thorough analysis of the final performance desired from the weld.

Now, we'd like to review several basic principles of aluminum metallurgy. You will recall that because of their composition, some alloys cannot be heat treated and some can.

The non-heat treatables, which have numbers beginning with 1, 3, 4 and 5, can generally, be welded with filler alloy and base alloy of the same basic composition. The heat treatables, which have numbers beginning with 2, 6 and 7, are more sensitive to what we call "hot-short cracking." As a result, a dissimilar filler alloy normally is used, often with a lower melting temperature than the base alloy.

How do you choose the correct filler alloy? What are the criteria? There are six. First, ease of welding or freedom from cracking; second, strength; third, ductility; fourth, corrosion resistance; fifth, service at sustained temperatures above 66° C. (150° F.); and sixth, color match of filler and base alloy after anodizing.

A look now at each of these criteria in greater detail:

W - Ease of Welding

The first standard for choosing correct filler alloy is ease of welding, or freedom from cracking. We previously stated that the non-heat treatable alloys are less sensitive to cracking than the heat treatables. It follows that the non-heat treatables will be easier to weld. In heat treatable alloys, where filler and base alloy must usually be dissimilar, the choice of correct filler requires careful consideration. Two tests have been used to determine the compatibility of the filler alloy with the heat treatable base alloy. The first is a simple continuous fillet weld test. If that shows no cracking, one can proceed to the more sensitive discontinuous weld test. Laboratory tests and practical experience confirm the following conclusions about the weldability of aluminum alloys: The high purity 1000 Series Alloys and Alloy 3003 are easy to weld with 1100 or 4043 filler alloys. High purity alloys can be welded with base alloy filler if desired. Alloys possessing from 1% to 2.5% magnesium, such as 3004 and 5052, are very sensitive to cracking when using base alloy filler. Alloys with 3.5% magnesium and more exhibit low sensitivity to weld cracking. The use of a 5% magnesium content filler, such as 5356, to weld 5052 can provide a higher magnesium percentage in the diluted weld metal and reduce sensitivity to weld cracking. The more magnesium an aluminum magnesium alloy contains, the less likely it is to crack; thus, the high magnesium content fillers 5356, 5183, and 5556 are commonly used to weld both wrought and cast aluminum magnesium base alloys.

The 6000 Series Alloys, which are heat treatable, all exhibit the same relative weldability. If welded with base alloy filler, all 6000 Series Alloys are very sensitive to weld cracking. These alloys must not be fused without a dissimilar alloy filler addition unless a compressive load can be applied to the weld. A compressive load applied by the clamping of two drawn beer barrel paths in a lathe can be used to overcome the welding solidification shrinkage stresses to permit welding without a filler addition. The 6000 Series Alloys can be welded with high magnesium content 5000 Series Fillers, or high silicon content 4000 Series Aluminum Filler Alloys.

The aluminum silicon alloys provide the least sensitivity to weld cracking and with either filler type provision must be made in the joint design for adequate filler dilution.

For any crack sensitive aluminum alloy it is preferred that the weld metal composition consist of at least 70% filler alloy. Reduced percentages of filler, as obtained in square-butt joints, generally result in cracking during weld solidification.

The 2000 Series Aluminum Copper Alloys are heat treatable alloys and exhibit a wide range of sensitivity to weld cracking. Alloy 2219 has the best weldability and can be welded with heat treatable filler alloy 2319, as well as with the aluminum silicon filler alloys. Alloys 2014 and 2036 exhibit fair welding characteristics with these fillers, but their lower melting points and wider melting range make them more sensitive to hot­short cracking in the weld transition zone. Alloy 2024 is a very poor choice for welding and is highly sensitive to cracking with standard filler alloys.  AlcoTec recommends that alloy 2024 not be welded due to its sensitivity to stress corrosion cracking.

Filler alloy 4145, a non-standard alloy, which is an aluminum silicon copper alloy with the lowest melting point of all the aluminum filler alloys, produces least weld cracking when used with base alloys 2014 and 2036, as well as with the 300 Series Aluminum Silicon Copper Casting Alloys.

Aluminum zinc alloys without copper, such as 7005, resist weld cracking better and provide better joint performance than aluminum zinc alloys with copper, such as 7075. The addition of copper reduces the melting point and increases the melting range so as to increase the sensitivity to hot-short cracking. Alloy 7075 is not suitable for arc welding.

S - Strength

The second standard for choosing correct filler alloy is strength. We know that the heat of welding softens aluminum alloys in the heat affected zone adjacent to the weld. In most butt welds, therefore, the heat affected zone of the base alloy dictates the strength of the joint. Often many filler metals can satisfy this strength requirement. On the other hand, the strength of a fillet weld depends on the composition of the filler alloy and the size of the fillet. The high magnesium content filler alloys produce the highest shear strength for fillet welds, while pure aluminum filler 1100 provides the lowest shear strength.

Since several fillers will develop the transverse butt weld strength for a base alloy combination, the strength rating comparisons used in filler selection guides are based upon the shear strength developed. If the filler does not develop the required butt weld tensile strength, it is not rated.

Since longitudinal shear strength values are lower than the transverse shear, the longitudinal values are used in establishing minimum allowables. Let's take a look at what choice of filler can mean. If 5,000 pounds per inch shear strength is required in welding 6061 alloy, either 4043 or 5556 fillers can be used. A single pass 1/4 inch fillet size is adequate with 5556, whereas a 7/16 inch fillet size is required with 4043. The 7/16 fillet will likely require three weld passes, so as to increase labor costs compared to the single pass 1/4-inch weld with 5556. In addition, the increased fillet size creates more weld solidification shrinkage to cause increased weld distortion and the possible need for a straightening operation. A few cents per pound extra paid for the stronger filler could save dollars in actual fabricating costs.

We'd like to call your attention to another important factor which can affect the weld joint strength. Non-heat treatable and heat treatable alloys react differently to the welding process. As you know, when you apply a torch or arc heat to aluminum, you create a heat affected zone, a small area on both sides of the weld in which the metal loses some strength. In a non-heat treatable alloy, the hard, or H-temper originally produced by cold rolling, is changed by the heat of the welding process to an annealed or 0-temper. These alloys are annealed by heating to 650°-700° F. and the time at temperature is unimportant, thus the portion adjacent to the weld which is heated to the annealing temperature is completely softened.

Several hours at annealing temperature are required to fully soften heat treatable alloys. When welding a heat treatable alloy, the heat affected zone is only partially annealed. The degree of annealing effect is both time and temperature dependent in these alloys. The faster the welding method and heat dissipation from the parts, the less the heat effect and the higher the as-welded strength. Automatic welding in a hold down fixture can provide both rapid travel speed and rapid chilling by clamping to produce high as-welded strength in heat treatable alloys. In either situation, the width of the heat affected zone depends on the welding process used, the speed of welding, and the type of fixturing, or backing, to dissipate the heat.

D - Ductility

The third criterion for choosing filler alloy is ductility. Non-heat treatable aluminum alloys exhibit excellent weld ductility when using like filler alloys. Highest ductility, on the order of 60% elongation, is observed with relatively pure aluminum alloys. The 5000 Series Alloys are also very ductile and exhibit 30%-40% weld elongation when welded with 5000 Series Fillers. This ductility is apparent when conducting guided bend tests to qualify welding procedures and operators. Welds in heat treatable alloys generally do not produce as much ductility as in the non-heat treatables. The 6000 Series Alloys can be welded with aluminum silicon or an aluminum magnesium alloy filler and exhibit medium ductility. The 5000 Series Fillers provide 50% more ductility than obtained with 4043 in the as-welded condition. When welding the 2000 Series Alloys, the aluminum copper filler alloy 2319 provides good ductility, twice that obtained with 4043. Excellent weld ductility results when welding the low copper content 7000 Series Alloys with the high magnesium content 5000 Series Filler Alloy. The heat treatable alloys can be resolution heat treated and aged after welding. Although strengths can be improved substantially, the weld ductility decreases as a result of post weld heat treatments.

C - Corrosion Resistance

The fourth basis for choosing a filler alloy is corrosion resistance. Most unprotected aluminum base alloy filler metal combinations are quite satisfactory for general exposure to the atmosphere. In cases where a dissimilar aluminum alloy combination of base and filler is used, and an electrolyte is present, it is possible to set up a galvanic action between the dissimilar compositions. The alloy which is anodic within the system will sacrifice itself to protect the other. The greater the difference in the solution potential of the alloys and the smaller the anode is in relation to the size of the cathodic member, the more rapid the sacrificial pitting type corrosive attack will be on the anode. Thus, a narrow weld of a highly anodic weld metal would be undesirable for immersed service.

Relative relationships of the aluminum alloy systems in water show the aluminum zinc 7000 Series Alloys to be the most anodic, while the aluminum copper 2000 Series Alloys are shown to be the most cathodic. Applying this to specific base alloy filler metal combinations can be important in selecting the proper filler alloy for immersed service. For instance, 4043 filler in a salt water solution exhibits the same potential as 6061 alloy, so no corrosive action would be expected. Filler alloy 5356 would be slightly anodic to 6061 alloy, so the weld would sacrifice itself to protect the base alloy over an extended period of time of immersed service. In contrast, 5356 welds in 7005 based alloy would be protected by the base material and the corrosion would not be as concentrated when a large surface protects the small anode. The difference in alloy performance can vary based upon the type of exposure. Filler alloy selection chart ratings are based on fresh or salt water only. For service other than fresh or salt water, contact AlcoTec. Specific chemical exposures may require special aluminum alloys for maximum performance. In other words, it may be desirable to control the level of impurities in an alloy if it is to be exposed to a specific chemical. This could apply to the filler metal as well as the base alloy. For instance, alloy 5654 has impurity controls for hydrogen peroxide service.

T - Service at Elevated Temperatures

The fifth criterion for choosing filler alloy is service at sustained temperatures above 66° C. (150° F.). Such temperatures are not recommended for 5000 Series Aluminum Magnesium Filler, or base alloys containing 3% or more magnesium. These temperatures can produce stress corrosion cracking, thus the high magnesium content filler alloys 5356, 5183, 5654 and 5556 are not suitable for sustained elevated temperature service. 5554 filler, with less than 3% magnesium content is suitable for elevated temperatures. The colder aluminum is, the stronger it becomes with no loss in ductility. All aluminum filler alloys are acceptable for cryogenic temperatures.

C - Color Match

The sixth and final basis for choosing correct filler alloy is color match. The color of an aluminum alloy when anodized or exposed to various environ­ments depends on its composition. A filler alloy should possess the same composition as the base alloy for best color match. Silicon in aluminum causes a darkening of the alloy when chemically treated. If 5% silicon alloy 4043 filler is used to weld 6063, and the assembly is anodized, the weld becomes black and is very apparent. A similar weld in 6063 with 5356 filler does not discolor during anodizing, so a good color match is obtained. Other elements in an alloy can also produce slight color changes. For instance, chromium can create a yellow or gold color shading. Thus, for best color match when anodizing, matching of significant elements in the filler and base alloys is desired.

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.  Alloy 6061-T6 has typically 45,000 PSI tensile strength prior to welding and typically 27,000 PSI in the as-welded condition.  Consequently, on occasion it is desirable to perform post weld heat treatment to return the mechanical strength to the manufactured component.  If post weld heat treatment is the option, it is necessary to evaluate the filler alloy used with regards to its ability to respond to the heat treatment.  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, there are some special filler alloys which have been developed to provide a heat treatable filler alloy which guarantees that the weld will respond to the heat treatment.  Filler alloy 4643 was developed for welding the 6xxx series base alloys and developing high mechanical properties in the post weld heat-treated condition.  This filler alloy was developed by taking the well-known alloy 4043 and reducing the silicon and adding 0.10 to 0.30 % magnesium.  This chemistry introduces Mg2Si into the weld metal and provides a weld that will respond to heat treatment.

Heat treatable filler alloys have been developed including 2319, 4009, 4010, 4145, 206.0, A356.0, C355.0 and 357.0 for the welding of heat treatable wrought and cast aluminum alloys.

Using the Filler Alloy Charts

To assist in selecting the proper filler alloy based upon the various criteria discussed, AlcoTec has assembled guides for choice of filler for the common base alloys. These guides are available in chart and slide rule form. Understanding and making use of the various aluminum filler alloy charts is simple once characteristic and rating symbols are explained.

The letters "W", "S", "D", "C", "T", and "M" listed at the top of the chart represent six filler alloy characteristics. "W" stands for ease of welding or freedom from weld cracking; "S" - strength of the weld; "D" - ductility; "C" - corrosion resistance in fresh or salt water; "T" - suitability for service at sustained temperatures above 66° C. (150° F.); "M" - color match after anodizing.

The letters "A", "B", "C", and "D" which appear in the body of the chart represent relative ratings of the characteristics at the top of the chart within each block. "A"is excellent; and "D" is acceptable, with "B" and "C" ranging somewhere in between.

Filler alloy combinations having no rating are not recommended. It should also be noted that the ratings do not cover the alloys when heat treated after welding.

Now that you have an understanding of the symbols, here is how to read the chart. First, select the base alloys to be joined -- one from the blue row at the top of the chart and another from the blue column on the left side of the chart. After locating the base alloy in the side column, move right across the chart until you reach the block directly under the base alloy in the top row. This intersecting block contains horizontal lines of letters, A-D, which represent the characteristic ratings of the filler alloys listed in the gray column down the left side of the chart. Again, the ratings correspond with the characteristics listed at the top of the columns. Let's take an example. Suppose you wanted to join base alloys 6061 and 3004 and you need to know what filler alloy would be most suitable in a given situation. First, find the intersecting block for base alloys 6061 and 3004. You will note that the first filler in the block, 4043, has three "A" excellent ratings, but also has a "C" rating and one "D" rating. When those ratings are matched with the corresponding characteristic symbols above, you will see that the filler alloy 4043 is the least sensitive to weld cracking, provides the best resistance to corrosion and meets the criteria for service at sustained temperatures above 150° F. However, the strength of the weld is the poorest compared to other listed filler alloys. Its ductility is poorer than 5356, 5556, and 5183, but better than 4145. If corrosion resistance is of utmost importance, filler alloy 4043 might be a good choice, but if ductility is critical, a better selection would be 5356 with an excellent rating as far as ductility is concerned.

Summary

We have seen that most aluminum alloys are weldable and that non-heat treatable alloys use similar filler alloys, while the heat treatables use dissimilar alloys. We have discussed the six criteria for choosing fillers. First, ease of welding, which is based on relative freedom from weld cracking; second, strength of the weld joint based on shear strength of fillet weld; third, ductility based on free bend elongation of the weld metal; fourth, corrosion resistance in a fresh or salt water environment; fifth, service at sustained temperatures above 66° C. (150° F.); and the sixth, color match of the weld relative to the base material after anodic treatment. Be certain to obtain the answers to all six criteria before recommending a filler alloy. In today's environment, we need to be especially careful.

This brief look at the criteria behind the ratings listed in the filler chart should allow you to better select the correct filler alloy for your application.

Conclusion

It must be concluded that final determination of the most suitable filler alloy can only be made after a full analysis of the welded components performance requirements.  Filler alloy selection for welding aluminum is an essential part of the development and qualification of a successful welding procedure qualification.