Learning about the alloys
June 30, 2009
The first step in learning about welding aluminum is learning about the various alloys themselves (cast and wrought)—the digits used in the aluminum designation system, the properties of the various alloys, and the considerations in matching a filler metal to a base material.
Selecting an appropriate filler alloy for welding aluminum differs from selecting a filler alloy for welding steels. In the case of steel, selection is primarily a matter of matching the tensile strength of the filler metal to that of the base alloy. For aluminum, many variables come into play, such as ease of welding, crack resistance, ductility, weld strength, corrosion resistance, service temperature, and the effect of postweld heat treatment. Some of these factors may be as important as, or more important than, tensile strength.
One of the first steps to successful aluminum welding, therefore, is to become acquainted with the many aluminum alloys, their characteristics, and the aluminum identification system. Understanding this system will help you to correlate the proper alloy with the service conditions of the finished component.
Aluminum Alloy Temper and Designation System In North America the Aluminum Association Inc. is responsible for the allocation and registration of aluminum alloys. It recognizes more than 600 aluminum alloys—more than 400 wrought and more than 200 in the form of castings and ingots. The chemical composition limits for these alloys are contained in the Aluminum Association's reference publications:
Aluminum alloys are grouped by the particular material's characteristics, such as its ability to respond to thermal and mechanical treatment and the primary alloying element.
In the four-digit wrought aluminum alloy identification system, the first digit indicates the principal alloying element (see Figure 1).
The second digit, if different from 0, indicates a modification of the specific alloy, and the third and fourth digits are arbitrary numbers given to identify a specific alloy in the series. For example, in alloy 5183, the number 5 indicates that it belongs in the magnesium series, the 1 indicates that it is the first modification to the original alloy 5083, and the 83 identifies the particular alloy in the 5xxx series.
The only exception to this numbering system is in the 1xxx series, commonly called pure aluminums because they are more than 99 percent aluminum. In this series, the last two digits provide the minimum aluminum percentage above 99 percent. For example, alloy 1350 contains a minimum of 99.50 percent aluminum.
The cast alloy designation system is based on three digits to the left of the decimal point and one to the right. The first digit indicates the principal alloying element (see Figure 2). The second and third digits are arbitrary numbers given to identify a specific alloy in the series.
The number following the decimal point indicates whether the alloy is a casting (.0) or an ingot (.1 or .2). A capital letter prefix indicates a modification to a specific alloy.
For example, in alloy A356.0, the capital A indicates a modification of alloy 356.0. The number 3 indicates that it is of the silicon plus copper and/or magnesium series. The 56 identifies the particular alloy within the 3xx.x series, and the .0 indicates that it is a final-shape casting and not an ingot.
Two distinctly different types of aluminum exist within the wrought and cast series. These are heat-treatable (those that can gain strength through the addition of heat) and non-heat-treatable alloys. This distinction can be important when considering the effects of arc welding on these materials.
The 1xxx, 3xxx, and 5xxx series wrought aluminum alloys are non-heat-treatable and are strain-hardenable only. The 2xxx, 6xxx, and 7xxx series wrought aluminum alloys are heat-treatable, and the 4xxx series consists of both heat-treatable and non-heat-treatable alloys. The 2xx.x, 3xx.x, 4xx.x, and 7xx.x series cast alloys are heat-treatable. Strain hardening generally is not applied to castings.
The heat-treatable alloys acquire optimum mechanical properties through thermal treatment, the most common being solution heat treatment and artificial aging. In solution heat treatment, the alloy is heated to an elevated temperature (around 990 degrees F) to put the alloying elements or compounds into solution. This is followed by quenching, usually in water, to produce a supersaturated solution at room temperature.
Solution heat treatment is usually followed by aging. Aging is the precipitation of a portion of the elements or compounds from the supersaturated solution in order to yield desirable properties. The aging process is divided into two types: aging at room temperature, which is called natural aging, and aging at elevated temperatures, about 320 degrees F, which is called artificial aging. Many heat-treatable aluminum alloys are used for welding fabrication in their solution heat-treated and artificially aged condition.
The non-heat-treatable alloys acquire optimum mechanical properties through strain hardening, which is induced by cold working.
The temper designation system addresses these material conditions through an extension of the alloy numbering system. This consists of a series of letters and numbers following the alloy designation number and connected by a hyphen. Examples include 6061-T6, 6063-T4, 5052-H32, and 5083-H112 (see Figure 3).
Tempers are further identified with a suffix containing a letter H or a letter T. A letter H indicates strain hardening and is followed by digits:
The first digit after the H indicates a basic operation:
The second digit after the H indicates the degree of strain hardening:
A letter T indicates thermal treatment; the T is followed by digits, which indicate a basic operation or a sequence of operations:
Additional digits indicate stress relief, such as Tx51 or Txx51 (stress relieved by stretching) or Tx52 or Txx52 (stress relieved by compressing).
The main differentiator among the various series is the alloying element or elements that, in turn, influence many of the series' characteristics.
1xxx. This series is non-heat-treatable and has an ultimate tensile strength from 10,000 pounds per square inch (PSI) to 27,000 PSI. They are weldable, but because of their narrow melting range, specialized welding procedures are necessary to produce acceptable welds. Their superior corrosion resistance makes them suitable in specialized chemical tanks and piping; their excellent electrical conductivity makes them suitable for busbar applications. They have relatively poor mechanical properties and are rarely used for general structural applications. These base alloys often are welded with matching filler material or with 4xxx filler alloys, depending on the application and performance requirements.
2xxx. This series is heat-treatable; the ultimate tensile strength range is from 27,000 PSI to 62,000 PSI. They have copper content from 0.7 to 6.8 percent. They are high-strength, high-performance alloys often used for aerospace and aircraft applications. They offer excellent strength over a wide range of temperatures.
Some are considered nonweldable by arc welding processes because of their susceptibility to hot cracking and stress corrosion cracking; others are arc welded successfully with the correct welding procedures. These base materials often can be welded with high-strength 2xxx series filler alloys designed to match their performance, but in some cases they can be welded with 4xxx series fillers containing silicon or a combination of silicon and copper, depending on the application and service requirements.
3xxx. These are non-heat-treatable alloys with ultimate tensile strength of 16,000 PSI to 41,000 PSI. The main alloying element is manganese, which varies from 0.05 to 1.8 percent. They have moderate strength, good corrosion resistance, good formability, and are suited for use at elevated temperatures. One of their first uses was pots and pans, and they are the major component today for heat exchangers in vehicles and power plants. Their moderate strength generally makes them unsuitable for structural applications. These base alloys are welded with 1xxx, 4xxx, and 5xxx series filler alloys, depending on their specific chemistry and particular application and service requirements.
4xxx. This series consists of both heat-treatable and non-heat-treatable alloys. The ultimate tensile strength varies from 25,000 to 55,000 PSI. They have silicon in amounts that vary from 0.6 to 21.5 percent. The silicon reduces the melting point and improves fluidity when molten. These characteristics are desirable for filler materials used for fusion welding and brazing; consequently, this series of alloys predominantly is used as filler material. Silicon by itself makes aluminum non-heat-treatable; however, adding magnesium or copper results in a heat-treatable alloy. Typically, these heat-treatable filler alloys are used only when a welded component is to be subjected to postweld thermal treatments.
5xxx. This non-heat-treatable series has an ultimate tensile strength of 18,000 to 51,000 PSI. They have magnesium additions from 0.2 to 6.2 percent. These have the highest strength of the non-heat-treatable alloys. In addition, these alloys are readily weldable and therefore are used for a variety of applications such as shipbuilding, transportation, pressure vessels, bridges, and buildings. Base alloys with less than approximately 2.5 percent magnesium often are welded successfully with the 5xxx or 4xxx series filler alloys. The base alloy 5052 generally is recognized as the maximum magnesium content base alloy that can be welded with a 4xxx series filler alloy. Because of problems associated with eutectic melting and the resulting poor as-welded mechanical properties, materials in this series at the high end of the magnesium scale should not be welded with 4xxx series fillers; 5xxx filler alloys that generally match the base alloy composition are suitable for these metals.
6xxx. These heat-treatable metals have an ultimate tensile strength of 18,000 PSI to 58,000 PSI. They contain a small amount of magnesium and silicon—around 1.0 percent. They are used widely throughout the welding fabrication industry, predominantly in the form of extrusions, and incorporated in many structural components. Solution heat treatment improves their strength. These alloys are solidification crack-sensitive, and for this reason should not be arc welded autogenously (without filler material). The filler metal dilutes the base material, thereby preventing hot cracking. They are welded with both 4xxx and 5xxx filler materials, depending on the application and service requirements.
7xxx. These heat-treatable alloys have an ultimate tensile strength from 32,000 PSI to 88,000 PSI. The main alloying element is zinc in amounts from 0.8 to 12.0 percent. They comprise some of the highest-strength aluminum alloys and often are used in high-performance applications such as aircraft, aerospace, and competitive sporting equipment. Like the 2xxx alloys, this series incorporates some alloys that are considered unsuitable candidates for arc welding and others that often are arc welded successfully. The commonly welded alloys in this series, such as 7005, are welded predominantly with the 5xxx series filler alloys.