Technical knowledge

Understanding the Alloys of Aluminum

With the growth of aluminum within the casting and cast parts fabrication industry, there are increasing requirements for those involved with developing aluminum projects to become more familiar with this group of materials. To fully understand aluminum, it is advisable to start by becoming acquainted with the aluminum identification / designation system, the many aluminum alloys available and their characteristics.

The Aluminum Alloy Temper and Designation System 

 In North America, The Aluminum Association Inc. is responsible for the allocation and registration of aluminum alloys. Currently there are over 400 wrought aluminum and wrought aluminum alloys and over 200 aluminum alloys in the form of castings and ingots registered with the Aluminum Association. The alloy chemical composition limits for all of these registered alloys are contained in the Aluminum Association’s Teal Book entitled “International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys” and in their Pink Book entitled “Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot. These publications can be extremely useful to the welding engineer when developing welding procedures, and when the consideration of chemistry and its association with crack sensitivity is of importance.

Aluminum alloys can be categorized into a number of groups based on the particular material’s characteristics such as its ability to respond to thermal and mechanical treatment and the primary alloying element added to the aluminum alloy. When we consider the numbering / identification system used for aluminum alloys, the above characteristics are identified. The wrought and cast aluminums have different systems of identification. The wrought system is a 4-digit system and the castings having a 3-digit and 1-decimal place system.

Wrought Alloy Designation System 

We shall first consider the 4-digit wrought aluminum alloy identification system. The first digit (Xxxx) indicates the principal alloying element, which has been added to the aluminum alloy and is often used to describe the aluminum alloy series, i.e., 1000 series, 2000 series, 3000 series, up to 8000 series (see table 1).

The second single digit (xXxx), if different from 0, indicates a modification of the specific alloy, and the third and fourth digits (xxXX) are arbitrary numbers given to identify a specific alloy in the series. Example: In alloy 5183, the number 5 indicates that it is of the magnesium alloy series, the 1 indicates that it is the 1^st modification to the original alloy 5083, and the 83 identifies it in the 5xxx series.

The only exception to this alloy numbering system is with the 1xxx series aluminum alloys (pure aluminums) in which case, the last 2 digits provide the minimum aluminum percentage above 99%, i.e., Alloy 13(50) (99.50% minimum aluminum).

WROUGHT ALUMINUM ALLOYS DESICNATION SYSTEM

Table 1

Cast Alloy Designation – The cast alloy designation system is based on a 3 digit- plus decimal designation xxx.x (i.e. 356.0). The first digit (Xxx.x) indicates the principal alloying elements, which has been added to the aluminum alloy (see table 2).

CAST ALUMINUM ALLOY DESIGNATION SYSTEM

Table 2

The second and third digits (xXX.x) 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  

Example: Alloy – A356.0 the capital A (Axxx.x) indicates a modification of alloy 356.0. The number 3 (A3xx.x) indicates that it is of the silicon plus copper and/or magnesium series. The 56 in (Ax56.0) identifies the alloy within the 3xx.x series, and the .0 (Axxx.0) indicates that it is a final shape casting and not an ingot.

The Aluminum Temper Designation System – If we consider the different series of aluminum alloys, we will see that there are considerable differences in their characteristics and consequent application. The first point to recognize, after understanding the identification system, is that there are two distinctly different types of aluminum within the series mentioned above. These are the Heat Treatable Aluminum alloys (those which can gain strength through the addition of heat) and the Non-Heat Treatable Aluminum alloys. This distinction is particularly important when considering the effects of arc welding on these two types of 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 consist 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 is not generally applied to castings.

The heat treatable alloys acquire their optimum mechanical properties through a process of thermal treatment, the most common thermal treatments being Solution Heat Treatment and Artificial Aging. Solution Heat Treatment is the process of heating the alloy to an elevated temperature (around 990 Deg. F) in order 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 a supersaturated solution in order to yield desirable properties.

The non-heat treatable alloys acquire their optimum mechanical properties through Strain Hardening. Strain hardening is the method of increasing strength through the application of cold working.T6, 6063-T4, 5052-H32, 5083-H112.

THE BASIC TEMPER DESIGNATIONS

Table 3

Further to the basic temper designation, there are two subdivision categories, one addressing the “H” Temper – Strain Hardening, and the other addressing the “T” Temper – Thermally Treated designation.

Subdivisions of H Temper – Strain Hardened

The first digit after the H indicates a basic operation:
H1–Strain Hardened Only.
H2 – Strain Hardened and Partially Annealed.
H3 – Strain Hardened and Stabilized.
H4 – Strain Hardened and Lacquered or Painted.

The second digit after the H indicates the degree of strain hardening:
HX2 – Quarter Hard          HX4 – Half Hard          HX6 – Three-Quarters Hard
HX8 – Full Hard                HX9 – Extra Hard

Subdivisions of T Temper – Thermally Treated

T1 –     Naturally aged after cooling from an elevated temperature shaping process, such as extruding.
T2 –     Cold worked after cooling from an elevated temperature shaping process and then naturally aged.
T3 –     Solution heat-treated, cold worked and naturally aged.
T4 –     Solution heat-treated and naturally aged.
T5 –     Artificially aged after cooling from an elevated temperature shaping process.
T6 –     Solution heat-treated and artificially aged.
T7 –     Solution heat-treated and stabilized (overaged).
T8 –     Solution heat-treated, cold worked and artificially aged.
T9 –     Solution heat treated, artificially aged and cold worked.
T10 –   Cold worked after cooling from an elevated temperature shaping process and then artificially aged.

Additional digits indicate stress relief.
Examples:
TX51 or TXX51 – Stress relieved by stretching.
TX52 or TXX52 – Stress relieved by compressing.

Aluminum Alloys And Their Characteristics 

 If we consider the seven series of wrought aluminum alloys, we will appreciate their differences and understand their applications and characteristics.

1xxx Series Alloys – (non-heat treatable – with ultimate tensile strength of 10 to 27 ksi) this series is often referred to as the pure aluminum series because it is required to have 99.0% minimum aluminum. They are weldable. However, because of their narrow melting range, they require certain considerations in order to produce acceptable welding procedures. When considered for fabrication, these alloys are selected primarily for their superior corrosion resistance such as in specialized chemical tanks and piping, or for their excellent electrical conductivity as in bus bar applications. These alloys have relatively poor mechanical properties and would seldom be considered for general structural applications. These base alloys are often welded with matching filler material or with 4xxx filler alloys dependent on application and performance requirements.

2xxx Series Alloys – (heat treatable– with ultimate tensile strength of 27 to 62 ksi) these are aluminum / copper alloys (copper additions ranging from 0.7 to 6.8%), and are high strength, high performance alloys that are often used for aerospace and aircraft applications. They have excellent strength over a wide range of temperature. Some of these alloys are considered non-weldable by the arc welding processes because of their susceptibility to hot cracking and stress corrosion cracking; however, others are arc welded very successfully with the correct welding procedures. These base materials are often welded with high strength 2xxx series filler alloys designed to match their performance, but can sometimes be welded with the 4xxx series fillers containing silicon or silicon and copper, dependent on the application and service requirements.

3xxx Series Alloys – (non-heat treatable – with ultimate tensile strength of 16 to 41 ksi) These are the aluminum / manganese alloys (manganese additions ranging from 0.05 to 1.8%) and are of moderate strength, have 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, however, often precludes their consideration for structural applications. These base alloys are welded with 1xxx, 4xxx and 5xxx series filler alloys, dependent on their specific chemistry and particular application and service requirements.

4xxx Series Alloys – (heat treatable and non-heat treatable – with ultimate tensile strength of 25 to 55 ksi) these are the aluminum / silicon alloys (silicon additions ranging from 0.6 to 21.5%) and are the only series that contain both heat treatable and non-heat treatable alloys. Silicon, when added to aluminum, reduces its melting point and improves its fluidity when molten. These characteristics are desirable for filler materials used for both fusion welding and brazing. Consequently, this series of alloys is predominantly found as filler material. Silicon, independently in aluminum, is non-heat treatable; however, a number of these silicon alloys have been designed to have additions of magnesium or copper, which provides them with the ability to respond favorably to solution heat treatment. Typically, these heat treatable filler alloys are used only when a welded component is to be subjected to post weld thermal treatments.

5xxx Series Alloys – (non-heat treatable – with ultimate tensile strength of 18 to 51 ksi) These are the aluminum / magnesium alloys (magnesium additions ranging from 0.2 to 6.2%) and have the highest strength of the non-heat treatable alloys. In addition, this alloy series is readily weldable, and for these reasons they are used for a wide variety of applications such as shipbuilding, transportation, pressure vessels, bridges and buildings. The magnesium base alloys are often welded with filler alloys, which are selected after consideration of the magnesium content of the base material, and the application and service conditions of the welded component. Alloys in this series with more than 3.0% magnesium are not recommended for elevated temperature service above 150 deg F because of their potential for sensitization and subsequent susceptibility to stress corrosion cracking. Base alloys with less than approximately 2.5% magnesium are often welded successfully with the 5xxx or 4xxx series filler alloys. The base alloy 5052 is generally 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 associated poor as-welded mechanical properties, it is not recommended to weld material in this alloy series, which contain higher amounts of magnesium with the 4xxx series fillers. The higher magnesium base materials are only welded with 5xxx filler alloys, which generally match the base alloy composition.

6XXX Series Alloys – (heat treatable – with ultimate tensile strength of 18 to 58 ksi) These are the aluminum / magnesium – silicon alloys (magnesium and silicon additions of around 1.0%) and are found widely throughout the welding fabrication industry, used predominantly in the form of extrusions, and incorporated in many structural components. The addition of magnesium and silicon to aluminum produces a compound of magnesium-silicide, which provides this material its ability to become solution heat treated for improved strength. These alloys are naturally solidification crack sensitive, and for this reason, they should not be arc welded autogenously (without filler material). The addition of adequate amounts of filler material during the arc welding process is essential in order to provide dilution of the base material, thereby preventing the hot cracking problem. They are welded with both 4xxx and 5xxx filler materials, dependent on the application and service requirements.

7XXX Series Alloys – (heat treatable – with ultimate tensile strength of 32 to 88 ksi) These are the aluminum / zinc alloys (zinc additions ranging from 0.8 to 12.0%) and comprise some of the highest strength aluminum alloys. These alloys are often used in high performance applications such as aircraft, aerospace, and competitive sporting equipment. Like the 2xxx series of alloys, this series incorporates alloys which are considered unsuitable candidates for arc welding, and others, which are often arc welded successfully. The commonly welded alloys in this series, such as 7005, are predominantly welded with the 5xxx series filler alloys.

ALUMINUM PROPERTIES

WHAT ARE THE MAJOR PROPERTIES OF ALUMINUM?

If you’ve ever wondered what properties of aluminum make it such a popular and versatile metal, you’re not alone. There are numerous characteristics that make aluminum and aluminum alloys one of the world’s most important materials in use across an impressive range of industries. This includes the appliance, architectural, aviation, and automotive industries, just to name a few.

Examining the physical, chemical, and mechanical properties of a material forms the basis of materials science. This makes it possible to predict behavior in particular environments and under stress. Such performance indicators help architects, fabricators, and designers select the correct material for a specific application.

Many outstanding properties of aluminum and aluminum alloys lead to a wide range of applications. For instance, of all metals, aluminum alloys are among the easiest to form and machine. Aluminum’s mechanical properties make it so. What other attributes drive the preference for aluminum products and materials?

The Major Properties of All Metals

Metals make up a majority of the elements on the periodic table. They are a class of elements distinguished by the following properties: ductility, malleability, hardness, conductivity, the ability to form alloys, and qualities of appearance.

These properties can be grouped as either physical, chemical, or mechanical and they can be expanded upon in further depth when considering specific alloy compositions and other factors, like temperature. The charts below refer to pure aluminum

Material Properties of Aluminum and Aluminum Alloys

Aluminum is a metal-like element with both metal and nonmetallic properties, situated in the boron and carbon family. Though aluminum is one of Earth’s most abundant elements, it must be sourced from bauxite ore and undergo a production process before becoming commercially pure, viable aluminum.

Aluminum is then classified according to alloyed elements in a numbered 4-digit series, 1xxx to 8xxx.

Commonly added elements include copper, magnesium, manganese, silicon, and zinc. With these, hundreds of alloy compositions exist.

These specific alloy compositions affect appearance and fabricability. The addition of elements improves strength, workability, corrosion resistance, electrical conductivity, and density compared with pure aluminum.

Physical Properties

Physical properties of aluminum relate to the observable form and structure, before any chemical alteration.

Takeaways for Physical Properties of Aluminum

Physical properties of aluminum help make sense of its applications. Looking at the chart above, we see that aluminum displays a good combination of strength, resistance to corrosion, and ductility. This helps explain how aluminum can exist in the form of foil and beverage cans, as well as piping and irrigation tubing.

Polished aluminum shows good reflectance over a broad range of wavelengths, which leads to its selection for a variety of decorative and functional uses, including appliances and lasers.

That aluminum is non-Ferro magnetic makes it suitable for electrical and electronics industries. The thermal conductivity of aluminum alloys is advantageous in heat exchangers, evaporators, electrically heated appliances and utensils, as well as automotive rims, cylinder heads, and radiators.

Its face-centered cubic structure contributes to excellent formability. Aluminum is also nontoxic and often used in food and beverage containers. According to The Aluminum Association it is also among the easiest to recycle of any of the structural materials.

Chemical Properties

A substance’s characteristic or behavior as it undergoes a chemical change or reaction. In other words, a substance’s atoms must be disrupted for the chemical properties to be observed. Observations of this disruption at the atomic level take place during and also following the reaction.

Takeaways for Chemical Properties of Aluminum

In some ways, the chemical properties of aluminum are unusual compared to other metals. For example, reactivity to both bases and acids is uncommon for metals. This becomes a factor worth considering when aluminum is used as a container for liquids. You have to be certain the aluminum will not dissolve. Hence why beverage cans have a thin liner to prevent corrosion.

Another quirky fact about aluminum is that aside from its powdered form, aluminum is nonpyrophoric. This means that in its powdered state, aluminum is flammable and considered a dangerous hazard, especially during processing when fine dust particles are common.

That aluminum combines so easily with oxygen directly impacts welding practices. The firm oxide layer that forms on the surface of aluminum melts at triple the temperature as the aluminum underneath. Therefore, deep intentional surface cleaning usually with acetone is needed prior to weld, and alternating current is required throughout the welding process.

Mechanical Properties

Mechanical properties note a materials relationship between stress and strain and measure the degree of elasticity in response to an applied load.

Takeaways for Mechanical Properties of Aluminum

Mechanical properties bear significantly on performance applications. This is particularly true when you consider how mechanical properties vary among aluminum alloys.

For instance, the trend for elongation across the aluminum alloy series is high for lower series alloys and low for higher series alloys. In other words, when comparing 1xxx series aluminum alloys to 7xxx series alloys, 1xxx series alloys will have significantly higher ductility.

This works inversely with tensile strength, hardness, and impact sensitivity, which will be lower among the lower series alloys. So, in that same comparison, the 1xxx series alloys will show much lower tensile strength, hardness, and impact sensitivity than their 7xxx counterparts.

Elevated temperatures also compromise aluminum even before they reach melting point. As a result, most aluminum alloys are not typically suggested for longtime service at higher temperatures. However, certain alloys have been specifically designed for high-temperature resistance, like the 2xxx aluminum-copper series.

The exceptional ability for aluminum to form alloys expands its reach across industries and applications. Without this essential capability, prime aluminum would be too soft and pliable for applications that require greater strength and durability.

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Aluminum alloys Categories

Aluminum Association system, most widely recognized in the United States, divided aluminum alloys into two general categories of wrought and cast alloys. A further differentiation for each category is based on the primary mechanism of property development. Their alloy identification system employs different nomenclatures for wrought and cast alloys, but divides alloys into families for simplification. For wrought alloys a four-digit system is used to produce a list of wrought composition families. The first digit is a number between 1 and 8 and specify the main alloying element.

Casting compositions are described by a three-digit system followed by a decimal value. The decimal .0 in all cases pertains to casting alloy limits. Decimals .1, and .2 concern ingot compositions, which after melting and processing should result in chemistries conforming to casting specification requirements.

Aluminum alloys application

Aluminum alloys are economical in many applications. They are used in the automotive industry, aerospace industry, in construction of machines, appliances, and structures, as cooking utensils, as covers for housings for electronic equipment, as pressure vessels for cryogenic applications, and in innumerable other areas.