Argentum Solutions, Inc.
Sterling guidance on corrosion and materials degradation
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David C. Silverman |
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Corrosion resistance of aluminum alloys depends both the constituents in the alloy and the heat treating process where applicable (or absence of heat treating where applicable). The non-heat treatable alloys tend to have a reasonably high resistance to general corrosion. The constituents tend to have little overall effect on corrosion relative to "pure" aluminum. The heat treatable alloys vary in corrosion resistance depending on the consituent. For example, the members of the 7XXX series that do not contain copper have greater corrosion resistance than the members of the 7XXX that do contain copper. The key point is that when speaking about corrosion of aluminum, both the alloy and its history have to be known. As a general rule, wrought aluminum alloys have far better machinability in terms of such parameters as decreased tool wear, decreased cutting force, and increased machining speed, as compared to steel or titanium. Machining of aluminum alloys depends on two characteristics, (1) the constituents present in the aluminum matrix and (2) the temper. Alloying elements and even impurities in very minute quantities can significantly affect the mechanical properties of aluminum. Alloying elements in aluminum can be in solution or remain out of solution as discrete particles. Alloying elements remaining in solution with aluminum after a heat treatment and stabilization of the heat treatment tend to have little effect on machining similar to their lack of effect on corrosion. But, if the element is allowed to leave solution as during a slow solidification, poor quenching, or over-aging, they can form hard particles that decrease tool life while, at the same time, increase other types of mechanical performance. For example, the same copper bearing aluminum alloy will machine differently depending on whether the heat treatment allows CuAl2 precipitates to form. This observation means that the machinability of certain aluminum alloys can depend markedly on the heat treatment. The American Standards Association adopted the numbering system for wrought aluminum and its alloys which had previously been established by the Aluminum Association. The designation is complex because it contains information about both composition and heat treatment. Information on both characteristics is required to make any judgment about mechanical or corrosion properties. UNS numbers (A9XXXX) begin with an A9. The next 4 digits (XXXX) are the same as the Aluminum Association numbers. But not every alloy with an Aluminum Association number has a UNS number. UNS numbers are not included here. Wrought Alloy Classification System for Composition This section discusses that portion of the classification that pertains to composition of wrought aluminum alloys. The numbering system is built around four digits written here as XXXX. The first digit (1 - 8) provides information about the main alloying constituent. A first digit of "9" signifies an alloy which is being developed. Once the alloy loses its experimental status, it no longer is specified as 9XXX. It becomes part of one of the other families. While it is being commercially evaluated that alloy designation is prefixed by an X as, for example, X2XXX. Finally, the prefix is dropped when the new alloy becomes a standard aluminum alloy. The second digit signifies the number of impurities which are controlled and which are registered with the Aluminum Association for that alloy. The last two digits serve no purpose other than to identify different alloys in the group registered. Below is a table of the major constituents and approximate concentration ranges. The ranges pertain to more than one designation. The table provides the approximate range across the entire family with no member of the family having that broadness. For example, virtually all of the 2XXX series fall in the range of 0.8 to 6.8 wt% copper but 2024 itself has a range of 3.8 to 4.9 wt% copper. An alloy of interest in the aircraft industry is designated 7075-T6 (more about the T6 designation below) with 5.1 to 6.1 wt% zinc, 1.2 to 2.0 wt% copper, 2.1 to 2.9 wt% magnesium, and 0.4 wt% silicon.
As in the case of steel, a number of additional specifications can exist to specify different forms such as bar, sheet, plate, or foil. These specifications can span different families of aluminum alloys in the same specification. For example, ASTM standard B209 can be specified for sheet and plate of alloys 3003, 5052, 6061, and 7075. In addition, there are Military specification numbers (MIL-X-XXXXX) and Federal Specifications (QQ-X-XXX/XX). They tend to include more than one family of aluminum alloys as specified by the Aluminum Association standards. The Aerospace Materials Specifications (XXXX) each cover one composition and temper. 1XXX The higher (>99 wt%) purity aluminum specified by this designation has found application as electrical conductors and heat sinks because of high thermal and electrical conductivity. This class of alloys has been used in the chemical industry as a material of construction because of better than average (for aluminum alloys) corrosion resistance. Moderate increases in strength may be achieved through strain-hardening. The alloy cannot be tempered by heat treatment. Though the alloy can be machined, chips can often be long with a string-like appearance. 2XXX Copper is the principal alloying element. Solution heat treatment must be performed for these alloys to attain optimum properties. When appropriately heat treated, the mechanical properties can be similar to and even exceed mild steel. These alloys can be extremely easy to machine relative to other aluminum alloys especially after being subjected to one of the tempers in the "T" category (see next section). Under some circumstances, their corrosion resistance can be inferior to other aluminum alloys because of the galvanic cell created between the copper containing inclusions and the aluminum. Aluminum 2024 is a common alloy that tends to find widespread use in the aircraft industry. 3XXX Manganese is the major alloying element. This class of alloys tend not to be heat treatable to enhance mechanical properties. There is a limit of about 1.5 wt% on the amount of manganese that can be added effectively to aluminum. Mechanical properties are enhanced usually by strain hardening, sometimes followed by a stabilization of the structure. This series of alloys is used in applications requiring moderate strength. Their machinability tends to be fair to poor with long, string-like chips being commonly formed. Resistance to general corrosion tends to be fairly good. 4XXX Silicon is the major alloying element. It can be added in substantial amounts. Its purpose is to cause a substantial lowering of the melting point without producing brittleness. This series of alloys is often used in welding wire because of the melting point. The alloys in this series tend not to be heat treatable. The alloys darken when anodized and, for that reason, can be found in architectural applications. There is little information on machinability of this alloy but it should be no worse than other non-heat treatable alloys. One member of this class, 4032-T6 is heat treatable and seems to be about as machinable as 2000 series aluminum. One of its applications is for pistons. 5XXX Magnesium is the major alloying element. Its presence creates a moderate to high strength alloy that cannot be heat treated. Mechanical properties are enhanced by strain hardening, sometimes followed by a stabilization of the structure. They offer better corrosion resistance (for aluminum alloys) to a number of atmospheres and have good mechanical properties. These properties tend to make these alloys choices for materials of construction in process industry applications. Their machinability tends to be fair. Overall, machining should be equal to or easier than that of the 3XXX series. 6XXX Silicon and magnesium are the major alloying elements. They are added in approximate proportions to form magnesium silicide. This component makes them heat treatable, very often being solution heat treated and either artificially aged or naturally aged to allow the precipitates to form. The most common alloy is 6061 used in a number of industries. Resistance to general corrosion tends to approach that of the non heat treatable alloys. Machinability depends on heat treatment regimen and tends to improve with heat treatment. Alloys in this group can be formed in the T4 (solution heat treated but not artificially aged) and then reach full T6 properties by artificial aging to that specification. 7XXX Zinc is the major alloying element. When coupled with a small amount of magnesium the alloy becomes heat treatable to very high strengths. A very common member of this group is 7075, often heat treated with the T6 temper. This alloy is commonly used by aircraft manufacturers for air-frame structures and other stressed parts. These alloys tend to have poorer corrosion resistance than some of the other aluminum alloys. Machinability depends on heat treatment. For example, 7075 in the "O" or annealed (lowest strength) condition has a worse machinability rating than in the "T6" or solution heat-treated and artificially aged (highest strength) condition. Cast Aluminum Alloys As in the case of wrought alloys, the mechanical characteristics of aluminum alloy castings depend on more than just the elemental composition. In this case, such properties depend on (1) alloy chemistry, (2) casting process (as it affects dimensions, solidification, surface characteristics, and soundness of part, (3) heat treatments that affect refinement and modification of the microstructure, and (4) the strength of the casting. The alloys of cast aluminum fall into the same two classes as wrought alloys, those that owe their mechanical properties to the alloying constituents and those that are heat-treated to improve their mechanical properties. The designation of a given alloy again requires the specification of the elemental composition and the specification of the heat treatment. Usually machinability is independent of the type of "T" temper used. But differences can exist if the alloy is subjected to a "T" versus an "F" temper with the "T" temper tending to make machinability better. These tempers are discussed in the next section.Aluminum casting alloys must contain, in addition to the strengthening elements discussed with wrought alloys, sufficient amounts of eutectic-forming elements that provide adequate fluidity. Though fluidity is generally not a problem, where high fluidity is required, aluminum-silicon alloys are used. There are basically six types of aluminum casting alloys, aluminum-copper, aluminum-copper-silicon, aluminum-silicon, aluminum-magnesium, aluminum-zinc-magnesium, and aluminum-tin. Cast Alloy Classification System for Composition A system of four digits written as XXX.X is used to identify aluminum and aluminum alloys in the form of castings and foundry ingot. The first digit (1 through 9, excluding 6) indicates the constituent added in the greatest percentage. The second two digits identify the aluminum alloy or indicate the aluminum purity. The last digit, separated from the others by a decimal point, indicates the product form, casting or ingot. The numeral "0" (XXX.0) indicates castings. The numeral "1" (XXX.1) indicates ingots that have specific chemical composition limits conforming to a standard of the Aluminum Association. The numeral "2" (XXX.2) indicates ingots that have chemical composition limits that differ from but fall within the limits used for XXX.1. A modification of the original alloy or impurity limits is indicated by a letter placed in front of the numerical designation. The letters are assigned in alphabetical order starting with A but omitting I, O, Q, and X. The letter "X" is reserved for experimental alloys. This prefix is dropped when the alloy is no longer experimental. The following table shows the alloy groupings and the approximate concentration ranges of the major alloying constituent. The ranges do not pertain to one designation. They are meant to provide the approximate range across the entire family with no member of the family having that broadness. For example, virtually all of the 3XX.X series fall in the range of 5 to 17 wt% silicon but 356.0 itself has a specification of 7 wt% silicon.
The temper classification system discussed in the previous section is applied to cast aluminum alloys. The temper will determine the hardness and may affect the machinability of the alloy. Therefore, as in the case of the wrought alloy, the temper should be specified with the alloy number because a number of the alloys can be subjected to more than one type of tempering depending on the application. Refer to the section entitled "Temper Designation System (Wrought & Cast Alloys)" below for the discussion on the heat treatment designation. 3XX.X Alloys An example of how the system works can be shown using the the 3XX.X alloys. This class of cast aluminum alloys is often encountered in the automotive industry. These alloys all contain relatively high levels of silicon with additions of copper, magnesium, or both. Some of the applications are alloy 319 for permanent mold cast intake manifolds and cylinder heads, alloy 356 for compressor bodies, some intake manifolds, and cast styled wheels (A356), alloy 380 for a variety of applications ranging from spacers to automatic transmission cases, and alloy 390 for cast engine blocks and some pump applications. The following table shows the chemical composition of some of the more important cast aluminum alloys found in automotive manufacture. The compositions pertain to United States specifications. Other countries can have slightly different limits on some of the components.
Temper Classification System (Wrought & Cast Alloys) The second part of the aluminum designation system is the temper which establishes many of the mechanical properties and can influence the corrosion properties. The same designation system is applied to both wrought and cast alloys. Wrought aluminum alloys fall into two categories, those that are heat-treatable and those that are not heat-treatable. As a general rule, the 2XXX, 4XXX, 6XXX, and 7XXX alloys may be strengthened by heat treatment. The 1XXX, 3XXX, and 5XXX alloys cannot be strengthened by heat treatment but rely on their alloying elements such as magnesium and manganese for strength. They may be strengthened by cold-working. Examples do exist that do not fit the general rule.The composition designation XXXX for wrought alloys and XXX.X for cast alloys only provides part of the designation because the post-fabrication heat treatment or strain-hardening can play as much or greater role in mechanical properties than the elemental composition alone. These treatments are part of the designation and must be included in order to fully predict and understand the behavior of the alloy. The designation of composition and temper together uniquely describe the mechanical and chemical properties of the alloy. A rather complex temper designation system was developed by the Aluminum Association to be used for all wrought and cast alloys. The number XXXX must be followed by the temper designation to completely specify the alloy. A full designation might appear as "XXXX-LABC" where "XXXX" is the four digit designation from before. The letter "L" is "F", "O", "H", "W", or "T". This letter is followed by one or more numbers (A, B, C) if the first letter is "H" or "T". The first letter "L" designates the type of temper. The meaning of the numbers depends on the letter preceding it. A complete specification might read, for example, 5052-H32, 7075-T6, or 7075-T651. Following are more detailed meanings of the letters and trailing numbers. The first letter that follows the composition specification (the letter "L" in "XXXX-LABC" for example) designates the type of temper as shown in the following table.
An "H" from the previous table is followed by one or more digits as shown in the next table. For example, a specification might read 3003-H12. Note that the 1XXX, 3XXX, and 5XXX alloys are found in either the "O" or "H" temper.
The first digit "X" in HXY shown above is followed by a second digit "Y" which indicates the degree of strain hardness in eighths (1 - 8) relative to the minimum value of the ultimate tensile strength in the annealed temper. The full hard condition (8) is equivalent to that obtained with about 75% cold reduction following a full annealing of the alloy. The three-quarters hard condition (6 as in H16) would be obtained with about 3/4 of 75% or 56% cold reduction. If a 9 appears as the second digit, the temper has achieved a minimum tensile strength that exceeds that of the HX8 temper. These types of hardening usually have no effect on machinability. Further strengthening of the non-heat treatable alloys (1XXX, 3XXX, and 5XXX) is done by various degrees of cold-working denoted by the "H" temper. Sometimes the H designation appears as HXYZ. The number corresponding to Z, when used, signifies a variation of the two digit temper. It is used when the degree of control of temper or the mechanical properties or both differ from but are close to those defined by the two digit H (XY in HXY) temper designation to which the number has been added. The temper "T" is subdivided into 10 specific sequences of treatment. This temper is found primarily with the wrought alloys 2XXX, 6XXX, and 7XXX. All "T" tempers have about the same effect on machinability. That is, they tend to make the wrought alloy more machinable relative to the "O" or "F" temper for the alloy. The meanings of the various values of "T" are shown in the following table.
Additional digits can appear in the "T" temper (Y, and Z in TXYZ). These digits indicate a variation in treatment that significantly alters the product characteristics that are or would be obtained using the basic treatment. The digit "Y" cannot be zero. The following table shows some of the variations that can exist. The "X" can be any of the above numbers (1-10). Again, these variations should have little effect on machinability.
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David C. Silverman, Ph.D. - Primary Consultant |