Enhance Strength of Aluminum Alloys: ASM's Guide to Materials & Methods

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In summary, Aluminum alloys can be strengthened through various methods including solid-solution formation, second-phase microstructural constituents, dispersoid precipitates, and strain hardening. The most common elements used for strengthening are copper, magnesium, manganese, silicon, and zinc, with zinc having the highest solid solubility in aluminum. Heat treatment, including solution heat treatment and artificial aging, can also be used to increase strength, but it is important to pay attention to the chemistry specifications of the alloy to ensure desired properties. Natural aging and artificial aging can both contribute to strengthening, but excessive heating can lead to over-aging and a decrease in strength. It is important to consider service conditions when selecting an alloy as they can affect both mechanical performance and corrosion behavior.
  • #1
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Periodically, I receive useful information on materials. This short note was sent from ASM.

Two most common methods to increase the strength of aluminum alloys are to:
  • Disperse second-phase constituents or elements in solid solution and cold work the alloy (non heat treatable alloys)
  • Dissolve alloying elements into solid solution and precipitate them as coherent submicroscopic particles (heat treatable or precipitation-hardening alloys)

Elements most commonly present in commercial aluminum alloys to provide increased strength, particularly when coupled with strain hardening by cold working or with heat treatment (or both) are copper, magnesium, manganese, silicon, and zinc. These elements all have significant solid solubility in aluminum, and in all cases, the solubility increases with increasing temperature. Of all the elements, zinc has the highest solid solubility in aluminum (a maximum of 66.4 at.%). The maximum solid solubility in aluminum alloys occurs at the eutectic, peritectic, or monotectic temperature.
 
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  • #2
One Minute Mentor said:
The predominant objective in the design of aluminum alloys is to increase strength, hardness, and resistance to wear, creep, stress relaxation, and fatigue. Strengthening in non-heat treatable alloys is achieved through solid-solution formation, second-phase microstructural constituents, dispersoid precipitates, and/or strain hardening. The principal alloys strengthened by alloying elements in solid solution are those in the Al-Mg (5xxx) series, ranging from 0.5 to 6 wt% Mg. These alloys often contain small additions of transition elements, such as Cr and Mn, and less frequently Zr, to control the grain or subgrain structure, and Fe and Si impurities, which usually are present in the form of intermetallic particles.

http://www.asminternational.org/Content/NavigationMenu/Magazines/HeatTreatingProgress/One_Minute_Mentor1/One-Minute_Mentor.htm [Broken]
 
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  • #3
Second-Phase Strengthening of Aluminum Alloys

One Minute Mentor said:
Elements and combinations that form predominantly second-phase constituents with relatively low solid solubility include Fe, Ni, Ti, Mn, and Cr, and combinations thereof. The presence of increasing volume fractions of intermetallic-compound phases formed by these elements and the elemental Si constituent formed by Si during solidification or by precipitation in the solid state during post-solidification heating also increase strength and hardness. These irregularly shaped particles form during solidification and occur mostly along grain boundaries and between dendrite arms. For alloys that consist of both solid solution and second-phase constituents and/or dispersoid precipitates, all of these microstructural components contribute to strength in a roughly additive manner.

http://asm.asminternational.org/ht-echarts/3046_OMM070607_figure.pdf [Broken]
 
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  • #4
Strain Hardening of Aluminum Alloys

One Minute Mentor said:
Strain hardening by cold rolling, drawing, or stretching is a highly effective way to increase the strength of non-heat treatable aluminum alloys. The increases in strength that accompany increasing reduction by cold rolling are obtained at the expense of ductility, as measured by percent elongation in a tensile test and by reducing formability in operations such as bending and drawing. All mill products can be supplied in the strain-hardened condition, although there are limitations on the amount of strain hardening that can be applied to products.

http://asm.asminternational.org/ht-echarts/3046_OMM071307_figure.pdf [Broken]
 
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  • #5
Heat-Treatable Aluminum Alloys

One Minute Mentor said:
Heat-treatable (precipitation-hardening) aluminum alloys for wrought and cast products contain elements that decrease in solubility with decreasing temperature, and in concentrations that exceed their equilibrium solid solubility at room temperature and moderately higher temperatures. Heat treatment for precipitation strengthening includes a solution heat treatment at a high temperature to maximize solubility, followed by rapid cooling or quenching to a low temperature to obtain a solid solution supersaturated with both solute elements and vacancies. The heat treatment is designed to maximize the solubility of elements that precipitate in subsequent aging treatments, which may include either natural aging or artificial aging.
Examples of heat treatment time and effect on strength - http://asm.asminternational.org/ht-echarts/3046_OMM072007_figure.pdf [Broken]

It's really important to pay attention to the chemistry specifications and ranges of particular elements in the alloy. Chemistry specs are written to allow some flexibility in manufacturing and avoid costly rejection. However, the effectiveness and effect of heat treatment, material properties and corrosion behavior can vary widely over the range of allowable composition. This matter is particularly critical when extending the performance range of a material.
 
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  • #6
Natural Aging in Aluminum Alloys

One Minute Mentor said:
Natural aging refers to spontaneous formation of a Guinier-Preston (G-P) zone structure during exposure at room temperature. Solute atoms either cluster or segregate to selected atomic lattice planes, depending on the alloy system, to form the G-P zones, which are more resistant to movement of dislocations through the lattice, and, therefore, are stronger. Of the binary alloys, aluminum-copper alloys undergo natural aging after being solution heat treated and quenched. The amounts by which strength and hardness increase become larger with time of natural aging and with the copper content of the alloy, from about 3% to the limit of solid solubility (i.e., 5.67%).

http://asm.asminternational.org/ht-echarts/3046_OMM072707_figure.pdf [Broken]


Here is a more general article on strengthening of metals, which features prominently a section on "Precipitation in Al-Cu Alloys".
http://www.key-to-metals.com/Article128.htm
 
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  • #7
Artificial Aging in Aluminum Alloys

One Minute Mentor said:
The rates and amounts of the changes in the strength and hardness of Al-Cu alloys can be increased by holding the alloys at moderately elevated temperatures (for alloys of all types, the useful range is about 120 to 230°C, or 250 to 450°F). This treatment is called artificial aging or precipitation heat treating. In the Al-Cu system, alloys having as little as 1% Cu, slowly quenched, start to harden after about 20 days at a temperature of 150°C (300°F). The alloys of this system having less than about 3% Cu show little or no natural aging after low cooling-rate quenching, which introduces little stress.

http://asm.asminternational.org/ht-echarts/3046_OMM080307_figure.pdf [Broken]
 
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  • #8
Artificial (Over)Aging Aluminum Alloys

One Minute Mentor said:
Artificial aging includes exposure to temperatures above room temperature to produce the transitional (metastable) forms of the equilibrium precipitate of a particular alloy system, which remain coherent with the solid-solution matrix, thus contributing to precipitation strengthening. Further heating at the temperatures that cause strengthening, or at higher temperatures causes the precipitates to grow, but even more importantly, to convert to equilibrium phases, which generally are not coherent. These changes soften the material, and if carried further, produce the softest or annealed condition.
This can affect corrosion behavior as well as mechanical performance.

http://asm.asminternational.org/ht-echarts/3046_OMM081007_figure.pdf [Broken]


When one is selecting an alloy one must also consider service conditions such as temperature, not only from consideration of corrosion and strength, but also the aging affects, which also change the corrosion and mechcanical performance.
 
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  • #9
Effect of Precipitation on Aluminum Alloy Properties

One Minute Mentor said:
Formability is the extent to which a material can be deformed in a particular process before the onset of failure. Aluminum sheet and aluminum shapes usually fail by localized necking or by ductile fracture. Precipitation-strengthened aluminum alloys usually are formed in the naturally aged (T4) condition, or in the annealed (O) condition, but only very rarely in the peak-strength (T6) condition where both the necking and fracture limits are low. Curves can be plotted for most of the precipitation-strengthened alloys in the 2xxx and 6xxx series showing the effect of a wide range of precipitation structures on some of the forming properties.

http://asm.asminternational.org/ht-echarts/3046_OMM081707_figure.pdf [Broken]
 
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  • #10
GP Zones in Precipitation Hardening Aluminum Alloys

One Minute Mentor said:
The exact size, shape, and distribution of Guinier-Preston (GP) zones depend on the alloy in which they form and on the thermal and mechanical history of the specimen. GP zones essentially are distorted regions on the matrix lattice, rather than discrete particles of a new phase having a different lattice. Thus, they are completely coherent with the matrix, imposing local but often large strains on it. These mechanical strains, as well as the presence of a locally solute-rich, sometimes ordered lattice, can account for large changes in mechanical properties of the alloy before any long-range microstructural changes occur. GP zones are metastable, and, thus, dissolve in the presence of a more stable precipitate. This dissolution causes a precipitate-free, visibly denuded region to form around the stable precipitate particles.
http://asm.asminternational.org/ht-echarts/3046_OMM083107_figure.pdf [Broken]
 
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  • #11
Solution Treating Time for Aluminum Alloys

One Minute Mentor said:
Heat treatment to increase the strength of aluminum alloys involves solution heat treatment (dissolution of soluble phases), quenching (development of supersaturation), and age hardening (precipitation of solute atoms). The time at the nominal solution heat treating temperature (soak time) required to effect a satisfactory degree of solution of undissolved or precipitated soluble phase constituents and to achieve good homogeneity of the solid solution is a function of microstructure before heat treatment. The time can vary from less than a minute for thin sheet product to as much as 20 h for large sand or plaster-mold castings. The time required to heat the load to the treatment temperature in furnace heat treatment also increases with section thickness and furnace loading, and, thus, total cycle time increases with these factors.

http://asm.asminternational.org/ht-echarts/3046_OMM090707_figure.pdf [Broken]


Students should consider membership in ASM International and TMS. They provide excellent resources and an opportunity to network with colleagues and potential employers.
 
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  • #12
Quenching Solution Treated Aluminum Alloys

One Minute Mentor said:
The objective of quenching solution treated components is to preserve the solid solution formed at the solution heat treating temperature by rapidly cooling to some lower temperature, usually near room temperature. This applies not only to retaining solute atoms in solution, but also to maintaining a certain minimum number of vacant lattice sites to assist in promoting the low-temperature diffusion required for zone formation. As a broad generalization, the highest strengths achievable and the best combination of strength and toughness are those associated with the most rapid quench rates. The effect of quench rate on mechanical properties also can depend on the desired temper. For example, in the underaged condition, a slow quench rate is more detrimental on ductility and fracture toughness. Strength would be more affected after near-to-peak aging. The relative effects of quench methods can be compared in terms of average quench rates.

http://asm.asminternational.org/ht-echarts/3046_OMM091407_figure.pdf [Broken]

See how the strength is affected by temper as well as quench rate, which is also affected by alloy composition.
 
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  • #13
Quench Rate vs. Properties of Solution Treated Aluminum Alloys

One Minute Mentor said:
Average quench rates are useful to compare experimental results from various quench methods. However, average quench rates only compare results in a "critical" temperature range, where precipitation is most likely to occur. This method is not entirely accurate because significant precipitation can also occur outside the specified critical temperature range of average quench rates. In addition, for high-strength alloys, toughness and corrosion resistance could be impaired without significant loss of tensile strength. Therefore, the more sophisticated comparison called quench-factor analysis is required for quantitative property prediction or property optimization.

http://asm.asminternational.org/ht-echarts/3046_OMM092107_figure.pdf [Broken]

The optimal quench rate will depend on composition because different intermetallics have different precipitation/solubility temperatures. Knowing the solubility temperatures of different phases, and in fact, knowing the phases which are present, is critical with respect to having a controlled and reproducable process.

Strength, toughness and corrosion resistance are all dependent on microstructure as well as composition, and in fact composition and microstructure go hand in hand. The other key factor is the thermo-mechanical process.
 
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  • #14
Effects of Quench Delay of Solution Treated Aluminum Alloys

One Minute Mentor said:
Whether the transfer of parts from the furnace to the quench is performed manually or mechanically, it must be completed in less than the specified maximum time. The maximum allowable transfer time or "quench delay" varies with the temperature and velocity of the ambient air and the mass and emissivity of the parts. Maximum quench delays can be determined from cooling curves that will ensure complete immersion before the parts cool below 400°C (750°F).

http://asm.asminternational.org/ht-echarts/3046_OMM092807_figure.pdf [Broken]

When developing a process and transferring from laboratory to production, things like transfer time (and interruptions) must be carefully considered and then monitored, especially where the process is not fully automated.
 
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  • #15
I am loving this weekly Aluminum factoid you're doing for us Astro. As someone who uses only a couple grades, I find it really interesting. Thanks!
 
  • #16
Water-Immersion Quenching of Aluminum Alloys

Thanks for the compliment Fred. I get this about every Friday, and I try to add something of value from my experience with different materials.

One Minute Mentor said:
Water-immersion quenching typically is controlled in practice by stipulating maximum quench-delay time and maximum water temperature. The first requirement controls the cooling rate during transfer and, for high-strength alloys, often is based on the criterion of complete immersion before the metal cools below 415°C (775°F). This specified temperature is based on a critical temperature for alloy 7075, which has one of the more severe C-curves. Therefore, the criterion for complete immersion of other alloys might be based on a temperature lower than the 415°C specification, depending on the characteristics of the particular C-curve.

http://asm.asminternational.org/ht-echarts/3046_OMM100507_figure.pdf [Broken]


The key is understanding the solution treatment temperature with respect to the temperature at beginning of quench and then the quench rate. Nucleation and precipitation of secondary phases are a function of temperature, and as second phases precipitate, the solid solution will be depleted of alloying elements. The longer the cooling time, the larger the second phase particles will become, so secondary phase particle size and distribution will be affected, which may have a dramatic impact on corrosion and mechanical performance.


The pdf file in the above link is a Time-Temperature curve. In general, I would caution any user not to necessarily take any T-T or phase diagrams as gospel. In my experience, somewhat inaccurate T-T curves and phase diagrams have made their way into the literature, e.g. one for Inconel-718 and one for Zr-Nb, so I would be suspect of any curve unless I have independent data. Inaccuracies arise from specs that may be too broad or which do not consider some subtle behavior or sensitivities which can be easily overlooked or unanticipated. Such sensitivities can be initial fine precipitates (nm scale), e.g. intermetallics such as silicides, which have very high dissolution temperatures and which may still be present if the solution temperature is not sufficiently high. Very fine precipitates then serve as nucleation sites for subsequent precipitation, and even for entirely different compositional phases. It is best (practice) to obtain independent and current data for verification of any T-T curve for one's particular alloy system.


Another aspect with respect to each of the processes mentioned in this thread is the separate and integrative qualification. Each step must be controlled within some range, but then the sequence of steps (integrated process) must be optimized and then controlled. Previous steps DO have an impact on subsequent steps, so the process/materials engineer must be cognizant of these matters. And this does not simply apply up to the point where product leaves the shop, but it applies to subsequent application (performance) in the field. On-going performance monitoring and feedback to the responsible engineering organization is crucial to safety and reliable (economical) use of materials and engineered products.
 
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  • #17
Spray Quenching of Aluminum Alloys

Sometimes one cannot quench a part/peice in a tank due to geometry or design consideration, or the heating is limited to the surface and the quenching must be done locally. So spray quenching is the answer.

One Minute Mentor said:
The quench rate for spray quenching is controlled by both the velocity and volume of water per unit area per unit time of impingement of the water on the workpiece; rate of travel of the workpiece through the sprays is an important variable. Local increases in temperature that occur within the first few seconds of quenching, caused by a phenomenon such as plugged spray nozzles, are particularly deleterious. The remaining "internal heat" could be sufficient to reheat the surface region, which causes a large loss in strength at the previously quenched surface. The loss of strength in the affected area of a heavy part is more severe than that caused by an inadequate quenching rate alone.

http://asm.asminternational.org/ht-echarts/3046_OMM101207_figure.pdf [Broken]

In addition to velocity and volume - the quenchant temperature is important.

This is an area where use of a good multiphysics (particularly heat transfer) is important, and becoming moreso.
 
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  • #18
Quench Severity for Aluminum Alloys

One Minute Mentor said:
Quench severity is commonly expressed in terms of an H-value (or Grossmann number), where the H-value is related to the thermal conductivity (k) of the parts(s) and the coefficient of heat transfer (C) between the quenchant and the part, related by the equation H = C/2k. Water can achieve cooling rates up to about 200°C/s (360°F/s) at the midplane of 25-mm (1 in.) thick plate.

Lower cooling rates are achieved by immersion in heated water, reducing the velocity of quenchant around the part, lowering surface tension (promoting boiling), and increasing the stability of the vapor film (lower thermal conductivity) around the part. Polymer quenchants retard cooling rates by the formation of films around the part.

http://asm.asminternational.org/ht-echarts/3046_OMM101907_figure.pdf [Broken]
 
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  • #19
I'm new to this forum and see a great amount of information.
We are aluminum forgers. One customer has required an upper and lower limit on hardness (90 to 110 BHN). We have only been required to hold a minimum limit by all other customers.
In doing my FMEA what in the heat treat process will impact hardness so that it would be greater than110?

Also, we have been doing an eddy current conductivity test in heat treat for years before me. What is the purpose of this and what in the heat treat process impacts conductivity?

Thank you for your time.

Regards
 

1. How can I enhance the strength of aluminum alloys?

There are several methods that can be used to enhance the strength of aluminum alloys. These include adding alloying elements such as copper, magnesium, or zinc, heat treatment processes such as quenching and tempering, and mechanical processes such as cold working.

2. What is the ASM's Guide to Materials & Methods?

The ASM's Guide to Materials & Methods is a comprehensive resource for engineers and scientists working with materials. It provides detailed information on materials properties, processing methods, and testing techniques for a wide range of materials, including aluminum alloys.

3. Is the ASM's Guide to Materials & Methods specific to aluminum alloys?

No, the ASM's Guide to Materials & Methods covers a wide range of materials, including metals, ceramics, polymers, and composites. However, it does have a specific section dedicated to aluminum alloys, providing in-depth information on their properties and methods for enhancing their strength.

4. Can the ASM's Guide to Materials & Methods be used by non-experts?

Yes, the ASM's Guide to Materials & Methods is designed to be accessible to both experts and non-experts. It provides clear explanations of materials science concepts and techniques, making it a valuable resource for anyone working with materials.

5. How frequently is the ASM's Guide to Materials & Methods updated?

The ASM's Guide to Materials & Methods is updated periodically to include the latest research and developments in the field of materials science. The most recent edition was published in 2019, and previous editions have been published every 5-10 years.

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