Heat treatment to increase strength of aluminum alloys is a three-step process: Solution heat treatment : dissolution of soluble phases Quenching : development of supersaturation Age hardening : precipitation of solute atoms either at room temperature (natural aging) or elevated temperature (artificial aging or precipitation heat treatment).
What is aluminium heat treatment?
Aluminum heat treatment is a process by which the strength and hardness of a specific subset of aluminum alloys, namely the wrought and cast alloys that are precipitation hardenable, are increased. Precipitation hardenable aluminum alloys include the 2XXX, 6XXX, 7XXX and 8XXX series.
What is precipitation heat treating or artificial aging of aluminum?
Aging at an elevated temperature is referred to as precipitation heat treating or as artificial aging. The typical change in hardness for artificial aging of aluminum is shown in Figure 1. Figure 1: Typical artificial aging curve for aluminum.
How to age aluminum heat-treatable alloy?
Artificial Aging of Aluminum Heat-Treatable Alloys (part 1) The process temperatures – 200°F up to approximately 400°F – can be selected for the artificial-age procedure. There can also be a treatment that combines cold working plus thermal accelerated aging. This will, of course, depend on the aluminum alloy and the specification requirements.
How to accelerate the aging process of solution-treated aluminum alloy?
In order to accelerate the aging process, therefore, heat is applied to the solution-treated aluminum alloy. The process temperatures – 200°F up to approximately 400°F – can be selected for the artificial-age procedure. There can also be a treatment that combines cold working plus thermal accelerated aging.
What is solution heat treatment of aluminum?
Solution treatment involves heating the aluminum to a temperature of 430-540°C (800-1000°F), at which alloying constituents are taken into solution (i.e., brought near their melting point) prior to a rapid quench.
What is solution heat treatment?
Solution treatment is a heat treating process that heats alloys to a specific temperature, sustaining that temperature long enough to cause one or more constituents to enter into a solid solution and then rapidly cooled to maintain the solution's properties.
What is natural aging in heat treatment?
Natural aging is a step in the heat treatment of aluminum alloys in which the metal is removed from the quench bath and allowed to gain its full strength at room temperature. In artificial aging, the metal is held at an elevated temperature for it to gain its full strength in a shorter period of time.
What is natural aging of aluminum?
Natural aging of aluminum occurs when the solid solution obtained after quenching starts to form precipitates immediately at room temperature. After quenching and any straightening, the supersaturated solid solution of aluminum wants to reach equilibrium. It does this through the process of precipitation hardening.
Is solution heat treatment the same as annealing?
The main difference between heat treatment and annealing is that heat treatment is used to obtain different desired properties (ex: increased strength, increased hardness, impact resistance, softening, increased ductility, etc.) whereas annealing is mainly done to soften a metal.
What is the purpose of solution annealing?
The purpose of solution annealing is to dissolve any precipitates present in the material, and transform the material at the solution annealing temperature into a single phase structure.
What is natural aging?
Ageing can occur at room temperature, which is known as natural ageing, or at elevated temperature, which is called artificial ageing. Natural ageing is a slow process in most types of age-hardenable alloys, and the effects of the ageing process may only become significant after many months or years.
What is natural aging process?
Aging is a gradual, continuous process of natural change that begins in early adulthood. During early middle age, many bodily functions begin to gradually decline. People do not become old or elderly at any specific age. Traditionally, age 65 has been designated as the beginning of old age.
What is meant by over aging of aluminum alloys?
At the stage when the precipitates transform from coherent to incoherent particles the maximum strength is reached. Over-ageing causes a deterioration in strength owing to coarsening of the incoherent particles. 8.11. Effect of ageing temperature on the tensile strength of an aluminium alloy.
What is the reason of aluminium extrusion aging?
The aging of aluminum alloys is significantly faster at elevated temperatures. This is usually achieved by a higher level of strength, than with natural aging. Artificial aging profiles of the 6xxx series alloys comprises heating to a temperature of about 180 ° C and holding at it for several hours, usually, 4-5.
What do you mean by ageing?
Aging is the sequential or progressive change in an organism that leads to an increased risk of debility, disease, and death. Senescence consists of these manifestations of the aging process.
What temperature is used to heat treat aluminum?
Depending on the exact process being used, furnace temperatures can range from 240 to 1000°F. It is important to keep in mind that the heat treating of aluminum is quite different ...
What happens to aluminum after it is heat treated?
After aluminum has been solution heat treated, the elements that dissolved will begin to precipitate out over time. This causes the grains to lock into position, which in turn increases the natural strength of the aluminum and is called aging.
How long does it take to quench an aluminum furnace?
If quenching is required as part of aluminum heat treatments, then it is critical to quench the part as soon as it comes out of the heat treatment furnace. A delay of more than 15 seconds can be very detrimental. Having a quenchant tank as close as safely possible to the heat treatment furnace is wise. Water at ambient temperature is typically used ...
What is the best way to quench aluminum?
Water at ambient temperature is typically used as a quenchant for aluminum alloys, but for more complex shapes with varying cross-sections other quenchants or methods may be considered. Other quenchant options for aluminum include: 1 Boiling water 2 Brine solutions 3 Forced air blasts 4 Still air 5 Polymers 6 Glycols 7 Fast quenching oils
What is hardening aluminum?
Aluminum alloys are subject to work hardening, also known as strain hardening. Strain hardening occurs when the aluminum alloy is being shaped by plastic deformation. The plastic deformation causes the grain structures within the aluminum to slide against each other along areas referred to as slip planes. As more and more plastic deformation takes place, there are fewer and fewer slip planes left that are easy to deform. As a result, more force is required to achieve further deformation. When a part has reached this state, it is said to be work hardened. In order to continue plastically deforming the material, the strain hardening has to be removed from the part.
What is annealing in casting?
Annealing also relieves internal stresses that can develop in a part during processes such as cold forging or casting, stabilize the dimensions of a part, and resolve issues that result from internal strains (such as warping). Also, annealing can be successfully performed on aluminum alloys that are considered non heat treatable alloys.
How long does it take for aluminum to harden?
The natural aging, or age hardening, process takes place at room temperature over a time period of four to five days, with 90% of the hardening occurring within the first day. Because of this effect, aluminum parts often need to be shaped rather quickly after going through a solution heat treatment process.
What is a non-heat treatable alloy?
These usually are referred to as the ‘heat-treatable’ alloys to distinguish them from those alloys in which no significant strengthening can be achieved by heating and cooling. The latter, generally referred to as ‘non-heat-treatable’ alloys, depend primarily on cold work to increase strength.
What is age hardening alloy?
Age-hardening alloys are precipitation strengthened indicated by heat-treatment tempers (T-tempers). Depending on the alloying elements, strengthening of aluminum can be achieved by heat treatment or by solid solution strengthening (often in conjunction with work hardening).
Why is aluminum used in aircraft?
This is because aluminum alloys can be heat-treated to relatively high strengths, while maintaining low weight. It is easy to bend and machine, and cost of material is low. Because of these advantages, it is the most common material used in aerospace today.
What is the effect of silicon additions on aluminum?
As an alloying element, the outstanding effect of silicon additions to aluminum and its alloys is the improvement in casting characteristics. Aluminum-silicon alloys that do not contain copper additions are used when good castability and good corrosion resistance are needed.
What is aluminum copper?
Aluminum-copper alloys containing 2 to 10 percent Cu, generally with other additions, form important families of alloys. The aluminum-copper system is the basis for the wrought 2xxx and cast 2xx.x alloys, and many other heat-treatable alloys contain copper. Most contain other alloying elements such as magnesium.
What is the impurity of aluminum?
Silicon is a ubiquitous impurity in commercial aluminum alloys. Impurity levels in electrolytic commercial aluminum in the range of 0.01 to 0.15 wt% Si, and the presence of iron greatly reduces the solubility of silicon in aluminum.
What aircraft uses heat treating material?
It is used in the manufacture of advanced commercial aircraft such as the Boeing 777, Airbus 380, and military aircraft such as the Boeing UCAV or the Boeing F/A-18 E/F. When used in conjunction with aluminum alloys, the term ‘heat treating’ is generally restricted ...
How can aluminium be strengthened?
Many wrought and cast aluminium alloys can be strengthened by solution treating and ageing to a variety of different tempers. The properties of heat treatable nickel-based alloys can be enhanced by selection of appropriate heat treating parameters.
What is the process of increasing the strength of an alloy?
Solution and age. Ageing is a process used to increase strength by producing precipitates of the alloying material within the metal structure. Solution treatment is the heating of an alloy to a suitable temperature, holding it at that temperature long enough to cause one or more constituents to enter into a solid solution ...
What is precipitation hardening?
Precipitation hardening: Stainless steels. Precipitation heat treatments strengthen materials by allowing the controlled release of constituents to form precipitate clusters which significantly enhance the strength of the component. Solution and age. Ageing is a process used to increase strength by producing precipitates ...
What temperature to heat treat alloy?
Solution heat treatment occurs at a range from 825°F to 980°F, with the specific temperature depending on the alloy.
How does solution heat treatment work?
In solution heat treatment the elements that cause age hardening dissolve, undissolved elements become spheroids, and the whole structure becomes homogenized. The quench traps dissolve elements in the solution that will later precipitate out and create the age hardening effect. Right after the quench the alloy is usually easy to work with, but as time passes, it will harden and become difficult to work.
How long does it take for an alloy to harden?
The elements that dissolve during the solution heat treatment process precipitate out over time after the quench and lock the grains into position, increasing the strength of the material. For naturally aged alloys, this happens at room temperatures and takes four or five days to reach full strength, but 90% of the hardening happens in the first 24 hours. The process starts immediately after the quench, but because it is a slow process there is a window to shape the alloy with relative ease immediately after quenching.
What happens when a part reaches homogenizing temperature?
Once the whole part reaches the homogenizing temperature it is allowed to cool slowly , resulting in a part that has a uniform internal structure ready to take advantage of other heat treatment processes or cold working.
What temperature is homogenized?
For many alloys the homogenizing temperature is 900°F to 1000°F, which is just under the melting point.
What is the process of hardening an alloy?
Some alloys require heat to fully precipitate out the dissolved elements and reach their maximum hardness. This process is called precipitation hardening. These alloys will harden some at room temperature, with the amount depending on the specifics of the alloy.
How long does it take to soak metal?
Soaking time is another key component of solution heat treatment, and it is a measure of time from when the coldest metal reaches the minimum limit of the desired range until quenching. For thin parts, the soaking time may be 10 minutes, while a heavy part may need 12 hours to soak.
How to stop aluminum hardening?
The aluminum hardening process can be stopped by placing aluminum parts in a freezer until they’re ready to be hit on the press again. After this secondary pressing, the parts go through an aging heat treatment process.
How long does it take for 6061 T4 aluminum to harden?
Because time and room-temperature air can also have a hardening effect, setting these newly treated aluminum parts in the open on the workroom floor for 96 hours will bring the parts to T4 hardness. 6061-T4 aluminum is part way to the hardest that this aluminum alloy can be.
How to make 6061 aluminum stronger?
The process of taking a 6061 aluminum part in O condition to a stronger, more durable condition begins by heat treating the parts at 985 degrees Fahrenheit for an hour in a drop bottom furnace. At that point, the basket containing the parts is dropped into a quench tank.
How long does it take to age aluminum 6061?
Aging aluminum 6061 is done between 350 to 500 degrees Fahrenheit, and—depending on thickness and other factors—the parts stay in the oven for 12-24 hours. Following this aging, the parts are now at a T6 condition.
What is the most common temper for aluminum?
In general, T6 is the most common temper for aluminum, and because 6061 aluminum is often used in heavy-duty structures, the strength and limited formability of metal at that temper is important.
Is 6061 aluminum ductile?
6061 aluminum is a ductile metal that can be stamped, cut, deep drawn, drilled, tapped, bent, welded, hydroformed and more. Magnesium and silicon are the major alloying elements in this metal, and 6061 is generally strong and corrosion resistant. It is heat treatable and crack resistant when stress is removed.
What is the process of natural aging?
In natural aging, the solid solution obtained after quenching starts to form precipitates immediately at room temperature. This process is termed natural aging, and the hardening during natural aging is attributed almost entirely to the homogenous precipitation of solute-rich GP zones and the clustering of vacancies.
What happens to mechanical properties during artificial aging?
During artificial aging, mechanical properties will improve . The yield strength will increase, as will the ultimate tensile. The yield strength will increase faster than the ultimate tensile strength. Because of this, the ductility decreases as the aging sequence progresses.
Why is soaking temperature slow?
Over-aging can result if the rate of approach to the soaking temperature is unusually slow, because of heavy compact loading, overloading the furnace, or use of a furnace with inadequate heating capacity. Typical artificial aging times for different alloys are shown in Table 1.
How does over aging affect tensile strength?
Over-aging decreases the tensile strength and increases the resistance to stress corrosion cracking. It also enhances the resistance to fatigue crack growth. It also imparts dimensional stability of the part. In artificial aging, the degree of precipitation and morphology of the precipitate is controlled by aging time and temperature.
What is precipitation hardening?
Precipitation hardening is the mechanism where the hardness, yield strength, and ultimate strength dramatically increases with time at a constant temperature (the aging temperature) after rapidly cooling from a much higher temperature (solution heat treat temperature). This rapid cooling or quenching results in a supersaturated solid solution ...
How does artificial aging affect precipitation?
In artificial aging, the degree of precipitation and morphology of the precipitate is controlled by aging time and temperature. Within limits, approximately equivalent effects can be obtained by shorter periods of time at higher temperatures or longer times at lower temperatures. A series of different transition precipitates can occur ...
Does artificial aging increase yield strength?
In general, the increase in yield strength that occurs during artificial aging increases faster than the ultimate tensile strength. This means that the alloys lose ductility and toughness. T6 properties are higher than T4 properties, but ductility is reduced.
What is heat treating aluminum?
The term “ heat treating ” for aluminum alloys is frequently restricted to the specific operations employed to increase strength and hardness of the precipitation-hardenable wrought and cast alloys. These usually are referred to as the “heat-treatable” alloys to distinguish them from those alloys in which no significant strengthening can be achieved by heating and cooling.
What is heat treating?
Heat treating in its broadest sense, refers to any of the heating and cooling operations are performed for the purpose of changing the mechanical properties, the metallurgical structure, or the residual stress state of a metal product. Aluminum Aging Furnace. When the term is applied to aluminum alloys, however, ...
What is precipitation hardening?
Precipitation hardening, also called age hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel and titanium, and some stainless steels.
How to take advantage of precipitation hardening reaction?
To take advantage of the precipitation hardening reaction, it is necessary first to produce a solid solution. The process by which this is accomplished is called solution heat treating, and its objective is to take into solid solution the maximum practical amounts of the soluble hardening elements in the alloy.
What are the elements used in precipitation strengthening?
Elements used for precipitation strengthening in typical aluminum and titanium alloys, make up about 10% of their composition. While binary alloys are more easily understood as an academic exercise, commercial alloys often use three components for precipitation strengthening, in compositions such as Al (Mg, Cu) and Ti (Al, V).
What is age hardening?
Age hardening : precipitation of solute atoms either at room temperature (natural aging) or elevated temperature (artificial aging or precipitation heat treatment). Precipitation strengthening is possible if the line of solid solubility slopes strongly toward the center of a phase diagram.
Why do alloys need to be heated?
Heating to decrease strength and increase ductility (annea ling) is used with alloys of both types; metallurgical reactions may vary with type of alloy and with degree of softening desired.
How long does it take for an alloy to age?
This will take time, ranging from a few days up to years to complete at room temperature, depending on the alloy composition/type.
What are the three critical considerations for artificial aging?
There are three critical considerations for the artificial-aging process: Process temperature selection. Process time at the selected temperature.
What is T1 after solution heat treatment?
After solution heat treatment, after natural aging and cold working temper, it is suitable for products whose strength is improved by cold working. T1. Cooled by the high-temperature molding process, and then naturally aged to a basically stable temper.
What is the highest degree of hardening?
Generally, the degree of hardening is divided into 8 levels, 1 is the lowest, 8 is the highest, and 9 represents a super-hard temper with a higher degree of work hardening than Hx8. H12. Work hardened to 25% hardness. H14. Work hardened to 50% hardness.
What is an unstable temper?
An unstable temper, which is only suitable for alloys that are naturally aged at room temperature after solution heat treatment. This temper code only indicates that the product is in the natural aging stage. ‘F’ is the free processing aluminum alloy temper.
Homogenizing
Annealing
Solution Heat Treatment
- Solution treating is typically performed in the 450 to 575°C (842 to 1067°F) range in air, followed by rapid quenching into cold water, hot water, boiling water (-T61 temper), water-polymer (glycol) solution, water spray or forced air. Natural ageing to the T4 temper will occur at ambient temperature for 2XXX, 6XXX, 2XX and 3XX alloys, with most re...
Natural Aging
Artificial Aging/Precipitation Hardening
- The process of shaping aluminum alloys causes the grain structures to slide against one other, along areas known as slip planes. After a while there are less easy slip planes, and increased force is required to shape the part. This state is referred to as work hardened. The annealing process resets the crystalline structure and creates a new batch of unused slip planes, making i…
Conclusion
- Solution heat treatment is similar to annealing, but it involves quenching, which is the rapid cooling of the alloy to preserve the distribution of the elements. In solution heat treatment the elements that cause age hardening dissolve, undissolved elements become spheroids, and the whole structure becomes homogenized. The quench traps dissolve elements in the solution tha…
Sources and Further Reading
- The elements that dissolve during the solution heat treatment process precipitate out over time after the quench and lock the grains into position, increasing the strength of the material. For naturally aged alloys, this happens at room temperatures and takes four or five days to reach full strength, but 90% of the hardening happens in the first 24...