Heat Treatment of Aluminum Alloys 

IntroductionWhen an aircraft designer specifies a particular material in a blueprint, not only is the type of alloy critical but so too is the Temper or Heat Treatment. Failure to meet the specified material heat treatment may result in production of a component that is drastically under strength or one that will rapidly deteriorate under adverse environmental conditions. For example, 2024-T3 has a tensile strength of approximately 60,000 pounds per square inch (psi). An error in material heat treatment process could result in installation of a component manufactured in 2024-O, with a tensile strength of only 30,000 psi, i.e., just half as strong as intended! It is essential that all concerned with the assembly and repair of aircraft structures have an understanding of the principles and process of heat treatment of aluminum alloys.

  

Alloy StructureAluminum alloy is an extremely complex structure, often comprising between six to nine different metal elements. In a molten state, during alloy production, some elements are dissolved in others (like cream in coffee) and some are not (like oil in water). As the molten alloy begins to cool a point will be reached when solidification begins. At that point “crystals” begin to form and, as cooling continues, more crystals develop, building on the first to form a metallic grain structure. Other compounds may also form within the alloy as it cools, some dissolved within the grain structure and others between the grains, along grain boundaries. The malleability, ductility and strength of the alloy are all dependent on the nature of the final alloy grain structure.

  

Heat TreatmentHeat treatment may be any one or combination of a series of operations involving the heating and cooling of alloys in the solid state. Its purpose is to alter the mechanical properties of the alloy, either to make the alloy soft for forming operations, or to achieve a specific mechanical strength. The heat treatment process achieves the required results by either permanent or temporary modification of the alloy grain structure. There are three heat treatment processes used to alter or improve the properties of aluminum alloys:

 a.     Annealingb.     Solution Heat Treatmentc.     Precipitation Heat Treatment (Artificial Aging).

 All three are used extensively in the aircraft manufacturing and repair industries and aircraft structural integrity is highly dependent on correct process and application.

    

Annealing Adjoining crystals within alloy structures can “slip” against each other along “slip planes” as the material is loaded or cold worked. An alloy in which slippage of the crystals has not yet occurred is found to be soft. As that soft metal is cold worked (hammered, rolled or stretched, all at room temperature) then adjoining crystals will move along a slip plane. Each slip plane has a limit to the amount of movement available and so further working will result in successive slippage along other planes. As more work is applied to the metal, the resistance to movement increases greatly and the material is said to “work harden”. Over-working will eventually cause the metal to fracture.The original workability of the alloy can be restored by heating the material to a point where a new crystal structure forms, complete with a new set of unworked slip planes. The critical temperature at which this occurs in an alloy is called the Recrystallization point and the process is called Annealing.

 Another critical factor in achieving correct annealing is the Soak Time. This is the minimum time over which the alloy must be held at the designated temperature for full Recrystallization to occur. The required Soak Time is dependent on alloy type and material thickness.

 Annealing is usually carried out in an air furnace and can be used to remove the effect of Cold Working non-heat treatable alloys or to remove the effects of Heat Treatment and Cold Working from heat treatable alloys. Following the annealing operation, aluminum alloys are designated as being at condition “0”, e.g., 2024-0.

      Note:       To change from Fahrenheit to Celsius subtract 32 and multiply by .5555       To change from Celsius to Fahrenheit multiply by 1.8 and add 32

 The full annealing process includes:

 a.     Heat to a pre-determined temperature

 b.     Soak at temperature for specific time

 c.     Cool in still air or at fixed cooling rate in the furnace

 The following Tables quote temperatures, soak times and cooling methods for typical alloys, as recommended by The Aluminum Association:

                                                                         Table 1                                                     To Anneal Heat Treated Material

 Alloy

 Soak Temperature

 Soak Time (hr)

 Cooling Rate

 201720246061

 413 C or 775 F413 C or 775 F 413 C or 775 F

 2-32-32-3

 (a)(a)(a)

7075 413 C or 775 F 2-3 (b)

                                                                         Table 2                                                     To Anneal Cold Worked Materials 

 Alloy

 Soak Temperature

 Soak Time (hr)

 Cooling Rate

 2017202460617075

 343 C or 650 F343 C or 650 F343 C or 650 F343 C or 650 F

 2-32-32-32-3

 (c)(c)(c)(b)

  

Cooling Rate Notes: 

a.     Cool at 28C per hour from the annealing temp down to 260C. Subsequent rate is unimportant.

 b.     Cool at uncontrolled rate down to 232C. Soak for 4 hrs at 232C followed by air cooling to

room temperature. 

c.     Cool at uncontrolled rate in air. 

To avoid costly heat treatment process prior to forming operations, materials are often procured in the Annealed (-0) condition. Even though some local work hardening may then occur during forming, to achieve full and homogeneous material strength, Heat Treatable alloys must next be Solution Heat Treated and Aged.

 

Solution Heat TreatmentWhen Heat Treatable alloys are raised above a particular high temperature, a Super-Saturated Solution of the alloy elements occurs, e.g., in 2024 all of the (4.4%) copper content will go into solid solution with the aluminum. However, at room temperature, an aluminum crystal will hold just 0.5% of its weight of copper and, if cooling from the high temperature is slow, the excess will precipitate out of solution. Now, if the alloy is heated to its super-saturated condition (approx. 500C) and then rapidly quenched in cold water, the copper fails to come out of solid solution as fast as the temperature changes. Instead, over a period of days, the alloy begins to Age, i.e., the copper atoms begin to move within the crystal structure to form compounds containing higher concentrations of copper. In time, these copper-rich compounds migrate to the metallic grain boundaries and they interfere with movement between slip-planes as load is applied to the material. This slip-plane interference results in a hardening effect similar to the Cold Working process already discussed.

 So, the Solution Heat Treatment process is a series of operations whereby the aluminum alloy is:

 a.     Heated to a specific temperature

 b.     Soaked at this temperature for a specific time

 c.     Rapidly quenched.  Natural Aging

       Following the quench, alloys Age to their full strength condition. All alloys begin the aging process in the same way and some achieve full strength within four days by aging at room temperature. These are known asNatural Aging alloys and actually achieve 90% full strength in 24 hrs. Examples of Natural Aging alloys are: 2017, 2117 and 2024. Other alloys begin to age but remain in an unstable state (-W temper) if left at room temperature and requires an Artificial Aging process to attain full strength (e.g., 7075). An interesting and useful feature of the Natural Aging process is that it can be retarded by refrigeration. Material that Naturally ages can be stored below freezing temperature in the "soft" or annealed condition.

 Solution Heat Treatment Temperature

       As discussed, the solution heat treatment temperature is critical and dependent on the alloy type. Typical heat treatable wrought alloys are listed in Table 3 together with their Solution Heat Treatment Temperature and Temper Designation after Heat Treatment

  

Alloy 

Solution Heat Treat Temperature

 Temper after Heat

Treatment

 20172024 (sheet)2117 (rivets)60617075 (sheet)

 502C or 935 F493C or 920 F502C or 935 F529C or 985 F482C or 900 F

 2017-T42024-T42117-T46061-T47075-W

 *Note:    This sample data is derived from the Aluminum Association handbook and does not

supersede approved Company Process Specifications for production purposes. 

Soak Time       Soaking Times are designed to ensure that an even temperature is attained throughout the item(s)

being Heat Treated. Exact times will depend on: 

a.     Type of furnace (air or salt bath) 

b.     Material thickness (time will be governed by requirements of thickest piece in the load) 

c.     Type of material (Alclad or bare) 

Air furnaces are prone to a temperature drop during loading and to compensate, the Soak Time does not begin until all furnace instruments have returned to their original setting, i.e., the temperature set before opening the door to load. Load distribution within an air furnace is also

critical and can affect the required Soak Times, particularly if it is not a circulating air type. As there are so many variables, no attempt will be made in this Module to quote specific Soak Times but they can vary between 10 min. for light sheet to 12 hrs. for heavy forgings. The learner must receive training specific to equipment and tasks to be encountered before attempting this critical heat treatment operation.  Quench Process

       Cold water is the most common quenching medium. The time interval between removal of material from the furnace and total immersion in the cold water is critical and should be kept to an absolute minimum. Any delay in the transfer will cause a drop in alloy temperature and partial precipitation of alloy elements from solution will occur. The consequence of a slow transfer and partial precipitation is an alloy that is highly susceptible to Intergranular and Stress Corrosion. Similar effects are caused if the temperature of the water is too high. The Quench bath should begin at a temperature not greater than 30C and should contain sufficient volume of water so as not to raise above 38C while Quenching. During the Quench, parts should be agitated to ensure an even transfer of heat into the water.

  

Distortion of Parts       Parts will naturally tend to distort during the Solution Heat Treatment process. Immediately

following the quenching operation, parts remain in a soft state for approximately 30 minutes and can be formed or straightened until hardened by the Natural Aging process.

 Heavy sections, such as large forgings, are often quenched in hot water to avoid severe distortion and cracking, caused by uneven temperature distribution in the section. This process is considered acceptable as thick forging material is not as prone to attack by corrosion as light sheet metal.

  

Re-Solution Heat Treatment       Parts that are un-clad, i.e., bare alloy, may be Solution Heat Treated repeatedly without harmful

effects. However, Alclad parts exhibit a diffusion of the pure aluminum cladding into the aluminum alloy if repeated heat treatments are applied. This results in a reduction in strength and increased likelihood of attack by corrosion. As a rule, Alclad material should not be re-heat treated but exceptions may be made for thicker section material.

  

Heat Treatment Equipment       Solution Heat Treatment is a precision process and records must be maintained of process

temperatures and soak times. The normal means of heating materials is by Air Furnace of Fused Salt Bath. In either case the temperature must be controlled to within ±6C by a controller equipped with a temperature/time pen recorder.

 If an air furnace is to be used then one equipped to circulate the air is preferable, so ensuring a more even temperature distribution through parts properly suspended in the furnace. Rivets to be

heat treated are normally loaded into a wire basket to allow adequate heat circulation and subsequent quenching.

 In addition to paper records of the heat treatment process, a test coupon, of the same material batch as the aircraft parts, should be heat treated along with the parts. On becoming fully Aged, this coupon should be tested for hardness to confirm the heat treatment process.

 

 Precipitation Heat Treatment

Precipitation Heat Treatment is defined as an Artificial Aging process. The treatment consists of taking parts previously Solution Heat Treated, e.g., 7075-W or 6061T4 and forcing the material to age by holding it at a prescribed temperature for a considerable period of time. During the aging period, some of the alloy compounds precipitate out of solution and end up at the grain boundaries to increase material strength by interfering with the slip-planes.

 Precipitation Heat Treatment may also be used to Over-Age certain products and so improve resistance to attach by Intergranular Corrosion. This is found most effective with the 2XXX (copper) series of alloys, where Artificial Aging is used to supplement Natural Aging, primarily to reduce material elongation under load by increasing Yield Strength.

 Process temperatures and soak times are critical if full alloy strength and corrosion resistance is to be assured. Several processes are employed but Table 4 presents typical process specifications, as quoted by the Aluminum Association:

                                                                         Table 4                                                         Precipitation Heat Treatment 

 Alloy

 Precipitation Temperature

 Soak Time

 Final Temper

 2024-T32024-T46061-T47075-W

 191C or 375 F191C or 375 F160C or 320 F121C or 250 F

 12 hr  9 hr18 hr24 hr

 -T81-T6-T6-T6

  

So, full Precipitation Heat Treatment involves the following process: 

a.     Heat the alloy to specified temperature 

b.     Soak the alloy at temperature for specified time 

c.     Air cool the alloy (under controlled conditions) to room temperature 

As for Solution Heat Treatment, paper records of the process are required and test coupons should be heat treated along with components.

  

Safety NotesAs all Heat Treatment necessarily entails working with high temperature equipment, care must obviously be taken to avoid burns. Thermal gloves must be worn when handling equipment.

 Working with a Salt Bath requires an extra level of awareness. Molten salt (Sodium Nitrite/Nitrate) does not have “steam” or give off a visible vapor and accidents have occurred through people inadvertently dipping their hand into a bath at 500C. The consequences are so drastic they can hardly be imagined!

 Explosions can be caused by introducing wet items into a Salt Bath. Water will expand to 1670 times its original volume as it turns into steam. At the temperatures in the salt bath it will turn into steam almost instantaneously. Ensure that all items are thoroughly dry.

 

HEAT TREATING ALUMINIUM

 

To understand the heat treatment and other ‘property improvement’ techniques used with aluminium and its alloys is necessary to understand what goes on inside the metal when it is first cast as either a pure metal or with the addition of alloys.

The aluminium molecule forms in the shape of a square with an atom on each corner and one in the center of each face of the square. The arrangement is called a face centered cubic cell. 

 

 

As the metal chills from the molten state, these cells form at random in the metal and the cells join together building up to form the grain, A similar building of cells can be observed on the surface of freezing water. Unlike water however shrinkage rather than expansion occurs.

 

UNIT CELLS BUILD UP AS TEMPERATURE DROPS

 

Continuing to build up the grains eventually interfere with adjacent grains giving an uneven grain shape. The rate of cooling affects the shape, configuration and size of the grains. Generally the axis of the grains is at random and therefore cannot be compared with the unidirectional grain in timber.

 

 

GRAIN BUILD UP CONTINUED AS TEMPERATURE DROPS UNTIL RESTRICTED BY FORMATION OF ADJACENT GRAINS

 

FINAL GRAIN STRUCTURE: AS SEEN UNDER MICROSCOPE.

 

When the metal is subjected to a load the grain structure will be deformed.  If the load exceeds the elastic limit the deformation will be permanent.

The deformation begins when the unit cells within the grain begin

to slide over one another with the application of the load and takes place along the planes between the individual cells. The ease of slip determines the hardness of the alloy.

I will discuss heat treatment using copper as the major alloying element as the example.

Copper forms the same type of face centered cubic cells. When added as an alloy to aluminium, some of the copper atoms displace aluminium atoms within the cell and make the slip more difficult and hence the metal harder.

When alloys are added and the metal is first cast, two types of material are formed, Intermetallic Compounds (chemical compounds) and Solid Solutions (mixtures). The Intermetallic compounds tend to lie in relatively large particles.  In this condition the metal is not very strong.

 

Before Solution Heat Treatment

CuAL2 Lies in large particles along the grain boundaries.

 

Aluminium can absorb small amounts of copper in solution at room temperatures.  With an increase in temperature more copper is absorbed in solution.

There is a similarity when one dissolves sugar or copper sulfate crystals in water. More crystals can be dissolved in hot water than in cold water. If as many crystals as possible are dissolved in hot water and the water allowed to cool then the crystals will precipitate out of the solution.

 

In solution heat-treating we are carrying out a similar process. The larger proportion of copper is absorbed in the aluminium at elevated temperatures.

Quenching in cold water then rapidly cools the hot metal. This holds the saturated solution of copper in the alloy.

 

After Solution Heat Treatment

 Cu AL2 is now dispersed in very fine particles uniformly

throughout the grain.

 

This condition however is only temporary as the copper still precipitates out of solution the process is known as age hardening because as the copper precipitates out in small particles, evenly, and these form in the slip planes making the metal harder and stronger. This process takes about 4 days.

 

 

In the mean time the metal is in a soft, workable condition for about 20 minutes.  It reaches 90% to 98% of its maximum hardness in 4 hours. This age hardening process may be halted if the newly treated metal is kept at temperatures below the freezing point of water.

Solution heat-treating puts the copper in the alloy in solid solution with the aluminium.  It is then soft.  After solution heat-treating, if the metal is allowed to reach temperatures above freezing, the metal age hardens to its strongest and hardest condition.

The age hardening process can be achieved artificially by Precipitation Heat-Treating (Artificial Ageing). The artificial ageing accomplished by precipitation heat-treating does a better and more even job than the natural method and with some alloys; the full hardness cannot be achieved without precipitation heat treatment.

The temperatures and soaking times shown on the tables are critical. If not then the solution and or precipitated alloy will contain pieces of free copper, which will cause intergranular corrosion.

 

Alloy Annealing Treatment Solution Heat Treatment (1) Precipitation Heat Treatment

Metal

Temperatur

e

Approx.

Time of

Heating

Hours

Temper

Designation

Metal

Temperature

F

Temper

Designation

Metal

Temperature

F

Approx.

Time of

Heating

Hours

Temper

Designation

1100 650   -0          2017 775 2-3 -0 940 -T4      2024 775 2-3 -0 920 -T4 375 7-9 -T86

2117 775 2-3 -0 940 -T4      3003 775   -0          5052 650   -0          6061 775 2-3 -0 970 -T4 350 6-10 -T6

7075 775 2-3 -0 870 -W 250 24-28 -T6

(1)     The time of heating varies with the product, the type of furnace and the size of load. For sheet metal heat treated in a bath of

molten salt, the time may range from 10 minutes for thin material to 60 minutes for thick. Time of several hours may be required in

air furnaces because the metal comes to temperature less rapidly. The material should be quenched from the solution heat-treating

temperature as rapidly as possible and with a minimum delay after removal from the furnace. Quenching in cold water is preferred

although less drastic chilling (hot or boiling water, air blast) is sometimes employed for bulky sections, such as forgings, to minimize

quenching stresses.

 

Quenching is also critical. Parts should be immersed directly from the furnace, edge first.  Cylindrical parts must be quenched end first.  Loss of heat between removal from the furnace and quenching will result in uneven treatment and lead to intergranular corrosion.

There are two types of furnaces used for aluminium heat treatment.  The air furnace, where hot air is the heating medium is the most satisfactory method for treating larger sections and the most usual production furnace in use.

Air furnaces are usually bottom loaded and located above the quench tank. Quench tanks should have a water spray on the surface to minimize the splash when parts are plunged into them from the furnace.

Salt baths are more easily controlled but because of their inherent dangers are generally reserved for small parts, rivets etc.

 

The Salt Bath

 

Salt baths contain a mixture of sodium nitrate and potassium nitrate. These salts melt at about 220 degrees C and in the molten condition they look like a pool of cool water. Hot salt can inflict nasty burns so don’t let appearances fool you, wear goggles, gloves, long sleeved overalls and leather aprons when using salt baths.

Jobs should be thoroughly dry before suspending in the salt bath. Water and salt definitely do not mix. Parts should be suspended so that they do not touch the sides or bottom of the bath.

If heating the bath from room temperature the surface of the salt should be broken. The bath should be cleaned regularly.

Magnesium rich alloys should not be treated in a salt bath.

In case of fire, evacuate the building and fight the fire from an external location, Water, in such cases cannot be used.

Non-heat treatable alloys may be annealed (softened) to enable

extensive working. All alloys harden when worked as the working changes the shape of the grains.