Sheet Metal Shop Basics Of Heat Treating Aluminum Alloys By David Sharp 1612 New York Ave., Manitowoc, Wis. /^RANGES IN THE PROPERTIES of aluminum alloys V_^ by heat treating are brought about by the solution and precipitation of certain alloy phases. These phases differ in composition according to the aluminum alloy in question but all harden by the same basic principle. At high temperatures the hardening phase is soluble in the aluminum and if given enough soak time will dissolve in it. At lower temperatures the hardening phase is not soluble and will precipitate out of solution. The size and distribution of these precipitated particles give the metal its hard or soft characteristics. If the metal is cooled slowly from solution temperature the hardening phase will precipitate out as large, widely spaced particles and the metal will be in the 0-temper (soft). By quenching the metal from solution temperature (usually in cold water for wrought products) the precipitation reaction is arrested and the hardening phase will stay in solution. Immediately after quenching the metal is nearly as soft as 0-temper. This is an unstable, supersaturated condition and in time the hardening phase will precipitate in a very fine dispersed form, causing the tensile strength and yield strength to increase. This room temperature precipitation is called natural aging and if the resulting structure is stable the metal is referred to as being in the T4 temper. Extreme care must be exercised to closely control the temperature of the metal when solution treating aluminum alloys. Overheating of the metal will result in melting of the eutectic phase at the grain boundaries. Once eutectic melting has taken place the metal will be brittle and virtually useless. The copper bearing alloys (2000 series) are especially susceptible to this form of damage. The danger lies in the fact that the condition cannot be detected visually. The following table lists the eutectic melting points of some of the more common copper bearing alloys.

Solution Temp.

Alloy 2014 ......... .......... 2017 . . . . . . . . . .......... 2024 . . . . . . . . . . . . . . . . . . . 2117 ......... ....... . 6061 ......... .......... (ASM

925°- 945°F 925°- 945°F 910°- 930°F 925°- 950°F 970°-1000°F

Eutectic Melting Temp. 950 °F 955°F 935 °F

Metals Handbook Vol. II)

tion of the hardening phase precipitate. Temperatures range from 240° to 375°F and soak times from 5 to 48

hours, depending on the alloy. The resulting stable condition is referred to as the T6 temper. In general, alloys which are stable in the naturally aged T4 condition can also be artificially aged to the T6 temper with a resulting increase in strength and decrease in formability. Typical Physical Properties of Aluminum Alloys Tens. Str. psi

Alloy 2014-T4 2014-T6 2017-T4 2024-T4 2024-T6 2024-T3 2024-T81 2117-T4 6061-T4 6061-T6

. . .... ........ ........ ........ . . . . .. ........ ........ ........ . . .... ........

62,000 70,000 62,000 68,000 69,000 70,000 70,000 43,000 35,000 45,000

Yield Str. psi

Elongation %

42,000 60,000 40,000 47,000 57,000 50,000 65,000 24,000 21,000 40,000

20 13 22

20 10 18 6 27 22 12

These values are only general. Different products and thicknesses of the same alloy will have different strength levels, ie.—forgings, extrusions, sheet. Alclad sheet will have slightly lower strength than unclad alloy. Other temper designations used for aluminum are: T-3—This temper is the same as the T4 temper except that the metal is given a controlled amount of cold work (usually about 1%) after solution heat treatment to improve the properties of the naturally aged condition. This cold work is applied by rolling or stretching, depending on the product. Alloy 2024 is commonly supplied in this temper. T8—This temper is the result of artificially aging metal in the T3 temper. That is, the metal is cold worked as described above, just after the solution heat treatment, and then is given the artificial aging treatment. Another digit following the 8 would indicate the amount of cold work, ie.—T81 has 1% cold work; T86 has 6% cold work. Alclad Aluminum

The stronger heat treatable aluminum alloys are not noted for their corrosion resistance. To protect sheet rolled of these alloys from corrosion the metal is often supplied with a metallurgical bonded cladding of a corrosion resistant aluminum. The cladding is usually high purity,

non-heat treatable aluminum. For example the cladding After quenching, some alloys such as 2024 are considerably hardened in a few hours and reach a stable T4 condition in a number of days. At extremely low temperatures the precipitation reaction after quenching is so slow that the soft, as quenched properties can be retained for some time. This is the basis for storing 2024 and 2017 rivets in dry ice between quenching and driving. Other alloys, such as 7075, keep increasing in strength for years after quenching unless they are artificially aged

to a stable condition. This precipitation heat treatment (artificial aging) is generally a low temperature, long time heating, which closely controls the size and distribu34

AUGUST 1967

on alclad 2024 is 99.50% minimum pure aluminum. The thickness of the cladding (2024 alloy) is 5% of the sheet thickness per side. Thus a sheet of .020" alclad 2024 will

have .001" of cladding on each side. Care must be used when heat treating alclad metal because at high temperatures the alloying elements that form the hardening phases are soluble in the aluminum cladding and will diffuse into it. If the metal is soaked at the solution temperature for too long a time the diffusion will reach such an extent that the corrosion resistance of

the cladding will be destroyed. From the T-18 Newsletter ®