For thermally hardenable alloys 6xxx reinforcing components are magnesium and silicon. therefore, to achieve the maximum potential of the alloy for hardening by aging, natural or artificial, in pressed profiles from 6xxx series alloys, for example, alloys 6060 and 6063, need to be sure, that before the aging operation, the solid solution contains the maximum possible magnesium and silicon content for this alloy.
T6 temper – full hardening
The usual way to maximize magnesium and silicon, available to highlight hardening phases during aging, consists, speaking scientifically, in heat treatment of alloy to solid solution. This is usually called just quenching.. The goal of quenching is to, to completely convert all (Mg, Si) -particles before aging operation.
For most alloys of the 6xxx series, the quenching heating temperature is in the range from 515 to 540 oC, with excerpt from 10 minutes to 1 hours depending on the thickness of the product and the type of furnace. Cooling after heating should be fast enough - in the air, compressed air, jets of water or immersion in water depending on the alloy, profile thickness and its shape. This is the condition of thermally hardened alloys., hardened with separate heating and then artificially aged is designated according to the international classification T6.
T5 state – incomplete hardening in the press
For all aluminum alloys of the 6xxx series and some alloys of the 2xxx and 7xxx series, the thermally hardened state T5 is provided, which is obtained by artificial aging profiles, which were accelerated cooled at the exit of the press.
International standards call this condition: "chilled from the temperature of thermoforming and artificially aged". Our standards, for example, GOST 8617 and GOST 22233, profiles in this state are called more definitely and slightly negatively: "incompletely hardened and artificially aged".
If there was no complete dissolution when pressing profiles, for example, magnesium and silicon in a solid solution of aluminum and / or cooling on the press was not fast enough, the, really, quenching will be incomplete. Therefore, the tensile strength requirements, for example, alloy 6063 in state T5 on 30 MPa below, than in the T6 condition.
Strength of aluminum profiles in T5 and T6 states
Strength aluminum alloys AD31, 6060 and 6063 in T5 states, T6 (T1 in domestic designations) with a wall thickness of up to 3 mm (for comparison) can be expressed by the following "formulas", which follow from the requirements for them standard GOST 22233-2001:
- for state T6 (T1):
6060-T6 = 190 MPa
AD31T1 (22) = 6060-T66 = 6063-T6 = 215 MPa
AD31T1 (25) = 6063-T66 = 245 MPa
- for T5 state:
6060-T5 = 160 MPa
AD31T5 = 6063-T5 = 175 MPa.
State T66 differs from the usual state of T6 in increased strength due to special control of manufacturing technology of profiles.
Full hardening in the press – state T6
The practice of manufacturers of aluminum profiles has long been shown, that complete quenching of the alloy can be achieved not only after separate heating, but also directly on the press. For this, it is necessary to ensure the complete dissolution of all (Mg2Si) -particles in the workpiece before or during pressing and to prevent the formation of new particles when cooling the profile. If such measures have been taken in the press, then after artificial aging a condition is achieved with a strength not lower than, and sometimes higher, than in the T6 condition.
T6 state during press quenching according to EN 755-2
The European standard EN 755-2 defines the requirements for the strength properties of extruded products, including, profiles. In tables with strength requirements for the part of thermally hardened alloys, the same note is present 5 for states T6 (and also T4 - for natural aging): "Properties can be obtained by quenching on the press". To these alloys, which can reach the state of T6 during hardening in the press, all 6xxx series alloys apply, including the most “running” – 6060, 6063, 6061, 6082 and 6005, as well as some alloys of the 2xxx series (2007, 2011, 2017A i 2030) and 7xxx series (7003, 7005 (analogue of our 1915), 7020 and 7022). This entry is EN 755-2 recorded and "legitimized" the long-term practice of obtaining the strength properties of the T6 state during hardening in a press instead of more expensive hardening with separate heating. Thus, if the pressed products after quenching on the press and artificial aging satisfy the requirements for the strength properties of the T6 state, then it is assigned a state of T6, although formally hardening did not take place with separate heating, and in the press.
T5 and T6 states in GOST and DSTU standards
Domestic requirements for the mechanical properties of extruded aluminum profiles determine interstate standards GOST 8716 and GOST 22233-2001, as well as Ukrainian DSTU B V.2.6-3-95 (GOST 22233-93). According to the manufacturing accuracy, profiles for enclosing structures are divided into three types: normal accuracy, increased accuracy and special (precision) accuracy. Normal accuracy generally corresponds to the accuracy of manufacturing general purpose profiles according to GOST 8617-81.
GOST 8617-81: states T5 and T1
Applies to general purpose extruded aluminum profiles made of AD0 grades, AD1, HELL and aluminum alloys, by series: AMts - D1, D16, AK4, AK6 – AMg2, AMg3, AMg5, AMg6 - AD31, AD33, AD35, AB – 1915, 1925 GOST 4784, as well as some alloys according to industry normative and technical documentation. T5 state is already present in this standard., instead of T6, its domestic equivalent is applied - state T1.
GOST 22233-2001: T5 states, T6, T64, T66 and T1
Combines requirements for extruded aluminum alloy profiles AD31, as well as its international analogues - alloys 6060 and 6063 for building envelopes (windows, facades, entrances to shops and t. P.), as well as combined ("warm") profiles of them. (On the features of the chemical composition of AD31 alloys, 6060 and 6063 in GOST 22233-2001 look. here). GOST 22233-2001 include requirements for the mechanical properties of profiles in T5 and T6 states for international alloys 6060 and 6063, as well as T5 and T1 (equivalent to T6) for the domestic AD31 alloy. In this case, the T6 state goes in two versions: "normal" – T6 and “reinforced” - T66, as well as unmatched, with increased ductility – T64. Similarly, the T1 state for the AD31 alloy also has two options: "normal" T1 (22) - "enhanced" – T1 (25).
DSTU B V.2.6-3-95 (GOST 22233-93): T5 and T1
Valid in Ukraine for extruded profiles made of alloys AD31 and 1915 for building envelopes, including, and in states T5 and T1. For combined ("warm") profiles in Ukraine there is a separate DSTU B V.2.6-30: 2006.
GOSTs about hardening in a press for T6 state
Any notes, as in EN 755-2, about the possibility of obtaining the T6 (or T1) state when quenching profiles on the press in accordance with GOST standards 8617-81, GOST 22233-2001 and DSTU B V.2.6-3-95 (GOST 22233-93) not. Therefore, what our standards, unlike European EN 755-2, do not give a formal basis for obtaining profiles in T6 or T1 states directly by quenching on a press.