The welding of aluminium and its alloys
CEN designation system
3.3.1 Alloy composition identification
A full listing of all of the British and European specifications dealing with any aspect of aluminium alloys, product forms, supply conditions and welding is given in Appendix A at the end of the book.
There are two methods in the CEN system for identifying aluminium alloys, one based on the numerical designation adopted by ISO and as recommended by the Aluminum Association, the other on the basis of chemical composition. The details of the European system are contained in the specification BS EN 573. This is divided into four parts as follows:
• Part 1 Numerical Designation System.
• Part 2 Chemical Symbol Based Designation System.
• Part 3 Writing Rules for Chemical Composition.
• Part 4 Form of Products.
In the European system the prefix ‘AB’ denotes ingots for remelting, ‘AC’ denotes a cast product, ‘AM’ a cast master alloy, the prefix ‘AW’ a wrought product. For the wrought alloys this is followed by the four digit number which uniquely identifies the alloy. The first digit indicates the main alloying element, with numbers 1 to 9 being used as follows:
AW 1XXX - commercially pure aluminium.
AW 2XXX - aluminium-copper alloys.
AW 3XXX - aluminium-manganese alloys.
AW 4XXX - aluminium-silicon alloys.
AW 5XXX - aluminium-magnesium alloys.
AW 6XXX - aluminium-magnesium-silicon alloys. AW 7XXX - aluminium-zinc-magnesium alloys. AW 8XXX - other elements e. g. lithium, iron.
AW 9XXX - no alloy groups assigned.
Except in the case of the commercially pure aluminium alloys, the last three digits are purely arbitrary and simply identify the specific alloy. In the case of the pure aluminium, however, the last two digits indicate the minimum percentage aluminium in the product to the nearest 0.01%, e. g. AW-1098- 99.98% Al, AW-1090-99.90% Al. The second digit gives the degree of control on impurities: a zero indicates natural impurity limits, a figure between 1 and 9 that there is special control of one or more of the individual impurities or alloying elements.
There are a total of 36 separate compositions of casting alloys, divided into 11 subsections as follows. It is worth mentioning that 29 of the alloys are based on the Al-Si system.
AC 2 1 XXX - Al Cu.
AC 4 1 XXX - Al SiMgTi.
AC 4 2 XXX - Al Si7Mg.
AC 4 3 XXX - Al Si10Mg.
AC 4 4 XXX - Al Si.
AC 4 5 XXX - Al Si5Cu.
AC 4 6 XXX - Al Si9Cu.
AC 4 7 XXX - Al Si(Cu).
AC 4 8 XXX - Al SiCuNiMg.
AC 5 1 XXX - Al Mg.
AC 7 1 XXX - Al ZnMg.
As with the wrought alloys the third and fourth digits identify the specific alloy in the group and are arbitrary.
Master alloys, which will not concern the shop-floor welding engineer, are identified with the prefix ‘AM’ followed by the number ‘9’, the second and third figures are the atomic number of the main alloying element, e. g. 14 for silicon, 29 for copper, the last two digits being chronological and issued in the order of registration of the alloy. For example, an aluminium-silicon master alloy could carry the designation AM 91404, identifying the alloy as being the fourth Al-Si alloy to be registered.
3.3.2 Temper designations
The mechanical properties of the alloys are affected not only by their chemical composition but also by their condition, e. g. annealed, cold worked, precipitation hardened. It is obviously important that this condition is clearly and unequivocally identified for both the designer and the welding engineer. To do this CEN has developed a system of suffixes that identify the amount of strain hardening the alloy has undergone or its heat treatment condition. There are five basic designations identified by a single letter which may be followed by one or more numbers to identify the precise condition.
The basic designations are as follows:
• F - as fabricated. This applies to wrought products where there is no control of the amount of strain hardening or the thermal treatments. There are no mechanical properties specified for this condition.
• O - annealed. This is for products that are annealed to produce the lowest strength. There may be a suffix to indicate the specific heat treatment.
• H - strain hardened (cold worked). The letter ‘H’ is always followed by at least two digits to identify the amount of cold work and any heat treatments that have been carried out to achieve the required mechanical properties.
• W - solution heat treated. This is applied to alloys which precipitation harden at room temperature (natural ageing) after a solution heat treatment. It is followed by a time indicating the natural ageing period, e. g. W 1h.
• T - thermally treated. This identifies the alloys that are aged to produce a stable condition. The ‘T’ is always followed by one or more numbers to identify the specific heat treatment.
The first digit after ‘H’ identifies the basic condition:
• H1 - strain hardened only.
• H2 - strain hardened and partially annealed. This applies to the alloys that are hardened more than is required and that are then annealed at a low temperature to soften them to the required degree of hardness and strength.
• H3 - strain hardened and stabilised. Stabilisation is a low-temperature heat treatment applied during or on completion of fabrication. This improves ductility and stabilises the properties of those strain-hardened alloys that soften with time.
• H4 - strain hardened and painted. This is for alloys that may be subjected to low-temperature heat treatment as part of a paint baking or adhesive curing operation.
The second digit after ‘H’ indicates the amount of strain hardening in the alloy. H18 is strain hardened only and in the most heavily cold worked condition. It is therefore the hardest and highest strength condition. Ductility will be very low and further cold work may cause the component to crack. Intermediate conditions are identified by the numbers 1 to 7 and are based on the strength relative to that of the annealed alloy, O condition and the H18 condition, e. g. an H14 alloy will have a strength halfway between the annealed and fully hard condition, H12 halfway between O and H14. There is an H9 condition in which the ultimate tensile strength exceeds that of the H8 condition by a minimum of 10N/mm2.
The third digit after ‘H’ is not mandatory and is used when the alloy requires special control to achieve the specific temper identified by the second digit or when some other characteristic of the alloy is affected. Examples of such characteristics are exfoliation corrosion resistance, seam welded tube or additional working after the final temper has been achieved, e. g. by embossing.
The ‘T’ designations are applied to those alloys that are age hardened, the first digit identifying the basic heat treatment:
• T1 - cooled from an elevated temperature-shaping treatment and naturally aged.
• T2 - cooled from an elevated temperature-shaping process, cold worked and naturally aged.
• T3 - solution heat treated, cold worked and naturally aged.
• T4 - solution heat treated and naturally aged.
• T5 - cooled from an elevated temperature-shaping process and artificially aged.
• T6 - solution heat treated and artificially aged.
• T7 - solution heat treated and overaged or stabilised.
• T8 - solution heat treated, cold worked and artificially aged.
• T9 - solution heat treated, artificially aged and cold worked.
More digits may be added to the designation to indicate variations in heat treatments or cold work. For example, TX51,510,511,52 or 54 all indicate those alloys that are stress relieved after heat treatment by some form of cold working such as stretching or restriking cold in the finish die. These additional digits are also used to indicate the temper condition of those alloys designated ‘W’.
The T7, artificially aged, temper designation may be supplemented by a second digit to indicate if the alloy is overaged and by how much. Other numbers are used to identify underaged conditions and increasing degrees of cold work etc.
The full details of these designations are contained in the specification EN 515 ‘Aluminium and Aluminium Alloys - Wrought Products - Temper Designations’.
