The welding of aluminium and its alloys

Mechanised/automatic welding

Automation or mechanisation of the TIG process can have a number of benefits. These include the ability to use faster travel speeds, resulting in less distortion and narrower heat affected zones; the better and more consistent control of the welding parameters enables very thin sheet material to be welded; there is a greater consistency in the weld quality; and it is possible to employ operatives with a lesser degree of skill and dexterity than is required for manual welding. There are, as ever, some drawbacks to the use of mechanisation, not least of which is the need to provide the welding fixture with far more accurate and consistent weld preparations than are required by the manual welder. Accurate joint fit-up and alignment is crucial to achiev­ing consistently high weld quality. Jigs and fixtures also need to be capable of holding the components within tight tolerances and of maintaining these tolerances as welding proceeds. As an example, autogenous welding of thin (say 3 mm) plate requires root gaps to be maintained at 0-0.025 mm and plate edges to be aligned to better than 0.05 mm if root penetration problems are to be avoided. Adding filler wire will assist in increasing the permissible tolerances but at the expense of welding speed. It is possible to develop welding procedures that will provide an acceptable unbacked root pass, but in many welding fixtures a removable backing bar is part of the clamping system. This greatly simplifies the task of setting up the joints accurately and in achieving a sound root and is to be recommended.

Although the parameters of welding current and voltage require con­trolling within small tolerance bands, the parameters of wire feed speed and travel speed are far more significant. Variations in wire feed speed may lead to either underfill if the feed speed slows or overfill and lack of fusion or penetration defects if the wire feed speed increases. Too slow a wire feed speed can also result in the wire ‘balling back’ and prevent a smooth melting of wire into the pool.

Automation or mechanisation of both AC-TIG and DCEN helium TIG welding may be achieved by adapting the manual techniques using con­ventional manual equipment attached to manipulating equipment such as crawler tractors. The task of mechanisation is simplified if the weld is

autogenous and a wire feed is not required, although this can be easily pro­vided from a spool of wire fed from a cold wire feed unit. The wire should be fed into the leading edge of the weld pool at a similar angle to that used in manual welding. Both the start of the wire feeding and carriage travel should be delayed until the weld pool is well established. When ending the weld the current should be tapered down and the wire feed speed adjusted to provide crater filling.

DCEN helium TIG is ideally suited to mechanisation since full advan­tage can be taken of the increase in travel speed, which may be up to 10 times that of an argon shielded AC-TIG weld. It is also possible to weld thick plates, up to 18 mm thick, in a single pass, square edge preparation with no filler metal, making this a very cost-effective method. The high travel speeds possible with the technique may lead to undercutting, partic­ularly if the welding current is increased in the expectation that this will permit even higher travel speeds to be achieved. Short arc lengths are nec­essary when autogenous welding, typically 0.8-1.5 mm, and in some cir­cumstances the electrode tip may be below the surface of the plate with the arc force depressing the weld pool surface. Contraction during cooling will cause upsetting to occur, resulting in a local thickening of the joint and providing sufficient excess weld metal that the joint is not underfilled.

The welding of aluminium and its alloys

Alloy designations: wrought products

Table A.4 BS EN BS EN Old BS/DTD Temperature (°C) numerical chemical number designation designation Liquidus Solidus IVIdUng range Al 99.99 1 660 660 0 AW-1080A Al 99.8 1A AW-1070A …

Principal alloy designations: cast products

Table A.3 BS EN numerical designation BS EN chemical designation Old BS number ANSI designation Temperature (°C) Liquidus Solidus Melting range Al 99.5 LM0 640 658 18 AC-46100 Al Si10Cu2Fe …

Physical, mechanical and chemical properties at 20°C

Table A.2 Property Aluminium Iron Nickel Copper Titanium Crystal structure FCC BCC FCC FCC HCP Density (gm/cm3) 2.7 7.85 8.9 8.93 4.5 Melting point (°C) 660 1536 1455 1083 1670 …

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