The welding of aluminium and its alloys

Mechanised and robotic welding

As MIG welding is a continuously fed wire process it is very easily mech­anised. The torch, having been taken out of the welder’s hand, can be used at welding currents limited only by the torch or power source and at higher travel speeds than can be achieved with manual welding. A typical robot MIG welding cell where the robot is interfaced with a manipulator for increased flexibility and a pulsed MIG power source is illustrated in Fig. 7.19. Greater consistency in operation means that more consistent weld quality can be achieved with fewer defects. The advantages may be sum­marised as follows:

• More consistent quality.

• More consistent and aesthetically acceptable bead shape.

• More consistent torch height and angle mean that gas coverage can be better and the number of defects reduced.

• Fewer stops and starts, hence fewer defects.

• Higher welding speeds means less heat input, narrower heat affected zones and less distortion.

• Higher welding current means deeper penetration and less need for large weld preparations with fewer weld passes and therefore fewer defects.

• Higher weld currents mean a hotter weld and reduced porosity.

7.19 Pulsed MIG power source interfaced with a robot and manipulator. Courtesy TPS-Fronius Ltd.

• The above advantages mean that less welding time is required and rework rates will be reduced, giving major improvements in productiv­ity and reductions in cost.

• There is no need for the skilled welder required for manual welding, a major advantage in view of the current shortage of highly skilled welders. The loading and unloading of the welding cell can be performed by unskilled workers, although knowledgeable and experienced engi­neers will be needed to programme and maintain the equipment.

There are some disadvantages to mechanised and robotic welding. Weld preparations need to be more accurate and consistent; more planning is needed to realise fully the benefits; capital expenditure will be required to purchase manipulators and handling equipment; maintenance costs may well be higher than with manual equipment and the full benefits of high deposition rates may only be achieved in the flat or horizontal-vertical posi­tion. Despite these problems there is an increased usage of mechanised and automated MIG equipment because of the financial benefits that may be achieved.

Thickness

(mm)

Joint type

Backing

Current

(A)

Voltage

(V)

Travel speed (mm/min)

12

Square edge

Temporary

400

26.5

380

12

Square edge

Permanent

450

29

350

19

Square edge

Temporary

540

33

275

19

Square edge

Two sided

465

29.5

380

25

Square edge

Two sided

540

33

275

32

Square edge (6mm sight V)

Two sided

530

33

275

To illustrate the cost benefits of mechanisation take as an example a 12 mm thick butt weld. Made using manual MIG this would require four passes to fill at a travel speed of around 175 mm/min, a total weld time of over 20 minutes per metre. A machine weld using argon as the shield gas could be made in a single pass at around 480 mm/min travel speed, a total weld time of just over 2 minutes. Using helium as the shielding gas would reduce this time even further. A set of typical parameters is given in Table 7.6.

Because of the higher duty cycle achievable with mechanised or auto­mated welding the power source, wire feeder and torch must be more robust and rated higher than those required for manual welding. Welding currents of 600 A or more may be used and this must also be borne in mind when purchasing a power source. The torch manipulator, whether this is a robot, a dedicated machine or simply a tractor carriage, must have sufficient power to give steady and accurate motion at a uniform speed with repeatable, precise positioning of the filler wire. Although at low welding currents con­ventional manual equipment may be adapted for mechanisation by attach­ing the torch to a manipulator, it is advisable to use water-cooled guns and shielding gas shrouds designed to provide improved gas coverage.

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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