The welding of aluminium and its alloys

Welding procedures and techniques

A set of outline welding procedures are given in Tables 7.2 and 7.3 for butt welding using either argon or helium as the shielding gas, and guidance on parameters for fillet welding is illustrated in Fig. 7.17. The parameters quoted form a starting point from which to develop a procedure specifically designed for the application. They are not to be regarded as hard and fast rules. Also included as Table 7.4 are suggested weld preparations for MIG welding of a range of plate thicknesses.

7.4.1 Arc starting

Because the wire is fed into the arc immediately that the arc is started there can be no preheating of the joint as possible with TIG. This results in shallow penetration and a humped weld bead on starting. Lack of fusion defects are often encountered - a ‘cold start’ - and weld bead shape may not be acceptable. To avoid these defects the welder should strike the arc some 25 mm ahead of the desired start point and then move back to the weld start before beginning to weld forward at a normal speed.

Arc starting may be achieved using a scratch start where the wire is allowed to protrude from the contact tip by 10 mm and brought to within 20 mm of the surface. The trigger is operated and at the same time the welding torch is moved to scrape the wire tip over the work surface. As soon as the arc is established the power source senses the change in voltage and starts the wire feed, the weld pool forms and welding can commence. A ‘running’ start is one where the wire begins to feed as soon as the trigger is operated and is short-circuited when it touches the workpiece, establish­ing the arc. The current surge on short-circuiting may cause arcing within the contact tip and spatter to adhere to the shroud and contact tip. These can lead to wire feeding problems.

As mentioned earlier, the new inverter power sources have a facility for a highly controlled arc start sequence. When the trigger is operated the wire is fed at a slow and controlled rate until the wire tip touches the workpiece. It is then retracted slightly and a pilot arc is ignited. Once this is stable the current is increased at a controlled rate, the wire speed increased to the desired feed rate and welding commences (Fig. 7.8). This gives a spatter - free start and a low risk of lack of fusion defects, a major improvement over the capabilities of older equipment.

Thickness Root gap/ Included angle Backing

(mm) face (mm) (degrees)

Voltage No. of

(V) passes

Filler diam. Travel speed

(mm) (mm/min)

3 face 1.6 1050 root/

TOC o "1-5" h z 3 reverse 800

3 face 1.6 850 root/

1 reverse 550

3 1.6 550 root/

500

4 face 1.6 850 root/

4 reverse 650

4 1.6 550 root/

450

4 face 1.6 900 root/

4 reverse 550

4 face 2.4 1000

1 reverse

5 2.4 1000

6 face 1.6 600

6 reverse

2.4 1000

2.4 1000

1. Where two welding parameters are specified in one entry the first refers to the requirements for the first pass.

2. Where a reverse side weld is specified it is necessary to grind the reverse of the root pass to ensure a sound joint.

3. When making a double sided joint it is recommended that the weld passes are balanced to reduce distortion.

7.4.2 Torch positioning

The angle at which the torch is presented to the joint is important in that an incorrect angle can result in air entrainment in the shielding gas and will also affect the degree of penetration. Ideally the torch should be normal to the surface and pointed forwards towards the direction of travel at an angle of between 10° and 15° from the vertical, the forehand angle (Fig. 7.18). As this angle increases penetration decreases and the amount of air entrained in the shielding gas gradually increases.

Arc length cannot be set by adjusting the voltage since this is a function of the resistance of the circuit as a whole. The arc length is set by the welder using both sight and sound, a correct arc length being characterised by a

Material

thickness

(mm)

Up to 1.6mm root gap. One or more runs from each side. Back-chipping recommended after first run

12.7-25.4mm

12.7-25.4mm

7.18 Torch position for MIG welding.

soft crackling sound similar to the sound of frying bacon. Too short an arc sounds harsh and gives excessive spatter while a long arc has a humming sound. The effect of changing the arc length is summarised in Table 7.5.

7.4.3 Ending the weld

If, when the weld is ended, the wire feed is abruptly stopped the weld pool will freeze and a shrinkage crater will form. If the weld pool is small this crater may be simply a shallow depression in the weld surface. In large weld pools the crater may extend down into the weld to form an elongated pore - piping porosity. As the weld continues to cool and contract then the asso­ciated shrinkage stresses may cause hot short or crater cracks to form. Any form of cracking is unacceptable and is to be avoided. Methods of elimi­nating this defect include the following:

• The use of run-off tabs on which the weld can be terminated, the tab being subsequently removed.

• Increasing the travel speed just before releasing the trigger. This causes the weld pool to tail out over a distance. It requires a high measure of skill on the part of the welder to produce acceptable results.

• Making a small number of brief stops and starts into the crater as the weld cools. This adds filler metal to the crater.

• As the trigger on the torch is released the wire feed speed and the welding current are ramped down over a period of time. The crater is fed with progressively smaller amounts of molten filler metal as it forms, resulting in the filling and elimination of the crater. This crater filling facility is standard on modern equipment and is the preferred method for avoiding piping porosity and crater cracks.