Welding Replacement Parts

The welding of the body elements can be carried out, depending on the requirements, with different methods:

• Spot welding
• Wire welding (MIG)

Equipment- 

The equipment used for these operations are:

• Welding machines with clamps - Squeeze Type Resistant Spot Welding (STRSW)
• Continuous wire welding - Metal Inert Gas (MIG)

General information on electric spot welding- 

During spot welding the heat required to melt the metal is provided by the resistance to the flow of current which the metal opposes.

Spot welding is carried out on panels where the join edges are overlapping and takes place through localized fusion of the metal that makes up the panel so no weld metal is required for this type of welding. In areas where three or more panels are overlapping, the spot welding must be repeated a second time.

The type of join that is produced is discontinuous; the weld spots must be correctly spaced in order to confer good mechanical resistance, in accordance with the precise instructions (see the tables below).

Spot welding- 

In the case of spot welding, check:

• That the arms are correctly aligned
• That the diameter of the ends of the electrodes are correct
• The flatness and correct matching of the edges of the weld
• The correct welding sequence.

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1. Correct welding sequence
2. Incorrect welding sequence
3. Size of the electrode

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Spot welder, geometric specifications of the electrode depending on the thickness of the panel and the welding sequence.

1. Table for determining the distance of the spot welds from the edges of the panels according to their thickness.
2. Table of reference values for the correct spacing of spot welds depending on the thickness of the panel.

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1. Area where welding should not be carried out

Seam welding (MIG)- 

When seam (MIG) welding, check:

• The speed of the wire
• The correct execution of the seam welding beads (alternation of welding sections)

In this welding system the electrode (an automatic advance continuous wire) is protected by an atmosphere of inert gases (this is where the initials MIG come from - Metal Inert Gas). - The continuous advance of the wire (which constitutes both the electrode and the weld metal), allows long welds to be made with no interruptions. The flow of inert gases sent to the welding area eliminates the air surrounding the fusion bath, preventing the oxidation of the metal. It also carries out the function of a protective coating for coated electrode welding. For these reasons, MIG welding differs from welding carried out with a coated electrode due to the absence of slag on the weld bead. Neither is it affected by the porosity that may be seen with normal arc welding.

Operating methods- 

In order to butt weld two panels correctly using a MIG welder: grind or dress the panel thoroughly, position the equipment so that when carrying out the welding the torch outer casing does not become twisted or folded in such a way that could obstruct the advance of the wire. Wear protective clothing and a suitable mask with an anti-glare visor (standard no. 8 glass for currents of 40-60A and no. 9 for currents of 80-200A). Be certain to have good electrical contact. Regulate the flow of gas in accordance with the instructions from the equipment's manufacturer. Make two tack welds at the ends of the join line and one in the middle, then make tack welds in between the two sections defined in this way. The spot welds should be 25 - 30 mm apart.

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The figure shows the correct application of the welding (1).

Carrying out welding (2) using an inert gas electrode (MIG) to join adjacent panels.

It is advisable to stitch weld initially because it is practical. The panel won't become distorted as a result of the build up of heat. Finish the welding process connecting the stitches to form a complete weld seam.

The difficulty in carrying out a single seam weld is that the torch/gun must be moved fairly slowly to produce proper penetration of the weld but sufficiently quick enough so as not to run the risk of "burning" through the panel.

Moving the torch/gun too quickly also leads to a poor fusion of the weld seam because, after grinding the seam, an insufficient layer of filler material remains to guarantee the strength of the joint.

After welding the panel, use a rigid disc grinder to grind the weld level with the panel.

At this point the alignment of the panels must be checked and, if necessary, corrected using a hammer and dolly block.

Then proceed with filling the gaps between the various spot welds through alternating welding sections, keeping the torch at an angle of about 60° (see previous diagram).

1. Tack welding the joint. The tack welds at the back of the panel are illustrated. Grinding the joint tack welds.
2. Welding.
3. The illustration shows two alternatives for the correct welding sequence in order to prevent the distortion of the panel.

The advantage of the welding operation is confirmed by the appearance of the rear of the weld seam which must show a series of spherical caps side by side for the entire length of the joint.

The MIG welder can also be used for joining overlapping panels where only one side is accessible.

To carry out spot welding, the panels must adhere perfectly and the torch must be perpendicular to the surface.

In this way the action of the electrode initially melts the first panel, then the second, where a crater is produced which is filled by the advance of the electrode.

In order to carry out this operation correctly, the welding machine must be regulated to current values that are suitable for the thickness of the panels being worked on with the torch resting on its supports and pressure on the surface to facilitate the panels being brought close together.

If the panels are thicker than 1.5 - 2.5 mm, make an initial 0.6 mm hole at the sites of the plug welds.

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1. Panel join area
2. Electrode
3. Plug weld
4. Torch/Gun
5. Hole (only for thick panels)

General rules for visible welding- 

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The following are important rules to be reckoned with:

• Incomplete penetration (p) should be 15-30% of the thickness of the panel (see figure below - point A incorrect welding, point B correct welding).

• The height of the seam (a) should be 60% of the thinnest thickness (s), the clearance (g) between the panels before welding should be 20% of the thinnest thickness for at least two thirds of the length (l) of the seam (see diagram below, point C and detail X). the diagram illustrates the value of the convexity (c) of the seam depending on the length (l) of the weld section.

• The convexity (e) of the shape of the weld seam section should depend on the length of the actual seam (see diagram below, point D).

Welding using brazing applies only if it is necessary to replace sheet metal that was previously welded with this type of welding and should only be carried out at the points recommended by the manufacturer.

This type of welding does not guarantee mechanical properties that are comparable with the other systems described previously and therefore does not apply to the joining structural components.

Air filtration and purification systems- 

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When carrying out work that produces fumes, gases and dusts which are dangerous to the mechanic, air filtration and purification systems must be used.

These systems are designed for the various requirements to handle different volumes of air that require purification. The operation of these systems is usually based on the combined action of mechanical and electro-static filtration and purification using activated charcoal.

Filtration takes place in two stages: mechanical elimination, using mesh filters, of the largest particles of dust, then ionization of the air flow, which statically charges the particles of dust. These are then eliminated from the flow through an electrostatic process.

The flow of air is then purified by passing through activated charcoal filters. In these systems, in addition to the specific efficiency, the ease of handling and noise levels have been improved and are never exceeds a level of discomfort.