Technology of welding of body metal joint

Poorly performed welding of the body metal together leads to local overheating and rapid through burning of thin sheet steel, especially in areas with broken zinc coating. When trying to connect spars or rapids without a gap and correct cutting of the edges, the effect of "warping" often occurs, when the part is deformed under the influence of thermal expansion, making further assembly of the body impossible without complex editing. Experienced craftsmen know that even a millimeter deviation in the setting of the current or the speed of the wire supply when working with a metal thickness of less than 0.8 mm is fraught with the formation of burns, which will subsequently have to be brewed in several passages, reducing the overall strength of the structure.

The main complexity of the process is the need for full weld root distillation with minimal heat, which requires the use of inverter semi-automatics with synergistic control or precise manual adjustment. Unlike a fly-flood joint, the joint does not have an additional area of metal to remove heat, so temperature This is becoming a critical factor in success. Incorrect selection of additive material or lack of protective argon atmosphere in certain modes of operation can lead to saturation of the seam with nitrogen and oxygen, turning the compound into a fragile structure, subject to instant destruction under vibrational loads.

Modern methods of body repair dictate their requirements for the quality of welded joints, especially when it comes to load-bearing safety elements. Use of the CO2 in its pure form or mixture with argon allows you to create a stable arc, but requires a welder to be highly qualified to control the welding bath. It is important to understand that the technology of the joint is not used everywhere, but only where the design of the unit does not allow overlapping of the sheets, or when it is required by factory standards for restoring the geometry of the body according to the manufacturer's specifications.

Preparing edges and gaps for the perfect joint

The quality of the future weld depends on 80% of how well the preliminary preparation of the connected surfaces is performed. The metal must be cleaned to a clean shine not only in the area of the future seam, but also on the adjacent area with a width of at least 20 mm on each side. The presence of an oxide film, paint, primer or anticorical residues leads to the porosity of the seam and a decrease in its mechanical strength, since these substances emit gases when heated, which remain inside the molten metal in the form of voids.

Particular attention should be paid to the geometry of the edges, which, ideally, should be smooth and fit tightly together along the entire length of the connection. If the metal is more than 1.5 mm thick, it is often required to perform V-cutting edge at an angle of 45-60 degrees, which provides deep melting of the root of the seam. For thin body panels with a thickness of 0.6-0.8 mm, cutting is not performed, but a minimum clearance is required, which is sometimes made equal to the diameter of the welding wire used.

To fix the parts in the correct spatial position before welding, it is necessary to use special clamps and rods, avoiding a point picket in a chaotic order. The grip joints should be located in increments of 50-70 mm and be performed with minimal thermal imposition, so as not to cause deformation of the thin sheet. Before the main broil, the places of the pickles are necessarily cleaned with a grinding wheel or file to remove slag and oxides.

⚠️ Attention: It is strictly forbidden to weld on surfaces treated with lead-containing primers or covered with bitumen mastics, since lead vapors are toxic, and bitumen completely destroys the structure of the seam when burned, making it porous and loose.

Edge-cleaning technique

Precise machining of the edges before welding is performed using a grinding machine with a petal circle or a special cutter. It is important not to overheat the edge during cleaning, as local overheating can change the structure of the metal and lead to its burnout during welding. The movement of the instrument must be progressive, without strong pressure, to maintain the geometry of the part.

Selection of equipment and welding modes

For high-quality welding of body metal, the most suitable equipment is a semi-automatic apparatus operating in a protective gas environment. The optimal choice is to use a mixture of argon and carbon dioxide in the proportion of 80/20 or 90/10, since pure argon gives too wide and shallow bath, and clean. CO2 Increases the sprinkling. Inverter current sources ensure the stability of the arc even at low currents, which is critical for working with a thin metal with a thickness of 0.6 mm.

The diameter of the welding wire is selected depending on the thickness of the welded parts: for the body metal, the standard is a wire with a diameter of 0.6 mm, less often 0.8 mm. The use of thicker wire will require an increase in current, which will inevitably lead to overheating and burning of the thin sheet. The wire feed rate should be synchronized with the melting rate so that the end of the wire is at a minimum distance from the bath, but does not touch the metal, causing sticking.

The voltage and current strength adjustment is performed experimentally on test samples of the same thickness and grade of steel as the repaired part. When welding, the joint current is usually set 10-15% lower than when welding to avoid sagging the bath and forming a hole. Modern devices are often equipped with a function of "two stroke / four stroke", where for bodywork preferable four-stroke mode, allowing you to separately control the fire of the arc and its completion.

The table below shows the approximate welding modes for the different thicknesses of the body metal using 0.8 mm wire and a mixture of gases:

Metal thickness, mm	Current power, A	Voltage, B	Feed speed, m/min
0.6 - 0.8	40 - 60	16 - 18	3.0 - 4.0
0.8 - 1.0	60 - 80	18 - 20	4.0 - 5.0
1.0 - 1.2	80 - 100	20 - 22	5.0 - 6.0
1.2 - 1.5	100 - 120	22 - 24	6.0 - 7.0

Welding technique

The welding process requires a high concentration and a steady hand from the operator, as a split second error can lead to a defect. The burner should be kept at an angle of 60-70 degrees to the plane of the part, tilting it towards the direction of movement ("angle forward") or perpendicular to the seam, but never deflecting back so that the gas does not cease to protect the welding bath. The movement should be uniform, without jerks and stops, with a constant speed corresponding to the melting speed of the edges.

There are several techniques of seam management, the choice of which depends on the gap and thickness of the metal. With a minimum gap, the "forward" method is used, when the welder leads the burner, controlling the formation of the seam in front of him. If the gap is increased or the cutting is required, oscillatory movements of the burner across the seam ("month", "triangle") can be used, however, on thin metal it is better to use direct conduct without transverse oscillations to minimize thermal application.

Particular attention is paid to the beginning and end of the seam: it is better to ignite the arc at a distance of 5-10 mm from the edge, quickly returning to the beginning to avoid burning the edge. Finish the seam is necessary by quickly moving the burner to the side or using the function of "crater welding", if it is provided by the apparatus, to prevent the formation of a terminal crack. After cooling the seam, it is necessary to knock down the slag (if it is formed) and clean the connection.

Corrosion protection and galvanizing

Body metal almost always has a zinc coating that burns in the welding zone, leaving the site vulnerable to electrochemical corrosion. Immediately after welding and cleaning the seam, it is necessary to restore anticorrosion protection using special zinc-based compounds. Application of zinc-containing soil It is a mandatory step, as it creates cathode protection, sacrificing itself for the sake of preserving the base metal.

For the treatment of internal cavities of joints, which is not possible to access by brush or spray gun, wax anticores with spraying under pressure are used. These compounds penetrate into microcracks and create an elastic film that does not crack when the body vibrations. The outer surfaces after priming are stained and painted, but the layer of zinc soil under the paint remains the most important barrier against rust.

Using a copper spray on the back of the seam during welding is also common practice to prevent splash adhesion and additional thermal protection, although its anti-corrosion properties are limited. The main thing is not to leave the weld open even for a short time, since fresh metal oxidizes instantly, especially in moist air.

⚠️ Warning: Never apply zinc-containing compounds over paint or rust – they only work when in contact with pure metal, otherwise the effect of the galvanic pair will not occur, and corrosion will continue under the protection layer.

Typical defects and ways to eliminate them

Even with the observance of technology, defects can occur that must be able to eliminate and eliminate. The most common problem is the porosity of the seam caused by insufficient supply of protective gas, draft in the working area or the presence of moisture on the wire. Pores reduce the cross section of the seam and are foci of corrosion, so such a site must be completely removed by grinding and digested, having previously eliminated the cause of the gas.

Cuts (grooves along the seam) are formed with too high current or long delay of the arc at the edges of the seam. They create a concentration of stresses and can become the beginning of the crack. Small cuts can be brewed with an additional passage with reduced current, and significant require grinding and complete digestion. Burns, in turn, are brewed by patching or multipass cooking with low current, starting from the edges of the hole.

Non-melting of the edges is a dangerous defect when the metal of the seam is not mixed with the base metal, but simply lay on top. This often happens with low current or high welding speed. You can check the presence of non-fusion visually (no characteristic risks of fusion) or using a magnifier. The defect is eliminated only by complete removal of the seam and repeated welding with the correct parameters.

FAQ: Frequently Asked Questions

Can I cook the body together without a gap?

Cook without a gap can only be very thin metal (up to 0.8 mm) with an ideal edge geometry, but for high-quality weld root broth on metal thicker than 1 mm, it is recommended to leave a gap equal to about half the diameter of the wire. The absence of a gap often leads to non-provar in the depth of the seam.

Do I need to remove zinc before welding?

It is not recommended to remove zinc mechanically directly in the seam zone, as this deprives the metal of protection. Zinc burns out during the welding process, forming oxides that can give pores. The best strategy is to cook with minimal clearance, and then qualitatively restore protection with zinc-containing soil.

Which gas is better: pure CO2 or mixture?

For bodywork, a joint mixture of argon and CO2 (80/20) is preferable, as it gives a softer arc, less spray and more stable combustion at low currents. Pure CO2 is more aggressive and prone to splashing, which is critical when working with thin metal.

How to treat the seam after welding inside the threshold?

For the internal cavities of the sills and spars after welding, it is necessary to use wax anticores with a sputter tube. They penetrate into all hard-to-reach places and create a durable protective film, unlike liquid soils, which may not leak deep.