How to Weld Spot Welds with a Semi-Automatic Welder Correctly and Without Burn-throughs

Spot and electro-rivet welding using a semi-automatic machine (MIG/MAG) is a fundamental skill without which quality body repair of a car or assembly of thin metal structures is impossible. The method of overlapping spot welding allows for reliable joining of parts while minimizing deformation from overheating. However, beginners often find this process difficult: accidental metal burn-through, lack of fusion, or a porous joint structure can ruin the entire job.

The secret to a perfect "electro-rivet" lies in the details. Everything is important: the correct setting of welding current and wire feed speed, the choice of shielding gas (pure carbon dioxide or argon mixture), and thorough surface preparation. In this practical guide, we will detail how to set up the semi-automatic machine, avoid typical errors, and achieve strong, aesthetic spots even on ultra-thin metal.

Preparing Equipment, Gases, and Consumables

The quality of the spot weld made by a semi-automatic machine directly depends on how thoroughly you have prepared the workspace and consumables. Even the most expensive welding apparatus cannot form a reliable "electro-rivet" if you try to weld over rust or use unsuitable wire.

Before pressing the torch button, you must assemble the correct "kit" of shielding medium and filler material, and thoroughly clean the mating surfaces until they shine. Below, we will examine which consumables guarantee maximum joint strength without pores and how to properly prepare the metal for overlapping welding to avoid defects even before the arc ignites.

Choosing Welding Wire and Shielding Gas (Carbon Dioxide vs. Flux Wire)

For quality spot welding (electro-riveting), especially in thin metal body repair (0.8–1.2 mm), the correct selection of consumables is critical.

• Gas Method (Classic): The optimal choice is copper-coated welding wire ER70S-6 (SW-08G2S) with a diameter of 0.8 mm and shielding gas carbon dioxide (CO2) or an argon-carbon dioxide mixture (Ar+CO2). Carbon dioxide provides reliable protection for the weld pool and a dense spot, while the argon mixture additionally reduces spatter and heat input, minimizing the risk of metal burn-through.
• Gasless Method (Flux Wire): Using self-shielded flux wire for spot welding on thin body metal is highly undesirable. It burns at elevated temperatures, which inevitably leads to deformation. Furthermore, the resulting slag contaminates the joint, requiring thorough cleaning of every tack weld.

Comparison Parameter	ER70S-6 Wire + CO2	Flux Wire (Gasless)
Burn-through Risk	Low (heat input is easy to control)	High (due to increased flux heating)
Spot Quality	Clean, flat, without pores or slag	Porous, requires mandatory slag removal
Application Area	Ideal for automotive body parts	Outdoors, for thick metal (from 2 mm)

Proper Cleaning and Preparation of Parts for Overlapping Welding

The quality of the weld spot directly depends on the cleanliness of the mating surfaces. Any residues of old paint, rust, primer, or zinc coating burn instantly upon heating, contaminating the weld pool. This leads to the formation of pores, spatter, and lack of fusion.

Step-by-step algorithm for preparing overlapping parts:

1. Mechanical Cleaning. Clean the mating areas of the blanks until they have a bright metallic shine on both sides. For this, it is best to use carbide discs or flap discs on an angle grinder (Ushm)—they effectively remove dirt without thinning the metal itself. The width of the cleaned area should be 15–20 mm wider than the area of the future overlap.
2. Degreasing. Wipe the prepared areas with a rag soaked in acetone or anti-silicone agent to remove traces of oil and grease.
3. Anti-corrosion Protection. If body repair is being performed, apply a special conductive welding primer based on zinc to the inner surfaces of the overlap. This will protect the hidden cavity from corrosion and will not interfere with the passage of welding current.

Ensuring perfect contact between the sheets is the key to the arc igniting instantly and the spot turning out dense and neat.

Setting Up the Semi-Automatic Machine for Spot Welding

Proper part preparation is only half the battle. To achieve a strong electro-rivet without burn-throughs or lack of fusion, you must meticulously set the parameters of the semi-automatic machine. Unlike forming a continuous seam, spot welding requires instantaneous and powerful heat input, so standard "universal" settings are not always applicable here.

The correct balance of parameters directly depends on the thickness of the mating blanks and the chosen wire diameter. We need to precisely adjust the key characteristics of the machine so that the arc has time to reliably join the sheets in fractions of a second, without turning the metal into a through-hole.

Adjusting Welding Current, Voltage, and Wire Feed Speed

A quality spot (electro-rivet) is formed in fractions of a second, so the accuracy of the semi-automatic machine's settings is critical. Unlike continuous seam welding, spot welding requires instantaneous and deep fusion without the risk of burning through the blank.

The main parameters are adjusted according to the thickness of the metal being welded. For classic body steel of 0.8–1.0 mm thickness using 0.8 mm wire, refer to the following values:

• Voltage (Volts): Determines the width of the weld pool. The optimal range is 16–18 V. At a low voltage, the spot will be convex and lack fusion; at a high voltage, a burn-through will occur instantly.
• Wire Feed Speed (Current): Directly determines the current strength. For thin metal, set the speed corresponding to a current of 60–80 A (on most machines, this is approximately 3–4.5 m/min).
• Inductance: If the machine has this regulator, increase its value. This will make the arc softer, reduce metal spatter, and help form a flatter spot.

Before welding finished parts, be sure to make several test spots on a similar piece of scrap metal for precise parameter adjustment.

Using the Tacking Mode and Setting the Spot Welding Timer (MIG-spot)

Using the MIG-spot (timer welding) mode is the best way to achieve absolutely identical spots and completely eliminate the risk of burn-through due to human error. In this mode, the machine welds for a strictly set time, automatically turning off the arc even if you continue to hold the torch button.

To set the spot welding timer, refer to the following parameters:

• For thin metal (0.6–0.8 mm): set the cycle time (Spot Time) in the range of 0.5–0.8 seconds. This is enough for reliable fusion of the sheets without deformation.
• For medium metal (1.0–1.5 mm): increase the time to 1.0–1.5 seconds.

If your semi-automatic welder is not equipped with MIG-spot functionality, you will have to perform tack welds manually. In this case, rely on visual inspection: as soon as the weld pool forms and joins both parts (which usually takes about 1 second), immediately release the trigger.

Spot and Rivet Welding Technology

The transition from theoretical semi-automatic settings to practical application requires an understanding of the process physics. Spot welding (MIG-spot) and creating rivets are the foundation of quality body repair and working with overlapping sheet metal. Unlike a continuous seam, spot welding minimizes thermal deformation of the parts, preserving the structure's geometry, and provides strength comparable to factory contact welding.

The success of this technological operation depends on two key factors: flawless mechanical fastening of the mating elements and precise adherence to the spacing between points. The slightest gap between the sheets will lead to burning through the top layer without fusing with the bottom one, so preparing and assembling the unit before welding plays a crucial role.

Step-by-Step Process for Rivet Welding in Body Repair

Rivet welding (simulating factory spot welding) is a basic method in body repair. The step-by-step algorithm is as follows:

1. Hole Preparation. Prepare holes with a diameter of 5–6 mm in the upper sheet to be welded (for metal thickness of 0.8–1.0 mm). Use a special bodywork punch or a drill bit. The bottom sheet must remain intact.
2. Cleaning and Tight Clamping. Clean the mating surfaces down to bare metal. Clamp the parts tightly with clamps in the welding zone. Any gap between the sheets will cause the top metal to burn through.
3. Torch Positioning. Hold the semi-automatic torch strictly perpendicular (at a 90-degree angle) to the plane of the part. Direct the welding wire precisely into the center of the prepared hole, lightly touching the bottom sheet.
4. Point Formation. Press the button on the torch. Start the welding on the bottom sheet to form a solid weld pool, and then move the arc lightly in a spiral motion to the edges of the upper sheet hole, filling it with metal. The process takes only 1–2 seconds.
5. Weld Protection. After turning off the arc, hold the torch over the point for 1–2 seconds while the shielding gas (carbon dioxide) is supplied to prevent oxidation and porosity.

Determining Optimal Point Spacing and Clamping Workpieces with Clamps

To achieve a strong connection without deformation, it is important to correctly calculate the spacing between points and ensure their tight mating.

Determining Point Spacing:

• For structural elements (sills, longitudinal beams, pillars), the optimal spacing is 30–40 mm. This ensures strength comparable to a factory weld.
• For attached panels (fenders, arches), the distance is increased to 50–70 mm.
• Points placed too closely (less than 20 mm) lead to excessive overheating and metal warping, while points that are too sparse reduce connection strength.

Clamping Workpieces with Clamps:
Any gap between the sheets to be welded is a guaranteed burn-through of the top layer. Use welding clamps (gripper clamps) or C-clamps for tight pressing. Fix the workpieces directly at the welding site. To avoid deformation, place points alternately from different sides or move from the center to the edges, repositioning the clamps as you advance.

How to Weld Thin Metal Without Burn-Throughs and Deformations

Working with thin sheet metal, especially in body repair, requires jeweler's precision from the welder. When the steel thickness does not exceed 1 mm, standard techniques often lead to immediate burn-through or warping due to excessive heat. A semi-automatic welder is ideal for such delicate tasks, but it requires special control over thermal impact.

To achieve a strong connection without deformation, it is necessary to understand the physics of heating thin workpieces. Next, we will examine why critical overheating occurs and what professional techniques can help you keep the weld pool under complete control.

Main Causes of Burn-Throughs and the Influence of Workpiece Thickness

When working with thin sheet metal (especially body steels from 0.6 to 1.2 mm thick), the risk of a through-burn increases manifold. The main reason for this defect is excessive heat input into the weld zone, which thin metal cannot physically handle due to low heat capacity and high melting speed.

Among the key factors provoking burn-throughs are the following:

• Mismatch between welding current and workpiece thickness. Excessive current instantly turns the weld pool into a through-hole.
• Gaps between parts. When welding overlapping sheets, the sheets must be pressed as tightly together as possible. Any air gap acts as a thermal barrier: the top sheet melts without having time to transfer heat to the bottom one, leading to immediate burn-through.
• Too slow torch travel speed. Holding the arc in one place longer than necessary (even by fractions of a second) guarantees the appearance of a hole.
• Incorrect wire diameter selection. Using wire with a diameter of 1.0–1.2 mm on thin metal requires increased currents. For body repair, it is optimal to use wire with a diameter of 0.8 mm.

Understanding these physical processes allows you to correctly adjust the machine settings and prepare the parts for work.

Professional Techniques: Correct Torch Angle and Intermittent Arc

To control the capricious thin metal, professionals use precise torch positioning techniques and strict control over heat input.

Torch Angle
When spot and rivet welding, hold the torch at an angle of 80–90 degrees (almost perpendicular) to the plane of the part. This position ensures a concentrated direction of heat and guarantees solid penetration of the bottom sheet without the risk of burning through the edges of the top one. If the torch is tilted too much, the arc will begin to 'blow' the molten metal sideways, leading to one-sided buildup and lack of fusion on the base. The wire protrusion in this case must be minimal—within 8–10 mm.

Intermittent Arc Technique
To prevent overheating, the intermittent arc method (welding with short tack welds) is used:

1. Ignite the arc precisely in the center of the future point for 0.5–1.5 seconds until the weld pool forms.
2. Extinguish the arc, pausing for 1–2 seconds to allow the metal to crystallize and cool slightly.
3. Repeat the cycle, transferring the heat to the adjacent area.

This rhythmic approach allows for complete control over the temperature in the weld zone and avoids deformation.

Beginner Mistakes and Weld Quality Control

Even with the ideal torch angle and calibrated arc time, novice welders often encounter defects in spot welds. Achieving a strong joint in body repair or metal structure assembly requires not only skill but also the ability to recognize defects in time. An unwelded rivet or a hidden pore can negate all the work done, making the structure vulnerable to loads and corrosion.

To ensure your welds are always reliable and aesthetically pleasing, it is necessary to thoroughly examine the nature of typical errors and master basic quality control methods for finished joints. Furthermore, working with a semi-automatic welder in a garage or home workshop requires strict adherence to safety rules, as the technician's health is the top priority of any technological process.

Why Pores, Lack of Fusion Occur and How to Fix Them

When spot welding and riveting, novice technicians most often encounter two defects: pores and lack of fusion.

Pores in a Spot Weld
The main cause of porosity is poor gas shielding or contamination. Gas bubbles form due to:

• Poor cleaning: Residues of paint, rust, or oil instantly burn off, releasing gases. Always clean the metal until it shines like bare metal.
• Gas problems: Insufficient carbon dioxide flow (less than 6–8 L/min), drafts in the room blowing away the protective environment, or a nozzle clogged with spatter.

Lack of Fusion (Incomplete Fusion)
Lack of fusion makes the joint brittle. It occurs if:

• Welding current is set too low: The arc power is insufficient to heat the bottom sheet of metal.
• Incorrect wire feed speed: If the wire is fed too quickly, it jams into the part, cooling the weld pool.
• Arc displacement: When welding a rivet, the arc must be directed strictly to the center of the hole for reliable joint fusion.

If a defect is found, completely grind or drill out the defective spot, re-prepare the surface, and repeat the welding with corrected parameters.

Safety Rules When Working in a Garage or Workshop

Adhering to safety procedures when spot welding in a garage is not a formality, but a guarantee of preserving your health and property. Working with a semi-automatic welder involves risks of electric shock, thermal burns, and fire, especially during automotive body repair.

Basic Rules for Safe Workshop Operation:

• Fire Safety: Before starting to rivet, ensure there are no flammable materials near the heating zone (insulation, mastic, interior trim, fuel lines). Always keep a working carbon dioxide or powder fire extinguisher nearby.
• Respiratory and Eye Protection: Use a welding mask with an automatic filter ("chameleon") for reliable eye protection against flashes during frequent tacking. When welding galvanized metal or using powder wire, always wear a respirator and ensure forced ventilation in the garage.
• Protective Gear: Work only in tight-fitting protective clothing made of canvas or heavy cotton (synthetics melt instantly from metal spatter) and leather gauntlets.
• Electrical Safety: Regularly check the integrity of the insulation on the torch cables and the ground clamp. Never work on a wet floor or in the rain.

Neglecting these simple rules can lead to serious injuries or the irreparable damage of a repairable part.

Conclusion

Mastering the technology of spot welding and riveting using a semi-automatic welder is a crucial skill for quality body repair and working with thin sheet metal. To ensure your joints are always strong, neat, and free of burn-throughs, it is enough to adhere to several golden rules:

• Thorough Preparation: Always clean the mating surfaces to bare metal and firmly clamp the parts with vices. Any gap or residue of rust will guarantee burn-through or pores.
• Precise Adjustment: Select the welding current, wire feed speed, and shielding gas flow rate (carbon dioxide or mixture) strictly according to the thickness of the workpiece. Do not neglect test tack welds on scrap pieces of similar metal.
• Process Control: Keep the torch at the correct angle, maintain the optimal wire stick-out, and do not overheat the metal. If your machine supports MIG-spot mode, be sure to use the built-in timer for perfect weld repeatability.

Don't be discouraged if the first results are far from ideal. Welding is a practical skill that requires muscle memory and patience. By spending a little time practicing, you can perform repairs of any complexity at a professional level right in your garage. Good luck with your work and reliable welds!