Most car frames are assembled using resistance spot welding in the factory, with MIG welding (also called gas metal arc welding) serving as the primary method for both production reinforcement and aftermarket repair. The specific technique depends on whether the vehicle uses a unibody or body-on-frame design, what metals are involved, and whether the work is being done on an assembly line or in a repair shop.
How Factory Assembly Differs From Repair
In a modern auto plant, robotic resistance spot welding does the bulk of the work. A typical unibody car has thousands of spot welds holding stamped steel panels together into a single structural shell. The process is fast, automated, and ideal for joining overlapping sheets of metal without adding filler material. Robots position electrodes on both sides of the joint, pass a high electrical current through the metal, and fuse the layers together in a fraction of a second.
Factories also use MIG welding, laser welding, and structural adhesives alongside spot welds. The Cadillac CT6, for example, combines 13 different materials that are welded, riveted, screwed, and bonded together to form its body structure. Laser welding has become increasingly common on assembly lines because of its speed and precision. One fabrication shop reported cutting welding time from 90 minutes to 10 after switching to automated laser welding, and automakers see similar efficiency gains at scale.
Repair shops, on the other hand, rely heavily on MIG welding. They can’t replicate the factory’s squeeze-type spot welding on an installed vehicle, so they use plug welds (drilling a hole in one panel and MIG welding through it) to mimic the original spot welds. Some shops also have single-sided spot welders designed for collision repair, but MIG remains the workhorse.
MIG Welding: The Most Common Choice
MIG welding dominates both production support and repair work on car frames because it’s relatively fast, works well on thin sheet metal, and produces clean welds with minimal cleanup. The process feeds a solid wire electrode through a gun while a shielding gas protects the molten weld pool from contamination. That shielding gas is what gives MIG welds their smooth, strong finish compared to stick welding, which leaves more slag behind.
For structural steel repairs, shops typically use ER70S-6 wire, a solid wire with a minimum tensile strength of 70,000 psi. The “S-6″ designation means it contains higher levels of manganese and silicon, which act as deoxidizers and help produce cleaner welds on metal that may have surface contaminants like mill scale or light rust. The most common shielding gas mixture is 75% argon and 25% CO₂, though some applications call for different ratios depending on wire diameter. Thinner wires (0.023″) sometimes run on pure CO₂, while thicker wires (0.035” and up) often pair with 95-98% argon and just 2-5% CO₂ for a cleaner bead and less spatter.
One former auto frame plant worker noted that car frames produced between 1984 and 1991 had extremely low carbon content (0.05% max) and were MIG welded at the factory with no pre-heat or post-heat treatment. That low carbon content made the steel easy to weld without cracking. Heavier truck frames, by contrast, used higher-carbon heat-treated steel that required specific weld procedures and matching high-strength filler.
TIG Welding: Precision Over Speed
TIG welding shows up in automotive work where appearance and precision matter more than speed. It uses a non-consumable tungsten electrode and gives the welder fine control over heat input and filler material. This makes it the preferred method for automotive restoration, custom fabrication, and working with thin or exotic metals. If you’ve seen a beautifully finished weld on a show car’s roll cage or exhaust system, it was almost certainly TIG welded.
The tradeoff is time. TIG welding is significantly slower than MIG, which is why factories and collision repair shops avoid it for structural frame work. It’s also more technically demanding, requiring the welder to coordinate both hands independently. For production-scale frame assembly, that combination of slow speed and high skill requirement makes it impractical.
Unibody vs. Body-on-Frame Construction
The type of frame your vehicle has changes what welding methods apply. Most modern cars, crossovers, and smaller SUVs use unibody construction, where the body and frame are a single integrated structure. This monocoque shell is made from dozens of stamped steel panels joined primarily by spot welds, with MIG welds and adhesives reinforcing key areas. Crush zones and load paths are engineered into the shape of the panels themselves.
Trucks and larger SUVs typically use body-on-frame construction, where the cab sits on top of a separate ladder-type frame made from two high-strength steel rails connected by cross-members. These frames are heavier gauge steel and are MIG welded or robotically welded at the factory. The solid foundation handles towing loads and resists the twisting forces that come with off-road use. Repairs to ladder frames generally involve MIG welding with high-strength filler wire, and any modifications like stretching a truck frame follow the same approach.
Welding High-Strength and Ultra-High-Strength Steel
Modern car frames increasingly use advanced high-strength steel (AHSS) and ultra-high-strength steel (UHSS) to reduce weight while maintaining crash protection. These steels range from 590 MPa to 1,500 MPa in tensile strength, and they require careful attention to heat input during welding. Too much heat changes the steel’s microstructure in the area surrounding the weld, called the heat-affected zone, which can weaken the metal and compromise its crash performance.
Honda’s repair guidelines illustrate how strict these requirements are. Standard MIG butt welding is approved for steel up to 780 MPa, but it must be done quickly to minimize the heat-affected zone while still achieving adequate penetration. At 980 MPa, butt welding is no longer approved, and only spot welds and plug welds are allowed. At 1,500 MPa (hot-stamped steel, commonly used in door reinforcement beams and B-pillars), plug welding is permitted only in specific locations outlined in the vehicle’s repair manual. Butt welding that steel is flatly prohibited because the heat would destroy its engineered strength.
These restrictions are why collision repair on newer vehicles must follow manufacturer-specific procedures. A technique that works perfectly on mild steel can create a hidden weak point in high-strength steel.
Welding Aluminum Frames
Aluminum-intensive vehicles like certain luxury sedans and sports cars present a different set of challenges. Aluminum conducts heat rapidly, which makes burn-through a constant risk on thinner sections. It also forms an oxide layer that melts at a much higher temperature than the base metal, requiring thorough cleaning and specific techniques to achieve a sound weld.
Automakers that use aluminum frames require MIG welders capable of pulse transfer mode for collision repair. Pulse welding rapidly alternates between high and low current, which controls heat input precisely enough to weld thin-gauge aluminum without burning through while also improving root fusion and reducing porosity on heavier sections. A standard short-circuit MIG welder is not sufficient. If your vehicle has an aluminum-intensive body, the repair facility needs pulse-capable equipment to meet manufacturer specifications.
Aluminum frames also cannot be mixed with steel welding equipment. Contamination from steel particles embedded in wire liners, drive rolls, or work surfaces can cause corrosion and weld failure, so dedicated aluminum tools are a requirement in any shop doing this work properly.