To understand what makes acceptable welds, think of this: When you replace factory welds, you have people's lives in your hands. Each weld must be made as originally designed. It can't be weaker, causing the vehicle to lose its crash worthiness, and it can't be stronger, compromising the vehicle's crash-safety design.
MIG welding – which involves weld placement and the subsequent cleaning, dressing and refinishing designed to make an undetectable repair – is among the most complex and critical tasks in structural repair.
Safety is critical. While burns are an obvious concern, exposure to ultraviolet light, especially to sensitive skin types, isn't always protected against properly. Although cautions are taken and protective gear is worn, eye and face protection often is inadequate. Ear protection, respiratory protection, shock hazards and fire protection must be addressed adequately.
The MIG welding process, using electrical resistance, heats a filler wire and the surrounding weld site to a temperature sufficient to melt steel. This temperature could, and often does, cause severe burns. All burns should be treated by a health-care professional.
To protect against burns, a technician must wear protective eyewear – safety glasses with side shields and a welder's helmet with protective filter lenses to protect the eyes from ultraviolet light (Fig 1). Technicians should protect their skin from ultraviolet rays (sunburn) by wearing long-sleeve, fire-resistant clothing, as well as protective welders gloves with long protective cuffs (gauntlets) (Fig 2). Don't wear pants with cuffs because they could catch slag. Wear shoes with protective coverings over the laces to keep slag from falling into them (Fig 3). The shirt's top button should be buttoned so the skin under the chin won't burn from the ultraviolet light (Fig 4).
Ultraviolet (UV) light, which is produced when welding, can exist without a technician feeling or seeing it. The light can produce first- and second-degree burns. The grainy, sandy feeling that occurs when the whites of the eyes are burned by light is a second-degree burn. Not all individuals respond to ultraviolet light the same. Some with fair skin are more susceptible to sunburn or welder's burn than individuals with darker complexion. Technicians who know they're susceptible to sunburn should take extra caution when welding.
Technicians who tilt their heads backward to use their bifocal lenses more efficiently may be exposing the skin below the helmet on the neck, and a burn often referred to as "the welder's V" can result.
For proper eye protecton technicians should wear welding helmets with safety glasses underneath with the appropriate filter lens. Filter lenses are rated from 4 to 12, and each individual, depending on skin type and the time of year, may require different darkening levels. Filter lens should be dark enough to protect the eyes while welding but light enough to see the welding area clearly. Automatic darkening lenses, which toggle rapidly back and forth between zero and the adjusted setting, protect eyes during welding (when the lenses will be in the darkest position) and affords clear sight when not welding. Auto-darkening helmets are more expensive than non-auto-darkening ones, but they allow for much clearer sight. Closing your eyes while welding doesn't afford sufficient protection to keep them from burning.
Hearing loss happens gradually throughout a long period of time, and it's not only as a result from the obvious loud noises. Lower, long-lasting noise also causes lasting damage. Ear protection during welding is neede to prevent hearing loss and protect against having hot slag roll down the ear canal. Often while welding inside a car, a technician must tilt his head in a way that welding spatter finds its way to the ear. Without ear protection, there's little a technician can do except listen to the sizzle and feel the burn. Use foam earplugs (Fig 5) for proper protecton.
Welding cars can produce toxic fumes. The fumes from burning galvanizing material during welding can be harmful, as are undercoating, paint and other contaminants that might linger even after a thorough cleaning. A respirator is recommended. See MSDS for the type.
A welder produces high voltage direct current; therefore, he should follow normal electrical precautions: Don't weld in a wet area or stand in water. Make sure the electrical ground is working properly before welding. Also, make sure all electrical connections are secured properly and without defects.
High-pressure gas cylinders, even the ones that aren't flammable, are still a potential danger during welding because the gas in the tank is stored under pressure. If the tanks were to be damaged, the sudden release of gas could cause an unsecured cylinder to explode. The most viable spot on the gas cylinder is the top, where a valve attaches (Fig 6). When the gas cylinder isn't in use, it should be stored with its safety cap screwed in place.
Technicians always should take precautions to protect themselves and others from fire during welding. Have the proper fire extinguisher on hand. Have a co-worker watch for any potential fire during welding. With welding safety equipment in place, sometimes it's difficult to realize a fire has started; therefore, a lookout should watch for fires and be prepared to put them out.
Gas metal arc welding, also called MIG, is the most common method of welding in the automotive collision repair industry. MIG welding machines can be set up to weld steel, aluminum and silicon bronze, although steel is the most common. The components of the MIG welder consist of a welding gun, power supply, electrodes, shielding gas and gas flow meter.
Although the welding machine is plugged into an alternating current, the welder uses direct current from a power supply to produce the weld. The electrode wire is commonly positive but not in all cases. The work clamp is generally the negative pole. DC electricity provides the heat needed to melt the electrode and pieces of metal that have been welded so they flow together forming a fusion weld.
Electrode wire will vary in size and material makeup, depending on what's being welded, and to lesser degree, the type of weld and the position of the joint. The three most common electrode sizes in collision repair are 0.023, 0.030 inches, and 0.035 inches.
Shielding gases are used in MIG welding to shield the weld and surrounding area so normal atmospheric gases, such as nitrogen and oxygen, don't interfere with and contaminate the weld as it's formed. The inert gases result in a more common name of metal inert gas welding. MIG welding was adapted from aluminum to steel welding, where a gas mixture of 75 percent argon and 25 percent carbon dioxide was needed. These percentages may vary, but 75:25 is common.
The gas flow meter takes high-pressure gas stored in a cylinder and reduces it to a usable level and flow rate to supply the welder. The gas doesn't flow continuously, but its on-off flow switch is activated each time the operator pulls the trigger, which activates a gas flow and the wire feed so the electrode is fed out at a specific, suitable speed. Gas flow meters (Fig. 8) are considered more accurate and supply a better stream of shielding gas to the welder than two-stage regulators.
Current, measured in amps, is the flow of electricity to the conductor. The proper amperage will differ depending on the type of metal, transfer wire size, shielding gas and base metal. However, charts help technicians find a starting point (Fig. 9). The wire feed dial on the welder also increases and decreases the current – the faster the wire feed, the higher the current.
Voltage, the force that pushes electricity through the wire electrode, is measured in volts. The higher the number, the greater force the potential electricity has. When welding, voltage affects the arc. When the length of the arc is set properly and the wire feed (amps) is correct, the arc metal transfer, along with a fusion of the metal, will be exact. After starting with the wire feed and the voltage as indicated by the chart (shown in figure 9), a technician should practice welding using a scrap piece of similar metal then evaluate each weld with visual and destructive testing to ensure the weld is strong.
The work clamp, or ground clamp, is attached to a vehicle, which completes the circuit. The work clamp should be placed as close to the welding area as possible to avoid any stray electrical surges from damaging the vehicle's electrical system. The work clamp should be placed in an area where a clean and secure ground can be attained. If the machine doesn't have a good ground, it might be difficult to fine tune the wire feed (amps) and voltage.
Stick-out is the amount of electrode wire that must stick out of the contact tip before the technician starts (Fig. 10). The wire stick-out length should range from 3/8 inch to ½ inch, without a large ball at the end of the welding wire. If a ball is noted, a technician should cut it off before starting.
Gun angle is the angle at which a welding technician holds the gun in relation to the work. As the gun angle changes, the shielding gas and the quality of the welds are affected. Gun angle is relative to the direction in which the weld is traveling. With the backhand method of welding, a technician drags the gun in the direction away from the weld pool. The forehand method is when a technician pushes the gun into the weld pool. Different welding positions – such as overhead, vertical and horizontal – require different gun angles and direction.
Welding speed is the rate at which the gun travels along the face of the weld. If the weld speed is too fast, the depth of the penetration will decrease. If the weld travel speed is too fast, the bead width may be too small, and poor penetration may occur.
The contact tip is a consumable item on a GMAW welder. The inside diameter of a contact tip is sized to the electrode wire passing through it. If the technician is welding with wire of 0.023 diameter, a 0.023 size tip should be used. If a smaller wire is passed through a large tip, electro-wobble will result in defects.
Welding techniques
Though a plug weld is the most commonly used weld in collision repair, it's not the only one. Technicians should be proficient at all of them. Each has its own specific advantages and disadvantages, which will indicate when it should be used (Fig. 11).
In a continuous weld, the arc is struck and a smooth, uninterrupted weld is applied in a steady, ongoing movement. The push, or forward, direction of travel is used, and a 10- to 15-degree travel of the angle should be maintained, with a 90-degree working angle. When thin body panels are welded, the continuous weld may cause warping.
The plug weld is one placed through a drilled or punched piece(s). The direction of travel is push or forward. The angle, which is 90 degrees to the work (the bottom of the hole where the base metal is thinnest), uses a travel angle of 10 to 15 degrees. The weld is started at the two o'clock position and pushed to the 10 o'clock position or until a hole is filled (Fig. 12).
A stitch weld is a series of short overlapping spot welds, which, when finished, create a continuous stream. Direction of travel, working angle and travel speed are the same as for continuous weld, which is often used on thin metals where warping or burn-through can be controlled.
In GMAW spot welds, the welder doesn't move. Therefore, no travel angle is used, and the work angle is 90 degrees. The trigger is pulled, and the arc is directed to penetrate both pieces of metal. This type of weld isn't often used in collision repair.
The GMAW tack weld, which is temporary, is used to hold fit-up pieces in place so a permanent weld can be applied. The length of the weld is generally 15 to 30 times wider than the base metal being held. There's little or no travel with a 90-degree working angle. The tack weld is temporary.
To make sure your welder is set up properly for the types of metal being welded, scraps from the surrounding area that have been removed should be used to practice welds before applying the permanent ones. Welds should be inspected for porosity, which indicates it's too weak and won't hold properly.
The weld should be checked for undercutting, a condition in which the weld doesn't completely fill the welded area. Overlap occurs when a weld laps over the base metal and has poor fusion. Insufficient penetration is when a weld doesn't completely go through the base metal.
The weld area should be inspected for excess weld spatter, which is normal. If the machine is set up properly and the weld is performed correctly, weld spatter can be minimized. Weld spatters are the No. 1 cause for painting in adjacent glass. All glass should be protected with a welding blanket or other means of protection. Painted glass isn't repairable and will need to be replaced.
If the practice welds pass inspection, they should be pulled apart to see how well they hold. The welded area should remain intact, and the steel adjacent to the weld will rip away as it's being destroyed.
