Welding in today’s shop

July 22, 2014
Personal, vehicle and shop safety can help to ensure the best weld every time. Get our tips.
Figure 1

Welding is a vast topic in collision repair, and like many other segments in the industry, ever changing. I aim to address the most common types of welding in the shop, such as Gas Metal Arc Welding (GMAW) — the correct name for what most people call MIG welding (Fig 1).

We’ll take a look both shop and personal safety; power for the welder; weld testing; MIG steel, MIG Aluminum and MIG silicon bronze welding; and finally the types of metal transfer of MIG welding.

Safety
In the shop you are likely to be using either a 110v or a 220v welder. Because of that power and the arc that is generated to form the weld, welders must protect themselves and fellow workers from the dangers of the welding process. First, one must be sure that the area set up for the technician to weld in is safe. The welding technician should follow all equipment makers’ recommendations and also check that the cables and connections are in good condition. The welder also needs to know and follow the local electrical codes for extension cords (more on cords later). Workers should never weld in a wet area or while wearing wet clothing, and should not use the welder as storage or a workbench. In keeping with general workplace safety rules, keep the area clean from clutter and debris. Working with the welding helmet down restricts vision enough without being sidelined by a slip or stumble over items in the way.

Personal protective equipment (PPE) such as a welding helmet, respirator and spark protection are necessary to prevent injury. For eye protection, a helmet with at least a grade 9 filter should be used to protect your eyes from ultraviolet damage. If the welding is done at a current above 60 amps, the filter must be darker (a higher number). If the operator is sensitive to ultraviolet exposure (indicated if he or she sunburns easily), a darker shade should be used. Additionally, a safety lens cover should be used over the filter lens. Though not necessary, the auto-darkening helmets are very helpful, as they switch from a nearly clear lens to the proper darkening shade as the arc is struck. 

Next is the respirator. Though you don't often think of a respirator in relation to welding, it is necessary in collision repair, where there are often fumes that burn off the vehicle. Even when the weld site is properly cleaned, fumes may be harmful, and in addition there may be natural fumes formed from the welding process. Wear an approved welding respirator, even in a well ventilated area. A welding fume extractor placed above the welding area is also a good idea, but it doesn't take the place of the respirator. Also, don't point a fan at the weld area because it may blow the shielding gas from the weld site and cause poor weld performance. 

Your skin, like your eyes, can be damaged from the UV rays produced from the welding process. Protect yourself from these and from spark burns by covering your skin with heavy flame resistant cloth or leather. Use heavy welding gloves with a long cuff and safety glasses with side shields under the helmet. Fasten the top button of your shirt to keep your neck from being burned with UV; and never keep a plastic butane lighter in your pocket, where a stray spark may cause it to explode. Don't weld in an area where there are flammable materials, and always have a fire extinguisher at hand for emergencies.

Shop Safety: Either have a spark and UV shield around the area you are welding to protect your fellow workers, or signal them by saying "welding cover" before you strike an arc.

Vehicle Safety: When a vehicle is being welding, the vehicle must be protected as well. Precautions include covering all glass from sparks around the weld area, disconnecting and isolating the negative battery cable, having the ignition switch in the locked position, disarming the passive restraint system and following the vehicle maker’s recommendation for removal of computer modules when welding or heating within 12 inches of the modules. 

Heat affect zone
When any metal is welded or even heating during repair, that area of metal that is heated but not melted is the heat affect zone. All metals are changed by this process of heating and cooling. Unfortunately some are affected more than others. Standard mild steel is affected least by heating and cooling, and years ago heat was commonly used to help repair this type of steel on vehicles. Even welding processes such as oxyacetylene torch welding and brazing that use large amounts of heat, thus creating a large heat affect zone, were commonly used.

As cars evolved, high strength steel (HSS) and ultra-high-strength steel (UHSS) were used, which are not only stronger, but also thinner. Lighter pieces can be used and remain as strong as or stronger than the older mild steel. Unfortunately, these steels are much more sensitive to heating and cooling, and so other ways of welding them together need to be used. All HSS steel is not the same, and the heat tolerance recommendations must be followed for both heating to repair and for the type of welding used. GMAW (MIG) welding is the most common welding type of repairing for these types of metals. MIG welding offers a very fast, powerful arch that affects a much smaller area of the base metal. Steels continued to have a reduced heat affect zone, and two-sided resistance spot welding became recommended by manufacturers for repairing vehicles.

As vehicle construction evolved, different types of metal were being welded together during manufacturing, and the welding of choice became laser welding, or welding with a laser beam. The beam provides a concentrated heat source allowing a narrow, deep weld and high welding rate with a smaller heat affect zone.

Power needed for welder
Each welder has different requirements of power to operate efficiently; and though some will operate on normal 220V single phase power (which is found in residential areas), three phase for larger machines may be required. Also, the power source at the wall should be tested to confirm that the correct power requirements are being met. Even if the supply power to the collision shop is at specifications, the power at the outlet may not be, due to insufficient sized wires.  Additionally, some facilities are often tempted to use power extension cords, which may not be recommended by the welder manufacturer or may not be code for the location of the facility. Cords also may diminish the current flow, which will produce insufficient welds. Setting up welders so they will perform at the manufacturer’s recommended performance level is critical.

Shielding gas
Shielding gas during GMAW welding protects the weld from oxygen, nitrogen and hydrogen, which will cause porosity (holes) in the weld. High-pressure cylinders are used to store this shielding gas; then it is delivered to the welder for welding. This gas also is a factor in controlling weld-pool heat, which is critical to controlling the heat affect zone (which will be covered later). 

There are two categories of gasses used for welding. The first is inert gas (a gas such as argon or helium that protects the weld but does not combine with the arc. This is a true MIG weld.) The second category is active gas; CO2 (carbon dioxide) is a common example of an active gas. It protects the weld, but also combines with the arc to contribute to the quality of the weld.

Figure 2
Figure 3

Shielding gas can be pure, or a mixture in various combinations can be used. A mixture of 75 percent argon and 25 percent CO2 is the most common recommended gas used in GMAW welding in collision repair. Pure argon, however, is recommended for welding aluminum and silicon bronze wire. There are also tri-mix shielding gases of 90 percent helium, 7.5 percent argon, and 2.5 percent CO2. This mix is commonly used for welding stainless steel.

When choosing the proper mix of shielding gas, the shop should follow the recommendations of the vehicle manufacturer for all of the welding processes, the wire to be used, and the shielding gas.

Test welds
Before any welding is done on a vehicle, a test weld should be performed. Even if you are a skilled and capable welder, you don't want to risk a bad weld on the vehicle; so take some of the similar metal that was removed from near the repair area and test the machine for proper settings and performance.

The test weld should be visually inspected for cracks, porosity (Fig 2), skips or voids, undercut or overlap (Fig 3), proper bead height and width for the weld. On the back of the weld, look for burn-through and for good penetration. If the test weld passes the visual test, then it should be destructive-tested. 

Destructive test: Place one side of the test weld in a vice and grab the other part of the test weld. By rocking the piece back and forth, destroy or break the two parts apart. A good weld is one in which the weld itself doesn't break when the surrounding base metal breaks.

After the welder is set up and a good test weld has been performed, the vehicle can be welded together.

Metal transfer modes
The metal transfer from the electrode to the base metal can be performed in a number of ways:  globular, short circuiting, spray and pulsed spray. 

Globular: Globular transfer is considered the least desirable in collision repair because of the high heat, poor weld surface and spatter. The metal transfer works by a ball of molten metal building up at the end of the electrode; when the misshaped ball either falls to the base metal by gravity or by short circuit, the weld surface is then uneven and a large amount of spatter is produced.   

Short circuiting (SCT): This method is often used in the collision industry. The lower heat makes it possible for lighter metals to be welded with less spatter and distortion, and with a smaller heat affect zone. In short-circuit, a ball of molten metal forms on the electrode. But instead of dropping to the base metal, a bridge is made and, for a fraction of a second, a short circuit is created, which extinguishes the arc. The arc quickly re-ignites and the process continues; the speed, about 100 times per second, makes it undetectable to the eye. SCT welding requires a finer "tuning" of the welder; that is, the amperage, voltage, wire feed and even the rate of travel are critical. To make sure that the welder is set properly for each application, a test weld should be performed.

Spray: The first metal transfer developed for MIG welding, spray uses higher current and voltages, and the electrode forms very small droplets that eventually vaporize the molten electrode, and it is transferred to the base metal. This virtually eliminates spatter, but because of the higher power, the heat is also very high and is therefore not suited for the lighter metals of vehicles. Also, because of the large weld pool, it is often limited to flat and horizontal welding positions.

Pulsed spray: A variation of the spray transfer method, pulsed spray uses a pulsed current, causing the molten droplets to be much smaller, allowing for lower current to be used. With less current, the heat affect zone is smaller and the weld pool is smaller; it also has a more stable arc and no spatter. These traits allow the material to be welded in all positions. Pulsed spray is somewhat slower than the others, and it does require pure argon for a shielding gas. It also requires a special power source capable of pulsing from 30 to 300 pulses per second. 

Pulsed MIG welding is particularly suited for welding not only steel, but also aluminum and silicon bronze. Many vehicle manufacturers now recommend a pulsed welder for these types of welding. 

FIgure 4
Figure 5

GMAW (MIG)
While some vehicles have this now, in the very near future, most pickups will have an aluminum body. (Ford will in 2015; GM will too, in 2018, it is rumored.) Thus the market share of aluminum repairs will increase very quickly. A technician may be asked to weld steel, aluminum and silicon bronze, sometimes all within the same week or even day. Does that mean you will need three different machines? Perhaps so; however, there is equipment set up for all three in one machine (Fig 4). Such welders have wire feed on one side that has aluminum and bronze feed, with steel in the other side to prevent steel-aluminum contamination. Two gasses (argon for aluminum and bronze, 75 percent argon and 25 percent CO2 gas for steel) are automatically changed when the weld is started (Fig 5). Some machines even have TIG and stick welding available with all five methods in one machine. The machines are pulsed-weld capable for steel, aluminum and bronze, which meet the recommendations for vehicle manufacturers.

Squeeze-type spot welding
As mentioned earlier, squeeze-type resistance spot welding, which had been the standard type of welding process used during manufacturing, became recommended for repair processes as aftermarket spot welders became more powerful. The operation could closely simulate the type of welding done in the factories. Because the newer HSS and UHSS steels were sensitive to heat, the welding process of choice became squeeze-type resistance spot welding (STRSW). This fast, efficient process significantly reduced the heat that was transferred to the second base metal that was not being joined, thus making it a stronger weld.

The number and location of spot welds recommended vary from maker to maker. Some give only general rules such as, "Avoid original weld locations,” or “Do not use single side welders.” Others recommend weld bond repairs, and still others may recommend replacing the original weld bond joints with twice as many spot welds, with no adhesive. As can be seen, it is incumbent upon the repair facility to find the recommendation from the vehicle maker so the welding repair can be completed correctly.

Conclusion
Welding in the collision repair industry is an ever-changing and ever-improving process. Though it is often frustrating that new equipment must be purchased and new techniques learned as vehicles evolve, we as technicians must also continually seek the needed skills that this changing industry requires. There is no such thing as knowing everything that is needed. Technicians must continue training, practice new skills and change with the industry. If we quit learning, we will soon be left behind.

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