Understanding Safe Welding

The primary focus of this series of articles will be Gas Metal Arc welding (G.M.A.W.), also known as MIG (Metal Inert Gas) or wire welding. It will include specific process information; machine selection criterion; weld metallurgy (technical jargon for the study of metals); basic design for racing applications; fabrication techniques; and above all, one of the most neglected areas, safety.

The process, MIG, was first patented in the U.S. around 1949 and primarily designed for welding aluminum. With the proliferation of MIG welding units available today, it is the most accessible process for many. Industry leaders are companies with very familiar names, i.e., Miller, Lincoln, and ESAB. With other lesser-known “off brands” taking their fair share of the market, the average consumer has many options.

Generally speaking, MIG welding produces heat from an electric arc established between a continuously fed, bare metal electrode (wire) and the base/parent metal (what you’re welding on) in conjunction with an external, inert (chemically, non-reactive) shielding gas of either argon, carbon dioxide, helium, or a mixture.

When the bare electrode melts, it forms the bead. The arc contains both amperage and voltage. To produce the soundest, highest quality welds possible, you must have the correct type of polarity and current, the correct shielding gas, correct burn-off rate, and the ability to set your machine accurately for the amount of voltage and amperage determined through wire feed speed. The welding technique is fairly simplistic. However, the setup is much more complex than many realize. It can be fatal if you have a miscalculation. The amperage and voltage must be synchronized.

MIG welding has five metal transfer processes (types) which could further complicate things for those lacking formal training. They include short circuiting (most often used for mild steel applications), spray-arc (used mainly for aluminum, but can be used for mild steel), pulsed-arc (which has limited use on race cars because of material thickness and is more complicated to set up), globular (the least desirable because it produces poor weld quality and soundness), and submerged arc, which is not a consideration for your needs. Most of the material that is found in racing today is light gauge. In the future, we may expound on welding chrome-moly, which is becoming increasingly more popular in dirt Late Model racing with the advent of the round tube chassis.

In order for you to distinguish between the first four processes, I will develop an auditory and visual concept for you.

First, I will address what the auditory sound should be. “Mentally” hear the sound of bacon frying in a skillet. That’s probably the closest thing we all can relate to when trying to determine if we are close on machine setup. The closer your machine is “dialed in,” the more the arc will crackle like frying grease.

Now, the visual representations for each transfer process. Short circuiting is often referred to as short-arc or “dip” transfer. The molten metal is transferred to the work when the wire feeds down and touches the metal surface. This completes the electrical circuit—“shorts out”— and melts the wire. It then recycles the process from 20-200 times per second and uses lower currents and voltages as well as smaller wire diameters.

For welding on race cars, MIG wire diameters range from .023 -.035-inch. Depending on the wire diameter and the material thickness, voltage ranges from 15-19 volts and 50-150 amps. This technique results in lower heat input, minimizing warpage and distortion, as well as reducing the Heat Affected Zone (HAZ). The HAZ (pronounced haze) area runs parallel to the weld area. This is the region of the grain structure of the base metal altered by the heat produced during welding. If you overheat this area, you will embrittle the weld zone, which results in increased hardness, an undesirable outcome. This will also reduce several of the mechanical “strength” properties including tensile, shear, fatigue, torsional, bending, ductility, elasticity, and elastic limit. This is just skimming the surface of this entire concept.

Great care should be exercised in setting the voltage and inductance controls in relationship to the wire feed speed, which are calibrated in inches per minute (IPM). Inductance controls the rate of current change caused by the surge in electrical current when the wire melts and becomes a part of the weld pool. Inductance aids in the control of splatter.

Spray arc produces molten teardrop shaped droplets, which detach from the end of the wire and spray across the arc column to the weld pool (puddle). Spray arc has an intensely hot and higher arc voltage, giving a higher deposition rate. If properly adjusted, the welds are almost splatter free, with deep penetration and high deposition rate. Generally, this isn’t a good transfer process because of the high heat input. This process is recommended for 1/8-inch and thicker, eliminating it from consideration for racing applications.

Pulsed arc is a combination of spray arc and globular. In reality, it is a pulsed, spray arc, rather than a continuous spray arc. This process produces pulsing approximately 60 times per second. Because of the metal transfer process requiring advanced techniques in setup, it is not as user friendly, considering all the variables.

Globular arc uses very low voltage and current values. The molten metal is transferred across the arc in large, ball-like globs. Gravity pulls the glob from the end of the wire once it has gained in mass 2-3 times the wire diameter. This has a highly unstable arc with lots of splatter and shallow penetration. Though some contend that penetration is deep, there is considerable discord over that subject. It is not acceptable when the primary consideration of weld strength and driver safety are paramount.

Current and polarity used for MIG welding is “Direct Current Reverse Polarity” or (DCRP). This is also commonly referred to as “Direct Current Electrode Positive”(DCEP). Without a huge section on electrical theory, this type of current inherently provides cleaning action. With the wire being positive, and the work negative, and given the electrons always flow to the positive, the electrons are traveling up to the wire, thereby reducing the heat input to the base metal. Approximately 30 percent of the deposited weld goes to the penetration value off the joint while 70 percent remains on the surface or “face.” There’s no need to worry about 1/3 of the weld penetrating, because that is sufficient to hold the joint together if constructed (configured) properly. This is another avenue of discussion about joint geometry, design, and fabrication styles and techniques.

Choosing the correct piece of equipment is the first step to successfully completing a job. There are basically three types of welding machines that will produce MIG welds. They are the old motor generators (either AC or DC), transformer-rectifiers, and the latest technology, inverters. Motor generators have several disadvantages, chief among them being the problem of cold lapping during the initial start of each new bead. Cold lapping is where the weld doesn’t penetrate the base metal and just lays on top of the material. Constant voltage, transformer-rectifier, was once the number one selection for MIG welding. Today, however, with inverter technology, you can achieve a better result and have a three-process machine all in one. The only downside is the lack of AC welding power to perform aluminum welding with TIG.

To optimize welding applications, you must have a unit with a wide range of flexibility. You need to select a power source that has a full range of voltage selection. MIG, unlike stick welding, requires the operator to marry both the selection of amperage, which is adjusted through the wire feed speed control, and the voltage.

Most people have had experience with stick welding where the desired amperage is set, and the machine automatically selects the correct amount of voltage based on your rheostat setting. MIG welding requires a person to be adept at setting both wire feed speed and voltage separately. The basic premise on which this process operates is that the wire feed speed is in direct concert with the amperage. Amperage is the source of heat, which melts both the wire and base metal. The higher the wire feed speed, which is calibrated in inches per minute (IPM), the higher the amperage. Voltage is the drive or push behind the amperage, which drives the molten metal into the base metal. The correct technical term is penetration. When you, as a welder, lack experience in setting this type of machine, a weld that looks acceptable on the surface may not have penetrated the base metal because of insufficient voltage.

Safety

Safety is of paramount importance, and is a most often overlooked and/or neglected aspect of welding. Most people think that when you’re “just welding,” the only concern for injury is your eyes from the welding “flash” arc. However, that isn’t the only thing that can be damaged by the invisible rays given off by the process. Safety glasses must be worn at all times, tight against the bridge of your nose. If you receive a number of flash injuries from your welding experiences, they may lead to blindness. Consider use of flash glasses, especially if you’re just beginning to weld. They take a little getting used to, especially if your work area doesn’t have sufficient lighting, but this additional layer of protection is beneficial. Glasses also protect your eyes from flying metal particles expelled during MIG welding.

Just like the sun, the welding arc gives off two rays called ultra-violet (UV) and infrared. When welding, skin injury can result from not wearing gloves, wearing a short sleeve shirt, or shorts. Your skin can be harmed by these rays from 20 feet away. Extended overexposure, as with the sun, causes premature aging. Various forms of skin cancer can also result.

Leather gloves should be worn to protect your hands from molten embers, sparks, and heat. They should have a long gauntlet (cuff) also made of leather to protect the forearm. A long sleeve shirt of 100 percent cotton should also be a part of your attire, as well as a pair of 100 percent cotton pants. Blue jeans are usually a preferred choice. Clothing of synthetic fibers, nylon, rayon, and polyester are highly flammable and should not be worn. Frayed material on clothing, even cotton, will catch fire. Grease or oil on clothing is a definite risk to personal safety.

You should wear high top, insulating boots (not tennis shoes) for welding. Burns infect easily, especially in areas not readily exposed to free air movement that tend to stay damp, like your feet.

Your welding helmet should be of sturdy construction, a fiber-metal type. Helmets are often referred to as “hoods.” The recommended shade lens is between 10-12 for MIG welding. However, your eyes will be the judge. If you see spots, like someone just took your picture, the lens is not dark enough. If you are straining to see, and have headaches as a result, that usually indicates that your lens is too dark. I personally use a shade 8 for most applications with a 1.5 shade set of flash glasses underneath my helmet.