The challenge for the automakers to make vehicles safe and strong as well as lightweight has resulted in material variations. Many of today’s steel-structured vehicles use steel for the outer panels that’s 0.70 millimeters thick. The structure is typically made of even thicker, higher strength steel.
This has resulted in I-CAR revising its Steel GMA (MIG) Welding Qualification Test to more accurately reflect the thicknesses of steels being used. The revised test was released on July 30, 2012 and features 10 welds made on steel coupons in two thicknesses. (Figure 1).
Based on a survey of some of the top-selling vehicles in North America, the two thicknesses selected were 22-gauge and 16-gauge zinc-coated steel. The 22-gauge varies in thickness between 0.68 and 0.81 millimeters. The 16-gauge varies between 1.4 and 1.6 millimeters. The original test coupons were a single thickness 18-gauge steel. The two thicknesses selected not only represent the steel found on current vehicles, but require a broader range of technician welding ability. If a technician properly passes the test, it will verify that they have the skills required to weld advanced structures.
Thin welding coupons were also included in the test based on several comments from participants indicating that they wanted to test on the kind of steel found on the exterior panels of many vehicles.
Mild Steel vs. High-Strength Steel
With the many different steels used in vehicles today, material selection and sourcing presented interesting challenges in revising the test. We asked ourselves, “If we select specific steel strengths that represent only one vehicle or OEM, how does this compare to other steels on other vehicle makes and models?”
To maintain a focus on weld quality and a consistent student experience, both of the coupons selected for use in the revised test were mild steel. We found little or no difference in welder settings when making GMA (MIG) welds on mild steel compared to welding on high strength steel (HSS), or even steel graded as ultra high strength steel (UHSS).
There’s another reason we went with mild steel only, and that’s because of the experiences we were having with destructive testing. A successful plug weld twist test on two mild steel coupons will invariably twist a nugget out of the base metal, or bottom coupon that does not have the punched hole. Any flaw in the plug weld, such as a skip or one spot around the hole where there’s no fusion, results in a nugget twisting out of the top coupon instead.
On a plug weld where the base metal is a grade of HSS and the top coupon is mild steel, the opposite occurs. A successful plug weld twist test pulls a nugget from the top mild steel coupon instead of the bottom coupon. It may be difficult to determine, then, whether the nugget twisted out of the top coupon because it’s weaker steel or because the weld is flawed in some way.
Plug Weld Tests
For the I-CAR Steel GMA (MIG) Welding Qualification Test, four of the 10 required welds are plug welds. These include a “thin-to-thin” plug weld (22 gauge to 22 gauge) in the vertical position. The plug weld is made in a 6-millimeter hole punched out of the top coupon. This represents, for example, plug welds on a pinchweld flange joining two exterior panels, such as along a wheelhouse opening. It could also represent plug welds made into a butt joint backing piece, such as along a rocker panel.
There’s also a “thick-to-thick” plug weld (16-gauge to 16-gauge) in the vertical position. The plug weld is made in an 8-millimeter hole punched out of the top coupon. This represents, for example, plug welds made into a butt joint with backing insert when sectioning a rail (Figure 2).
For the destructive test on these two welds, I-CAR requires a tearout hole in the bottom base metal coupon that’s at least 5 millimeters, but no greater than 10 millimeters. The I-CAR Steel GMA (MIG) Welding Qualification Test gauge can be used to measure the tearout hole. A tearout hole close to the size of the plug weld itself signifies a strong plug weld, but too large of a tearout indicates too much heat.
There’s also a 22-gauge to 16-gauge or “thin-to-thick” plug weld in both the vertical and overhead positions. These represent, for example, plug welds along a vertical pinchweld joining an outer B-pillar to a thicker reinforcement. With this example, the thicker steel on a vehicle may be HSS or stronger, as is common on B-pillar reinforcements. Again, in the I-CAR test, we wanted a nugget to twist out of the bottom coupon, so both coupons are mild steel.
We found in our research that the more heat applied when making the plug weld, the larger the tearout hole. Since excessive heat is something to avoid when welding grades of HSS, we require a tearout hole from the bottom thicker coupon, but that hole should be a maximum 5 millimeters, rather than a minimum 5 millimeters like the rest of the plug welds.
Other Test Welds
In addition to plug welds, there are open butt joints, butt joints with backing and fillet or lap welds required in the qualification test. All of these welds are done on materials of the same thicknesses (Figure 3).
There’s a vertical open butt joint using two thin coupons which, for example, represents a common joint required on pillars and rocker panels on Toyota vehicles. There’s also an overhead butt joint with backing using three thick coupons. This represents, for example, a common joint when sectioning a front lower rail on a Chrysler vehicle. There are two thin-to-thin fillet welds required, both a vertical and overhead. These represent the joints where an exterior panel is lapped over another exterior panel.
Administering the Test
As has been the case for several years, an I-CAR test administrator will administer the test at your repair facility using your own welding equipment. This helps ensure you’re familiar with the equipment and surroundings. Also, the tips on maintenance and tuning the welder will give you intimate knowledge of your welding equipment. The test administrator provides the test coupons and a weld positioner for making the welds in the vertical and overhead positions. A check-off form, given to the repair facility prior to the test day, ensures that the facility has the right equipment and materials for the test. As an example, a large vise is required for destructive testing of the welds (Figure 4).
Like any I-CAR event, the Steel GMA (MIG) Welding Qualification Test is administered in a supportive learning environment. The test administrator leads a practice session, as much as is required, and offers guidance throughout to ensure all of the technicians making the welds are successful. By the end of the session, everyone participating in the event will know more about GMA (MIG) welding and be a better welder.
To learn more about the Steel GMA (MIG) Welding Qualification Test (WCS03), or to request a welding event in your area, visit the I-CAR website and click on “In-Shop Welding Qualification Series” under the “Quick Links” tab on the home page.
Article contributed by I-CAR.