Introduction to Resistance Welding

In simplest terms, welding is a process by which two or more pieces of metal are joined together by applying heat and pressure. Traditionally, blacksmiths and craftsmen would heat metals in a furnace and then weld them by hammering the red-hot metals together. By hammering the metals as they cooled, the weld would be made stronger. The heating and hammering method is known as "forge welding." While forge welding worked quite well for most of the welding done back then, today's new advanced metals and welding requirements are a little more advanced. Just imagine an automobile manufacturer or collision repair facility using this process.

As time passed and the requirements for more efficient welding methods were needed, engineers and those craftsmen involved in welding were forced to invent better processes. One such person was a professor and electrical engineer by the name of Elihu Thompson (of Thompson-Houston Electric Co., which was purchased by General Electric Co. in 1882). In 1885, Professor Thompson invented a process called electric resistance welding, due to the need of heat control on the metal being welded. At that time, MIG/GMAW (Metal Inert Gas/Gas Metal Arc Welding) and Gas Welding were the most popular welding methods, as both use filler wire and higher temperatures to complete a weld. As time went on, Professor Thompson's method progressed into what we know of today as "Squeeze Type Resistance Spot Welding" (STRSW), or the shortened version, "Resistance Spot Welding" (RSW). Today's resistance welders work almost exactly on the same principles laid out by Professor Thompson. RSW is so aptly named because it is the resistance between the contact surfaces of the metals being welded that generates the heat to fuse them together, along with the pressure being applied.

Obtaining the best results for a proper RSW requires a thorough understanding of the materials being welded, control of the heat and pressure being applied at the weld point, the proper clamping, electrode tip maintenance and the amount of electrical current provided to the equipment. On earlier STRSW equipment, there were a lot of variables the technician needed to adjust and fine tune, such as the weld current, squeeze time, weld time, hold time, electrode force (pressure), material thickness and surface condition and cleanliness of the materials being welded. Fortunately for us, today's resistance spot welders have eliminated much of these adjustments, and some even do everything automatically.

Weld Cycle

There are three major processes to complete a RSW: Squeeze Time (Pressure), Weld Time (Current) and Hold Time (Cooling/Solidification).

1. Squeeze Time (Pressure) is when the electrodes come together and build up to a specified amount of force before the current (Weld Time) is applied.
2. Weld Time (Current) is when the current is actually passing through the work pieces (metal) and enough resistance is built up to create enough heat (2400°F) to melt the pieces together without filler wire, like the MIG/GMAW method.
3. Hold Time (Cooling/Solidification) — While the electrode force (pressure) is still being applied and the current has ceased (off), the weld nugget cools and the metals are forged under the force of the electrodes. The electrode force continues to hold the work pieces together until the weld solidifies and cools and the weld nugget reaches its maximum strength.

Surprisingly, this whole welding process takes approximately three to five seconds. Just think how much time it takes to make a MIG/GMA weld; in some cases, you still have to dress the weld, and that is not necessary with STRSW.

Today's STRSW

Today's welders make welding easier and the technicians more efficient, but why are we using the STRSW process? Many OEMs are requiring their use in their specific repair procedures, such as Mercedes Benz, Audi and BMW, to name a few, due to the advanced steels being used. Here are some of the advantages of STRSW: Less heat affect zone (HAZ); less burn damage to adjacent panels; drilling holes is not required; replication of OEM weld appearance and strength; no dressing (grinding) required; no filler lens (welding helmet) required; simple to master and an almost undetectable repair.

Let's look at some of the features of today's welders. After purchasing your STRSW, you will want to start welding right away, but some practice and set up will be needed. After having an electrician run the wiring and installing the proper receptacles (wall plugs/outlets), you will be ready to start practicing with your STRSW. Due to the OEM requirements to weld today's advanced high strength steels, it is recommended to purchase a three-phase inverter welding machine that is water-cooled, such as the ProSpot i4, Car-O-Liner CR510, CTR12000 Vision or the Elektron M100W. Three-phase RSW usually requires a four-gauge wire, 60 amp breaker and 30-foot limit on the extension cord; make sure your electrician follows the proper equipment maker's recommendations. After installation, read the equipment maker's user manual; it is also recommended to take a formal course on STRSW from I-CAR (WCS04), P&L's RSW course or a similar course to educate you about the process and operation of the RSW method. Many of today's machines will ask you to choose the type of substrate (type of steel) you will be welding, such as HSS/Galv. (High Strength Steel/Galvanized), Mild Steel, Weld-Bond (using adhesives), Boron Steel, AHSS (Advanced High Strength Steel), Pulse Mode, OEM Specific Settings and even Custom Modes for your own preference. The next step is to choose your material thickness; it is important to know if the setting is based on the total thickness or the total thickness divided by two (check with your manufacturer). The last step will be to set the inlet pressure of the machine; generally, the starting point is 60psi and moving up to a maximum of 90psi. Some STRSWs, such as the Car-O-Liner CTR12000 Vision, use a sensor to automatically set the type of substrate, weld time and pressure settings. This basically eliminates technician error.

Welding Procedures

The following is a general list of procedures to follow to ensure proper resistance welds after the replacement panel has been prepared for installation:

1. Check the OEM replacement procedures to see what welds are required for each area of the panel being replaced and the mating flange preparation. Some OEMs require STRSW to be used on all flanges where the arms can access both sides of the flange and in areas inaccessibly utilizing MIG/GMAW plug welds. Other OEMs require rivet-bonding in areas inaccessible by the arms and no MIG/GMAW at all. Some OEMs require weld-bonding for installation of replacement panels. (For OEM repair procedures, check the OEM website or www.ALLDATACollision.com.)

2. Set the machine settings (unless it is fully automatic like the CTR1200 Vision) to the OEM specifications and check that electrode tips are clean and in good condition. Some machines have sharpeners to clean and reshape the tip, while others have replaceable electrode tips which should be changed every job and charged for on the estimate.

3. Now for the most important step — practice welds. Cut some metal coupons (pieces) from the damaged or unused portion of the replacement part. Set the coupons up exactly the same way the flanges are set up on the vehicle. Clamp them together and weld. After the coupons cool off (a minute or two), destructively test them by peeling them apart. Measure the tear-out hole, which needs to be five times the metal thickness of the thicker piece for a two-layer flange weld and the middle thickness on a multiple layer flange. After passing the visual and destructive test, take a photo of the coupons and attach the photos to the file and save the coupons for liability protection. Now, you are ready to weld.

Hopefully, this article has brought to your attention the importance of the STRSW process and the procedures to ensure a proper weld. Look for a future article on advanced joining techniques and procedures. Feel free to contact us at any time if you have any questions that we could help with.

Larry Montanez is a former I-CAR instructor and is co-owner of P&L Consultants with Peter Pratti Jr. P&L Consultants work with collision repair shops on estimating, production and proper repair procedures. P&L conducts repair workshops on MIG & resistance welding, measuring for estimating and advanced estimating skills. P&L also conducts investigations for insurers and repair shops for improper repairs. P&L can be reached by contacting Larry at (718) 891-4018 or larrygoju@aol.com.

Jeff Lange, PE is president of Lange Technical Services, Ltd. of Deer Park, N.Y. (www.LangeTech.net). Jeff is a licensed New York State professional engineer who specializes in investigating vehicle and component failures. Lange Technical Services, Ltd. is an investigative engineering firm performing forensic vehicle examinations and analysis for accident reconstruction, products liability and insurance issues. Jeff can be reached at 631-667-6128 or by e-mail at Jeff.Lange@LangeTech.net.