Machine Design
What’s the Difference Between Resistance-Welded Fasteners and Arc-Welded Studs?

What’s the Difference Between Resistance-Welded Fasteners and Arc-Welded Studs?

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Welding is often used to permanently fasten two pieces of metal together, often by robotic welders. Screws and nuts are the most common welded fasteners, but pins and unthreaded studs are often used as locating or bearing surfaces rather than fasteners. There are two general groups of welded fasteners: resistance welded ones and arc-welded studs.

So what’s the difference?

Resistance-Welded Fasteners

Resistance-welded fasteners are internally or externally threaded parts meant to be permanently fused in place by standard welding equipment.

The two methods used in resistance welding are:

Projection welding: In this method, heat from the welder gets focused on a fastener’s embossed or coined projections. This fuses the projections with the surface of the metal base and forms a weld. A press-type welder with electronic controls is recommended for this task because it provides positive electrode alignment and equalized welding pressures.

Spot welding: In this welding process, current is sent through the entire area under the electrode to fuse the fastener and metal base together. It is often done with a rocker-arm type of spot welder because it can handle a variety of different types of welds and fastener designs. The equipment needed for spot welding is less expensive than that needed for projection welding. But projection welding is more versatile and gives engineers more latitude in their designs.

For good results, both the part and material must be suitable for resistance welding. Low-carbon 1010 steel is the most commonly used material for spot welded fasteners. Parts must also be portable because they must be taken to the welding machine. Portable welders are not recommended for use on these types fasteners.

To justify the costs of welding and make it economically feasible, production volumes should number at least 1,000.

The most common use of resistance-welded fasteners is with sheet metal parts measuring 0.03 to 0.125-in thick. But any size fastener can be welded to material of any thickness if the materials are compatible, the welding properly controlled, and the fastener well designed.

 

Arc-Welded Studs

In stud welding, the heat from an electric arc created between the fastener and the part to which it is being joined melts metal on both components. The two parts are then brought together under pressure. When the parts cool, the two parts are fused and the joint is complete.

Both the fastener and material must be weldable and one end of the fastener must be designed to be heated and fused to another part.

Stud welds are leak-proof, pressure-tight and can be made using automatic and semiautomatic equipment. Automatic welders can turn out 60 welds per minute.

There are two general methods of stud welding: electric arc and capacitor discharge.

Electric-arc stud welding: This is the most common stud-welding method and it is largely semiautomatic. In the process, dc current from a motor-generator or transformer-rectifier passes in an arc from the stud (electrode) to the metal plate and creates heat. The weld cycle is a function of stud diameter and varies from 0.1 to 1.5 seconds.

When the weld is complete, the entire cross-sectional area of the stud is fused to the base metal, making a strong bond. For the best results, the base metal or plate should be heavy enough to support the full strength of the welded fastener. However, lighter-gauge metal materials are also often arc-welded. To avoid burn through, one rule of thumb says the plate should be at least 20% as thick of the diameter of weld at its base. For a weld that is as strong as the faster will allow, the plate should be at least one-third as thick as the diameter of the weld at its base.

The most commonly used faster for electric-arc stud welding are made of low carbon steel that has a minimum tensile strength of 60,000 psi and a minimum yield strength of 50,000 psi. High-grade fasteners comparable in strength to SAE grade 5 bolts are also used.

This form of arc welding can be used on round and angled surfaces, as well as flat ones because  it relies on the stud‘s ferrule to provide the molted metal that harden into create the weld.

Capacitor-discharge stud welding: In this process, the arch generated by the rapid discharge of electricity from a capacitor generates the metal –melting heat. Pressure applied during or immediately after the discharge completes the joint. Just as in arc welding, the heat is created by current passing in an arc from the stud to the plate.

One advantage of the capacitor-based welding is that it can weld studs to thin materials without thermally distorting or discoloring them and without burn-through. The weld is not that deep into the plate, so dissimilar metals can be welded without metallurgical problems. Plates can be as thin as 0.016 in for steel and 0.04 in. for aluminum.

Fasteners used in capacitor–discharge welding are generally made from annealed C-1008 or C-1010 steels. Tensile strengths range from 40,000 to 50,000 psi. Nonferrous fasteners are made from magnesium-aluminum and silicon aluminum, and austenitic steel.

To get the most from this form of welding, the metal plate should be flat or nearly flat.

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This file type includes high resolution graphics and schematics when applicable.
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