Fasteners smaller than 3/32 in. in diameter and bolts or unthreaded pins up to 1¼ in. in diameter can be successfully welded. Fastener lengths vary from ¼ to 40 in., depending upon end use. Fasteners may be made of mild steel, stainless steel, alloy steel, aluminum, brass, bronze, or magnesium.
Welded stud fasteners may be used to replace studs normally secured by drilling and tapping, arc welding, resistance welding, or brazing. The studs may be placed, where required, without regard to clearances behind the plate, and may be secured to exposed surfaces after other assembly operations have been completed. The base metal must be weldable. Straight low-carbon steels or austenitic (300 Series) stainless steels, except free-machining grades, produce good weld results with normal techniques. Other steel alloys can be welded, but heat treatment may be required to develop full weld strength. Many aluminum, brass, and copper alloys can also be welded. Weld quality with special materials may have to be determined through testing of prototype samples.
The most common fasteners used in electric-arc welding are made from low-carbon steels, with a minimum tensile strength of 60,000 psi and a minimum yield strength of 50,000 psi. Special high-strength fasteners, comparable in strength to SAE grade 5 bolts, are also available. Capacitor-discharge fasteners are generally made from C-1008 to C-1010 steels in the annealed condition. Tensile strengths are 40,000 to 50,000 psi. Austenitic stainless steel, magnesium-aluminum, silicon-aluminum, and other nonferrous fasteners are made most commonly from materials in the as-rolled condition.
Fasteners for capacitor-discharge welding are most effectively used on flat or nearly flat surfaces. Since point contact is not needed for drawn-arc capacitor-discharge welding, fasteners can be welded to round or contoured surfaces. Electric-arc stud welding is adaptable to round or angled surfaces, since it depends on the ferrule for the formation of a relatively large pool of molten metal. The ferrule must be made to fit the contour in question.
Weld fasteners should be considered when:
- Assembly requires any resistance welding of parts, sections, or braces. A fastener can often be welded in place along with the other parts to speed up production and assembly.
- Fasteners must be mounted in a location where wrenching or assembly operations would be difficult or impossible with conventional fasteners.
- Assembly requires a threaded section of a sheet-metal part or plate member. Welding a nut in place to serve as a thread anchor is often more convenient, less costly, and faster than forming and tapping a hole.
- Hidden fasteners are required in blind locations. Weld fasteners, either screws or nuts, can be readily attached to the enclosed side of a sheet-metal or plate section.
- Loosening of a fastener under vibration or shock is a critical problem.
- Permanent fastener attachment is required to avoid loose parts that might fall into equipment or get lost in assembly.
- A hermetically sealed fastener is required. Use of sealing-type weld fasteners can eliminate the need for separate sealing elements.
Where clearance conditions or part shape indicate a need for special welding electrodes, consider reversing the fastener assembly. That is, if a weld screw with a conventional nut has been specified, try changing over to a weld nut and a conventional screw.
Through-hole fasteners eliminate the need for locating templates, jigs, and fixtures. Where through-hole screws or nuts are used, the diameter of the hole in which the fastener is located should be at least 0.010 to 0.015 in. larger than the fastener's major diameter. This clearance is generally considered optimum for handling ease and weld-spatter elimination. A smaller clearance will hinder positioning of the fastener. A larger clearance may lead to misalignment and faulty welds.
When multiple fasteners are required on a single assembly, standardizing on one size should be stressed. Although this standardization may lead to overdesign in some places, it usually is justified by the advantages gained in materials handling, assembly, production, and inventory simplification.