Studwelding is a fast, reliable and accurate method of attaching a metal fastener to a metal part using an electric arc.
Studwelding uses direct current electricity to create an arc between a fastener or stud and the parent material. The parent material or work piece can vary from thin sheet to thick plate. The arc melts the end of the stud and creates a molten pool in the parent material. The weld is completed when the stud is forged into the molten pool under pressure. Usually studs are externally threaded, plain pins (unthreaded) or internally threaded tapped bosses (pads). The equipment required depends on the diameter of the stud to be attached, the condition and thickness of the parent material and the type of material. Typical materials are mild steel, stainless steel, aluminium alloy, pure aluminium and lead free brass. The stud is held in a handtool or welding head which is used to initiate the weld and apply the forging pressure. The handtool and the parent material are connected to a power source (the controller). Studwelding can be used to weld studs from M2.5 to M24.
There are three types of studwelding: Drawn Arc (DA), Short Cycle (SC) and Capacitor Discharge (CD). DA is used for larger diameters whereas SC and CD are used for smaller diameters. The weld duration varies from a few milliseconds for CD to more than a second for large diameter DA. For DA and SC three phase mains power is transformed and rectified, for CD single phase power is used to store energy in capacitors. DA equipment is larger and less portable than SC equipment. The lightest and most portable equipment is CD. The DA process requires the use of a ceramic ferrule (arc shield), to contain and shape the weld fillet, this is not reusable. Weld spatter is evident with the SC process and this can be minimised using a shrouding gas. CD is by far the cleanest process with only a small witness of molten metal visible around the flange. The DA weld zone extends into the parent material to a depth of one third of the stud diameter, so plate thickness will determine where it can be used in order to avoid burn through. The SC weld zone enable sheets as thin as 1.5mm to be used whereas CD can be used on sheets as thin as 0.7mm.
The three types of studwelding are used in different circumstances.The DA process is the most tolerant of surface imperfections. As long as a pilot arc can be established then rust, mill scale, oil and dirt will not prevent a weld. DA studs have a fluxed weld end and a length allowance for burn off. A good weld is represented by an even fillet and the correct length reduction of the stud. The SC process is also tolerant of surface imperfections and does not require a fluxed stud nor a ferrule. The CD process is ideal for thin gauge sheet where reverse marking can be minimised or completely eliminated. For CD the sheet surface should be clean and flat and the weld stud must have a precise pip and cone angle on the weld end. CD studs and CD equipment are lowest in cost and the process is the fastest. Handheld CD Machines can weld up to 6 studs per minute and automation can increase this to over 20 studs per minute. CD is easy to jig as it does not require ferrules nor shrouding gas and consequently is the most accurate process. The short duration pulse of energy in CD welding imparts local heating only to the weld area thus avoiding heat distortion. The welds are clean and require no finishing. The molten metal of a stud weld creates complete fusion across the whole diameter area of the stud which makes the joint stronger than either the stud or the parent material. A destruction test demonstrates that either a slug of the parent material is pulled out (on thinner gauge sheet) or the stud breaks in the heat affected zone above the weld. Reliable, repeatable stud welds are achieved under consistent conditions of applied power and weld time.