Stud Welding

Every day, millions of welding studs are joined with stud welding processes in many areas of the metalworking industry. The easy handling of the equipment technology, the large variety of standardized welding studs available (threaded studs, pins, studs with internal threads, and headed studs, etc.), and the use of welding elements according to customer standards contribute to the growing popularity of stud welding. Examples of this can be found in automotive engineering, where fir tree studs or paint clearing threaded studs are used in their millions. Furthermore, switch cabinet and equipment manufacturing often have welded earthing angles. In the construction industry, stud welding is used for fastening insulation.

The “stud to sheet” welded joint has decisive design and economic benefits compared with other welding and fastening methods such as:

• Gas shielded metal arc welding (MIG/MAG/WIG)
• Clinching / riveting,
• Gluing, or
• Screwing

How does stud welding work?

In stud welding, an arc is ignited between one end face of the stud and the workpiece. Both joining partners are melted and then joined under low contact pressure. The stud welding process usually takes less than 1s.

The ultimate goal is that the welded stud holds and that fracture occurs outside the weld zone when the stud is put under a load. To achieve this, the welding studs must meet a variety of different requirements:

1. The main task of the stud is to withstand the mechanical load and/or fulfil the function specified by the design.

2. The stud material and material surfaces must be suitable for welding.

3. The stud must achieve a required and sufficient welding quality.

4. Cost-efficient production (mass production via pressing/cold forming) and availability.

5. The stud should be easy to store and transport (e.g., protection by copper-plated surfaces, see also welding suitability of different materials below.)

6. The stud should be in conformity with technical standards, e.g., DIN EN ISO 13918 [1] or technical rules, e.g., those of the German Institute for Structural Engineering (DIBt).

7. The stud may have to meet additional customer requirements

Depending on the customer, the component, the material and the process requirements, welding studs with different geometric forms and materials can be used in the various stud welding processes, which is crucial for achieving the right quality. The optimum working ranges differ, among other things, in the diameter of the welding element, the materials and component surfaces used, the sheet thickness and working position, the customer's quality criteria and the process requirements (automation, quality and reproducibility, workshop or site conditions), etc.

Depending on the external stud geometry, the stud welding processes or the weld pool protection, different types of studs have been developed with different significance.

Weld pool protection

As commercially available welding elements for stud welding, in principle 3 types of studs are offered as standard types according to DIN EN ISO 13918 [1], which differ in geometry with regard to the stud function.

Rule of thumb:

• The welding process determines the welding geometry.
• The ignition geometry of the stud influences the process through its shape.
• The component function determines the external geometry.

For the actual welding process, the geometric shape outside the welding plane and the stud length are of no significance. The design of the required welding geometry or stud tip depends on the welding process used. The longer the welding time, the greater the melting volume and the more tapered the stud tip. For example, the ignition cone of welding elements for tip ignition is flatter than that of welding elements for drawn arc ignition.

Drawn arc stud welding - Auxiliary ceramic ferrule

Drawn arc stud types with a ceramic ferrule feature a pressed-in aluminum ball at the tip to more easily ignite the arc and deoxidize the weld pool. The ceramic ferrule is a decisive factor in drawn arc studwelding with ceramic ferrule. In accordance with the standard, ceramic ferrules must always be supplied by the supplier as a unit to match the stud. Ceramic ferrules may only be used in dry conditions. Damp ceramic ferrules lead to pores and thus to a deterioration of the welding result.

A ceramic ferrule is used

• for standard stud types, pins or headed studs larger than 6 mm in diameter (welding diameter),
• when welding in forced positions (overhead position or vertical wall), and
• when welding under construction site conditions.

Disadvantage: Welding with ceramic ferrules is not suitable for automated series production due to the handling, feeding and associated technical effort.

Welding elements for stud welding with tip ignition (CD) - ignition tip

There are good reasons that the ignition tip in tip ignition is called the "ignition tip", since its shape determines the arc ignition process and thus the procedure. It must not be damaged or deformed and has a narrow qualitative tolerance in terms of length and diameter in DIN EN ISO 13918. The ignition tip is therefore not a "centering tip". For positioning the stud on the component, the stud position should only be marked using scribe marks. Deep marking, especially the use of manual center punches, shortens the welding time. The use of automatic center punches is recommended to obtain uniformly pronounced (shallow) marks.

The ignition tip on studs for tip ignition is responsible for the ignition process and the welding time; it is not a centering tip for positioning.

CE marking of studs

Unfortunately, you can always find data or certificates that give the impression that studs are CE certified. Welding studs, in particular threaded studs, are regulated in DIN EN ISO 13918 [1]. Since this standard is not a harmonized standard, welding studs according to DIN EN ISO 13918 [1] may not be marketed with a CE mark. As proof of compliance of the geometric, mechanical and chemical properties with the requirements of the standard, these studs must be supplied with acceptance test certificate 3.1 to DIN EN 10204.

An exception is the CE marking of (welded) construction products according to DIN EN 1090 "Execution of steel structures and aluminum structures", e.g., "headed studs on plate". This CE marking means in principle that the conformity verification procedures defined for this construction product with regard to conformity with the technical specification on which the CE marking is based have been successfully undergone and that the construction is thus such that it can meet the essential requirements when properly designed and executed.

Note: For welding tasks that are carried out in the regulated (e.g., building inspection) area, only studs that are listed in and comply with the applicable standardization – DIN EN ISO 13918 - should be used. When using non-standardized, manufacturer-specific welding elements, the end customer or the certification authorities should always be consulted.

Common diameters are:

The stud length is limited by the following criteria:

Minimum stud length: The minimum stud length depends on 1) the required insertion depth for sufficient fixation of the stud in the stud holder, 2) the required projection for the required formation of the weld bead, and 3) the material- and diameter-dependent safety tolerance.

Minimum stud length: The maximum stud length is theoretically unlimited, but depends on the equipment required for welding. The leg assembly must be long enough to adequately support the stud and the gun must be able to move the (heavier) stud weight to match the welding task.

Ratio of stud length to stud diameter: Especially in applications requiring automatic separation and feeding, the ratio of stud diameter or flange diameter to stud length must not be 1:1 in order to avoid rotation of the stud during separation or feeding.

Concerning the selection of materials, the same rules apply as in conventional arc welding.

• Materials of the same type should always be welded.
• As regards unalloyed steels, the institute for standardization only mentions "4.8 - suitable for welding”. According to DIN EN ISO 898, 4.8 means a tensile strength of 420 N/mm2, yield strength 340N/mm² at 14% yield strength. The suitability of unalloyed or low-alloy steels for fusion welding is defined by the carbon content and the further chemical analysis. Due to the extremely rapid temperature rise and cooling processes (arcing time <1s), there is a risk of hardening due to embrittlement in steel. The carbon content of the materials to be welded (stud and base material) should therefore be C <0.17%.

When a weld has been executed in a careful, proper and professional manner, it can be assumed that the welded joint can withstand a greater static load than the stud or the component. When the load limit is exceeded, the fracture therefore occurs outside the weld zone – either in the stud or in the sheet base material.

Therefore, the characteristic values of stud and sheet are decisive for the strength calculation; the load-bearing capacity of the weld does not need to be taken into account in the calculation. The breaking force can thus be quickly calculated using the minimum tensile strength of the materials, see also notes on the calculation of stud welded joints in DVS 0967 [3]. When calculating stud welded joints, a distinction must be made depending on the application and the applicable regulations. A distinction is made between, among other things, static or dynamic stress, compression, tension, bending or torsion. The studs must therefore be designed in such a way that the serviceability and load-bearing safety of the entire component are ensured.

Welding studs manufactured on an industrial scale and in compliance with the applicable technical standards and rules are therefore by no means just a simple screw or piece of wire, but an important, decisive link for ensuring welding quality and function.

System solution "Process responsibility / quality = equipment technology + welding element"

The user should therefore obtain detailed technical information and advice regarding the technical possibilities and areas of application when using them. Especially the accompanying technical standards, such as DIN EN ISO 13918 "Welding - Studs and ceramic ferrules for arc stud welding", the standard designations for welding elements and technical criteria defined therein should form the basis between the customer (purchasing) and the producer / supplier in order to define the corresponding product and the applicable quality on the one hand, and to avoid possible problems due to incorrect information caused by manufacturer-specific designations on the other hand.