Stud welding: everything you need to know

What - how - on what - with what - 12 questions - 12 answers - the most important topic sections give you a quick, concise and easily understandable insight into the subject of stud welding.

You want to place a screw or bolt onto or through sheet metal and have no idea?
You want to save time and money when welding and don't want to see the welding spot?
-> "Stud Welding creates invisible connections".
Stud Welding? How does this actually work and what do I need for it?
What can be achieved with stud welding?
Don't know which method will best solve your problem?
Want to know how strong the welded joint is?
You don't want to invest a lot of time and effort in an elaborate welder's examination - stud welding can be performed by anyone - procedures and equipment technology are quick to learn and easy to master...
Time is money - but how long does the welding process actually take?
Weld studs? What dimensions and shapes are there for my application?

Looking for new ways in your production, need more effectiveness and efficiency in your production and require a high quality welded joint? We offer you solutions - welding processes - welding elements and equipment technology for your application.

Drawn Arc Stud Welding ("stud welding") is a particularly economical welding process for joining round-shaped metallic parts (studs / welding studs / welding elements) with metallic workpieces such as sheet metal, profiles and pipes. Every day, millions of welding studs are joined via stud welding processes in many areas of the metal industry. The simple handling of the equipment technology plays a significant role in the spread of stud welding. The decisive factors, however, are the design and economic advantages (see Table 1) for the connection "screw to sheet metal" compared to other welding and fastening methods such as:

• Metal inert gas welding process (MIG/MAG/WIG)
• Clinching / Riveting
• gluing

Stud welding is in many areas the most economical joining technique for fastening screw- or stud-shaped components and often the only technical solution for joining elements to thin sheet metal.

Stud welding can be used everywhere in metal construction and the metalworking industry and has proven itself for decades in welding threaded studs, internal threaded bushes or pins securely and cost-effectively to sheet metal - "Stud welding creates invisible connections".

Range of application extends from steel construction, bridge construction, composite construction and facade construction to power station, industrial furnace construction, general mechanical engineering and apparatus construction, vehicle construction and shipbuilding, boiler construction, tank construction and plant engineering to the household and electrical industries.

Areas of application in metal construction

- Enclosures, apparatus and switch cabinet construction
- Electrical industry
- Vending machines (beverage, food, cigarette vending machines)
- Household appliances, commercial kitchens, food industry, catering
- Laboratory and medical technology
- Fittings
- Facade engineering
- Insulation technology (climate insulation)
- "unmarked rear side"
- Vehicle construction
- Door, window and facade construction
- Steel construction
- Shipbuilding
- Thermal insulation

Definition

Stud welding refers to the joining of circular-shaped parts (studs) with flat workpieces by means of a welding arc and the application of a pressing force. The zones are joined in the liquid state of the welding zone. No filler material is used.

During stud welding, an arc is ignited between one end 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 a second.

Stud welding can be used for both round and rectangular cross-sections. In addition to the stud welding which is widely used and standardised in DIN EN ISO 13918, a large number of welding elements according to customer-specific or factory standards can be found in the various metal-processing industries.

• A special (welding) power source -> power unit The power source implements the control system for providing welding energy and coordinating the movement device
• A special movement device -> welding gun or welding head
• Welding current cables and connections
• Chuck for holding the stud
  
• Welding element -> stud
• Technical accessories (e.g. foot assembly for shielding gas)

Equipment technology and processes are easy to master

The process runs automatically. In this sense, there is no "welder" but an operator. The operator has no direct influence on the welding process, as is the case with manual electrode and MSG welding.

As with all joining and welding processes, the achievement of sufficient joining quality in arc stud welding is also influenced by a large number of parameters. In addition to a certain manual dexterity and training of the operator, it is therefore important that the process runs smoothly and that the welding task is taken into account.

An extensive range of stud welding equipment and accessories are offered to solve customer-specific welding tasks "stud to metal". The range extends from simple, manual equipment with stud welding guns to comprehensive CNC stud welding systems.

The best possible basis for ensuring the process and quality of the welded product is provided if the supplier can offer a system solution "process responsibility / quality = equipment technology + welding element". In case of questions or problems, there is only one contact point for the quality-influencing criteria "equipment technology" and "welding element".

However, the product liability of the manufacturer can only refer to the delivered devices and / or studs, but not to the quality of the welded product (studs to sheet metal). This is determined by many influencing factors during the welding process.

Depending on the heat input, various processes and process variants with different significance have developed. The different processes of drawn arc stud welding can be differentiated according to:

Type of arc ignition

• capacitor discharge stud welding
• drawn arc stud welding

Both processes differ in the ignition geometry of the bolts, the process sequence, the equipment technology and (partly) in the field of application. Both processes use direct current - but different energy sources, see Figure 2.

Type of energy source used

• Capacitor discharge
• Transformer / rectifier, inverter

the length of the welding time

• approx. 1 - 3 ms -> capacitor discharge
• approx. 5 - 100 ms -> short-cycle ignition
• > 100 ms -> drawn arc ignition

or the welding pool protection used

• without weld pool protection (NP - No Protection)
• with inert gas (SG - Shielding Gas)
• with ceramic ring (CF - Ceramic ferrule)

Depending on the customer, component, material and process requirements, different stud welding processes can be used and their decisive quality criteria can be used. The optimum working ranges of the different stud welding processes differ, among other things, in the diameter of the welding element, in the materials and component surfaces used, the sheet thickness and working position, the required connected load and the process requirements (automation, quality and reproducibility, workshop or construction site conditions), etc.

If the welding is carried out carefully, properly and professionally, it can be assumed that the welded joint can withstand a greater static load than the stud or component. The fracture occurs when the load limit is exceeded outside the welding zone in the stud or in the base sheet material.

Therefore, the characteristic values of stud and plate are decisive for the strength calculation; the load-bearing capacity of the weld does not have to be taken into account mathematically.

The breaking force can thus be calculated from the minimum tensile strength of the materials, see also Notes on the calculation of stud welds in DVS 0967.

When calculating stud welded joints, a distinction must be made depending on the case of application and the applicable rules and regulations. A distinction is made between static or dynamic loads, compression, tension, bending or torsion. The design of the studs must therefore be carried out in such a way that the serviceability and load safety of the entire component are guaranteed.

For bolts according to DIN EN ISO 13918, the characteristic values for calculation - the yield strength (Rp / fy,b,k) and the tensile strength (Rm / fu,b,k) - are specified in the applicable material tables.

The geometric characteristics outside the welding plane and the stud length are irrelevant for the welding process.

The design of the required welding geometry or stud tip depends on the welding process used. The longer the welding time and thus the melting volume, the more tapered the stud tip is. For example, the ignition cone of welding elements for tip ignition is flatter than that of welding elements for drawn arc ignition.

The characteristics outside the welding plane and the stud length are irrelevant for the welding process.

The design of the stud base geometry or stud tip depends on the welding process used. The longer the welding time and thus the melting volume, the more tapered the stud tip is. For example, the ignition cone of welding elements for tip ignition is flatter than that of welding elements for drawn arc ignition.

The stud designation then consists of the combination "code letter geometry" and "code letter method".

Thus, for example, a PT stud is a threaded stud (external thread) for tip ignition.

Note:

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

Stud types for drawn arc ignition with ceramic ring have a pressed-in aluminium ball at the tip in order to ignite the arc more easily and to deoxidise the weld pool.

A ceramic ring is used

• for studs larger than 12 mm diameter (welding diameter)
• when welding in constrained positions (vertical wall, overhead)
• when welding under construction site conditions
• Disadvantage: not suitable for automated series production

The ceramic ring must be selected to match the stud and is generally always delivered with the welding element.

Industry Specific / Customer Specific

In addition, there are a large number of non-standardized, industry-specific welding elements. The welding and functional surface geometry is designed according to the application. Examples of this can be found in automotive engineering, where millions of fastening elements with coarse threads or lacquer grooves are used, in switch cabinet and device construction with welded earthing angles or in power plant and plant construction with fastening elements for thermal insulation.

The pin length is limited by the following criteria:

Minimum stud length: The minimum stud length results from the required insertion depth for sufficient fixing of the stud in the stud holder plus the required projection for the required formation of the welding bead as well as a material and diameter-dependent safety tolerance.

Maximum stud length: The maximum stud length is theoretically unlimited, but depends on the equipment required for welding. The stand must be long enough to support the stud sufficiently and the gun must be able to move the (higher) stud weight according to the welding task.

Stud length to stud diameter ratio: Especially for applications that require automatic separation and feeding, the stud diameter or flange diameter to stud length ratio must not be 1:1 in order to avoid rotating the stud during separation or feeding.

In principle, the same rules apply to the materials as to conventional arc welding processes

• Always weld materials of the same type.
• Due to the extremely fast temperature rise and cooling processes (arc burning time < 1s), there is a risk of hardening of steel due to embrittlement. The carbon content C of the materials to be welded (stud and base material) should therefore be < 0.17%.

Compared to work piece handling or feeding times for the welding element, the actual welding process is very short.

The surface of the materials must be clean and metallically bright.

The electrical conductivity must be given attention in particular at the contact points - stud / work piece and ground connection / work piece.

Paint, rust, scale, grease or oil or other coatings unsuitable for welding (e.g. anodised) must be removed from the weld area (mechanically or chemically).

In particular, galvanized component surfaces must be checked for weldability.

With very short welding times of < 50 ms, the surface must be cleaned particularly carefully.

In the case of aluminium, existing oxide layers should be removed.

Rear side markings

Even with such short welding times, deformations or markings on stainless steel or aluminium up to a sheet thickness of 4 mm often cannot be avoided. Visibility is strongly dependent on the structure of the reverse side, especially on highly polished or lacquered sheets. In the sheet thickness range of 0.6 mm, the shrinkage of the weld pool always results in a thermally induced indentation.

Thermal rear side markings, such as tarnish points, do not usually occur in capacitor discharge ignition, but only in drawn arc ignition, where welding times are significantly longer. Characteristics and visibility depend, among other things, on the material thickness, the time energy input / stud diameter and the material.

The user's desire to reduce costs while simultaneously increasing product quality and process reproducibility quickly leads to the topic of automation.

In addition to the criteria listed in Table 8, it is necessary to define customer-specific characteristics in the specifications for an automation system, such as

• Stud dimensions and geometries
• Number of different studs (dimensions, material)
• Number of (different) work pieces (number, dimensions, heights)
• Traversing speeds, position and repeat accuracy
• Machine safety (housing, noise protection)
• Workplace requirements and ease of use

HBS - everything from a single source

HBS supplies a complete range - from manual welding guns with automatic stud feed and semi-automatic systems to fully automatic stud welding systems and robot applications; from technical advice to on-site service, from stud to complete system.

Request the HBS product overview "Automatic stud welding systems".

[1] DIN EN ISO 13918 (1998)
Schweißen - Bolzen und Keramikringe zum Lichtbogenbolzenschweißen

[2] DIN EN ISO 14555 (2007)
Schweißen Lichtbogenbolzenschweißen von metallischen Werkstoffen

[3] DIN EN ISO 14175 (alt: DIN EN 439)
Schweißzusätze – Schutzgase zum Lichtbogenschweißen und Schneiden

[4] Merkblatt DVS 0902
Lichtbogenbolzenschweißen mit Hubzündung
DVS-Verlag, Düsseldorf

[5] Merkblatt DVS 0903
Kondensatorentladungs-Bolzenschweißen mit Spitzenzündung
DVS-Verlag, Düsseldorf

[6] Merkblatt DVS 0904
Hinweise für die Praxis – Lichtbogenbolzenschweißen
DVS-Verlag, Düsseldorf

[7] Merkblatt DVS 0967
Berechnung von Bolzenschweißverbindungen
DVS-Verlag, Düsseldorf

[8] R. Trillmich; W. Welz
Bolzenschweißen, Grundlagen und Anwendung
DVS Fachbuchreihe Schweißtechnik, Band 133,
DVS-Verlag, Düsseldorf, 1997 / DVS Media GmbH, Düsseldorf, 2014

[9] N.N.
Firmenunterlagen
HBS Bolzenschweiss-Systeme GmbH & Co.KG, 85221 Dachau

[10] SFI aktuell. (2015). SFI aktuell, DVS Media GmbH, Düsseldorf

[11] Klier, R.; Nitschke-Pagel,Th; Parsi, A.
WEKA Media - Praxiswissen Schweißaufsicht