Arc Stud Welding Process Explained

Arc Stud Welding Process Explained

Welding Material and Stud

(1) Welding Material

Metals easily welded by other arc welding methods are suitable for stud arc welding, including low carbon steel, low alloy high strength steel, stainless steel, and aluminum alloy. Table 7-2 provides the combinations of welding materials and stud materials.

Table 7-2 Combination of Welding Materials and Stud Materials for Stud Arc Welding

Welding MaterialStud Material
Low Carbon SteelLow Carbon Steel, Austenitic Stainless Steel
Austenitic Stainless SteelLow Carbon Steel, Austenitic Stainless Steel
Aluminum AlloyAluminum Alloy

The minimum thickness for stud welding is related to the diameter of the stud end. To maximize the strength of the fastener, prevent burn-through, and minimize distortion, it’s recommended that the thickness of the workpiece be no less than one-third of the stud diameter. When strength is not the primary requirement, the thickness should not be less than one-fifth of the stud end diameter.

The recommended minimum thickness of the workpiece for stud arc welding is listed in Table 7-3.

Table 7-3 Recommended Minimum Workpiece Thickness for Stud Arc Welding

Stud End Diameter
(Without Backing Plate)
Without Backing PlateWith Backing Plate
φ15. 93.8
φ19. 14.7
φ22. 26.4
φ25. 49.5

(2) Stud

The most commonly used stud materials in the industry are low carbon steel, high strength steel, stainless steel, and aluminum, with their minimum tensile strengths listed in Table 7-4.

The shape of the stud must allow the welding torch to grip and weld smoothly, its end diameter restricted by the thickness of the workpiece.

Table 7-4 Minimum Tensile Strength of Stud Arc Welding Joints

Stud MaterialStud End Diameter Range
Minimum Tensile Strength of Joint
Low Carbon Steelφ3 ~ φ32415
High Strength Steelφ3 ~ φ32830
Stainless Steelφ3 ~ φ19585
Aluminumφ6 ~ φ13275

Requirements for stud sizes in stud arc welding are as follows:

1) The stud length must be greater than 20mm for welding. The stud length should consist of the grip length, ceramic ring height, and welding allowance. The ceramic ring height inserted into the stud during welding is usually around 10mm, with a welding allowance of 3~5mm, and a grip length of 5~6mm.

The welding allowance refers to the difference between the original stud length and the distance from the workpiece surface to the stud end after welding, which is the shortening caused by welding melting, pool insertion, and plastic deformation during pressurization.

This length must be considered when designing the stud length, hence called the welding allowance. Table 7-5 lists the typical shortening of the stud during stud arc welding.

Table 7-5: Typical Shortening Amount of Studs during Arc Stud Welding

Stud Diameter/mm5 ~ 1216 ~22≥25
Length Shortening Amount/mm355 ~6

2) The stud diameter is usually greater than 6mm and less than 30mm, otherwise, the welding difficulty will increase and might even be impossible.

3) The stud end should be machined into a taper shape, with the taper degree defined by professional standards, and large deviations are allowed. Tapering is for facilitating short circuit arc initiation, concentrating the current lines during short circuit power-on, and enhancing the role of thermal ionization and thermal emission during the arc generation process.

For steel stud welding, a certain amount of flux is often placed at the center of the stud end (within about 2.5mm of the welding point) for deoxidation and arc stability. Figure 7-8 shows four methods of fixing the flux to the stud end, with Figure 7-8c being the most commonly used. For studs with a diameter less than 6mm, flux is not required unless there is a specific application.

Aluminum studs should have a pointed end to facilitate arc initiation, but no flux is needed. To prevent the weld metal from oxidizing and to stabilize the arc, inert gas protection is required. The bottom end of the stud to be welded is usually round, but square or rectangular shapes are also used. The aspect ratio of the rectangular stud end should not exceed 5.

Figure 7-8: Methods of Fixing Welding Flux at the End of Stud Welding

a) Encapsulated Particles
b) Coating
c) Inlaid Solid Flux
d) Casing Solid Flux

(3) Ceramic Ferrule

Stud arc welding generally uses ceramic ferrules, which are fitted onto the stud end to be welded before welding, and held in place by the chuck on the welding torch. The function of the ferrule is to concentrate the arc heat in the welding area, prevent air from entering the welding area, reduce the oxidation of the molten metal, confine the molten metal within the welding area, and shield the arc light.

There are two types of ferrules: disposable and semi-permanent. The former is used once and is often made of ceramic material, which is easy to break after welding. The latter can be reused a certain number of times and replaced before the welding quality becomes unacceptable.

The one-time use ferrule doesn’t need to slide out along the stud body after welding, so the shape of the stud is unrestricted, hence its wide application.

The ferrule is cylindrical, with its bottom matching the surface of the base metal to be welded and serrated, facilitating the venting of gas from the welding area. Its internal shape and size should be able to accommodate the fillet weld formed at the bottom end of the stud due to the extrusion of the molten metal.

The ferrule should match the stud size and ensure certain tolerance requirements. For a stud with a diameter of 22mm and a length of 200mm, the size and tolerance of the ferrule are shown in Figure 7-9. The ferrule should be kept dry and clean, and dried at 120°C before use.

Welding Parameters

The welding arc, as the heat source of arc stud welding, its power is determined by the product of arc voltage, welding current, and arcing time. The selection of welding parameters is the determination of these three parameters. During welding, the arc voltage is determined by the lifting height of the welding gun.

Once the lifting height is set, the arc energy is determined by the welding current and the welding time. For studs of a certain diameter, the welding current range is fixed, and the corresponding time parameters can only be selected within this range.

Since the same input energy can be obtained from different combinations of welding current and welding time, within a certain range, changing the welding time can possibly compensate for low or high welding current.

The relationship between current and time for welding low-carbon steel studs of various diameters is shown in Figure 7-10. For a given stud size, there is a fairly wide range of weldability, and the most suitable welding current and welding time must be selected within this range.

Figure 7-9: Matching of Ferrule and Stud Sizes
Figure 7-10: Relationship Between Current and Time in Low Carbon Steel Arc Stud Welding

In actual production, welding conditions are generally determined through testing based on the data recommended by the manufacturer in the equipment instruction manual. They can also be estimated using empirical formulas.

In the formula:

  • Welding current (A);
  • TW is the arc burning time (ms);
  • d is the stud diameter (mm).

When welding with carbon steel arc studs, pay attention to the carbon content and the thickness of the base material. Generally, preheating is not necessary for welding if wc≤0.3%. If the thickness of the base material is less than 3mm, preheating is not necessary even if wc>0.3%.

As the carbon content increases, to prevent welding cracks, it is necessary to preheat appropriately, especially for high carbon steel (wc≥0.45%), preheating and post-weld heat treatment are required, with a typical preheating temperature of 370℃, and heat treatment temperature of 650℃.

For low alloy high-strength steel, when wc>0.15%, preheating is also necessary to improve the toughness of the joint.

When welding stainless steel studs to low carbon steel base materials with stud welding, the chromium in the studs is diluted by the base material, causing the weld metal to harden, especially when the mass fraction of carbon in the base material is greater than 0.2%, it is more severe. At this time, stainless steel studs with higher chromium and nickel content should be used.

Aluminum alloy arc stud welding requires argon gas protection. Compared with steel stud welding, longer arcs, longer welding times, and lower welding currents are required when selecting parameters. Preheating and post-heating are both performed by local gas flame treatment, and only when large-area dense welding is performed is full-scale preheating and post-weld heat treatment performed.

Compared with conventional arc welding, because the heating and cooling rates of the welding area are both high in arc stud welding, the effect of the thermal cycle on the performance of the heat-affected zone is smaller. The structural steel used for shipbuilding and construction is tempered before leaving the factory, so preheating at 370℃ is required before stud welding.

Key Points of Welding Operation

(1) Surface Cleaning of the Weldment

The ends of the studs and the surface of the weldment should always be kept clean, free of paint, scale, oil and water stains, etc., and a small amount of rust is allowed.

(2) Positioning

The positioning method is determined based on the predetermined use and requirements of the stud. When high precision is required, it is recommended to use special positioning fixtures or fixed stud welding equipment.

With a hand-held stud welding gun, the simplest and most common positioning method is to draw lines and drill center holes on the workpiece with a template, and then place the tip of the stud at the center hole marking to position the stud. The positioning error of this method can be kept within ±1.2mm.

When there are many studs, positioning welding can be performed directly with the holes in the template, without pre-marking on the workpiece. During welding, the positioning can be done by putting the ring into the hole of the template. The ring material itself has manufacturing errors, so the positioning error of the stud is generally ±0.8mm.

(3) Implementing Welding

Steel stud welding is done with direct current electrode positive, and aluminum and its alloy stud welding is done with direct current electrode negative. Before welding, adjust the lifting amount of the welding gun, the length of the stud protruding from the ring, the welding current, and the arcing time in advance.

During welding, keep the welding gun vertical to the surface of the workpiece, do not move or shake the welding gun during the welding process, and do not lift the gun immediately after welding to prevent pulling up the stud (dewelding).

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