Choosing the Right Welding Method for Threaded Studs

Choosing the Right Welding Method for Threaded Studs

Selection of Stud Welding Methods

Arc stud welding, capacitor discharge stud welding, and short cycle stud welding each have their common and optimal application ranges.

The more challenging choice is among the three methods of capacitor discharge stud welding, namely pre-contact, gap reservation, and draw arc methods. The choice of welding method is based on the thickness of the workpiece, the material, and the size of the fastener.

1) For force-bearing joints with a stud diameter larger than 8mm, the arc stud welding method is suitable. Although arc stud welding can weld studs with a diameter of 3-30mm, when the stud diameter is less than 8mm, other methods such as capacitor discharge stud welding or short cycle stud welding are more appropriate.

2) The thickness δ of the workpiece and the stud diameter d have a certain proportional relationship. For arc stud welding, d/δ=3-4; for capacitor discharge stud welding and short cycle stud welding, this ratio can reach 8-10. Therefore, when the plate thickness is less than 3mm, it’s better to use capacitor discharge stud welding or short cycle stud welding, and arc stud welding is not recommended.

3) For carbon steel, stainless steel, and aluminum alloys, arc stud welding, capacitor discharge stud welding, and short cycle stud welding can all be used. However, for aluminum alloys, copper, coated thin steel plates, or welding of dissimilar metal materials, capacitor discharge stud welding is the best choice.

According to the above principles, if capacitor discharge stud welding is determined to be the best welding method, then the three welding methods within capacitor discharge stud welding have different application ranges and should be selected according to the following principles:

①Pre-contact welding is only suitable for mobile equipment, and is mainly used for welding carbon steel and welding carbon steel studs to coated steel plates;

②Gap reservation welding can be used with handheld or fixed equipment, used for welding carbon steel, stainless steel, and aluminum alloys. Like the draw arc method, this method can also weld dissimilar metal materials, and inert gas protection is not needed when welding aluminum;

③The draw arc method has the same materials and equipment as the gap reservation method, but the stud does not need a special protrusion. This method is most suitable for batch welding with an automatic feeding system, and inert gas protection is needed when welding aluminum;

④For thin plates with a thickness of less than 1.0mm, where the joint is required to have a protrusion on the back, the gap reservation and pre-contact welding methods should be used.

Inspection of Welding Quality

Inspection Methods for Stud Welding Joint Quality

The inspection methods for stud welding joint quality mainly include visual inspection, metallographic inspection, and mechanical performance testing.

(1) Visual Inspection

As arc stud welding is prone to the following problems:

① the stud is not inserted into the molten pool and is suspended;

② insufficient heat;

③ overheating;

④ magnetic bias blowing;

⑤ the stud is not perpendicular to the workpiece, etc.

Therefore, a visual inspection of the arc stud welding joint of steel studs must be carried out, mainly to inspect the continuity, uniformity, and fusion of the weld seam at the end of the stud, to determine whether the weld seam has defects.

Visual inspection of the joints of capacitor discharge stud welding and short cycle stud welding is almost meaningless, because the molten pool is very shallow, the joint is a plastic connection, and there is no recrystallized weld seam.

(2) Metallographic Inspection

For the metallographic structure, it is only necessary to perform a macroscopic joint metallographic structure analysis on the arc stud welding joint to check the fusion situation and defects such as cracks. For capacitor discharge and short cycle stud welding, it is not necessary to perform a metallographic inspection.

(3) Mechanical Performance Testing

Whether testing is needed is determined according to usage conditions. Mechanical performance testing should be performed on the process evaluation samples before welding production to determine the best welding process, and also random inspections should be conducted on the production site.

Mechanical performance testing methods include on-site hammering, on-site bending tests, joint tensile and torque tests. For capacitor discharge stud welding and short cycle stud welding, when welding non-load-bearing joints, generally only hammering and bending tests are performed.

In car manufacturing, there are about 150 studs of different sizes on Audi sedans, but they are all connection joints, not load-bearing, so general joint tensile and torque tests can be skipped, and only hammering and bending tests need to be performed. In some furnace or boiler manufacturing, studs are just used to fix the insulation material, so no mechanical tests are needed.

The bending test involves inserting a homemade socket into the stud of the joint for bending, with steel studs bent 75°, aluminum alloy studs bent 15°, and visually observing no cracking as acceptable. Hammering is also considered acceptable if no visible cracking occurs.

The torque test applies a predetermined load with a torque wrench to determine if the strength requirements have been met. Specific requirements should be determined according to product technical conditions or enterprise quality management standards.

Defects in Stud Welding and Their Correction Methods

Before production, the welding process should be evaluated, typically using the same process parameters and operating procedures as in production, and then conducting bending, twisting, or tensile tests on the test pieces.

A simple bending test can be performed by hammering the stud to bend it or using a piece of pipe to bend it, as shown in Figure 7-19.

Figure 7-19 Bending Test of Stud

According to the given bending angle value in the quality standard that should not result in damage, if the actual measured bending angle is less than the α value, the test piece is qualified.

For weldments with low bending resistance requirements, a bending angle within the range of 10°-15° can be evaluated as qualified. For visible defects, as shown in Figure 7-20, visual inspection is mainly used in field production to judge the welding quality.

Figure 7-20 Visible Defects in Arc Stud Welding Joint

a) Good weld formation
b) Not inserted
c) Not perpendicular
d) Insufficient pressure applied
e) Insufficient heat
f) Excessive heat

Refer to Table 7-10 for defects and their correction methods in arc stud welding.

Table 7-10 Defects and Correction Methods of Arc Stud Welding

Serial NumberVisual InspectionDestructive Inspection
General AppearancePossible CausesCorrection MethodsAppearance of DestructionPossible CausesCorrection Methods
1The weld seam is uniform, shiny, and intact, with the post-weld stud length within tolerance.
Correct Welding ParametersNo Correction NeededMaterial Tear
Correct Welding ParametersNo Correction Required
2The weld seam diameter has decreased, and the stud length is excessive.
Inappropriate immersion length for the weldment or the lifting height is too high.Increase the immersion size, verify the centering of the ceramic ring, check the lifting height, and reduce the welding current or time.The weld seam is damaged after appropriate deformation.
Correct Welding ParametersNo Correction Required
3The weld seam diameter has decreased and is irregular; the stud length is too long.
The welding parameters are too low and the protective ceramic ring is damp.Increase the welding current or duration, and dry the ceramic ring in the oven.The tear is within the weld seam.
The welding parameters are too low; the material is not suitable for stud welding.Increase the welding current or duration, and check the chemical composition of the material.
4The weld seam is asymmetrical at the raised area, resulting in undercutting.
This issue is due to the arc blow effect and incorrect ceramic ring centering.Refer to item number 3.The damage is in the heat-affected zone; the light gray damaged surface shows no appropriate deformation.
The parent material contains too much carbon, making it unsuitable for stud welding.Increase the welding duration; preheat as necessary.
5The reinforcement height of the weld seam has decreased, exhibiting a significant amount of lateral spatter. The stud length is too short after welding.
The welding parameters are too high, and the immersion speed into the workpiece is too fast.Reduce the welding current or duration, and adjust the immersion speed into the workpiece and the welding torch damper.There’s a disruption at the weld seam, exhibiting a metallic sheen.
The stud welding flux content is too high, and the welding time is too short.Verify the number of weld studs and increase the welding duration.
6The parent material exhibits a grid-like tear.
Non-metallic inclusions are present in the parent material, making it unsuitable for stud welding.Refer to relevant standards and choose a parent material with the best possible toughness.

Please refer to Table 7-11 for defects and their corrections in capacitor discharge stud welding.

Table 7-11: Defects of Capacitor Discharge Stud Welding and Their Correction Methods

Serial NumberVisual InspectionDestructive Inspection
General AppearancePossible CausesCorrection MethodsAppearance of DestructionPossible CausesCorrection Methods
1There are no visible defects around the small spatter of the welded joint.
Correct Welding ParametersNo Correction NeededMaterial Tear
Correct Welding ParametersNo Correction Required
2There is a gap between the flange and the parent material.
Inappropriate power, insufficient spring pressure, and unsuitable support for the parent metal.Increase the power, correct the spring pressure, and provide suitable support.The stud is damaged on the flange.
Correct Welding ParametersNo Correction Required
3Significant spatter around the weld seam.
The power is too high, while the spring pressure is low.Reduce the power and increase the spring pressure.Damage is located at the weld seam.
Inappropriate power, unsuitable pressure, and the stud/base material combination is disproportionate.Increase power, heighten pressure, and alter the stud or the base material.
4The welding spatter strays from the center.
The arc is blowing off course.Take corresponding measures.The backside of the workpiece deforms after welding.
The power is too high, the pressure is too large, the welding method is not suitable, and the base material is too thin.Decrease the power, reduce the pressure, use a welding method that includes a pre-set gap, and increase the thickness of the base material.
See Table 7-12 for defects of short-cycle stud welding and their correction methods.

Table 7-12: Defects of Short-Cycle Stud Welding and Their Correction Methods

Serial NumberVisual InspectionDestructive Inspection
General AppearancePossible CausesCorrection MethodsAppearance of DestructionPossible CausesCorrection Methods
1The reinforced weld seam is regular with no visible defects.
Correct Welding ParametersNo Correction NeededTorn from the base material.
Correct Welding ParametersNo Correction Required
2Localized weld seam.
The welding current is low or the duration is short, and the polarity is incorrect.Increase the welding current or duration, and adjust the polarity.The stud is damaged after appropriate deformation.
Correct Welding ParametersNo Correction Required
3There’s a large, irregular weld bead protrusion.
The welding duration is too long, resulting in an excessively high elevation of the stud.Shorten the welding time and maintain a distance of 1.2mm from the end face of the welding torch shield to the end face of the stud.The damaged area is within the heat-affected zone.
The parent material has too high a carbon content and is not suitable for stud welding.Verify the parent material.
4There are gas pockets within the reinforced weld seam.
The welding duration is too long, the welding current is too low, and the weld pool has oxidized.Shorten the welding time, increase the welding current, and provide appropriate shielding gas.The weld penetration is insufficient.
The heat input is too low, and the welding polarity is incorrect.Increase heat input and correct the welding polarity.
5The weld seam protrusion is asymmetrical.
Arc blow effect.Take appropriate measures.

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