Spot welding resistance parameters

Resistance spot welding is a joining process where heat is generated by resistance to the flow of electric current through the parts to be joined, concentrated in a small area by copper electrodes. The fundamental parameters that determine weld quality are welding current, application time, force exerted by the electrodes, thickness and type of base material, and electrode geometry. Precise control of these parameters is essential to obtain a nugget of adequate size and satisfactory mechanical properties, avoiding defects such as spatter, porosity, or lack of fusion.

Process Parameters

Typical parameters vary significantly with material and sheet thickness. The following table provides indicative values for the most common combinations, including current, welding time, and electrode force. Water-cooled copper electrodes and a mains frequency of 50/60 Hz are assumed.

Material	Sheet Thickness	Welding Current	Welding Time	Electrode Force	Typical Spot Diameter
Low carbon steel (DC01)	0,8 mm / 0.031 in	8 – 10 kA	0,14 – 0,20 s (8 – 12 cycles at 60 Hz)	2000 – 2500 N / 450 – 562 lbf	4 – 5 mm / 0.157 – 0.197 in
Low carbon steel (DC01)	1,0 mm / 0.039 in	9 – 11 kA	0,18 – 0,24 s (11 – 14 cycles at 60 Hz)	2200 – 2800 N / 495 – 630 lbf	5 – 6 mm / 0.197 – 0.236 in
Low carbon steel (DC01)	1,5 mm / 0.059 in	10 – 13 kA	0,26 – 0,34 s (16 – 20 cycles at 60 Hz)	3000 – 4000 N / 674 – 899 lbf	6 – 7 mm / 0.236 – 0.276 in
Stainless steel AISI 304	0,8 mm / 0.031 in	6 – 8 kA	0,10 – 0,16 s (6 – 10 cycles at 60 Hz)	2500 – 3000 N / 562 – 674 lbf	4 – 5 mm / 0.157 – 0.197 in
Stainless steel AISI 304	1,2 mm / 0.047 in	7 – 9 kA	0,14 – 0,20 s (8 – 12 cycles at 60 Hz)	3500 – 4500 N / 787 – 1012 lbf	5 – 6 mm / 0.197 – 0.236 in
Aluminum AA 1050 (with oxide cleaning)	1,0 mm / 0.039 in	18 – 22 kA	0,08 – 0,12 s (5 – 7 cycles at 60 Hz)	3000 – 4000 N / 674 – 899 lbf	5 – 6 mm / 0.197 – 0.236 in

The electrical resistance at the sheet interface, responsible for the heat generated according to Joule’s law, also depends on the material and its surface condition. For dry and clean carbon steel, the typical contact resistance before thermal collapse is on the order of 100 – 200 µΩ, while for clean aluminum it can be less than 50 µΩ, which requires higher currents to achieve fusion.

Typical Tolerances

The dimensional tolerances of spot welding affect positioning, spot diameter, and penetration. The spot weld diameter for sheets from 0.5 to 3 mm is typically maintained within a variation of ±0.5 mm / ±0.020 in from the nominal value. Nugget penetration should reach between 30% and 70% of each sheet thickness, with an allowable scatter of ±10%. Regarding location, the minimum distance from the center of the spot to the sheet edge is 2 times the spot diameter; for example, for a 5 mm / 0.197 in spot, the distance to the edge must not be less than 10 mm / 0.394 in. The spacing between adjacent spots must not be less than 10 times the thickness of the thinnest sheet, meaning that for a 1 mm / 0.039 in sheet the minimum separation is 10 mm / 0.39 in.

Applicable Materials

Spot welding is applicable to a wide range of conductive metals. The most suitable materials are:

• Low carbon steel (carbon content < 0,15 %). It is the most favorable material, as it forms ductile nuggets with low tendency to cracking.
• Austenitic stainless steels (300 series) and ferritic. They are weldable with slightly reduced current parameters compared to carbon steel, although austenitic grades require higher electrode force due to their higher electrical resistance and lower thermal conductivity.
• Aluminum and its alloys. They require much higher power (currents ≥ 18 kA), atmosphere and oxide-free surfaces, which usually demands immediate chemical or mechanical cleaning before welding.
• Coated steels (galvanized, chrome-plated, nickel-plated). Weldability depends on the coating; nickel and chromium coatings weld relatively well, while tin and zinc coatings require special preparation to avoid electrode contamination. Galvanized sheets tend to alloy with copper electrodes, reducing their service life.

Dissimilar materials are not spot weldable under normal conditions due to differences in melting point, thermal conductivity, and electrical resistance.

Advantages

The spot welding process offers significant advantages in high-volume production: it uses no filler material or shielding gas, so operating costs are low. Welding time is extremely short, between 0.01 s and 0.63 s, allowing very fast work cycles. Energy is concentrated exclusively at the joint point, minimizing thermal distortion and overall energy consumption. It is easily automated and robotizable, being the predominant method in automobile body manufacturing. Additionally, multiple thicknesses can be welded simultaneously, as long as the thickness ratio does not exceed 3:1.

Limitations

The main limitation is that it is only applicable to lap joints, not to butt or angle configurations. Electrode access to the welding zone must be possible from both sides of the sheets, which restricts complex geometries or parts with closed cavities. The practical maximum thickness in steel is approximately 3 mm / 0.118 in, and although machines capable of welding up to 32 mm exist, the required currents and deformations make them scarcely viable for general use. Welding produces surface indentations and local hardening, which can compromise aesthetic finish and fatigue behavior. Copper electrodes suffer progressive wear and must be replaced or dressed periodically, especially when welding coated materials.

Parameter Selection Guide

The selection of welding parameters follows a sequence starting from the material and sheet thickness:

1. Determine the target spot diameter, typically between 3 mm / 0.118 in and 12.5 mm / 0.50 in, based on the thinnest thickness (approximately 5 times the thinnest sheet thickness).
2. Set the welding current according to material-specific tables. As a general rule, for carbon steel between 8 and 13 kA are required for thicknesses of 0.8 to 1.5 mm. For austenitic stainless steel, reduce the current by 15% to 25% compared to carbon steel of the same thickness.
3. Set the welding time that ensures complete fusion without overheating. Times range from 0.08 s to 0.63 s, being shorter for thin thicknesses and low-resistivity materials.
4. Adjust the electrode force to ensure uniform electrical contact and contain the molten metal. Typical values range from 1500 N to 5000 N (337 to 1124 lbf), increasing with material thickness and resistivity.
5. Select the electrode material and geometry, generally copper-chromium or copper-beryllium with a face diameter of 4 to 8 mm, depending on the desired spot size and material type (spherical tip for aluminum, flat or truncated cone for steel).

During setup, a sample is welded and the nugget is verified by peel test or macrography, adjusting current, time, and force if necessary.

Frequently Asked Questions (FAQ)

What thicknesses can be spot welded?

Spot welding is typically applied to sheets of 0.5 mm to 3 mm (0.020 to 0.118 in) thickness, although machines capable of welding up to 32 mm (1.25 in) with special power supplies exist.

What current is needed to weld 1 mm steel?

The recommended current for low carbon steel of 1 mm (0.039 in) thickness is between 9 kA and 11 kA, with welding times of 0.18 to 0.24 seconds.

What is the typical diameter of a spot weld?

The spot diameter varies between 3 mm and 12.5 mm (0.118 to 0.5 in); for 1 mm (0.039 in) sheets, spots of 5 to 6 mm (0.197 to 0.236 in) are obtained.

How long does a spot welding cycle last?

A complete cycle, including squeeze, welding, and hold, lasts between 150 ms and 800 ms, with the current flow phase limited to 10–630 ms (0.01–0.63 s).

What force do the electrodes exert during welding?

Electrode force varies between 1500 N and 5000 N (337 to 1124 lbf) for sheet thicknesses of 0.5 to 3 mm, being higher in materials with high electrical resistance such as stainless steels.

What is the minimum distance between spot welds?

The minimum spacing between centers of adjacent spots must be 10 times the thickness of the thinnest sheet; thus, in a 1 mm (0.039 in) sheet, the spacing is at least 10 mm (0.39 in).

References

• engineeringtoolbox.com: https://www.engineeringtoolbox.com/welding-temperatures-d_2186.html
• efunda.com: https://www.efunda.com/processes/metal_processing/welding.cfm
• manufacturingguide.com: https://www.manufacturingguide.com/en/motstandsbultsvetsning