RSW Parameter Guidelines

 

Parameter Guidelines

In summary, Tables 1 and 2 provide the AWS C1.1 Spot Welding Parameter Guidelines link to Recommended Practices for Resistance Welding. These general guidelines can be used to approximate which parameters can be used to begin the Resistance Spot Welding (RSW) process of a specific part thickness. From the recommended parameters, changes can be made on a specific stack-up to ensure an acceptable strength and nugget size for a particular application. Additionally, more complicated RSW parameter guidelines using a pulsation welding schedule with AC 60 Hz for welding AHSS is included in Table 3.

Table 1: Spot welding parameters for low-carbon steel 350-700 MPa (AHSS).A-14

Table 1: Spot welding parameters for low-carbon steel 350-700 MPa (AHSS).A-14

  1. Use of coated parameters recommended with the presence of a coating at any faying surface.
  2. These recommendations are based on available weld schedules representing recommendations from resistance welding equipment suppliers and users.
  3. For intermediate thicknesses parameters may be interpolated.
  4. Minimum weld button shear strength determined as follows:
    • ST = ((-8.83×10-7 × S2 + 1.34×10-3 × S + 1.514) × S × 4t1.5)/1000
    • ST = Shear Tension Strength (kN)
    • S = Base Metal Tensile Strength (MPa)
    • t = Material Thickness (mm)
  5. Metal thicknesses represent the actual thickness of the sheets being welded. In the case of welding two sheets of different thicknesses, use the welding parameters for the thinner sheet.
  6. Welding parameters are applicable when using electrode materials included in RWMA Classes 1 , 2, and 20.
  7. Electrode shapes listed include: A-pointed, B-domed, E-truncated, F-radiused. Figure 2 shows these shapes.
    • The use of Type-B geometry may require a reduction in current and may result in excessive indentation unless face is dressed to specified diameter.
    • The use of Type F geometry may require an increase in current.
  8. Welding parameters are based on single-phase AC 60 Hz equipment.
  9. Nugget diameters are listed as:
    • Minimum diameter that is recommended to be considered a satisfactory weld.
    • Initial aim setup nugget diameter that is recommended in setting up a weld station to produce nuggets that consistently surpass the satisfactory weld nugget diameter for a given number of production welds.

 

"Table

Table 2: Spot welding parameters for low-carbon steel >700 MPa (AHSS). A-14

  1. Use of coated parameters recommended with the presence of a coating at any faying surface.
  2. These recommendations are based on available weld schedules representing recommendations from resistance welding equipment suppliers and users.
  3. For intermediate thicknesses parameters may be interpolated.
  4. Minimum weld button shear strength determined as follows:
    • ST = ((-8.83×10-7 × S2 + 1.34×10-3 × S + 1.514) × S × 4t1.5)/1000
    • ST = Shear Tension Strength (kN)
    • S = Base Metal Tensile Strength (MPa)
    • t = Material Thickness (mm)
  5. Metal thicknesses represent the actual thickness of the sheets being welded. In the case of welding two sheets of different thicknesses, use the welding parameters for the thinner sheet.
  6. Welding parameters are applicable when using electrode materials included in RWMA Classes 1, 2, and 20.
  7. Electrode shapes listed include: A-pointed, B-domed, E-truncated, F-radiused. Figure 2 shows these shapes.
    • The use of Type-B geometry may require a reduction in current and may result in excessive indentation unless face is dressed to specified diameter.
    • The use of Type F geometry may require an increase in current.
  8. Welding parameters are based on single-phase AC 60 Hz equipment.
  9. Nugget diameters are listed as:
    • Minimum diameter that is recommended to be considered a satisfactory weld.
    • Initial aim setup nugget diameter that is recommended in setting up a weld station to produce nuggets that consistently surpass the satisfactory weld nugget diameter for a given number of production welds.

 

Table 3: AHSS bare-to-bare, bare-to-galvanized, Galvanized-to-galvanized RSW parameters for pulsating AC 60 Hz.

Table 3: AHSS bare-to-bare, bare-to-galvanized, Galvanized-to-galvanized RSW parameters for pulsating AC 60 Hz.A-14

 

 

Heat, Material and Thickness Balances

The heat input in Resistance Spot Welding (RSW) is defined as:

Heat Input = I2Rt

where: I is welding current
R is total resistance, and
t is weld time

The heat input must be changed depending on the gauge and grade of the steel. Compared to low strength steel at a particular gauge, the AHSS at the same gauge will need less current. Similarly, the thin gauge material needs less current than thick gauge. Controlling the heat input according to the gauge and grade is called heat balance in RSW.

For constant thickness, Table 1 shows steel classification based on strength level. With increasing group numbers, higher electrode force, longer weld time, and lower current are required for satisfactory RSW. Material combinations with one group difference can be welded with little or no changes in weld parameters. Difference of two or three groups may require special considerations in terms of electrode cap size, force, or type of power source.

 

Table 1: Steel classification for RSW purposes.A-11

Table 1: Steel classification for RSW purposes.A-11

 

For a particular steel grade, changes in thickness may require adoption of special schedules to control heat balance. When material type and gauge are varied together, specific weld schedules may need to be developed. Due to the higher resistivity of AHSS, the nugget growth occurs preferentially in AHSS. Electrode life on the AHSS-side may be reduced due to higher temperature on this side. In general, electrode life when welding AHSS may be similar to mild steel because of lower operating current requirement due to higher bulk resistivity in AHSS. This increase in electrode life may be offset in production due to poor part fit up created by higher AHSS springback. Frequent tip dressing will maintain the electrode tip shape and help achieve consistently acceptable quality welds.

 

 

Welding Current Mode

AHSS can be welded with power sources operated with either AC or DC types (Figure 1). Middle-Frequency Direct-Current (MFDC) has an advantage over conventional AC due to both unidirectional and continuous current. These characteristics assist in controlling and directing the heat generation at the interface. Current mode has no significant difference in weld quality. It should be noted that both AC and DC can easily produce acceptable welds where thickness ratios are less than 2:1. However, some advantage may be gained using DC where thickness ratios are over 2:1, but welding practices must be developed to optimize the advantages. It also has been observed that nugget sizes are statistically somewhat larger when using DC welding with the same secondary weld parameters than with AC. Some studies have shown that welding with MFDC provides improvements in heat balance and weld process robustness when there is a thickness differential in AHSS (as shown in Figure 2). DC power sources have been reported to provide better power factors and lower power consumption than AC power sources. Specifically, it has been reported that AC requires about 10% higher energy than DC to make the same size weld.L-7

 

 

Figure 1: Range for 1.4-mm DP 350/600 CR steel at different current modes with a single pulse.L-2

Figure 1: Range for 1.4-mm DP 350/600 CR steel at different current modes with a single pulse.L-2

 

Figure 2: Effect of current mode on dissimilar-thickness stack-upL-2

Figure 2: Effect of current mode on dissimilar-thickness stack-up.L-2

 

Consult safety requirements for your area when considering MFDC welding for manual weld gun applications. The primary feed to the transformers contains frequencies and voltages higher than for AC welding.

 

 

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