RSW Joint Performance Testing
Acceptable weld integrity criteria vary greatly among manufacturers and world regions. Each AHSS user needs to establish their own weld acceptance criteria and the characteristics of AHSS resistance spot welds. AHSS spot weld strength is higher than that of the mild steel for a given button size (See figure in Process Simulation). It is important to note that partial buttons (plugs) or interfacial fractures do not necessarily characterize a failed spot weld in AHSS. Interfacial fractures may be typical of smaller weld sizes in Mild steel or in all weld sizes in AHSS. Reference is made to the new Specification AWS/ANSI D8.1 Weld Quality Acceptance (Figures 1 through 4).
Figure 1: Load-bearing capacity of spot welds on various cold-rolled steelsL-2 (Steel type, grade, and any coatings are indicated on the bars.).
Figure 2: Representative minimum shear tension strength values for Group 3 steels.A-13
Figure 3: Representative minimum shear tension strength values for Group 4 steels.A-13
Figure 4: Minimum Cross-Tension Strength (CTS) values for Group 2, 3, and 4 steels.A-13
Resistance Spot Welding
Judging Weldability Using Carbon Equivalence (CE)
Existing Carbon Equivalent (CE) formulas for Resistance Spot Welding (RSW) of steels do not adequately predict weld performance in AHSS. Weld quality depends on variables such as thickness, strength, loading mode, and weld size. New formulae are proposed by various entities. Because there is no universally accepted formula, use of any one CE equation is not possible and users should develop their own CE equations based on their experience.
Part Fit-Up
Resistance welding depends on the interfacial resistance between two sheets. Good and consistent fit-up of parts is important to all resistance welding. Part fit-up is even more critical to the welding of AHSS due to increased YS and greater springback. In case of poor or inconsistent part fit-up, large truncated cone electrodes are recommended for both AHSS and conventional steels. The larger cap size will have large current range, which might compensate for the poor part fit-up. Also, progressive electrode force and upslope can be used to solve poor part fit-up.
Resistance Spot Welding
Although there are differences in weld process depending on weld tip material and shape (truncated cone and dome shape), AHSS can be welded with all weld tip shapes and materials. Dome-shaped electrodes ensure buttons even at lower currents due to higher current densities at the center of the dome shape (Figure 1). The curve of dome-shaped electrodes will help to decrease the effect of electrode misalignment. However, dome electrodes might have less electrode life on coated steels without frequent tip dressing. Due to round edges, the dome electrode will have fewer tendencies to have surface cracks when compared to truncated electrode.
Figure 1: Effect of electrode geometry on current range
using AC power mode and single pulse.L-2
Resistance Spot Welding
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
- Use of coated parameters recommended with the presence of a coating at any faying surface.
- These recommendations are based on available weld schedules representing recommendations from resistance welding equipment suppliers and users.
- For intermediate thicknesses parameters may be interpolated.
- 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)
- 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.
- Welding parameters are applicable when using electrode materials included in RWMA Classes 1 , 2, and 20.
- 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.
- Welding parameters are based on single-phase AC 60 Hz equipment.
- 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 2: Spot welding parameters for low-carbon steel >700 MPa (AHSS). A-14
- Use of coated parameters recommended with the presence of a coating at any faying surface.
- These recommendations are based on available weld schedules representing recommendations from resistance welding equipment suppliers and users.
- For intermediate thicknesses parameters may be interpolated.
- 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)
- 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.
- Welding parameters are applicable when using electrode materials included in RWMA Classes 1, 2, and 20.
- 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.
- Welding parameters are based on single-phase AC 60 Hz equipment.
- 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.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
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 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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Resistance Spot Welding
In general, if any type of AHSS [Dual-Phase (DP), Transformation-Induced Plasticity (TRIP), Complex Phase (CP), Ferrite Bainite (FB), or Martensitic (MS)] is used for the first time, the user should take the welding schedules applied to mild steel and then:
- Increase the electrode force by 20% or more depending on Yield Strength.
- Increase weld time as appropriate.
If these changes are insufficient, try these additional changes:
- A multi-pulse welding schedule (several pulses or post heating).
- Larger tip diameter and/or change the type of electrode.
- Increase the minimum weld size.
When resistance welded, AHSS require less current than conventional mild steel or HSLA because AHSS have higher electrical resistivity. Therefore, current levels for AHSS are not increased and may even need to be reduced depending on material chemical composition. However, most AHSS grades may require higher electrode forces for equivalent thickness of mild steels because electrode force depends on material strength. If thick mild steel or HSLA steel (of the same thickness) is replaced by an equivalent thickness of AHSS, the same forces may be required during assembly welding.
AHSS often have tighter weld windows (welding parameters that give acceptable welds) when compared to mild steels, as shown in the Figure 1.
Figure 1: Schematic weld lobes for AHSS, HSLA, and mild steel with a shift to lower currents with increased strength.
The current range (kA) for AHSS of 600-1400MPa during RSW is shown in Figures 2 and 3. The process window for Resistance Spot Welding of AHSS is influenced by the electrode force and welding time used in a major way. The current range increases by an average of 500 A for every additional 500 N of electrode force (Figure 2). The current range also increases by an average of 250 A for each additional 40 ms of welding time (Figure 3). Extra amounts of electrode force and welding time lead to increased current range, allowing for a wider process window.
Figure 2: RSW with AHSS, current range for varying electrode force (Cap Type B 16/6, 6-mm tip diameter, single pulse, 340-ms weld time, 250-ms hold time, plug failures.).I-6
Figure 3: RSW with AHSS, current range for varying weld time (Cap Type B 16/6, 6-mm tip diameter, single pulse, 3.5-kN electrode force, 250-ms hold time, plug failures.).O-1
A more extensive weld studyT-5 of three DP HDGA (45/45 g/m2) coated steels showed similar welding behavior for all three steels. The 1.6-mm-thick steels were DP 340/590, DP 420/780, and DP 550/980. To characterize the welding behavior of the steels, useful current ranges and static weld tensile tests were performed. The useful current range is the difference between the welding current required to produce a minimum button size (Imin) and the current that causes expulsion of weld metal (Imax). In this study, the 4√t as the minimum button diameter was used, where “t” is the nominal sheet thickness. This is generally used in the automotive and steel industries. The weld current range was 2.2 kA for the DP 340/590 and DP 420/780 and 2.5 kA for the DP 550/980 steel (Figure 4). These current ranges are sufficiently wide to weld successfully the DP steels. The study also found no weld imperfections, which means these three DP steels are weldable with simple, easy to use welding parameters.
Figure 4: Welding current ranges for 1.6-mm DP HDGA steels with minimum tensile strengths of 590, 780, and 980 MPa.T-5
Average reported weld hardness was 380 HV (Vickers Hardness) for the DP 340/590 and 415 HV for the other two. Again, all three DP steels had similar weld hardness distributions. The study also concluded that weld fracture mode alone is not a good indicator of weld integrity and performance. The load to fracture should be considered more important in judging weld integrity.
A second studyT-6 compared two 1.6-mm-thick HDGA (45/45 g/m2) steels: DP 420/700 and TRIP 420/700. The weld current range for 18 cycles weld time was similar: 1.4 kA for the DP 420/700 and 1.5 kA for the TRIP 420/700. The average weld hardness was 400 HV for both steels. The study concluded that acceptable welds with no imperfections can be produced in both steel grades. Both steel grades are readily weldable with easily adoptable welding parameters. Weld tensile strength differences between the two steels were small and not considered statistically significant.
Weld schedules (Figure 5) with pulsed current profiles for AHSS can have weld-current ranges similar to mild steel. Even though there is no increased tendency for weld expulsion with AHSS avoiding weld expulsion is highly desirable with AHSS. Loss of nugget material can affect weld-nugget size and strength.
Figure 5: Schematics of optimized weld schedules for AHSS.B-1
Post annealing (tempering pulse weld schedule) of TRIP steel may alter weld fracture mode and weld current range (Figure 6). However, since studies have shown that the occurrence of partial or IF fractures does not necessarily indicate poor weld quality, the use of pulsed current is not required to improve weld quality. Further, the effect of current pulsing on tensile and fatigue properties, as well as the electrode tip life, is not known. Therefore, users should perform their own evaluations regarding the suitability of such modified parameters.
Figure 6: Post annealing may enlarge weld current range.B-1
Additional work using Quench and Partition (Q&P) 980 showed less current required than conventional steels because it has higher electrical resistivity. Due to ultra-high base metal (BM) strength, it needs higher electrode force than conventional steels which have equivalent thickness. The weld lobe of 1.6-mm Q&P 980 is shown in Figure 7 with the pulsed weld time and force of 5.8 kN. The yellow zone of this figure shows the fracture mode of full button (FBF) when peel tested. Some pictures of these weld spots’ fracture mode are captured in Figure 8, for diameters of 6.0 to 7.7 mm.
Figure 7: Pulsed current profile and weld lobe of 1_6-mm Q&P 980.B-4
Figure 8: Fracture mode of weld spots in yellow zone.B-4
Hardness measurements and cross sections through the spot weld different zones can be identified as depicted in Figure 9. In a first step, the spot-welded joint can be subdivided into three zones: weld, Heat Affected Zone (HAZ), and BM. The weld is covered by the HAZ, where the melting temperature is not reached but high enough to change the microstructure. This region is dominated by inhomogeneous properties due to the different temperature and cooling gradients. Considering the hardness measurements of AHSS 3 sample, even a softening in the HAZ compared to the BM can be observed. Finally, the HAZ is surrounded by the BM, which does not show any local changes within the structure. These modifications of microstructure in the HAZ and weld are essential for the load-bearing capacity because the strength and ductility are drastically changed in comparison to the BM. Normally a high hardness is related to high strength and less ductility.
Figure 9: Hardness distribution through spot welds of various strength steels.P-7
Weld spot micrograph and microhardness of 1.6-mm Q&P 980 is shown in Figure 10, in which no weld defects, such as cracks, shrinkage void, pore, no fusion, deep indentation, etc. were found. Hardness testing is typically performed as shown in Figure 11 (diagonal traverse across the weld from BM of top coupon to BM of bottom coupon) using a suitable instrument for micro-indentation hardness testing (Vickers or Knoop).
Figure 10: Weld spot micrograph and microhardness of 1.6-mm DP 980.B-4
Figure 11: Typical cross-sectioned weld and hardness traverse.A-13
The different hardness values seen in a typical cross-sectioned weld depict different microstructural regions.P-8 Figure 12 shows the temperature distribution of a typical 2T weldment of hot stamped boron steel. At the solidified weld nugget, we see the highest temperatures and steadily decrease toward the unaffected base metal. The weld metal, coarse grain heat affected zone, fine grain heat affected zone, and unaffected base metal are made up of martensitic microstructure. The base metal has this microstructure due to the heat treatment (hot stamping) process. The weld metal, coarse grain heat affected zone, and fine grain heat affected zone are exposed to austenitizing temperature upon welding and are cooled rapidly reforming the martensite microstructure. The subcritical heat affected zone has a unique microstructure of over-tempered martensite. In this region, the peak temperature re below the Ac1, causing the base metal martensitic microstructure to decompose into ferrite and cementite. Micrographs of the different weld regions can be seen in Figure 13.
Figure 12: Temperature distribution of a typical 2T RSW of hot stamped boron steel.P-8
Figure 13: Different microstructures seen throughout the HAZ of a 2T hot stamped boron steel joint.P-8
