Resistance Spot Welding of Automotive AHSS

June 27, 2019

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Compared to conventional steels, advanced high strength steels (AHSS) contain higher alloy additions with complex microstructures consisting of multiple phases. All AHSS grades rely on some combination of processing at high temperatures (thermomechanical processing) in the austenite phase field, followed by controlled cooling. The ability to combine alloying with thermomechanical processing allows steelmakers to customize microstructures, and, therefore, customize material properties to optimally combine strength and ductility. Further, AHSS grades attain additional strength by strain hardening.

Figs. 1 & 2—To increase the operating-process window when welding AHSS, fabricators can use higher electrode force (left) and/or longer weld time.

The numerous AHSS families offer strengths ranging from approximately 500 to more than 2000 MPa, and percentage elongations as high as 60 percent to less than 10 percent. Here, we address following AHSS grades: TRIP (transformation-induced plasticity); DP (dual phase); CP (complex phase); MS (martensitic); TWIP (twinning-induced plasticity); and HF (hot formed) steels. These steels typically are classified (or graded) by the steel type and tensile strength (in MPa), and not by their composition. For example, DP 500 has a tensile strength of 500 MPa. Further, steel-manufacturing researchers are focused on a new family of steels, referred to as 3rd-generation AHSS, to increase ductility even more while maintaining high strength.

Higher Resistivity Means Less Welding Current Needed

AHSS grades, due to their relatively high alloy content, have higher resistivity compared to conventional steels. As a result, they typically require lower weld current during spot welding to produce a weld nugget. In addition, the higher yield strength of AHSS grades requires the use of higher electrode force (20-percent higher, or more) to produce proper contact between the workpieces. And, in some cases, when compared to welding conventional steels, the fabricator may have to use larger-diameter electrodes and create larger welds. As a guide to minimum nugget size, 5 x t1/2 (t, sheet thickness in mm) may be a better target, compared to 4 x t1/2 for welds with conventional steels.

Another consideration when welding AHSS: The current range (or process window, a measure of process robustness) tends to be narrower than with conventional steels, making it somewhat difficult to minimize nugget size without expulsion. In order to increase the operating-process window, fabricators can use higher electrode force and/or longer weld times (Figs. 1 and 2).

Those looking for RSW parameter guidelines for AHSS can refer to the American Welding Society document, AWS C1.1—Recommended Practices for Resistance Welding. Data are based on the strength of the steel being welded.

Hardenability and Its Effect on Weld-Quality Evaluation

Again because of their high alloy content, AHSS grades tend to be much more hardenable (likely to form martensite) relative to conventional steels. To assess a steel’s hardenability, metallurgists refer to carbon equivalence (CE)—a high CE indicates a high probability for martensite formation.

Fig. 3—Microhardness traverses of spot welds reveal much higher hardness values for AHSS compared to conventional high-strength low-alloy steels. Source: WorldAutoSteel

Note: While many CE formulas find use for evaluating AHSS grades, their use and accuracy are not nearly as well established as they are for conventional steels. But regardless of the formula used, CEs for AHSS grades likely will exceed those for conventional steels, likely causing the formation of hard martensite during RSW.

For example, the WorldAutoSteel organization used a CE formula called the Yurioka Equation to determine the CEs of some TRIP and DP steels (in the range of 1000-MPa tensile strength). These steels approached CEs of 0.6 –extremely high when compared to that for high-strength low-alloy steels (Fig. 3).

While automakers have successfully resistance welded AHSS grades for the last few years, the high strength and hardness often will affect spot-weld failure modes during weld-quality evaluations using the typical peel and chisel testing methods. A well-established industry standard associated with peel testing of conventional steels states that an acceptable peel test “pulls a nugget” or a “full button.” However, with AHSS, full-button pulls are less likely due to the high CEs that likely will produce hard weld nuggets. Compounding this fact, the higher yield strengths of the material will tend to produce greater stresses concentrating at the edge of the nugget during a peel or chisel test.

Fig. 4—Conventional peel and chisel testing of spot welds in AHSS sheet likely will produce interfacial or partial interfacial failure modes, which can occur even when weld strength is acceptable for the intended application.

In the end, conventional peel and chisel testing of spot welds in AHSS sheet likely will produce interfacial or partial interfacial failure modes (Fig. 4). And, these failure modes can occur even when weld strength is acceptable for the intended application. Even full interfacial failures can exhibit high strength, although it may sometimes be challenging to differentiate between an interfacial failure and a “stuck” weld condition (which refers to an unfused bond of unacceptable strength).

Improving AHSS Welds

Fig. 5—To improve nugget failure modes for AHSS, by softening the martensite, fabricators add a temper cycle at the end of the RSW process.

To improve nugget failure modes for AHSS, the hard martensite must be softened. Adding a temper cycle at the end of the weld (Fig. 5) offers a simple and effective approach to accomplishing this. This requires including a sufficient amount of quench time prior to tempering, to allow the complete transformation to martensite; the amount of tempering time and current dictates how much softening occurs.

Of course, the quench time adds cycle time to each weld, so production requirements mandate that this time be kept as short as possible. In addition, there are other approaches to reduce the cooling rate, including current pulsing and current sloping, and use of longer weld times and shorter hold times. MF