Page 35 - MetalForming July 2019

Higher Resistivity Means Less Welding Current Needed
AHSS grades, due to their relatively high alloy content, have higher resis- tivity 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 elec- trode force (20-percent higher, or more) to produce proper contact between the workpieces. And, in some cases, when compared to welding conven- tional steels, the fabricator may have to use larger-diameter electrodes and create larger welds. As a guide to min- imum nugget size, 5 x t1/2 (t, sheet thickness in mm) may be a better tar- get, compared to 4 x t1/2 for welds with conventional steels.
Another consideration when weld- ing 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 win- dow, fabricators can use higher elec- trode 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 equiva- lence (CE)—a high CE indicates a high probability for martensite formation.
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
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 eval- uations 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. Com- pounding this fact, the higher yield strengths of the material will tend to produce greater stresses concentrating
Fabrication: Welding Well
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(or graded) by the steel type and tensile strength (in MPa), and not by their composi- tion. For example, DP 500 has a tensile strength of 500 MPa. Further, steel-manufac- turing researchers are focused on a new fam- ily of steels, referred to as 3rd-generation AHSS, to increase duc- tility even more while maintaining high strength.
Fig. 3—Microhardness traverses of spot welds reveal much higher hardness values for AHSS compared to conventional high-strength low-alloy steels. Source: WorldAutoSteel
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 organi- zation used a CE formula called the Yurioka Equa- tion 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
Interfacial Fracture Interfacial Fracture With Full Interfacial Fracture With Button Pull Partial Thickness Fracture
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Fig. 4—Conventional peel and chisel testing of spot welds in AHSS sheet likely will pro- duce interfacial or partial interfacial failure modes, which can occur even when weld strength is acceptable for the intended application.
Hardness [HV0.1]