Weldability study of dual phase and transformation induced plasticity automotive steels
Resistance spot welding studies at varying current with an 8 mm electrode on steel grades DP980, DP690, TRIP980 and TRIP780, established that peak strength with sound weld could be achieved at a current range of 9 to 10 kA, where the nugget diameter was between 6.9 and 7.5 mm. The joint eff...
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| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
University of Belgrade, Technical Faculty, Bor
2023-01-01
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| Series: | Journal of Mining and Metallurgy. Section B: Metallurgy |
| Subjects: | |
| Online Access: | https://doiserbia.nb.rs/img/doi/1450-5339/2023/1450-53392300014A.pdf |
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| Summary: | Resistance spot welding studies at varying current with an 8 mm electrode on
steel grades DP980, DP690, TRIP980 and TRIP780, established that peak
strength with sound weld could be achieved at a current range of 9 to 10 kA,
where the nugget diameter was between 6.9 and 7.5 mm. The joint efficiency,
measured as hardening ratio was highest in TRIP690 at 2.22 and for higher
strength grade it was between 1.37 to 1.51. The softening ratio in the HAZ
associated with tempering of preexisting martensite, was 0.86 to 0.9. Higher
fusion strength was associated with alloying content that increased
resistance that increased weld pool fusion that enhanced the nugget diameter
and hence the strength. The TRIP steel at high cooling rate showed
martensite and retained austenite and the DP steel showed martensite content
in higher grade and dispersed ferrite with martensite in the lower DP grade.
The Tensile shear strength was lowest (16kN) for DP780, while for other
grades it is between 20 to 22 kN. The strength enhancement in TRIP steel was
due to higher weld diameter at higher current, where partial pull out
failure mode was observed. At lower current, the interfacial pull out
failure mode was observed with poor fusion and at high currents partial pull
out at HAZ is observed. The Coach peel strength was highest at 3.2 kN for
DP780, 2.2 kN for DP980, 2 kN for TRIP 690 and 1.7kN for TRIP 980 and
follows the trend in TSS. The observed failure may be attributed to the high
fusion strength in TRIP steel due to sound nugget and a microstructure of
martensite with retained austenite. The lower strength with DP steels is
attributed to low alloyed lath martensite in DP 980 and the lower grade
DP780 is softer associated with ferrite dispersion in martensite. At very
high currents, the fusion is good but there is pull out at HAZ due to the
softening associated with the tempering of preexisting martensite that
influenced the failure at HAZ failure mode. |
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| ISSN: | 1450-5339 2217-7175 |